Frequency up-converter

For second harmonic generation (SHG), the tensor is y(2)(—2co co, co) (useful for frequency doubling and parametric down-conversion) while for the linear electrooptic or Pockels71 effect the tensor is y(2)(— co co, 0) (useful for Q-switching of lasers, for phase or amplitude modulators, and for beam deflectors) for optical rectification the tensor is y 2>(0 00, —co) for frequency mixing the tensor is y(2)(— co3 oolr co2) (useful for frequency up-converters, optical parametric oscillators, and spectroscopy). 

Frequency up-converted lasing is an important research area and has become more promising in recent years because this technique provides several main advantages when compared to other coherent frequency up-conversion techniques ... 

Fig. 139 Frequency up-converted light emission from powdery grains of DCM organic dye molecules, (a) Illustration of the emission mechanism, (b) light spots emitted from the grains placed in a quartz conteunec...

Fig. 1.40 Spectral profiles of the frequency up-converted light. Curve A is the spectrum obteiined by applying near-infrtired light with a wavelength of 805 nm. Curve B is the conventional fluorescence spectrum...

Figure 1.41a shows the relation between the incident light intensity I used for excitation and the frequency up-converted light intensity 7uc emitted from the powdery grains of the DCM organic dye molecules. Experimental results are presented for the wavelength components of 650 and 690 nm, to which the curve... 

Fig. 1.41 (a) Relation between the incident light intensity and the frequency up-converted iight intensity, (b) Illustration of the two-step excitation and subsequent light emission, (c) Efficiency of the frequency up-conversion as a function of the incident light power density. For comparison, the efficiency of the second harmonic generation from a KDP crystal is tilso shown... 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

A hysteresis loop can formally be drawn for the interconversion of a photo-chromic substance between two states A and B characterized by two well separated absorption bands as shown in Figure 26 sweeping the frequency up from vc to Va converts the system from state A to B when vc reaches the absorption band of A the system remains in state B if Va goes back to v0 sweeping vc to Vb converts the system from state B to A, where it remains when V goes back to VQ. Such state interconversion curves are also characterized by the non-linearity of the response with respect to scanning the triggering stimulus. 

Both TCSPC and frequency-domain fluorimetry are limited in time resolution by the response of available detectors, typically >25 ps. For cases in which higher time resolution is needed, fluorescence up-conversion can be used (22). This technique uses short laser pulses (usually sub-picosecond) both to excite the sample and to resolve the fluorescence decay. Fluorescence collected from the sample is directed through a material with nonlinear optical properties. A portion of the laser pulse is used to gate the fluorescence by sum frequency generation. The fluorescence is up-converted to the sum frequency only when the gate pulse is present in the nonlinear material. The up-converted signal is detected. The resolution of the experiment therefore depends only on the laser pulse widths and not on the response time of the detectors. As a result, fluorescence can be resolved on the 100-fs time scale. For a recent application of fluorescence up-conversion to proteins, see Reference 23. 

A prominent example of the latter is harmonic generation or frequency up-conversion. Here, a high-quality long wavelength laser beam is converted to the soft X-ray region through the nonlinear response of certain transmitting... 

The DP-CP can be used to convert optical energy to electrical energy with frequency up-conversion. This section reviews two examples. 

The medium subjected to cavitation may be a macroscopically homogeneous liquid (before cavitation occurs),or a heterogeneous medium made of immiscible liquids or a solid and a liquid. Any liquid can be subjected to cavitation, which a priori can be triggered by any kind of pressure wave, from infrasonic frequencies up to several MHz. For example, bromine atoms released in brominated water cavitating at infrasonic frequency (<16 Hz) convert maleic to fumaric acid via a mechanism described in Ch. 2 (p. 65). Sonoluminescence, usually observed between 20 kHz and 2 MHz (Fig. 1), was also studied by... 

Most of the practical transmitters in use today are special variations on these canonical forms. Practical transmitters may perform the RF operations at some convenient lower RF frequency and then up-convert to the desired operating frequency. In the case of RF signals that contain no AM, frequency multipUers may be used to arrive at the operating frequency. Of course, power amplifiers are usually required to bring the output power level up to the specified value. If the RF signal contains no amplitude variations, class C ampMers (relatively high efficiency) may be used otherwise, class B amplifiers are used. 

The differences in transmitter design between low VHF, high VHF, and UHF are mainly in the up-converter and the output power level Low VHF are channels 2-6, and high VHF are channels 7 13. UHF are channels 14—83, except for channels 70 3, presently assigned to mobile radio service. Present-day practice is to use an exciter operating at or near the intermediate frequency (IF) followed by an up-converter, which translates the IF signal to the desired channel frequency. In most cases, distortion corrections can be done at IF, although some are done at baseband and RF (see Table 16.9). 

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