Recent Advances in photonic devices for Analog Fiber Link: Modulator Technologies

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Daniella Angela Carter
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1 Networking the World TM ecent Advances in photonic devices for Analog Fiber Link: Modulator Technologies P. K. L. Yu, X.B. Xie*, G. E. Betts**, I. Shubin, Clint Novotny***, Jeff Bloch, W. S. C. Chang Department of ECE University of California, San Diego * Now with CEOL, Univ. of Central Florida ** With Photonic Systems Inc. Boston, Mass. USA *** Now with BAE Systems

2 Outline of Presentation Introduction: Analog fiber link Electroabsorption Modulator Multiple Quantum Wells and Gain Saturation E-O effect in InP nanowires Conclusion

3 Analog Fiber-Optic Link F Input F Output Transmitter Optical Fiber eceiver Direct Modulation F Input External Modulation F Input Laser Optical Output Laser Optical Modulator Optical Output Transmitter Transmitter

4 Analog Fiber-Optic Link Applications CATV Antenna emoting Phased Array Antenna

5 Externally Modulated Analog Fiber Optics Link DC Externally Modulated Link F in Bias Network Laser EIM Optical Fiber Photodetector Bias Network DC F out EIM= External Intensity Modulator Avoids the relaxation oscillation and reduces the chirp of the direct modulated laser diode; good for wide bandwidth modulation. Link F gain, G ~ (P opt) )

6 Important Analog Link Parameters 1. F Gain: Output F power/input F power. Bandwidth: 3 db F gain cut-off frequency bandwidth 3. Noise Figure: Input SN/Output SN 4. Spurious Free (or Intermodulation free) Dynamic ange: F power range above noise and intermod distortions

7 F Gain of the External Intensity Modulated link P opt 1 T(V) Transmitter P opt-d Detector P opt t ff [T(V b ) + T (V b ) ac cost] 1/ d optical fiber 0 V b + ac cost V d P opt L[T(V b ) + T (V b ) ac cost] I G P F out P F in P opt T( Vb ) Lf d Modulator Detector G P opt t ff in V L f d out where equivalent V V = /( dt/dv),

8 External Modulator Candidates Electro-optic Modulator: (a) Lithium Niobate (b) Semiconductor (c) Polymer (large r s) Semiconductors typically have smaller EO coefficients; one can also exploit the effects near a bandgap. We will describe those in nanowires Electrooptic Modulator F Input Optical Input Optical Output t Optical Transmission MZM Optical Transfer Curve V Bias Voltage (V)

9 State-of-the-Art LiNbO 3 Externally Modulated Link Link Gai in (db) Noise Figure (db) Link Frequency (GHz) * Courtesy of Ed Ackerrman, PSI

10 Outline of Presentation Introduction: Analog fiber link Electroabsorption Modulator Multiple Quantum Wells and Gain Saturation E-O effect in InP nanowires Conclusion

11 Electroabsorption Modulator Bias and F Signal p i n Franz-Keldysh Effect (FKE) Quantum Confined Stark Effect (QCSE) E c E v x x

12 Semiconductor Electroabsorption Waveguide Modulators Modulator DC photocurrent I m is caused by electroabsorption I m P i n I m

13 Outline of Presentation Introduction: Analog fiber link Electroabsorption Modulator Multiple Quantum Wells and Gain Saturation E-O effect in InP nanowires Conclusion

14 Broadened optical absorption spectra of a quantum well SQW Ab bsorption (cm -1 ) E=30 kv/cm 50 kv/cm 70 V/cm 90 V/cm 130 V/cm Wavelength (nm)

15 Design Strategy for achieving High Link Gain G P opt t V ff in L f d out To overcome the C bandwidth limit with minimum reduction of the modulation efficiency. To achieve high F link gain, high power operation with good coupling to fiber is needed. Low optical residual propagation loss to ensure small insertion loss. Large optical/microwave field interaction volume to Large optical/microwave field interaction volume to ensure low V, hence high F link gain.

