Imaging System of a Bose-Einstein Condensation

Experiment, and its Automation

Fabien Lienhart

U.C BerkeleyPhysics departmentStamper-Kurn’s group

August, 29th 2003

Plan

• Recent steps forward in the BEC experiment

• The Imaging System

• Automation of the imaging system: Visual Basic in WinView

Improvements and breakthroughs on the BEC

experiment

• Ultrahigh Vacuum fixed• MOT laser brighter and monomode• Polarization Gradient cooling set• First efficient magnetic trappings• Next (last?) step…

Back to a Ultra-High Vacuum

Problem: water leaking from a coil in the vacuum chamber

Open the chamber, fix the leak, close the chamber and Back to the vacuum

•Rotary pump

•Turbomolecular pump

•1 week baking-out, monitored by a Rare Gas Analyser

•Gauge limit

10-3

10-7

10-11

Torr

MOT LaserHow things work… Problem

Ugly beam which could be stronger

Solutions•Tapered amplifier: up to 300 mW•Polarization maintaining

optical fiber

Results •After the fiber: - 70 mW

- monomode - good shape

•Up to 3 billion atoms trapped

Optimizing pg-coolingHow things work…

Easy case of lin –lin lightImprovements• Better beam shape• Transition between the two pg-cooling steps found: optimization of T and N

Results30 K reached

(Time of flight measurement)

First efficient magnetic trappings

How Ioffe-Pritchard trap works…

Results• More than 600 million atoms trapped• Lifetimes ranking from 15s (low Bias-Field) to 50s• Adiabatic compression achieved

The Imaging System

• The constraints• The experimental device• Characterization of the system• Results and future work

The Constraints

• Requirements of the system- Resonant light has to be used: 2-to-3 beam- Top-bottom axis- Keep the polarization of the light- Three very different magnifications: ~ ½, 5 and

16

• The difficulties- Top-bottom axis is already crowded!- Only a few gold mirrors can be used at 45o

- Precise magnification needs to be known (quantitative imaging) and no way to place a fine object in the center of the trap!

The Experimental Device

Characterization of the System

ResultsMag ½Magnification: 0.510 0.015Resolution: 80 m (close to Camera limit)Not to sensitive to the position of the MOTBut sensitive to the angle of the last lens (distortion)

Mag 5Magnification: 4.60 0.01Resolution: 8.7 m (close to diffraction limit)No distortionResolution very sensitive to the position of the cloud

Mag 16Magnification: 12.0 0.6Resolution: 14 mPosition of the camera affects the magnification (.35/cm)

without really changing the resolutionVery sensitive to the angle of the last lens (distortion)

Automation of the Imaging System

• How WinView works

• Adding buttons with WinView

• Example of routine: rotating the images

• Future work

How WinView works

•WinView controls the camera•Automating WinView:

Using MacrosEasy, but limited and buggy

Or…

Adding buttons with Visual Basic1. Write your script in VB

Object Oriented ProgrammingClasses which implement WinView

2. Transform it into a DLL

3. Register the DLL

4. A new icon should appear inWinView’s taskbar

The implemented buttons

• Close All - closes all the windows

• AutoSave - saves all the windows with the date

• QuickASCII - saves the image as a text

• AbsorptionLoop - cycles absorption pictures

• RotateFrame - rotates the pictures

Example: the rotation

ProblemMatrix index must be integer numbersWhich is not the case after rotation

SolutionsImplementation of various algorithms

1. Closest neighbor2. Gaussian interpolation3. Bicubic interpolation4. -Spline method: 2 ideas

leading to the best results

Idea 1 - Three-pass rotation

Idea 2 – Efficient 1D interpolationCubic b-Spline

Third order Basis Piecewise polynomials

Advantages- Normalized contributions- Compact support -> local control- Interpolated function is C2

- Fast implementation: z-transform of the convolution givesan efficient recursive algorithm

Performances of the different methods

A. OriginalB. Clothest neighborC. Gaussian interpolationD. Bicubic interpolationE. -Spline method

0

1

2

3

4

5

6

B C D E

Time (in s) taken for a rotation

Conclusion

Work achieved- Imaging system: characterization and limits of the system- WinView: add-ins

In the next month-Imaging system:

- way to easily calibrate the system- try different lens for mag ½ and 16

- WinView: gaussian fit of the profile of the cloud