AWG technologies for dense WDM applications

This paper reviews the recent technologies supporting arrayed waveguide gratings (AWGs) based on planar lightwave circuits. From the practical point of view, I describe new technologies such as an athermalization and the high-contrast waveguide that includes the mode field transfer needed for low connection loss with conventional optical fiber. As AWG applications, I present new types of AWG modules such as a dynamic gain equalizer and a variable optical attenuator (VOA) MUX/DEMUX. Both of these will play an important role for next-generation wavelength-division multiplexing networks.     

DWDM: shaping the future communications network

This paper describes the increase in the number of optical carriers per single fiber, a technology known as wavelength division multiplexing (WDM). This successful technology takes advantage of existing glass or synthetic fiber. WDM greatly simplifies the traditional signal regeneration since optical amplification is much simpler and more cost effective than single channel amplification. However, network bandwidth elasticity is best addressed with WDM. However photonic devices performing in the low-loss spectral band have enabled more than a single wavelength in the same fiber. Thus, DWDM technology exhibits an inherent flexibility.     

N/spl times/N arrayed waveguide gratings with improved frequency accuracy

The limitations imposed by the frequency standards on the performance of N/spl times/N arrayed waveguide gratings (AWGs) are reviewed for different device layouts. An improved design that allows for an enhanced frequency accuracy of the transmission response of the device is presented and experimentally verified. The maximum deviation from the frequency standards is reduced by almost a factor of two in a 32 /spl times/ 32 AWG with 100 GHz channel spacing.     

Acoustic-optic filters

Optical filters are an important building block in the next generation of optical communication systems. Optical networks seem to be headed towards densely packed wavelength-division multiplexing (WDM) systems. These WDM systems require optical filters for the purpose of wavelength selection and switching. Numerous optical filters have been proposed including acousto-optical filters, electro-optical filters, and Fabry-Perots. Acoustically tunable optical filters (ATOFs) are considered promising. ATOFs are not only an important WDM device; they have great potential in many other applications such as optical spectroscopy. The acousto-optic tunable filter has a unique combination of desirable properties. These properties make it a potentially important building block     

Highly spectral-efficient optical code-division multiplexing transmission system

A highly spectral-efficient transmission system based on optical code-division multiplexing (OCDM) technique is investigated. To meet the rapid increase in the demand of data bandwidth, spectral efficiency is becoming a key factor in optical transport systems. Several modulation formats along with the optical receiver design have been proposed to upgrade the spectral efficiency. OCDM is one of the promising techniques for this purpose. OCDM is the other class of multiplexing technique than optical time-division multiplexing and wavelength-division multiplexing (WDM). OCDM provides unique features such as asynchronous transmission, secure communication, soft capacity on demand, and high degree of scalability. In this paper, we apply OCDM technique to the highly spectral-efficient transmission system by quaternary phase-shift keying optical encoding/decoding along with ultrafast optical time-gating and optical hard thresholding. As a result, a transmission of 6.4 Tbit/s OCDM/WDM (4 OCDM/spl times/40 WDM/spl times/40 Gbit/s) using only C-band wavelength region is experimentally demonstrated with 1.6-bit/s/Hz spectral efficiency.     

Calculation of dispersion in arrayed waveguide grating demultiplexers by a shifting-image method

It is shown that, as a first approximation, chromatic dispersion in demultiplexers based on arrayed waveguide gratings (AWGs) is proportional to the local spatial curvature of the wavefronts of the field at the output of the input guide, whose image shifts across the output guide as frequency changes. More accurate results can be obtained by convolving the input field with the AWG impulse response and the output guide mode. This approach to the calculation of dispersion provides physical insights from the field of imaging, not available with the commonly used method which relies on a Fourier series approach. Spherical wavefronts lead to constant dispersion. The use of a parabolic taper at the end of the input guide can lead to large amounts of dispersion, varying across the filter passband; this case is treated in detail. The method can also be adapted for use with demultiplexers based on reflective echelle gratings.     

Optical delay line based on arrayed waveguide gratings' spectral periodicity and dispersive media for antenna beamforming applications

An optical delay line (ODL) based on the free spectral range periodicity of arrayed waveguide gratings (AWGs) and dispersive media is analyzed with emphasis on the AWG physical features that limit its performance as a photonic antenna beamforming network. This ODL presents multiple simultaneous true-time delay, straightforward multibeam capability and a drastic cost reduction in comparison with previously reported schemes. Moreover, a simulation model of the ODL that takes into account the AWG frequency misalignment is presented. Finally, experimental results are also provided for 40-GHz electrical signals.     

