Using Thermal Profiling to Monitor Optical Feedback in Semiconductor Lasers
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. When semiconductor lasers are integrated into photonic integrated circuits (PICs), the lack of direct optical access compounds this difficulty because traditional optical methods of monitoring and controlling optical feedback are unavailable. 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, so this technique is ideally suited for lasers integrated into PICs. We examined the surface temperature (Ts), heat sink temperature (Ths), and ambient temperature (Ta) of a 5 quantum well InGaAsP/InP cleaved-facet laser coupled to a 38 cm long external optical cavity using 25x25 micrometer^2 NIST-traceable microthermocouples. We combined our experimental measurements with a total energy balance model for the laser: Prad = Pel - Pcond - Pconv ≡ IV - (Ts-Ths)/ZT - Aeffh(Ts-Ta) (1) 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=15.0K/W and area-weighted heat transfer coefficient Aeffh=2.8mW/K were determined experimentally while operating the laser below threshold. Monitoring the measured difference, Ts-Ths, as the bias current to the laser was increased, we observed the expected rise in the surface temperature proportional to the electrical power below threshold with Prad=0. Above threshold, reduction in the slope occurs as emitted photons remove energy from the laser.2 When the laser is exposed to optical feedback, a fraction of the reflected light is coupled into the lasing mode, while the remaining fraction (1-p) is absorbed at the facet. Thus, the radiated power is: Prad = Pout PoutRext(1-p), (2) where Pout is the optical power emitted and Rext=91% is the reflectivity of the external cavity. From our temperature measurements, using equation 1 and 2, we found the radiated power emitted by the laser without recourse to direct optical measurements. Comparing our results from this method and direct optical measurements we found a strong quantitative agreement for both cases, with and without feedback.