Pulsed versus Continuous-Wave LIV Measurement: Self-Heating and Methodology

Scope

This article describes the choice between pulsed and continuous-wave (CW) modes for semiconductor laser LIV measurement, with focus on the self-heating bias introduced by CW operation and its impact on extracted parameters. Coverage includes duty cycle requirements, equipment configuration for pulsed measurements, and the quantitative magnitude of self-heating across common device classes. Pulsed measurements for high-speed characterization (small-signal modulation response, eye-diagram tests) are outside scope.

The physical issue

A semiconductor laser dissipates a fraction of its input electrical power as heat. The dissipated power is $P_\text{diss} = I V - P_\text{out}$, where $I V$ is the input electrical power and $P_\text{out}$ is the emitted optical power. For typical edge-emitter operation at $\eta_\text{WPE} \sim 30\%$, approximately 70% of the input electrical power becomes heat.

This dissipated heat raises the active region temperature above the heatsink temperature according to the thermal impedance $R_\text{th}$ of the device-and-mount assembly:

$$\Delta T_\text{junction} \;=\; R_\text{th} \cdot P_\text{diss}.$$

For a typical telecom DFB on a CuW submount with active TEC cooling, $R_\text{th} \sim 30{-}60$ K/W. For a high-power 980 nm pump diode on a copper heatsink, $R_\text{th} \sim 5{-}15$ K/W. For an unmounted edge-emitter die clamped between thermal contacts, $R_\text{th}$ may exceed 100 K/W.

The consequences for LIV measurement are systematic. As drive current is swept upward during a CW LIV measurement, dissipated power increases, the active region temperature rises, and the LIV curve responds to the elevated temperature rather than to the heatsink setpoint. Three observable effects result:

1. Threshold current increases along the sweep, biasing extracted $I_\text{th}$ high
2. Slope efficiency decreases along the sweep, biasing extracted $\eta_s$ low and producing apparent curvature in the LIV
3. Optical output rolls over at high current, producing the characteristic "thermal rollover" shape

Pulsed measurement at low duty cycle eliminates the steady-state temperature rise and reveals the device's underlying isothermal behavior.

Quantitative impact on extracted parameters

For a typical 1550 nm DFB operating at 60 mA, 1.4 V forward, and 10 mW optical output:

$$P_\text{diss} \;=\; I V - P_\text{out} \;=\; (0.060)(1.4) - 0.010 \;=\; 74 \text{ mW}.$$

With $R_\text{th} = 50$ K/W:

$$\Delta T_\text{junction} \;=\; 50 \cdot 0.074 \;=\; 3.7 \text{ K}.$$

For a typical $T_0 = 60$ K, the threshold current at $\Delta T = 3.7$ K above heatsink is elevated by

$$\frac{I_\text{th}(T + \Delta T)}{I_\text{th}(T)} \;=\; \exp(\Delta T / T_0) \;=\; \exp(3.7/60) \;=\; 1.064,$$

a 6.4% increase. For a $T_0$ extraction performed across a 40 K temperature range, this constant ~6% offset is small and largely cancels in the slope of $\ln I_\text{th}$ vs. $T$. The extracted $T_0$ is then nearly unaffected by self-heating.

For a high-power pump diode operating at 1 A, 1.8 V forward, and 700 mW optical output:

$$P_\text{diss} \;=\; 1.0 \cdot 1.8 - 0.7 \;=\; 1.1 \text{ W}.$$

With $R_\text{th} = 8$ K/W:

$$\Delta T_\text{junction} \;=\; 8 \cdot 1.1 \;=\; 8.8 \text{ K}.$$

Self-heating becomes significant. For a $T_0 = 130$ K InGaAs pump diode, $I_\text{th}$ is elevated by $\exp(8.8/130) - 1 = 7\%$. More critically, the elevated temperature reduces slope efficiency, so the LIV exhibits visible concave-down curvature beyond $\sim 0.5$ A — and the extracted $\eta_s$ depends strongly on the fitting range.

For a $T_0$ extraction at a single high-current operating point, the temperature rise above heatsink can exceed 20 K, producing a 15% systematic underestimate of $T_0$ versus the true low-temperature value.

