We propose and realize the first classical ghost-imaging (GI) experiment in the frequency or wavelength domain, thus performing ghost spectroscopy using thermal light exhibiting photon bunching. The required wavelength correlations are provided by light emitted by spectrally broadband near-infrared amplified spontaneous emission of a semiconductor-based superluminescent diode. They are characterized by wavelength-resolved intensity cross-correlation measurements utilizing two-photon-absorption interferometry. Finally, a real-world spectroscopic application of this ghost spectroscopy with a classical light scheme is demonstrated in which an absorption band of trichloromethane (chloroform) at 1214 nm is reconstructed with a spectral resolution of 10 nm as a proof-of-principle experiment. This ghost-spectroscopy work fills the gap of a hitherto missing analogy between the spatial and the spectral domain in classical GI modalities, with the expectation of contributing towards a broader dissemination of correlated photon ghost modalities, hence paving the way towards more applications which exploit the favorable advantages.

In this paper, we numerically investigate the effect of optical injection locking on the noise properties of mid-infrared quantum cascade lasers. The analysis is carried out by means of a rate equation model, which takes into account the various noise contributions and the injection of the master laser. The obtained results indicate that the locked slave laser may operate under reduced intensity noise levels compared with the free running operation. In addition, optimization of the locking process leads to further suppression of the intensity noise when the slave laser is biased close to the free-running threshold current. The main factors that significantly affect the locking process and the achievable noise levels are the injected optical power and the master-slave frequency detuning.

By exploiting polarization correlations of light from a broadband fiber-based amplified spontaneous emission source we succeed in reconstructing a hidden polarization in a ghost polarimetry experiment in close analogy to ghost imaging and ghost spectroscopy. Thereby, an original linear polarization state in the object arm of a Mach–Zehnder interferometer configuration which has been camouflaged by a subsequent depolarizer is recovered by correlating it with light from a reference beam. The variation of a linear polarizer placed inside the reference beam results in a Malus law type second-order intensity correlation with high contrast, thus measuring a ghost polarigram.

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincaré sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient $g^{(2)}(\tau )$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for $g^{(2)}(\tau )$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.

This study investigates chaotic and spiking dynamics of mid-infrared quantum cascade lasers operating under external optical feedback and emitting at 5.5 $\mu m$ and 9 $\mu m$ . In order to deepen the understanding, the route to chaos is experimentally studied in the case of continuous-wave and current modulation operation. The non-linear dynamics are analyzed with bifurcation diagrams. While for quasi-continuous wave operation, chaos is found to be more complex, pure continuous wave pumping always leads to the generation of a regular spiking induced low-frequency fluctuations dynamics. In the latter, results show that by combining external optical feedback with periodic forcing and further induced current modulation allows a better control of the chaotic dropouts. This work provides a novel insight into the development of future secure free-space communications based on quantum cascade lasers or unpredictable optical countermeasure systems operating within the two transparency atmospheric windows hence between 3 $\mu m$ - 5 $\mu m$ and 8.5 $\mu m$ - 11 $\mu m$

We present a semiconductor laser system tailored to exhibit unusual spectral broadband emission dynamics. We study the dynamics properties and its physical origin in detail and discuss the potential of this system as a high-power incoherent laser light source. Our semiconductor laser (SL) system comprises a particularly long edge-emitting laser of 1.6 mm length and a short external cavity of comparable length. We have adjusted for resonant coupling conditions between both cavities, such introducing strong modal coupling. By varying the pumping or the optical feedback phase, we obtain a characteristic cyclic scenario evolving from stable emission via a period-doubling cascade to chaos and back to stable emission. We find distinct differences to the short-cavity regime of conventional nonresonant SL systems reported so far. The most prominent difference is the onset of chaotic intensity dynamics in conjunction with pronounced multimode dynamics of high optical bandwidth exceeding $\sim 7\mathrm{nm}$, therefore comprising more than 100 lasing longitudinal modes. In that sense, the presented system represents an excellent nonlinear dynamical model system offering well-controllable generation of distinct multimode dynamics. Furthermore, we demonstrate that the nonlinear dynamics properties allow for controlled adjustment of the coherence length in a wide range between $\sim 130\mu \mathrm{m}$ and $\sim 8\mathrm{m}$. This property facilitates application in novel measurement technology in which (in)coherence properties are of importance.