The impact of phase errors on arrayed waveguide gratings

The impact of phase errors on arrayed waveguide gratings (AWGs) due to waveguide length deviation in AWGs has been analyzed theoretically. A statistical model to estimate the influence of phase errors in silica-based arrayed waveguide multiplexers was developed, using Fourier optics and scalar diffraction theory. In addition, the insertion loss and crosstalk of AWGs caused by phase errors were analyzed. To maintain the adjacent crosstalk at less than -25 dB, a maximum standard deviation /spl sigma//sub max/ of phase errors to the design of AWGs is obtained.     

Field demonstration of in-line all-optical wavelength conversion in a WDM dispersion managed 40-Gbit/s link

The development of wavelength-division multiplexing (WDM) all-optical transport networks is an interesting solution to increase the capacity of long-haul transmission systems and to solve the route-exhaust problems of metropolitan networks, driving down the cost of that traffic. Routing can be achieved using a transparent device able to select and interchange wavelengths, such as an all-optical wavelength converter. In this paper, an optical transport network over an embedded link located between Rome and Pomezia in Italy is emulated. The transmission has been realized along a WDM, 5/spl times/100 km long, dispersion managed link at 40 Gb/s. The in-line rerouting process has been controlled by means of an all-optical wavelength converter realized with a periodically poled lithium niobate waveguide. Moreover, a polarization-independent scheme for the converter has been exploited to allow the in-line signal processing. This scheme is based on the counterpropagation of TE and TM signal components along the same guide and results extremely compact. In this paper it is demonstrated that wavelength conversion and rerouting add no penalty with respect to the simple transmission along the embedded cable. This result seems to be another step toward the feasibility of true all-optical networks.     

Large-Scale InP Photonic Integrated Circuits: Enabling Efficient Scaling of Optical Transport Networks

An overview of commercially available large-scale photonic integrated circuits (PICs) in indium phosphide is provided. Results of 100-Gb/s PICs that are field-deployed in wavelength-division-multiplexing (WDM) networks is provided, along with development results showing scaling of both channel count and channel bit rate to implement next-generation PICs with an aggregate capacity of 1.6 Tb/s. Use of PIC-enabled WDM systems allows affordable optical-electrical-optical (OEO) conversion and the implementation of "digital" optical networks with enhanced sub-wavelength reconfigurable bandwidth management, digital performance monitoring, and protection features. PICs will enable the continued capacity scaling required for next-generation IP core networks and support of high-bandwith 40-G and 100-GbE service connectivity between core routers     

Modelling PBG devices to DWDM filters design

The fibre optic transmission systems require a bandwidth of about 25 THz in telecommunications networks, for which it is necessary to resort to dense wavelength division multiplexing (DWDM) systems. These systems need optical filters to broadcast selectively or not in a given wavelength band. A new very promising technology for these applications is the photonic crystals with forbidden bandgap (photonic bandgap (PBG)). In this paper, we propose a model of PBG devices to design DWDM filters on PBG materials. Copyright © 2004 John Wiley & Sons, Ltd.     

Monolithically integrated eight-channel WDM modulator with narrow channel spacing and high throughput

We demonstrate an eight-channel wavelength division multiplexing (WDM) modulator module that monolithically integrates arrayed waveguide gratings and semiconductor optical amplifiers and electroabsorption optical modulators arrays. The compact module can generate individual optical signals for each WDM channel with low optical and electrical crosstalk. We show two configurations for the narrow channel spacing of 25 GHz and high throughput of beyond 80 Gb/s. Combining this WDM modulator with a multi-wavelength light source is a promising approach to creating a compact WDM optical transmitter.     

Optical multiplexing technologies for access-area applications

This paper describes optical multiplexing technologies expected to be applied to access-area networks. First, these technologies are broadly categorized into wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA), and possible variants are described in a comparison to time-division multiple access (TDMA). Next, their typical performances reported so far are overviewed in relation to the light sources utilized. While it is well known that WDM/OCDMA exhibits higher performance with laser sources, it is pointed that WDM/OCDMA with incoherent light sources are interesting alternatives for access-area applications. Two multiplexing methods that use incoherent light sources are introduced in more detail; one is spectrum-sliced WDM and the other is coherence-multiplexed CDMA. After the introduction of transmission experiments, their performance limitations are discussed.     

