@article{KietheHeuerJechow2017,
  author    = {Kiethe, Jan and Heuer, Axel and Jechow, Andreas},
  title     = {Second-order coherence properties of amplified spontaneous emission from a high-power tapered superluminescent diode},
  series = {Laser physics letters},
  volume    = {14},
  journal   = {Laser physics letters},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {1612-2011},
  doi       = {10.1088/1612-202X/aa772c},
  pages     = {4},
  year      = {2017},
  abstract  = {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 optical feedback. When operated in an external cavity, the degree of second-order coherence changed from the almost thermal case of g((2))(0)approximate to 1.9 towards the mostly coherent case of g((2)) (0) approximate to 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-slow(2) < 1.6. In this case, the degree of second-order coherence 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.},
  language  = {en}
}
@article{KurzkeKietheHeueretal.2017,
  author    = {Kurzke, Henning and Kiethe, Jan and Heuer, Axel and Jechow, Andreas},
  title     = {Frequency doubling of incoherent light from a superluminescent diode in a periodically poled lithium niobate waveguide crystal},
  series = {Laser physics letters},
  volume    = {14},
  journal   = {Laser physics letters},
  publisher = {IOP Publ.},
  address   = {Bristol},
  issn      = {1612-2011},
  doi       = {10.1088/1612-202X/aa6889},
  pages     = {5},
  year      = {2017},
  abstract  = {The amplified spontaneous emission from a superluminescent diode was frequency doubled in a periodically poled lithium niobate waveguide crystal. The temporally incoherent radiation of such a superluminescent diode is characterized by a relatively broad spectral bandwidth and thermal-like photon statistics, as the measured degree of second order coherence, g((2))(0)= 1.9 +/- 0.1, indicates. Despite the non-optimized scenario in the spectral domain, we achieve six orders of magnitude higher conversion efficiency than previously reported with truly incoherent light. This is possible by using single spatial mode radiation and quasi phase matched material with a waveguide architecture. This work is a principle step towards efficient frequency conversion of temporally incoherent radiation in one spatial mode to access wavelengths where no radiation from superluminescent diodes is available, especially with tailored quasi phase matched crystals. The frequency doubled light might find application in imaging, metrology and quantum optics experiments.},
  language  = {en}
}
@article{MayerMatthaeiHeueretal.2019,
  author    = {Mayer, Dennis and Matthaei, Christian T. and Heuer, Axel and G{\"u}hr, Markus},
  title     = {Kagome-fiber prism compressor combination for Yb},
  series = {Journal of optics},
  volume    = {21},
  journal   = {Journal of optics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2040-8978},
  doi       = {10.1088/2040-8986/aafdf6},
  pages     = {6},
  year      = {2019},
  abstract  = {Spectral broadening in hollow-core fibers is an important tool for pulse compression of low-peak power laser pulses, especially for Yb-based lasers. Here, we present a pulse compression scheme to reduce the pulse duration of a commercial Yb:KGW laser operating at 100 kHz repetition rate and 40 mu J pulse energy from 390 to 38 fs. The spectral broadening is accomplished using a krypton-filled Kagome-type fiber. We report broadened spectra for variable Kr-pressures and input powers. At optimal settings of 8 bar Kr-pressure and 3.3 W input power, the bandwidth of the pulse at the -10 dB level increased from 9.5 to 85 nm corresponding to a Fourier limit of 26 fs. A simple SF10 prism compressor is used to reduce the accumulated chirp and shortens the fiber output from about 500 to 38 fs. In addition to the spectral broadening, a pressure dependent change of the polarization is observed.},
  language  = {en}
}
@article{MenzelHeuerMilonni2019,
  author    = {Menzel, Ralf and Heuer, Axel and Milonni, Peter W.},
  title     = {Entanglement, Complementarity, and Vacuum Fields in Spontaneous Parametric Down-Conversion},
  series = {Atoms},
  volume    = {7},
  journal   = {Atoms},
  number    = {1},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2218-2004},
  doi       = {10.3390/atoms7010027},
  pages     = {14},
  year      = {2019},
  abstract  = {Using two crystals for spontaneous parametric down-conversion in a parallel setup, we observe two-photon interference with high visibility. The high visibility is consistent with complementarity and the absence of which-path information. The observations are explained as the effects of entanglement or equivalently in terms of interfering probability amplitudes and also by the calculation of a second-order field correlation function in the Heisenberg picture. The latter approach brings out explicitly the role of the vacuum fields in the down-conversion at the crystals and in the photon coincidence counting. For comparison, we show that the Hong-Ou-Mandel dip can be explained by the same approach in which the role of the vacuum signal and idler fields, as opposed to entanglement involving vacuum states, is emphasized. We discuss the fundamental limitations of a theory in which these vacuum fields are treated as classical, stochastic fields.},