16 Peripheral Coupled Waveguide Electro-absorption Modulator Small confinement factor!! By placing the active absorption layer in the evanescent portion of the optical mode, we can decouple the optical waveguide design & electroabsorption material design.

17 Optical Mode and Confinement Factor in EAM Typical EAM PCW EAM Confinement factor : the ratio of optical power within the active absorption layer. Smaller confinement factor Larger optical mode Smaller scattering loss Decoupling between optical and microwave waveguide

18 educing Insertion Loss Large optical mode improves fiber to EAM coupling to be around db per facet; Submerged mode reduces scattering loss; Small confinement factor reduces propagation loss with best result of 0.8 db/mm; Best fiber-to-fiber loss was measured to be 4 db.

19 Absorption along EAM Waveguide EAM Waveguide Absorption Profile C.F. = 0.08 C.F. = 0.10 C.F. = 0.15 Normazlied Absorbed Opt tical Power pe er Unit Normazled Length Normalized Absorption Coefficient L= Normalized Length

20 PCW EAM Waveguide Design Optical Waveguide Microwave Waveguide ement Factor Confin Optical Waveguide Width (micron) Confinemen nt Factor Microwave Waveguide Width (micron)

Peripheral Coupled Waveguide EA modulator W 1 =1.5m, W =W 1 +4m W 1 =m, W =W 1 +4m W 1 =m, W =W 1 +8m Confinement factor. =.64% 4.6% 3.44% 0.35 0.08 0.35 0.08 0.35 0.08 0.30 0.

21 Peripheral Coupled Waveguide EA modulator W 1 =1.5m, W =W 1 +4m W 1 =m, W =W 1 +4m W 1 =m, W =W 1 +8m Confinement factor. =.64% 4.6% 3.44% Modu ulator Current (ma) Detector Current (m ma) Modu ulator Current (ma) Detector Current (m ma) Modu ulator Current (ma) Detector Current (m ma) everse Bias Voltage (V) everse Bias Voltage (V) everse Bias Voltage (V) 0.00 Propagation loss =0.97dB.09dB 1.43dB (length = 1. mm)

22 Fabricated PCW EAM MQW n-metal p-metal BCB BC B n-ingaasp InP p-inp n-metal

23 High Power EAM Link gain higher close to transparency EAM Optical Transfer Curves. mw 45 mw 15 mw 90 mw 590 mw Normalized Transmission in (db) F Link Ga Link Gain vs. Optical Power Input Optical Power (dbm) Normalized Transmission n everse Bias Voltage (V) 590 mw input optical power ma photocurrent Photocurren nt (ma)

24 Gain Limitation of EA modulator Analog fiber link Small-signal Equivalent circuit of EA Modulator: Effect of Modulator Photocurrent

25 Analysis of photocurrent feedback effect on Gain The modulator photocurrent at the biasing point is given by: For simplicity, consider low frequency modulation, the effect of C p, C M L s can be neglected, defining m as the modulator photocurrent efficiency: The modulator photocurrent at the biasing point is given by: m e M IN B M IN P v V p t p i ) 1 ( We can define an effective small-signal ac photocurrent resistance P : e P t V It is seen that as power go up, P decrease in value, therefore the link gain saturates under high power, reaching a limit independent of power or V e : p L t I M 1 1 L V p G 4 S D M D O Limit t G 1 1 S L S L M P e V 1 L M S M M

26 Gain Saturation G. E. Betts et al., PTL, 006

27 Experimental Verification of Photocurrent Effect M =0.8 A/W D = 0.8 A/W D t I = t O = -3 db V = 0.85 V ) Gain (db Measured dgi Gain Calculated Gain Photocurrent 10 Modulator Photocurren nt (mw) Laser Power (mw) 1 Measured gain closely matches gain from model.

28 PCW EAM SFDs Multi-octave SFD Sub-octave SFD At 80 mw optical input power, Multi-octave SFD of 118 db-hz /3, sub-octave SFD of 13 db-hz 4/5.

