We adopted a single-mode, single-wavelength volume holographic grating (VHG) wavelength-stabilized wavelength laser diode (LD) as a pumping LD for an end-pumped microchip Nd:YAG and Nd:YVO4 lasers we developed during CW and pulse operations. Higher optical-optical and slope efficiencies during CW operation have been obtained than when using a VHG LD experimentally. Output laser power is insensitive to the temperature of the LD when using a wavelength-stabilized LD and can remain stable and almost constant until the temperature of LD increases up to 40°C. The improved optical-optical conversion efficiency of 58% for the Nd:YVO4 laser has been obtained and calculated the output laser power during CW operation and compared it with the experimental results. We found that the output laser power of the Nd:YVO4 laser using the VHG wavelength-stabilized LD was more than twice as high as that using an LD without VHG. When the ambient temperature increases, the difference in output laser power should be large. In the future, a low-cost end-pumped microchip laser that does not require a temperature control should be developed.
Solid-state lasers are compact and efficient and have high beam quality, and the application of laser diode- (LD-) pumped lasers to the industrial field has expanded. Various end-pump microchip solid-state lasers under CW and pulse operations have been developed [
The possibility of a laser using a neodymium-doped yttrium orthovanadate (Nd:YVO4) crystal was reported in 1966 [
Research has been conducted to stabilize the output frequency by using the feedback of the partial output light of an LD to semiconductors [
The CW output laser power for a microchip laser is calculated as follows [
The Nd:YAG parameters for calculating the output laser power are described below. We used equations (
The focused spot diameter of LD light on the laser materials was assumed to be 230
The experimental setup for the two microchip lasers is shown in Figure
Experimental setup for (a) Nd:YAG and (b) Nd:YVO4 lasers.
The LD was set tightly in a copper holder. A temperature-control device with a Peltier cooler and electronic controller (TED200C, Thorlabs) was used to control the LD temperature.
A thermistor for the temperature measurement was installed outside the LD can. The output light from the LD entered once through a single lens with AR coating (
A Cr4+:YAG crystal with a diameter of 10 mm was set near the laser materials, as shown in Figure
The experimental results for generating a CW laser are shown in Figure
Output laser power of Nd:YAG laser during CW operation.
For the Nd:YAG laser using the LD without VHG, the threshold for the pump power was 70 mW, slope efficiency was 40%, and maximum output conversion efficiency was 33%, as shown in Figure
In the range of pump power (below 370–380 mW) where the thermal shift of the LD without VHG shows good superposition with the absorption line of Nd, the slope efficiency is higher without LD stabilization. This is because the peak wavelength of the VHG LD light is a little shorter than that of the absorption band for the Nd:YAG. The peak wavelength of the output light for the LD without VHG is almost the same as that of the absorption band for the Nd:YAG.
The threshold was also 60 mW for the Nd:YAG laser using the VHG LD, but the slope efficiency was 43% and maximum output conversion efficiency was 37%. The conversion efficiency and slope efficiency increased slightly. The maximum output laser power was 200 mW.
The peak wavelength of the LD light shifts to the longer side as the temperature of LD increases. Thus, increasing the absorbed power of the LD light resulted in increasing the output laser power slightly and nonlinearly.
For the Nd:YVO4 laser using LD without VHG, the threshold for the pump power was 20 mW, slope efficiency was 47%, and conversion efficiency at the maximum output was 36%, as shown in Figure
Output laser power of Nd:YVO4 laser during CW operation.
When we changed the resonator length for the Nd:YVO4 laser using the VGH LD to 5 mm, the threshold was 20 mW, slope efficiency was 59%, and maximum output conversion efficiency was 58%, as shown in Figure
Normally, the output laser power of a four-level laser oscillator (such as the Nd:YAG laser) is calculated to change almost linearly with the pump power. However, when the pumping power was high in the experiment, the temperature of the laser medium increased and the output was saturated. When the pump power was about 500 mW in this experiment, the temperature of the laser medium was low, so the above does not apply. The difference in laser power for
The conversion and slope efficiencies increased using the VHG LD, and the obtained maximum output laser power was 290 mW. When using the VHG LD, the threshold decreased by 50%, and the slope efficiency and output power also increased. This is probably because the bandwidth of the output light from the LD narrowed, spatial mode improved, the divergence angle of the LD light decreased, and the focusing diameter of the pumping LD light decreased.
Both LD output power characteristics used in this experiment do not change with an increase in temperature and show only a slight decrease from 25°C to 35°C at the output power of 500 mW. Even if the temperature increases 10°C, the output power will not decrease significantly for the LD current. Exhausting the heat of the LD cannot catch up and the chip temperature increases to around 30°C. The peak wavelength of the LD light shifts to a long wavelength and moves away from the peaks of the absorption band for the Nd:YAG and Nd:YVO4, so the laser output decreases. When a VHG LD is used, the output power increases linearly with the pump power. This also shows that the pump power does not drop as a function of the LD current.
