Introduction to WDM Optics Technology in High-Speed Optical Transceivers

There are two primary methods for increasing bandwidth in optical modules: 1) Increasing the bit rate of each channel, either by directly raising the baud rate or by maintaining the baud rate and using more complex modulation techniques (such as PAM4); 2) Increasing the number of channels, such as by adding parallel optical fibers or using Wavelength Division Multiplexing (WDM), including CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing). WDM technology enables the transmission of multiple wavelength signals over a single optical fiber, which exponentially increases the transmission capacity of the fiber. This technology has been widely adopted for medium- to long-distance optical communications and interconnections in data centers.

There are two main technologies used for implementing WDM optics technology in optical modules: Thin-Film Filters (TFF), based on free-space optics, and Arrayed Waveguide Grating (AWG), based on Planar Light Circuit (PLC) technology, as well as Echelle Diffraction Grating (EDG) and cascaded Mach-Zehnder Interferometer (MZI) arrays. Among these, TFF (using the Z-Block technology) and AWG are the most commonly used and representative MUX/DEMUX subcomponents.

TFF technology in optical modules typically utilizes the Z-block method. It is based on a free-space optics design, combined with collimators, and incorporates four CWDM wavelength filters to perform multiplexing and demultiplexing. The transmission wavelengths of each filter are 1271nm, 1291nm, 1311nm and 1331nm respectively.

In order to simplify the packaging process and reduce the size and cost, CWDM4 AWG chip based on integrated optical technology has been developed. AWG is the abbreviation of arrayed waveguide grating, which has been used in telecommunication network for a long time. Chips based on CWDM4-AWG technology have now matured and are widely used in 100Gbps CWDM4 QSFP28 products.

The earliest CWDM4 AWG chip has input/output ports at both ends, as shown in Figure below. In order to wind the fiber easily and integrate it into the fiber transceiver module, CWDM4 AWG chip with one side input/output has been developed. The input port is wound to the output port by bending the waveguide, as shown in Figure below. This design further simplifies the coupling process between waveguide and fiber array. Of course, due to the limited width of the chip, the bending radius of the waveguide is less than 1 mm, which will lead to a certain bending loss.

In a CWDM4 fiber transceiver module, two CWDM4 AWG chips are needed. One is used for multiplexing transmission of optical signals, the other is used for demultiplexing reception of optical signals. At present, the CWDM4 AWG chip at the transmitting end mainly adopts the unilateral input / output structure shown in Figure below, while at the receiving end, each wavelength of demultiplexing will eventually be detected by the photodetector, and there is no need to couple to the single-mode fiber to continue transmission. For this reason, the CWDM4 AWG chip at the receiving end usually adopts the input/output structure on both sides as shown in Fig. below. The output port adopts multimode optical waveguide, and the output end face is polished into a 45° inclined plane to realize the 90° turning of the light beam, which is incident on the photodetector array, and the latter is directly mounted on the PCB board.

Both Z-block and AWG have their respective advantages and disadvantages. Z-block technology offers the benefits of low loss and good channel quality. CWDM4 modules based on Z-block technology can support 100G or higher-speed signal transmission over distances of 10 kilometers or more. In terms of application trends, AWG is commonly used in the receiver end of traditional optical modules, offering excellent cost and packaging advantages. Currently, both of these solutions are being applied by various manufacturers.

Key Components in TFF Technology will be introduced in the bellowing.
Z-block
The MUX/DEMUX component is one of the most crucial parts of high-speed optical modules, and the Z-block is the core device within this component.

The diagram below shows the typical structure of a Z-block. In the center is a processed rhombohedral prism (also a parallelogram-shaped glass substrate), with the backside of the prism coated with a high-reflection film. On the opposite side, WDM filters for different wavelengths are applied. Each filter only allows light signals of the current channel's wavelength to pass through, while reflecting the other wavelengths, thereby selecting a specific wavelength beam for transmission.

The light signals emitted from the four collimators on the right pass through their corresponding filters, undergo different numbers of reflections, and then reach the common collimator on the left, where they are coupled into the output optical fiber. This process achieves the optical path multiplexing (MUX). For example, the collimated light beams containing four wavelengths enter from the incident side at specific angles as designed. The 1271 channel passes directly through Filter 1 and exits through the anti-reflection coating area of the rhombohedral prism. The 1291 signal passes through Filter 2 and is incident on the reflective coating area of the prism, where it is reflected onto Filter 1. Filter 1 then reflects it back to the prism’s anti-reflection coating area, from which it exits. Similarly, the 1311/1331 signals undergo multiple reflections and ultimately exit through the anti-reflection coating area of the block. The entire optical path inside the block forms a "Z" shape, which is why it is called the Z-block.

