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The influence of an ultrathin GaP (or GaAs) sublayer on the nonradiative intraband relaxation in InAs/InGaAsP/InP quantum dots (QDs) is investigated. It is found that, based on our studies, the QDs with some heights (e.g., 1.5 nm) and GaP sublayer thicknesses (e.g., 1.03 monolayers) present the first excited state (ES) with higher state degeneracy with respect to ground state (GS), which suggests that the Auger relaxation is triggered more easily. We also find that the energy difference of the ES and GS decreases with increasing sublayer thickness, which suggests that the electron–phonon interaction is affected. This work further presents a study of intraband relaxation for an InAs/InP QD with a GaP or GaAs sublayer. It is found that there is a critical thickness of the GaP sublayer: When the sublayer is less than the critical thickness, the intraband relaxation is only determined by one-longitudinal optical (LO) phonon or two-LO phonons, which is dependent on QD heights. However, with the GaAs sublayer, QDs do not have the above feature. This finding may be helpful for designing and optimizing high-speed QD devices.

A broadband aperture circularly polarized (CP) antenna based on substrate integrated gap waveguide (SIGW) for millimeter applications is presented. An inclined aperture is etched on the metal layer of the SIGW and excited by transmission microstrip line of SIGW. The mechanism of CP radiation is analyzed briefly in the paper. The simulated results show impedance bandwidth from 24.8 to 31.7 GHz (24.4%) and 3-dB axial ratio bandwidth from 27.3 to 28.8 GHz (5.3%). The design is validated.

The two-state lasing conditions at room temperature in InAs/InP quantum dot (QD) lasers under a continuous wave electrical bias current are studied. It is found that excited state (ES) lasing is promoted by moderately decreasing spacer thickness and increasing dot-size dispersion in a QD stack, and the physical origins are due to the increased bottleneck effect and inhomogeneous broadening. Moreover, it is proved theoretically that ground state (GS) lasing with high inhomogeneous broadening may result in high phase and intensity noise at a high bias current. Therefore, an appropriate spacer thickness together with appropriate inhomogeneous broadening is critical to the GS and/or ES lasing, which helps an optimal design of laser geometry.

In this paper., a seventh-order filter using substrate integrated gap waveguide (SIGW) technology., i.e. SIGW filter., is proposed for the first time., which has high selectivity and wide stopband. The SIGW filter includes three-layer substrates. The top layer substrate realizes a perfect magnetic conductor (PMC) using periodic plated vias and metallic patches. It is applied for packaging substrate integrated waveguide (SIW) filter to avoid interference and losses induced by external radiation emission and internal radiation leakage. The middle layer substrate generates a gap layer. In the base/bottom layer, cascaded resonators of the SIW filters are formed by properly etching slots on the metal plane and placing plated vias in the middle of this layer. The higher-order modes of TE 103 and TE104 are suppressed using slots designed in the base layer at the third-order cavity, which leads to wide stopband. Moreover, the effect of the SIGW-based PMC structure on bandwidth and transmission zeroes is studied. The measured results demonstrate that the proposed SIGW filters perform well in the both passband and stopband.

This paper proposed a wideband 3dB coupler based on substrate integrated gap waveguide. The proposed coupler takes the branch-line coupler as the prototype and the simulations have done. According to the simulation results, the proposed coupler achieved the equal power division and the isolation are all above 20dB from 23.59GHz to 29GHz. Moreover, the phases between through port and coupled port differ 90 deg and the losses for both the through port and isolated port are about 1dB. In addition, the comparison of the couplers with different tilt angle coupling lines has been done. And with the simulation results, when the tilt angle equals 0, the bandwidth of isolation above 20dB is about 3 times of the tilt angles equaling $\pi/4,4\pi/25$ and $\pi/6$ .

In this paper, a six order bandpass SIW filter is presented which can realize efficiently by using the technology of electric and magnetic couplings. It is made up of a layer substrate. The six order SIW filter is a narrow bandpass microwave filter through electric coupling and magnetic coupling of SIW resonators or cavities. The cascaded resonators of the filter are formed by properly etching the slots on the top metal plane of a SIW cavity. The slots create the electric coupling. The magnetic coupling is formed by period metallic vias in the middle of a SIW cavity. Furthermore, the electric and magnetic couplings achieves two transmission zero points. The electric coupling have influence on the upper transmission zero point, and magnetic coupling have influence on the lower transmission zero point. It is demonstrated that the SIW filter has a bandwidth of 1.5 GHz at a center frequency of 15 GHz. The measured results have a good agreement with the simulated results.

