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Single-frequency Lasers

Definition: lasers emitting radiation in a single resonator mode

German: einmodige Laser

Category: lasers

How to cite the article; suggest additional literature

A single-frequency laser (sometimes 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:

Most single-frequency lasers operate continuously, but there are also Q-switched single-frequency lasers, which do not exhibit mode beating and thus exhibit very clean pulse shapes and low noise.

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.

Applications

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 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.

Suppliers

The RP Photonics Buyer's Guide contains 34 suppliers for single-frequency lasers. Among them:

Bibliography

[1]M. Fleming and A. Mooradian, “Spectral characteristics of external-cavity controlled semiconductor lasers”, IEEE J. Quantum Electron. 17 (1), 44 (1981)
[2]K. Kobayashi and I. Mito, “Single frequency and tunable laser diodes”, J. Lightwave Technol. 6 (11), 1623 (1988)
[3]J. J. Zayhowski and A. Mooradian, “Single-frequency microchip Nd lasers”, Opt. Lett. 14 (1), 24 (1989)
[4]J. J. Zayhowski, “Limits imposed by spatial hole burning on the single-mode operation of standing-wave laser cavities”, Opt. Lett. 15 (8), 431 (1990)
[5]R. Paschotta et al., “Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning”, Opt. Lett. 22 (1), 40 (1997)
[6]K. I. Martin et al., “Stable, high-power, single-frequency generation at 532 nm from a diode-bar-pumped Nd:YAG ring laser with an intracavity LBO frequency doubler”, Appl. Opt. 36 (18), 4149 (1997)
[7]Y. Takushima et al., “Polarization-stable and single-frequency fiber lasers”, J. Lightwave Technol. 16 (4), 661 (1998)
[8]A. Liem et al., “100-W single-frequency master-oscillator fiber power amplifier”, Opt. Lett. 28 (17), 1537 (2003)
[9]K. H. Ylä-Jarkko and A. B. Grudinin, “Performance limitations of high-power DFB fiber lasers”, IEEE Photon. Technol. Lett. 15 (2), 191 (2003)
[10]A. Polynkin et al., “Single-frequency fiber ring laser with 1 W output power at 1.5 μm”, Opt. Express 13 (8), 3179 (2005)

(Suggest additional literature!)

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
and other articles in the category lasers

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