What is a TO header?
A TO (transistor outline) header is a metal package with glass-to-metal sealed electrical leads and an optical aperture for a lens or window. Originally developed for transistors, TO headers are now the dominant packaging format for single-mode and multimode laser diodes, photodetectors, and VCSELs in telecom, datacom, industrial, and sensing applications.
The designator (TO18, TO46, TO56, TO60) specifies the base diameter, pin configuration, and standardized mechanical outline. Choosing the wrong one can mean redesigning your optical assembly — so this decision is worth getting right early.
Comparison by package type
| Package | Base ⌀ | Data rate | Typical pins | Primary application |
|---|---|---|---|---|
| TO18 | 5.6 mm | ≤2.5G | 2–3 | Single-mode DFB, FP laser, industrial laser |
| TO46 | 4.7 mm | ≤2.5G | 2–3 | VCSEL, low-speed sensor, compact emitter |
| TO56 | 5.6 mm | Up to 10G | 3–4 | 10G DFB, datacom, CATV |
| TO60 | 5.6 mm | Up to 25G | 4+ | 25G EML, coherent, high-speed datacom |
| TO5 | 9.0 mm | DC–low MHz | 3–8 | Detector, sensor, IR emitter |
| TO8 | 12.7 mm | DC–low MHz | 3–8 | High-power sensor, detector array |
TO18 — the workhorse
TO18 is the most widely used laser diode package in the industry. Its 5.6 mm base diameter accommodates most single-mode DFB and FP laser die sizes and is compatible with the widest range of optical assembly hardware (collimating lenses, fiber pigtails, free-space coupling optics). Data rates up to 2.5G are supported without special high-frequency design considerations.
Standard part number: FL-TO18-2G
Best for: Telecom (2.5G CWDM/DWDM), industrial laser systems, pump laser modules, sensing.
TO46 — compact, VCSEL-optimized
TO46's smaller 4.7 mm base makes it suitable for space-constrained designs and compact VCSEL-based emitters. The package is widely used in consumer-grade sensing, proximity detection, and low-speed multimode datacom. It is not suitable for high-speed single-mode applications due to lead inductance at the smaller form factor.
Best for: VCSEL arrays, ToF sensors, low-speed industrial emitters, consumer optical systems.
TO56 — 10G standard
TO56 shares the 5.6 mm base with TO18 but adds a fourth pin for monitoring photodiode (MPD) current and uses a lead configuration optimized for 10G bandwidth. The package is the standard choice for 10G SFP+ transceivers and is widely specified in CATV and 10G datacom applications. The high-frequency lead design requires attention to impedance matching on the PCB.
Standard part number: FL-TO56-10G
Best for: 10G SFP+, CATV, 10G CWDM/DWDM TOSA modules.
TO60 — 25G and beyond
TO60 is the high-speed variant for 25G and EML (electro-absorption modulated laser) applications. The lead configuration is designed for impedance-controlled transmission at 25G, and the package supports advanced modulation formats used in coherent and PAM4 systems. FerraLink's FL-TO60-25G is qualified for 25G TOSA applications in SFP28 and QSFP28 transceivers.
Standard part number: FL-TO60-25G
Best for: 25G SFP28, QSFP28, 100G-LR4 TOSA modules, EML-based optical transmitters.
Optical window selection
The cap and window choice significantly affects optical coupling efficiency and environmental protection:
| Window type | Transmission | Best for |
|---|---|---|
| Flat glass (uncoated) | ~92% | Low cost, non-critical coupling |
| Flat glass (AR coated) | >99% | High-efficiency coupling, reduced back-reflection |
| Sapphire flat | >96% | Harsh environment, scratch resistance |
| Ball lens | Focused beam | Fiber pigtailing, collimation |
| Aspheric lens | Optimized NA | High-NA coupling, diffraction-limited beam |
FerraLink pricing
FerraLink TO headers are sourced directly from certified manufacturers at 1/3 of typical US supplier pricing and 1/2 of Japanese supplier pricing. All parts ship with glass-to-metal seal qualification data and can be supplied with or without optical windows and caps.
Individual sample pricing starts at $25. A 10-piece testing box covering TO18, TO56, and TO60 variants is available at $250.