EE-566 Presentation Topic: Fiber Bragg Gratings Presented By: Eric Glauber Date: 10/29/03

Fiber Bragg Grating: Introduction The Fiber Bragg Grating (FBG) is a fiber optic passive component exhibiting basic functional attributes of reflection and filtering. FBG’s are relatively simple to manufacture, small in dimension, low cost and exhibit good immunity changing ambient conditions and EM radiation. FBG’s have replaced bulk optic mirrors & beam splitters in equipment which increases system stability and portability.

Fiber Bragg Grating: Introduction FBG’s are commercially used in the areas of Telecommunications and Sensors: Telecommunications Fiber Lasers Fiber Amplifiers Fiber Filters Dispersion Compensators Optical Fiber Phase Conjugator WDM Multiplexers Demultiplexers Sensors Strain Sensors Temperature Sensors Chemical Sensors Accelerometers

Fiber Bragg Grating: Theory 1978 – Hill et. all Phenomenon of photosensitivity in optical fibers Exposed Ge-doped core fibers to intense light at 488 or 514 nm Induced permanent refractive index changes to the core. ‘The sinusoidal modulation of the index of refraction in the core due to spatial variation in the writing beam gives rise to a refractive index grating that can be used to couple energy of the fundamental guide mode to various guided and lossy modes.

Fiber Bragg Grating: Theory FBG is a longitudinal periodic variation of the index of refraction in the core of an optical fiber. The spacing of the variation is determined by the wavelength of the light to be reflected. Bragg Wavelength

Fiber Bragg Grating: Theory The Bragg Condition is the result of two requirements: Energy Conservation: Frequency of incident radiation and reflected radiation is the same. Momentum Conservation: Sum of incident wave vector and grating wave vector equal the wave vector of the scattered radiation. K + ki = kf The resulting Bragg Condition is: lB = 2L neff The grating reflects the light at the Bragg wavelength (lB) lB is a function of the grating periodicity (L) and effective index (neff). Typically; lB= 1.5 mm, L = 0.5mm The Bragg wavelength is approx. 2-3 x the periodicity. 1300 – 1500nm (infrared)

Fiber Bragg Grating: Theory The spectral component reflected (not transmitted) typically has a bandwidth of 0.05 – 0.3 nm. A general expression for the approximate Full Width Half Maximum bandwidth of a standard grating is given by (S = grating parameter (.5 to 1), N = numbers of grating pains): Δλ =λ B S( (Δn/2n0)2 + (1/N)2 )1/2

Fiber Bragg Grating: Theory The shift in Bragg Wavelength with strain and temperature can be expressed using: DlB = 2nL({1-(n2/2)[P12 – n(P11 + P12)]}e + [a + (dn/dT)/n]DT Where: e = applied strain Pi,j = Pockel’s coef. of the stress-optic tensor n = Pisson’s ratio a = coef. of thermal expansion DT = temperature change [P12 – n(P11 + P12)] ~ 0.22 The shift in Bragg Wavelength is approximately linear with respect to strain and temperature. Delta-Bragg-Wvlgth/Bragg-Wvlgth = Delta-Grating/Grating + Delta-index/index 1) When fiber is strained, Grating period inc. and index of refraction decreases. Thus they have contrasting effects on Bragg Wavelength. 2)With a perturbation, typically the change in grating index of refraction has the largest effect.

Fiber Bragg Grating: Theory The measured strain response at a constant temperature is found to be: (1/lB)dlB/ de = 0.78 x 10-6me-1 Sensitivity Rule of thumb at lB = 1300nm: 0.001nm/me

Fiber Bragg Grating: Theory The measured temperature response at a constant strain is found to be: (1/lB)dlB/ dT = 6.67 x 10-6 oC-1 Sensitivity Rule of thumb at lB = 1300nm: 0.009nm/ oC

Fiber Bragg Grating: Theory – Blazed Grating Bragg grating planes are tilted at an angle to the fiber axis. Light which otherwise would be guided in the fiber core, is coupled into the loosely bound, guided cladding or radiation modes. The bandwidth of the trapped out light is dependent on the tilt angle of the grating planes and the strength of the index modulation. As shown above, the vector diagram is a result of the conservation of momentum and conservation of energy requirement. The results of applying the law of cosines yealds: Cos(θb) = ׀K׀/2v

Fiber Bragg Grating: Theory – Chirped Grating Bragg grating has a monotonically varying period as illustrated above. These gratings can be realized by axially varying either the period of the grating or the index of refraction of the core or both. The Bragg Condition becomes: λB = 2neff(z)Λ(z) The simplest type of chirped grating is one which the grating period varies linearly with axial length: Λ(z) = Λ0 + Λ(z)

Fiber Bragg Grating: Manufacturing 1989 Meltz et. all. Grating written into core by holographic side exposure method Exposure with two beam interference pattern of UV light at 244nm Focal spot is approx. rectangular, 4mm L X 125 mm W.

Fiber Bragg Grating: Manufacturing Split Beam Interferometer Method Insert Fig. 1, Meltz 1989

Fiber Bragg Grating: Manufacturing   Strongest gratings were written with244nm pulsed radiation with an average power of 4-20 mW. Fibers: Core dia. 2.2-2.6 micrometers N.A. 0.17-0.24 GeO2 doping of 5-12.5mol% Bragg wavelength – 577-591nm Application time: Grating are observed to form quickly at power levels of 10mW or higher Ie. 10 sec exposure @ 23mW.

Fiber Bragg Grating: Manufacturing Novel interferometer technique using a right angled prism. Inherently more stable -because beams are perturbed similarly by any prism vibration.

