mohammad h. asghari

Research Interests

 Optical signal processing and its applications in applied and fundamental sciences, silicon photonics, optical signal characterization, optical computing, fiber-optics telecommunications, ultrafast imaging and optics for biomedical applications.

A. Wideband data conversion

Stereopsis-Inspired Time-Stretched Amplified Real-Time Spectrometer (STARS)

Wideband analog-to-digital conversion is one of the most critical problems faced in communication, instrumentation, and radar systems. Complex-field characterization of optical signals with durations ranging from sub-picoseconds to tens of nanosecond durations is also highly desired for a variety of applications from telecommunications to information processing and applied sciences. Solutions based on nonlinear optical gating or spectral shearing interferometry require high intensity signals hence they do not offer high sensitivity. In addition, they are well suited for characterization of ultrashort optical signals (e.g. femtosecond and picosecond pulses) while it is challenging to employ these techniques for characterization of optical signals with longer durations. Referencing methods for characterization of optical waveforms have been used to measure optical signals with durations in ns regime, however in these applciations a highly stable reference well synchronized with the signal under test is required which significantly increases their complexity.

In this work, we introduced and experimentally demonstrated a single-shot real-time optical vector spectrum analyzer (VSA). This simple and powerful instrument combines amplified dispersive Fourier transform with stereopsis reconstruction algorithm and is inspired by binocular vision in biological eyes. Moreover, a dynamic time-stretch concept is employed to dramatically enhance the phase reconstruction accuracy and dynamic range for ultrashort optical signals (> 30 times). We show that using a non-iterative analytical expression, the phase profile of the input signal can be reconstructed using intensity-only measurements. The proposed method is experimentally proved by fully characterizing the time-varying amplitude and phase of single-shot THz-bandwidth optical signals, with durations ranging from sup ps to 35,000 ps, with ultra-small to ultra-large temporal phase variations and at 25 MHz update rate. We have also used this instrument to characterize the amplitude and phase of a pre-chirped 40 Gbps DQPSK optical signal using a 1.5 GHz digitizer and without using a reference signal.

 

Reconstructed group delay (GD) profiles (solid lines) of signal under test  for different evaluated dispersion compensating fiber (DCF) sections compared to the corresponding simulated ideal GD profiles (dotted lines).  Reconstructed energy spectrum of the SUT is also plotted in the inset.

Experimental setup for single-shot and real-time demodulation of optical telecommunication signals using the proposed method. PC: Polarization controller, RF: Radio frequency.

Real-time single-shot complex-field characterization of 40 Gbps QPSK optical signal using a 1.5 GHz electronic digitizer using our method.

 

References:

Mohammad H. Asghari and Bahram Jalali, "Stereopsis-inspired time-stretched amplified real-time spectrometer (STARS)",  IEEE Photonics Journal, Vol. 4, pp. 1693-1701 (2012), Invited.

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B. Fastest memory unit in the world

Photonic Integrator-Based Optical Memory Unit

Ultra high-speed loadable and erasable optical memory units offering ultrafast (i.e. picosecond) switching/transition times and ultra-long (i.e. nanosecond) life-times are increasingly required for many applications, including ultra-fast all-optical computing and information processing circuits, and in ultra-high bit-rate optical telecommunications. A critical common drawback of previously demonstrated optical memory concepts concerns their limited operation bandwidth; the fastest optical memory units reported to date are limited to switching times longer than ~20ps (operation bandwidths < 50GHz).
In this work, we proposed and experimentally demonstrated an ultrahigh-speed loadable and erasable linear optical memory unit with switching times down to the picosecond regime, corresponding to effective operation bandwidths of a few hundreds of GHz. Our proposal is based on a conceptually novel design for a 1-bit optical memory unit using an ultra-fast photonic time integrator. As a proof-of-concept experiment, a 1-bit fiber Bragg grating based optical memory element is implemented, demonstrating an unprecedented switching time of ~1.4ps (~700 GHz speed).

Conceptual working principle of a memory unit.
 

Experimental proof-of-concept setup for the fastest memory unit in the world (700 GHz speed).

