PT-25-813_Docs1_SOW - MOT Collimators-Redacted.pdf

Statement of Work MOT Beam Collimators

Clock Development Division, Precise Time Department U. S. Naval Observatory

BACKGROUND

The United States Naval Observatory (USNO) is the authoritative source of time for the Department of Defense. For this purpose the USNO’s Precise Time Department maintains an ensemble of high-performing atomic clocks that is used to generate precise time. Because of the lack of commercial vendors for the highest performing clocks, stocking the clock ensemble with systems based on cutting-edge technology requires that these systems be designed and built in-house.

To that end, USNO seeks to procure quantity 13 fiber beam expanders to generate the laser beams for a magneto-optical trap (MOT), according to the design used in the original fountain build. The specifications of these MOT collimators are listed below.

REQUIREMENTS

The MOT beam-expanding collimators for the USNO fountains must meet the following specifications:

1) The collimator must be designed for a ~.085-.090 numerical aperture1 PM fiber with FC/APC connector, with 2mm narrow key (and laser polarization) aligned to the slow axis of the fiber. The collimator must be designed specifically for the PM fiber that we will use, which will be provided along with a fiber-coupled test source for final adjustments. A polarizing beam splitting cube should be the first optical element, helping to define the polarization of the output light and allowing the collimator to be used with a fiber connected to either of two ports. This requires correct orientation of the cube and the fiber bulkhead receptacles; see figure 1.

2) The output laser beam must have a Gaussian intensity profile with a diameter (2 x beam waist) of 23-25 mm, and the clear aperture of the collimator must be at least 38.1 mm (1.5”), corresponding to transmission of 99% of the light.

3) The output laser beam must be well collimated, diffraction limited at 780 nm, and make an angle of less than 1 milliradian with respect to the housing flange. We would like the collimator tested with a shear plate collimation tester, with typical collimation resolution of 220 microradians or better (see for example Melles Griot part number 09 SPM 001), and the test data provided with the parts. A test source and the fiber to be used will be provided for final adjustments.

4) The optics must be contained in a black anodized aluminum housing with an output flange for mounting to our apparatus. The dimensions of the flange and its orientation with respect to the fiber bulkhead receptacles must be identical to those shown in figure 3. The output side of the flange must have an o-ring groove with dimensions as shown, to accommodate Marco Rubber AS568A-133 type o-ring.

5) The total length from fiber output to the far end of the mounting flange must be 75 mm (or less), as shown in figures 1 and 3.

6) All materials used in the collimator must be completely nonmagnetic. The housing must be made of anodized aluminum, the fiber bulkhead receptacles made of anodized aluminum and ceramic, and the screws for joining components made of nonmagnetic brass (another strong, nonmagnetic material may be acceptable for screws; discuss alternatives with technical contact first).

7) All optical surfaces that are exposed to air must be antireflection coated for 780 nm, resulting in reflectivity of 0.25% per surface.

1 Numerical aperture (NA) is here defined as the sine of the angle from the center of the fiber to where the intensity drops to 1/e2 of the peak value.

Figure 1 Schematic of USNO fiber collimator.

Figure 2 Drawing of collimator now in use, showing two input ports with FC/APC bulkhead receptacles. All metal is anodized aluminum.

Figure 3 Drawing of collimator now in use. Dimensions for output flange hole patterns and o-ring groove are shown, as are the correct orientation of the flange hole patterns with respect to the fiber bulkheads.