Low-cost, broadly tunable (375–433 nm & 746–887 nm) Cr:LiCAF laser pumped by one single-spatial-mode diode

Abstract

We describe a low-cost, compact, and efficient Cr(3+):LiCaAlF(6) (Cr:LiCAF) laser that is broadly tunable in the near infrared (746-887 nm) and blue/ultraviolet (375-433 nm) regions of the optical spectrum. The pump source is a 660 nm single-mode-diode with 145 mW of output power. A melt-grown Cr:LiCAF crystal with an extremely high figure of merit above 2000 was used as the gain medium. This enabled the construction of a high-Q-cavity with continuous wave (cw) lasing thresholds as low as 3 mW, slope efficiencies as high as 54%, and output powers up to 63 mW. The stored intracavity power levels were above 30 W. By placing a beta-barium-borate crystal at a second intracavity focus, we could obtain tunable cw blue/ultraviolet radiation with output powers up to 3.5 mW around 400 nm. When mode-locked using a saturable Bragg reflector around 800 nm, the Cr:LiCAF laser produced 95 fs pulses with average powers of 33 mW and peak powers of 3.58 kW at a repetition rate of 85.5 MHz. Extracavity second harmonic generation enables generation of femtosecond pulses around 400 nm. These results demonstrate the possibility to grow Cr:LiCAF crystals with passive losses below 0.15% per cm, which enables construction of low-cost and complexity systems that are pumped only with one low-power single-mode-diode.

Tapered diode-pumped continuous-wave alexandrite laser

“…In conflict with theory, the obtained slope efficiencies at high output coupling would start to decrease. A similar effect is also observed in Cr:colquiriites, where it is due to the presence of Auger energy upconversion processes [2]. Here, the decrease in slope efficiency is a side Fig.…”

“…The remaining 0.2% loss should be attributed to the alexandrite crystal, where in each pass one roughly gets a loss of 0.1%. For the 10-mm-long alexandrite crystal, this corresponds to a quite low loss level of only about 0.1% per centimeter (cm), which is similar to what can be achieved with Cr:LiCAF (0.14% per cm [2]) and Cr:LiSAF (0.2% per cm [65]), and much better than typical Ti:sapphire crystals (2% per cm [11]). This also shows itself in the crystal figure of merit (FOM) value, which is defined as the ratio of the absorption coefficient at the pump wavelength to the absorption coefficient at the lasing wavelength.…”

“…We suspect that a similar mechanism might also be at work in the alexandrite gain medium. More details on the effect of Auger upconversion on laser slope efficiency can be found in [2]. The cavity loss estimate using Findlay-Clay (0.4 0.1%) and Caird (0.3 0.1%) analyses are in relatively good agreement with each other.…”

“…This also shows itself in the crystal figure of merit (FOM) value, which is defined as the ratio of the absorption coefficient at the pump wavelength to the absorption coefficient at the lasing wavelength. The alexandrite crystal that was used in this study has an estimated FOM of about 3000 (≅ 3∕0.001), which is relatively high in comparison with Cr:LiCAF (2150 [2]), Cr:LiSAF (1500 [65]), and Ti:sapphire (150 [11]). On the other hand, Scheps et al reported a passive loss level of 0.06% per cm from a 0.2% chromium-doped alexandrite crystal (the crystal probably had even lower losses since they have assumed that the cavity optics did not contribute to the resonator losses) [36].…”

“…If these active laser media also posses suitable absorption bands that allow direct diode pumping, this brings many additional advantages, such as lower cost [1,2], compactness [3,4], higher electrical-to-optical conversion efficiency [5,6], and lower noise [7]. Ti:sapphire has the broadest gain bandwidth among all the solid-state laser media, facilitating tuning from 680 to 1180 nm [8], as well as direct generation of pulses as short as 5 fs [9].…”