Laser system

We use a laser system based on home made diode lasers. It provides the optical fields for laser cooling, optical pumping and absorption imaging. One additional laser is used for the optical readout of cantilever motion. The system is compact and fits on the area of 1.4×1.2 m2. We cover it with a foam board box to reduce dust, acoustic vibrations and air circulation. To decouple the beam alignment at the vacuum cell from the laser system, all beams are coupled to polarization maintaining single-mode fibers11.

For the manipulation and detection of atoms we have to derive four different frequencies of laser light close to the D2 line of 87Rb at 780.24 nm. An overview of the involved transitions is shown in figure 4.6. We use three different lasers that typically run with Sharp diodes12 _{with 120 mW nominal output power. Two of the}

lasers contain a grating in Littrow configuration [256] to reduce the linewidth. Their frequency is stabilized by Doppler-free saturation spectroscopy in a Rb vapour cell [257]. We generate a locking signal by frequency modulation of the laser at 110 MHz [258]. The feedback of the lock regulates the grating position with a piezo (integral path) and the laser current (proportional path), which results in a laser linewidth of ∼300 kHz.

An additional laser is used for the independent readout of the cantilever. We oper- ate it with a diode running at 830 nm to be far off resonant from optical transitions. This avoids scattering by the atoms, and at low intensity, dipole forces remain negli- gible. It is thus possible to continuously monitor the cantilever, also in the presence of atoms nearby.

A schematic overview of the system is shown in figure 4.7. The following beam lines constitute the setup:

Cooling To obtain sufficient power for laser cooling we use a master-slave con- figuration. The master laser is grating stabilized and frequency locked to the F = 2 → F0 = (2,3) crossover resonance. We feed the light through a double-pass AOM to change the frequency shift without altering the beam alignment. For cool- ing we use a maximal red detuning of ∆ =−11γ while for imaging in the presence of magnetic fields we use blue detuning up to +3γ, where γ = Γ/2π = 6.065 MHz the natural linewidth of the D2 line. The beam is then injected into a slave laser without grating. It provides the full output power of the diode while the frequency stability is inherited from the master laser. We typically run the slave with a cur-

11_{Thorlabs PM-780HP}

4.2 Vacuum, Lasersystem, Electronics 73 λ = 780.24 nm Γ/2π = 6.065 MHz 266.7 156.9 72.2 6834.7 repump pump MOT _detection

[MHz] 52_P3/2 52_S1/2 F = 3 F = 2 F = 1 F = 0 F = 2 F = 1

master lock point slave lock point

Figure 4.6.: Level scheme of the 87Rb D2 line and used laser frequencies. The locking point of the two spectroscopy stabilized lasers is indicated.

rent of 120 mA and obtain up to 80 mW optical power after an optical isolator. A switching AOM is used to adjust the power and for fast switching. Finally, the light is split into four paths, each of which is coupled into a polarization maintaining single-mode fiber that guides it to the experiment. Two fibers provide the beams that are reflected on the chip under 45◦ and carry 15 mW each, and the remaining two provide horizontal beams carrying 5 mW each.

Imaging A small fraction of the slave light is split to a path with an additional switching AOM to provide light for absorption imaging. We use two axis for imaging to be able to cover both the cooling and the cantilever region. The power is chosen such that the peak intensity in the collimated beam after the fiber is a small fraction (< 20 %) of the saturation intensity Is = 1.67 mW/cm2 of the cycling transition F = 2 →F0 = 3.

Repumping The cooling light also drives the non-resonant transition F = 2 →

F0 = 2 with a probability ∼ 1/2000 for each scattering event. This populates the F = 2 excited state which can decay also to the F = 1 ground state. To bring population in this state back to the cooling cycle we use a grating stabilized repump laser. It is locked to the F = 1 → F0 = (1,2) crossover and shifted to resonance with a switching AOM that is also used to set the power level. We overlap the beam with the 45◦ cooling beam on a polarizing beam splitter. After the fibers we have a power of 5 mW each.

AOM 110MHz rf-PD slow PD Rb Rb slow PD rf-PD slow PD Rb Master Slave Repump PBS PBS NPBS PBS MOT

hor. MOThor. MOT 45° MOT 45° Det. X Det. Y AOM PBS PBS PBS PBS f=300 f=150 f=50 f=100 f=50 f=150 AOM 80MHz AOM 80MHz AOM 80MHz AOM 80MHz f=500 f=100 Readout M O A f=150 f=50 AOM 80MHz

Figure 4.7.: Diode laser system used for cooling, optical pumping, imaging, and cantilever readout. All beams are controlled with AOMs and electromechanical shutters, and are coupled to polarization maintaining single-mode fibers.

Optical pumping Light for optical pumping is derived from the unshifted master laser with a typical power of 17 mW. The pumping light is frequency shifted to the F = 2→ F0 = 2 transition by an AOM in double-pass configuration. The light is overlapped with the imaging beam for the y-axis on a non-polarizing beam splitter and coupled in the same fiber.

Fast switching within less than one microsecond is accomplished by switching AOMs for each beam. However, residual scattering into all diffraction orders also without RF driving of the AOM crystal leaves some light in the fibers and thereby reduces the magnetic trap lifetime. Electro-mechanical shutters13 _{with a transient}

time of 1 ms and a delay of 5−7 ms are used to completely block all beams during magnetic trapping phases.

4.2 Vacuum, Lasersystem, Electronics 75