The document discusses the increasing demand for high-speed optical modules driven by the growth in AI workloads and the need for greater network bandwidth in data centers. It outlines two main methods for enhancing bandwidth: increasing bit rates per channel and expanding the number of transmission channels using parallel optics and wavelength division multiplexing (WDM) techniques. Additionally, it highlights the importance of integrating optical components and advanced technologies to improve transmission efficiency and reduce costs.

1. 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.

2. 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

3. 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.

4. 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.

5. 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.

6. 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.