Laser oscillation <Injection Current-Optical Output (I-L) Characteristics>

What are Injection Current-Optical Output (I-L) characteristics?

The figure below show the Injection Current-Optical Output (I-L) characteristics.
Laser diodes oscillate when the gain obtained by amplification exceeds the internal and mirror losses. In other words, an oscillation current threshold exists.
The maximum output is limited by kink (bending of the current-optical output line), COD (end face light breakdown), and thermal saturation due to temperature.

[Injection Current-Optical Output (I-L) Characteristics]

[Application Examples by Semiconductor Laser Function]

Parameter Symbol Definition
Threshold Current Ith In Fig. 2, A is classified as a spontaneous emission region and B is defined as a laser oscillation region.
The current value at which laser oscillation starts is referred to as the threshold current.
Operating Current Iop The forward current required to produce a specified light output.
Operating Voltage Vop The forward voltage required to produce a specified light output.
Differential Efficiency η The increase in light output per unit of drive current.
Shows the slope of the light output line with respect to the forward current in the laser oscillation region.
Monitor Current Im The output current of the built-in photodiode at a specified light output.

Injection Current-Light Output (I-L) Characteristics: Measurement Method

An optical power meter is used to measure the light output of the laser diode.
The light receiving surface is positioned so that the total light flux of the laser beam enters the light receiving surface of the optical power meter.
The light receiving surface is tilted by 5-20° with respect to the optical axis to prevent reflected light from the light receiving surface of the optical power meter from returning to the laser diode.
The IL characteristics represent the relationship between the forward current (IF) and light output (Po), making it possible to obtain the threshold current (Ith) and operating current (Iop).
The monitor current (Im) is the output current when the laser beam emitted from the rear face of the laser chip is monitored by the built-in photodiode.

[Measurement Image]

Note:
Since these measurements are largely dependent on temperature, measurements are made with a heat sink attached to the laser diode and the case temperature controlled using a temperature controller.