Raman Shifting Device

This invention relates to Raman Converters and to Raman Laser Systems comprising such converters.

Raman shifting devices include a Raman scattering medium, usually a high pressure gas cell. In such devices laser radiation at a given wavelength is converted to a radiation which has a longer wavelength by a process of Stimulated Raman scattering. This longer wavelength radiation is termed stokes Radiation or Raman-shifted Radiation. The scattering medium is chosen for conversion of the laser radiation to the desired wavelength. By using methane which has a frequency shift of 2916 cm.sup.-1 it is possible to convert the 1.06 micron radiation of the Neodymium:YAG laser to 1.54 micron which is an "eyesafe" wavelength.

Several conversion schemes are known in the prior art. For example in U.S. Pat. No. 3,668,420 issued to Vanderslice in 1972, the Raman converter used includes two mirrors which form a resonator for the shifted wavelength. In European Patent No. 63205, Bess and Ruger (1985), a single mirror is used to form a "semi resonator". A more comprehensive description is given by Parazzoli et al in IEEE Journal of Quantum Electronics Vol. 24 (1988) p. 872. In both of these designs the mirrors used must be aligned with respect to the pump source radiation in order to reflect the wavelength-shifted radiation to the region in which the pump source radiation is focused. This alignment is critical for the suppression of the competing process of stimulated Brillouin scattering and must be precisely maintained thereby complicating the mechanical construction of the converter.

In an attempt to overcome this drawback, Bruesselbach and Whirst described in their international patent application WO 86/02784 a design in which the mirror of the Raman semi-resonator is also the output mirror of the laser source. This design does not guarantee however an automatic alignment of the Raman converter since the laser beam may deviate from the normal to the output mirror as a result of thermal effects in the active medium or in the Q-switching material or due to misalignment of the laser mirrors.

It is an object of this invention to overcome the above mentioned drawbacks and to provide a simple Raman converter which does not need precise alignment of optical elements.

These advantages are realized according to the present invention by using a backward Raman converter for Raman-shifting the beam of a pump laser. This backward Raman converter includes a Raman medium in which the pump laser beam interacts with a Raman medium to generate Raman-shifted radiation, a dichroic coupler positioned between said pump laser and one side of said Raman medium for directing said pump laser beam to said Raman medium, and reflection means positioned on the other side of said Raman medium for reflecting said Raman-shifted radiation back into said Raman cell where it is amplified by interacting with said Raman medium and the incoming pump laser beam. The amplified Raman-shifted beam is coupled out of the system by said dichroic coupler.

FIG. 1 is a schematic illustration of a Raman converter according to the present invention.

FIG. 2 is a schematic illustration of a preferred embodiment of a Raman converter according to the present invention.

The following description, which refers to certain types of lasers, etc., is by way of illustration only and ought to be construed in a non-limitative manner.

A more detailed description of the invention will be given now with reference to FIG. 1. A Raman converter 11 of the invention is illustrated in operation with a pump laser 10, which is a Neodymium:YAG laser producing a pump laser beam 12 at a wavelength of 1.06 micron. The dichroic coupler 16 consists of a glass plate with a dichroic dielectric coating transmitting the pump laser beam 12 and reflecting the beam at the Raman-shifted wavelength of 1.54 micron.

Raman cell 20 is a container with windows 24 and 26, containing methane gas 22 at a high pressure. Within said Raman cell 20, the pump laser beam interacts with molecules of the methane gas 22 to produce by stimulated Raman scattering Raman-shifted radiation 30 at a wavelength of 1.54 micron, which is reflected by reflector 33 back to the Raman cell 20 where it is amplified by backward stimulated Raman scattering of the incoming pump laser beam 12. The amplified Raman-shifted beam 36 is reflected by the dichroic coupler 16 as Raman-shifted output beam 38.

In this Raman converter the conversion is essentially performed by backward Raman scattering within the gas 22, this scattering being enhanced by a feedback of the forward scattered Raman-shifted radiation 30. This converter may be also interpreted as a kind of "semi-resonator" converter, as only a single reflector is used for feedback of the Raman-shifted radiation. A preferred embodiment of the invention comprises a retro-reflector as reflecting means rendering the converter insensitive to possible tilting of the reflecting means or to possible wandering of the pump beam. In this way the mechanical construction of the converter is simplified.

