Composite Second-order Distortion Research Articles

Chirp Compensation of Directly Modulated 3s-DBR Laser for WDM-RoF-Based Mobile Fronthaul

In this letter, we propose and demonstrate through simulations, a chirp compensation scheme with a directly modulated three section distributed Bragg reflector (3s-DBR) laser, for use in a WDM-RoF-based mobile fronthaul network to eliminate the signal distortion caused by the interplay between laser chirp and fiber dispersion, including RF power fading and the composite second-order (CSO) distortion. Employing the phase section characteristics of the 3s-DBR laser, the frequency chirp of the device is reduced with reverse modulation current injection into the phase section. The simulation results show that with the chirp compensation scheme, reduced DBR laser chirp can be achieved, and error vector magnitude (EVM) of a 12 channel 211-MHz bandwidth filtered-OFDM (f-OFDM) signal with 64-QAM after transmission over 20-km standard single mode fiber (SSMF), can still satisfy the 3rd generation partnership project’s (3GPP) requirement.

Adaptive Blind CSO Cancellation Technique for RoF Systems Implemented by Using DMLs

We report on the adaptive cancellation technique of the dispersion-induced composite second-order (CSO) distortions for use in radio-over-fiber (RoF) systems implemented by using directly modulated lasers (DMLs). The proposed technique utilizes the magnitude of the CSO distortions measured at outside the signal’s bandwidth as a cost function of the steepest descent algorithm. As a result, this technique can blindly cancel out the CSO distortions without the necessity of measuring any device and fiber parameters used in the RoF system. The effectiveness of the proposed technique is confirmed in the RoF link implemented by using a 1.55- $\mu \text{m}$ DML and is capable of transporting 24 100-MHz filtered-orthogonal-frequency-division-multiplexing signals in 64 quadrature amplitude modulation format.

RoF-Based Mobile Fronthaul Networks Implemented by Using DML and EML for 5G Wireless Communication Systems

We investigate the feasibility of implementing a mobile fronthaul network (MFN) based on the radio-over-fiber (RoF) technology for the fifth generation wireless communication systems cost effectively by using either directly modulated lasers (DMLs) or electroabsorption modulated lasers (EMLs) operating in the 1.55- μ m window. The results show that the performance of the RoF-based MFN implemented by using DMLs is primarily limited by the composite second-order (CSO) distortions arising from the interplay between the DML's adiabatic chirp and fiber's chromatic dispersion. Thus, when we implement the RoF-based MFN by using the currently available commercial DML, its reach could be limited to ∼5 km due to the CSO distortions. To increase this reach, we can utilize the DML having a very small adiabatic chirp (or the optical dispersion compensation or CSO cancelation technique). On the other hand, when we implement the RoF-based MFN by using EML, its performance is limited mostly by the relatively poor linearity and low output power of EML rather than the chirp. For example, if we increase the optical modulation index to compensate for the low output power of EML, the distortions caused by the EML's nonlinear transfer curve can also be increased. Thus, for the use in the RoF-based MFN, it would be highly desirable to increase the output power of EML as much as possible. For a demonstration, we successfully transmit 24 100-MHz-bandwidth filtered orthogonal-frequency-division-multiplexing signals over 20 km of the standard single-mode fiber by using an EML transmitter with 7-dBm output power.

Based on FP LDs with split band technique in two-channel fiber optical CATV transport system

This study proposes a two-channel fiber optic cable television (CATV) transport system based on Fabry–Perot laser diodes (FP-LDs) with a split band technique. To reduce the interference between channels, we apply the split band technique to two channels with different frequencies by using two converters at each moment. In this two-channel transmission scheme, composite second-order (CSO) and composite triple-beat (CTB) distortions induced by the systems are possibly confined in unused channels and result in good transmission performances. Through a 40 km standard single-mode fiber transmission, excellent performances of carrier-to-noise ratio (≥50 dB), CSO (≥70 dB), and CTB (≥72 dB) are obtained using the proposed fiber optic CATV transport systems. The proposed systems employing FP-LDs with a split band technique are also simpler and more cost effective than conventional externally modulated systems.

