Type IIA orientifold compactification on SU(2)-structure manifolds

We investigate the effective theory of type IIA string theory on six-dimensional orientifold backgrounds with SU(2)-structure. We focus on the case of orientifolds with O6-planes, for which we compute the bosonic effective action in the supergravity approximation. For a generic SU(2)-structure background, we find that the low-energy effective theory is a gauged N=2 supergravity where moduli in both vector and hypermultiplets are charged. Since all these supergravities descend from a corresponding N=4 background, their scalar target space is always a quotient of a SU(1,1)/U(1) x SO(6,n)/SO(6)xSO(n) coset, and is therefore also very constrained.Comment: 31 pages; v2: local report number adde

Tunable two color emission in a compact semiconductor ring laser with filtered optical feedback

We report on an integrated approach to obtain two color emission from a semiconductor ring laser with filtered optical feedback. This feedback is realized on-chip by employing two arrayed waveguide gratings to split/recombine light into different wavelength chan nels. Semiconductor optical amplifiers are used in the feedback loop to control the feed back strength of each wavelength channel independently. Results show that the effective gain of the d(tferent anodes is the key parameter which has to be balanced to obtain two color emission. This can be aclueved by tuning the injection current in each amplifie

Switchable multiwavelength emission using semiconductor ring laser with optical filtered feedback

Email Print Request Permissions Summary form only given. Single laser chips that emit multiple wavelengths simultaneously are interesting for a range of applications including wavelength division multiplexing, optical instrument testing and optical sensing. A number of approaches have been proposed to achieve multiple wavelength emission (MWE) by e.g. using multiple lasers, but they tend to be bulky and/or expensive. Some of these structures need thermal tuning of the emission wavelengths, which is relatively slow and requires precise control of the chip temperature. In this work we report on a novel integrated approach in order to obtain MWE from a single semiconductor laser based on on-chip filtered optical feedback. The layout of our device is shown in Fig.1(a). It consists of semiconductor ring laser (SRL), two arrayed waveguide gratings which are used to split/recombine light into 4 different wavelength channels, four semiconductor optical amplifier gates and passive and active waveguides to connect these different components.We can select either triple wavelength emission, dual wavelength emission (DWE) or single longitudinal mode emission (SME) by properly adjusting the currents in the semiconductor optical amplifier gates of the feedback loop. An advantage of our device is that we can select the lasing longitudinal modes, and thus the emitted wavelengths, in a simple manner by changing the current in the feedback amplifiers. Wavelength selection is done in a non-thermal fashion, which can in principle be done fast. MWE is achieved in a SRL, which has the additional advantage that it can easily be integrated with other photonic components on a chip.Experimentally the device output without feedback is multi-mode above the threshold current (64 mA). SME can be achieved by pumping one gate with a suitable current [1]. When current is applied to two gates at the same time while the SRL is biased above threshold current, SRL shows DWE for a range of currents on gate 4 and gate 2. This DWE can b- observed in the optical spectrum shown in Fig.1(b), at the top of this figure we show a schematic plot of the filter passband of each of the gate channels. The selected longitudinal modes are spectrally positioned within the arrayed waveguide gratings filter passbands corresponding to gate 2 and gate 4 which are chosen to be pumped. The two peak wavelengths are ¿1 = 1580.788 nm (gate 4 channel), ¿2 = 1583.288 nm (gate 2 channel). This DWE can be explained by the fact that a suitable amount of feedback cancels the gain difference between the wavelength channels due to fabrication and material dichroism. By increasing or decreasing the current injected in one of the pumped gates, we notice switching from two modes in the output to one of them. By pumping three gates instead of two and by precise adjustment of the currents in the gates, triple wavelength emission was observed with similar switching behavior to DWE and then to SME just by changing one of the gate currents. In this contribution we will further discuss the precise behavior of the MWE from the device. We will also show results from numerical simulations based on two directional-mode model[2] extended with Lang-Kobayashi terms to take into account for optical feedback. Some of these numerical results are shown in Fig.1(c). We plot in this figure the maxima of the intensities of the three modes when the feedback phase is equal to 0.5p for each mode. The first modes feedback strength ¿1 is fixed while the feedback strength ¿2 and ¿3 of the second and third modes are kept equal ¿2 = ¿3 and are increased simultaneously. As can be seen from Fig1.(c), the device output is changing from SME when (¿2 = ¿3) ¿1. The numerical results are in qualitative agreement with the

Fast random bit generation based on a single chaotic semiconductor ring laser

Here, we numerically and experimentally demonstrate that, by combining two post-processing methods (multi-bit extraction and bitwise OR-exclusive (XOR) operations). in a single chaotic semiconductor ring laser (SRL), it is possible to generate true random bits with a bit rate up to 40 Gb/s from a chaos bandwidth of ˜ 2 GHz, thanks to the device ability of lasing in two directional modes and the fact that the two mode signals have low correlations. In addition, SRLs can be easily implemented on chip

Simulations of fast switching between longitudinal modes of the novel semiconductor tunable laser with filtered optical feedback

A system of delay differential rate equations is introduced as a tool to investigate switching dynamics induced by on-chip filtered feedback in Photonic Integrated Circuits. We show different switching mechanisms between lasing modes that can be induced by modulation of the frequencies of the feedback filter. Finally a novel integrated tunable laser is presented which operates according to such control scheme

Novel integrated tunable laser using filtered feedback for simple and very fast tuning

We present a novel integrated tunable laser based on filtered feedback, which combines a simple tuning method with ns switching speed

Discretely tunable laser based on filtered feedback for telecommunication applications

A novel discretely tunable laser based on filtered feedback is presented. The semiconductor device consists of a Fabry - Perot laser with deeply etched broadband distributed Bragg reflector mirrors. Single-mode operation is achieved by using feedback from an integrated filter. This filter contains an arrayed waveguide grating wavelength router and a semiconductor optical amplifier gate array. Design, simulation, and the first characterization results of this new integrated filtered-feedback tunable laser device are presented. It shows a combination of a simple and robust switching algorithm with good wavelength stability. A rate equation model predicts that a properly designed device can switch within 1 ns. The fast switching and reduced control complexity makes the device very promising for various advanced applications in optical telecommunication networks

Low-frequency fluctuations in vertical-cavity surface-emitting lasers with polarization selective feedback: experiment and theory

We present theory and measurements on a vertical-cavity surface-emitting laser (VCSEL) biased close to its solitary threshold and subject to polarization selective external optical feedback. The system shows low-frequency fluctuations (LFF) in the selected polarization mode (PM). Below solitary laser threshold, the orthogonal PM remains silent, while it only responds after a dropout event in the main mode above threshold. Our calculations show good agreement with the measurements and identify a type of synchronization between the low-frequency dynamics of the two PMs