WO1988001798A1

WO1988001798A1 - Pre-heat control system for a laser

Info

Publication number: WO1988001798A1
Application number: PCT/GB1987/000595
Authority: WO
Inventors: Roland Henry Pratt; Mark Adrian Vincent Chapman
Original assignee: Renishaw Plc
Priority date: 1986-08-30
Filing date: 1987-08-24
Publication date: 1988-03-10
Also published as: GB8621037D0

Abstract

A laser is pre-heated by a heater coil (18) which surrounds part of the laser cavity (10). The heater changes the length of the laser cavity during operation due to thermal expansion of the cavity. Changes in length of the cavity vary the frequency of the modes of the laser and during the heating period the modes can be seen to ''pass through'' the power curve of the laser. The frequency with which the modes pass through the power curve is related to the difference between the temperature of the cavity and the temperature of its surroundings. A heater control circuit (20) is provided which monitors the intensities of the mode or modes in the cavity as the cavity length changes and produces a pulsed output indicative of the mode passing frequency. Means are provided for controlling the duration of the pre-heat period using information derived from said pulsed output.

Description

PRE-HEAT CONTROL SYSTEM FOR A LASER

The present invention relates to a pre-heat control system for a laser.

The frequency of a laser beam is affected by changes in the length of the cavity in which the lasing action takes place. The laser generates heat once it is switched on and this heat expands the laser cavity thus changing the frequency of the output beam during a warm-up period before it reaches its operating temperature.

Also, during any period of operation, the laser will receive heat from, or pass heat to, the surrounding atmosphere depending on the difference in temperature between the laser and the surrounding atmosphere.

Since for most applications of the laser it is essential that the laser operates with a stable intensity or frequency, the laser cannot be used during its warm-up period, which may be a matter of an hour or more, nor can it be used at all without some form of frequency or intensity stabilisation. Thus it is an important requirement in applications of lasers to be able to pre-heat the laser to its operating temperature quickly and to stabilise the frequency or intensity of the beam at that temperature.

One example of a system for controlling the wavelength or intensity of a laser is disclosed in UK Patent No. 1,448,676. In this Patent there is disclosed a temperature control device which extends over at least a part of the operational section of the laser cavity and which varies the temperature of the laser cavity in dependence upon the detected wavelength or intensity of the emitted laser beam to minimise variations thereof.

Such a temperature control device in the form of a heating element can be used for pre-heafcing the laser to its operational temperature.

A problem arises however, in providing a simple automatic control for the pre-heater. Because of the heat dissipated by an operating laser it always operates at a temperature in excess of the ambient temperature. For a Helium-Neon laser this may be around 30°C excess. Thus for automatic control of the laser after the warm-up period, the heater must still be capable of providing a significant level of positive heat input, (i.e. to heat the laser at a rate greater than it is losing heat to the environment), and also the laser must have been pre-heated to a temperature above its normal operating temperature. Direct measurement of the ambient temperature and the laser temperature in order to set the pre-heat level accurately would require additional sensors and electronic circuitry which adds to the cost of the system.

Alternatively, measurement of the ambient temperature may be avoided by pre-heating the laser to a pre-set temperature which is above the temperature which the laser alone would reach when operating in the warmest anticipated environment. This means that the laser will typically be operating at a higher temperature than necessary thus shortening the life of the laser. This method can also cut down the control range of the heater, or alternatively require a larger heater to be used.

It is an object of the present invention to provide a pre-heat control system for a laser which overcomes this difficulty.

This object is fulfilled in accordance with the invention as claimed in the appended claims by controlling the duration of the pre-heating period from signals derived from the laser beam itself.

The advantage of the invention is that the necessary signals can be readily generated from photo-diodes in the laser which are already available and are used for frequency stabilisation of the laser beam, and that no direct measurement of either the laser temperature or the ambient temperature is required.

An example of the invention will now be more particularly described with reference to the accompanying drawings in which:

Fig 1 is a representation of the power curve for a gas laser beam output showing two resonant modes,

Fig 2 is an illustration of the major components of the laser and a stabilisation system in accordance with the present invention,

Fig 3 is a circuit diagram showing the major elements of the control circuits of the stabilisation system of Fig 2 and,

Figs 4a to 4c illustrate the outputs of three of the control circuits of Fig 3. For most lasers light will be emitted at a number of frequencies which will lie within the frequency band of the light output of the power curve.

