A record performance metric waveguide grating router (WGR) design with a 200 GHz free spectral range (FSR) capable of resolving sub-GHz resolution spectral features is developed for a fine resolution photonic spectral processor (PSP). The WGR’s FSR was designed to support subchannel add/drop from a superchannel of 1 Tb/s capacity. Due to fabrication imperfections, we introduce phase corrections to the light beams emerging from the 250 waveguides of the WGR output using a liquid crystal on silicon (LCoS) phase spatial light modulator (SLM) placed in an imaging configuration. A second LCoS SLM is located at the Fourier plane, for arbitrary spectral amplitude and phase manipulations. The PSP is utilized in multiple system transmission experiments, such as flexible spectral shaping and subcarrier drop demultiplexing with sub-GHz spectral resolution.
As traffic volumes carried by optical networks continue to grow by tens of percent year over year, we are rapidly approaching the capacity limit of the conventional communication band within a single-mode fiber. New measures such as elastic optical networking, spectral extension to multi-bands, and spatial expansion to additional fiber overlays or new fiber types are all being considered as potential solutions, whether near term or far. In this tutorial paper, we survey the photonic switching hardware solutions in support of evolving optical networking solutions enabling capacity expansion based on the proposed approaches. We also suggest how reconfigurable add/drop multiplexing nodes will evolve under these scenarios and gauge their properties and relative cost scalings. We identify that the switching technologies continue to evolve and offer network operators the required flexibility in routing information channels in both the spectral and spatial domains. New wavelength-selective switch designs can now support greater resolution, increased functionality and packing density, as well as operation with multiple input and output ports. Various switching constraints can be applied, such as routing of complete spatial superchannels, in an effort to reduce the network cost and simplify the routing protocols and managed pathway count. However, such constraints also reduce the transport efficiency when the network is only partially loaded, and may incur fragmentation. System tradeoffs between switching granularity and implementation complexity and cost will have to be carefully considered for future high-capacity SDM–WDM optical networks. In this work, we present the first cost comparisons, to our knowledge, of the different approaches in an effort to quantify such tradeoffs.
Spectrally dispersed light from a fine resolution waveguide grating router (WGR) of 25-GHz free spectral range that radiates to free space is spatially filtered at ~1 GHz optical resolution and 50 MHz spectral addressability using a liquid crystal on Silicon (LCoS) spatial light modulator (SLM). Fabrication imperfections leading to phase errors on the 32 waveguide arms of the WGR are corrected using a UV pulsed laser to inscribe permanent optical path changes to the waveguides. WGR phase errors are permanently trimmed waveguide-by-waveguide with an excimer laser by inducing stress in the glass cladding above the waveguide for coarse setting and using the photosensitivity effect for fine setting. The WGR was then mated with an LCoS SLM located at the Fourier plane to form a photonic spectral processor, for arbitrary spectral amplitude and phase manipulations.
We present here the work performed in the EU-funded flexible optical cross-connect (FOX-C) project, which investigates and develops new flexible optical switching solutions with ultra-fine spectral granularity. Thanks to high spectral resolution filtering elements, the sub-channel content can be dropped from or added to a super-channel, offering high flexibility to optical transport networks through the fine adaptability of the network resources to the traffic demands. For the first time, the FOX-C solutions developed in the project are investigated here and evaluated experimentally. Their efficiency is demonstrated over two high spectral efficiency modulation schemes, namely multi-band orthogonal frequency division multiplexing (MB-OFDM) and Nyquist WDM (N-WDM) formats. Finally, in order to demonstrate the relevance of the FOX-C node concepts, a networking study comparing the economic advantages of the FOX-C optical aggregation solution versus the electronic one is performed.
