Amplitude Modulation Mode Research Articles

Mode locking of a continuous wave Nd:glass laser pumped by a multistripe diode laser

The performance of a continuous wave actively mode-locked Nd: phosphate glass laser longitudinally pumped by a multistripe diode laser is described. The laser operates at 1.054 μm; the pump threshold and the slope efficiency are found to be 120 mW and 11%, respectively. The shortest pulse duration is 7 ps, which appears to be approximately twice as short as predicted by the theory of amplitude modulation mode locking. We explain the improved performance by additional frequency modulation due to the nonlinear index of the active material.

Critical bandwidth for modulation detection

Two sinusoidal carriers separated by frequency F were each sinusoidally amplitude modulated (AM). Subjects were asked to discriminate between a complex in which the two AM signals had the same phase and a complex in which the phase spectra of the lower frequency AM components were delayed relative to the upper frequency AM components. The threshold value of the phase shift was measured from three-point psychometric functions obtained from a two-alternative, forced-choice task. A broadband-noise background and random variation in overall level were used to force the two normal-hearing listeners to attend to the temporal structure of the two-tone AM complexes. The frequency separation between the two carriers (F), the rates of AM modulation, and the form of the phase shift (entire waveform or envelope delay) were parameters in the study. The results indicate that the threshold for detecting the phase shift is small when the two carriers are within a critical band. However, listeners are still quite sensitive to the phase shifts for very large frequency separations between the carriers. The results will be discussed in terms of the auditory system's ability to use temporal modulation to identify common sources in a complex sound. [Work supported by AFOSR-Life Sciences and the NINCDS.]

Measurement of small standing wave ratio and losses in dielectric strip lines

The block diagram by means of which the proposed method is implemented is shown in Fig. i. The tested DSL element 6 containing a polyethylene waveguide 5 on a Teflon backing 7 is connected through a matching transformer 3 and a waveguide-to-strip line junction 4 to a directional coupler 2 aligned onto the reflected wave. The SHF signal generator 1 operates in an amplitude-modulation mode. Through detector 8, the directional coupler 2 is connected with a meter 9.

Auditory spectral filtering and monaural phase perception.

The fundamental relation of limited auditory frequency resolution to mortar phase perception is studied. Old and new results are discussed for experiments employing sinusoidally amplitude-modulated (AM) carrier tones and quasi-frequency modulated (QFM) carrier tones, which are related by a phase transformation. One experiment concerns AM and QFM modulation thresholds. A second experiment considers the quality differences between AM and QFM. Both experiments show that phase effects disappear for stimulus bandwidths that exceed a value roughly proportional to the critical band at the carrier frequency. The proportionality factor depends upon carrier level and differs greatly between experiments. A unified psycho-physical account is given with a model consisting of a quasilinear bandpass analyzing filter followed by ideal envelope detection, and concluding with either a peak-to-minimum or peak-threshold decision. Parameter values in the model are evaluated from the phase-perception data, and these values support the assumptions that underlie the model. It is shown that the critical-bandlike experimental data do not provide a measure of the commonly conceived critical band. Rather, the critical-band scale merely normalizes the analyzing-filter parameters. This work indicates that the model employed offers wider application.