**4.5 Optical layout, detectors and IR camera**

The main part of the magneto optical trap optics was purchased as one single item 21. Our optical layout (Fig.13) include a larger list of parts. The rays coming from each laser are vertically polarized. After leaving the first optical glass divider (OGD1 in Fig.13) each laser beam is driven to a polarizing beamsplitter cube. The polarization of the repumping laser is rotated in 90 degrees by means of a half wave plate and becomes horizontally polarized before entering the polarizing beam splitter cube. The polarization of the cooling laser is maintained vertical and reflected by the beamsplitter cube. By this mean, the cooling laser and the repumping laser become collinear. Both lasers were driven over a line of holes of the optical top and continued collinear at least at 4 meters from the exit of the polarizing beam splitter cube. The polarization of the cooling laser was orthogonal to the polarization of the

 20 MDC-Vacuum, Model 150008

<sup>21</sup> Toptica Photonics, Model MOT-Optics

168 Quantum Optics and Laser Experiments

Fig. 11. Left: 55x55x55 mm glass cell, right: glass cell constructed with 35x50x4 mm plates.

inch conflate type adapter and 7 optical windows with 1 inch useful area 20.

Fig. 12. Side and top view of observation cell.

**4.5 Optical layout, detectors and IR camera** 

20 MDC-Vacuum, Model 150008 21 Toptica Photonics, Model MOT-Optics

The third version (Fig. 12) consisted in a cell prepared by a glass blower. The cell has a 2.75

The main part of the magneto optical trap optics was purchased as one single item 21. Our optical layout (Fig.13) include a larger list of parts. The rays coming from each laser are vertically polarized. After leaving the first optical glass divider (OGD1 in Fig.13) each laser beam is driven to a polarizing beamsplitter cube. The polarization of the repumping laser is rotated in 90 degrees by means of a half wave plate and becomes horizontally polarized before entering the polarizing beam splitter cube. The polarization of the cooling laser is maintained vertical and reflected by the beamsplitter cube. By this mean, the cooling laser and the repumping laser become collinear. Both lasers were driven over a line of holes of the optical top and continued collinear at least at 4 meters from the exit of the polarizing beam splitter cube. The polarization of the cooling laser was orthogonal to the polarization of the repumping laser. The combined laser beams were simultaneously expanded by a laser beam expander consisting of a *f* = 50 mm lens followed by two *f* = 300 mm. The diameter of the three lenses was 25 mm. The diameter of the lasers was nearly 3 mm and at the exit it was 12 mm giving an expansion of 4x. The laser disk was rounded by an iris diaphragm.

Fig. 13. Combination of repumping and cooling laser beams followed by simultaneous beam expansion. OGD = optical glass divider, L = lenses, HWP = half wave plate, PBSC = polarizing beam splitter cube, ID = iris diaphragm.

After passing the iris diaphragm, both lasers were divided in a 0.3/0.7 divider. Most of the laser power (70%) was directed to the horizontal plane (Fig. 14). A polarizing beam splitter cube divided both lasers equally. Each pair of beams that leaved the polarizing beam splitter cube were divided again by means of two non polarizing beam splitter cubes. By this method it was possible to obtain two sets of counter propagating pairs of beams. In each leg of this arrangements quarter wave plates to with the correct circular polarizations. We installed a surveillance IR camera to observe the cloud and an IR CCD 22 with a 50 mm lens.

<sup>22</sup> Altec Vision, Model PL-B771U

Cold Atoms Experiments: Influence of Laser Intensity Imbalance on Cloud Formation 171

M

vertical plane

beam divider 66% from horizontal plane

6

Fig. 15. Beam division in the vertical plane and use of quarter wave plates to obtain the correct circular polarization. QWP = quarter wave plate, PBSC = polarizing beam splitter

PBSC

HWP QWP

M 2 6.5 10 10

ID

optical top

ID

M

A rubidium getter 23 is used to introduce the neutral atoms into the vacuum chamber. The main feature of this getter is that it allows introducing a controlled amount of atoms. The rubidium is released as a vapour when a current flows through the getter. The current required to release the necessary amount of neutral atoms is close to 3.7A. A diagram of the getter is shown in Fig.16. The getter is contained in a chamber with a trapezoidal section and released from a small aperture at the upper part. When the getter cools down, condensation and solidification of the material closes the exit. To start the vapour emission it is necessary to increase the current to 8A during nearly 2 seconds. The pulse duration should be controlled precisely by means of a programmable current power supply 24 to avoid the

The code for the power supply was made with Labview6.0. The code set 5 s at 3 A, 2 s at 8 A, 4 s at 6 A and fixed the current at 3.7 A the rest of the time. Several getters were soldered to pair of pins of an 8 pin conflate flanged power feedthrough 25. Care was taken to label the

**4.6 Introduction of neutral atoms using a rubidium getter** 

cube, M = mirror.

destruction by melting of the getter.