The laser cavity will resonate in a large number of longitudinal modes which are separated by a difference in frequency Δ f which is related to the cavity length by the expression Δ f = c

2L where c is the velocity of light in the cavity, and L is the length of the cavity.

By appropriate choice of the cavity length therefore, the separation of the modes can be arranged to be such that only one or two modes will resonate within the frequency band covered by the power curve.

However, both the frequency and mode selection of the laser emission are sensitive to changes in the length of the laser cavity, as will be described in more detail hereinafter. Thus for most practical applications the frequency, wavelength, or intensity of an emitted laser beam has to be stabilised against variations caused by changes in the laser cavity length due, for example to changes in temperature of the laser cavity.

Referring now to Fig. 1 a curve is shown which is a plot of frequency along the horizontal axis and intensity along the vertical axis. For simplicity of description only two resonant modes M1 and M2 of the laser are shown in full lines having frequencies fl and f2 with intensities I1 and I2.

The frequencies f1 and f2 are dependent on the length of the laser cavity. Thus, for example, a change in cavity length such as to increase the frequencies of the lodes will cause the full lines to move to the right. If the modes M1 and M2 are selected so that the two frequencies f1 and f2 are always on opposite sides of the peak P of the power curve, the movement to the right will cause the frequencies of both modes to increase while the intensity of mode Ml increases and the intensity of mode M2 decreases. As the cavity length continues to change the intensity of mode M2 will continue to decrease until the point is reached at which the cavity length will no longer support resonance of mode M2 at the increased frequency, and a new mode M3 (not shown) will occur at a new lower frequency at the left hand side of the power curve. This mode M3 will be spaced from the mode M1 by a similar frequency difference Δ f.

This cycle will occur every time the cavity length has changed by one half of the wavelength of the emitted ligfit. Thus it can be seen that the modes "pass through" the power curve as the cavity length continues to change. It is a feature of the laser that all adjacent modes have orthogonal planes of polarisation.

In addition to the changes in frequency due to changes in cavity length, any increase in output power of the laser will predominantly change the height of the power curve i.e. the intensity of the emitted light, while having an insignificant effect on the width and general form of the curve. This change produces the curve shown in dotted lines in which the height of the curve has i n creas ed from h t o h1 . S im i l ar ly a decrease in output power will reduce the height of the curve.

It can therefore be seen that any control system arranged to restore the value of one of the intensities of the two modes to its original value, or to restore the original difference in values of the two intensities, other than by restoring the power output to its original value , wi ll necessarily change the frequency of the mode under the new power curve.

We have now found that if the controller is arranged to maintain the ratio of the intensities I1 to I2 constant, the frequencies f1 and f2 will remain constant regardless of the change in height of the power curve due to power output drift.

Referring now to Fig 2 there is illustrated a laser 10 designed to produce an output beam 12 having two linearly polarised modes, which in accordance with the operation of the laser cavity have defined orthogonal planes of polarisation. For the purposes of this description the laser 10 will be referred to as a Helium-Neon gas laser, but many other types of laser may benefit from a control system of the present invention.

Two reference beams 13 and 14 are taken from the beam 12 via beam splitters 15 and 16 respectively, and are directed through orthogonally orientated polaroids P1 and P2 on to two photo-diode detectors D1 and D2 which each provide an electrical signal proportional to the intensity of a different one of the two modes in the light emitted by the laser. It will be understood that the reference beams could be produced from a beam taken from the back of the laser by passing the beam through one or more beam splitters which may be inside or outside of the laser housing.

The basic control element for stabilising the frequencies of the laser beam is a heater coil 18 which is wound around part of laser cavity to control the temperature thereof, and therefore the cavity length, in accordance with signals received from a control system 20 which is described in more detail in Fig 3.

Referring now to Fig 3 the photo-diodes D1 and D2 are shown with laser beams directed thereon which represent the two orthogonally polarised modes respectively. The two photo-diodes produce electrical outputs which are passed to variable gain amplifiers 21 and 22, the outputs of which are voltages (V1 and V2) which are proportional to the intensities of the two modes respectively. The ratio of the two voltages V1 and V2 can be set to a desired pre-determined level by varying the gain of the two amplifiers, thus setting the ratio of the intensities at which the laser is to be stabilised.

The outputs V1 and V2 are passed as inputs into a differential amplifier 24 which produces an output voltage V3 which is proportional to the difference between the output voltages V1 and V2. This constitutes an error signal, the magnitude of which depends on the difference of the intensities of the two beams from their set ratio, and the sign of which depends on the polarisation planes of the modes and their respective intensities. Thus as the modes "pass through" the power curve, a sign change will occur every time the cavity length has changed by one half of the wavelength of the emitted light.

