Figure of merit for photonic differentiators

Optical Computing: Status and Perspectives

For many years, optics has been employed in computing, although the major focus has been and remains to be on connecting parts of computers, for communications, or more fundamentally in systems that have some optical function or element (optical pattern recognition, etc.). Optical digital computers are still evolving; however, a variety of components that can eventually lead to true optical computers, such as optical logic gates, optical switches, neural networks, and spatial light modulators have previously been developed and are discussed in this paper. High-performance off-the-shelf computers can accurately simulate and construct more complicated photonic devices and systems. These advancements have developed under unusual circumstances: photonics is an emerging tool for the next generation of computing hardware, while recent advances in digital computers have empowered the design, modeling, and creation of a new class of photonic devices and systems with unparalleled challenges. Thus, the review of the status and perspectives shows that optical technology offers incredible developments in computational efficiency; however, only separately implemented optical operations are known so far, and the launch of the world's first commercial optical processing system was only recently announced. Most likely, the optical computer has not been put into mass production because there are still no good solutions for optical transistors, optical memory, and much more that acceptance to break the huge inertia of many proven technologies in electronics.

Terahertz-frequency temporal differentiator enabled by a high-Q resonator

Terahertz (THz) fundamental "building blocks" equivalent to those used in multi-functional electronic circuits are very helpful for actual applications in THz data-processing technology and communication. Here, we theoretically and experimentally demonstrate a THz temporal differentiator based on an on-chip high-quality (Q) factor resonator. The resonator is made of low-loss high-resistivity silicon material in a monolithic, integrated platform, which is carefully designed to operate near the critical coupling region. The experiment demonstrates that the device can perform the first-order time derivative of the input signal electric field complex envelope at 214.72 GHz. Our investigation provides an effective approach for terahertz pulse re-shaping and real-time differential computing units.

Reconfigurable microwave signal processor with a phase shift of π

We propose and experimentally demonstrate a reconfigurable microwave signal processor, with a bandwidth up to tens of gigahertz. In this technique, any microwave signal processing function with a phase shift of π could be performed by shaping the input optical intensity spectrum. The phase shift of π is implemented by using a differential detection. Thanks to the broad bandwidth provided by the incoherent optical source and the high resolution of the user-defined optical filter, the frequency response of our approach could be in a great agreement with that of an ideal signal processing function. In the experiment, temporal intensity Hilbert transformations and temporal intensity differentiations of Gaussian-like pulses with widths of 125ps, 85ps and 68ps are accurately achieved.

Bandwidth-adaptable silicon photonic differentiator employing a slow light effect

A photonic differentiator (DIFF) plays a crucial role in photonic circuits. Despite the fact that a DIFF having a terahertz bandwidth has been reported, the practical bandwidth is limited to being a bandpass response. In this Letter, we propose the concept of a bandwidth-adaptable DIFF, which exploits the slow light effect in a photonic crystal waveguide (PhCW) to overcome the inherent bandwidth limitation of current photonic DIFFs. We fabricated a PhCW Mach-Zehnder interferometer (PhCW-MZI) on the silicon-on-isolator material platform to validate our concept. Input Gaussian pulses with full width to half-maximums (FWHMs) ranging from 2.7 to 81.4 ps are accurately differentiated using our PhCW-MZI. Our all-passive scheme circumvents the bandwidth bottlenecks of previously reported photonic DIFFs and can greatly broaden the application area of photonic DIFFs.

Reconfigurable photonic temporal differentiator based on a dual-drive Mach-Zehnder modulator

We propose and demonstrate a reconfigurable photonic temporal differentiator based on a dual drive Mach-Zehnder modulator (DDMZM). The differentiator can be reconfigured to different differentiation types by simply adjusting the bias voltage of DDMZM. Both simulations and experiments are carried out to verify the proposed scheme. In the experiment, a pair of polarity-reversed field differentiation and a pair of polarity-reversed intensity differentiation are successfully generated. The differentiation accuracy and conversion efficiency versus the time delay are also investigated.

