Pulse tests keep laser diodes cool

Laser diodes, the devices that illuminate telecom fiber-optic cables, require light-current-voltage (L-I-V) measurements during production. These measurements let you characterize a laser diode's light output and forward voltage as a function of input current. You can use the measurements to correlate optical and electrical characteristics with semiconductor process information. You often can use the data to:

• indicate a pass/fail condition to weed out bad parts,
• alert process engineers to possible deviations in production, and
• sort devices according to optical performance and electrical characteristics.

The earlier in production you perform the testing, the sooner you can take corrective action.

Testing a laser diode involves applying power and measuring its characteristics. But the voltage across a laser diode and the current passing through it will cause the chip to self-heat. Self-heating causes unwanted changes in the device's characteristics—forward voltage, resistance, efficiency, and threshold current—that adversely affect its performance. Commercial laser-diode modules (LDMs) include heat sinks and thermoelectric coolers (TECs, also called Peltier devices) that maintain a laser diode's temperature to within 0.005°C, and thus maintain required operating conditions.

Testing laser diodes that exist on a wafer or in die form presents a challenge. These devices lack TECs, so maintaining a constant, controlled temperature proves impractical, if not impossible. To test laser diodes before mounting them on carriers, you can use a pulsed current test system (Figure 1) that consists of a pulse source, current-to-voltage (I-V) converters, facet detectors, and a digital oscilloscope.
 

To test a laser diode, you must subject it to an increasing forward current while you measure the DUT's forward voltage and optical output power. You also must measure the DUT's optical output power with the photodiodes and I-V converters. From those measurements, you can plot laser characteristics, such as threshold current (I_TH in Figure 2. You derive I_TH by taking the second derivative of the DUT's optical output power (L) with respect to forward current (I_F).

Figure 2. To calculate a laser diode’s threshold current (ITH), calculate the second derivative of the DUT’s light output as a function of input current. The spike indicates the threshold current.

Figure 3. In a kink test, you must find the derivative of light output to input current. A negative slope indicates a kink.

To perform an L-I-V pulse test, you must deliver current pulses with suitable magnitude, duration, duty cycle, and rise and fall times. Typical duration ranges from 500 ns to 1 µs with a 1% duty cycle.

You can use either of two common methods of delivering current pulses: a pulsed constant-current source coupled directly to the laser diode, or a pulsed constant-voltage source that drives a known resistance. The pulsed current source is more accurate.

During a pulsed current test, you measure the laser diode's optical output power with a series of current pulses. Increase the amplitude of each successive pulse so you "sweep" the current from no current to the DUT's maximum current level in 1-mA steps. For telecom transmitter lasers, the pulse will reach several hundred milliamps in amplitude; pump lasers used in erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers may need pulses up to 5 A.

The rise and fall times of the current pulses must be fast enough to maximize the flat time at the top of the pulse. Make the sum of the rise time and the fall time less than 30% of the total pulse width to allow for signal settling time and flat time at the top. For a 500-ns pulse, that equates to a maximum of about 75 ns each for the rise time and fall time. Figure 4 illustrates the timing for typical current pulses.

Figure 4. Drive a la ser diode with a current pulse with a width from 500 ns to 1 ms.

Pulse Delivery

The speed of an electrical pulse through a material is a function of the material's impedance. The change in propagation speed as a signal passes between different materials or impedances causes coupling loss and reflections. Reflections can result in constructive or destructive interference. When using high-speed pulses in laser diode testing, you must optimize signal coupling and minimize unwanted reflections. Otherwise, you'll get gross measurement errors, and you could damage the DUT. Use impedance-matching circuits in your measurement system to minimize these problems.

Typical voltage pulse sources, as well as many current pulse sources, have a characteristic 50-Ù output impedance. That's a good match for a standard 50-Ù coax cable, and it minimizes signal distortion when it's terminated with 50 Ù. A typical laser diode, however, has a characteristic impedance around 2 Ù. Connecting a 50-Ù coax directly to the 2-Ù laser diode results in a severe impedance mismatch.