The effect that this has on the error signal V3 is illustrated in Fig. 4a by the alternate positive and negative slopes of the error signal as the cavity length changes. The control system can be arranged to stabilise at the zero crossing point on a positive or negative slope by selection of the polarity of the error signal, for example, by putting a switch x in the lines V1 and V2 between the amplifiers 21 and 22 and the subtractor 24, to reverse the inputs to the subtracter 24. This feature allows the relative polarisation planes of the two stabilised frequencies to be swapped if required.

The relative polarisation planes of the two frequencies could be swapped in other ways for example by swapping the polaroids P1 and P2 over, or by further electronic processing the error signal V3.

The error signal V3 is passed to a proportional and integral control circuit 26 (known per se) which derives a control signal 28 from the input error signal V3. The amplifier 24 and the circuit 26 constitute a scheduling means for producing the control signal. The control signal 28 is passed via a part of a pre-heat control unit 29 (which is more fully described below) through a power transistor 30 to the heater coil 18.

The heater coil heats the laser tube in accordance with the signals received from the control system to maintain the length of the laser cavity to minimise any changes in the ratio of the intensities of the two beams. Since the power curve defines the frequencies of the two modes in dependence upon their intensities, it can be seen that stabilisation of the ratio of the intensities will stabilise the two frequencies.

The remainder of the control system shown in the drawing consists of a laser pre-heating system which includes the control unit 29 which is switched on to pre-heat the laser to reduce its warm up time, and further control circuits 34 and 36 which determine the rate of heat input during, and the duration of, the pre-heat cycle. The objective of the control system is to switch from pre-heat mode to control mode when the laser is at its required temperature in excess of ambient temperature. At this point the heater must still be capable of providing sufficient heat input to satisfy the control mode operation.

As the laser warms up to its operating temperature after it has been switched on, the laser cavity length will increase. As described above with reference to Fig 1, the increase in cavity length will cause the modes of operation of the laser to "pass through" the power curve. Between mode changes the increase in cavity length will cause a cyclic change in the frequencies and, hence the intensities of the modes under the power curve. The cyclic change of intensities will be picked up by the two photo-diodes D1 and D2.

We have devised a control system which makes use of this information from the photo-diodes to produce rapid heating of the laser cavity up to the required temperature above the ambient temperature without recourse to measuring the ambient temperature or the temperature of the laser directly. The system relies on the fact that the rate of increase in temperature of the laser cavity is dependant on the difference between the rate of heat input from the heater 18, and the rate of heat loss to the surroundings from the laser cavity.

Thus with a constant rate of heat input to the laser, the higher the temperature of the laser above ambient temperature the slower will be its rate of increase in temperature. The rate of increase in temperature is therefore indicative of the difference between the temperature of the laser and that of its surroundings. Since the frequency of the cyclic change of intensities is directly related to the rate of increase in the temperature of the laser cavity, it can be seen that measurement of the frequency of this cyclic change provides a measure of the rate of change of temperature of the cavity. Also it can be seen that the actual temperature rise of the laser tube is proportional to the number of modes which have "passed through" the power curve since the heater was turned on.

Measurement of said cyclic frequency is performed by control circuit 34 which is basically a window comparator used to detect zero crossings of the error signal V3. The window comparator includes two comparators 40 and 42, each of which is arranged to receive the error signal V3 and compare it with a reference voltage respectively referenced V ref 1 and V ref 2. The window comparator is arranged to produce an output V4 in the form of a pulse when the error signal is small. During the warm-up period the error signal V3 cycles about zero volts (see Fig 4a), and the pulsed output signal V4 will be generated each time the error signal crosses the zero volts reference (see fig 4B). It will be clear that the frequency of the V4 pulses will vary as the frequency of the cyclic error signal varies.

Control circuit 36 is a missing pulse detection circuit (known per se) which is set to provide a change in output signal when the pulse rate of the pulsed output V4 falls below a threshold level. The output of control circuit 36 is used therefore initially to override the heater control signal 28 from circuit 26 and to allow the pre-heat control unit 29 to supply power continuously to the heater coil. Once the threshold rate of pulsed output V4 is reached however, the output of control circuit 36 switches the control circuit 29 out, and control of the heater coil 18 is then taken over by control circuit 26.

It will be clear from the above that no addit ional sensors or associated circuitry are required for measurement of ambient temperatures with the pre-heat control circuit of the present invention, while the laser is nevertheless controlled to operate at its optimum temperature in excess of the ambient temperature.

