Spectral Shaper Research Articles

Photonic implementation of a highly reconfigurable wideband RF spectral shaper

In emerging Radio Frequency (RF) systems, the capability to dynamically process a wide range of frequency bands is highly desired due to the multi-frequency operation in multi-function RF systems and the large signal bandwidth usage associated with data-intensive applications. As the operation bandwidth increases, the frequency response uniformity of both RF components and microwave transmission medium degrades, that leads to the need of broadband RF spectrum equalization to support wideband high-quality services. Therefore, a wideband and reconfigurable RF spectral shaper is highly desired for dynamically manipulating the RF spectrum from low- to high-frequency band. Unfortunately, it is very challenging for conventional RF electronics to achieve dynamic spectral control over a wide operation bandwidth. In this paper, we take advantages of photonics and demonstrate a highly-reconfigurable RF spectral shaper that can manipulate an RF spectrum of tens of GHz wide. The proposed scheme is based on a microwave photonic filter architecture with multiple tunable, reconfigurable and switchable passbands. By manipulating the shape, bandwidth, and frequency of the passbands, highly reconfigurable wideband frequency responses for spectral equalization are experimentally achieved, covering the entire 0 to 10 GHz frequency range with adjustable attenuation up to 40 dB. Various RF equalization functions including tunable positive/negative slope, non/inverted parabolic, and multi-point spectral control with tunable floor are experimentally demonstrated using the proposed system. Several of the demonstrated RF functions have also been applied to arbitrary pulse shaping. The RF spectral shaper can be tuned to adapt to different scenarios dynamically, which could benefit a variety of applications including RF signal processing systems and communication networks.

Simultaneous Generation of WDM Chirped Microwave Waveforms Using Integrated Spectral Shapers in Silicon Photonics

We demonstrate a simultaneous generation of wavelength-division-multiplexed chirped microwave waveforms using an optical spectral shaping followed by a wavelength-to-time mapping. We design two types of integrated spectral shapers based on parallel distributed Fabry–Perot cavities or an arrayed waveguide grating Sagnac interferometer incorporating linearly chirped waveguide Bragg gratings. The generated waveforms have different central RF frequencies and the opposite signs of RF chirp.

Photonic generation of FCC-compliant UWB pulses based on modified Gaussian quadruplet and incoherent wavelength-to-time conversion

A novel scheme for the generation of FCC-compliant UWB pulse is proposed based on modified Gaussian quadruplet and incoherent wavelength-to-time conversion. The modified Gaussian quadruplet is synthesized based on linear sum of a broad Gaussian pulse and two narrow Gaussian pulses with the same pulse-width and amplitude peak. Within specific parameter range, FCC-compliant UWB with spectral power efficiency of higher than 39.9% can be achieved. In order to realize the designed waveform, a UWB generator based on spectral shaping and incoherent wavelength-to-time mapping is proposed. The spectral shaper is composed of a Gaussian filter and a programmable filter. Single-mode fiber functions as both dispersion device and transmission medium. Balanced photodetection is employed to combine linearly the broad Gaussian pulse and two narrow Gaussian pulses, and at same time to suppress pulse pedestals that result in low-frequency components. The proposed UWB generator can be reconfigured for UWB doublet by operating the programmable filter as a single-band Gaussian filter. The feasibility of proposed UWB generator is demonstrated experimentally. Measured UWB pulses match well with simulation results. FCC-compliant quadruplet with 10-dB bandwidth of 6.88-GHz, fractional bandwidth of 106.8% and power efficiency of 51% is achieved.

Photonic linear chirped microwave signal generation based on the ultra-compact spectral shaper using the slow light effect.

A novel concept to generate a linear chirped microwave signal is proposed and experimentally demonstrated. The frequency to time mapping method is employed, where the photonic crystal waveguide Mach-Zehnder interferometer structure acts as the spectral shaper thanks to the slow light effect. By optimizing the structural parameters of the photonic crystal waveguide, a linear chirped microwave signal with the time-bandwidth product of about 30 is experimentally obtained. The impact of the slow light photonic crystal waveguide on the generated linear chirped microwave signal is also investigated. The utilization of the slow light effect brings in significant advantages, including the ultra-small footprint of 0.096 mm2 and simple structure to our scheme, which may be of great importance towards its potential applications.

