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Research topics

 

General research interests

 
  • Ultrafast photonics;
  • Optical signal processing;
  • Optical pulse shaping;
  • Ultrafast optical signal measurement;
  • Fourier optics and imaging;
  • All-fiber technologies;
  • Integrated waveguide devices;
  • Silicon photonics;
  • Fiber/waveguide Bragg and long-period gratings;
  • Optical telecommunications;
  • Optical computing;
  • Ultrafast information processing;
  • Light interferometry;
  • Ultra-broadband microwave engineering.

 

Overview of scientific contributions and ongoing research activities

 

 

  • Proposal and demonstration of innovative methods for time-domain optical signal processing using all-fiber and integrated-waveguide (silicon photonics) grating devices, including pulse repetition rate multiplication, (sub-)picosecond optical pulse shaping, high-speed advanced optical coding, ultrafast photonic temporal differentiation and integration and real-time optical Hilbert transformation.

 

  • Fundamental studies on space-time duality theory, e.g. innovative time-lens concepts, and proposal and demonstration of novel optical pulse processing techniques based on this theory, including temporal self-imaging (Talbot) phenomena (on periodic optical pulse trains), real-time Fourier transformation, simplified temporal imaging, temporal zone plates, and time-domain holograms.

 

  • Proposal and demonstration of novel photonic temporal signal processing methods based on time-frequency optical dualities, including spectral self-imaging phenomena (on periodic optical frequency combs) and time-to-frequency conversion.

 

  • Proposal and demonstration of novel architectures for programmable ultrafast optical pulse shaping and processing based on (i) concatenated interferometers, and (ii) all-fiber time-domain linear filtering.

 

  • Application of optical waveform processing/shaping techniques for ultrahigh-bit-rate optical switching, all-optical computing, nonlinear optics studies, ultrafast optical clock generation, and full characterization of complex, low-power ultrafast optical waveforms.

 

  • Proposal and development of innovative high-performance optical pulse interferometry schemes for full characterization of optical signals and devices, biomedical imaging, and laser range finding.

 

  • Proposal and development of innovative ultra-broadband microwave signal processors and generators using fully-electronic technologies (microstrip electromagnetic band-gaps) and incoherent light processing-based platforms.

 

A few samples of research contributions by our group

 

Time-Domain Holograms for Generation and Processing of

Temporal Complex Information by Intensity-Only Modulation Processes

 

The concept of time-domain holography has been introduced and analytically described for first time, to the best of our knowledge, which can be interpreted as a new milestone within the context of the so-called space-time duality. For this purpose, two main steps have been developed, namely, the signal recording stage and the signal retrieval stage (Fig. 1). In the signal recording stage, an interferogram is created by inserting the information signal and a reference CW signal in a heterodyne detection configuration. By applying holographic concepts, a dual photodetector is used, instead of the balanced photodecector typically used in heterodyne detection. Alternatively, the modulating interferogram signal can be also computationally designed for generation of a desired, user-defined optical complex waveform using the described single intensity-modulation scheme. This process can be interpreted as the time-domain counterpart of CGH (Fig. 1(a)). In the signal retrieval stage, the optical complex waveform is created by inserting the previously detected temporal hologram (or the computationally generated hologram created using an arbitrary waveform generator) as a modulating signal in an electro-optical (Mach-Zhender) modulator. At the output of the modulator, several terms spectrally separated appear, exactly as in the spatial-domain holography. One of these terms is an exact copy of the information signal, and can be easily filtered in (selected) using a band-pass optical filter (Fig. 1(b)).

This novel concept has been experimentally demonstrated through the generation and subsequent retrieval of user-defined, complex optical temporal waveforms using the predicted, simpler schemes based on time-domain equivalents of holographic concepts. In particular, Fig. 2 shows an example of the experimentally generated optical waveforms, namely, a 1024-symbol 3Gbps optical data stream under a 16-QAM-modulation format.

 

 

Temporal Zone Plates for Linear Optical Pulse Compression

 

 

Fig. 1 Space-time duality. (a) Light focusing by a spatial phase zone plate. (b) Pulse compression by a temporal phase zone plate. Fig. 2 Ideal and experimentally measured electronic waveforms for temporal phase modulation in the implemented temporal GPZPs for orders (a) n=1, (b) n=2, and (c) n=3. Fig. 3 Temporally compressed intensity waveforms in the ideal case, simulation, and experiment using temporal phase zone plates of orders (a) n=1, (b) n=2, and (c) n=3. (d)-(f) show a closer view of the compressed optical pulses in (a)-(c). All waveforms are represented in normalized units.


Temporal zone plates do not exhibit the limiting tradeoff between temporal aperture and frequency bandwidth (temporal resolution) of conventional linear time lenses. As a result, these zone plates can be ideally designed to offer a time-bandwidth product as large as desired, practically limited by the achievable temporal modulation bandwidth (limiting the temporal resolution) and the amount of dispersion needed in the target processing systems (limiting the temporal aperture). There are two kinds of temporal zone plates – temporal intensity/phase zone plates, which can be realized by temporal intensity/phase modulation. For example, temporal phase zone plates, which are time-domain analog of spatial phase zone plates, are illustrated in Fig.1.


