Ultrafast Coherent Optical Signal Processing using Stabilized Optical Frequency Combs from Modelocked

Transcription

1 Ultrafast Coherent Optical Signal Processing using Stabilized Optical Frequency Combs from Modelocked Diode Lasers Peter J. Delfyett CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida University of California Santa Barbara, CA December 5, 2012

2 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques Summary and Conclusions 2

3 Motivation Why Diode Based Fiber Lasers? Diode lasers are small (100 s microns), electrically efficient (>70%), wavelength agile (300 nm to >10 microns via bandgap engineering). Robust, no moving / mechanical parts Broad bandwidth potential for large tuning bandwidth. Operates over very broad temperature ranges. Cost effective, direct electrically (battery) pumped. Can engineer the cavity Q to be >> than conventional cavities Potential for photonic integrated circuits, e.g., electronics, lasers, modulators & detectors full functioning optoelectronic systems on a chip computing & signal processing at the speed of light! 3

4 Applications Enabled By Optical Frequency Combs Sensing, Detecting and Response Synthetic Aperture Imaging Advanced Waveform Generation/Measurement Ultrawideband Communications 4

5 Power Power Amplitude Power Power Power Harmonic Modelocked Lasers Schematic Representations Time Interleaved Pulse Trains T C =L/c Time Overlaid Pulse Trains T= 1/f ML e it 2 Time Overlaid Supermode Spectra c/l 5 f ML Optical Frequency Time e i e i2 Amplitude Power Time Interleaved Supermode Spectra E() E(-) E(-2) f ML c/l Optical Frequency e it2t Time A 1 =1 A 2 =1 A 3 =0.5

6 Watts/Hz db/hz Intensity Intensity Intensity of Optical Pulse Train Optical Pulse Train Intensity (a) Intensity of Optical Pulse Train Optical Pulse Train Intensity (b) Time Optical Spectrum Optical of of Pulse Pulse Train Train (c) Time 20 RF Power Spectrum of Pulse Train RF Power Spectrum of Pulse Train (d) Supermode Noise Spurs Frequency Frequency

7 7 Low Noise Modelocked Diode Lasers Via Stabilization of the Frequency Comb

8 Power Power Fundamentally Modelocked Lasers c/l RF Power Spectrum Optical Frequency Frequency T=100 ps Time RF Power Spectrum Corner Frequency f mod =c/l =10 GHz 8 L 1 pulse in the cavity SOA ~ System Noise Floor Log Frequency Corner frequency moves to large offset frequencies w/ short cavities

9 Power Power Harmonically Modelocked Lasers 10GHz Coupled Modes c/l RF Power Spectrum Optical Frequency T=100 ps Time RF Power Spectrum Corner Frequency Supermodes f mod =Nc/L =10 GHz 9 L N pulses in the cavity N Independent longitudinal mode groups SOA ~ Example: Ring Laser Mode Spacing=10 MHz f mod = 10 GHz N=1000 System Noise Floor Log Frequency

10 Power Power Power Power Harmonic Modelocking & Supermode Suppression 10GHz 10GHz Optical Frequency T=100 psec Optical Frequency T=100 psec Time Time L T=100 psec L Supermode Suppression Filter =10GHz SOA SOA F mod =nc/l 10 = 10GHz ~ F mod =nc/l =10GHz ~

11 Transmission Transmission Nested Optical Cavities I T T2 i 0.0 I2 1 Rexp idi i i 1 R2 exp id R 1 =R 2 =90%; T 1 =T 2 =10%; FSR 2 / FSR 1 = (a) Frequency (b) Frequency Cavity Product Identical to R=99%; T=1% 11

12 Harmonically Mode-locked Lasers & Supermode Suppression Laser cavity DC DCF DCF PC IM PC FL PC etalon GHz SOA PC I FL SOA: semiconductor optical amplifier PC: polarization controller IM: intensity modulator I: isolator DCF: dispersion compensating fiber FL: fiber launcher Modulation rate Etalon transmission Mode spacing The etalon free spectral range must match the mode-locking rate. Laser cavity modes must coincide with etalon transmission peaks. ν 12

