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This letter presents analog and digital pre-distortion hybrid linearization of a broadband radio-over-fiber transmission system. Analog pre-distortion (APD) is used to mainly compensate for out-of-band nonlinear distortion, and digital pre-distortion (DPD) is used to mainly compensate for in-band nonlinear distortion and memory effect. Thus, by combining APD and DPD, all the quiescent distortions and memory effect can be suppressed effectively. The hybrid APD and DPD linearization is evaluated in three different scenarios: two 20-MHz long-term evolution (LTE) signals located at 800 and 900 MHz, and located at 800 and 840 MHz, and three 20-MHz LTE signals located at 800, 850, and 900 MHz. All three experiments show that the hybrid linearization results in better improvement in error vector magnitude, adjacent-channel power ratio, and third-order intermodulation distortion than the APD-only and DPD-only.

A novel uni-traveling-carrier photodetector (UTC-PD), bonded to high-reflectivity (HR) distributed Bragg reflector (DBR) mirrors, referred to as HR-UTC-PD, is first developed to overcome the bandwidth-responsivity tradeoff in UTC-PDs at various wavelengths. Grubbs' test is adopted, for the first time, to measure the bonded thickness of random locations for evaluating the reflected light loss generated in the bonded region. Compared with a standard UTC-PD without HR DBR mirrors, the maximum improvement of HR-UTC-PD responsivity (0.88A/W at 1560 nm) is 24.4% under a bias voltage of 3 V. The obtained 3-dB bandwidth for a 40-μm-diameter PD, which is neither increased nor degraded with the HR DBR mirrors, is 13.87 GHz at a bias voltage of 6 V and a 1550 nm wavelength. The saturation current up to 50 mA at 1-dB compression point is obtained at the frequency of 10 GHz, under a 50-Ω load and a 6 V bias voltage.

This letter presents a novel analog predistortion circuit based on two Schottky diodes that can suppress both the third- and fifth-order nonlinear distortions simultaneously. This circuit is simple, of low cost, and of low power consumption. The predistortion circuit is verified in two radio over fiber (RoF) transmission systems. It is shown that spurious free dynamic range is improved significantly from 1 to 6 GHz. Using the circuit for Wi-Fi wireless signal over RoF, error vector magnitude is improved by up to 3.9 dB.

A low-cost broadband analog predistortion circuit (PDC) is designed and experimentally verified in a radio over fiber (RoF) system. A miniature hybrid microwave integrated circuit technique is used to reduce the size and parasitics of PDC. Only two zero bias GaAs beam lead detector diodes and capacitors are used in the PDC. The diodes can be biased at due to zero bias characteristic so that power consumption is reduced. No broadband matching network is required because of high series resistance of the zero bias diodes. The linearization performance is evaluated using an RoF system without and with the PDC. It is shown that the PDC improves the input 1-dB compression power by 0.4-2.2 dB from 7 to 18 GHz; and spurious free dynamic ranges are improved by ~ 10 dB from 7 to 14 GHz and ~ 6 dB from 15 to 18 GHz, limited by fifth order nonlinear distortion.

We experimentally demonstrate a low-cost broadband predistortion circuit (PDC) for the linearization of a single-drive x-cut Mach-Zehnder modulator (sd-xMZM) in broadband analog radio-over-fiber (RoF) links over 3-5 GHz. By tuning the bias current, the nonlinear characteristic of the PDC can be adjusted to linearize different types of nonlinear components in broadband analog RoF links over 3-5 GHz. By adding the PDC in front of the sd-xMZM, input-refereed 1 dB compression point (P1dB) is improved by 2 dB. By evaluating with the two-tone signal, the PDC allows 14 dB improvement in spurious free dynamic range at 4 GHz, and the third-order intermodulation distortion is suppressed by more than 6 dB over 3-5 GHz. Moreover, the error vector magnitude of an orthogonal frequency division multiplexing ultra-wideband signal is improved by more than 2.5 dB.

Tunable terahertz beat signal generation is demonstrated by using a C-band InAs/InP quantum-dot (QD) mode-locked laser combined with external cavity of two fiber Bragg gratings (FBGs), where one of the FBGs is tunable in wavelength. Beat signals with ultra-high repetition rates quasi-continuously from 1 to 2.21 THz are observed between the two modes, which are phase-correlated due to intracavity four-wave mixing effect in the QD waveguide.

We experimentally investigate optical mixed polarization for the linearization of an electro-absorption modulator (EAM) in radio-over-fiber links. A linear polarizer is used before and after a polarization-dependent EAM. Thus, both transverse-electric and transverse-magnetic polarized lights are simultaneously modulated by different amounts. By carefully optimizing two polarization angles, the third-order intermodulation distortion (IMD3) is suppressed. The linearization leads to suppression of the IMD3 by more than 16 dB and improvement of spurious-free dynamic range by 8.1 and 9.5 dB for back-to-back and after 20 km of fiber transmission, respectively, compared to the nonlinearization case.

Performance of millimeter-wave (MMW) multiband orthogonal frequency-division-multiplexing (OFDM) ultra-wideband (UWB) signal generation using optical frequency up-conversion and transmission over fiber is experimentally investigated. In this work, we propose an optical frequency quadrupling technique using two cascaded Mach-Zehnder modulators (MZMs) biased at quadrature and driven by the same local oscillator frequency but with 180° phase shift between the two MZMs. We demonstrate 30-GHz MMW wireless that carries three bands of OFDM UWB signals.

We demonstrate a broadband supercontinuum (SC) generation that covers an optical spectrum from 1300 to 2000 nm with output power of 1.3 W. The SC generation is realized using a fiber ring cavity that mainly consists of erbium-ytterbium-doped fiber pumped by multimode 975-nm pump lasers, and highly nonlinear fiber (HNLF). Moreover, we investigate the role of four-wave mixing (FWM) in the SC generation by using two different HNLFs, one having normal and the other having anomalous dispersion. It is found that FWM plays a critical role for broadening and flattening of the SC generation in addition to stimulated Raman scattering.

