Fabry-Perot oscillations

Figure 25.4 shows the MIR absorption spectra at normal incidence of a free standing 7 pm thick PPX frlm. The peaks occurring in the spectram are superimposed by an oscillatory modulation of defined period. Whereas the individual peaks are related to vibronic excitations of the polymer, the periodic modulation of the baseline can be attributed to Fabry-Perot oscillations caused by refractive index changes at the PPX interfaces. 

Figure 25.4 MIR absorption spectrum measured on a free standing 7 pm thick poly-paraxylylene (PPX) film. The sharp peaks originate from vibronic excitations of the polymer and the periodic modulation attributed to Fabry-Perot oscillations.

Fig. 2. A typical pulsed, tunable dye laser, of the Hansch design, in an oscillator— amplifier configuration. Suitable pump lasers are Nj, excimer and Nd YAG-harmonics. The broken line represents a pressure chamber which is required to scan the wavelength if an air spaced Fabry—Perot etalon is used. With solid etalons, tuning is achieved by tilting the etalon and the pressure chamber is not required.

Fig. 1. 249.9-GHz FIR-ESR spectrometer. A, 9-T magnet and sweep coils B, phase-locked 250-GHz source C, 100-MHz master oscillator D, Schottky diode detector E, resonator and modulator coils F, 250-GHz quasioptical waveguide G, power supply for main coil (100 A) H, current ramp control for main magnet I, power supply for sweep coil (50 A) J, OC spectrometer controller K, lock-in amp for signal L, field modulator and lock-in reference M, Fabry-Perot tuning screw N, vapor-cooled leads for main solenoid O, vapor-cooled leads for sweep coil P, He bath level indicator Q, He transfer tube R, bath temperature thermometer S, " He blow-off valves. [From Lynch et al. (1988), by permission of the AIP.]...

The unfiltered OLED shows a deep absorption peak due to the Fabry-Perot resonance of the naturally-occurring weak microcavity, and the filtered OLED shows oscillations in the reflectance due to the same effect. Lower reflectance filters could be designed with more layers in the DBR, at the expense of added complexity. 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

Surin used the Orotron oscillator as a tuneable source of coherent MMW radiation to study the spectroscopy of SiH4 and ND3 in the 90-160 GHz range. The gas was introduced into a cell placed in a Fabry-Perot cavity 10. The absorption signals were detected from variation of the Orotron electron beam current. No phase or frequency lock schemes were necessary and resolution achieved was sufficient to resolve the lines Doppler profiles sensitivity was estimated as 3-5 X 10" m . 

As mentioned above, the gain medium of every kind of laser is associated with an optical resonator, whose structure will be detailed below. The simplest Fabry-Perot-type resonator comprises only two mirrors, between which the gain medium is situated. The feedback structure imposes two basic properties upon the oscillating laser field. It defines not only the allowed resonant frequencies within the gain medium s emission spectrum but also the spatial characteristics of the laser beam. 

A nice example of the influence of laser heating on reorientation was found by Cheung et They observed in a Fabry-Perot resonator filled with a nematic liquid crystal self-oscillation. It was shown that the oscillation resulted from the competition between the fast thermal and slow orientational mechanisms. Thermal processes are fast of course only in comparison with reorientation. In semiconductors similar self-oscillations occur in which heating is the slow process and optical nonlinearities of electronic origin play the role of the fast mechanism. ... 

If a high pulse energy is needed while retaining a small linewidth and a high beam quality, it is advisable to utilize an oscillator/amplifier arrangement, as mentioned above. In Fig. 8.22 two examples are given. In order to achieve a very small linewidth ( 100 MHz) an external Fabry-Perot interferometer can be used to filter the oscillation output before amplifica-... 

Figure 7 On-line in situ Raman spectra displaying the growth of ZnS Sei, on (100) GaAs recorded at room temperature and at an excitation energy of 2.808 eV. The spectra show intensity oscillations due to Fabry-Perot interference of the LO fundamentals of the substrate and the ZnSSe layer, as well as of their overtones. (Reproduced from Ref. 121.)...

Fig. 3.1. Experimental arrangement described by Kachi and Kon [3.33]. The CO2 pump laser cavity is formed by a mirror and a grating at the Littrow angle (autocollimation). Output coupling occurs via the zeroth order reflection from the grating. Note the hinge mount which ensures that the output beam is in a fixed direction for all wavelengths. The submillimeter laser resonator used here is of the Fabry-Perot type. A small amount of the radiation is coupled out to the detection system with a Michelson interferometer used to select the desired line when two or more lines oscillate. From [3.33]...

The measurements done by Guo et al. [25] showed decay in the enhancement beyond approximately 80 nm. Their FDTD calculations instead showed a continued oscillation in the average field-squared at the position of the fluors out to at least 600 nm these oscillations correlate with Fabry-Perot resonances within the oxide, an observation which has been made previously by Andrewartha [59] for a similar type of structure (a "bottle grating interestingly calculations of the reflectivity from this related structure showed similar sharp oscillations [59]. The calculated oscillations originate from the interference between diffraction from the nanowire array and multiple reflections from the aluminum... 

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