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.

In addition to microstrip line and basic grounded coplanar waveguide (GCPW) line circuits with perfect magnetic conductor (PMC) package, we further study the PMC package for a real component—a fifth order bandpass filter designed based on substrate integrated waveguide (SIW) with transitions to GCPWs. In particular, a simplified design of the PMC package is proposed for such a hybrid circuit. A fabricated SIW-GCPW filter was characterized without a lid, and with a traditional smooth lid, a modified smooth lid, a complete PMC lid and a simplified PMC lid. Both simulated and measured S-parameters show that the filter outband rejection using the proposed simplified PMC lid is better than the traditional and existing lids.

A new low-loss quasi-transverse electromagnetic (TEM) microstrip line, called as a self-packaged microstrip line, is proposed for microwave and millimeter-wave applications. It is fully based on the low-cost PCB process, but without the issues of radiation and surface waves by using a top layer perfect magnetic conductor (PMC) substrate. The self-packaged microstrip line consists of three layer substrates: a bottom layer to place the metal strip, a top layer to build the PMC shielding with the periodic plated vias, and a middle layer to separate the PMC layer from the metal strip and base ground plane. The ground plane works as an ideal PEC creating a PEC-PMC structure. It is evident that, within the band of interest, the space radiation, surface waves, and cavity resonances appearing in the standard microstrip line (without/with a bulky metallic shielding box) can be efficiently suppressed by the top-layer PMC shielding when a proper gap height is implemented [1]-[4]. As a result, a thicker bottom layer substrate can be applied in the packaged microstrip line to reduce the conductor losses; and no issues of interference or crosstalk will arise between the adjacent circuits/components [5], [6]. Therefore, this self-packaged microstrip line is very helpful to improve the microstrip circuit performance, such as the gain, bandwidth and radiation efficiency of the microstrip antenna, and the passband/stopband characteristics and compactness of the microstrip filter, in particular at high frequencies, due to the reduced attenuation and the absence of surface waves (and/or radiation) [7]-[9]. Indeed, the suppression of the surface waves in the substrate can also be realized by electromagnetic bandgap (EBG) or defect ground structure (DGS), because of the high-impedance surface [10], [11]. However, an additional metal shielding that surrounds a microstrip-EBG/DGS circuit is still required to prevent the space radiation. Otherwise, significant radiation will increase the insertion loss and additionally cause the unwanted RF interference among the adjacent circuit elements. Moreover, such packaging is very complicated, since a recessed region is required in the metal bearing carrier under the defected ground slots, and the resonance frequency of the DGS depends on the dimensions of this region [12], [13]. So, it can be seen that the proposed novel self-packaged microstrip line perfectly merges both the merits of suppressing the radiation in space and the surface waves in the substrate, which makes it very attractive for the high cost-effective circuit design in both performance improvement and size reduction.

In this paper, a new all-fiber mode converter was proposed, which can achieve LP 01 to LP 02 mode conversion and vice versa. The proposed mode converter is based on a tapered core and four bipyramids combined structure with annular distribution; the tapered core is used to excite the high-order modes while the bipyramids combined structure curb the undesired high-order modes and make the LP02 dominate. A analysis shows that conversion efficiency of 89% and extinction ratio of12dB over the whole C-band; and the performance of the mode converter is not sensitive to minor changes in structural parameters. The proposed mode converter also can achieve LP 02 back to LP 01 .

This paper proposes a novel multi-stub ultra-wide band planar monopole antenna. The radiation patch of the antenna is composed of a pair of back-to-back E-shape-like stub elements. The E-shape-like stub element is the extension of the E-shape stub element, adding two semi-circle elements at the back of the E-shape stub element and extending the top of the E-shape stub element with an inverted L-shape element with a “suspending hammer” element. Two semi-circle slots etched on the rectangular ground which is extended make the ground with two step-like structures to realize ultra-wideband. The antenna is printed on a substrate of the dielectric constant 4.6. The experiment results show that the bandwidth of the antenna is 3.2GHz–18.7GHz for S11 of less than −10dB.