13 Setup Ultra-low noise osc. at GHz DC Output PDH Loop PC IM I OPS PID PC SOA VOD PC PM I PC Free Space Optics Cir PBS PC 640 MHz PD Laser Cavity FPE PC O PS I: isolator SOA: semiconductor optical amplifier OPS: Optical phase shifter PD: photodetector PC: polarization controller IM: intensity modulator PBS: polarization beam splitter FPE: Fabry-Perot etalon PID: PID controller PM: phase modulator Cir : optical circulator OPS: Optical Phase Shifter VOD: Variable Optical Delay DCF: Dispersion Comp. Fiber PDH: Pound Drever Hall 13

14 Actively MLL with intracavity 1000 Finesse etalon Laser is constructed on a optical breadboard and thermally and acoustically isolated with foam insulation. 14

15 Actively MLL with intracavity 1000 Finesse etalon Sampling scope and autocorrelation traces The pulses are compressed to 1.1 ps autocorrelation FWHM by using a dual grating compressor. 15

16 Actively MLL with intracavity 1000 Finesse etalon Optical spectrum High Resolution Comb Line The 10 db spectral width of the optical spectrum is ~8.3nm. The comb line has a ~50dB signal-to-noise ratio 16

17 Actively MLL with intracavity 1000 Finesse etalon Timing jitter and amplitude noise: Integrated timing jitter (1 Hz 100 MHz) is ~3fs and up to Nyquist it is 14fs. 17 Integrated amplitude noise (1 Hz 100 MHz) is 230ppm. Note the overall dynamic range of the measurement 10 16

18 Amplitude (dbm) Actively MLL with intracavity 1000 Finesse etalon Optical linewidth/stability measurement. High Resolution Spectrum Analyzer MLL CW laser PC OSA RFSA Stability CW laser Stabilized Frequency Comb lines Frequency (GHz) The linewidth of the laser with the 1000 Finesse etalon was measured as ~ 500 Hz (Note the relative ratio of the carrier frequency to the linewidth ~ ) Stability of 150 khz over 30 sec (NB: Measurements are limited by the CW laser linewidth & stability) 18

19 Low Noise Modelocked Diode Lasers The Effect of Intracavity Power 19

20 Gain (db) SCOW Amplifier SCOWA Slab-Coupled Optical Waveguide Amplifier P out (dbm) 1 A 2 A 3 A 4 A 20 J. J. Plant, et. al. IEEE Phot. Tech. Lett., v. 17, p.735 (2005) W. Loh, et. al. IEEE J. Quant. Electron., v. 47, p. 66 (2011)

21 Etalon stabilized HMLL Experimental setup Ultra-low noise oscillator GHz Laser Output PC PC IM ISO PZT SCOWA VOD PID PC ISO PM LPF OC PC PS PC DBM FPE (FSR = GHz) PC PBS CIR 500 MHz PD Pound-Drever-Hall Loop Optical Path Electrical Path CIR: Circulator DBM: Double Balanced Mixer FPE: Fabry-Perot Etalon ISO: Isolator LPF: Low-Pass Filter OC: Output Coupler (Variable) PC: Polarization controller PD: Photodetector PID: Proportional-Integral-Differential Controller PM: Phase Modulator PS: Phase Shifter PZT: Piezoelectric Transducer (Fiber Stretcher) SOA: Semiconductor Optical Amplifier (SCOWA) VOD: Variable Optical Delay 21 Long fiber cavity provides narrow resonances Fabry-Pérot Etalon provides wide mode spacing Pound-Drever-Hall loop locks both cavities An ultra-low noise oscillator is used to drive the laser I. Ozdur, et. al., PTL, v. 22, pp (2010) F. Quinlan, et. al., Opt. Express 14, (2006)

