High-Speed Directly Modulated Lasers

Transcription

1 High-Speed Directly Modulated Lasers Tsuyoshi Yamamoto Fujitsu Laboratories Ltd. Some parts of the results in this presentation belong to Next-generation High-efficiency Network Device Project, which Photonics Electronics Technology Research Association (PETRA) contracted with New Energy and Industrial Development Organization (NEDO).

2 Outline Direct modulation of semiconductor lasers Frequency response of semiconductor lasers Limiting factors of modulation bandwidth Approaches for high-speed modulation High-speed distributed reflector lasers High-speed direct modulation 25-Gbps direct modulation 40-Gbps direct modulation Other approaches Summary 1

3 Direct Modulation of Semiconductor Lasers Current Light output Optical signal Light Threshold current: I th Current Electrical signal Simplest way to generate intensity-modulated optical signal 2

4 Transmitters for Optical Fiber Communication Transmission distance (km) CW laser + LiNbO 3 modulator EML Directly modulated laser (DML) 1 1G 10G 100G Modulation speed (bps) DML: Small size & low power consumption Attractive for short-reach application EML: Electroabsorption modulator integrated laser 3

5 Directly Modulated Lasers in Photonic Networks SONET/SDH 1.3-µm FP and DFB LDs 155 Mbps-10 Gbps, <10 km CWDM/DWDM (with TEC) 1.5-µm DFB LDs 2.5 Gbps, <100 km Access GE-PON, G-PON Downstream: 1.49-µm DFB LDs Upstream: 1.3-µm FP or DFB LDs 10GE-PON, XG-PON Upstream: 1.27-µm DFB LDs LAN (GbE, 10GbE), SAN(FC) 0.85-µm VCSELs, << 500 m 1.3-µm FP and DFB LDs, < 10 km 4

6 Optical Signal from Directly Modulated Lasers Electrical input Current Time Optical output Light 5 Time Features Overshoot on the leading edge and oscillation Increase in jitter under large extinction ratio condition Wavelength chirping (adiabatic and transient chirp) Optical signal generated from DML is not ideal. This is mainly due to change of carrier density in the active layer.

7 10-Gbps Direct Modulation Example of 10-Gbps eye pattern (with filter) To suppress influence of the overshoot, setting relaxation oscillation frequency (f r ) much higher than 10 GHz Filtering the oscillation by receiver with limited bandwidth Driving current is usually determined by f r, not by output power. To suppress jitters related with turn-on delay, utilizing low extinction ratio less than 7dB 6

8 Modulation Speed of Directly-Modulated Lasers Frequency response of a semiconductor laser R(f) = f r 2 ((f 2 -f 2 r ) 2 + f 2 γ 2 /(2π) 2 ) 1/2 (1+(2π fcr) 2 ) 1/2 1 f r :Relaxation oscillation frequency γ :Damping coefficient ( Kf r 2 ) C:Capacitance R:Resistance Limiting factors of bandwidth Response (db) f r :15 GHz K : 0.3 ns CR bandwidth:20 GHz Frequency (GHz) Relaxation oscillation frequency ~ 1.55 f r Damping ~ 8.89/Κ CR time constant ~ 1/(2πCR) 7

9 How to Increase f r? f r ( ) 1/2 L W N w L w Decreasing active-region volume Increasing differential gain (dg/dn) (I-I th ) Γ : Optical confinement factor dg/dn : Differential gain L : Active region length W : Active region width N w : Number of wells L w : Well thickness I : Injection current I th : Threshold current Length (L ), Width (W) Γ dg/dn *Reduction of thickness (N w, L w ) is mostly compensated by reduction of optical confinement factor (Γ). Selection of material system Lowering threshold gain Setting of detuning 8

10 To Reduce Active-Region Volume: Length Cavity structure is a key issue in reduction of active-region length. Active region λ/4 shift DFB DFB DFB DFB Integration - - Waveguide DBR mirrors λ/4 shift Facet coating AR/AR AR/HR AR/HR AR/AR Short active Difficult Difficult Easy Easy region (< 150 µm) Butt-joint regrowth No No Yes Yes Threshold gain High Medium Medium Low Single-mode yield Good Fair Fair Good For short active region (< 150 µm), waveguide integration is necessary. HR coating reduces threshold gain but deteriorates single-mode yield. 9

11 To Reduce Active-Region Volume: Width Active-region width depends on waveguide structure. Ridge waveguide structure Buried heterostructure (BH) Active layer Active layer Active region Width Defined by ridge width and current spreading Relatively wide (Usually > 2 µm) Defined by mesa width Narrow (Usually < 1.5 µm) Fabrication Etching of ridge Etching of mesa and regrowth Point Control of guided optical mode and current spreading Suppression of leakage current at regrowth interface and current blocking structure 10