The optical properties of a vertical multilayer stack of semiconductor self-assembled quantum dots (QDs) depend on the coupling status. We present an improved modeling method for analysis of QD stacks, consisting of several separate steps in the elastic strain analysis, to reproduce the realistic unidirectional strain accumulation along the QD growth direction from lower and upper dots. Based on this modeling method, by varying the spacer thickness, we systematically study a large variety of QD stack structures, for better understanding the influence of strain and electronic coupling mechanisms. A “quasi continuum band” or highly degenerate excited state (ES) is found in the closely multi-stacked QDs, probably accounting for the occurrence of distinct blueshifting of the photoluminescence peak wavelength for the decreasing spacer thickness. The enhanced ES emission from this type of stacking QD modeled by using rate equations may pave a way for designing potential high-performance laser devices operating on ES.

This paper proposes a novel all-optical fiber mode converter for mode conversion from LP01 to LP02 and vice versa. The mode converter is formed by connecting a single-mode fiber, a taper-core multi-mode fiber, and a few-mode fiber (FMF) together. The taper fiber core is designed to convert LP01 mode to LP02. The few-mode fiber is used to cut off the modes that are higher than LP02. It is shown that the proposed mode converter provides 20 dB extinction ratio and low insertion loss in a very broad optical bandwidth of 200 nm, from 1465 to 1665 nm. By further optimizing the FMF radius only for each of the O-, E-, S-, C-, U-, etc. bands, the mode converter can apply to any band of interest with higher performance. It is found that the performance of the mode converter has a large tolerance to structural parameters. The mode converter has the same performance in reciprocal operation, i.e., LP02 to LP01.

A short pulse train with pulsewidth < 1 ps 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.

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 [1]?[2]. 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 application.

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 investigate by theory and experiment the performance improvement of an orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) transmitted in a radio-over-fiber (RoF) system using a linearized mixed polarization single-drive X-cut Mach-Zehnder modulator (sd-xMZM). The third-order nonlinearity is suppressed due to the polarization dependent electrooptic coefficients of an X-cut MZM. It is found that the improvement of more than 4 dB in 1-dB compression RF power P1 dB is predicted in theory and verified experimentally. In addition, the adjacent channel power ratio and third-order intermodulation distortion to carrier power ratio of an OFDM UWB signal transmitted by an RoF link using mixed polarization sd-xMZM are expressed in closed forms. Finally, it is verified that the bias voltage can be optimized to improve the error vector magnitude by up to ~ 9 dB for an OFDM UWB signal transmission-over-fiber link using mixed polarization sd-xMZM.

Linewidth enhancement factor (LEF) of InAs/InP quantum dot (QD) multi-wavelength lasers (MWLs) emitting around 1.5 μm is investigated both above and below the threshold. Above the threshold, LEFs at three different wavelengths around the gain peak of 1.53 μm by the injection locking technique are obtained to be 1.63, 1.37 and 1.59. Then by Hakki–Paoli method LEF is found to decrease with increased current and shows a value of less than 1 below the threshold. These small LEF values have clearly indicated that our developed InAs/InP QDs are perfect and promising gain materials for QD MWLs, QD mode-locked lasers (QD MLLs) and QD distributed-feedback (QD DFB) lasers around 1.5 μm.

In this paper, a new-version microstrip line is proposed, named packaged microstrip line. It is comprised of three dielectric substrates. This novel microstrip line is no longer suffering from the issues of radiation losses, surface waves, or cavity resonances that exist in the standard microstrip line without or with a metallic shielding box. Instead of the packaging way based on the standard air gap waveguide technology, this novel packaged microstrip line has the following merits: greatly reduced weight and size, easy manufacturing, constant gap height and more flexibility in design. We present how to adopt a proper dielectric substrate for each layer of the packaged microstrip line considering both stopband and line losses, and how to design the line width for a given characteristic impedance. One prototype of this proposed novel packaged microstrip lines is fabricated and tested, showing a good agreement with the simulated results.

