GB2037064A

GB2037064A - Lasers

Info

Publication number: GB2037064A
Application number: GB7942361A
Authority: GB
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 1978-12-15
Filing date: 1979-12-07
Publication date: 1980-07-02
Also published as: DE2949906A1; JPS5586178A; FR2444351A1; AU5376979A

Abstract

To correct for the effects of thermal birefringence in a laser rod (2), the rod is arranged between end mirrors (3) and (4) on a first axis (11), with a quarter wave plate (6) on one side of the rod and a polarising prism (8), on the other side, the polarising prism reflecting light out of the cavity along a second axis (12) to a third mirror (9), and each axis (11), (12) including separate Q switches (7) and (10) arranged to operate in synchronism. <IMAGE>

Description

SPECIFICATION Thermally compensated laser This invention relates to a thermally compensated laser.

BACKGROUND OF INVENTION In a solid state laser such as the Neodynium doped Yttrium Aluminium Garnet Laser, thermal gradients in the material operating at high average power input induce thermal birefringence and thermal lensing.

Thermal lensing can be allowed for by suitable choice of laser mirror curvatures. Thermal birefringence is a more difficult problem.

In high average power lasers the rod is usually water cooled. This introduces thermal gradients in the rod, the rod becoming hotter in the centre than at the edges. Since the rod is usually cylindrical, the thermal gradient is radial, and it induces optical birefringence so that light polarised in the radial direction propagates at a different velocity to light polarised in the tangential direction. In a laser containing a polarising element such as a polarising prism or Brewster plate, the plane polarised beam transmitted by the polarising element is, on emerging from the rod, elliptically polarised. The degree of ellipticity and the orientation of the major and minor axes vary from point to point over the rod surface.

Without compensation this causes a severe drop in the laser output as a component of the light is subsequently rejected by the polarising element.

The basis of a thermally compensated design is to achieve a 90 rotation of the plane of polarisation on each alternate pass through the laser rod. This has the effect of interchanging the radial and tangential polarisation components so that the ellipticity induced in one pass is cancelled in the next pass.

This can be achieved, for instance, in the method suggested by W.C. Scott and M De Wit in Applied Physics letters, Vol 18, No 1, 1 Jan 1971, pp 3-4. In it two identical laser rods and pumping cavities have a half wave plate introduced between them rotating the plane of polarisation by 90 .

The disadvantage of that system is that two laser rods are required with their separate pumping, the rods being separated by half a wave plate thus adding materially to design complication.

In another design for thermal compensation G D Ferguson of the United States Navy, Washington, as described in United States Patent No 4,068,190 issued on January 10, 1978, has achieved the same result by spatial separation of the two orthogonal polarisation components. In this design a large and expensive block of calcite is used to achieve the spatial separation. The two orthogonally polarised beams then propagate along parallel paths through a Q switch to a mirror and back into the system. A quarter wave plate is again used to interchange the polarisation components. This system requires a large and expensive calcite block for polarisation separation and a large aperture Q switch in order to cope with the two beams propagating side by side.

Another technique used for a different purpose but with some of the features needed to overcome thermal birefringence was outlined in a paper by P E Oettinger, published in the Journal of Appiied Physics Vol. 46, No 12, December, 1 975 under the title "Three Mirror Double Pass Laser".

In that design, the necessary rotation of the plane of polarisation is achieved on each alternate pass through the laser medium.

In the presence of thermal birefringence as occurs when the laser is operated at high repetition rates, it is found that laser action can occur even with the added mirror covered, due to the coupling of the orthogonal polarisation states induced by thermal birefringence.

When the added mirror is uncovered and the Q switch is in the "off' state this leads to "pre-lasing", depleting the inverted population prior to the opening of the Cl switch, resulting in a drop in output power of the Cl switch pulse.

OBJECT OF THIS INVENTION An object of this invention is to provide a unit which can utilize a standard design without the need of a large aperture and which generaily will be of relatively simple construction and which will give an effective Q switched pulse.

SUMMARY OF INVENTION The method of this invention uses a thermally compensated laser beam in which a 90 rotation of the plane of polarisation occurs on each alternate pass through the laser rod whereby to interchange the radial and tangential polarisation components so that the ellipticity induced in one pass is cancelled in the next path and in which a beam splitter is used to produce a first axis through the laser rod and a second intercepting axis at an angle thereto, characterised by a Q switch on each said axis synchronously timed whereby to prevent coupling of the polarisation states to prevent pre-lasing.

According to this invention a laser rod is pumped in the usual way and has a mirror positioned adjacent each end, but on one side of the rod, a quarter wave plate is disposed between the end of the rod and the mirror, and on the other side a polarising prism and Q switch are interposed between the end of the rod and the mirror, and adjacent to the polarising prism, outside of the axis of the rod and mirrors, is a further Q switch and mirror which reflects light from the polarising prism back through the polarising prism to the rod.

