Photonic Signal Processing Research Articles

Optical single sideband modulation based on a high-order birefringent filter using cascaded Solc-Sagnac and Lyot-Sagnac loops.

We propose and experimentally demonstrate a simple and flexible photonic approach to implementing single sideband (SSB) modulation based on optical spectral filtering. The high-order birefringent filter is realized through the cascaded Solc-Sagnac and Lyot-Sagnac loops. By adjusting the rotation angle of the polarization controller (PC), the notch position to remove undesired sidebands changes. The frequency for SSB modulation varies accordingly. The periodical response of the filter spectrum allows both the carrier wavelength and the optical carrier to sideband ratio (OCSR) to be tunable. SSB modulation over a frequency range from 5 to 40 GHz and tunable OCSR ranging from -9.174 to 34.408 dB are obtained. The significant merits of the proposed approach are the simple structure, easy operation, large frequency range, tunable OCSR, and wavelength independence. The approach has potential applications in optimizing the transmission performance of photonic microwave signal processing systems.

Optical spectroscopy and decoherence studies ofYb3+:YAG at 968 nm

The $^{2}F_{7/2}\ensuremath{\leftrightarrow}^{2}F_{5/2}$ optical transitions of $\mathrm{Y}{\mathrm{b}}^{3+}$ doped into ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5}{\mathrm{O}}_{12}$ (YAG) were studied for potential quantum information and photonic signal processing applications. Absorption and fluorescence spectroscopy located the energy levels of the ground $^{2}F_{7/2}$ and excited $^{2}F_{5/2}$ manifolds, allowing inconsistencies between previous assignments of crystal field splittings in the literature to be resolved. These measurements reveal an unusually large splitting between the first and second levels in both the ground and excited multiplets, potentially providing for reduced sensitivity to thermally induced decoherence and spin-lattice relaxation. Spectral hole burning through two-level saturation was observed, determining the excited state lifetime to be 860 \ensuremath{\mu}s and resolving ambiguities in previous fluorescence measurements that were caused by the large radiation trapping effects in this material. Optical decoherence measurements using two-pulse photon echoes gave a homogeneous linewidth of 18 kHz for an applied magnetic field of 1 T, narrowing to 5 kHz at 2.5 T. The observed decoherence was described by spectral diffusion attributed to $\mathrm{Y}{\mathrm{b}}^{3+}\mathrm{\ensuremath{-}}\mathrm{Y}{\mathrm{b}}^{3+}$ magnetic dipole interactions. Laser absorption determined an inhomogeneous linewidth of 3.6 GHz for this transition in this 0.05%-doped crystal, which is narrower than for any other rare-earth-ion transition previously studied in the YAG host. The temperature dependence of the transition energy and linewidth of the lowest $^{2}F_{7/2}$ to lowest $^{2}F_{5/2}$ transition centered at 968.571 nm measured from 4 K to 300 K was well described by phonon scattering at higher temperatures, with an additional anomalous linear temperature-dependent broadening at temperatures below 80 K. Two magnetically inequivalent subgroups of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions were identified when a magnetic field was applied along the $\ensuremath{\langle}111\ensuremath{\rangle}$ axis, as expected for the ${D}_{2}$ sites in the cubic symmetry crystal, with ground and excited state effective $g$-values of ${g}_{\mathrm{g}}=3.40$ (3.34) and ${g}_{\mathrm{e}}=1.04$ (2.01), respectively. Together with the convenient diode laser wavelength of this transition, our study suggests that $\mathrm{Y}{\mathrm{b}}^{3+}$:YAG is a promising material system for spectral hole burning and quantum information applications.

