WO1993026067A1

WO1993026067A1 - Raman device

Info

Publication number: WO1993026067A1
Application number: PCT/AU1993/000286
Authority: WO
Inventors: James Richards
Original assignee: The Commonwealth Of Australia
Priority date: 1992-06-17
Filing date: 1993-06-17
Publication date: 1993-12-23

Abstract

An arrangement for a Raman laser shifter which uses a pressure cell (20) and optical elements (21-24) where the optical elements comprise mirrors (23, 24) and substantially pressure invariant lenses (21, 22) and which, in folded cavity design, is no longer than 50 mm and can thus be incorporated into existing laser systems.

Description

RAMAN DEVICE

This invention relates to laser devices and in particular to a compact Raman laser incorporating novel optical elements. A Raman laser is a laser device incorporating a Raman scattering cell to shift the output of a pump laser to a different frequency.

BACKGROUND ART

Frequency shifting devices utilising high pressure cells of Raman active gases are well known. In one form, these prior art devices consist of a linear high pressure cell with a window at each end. The output of a laser is focussed into the cell and the frequency shifted beam (as well as the unshifted fundamental beam) emerges from the other end. In some devices the focussing elements are incorporated as the windows of the cell although it is more common for the windows to be inactive elements and the lenses to be somewhat separated from the cell.

The most common gases used in these devices are Hydrogen (H₂),

Deuterium (D₂) and Methane (CH₄). These gases are contained at pressures in the vicinity of 30 atmosphere. Such high pressures are necessary for an appreciable frequency shifting effect to be observed.

In the prior art devices described above the output from the Raman cell normally consists of a number of frequency shifted beams corresponding to the stokes or antistokes shifts of the scattering medium. Variation of the pressure in the cell results in changes in the relative conversion efficiency into each stokes or antistokes beam.

In another form the prior art devices include mirrors which provide optical feedback at one of the stokes (or antistokes) frequencies. This considerably enhances the conversion efficiency of the device at the selected wavelength. The mirrors are normally dichroic in that they transmit the pump (or fundamental) radiation while reflecting the Raman shifted radiation. One mirror typically has a reflectivity at the Raman frequency of nominally 100 % and the other mirror has an output coupling of around 50 %. A common feature of Raman lasers is that the Raman output is very dependent on the pressure of the gas in the cell. The problem is that changing the pressure of the gas in the cell will change the refractive index in the cell and therefore change the focal length of the lens focussing the pump radiation into the cell. It has therefore been necessary to either design Raman cells to operate at fixed pressures or to have the lens separate from the cell so that they may be adjusted as required.

DISCLOSURE OF THE INVENTION

It is an intended object of this invention to reduce this problem.

According to this invention there is proposed a Raman laser comprising a pressure cell and optical elements wherein the optical elements comprise a pair of substantially pressure-invariant lenses and a pair of mirrors, and wherein the pressure cell contains a Raman active gaseous laser medium to effect a Raman interaction.

In preference the pair of pressure invariant lenses are meniscus lenses having a second surface of radius

R, n

^R2 = ^— n - 1 where Ri is the radius of a first surface and n is the refractive index of the lens material.

An application of this invention is the conversion of the output of a Nd:YAG laser at 1.06 micron to an eye-safe wavelength of 1.54 micron. Conversion of 1.064 micron laser output from a Nd:YAG laser to the eye-safe region at 1.54 micron is a well established technique. It forms one of the two practical methods of generating eye safe radiation. The other technique is the use of Eπglass as the lasing material. The main advantage of the Raman shifting approach is that far higher repetition rates are possible than for Er:glass and that it is possible to convert the many existing YAG systems to eye safe operation.

For many conversions there is a space limitation which requires as small a Raman shifting device as possible. Another problem with prior art devices is that they are reasonably long. This is primarily due to geometric constraints on the focussing of the pump radiation into the Raman cell and the damage threshold of optical materials. Recent laser developments have resulted in very compact devices and there is therefore a need for a compact Raman shifting device which is of the order of 75 mm long.

The lens used to focus the pump beam into the pressure cell can be designed to provide a variety of features. Factors that must be considered are the overall cell length, spherical aberration and degree of astigmatism. These factors are well known but it has been discovered that another property of the lenses is also significant for a practical Raman shifting device. This factor is the dependence of the focal properties of the two lenses on the pressure of gas in the cell. As the cell pressure changes, either through temperature changes or gas leaks, the refractive index of the gas also varies and this can cause changes to the focal properties of the optical elements.