 

Experimentally measured normalized intensities of the input and output signals from the optical memory unit characterized with FTSI technique : (a) to (c): dashed lines: input Set (S) and Reset (R) pulses with different relative time delays; solid lines: corresponding output signals from the optical memory unit. Inset in (c) is zoomed plot over the time interval between 64ps to 75ps.

References:

Mohammad H. Asghari and Jose Azana, "Photonic integrator-based optical memory unit," IEEE Photonics Technology Letters, Vol. 23, pp. 209-211 (2011).

Mohammad H. Asghari, Yongwoo Park, and Jose Azana, "New design for photonic temporal integration with combined high processing speed and long operation time window," Optics Express, Vol. 19, pp. 425-435 (2011).

Laser Focus World (LFW) magazine review: Click here

 

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C. Designing photonic devices

High-Order Passive Photonic Temporal Integrators

An Nth-order temporal integrator (where N = 1, 2, 3 . . . refers to the integration order) is a device that calculates the Nth cumulative time integral of an input signal. Temporal integrators are fundamental basic blocks in many signal processing operations of interest, e.g., in computing, control, and communication networks. As compared with their electronic counterparts, photonic temporal integrators can provide much higher operation bandwidths, i.e. higher processing speeds. Photonic temporal integrators have already been proposed for various interesting applications, including ultrafast pulse shaping and all-optical memories. Higher order integrators are key building blocks in a large number of signal processing circuits. A relevant example of application of these devices is that of computing systems devoted to solving ordinary differential equations (ODEs). Linear ODEs can be solved in real-time using a suitable combination of first and high-order integrators, adders and multipliers. Realizing these operations all-optically would translate into processing speeds well beyond the reach of present electronic digital or analog computing solutions.

We experimentally demonstrate, for the first time, an ultrafast photonic high-order (second-order) complex-field temporal integrator. The fabricated device uses a single apodized uniform-period fiber Bragg grating (FBG) and it is based on a general FBG design approach for implementing optimized arbitrary-order photonic passive temporal integrators. Using this same design approach, we also fabricate and test a first-order passive temporal integrator offering an energetic-efficiency improvement of more than one order of magnitude as compared with previously reported passive first-order temporal integrators. Accurate and efficient first and second-order temporal integrations of ultrafast complex-field optical signals (with temporal features as fast as ~2.5-ps) are successfully demonstrated using the fabricated FBG devices.

Experimental setup for temporal characterization of the fabricated FBG-based first and second-order ultrafast photonic temporal integrators.

Reflection temporal responses to the input optical signal with the measured temporal envelope plotted in (a). Experimentally recovered amplitude (blue solid line) and phase (green dotted line) temporal profiles of the reflected optical waveform from (b) first-order and (c) second-order temporal integrator, compared to the ideal output amplitude profile, i.e. numerical first-order and second-order cumulative time integration of the signal in (a) (red circles). The operation time window in each case is indicated with a gray hatched box.

 

References:

Mohammad H. Asghari, Chao Wang, Jianping Yao and Jose Azana, "High-order passive Photonic temporal integrators," Optics Letters, Vol. 35, pp. 1191-1193 (2010).

Mohammad H. Asghari and Jose Azana, "On the design of efficient and accurate arbitrary-order temporal optical integrators using fiber Bragg gratings ," IEEE/OSA Journal of Lightwave Technology, Vol. 27, pp. 3888-3895 (2009)

Mohammad H. Asghari and Jose Azana, "Proposal and analysis of a reconfigurable pulse shaping technique based on multi-arm optical differentiators ", Optics Communications, Vol. 281, Issue 18, 15, pp. 4581-4588 (2008).

Mohammad H. Asghari and Jose Azana, "Design of all-optical high-order temporal integrators based on multiple-phase-shifted Bragg gratings ," Optics Express, Vol. 16, Issue 15, pp. 11459-11469 (2008).

Mohammad H. Asghari and Jose Azana, "Proposal for arbitrary-order temporal integration of ultrafast optical signals using a single uniform-period fiber Bragg grating ", Optics Letters, Vol. 33, Issue 13, pp. 1548-1550 (2008).