Adjustable Aperture Based on the Phase Modulation of Spatial Light Modulator

In this paper, we propose an adjustable aperture based on the phase modulation of a spatial light modulator (SLM). Two polarized beam splitters (PBSs) are used to control the polarized light and one SLM is used to modulate the phase of the polarized light. A specific grayscale is produced, where the middle grayscale is with phase of 0 and the other is with phase of $\pi$ . When the light passes through the first PBS and the SLM, the polarized state of p-light is changed to s-light by the grayscale with phase of $\pi$ , so only middle light can pass through the second PBS and is detected by the camera. By controlling the size of the middle grayscale with phase of 0, the device can realize the function of adjustable aperture. Our experiment shows that the aperture can be tuned between 0 and full size, meaning that it can reach the fully open and closed state. The results verify its feasibility. The adjustable aperture has wide applications in imaging system, fiber-optic communication, and photonic signal processing.

Design of photonics half subtractor logics for photonics signal processing

This paper deals with photonics half subtraction design for photonics signal processing by using SOA-MZI configuration. The simulation is used to design the half subtraction operation at 1Gb/s. The photonics half subtractor logic is implemented by using phase modulation of signals in SOA-MZI. Simulative realization of optical half subtraction is obtained by using three SOA-MZI structure which lead revolution in signal processing for high speed operation. The photonics half subtractor is obtained with good quality of signal. The clear eye diagram and spectrums are observed for each output of subtractor and borrows.

A fully reconfigurable photonic integrated signal processor

Photonic signal processing has been considered a solution to overcome the inherent electronic speed limitations. Over the past few years, an impressive range of photonic integrated signal processors have been proposed, but they usually offer limited reconfigurability, a feature highly needed for the implementation of large-scale general-purpose photonic signal processors. Here, we report and experimentally demonstrate a fully reconfigurable photonic integrated signal processor based on an InP–InGaAsP material system. The proposed photonic signal processor is capable of performing reconfigurable signal processing functions including temporal integration, temporal differentiation and Hilbert transformation. The reconfigurability is achieved by controlling the injection currents to the active components of the signal processor. Our demonstration suggests great potential for chip-scale fully programmable all-optical signal processing. Scientists experimentally demonstrate a fully configurable photonic integrated signal processor based on an InP–InGaAs material system by controlling the injection currents to the active components.

Advances in High-speed and Adaptive Microwave Photonic Signal Processing

Photonic signal processing, using photonic approaches to condition wideband microwave signals, is attractive due to the inherent advantages of high time-bandwidth product and immunity to electromagnetic interference. In this keynote paper, we describe recent advances in wideband microwave photonic signal processing. This includes programmable microwave photonic phase shifters and true-time delay elements for phased array beamforming, and single bandpass tunable filtering approaches that overcome spectral periodicity. We also present optoelectronic oscillators with wideband frequency operating range, high-resolution multiple frequency microwave photonic measurement systems, and photonic RF memory structures that can realise a long reconfigurable storage time with wide instantaneous bandwidth.

Ultra-Wideband and Adaptive Photonic Signal Processing of Microwave Signals

Microwave photonic techniques are attractive for processing high bandwidth signals, overcoming inherent electronic limitations. These processors provide new capabilities for realizing adaptive processing over extremely wide microwave frequency ranges, ultra-wideband operation, and electromagnetic interference immunity. In-fiber signal processors are inherently compatible with fiber-wireless systems, and can provide connectivity with in-built signal conditioning. Recent new methods in wideband microwave photonic processing are presented, including ultra-wideband phase shifters and true time delays for phased array beamforming, widely tunable single passband filters, switchable microwave photonic filters, wideband photonic mixers, high-resolution multiple frequency microwave photonic measurement systems, and photonic RF memory structures.

