Flatsurface buried heterostructure distributed feedback lasers operating in a singlelongitudinal mode at 1.5 μm with low chirpStefan Nilsson, Tawee TanbunEk, and Björn Broberg Citation: Applied Physics Letters 51, 2082 (1987); doi: 10.1063/1.98977 View online: http://dx.doi.org/10.1063/1.98977 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/51/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Measurements of the nonlinear damping factor in 1.5 μm distributed feedback lasers Appl. Phys. Lett. 54, 90 (1989); 10.1063/1.101225 Origin of modulationinduced mode partition and Gb/s system performance of highly singlemode 1.5 μmdistributed feedback lasers Appl. Phys. Lett. 52, 1762 (1988); 10.1063/1.99635 Measurements of dynamic photon fluctuations in a directly modulated 1.5μm InGaAsP distributed feedback laser Appl. Phys. Lett. 47, 448 (1985); 10.1063/1.96142 1.55μm InGaAsP distributed feedback vapor phase transported buried heterostructure lasers Appl. Phys. Lett. 47, 12 (1985); 10.1063/1.96410 Frequency stabilization of 1.5μm InGaAsP distributed feedback laser to NH3 absorption lines Appl. Phys. Lett. 45, 826 (1984); 10.1063/1.95432

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Flat",surface buried heterostructure distributed feedback lasers operating in a single longitudinal mode at 1.5 I-Lm with ~ow chirp

Stefan Nilsson,a) Tawee Tanbun-Ek,a) and Bjorn Broberg8)

Institute of Microwave Technology, Box 70033, S-lOO 44 Stockholm, Sweden

(Received 18 August 1987; accepted for publication 19 October 1987)

Distributed feedback lasers emitting in a stable dynamic single mode at 1.5 p.m wavelength in a temperature range of more than 50 "C have been developed using the so-caned flat -surface buried heterostructure, Threshold currents as low as 14 rnA in cw (8 rnA in pulse) operation and output powers as high as 20 m W /facet were obtained. By detuning the Bragg wavelength to the shorter wavelength side of the gain peak, and by reducing the confinement factor, a wavelength chirp as a function of modulation current as low as 0.003 nm/rnA has been obtained. This value corresponds to a total chirp of 0.1-0.2 fim under normal modulation conditions.

In optical fiber communication systems operating at the wavelength of 1.55 flm, dynamic single-mode lasers, such as distributed feedback (DFB) lasers, are essential due to wavelength dispersion in the fiber. In order to minimize the penalty caused by dispersion, the wavelength of the single mode needs to be stable throughout the optical pulses. This stability is generally limited by a dynamic linewidth broad­ening, or chirp. !

Theoretically it has been predicted that the chirp in DFB lasers can be reduced by detuning the Bragg wave­length to the shorter wavelength side of the gain peak? At the same time the high-frequency modulation response im­proves since the differential gain increases.'

The maximum amount of detuning possible is generally determined by the limit where the threshold gain difference between the DFB mode at the Bragg wavelength and the Fabry-Perot-like modes at the gain peak is smaner than what is required to ensure dynamic single-mode operation. A detuning of about 10 nm has been reported, giving im­proved modulation characteristics,~.4 In our experiments, we have obtained stable dynamic single-mode operati.on with a 20-nm detuning due to a high coupling strength of the grating.

Another factor that might contribute to a low chirp is a narrow active region which causes a smaller confinement factor of the light, Le., a smaller fraction of the light is affect­ed by the change of the refractive index in the active region during the modulation.

Recently, a new structure calied fiat-surface buried het­erostructure (FER) has been reported.5 It has shown to be suitable for the fabricaiion of high performance lasers due to the possibility of achieving very narrow active stripe..q with good control, and at the same time thick and efficient cur­rent blocking layers. We have developed a 1.5-p:m-wave­length DFB version of this structure, and to our knowledge this is the first presentation of such lasers.