FIG. 2 illustrates a preferred embodiment of the invention. It is known that for a good conversion efficiency in a compact Raman shifting device it is necessary to focus the pump laser beam within the Raman cell to produce there the high power density needed for efficient Raman interactions. This is attained in the Raman converter of the present invention by focusing means comprising a pair of lenses 54 and 56 positioned one on each side of the Raman cell 20. Alternatively, lenses 54 and 56 may respectively replace the windows 24 and 26 of the Raman cell 20. The reflecting means is a corner reflector 58 consisting a glass prism with three mutually perpendicular faces. Three total internal reflections of the incident beam at these faces result in retro-reflection of that beam. An alternative to this type of retro-reflector is the hollow retro-reflector consisting of three mirrors mounted perpendicular to each other. In operation, the pump laser beam 12 transmitted through the dichroic coupler 16 is focused by lens 54 to a focal region 60 within the Raman cell 20 to produce there a high power density of the order of several Gigawatts per square centimeter for an efficient Raman interaction to take place. It is in this focal region 60 that the Raman-shifted radiation 62 is generated. This radiation diverges and is collimated by the lens 56 to collimated beam 64 that is reflected backwards by corner reflector 58, and after such reflection, is focused by lens 56 to the focal region 60 where it is amplified by backward stimulated Raman scattering of the pump laser beam 12. The amplified Raman-shifted beam 66 diverges and is collimated by lens 54 to collimated beam 68 which is reflected by the dichroic coupler 16 resulting in Raman-shifted output beam 69.

In operation, even a weak reflected Raman-shifted radiation 62 is sufficient to enhance backward Raman scattering in the focal region. As a result, most of the Raman scattering is in the backward direction. In this way the pump laser beam 12 is attenuated before reaching the focal point, thereby enabling operation of the converter at higher energies before the occurrence of electrical breakdown in the focal region 60. For some applications this may be a significant advantage compared with the prior art designs.

The advantage of using a corner reflector as reflecting means is that it does not need exact alignment and irrespective of its orientation it reflects the collimated beam 64 back on itself so that it is focused exactly to the focal region 60 for an additional interaction. As was verified experimentally this insensitivity to misalignment results in a significant improvement in the energy stability of the Raman shifted output beam. According to the invention, reflecting means such as a mirror, a porro prism or other types of roof reflectors known in the art can be used.

Additional improvements can be applied to the Raman converter:

As is well known, the efficiency of Raman-shifting devices depends strongly on the competition between the Raman scattering and the Brillouin scattering processes. Depending on the type of Raman medium used and on the wavelength of the pump laser, it may be sometimes advantageous to add between the Raman cell 20 and reflecting means 33 in FIG. 1 a filter that removes the remaining radiation at the pump wavelength thereby preventing its back reflection to the focal region 60. Such a feedback at the pump wavelength is undesirable since it may enhance the Stimulated Brillouin scattering thereby reducing the Raman conversion efficiency. Such an undesirable feedback at the pump wavelength can be prevented by using a suitable absorber, or by a reflective filter or by the use of dispersive elements such as a grating or prism. If a reflective filter is positioned between Raman cell 20 and reflecting means 33 in FIG. 1, the reflection at the pump wavelength can be utilized as a secondary output beam thereby providing a dual wavelength laser system. Another solution can be provided by using as reflecting means 33 a reflector which selectively reflects the stokes shifted radiation at 1.54 micron but not the radiation at 1.06 micron.

In another modification of the preferred embodiment, a phase conjugating reflector is used as the reflecting means 58 in FIG. 2. Like the corner reflector, the phase conjugating reflector provides retro-reflection of the Raman shifted radiation 62. Its advantage is that, due to a reversal of the radiation wavefront, any wavefront deformation induced by the first passage of the Raman shifted radiation through the focal region 60 within the Raman medium, is cancelled after a second pass along the same path. In this way an undistorted output beam is obtained even in cases where the Raman medium induces optical distortions due to its heating for example. Various types of phase-conjugating reflectors are known in the prior art. In such devices a phase-conjugated beam is produced by a nonlinear optical interaction of the incident beam with the phase-conjugating material. Stimulated Brillouin scattering (SBS) is one of the nonlinear interactions known in the prior art to produce phase conjugation of laser beams. This type of phase conjugation can be easily combined with the converter described in FIG. 2. This can be done for example by replacing window 26 with a short focus concave mirror which focuses the Raman-shifted radiation 62 into a small focal point within the gas from which stimulated Brillouin backscattering provides the phase-conjugate reflection.

A Raman converter was built in accordance with FIG. 2. The pump laser used was a Q-switched Nd:YAG laser producing 120 milliJoules per pulse of 16 nsec of 1.06 micron radiation. Using high pressure methane as the Raman medium, a corner reflector as the reflecting means and a dichroic mirror as the dichroic coupler, a Raman shifted output beam was generated with 40 milliJoules per pulse.

One embodiment of the invention provides a Raman converter that is insensitive to optical misalignment or to wandering of the pump laser beam. It should be understood that the invention is not limited to the embodiments described herein but that various modifications and changes in the nature and arrangement of parts can be resorted to without departing from the scope and spirit of the invention.