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Optical Isolator: Application to Photonic Integrated Circuits

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1 Optical Isolator: Application to Photonic Integrated Circuits
Tetsuya MIZUMOTO Dept. of Electrical and Electronic Eng. Tokyo Institute of Technology IEEE Photonics Soc. distinguished lecture T.Mizumoto

2 Outline Bulk optical isolator magneto-optic (Faraday) effect
operation principle Waveguide optical isolator TE-TM mode conversion isolator nonreciprocal loss (active) isolator nonreciprocal phase shift isolator integration (direct bonding) Non-magneto-optic approach Here is the outline of my talk. This course is composed of three parts. The first parts introduces you to the bulk nonreciprocal devices. That is, the lightwave propagates as a beam which is not confined in the transverse directions.As an important principle of operation, I describe the Faraday effect in a magneto-optic material. It is followed by the illustration of operational principle of isolators and circulators. In the second part, I will focus on the waveguide isolators. I address several types of waveguide isolators, which includes TE-TM mode conversion type, nonreciprocal loss isolator, interferometric isolator and semi-leaky waveguide isolator. Among them, I focus on the interferometric isolator, which uses nonreciprocal phase shift. I also address the integration of the isolator with active devices. In the third part, waveguide circulators are introduced to you. Basically, they can be a modification of waveguide isolator. And in the last part, non-magneto-optic approach for waveguide nonreciprocal device will be touched upon. IEEE Photonics Soc. distinguished lecture T.Mizumoto

3 What happens? Photon injection  photon-generated carrier
 disturbs carrier distribution (amplitude-noise)  carrier-induced index change (phase-noise) Isolator This slide explains what happens if there exists reflection external to the laser cavity. When lightwave is unintentionally injected into LD, it generates carriers in the cavity. This disturbs the carrier distribution, and hence, the amplitude noise is generated in LD output. Also, the carrier induces the change of refractive index inside of cavity. This causes the phase noise of LD output. In order to avoid these unwanted effects, it is essential to use an optical isolator for preventing the unintentional injection of light. IEEE Photonics Soc. distinguished lecture T.Mizumoto

4 Magneto-optic material
Requirement - large magneto-optic (MO) effect --> 1-st order MO effect (Faraday rotation) - low optical absorption - temperature insensitive Rare earth iron garnet (R3Fe5O12) Y3Fe5O12 (YIG) --> (Y3-xBix)Fe5O12, (Y3-xCex)Fe5O12 enhancement of Faraday rotation Before talking about the isolator and circulator operation, let me introduce the magneto-optic material. From the view point of practical application, the MO materials are required to have these characteristics. First of all, large magneto-optic effect is preferable to reduce the required length of MO part. The optical absorption of MO material results in the insertion loss of devices. Thus, the absorption must be minimized. Also, the temperature dependence of MO effect and absorption result in the temperature dependent characteristics. Rare earth iron garnets are the best among several candidates of MO materials. The composition of garnet is denoted by this expression, and the yittrium iron garnet, YIG, is the most popular. When a part of rare earth is substituted by Bi and Ce, the MO effect is enhanced. The crystal structure of rare earth iron garnet is shown in these figures. It is composed of 8 fundamental unit cells, one of which is shown here. IEEE Photonics Soc. distinguished lecture T.Mizumoto

5 Characteristics of Y3-xCexFe5O12 (Ce:YIG)
Just as an example of the wavelength dependence of MO effect, this figure shows the measured Faraday rotation of Ce-substituted YIG. In the fiber optic communication wavelength range, it has some thousands or tens thousands degree per cm. It is enhanced in accordance with the substitution level. If you look at the absorption, it also increases in accordance with the substitution level. Roughly speaking, Ce:YIG has the Faraday rotation of 7500 deg/cm and 4500 deg/cm at 1.31 and 1.55 um wavelengths. The absorption is less than 10 dB/cm at these wavelengths. Spectra of Faraday coefficient Spectra of optical absorption M. Gomi, et al., J. Appl. Phys., 70(11), (1991). IEEE Photonics Soc. distinguished lecture T.Mizumoto

6 Bulk isolator Bulk isolator, in either beam interface or fiber interface, uses rotation of polarization. Basic configuration Namiki Input and output : same polarization Now let’s turn to the explanation of bulk isolators. As I said, optical isolators are indispensable to protect optical active devices from unwanted reflected light. The isolator that is commercially available nowadays is basically composed of bulk optics, and uses the rotation of polarization for its operation. The essential function of nonreciprocity is provided by the Faraday rotation. It rotates the plane of polarization in different directions depending on the propagation direction. So, the lightwave propagating in the forward direction can pass the output polarizer, but the backward wave cannot. If you want to have an identical polarization in input and output, you have to add an additional rotator that provides 45 deg rotation irrespective of propagation direction. Then, both input and output polarizations become horizontal. Obviously, this type of isolator has a polarization dependent characteristic. That is, if a vertical polarization is incident in the forward direction, it is blocked. IEEE Photonics Soc. distinguished lecture T.Mizumoto

7 Bulk isolator birefringent plates polarization independent operation
Fiber in-line isolator --> Walk-off FDK Isolation>35dB, IL<0.6dB A polarization-independent operation can be achieved by adding birefringent crystals as is shown in this figure. In the forward direction, polarization components are rotated by 45 deg in a Faraday rotator and by another 45 deg in a half-wave plate. They are combined in a birefringent plate. In the backward direction, the rotations in the Faraday part and the half-wave plate are canceled. Two waves are refracted so that they are not directed to the aperture of fiber. In these bulk isolators, it is needed to align precisely-prepared optical elements carefully. This is a labor intensive work, and hence, the isolator becomes a costly component. Waveguide optical isolators are desired for reducing costs as well as for possible integration with optical active devices. Kyocera Isolation>30dB, IL<2.5dB T.Matsumoto (NTT), Trans. IECE, J62-C, (1979). IEEE Photonics Soc. distinguished lecture T.Mizumoto

8 TE-TM mode conversion type
Translate Faraday isolator into waveguide one. Faraday part Cotton-Mouton Mode selector Magnetooptic waveguide M qm TE-TM mode conversion Isolation:12.5 dB, l=1150 nm Length: 6.8 mm Even though you succeeded in fabricating a garnet waveguide, there remains a critical problem, i.e., the phase matching between two orthogonally polarized modes. I will explain this matter in detail by using a planar-type waveguide Faraday rotator. In the early stage of research, the waveguide version of bulk isolator shown here was studied. This isolator uses TE/TM mode conversion that is similar to the Faraday rotation in a bulk isolator. Compared with the Faraday bulk isolator, the waveguide version has difficulty. That is, in the bulk isolator, the plane of polarization rotates in a linearly polarized state. However, it does not in the waveguide, since the waveguide inherently has birefringence due to the waveguide configuration. In order to obtain a proper isolator operation, the phase matching must be taken between TE and TM modes. The phase matching issue causes the necessity for precise control of waveguide parameters for controlling the waveguide birefringence. K. Ando, T. Okoshi and N. Koshizuka (present AIST), Appl. Phys. Lett., 53(1), 4 (1988). IEEE Photonics Soc. distinguished lecture T.Mizumoto

9 TE-TM mode conversion Phase matched: d=bTE-bTM=0 Phase mismatched:
rotates in a linearly polarized state Phase mismatched: Faraday rotation in a birefringent medium When delta=0, that is, TE and TM modes are phase matched, they propagates in phase. So, they constitutes a linear polarization at an arbitrary position. On the other hand, when there exists a phase mismatch between TE and TM modes, there exists an out-of-phase component. Therefore, the synthesized polarization becomes elliptic. This means that, even though the principal axis of polarization rotates by 45deg, the undesired component remains, which is perpendicular to the desired one. So, even if the 45 degree component is blocked by a polarizer in the backward direction, the undesired component passes the polarizer. Thus, the perfect isolation can never be realized. That is why, we have to realize a phase matching between TE and TM modes to obtain a good isolator performance, if we use the rotation of polarization. Birefringence-free (phase matching) is essential to isolator operation. IEEE Photonics Soc. distinguished lecture T.Mizumoto

