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Register a new userSecond-order coherence properties of amplified spontaneous emission from a high-power tapered superluminescent diode
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We study the degree of second-order coherence of the emission of a high-power multi-quantum well superluminescent diode with a lateral tapered amplifier section with and without feedback. When operated in an external cavity, the degree of second-order coherence changed from the almost thermal case of g$^{(2)}$(0)$approx$1.9 towards the mostly coherent case of g$^{(2)}$(0)$approx$1.2 when the injection current at the tapered section was increased. We found good agreement with semi-classical laser theory near and below threshold while above laser threshold a slightly higher g$^{(2)}$(0) was observed. As a free running device, the superluminescent diode yielded more than 400 mW of optical output power with good spatial beam quality of $M^2_{slow} < 1.6$. In this case, the DSOC dropped only slightly from 1.9 at low powers to 1.6 at the maximum output power. To our knowledge, this is the first investigation of a high-power tapered superluminescent diode concerning the degree of second-order coherence. Such a device might be useful for real-world applications probing the second order coherence function, such as ghost imaging.
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Based on the second-order quantum interference between frequency entangled photons that are generated by parametric down conversion, a quantum strategic algorithm for synchronizing two spatially separated clocks has been recently presented. In the reference frame of a Hong-Ou-Mandel (HOM) interferometer, photon correlations are used to define simultaneous events. Once the HOM interferometer is balanced by use of an adjustable optical delay in one arm, arrival times of simultaneously generated photons are recorded by each clock. The clock offset is determined by correlation measurement of the recorded arrival times. Utilizing this algorithm, we demonstrate a proof-of-principle experiment for synchronizing two clocks separated by 4km fiber link. A minimum timing stability of 0.4 ps at averaging time of 16000 s is achieved with an absolute time accuracy of 59.4 ps. The timing stability is verified to be limited by the correlation measurement device and ideally can be better than 10 fs. Such results shine a light to the application of quantum clock synchronization in the real high-accuracy timing system.
We present a fully quantum mechanical treatment of optically rephased photon echoes. These echoes exhibit noise due to amplified spontaneous emission, however this noise can be seen as a consequence of the entanglement between the atoms and the output light. With a rephasing pulse one can get an echo of the amplified spontaneous emission, leading to light with nonclassical correlations at points separated in time, which is of interest in the context of building wide bandwidth quantum repeaters. We also suggest a wideband version of DLCZ protocol based on the same ideas.
Population inversion on the 5D-6P transition in Rb atoms produced by cw excitation at different wavelengths has been analysed by comparing the generated mid-IR radiation at 5.23 um originated from amplified spontaneous emission and isotropic blue fluorescence at 420 nm. A novel method of detecting two-photon excitation in atomic vapours using ASE is suggested. We have observed directional co- and counter-propagating emission at 5.23 um. We find that the power dependencies of the backward- and forward-directed emission can be very close, however their spectral dependencies are not identical. The mid-IR emission in Rb vapours excited by nearly counter-propagating beams at 780 and 776 nm does not exactly coincide spatially with the applied laser beams. The presented observations could be useful for enhancing efficiency of frequency mixing processes and new field generation in atomic media.
We have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32 1/cm for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300 ps and carrier capture times of 15 and 40 ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into the active region from the uppermost ZnMgSSe cladding layer, yielding a longer rise time for the pump-probe signals for this sample.
We study spontaneous emission from a rubidium atom into the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber. We show that the spontaneous emission rate depends on the magnetic sublevel, the type of modes, the orientation of the quantization axis, and the fiber radius. We find that the rate of spontaneous emission into the TE modes is always symmetric with respect to the propagation directions. Directional asymmetry of spontaneous emission into other modes may appear when the quantization axis does not lie in the meridional plane containing the position of the atom. When the fiber radius is in the range from 330 nm to 450 nm, the spontaneous emission into the HE$_{21}$ modes is stronger than into the HE$_{11}$, TE$_{01}$, and TM$_{01}$ modes. At the cutoff for higher-order modes, the rates of spontaneous emission into guided and radiation modes undergo steep variations, which are caused by the changes in the mode structure. We show that the spontaneous emission from the upper level of the cyclic transition into the TM modes is unidirectional when the quantization axis lies at an appropriate azimuthal angle in the fiber transverse plane.
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