Polarization-maintaining Fiber Filter WDM

1. Definition
Polarization-maintaining Fiber FWDM(Filter Wavelength Division Multiplexin ) is the optical passive device, it can couple two or more different wavelengths of light into the same optical fiber transmission. It can also be a plurality of different wavelengths of light in the same optical fiber were separated into different optical fiber transmission.

2. Characteristic
1). To combine or separate two (usually two) or more beams of light of different wavelengths
2). Wide operating bandwidth, low insertion loss, high channel isolation, high stability and reliability.
3). Produced by dielectric film filter technology, it provides a variety of wavelength combinations in the wavelength range of 780 nm-20550 nm.

3. Description
1). Working principle of Filter WDM- Multilayer Dielectric Thin-film Filter
A thin-film resonant cavity filter (TFF) is a Fabry-Perot interferometer, or etalon, where the mirrors surrounding the cavity are realized by using multiple reflective dielectric thin-film layers.
This device acts as a bandpass filter, passing through a particular wavelength and reflecting all the other wavelengths. The wavelength that is passed through is determined by the cavity length.
To improve the reflection efficiency, the thin-film resonant multicavity filter (TFF) chip usually adopts multilayer film (generally 50-100 layers). By using the interference of multilayer dielectric film, one wavelength passes through and other wavelengths reflect.

Figure 1. Working principle of thin-film WDM

Figure 2. Schematic diagram of structure and wavelength selection of thin-film filter

Figure 3. Schematic diagram of structure and wavelength selection of thin-film filter

When two different wavelength ranges of light are input from the common, WDM devices will allow one beam of light with the specific wavelength range to be transmitted and output from Pass port; The remaining light will be reflected and coupled to the Reflect port output, this is the process of wavelength division. According to the principle of reversible optical path. When different wavelengths of light input from Pass port and Reflect port, both lights will be coupled and output form the Common port by the WDM device, This is the process of optical wavelength multiplexing coupling.
Compared with fused WDM, the main feature of Filter WDM is that the wavelength range is more flexible. The user can specify any range of transmitted and reflected wavelengths.

2)Polarization state of PM FWDM
In the polarization-maintaining fiber transmission system, the PM fiber FWDM can usually transmit light of any polarization state, that is to say, the PM FWDM is not sensitive to the polarization state. So, we usually default is that PM FWDM is both axis working.
However, if the user requests, we can integrate a 0-degree polarizer to improve the extinction ratio of polarized light. Since the polarizer can only be placed after the Thin-film Filter, that is, only the Pass wavelengths can be polarized, and the polarization state of the Reflect wavelengths does not change. Generally, pass wavelengths as a seed signal source, such as 1064nm or 1550nm, which is necessary in many laser systems to polarize.
It should be noted that due to the power limitation of the polarizer, it is only suitable for low power, and it is not recommended to have too high pulse energy. If it must be used for high average power, such as > 2W, and applications with pulses, PBS polarizing is required, but the production cost will be higher.

Figure 4. Schematic diagram of structure of WDM with pass signal add polarizer.
 

2). key parameter

a. IL (Insertion Loss)
Insertion loss refers to the additional loss caused by passing WDM, which is defined as the ratio of the optical power of the input and output ports of the passive device:

As shown in the formula above, Pin is the power of the inverted input, Pout is the power of the inverted output. The performance of the device requires the insertion loss of forward incident light to be as small as possible

Figure 5. Schematic diagram of Insertion Loss test of WDM.

Taking the above figure 5 as an example, the blue light input is 100 mW, the output port is the Pass port; the red light input is 100 mW, and the output port is the Reflect port. It can be seen that the input optical power at the Pass pin = 100 mW, and the output optical power Pout = 90 mW, then the insertion loss IL of Pass port is:

IL= 10 × log (100/90)
= 10 × 0.046
= 0.46 dB

It can be known that the input optical power at the Reflect port Pin=100 mW, and the output optical power Pout= 95 mW, so the insertion loss IL of the Reflect port is:

IL= 10 × log (100/95)
= 10 × 0.022
= 0.22 dB

Note that the Insertion loss for the Reflect port will be better than the Pass port.

b. ISO (Isolation)

Isolation refers to the ability of one optical path of WDM to isolate the light in other optical paths. Each port of the WDM is designed to have low insertion loss (i.e., high transmission) at the desired wavelength while suppressing the signal at the specified wavelength of the other port, which minimizes cross talk between the ports. Therefore, isolation is specified as the insertion loss of these undesired wavelengths. High dB values of isolation are ideal for signal separation applications using a WDM.

Figure 6. Optical path diagram of isolation test of the Pass port in WDM

Taking Figure 6 as an example, Pass port is the transmission channel port of blue light and Reflect port is the transmission channel port of red light. Theoretically, the red light input from Common port should be totally from Reflect port, However, the WDM can not completely isolate the red light output to the Pass port. To characterize the isolation ability of WDM to unexpected light, the concept of channel isolation is introduced.
Definition: the decibel of the ratio of the power value of the unexpected incident optical signal to the power value of the input optical signal:

As shown in the formula above, Pin is the input power, Pout is the output power of unexpected optical port. The WDM device requires that the higher the isolation value of the undesired light, the better the performance. At the same time, please note that the isolation indicators of the Pass port and the reflection port are inconsistent. Usually, the isolation on the Pass port will be higher.

Taking Figure 6 as an example, we know that the input optical power of red light Pin =100 mW, the output power for the unexpected light Pass port Pout=0.1 mW, and the ISO of the Pass port is:

IL= 10 × log (100/0.1)
= 10 × 3.00
= 30.0dB

As shown in the Figure 7, the input optical power of optical blue light Pin =100 mW, the output power for the unexpected light Reflect port Pout=2 mW, and the ISO of the Reflect port is:

IL= 10 × log (100/2)
= 10 × 1.699
= 16.99 dB

 Figure 7. WDM Optical path diagram of isolation test of the Reflect port

c. Polarization Extinction Ratio (PER)
The polarization extinction ratio (PER) is a measure of how well a polarization-maintaining (PM) fiber or device can prevent cross coupling between the different polarization axes of the fiber. External stress on a fiber from sources such as heating, bending, or pulling can cause the PER to change.
Rotating Polarizer Method:
Rotating Polarizer Method is the most common method for measuring PER uses a low-coherence (unpolarized or circularly polarized) broadband light source and measures the extinction ratio with a linear polarizer and power meter.

The PER is measured using the following test measurement setup:

Figure 5. schematic diagram of measurement of polarization extinction ratio setup

Connect the components as shown above. Note that it is necessary to ensure that the panda eye of the PM fiber is perfectly aligned.

Adjust the rotatable polarizers sequentially until a minimum power value is measured by the power meter. Record the measured value as Pmin(dB).
Rotate the rotatable polarizers mount by 90°. Then record the measured value as Pmax(dB).
After Pmin and Pmax are measured, the extinction ratio can be calculated: PER(dB)= Pmax(dB) - Pmin(dB)

DK Photonics can provide a complete set of equipment/devices for the above measurement of extinction ratio setup, if you need it, please contact our sales: sales@dkphotonics.com.