9:OOam - 9:15am Thl.3 High-Temperature Modulation Dynamics of

1.3 pm A1,GrayIn~-,yAs/InP Compressive-Strained Multiple-Quantum-Well Lasers

C. E. Zah, M. C. Wang("), R. Bhat, T. P. Lee, S. L. Chuang'") 2. Wagcb), D. Darby@), D. Flanders", and J. J. Hsieh@"

Blellcore, NVC3X361,331 Newman Springs Rd., Red Bank, NJ 07701, USA (") Dept. of ]Electrical and Computer Engineering, Univ. of Illinois, Urbana-Champaign, IL 6 180 1

') Lasertron, Burlington, MA 01803

Abstract: Intrinsic small-signal modulation responses of 1.3 pm Al,Ga In As/InP compressive-strained multiplequantum-well lasers with three different barrier layers me invest&at%-&or the lasers with a barrier bandgap wavelength of 1.01 pm, k factors are determined to be 0.24425 ns, and thermal-limited 3-dB bandwidths of 19.5, 15, and 13.9 GiHz are measured at 25,65 and 85 OC, respectively.

For loop applications, it is desirable to use uncooled semiconductor lasers that can perform well in an extreme temperature environment. They are more economical and reliable than the lasers requiring cooling against the variations in the ambient temperature. The conventional GaIdsP-based multiplequantum-well lasers suffer from the compromise between poor electron confinement and nomniform hole distribution resulting from the small band offset ratio (-40%) of the conduction band. It has been expected that the carrier confinement in the AlGaInAs-based lasers should be improved since the band offset ratio of the conduction band is about -72% in this material system. Receitly, we have demonstrated that the 1.3 pm Al,Ga Inl-,- As/InP compressive-strained multiplequantum-well lasers have excellent light-current performance and k g h reiiability over the temperature range from -40 to 85 OC.ly2 In this work, we investigate the barrier height effects on the high-temperature modulation dynamics of Al,GayIn~~,~yAs/InP compressive-strained multiplequantum-well lasers for the first time.

The devices used in this study are three kinds of 1.3 pm Al,GayInl,,,,,As/InP compressive-strained five- quantum-well ridge-waveguide lasers, each employing five quantum-wells in the middle of the graded-index separate-confinement hleterostructure (GRINSCH) region. Their structures are described in Table I. The bandgap wavelengths of the barrier layers are 0.94, 1.01 and 1.17 pm, respectively. On either side of the multiplequantum- well region, a linearly graded AlGaInAs waveguide layer of 100 nm is grown, which begins at the barrier composition and ends ,at a quaternary layer of 0.96 pm bandgap wavelength for Device A or a ternary layer of Al.48In.52As for Devices B and C. The fabrication details can be fomd in Ref. [3] and [4]. The cavity is 300 pm long and the rear facet 11s high-reflection coated with a reflectance of 70 %.

The intrinsic small-signal modulation responses are measured in the range of 0.1-20 GHz by a parasitic- free optical modulation technique. The 1.3 pm laser diode under test is optically modulated by injecting a 0.98 pm light externally modulated by an optical modulator inside a lightwave network analyzer. The output of the 1.3 pm laser diode is separated from the reflected 0.98 pm light by a WDM ~ o u p l e r . ~ The intrinsic small-signal modulation responses are measured at various bias currents and three heat sink temperatures. As shown in Fig. 1, the solid lines are the tlheoretical fittings to the measured data for Device B shown in dots. Its maximum intrinsic 3-dB bandwidths are measured to be of 19.5, 15, and 13.9 GHz at 25, 65 and 85 OC, respectively. Power roll-off caused by heating prevents further increase of the modulation bandwidth. The device characteristics are summarized in Table 11. It includes the differential gain gn, the nonlinear gain factor t; and the k factor, derived from the relaxation resonance frequency& and the nonlinear damping factor y which are obtained by fitting the experimental data.

