Laser slope efficiency

We have used a femtosecond-written Nd YAG ceramic optical waveguide as an active media to achieve continuous wave 1.06 pm laser operation. We have obtained output laser power of 40 mW and with a laser slope efficiency in excess of 40%. Single mode and stable laser oscillation have been achieved by using the natural Fresnel reflection for optical feedback without the requirement of any kind of mirror or reflective component. 

From the laser curve of Figure 4, it is also possible to get a rough estimation of the optical looses of the fabricated waveguide. Assuming a complete absorption of the launched pump power and a 100% pumping efficiency, the laser slope efficiency (r iaser) can be approximately written as ... 

The variation in CW laser emission efiftciency % of Nd YAG with the pump-to-laser volume superposition efiftciency describes the dependence of the efficiencies of de-excitation processes on It has been demonstrated that a decreased volume supeiposition means that the higher the laser threshold, the lower the laser slope efficiency. At the same time, there is an increase in the heat load parameter tjh. 

However, a major part of this power improvement was also due to massive gains in diode laser efficiency. Diverse commercial lasers, based upon single crystals, are typically no more than 10% to 20% in efiiciency, but usualty much less. It is typical for a crystal laser to have a "slope" efficiency of 2-5% with outputs less than 10 milliwatts. InGaAsP diode lasers are efficient by these standards with efficiencies greater than 20%. Strained layer quantum-weU structures have been measured with efficiencies in excess of 85% at 1500 nm. wavelength. Typical threshold currents for... 

The variation of % with j/v determines the slope of the change in the thermal power Pneat above the laser threshold, >/h,aih- For instance, the heat load parameter of 1 at.% Nd YAG in the absence of laser emission, which determines the slope of Pneab i e., 7h,bfli, below the threshold, calculated with the emission quantum efficiency /qe = 0.8 is 0.377. Above the threshold, for CW 1064 nm laser emission pumped at 809 nm, the values of the slope /h,ath are 0.24, 0.254, 0.267, and 0.281, for i/v = 1, 0.9, 0.8, and 0.7, respectively. If the slope efficiency is lower than that predicted by the measured residual losses, the outcoupling losses, and the quantum defect, it means that the pump-to-laser volume superposition is not sufficiently good. 

It was found that the ceramics absorbed about 64 % of the pump power irradiated on its surface. The OCs with transmissions of 3, 3.9, 6, and 10 % at 1064 nm were used for laser output experiments. The laser threshold was increased firom 0.345 to 1.03 W as the transmission of the OC was increased. The maximum output power of 0.356 W was obtained at an absorbed pumping power of 1.96 W, as the OC had Toe 10. An optical optical efficiency of 18.2 % and a slope efficiency... 

A similar laser, with diode end-pumping, could produce pulses of 128 ns at a repetition rate of 230 kHz, with a slope efficiency of 9 % [151, 152]. One of the examples was a LD-end-pumped passively g-switched Nd YAG ceramic laser, operated at 1319 nm with a V YAG saturable absorber [151]. An average output power of 1.8 W was achieved at the pump power of 23.7 W, which corresponded to an optical conversion efficiency of 7.8 % and a slope efficiency of 9 %. The minimum pulse width of 128 ns at a pulse repetition rate of 230 kHz was obtained with a T = 2.8 % OC at the pump power of 23.7 W. Another example was a side-pumped Nd-doped Gd(0.6)Y(0.4)VO(4) bounce laser, which was combined with a V YAG saturable absorber crystal [152]. It offered a passively g-switched output of 6.5 W at 1.3 pm. Output powers of 6.5 and 6 W were observed at a maximum pump level for the multi- and TEM(00)-mode operations, demonstrating optical-to-optical efficiencies of 17.5 and 16.2 %, respectively. 

Similarly, with 1532 nm volume Bragg grating stabilized (Er, Yb) fiber laser CW pumping, 13.8 W 1645 nm radiation with a slope efficiency of 54.5 % in incident power could be obtained by using a 1 at.% Er YAG ceramic rod [183]. In this study, polycrystaUine Er YAG ceramics with 0.5 and 1.0 at.% Er " doping concentrations, prepared by using solid-state reaction method, were cut and polished to have a dimension of 2 x 3 nun in cross section and 14 mm in length/... 

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