Loss-imbalance equalization in arrayed waveguide-grating (AWG) multiplexer cascades

The optimized configuration of an N-channel frequency-selective 2/spl times/2 switch based on an arrayed-waveguide grating (AWG) (2N+1)/spl times/(2N+1) multiplexer is proposed that offers significant crosstalk reduction and loss-imbalance equalization. The optimization is achieved by employing fold-back optical paths instead of loop-back paths. A switch constructed with a silica-based AWG 16/spl times/16 multiplexer demonstrates coherent crosstalk of less than -30 dB in the transmitting frequency-division-multiplexed (FDM) channels; the gain in optimization is more than 15 dB. The largest loss-difference among the seven FDM channels is simultaneously reduced from 4.5 dB to 1.6 dB. These improvements will extend the cascadable number of nodes in all-optical FDM networks. >

Utilizing T-band (1000 nm to 1260 nm) for optical communications is promising for short reach, and large capacity networks, such as data centers or access networks. It is feasible to use this with low-cost coarse wavelength division multiplexing (WDM). However, a tunable wavelength light source is necessary for such applications. In this paper, we propose a new configuration for an external cavity laser, which uses a silica-based arrayed waveguide grating (AWG) for the wavelength selecting element. The external cavity laser consists of a gain chip with high reflection (HR) and anti-reflection (AR) coated facets, coupling lenses, an AWG with AR/HR coatings, and an output fiber. The AWG has 17 connection ports, which correspond to 17 wavelengths with a channel spacing of 1.67 nm. The width of the connection port waveguides was optimized to achieve high coupling efficiency. The AWG chip size is 15 mm x 30 mm. The active layer in the gain chip has InAs quantum dots. The spontaneous emission 3-dB bandwidth was 48 nm (1108 nm to 1156 nm) when a current of 150 mA was injected into the gain chip. The lasing wavelength of the external cavity laser was successfully tuned from 1129.9 nm to 1154.4 nm by selecting the connection ports of the AWG. The typical threshold current was about 130 mA.

Na/sub x/CoO/sub 2-/spl delta// (0.5/spl les/x/spl les/1) crystals were grown by a flux method. The largest Na/sub x/CoO/sub 2-/spl delta// crystal size was 10/spl times/10/spl times/0.5 mm, and the crystal phase depended on the composition of the flux. Crystals grown from a flux of NaCl mixed with Na/sub 2/CO/sub 3/ and B/sub 2/O/sub 3/ and crystals grown from a flux of Na/sub 2/CO/sub 3/ mixed with B/sub 2/O/sub 3/ were identified as Na/sub 0.6/CoO/sub 2-/spl delta// (/spl gamma/ phase) and Na/sub 0.9/CoO/sub 2-/spl gamma// (/spl alpha/ phase), respectively. The in-plane (ab-axis) thermoelectric properties of each phase were measured. The electrical resistivity (/spl rho/) of the /spl gamma/ phase and the a phase showed the almost same metallic temperature dependence except for an unusual increase of /spl rho//sub /spl alpha// at 5-20 K and the different magnitude. The value of /spl rho//sub /spl gamma// and /spl rho//sub /spl alpha// were 1.3 m/spl Omega/cm and 30 m/spl Omega/cm at 300 K, respectively. The Seebeck coefficient (S) of each phase exhibited the different S(T) behavior and the different magnitude, and the values were 100 /spl mu/V/K and 150 /spl mu/V/K at 300 K, respectively.

We propose a new compensation technique for the second-order temperature dependence in a silica-based arrayed-waveguide grating (AWG) multi/demultiplexer with a resin-filled groove that realizes a wide operating temperature range. We newly employ an additional interferometer in the input port and control the optical field perturbation by using a first-mode lightwave at the entrance to the first slab waveguide. We employ the design to fabricate a 32-channel 100-GHz-spacing athermal AWG that is as compact as a conventional AWG, and demonstrate a reduction in the passband wavelength variation from 70 to 22 pm over an extended - 40degC to 80degC temperature range.

