Heterodyne Detection

Heterodyne detection, also known as coherent detection, is a method used in Free Space Optics (FSO) to enhance signal detection. It works by combining an incoming optical signal with a locally generated reference signal, known as a local oscillator (LO), at the receiving end. This allows for the detection of weak signals with greater sensitivity, which is especially useful in long-distance or noisy environments, like satellite communication or long-distance FSO links.

The frequency of the receiving detector’s LO is slightly different (offset) from the incoming signal at a fixed amount. The incoming optical signal and reference signal are combined inside the detector. When combined they form an interference pattern—similar to what happens when you drop two pebbles into a pond, and each pebble creates ripples that spread out. Where the ripples from both pebbles meet, they combine to form a new pattern of waves. In some places, the waves might get bigger (constructive interference), and in other places, they might cancel each other out (destructive interference).

In heterodyne detection, the interference created by combining the known reference signal (the LO) and the incoming optical signal produces a new wave, called the intermediate frequency (IF). The IF contains both the amplitude and phase information of the original signal, which are like the size and timing of the ripples. By measuring this interference pattern, the detector can recover the original data with greater accuracy. This mixed signal is then detected by a photodetector, which converts it into an electrical signal. The IF signal is further processed to extract the original data sent by the FSO system.

In simpler terms, inside the detector, a known reference signal (LO) is created. Since the properties of the LO are known, any differences between the IF (which is a mix of the LO and the incoming signal) must come from the incoming signal. By measuring this difference, the detector is able to recover the incoming signal more effectively than it could by detecting it directly. This technique amplifies the signal and improves the signal-to-noise ratio (SNR).

Additionally, the narrow frequency range of the LO allows for the use of narrowband filters to remove noise from the incoming signal, improving clarity. Heterodyne detection also enables more advanced modulation schemes, where changes in phase and frequency can be measured with higher precision.

The amplitude, or the strength of the wave, and its phase, or timing, are referred to as quadratures. Quadratures are two different ways of describing parts of the wave’s motion. For example, the motion of a swing on a swing set could be described by how far forward or backward it is, which is similar to the amplitude of a wave. You could also describe how fast the swing is moving at any given moment, which is related to the phase of the wave. Knowing only one value wouldn’t fully describe the swing’s motion—its speed could be zero either at rest or when it’s at its highest point moving forward or backward.

In quantum communication, heterodyne detection is valuable because it can measure both the amplitude and phase (or both quadratures) simultaneously, which is essential for certain protocols like Continuous-Variable Quantum Key Distribution (CV-QKD) that require complete information about a quantum state.

Homodyne Detection

Homodyne detection differs from heterodyne detection in that the frequency of the LO is identical (or very close) to that of the incoming signal. As a result, there is no intermediate frequency to measure. Instead, the signal is directly converted from an optical signal to an electrical signal without needing to process a frequency offset.

The advantage of homodyne detection is that it simplifies the receiver design and can provide higher theoretical sensitivity since all of the signal’s power is used directly for detection. However, it requires very tight synchronization (phase locking) between the LO and the incoming signal, making it more sensitive to phase noise and jitter.

In quantum communication, homodyne detectors can only measure one quadrature (either amplitude or phase) at a time. While this limits the ability to measure both quadratures simultaneously, it results in less measurement noise, which can be beneficial for protocols that prioritize precision, such as Discrete-Variable Quantum Key Distribution (DV-QKD).

References

https://www.rp-photonics.com/optical_heterodyne_detection.html

https://physics.stackexchange.com/questions/394228/how-does-homodyne-detection-work-in-quantum-optics

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=908299