Raman Lasers

The Raman-active medium is usually either an optical fiber or a bulk crystal, but sometimes also a gas. The long interaction length in fibers makes it easy to exceed the 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. 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.

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 can be bridged. This technique can be used e.g. 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 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 [11]. It is even possible to do further nonlinear frequency conversion, e.g. with an intracavity frequency doubler [20]. The optimization of such devices for high output power is not easy, however, 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 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 [29].

Raman lasers based on waveguides in silicon (not silica) on a chip (silicon lasers) have been demonstrated [9]. 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 [10].

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 [4]. Low-threshold devices have also been made with photonic crystal fibers, where the small holes are filled with a gas [22].

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.

Examples of applications of Raman lasers are:

• A fiber Raman laser, or possibly a cascade of Raman fiber lasers, can be used for pumping an erbium-doped fiber amplifier.
• 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.

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