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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 20m x 20m 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
  ATsurf 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 15m x 500m
=1.55m 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.
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