GB2057181A - Improvements in or relating to gas laser arrangements - Google Patents

Abstract

A gas laser arrangement has a plasma tube (1) positioned between two mirrors (2, 3) which are aligned on its longitudinal axis to facilitate laser operation. A magnet means (7) is positioned alongside the tube (1) and is effective to produce an axial magnetic field which is substantially parallel with the longitudinal axis of the laser. The magnetic field is so arranged to reduce interaction between operating modes of the laser thereby to reduce noise content of the laser generated radiation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas laser arrangements This invention relates to gas laser arrangements.

Conventional low power Helium-Neon lasers normally oscillate simultaneously in several longitudinal modes even if they have been designed to oscillate only at one fluorescent line and in the lowest order spatial mode. In practice, these longitudinal modes heterodyne together at a photodetector and produce modulation products which occupy the R.F. and L.F. frequency bands as well as the expected frequencies, usually at V.H.F., which are given by their nominal frequency differences. When these lasers are employed in sensitive interferometric systems, these modulation products can cause significant noise in the signal bandwidths. Commonly, the noise will strongly fluctuate as the temperature varies due to warm-up and ambient changes. Similar problems will occur when homodyne or heterodyne detection of weak signals is attempted.

Helium-Neon lasers which are available commercially suffer from a number of disadvantages which make them less than ideal sources. Some of these disadvantages are: i) The oscillation and emission of power at more than one discrete wavelength (Longitudinal mode) within the bandwidth of each fluorescent line.

ii) The uncontrolled frequency drifting of these longitudinal modes with time even in reasonably stable environments due to small temperature changes.

iii) The fluctuations in relative power levels of each mode and small changes in frequency spacing between them as frequency drifting occurs, both broadly due to coupling between the modes.

In addition to the above, an incorrectly designed laser will probably oscillate in more than one wavelength band, the centre of each band corresponding to a characteristic fluorescent line for the gas e.g. 1.15,1.16,1.17,1.19 um for Helium-Neon.

Also it may oscillate in spatial modes of higher order than the transverse mode (TEM model) which is normally required.

The effects on a coherent detection system of the above imperfections can be summarised as follows: i) If the power is emitted in more than one mode within a given fluorescent linewidth, the coherence length of the laser is greatly reduced. If, for example, two equal power longitudinal modes are oscillating, interferometry becomes impossible for path length differences which are integral multiples of the laser resonator length plus one half. So, the coherence function is periodic, rather than declining with path difference, but neverthless can be a significant practical problem.

ii) When three or more modes are oscillating within a single line width they will in general interact in the photodetector. Although the difference frequencies between the modes in most cases occupy the VHF or UHF spectrum, further mixing can take place in the detector between these frequencies. The second order difference frequencies appear in the RF and LF band down to nearly zero Hertz and severely corrupt the required electronic signals.

iii) Because of interaction between the modes, the frequency differences can vary as the modes drift through the fluorescent lines. In turn, the second order difference frequencies will fluctuate strongly, eliminating the practicality of filtering them out.

iv) The magnitudes of the second order differ ence frequencies will also fluctuate strongly as drifting and mode coupling introduce sudden changes in the relative amplitudes of the longitudin al modes, further corrupting the required signals.

The present invention seeks to provide a gas laser capable of producing a signal in which the noise content of the generated signal due to interaction between oscillation modes is reduced.

According to the present invention a gas laser arrangement comrpises a plasma tube positioned between two mirrors aligned on its longitudinal axis to facilitate laser operation, magnet means positioned alongside the tube and effective to produce an axial magnetic field substantially parallel with the longitudinal axis of the laser so arranged to reduce interaction between operating modes of the laser thereby to reduce noise content of the laser generated radiation. The axial magnetic field results in a reduction of the amplitude of side modes and an increase in power in a single required oscillating mode thereby facilitating single mode operation.

Magnetic field adjustment means may be provided for controlling the axial magnetic field produced by the magnet means thereby to facilitate adjustment of the operating characteristics of the arrangement in dependence upon the strength of the axial magnetic field. The adjustment means enables optimum reduction of interaction between operating modes to be set.

The magnet means may comprise a plurality of electromagnets or permanent magnets. Said adjustment means may comprise adjustment screws operatively associated with the magnets for adjusting the position of the magnets relative to the tube. Alternatively, the magnetic means may comprise a solenoid which surrounds the plasma tube.

