1. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Dynamics & Modulation Properties of Multi-Transverse-Modes Semiconductor Vertical-Cavity Surface-Emitting Lasers

2. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2 Outline ¬VCSEL - an introduction ­Single-mode VCSEL dynamics ­Multi-transverse-modes VCSEL dynamics ¯Dynamic response to an optical, parasitic-free excitation °Characterization and dynamics of VCSEL grown on a patterned wafer ±Summary

3. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3 VCSEL Vs. Edge Emitting Laser n No need for cleavage: u 2-D arrays u Cheaper device u On chip testing n Length cavity  single longitudinal mode. n Epitaxial mirrors R=0.999  high photon density. n Symmetric “wavequide” with broad lateral area: u High order transverse modes. u Easy coupling to a multi-mode fiber. N P I Current Light (b) N P I Top Mirror Bottom Mirror Current Light (a) VCSEL Edge Emitting

4. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4 (b) Bottom emitting mesa (d) Intra-cavity mesa (f) Buried ion layer (h) Grown on a patterned wafer (g) Oxide confined (c) Top emitting mesa (e) Ion implanted device VCSEL Device Geometries P N I Oxide Isolator Dielectric Mirror (a) Etched well

5. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 5 VCSEL Main Characteristics n Thermal Red Shift. n Substrate feedback induced ripples on L-I curve n Multi-Transverse modes appearance A6 B C D E F A5 A4 A3 A2 A1 Spectrally Resolved Near Field

6. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 6 Ion-Implantation-Based VCSEL Advantages u Easier and cheaper to manufacture. u Large area contact pads. u Planar surface. n Surrounding material: u Better heat dissipation u Less recombination centers at the periphery  Higher efficiency n Gain guided mechanism - fewer transverse modes Advantage ? n Fabrication:

7. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 7 VCSEL Main Application - Optical Interconnections Systems n The problem: u Multi-mode fibers tend to generate modal noise n The solution: u Usage of a less coherent light source: i.e. multi-mode VCSEL  ?What are the modulation characteristics of a multi-mode VCSEL ? n Optical interconnection systems are based on: u Array of independent VCSEL u Multi-mode fiber ribbons

8. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 8 The Experimental Set-up Variable Attenuator CCD VCSEL Fast GaInAs Detector Bias - T DC Current Source Temp. Controller Two Options RF Generator RF spectrum Analyzer Network Analyzer Removable Silicon PIN Detector X-Y Recorder L-I curve microscope Imaging Spectrometer BS RF probe Near Field Image Spectrally Resolved Near Field Removable Mirror Near Field Image

9. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 9 I - Modulation of a Single Mode VCSEL n Direct modulation of semiconductor laser. n Modulation of a 10  m diameter VCSEL defined by buried proton layer - experimental: u MCEF - modulation coefficient efficiency factor u Max -3db B.W. & Intrinsic max B.W. u Novel study of the transport time across the device

10. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1010 Laser Dynamics - Basic Model n 2 conjugate poles response - resonance & damping factor. I S PN Injection RL C Vout Vin Equivalent Circuit n Assumptions: u Neglecting transport effects u Lumped QWs - uniform carrier density u Single lasing mode

11. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis1 Laser Dynamics - Including Transport effects n 3 poles response - roll-off pole in addition the to resonance & damping factor. I S PNN Equivalent Circuit Vout RL C Vin n Assumptions: u Single lasing mode u Lumped QWs - uniform carrier density - N TT CC u Lumped barrier - uniform carrier density - N B Adding time constant,  s, which consists of:  t ;  c ;  parasitic Rp Cp x1

12. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1212 Laser Dynamics - Small Signal Analysis n Rate equations: Small Signal Analysis  n Higher photon density in VCSEL  larger B.W. n At higher injection levels,  limits max. B.W.

13. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1313 Modulation of a 10  m Diameter VCSEL (Single Mode Operation Regime) n Max B.W. - 14.5 GHz ; limited by the emerging of multi-mode lasing regime. n All curves were fitted to the a 3 pole transfer function, extracting: B.W. ; F r ;  ;  s

14. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1414 Extracting Modulation Coefficient Efficiency Factor n As long as: u  <<  R u The roll-off pole influence can be neglected  n Since  n MCEF = 7.38 GHz /  mA The best reported for ion implanted VCSEL ?What are the limiting factors ? (beside multi-mode lasing)

15. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1515 Maximum Intrinsic Modulation B.W. n When: u The roll-off pole influence can be neglected u However,  ~  R n Assuming  Maximum B.W. Is achieved at:  =  2*  R  n K = 0.11 nSec  Maximum Intrinsic f -3dB = 80 GHz The best reported for VCSEL ?Yet, What is the influence of the transport effects …

16. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1616 Transport Effect on the Modulation Response

17. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1717 Extracting the Transport Time: n The roll-off pole time constant is composed of: u The intrinsic transport & capture time. u The diode & Bragg Mirrors, current depended, RC time constant n Phenomenological approximation: Carrier’s Transport & Capture time constant  trans = 15pSec Extracted for VCSEL for the first time !

18. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1818 I - Modulation of a Single Mode VCSEL Conclusions n Medium area, ion implanted VCSEL exhibit high modulation B.W., As long as single mode operation is maintained. n The MCEF & the max. B.W., are the highest measured for ion implanted device. n An intrinsic max B.W. Of 80GHz was demonstrated. The carrier transport time was extracted:  trans = 15psec, and its limitations on modulation B.W. were as illustrated.

19. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 1919 II - Modulation of a Multi-mode VCSEL n The Theoretical Model. u The model u Small signal modulation frequency response for different mode combinations n Experimental Results u Modulation of a 20  m VCSEL defined by buried proton layer : F Frequency response of a multi-mode VCSEL modulation F 2nd harmonic distortion u Modulation of a VCSEL array Y. Satuby and M. Orenstein, “Modulation Characteristics and Harmonic Distortion of VCSEL Arrays and Multi Transverse Mode VCSELs”, LEOS Annu. Meeting, Nov. 1997, ThA2

20. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2020 The Model n Intensity distribution of the modes is assumed to be known. n One parameter rate equation for the photon number of each mode. n Rate + Continuity equation for a two dimensional distribution of the carrier density - N(x,y) u Modal gain is attributed to the overlap between the gain distribution and the mode profile u Device geometry is defined through J(x,y) u Photon density is the incoherent sum for all modes

21. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2121 Example - Two Non-Overlapping Transverse Modes n 20um diameter device n LP mn modes are assumed, (according to experimental results): u LP 21 u LP 01 - smaller in diameter (compare to device diameter) due to: F Spatial hole burning (self focusing) F Thermal lensing n I=14mA  0.7 mW 0.43 mW ?How does the Dynamic response look like …

22. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis2 Dynamics of Two Non-Overlapping Transverse Modes Impulse ResponseFrequency Response n The modes behave as two independent lasers. ?How do current level & diffusion coefficient modify the dynamic response ?

23. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2323 n As current increases the power of each mode increases linearly n f r of each mode changes according to the power of the mode

24. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2424 n Diffusion coefficient is not well known. Thus, calculation are made for a wide range of it n As diffusion coefficient increases, (at constant current of 14mA), the basic mode becomes dominant n f r of each mode changes according to the power of the mode

25. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2525 Dynamics of Two Overlapping Transverse Modes n The modes behave as “coupled” oscillators. Impulse ResponseFrequency Response ?How do current level & diffusion coefficient modify the dynamic response ? n According to experimental results, the modes of a non-linear laser cavity are taken as: u LP 01 u A combination of LP 02 +LP 21 n I=15mA, D=30

26. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2626 n When the higher mode emerges, the power of the basic mode is almost clamped. n The resonance frequencies can not be related to a specific mode n The resonance frequencies do not follow the power of the modes - an “Avoided Crossing” phenomena is observed: Despite of crossing of the photon density of the two modes, the resonance frequencies do not cross

27. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2727 n As Diffusion Coefficient increases, (at constant current of 15mA), the basic mode becomes dominant n The “Avoided Crossing” is illustrated again

28. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2828 20  m Diameter VCSEL Defined by Buried Proton Layer (Higher Dose) - Experimental B C D A E F G H Frequency Response Spectrally Resolved Near Field L - I Curve

29. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 2929 20  m Diameter VCSEL Defined by Buried Proton Layer (Lower Dose) - Experimental Frequency ResponseSpectrally Resolved Near Field L - I Curve A B C D E F G H mm 20 n Lower dose  A wider active area n B, D, F are local minima on the L-I curve

30. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3030 20  m Diameter VCSEL Defined by Buried Proton Layer (Lower Dose) - 2nd Harmonic Distortion - Experimental n Single mode operation, 2nd harmonic level is -24dbc n Two transverse mode regime - 2nd harmonic peaks at: u Excitation at the two resonance frequencies u Excitation at half the resonance frequencies u Excitation at half the notch frequency

31. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3131 Modulation of a VCSEL Array - Experimental n Multi-mode operation is maintained throughout the whole L-I curve L - I Curve n Array is defined using mirror patterning n Triangular array - producing modes similar to the large area VCSEL

32. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3232 Frequency ResponseSpectrally Resolved Near Field n Modulation response with two resonance was measured - regardless of local minima or maxima on the L-I curve n Modulation response with three resonance was obtained for three mode operation n 2nd Harmoic Distortion peaks: u At the resonances & their half frequencies u At half the notch frequency (stronger response than excitation at the notch itself) A C D E B mm 20 Array Modulation - Continue

33. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis3 II - Modulation of a Multi-mode VCSEL Conclusions n A theoretical model for the dynamics of multi-transverse-mode VCSEL was presented: u A multi-mode laser is characterized by a multi-resonance frequency response to a small signal current modulation u For two modes - one contained in the other, the resonance frequencies exhibited an “avoided crossing” like phoneme as modal power changed n Experimental results demonstrated: u The multi-resonance behavior for multi-mode VCSEL u A “flattened” frequency response for multi-higher-transverse-mode operation regime n Modulation of a VCSEL array further confirmed the results n A strong second harmonic distortion was measured, when frequency response was not spectrally uniform

34. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3434 III - Parasitic-Free Response to a Pulsed Optical Excitation of a Large Area VCSEL BS CCD x50 BS Electrical Pulser Fast GaInAs Detector Variable Attenuator CCD VCSEL Fast Sampling Oscilloscope microscope Optical Spectrum Analyzer Pulsed Ti-Sa Laser

35. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3535 Parasitic-Free Response Along the Current Pulse n 150nSec 80mA current pulse n Excitation by 1pSec 810nm pulses n Two time constants: u Relaxation-oscillation of 8 GHz u Second pulse generation after 0.35nSec (3 GHz ) n Second pulse generation is time depended

36. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3636 IV - Characterization and Dynamics of VCSEL Grown on a Patterned Wafer n A novel method of “ready to use” VCSEL fabrication n Unique modal behavior n Dynamic properties: u Theoretical analysis u Experimental results M. Orenstein, Y. Satuby, U. Ben-Ami, J. P. Harbison, “Transverse modes and lasing characteristics of selectively grown vertical cavity semiconductor lasers”. Appl. Phys. Lett. 69(1996), pp. 1840-1842.

37. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3737 Selective Growth Over Openings in a Si 3 N 4 Mask n A novel “ready to use” VCSEL structure grown by MBE over GaAs patterned wafer u Over the Si 3 N 4 layer an insulating polycrystalline material was grown. u Through the 20  m  20  m openings growth of a monocrystalline VCSEL structure was achieved. n Unisotropic growth process, material is less packed along (011) direction n The only required process, is the formation of contact layers SEM pictures of cleaved device’s facets

38. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3838 Top View of the Selective Grown VCSEL n Top view: u (a) Optical photo u (b) AFM scan of a single VCSEL u (c) Corresponding height profile along the [011] axis n The final device area is 15  m  15  m due to 2  m migration of the interfaces

39. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 3939 Pulsed Operation Characteristics n The dominant mode was always a one dimensional transverse mode aligned along 011 axis. with 3-5 lobes (2) ­TEM 30 lasing Mode (3) ®TEM 31 lasing Mode n Pulsed L-I Curve, I th =7mA,  =14% ?What will SRNF image revile at higher current levels ? (1) ¬Spontaneous emission n Near field patterns:

40. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4040 Transverse Modes During Pulsed Operation ( I ) ( II ) ( III )  m n A 10nSec current Pulse to avoid thermal wavelength sweeping ( I ) 23 mA ( II ) 40mA ( III ) 58mA n The TEM 30 & TEM 00 modes, polarized perpendicularly to each other, are the dominant modes n Non-typical, the lower modes emerge at higher current levels SRNF Images Remark: At CW operation, the lower-order modes are the dominant !

41. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4141 CW Operation of the Selective Grown VCSEL n A typical CW L-I curve is achieved n V-I curve demonstrates a typical 50  resistance n The fundamental modes become the dominant ones ?How would the dynamics & modulation response look like ?

42. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4242 Theoretical Response n The model described earlier was used. n A 15  m  15  m square current injection profile n Modes TEM 00 & TEM 10 were assumed. (highly overlapping modes) n Single Resonance Response

43. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4343 Experimental Response 20mA 18mA n A single resonance response in accordance to theory n Multi-transverse TEM m0 modes operation

44. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis4 Carrier Life Time Measurement ?Does the polycrystalline material induce shorter life time, due to traps at the periphery ? Carrier life time  nr =1.8 nsec, as for proton implanted VCSEL n Using the large signal response relation:

45. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4545 IV - VCSEL Grown on a Patterned Wafer Conclusions n A simple selective growth method for VCSEL fabrication was demonstrated. n The lasers exhibited similar characteristics to VCSEL fabricated using conventional methods n A unique transverse mode behavior, attributed to strain induced by the growth boundaries was observed. n The traps induced by the growth process at the boundaries, did not modify carrier life time n The modulation scheme for such a modal behavior was calculated & measured to yield a single resonance frequency response

46. Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis 4646 Summary n The dynamics of a single mode operated VCSEL was analyzed, and transport time across the device was measured n The dynamics of a multi-transverse-mode VCSEL was studied: u A theoretical model has been presented, and a number of cases were examined : F Two non-overlapping modes respond as two independent lasers F Two modes, one contained in the other acts as two “coupled oscillators” having two resonance response F In case of two highly overlapping modes, single resonance modulation response is expected u Experimental results confirmed the results n The use of optical excitation to achieve a dynamic parasitic-free VCSEL’s response was illustrated n A VCSEL fabricated by novel method of using selective growth was introduced and characterized