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).
1
Outline
Direct modulation of semiconductor lasers
Frequency response of semiconductor lasers
High-speed direct modulation
Summary
25-Gbps direct modulation40-Gbps direct modulation
Other approaches
Limiting factors of modulation bandwidthApproaches for high-speed modulationHigh-speed distributed reflector lasers
2
Current
Ligh
t out
put
Electrical signal
Optical signal
Threshold current: Ith
Direct Modulation of Semiconductor Lasers
Current
Light
Simplest way to generate intensity-modulated optical signal
3
DML: Small size & low power consumptionAttractive for short-reach application
Transmitters for Optical Fiber Communication
CW laser + LiNbO3 modulator
Directlymodulated laser (DML)
EML
10
100
1000
1G 10G 100G
Tran
smis
sion
dis
tanc
e (k
m)
Modulation speed (bps)
1
EML: Electroabsorption modulator integrated laser
4
Directly Modulated Lasers in Photonic Networks
LAN (GbE, 10GbE), SAN(FC)0.85-µm VCSELs, << 500 m1.3-µm FP and DFB LDs, < 10 km
AccessGE-PON, G-PON
Downstream: 1.49-µm DFB LDsUpstream: 1.3-µm FP or DFB LDs
10GE-PON, XG-PONUpstream: 1.27-µm DFB LDs
SONET/SDH1.3-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
5
Optical Signal from Directly Modulated Lasers
Time
TimeC
urre
ntLi
ght
Electricalinput
Opticaloutput
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.
Time
Ligh
t
6
10-Gbps Direct Modulation
Example of 10-Gbps eye pattern
• To suppress influence of the overshoot, setting relaxation oscillation frequency (fr ) much higher than 10 GHz
(with filter)
• To suppress jitters related with turn-on delay, utilizing low extinction ratio less than 7dB
Driving current is usually determined by fr , not by output power.
Filtering the oscillation by receiver with limited bandwidth
7
-15
-12
-9
-6
-3
0
3
6
9
0 5 10 15 20 25 30
((f 2-fr2)2+ f 2γ 2/(2π)2)1/2
Modulation Speed of Directly-Modulated Lasers
|R(f)| =
Frequency response of a semiconductor laser
(1+(2π fCR)2)1/2fr
2
fr :Relaxation oscillation frequencyγ :Damping coefficient (∝Kfr
2)C:CapacitanceR:Resistance
1
Frequency (GHz)
Res
pons
e (dB
)
fr :15 GHzK :
0.3 nsCR bandwidth:20 GHz
Limiting factors of bandwidth• Relaxation oscillation frequency ~ 1.55 fr
• Damping ~ 8.89/Κ• CR time constant ~ 1/(2πCR)
8
• Increasing differential gain (dg/dn)
• Decreasing active-region volume
L W Nw Lw
How to Increase fr ?
fr ∝( )1/2(I-Ith )
Γ : Optical confinement factor dg/dn : Differential gainL : Active region length W : Active region widthNw : Number of wells Lw : Well thicknessI
: Injection current Ith : Threshold current
Length (L ), Width (W)
Γ dg/dn
Selection of material systemLowering threshold gainSetting of detuning
*Reduction of thickness (Nw , Lw ) is mostly compensated by reduction of optical confinement factor (Γ).
9
To Reduce Active-Region Volume: Length
Active region λ/4 shift DFB DFB DFB DFBIntegration - - Waveguide DBR mirrors
Facet coating AR/AR AR/HR AR/HR AR/ARShort active region (< 150 µm)
Difficult Difficult Easy Easy
Butt-joint regrowth No No Yes YesThreshold gain High Medium Medium LowSingle-mode yield Good Fair Fair Good
Cavity structure is a key issue in reduction of active-region length.
• For short active region (< 150 µm) , waveguide integration is necessary.• HR coating reduces threshold gain but deteriorates single-mode yield.
