Using Thermal Profiling to Quantify Optical Feedback into Semiconductor Lasers Evelyn Kapusta1, Dietrich Lüerßen1,2, and Janice Hudgings1 1. Department of Physics, Mount Holyoke College, South Hadley, MA 01075;
[email protected] 2. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139
Abstract: Thermal profiling is an ideal technique for monitoring optical feedback into semiconductor lasers in photonic integrated circuits. Quantitative measurements of optical output power, optical feedback magnitude, and threshold current shift are obtained without optical measurements.
2005
Optical Society of America
OCIS codes: (140.5960) Semiconductor lasers; (000.2170) Equipment and techniques; (120.6780) Temperature
Semiconductor lasers are extremely sensitive to back-reflections that arise in practical applications, potentially resulting in mode hopping, coherence collapse, strong excess noise, and chaotic dynamics [1]. Prior work has demonstrated that the magnitude of optical feedback into a semiconductor laser can be quantified by monitoring the resulting reduction in threshold current or redshift in the lasing wavelength [2,3]. When a semiconductor laser is integrated into a photonic integrated circuit (PIC), the lack of direct optical access to the laser hinders traditional optical methods of monitoring and controlling optical feedback. In this work, we show that the magnitude of optical feedback into a semiconductor laser can be quantified based on surface temperature measurements alone, without recourse to direct optical measurements. Therefore, the technique can provide vital information on optical feedback into lasers that are integrated into PICs. We studied optical feedback into a multiple quantum well InGaAsP/InP cleaved-facet laser coupled to a 38 cm long external optical cavity. The amount of optical feedback into the laser was controlled by placing a variable attenuator in the external cavity. We monitored the surface temperature (Ts), heat sink temperature (Ths), and ambient temperature (Ta) using 25x25 µm2 NIST-traceable microthermocouples with an accuracy of 200 mK and a precision of 10 mK. We define the feedback power ratio R as the ratio of the power coupled back into the lasing mode to the output power of the laser: R = Preflected/Pout = ρ2T2ATTNRMIRROR = ρ2Req, (1) where ρ is the coupling efficiency between the external cavity and the laser. TATTN is the one-way transmittance through the variable attenuator in the external cavity, and the reflectivity RMIRROR of the external mirror is 91%. Req is the combined reflectivity of the external cavity. From the thermal measurements, we can find the optical output power of the laser by means of a total energy balance model for the laser: (2) Prad = Pel - Pcond - Pconv ≡ IV - (Ts-Ths)/ZT - Aeffh(Ts-Ta) The electrical power Pel generated in the laser is dissipated through conduction Pcond, convection Pconv, and radiative power Prad due to photons emitted by the laser. The thermal impedance ZT=15.0 K/W and area-weighted heat transfer coefficient Aeffh=2.8mW/K are determined experimentally while operating the laser below threshold [4]. Figure 1a shows that below threshold (Prad=0), the surface temperature increases linearly with electrical power. Ts-Ths exhibits a kink at threshold due to the sudden increase in optical power; as photons are emitted, they remove energy from the laser [4]. In Figure 1b, we plot the difference between the linear fit to the data below threshold and the experimental data over the full range of bias currents. From this plot, threshold currents of 58 mA with feedback and 65 mA without feedback can be clearly identified; these threshold currents are in quantitative agreement with the L-I curves shown in Figure 2. When the laser is exposed to optical feedback, a fraction ρ of the reflected light is coupled into the lasing mode, while the remaining fraction (1-ρ) is absorbed at the facet. Thus, the radiated power is: (3) Prad = Pout – PoutReq(1-ρ), where Pout is the optical power emitted from the laser. This absorption of a fraction of the back-reflected light is visible in the thermal results in Figure 1. It shows as a slight increase in the surface temperature of the laser with optical feedback relative to the free-running case. From the temperature measurements, using equation 2, we find the radiated power emitted by the laser without recourse to direct optical measurements. As shown in Figure 2, there is quantitative agreement between this method and direct optical measurements for both cases, with and without feedback. While the no-feedback case has no
1.2
Optical Output Power (mW)
Deviation from linear fit in(a) (K)
TS - THS
(K)
adjustable parameters, the case with optical feedback allows the determination of a coupling efficiency of ρ=20±5%. This coupling efficiency agrees well with an independent determination of ρ=15% based on the magnitude of the shift in threshold current with feedback [3].
