EP1012540A1

EP1012540A1 - Optical transducer, method and laser diode arrangement

Info

Publication number: EP1012540A1
Application number: EP97919531A
Authority: EP
Inventors: Ian Hugh White; Roger Phillip Griffiths
Original assignee: Rdp Electronics Ltd
Current assignee: Rdp Electronics Ltd
Priority date: 1996-04-23
Filing date: 1997-04-23
Publication date: 2000-06-28
Also published as: AU2397097A; WO1997040344A1

Abstract

An optical transducer for determining distance or displacement of a mirror (3) comprises a laser diode (1) which is excited at constant current. The spectrum of the voltage signal across this laser diode is analysed and the spacing between this laser diode and the mirror (3) is determined from the formula f = cm/2L where f is the frequency; c is the speed of light; m is the mode number and L is the effective optical length of the cavity. A force or other transducer can be coupled to the mirror (3) to enable force or other parameters to be determined. In another embodiment the laser beam is conducted to a remote optical cavity of variable length by an optic fibre and the frequencies in the remote cavity are measured by analysing the voltage signal across the laser diode.

Description

OPTICAL TRANSDUCER. METHOD AND LASER DIODE ARRANGEMENT

In general, optical displacement sensors, particularly those of non-contact form, exhibit excellent operating characteristics when used for long range applications, such as in rangefinding, or in very short range applications of a millimetre or less, when very accurate interferometric techniques can be used. In the intermediate region however, it has been difficult to determine simple robust optical techniques for absolute position sensing. Various advanced interferometric techniques have been developed but these typically measure displacement from some known point rather than absolute values, or are complex and expensive. Triangulation schemes have been adopted but these have limited accuracy unless very precisely constructed, and the alignment tolerances between target and sensor head are severe.

An object of the present invention is to provide an apparatus and method in which at least some of the above disadvantages are overcome.

A further object is to provide a method and apparatus which can detect or measure parameters other than length.

In one aspect the invention provides transducer apparatus comprising: a) means for generating a laser beam in a laser cavity whose optical length is a function of an external parameter; b) means lor measuring a frequency of a lasing mode in said laser cavity, and c) output means coupled to an output signal from said measuring means and arranged to indicate said parameter.

In one embodiment the external parameter is a distance or displacement which determines or alters the length of the optical cavity. In other embodiments the optical length (defined as L.n, where L is the length of the cavity and n is the refractive index of the material of the cavity) is affected by changes in refractive index in, e.g. a body of material whose refractive index is sensitive to temperature. Hence temperature can be measured. In further embodiments a movable element (e.g. a movable reflector) of the optical cavity is mechanically coupled to a transducer, e.g. a force transducer to enable force or any other variable detectable by the transducer (velocity for example) to be detected or measured.

In another aspect the invention provides a method of detecting or measuring a parameter comprising the steps of generating a laser beam in a laser cavity whose optical length is a function of said parameter, measuring a frequency of at least one lasing mode in said laser cavity, and generating an output signal which is dependent on said measured frequency and indicative of said

parameter.

Preferably the frequency of one or more external cavity modes is measured. In a particularly preferred embodiment the laser beam is transmitted via a fibre optic cable, e.g. through a hazardous, electπcally noisy or electrically sensitive environment, to an external cavity whose optical length is sensitive to the parameter of interest. The laser beam is preferably generated by a laser diode.

In another aspect the invention relates to diode laser apparatus having means for monitoring the electro-optical state of the laser diode. Such apparatus can optionally incorporate the transducer apparatus of the first aspect of the mvention but is also applicable to laser diodes which are not coupled to transducers.

Accordingly in a third aspect the invention provides an RF diode laser arrangement comprising RF coupling means arranged to couple an RF output of a laser diode to electπcal monitonng circuitry, the electrical monitoπng circuitry being arranged :n use io monitor an electro-optical condition withm the lasei diode.

