ions. The dark regions imaged by the EMCCD camera are noisier than those imaged by the SPC camera, whereas the (bright) ions themselves show comparable (low) noise.
2.5. Laser system
The experiments presented in this thesis are performed with24Mg+ ions that are laser cooled on the D2 (3s1/2 −3p3/2) transition at 279.6 nm. One of the reasons for choosing this ion was the possibility to generate this wavelength with a convenient all-solid state laser system. Recently, Yb fiber lasers have become available that emit 2 W and more at 1120 nm with a specified linewidth below 100 kHz. With two subsequent resonant second harmonic generation stages (SHG, developed by Frank Markert as part of his diploma thesis [35] and published in [36]) we are able to convert up to 10% of the infrared light into the UV. In total, this gives us a turn-key all solid state laser system providing up to a 200 mW of narrow band UV light. This is plentiful: The saturation intensity of the D2 transition is
Is=
2π2c~Γ
3λ3 = 2480 W/m
2, (2.16)
so if the beam is focused to a diameter of 2w0 = 0.5 mm less than 0.5 mW saturate the transition.
However, the operating wavelength at 1120 nm is near the long wavelength end of the Yb gain profile and thus technically challenging. Unfortunately we had our share of the bad luck several groups around the world had using similar fiber lasers: Both our fiber lasers (from two different companies) failed and could not be repaired by the manufacturer. We therefore installed a dye laser that serves three purposes. It can replace the fiber laser by generating 560 nm, simply omitting the first SHG stage. Second, without changing the dye it can be tuned to provide light for photoionization: Neutral Mg atoms are ionized by two 285 nm photons, which can be generated by frequency doubling 570 nm. Finally, it can be tuned to excite the D1 transition (3s1/2−3p1/2) at 280 nm which is outside the tuning range of the fiber laser.
2.5.1. The fiber laser
The fiber laser (Koheras Boostik) utilizes Yb as gain medium and emits up to 2 W of single- frequency and single-mode light at 1118.54 nm. The total output power is about 2.2 W due to an additional 200 mW of amplified spontaneous emission (ASE) that gives a broad incoherent background peaked about 50 nm below the signal. The linewidth is specified to be 100 kHz or less. The output fiber is not polarization maintaining, so the output is in general elliptically polarized. A combination of a λ/4 and λ/2 wave plate transforms the output to linear polarization. After turning on the laser the polarization changes due to thermal and other effects, but after about one hour it returns to the previous’ day polarization state and remains there, given the fiber has not been moved. Due to its sensitivity to back-reflections a 60 dB optical isolator is placed just after the wave plates. The output wavelength can be controlled by changing the temperature or by applying a voltage to a piezo which dilates the oscillator fiber. The piezo has a tuning range of 64 GHz which is sufficient to scan across all Mg+ isotopes: The splitting 24Mg+− 26Mg+ ∼= 3 GHz. The tuning sensitivity of the piezo has been measured to be 719 MHz/V.
IN 1120nm I/O 560nm OUT 280nm LBO BBO Diag. MM Diag. AC HC HC MM
Figure 2.22.: Image of the two second-harmonic generation cavities used to generate light at 280 nm. If the Yb fiber laser is used the port visible to the left is used as input for 1120 nm which is frequency- doubled resonantly to 560 nm in lithium borate (LBO). If the dye laser is used, the 560 nm are fed in via the port in the middle. A second resonant SHG stage frequency doubles the green light to 280 nm in aβ−barium borate crystal (BBO). The 560 nm port is also used as output: If the fiber laser is used and not referenced to the frequency comb, a few milliwatt in the green are sampled and sent to an iodine spectrometer for frequency reference. The mode-matching lenses are labeledMM, the optics for the H¨ansch-Couillaud locksHC, Diag. are photodiodes for intensity monitoring andAC is the astigmatism compensation of the UV beam.
2.5.2. The dye laser
Photoionization of neutral magnesium atoms requires light at 285 nm, which cannot be provided by frequency-quadrupling an Yb fiber laser as described above. Also, the D1 (3s1/2 −3p1/2) transition at 280 nm is outside the tuning range. For these reasons and as backup system for the fiber laser a dye laser was set up by V. Batteiger and H. A. Sch¨ussler.
The dye laser is a commercial Coherent 699/21, operating on Rhodamine 19 and pumped at 532 nm by a Coherent Verdi V10. For photoionization it is set to 570 nm, for laser cooling/ spectroscopy near 560 nm. The dye recipe we use is11: 1 g Rhodamine 19 solved in 250 ml benzyl alcohol and subsequently mixed with 1.75 l of ethylene glycol. The efficiency can be increased significantly by chemically tuning the dye mixture. Adding 8 g potassium hydroxide solved in 80 ml methanol nearly triples the output power at 560 nm. Pumped with 5 W we obtain up to 520 mW at 560 nm, at 7.2 W pump the output is about 1 W. A fresh dye mixture lasts under typical conditions about 8 weeks.