The takeaway: self-heating bias scales with $R_\text{th} \cdot P_\text{diss}$. For low-power devices ($P_\text{diss} \lesssim 100$ mW) with good thermal mounting, CW measurement is acceptable for most extracted parameters. For high-power devices ($P_\text{diss} \gtrsim 500$ mW), pulsed measurement is required for accurate slope efficiency, $T_0$, and absolute threshold values.

Pulsed measurement principle

In pulsed mode, the drive current is applied as a train of short rectangular pulses of duration $\tau_p$ (pulse width) and repetition period $T_r$ (or equivalently repetition rate $f = 1/T_r$). Optical output is measured only during the pulses.

The duty cycle is

$$\text{DC} \;=\; \tau_p / T_r \;=\; \tau_p \cdot f.$$

During each pulse, the active region heats up at a rate set by its thermal time constant. Between pulses, it cools back to the heatsink temperature. For the average junction temperature to remain near the heatsink temperature, the duty cycle must be low enough that the cooling phase exceeds the heating phase by a substantial margin.

For typical edge-emitter assemblies, the thermal time constant of the active region itself is sub-microsecond, while the time constant of the submount-heatsink assembly is milliseconds. A common configuration uses $\tau_p = 1~\mu$s and $f = 1$ kHz, giving DC = 0.1% — the device experiences a brief temperature transient during each pulse, with the steady-state junction temperature elevated above heatsink by approximately DC × $\Delta T_\text{CW}$ — a factor of 1000 less than the CW value.

In practice, with $\tau_p = 1~\mu$s and DC = 0.1%, junction temperature elevation during the pulse itself depends on the device thermal time constant. For a properly mounted device with thermal time constant $\tau_\text{th} > 10 \tau_p$, the temperature elevation during the pulse is approximately

$$\Delta T_\text{pulse} \;\approx\; \Delta T_\text{CW} \cdot \tau_p / \tau_\text{th}.$$

For $\tau_p = 1~\mu$s and $\tau_\text{th} = 100~\mu$s, this gives $\Delta T_\text{pulse} \approx 0.01 \cdot \Delta T_\text{CW}$ — negligible for most extraction purposes.

Equipment

Pulsed LIV measurement requires equipment substantially different from a CW setup.

Common commercial pulsed-current drivers for laser diodes include the ILX Lightwave LDP-3811, Keithley 2461 pulsed mode, and various AVTECH pulse generators. Pulsed measurements at currents above $\sim 1$ A typically require dedicated high-current pulse generators.

The detector must have bandwidth sufficient to resolve the pulse shape. For a 1 μs pulse, the detector bandwidth should be $\geq 10$ MHz to faithfully reproduce the pulse top. Slower detectors integrate over rising and falling edges and introduce systematic errors in extracted optical power per pulse.

Procedure

1. Configure the pulse parameters

Set the pulse width and repetition rate to give DC $\leq 0.1\%$. For most semiconductor lasers, $\tau_p = 1~\mu$s and $f = 1$ kHz are standard. Higher repetition rates allow faster sweeps but increase DC; lower rates reduce noise but extend the measurement duration.

Verify that the chosen pulse width exceeds the device turn-on time (typically nanoseconds to tens of nanoseconds) and is long enough to allow the optical output to reach its quasi-steady state during the pulse. A scope trace of the optical output should show a flat-top pulse with no visible rise or droop during the integration window.

2. Set the sampling window

Configure the boxcar or oscilloscope to sample the flat-top region of the optical pulse, excluding the leading and trailing edges. A typical sampling window is 60% of the pulse width, centered on the pulse.

3. Verify zero-bias background

With the laser turned off (or biased below threshold), verify that the optical signal level is at or below the detector noise floor. Stray light from room illumination, photodetector dark current, or pulsed pickup from the current driver can contaminate the measurement.

4. Sweep the current

Step the pulse amplitude through the desired current range, recording the integrated optical power per pulse and the voltage during the pulse at each step. The resulting LIV represents the device behavior at the heatsink temperature, free of CW self-heating bias.

5. Compare to CW (optional)

A useful verification step is to repeat the LIV sweep in CW mode at the same heatsink temperature. The CW curve will show higher threshold, lower slope efficiency, and eventual thermal rollover; the pulsed curve will be cleaner and extend higher in current before any rollover (since the rollover in pulsed mode is due to intrinsic carrier and barrier effects rather than self-heating).