Preferred features are defined in the dependent claims

Preferred embodiments are described below by way of example only with reference to Figures 1 to 19 of the accompanying drawings, wherein:

Figure 1 is a schematic block diagram of one embodunent ol the invention; Figure 2 shows the RF spectrum of the voltage across the laser diode m Figure 1 , at different levels of excitation current;

Figure 3 is a plot of measurement error: cavity length obtained from the apparatus of Figure 1 ; using two different methods of signal processing;

Figure 4 is a schematic block diagram of another embodiment of the mvention;

Figure 5 shows the frequency spectrum of the signal developed across the laser diode in the embodiment of figure 2, at different values of displacement;

Figure 6 is a schematic block diagram of a further embodiment;

Figure 7 is a schematic block diagram of a further embodiment,

Figure 8 is a schematic block diagram of a further embodiment;

Figure 9 is a schematic block diagram of a further embodiment suitable for measunng the thickness of a film of matenal or for measunng changes in refractive index and due, e.g. to temperature changes;

Figure 10 is a schematic block diagram of a further embodiment suitable for detecting sound or vibration Figure 1 1 is a schematic block diagram of a diode laser arrangement m accordance with the third aspect of the invention;

Figure 12 is a diagram showing the transfer function of two filters used in the embodiment of Figure 11 ;

Figure 13 is a plot of the output of RF detector 42 (Figure 1 1) over time;

Figure 14 is a poweπfrequency plot of the output of filter 35 (Figure 11);

Figure 15 is a schematic block diagram of a further embodiment in accordance with the third aspect of the invention;

Figure 16 is a plot of frequency: cavity length for the embodiment of figure

15;

Figure 17 is a schematic block diagram of a further embodiment in accordance with the third aspect of the invention;

Figure 18 is a schematic block diagram of a further embodiment m accordance with the third aspect of the invention; and

Figure 19 is a schematic block diagram of a further embodiment in accordance with the third aspect of the invention. Referring to Figure 1 , a laser diode 1 is driven at constant current and generates a laser beam 4 in an external cavity defined by the front exit face of the laser diode and a plane mirror 3. Mirror 3 is movable from left to nght in Figure 1 as shown by the double-headed arrow. A lens 2 or other beam-shaping element collimates the laser beam which has external cavity frequency modes, f_m = ( 1 + α) cm/2L where c is the speed of light in free space, m is the mode number, L is the effective optical length of the external cavity and α depends on relative reflectivities of the laser facets. The lens 2 may not be necessary in some cases. The path between the laser 1 and photodiode 5 is not part of the external cavity and does not affect the frequency modes.

The above frequencies can be detected either by a photodetector 5 which analyses the beam exiting from the rear face of the laser diode or, preferably, by analysing the A.C. components of the voltage signal across the photodiode, e.g. b) digitising the signal in an analogue -to -digital converter 7, and processing the digitised signal in a digital signal processor 8 (including generating a Founer transform to determine the frequency components). AlternaUvely standard microwave instrumentation could be used to transform the frequency spectra into the time domain. Then simple purpose built circuitry can process it

The frequency spacing D between adjacent modes or the fundamental or a harmonic frequency can be determined and hence the spacing between the front end face of the laser diode 1 and mirror 3 can be determined, either by calibration or bv theoretical analysis The result is displaved on a displav 9. Figure 2 shows the frequency spectrum of the voltage across a particular laser diode a) at an excitation current of 8.2 mA and b) at an excitation current of 8.6 mA. As the excitation current is increased, higher modes appear and the spectrum shows strong components due to the beating of the modes together. The cavity length can be deteπnined either by measuring the absolute values of the mode frequencies or the difference D between them.

