There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Low-phase-noise microwave oscillators are important to a wide range of subjects, including
communications, radar and metrology. Photonic-based microwave-wave sources now provide
record, close-to-carrier phase-noise performance, and compact sources using microcavities
are available commercially. Photonics-based solutions address a challenging scaling problem
in electronics, increasing attenuation with frequency. A second scaling challenge, however, is
to maintain low phase noise in reduced form factor and even integrated systems. On this
second front, there has been remarkable progress in the area of microcavity devices with
large storage time (high optical quality factor). Here we report generation of highly coherent
microwaves using a chip-based device that derives stability from high optical quality factor.
The device has a record low electronic white-phase-noise floor for a microcavity-based
oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of
a photonic-based, microwave frequency synthesizer. The synthesizer performance is comparable
to mid-range commercial devices.

Microwave synthesizer using an on-chip Brillouin oscillator

Optical frequency division by using frequency combs has revolutionized time keeping and the generation of stable microwave signals. We demonstrate optical frequency division and microwave generation by using a tunable electrical oscillator to create dual combs through phase modulation of two optical signals that have a stable difference frequency. Phase-locked control of the electrical oscillator by means of optical frequency division produces stable microwaves. Our approach transposes the oscillator and frequency reference of a conventional microwave frequency synthesizer. In this way, the oscillator experiences large phase noise reduction relative to the frequency reference. The electro-optical approach additionally relaxes the need for highly linear photodetection of the comb mode spacing. As well as simplicity, the technique is also tunable and scalable to higher division ratios.

/pdf/electro-optical-frequency-division-and-stable-microwave-2ki1msk1xo.pdf

Electro-optical frequency division and stable microwave synthesis

Tunable dual-frequency oscillation is demonstrated in a vertical external-cavity surface-emitting laser. Simultaneous and robust oscillation of the two orthogonally polarized eigenstates is achieved by reducing their overlap in the optical active medium. The class-A dynamics of this laser, free of relaxation oscillations, enables one to suppress the electrical phase noise in excess that is usually observed in the vicinity of the beat note.

Experimental demonstration of a tunable dual-frequency semiconductor laser free of relaxation oscillations

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

The general field of the invention is that of fiber-optic sensors comprising at least one measurement optical fiber having an optically pumped doped amplifying medium, the optical characteristics of which are sensitive to a physical quantity, the fiber having at least one Bragg grating The fiber is designed so as to generate, in the amplifying medium, two optical waves having different optical frequencies that propagate in the same direction after reflection on the Bragg grating and are emitted by the amplifying medium, the two optical frequencies depending on the physical quantity The two waves may be generated using either a birefringent polarization-maintaining fiber or a DBR (Distributed Bragg Reflector) laser cavity Notably, this sensor may be used as a hydrophone

Self-referenced optical fibre sensor and related sensor network

The time-reversal (TR) protocol we implement in an erbium-doped YSO crystal is based on photon echoes but avoids the storage of the signal to be processed. Unlike other approaches implying digitizing or highly dispersive optical fibers, the proposed scheme reaches the μs range and potentially offers high bandwidth, both required for RADAR applications. In this Letter, we demonstrate faithful reversal of arbitrary pulse sequences with 6 μs duration and 10 MHz bandwidth. To the best of our knowledge, this is the first demonstration of TR via linear filtering in a programmable material.

/pdf/time-reversal-of-optically-carried-radiofrequency-signals-in-45t8rkztll.pdf

Time reversal of optically carried radiofrequency signals in the microsecond range.

The photodiode impact on the phase noise of an optical link, and particularly its ability to convert the laser amplitude noise into microwave phase noise, is studied and modeled. The model involves a nonlinear RC cell which describes a photogeneration delay which is a function of the optical power. The whole system is then implemented on a microwave CAD software, and the link gain and phase noise performance are computed.

/pdf/photodiode-nonlinear-modeling-and-its-impact-on-optical-12u5uvfpem.pdf

Photodiode nonlinear modeling and its impact on optical links phase noise

Interlaced spin grating is a scheme for the preparation of spectrospatial periodic absorption gratings in an inhomogeneously broadened absorption profile. It relies on the optical pumping of atoms in a nearby long-lived ground state sublevel. The scheme takes advantage of the sublevel proximity to build large contrast gratings with unlimited bandwidth and preserved average optical depth. It is particularly suited to Tm-doped crystals in the context of classical and quantum signal processing. In this paper, we study the optical pumping dynamics at play in an interlaced spin grating and describe the corresponding absorption profile shape in an optically thick atomic ensemble. We show that, in Tm:YAG, the diffraction efficiency of such a grating can reach $18.3%$ in the small-angle and $11.6%$ in the large-angle configuration when the excitation is made of simple pulse pairs, considerably outperforming conventional gratings.

Loïc Morvan

Papers

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

Self-referenced optical fibre sensor and related sensor network

Time reversal of optically carried radiofrequency signals in the microsecond range.

Photodiode nonlinear modeling and its impact on optical links phase noise

Interlaced spin grating for optical wave filtering