Accordingly in another form of the invention there is proposed a Raman laser comprising a pressure cell and optical elements wherein the optical elements comprise a pair of substantially pressure-invariant lenses, a pair of mirrors and a pair of beam steering elements, and wherein the pressure cell contains a Raman active gaseous laser medium for producing a Raman interaction.

In preference the pair of beam steering elements are adapted to work in cooperation to fold a beam of radiation within the pressure cell such that the optical path length of the radiation is greater than a length of the cell.

Folded cavity designs have proved inefficient in the past due to unacceptable losses associated with beam steering mirrors. This problem is addressed in the present invention by the use of displaced porro prisms. The porro prisms are preferably formed from BK7 glass and are anti-reflection coated on the entrance and exit faces. The porro prisms use total internal reflection (TIR) to achieve high reflectivity from the internal faces.

In an alternative form the beam steering elements are adapted to rotate and displace a beam by two TIR reflections perpendicular to each other. This is achieved in a block cut similar to the porro prisms but with the second angled face at 90 degrees to the first.

In preference the pressure invariant lenses are anti-reflection coated with all anti-reflection coatings being designed for less than 0.3 % reflectivity at the wavelength of the pump radiation and at the wavelength of the Raman shifted radiation.

In preference the pair of mirrors form an optical cavity which provides feedback at the Raman shifted wavelength. One mirror is nominally 100 % reflective at the pump wavelength and the other mirror is an output coupler at the Raman shifted wavelength.

In preference the Raman active medium is methane gas. Methane provides a Raman shift which produces an output of 1.54 micron from a pump wavelength of 1.06 micron. In one form the Raman active medium is flowed through the pressure cell.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of this invention a preferred embodiment will now be described with reference to the attached drawings in which :

FIG. 1 shows a cross-sectional view of a first embodiment of

Raman laser incorporating an in-line design;

FIG.2 is a schematic diagram of a second embodiment of Raman laser incorporating a folded cell design;

FIG. 3 shows the pressure invariance of the lens design;

FIG. 4 shows a plot of Raman laser output versus pump input;

FIG. 5 shows a plot of Raman shifted output energy versus methane pressure; and

FIG. 6 shows a schematic of a third embodiment of a compact

Raman laser.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the figures in detail there is shown in FIG 1 a conventional in- line Raman cell design 1. The cell is of metal construction and is designed to contain gas at up to 80 atmospheres. Laser radiation is represented by lines 2. The radiation is focussed into the cell by lens 3 and collected by lens 4. Lenses 3 and 4 are designed to have a pressure invariant focal length.

The focal properties of a 75 mm focal length lens are shown in FIG 3. The beam divergence is plotted as a function of the curvature R-i of the surface 5 of lens 3. Since the focal length and R-| are given the curvature R₂ of surface 6 is determined. Fig 3 shows that there is a curvature which produces zero gas induced beam divergence at pressures of 500 psi (36 atmospheres) and 1000 psi (72 atmospheres).

The Raman laser of FIG 1 shows a pair of mirrors 7 and 8. The mirrors are coated for a pump radiation of 1.06 micron and a Raman generated output of 1.54 micron corresponding to the first Stokes shift of methane. The input surface 9 of mirror 7 is anti-reflection coated for less than 0.3 % reflection at 1.06 micron. The other surface 10 is coated for greater than 95 % transmission at 1.06 micron and greater than 99 % reflection at 1.54 micron. The surface 11 of mirror 8 is coated for greater than 95 % reflectivity at 1.06 micron and 50 % reflectivity at 1.54 micron. The surface 12 of mirror 8 is anti-reflection coated for less than .35 % reflectivity at 1.54 micron.

In practice an optical isolator is positioned between the pump laser and the

Raman laser. The optical isolator prevents feedback from the Raman laser from interfering with the stable operation of the pump laser. This is particularly crucial for the embodiment described above which reflects pump radiation at surface 11.

The cell 1 is filled and emptied through port 13.

The efficiency of the Raman laser of FIG 1 is shown in the graph of FIG 4. The graph shows the output of a 160 mm long Raman laser when pumped by a multimode (3 mrad divergence), 6 mm diameter, 10 nsec pulse length Nd:YAG laser. The maximum energy conversion efficiency is 37.6 % which corresponds to a quantum efficiency of 55 %.

