Birefringent crystals are widely used to fabricate optical passive devices such as optical isolator, optical circulator, polarization beam combiner/splitter, optical interleaver. The mostly used crystals are uniaxial, such as YVO4 and LiNbO3.
Birefringence in Uniaxial Crystal Wave normal K is the normal of equalphase surface which describes the transmission of wave phase. Ray S is the normal of wave front which describes the transmission of optical energy. When light incidents on the air-crystal (uniaxial) interface, there are usually two refractive rays, o-ray So and e-ray Se. The direction of each ray depends on the direction of corresponding wave normal.
Optical isolators are widely used in optical fiber communication systems, optical fiber sensing systems and fiber lasers. The basic and common principle for optical isolators is Faraday effect. However, the device structures and characteristics are variable, which are detailed as follow.
Free-space Optical Isolator The structure of a free-space optical isolator is shown in Fig.1, which comprises two polarizers, a Faraday rotator (FR) and a magnet ring. The transmission axes of the two polarizers are aligned with 45º angle and the FR has a fixed rotatory angle of 45º in a saturated magnetic field.
Theoretically, the fiber is a circular core which should not produce birefringence, and the polarization state in such a fiber will not change during transmission. In practice, there is always some external forces or other reasons which makes the fiber uneven or curved, and will cause a small amount of birefringence. When the fiber is subject to any external interference, such as wavelength, bending of the fiber, temperature and other factors, the polarization state of light will become chaotic when transmitted in polarization-maintaining fiber.
In the first chapter (WDM Devices — AWG with Flat Response), the reasons for the Flat Response required, cause for Gaussian Passband, and three main passband optimization proposals are introduced in brief. This chapter is about two other passband optimization proposals.
4) Shaping of Phase Transfer Function Let’s review the proposals of adding MMI at the input and taper at the output. The core feature is to flatten the focused optical field or the eigen mode of the output waveguide. Thus the correlation function between the two optical fields is flattened. Anyway, the correlation between two mismatched optical fields will introduce excess power loss. The more is the mismatch, the more is the power loss. The AWG designers need to balance the passband width and the loss penalty.
Why Is Flat Response Required? In the all optical network (AON), the optical signals passed tens of nodes before reaching the destination node, as shown in Fig.1. The ROADM nodes are usually composed of wavelength selective switches (WSS), multiplexers/demultiplexers and optical switches. The wavelength multiplexers/demultiplexers are optical filters, including TFF-based WDM devices, arrayed waveguide gratings (AWG) and optical interleavers.
Why is AWG demanded? As we know, DWDM technology enables transmission of dozens of wavelengths in a single fiber, which expands the capacity of optical fiber communication enormously. The first mux/demux modules for DWDM system are based on thin-file filters (TFFs), as shown in Fig.1 and Fig.2. Both are designed in serial structure. Different wavelengths travel different number of devices in the module and result in different power loss. The loss uniformity degrades with increment of port number. Meanwhile, the maximum loss at the last port is another limitation on the port number. Thus the TFF-based WDM modules are usually limited to be ≤16 channels.
What’s CDC-F ROADM? A ROADM node has a network node interface (NNI) and a user network interface (UNI). The NNI interconnects DWDM signals from/to multiple directions. The DWDM signals are switched between different directions in wavelength granularity. The UNI downloads signals designated to the node and uploads signals from the node in wavelength granularity. In order to realize non-blocking switching and adding/dropping of wavelengths, the new generation of ROADM nodes are required to be colorless, directionless and contentionless (CDC ROADM).