Widely-Tunable Electroabsorption-Modulated Sampled Grating DBR Laser Integrated with

Semiconductor Optical Amplifier

Y. A. Akulova, C. Schow, A. Karim, S. Nakagawa, P. Kozodoy, G. A. Fish, J. DeFranco, A. Dahl, M. Larson, T. Wipiejewski, D. Pavinski, T. Butrie, L. A. Coldren

Agility Communications, Inc., 600 Pine Ave, Santa Barbara, CA 93117 yakulova@agility.com

Abstract: We report on a sampled grating DBR laser monolithically integrated with an electroabsorption modulator and semiconductor optical amplifier. A fiber coupled time-averaged power in excess of 3 dBm across a 40 nm tuning range and 2.5 Gb/s transmission over 200 km of standard fiber are achieved. © 2001 Optical Society of America OCIS codes: (250.5300) Photonic integrated circuits; (060.2330) Fiber optics communications

1. Introduction Tunable lasers are desired for optical networking applications ranging from one time wavelength provisioning and sparing to dynamic wavelength provisioning in re-configurable optical add/drop multiplexers, photonic cross-connects, and all-optical regenerators. Several tunable laser technologies with direct or integrated modulation (≥2.5 Gb/s) have been demonstrated [1-4]. Among those only the Sampled Grating Distributed Bragg Reflector (SG-DBR) laser architecture combines the advantages of wide tuning range, high output power, simplicity for integration with other components [4-6], and high reliability [7]. In this paper we report on a tunable 2.5 Gb/s transmitter based on a SG-DBR laser monolithically integrated with a semiconductor optical amplifier (SOA), and an electro-absorption modulator (EA) modulator, and demonstrate transmission over distances required in long-reach metro applications. 2. Device design and fabrication As illustrated in Fig. 1, the device consists of a four-section SG-DBR laser, an SOA, and an EA modulator, all integrated on the same InP chip. The SOA compensates on-state modulator loss and cavity losses caused by free carrier absorption in the tuning sections and allows power leveling with insignificant wavelength deviation. The integration of the laser and SOA active regions with the tuning and modulator sections of the device has been accomplished by using an offset quantum-well structure [4]. In this simple integration technology the active region of the modulator uses the same bulk quaternary waveguide as the tuning sections of the laser. The Franz-Keldysh effect in the bulk waveguide material provides for larger spectral bandwidth as compared to the quantum-confined Stark effect. The composition of the bulk waveguide can be optimized to achieve high tuning efficiency for the laser and a target extinction ratio over the required wide spectral bandwidth for the modulator. An angled waveguide and wide-band anti-reflection coating at the output of the device were used to suppress the optical feedback to the SGDBR laser.

Fig. 1. Schematic of SG-DBR laser integrated with SOA and EA modulator.

Light Out

Front Mirror Gain Phase Rear MirrorAmplifier Modulator

MQW active regionsQ waveguide Sampled-gratings

3. Results and discussion The device was packaged in a cooled butterfly package with a co-planar RF input. The package was mounted into a transmitter assembly with dc current and voltage drivers. The ITU channel selection is done using a look up table for the laser and SOA currents and the dc bias voltage for the modulator. More than 100 consecutive 50 GHz spaced ITU channels with fiber coupled output powers > 10 mW and SMSR greater than 40 dB were demonstrated for Igain = ISOA = 150 mA and Vmod = 0 V (Fig. 2a). Relative intensity noise is less than –145 dB/Hz and unmodulated time-average linewidth is below 20 MHz across the tuning range.

The extinction ratio (ER) characteristics of EA-modulators are strongly dependent on the detuning between the lasing and absorption-edge wavelengths. To provide uniform RF ER over a wide spectral bandwidth, the dc bias on the modulator section has been adjusted for each channel. Power leveling was accomplished by adjusting SOA current in the range of 40-100 mA. Time-averaged power > 3 dBm and RF ER > 10 dB was simultaneously obtained across the tuning range with 3 V peak-to-peak modulation (Fig. 2b).

Fig. 2. a) Fiber coupled power and SMSR across the tuning range. b) RF extinction ratio and fiber coupled time averaged optical power vs. wavelength for a fixed 3V pk-pk rf drive voltage.

