In 2020, the state proposes to speed up the construction of new infrastructure such as 5G. China’s three major operators have begun to build 5G networks on a large scale, and WDM devices in the 5G era are also ushering in new opportunities. Multi-antenna technology has a huge driving force for system bandwidth. The marginalization trend of metropolitan area network WDM/OTN and 5G bearer requirements have driven the development of WDM devices, bringing new demand for millions of WDM devices a year to the WDM device market.

A WDM device is a device that synthesizes and separates optical wavelengths. The one that synthesizes is called a multiplexer, and the one that separates is called a demultiplexer. The next three years will be the peak period of 5G construction. With the large-scale deployment of 5G base stations, the demand for WDM devices is expected to reach its peak in 2022.

For the current 5G network construction scheme, including the CWDM scheme that has been put into the network in large quantities, the MWDM scheme proposed by China Mobile in the future, the LWDM scheme proposed by China Telecom, and the DWDM scheme proposed by China Unicom.

Among various WDM technologies, DWDM technology utilizes the wide bandwidth and low loss characteristics of single-mode fiber, uses multiple wavelengths as carriers, and allows each carrier channel to be transmitted simultaneously in the fiber. Compared with the general single-channel system, dense WDM (DWDM) not only greatly improves the communication capacity of the network system, fully utilizes the bandwidth of the optical fiber, but also has many advantages such as simple capacity expansion and reliable performance, especially it can be directly connected to Into a variety of business makes its application prospects are very bright. Its structure and function are very similar to traditional WDM devices in appearance (see the figure below). On the left is a dual-fiber collimator with a filter attached to the front end. On the right is a single-fiber collimator. Incident λ1 λ2…λn, after the action of the filter, the other end of the dual fiber exits λ1 λ2 λm-1, λm+1…λn, and the single fiber end exits λm, thus separating the signal λm.

Among the three-port WDM devices, the production process of DWDM products is relatively complicated, and the adjustment of the filter is the most critical. This article will focus on the assembly process of DWDM filter products, and briefly explain the key points encountered in the assembly process: the impact of the incident light angle on the shift of the center wavelength of the product and the transmission bandwidth.

1. Center wavelength shift:

The central wavelength is the core index of the filter device. The figure below shows the optical path of a typical three-port DWDM product on the input side of dual fibers. As shown in the figure, the geometric centerline of the axial direction of the dual optical fibers constitutes the optical axis, which is represented by a dotted line in the figure. Assuming that the distance away from the optical axis of the input fiber is D, there will be a deflection angle θ between the light beam at the exit end of the lens and the film surface (optical axis) of the filter. Theoretical deduction shows that the relationship between the filter transmission center wavelength and this angle is:,

As θ increases, the transmission center wavelength will change to a shorter wavelength;

As θ decreases, the transmission center wavelength will change to a longer wavelength;

This wavelength change can be expressed as an approximate calculation formula:

which approximately has

Among them, λ0 is the central wavelength when the filter is incident at 0 degrees; Ne is the equivalent refractive index, with a value of 1.64; D is the distance from the optical fiber to the optical axis; θ(D) is the incident angle of the beam on the surface of the film.

Using this formula, the figure below shows the relationship between the incident angle changing from 0° to 3° (the abscissa in the figure is radians) and the center wavelength of the filter

Ordinate: Unit: nm Abscissa: Unit: rad

In actual application, the incident spectrum of the filter at different angles is as follows, where the incident angle on the right is 1.8 degrees, and the incident angle on the left is 2.5 degrees:

Ordinate: Unit: dB Abscissa: Unit: nm

In practice, according to the above principles, Feiyu automatically scans and distributes fibers through self-developed coupling software, and can control the center wavelength offset of DWDM devices to the best by changing the distance D from the optical axis of the dual fibers.

As can be seen from the following statistical chart, the central wavelength scanning data of 5K Feiyu Fiber 100G DWDM finished products are randomly selected:

Abscissa: Unit: nm

The central wavelength offset of 98.9% of the products is between -0.05 and +0.05, and the central wavelength offset of 99.9% of the products is between -0.07 and +0.07

two. Transmission bandwidth

1. Influence of Incidence Angle on Transmission Bandwidth

The increase of the incident angle causes the interference effect of the filter film layer to deteriorate, and the bandwidth of the transmission line decreases.

The figure below shows the attenuation of the 100G C55 transmission spectrum at different incident angles (the three spectral lines are 1.8-degree incidence (top), 2.5-degree incidence (middle) and 3-degree incidence):

Ordinate: Unit: dB Abscissa: Unit: nm

The following table shows the bandwidth change (unit nm) s of randomly selected Feiyu 100G C55 filters at different angles

In summary, as the incident angle increases, the transmission bandwidth decreases.

2 Influence of filter position on transmission bandwidth

The commonly used 1.4×1.4mm filter is cut from a large piece. The cutting process will cause residual stress on the edge of the filter, forming a stress zone. The closer to the edge of the filter, the more obvious this phenomenon is.

The figure below is the transmission spectrum of the 100G C55 transmission spectrum at different incident positions of the filter (the upper spectrum is the central incidence, and the lower is the eccentric incidence):

Ordinate: Unit: dB Abscissa: Unit: nm

Through experimental testing, there are such data (in nm)