Figure 3 shows plots 10 and 1 1 indicating preliminary measurements using the sensors of Figure 1. Plot 10 was obtained by calculating the measurement error achieved whilst measuring the beat frequency D between different modes and plot 11 was obtained by calculating the error incurred by measuring the absolute value of the second harmonic of the fundamental frequency. A measurement range of up to 1.5 metres has been demonstrated with accuracies of 0.2% of range easily achievable. Further, careful sensor design should enable much higher accuracies to be achieved. Displacement sensitivities of 25 micrometres have been demonstrated although the ultimate displacement resolution is expected to be 1 micrometre or better. A wide range of laser diodes can be utilised, including edge emitting and surface emitting laser diodes. Other types of laser could also be used, (for example multicontact laser diodes) as could devices in which the reflectivity of the front facet is extremely low. Also the position of the photodiode need not be as shown in Figure 1. A portion of the optical beam could be tapped off using a fibre optic coupler, allowing the photodiode to monitor the beam directly. Figure 4 shows a further embodiment in which the laser diode 1 is mounted immediately adjacent the electronic circuitry 13 (e.g. comprising an analogue digital converter and digital signal converter as shown in Figure 1) and is coupled by a fibre optic cable 30 to an external cavity defined by a movable plane mirror 3 and the far end of the optic fibre cable. The optic fibre can be greater than 50 metres in length. The external cavity is provided with a probe 15 which is directly coupled to mirror 3 and the resulting assembly is mounted on PTFE bearings 14 to enable it to move from left to right as shown by the double headed arrow. The right hand end of the probe can accordingly be placed in contact with a surface whose position is to be determined and the frequency spectrum across the laser diode 1 will vary according to the position of the probe 15.

As a result, one is provided with a sensor in which the sensor head is only optically addressed with very low optical powers and hence can be regarded as safe.

The arrangement of Figure 4 can be operated in either of two modes. Firstly, the frequencies or beat frequencies of the long external cavity mode (i.e. those modes generated in the optic fibre between the laser diode 1 and the mirror 3! can be measured. However, this method of operation has the disadvantage that the cavity is very long and hence accuracy is degraded by any unwanted changes in the cavity, due. e.g. to flexing of the optic fibre. Accordingly, it is preferred to analyse at high resolution a secondary cavity caused by the modes defined by reflections which occur at the far end of the optic fibre (indicated at 40) and the mirror 3. If the reflections at the fibre-air interface 40 are not sufficient a partial mirror may be used to create this secondary cavity. By measuring the modal frequencies in this particular cavity, one can determine the distance between the mirror 3 and the surface 40 with high resolution.

Figure 5 shows different spectra of the frequencies due to these modes which are measured across the laser diode 1. Figure 5(a) shows the spectrum at a datum position of mirror 3 and Figures 5(b) and (d) show the spectra at positions x + 90 mm x + 180 mm and x + 300 mm respectively. It will be seen that the spacing D¹ between the peaks of these envelopes is directly related tc the position of mirror 3.

Figure 6 shows a further embodiment in which the plane mirror is mounted on PTFE bearings in a similar arrangement to that shown in Figure 4. However, the laser diode 1 is incorporated in the cavity. Since the cavity is sealed rhere ^{3 S no} danger of escape of laser radiation.

Figure 7 shows a further embodiment in which the position of an external assembly comprising an annular magnet assembly 17 mounted on linear bearings 16 is determined. Mirror 3 is mounted on an internal bearing assembly within a tube 1 8. This internal beaπng assembly comprises bearings 14¹ and (e.g. of PTFE) includes ferromagnetic material which ensures that the mirror 3 follows the movements of the annular magnet assembly 17 which are indicated by the double headed arrow.

In order to avoid the necessity of accurate alignment of any reflecting target (such as the mirror 3) with the optical axis of the cavity, a retroreflector can be mounted on the target surface and an embodiment utilising this principle is illustrated in Figure 8. A laser diode 1 is mounted in a sensor head 12 which also carries a plane mirror 31, and a retroreflector 10 is on a target 11. The movement of this target 1 1 and hence the retroreflector 10 is indicated by the double headed arrow. A laser beam 4 is confined within a folded cavity defined by the exit face of laser diode 1 , the reflecting surfaces of retroreflector 10 and the surface of mirror 31. It has been found that the orientation of the target 1 1 can vary by up to 50 degrees before the accuracy of the position measurement of the target is impaired.

