Glossary entry

Balanced detection

A photodetection scheme using two photodiodes with the difference of their photocurrents taken as the output. Cancels common-mode signals and noise, doubling sensitivity for coherent detection.

A balanced photodetector consists of two photodiodes in series (or with their outputs subtracted electronically). Signal is split equally between the two diodes via an optical hybrid; the photocurrents are subtracted:

i_\text{balanced} \;=\; i_1 - i_2 \;=\; \mathcal{R} (P_1 - P_2).

For coherent detection, the optical hybrid divides the input as:

P_1 \;=\; \frac{1}{2}|E_S + E_{LO}|^2, \qquad P_2 \;=\; \frac{1}{2}|E_S - E_{LO}|^2.

The difference cancels the large DC LO offset and doubles the beat signal:

i_\text{balanced} \;\propto\; 2 \, \text{Re}\!\left[E_S \, E_{LO}^*\right].

Why balance helps.

Noise/signal source	Single-detector	Balanced
Coherent beat signal	$\propto \sqrt{P_S P_{LO}}$	$2 \sqrt{P_S P_{LO}}$ — doubled
LO intensity noise (common-mode)	Limits sensitivity	Cancelled to first order
LO RIN	Adds to noise floor	Cancelled to first order
Detector dark current	Adds noise	Both detectors contribute, but signal also doubled
Detector shot noise	$\propto \sqrt{P_{LO}/2}$	$\propto \sqrt{P_{LO}}$ (twice the photons, but added in quadrature)

The 3 dB SNR improvement comes from: signal doubling (6 dB) minus shot noise doubling (3 dB) = 3 dB net.

Critical hybrid balance. Balanced detection works only when the optical hybrid splits signal and LO with high common-mode rejection. Typical commercial balanced detectors specify $\sim$ 25 – 30 dB common-mode rejection ratio (CMRR) at the operating wavelength. Below this, residual imbalance leaves LO intensity noise contaminating the output.

Applications.

Use case	Why balanced
Coherent telecom receivers	Standard architecture for QPSK and higher-order coherent reception
Optical homodyne measurement	LO RIN suppression is essential for shot-noise-limited detection
RIN measurement	Self-balanced configurations measure laser RIN by canceling correlated noise
OCT (swept-source)	LO from reference arm, signal from sample arm — balanced detection rejects LO sweep-induced fluctuations
Quantum-noise-limited measurements	Required for sub-shot-noise quantum metrology

Practical balanced detectors are commonly packaged with both diodes on a single substrate, with closely-matched responsivity and bandwidth, and integrated transimpedance amplifiers operating in differential mode. Modern coherent telecom transceivers integrate four balanced detector pairs (in-phase and quadrature, for two polarizations) into a single silicon photonic IC.

The balanced detection scheme is to coherent receivers what differential signaling is to high-speed digital electronics — common-mode noise rejection is the architectural advantage in both cases.