What is coherent optics?
At its most basic, coherent optical transmission is a technique that uses modulation of the amplitude and phase of the light, as well as transmission across two polarizations, to enable the transport of considerably more information through a fiber optic cable. Using digital signal processing at both the transmitter and receiver, coherent optics also offers higher bit-rates, greater degrees of flexibility, simpler photonic line systems, and better optical performance.
Homodyne Detection
Homodyne detection is a method of extracting information encoded as modulation of the phase and frequency of an oscillating signal, by comparing that signal with a standard oscillation that would be identical to the signal if it carried null information. “Homodyne” signifies a single frequency, in contrast to the dual frequencies employed in heterodyne detection. When applied to processing of the reflected signal in remote sensing for topography, homodyne detection lacks the ability of heterodyne detection to determine the size of a static discontinuity in elevation between two locations. Homodyne detection is more readily applicable to velocity sensing.
Homodyne Detection in Optic
In optical interferometry, homodyne signifies that the reference radiation (the local oscillator) is derived from the same source as the signal before the modulating process. For example, in a laser scattering measurement, the laser beam is split into two parts. One is the local oscillator and the other is sent to the system to be probed. The scattered light is then mixed with the local oscillator on the detector. This arrangement has the advantage of being insensitive to fluctuations in the frequency of the laser. Usually the scattered beam will be weak, in which case the (nearly) steady component of the detector output is a good measure of the instantaneous local oscillator intensity and therefore can be used to compensate for any fluctuations in the intensity of the laser.
Key Benefits
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- Noise reduction to shot noise limit As with any small signal amplification, it is most desirable to get gain as close as possible to the initial point of the signal interception: moving the gain ahead of any signal processing reduces the additive contributions of effects like resistor Johnson-Nyquist noise, or electrical noises in active circuits. In optical heterodyne detection, the mixing-gain happens directly in the physics of the initial photon absorption event, making this ideal. Additionally, to a first approximation, absorption is perfectly quadratic, in contrast to RF detection by a diode non-linearity.
- Mapping optical frequencies to electronic frequencies allows sensitive measurements The difference frequency linewidth can be much smaller than the optical linewidth of the signal and LO signal, provided the two are mutually coherent. Thus small shifts in optical signal center-frequency can be measured: For example, Doppler lidar systems can discriminate wind velocities with a resolution better than 1 meter per second, which is less than a part in a billion Doppler shift in the optical frequency. Likewise small coherent phase shifts can be measured even for nominally incoherent broadband light, allowing optical coherence tomography to image micrometer-sized features. Because of this, an electronic filter can define an effective optical frequency bandpass that is narrower than any realizable wavelength filter operating on the light itself, and thereby enable background light rejection and hence the detection of weak signals.