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Presented at 2002 Conference on Lasers and Electro-Optics Experimental Determination of Heat Flow in Semiconductor Lasers K. P. Pipe and Rajeev J. Ram Research Laboratory of Electronics, Massachusetts Institute of Technology, Room 26-459, Cambridge, Massachusetts, 02142-1363 Telephone: (617) 258-0273 / Fax: (617) 258-7864 / Email: [email protected] and [email protected] Abstract: This work discusses a comprehensive model and experimental data regarding heat generation and transport in semiconductor lasers; surface temperature is related to optical device parameters such as threshold and optical efficiency. This technique can be used to perform waferscale testing of photonic integrated circuits. 2001 Optical Society of America OCIS codes: (140.6810) Thermal Effects; (140.5960) Semiconductor Lasers; (120.6780) Temperature; (120.1880) Detection; (250.5300) Photonic integrated circuits The strong temperature dependence of a semiconductor laser's performance is clearly apparent in a comparison of pulsed versus continuous-wave operation; the former leads to lower threshold current [1,2], higher quantum efficiency [2], more stable performance over varying ambient temperature [3], and the ability to operate at longer wavelengths [3]. This relationship motivates a careful study of heat generation and transport in optical devices [47]. Here we describe the relevant thermal processes and relate them to experimental data taken on an edge-emitting laser diode using a 20m x 20m NIST-calibrated microthermocouple with 10 mK accuracy. An optical device is typically fabricated on a thick substrate that is in contact with a heat sink that maintains the preset temperature of a nearby thermistor. The heat generated by the bias power IV (neglecting the voltage drop in the bias probes and at the heat sink) is balanced by the energy transport processes of thermal conduction (Pcond), convection (Pconv), and radiation (Prad), which can be written for a laser above threshold with this geometry as: IV Pcond Pconv Prad Ak (T T ZT surf I Tamb spon h q th stim I I h q th ATsurf 4 (1) where T = Tsurf - Ths is measured between the surface of the laser and a nearby point on the heat sink (cf. Fig. 1), ZT is the thermal impedance, Tamb is the ambient temperature, A is the device area, k is the convection heat-transfer coefficient, and is the Stefan-Boltzmann coefficient. These measurements were taken on a 15m x 500m =1.55m 5-QW InGaAsP/InP ridge-waveguide laser (Ith = 90mA, differential efficiency stim=0.28) that was placed at the edge of a 3cm x 3cm Peltier-cooled gold-coated copper block. The thermistor used to set the control temperature of the heat sink was located inside the block approximately 2mm from the laser. In most devices, the conduction term Pcond dominates thermal dissipation, and ZT is only a weak function of temperature (through the temperature dependence of the thermal conductivity); surface temperature is often wellapproximated by a linear fit: Tsurf Ths + IVZTo [8]. However, since the neglected terms are often close in magnitude to the optical output (cf. Fig. 2), they must all be taken into consideration when deriving optical parameters such as the efficiency from a given T. For example, a second-order fit for ZT (T) allows for an accurate determination of optical power Prad = IV – Pcond - Pconv (cf. Fig. 3) that relies only on measurement of Tsurf, Ths, and Tamb. Although the edge-emitting device measured here has a small threshold current density (~250 A/cm2), its large area and 5 quantum wells lead to a large threshold current and a small wall-plug efficiency. In a device with greater efficiency (such a surface-emitting geometry), the optical power is expected to dominate, leading to the simplification of Eq. 1. A comprehensive model that incorporates all the terms of Eq. 1 could be used to characterize an optical device (or calibrate an optical detector) without any invasive probing or calibration. This would be useful in situations where