Tool · Laser characterization

Junction Temperature Calculator

How much hotter than its heat sink does the active region of this laser run, and what thermal resistance does that imply? In one mode the calculator converts a measured shift of the lasing wavelength between pulsed and CW operation into a junction temperature rise ΔT_j = Δλ / (dλ/dT) and divides it by the dissipated power to give R_th; in the other it predicts T_j from a known R_th and case temperature. Background: wavelength temperature coefficient, DFB lasers, and wall-plug efficiency.

Presets

The first preset is the worked example. The 1310 nm case uses dλ/dT = 0.078 nm/°C, the computed value for a 1310 nm DFB in the glossary table, with example drive values. The pump case dissipates 2 W at 30 K/W, a 60 °C rise, the figure quoted in the companion article.

Drive current (mA)

Forward voltage V_f (V)

Optical power out (mW)

Dissipated power P_diss = I·V_f − P_opt = 178 mW

Wavelength tuning dλ/dT (nm/°C)

From the pulsed calibration; ~0.09–0.10 for 1550 nm DFBs.

Measured shift Δλ, CW − pulsed (nm)

Case (stage) temperature (°C)

The stage temperature at which both the pulsed and CW spectra were taken.

Result

Junction temperature rise ΔT_j: 6.5 °C
Junction temperature T_j: 31.5 °C
Thermal resistance R_th: 36.4 K/W

Junction temperature vs. drive current

Uses the extracted R_th, a constant V_f, and an optical power that scales with current so that P_opt/(I·V_f) keeps its entered value; the dot is the entered operating point. Under these assumptions the line is straight. A real laser emits almost nothing below threshold, and its V_f rises with current through the series resistance, so the true curve departs from this line away from the operating point.

Method, calibration procedure, and the systematic errors to avoid are covered in the companion article Measuring laser diode junction temperature.

The address holds the mode and every input.

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The model

The heat deposited in the chip is taken as the electrical input less the light that leaves it:

P_{diss} = I V_{f} - P_{opt}

In the extraction mode, the lasing wavelength of a DFB laser is first calibrated against stage temperature under short, low duty cycle pulses, which gives the coefficient $d λ / d T$ with negligible self-heating. The same laser is then run CW at the same stage temperature, and the red shift $Δ λ$ of the CW line relative to the pulsed line is read as a temperature rise:

Δ T_{j} = Δ λ / (d λ / d T)

R_{th} = Δ T_{j} / P_{diss}

With the stage temperature entered as the case temperature, the extraction mode also reports $T_{j} = T_{case} + Δ T_{j}$ .

The prediction mode runs the second relation backwards, $T_{j} = T_{case} + R_{th} P_{diss}$ .

The method relies on the DFB wavelength following the grating, whose effective index changes with the temperature of the guided-mode region; the result is therefore an average over that region rather than the peak temperature at the hottest point. Above threshold the carrier density is nearly clamped, so the carrier-induced index change between pulsed and CW operation is small, though not zero. A Fabry-Perot laser does not suit the method, because its spectrum follows the gain peak and hops between modes. If the power entered is front-facet power only, the rear-facet light is counted as heat, which overstates $P_{diss}$ and understates $R_{th}$ . The model is linear, so it assumes $R_{th}$ does not change with temperature over the range involved.

The plot of junction temperature against drive current uses the thermal resistance of the active mode, extracted or entered, and holds $V_{f}$ and the ratio $P_{opt} / (I V_{f})$ at their entered values, so that $T_{j} (I) = T_{case} + R_{th} P_{diss} I / I_{0}$ is a straight line through the operating point $I_{0}$ . Near threshold, where the laser emits little light, and at high current, where the series resistance raises $V_{f}$ , a measured curve departs from this line.

Worked example, from the default inputs. At 150 mA and 1.45 V the electrical input is 217.5 mW; less 40 mW of light, the dissipated power is 177.5 mW, shown rounded as 178 mW. A shift of 0.62 nm at 0.096 nm/°C gives $Δ T_{j} = 6.46$ °C (shown as 6.5 °C), and dividing by 0.1775 W gives $R_{th} = 36.4$ K/W. With the default 25 °C stage, the extracted junction temperature is 31.5 °C, and the plot reaches 25 + 2 × 6.46 = 37.9 °C at 300 mA. In the prediction mode, the default 36 K/W and 25 °C case give a rise of 36 × 0.1775 = 6.4 °C and $T_{j} = 31.4$ °C.

References: L. A. Coldren, S. W. Corzine and M. L. Mashanovitch, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012). The companion article Measuring laser diode junction temperature gives the calibration procedure.