Laser Focus World (LFW) magazine review: Click here

 

All-optical Hilbert Transformer Based on a Single Phase-Shifted Fiber Bragg Grating

A time-domain Hilbert transformer also referred to as a quadrature filter or a wide-band π phase shifter, is a device that calculates the Hilbert transform of an arbitrary input temporal signal. Hilbert transformers can be routinely implemented in the electronic domain, either as analog or digital filters, and they are fundamental devices for numerous applications, e.g. in communications, computing, information processing, signal analysis and measurement etc.. A similar range of applications could be expected for a photonic implementation of the Hilbert transformer, i.e. photonic Hilbert transformer (PHT), with the essential difference that such a device would enable processing signals directly in the all-optical domain and at speeds (operation bandwidths) well beyond the reach of electronic technologies. Previous microwave PHT solutions are limited to operation bandwidths < 40GHz.

In this work, the first all-fiber design for implementing an all-optical temporal Hilbert transformer is proposed and numerically demonstrated. We show that an all-optical Hilbert transformer can be implemented using a uniform-period fiber Bragg grating (FBG) with a properly designed amplitude-only grating apodization profile and a single π phase-shift in the middle of the grating length. All-optical Hilbert transformers capable of processing arbitrary optical waveforms with bandwidths up to a few hundreds of GHz can be implemented using readily feasible FBGs.

Schematic diagrams showing the base-band spectral transfer function (a) and envelope of the temporal impulse response (b) of an ideal PHT (dotted curves) and the physically realizable PHT proposed in this paper (solid curves).

 

Spectral and temporal responses of a Photonic Hilbert transformer (PHT) based on a 2-cm long mid-strength uniform-period FBG with the apodization profile plotted in the inset of (b): (a) Reflectivity as a function of optical frequency deviation (around 193THz); the insets show the phase change of the reflection FBG spectral response and corresponding amplitude in dB around the central frequency; (b) Envelope amplitude of the reflection temporal impulse response (solid curve), shown in normalized units [n.u.]. The impulse response amplitude of an ideal bandwidth-limited PHT is also shown (red circle dots).

 

References:

Mohammad H. Asghari and Jose Azana, "All-optical Hilbert transformer based on a single phase-shifted fiber Bragg grating: design and analysis ," Optics Letters, Vol. 34, pp. 334-336 (2009).

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D. Optical signal characterization

Complex-Field Measurement of Dynamic Optical Waveforms based on Real-Time Spectral Interferometry

Future progress in a wide range of fields essentially depend on the development of improved temporal waveform measurement methods, capable of providing the stringent performance that is required to capture ultrafast phenomena, i.e. with resolutions down to the sub-picosecond range, in an entirely dynamic fashion, i.e. as these phenomena evolve at ultrahigh speeds. The capability of performing such advanced measurements is important for applications in which random (non-repetitive), rapidly-changing ultrafast waveforms need to be fully characterized and evaluated. These include real-time monitoring in ultrahigh-bit-rate optical telecommunication, computing and information processing systems; testing of electronic and photonic materials, devices and sub-systems; and observation and analysis of a large variety of ultrafast dynamic events in physics, biology, chemistry etc.

Several methods are now available for single-shot measurement of the complex field (amplitude and phase profiles) of optical waveforms with resolutions down to the sub-picosecond range. As a main critical limitation, all these techniques exhibit measurement update rates typically slower than a few Hz. It would be very challenging to directly upgrade the update rate of any of these available methods beyond a few kHz. By combining spectral interferometry with dispersion-induced real-time optical Fourier transformation, here we demonstrate single-shot complex-field measurements of optical waveforms with a resolution of ~400 fs over a record length as long as ~350 ps, corresponding to a large record-length-to-resolution ratio of ~900. This performance is achieved at measurement updated rate of ~17 MHz, i.e. at least one thousand times faster than with any previous single-shot complex-field THz-bandwidth optical signal characterization method.