detectors are not available or are not able to be positioned in the light path, such as in integrated photonic systems where optical elements emit light laterally into adjacent fabricated components. Similar work has been done using photothermal reflectance measurement of facet temperature [9], although without a detailed thermal model to justify efficiency characterization. One way to determine the parameters of Eq. 1 is to carefully measure above threshold (with, for example, a lock-in amplifier) where the operation of the laser is more stable. The investigation of heat flow in a semiconductor laser diode performed here leads to the conclusion that a thermal probe is useful for parameter extraction; the model used, however, must be quite detailed in devices with low wall-plug efficiency. 32 Measured Surface Temperature Measured Heat Sink Temperature 30 Temperature ( o C) 28 T 26 24 Tamb 22 20 18 Ttherm 16 0 50 100 150 Current (mA) Fig. 1. Measured variation of surface and heat sink temperatures. Also shown are the controlled ambient temperature and the set temperature of the heat sink thermistor; note that the large-area heat sink has a thermal gradient between the thermistor and the laser. 250 2 Power (mW) 200 150 Power (mW) 10 0 10 -2 10 0 50 100 150 Current (mA) IV ZT (T=Tamb) ZT (T) Convection Stim. Optical 100 50 0 0 50 100 150 Current (mA) Fig. 2. Heat introduced (solid markers) and removed (open markers) through bias power, thermal conduction (at constant ZT and ZT (T)), convection, and directly measured optical power. The blackbody term is negligible. INSET: Log scale. 4 T (oC) 3 Optical Power (mW) 15 Direct Op tical Power Mete r Derived fr om Thermal Pro be 10 5 0 0 50 100 150 I (mA) 2 1 Threshold Measured T Linear Fit Below Threshold 0 0 50 100 150 200 I*V (mW) Fig. 3. Measured T vs. I, showing deviation caused by emission of optical power above threshold, which appears as a discontinuity (proportional to stim) in surface temperature. INSET: Optical power, as measured by both a direct optical power meter and a thermal probe. 1. H. J. Yi, J. Diaz, I. Eliashevich, M. Stanton, M. Erdtmann, X. He, L. J. Wang, M. Razeghi, "Temperature dependence of threshold current density Jth and differential efficiency d of high-power InGaAsP/GaAs (=0.8 m) lasers." Appl. Phys. Lett. 66, 253-5 (1995). 2. Milind R. Gokhale, J. Christopher Dries, Pavel V. Studenkov, Stephen R. Forrest, Dmitri Z. Garbuzov, "High-power high-efficiency 0.98-m wavelength InGaAs-(In)GaAs(P)-InGaP broadened waveguide lasers grown by gas-source molecular beam epitaxy." IEEE J. Quantum Electron. 33, 2266-76 (1997). 3. H. K. Choi and G. W. Turner, "Mid-infrared semiconductor lasers based on antimonide compounds" in Optoelectronic Properties of Semiconductors and Superlattices, M. O. Manasreh, ed. (Gordon and Breach, Amsterdam 1997). v. 3, p. 369. 4. N. Bewtra, D. A. Suda, G. L. Tan, F. Chatenoud, J. M. Xu, "Modeling of quantum-well lasers with electro-opto-thermal interaction." IEEE J. Select. Top. Quantum Electron. 1, 331-40 (1995). 5. Paul R. Berger, Niloy K. Dutta, Kent D. Choquette, Chulam Hasnain, Naresh Chand, "Monolithically Peltier-cooled vertical-cavity surfaceemitting lasers." Appl. Phys. Lett. 59, 117-9 (1991). 6. M. Bertolotti, G. L. Liakhou, R. Li Voti, R. P. Wang, C. Sibilia, A. V. Syrbu, V. P. Yakovlev, "An experimental and theoretical analysis of the temperature profile in semiconductor laser diodes using the photodeflection technique." Meas. Sci. Technol. 6, 1278-90 (1995). 7. K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal thermoelectric heating and cooling in heterostructure diode lasers.” Conference on Lasers and Electro-Optics, Baltimore, MD, May 2001. 8. S. G. Patterson, E. K. Lau, K. P. Pipe, and R. J. Ram, “Temperature characteristics of bipolar cascade lasers.” Appl. Phys. Lett. 77, 172-4 (2000). 9. S. Dilhaire, S. Jorez, L. Patiño-Lopez, W. Claeys, and E. Schaub, “Laser diode light efficiency determination by thermoreflectance microscopy.” Microelectronics J. 32, 899-901 (2001).