Operation bandwidth optimization of photonic differentiators

We theoretically investigate the operation bandwidth limitation of the photonic differentiator including the upper limitation, which is restrained by the device operation bandwidth and the lower limitation, which is restrained by the energy efficiency (EE) and detecting noise level. Taking the silicon photonic crystal L3 nano-cavity (PCN) as an example, for the first time, we experimentally demonstrate that the lower limitation of the operation bandwidth does exist and differentiators with different bandwidths have significantly different acceptable pulse width range of input signals, which are consistent to the theoretical prediction. Furthermore, we put forward a novel photonic differentiator scheme employing cascaded PCNs with different Q factors, which is likely to expand the operation bandwidth range of photonic differentiator dramatically.

Time-delay to intensity mapping based on a second-order optical integrator: application to optical arbitrary waveform generation

We propose and validate experimentally a time-delay to intensity mapping process based on second-order optical integrators. This mapping provides dynamic control of the intensity modulation profile of a waveform based on a purely passive and linear process. In particular, we can realize linear intensity control by tuning the time-delay between two optical pulses launched into a second-order optical integrator. We suggest and experimentally prove the use of this mapping process for reconfigurable optical arbitrary waveform generation.

Fractional-order photonic differentiator using an on-chip microring resonator

A tunable temporal photonic fractional differentiator using a silicon-on-isolator (SOI) electrically tuned microring resonator (MRR) is proposed and experimentally demonstrated. Through changing the voltage applied on the MRR, the fractional order of the photonic differentiator can be continuously tuned. The proposed fractional-order differentiator is demonstrated experimentally with Gaussian pulse injection and rectangular pulse injection, respectively. The small deviation shows the feasibility of our photonic differentiator with an integrated silicon MRR.

Use of photonic crystal cavities for temporal differentiation of optical signals

We show that general-form optical cavities are able to perform the temporal differentiation of optical signals. Analytical relationships to account for the scattering losses for such a cavity's characteristics are deduced on the basis of temporal coupled-mode theory. A compact nanocavity-aided differentiator based on a ridge photonic crystal waveguide is designed.

Compact, flexible and versatile photonic differentiator using silicon Mach-Zehnder interferometers

We propose and experimentally demonstrate the flexibility and versatility of photonic differentiators using a silicon-based Mach-Zehnder Interferometer (MZI) structure. Two differentiation schemes are investigated. In the first scheme, we demonstrate high-order photonic field differentiators using on-chip cascaded MZIs, including first-, second-, and third-order differentiators. For single Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.5%. In the second scheme, we demonstrate multifunctional differentiators, including intensity differentiator and field differentiator, using an on-chip single MZI structure. These different differentiator forms rely on the relative shift between the probe wavelength and the MZI resonant notch. Our schemes show the advantages of compact footprint, flexible functions and versatile differentiation forms. For example, high order field differentiators can be used to generate complex temporal waveforms, such as high order Hermite-Gaussian waveforms. And intensity differentiators are useful for ultra-wideband pulse generation.

High-order photonic differentiator employing on-chip cascaded microring resonators

We propose and experimentally demonstrate a high-order photonic differentiator using on-chip complementary metal oxide semiconductor-compatible cascaded microring resonators, including first-, second-, and third-order differentiators. All the microring resonator units have a radius of 150 μm and a free spectral range of 80 GHz. The microring resonator can implement the first-order derivative of the optical field near its critical coupling region. Hence higher-order differentiation can be obtained by cascading more microring units on a single chip. For the periodical Gaussian optical pulse injection, the average deviations of all differentiators are less than 6.2%. The differentiation of pseudo-random bit sequence signals at 5 Gbit/s is also demonstrated. Our scheme is a compact and low-power-consumption solution since the cascaded microring units are fabricated with compact size on the silicon-on-insulator substrate.