NICE-OHMS of Oxygen at Cryogenic Temperatures
8.5 Optical setup
8.6.5 Scaling with input power
The present NICE-OHMS setup has been optimized for an input power of 1 mW in front of the cryostat in order to reduce the heat load on cryogenic optics due to light absorption or scattering. Traces can nonetheless be recorded also at
158 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES
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1 mW
2 mW
3 mW
4 mW
5 mW
6 mW
7 mW
8 mW
9 mW
10 mW
Detuning from 761.7179nm [GHz]
NICE-OHMSSignal[arb.u.]
Figure 8.33:Experimental NICE-OHMS signals at increasing input power. The scal-ing of all axes is the same.
higher input powers, as Figure 8.33 demonstrates. The signals show a linear scaling without a change in quality. Figure 8.34 summarizes the amplitudes, which slightly start to saturate above an input power of 8 mW. All electronic amplification can hence be regarded as linear up to that point, so that the 1 mW signals should be free of any electronic distortion.
The use of higher input powers could be desirable in future measurements to reduce the shot noise or saturate the sample between the cavity mirrors for Doppler-free spectroscopy. The latter is in fact probably the most natural field
RESULTS 159
0 2 4 6 8 10
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Originalsignalamplitude[Vpp]
Incident power [mW]
Figure 8.34:NICE-OHMS signal amplitude as a function of input power.
of application for the NICE-OHMS technique. Although the circulating power has been increased to above 60 W (!) by using input powers of up to 10 mW, no sign of sample saturation was observed with the current setup for the A band transitions of O2. Due to the large cavity enhancement factor ofP ≈6412, even higher input powers should not be used to avoid optical damaging of the cavity mirrors.
As a side note it should be mentioned that the high circulating power is nei-ther problematic for the cryogenic environment nor for the potentially trapped sample between the cavity mirrors. Since it is achieved on the basis of low mir-ror loss, the associated absorptive heating of the mirmir-rors stays minimal. It will also not lead to unreasonable light-induced loss from the trap, since the NICE-OHMS technique will only be applied to species with extremely low absorption cross sections.
8.6.6 Detection limit
To estimate the available detection limit, room-temperature NICE-OHMS sig-nals have been recorded for the R1Q2 transition at various oxygen pressures, covering almost three orders of magnitude. From this data, the individual signal-to-noise ratio was obtained by comparing the respective amplitudes to that of a 7.2×10−6mbar baseline, yielding the values plotted in Figure 8.35. The individ-ual pressures have been converted into peak absorption coefficients by making use of the ideal gas equation and (7.20), taking Sif= 1.074×10−19MHz/cm−2 from the HITRAN database (Table D.1).
Also shown is a theoretical fit, which was simply obtained by multiplying (7.21) with an appropriate electronic gain factor. All other required quantities in this equation have just been put in as measured, i. e. αp as calculated from the pressure reading andr = 99.9965 % as well asL= 86.5 mm for the current cavity. The curve yields a detection limit of 1.02×10−6%/cm, slightly better
160 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES
10-7 10-6 10-5 10-4 10-3
1 10 100
S/N
Peak absorption [%/cm]
Figure 8.35:Experimental signal-to-noise ratio for NICE-OHMS detection.
than the level of sensitivity estimated to be required in a buffer-gas loading experiment (Table 6.5). This result is more than two orders of magnitude below the CEAS detection region as obtained from equation (7.18), and thus has to be attributed to the use of FM spectroscopy techniques.
The obtained sensitivity is far from being limited by electronic or shot noise, as is obvious from the baseline recorded at 7.2×10−6mbar (Figure 8.36). It shows systematic, wavelength-dependent features, which at least partially have to be attributed to spurious etalon effects. Similar limitations have in fact been observed in the experiment of Gianfrani et al. [107]. To some extend, they are also caused by RF interferences at the NICE-OHMS modulation frequency, though, as Figure 8.37 illustrates.
Fundamentally, the detection sensitivity attainable with this setup is only limited by shot noise of the photons arriving at the NICE-OHMS detector.
Assuming shot noise limited operation, the minimum detectable integrated
ab--4 -2 0 2 4
-2 -1 0 1 2
NICE-OHMSSignal[mV]
Deviation from 761.7173 [GHz]
7.2£10-6mbar
Figure 8.36: Limiting baseline. The residual features seen here are not caused by oxygen absorption lines.
CONCLUSION 161
0 10 20 30 40 50
0.0 0.5 1.0 1.5 2.0
SignalLevel[V]
Time [s]
NICE-OHMS
DC
Figure 8.37: Observation of RF interferences at 1.734 GHz. The changing level of the NICE-OHMS signal at constant laser frequency was generated by people moving through the laboratory close to the experiment. Since the observed DC signal level does not change, the variations cannot be attributed to scattered light randomly hitting the photodiode. No low-pass filtering was applied to the raw signals here, so that the noise level is comparatively high.
sorption would theoretically have been [61]
(αpL)min =
√2 J0(β)J1(β)
π 2F
2eB ηPout
.
With a modulation index of β = 0.967, an empty-cavity finesse of F = 89 105, a detection bandwidth of B = 100 Hz, a specified photodiode responsivity of η = 0.2 A/W, and a transmitted power of Pout = 60W, a detection limit of αp = 1.41×10−9%/cm should thus in principle be achievable with the current cavity length of L= 86.5 mm.
8.7 Conclusion
With the installation of NICE-OHMS, the available detection sensitivity has indeed been pushed into a regime adequate for the observation of magnetically trapped, dilute oxygen samples. Since O2 has particularly weak lines in the considered optical region, the setup should also immediately be useful to study a great variety of alternative molecular species.
To allow detection with a signal-to-noise ratio significantly above one, fur-ther improvements are necessary. An appropriate RF shielding of all 1.7 GHz components could already yield sufficient results. Moreover, an additional mod-ulation scheme can be employed to further reject noise and baseline drifts. By superimposing a dither at low audio frequencies onto the voltage applied to the
162 NICE-OHMS OF OXYGEN AT CRYOGENIC TEMPERATURES cavity piezo, the laser frequency will be modulated around the molecular reso-nance [112]. Subsequent lock-in detection after demodulation atωNO will then convert the usual dispersion lineshape into its derivative. It should in particular be less sensitive to etalon effects.
A more fundamental problem is the adsorption of molecules on the mirror surfaces at cryogenic temperatures, as observed for oxygen at 4.2 K. The accom-panying decrease in finesse virtually makes the resonator useless for detection.
Even if the mirrors take no damage, a recovery would require a cumbersome warm-up of the cryostat above the melting point of the investigated substance.
Therefore, sophisticated measures apparently have to be taken to prevent any molecules from sticking to the mirrors.
The presence of a helium buffer-gas on the other hand seems to be no issue, as long as gas discharges due to the high piezo voltages can be avoided. This either requires a reduction of the scan range or a thorough electrical insulation of all conductors in the experimental cell.