Acceptable Light Levels for Fibers and the Optical Power Budget

Key Takeaways

• For the reliable operation of fiber optic communication systems, the receiver requires minimum power throughout the service time of the system.

• The optical power budget is the minimum light energy required for transmitting signals successfully to the receiver through fiber optic fibers.

• The maximum length of a fiber optic cable is limited by the transmitter’s output power and the receiver’s sensitivity.

Today, media conversion is facilitated by the integration of fiber optic cables and copper, metal-based Ethernet architectures. This fiber optic and ethernet combo is a cost-effective method of media conversion and can be easily optimized and modified with emerging technologies. 

The effectiveness of media conversion is enhanced with the knowledge of acceptable light levels for fibers. Usually, reverse calculations are preferred for determining these levels. The calculation starts from the receiver end to the optical fiber attenuation losses, and finally, to the transmitter. In this article, we will discuss the connection between acceptable light levels for fibers and the optical power budget. 

The Acceptable Light Levels for Fibers

To realize fiber optic communications, transmitters, receivers, and fiber optic cables are required. Fiber optic cables carry optical signals from the transmitter end to the receiver end, enabling data transfers. For the reliable operation of fiber optic communication systems, the receiver requires minimum power throughout the service time of the system. Receiver sensitivity is the parameter that defines the minimum average power demand of the receiver. Transmitter light levels should match this minimum receiver power after supplying the fiber optic attenuation losses. With this concept, comes the term “optical power budget.”

Optical Power Budget

To ensure the correct operation of fiber optic communication systems without any loss of data or downtime, transmitters should supply the input signals above the optical power budget. By satisfying the optical power budget, fiber optic communications exhibit smooth and reliable operations.

The optical power budget is the minimum light energy required for transmitting the signals successfully to the receiver through fiber optic fibers. The transmitters have specified average launch power, which provides information about the optical power capability in d_Bm. 

P_B = P_T - P_R

In the equation above, note that PB is the optical power budget, P_Bis the minimum transmitter power, and P_B is the minimum receiver sensitivity. The equation can be expressed in dB or d_Bm.

According to the equation, the minimum average transmitter power needs to be greater than the optical power budget to ensure the reliability of fiber optic communications. If the transmitter is not capable of supplying the optical power budget, the fiber communication network faces signal distortions due to inadequate light levels.

The optical power budget is influenced by the distance between the transmitter and receiver, as the losses in fiber optic cables are dependent on their length. The length determines up to what distance the fiber optic cable can be extended from the transmitter without disturbing reliable communications. The maximum length of fiber optic cables is limited by the transmitter’s output power and receiver’s sensitivity. 

Calculating the Optical Power Budget 

Calculating the optical power budget is important in fiber optic communications, as the acceptable input light levels of the fiber are dependent on that value. There are several factors affecting the optical power budget of fibers:

1. Fiber loss - The losses incurred when light is transmitted through the fiber are called fiber losses. They are expressed as dB per distance. A loss is equal to the product of the loss factor (fiber specification) and the distance. 
2. Fiber optic connector loss - The losses in the connectors in the fiber optic network are a factor influencing the power budget. Depending on the type of connector (either single-mode or multi-mode connector), the loss value varies. 
3. Type and count of splices - The type and count of splices add losses to the fiber optic network. To reduce losses, mechanical splices are replaced by fusion splices. 
4. Power margin - A window for accommodating aging of the transmitters, fiber, and receivers comes with a power margin. The power margin is included in the power budget calculation to account for mechanical twisting or bending of the fiber and additional devices. The margin is effective in compensating for link degradation and losses. 

The acceptable light levels for fiber optic communications are dependent on the optical power budget and receiver sensitivity. The power budget value is influenced by the losses incurred to the input light levels on their way to the receiver end. Cadence offers a suite of design and analysis tools that can assist in determining the power budget before designing fiber optic communication transceivers. 

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