Tim Hallmark; Coherent Inc

Tim Hallmark; Coherent Inc.

Notes from 1/16/2015 Visit

General Laser Care:

1)  Do not leave laser on with shutter closed longer than 15 min at a time. Thermal effects will mess up the alignment of the laser.

2)  Warm the laser up for ~30 min prior to use, open the shutter!

3)  Filter on front panel of the control/power supply box should be vacuumed occasionally to maintain proper cooling!

4)  Chillers: Leave on all the time if laser has a service contract, otherwise turn chiller off when not in use.

MIRA care:

Installation notes:

Verdi pump lasers base plates:

Rear screw on base plate mount should be tightened slightly more than finger tight, tighten the front screw firmly. Failure to follow this helpful hint will result in misalignment as the laser heats up. Small alignment alterations here will result in large alignment alterations within the MIRA.

MIRA mount

Mount the MIRA fit as tightly as possible.

Daily Tuning:

1)  Peak power by adjusting the knobs labeled 1 in the MIRA diagram above. The major adjustment will likely be in the horizontal, knob is labeled with H.

2)  After peaking power with knobs labeled 1 in diagram, adjust knobs labeled 2 in the MIRA diagram. Again adjustment will be mostly horizontal.

3)  If daily tuning doesn’t provide the expected output power proceed with the following cleaning protocol.

MIRA cleaning protocol

General notes:

Cleaning with hemostats, lens paper, and acetone works well!

Tim used same lens paper on multiple optics and scrubbed hard if necessary.

If power does not improve after cleaning, clean it again!

Hooking up some sort of positive pressure to the Mira cavity could increase the time between required realignments.

Primary dirty optic culprits:

1)  Verdi window

2)  Mirror between the Verdi and the MIRA

3)  MIRA input window before opening the laser

If power still remains low then proceed to clean the following:

Open the laser cavity, top panels are easily removed.

All optics in the laser cavity can be cleaned (including the ‘butterfly’ starter and prisms) with acetone. DO NOT CLEAN the CRYSTAL! Be careful when cleaning the prisms, only clean the surfaces that light interacts with; do NOT touch the foil with Acetone. Optics that have a slightly horizontal surface are likely dirtier and will improve power.

While laser cavity is open, double check the crystal mount is cold by touching the mount.

When cleaning doesn’t increase output power:

1)  Check the alignment and adjust!

a.  Gently lean a piece of paper inside the laser cavity, leaning upright as shown in the diagram by the white rectangle.

b.  There should be 2 green spots on the paper, one large and one small. The centers of these 2 spots should overlap. If they do not, adjust the mirror between the Verdi and the MIRA first, if this does not work to overlap the spots, then adjust either knob marked with Xs in the MIRA cavity. It is possible that you will need to adjust both the mirror between the Verdi and the MIRA as well as the knobs marked with Xs on the MIRA diagram.

2)  Prism B should be adjusted such that the beam just barely passes through the tip of the prism.

3)  DO NOT EVER ROTATE the PRISMs in the MIRA cavity to align the system.

4)  DO NOT TOUCH the micrometer positions of Mirror 1 and Mirror 2 to align the laser cavity. If these are accidently moved, the positions of the last service date or the initial factory positions can be found on the service data sheet. Ask Kevin for this information!

Tuning the Wavelength:

1)  Flip the switch on the box to CW mode

2)  Adjust the wavelength know to the desired wavelength

3)  Adjust the prism (Triangle labeled A) on MIRA diagram to compensate.

1. Optimal prism position is important for optimal pulse widths.
2. Completely remove prism from the beam path (turn the knob until power diminishes)
3. Slowly put the prism back into the beam path. At 800 nm, the optimal prism position is approximately 7 turns. This will change for other wavelengths! Optimal prism position can be determined by examining the beam profile, the beam should be roughly 8-10nm wide. Another approach is to look at the CW amount, when the box switch is flipped to modelock. In this mode, the optimal prism location is just immediately before you start to get CW.

Note: It is possible for MIRA laser to ‘double pulse’, two pulses circulate simultaneously. Laser output can be checked using a fast photodiode and an oscilloscope. (Are pulses ~80 MHz or is it nearly twice that?) Alternatively you can use the spectrum to investigate.

To check the spectrum, as the output aperture in moved in one direction you will the following progression:

Lots of CW breakthrough

The second taller peak is probably double pulsing. You want the first, shorter peak that occurs just before CW breakthrough!

OPO/Chameleon on the ULTRA system

OPO is currently in linear configuration.

OPO is possibly fan-poled; Tim will be sending Kevin additional information including the proper manual in the coming weeks.

If OPO is in fact fan-poled, OPO can be pumped at any wavelength, output power will be dependent on pump wavelength and output wavelength. Tables will be provided in additional information.

If the SHG crystal is ever installed (currently it resides in an air-tight container in the cabinet behind ULTRA), the OPO controller and Chiller need to be ON at all times! If this does not happen the SHG crystal will absorb moisture from the air and will be permanently damaged!

Currently the SHG crystal is not installed; therefore it is not necessary to run the chiller.

Tim will be assisting us in the coming weeks to determine the status of the software issues with the OPO tenability. I seem to remember the software moving the PPL position to adjust the wavelength.

PPL crystal is the main determinant of the output wavelength in linear configuration.

The piezo is important if the system is ever put into circular configuration.

Other Notes:

Tim likes the RTE-111 chillers.