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Message ID: 3040     Entry time: Wed Jun 2 22:25:39 2010
Author: Kevin 
Type: Update 
Category: PSL 
Subject: Low Power 2W Beam Profile 

Koji is worried about thermal lensing introducing errors to the measurement of the 2W beam profile so I measured the profile at a lower power.

I used the same setup and methods used to measure the profile at 2W (see entry). This measurement was taken with an injection current of 1.202 A and a laser crystal temperature of 25.05° C. This corresponds to approximately 600 mW (see power measurement).

The data was fit to  w = sqrt(w0^2+lambda^2*(x-x0)^2/(pi*w0)^2) with the following results

For the horizontal beam profile:

reduced chi^2 = 2.7

x0 = (-203 ± 3) mm

w0 = (151.3 ± 1.0) µm

For the vertical beam profile:

reduced chi^2 = 6.8

x0 = (-223 ± 6) mm

w0 = (167.5 ± 2.2) µm

In the following plots, the blue curve is the fit to the vertical beam radius, the purple curve is the fit to the horizontal beam radius, * denotes a data point from the vertical data, and + denotes a data point from the horizontal data.

The differences between the beam radii for the low power and high power measurements are

Δw0_horizontal = (38.3 ± 1.2) µm

Δw0_vertical = (43.5 ± 2.4) µm

Thus, the two measurements are not consistent. To determine if the thermal lensing is in the laser itself or due to reflection from the W2 and mirror, we should measure the beam profile again at 2W with a razor blade just before the W2 and a photodiode to measure the intensity of the reflection off of the front surface. If this measurement is consistent with the measurement made with the beam scan, this would suggest that the thermal lensing is in the laser itself and that there are no effects due to reflection from the W2 and mirror. If the measurement is not consistent, we should do the same measurement at low power to compare with the measurement described in this entry.

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