The control circuit 36 described above may alternatively be a microprocessor or other counter/timer circuit, which would allow more sophisticated analysis of the zero crossings of the signal V4.

For example, the initial rate of rise of tube temperature can be inferred from the initial zero crossing frequency, which can then be used as an indication of the initial temperature of the tube relative to ambient temperature. This gives an indication as to whether or not the tube is already warm perhaps from a previous period of operation and can be used to provide a signal to limit the pre-heat cycle to a specific period of time before stabilising it in order to prevent excessive heating of the tube.

In addition, the total number of zero crossings may be counted to provide a measure of the rise in tube temperature from the start of the heating cycle. This can be used as an alternative method of limiting the pre-heat cycle by controlling the heater to operate only until a certain number of zero crossings have been counted after switching the laser on, using the output of the frequency monitoring circuit to give an indication of the intial temperature of the tube relative to ambient temperature.

In an alternative embodiment of the invention the laser may be a single frequency laser. In this case the error signal V3 will not necessarily cycle around zero as the modes pass through the gain curve. Thus the control circuit may be modified to include a circuit which will artificially create zero crossings in the error signal.

The word laser as used throughout this specification is to be understood in its broadest sense, i.e. to encompass devices generating electro-magnetic radiation throughout the spectrum from the far infra-red to the ultra-violet.

Claims

CLAIMS:

1. Laser beam generating apparatus comprising a laser beam generator, a heater for heating the laser cavity and a heater control system, characterised in that said control system comprises means for initiating a pre-heat cycle on the laser, and means for detecting the cyclic variations in intensity of a resonant mode or modes of the laser output as the length of the laser cavity changes during the pre-heat cycle and for producing an output dependent upon the frequency of said cyclic variations and a control circuit which receives the output of the detector means and determines the duration of the pre-heat cycle.

2. Laser beam generating apparatus as claimed in Claim 1 in which the means for detecting the cyclic variations in intensity comprises a photo-diode detector which produces and output voltage in dependence on the intensity of a resonant mode of the laser and means for receiving the output voltage of the photo-diode detector and which produces an output therefrom which is proportional to the intensity of the mode.

3. Laser beam generating apparatus as claimed in Claim 1 or Claim 2 and in which the laser beam generator produces a dual frequency laser beam and the means for detecting the cyclic variations in intensity comprises two photo-diode detectors each of which respectively produces an output voltage in dependence upon the intensity of one of the two modes, and an error signal generator which receives the output voltages of the two photo-diode detectors and produces an output proportional to the difference in the intensities of the two modes.

4. Laser beam generating apparatus as claimed in Claim 3 and in which a differential amplifier is used to produce said error signal from the output voltages of the photo-diodes.

5. Laser beam generating apparatus as claimed in Claim 3 or Claim 4 and in which the control system additionally comprises a pulse generator for receiving the error signal and for producing a pulsed output therefrom.

6. Laser beam generating apparatus as claimed in Claim 5 and in which the pulse generator provides a pulse each time the error signal approaches zero.

7. Laser beam generating apparatus as claimed in Claim 5 or Claim 6 and in which the control circuit for determining the duration of the pre-heat cycle comprises a missing pulse detection circuit which provides a signal for terminating the pre-heat cycle when the pulse frequency drops below a pre-determined threshold.

8. Laser beam generating apparatus as claimed in Claim 5 and in which the control circuit comprises a micro-processor which receives the output of the pulse generator and is programmed to determine from the initial frequency of the pulses received an indication of the initial laser temperature, and to determine the duration of a pre-heat cycle which will provide a pre-determined temperature difference between the laser and ambient temperature in dependence upon the initial laser temperature indicated.

9. Laser beam generating apparatus as claimed in

Claim 8 and in which the micro-processor is programmed to provide a signal for terminating the pre-heat cycle after a time interval sufficient to achieve the pre-determined temperature difference.

10. Laser beam generating apparatus as claimed in Claim 8 and in which the micro-processor is programmed to provide a signal for terminating the pre-heat cycle after receipt of a further pre-determined number of pulses from the pulse generator to achieve the pre-determined temperature difference.

11. Laser beam generating apparatus as claimed in Claim 5 and in which the control circuit comprises a counter/timer circuit which receives the output of the pulse generator and which determines the initial frequency of the pulses received, and provides a signal for terminating the pre-heat cycle after a further pre-determined time interval or pulse count.