In-phase/in-quadrature demodulator for spectral information of interference signal
View Patent ↗Sinusoidal in-phase and in-quadrature signals at a given spatial frequency are combined with the irradiance signals generating a correlogram of interest and integrated over the length of the correlogram data-acquisition scan. The integration outputs are then used to calculate the amplitude and the phase of the correlogram signal at the selected spatial frequency, thereby producing targeted spectral information. The signal generator used to generate the in-phase and in-quadrature sinusoidal signals may be scanned advantageously through any desired range of spatial frequencies, thereby producing corresponding amplitude and phase spectral information for the correlogram. Because the procedure produces spectral information independently of the number of data frames acquired during the interferometric scan, it is materially more rapid than conventional FFT analysis.
1. A method for extracting spectral information from an interferogram produced by a plurality of interference signals, the method comprising the following steps:
mixing the interference signals with in-phase sinusoidal signals having a predetermined spatial frequency and integrating a resulting in-phase mixed signal, thereby producing an in-phase integrated output;
mixing the interference signals with in-quadrature sinusoidal signals having said predetermined spatial frequency and a relative phase shift of substantially 90 degrees and integrating a resulting in-quadrature mixed signal, thereby producing an in-quadrature integrated output;
combining said in-phase and in-quadrature integrated outputs to calculate a spectral function corresponding to said predetermined spatial frequency.
2. The method of claim 1 , wherein said spectral function is amplitude.
3. The method of claim 1 , wherein said spectral function is phase.
4. The method of claim 1 , further comprising repeating each step for a range of spatial frequencies, thereby producing corresponding spectral function data.
5. The method of claim 2 , further comprising repeating each step for a range of spatial frequencies, thereby producing corresponding spectral amplitude data.
6. The method of claim 3 , further comprising repeating each step for a range of spatial frequencies, thereby producing corresponding spectral phase data.
7. Interferometric apparatus for extracting spectral information from an interferogram produced by a plurality of interference signals, said apparatus comprising:
a signal generator for generating in-phase sinusoidal signals having a predetermined spatial frequency and in-quadrature sinusoidal signals having said predetermined spatial frequency and a relative phase shift of substantially 90 degrees;
a signal mixer for mixing the interference signals with said in-phase sinusoidal signals and the interference signals with said in-quadrature sinusoidal signals, thereby producing an in-phase mixed output and an in-quadrature mixed output;
an integrator for integrating the in-phase mixed output and the in-quadrature mixed output, thereby producing an in-phase integrated output and an in-quadrature integrated output; and
a processor including a computer readable medium storing instructions for combining said in-phase and in-quadrature integrated outputs to calculate a spectral function corresponding to said predetermined spatial frequency.
8. The apparatus of claim 7 , wherein said spectral function is amplitude.
9. The apparatus of claim 7 , wherein said spectral function is phase.
10. The apparatus of claim 7 , further including an interferometer to produce said plurality of interference signals.
11. The apparatus of claim 10 , wherein said interferometer is a vertical scanning interferometer.
12. A computer program product encoded in an information storage medium and usable with a programmable computer processor, said product being programmed to mix interference signals with in-phase sinusoidal signals having a predetermined spatial frequency and integrate a resulting in-phase mixed signal, thereby producing an in-phase integrated output; to mix the interference signals with in-quadrature sinusoidal signals having said predetermined spatial frequency and a relative phase shift of substantially 90 degrees and integrate a resulting in-quadrature mixed signal, thereby producing an in-quadrature integrated output; and to combine said in-phase and in-quadrature integrated outputs to calculate a spectral function corresponding to said predetermined spatial frequency.
13. The product of claim 12 , wherein said spectral function is amplitude.
14. The product of claim 12 , wherein said spectral function is phase.