High Speed Coherent Optical Fiber Communication

Transcription

1 High Speed Coherent Optical Fiber Communication B.Bala Subbanna,M.E,(Ph.D) Associate Professor, Skd Eng College,Gooty. Dr.Stephen Arputha Raj MIE,M.E,M.S,Ph.D,Dean, Amet University. Abstract: The first proposal of coherent optical communications using heterodyne detection was done by De Lange in 1970 [1], it did not attract any attention because the IMDD (intensity modulation and direct detection scheme became mainstream in optical fiber communication systems during the 1970s. On the other hand, in 1980, Ok oshi and Kikuchi and Fabre and Le Guen independently demonstrated precise frequency stabilization of semiconductor lasers, which aimed at optical heterodyne detection for optical fiber communications. Figure 2.1 shows the heterodyne receiver proposed in for frequency-division multiplexed (FDM) optical communication systems. Each FDM channel was selected by heterodyne detection with multiple local oscillators (LOs) prepared in the receiver. The center frequency drift of semiconductor lasers for a transmitter and a local oscillator could be maintained below 10 MHz. Even when the frequency drift of the transmitter laser was suppressed, the carrier phase was fluctuating randomly due to large phase noise of semiconductor lasers. Therefore, narrowing spectral line widths of semiconductor lasers was the crucial issue for realizing stable heterodyne detection. The method of measuring laser line widths, called the delayed self-heterodyne method, was invented, and the spectral property of semiconductor lasers was studied extensively. It was found that the line width of GaAlAs lasers was typically in the range of 10 MHz. Such a narrow line width together with the precisely controlled center frequency accelerated researches of coherent optical communications based on semiconductor lasers, digital signal processing for coherent receivers. Outputs from the homodyne receiver comprising phase and polarization diversities are processed by digital signal-processing circuits, restoring the complex amplitude of the signal in a stable manner despite fluctuations of the carrier phase and the signal SOP. Symbol-by-symbol control of such time-varying parameters in the digital domain can greatly enhance the system stability compared with optical control. Keywords: IMDD, POS,CD,HOMODYNE RECEIVER,COHARENT RECEIVER,SOP,PSK,QPSK. 1.INTRODUCTION: The research and development in optical fiber communication systems started in the first half of the 1970s. Such systems used intensity modulation of semiconductor lasers, and the optical signal intensity transmitted through an optical fiber was detected by a photodiode, which acted as a square-law detector. This combination of the transmitter and the receiver is called the intensity modulation and direct detection (IMDD) scheme, which has been commonly, employed in optical communication systems up to the present date. Such IMDD scheme has a great advantage that the receiver sensitivity is independent of the carrier phase and the state of polarization (SOP) of the incoming signal, which are randomly fluctuating in real systems. 2. METHODS: Coherent Omdd Modulation parameters I and Q or Amplitude and Phase Intensity Detection method Heterodyne or homodyne detection Direct detection. Adaptive control Necessary (carrier phase and SOP) not necessary. Working Principle of Coherent Optical Detection: The coherent receiver measures the complex amplitude of the optical signal with the shot-noise-limited sensitivity and how information on the state of polarization can be extracted by the use of polarization diversity the configuration of the coherent optical receiver. The fundamental concept behind coherent 96

2 detection is to take the product of electric fields of the modulated signal light and the continuous-wave (CW) local oscillator (LO). Let the optical signal incoming from the transmitter be Es (t) = As (t) exp( jωs t), (1) where As (t) is the complex amplitude and ωs the angular frequency. Similarly, the electric field of LO prepared at the receiver can be written as ELO(t) = ALO exp( jωlot), (2) where ALO is the constant complex amplitude and ωlo the angular frequency of LO. We note here that the complex amplitudes As and ALO are related to the signal power Ps and the LO power PLO by Ps = As 2 /2 and PLO = ALO 2 /2, respectively. Balanced detection is usually introduced into the coherent receiver as a means to suppress the dc component and maximize the signal photocurrent. The concept resides in using a 3-dB optical coupler that adds a 180 phase shift to either the signal field or the LO field between the two output ports. When the signal and LO Potential of Coherent Optical Communication Systems : Coherent optical communication promises ultimate performance and dispersion tolerance in upgraded or newly built fiber links. Many offline experiments have already been conducted to evaluate the possibilities offered by this technology. Most of them concentrated on the evaluation of polarizationmultiplexed quadrature phase shift keying (QPSK) systems. But also 8-level phase shift keying (8-PSK) [4] and quadrature amplitude modulation (QAM) have already been investigated. The results of these experiments clearly demonstrate the potential of coherent optical communication. For polarization multiplexed QPSK transmission data rates up to 86 Gbit/s have been reached [6]. In another experiment 42.8 Gbit/s polarization multiplexed QPSK data was transmitted over 6400 km of single mode fiber (SMF) with purely electronic dispersion compensation. This represents the longest reach achieved for transmission without optical dispersion compensation for any 10 GBaud system operating over standard fiber as well as for all 40Gbit/s systems. The highest number of bit/symbol achieved in a coherent transmission experiment was 12 bit/symbol in a polarization multiplexed 64 QAM test bed [8], reaching a spectral efficiency as high as 6 bit/s/hz Recent Real Time Coherent Transmission Experiments: The first real time coherent synchronous QPSK transmission was published in Mbit/s QPSK data was received in a digital receiver using a phase-locked-loop (PLL) to compensate for the intermediate frequency and phase offset between the signal laser and the local oscillator laser. The drawback of the PLL approach is that external cavity lasers (ECL) are required. The PLL can only lock, if the laser line width time s symbol rate ratio is below Another approach which is more tolerant against laser phase noise and allows to use standard DFB lasers is the feed-forward carrier recovery [12]. The first real time coherent transmission system using this kind of receiver was demonstrated in 1992 [13], with a throughput of 565 Mbit/s of PSK modulated data. The data was recovered synchronously in a coherent receiver with analog carrier recovery and standard DFB lasers. \ 97