The laser was fabricated by liquid phase epitaxy (LPE) in three steps. A first-order grating (235-nm pitch) was made by holographic photo electrochemical etching6 direct­lyon the substrate. In the first LPE step a waveguide layer

ol Present address: Swedish Institute of Microelectronics, Box 1084, s-164 21 Kista, Sweden.

(AI!; = 1.3pm), an active layer (Ag = L53,um),andananti­meltback layer (A g = 1.3 /-tm) were successively grown at 600 T. In order to preserve the grating, the substrate was kept in a cold zone of the furnace until just before growth. A grating depth of more than 60 nrn after growth was easily obtained. Next, a SiN" etch mask with 2-pm-wide stripes in the ( 110) direction perpendicular to the grating was formed. Wet etching was performed to get 0.5-1 pm wide active stripes. Then the wafer underwent the second LPE growth of the p-n IuP current blocking layers, each about 1,1m thick. This step is rather uncritical because the reverse p-n junction will automatically be located in a proper position. After this the SiN" was removed and ap-InP embedding layer and a quaternary contact layer were grown to obtain a flat surface (Fi.g. O. A new SiNx film was deposited and S-p-m-wide electrode openings were formed followed by Zn diffusion and Au-Zn electroplating and evaporation of the electrodes. The wafer was cleaved into chips with cavity lengths be­tween 250 and 400 pm. The lasers were honded on diamond heat sinks with Sn-Pb solder both inp-side down andp-side up configuration.

The distribution of threshold currents as measured in pulsed operation is shown in Fig. 2. The average value was 19 rnA, while the lowest value was 8 rnA. The threshold currents as measured in cw operation were in the range of 14-40 mAo A differential quantum efficiency of up to to •......... (-; . .- ....... . .... .-'.......... "': . . . . , ..... .- . .. .- . .-

':.; :', .. .. ... . ..'.,. ..

FIG. 1. SEM view of a flat-surface buried heterostructure (FBH) DFB laser.

2082 Appl. Phys. lett. 51 (25). 2i December 19B7 0003-6951/87/512082-03$01.00 «:) i9S7 American institute of Physics 2062

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lL. o a:: w ~ 5 :::J Z

.i o

r--N:::38

~h=i9 rnA ,..--

LS-308 ("II =1.5

FBH -OFB

! r--' I-

t--

5 10 15 20 25 30 35 40 THRESHOLD CURRENT(mA)

FIG. 2. Distribution ofthe pulsed threshold currents of38 randomly chosen FBH-DFB lasers.

14%/facet and a maximum output power of 20 mW were obtained. The maximum cw temperature was at least 80°C. The FBH-DFB lasers operate in a single longitudinal mode (Fig. 3) with a side-mode suppression ratio better than 30 dB and a coupling coefficient exceeding 100 em - 1 as mea­sured from the width of the stop band.

In the chirp measurements, the laser was biased to give 2-m W output power and was directly modulated by a sinu­soidal signal. The chirp was measured as the width of the lasing mode at -- 10 dB of the spectral peak, which gives an

CW (p-side up) 20·C

FBH-OFB laser

lill :=17 rnA

20 40 60 80 DC CURRENT (rnA)

, !

1. 5060 1. 5130 1.5200 WAVELENGTH ( "m )

100

FIG. 3. Light-current characteristics of a FEB, DFE laser togdher with its spectra at different current injection levels.

2083 Appl. Phys. Lett., Vol. 51, No. 25,21 December 1987

10 20 30 40 MODULA Tl ON CURRENT Ipp< mAl

FIG. 4. Measured wavelength chirp at .- 10 dB of the spectral peak as a function of modulation peak-to-peak curn:nt. Ito was 26 rnA.

appreciably greater and more significant value than the com­monly used full width at half-maximum (FWHM). A plot of the chirp versus modulation current shows a linear de­pendence with an increment 0[0.003 nm/rnA (Fig. 4). This value is lower than what we usually obtain with good match­ing between the Bragg wavelength and the gain peak, and is also lower than what is reported in the literature. 7 A chirp of about 0.12 nm was measured at a modulation current of 35 rnA which corresponds to about 40% modulation depth.

