METHOD AND APPARATUS FOR IMPROVED SIGNAL TO NOISE RATIO IN RAMAN SIGNAL DETECTION FOR MEMS BASED SPECTROMETERS

• US 20080018890A1
• Filed: 07/24/2006
• Published: 01/24/2008
• Est. Priority Date: 07/24/2006
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Maity, Sandip, Goravar, Shivappa, Chandrasekaran, Shankar, Banerjee, Ayan, Zribi, Anis, Que, Long, Kennerly, Stacey, Claydon, Glenn
• Granted Patent: US 7,586,602 B2
• Time in Patent Office: Days
• US Class Current: 356/301
• CPC Class Codes: G01J 1/44 Electric circuits for command of an exposure part G03B7/02
• CPC Class Codes: G01J 3/02 Details
• CPC Class Codes: G01J 3/0229 using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
• CPC Class Codes: G01J 3/0232 using shutters
• CPC Class Codes: G01J 3/0256 Compact construction
• CPC Class Codes: G01J 3/0264 Electrical interface; User interface
• CPC Class Codes: G01J 3/0272 Handheld
• CPC Class Codes: G01J 3/0286 Constructional arrangements for compensating for fluctuations caused by temperature, humidity or pressure, or using cooling or temperature stabilization of parts of the device; Controlling the atmosphere inside a spectrometer, e.g. vacuum
• CPC Class Codes: G01J 3/0297 Constructional arrangements for removing other types of optical noise or for performing calibration
• CPC Class Codes: G01J 3/1895 using fiber Bragg gratings or gratings integrated in a waveguide
• CPC Class Codes: G01J 3/4338 Frequency modulated spectrometry
• CPC Class Codes: G01J 3/44 Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
• CPC Class Codes: G02B 26/04 by periodically varying the intensity of light, e.g. using choppers shutters, diaphragms for cameras G03B9/00; devices for eliminating or reducing the effect of flicker in projection systems G03B21/40
• CPC Class Codes: H01L 31/107 the potential barrier working in avalanche mode, e.g. avalanche photodiodes

Abstract

A method of Raman detection for a portable, integrated spectrometer instrument includes directing Raman scattered photons by a sample to an avalanche photodiode (APD), the APD configured to generate an output signal responsive to the intensity of the Raman scattered photons incident thereon. The output signal of the APD is amplified and passed through a discriminator so as to reject at least one or more of amplifier noise and dark noise. A number of discrete output pulses within a set operational range of the discriminator is counted so as to determine a number of photons detected by the APD.

Claims

1. A method of Raman detection for a portable, integrated spectrometer instrument, the method comprising:

directing Raman scattered photons by a sample to an avalanche photodiode (APD), said APD configured to generate an output signal responsive to the intensity of said Raman scattered photons incident thereon;

amplifying said output signal of said APD and passing said amplified output signal through a discriminator so as to reject at least one or more of amplifier noise and dark noise; and

counting a number of discrete output pulses within a set operational range of said discriminator so as to determine a number of photons detected by said APD.

2. The method of claim 1, further comprising cooling said APD by thermoelectric cooling.

3. The method of claim 2, wherein said APD is biased in a sub-Geiger mode of operation.

4. The method of claim 1, further comprising passing said output signal of said APD through a preamplifier so as to shape an output current pulse from said APD to a NIM standard pulse.

5. The method of claim 1, further comprising chopping an input optical beam prior to Raman scattering.

6. The method of claim 5, wherein said chopping is implemented through frequency modulation of an acousto-optic modulator.

7. The method of claim 5, wherein said chopping is implemented through frequency modulation of a deflectable cantilever positioned within the path of said input optical beam.

8. An apparatus for Raman detection within a portable, integrated spectrometer instrument, comprising:

an avalanche photodiode (APD) integrated on a chip, said APD configured to receive Raman scattered photons by a sample and generate an output signal responsive to the intensity of said Raman scattered photons incident thereon;

an amplifier integrated on said chip, said amplifier configured to amplify said output signal of said APD and pass said amplified output signal through a discriminator integrated on said chip, so as to reject at least one or more of amplifier noise and dark noise; and

a digitizer integrated on said chip, configured to count a number of discrete output pulses within a set operational range of said discriminator so as to determine a number of photons detected by said APD.

9. The apparatus of claim 8, wherein said APD is cooled by thermoelectric cooling.

10. The apparatus of claim 8, wherein said APD is biased in a sub-Geiger mode of operation.

11. The apparatus of claim 8, further comprising a preamplifier integrated on said chip, said preamplifier configured to shape an output current pulse from said APD to a NIM standard pulse.

12. The apparatus of claim 8, further comprising a beam-interrupting mechanism configured to chop an input optical beam prior to Raman scattering.

13. The apparatus of claim 12, wherein said beam-interrupting mechanism comprises a frequency modulated acousto-optic modulator.

14. The apparatus of claim 12, wherein said beam-interrupting mechanism comprises a frequency modulated, deflectable cantilever positioned within the path of said input optical beam.

15. A method of Raman detection for a portable, integrated spectrometer instrument, the method comprising:

directing an input optical beam incident upon a sample to be measured, said input optical beam modulated at a heterodyne frequency;

directing scattered photons by said sample through receiving optics so as to filter Rayleigh scattered photons and pass Raman scattered photons through a tunable filter; and

detecting said passed Raman scattered photons at a wavelength passed by said tunable filter through demodulation at said heterodyne frequency.

16. The method of claim 15, further comprising modulating said input optical beam with an acousto-optic modulator.

17. The method of claim 15, further comprising modulating said input optical beam with a deflectable, micro electromechanical system (MEMS) cantilever positioned within the path of said input optical beam.

18. The method of claim 17, wherein an applied voltage across said cantilever and a ground plate causes said cantilever to deflect out of the path of said input optical beam.

19. The method of claim 15, wherein said passed Raman scattered photons are detected using photon counting.

20. The method of claim 19, wherein said photon counting is implemented using an avalanche photodiode (APD).

21. An apparatus for Raman detection within a portable, integrated spectrometer instrument, comprising:

an optical source for directing an input optical beam incident upon a sample to be measured;

a beam-interrupting mechanism configured to modulate said input optical beam at a heterodyne frequency;

receiving optics configured to collect photons scattered by said sample so as to filter Rayleigh scattered photons and pass Raman scattered photons through a tunable filter; and

a photon detector configured to detect said passed Raman scattered photons at a wavelength passed by said tunable filter through demodulation at said heterodyne frequency.

22. The apparatus of claim 21, wherein said beam-interrupting mechanism further comprises an acousto-optic modulator.

23. The apparatus of claim 21, wherein said beam-interrupting mechanism further comprises a deflectable, micro electromechanical system (MEMS) cantilever positioned within the path of said input optical beam.

24. The apparatus of claim 23, wherein an applied voltage across said cantilever and a ground plate causes said cantilever to deflect out of the path of said input optical beam.

25. The apparatus of claim 21, wherein said passed Raman scattered photons are detected using photon counting.

26. The apparatus of claim 25, wherein said photon counting is implemented using an avalanche photodiode (APD).