Versatile shaping of optical frequency combs and applications

Optical frequency combs are a series of phase-locked and equidistant laser lines in the spectral domain. In the time domain, they correspond to periodic pulse trains by the Fourier relation. The ability to shape optical combs and pulse trains is pivotal to many applications in optical communications and microwave photonics. For communication purposes, the information can be multiplexed in either time or frequency, by associating the data symbols to the comb lines or optical pulses. In particular, shaping optical pulses into sinc profile is of special interest, as it achieves high spectral efficiency when multiplexed. In regards to microwave photonics, frequency combs also have emerged as useful tools for processing radiofrequency (RF) signals in parallel. Comb-based RF photonic filter is one of the examples. Optical combs combined with dispersive propagation could construct filtering functions in the RF domain. Additionally, the shaping of comb spectra enables reconfiguration of the synthesized RF photonic filters. The thesis presents results on various shaping techniques for the generation and applications of optical frequency combs. Both electro-optic combs and integrated microcombs are explored in the study, while their pulse shaping takes place either in the generation stage or externally. The first part of the thesis deals with optical sinc pulse shaping. A simple and flexible sinc pulse generator is demonstrated based on a single electro-optic modulator. Rectangular spectra of optical sinc pulses are harnessed to shape RF filters with sinc responses. Moreover, the method of sinc pulse shaping can fulfill the add-drop functionalities for superchannels multiplexed from sinc pulses. In the second part, temporal Talbot shaping of frequency combs is addressed. The temporal Talbot effect multiplies the repetition-rates of optical pulse trains in time. A novel temporal Talbot multiplier is demonstrated in a conventional optical tapped delay line structure. Furthermore, such shaping concept is extended for the demonstration of azimuthal Talbot effect. When the orbital angular momentum modes are superimposed with Talbot phases, the light petal is self-imaged in the azimuthal angle. Lastly, the third part exploits the internal shaping of soliton microcombs for the use in RF photonic filters. Versatile soliton states and thereby various microcomb spectra, are generated in a microresonator on demand. Such optical spectra could reconfigure their corresponding RF filters. Among others, perfect soliton crystals and two-soliton states are utilized, which respectively divide and translate their filter passband frequencies.

Optical Signal Processing and Pulse Shaping for Wavelength Multiplexed High Speed Communication Systems

Mohammad Amin Shoaie

The steady growth of capacity demand in telecommunication networks has sparked the development of various photonic devices for ultrafast optical signal processing functions to meet the requirements of future flexible fiber networks in general and backbone in particular. Although these photonic devices expand the electrical bandwidth operation, they mostly operate at single wavelength and hence remain non-viable solutions for practical implementation in WDMnetworks that are considered as the major technology for high speed communications. Another key challenge of future optical networks is the ability tomerge channels in time and frequency domain in the most efficient way in order to reach the theoretical Nyquist limit of transmission links. A promising technique is the use of sinc-shaped Nyquist pulses that enable multiplexing channels in time domain with no inter-symbol interference (ISI) while exhibiting a rectangular spectrumthat alleviates the need for guard-band. The sinc pulse is indeed the basic building block in most theoretical papers that have estimated overall capacity limits, and intense efforts are being made to generate optical Nyquist pulses beyond the limit of electronics that can directly be used at the physical layer. Within the above context, two approaches, referred to as optical signal processing of WDM networks and generation/detection of Nyquist superchannels, have been studied in this thesis. The first addressed problem is simultaneous signal processing of WDMchannels. We present two principal blocks required for routing and transporting data in WDM networks, both based on dual-pump fiber optical parametric amplifier (FOPA) with (sinusoidally) modulated pumps. We show that this scheme can be designed to operate simultaneously on WDM channels at any desired wavelength range. The former block enables simultaneous wavelength conversion and time compression which is a necessary functionality in connecting dissimilar rate WDM networks. The latter processing block is all-optical 3R regeneration (reamplification, reshaping, retiming) which is crucial for maintaining pulse quality along long-haul WDMlinks. We use theoretical analysis supported by experimental results to demonstrate the efficiency of the proposed technique. The second problem that we investigate is the generation and detection of WDM-Nyquist superchannels. We developed a simple technique based onMach-Zehnder modulators (MZM) to generate a sinc-shaped Nyquist time window by direct synthesis of a rectangular, phase locked frequency comb. We show the produced pulses have exceptional quality as well as high tunability in terms of pulse width and repetition rate. We also further demonstrate a noncoherent method based on the proposed technique to performreal-time demultiplexing of WDM-Nyquist superchannels, simultaneously in time and frequency. The experimental results that are proved by mathematical analysis are employed to demonstrate the effectiveness of the proposed methods.

