Astigmatic Difference (As)

What is Astigmatic Difference (As)?

For laser diodes, the apparent focal positions are different in the horizontal direction and the direction perpendicular to the junction. The distance between these two focal points is defined as the astigmatic difference (As).

[Astigmatic Difference (As)]

Astigmatic Difference (As): Measurement Method

Astigmatic difference is measured by the knife edge method as shown in Fig. 1.
The laser diode beam is cut from one side with a knife edge.

Fig. 1

By plotting the amount of light passing through the knife edge and knife edge position (X), the integral value of the laser beam can be obtained. [Fig. 2]

Fig. 2

Differentiating this allows the beam diameter (full width at half maximum) to be measured. [Fig. 3]

Fig. 3

The beam diameter is measured by moving the end face position (Z) of the laser diode.
The end face position and beam diameter in the horizontal direction and direction perpendicular to the junction of the laser diode are measured. [Fig. 4]
The astigmatic difference is obtained from the position of the minimum value of the 2 beam diameters.

Fig. 4