This paper studies the asymptotic characteristics of optimum uniform scalar quantizers as N, the number of levels, becomes large. It is shown that the length of the support region increases as (ln N)/sup 1//spl alpha//, when applied to a random variable with a generalized Gaussian density of the form p(x)=ae(-b|x|/sup /spl alpha//). Moreover, the mean-squared error is asymptotically well approximated by /spl Delta//sup 2//12, where /spl Delta/ is the step size, and decreases as (ln N)/sup 2//spl alpha///N/sup 2/. >

This paper describes the performance improvement of 10-Gb/s four-channel WDM dispersion-supported transmission systems by using broadened passband arrayed-waveguide grating demultiplexers. Multimode interference couplers are assumed to be used to broaden the passaband of these demultiplexers. Performance Improvement of 40-Gb/s Capacity Four-Channel WDM Dispersion-Supported Transmission by Using Broadened Passband Arrayed-waveguide Grating Demultiplexers Mario M. Freire Department of Mathematics and Computer Science, University of Beira Interior Rua Marques d'Avila e Bolama, P-6200 Covilha, Portugal Telephone: +351 75 3101700, Fax: +351 75 26198, E-mail: mfreire@ alpha.ubi.pt Henrique J. A. da Silva Department of Electrical Engineering, Pole II of University of Coimbra Pinhal de Marrocos, P-3030 Coimbra, Portugal Telephone: +351 39 7006248, Fax: +351 39 7006247, E-mail: hjas@ci.uc.pt Summary Integrated N×N wavelength multiplexers based on arrayed-waveguide gratings (AWG) are being developed for wavelength division multiplexing (WDM) systems and networks, since they can offer four basic functions: multiplexing, demultiplexing, add/drop multiplexing, and N×N full-interconnection [1]. The use of this technology for demultiplexing in high density WDM systems may impose very stringent requirements on the alignment of signal wavelengths and demultiplexers due to their peaked (Gaussian-like) spectral passband. However, the passband of these demultiplexers may be broadened by the use of multimode interference couplers (MMI), as proposed and demonstrated in [2]. In this paper, we describe the performance improvement due to the use of broadened passband AWG demultiplexers in 40-Gb/s capacity four-channel WDM dispersion-supported transmission (DST) systems with 0.5 nm channel spacing. The block diagram of the simulated 10-Gb/s four-channel WDM-DST system is shown in Fig. 1. The system model is broadly similar to the one reported in [3] and includes a rate equation model for quantum-well lasers, and the frequency responses of a PIN photodiode and of a standard singlemode fiber (SMF) with chromatic dispersion of 17 ps/(nm.km) at 1532 nm. The frequency response of the AWG filter is assumed to be Gaussian and the frequency response of the broadened passband AWG filter is modeled by the double Gaussian approximation [2] with r = 2 , which is the ratio between the peak separation and the FWHM of each simple Gaussian profile. The optical amplifiers (EDFAs) have been considered as linear with a noise figure of 6 dB and an equivalent noise bandwidth of 1 THz. A RC lowpass filter was used to provide the 3-dB bandwidth required by the DST method [4]. The receiver sensitivity was estimated taking into account the signal-ASE and ASE-ASE beat noises, the thermal and shot noises, as well as the contribution due to frequency-to-intensity conversion of laser phase noise after propagation via dispersive fibers [5]. Fig. 2 shows the receiver sensitivity for channel 2 versus fiber length for single-channel DST, without optical filters, and for four-channel WDM-DST using an arrayed-waveguide grating demultiplexer with (BP-AWG) and without (AWG) broadened passband. For comparison purposes, the receiver sensitivity of four-channel WDM-DST using a three-mirror Fabry-Perot demultiplexer (TMF) is also shown. For distances larger than 80 km, the performance of four-channel WDM-DST systems using three-mirror Fabry-Perot demultiplexers is close to the performance of single-channel DST systems without optical filters. Compared with the performance of single-channel DST without optical filters, the use of conventional arrayed-waveguide grating filters as demultiplexers in four-channel WDMDST degrades the system performance at least 0.6 dB for link lengths larger than 11 km. However, the use of broadened passband AWG demultiplexers instead of the conventional AWG demultiplexers improves the performance of the four-channel WDM-DST system at least 1.5 dB, for link lengths larger than 11 km (see Fig. 2). Detailed BER characteristics for channel 2 are displayed in Fig. 3 for four-channel WDM-DST via 204 and 253 km SMF. For the 204 km transmission simulation, sensitivities of -23.886, -23.169 and -24.827 dBm have been estimated for TMF, AWG and BP-AWG demultiplexers, respectively. The BER curves for 253 km clearly indicate an error floor due to laser phase noise. In conclusion, the use of broadened passband AWG demultiplexers improves the performance of four-channel WDM-DST systems with 0.5 nm channel spacing, while conventional AWG demultiplexers are less suitable to operate at this channel spacing.