A further embodiment is shown in Figure 9, wherein the laser beam 4 is directed to an external cavity defined by a body of material 20 located on a support 32. Hence external cavity is defined by the body of material 20 and the modes of the operation of the beam 4¹ can be analysed to determine the thickness in a manner similar to that illustrated in Figure 5 above.

The body of material 20 can be, for an example, a thin film of light transmissive material. Alternatively it could be a bodv of maierial whose reflective index is sensitive to temperature. Since the optical length of the cavity determines the frequency spectrum of the signal across laser diode 1 , any changes in refractive index and hence changes in temperature can be measured or detected remotely

Figure 10 shows a further embodiment of the invention wherein the refractor defining the far end of the cavity is a lightweight element 3, which may be a freely suspended diaphragm or a ribbon for example. Sound from a sound source S will cause it to oscillate and hence to vary the length of the cavity (in a variant, variations in the refractive index of a body of material can be detected) and such variations can be detected and analysed by electromc circuitry 13. For example, the frequency spacing between different modes can be determined at a rapid rate (e.g. 40 kHz or greater) and the resulting output signal can be differentiated to generate a digital signal representative of the instantaneous velocity of the element 3¹. In this manner the arrangement can function as a digital microphone.

In a vanant of the above embodiment, the laser could be dnven in sucn a manner as to have a narrow frequency hnewidth. Oscillation o! the element 3 due to sound from sound source S will cause a vanation in the hnewidth. This variation can be measured, enabling the arrangement to function as a digital microphone.

Figure 1 1 shows a laser diode arrangement compπsing circuitry 13 suitable lor use with the diode laser 1 of any of the preceding embodiments. Lasei 1 is /GB97/01123

12

connected to a DC and RF port of a bias T network 33 which feeds a DC bias to the laser from a suitable constant current source 34 (connected to its DC port) and outputs an RF signal from laser 1 via its RF port to circuitry comprising RF amplifier 45, bandpass filter 35 and a further RF amplifier 36.

An automatic gain control (AGC) system (37 - 40) is utilised, if required, to maintain the power of the RF at the bandpass filter 35 at a constant level. The output from the bandpass filter is then filtered by a high pass 'shaping' filter 41. This may be any filter whose transfer function vanes as a function of frequency over the passband of the bandpass filter. An example of this is a simple high pass filter whose transition region coincides with the passband of the bandpass filter as shown in Figure 12.

Thus it can be seen that the amplitude of the RF output of the shaping filter is dependent upon the frequency of the mode in question. Therefore an RF power meter or other suitable detector device 42, such as for example, a RF Schottky detector diode is used to provide a measure of the frequency of the particular external cavity mode. This feeds a DC amplifier and a data acquisiuon card 44 in a personal computer (not shown).

An example of a typical output from such a power detecting component is given m Figure 13. Prior calibration may be utilised to provide a look-up table in order to calibrate the output of the RF power detecting component. Digital signal processing (DSP) lineaπsation techniques may then be used m some applications The filter decoding method has been demonstrated to be capable of achieving resolutions of less than 1 micrometre. However, with more specialised filter systems, specifically designed for small range / high resolution operation, this figure can be substantially improved. Therefore it is envisaged that certain embodiments may provide sub-micron accuracy in the future.

A further embodiment in which decoding of the output of the optical displacement sensor is performed directly is shown in Figure 15. The peak frequency of the RF signal produced by the sensor is measured using a frequency meter 46. This embodiment provided a digitally coded output signal proportional to the length of the external cavity.

This embodiment could consist of either a single or multichannel system. Each channel could monitor the frequency of a particular cavity mode generated by the sensor. To achieve this each channel would be pre-filtered with a bandpass filter 35 to isolate the particular mode of interest. Figure 14 shows a typicai frequency spectrum produced after bandpass filtenng. In this case the fundamental cavity mode has been isolated.

