Bose 2.2 User Manual download pdf (Page 48)

3.1.3 Laser Systems

We now give a brief overview of the lasers that are involved in the production process of

the BEC.

425 nm MOT laser system

The laser light at wavelength

= 425

, resonant with the

↔

transition, is

used for several purposes on the experiment: radial cooling in oven chamber (120

Zeeman slower (100

), magneto-optical trap (radial: 2

, axial: 10

), and

imaging (10

). We produce the blue light in a multi-step process: a diode-pumped

solid state (DPSS) laser

delivers 18

of green light at a wavelength

= 532

, which

we use to optically pump a Ti:sapphire crystal inside a ring cavity

. At the output of the

Ti:sapphire system we obtain around 3

of infrared light at

= 851

, with a narrow

linewidth of ∆

ν ∼

kHz

. Finally, the infrared light is frequency-doubled by using a

lithium triborate (LBO) crystal inside a home-made monolithic ring cavity

[149, Ch.5]

from which we obtain typically around 800 mW of blue light

The laser frequency is actively stabilized by performing Doppler free polarization spec-

troscopy

[151] with an electronic feedback to the external cavity of the Ti:sapphire

system

. The resulting laser linewidth ∆

425

∼

MHz

is well below the natural linewidth

)

∼

MHz

of the

excited state. It is therefore suﬃciently small for the cooling

and the imaging of the atomic cloud.

663 nm repump laser system

The repump laser system operating at

= 663

is a home-built external-cavity

diode laser. It is frequency stabilized by a passively stable reference cavity

using the

Pound-Drever-Hall (PDH) stabilization scheme [152, 153]. The output power of the laser

(

= 10

) is split into two beams, one going to the experiment (

= 7

) and one

used for the stabilization. The long-term frequency stability of the repump laser depends

on the thermal expansion properties of the reference cavity and exhibits a drift of around

MHz

per hour. Therefore, the laser frequency has to be readjusted several times per

day. Recently, a new cavity has been built [154] using the material SuperInvar

which

shows a low coeﬃcient of thermal expansion. In addition, the new device shows a much

higher ﬁnesse than the one currently used, and can be actively stabilized in length via a

piezoelectric actuator.

Coherent, Inc.: Verdi V18.

Coherent, Inc.: MBR 110

The length of the doubling cavity is actively stabilized using the Hänsch-Couillaud locking technique [148],

which does not require a frequency modulation of the incoming laser light.

The total blue power speciﬁed here includes the main beam after the doubling cavity and two separately

outcoupled beams used for imaging and spectroscopy.

We use a hollow cathode lamp [150] to provide the atomic chromium vapour in the spectroscopy setup.

See [113, Ch.3.2.1] for the latest improvements of the light detection electronics.

Reference cavity: length l = 1 m, free spectral range FSR = 75 MHz (TEM01 mode).

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