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In our practical applications, we have discovered a phenomenon, most of 1.25G fiber SFP module types use PIN detector instead of APD detectors, why?
There are mainly two reasons:
- The sensitivity of PIN receivers is sufficient to meet the transmission standards of 1.25G (such as short to medium distance)
- APD has a higher cost and requires complex bias circuits, not cost-effective to use at low rates. It is mainly used for long-distance communication with transmission rates of 10G or higher.
Below is the core difference comparison:
| Receiver type | PIN (photodiode) | APD (Avalanche Photodiode) |
| Receiver sensitivity | Low (sufficient for 1.25G short distance) | Extremely high (capable of amplifying weak light signals) |
| Working Voltage | Low (usually 3.3V is sufficient) | High (tens of volts to hundreds of volts) |
| Cost | Cheap and mature craftsmanship | Expensive and complex manufacturing process |
| Circuit complexity | Simple (simply zoom in) | Complex (requiring additional APC/ATC bias compensation circuit) |
Specific Differences between PIN and APD
Cost and structural differences
- PIN receiver: simple structure, mature technology, and very low production cost.
- APD receiver: The structure is complex, requiring extremely high process control, and the chip itself is expensive.
- Peripheral circuit: APD operation requires high voltage bias of tens or even hundreds of volts, requiring additional design of high voltage boost circuit and temperature compensation circuit, which significantly increases the overall hardware cost.
Performance requirement matching
- The demand for 1.25G: 1.25G optical modules (such as Gigabit Ethernet, Fiber Channel) are typically used for short distance or intermediate transmission within 20 kilometers.
- Sensitivity sufficient: Within this distance range, the sensitivity of the PIN receiver (usually around -20dBm) fully meets the signal reception requirements.
- Overuse: The core advantage of APD is its ultra-high sensitivity (up to -30dBm or less), which is used for long-distance (over 40 kilometers) transmission. Using APD over short distances is considered excessive performance.
Saturated optical power and burning risk
- Low overload point: APD is very delicate, and a slightly higher input optical power (usually exceeding -6dBm to -10dBm) can reach saturation or even burn out. If used for short distance transmission, a fiber optic attenuator must be added.
- PIN is more durable: The saturation optical power of the PIN receiver is usually above 0dBm, with strong resistance to optical shock. It does not require additional optical attenuators in short distance networks, making it safer and more convenient to use.
Power consumption and heat
- PIN: Low operating voltage (usually 3.3V), extremely low power consumption, and low heat generation.
- APD: High voltage driving significantly increases the overall power consumption and heat generation of the module, which is not conducive to the miniaturization and high-density deployment of optical modules.
When will 1.25G SFP module also use APD?
Only when the 1.25G optical module needs to achieve ultra long distance transmission (such as 40km, 80km or above), due to the huge fiber loss, the optical signal reaching the receiving end is very weak, and an APD receiver is used to obtain “avalanche gain”, thereby improving the receiving sensitivity to -30dBm to -37dBm.
Conclusion
As a mature and cost sensitive product, choosing a PIN receiver with low cost, simple circuit, and not easily burned out is the optimal solution for the market and technology selection of 1.25G optical modules, while meeting performance standards. APD detectors become a necessary choice only in higher transmission rates such as 10G/40G/100G, or in ultra long distance (40km+) links where receiver sensitivity decreases due to increased rates.
FAQ
The SFP-1.25G-ZX100 module I bought has a PIN detector as stated in the datasheet, Tx power 0 ~ +5dB, receiver sensitivity -28~-3 dBm. Is it using a PIN receiver? Is there an error in the description?
According to your description, there is a high probability that an APD receiver has been used for 100KM, as conventional PIN receivers are almost impossible to achieve -28dBm without expensive optical amplification or extreme processes. This has already entered the working threshold of APD receivers.
Do I need a PIN or APD detector for a 120km gigabit optical module?
At a speed of 1.25G gigabit, the typical APD sensitivity ranges from -28dBM to -32dBm, while the limit of PIN receivers is around -20dBm to -24dBm, so APD detectors must be used.
For a 1.25G-40km optical fiber transceiver, is it recommended to use PIN or APD detectors?
For optical fiber transceiver 40km long reach, it is strongly recommended to use PIN detectors instead of APDs. Because the link budget is duplicated, there is no need to add optical attenuation, and the cost is lower.
Read More:
The differences between DFB, DML, and EML lasers
Difference Between PIN and APD Detector Modules
Differences SFP transmitter FP, VCSEL, DFB, DML, and EML
Why do some SFP-10G-ER 40KM optical modules use PIN detectors
