TO56 and TO60: what the headers offer
TO56 (5.6 mm diameter) and TO60 (6.0 mm diameter) are the two most common transistor outline formats for single-mode DFB and distributed Bragg reflector (DBR) laser diodes used in telecom, sensing, and datacom applications. They accommodate lasers up to ~1 W CW output at 10G, 25G, and beyond, with isolated or non-isolated pin configurations, and with or without monitor photodiodes.
The critical distinction is whether the header includes an integrated thermoelectric cooler (TEC) mounted beneath the laser chip submount. A TEC actively pumps heat away from the laser junction, holding it at a stable setpoint temperature regardless of ambient conditions. An uncooled header relies entirely on passive heat dissipation through the case wall and the carrier board.
Why temperature matters for laser diodes
All semiconductor laser diode parameters drift with temperature:
| Parameter | Typical drift rate | Consequence if uncontrolled |
|---|---|---|
| Lasing wavelength | 0.08–0.12 nm/°C (DFB) | Channel collision in DWDM; out-of-band for etalon locking |
| Threshold current | +0.5–1.5 mA/°C | Output power drop; PID overcorrects, oscillation |
| Slope efficiency | −0.003–0.01 W/A/°C | Lower output at same drive current |
| SMSR (side mode suppression) | Degrades above Tj | Multimode operation, spurious spectral lines |
| Device lifetime (MTTF) | Halved per ~10°C rise (rule of thumb) | Reliability failure in field conditions |
For DFB lasers used in DWDM systems, where channel spacing is 100 GHz (0.8 nm at 1550 nm) or 50 GHz (0.4 nm), a 5°C ambient swing can shift the emission wavelength by 0.5 nm — enough to exceed the ITU-T channel mask. Temperature control is not optional in this application.
TEC-cooled TO headers: how they work
A TEC module (Peltier element) placed between the header floor and the laser submount uses electrical current to pump heat from the cold (laser) side to the hot (case) side. A thermistor on the cold plate provides a feedback signal to a temperature controller IC, which adjusts TEC drive current to maintain setpoint.
Typical TEC specifications in a TO60 cooled header:
| Parameter | Value |
|---|---|
| TEC Qmax (max heat pumping) | 0.5–1.5 W |
| Max temperature differential (ΔT) | 30–50°C |
| Operating voltage | 0.5–2.5 V |
| Max current (Imax) | 0.7–1.5 A |
| Thermistor type | NTC 10 kΩ at 25°C |
| Pin count (cooled header) | 6–8 pins (+ TEC+ / TEC− / thermistor) |
The TEC itself consumes power: for every 1 W of laser heat removed, the TEC may consume 1.5–3 W of electrical power, depending on efficiency (COP). At cold plate setpoints well below ambient — as in outdoor applications — TEC power can exceed laser drive power. This is a critical system budget consideration.
Uncooled TO headers: advantages and tradeoffs
Uncooled TO56/TO60 headers eliminate the TEC and thermistor, using 2–4 pins instead of 6–8. They are smaller, lighter, less expensive, and require no temperature controller IC on the driver board. They are the right choice when:
- The laser application is narrowband and wavelength stability is not critical (pump lasers, power delivery, illumination).
- The device operates in a temperature-controlled enclosure (instrument chassis, server rack at 0–70°C).
- The system already performs current modulation to compensate for optical power drift — and wavelength shift is acceptable.
- Cost and power budget are constrained and the application can tolerate ±1–3 nm wavelength variation.
- High-speed direct modulation (10G+ NRZ, PAM4) is required — TEC adds parasitic capacitance and loop latency.
Many industrial fiber-coupled pump lasers at 793 nm, 915 nm, and 976 nm use uncooled TO headers because their Yb-doped fiber load is spectrally broad enough to accommodate ±5 nm wavelength drift, and the high optical power required makes TEC overhead prohibitively expensive.
Decision matrix: cooled vs. uncooled TO header
| Application | Recommended header | Reason |
|---|---|---|
| DWDM telecom (C-band, L-band) | Cooled TO60 | ±0.05 nm wavelength stability required |
| 2.5G BIDI access modules | Uncooled TO56 | Wide spectral range; cost-sensitive |
| FMCW lidar (1550 nm coherent) | Cooled TO60 | Phase noise degrades with Tj fluctuation |
| ToF pulsed lidar (905 nm, 1064 nm) | Uncooled TO56 | Intensity, not wavelength, is the parameter |
| Spectroscopy / gas sensing (TDLAS) | Cooled TO56/TO60 | Wavelength tuning precision is the measurement |
| Industrial CW fiber laser pump | Uncooled TO60 | Power delivery; high output; TEC budget prohibitive |
| Medical / diagnostic fluorescence | Often uncooled | Depends on emission filter bandwidth |
| High-speed 25G datacenter SFP28 | Usually uncooled | Temperature-compensated modulation firmware |
TEC power budget: a worked example
Consider a 25G DFB laser at 1550 nm, 10 mW output, 100 mA drive current, 1.5 V forward voltage operating in a module that may reach 70°C ambient (industrial temperature grade).
The TEC in this case consumes nearly 3× the laser power. In a battery-powered application or a high-density module where thermal headroom is limited, this power budget may be decisive. In a pluggable telecom module with ample power rail headroom, it is routine.
FerraLink TO header options
FerraLink supplies both cooled and uncooled TO headers in TO18, TO46, TO56, and TO60 formats, with glass-to-metal hermetic seals, custom pin configurations, and optional optical windows (sapphire, ball lens, AR-coated, aspheric). Standard part numbers:
Custom pin count, glass seal material, and cap/window options available. Contact us with your optical power, wavelength, modulation rate, and operating temperature range.