Study on the Working Bandwidth of ORTE Optical Fiber Isolator

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2022-07-28

As a core optical passive device, the online optical fiber isolator has extremely low insertion loss, high isolation and high reliability. It is mainly used in EDFA, ASE light source and other equipment to isolate the reverse light from the light source. Influence.

So other than that, do you know why there are no full-bandwidth fiber optic isolators on the market? Let's talk about some alternative knowledge of online fiber optic isolators.

What is a fiber optic isolator?

Optical fiber isolators are divided into online (polarization-independent) optical isolators and free-space (polarization-dependent) optical isolators. The light of a specific wavelength will have lower loss when passing through the optical fiber isolator in the forward direction, while the reverse direction will pass through the optical fiber isolator. The device will have a lot of loss, which is what we often call isolation.

Therefore, a fiber optic isolator is a fiber optic passive device that can conduct forward and reverse isolation of light of a specific wavelength, similar to the function of a diode in an electronic device.

Full Bandwidth Fiber Optic Isolators

For the optical communication industry, the full bandwidth generally refers to the working bandwidth of 1260nm~1620nm. To realize a full bandwidth isolator, the full bandwidth of the material must be realized first, and the core materials that affect the performance of the isolator are birefringent crystals and Faraday rotators.

The core parameters of the birefringent crystal are the optical axis direction and the coating, the optical axis direction has nothing to do with the wavelength, and the design of the coating layer parameters can easily meet the parameter requirements of the 1260nm~1620nm bandwidth, so for the birefringent crystal to achieve full The bandwidth function is relatively easy.

The core parameters of the Faraday rotator are the rotation angle, the coating, and the absorption of light energy by its own material. The Faraday rotator will have serious light absorption when the wavelength is too long or too short, resulting in greater insertion loss of the device. As shown in Figure 1 below, it is the corresponding relationship between the insertion loss and the wavelength of the Japanese GLB optical rotator at short wavelengths. It can be seen from the figure that the insertion loss will increase linearly when the wavelength is less than 1100nm. Figure 2 below shows the corresponding relationship between the insertion loss and the wavelength of various types of Faraday optical rotators commonly used by us at long wavelengths. It can be seen that the GLB optical rotator has a small insertion loss at 1300nm~1650nm, so the optical energy absorption of the optical rotator is basically Implementation of full bandwidth isolators is not restricted. The problem of coating is the same as that of birefringent crystals, which can easily meet the parameter requirements of 1260nm~1620nm bandwidth through the design of film layer parameters.

Regarding the rotation angle of the last core parameter of the Faraday rotator, the following table shows the wavelength correlation coefficient of the commonly used 45-degree Faraday rotator. The so-called wavelength correlation coefficient is the variation of the Faraday rotator's optical rotation angle with the wavelength, expressed in deg/nm.

We can see that for every 1nm difference in wavelength, the rotation angle will deviate by about 0.07° relative to 45°. If we want to make a full-bandwidth isolator, we will choose an optical rotator close to the center wavelength of the bandwidth. The 1480nm optical rotator in the table below is relatively close. We will use the 1480nm optical rotator and the deflection of 0.07°/nm to calculate the full bandwidth. The maximum angular deviation at this time: 1480nm-1260nm=220nm, 220nm×0.07°/nm=15.4°.

According to the characteristics of the optical rotator, the optical rotation angle will increase to short wavelengths, so the actual rotation angle of the optical rotator is 60.4° at 1260nm. Due to the large difference in the relative optical axis of 45° from the birefringent crystal, the forward birefringent light ( O light and e light) have too much deflection angle, and the deflection angle of reverse birefringent light is too small, which will cause the insertion loss of the full-bandwidth isolator to increase at the short-wave and long-wave positions and reduce the isolation, which cannot be used in the system.

Ultra-Long Bandwidth Fiber Optic Isolators

Due to the influence of the material, we cannot realize the function of the full bandwidth of the isolator, but we can realize the function of the ultra-long bandwidth isolator according to the characteristics of the optical rotation angle of the optical rotator and through the cooperation of the optical axis angle of the wedge angle plate. The following is our ultra-long bandwidth The parameter index of the isolator and the distribution of key parameters.