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Laser diodes, the devices that illuminate telecom fiber-optic cables, require light-current-voltage (LIV) measurements during production. These measurements let you characterize a laser diode’s light output and forward voltage as a function of input current.

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. Laser diode self-heating causes unwanted changes in the device’s characteristics—forward voltage, resistance, efficiency, and threshold current—that adversely affect the laser’s reliability.

To perform a LIV test on 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.

L-I-V Test Sweep

 By applying increasing current to the laser diode so it that emits light, the optical output is measured together with the voltage drop across the diode element. The resulting LIV curve reveals important clues about the quality of manufacture and the performance of the laser diode, enabling a pass/fail decision to be met. 

Forward voltage test. The forward voltage (VF) test verifies the forward DC characteristics of the LD. Current (IF) is swept, and voltage drop across the LD is measured. Some high-power LDs may require IF sweeps up to 2–3 A, usually in increments of 1 mA.

Most need sweeps up to 1 A, with 0.5 mA or 0.25 mA steps. Time per test sweep should be in the range of a few milliseconds. The typical measurement range is 0–10 V, and microvolt-level resolution is required.

The resulting LIV curve reveals important clues about the quality of manufacture and the performance of the laser diode, enabling a pass/fail decision to be met.

Types of Characterizations

For example, following laser diode characterization can be determined based on the LIV measurement:

  • Power vs. current
  • Power vs. voltage
  • Threshold current
  • Series resistance
  • Slope efficiency