Time- and spectral-domain holography for high-speedudprocessing of optical signals.

摘要

The ever-increasing traffic data requirements in telecommunication services lead to a continuousudneed for higher transmission capabilities. In this scenario, fiber-optic communications have proven toudbe a very promising way of achieving such high bitrates. Nowadays, wavelength division multiplexingud(WDM) systems support transmission capabilities of about 10 Tbps, through multiplexing of severaludhundred wavelengths with a single channel bit rate of ∼ 40 Gbps. For pre- and post- processing ofudinformation, WDM requires opto-electronic signal conversion circuits individually set and operatedudfor each different wavelength channel; therefore this evolution leads to an impractical increase ofudcircuitry complexity and power consumption. Moreover, with the transmission-capacity increaseudin WDM systems, coherent technologies have attracted a large interest over the recent years. Theudmotivation lies in finding methods for achieving the growing bandwidth demand with multileveludcomplex modulation formats. To implement high-order complex (amplitude and phase) modulationudformats, the optical in-phase and quadrature (IQ) components of the information signal need to beudsynthesized, processed and detected independently, additionally requiring proper synchronizationudof these two IQ optical paths.udTherefore, in spite of the fact that the combination of WDM and coherent technologies enables audmore efficient use of the available spectrum, it also hinders the required circuitry in the transmitter,udreceiver and intermediate network nodes. In this Thesis, we present and experimentally demonstrateudnew concepts and signal processing techniques that remarkably simplify the required electro-opticaludcircuitry (and consequently the power consumption) in coherent optical systems. Furthermore, weudalso develop new ultrafast all-optical signal processors, able to process the information directly inudthe optical domain at ultrafast speeds (ideally, with speeds into the THz regime). These opticaludprocessing systems are becoming increasingly important for a myriad of scientific and engineeringudapplications, including not only high-speed optical telecommunications but also optical computingudsystems, ultrafast biomedical imaging, or ultrafast measurement and characterization systems.udTheir fundamental goal is to avoid current electronic-based processing, which severely limits theudoperation speeds below a few tens of GHz and entails a bottleneck for the effective use of the highudbandwidth intrinsic to optics.udThe problem of simultaneously controlling the amplitude and phase of a complex electromagneticudsignal has long been solved in the spatial domain. Holography was developed as a lenslessudinterferometric imaging system that was able to record and subsequently reconstruct the originaludcomplex-valued information signal, in spite of the recording medium being sensitive to intensity-onlyudvariations. Holographic systems have been widely applied in a vast number of fields, such as 3Dudimaging, spatial-domain signal processing, microscopy or security. The basics of classical (spatialuddomain holography) are reviewed in Chapter 3, paying special attention to those concepts that willudserve as foundations for the original ideas presented through this work.udIn this Thesis, we propose and formulate for the first time, to the best of our knowledge, theudexact time-domain counterpart of spatial domain holography, by means of the space-time duality.udThis method, which is described in Chapter 4, enables simultaneous control of the amplitude andudphase of a temporal optical waveform with complex envelope using a simple setup composed ofuddevices sensitive to intensity-only or phase-only variations. To prove its effectiveness, several applicationsudof time-domain holography are experimentally demonstrated and the results are presentedudin this dissertation. First, as a proof of concept, we demonstrate generation of complex-modulatedudoptical waveforms using a simple setup mainly composed of an electro-optical intensity modulatorudand a band-pass filter. Then, these complex-envelope waveforms are detected (in amplitudeudand phase) using a heterodyne scheme based on an intensity-only photodetector. Additionally, weudpropose and implement a simplified scheme to perform electro-optical temporal phase conjugation.udThis holographic method greatly simplifies previous electro-optical approaches, avoiding the needudfor detection and subsequent processing of the phase of the optical signal prior to the electronicbasedudconjugation process. Instead, the proposed approach uses intensity-only photodetection andudmodulation components, combined with a band-pass filter, thus reducing the complexity and potentialudcost of the setup, minimizing errors and simplifying the procedure. Finally, we demonstrateudwavelength conversion of complex-envelope optical signals based on time-domain holography. Inudthis case, an all-optical approach based on nonlinear cross-phase modulation is used. This techniqueudexhibits important advantages with respect to all previous approaches that typically useudfour-wave mixing, as it avoids the stringent phase-matching condition and requires at least oneudorder of magnitude less power in the employed pump signals.udUsing the Fourier-transform property of duality between the time domain and the frequencyuddomain, we also propose and formulate, for the first time, the concept of spectral-domain holography,udwhich is described in Chapter 5. This novel concept enables the simultaneous control of theudamplitude and phase of an optical spectral response by just manipulating the amplitude spectrum.udSpectral-domain holography is applied to the design of two kinds of signal processors. First, weudimplement complex-valued and non-symmetrical optical pulse shaping using a scheme based onudtime-domain spectral shaping, which achieves temporal resolutions in the sub-picosecond regimeudbut has been typically restricted to symmetric and intensity-only pulse shaping operations. In thisudscheme, the modulating signal that performs the spectral shaping is a spectral hologram, enablingudthe synthesis of complex-envelope output waveforms using a setup identical to that of previousudspectral shaping methods. The proposed methodology can be considered as the time-domain counterpartudof (spatial domain) Vander-Lugt filters. Then, we apply spectral-domain holography to theudimplementation of non-minimum-phase optical pulse processors using fiber Bragg gratings (FBGs)udoperating in transmission, which can be considered as optical linear filters with a minimum-phaseudspectral response. In this case, the complex-valued spectral response of the target filter is encodedudin an amplitude-only spectral response (the spectral hologram). The use of FBGs operating inudtransmission has well-known advantages with respect to the reflective configuration. In this Thesis,udwe present and experimentally demonstrate an additional extraordinary advantage: an FBGudoperating in transmission is able to implement signal processing functionalities with bandwidthsudwell in the THz regime (one order of magnitude higher than conventionally achieved bandwidths)udthanks to the degree of freedom available in choosing the spectral phase in reflection. In particular,udwe propose the use of a quadratic spectral phase in reflection, which translates into a linear chirp,udallowing the increase of the grating’s operation bandwidth without increasing the grating spatialudresolution.udThe novel concepts of time- and spectral-domain holography can be foreseen as powerful toolsudfor the development of new techniques for the generation, measurement and processing of ultrafast complex-envelope optical temporal waveforms. In this Thesis, we have demonstrated interestingudmethods aimed at (i) simplifying the current required setup in coherent systems, and (ii) allowingudthe implementation of simpler, arbitrary ultrafast optical signal processing devices, which are keyudcomponents for future, low power-consumption high-capacity telecommunication networks. Moreover,udthe vast number of applications of spatial-domain holography allows us to predict a similarudbroad range of applications for the time/spectral-domain holography.