10 Waveguide isolators type mechanism mode conversion filed shift
guided TE / guided TM (Faraday & Cotton-Mouton) transversely radiated TE / guided TM (with TM nonreciprocal phase shift) guided TE / radiated TM (semi-leaky) nonreciprocal phase shift (interferometer) nonreciprocal loss (active) There have been proposed several types of optical isolators. This table summarizes waveguide optical isolators investigated so far. Basically, they are classified into two categories. The first one uses a mode conversion, while the other uses field displacement. The first one, which I explained in previous slides, uses the mode conversion between guided TE and TM modes. As I mentioned already, this configuration requires a precise phase matching between guided modes. Second type uses the conversion between a guided TM mode and transversely radiated TE modes. In the case of semi-leaky isolator, mode conversion occurs between guided TE and radiation TM modes in a wide range of waveguide parameters, since radiation mode has a continuous spectrum. In the second category, it is not necessary to care about the phase matching, since the device works in a single polarization. They are classified into two groups. The first group uses the nonreciprocal phase shift, while the second one uses the nonreciprocal loss. Details will be described in next slides. IEEE Photonics Soc. distinguished lecture T.Mizumoto

11 Mode conversion: transversely leaky mode
Nonreciprocal radiation (TM phase shift) Nonreciprocal radiation isolator uses conversion between guided TM- and transversely radiation TE-modes. This diagram shows the propagation constants of associated modes. Here, note that TM mode experiences the nonreciprocal phase shift. That is, the forward TM mode has a different propagation constant from backward TM mode. By proper setting of waveguide dimensions, the waveguide can be set so that the propagation constant of backward TM mode is below the transverse cutoff of TE mode. So, the backward TM mode couples to transversely radiation TE mode. Since the forward TM mode has a different propagation constant from backward wave, it can be set beyond the transverse cutoff of TE mode. Then, the device works as follows, In the forward direction, TM mode is guided in the waveguide without mode conversion. In the backward direction, TM mode can couple to radiation TE mode due to a magnetization component along the propagation direction, i.e. Faraday effect. Since the converted TE mode is radiated away, backward wave is not transmitted. The device can function as an isolator. Performance: - Isolation: 27 dB (l=1535 nm, L=4.1 mm) - wavelength sensitive (7 dB at l=1515 nm) T. Shintaku (NTT), Appl. Phys. Lett., 73(14), 1946 (1998). IEEE Photonics Soc. distinguished lecture T.Mizumoto

12 Semi-leaky isolator: operation principle
Anisotropy of LiNbO3 LiNbO3 mode conversion  reciprocal Magneto-optic mode conversion  nonreciprocal (changes its sign for F/B)  Semi-leaky waveguide TE mode guided Forward -k(Ce:YIG)+k(LiNbO3)=0 Backward k(Ce:YIG)+k(LiNbO3)≠0 TM mode  unidirectional mode conversion radiated Semi-leaky isolator is attractive; - relaxed fabrication tolerance - simple mono-section structure - easy control of magnetization As an another application of wafer bonding, we briefly explain a semi-leaky isolator. The operation principle of semi-leaky optical isolator is shown in this slide.There are two points to be mentioned. The first one is an anisotropy of LiNbO3, which produces a semi-leaky propagation characteristic. By arranging the crystal orientation as is shown here, TE mode experiences an extraordinary refractive index which is smaller than the index of guiding layer. On the other hand, TM modes are influenced by the ordinary index. The ordinary index of LiNbO3 is higher than the guiding layer index. Thus, TE mode is confined in the guiding layer, while TM mode is radiated into LiNbO3 cladding region. This is the semi-leaky guiding characteristic. The second point is a mode conversion. There are two kinds of mode conversion. A magneto-optic mode conversion is produced by Ce:YIG guiding layer. It is a nonreciprocal conversion. That is, it changes its sign depending on the propagation direction. Another kind of mode conversion is provided by LiNbO3. When lightwave propagates with an offset angle with respect to the LiNbO3 crystal axis, mode conversion occurs between TE and TM modes. Let me remind you that this is a reciprocal conversion. Therefore, by choosing the offset angle properly, it is possible to cancel two kinds of mode conversion in the forward direction. In the backward direction, a certain amount of mode conversion remains because of the nonreciprocal nature of magneto-optic mode conversion. So, TE mode propagating in the forward direction is truly guided, while the reflected TE mode is converted into TM mode and is radiated into the cladding region. Thus, the device works as an isolator. Since the conversion occurs between guided and radiated modes, large fabrication tolerance is expected. Moreover, this isolator has the advantage of simple structure. However, there remains one problem, i.e., uniform and tight bonding between LiNbO3 cladding and magneto-optic guiding layer. Our approach to overcome this is to use wafer direct bonding. - but, uniform and tight LiNbO3 / garnet contact is needed. S.Yamamoto, et al (Osaka U.), IEEE QE, 12, 764 (1976).  direct bonding IEEE Photonics Soc. distinguished lecture T.Mizumoto

13 Nonreciprocal loss (active) isolator
Active group: U.Tokyo, AIST, Ghent U. Contrary to these isolators, so called active isolators was proposed by the group of University of Tokyo and AIST in Japan. Its performance has been demonstrated by the group of Univ. Tokyo. This type of isolator uses nonreciprocal loss compensation configuration. That is, due to the large magneto-optic effect of ferro-magnetic material, the nonreciprocal displacement of optical field is provided. That is, the field penetrates into ferro-magnetic material more in the backward wave. This brings about remarkable difference in optical attenuation between forward and backward propagating waves. The loss experienced by the forward propagating wave is compensated for by the gain of semiconductor optical amplifier. The remaining loss for the backward wave provides the optical isolation. The group of university of Tokyo reported an isolation of 14.7dB/mm for TE mode as is shown here. Isolation: 14.7 dB/mm Insertion loss: 14.1 dB/mm (I=150 mA) H.Shimizu and Y.Nakano (U.Tokyo), JLT, 24, (2006). IEEE Photonics Soc. distinguished lecture T.Mizumoto

14 Integration with active devices
compatible waveguide structure material & dimensions nonreciprocal loss (active) excellent compatibility to active devices 4 dB isolation at l= nm 4 dB 15OC active isolator 0.7 mm DFB LD 0.3 mm 90mA 150mA Also, it has been successfully demonstrated that this isolator can be integrated with LD. This slide explains an excellent demonstration done by the group of Univ. Tokyo. They fabricated a DFB LD integrated with an active isolator. It is shown that the isolator exhibits an isolation of 4dB from the observation that the LD output changes by 4dB when reversing the direction of applied magnetic field. H. Shimizu and Y. Nakano (U.Tokyo), IEEE PTL, 19, (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

15 Comparison: passive and active isolators
type Passive Active Integration type dependent excellent Noise none ASE Power consumption current injection to SOA Polarization dependence yes, but can be overcome yes The comparison between such an active isolator and a conventional passive isolator is summarized in this table. As for the integration with optical active devices, the active isolator has the great advantage of waveguide compatibility. The isolator itself has the structure of active device. When we think about the noise performance, the passive isolator generates no noise, while the active one could give an ASE noise due to optical amplification. For power consumption, the passive isolator requires completely none. On the other hand, the active isolator requires the current injection to compensate the large optical absorption of ferro magneto-optic material. IEEE Photonics Soc. distinguished lecture T.Mizumoto

