GB2108348A - Doppler lidar - Google Patents
Doppler lidar Download PDFInfo
- Publication number
- GB2108348A GB2108348A GB08229505A GB8229505A GB2108348A GB 2108348 A GB2108348 A GB 2108348A GB 08229505 A GB08229505 A GB 08229505A GB 8229505 A GB8229505 A GB 8229505A GB 2108348 A GB2108348 A GB 2108348A
- Authority
- GB
- United Kingdom
- Prior art keywords
- frequency
- radiation
- transmitter
- optical system
- laser
- Prior art date
- Legal status (The legal status 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 status listed.)
- Granted
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/10—Systems determining position data of a target for measuring distance only using transmission of interrupted, pulse-modulated waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/50—Systems of measurement based on relative movement of target
- G01S17/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Optical Radar Systems And Details Thereof (AREA)
Abstract
In a doppler lidar, light of frequency f1 is emitted from both ends (3, 4) of a laser (1). The transmitted beam (25) is periodically translated and frequency shifted by an acousto-optical device (7, 8, 9, 10) so as to produce a sequence of pulses of frequency fD = f1 a long beam (27, 29). The received beam (30, 31) of frequency f2 is mixed with the heterodyne beam (26,32) by half-silvered mirror (19). The composite beam falls on a detector (20) and a signal of frequency f1-f2 passes through a filter (24) to processing circuitry (23) to yield range and/or speed information. In order to prevent backscattered light of frequency fD passing through the laser (1) and mixing with the heterodyne beam (26,32) the frequency fD must not equal kc DIVIDED 2L, and preferably equals (2k+1)c DIVIDED 4L, where L is the length of the resonant cavity (2) of the laser (1), k is an integer, and c is the speed of light. <IMAGE>
Description
SPECIFICATION
Transmitting-receiving laser device for heterodyne detection
The present invention relates to a transmittingreceiving laser device for heterodyne detection.
It is known that it is possible to measure the distance of a target by directing towards it a pulse from a laser transmitter and by measuring the interval of time which elapses between the release of the pulse and the return of an echo of this pulse from the target.
In heterodyne detection devices, the transmission frequency of the laser transmitter is slightly out of phase relative to that of a local oscillator, and the echo reflected by the target is directed towards a photoelectric receiver which also receives the output from the local oscillator. The receiver then produces signals which are modulated at a frequency which is equal to the difference between the frequency of the echo signal and that of the local oscillator. The result is a considerable increase in the ratio of signal to noise at reception.
In order to avoid problems of synchronisation, a single laser is often used to perform the functions of transmitter and of local oscillator.
For this, the energy of the beam from a single laser is divided into two parts with the aid of an optical blade arranged at 45 degrees to the path of the beam. The first part passes through an optical system which produces a pulse which is slightly out of phase relative to that of the laser, this pulse being directed towards the target. The second part is directed towards the receiver in order to be mixed with the echo reflected by the target.
However, the device described above has one disadvantage. The optical system causes parasitic reflections of the phase displaced signal. These reflections spread in the opposite direction to the transmitting beam towards the laser and are reflecting by the optical blade onto the receiver. This results in high level jamming which interferes greatly with the measurements required.
The object of the present invention is to mitigate this disadvantage.
According to the present invention a transmittingreceiving laser device for heterodyne detection, comprising:
a continuous laser transmitter capable of producing radiation offrequencyf1,
an optical system arranged at the output of the transmitter for receiving a first part of the radiation of Frequency f1 and to produce in response a radiation pulse the frequency of which is different to f1, for transmission to a target, the optical system comprising at least one dioptical surface which is capable of returning to the transmitter parasitic reflections of the output pulse from the optical system,
a photoelectric detector arranged close to the transmitter in order to receive an echo, of frequency f2, from the target,
means for directing to the detector a second part of the radiation of frequency f1 in order to produce heterodyne detection, the detector being suitable for producing in response to said echo of frequency f2 and to the second part of the radiation of frequency f, an electrical signal which is modulated at a frequency of f1-f2, and a circuit for processing the modulated electrical signal produced by the detector, is characterised in that the lasertransmitter comprises a resonant optical cavity comprising two opposite partially transparent mirrors, such that in operation the first part of the radiation from the transmitter issues from a first mirror and the second part of this radiation issues from the second mirror, and that the frequency of the output pulse from the optical system is equal to a predetermined value different from
kc 2L where L is the distance between the first and the second mirrors, c is the speed of the light and k is any integer.
A particular embodiment of the present invention will now be described by way of example with reference to the accompanying drawing which shows a schematic representation of the device.