12 To Obtain Large dg/dn : Material System AlGaInAs quantum well (QW) InGaAsP quantum well (QW) Superior band diagram of AlGaInAs QW Large conduction band offset (ΔE c ) Small valence band offset (ΔE v ) Better electron confinement Increase in differential gain 11

13 f r of AlGaInAs and InGaAsP lasers AlGaInAs quantum-well laser InGaAsP quantum-well laser f r (GHz) (I-I th ) 1/2 (ma 1/2 ) f r (GHz) , 50, 75, 85ºC 25, 50, 75, 85ºC FP laser L = 300 µm FP laser L = 300 µm (I-I th ) 1/2 (ma 1/2 ) Larger f r and smaller temperature dependence in AlGaInAs QW laser Ref. T. Ishikawa et al., Proc. of 10th Int. Conf. on Indium Phosphide and Related Materials, pp ,

14 To Obtain Large dg/dn : Design of Threshold Gain Decrease in active-region length Γg th 1/L Gain, g Decrease in dg/dn Increase in threshold gain, Γg th if too much Carrier density, n Decrease in differential gain Keeping threshold gain sufficiently low in short active region Increase optical confinement factor Large number of quantum wells Increase optical feedback of reflector Large coupling coefficient of grating ( > 100 cm -1 ) High-reflection coating or integrated mirror 13

15 To Obtain Large dg/dn : Setting of Detuning Detuning, Δλ Difference between lasing wavelength (λlaser ) and gain peak wavelength (λgain ) Gain, g Larger dg/dn Generally, λlaser shorter than λgain provides larger dg/dn. Wavelength, λ n 2 > n 1 In wide temperature range operation Δλ depends on temperature. Gain, g Low temperature nm/K λ Laser λ Laser High temperature Optimization to support whole temperature range is necessary nm/k Wavelength, λ 14

16 f r of Lasers with Different Detuning (Example) 1.55-µm AlGaInAs distributed reflector lasers with 75-µm long active region nm 5.0 GHz/mA 1/ nm 4.5 GHz/mA 1/ fr (GHz) GHz/mA 1/ (I - I th ) 1/2 (ma 1/2 ) fr (GHz) GHz/mA 1/ (I - I th ) 1/2 (ma 1/2 ) Ref. A. Uetake et al., 22nd Annual Meeting of IEEE Photonics Society, ThBB3,

17 Influence of Damping Damping Suppression of relaxation oscillation mainly due to nonlinear gain effect Intrinsic bandwidth of semiconductor laser Damping K factor: K ~ γ /f r 2 Major factors to determine K Nonlinear property of gain material itself dg/dn Photon lifetime 16 Response (db) K : 0.25 ns CR bandwidth: 40 GHz f r :15 GHz 20 GHz 25 GHz 30 GHz Frequency (GHz) Selection of material Optimization for f r Room for design Large K factor prevents high speed modulation, but in some cases appropriate damping suppresses overshoot due to relaxation oscillation.

18 CR Time Constant Design as small as possible Resistance Cladding layers Optimization of doping profile Interface of heterostructure Decreased band discontinuity and optimized doping Electrode contact (increase with reduced active-region area) Capacitance (almost independent of design of active layer and cavity) Accompanied by pn junction Utilizing semi-insulating current blocking structure in BH lasers Reduced area of upper cladding layer Bonding pad Small size 17

19 High-Speed Distributed Reflector laser Introducing concept of Distributed Reflector (DR) laser* to short-cavity lasers DBR mirror DBR mirror DFB region AlGaInAs MQW active layer AR coating AR coating Integrating DBR mirrors on both sides of the DFB active region Reducing active-region length beyond limit of cleaving process Avoiding influence of phase variation at facets by integrated mirrors and AR coatings Increasing optical feedback to decrease threshold gain in shortcavity lasers *Ref. J-I. Shim et al., IEEE J. Quantum Electron., vol. 27, pp ,

20 AlGaInAs DR Laser with 100-µm-long Active Region 50 µm 100 µm 100 µm Output power (mw) ºC 50ºC 70ºC 85ºC I th = 3.6 ma (25ºC) 11.0 ma (85ºC) Current (ma) Intensity (db) Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB ºC 50ºC 70ºC 85ºC -60 I = 80 ma Wavelength (nm) 19

21 AlGaInAs DR Laser with 100-µm-long Active Region 50 µm 100 µm 100 µm GHz/mA 1/2 10 I = 80 ma fr (GHz) ºC 50ºC 70ºC 85ºC Response (db) ºC 85ºC 25ºC 50ºC GHz/mA 1/ (I-I th ) 1/2 (ma 1/2 ) -20 f 3dB = 28.3 GHz (25ºC) 20.8 GHz (85ºC) Frequency (GHz) Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB3. 20