16 Waveguide isolator: nonreciprocal phase shift
Interferometer type - Isolation: 19 dB (l=1540 nm, L=8.0 mm) J. Fujita, M. Levy and M. Osgood, Jr. (U.Columbia), Appl. Phys. Lett., 76(16), 2158 (2000). - Isolation: 25 dB (l=1600 nm, L=4.0 mm) Y. Shoji and T. Mizumoto (Tokyo Tech), Optics Express, 15, (2007). - wavelength insensitive designed to cover both 1.31/1.55 mm in a single chip Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007). - polarization independent not by polarization diversity scheme Y. Shoji and T. Mizumoto (Tokyo Tech.) et al, JLT, 25(10), (2007). This slide shows a typical configuration of interferometric isolator. Basically it is a kind of Mach-Zehnder interferometer. When we apply a magnetic field transversely to the light propagation direction in the film plane, it generates the nonreciprocal phase shift in TM mode. By combining this effect with Mach-Zehnder interferometer, the isolator operation is obtainable. The performance of this isolator is demonstrated by the group of Columbia University. Recently, we demonstrated that the device works in a wide wavelength range with large isolation. The reported isolation is up to 25dB. IEEE Photonics Soc. distinguished lecture T.Mizumoto

17 Interferometric isolator: operation principle
Single polarization operation → No need for phase matching → Fabrication tolerant Simple in-plane magnetization This figure illustrates how the interferometric isolator works. The input light is divided into two waves with equal amplitude at tapered coupler. The two waves experience nonreciprocal phase shift with opposite sign. This results in -90deg phase difference between two arms of interferometer. This phase difference is canceled by 90 deg reciprocal phase shift, which is provided by an optical path difference of a quarter wavelength. At the output coupler, two waves are combined in-phase and are coupled out from the central waveguide. In the reverse direction, the nonreciprocal phase shift changes its sign, and thus, destructive interference occurs at the left coupler. Due to the symmetric structure of the coupler, the output from the central port is prohibited. Therefore, the input port is isolated from a reflected light. The phase matching between orthogonal polarized modes is not necessary in this isolator. As I mentioned previously, this brings about the advantage of relaxed fabrication tolerance compared with the mode conversion isolator. IEEE Photonics Soc. distinguished lecture T.Mizumoto

18 Nonreciprocal phase shift
y z x 1st–order MO effect linear in b The nonreciprocal phase shift plays an essential role in the nonreciprocal performance of the interferometric isolator. This slide explains the nonreciprocal phase shift that the lightwave propagating in the magneto-optic waveguide experiences. When the waveguide is composed of a magneto-optic guiding layer, whose magnetization is aligned in a film plane transversely to the light propagation direction, the characteristic equation of TM mode is given by this equation. A linear term with respect to the propagation constant appears in this equation. Therefore, if you change the sign of beta, i.e, whether it propagates in positive direction or negative direction, the absolute value of propagation constant varies. The difference between beta+ and beta- is defined as a nonreciprocal phase shift per unit distance. Nonreciprocal phase shift = (b+-b-) (m-1) IEEE Photonics Soc. distinguished lecture T.Mizumoto

19 Nonreciprocal phase shift
Nonreciprocal phase shift = (b+-b-) (m-1) 0.2 0.4 0.6 0.8 1 1.0 2.0 Thickness of Ce:YIG guiding layer [mm] NPS/(p/2) [mm-1] l=1550nm TM0 mode d (CeY)3Fe5O12 SGGG (n=1.94) SiO2 (n=1.45) cutoff Physical understanding of this effect is given by the explanation that the field penetration into cladding regions varies depending on the propagation direction. So, the nonreciprocal phase shift depends on the thickness of guiding layer. This figure shows the calculated nonreciprocal phase shift as a function of the guiding layer thickness. Ce-substituted yttrium iron garnet is assumed as a guiding layer. This magneto-optic material has a Faraday rotation as large as -4500deg/cm at 1550nm wavelength. As a top cladding layer, we deposit SiO2, and we assume the garnet substrate as an under-cladding layer. The nonreciprocal phase shift takes a maximum value when the waveguide is close to cutoff. IEEE Photonics Soc. distinguished lecture T.Mizumoto

20 Interferometric isolator: calculated performance
1.45 1.5 1.55 1.6 1.65 0.1 0.2 0.3 0.4 0.5 Wavelength (mm) Forward loss (dB) 1.45 1.5 1.55 1.6 1.65 10 20 30 40 50 Wavelength (mm) Backward loss (dB) This slide shows the typical characteristics of interferometric isolator. The forward and backward losses are calculated for the device shown here. It consists of a Ce:YIG guiding layer grown on a substituted GGG substrate. Although it is not shown explicitly in this figure, the over cladding layer is composed of SiO2. You can see that the theoretical insertion loss is below 0.1 dB in the wavelength range shown here. Let me remind you that the material losses are ignored in the calculation. Also, the backward loss > 20 dB is obtainable in the wavelength range between 1.51 and 1.59 um. If you need 30 dB isolation, the bandwidth is rather limited into +/- 15 nm around 1.55 um. Although you might think that the operation bandwidth is rather limited, it is quite wide compared with the TE/TM mode conversion isolator. IEEE Photonics Soc. distinguished lecture T.Mizumoto

21 Interferometric isolator: wideband operation
dependences : MO effect waveguide dispersion wideband design Phase shift q R l N (backward) (forward) q (backward) p p/2 -p/2 3p/2 2p q (forward) Conventional design Phase shift q R l N (backward) (forward) q (backward) p p/2 -p/2 q (forward) From a practical view point, it is highly desired for the isolator to operate in a wide wavelength range. The wavelength dependence of the interferometric isolator is mainly determined by two factors, the wavelength dependence of the nonreciprocal phase shift and that of the reciprocal phase shift. The former is mainly governed by the wavelength dependence of Faraday rotation. which is reduced at longer wavelength region. The latter is determined by the dispersion of waveguide. In the conventional design, the wavelength dependence of nonreciprocal phase shift is superposed to that of reciprocal one. We proposed to cancel two wavelength dependences by introducing a proper design. That is, by introducing 3pai/2 phase bias instead of pai/2 and reversing the sign of nonreciprocal phase shift, the wavelength dependence of the nonreciprocal phase shift is canceled by that of reciprocal one. Cancellation of wavelength dependences in backward propagation Y.Shoji and T.Mizumoto (Tokyo Tech.), Appl. Opt., 45, 7144 (2006). IEEE Photonics Soc. distinguished lecture T.Mizumoto

22 Wideband design: experimental results
measured with a reference of  straight waveguide (5dB loss) Wideband design Conventional design 1500 1550 1600 1650 30 20 10 Wavelength (nm) attenuation (dB) forward backward 1500 1550 1600 1650 30 20 10 Wavelength (nm) attenuation (dB) forward backward The slide shows the experimental results on the isolator performance for the broad-band and conventional design. The isolator consists of a Ce:YIG guiding layer. Both the isolators are fabricated on the same chip. As is shown clearly, the broad-band design gives larger isolation in wider wavelength range. Larger isolation in wider wavelength range Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

23 Ultra-wideband design
1.3 1.4 1.5 1.6 50 40 30 20 10 Wavelength [mm] attenuation [dB] 1.55 mm Forward mm Forward 1.55 mm Backward mm Backward Wideband design covers fully 1310 nm / 1550 nm bands and more. Isolation > 40 dB : @ nm Further broadening of operation band width is realizable with a proper design of reciprocal phase shifter. That is, by adjusting the waveguide width and length properly, we can cancel the wavelength dependence of Faraday rotation by that of reciprocal phase difference between two arms. As a result, the isolator that covers both 1.31 and 1.55 um wavelength range with an isolation over 40dB in a single chip. Such a ultra wideband design is never possible in a bulk isolator. This successful design is due to that we have additional freedom of controlling waveguide dispersion characteristics. Y. Shoji and T. Mizumoto (Tokyo Tech.), Optics Express, 15, 639 (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

