Temperature dependence of Alexandrite effective emission cross section and small signal gain over the 25-450 °C range

Abstract

We present detailed measurements of effective emission cross section spectra of the Alexandrite gain medium in the 25-450°C temperature range and provide analytic formulas that can be used to match the measured spectra. The measurement results have been used to investigate the wavelength and temperature dependence of small signal gain, as well as gain bandwidth relevant for ultrafast pulse generation/amplification. We show that the estimated laser performance based on the measured spectroscopic data provides a good fit to the results in the literature. We further discuss the need for a detailed measurement of excited-state absorption cross section in future studies.

Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

“…• Alexandrite possesses an intrinsic slope efficiency of 65% [42], and has broad emission bands enabling tunable lasing in the 700-860 nm region [15,16]. Alexandrite's broad gain spectrum could potentially generate sub-20-fs pulses centered around 750 nm [43]. Mode-locking of Alexandrite using Saturable Bragg Reflectors (SBRs) [29], Kerr-lensing [30,31], and graphene saturable absorbers [32] has recently been demonstrated, resulting in generation of down to 70-fs level pulses so far [31].…”

“…The temperature dependence of effective emission cross section of Alexandrite at the lasing wavelength (∼765 nm) has been estimated using [43]:…”

“…• The gain product of Alexandrite at room-temperature is similar to Ti:Sapphire (σ em τ f : 130 x10 -26 cm 2 s), which is quite low compared to Yb:YAG. On the other hand, due to the peculiar energy structure of Alexandrite, the σ em τ f -product increases with temperature, and reaches a value of 163 x10 -26 cm 2 s at 100 °C (specified values are for a wavelength of 765 nm) [43]. Due to this interesting property, Alexandrite lasers usually provide better laser performance at elevated temperatures (cw lasers provide optimum performance around 80-100°C) [17,34,35,78].…”

“…The ratio (f l ) of ESA cross section at the lasing wavelength (σ l,esa ) to emission cross section (σ em ) is relatively low around the gain peak and has a value of 0.05 at room temperature [60]. However, ESA cross section is as strong as the emission cross section at the edges of the emission spectrum, which actually limits the tuning range [43]. Moreover, ESA cross section at the lasing wavelength region is shown to increase with temperature, and the rate of increase in ESA looks higher than the rate of increase observed in emission cross section [43,79].…”

“…However, ESA cross section is as strong as the emission cross section at the edges of the emission spectrum, which actually limits the tuning range [43]. Moreover, ESA cross section at the lasing wavelength region is shown to increase with temperature, and the rate of increase in ESA looks higher than the rate of increase observed in emission cross section [43,79]. • For efficient laser operation, the chromium doping of Alexandrite crystals are kept within the 0.01-0.2% range, due to problems in obtaining uniform doping throughout the laser gain media at higher doping levels.…”