We propose a novel and cost-effective frequency down-conversion technique for millimeter-wave (mm-wave)-over-fiber uplinks using an electroabsorption modulator (EAM) at the base station. It is found that the distributed optical local oscillator light from a central station can be directly modulated by the data signal down-converted from mm-wave inside the EAM and reused as the uplink optical signal with proper filtering. It is also demonstrated that the uplink mm-wave can be down-converted to either an intermediate frequency or the baseband. We numerically validate the feasibility of such a scheme and experimentally demonstrate the error-free performance by a total 40 km of fiber delivery with 1-dB receiver power penalty.

We propose and investigate a linearized optical single-sideband (OSSB) Mach-Zehnder modulator (MZM) for radio-over-fiber systems. The linearized MZM makes use of a dual electrode z-cut LiNbO3 MZM with linear polarizers placed before and after it, respectively. By carefully selecting the angles of the two linear polarizers, third-order intermodulation and second-order distortion can be suppressed significantly. More importantly, it will be shown that with this linearized modulator, not only can OSSB modulation be obtained, but also an improvement in spurious free dynamic range of 15 dB is achieved as compared to the conventional MZM.

We propose a new technique to reduce the bending- induced linear birefringence (LB) by fiber polarization rotator (FPR) in the reflective fiber-optical current sensor (FOCS). An accurate Jones matrix model was established. Such LB influence on FOCS performance is evaluated based on this model. A comparison was made between the proposed FPR scheme and the conventional scheme on measurement errors. Simulation shows that the proposed scheme has a large improvement on such LB reduction by an order of magnitude.

We present a novel radio over fiber (RoF) balanced system that uses only one wavelength and one fiber. In this balanced system, the upper-side band (USB) and lower-side band (LSB) subcarriers, with an electrical phase difference of 180/spl deg/ between them, carry the same signal data. Thus, balanced detection of USB and LSB subcarriers with optical carriers will suppress even order nonlinear distortion. Simulation shows that the proposed balanced system outperforms previous balanced RoF systems in the reduction of second-order nonlinear distortion.

A novel photonic frequency down-shifting technique for millimeter-wave-band radio-over-fiber (RoF) systems is proposed and verified by simulation. The frequency shifting is based on subcarrier modulation (SCM). An optical carrier with a subcarrier is injected into the frequency shifter consisting of a Mach-Zehnder modulator (MZM) or electroabsorption modulator (EAM) driven by a radio frequency sinusoidal signal. The frequency-shifted optical carrier with a frequency-shifted subcarrier is, thus, generated by SCM modulation.

Distributed fiber Raman amplifier (DFRA) with incoherent pumping is investigated and its performance is compared to that with the conventional coherent pumping. It is shown that increasing the spectral bandwidth of incoherent pumping source can reduce the Raman gain ripple significantly, and degrade noise figure slightly at short wavelengths, compared to coherent pumping. To achieve the same gain flatness, the number of pumps for broad-band DFRAs with incoherent pumping can be significantly reduced compared to the coherent pumping.

Polarization-mode dispersion (PMD) penalty measurement on a 10-Gb/s deployment-ready dense wavelength-division-multiplexing (DWDM) long-haul transmission system is reported in this letter. We investigate the impact of self-phase modulation (SPM) and cross-phase modulation (XPM) on PMD penalty in the DWDM system. It is confirmed that SPM can suppress the PMD penalty, and XPM-induced polarization-scattering in DWDM systems can reduce the PMD impairment a little bit further. Our measurements suggest that it is necessary to carry out the PMD penalty test on DWDM systems rather than back-to-back systems in order to accurately allocate transmission margin of the systems.

We demonstrate a simple black box model to evaluate both saturated gain and corresponding noise figure. This approach is simple and very suitable for system modeling with satisfied accuracy.

We point out that the use of polarization-division multiplexed solitons is only preferable to a copolarized pulse train only if the system is interaction-limited and the polarization-mode dispersion (PMD) is sufficiently weak. If the PMD is stronger, copolarized solitons perform better than polarization-division multiplexed ones. A simple analytical formula for when this occurs is derived.

This letter presents guidelines of how to design actively mode-locked fiber ring lasers to generate nearly transform-limited soliton pulses. The minimum fiber cavity length and/or dispersion, with which nearly transform-limited soliton pulses can be generated, need to satisfy an additional condition besides two previously known conditions of stability.

This letter presents a detailed investigation of how solitons broaden in the presence of polarization-mode dispersion. For a given amount of polarization-mode dispersion and pulsewidth there exists an optimal choice of group-velocity dispersion, for which solitons are most resistant to the effect of polarization-mode dispersion. We also optimize the soliton power enhancement factor and find that solitons can propagate roughly twice the distance compared to linear pulses in the absence of group-velocity dispersion.

In this letter, we predict transmission over more than 6000 km using standard fiber with the application of mid-way phase-conjugation in a 1.55-/spl mu/m, 10-Gb/s IM/DD system with in-line amplifiers for the power penalty at BER=10/sup -9/, which is less than 6 dB; the system must operate with an average power into the fiber within the range of -5 to 5 dBm. >

A coherent optical heterodyne CPFSK receiver with a limiter-discriminator (LD) detector, which cannot be degraded by quantum phase noise of lasers when the frequency deviation ratio h is equal to 1, is proposed. The receiver can give the same bit error rate as the ideal digital DPSK system. In addition, 1/2 exp (-SNR) can be considered as the upper bound of the minimum error rate of the CPFSK-LD detector. >