Optical transceiver / transponder

An optical transceiver and an optical transponder both bridge electrical signals to optical fiber, but they differ in the complexity of signal processing they perform.

Transceiver (short for transmitter + receiver). A passive-DSP electro-optical module:

• Accepts electrical data input (typically NRZ or PAM4 binary)
• Modulates a laser to produce the optical output
• Receives optical input on a photodetector
• Outputs the recovered electrical signal

Standard datacom transceivers: SFP, SFP+, SFP28, QSFP+, QSFP28, QSFP56, QSFP-DD, OSFP.

Transponder. Includes a transceiver plus a digital signal processor (DSP) that 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 mode demultiplexing
• Adaptive equalization

Coherent transponders process 100G – 1.6T per port. The DSP component is typically larger and dissipates more power than the optical transmit/receive components combined.

Pluggable form factors and capacities.

Form factor	Typical capacity	Power	Application
SFP+	10 Gb/s	$< 1$ W	10G Ethernet, datacom
SFP28	25 Gb/s	$< 1.5$ W	25G Ethernet, 5G fronthaul
QSFP28	100 Gb/s (4×25 NRZ)	3 – 4.5 W	100G Ethernet, datacom
QSFP56	200 Gb/s (4×50 PAM4)	3 – 5 W	200G Ethernet
QSFP-DD	400 Gb/s (8×50 PAM4)	10 – 15 W	400G Ethernet datacom
OSFP	400 Gb/s and beyond	12 – 18 W	400G/800G, often coherent
CFP2-DCO	100 – 200 Gb/s coherent	12 – 20 W	Coherent metro and access
QSFP-DD ZR / ZR+	400G coherent	14 – 18 W	DCI and metro coherent
OSFP800	800 Gb/s (8×100 PAM4 or 2×400G coherent)	15 – 25 W	Datacenter switch interfaces, AI fabric
OSFP1600	1.6 Tb/s	25+ W	Emerging 2025-2027

Transponder cards vs pluggables. Historically, transponders were large rack-mount line cards: optics + DSP + FEC chips on a $\sim$ 30 × 20 cm board. With Moore's law shrinking DSP silicon and pluggable form factors expanding power budgets, coherent transponders have migrated into pluggables (QSFP-DD-ZR was the breakthrough, OSFP makes everything else possible). Vendor-side line-card transponders persist for very high-end long-haul transmission ($> 800$ km, $> 600$ Gb/s) where pluggable thermal limits become binding.

Transmit and receive subsystem architectures.

For direct-detection datacom (NRZ or PAM4):

• TX: directly-modulated VCSEL or DFB or external modulator + DFB; Ge-on-Si PD receiver
• RX: limiting amplifier + clock-and-data recovery + signal regeneration

For coherent telecom:

• TX: low-linewidth ITU-grid laser + dual-polarization I/Q modulator (typically LNOI or thin-film LN) + driver amplifiers
• RX: matching low-linewidth LO laser + polarization-diverse 90° hybrid + balanced photodetectors + analog-to-digital converters at 2× symbol rate
• 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.

Cost trajectory. 400G ZR coherent pluggables launched at ~$5000 per unit in 2020; current prices (2026) are $1000 – $2500. 800G ZR / ZR+ launched 2024 – 2025 at $3000 – $5000 per unit; expected to drop 30 – 50 % over 2025 – 2027 as production scales.

References: OIF (Optical Internetworking Forum) implementation agreements (400ZR, 800LR, 800ZR) for the application-layer specifications; Cisco Optical Networking 800G Architecture Reference for the pluggable-vs-transponder design rationale.