Frequency stabilization

The foremost of the modem teclmiques is tlie use of lasers as spectroscopic tools. Lasers are extremely versatile light sources. They can be designed with many usetlil properties (not all in the same instmment) such as high intensity, narrow frequency bandwidth with high-frequency stability, tunability over reasonable frequency ranges, low-divergence beams which can be focused into very small spots, or pulsed beams with... 

The second compensating pole is just used to maintain high frequency stability by depressing the gain above the cross-over frequency. 

At 10Hz in a typical Nd-YAG laser 1000Hz/- /Hz, and the typical finesse asymmetry is of the order of one percent. In order to detect a gw signal the laser frequency noise has to be lowered by six orders of magnitudes (compared to the noise of a free running laser), and the two arms made as identical as possible. In order to achieve this complex frequency stabilization methods are employed in all interferometric detectors, and in order to insure the perfect symmetry of the interferometer, all pairs of Virgo optical components are coated during the same run (both Fabry-Perot input mirrors then both end mirrors are coated simultaneously). 

H. Schnitzler, U. Frohlich, T.K.W. Boley, A.E.M. Clemen, J. Mlynek, A. Peters, and S. Schiller, All-solid-state tunable continuous-wave ultraviolet source with high purity and frequency stability. Applied Optics 41(33), 7000-7005 (2002). 

Section IV explains a new approach to high resolution spectroscopy based on various kinds of saturation effects. Some of the experiments are performed inside the laser resonator, which implies the presence of coupling phenomena between the absorbing molecules under investigation and the laser oscillation itself. These feedback effects can be used for high-precision frequency stabilization and to measure frequency shifts and line profiles with an accuracy never... 

With normal efforts for frequency stabilization in the optical frequency region, time-averaged laser linewidths of about 1 Mc/sec have been obtained 3 ), compared to 1000 Mc/sec in case of spontaneous fluorescence lines. 

Growing better crystals can certainly improve threshold and output power. As soon as the problems of frequency stabilization have been solved satisfactorily the parametric oscillator will surpass conventional devices in its intensity and resolving power... 

These hole-burning effects in intracavity absorbers have attracted increapd interest because they are, besides their application to high-resolu on spectroscopy, surprisingly useful for extreme frequency stabilization of gas lasers, as will be shown in the next section 339a)... 

The frequency stability obtained up to now is already better than 1x10 and therefore exceeds the accuracy of the optical wavelength standard (red krypton line) by more than three orders of magnitude 333b, c)... 

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

The increased frequency stability of single-mode lasers offers new possibilities in high-resolution spectroscopy, and new techniques for measuring frequencies up to 10 Hz have enabled absolute frequency determinations of infrared laser lines. This opens the way for much more accurate measurements of the speed of light, the basic parameter involved in most spectroscopic quantities. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Normally an oscillator circuit Is designed such that the crystal requires a phase shift of 0 degrees to permit work at the series resonance point. Long-and short-term frequency stability are properties of crystal oscillators because very small frequency differences are needed to maintain the phase shift necessary for the oscillation. The frequency stability Is ensured through the quartz crystal, even If there are long-term shifts In the electrical values that are caused by phase jitter due to temperature, ageing or short-term noise. If mass Is added to the crystal. Its electrical properties change. 

Recent work on a production basis involving 5-fim spectra of 12CD3F has verified line-center frequency stability under deconvolution. Large numbers of experimental records of 12CD3F, simultaneously recorded with CO in the sample, were calibrated before and after deconvolution (point-simultaneous methods, Jansson algorithm). No systematic differences were detected in comparisons of the before and after frequencies of nonblended absorption lines. That is, the variance was consistent with the optomechanical precision of the spectrometer and the mean deviation summed to approximately zero, validating the frequency calibration of the deconvolved data. 

Advantages Good long-term frequency stability, moderate distortion, easily adjusted,... 

Advantages Low part count, moderate frequency stability Disadvantages Harmonic distortion not controlled... 

Another recent notable technical advance has been the development of a pulsed Orotron source currently being used and tested in the 360 GHz system at Berlin. This electron-beam device (Smith Purcell free electron laser) has feedback via a high-Q Fabry-Perot cavity and thus features good frequency stability as well as pulse output powers at 360 GHz in the many tens of mW. 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

For 13C experiments, the sample is usually prepared by dissolving the compound to be investigated in a deuterated solvent, which is usually required for field/frequency stabilization. For calibration, a small amount of a reference compound (usually tetra-methylsilane) is added to the sample. 

Any heteronuclear signal of a solvent or an added reference substance can be used for referencing I3C shifts. For example, 13C shifts can be directly measured relative to a deuterium signal of the deuterated solvent usually required for field/frequency stabilization. However, homonuclear shift references such as the l3C signals of tetramethylsilane (TMS), carbon disulfide, benzene, cyclohexane, 1,4-dioxane or the easily localizable mul-tiplet signals of deuterated solvents (Fig. 2.22) are predominantly applied in 13C NMR. 

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