Category Archives: Technical Forum

Crystal-based Devices— Basics for Crystal Optics

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.

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Crystal-based Devices—Optical Isolators

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.

Fig.1 Structure of free-space optical isolator
Fig.1 Structure of free-space optical isolator
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360° Ferrule Rotation Adjustable Polarization Maintaining(PM) Optical Fiber Connector

What is Polarization Maintaining(PM) fiber?

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.

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WDM Devices — AWG with Flat Response (2)

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 undefined or the eigen mode undefinedof the output waveguide. Thus the correlation function undefined 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.

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WDM Devices — AWG with Flat Response

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.

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WDM Devices — Arrayed Waveguide Grating

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.

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The Structures of 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).

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What is All Optical Network (AON)?

Review of Optical Fiber Communication
Based on industrial view, the development of optical fiber communication has experienced four stages and is now in the fifth stage. In 1970, the emergence of optical fiber with low loss and laser diode operating at room temperature initialized optical fiber communication.

However, the wide application of optical fiber communication was in 1990s. USA government released the plan named “National Information Infrastructure (NII)” in 1993. Optical fiber communication technologies were important parts supporting NII and were developed rapidly. The symbolic technology in the period is DWDM, which expands the transmission capacity of optical fiber communication by tens of times. The development of optical fiber communication slowed down in 2001 with the burst of internet bubble.

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WDM Devices — TFF-based WDM Devices

As we know, optical fiber communication is one of the enabling technologies for Internet and changed the world. The advantage of optical fiber communication is to transmit dozens of wavelengths in a single optical fiber, which is called wavelength-division multiplexing (WDM). The basic devices for WDM transmission are optical filters, which can be realized by fused biconical taper (FBT), thin film filter (TFF), arrayed waveguide grating (AWG) and optical interleaver. TFF and AWG are most commonly used in the WDM system. This paper discusses TFF-based WDM devices.

Thin Film Filter
Fabry-Perot interferometer (FPI) is a commonly used interferometer for spectrum filtering. The structure of a FPI is shown in Fig.1, which consists of two glass plates spaced by a spacer with precise thickness. The inner surfaces of the plates are coated for partial reflection and the outer surfaces are usually anti-reflection (AR) coated.

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MEMS Optical Devices — MEMS OXC

Application of OXC
Optical cross connect is a matrix switch usually with N×N ports. The OXC can be used to construct a CDC ROADM (Colorless, Directionless, and Contentionless Reconfigurable Optical Add/Drop Multiplexer), as shown in Fig.1 [1].

Fig.1 CDC ROADM based on WSS and OXC [1]

OXC Constructed by 1×N Optical Switches
The OXC can be constructed by 1×N optical switches, as shown in Fig.2. It requires 2N 1×N optical switch to construct a N×N OXC. Thus the size and cost of the OXC module increase rapidly with the incensement of port number N. OXC of this structure is usually limited to below 32×32 ports.

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