You can reduce the mismatch if you place a resistor in series with the laser diode. The optimal resistance value you should use is the transmission line impedance less the laser diode characteristic impedance, or approximately 48 Ù.

Unfortunately, this technique has a negative side effect. To overcome the added resistive load, the pulse source must generate a voltage equal to the resistance of the load multiplied by the desired current. You need 250 V to drive 5 A through a 50-Ù load. That voltage poses a safety hazard to people, fixtures, and components. A poor electrical contact between the DUT probe and a laser chip can produce a 250-V arc followed by 5 A. The laser diode likely won't survive.

To avoid this mismatch, use a pulse source and transmission line with a characteristic impedance identical to that of the laser diode. The potential required to drive a 5-A pulse through a 2-Ù impedance reduces to 10 V.

The distance that an electrical pulse travels also affects the quality of the signal at the laser diode. And any reflection from the transmission line's termination travels back up the line. The drive circuit either absorbs it or reflects it again. If the drive circuit and laser diode generate reflections, the length of the transmission line strongly affects the settling time of the signal. Make all cables as short as practical.

When you perform pulse tests, you must verify the input pulses' integrity at the DUT. You can use a shunt resistor in series with the DUT. Use an oscilloscope under computer control to monitor the current pulse. The resistor's value must be low enough that it doesn't significantly affect the laser diode's input signal. "Low enough" means 10% or less of the laser diode's 2-Ù resistance. Choose a shunt resistor with a capacitance and inductance low enough to keep the overall load impedance 90% resistive. Don't use a common wire-wound resistor; it will create a high-impedance path for the high-frequency components of the current pulse.

Figure 5. A typical test system uses a current-pulse generator, a DMM, and an oscilloscope to excite a laser diode, measure the input current, and measure the DUT’s optical output power.

A scope probe can alter a laser diode's electrical characteristics because it appears to the transmission line as an unshielded, unterminated conductive stub. The probe changes the impedance at the probe connection, which produces reflections from the unterminated stub that can produce significant undershoot in the current pulse.

In current-pulse tests, you must capture the pulsed optical output of the laser diode, particularly its peak values. Then, you must time correlate the measurements to those of the laser diode's input current and forward voltage.

You can use a scope to display the input current by measuring the voltage across the shunt resistor, then use the instrument to calculate the current. To measure the DUT's optical output power, you need a photodiode with an electrical output that connects to a scope channel through an I-V converter. You can use the raw data to derive parameters such as the turn-on current and the knee. Follow these steps:

1. Set the oscilloscope to trigger on the pulse source's external trigger output.
2. Set the I-V converters to a suitable range based on I-V sweep values.
3. Configure the pulse source for the first current step (typically 0.25 mA).
4. Program the PC to trigger the pulse source over the IEEE 488 bus.
5. Use the DSO to capture the forward voltage drop and optical output of the laser diode.
6. Download waveforms from the DSO to the host computer over the IEEE 488 bus.
7. Use the PC to analyze each trace. Identify the flat portion of the input pulse, and calculate the corresponding value.
8. Through the IEEE 488 link, reset the pulse source to generate the next pulse in the sweep.
9. Arm the DSO and trigger the pulse source.
10. Repeat steps 5 through 9 up to 2000 times per L-I-V sweep.
11. Calculate I_TH and look for kinks in d L /d I_F

You also can add a boxcar averager (BCA) between the current-to-voltage converters and the DSO. The BCA amplifies and integrates the signal over many pulses, which suppresses noise. With a BCA, the scope acts simply as an analog-to-digital converter. By integrating and averaging a series of pulses of the same magnitude, a BCA improves the signal-to-noise level and eliminates problems associated with individual pulse analysis using a computer algorithm on the DSO output. A BCA accelerates testing and improves the resolution of optical power measurements in the presence of noise.