Optical Wavelength Interleaving

Transcription

1 Advances in Wireless and Mobile Communications. ISSN Volume 10, Number 3 (2017), pp Research India Publications Optical Wavelength Interleaving Shivinder Devra Assistant Professor, Department of Electronics Technology, Guru Nanak Dev University, Amritsar , Punjab, India. Abstract An optical wavelength interleaver is a periodic optical filter that combines or separates a number of dense wavelength-division multiplexed (DWDM) signals. The free spectral range (FSR) of core elements of the filter determines its period. An optical interleaver is a 3-port passive fiber-optic device which is used to multiplex odd and even channels i.e. two sets of dense wavelengthdivision multiplexing (DWDM) channels into a composite signal stream in an interleaving way. For example, the two multiplexed signals with 100 GHz spacing are taken in an optical interleaver and get interleaved with each other that creates a denser DWDM signal with channels spaced 50 GHz apart. Similarly denser composite signals with the channel spacing of 25 GHz or 12.5 GHz can be created with the repetition of this process. An optical deinterleaver is an interleaving process used in a reverse direction which separates a denser DWDM signal into odd and even channels. For example, in most DWDM equipment, the standard channel spacing is 100 GHz. But spacing the signal-carrying frequencies every 50 or even 25 GHz can double or even quadruple the number of channels per fiber. Thus, optical interleaver can expand the number of channels per fiber, and devices and/or networks can be upgraded without requiring that all devices be upgraded. The principle of an optical interleaver is based on the multiple-beam interference with stepphase Michelson interferometer and Birefringent crystal networks as the two available approaches at present. The step-phase Michelson interferometer is based on Michelson interferometer combined with Gires-Tournois interferometer. In this paper, the different techniques of interleaving of the number of wavelengths along with their merits and demerits have been reviewed.

2 512 Shivinder Devra Keywords: Wavelength division multiplexing (WDM), Free spectral range (FSR), Finite impulse response filters (FIR), Infinite impulse response filters (IIR). I. INTRODUCTION Optical interleaver is an optical device that is used to multiplex and de-multiplex two or more wavelengths in wavelength division multiplexed (WDM) system [1]. It is implemented using digital signal processing techniques to realize FIR or IIR filters [2], [3], [4], [5]. Optical Interleaver is basically an optical filter with the periodic nature that reduces the number of Fourier components required for a flat pass-band (determined by the wavelength range to be transmitted over the fiber) and highisolation rejection band (outside the required range) [6], [8], [9]. The period is determined by the free-spectral range of the core elements, where narrower channel spacing is achieved by a longer optical path [2]. The various interleaver function designs are shown in fig. 1. Fig. 1. Types of Interleaver functions. All filter functions are periodic in frequency (a) Original interleaver: even and odd channels are separated onto two different ports. (b) Separation of channels out to 1:4 or higher. (c) Banded interleaver, separates even and odd bands of channels, more difficult than 1:2 interleaver because of higher filter rolloff. (d) Asymmetric interleaver separates one channel in N [2].

3 Optical Wavelength Interleaving 513 II. INTERLEAVER DESIGNS Interleaver is a bidirectional device which can act both as multiplexer and demultiplexer with the advantage of high bandwidth utilization. The three most commonly used designs are: 1. Mach-Zehnder interferometer (Lattice filter made with planar waveguides). 2. Michelson interferometer (Gires-Tournois (GT) based interferometer). 3. Arrayed-waveguide router (AWG). 1. Mach-Zehnder Interferometer (MZI): It is a FIR filter with only zeros in the transfer function and having linear phase which is the major advantage offered by it. It is typically used for 1:2 interleaving and can be cascaded to form 1:2 N filters. As shown in the fig. 2 one path experiences an extra delay, but by changing splitting ratios (ki) and delay lengths (ΔL), various filtering functions can be obtained [2]. The coupling ratio of the circuit as a function of individual coupling ratio ( ) is = ( ) cos 2 ( L / ) (1) Fig. 2. The Mach-Zehnder unit cell (a) The differential delay is imparted by the pathlength difference (ΔL) of the arms between the power couplers. Phase is adjusted permanently by the heating pads. (b) Power coupler structure for relaxing the fabrication tolerance. Coupling dependence is made fabrication-insensitive L from fabrication-sensitive by the Cascade of four couplers with equal coupling ratios and three appropriately designed delays [2].

4 514 Shivinder Devra Depending upon the coupling ratio, the power in different branches is distributed. When L = /6 and =50%, the total coupling ratio is 50%. Remains stabilized between 48-50% even when varies between 30-70% [2]. 2. Michelson Interferometer Unlike Mach-Zehnder interferometer which is FIR filter, it is IIR filter. As a result it could not be designed as a linear phase interleaver [2], but it offers advantages such as compact size and higher isolation. It is basically a Gires-Tournois based filter called Gires-Tournois interferometer (GTI) as shown in fig. 3. Fig. 3. Interference Gires-Tournois based interleaver [2]. A GT cavity is loaded into each arm of a Michelson interferometer, the phase difference being an eighth wave. The cavities are detuned from one another by an aggregate half-wave so one is anti-resonant when the other is resonant [2]. The tuning plates located in the air-gap cavities help tune the final filter shape. The cavity length sets the FSR of the interleaver. The 50/50 power splitter interferometrically combines the light returning from arms (1) and (2). The phase shift 1 imparted on path (1) is 1 = -2 tan -1 ( ( 1+ R 1/2 / 1- R 1/2 ) * tan ( kl) ) (2) Where R is the amplitude squared reflection, K is the wave number, L is the cavity length. The overall phase difference between the two arms is = -2 tan -1 ((1 + R 1/2 /1 R 1/2 ) tan (kl)) +

