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Heterodyne Laser Frequency Modulation Spectroscopy With Automatic Phase Correction

IP.com Disclosure Number: IPCOM000099605D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 98K

Publishing Venue

IBM

Related People

Horne, DE: AUTHOR [+3]

Abstract

This disclosure provides a means for performing quantum-limited laser FM spectroscopy [1] with low optical power and excess-noise-free photodiode detectors, thus avoiding the excess noise limitations resulting from the use of avalanche photodiodes that are normally required when the sample under study can only tolerate low optical powers.

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Heterodyne Laser Frequency Modulation Spectroscopy With Automatic Phase Correction

       This disclosure provides a means for performing
quantum-limited laser FM spectroscopy [1] with low optical power and
excess-noise-free photodiode detectors, thus avoiding the excess
noise limitations resulting from the use of avalanche photodiodes
that are normally required when the sample under study can only
tolerate low optical powers.

      The solution presented here uses heterodyne detection, in which
a high power, frequency-shifted local oscillator beam is brought
along a separate path to the photodiode detector and only a low-power
phase- modulated beam passes through the sample.  Referring to Fig.
1, the single-frequency light from a tunable laser is split by
beamsplitter BS1 into two beams.  One beam (the "FM" beam) propagates
through an electro-optic phase modulator EO driven at frequency nm to
produce FM sidebands as is well-known in the art.  Only two sidebands
are shown, spaced +nm and -nm from the laser frequency nL . This beam
is attenuated to the mW level, as required, and is sent through the
sample of interest.  The other beam (the "LO" beam) is sent through
an acousto-optic modulator AO driven at nAO, and the first
Bragg-diffracted beam is selected with an aperture, producing a beam
of frequency nL + nAO as is well-known in the art.  (The other
first-order beam at nL - nAO may also be used.)  This beam does not
propagate through the sample and should be in the mW power range in
order to maintain quantum-limited sensitivity at the detector.

      The two beams are then recombined on beamsplitter BS2 and
detected in a balanced detector that is insensitive to amplitude
fluctuations in the LO beam as is well-known in the art (2). The
detectors can be PIN or other photodiodes that have no excess noise.
The electrical difference between the two photocurrents (produced in
a hybrid T or 180- degree power combiner) then produces an RF signal
that is unaffected by the low-frequency amplitude noise in the LO
beam.

      The spectrum of the RF currents after amplifier A1 then
contains components at nAO, nAO + nm, nAO - nm .  These three signals
form an RF copy of the FM spectrum of...