Miniature lasers, sometimes referred to as microlasers or nanolasers, are lasers which are designed to have substantially smaller dimensions than traditional lasers – a few millimeters or sometimes even well below 1 mm.
While microlasers are often the result of traditional ways of miniaturization, an extreme case are nanolasers, with dimensions even well below one wavelength. Those are based on special physical operation principles, as e.g. traditional types of laser resonators cannot be realized with such small size.
Some lasers may be called miniature lasers despite relatively large dimensions, if they are at least rather compact compared with other lasers of the same type. For example, there are miniature mode-locked bulk lasers achieving extremely high pulse repetition rates.
Main Reasons for Miniaturization
Reasons for the miniaturization of lasers are typically some of the following, mostly related to certain aspects of laser applications:
- Integration: Compact dimensions allow one into integrate lasers in devices for which very little space is required, or little weight can be tolerated. For example, in the future there may be wearable laser projection displays, with a micro-laser fixed to some eyewear. Also, there are tiny sensors requiring an ultra-compact laser source. Another application area with stringent space requirements is laser-based optical communications on computer chips. In some cases, tiny lasers need to be integrated in photonic integrated circuits.
- Cost: While the development of a miniature laser source may be expensive, mass production can be rather cheap. In particular, cost-effective mass product may result from lasers which can be fabricated with wafer technologies: many lasers devices can be fabricated together on a single wafer, and are separated only towards the end of the product process. In that way, miniaturization may be an essential step for establishing some new laser applications in mass markets.
- Power consumption: Mainly due to their tiny mode area in the laser gain medium, many miniature lasers require only a minimal electric power.
Design and Construction
Not all types of lasers are well suited for miniature designs. For example, a helium–neon laser can hardly be made very small, since its laser gain media generates only a fairly low gain per unit length. The following list contains some typical conditions on a laser type for a good potential of miniaturization:
- The laser gain medium needs to provide a sufficiently high gain within a quite limited path length. (To some extent, the gain requirement can be mitigated by reducing the propagation losses in the laser resonator).
- The gain medium and the required additional means (for pumping, cooling, etc.) need to be made rather small.
- The cooling requirements should be quite moderate, i.e., the laser can typically not generate a high output power.
- It must be possible to design a small enough laser resonator. For example, that may be challenging if, for some reason, a substantial mode area of the resonator modes of a bulk-optical laser resonator is required, which would lead to a Rayleigh length far longer than the allowed resonator dimensions. (Working with a waveguide may solve that problem.)
- Ideally, one should not require large optics for beam conditioning, or a Faraday isolator protecting the laser against back-reflections, since otherwise the advantages of a tiny laser may be spoiled. Similarly, it is often not practical to incorporate means for nonlinear frequency conversion.
- Similarly, it should not be necessary to incorporate optical elements in the laser resonator which require much space.
- It is usually not acceptable to require any alignment or other maintenance after deployment.
Diode Lasers and Other Semiconductor Lasers
Diode lasers are particularly suitable due to a combination of favorable aspects:
- a high gain per unit length, making a resonator length of hundreds or even tens of microns sufficient
- very compact pumping arrangement (electrical pumping through some electrodes)
- the use of a waveguide
Even quite high output powers are possible from lasers with dimensions below 1 mm. A wide range of emission wavelengths are possible through the use of material and design details. Special spectral properties, such as narrow linewidth emission or wavelength tuning capability, can also be realized with rather compact means.
Apart from traditional edge-emitting semiconductor lasers, there are also other kinds such as surface-emitting semiconductor lasers, which may be made similarly compact while offering high output power in combination with high beam quality.
Other Miniature Lasers
Some types of miniature lasers are made within photonic integrated circuits. Basically always, they are based on waveguides where some laser gain is provided, e.g. through dopants or through coupling to some other gain medium via evanescent waves. Even in silicon photonics, it is possible to make lasers, despite some difficulties.
Nanolasers require special techniques, for example, for realizing extremely compact laser resonators. For example, they can be based on surface plasmons at metal structures, on nanofibers, or on photonic crystals with strong confinement of light.
Some types of non-semiconductor solid-state lasers can at least be made relatively compact, although usually not as small as diode lasers:
- Microchip lasers have a rather short monolithic laser resonator, even in combination with a passive Q switch for pulse generation.
- Even traditional types of bulk lasers concerning the type of resonator (made from discrete optical elements) can be made rather small, with overall dimensions of a few millimeters. For example, such lasers are made to achieve mode locking with ultra-high pulse repetition rate.
- There are fiber lasers requiring only a couple of millimeters of active fiber.
Typical Features of Miniaturized Lasers
Although different types of miniature lasers differ in many respects, some typical features are found:
- For various reasons, the available output powers are rather small. On the other hand, miniature lasers can often be operated with quite little electrical power, which is particularly advantageous for battery-powered mobile devices.
- As the laser resonators are very short, their free spectral range is large. This makes it easier to achieve stable single-frequency operation (without random mode hopping). In the case of pulse generation with mode locking, the pulse repetition rate is necessarily extremely high, and the output pulse energies are rather small.
- Their compact setup makes many miniature lasers quite robust, e.g. against mechanical shocks.
- The combination of low output power and a short resonator may lead to increased levels of laser noise (e.g. an increased linewidth), although the compact and rugged setup may reduce the influences of acoustic noise.
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