Rohm Breadcrumb


Laser Beam Shape

Far Field Pattern (FFP)

The intensity distribution of light emitted to a place far enough away from the chip end face is called the far field pattern.
Laser Beam is not perfectly linear, but advances while spreading out by diffraction.
Since the resonators (active layer and stripe) within the chip comprise of up to several tens of nm in the vertical direction and several um in the horizontal direction, in general the far field pattern will be larger in the direction perpendicular to the active layer than in the horizontal direction.

[Far Field Pattern Image] [Far Field Pattern Image]

Far Field Pattern: Measurement Method

The APC driver is used to keep the light output of the laser beam constant.
The light receiving surface of the photodiode is moved on the circumference around the light emitting point towards the light emitting surface of the laser chip, then the angle and light intensity are measured with the front as 0°.

[Measurement Image] [Measurement Image]
APC Driver:

Auto Power Control refers to output constant drive.
In LEDs, ACC (constant current drive) is generally used, but in LD applications this can result in unstable output. Therefore, a typical laser diode has a built-in photodiode for monitoring which is installed in the package to receive light from the rear end of the chip.
A drive circuit that feeds back the output of this photodiode (Im: monitor current) so as to be constant is called an APC driver (circuit).