The laser system for photoionization of magnesium 24Mg atoms [109, 97] (31S0→31P1→continuum,
λvac=285.2965 nm) and for cooling 24Mg+ions (32S1⁄2→32P3⁄2,λvac=279.6352 nm) [9, 76] consists of two dye cw ring lasers from Coherent whose output is transfered to the femtosecond beam line via optical fibers. There, it is frequency doubled in home-built second harmonic generation enhancement resonators based on bbo.
Rhodamin 1927 diluted in ethylene glycol is used as lasing dye. A small volume (≈5 mℓ) KOH
solution (0.45 g dissolved in 50 mℓ methanol) is added to the dye solution of the cooling laser to
shift the emission spectrum of the dye towards shorter wavelengths and thereby enhance the laser output power. Both lasers are pumped with 3.5 W of a frequency doubled Nd:YAG laser28 yielding
about 500 mW at 570 nm and 300 mW output power at 560 nm. Under these conditions, the dye solution had to be exchanged after two to three months of regular use.
In future, the dye lasers are planned to be replaced by a solid state laser system operating in the infrared: one erbium doped fiber laser29 at 1120 nm and a diode laser30 at 1140 nm. For the fiber lasers, efficient frequency conversion to the ultraviolet has already been demonstrated in our group [56]. The diode laser system is currently about to be finalized [47].
a. frequency stabilization
The laser frequency of the dye-lasers (line width≈ 0.5 MHz) is stabilized to a heated reference
cavity in order to correct for high-frequency noise. Long-term absolute frequency drifts were taken care of by stabilizing the laser frequency to molecular transitions in iodine. To this end, Doppler-free saturated absorption spectroscopy [158] was set up for the cooling laser, Doppler-free polarization spectroscopy [178] for the photoionization laser. Further details about the experimental realizations are given in fig. 2.18 on page 45. Low noise data are presented in fig. 2.19 on page 46.
The polarization spectroscopy gets along with less electronic equipment compared with absorp- tion spectroscopy. Neither acusto-optic modulators nor any lock-in technique is involved. However, one has to trade-off simplicity with a 500 times weaker signal since there is no amplification stage in the polarization setup. Nonetheless, this turned out to be no limitation since the mainly thermal
27 LC5750, Lambda Physik 28 Millenia XP, SpectraPhysics 29 orange one, Menlo Systems
2.7. Ion trapping laser system λ/2 λ/4 λ/4 λ/2 PBS periscope lens f=5 00 m m peri scope lens f=250mm frequency doubling magnesium photo-ionization laser
285 nm frequency doubling magnesium cooling laser
280 nm
RF build up chamber 3
chamber 1 diode lasers for barium
photo-ionization and repumper
from fs laser 70 m fiber coupling ob serv at ion observation neutral density filter
sh ut ter sh ut ter photodiode vacuum pump vac uu m p um p
vacuum apparatus footprint: 130 cm × 110 cm separate laboratory dye laser 560 nm dye laser 570 nm pump 532 nm
Figure 2.17.: Beam path schematic for 24Mg+ photoionization and cooling contained on a 1.8 m×1.2 m optical table. The laser light for the two frequency doubling stages is supplied
by through 70 m long optical fibers that are coupled to two dye lasers in a distant laboratory in the same building. Both uv output beams are overlapped on a polarizing beam splitter from where the light can be directed to chamber 1 and 3 with adjustable power. A flipper mounted neutral density filter in the beam path to chamber 1 is used to repeatedly reduce the intensity in this arm during the automatic loading scheme; see appendix A.1.1.a on page 124. Mechanical shutters in the respective beam paths allow to block the light. The ions in chamber 1 are irradiated with linearly and parallel to the optical table polarized light focused by a 250 mm lens. Their fluo- rescence is detected “from above” the apparatus perpendicular to the optical table. In chamber 3 the cooling laser light is polarized linearly and perpendicular to optical table. It is focused by a 500 mm lens and detection is “from the side” parallel to the optical table. Periscopes lift the beam from 100 mm outside to 254 mm in order to match the height of the rf-guide inside the vacuum. tiamo was connected to the femtosecond laser system fp2 (see section 3.1 on page 55) at chamber 2 for the majority of the experiments of this thesis. The pulsed beam path is shown in red. A photodiode in front of chamber 3 is used to monitor the pulse energy. The laser system for photoionization and cooling of 138Ba+is not shown explicitly for visibility reasons. It will be described in the thesis of Günther Leschhorn.