The fiber transmission experiments were performed using integrated SGDBR-SOA-EA modulator chip mounted

on RF ceramic carrier. Bit error rate characteristics for three representative channels are shown in Fig. 3. Error-free transmission has been demonstrated for 200 km of standard single mode fiber. The dispersion penalty is < 0.3 dB at 1560 nm and <1.5 dB at 1530 nm. Further optimization of EA modulator operating parameters should result in lower dispersion penalty at short wavelength range. 4. Summary In summary, we have demonstrated a widely-tunable, 2.5 Gb/s transmitter based on a SG-DBR laser monolithically integrated with a SOA and EA modulator. Time-averaged powers in excess of 3 dBm and RF extinction ratio > 10 dB across a 40 nm tuning range have been achieved. Error-free transmission at 2.5 Gb/s has been demonstrated for 200 km of standard single mode fiber.

6

8

10

12

14

16

0

10

20

30

40

50

60

1520 1530 1540 1550 1560 1570 1580

Fibe

r Cou

pled

Pow

er, d

Bm

SM

SR, dB

Wavelength, nm

Igain

= 150 mA

ISOA

= 150 mAV

mod = 0 V

a)

0

2

4

6

8

10

12

0

1

2

3

4

5

1520 1530 1540 1550 1560 1570 1580

RF

ER, d

B

Time Averaged Pow

er, dBm

Wavelength, nm

b)

Fig. 3. Bit error rate curves for 0 and 200 km of standard single-mode fiber spans for three different wavelengths (2.5 Gb/s NRZ, 231-1 PRBS).

5. References [1] J. E. Johnson, L. J.-P. Ketelsen, D. A. Ackerman, J. M. Geary, W. A. Ausous, F. S. Walters, J. M. Freund, M. S. Hybertsen, K. G. Glogovsky, C. W. Lentz, C. L. Reynolds, R. B. Bylsma, E. J. Dean, and T. L. Koch, “Electroabsorption-Modulated Wavelength-Selectable Lasers”, presented at Integrated Photonics Research, Monterey, CA, paper ItuC1, 2001. [2] W. Yuen, G.S. Li, R.F. Nabiev, M. Jansen, D. Davis, C. J. Chang-Hasnain, “Electrically-Pumped Directly-Modulated Tunable VCSEL for Metro DWDM Applications”, IEEE/LEOS Summer Topical Meetings, Invited paper TuA1.2, Copper Mountain, CO, 2001. [3] M. Jiang, C-C. Lu, P. Chen, J.-H. Zhou, J. Cai, K. McCallion, K. J. Knopp, P. D. Wang, M. Azimi, D. Vakhshoori, “Error Free 2.5 Gb/s transmission over 125 km conventional fiber of a directly modulated widely tunable vertical cavity surface emitting laser”, OFC 2000, Baltimore, MD, 2000. [4] B. Mason, G. A. Fish, S. P DenBaars, and L. A. Coldren, “Widely tunable Sampled Grating DBR Laser with Integrated Electroabsorption Modulator”, IEEE Photnics Technology Letters, 11(6), 638-40, 1999. [5] B. Mason, J. Barton, G. A. Fish, L. A. Coldren, S. P. DenBaars, “Design of Sampled Grating DBR Lasers with integrated Semiconductor Optical Amplifiers”, IEEE Photnics Technology Letters, 12(7), 762-4, 2000. [6] J. Barton, L. Coldren, and G. Fish, “Tunable Laser using Sampled Grating DBRs”, IEEE/LEOS Summer Topical Meetings, Invited paper TuA2.1, Copper Mountain, CO, 2001. [7] F. Delorme, G. Terol, H. de Bailliencourt, S. Grosmaire, P. Devoldere, “Long-tern wavelength stability of 1.55 -µm tunable distributed Bragg reflector lasers,” IEEE Journal of Selected Topics in Quantum Electronics, 5(3), 480-6, 1999.

-38 -36 -34 -32 -30 -28 -26

1530 nm B-to-B1530 nm 200 km1540 nm B-to-B1540 nm, 200 km1560 nm, B-to B1560 nm, 200 km

Bit E

rror R

ate

Average Received Power [dBm]

10-06

10-10

10-08

10-04

10-11

10-03

10-09

10-07

10-05

2-2.5 V Bias, 3-3.3 V

p-p

Widely-Tunable Electroabsorption-Modulated Sampled Grating DBR Laser Integrated with

Semiconductor Optical Amplifier

Y. A. Akulova, C. Schow, A. Karim, S. Nakagawa, P. Kozodoy, G. A. Fish, J. DeFranco, A. Dahl, M. Larson, T. Wipiejewski,

D. Pavinski, T. Butrie, L. A. Coldren

Agility Communications, Inc.

