[Show abstract][Hide abstract] ABSTRACT: A mode converter between LP01 and LP0m modes is proposed using two stages of tapers. The first stage is formed by an adiabatically tapering a circular fiber to excite the desirable LP0m mode. The second stage is formed by inserting an inner core (tapered from both sides) with a refractive index smaller than the original core. This second stage is used to obtain low insertion loss and high extinction ratio of the desired LP0m mode. Three converters between LP01 and LP0m, m=2, 3, and 4, are designed for C-band, and simulation results show that less than 0.24, 0.54 and 0.7 dB insertion loss and higher than 15, 16, and 17.5 dB extinction ratio over the entire band were obtained for the three converters, respectively.
Full-text · Article · Nov 2015 · Optics Communications
[Show abstract][Hide abstract] ABSTRACT: We investigate the application of perfect magnetic conductor (PMC) packaging for a nonsmooth perfect electric conductor surface, caused by metalized via holes. The PMC shielding is artificially built by a metal lid of pins, based on the gap waveguide (GW) technology. As a foundation, the effects of via hole in a unit cell are studied first considering its diameter, substrate thickness, pin size, pin height, as well as relative position between the via hole and the pin. The via adjacent space and pin rows are also considered for a general grounded coplanar waveguide along with ground vias. The effectiveness of package for such a transmission line containing metalized via holes is then verified experimentally. It is shown that the measured performance agrees well with the simulation. This paper presents a guideline for the GW-based PMC packaging technology.
No preview · Article · Oct 2015 · IEEE Transactions on Components, Packaging, and Manufacturing Technology
[Show abstract][Hide abstract] ABSTRACT: We investigate the performance of multiband OFDM ultra wideband (UWB) signal over fiber system considering the impact of RF noise. A compensation technique is proposed to reduce RF noise impact.
[Show abstract][Hide abstract] ABSTRACT: The dual wavelength linearization (DWL) technique using two lasers with different wavelengths is for the first time comprehensively studied theoretically and experimentally to improve output signal power and suppress second- and third-order nonlinearities in radio-over-fiber (RoF) systems using an electro-absorption modulator (EAM). Spurious-free dynamic ranges (SFDRs) are calculated and measured. Both the calculation and experiments show that not only the RF signal power is significantly increased, but also both the second- and third-, as well as fifth-order nonlinearities are suppressed. For our considered EAM, when the second-order nonlinearity is suppressed maximally, the SFDRs with respect to second-order harmonic distortion and second-order intermodulation distortion are improved by 11.5 and 8.5 dB, respectively. In the mean time, the SFDRs with respect to third-order harmonic distortion (HD3) and third-order intermodulation distortion (IMD3) are improved by 1.8 and 1.3 dB, respectively. When the third order is suppressed maximally, the SFDRs with respect to HD3 and IMD3 are improved by 8.1 and 20.4 dB, respectively. The fifth-order intermodulation distortion is also suppressed at the same time. The RoF transmissions for WiFi signals are also verified. As a result, 3.5 dB at 2.4-GHz improvement and 2.8 dB at 5-GHz improvement of error vector magnitude are obtained for an RoF system by using the DWL technique.
No preview · Article · Aug 2015 · IEEE Transactions on Microwave Theory and Techniques
[Show abstract][Hide abstract] ABSTRACT: In this paper, a mode converter between mode LP 01 and LP 0m (m>1) is proposed based on two-stage tapered fiber. Each LP 01-LP 0m converter is obtained by changing the parameters of a generic structure. Three converters between LP 01 and LP 0m , m=2, 3, & 4, are designed for C-band, with less than 0.7-dB insertion loss and higher than 15 dB extinction ratio over the entire band.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a double fiber taper-based mode converter that converts LP<sub>01</sub> to LP<sub>02</sub> and vice versa. The first taper is used to convert LP<sub>01</sub> to some higher order LP<sub>0m</sub> (m>1) modes while the second taper suppresses the undesired higher order modes (m>2) and results in LP<sub>02</sub> overwhelmingly dominated. A simulation shows that conversion efficiency of almost 100% at the central wavelength of O-, S-, and C-Band, and above 98% over the S- and C-Band was achieved. Moreover, suppression of undesired higher order modes is more than 10 dB over the whole O-, S-, and C-Band. In particular, the suppression is more than 19 dB for the whole C-Band. The analysis also shows that the performance of the mode converter is not sensitive to slight variations of the converter's parameters. The same converter also can be used to convert LP<sub>02</sub> back to LP<sub>01</sub>. It is shown that conversion efficiency higher than 99% and suppression of undesired higher order modes higher than 30 dB can be obtained. Further, a (de)multiplexer for an LP<sub>02</sub> and an LP<sub>01</sub> mode was designed using the mode converter combined with a symmetric directional coupler. The multiplexer is broadband and has an insertion loss less than 0.5 dB in the C-Band.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a double fiber taper-based mode converter that converts LP01 to LP02 and vice versa. The first taper is used to convert LP01 to some higher order LP0m (m > 1) modes while the second taper suppresses the undesired higher order modes (m > 2) and results in LP02 overwhelmingly dominated. A simulation shows that conversion efficiency of almost 100% at the central wavelength of O-, S-, and C-Band, and above 98% over the S- and C-Band was achieved. Moreover, suppression of undesired higher order modes is more than 10 dB over the whole O-, S-, and C-Band. In particular, the suppression is more than 19 dB for the whole C-Band. The analysis also shows that the performance of the mode converter is not sensitive to slight variations of the converter's parameters. The same converter also can be used to convert LP02 back to LP01. It is shown that conversion efficiency higher than 99% and suppression of undesired higher order modes higher than 30 dB can be obtained. Further, a (de)multiplexer for an LP02 and an LP01 mode was designed using the mode converter combined with a symmetric directional coupler. The multiplexer is broadband and has an insertion loss less than 0.5 dB in the C-Band.