λ/4 shift
10
Ridge waveguide structure Buried heterostructure (BH)
Active region Defined by ridge widthand current spreading
Defined by mesa width
Width Relatively wide(Usually > 2 µm)
Narrow(Usually < 1.5 µm)
Fabrication Etching of ridge Etching of mesa and regrowthPoint Control of guided optical mode
and current spreadingSuppression of leakage current
at regrowth interface and current blocking structure
To Reduce Active-Region Volume: Width
Active-region width depends on waveguide structure.
Active layer Active layer
11
Better electron confinement Increase in differential gain
To Obtain Large dg/dn : Material System
AlGaInAs quantum well (QW)
Superior band diagram of AlGaInAs QW
InGaAsP quantum well (QW)
• Large conduction band offset (ΔEc )• Small valence band offset (ΔEv )
12
fr of AlGaInAs and InGaAsP lasers
AlGaInAs quantum-well laser InGaAsP quantum-well laser
Ref. T. Ishikawa et al., Proc. of 10th Int. Conf. on Indium Phosphide and Related Materials, pp. 729-732, 1998.
0 102 6 84(I-Ith )1/2 (mA1/2)
0 102 6 84(I-Ith )1/2 (mA1/2)
14
0
12108642
f r(G
Hz)
14
0
12108642
f r(G
Hz)
FP laserL = 300 µm
25, 50, 75, 85ºC 25, 50, 75, 85ºC
FP laserL = 300 µm
Larger fr and smaller temperature dependence in AlGaInAs QW laser
13
To Obtain Large dg/dn : Design of Threshold Gain
Gai
n, g
Carrier density, n
Decrease in active-region length
Increase in threshold gain, Γgth
Decrease in differential gain
if too much
Γgth∝ 1/L
• Increase optical confinement factorLarge number of quantum wells
• Increase optical feedback of reflectorLarge coupling coefficient of grating ( > 100 cm-1)High-reflection coating or integrated mirror
Decrease in dg/dn
Keeping threshold gain sufficiently low in short active region
14
To Obtain Large dg/dn : Setting of Detuning
Wavelength, λ
Gai
n, g
n1n2
Difference between lasing wavelength (λLaser ) and gain peak wavelength (λGain )
>
In wide temperature range operation
Lowtemperature
High temperature
Wavelength, λ
λLaserλLaser
Δλ
depends on temperature.
0.4 - 0.5 nm/K
0.08 - 0.1nm/K
Detuning, Δλ Larger dg/dn
Gai
n, g
Generally, λLaser shorter than λGain
provides larger dg/dn.
Optimization to support whole temperature range is necessary.
15
(I - Ith )1/2 (mA1/2)0 2 4 6 8
0
5
10
15
20
f r(G
Hz)
25℃50℃70℃85℃
4.5 GHz/mA1/2
3.5 GHz/mA1/2
(I - Ith )1/2 (mA1/2)0 2 4 6 8
0
5
10
15
20
f r(G
Hz)
5.0 GHz/mA1/2
25℃50℃70℃85℃
Δλ @25ºC= - 4.2 nm
fr of Lasers with Different Detuning (Example)
3.4 GHz/mA1/2
Δλ @25ºC= + 5.5 nm
1.55-µm AlGaInAs distributed reflector laserswith 75-µm long active region
Ref. A. Uetake et al., 22nd Annual Meeting of IEEE Photonics Society, ThBB3, 2009.
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Influence of Damping
-15
-12
-9
-6
-3
0
3
6
9
12
15
0 10 20 30 40
Frequency (GHz)
Res
pons
e (d
B)
K : 0.25 nsCR bandwidth: 40 GHz
fr :15 GHz20 GHz
25 GHz
30 GHz
Suppression of relaxation oscillationmainly due to nonlinear gain effect
Damping
Intrinsic bandwidth of semiconductor laser
Damping K factor: K ~ γ /fr2
Major factors to determine K• Nonlinear property of gain material itself• dg/dn • Photon lifetime
Large K factor prevents high speed modulation, but in some casesappropriate damping suppresses overshoot due to relaxation oscillation.