This spectrum consists of both measurement noise inherent in the sensor output signal and amplifier noise introduced by the signal processing.

Frequency meters operate on the principle that dunng a defined gate time the numbers of transitions or crossings between threshold levels are counted. Therefore, the output of the frequency meter is dependent upon not only the frequency of the signal mode of interest, but also the noise distribution in the passband. In order to alleviate the effects of noise, as shown in Figure 15, the amplified sensor output signal is bandpass filtered by filter 35 to select the one peak of interest, amplified by RF amplifier 36 and only then is its frequency measured with frequency meter 46.

A typical output obtained with such a system is shown in Figure 16. Typical results show that the output accuracy is better than 1 part in 10000 for a 1 second gate time. However, as this example details a generalised system, this suggests that better resolution is potentially obtainable.

The digital output from the frequency meter may not provide a direct measurement of the peak signal frequency, due to the presence of the system noise, m this case the output could be compared to previously obtained calibration data for linearisation purposes. DSP (Digital Signal Processing) techniques could also be used to enhance performance.

Figure 17 shows a phase locked-loop based system. This could in fact be a slight variant on the frequency meter decoding system of Figure 15. In this embodiment, a phase-locked loop comprising a phase -sensitive detector 50 coupled to an input of a controller 47 is used to lock a local oscillator 48 to the peak frequency of the input signal The frequency of this local oscillator is then measured using a frequency meter 46 which has an input connected to an output of a splitter .

Again the system has the potential of being single or multi- chanelled, with a bandpass filter 35 used to select the particular mode of interest

In this decoding embodiment, the measurement is made upon a clean local oscillator signal, and therefore this system possesses inherently better noise immunity than the simple frequency counter system. Thus, once the phase- locked loop has achieved its initial lock, it should be able to track the position of the external cavity mode as long as it stays within the passband of the bandpass filter. If the lock is held, then an extremely accurate measurement may be made since the frequency measurement is made upon the extremely low noise local oscillator signal which originated from the laser.

The embodiment of Figure 18 utilises heterodyne decoding. An amplifier 45 feeds the signal to an image-rejection bandpass filter 35' and thence to an input of a mixer 51 whose outer input is coupled to the output of a local oscillator 48 The frequency of local oscillator 48 is varied in a sawtooth fashion by a ramp generator 55.

Low pass filter 35' blocks any unwanted image frequencies which may degrade the heterodyne process. In the most simple embodiment of the heterodyne system the local oscillator is swept periodically in a sawtooth fashion and its output mixed with the input signal. As the input to the local oscillator is swept through its range of frequencies, different input frequencies are successively mixed to pass through an IF amplifier 52 and bandpass Filter 53. The signal is then detected by a detector 54. Therefore the frequency spectrum can be output as a time domain waveform. With the use of simple timing circuitry (not shown) which correlates the instantaneous frequency of local oscillator 48 with the instantaneous amplitude of the signal detected by detector 54, a measurement can then be made of the frequency separation of the external cavity modes, thus providing position information.

In some applications the use of timing circuitry may not provide a precise enough measurement. In this case a FFT based system could be used. In this variant, the output from the heterodyne system would be input to an A/D converter and acquired by a computer for digital signals processing (DSP). An FFT or other suitable algonthm could be performed which would be used to assess the repetition rate of the time domain signal. This is m reality the frequency separation of the time domain signal. The resolution of such a system may be enhanced using standard DSP techniques such as zero padding. To date this embodiment has been used in an off-line situation and has produced results accurate to better than 0.2% of full measurement range.

