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Suppression of Residual Amplitude Modulation in Laser Frequency Modulation Measurement Techniques

IP.com Disclosure Number: IPCOM000061546D
Original Publication Date: 1986-Aug-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Gehrtz, M: AUTHOR [+2]

Abstract

The sensitivity inherently available in FM laser measuring techniques is greatly reduced by Residual Amplitude Modulation (RAM). RAM produces a non-zero background signal that can mask the FM measurement signal and contains low-frequency laser noise. Optical and electronic methods for suppressing RAM in FM laser measuring are described, both of which result in commensurately enhanced sensitivity. Optically, a frequency modulated laser beam is split into a sample beam and a reference beam which are directed into separate optical paths. The phase of the modulated sample beam is adjusted to cancel RAM when recombined with the reference beam and directed onto a photodetector. One implementation of the optical suppression technique is shown in the figure.

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Suppression of Residual Amplitude Modulation in Laser Frequency Modulation Measurement Techniques

The sensitivity inherently available in FM laser measuring techniques is greatly reduced by Residual Amplitude Modulation (RAM). RAM produces a non-zero background signal that can mask the FM measurement signal and contains low- frequency laser noise. Optical and electronic methods for suppressing RAM in FM laser measuring are described, both of which result in commensurately enhanced sensitivity. Optically, a frequency modulated laser beam is split into a sample beam and a reference beam which are directed into separate optical paths. The phase of the modulated sample beam is adjusted to cancel RAM when recombined with the reference beam and directed onto a photodetector. One implementation of the optical suppression technique is shown in the figure. Two beams of FM laser light are produced by beam splitter 11 from light received from FM laser source 10. The sample beam is directed along path 20 through sample 12. The reference beam is directed along path 22 through variable attenuator 13 and polarization rotator 14. Beam splitter 15 recombines the sample and referenced beams and transmits the recombined beam to photodetector 16. If sample 12 were not in path 20, photocurrents of the sample and reference beams are given by:

(Image Omitted)

In these relations, the subscripts r and s refer to reference and sample, respectively, ir and is are the photocurrents, Ir and Is are beam intensities, and lr and ls are the path lengths of paths 22 and 20, respectively. In addition, R is the RAM index, lrf is the radial frequency of the RF oscillator producing the FM laser light, wR is the phase shift between the RAM and the FM generating oscillator, h is the photodiode sensitivity, and c is the speed of laser light in the path medium. In all-optical RAM suppression, the path length difference is given by: Wl = ls - lr
(3) which is set by positioning sample beam path steering mirrors 17 and 18. When the sample beam is adjusted for a 180 degree RF-phase-shift with respect to the reference beam, the two sine terms in equations (1) and (2) have opposite signs and the path length difference is equal to one-half the RF wavelength, namely,

(Image Omitted)

When the intensities of the two beams are adjusted to the same value using the variable attenuator 13, the total photocurrent of the recombined beams is given by: The total photocurrent is constant in time and with homodyne detection yields zero signal. In the all-optical double-beam RAM suppression method described, interferometric effects on the...