Nonplanar Ring Oscillators
A nonplanar ring oscillator is a laser oscillator with a special monolithic design, which is very stable, compact and rugged. The resonator of an NPRO laser (Figure 1) consists of a single laser crystal (typically made of Nd:YAG) within which the laser light circulates. On one of the internal surfaces (the front face, which can have a slightly convex shape), there is a partially reflective dielectric mirror coating which is highly transmissive for the pump light (blue beam in Fig. 1) and serves as the output coupler mirror of the resonator. On all other internal surfaces, total internal reflection occurs.
The front face of the crystal has a dielectric coating, serving as the output coupler and also a partially polarizing element, facilitating unidirectional oscillation. The blue arrow indicates the pump beam, normally generated with a laser diode.
Such lasers have originally been called MISERs = monolithic isolated single-mode end-pumped rings, but later the term NPRO = nonplanar ring oscillator became more common.
Most NPROs are made of Nd:YAG. It is possible, however, to use other materials, such as Yb:YAG. In that case, reabsorption losses in unpumped regions can be eliminated by using a composite laser crystal, consisting of Yb:YAG and undoped YAG pieces which are bonded to each other.
An NPRO is usually pumped with a high-brightness laser diode. With that, it can reach an output power of some hundreds of milliwatts or sometimes even several watts. NPROs are often used as master lasers for high-power single-frequency MOPA devices.
Importance of a Nonplanar Resonator
An important detail is that the ring resonator of an NPRO is nonplanar, i.e., that the beam path is not in a single plane. That causes a slight rotation of the polarization direction in each round-trip. If a small magnet is attached to the laser crystal, its magnetic field can cause an additional polarization rotation via the Faraday effect. For one of the two oscillation directions, the two polarization rotations partly cancel, leading to a lower optical loss when the beam hits the output coupler face of the crystal ( because the coating has a slightly polarization-dependent reflectivity). The other oscillation direction leads to a higher loss and is thus firmly suppressed. In this way, one easily obtains unidirectional operation, thus avoiding any standing-wave patterns (except very near to the reflection points), which would cause spatial hole burning. For this reason, with nonplanar ring oscillators one easily obtains stable single-frequency operation.
Spectral Properties and Laser Noise
Given that the round-trip length is short (typically between a few centimeters and 10 cm), the NPRO cavity has a relatively large free spectral range, which allows for continuous (mode-hope free) frequency tuning over several gigahertz. Tuning can be achieved with a piezoelectric transducer pressing on the crystal, by changing the crystal temperature with a Peltier element, or by adjusting the pump power. There have also been nonplanar ring lasers containing an electro-optic crystal for tuning.
Due to the stable mechanical setup, the low optical losses of the laser resonator and the low noise of a laser diode as pump source, the laser noise of an NPRO can be very small. Typical linewidths are in the region of a few kilohertz.
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