Semiconductor diode laser spectrometer arrangement and method

• US 20050157303A1
• Filed: 04/08/2003
• Published: 07/21/2005
• Est. Priority Date: 04/09/2002
• Status: Active Grant

• Current Assignee: Emerson Process Management Limited
• Original Assignee: Cascade Technologies Ltd. (Emerson Electric Company)
• Inventors: Normand, Erwan, Duxbury, Geoffrey, Langford, Nigel
• Granted Patent: US 7,283,243 B2
• Time in Patent Office: Days
• US Class Current: 356/437
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G01J 3/4338 Frequency modulated spectrometry
• CPC Class Codes: G01N 2021/399 Diode laser
• CPC Class Codes: G01N 21/031 Multipass arrangements
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: H01S 5/06216 Pulse modulation or generation
• CPC Class Codes: H01S 5/0622 Controlling the frequency of the radiation
• CPC Class Codes: H01S 5/3402 intersubband lasers, e.g. transitions within the conduction or valence bands

Abstract

A method apparatus for sensing gases using a semiconductor diode laser spectrometer, the method comprising: introducing a sample gas into a non-resonant optical cell (17); applying a step function electrical pulse (19) to semiconductor diode laser (20) to cause the laser (20) to output a continuous wavelength chirp for injecting (16a) into the optical cell injecting (16a) the wavelengh chirp into the optical cell (17); using the wavelength variation provided by the wavelength detecting (23) light emitted from the cell (17), wherein a chirp rate is selected to substantially prevent light interference occuring in the optical cell (17).

Claims

1. A method for sensing gases using a diode laser spectrometer, the method comprising:

introducing a sample gas into a non-resonant optical cell having reflecting elements;

applying a step function electrical pulse to a semiconductor diode laser to cause the laser to output a continuous wavelength chirp for injecting into the optical cell;

injecting the wavelength chirp into the optical cell;

using the wavelength variation provided by the wavelength chirp as a wavelength scan, and detecting light emitted from the cell, wherein the method further includes using a chirp rate such that there is a time delay between spots on the reflecting elements sufficient to prevent light interference occurring in the optical cell.

2. A method as claimed in claim 1, wherein the duration of the pulse applied to the semiconductor diode laser is equal to or less than one microsecond.

3. A method as claimed in claim 1, wherein the duration of the pulse is less than the duration necessary for the optical output power to become zero after the drive pulse has been applied.

4. A method as claimed in claim 1 further involving varying the rate of change of wavelength per unit time.

5. A method as claimed in claim 4 wherein varying the rate of change of the wavelength per unit time involves varying the amplitude of the current/voltage drive pulse.

6. A method as claimed in claim 1 comprising adjusting the wavelength scan length.

7. A method as claimed in claim 6 wherein adjusting the wavelength scans involves varying the duration of the current/voltage drive pulse.

8. A method as claimed in claim 1 comprising temperature varying the semiconductor diode laser temperature.

9. A method as claimed claim 1, wherein the semiconductor diode laser has output radiation having wavelengths in the region of 1 μ

m to 14 μ

m.

10. A method as claimed in claim 1 wherein the semiconductor laser is a quantum cascade laser.

11. A method as claimed in claim 1, wherein the cell is a Herriot cell.

12. A method as claimed in claim 1, wherein the amount of radiation absorbed is determined using an amplitude measurement of radiation transmitted through the sample and an amplitude measurement of a reference pulse.

13. A semiconductor diode laser spectrometer for measuring absorption by a sample, the spectrometer comprising a semiconductor diode laser;

a non-resonant optical cell for containing a sample gas and having reflecting elements at either end thereof;

an electric pulse generator adapted to apply a substantially step function electrical pulse to the laser to cause the laser to introduce a continuous wavelength chirp into the sample cell, and a detector for detecting light output from the cell and adapted to use the wavelength variation of the wavelength chirp as a wavelength scan, wherein the chirp rate used is such that there is a time delay between spots on the reflecting elements sufficient to prevent light interference occurring in the optical cell.

14. A spectrometer as claimed in claim 13, wherein the duration of the electrical pulse is equal to or less than 1 microsecond.

15. A spectrometer as claimed in claim 13, further comprising means for varying the rate of change of wavelength per unit time of the chirp.

16. A spectrometer as claimed in claim 15 wherein the means for varying the rate of change of the wavelength are operable to vary the amplitude of the current/voltage drive pulse.

17. A spectrometer as claimed in claim 13 further comprising means for adjusting the wavelength scan length.

18. A spectrometer as claimed in claim 17 wherein the means for adjusting the wavelength scan are operable to vary the duration of the electrical pulse.

19. A spectrometer as claimed in claim 13 further comprising means for varying a starting wavelength point of the wavelength scan.

20. A spectrometer as claimed in claim 19, wherein the means for varying a starting wavelength point are operable to vary the semiconductor diode laser base temperature.

21. A spectrometer as claimed in claim 20, wherein the means for varying the temperature of the semiconductor diode laser comprise a thermoelectric heater/cooler or means for adjusting the duty cycle or the pulse repetition frequency of the repeated current/voltage drive pulses applied to the electrical contacts of the laser diode or means for adjusting the pulse amplitude of the current/voltage drive pulses or means for adjusting the base DC level of the current/voltage drive pulses applied to the electrical contacts of the laser diode.

22. A spectrometer as claimed in claim 13, wherein a beam splitter is provided to split radiation output from the laser into two components, the first component for passing through the sample and the a second component that does not pass through the sample.

23. A spectrometer as claimed in claim 13, wherein the semiconductor diode laser emits radiation having wavelengths in the region of 1 μ

m to 14 μ

m.

24. A spectrometer as claimed in claim 13, wherein the optical cell is a Herriot cell.

25. A spectrometer as claimed in claim 13, wherein the chirp has a frequency variation of about 60 GHz.

26. A spectrometer as claimed in claim 13, wherein the applied pulse has a duration that is greater than 150 ns, in particular greater than 200 ns.

27. A spectrometer as claimed in claim 13, wherein the applied pulse has a duration that is in the range of 150 to 300 ns, preferably 200 to 300 ns.