We report a method for locking the output wavelength and reducing the spectral linewidth of diode lasers by feeding back light to the emitters from a wavelength selective external optical cavity. Ten years ago our team developed a stepped-mirror that allowed a single external cavity to lock the wavelength of a stack of diode array bars by equalizing path lengths between each emitter and the grating. Here we report combining one such step-mirror external cavity with an array of power dividers, each sending a portion of this feedback power to a separate diode array bar stack.
External cavity diode laser systems are well-suited for diode pumped alkali laser (DPAL) systems due to their high power efficiency and excellent wavelength control under changing thermal loads. By conditioning the characteristics of feedback power, external cavities can narrow the spectral bandwidth and limit transverse modes of diode laser bars. Existing configurations typically use low-efficiency diffraction gratings at the Littrow angle to send back to the diodes a small fraction of the power, while directing the majority of the power forward in the output beam. We previously reported that a stepped mirror allows a single external cavity to condition the output beams of a stack of diode array bars. In this report, we describe a new approach that could use a single external cavity to condition the output beams of several hundred diode array bars. A high efficiency grating is used to feedback essentially all the power in the external cavity, and power splitters then distribute the power to multiple diode array stacks. A 384 bar module capable of 20 kW power output into a modelimited slowly diverging beam with a spectral width below 0.050 nm has been designed and proposed for use in a DPAL. A 50 bar 3 kW prototype is currently being assembled.
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