In this figure, a laser transmitter 1 of the continuous type comprises a laser tube 2 the ends of which are closed respectively by two opposite mirrors 3 and 4which are centered on an axis 6 and which form a resonant optical cavity. According to one arrangement of the invention, the two mirrors 3 and 4 are partially transparent. The tube 2 contains an active laser gas which is capable of being excited by known means schematically represented at 5.
The device includes a germanium crystal 7 positioned on the axis 6 in such a way that an inlet face of the crystal receives the laser radiation issuing from the cavity by the mirror 4. The crystal 7 is provided with two electro-acoustic transducers 8 and 9 which are connected to an electrical supply circuit 10. A diphragm 11 is arranged in such a way asto allow the radiation issuing from the other face of the crystal 7 to pass through its opening 12. This radiation then passes through an afocal optical transmitting system composed of a divergent lens 13 and of a convergent lens 14 centered on an axis 15 parallel to the axis 6.
An afocal optical receiving system is arranged on an axis 16 parallel to the axis 15 close to the afocal optical transmitting system. The receiving system also comprises a divergent lens 17 and a convergent lens 18.
An optical blade 19 is inclined at 45 to the axis 16 in such a way as to reflectthe received radiation towards a photoelectric detector 20 through a focusing lens 21.
A reflecting mirror 22 is inclined at 45 to the axis 6 in such a way as to reflect to the lens 21 and the detector 20 through the optical blade 19, the laser radiation issuing from the cavity by the mirror 3.
The electrical output of the detector 20 is connected to a processing circuit 23 through a filter 24.
The device described above and illustrated in the drawing functions in the following manner.
The continuous laser radiation of frequency f, transmitted by the transmitter 1 comprises two beams 25 and 26 issuing respectively from the two
partially transparent mirrors 4and 3 of the resonant
optical cavity.
The circuit 10 applies a series of elearical pulses in
succession to the input of the two tmnsducers 8 and
9 in such a way as to create an acoustic wave which
spreads in the crystal 7. When the transducers 8 and
9 are energised, the interaction of the acoustic wave
and the beam 25 penetrating the admission face of
crystal which is inclined to the axis 6 changes the
phase of the beam. The transducers 8 and 9 are
arranged in such a waythatthe beam 27 issuing from the crystal 7 is constantly parallel to the beam
25. The beam 27 passes through the opening 12 of the diaphragm 1.
When the transducers 8 and 9 are not energised, that is, in the intervals of time which separate the
pulses from the circuit 10, the beam 25 passes through the crystal 7 in accordance with a path illustrated by broken line in the figure, to be absorbed by the solid part of the diaphragm 11. The beam 27 is formed therefore by a series of successive pulses of laser radiation of frequency fD which is out of phase relative to f1. The beam 27 then passes through the afocal system 13-14which produces a beam 29 which is less divergent but of larger cross section.
The beam 29 is then directed towards a target by known means (not shown).
The target reflects part of the energy of the laser pulses in the form of a beam 30 which impinges on the lens 18 of the afocal receiving system. The beam 31 of frequency f2 from the lens 17 is reflected by the
optical blade 19 and is then focused by the lens 21
onto the sensitive surface of the detector 20.
The beam 26 of frequency f1 from the mirror 3 of the laser cavity is reflected by the mirror 22 to form a
beam 32 which passes through the optical blade 19 to be likewise focused by the lens 21 onto the sensitive surface of the photoelectrical detector 20. This detector, which is of the quadratic type, produces at its output an electrical signal modulated at the frequency f2-f1. The filter 24 is of the band pass type and enables the modulating signal to be extracted.
The device shown in the figure is designed to measure the radial speed of a target. In this case, the circuit 23 enables this speed to be determined as a function of the variations in the frequency f241 detected, these variations resulting directly from those of the frequency f2 by Doppler effect.
It is difficult to prevent parasitic reflections, either on the exit face 33 of the crystal 7 or on a dioptric surface of the afocal emission system 13, 14. The effect of these reflections is to return a fraction of the energy of the output radiation from the crystal 7 in a direction which is opposite to that of transmission, that is, towards the transmitter 1, to the exit face 34 of the mirror 4.
In orderto prevent these reflections from passing through the cavity 3-4 to mix with the beam 26 and to affect the sensitive surface of the detector 20, the difference of frequency between the input to and the output from the crystal 7 is determined in such a way that the beam of frequency fed can be attenuated by the laser cavity. It is sufficient therefor that fD be dif ferentfrom kc
2L
where c is the speed of light
L is the distance between the mirrors and k is an
integer.
The phase displacement is preferably chosen in
such a way that is appreciably equal to
(2k + 1) c
4L
In these conditions, the parasitic pulses are greatly attenuated and their level after they have passed through the cavity from the mirror 4to the mirror 3 is sufficiently low so as not to interfere with the measurements required.
The present invention can be utilised in the production of a device for measuring the speed of a target; this device can also comprise additional known means for measuring the distance of the target.
