. (2010) Chromatic dispersion compensation in coherent transmission system using digital filters. Optics Express, 18 (15 Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way.Publisher's statement: © 2010 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. http://dx.doi.org/10.1364/OE.18.016243 A note on versions: The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP URL' above for details on accessing the published version and note that access may require a subscription. Abstract:We present a comparative analysis of three popular digital filters for chromatic dispersion compensation: a time-domain least mean square adaptive filter, a time-domain fiber dispersion finite impulse response filter, and a frequency-domain blind look-up filter. The filters are applied to equalize the chromatic dispersion in a 112-Gbit/s non-return-to-zero polarization division multiplexed quadrature phase shift keying transmission system. The characteristics of these filters are compared by evaluating their applicability for different fiber lengths, their usability for dispersion perturbations, and their computational complexity. In addition, the phase noise tolerance of these filters is also analyzed.
It is found that dissipative types of stable soliton structures can exist in nonlinear optical media with broadband gain and group-velocity dispersion (GVD). These structures resemble ionization or combustion waves and are essentially self-accelerating pulses with a stationary-envelope form and a permanently shifting wave spectrum. Contrary to the conservative solitons, the dissipative ones exist for any sign of GVD. Being an attractor in the development of arbitrary initial distributions, the dissipative structures cause the fundamental Schrodinger solitons to disappear in the course of evolution in weakly nonconservative systems. PACS number(s): 42.50.Rh, 42.81. Dp, 42.65.ReRecent progress in developing ultrashort-laser techniques attracts the interest of studying interaction between powerful optical pulses and broadband active media. This problem is of particular importance for the dynatnics of femtosecond laser generators [1], amplifiers of supershort pulses [2], and nonlinear active optical fibers [3]. A common theoretical aspect in these applications is the long-term evolution of wave packets in dispersive active media that exhibits simultaneously both conservative and dissipative nonlinearities.It is we11 known that the propagation of wave pulses in a lossless and dispersive dielectric medium with its self-action incorporated is described by the nonlinear Schrodinger (NLS) equation, which normally includes the conservative cubic nonlinearity of the refractive index. The interplay between nonlinearity and group-velocity dispersion (GVD) then may lead to soliton generation [4] or quasishockwave formation [5]. On the other hand, in two-level optical media solitons of induced transparency can be formed due to the nonconservative character of the resonant light-matter interaction [6]. Specific nonconservative mechanisms of soliton generation, caused by a delicate balance between saturable absorption and amplification and/or transverse phase modulation and gain inhomogeneity, play an important role in the passively modelocked short-pulse laser operation [1,7,8].The purpose of the present paper is to show that the long-term evolution of a wave pulse propagating in a nonlinear two-level optical medium containing broadband gain elements results in the formation of new types of single soliton structures (named here dissipative opti-
The Bit-Error-Ratio (BER) floor caused by the laser phase noise in the optical fiber communication system with differential quadrature phase shift keying (DQPSK) and coherent detection followed by digital signal processing (DSP) is analytically evaluated. An in-phase and quadrature (I&Q) receiver with a carrier phase recovery using DSP is considered. The carrier phase recovery is based on a phase estimation of a finite sum (block) of the signal samples raised to the power of four and the phase unwrapping at transitions between blocks. It is demonstrated that errors generated at block transitions cause the dominating contribution to the system BER floor when the impact of the additive noise is negligibly small in comparison with the effect of the laser phase noise. Even the BER floor in the case when the phase unwrapping is omitted is analytically derived and applied to emphasize the crucial importance of this signal processing operation. The analytical results are verified by full Monte Carlo simulations. The BER for another type of DQPSK receiver operation, which is based on differential phase detection, is also obtained in the analytical form using the principle of conditional probability. The principle of conditional probability is justified in the case of differential phase detection due to statistical independency of the laser phase noise induced signal phase error and the additive noise contributions. Based on the achieved analytical results the laser linewidth tolerance is calculated for different system cases.
Bit-Error-Ratio (BER) of intensity modulated optical orthogonal frequency division multiplexing (OFDM) system is analytically evaluated accounting for nonlinear digital baseband distortion in the transmitter and additive noise in the photo receiver. The nonlinear distortion that is caused by signal clipping and quantization is taken into consideration. The signal clipping helps to overcome the system performance limitation related to high peak-to-average power ratio (PAPR) of the OFDM signal and to minimize the value of optical power that is required for achieving specified BER. The signal quantization due to a limited bit resolution of the digital to analog converter (DAC) causes an optical power penalty in the case when the bit resolution is too low. By introducing an effective signal to noise ratio (SNR) the optimum signal clipping ratio, system BER and required optical power at the input to the receiver is evaluated for the OFDM system with multi-level quadrature amplitude modulation (QAM) applied to the optical signal subcarriers. Minimum required DAC bit resolution versus the size of QAM constellation is identified. It is demonstrated that the bit resolution of 7 and higher causes negligibly small optical power penalty at the system BER=10⁻³ when 256-QAM and a constellation of lower size is applied. The performance of the optical OFDM system is compared to the performance of the multi-level amplitude-shift keying (M-ASK) system for the same number of information bits transmitted per signal sample. It is demonstrated that in the case of the matched receiver the M-ASK system outperforms OFDM and requires 3-3.5 dB less of optical power at BER=10⁻³ when 1-4 data bits are transmitted per signal sample.
Er 3+ photoluminescence from Er-doped amorphous SiO x films prepared by pulsed laser deposition at room temperature: The effects of oxygen concentration Appl. Phys. Lett. 82, 3436 (2003); 10.1063/1.1573335 Infrared-to-violet upconversion from Yb 3+ and Er 3+ codoped amorphous fluoride film prepared by pulsed laser deposition J. Appl. Phys. 92, 6936 (2002); 10.1063/1.1521801Pulsed laser deposition of Cu:Al 2 O 3 nanocrystal thin films with high third-order optical susceptibility Photoluminescence with the bandwidth of 45 nm ͑1523-1568 nm at the level of 3 dB͒ was observed in amorphous Er 2 O 3 films grown on quartz substrate by pulsed laser ablation of erbium oxide stoichiometric target. Optical transmission spectrum has been fitted to Swanepoel formula ͓J.Phys. E 16, 1214 ͑1983͔͒ to determine dispersion of refractive index and to extract resonance absorption peaks at 980 and 1535 nm. The maximum gain coefficient as high as 800 dB/ cm at 1535 nm was estimated using McCumber theory and experimental spectrum of the resonance absorption. For 5 mm long waveguide amplifier with erbium doping confinement factor of 0.1, the theory predicts the spectral gain of 18 dB with 1.2 dB peak-to-peak flatness in the bandwidth of 31 nm ͑1532-1563 nm͒ when 73% of Er 3+ ions are excited from the ground state to the 4 I 13/2 laser level. Strong broadband photoluminescence at room temperature and inherently flat spectral gain promise Er 2 O 3 films for ultrashort high-gain optical waveguide amplifiers and integrated light circuits.
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