[Show abstract][Hide abstract] ABSTRACT: Accurately modeling the memory effects is critical in modeling and nonlinearity pre-compensation for wideband radio frequency (RF) power amplifiers (PAs) with strong memory effects. In this paper, the memory effects are divided into two categories: lagging memory effect and leading memory effect, which are defined based on the input samples located after and before the current output samples, separately. Considering the above two types of memory effects, a generalized two-box cascaded (GTBC) nonlinear behavioral model for RF PAs with strong memory effects is proposed: one box for a static nonlinearity block that is used to include the strong static nonlinearities and the other box for a dynamic nonlinearity block which is used to predict the mild dynamic nonlinearities. The modeling accuracy of the proposed model, in terms of the normalized mean square error (NMSE) and the memory effect intensity (MEI), is compared to the generalized memory polynomial (GMP), the augmented Hammerstein (AH) and the enhanced Hammerstein (EH) models for a 450-470 MHz Doherty PA with a three-carrier WCMDA input signal and a 895-935 MHz inverse Class-F Doherty PA with a single-carrier FDD-LTE OFDM input signal. The validation demonstrates that the proposed model leads to high-accuracy with fewer modeling coefficients and floating point operations (FLOPs).
No preview · Article · Dec 2014 · IEEE Transactions on Microwave Theory and Techniques
[Show abstract][Hide abstract] ABSTRACT: A linearization technique using two lasers with different wavelengths is further studied to improve output signal and suppress second and third order nonlinearities in radio-over-fiber (RoF) systems simultaneously, if an optical device or a subsystem generates nonlinear distortion and also has wavelength dependent transmission characteristic. In this paper, for an RoF system using an EAM that has wavelength dependent transmission, it is found that the optimum suppression of second order nonlinearity can be achieved by adjusting the power ratio of the two lasers. Compared to using single laser at 1552.6 nm, 1.6 dB improvement of signal power, 23 dB suppression of second order nonlinearity, and 2.1 dB suppression of third order nonlinearity are obtained experimentally. More than 11.5 dB improvement of spurious free dynamic range, 3 dB improvement of input power at 1 dB compression (P1dB), and 4 dB improvement of output P1dB are achieved.
[Show abstract][Hide abstract] ABSTRACT: A low-cost broadband predistortion circuit using zero bias diodes is designed to remove 3rd order intermodulation distortion of electro-absorption modulator in radio-over-fiber system. Experiments show that input power at 1-dB compression point is improved by 0.8-3.8 dB from 8 to 17 GHz; and spurious free dynamic range is improved by ~9 dB from 7 to 14 GHz and ~4 dB from 15 to 18 GHz, limited by 5th order intermodulation distortion. The power consumption is reduced and no matching network is required because of characteristics of zero bias diode.
[Show abstract][Hide abstract] ABSTRACT: Optical and electrical linearization techniques are reviewed for broadband radio over fiber transmission. The optical linearization includes optical mixed polarization and two-wavelength linearization, and the electrical linearization includes analog pre-distortion circuit. The optical linearization leads to more than 10-dB improvement of spur-free dynamic range (SFDR). The analog pre-distortion circuit that is simple and of low-cost results in about 10 dB improvement of SFDR.
