Optical fiber bundles converting pulsed lasers into continuous waves for improved measurement linearity in spectrophotometry

Laser sources have been well adopted in spectrophotometry and optical materials measurements with excellent advantage of high spectral output power and hence large signal-to-noise ratio compared with traditional lamp sources. However, continuous wave lasers normally have very small tuning spectral range, due to either the limited bandwidth of the gain profile or low conversion efficiency due to the limited amount of pump power. Pulsed laser sources can be generated via nonlinear processes such as optical parametric oscillation or amplification and so on to cover a wide spectral range. For instance, supercontinuum laser sources have been developed with above 1 W/nm collimated spectral power over the visible to mid-infrared range. The output of the supercontinuum laser sources can be extended to cover UV range via second harmonic generation. Normally the spectral bandwidth of the output can be from tens to thousands nm and a monochromator or its equivalents can be used to select or tune the desired working wavelengths. The repetition rate of the laser sources can be from a few Hz to more than 1 GHz, with reduced pulse interval and efficiency due to more distributed pulse energy and hence peak power. A pulsed laser source with an original pulse width of ~ 130 fs, a repetition rate of ~ 80 MHz, and the spectral range from 280 to 2000 nm is applied for spectrophotometry calibration and research works have been devoted to converting the pulsed laser into continuous wave in order to improve the measurement linearity. Optical fiber bundles were used to divide each laser pulse into hundreds of small pulses via different optical path length and then recombine them for a temporally further distributed pattern. Two different types of optical fibers were adopted for the bundle, one for 280 nm ~ 700 nm wavelength range, and the other for 700 nm ~ 2000 nm. The numerical aperture of the optical fiber is 0.22 and the core diameter is ~ 200 μm. Each bundle has 100 pairs of optical fibers with different lengths. The refractive indices were extrapolated over the wide spectral range and used to calculate the optical fiber lengths for uniform time interval after recombination. The optical losses of the optical fibers over the wide spectral range were evaluated. After taking the spectral optical losses, fiber lengths, and input beam spatial power distribution into account, the arrangement of the optical fibers with different lengths at the input terminal was optimized for overall high performance over the two desired working spectral range. The time interval, relative power, spatial distribution of the distributed pulses throughout the designed optical fiber bundles were validated using a highspeed photodetector and oscilloscope. The optical fiber bundles can be useful for a variety of spectrophotometry calibration and optical materials measurement works using pulsed laser sources.