WO1988001799A1

WO1988001799A1 - Frequency stabilised laser and control system therefor

Info

Publication number: WO1988001799A1
Application number: PCT/GB1987/000596
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: GB8621036D0

Abstract

In a multiple frequency laser the output frequencies are stabilised by a control system including a heater (18) for heating the laser cavity and thus varying its length, and a heater control circuit (20) which includes means for monitoring the intensities of the modes (P1, D1, P2, D2) in the cavity. Adjacent modes of the laser are orthogonally polarised and the control system further includes means for maintaining constant the ratio of the sums of the intensities of the modes in the two orthogonal polarisation planes in order to stabilise the laser frequencies. In the described embodiment the laser is a dual frequency laser having two orthogonally polarised modes and the control system maintains the ratio of the intensities of the two modes constant at a pre-determined value.

Description

FREQUENCY STABILISED LASER AND CONTROL SYSTEM

THEREFOR

The present invention relates to frequency stabilised lasers and to control systems therefor.

The frequency of a laser beam emission is so high that direct aeasurement of frequency is impractical. Thus, currently, lasers are stabilised by control systems which control the intensity of the laser beam. Since there are direct relationships, by virtue of the laser power curve, between the intensity of the laser light and its frequency, control of intensity gives a measure of control over the frequency.

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 dependance upon the detected wavelength or intensity of the emitted laser beam to minimise variation thereof. The disclosure also includes the proposal that the intensities of two longitudinal modes of a dual mode laser output may be maintained stable.

In applications of lasers in interferometry however, it is not sufficient simply to control the intensities of the two longitudinal modes of the laser to be stable (i.e. fixed in position under the power curve), because changes in power output of the laser will change the shape of the power curve and hence the intensities of the output of the two modes of the laser beam.

Under these circumstances, as will be described in more detail below, controlling the laser output to return the intensity to its fixed value will cause the frequencies of the modes to change.

It is an object of the present invention to provide a control sys tem for s tabi lis ing the two frequencies of a dual frequency laser beam by eliminating or substantially reducing, the changes in the two frequencies due to the drift in the output power of the laser.

Also a requirement often arises for some experimental purposes, not only for the frequencies of a dual frequency laser beam to be stable, but also for the two frequencies to be tuned within a range while maintaining stability of each.

It is another object of the invention to provide apparatus having the capability of tuning the two frequencies of a dual frequency laser beam and subsequently maintaining the new frequencies stable at the desired points within the tuning range.

These objects are achieved in accordance with the invention as defined in the claims appended hereto, by providing a control system for stabilising the frequencies of a multiple frequency laser beam which controls the ratio of the sum of the intensities of the modes in the two othogonal polarisation planes to a pre-determined level.

The advantages of such a system of control are that the drift in the intensities due solely to the drift in output power of the laser will not affect the frequencies of the modes, and that the laser can be tuned to different frequencies simply by changing the pre-determined ratio of intensities which the control system is set to control.

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

Fig 1 is a representat i on 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 =

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 f1 and f2 with intensities I1 and 12.

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 modes 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, themovement to the right will cause the frequencies of both modes to increase while the intensity of mode M1 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 light. 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 increased from h to h1. Similarly 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, will 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 Dl 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 subtracter 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 reIative 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 Dl 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.

Although the main control device described is a heater coil, other methods of varying the length of the laser cavity to maintain the ratio of the intensities constant may be used, for example a piezo-electric device or other mechanical fluidic or magnetic means. A Peltier Cooler may be added to the control circuit to give a greater range of control but at additional cost.

Since adjacent modes have orthogonal planes of polarisation, the principle of stabilisation of the present invention can be used in lasers with more than two modes. In this case the ratio being maintained would be the sum of the intensities of the modes in each of the two orthogonally polarised planes.

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. Apparatus for generating a laser beam having multiple stabilised frequencies, comprising a laser beam generator for producing a beam having a plurality of resonant modes of different frequencies and polarised in two orthogonal planes, and a closed loop control system which is arranged to control an operational characteristic of the laser to maintain constant the ratio of the sum of the. intensities of the modes in the two orthogonal polarisation planes thereby providing stabilisation of the frequencies of the laser beam.

2. Apparatus according to claim 1 and in which the closed loop control system comprises: detector means for detecting the intensities of the two modes in the two orthogonally polarised planes and for producing two output signals each representative of the total intensity" of the radiation in a different one of the polarisation planes, scheduling means for generating a signal indicative of the ratio of the two signals from the detector means and for generating a control signal indicative of a variation of said ratio from a pre-determined ratio and control means arranged to receive the control signal from the scheduling means and to vary the operational characteristic of the generator in accordance therewith in the sense of reducing the said variation of said ratio.

3. Apparatus according to claim 2 and in which the laser beam generator produces a dual frequency laser beam, and the closed loop control system maintains the ratio of the intensities of the two orthogonally polarised modes constant.

4. Apparatus according to claim 1 or claim 2 and in which the operational charactarisitc of the generator is the length of the laser cavity.

5. Apparatus according to claim 4 and in which the control means for varying the length of the laser cavity is a temperature controller.

6. Apparatus according to claim 5 and in which the temperature controller is a heater coil extending over at least a portion of the laser cavity.

7. Apparatus according to claim 2 and in which the scheduling means includes a differential amplifier which produces an output error signal which is proportional to the difference between the detector output signals, and a proportional and integral control circuit which receives the error signal and produces the control signal for the control means.