After magnetic transport we wish to transfer into a crossed optical dipole trap ('crossed ODT' or 'CDT') for further cooling before loading into the optical lattice. For more information about the theory of optical dipole traps see: Optical dipole traps for neutral atoms
The dipole trapping beam is generated by a fiber amplifier and a narrow linewidth source. The source is The Rock from NP Photonics (datasheet), and the fiber amplifier is a 40W 1.0 μm Single Frequency PM Fiber Amplifier from Nufern (datasheet).
After the amplifier, we use two AOMs to diffract out two power controlled beams in order to form a crossed dipole trap. These diffracted beams are then brought to the upper level of the experiment via two periscopes, shaped with telescopes and prisms, and then focused into the science chamber.
The control box for The Rock has three switches for relative intensity noise suppression (RIN) On/Off, Laser On/Off, and constant current (ACC) versus constant power (ACP). There are two LEDs to indicate the presence of a 2nd cavity mode (red) and temperature stability (green). The sequence for turning on The Rock is:
- - Laser is off from the previous day, switches should be set to: RIN Off, Laser Off, ACC
- - Switch Laser to On and wait for the grating temperatures to stabilize. After ~20 minutes the green LED will light, and hopefully the red LED will be off
- - Switch to RIN On to stabilize power (used to also switch to APC, but this was seen to add noise - now leave on ACC)
The output of The Rock is ~150mW. More details can be found here: The Rock
The key on the front of the fiber amplifier has 3 stages as pictured:
The amplifier is controlled via computer using USB. The recommended start procedure is:
- - Key in position 0
- - Couple an input signal of 50-200mW which is polarized along the slow axis of a PM fiber (some of this light will be transmitted through the amplifier and out through the output isolator, even when the power is off)
- - Turn on water cooling at T=23+/-3 degrees Celsius
- - Open NuFern_SFA_Rack_v1.0
- - Turn key to position 1 and pull out emergency stop button (if needed)
- - Turn key to position 2, GUI changes from 'Not Ready' to 'Ready'
- - Press 'Enable' on GUI, preamplifier is now on and ~800mW of power should be observed from the output isolator
- - Increase amplifier power from 0-70% using GUI
- - Nufern output power is now ~45W
More information about the amp is available here: Nufern Amp
The AOMs used for the two crossed ODT beams are the Gooch and Housego I-FS080-2S2G-3-LV1. The two AOMs are labelled Dipole 1 and Dipole 2, and are driven by two Crystal Technologies 80MHz drivers (fixed frequency) controlled by two ALPS boxes for power control. The Dipole 1 and Dipole 2 AOMs diffract into the +1st and -1st order, respectively, giving frequency shifts of +80MHz and -80MHz. The layout of these AOMs is shown below:
(old picture) The dipole beams are now fiber coupled. The layout is the same as above, but instead of periscopes, each beam goes though a 2X expanding telescope and then a fiber coupler.
More information about the G&H AOMs is available here: G&H AOM
On the upper level, we use a PBS and pickoff to tidy the polarization and monitor the beam power. The beam then passes through a 2:1 reducing telescope (using fused silica lenses for high power performance), as well as a prism pair to make the beams elliptical. We would like the focus of the beams at the atoms to be roughly 60$\mu m$ in the vertical direction and 170$\mu m$ in the horizontal direction. The positions of the telescopes and prisms were decided by using Gaussian Beam Propagation, trying to maximally overlap the foci of both axes of the elliptical beams. Details of the layout can be found in the following .ppt file: Elliptical Beams