Eisenstein Cohomology and ratios of critical values of Rankin-Selberg L-functions

This is an announcement of results on rank-one Eisenstein cohomology of GL(N), with N > 1 an odd integer, and algebraicity theorems for ratios of successive critical values of certain Rankin-Selberg L-functions for GL(n) x GL(n') when n is even and n' is odd.Comment: Final version to appear in Comptes Rendus Math. Acad. Sci. Pari

Observation of relaxation resonance effects in the field spectrum of semiconductor lasers

Subsidiary maxima are observed in the field spectra of single mode semiconductor lasers. Measurements of their power dependence show they are linked to the relaxation resonance. We attribute these maxima to combined phase and amplitude fluctuations at the relaxation resonance. A theoretical calculation of the field spectrum using the results of a noise analysis incorporating carrier dynamics agrees very well with observations

Picosecond Mode-Locking And X-Band Modulation Of Semiconductor Lasers

Recent developments in two areas of high speed semiconductor lasers will be addressed: (1) passive mode-locking of a segmented-contact semiconductor laser with a reliable, controllable saturable absorber which produces stable picosecond optical pulses, and (2) realization of very high frequency (X-band) direct analog modulation of a semiconductor laser diode

Longitudinal mode spectrum of GaAs injection lasers under high-frequency microwave modulation

Experimental observations of the lasing spectrum of a single mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single longitudinal mode for optical modulation depths up to ~80%, beyond which the lasing spectrum becomes multimoded, whose envelope width increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which enables one to predict the dynamic lasing spectrum of a laser from its cw lasing spectra at various output powers

Bistability and pulsations in cw semiconductor lasers with a controlled amount of saturable absorption

Experimental results of a buried heterostructure cw laser with a controllable amount of saturable absorption introduced by a segmented contact are presented. With no absorption the laser is stable and has a linear output characteristic. Increasing of the saturable absorption by changing the pump current through the control segment causes the light output of the device to pulsate and to show bistable and hysteretical behavior. The introduction of a controllable amount of saturable absorption suggest the usefulness of this device in generating extremely short pulses, for example, by passive mode locking and as a bistable optical device

Bistability and negative resistance in semiconductor lasers

Experimental results of a buried heterostructure laser with a segmented contact to achieve inhomogeneous gain are presented. Measurements reveal a negative differential resistance over the absorbing section. Depending on the source impedance of the dc current source driving the absorbing section, this negative resistance can lead to (i) bistability with a very large hysteresis in the light-current characteristic without self-pulsation or (ii) a small hysteresis with self-pulsations at microwave frequencies. An analysis, which includes the electrical part of the device, leads to an explanation of self-pulsations in inhomogeneously pumped lasers without having to rely on a sublinear gain dependence on injected carrier concentration

Dynamical switching characteristics of a bistable injection laser

The switching characteristics of a bistable injection laser with very large hysteresis is examined. Switch-on delays are shown to exhibit a "critical" part and a "noncritical" part, both of which can be reduced by increasing the overdrive current. It is possible to obtain fairly fast switching time (<20 ns) with a strong overdrive. Nominal delays of 100–200 ns result under moderate overdrives. These long time scales are due to long carrier lifetimes in the carrier-depleted absorption section, a property intrinsic to these bistable injection lasers

Graded collector heterojunction bipolar transistor

A graded collector heterojunction bipolar transistor is proposed. The graded collector improves device speed performance at high current densities by reducing the influence of the Kirk effect