In this paper, a novel comprehensive transmitter architecture has been proposed, which enables the concurrent transmission of two separate frequency bands by carrier aggregation. For different frequencies, the proposed transmitter can simultaneously employ 1-bit Low-pass and Band-pass delta-sigma modulators structures, or employ hybrid structures. Carrier frequencies are equal to oversampling frequencies of modulators. Besides, signals on two carriers are not interfered with each other to ensure transmission dual-band signal. Performance analysis is simulated by using Matlab/Simulink. The simulation results achieve high signal-to-noise radio and show the effectiveness of the proposed architecture.

A small CPW fed triple band-notched (Ultra Wide Band) UWB antenna is presented. The proposed antenna consists of a shovel-shaped radiating patch with three C-shaped slots, a feed line and a modified ground plane. By etching three C-shaped slots with variable dimensions on the radiating patch, frequency band-stop performance is generated and we can control its band-notched frequency and bandwidth through modifying the dimensions of C-shaped slot. The proposed antenna has a very compact size of 14×22×1.6mm3. Simulated VSWR shows that the proposed antenna could operate from 2.88 to 10.67 GHz with VSWR less than 2, except 3.20 to 3.97GHz, 5.26 to 5.75GHz and 7.57 to 8.37 GHz.

To miniaturize the conventional air gap waveguide (GW), a substrate integrated gap waveguide (SIGW) is proposed. Moreover, SIGW makes up the defects in the microstrip-ridge air GW, such as inconstant gap height, holes in strip, as well as strip electroless nickel immersion gold (ENIG) coating. To verify the proposed SIGW, a prototype was designed and manufactured using the multi-layer printed circuit board (PCB) technology. It contains a very simple transition between SIGW and microstrip lines for measurements. Further, the experimental results show that the measured performance agrees well with the simulated.

A low-cost linearization technique is proposed to improve RF signal power and suppress third order intermodulation distortion (IMD3) in radio-over-fiber (RoF) transmission systems. A directly modulated laser (DML) and an electro-absorption modulator integrated laser (EML) in C-band are both used for optical subcarrier modulation. The IMD3s induced by the two optical subcarrier modulations are suppressed by each other through adjusting the bias voltage of the electro-absorption modulator in the EML. In our initial experiments, the spurious free dynamic range (SFDR) is improved by more than 3.5 dB and output power at 1 dB compression point is improved by 4.3 dB.

We propose and analyze a photonic-assisted periodic triangular-shaped pulses generator, in which a continuous wave (CW) laser is connected to a dual electrode Mach-Zehnder modulator (De-MZM), and then the De-MZM is connected with a normal dispersion fiber or element. A radio frequency (RF) sinusoid directly drives the De-MZM to obtain optical subcarrier modulation with optical carrier suppression. By carefully setting the physical parameters, the generated harmonics of optical intensity can be corresponding to the Fourier components of typical periodic triangular pulses. Numerical simulations reveal that modulation index must be set to 2.305 and bias voltage drift must be within a range from from the minimum transmission of the De-MZM. In addition, the dispersion must be set correctly to suppress the primary distortion of 4th order harmonics.

A novel class of low-cost, small-footprint and high-gain antenna arrays is presented for W-band applications. A 4 × 4 antenna array is proposed and demonstrated using substrate-integrated waveguide (SIW) technology for the design of its feed network and longitudinal slots in the SIW top metallic surface to drive the array antenna elements. Dielectric cubes of low-permittivity material are placed on top of each 1 × 4 antenna array to increase the gain of the circular patch antenna elements. This new design is compared to a second 4 × 4 antenna array which, instead of dielectric cubes, uses vertically stacked Yagi-like parasitic director elements to increase the gain. Measured impedance bandwidths of the two 4 × 4 antenna arrays are about 7.5 GHz (94.2-101.8 GHz) at 18 ± 1 dB gain level, with radiation patterns and gains of the two arrays remaining nearly constant over this bandwidth. While the fabrication effort of the new array involving dielectric cubes is significantly reduced, its measured radiation efficiency of 81 percent is slightly lower compared to 90 percent of the Yagi-like design.