22 Relative Power (db) L(f) (dbc/hz) Integrated Timing Jitter (fs) Power (dbm) Power (dbm) Time (35 s) Etalon-based Ultralow-noise Frequency Comb Source Optical Spectrum Wavelength (nm) ~60 db Radio-Frequency Spectrum Span: 100 MHz Res. BW: 3 khz Frequency (GHz) High-Resolution Optical Spectrum Span: 1 GHz Res. BW: 1 MHz ~60 db Frequency (100 MHz/div) Optical Frequency Stability Measurement Etalonstabilized laser ( GHz) Etalonstabilized laser ( GHz) Real-time Spectrum Analyzer Single sideband phase noise spectrum Residual Phase Noise Noise Floor Poseidon Oscillator Absolute Noise k 10k 100k 1M 10M 100M Frequency Offset (Hz) Real-time spectrogram Frequency Offset (MHz)

23 AC Trace (a.u.) Relative Power (db) Oscillator characterization Intensity Autocorrelation RF Power Spectrum Compressed AC Transform Limited AC p = 930 fs Span: 100 MHz Res. BW: 3 khz Delay (ps) Frequency (GHz) Pulses are compressible to close to the transform limit Photodetected RF tone has >90 db dynamic range 23

24 Optical Power (dbm) Amplification Output power and spectral characteristics Directly from MLL I=4A, P out =214 mw I=4A, P out =320 mw Wavelength (nm) 24

25 L(f) (dbc/hz) Integrated Jitter (fs) Timing Jitter SSB Phase Noise Comparison (ii) (iii) (iv) (i) All-anomalous Cav. and SCOWA 10 (ii) Disp. Comp. Cav. (iii) All-anomalous and Covega (iv) Poseidon Oscillator 8 Noise Floor -110 (i) k 10k 100k 1M 10M 100M Frequency Offset (Hz) 0 25

26 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques High Precision Laser Radar w/ Unambiguous Ranging & Velocimetry Summary and Conclusions 26

27 General Ideas for OFC Modulation Desirable Modulator Qualities for real time OFC applications: - Linear modulation transfer function - Large modulation bandwidth - Low Insertion Loss (negative..?) - Low V π - Good power handling capability - Comb filtering, tunable, arrays Current methods of modulating light intensity: Direct modulation of diode driving current Frequency chirp External modulation: Electro-optic modulators (EOM) Nonlinear modulation transfer function and Relatively high V π Electro-absorption modulators (EAM) Poor optical power handling, High insertion loss and Sensitive to temperature and wavelength Proposed concept for OFC modulation: Injection locking a resonant cavity w/ gain (VCSEL) arcsine phase modulation NB: Linear intensity modulator in an interferometric configuration 27

28 Injection-Locked Resonant Cavity as an Arcsine Phase Modulator ω 1 Master laser Adler s equation*: ω 0 Slave laser φ ω π 2 ω 1 ω 0 φ ω = sin 1 ω ω m ω = ω 1 ω 0 2ω m : locking range ω 0 ω = ω 1 ω 0 ω 1 π 2 *A. E. Siegman, Lasers, 1986 Locking range 28

29 Resonant Cavity Interferometric Modulator Comparison to a Conventional MZ Modulator Resonant cavity linear modulator I in sin ( ( )) f(t) ~ 1 f t I 0,ω 1 f ( t) 1 Iout Iin ( ) 2 T(V) Phase response Stable locking range Calculate SFDR I out π/2 V Electro-optic Mach-Zehnder modulator T(V) I in f(t) ~ 0 V ( t) / V 1 cos( ) I out I in ( ) 2 I out V 29

30 Phase Modulation & Filtering -Channel selection concept Ch. 1 Ch. 2 VCSEL Bias T DC current= I 1 Ch. N Comb Modulated Output AC Modulation; f 1, P(f) ω= ω + f 1 I(ω) 0 Filtering & Modulation f 1 f Ch. 1 DC=I 1 Ch. 2 DC=I 2 ω RF Spectrum Optical Spectrum 30

31 Frequency (THz) Phase Modulation & Filtering -Channel selection concept Ch. 1 Ch. 2 VCSEL Bias T DC current= I 2 Ch. N Comb Modulated Output AC Modulation; f 2 P(f) ω= ω + f 2 Filtering & Modulation I(ω) 0 f 2 f Frequency vs. Current Measurement Linear fit Ch. 1 DC=I 1 Ch. 2 DC=I 2 ω RF spectrum Slope ~ 50 GHz/mA Optical Spectrum DC Driving Current (ma) 31

