Optical All-pass Filters Research Articles

Reconfigurable second-order optical all-pass filter

Abstract The optical all-pass filter (APF), which exhibits a constant amplitude response and a variable phase response, is a key to manipulating the optical phase without inducing signal amplitude distortion. High-order APFs are significantly demanded because they can afford large time delays and phase shifts. However, to date, only first-order APFs based on lossy waveguides have been reported. Although high-order APFs can be simply obtained by cascading multiple first-order APFs, the complexity and size are increased. To solve this problem, we propose and demonstrate a second-order APF using Mach–Zehnder interferometer-assisted microring resonators. The device is fabricated based on a silicon-on-insulator platform. Based on the second-order APF, an adjustable time delay between 553 and 948 ps is obtained, and the corresponding amplitude variation is less than 1.7 dB. Meanwhile, a microwave photonic phase shifter is also obtained based on the APF. The microwave phase shift can be adjusted from 0 to 3.27π, with an RF power variation within 2.4 dB. Additionally, the second-order APF can be reconfigured to a first-order APF, which significantly enhances its flexibility. The reconfigured first-order APF can realize an adjustable time delay between 257 and 429 ps, and the amplitude variation is less than 0.9 dB. The proposed high-order APF provides a novel approach to manipulating optical signals.

Optical All-Pass Filter Realized by Self-Compensation of Loss

Filters are key devices in optical systems and are usually applied to signal suppression or selection based on their amplitude frequency responses. Different from amplitude filtering, an all-pass filter (APF) is a unique type of filter that exhibits a uniform amplitude and variable phase in the frequency response. Traditionally, an APF is designed to be ideal and waveguide loss as well as coupling loss are omitted, which is impossible in practice. Here we demonstrate a method of realizing an APF even when waveguide loss and coupling loss exist, and a feasible structure for realizing the APF with waveguide loss and coupling loss is proposed. The method indicates that an APF can be realized by introducing self-compensation of loss to the structure of an ideal single-stage APF, and further, an APF based on silicon-on-insulator foundry is fabricated and demonstrated. The method of self-compensation of loss can also be used to realize APFs in the electrical domain. We anticipate that our work will inspire realizations of more APF-related devices that once seemed impossible in practice, as well as loss related devices or structures.

Optical All-Pass Filter in Silicon-on-Insulator

Optical all-pass filters, which exhibit constant amplitude response and pure phase variation, are extremely desired when only optical phase manipulation is required. However, many reported all-pass...

Synthesis of Optical IIR Allpass Filters Using Layer Peeling Algorithm Based on Eigenvalue Problem

We propose a layer peeling algorithm with the eigenvalue problem to design optical IIR allpass filters with an equiripple phase response in the Chebyshev sense. The layer peeling algorithm is used to determine the coupling coefficients of the optical IIR allpass filter structures from the transfer function in the z-transform domain. The eigenvalue problem is used to design the transfer function with the optimal phase response of the optical IIR allpass filter. To verify the proposed method, we design a Hilbert transformer, which is useful in optical communications and signal processing.

Magneto-Optic Interferometric Switch With Resonator Configuration

All-optical resonators are candidates for next-generation, integrated, optical switching devices for communication purposes. Their simplicity in design can allow for easy integration using standard CMOS fabrication processes. Other switch configurations, such as the Mach-Zhender and the Sagnac, have been implemented but are more complicated. Traditionally, optical resonators have been used as optical all-pass filters. However, by including a Faraday rotator in the resonating loop with an actuating field, an all-optical switch can be realized. Optical resonator switches have often been proposed and implemented utilizing the electro-optic effect. This study reveals that magneto-optic equivalents show promise as alternatives. Here, a magneto-optical switch in a resonator configuration is proposed and implemented with theoretical analysis using the Jones calculus technique.

A Novel Broadband Photonic RF Phase Shifter

A novel broadband photonic RF phase shifter is proposed that employs single-sideband modulation of the broadband RF signal onto an optical carrier. The phase of the carrier is shifted relative to the modulated signal using an optical allpass filter. Ring-resonator-based optical allpass filters are tuned across the carrier frequency to yield an RF phase shift of over 360 degrees. Results from a proof-of-concept experimental demonstration are presented for modulation frequencies in the range of 2-4 GHz.

Compact all-pass filters in photonic crystals as the building block for high-capacity optical delay lines.

Optical all-pass filters, which generate strong on-resonance optical delay while maintaining a unity transmission coefficient throughout the entire resonant bandwidth, are of great importance for constructing delay lines in optical buffer applications. We provide an analysis of optical delay lines based upon cascading multiple stages of all-pass filter structures. We show that the maximum capacity of such delay lines is determined primarily by the dimensions of each stage. Motivated by this analysis, we describe compact optical all-pass filters in two-dimensional photonic crystals. The accidental degeneracy of the cavity modes introduces a strong group delay and dispersion while maintaining total transmission.

Mechanically switchable photonic crystal filter with either all-pass transmission or flat-top reflection characteristics.

We theoretically introduce a new type of optical all-pass filter based on guided resonance in coupled photonic crystal slabs. The filter exhibits near-complete transmission for both on- and off-resonant frequencies and yet generates large resonant group delay. We further show that such a filter can be mechanically switched into a flat-top band rejection filter.