The idea behind flexible optical transmission is to optimize the use of fiber capacity by flexibly assigning spectrum and data rate adapted to the needs of end-to-end connection requests. Several techniques have been proposed to this end. One such technique is based on the utilization of Nyquist-shaping filters with the aim of reducing the required channel spacing in flexible single-carrier and super-channel optical transmission systems. Nonetheless, the imperfect shape of the filters used at the bandwidth-variable transceivers and wavelength-selective switches compels the necessity to allocate a certain spectral guard band between (sub-)channels. Bearing this is mind, in this paper, we focus on the evaluation of the network-level performance, in terms of the filter characteristics and the WDM frequency-grid granularity, of flexible Nyquist-WDM-based transmission. We demonstrate that a granularity of 6.25 GHz offers a good compromise between network performance and filter requirements for spectrum assignment to single-carrier and super-channel signals. However, for subchannel allocation within a super-channel, granularities as fine as 3.125 GHz are required to take advantage of filters with resolutions in the region of 1-1.2 GHz. Finer filter resolutions and frequency slot granularities provide negligible performance improvement.
We propose a novel architecture for all-optical add-drop multiplexing of OFDM signals. Sub-channel extraction is achieved by means of waveform replication and coherent subtraction from the OFDM super-channel. Numerical simulations have been carried out to benchmark the performance of the architecture against critical design parameters.
Efficient mode division multiplexing using the spatial aperture sampled concept from single-modefibers to a few-mode fiber is extended to circular step index fibers supporting up to ten spatial modes and to annular refractive index profile fibers supporting nine optical orbital angular momentum modes (mode counts double when considering polarization states for each case). Each sampling beam aperture is spatially shaped for lower average coupling losses and mode dependent losses or a balance thereof. The optimization demonstrates the scalability and consistent low losses for the aperture-sampled mode multiplexer for increasing mode counts, as opposed to the phase hologram-based mode conversion technique. The aperture-sampling approach is also found to be robust to small input fiber alignment errors and fiber geometrical distortions.
Over the last few decades, network traffic has consistently grown at an exponential rate and was efficiently satisfied using WDM and more efficient coding schemes requiring coherent detection. There is no indication that the network traffic growth trend will cease anytime soon, and we are nearing the day when the capacity of the ubiquitous single-mode fiber will be fully exploited. Space-domain multiplexing (SDM) for high-capacity transmission is the promising solution with the scaling potential to meet future capacity demands. However, there is still a large technological gap between current WDM optical communication system designs and SDM network implementations. In this article we lay the foundation of switching node designs for future WDM-SDM optical networks.
The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The free-space optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.
Accurate amplitude and phase measurements of ultrashort optical waveforms are essential for their use in a wide range of scientific disciplines. Here we report the first demonstration of full-field optical reconstruction of ultrashort waveforms using a time-to-space converter, followed by a spatial recording of an interferogram. The algorithm-free technique is demonstrated by measuring ultrashort pulses that are widely frequency chirped from negative to positive, as well as phase modulated pulse packets. Amplitude and phase measurements were recorded for pulses ranging from 0.5 ps to 10 ps duration, with measured dimensionless chirp parameter values from −30 to 30. The inherently single-shot nature of time-to-space conversion enables full-field measurement of complex and non-repetitive waveforms.
Phase modulated sub-picosecond pulses are converted by a time-to-space processor to quasi-monochromatic spatial beams that are spatially demultiplexed and coherently detected in real-time. The time-to-space processor, based on sum-frequency generation (SFG), serves as a serial-to-parallel converter, reducing the temporal bandwidth of the ultrashort pulse to match the bandwidth of optoelectronic receivers. As the SFG process is phase preserving, we demonstrate homodyne coherent detection of phase modulated temporal pulses by mixing the demultiplexed SFG beam with a narrow linewidth local oscillator (LO) resulting in single-shot phase detection of the converted pulses at a balanced detector. Positively and negatively phase-modulated signal pulses are individually detected and LO shot noise limited operation is achieved. This demonstration of real-time demultiplexing followed by single-shot full-field detection of individual pulses, highlights the potential of time-to-space conversion for ultrahigh bit rate optical communications and data processing applications.
Variable optical attenuation (VOA) for three-mode fiber is experimentally presented, utilizing an amplitude spatial light modulator (SLM), achieving up to −28dB uniform attenuation for all modes. Using the ability to spatially vary the attenuation distribution with the SLM, we also achieve up to 10dB differential attenuation between the fiber’s two supported mode group (LP01 and LP11). The spatially selective attenuation serves as the basis of a dynamic mode-group equalizer (DME), potentially gain-balancing mode dependent optical amplification. We extend the experimental three mode DME functionality with a performance analysis of a fiber supporting 6 spatial modes in four mode groups. The spatial modes’ distribution and overlap limit the available dynamic range and performance of the DME in the higher mode count case.
