Two fundamentally different laser requirements
Pulsed ToF lidar sends a short, intense pulse of light and measures the round-trip travel time. The laser is typically a high-peak-power edge-emitting device at 905 nm (Si APD compatible) or 1064 nm, operating in pulsed mode with duty cycles well below 1%. Peak currents can reach 20–50 A for 1–5 ns pulses.
FMCW lidar emits a continuous, frequency-chirped signal — typically a single-mode DFB or external cavity laser at 1550 nm — and measures range and velocity from the beat frequency between emitted and reflected light. The laser runs CW, at modest optical power (1–30 mW), and must maintain coherence over the chirp duration. Phase noise, linewidth, and frequency stability are the critical parameters.
These two operating modes lead to radically different packaging requirements.
Laser source comparison
| Parameter | Pulsed ToF lidar | FMCW coherent lidar |
|---|---|---|
| Wavelength | 905 nm, 1064 nm | 1550 nm |
| Laser type | Gain-switched Fabry-Pérot or bar | Single-mode DFB or ECL |
| Operating mode | Pulsed (1–50 ns, <1% duty cycle) | CW chirped |
| Peak current | 20–50 A | 50–150 mA |
| Average power | 50–500 mW | 1–30 mW |
| Linewidth requirement | Not critical | <1 MHz (instantaneous) |
| Wavelength stability | ±2–5 nm acceptable | <1 pm critical |
| Phase noise | Irrelevant | Critical — determines velocity resolution |
| Temperature sensitivity | Low | High — requires TEC or compensation |
Submount material: SiC for pulsed, ALN for FMCW
The submount material choice follows directly from the laser operating mode.
For pulsed lidar at 905 nm and 1064 nm, the laser operates at very high peak currents (20–50 A) for nanosecond durations. The instantaneous power dissipated in the junction during a 5 ns pulse at 30 A and 2 V forward voltage is 60 W — for 5 nanoseconds. The submount must absorb and spread this thermal spike faster than the pulse duration to prevent micro-cracking and junction damage. Silicon carbide at 350–400 W/m·K outperforms ALN in this transient thermal spreading role. SiC is also electrically semi-insulating, making it compatible with both p-side-down and n-side-down die attachment.
For FMCW lidar at 1550 nm, the laser is a low-power CW DFB device — similar to a telecom DFB. Heat load is modest (50–200 mW), and the dominant concern shifts from thermal spreading to CTE matching and wavelength stability. ALN at 4.3–4.6 ppm/°C is the best match for InP-based DFB lasers (4.5 ppm/°C). Combined with a TEC-cooled TO60 or butterfly package, ALN delivers the junction temperature stability needed to maintain coherence and chirp linearity.
Pulsed ToF
SiC Submount
Peak power handling, transient thermal spreading, 350–400 W/m·K
FMCW
ALN Submount
CTE match for InP DFB, steady-state thermal stability, 170–210 W/m·K
Package type requirements
Package format is driven by the optical power level, beam geometry, and environmental requirements.
| Requirement | Pulsed ToF lidar | FMCW coherent lidar |
|---|---|---|
| Package format | TO56 / cavity package / bar package | TO60 TEC / butterfly / coaxial |
| Hermeticity | Required (MIL-STD-883 leak rate) | Required — phase noise degrades with humidity |
| Window type | AR-coated flat (broadband) | AR-coated + optional isolator (OI) |
| Fiber coupling | Rare (free-space emitter) | Common (PM fiber output, FC/APC) |
| TEC | Usually uncooled | Almost always TEC-cooled |
| Monitor PD | Sometimes (power monitoring) | Required for APC feedback |
| Thermal resistance target | <5°C/W (case to die) | <3°C/W (case to die, TEC must manage rest) |
Optical window selection
The optical window of a hermetic package has a disproportionate impact on performance for both lidar types.
For pulsed 905 nm lidar, the window must transmit efficiently at 905 nm without etalon effects that distort the pulse shape. A flat AR-coated borosilicate or fused silica window with broadband AR coating (800–1100 nm) at reflectance <0.5% per surface is standard. Sapphire windows are used in high-reliability automotive applications for scratch and impact resistance.
For FMCW 1550 nm coherent lidar, back-reflections into the laser cavity are a major source of phase noise. Even a 0.1% back-reflection can corrupt the coherence of a narrow-linewidth DFB. The window must have an ultra-low-reflection AR coating (<0.1% at 1550 nm), and — in demanding applications — an optical isolator (30–40 dB isolation) is placed between the laser and the window. This is a key reason many FMCW systems use butterfly packages with integrated PM fiber coupling and built-in isolators.
Environmental and reliability requirements
Both lidar types are typically deployed in automotive, industrial, or outdoor environments where temperature cycling, vibration, humidity, and dust are real operating conditions. The key differences:
- Pulsed lidar at 905 nm may use Si APD detectors at the receiver, which are sensitive to wavelength only within ~10 nm. The package must maintain beam collimation and hermeticity but does not require the wavelength precision of FMCW.
- FMCW lidar relies on the coherent interference between emitted and received light. Any frequency excursion during the chirp — from vibration-induced current noise, thermal transient, or mechanical deformation of the fiber-to-package coupling — directly degrades the range and velocity measurement.
- Automotive qualification (AEC-Q101 equivalent) requires both types to pass −40°C to +125°C temperature cycling for typically 1,000 cycles, plus damp-heat (85°C/85% RH, 1,000 hours) for hermeticity and HTOL (high-temperature operating life) for electromigration.
- Metal-ceramic hermetic packages from FerraLink meet MIL-STD-883 Method 1014 leak rate requirements (≤1.0×10⁻³ Pa·cm³/s) for both pulsed and FMCW applications.
Summary: packaging decisions by lidar type
FerraLink lidar packaging components
FerraLink supplies the submounts, TO headers, hermetic packages, and optical transceivers used in both pulsed ToF and FMCW lidar systems:
- SiC submounts (350–400 W/m·K) for pulsed emitter bars and high-peak-power single emitters
- ALN submounts with AuSn predeposited for FMCW DFB laser die attach
- TO56 and TO60 headers (cooled and uncooled, with AR-coated windows)
- Cavity hermetic packages for multi-emitter lidar bars and VCSEL arrays
- Butterfly packages for fiber-coupled FMCW emitters