32 Static phase (radian) Power (dbm) Fundamental & intermodulation power (dbm) CW laser Iso Linear Modulator Experimental Results GHz 1 GHz VOA + PZT PC RF Bias Tee VCSEL CIR PC PID I DC 50/50 90/10 PD VCSEL: vertical cavity surface emitting laser Iso: isolator VOA: variable optical attenuator PC: polarization controller PZT: piezoelectric transducer PD: photo detector PID: proportional-integrated-differential controller CIR: circulator OSA: optical spectrum analyzer RFSA: RF spectrum analyzer EDFA High-res OSA PD RFSA SFDR = 130 db.hz 2/3 Fundamental IM3 Noise floor Spur free dynamic range of ~130 db.hz 2/3 Very low V π of ~ 2.6 mv Multi-gigahertz bandwidth (~ 5 GHz) Possible gain Measurement 0 Fit DC Current Deviation (ma) RF Input (dbm) db Frequency (GHz) 32

33 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques High Precision Laser Radar w/ Unambiguous Ranging & Velocimetry Summary and Conclusions 33

34 Direct demodulation of phase modulated signals Operating principle: Detecting light-induced changes in the forward voltage of an optically injection locked VCSEL operating above threshold. Physical origin: Voltage change is due to the change in the carrier density in the active region of the VCSEL when driven by an external phase modulated light. I (ω) & ψ(ω) V(ω) ω l ω o ω h ω Δω Locking range N. Hoghooghi, et. al, IEEE Photonics Technology Letters, 22(20), pp ,

35 Channel filtering concept Ch. 1 Ch. 2 VCSEL Bias T AC voltage DC voltage Ch. N I(ω) π π 0 0 π π 0 0 Phase detector P(f) Ch. 1 f mod =f 1 Ch. 2 f mod =f 2 ω f 1 f 2 f Optical spectrum RF spectrum 35

36 Demodulation & channel filtering with an injection-locked VCSEL Nx1 combiner Power (db) Ch.2 (0.9 GHz) Ch.1 (0.8 GHz) Optical path Electrical path RFSA CW laser 12.5 GHz IM WDM filter PC PC PM PM PC CIR RF Bias T DC VCSEL PC PM OSA Ch.3 (1 GHz) 0-10 VCSEL PC: polarization controller PM: phase modulator IM: intensity modulator CIR: circulator OSA: optical spectrum analyzer RFSA: RF spectrum analyzer Ch.3 Ch.2 Ch Wavelength (nm) 36

37 Power (dbm) Power (dbm) Power (dbm) Power (db) Power (db) Power (db) Optical spectra Experimental results of three channel system Ch.1 Ch.2 Ch Wavelength (nm) Wavelength (nm) Wavelength (nm) Corresponding detected RF spectra RBW 30 khz Span 270 MHz SNR ~ 60 dbc/hz -65 RBW 30 khz -70 Span 270 MHz SNR ~ 62 dbc/hz RBW 30 khz Span 270 MHz SNR ~ 60 dbc/hz Frequency (MHz) Frequency (MHz) Frequency (MHz) First demonstration of direct demodulation and channel filtering of phase modulated signals with SNR of 60 dbc/hz. 37

38 Linear Modulator Concept for Pulsed Light - A resonant cavity (Fabry-Perot) with multiple resonances, injection locked by a modelocked laser as the frequency comb. - By simultaneous modulation of the period combs, one imparts arcsine phase modulation on each injected comb. Received RF signal 1/f rep Fabry-Perot Laser FSR=f rep Optical Frequency MLL FP FP resonances Corresponding phase responses Injected comb lines from the MLL Imparted phase on each injected combs 38

39 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques Summary and Conclusions 39

40 Multi-heterodyne detection of frequency combs Motivation Mode-locked laser f A W D M WDM f AM AM Extremely complex arbitrary waveforms can be generated with frequency combs Instantaneous bandwidth in the order of several THz PM PM f W D M A dt ~ 1/BW BW f 40

