OPO Bandwidth

Using an optical beat note between frequency-doubled signal output and the OFC, the frequency of the signal can be measured and stabilized. The quality of the stabilization depends on the frequency bandwidth of the PZT mirror inside the OPO cavity, since only noise contributions within this bandwidth are compensated. Before assembling the cavity, we made measurements of the frequency response of the PZT mirror assembly

Chapter 9. Experiment

using the phase shift measurement setup described in Section 5.3.2. One of the frequency synthesizer (based on the AnlalogDevices AD9854) outputs is amplified up to±32 V with an offset of 33 V (SVR 150-1), which is afterward connected to the PZT. A modulation of the PZT results in an amplitude modulation in the frequency measurement setup, which is monitored with an oscilloscope through a photodiode. The second frequency synthesizer output supplies the reference signal for the second oscilloscope channel. In the following, the modulation frequency is changed stepwise over the range from 30 kHz to 600 kHz. The demodulation of the signal is performed digitally with an FFT for each frequency step (Section 5.3.2). The frequency dependent phase differences relative to the reference signal are displayed for different mounting blocks of the PZT actuator in Figure 9.12a and 9.12b.

The first PZT mounting block follows the basic design of a lead filled copper cone with a

(a) (b)

Figure 9.12:(a) Various iterations of the PZT actuator mount, which determines the frequency dependent phase shift between the modulation applied to the PZT and its response. (b) The best design found is a 0.5 inch conical mounting block made of tungsten.

short cylindrical section near the wide end. The length and the diameter of the mounting block are 1 inch, while the diameter of the flat tip is 5 mm. These specifications correspond roughly to a device described in the literature, which promises a resonance-free bandwidth of 180 kHz^[204].

However, we were unable to replicate the results presented in that work, despite constructing multiple variants of their device. Our implementations of this design always resulted in numerous resonances, starting from just above 30 kHz. We tried slightly varying shapes of the design of the copper shell, used a mixture of tin and flux on the inside of the copper shell to improve adhesion and even tried absorbing impurities from the molten lead with a slice of potato. Soldering the PZT onto the mounting block instead of gluing it also

9.6. Molecular Beam had no effect on the measurement result. Replacing the copper lead union with a solid lead block of the same shape improved the resonance structure somewhat by fewer resonance peaks in the important low-frequency range up to 100 kHz. Encouraged by this result, we replaced the lead mount with a cylindrical stainless steel mount of 1 inch diameter and length. Using this block, the first resonance shows up around 90 kHz (Figure 9.12a).

Changing the shape of the cylinder into the conical shape used before, shifts the first resonance peak further up to 100 kHz and simultaneously reduces the size of the following resonance. Further, reducing the dimension of the block to 0.5 inch leads to the largest improvement, with the first resonance peak up to 190 kHz. Finally, constructing the block out of tungsten instead of stainless steel reduces the size of the first resonance, although its location shifts again to a lower frequency around 175 kHz (Figure 9.12b).

After these measurements, we selected the tungsten mounting block for the OPO.

Thus, the OPO PZT actuator can compensate noise contributions up to a bandwidth of 175 kHz. If the PLL is able to handle the phase shift of the first resonance without inducing oscillations the bandwidth limit could be pushed up to 250 kHz, corresponding to the second resonance peak.

9.6 Molecular Beam

Equally important as the precision laser system is a rotationally and vibrationally cold source of OH or OD molecules. The basics of a molecular beam have been discussed previously (Section 7.1), which allows us to focus now on the technical details. The stainless steel bubbler containing the white fuming nitric acid (> 95 % HNO₃) is electropolished from the inside, with a maximum capacity of 160 ml (Wilhelm Schmidt GmbH JEX0.15). It is filled with glass with approximately 10 ml of nitric acid soaked into it, which maximizes the number of exposed molecules to the incoming xenon gas. If the bubbler is left at room temperature, the pulsed solenoid valve (Parker Series 9 General Valve) was found to clog after a few hours. Opening the valve reveals a black substance, which is probably identical to the corrosion products found on the inside of the bubbler. Therefore, the bubbler is cooled to −15^◦C, which prevents condensation inside the valve and has little effect on the OH density^[205]. The valve is operated at 10 Hz with a pulse width of approximately 100µs. A 6 mm long quartz capillary is attached to the 1 mm nozzle of the valve. A 8 mJ, 10 ns pulse from an argon fluoride (ArF) excimer laser at 193 nm (GAM EX5/250-180) is weakly focused on to the tip of the capillary and dissociates nitric acid molecules into OH and NO₂ before the supersonic expansion (Figure 9.13). The expansion of the OH molecules results in rotational cooling of the molecules. The translation energy experiences no cooling. The translational degrees of freedom also experience cooling, resulting in a narrow velocity distribution, but the enthalpy of the molecules before the expansion is mostly converted into a large mean forward velocity of the molecules. A time of flight

Chapter 9. Experiment

Figure 9.13:Schema of the molecular beam creation and detection setup. Nitric acid (HNO3) get seeded with xenon (Xe) and propagates through a pulsed valve (PV) into a fused silica nozzle (N). An intense UV pulse of an argon fluoride (ArF) excimer laser dissociates the molecule inside the nozzle, leading to a supersonic expansion of the OH in the first vacuum chamber. Afterwards, a skimmer (S) selects a narrow velocity class of the OH for detection inside a second vacuum chamber. The 308 nm laser beam passes first an attenuator (AT), a pinhole (PH), a lens (L1), a Brewster window and a light baffle (LB1), before exciting the OH. Doppler reduced detection requires excitation of the OH also from the opposite direction. Therefore, a retroreflection mirror (M) sends the beam back. The reflected beam is aligned to maximize the signal on the photodiode (PD). Finally, a photomultiplier (PMT) detects the OH fluorescence light, after it passes a lens (L2) and two color filters (F1, F2).

measurement estimates the molecular velocity to approximately 340 m/s, which is largely determined by the heavier xenon atoms.

A skimmer with an aperture of 4 mm selects a small fraction of the OH molecules before they enter the second differential pumped vacuum chamber. The OH molecules propagate inside the second chamber through a 480 mm long traveling-wave Stark decelerator, with a 4 mm circular profile^[206]. For all experiments in this thesis, the decelerator electrodes are grounded, resulting in a negligibly small electric field strength in the spectroscopy region after the decelerator. Additionally, the electrodes act as a geometric aperture and limit the transverse velocity spread to a full width at half maximum (FWHM) of around 2.5 m/s. After the Stark-decelerator the molecules are excited by the 308 nm beam of the UV spectroscopy laser (Section 9.4). Some part of the reemitted fluorescence light is collected with a fused silica lens (D = 50 mm) and filtered by two color filters before reaching the on-axis photomultiplier tube (PMT) (ET Enterprises 9829QSB). In detail, a UG5 color filter right behind the lens and a UG11 color filter in front of the PMT both help reduce the intensity of the photodissociation pulse as well the intensity of visible light. The transmission maximum (>90 %) of both filters is around 330 nm.

However, the strong dissociation pulse still limits the SNR of the fluorescence signal.

This has been improved somewhat by suppressing the gain of the PMT for a 20µs interval during the time of the dissociation pulse. In detail, electrons emitted from the

photo-9.7. UV Spectroscopy Measurement