24 Photonic integrated circuit: device and material
LD, SOA III-V semiconductor modulator, SW LiNbO3, III-V semiconductor l-MUX/DeMUX Silica Isolator Magneto-optic material I would like to remind you that there are several key materials to realize a specific function in photonic devices. Obviously, III-V compound semiconductors are essential for optical active devices like LD and amplifiers. As for high-speed modulator or switch, EO effect of LiNbO3 or elctrco-absorption of III-V is used. For passive devices, silica is the best material because of its low absorption. For nonreciprocal functions, magneto-optic effect is essential and MO garnet is the best, since it has high MO effect with low absorption in a fiber optic communication wavelength range. Another thing, I have to point out is the alignment. Since optical waveguide components used in photonic integrated circuit are structured with micron - submicron waveguide dimensions, alignment of these waveguides is a really hard task. We have to use a lithography process for making the waveguides. When you prepare individual components and try to mount them on a common platform, alignment becomes a critical issue. It is really a hard task because of its small dimensions both in horizontal and vertical directions. photonic integrated circuit waveguide alignment  lithography process materials  to be grown (deposited) on a common platform IEEE Photonics Soc. distinguished lecture T.Mizumoto

25 Our approach: integration of isolator and LD
compatible waveguide structure material & dimensions Single polarization operation Direct bonding LD integrated with isolator This slides illustrates our idea to integrate an isolator with a laser diode. By using a crystal growth technique, like a two step growth or a selective growth technique, we can prepare the guiding layer of isolator together with the active layer of laser diode on a single substrate. This guarantees a vertical waveguide alignment between two devices. The in-plane alignment is achieved by using any kind of lithography process. In order to obtain nonreciprocal function in this device, a magneto-optic garnet is directly bonded as a top cladding layer. Common semiconductor guiding layer (selective growth & mask process) H. Yokoi and T.Mizumoto (Tokyo Tech.), Electron. Lett., 33, 1787 (1997). IEEE Photonics Soc. distinguished lecture T.Mizumoto

26 III-V waveguide isolator
When we consider to integrate an isolator with optical active devices, the waveguide of isolator must be compatible with that of active devices in terms of material and dimensions. Since the optical active devices are constructed with III-V semiconductors, it is essential to use a semiconductor waveguide in the isolator. The nonreciprocal phase shift is obtained even in the structure, where the magneto-optic garnet is used as a top cladding layer and the guiding layer is composed of III-V. This structure is obviously suitable for integrating the device with optical active devices. IEEE Photonics Soc. distinguished lecture T.Mizumoto

27 Nonreciprocal phase shift
y z x Nonreciprocal phase shift = (b+-b-) (m-1) 1st–order MO effect qF=-4500deg/cm linear in b The nonreciprocal phase shift is provided, even if the magneto-optic material is used in a upper cladding layer instead of a guiding layer. The characteristic equation for TM mode propagating in this layered structure is given here. Linear terms with respect to the propagation constant appear in this equation. Therefore, the absolute value of propagation constant varies whether it propagates in positive direction or negative direction. The nonreciprocal phase shift is defined by this equation. This figure shows the calculated nonreciprocal phase shift as a function of the guiding layer thickness. The guiding layer is assumed to be GaInAsP whose bandgap wavelength is 1250nm or 1410nm. The magneto-optic material loaded as a top cladding layer is a Ce-substituted yttrium iron garnet, which has a Faraday rotation as large as -4500deg/cm at 1550nm wavelength. The nonreciprocal phase shift takes a maximum value when the waveguide is close to cutoff. By comparing two guiding layers, larger nonreciprocal phase shift is obtainable for the guiding layer having higher refractive index. IEEE Photonics Soc. distinguished lecture T.Mizumoto

28 Bonding garnet on III-V
InP GaInAsP n(garnet) < n(III-V)  Evanescent field is to be used in MO garnet. direct bonding with no gap in-between III-V MO garnet crystal structure zinc blende garnet lattice constant (A) 5.869 (InP) 12.54 thermal expansion (K-1) 4.56 X 10-6 (InP) 9.20 X 10-6 refractive index 3.2 – 3.5 2.2 Since the refractive index of garnet is lower than III-V, we are forced to use an evanescent field penetrating into the garnet. It is required to achieve a sufficient contact between garnet and III-V with no gap in-between. As you know, epitaxial growth of heterogeneous crystals is usually difficult due to mismatch of physical properties. When we think about the case of III-V and garnet, they are completely different as shown in this table. In spite of the challenging works done by Dr. Razeghi and Dr. Heisma, the growth of garnet on III-V or vice vesa is still a tough work. Also, we tried to make a crystal growth of III-V on garnet, and vice versa, but, unfortunately, we failed. In stead, We developed the direct bonding of garnet onto III-V. Challenging: epitaxial growth of III-V on garnet done by Dr. M. Razeghi (Thomson), JAP, 59, 2261 (1986) and Dr. J. Haisma (Philips), J. Cryst. Growth, 83, 466 (1987) IEEE Photonics Soc. distinguished lecture T.Mizumoto

29 Surface activated bonding
The hydrophilic bonding technique was not sufficient for our purpose in terms of bonding strength and uniformity. So, we turned out direction to another approach of surface activated wafer bonding to achieve a rigid contact. The surface activated bonding is conducted in a vacuum chamber. After a suitable surface activation process, two wafers are contacted in a vacuum chamber. Sometimes, wafers are pressed, and annealed succeedingly. There are several surface activation processes like Ar ion beam irradiation and Ar plasma exposure. We found that oxygen plasma exposure was effective for the combination of garnet and III/V, and we succeeded in bonding the garnet on GaInAsP with 1MPa at 250 deg C for 1h. Surface activation in vacuum chamber IEEE Photonics Soc. distinguished lecture T.Mizumoto

30 Direct bonding: garnet on GaInAsP/InP waveguide
Ce:YIG / GaInAsP Ce:YIG GaInAsP Bonding strength Fracture in an InP substrate at a tensile > 0.5 MPa This slide shows that Ce:YIG is successfully bonded onto a GaInAsP waveguide. Here, the waveguide pattern was made on guiding layer with E-beam lithography and CH4/H2 reactive iron etching. This picture shows a cross sectional SEM image of bonded sample. A garnet layer is successfully bonded on the top surface of semiconductor waveguide. We also checked the bonding strength in a sample. The tensile test revealed that the bonded sample exhibited fracture in an InP substrate with a tensile > 0.5MPa without debonding. So, it can be concluded that the bonding strength is sufficient to make an optical device. Please note that the low temperature heat treatment is essential for bonding dissimilar crystals to avoid the problems associated with mismatch in thermal expansion coefficients. Low temperature heat treatment T.Mizumoto, et al, ECS Meeting, 1258 (2006). IEEE Photonics Soc. distinguished lecture T.Mizumoto

31 Si-waveguide isolator
L=364mm R=2.5mm MMI The same idea of interferometric isolator together with a direct bonding technique can be applied to a Si-waveguide isolator. If we apply the optimum design of Si wire waveguide, the isolator can be constructed with a total length of 360um. This is the world smallest isolator. IEEE Photonics Soc. distinguished lecture T.Mizumoto