We report on a study of terahertz (THz) generation using implanted InGaAs photomixers and multi-wavelength quantum dot lasers. We carry out InGaAs materials growth, optical characterization, device design and fabrication, and photomixing experiments. This approach is capable of generating a comb of electromagnetic radiation from microwave to terahertz. For shortening photomixer carrier lifetime, we employ proton implantation into an epitaxial layer of lattice matched InGaAs grown on InP. Under a 1.55 μm multi-mode InGaAs/InGaAsP quantum dot laser excitation, a frequency comb with a constant frequency spacing of 50 GHz generated on the photomixer is measured, which corresponds to the beats of the laser longitudinal modes. The measurement is performed with a Fourier transform infrared spectrometer. This approach affords a convenient method to achieve a broadband multi-peak coherent THz source.

We present theoretical analysis of simultaneous photonic frequency downconversion, based on four-wave mixing (FWM), and optical subcarrier modulation in an electroabsorption modulator (EAM) for broadband millimeter wave (mm-wave) over fiber uplinks. A model of FWM generation in an EAM is established. The FWM efficiency with an EAM at dc bias with and without RF modulation is analyzed, including the impact of bias voltage, modulation voltage, and frequency detuning. The calculated FWM efficiency agrees well with the measured. The signal-to-noise ratio (SNR) of photonic frequency downconverted RF signal is also analyzed, considering the nonlinear distortion due to the EAM nonlinearities. Moreover, calculated error vector magnitude using theoretical SNR is compared with the measured for multiband orthogonal frequency division multiplexing ultrawideband mm-wave signals.

A compact disc ultrawideband (UWB) antenna with quintuple band rejections is proposed and demonstrated, which has a portrait shape and is therefore “portrait antenna.” The quintuple band-notched functions are achieved by utilizing C-shaped slots, nested C-shaped slots, U-shaped slots, and open-circuit stubs. It is demonstrated that the portrait antenna operating from 2.45 to 12 GHz (VSWR ; 2) of 3.27-3.57, 5.01-5.45, 5.55-6.05, 7.05-7.45 and 7.83-8.19 GHz are implemented simultaneously for rejecting WiMAX, WLAN, downlink of X-band satellite communication, and ITU 8-GHz band signals. Moreover, the antenna has a simple structure and a compact size.

A novel configuration using only one Mach–Zehnder interferometer (MZI) for photonic-assisted instantaneous microwave frequency measurement is proposed. The amplitude comparison function (ACF), related to the input microwave frequency while independent of the input optical power and modulate index, is achieved by using a ratio of low-pass to bandpass frequency responses introduced by intensity and phase modulation with a shared MZI. The microwave frequency can be estimated by the measured ACF. A proof-of-concept experiment for measurement of RF from 5 to 10 GHz is successfully demonstrated with the measurement errors less than ± 0.2 GHz.

We analyze, in theory, simulation, and experiment the spurious-free dynamic range (SFDR) for a radio-over-fiber (RoF) system using a polarization-dependent electro-absorption modulator (EAM) that is linearized using mixed polarization. The mixed-polarization EAM makes use of a polarization-dependent EAM, where a linear polarizer is placed in front of and behind the optical modulator. It is found that improvements of more than 8.1 and 9.5 dB in SDFR for back to back and after 20 km of fiber transmission, respectively, are predicted in theory and simulation and verified experimentally. Suppression of second-order nonlinear distortion is also investigated by simulation and experiment. Moreover, closed forms of the adjacent channel power ratio and third-order intermodulation distortion to carrier power ratio of an orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) signal are presented for an RoF link using mixed-polarization EAM. Finally, mixed-polarization EAM is verified for improving the error vector magnitude of the OFDM UWB signal over fiber by more than 3 dB.

Coherence multiplexing (CM) technique for analog signal transmission was proposed but has not been studied comprehensively. In this paper, we investigate RF signal transmission over CM systems (RF over CM) in theory, simulation, and experiments. The transmission performances are analyzed using a single-tone RF signal, and theoretical SNR is developed. It is found that RF over CM systems are limited in performance by both beat noise and fiber chromatic dispersion due to the use of a broadband light source. A broader light source improves the system performance only if fiber dispersion of the system is negligibly small. Otherwise, the performance is seriously degraded by fiber dispersion. The theoretical model is verified experimentally. Finally, an orthogonal frequency division multiplexing ultra-wideband wireless signal over a CM system is verified. It is concluded that CM systems cannot be used for high-frequency RF signal transmission.