EPFL2016

Wave propagation in periodic buckled beams

Florian Paul Robert Maurin

Folding of the earth's crust, wrinkling of the skin, rippling of fruits, vegetables and leaves are all examples of natural structures that can have periodic buckling. Periodic buckling is also present in engineering structures such as compressed lattices, cylinders, thin films, stretchable electronics, tissues, etc., and the question is to understand how wave propagation is affected by such media. These structures possess geometrical nonlinearities and intrinsic dispersive sources, two conditions which are necessary to the formation of stable, nonlinear waves called solitary waves. These waves are particular since dispersive effects are balanced by nonlinear ones, such that the wave characteristics remain constant during the propagation, without any decay or modification in the shape. It is the goal of this thesis to demonstrate that solitary waves can propagate in periodic buckled structures. This manuscript focuses specifically on periodically buckled beams that require either guided or pinned supports for stability purposes. Buckling is initially considered statically and investigations are made on stability, role played by imperfections, shape of the deflection, etc. Linear dispersion is analyzed employing the semi-analytical dispersion equation, a new method that relates the frequency explicitly to the propagation constant of the acoustic branch. This allows the quantification of the different dispersive sources and it is found that in addition to periodicity, transverse inertial and coupling effects are playing a dominant role. Modeling the system by a mass-spring chain that accounts for additional dispersive sources, homogenization and asymptotic procedures lead to the double-dispersion Boussinesq equation. Varying the pre-compression level and the support type, the main result of this thesis is to show that four different waves are possible, namely compressive supersonic, rarefaction (tension) supersonic, compressive subsonic and rarefaction subsonic solitary waves. For high-amplitude waves, models based on strongly-nonlinear PDEs as the one modeling wave propagation in granular media (Hertz power law) are more appropriate and adaptation of existing work is done. Analytical model results are then compared to finite-element simulations of the structure and experiments, and are found in excellent agreement. In this thesis, in addition to the semi-analytical dispersion equation, two other new methods are proposed. For periodic structures by translation with additional glide symmetries (e.g. buckled beams), Bloch theorem is revisited and allows the use of a smaller unit cell. Advantages are dispersion curves easier to interpret and computational cost reduced. Finally, the last contribution of this thesis is the use of NURBS-based isogeometric analysis (IGA) to solve the extensible-elastica problem requiring at least C1-continuous basis functions, which was not possible before with classical finite-element methods. The formulation is found efficient to solve dynamic problems involving slender beams as buckling.

EPFL2015

Monolithic piezoelectric control of soliton microcombs

Miles Henry Anderson, Jijun He, Tobias Kippenberg, Junqiu Liu, Tianyi Liu, Erwan Guillaume Albert Lucas, Arslan Sajid Raja, Rui Ning Wang, Wenle Weng

High-speed actuation of laser frequency(1)is critical in applications using lasers and frequency combs(2,3), and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis(4), frequency division(5)and optical clocks(6). Soliton microcombs(7,8)have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations(9,10). Yet integrated microcombs using thermal heaters have limited actuation bandwidths(11,12)of up to 10 kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components(13). By monolithically integrating AlN actuators(14)on ultralow-loss Si(3)N(4)photonic circuits(15), we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz-substantially exceeding bulk piezo tuning bandwidth-that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb's carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine(16,17)for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances(18), resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal-oxide-semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs. By monolithically integrating piezoelectric actuators on ultralow-loss photonic circuits, soliton microcombs-a spectrum of sharp lines over a range of optical frequencies-can be modulated at high speeds with megahertz bandwidths.

NATURE PUBLISHING GROUP2020