NICE-OHMS of Oxygen at Cryogenic Temperatures
8.3 Experimental concept
Figure 8.8 gives a summarizing overview of the actual implementation of NICE-OHMS in this work, listing all vital functional components required for its oper-ation. The optical layout is in principle quite similar to the schematic of Figure 8.5. It requires a minimal set of only 2 EOMs and 1 AOM for all modulation and stabilization purposes, making it one of the simplest NICE-OHMS setups possible. As thus all modulators have to perform several tasks, the driver and stabilization electronics accordingly gets more involved.
To be as flexible as possible in the choice of wavelength and also avoid all output power issues, a Ti:Sapphire laser (Coherent 899) has been chosen as light source. It provides extremely simple course wavelength tuning and an ample 1.8 W around 761 nm. Frequency scans can easily be initiated through the internal scan control or via an external control voltage. The laser thus serves as the long-term frequency reference in this experiment.
Since the Ti:Sapph lacks sufficiently fast internal feedback capabilities, sta-bilization to the high-finesse resonator requires external high-bandwidth fre-quency actuators. Pound-Drever-Hall locking to the cavity is therefore realized via the cavity’s piezo on long timescales as well as the AOM and EOM 2 on shorter timescales, following a scheme originally considered by Hall et al. [116].
The AOM at the same time serves to stabilize the laser power in front of the cavity. By placing the power monitor at the latest possible position in the beam path, the high-bandwidth amplitude stabilization also removes any power variations resulting from the intermediate optics, such as etalon effects.
The laser system and the actual experiment are situated on two separate optical tables connected with an 8 m long singlemode fiber. Besides a mechanical isolation of the two parts, the use of a fiber has the additional benefit of always providing an almost perfect and spatially stable TEM00 mode for efficient and reliable coupling into the resonator. Any beam steering from a realignment of the Ti:Sapph resonator or adjustments to the AOM frequency is converted into power fluctuations, which will be taken out by the intensity stabilization.
EOM 1 is used to modulate the laser light at the NICE-OHMS frequency,
132 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES
¸/4 AOM EOM1 (NICE-OHMS)EOM2 (Stabilization) PZT
NICE-OHMS Detector¸/4
Ti:Saph ACDC
Lock Detector
Power Monitor
Intensitystabilization P,I,Offset,Gain
AOMDriver AOM
Frequency Amplitude Fiberlink NOModulation NOSignalEOM DCSignalNODet.
Frequency
FSRTracking DetectorNOFreq. furthersignal processing
OpticalSystem FrequencyScan Functiongenerator SRS Model DS345 2Hz
Cryogenic environment Lowpass 5MHz
RFamplification MCZFL-500LN Power Splitter EOMDriver EOMModulation Phaseadjust
Laser-CavityLock Modulation Detector
EOM AOMPiezo
Figure 8.8: Conceptual overview of the complete NICE-OHMS setup.
CRYOGENIC SETUP 133 which in turn has additional sidebands for locking to the free spectral range of the cavity. All required error signals are obtained from a single high-bandwidth detector picking up the cavity reflection. Its output is amplified and then split to be further processed for FSR tracking as well as laser-cavity stabilization.
8.4 Cryogenic setup
One of the major challenges in the application of NICE-OHMS to sample detec-tion in a buffer-gas loading experiment is the operadetec-tion of a high-finesse optical resonator in a buffer-gas filled experimental cell inside the dilution refrigerator.
The cavity mirrors cannot be placed outside the cryostat, as the residual loss on any intermediate optics like windows would significantly reduce the attainable finesse. This situation is simulated here by placing the cavity inside a copper cell attached to the cold plate of a simple LHe cryostat.
Apart from the presence of confining magnetic fields, the chosen cryogenic environment already allows to gain valuable experience on the cavity behavior and test techniques like capillary injection under realistic conditions as expe-rienced in future trapping experiments. In the present setup, the experimental volume can be cooled to temperatures anywhere between 300 and 4.2 K, and gas-phase samples may be injected for spectroscopy through a heatable thin-walled tube.
8.4.1 Cryostat
The main components of the utilized commercial CryoVac cryostat are illus-trated in Figure 8.9. With a total height of about 1 m it is relatively small and can easily reside on its outer vacuum tank directly on top of an optical table.