  language  = {en}
}
@article{MenzelMarxPuhlmannetal.2019,
  author    = {Menzel, Ralf and Marx, Robert and Puhlmann, Dirk and Heuer, Axel and Schleich, Wolfgang},
  title     = {The photon},
  series = {Journal of the Optical Society of America : B, Optical physics},
  volume    = {36},
  journal   = {Journal of the Optical Society of America : B, Optical physics},
  number    = {6},
  publisher = {Optical Society of America},
  address   = {Washington},
  issn      = {0740-3224},
  doi       = {10.1364/JOSAB.36.001668},
  pages     = {1668 -- 1675},
  year      = {2019},
  abstract  = {We investigate the role of the spatial mode function in a single-photon experiment designed to demonstrate the principle of complementarity. Our approach employs entangled photons created by spontaneous parametric downconversion from a pump mode in a TEM01 mode together with a double slit. Measuring the interference of the signal photons behind the double slit in coincidence with the entangled idler photons at different positions, we select signal photons of different mode functions. When the signal photons belong to the TEM01-like double-hump mode, we obtain almost perfect visibility of the interference fringes, and no "which slit" information is available in the idler photon detected before the slits. This result is remarkable because the entangled signal and idler photon pairs are created each time in only one of the two intensity humps. However, when we break the symmetry between the two maxima of the signal photon mode structure, the paths through the slits for these additional photons become distinguishable and the visibility vanishes. It is the mode function of the photons selected by the detection system that decides if interference or "which slit" information is accessible in the experiment.},
  language  = {en}
}
@article{MenzelPuhlmannHeuer2017,
  author    = {Menzel, Ralf and Puhlmann, Dirk and Heuer, Axel},
  title     = {Complementarity in single photon interference - the role of the mode function and vacuum fields},
  series = {Journal of the European Optical Society-Rapid},
  volume    = {13},
  journal   = {Journal of the European Optical Society-Rapid},
  publisher = {Springer},
  issn      = {1990-2573},
  doi       = {10.1186/s41476-017-0036-x},
  pages     = {7},
  year      = {2017},
  abstract  = {Background In earlier experiments the role of the vacuum fields could be demonstrated as the source of complementarity with respect to the temporal properties (Heuer et al., Phys. Rev. Lett. 114:053601, 2015). Methods Single photon first order interferences of spatially separated regions from the cone structure of spontaneous parametric down conversion allow for analyzing the role of the mode function in quantum optics regarding the complementarity principle. Results Here the spatial coherence properties of these vacuum fields are demonstrated as the physical reason for complementarity in these single photon quantum optical experiments. These results are directly connected to the mode picture in classical optics. Conclusion The properties of the involved vacuum fields selected via the measurement process are the physical background of the complementarity principle in quantum optics.},
  language  = {en}
}
@article{NiebuhrHeuer2017,
  author    = {Niebuhr, Mario and Heuer, Axel},
  title     = {Phase measurement and far-field reconstruction on externally coupled laser diode arrays},
  series = {Optics express},
  volume    = {25},
  journal   = {Optics express},
  number    = {13},
  publisher = {Optical Society of America},
  address   = {Washington, DC},
  issn      = {1094-4087},
  doi       = {10.1364/OE.25.014317},
  pages     = {14317 -- 14322},
  year      = {2017},
  abstract  = {Passive coherent combination of several discrete low power laser diodes is a promising way to overcome the issue of degrading beam quality when scaling single emitters to > 10W output power. Such systems would be an efficient alternative to current high power sources, yet they suffer from fatal coherence loss when operated well above threshold. We present a new way to obtain detailed coherence information for laser diode arrays using a spatial light modulator to help identify the underlying decoherence processes. Reconstruction tests of the emitted far-field distribution are conducted to evaluate the performance of our setup.},
  language  = {en}
}