All-optical processing to optical and radio frequency (RF) signals

Signal processing is always the heart of the overall information technology and industry, providing enabling solutions for the processing, understanding, learning, retrieval, mining, and extraction of information from different signals. Regarding the all-optical signal processing, it dates back to the 1980s when the electronic bottleneck was considered as a big obstacle to the bandwidth and capacity of optical communications. Fortunately, all-optical signal processing provides intrinsic wide instantaneous bandwidth and large time-bandwidth product (TBP), powered by optics or photonics technology. Although we are happy to see various breakthrough achievements in electronic signal processing over past decades, all-optical signal processing are still highly required today along with the exponential growth of data capacity, the ubiquitous access, and the realtime interactions. There are some increasing motivations listed as follows, all-optical storage and switching to for large-capacity and energy-efficient networks, broadband and complex microwave signal processing for 5G and beyond systems and defense scenarios, and innovative materials and devices for quantum information processing. In particular, all-optical signal processing is of great significance in both optical and radio frequency (RF) domains, providing intrinsic advantages on large bandwidth to circumvent the electronic bottleneck. Here, latest and state-of-art advances on all-optical processing to both optical and RF signals are comprehensively reviewed, with warped transform and compression, integrator and differentiator, optical memory, fully photonics-based radar, and nonlinear photonic RF signal processing being highlighted in details. Moreover, advances on photonic integrated circuits (PICs) for all-optical processing are demonstrated, especially the CMOS-compatible PICs and high-density nanophotonic integration. Optical signal processing refers to a wide variety of techniques and applications, including compression, transform, analog/logic operations, arbitrary waveform processing, compensation and equalization, wavelength and format conversion, all-optical storage/switching/ memory [1–10]. Here, we will concentrate on the latest advances on warped transforms and compression, differentiation and integration, and optical memory. First of all, a concept of the photonic hardware accelerator is proposed to process wideband waveforms [1]. In principle, nonlinear group delay dispersions (sub-linear or super-linear ones) are introduced to perform warped transform or anamorphic stretch transform, such that wideband waveforms are transformed and processed according to the non-uniform entropy of their spectra. Potential applications include non-uniform Fourier domain sampling, data compression, Big Data predicament in biological cell screening, signal-to-noise ratio (SNR) enhancement, spectrotemporal encoding, and so on. For example, a compression in the TBP of a signal is achieved to circumvent the Big Data problem in high-throughput measurements and ultrafast imaging [1, 2], without sacrificing the spectral resolution or bandwidth. The authors demonstrated a modulation bandwidth compressed by 500 times, while the input optical signal had a 1,000-GHz modulation bandwidth and a 50-ps duration. Consequently, the TBP and then the digital data size were reduced by 2.73 times. These requirements are already available in optics X. Zou (&) W. Pan L. Yan Center for Information Photonics and Communications, Southwest Jiaotong University, Chengdu 610031, China e-mail: zouxihua@swjtu.edu.cn

Si <sub>3</sub>N <sub>4</sub>-Based Integrated Optical Analog Signal Processor and Its Application in RF Photonic Frontend

Digital signal processing has achieved great success in the field of signal processing over the past several decades. However, as the bandwidth requirement increases, the power consumption and effective number of bits (ENOB) of the analog-to-digital convertor (ADC) have become bottlenecks. One solution is returning to analog and applying microwave photonic technologies, which shows potential for multiband signal processing. In this paper, a programmable integrated analog photonic signal processor based on cascaded Mach-Zehnder interferometers (MZIs) and a channelized filter has been proposed. Different shapes of the signal processor can be acquired for different applications. The highest processing resolution is 143 MHz, and the processing range of the signal processor can be higher than 112.5 GHz. An application of the signal processor for the signal extraction in a radio frequency (RF) photonic frontend operating from L-band to K-band is presented.

Programmable photonic signal processor chip for radiofrequency applications

Integrated microwave photonics, an emerging technology combining radio frequency (RF) engineering and integrated photonics, has great potential to be adopted for wideband analog processing applications. However, it has been a challenge to provide photonic integrated circuits with equal levels of function flexibility as compared with their electronic counterparts. Here, we introduce a disruptive approach to tackle this need, which is analogous to an electronic field-programmable gate array. We use a grid of tunable Mach–Zehnder couplers interconnected in a two-dimensional mesh network, each working as a photonic processing unit. Such a device is able to be programmed into many different circuit topologies and thereby provide a diversity of functions. This paper provides, to the best of our knowledge, the first ever demonstration of this concept and shows that a programmable chip with a free spectral range of 14 GHz enables RF filters featuring continuous, over-two-octave frequency coverage, i.e., 1.6–6 GHz, and variable passband shaping ranging from a 55 dB extinction notch filter to a 1.6 GHz bandwidth flat-top filter.

