Photodetector Series: Introduction to Balance Photodetector

Introduction to Balance Photodetector (Optoelectronic Balance Detector)
Balance Photodetector can be divided into fiber optic coupling type and spatial optical coupling type according to the optical coupling method. Internally, it consists of two highly matched photodiodes, a low-noise, high bandwidth transimpedance amplifier circuit module, and an ultra-low noise power module. It has the characteristics of high common mode rejection ratio, ultra-low noise, and high bandwidth, and is widely used in the field of coherent optical communication. It has become a research hotspot for enterprises and universities in various countries in recent years.
Working principle of Balance Photodetector (Optoelectronic Balance Detector)
The Balance Photodetector uses two photodiodes in reverse bias state as the light receiving unit. When receiving a light signal, the photocurrent generated by two photodiodes is subtracted and coupled to a transimpedance amplifier to convert the current signal into a voltage signal for output. The use of self reducing structure can effectively suppress the common mode signal introduced by local oscillator light and dark current, increase the differential mode signal, and to some extent improve the detection ability of weak light signals.
Advantages: High common mode rejection ratio, high sensitivity, and high detection bandwidth can meet various application scenarios.
Disadvantages: Low saturated optical power, only suitable for weak light detection, integration needs to be improved.

FIG: Working principle diagram of balance detector
Performance parameters of Balance Photodetector (Optoelectronic Balance Detector)
1. Responsiveness
Responsiveness refers to the efficiency of a photodiode in converting light signals into photocurrent, which is the ratio of photocurrent to light power. Choosing a photodiode with higher responsivity can effectively improve the sensitivity of the Balance Photodetector.
Responsiveness refers to the efficiency of a photodiode in converting light signals into photocurrent, which is the ratio of photocurrent to light power. Choosing a photodiode with higher responsivity can effectively improve the sensitivity of the Balance Photodetector.
2. Bandwidth
The bandwidth represents the signal frequency at which the output signal amplitude of the Balance Photodetector decays by -3dB, and is related to the parasitic capacitance of the photodiode, the size of the transimpedance, and the gain bandwidth product of the operational amplifier.
3. Common mode rejection ratio
Common mode rejection ratio is used to measure the degree of suppression of common mode signals by balanced detectors, and commercial products generally require a minimum common mode rejection of 25dB.
4.NEP
Noise Equivalent Power: The input signal power required at a signal-to-noise ratio of 1, which is an important parameter for measuring the noise performance of a system. The main components of balanced detector noise are optical scattering noise and electrical noise.

Application of Balance Photodetector (Optoelectronic Balance Detector)
In recent years, Balance Photodetector have been widely used in fields such as laser wind radar, laser vibration measurement, fiber optic sensing, weak light coherent detection, spectral detection, gas detection, etc. The research on the high speed, high bandwidth, low noise, high common mode rejection ratio, and high sensitivity of balanced detectors has made breakthroughs and is developing towards high integration and low power consumption to meet different application scenarios.