Power-independent linewidth

In Fig. 2 we show data taken with similar TJS lasers at three temperatures for AVjujjjjj as a function of inverse power. The characteristic linear dependence of the Lorentzian linewidth on inverse power is clearly evident. A representative value for the linewidth-differential power product for (GaAl)As lasers at room temperature is Avpyjjjj AP 100 MHz mW. As the temperature is reduced, a power-independent linewidth component becomes increasingly evident. This is observed to increase from about 2 MHz at 273 K to over 11 MHz at 77 Additional measurements on these TJS de-... 

Zeiger has recently published a theoretical examination of the effect of traps on the SL linewidth. Given a sufficient density of trap states within the optical mode volume, a result similar to the observed behavior is obtained if the trap decay rates are at least of the order of the power-independent linewidth. The motivation for the trapping theory came in part from the widely observed l/f behavior in the low frequency current, amplitude and frequency noise spectra of SL s. This is discussed in more detail shortly. Similar behavior in the current fluctuation noise in semiconductors has been related to the presence of trap levels. 

Another fundamental process in semiconductors that may contribute to the observed power-independent linewidth is light scattering. Among the many scattering mechanisms that have been observed in GaAs, those that are related to a random process (i.e., carrier density variations, temperature fluctuations, etc.) and produce radiation that couples to the laser mode may increase the noise in the mode above that due to spontaneous emission. However, as opposed to the spontaneous light, the on-line scattered component will increase with the power in the mode above threshold and therefore contribute a power-independent phase fluctuation rate. Various Rayleigh... 

The frequency noise power spectral density of a SL typically exhibits a 1/f dependence below 100 kHz and is flat from 1 MHz to well above 100 MHz. Relaxation oscillations will induce a pronounced peak in the spectrum above 1 GHz. The "white" spectral component represents the phase fluctuations that are responsible for the Lorentzian linewidth and its intensity is equal to IT times the Lorentzian FWHM.20 xhe 1/f component represents a random walk of the center frequency of the field. This phase noise is responsible for a slight Gaussian rounding at the peak of the laser field spectrum and results in a power independent component in the linewidth. Figure 3 shows typical frequency noise spectra for a TJS laser at two power levels. 

Figure 5 shows three data points for the linewidths measured for a single mode PhSi-xSe diode laser (indicated by circles) in addition to the linewidth of the laser illustrated in Figure it (indicated by a square). Figure 5 shows the linewidths plotted as a function of the inverse of the single-ended laser power output. A linear extrapolation of the three data points of the single mode laser to zero inverse power indicated negligible linewidth intercept. Hence, the power-independent... 

If one had two species in the sample with different chemical shifts and significantly different T2S, the spikelet echo sequence could demonstrate the existence of both species, independent of the pulse repetition rate. The resulting spectrum would be a composite of the spikelet spectra of the two individual species with different breadths (widths or extents of the power spectra) and different linewidths for each of the spikes or lines. This capability of distinguishing species based upon their T2S proved to be important in the identification of disordered phases or components of adsorbate material on the surface. 

The linewidth Av of a single-mode color-center laser is mainly determined by fluctuations of the optical path length in the cavity (Sect. 5.4). Besides the contribution Avni caused by mechanical instabilities of the resonator, temperature fluctuations in the crystal, caused by pump power variations or by temperature variations of the cooling system, further increase the linewidth by adding contributions Avp and Avf Since all three contributions are independent, we obtain for the total frequency fluctuations... 

Fig. 13. (Upper) ip-NMR spectrum (121.5 MHz) of the purple membrane from Halobacterium cutirubrum obtained with high-power H decoupling at IS C spectral width 125 kHz acquisition time 6 ms recycle time 1 s pulse width 45° decoupler on 4 fts before acquisition and during acquisition but off during remainder of cycle Fourier transform of 16,384 data points after zero iilling. (Lower) Computer simulation of the above in terms of two axially symmetric powder patterns characterized by effective chemical-shift anisotropies of -I-61 and 4-18.5 ppm the angular-independent and -dependent linewidths were 300 and 200 Hz, and 350 and 250 Hz, for the phosphomonoester and phosphodiester, respectively. The broken curves are the powder patterns for the two types of phosphate ester present. From Ekiel et al. (1981).

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