Parallel Optics and WDM Optics in High-speed Optical Modules
                      
Parallel Optics and WDM Optics in High-speed Optical Modules 
With the exponential growth in AI model training and inference demands, the need for 
computing power has surged, driving a significant increase in network bandwidth 
requirements for AI data centers. This trend has accelerated the development of high-speed 
optical modules, which serve as critical components in data centers and high-performance 
computing systems by enabling high-speed, high-capacity data transmission.   
 
There are two primary methods for increasing bandwidth in optical modules: 
1) Increasing the bit rate per channel, either by directly raising the baud rate or by 
maintaining the baud rate while adopting more advanced modulation schemes such as PAM4.   
2) Expanding the number of transmission channels, either by increasing the number of 
parallel optical fibers or by employing Wavelength Division Multiplexing (WDM) techniques 
such as CWDM and LWDM.   
 
Based on the transmission method, optical modules can be classified into parallel optical 
modules and WDM optical modules. Parallel optical solutions are particularly cost-effective 
for short- to medium-distance transmissions, whereas WDM solutions are more advantageous 
for long-distance transmissions as they significantly reduce fiber costs.   
 
Parallel optics transmission 
For parallel optics transmission, parallel optical modules at both ends of the link contain 
multiple transmitters and receivers, utilizing multiple optical fibers to transmit and receive 
signals through multiple paths. Common parallel optical module types include SR4, SR8, 
PSM4, DR4, and DR8.   
 
 
MT(MPO) and fiber array (FA) assemblies are key components for parallel optical 
interconnections, which can be integrated into optical modules to connect external and 
internal optical connections. The compact size and multi-channel capability of MT ferrules 
make them ideal for high-speed parallel optical transmission. Since different optical module 
types and manufacturers have unique internal structures, MT-FA and MT-MT jumpers are 
highly customized. Various configurations include MT-FA, MT-2×Mini MT, MT-FA with 
isolators, and MT-FA with lens arrays.   
 
FA with isolators or lens array products are widely used in high-speed optical modules. The 
primary function of an isolator is to effectively prevent optical signal reflections, allowing 
only unidirectional light transmission based on the non-reciprocal properties of Faraday 
rotation.  In traditional optical modules, isolators are typically used as standalone 
components. Integrating a fiber array with an isolator simplifies module design, optimizes 
space utilization, and reduces coupling time while ensuring high-quality signal transmission. 
Lenses play a crucial role in optical transceiver modules by facilitating optical coupling. 
Since laser emissions are divergent, lenses help collimate or focus the light beam to improve 
transmission efficiency. On the receiving end, as data rates increase, photodiodes (PD) 
receiving area becomes smaller, making direct fiber-to-PD coupling less efficient. This is 
where lens arrays become essential, as they effectively improve coupling efficiency by 
focusing light onto the PD. Additionally, they simplify optical module packaging, reduce 
manufacturing steps, and lower production costs. Both FA and lens arrays can be customized 
 
based on beam profile, angles, and specific customer requirements to achieve precise optical 
coupling.   
 
 
WDM optics transmission 
WDM technology enables the transmission of multiple wavelength signals over a single 
optical fiber, significantly increasing fiber transmission capacity. It has been widely adopted 
for medium- to long-distance optical communication and data center interconnects. Typical 
WDM optical module types include FR4, FR8, and LR4.   
 
 
 
WDM can function as either a multiplexer (MUX) or a demultiplexer (DEMUX). A Demux 
separates incoming multi-wavelength WDM signals into individual wavelengths, and couples 
them into photodiodes for optical-to-electrical conversion. A Mux combines different 
wavelength signals from multiple laser sources into a signal, which is then efficiently coupled 
into a single output fiber. 
 
WDM optical components consist of several discrete functional components. On the 
receiving end, these include fiber collimators, WDM blocks, mirrors, lens arrays, and prisms, 
while the transmitting end typically includes collimators, isolators, and WDM blocks. 
Precision alignment is required between these discrete components to ensure optical 
performance. As the demand for high-speed and high-density modules grows, integration has 
become increasingly important, which can simplify optical coupling processes, enhance 
production efficiency and improve product consistency.   
 
There are two main technologies used for implementing WDM optics technology in optical 
modules: 
1) Thin-Film Filters (TFF), based on free-space optics; 
2) Planar Lightwave Circuit (PLC)-Based Solutions, including Arrayed Waveguide Gratings 
(AWG), Echelle Diffraction Gratings (EDG), and Cascaded Mach-Zehnder 
Interferometers (MZI).   
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. 
 
 
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. 
 
A typical 400G Rx optical integrated subassembly based on Z-block technology which 
integrates all key components of 400G ROSA, including receptacle, collimator, Z-block, lens 
array, prism, and baseplate. With just one coupling step to assembly into transceivers, greatly 
simplifying the process. 
 
 
 
 
Z-block technology offers the benefits of low loss and good channel quality but involves 
complex manufacturing processes. Several key assembly control factors must be considered:   
- Z-block surface dimensional accuracy: Critical for maintaining collimator beam quality. 
- Focal spot position tolerances (X/Y/Z axis): Optical simulation techniques are required to 
ensure precise alignment with the PD. 
- Mechanical and environmental reliability: High shear force and HAST requirements ensure 
product reliability and stability in applications.   
 
HYC provides fully customized optical coupling solutions for optical transceivers, covering 
both PD-to-Fiber and LD-to-Fiber connections, including 2-channel, 4-channel, 2×4-channel 
LAN-WDM/CWDM Block and BIDI Block, tailored for ROSA, TOSA, BOSA, etc. 
 
As optical networks and data centers advance toward ultra-high speeds, greater capacity, and 
higher integration, optical transceiver modules are adopting more compact and highly 
integrated solutions. This trend is fueling a rapid surge in demand for both parallel optical 
subassemblies and WDM optical subassemblies. 
 
 
About HYC Co., Ltd 
Founded in 2000, HYC is a leading global manufacturer of innovative and reliable passive 
optical components. HYC designs, develops, manufactures, and sells a comprehensive line of 
passive optical devices that enables 5G/6G, data center, data communication, FTTH, 
aeronautical communication networks.