Homework 2 - Nonobviousness Analysis - Selected Patent
3,463,924 (Culshaw et al.): Opposite Circularly-Polarized Optical Heterodyne Detection System
This patent describes a helium-neon laser that uses a magnetic field to produce two oppositely polarized oscillating components with a wide difference between their frequencies. These two beam components can be split and recombined to produce optical heterodyne signals. It was intended to be used for measurements and communications. A coil is wrapped around the laser and attached to a battery, which generates the magnetic field used to produced the oppositely polarized oscillation modes. Quarter wave plates and polarizing beam cubes are used to split and recombine the two beam components for measurements. The test beam is reflected off a vibrating mirror, and the Doppler shifts of the vibrating mirror can be measured by a photodetector.
The patent claims that the system is an improvement on prior art for the following reasons:
- the large difference between the frequencies of the two components makes the system more stable
- in measuring vibrations, both the sign and the magnitude of the measured Doppler shift can be determined, whereas in prior art only the magnitude could be determined
- low frequency noise can be filtered out by intermediate frequency filtering