Encyclopedia > letter S > single-frequency lasers

Single-frequency Lasers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: lasers emitting radiation in a single resonator mode

More general term: narrow-linewidth lasers

Category: article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/6h2 Cite the article: BibTex plain text HTML Link to this page! LinkedIn

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.

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 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.

More to Learn

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

Suppliers

The RP Photonics Buyer's Guide contains 67 suppliers for single-frequency lasers. Among them:

Menlo Systems

Menlo Systems offers ultrastable frequency-stabilized lasers at basically any wavelength. We supply fully characterized systems with linewidths < 1 Hz and Allan deviations of 2 × 10⁻¹⁵ (in 1 s) as well as modules and components allowing for state-of-the-art systems tailored to your requirements.

CNI Laser

CNI offers single-frequency lasers with wavelengths from the UV to the infrared region. They are widely used in many scientific and industrial applications. The spectral linewidth can be less than 0.00001 nm.

CNI successfully provided custom made lasers for 670.776 nm, 589.159 nm, 589.756 nm and even Q-switched SLM lasers to customers. The frequency drift over 8 hours was <200 MHz.

MPB Communications

MPBC line of single frequency lasers provide low phase and intensity noise, long coherence length and high reliability for applications in metrology, quantum technologies and atomic clocks.

MPBC provides single-frequency (SF) Seed fiber lasers with a linewidth < 50 kHz,and SF lasers with a linewidth < 100 kHz.

● The SF Seed wavelength range covers 910 to 1950 nm and provides up to 50 mW of output power at selected wavelengths.

● The SF Visible wavelength range covers 460 to 790 nm and provides up to 5 W of output power at selected wavelengths. Higher powers up to 10 W are available from 532 to 671 nm.

● The NIR SF wavelength range covers 920 to 1900 nm with a fiber-coupled output power up to 30 W.

ALPHALAS

ALPHALAS offers CW or pulsed single-frequency lasers (single longitudinal mode) with TEM₀₀ beam profile at most of the standard laser wavelengths. Various proprietary and standard technologies including monolithic non-planar ring oscillators (NPRO), DFB and unidirectional ring laser designs are used to achieve stable single-frequency operation with a very low intensity noise. The output power in CW mode ranges from several tens of mW to > 10 W at 1030 nm and 1064 nm. The single-frequency Q-switched lasers offer the unique combination of very narrow spectrum (transform-limited pulses) with very low amplitude noise due to the absence of mode beating.

Applications include high-resolution spectroscopy, interferometry, holography, optical metrology, meteorology, efficient Brillouin- and Raman-shifted generation, Raman spectroscopy and optical trapping, to mention just a few of them.

RPMC Lasers

Serving North America, RPMC Lasers offers single longitudinal mode (SLM) lasers from UV to SWIR (≈266 nm – 2300 nm), delivering absolute spectral purity and precise wavelength control for spectroscopy, metrology, and telecom applications.

Advanced stabilization via SLM DPSS, HeNe, DFB, and VBG diodes, plus wavelength combiners, ensures minimal frequency drift with temperature/current control and integrated TEC for peak single-frequency performance.

Tailored OEM and turnkey solutions – free-space or fiber-coupled – feature compact, rugged designs for lab, industrial, or portable use, customizable to meet specialized scientific and research demands.

Let RPMC help you find the right SLM laser today!

AdValue Photonics

AdValue Photonics has the AP-SF single-frequency fiber laser emitting in the 2-μm wavelength region with a linewidth around 10 kHz. The AP-SF1 is an amplified version with 5 W output power. Both come with a turn-key benchtop housing.

Eblana Photonics

Eblana Photonics Discrete-Mode technology platform delivers unrivaled wavelength uniformity and stability which is critical for many scientific and industrial applications. Eblana’s DM laser exhibit high SMSR and excellent tuning performance over a wide wavelength range, with products available from 650 nm – 12 μm.

Lumibird

With the CVFL, CYFL and CEFL kilo models, Lumibird offers CW fiber lasers with very narrow linewidth down to 1 kHz. These single frequency lasers emits at 1054/1083 nm for the ytterbium version, in the 1.5-µm range for the erbium version and at frequency converted wavelengths for the CVFL model. These lasers are specifically designed for applications which require high precision such as LIDAR, atomic spectroscopy, or atom cooling.