We have designed, grown and fabricated InAs/InP quantum dot (QD) waveguides as the gain materials of mode-locked lasers (MLLs). Passive InAs/InP QD MLLs based on single-section Fabry–Perot (F–P) cavities with repetition rates from 10 GHz to 100 GHz have been demonstrated in the C- and L-band. Femtosecond (fs) pulses with pulse duration of 295 fs have been achieved. The average output power is up to 50 mW at the room temperature of 18 °C. By using the external fiber mixed cavities fs pulse train with a repetition rate of 437 GHz has been generated. We have also discussed the working principles of the developed QD MLLs.

A traveling-wave electrooptical modulator that makes use of a mode coupling mechanism between a substrate integrated waveguide (SIW) and coplanar waveguide (CPW) is proposed for millimeter-wave (mmW) radio-over-fiber applications. In this structure, SIWs are added at the input/output ports of conventional modulators from which mode conversion and signal transfer take place between lossy CPW traveling-wave electrodes and low-loss mmW SIW terminals. Such a gradual coupling from SIW to CPW in the modulation path creates a nonuniform field overlap integral and gives the ability of using the modulator at higher frequency. A laser micromachining technique for a transparent LiNbO3 substrate and a titanium in-diffusion technique for achieving a good optical field confinement are used to fabricate the SIW and optical waveguide, respectively. The bending structure of the SIW, fabrication of pad resistors, and broadband transitions are considered in the proposed modulator. 7-GHz bandwidth was measured for the proposed modulator at 60 GHz.

Enabling technologies for optical wireless broadband access networks have been explored and developed with respect to optical components, modules, transmission systems, and networks. In this paper, we review the most significant accomplishments reported during 2010 with emphasis on radio-over-fiber (RoF) technology that is critical for the deployment of optical wireless broadband access networks in the near future.

We show that an optical modulator that consists of an integrated dual parallel Mach-Zehnder modulator (dMZM) can be used for obtaining not only optical single-side band modulation but also tunability of optical carrier to sideband ratio (OCSR) simultaneously. Such a modulator will be vital for optimizing the performance of radio over fiber links by improving modulation efficiency and receiver sensitivity and by removing fiber chromatic dispersion induced RF power fading. It is shown that a wide range of OCSR tunability can be obtained by altering bias voltage of dMZM, and optimum OCSR, to maximize the output RF power, depends on RF modulation index and extinction ratio of the integrated dMZM. Good agreement between theory, simulation, and experiment is obtained. For typical extinction ratio and low modulation index, it is found that an OCSR of 0 dB is optimum to maximize RF carrier power. However, for multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio, the best error vector magnitude (EVM) of -21.8 dB is obtained experimentally at an OCSR of ~5.4 dB due to avoidance of clipping induced nonlinear distortion.

This letter presents a compact, uniplanar, circularly polarized, coplanar waveguide (CPW) spiral antenna for wideband applications. The antenna is directly fed by a 50-Omega CPW from the outside edge of the spiral, thus a balun for matching is not required. This feed provides the capability to have an entire uniplanar array of spirals. It is found that a thick substrate of high dielectric constant absorbs most of the radiated power and results in a unidirectional radiation pattern, which is desirable in many applications. Therefore, this antenna structure is fabricated on a substrate of high dielectric constant, resulting in miniaturization and capability for integrated system applications. It is experimentally verified that the proposed spiral antenna has stable radiation pattern, and its axial ratio is less than 3 dB over the frequency range of 11.4-17.5 GHz.

