Specifications and performance

Wavelength tuning and performance measured at installation (March 2020) are available in the following documents:

Input beam requirements

Switching the 800 nm pump beam path to the TOPAS arm can be performed as described here.

The input beam diameter at 1/e^2 should be between 6 and 10 mm. To achieve this, a down telescope with curved mirrors is installed after the pick-off by LT4. The mirrors are from Eksma with radii R1 = -1500 mm and R2 = +600 mm. The beam can be walked into the TOPAS via two mirrors.

The leakage through the last mirror is sent to a beam block and monitored by a camera. Inspecting the beam size at this point and in the far field by propagating the beam back into the main PP 800 nm path and looking at 8CAM5 is a good way to check the alignment of the telescope.

Note

The TOPAS requires horizontal polarization. The PP laser entering the ILH is vertically polarized, so a half wave plate is required to turn the polarization by 90 degrees. It is usually inserted in between the two walk-off mirrors.

Alignment

Note

To start with TOPAS alignment, homing of all motors is necessary, especially if the device was switched off or disconnected from the control computer. Simply click “reset all” in the WinTopas4 software on the left panel.

SHG with fresh pump

One of the first alignment procedures is to generate second harmonic (400 nm) from the pump beam only in the NirUVis box directly. The conversion efficiency is expected to be similar to what the OPA can deliver, around 30%.

  • block the seed in the preamplifier arm
  • insert mirror M12: in this configuration 20% of the pump should be transmitted through M12 and exit the TOPAS.
  • measure the power of the pump before NirUVis box.
  • select wavelength of 900 nm: this sets crystal #4 (second harmonic of Idler, SHI) on NRV-Crystal 2 motor.
  • select WS5 on NRV-WS-Wheel 1 and WS6 on NRV-WS-Wheel 2 for maximum reflection of 400 nm.
  • adjust the phase-matching angle of the crystal by rotating NRV-Crystal 2 motor.
  • measure the SHG power at the output of the NirUVis box.