Narrow-linewidth Lasers
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: single-frequency lasers with a narrow optical emission spectrum
More general term: lasers
More specific term: frequency-stabilized lasers
Categories:
DOI: 10.61835/bsr Cite the article: BibTex plain textHTML Link to this page LinkedIn
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.
Some lasers do not only need to have a small linewidth, but also exhibit very high stability of the optical free. See the article on frequency-stabilized lasers.
Types of Narrow-linewidth Lasers
The most important types of narrow-linewidth lasers are the following:
- Among semiconductor lasers, distributed feedback laser diodes (DFB lasers) and distributed Bragg reflector lasers (DBR lasers), operating e.g. in the 1.5- or 1.0-μm wavelength region, are the most common. Typical operation characteristics are an output power of some tens of milliwatts (or possibly somewhat above 100 mW) and a linewidth of several megahertz.
- Significantly smaller linewidths from semiconductor lasers are possible e.g. by extending the resonator with a single-mode fiber containing a narrow-bandwidth fiber Bragg grating, or with other types of external-cavity diode lasers. In such a way, ultra-narrow linewidths of a few kilohertz or even below 1 kHz can be achieved.
- Small fiber lasers in the form of distributed feedback lasers (with the resonator formed essentially by a special fiber Bragg grating) can generate tens of milliwatts of output power with a linewidth in the region of a few kilohertz.
- Higher output powers can be generated with longer distributed Bragg reflector lasers (DBR fiber lasers) or unidirectional fiber ring lasers, and also by using a fiber amplifier.
- Diode-pumped solid-state bulk lasers, e.g. in the form of nonplanar ring oscillators, can also have linewidths of a few kilohertz, combined with relatively high output powers of the order of 1 W. Although a 1064-nm wavelength is typical, other wavelengths e.g. in the 1.3- or 1.5-μm wavelength regions are also possible.
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.
Physical Description of Narrow Linewidth Light
The light output of a narrow linewidth single-frequency laser may be described as follows:
- Since it is essentially in a single propagation mode, it can be fully described by a time-dependent complex amplitude of that mode. The spatial dependence is defined by the mode; the time- and location-dependent amplitude is the product of the mentioned time-dependent mode amplitude and the spatial mode profile.
- The evolution of that complex amplitude is random. There is some intensity noise, related to fluctuations in the modulus of the amplitude, and phase noise: – The intensity noise can be described with an autocorrelation function of the intensity, the Fourier transform of which delivers the power spectral density of intensity noise; see also the article on relative intensity noise. – For the phase noise, one has an autocorrelation function of the optical phase and again a power spectral density obtained by Fourier transform. The optical phase undergoes an unbounded random walk, since (in contrast to the intensity) there is no “restoring force” for the phase. The power spectral density is proportional to <$f^{-2}$> in simple random walk models.
- The laser linewidth is normally mostly determined by phase noise.
- The mathematical relation from phase noise spectrum to the field spectrum, from which one obtains the linewidth, is complicated. For the simple random walk as explained above, the field spectrum becomes Lorentzian.
If only the random walk of phase caused by spontaneous emission in the laser gain medium were present, one would obtain the Schawlow–Townes linewidth, which is usually very small. The real linewidth of a laser is usually far larger, and dominated by additional technical noise sources, e.g. related to mechanical vibrations and thermal fluctuations. Particularly if vibrations are important, the mathematical structure of the phase noise becomes more complicated.
For multimode lasers, the description is again more complicated, as one optical phase needs to be associated with each spectral component, and there are generally fluctuations in power and other interactions between those.
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.
- Holography requires either continuous-wave or pulsed single-frequency lasers for generating highly coherent light.
- Normally less demanding in terms of linewidth are applications in optical fiber communications, e.g. in transmitters or for test and measurement purposes.
More to Learn
Encyclopedia articles:
Bibliography
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(Suggest additional literature!)
Suppliers
The RP Photonics Buyer's Guide contains 101 suppliers for narrow-linewidth lasers. Among them:
Geola

Geola's Single Longitudinal Mode (SLM) lasers are available with:
- pulse energies from 2 mJ to 10 J
- pulse durations of 1 ns to 400 ns
- standard repetition rate – single shot to 50 Hz
- fundamental wavelength at either 1047, 1053, 1064, 1079, 1313, 1319, 1338 or 1342 nm
O-E Land

O/E Land offers the OEFLS-NL-100 single-frequency fiber laser, emitting at 1520–1580 nm with <5 kHz linewidth. You can get 20 mW or even >50 mW output power.
Frankfurt Laser Company