32 Nonreciprocal phase shift in SOI WG
Nonreciprocal phase shift (NPS):Db = b+ - b- External magnetic field SiO2 Ce:YIG Si x y z Ex TM mode Lp/2 (Min) ~300mm @0.2-mm-thick The good thing of SOI-based waveguide is that the under-cladding layer, SiO2, has a lower refractive index than an upper cladding layer of magneto-optic garnet. Because of this, an optical field is pushed up, and larger portion of optical field penetrates into garnet layer. Thus, larger nonreciprocal phase shift can be expected in an SOI waveguide compared with the case of III-V. This figure shows the comparison of nonreciprocal phase shift between III-V and SOI waveguide. In both waveguides, Ce:YIG is used as a upper cladding magneto-optic layer. And, the guiding layers have the same refractive index. Clearly shown is that larger nonreciprocal phase shift is obtained in SOI-based waveguide. If we compare the maximum values, 16 times larger phase shift is obtainable in an SOI waveguide. Thanks to this, the required length of phase shifter is reduced to 250um when we use 0.2um-thick Si guiding layer. CeY2Fe5O12 (Ce:YIG): QF = deg/cm H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42, (2003) IEEE Photonics Soc. distinguished lecture T.Mizumoto

33 Si-waveguide optical isolator
Si rib waveguide Ce:YIG SGGG 4.0mm SOI Ce:YIG H.Yokoi, et al (Tokyo Tech.)., Applied Optics, 42, (2003) 300 Si SiO2 2mm 10nm Ce:YIG 300nm We fabricated the SOI-based waveguide isolator. Here, we modified the waveguide structure into a rib structure to avoid the large propagation loss of Si wire waveguide. The Si rib waveguide is fabricated in a 300-nm thick Si guiding layer. The waveguide width is 2um and the height of rib is 10nm. Although we used a rib waveguide instead of Si-wire, the total length of interferometer is less than 4mm. This picture shows the fabricated device. Bonding condition  Anneal: 250 oC Press: 5 MPa, 1 hour Rib waveguide for reducing propagation loss (trial fabrication) IEEE Photonics Soc. distinguished lecture T.Mizumoto

34 Measurement setup CW CCW TM mode
ASE source N S Spectrum Analyzer PMF TM mode IR camera TV monitor Polarizer lens Optical switch CW CCW Sample 3-pole magnet --> anti-parallel magnetic field (S-N-S or N-S-N) 2X2 optical SW --> reverses propagation direction (CWCCW) The fabricated device was characterized using the measurement set-up shown here. We used an ASE light source and TM mode was launched into an isolator through a PMF directly coupled to the chip. The output light was probed by another PMF and was measured with an OSA. We applied an external magnetic field with a pair of miniature magnets as is shown in the figure. To measure the forward and backward transmittance, we exchanged the propagation direction by using an optical SW. IEEE Photonics Soc. distinguished lecture T.Mizumoto

35 First demonstration of Si-waveguide isolator
1530 1540 1550 1560 1570 -70 -60 -50 -40 Wavelength (nm) Transmittance (dB) w/o H field CCW CW Mag: N-S-N The interference reverses as the propagation direction is reversed. N-S-N S-N-S 1530 1540 1550 1560 1570 -70 -60 -50 -40 Wavelength (nm) Transmittance (dB) Isolation: 21dB CW CCW Mag: S-N-S This slides shows the experimental results. The transmitted spectrum are shown for two counter-propagating directions. You easily observe that transmittance differs depending on the propagation direction. For example, it is high for CW direction at 1560 nm, while it is low for CCW direction at that wavelength. When the magnetic field is not applied, the transmittance do not change remarkably. Also, the situation is reversed, when we reverse the direction of applied magnetic field. This experimental results reveals that nonreciprocal phase shift reverses the interference as is expected. If we measure the isolation that is defined by the ratio of backward to forward transmittance, it is up to 21 dB at 1560 nm. The interference reverses as the magnetic field directions are reversed. First demonstration of Si waveguide isolator ! Y. Shoji, T. Mizumoto (Tokyo Tech), et al. APL, 92, (2008). IEEE Photonics Soc. distinguished lecture T.Mizumoto

36 Si-waveguide isolator: insertion loss
MZI Single WG Ce:YIG upper clad 2.0 mm 4.0 mm 1530 1540 1550 1560 1570 -70 -60 -50 -40 wavelength (nm) transmittance (dB) (a) (b) (c) 21dB Isolation (a) Coupling loss between fiber and waveguide x2 : 37 dB (b) Propagation loss : 4 dB Si waveguide (2.5 dB / 4 mm) + Absorption of Ce:YIG (0.2 dB) + reflection at bonding boundary (0.65 dB x2) (c) Excess loss of MZI : 4 dB This slide summarized the loss measurement. We fabricated a single S-shaped waveguide along with the isolator. The garnet cladding layer was bonded equally on this waveguide. You can check that the transmittance of a single waveguide does not change even when the magnetic field is reversed. First, the fiber-waveguide coupling loss was measured to be 37dB for two junctions. A single waveguide has a propagation loss of 4 dB, which includes 2.5 dB propagation loss for 4-mm-long Si waveguide, 0.2dB absorption of garnet, and estimated 0.65dB reflection loss at the boundary of garnet and air cladding regions. The isolator has 4 dB additional loss compared with this S-shaped waveguide. So, in total, the insertion loss of the isolator was 8 dB excluding the fiber-waveguide coupling loss. We believe that the loss can be reduced by improving the fabrication technique. Insertion loss of the isolator ((b)+(c)) : 8 dB IEEE Photonics Soc. distinguished lecture T.Mizumoto

37 Non-magneto-optic approach
“Indirect photonic transition” -3 -2 -1 1 2 3 0.2 0.4 0.6 0.8 kz (2p/q) w (2pc/a) W w1 w2 k1 k2 -k1 -k2 Backward: Mode-1 (w1, -k1) is coupled to mode-2 (w2, -k2). (-k1 - q = -k2 , w2-w1=W : phase-matched) --> transition mode-2 (w2, -k2) filtered out Forward: Mode-1 (w1, k1) is uncoupled to mode-2 (w2, k2). (k1 - q > k2 , phase-mismatched) --> no transition Up to here, I describe the optical nonreciprocal devices that are based on the magneto-optic effect. There is another approach that does not use a magneto-optic effect. The idea shown here was proposed by Prof. S. Fan. The idea is based on an indirect photonic transition. It uses the mode coupling between two modes of different frequencies. They are phase-mismatched by q. The phase mismatch is compensated for by some means in backward direction so that the mode-1 is converted into mode-2. The converted mode-2 can be filtered out selectively, if the frequency is different from mode-1. This conversion, however, does not occur in the forward direction if the coupling vector q is invariant. If this is the case, the forward mode-1 is transmitted as it is. Zongfu Yu and Shanhui Fan (Stanford), Nature Photonics, 3, (2009). IEEE Photonics Soc. distinguished lecture T.Mizumoto

38 Non-magneto-optic approach
Traveling wave (dynamic) modulation 0-th 1-st Example (l=1550 nm): d/e=5x10-4, f=20 GHz w=0.27 mm, L=2.19 mm Backward: effective coupling e(z,t)=d cos(W t - (-q)z) -k1 - q = - k2 w2-w1=W As an example of phase matching mechanism, they propose to use an electro-optic modulation by a traveling wave. If you put an electrode in the half part of waveguide, it produces an effective coupling between fundamental mode at w_1 and 1st-order mode at w_2. They have even and odd distribution functions in a transverse direction, respectively. As is expressed in this figure and equation, the perturbation induced by an E field travels in the negative z direction, which generates an effective coupling between two modes propagating in this direction. When the lightwave propagates in the opposite direction, the coupling does not occur effectively because of phase mismatch due to an invariant coupling vector q. These parameters are introduced as a typical example in the paper. Such a technique is interesting because it does not require any magneto-optic material, which is generally hard to be implemented in the integrated photonic circuits based on III-V and Si. Z. Yu and S. Fan (Stanford), Nature Photonics, 3, (2009). IEEE Photonics Soc. distinguished lecture T.Mizumoto

