1542 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 11, NOVEMBER 2002 A Single WDM Multi/Demultiplexer for All SMF Transmission Windows in 1280–1680 nm Alexandros Stavdas, Member, IEEE Abstract—A new advanced design methodology for multi/de- multiplexers based on holographic concave gratings is presented. The proposed design is benchmarked against a single grating operating in the entire useful range of the single-mode fiber (1280–1680 nm). In addition, a new design for a grating serving as a 1000 wavelength-division-multiplexing channel demultiplexer within the 1450–1650 nm spectral range is given. It is shown that the new mount offers considerably improved performance. Index Terms—Concave grating, demultiplexers, diffraction grating, high capacity systems, SMF transmission windows, WDM. I. INTRODUCTION W AVELENGTH-DIVISION multiplexing (WDM) has significantly bolstered optical networking evolution and will continue to underpin the next steps of this revolution. The upcoming major challenges include the introduction of WDM in metropolitan and access networks as well as the exploitation of all the available transmission bands of the single-mode fiber (SMF). The latter is an important milestone for attaining transmission capacities of 10 Tb/s [1] and beyond. In particular, advances in single-mode fiber design led to the introduction of fiber types like AllWave offering the possibility of exploiting the 1400-nm low-dispersion region and/or intro- ducing coarse-WDM in the access network. Also, there are still legacy systems in the 1.3- m window and the need to remain operational concurrently with dense WDM (DWDM) in the 1.55- m region is acute. Despite these developments, a single optical multiplexer covering the spectral range 1280–1680 nm has not been reported yet. The aim of this work is to present the design methodology and the simulated performance of a demultiplexer based on a holographic concave grating offering 500 channels with 0.8-nm channel spacing covering the entire aforementioned spectrum. Also worth mentioning is a feature of this device, its scalability. The same concave grating could be deployed for a 40-channel demultiplexer within the C band as well as for a demultiplexer covering the entire 1280–1680 nm band. Hence, it could equally serve either a DWDM core network or a coarse-WDM access network. In Section II the design methodology of such a grating is presented and in Section III its simulated performance. In the same section, the application of the proposed method for im- Manuscript received April 22, 2002; revised July 24, 2002. The author is with the National Technical University of Athens, De- partment of Electrical and Computer Engineering, Athens, Greece (e-mail: astavdas@cc.ece.ntua.gr). Digital Object Identifier 10.1109/LPT.2002.803873. proving the performance of a 1000-channel demultiplexer in the 1450–1650–nm spectral range is illustrated. II. DESIGN METHODOLOGY The large optical aberrations of concave gratings that degrade image quality have been extensively studied for ruled [2] or holographic [3] spectrographs. The reader interested could find a list of references in [4]. The design methodology of holo- graphic concave grating multi/demultiplexers is presented in [5] and [6]. Here, the notation and terminology of [6] is adopted. In concave grating design, one of the quantities, wavefront variance (WV), Strehl ratio, or the Rayleigh criterion is used as a merit function for assessing image quality. The relation be- tween these quantities is presented in [6]. Here, the WV, given by [6, eq. (6)], will be used in the subsequent optimizations. The minimization of WV for a single wavelength is not a suf- ficient condition for ensuring good optical performance across a reasonable spectral range since the aberrations increase lin- early away from the stigmatic point. Thus in [5], the reconstruc- tion geometry satisfying first-order stationarity conditions with respect to wavelength were readily incorporated in the design. In these mounts the optical aberrations of the wavelength scale only according to where is the aberration corrected (central wavelength) in the band under consideration [6, eq. (3)]. A comparison between the simulated performance obtained from the methodology of [5] and experimental results is presented in [8] and [9]. For even wider bandwidth demulti- plexers, the only viable approach in the framework of first-order stationarity conditions is a design allowing trading off the per- formance of the central wavelength to those located at spectral edges [6]. Nevertheless, for designing a grating with uniform performance across 1280–1680 nm, the condition (first order stationarity) could not prevent the wavelengths in the outer parts of the spectrum from being seriously degraded by the aberrations. In [7], the general methodology for obtaining second order stationarity mounts is presented. Nevertheless, for highly cor- rected systems apart from [7, eqs. (18) and (19)], the conditions of zero and stationary meridional focus ( ) should also be introduced. Here, the methodology of [7] is extended in the following way. With reference to Fig. 1, if is the grating constant, is the angle of diffraction for , the reciprocal linear dispersion is whilst the condition for second degree stationarity of , i.e., is met when . The latter two equations are solved with 1041-1135/02$17.00 © 2002 IEEE