Chun-Yu Cho

High-power blue LEDs, up to 7.3 W per chip operating in the pulsed regime, provide a compelling alternative to conventional near-IR pumps for solid-state lasers. We report the first, to the best of our knowledge, direct blue LED-pumped Pr:YLF laser, generating a peak output power up to 8.4 W at 640 nm. This compact approach offers a low-cost, scalable pathway for high-power visible lasers, while enabling downstream intracavity conversion techniques including second-harmonic generation (SHG) for UV emissions and optical parametric oscillation (OPO) for tunable near-IR generation.

This study establishes the criteria for achieving high-beam-quality output in monolithic, diffusion-bonded Nd:YAG/Cr ⁴⁺ :YAG lasers. In this monolithic architecture, the cavity mode size is constrained by the selected pump mode size, establishing a fixed pump-to-cavity mode size ratio. The high-beam-quality output can be ensured by maintaining a sufficiently low pump-to-cavity mode size ratio. In this work, a pump-to-cavity mode size ratio < 1.25 is determined to achieve an M ² beam quality factor < 2. These findings are validated through comprehensive experimental analysis across diverse configurations. Furthermore, we investigate the feasibility of energy scaling by optimizing crystal length and mitigating thermal effects. This finding offers a framework for the design of high-performance, passively Q-switched monolithic lasers.

This work shows the feasibility of demonstrating various yellow laser outputs with common solid-state laser materials. The Nd:YAG laser is able to achieve wavelength adjustment by using the birefringence filter. By further including the intracavity stimulated Raman scattering, the fundamental wavelength can be extended. Up to fifteen different wavelengths in a range from 1144 nm to 1194 nm can be successfully generated by using different Raman crystals. Finally, the yellow laser, especially that at 577 nm, can be generated via second harmonic generation.

The temperature-dependent second threshold criteria for the Nd:YVO₄/Cr⁴⁺:YAG passively Q-switched (PQS) laser is explored. The emission cross section of the Nd:YVO₄ crystal is found to decrease as temperature elevates, which is feasible to reach the PQS second threshold with a flexible cavity design. We derive theoretical analysis and experimentally verify it by demonstrating the cavity mode size ratio variation between those on the gain medium and the saturable absorber. With the successful demonstration of reaching the second threshold condition by increasing the Nd:YVO₄ temperature, an application of the intracavity stimulated Raman scattering is further explored to show the benefit of this method.

The stimulated Raman scattering (SRS) from the Nd:YVO₄ gain medium is originally employed to eliminate the self-pulsing effect from the intracavity optical parametric oscillator (OPO) for achieving a continuous-wave (CW) mid-infrared output. The SRS with a second-order pump-wave power depletion is applied as the damping for the coupling between OPO pump-wave relaxation oscillation and signal-wave depletion. The SRS threshold conditions for different cavity and diode-pumped mode size designs are theoretically and experimentally explored. By using different resonators, the CW mid-infrared output from 120 mW to 1.56 W at the diode pump power from 3 to 19.3 W can be successfully demonstrated.

A low-threshold efficient setup for CW wavelength versatile vortex generation is proposed. The intracavity optical parametric oscillator is utilized for low-threshold operation. By using the pump-wave off-axis pumping for the signal-wave oscillator, high-order Hermite–Gaussian modes at the eye-safe region can be obtained, which can further generate a vortex via an astigmatism mode converter. The overall laser performance is additionally verified to be better by comparing it with the diode off-axis pumping method where the TEM_1,0 pump wave was generated first for the signal-wave TEM_1,0 mode oscillation. This method is expected to be further applied for more complicated structured light generation.

The design criteria of the wavelength-selectable Nd:YAG laser by controlling a birefringence filter (BRF) rotational angle and temperature are explored. Theoretically, the angle and temperature-dependent BRF transmission are applied to the threshold pump power equation to verify the achievable output wavelength. It is found that by using a thicker BRF with a 3.3-mm length instead of the typical <1-mm design, the temperature-varying range can be narrower, which can be controlled by the thermoelectric module. A selectable wavelength at 1077 nm or 1122 nm can then be achieved for further green-yellow selectable output via SHG. The output performance of the Nd:YAG laser with BRF is experimentally verified, where eight emission peaks can be selected. The green-yellow selectable output is finally obtained with SHG by varying the BRF temperature only.

To reach the second threshold criterion for the Nd:YVO₄ passively Q-switched laser with a Cr⁴⁺:YAG saturable absorber, the method of temperature-elevating for decreasing emission cross section with a proper gain-medium-to-absorber-mode-size-ratio resonator is explored.