We propose and demonstrate a novel and simple predistortion circuit using reflective antiparallel diodes for linearization of electro-absorption modulators (EAMs) in multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio-over-fiber systems for UWB signals from 3.1 to 4.8 GHz. In this novel predistortion circuit, neither amplifiers, nor phase shifters are used, while only two antiparallel connected diodes are used, and the predistortion signal is adjusted by tuning the diodes' dc-bias current. The simple architecture makes the circuit cost effective and suitable for UWB system applications. By adding the predistortion circuit before the EAM, more than 7-dB suppression in third-order intermodulation distortion and 11-dB improvement in spurious-free dynamic range are achieved over the frequency range of 3.1-4.8 GHz, i.e., bandgroup 1 of MB-OFDM UWB. Correspondingly, 1-dB improvement in error vector magnitude is obtained experimentally when the designed predistortion circuit is used.

In this paper, a universal LP01 to any LPlm mode converter structure is proposed based on a circular waveguide. As examples, nine mode converters from LP01 to LP02, LP03, LP04, LP05, LP06, LP07, LP11, LP21 and LP31 are designed. It is shown that an insertion loss of less than 3 dB is achieved over the entire C-band for all the mode converters. Furthermore, two mode (de)multiplexers based on directional couplers are demonstrated, showing an insertion loss less than 2.5 dB and a crosstalk less than −10 dB over the C-band.

We propose and demonstrate a novel and cost-effective photonic down-conversion scheme for millimeter wave (mm-wave) over fiber (MoF) uplinks using an electro-absorption modulator (EAM) at a base station. Two wavelengths with frequency difference of flo, used as dual wavelength, are incident to an EAM that is driven by an mm-wave signal at fmm to obtain optical subcarrier modulation. Due to four-wave mixing (FWM), two new wavelengths are generated inside the EAM and used for the optical carriers for the uplink. One of the generated two wavelengths and one of two mm-wave signal subcarriers is separated by fmm-flo in frequency, thus down-converted. Furthermore, down-conversion with tunability of optical carrier to optical subcarrier power ratio can be achieved by simply controlling the dual wavelength power into the EAM. We experimentally investigate the efficiency of the FWM versus EAM reverse bias voltage and input optical power. The performance of the proposed scheme for down-conversion of a 30 GHz multiband (MB) orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) is also investigated in terms of error vector magnitude (EVM) versus RF and LO modulation index, and received optical power at the photodetector.

Reference [1, Fig. 8] shows the strain-induced band edge modification (−0.65 eV in CB, −0.41 eV in HH, and +0.1 eV in LH, respectively) in the GaP sublayer, and in fact the non-strain band edge plus strain induced band edge modification should be shown. However, this error does not affect the model and conclusion. The corrected Fig. 8 is shown below.

In this paper, a novel framework for designing optimized photonic crystal (PhC) sensors has been proposed. The complexity of such structures has resulted in the lack of an analytical method to design the structures. Therefore, this framework aims to provide a comprehensive and automatic method to find the best values for the structural parameters without human involvement. The framework is explained with an example of designing a PhC liquid sensor. In the framework, an optimizer called the "multi-objective gray wolf optimizer" is utilized. However, a diverse range of multi-objective optimizer algorithms could be utilized. The results show that the proposed framework can design any kind of PhC sensor. Simplicity, being straightforward, and no human involvement are the advantages of the proposed framework. In addition, a significantly wide range of optimal designs will be found that are suitable for general and specific applications.

Substrate integrated waveguides (SIW) are built up of periodically arranged metallic via-holes or via-slots. The leakage loss of SIW structures increases with the distance between the via-holes or via-slots. An open periodic waveguide with a large via distance supports the propagation of leaky-wave modes and can thus be used for the design of a leaky-wave antenna. In this paper, this leakage loss is studied in detail and used to design a periodic leaky-wave antenna. The proposed concept represents an excellent choice for applications in the millimeter-wave band. Due to its versatility, the finite difference frequency domain method for periodic guided-wave or leaky-wave structures is used to analyze the characteristics of the proposed periodic leaky-wave antenna. Two modes (TE10 and TE20) are investigated and their different leaky-wave properties are analyzed. Based on the proposed leaky-mode analysis method, a novel periodic leaky-wave antenna at 28-34 GHz is designed and fabricated.