Plasma-induced quantum well intermixing for monolithic photonic integration

Plasma-induced quantum well intermixing (QWI) has been developed for tuning the bandgap of III-V compound semiconductor materials using an inductively coupled plasma system at the postgrowth level. In this paper, we present the capability of the technique for a high-density photonic integration process, which offers three aspects of investigation: 1) universality to a wide range of III-V compound material systems covering the wavelength range from 700 to 1600 nm; 2) spatial resolution of the process; and 3) single-step multiple bandgap creation. To verify the monolithic integration capability, a simple photonic integrated chip has been fabricated using Ar plasma-induced QWI in the form of a two-section extended cavity laser diode, where an active laser is integrated with an intermixed low-loss waveguide.     

Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology

We give an overview of recent progress in passive spectral filters and demultiplexers based on silicon-on-insulator photonic wire waveguides: ring resonators, interferometers, arrayed waveguide gratings, and echelle diffraction gratings, all benefit from the high-index contrast possible with silicon photonics. We show how the current generation of devices has improved crosstalk levels, insertion loss, and uniformity due to an improved fabrication process based on 193 nm lithography.      

Design and applications of silica-based planar lightwave circuits

Planar lightwave circuits (PLCs) are waveguide devices that integrate fiber-matched optical waveguides on silicon or glass substrate to provide an efficient means of interaction for the guided-wave optical signals, PLCs provide various important and functional devices for optical wavelength-division multiplexing, time-division-multiplexing systems, and subscriber networks. This paper reviews the recent progress and future prospects of PLC technologies including arrayed-waveguide grating multiplexers, optical add-drop multiplexers and hybrid optoelectronics integration technologies     

Statistical analysis of correlated phase error in transmission characteristics of arrayed-waveguide gratings

Transmission characteristics of arrayed waveguide gratings (AWGs) are significantly influenced by phase errors in a waveguide array, which are easily caused by the variation of fabrication process. In this paper, the statistical analysis of transmission characteristics is presented using correlation and spectral density of phase errors. A simple statistical approach, based on a simplified model of a waveguide array, is newly introduced to clarify statistical behavior of phase errors in AWGs. Spectral density is modeled from measured phase errors, and then average transmission characteristics are simulated for AWGs with different channel spacing. Average transmission characteristics, simulated with the spectral density model, almost describe measured results in the cases of various channel spacing. It is also indicated that an AWG may have larger adjacent-channel crosstalk with narrower channel spacing because the adjacent-channel crosstalk is affected by phase errors with a smaller spatial frequency.     

Tuning in to RF MEMS

RF MEMS technology was initially developed as a replacement for GaAs HEMT switches and p-i-n diodes for low-loss switching networks and X-band to mm-wave phase shifters. However, we have found that its very low loss properties (high device Q), its simple microwave circuit model and zero power consumption, its high power (voltage/current) handling capabilities, and its very low distortion properties, all make it the ideal tuning device for reconfigurable filters, antennas and impedance matching networks. In fact, reconfigurable networks are currently being funded at the same level-if not higher-than RF MEMS phase shifters, and in our opinion, are much more challenging for high-Q designs.     

FDTD analysis of switching characteristics in magnetooptic functional devices using magnetostatic surface waves

Collinear magnetooptic interaction with magnetostatic surface waves (MSSW) can be used for wavelength‐selective switches, wavelength filters, and frequency shifters in wavelength‐division‐multiplexed (WDM) photonic networks and optical processing systems. The switching efficiency can be improved with a multilayer waveguide structure. To investigate the dynamic switching characteristics, the FDTD method was employed. The mode conversion between TE and TM mode was successfully demonstrated with FDTD simulation. The filtering characteristics were also evaluated. The FDTD results were compared with the result from the coupled mode theory, and good agreement was obtained. Switching of an optical pulse was also demonstrated by the FDTD method. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(1): 40–47, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20501     

Optimal routing and wavelength assignment for survivable multifibre WDM networks

Multifibre optical networks use a bundle of fibres to realize a link between two optical nodes. Such networks can offer significant economic benefits over single-fibre networks because of their ability to relax the restrictions imposed by the wavelength continuity constraint and their potential for handling future growth. This paper introduces two new and efficient integer linear program (ILP) formulations for dynamic wavelength allocation in survivable multifibre wavelength-division multiplexing (WDM) networks, using dedicated and shared protection. Single-fibre networks, both with and without wavelength conversion, can be treated as a special case of these formulations. The new formulations have been tested on several well-known WDM networks, and the results have been compared to those for single-fibre networks. A simple heuristic for dynamic lightpath allocation is also proposed, and its performance is validated by a comparison of the results to optimal solutions generated by the ILPs. Experimental results demonstrate that the new ILPs are feasible for current networks under low-to-medium traffic. For very large or highly congested networks, the heuristic can be used.