3 More than a decade later, in June 2006, the first real time QPSK transmission with standard DFB lasers was published [14]. This time a digital synchronous coherent receiver with feed-forward carrier recovery was applied for a data rate of 800 Mbit/s. The system was realized using commercially available ADCs interfacing with a field programmable gate array (FPGA) for signal processing. In November 2006 the maximum data rate for coherent synchronous QPSK transmission was already pushed to 4.4 Gbit/s. Another half year later the spectral efficiency for realtime systems was doubled by a polarizationmultiplexed QPSK transmission system with fast electronic polarization tracking at a data rate of 2.8 Gbit/s, still realized with commercially available electronic components. But in July 2007 Nortel announced the first all time implementation of a polarization multiplexed coherent QPSK system with a data rate of 40 Gbit/s, implemented this time in an application specific integrated circuit (ASIC). 3.RESULTS AND DISCUSSIONS: The amplitude versus frequency variations are shown below. The polarization modes with dispersions are showing below. The Diagram of Optical Fiber Cable: 98

4 Flow Chart for Maximum Sharpness of The Signal. 99

5 The amplitude versus frequency variations in the signal floware shown below. 100

6 4. Conclusion: The capabilities of coherent optical technology are essential technology for optical transmission systems in the range of 100Gb/S generation. And realize a stability that is adequate for real systems operations. 5.Referrences: Bennet, S. Fibre Optic gyro system keeps bus riders informed, Photonics Spectra, August 1996, pp Burns. W.K. Fiber Optic Gyroscopes Light is better, Optics and Photonics News, May 1998, pp Chynoweth, A.G. Lightwave Communications: The fiber lightguide, Physics Today, 29 (5), 28, Farmer, K.R., and T.G. Digges. A miniature fiber sensor, Photonics Spectra, August 1996 Fiber Optic Technology Put to Work Big Time, Photonics Spectra, August 1994, p Gambling, W.A. Glass, light, and the information revolution, Ninth W.E.S. Turner Memorial Lecture, Glass Technology, Vol. 27 (6), 179, Ghatak, A., I.C. Goyal, and R. Varshney. Fiber optica: A software for characterizing fiber an integrated optic waveguides. New Delhi: Viva Books, Ghatak, A., and K. Thyagarajan. Introduction to fiber optics. Cambridge: Cambridge University Press, Grifford, R.S., and D.J. Bartnik. Using optical sensors to measure arterial blood gases, Optics and Photonics News, March 1998, pp Hotate, K. Fiber Optic Gyros, Photonics Spectra, April 1997, p Ishigure, T., E. Nihei, and Y. Koike. Optimum refractive index profile of the graded index polymer optical fiber, toward gigabit data links, Applied Optics, Vol. 35, 1996, pp Kao, C.K., and G.A. Hockham. Dielectric-fibre surface waveguides for optical frequencies, Proc. IEEE, Vol. 113 (7), 1151, 1966 Kapron, F.P., D.B. Keck, and R.D. Maurer. Radiation losses in glass optical waveguides, Applied Physics Letters, Vol. 17, 423, Katzir, A. Optical Fibers in Medicine, Scientific American, May 1989, pp Keiser, G. Optical Fiber Communications. New York: McGraw Hill, Koeppen, C., R.F. Shi, W.D. Chen, and A.F. Garito. Properties of plastic optical fibers, Journal of the Optical Society of America, B Vol. 15, 1998, Koike, Y., T. Ishigure, and E. Nihei. High bandwidth graded index polymer optical fiber, IEEE Journal of Lightwave Technology, Vol. 13, 1995, pp Maclean, D.J.H. Optical Line Systems. Chichester: John Wiley, Marcou, J., M. Robiette, and J. Bulabois. Plastic Optical Fibers, Chichester: John Wiley and Sons, Marcuse, D. Loss analysis of single mode fiber splices, Bell Systems Tech. Journal, Vol. 56, 703, Miya, T., Y. Terunama, T. Hosaka, and T. Miyashita. An ultimate low loss single mode fiber at 1.55 mm, Electron. Letts., Vol. 15, 106,