Frankfurt Laser Company offers various kinds of wavelength-stabilized laser diodes which all exhibit a narrow emission linewidth. Those based on DFB or DBR lasers even exhibit single-frequency operation.
HÜBNER Photonics

HÜBNER Photonics offer narrow-linewidth lasers in the Cobolt 08-01 Series. All lasers in the 08-01 Series are either frequency stabilized diode lasers or single longitudinal mode diode-pumped solid-state lasers with excellent noise and power stability. This product series is intended for applications like Raman spectroscopy.
Alpes Lasers

Alpes Lasers offers single-mode, narrow-linewidth tuneable DFB lasers with wavelengths from 4 to 14 μm and powers up to hundreds of milliwatts. In CW mode, our DFBs will typically exhibit linewidths on the order of MHz.
AeroDIODE

SHIPS TODAY: AeroDIODE offers fiber-coupled DFB laser diodes (1030 nm, 1064 nm, 1310 nm, 1550 nm or many other devices with 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 10s kHz are obtained with our dedicated ultra-low noise driver.
See also our tutorial on Fiber coupled laser diodes.
CSRayzer Optical Technology
CSRayzer provides narrow linewidth laser diode modules, or narrow linewidth lasers, with different grades of linewidth: 200 kHz, 100 kHz or 3 kHz.
Edmund Optics

Edmund Optics offers narrow-linewidth lasers, e.g. in the form of metrology lasers with ±0.002 nm wavelength stability.
TOPTICA Photonics

All of TOPTICA’s tunable diode lasers offer a narrow linewidth of typically 100 kHz, corresponding to coherence lengths of almost 1 km. By stabilizing these lasers even further with TOPTICA’s locking electronics, linewidths below 1 Hz are possible.
Vexlum

VEXLUM lasers are designed for narrow-linewidth operation, utilizing an external cavity configuration with frequency-selecting elements such as a birefringent filter and etalon. These lasers offer:
- Narrow free-running linewidths: < 100 Hz (instantaneous), < 10 kHz (10 μs), < 100 kHz (100 μs)
- Virtually no amplified spontaneous emission (ASE) background (SNR > 70 dB) due to VECSEL technology.
MPB Communications

MPBC’s narrow linewidth lasers provide precise and stable output in the visible and NIR wavelength ranges for applications in medical diagnostics, imaging and advanced technologies.
Visible and NIR CW lasers are offered with 100 pm linewidth and a diffraction-limited beam.
- The visible wavelength range covers 465 to 775 nm and provides up to 5 W of output power at selected wavelengths.
- The NIR wavelength range covers 930 to 2000 nm with a fiber-coupled output power up to 100 W.
MPBC also 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

Lasers with intracavity spectral selecting components which generate narrow linewidth laser radiation are offered by ALPHALAS. Special configurations deliver a single longitudinal mode (single frequency) at the standard laser wavelengths like 1030 nm, 1053 nm, 1064 nm as well as customer-specific wavelengths. In addition, nonplanar ring oscillators (NPRO) generate single-frequency laser radiation at 1064 nm with powers up to 100 mW that can be boosted up to 10 W in a MOPA configuration.
Injection-seeded high-power Q-switched lasers of the series PULSELAS-A-SF deliver single-frequency, 10 ns long pulses with up to 10 W average power at 10 kHz repetition rate.
Sacher Lasertechnik

Sacher Lasertechnik offers single-mode narrow-linewidth distributed feedback lasers emitting between 760 nm and 2800 nm. Free space and fiber-pigtailed versions available.
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.
Stuttgart Instruments

The Stuttgart Instruments (SI) Piano Laser System is a narrowband very precise infrared light source for a wide range of analytical applications in spectroscopy and microscopy to study the chemical and structural composition and behavior of materials on the nanoscale. In this context, the SI Piano is characterized by its outstanding precision, spectral flexibility and unbeatable reproducibility; packaged in a compact and comparably low-cost design. This enables new research approaches for the materials of the future, and also achieves transmission from science to industry
eagleyard Photonics

Single frequency laser radiation, precisely balanced in narrow linewidth and tunability, will excite, detect or manipulate atoms in spectroscopy, such as rubidium, cesium, potassium, calcium, lithium, strontium and others.
Toptica eagleyard offers among others single frequency lasers for resonance-matching narrow-tuning stand-alone emitters.
Quantifi Photonics

Quantifi Photonics' Laser 1000 Series is a fully-compliant micro-ITLA laser with up to four channels in a compact benchtop or PXI form factor. It offers 0.01 pm resolution tunability across C or L bands, exceptional power accuracy up to 16.5 dBm, and optional dither suppression. It is a versatile and cost-effective general-purpose instrument.
Spectra Quest Lab