One further embodiment of the heterodyne decoding principle is shown in Figure 19 and is arranged to perform heterodyne peak tracking. In this embodiment, the input signal is subiected to initial signal processing (if required), by an automatic gain controlled amplification block 56 and is then bandpass filtered by a filter 35' in order to produce a signal which consists of only one external cavity mode which remains at a constant power at the input to a mixer 51 of the heterodyne stage. Heterodyne detection is then carried out in such a way that the mixer output IF frequency corresponding to the external cavity peak remains at the IF output of the mixer. In order to do this, the amplitude of the amplified IF output signal from IF amplifier 52 is analysed and connected in a feedback loop comprising an IF bandpass filter 53, power detector 54 and peak detection controller 57 which controls the frequency of a local voltage controlled oscillator (VCO) 48. The peak detection controller 57 senses changes in the detected power and changes the control voltage of the local oscillator in such a manner as to move its output frequency towards the peak of the filtered RF input signal. The frequency of the voltage controlled oscillator is then measured using a frequency meter arrangement similar to that of Figure 17 providing a stable, digital output.

Claims

1. Transducer apparatus comprising: a) means (1) for generating a laser beam (4) in a laser cavity whose optical length is a function of an external parameter; b) means (7, 8; 13; 35, 42; 46; 46, 47, 48, 50; 48, 54, 55) for measuring a frequency of an alternating signal corresponding to at least one lasing mode in said laser cavity; and c) output (9) means coupled to an output signal from said measuring means and arranged to indicate said parameter.

2. Apparatus as claimed in Claim 1 , wherein said generating means (1) is coupled to an enclosed optical cavity having a movable reflector (3) therein, the position of the movable reflector determining the optical length of said enclosed optical cavity and a displacement probe (15) outside said enclosed optical cavity being mechanically coupled to said movable reflector.

3. Apparatus as claimed in Claim 2, wherein said displacement probe (15) is coupled to said movable reflector (3) by magnetic means (17).

4. Apparatus as claimed in Claim 1, wherein said laser beam generating means (1) is arranged to transmit said laser beam to an external laser cavity whose optical length is a function of the parameter to be measured.

5. Apparatus as claimed in Claim 4, comprising an optic fibre (30) which optically couples said laser beam generating means to said external laser cavity.

6. Apparatus as claimed in any of Claims 4 or 5, wherein a retroreflector (11) defines an end of a laser cavity.

7. Apparatus as claimed in any preceding Claim, wherein said laser beam generating means comprises a laser diode (1) and said measuring means (7, 8; 13; 35, 42; 46; 46, 47, 48, 50; 48, 54, 55) is arranged to measure an alternating electric signal in said laser diode.

8. Apparatus as claimed in any preceding Claim, wherein said laser cavity is arranged to lase in two or more modes and said measuring means (7, 8) is arranged to measure the frequency of a mode which is a harmonic of the fundamental mode or to measure the frequency interval between lasing modes.

9. Apparatus claimed in any preceding Claim, wherein said laser beam has a microwave frequency.

10. Apparatus as claimed in any preceding Claim, which is arranged to measure distance and /or displacement and /or velocity and /or ^rreleration.

11. Apparatus as claimed in any preceding claim, comprising a movable element (3) which determines said optical length, said movable element being coupled to a force transducer (60).

12. Apparatus as claimed in Claim 11 , which is arranged to measure weight or pressure.

13. Apparatus as claimed in any preceding claim, comprising diaphragm means (31) arranged to determine said optical length.

14. Apparatus as claimed in Claim 13, wherein said diaphragm means (31) is responsive to sound, the apparatus being arranged to function as a microphone.

15. Apparatus as claimed in any of Claims 1 to 9, wherein the laser cavity whose optical length is a function of the parameter to be measured includes an element or body of material (20) whose refractive index is dependent upon said parameter.

16. A method of detecting or measuring a parameter comprising the steps of generating a laser beam in a laser cavity whose optical length is a function of said parameter, measuring a frequency of at least one lasing mode in said laser cavity, and generating an output signal which is dependent on said measured frequency and indicative of said paramerer.

17. A method as claimed in Claim 16, wherein said laser beam is generated in an enclosed optical cavity having a movable reflector (3) therein and the position or displacement of a probe ( 15) coupled to the movable

reflector of said enclosed optical cavity is determined by measuring said frequency, said probe extending outside said enclosed optical cavity.

