Dr. Efi Rotem, Ophir Photonics CTO
VCSELs were first used in the telecom industry, and today are widely used as light sources in sensing applications. Low power (mW) applications include face and gesture recognition, proximity sensors, and augmented reality displays. High power (Watts) applications include LIDARs for robotics, UAVs and autonomous vehicles. These new applications run on batteries and will need to minimize power consumption. Therefore characterization of VCSEL power, beam profile, noise, etc. is critical. Often VCSELs are coupled with passive optics or scanning optics to generate very wide beams. Additionally, VCSELs are often used in pulsed mode. These factors make VCSEL test and measurement a challenging task. Our customers have turned to Ophir equipment to get reliable power and beam profile data on their VCSELs.
- Wavelength range: 650-1080 nm, 1550 nm
- Power range: mW to Watts
- High modulation bandwidth
An Integrating sphere is used to collect light at high angles. Input port adapters for ±40°, ±60°, and ±85° beam angles are available. A calibrated photodiode detector attached to the sphere is used to measure the optical power. If the laser wavelength is not accurately known, a measurement uncertainty of ±0.2%/nm at 650-1000 nm, and ±1%/nm at 1000-1080 nm and 1550 nm, is introduced.
Additional ports on the integrating sphere are used for connecting other instruments such as spectrometers and fast photodiode detectors to make multiple types of measurements simultaneously. Integrating sphere power sensors cover power ranges from <1 μW up to 30 W.
For power levels from 100mW and up, thermopile sensors can also be used. Thermopile sensors are more compact and easier to use than integrating sphere sensors. As they operate on the principle of absorbing laser radiation and transforming it into heat, Ophir sensors with the new LP2 coating have very little sensitivity to incidence angle making them ideal for measuring the power of divergent laser beams. The black coating is also spectrally flat over hundreds of nanometers. Therefore, measurement error due to wavelength uncertainty is smaller than 0.01%/nm. Thermopile sensors provide the best combination of sensor area and beam angle acceptance. Table 1 shows a comparison between thermopile and integrating sphere power measurement. Figure 2 shows the angle dependence of the LP2 coating.