The pressure invariant lenses of the Raman laser also produce less variation in output energy with pressure than conventional lenses. FIG 5 shows that the output of the Raman laser is essentially invariant over a range of 500 psi to 1500 psi. This is particularly useful in applications requiring stable operation over long periods without maintenance. Some leakage of Raman active gas can be tolerated without performance of the laser being seriously degraded.

A second embodiment of a Raman laser incorporating pressure invariant lenses is shown in FIG 2. In many applications a compact Raman laser is required and this can best be achieved by a folded cavity design. There is shown a schematic of a compact Raman laser consisting of a pressure cell 20. As with the in-line cell there are a pair of pressure invariant lenses 21 and 22. An optical cavity is formed by mirrors 23 and 24 which have similar coatings to those described above.

A third embodiment of a compact Raman laser is shown in FIG 6. In this embodiment most of the optical path length is transverse to the axis of the pump laser. In this way a very short axial path length is achieved although there is a lateral displacement of the Raman laser output compared to the pump laser beam. This embodiment only requires a 30 mm long space at the output of the pump laser.

In this case two TIR reflections are used to displace and rotate the beam by 90 degrees. The pump beam 30 enters the device through dichroic mirror 31 which is coated as above. The beam enters a glass block 32 which has two

TIR faces 33 and 34 which turn and displace the beam. The entrance face 35 of the block is a piano-vex lens optically contacted to the block. The exit face 36 of the block is a piano-cave lens optically contacted to the block. The piano-vex lens and piano-cave lens combination provide the pressure- invariant focussing as before. The curvature of the face 36 is 30 mm and equal to the focal length to the centre of the gas cell 37.

The gas cell 37 is a pressure vessel as before except in this embodiment there is shown a flowing gas cell arrangement. Gas enters the cell 37 through port 38 and exits through port 39. The cell is sealed on 'O' rings 40 by the blocks 32.

The Raman output 42 exits the cell by a similar block to that described. An output coupler 41 forms the laser cavity with the input coupler 31. Optical alignment of the Raman laser is carried out by tilt adjustments which aligns the positions of the TIR blocks 32. The third embodiment results in a device with a length of 30 mm and a lateral beam displacement of 82.5 mm.

An advantage of this embodiment is that the s and p components of the beam are interchanged between the two TIR reflections hence after passing through the unit there is no resultant phase change, hence a circularly polarised input beam remains circularly polarised. This is not achieved using the displaced porros of the second embodiment.

Claims

1. A Raman laser comprising a pressure cell and optical elements wherein the optical elements comprise a pair of substantially pressure- invariant lenses and a pair of mirrors and wherein the pressure cell contains a Raman active gaseous laser medium to effect a Raman interaction.

2. A Raman laser as in Claim 1 wherein the substantially pressure- invariant lenses are meniscus lenses having a second surface of radius

R, n

^R2² = - n^-—1 where Ri is the radius of the first surface and n is the refractive index of the lens material.

3. A Raman laser as in any one of the preceding claims wherein there is further included a pair of beam steering elements.

4. A Raman laser as in the immediately preceding claim further characterised in that the pair of beam steering elements are adapted to work in cooperation to fold a beam of radiation within the pressure cell such that the optical path length of the radiation is greater than a length of the cell.

5. A Raman laser as in either of the last two preceding claims wherein the pair of beam steering elements are comprised of displaced porro prisms.

6. A Raman laser as in anyone of the preceding claims 1-4 wherein there is included a pair of beam steering elements adapted to rotate and displace a beam by two TIR reflections perpendicular to each other.

7. A Raman laser as in any of the preceding claims wherein the substantially pressure invariant lenses have less than 0.3 % reflectivity at the wavelength of the pump radiation and at the wavelength of the Raman shifted radiation.

8. A Raman laser as in any one of the preceding claims wherein the pair of mirrors form an optical cavity which provides feedback at the Raman shifted wavelength.

9. A Raman laser as in the last preceding claim wherein one of the mirrors is substantially 100 % reflective at the pump wavelength and the other mirror is an output coupler at the Raman shifted wavelength.

10. A Raman laser as in any of the preceding claims wherein the active gaseous medium is methane gas.