Selection of materialOptimization for frRoom for design
17
CR Time Constant
Capacitance
Resistance
• Cladding layersOptimization of doping profile
• Interface of heterostructureDecreased band discontinuity and optimized doping
• Electrode contact
• Accompanied by pn junctionUtilizing semi-insulating current blocking structure in BH lasersReduced area of upper cladding layer
• Bonding padSmall size
Design as small as possible
(almost independent of design of active layer and cavity)
(increase with reduced active-region area)
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High-Speed Distributed Reflector laser
AR coating
DBR mirror DBR mirror
DFB region
AR coating
• 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 short-cavity lasers
AlGaInAs MQW active layer
*Ref. J-I. Shim et al., IEEE J. Quantum Electron., vol. 27, pp. 1736-1745, 1991.
Introducing concept of Distributed Reflector (DR) laser*to short-cavity lasers
Integrating DBR mirrors on both sides of the DFB active region
19
AlGaInAs DR Laser with 100-µm-long Active Region
25ºC
50ºC
70ºC
85ºC
Current (mA)0 20 40 60 80
15
Out
put p
ower
(mW
)
Ith = 3.6 mA (25ºC)11.0 mA (85ºC)
Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB3.
100 µm50 µm 100 µm
-70
10
0
-10
-20
-30
-40
-50
-60In
tens
ity (d
B)
10
5
01310 1315 1320 1325 1330
Wavelength (nm)
25ºC
50ºC
I = 80 mA
70ºC
85ºC
20
AlGaInAs DR Laser with 100-µm-long Active Region
3.8 GHz/mA1/2
3.0 GHz/mA1/2
25ºC50ºC70ºC85ºC
Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB3.
0 10 20 30 40Frequency (GHz)
-20
-10
0
10
Res
pons
e (d
B)
I = 80 mA
f3dB = 28.3 GHz (25ºC)20.8 GHz (85ºC)
25ºC50ºC
70ºC85ºC
0
5
10
15
20
0 2 4 6 8(I-Ith )1/2 (mA1/2)
f r(G
Hz)
100 µm50 µm 100 µm
21
Recent Reports of 25-Gbps direct modulation
Materialsystem
Cavity Wave -guide
Active region length
Temp.[ºC]
Publication
Fujitsu/OITDA AlGaInAs DFB BH 150 µm 70 Electron. Lett. 2008
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 CLEO2010Fujitsu/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.
22
Ibias = 42 mAImod = 40 mAp-p
25ºC 70ºC 80ºC
10 ps/div.
Ibias = 47 mAImod = 40 mAp-p
Ibias = 55 mAImod = 40 mAp-p
50ºC
Ibias = 62 mAImod = 40 mAp-p
25-Gbps Eye Patterns of 1.3-µm DFB Laser
NRZ signal, PRBS = 231-1Dynamic extinction ratio: 5.0 dB 150 µm
Ref. K. Otsubo et al., J. Select. Topics Quantum Electron.,vol. 15, pp. 687-693, 2009.
23
0
1
2
3
0 5 10 15Transmission distance (km)
Pen
alty
(dB
)25 Gbps, ER = 5.5 dBBER of 10-12
Penalty from back-to-back at 25ºC
25ºC: λLaser = 1325.08 nm70ºC: λLaser = 1329.62 nm
Single Mode Fiber Transmission using 25-Gbps DML
Δλ @25ºC = +15.1 nm
Ref. K. Otsubo et al., J. Select. Topics Quantum Electron.,vol. 15, pp. 687-693, 2009.
24
-1
0
1
2
3
-40 -30 -20 -10 0 10 20Chromatic dispersion (ps/nm)
Pen
alty
(dB
)
1303 nm1322 nm
1296 nm 25ºCSMF
λLaser = 1296, 1303, 1322 nm
6.5-26 km25 Gbps
-32 ~ +15 ps/nm
AlGaInAs DFB LDs
-28 ps/nm +9 ps/nm
Worst case of 10-km transmissionwithin LAN-WDM grid
Influence of Fiber Dispersion in 25-Gbps DML
Experimental setup
Ref. K. Otsubo et al., J. Select. Topics Quantum Electron.,vol. 15, pp. 687-693, 2009.