Spectra Quest Labs offers tunable single frequency lasers (linewidth < 100 kHz) up to 100 mW with ASE-suppressed optical output (SMSR > 80 dB) through a PM fiber:
- λ-Master offers precise wavelength control and is suitable for laboratory users.
- λ-Rapid offers fast wavelength sweep.
- λ-Lock is suitable for manual wavelength control.
All are equipped with an analog wavelength tuning function using PZT and LD current modulation function (BW: 5 MHz) and can also perform analog wavelength tuning and frequency locking.
Radiantis

Titan is a continuous-wave (CW) broadly tunable laser system provided by Radiantis. With high average power (>5 W), this laser system offers tuning across 1450 – 4000 nm with linewidths of the order of 30 MHz (depending on the wavelength). Wavelength tuning is automated.
NKT Photonics

Koheras single-frequency fiber lasers are longitudinally single-mode and offer extremely low phase and intensity noise levels. Available in the erbium, ytterbium, or thulium wavelength ranges and with frequency-conversion to many other bands. Reliability is our highest priority. The all-fiber DFB design ensures robust and reliable operation for thousands of hours, prioritizing reliability. Koheras lasers are highly stable and mode-hop-free, even under changing environmental conditions. We also offer shot-noise-limited lasers for applications requiring exceptionally low-intensity noise.
ID Photonics

Introducing our CoBrite tunable narrow linewidth laser ecosystem for 1515 nm to 1625 nm. Designed with simplicity and versatility in mind, CoBrite offers multiple chassis options and laser variants, making it an essential tool for researchers in the lab and on the production floor. With tunability options in the C-band, L-band, or C+L band, from a single source or in configurations of more than 100 ports, there's a CoBrite configuration to suit virtually every application. We keep expanding the CoBrite ecosystem to address new applications and market requirements when they arise.
CNI Laser

CNI offers narrow-linewidth diode lasers with the characteristic of ultra narrow spectral linewidth <0.03 nm, ideal for applications like DNA sequencing, flow cytometry, digital imaging, analytical chemistry, particle measurement, confocal microscopy, Raman spectroscopy and many other fields.
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.
Exail

Exail’s single-frequency fiber lasers are based on UV Bragg grating technology applied to active rare-earth photosensitive fibers. The ultra-short cavity and the phase-shifted design permit ultra-narrow linewidth and robust mode-hop-free laser operation, ideal for various sensor applications (1.5 and 2 µm available upon request).
Benefits and features:
- wavelength range 1530 – 1565 nm and 2 µm
- output power: >10 mW (> 10 µW for low noise version)
- single longitudinal mode, mode-hop-free
- narrow linewidth, low phase noise, SMSR >50 dB
- linear polarization, PM available
- WDM compatible
- low optical feedback sensitivity
- 125 or 80 μm clad diameter
Eblana Photonics

Eblana Photonics’ EP1550-0-NLW Series laser diodes have advanced capabilities with extremely narrow linewidths in comparison to equivalent DFB devices. This laser has a free running linewidth of 100 kHz or less, in a monolithic chip with no external cavity or other linewidth narrowing structures. Eblana’s patented Discrete-Mode technology allows for narrow linewidth laser diode designs at low operating currents.
Menlo Systems

Menlo Systems offers ultrastable, frequency stabilized lasers at basically any wavelength. We supply fully characterized systems with linewidths below 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.
RPMC Lasers

Serving North America, RPMC offers a wide range of narrow linewidth lasers, laser diodes, and modules with wavelengths from 355 nm to 16 μm. These offerings include DFB diode lasers, external cavity VBG diode lasers, ultrafast lasers, microchip lasers, quantum cascade lasers, and DPSS lasers. All are available in both OEM and turnkey packages. Standard and custom options available. Let RPMC help you find the right laser today!
Questions and Comments from Users
2022-08-30
How can we simulate the CW laser with a finite linewidth? To be more specific, what's the effective expression for electromagnetic field in time domain E(t)? I once tried to use certain spectrum to do the Fourier transform, however it turned out to be a pulse (wave packet) in time domain, not a continuous wave.
The author's answer:
Yes, the finite linewidth is related to a random evolution of the electric field. That may be simulated numerically e.g. with a random walk of the optical phase.
2021-02-27
Can the narrow-linewidth semiconductor laser or the external cavity coupled narrow-linewidth diode laser be simulated by any commercial software, for example, the RP photonics software?
The author's answer:
I am not sure what exactly you want to simulate and investigate. Maybe the stability of single-mode operation during wavelength tuning? That would require a fairly specialized model – probably some custom development.