Bose 2.2 User Manual download pdf (Page 49)

427 nm optical pumping laser system

The setup of the optical pumping laser system (

= 427

) is similar to the MOT

laser system, however, involving much lower laser powers. We produce around 60

of light at a wavelength

= 855

from an external-cavity diode laser

, which is

frequency stabilized to the same reference cavity as the repump laser, using as well the

PDH technique. The infrared light is frequency doubled by a potassium niobate (KNbO

)

crystal

inside a home-made ring-cavity [155]. We obtain around 10

of blue light at

the output of the cavity. The light level is then actively stabilized to around

= 200

which is the optimum value for the optical pumping using the

↔

transition

(see [73, Ch.4.8] for details of the optical pumping process).

1076 nm optical dipole trap laser system

The laser light at

= 1076

for trapping the atoms in the crossed ODT is provided by

an ytterbium ﬁber laser

, which we operate at an output power

= 61

. Behind the

exit of the ﬁber, the light is split into the horizontal (’ODT1’) and the vertical (’ODT2’)

trapping beams, separately controlled in intensity by AOMs. During the BEC production

sequence, we use the maximum powers

= 17

and

= 9

in ODT1 and ODT2,

respectively. The two linearly polarized laser beams are crossing under an angle of 90

◦

with their polarization directions being perpendicular to each other to avoid interferences.

The waists of the Gaussian laser beams are

= 30

and

= 50

for the ODT1 and

the ODT2, respectively. We thus obtain a maximum trap depth of

ODT

|/k

∼

250

in the crossed conﬁguration [74].

An issue that occurs when dealing with high laser powers are thermal expansion eﬀects in

the lenses that are placed in the optical path. To keep these eﬀects as low as possible, we

are using thin quartz lenses with a thermal expansion coeﬃcient around 10 times lower

than standard lenses made of the material BK7 [74].

In addition, to minimize the movement of the laser beams during the intensity ramps

we have developed a two-frequency driver for the AOMs [156]. Compared to the standard

single frequency operation, we are able to decrease the beam displacement by a factor of

around 20. Recently, we have exchanged the analogue power control by a digital one; this

system shows the same performance while being more user-friendly [122, Ch.A.1].

Laser diode: ‘LD-0850-0100-1, SDL-5411-G1’, supplier: Toptica Photonics AG.

Here, we use KNbO

instead of LBO to enhance the conversion eﬃciency at such low input laser powers.

The temperature of the crystal is actively stabilized (

T ∼

◦

), also in contrast to the LBO crystal,

where this is not needed.

IPG Photonics, model: YLR-100-LP.

The beam movement during the intensity ramps is induced by a heating of the AOM crystal when

increasing the RF driving power.

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Bose 2.2 User Manual Page 49

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