The free-running linewidth is rather large. A measurement of a beat note with a fiber laser yielded a full-width half maximum of 3.6 MHz.
2.5. LASER SYSTEM 45 AOM f QWP M L PBS f
Figure 2.23.: Schematic of a double-pass acousto-optic modulator. The linearly polarized input beam is partially deflected by the AOM. The first order beam is selected with an aperture and sent to a retro- reflector. A lensL placed one focal lengthf away from the AOM translates angular variations into parallel displacements, so that the reflected beam enters the AOM at the same angle as the original beam. The mirror M is also placed one focal length away from the lens, so it forms a telescope with magnification 1 and the beam stays collimated. To separate the input beam from the two times deflected beam a quarter wave plate is placed between the AOM and the mirror. After two passes it rotates the linear polarization by 90◦, so the polarizing beam splitter (PBS) reflects only the desired light.
2.5.3. The SHG stages
Both SHG stages are bow-tie cavities stabilized using the H¨ansch-Couillaud method [37]. The first SHG stage utilizes a 15 mm long lithium borate (LBO) crystal as nonlinear medium and is phase-matched non-critically by heating it to T ∼86.5◦C. Up to 900 mW at 560 nm are obtained. Thanks to temperature phase matching the output beam is nearly Gaussian, so no beam shaping is necessary. A biconvex lens focuses the light into the second cavity. In addition, aλ/2 wave plate together with a polarizing beam splitter allows to sample a variable amount of light for diagnostics and frequency stabilization. Alternatively, if the dye laser is used, this serves as a “port” to feed the green light into the system.
In the second stage, a 10 mm long β-barium borate (BBO) crystal converts the light into the UV near 280 nm. Phase matching is obtained critically, that is, by angle tuning. Up to 200 mW of UV light are generated. However, the output is strongly astigmatic and shows interference fringes. A telescope of two cylindrical lenses is used to obtain a symmetric beam profile, which is additionally filtered spatially by a 100µm pinhole. Typical day-to-day power levels measured after the pinhole are between 10 mW and 20 mW. The setup is shown in Fig. 2.22.
The SHG stages worked very reliably. Once locked the system required no further attention, cleaning and alignment was necessary every 2-4 days only.
2.5.4. Double pass acousto-optic modulators
The experiments presented in the following chapters require two laser beams with indepen- dently controllable and stable intensities and frequencies. This is achieved using acousto-optic modulators (AOM). In these devices the incident light beam is diffracted by an acoustical wave in a crystal. The frequency of the first-order diffracted beam is shifted exactly by the acoustic frequency±νaom. In addition it is deflected by an angle that depends on νaom. To
11
Figure 2.24.: The atomic oven used in both the 6rod trap and the endcap trap experiments. The image shows a test set-up with a ceramic “target” attached to the oven in the same distance as the trap center will be. The tantalum tube is held in place by the heating wire which acts like a spring and pushes the tube into a milled guiding structure.
compensate the angular deviations of the deflected beam and increase the scanning range, the AOMs were used in a double-pass (DP) configuration. A schematic of a DP-AOM is shown in Fig. 2.23.
Specifically, we used Brimrose QZF-150-100 shifters with a center frequency of 150 MHz and a specified tuning range of about 100 MHz. With 2 W of RF drive power a single pass diffraction efficiency of up to 70 % is achieved. Due to the rotation of the polarization the second pass is less efficient. In the double-pass configuration the total efficiency was up to 30 % but varied strongly as function of drive frequency. The usable frequency range was slightly below specification between 100 and 190 MHz, so in total the beam could be scanned across 180 MHz.
The DP-AOMs simultaneously served as “noise eaters”, i.e. for intensity stabilization. For this purpose aλ/2 retarder and a polarization beam splitter was installed in the output beam path that allow to sample a variable amount of light which is directed onto a photodiode. This provides an error signal for feedback on the RF drive power.
The retro-reflector of the DP-AOM ensured good beam pointing stability. A CCD beam profiler located 3 m away from a DP-AOM could not detect any shifts of the center-of-gravity of the deflected beam. However, the beam profile was found to change slightly as the frequency of the beam is scanned. This turned out to be an important systematic uncertainty for the spectroscopy experiments described in the following chapter: Ions sample the intensity only at one point within the profile, so that even though the integrated intensity is stabilized, the ion will be subject to a systematically varying intensity as the frequency of the DP-AOM is scanned. This effect is removed by spatially filtering the output of the DP-AOM with a pinhole and sampling light for stabilization after it.