23 Saes Getters, Model RB/NF/3.4/12 FT10+10

24 Instek, Model PSM-2010

25 Kurt K. Lesker, Model EFT0084033

The small part of the optical power (0.3 that was obtained at the 0.70/0.30 beam divider was directed vertically to the optical top as shown in Fig. 15 and directed parallel to the horizontal plane to a half wave plate that rotated both lasers in nearly 45º. A polarizing beam splitter cube disposed after the half wave plate divided the beam in two parts with the same intensity. One part went upwards and the other crossed the polarizing beam splitter cube and was directed by means of two mirrors in the counter propagating downward direction. Two quarter wave plates were used to obtain the correct circular polarization. With our experimental conditions we tried to balance the power from every ray as best as possible.

Fig. 14. Beam division in the horizontal plane and use of quarter wave plates to obtain the desired circular polarization. HWP = half wave plate, PBSC = polarizing beam splitter cube, NPBSC = non polarizing beam splitter cube, M = mirror, BD = beam divider 0.3 to vertical plane.

170 Quantum Optics and Laser Experiments

The small part of the optical power (0.3 that was obtained at the 0.70/0.30 beam divider was directed vertically to the optical top as shown in Fig. 15 and directed parallel to the horizontal plane to a half wave plate that rotated both lasers in nearly 45º. A polarizing beam splitter cube disposed after the half wave plate divided the beam in two parts with the same intensity. One part went upwards and the other crossed the polarizing beam splitter cube and was directed by means of two mirrors in the counter propagating downward direction. Two quarter wave plates were used to obtain the correct circular polarization. With our experimental conditions we tried to balance the power from every ray as best as

Fig. 14. Beam division in the horizontal plane and use of quarter wave plates to obtain the desired circular polarization. HWP = half wave plate, PBSC = polarizing beam splitter cube, NPBSC = non polarizing beam splitter cube, M = mirror, BD = beam divider 0.3 to vertical

BD

QWP QWP

surveillance camera

M M M

QWP

6

CCD

M

PBSC

3

QWP

NPBS

HWP

2. 2 3

3

NPBS

M

possible.

plane.

Fig. 15. Beam division in the vertical plane and use of quarter wave plates to obtain the correct circular polarization. QWP = quarter wave plate, PBSC = polarizing beam splitter cube, M = mirror.

### **4.6 Introduction of neutral atoms using a rubidium getter**

A rubidium getter 23 is used to introduce the neutral atoms into the vacuum chamber. The main feature of this getter is that it allows introducing a controlled amount of atoms. The rubidium is released as a vapour when a current flows through the getter. The current required to release the necessary amount of neutral atoms is close to 3.7A. A diagram of the getter is shown in Fig.16. The getter is contained in a chamber with a trapezoidal section and released from a small aperture at the upper part. When the getter cools down, condensation and solidification of the material closes the exit. To start the vapour emission it is necessary to increase the current to 8A during nearly 2 seconds. The pulse duration should be controlled precisely by means of a programmable current power supply 24 to avoid the destruction by melting of the getter.

The code for the power supply was made with Labview6.0. The code set 5 s at 3 A, 2 s at 8 A, 4 s at 6 A and fixed the current at 3.7 A the rest of the time. Several getters were soldered to pair of pins of an 8 pin conflate flanged power feedthrough 25. Care was taken to label the

<sup>23</sup> Saes Getters, Model RB/NF/3.4/12 FT10+10

<sup>24</sup> Instek, Model PSM-2010

<sup>25</sup> Kurt K. Lesker, Model EFT0084033

Cold Atoms Experiments: Influence of Laser Intensity Imbalance on Cloud Formation 173

and was focused with an *f* = 200 mm lens to the interferometer. The light reflected from the interferometer becomes horizontally polarized after passing twice the quarter wave plate

and was reflected by the polarizing beam splitter cube into a fast photodiode 28.

G PZT

LD laser

Fig. 17. Optical setup for Pound Drever Hall stabilization method.

L

FPI L

where *R* is the mirror reflectivity and FSR

 

is the modulation amplitude,

frequency / 2 20 MHz 

two weak sidebands as

where 

Fig.18.

 / 2 20 MHz 

28 Thorlabs, Model PDA10-EC

The reflected electric field from a Fabry Perot interferometer is given by

1 1 e

QWP PBSC 2M

11 2 3 14

OI

 2 / 

1

<sup>1</sup> <sup>2</sup> ( )

 

*n*

*L w*

*i r i i e R E E R*

PD

4

 2 2 0 0 0 0

> 2 0

<sup>1</sup> ( ) <sup>2</sup>

is a Lorentzian function, and the laser linewidth. A modulated spectra for

1

( ) ( )( ; ) ( ) ( ; ) ( ; ) *<sup>n</sup>*

*I JL J L nL n*

   

2M

(24)

2

modulation frequency and laser linewidth 10 MHz is depicted in

. The incident laser amplitude can be written as a carrier with

(23)

M

OGD

6

 

. The laser is modulated at a

(25)

earths. The soldering was made by means of thermocouple point soldering device. This uses three 5.1 mF, 350 V electrolytic capacitors in parallel. 70 V is enough to sold the parts. We used only one getter for more than 100 hours and it is still working.

Fig. 16. View of the rubidium getter.
