Single-frequency Lasers
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: lasers emitting radiation in a single resonator mode
Category: 
- lasers
- narrow-linewidth lasers
- frequency-stabilized lasers
- single-frequency lasers
- narrow-linewidth lasers
Related: single-frequency operationsingle-mode operationmode hoppinglinewidthnarrow-linewidth lasersdistributed Bragg reflector laserslaser diodesfiber lasersinjection lockingtwisted-mode techniquestabilization of lasersfiber-optic sensors
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DOI: 10.61835/6h2 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What are Single-frequency Lasers?
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 substantially 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.
Characterization of Spectral Purity
Spectral purity is a central performance figure of single-frequency lasers. While the term “linewidth” is commonly used to quantify the spectral purity of a single-frequency laser, a single number is often insufficient to fully characterize the frequency noise. The spectral width is determined by both white noise (leading to a Lorentzian lineshape and the so-called intrinsic linewidth) and technical noise sources such as mechanical vibrations and temperature fluctuations (often following a ($1/f$) behavior), which broaden the line over longer observation times. Therefore, specifications should ideally distinguish between the instantaneous linewidth (short measurement time) and the integrated linewidth (measured over a specified longer duration, e.g. 1 ms or 1 s). The side-mode suppression ratio (SMSR) is an additional frequently used figure. The comprehensive description is with the phase noise power spectral density (PSD) as a function of noise frequency; this is typically obtained by beat note measurements between two lasers.
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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a single-frequency laser?
A single-frequency laser is a laser that operates on a single resonator mode. It emits quasi-monochromatic radiation with a very narrow linewidth and low phase noise. By eliminating mode competition noise, it can also achieve very low intensity noise.
Which kinds of lasers can operate on a single frequency?
Various lasers can be designed for single-frequency operation. Common examples are distributed feedback lasers (DFB lasers), certain fiber lasers, diode-pumped solid-state bulk lasers (e.g., in a ring configuration), and vertical cavity surface-emitting lasers (VCSELs).
How can one obtain high optical powers from a single-frequency source?
A common method is to use a master oscillator power amplifier (MOPA), where a low-power single-frequency laser acts as a seed for a power amplifier. Another method is injection locking of a high-power laser with a low-power single-frequency laser.
Why must single-frequency lasers often be protected from back-reflections?
Single-frequency lasers can be very sensitive to optical feedback. Even a minute amount of power reflected back into the laser can cause strongly increased noise or even chaotic multimode operation. Protection is typically provided by Faraday isolators.
For which applications are single-frequency lasers used?
They are used where a very narrow spectral width or low noise is essential. Key applications include optical metrology, interferometry, high-resolution laser spectroscopy (like LIDAR), optical fiber communications, and for seeding enhancement cavities for nonlinear frequency conversion.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 75 suppliers for single-frequency lasers. Among them:
The SLIM LINER — a high spectral purity laser source — is a single-frequency, ultra-narrow linewidth laser that is based on a Self-Adaptive Photonic Oscillator (SAPO) technology developed by the Institut Foton at Université de Rennes in France. A pump laser is optically locked onto a cavity using stimulated Brillouin scattering that offers an extremely narrow gain bandwidth, naturally favoring a high spectral purity, with a frequency noise as low as 0.0005 Hz²/Hz at 200 kHz Fourier frequency.
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.
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.
See also our tutorial on fiber-coupled laser diodes.
HÜBNER Photonics specializes in high-performance single frequency lasers utilizing either diode-pumped solid state (DPSS) or fiber laser technology:
The Cobolt 04-01, 05-01, and 08-01 Series lasers are designed to deliver exceptional wavelength stability and accuracy, along with power stability. They are particularly suited for applications requiring a highly coherent light source. Key features include:
- Excellent wavelength stability
- High accuracy and power stability
- Ideal for applications needing coherent light sources
The Ampheia fiber laser is an ultra-low noise, single frequency, fiber laser system which is affordable despite outstanding laser performance. Key features:
- Up to 50 W at 1064 nm and 5 W at 532 nm, CW, single-frequency emission
- Ultra-low relatively intesity noise (RIN) and perfect beam quality
- Single-stage fiber amplifier with integrated seed laser
- Optical signal to noise ratio (OSNR > 70 dB)
- Robust and maintenance free
- Easy installation and user-friendly interface
Menlo Systems offers ultrastable frequency-stabilized lasers at basically any wavelength. We supply fully characterized systems with linewidths < 1 Hz and Allan deviations of less than 9.6 · 10−17 (in 1 s) in certain systems as well as modules and components allowing for state-of-the-art systems tailored to your requirements.
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.
Serving North America, RPMC Lasers offers single longitudinal mode (SLM) lasers from UV to SWIR (≈266–2300 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!
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.
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.
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.
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!
Innolume’s single-frequency Distributed Feedback (DFB) lasers, available in the 968–1330 nm spectral range, deliver up to 250 mW of optical power with a narrow linewidth of ~1 MHz and exceptional side-mode suppression ratio (SMSR) reaching 55 dB.
Engineered for stable single longitudinal mode operation, these lasers incorporate an internal diffraction grating for Bragg reflection and offer < 0.1 nm wavelength tuning precision via temperature and current control.
They are available with polarization-maintaining or single-mode fiber, a 900 µm loose tube, and a variety of connector options including FC/PC, SC/APC, and APC ferrule, supporting seamless integration into complex photonic systems.
ModeCW is a continuous wave, single-frequency fiber laser system delivering up to 6 W in the 1550–1570 nm spectral range with an ultra-low-noise RIN lower than 0.006%. It can serve as a perfect pumping source of Cr:ZnS/Se oscillators.
Main features:
- Integrated single-frequency seed source (linewidth < 80 kHz)
- Linear polarization (all-PM design), excellent single-mode beam
- High output power (up to 6 W)
- Active output power stabilization
- Excellent noise properties (integrated RIN lower than 0.006% in the 1 Hz – 1 MHz range)
- Maintenance-free, button-operated
- Single-box solution, 19” rack 3U
- Single-mode fiber output
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
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.
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.
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(Suggest additional literature!)