Definition: lasers based on Raman gain rather than on laser gain from stimulated emission
Categories: nonlinear optics, laser devices and laser physics
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Author: Dr. Rüdiger Paschotta
A Raman laser is a light source similar to an ordinary laser, but with a nonlinear amplifier medium based on Raman gain (stimulated Raman scattering) rather than on stimulated emission from excited atoms or ions. The main attraction of this type of device is that essentially any Raman laser wavelength can be achieved with a suitable choice of the pump wavelength, provided that both wavelengths are within the transparency region of the material and a sufficiently high nonlinearity and/or optical intensity are reached.
The term Raman lasers is sometimes also used for lasers of other types which are suitable for Raman spectroscopy, but this is not the topic of this article.
Different Types of Raman Lasers
The Raman-active medium essentially have to provide a χ(3) nonlinearity with a pronounced delayed nonlinear response. It is usually either an optical fiber, another type of waveguide, or a bulk crystal, but sometimes one uses a gas.
Raman Fiber Lasers
The long interaction length in fibers makes it easy to exceed the laser threshold, particularly if a low-loss laser resonator is built. Continuous-wave operation is then possible with pump and output powers typically of the order of hundreds of milliwatts to several watts, but Raman fiber lasers operating at much higher power levels are also possible. Within the spectral range of the Raman gain, the Raman laser wavelength can be selected with a fiber Bragg grating, which also leads to a small emission bandwidth, although single-frequency operation is difficult to achieve due to the nonlinear interaction.
The setup of a fiber Raman laser can be fairly simple; one would typically use fiber Bragg gratings as end reflectors to form the Raman laser resonator, and pump the device by injecting pump light from usually one side. For high-power devices, one may also use a double-clad fiber, where the pump light is injected into the pump cladding, which can be substantially larger than the fiber core. It is also possible to use nested fiber Bragg gratings in order to realize Raman conversion in several steps, overall achieving a larger wavelength shift.
Cascaded Raman fiber lasers can be built with nested pairs of fiber Bragg gratings. Oscillation on one Raman order is used for pumping another order, so that larger frequency offsets (wavelength shifts) can be bridged. This technique can be used, for example, to make a 1480-nm pump source for erbium-doped fiber amplifiers, which itself is pumped with a 1064-nm solid-state bulk laser.
Raman fiber lasers are often pumped with rare-earth-doped fiber lasers. However, direct diode pumping is also possible since sufficiently high-brightness laser diodes have become available .
Most Raman fiber lasers are based on ordinary silica fibers and use a Raman frequency shift around 13 THz. However, one may use other materials, such as phosphosilicate glass, to realize either a much larger Raman frequency shift around 40 THz or a very small shift of only ≈3 THz . The latter leads to a very small quantum defect and low heat load.
As Raman amplification works only at high intensities, other nonlinearities such as the Kerr effect or four-wave mixing are also unavoidably strong in such devices. This can make it difficult to obtain, e.g., narrow-linewidth operation of a Raman laser.
Other Raman Waveguide Lasers
Raman lasers based on waveguides in silicon (not silica) on a chip (silicon lasers) have been demonstrated . Such a silicon Raman laser is possible despite the short interaction length, because silicon has a very high Raman gain coefficient, and the waveguides used have a very strong mode confinement.
A detrimental effect can be two-photon absorption.
Note that a silicon Raman laser still requires an external pump laser (which is difficult to realize with silicon), but makes it possible to reach longer wavelengths than otherwise possible with silicon.
Continuous-wave Raman lasers have also been demonstrated with toroidal microcavities based on silica .
Bulk Raman Lasers
Raman lasing is also sometimes used in solid-state bulk lasers. Here, the Raman-active medium can be a crystal (e.g. made of barium nitrate, potassium gadolinium tungstate = KGW or synthetic diamond) placed in a separate cavity, or it can be an additional Raman crystal within the laser resonator (intracavity Raman conversion). Sometimes, even the laser crystal itself can be used (self-Raman conversion), e.g. in an ytterbium-doped tungstate or vanadate crystal as the Raman-active medium.
Due to the short interaction length (compared with fibers) of only a few centimeters, the Raman threshold is high and can typically only be exceeded in Q-switched operation, with pulse durations in the nanosecond range. However, continuous-wave operation is possible with optimized low-loss resonators .
It is even possible to do further nonlinear frequency conversion, e.g. with an intracavity frequency doubler .
The optimization of such devices for high output power is not easy, particularly due to strong thermal lensing in the laser crystal and the Raman crystal. Also, there can be nontrivial demands on the dielectric mirror coatings, which have to fulfill specifications at three or more wavelengths.
Raman Gas Lasers
Gases can also be used as Raman gain media. Raman gas lasers usually have a high threshold pump power, but it is possible to achieve a low threshold power by using a high-finesse laser resonator  and/or by realizing a long propagation length. Low-threshold devices have also been made with photonic crystal fibers, where the small holes are filled with a gas . This kind of device offers the combination of small mode area and long interaction length, and can thus work with particularly low threshold pump powers.
Examples of applications of Raman lasers are:
- Fiber Raman lasers, or more often cascades of Raman fiber lasers, are used for in-band pumping erbium-doped fiber amplifiers around 1480 nm. For such cascades with a lower number of Raman shifting steps, one uses phosphosilicate fibers, exhibiting a quite large Raman frequency shift.
- A 589-nm Raman laser can be used for a laser guide star in the form of a sodium beacon for atmospheric correction in an astronomical observatory.
- Raman lasers may be useful as part of RGB sources for digital projection displays.
The RP Photonics Buyer's Guide contains 7 suppliers for Raman lasers. Among them:
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See also: Raman amplifiers, Raman scattering, laser guide stars, The Photonics Spotlight 2012-03-12
and other articles in the categories nonlinear optics, laser devices and laser physics
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