Passive silicon photonic devices for microwave photonic signal processing

We present our recent progress on microwave signal processing (MSP) using on-chip passive silicon photonic devices, including tunable microwave notch filtering/millimeter-wave (MMW) signal generation based on self-coupled micro-resonators (SCMRs), and tunable radio-frequency (RF) phase shifting implemented by a micro-disk resonator (MDR). These schemes can provide improved flexibility and performances of MSP. The experimental results are in good agreement with theoretical predictions, which validate the effectiveness of the proposed schemes.

Microwave photonic delay line signal processing.

This paper provides a path for the design of state-of-the-art fiber-optic delay lines for signal processing. The theoretical forms for various radio-frequency system performance metrics are derived for four modulation types: X- and Z-cut Mach-Zehnder modulators, a phase modulator with asymmetric Mach-Zehnder interferometer, and a polarization modulator with control waveplate and polarizing beam splitter. Each modulation type is considered to cover the current and future needs for ideal system designs. System gain, compression point, and third-order output intercept point are derived from the transfer matrices for each modulation type. A discussion of optical amplifier placement and fiber-effect mitigation is offered. The paper concludes by detailing two high-performance delay lines, built for unique applications, that exhibit performance levels an order of magnitude better than commercial delay lines. This paper should serve as a guide to maximizing the performance of future systems and offer a look into current and future research being done to further improve photonics technologies.

Continuously Tunable Flat-Passband Microwave Photonic Notch Filter Based on Primary and Secondary Tap Distribution Impulse Response

Abstract: A new microwave photonic signal processor that has the ability to realize a continuously tunable high-resolution notch filter response is presented. It is based on designing the filter impulse response to have a primary and secondary tap distribution. With a proper design on the tap amplitudes, the notch filter passband can be flattened. The filter notch frequency can be also continuously tuned by controlling the phase shift of the secondary taps via a diffraction-based Fourier-domain optical processor. The notch filter has a robust response and high signal-to-noise-ratio (SNR) performance. Experimental results demonstrate that the new microwave photonic notch filter can simultaneously realize a flat passband of only a 1-dB ripple, a large free spectral range of 4.7 GHz, and a notch depth of over 40 dB while tuning the notch frequency.

Optical attenuator based on phase modulation of a spatial light modulator

In this Letter, we propose an optical attenuator based on the phase modulation of a spatial light modulator (SLM). In this system, we use two polarized beam splitters (PBSs) to control the polarized light and one SLM to modulate the phase of the polarized light. In the initial state, the light beam is divided into p-light and s-light when it passes through the first PBS. When the light passes through the second PBS, s-light is reflected and p-light is detected by the CCD camera. By loading different grayscales on the SLM, p-light changes its polarized state to s-light. The light power can be attenuated during the loading process. Our experiment shows that the system can obtain a wide optical attenuation from 1–27.2 dB. When loading two grayscales, the SLM has a fast switching time of 25 ms under a low actuated voltage of 5.5 V. The response time of the optical attenuator depends on the switching time of the SLM. Therefore, the system can also have a fast response time. By using the method of spatial multiplexing and adding two mirrors in the system, it can also be extended into a 1×2 optical switch. The results verify its feasibility. The optical attenuator has wide applications in photonic signal processing and fiber-optic communication.

On-chip mode-locked laser diode structure using multimode interference reflectors

We report, for the first time to our knowledge, an on-chip mode-locked laser diode (OCMLLD) that employs multimode interference reflectors to eliminate the need of facet mirrors to form the cavity. The result is an OCMLLD that does not require cleaved facets to operate, enabling us to locate this OCMLLD at any location within the photonic chip. This OCMLLD provides a simple source of optical pulses that can be inserted within a photonic integrated circuit chip for subsequent photonic signal processing operations within the chip (modulation, optical filtering, pulse rate multiplication, and so on). The device was designed using standardized building blocks of a generic active/passive InP technology platform, fabricated in a multi-project wafer run, and achieved mode-locking operation at its fundamental frequency, given the uncertainty at the design step of the optical length of these mirrors, critical to achieve colliding pulse mode-locked operation.