29 Outline of Presentation Introduction: Analog fiber link Electroabsorption Modulator Multiple Quantum Wells and Gain Saturation E-O effect in InP nanowires Conclusion

30 Electrooptic Coefficient Electrooptic effect: 1 n r ijk E k s ijkle k E l ij Linear electrooptic coefficient, r, of quantum dots: 1 CdSe (dispersed in polymer) 5-60 pm/v InAs (QDs grown on GaAs substrate) 43 pm/v In 0.4 Ga 0.6 As (grown on GaAs substrate)5.8 pm/v QD systems exhibit 1- orders of magnitude enhancement over bulk electrooptic coefficient, due to quantum confinement effects and surface effects In the same token, it would be of much interest to examine the electrooptic coefficient of nanowires 1 F. Zhang, L. Zhang, Y. X. Wang, and. Claus, Appl. Opt. 44, 3969 (005). S. Ghosh, A. S. Lenihan, M. V. G. Dutt, O. Qasaimeh, D. G. Steel, and P. Bhattacharya, J. Vac. Sci. Technol. B 19, 1455 (001).

31 InP Nanowire Growth 1) Heat Sample in MOCVD reactor under PH 3 flow ) Start TMIn flow 3) Indium droplets form 4) Nanowire begins to grow InP Substrate

32 Optimized Growth T g =450 o C V/III ~ 5 Wires Cou unted Diameter (nm) Very uniform diameter, length High density (~10 9 NWs/cm )

33 Test Structure NWs under test Probe Gold SiO ITO SiO PMMA Glass Slide

34 Measurement esults Diameter (nm) Fill Factor r (pm/v) n 3 r (pm/v) InP NW % Bulk InP N/A N/A Bulk r =341 = N/A N/A n e.14 n e3 r 33 - n o3 r 13 = LiNbO 3 r 13 = 10.3 n o =. NW electrooptic coefficient exhibits an enhancement of 1- orders of magnitude over bulk InP Largest figure of merit is 0 times larger than LiNbO 3 This fabrication technique provides a method to transfer a layer of aligned NWs to a host substrate. A waveguide with embedded NWs could provide adequate phase modulation.

5x10 17, 00nm InGaAs, Si, 1.0x10 16, 00nm InGaAsP,Q1.4, undoped, 15nm InGaAsP,Q1.1, undoped, 15nm InP, Si, 1x10 16, 5nm InP, Si, 1.

35 High Material Power Photodiode Structure InGaAs,,p +, Zn,.0x10 19, 50nm InP, p+, Zn, 3x10 18, 1000nm InGaAs, Zn, x10 18, 100nm InGaAs, Zn, 1x10 18, 150nm InGaAs, Zn,, 5x10 17, 00nm InGaAs, Zn,.5x10 17, 00nm InGaAs, Si, 1.0x10 16, 00nm InGaAsP,Q1.4, undoped, 15nm InGaAsP,Q1.1, undoped, 15nm InP, Si, 1x10 16, 5nm InP, Si, 1.0x10 16, 600nm InP, n+, Si, 1.0x10 19, 1000nm InGaAs, n+, Si, 1.0x10 19, 0nm InP, n+, Si, 1.0x10 19, 00nm InP, semi-insulating substrate, Double side polished The material structure of the hybrid photodiode demonstrated by Professor Joe Campbell s team at the University of Texas

36 E-field of MUTC structure including the 50 load Doping conc(#/cm 3 ) InP Layer P absorber Lightlydoped n layer doping Electric Field(V/cm) i layer n layer E Field um

37 Conclusion Major advances in link gain has been made in links using traditional modulator such as lithium niobate MZM modulator The electroabsorption modulator (EAM) can be designed to have low optical loss and high power properties Nonetheless, electroaborption modulators can achieve high SFD. InP nanowires have great potential for effective electro-optic modulation.

HIGH-EFFICIENCY MQW ELECTROABSORPTION MODULATORS

HIGH-EFFICIENCY MQW ELECTROABSORPTION MODULATORS J. Piprek, Y.-J. Chiu, S.-Z. Zhang (1), J. E. Bowers, C. Prott (2), and H. Hillmer (2) University of California, ECE Department, Santa Barbara, CA 93106