Simultaneously Generating Multiple Chirped Microwave Pulses With Superimposed FBGs

We demonstrate simultaneous generation of two different chirped microwave waveforms using an optical spectral shaper based on two superimposed linearly chirped fiber Bragg gratings in a Sagnac loop and wavelength-to-time mapping. By tuning the delay time in the Sagnac loop, we can simultaneously tune the central frequency of both waveforms and change the sign of the RF chirp for both waveforms together, from a negative chirp to a V-shape with both positive and negative chirps to a positive chirp.

Corrections to “Silicon-Based On-chip Electrically-Tunable Spectral Shaper for Continuously Tunable Linearly Chirped Microwave Waveform Generation” [Oct 16 4664-4672

Presents corrections to the paper, “Silicon-based on-chip electrically-tunable spectral shaper for continuously tunable linearly chirped microwave waveform generation,” (Zhang, W. and Yao, J.), J. Lightw. Technol., vol. 34, no. 20, pp. 4664–4672, Oct. 2016.

Silicon Photonics for Microwave Photonics Applications

We provide an overview of recent work on developing integrated microwave photonic subsystems in silicon photonics for generating chirped microwave waveforms as well as for providing optical true time delay. First, we describe on-chip spectral shapers based on Bragg gratings (BGs), including a distributed Fabry–Perot cavity, a Michelson interferometer incorporating identical chirped BGs, and a Sagnac loop incorporating a chirped BG, for use in wavelength-to-time mapping systems. The performance of these on-chip spectral shapers is compared to those based on microring resonators as well as fiber or free-space configurations. Second, we consider the development of an index variable optical true time delay line (OTTDL) using subwavelength grating (SWG) waveguides. For SWG waveguides of the same length, incremental delays can be obtained by tailoring the group index via control over the duty cycle. We compare the performance of SWG waveguide OTTDLs to other length variable implementations in silicon.

Investigation of fiber Bragg grating as a spectral notch shaper for single-pulse coherent anti-Stokes Raman spectroscopy

We experimentally demonstrate compact and efficient single-pulse coherent anti-Stokes Raman spectroscopy (CARS) via spectral notch shaping implemented with a fiber Bragg grating. We show that a fiber Bragg grating can serve as a narrowband notch filtering component on a 90nm broadband femtosecond pulsed laser without spectral distortion. Finally, we obtain CARS spectra of various samples in the fingerprint region of molecular vibrations. This scheme has potential for compact implementations of all-fiber single-pulse multiplex CARS due to its compatibility with fiber optics.

A Photonic Approach to Linearly Chirped Microwave Waveform Generation With an Extended Temporal Duration

A photonic approach to linearly chirped microwave waveform generation with an extended temporal duration is proposed and experimentally demonstrated. The linearly chirped microwave waveform generation is realized based on spectral-shaping and wavelength-to-time mapping, in which a Fabry–Perot (FP) interferometer with a linearly increasing or decreasing free spectral range is used to as a spectral shaper, and the wavelength-to-time mapping is realized using a recirculating dispersive loop with a large equivalent dispersion coefficient realized by allowing the spectrally shaped optical pulse to travel in the dispersive loop multiple times. The generation of two linearly chirped microwave waveforms at two different frequency bands with two temporal durations of 25 and 42 ns and a time-bandwidth product (TBWP) of 210 is experimentally demonstrated.

Silicon-Based On-Chip Electrically-Tunable Spectral Shaper for Continuously Tunable Linearly Chirped Microwave Waveform Generation

A silicon-based on-chip electrically-tunable spectral shaper for the generation of a tunable linearly chirped microwave waveform (LCMW) based on spectral shaping and wavelength-to-time (SS-WTT) mapping is designed, fabricated, and demonstrated. The on-chip spectral shaper has a Michelson interferometer structure with two linearly chirped waveguide Bragg gratings (LC-WBGs) incorporated in its two arms. Due to the wavelength-dependent length difference between the two arms of the interferometer, the spectral response of the spectral shaper exhibits a wavelength-dependent free spectral range, which is required for the generation of an LCMW based on SS-WTT mapping. To enable electrical tuning of the spectral response, a lateral PN junction is introduced to each of the waveguides where the LC-WBGs are inscribed. Thanks to the plasma dispersion effect, the spectral response of the spectral shaper can be tuned by changing the bias voltages applied to the PN junctions, which would lead to the tuning of the generated LCMW. A theoretical analysis on the LCMW generation is performed, which is verified by an experiment in which an electrically-tunable spectral shaper is fabricated using a CMOS-compatible process with 248-nm deep ultraviolet lithography. By independently controlling the bias voltages to the PN junctions, a continuous tuning of the generated LCMW is demonstrated.