25
Four 1.3-µm DR Lasers for LAN-WDM Application
50ºC, CW2 mW
1285 1290 1295 1300 1305 1310 1315
Wavelength (nm)
Rel
ativ
e in
tens
ity
λ1 λ2 λ3 λ4
20 dB
Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference(OECC2010), 6D1-4.
125 µm25 µm 100 µm
26
0
5
10
15
0Current (mA)
Pow
er (m
W)
40 80
λ1 λ2 λ3 λ4
25ºC50ºC
85ºC
25ºC50ºC
85ºC
25ºC50ºC
85ºC
25ºC50ºC
85ºC
λ1 λ2 λ3 λ4
Ith (mA) @ 25ºC 6.3 5.7 5.4 5.1Ith (mA) @ 50ºC 10.0 9.2 8.5 8.0
0 40 80 0 40 80 0 40 80
Light-Current Characteristics of LAN-WDM DR Lasers
Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference(OECC2010), 6D1-4.
27
0
5
10
15
25
0 2 4 6 8(I - Ith )1/2 (mA1/2)
f r(G
Hz)
2050ºC
fr of LAN-WDM DR Lasers
λ1λ2λ3λ4
λ1 λ2 λ3 λ4
fr (GHz) @ I = 50 mA 20.6 21.1 21.8 21.5
3.6 GHz/mA1/2
Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference(OECC2010), 6D1-4.
28
NRZ signal, PRBS = 231-1Dynamic extinction ratio: ~ 6 dB
25.8-Gbps Operations of LAN-WDM DR Lasers at 50ºC
λ1
= 1295.73 nm λ2
= 1300.03 nm λ3
= 1304.65 nm λ4
= 1309.25 nm
10 ps/div.
(MM: Mask margin)
MM = 17 %
Ibias = 46 mAImod = 46 mAp-p
MM = 16 %
Ibias = 44 mAImod = 46 mAp-p
MM = 18 % MM = 19 %
Ibias = 43 mAImod = 46 mAp-p
Ibias = 43 mAImod = 46 mAp-p
Ref. K. Otsubo et al., 15th OptoElectronics and Communications Conference(OECC2010), 6D1-4.
125 µm25 µm 100 µm
29
Wave- length
Material system
Cavity Wave- guide
Active region length
Temp.[ºC]
Publication
NTT 1.3 µm InGaAsP DFB 25 OFC2002KTH 1.55 µm InGaAsP DBR BH 145 µm RT IPRM2003
Alcatel 1.55 µm DFB RT ECOC2003NTT 1.55 µm InGaAsP DFB 25 ECOC2004
TU Eindhoven 1.55 µm InGaAsP DFB 25 JLT, 2005Hitachi 1.3 µm AlGaInAs DFB Ridge 100 µm 25 OFC2006Hitachi 1.3 µm GaInNAs FP Ridge 200 µm 5 ECOC2006Hitachi 1.3 µm AlGaInAs DFB Ridge 100 µm 60 PTL, 2007
Fujitsu/OITDA 1.3 µm AlGaInAs DFB BH 150 µm 50 ISLC2008Fujitsu/OITDA 1.3 µm AlGaInAs DR BH 100 µm 40 OFC2009Fujitsu/OITDA 1.55 µm AlGaInAs DR BH 75 µm 85 LEOS2009Fujitsu/PETRA 1.3 µm AlGaInAs DR BH 100 µm 85 ISLC2010
NTT 1.3 µm AlGaInAs DFB(+WG)
Ridge 100 µm 60 OFC2011
Recent Reports of 40-Gbps direct modulation
• Modulation speeds are 40 to 43 Gbps.
30
Ibias = 28.6 mAImod = 40 mAp-p
25ºC 70ºC 85ºC
10 ps/div.
Ibias = 30.7 mAImod = 40 mAp-p
Ibias = 33.8 mAImod = 40 mAp-p
50ºC
Ibias = 38.2 mAImod = 40 mAp-p
40-Gbps Eye Patterns of 1.55-µm DR Laser
NRZ signal, PRBS = 231-1Dynamic extinction ratio: 5.0 dB 75 µm50 µm 100 µm
Ref. A. Uetake et al., 22nd Annual Meeting of IEEE Photonics Society, ThBB3, 2009.