If electromagnets or a solenoid are used, current adjustment may be used to control the magnetic field.

The mirrors may comprise an output mirror which is adapted to pass therethrough some laser light so as to provide an output signal and to reflect the majority of light it receives back through the tube to the other mirror and the other mirror may also be arranged to pass some laser radiation to an auxiliary resonator mirror.

The auxiliary resonator mirror may be coupled to adjustment means whereby it is positionally adjustable along said longitudinal axis thereby to reduce the amplitude of side modes of the generated signal.

The adjustability of the resonator mirror can enable side modes to be completely eliminated and results in an even greater increase in power available at the required oscillation mode. The said auxiliary resonator mirror may be arranged to pass some light to detector means and the adjustment means may be a transducer responsive to a signal derived from the detector means thereby to define a closed loop servo system whereby the position of said resonator mirror is adjusted automatically in dependence upon signals received by the detector means.

The said other mirror and the said resonator mirror may be substantially identical in construction.

The output mirror may be coupled to a transducer whereby selection of the frequency of the dominant mode corresponding to maximum power is facilitated.

The laser may be a Helium-Neon laser and it may be arranged to operate by suitable choice of mirrors at 1.15 micro metres. The invention as applied to a laser operating at this wavelength is particularly suitable for use in communications along optical fibres as this wavelength is particularly efficient in terms of long distance communication due to low losses in the fibres and also permits wide bandwidth transmission.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure lisa generally schematic front view of a laser arrangement according to the present invention; Figure 2 is a generally schematic front view of a laser arrangement according to an alternative aspect of the invention; Figure 3a, Figure 3b, Figure 3c, Figure 3d and Figure 3e are wave form diagrams showing the gain of a laser according to the present invention under various operating conditions; Figure 4 is a generally schematic sectional side view of a mirror arrangement fcr use in the laser as shown in Figure 2; and Figures 5a, 5b and 5c are wave form diagrams illustrating operation of the laser as shown in Figure 2.

Figure 6 is a schematic block diagram of a closed loop control arrangement for control of the selected mode of operation.

Before turning to the drawings some consideration will now be given to the present state of the art apertaining to helium neon lasers and the problems associated therewith.

Referring to Figure 1 the laser arrangement comprises a plasma tube 1 disposed between an output mirror 2 controllable by a mirror drive control 2a and a rear mirror 3. The output mirror 2 is mounted on a transducer which might comprise a piezoelectric crystal the transducer being adapted to be driven so asto move in the longitudinal axis of the plasma tube for modulation purposes. In order to minimise temperature effects on the arrangement thus far described the mirrors are mounted on support plates 4 and 5 which are mechanically coupled by means of Invar rods 6a, 6b and 6c. Arranged alongside the plasma tube 1 are a number of permanent magnets 7 which are connected between screw adjusters 8 and 9 at each end thereof the screw adjusters 8 and 9 being connected to the Invar rods 6b and 6c respectively.The magnets produce an axial magnetic field along the longitudinal axis of the laser plasma tube 1 and the adjustment screws 8 and 9 serve to facilitate adjustment of the positional relationship between the plasma tube 1 and the permanent magnets 7.

If no magnets were used in the laser arrangement shown in Figure 1 a gain/frequency characteristic would result as shown in Figure 3a and it can be seen that there are abrupt gain changes as the oscillating modes move through the gain curve. This movement is caused, by temperature and other environmental changes, to produce frequency hopping or mode locking.

If magnets are used which are clamped hard against the plasma tube 1, then a frequency charac teristicwith gain will result as shown in Figure 36 and it can be seen that although the shoulders present in the waveform of Figure 3a are not present, the fluorescent line is wide and therefore several frequency modes of operation may be supported which is not desirable.

By adjustment of the magnets, a gain/frequency response curve may be produced as shown in Figure 3e wherein the magnets are adjusted to show the so called "Lamb Dip" at the central operating frequency 10 and to show the side lobes 11 and 12. However by judicious adjustment of the magnets 7, a gainl frequency response characteristic may be produced as shown in Figure 3dwherein the magnets have been adjusted for maximum power in the central mode operating frequency 13. Figure 3e shows a typical output response from the laser wherein a principal output peak 14 is shown at approximately 1.1523 micro metres wavelength with side mode frequencies 15 and 16 which are each 385 MHz from the central frequency 13.

