What is a balanced photodetector

What is a balanced photodetector
Balanced Photodetector (BPD photodetector) is a precision optoelectronic detection device based on differential detection technology, mainly used to extract weak optical signals in complex noise environments, thereby greatly improving the signal-to-noise ratio (SNR) and receiving sensitivity of the system.
1、 Core structure
A balanced photodetector mainly consists of the following three parts:
1.1 Two matched photodiodes (PDs): Receive two coherent or complementary incident light signals, requiring highly matched performance such as responsivity, dark current, and response time of the two diodes.
1.2 Differential amplifier (usually a low-noise transimpedance amplifier TIA): converts two photocurrent signals into voltage signals and performs differential amplification (subtraction) processing.
1.3 Related circuits and interfaces: including an optical splitter (used to split input light into two paths), a monitoring amplification channel (used to monitor the independent optical power of two paths), and a differential output terminal.
2、 Working principle
2.1 Optical signal splitting and conversion: The input optical signal is split into two paths by an optical splitter and input into two photodiodes respectively. These two optical signals are usually in opposite phases (such as complementary signals generated by optical mixers/interferometers).
2.2 Differential subtraction: After converting two optical signals into two currents, they are input into a differential amplifier for subtraction (V_out ∝ I1- I2).
2.3 Noise cancellation and signal retention: Common mode noise (such as laser intensity noise, background light interference, power ripple, etc.) is simultaneously superimposed on two signals, subtracted, and mutually cancelled out (approaching 0); And the weak differential signal (useful signal) is subtracted to obtain superposition, which is effectively amplified and extracted.
3、 Core advantages
3.1 High Common Mode Rejection Ratio (CMRR): It can significantly suppress common mode noise (with a suppression capability of over 30-70dB), effectively solving the problems of laser relative intensity noise (RIN) and background light interference.
3.2 High signal-to-noise ratio and high sensitivity: By differential amplification, the relative proportion of weak signals is significantly increased, making it suitable for detecting extremely weak light signals.
3.3 High linearity and wide bandwidth: Optimized circuit design, capable of accurately reproducing high-speed optical signals, with a response bandwidth up to the GHz level.
4、 Typical application scenarios
4.1 Coherent Optical Communication: Used in coherent receivers to detect the beat frequency signal obtained by mixing signal light with local oscillator light, improve reception sensitivity, and suppress local oscillator light noise.
4.2 Lidar: Used to detect the beat frequency between weak echo signals and local oscillator light, enhance weak echo detection capability, and improve the ability to resist background light and laser intensity noise.
4.3 Quantum Optics and Quantum Communication: Used for quantum state measurement, compressed light detection, quantum random number generation, etc., to achieve particle noise limited detection.
4.4 Precision optical measurement: used for optical coherence tomography (OCT system), heterodyne/homodyne interferometry, spectral analysis, etc., to improve the detection sensitivity of interference signals and weak optical signals.