The Construction of a Continuous Wave Cavity Ringdown Spectrometer

Henry Timmers


A continuous wave Cavity Ringdown Spectrometer has been constructed and implemented using two distinct designs. The first design injected infrared laser light into a half-symmetric spherical cavity using super mirrors to create the reflecting surfaces for the cavity. The curved super mirror of the cavity was dithered at a sinusoidal frequency much slower than the ringdown time, allowing the production of ringdown events as the cavity passed over resonant modes. From the decay constants of the ringdown events, a value of R = 0.9994 � 0.0002 was measured for the reflectance of the cavity super mirrors. This first method produced low signals and inconsistent measurements for the ringdown decay constants, so a second set-up was constructed to incorporate optical feedback into the design. The second design injected light into the cavity using a pellicle beam splitter and dithered an intra-cavity mirror at a sinusoidal frequency much slower than the ringdown time, effectively changing the phase of the injected light. When the injected light moved in-phase with the ringdown cavity, the optical feedback of the system would increase by many orders of magnitude and interfere with the operation of the laser diode. This caused the laser diode light to lock to the ringdown cavity modes and build up to an maximum amplitude within the cavity. When this occurred, the laser was shut off using a system of electronics and the injected light was allowed to decay out of the cavity, producing ringdown events. The decay constants measured from this set-up produced a value of RD = 0.9964 � 0.0009 for the reflectance of the distributed Bragg Reflector. Once the optical feedback method of Cavity Ringdown Spectroscopy is fully optimized, the set-up can be used to accurately measure the reflectivity of semiconductor mirrors as well as the absorption of a sample of graphene.