Optical sources are key components of optical communication systems, making it crucial to ensure their proper functioning. This is typically achieved in two ways: through a single measurement or test to verify that the device meets catalog specifications, or by monitoring certain operating parameters (optical power, wavelength, etc.).
For all these purposes, a wide range of instrumentation is available on the market for measuring and characterizing optical sources in general: DFB lasers, Fabry-Perot lasers, LEDs, VCSELs, etc. Depending on the type of measurement to be performed, the application, and the working environment (manufacturing plant, research laboratory, operator's control room, etc.), one solution or another, as well as a specific measurement setup, must be chosen. This article will analyze some of the commercially available solutions that allow the characterization of the main operating parameters of a laser source. More specific instrumentation also exists for other types of measurements, although that is beyond the scope of this article.
Optoelectronic Characterization of a Laser Diode.
The typical appearance of a laser diode module is shown in Figure 1. It basically consists of the laser source with an integrated photodiode to monitor the optical power, as well as a thermistor and a TEC (thermoelectric cooler) to control the device's operating temperature. High-speed modules also incorporate an integrated modulator where the digital signal to be transmitted is injected, although in our case we will focus on the DC characterization of the device. For this, a measurement setup like the one shown in Figure 2 can be used. This is a light-current-voltage (LIV) test system from Keithley Instruments. The Model 2420 unit powers the laser diode, performing a current sweep from 0 to 3 A in programmable steps. For each step, the 2420 unit stores the current and voltage measurements, while the 2502 unit measures and stores the current flow in the photodiodes. Once the scan is complete, both devices download the data to the PC for further analysis. Using appropriate software, the device's operating curves can then be plotted and characteristic parameters calculated. An example of these results is shown in Figure 3.
The DC test measurements typically performed on a laser diode can be summarized as follows: forward voltage, threshold current, light intensity, slope efficiency (dL/dI), photodiode reverse voltage (monitor), photodiode current, and photodiode dark current. Some of these measurements are discussed in more detail below.
Forward Voltage Test
: The forward voltage (VF) test verifies the forward bias characteristics of the laser diode. It consists of performing a sweep of the bias current (IF) and measuring the voltage drop across the diode. In most cases, the sweep is performed up to 1 A in 0.5 or 0.25 mA steps, although some high-power laser diodes require currents of up to 2–3 A in 1 mA increments. Forward voltage measurements are in the 0–10 V range with microvolt resolution. The sweep duration is on the order of a few milliseconds.
Threshold Current Test:
The laser threshold current (Ith) is defined as the current at which the device enters laser mode and begins emitting. Its value can be obtained by twice differentiating the LI curve and calculating the position of its maximum. Figure 4 graphically represents the calculation procedure.
Light Intensity Test
: Light intensity can be measured in two ways: AC and DC. AC measurement uses a simple optical power meter connected to the laser diode output. DC measurement, on the other hand, involves placing a reverse-biased photodiode at the laser output and measuring the photodiode's DC current with a high-precision multimeter (picoammeter). When optical power measurements are required, a calibrated integrating sphere and detector are used. The system software uses the measured electrical current value along with the photodiode calibration data to calculate the optical power at the module's output. The typical photodiode current range is 0–3 mA with a 0.1 mA resolution. The main advantage of DC measurement is that it is faster than AC measurement.
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Slope efficiency test
, also called the "kink test," verifies the linearity of the laser's linearity (LI) curve. This involves taking the first derivative of the curve to detect any "peaks" that could affect the laser's modulation response. The dL/dI graph should be practically horizontal within the laser's operating range. Otherwise, the device may be considered defective. Figure 5 illustrates the process of detecting a defective laser.
Photodiode Testing:
The main measurements typically performed on the monitor photodiode are reverse breakdown voltage and dark current. The reverse breakdown voltage limits the maximum voltage that can be applied to the reverse-biased photodiode without destroying the junction. This value is estimated by supplying the device with a current of -0.01 mA and measuring the resulting voltage across the junction. The dark current, on the other hand, can be obtained by blocking the laser diode current (it must also be ensured that no other photons strike the photodiode), biasing the photodiode junction, and measuring the resulting current flow.
Temperature Dependence:
Most of the measurements described above are temperature-dependent. Therefore, laser diode characterization is usually performed at both the nominal temperature and the extreme operating temperatures specified (typically -40, 25, and 85 °C). In other cases, the LIV test is performed at a range of temperatures: 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, and 30 °C. The laser module's thermistor and TEC (Figure 1) are used to monitor the device's operating temperature. Test measurements are also performed to verify the correct operation of these components. These measurements essentially consist of modulating the operating temperature point and verifying the thermistor's resistance after the new temperature has been reached. Microampere currents are used to measure the thermistor's resistance to minimize power dissipation.
Author:
Francisco Ramos Pascual. PhD in Telecommunications Engineering
Full Professor at the Polytechnic University of Valencia