41 Optical Power Spectral Density RF Power Spectral Density Multi-heterodyne Detection of Frequency Combs (Optical Sampling) (a) (b) f rep PLL Comb Source Comb Source f (1) rep D LPF RFSA Diagnostics Oscilloscope δ f (2) rep ν Photodetection δ ν Δ Δ+δ Δ+2δ Δ f (1) rep ½f (1) rep f 2f (1) rep f (2) rep 41 Each pair of comb-lines generates a unique RF beat-note The RF beat-note retains the relative phase between the comb-lines

42 Multi-heterodyne detection of frequency combs Experimental results Mode-locked laser combs Power (5 db/div.) Power (dbm) Power (dbm) Optical spectra First two sets of RF beat notes Wavelength (nm) Effective repetition rate detuning ~600 khz Total Optical BW ~ 17nm ~2.12THz Compression factor ~ 17,000x Frequency (MHz) 10 GHz spacing optical comb is mapped into a 600 khz spacing RF comb Frequency (MHz) 42

43 Pulse Combs Time Domain Experimental Results (10 GHz & 250 MHz) Voltage [mv] Amplitude (a.u.) Time domain waveform Stretched Dispersion Normal 0-10 Direct output time [s] 0 5 Time (μs) Compressed Anomalous As the optical pulse is stretched and compressed, the RF waveform does the same Optical waveform is mapped to RF waveform 43

44 Phase Modulated CW Combs Experimental Results ~10 GHz CW Laser Phase Modulator RFSA Erbium Fiber Mode-locked Laser Real time oscilloscope 44

45 Instantaneous Frequency (MHz) Amplitude (mv) Amplitude (a.u.) Phase () Multi-heterodyne detection of frequency combs Experimental results Phase modulation combs Time domain waveform Time (s) Instantaneous frequency 60 Fourier transform Frequency (MHz) Time (s) The optical waveform chirp is mapped to the RF waveform Spectral phase information can be retrieved 45

46 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques Summary and Conclusions 46

47 Intensity Intensity Intensity Optical DACs using Frequency Comb Filtering Static Approach T 1/T Time Frequency T 1/T Time Frequency 47

48 Optical DACs using Frequency Comb Filtering Dynamic Approach WDM DeMux Modulator Array Maximum Modulation Rate F~ WDM Mux Modelocked Comb Generator N Combs Comb Spacing Ultra-Pure CW channels Modulated CW Channels Arbitrary Waveform Instantaneous Bandwidth Nx Temporal Gate 48 Pulse Shaping at the Highest Possible Spectral Resolution Challenges: Some waveforms require phase modulation well beyond 2

49 A Novel Concept for Ultra-High-Speed Optical Pulse Shaping Phase / Amplitude > 10 6 increase in the refresh rate!!! Our novel idea is different than the conventional approaches in 4 ways: Instead of manipulating the existing optical combs, we regenerate the optical combs with the desired amplitudes and phases The refresh rate is limited by the modulation speed of the VCSELs (10s of GHz) The channel count can easily be scaled by going from 1-D array into 2-D array geometry Simultaneous modulation and amplification 49 49

50 High-speed Reconfigurable Optical Arbitrary Waveform Generation Four optical comblines, independently modulated and coherently combined Wavelength demux and mux pair 6.25 GHz channel spacing Each modulator Injection-locked VCSEL with current modulation 50

51 Power (dbm) Voltage (mv) Experimental Setup Optical Frequency Comb Source Generated by modulation of CW laser Adjust DC bias voltages, RF phases and amplitudes to achieve five combs of 0 equal power ps ~30 ps Wavelength (nm) Time (ns)

52 Experimental Setup Demux, Mux Specifications Essex Hyperfine WDM filters Fiber-pigtailed input and outputs Channel spacing of 6.25 GHz Adjacent channel isolation ~ 22 db Gaussian shaped passband 3 db channel bandwidth ~ 3.5 GHz Mux, Demux are a matched pair 52

53 Power (dbm) Voltage (V) Voltage (V) Intensity Profile of Rapidly Updated Optical Waveforms VCSEL RF frequency (MHz) VCSEL RF frequency (MHz) m 100m 0 200m n 2.56n 3.84n 5.12n 6.40n 7.68n 8.96n Time (s) 100m 0-20 Time (s) 5.120n 6.400n Photodetected RF spectrum G 12.50G 18.75G 25.00G Frequency (Hz)