A comparison made between Devices A, B and C suggests that the differential gain gn is similar in Devices B and C, but lower in Device A. More importantly, the temperature sensitivity of Device A is also worse. By contrast, the nonlinear gain factor E and its temperature sensitivity are found to be similar among the three devices, within experimental error. As a result, the k factor is larger in Device A, and increases faster as the temperature increases. To further understand the origin of the temperature sensitivity of %e differential gain in these three lasers, spontaneous emission spectra were measured at high temperature (75 C). The spontaneous emission was measured through a window on the back side of the chips. As shown in Figs. 2, three transition peaks can be identified in the spontaneous emission spectra of Device A at 18 mA bias current. They are attributed to the band to band transitions in the well, the heavy hole band to the conduction band (1.30 pm) and the light hole band to the conduction band (1.23 pm), and in the barrier (1.17 pm). At room temperature, the lasing mode is due to the transition between the heavy hole band and the conduction band which dominates the other two transitions. With the increase of the bamer height in Devices B and C, the separation between the heavy-hole and the light-hole

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subbands increases. Most of the valence holes stay in the first heavy-hole state, which is codlrmed by the clean spectra of Device C in Fig. 2

To reduce the temperature sensitivity of lasers, it is important to confine the carrier well at high ~empe~ature by adopting a sufficient hlgh barrier and a large compressive-strain in the well such as Devie B.

has smaller k factor, higher Merentid gain and smaller nonlinear gain factor than the InGaAsPAnP well lasers over the temperature range from 25 to 85 "C5 This is attributed to the favorable band nl-x-yAs/InP materials system which enables the design of lasers with deep quantum wells.

nl-x-yAsDnP materials system is a better choice for making hgh speed lasers. *, Tech. Dig. of Optical Fiber Comm. ThG1, 204-205, San Jose, CA, February 20-25, 9994.

12) Z. Wag, et al., Tech. Dig. of Optical Fiber Comm. Wl4, 144-145, San Jose, CA, February 20-25, 1994,

et al., 7th Znt. Con$ on M O V E , Yokohama, Japan, May 31-June 3, 1994. awa, et al., Appl. Phys. Lett., 61 (4), 396-398, 1992.

Well Composition Al. 17&. 1571n.673AS A1.153Ga.1141n.733AS Al.143Ga.1061n.751AS Strain 0.99 Yo 1.4 Yo 1.53 Yo

1.77 % 0.93 Yo 1.03 Yo Barrier Composition Al.2Ga.2731n.527As A1.303Ga. 1651n.532AS Al.368Ga. 11 11n.521As Banier Thickness 10.1 nm 10.0 nm 10.2 nm

aR 1.17 pm 1.01 pm 0.94 pm

Well Thickness 7.1 nm 5.2 nm 6.2 nm 17W

Table I1 Device characteristics of 1.3 pm Al,Ga,.In1 +,,AS compressive-strained quantum well lasers GHz L-

Device T, OB: Ith, mA h, pm ai, cm-1 fi' Jmw k, ns g,, 101%m2 E, 10-l7 cm3 A 25 14 1.29 11.6 1.56 0.3 1 3.35 1.36

65 36 1.31 16.1 1.15 0.37 1.62 0.92 25 13 1.34 8.6 1.7 0.24 5.82 1.35

B 65 21 1.36 10.7 1.49 0.24 4.18 1.04 85 28 1.37 11.3 1.36 0.25 3.38 0.93 25 14 1.34 4 1.49 0.27 5.57 1.41

C 65 24 1.36 4.34 1.34 0.27 4.38 1.12 85 33 1.37 3.48 1.13 0.28 3.16 0.85

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1

10

Wavebngth (nm)

Fig. 1 Small-signal modulation responses of Device Fig.2 Measured spontaneous emission spectra of Device A and C at 75 OC with a bias current of 18 mA.

B at 25,65 and 85 O C with bias currents of 153, 137 and 150 mA, respectively.

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