5 Optical Wavelength Interleaving tan -1 (( 1 + R 1/2 /1 R 1/2 ) tan (kl + ) ) - (3) In the output stage, the 50/50 splitting cube mixes the phase response of the two arms giving output intensities are as: I1 = Iocos 2 ( / 2) (4) I2 = Io sin 2 ( / 2) (5) It is bidirectional i.e. if we enter the wavelengths from arms 1 and 2 then it combines or multiplexes the wavelengths from these arms. Temperature dependent cavity phase change and FSR change is avoided by making cavities air-gap type. Ultralow expansion materials maintain the mirror gap [2]. GT filters have infinite impulse response with corresponding poles in the transfer function introducing some chromatic dispersion [3], [4] which can be reduced via cascading with it second complementary GTI [2]. 3. Arrayed waveguide router (AWG) Arrayed waveguide gratings (AWG) are widely used as optical demultiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of increasing the transmission capacity of optical networks significantly by multiplexing a large number of wavelengths into a single optical fiber[8]. The fundamental principle behind the operation of these devices is that light waves of different wavelengths interfere linearly with each other which means that, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber with negligible crosstalk between the channels thereby increasing the capacity of the optical networks. The Arrayed waveguide routers multiplex channels of several wavelengths carrying the required information onto a single optical fiber at the transmission end and they demultiplex these information carrying wavelengths to retrieve individual channels of different wavelengths at the receiving end of an optical communication network[8], [9], [10], [11], [12], [13]. Unlike a Mach-Zehnder, an AWG have many outputs for a single input so it is more suitable for de-multiplexing. The FSR of the grating is given by: FSR = c / ng L (6) ng is the group index of the waveguides at the center-channel wavelength and L is the path difference between adjacent grating arms [2], [5], [6]. In particular FSR = N * C (7) N is the total channels with channel spacing of C.

6 516 Shivinder Devra III. DISCUSSIONS The different designs of interleaving optical wavelengths have their own advantages and disadvantages. Some of these designs are implemented using FIR filters and some using IIR filters accordingly their design complexity is raised. Table 1. Comparison between the various designs of wavelength interleaver. Design Filter Complexity Practically used as Mach-Zehnder Interferometer FIR High 1:2 interleaver Michelson Interferometer IIR Low Both mux and demux Array Waveguide Router Filters FIR Highest 1:N interleaver IV. CONCLUSIONS There are varieties of interleaver designs developed till now but no one is considered to be the best one. Each design has its own unique features and implementation cost. The final selection of particular design is determined by the application in use. An application may require more than one design cascaded to another. ACKNOWLEDGMENTS I would like to thank C.H. Cheng, S.Cao, J. Chen, J.N. Damask, C.R. Doerr, L. Guiziou, G. Harvey, Y. Hibino, H. Li, S. Suzuki, K.Y. Wu, P. Xie, G. Keiser and J.M. Senior in respect that their work, papers etc helped me a lot in writing my full paper on optical wavelength interleaving. REFERENCES [1] Cheng, C.H., Signal Processing for Optical Communication, IEEE Signal Processing Magazine, vol. 23, no. 1, pp , [2] Cao, S., Chen, J., Damask, J.N., Doerr, C.R., Guiziou, L., Harvey, G., Hibino, Y., Li, H., Suzuki, S., Wu, K.Y., Xie, P., 2004, Interleaver Technology: Comparisons and Applications Requirements, Journal of Lightwave Technology, vol. 22, no. 1, pp [3] Arumugam, M., 2001, Optical Fiber Communication An Overview, Journal of Physics, Indian Academy of Sciences, vol. 57, no. 11, pp [4] Knipp, D., 2007, Photonics and Optical Communication, International University Bremen, Course Number , pp [5] Gibson, J.D., 1993, Principles of Analog and Digital Communications, Edition 2nd, Prentice Hall International Series.

7 Optical Wavelength Interleaving 517 [6] Semenova, D.Y., 2003, Optical Communication System, School of Electronics and Communication Engineering, pp [7] Takahashi, H., Suzuki, S., Kato, K., Nishi, I., 1990, Arrayed-waveguide grating for wavelength division multi/demultiplexer with nanometer resolution, Electron. Lett.26, [8] Okamoto, K., Yamada, H., 1995, Arrayed-waveguide grating multiplexer with flat spectral response, Opt. Lett. 20, [9] Murtaza, A., Renuka, P., 2010, Signal Processing Overview of Optical Coherence Tomography Systems for Medical Imaging, White Paper SPRABB9, Texas Instruments, no. 6. [10] Pedrotti, F. L., Pedorotti, L. S., 2007, Introduction to Optics,Pearson Education. [11] Keiser, G., 2000, Optical Fiber Communication, McGraw-Hill International series, Third Edition. [12] Senior, J. M., 2005, Optical Fiber Communications Principles and Practice Harlow Pearson 2005 Prentice Hall International Series in optoelectronics, second edition, no. 3. [13] Mynbaev, D. K., 2001, Fiber Optic Communication Technology, Prentice Hall International Series.

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