HÜBNER Photonics

HÜBNER Photonics offer single frequency lasers based on diode pumped solid state technology (DPSS) in the Cobolt 04-01, 05-01 and 08-01 Series. In addition to the single frequency, all lasers have excellent wavelength stability and accuracy as well as power stability. Ideal for any application needing a highly coherent light source.

NKT Photonics

Our Koheras narrow linewidth, single-frequency fiber lasers are ultra-low noise sources. We have based these lasers on a DFB design to give you a robust and reliable operation. Koheras offers unprecedented low phase and intensity noise levels at rubidium, strontium, barium, and ytterbium wavelengths. It has high stability and mode-hop-free inherent single-frequency output – even when exposed to changing environmental conditions.

QPC Lasers

QPC Lasers solutions include eye-safe MOPA diode lasers offering peak single-mode powers up to 100 W for applications including automotive LIDAR, Doppler anemometry and free space communications.

AeroDIODE

SHIPS TODAY: AeroDIODE offers fiber-coupled DFB laser diodes (1030 nm DFB, 1064 nm DFB, 1310 nm DFB, 1550 nm DFB or many other devices at a wavelength between 1250 nm and 1650 nm). They are offered as stock items or associated with a CW laser diode driver or pulsed laser diode driver. They are compatible with our high speed nanosecond pulsed drivers or low noise laser diode driver for single frequency emission. Typical narrow linewidth of a few tens of kHz are obtained with our dedicated ultra-low noise driver.

Vexlum

VEXLUM lasers are designed for narrow-linewidth operation, featuring an external cavity configuration with frequency-selecting elements such as a birefringent filter and etalon/EOM. Key specifications include:

Narrow free-running linewidths: < 100 Hz (instantaneous), < 10 kHz (10 μs), < 100 kHz (100 μs)
Sub-Hz linewidth achievable by locking to a frequency comb
Minimal amplified spontaneous emission (ASE) background (SNR > 70 dB) due to VECSEL technology

TOPTICA Photonics

TOPTICA's single-frequency diode lasers employ state-of-the-art diode technology to achieve the highest single-frequency output power of any direct diode-based system. For example, the TopMode 405 nm model offers as much as 100 mW – an industry record!

Bibliography

[1]	M. Fleming and A. Mooradian, “Spectral characteristics of external-cavity controlled semiconductor lasers”, IEEE J. Quantum Electron. 17 (1), 44 (1981); https://doi.org/10.1109/JQE.1981.1070634
[2]	K. Kobayashi and I. Mito, “Single frequency and tunable laser diodes”, IEEE J. Lightwave Technol. 6 (11), 1623 (1988); https://doi.org/10.1109/50.9978
[3]	J. J. Zayhowski and A. Mooradian, “Single-frequency microchip Nd lasers”, Opt. Lett. 14 (1), 24 (1989); https://doi.org/10.1364/OL.14.000024
[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); https://doi.org/10.1364/OL.15.000431
[5]	R. Paschotta et al., “Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning”, Opt. Lett. 22 (1), 40 (1997); https://doi.org/10.1364/OL.22.000040
[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); https://doi.org/10.1364/AO.36.004149
[7]	Y. Takushima et al., “Polarization-stable and single-frequency fiber lasers”, J. Lightwave Technol. 16 (4), 661 (1998); https://doi.org/10.1109/50.664080
[8]	A. Liem et al., “100-W single-frequency master-oscillator fiber power amplifier”, Opt. Lett. 28 (17), 1537 (2003); https://doi.org/10.1364/OL.28.001537
[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); https://doi.org/10.1109/LPT.2002.806827
[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); https://doi.org/10.1364/OPEX.13.003179
[11]	S. Fu et al., “Review of recent progress on single-frequency fiber lasers”, J. Opt. Soc. Am. B 34 (3), A49 (2017); https://doi.org/10.1364/JOSAB.34.000A49
[12]	J. Zhang et al., “Near thermal noise limit, 5 W single frequency fiber laser base on the ring cavity configuration”, Opt. Express 32 (1), 104 (2024); https://doi.org/10.1364/OE.507390

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.