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How to test a pulsed laser diode

Pulsed laser diodes are tested using low-duty-cycle LIV (Light-Current-Voltage) measurements to evaluate optical output, threshold current, and thermal response without damaging the device.

Key Steps for Testing

1. Prepare the Device Under Test (DUT)

  • Ensure the laser diode is properly mounted on a carrier or submount to facilitate handling and thermal management.
  • For VCSELs, testing can occur at the wafer stage; for edge-emitting diodes, testing is done at the bar stage before dicing.
  • Use appropriate shielding to prevent accidental exposure to the laser beam, especially for IR or UV diodes, which are invisible to the eye . 2. Set Up Pulse LIV Measurement
  • Apply a low-duty-cycle pulsed current to the diode to minimize heating.
  • Simultaneously measure the forward voltage and optical power output at each current step.
  • Ensure the diode temperature is tightly controlled, as LIV characteristics are temperature-dependent . 3. Analyze Key Parameters
  • Threshold Current (Ith): The minimum current at which lasing begins.
  • Slope Efficiency: The rate of optical power increase per unit of current above threshold.
  • Quantum Efficiency: Ratio of emitted photons to injected electrons.
  • Kink Detection: Identify any negative slope in the optical power vs. current curve, which may indicate defects . 4. Thermal and Transient Considerations
  • Pulsed testing is essential for early-stage diodes without temperature control to prevent thermal runaway.
  • Low-duty-cycle pulses allow evaluation of thermal response and transient behavior without damaging the diode . 5. Optional DC Testing
  • Once the diode is mounted in a module with thermal control (e.g., heat sink or TEC), DC LIV testing can be performed to compare with pulsed results.
  • Some diodes may pass DC tests but fail pulsed tests due to temperature-induced characteristic changes . 6. Safety Precautions
  • Never look directly into the laser beam.
  • Use proper laser safety goggles rated for the diode's wavelength.
  • Ensure the test setup prevents accidental exposure to stray reflections . 7. Additional Checks
  • For high-power diodes, measure both front and rear facet optical power.
  • Correlate electrical and optical data with manufacturing process information to detect early failures or defects . By following these steps, you can safely and effectively test pulsed laser diodes, ensuring accurate characterization of their optical and electrical performance while minimizing the risk of damage.
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