To demonstrate the introduced temporal zone plate concept, we set up a linear optical pulse compression experiment, in which 1st-order, 2nd-order, and 3rd-order temporal phase zone plates are used. The electronic waveforms, which are used to drive the electro-optic phase modulator, are shown in Fig. 2. The temporal apertures for order 1, 2, and 3, which equal to the temporal duration of the electronic waveforms, are 1.88 ns, 3.85 ns, and 5.77 ns, respectively. The corresponding compressed pulse waveforms are shown in Fig. 3. There is a good agreement between numerical simulation (in which the experimental limitation is considered so that it is slightly deviated from ideal profile) and experiment. For the cases of order n=1, 2, 3, the FWHM of the compressed temporal pulses in the experiments are 38.6 ps, 23.9 ps, and 25.5 ps, respectively. Thus the time-bandwidth products for these three experiments are 50, 161, and 226, respectively. They are much higher than the time-bandwidth product (typically ? 5) for conventional time lens. The linear pulse compression experiments demonstrated here indicate that the temporal phase zone plate concept is a very promising approach to increase the energetic efficiency of previous schemes based on electro-optic time lenses by acting over longer input signal durations (higher input signal energies), leading to significantly increased output pulse peak powers.
 

 

 

Tsymbol/s Optical Signal Processing and Synthesis Based on

Superluminal Space-to-Time Mapping in Fiber Long Period Gratings (LPGs)

 

 

(a) Illustration of our proposed and experimentally demonstrated novel fiber-optics approach for ultrafast (~Tbit/s) optical code generation using long period gratings. The proposed concept of "superluminal space-to-time mapping in LPGs", i.e. v>>Light-Speed, opens up a promising new avenue to overcome the fundamental time-resolution limitations of present in-fiber and on-chip optical waveform generation (shaping) and processing devices, which are intrinsically limited by the achievable spatial resolution of fabrication technologies. The proposed approach enables processing/synthesis of optical waveforms with temporal features orders of magnitude faster (shorter) than those achievable using Bragg grating devices assuming the same practical spatial resolution limitations during grating fabrication. (c) Experimental demonstration details for 3.5 Tbit/s complex pulse data generation. For more details see the following references:


Theory of superluminal space-to-time mapping [1].
Tsymbol/s optical Coding [2,3].
THz-bandwidth optical Hilbert transformers [4].
THz-bandwidth optical temporal differentiators [5].

 

  1. R. Ashrafi, M. Li, S. LaRochelle, J. Azaña, “Superluminal space-to-time mapping in grating-assisted co-directional couplers,” Opt. Express, vol. 21, pp. 6249-6256 (2013).
  2. R. Ashrafi, M. Li, and J. Azaña, “Tsymbol/s optical coding based on long period gratings,” IEEE Photon. Technol. Lett., vol. 25, pp. 910-913 (2013).
  3. R. Ashrafi, M. Li, N. Belhadj, M. Dastmalchi, S. LaRochelle, J. Azaña, “Experimental demonstration of superluminal space-to-time mapping in long period gratings,” Opt. Lett., vol. 38, pp. 1419-1421 (2013).
  4. R. Ashrafi, J. Azaña, “THz-bandwidth real-time photonic Hilbert transformers based on long-period gratings,” Opt. Lett., vol. 37, pp. 2604-2606 (2012).
  5. R. Ashrafi, M. Li, and J. Azaña, “Coupling-strength-independent long-period grating designs for THz-bandwidth optical differentiators,” IEEE Photonics Journal, vol. 5, p. 7100311 (2013).

 

 

On-Chip Ultrafast Photonic Temporal Integrator

 

All-optical analog circuits for computing, information processing and networking could overcome the severe speed (i.e. bandwidth) limitations presently imposed by the use of electronics. However, in photonics, there are very few fundamental 'building blocks' equivalent to those used in electronics (e.g. differentiators, integrators, memory elements etc.) to build up complex circuits for advanced analysis, processing and computation. Realizing these photonic building blocks in a monolithic platform - ideally compatible with CMOS technology - represents a crucial step for the future development of ultrafast computing and information processing circuits on a chip. As a very relevant example, a temporal integrator is one of these fundamental analog signal-processing devices. Recently, we have reported the first monolithic photonic integrator based on a passive micro-ring resonator in a CMOS compatible platform, performing an unprecedented time-bandwidth product of ~100 (~200-GHz processing bandwidth and ~800-ps integration time window), see

 

 

Plot (a) shows a schematic of the used integrated micro-ring resonator device, illustrating its working principle as a photonic temporal integrator. In the plots (b), (c), and (d), the input/output system time-domain response is represented. More specifically, the cumulative time integral has been performed on different waveforms: i) the optical pulse directly generated by the laser source (b, inset-2); ii) a sequence of two pi-shifted pulses with a temporal delay of 275ps (c, inset); and iii) a strongly chirped optical pulse with a field-amplitude time duration around 1,340ps, corresponding to an intensity time-width of ~800ps (d, inset). This latter pulse was generated by propagation of the original laser pulse through a highly dispersive chirped fiber Bragg grating (FBG). The corresponding experimental (black curve) and theoretical (red curve) time integrals are represented in the main plots.  Inset b-2) depicts the experimental impulse response obtained by using a fast (~8ps) amplified photo-detector. 