18. A method as claimed in Claim 17, wherein said laser beam is transmitted to an external laser cavity whose optical length is a function of said parameter.

19. A method as claimed in Claim 18, wherein a retroreflector (11) defines an end of said external laser cavity.

20. A method as claimed in any of Claims 16 to 19, wherein the frequency of a harmonic of the fundamental lasing frequency or the frequency interval between lasing modes is measured.

21. A method as claimed in Claim 18, wherein said external cavity is defined by a film of light-transmissive material (20) and the thickness of said film is measured.

22. A method as claimed in any of Claims 16 to 20, wherein the measured frequency is analysed to determine distance and/or displacement and/ or velocity and /or acceleration.

23. A method as claimed in Claim 22, wherein sound or vibration is detected or analysed by determining changes in the optical length of a cavity susceptible to sound or vibration.

24. A method as claimed in any of Claims 18 to 20, wherein said optical length is determined by a movable element (3) which is coupled to a force transducer (60), whereby the force acting on said transducer is detected or measured.

25. A method as claimed in any of claims 16 to 24, wherein said laser beam is generated remotely from the optical cavity whose optical length is a function of said parameter.

26. Transducer apparatus substantially as described hereinabove with reference to any of Figures 1 to 10 of the accompanying drawings.

27. A method of detecting or measunng a parameter substantially as described hereinabove with reference to any of Figures 1 to 10 of the accompanying drawings.

28. An RF diode laser arrangement comprising RF coupling means (33) aπanged to couple an RF output of a laser diode (1) to electrical monitoring circuitry ( 13) , the electrical monitoring circuitry being arranged in use to monitor an electro-optical condition within the laser diode.

29. A diode laser arrangement according to Claim 28 wherein said electrical monitoring circuitry ( 13) is arranged to monitor the frequency of a lasing mode or the frequency interval between two or more lasing modes within the laser diode ( 1).

30. A diode laser arrangement according to Claim 29 wherein said electrical monitoring circuitry includes a local oscillator (48) having a variable output frequency which is coupled to an output of the laser diode ( 1) in such a manner as to ensure a predetermined relationship between said output frequency and the frequency of said lasing mode or said frequency interval.

31. A diode laser arrangement according to claim 30 wherein said local oscillator (48) is included in a phase-locked loop (47, 48, 50) which is coupled to an RF output of the laser diode.

32. A diode laser arrangement according to any of Claims 28 to 31 , further comprising a mixer (51) arranged to heterodyne said RF output with an RF signal and detecting means (54) arranged to detect tiie output signal of the mixer.

33. A diode laser arrangement according to Claim 32, further comprising means (55) for varying the frequency of said RF signal in a predetermined manner and detecting means (54) arranged to generate a time domain output signal.

34. A diode laser arrangement according to Claim 33, further comprising a feedback loop comprising said mixer (51), a detector (54) coupled to an output of the mixer, and a variable frequency local oscillator (48) having a control input coupled to the output of the detector and having an output coupled to an input of the mixer.

35. A diode laser arrangement according to any of claims 28 to 34, further comprising filter means (41) having a slope in its transfer function at the frequency of a lasing mode of the arrangement and a detector (42) responsive to the output of the filter means and sensitive to said frequency by virtue of said slope.

36. A diode laser arrangement according to any of Claims 28 to 36. further comprising digital frequency detection means (46) arranged to generate a digital output.

37. A diode laser arrangement according to any of Claims 28 to 36, wherein said laser diode ( 1) is coupled to an RF and DC input port oi^" a bias T network (33) and said monitoring circuitry (13) is coupled to an RF port of the bias T network.

38. A diode laser arrangement as claimed in any of Claims 28 to 37, wherein said laser diode (1) is incorporated in transducer apparatus as claimed in any of Claims 1 to 15 or 26.

39. A diode laser arrangement substantially as described hereinabove with reference to Figures 1 1 to 18 of the accompanying drawings.