31
Ibias = 32 mAImod = 36 mAp-p
25ºC 70ºC 85ºC
10 ps/div.
Ibias = 33 mAImod = 34 mAp-p
Ibias = 54 mAImod = 54 mAp-p
50ºC
Ibias = 63 mAImod = 54 mAp-p
40-Gbps Eye Patterns of 1.3-µm DR Laser
NRZ signal, PRBS = 231-1Dynamic extinction ratio: 5.0 dB
Ref. T. Yamamoto et al., 22nd Int. Conf. on Semiconductor Lasers (ISLC2010), ThB3.
100 µm50 µm 100 µm
32
40 Gbps Bit Error Rate Characteristics (Back-to-Back)
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
BTB (25ºC)BTB (50ºC)BTB (70ºC)
Ref. T. Simoyama et al., OFC/NFOEC2011, OWD3.
100 µm50 µm 100 µm
λLaser @25ºC= 1309.4 nm
33
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
BTB (25ºC)BTB (50ºC)BTB (70ºC)5km (25ºC)5km (50ºC)5km (70ºC)
40-Gbps transmission over 5-km fiber
Ref. T. Simoyama et al., OFC/NFOEC2011, OWD3.
34
40-Gbps Eye Patterns after Fiber Transmission
backto
back
After5km
After10km
25ºC 70ºC50ºC 10 ps/div.
Ibias = 45 mA, Imod = 60 mAp-p
Ibias = 51 mA Imod = 60 mAp-p
Ibias = 59 mA Imod = 60 mAp-p
Ref. T. Yamamoto et al., 23rd Int. Conf. on Indium Phosphate and Related Materials(IPRM2011), MO-1.1.1.
35
Other Approaches (1)
Utilizing frequency modulation
Cavity Wavelength Active region length
Speed Transmissio n distance
Publication
NEC/AZNA DFB 1.55 µm 43 Gbps 100 km ECOC2007NTT DBR 1.53 µm 80 µm 25 Gbps 40 km PTL, 2008NTT DBR 1.53 µm 180 µm 40 Gbps 20 km OFC2009
Approach for keeping the carrier density in the active region constantunder modulation to avoid relaxation oscillationConversion to intensity modulation by narrow bandwidth optical filterPotential of longer transmission compared with zero-chirp light source
• *Driving DFB laser under high-bias and small extinction ratio condition• Modulating phase section of DBR laser
[Methods]
*Ref. Y. Matsui et al., IEEE Photon. Technol. Lett., vol. 18, pp. 385-388, 2006.
IMsignal
FMsignal
Act.PhaseIV
36
Other Approaches (2)
Light injection to the active region
DFB IFB
AR HR
Integrated laser: Passive Feedback DFB Laser (by Fraunhofer HHI)Frequency
Res
pons
e
Conventional
New resonanceby light injection
Relaxationoscillation
Generation of new resonance peak at much higher frequency than relaxation oscillation by photon-photon interaction
Ref. U. Troppenz et al., 32nd European Conference on Optical Communication (ECOC2006), Th4.5.5.U. Troppenz et al., 35th European Conference on Optical Communication (ECOC2009), Paper 8.1.4.
Feedback the output light of DFB laserwith controlling the phase by IFB section
Up to 40-Gbps modulation in 1.3 and 1.55 µm
37
• Limiting factors of modulation speedfr , damping, CR time constant
Most importantLarge differential gain & small active region
• High-speed direct modulation25-Gbps operation up to 100ºC25-Gbps operation at 50ºC for 4 lasers on LAN-WDM grid
40-Gbps operation up to 85ºC40-Gbps transmission over 10-km SMF up to 70ºC
Summary
High-speed directly modulated laser (DML)
Short-cavity AlGaInAs quantum well lasers
DMLs are promising for future high-speed data transmission.
38
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