39 Summary Optical isolators for photonic integrated circuits
★ Mode conversion isolator requirement of phase matching  limited fabrication tolerances ★ Interferometric isolator single polarization operation  no need for phase matching ultra-broad band operation (1.31/1.55 mm in a single chip) integration with active devices  Ce:YIG/ III-V, Ce:YIG/ Si low-temperature direct bonding first demonstration of Si waveguide isolator  21 dB isolation ★ Non-magneto-optic approach attractive (less restricted by material), but still challenging IEEE Photonics Soc. distinguished lecture

40 IEEE Photonics Soc. distinguished lecture

41 IEEE Photonics Soc. distinguished lecture

42 Semi-leaky isolator: performance
1.5 mm 4.5 mm External magnetic field (Electromagnetic Coil) Power meter PMF Tunable laser l=1550 nm Polarizer constant coupling loss (-15 dB/facet) 20.2 dB W=3 mm Measured isolation : 20.2 dB / 1.5 mm=13.5 dB/mm We measured isolator performance using this experimental setup. TE mode is launched through a PMF. A magnetic field was applied along the light propagation direction to produce a magneto-optic mode conversion. Measured transmittance loss is plotted as a function of applied magnetic field. Note that the loss includes the coupling loss between waveguide and fiber. By comparing the transmittance between +/- 200Oe, we obtain 20.2dB loss difference. Since the device length was 1.6mm, this corresponds to the isolation of 12.6dB/mm, which is comparable to the theoretical prediction. From these observations, we can conclude that the wafer bonding provides tight contact sufficient for this kind of application. T.Mizumoto et al, IEICE Trans, J89-C, 423 (2006). T.Mizumoto et al, OFC2007, OThU4 (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

43 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

44 Circular polarization
Faraday effect Dielectric tensor Circular polarization CW: CCW: This slide explains an important magneto-optic effect, Faraday rotation. Since most optical isolators and circulators use this effect, it is quite important to understand the operation principle of optical nonreciprocal devices. When a magneto-optic material is magnetized along the light propagation direction, z direction in this case, the dielectric constant of this material is described by a tensor form shown here. It has off-diagonal components. They are pure imaginary values if losses are negligible. When circularly polarized light is propagating in this material, it experience different dielectric constants depending on the circulating direction. That is, for clock-wise circular polarization, the magneto-optic material works as an isotropic dielectric material that has a permittivity constant given by this equation. For the other circularly polarized light, the material exhibits a different dielectric constant. Therefore, two circularly polarized waves propagate in different velocity. Considering this situation, the Faraday rotation can be understood as is explained in the next slides. IEEE Photonics Soc. distinguished lecture T.Mizumoto

45 Faraday effect Linearly polarized wave --> two circular polarized components CW circular polarized CCW circular polarized A linearly polarized wave can be de-composed into two circularly polarized waves with equal amplitudes. Please note that two circularly polarized waves propagates with different propagation constants beta+ and beta-, respectively. After propagating a distance z, two waves have different phases. By combining two waves, we find the polarization state at the position z. By adding x components and y components, respectively, we obtain these expressions. Here, you observe that the combined x component has the cosine dependence on z in amplitude. On the other hand, y component has a sine dependence. Thus, it can be understood that the wave becomes linearly polarized light and polarized direction is rotated by an angle of theta with respect to the incident polarization. As is clearly shown in this equation, the angle of rotation theta increases in proportion to the propagation distance z. IEEE Photonics Soc. distinguished lecture T.Mizumoto

46 Reversal of propagation direction
Faraday effect Reversal of propagation direction Reversal of H-field Backward Forward Reversing the propagation direction can be considered that the direction of H field is reversed as is shown in this slide. Since the sign of off-diagonal components of magneto-optic dielectric tensor is changed due to revered magnetization, the rotation direction of polarization is also reversed. So, in the forward direction, the plane of polarization is rotated in the clockwise direction, while in the backward wave, it rotates in counter clockwise direction when you observe it along the propagation direction. IEEE Photonics Soc. distinguished lecture T.Mizumoto

47 Waveguide Faraday rotator
E. Pross, et al. (Philips) , APL, 52(9), 682 (1988). When only the Faraday rotator is replaced with a waveguide one, it becomes a primitive version of waveguide isolator. There were several reports on the waveguide Faraday rotator. Here, it should be noted that the garnet material is hard to be etched. It requires high-temperature phosphoric acid, if you apply wet chemical etching. As a dry etching process, Ar sputter etching or Cl-based reactive iron etching can be applied. After forming the waveguide core, a over-cladding layer is grown, as is shown in these slides. Since the refractive index of garnet is rather high, around 2.2, the over cladding layer has to have a similarly high refractive index. If you grow the garnet having a slightly different composition from the waveguide core, it can be used as a over-cladding layer. This picture shows an output near field pattern. N. Sugimoto, et al. (NTT) , APL, 63(9), 2744 (1993). IEEE Photonics Soc. distinguished lecture T.Mizumoto

48 Isolator Isolator - two-port device #1 #2 - includes loss mechanism
non-unitary matrix --> lossy The first slide explains the function of isolator. Ideally, it transmits the lightwave from port-1 to port-2 without any loss of lightwave energy. And, in the reverse direction, the lightwave is blocked completely. So, the scattering matrix that characterizes the device function becomes as follows. You can check easily that this matrix does not satisfy the unitary condition. That is, the device is essentially lossy. Any kind of losing optical energy should be implemented to achieve this function. IEEE Photonics Soc. distinguished lecture T.Mizumoto

49 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

50 Circulator Circulator #1 #2 - many-port device - lossless device #3
Let’s look at a circulator. The function of optical circulator is described by the scattering matrix shown here. The input from port-1 exits from port-2, and the input from port-2 appears at port-3, from port-3 to port-1. That is, input / output port relationship is circulating. Thus, the device can be loss-free. Actually, it can be confirmed by checking that the scattering matrix satisfies the unitary condition. unitary matrix --> lossless IEEE Photonics Soc. distinguished lecture T.Mizumoto

51 H.Iwamura, et al, Electron. Lett., 15, 830-831 (1979).
Optical circulator H.Iwamura, et al, Electron. Lett., 15, (1979). Optical circulator can be constructed by combining the rotation of polarization plane in a Faraday rotator and the polarization beam splitters. This slide shows its principle of operation. Assume that the input lightwave is composed of an arbitrary polarization. It is split into to two polarization components, i.e., vertical and horizontal polarizations, as denoted by red and blue marks. Two components propagate different path. They experience 45deg Faraday rotation and another 45deg rotation that is not dependent on propagation direction. So, vertical polarization is converted into horizontal polarization, while the horizontal one is converted into vertical. By virtue of output PBS, two polarizations are combined into one beam, and output from port-2. In the reverse direction, the input from port-2 is split into horizontal and vertical polarizations in PBS. Since the rotation of polarization in reciprocal rotator and Faraday rotator are canceled, horizontal and vertical polarizations pass as they entered rotator section. They are combined in the left PBS, and output from port-3, instead of port-1. You easily understand that the device works for any polarization state, i.e., polarization-independent operation. Also, as is shown in this illustration, two beams traverse the optical paths of same distance. Therefore, in principle, there is no polarization mode dispersion. - uses rotation of polarization - polarization independent operation IEEE Photonics Soc. distinguished lecture T.Mizumoto

52 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach Let’s move on to the waveguide isolators. IEEE Photonics Soc. distinguished lecture T.Mizumoto

53 TE-TM mode conversion Faraday rotation TE-TM mode conversion
qF : Faraday rotation, G : field confinement factor Phase mismatch The rotation of polarization can be understood as a conversion between TE and TM modes in a waveguide. If you launch a TE mode at the input port, it is converted into TM in a MO waveguide. After being propagated in a distance z, TE and TM components are described by these equations. Here, delta denotes a phase mismatch between TE and TM modes, and k indicates the conversion coefficient due to a magneto-optic effect, which is proportional to a Faraday rotation. The polarization state at an arbitrary position in the waveguide Faraday rotator can be understood from these equations. IEEE Photonics Soc. distinguished lecture T.Mizumoto

