Coherent differential absorption lidar (dial)

• US 7,397,568 B2
• Filed: 09/08/2003
• Issued: 07/08/2008
• Est. Priority Date: 09/10/2002
• Status: Active Grant

• Current Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Original Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Inventors: Harris, Michael, Willetts, David V, Bryce, David J, Pearson, Guy N
• Primary Examiner(s): Lee; Hwa S
• Application Number:
  US10/527,329
• Publication Number:
  US 20060011840A1
• Time in Patent Office: 1,765 Days
• Field of Search: 356/28, 356/28.5, 356/484, 356/437
• US Class Current: 356/484
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01S 17/87 Combinations of systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 17/88 Lidar systems specially adapted for specific applications
• CPC Class Codes: G01S 7/4818 using optical fibres
• CPC Class Codes: G01S 7/487 Extracting wanted echo signals , e.g. pulse detection
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: Y02A 90/10 Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Abstract

A coherent differential absorption lidar (DIAL) device 2 comprises a transmit portion 4 for directing a combined light beam to a remote target and a receive portion 5 for receiving light returned from the remote target and for coherently mixing the received light with its associated local oscillator beam. The combined beam comprises at least two component light beams of discrete wavelengths. The device further comprises a signal correction means. The signal correction means comprising a means for extracting a portion of each component light beam from the transmit portion, a means for introducing a frequency difference 62 between each extracted component light beam and its associated local oscillator beam and a means for directing the extracted beam into the receive portion. This provides an additional correction signal thereby improving device performance.

Claims

1. A coherent differential absorption lidar (DIAL) device comprising:

a transmit portion for directing a combined light beam, comprising at least two component light beams of discrete wavelengths, to a remote target and providing a local oscillator beam associated with each component light beam,

a receive portion for receiving light returned from the remote target and for coherently mixing the received light with its associated local oscillator beam, and

a signal correction means comprising;

a means for extracting a portion of each component light beam from the transmit portion,

a means for introducing a frequency difference between each extracted component light beam and its associated local oscillator beam and

a means for directing each extracted component light beam into the receive portion.

2. A DIAL device according to claim 1, and further comprising a means for introducing a frequency difference between each of the at least two component light beams and the associated local oscillator beam.

3. A DIAL device according to claim 1 wherein the combined light beam is routed through a fibre optic cable prior to transmission to the remote target.

4. A DIAL device according to claim 1 wherein the transmit portion focuses light on the remote target using a first optical arrangement and the receive portion collects light from the remote target using a second optical arrangement.

5. A DIAL device according to claim 1 wherein each of the at least two component light beams is generated by a discrete laser source.

6. A DIAL device according to claim 1 wherein the transmit portion comprises one laser source, a means for dividing the light beam output by the laser source into at least two component light beams and a means for introducing a frequency difference between said component light beams.

7. A DIAL device according to claim 1 wherein one or more of the means for introducing a frequency difference comprises an acousto-optic modulator.

8. A DIAL device according to claim 1 and further comprising at least one polarisation controller configured so as to control the polarisation state of the received light and/or the extracted component light beam with respect to the polarisation state of the associated local oscillator beam.

9. A DIAL device according to claim 1 wherein the transmit portion further comprises at least one optical amplifier to amplify the intensity of one or more of the at least two component light beams.

10. A DIAL device according to claim 1 wherein the signal correction means additionally comprises at least one delay line.

11. A DIAL device according to claim 1 in wherein the wavelength of one of the at least two component light beams is selected to coincide with a peak in absorption of a gas species of interest.

12. A method of providing a normalisation signal in a coherent DIAL device having a transmit portion for directing a combined light beam comprising at least two component light beams of discrete wavelengths to a remote target and providing a local oscillator beam associated with each component light beam and a receive portion for receiving light returned from said remote target and for coherently mixing the received light with said associated local oscillator beam, the method comprising the steps of;

extracting a portion of each component light beam from the transmit portion of the device,

introducing a frequency difference between each extracted component light beam and said associated local oscillator beam, and

inputting each said extracted component light beam into the receive portion of the device.