54 Power (dbm) Intensity Profile of Rapidly Updated Optical Waveforms VCSEL RF frequency (MHz) Optical Spectrum -10 VCSEL 3 IL trc -20 VCSE Freq.( RF (d Frequency (THz)

55 Reconfigurable Cross Connect Switch / Pulse Shaping Code Reconfiguration DC 3 DC 1 DC 4 DC 2 DC 1 DC 3 DC 2 DC 4 Information from any wavelength can be arbitrarily switched between channelsat rates approaching channel spacing. 100 times faster that the existing MEMS technology. 55

56 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques Summary and Conclusions 56

57 Matched Filtering using OFC s 57

58 Comparison to OCDMA Optical Code Division Multiple Access (OCDMA) Spectral modulation, temporal spread Decoding: Needs non-linear optical thresholding because of slow response time of photodetectors Heritage and Weiner, IEEE JSTQE, 2007 Coherent detection technique is linear Requires less optical power Jiang et al., IEEE PTL,

59 Complete Experimental Setup 59

60 Interference using Orthogonal Codes Using orthogonal codes gives best contrast between different binary sequences λ PD 0,0,0, λ 0,0,0, λ Code A Code B Code C Code D λ PD 0,π,0,π λ 0,0,0, λ Differential signal - PD λ Differential signal - PD λ 60

61 Summary of Results, of Matched Filtering Q match match mismatch mismatch 11.5 BER erfc Q 30 61

62 Outline Motivation Background Key Technologies Stabilized Optical Frequency Combs Arcsine Phase & Linear Intensity Modulators w/ Comb Filter Direct Phase Detection (w/o external local oscillator) w/ Comb Filter Applications Arbitrary Waveform Measurements Arbitrary Waveform Generation Pattern Recognition using Matched Filtering Techniques Summary and Conclusions 62

63 Summary Demonstrated key technologies and applications using OFC s Key Technologies Stabilized optical frequency combs (1.5 fsec jitter; <1kHz, 10Hz) Lowest noise mode-locked comb source at 10 GHz & 1550nm Linear intererometric intensity modulators & channel filtering First linear interferometric modulator (130 db/hz 2/3 SFDR, V π =2.6mV) Direct phase detection and channel filtering (>60 dbc/hz) Applications Arbitrary waveform measurements (A to D Converter ) Reconstruction of Incoherent (Independent) Sources Arbitrary waveform generation (D to A Converter) Fastest true real-time waveform generation (Mod Rates: >3GHz; IB: >22 GHz) Matched filtering w/ differential photodetection (BER=10-30 ) 63

64 64

65 Phase Fundamentals of Injection Locking Using VCSELs as Modulators Optical intensity and phase response vs. Δω Δω controlled via current modulation of VCSEL Intensity change is small Phase difference between master and slave light, φ 0 : 1 1 f0 sin tan L 1 2 f0 cot A.E. Siegman, 65 Lasers, Chap. 29, University Science Books, 1986 F. Mogensen, et al., IEEE J. Quantum Electronics., vol. 21, 1985 Master Laser Phase response Output intensity ω 1 ω fr Locking range = ω L Δω =ω 1 ω fr α Linewidth enhancement factor Slave Laser (VCSEL) ω

66 Resonant Cavity Interferometric Modulator - Theory Injection-locked laser phase response φ-φ 1 π/2 Locking Range ω o ( ) sin ( ) 1 ω 0 : slave frequency m ω 1 : master frequency ω 1 Iin Linear Modulator sin 1 ( f ( t)) Iout -π/2 Put them together Mach-Zehnder Interferometer I out I in 1 cos(sin ( 1 ( f ( t)) / 2) ) 2 I in 1 f ( t) ( ) 2 I in I in 1 cos( ) ( ) 2 66