Optical decoherence studies ofTm3+:Y3Ga5O12

Decoherence of the 795 nm $^3$H$_6$ to $^3$H$_4$ transition in 1%Tm$^{3+}$:Y$_3$Ga$_5$O$_{12}$ (Tm:YGG) is studied at temperatures as low as 1.2 K. The temperature, magnetic field, frequency, and time-scale (spectral diffusion) dependence of the optical coherence lifetime is measured. Our results show that the coherence lifetime is impacted less by spectral diffusion than other known thulium-doped materials. Photon echo excitation and spectral hole burning methods reveal uniform decoherence properties and the possibility to produce full transparency for persistent spectral holes across the entire 56 GHz inhomogeneous bandwidth of the optical transition. Temperature-dependent decoherence is well described by elastic Raman scattering of phonons with an additional weaker component that may arise from a low density of glass-like dynamic disorder modes (two-level systems). Analysis of the observed behavior suggests that an optical coherence lifetime approaching one millisecond may be possible in this system at temperatures below 1 K for crystals grown with optimized properties. Overall, we find that Tm:YGG has superior decoherence properties compared to other Tm-doped crystals and is a promising candidate for applications that rely on long coherence lifetimes, such as optical quantum memories and photonic signal processing.

Special issue of optical and quantum electronics on information photonics

The processing of information using photonics technology is the key tomany emerging applications, for example bioand environmental-sensing, in addition to more conventional uses for data transmission in the rapidly evolving information society. Information photonics thus encompasses the development of new theories, models, devices, systems and applications. This Special Issue presents some recent developments in this exciting field. Its scope includes the design and realization of novel photonic waveguide and signal processing structures, the design, fabrication, and testing of photonic crystals and polarisation maintaining optical fibres, novel photonic configurations incorporating metamaterials, and the use of photonics in bio-sensing and medical diagnostics. The Guest Editors for the Special Issue thank all authors and reviewers for their expert contributions.

From spectral holeburning memory to spatial-spectral microwave signal processing

Many storage and processing systems based on spectral holeburning have been proposed that access the broad bandwidth and high dynamic range of spatial-spectral materials, but only recently have practical systems been developed that exceed the performance and functional capabilities of electronic devices. This paper reviews the history of the proposed applications of spectral holeburning and spatial-spectral materials, from frequency domain optical memory to microwave photonic signal processing systems. The recent results of a 20 GHz bandwidth high performance spectrum monitoring system with the additional capability of broadband direction finding demonstrates the potential for spatial-spectral systems to be the practical choice for solving demanding signal processing problems in the near future.

Scaling computation with silicon photonics

Abstract

Optical single sideband polarization modulation for radio-over-fiber system and microwave photonic signal processing

An approach to implementing optical single sideband (OSSB) polarization modulation, which is a combination of two orthogonally polarized OSSB modulations with complementary phase differences between the optical carrier and the sideband, is demonstrated based on two cascaded polarization modulators (PolMs). The two PolMs are driven by two RF signals that are 90° out of phase. By properly adjusting the polarization state between the two PolMs, OSSB polarization modulation with large operation bandwidth can be realized. An experiment is performed. OSSB polarization modulation with an operation bandwidth from 2 to 35 GHz is successfully demonstrated. The spectral profile of the OSSB polarization-modulated signal is observed through an optical spectrum analyzer, and its complementary phase properties are analyzed by sending the signal to a photodetector (PD) for square-law detection. Due to the complementary phase differences between the optical carrier and the sideband along the two polarization directions, no microwave frequency component is generated after the PD. The generated OSSB polarization-modulated signal is transmitted through 25 and 50 km single-mode fiber with 50 Mbaud 16 quadrature amplitude modulation baseband data to investigate the transmission performance of the proposed system in radio-over-fiber applications, and very small error vector magnitude degradation is observed. OSSB polarization modulation is also employed to realize a microwave photonic phase shifter. A full-range tunable phase shift is obtained for 2 and 35 GHz microwave signals.