Claims (6)
1. A transmitting-receiving laser device for heterodyne detection, comprising:
a continuous laser transmitter capable of producing radiation offrquencyf1, an optical system arranged at the output of the transmitter for receiving a first part of the radiation of frequency f1 and to produce in response a radiation pulse the frequency of which is different to f1, for transmission to a target, the optical system comprising at least one dioptical surface which is capable of returning to the transmitter parasitic reflections of the output pulse from the optical system,
a photoelectric detector which is arranged close to the transmitter in orderto receive an echo, of frequency f2, from the target,
means for directing to the detector a second part of the radiation offrequencyf, in order to produce heterodyne detection, the detector being suitable for producing in response to said echo of frequency f2 and to the second part of the radiation of frequency f1, an electrical signal modulated at the frequency of fl-f2, and a circuitfor processing the modulated electrical signal produced by the detector, characterised in that the laser transmitter comprises a resonant optical cavity comprising two opposite pa rtially transparent mirrors such that in operation the first part of the radiation from the transmitter issues from a first of these mirrors and the second part of the radiation from the transmitter issues from the second mirror and that the frequency of the output pulse from the optical system is equal to a predetermined value different from
kc
2L where L is the distance between the first and the second mirror, c is the speed of the light and k is an integer.
2. A laser device according to claim 1, characterised in that the predetermined value is appreciably equalto (2k+1)c 4L
3. A laser device according to claim 1, characterised in that the optical system comprises a crystal through which the radiation of frequency 1 passes, this crystal being provided with at least one electrooptical transducer arranged to be supplied by an electrical control circuit, the output of the crystal constituting the output pulse for transmission to a target.
4. A laser device according to claim 3, characterised in that said optical system also comprises an afocal optical system through which the radiation from the crystal is directed.
5. A laser device according to claim 1, 2, 3 or 4, characterised in that the processing circuit comprises a band pass filter which is centered on the frequency f2-f1.
6. A laser device substantially as hereinbefore described with reference to the accompanying drawing.
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR8119452A FR2514959A1 (en) | 1981-10-16 | 1981-10-16 | HETERODYNE DETECTION TRANSMITTER-RECEIVER LASER DEVICE |
Publications (2)
Publication Number | Publication Date |
---|---|
GB2108348A true GB2108348A (en) | 1983-05-11 |
GB2108348B GB2108348B (en) | 1984-12-12 |
Family
ID=9263087
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
GB08229505A Expired GB2108348B (en) | 1981-10-16 | 1982-10-15 | Doppler lidar |
Country Status (2)
Country | Link |
---|---|
FR (1) | FR2514959A1 (en) |
GB (1) | GB2108348B (en) |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4552456A (en) * | 1981-10-31 | 1985-11-12 | Nissan Motor Company, Limited | Optical pulse radar for an automotive vehicle |
EP0821246A1 (en) * | 1996-07-23 | 1998-01-28 | Commissariat A L'energie Atomique | Self-mixing laser velocimeter |
CN101166947B (en) * | 2005-02-14 | 2010-05-19 | 数字信号公司 | System and method for providing chirped electromagnetic radiation |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN107966707B (en) * | 2017-12-15 | 2024-08-30 | 杭州欧镭激光技术有限公司 | Laser ranging system |
Family Cites Families (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2254996A5 (en) * | 1973-12-18 | 1975-07-11 | Minisini Pierre | Method of measuring small phenomena - involves comparing beam modified by phenomenon with reference |
-
1981
- 1981-10-16 FR FR8119452A patent/FR2514959A1/en active Granted
-
1982
- 1982-10-15 GB GB08229505A patent/GB2108348B/en not_active Expired
Cited By (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4552456A (en) * | 1981-10-31 | 1985-11-12 | Nissan Motor Company, Limited | Optical pulse radar for an automotive vehicle |
EP0821246A1 (en) * | 1996-07-23 | 1998-01-28 | Commissariat A L'energie Atomique | Self-mixing laser velocimeter |
FR2751755A1 (en) * | 1996-07-23 | 1998-01-30 | Commissariat Energie Atomique | LASER VELOCIMETER WITH AUTODYNE DETECTION |
US5825465A (en) * | 1996-07-23 | 1998-10-20 | Commissariat A L'energie Atomique | Autodyne detection laser velocimeter |
CN101166947B (en) * | 2005-02-14 | 2010-05-19 | 数字信号公司 | System and method for providing chirped electromagnetic radiation |
Also Published As
Publication number | Publication date |
---|---|
FR2514959B1 (en) | 1983-12-02 |
FR2514959A1 (en) | 1983-04-22 |
GB2108348B (en) | 1984-12-12 |
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Legal Events
Date | Code | Title | Description |
---|---|---|---|
PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19921015 |