drifts occur on a minute time scale. A slow integrator circuit adapted to that time scale averages over noise contributions and allows to keep the photoionization laser frequency drift sufficiently small.
b. optical fiber link
The tiamo approach of connecting an ion trap to a femtosecond laser setup and one day to a X-ray beam line requires a movable, preferentially small apparatus. By contrast, the stability require- ments on the cooling and photoionization laser system imply a laboratory with sufficient space, air condition and otherwise clean and stable environment. Thus, it was opted for to separate the uhv apparatus from the laser system and transfer the laser light via optical fibers to the actual experiment. It is expected that a similar procedure will be followed in future applications of tiamo at big accelerator facilities.
For most of the time, the uhv apparatus has been connected to the femtosecond beam line situated one floor beneath and about 30 m away from the laser laboratory which necessitated to install two 70 m long single mode optical fiber links31 equipped with precisely adjustable fiber couplers32. In parallel with the two main optical links also one backup optical fiber, three coaxial cables and one 2×10 data cable were installed in order to be prepared for further communication
needs between the two laboratories.
The manufacturer discourages using the optical fibers at lengths exceeding a few meters and powers above some tens of milliwatts [55]. However, after more than one year, no degradation was observed even at input powers as high as 500 mW. Nonetheless, whereas the transmittance of the optical link at low input power (<200 mW) is about 55 % including in- and out-coupling losses, it
drops significantly at input powers above 300 mW. It is supposed that stimulated Brillouin backscat- tering [159] causes the reduced transmittance. For this reason, the maximum transmissible power is limited to 135 mW. Figure 2.20 on page 47 illustrates the attenuation effect and demonstrates that no hint for degradation of the optical fibers has been observed by the end of this thesis.
c. frequency doubling
The second harmonic of the visible laser radiation transfered via the optical fiber link must be generated on the table that also carries the vacuum apparatus since highly uv transparent single mode optical fibers are not available. The frequency is converted using the birefringent non-linear optical crystal β-barium borate (bbo) in an enhancement resonator. The basic idea is to exploit a high-finesse resonator’s ability to store an enhanced amount of circulating energy. The related power is focused to a tiny beam waist inside the non-linear crystal and can result in second harmonic generation efficiencies of a few ten percent. A detailed description of the highly optimized frequency doubling resonators that we developed in close collaboration with tiamo’s partner group qsim and the laser spectroscopy division at the mpq is reported by Friedenauer et al. [56] and briefly repeated in fig. 2.21a on page 48. Although these resonators were developed for erbium doped fiber lasers operating in the infrared, the second frequency conversion stage of the mentioned reference
31 purchased as F-SA-C from Newport, cut-off wavelength 488 nm, Fibercore 32 60FC-4-A4.5S-01, Schäfter-Kirchhoff
2.7. Ion trapping laser system dye laser 560 nm iodine cell integrator λ/2 wave plate
single mode fiber
f=+100mm lens PBS AOM prism half mirror glas substrate VCO rf-amplifier lock-in amplifier reference signal pump Nirvana photo-receiver
(a)Absorption spectroscopy. Signal and reference beams (thin lines) are picked by a glass substrate (5 % each) from the input beam (5 mW), pass through an 10 cm long iodine cell (Sacher Lasertechnik) and hit a differential photodetector. The pump beam (thick line) passes through an aom-setup (atm- 801a1, IntraAction) where it undergoes a double-pass 2×93 MHz frequency red shift. Its frequency is adjusted with the voltage controlled oscillator (vco) and additionally frequency modulated with a 20 kHz reference generated by the lock-in amplifier (Model 116, Princeton Applied Research). After overlapping the pump with the signal beam on a pbs, the pump beam saturates the iodine transitions for the counterpropagating signal beam. The detector output being the difference between signal and reference power is demodulated using the lock-in amplifier and converted into a feed-back signal to the laser by an integrator circuit. A typical differential error signal is depicted in fig. 2.19a on the next page.
