Traffic Sensing and Monitoring Apparatus

• US 20080277568A1
• Filed: 10/18/2006
• Published: 11/13/2008
• Est. Priority Date: 10/25/2005
• Status: Active Grant

• Current Assignee: Optasense Holdings Limited (Luna Innovations, Inc.)
• Original Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Inventors: Hill, David John, Crickmore, Roger Ian
• Granted Patent: US 7,652,245 B2
• Time in Patent Office: Days
• US Class Current: 250/227.12
• CPC Class Codes: G01D 5/35361 using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
• CPC Class Codes: G08G 1/02 using treadles built into the road pads or other sensitive devices responsive to passage of vehicles E01F11/00

Abstract

Traffic sensing and monitoring apparatus (100) comprises a length of optical fibre (112), means (102, 103, 104, 106, 107, 109, 119) arranged to introduce a series of pulse pairs into an input end of the length, each of the pulse pairs comprising first and second pulses of radiation having a frequency difference Ω and the second pulse being delayed with respect to the first by a delay τ, and the apparatus further comprising a photodetector arranged to detect radiation which is Rayleigh-backscattered within the length of fibre towards the input end to generate an output signal in response thereto. Apparatus of the invention allows sensing and monitoring of traffic at any position along a long length of road (e.g. 5 km) with lower installation and maintenance costs per unit length compared to prior art systems providing the same spatial resolution.

Claims

1. Traffic sensing and monitoring apparatus comprising a length l of optical fibre, a light source arranged to introduce a series of pulse pairs into an input end of the length, each of the pulse pairs comprising first and second pulses of radiation having a frequency difference Ω

and the second pulse being delayed with respect to the first by a delay τ

, and the apparatus further comprising a photodetector arranged to detect radiation which is Rayleigh-backscattered within the length of fibre towards the input end and to generate an output signal in response thereto.

2. Apparatus according to claim 1 wherein the light source arranged to introduce a series of pulse pairs into the length of optical fibre is arranged to operate at a rate less than or equal to v/21, preferably substantially v/21, where v is the speed of the pulses in the length of optical fibre.

3. Apparatus according to claim 1 wherein vτ

/2 substantially corresponds to the length of a unit of traffic to be sensed and monitored.

4. Apparatus according to claim 1 further comprising an interrogation system arranged to obtain a set of signal samples by sampling the photodetector'"'"'s output signal at a particular time to after the first pulse of each pulse pair enters the length l of optical fibre, and to process the signal samples to generate an output signal corresponding to the frequency or phase of a signal composed of said samples.

5. Apparatus according to claim 4 wherein the interrogation system is arranged to obtain sets of signal samples by sampling the photodetector output signal at times t₀−

Δ

t, t₀ and t₀+Δ

t after the first pulse of each pulse pair enters the length l of optical fibre and to process the set having samples corresponding to the largest signal with a frequency of approximately Ω

.

6. Apparatus according to claim 5 wherein the interrogation system is arranged to change the set which is processed from a first set to a second set when the samples of the second set correspond to a signal exceeding a signal to which samples of the first set correspond by at least a predetermined amount.

7. Apparatus according to claim 5 wherein the interrogation system is arranged such that when the set which is processed is changed from a first set to a second set, both sets are processed for a period to generate first and second output signals, the interrogation system being further arranged to evaluate the difference between the second and the first output signals and to apply a corresponding offset to the second output signal to reduce any discontinuity between the first and second output signals.

8. Apparatus according to claim 5 wherein Δ

t≦

τ

/10.

9. Apparatus according to claim 4 wherein the interrogation system is arranged to obtain a plurality of sets of signal samples by sampling the photodetector'"'"'s output signal at each of a plurality of times after the first pulse of each pulse pair enters the length 1 of optical fibre, and to process the samples of each set to generate respective output signals each corresponding to the frequency or phase of a signal composed of samples of a given set.

10. Apparatus according to claim 9 wherein consecutive times are separated in time by at least vτ

/2.

12. Apparatus according to claim 1 where the length of optical fibre is a length of polarisation-maintaining fibre and the light source arranged to introduce the series of pulse pairs into the input end of the length of optical fibre is arranged to launch polarised pulse pairs into said length of fibre such that radiation individual pulses is polarised along a principal axis of the length of fibre.

13. Apparatus according to claim 1 wherein the light source arranged to introduce a series of pulse pairs into an input end of the length of optical fibre comprises first and second acousto-optic modulators (AOMs) arranged to receive at least partially coherent radiation and to output, respectively, a first pulse of radiation of frequency ω

₁ and a second pulse of radiation of frequency ω

₂, wherein ω

₁ω

₂=ω

means for delaying the second pulse with respect to the first, and means for coupling the pulses into an optical fibre.

14. Apparatus according to claim 13 further comprising a laser and means arranged to input first and second portions of the laser'"'"'s output into the first and second AOMs respectively.

17. Apparatus according to claim 14 and further comprising an optical modulator arranged to receive the laser'"'"'s output and to output pulsed radiation to said inputting means such that each AOM receives a portion of each pulse output by the modulator and wherein each AOM is arranged to increase the extinction ratio of a pulse input to it outside a time window which includes the pulse.

18. Apparatus according to claim 17 wherein the modulator is an integrated optical modulator or a semiconductor optical amplifier.

20. Apparatus according to claim 14 wherein the laser is a pulsed laser.

22. Apparatus according to claim 1 wherein Ω

is in the range 40 kHz≦

Ω

≦

40 MHz.

11. (canceled)

15-16. -16. (canceled)

19. (canceled)

21. (canceled)

23. (canceled)

24. A method of sensing and monitoring traffic, the method comprising the steps of

(i) providing a length of optical fibre on or below a surface for carrying vehicles such that the weight of a vehicle using the surface may act on said length;

(ii) introducing a series of pulse pairs into an input end of the length, each of the pulse pairs comprising first and second pulses of radiation having a frequency difference Ω and

the second pulse being delayed with respect to the first by a delay r; and

(iii) detecting radiation which is Rayleigh-backscattered within the length of fibre towards the input end to generate an output signal in response thereto.

25. A method according to claim 24 wherein pulse pairs are introduced into said length of optical fibre at a rate less than or equal to v/21, preferably substantially v/21, where v is the speed of the pulses in the length of optical fibre.

26. A method according to claim 24 wherein vτ

/2 substantially corresponds to the length of a unit of traffic to be sensed and monitored.

27. A method according to claim 24, the method further comprising the steps of:

(i) sampling the photodetector'"'"'s output at a particular time to after the first pulse of each pulse pair enters the length l of optical fibre; and

(ii) processing samples obtained in step (i) to generate an output signal corresponding to the frequency or phase of a signal composed of said samples.

28. A method according to claim 27, the method comprising the steps of:

(i) sampling the photodetector'"'"'s output signal at times t₀−

Δ

t, to and t₀+Δ

t after the first pulse of each pulse pair enters the length l of optical fibre to generate respective sets of samples; and

(ii) processing the set having samples corresponding to the largest signal with a frequency of approximately Ω

.

29. A method according to claim 28 wherein Δ

t≦

τ

/10.

30. A method according to claim 27, the method comprising the steps of:

(i) obtaining a plurality of sets of signal samples by sampling the photodetector'"'"'s output signal at each of a plurality of times after the first pulse of each pulse pair enters the length l of optical fibre; and

(ii) processing the samples of each set to generate respective output signals each corresponding to the frequency or phase of a signal composed of samples of a given set.

31. A method according to claim 30 wherein consecutive times are separated in time by at least vτ

/2.

32-33. -33. (canceled)