Fiber Bragg Grating: Manufacturing Phase Mask Technique. UV is diffracted into –1,0,1 orders by relief grating. Input mask is wavelength specific. Different lB require different phase masks. Silica plate with relief grating generated by e-beam exposure and plasma etching. The two first orders undergo total internal reflection at the glass air interface of the rectangular prism and interfere on the bare fiber surface

Fiber Bragg Grating: Manufacturers Advanced Optics Solutions GMBH Blue Road Research 3M Optical OEM Systems Alcatel Optronics Boeing Gould Fiber Optic MPB Communications OZ Optics Limited TeraXion Inc. Oxford Lasers Inc. Thorlabs Inc.

Fiber Bragg Grating: Manufacturers FBG custom design software:

Fiber Bragg Grating: Manufacturers FBG order form: Insert FBG order form

Fiber Bragg Grating: Manufacturers

Fiber Bragg Gratings - Application Mach-Zehnder interferometric Band Pass Filter: This arrangement has two identical FBG’s. The relative phases of the two transmitted signals is adjusted to maximize the output from either port 3 or 4 and thus avoiding the 3-db loss of the couplers. This arrangement is difficult to manufacture because the performance of the filter is strongly affected by the spectral characteristics of the two couplers. Also affecting the filter performance is any imbalance int the power splitting of the couplers or the relative phases of the signals. Tap: Light is emitted form the

Fiber Bragg Gratings - Application Chirped Grating used for Dispersion Compensation: The operating principle is that the grating reflects different wavelength components of the signal from different sections along the grating. As shown below, short wavelengths are reflected first at the near end and longer wavelengths are reflected at a later time at the far end of the grating. Shown below is a simple expression for the group delay dispersion of a linear chirped grating of length L. Tap: Light is emitted form the

Fiber Bragg Gratings - Application Chirped Grating used for Dispersion Compensation: Shown below is a system which transmits a 2 pico-second pulse which is compensated using a linearly chirped Bragg grating. Shown to the lower right is a graph of the compensated and uncompensated signal. Tap: Light is emitted form the Other more complicated grating structures are avaliable. For example cubic dispersion compensation is important in long distance, high bit rate transmission systems.

Fiber Bragg Gratings - Application Mach-Zehnder DWM – Multiplexer / Demultiplexer: This is the Mach-Zehnder arrangement for a WDM application. Tap: Light is emitted form the Extraction of channel λk Insertion of channel λk

Fiber Bragg Gratings - Application Frustrated Coupler Drop Add Multiplexer: This is composed of a mismatched coupler with a Bragg Grating written into one of the cores. Power would not normally be transferred due to the mismatch of the core. With the Bragg grating, power transfer of the guided mode from port 1 to port 4 can happen if the sum of the propagation constants of the LP01 modes of each core satisfies the Bragg condition: (β01(λ12,1) +β01(λ12,2)) = 2Π/Λ This cross coupling transfers the guided optical power at λ12 in core 1 into back propagating optical power in core 2. Tap: Light is emitted form the

Fiber Bragg Gratings - Application Chemical Sensor: FBG is blazed into the core at a tilt angle y. The FBG Tap radiates a beam out of the core and cladding at an angle gB. Tap: Light is emitted form the

Fiber Bragg Gratings - Application Chemical Sensor (Continued): FBG Tap excites fluorescent layer which in turn emits light which is in turn collected by the core (lf1). Insert Insert fig. 4, chem sensor

Fiber Bragg Gratings – Application, WDM In WDM, each FBG sensor is assigned a portion of the source spectrum. Enables quasi-distributed sensing of strain, temperature, chemical, etc…. No. of FBG is a function of: Source profile width Grating operational bandwidth Insert Fig. 2, Kersey, 1997

Fiber Bragg Gratings – Application, WDM Approx 20 strain sensors can be multiplexed along a single fiber (peak strains of +1000me) Insert Fig. 2, Kersey, 1997

Fiber Bragg Gratings – Application, WDM Time & Wavelength Division Multiplexing: Insert Fig.9, Kersey, 1997 The literature discusses a 3x3 array (9 elements) The literature states up to 100 element arrays are possible

Fiber Bragg Gratings – References Othonos, Andreas and Kalli, Kyriacos, “Fiber Bragg Gratings – Fundamentals and Applications in Telecommunications and Sensing”, Artech House, Inc, 1999. Hill, K.O., et al., “Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication,” Appl. Phys. Letter, Vol.32 (647-651) 1978.  Kersey, Alan D., et al., “Fiber Grating Sensors,” Journal of Lightwave Technology, Vol. 15, No. 8, August 1997. Maher, M.H. and E.G. Nawy, “Evaluation of Fiber Optic Bragg Grating Strain Sensor in High Strength Concrete Beams,” Fiber Optic Sensors for Construction Materials and Bridges, pp. 120-133, Farhad Ansari, editor, Technical Publishing Company, Inc., 1998. Pallas-Areny, R. and J.G. Webster, Sensors and Signal Conditioning, second Ed., John Wiley & Sons, Inc., 2001. Meltz, G., W.W. Morey and W.H. Glenn, “Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method,” Opt. Lett, 14, (823-825) 1989.  Webster, J.G., The Measurement Instrumentation and Sensors Handbook, RC Press, 1999. http://www.photonics.com/directory/bg/xq/asp/url.viewcat/bgpsa.30125/qx/categories.html http://www.aos-fiber.com/ Insert Fig.9, Kersey, 1997 The literature discusses a 3x3 array (9 elements) The literature states up to 100 element arrays are possible