As the characteristics of semiconductor lasers generally are dependent on temperature, we have investigated the modulation behavior at different temperatures. We have found that a single longitudinal mode was maintained in a wide temperature range ( > 50°C) both in static and dynam­ic conditions as shown in Fig. 5. We did not observe any

1. 5180

E ::l.

~1.5170 -<

:r: t5 i .5160 z w ...J W ~ 1.515 3

-STATIC

",.·-.Dynamic peal< 'A F Z

50~ a: :::J u

4°0 ....l o :r:

30~ 0:: :r: I-

~~~--b-~~~~~~20

20 30 40. 50 T8\{PERATURE ( C)

FIG. 5. Temperature dependence of the lasing waVelength, both in static and dynamic conditions, of a FBH-DFB laser together with its threshold current.

Nilsson, Tanbun-Ek, and Broberg 2083

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>-f-

\11 z W f-z -l « u ;:: a.. 0

0 to 2.0 3.0 TIME ( ns)

@ 1~ 2.'5 ns

® 2.25

® 2.0

0]L_ 1.75

<!') t-· 1.5

®JL 125

(0J '1.0

0.75

CD-l 0.5

I (J) ---N-- Q 25

1.5~5Hi2 WAVELifNGTH( pm}

FIG. 6. Time-resolved spectra within an optical pulse of a FBH-DFB laser.

significant dependence of the chirp on the operating tem­perature. Both the static and the dynamic time-averaged peak wavelengths of the lasers increased by 0.075 nm/K with temperature. The latter was a few hundredths of a nan-

2084 AppL Phys. Lett, Vol. 51, No, 25, 21 December 1987

omcter shorter than the static wavelength. The temperature dependence of the threshold current can be characterized by the To value, which was typically 47 K.

In order to investigate the nature of the chirp in detail, we recorded time-resolved spectra in different parts of the optical pulse when the laser was modulated at a frequency of about 250 MHz (Fig. 6). The laser was biased at 2 m W power and the modulation current was about 45 rnA p-p. As can be seen, the laser operated in a stable single longitudinal mode in an parts of the optical pulse, and a shift toward longer wavelength (red shift) is observed. The maximum swing of the wavelength within the pulse was measured to be about 0,2 nm.

In conclusion, by detuning the Bragg wavelength 20 nrn to the shorter wavelength side of the gain peak, and by reduc­ing the confinement factor of the waveguide, an increment of the chirp with modulation current as low as 0,003 nm/rnA has been obtained. This corresponds to a total chirp of 0.1-0.2 nm at normal modulation conditions. The FBH laser structure has been found to be very attractive as it enables combination of narrow active stripes, giving low threshold currents (14 mA cw, 8 mA pulsed), with efficient current blocking, giving high power (20 m W cw).

We acknowledge Lennart Biicklin for fabricating the gratings and Berti! Holmberg and Lena Biickbom for valu­able help with the processing.

lK, Kishino, S. Aoki, and Y. Suematsu, IEEE J. Quantum Electron. QE. 18,343 (1982).

2L, D. Westbrook, Electron. Lett. 21,1018 (1985). 'K. Kamite, H. Sudo, M. Yano, H. Ishikawa, and H. Imai, 10th IEEE Int. Semiconductor Laser Conf., Kanazawa, Japan, October 14-17, 1986, p. 180.

4K. Kamite, H, Sudo, M. Yano, H. Ishikawa, and H, 1mai, IEEE J. Quan­tum Electron. QE-23, 1054 (1987).

5H. Ishikawa, H. Soda, K. Wakao, K. Kihara, K. Kamite, Y. Kotaki, M. Matsuda, H. Sudo, S. Yamakoshi, S. I~ozumi, and H. 1mai, IEEE J. Lightwave Technol. LT·5, 848 (l987).

6L, Biicklin, Electron. Lett. 23. 658 (1987). 7G. P. Agrawal, N. A. Olsson, and N. K. Dutta, Appl. Phys. Lett. 45,119 (1984).

Nilsson, Tanbun-Ek, and Broberg 2084

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