67 Resonant Cavity Interferometric Modulator - Comparison to a conventional MZ modulator Resonant cavity linear modulator I in sin ( ( )) f(t) ~ 1 f t I 0,ω 1 f ( t) 1 Iout Iin ( ) 2 T(V) Phase response Stable locking range Calculate SFDR I out π/2 V Electro-optic Mach-Zehnder modulator T(V) I in f(t) ~ 0 V ( t) / V 1 cos( ) I out I in ( ) 2 I out V 67

68 Phase Modulation & Filtering -Channel selection concept Ch. 1 Ch. 2 VCSEL Bias T DC current= I 1 Ch. N Comb Modulated Output AC Modulation; f 1, P(f) ω= ω + f 1 I(ω) 0 Filtering & Modulation f 1 f Ch. 1 DC=I 1 Ch. 2 DC=I 2 ω RF Spectrum Optical Spectrum 68

69 Frequency (THz) Phase Modulation & Filtering -Channel selection concept Ch. 1 Ch. 2 VCSEL Bias T DC current= I 2 Ch. N Comb Modulated Output AC Modulation; f 2 P(f) ω= ω + f 2 Filtering & Modulation I(ω) 0 f 2 f Frequency vs. Current Measurement Linear fit Ch. 1 DC=I 1 Ch. 2 DC=I 2 ω RF spectrum Slope ~ 50 GHz/mA Optical Spectrum DC Driving Current (ma) 69

70 Power (dbm) Voltage(V) Static phase (radian) Linear interferometric modulator setup CW laser Iso Optical path VOA Electrical path PZT PC VCSEL: vertical cavity surface emitting laser Iso: isolator VOA: variable optical attenuator PC: polarization controller PZT: piezoelectric transducer RF Bias Tee VCSEL CIR PC Piezo driver I DC 50/50 PD PD: photo detector CIR: circulator High-res OSA: High resolution optical spectrum analyzer RFSA: RF spectrum analyzer High-res OSA RFSA 0 Measurement 0 Fit V 0 π ~ 2.6 mv DC Current Deviation (ma) Time(sec) -10 db bandwidth 10 db ~5 GHz Frequency (GHz) 70

71 How to measure linearity of a modulator? -Two-tone experiment V(t) I in Modulator I out = ω 1 ω 2 2ω 1 2ω F 2ω 1 ω 2 ω 1 ω 2 2ω 1 2ω 2 3ω 1 3ω 2 2ω 2 ω 1 Spur-free dynamic range (SFDR) Noise floor ω 3ω 1 3ω 2 + 2ω 1 -ω 2 2ω 2 -ω 1 N. Hoghooghi and P. J. Delfyett, IEEE Journal of Lightwave Technology, 29(22), pp ,

72 Analog link employing linear modulator GHz 1 GHz + RF Bias Tee I DC VCSEL High-res OSA CW laser Iso VOA PC CIR 50/50 90/10 1 km of fiber EDFA PD RFSA PZT PC PID PD VCSEL: vertical cavity surface emitting laser Iso: isolator VOA: variable optical attenuator PC: polarization controller PZT: piezoelectric transducer PD: photo detector PID: proportional-integrated-differential controller CIR: circulator OSA: optical spectrum analyzer RFSA: RF spectrum analyzer Optical path Electrical path 72

73 RIN [dbc/hz] Integrated RMS RIN (%) Power (dbm) Fundamental & intermodulation power (dbm) Spur-free dynamic range measurement of the link Sample RF spectrum Frequency (GHz) SFDR = 130 db.hz 2/3 Fundamental IM RIN Frequency Offset [MHz] RF Input (dbm) Noise floor Power of the fundamental is a factor of >3,000,000^2 higher than third-order intermodulation power. Order of the magnitude better than DARPA project goal. 73

74 Modelocking Basics A Review Optical Cavity Allowed Modes c/2l L Laser Medium Spontaneous Emission Spectrum Laser Cavity Laser Spectrum 74

75 Modelocking Basics A Review E-Field Spectrum E-Field Modulated E-Field Modulator m P =2/ o - m o E-Field Spectrum o o + m Modelocked Spectrum c= o =2 75 T=2L/c

