Optical Module PCB

Optical Module PCB

Optical modules are high-speed electrical-to-optical interface devices that convert gigabit electrical signals into photoelectric signals. The demand for these modules is rapidly increasing due to big data, 5G, blockchain, artificial intelligence and cloud computing.

To reduce power consumption, OBO and CPO technologies move the optical engine close to the switch chip, which shortens the electrical connection distance and improves the quality of high-speed signal.

The Optical Sub-Assembly (ROSA)

Optical modules use light to transmit and receive electrical signals. They consist of optoelectronic devices, functional circuits, and optical interfaces. They can convert electrical signals into optical signals and vice versa, which makes them useful in high-speed networks and data centers. They can also be used to connect Optical Module PCB servers and switches. Typical optical modules include the XFP (10-Gigabit Small Form Factor Pluggable), CFP, CWDM, GBIC, and QSFP+.

The ROSA (Receiver Optical Sub-Assembly) of an Optical Module PCB is made up of photodetectors, amplifiers, and other components that enable it to receive and process optical signals. The photodetectors convert optical signals into electronic signals through the photoelectric effect. They can be either PIN or avalanche photodiodes. The avalanche photodiodes have higher receiver sensitivity than PIN photodiodes and can support a wider wavelength range.

The amplifiers provide additional power to the signals. This ensures that they can be transmitted over long distances. The amplifiers also improve the signal-to-noise ratio of the amplifier by reducing noise from the environment. The amplifiers can be either pre-amplified or post-amplified. A pre-amplified amplifier can be used to boost a low-level signal while a post-amplified amplifier can enhance the strength of a weaker signal. Both types of amplifiers can reduce the jitter and SNR of an optical transmission system by eliminating gaps in the signal.

The Optical Engine

A key part of the OE is the optical transceiver, which converts digital electrical signals into light signals. This requires highly-specialized compound semiconductor materials – like Indium Phosphide and Gallium Arsenide – fabricated into a variety of active devices, including lasers and detectors. These devices are connected to each other via waveguides, which then direct the light through “passive” devices (such as multiplexers and de-multiplexers) and ultimately into the fiber. The reverse process, which turns light back into electronic signals, is also part of the OE.

A significant problem with high-speed optical communication is the degradation of the signal caused by the long distance between the engine and the switch chip. To reduce this, the engine is moved closer to the switch chip, a solution known as onboard optics (OBO). This approach is also sometimes called co-packaged optics (CPO), and it shortens the electrical connection distance between the engine and the switching chip to near zero.

AT&S has invested in a robust simulation process for thermal performance, warpage, stress, high-speed signal loss, and electromagnetic compatibility (EMC). AT&S’s strong R&D team and simulation capabilities allow us to quickly and reliably provide customers with the results they need to make informed design decisions. Moreover, AT&S’s advanced modeling and simulation software provides the ability to run virtual tests to validate thermal performance and other design parameters.

The Switch Chip

Optical Module PCBs use switch chips to convert photoelectric signals into electrical ones. The switch chip also provides a control interface. The outputs of the switch chip are either a differential LVPECL data signal or an output containing both a data and a signal detect. The latter signal is used for error detection.

Traditionally, the optical interface of an optical module has been connected to a copper-based MAC via a PHY. However, the latest systems are utilizing parallel optical transceivers on the board to support hundreds of Gbps of data rates. The data paths run over fiber off the board to a daughtercard or backplane, and back to the MAC via multiple differential lanes.

The most common optical modules in the market are XFP, CWDM, CFP and QSFP+. The most recent versions of these modules are designed to support backhaul in 5G access networks. They will need to support a variety of interface protocols, including eCPRI, CPRI, NG-PON, and OTN, as well as modulation formats such as NRZ, ERZ, and PAM4.

These optical modules are packaged in surface-mount components with a BGA footprint or as pre-packaged daughtercards. The latter are used in military embedded computing applications and follow Optical Module PCB Supplier a form factor standard such as VITA 57.1 or VPX.

The Electrical Interface

As data rates have increased from 100G to 400Gbps, the demand for faster and farther has driven a significant change in the electrical interface. To avoid high-speed signal loss, careful consideration is required to ensure that the PCB layout provides good impedance matching.

In the early stages, optical modules used simple passive circuits to provide a low cost, high-speed connection from copper to optical fiber. However, as speeds increased the interface was upgraded to a retimed digital interface. The Optical Internetworking Forum (OIF) defined the standard known as the Common Electrical Interface (CEI).

The CEI uses multiple layers of circuitry on a flex board to connect the module to the PCB. The circuitry is designed to be compatible with the optical device, and includes the necessary impedance matching resistors and filter capacitors for proper operation. The flex circuit also incorporates the necessary ground plane for efficient thermal transfer between the module and the host PCB.

As an experienced PCB manufacturer, AT&S works closely with global, renowned optical module manufacturers to empower their high-speed design and manufacturing processes. Our strong R&D and simulation capabilities enable thermal performance analysis, warpage, stress, electromagnetic compatibility (EMC) and high-speed signal loss prediction. We have successfully developed and characterized both 400G and 800G optical module PCBs, with preliminary research on 1.6T optical modules underway.