The laser arrangement as shown in Figure 1 is satisfactory for some applications, it is nevertheless desirable for some other applications to further suppress the side mode shown as 15 and 16 in Figure 3e. To facilitate further side mode suppression, an arrangement is provided as shown in Figure 1 wherein parts corresponding to Figure 1 bear the same numerical designations. The plasma tube 1 and magnets 7 with their associated adjusters 8 and 9 are substantially identical to Figure 1.The rear mirror 17 however, is arranged to passsome laser a light through an aperture in the support 5 to a Fabry-Perot interferometer auxiliary resonator comprising a resonator mirror 18 which in this embodi- - ment is connected to a transducer 19 which may have a voltage applied to it on lines 20 for operating a mirror drive whereby its position along the longitu dinal axis of the plasma tube 1 may be adjusted.

Operation of a Fabry-Perot interferometer is well known and the arrangement of the mirrors 2, 17 and 19 is shown more clearly in Figure 4. By adjustment of the distance L2 between the mirrors 17 and ISthe phase of coherent light in the auxiliary resonator may be adjusted such that the side modes 15 and 16 shown in Figure 3e are suppressed. At the position of maximum suppression the amplitude of the fundamental or main frequency of the laser 14 which is transmitted by mirror 18 will beata minimum and by arranging that the mirror 18 passes a small amount of light to a detector 21 a servo system is provided whereby a control signal is applied to the detector 21 so as to maintain the position of the mirror 18 so that a minimum signal at the fundamental frequency is received by the detector 21.The optimum dimension at a particular frequency is L2 = L1 where the minima in reflectivity corespond exactly to the side modes (compare Figures 5a and 5b). If for the sake of compactness L2 must be reduced, the reflectivity will be greater for the side modes (compare 5b and 5c).

There are two separate requirements which must be satisfied to achieve acceptable single mode operation. Firstly, a single mode must be selected and secondly the mode must be positioned on the peak of the gain curve. To examine the first requirement we can assume that conditions are stable enough for the selected mode to remain fixed at the peak of the gain curve. Referring now to the control system for the third mirror as shown in Figure 6. The signal obtained from a photo detector 25 at the rear at the laser varies as mirror 18 is adjusted because the amount of optical power transmitted through mirrors 17/18 changes. When the unwanted modes have been suppressed, the power drops and falls to a minimum midway between the two positions of mirror 18 corresponding to the onset of suppression.

However, between these two positions, the main laser output is essentially unaffected. Therefore, a feedback signal can be derived from the rear of the laser using synchronous detection as shown in Figure 6. Mirrors 18 is mounted on a piezoelectric transducer and is made to oscillate in position in response to an oscillator 26 coupled thereto via a summing amplifier 30 and the resulting oscillations in the photodetector output are synchronously detected in a phase sensitive detector 27 to give an error signal which is fed via a low pass filter 28 to an integrator 29 the output of which is coupled to a second input of the summing amplifier 30 and thereby controls the means position of mirror 18. No modulation of the main output beam should be evident.

In order to position the selected mode at the peak of the gain curve a similar arrangement can be employed. In this case, mirror 2 would be made to oscillate in position at a different frequency from mirror 18. If this technique were used, clearly some intensity modulation of the output beam would occur. However, this could be quite low and probably could be at a modulation frequency which was not of interest in the equipment in which the laser was operating. This means of control only works when a single mode is present. With more than one mode, the constant total power is merely shifted between modes as the resonator is scanned.

For some applications, automatic control would not be necessary. Infrequent manual adjustment has been found quite acceptable. If the resonatorstruc- ture is built with suitable materials (e.g. super Invar) and the operating temperature remains reasonably constant, only occasional trimming is found to be necessary.

By utilising a laser arrangement according to the present invention mode interaction effects are minimised whilst the gain at the centre weavelength is optimised as shown in Figure 3d. By using the auxiliary resonator arrangement requiring a third auxiliary resonator mirror, specified characteristics can be obtained whereby a single mode may be emphasised and sub-modes suppressed. This re sults in maximum output power in the selected mode. Adjustment for optimum conditions is simply achieved by adjustment of the mirror 18 and is not critical. It will be appreciated that by utilising the detector 21 which receives its light via the mirror 18 the main output which is obtained via output mirros 2 is unaffected and unwanted modes may thereby be automatically suppressed over a relatively large part of the free spectral range of the laser.Although in some applications it is desirable to use the additional mirror 18 and the auxiliary resonator, under suitable conditions of gas mixture pressure disharge current, adjustment of the axial magnetic field alone by means of the adjusters 8 and 9 may be affected to produce either a single laser mode, or adequately suppressed side modes.

