Single-frequency Lasers
Definition: lasers emitting radiation in a single resonator mode
More general term: narrow-linewidth lasers
German: einmodige Laser
Category: laser devices and laser physics
Author: Dr. Rüdiger Paschotta
How to cite the article; suggest additional literature
URL: https://www.rp-photonics.com/single_frequency_lasers.html
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.
Suppliers
The RP Photonics Buyer's Guide contains 64 suppliers for single-frequency lasers. Among them:


Frankfurt Laser Company
Frankfurt Laser Company offers various kinds of wavelength-stabilized laser diodes. Those based on DFB or DBR lasers exhibit single-frequency operation.


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.


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.


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 requirehigh precision such as LIDAR, atomic spectroscopy, or atom cooling.


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.


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.


Eblana Photonics
Eblana Photonics Discrete-Mode technology platform delivers unrivalled 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 – 2400 nm.


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.


MPB Communications
MPBC single-frequency fiber lasers are sources of single-frequency polarized light offering low phase and intensity noise, narrow spectral linewidth and long coherence length. They are comprised of a single-frequency near-infrared Yb-doped fiber laser with a single-mode narrow-band output and electronics which provide temperature stabilization as well as active stabilization of the laser output power. Available in the NIR at 1028 and 1064 nm with output power up to 10 W, or in the visible at 514 nm and 532 nm with 200, 500 or 1000 mW of output power.


RPMC Lasers
RPMC offers a wide range of single-frequency lasers, exhibiting narrow linewidths, with wavelengths from 349 nm to 16 μm. These offerings include DFB and DBR diode lasers, external cavity VBG diode lasers, fiber lasers, and DPSS lasers, available with both free-space and fiber-coupled output, and many packaging options including TO Can, butterfly, HHL, OEM modules, and turnkey packages.


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.


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.
Bibliography
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[5] | R. Paschotta et al., “Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning”, Opt. Lett. 22 (1), 40 (1997), DOI: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), DOI:10.1364/AO.36.004149 |
[7] | Y. Takushima et al., “Polarization-stable and single-frequency fiber lasers”, J. Lightwave Technol. 16 (4), 661 (1998), DOI:10.1109/50.664080 |
[8] | A. Liem et al., “100-W single-frequency master-oscillator fiber power amplifier”, Opt. Lett. 28 (17), 1537 (2003), DOI: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), DOI:10.1109/LPT.2002.806827 |
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[11] | S. Fu et al., “Review of recent progress on single-frequency fiber lasers”, J. Opt. Soc. Am. B 34 (3), A49 (2017), DOI:10.1364/JOSAB.34.000A49 |
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
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