Reza Ashrafi

Fiber-Based Photonic Generation of High-Frequency Microwave Pulses With Reconfigurable Linear Chirp Control

A fiber-based approach for linear and continuous chirp control in photonics microwave pulse generation is proposed and demonstrated. The technique is based on spectral pulse shaping combined with nonlinear frequency-to-time mapping (FTM). Particularly, the proposed fiber-optics method provides an unprecedented arbitrary chirp-rate control by combining spectral shaping induced by dispersion unbalance in a fiber interferometer, resulting in a nonlinear-chirp frequency interference pattern, with nonlinear FTM. Full linear chirp reconfigurability, including positive, negative and zero chirp rates, is achieved from a single fiber-optics platform by simply varying the relative delay between the interferometer arms. Moreover, a simple balanced photodetection strategy is implemented to achieve dc-free microwave pulses with significantly improved noise figures. High-quality dc-free gigahertz-frequency microwave sinusoids were successfully generated with central frequencies ranging from 100 MHz to 25 GHz and chirp rates ranging from -160 MHz/ns to +160 MHz/ns.

Single-shot photonic time-intensity integration based on a time-spectrum convolution system

Real-time and single-shot ultra-fast photonic time-intensity integration of arbitrary temporal waveforms is proposed and demonstrated. The intensity-integration concept is based on a time-spectrum convolution system, where the use of a multi-wavelength laser with a flat envelope, employed as the incoherent broadband source, enables single-shot operation. The experimental implementation is based on optical intensity modulation of the multi-wavelength laser with the input waveform, followed by linear dispersion. In particular, photonic temporal intensity integration with a processing bandwidth of 36.8 GHz over an integration time window of 1.24 ns is verified by experimentally measuring the integration of an ultra-short microwave pulse and an arbitrary microwave waveform.

Terahertz bandwidth all-optical Hilbert transformers based on long-period gratings.

A novel, all-optical design for implementing terahertz (THz) bandwidth real-time Hilbert transformers is proposed and numerically demonstrated. An all-optical Hilbert transformer can be implemented using a uniform-period long-period grating (LPG) with a properly designed amplitude-only grating apodization profile, incorporating a single π-phase shift in the middle of the grating length. The designed LPG-based Hilbert transformers can be practically implemented using either fiber-optic or integrated-waveguide technologies. As a generalization, photonic fractional Hilbert transformers are also designed based on the same optical platform. In this general case, the resulting LPGs have multiple π-phase shifts along the grating length. Our numerical simulations confirm that all-optical Hilbert transformers capable of processing arbitrary optical signals with bandwidths well in the THz range can be implemented using feasible fiber/waveguide LPG designs.

Active Fabry-Perot cavity for photonic temporal integrator with ultra-long operation time window

In this paper, a photonic temporal integrator based on an active Fabry-Perot (F-P) cavity is proposed and theoretically investigated. The gain medium in the F-P cavity is a semiconductor optical amplifier (SOA) with high gain coefficient. Key feature of the proposed photonic integrator is that the length of integration time window is widely tunable and could be ideally extended to infinitely long when the injection current is approaching lasing condition. Based on an F-P cavity with practically feasible parameters, a photonic temporal integrator with an integration time window of 160 ns and an operation bandwidth of 180 GHz is achieved. The time-bandwidth product of this photonic temporal integrator is 28,800, which is about two-orders of magnitude higher than any previously reported results. Gain recovery effect has been also considered and analyzed for the impact on performance of the photonic integrator, followed by the simulation results of the impact of gain recovery.

Ultrafast Optical Arbitrary-Order Differentiators Based on Apodized Long-Period Gratings

We propose a novel, optimized design for arbitrary-order optical differentiation based on a uniform-period especially apodized long-period fiber or waveguide grating (LPG) operated in transmission. We show that the LPG solution can be optimized to utilize the entire grating resonance bandwidth for optical differentiation by properly customizing the LPG apodization profile through a discrete inverse-scattering grating synthesis technique. This strategy leads to a significantly increased processing speed and a maximized energetic efficiency as compared with previous unapodized LPG-based optical differentiator designs. As an example, optimized first-, second-, and third-order optical differentiators are designed using apodized LPGs implemented in standard single-mode fiber (SMF). The designed passive devices are practically feasible and offer an unprecedented operation bandwidth of 12 THz, which is capable of accurately processing time features as short as ~100 fs, and an optimal energetic efficiency, which reaches a peak power spectral response of nearly 100% within their operation band.