Using low-temperature plasma-enhanced chemical vapor deposition, a silica-based arrayed-waveguide grating (AWG) filter has been monolithically integrated with germanium photodiodes on a silicon-based photonic platform. All the outputs of a 16-channel AWG with 200- GHz spacing have been equipped with germanium photodiodes. All wavelength channels of the integrated device were confirmed to function as a wavelength-division-demultiplexing receiver for 1.25-Gbps PRBS signals. In addition, we have successfully mounted multi-channel transimpedance amplifiers and limiting amplifiers on the integrated photonic device using flipchip bonding technology.

We consider the relative stability of time invariant and time varying unnormalized lattice filters. First, we consider a set of lattice filters whose reflection parameters /spl alpha//sub i/, obey |/spl alpha//sub i/|/spl les//spl delta//sub i/, and provide necessary and sufficient conditions on the /spl delta//sub i/ that guarantee that each time invariant Lattice in the set has poles inside a circle of prescribed radius 1//spl rho/ 1//spl rho/.

Summary form only given. The feasibility of cavity-controlled spontaneous emission of a motionless two level atom is analyzed quantum mechanically and nonperturbatively. It is proposed to control the emission by putting the excited atom into the combined field of two lossless modes with their frequencies /spl omega//sub 2,1/ detuned symmetrically about the atomic one /spl omega//sub /spl alpha//, i.e. /spl omega//sub 2,1/=/spl omega//sub /spl alpha///spl plusmn//spl Delta/. The controlling /spl omega//sub 2,1/ modes are isolated from the free radiation field by an one-dimensional ideal cavity (or by two crossed ones) at zero temperature. These modes are taken to be linearly polarized in the same plane. Because the wavelength and cavity dimensions are assumed to be of the same order, the retardation effects are neglected. The spontaneous emission is treated as a radiative decay of the macromolecule-like atom+/spl omega//sub 2,1/ modes-subsystem (the H/sub s/-one) under its interaction with the free H/sub k/-field.

The structure of the semiconducting ReSi/sub 2-/spl delta// phase formed in binary rhenium silicide thin films has been studied using X-ray powder diffraction. The films were deposited by magnetron cosputtering onto unheated, oxidized Si-wafers. Subsequent annealing results in crystallization of the amorphous films. By means of Rietveld analysis the unit cell dimensions and the atomic occupation factors of the crystalline phases have been determined. After annealing the film at T=973 K a ReSi/sub 2-/spl delta//-phase phase was formed the chemical composition of which has been determined to be ReSi/sub 1.55/. Temperature enhancement gave rise to increase the occupation factors of Si resulting in an chemical composition of ReSi/sub 1.78/ at T=1073 K. This phase crystallizes with primitive space group P1 and a=0.3138 nm, b=0.3118 nm, c=0.7688 nm and /spl alpha/=89.83.

An implementation method of clutter simulations in a new airborne phased-array radar video simulator is discussed. The means of zero memory nonlinearity (NMNL) is adopted to simulate clutter, whereby let Gaussian white noise pass through a low-pass filter to get the power spectral characteristics, then NMNL is used to achieve the desired amplitude distribution characteristics. The simulated clutter is divided into three parts: the main-beam clutter, the side lobe clutter and the altitude-line clutter. Three channels are adopted to simultaneously simulate the sum channel clutter (/spl Sigma/), the azimuth difference channel clutter (/spl Delta//spl alpha/) and the pitching difference channel clutter (/spl Delta//spl beta/).