Tel: (805) 690-1758yakulova@agility.com

1/23/2003 11:43 AM

2

OutlineIntroduction to Widely-Tunable SG-DBR Lasers

– System requirements

Device design

– SG-DBR Lasers: Theory of Operation

– Integration with other components

CW operation SGDBR-SOA-EAM

High-speed performance

Summary

1/23/2003 11:43 AM

3

Requirements for Tunable Lasers

Output Power– External Mod. – 10-20 mW– Integrated Mod. – 0-5 dBm

Tuning Range– Replacement – 8-32 nm– Enabling – >32 nm

Tuning Speed– Sparing – 1 sec– Restoration – <10 ms– Packet Switching – <10 ns

Wavelength Control– 100 GHz – ±5 GHz– 50 GHz – ±3 GHz– 25 GHz – ±1 GHz

1/23/2003 11:43 AM

4

Sampled Grating Mirror DesignFront Mirror Gain Phase Rear Mirror

SG-DBR

ΛΛΛΛs

d

ΛΛΛΛBeff

CB n2

λ=ΛSg

m n Λ=∆

2

2λλ

∆λB

Repeat Mode Spacing

∆λF

5-10X Tuning Range of DBR

Reliable, Manufacturable InPTechnology

Can Cover C band, L band or C + L

Easily Integrates Monolithically with Other Components (e.g. EAM, SOA)

1/23/2003 11:43 AM

5

Device design and integration technology

Monolithic InP chip/ same material structure and process as for SGDBR aloneWaveguide common to SGDBR, SOA , and EA modulator Optimized for RF ER and chirp over 40 nm tuning range, and FM efficiency of the tuning sectionsSOA Breaks Power/Tuning Range Tradeoff + VOA function

Light Out

Front Mirror Gain Phase

Rear Mirror

SG-DBR Laser

AmplifierEA

Modulator

MQW active regionsQ waveguide

1/23/2003 11:43 AM

6

TEMLA – Tunable ElectroabsorptionModulated Laser Assembly

Close loop control:PowerWavelengthMirror controlTemperature

Modulator driver:Vpp

VdcCompact footprint and industry standard pinoutMicroprocessor based control system

■ Integral wave-locker (50 GHz etalon)Tuning of the laser and modulator driver for the desired channel < 10 mS

■ VOA− Blanked output power –30 dBm− VOA dynamic range > 10 dB

Light out Front Mirror Gain Phase Back Mirror

Wavelength Locking

Mirror Control (channel selection)

PowerMonitor

Wavelength Locker

AmplifierModu-lator

RF Signal and DC

BiasPowerControl

1/23/2003 11:43 AM

7

CW Performance of SGDBR-SOA-EAM

>100 50 GHz ITU ChannelsFiber coupled power > 10 dBmSMSR > 40 dB

6

8

10

12

14

16

0

10

20

30

40

50

60

1520 1530 1540 1550 1560 1570 1580

Fibe

r Cou

pled

Pow

er, d

Bm

SMSR

, dB

Wavelength, nm

Igain

= 150 mAISOA

= 150 mAV

mod = 0 V

0

10

20

30

40

0 50 100 150

Chi

p po

wer

, mW

Igain

, mA

ISOA

, mA

150125100

1/23/2003 11:43 AM

8

CW close loop performance

Fiber coupled power = 10 dBm +/- 0.05 dBWavelength deviation <+/- 3.5 pm

0

5

10

15

-0.02

-0.01

0

0.01

0.02

0.03

1525 1535 1545 1555 1565

Fibe

r cou

pled

pow

er, d

Bm ITU w

avelength error, nm

Wavelength, nm

1/23/2003 11:43 AM

9

RIN and Linewidth of SOA Amplified SG-DBR

-154 -152 -150 -148 -146 -144 -142.01

.1

1

51020305070809095

99

99.9

99.99Worst Case RIN over (0.1 GHz -10 GHz)640 channels sampled over 15 parts

Mean Value: 146 dB/HzStandard Deviation: 1.7 dB/Hz

RIN (dB/Hz)

Perc

ent

SOA does not degrade RIN & LinewidthRIN ~ -146 dB/HzLinewidth measured by self-homodyne technique at subsystem level <10 MHzLinewidth ~ 1 MHz using FM Noise Density measurement(more accurate than self-homodyne method).

