Optical transceiver / transponder
A transceiver is a module that combines an optical transmitter and receiver and converts between electrical and optical signals. A transponder converts a client signal to a line-side wavelength through an optical-electrical-optical stage, usually adding forward error correction and transport framing.
An optical transceiver and an optical transponder both bridge electrical signals and optical fiber, but they sit in different places in a network. A transceiver is the module that turns a host's electrical signal into light and back. A transponder takes a client signal, often from a short-reach transceiver, converts it to electrical form and retransmits it on a wavelength and format suited to the transport network: an optical-electrical-optical (O-E-O) conversion between a client side and a line side.
Transceiver (short for transmitter + receiver). A module that converts between an electrical host interface and optical fiber:
- Accepts electrical data input (typically NRZ or PAM4)
- Modulates a laser to produce the optical output
- Receives optical input on a photodetector
- Outputs the recovered electrical signal
Many high-speed transceivers contain a DSP: 400G and 800G PAM4 modules equalize and retime the signal, and coherent modules carry a full coherent DSP with forward error correction.
Standard datacom transceivers: SFP, SFP+, SFP28, QSFP+, QSFP28, QSFP56, QSFP-DD, OSFP.
Transponder. A transponder has a client side, which connects to routers or switches through standard short-reach optics, and a line side, which transmits on a wavelength of the transport network, such as a channel of a DWDM grid. A muxponder combines several client signals onto one line signal. On a coherent line side, the DSP performs:
- Forward error correction (FEC) encoding on outgoing data, decoding on incoming data
- Constellation mapping for high-order modulation (QPSK, 16-QAM, 64-QAM)
- Chromatic dispersion compensation
- Carrier phase recovery
- Polarization tracking and polarization demultiplexing
- Adaptive equalization
Coherent line interfaces carry 100 Gb/s to 1.6 Tb/s per wavelength.
Pluggable form factors and capacities.
| Form factor | Typical capacity | Application |
|---|---|---|
| SFP+ | 10 Gb/s | 10G Ethernet, datacom |
| SFP28 | 25 Gb/s | 25G Ethernet, 5G fronthaul |
| QSFP28 | 100 Gb/s (4×25 Gb/s NRZ) | 100G Ethernet, datacom |
| QSFP56 | 200 Gb/s (4×50 Gb/s PAM4) | 200G Ethernet |
| QSFP-DD | 400 Gb/s (8×50 Gb/s PAM4) | 400G Ethernet, datacom |
| QSFP-DD or OSFP, 400ZR / ZR+ | 400 Gb/s coherent | Data center interconnect and metro |
| CFP2-DCO | 100 – 400 Gb/s coherent | Coherent metro and regional links |
| OSFP | 800 Gb/s (8×100 Gb/s PAM4) or 800G coherent | Datacenter switch interfaces, AI cluster fabrics |
| OSFP | 1.6 Tb/s (8×200 Gb/s PAM4) | Newest datacenter and AI cluster links |
Power grows with rate and with reach: a coherent module draws more power than a short-reach module in the same form factor, and each MSA defines the power classes a host must be able to cool.
Transponder cards vs pluggables. Transponders were traditionally line cards in a transport shelf, with the optics, DSP and FEC on one board. As CMOS scaling shrank coherent DSPs and pluggable power budgets grew, the line-side function moved into pluggable modules such as 400ZR in QSFP-DD and OSFP, which plug directly into routers and switches and remove the separate transponder. Line-card transponders remain for long-haul and subsea links and for the highest rates per wavelength, where the DSP and optics exceed what a pluggable can power and cool.
Transmit and receive subsystem architectures.
For direct-detection datacom (NRZ or PAM4):
- TX: directly modulated VCSEL (850 nm) or DFB, or a DFB with an external modulator (an EML or a silicon photonic modulator)
- RX: photodiode (GaAs at 850 nm; InGaAs or germanium-on-silicon at 1310 nm) and transimpedance amplifier, followed by a limiting amplifier and clock-and-data recovery in NRZ modules, or by a DSP that equalizes and retimes the signal in PAM4 modules
For coherent telecom:
- TX: narrow-linewidth tunable laser on the ITU grid + dual-polarization I/Q modulator (silicon photonic, InP, or lithium niobate, including thin-film LNOI) + driver amplifiers
- RX: narrow-linewidth local oscillator (in many pluggables, the transmit laser split in two) + polarization-diverse 90° hybrid + balanced photodetectors + analog-to-digital converters sampling at between one and two samples per symbol
- DSP: chromatic dispersion compensation + adaptive equalization + carrier recovery + symbol decoding + FEC decoding
MSA (multi-source agreement) standards. Pluggable form factors are defined by industry MSAs (CFP-MSA, QSFP-DD-MSA, OSFP-MSA) so that any compliant module from any vendor works in any compliant host system. The MSA defines mechanical envelope, electrical interface, optical interface, management protocol (typically I²C-based CMIS), and power budget.
References: OIF (Optical Internetworking Forum) implementation agreements for 400ZR, 800ZR and 800LR; the QSFP-DD, OSFP and CFP MSA specifications for the form factors.