22 Recent Reports of 25-Gbps direct modulation Material system Cavity Wave -guide Active region length Temp. [ºC] Publication Fujitsu/OITDA AlGaInAs DFB BH 150 µm 70 Electron. Lett NTT AlGaInAs DFB Ridge 200 µm 85 OFC2009 Finisar AlGaInAs DFB Ridge 200 µm 45 OFC2009 Sumitomo AlGaInAs DFB Ridge 250 µm 25 IPRM2009 Hitachi/OITDA AlGaInAs DFB Ridge 160 µm 95 ECOC2009 Hitachi/PETRA AlGaInAs DFB Ridge 150 µm 100 CLEO2010 Fujitsu/PETRA/Univ. of Tokyo/QD Laser Quantum dot FP High mesa 400 µm RT CLEO2010 Fujitsu/PETRA AlGaInAs DR BH 125 µm 50 OECC2010 Mitsubishi AlGaInAs DFB(+WG) BH 150 µm 50 ISLC2010 Wavelengths are 1.3 µm in all reports. Modulation speeds are 25 to 26 Gbps. 21

23 25-Gbps Eye Patterns of 1.3-µm DFB Laser NRZ signal, PRBS = Dynamic extinction ratio: 5.0 db 150 µm 25ºC 50ºC 70ºC 80ºC 10 ps/div. I bias = 42 ma I bias = 47 ma I bias = 55 ma I bias = 62 ma I mod = 40 ma p-p I mod =40mA p-p I mod =40mA p-p I mod =40mA p-p Ref. K. Otsubo et al., J. Select. Topics Quantum Electron., vol. 15, pp ,

24 Single Mode Fiber Transmission using 25-Gbps DML Penalty (db) Gbps, ER = 5.5 db BER of Penalty from back-to-back at 25ºC = nm 25ºC: λ Laser = nm 70ºC: λ Laser = nm Transmission distance (km) Ref. K. Otsubo et al., J. Select. Topics Quantum Electron., vol. 15, pp ,

25 Influence of Fiber Dispersion in 25-Gbps DML Experimental setup AlGaInAs DFB LDs 25 Gbps λ Laser = 1296, 1303, 1322 nm SMF km Penalty (db) Worst case of 10-km transmission within LAN-WDM grid 1296 nm 25ºC 1303 nm 1322 nm -32 ~ +15 ps/nm -28 ps/nm +9 ps/nm Chromatic dispersion (ps/nm) Ref. K. Otsubo et al., J. Select. Topics Quantum Electron., vol. 15, pp ,

26 Four 1.3-µm DR Lasers for LAN-WDM Application Relative intensity 50ºC, CW 2 mw 20 db λ 1 λ 2 λ 3 λ 4 25 µm 125 µm 100 µm Wavelength (nm) Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference (OECC2010), 6D

27 Light-Current Characteristics of LAN-WDM DR Lasers 15 λ 1 λ 2 λ 3 λ 4 Power (mw) ºC 50ºC 85ºC 25ºC 50ºC 85ºC 25ºC 50ºC 85ºC 25ºC 50ºC 85ºC Current (ma) λ 1 λ 2 λ 3 λ 4 I th 25ºC I th 50ºC Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference (OECC2010), 6D

28 f r of LAN-WDM DR Lasers ºC 3.6 GHz/mA 1/2 f r (GHz) λ 1 λ 2 λ 3 λ (I - I th ) 1/2 (ma 1/2 ) λ 1 λ 2 λ 3 λ 4 f r I = 50 ma Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference (OECC2010), 6D

29 25.8-Gbps Operations of LAN-WDM DR Lasers at 50ºC NRZ signal, PRBS = Dynamic extinction ratio: ~ 6 db 25 µm 125 µm 100 µm λ 1 = nm λ 2 = nm λ 3 = nm λ 4 = nm MM = 17 % MM = 16 % MM = 18 % MM = 19 % I bias = 46 ma I mod = 46 ma p-p I bias = 44 ma I mod = 46 ma p-p I bias = 43 ma I mod = 46 ma p-p 10 ps/div. I bias = 43 ma I mod = 46 ma p-p (MM: Mask margin) Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference (OECC2010), 6D