54 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

55 Nonreciprocal phase shift
y z x MO perturbation Nonreciprocal phase shift This effect can also be understood by the coupling between transverse and longitudinal E fields of TM modes through the off-diagonal elements of dielectric tensor. The difference between forward and backward propagation constants is given by this equation. Let me remind you that the phase shift becomes zero, when the distribution of H field is symmetric in the magneto-optic material. So, it is essential to realize the asymmetry of H field distribution in a MO material. IEEE Photonics Soc. distinguished lecture T.Mizumoto

56 Interferometric isolator: polarization-independent
N: nonreciprocal phase shifter provides NPS only for TM mode MC: mode converters provide TE-TM mode conversion As you understand from the operation principle of nonreciprocal phase shift, the interferometric isolator has a disadvantage of polarization dependent operation. The nonreciprocal phase shift is provided only for TM mode in the configuration explained here. Of course, it is preferable for the device to work for both polarizations. We proposed the polarization independent configuration of the interferometric isolator as is shown here. Here, let me remind you that the nonreciprocal phase shift is given only for TM mode. The operation principle is explained separately for TE and TM mode in forward and backward directions. Figures (a) and (b) explain the backward operation. The input TM mode is split into two with equal amplitude. One of them is converted into TE and passes the nonreciprocal phase shifter. It does not experience the magneto-optic phase shift, but experiences the phase shift of pai in the phase shifter located in the last part. The other wave propagating lower arm, which is TM mode, experiences the magneto-optic phase shift. The path lengths are set so as to provide the same phase shift for TE and TM modes in this section. TM mode is converted into TE, and it interfere destructively in the output coupler with the other branch. The structure provides a similar operation for TE mode. In the backward direction, TE mode experiences the pai phase shift in the upper branch, while it is converted into TM in the lower branch. The TM mode experiences the pai magneto-optic phase shift. Here, please remember that the phase shift is different from the case of forward direction. Since the two branch provides the same phase shift, two waves interfere constructively after being converted into TM mode. Similar operation can be applied to TM mode incidence. Y. Shoji and T. Mizumoto (Tokyo Tech.) et al, JLT, 25(10), (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

57 Hydrophilic bonding ・Si / Si ・Si/SiO2 / Si, ・III-V(GaAs,InP) / Si,
・III-V(GaAs, GaP, InP, InAs) / III-V ・Ce:YIG / III-V ・Ce:YIG / SiO2 ・Ce:YIG / LiNbO3 Issues to be considered ・surface treatment → hydrophilic ・mismatch in thermal expansion coefficient → low temperature heat treatment First, we developed a hydrophilic direct bonding technique for the wafer combination of III-V and garnet. This slide illustrates the mechanism of hydrophilic wafer bonding. This technique has been primarily applied to the combination of semiconductors. Also, we applied this technique to the combinations of silica and garnet, LiNbO3 and garnet. This technique provides wide range of variation in material combination used in integrated optics. Let me explain the hydrophilic wafer bonding process we used. With appropriate treatment, the surfaces of wafer become hydrophilic. That is, a lot of hydroxyl groups are adsorbed onto the surfaces. When they are brought into contact at room temperature, hydrogen bonding occurs between the hydroxyl group. With the successive heat treatment, dehydration condensation takes place. This strengthens bonding. And, finally, two wafers are bonded through their dangling bonds that is originally located on the wafer surfaces. IEEE Photonics Soc. distinguished lecture T.Mizumoto

58 Hydrophilic bonding: fabrication
Let's move onto the device fabrication. A waveguide pattern was made on GaInAsP guiding layer with E-beam lithography and CH4/H2 reactive iron etching. The surface of wafer is exposed to weak O2 plasma in 30 sec and dipped into de-ionized water. Ce:YIG was grown on a substituted garnet substrate by using sputter epitaxy. The surface of Ce:YIG was slightly etched in phosphoric acid or was exposed to O2 plasma, and was dipped into de-ionized water. The two wafers are brought into contact at room temperature. And the bonded sample was loaded into annealing furnace. The sample was heated at 220deg in H2 atmosphere for 3hours with a weight shown here. Please note that the low temperature heat treatment in essential in the case of garnet. The garnet layer is reduced and its absorption is increased dramatically when the heat treatment is carried out at higher temperature than 450deg in H2 atmosphere. Also, as you can understand from the mechanism of wafer bonding, the crystal orientation of wafers does not matter for bonding wafers. The garnet layer is oriented in (111), while the semiconductor layer (100), in this case. IEEE Photonics Soc. distinguished lecture T.Mizumoto

59 Semiconductor waveguide isolator: demonstration
H. Yokoi, et al (Tokyo Tech.), Appl. Opt, 39, 6158 (2000). We measured the isolator performance in a fabricated device. Since reversing the direction of external magnetic field is equivalent to reversing the propagation direction, we measured the isolation by reversing the magnetic field. When we apply a small magnet in this way, a fraction of lightwave is observed to exit from the central output port. When we reverse the direction of magnetic field, the output from the same port is completely blocked. It is demonstrated that the device exhibits 5 dB isolation. When the isolator operates ideally, all the lightwave is output from the central waveguide. However, as you observe in this picture, there is a lot of leakage from the outer waveguides. Such a poor performance is due to that the nonreciprocal phase shift as well as reciprocal one were inappropriate in a fabricated device. As for the nonreciprocal part, the garnet layer was magnetized incompletely. That is, the whole length of nonreciprocal phase shifter, which is 6mm, was not magnetized properly because the length of magnet was only 3mm. This can be improved by using a large magnet, or by reducing the length of nonreciprocal phase shifter. As for reciprocal part, a post-fabrication adjustment should be incorporated. IEEE Photonics Soc. distinguished lecture T.Mizumoto

60 Calculated characteristics
perfectly balanced Lasym =111 mm Forward -10 -20 Transmission loss (dB) Backward -30 -40 slightly unbalanced Lasym =0 mm -50 1530 1540 1550 1560 1570 This slide shows the calculated characteristics of SOI waveguide optical isolator. When the interferometer is fabricated as is designed, the backward loss greater than 30dB is obtainable in full C-band. Also, the insertion loss below 0.2dB is expected in this wavelength range. However, when there exists fabrication error, they are degraded as shown by broken lines. When equivalent unbalance of 111 um exists in cladding layer between two arms of interferometer, the maximum available isolation is reduced to 30dB. Also, it becomes more wavelength sensitive. Wavelength (nm) Ideal MMI couplers Y.Shoji, T.Mizumoto, et al., APL, 92, (2008) IEEE Photonics Soc. distinguished lecture T.Mizumoto

61 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

62 Mode conversion: semi-leaky
TE mode Semi-leaky type guided proposed by S. Yamamoto, et al (Osaka U.), IEEE QE, 12, 764 (1976). Mode conversion - TE-guided and TM-radiation modes - MO and LN mode conversions TM mode Isolation: 20.2 dB (l=1550 nm, L=1.5 mm) - fabrication tolerant - wavelength insensitive radiated A semi-leaky isolator has the great advantage of large fabrication tolerances, because it uses conversion between guided and radiation modes. We reported an isolation of 20dB in a 1.5mm-long device. Also, it provides sufficient performance in a wide wavelength range. Detail will be explained later. T. Mizumoto et al.(Tokyo Tech), OFC 2007, OThU4 (2007). IEEE Photonics Soc. distinguished lecture T.Mizumoto