Experimental setup used for demonstration of the proposed real-time and single-shot complex-field optical signal measurement method, specifically illustrating the use of a balanced temporal interleaving scheme to physically suppress the DC component of the measured interference pattern.

 

Real-time and single-shot complex-field characterization of an interference optical signal (SUT) having a time-bandwidth product of ~900: Recovered amplitude (a) and phase (b) time-domain profiles of the experimentally measured SUT (blue, solid) compared to the theoretically simulated phase profile (red, dashed). Insets are zoomed plots over the time interval between 210 ps to 220 ps. The measured spectral amplitude of the optical SUT is plotted in the inset of (a).

 

(a) Experimental setup for generating rapidly-changing ultrafast optical signals by intensity modulation of dispersed broadband pulses using an electro optic modulator (EOM) driven by a synchronized train of electronic pulses in which the DC bias level is rapidly swept (the bias is driven by a 1.6-MHz electrical sinusoids). Amplitude (b) and phase (c) time profiles of 30 rapidly-changing ultrafast waveforms as measured at the EOM output with an update rate of ~17 MHz, expanding over a total duration of ~1.773μs. Results corresponding to the individual characterization of 3 of these ultrafast waveforms at the measurement times of 236.4 ns, 354.6 ns and 827.4 ns are plotted in (d).

 

References:

Mohammad H. Asghari, Yongwoo Park and Jose Azana, "Complex-field measurement of ultrafast dynamic optical waveforms based on real-time spectral interferometry," Optics Express, Vol. 18, pp. 16526-16538 (2010).

Laser Focus World (LFW) magazine review: Click here

 

 

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E. Wideband Microwave Photonics

Photonic Temporal Integration of Broadband Intensity Waveforms over Long Operation Time Windows

A temporal integrator is a block that gets an arbitrary time-domain signal at its input port and generates the cumulative temporal integration of this input waveform at the output port. Temporal integrators are fundamental devices for implementing a wide variety of signal processing operations of interest, e.g., in computing, control, and communication networks [1]. In contrast to complex-field ("coherent") photonic temporal integrators, a photonic intensity integrator (i.e. "incoherent" optical integrator) operates on the time-domain intensity of the incoming waveform. As such, this device is particularly attractive for processing broadband microwave/millimetre-wave. Intensity photonic integrator (operating in tens of GHz bandwidth regime) is the bridge for temporal integration from electronic to all-optical processing speeds and it fills the empty processing bandwidth range between the electronic and all-optical solutions. Intensity integrators can enable ultrafast signal processing blocks with higher processing speeds (and cheaper) than their current electronic counterparts and still compatible with electronic circuits.

In this work, we proposed and experimentally demonstrated a new design for implementing a photonic intensity integrator simultaneously offering a high processing speed and a long operation time window. Our proposal is based on cascading a discrete-time photonic integrator and a high-speed analog time-limited photonic intensity integrator, both implemented in fiber-optics platforms. We have extended the operation time window of a time-limited intensity integrator with a ~36 GHz processing speed by eight times, from ~0.5 ns (original value) to ~ 4 ns, demonstrating the largest processing time bandwidth product TBP >144 ever reported for high-speed photonic intensity integrators, also significantly outperforming available electronic technologies.

Conceptual diagram of the proposed ultrafast photonic intensity integrator design through illustration of its temporal impulse response.

 

Frequency transfer function of the experimentally demonstrated new photonic intensity integrator (solid line) compared to an ideal photonic intensity integrator  (circles).

 

Experimentally measured temporal response (solid lines) of the proposed photonic intensity integrator to (top plot) an intensity double pulse and (bottom plot) a microwave signal with quadratic chirp (dotted lines). Ideal outputs are plotted with circles.

References:

Mohammad H. Asghari, Yongwoo Park, and Jose Azana, "Photonic temporal integration of broadband intensity waveforms over long operation time windows", Optics Letters, Vol. 36, pp. 3557-3559 (2011).

