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Narrow-linewidth Lasers

Definition: single-frequency lasers with a narrow optical emission spectrum

German: Laser mit geringer Linienbreite, schmalbandige Laser

Categories: fluctuations and noise, lasers

How to cite the article; suggest additional literature

A number of laser applications (see below) require lasers with a very small optical linewidth, i.e., with a narrow optical spectrum. The term narrow-linewidth lasers usually applies to single-frequency lasers, i.e., lasers oscillating on a single resonator mode with low phase noise and thus with high spectral purity. Typically, such lasers also exhibit low intensity noise.

Types of Narrow-linewidth Lasers

The most important types of narrow-linewidth lasers are the following:

Essential Factors for a Narrow Laser Linewidth

For achieving a narrow emission bandwidth (linewidth) from a laser, several issues of laser design have to be observed:

  • First, single-frequency operation needs to be achieved. This is easiest when using a gain medium with small gain bandwidth and a laser resonator with short length (leading to a large free spectral range). The goal should be long-term stable single-frequency operation without mode hopping.
  • Second, external noise influences must be minimized. This requires a stable resonator setup (preferably a monolithic one), possibly with special protection against mechanical vibrations. An electrically pumped laser should be operated with a low-noise voltage or current source, and an optically pumped laser should have a pump source with low intensity noise. Furthermore, any optical feedback must be avoided, e.g. by using a Faraday isolator. Ideally, external noise influences will become lower than internal noise, e.g. from spontaneous emission in the gain medium. This is often easily achievable at high noise frequencies, but not at low noise frequencies which are most important for the linewidth.
  • Third, the laser design should be optimized so that the laser noise and in particular the phase noise are minimized. A high intracavity optical power and long resonator can be beneficial, although stable single-frequency operation is more difficult to achieve with a longer resonator.

Of course, the design optimization requires that the relative importance of different noise sources is known, because different measures can be required depending on which noise source is dominant. For example, measures which minimize the linewidth according to the Schawlow–Townes equation will not necessarily minimize the actual linewidth, if this is determined e.g. by mechanical noise.

Noise Characterization and Specification

Both the characterization and the specification of the noise of narrow-linewidth lasers are far from trivial issues. Various measurement techniques are discussed in the article on linewidth, and particularly for linewidth values of a few kilohertz or less such measurements are demanding. Furthermore, a linewidth value alone can not be considered a complete noise characterization; it is preferable to have a complete phase noise spectrum, apart from information on relative intensity noise. At least, the linewidth value should be specified together with a measurement time, and possibly with some information concerning frequency drifts for longer time intervals.

Of course, different applications have different requirements, and it should be checked in detail how tight noise specifications should really be demanded in any particular case.

Applications of Narrow-linewidth Lasers

  • A particularly important field of application is the area of sensors, e.g. fiber-optic sensors for strain and/or temperature, various types of interferometric sensing, trace gas detection with differential absorption LIDAR (DIAL), or wind speed measurements with Doppler LIDAR. Linewidths of only a few kilohertz are required for some fiber-optic sensors, whereas 100 kHz can be sufficient for, e.g., LIDAR measurements.
  • Optical frequency metrology requires sources with very narrow linewidth, often achieved with stabilization techniques.
  • Normally less demanding in terms of linewidth are applications in optical fiber communications, e.g. in transmitters or for test and measurement purposes.

Suppliers

The RP Photonics Buyer's Guide contains 39 suppliers for narrow-linewidth 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]S. P. Smith et al., “Narrow-linewidth stimulated Brillouin fiber laser and applications”, Opt. Lett. 16 (6), 393 (1991)
[4]N. Uehara and K. I. Ueda, “193-mHz beat linewidth of frequency-stabilized laser-diode-pumped Nd:YAG ring lasers”, Opt. Lett. 18 (7), 505 (1993)
[5]Y. Shevy and H. Deng, “Frequency-stable and ultranarrow-linewidth semiconductor laser locked directly to an atom-cesium transition”, Opt. Lett. 23 (6), 472 (1998)
[6]B. C. Young et al., “Visible lasers with subhertz linewidths”, Phys. Rev. Lett. 82 (19), 3799 (1999)
[7]R. M. Williams et al., “Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers”, Opt. Lett. 24 (24), 1844 (1999)
[8]St. A. Webster et al., “Subhertz-linewidth Nd:YAG laser”, Opt. Lett. 29 (13), 1497 (2004)
[9]J. Geng et al., “Narrow linewidth fiber laser for 100-km optical frequency domain spectroscopy”, IEEE Photon. Technol. Lett. 17 (9), 1827 (2005)
[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)
[11]H. Stoehr et al., “Diode laser with 1 Hz linewidth”, Opt. Lett. 31 (6), 736 (2006)
[12]J. Geng et al., “Dual-frequency Brillouin fiber laser for optical generation of tunable low-noise radio frequency/microwave frequency”, Opt. Lett. 33 (1), 16 (2008)
[13]S. Vogt et al., “Demonstration of a transportable 1 Hz-linewidth laser”, Appl. Phys. B 104 (4), 741 (2011)

(Suggest additional literature!)

See also: linewidth, single-frequency lasers, laser applications, laser noise, noise specifications, spectroscopy
and other articles in the categories fluctuations and noise, lasers

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