WO1997040344A1 - Transducteur optique, et procede et ensemble diode laser associes - Google Patents

Transducteur optique, et procede et ensemble diode laser associes Download PDF

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Publication number
WO1997040344A1
WO1997040344A1 PCT/GB1997/001123 GB9701123W WO9740344A1 WO 1997040344 A1 WO1997040344 A1 WO 1997040344A1 GB 9701123 W GB9701123 W GB 9701123W WO 9740344 A1 WO9740344 A1 WO 9740344A1
Authority
WO
WIPO (PCT)
Prior art keywords
laser
frequency
cavity
diode
output
Prior art date
Application number
PCT/GB1997/001123
Other languages
English (en)
Inventor
Ian Hugh White
Roger Phillip Griffiths
Original Assignee
R.D.P. Electronics Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB9608370A external-priority patent/GB2313662B/en
Priority claimed from GB9704937A external-priority patent/GB2323161A/en
Application filed by R.D.P. Electronics Ltd. filed Critical R.D.P. Electronics Ltd.
Priority to EP97919531A priority Critical patent/EP1012540A1/fr
Priority to AU23970/97A priority patent/AU2397097A/en
Publication of WO1997040344A1 publication Critical patent/WO1997040344A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light

Definitions

  • 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.
  • 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.
  • 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.
  • the external parameter is a distance or displacement which determines or alters the length of the optical cavity.
  • 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.
  • a movable element e.g. a movable reflector
  • 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.
  • 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
  • the frequency of one or more external cavity modes is measured.
  • the laser beam is transmitted via a fibre optic cable, e.g. through a hazardous, elect ⁇ cally 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.
  • the invention in another aspect 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.
  • the invention provides an RF diode laser arrangement comprising RF coupling means arranged to couple an RF output of a laser diode to elect ⁇ cal monitonng circuitry, the electrical monito ⁇ ng circuitry being arranged :n use io monitor an electro-optical condition withm the lasei diode.
  • 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 powe ⁇ frequency 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
  • 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.
  • FIG 19 is a schematic block diagram of a further embodiment in accordance with the third aspect of the invention.
  • 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.
  • 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 dete ⁇ nined 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.
  • FIG. 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.
  • 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.
  • 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.
  • a partial mirror may be used to create this secondary cavity.
  • 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 D 1 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.
  • 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 bea ⁇ ng assembly comprises bearings 14 1 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.
  • 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.
  • FIG. 9 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 4 1 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.
  • 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 3 1 . In this manner the arrangement can function as a digital microphone.
  • 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 comp ⁇ sing circuitry 13 suitable lor use with the diode laser 1 of any of the preceding embodiments.
  • Lasei 1 is /GB97/01123
  • 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.
  • 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).
  • m Figure 13 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) linea ⁇ sation techniques may then be used m some applications.
  • DSP digital signal processing
  • 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.
  • FIG. 15 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.
  • 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.
  • 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
  • FIG 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.
  • 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 .
  • 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.
  • 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.
  • the local oscillator is swept periodically in a sawtooth fashion and its output mixed with the input signal.
  • 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.
  • 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.
  • timing circuitry may not provide a precise enough measurement.
  • a FFT based system could be used.
  • the output from the heterodyne system would be input to an A/D converter and acquired by a computer for digital signals processing (DSP).
  • DSP digital signals processing
  • 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.
  • FIG. 19 One further embodiment of the heterodyne decoding principle is shown in Figure 19 and is arranged to perform heterodyne peak tracking.
  • the input signal is subjected 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.
  • 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.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

Un transducteur optique permettant de déterminer la distance ou le déplacement d'un miroir (3) comprend une diode laser (1) qui est excitée à intensité constante. On analyse le spectre du signal de tension dans ladite diode laser, et on calcule l'écartement entre elle et le miroir (3) à partir de la formule f = cm/2L, où f est la fréquence, c la vitesse de la lumière, m le numéro du mode et L la longueur optique efficace de la cavité. Une force, ou un autre transducteur, peut être couplée au miroir (3), ce qui permet de calculer ladite force ou d'autres paramètres. Dans un autre mode de réalisation, on amène par une fibre optique le faisceau laser à une cavité optique éloignée, de longueur variable, et on calcule les fréquences dans ladite cavité en analysant le signal de tension dans la diode laser.
PCT/GB1997/001123 1996-04-23 1997-04-23 Transducteur optique, et procede et ensemble diode laser associes WO1997040344A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP97919531A EP1012540A1 (fr) 1996-04-23 1997-04-23 Transducteur optique, et procede et ensemble diode laser associes
AU23970/97A AU2397097A (en) 1996-04-23 1997-04-23 Optical transducer, method and laser diode arrangement

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GB9608370A GB2313662B (en) 1996-04-23 1996-04-23 Optical transducer and method
GB9608370.4 1996-04-23
GB9704937A GB2323161A (en) 1997-03-10 1997-03-10 Laser cavity optical transducer
GB9704937.3 1997-03-10

Publications (1)

Publication Number Publication Date
WO1997040344A1 true WO1997040344A1 (fr) 1997-10-30

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EP (1) EP1012540A1 (fr)
AU (1) AU2397097A (fr)
WO (1) WO1997040344A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012013759A3 (fr) * 2010-07-28 2012-03-22 Sms Siemag Ag Dispositif de mesure et cage de laminoir ainsi que procédé d'exploitation des deux dispositifs

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5871401A (ja) * 1981-10-24 1983-04-28 Mitsubishi Electric Corp 微小変位測定装置
EP0402691A1 (fr) * 1989-05-29 1990-12-19 Rainer Dipl.-Ing. Thiessen Senseur à laser avec résonateur externe
EP0605847A1 (fr) * 1993-01-05 1994-07-13 Motorola, Inc. Capteur à réflexion d'ondes électromagnétiques
WO1995013638A1 (fr) * 1993-11-08 1995-05-18 International Business Machines Corporation Dispositif laser hybride a semi-conducteurs a cavite externe couplee

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5871401A (ja) * 1981-10-24 1983-04-28 Mitsubishi Electric Corp 微小変位測定装置
EP0402691A1 (fr) * 1989-05-29 1990-12-19 Rainer Dipl.-Ing. Thiessen Senseur à laser avec résonateur externe
EP0605847A1 (fr) * 1993-01-05 1994-07-13 Motorola, Inc. Capteur à réflexion d'ondes électromagnétiques
WO1995013638A1 (fr) * 1993-11-08 1995-05-18 International Business Machines Corporation Dispositif laser hybride a semi-conducteurs a cavite externe couplee

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
"Diodes laser: une application originale dans les capteurs.", MESURES, RÉGULATION, AUTOMATISME, vol. 49, no. 9, 1 June 1984 (1984-06-01), PARIS, FR., pages 63 - 67, XP002029605 *
PATENT ABSTRACTS OF JAPAN vol. 7, no. 164 (P - 211)<1309> 19 July 1983 (1983-07-19) *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012013759A3 (fr) * 2010-07-28 2012-03-22 Sms Siemag Ag Dispositif de mesure et cage de laminoir ainsi que procédé d'exploitation des deux dispositifs

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Publication number Publication date
EP1012540A1 (fr) 2000-06-28
AU2397097A (en) 1997-11-12

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