Testing diode lasers with direct current is a complex operation. Extremely tight temperature control is required to maintain a specific output wavelength and to avoid destructive heating. It is best to test with a pulsed stimulus even when devices will be powered in the field with steady-state DC. Steady power generates heat in the diode laser, which causes wavelength drift. Packaged diode laser modules include thermoelectric coolers (TECs) for thermal stability, but at this stage of testing, the diode laser is still in raw form. Pulsed measurement techniques solve these problems by eliminating excessive thermal stress on the device under test. By designing tests that can return the desired measurement data using only a brief pulse to stimulate the diode laser, you can avoid heating without any extreme thermal stabilization measures.
Testing short-duration optical pulses using commercial optical power meters can be problematic, however. Typically, such meters are designed for high-precision measurements that require many seconds of integration time per reading. The thousands of pulses accumulated require special computational algorithms (for example, test instrument firmware or external PC-based test programs that calculate peak optical power based on the assumption that average power is a function of the duty cycle of the current pulse driving the laser). It's also assumed that the integral of the noise signal is zero.
The Pulsed L-I-V Test System uses conventional instruments and computer control to minimize thermal stress on the device under test.
By using a combination of rack-mounted instruments and custom software running on a PC controller, you can perform pulsed light-current-voltage (L-I-V) testing faster and more accurately. A typical system (see figure) includes a laser driver (pulse source), optical measurement components, a pair of high-speed voltage-to-current converters, and a high-speed multichannel digital sampling oscilloscope (DSO).
Typically, the user sets the oscilloscope to trigger on the pulse source's external trigger output. The current-to-voltage converter is set to a suitable range based on I-V sweep values (voltage drop across the diode versus drive current), and the pulse source is configured for the first current step (typically 0.25 mA).
The PC triggers the pulse source via a general purpose interface bus (GPIB), and the oscilloscope captures the forward voltage drop and optical output of the diode laser.
Once you download the waveforms from the scope to the computer, you can use software to identify the flat portion of the pulse, average the samples over their flat time, and calculate the corresponding value for voltage or current, as desired. Next, the pulse source resets to generate the next pulse in the sweep. This process should be repeated up to 1000 times per L-I-V sweep.
The analysis process described above takes roughly 10 to 20 seconds per pulse. A 500-point pulsed L-I-V sweep can require up to several minutes. Although this rate would theoretically allow you to test up to 3000 parts per day, typical production-line irregularities will reduce actual throughput to about 2500 parts per day (85% of theoretical capacity).
The total cost of equipment, software development, and system integration for a production test system like this is roughly $100,000 to $150,000. Robotic handling of components can double that figure.
Diode laser manufacturers are always looking for faster, more cost-effective testing methods. Systems based on pulsed-source measure units can combine multichannel data acquisition, dedicated timing circuitry, high-speed current-to-voltage converters, and digital signal processing (DSP) to emulate DSO functionality. Internal test sequencing memory permits the DSP to orchestrate pulsed L-I-V sweeps, without further interaction with other equipment or the control computer, using test parameters that have been downloaded via the GPIB. With this design, the system can also provide control signals directly to the component handler via a digital I/O port.
Equipment integrating a high-speed DSP can also perform fast analysis of captured pulse measurements by accomplishing the analysis process during the off-time between pulses. Up to 15,000 devices per day can be tested, even with the assumption of only 85% system utilization. oe
Paul Meyer is a product marketer in the Optoelectronic Component Test Group at Keithley Instruments Inc., Cleveland, OH.