A Raman laser is a specific type of

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

in which the fundamental light-amplification mechanism is

stimulated Raman scattering Raman scattering or the Raman effect () is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by ...

. In contrast, most "conventional" lasers (such as the

ruby laser A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960. Ruby lasers produce pulses of ...

) rely on stimulated electronic transitions to amplify light.

Specific properties of Raman lasers

Spectral flexibility

Raman lasers are optically pumped. However, this pumping does not produce a

population inversion In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited ene ...

as in conventional lasers. Rather, pump photons are absorbed and "immediately" re-emitted as lower-frequency laser-light photons ("Stokes" photons) by

. The difference between the two photon energies is fixed and corresponds to a vibrational frequency of the gain medium. This makes it possible, in principle, to produce arbitrary laser-output wavelengths by choosing the pump-laser wavelength appropriately. This is in contrast to conventional lasers, in which the possible laser output wavelengths are determined by the

emission lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identi ...

of the gain material. In

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...

s made of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

, for example, the frequency shift corresponding to the largest Raman gain is about 13.2 THz. In the

near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

, this corresponds to a wavelength separation between pump light and laser-output light of about 100 nm.

Types of Raman lasers

The first Raman laser, realized in 1962, by Gisela Eckhardt and E.J. Woodbury used

nitrobenzene Nitrobenzene is an organic compound with the chemical formula C6H5 NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale from benzene as a precursor ...

as the gain medium, which was intra-cavity-pumped inside a

Q-switching Q-switching, sometimes known as giant pulse formation or Q-spoiling, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatt) peak power, much hi ...

. Various other gain media can be used to construct Raman lasers:

Raman fiber lasers

The first

continuous-wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...

Raman laser using an optical fiber as the gain medium was demonstrated in 1976. In fiber-based lasers, tight spatial confinement of the pump light is maintained over relatively large distances. This significantly lowers threshold pump powers down to practical levels and furthermore enables continuous-wave operation. In 1988, the first Raman fiber laser based on fiber Bragg gratings has been made. Fiber Bragg gratings are narrow-band reflectors and act as the mirrors of the laser cavity. They are inscribed directly into the core of the optical fiber used as the gain medium, which eliminates substantial losses that previously arose due to the coupling of the fiber to external bulk-optic cavity reflectors. Nowadays, commercially available fiber-based Raman lasers can deliver output powers in the range of a few tens of Watts in continuous-wave operation. A technique that is commonly employed in these devices is ''cascading'', first proposed in 1994: The "first-order" laser light that is generated from the pump light in a single frequency-shifting step remains trapped in the laser resonator and is pushed to such high power levels that it acts itself as the pump for the generation of "second-order" laser light that is shifted by the same vibrational frequency again. In this way, a single laser resonator is used to convert the pump light (typically around 1060 nm) through several discrete steps to an "arbitrary" desired output wavelength.

Silicon Raman lasers

More recently, Raman lasing has been demonstrated in

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

-based integrated-optical waveguides by Bahram Jalali's group at the University of California in Los Angeles in 2004 (pulsed operation) and by Intel in 2005 (continuous-wave), respectively. These developments received much attention because it was the first time that a laser was realized in silicon: "classical" lasing based on electronic transitions is prohibited in crystalline silicon due to its indirect bandgap. Practical silicon-based light sources would be very interesting for the field of

silicon photonics Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commo ...

, which seeks to exploit silicon not only for realizing electronics but also for novel light-processing functionality on the same chip.

References

External links

"Raman Lasers", in the Encyclopedia of Laser Physics and Technology
{{Raman spectroscopy, state=autocollapse Laser science Raman scattering