76 Coherent Optical Signal Processing & Communications using Optical Frequency Combs What are optical frequency combs? Coherent, stabilized cw optical frequencies generated on a periodic frequency grid, (e.g., a set of longitudinal modes from a modelocked laser). Modulator P =2/ Modelocked Spectrum Why re-visit coherent communications/signal processing? Allows the use of E(t) as compared to I(t) high spectral efficiency. (80x -200xincrease) Coherent combs of stabilized optical frequencies are easily obtainable from modelocked lasers. Channel conditioning can be done simply ((frequency stabilization of the entire comb as compared to individual lasers). Sets of combs at separate locations can be made coherent (frequency and phase) 76 T=2L/c Optical Frequency Combs

77 Ultrafast Photonics Group Systems Applications Optical Networks for Signal Processing & Communications Optical Sampling for A-to-D Converters Arbitrary Waveform Generation Precision Laser Radar Fundamental Physics Quantum Dot Ultrafast Light- Matter Dynamics New Device Development Q-Dot Optical Amplifiers Modulators & Photodetectors Active Optical Filters

78 Stabilized Comb Source Specs Simultaneous optical frequency stabilization and supermode suppression of a GHz harmonically mode-locked laser with: 1.1ps pulse width with and 50 db suppresion to the next observable optical mode. 500 Hz optical linewidth and sub 150 khz maximum frequency deviation in 30 seconds. 3 fs integrated timing jitter from (1 Hz 100 MHz) and 14 fs timing jitter extrapolated to Nyquist (1 Hz 5.14 GHz). Ozdur I., et al, A semiconductor based 10-GHz optical comb source with 3 fs integrated timing jitter (1Hz-100MHz) and ~500 Hz comb linewidth Photonic Technology Letters Vol. 22, No. 6, March 15,

79 Frequency Offset (MHz) Power (dbm) Rel. Power (db) Oscillator characterization Optical Spectra Optical Spectrum Wavelength (nm) Single comb-line beat-note 2 khz FWHM Lorentzian 1 khz FWHM Lorentzian Span: 2 MHz Res. BW. 100 Hz Frequency Offset (MHz) Spectrogram time (s) 79

80 L(f) (dbc/hz) Integrated Timing Jitter (fs) M(f) (dbc/hz) Integrated AM Noise (%) Phase and amplitude noise Single Sideband Phase Noise Directly from MLL Amplified (P out ~ 200 mw) k 10k 100k 1M 10M 100M Frequency Offset (Hz) Pulse-to-pulse energy fluctuations Directly from MLL Amplified k 10k 100k 1M 10M 100M Frequency Offset (Hz)

81 AC Trace (a.u.) Relative Power (db) Oscillator characterization Pulses Compressed AC Transform Limited AC p = 930 fs Span: 100 MHz Res. BW: 3 khz Delay (ps) Frequency (GHz) Pulses are compressible to close to the transform limit Photodetected RF tone has >90 db dynamic range 81

82 Conclusions An optical comb source has been built with: Stable (instability < THz over 60 s), low line-width (< 1 khz) optical comb High repetition rate (10 GHz) optical pulse-train Short pulses generated from a dispersion compensated cavity (τ p <1 ps) Power Amplification with a Slab-Coupled Optical Waveguide Amplifier yields High optical power (up to 390 mw, > 5 mw per comb-line) No evident degradation in Phase (14 fs jitter integrated to Nyquist) and Amplitude Noise (< 0.03%, 1 Hz to 100 MHz) 82

83 Linear Intensity Modulator System Configuration -Iso: Isolator -PC: Polarization Controller -PS: Optical Phase Shifter -VOA: Variable Optical Attenuator -TEC: Temperature Controller -Cir : Circulator -VCSEL: Vertical Cavity Surface Emitting Laser -RFSA: Radio Frequency Spectrum Analyzer -OSA: Optical Spectrum Analyzer 83

84 Concept of Photonic Arbitrary Waveform Generation Static Fourier Analysis f K A0 ( t) Ak cos( k0t k ) 2 k 1 k : periodic frequency components A k : amplitude of the k th frequency component α k phase of the k th frequency component Performance Characteristics Limited to periodic signals Minimum periodicity ~ Mode spacing - filter spacing Accuracy determined by number of combs 84