Types of Optics

Fully-retimed Optics

Fully-retimed optics are traditional optical modules designed to ensure the highest levels of signal integrity and performance. Fully-retimed optics utilize re-timing mechanisms for both the transmit (Tx) and receive (Rx) signals. That is, these optics operate with two DSPs. Re-timing involves regenerating the clock signals for both directions of data flow, minimizing jitter and other signal distortions. This approach results in a cleaner and more robust signal, facilitating high-speed data transmission with low latency and high reliability. Fully-retimed optics are especially critical in applications where data integrity and low latency are critical, such as in high-performance computing environments and data center interconnects (DCI).

Fully-retimed offers the following advantages:

• Highest signal integrity and reliability—Fully retimed optics eliminate jitter and ensure the highest level of signal clarity by regenerating clock signals on both transmission and reception paths.
• Low latency—By minimizing signal distortions and maintaining signal integrity, fully retimed optics support low-latency communication essential for time-sensitive applications.

Fully-retimed optics could pose the following challenges:

• Higher power consumption—The use of DSPs for both Tx and Rx signals increases the power requirements of the module.
• Increased cost—Incorporating two DSPs and associated retiming mechanisms makes fully retimed optics more expensive compared to solutions like Linear Receive Optics (LRO) or Linear Pluggable Optics (LPO).

Half Retimed Optics or Linear Receive Optics

Half retimed optics or LRO applies re-timing mechanisms to only one direction of data flow, mostly to the transmit (Tx) signals. That is, LRO modules operate with a single DSP. The receive (Rx) signals are typically handled by the host system in an LRO implementation. This approach offers a balance between performance and cost. By retiming only one direction, LRO modules still enhance signal quality and reduce jitter, although not as effectively as fully retimed optics. These optics are ideal for scenarios where moderate signal integrity improvement is adequate, providing a cost-effective solution for less critical datapath within a network.

LRO offers the following advantages:

• Reduced power draw—By eliminating the retiming function on the receive side, LRO modules consume less power compared to fully retimed modules. While the power savings are not as substantial as those of LPO modules, they still offer significant reductions.
• Cost efficiency—The absence of retiming circuitry on the receive side simplifies the receiver module and reduces its cost compared to fully retimed modules. Although the cost reduction is not as pronounced as in LPO modules, it is still beneficial.
• Improved interoperability—LRO modules reduce the overall risk to link performance by concentrating retiming within a single DSP mid-span between hosts. This configuration minimizes interoperability challenges and simplifies integration.

LRO could pose the following challenges:

• Moderate signal integrity—While LRO enhances signal integrity, it does not achieve the same level of improvement as fully retimed optics. The host system must ensure it can manage the signal recovery effectively.
• Compromise in savings—LRO represents a compromise solution, offering roughly half the power and cost savings compared to LPO interfaces. Although it provides some benefits, it does not fully maximize efficiency.

Linear Pluggable Optics

Linear optics or linear pluggable optics (LPO) rely on direct detection and analog signal processing for transmission. An LPO does not incorporate full retiming mechanisms, such as DSP circuitry. That is, LPO modules operate without any DSP. The latest generation of optical transceivers including 400GbE, 800GbE, and 1.6 T use LPO modules. Unlike traditional fully retimed optical modules, LPO transceivers depend on the host to handle retiming and signal conditioning. By omitting the DSP, LPO achieves lower power consumption and higher energy-efficient while still supporting high-speed data transmission.

LPO offers the following advantages:

• Lower power consumption—Removing the retimers reduces energy usage. Retimers are energy-intensive, and their absence in the module leads to significant power savings.

• Cost efficiency—LPO reduces module costs by eliminating the DSP. DSP accounts for over one-fourth cost of a typical transceiver module.

LPO could pose the following challenges:

• Signal integrity challenges—Ensuring robust link signal integrity is more challenging with LPO systems. The system must support approximately 16 dB of loss from the host switch to the module on both transmit and receive sides, along with several dB of optical loss.

• Interoperability issues—Connecting an LPO module with another LPO switch from a different manufacturer is a challenge at 100 Gbps per channel. This difficulty increases at 200 Gbps per channel.

Possible workarounds to mitigate these challenges include:

• Book-ended solution—This approach involves using one specific manufacturer's hardware on both sides of the link, simplifying implementation but limiting flexibility and leading to vendor lock-in.

• Engineered link—Custom designing connections for specific setups avoids vendor lock-in but increases the complexity and costs, making large-scale deployments less feasible.

Coherent Optics

Coherent optics utilizes advanced modulation formats and DSP to achieve high data transmission rates over long distances with exceptional signal integrity. Coherent optics employ sophisticated techniques such as phase modulation, amplitude modulation, and polarization multiplexing to encode data onto light waves. The use of coherent detection allows for precise recovery of the transmitted signals, even in the presence of significant dispersion and noise. Coherent optics support high spectral efficiency, making them ideal for modern high-capacity optical transport networks.

Note:

Juniper does not currently support coherent optics (ZR/ZR+) for 800 Gbps data transmission.