a)
1.0 0.8 0.6 0.4
0.00 -0.01 b) -0.02 -0.03 -0.04 -0.05 -0.06 0
58 mA 65 mA / /
data/fit without optical feedback data/fit with optical feedback
20
40
60
80
5
5
a) no feedback
b) optical feedback
4
4
3
3
2
2
58mA
65mA
1
1
0 -1
0 direct optical data fromthermal data
0
20
40
60
80
0
20
40
60
80
-1
Electrical Input Power (mW)
100
Electrical Bias Power (mW)
Figure 2: Quantitative agreement is obtained between direct optical measurement of the LI curves (solid lines) and the output optical power predicted by the thermal profiling data (dots).
Figure 1: (Ts-Ths) is proportional to the electrical bias power, except when energy is removed by photons (a). The difference between the linear dependence of (Ts-Ths) on the electrical power and (Ts-Ths) over the entire range of bias currents is shown in (b).
As can be seen in Figure 1b and Figure 2, the reduction in threshold current due to optical feedback can be quantified from the thermal measurements. Using this technique, we measured the normalized threshold current of a second laser for various feedback strengths; this laser is similar to the one used to obtain Figures 1 and 2. The comparison between the results obtained through thermal profiling and traditional optical methods is shown in Figure 3. Based on the work in references 3 and 5, the reduction in threshold current from the long extended cavity can be described by: I th I th 0
=
(
2αl − ln R1 Reff
),
2αl − ln ( R1 R2 )
(4)
where Ith0 is the threshold current of the laser without feedback, α is the absorption and scattering losses inside the cavity, and l is the length of the laser cavity [2]. R1 and R2 are the reflectivities of the laser mirrors, and R eff =
(
R2 +
R
) / (1 + 2
R 2R
) is the effective reflectivity of the extended-cavity measured at the output mirror 2
of the laser. The feedback power ratio R is as defined above in equation (1). The standard parameters for an edgeemitting semiconductor laser such as the one used in this model are αl = 0.125 and
R1R 2 =0.32 [6]. Fitting this
model to the experimental data in Figure 3, we obtain a coupling efficiency of ρ =15±2%. This value is expected to differ slightly from the one quoted above because of the use of a different laser. We have demonstrated that thermal profiling allows us to determine the amount of optical feedback into a semiconductor laser without recourse to direct optical measurements. The optical output power of the laser can be quantitatively determined from the thermal measurements for both a free-running laser and a laser exposed to optical feedback; in both cases, the LI curves determined by means of thermal measurements alone are in excellent agreement with the direct optical measurements. Furthermore, the shift in threshold current due to optical feedback is determined from the thermal measurements, enabling us to precisely quantify the feedback power ratio R and the coupling efficiency ρ. Because this technique does not rely on direct optical measurements, it is ideally suited for monitoring the optical feedback into semiconductor lasers integrated into photonic integrated circuits.
normalized threshold current I /I th th0
1.00 0.96 0.92 0.88 0.84 0.0
0.2
0.4
0.6
0.8
1.0
reflectivity of the external cavity Req
Figure 3: Normalized threshold current obtained from thermal measurements (dots) versus direct optical measurements (crosses). Solid line shows the fit of equation (4) to the data with a coupling efficiency of ρ=15%.
References: 1. K. Petermann, “External optical feedback phenomena in semiconductor lasers,” IEEE J. Sel. Topics in Quant. Electron. 1, p. 480-489 (1995). 2. S. Jiang, et al, “Influence of external optical feedback on threshold and spectral characteristics of VCSELs,” IEEE Photon. Tech. Lett. 6, 34-36 (1994). 3. P.A. Judge, C.H.L. Quay, and J.A. Hudgings, “Robustness to optical feedback of oxide-confined versus proton-implanted vertical-cavity surface-emitting lasers,” App. Phys. Lett. 81, 3933-3935 (2002). 4. K.P. Pipe and R.J. Ram, “Comprehensive heat exchange model for a semiconductor laser diode,” IEEE Photon. Tech. Lett 14, 504-506 (2003). 5. J.H. Osmundsen and N. Gade, “Influence of optical feedback on laser frequency spectrum and threshold conditions,” IEEE J. Quant. Electron. 19, 465-469 (1983). 6. L.A. Coldren and S.W. Corzine, Diode Lasers and Photonic Integrated Circuits (John Wiley and Sons, 1995).
