Frequency modulation, laser spectroscopy

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

Remarks Steimle et al. [OOSte] have applied transient frequency modulation absorption spectroscopy to PtC and TiS molecules as produced in a laser ablation supersonic expansion source, and discussed the results in comparison with LIF detection techniques. 

The advantage of this derivative spectroscopy [2] with a frequency-modulated laser is the possibility for phase-sensitive detection, which restricts the frequency response of the detection system to a narrow frequency interval centered at the modulation frequency Q. Frequency-independent background absorption from cell windows and background noise from fluctuations of the laser intensity or of the density of absorbing molecules are essentially reduced. Regarding the signal-to-noise ra-... 

Frequency-Modulation Spectroscopy. Frequency-modulation spectroscopy (tins) is a high sensitivity null-background infrared technique for measuring absorbances down to 10 with fast acquisition speeds. Fms involves frequency-modulating a laser source at COq to produce a carrier frequency having sidebands at cJq where is an integral multiple of the modulation frequency. Dye lasers and many other single-line sources can... 

The rapid progress in recent years in the spectroscopy of the hydrogen atom has renewed pressure for a much better optical frequency standard. This in itself would not be enough to solve the measurement problem. New techniques of comparing optical frequencies are needed. He have developed methods of modulating lasers which can be used for frequency differences in excess of 2THz. 

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

The second scheme to be treated is based on a frequency modulation of the monochromatic incident wave. It was not designed specifically for laser spectroscopy, but was taken from microwave spectroscopy where it is a standard method. The laser frequency co] is modulated at the modulation frequency 2, which changes coi periodically from cul — Acol/ to cul + Acul/2. When the laser is tuned through the absorption spectrum, the difference APr = Py(col Al/2) is detected with a lock-in amplifier (phase-sensitive detector) tuned to the modulation frequency (Fig. 1.4). If the modulation sweep Acol is sufficiently small, the first term of the Taylor expansion... 

Fig. 1.4 Absorption spectroscopy with a frequency-modulated single-mode laser is sinusoidally modulated at a modulation frequency Q, the Taylor expansion yields...

M.W. Sigrist (ed.). Tunable diode laser spectroscopy. Appl. Phys. B (2008) (Special Issue) J.A. Silver, Frequency modulation spectroscopy for trace species detection. Appl. Opt. 31, 707 (1992)... 

This paper reports a sensitive optical interferometric technique dual frequency modulation (DFM) for measuring and stabilizing a laser frequency by comparison, in a single step, to a radio frequency (rf) standard. Conversely, a low-noise rf source can be stabilized by a laser frequency reference. A prototype has demonstrated a resolution of 2 parts in 10 , but devices currently under development should have a resolution of 10 and an absolute accuracy of 10" . The method may be competitive with the optical frequency synthesis chain in accuracy and its simplicity suggests its convenient use in metrology, high-precision optical spectroscopy, and gravity wave detection. 

Figure 6.8 Absorption spectroscopy using a frequency-modulated narrow bandwidth laser...

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Using the dispersion profiles of Doppler-free molecular lines in polarization spectroscopy (Sect. 7.4), it is possible to stabilize a laser to the line center without frequency modulation. An interesting alternative for stabilizing a dye laser on atomic or molecular transitions is based on Doppler-free two-photon transitions (Sect. 7.5) [5.77]. This method has the additional advantage that the lifetime of the upper state can be very long, and the natural linewidth may become extremely small. The narrow Is —2s two-photon transition in the hydrogen atom with a natural linewidth of 1.3 Hz provides the best known optical frequency reference to date [5.76]. 

---