Fernández Ruiz, María del Rosario
(2016).
Time- and spectral-domain holography for high-speed
processing of optical signals.
Thèse.
Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en télécommunications, 212 p.
Résumé
The ever-increasing traffic data requirements in telecommunication services lead to a continuous
need for higher transmission capabilities. In this scenario, fiber-optic communications have proven to
be a very promising way of achieving such high bitrates. Nowadays, wavelength division multiplexing
(WDM) systems support transmission capabilities of about 10 Tbps, through multiplexing of several
hundred wavelengths with a single channel bit rate of ∼ 40 Gbps. For pre- and post- processing of
information, WDM requires opto-electronic signal conversion circuits individually set and operated
for each different wavelength channel; therefore this evolution leads to an impractical increase of
circuitry complexity and power consumption. Moreover, with the transmission-capacity increase
in WDM systems, coherent technologies have attracted a large interest over the recent years. The
motivation lies in finding methods for achieving the growing bandwidth demand with multilevel
complex modulation formats. To implement high-order complex (amplitude and phase) modulation
formats, the optical in-phase and quadrature (IQ) components of the information signal need to be
synthesized, processed and detected independently, additionally requiring proper synchronization
of these two IQ optical paths.
Therefore, in spite of the fact that the combination of WDM and coherent technologies enables a
more efficient use of the available spectrum, it also hinders the required circuitry in the transmitter,
receiver and intermediate network nodes. In this Thesis, we present and experimentally demonstrate
new concepts and signal processing techniques that remarkably simplify the required electro-optical
circuitry (and consequently the power consumption) in coherent optical systems. Furthermore, we
also develop new ultrafast all-optical signal processors, able to process the information directly in
the optical domain at ultrafast speeds (ideally, with speeds into the THz regime). These optical
processing systems are becoming increasingly important for a myriad of scientific and engineering
applications, including not only high-speed optical telecommunications but also optical computing
systems, ultrafast biomedical imaging, or ultrafast measurement and characterization systems.
Their fundamental goal is to avoid current electronic-based processing, which severely limits the
operation speeds below a few tens of GHz and entails a bottleneck for the effective use of the high
bandwidth intrinsic to optics.
The problem of simultaneously controlling the amplitude and phase of a complex electromagnetic
signal has long been solved in the spatial domain. Holography was developed as a lensless
interferometric imaging system that was able to record and subsequently reconstruct the original
complex-valued information signal, in spite of the recording medium being sensitive to intensity-only
variations. Holographic systems have been widely applied in a vast number of fields, such as 3D
imaging, spatial-domain signal processing, microscopy or security. The basics of classical (spatial
domain holography) are reviewed in Chapter 3, paying special attention to those concepts that will
serve as foundations for the original ideas presented through this work.
In this Thesis, we propose and formulate for the first time, to the best of our knowledge, the
exact time-domain counterpart of spatial domain holography, by means of the space-time duality.
This method, which is described in Chapter 4, enables simultaneous control of the amplitude and
phase of a temporal optical waveform with complex envelope using a simple setup composed of
devices sensitive to intensity-only or phase-only variations. To prove its effectiveness, several applications
of time-domain holography are experimentally demonstrated and the results are presented
in this dissertation. First, as a proof of concept, we demonstrate generation of complex-modulated
optical waveforms using a simple setup mainly composed of an electro-optical intensity modulator
and a band-pass filter. Then, these complex-envelope waveforms are detected (in amplitude
and phase) using a heterodyne scheme based on an intensity-only photodetector. Additionally, we
propose and implement a simplified scheme to perform electro-optical temporal phase conjugation.
This holographic method greatly simplifies previous electro-optical approaches, avoiding the need
for detection and subsequent processing of the phase of the optical signal prior to the electronicbased
conjugation process. Instead, the proposed approach uses intensity-only photodetection and
modulation components, combined with a band-pass filter, thus reducing the complexity and potential
cost of the setup, minimizing errors and simplifying the procedure. Finally, we demonstrate
wavelength conversion of complex-envelope optical signals based on time-domain holography. In
this case, an all-optical approach based on nonlinear cross-phase modulation is used. This technique
exhibits important advantages with respect to all previous approaches that typically use
four-wave mixing, as it avoids the stringent phase-matching condition and requires at least one
order of magnitude less power in the employed pump signals.
Using the Fourier-transform property of duality between the time domain and the frequency
domain, we also propose and formulate, for the first time, the concept of spectral-domain holography,
which is described in Chapter 5. This novel concept enables the simultaneous control of the
amplitude and phase of an optical spectral response by just manipulating the amplitude spectrum.
Spectral-domain holography is applied to the design of two kinds of signal processors. First, we
implement complex-valued and non-symmetrical optical pulse shaping using a scheme based on
time-domain spectral shaping, which achieves temporal resolutions in the sub-picosecond regime
but has been typically restricted to symmetric and intensity-only pulse shaping operations. In this
scheme, the modulating signal that performs the spectral shaping is a spectral hologram, enabling
the synthesis of complex-envelope output waveforms using a setup identical to that of previous
spectral shaping methods. The proposed methodology can be considered as the time-domain counterpart
of (spatial domain) Vander-Lugt filters. Then, we apply spectral-domain holography to the
implementation of non-minimum-phase optical pulse processors using fiber Bragg gratings (FBGs)
operating in transmission, which can be considered as optical linear filters with a minimum-phase
spectral response. In this case, the complex-valued spectral response of the target filter is encoded
in an amplitude-only spectral response (the spectral hologram). The use of FBGs operating in
transmission has well-known advantages with respect to the reflective configuration. In this Thesis,
we present and experimentally demonstrate an additional extraordinary advantage: an FBG
operating in transmission is able to implement signal processing functionalities with bandwidths
well in the THz regime (one order of magnitude higher than conventionally achieved bandwidths)
thanks to the degree of freedom available in choosing the spectral phase in reflection. In particular,
we propose the use of a quadratic spectral phase in reflection, which translates into a linear chirp,
allowing the increase of the grating’s operation bandwidth without increasing the grating spatial
resolution.
The novel concepts of time- and spectral-domain holography can be foreseen as powerful tools
for the development of new techniques for the generation, measurement and processing of ultrafast complex-envelope optical temporal waveforms. In this Thesis, we have demonstrated interesting
methods aimed at (i) simplifying the current required setup in coherent systems, and (ii) allowing
the implementation of simpler, arbitrary ultrafast optical signal processing devices, which are key
components for future, low power-consumption high-capacity telecommunication networks. Moreover,
the vast number of applications of spatial-domain holography allows us to predict a similar
broad range of applications for the time/spectral-domain holography.
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