With the Internet, high-speed IP data traffic growth, resulting in demand for transmission bandwidth is increasing. Though DWDM (Dense Wavelength Division Multiplexing) technology as the most effective way to solve the line bandwidth expansion, but CWDM (coarse wavelength division multiplexing) technology over DWDM system has advantages in cost and maintainability.

CWDM and DWDM are the property of WDM technology, can be coupled to different wavelengths of light to the single fiber transmission together.

The latest CWDM ITU standard G.695, specifies from between 1271nm to 1611nm 20nm intervals of 18 wavelength channels, taking into account the impact of the water peak ordinary G.652 fiber, generally use 16 channels. Because large channel spacing, so multiplexing wave devices and laser devices cheaper than DWDM.

DWDM channel spacing according to need 0.4nm, 0.8nm, 1.6nm different intervals, intervals smaller, the need for additional wavelength control device, so the device based on DWDM technology based equipment prices compared to CWDM technology is high.

PIN photodiode is very high doping concentration in the P-type, between the N-type semiconductor, add a layer of lightly doped N-type material, called the I (Intrinsic) layer. Because it is lightly doped electron concentration is very low, the diffusion after the formation of a wide depletion layer, this can increase the speed of their response and conversion efficiency.

APD avalanche photodiode, it not only has the optical / electrical conversion effect, and having an internal amplification effect, the amplification by the avalanche multiplication effect is completed inside the tube. There is a gain of APD photodiode in the optical receiver sensitivity requirements of the occasion, the use of APD help extend the system of transmission distance.