Introduction
Laser diodes are among the most widely used optical sources in modern photonics. They are found in applications ranging from optical communication and spectroscopy to medical devices and industrial sensing as well as cutting and welding with high power lasers in manufacturing environments. To evaluate their performance and reliability, engineers rely on a standardized testing method known as LIV characterization.
LIV measurements analyze the relationship between light output (L), electrical current (I) and voltage (V) of a laser diode. By measuring these three parameters simultaneously, researchers can determine key performance characteristics such as threshold current, efficiency and optical output power.
This article explains how Laser Diode LIV Characterization works, what information it provides and how accurate measurements can be achieved in laboratory and production environments.
Table of Contents
What Is LIV Characterization?
LIV characterization measures the optical and electrical behavior of a laser diode as the drive current is varied.
During the test:
- A controlled current source drives the laser diode.
- Optical power output is measured using a photodiode or integrating sphere.
- The voltage across the laser diode is recorded simultaneously.
The result is a set of curves that describe how the laser device behaves under different operating conditions.
These curves are essential for understanding the performance and efficiency of a laser diode.
The Three Components of LIV Measurement
Light Output (L)
The light output represents the optical power emitted by the laser diode. It is typically measured in milliwatts or Watts, depending on the maximum power of the device.
Optical power is usually measured with:
- photodiode based optical power meters
- integrating spheres
This measurement reveals how efficiently the laser converts electrical power into light.
Drive Current (I)
The drive current is the electrical current supplied to the laser diode.
During an LIV test, the current is gradually increased from zero to the maximum of the device’s operating range. This controlled sweep allows engineers to observe how the optical output responds to increasing current.
Accurate current control is essential because even small fluctuations can significantly affect laser performance.
Voltage (V)
The voltage across the laser diode provides information about its electrical behavior and internal resistance.
Voltage measurements are used to calculate:
- electrical power consumption
- device efficiency
- thermal behavior
Combining voltage with optical power measurements enables complete characterization of device performance.
The Derived Results from LIV Measurement
Threshold Current
One of the most important parameters obtained from LIV characterization is the threshold current.
The threshold current is the minimum current at which the laser diode begins producing coherent laser emission.
Below this current level:
- the device behaves like a light-emitting diode
- light output increases slowly
Above the threshold current:
- stimulated emission dominates
- optical output increases rapidly
Determining the threshold current is critical for evaluating laser efficiency and device quality.
From Figure 1 it is clear that the definition of threshold current is not trivial. There are various algorithms in use. A very powerful algorithm uses the second derivative of the L-I curve, whereby the threshold is defined as the current at the local maximum.
Slope Efficiency
Slope efficiency describes how efficiently a laser diode converts electrical current into optical output once the threshold current has been reached. It is calculated from the slope of the light-versus-current curve above the threshold region.
High slope efficiency indicates:
- efficient device design
- low internal losses
- good optical coupling in the case of fibre coupled devices
Slope efficiency is a key performance metric in laser diode development.
The slope efficiency and its first derivative give insight into the thermal stress of the device under test.
Furthermore, indications of crystal defects leading to lifetime reduction or coupling mismatch in fibre coupled systems can be inferred from the slope efficiency and its first derivative (so-called “kinks”).
Electrical Power and Efficiency
By combining current and voltage measurements, engineers can determine the electrical power supplied to the laser diode.
Comparing electrical power with optical output power provides the overall device efficiency.
This information is important for:
- thermal management
- system design
- power consumption optimization
Measurement Setup for LIV Characterization
A typical LIV measurement setup includes several specialized instruments.
Key components include:
- laser diode driver with precise current control
- photodiode detector or integrating sphere
- transimpedance amplifier (photodiode amplifier)
- voltage measurement electronics
- digitization and computer communication electronics
- data acquisition software
Together, these components enable automated measurements that produce accurate and repeatable LIV curves.
Common Measurement Challenges
Several factors can affect the accuracy of LIV measurements.
Current Accuracy
The most important parameter influencing the accuracy of an LIV measurement is the accuracy of the driven current. For pulsed systems, this means that the current pulse must quickly reach the desired drive current without overshoot and ringing and then remain stable for the duration of the measurement at that current. This is a very demanding requirement.
Thermal Effects
Laser diode performance is strongly influenced by temperature. Without proper thermal control, measurement results may vary significantly.
Thermoelectric coolers and temperature controllers are often used to maintain stable device temperature.
Optical Alignment
If the optical detector does not capture the full output beam, the measured optical power may be lower than the true value.
Integrating spheres are often used to capture widely divergent beams and eliminate alignment sensitivity.
Applications of LIV Characterization
LIV testing is used in many photonics fields.
Common applications include:
- laser diode development
- semiconductor device research
- optical communication component testing
- quality control in laser manufacturing
Engineers rely on these measurements to verify that devices meet required specifications.
LIV Measurement Solutions from Artifex Engineering
Artifex Engineering manufactures specialized instrumentation for laser diode characterization, including dedicated, compact, turnkey systems designed for precise LIV measurement.
- LIV100 is a powerful short-pulse laser diode characterization system designed for testing laser diodes and LEDs at the chip, bar, or submount level. Its extremely fast rise time with virtually no overshoot enables accurate measurements of thermally “naked” devices while minimizing thermal stress on the device under test.
- LIV120 is a versatile CW/QCW laser diode tester designed for reliable characterization and testing of submounted or packaged laser diodes and LEDs.
Together, these tools enable reliable evaluation of laser diode performance in research laboratories and industrial testing environments.
Conclusion
LIV characterization is one of the most important techniques for evaluating laser diode performance. By measuring optical output, drive current and voltage simultaneously, engineers can determine critical parameters such as threshold current, slope efficiency and overall device efficiency.
Accurate LIV measurements require carefully designed instrumentation and stable measurement conditions. With the right equipment and methodology, researchers can obtain precise data that supports the development of high-performance photonic devices.
About the Author:
Dr. Steven Wright
CEO & Doctor of Natural Sciences (Dr. rer. nat.)
Dr. Steven Wright is the founder and CEO of Artifex Engineering. Born in Canada, he holds a Ph.D. in Natural Sciences and has worked in photonics for more than 30 years, learning the field from the ground up and applying that experience to building Artifex. Still hands-on today, he regularly works alongside the team in the lab, staying close to the science.
