Polarization Selectivity of the Thin-Metal-Film Plasmon-Assisted Fiber-Optic Polarizer.

The interaction between light and metallic nanostructures leads to many impressive achievements and has a wide range of applications. The thin-metal-film plasmon-assisted fiber-optic polarizer is one of the essential applications. However, the polarization mechanism and the transmitted polarization of the plasmon-assisted polarizer have given rise to controversy over the past decade. Which of the polarizations is preferentially transmitted through the polarizer? The transverse electric polarization or the transverse magnetic polarization? Here, special emphasis is placed upon the polarization mechanism and the transmitted polarization of thin-metal-film plasmon-assisted fiber polarizers. We first investigate the polarization mechanism of the polarizers theoretically and numerically. Furthermore, a novel approach is proposed to demonstrate the transmitted polarization in the plasmon-assisted fiber polarizers experimentally. We demonstrate that the polarization mechanism is based on the polarization selective absorption of the metallic material, and the transverse electric polarization is the only transmitted polarization of the metallic plasmon-assisted polarizer. Moreover, the plasmon-assisted polarizer can offer a high polarization extinction ratio (33.1 dB) and a low insertion loss (1.1 dB) at room temperature and have excellent temperature stability in the range of -48 to 82 °C. Experimental results agree well with our theoretical and numerical analyses. The findings clarify the confusion about the polarization mechanism and the transmitted polarization of metallic plasmon-assisted fiber polarizers over the past decade, providing new ground for the exploration of polarization-sensitive optical systems, with good potential applications in the fields of optical sensors, plasmonic lasers, coherent optical communications, and biosensor systems.

Long-term stability improvement of tunable optoelectronic oscillator using dynamic feedback compensation.

We propose and demonstrate a novel method to improve the long-term stability of the wideband tunable optoelectronic oscillator (OEO) where a dynamic compensation scheme is achieved to offset the parameter variation of the fiber. In our method, a 900 MHz calibration signal transmits in the fiber link of the OEO's feedback loop for establishing a servo system which can extract the time delay of the feedback loop. The time delay varies with the external environment because refractive index and length of the fiber fluctuate with ambient temperature variations. Taking the extracted information as the reference, the wavelength of the tunable laser used in the OEO can be controlled precisely and continually to offset the random delay fluctuation in the fiber. Consequently, the long-term stability of the microwave signal generated by the OEO can be optimized. The experiment results show that Allan deviation achieved in 1000-s averaging time is improved more than two orders of magnitude when the tunable OEO worked at 2.4 GHz.

Investigation on Nyquist pulse generation using a single dual-parallel Mach-Zehnder modulator.

The generation of Nyquist pulses with a dual parallel Mach-Zehnder modulator (DPMZM) driven by a single RF signal is demonstrated theoretically and experimentally. A complete theoretical analysis is developed and the limitation of the proposed scheme is also discussed. It is theoretically proved that Nyquist pulses with a spectrum of 5 flat comb lines can be generated using a single DPMZM, which is also verified with simulation. 7 flat comb lines in frequency domain can also be obtained if a large RF driving voltage is applied to DPMZM but the generated waveforms won't present a sinc-shape. This scheme is further investigated experimentally. 40 GHz Nyquist pulses with full-width-at-half-maximum (FWHM) less than 4.65 ps, signal-to-noise ratio (SNR) better than 29.5 dB, and normalized root-mean-square error (NRMSE) less than 2.4% are generated. It is found that a tradeoff exists between the insertion loss of the DPMZM and the deviation of generated pulses. The tunability of repetition rate is experimentally verified by generation of 1 GHz to 40 GHz Nyquist pulses with SNR better than 28.4 dB and NRMSE less than 6.15%.

Low overhead slipless carrier phase estimation scheme.

Two slipless schemes are compared with application to single carrier 30 Gbaud quadrature phase shift keying (QPSK) system. An equivalent linewidth model considering the phase noise induced by both the laser linewidth and fiber nonlinearity is applied in the performance analysis. The simulation results show that it is possible to mitigate cycle slip (CS) using only 0.39% pilot overhead for the proposed blind carrier phase recovery (CPR) + pilot-symbols-aided phase unwrapping (PAPU) scheme within 1 dB signal-to-noise ratio (SNR) penalty limit at the bit error ratio (BER) of 10(-3) with 4 MHz equivalent linewidth.

Stable fiber-optic time transfer by active radio frequency phase locking.

In this Letter we demonstrate a fiber link capable of stable time signal transfer utilizing our active long-distance radio frequency (RF) stabilization technology. Taking advantage of the chromatic dispersion in optical fiber, our scheme compensates dynamically the link delay variation by tuning the optical carrier wavelength to phase lock a round-trip RF reference. Since the time signal and the RF reference are carried by the same optical carrier, a highly stable time transfer is achieved at the same time. Experimentally, we demonstrate a stability of the time signal transfer over 50-km fiber with a time deviation of 40 ps at 1-s average and 2.3 ps at 1000-s average. The performance of the RF reference delivery is also tested, with an Allan deviation of 2×10(-15) at 1000-s average. According to our proposal, a simultaneous stable time and frequency transfer is expected.

Stokes and anti-Stokes differential pulse pair based distributed Brillouin fiber sensor with double-sideband probe wave.