DSP-based CSO cancellation technique for RoF transmission system implemented by using directly modulated laser.

We propose and demonstrate a simple composite second-order (CSO) cancellation technique based on the digital signal processing (DSP) for the radio-over-fiber (RoF) transmission system implemented by using directly modulated lasers (DMLs). When the RoF transmission system is implemented by using DMLs, its performance could be limited by the CSO distortions caused by the interplay between the DML's chirp and fiber's chromatic dispersion. We present the theoretical analysis of these nonlinear distortions and show that they can be suppressed at the receiver by using a simple DSP. To verify the effectiveness of the proposed technique, we demonstrate the transmission of twenty-four 100-MHz filtered orthogonal frequency-division multiplexing (f-OFDM) signals in 64 quadrature amplitude modulation (QAM) format over 20 km of the standard single-mode fiber (SSMF). The results show that, by using the proposed technique, we can suppress the CSO distortion components by >10 dB and achieve the error-vector magnitude performance better than 6% even after the 20-km long SSMF transmission.

Simultaneous dispersion and non-linearity compensation with mid-span optical phase conjugation and distributed Raman amplifier for a sub-carrier multiplexed optical transmission link

Optical phase conjugation (OPC) and distributed Raman amplifier (DRA) combination (OPC–DRA) is demonstrated as a potential enabling solution for simultaneous reduction of fiber non-linearities and dispersion compensation of a sub-carrier multiplexed (SCM) optical transmission link. The present work is focused on the use of OPC–DRA combination for system performance improvement in terms of composite second order distortion (CSO) and carrier to noise ratio (CNR) of the SCM link. The analysis further shows that, introduction of DRA with proper pumping scheme significantly reduce fiber non-linearity resulting in improvement of the system performance in terms of CNR, compared to the situation where only mid-way optical phase conjugation is used.

Analysis of dispersion compensation by chirped fiber grating in directly and externally modulated CATV systems

The dispersion compensation effect of the chirped fiber grating (CFG) is analyzed theoretically, and analytic expressions are derived for composite second-order (CSO) distortion in analog modulated sub-carrier multiplexed (AM-SCM) cable television (CATV) systems with externally and directly modulated transmitters. Simulations are given for the two kinds of modulations and for standard single mode fiber and non-zero dispersion shift fiber (NZDSF) systems. The results show that CFG could be used as a dispersion compensator in directly modulated systems, but its dispersion coefficient should be adjusted much more precisely than the externally modulated system. The requirements for the NZDSF system could be loosened much. It is proposed that directly modulated source may be used as a transmitter in CATV systems combined with tunable CFG dispersion compensator being adjusted precisely, which may be more cost-effective than externally modulation technology.

Computer-aided analysis of distortions in SCM systems with EDFAs including chirping effect

In this paper, we have applied an equivalent electrical circuit model of erbium-doped fiber amplifier (EDFA) including chirping effect to analyze the distortions in subcarrier multiplexing (SCM) systems. The model is simple and convenient for analyzing the distortions and performance of lightwave transmission systems using EDFAs, through the aid of a SPICE circuit simulator. The performance evaluation is shown of a 42-channel SCM EDFA system, including the composite second-order distortions (CSO), the allowable modulation index, and the limit on the number of channels.

Improvement of CSO/CTB Performances Employing Up-Converted and Polarization Modulation Techniques

A two-wavelength wavelength-division multiplexing (WDM) community antenna television (CATV) transport system that employed up-converted and polarization modulation techniques to reduce the composite second-order (CSO) distortion and optical nonlinearities is proposed and demonstrated. In contrast to a two-wavelength WDM CATV transport system using the intensity modulation technique, good performances of CSO and composite triple beat were obtained, accompanied by acceptable carrier-to-noise ratio value in our proposed system over a 100-km single-mode fiber transmission.

