Compact spectrometer for precision studies of multimode behavior in an extended-cavity diode laser

We have built a compact, inexpensive, high-precision spectrometer and used it to investigate the tuning behavior of a grating stabilized extended-cavity diode laser (ECDL). A common ECDL design uses a laser chip with an uncoated (partially reflecting) front facet, and the laser output exhibits a complicated pattern of mode hops as the frequency is tuned, in some cases even showing chaotic dynamics. Our grating spectrometer (based on a design by White {\&} Scholten) monitors a span of 4000GHz (8nm at 780nm) with a linewidth of 3GHz, which with line-splitting gives a precision of 0.02GHz in determining the frequency of a laser mode. We have studied multimode operation of the ECDL, tracking two or three simultaneous chip cavity modes (spacing \textasciitilde 30GHz) during tuning via current or piezo control of the external cavity. Simultaneous output on adjacent external cavity modes (spacing \textasciitilde 5GHz) is monitored by measuring an increase in the spectral linewidth. Computer-control of the spectrometer (for line-fitting and averaging) and of the ECDL (electronic tuning) allows rapid collection of spectral data sets, which we will use to test mathematical simulation models of the non-linear laser cavity interactions.