ETMX HWS alignment and mode-matching activities - we have a functioning Hartmann sensor with green LED source

[Aidan, Danny, TJ]

 Summary

We finished setting up the ETMX HWS. The alignment may have been bumped in the final steps as the HWS spot pattern envelope is smaller than the intensity distribution. ALS leaks onto the HWS camera. Photos of the layout are attached.

Alignment

We finished aligning the HWS source beam to the ETM on HWSX. Once again, we could see what looked like the AERM aperture.

We adjusted the picomotors (manually) to optimize the alignment of the intensity pattern to the shadow of the AERM aperture. We struggled to get the intensity moved up in pitch - it remained a little low relative to the aperture but horizontally it looked centered.

When we closed the iris on the fiber launcher (that is nominally centered on the intensity distribution), it was centered close to the center of the AERM shadow.

We readjusted the lens position after the fiber launcher so that the beam was focussed approximately 1.3m along the optical axis (around the location of the first lens on the outgoing beam).

We tried the ETM pitch modulation technique to optimize the imaging but the results were inconclusive due to non-linear fluctuations in the Y-centroid signal on the HWS camera. We estimate that the conjugate plane of the HWS is within 50m of the ETM (or the HWS is within 10cm of the conjugate plane of the ETM). One option for improving this is to simply try to get the AERM aperture as sharp as possible but moving the HWS along the optical axis.

We installed the Hartmann plate on the sensor and started the HWS software. Closer inspection of the HWS pattern shows that the lower right section (in the attached images) has disappeared. It's possible that we might have clipped the beam somewhere in the final steps of installing the plate. We should remove the plate and re-check the alignment.

Optical layout (distances between optics)

Optics	Distance
M1A to M1B	118 mm
M1B to L1 (lens)	118 mm
Lens to M1	28 mm
M1 to L2 (lens)	498 mm
L2 (lens) to M3	314 mm
M3 to BS2 (now a mirror)	98 mm
BS2 to L3 (lens)	585 mm
L3 (lens) to M3	148 mm
M3 to M4	65 mm
M4 to HWS-BS	656 mm
HWS-BS to M6	324 mm
M6 to M7	80 mm
M7 to HWS	86 mm
HWS-BS (front) to MS1	130 mm
MS1 to L_fiber	265 mm
HWS-BS (front) to POL	145 mm

 

HWS return beam images

Fiber launcher Iris open

Fiber launcher iris partially closed

 

Hartmann sensor spots

 

 

ALS leakage onto HWS camera

The ALS beam leaks onto the HWS camera as shown in the attached image. Like the ETMY ALS beam there is probably some small polarization rotation inside the vacuum system.

 

Spherical power measurement

The initial spherical power (over approximately 5000s) shows very low noise (perhaps 3uD - 6uD RMS). The ALS laser was unshuttered at the end of this time and saturated the camera.