 

Implementation of Broadband Microwave Arbitrary-Order Time Differential Operators using a Reconfigurable Incoherent Photonic Processor

Photonic differentiators are basic building blocks for all-optical signal processing and computing capable of overcoming the processing speed limitations of equivalent conventional electric circuits. Coherent differentiation has shown to be of scientific and practical significance for a wide range of applications, including generation of high-order Hermite-Gaussian pulse waveforms, ultrashort pulse shaping, and direct phase reconstruction of arbitrary optical signals. On the other hand, incoherent photonic differentiators, operating on intensity time waveforms, have proven particularly useful for ultra-wide band (UWB) microwave signal generation and processing. This last type of photonic differentiators can be considered as a direct all-optical equivalent to conventional electronic temporal differentiators since amplitude-only signals are actually processed in the electronic domain. This is potentially important to ensure full compatibility with electronic solutions in future photonic-based ultra-high-speed information processing and computing applications. High-order intensity differentiation (HOD) is also of significant interest for a range of applications, such as reconfigurable UWB pulse shaping based on the combination of differently weighted HODs and implementation of arbitrary differential equation operators. The latter application is particularly interesting because one could for instance implement differential operators modeling the behavior of ultrahigh-frequency RLC processing circuits. This equivalence could be exploited in many different ways. For instance, classical filter design methods could be directly used for designing microwave photonic filters e.g. aimed at realizing different signal processing functionalities on microwave signals.

In this work, we discussed and experimentally demonstrated the design of a reconfigurable incoherent optical signal processing system aimed to implement arbitrary differential equation operators; we show that any differential operator, including first and high-order time differentiators, can be implemented using the same programmable platform. Targeting the implementation of a fully reconfigurable photonic signal processing platform, we propose the use of a discrete-time microwave photonic filtering scheme in which the desired differentiation operator can be embedded in the source power spectrum as determined by the corresponding finite-difference-time-domain (FDTD) equations based on the well-known Euler's approximation. We note that a backward time-difference scheme based on the Taylor's series expansion has been previously used for coherent temporal differentiation and both are commonly used approaches for approximating the differentiations of a given temporal function. The successive photonic time-derivatives of Gaussian-like pulse intensity waveforms as short as 40-ps (full-width-at-half-maximum, FWHM) are demonstrated up to the fourth-order using the same processing platform, which exhibits an effective processing bandwidth exceeding tens of Gigahertz. Finally, a linear 'second-order differential operator' conceived to emulate a high-frequency series RLC circuit is also implemented and successfully tested.
 

Schematic of the proof-of-concept experimental setup used for reconfigurable implementation of arbitrary temporal differential operators. The spectrum of the incoherent light source is also shown on the left. MOD: electro-optic Mach-Zehnder modulator. SMF: standard single-mode fiber. DEMUX: wavelength-division-demultiplexing filter. MUX: wavelength-division-multiplexer filter. Multi-ch attn: multi-channel attenuator. BD: differential balanced photoreceiver.

 

Third and Fourth-order photonics derivatives of a 72-ps(FWHM) input electric Gaussian pulse. (a) and (c) are spectral density profiles shaped according to the finite difference codes of the third and the fourth-order derivatives; (b) and (d) are numerical (solid dots) and experimental curves (black lines) of the derivative outputs. The inset of (b) shows the measured 72-ps input pulse waveform.

 

Schematic of the equivalent RLC circuit emulated through the use of the proposed photonics platform.

 

(a) Measured input modulation signal. (b) Measured output waveform (red curve) after the photonics RLC filter. A numerical differentiation (black curve) of the input signal with the RLC filter operator is also shown for comparison.

 

References:

Yongwoo Park, Mohammad H. Asghari, Robin Helsten, and Jose Azana, "Implementation of broadband Microwave arbitrary-order time differential operators using a reconfigurable incoherent photonic processor," IEEE Photonics Journal, Vol. 2, pp. 1040-1050 (2010).

Mohammad H. Asghari and Jose Azana, "Proposal and analysis of a reconfigurable pulse shaping technique based on multi-arm optical differentiators ", Optics Communications, Vol. 281, Issue 18, 15, pp. 4581-4588 (2008).