What are the practical uses of diodes in fiber optic communication equipment?

一, Light source driving and light signal emission
1. Electro optic conversion of semiconductor laser diodes (LDs)
Semiconductor laser diodes (LDs) achieve electro-optical conversion by injecting current and are the core light source of fiber optic communication. Its working principle is based on stimulated radiation. When the current exceeds a threshold, electron hole pairs recombine and release photons, forming coherent laser output. LD has the following technical characteristics:
Narrow spectral line width: Typical spectral width of 1-5nm, effectively suppressing pulse broadening caused by fiber dispersion, supporting high-speed transmission above 10Gbps.
High coupling efficiency: Through direct coupling or lens focusing, the coupling efficiency between LD and fiber can reach 90%, ensuring efficient injection of optical power.
Threshold characteristic: Pre bias current is required to reduce carrier establishment delay and improve modulation rate. For example, DFB lasers typically have a threshold current of 10-30mA at a wavelength of 1550nm.
2. Integrated design of laser diode components
Modern LD components integrate opto isolators, monitoring photodiodes (PDs), thermistors (RTs), and thermoelectric coolers (TECs) to form a closed-loop control system:
Optical isolator: prevents reflected light from interfering with LD and reduces noise.
Monitoring PD: Real time monitoring of LD output power, feedback to automatic power control (APC) circuit to ensure output stability.
TEC temperature control: maintains LD operating temperature through Peltier effect, compensates for temperature drift of threshold current (typical value: threshold current increases by 1-2 times between 20 ℃ and 50 ℃).
二, Optical signal detection and reception
1. The photoelectric conversion of PIN photodiodes
PIN photodiodes absorb photons through a reverse biased depletion layer, generating electron hole pairs and creating a photocurrent. Its core advantages include:
Low noise: Typical dark current value<1nA, suitable for detecting weak light signals.
Wide spectral response: silicon-based PIN diodes cover the 400-1100nm wavelength range, while indium gallium arsenide (InGaAs) - based diodes can be extended up to 1650nm.
High speed response: When the junction capacitance is less than 1pF, the bandwidth can reach GHz level and support speeds of over 40Gbps.
2. Gain mechanism of avalanche photodiode (APD)
APD achieves internal current gain (M=10-100) through carrier avalanche multiplication effect under high electric field, significantly improving receiving sensitivity:
Sensitivity advantage: At 10 ^ -12 W optical power, the signal-to-noise ratio (SNR) of APD is 10-20dB higher than that of PIN diodes.
Noise trade-off: The multiplication process introduces excess noise, and it is necessary to optimize the balance between the multiplication factor M and the noise. For example, InGaAs APD has a typical excess noise factor of F=2-3 at a wavelength of 1550nm.
3. Industrial applications of silicon photodiodes (Si PD)
Taking the S1223-01 Si PD as an example, its technical parameters include:
High sensitivity: quantum efficiency>90%, suitable for low light environments.
Wide dynamic range: Linear response covering optical power from -40dBm to 0dBm.
Stability: Long term working drift<0.5%/year, meeting industrial grade reliability requirements.
三, System control and noise suppression
1. Power management of voltage regulator diodes
In the LD driver circuit, a voltage regulator diode (such as ZMM3V3) provides an accurate 3.3V reference voltage to ensure current source stability:
Low leakage current: Reverse leakage current<2 μ A @ 1V, reducing power consumption.
High precision: The voltage regulation value deviation is less than ± 1%, ensuring the consistency of LD output power.
2. Surge protection of transient suppression diode (TVS)
TVS diodes (such as SMF10CA) are used to absorb power surges and protect LDs from voltage spikes:
Quick response: response time<1ns, clamp voltage<17V.
High power endurance: peak pulse power up to 200W@10 /1000μs.
3. Rectification and protection of Schottky diodes
In LD bias circuits, Schottky diodes provide low forward voltage drop (< 0.7V@5A )Characteristics of high-speed switch:
Low loss: reduces circuit heating and improves efficiency.
Reverse recovery time:<10ns, suitable for high-frequency modulation.
https://www.trrsemicon.com/transistor/npn-transistors-2sc2412-factory.html