FM Noise Density MeasurementRIN Measurement

106

107

108

0 1 2 3 4 5 6 7 8

Freq

uenc

y N

oise

, (H

z2 /Hz)

Frequency (GHz)

1 MHz Linewidth

5 MHz Linewidth

Pout = 10 mW

Pout = 10 mW

1/23/2003 11:43 AM

10

High-speed performance: RF ER & Chirp

Chirp mapped out using Time Resolved SpectroscopyCurved waveguide and multilayer AR coating eliminate optical cross-talk Low on-chip electrical cross-talkEA modulator chirp can be adjusted by Vdc and Vp-p

RF ER > 10 dB, chirp < 0.2 A over wide tuning range

0.1

0.15

0.2

0.25

1 1.5 2 2.5 3 3.5

Chi

rp p

k-pk

, Ang

stre

ms

Vdc, V

1525 nmV

dc = 2.7 V

1543 nmV

dc = 2.7 V

1561nmV

dc = 3.3 V

6

8

10

12

14

1 1.5 2 2.5 3 3.5

RF

ER, d

B

Vdc, V

1525 nm 1545 nm

1560 nm

Vpp

3 V and 3.3 V

1/23/2003 11:43 AM

11

Modulated Performance: RF ER & Pave& VOA Operation

0

2

4

6

8

10

12

0

1

2

3

4

5

1520 1530 1540 1550 1560 1570 1580

RF

ER, d

B

Time A

veraged Power, dBm

Wavelength, nm

Igain

= 150 mAISOA

- variableV

mod p-p = 3 V

Vmod dc

-variable

Time-averaged power 3 dBm and RF ER > 10 dB across C-bandOutput power dynamic range of ~20 dB w/ small change in SMSR andWavelength (open loop operation)

15 28 nm15 30 nm15 34 nm15 38 nm15 42 nm

1 5 46 n m1 5 50 n m1 5 54 n m1 5 58 n m1 5 62 n m

-40

-20

0

20

40

-0 .04

-0 .02

0

0 .02

0 .04

Wav

elen

gth

Dev

. (pm

)3 6

40

44

48

52

-10 -5 0 5 10SM

SR (d

B)

O u tp ut P ow er (dB m )

1/23/2003 11:43 AM

12

Transmission characteristics1528 nm

1540 nm

1550 nm

1560 nm

PRBS 231-1 at 2.5 Gb/s4th order Bessel-Thomson filterSONET mask with 25% margin

< ~ 1 dB DP over 200 km NDSFSupports OC-48 with FEC

-36 -35 -34 -33 -32 -31 -30 -29

1530 nm 0 km1530 nm 200 km1540 nm 0 km1540 nm 200 km1550 nm 0 km1550 nm 200 km1560 nm 0 km1560 nm 200 km

Bit

Erro

r Rat

e [E

rror

s/s]

Average Received Power [dBm]

10-06

10-10

10-08

10-04

10-11

10-09

10-07

10-05

2.488 GB/s, 231-1200 km, SMF28

10-12

10-13Unfiltered 2.7 Gb/s

1/23/2003 11:43 AM

13

OC-192 Operation of EAM

Integration technology compatible with higher bit rates> 10 dB RF ER across C-band

PRBS 231-1, Vp-p = 3V

1/23/2003 11:43 AM

14

SummarySG-DBR lasers meet the requirements of many market segments: from Metro to Ultra-Long Haul.Monolithic SGDBR-SOA-EAM chip using platform technology– High yield– Low cost– High Volume

SGDBR-SOA-EAM characteristics:– Wide Tuning (>40 nm)– High Power (>10 mW CW)– 3 dBm time averaged power and >10 dB RF ER across C-band– Support OC-48 with FEC

Fully functional widely-tunable Transmitter with integrated wavelength locker and close loops control (power, wavelength, mode, temperature)SGDBR-SOA-EAM integration technology is compatible with higher output power and bit rates