This overcomes the previous problems when the two Q switches are synchronised, such as by driving them in parallel, preferably electrically. The introduction of the second Q switch prevents the coupling of the polarisation states and pre-lasing cannot occur.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a typical assembly used in this invention, and Figure 2 shows a schematic light path the two-way arrows showing polarisation in the plane of the paper and the dots showing polarisation at right angles to the plane of the paper.

DESCRIPTION OF THE PREFERRED EMBODI MENT In the drawing designated Fig. 1 the laser rod 2 is disposed between a first mirror 3 and a second mirror 4 and is pumped by a source 5.

Between the mirror 3 and the end of the rod 2 is a quarter wave plate 6. This can be on the other side of the rod 2.

At the other end of the rod 2, interposed between the rod and a Q switch 7 is a polarising prism 8 which reflects light to a third mirror 9 through a second Q switch 10 which mirror 9 reflects it back to the prism 8.

In the drawing, the rod axis is designated 11 and the prism axis 1 2.

In Fig. 2 the various integers are given the same reference numerals.

It will be noted that the quarter wave plate 6 and mirror 3 combination rotates the plane of polarisation through 90 after reflection.

Now light returning through the system in the wrong plane of polarisation for the polarising prism 8 is reflected out of the cavity, and the third mirror 9 is therefore introduced to reflect the light back into the system but through the second Q switch 10. The light then returns through the rod and the quarter wave platemirror combination and again the plane of polarisation is rotated by a further 90 . On its return to the polarising prism the plane of polarisation is now correct for transmission through the prism so that the light now travels to the second mirror 4 and completes the traverse of the cavity.

Fig. 2 shows diagrammatically the path of light through the system together with the polarisation state at each stage with the two Q switches in the "open" state and with the two-way arrows showing polarisation in the plane of the paper and the dots showing polarisation at right angles to the plane of the paper as earlier stated. The system achieves the desired objective of a 90 rotation of the plane of polarisation on each alternate path through the laser rod, thus providing compensation for thermal birefringence. The output of the laser in this configuration has been found to be independent of the average power input.

In the present system, the polarising prism may be of standard design and does not need a large aperture, nor do the Cl switch apertures have to be large. The only components required in addition to a standard laser design are the quarter wave plate, an additional mirror and a second Q switch.

Output is possible through each of the mirrors 3, 4 and 9 but also on the axis 1 2 as indicated by the arrow if the device is operated in the pulse transmission mode.

Claims

1. The method of producing a thermally compensated laser beam in which a 90 rotation of the plane of polarisation occurs on each alternate pass through the laser rod whereby to interchange the radial and tangential polarisation components so that the ellipticity induced in one pass is cancelled in the next path and in which a beam splitter is used on a first axis through the laser rod and a second intercepting axis at an angle thereto, characterised by a Cl switch on each said axis synchronously timed whereby to prevent coupling of the polarisation plates to prevent prelasing.

2. A thermally compensated laser of the type comprising a pumped laser rod with a first and second mirror on the axis of the rod and including a quarter wave plate and a polarising prism and a Q switch on the said axis between the said mirrors, and including a third mirror on a second axis of the said polarising prism, characterised by a further Cl switch on the said second axis and means to synchronise the said further Cl switch with the Cl switch on the first axis, whereby to correct for the effects of thermal birefringence.

3. A thermally compensated laser according to Claim 2 which comprises a laser rod and pumping means therefor arranged between a first and a second mirror disposed on the transmission axis of the said laser rod one on each side of the said laser rod, a quarter wave plate also on the said axis on one side of the said laser rod, a polarising prism also on the said axis on one side of the said laser rod, a further axis passing through the said polarising prism with a third mirror on the said further axis, characterised by a pair of synchronised Q switches one on the first said axis and the other on the said further axis, whereby to correct for the effects of thermal birefringence.

4. A thermally compensated laser according to Claim 2 which comprises a laser rod and pumping means therefor arranged between a first and a second mirror disposed on the transmission axis of the said laser rod one on each side of the said laser rod, a quarter wave plate also on the said axis on one side of and adjacent to the said laser rod, a polarising prism also on the said axis on one side of the said laser rod, to project light along a second axis passing through the said polarising prism, and a third mirror on the said second axis, characterised by a pair of synchronised Cl switches one on the first said axis between the said polarising prism and the said second mirror and the other on the said second axis between the said polarising prism and the said third mirror.

5. A thermally compensated laser according to Claim 2 or 3 wherein the said Q switches are synchronised electrically.

6. A method of producing a thermally compensated laser beam substantially as described.

7. A thermally compensated laser substantially as hereinbefore described with reference to the accompanying drawings.