Modulation bandwidth

Range resolution is the ability of a radar system to distinguish two closely spaced targets at differing ranges. In monostatic radar this is primarily a function of pulse length or modulation bandwidth. Azimuth... 

The bandwidth is seen to vary from 500Hz to 22.2kHz. This is important in two respects. The first is that by no means all of the 150kHz modulation bandwidth is being used (in this case it is only 15% of the available bandwidth). Secondly, as the bandwidth is a function of time the performance of the radar system will also be a function of time. 

Electrical measurements are expressed in terms of modulation bandwidth rather than observation times. It can be shown that the bandwidth B corresponding to an observation time Tq is the reciprocal of 2tq. Thus,... 

The modulation bandwidth of the semiconductor injection laser given by Eq. (15), is determined by the gain coefiicient, photon lifetime, and the steady-state photon density. These parameters are influenced by the structure of the laser, the operating temperature, and the laser diode material system. For a given laser, increasing the injection current, as shown in Fig. 16, can increase the bandwidth. However the maximum current is limited by the... 

Another method of increasing the modulation bandwidth is to decrease the photon lifetime. This is most easily accomplished by decreasing the laser diode cavity length. This, however, increases the laser threshold current level, and as a result, lasers with extremely low threshold current levels are required for this method. Utilizing a 40-/rm-long AlGaAs multiple quantum well laser, a modulation bandwidth of 50 GHz has been achieved. 

In addition to excessive current pumping and shortening the laser diode cavity length as a means to increase the modulation bandwidth, the differential in coefficient can be increased. This can be achieved by lowering the operation temperature of the laser diode. For example, the bandwidth of a GaAs buried heterostracture laser which is 7 GHz at room temperature (25°C) can be extended to 9.5 GHz at —50°C for a 175-/nm-long cavity. Similarly, the bandwidth of a InP constricted mesa laser can be increased from 16 GHz at 20°C to 26.5 GHz at -60°C. The drawback with this method is that there is a decrease in the modulation efficiency due to an increase in the series resistance of the diode which occurs at low temperatures. 

Higher differential gain coefficients can also be obtained by utilizing semiconductor quantum well strac-tures. Due to the confinement of carriers in a direction of the quantum well growth, the density of states become stepwise and increases the differential gain coefficient. This staircase density of states distribution increases the modulation bandwidth by a factor of two. It is expected that another factor of two increase in the modulation bandwidth will be obtainable with quantum wire stiuctures, which have a quasi-discrete density of states distribution. 

Other phenomena that influence the modulation dy-nanucs of semiconductor lasers include the spontaneous emission, gain saturation, and the external electrical connections to the device. These effects normally manifest themselves as a suppression of the relaxation oscillations, which reduces the peak of the modulation response and also reduces the modulation bandwidth. 

The modulation bandwidth A/ of an acousto-optic modulator is determined primarily by the angular spread of the optical beam. The finite angular distribution of the wave vector allows the Bragg diffraction to occur over a finite range of acoustic frequencies. By differentiating Eq. (3.30), the expression for the bandwidth is... 

A typical diode laser frequency response for small amplitude modulation is shown in Fig. 9.44. As can be seen from this plot, the modulation bandwidth extends from DC to the relaxation resonance. The frequency of the relaxation resonance, /reiax> is proportional to the square root of bias current above threshold and inversely proportional to the geometric mean of the lifetimes of the carriers in the inverted population, z ,... 

Recently, optical loss due to optical mode size mismatch between polymer EO waveguide modes and silica frber modes have been dramaticaUy reduced exploiting reactive ion etched tapered transitions (5). This achievement together with the demonstration of integration of polymeric electro-optic circuitry with VLSI semiconductor electronics (6) and very large polymeric modulator bandwidths (7) represent dramatic advances in the utilization of lymeric electro-optic materials. 

Medium effects (corrected for the bulk-susceptibility effect) have been reported for four commonly used solvent-internal reference systems in a paper by Stephen and his co-workers, who point out that the difference in chemical shifts between internal and external reference signals may be as great as 0.54 p.p.m. A novel type of noise-modulated, heteronuclear decoupling has been used by Evelyn and Hall to simplify the n.m.r. spectra of sugar derivatives containing the — CHgF group. By suitable adjustment of the modulation bandwidth and the radio-frequency intensity, the resonances of the coupled nuclei are broadened sufficiently... 

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