Frequency-feedback cavity enhanced spectrometer
A spectrometer comprising an optical cavity, a light source capable of producing light at one or more wavelengths transmitted by the cavity and with the light directed at the cavity, a detector and optics positioned to collect light transmitted by the cavity, feedback electronics causing oscillation of amplitude of the optical signal on the detector at a frequency that depends on cavity losses, and a sensor measuring the oscillation frequency to determine the cavity losses.
1. A spectrometer comprising:
an optical cavity;
a light source capable of producing light at one or more wavelengths transmitted by said cavity and with the light directed at said cavity;
a detector and optics positioned to collect light transmitted by said cavity;
feedback electronics causing oscillation of amplitude of the optical signal on said detector at a frequency that is less than the free spectral range of the cavity and that depends on cavity losses; and
a sensor measuring the oscillation frequency to determine the cavity losses.
2. The spectrometer of claim 1 wherein said light source is an LED.
3. The spectrometer of claim 1 wherein said light source is a SLED.
4. The spectrometer of claim 1 wherein said light source is a laser.
5. The spectrometer of claim 1 wherein said light source is a lamp.
6. The spectrometer of claim 1 wherein amplitude of said light source is modulated.
7. The spectrometer of claim 1 wherein wavelength or frequency of said light source is modulated so as to modulate the light transmitted by the cavity.
8. The spectrometer of claim 1 wherein said cavity comprises at least two mirrors enclosing a gaseous sample.
9. The spectrometer of claim 1 wherein said cavity comprises at least two mirrors enclosing a liquid sample.
10. The spectrometer of claim 1 wherein said cavity comprises at least two mirrors enclosing a suspension.
11. The spectrometer of claim 1 wherein said cavity comprises one or more surfaces that use total internal reflection arranged so that an evanescent field is in contact with a sample.
12. The spectrometer of claim 1 wherein said cavity comprises an optical fiber made so that an evanescent field of said optical fiber can interact with a sample.
13. The spectrometer of claim 1 wherein said cavity comprises an integrating sphere.
14. The spectrometer of claim 1 additionally comprising one or more amplifiers and/or phase shifters arranged to create self-oscillation at a target frequency.
15. The spectrometer of claim 1 additionally comprising a phase locked loop.
16. The spectrometer of claim 15 wherein said phase locked loop generates a clock signal that is an integer ratio of the oscillation frequency.
17. The spectrometer of claim 16 wherein said clock signal is used to generate a phase-shifted modulation frequency.
18. The spectrometer of claim 17 wherein said clock signal produces a number of preset phases.
19. The spectrometer of claim 18 wherein said preset phases are chosen to keep the oscillation frequency within a target range of frequencies.
20. The spectrometer of claim 18 wherein the oscillation frequency is measured at a number of preset modulation phases and the resulting frequencies are combined to determine an estimate of cavity loss that is substantially independent of multiexponential behavior of the cavity.