Microchip laser

A microchip laser is a monolithic solid-state laser made from a thin, polished wafer of gain crystal, typically Nd:YAG or Nd:YVO₄ between about 0.2 and 2 mm thick, with dielectric mirror coatings deposited directly on its two flat faces. A laser diode at 808 nm pumps it through the input coating, which transmits the pump and reflects the 1064 nm laser light. Large wafers are coated once and diced into chips about a millimeter across, so the laser has no alignment and very few parts. Continuous-wave microchips give single-frequency output from milliwatts to about a watt; passively Q-switched versions give pulses of a few hundred picoseconds to about a nanosecond with energies from a few to tens of microjoules.

Single-frequency operation

The cavity is so short that its longitudinal mode spacing exceeds the gain bandwidth. For a cavity of length $L$ and index $n$,

$$\Delta\nu_\text{FSR} = \frac{c}{2nL}.$$

A 0.5 mm Nd:YAG chip ($n$ = 1.82) has a spacing of 165 GHz. The gain bandwidth of Nd:YAG at 1064 nm is about 0.5 nm, or 132 GHz, so only one longitudinal mode lies near the gain peak and the laser runs single-frequency without any intracavity filter. The condition $\Delta\nu_\text{FSR}$ greater than the gain bandwidth holds for Nd:YAG chips shorter than about 0.62 mm. This is the design rule introduced by Zayhowski and Mooradian in 1989; Nd:YVO₄, with its higher index and stronger pump absorption, permits thin chips at good efficiency.

The flat-flat cavity is not stable by itself in the resonator-stability sense. The transverse mode is defined by thermal lensing and gain guiding from the focused pump, so the beam is usually a good TEM₀₀ mode with a waist of tens of micrometers, but its size depends on pump power.

Passive Q-switching and sub-nanosecond pulses

Bonding or coating a saturable absorber onto the gain chip, most often Cr⁴⁺:YAG for 1 µm lasers, makes a passively Q-switched microchip. The saturable absorber holds off lasing until the stored gain exceeds the combined loss, then bleaches and releases a single pulse. The pulse duration scales with the cavity round-trip time, so the short cavity gives short pulses. For a 1.5 mm Nd:YAG and Cr⁴⁺:YAG assembly the round trip is

$$t_\text{rt} = \frac{2nL}{c} \approx 18\ \text{ps},$$

and a 500 ps pulse lasts about 27 round trips. Pulse widths from a few hundred picoseconds to about a nanosecond are typical, and the shortest chips, Q-switched with semiconductor saturable absorbers, reach below 100 ps. A 10 µJ, 500 ps pulse has a peak power of about 20 kW (18.8 kW for a Gaussian shape), enough for direct frequency conversion and for laser-induced breakdown. Repetition rates range from about 1 kHz to hundreds of kHz, set by pump power, because each pulse fires when the gain reaches threshold.

Shorter pulses can also be extracted by cavity dumping a Q-switched microchip or by active Q-switching with an electro-optic element in a slightly longer composite cavity.

Applications

• Ranging and lidar, where sub-nanosecond pulses give centimeter-scale resolution from a small, robust source.
• Laser-induced breakdown spectroscopy, micromachining and marking with high peak power at modest average power.
• Seed sources for fiber and solid-state amplifiers, and pump sources for picosecond optical parametric devices.
• Frequency-doubled green sources: a Nd:YVO₄ chip bonded to a KTP doubling crystal is the green laser pointer and many low-cost DPSS modules.
• Single-frequency continuous-wave references and seeds for interferometry and spectroscopy.

Limitations and pitfalls

• Timing jitter. A passively Q-switched chip fires when the gain crosses threshold, so pulse timing is set by pump noise and spontaneous emission. Jitter relative to an external trigger is far larger than the pulse width unless the pump is pulsed or the switch is active.
• Power scaling. The small pumped volume limits average power to a few watts at most; thermal lensing changes the mode size and, in Q-switched devices, the pulse energy and repetition rate.
• Mode hops with temperature. The single mode tunes with chip temperature through thermal expansion and the thermo-optic effect until the next mode reaches the gain peak, so single-frequency chips are temperature controlled.
• Coating damage. The intracavity fluence at the coatings is high for sub-nanosecond pulses, which bounds the usable pulse energy for a given mode size.

Common questions

Why are microchip lasers single-frequency?

Because their mode spacing, $c/(2nL)$, is larger than the gain bandwidth, so only one cavity mode has net gain. Longer cavities have several modes under the gain curve and need an etalon or a ring cavity to select one.

How do microchip lasers produce sub-nanosecond pulses?

The Q-switched pulse duration is a few tens of cavity round trips, and a millimeter-scale cavity has a round trip near 20 ps. A conventional Q-switched laser with a 10 cm cavity has a round trip of about 0.7 ns and produces pulses of about 10 ns or longer.

References: J. J. Zayhowski and A. Mooradian, "Single-frequency microchip Nd lasers," Opt. Lett. 14, 24 (1989); J. J. Zayhowski, "Passively Q-switched Nd:YAG microchip lasers and applications," J. Alloys Compd. 303–304, 393 (2000); W. Koechner, Solid-State Laser Engineering, 6th ed. (Springer, 2006); A. E. Siegman, Lasers (University Science Books, 1986).