A wavelength tunable, quasi-cw laser source for high accuracy spectrometric measurement in the 200 nm to 500 nm region

Harmonic generation of the output of a mode-locked titanium sapphire laser is a highly effective means for producing high power, quasi-cw, wavelength tunable radiation in the 200-500 nm spectral range. The laser is not truly cw, consisting of pulses at approximately 100 MHz. This paper investigates the possibility of stretching these pulses in time using a multimode optical fiber to achieve a truly cw signal: stretching of a 1.5 ps pulse to around 1 ns has been demonstrated. Non-linearity effects in the responsivity of different optical radiation detectors due to the pulsed nature of the radiation have been investigated, and no evidence for non-linearity has been seen for either silicon or GaAsP detectors at wavelengths of 300 or 360 nm within the measurement uncertainties (< 0.15%). Finally, data are presented indicating that the use of mode-locked radiation in transmittance spectrophotometry avoids the problem of interference between front and back surface reflections which has previously hindered high accuracy transmittance measurement using lasers.