dye laser 570 nm iodine cell integrator Glan polarizer PBS λ/4 wave plate λ/2 wave plate λ/2 wave plate single mode fiber
f=+50mm lens f=+63mm lens half mirror PBS signal pump Nirvana photo-receiver
(b)Polarization spectroscopy. After a collimation telescope (50 mm and 63 mm lens), the input beam (<5 mW) is split into pump and signal beam with adjustable power ratio by a polarizing beam splitter (pbs) and a half wave plate. The signal beam is further polarization cleaned by a Glan polarizer and finally passes through an 7.5 cm long iodine cell (Sacher Lasertechnik). The pump beam becomes circular polarized after passing through a quarter wave plate and is sent to the iodine cell counter- propagating to the signal beam. The signal is rotated by another half wave plate such that it splits into orthogonal polarization components at a pbs whose power difference is detected using a differential photo receiver (Nirvana, New Focus). An integrator circuit generates a feedback signal to the laser. A typical error signal is depicted in fig. 2.19b on the next page.
Figure 2.18.:Doppler-free spectroscopy setups. (a) shows the setup used to stabilize the cooling dye-laser, (b) shows the setup used to stabilize the photoionization dye-laser.
-500 -400 -300 -200 -100 0 100 200 300 frequency in the green/MHz
24Mg+
(a)Absorption spectroscopy iodine lines used in the cooling laser stabilization, the dashed line marks the atomic transitions reported in [9]. The frequency difference (2×114 MHz in the uv) between the lock point (5.360 413 53⋅1014Hz) marked by an arrow and the reference is composed of a frequency shift induced by the aom (2×93 MHz) in the spectroscopy setup (see fig. 2.18a on the previous page) and a frequency red shift (42 MHz) of about one natural line width required for laser cooling. The isotopic frequency shifts of 25Mg+ (1621 MHz) and 26Mg+(3087.6 MHz) [9] make the respective atomic transition frequencies lie far outside to the blue of the shown iodine absorption lines.
0 200 400 600 800 1000
frequency in the green/MHz
24Mg 25Mg 26Mg
(b)Polarization spectroscopy iodine lines R115(20-1) used in the photo-ionization laser stabilization, the dashed line marks the atomic resonance of neutral 24Mg. The lock point (5.254 052 88⋅1014Hz) marked by an arrow is red-shifted relative to the reference in order to reduce residual photoionization of 25Mg and 26Mg (isotopic frequency shifts 2×371.9 MHz and 2×707.7 MHz in the uv, dotted lines). All frequency values are taken from [14].
Figure 2.19.:Doppler-free iodine transition lines used for frequency locking as seen on an oscillo- scope. The actual lock point is indicated by an arrow, the position of the transition by a dashed line. The frequency calibration of thex-axis is based on data from Iodine Spec from Toptica Photonics with the largest uncertainty given by the dye laser scan being not strictly linear.
2.7. Ion trapping laser system 20 40 60 80 100 120 0 50 100 150 200 250 300 350 400 out put po w er/ m W input power/mW sep 2009 mar 2010
Figure 2.20.:Output power of the optical fiber link connecting the tiamo setup with the photoionization laser as function of the in- put power measured twice with a tempo- ral delay of six months. No degradation of the transmission characteristic was observed. The higher transmittance in the later measure- ment indicates an improved fiber coupling. Increasing the input power above 350 mW does not yield more laser power at the exper- iment. Brillouin backscattering losses inside the optical fiber are assumed to saturate the transmission [159].
could be adopted without major modifications. tiamo’s dye laser system operating in the visible superseded the otherwise first ir→vis frequency doubling stage. There has been another infrared fiber laser available for tiamo as well but due to in the meantime resolved stability problems and temporal reasons, the dye laser solution has been opted for.
Typical uv output powers from the second harmonic stages measured about 1.5 mW at 280 nm
and 4 mW at 285 nm. Although higher values should be obtainable in principle, the experimental needs allowed to save the tedious daily optimization routines required for maximum output since less than 100 µW were actually used for laser cooling.
The uv cooling and photoionization beams were overlapped on a polarizing beam splitter. Ro- tating the polarization of the light with the half wave plates depicted in fig. 2.17 on page 43, allowed to adjust the power going into the beam paths leading to chamber 1 and 3. Focusing lenses right in front of the vacuum chamber generated a beam waist of approximately 80 µm in the respective trapping regions. The polarization direction of the cooling laser light is chosen to maximize the amount of collectible fluorescence photons along the observation direction; see section 2.6 on page 37 for a detailed discussion.