By matching the mirror 18 to the mirror 17 as detailed as follows in (iii) it is possible to use readily available proprietory mirrors.

Much effort has been expanded by various research groups over the years since lasers were first produced to make them approach the ideal for an optical source. The techniques just before described improve on past efforts in the following ways.

i) The influence of the magnetic field mentioned above facilitates the achievement of relatively large output power in a single mode. It has always been possible to select a single mode by reducing the excitation of the laser so that only one mode would oscillate. But obviously the output power is reduced greatly as a result. It has been thought in the past that the power that would have been emitted in the suppressed modes would not be transferrable to the selected mode due to a phenomenon called inhomogenous broadening in Helium-Neon lasers.

The application of the correct axial magnetic field has been chosen to produce a large degree of coupling so that nearly all (90%) the power within the suppressed modes is transferred to the selected one.

ii) The three mirror cavity is not new in concept.

It relies on there being two resonators coupled together, each with eigen frequencies at different spacings. This fact together with the limited bandwidth available within the fluorescent line means that conditions for oscillation are only correct at one of the possible eigen frequencies. The present design, however, uses a new philosophy, namely the use of a pair of mirrors of equal reflectivity to constitute the rear composite mirror of the laser. The reflectivities are designed to obtain a suitable compromise between high reflectivity at the selected wavelength and low reflectivity, and so relatively high loss, at the other mode wavelengths (see Figure 2). This approach is most useful for lasers where about 3 to 5 modes can oscillate within the fluoresent line (in Figure 3e, three modes are shown). This situation represents a very common group of lasers of approxiately 40 cm length giving a few mW output power and being compact enough to fit in laboratory industrial equipment. With longer lasers, other approaches to mode selection become more suit able. The distance of the third mirror from the back of the standard laser is clearly important in compact systems, and this enters into the design trade offs including reflectivity mentioned above, as can be seen from Figures 5b and 5c.

The importance of making the reflectivity of mirrors 2 and 3 equal is that it enables the highest gain/loss ratio for the selected/rejected modes to be achieved. It should be noted that the concept of an oscillation threshold (commonly designated loop gain = 1) is vital to the operation of the three mirror technique, particularly when L2 must be less than the ideal value of LI/L2.

iii) The coupling of the third mirror to the laser resonator depends on two things, firstly choice of ahe.correct reflectivity and secondly the choice of the correct radius of curvature of the mirror to spatially match with low loss into the oscillatory mode. These requirements present difficult practical problems. To be able to vary reflectivity and third mirror position independently to find the optimum design, the mirror curvature must also be varied independently.

Thus a large number of different combinations of mirror could be required. This problem has been significantly relieved by the use of readily available mirrors with lens surfaces on the backs of the mirror substrate which transform the beam wavefronts to plane parallel at these surfaces. This has been made use of in the following way (see Figure 4). The third mirror 18 of - his type has been positioned with the back surface facing rhe laser. The second laser mirror 17 presents its back surface to the third mirror 18.Thus, for the sort of distance likely - ten to twenty centimetres - a high degree of matching is achieved with a range of different refleczi;ty mirrors which can have any reasonable radii of curvature.

iv) The fact that a very simple laser can be made with a single mode or one dominant mode by empioying the magnetic field technique only is significant for io. cost systems using homodyne detection. Some reduction in available power may be required, but not nearly so much as would be to select one mode without the field effect. Servo control may be used here to keep the mode at the peak of the fluorescent line.

v) The servo control system outlined above is inherently simple and does not significantly influence the output of the laser. it is situated at the rear of the laser and relies on very small, very low frequency amplitude pertubations which would often be below the signal band of interest, and in any case are less than those pertubations commonly present in conventional laser output.

vi) The greatly improved thermal stability of the laser resonators allows the use of a predetermined control to correct for changes in temperature. This improvement could be used to make inexpensive systems with perfectly adequate reliability.

