DPSS laser (diode-pumped solid-state laser)

A diode-pumped solid-state (DPSS) laser uses one or more laser diodes as the pump source for an ion-doped crystal or glass. The diodes emit in a band a few nanometres wide that can be placed on a strong absorption line of the dopant, such as 808 nm or 885 nm for neodymium and 940 nm or 969 nm for ytterbium, where a flashlamp spreads its output across hundreds of nanometres. Nearly all the pump light that reaches the crystal is then absorbed where it is useful. DPSS lasers range from green laser pointers of a few milliwatts to multi-kilowatt industrial sources, and they include most compact Q-switched and mode-locked lasers at 1064 nm and its harmonics.

End pumping and side pumping

In end pumping, the diode output is collimated and focused along the cavity axis into a small spot in the crystal, overlapping the TEM₀₀ laser mode. Good overlap gives low threshold, high efficiency and near-diffraction-limited output, and it is the usual design up to a few tens of watts; the limit is the heat concentrated near the input face. In side pumping, diode bars surround a rod or slab, spreading the heat over a larger volume at the cost of weaker mode overlap and lower beam quality, and it scales to kilowatts. Fiber-coupled pump modules, with the diode light delivered through a multimode fiber of a few hundred micrometres core, separate the heat-generating diodes from the laser head.

Threshold and efficiency

For an end-pumped four-level laser with Gaussian pump and mode radii $w_p$ and $w_0$, the absorbed pump power at threshold is

$$P_\text{th} = \frac{\gamma}{\eta_p}\,\frac{h\nu_p}{\tau}\, \frac{\pi\,(w_0^2 + w_p^2)}{2\sigma},$$

where $\gamma$ is the single-pass logarithmic loss, $\eta_p$ the pump quantum efficiency, $\tau$ the upper-state lifetime and $\sigma$ the emission cross section. For Nd:YAG ($\sigma = 2.8 \times 10^{-19}$ cm², $\tau = 230$ µs) pumped at 808 nm, with a 5 % output coupler and 0.5 % internal loss per pass ($\gamma = 0.031$), $\eta_p = 0.9$ and both radii 200 µm, the threshold is 0.16 W. Above threshold the slope efficiency is the product of the pump efficiency, the quantum defect ratio $808/1064 = 0.76$ and the fraction of the loss that is useful output coupling, 0.84, giving 0.57. A 2 W absorbed pump then yields about 1.05 W, ignoring mode-overlap and thermal corrections. Measured optical-to-optical efficiencies of end-pumped Nd:YVO₄ lasers above 50 % are common.

The overall wall-plug efficiency multiplies this by the electrical-to-optical efficiency of the pump diodes, typically about 50–60 % for high-power 808 nm and 9xx nm bars, plus the power drawn by coolers and drivers.

Green lasers and harmonics

Many DPSS lasers are frequency-converted. In a green laser pointer or a 532 nm laboratory module, an 808 nm diode pumps Nd:YVO₄ and a KTP or LBO crystal inside the cavity performs second-harmonic generation of the 1064 nm light, where the circulating intensity is highest. Q-switched DPSS lasers with external harmonic crystals produce 355 nm and 266 nm for micromachining and marking, and continuous single-frequency DPSS lasers at 532 nm pump Ti:sapphire lasers.

Pitfalls

Diode wavelength shifts with temperature by roughly 0.3 nm/K, so a 10 K change moves the pump 3 nm, comparable to the width of the 808 nm Nd:YAG absorption line; the diodes are held on a thermoelectric cooler or water-cooled plate, or wavelength-stabilized with a volume Bragg grating, to keep the output constant. Diode aging changes both power and wavelength. The concentrated pump in end-pumped designs produces a strong thermal lens, so the mode size, and sometimes the stability of the cavity, depends on pump power. Cheap green pointers often omit the infrared filter after the doubling crystal and emit unconverted 1064 nm and 808 nm light that is invisible yet can be stronger than the green.

Common questions

What does DPSS mean?

Diode-pumped solid-state: a solid-state laser whose pump is a semiconductor laser. The emitted wavelength is set by the doped crystal and any harmonic conversion; the diode wavelength only has to match an absorption line.

Why not use the diode laser directly?

Diode output is spectrally broad and, for high-power bars, has poor beam quality and cannot store energy for long. The crystal converts it into a narrow-linewidth, near-diffraction-limited beam, and its microsecond storage time allows Q-switched pulses of high peak power.

References: W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006); O. Svelto, Principles of Lasers 5th ed. (Springer, 2010); T. Y. Fan, R. L. Byer, Diode laser-pumped solid-state lasers, IEEE J. Quantum Electron. 24, 895 (1988).