DUAL-CHIRP ARBITRARY MICROWAVE WAVEFORM GENERATION BY USING A DUAL PARALLEL MACH-ZEHNDER MODULATOR FEEDING WITH RF CHIRP SIGNAL

In this paper, dual-chirped arbitrary microwave waveform has been generated through photonics, incorporated with single dual parallel mach-zehnder modulator (DPMZM) inbuilt mach zehnder interferometer (MZI) structure. We have taken two cases of chirping, i.e., linear and nonlinear chirps. A case of linear chirping has been explored previously. However, to the best of the authors' knowledge effect of nonlinear chirping in this paper is evaluated for the first time. Other photonics approaches are also available, such as spectra shaping and wavelength to time mapping. But due to fixed spectral response of spectral shaper, center frequency of linear chirp generated waveform is fixed. To get large center frequency again we have to use large number of spectral shapers which will increase the system complexity. DPMZM avoids such difficulties. These MZMs are biased at the minimum transmission point to get carrier suppressed modulation. Product modulator (PM) is cascaded to the lower arm of DPMZM. Here by using DPMZM we get two advantages. First we have two complimentarily chirped microwave waveforms and second up conversion of the frequency of microwave carrier. A dual-chirped microwave waveform with centre frequency 6 GHz with bandwidth 200 MHz and 2 GHz is generated. The paper gives specific details about various performance parameters such as input signal frequency and power, output signal parameters viz output frequency, chirp rate, chirp bandwidth, time bandwidth product (TBW), etc. The overall model and its performance parameters are computed through MATLAB simulation.

Spectrum slicer for snapshot spectral imaging

We propose and demonstrate an optical component that overcomes critical limitations in our previously demonstrated high-speed multispectral videography—a method in which an array of periscopes placed in a prism-based spectral shaper is used to achieve snapshot multispectral imaging with the frame rate only limited by that of an image-recording sensor. The demonstrated optical component consists of a slicing mirror incorporated into a 4f-relaying lens system that we refer to as a spectrum slicer (SS). With its simple design, we can easily increase the number of spectral channels without adding fabrication complexity while preserving the capability of high-speed multispectral videography. We present a theoretical framework for the SS and its experimental utility to spectral imaging by showing real-time monitoring of a dynamic colorful event through five different visible windows.

Design and Applications of In-Cavity Pulse Shaping by Spectral Sculpturing in Mode-Locked Fibre Lasers

We review our recent progress on the realisation of pulse shaping in passively-mode-locked fibre lasers by inclusion of an amplitude and/or phase spectral filter into the laser cavity. We numerically show that depending on the amplitude transfer function of the in-cavity filter, various regimes of advanced waveform generation can be achieved, including ones featuring parabolic-, flat-top- and triangular-profiled pulses. An application of this approach using a flat-top spectral filter is shown to achieve the direct generation of high-quality sinc-shaped optical Nyquist pulses with a widely tunable bandwidth from the laser oscillator. We also present the operation of an ultrafast fibre laser in which conventional soliton, dispersion-managed soliton (stretched-pulse) and dissipative soliton mode-locking regimes can be selectively and reliably targeted by adaptively changing the dispersion profile and bandwidth programmed on an in-cavity programmable filter. The results demonstrate the strong potential of an in-cavity spectral pulse shaper for achieving a high degree of control over the dynamics and output of mode-locked fibre lasers.

Photonic Generation of Linearly Chirped Microwave Waveforms Using a Silicon-Based On-Chip Spectral Shaper Incorporating Two Linearly Chirped Waveguide Bragg Gratings

Photonic generation of linearly chirped microwave waveforms using a silicon-based on-chip spectral shaper is proposed and demonstrated. The on-chip optical spectral shaper has a Mach–Zehnder interferometer (MZI) structure, in which two identical linearly chirped waveguide gratings (LC-WBGs) with opposite chirp rates are incorporated into the two arms. The LC-WBGs are fabricated on two rib waveguides by linearly varying the rib widths along the gratings to produce linear chirps. By adding an offset waveguide to one arm of the MZI and controlling the length of the offset waveguide, the spectral response of the shaper can be controlled to have a symmetrical, linearly increasing, or linearly decreasing free spectral range, which is needed for the generation of a linearly chirped microwave waveform based on spectral-shaping and wavelength-to-time mapping. The proposed device is fabricated using a CMOS-compatible process with 193-nm deep ultraviolet lithography. A theoretical analysis on the chirped microwave waveform generation is performed, which is verified by an experiment. Two linearly chirped microwave waveforms with symmetrical and uniform chirp profiles are experimentally generated using two fabricated spectral shapers with different lengths of the offset waveguides.