Optical delay lines based on optical filters

Optical delay lines have some important applications, notably in optical communication systems and in phased arrays. These devices are based on the concept of optical group delay, which, in turn, can be understood as the property of an optical filter. Optical filters are well-understood devices and, in particular, their dispersive properties determine the group delay response. We review these dispersive properties and point out some of the inherent tradeoffs involved in generating large group delay. Fiber Bragg gratings and recent results on optical all-pass filters are used as examples.

General IIR optical filter design for WDM applications using all-pass filters

A general design algorithm is presented for infinite impulse response (IIR) bandpass and arbitrary magnitude response filters that use optical all-pass filters as building blocks. Examples are given for an IIR multichannel frequency selector, an amplifier gain equalizer, a linear square-magnitude response, and a multi-level response. Major advantages are the efficiency of the IIR filter compared to finite impulse response (FIR) filters, the simplicity of the optical architecture, and its tolerance for loss. A reduced set of unique operating states is discussed for implementing a reconfigurable multichannel selection filter.

A tunable dispersion compensating MEMS all-pass filter

A tunable dispersion compensating filter based on a multistage optical all-pass filter with a microelectromechanical (MEM) actuated variable reflector and a thermally tuned cavity is described. A two-stage device was demonstrated with a tuning range of /spl plusmn/100 ps/nm, 50-GHz passband and a group delay ripple less than /spl plusmn/3 ps. The device has negligible polarization dependence and is suitable for single or multiple channel compensation. An off-axis, two-fiber package with an excess loss <2 dB/stage avoids the need for a circulator. By cascading four stages, a passband to channel spacing ratio of 0.8 is obtained that allows both 40 Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) signals to be compensated.

Integrated all-pass filters for tunable dispersion and dispersion slope compensation

New integrated optical all-pass filters are presented that can be used for tunable dispersion compensation, dispersion slope compensation, and as building blocks in tunable bandpass filters. The dispersion slope compensation capability is demonstrated using ring resonators in /spl Delta/=2% Ge-doped silica planar waveguides. In addition, a tunable four-stage filter with free-spectral range (FSR)=25 GHz, a passband width of 14 GHz (0.56/spl times/FSR), and D=1800 ps/nm is reported.

General optical all-pass filter structures for dispersion control in WDM systems

All-pass filters (APFs) are devices that allow phase correction or equalization without introducing any amplitude distortion. An optical implementation of such devices is very attractive since they can be used for dispersion compensation. In contrast to other dispersion control devices, optical APFs can correct any order of dispersion. This can be achieved by careful design of multistage APFs to approximate a target phase profile. However, large dispersion is usually narrow band or requires many filter stages. These performance tradeoffs and the general phase properties of optical APFs are reviewed and clarified in the first part of this paper. In the second part, a general design methodology of optical APFs is introduced. We show that any all-pass structure may be constructed from simple N-port devices (such as directional couplers or Mach-Zehnder interferometers) with N-1 outputs fed back to any of the N-1 inputs. The feedback paths may contain delays or further APFs (recursive design). This set of design rules allows for constructing complex all-pass filters of any number of stages starting with very simple elements. We use this technique to demonstrate a number of optical all-pass structures that may be implemented in planar waveguide or using thin-film filter technology.

A multiport frequency band selector with inherently low loss, flat passbands, and low crosstalk

Conventional waveguide grating routers trade loss for passband flatness and use many grating arms to reduce crosstalk. A frequency band selector with low loss, flat passbands, and low crosstalk is analyzed using multibranch interferometers having a small number of arms. An optical all-pass filter is added to each arm to flatten the phase response around each passband. The bandwidth over which the crosstalk loss is less than 30 dB is improved by an order of magnitude over conventional architectures. Simulations show that the design is quite insensitive to loss and variations in the all-pass filter design parameters.

Efficient architectures for exactly realizing optical filters with optimum bandpass designs

Butterworth, Chebyshev, and elliptic bandpass filter designs are optimal in the sense of band flatness or equiripple characteristics. A new architecture using optical all-pass filters is presented which can realize these designs exactly and efficiently using either ring resonators or reflectors such as Bragg gratings or thin-film interference filters. Design examples are given for a seventh- and eighth-order elliptic filter, and the new architecture is shown to be tolerant to loss. Previously, reflective filters could only approximate optimal responses. An order of magnitude improvement in transition width is demonstrated for an elliptic filter compared to an optimized transmission response for an individual thin-film filter.

Optical all-pass filters for phase response design with applications for dispersion compensation

Lossless all-pass optical filters are introduced, which can approximate any desired phase response while maintaining a constant, unity amplitude response. Architectures using cascade and lattice structures based on ring resonators and cavities defined by reflectors are discussed. Two applications are presented: 1) for fiber dispersion compensation and 2) for compensation of the nonlinear phase response of narrow bandpass optical filters such as thin-film or Bragg grating filters. All orders of dispersion can be compensated in principle, and the filters are periodic so multiple channels can be compensated with a single device. The architectures are very compact compared to alternatives such as chirped Bragg gratings.

Coherent-state optical amplifier: a proposal

A new kind of phase-insensitive optical amplifier is proposed whose output is in the coherent state (ideal laser light) if the input is in the coherent state. In-phase and quadrature modulations are preserved in absolute values. Both modulations can be tapped off. The amplifier employs conventional optical amplifiers, electrical feedback, and all-pass filters. Remarkably, these properties hold when either linear or nonlinear optical amplifiers are employed.