Pulse Energy Measurement of Divergent Laser Beams/LiDAR
Laser diodes used in distance measuring devices have a diverging elliptical beam profile with typical peak powers of up to 100 W. They are operated in a short pulse width, low pulse frequency mode with low average power. For quality assurance, the peak power and the pulse waveform are of primary interest. These two optical parameters cannot be measured with a single detector. Therefore, pulse energy and pulse waveform must each be measured separately.
The pulse waveform is measured using fast, small-area photodiodes terminated with a low-impedance shunt resistor. The temporal voltage curve across the resistor is measured and recorded with a digital oscilloscope. Rise times of less than a nanosecond are possible. The responsivity of this type of detector is very low because of the low-resistance circuitry. Additionally, the small area of the photodiode can only detect a portion of the entire laser beam profile. The photodiode and laser beam must be carefully aligned due to non-uniformity in the laser beam profile. This method is entirely suitable for the measurement of the relative pulse waveform, which is needed together with the pulse energy to calculate the peak power.