Raman Gain Medium

This invention relates to Raman gain media for use in optical lasers and amplifiers.

Stimulated Raman scattering can be viewed as a modulation process produced by molecular vibrations. In the process, the application of an incident radiation, i.e., pump radiation, results in the generation of upper and lower sidebands. In particular, it is the frequency shift to longer wavelengths that is of special interest in that it provides a means for generating coherent radiation in the 1 .mu.m to 1.8 .mu.m range. This range includes the 1.3 .mu.m and 1.55 .mu.m wavelengths which are of particular interest in long distance optical transmission systems.

Raman signal sources currently available include silica Raman fiber lasers which are pumped by mode locked and Q-switched Nd:YAG lasers having 100 picoseconds pulse widths. While silica fiber lasers are reliable and simple, their outputs are not continuously tunable with currently available pump sources. In addition, the silica Raman shift is relatively small (i.e., 400 cm.sup.-1). (See, for example, U.S. Pat. No. 3,705,992, issued Dec. 12, 1972.) As a consequence, one cannot obtain very short (i.e., 1 picosecond) Raman shifted output pulses in the infrared region inasmuch as the short picosecond sources are only available in the visible region.

Alternative sources are the tunable F-center lasers, which are more difficult to operate and are fairly short lived, and the Raman gas lasers. The gain medium in the latter are one or more gases under pressure. Typically, each gas has a different Raman shift, thereby providing a plurality of output signals over the band of interest. The problem with these arrangements resides in the small interaction path which, in turn, results in only modest output power levels unless very large pump powers are used.

A new Raman gain medium, in accordance with the present invention, comprises a lightguide, such as an optical fiber, into which one or more gases have been diffused and reside therein in molecular form. When used as a Raman laser, such a gas-in-glass medium combines the advantages of the prior art Raman fiber lasers and the Raman gas lasers. By using a lightguide as the host, long, stable interaction lengths, and correspondingly higher powers, are obtainable. By using a gas as the active material, the desired Raman shift can be more readily obtained. In particular, Raman shifts into the far infrared region can be achieved.

FIG. 1 shows a Raman oscillator employing a gas-in-fiber gain medium; and

FIG. 2 shows a Raman amplifier employing a gain medium in accordance with the invention.

Referring to the drawings, FIG. 1 shows a laser 10 employing a Raman gain medium 11 in accordance with the present invention. As in the typical laser, the gain medium is disposed within a resonant cavity formed by a pair of mirrors 12 and 13, at least one of which, i.e., 13, is partially transmissive at the wavelength of the laser output signal. The other mirror 12 is highly reflective at the wavelength of the output signal but highly transmissive at the wavelength of the input pump signal. (For examples of fiber Raman laser configurations see U.S. Pat. Nos. 4,039,851 and 4,063,106.)

In accordance with the illustrative embodiment of FIG. 1, the gain medium 11 comprises a length of optical fiber into which molecular gas is diffused, and remains therein in molecular form. In particular, since the gain medium in the instant invention is based upon the vibrational Raman shift, a diatomic gas molecule is employed. Larger multiatomic gas molecules can be employed provided there is room for them in the host interstice. Examples of gases that have been diffused into glass and can be used are H.sub.2, O.sub.2, N.sub.2, and their isotopes. (See, for example, the article entitled "Properties and Structure of Vitreous Silica II" by R. Bruckner, published in the Journal of Non-Crystalline Solids 5 (1971), pp. 177-216.)

The host can be any type of glass or crystal material. However, in all cases laser or amplifier operation is limited to only that part of the spectrum for which the host material is transparent.

In an illustrative embodiment of the invention a hydrogen (H.sub.2) impregnated silica fiber was employed. The fiber, 485 meters long, comprised a 3.9 .mu.m diameter core of GeO.sub.2 -P.sub.2 O.sub.5 doped silica surounded by a silica cladding having a 63 .mu.m outside diameter. The attenuation for this fiber was 8 dB/km at 0.647 .mu.m and 1.6 dB/km at 0.883 .mu.m. Hydrogen was diffused into the fiber at a pressure of 7300 psi at room temperature for a period of three weeks. The resulting H.sub.2 concentration in the silica, held interstitially, is the equivalent of about 15 atmospheres.

The stimulated Raman gain coefficient, .gamma., for any particular medium can be determined experimentally by applying a pump, and measuring the power gain at the Raman shifted wavelength with a probe signal. If P.sub.in and P.sub.o are the measured input and output powers of the probe, and P.sub.p the pump power, .gamma. can be computed from the following relationship:

assuming negligible attenuation of the pump, where P1 A is the effective mode cross sectional area;

and

L is the interaction path length.

For the particular medium described above, a krypton laser source, operating at a wavelength of 0.647 .mu.m, was used as the pump, and an injection laser, operating at the Raman shifted wavelength of 0.883 .mu.m, provided the input probe signal. The resulting measurements and calculation yielded a .gamma. of 2.5.times.10.sup.-12 cm/W. Taking into account the relative line widths of H.sub.2 in silica and H.sub.2 as a gas, and the relative interaction lengths of the fiber and a conventional gas cell, an enhancement of 2.times.10.sup.4 in gain is realized by this fiber medium as compared to a conventional gas cell.

Because the gas that has been diffused into the fiber can just as readily diffuse out, the gain medium is advantageously kept within an enclosure that is either pressurized or cooled in order to retard the out-diffusion process. If cooled sufficiently, some increase in gain can also be realized. For example, at liquid nitrogen temperatures, an increase in the gain of about 50 percent can be obtained. An alternative to either of the above is to coat the fiber with an inert layer of material that is impervious to the diffused gas.

A gas-in-fiber gain medium can also be used as an amplifier 20, rather than an oscillator, as illustrated in FIG. 2. As in the oscillator, the gain medium 21 is advantageously disposed in a suitably maintained enclosure. The signal to be amplified, along with the pump signal is injected into one end of the fiber and the amplified signal obtained at the other end. Suitable mirrors and lenses 23, 24 and 25 are included for coupling in an out of the amplifier.

Because Raman shifts into the far infrared region are obtainable in a gain medium of the type described hereinabove, IR fibers are advantageously employed. These include antimony doped GeO.sub.2 fibers, polycrystalline ThCl.sub.2 fibers, and single crystal fibers made of sapphire, YAG or yttria. The gases can be selected from among the group including hydrogen, nitrogen and oxygen and isotopes of these gases, and larger multiatomic molecules that can be accommodated by the host.