10115 - 10:3C

Double-fused 1Spm vertical cavity lasers operating continuous wave up to 71°C

ThA8

K. A. Black”, N. IM. Margalitb, E. R. Hegblomb, P. Abrahamb, Y-J. Chid, J. Piprekb, J. E. Bowersb, and E. L. H u ~ . ~

‘Materials Department bElectrical and Computer Engineering Department University of California, Santa Barbara, CA 93 106

Abstract

We report record high temperature performance of long wavelength vertical cavity lasers, operating continuous-wave up to 71°C. A record high long wavelength characteristic temperature (To) of 132K has been measured over an operating range of -75°C to 35’C.

Long wavelength vertical cavity surface emitting lasers (VCSELs) are attractive sources for fiber optic communication, offering an economic alternative to their in-plane counterparts with their capability for on wafer testing and ease of packaging. However, enhanced high temperature performance is necessary for such applications to be realized. Recent design improvements such as the incorporation of a selective laterally oxidized current aperture and a top emitting structure has led to vastly improved high temperature operating characteristics[l] and output powers[2]. In this paper, we report the operation of a 1.5pm doublefused VCSEL up to 71°C. We attribute the enhanced thermal characteristics of the device to a combination of novel active region and improved mirror design, as well as a lowered fused junction voltage.

The device structure consists of two MBE grown Al(Ga)As/GaAs mirrors on GaAs substrates fused to an InGaAsPhP active region grown on an InP substrate by MOCVD. A schematic of the device is shown in Fig. 1. Details of the fusion conditions are reported elsewhere[ 11. The top p-mirror consists of 25.5 quarter wave periods of GaAs/Al,,G%,,As with a single 40nm Al,,,Ga,,As layer for selective lateral oxidation placed 145nm from the GaAshnP fused junction. The ten closest mirror periods to the active region have a relatively low bulk carbon doping of 2x10”/cm3 to minimize optical loss; the remaining mirror periods have a bulk carbon doping of 7x1017/cm3 for lower resistance. ’The interfaces are parabolically graded with a pulsed doping of 3x10%m3. Prior to fusion, a 30nm GaAs layer was regrown with a Be doping of 5x10%m3. The bottom mirror is an undoped 30 period GaAs/AlAs quarter wave stack. The active region consists of six 7 nm 1%-strained quantum wells (QWs) with seven 7 nm thick barriers. On either side of the active region is a strain compensating Ino,,G~,,P layer and a 300nm InP cladding.

The room temperature (RT) photoluminescence of the active region is at 1542nm and the lasing mode is at 1507nm. Figure 2 shows the high temperature L-I curves of a device with a 6pm oxide opening. The threshold current at RT is 2.lmA, and the threshold voltage is 2.4V. The improved temperature performance of this device cannot be attributed to the misalignment of the gain peak and optical mode at RT, as the gain-offset only increases with temperature. The gain peak and mode wavelength shifts with temperature are 0.54 and 0.11 nm/K respectively[4]. The zero gain ofifset is calculated to be at 220K. To further analyse the temperature sensitivity of the device, the temperature depenldence of threshold current on temperature was measured from -190°C to 70°C, as is shown in Fig. 3. There are three major .factors contributing to the enhanced performance of this device over those previously fabricated. The use of carbon doping enables 90%Al(Ga)As/GaAs mirrors to be used in the top mirror as opposed to the Be doped 67%AI(Ga)As/GaAs mirrors previously used, reducing the loss. The low operating voltage of the device also contributes to reduced device heating. Finally, the introduction of an In,,Ga,,,P barrier layer to the active region acts both as a strain compensation mechanism far compressively strained QWs, and to increase the confinement energy of the electrons in the active region by about 30 meV compared to InP. Finally, the increased confinement energy of the InGaP barrier acts to reduce carrier leakage out of the active region. Additional improvements in device performance can be expected by better alignment of the gain peak and mode at RT. 113- N. M. Margalit, J. Piprik, S . Zhang, D. I Babic, K. Streubel, R. P. Mirin, J. E. Bowers, and E. L. Hu, “64°C continuous-wave operation of 1.Spm vertical-cavity laser”, IEEE J. Sel. Top. Quantum Electron., 1997, 3, 359-365 [2]- N.M. Margalit, A.K. Black, Y.J. Chiu, E.R. Hegblom, K. Streubel, P. Abraham, M. Anzlowar, J.E. Bowers, E.L. Hu, “Top-emitting double-fiused 1.5pm vertical cavity lasers”, Electronics Letters vol. 34, No. 3 (1998) 285-7 [3]- J. Piprek, Y.A. Akulova, D.I. Babic, L.A. Coldren, J.E. Bowers, “Minimum temperature sensitivity of 1.55 pm vertical-cavity lasers at -30 nm gain offset”, Applied Physics letters vol. 72, No. 15 (1998) 1814-6

(0-7803-4223-2/98/$‘I0.010 1 998 IEEE)

A record clharacteristic temperature of 132K is measured.

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+ Power out

n-metal contact InGaAsP MQ active region

Fig. 1: Schematic of double fused 1.5pm VCSEL.

0.05

0.04

% 0.03

b v

g 0.02 a

0.01

0 0 2 4 6 8 10 12

1 (“4)

Fig. 2: C-w L(1) characteristics as a function of temperature.

Wide temperature range operation of 1.5pm VCSEL

a E v .Ei

Y

T,:.132K _r’

-200 -150 -100 -50 0 50 100

Temperature (“C)

Fig. 3: Threshold current as a function of temperature.

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