Method for balancing detector output to a desired level of balance at a frequency

• US 6,574,031 B1
• Filed: 12/28/1999
• Issued: 06/03/2003
• Est. Priority Date: 12/08/1997
• Status: Expired due to Fees

• Current Assignee: United States Incorporated, United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Sachse, Glenn W.
• Primary Examiner(s): Ben, Loha
• Application Number:
  US09/476,347
• Time in Patent Office: 1,253 Days
• Field of Search: 359/276, 359/279, 359/320, 359/325, 359/246, 359/245, 359/259, 356/364, 356/437, 250/343, 250/339.13
• US Class Current: 359/276
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

A multi-gas sensor is provided which modulates a polarized light beam over a broadband of wavelengths between two alternating orthogonal polarization components. The two orthogonal polarization components of the polarization modulated beam are directed along two distinct optical paths. At least one optical path contains one or more spectral discrimination elements, with each spectral discrimination element having spectral absorption features of one or more gases of interest being measured. The two optical paths then intersect, and one orthogonal component of the intersected components is transmitted and the other orthogonal component is reflected. The combined polarization modulated beam is partitioned into one or more smaller spectral regions of interest where one or more gases of interest has an absorption band. The difference in intensity between the two orthogonal polarization components is then determined in each partitioned spectral region of interest as an indication of the spectral emission/absorption of the light beam by the gases of interest in the measurement path. The spectral emission/absorption is indicative of the concentration of the one or more gases of interest in the measurement path. More specifically, one embodiment of the present invention is a gas filter correlation radiometer which comprises a polarizer, a polarization modulator, a polarization beam splitter, a beam combiner, wavelength partitioning element, and detection element. The gases of interest are measured simultaneously and, further, can be measured independently or non-independently. Furthermore, optical or electronic element are provided to balance optical intensities between the two optical paths.

Claims

1. A method for electronically processing detector output to digitally balance optical intensities to a desired level of balance at a frequency f, comprising the steps of:

amplifying the detector output, wherein the detector output comprises the baseband signal I and the difference signal Δ

I(t);

digitizing the amplified detector output;

dividing the digitized amplified detector output in real time by the balance function β

(t), where β

(t)=1+α

sin(2π

ft+φ

), where phase φ

is in phase with the Δ

I(t) signal at frequency f, and where α

is adjusted by a computer across time to achieve the desired level of balance;

synchronously demodulating the function V(t)/β

(t) at frequency f, wherein V(t) is the digitized amplified detector output;

digitally low pass filtering the digital demodulated signal to reduce the bandwidth;

averaging the V(t)/β

(t) signal using a digital low pass filter identical in characteristics to the demodulated filtering; and

transmitting the digital demodulated difference signal and the averaged baseband signal to the computer for adjustment of α

across time to achieve the desired level of balance.

2. A method for electronically processing detector output to digitally balance optical intensities to a desired level of balance at a frequency f, comprising the steps of:

amplifying the detector output, wherein said detector output comprises the baseband signal I and the difference signal Δ

I(t);

digitizing the amplified detector output;

multiplying the digitized amplified detector output in real time by the balance function γ

(t), where γ

(t)=1/β

(t) and β

(t)=1+α

sin(2π

ft+φ

), where γ

(t)=1-α

sin(2π

ft+φ

) for small α

, where phase φ

is in phase with the Δ

I(t) signal at frequency f, and where α

is adjusted by a computer across time to achieve the desired level of balance;

synchronously demodulating the function V(t)/γ

(t) at frequency f, wherein V(t) is the digitized amplified detector output;

digitally low pass filtering the digital demodulated signal to reduce the bandwidth;

averaging the V(t)/γ

(t) signal using a digital low pass filter identical in characteristics to the demodulated filtering; and

transmitting the digital demodulated signal and the averaged signal to the computer for adjustment of α

across time to achieve the desired level of balance.

3. A method for processing detector output to balance optical intensities to a desired level of balance at a frequency f, comprising the steps of:

amplifying the detector output, wherein the detector output comprises the baseband signal I and the difference signal Δ

I(t), and wherein the amplified detector output is the analog function denoted V(t) having DC and AC components;

analog dividing the amplified detector output V(t) output in real time by the balance function β

(t), where β

(t)=1+α

sin(2π

ft+φ

), where phase φ

is in phase with the Δ

I(t) signal at frequency f, and where α

is adjusted by a computer across time to achieve the desired level of balance;

synchronously demodulating the function V(t)/β

(t) at frequency f, wherein the synchronous demodulation is analog;

separately low pass filtering the DC and demodulated AC components of the V(t)/β

(t) signal;

converting the filtered DC and AC signals from analog to digital representation; and

transmitting the digital representation to the computer for processing.

4. A method for electronically processing detector output to balance optical intensities to a desired level of balance at a frequency f, comprising the steps of:

amplifying the detector output, wherein the detector output comprises the baseband signal I and the difference signal Δ

I(t), and wherein the amplified detector output is the analog function denoted V(t);

analog multiplying the amplified modifier output in real time by the balance function γ

(t), where γ

(t)=1/β

(t) and β

(t)=1+α

sin(2π

ft+φ

), where γ

(t)=1-α

sin(2π

ft+φ

) for small α

, where phase φ

is in phase with the Δ

I(t) signal at frequency f, and where α

is adjusted by a computer across time to achieve the desired level of balance;

synchronously demodulating the function V(t)/γ

(t) at frequency f wherein the synchronous demodulation is analog;

separately low pass filtering the DC and demodulated AC components of the V(t)/γ

(t) signal;

converting the filtered AC and DC signals from analog to digital representation; and

transmitting the digital representation to the computer for processing.