This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

Publisher Summary The widespread availability of spatially and temporally coherent laser sources makes the production of optical vortices inevitable in any experiment involving scattered laser light. The realization of quantized vortices is not specific to optics: these objects occur in all spatial scalar fields. Although optical vortices are often referred to as “points of phase singularity within a cross section of the field,” physical optical fields extend over three dimensions, and the phase singularities are actually lines of perfect destructive interference that are embedded in the volume filled by the light. Optical vortices are examples of the singularity lines within all complicated scalar fields. By comparison, electromagnetic vector fields do not generally have nodes in all components simultaneously. However, vector fields possess singularities associated with the parameterization of elliptical and partial polarization rather than phase. Polarization singularities are present in many situations, ranging from sunlight to the light transmitted by birefringent materials. Their descriptors are more complicated than their scalar counterpart in that they have both handedness and additional categorization. The study of optical vortices and orbital angular momentum has led to a recognition that the energy flow—characterized by the Poynting vector—has features not immediately apparent from the intensity alone, nor from global properties of a beam.

Singular optics: optical vortices and polarization singularities

Spider silks outrival natural and many synthetic fibers in terms of their material characteristics. In nature, the formation of a solid fiber from soluble spider silk proteins is the result of complex biochemical and physical processes that take place within specialized spinning organs. Herein, we present natural and artificial silk production processes, from gene transcription to silk protein processing and finally fiber assembly. In-vivo and in-vitro findings in the field of spider silk research are the basis for the design of new proteins and processing strategies, which will enable applications of these fascinating protein-based materials in technical and medical sciences.

Spider Silk: From Soluble Protein to Extraordinary Fiber

As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material. Herein we report the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium-manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets

We consider reflection and transmission of polarized paraxial light beams at a plane dielectric interface. The field transformations taking into account a finite beam width are described based on the plane-wave representation and geometric rotations. Using geometrical-optics coordinate frames accompanying the beams, we construct an effective Jones matrix characterizing spatial-dispersion properties of the interface. This results in a unified self-consistent description of the Goos–Hanchen and Imbert–Fedorov shifts (the latter being also known as the spin Hall effect of light). Our description reveals the intimate relation of the transverse Imbert–Fedorov shift to the geometric phases between constituent waves in the beam spectrum and to the angular momentum conservation for the whole beam. Both spatial and angular shifts are considered as well as their analogues for higher-order vortex beams carrying intrinsic orbital angular momentum. We also give a brief overview of various extensions and generalizations of the basic beam-shift phenomena and related effects.

https://iopscience.iop.org/article/10.1088/2040-8978/15/1/014001/pdf

Goos-Hanchen and Imbert-Fedorov beam shifts: an overview

We describe the stabilization of the beatnote of an Er,Yb:glass dual-frequency laser at 1.5 mum with and without an external microwave reference. In the first case, a classical optical phase-locked loop (OPLL) is used, and absolute phase noise levels as low as -117 dBrad2/Hz at 10 kHz from the carrier are reported. In the second case one or two fiber-optic delay lines are used to lock the frequency of the beatnote. Absolute phase noise levels as low as -107 dBrad2/Hz at 10 kHz from the carrier are measured, fairly independant of the beatnote frequency varying from 2 to 6 GHz. An analysis of the phase noise level limitation is presented in the linear servo-loop theory framework. The expected phase noise level calculated from the measurement of the different noise sources fits well with the predictions.

Dual-Frequency Laser at 1.5 $\mu$ m for Optical Distribution and Generation of High-Purity Microwave Signals

An optical source of microwaves with very low phase noise for communications and radar systems is realized and tested. It is obtained by dual-frequency operation of a diode pumped Er,Yb:glass laser. An electrooptic crystal inserted inside the resonator permits both to tune the frequency difference between orthogonally polarized eigenstates, and to turn the laser into a voltage controlled oscillator. An optical phase-locked loop is then implemented in the GHz range, resulting in a measured instrument limited 3 dB-linewidth of 10 Hz. The phase noise is shown to be -100 dBc/Hz at 10-kHz offset.

Offset phase locking of Er,Yb:glass laser eigenstates for RF photonics applications

Dual tunable wavelength operation of Er,Yb:phosphate glass laser is reported. The spatial separation of the laser eigenstates and the use of two properly designed intracavity etalons permit one to tune the two wavelengths independently from 1540.5 to 1562.7 nm. The generated beat note, monitored using a Michelson interferometer, is experimentally shown to be adjustable from dc to 2.7 THz. The linewidth of this beat note is found to be less than 10 kHz. Several applications are discussed.

Dual tunable wavelength Er,Yb:glass laser for terahertz beat frequency generation

The nonlinear Goos-H\"anchen effect is experimentally isolated for the total reflection of a Gaussian beam at an interface between a linear medium and a negative Kerr-type nonlinear medium. The evolution of this nonlinear spatial shift versus light intensity is measured for both TE and TM polarizations. The results are in good agreement with a simple model based on nonlinear Artmann's formulas.

Measurement of the Nonlinear Goos-Hänchen Effect for Gaussian Optical Beams.

A two-propagation-axis solid-state laser is shown to provide a widely tunable optical microwave source. The spatial separation of the laser eigenstates is shown to enable an etalon to act as a coarse tuner, forcing oscillation in any nonadjacent cavity modes. The frequency difference between opposite helicoidal eigenstates operating in nonadjacent cavity modes can then be tuned continuously. The beat note from such a solid-state laser is shown to vary from dc to 26 GHz, i.e., 30 times the laser free-spectral range, and is limited only by the free-spectral range of the etalon.

A. Le Floch

Papers

Dual-Frequency Laser at 1.5 $\mu$ m for Optical Distribution and Generation of High-Purity Microwave Signals

Offset phase locking of Er,Yb:glass laser eigenstates for RF photonics applications

Dual tunable wavelength Er,Yb:glass laser for terahertz beat frequency generation

Measurement of the Nonlinear Goos-Hänchen Effect for Gaussian Optical Beams.

Tunable optical microwave source using spatially resolved laser eigenstates.