Mark E. Bridges

Design of a wavelength-tunable light source using an acousto-optic tunable filter

We report on the design and performance of a wavelength-tunable light source for illumination of fluorescent samples. The requirements included a maximum possible irradiance at the sample, rapid and continuous tuning between wavelengths over the range of 400-800 nm with 10-40 nm wide spectral output pass bands, 3 dB out-of-band spectral rejection, and coupling of the light into a fiber bundle for delivery to the sample. We selected an acousto-optic tunable filter (AOTF) for spectral tuning and a xenon arc lamp as the input source in this application. The tuning range of the AOTF was 450-750 nm, necessitating the use of additional narrow-band interference filters to tune across the entire 400-800 nm range. The AOTF had a 4.2° separation angle between the diffracted and non-diffracted beams. In order to achieve the best out-of-band rejection possible and to avoid overlap of the beams, the input source must diverge less than half the separation angle. The active aperture of the AOTF was 10 mm × 10 mm. The optical design included separate beam paths for the AOTF and interference filters that resulted in optimizing the throughput for each case and matching both beams to the numerical aperture of the fiber bundle. We discuss the design process, our prototype system, and issues to be addressed for use of an AOTF in fluorescent excitation applications.