(1) Field of the Invention

THIS INVENTION relates to a method of, and apparatus for, computer controlled tuning of lasers.

(2) Prior Art

In recent years, the range of applications for lasers has increased markedly. In industry, lasers are used in areas such as cutting, welding, surveying and gas analysis, while in medicine, they are used as scal¬ pels and for welding detached retinas.

One major problem has been to accurately tune the lasers to operate at selected frequencies (or wave¬ lengths), each laser having a characteristic set of lines within its gain envelope. For example, it has not been possible to accurately tune C0 _? type lasers at all of the. characteristic lines in their- gain envelope. Generally each laser has been set to operate at a fixed wavelength. It is well known that different gases absorb light at different wavelengths in characteristic patterns which provide a "signature" for the gases. These patterns enable the existence and/or concentration of the gases to be detected e.g. for leak detection purposes in factories.

Where a gas has to be tested at two or more wave¬ lengths, two or more accurately tuned, highly stabilized lasers must be employed, each tuned to a particular wavelength. The equipment must be highly stabilized as variations in temperature and pressure can change the wavelength of the laser light. The change in wavelength may be such that the lasers operate at characteristic lines other than for which they are allegedly tuned and so false qualitative and/or quantitative measurements may be made in relation to the gas.

The equipment necessary to maintain the stabi¬ lity of the lasers prevents them from being readily portable or versatile and so has restricted the range of potential industrial applications. SUMMARY OF ^" THE PRESENT INVENTION

It is an object of the present invention to provide computer controlled tuning for lasers where the lines in the gain envelope can be identified.

It is a preferred object to provide such tuning which enables the laser to be tuned to selected lines.

It is a further preferred object to provide such tuning where the whole system is temperature independent and self-correcting.

Other preferred objects of the present invention will become apparent from the following description.

In one aspect, the present invention resides in a method for tuning a laser of the type having a cavity, a front window, a Brewster window and a grating, the method including the steps of:

(a) mounting the grating on a mounting movable relative to the cavity;

(b) moving the grating to a selected angular position and/or cavity length relative to the cavity; (c) measuring the laser power output using a first detector and moving the grating to optimise the laser power output so that the laser is tuned to a characteristic line;

(d) measuring the angular position and cavity length of the grating corresponding to the character¬ istic line;

(e) measuring the output from a photoacoustic cell or second detector located in at least a portion of the output beam from the laser; (f) repeating steps (b) to (e) until the gain

envelope of the laser has been traversed; and

(g) from the measurements in step (3), establish the identity of each line sampled in the gain envelope and its respective grating position. In a second aspect the present invention resides in an apparatus for tuning a laser of the type comprising a cavity, a front window, a Brewster window and a grating, the apparatus including: a mounting for the grating movable to vary the angular position and the cavity length of the grating relative to the cavity; means to move the mounting; a first detector to measure the power output from the laser; a photoacoustic cell or second detector located in at least a portion of the output beam from the laser; means to measure the output from th ^' e ^' photo- acoustic cell or second detector; and means to control the movement of the mounting, and thereby the grating, dependent on the measurement of the output power of the laser by the first detector and the measured output of the photoacoustic cell or second detector. Preferably the laser grating is mounted on a turn-table. rotatable (e.g. by a stepping motor) to vary the angular position of the grating relative to the cavity axis, the turn-table being longitudinally movable (e.g. by a piezoelectric drive) along, or parallel to the cavity axis to vary the cavity length.

Preferably, in step (c), the output from the laser is measured by a first detector which registers the intensity of the beam reflected from the Brewster window in the laser tube. Preferably a beam chopper-splitter is provided

outside the cavity, spaced from the front window, to provide a chopped output beam at the laser frequency and proportional to its power. Preferably the beam reflected by the chopper-splitter is reflected by a mirror onto a photoacoustic cell (containing e.g. ammonia or ethylene) to measure the laser output in step (e).

Preferably the system is controlled by a compute /microprocessor capable of driving the stepping motor, the piezoelectric driver and registering the outputs from the detector and the photoacoustic cell. Suitable software is provided to control the system. BRIEF DESCRIPTION OF THE DRAWINGS To enable the invention to be fully understood, a preferred embodiment will now be described with reference to the accompanying; drawings, in which:

FIG. 1 is a schematic layout of the control system;

FIG. 2 is a graph of the CO, laser gain curve; and

FIG. 3 is a graph of the absorption character¬ istic of ammonia relative to the lines of the CO gain curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG.. 1, the laser 10 has a cavity 11 with a front window 12 and a Brewster window 13.

The grating 14 is mounted on a turn-table 15 which is rotatable by a micrometer driven by a stepping motor 16 to vary the grating angle and which is movable along (or parallel to) the cavity axis, to vary the cavity length, by a piezoelectric device 17.

The laser power output is monitored by a detector 18 which registers the intensity of the beam reflected from the back of the Brewster window 13.

A beam chopper-splitter 19 is provided in front of the front window 12 and is driven by a motor 20. The face of the chopper-splitter 19 is mirrored to reflect a chopped output beam to a mirror 21, which reflects the beam to a photoacoustic cell 22 containing e.g. ammonia or ethylene at a low partial pressure to to calibrate the frequency (or wavelength) of the parti¬ cular line.

The stepping motor 16, piezoelectric device 17, detector 18 and photoacoustic cell 22 are connected to a computer or micropressor C.

The software in the computer C causes the stepping motor 16 to drive the grating 14 to a selected angular position and then varies the cavity length (via the piezoelectric device 17).

By monitoring the laser power via detector 18, the grating position can beoptimised for a particular laser characteristic line.

The output from the photoacoustic cell 22 is monitored and the steps are repeated until the gain envelope of the laser (FIG. 2) has been traversed.

By using the measurings obtained by the above steps, and by reference to the known absorption co¬ efficients of the gas in the photoacoustic cell 22, the identity of each characteristic line in the gain envelope can be established for reference against the respective grating angle and cavity length therefor.

Should an operator wish to measure the absorption coefficient of a gas at a line e.g. identi- fied as P10, he can type the line identification into the computer and the computer will operate the stepping motor 16 and piezoelectric drive 17 to position the grating 14 to so tune the laser to that line. To check that the laser is tuned at line P10, the operator can move the grating to tune' the. laser to line P6 where high

absorption of the light occurs in the photoacoustic cell and then move the grating back towards its initial position, ,traversing 4 characteristic lines. Turning the laser to line P12 and then returning the grating to its initial position provides a confirma- atory check that the laser is tuned to line P10 as required.

In the graph of FIG. 3, high absorption of the light in the ammonia occurs near lines P12, Pl8 and P38 and low absorption at lines P30 and P40. By measuring the relative absorption at each file line of the CO laser, a fingerprint would establish the presence of ammonia while the differential absorption at say lines Pl8 and P30 would establish the concen- tration of the ammonia.

While the present invention can have particular application for low power, low gain short cavity lasers, the computer controlled tuning-method and apparatus is suitable for most, if not all, types of lasers and provides a compact, stable and efficient tuning facility, Various changes and modifications may be made to the embodiment described without departing from the scope of the present invention as defined in the appended claims.