Bragg laser system for drop tower operation

Stringent mechanical requirements are imposed on the laser system, since it will be objected to critical thermal and mechanical loads (see Sec. 5.1). The other main requirement is obviously given by the capability of driving two-photon transitions between different momentum states of ⁸⁷Rb.

For anti-parallel Bragg beams and a perfect resting point source, first-order Bragg diffraction is resonant at a detuning of δ = 2¯hk²/m = 2π·15.08 kHz. Since this is only an ideal case, the frequency difference δ should be a variable parameter to compensate any shifts of the resonance. Continuously illuminated atoms undergo Rabi oscillations between the coupled momentum states. Therefore, the Bragg pulse durationτ obviously needs to be tunable to optimize the beam splitter efficiency. The effective Rabi frequency (see Sec. 3.2.2), can be calculated as

Ωeff= 3πc²ΓI

∆¯hω³ . (3.39)

Therefore, the absolute intensity I of the beams as well as the detuning ∆ to the

|5S_1/2, F = 2i → |5P_3/2F = 3i transition of ⁸⁷Rb should be an adjustable parameter.

We now want to focus on the technical realization. A schematic and a photograph of the assembled Bragg laser system are given in Fig. 3.6.

Distributed feedback (DFB) diode laser source

The light source is a distributed feedback (DFB) laser diode, based on a III-V com-pound semiconductor (GaAs) integrated into a hermetic TO3 housing filled with a technical gas (Eagleyard, EYP-DFB-0780-00080-1500-TOC03-0000). It features a Bragg grating which is implemented by a periodic variation of the index of

refrac-Figure 3.6:Miniaturized Bragg laser system for drop tower operation. The system is based on a distributed feedback (DFB) diode laser, miniaturized optics and opto-mechanics mounted onto an 270 x 310 mm² aluminum breadboard with M3 threads (1 cm spacing). Further details in text.

tion and extends over the total length of the semiconductor resonator². The grating narrows the linewidth of the emission and guarantees single frequency emission by se-lecting a single longitudinal laser mode. Tuning of the emitted light is accomplished by modulating either the diode’s current (dλ/dI ≈ 0.003 nm/mA) or temperature (dλ/dT ≈0.06 nm/K), the latter of which is possible since the chip is soldered onto a thermoelectric cooler (TEC).

Due to the absence of any critical opto-mechanical component, DFB diodes compro-mise a monolithic structure and permit high long-term stability and reliability. The maximum forward current and corresponding optical output power given by the manu-facturer areI_max = 200 mAand P_max ≈100 mW. The laser threshold and slope have been measured to beI_th= 37 mAandS= 0.74W/A, respectively. The emission spec-trum reached the D₂-line of⁸⁷Rb for a temperature of22.0^◦C. Standard specifications are summarized in Tab. 3.1.

Opto-mechanical design and beam paths

The light emitted by the DFB diode (see Fig. 3.6) is collimated with an aspheric lens (f = 2.2 mm) and passes an optical isolator (OI, 30 dB isolation), whose transmission was measured to be0.78. In this module, the used opto-mechanical components (mirror mounts, optics holders) are self-made constructions, mostly based on aluminum alloys and stainless steel.

After passing a half-wave plate (λ/2), the beam is split into two paths at a polariza-tion beam splitter cube (PBS). One path is used for absolute frequency stabilizapolariza-tion.

A fraction of Bragg laser light is overlapped with reference light (emerging from fiber collimator (FC) 1) using another PBS. As a reference, we use the cooling laser (see Sec. 2.4.3), stabilized to the |5S_1/2, F = 2i → |5P_3/2F = 3i transition of ⁸⁷Rb. The

2Another laser type with an integrated Bragg grating is the distributed Bragg reflector (DBR) diode laser. Here, the grating is not situated in the vicinity of the active medium but implemented aside the gain section acting as a local reflector.

parameter unit min. typ. max.

spectral width (FWHM) ∆ν MHz 2

temperature coefficient dλ/dT nm/K 0.06 current coefficient dλ/dI nm/mA 0.003 typ. Output power @ I = 180 mA mW 80

slope efficiency S W/A 0.6 0.8 1

threshold current I_th W/A 70

Table 3.1:Specifications of the DFB-diode used in the Bragg laser system at the begin of life, adapted from [183]. Remark: By using the QUANTUS-I laser electronics, we usually measured FWHM linewidths of about∆ν≈5 MHz.

beat signal is detected with a fast GaAs-based photodetector (Hamamatsu G4176-03).