Composite second-order performance improvement in optical fibre CATV transmission system using chirped fibre grating

Theoretically, we analyse the dispersion compensation characteristics of the chirped fibre grating (CFG) in an optical fibre cable television (CATV) system and obtain the analytic expression of the composite second-order (CSO) distortion using the time-domain form of the field envelope wave equation. The obtained result is in good agreement with the numerical simulation result. Experimentally, we verify the result by making use of the tunable characteristics of CFG to change the dispersion compensation amount and obtain an optimal CSO performance in a 125km fibre transmission link. Both the theoretical and experimental results show that the CSO performance can be improved by properly choosing the dispersion compensation amount for a certain fibre transmission link.

Multichannel Single-Sideband SCM/DWDM Transmission Systems

To understand the transmission limitation of multiple narrow single-sideband subcarrier-multiplexed (SSB/SCM) signals, this paper first presents a closed-form analysis to predict Mach-Zehnder intensity modulator-induced composite triple beat (CTB), and linear-fiber-dispersion-induced composite second-order (CSO) and CTB distortions. To combine SSB/SCM with dense-wavelength-division-multiplexing (DWDM) systems, analytical and numerical tools, which are not constrained by any wavelength spacing and modulation frequencies, are used to analyze cross-phase modulation-induced crosstalk. All the analytical and numerical results are verified by computer simulations. Several multichannel SSB/SCM/DWDM systems with transport capacities of 10 or 20 Gb/s per wavelength, with a wavelength spacing of 25, 50, and 100 GHz, are also studied in this paper to understand the fundamental transmission limitations.

Measurement of gain tilt and its contribution to composite second-order distortion in the analog-digital transmission systems with Er/Yb fiber amplifiers.

Fiber-optic-based analog or digital community antenna television systems experience composite second-order (CSO) distortion caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog-digital transmission system with a high-power erbium/ytterbium-codoped fiber amplifier has been experimentally measured and its contribution to the CSO distortion of the system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO distortion of the transmission system with and without the amplifier. The dependence of the gain tilt on the modulation frequency and wavelength of the input light is also investigated.

Application of SPICE simulation to study WDM and SCM systems using EDFAs with chirping

The authors have proposed an equivalent electrical circuit model of erbium-doped fiber amplifiers (EDFAs), including the chirping effect for teaching or studying the distortions and performance of lightwave transmission systems. Through the aid of a SPICE circuit simulator, its validity is demonstrated by analyzing the cross modulations of an eight-channel wavelength division multiplexing system. The intermodulations in an amplitude modulation-subcarrier multiplexing (SCM) system are demonstrated. The channel-number dependence of the composite second-order distortion in a 42-channel SCM system is shown, and its applications to analyze other systems are also discussed.

CSO distortions due to the combined effects of self- and external-phase modulations in long-distance 1550-nm AM-CATV systems

We found that the combined effects of self- and external-phase modulations must be considered in order to precisely predict the composite second-order distortions in a long-distance 1550-nm externally modulated AM-cable TV (CATV) system, especially when the applied phase modulation index and modulating tone frequency to the integrated phase modulator are high. This result has important implications to the optimum design of 1550-nm transmitter for long-distance AM and QAM-CATV systems.

Bit-error rate of M-QAM signal and its analysis model for composite distortions in AM/QAM hybrid transmission

In cable television (CATV) systems that deliver digital video services by employing a hybrid fiber and coaxial network, it is important to consider digital channel deterioration due to composite distortions, as well as the amplitude-modulation vestigial-sideband channel. Furthermore, it is essential to clearly estimate a digital channel's quality in designing CATV transmission systems. However, there have been few studies on statistical models and an analysis method to estimate the bit-error rate (BER) of M-ary quadrature amplitude-modulation signals for composite distortions. This paper shows statistical properties of composite distortions and a new BER analysis method to handle them. In the evaluations, the amplitude distributions of composite distortions are given by Weibull distributions with skewness levels of 2.3 for composite second-order distortion and 1.5 for composite triple-beat distortion. We propose a new BER analysis method that uses the Weibull distributions for the composite distortion statistical features and demonstrate the effectiveness of the new BER analysis method in estimating BER by comparing its measurement results to calculation results.