The wavelength division demultiplexing receiving optical path of the Z-block module is shown in Figure below. The optical signal at the common end is input from the left collimator. The optical signal of each channel passes through the corresponding filter plate after different reflection times, and then focuses on the corresponding unit on the photodetector array through the microlens. The photodetector array is mounted on the PCB board, as shown in Figure. In the horizontal plane, the beam which is decomposed and multiplexed needs to pass through a right-angle prism to achieve 90-degree turn, and then incident on the photodetector along the vertical direction. The size of active region of photodetector is usually only Φ50μm, and the diameter of collimated beam transmitted in Z-block is much larger than that. Therefore, microlens is needed to focus, and the microlens needs to be adjusted up, down, left and right in the cross section of the vertical optical path, so as to aim the focused spot at the source region of photodetector. This adjusting focusing process also increases the complexity of Z-block assembly process.

How many times can a light beam be reflected? In theory, a light beam can be reflected an infinite number of times. However, due to light scattering and the absorption characteristics of materials, the number of reflections is limited. Eventually, the light will either be scattered or absorbed. Currently, the most commonly used Z-block configuration is the 4-channel version, which is constrained by optical performance and assembly yield. A light beam typically undergoes no more than four reflections on the Z-block filters. As the number of channels increases, the parallelism between the beams deteriorates, and the spot quality worsens, which negatively impacts coupling efficiency.

In the current market, 800G solutions more commonly adopt the 8×100G configuration. Z-block technology is still widely used in optical modules such as 800G FR8 and LR8. There are several different types of 800G solutions, with a few common configurations:

Fiber Collimators

Fiber collimators are used to collimate input signal light, converting the output light from a fiber into a collimated light beam with a specified beam diameter or spot size in free space. They can also be used in reverse, to focus light into the fiber. Typically, a fiber collimator consists of a fiber tip, a collimating lens, and a housing. When a laser is emitted from a waveguide, it is usually a Gaussian beam with a large divergence angle. As it propagates through free space, the spot quickly diverges and increases in size, making it difficult to integrate with other optical components in free space. This is where a collimator is required. When the beam exits the collimator, the collimating lens ensures that the beam is parallel or focused. The collimating lens can be a C-lens, GRIN-lens, spherical lens, or aspherical lens, among others.

Optical Isolator
An optical isolator is a passive optical device that allows light to pass in only one direction. Its working principle is based on the non-reciprocal nature of Faraday rotation. Optical isolators are made using the Faraday effect, where when plane-polarized light is incident along the direction of a magnetic field in a non-optically active material, the polarization plane of the light will rotate by an angle θ. If the reflected light passes through the Faraday device again, the polarization plane will rotate by an additional angle of 2θ. In simple terms, an optical isolator only allows light to pass in the same direction and isolates the reflected light from the fiber, thereby protecting the laser from interference caused by back-reflected light. An optical isolator typically consists of three parts: polarizing filters on both sides (input and output) and a Faraday rotator in the center.

Working Principle:
When light passes through the first input polarizer, it becomes vertically polarized and reaches the Faraday rotator in the middle. The rotator will only rotate the light by 45° in one direction. The rotated light then aligns with the angle of the polarizer placed after the rotator, allowing the light to continue through and be output.

When light from the opposite direction passes through the output polarizer and enters the rotator, it is rotated by 45° in the same direction again. The rotated light then reaches the previous polarizer. Since the polarization direction is now different, the light cannot pass through and is blocked. This isolates the light and prevents the transmission of optical signals in the reverse direction.

In optical transceivers, WDM multiplexing and demultiplexing are typically achieved using discrete components, including fiber collimators, WDM filters, mirrors, lenses, isolators, and others. However, the assembly efficiency is relatively low. With Z-block free-space technology, components such as lenses, collimators, and isolators can be integrated into a single system. Through precise optical path design and optimization, coupling efficiency is significantly improved.

HYC’s integrated optical components are primarily used in 400G/800G FR/ER/LR high-speed optical transceivers. The RX side integrates components such as receptacles, collimators, Z-blocks, lens arrays, isolators, and prisms. These components can be easily coupled in one simple step for assembly into the optical transceiver module, greatly simplifying the assembly and coupling process. The core technology of the product lies in optical simulation, integrating precision optical coupling assembly and testing, as well as back-end processing capabilities for optical components. This design ensures optimal coupling components, rapid coupling, and minimal insertion loss. HYC can participate in joint development during the customer product development stage, offering full optical path simulation, Z-block dimensional control, analysis of different Gaussian beam distributions based on customer designs, beam quality convergence, and position tolerance analysis. Additionally, HYC provides customized services in precision optical coupling assembly and testing, as well as reliability control.