Figure of merit for photonic differentiators

We introduce a universal figure of merit to evaluate the processing speed (operation bandwidth) performance of arbitrary-order optical differentiators. In particular, we define the maximum-to-minimum bandwidth ratio (MMBR) as a main figure of merit of these devices, which essentially informs about the broadness of the acceptable input pulse bandwidth range. We derive and numerically confirm a general analytical expression for the MMBR of an arbitrary optical differentiator, showing that this can be expressed simply as a function of the differentiators amplitude resonance depth. The device MMBR can be improved by increasing the filters resonance depth, depending also on the differentiation order; in particular, the MMBR quickly deteriorates as the differentiator order is increased. In our analysis, photonic differentiators are considered in two main groups, namely (i) non-minimum phase and (ii) minimum phase optical filtering implementations. The derived analytical expression for the device MMBR is generalized for these two different solutions, and the validity of the obtained analytical estimates is verified through numerical simulations, including results for the cases of 1st-, 2nd-, and 3rd-order differentiators.

Experimental demonstration of superluminal space-to-time mapping in long period gratings

We experimentally demonstrate a superluminal space-to-time mapping process in grating-assisted (GA) codirectional coupling devices, particularly fiber long period gratings (LPGs). Through this process, the grating complex (amplitude and phase) apodization profile is directly mapped into the devices temporal impulse response. In contrast to GA counterdirectional couplers, e.g., Bragg gratings, this mapping occurs with a space-to-time scaling factor that is much higher than the propagation speed of light in vacuum. This phenomenon has been used for synthesizing customized complex optical pulse data sequences with femtosecond features (3.5 Tbit/s data rate) using readily feasible fiber LPG designs, e.g., with subcentimeter resolutions.

Superluminal space-to-time mapping in grating-assisted co-directional couplers.

A superluminal space-to-time mapping process is reported and numerically validated in grating-assisted (GA) co-directional couplers, e.g. fiber/waveguide long-period gratings (LPGs). We demonstrate that under weak-coupling conditions, the amplitude and phase of the grating complex apodization profile of a GA co-directional coupling device can be directly mapped into the devices temporal impulse response. In contrast to GA counter-directional couplers, this mapping occurs with a space-to-time scaling factor that is much higher than the propagation speed of light in vacuum. This phenomenon 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. We numerically demonstrate the straightforward application of the phenomenon for synthesizing customized femtosecond-regime complex optical waveforms using readily feasible fiber LPG designs, e.g. with sub-centimeter resolutions.

Subwavelength grating enabled on-chip ultra-compact optical true time delay line

An optical true time delay line (OTTDL) is a basic photonic building block that enables many microwave photonic and optical processing operations. The conventional design for an integrated OTTDL that is based on spatial diversity uses a length-variable waveguide array to create the optical time delays, which can introduce complexities in the integrated circuit design. Here we report the first ever demonstration of an integrated index-variable OTTDL that exploits spatial diversity in an equal length waveguide array. The approach uses subwavelength grating waveguides in silicon-on-insulator (SOI), which enables the realization of OTTDLs having a simple geometry and that occupy a compact chip area. Moreover, compared to conventional wavelength-variable delay lines with a few THz operation bandwidth, our index-variable OTTDL has an extremely broad operation bandwidth practically exceeding several tens of THz, which supports operation for various input optical signals with broad ranges of central wavelength and bandwidth.

Coupling-Strength-Independent Long-Period Grating Designs for THz-Bandwidth Optical Differentiators

A novel design of THz-bandwidth all-optical arbitrary-order temporal differentiators using long period fiber/waveguide gratings (LPGs) is proposed and numerically demonstrated. The proposed technique is based on the first-order Born approximation approach in LPGs. We show that an N th-order optical differentiator can be implemented based on an LPG incorporating N π-phase shifts along its length and operating in the cross-coupling mode. The proposed design has a strong tolerance against practical fluctuations in the grating parameters, e.g., as induced during the fabrication process or by environmental fluctuations. In particular, this LPG design solution is essentially insensitive to variations in the grating coupling strength.