High-Speed Directly Modulated Lasers - Semantic …...High-Speed Directly Modulated Lasers Tsuyoshi Yamamoto Fujitsu Laboratories Ltd. Some parts of the results in this presentation

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.

16

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)

18

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


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.



50ºC


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


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


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


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


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 %


MM = 16 %


MM = 18 % MM = 19 %




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.



50ºC


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


25ºC 70ºC 85ºC

10 ps/div.



50ºC


40-Gbps Eye Patterns of 1.3-µm DR Laser

NRZ signal, PRBS = 231-1Dynamic extinction ratio: 5.0 dB


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

Documents

High-Speed Directly Modulated Lasers - Semantic …...High-Speed Directly Modulated Lasers Tsuyoshi Yamamoto Fujitsu Laboratories Ltd. Some parts of the results in this presentation