This is what I already told to Kevin and Rana:
A direct output beam is one of the most difficult measurements for the mode profiling.
I worried about the thermal lensing.
Since most of the laser power goes through the substrate (BK7) of the W2 window, it may induce thermal deformation on the mirror surface.
An UV fused silica window may save the effect as the thermal expansion coefficient is 0.55e-6/K while BK7 has 7.5e-6.
In addition to the thermal deformation issue, the pick-off setup disables us to measure the beam widths near the laser aperture.
I rather prefer to persist on the razor blade then use the pick off between the blade and the PD.
I also confess that the description above came only from my knowledge, and not from any scientific confirmation including any calculation.
If we can confirm the evidence (or no evidence) of the lensing, it is a great addition to my experience.
[Rana, Kiwamu, Kevin]
The Innolight 2W beam profile was measured with the beam scan. A W2-IF-1025-C-1064-45P window was used to reflect a small amount of the main beam. A 5101 VIS mirror was used to direct just the beam reflected from the front surface of the W2 down the table (the beam reflected from the back surface of the W2 hit the optic mount for the mirror). A razor blade beam dump was used to stop the main transmitted beam from the W2. The distance from the laser was measured from the front black face of the laser to the front face of the beam scan (this distance is not the beam path length but was the easiest and most accurate distance to measure). The vertical and horizontal beam widths were measured at 13.5% of the maximum intensity (each measurement was averaged over 100 samples). These widths were divided by 2 to get the vertical and horizontal radii.
The mirror was tilted so that the beam was close to parallel to the table. (The center of the beam fell by approximately 2.1 mm over the 474 mm that the measurement was made in).
The measurement was taken with an injection current of 2.004 A and a laser crystal temperature of 25.04°C.
This data was fit to w = sqrt(w0^2+lambda^2*(x-x0)^2/(pi*w0)^2) with lambda = 1064nm with the following results
For the horizontal beam profile:
reduced chi^2 = 4.0
x0 = (-138 ± 3) mm
w0 = (113.0 ± 0.7) µm
For the vertical beam profile:
reduced chi^2 = 14.9
x0 = (-125 ± 4) mm
w0 = (124.0 ± 1.0) 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.