In optical communication systems, optical modules are the core of optical signal transmission, and their performance is crucial to network stability and reliability. However, when long-distance optical modules are directly connected to short-distance optical fibers without attenuation, the optical components at the receiving end are easily damaged. This involves complex optical power management and engineering considerations.

I. Optical Power Overload: The "Fatal Threat" to Receivers

To compensate for signal attenuation over long transmission distances, long-haul optical modules (such as 40km and 80km modules) transmit at higher optical power. A 40km single-mode module can reach +2dBm, while the receiver's overload threshold is often only -3dBm.

If directly connected to a short-haul fiber (such as a 10km fiber), the optical signal attenuation is insufficient, and the optical power at the receiver far exceeds the limit. This can permanently damage components such as the photodiode (PD) and avalanche photodiode (APD), resulting in reduced module sensitivity, increased bit error rates, or even complete failure.

II. Dynamic Range Imbalance: The "Invisible Killer" of Signal Quality

The dynamic range of an optical module is the difference between transmit and receive sensitivity. For example, a 40km module with a dynamic range of 20dB (transmit +2dBm, receive sensitivity -18dBm) can tolerate optical power fluctuations between -18dBm and +2dBm. However, over short fiber connections, the actual received optical power may exceed the upper limit.
For example, if the total line loss is 15dB, the receive end's power level at -13dBm, while not overloaded, is close to the edge of the dynamic range. This excessive power can cause signal distortion, exacerbating intersymbol interference and increasing the bit error rate. For high-speed transmission (above 10Gbps), the bit error rate can soar from 10⁻¹² to 10⁻⁹, leading to service interruption.

III. Engineering Practice: Concerns about Lack of Safety Margin

In optical communications, the received optical power must be at least 3dB higher than the receiving sensitivity (i.e., the power margin) to account for increased losses due to factors such as temperature fluctuations and fiber aging. For example, if the module's receiving sensitivity is -24dBm, the actual received optical power should be controlled within -21dBm.
However, when a long-distance module is directly connected to a short-distance fiber, this safety margin may be lost. For example, if a 100km module is connected to a 50km fiber, with a transmit power of +3dBm and a total loss of 11dB, and a receive power of -8dB, this may appear safe, with a 16dB margin. However, if the fiber loss decreases by 2dB, the received optical power will rise to -6dBm, causing device performance degradation over long-term operation.

IV. Attenuator: The "Control Valve" for Optical Power

To solve the above problems, an optical attenuator is an essential component for connecting long-distance modules to short-distance optical fibers. Its functions are as follows:

1. Power control: Reduce the received optical power to a safe range, such as attenuating -2dBm to -8dBm, to avoid overload damage.

2. Signal optimization: Adjust the optical power to the center of the dynamic range, such as attenuating -13dBm to -18dBm, to reduce distortion.

3. Margin reservation: Reserve more than 3dB of power adjustment space for the system to cope with future power loss increases.

For example, a 40km module connected to a 10km fiber has a transmit power of +2dBm, a total loss of 3dB, and a receive power of -1dBm. After inserting a 10dB attenuator, the received optical power drops to -11dBm, below the overload threshold but above the receive sensitivity. This leaves a 7dB power margin, ensuring long-term stable system operation.

Conclusion: The Inevitable Transition from "Direct Connection" to "Controllable"

The risks of directly connecting long-distance optical modules to short-distance optical fibers stem fundamentally from unbalanced optical power management. As optical communication systems evolve toward higher speeds and longer distances, the gap between optical module transmit power and receive sensitivity widens, necessitating even higher levels of power control accuracy.
The proper use of optical attenuators enables precise control of optical power, protecting receiving components, optimizing signal quality, and improving network reliability and stability. This is an essential engineering choice and a reflection of the "safety first" principle in optical communications.