 

 

Integrated Microwave Photonics Research

 

Fig. 1. Photo of a SOI chip including a set of WBGs (a). Zoom of the WBGs mask layout (b). Single WBG (c, d). SEM image of the strip waveguide with sidewall corrugations (e)

 

Fig. 2. Envisioned schematic of a general purpose reconfigurable MWP signal processor based on CDCs. EOM: electro-optic modulator; PD: photodetector.

 

Microwave photonics (MWP) signal processing has attracted a great deal of attention in recent years as an enabling technology for a number of functionalities not attainable by purely microwave solutions. Especially promising are the benefits expected by the transition towards a fully-integrated implementation of multiple MWP signal processing functionalities, represented by the rapidly expanding field of integrated microwave photonics (IMWP), using Photonic Integrated Circuits (PICs). This perspective provides numerous advantages especially in terms of performance, compactness, and potential low-cost when employing CMOS compatible fabrication equipment with high yield and low unit cost, as offered by the silicon photonics platform. A number of IMWP signal processor architectures have been reported to date.
In this context, integrated waveguide Bragg grating (WBG) devices constitute a particularly attractive approach thanks to their compactness and flexibility in producing arbitrarily defined amplitude and phase responses, by directly acting on coupling coefficient and perturbations of the grating profile.
Our research activities focus on the exciting possibilities offered by the silicon photonics platform in the field of MWP, potentially enabling integration of highly-complex active and passive functionalities with high yield on a single chip, with a particular focus on the use of WBGs as basic building blocks for linear filtering operations.
To date we reported several first-in-the-world demonstrations of novel RF signal processing functionalities based on the use of very simple uniform and non-apodized phase-shifted WBGs realized in collaboration with the University of British Columbia, the Institute of Semiconductors, Chinese Academy of Sciences and CMC Microsystems.

 

  • M. Burla, L. R. Cortés, M. Li, X. Wang, L. Chrostowski, and J. Azaña, “Integrated waveguide Bragg gratings for microwave photonics signal processing,” Opt. Express, vol. 21, pp. 25120-25147 (2013).
  • M. Burla, M. Li, L. Romero Cortés, X. Wang, L. Chrostowski, and J. Azaña, “2.5 THz Bandwidth On-Chip Photonic Fractional Hilbert Transformer based on a Phase-Shifted Waveguide Bragg Grating”, in Proc. of the 2013 IEEE Photonics Conference (IPC 2013), Bellevue, WA, USA, 9-13 Sept 2013, paper WD2.2.
  • M. Burla, L. Romero Cortés, M. Li, X. Wang, L. Chrostowski, and J. Azaña, “On-Chip Ultra-Wideband Microwave Photonic Phase Shifter and True Time Delay Line based on a Single Phase-Shifted Waveguide Bragg Grating”, in Proc. of the 2013 IEEE International Topical Meeting on Microwave Photonics (MWP 2013), Alexandria, VA, USA, Oct. 28-31, 2013.

 

 

Incoherent Lightwave-Based Ultra-broadband Microwave Signal Processing

 

 

A wide range of time-domain signal processing operations are based on the use of large amounts of group-velocity dispersion (GVD) over time-limited waveforms. Dispersion-engineering has been extensively used in the optical domain for applications such as real-time Fourier transformation (RTFT), real-time reflectometry and interferometry, pulse repetition rate multiplication, temporal imaging etc. Similar concepts have also proved extremely useful in other frequency regions, including the microwave domain. However, in the microwave domain, there is an urgent need for devices capable of inducing a large amount of GVD, namely above  a few ns2, over a wide frequency range (0 ~ 100 GHz). Plot (A) shows an schematic of a recently demonstrated fiber-optics incoherent-light configuration for inducing extraordinary GVD amounts on electrical (RF) waveforms (double-pulse in the illustration). This system is capable of fullfiling the above defined stringent GVD-bandwidth requirements. In the illustrated example, a GVD equivalent to that of 185,000km of standard single-mode fiber (SMF) is induced on the input microwave signal by propagation through a section of only 120km of SMF. (B) Results corresponding to an example of real-time Fourier transformation (linear frequency-to-time mapping) of a nanosecond-long input microwave signal with a full bandwidth approaching 20GHz (input signal shown in the top plot). The bottom plot shows the measured output temporal waveform (blue curve, bottom axis) as compared with the signal input spectrum (green curve, top axis). More details can be found at