Optical feedback

All lasers have the following three fundamental components (see Figure 1). The gain medium, the business end of the laser, is the medium in which excited states are created which will produce stimulated emission. The exciter is the source of energy for production of the excited states in the gain medium. Finally, the optical resonator determines the directionality, wavelength selectivity, optical feedback, polarization, and other characteristics of the stimulated emission from the gain medium. 

All these waveguiding films lack one important feature that would be necessary for true lasing They do not have a resonator for optical feedback that would lock the optical modes traveling in the gain direction. The thin-film waveguide confines the optical modes in one direction (in the vertical), but in the other two dimensions the modes have translational and rotational symmetry. The incorporation of resonator structures into the thin films in order to get true organic solid-state lasers will be described next. 

The gratings can also be made in situ by holographic irradiation as was demonstrated for low molecular stilbenes in a polystyrene matrix [197]. Here, the spatial modulation of gain dominates over the refractive index modulation in its contribution to optical feedback. The principles of holographic irradiation will be described in Section VIII, which discusses photosensitive materials. 

The emission linewidth of LEDs is typically 10 nm, which is quite broad in comparison to that achievable with laser diodes. Narrower linewidths down to ca. 0.9 nm can be obtained using resonant cavity LEDs.(61) Superluminescent LEDs have been produced with low spectral ripple (less than 10% at wavelengths down to 670 nm) by suppressing the optical feedback in laser diode junctions.(62)... 

An optical resonator system, which provides the optical feedback. 

Wyatt and Phillips (1972) commercialized an electrostatic balance with electro-optic feedback control and light-scattering capabilities the device is shown in Fig. 2. The dc field was generated by a lower electrode and an electrified pin mounted in the upper plate. The particle was illuminated by a laser beam, and light scattered from the particle hit the edge of an adjustable... 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

In the work by Hales and co-authors [83] the prehistory distribution was observed experimentally using a semiconductor laser with optical feedback. Near the solitary threshold, the system was unstable after a period of nearly steady operation, the radiation intensity decreased then it recovered comparatively quickly, growing to regain its original value decreased again and the cycle repeated. In the experiment, the output intensity was digitized with 1 ns resolution. The pi, obtained in [83] from 1512events is shown in Fig. 9. The results were compared with the results of numerical simulation for the system (17). 

D. Fish, S. Deane, J. Shannon, A. Steer, N. Young, 35.2 Improved Optical Feedback for AMOLED Display Differential Ageing Compensation, SID Symposium Digest of Technical Papers 2004, 35, 1120-1123. 

Fig. 3. (a) Spectral power density of the beat note between the extended-cavity master laser and the slave laser, with or without optical feedback (b) Spectral power density of the beat note between the two phase-locked diode lasers... 

Fig. 4. (A) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under low surface pressure. (B) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure. (C) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure, collected using the optical feedback approach. Reproduced with permission from Ref. [19]. Copyright 1999 Blackwell Publishing.

Laser-diode feedback. This detection technique utilizes the great sensitivity of lasers to optical feedback and is used to detect movement of the cantilever. Laser light reflected off the back of the cantilever is returned back into the laser 120 -22],... 

A laser is a radiation source which produces a very high spectral radiance in a small spectral range at a fixed wavelength. A laser combines a radiation source with spectral isolation of its radiation - two important components of a spectrometer. The word laser is an acronym which stands for light amplification by stimulated emission of radiation. The essential elements of a laser are an active medium a pumping process to produce a population inversion and a suitable geometry or optical feedback elements (Moore et al., 1993). Most lasers are essentially Fabry-Perot interferometers whose cavities contain... 

Fig. 4.25. Increase of the density of Rh-dUTP in the oscillating field (4 kV cm at 4 Hz) observed in the confocal element between the tips and decay of the concentration as the field is switched off [50]. (a) Concept of the quadruple trap with a seven-element detector projected into the cavity (diameter 6 pm) surrounded by four electrodes (metal tips or microcapillaries). The observed particle undergoing diffusion will be repositioned by the electric field whose magnitude and direction is computed from the optical feedback (b) [51]...

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