63 Semi-leaky isolator: design
Ce:YIG qF=-4500 deg/cm To cancel mode conversion in forward direction  offset angle of LiNbO3 Isolation = 14.1 dB/mm for 50dB isolation : L=3.5 mm Mode conversion in backward direction  isolation This figure shows the design of semi-leaky isolator. We use Ce substituted YIG as a magneto-optic guiding layer. For a given thickness of guiding layer, the offset angle of LiNbO3 crystal axis is determined to cancel the mode conversion in forward direction. For 1.19um-thick guiding layer, which will be used in the following experiment, the offset angle is determined to be 20 deg. This gives the mode conversion in the backward direction, and determines the isolation. In this particular case, the isolation of 14.1dB/mm can be expected. Since the isolation is given by the radiation loss, it increases in proportion to the device length. If you want isolation of 50dB, the device length is to be 3.5mm in this particular design. IEEE Photonics Soc. distinguished lecture T.Mizumoto

64 Semi-leaky isolator: calculated performance
1500nm < l < 1600nm: Isolation >12.5dB/mm Forward loss < 0.09dB/mm Next slide shows the dependence of characteristics on the deviation of guiding layer thickness. When the waveguide parameters deviate from designed values, the condition for canceling the forward mode conversion is not satisfied. This brings about the forward loss. For +-0.1um deviation, the increase in the insertion loss is below 0.1dB/mm. Even in this situation, the isolation over 13dB/mm is obtainable. This fairly large tolerance is attributable to that the guided TE mode is coupled to the radiation TM mode that has continuous spectrum. Next slide shows the wavelength dependence. The wavelength dependence of material parameters like refractive indices and Faraday rotation is taken into the calculation. For the wavelength range from 1.50 to 1.60 um, the insertion loss below 0.1dB/mm is expected, and the isolation over 12.5dB/mm is obtainable. IEEE Photonics Soc. distinguished lecture T.Mizumoto

65 Semi-leaky isolator: fabrication
4.5mm Bonding completed Time : 3 min Anneal : none (RT) Positioning : ~ 10 min High vacuum Sample set Pressure : 4.0 Pa (= 3.0x10-2 Torr) Gas flow : O2 2 sccm Ar 20 sccm RF power : 250 W Time : 5 min RF plasma : Ar + O2 Press : ~ 1MPa Vacuum : 6.0x10-7 Pa x-cut LiNbO3 Ce:YIG waveguide&terrace Garnet No.CY0523 The isolator was fabricated with pieces of x-cut LN and Ce:YIG waveguide using surface activated bonding technique. The terrace region was prepared aside of waveguide pattern to provide a sufficient bonding area. We apply a plasma generated in a gas mixture of Ar and O2 for 5min. Two wafers were mated in a vacuum chamber and were pressed with a pressure of 1MPa for 3min. Again, this time, low temperature treatment is essential to circumvent the problem associated with the difference in thermal expansion coefficients between garnet and LiNbO3. We succeeded in bonding at room temperature. IEEE Photonics Soc. distinguished lecture T.Mizumoto

66 Semi-leaky guiding characteristics
partially guided radiated TE mode TM mode We examined a semi-leaky guiding characteristic in a fabricated device. This slide shows the output near field pattern for TE mode input. You can see that the lightwave is partly radiated in the cladding region. Next slide shows the output for TM mode. Contrary to the previous one, the output of TM mode is rather weak. This is due to that TM mode is not confined in the guiding layer. That is the waveguide does exhibit the semi-leaky guiding characteristic. Semi-leaky guiding characteristic IEEE Photonics Soc. distinguished lecture T.Mizumoto

67 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

68 Waveguide optical circulator: TE-TM Mode conversion
Let’s move on to a waveguide circulator. This slide shows the waveguide circulator that uses the rotation of polarization together in a interferometer configuration. It consists of two 3-dB wavelength insensitive couplers, two half-wavelength plates and Faraday rotators. Basically, the device itself is a Mach-Zehnder interferometer. Its principle of operation is explained in the next slide. N. Sugimoto, et al. (NTT), IEEE PTL, 11, (1999). - uses TE-TM mode conversion (rotation of polarization plane) IEEE Photonics Soc. distinguished lecture T.Mizumoto

69 Waveguide optical circulator: operation principle
#4 #3 #2 #1 #4 #1 #4 #2 The state of polarization is shown in this slide at several points along the waveguides. The incident polarization is decomposed into vertical and horizontal polarization components. They are split into two in the 3-dB coupler. In the upper waveguide, the half-wave plate provides -45 deg rotation. It is followed by the +45 deg Faraday rotator. Thus the overall rotation becomes zero. In the lower waveguide, the +45 deg Faraday rotation is cancelled by the l/2 plate. So, two waves become in-phase and are coupled into the cross-port. In the reverse propagation, the sign of Faraday rotation is reversed. For example, if you look at the vertical component in upper waveguide, it is rotated by 45 deg in the clockwise direction. By virtue of half-wavelength plate, it is further rotated by 135 deg in the counter-clockwise direction. In the lower waveguide, the polarization plane is rotated by 45 deg in the clockwise direction in half-wavelength plate. It is rotated by another 45 deg in clockwise direction due to Faraday rotation. So, the waves propagating in lower branch becomes out-of-phase to the upper waveguide. The horizontal polarization behaves similarly. By combining these waves in a 3-dB coupler, they are output from the lower port, which is different from the forward direction. As a result, the device works as a circulator. IEEE Photonics Soc. distinguished lecture T.Mizumoto

70 Waveguide optical circulator: performance
#1 #2 #3 #4 Measured transmittance (dB) out in #1 #2 #3 #4 -- The measured characteristics of this circulator are summarized in this table. The insertion loss is around 3 dB, while the isolation is around 18 dB and dB in the other direction. These performances are reported by the group of NTT. N. Sugimoto, et al. (NTT), IEEE PTL, 11, (1999). IEEE Photonics Soc. distinguished lecture T.Mizumoto

71 Waveguide optical circulator: Interferometric circulator
Direction-A (in-phase interference) T. Mizumoto, et al. (Tokyo Tech.), EL, 26, (1990). We can construct the circulator by using the nonreciprocal phase shift. Here, the 3 dB MMI dividers / couplers used in an interferometric isolator are replaced by 3 dB directional couplers as shown in this slides. The operational principle can be understood easily by considering that two waves propagating in two arms become in-phase from left to right and out-of-phase from right to left. Thus, the lightwave is transmitted into cross polarization when propagating from left to right, while it is transmitted in bar ports in the opposite direction. Direction-B (out-of-phase interference) IEEE Photonics Soc. distinguished lecture T.Mizumoto

72 Outline Part-1: Bulk nonreciprocal devices
magneto-optic effect (Faraday rotation) operation principle of isolators and circulators Part-2: Waveguide isolators operational principles, design and characterization TE-TM mode conversion isolators Nonreciprocal loss isolator Interferometric isolator Semi-leaky waveguide isolator Part-3: Waveguide circulators Part-4: Non-magneto-optic approach IEEE Photonics Soc. distinguished lecture T.Mizumoto

73 Summary 2 Interferometric isolator - single polarization operation
--> no need for phase matching - ultra-wide band operation (1.31 / 1.55 mm in a single chip) - integration with active devices --> Ce:YIG/ III-V, Ce:YIG/ Si low-temperature direct bonding - first demonstration of Si waveguide isolator --> 21 dB isolation Semi-leaky waveguide isolator - highly fabrication tolerant - LN/Ce:YIG direct bonding - 20 dB / 1.5 mm IEEE Photonics Soc. distinguished lecture

74 Summary 3 Waveguide circulator Hybrid Faraday rotation type
MZ interferometer Non-magneto-optic approach dynamic modulation - indirect photonic transition of eigen modes dependent on propagation direction IEEE Photonics Soc. distinguished lecture


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