An ultra-stable VCSEL light source

We demonstrate a miniature VCSEL-based light source with optical power output that varies by less than 50 ppm/degrees C over a 40 degrees C temperature range. This represents a ten-fold improvement in control accuracy over what is achieved by the best available light sources with semiconductor emitters. A single-mode, polarization-locked, 670-nm VCSEL (Vixar, Inc.) is used to demonstrate the feasibility of the light source. The critical component of our control system is a beamsplitter formed by a wedge of fused silica with a weakly-polarizing, interference coating. A wedge shape eliminates fluctuations in optical power caused by interference fringes and the normal incidence of the exit beam removes polarization dependency as well as the need for an anti-reflective coating. The beamsplitter is in a closed control loop that enables compensation for control errors caused by changes in photodetector responsivity, emission wavelength, and beam divergence with temperature. We also show that careful attention to optical and mechanical alignment is essential to realize the performance enhancements reported herein. Angular tolerances of +/- 0.05 degrees are required and most coating houses lack the metrology capabilities to characterize the TCB coating with such high tolerances. A discrete-wavelength reflectometer was developed for this purpose.