Monolithic Solid-state Lasers

Author: the photonics expert Dr. Rüdiger Paschotta

Although most solid-state lasers consist of a number of discrete elements (e.g. of a laser crystal or glass, some laser mirrors, and possibly additional intracavity optical elements), there are some types of lasers which are monolithic. For monolithic lasers according to a strict definition, the whole laser resonator consists only of some piece of crystal or glass. The resonator is then closed either with dielectric mirror coatings on the surfaces, or with total internal reflection. A somewhat relaxed definition allows for reflections from additional optical elements, and even for additional components within the laser resonator, provided that these elements are rigidly attached (e.g. bonded) to the gain medium.

Normally, one ignores the need of a separate laser diode for pumping, although this makes the complete laser setup clearly non-monolithic.

Types of Monolithic Lasers

There are monolithic lasers of different kinds; some typical examples are listed in the following:

• Microchip lasers have a linear resonator, with the gain medium being a thin slice of crystal or glass material. Such lasers are available as compact single-frequency lasers, also in Q-switched form or with intracavity frequency doubling (→ green lasers).
• Nonplanar ring oscillators (NPROs) are monolithic lasers where the laser radiation circulates along a nonplanar ring in a single laser crystal (e.g. of Nd:YAG). In most cases, these are single-frequency lasers, and they can deliver up to a few watts of output power.
• Waveguide lasers are also often monolithic. Such a laser can be based on, e.g., a channel waveguide in lithium niobate (LiNbO₃). The nonlinearity of this material allows for further functions, such as electro-optic modulation for Q switching, mode locking, or wavelength tuning.
• Fiber lasers might also be called monolithic when all fiber components (for example, a fiber coupler, a gain fiber, a fiber-coupled pump laser diode and two fiber Bragg gratings as end mirrors) are spliced together, such that the laser becomes alignment-free.
• Most semiconductor lasers, e.g. laser diodes, are monolithic.

Typical Properties

A common property of monolithic lasers is that they have a very stable and compact setup. That can be helpful for obtaining stable single-frequency operation, for example, and a low sensitivity to vibrations.

Monolithic laser designs often allow for fairly low intracavity losses (possibly well below 1%), leading to a low threshold pump power and relatively small linewidth (even though carefully designed lasers with longer resonators can have a still narrower linewidth).

Another consequence of the typically short resonator is a high relaxation oscillation frequency. Quantum-limited laser noise performance may thus be achieved only at relatively high noise frequencies.

A practical limitation is that a monolithic laser setup normally does not allow the insertion of additional intracavity optical components (although special designs allow for that [14]). Also, it is usually not possible to modify various design parameters without fabricating a whole new laser device.

More to Learn

Encyclopedia articles:

• solid-state lasers
• microchip lasers
• waveguide lasers