Summary form only given. In optical burst switching (OBS) networks, a short switching time is very important for handling fine grain packets such as TCP ACK packets, which are only 40 bytes in length. At data rate of 10 Gbps, such a data burst will occupy a period of only 32 ns. Therefore OXC (optical cross connector) with fast switching speed, for example < 100 ns, are generally required for OBS networks. There are many methods to configure an OXC. Tunable optical filters may used to build large scale OXC. In an optical filter, the tuning ability is usually achieved by introducing extra phase shift via the electro optic, thermo optic, or piezoelectric effect, etc. Up to date, the most commonly used tunable filter structures are interferometric such as Fabry-Perot (FP) and Mach-Zehnder (MZ) interferometers. They are usually poor in the performance of passband flatness and the crosstalk, and require robust electronics to stabilize the central wavelength. Hence, they are rarely practically used as switching elements in the dense wavelength division multiplexing (DWDM) optical networks. In recent years, digitally tunable optical filter (DTOF) has received increasingly attention. Structures based on the arrayed waveguide grating (AWG) and on-off switches have been reported. DTOF based on AWG may require precision thermal control for AWG usually suffers the problem of temperature dependence. In this talk, we present a novel approach to realize the DTOF (digitally tunable optical filter) based on DWDM thin film filters (TTF) and semiconductor optical amplifier (SOA). Apart from the outstanding feature being consistent with the wavelength comb suggested by international telecommunication union-telecommunication (ITU-T), the proposed DTOF is very stable in temperature performance. Furthermore it requires less SOAs in comparison with the reported structures.

In this paper, an optical add-drop filter (ADF) based on a 2D square lattice photonic crystal (PC) is designed. Band diagram of considered ADF is analyzed with Plane Wave Expansion (PWE) technique and simulations on light propagation through it have been carried out using 2D Finite Difference Time Domain (FDTD) technique. The 2D square lattice PC consists of 35*29 infinitely long circular silicon rods of refractive index 3.45 in air background. The ADF design consists of two defect waveguides Bus and Drop which are coupled to each other through a square ring consisting of an octagon shape core. The proposed ADF is a compact structure of 286.19µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . with resonance at wavelength 1549.2nm, a bandwidth of 2 nm, a high quality factor of 774.59, and a dropping efficiency of 90%. The effect of refractive index variation (of different parts of the design) on the performance of ADF is also investigated.

In future optical networks, versatile functionalities will be required for the optical network subsystems to fully utilize the spectral resources with low energy consumption. The key technologies are the spectral efficient MUX/DEMUX technique and flexible control of optical channels with high frequency granularity. An orthogonal frequency division multiplexing (OFDM) and Nyquist wavelength division multiplexing (N-WDM) are the most promising candidates of spectral efficient multiplexing techniques, and all-optical (AO) processing is expected to reduce the energy consumption. In an AO-OFDM systems, discrete Fourier transform (DFT) and inverse DFT (IDFT) are performed in optical domain by specially designed arrayed waveguide gratings (AWGs). In our experiment, 12.5 GHz spaced AO-OFDM system has been successfully demonstrated with no guard interval. In N-WDM systems, the Nyquist signal is generated by using carrier-suppressed return-to-zero (CS-RZ) signal and optical Nyquist filtering, which is achieved with two flat-top AWGs and optical interleaver, and the 25 Gbaud signals are successfully multiplexed in the experiment. Although both AO-OFDM and N-WDM can achieve the highest spectral efficiency, N-WDM is more suitable for flexible optical networks. This is because the N-WDM channels have less spectral overlap with the other channels than AO-OFDM, owing to its rectangular shaped compact spectrum. Therefore, N-WDM channel can be easily multiplexed and demultiplexed by optical filters. At an optical network node, channel defragmentation is indispensable technology to flexibly control the optical channels. We have experimentally demonstrated a format independent optical channel defragmentation with N-WDM signal. We believe these technologies are promising for future flexible optical networks.

PHASARs or Arrayed Waveguide Grating (AWG) multiplexers for application in Dense Wavelength Division Multiplexing optical networks have been realized using silica-waveguide technology. Using waveguide tapers, a flat-top passband characteristic is obtained. Using Fourier transform spectroscopy employing a low coherence interferometer, the power-distribution coefficients and the phase error distribution in such components is evaluated. Because of the coherence length of the used light source being smaller than the optical path length difference in the PHASAR, it is possible to resolve individual waveguides of the array. Thus, the transfer functions of the individual optical paths are accessible to the measurement, enabling the full simulation of the PHASAR transmission spectrum. It is demonstrated that the measured phase error distribution in tapered PHASAR devices consists of contributions from both tapered waveguides and arrayed waveguide grating regions. An evaluation method suitable for investigating both contributions separately is developed. Phase shift introduced by the waveguide tapers is in good agreement with simulation calculations based on the Beam Propagation Method. An analytic transmission calculation based on measured phase and power-distribution coefficients enables a full simulation of PHASARs including insertion loss, bandwidth and crosstalk performance. Excellent agreement with transmission measurements is performed.