# Functions and normal working ranges of the bias current, cooling current, and back facet current of OA boards on BWS 1600G devices

Question:
What are the functions and normal working ranges of the bias current, cooling current, and back facet current of OA boards on BWS 1600G devices?
Analysis:
The bias current of an OA board is actually the drive current of the laser on the OA board. A laser is actually a photodiode. Therefore, the drive current is also called bias current. Due to the differences between pump lasers, the bias current also varies greatly for lasers of the same type (for example, 100 mW 980 nm pump lasers). The bias current ranges for different pump lasers are as follows:
100 mW 980 nm pump laser: 250 mA
150 mW 980 nm pump laser: 350 mA
160 mW 1480 nm pump laser: 700 mA
Strictly speaking, cooling current should be temperature control current, which is classified into cooling current and heating current. The laser core temperature must be kept at 25°C; otherwise, the output wavelengths and output power are affected. In normal cases, the laser emits heat. Therefore, the temperature control performance is cooling. When the environment temperature is low, the temperature control performance may be heating, and the temperature control current may be 0. To be specific, laser heating and heat dissipation are dynamically balanced. The heating current and cooling current vary depending on changes of the ambient temperature and input optical power. Generally, the current values are as follows (the cooling current is a positive value and the heating current is a negative value):
980 nm pump laser: 1.6 A (cooling current), 0.4 A (heating current)
1480 nm pump laser: 2.2 A (cooling current), 0.8 A (heating current)
Back facet current: Each laser uses an internal optical path to reflect a part of output light to the internal PIN tube for check and then outputs the back facet current. The original purpose is to check laser working status based on the back facet current. However, the detected laser back facet current is inaccurate. Therefore, only the circuit is checked and the check result is reported. The only requirement for the back facet current is that the current value is not 0.