The advantages of the present techniques apply primarily although not exclusively to a specific power range of Helium-Neon laser operating at 1.15 um wavelength. They are that a readily available laser can be relatively easily modified to give single mode operation with very little or no penalty in power or space needed and without great complexity or hardware expense.

As previously stated, the requirement for the single mode laser concerns a compact interferometric type of instrument working at 1. ï 5 lim and the techniques were developed for this. Other areas of application could use the techniques either at this wavelength or at other wavelengths at which the techniques may apply (e.g. 0.633 and 3.39 'ism).

Applications at 1.15 um would include the potentially important fibre optic systems. Ideally, operation at slightiy longer wavelengths would be desirable to achieve minimum loss and material dispersion. However, the current lack of suitable detectors and reasonably priced laser sources means that a 1.15 um laster with germanium detector is highly attractive. The ease of modulation of a gas laser together with a high coupling efficiency in a low order mode to the fibre gives it advantages over I.e.ds, and possibly diodes, at high frequencies.

With a single mode, homodyne or heterodyne detection can be achieved with the usual large increase in sensitivity over direct detection. Frequency locking of similar lasers with a constant offset over the range 0 Hzto greater than 1 GHz should be easily available. Clearly, this does not only apply to fibre optic systems. Any wide bandwidth communication system, for example, may benefit from the technique developed. In most cases, the low average power is quite sufficient for all but long range line of sight systems.

Claims (15)

1. A gas laser arrangement comprising plasma tube positioned between two mirrors aligned on its longitudinal axis to facilitate laser operation, magnet means positioned alongisde the tube and effective to produce an axial magnetic field substantially parallel with the longitudinal axis of the laser so arranged to reduce interaction between operating modes of the laser thereby to reduce noise content of the laser generated radiation.

2. A laser arrangement as claimed in claim 1, comprising field adjustment means for controlling the axial magnetic field produced by the magnet means thereby to facilitate adjustment of the operating characteristics of the arrangement in dependence upon the strength of the axial magnetic field.

3. A laser arrangement as claimed in claim 2, wherein the adjustment means comprises adjustment screws operatively associated with the magnet means for adjusting the position of the magnet means relative to the tube.

4. A laser arrangement as claimed in any one of the preceding claims, wherein the magnet means comprises a plurality of electromagnets.

5. A laser arrangement as claimed in claim 1,2, or 3, wherein the magnet means comprises a plurality of permanent magnets.

6. A laser arrangement as claimed in claim 1,2 or 3, wherein the magnetic means comprises a sole- noid which surrounds the plasma tube.

7. A laser arrangement as claimed in claim 4 or 6, comprising current adjustment means coupled with the magnet means and arranged to control the magnetic field produced thereby.

8. A laser arrangement as claimed in any one of the preceding claims, wherein said mirrors comprise an output mirror adapted to pass therethrough some laser radiation so as to provide an output signal and to reflect the majority of radiation it receives back through the tube to the other mirror and said other mirror is adapted to pass therethrough some laser radiation, the arrangement further comprising an auxiliary resonator mirror arranged to receive the radiation which passes through said other mirror.

9. A laser arrangement as claimed in claim 8, wherein the resonator mirror has a shape which is matched to the shape of the wavefront received through said other mirror.

10. A laser arrangement as claimed in claim 8 or 9, including adjustment means coupled to the resonator mirror and effective to positionally adjust the resonator mirror along said longitudinal axis thereby to reduce the amplitude of side modes of the generated signal.

11. A laser arrangement as claimed in claim 10, wherein the adjustment means comprises a transducer and the resonator mirror is adapted to pass some laser radiation, the arrangement further comprising a detector means arrange to receive radiation passed by the resonator mirror, which detector means is coupled with the transducer to define a closed loop system whereby the position of said resonator mirror is adjusted automatially in dependence upon signals received by the detector means.

12. A laser arrangement as claimed in any one of claims 8 to 11, wherein said other mirror and said resonator mirror are substantially identical in construction.

13. A laser arrangement as claimed in any one of the preceding claims, including a transducer coupled with said output mirror thereby to facilitate selection of the frequency of the dominant mode corresponding to maximum power.

14. A laser arrangement as claimed in any one of the preceding claims wherein the plasma tube contains Helium and neon gas and is arranged to operate at 1.15 micro metres wavelength.

15. A laser arrangement substantially as described herein with reference to and as illustrated in the drawings.