7,977 research outputs found
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 pulsations in semiconductor lasers with inhomogeneous current injection
Bistability and pulsation at microwave frequencies are observed in CW GaAs semiconductor lasers with inhomogeneous current injection. Inhomogeneous current injection is achieved with a segmented contact structure. Crucial to the understanding of the characteristics of this device is the discovery of a negative differential electrical resistance across the contacts of the absorbing section. Depending on the electrical bias condition, this negative differential resistance leads to bistability or light-jumps and self pulsations. A simple model based on conventional rate equations with a linear gain dependence on carrier density explains the observed behavior and suggests a new mechanism in inhomogeneously pumped diode lasers for light-jumps and pulsations which does not depend on the condition for the usually proposed repetitively Q-switching. Investigation of the switching dynamics of this bistable optoelectronic device reveals a delay time which is critically dependent on the trigger pulse amplitude and typically on the order of a few nanoseconds with power-delay products of 100 pJ. The observed critical slowing down and its origin is discussed. We also report on the characteristic of this laser coupled to an external optical cavity and we demonstrate successfully that this bistable laser can be used as a self coupled stylus for optical disk readout with an excellent signal to noise ratio
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
Families of lattice polarized K3 surfaces with monodromy
We extend the notion of lattice polarization for K3 surfaces to families over
a (not necessarily simply connected) base, in a way that gives control over the
action of monodromy on the algebraic cycles, and discuss the uses of this new
theory in the study of families of K3 surfaces admitting fibrewise symplectic
automorphisms. We then give an application of these ideas to the study of
Calabi-Yau threefolds admitting fibrations by lattice polarized K3 surfaces
Passive mode locking of buried heterostructure lasers with nonuniform current injection
In this letter we report on a novel method to passively mode lock a semiconductor laser. We present experimental results of GaAlAs buried heterostructure semiconductor laser with a split contact coupled to an external cavity. The split contact structure is used to introduce a controllable amount of saturable absorption which is necessary to initiate passive mode locking. Unlike previous passive mode locking techniques, the method presented does not rely on absorption introduced by damaging the crystal and is consequently inherently more reliable. We have obtained pulses with a full width at half-maximum of 35 ps at repetition frequencies between 500 MHz and 1.5 GHz
Calabi-Yau Threefolds Fibred by Mirror Quartic K3 Surfaces
We study threefolds fibred by mirror quartic K3 surfaces. We begin by showing
that any family of such K3 surfaces is completely determined by a map from the
base of the family to the moduli space of mirror quartic K3 surfaces. This is
then used to give a complete explicit description of all Calabi-Yau threefolds
fibred by mirror quartic K3 surfaces. We conclude by studying the properties of
such Calabi-Yau threefolds, including their Hodge numbers and deformation
theory.Comment: v2: Significant changes at the request of the referee. Section 3 has
been rearranged to accommodate a revised proof of Proposition 3.5 (formerly
3.2). Section 5 has been removed completely, it will instead appear as part
of Section 5 in arxiv:1601.0811
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