Model 2602 SourceMeter Instrument Performing LIV Test Sweep

Forward Voltage Test
The forward voltage (VF) test verifies the forward DC characteristics of the laser diode. During the measurement, the current IF is swept and the voltage drop across the laser diode is measured.
Some high-power laser diodes may require a current sweep range of 2~3A, with a step size of 1mA. In more cases, the current is generally swept to 1A with a step size of 0.5mA or 0.25mA. The step time of the test sweep should be on the order of several milliseconds, and the voltage measurement range is generally 0~10V with an accuracy of μV.
VCSELs are generally low-power devices, and the current sweep is up to 30mA with a step size of about 1μA.
In the forward voltage test, one channel of the Model 2602 Dual Channel Source Meter can be used as a current source connected to the laser diode while measuring the corresponding voltage drop.

Light Intensity Measurement
The light intensity (L) measurement verifies the light output of the laser diode. As the drive current increases, the light output power also increases. The output of the light intensity measurement is generally in milliwatts.
In the DC-based light intensity measurement, a reverse-biased photodiode is connected to the output of the laser diode. The photodiode absorbs the optical radiation of the laser diode and converts it into an electric current. The DC current of the photodiode is measured by a picoammeter or an electrometer (a high-precision DC multimeter). However, the photocurrent can also be measured by a Source-Measure Unit (SMU) that provides a suitable low current measurement range. Generally, a range of 100nA is more than sufficient for most laser diodes. The
measured photocurrent is then used to infer the optical power of the device under test. Optical power measurement requires a calibrated detector or integrating sphere. The calibration information, i.e. the responsivity R, is a wavelength-dependent value that can be determined during the calibration process.
To derive the optical output power from the measured photocurrent, the following formula is used:
L = Ip/R
where L is the output power of the light source (watts), Ip is the detector current, i.e. the photocurrent (amperes), and R is the responsivity of the detector at the corresponding wavelength (amperes/watt)*.
*The responsivity curve can be obtained after calibration of the detector or integrating sphere/detector assembly.

The current measured by the detector is divided by the responsivity of the detector at the wavelength of interest, and the result is the optical power impinging on the detector.