Chirped Microwave Pulse Generation Using an Integrated SiP Bragg Grating in a Sagnac Loop

We demonstrate an integrated spectral shaper based on a Sagnac loop incorporating a chirped Bragg grating in silicon photonics for the photonic generation of chirped microwave pulses. The technique is based on optical spectral shaping combined with linear frequency-to-time mapping. By tuning the central wavelength of the input optical pulse, we obtain chirped microwave pulses with central frequencies ranging from $\sim 10$ GHz to $\sim 30$ GHz and an RF chirp rate of $\sim 20$ GHz/ns with both positive and negative signs.

Independent manipulation of the phase and amplitude of optical sidebands in a highly-stable RF photonic filter.

Microwave photonic cancellation notch filters have been shown capable of achieving ultra-high suppressions independently from the strength of optical resonant filter they use, making them an attractive candidate for on-chip signal processing. Their operation, based on destructive interference in the electrical domain, requires precise control of the phase and amplitude of the optical modulation sidebands. To date, this was attainable only through the use of dual-parallel Mach-Zehnder modulators which suffer from bias drifts that prevent stable filter operation. Here we propose a new cancellation filter topology with ease of control and enhanced stability using a bias-free phase modulator and a reconfigurable optical processor as the modulation sidebands spectral shaper. We experimentally verify the long term stability of the novel filter topology through continuous real-time monitoring of the filter peak suppression over 24 hours.

Spectrum-Sliced Microwave-Photonic Filter Based on Fourier Transform of Modified Optical Spectrum

A new technique is presented to reconfigure the dispersion-induced RF fading in a spectrum-sliced microwave photonic filter to an arbitrary spectral shape, including the flat-top response with rectangular shape and operating frequencies beyond conventional baseband. The technique is based on the Fourier transform of the modified optical source power spectrum, which is realized by applying a designed optical amplitude and phase manipulation using a diffraction-based spectral pulse shaper to the ports of a dual-output Mach-Zehnder electro-optic modulator in combination with a broadband light source. This scheme is fundamentally different from earlier finite-impulse-response-based spectrum-sliced microwave-photonic filters that require specific slicing elements for optical taps, but offers the same flexibility in realizing various filter passband characteristics with the added benefit of widely shape-invariant tuning and a single passband response. Experimental results verify the technique and demonstrate a continuously tunable dispersion-induced RF fading up to 20 GHz, exhibiting shape invariance and a flat-top response.

Photonic generation of chirped microwave and millimeter wave pulses based on optical spectral shaping and wavelength-to-time mapping in silicon photonics

We provide an overview of photonic generation of chirped microwave and millimeter wave pulses based on optical spectral shaping followed by wavelength-to-time mapping. We summarize results obtained using bulk optic/benchtop and all-fiber spectral shapers, and discuss recent developments on integrated versions in silicon photonics. In particular, we describe devices based on microring resonators and present new results obtained using integrated spectral shapers incorporating chirped Bragg gratings.

Generating Chirped Microwave Pulses Using an Integrated Distributed Fabry–Pérot Cavity in Silicon-on-Insulator

We demonstrate photonic generation of chirped microwave pulses (CMPs) using an integrated distributed Fabry-Pérot cavity (DFPC) in silicon-on-insulator as a spectral shaper. The DPFC is based on a pair of spatially offset chirped Bragg gratings and has a response with varying free spectral range. The spectral shaper is used to filter broadband pulses from a mode-locked fiber laser, and CMPs are obtained at the output of a photodetector following a wavelength-to-time mapping process in a dispersive medium. We generate CMPs with a central frequency of up to 20 GHz, a radio-frequency (RF) chirp of 0.012 GHz/ps, and a time-bandwidth product of 14.5.

Selective control of excitation pathways of upconverted fluorescent states using a broadband laser and a spectral mask

We present a new technique to study materials’ response to broadband, optical excitation using upconversion through excited state absorption. We use the broadband spectrum from an ultrafast laser coupled to a spectral shaper to manipulate our excitation bandwidth and selectively control which transitions occur in the ground state and excited state absorptions. By analyzing the effects of spectral shaping of the excitation laser on the emission yield of fluorescent light, we can test models of the electron population densities and transition probabilities to recover electron energy-level-specific information about the system being studied. Here, we apply this technique to Y2O3:Er3+.