The effect of interleaver depth and QAM channel frequency offset on the performance of multichannel AM-VSB/256-QAM video lightwave transmission systems

The effect of a convolutional interleaver with variable depth in a quadrature amplitude modulator (QAM) modem to combat burst-errors generated by bursty composite second-order (CSO) distortions in multichannel AM-VSB/256-QAM video lightwave transmission systems has been analyzed. It is shown that by offsetting the QAM channel frequency relative to the dominant CSO distortions with I=204, J=1 interleaver, the 256-QAM coded bit-error rate was reduced by more than 500 times even in the presence of large CSO distortion levels (>-60 dBc) compared with the no interleaver and QAM frequency offset case. These results should help in the selection of the proper interleaver depth as well as the QAM channel frequency plan, depending on the magnitude of the CSO/CTB distortions in the CATV network.

Compensation for fiber-induced composite second-order distortion in externally modulated lightwave AM-SCM systems using optical-phase conjugation

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required.

Extraction of semiconductor intrinsic laser parameters by intermodulation distortion analysis

A new technique is described for the extraction of the intrinsic semiconductor parameters from intermodulation measurement of composite second-order (CSO) and composite third-order (CTB) distortion products. The effect of the parasitics elements are eliminated, by taking the distortion measurements at constant optical modulation depth, which enables one to obtain the laser parameters on packaged laser diodes. An optimization algorithm is then described that extracts the relevant laser parameters from the distortion data. This method was applied to a commercially available packaged laser diode and very good agreement was obtained between the measured and theoretical results.

1.3-μm uncooled DFB lasers with low distortion for CATV application

Strained-layer multiquantum-well distributed-feedback (MQW-DFB) lasers at a wavelength of 1.3 /spl mu/m operating from -40/spl deg/C to 85/spl deg/C without any coolers are demonstrated. Extremely low threshold current of 17 mA at 85/spl deg/C and operation current of 37 mA at 5 mW and 85/spl deg/C are obtained. To the best of our knowledge, each of them is the lowest value so far reported in InGaAsP-InP based DFB lasers. The lasers also realize low distortion of less than -50 dBc at 65/spl deg/C in 78-channel composite second order distortion (CSO) measurements. An experimental estimation of the leakage current in buried heterostructure presents that the distortion at high temperatures is mostly dominated by the carrier overflow from an active layer, not by the leakage current flowing through current blocking layers. On the basis of the leakage current analysis, a laser structure including the active layer and the current blocking layer is chosen to achieve low distortion over a wide-temperature-range. The uncooled distributed-feedback (DFB) laser we have developed is very useful for several channel transmission in CATV systems.

Reduction of dispersion-induced distortion in SCM transmission systems by using predistortion-linearized MQW-EA modulators

The application of low-chirp MQW (multiquantum well)-EA (electro-absorption) modulators to subcarrier multiplexing (SCM) optical transmission systems is studied. The authors show that a third-order predistortion circuit is feasible for compensating the nonlinearity of this type of modulator. The degree of frequency chirping per optical intensity modulation depth of the modulator is theoretically determined to be about 1.4 MHz. A 42-channel AM-SCM signal transmitter with the predistortion circuit realizes both composite second-order distortion (CSO) and composite triple beat distortion (CTB) values under -57 dBc after 200-km-long transmission. The authors confirm dispersion-induced distortion of the MQW-EA modulator is as small as that of the LiNbO/sub 3/ modulator. Carrier-to-noise ratios (CNRs) of 45.2 dB for channel 1 and 43.8 dB for channel 42 were measured after 100 km transmission. It is found that the deterioration of a CNR is caused by the cascaded erbium doped fiber amplifiers (EDFAs), Rayleigh backscattered power and the optical phase noise. Theoretical CNRs show that the output power of the modulator should be higher to improve CNR.