Four-Level Pulse Width Modulation for Fiber Optic Communications (original) (raw)
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Fifth order versus third order intermodulation distortion
2007
The third order intermodulation distortion (IMD 3 ) rather than the fifth order intermodulation distortion (IMD 5 ) is often considered when examining the performance of an analogue fibre optic system. This paper aims at looking into the possibility of the IMD 5 being comparable to the IMD 3 . The IMD 5 generated when the laser is intensity-modulated by the multiple carriers signal is simulated and compared to the IMD 3 . Individual IMD 5 level is found to be lower than the IMD 3 when the RF powers of the individual carriers are low or the modulation index per channel is low. However the composite IMD 5 level at channel position could exceed the composite IMD 3 especially when modulation index increases. This is contributed by the number of IMD 5 being greater than that of the IMD 3 based on counting. Hence indicating the IMD 5 should not be neglected in examining the performance of the analogue fibre optic system driven by multiple carriers.
Nonlinear Distortion in Transmission of Higher Order Modulation Formats
IEEE Photonics Technology Letters, 2000
Transmission performance of a range of modulation formats including differentially encoded binary-, quaternary-, 8-, and 16-phase-shift-keying (DBPSK, DQPSK, D8PSK, and D16PSK) as well as 8-and 16-quadrature amplitude modulation (D8QAM and D16QAM) over optically compensated and DCF-free links are compared. Four different pulse-shapes are considered in a single-channel environment and the relative impact of intrachannel nonlinearities and nonlinear phase noise are analyzed.
SSRN Electronic Journal
We present numerical modeling and simulation on the effects of standard single-mode fiber (SSMF) properties and different dispersion management approaches on the distortions of laser diode. These distortions are associated with the two-tone modulation of laser diode for use in radio over fiber systems. The fiber properties include attenuation and dispersion. The dispersion management approaches include the use of nonzero dispersion-shifted fiber (NZ-DSF), fiber Brag grating (FBG), and dispersioncompensating fiber (DCF). The laser is directly modulated with two analog frequencies of 25 and 25.1 GHz at different modulation depths (m). The modulated laser signals are then propagated through SSMF at different lengths (L). The investigated laser signal distortions include the 2nd harmonic distortion (HD2), and the 2nd and 3rd intermodulation distortions (IMD2 and IMD3, respectively). The results reveal that all laser distortions are exacerbated as the fiber length increases, which is mainly due to the chromatic dispersion, while fiber attenuation has no effect. The use of dispersion management approaches gives almost similar effects on the reduction of IMD3 when L = 1 km, regardless of the value of m. Up to m = 0.3, DCF is the most effective approach for reducing all distortions over the entire fiber length range, while NZ-DSF is the least. When L = 2 km, FBG is the most effective approach for reducing both IMD2 and HD2 when m ≥ 0.4, whereas when L is increased to 6 km, DCF is the most effective approach up to m = 0.5.
SUPPRESSION OF NONLINEARITY INDUCED DISTORTIONS IN RADIO OVER FIBER LINKS
Radio over fiber is an analog-optical link proposed as a promising cost-effective solution to meet the explosive demand for broadband, interactive and multimedia services over wireless media. RoF provides functionally simple base stations that are interconnected to a control station through an optical fiber, exhibiting the exciting features of transparency to bandwidth, modulation techniques, centralized sharing of resources, multiuser-multiservice operation and immunity to electromagnetic interference. But harmonic and intermodulation distortions in laser diode and semiconductor optical amplifier affect the signal quality to a greater extent. So necessary actions must be taken to improve the signal to noise ratio. For linearizing the laser diode, a suitable predistorter circuit topology is modeled using sinusoidal and QPSK input. Similarly, the nonlinearities in SOA can be suppressed by the introduction of feedforward technique. The individual effect of predistortion in laser and feedforward in SOA are investigated and the combined effect is also investigated in this paper.
Optical and Quantum Electronics, 2007
Harmonic and intermodulation distortion modeling in IM-DD multi-band radio over fiber links exploiting injection locked lasers g i o v a n n i t a r t a r i n i 1 , a l e s s a n d r a l e n a 1 , d a v i d e p a s s a r o 2 , l o r e n z o r o s a 2 , s t e f a n o s e l l e r i 2, * , p i e r f a c c i n 3 and e n r i c o m a r i a f a b b r i 3 Abstract. A comprehensive numerical tool has been developed for the evaluation of the performances of Radio over Fiber (RoF) links intended for wireless signal distribution.At the transmitter end an appropriate set of rate equations allows to model the optical source as a solitary laser or as an appropriately injection locked laser. The optical channel is modeled putting into account the combined effect of fiber dispersion, laser source non ideal performances (e.g. non-linear effects, frequency chirp), and quadratic detection of the receiving photodiode. The simulation model developed can be a useful tool at the design stage allowing a preliminary evaluation of the characteristics of real RoF links.
PERFORMANCE ANALYSIS OF RADIO-OVER-FIBER SYSTEM AGAINST SECOND ORDER INTERMODULATION DISTORTION
A dual electrode Mech-Zehnder Modulator (MZM) for modulating two Radio Frequency (RF) signals in Radio-over-Fiber (RoF) system has been studied and analyzed to determine the effect of second order intermodulation product term on the Noise to Signal Ratio (NSR) of the system. The results have been investigated at different fiber length as well as for different input frequencies. Results indicate that noise becomes very large at modulation index of 1.7 and input RF frequency 28GHz and 32GHz.
IEEE/OSA Journal of Lightwave Technology, 2007
In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two-and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.
Journal of Lightwave Technology, 2000
In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two-and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.
IEEE Photonics Technology Letters, 2000
A self-pulsating laser is used to generate a multicarrier (five radio frequency (RF) channels) microwave optical signal for use in a hybrid radio/fiber system. The self-pulsation is achieved by external light injection into the laser diode. By varying the RF channel spacing, we have been able to estimate the degradation in system performance due to intermodulation distortion (caused by the nonlinear dynamic response of the laser). The power penalty on the central RF channel is found to be 3.2 dB for operation at the RF band around the laser self-pulsation frequency of 18.5 GHz. We have also characterized the performance of the multicarrier hybrid radio/fiber system in the frequency band corresponding to the inherent relaxation frequency of the laser.