Diode-pumped Solid-state Laser - Common DPSS Processes

Common DPSS Processes

The most common DPSS laser in use is the 532 nm wavelength green laser pointer. A powerful (>200 mW) 808 nm wavelength infrared GaAlAs laser diode pumps a neodymium-doped yttrium aluminium garnet (Nd:YAG) or a neodymium-doped yttrium orthovanadate (Nd:YVO₄) crystal which produces 1064 nm wavelength light from the main spectral transition of neodymium ion. This light is then frequency doubled using a nonlinear optical process in a KTP crystal, producing 532 nm light. Green DPSS lasers are usually around 20% efficient, although some lasers can reach up to 35% efficiency. In other words, a green DPSS laser using a 2.5 W pump diode would be expected to output around 500-900 mW of 532 nm light.

In optimal conditions, Nd:YVO₄ has a conversion efficiency of 60%, while KTP has a conversion efficiency of 80%. In other words, a green DPSS laser can theoretically have an overall efficiency of 48%.

In the realm of very high output powers, the KTP crystal becomes susceptible to optical damage. Thus, high-power DPSS lasers generally have a larger beam diameter, as the 1064 nm laser is expanded before it reaches the KTP crystal, reducing the irradiance from the infrared light. In order to maintain a lower beam diameter, a crystal with a higher damage threshold, such as LBO, is used instead.

Blue DPSS lasers use a nearly identical process, except that the 808 nm light is being converted by an Nd:YAG crystal to 946 nm light (selecting this non-principal spectral line of neodymium in the same Nd-doped crystals), which is then frequency-doubled to 473 nm by a beta barium borate (BBO) or lithium triborate (LBO) crystal. Because of the lower gain for the materials, blue lasers are relatively weak, and are only around 3-5% efficient. In the late 2000s, it was discovered that bismuth triborate (BiBO) crystals were more efficient than BBO and LBO and do not have the disadvantage of being hygroscopic, which degrades the crystal if it is exposed to moisture.

Violet DPSS lasers at 404 nm have been produced which directly double the output of a 1,000 mW 808 nm GaAlAs pump diode, for a violet light output of 120 mW (12% efficiency). Initially, these lasers out-performed gallium nitride (GaN) direct 405 nm Blu-ray diode lasers. As direct 405nm diode technology progressed (primarily for use in Blu-ray disc writers) output powers of greater than 500mW have become possible, exceeding the output powers possible from directly doubled 404nm DPSS lasers. Further, the frequency-doubled violet lasers have a considerable infrared component in the beam, resulting from the pump diode.

Yellow DPSS lasers use an even more complicated process: A 808 nm pump diode is used to generate 1,064 nm and 1,342 nm light, which are summed in parallel to become 593.5 nm. Due to their complexity, most yellow DPSS lasers are only around 1% efficient, and usually more expensive per unit of power.

Another method is to generate 1,064 and 1,319 nm light, which are summed to 589 nm. This process is more efficient, with about 3% of the pump diode's power being converted to yellow light.