30 Recent Reports of 40-Gbps direct modulation Wavelength Material system Cavity Waveguide Active region length Temp. [ºC] Publication NTT 1.3 µm InGaAsP DFB 25 OFC2002 KTH 1.55 µm InGaAsP DBR BH 145 µm RT IPRM2003 Alcatel 1.55 µm DFB RT ECOC2003 NTT 1.55 µm InGaAsP DFB 25 ECOC2004 TU Eindhoven 1.55 µm InGaAsP DFB 25 JLT, 2005 Hitachi 1.3 µm AlGaInAs DFB Ridge 100 µm 25 OFC2006 Hitachi 1.3 µm GaInNAs FP Ridge 200 µm 5 ECOC2006 Hitachi 1.3 µm AlGaInAs DFB Ridge 100 µm 60 PTL, 2007 Fujitsu/OITDA 1.3 µm AlGaInAs DFB BH 150 µm 50 ISLC2008 Fujitsu/OITDA 1.3 µm AlGaInAs DR BH 100 µm 40 OFC2009 Fujitsu/OITDA 1.55 µm AlGaInAs DR BH 75 µm 85 LEOS2009 Fujitsu/PETRA 1.3 µm AlGaInAs DR BH 100 µm 85 ISLC2010 NTT 1.3 µm AlGaInAs DFB (+WG) Modulation speeds are 40 to 43 Gbps. Ridge 100 µm 60 OFC

31 40-Gbps Eye Patterns of 1.55-µm DR Laser NRZ signal, PRBS = Dynamic extinction ratio: 5.0 db 50 µm 75 µm 100 µm 25ºC 50ºC 70ºC 85ºC 10 ps/div. I bias = 28.6 ma I bias = 30.7 ma I bias = 33.8 ma I bias = 38.2 ma I mod = 40 ma p-p I mod =40 ma p-p I mod =40 ma p-p I mod =40 ma p-p Ref. A. Uetake et al., 22nd Annual Meeting of IEEE Photonics Society, ThBB3,

32 40-Gbps Eye Patterns of 1.3-µm DR Laser NRZ signal, PRBS = Dynamic extinction ratio: 5.0 db 50 µm 100 µm 100 µm 25ºC 50ºC 70ºC 85ºC 10 ps/div. I bias = 32 ma I bias = 33 ma I bias = 54 ma I bias = 63 ma I mod = 36 ma p-p I mod =34mA p-p I mod =54mA p-p I mod =54mA p-p Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB3. 31

33 40 Gbps Bit Error Rate Characteristics (Back-to-Back) BTB (25ºC) BTB (50ºC) BTB (70ºC) 50 µm 100 µm 100 µm λ nm Ref. T. Simoyama et al., OFC/NFOEC2011, OWD3. 32

34 40-Gbps transmission over 5-km fiber BTB (25ºC) BTB (50ºC) BTB (70ºC) 5km (25ºC) 5km (50ºC) 5km (70ºC) Ref. T. Simoyama et al., OFC/NFOEC2011, OWD3. 33

35 40-Gbps Eye Patterns after Fiber Transmission back to back 25ºC 50ºC 70ºC 10 ps/div. After 5km After 10km I bias = 45 ma, I mod = 60 ma p-p I bias = 51 ma I mod = 60 ma p-p I bias = 59 ma I mod = 60 ma p-p Ref. T. Yamamoto et al., 23rd Int. Conf. on Indium Phosphate and Related Materials (IPRM2011), MO

36 Other Approaches (1) Utilizing frequency modulation Approach for keeping the carrier density in the active region constant under modulation to avoid relaxation oscillation Conversion to intensity modulation by narrow bandwidth optical filter Potential of longer transmission compared with zero-chirp light source [Methods] *Driving DFB laser under high-bias and small extinction ratio condition Modulating phase section of DBR laser V Phase I Act. FM signal IM signal Cavity Wavelength Active region length 35 Speed Transmissio n distance Publication NEC/AZNA DFB 1.55 µm 43 Gbps 100 km ECOC2007 NTT DBR 1.53 µm 80 µm 25 Gbps 40 km PTL, 2008 NTT DBR 1.53 µm 180 µm 40 Gbps 20 km OFC2009 *Ref. Y. Matsui et al., IEEE Photon. Technol. Lett., vol. 18, pp , 2006.

37 Other Approaches (2) Light injection to the active region Relaxation oscillation New resonance by light injection Generation of new resonance peak at much higher frequency than relaxation oscillation by photon-photon interaction Response Conventional Integrated laser: Passive Feedback DFB Laser (by Fraunhofer HHI) Frequency DFB IFB Feedback the output light of DFB laser with controlling the phase by IFB section AR HR Up to 40-Gbps modulation in 1.3 and 1.55 µm Ref. U. Troppenz et al., 32nd European Conference on Optical Communication (ECOC2006), Th U. Troppenz et al., 35th European Conference on Optical Communication (ECOC2009), Paper

38 Summary High-speed directly modulated laser (DML) Limiting factors of modulation speed f r, damping, CR time constant Most important Large differential gain & small active region Short-cavity AlGaInAs quantum well lasers High-speed direct modulation 25-Gbps operation up to 100ºC 25-Gbps operation at 50ºC for 4 lasers on LAN-WDM grid 40-Gbps operation up to 85ºC 40-Gbps transmission over 10-km SMF up to 70ºC DMLs are promising for future high-speed data transmission. 37

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