Utilization of a modulated laser beam in heterodyne interferometry

A method of observing an electro-optical frequency response characteristic of a workpiece, comprising

• (a) irradiating a molecular (or atomic) population of a portion of the workpiece with first and second laser beams respectively having first and second wavelengths both in a range of approximately 250 nm to approximately 4000 nm whereby to subject said population to a composite carrier beam of a mean frequency midway between the first and second wavelengths, said carrier beam being amplitude modulated at a beat frequency equal to the difference between the first and second wavelengths whereby to obtain from said population a response beam modulated at the beat frequency;
• (b) multiplying the composite modulated carrier beam with said response beam and detecting the average of such multiplication over time;
• (c) introducing a variable length time delay into one of the beams to cause a shift in the relative phase of the carrier and response beams and varying this length to obtain a plot of different values for said averages as a function of the relative phase;
• (d) ascertaining the maximum and minimum values of said averages;
• and
• (e) measuring the degree of modulation of said response beam by measuring the modulation depth of said plot of the different values for said averages, said modulation depth being given by the ratio between the amplitude of, and the mean value of, said maximum and minimum values.

A method of measuring a beat frequency of two laser beams of different wavelength, comprising

• (a) combining first and second laser beams respectively having first and second wavelengths both in a range of approximately 250 nm to approximately 4000 nm but different from each other to generate a carrier beam of a mean frequency halfway between the first and second wavelengths, said carrier beam being modulated at a beat frequency equal to the difference between the first and second wavelengths;
• (b) splitting said carrier beam into two portions;
• (c) multiplying said two beam portions together and detecting the average of such multiplication over time;
• (d) introducing a shift into the relative phase of the beam portions and varying this phase shift to obtain a plot of different values for said averages as a function of the relative phase;
• and
• (e) measuring the spacing between two consecutive maximum or minimum values of said averages to measure the beat frequency.

A method of measuring distance comprising

• (a) combining first and second laser beams respectively having first and second wavelengths both in a range of approximately 250 nm to approximately 4000 nm but different from each other to generate a carrier beam of a mean frequency halfway between the first and second wavelengths, said carrier beam being modulated at a beat frequency equal to the difference between the first and second wavelengths;
• (b) splitting said carrier beam into two portions;
• (c) multiplying said two beam portions together and detecting the average of such multiplication over time;
• (d) locating said distance in one of said beam portions to introduce a relative phase shift between the beam portions being multiplied and obtaining a further average of the multiplication with said phase shift applied to the beam portions;
• and
• (e) comparing the value of this further average with the value of the average obtained without the phase shift to determine the length of the phase shift and hence said distance.

A method of communication comprising

• (a) combining first and second laser beams respectively having first and second wavelengths both in a range of approximately 250 nm to approximately 4000 nm but different from each other to generate a carrier beam of a mean frequency halfway between the first and second wavelengths, said carrier beam being modulated at a beat frequency equal to the difference between the first and second wavelengths;
• (b) splitting said carrier beam into two portions;
• (c) multiplying said two beam portions together and detecting the average of such multiplication over time;
• (d) introducing a time delay into one of the beam portions being multiplied together;
• and
• (e) digitally switching back and forth at least one of the beat frequency, the mean frequency and the length of the time delay to introduce a relative phase shift into the beam portions being multiplied to obtain different values for said averages that represent the respective digital values.

Apparatus for observing an electro-optical frequency response characteristic of a workpiece, comprising

• (a) a pair of lasers for subjecting a molecular (or atomic) population of a portion of the workpiece to a composite carrier beam of a mean frequency midway between wavelengths of the lasers amplitude modulated at a beat frequency equal to a difference between said wavelengths to obtain from said population a response beam modulated at the beat frequency;
• (b) means for multiplying the composite carrier beam with said response beam, including means for detecting the average of such multiplication over
• c) time delay means located in one of said beams;
• (d) means for varying the length of said time delay and for plotting different values for said averages for different lengths of time delay;
• and
• (e) means for measuring the modulation depth of said plot for the different values of said averages as a ratio between the amplitude of, and the mean value of, the maximum and minimum values of, said plot to provide a measure of the degree of modulation of the response beam.

Apparatus for measuring distance, comprising

• (a) a pair of lasers for generating a composite carrier beam of a mean frequency midway between wavelengths of the lasers amplitude modulated at a beat frequency equal to a difference between said wavelengths;
• (b) means for splitting said composite beam into two portions;
• (c) means for multiplying said two beam portions together and for detecting the average of such multiplication over time;
• (d) means for locating said distance in one of said beam portions to introduce a relative phase shift between the beam portions being multiplied for obtaining a further average of the multiplication with said phase shift applied to the beam portions;
• and
• (e) means for comparing the value of this further average with the value of the average obtained without the phase shift for determining the value of the phase shift and hence said distance.

Apparatus for digital transmission of data on a laser beam, comprising

• (a) a pair of lasers for generating a composite carrier beam of a mean frequency midway between wavelengths of the lasers amplitude modulated at a beat frequency equal to a difference between said wavelengths;
• (b) means for splitting said composite beam into two portions;
• (c) means for multiplying said two beam portions together and for detecting the average of such multiplication over time;
• (d) digital input means for switching back and forth at least one of the beat frequency, the mean frequency and the length of the time delay for introducing a relative phase shift into the beam portions being multiplied to obtain different values for said averages that represent the respective digital values.