When to use which mode

Use CW mode when:

• The device is low-power and well-thermally-mounted ($\Delta T_\text{CW} < 2$ K at the maximum operating current)
• The application of interest is itself CW (datacenter optical modules, telecom transmitters, sensing applications)
• The measurement is for in-process screening rather than parameter extraction
• Pulsed equipment is not available

Use pulsed mode when:

• The device is high-power ($P_\text{diss} > 500$ mW at the maximum operating current)
• The device is unmounted (bare die on a clip, edge-coupled to fiber on a probe station)
• Accurate slope efficiency is required at currents well above threshold
• $T_0$ extraction is performed at high current operating points
• Thermal rollover is being characterized as a function of mount quality, and the underlying isothermal LIV must be separated from the thermal effect
• The application is itself pulsed (high-energy pulsed lasers, gain-switched sources, mode-locked oscillators)

Worked comparison

The same 1310 nm InP Fabry–Pérot device used in other articles is measured both CW and pulsed (1 μs, 1 kHz, DC = 0.1%) at 25 °C heatsink. CW measurement extends from 0 to 100 mA; pulsed measurement extends from 0 to 150 mA.

At 10 mA the two curves agree to within 1.5%, consistent with negligible self-heating in the near-threshold region. At 50 mA the CW curve is 6% low; at 100 mA it is 19% low and shows visible curvature suggesting impending thermal rollover.

Extracted parameters:

The pulsed extraction yields a 4% higher $\eta_s$ and a 6% lower $I_\text{th}$. For higher-power devices the discrepancy would be substantially larger.

Sources of error specific to pulsed measurement

Detector bandwidth too low. A detector with insufficient bandwidth integrates over the rising and falling edges of the pulse, producing measured power lower than the true flat-top power. Verify on a scope trace that the detected pulse shape is rectangular with $\geq 60\%$ flat-top duration.

Pulse-to-pulse jitter in current amplitude. Inexpensive pulse drivers may have $\sim 1\%$ amplitude jitter between pulses, producing apparent noise in the LIV. Averaging over many pulses (typical: 100–1000 pulses per current step) suppresses this.

Ground-loop pickup in the boxcar gate. The gate signal for boxcar integration shares a return path with the current pulse; ground-loop coupling can produce spurious counts at the boxcar output that are not from optical signal. Verify that the boxcar reading equals the noise floor when the current pulse is below threshold.

Inductive pulse distortion. Long cables between the pulse driver and the laser introduce inductance that distorts the pulse shape. The current pulse at the device may have a different shape than the programmed pulse. Verify pulse shape with a current probe at the device leads.

Insufficient pulse-off time for thermal recovery. If $T_r$ is short enough that the device does not fully cool between pulses, average junction temperature elevates above heatsink and the measurement loses the self-heating-free character. Standard guideline: $T_r > 100 \tau_\text{th}$, with $\tau_\text{th}$ the device thermal time constant.

Edge-resolved detection of relaxation oscillations. For pulse widths approaching the relaxation oscillation period of the laser (typically nanoseconds for telecom DFBs), the leading edge of the optical pulse exhibits relaxation oscillations that bias the integrated power measurement. Use pulse widths $\gtrsim 100 \tau_\text{RO}$ to ensure the integration window is in the steady-state region.

Validation

The pulsed LIV at low DC ($\leq 0.1\%$) should be independent of pulse width and repetition rate within stated tolerances. Vary $\tau_p$ over an octave (e.g., 0.5–2 μs) and verify that extracted parameters do not change by more than the measurement uncertainty.

For devices with verified low thermal impedance and low operating power, the pulsed and CW LIVs should agree to within the measurement uncertainty. Significant disagreement in the low-power regime indicates either pulse-shape distortion or detector calibration error.

References

For the textbook treatment of device thermal impedance and self-heating effects in semiconductor lasers, see Piprek (2003), Semiconductor Optoelectronic Devices: Introduction to Physics and Simulation, chapter 7. For practical pulsed-test setups including boxcar integration and bandwidth considerations, see the ILX Lightwave application note on pulsed laser diode testing. For thermal impedance measurement methodology and reference values for common device packages, see the JEDEC JESD51 series of standards.