[Show abstract][Hide abstract] ABSTRACT: A short pulse train with pulsewidth was generated in a quantum dot mode-locked laser (QD MLL). Due to the short dispersion length, it is required to include group-velocity dispersion (GVD) in modeling pulse train generation and evolution from QD MLLs. On the other hand, Kerr effect is also required to consider due to high peak power density in the laser cavity, and its induced self-phase modulation (SPM) also contributes to the pulse evolution. In this paper, a time domain traveling wave model, including the effect of GVD and SPM, combined with rate equations, is established to model the pulse evolution in a single-section QD MLL. It is shown that the pulse evolution calculated by this model is in reasonable agreement with the experiments. The contribution to the pulse evolution by the GVD and SPM impact is discussed.
No preview · Article · Dec 2013 · IEEE Journal of Quantum Electronics
[Show abstract][Hide abstract] ABSTRACT: The Mach-Zehnder modulator (MZM) has been widely used for broadband photonic analog links and high-speed digital optical fiber communication systems because it possesses large modulation bandwidth, low driving voltage, and low chirp. The MZM is a very important optical modulator for photonic applications. In an MZM, the input light is split into two paths, each of which is modulated by an electrical signal. Then the two arms are combined to generate an intensity-modulated light or a phase-modulated light at the output of the MZM. An MZM can be made of lithium niobate (LiNbO3), gallium arsenide (GaAs), or indium phosphide (InP), materials that exhibit some anisotropy in their dielectric properties. Theoretically, the relation of output optical field and driving voltage is a cosine function, i.e., nonlinear transfer function. For RF photonics, the MZM has typically two applications: optical harmonic generation for optical frequency multiplication and optical subcarrier modulation for optical signal modulation. For optical frequency multiplication, high transfer-function nonlinearity is preferred. In contrast, high transfer-function linearity is preferred for optical subcarrier modulation. It is well known that a cosine transfer function can present high or low nonlinearity dependent on operation voltage. Specifically, bias voltages determine the degree of nonlinearity or linearity of the MZM transfer function. For optical frequency multiplication such as millimeter-wave generation, the MZM should be biased at some specific bias points, such as minimum transmission, maximum transmission, and quadrature bias points, to enhance nonlinearity ?. For optical subcarrier modulation, biasing an MZM in its linear region such as quadrature bias points allows transmitting broadband RF signals with multioctave bandwidth and improves spurious free dynamic range (SFDR). Therefore, care must be taken to maintain and control the MZM bias point for a specific applicat- on.
No preview · Article · Nov 2013 · IEEE Microwave Magazine
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Nov 2013 · IEEE Photonics Technology Letters
[Show abstract][Hide abstract] ABSTRACT: A novel UWB antenna is presented, which is formed by the modified Sierpinski-carpet fractal with the 50Ω micro-strip. Four resonant frequencies and three operation frequency bands can be observed via measurement between 1GHz-20GHz when return loss is smaller than -10 dB. The bandwidth of lower frequency band is from 1.1GHz to 10.8 GHz.
[Show abstract][Hide abstract] ABSTRACT: A novel high isolation MIMO antenna for WLAN and WiMax operation is proposed. The proposed antenna consists of two back-to-back G-shaped monopoles placed symmetrically with respect to a mushroom-like EBG structure. The measured results show that the proposed MIMO antenna covers WLAN (2350-2550 MHz) and WiMax (3500-3690 MHz) bands with high isolation. The antenna can provide pattern diversity to combat multipath fading.
[Show abstract][Hide abstract] ABSTRACT: A novel statistical model, called RM model, on the mobile channels is proposed. The envelope of the received signal is modeled as a Rician signal with exponent M. The Probability density function (PDF) in dB is derived by using signals in dB. The expectation of received signals (in dB) is in directly proportional to M.
[Show abstract][Hide abstract] ABSTRACT: In this paper, physically-based simulations are carried out to investigate and design broadband and high-output power uni-traveling carrier (UTC) photodiodes. The physical model is first verified by comparison to experimentally measured results. The graded-bandgap structure, which can induce potential gradient, is considered to be used in the absorption layers. It is shown that the electric field in the absorption layer is increased by the gradient, thus the performance of bandwidth and saturation current is improved by 36.6% and 40% respectively for our considered photodiode. Moreover, a modified graded-bandgap structure is proposed to further increase the electric field, and an additional 9.5% improvement in bandwidth is achieved. The final proposed UTC-PD structures will result in 399-GHz bandwidth and 49-mA DC saturation current.
[Show abstract][Hide abstract] ABSTRACT: We propose and investigate the suppression of RoF nonlinearity using a semiconductor optical amplifier combined with simplified mixed polarization. It is found that EVM of OFDM UWB can be improved by up to 10.2dB.