It has two separate dewars, where the lower one can be filled from the upper one via a needle valve operable from room temperature. This design in prin-ciple allows continuous pumping on the lower dewar for a further reduction of the bath temperature. Thermal radiation is blocked by an outer vapor-cooled shield and an inner shield connected to the upper dewar. The remaining en-closed experimental volume is about 12 cm high and 19 cm in diameter. Optical access is restricted to a single horizontal axis with a free diameter of about 18 mm. Tilted AR coated windows are placed on the outer vacuum can and the experimental cell, while the ports at the two intermediate radiation shields are left open. The lower part of the opened cryostat can be seen together with the attached experimental cell in Figure 8.10.
8.4.2 Experimental cell
The experimental cell (Figure 8.11) has been machined completely out of OFHC copper for good thermal conductance and temperature homogeneity. Both win-dow flanges as well as the major cell access have been sealed with indium,
134 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES
Recovery and optional pumping LHe fill line
Vapor-cooled shield 4K shield
Upper LHe tank (7l)
Lower LHe tank (2.3l) He recovery
Thermal copper ribbon link
Experimental cell with resonator Thermally isolating G-10 spacer
Needle valve
Transmission
 30 cm
(total height: 98 cm)
Figure 8.9: Layout of the LHe cryostat containing the NICE-OHMS cell.
CRYOGENIC SETUP 135
Experimental cell
Cell window Pumping line Capillary injection line Lower LHe tank
Cold plate
Figure 8.10: Close-up of the lower cryostat with experimental cell.
ensuring simple reopening for further improvements or alternative usage. As in the chromium buffer-gas loading experiment, all capillary connections as well as the electrical feedthrough to the cell have been kept removable with Swagelok VCR fittings.
A G-10 tube attached to the cryostat cold plate holds the experimental cell in place. Since the tube has a very low thermal conductivity, the actual thermal contact to the LHe bath can be tuned to any desired level by establishing additional connections. This makes the setup customizable to situations where a heating of the complete cell above the base temperature of the cryoliquid is intended. At present, the thermal link is realized with two flexible copper ribbons.
Two thin-walled, 1/8” stainless steel tubes (seen in Figure 8.10) connect the cell volume to a simple room-temperature gas-handling system for pumping as well as sample and helium buffer-gas injection. They have been thermally an-chored only at the vapor-cooled shield of the cryostat to reduce the probability of potential probe gas solidification on the way to the experimental cell. For the same purpose, one of the capillaries can be heated on the last few centimeters, where its diameter is further reduced to about 1 mm. The heater is realized with twisted pair resistive Manganin wire 0.1 mm in diameter and a total length of
136 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES
Thermal anchoring to cryostat cold plate
Pt-100 sensor
Electrical feedthrough
Heated capillary Pumping line
Figure 8.11:NICE-OHMS experimental cell. Its outer dimensions are approximately 15×7×6cm (L×W ×H), excluding protruding and attached parts.
about 10 m, giving a total resistance of about 560 Ω at room temperature. To reduce possible heat transfer to the cell, the final connection is made with larger diameter thin-walled stainless tubing, which has low thermal conductivity and at the same time is safe from clogging.
8.4.3 Thermometry
For thermometry, a Pt-100 sensor is attached to the cell top. By measuring its resistance at the LHe temperature, it was possible to calibrate it down to 4.2 K.
The calibration data is well fitted by the conversion formula lnT = 1.83516 + 1.8878·lnR−0.70962·(lnR)2
+ 0.15023·(lnR)3−0.0101·(lnR)4 , (8.4) whereT is the temperature in Kelvin and Rthe sensor resistance in Ohm. The knowledge of (8.4) allowed to also calibrate the temperature dependence of the capillary heater, which is important to assure capillary temperatures above the melting point of the respective probe gas. This was accomplished during a warm-up of the cryostat from 4.2 K, in which helium gas in the isolation vacuum should have guaranteed roughly equal temperatures of the cell and the capillary.
The resulting calibration data is plotted in Figure 8.12.
Although it is hard to fit to an analytical curve, the data should serve to roughly estimate the capillary temperature from the measured heater resis-tance. The latter can be derived from a standard four-terminal measurement at
OPTICAL SETUP 137
500 510 520 530 540 550 560
10 100
Temperature[K]
Resistance [Ohm]
Figure 8.12: Temperature dependence of the resistive capillary heater.
arbitrary excitation currents, as the heater provides appropriate connections.
Figure 8.12 also shows that the resistance drops only slightly over the complete range of temperatures, so that it remains sufficiently high at 4.2 K for efficient heating.