In this paper, we present a very compact ultra-wideband (UWB) filter (patent pending), which is integrated in a commercial leaded package. This UWB filter is designed through quasi-equally allocating the first three resonant frequencies of the stub-loaded resonator. Input/output excitation is realized by a technique of wire-bonded interdigital capacitors for obtaining strong wideband coupling. After formulating the resonance conditions and analyzing the characteristics of the stub-loaded resonator, an equivalent-circuit topology is developed and a single-stage prototype is fabricated. Measured results agree well with the results generated from both circuit and full-wave simulations. In order to improve the selectivity, two stages of stub-loaded resonators are cascaded. A two-stage prototype is then assembled in a commercial leaded package and measured with a test fixture. Both simulation and measurement results validate our proposed design methods and packaging solutions. This filter is the first demonstration of integrated UWB filters with a commercial package, which can be conveniently applied in UWB communication and radar systems.

Multiband (MB) orthogonal frequency-division multiplexing (OFDM) ultra-wideband (UWB) wireless, which provides high data rate access, is required to be distributed by using optical fiber. The performance of MB-OFDM UWB over fiber transmission system is investigated considering optical modulation and demodulation impact. Theoretical analysis of the effect of fiber dispersion, optical transmitter, and optical receiver response on system performance is carried out considering amplitude and phase distortion. Experiments are conducted and verified by our theoretical analysis and good agreement is obtained. It is found that RF modulation index of ~4% is optimum for optical transmitter with Mach-Zehnder modulator, and optical receiver with Chebyshev-II response is the best for MB-OFDM UWB over fiber. Compared to back-to-back UWB over fiber, optical transmission is mainly limited by laser phase noise converted relative intensity noise and phase distortion induced by fiber dispersion when optimum modulation index is used. Higher modulation index is limited by amplitude and phase distortion to OFDM signal induced by optical transmitter and receiver response nonlinearities and fiber dispersion and the spectral mask. It is also found that highly received optical power is required for transmission of MB-OFDM UWB signal over fiber.

We analyze in theory, simulation, and experiment the spurious-free dynamic range (SFDR) for a radio-over-fiber system using an optical single sideband (OSSB) Mach-Zehnder modulator (MZM) that is linearized using mixed polarization. The mixed polarization OSSB MZM makes use of a dual-electrode z-cut LiNbO3 MZM, where a linear polarizer is placed in front and in behind the optical modulator, respectively. It is found that intermodulation distortion can be suppressed significantly regardless of modulation index. Thus intermodulation induced power fading and crosstalk via fiber chromatic dispersion for a given RF carrier is reduced compared to using the conventional OSSB MZM. Improvements of ~ 13 dB in SDFR is predicted in theory, simulation, and demonstrated experimentally.

This paper proposes a double fiber taper-based mode converter that converts LP(01) to LP(02) and vice versa. The first taper is used to convert LP(01) to some higher order LP(0m) (m1) modes while the second taper suppresses the undesired higher order modes (m2) and results in LP(02) 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(02) back to LP(01). 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(02) and an LP(01) 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.

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.

A novel coplanar waveguide (CPW)-fed compact ultrawideband (UWB) microstrip antenna is proposed for ultrawideband applications. By etching a mixed slots structure on the surface of the radiating patch, the area of it was greatly reduced and the impedance bandwidth was extended. The mixed slots include two semicircle and one rectangular slots. Similarly, through etching a pair of quarter-circle slots and two pairs of mutually perpendicular rectangular slots, it improved the impedance match and minimized the proposed monopole antenna. The antenna has a very compact size of 20× 15 ×1.6mm3. Simulated results show that the proposed antenna could operate from 3.1 to 11.1 GHz for S11 ≤ −10 dB with stable radiation patterns.

A novel broadband analog predistortion circuit (PDC) is designed to linearize radio-over-fiber (RoF) systems. Dual Schottky diode is used as predistorter. The PDC with high bandwidth and small dimension is achieved by removing quarter-wavelength transformers and power dividers. Measured S-parameters show that the PDC can work from 10 MHz to up to 40 GHz. The PDC is verified using two-tone and WiFi signals in an RoF system. The spur-free dynamic range with respect to third order intermodulation distortion is improved by more than 10 dB from 1 to 5 GHz and more than 5 dB from 1 to 30 GHz, and it is found that the RoF system is limited by fifth order nonlinearity. Error vector magnitude is improved by 1 dB for WiFi signals at 2.4 and 5 GHz by using the PDC.