The calculated results of the optimum coupling reflectivity of the output mirror and conversion efficiency are shown in Figure
Calculated output laser power and conversion efficiency for the reflectivity of output mirror: (a) Nd:YAG and (b) Nd:YVO4.
The experimental results for the stability of laser power with respect to LD temperature are shown in Figure
Stability of output laser power for LD temperature: (a) Nd:YAG and (b) Nd:YVO4.
The calculated absorption rate of laser materials as a function of the LD temperature obtained from the experimental results is shown in Figure
Absorption ratio of laser materials as a function of LD temperature: (a) Nd:YAG and (b) Nd:YVO4.
The calculated results of the
Calculated small signal gain coefficient and laser large gain: (a) Nd:YAG and (b) Nd:YVO4.
In fact, the peak wavelength of the LD light shifts due to increasing the temperature of the LD. However, since the absorption band was wide for the Nd:YVO4, the output could be maintained against the temperature change of the LD. In addition, the Nd:YAG had a narrow absorption band and thus exhibited a slight decrease in laser output. However, it was shown that the VHG LD is effective for maintaining the output laser power even at high temperatures when using the Nd:YAG.
The experimental results for pulse laser generation are shown in Figure
Nd:YAG laser under pulse operation: (a) average output laser power and (b) repetition rate.
For the Nd:YAG laser using the LD without VHG, the threshold for pumping power was 160 mW, slope efficiency was 18%, and maximum output conversion efficiency was 13%, as shown in Figure
The repetition rate varied from 7 to 36 kHz for the Nd:YAG laser using the LD without VHG. It also varied from 2 to 60 kHz when using the VHG LD, as shown in Figure
For the laser using the Nd:YVO4 laser using the LD without VHG, the threshold for the pump power was 160 mW, slope efficiency was 50%, and conversion efficiency at the maximum output was 30%, as shown in Figure
Nd:YVO4 laser under pulse operation: (a) average output laser power and (b) repetition rate.
The repetition rate varied from 400 to 800 kHz for the Nd:YVO4 laser using the LD without VHG. It also varied from 400 to 900 kHz for this laser using the VHG LD, as shown in Figure
Experimental results for the stability of the output laser power with respect to LD temperature are shown in Figure
Stability of laser power for LD temperature: (a) Nd:YAG and (b) Nd:YVO4.
For the Nd:YVO4 laser using the LD without VHG, the average output laser power was 20 mW at 23°C, and the pump power was 200 mW, as shown in Figure
These experimental results indicate that microchip lasers can be driven by simply cooling LDs to a temperature at which the LDs will not be damaged, even without a temperature controller. This enables us to simplify the device. Increasing the pump power of the LD light and obtaining a higher output laser power are for future work.
These microchip lasers are intended for operating up to around 40°C. Because the heat generated in the laser material is below 0.5 W, the temperature of the laser material is 10°C higher than the ambient temperature. Thus, the optical parameters of the laser materials should not change. However, when the output power of the LD light significantly increases, the laser material generates a large amount of heat and the optical parameters change. Thus, cooling the laser material and keeping the temperature constant are necessary.
In the case of CW laser emission, the slope efficiency depends on pump absorption efficiency, quantum efficiency, pump and resonator mode matching, quantum defect, output mirror transmission, and residual optical loss. In the case of laser materials with high quality, laser material and pump characteristics enable slope efficiency close to the quantum defect by selecting the resonator properly. An experimental result on the conversion efficiency in [
In our study, the intensity of pump light was low, the laser gain was not high as fiber lasers, the ratio of mode utilization was low, the reflectance of the output mirror was high, and the extraction efficiency was low, so the conversion efficiency was limited to 60%. However, the conversion efficiency will be higher than that of the standard Nd:YVO4 microchip laser [
We developed end-pumped microchip Nd:YAG and Nd:YVO4 lasers pumped using a VHG LD as a pumping LD. We experimentally obtained high optical-optical conversion efficiency of close to 60% for the Nd:YVO4 laser during CW operation. The output laser power is stable for the LD temperature of close to 40°C when using the VHG LD. The output laser power was calculated numerically to compare the experimental results. These experimental results will lead to the development of end-pump microchip lasers that do not require a temperature controller. The output laser power of the Nd:YVO4 laser using the VHG LD was more than twice as high as that using the LD without VHG.
No data were used to support this study.
The authors declare no conflicts of interest.