A single-frequency laser (rarely called a single-wavelength laser) is a laser which operates on a single resonator mode, so that it emits quasi-monochromatic radiation with a very small linewidth and low phase noise (see also: narrow-linewidth lasers). Because any mode distribution noise is eliminated, single-frequency lasers also have the potential to have very low intensity noise. In nearly all cases, the excited mode is a Gaussian mode, so that the output is diffraction limited.
Particularly in low-power single-frequency lasers such as laser diodes, there is some small amount of optical power in various resonator modes, even though one mode is clearly dominating. This is because such modes may be only slightly below the laser threshold, so that spontaneous emission can already generate some substantial power. The mode suppression ratio (MSR) is then defined as the power of the lasing mode divided by that in the next strongest mode. It can be optimized by making the laser resonator more frequency-selective.
Single-frequency lasers can be very sensitive to optical feedback. Even if less than a millionth of the output power is sent back to the laser, this may in some cases cause strongly increased phase noise and intensity noise or even chaotic multimode operation. Therefore, single-frequency lasers have to be carefully protected against any back-reflections, often using one or two Faraday isolators.
Types of Single-frequency Lasers
Details of the physics of single-frequency operation are discussed in the corresponding article; the present article discusses the most important types of single-frequency lasers, which differ very much in terms of output power, linewidth, wavelength, complexity and price:
- Some low-power laser diodes, in particular index-guided types, usually emit on a single mode. Stable single-mode operation is often achieved with distributed feedback lasers (DFB lasers) or distributed Bragg reflector lasers (DBR lasers). Typical linewidths are in the megahertz region (→ Schawlow–Townes linewidth, linewidth enhancement factor). Significantly smaller linewidths are possible e.g. by extending the resonator with a single-mode fiber containing a narrow-bandwidth fiber Bragg grating, or with external-cavity diode lasers.
- Special kinds of fiber lasers allow for single-frequency operation. Some of these are based on unidirectional ring laser designs, others have linear resonators and very short (highly doped) fibers. In any case, at least one fiber Bragg grating is usually used. Linear fiber lasers are sometimes realized as distributed feedback lasers. Very narrow linewidths of a few kilohertz can be achieved (particularly with devices having somewhat longer resonators), whereas output powers vary between a few milliwatts and several watts – in combination with a single-frequency fiber amplifier even more.
- Diode-pumped solid-state bulk lasers can be forced to operate on a single mode; this is often achieved with unidirectional ring laser designs, often with an intracavity filter, and sometimes with the twisted-mode technique. Output powers can reach the multi-watt level, and the linewidth can be as low as a few kilohertz.
- Vertical cavity surface-emitting lasers (VCSELs) have very short monolithic laser resonators, thus huge cavity mode spacings, and easily emit a few milliwatts on a single mode. The linewidth is at least a few megahertz, but the tuning range (without mode hops) can be very large.
- A helium–neon laser can easily emit a single frequency, if its laser resonator is made short enough (of the order of 20 cm), because the gain bandwidth is small.
Methods for Higher Output Powers
For higher output powers, master oscillator power amplifier configurations are often used. An alternative with potentially lower laser noise is to use injection locking of a high-power laser with a single-frequency low-power seed laser.
Typical applications of single-frequency lasers occur in the areas of optical metrology (e.g. with fiber-optic sensors) and interferometry, optical data storage, high-resolution laser spectroscopy (e.g. LIDAR), and optical fiber communications. In some cases such as spectroscopy, the narrow spectral width of the output is directly important. In other cases, such as optical data storage, a low intensity noise is required, thus the absence of any mode beating noise.
Single-frequency sources are also attractive because they can be used for driving resonant enhancement cavities, e.g. for nonlinear frequency conversion, and for coherent beam combining. The latter technique is currently used to develop laser systems with very high output powers and good beam quality.
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See also: single-frequency operation, single-mode operation, mode hopping, linewidth, narrow-linewidth lasers, distributed Bragg reflector lasers, laser diodes, fiber lasers, injection locking, twisted-mode technique, stabilization of lasers, fiber-optic sensors
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