We generate transform-limited WDM optical sampling pulse bursts by filtering ultrashort pulses from a mode-locked laser. A phase spatial light modulator (SLM) is used in a biased pulse shaper to circumvent the need to modulate with 2π phase wraps, which are known to limit the phase response. The arrangement compresses and retimes user-selectable bandwidths from the optical short pulse source with precise control of pulse bandwidth, pulse stream rates, and duty cycle.
We introduce a next-generation long-reach access optical network (35 dB loss budget 2 dB margin) delivering up to 40G/40G per passive 1∶256 optical distribution network, supportingsymmetrical1 Gb∕s rates perhome user or up to 40 Gb∕s for business users (e.g., enterprises, antenna sites). The proposed system is based on a novel spectrally efficient orthogonal frequency division multiplexing/ wavelength division multiplexingOFDM/WDMarchitecture symmetrically using 16-QAM OFDM polarization diversity in both the downstream and upstream in order to serve low-cost energy-efficient symmetric 1 Gb∕s optical network units (ONUs), which are self-coherent, laserless, colorless, and tunable-filter-free. Each ONU comprises a standard semiconductor optical amplifier (SOA), a silicon-based photonic integrated circuit (PIC), and mixed-signal electronic integrated circuits (ICs) performing the signal processing at a relatively slow rate as compared with the overall passive optical network (PON) throughput: digital to analog converters (DACs) and analog to digital converters (ADCs) at 417 MS∕s for the home user ONUs.
We report a two-span, 67-km space-divisionmultiplexed (SDM) wavelength-division-multiplexed (WDM) system incorporating the first reconfigurable optical add–drop multiplexer (ROADM) supporting spatial superchannels and the first cladding-pumped multicore erbium-doped fiber amplifier directly spliced to multicore transmission fiber. The ROADM subsystem utilizes two conventional 1 × 20 wavelength selective switches (WSS) each configured to implement a 7 × (1 × 2) WSS. ROADM performance tests indicate that the subchannel insertion losses, attenuation accuracies, and passband widths are well matched to each other and show no significant penalty, compared to the conventional operating mode for the WSS. For 6 × 40 × 128-Gb/sSDM-WDMpolarization-multiplexed quadrature phaseshift- keyed (PM-QPSK) transmission on 50 GHz spacing, optical signal-to-noise ratio penalties are less than 1.6 dB in Add, Drop, and Express paths. In addition, we demonstrate the feasibility of utilizing joint signal processing of subchannels in this two-span, ROADM system.
We propose a new type of photonic analog-to-digital converter (ADC), designed for high-resolution (>7 bit) and high sampling rates (scalable to tens of GS/s). It is based on encoding the input analog voltage signal onto the phase of an optical pulse stream originating from a modelocked laser, and uses spatial oversampling as a means to improve the conversion resolution. This paper describes the concept of spatial oversampling and draws its similarities to the commonly used temporal oversampling. The design and fabrication of a LiNbO3/silica hybrid photonic integrated circuit for implementing the spatial oversampling is shown, and its abilities are demonstrated experimentally by digitizing gigahertz signals (frequencies up to 18GHz) at an undersampled rate of 2.56GS/s with a conversion resolution of up to 7.6 effective bits. Oversampling factors of 1-4 are demonstrated.
We present a new micro-electro-mechanical system (MEMS) spatial light modulator (SLM) with a two-dimensional array of tightly-spaced square micromirrors (or pixels) designed to sag (or piston motion). This diffractive MEMS modulator is to be used for independently applying amplitude attenuation and phase control to spectrally-dispersed light along one dimension. The spectral phase and amplitude modulator operate in conjunction with a dispersive optical setup, where spatially resolved frequency components are to be incident onto and independently modulated by the device. The MEMS design is based on two common actuators per array column, in order to set the two degrees of freedom of amplitude and phase for every spectral component. This MEMS SLM is thus optimal in actuator/electrode count, especially when compared to conventional SLM where each pixel is independently actuated. The MEMS sag range is compatible with near-IR wavelengths used in the fiber-optic communication band.