The other path is used to generate the optical lattice and therefore the beam is again split into two paths, each passing an acousto-optical modulator (AOM). These devices (Crystal technology, 3080-122) are switchable on a ns-timescale and driven with 80MHz and 80MHz + δ, respectively. In this way, they generate the required frequency differenceδfor the two beams. After passing the AOMs, the light is coupled into polarization maintaining single-mode optical fibers (SuK, PMC-850-5,1-NA013-3-APC-400-P) with commercial, miniaturized laser beam couplers (SuK 60SMS series, 8^◦ polished), and finally guided to the experiment.

The light from both fibers is collimated to a Gaussian beam with a diameter of 0.65 cm (FWHM) using a single-lens telescope each, attached at opposites sides of the vacuum chamber and pointing along the x-direction (see Fig. 3.5). Both beams are equally linearly polarized, thus forming an optical grating at the position of the atoms. Fiber-coupling and AOM diffraction efficiencies (cw) are both aroundη = 70%, resulting in typical values of total optical power for the Bragg lattice of about P_typ = 10 mW (cw)@Ityp= 100 mA.

To verify the mechanical stability of the whole setup, the laser system was success-fully tested in a self-built mini drop tower providing a drop altitude of approximately 1 m (designed and built by K. Moehle during a research assistant period in 2008). This tower consists of a platform, on which laser test assemblies can be mounted and which is identical to the used platforms in the drop capsules [107]. It is guided via two stain-less steel metal rods and can be elevated by a mechanical winch to a height of 1 m and subsequently be dropped from that distance. At the bottom, a combination of foam sheets decelerates the assembly with typical loads of up to 50 g which are comparable to the expected ones for the QUANTUS-I apparatus in the Bremen drop tower.

Stabilization and Switching electronics

To prevent excitations and atom losses which lead to decoherence, the absolute laser frequency of the Bragg beams has to be sufficiently detuned from atomic resonances.

This is realized by an offset lock stabilization, as depicted in Fig. 3.7 (top).

The detected beat signal (G4176-03) between Bragg and cooling laser is typically around ∆ = 640 MHz. It is first amplified (ZJL-7G), passes a directional coupler

Bias-Tee

Figure 3.7: Stabilization path of the Bragg laser system in QUANTUS-I. Details in text.

(ZFDC-10-2S), then divided by a programmable divider and compared with a stable reference oscillator (PXI-5404, 80 MHz) in a digital phase frequency detector (HMC 440QS16G). Subsequently, an error signal is fed into a lockbox which generates a control signal for the current controller driving the Bragg laser diode. In this way, the frequency of the Bragg laser source can be stabilized to a fixed detuning∆, which can be adjusted by changing the LO frequency or the divider scaling.

The electronics for driving the AOMs are depicted in Fig. 3.7 (bottom). Here, two frequency generator cards (PXI-5404), phase-locked to a 10MHz internal reference, are used to drive one AOM each. The PXI-5404 is a 100 MHz frequency generator with a1.07µHzfrequency and 12 Bit vertical resolution. By following the right path, the 80 MHz +δ output passes a power splitter (ZFSC-2-2-S+) from which one output (OUT 2) is used as the LO for offset lock stabilization. On the left-hand side, the 80 MHz output has to be attenuated (VAT-3) such that both RF signals remain at the

same amplitude for subsequent amplification.

They pass a directional coupler (ZFDC-10-2S) and are fed into one RF switch each (ZYSWA-2-50DR), capable of switching times of about 20 ns (10−90%). A micro-controller (Atmel ATmega8) generates a TTL signal to simultaneously trigger both switches with µs resolution. This device controls our Bragg pulse duration. Finally, each output is amplified to about 1 W (ZHL32A) and fed into the AOMs (3080-122).

In a later stage of the experiment, we exchanged this pulse generation concept with a state-of-the-art timing processor (PulseBlaster DDS-II-300), providing two indepen-dent analog output channels ranging in frequency from 5 kHz to 100 MHz with sub-Hertz resolution. Various envelopes can now be programmed with 300 MHz sampling rate and 14 Bits sampling precision³. This board gets triggered from the experimental sequence (LabView routine) and is programmed via USB.