We propose and demonstrate a distributed Brillouin fiber sensor using Stokes and anti-Stokes differential pulse pair based on double- sideband probe wave, in which the two sidebands of probe wave are used to balance the power of two pump pulses. The spatial resolution is determined by the slightly width difference of the two balanced pulses, without Brillouin gain spectrum broadening. The pulses perform gain-loss process in optical field before the probe signal being detected, without any post-processing or extra measurement time. The proposed technique can achieve high spatial resolution, natural Brillouin gain spectrum linewidth, normal measurement time and long sensing range simultaneously.

Experimental investigation on multi-dimensional digital predistortion for multi-band radio-over-fiber systems.

The recently-proposed multi-dimensional digital predistortion (DPD) technique is experimentally investigated in terms of nonlinearity order, memory length, oversampling rate, digital-to-analog conversion resolution, carrier frequency dependence and RF input power tolerance, in both directly-modulated and externally-modulated multi-band radio-over-fiber (RoF) systems. Similar characteristics of the multi-dimensional digital predistorter are identified in directly-modulated and externally-modulated RoF systems. The experimental results suggest implementing a memory-free multi-dimensional digital predistorter involving nonlinearity orders up to 5 at 2 × oversampling rate for practical multi-band RoF systems. Using the suggested parameters, the multi-dimensional DPD is able to improve the RF input power tolerance by greater than 3dB for each band in a two-band RoF system, indicating an enhancement of RF power transmitting efficiency.

Phase-conjugation-based fast RF phase stabilization for fiber delivery.

In this paper, we propose a phase-conjugation-based fast radio frequency (RF) phase auto stabilization technique for long-distance fiber delivery. By phase conjugation at the center site, the proposed scheme pre-phase-promotes the RF signal with the shift which is acquired by round-trip transferring another RF whose frequency is half of the one to be sent. Such phase pre-promotion is then used to counteract exactly the following retard induced by one-way delivery. Different from the previous phase-locking-loop-based schemes, the proposed open-loop design avoids the use of any tunable parts and dynamic phase tracking, enabling a fast phase stabilization at the remote site. An end-less compensation capacity can also be achieved. Our design is analyzed by theory. Experimentally, the new scheme is verified by transferring a frequency of 2.42 GHz through a 30-km optical fiber link. Significant phase drift compression is observed. The rapid phase stabilization is verified by introducing sudden time delay change into the link. The recovery time equals to the round-trip time of the link plus the transitional duration of the delay change, which is much shorter than the traditional trial-and-error phase locking loop. Important issues of the system design are discussed.

Linear dissipative soliton in an anomalous-dispersion fiber laser.

We report on the generation of linear dissipative soliton (LDS) from an erbium-doped actively mode-locked fiber laser. We show that depending on the down-chirping effect of quadratic phase modulation, instead of the fiber nonlinear Kerr effect in an all-normal-dispersion (ANDi) cavity, stable LDS can be realized in the linear dissipative system. The DS operation of ANDi laser and LDS operation of anomalous dispersion laser are experimentally investigated and compared, and the formation mechanisms of the DS and LDS are discussed. Finally, optical frequency comb generated by the LDS laser is demonstrated.

Multifrequency radio frequency sensing with photonics-assisted spectrum compression.

We propose and demonstrate a multifrequency radio frequency (RF) spectrum estimation technology. With photonic assistance, a multifrequency RF signal ranging from 0 to 1 GHz is highly spectrally compressed and sensed using a single analog-to-digital converter with analog bandwidth of 42.6 MHz. By calculating the cross correlation of pseudo random binary sequence and the encoded signal, up to 40 RF tones are precisely recognized without large computational load. The employment of optical mixing decreased the cost and increased the performance compared to its electrical counterpart. The theory and performance are also discussed.

Performance analysis for IEEE 802.11 distributed coordination function in radio-over-fiber-based distributed antenna systems.

In this paper, we analyze the performance of IEEE 802.11 distributed coordination function in simulcast radio-over-fiber-based distributed antenna systems (RoF-DASs) where multiple remote antenna units (RAUs) are connected to one wireless local-area network (WLAN) access point (AP) with different-length fiber links. We also present an analytical model to evaluate the throughput of the systems in the presence of both the inter-RAU hidden-node problem and fiber-length difference effect. In the model, the unequal delay induced by different fiber length is involved both in the backoff stage and in the calculation of Ts and Tc, which are the period of time when the channel is sensed busy due to a successful transmission or a collision. The throughput performances of WLAN-RoF-DAS in both basic access and request to send/clear to send (RTS/CTS) exchange modes are evaluated with the help of the derived model.

High-repetition-rate, stretch-lens-based actively-mode-locked femtosecond fiber laser.

We propose and demonstrate a novel actively mode-locked fiber laser based on a stretch-type time-lens. The pulse generated by this scheme has high repetition rate and large bandwidth while no nonlinearity is participated. A 10-GHz chirped pulse train with 18-ps duration and 11.6-nm bandwidth is obtained, which is then extra-cavity compressed down to 825 fs. The pulse characteristics dependent on the cavity dispersion and time-lens strength are discussed. Pulse propagation in the laser is similar with dissipative soliton in all-normal-dispersion laser. The results demonstrate that the stretch-lens inside the actively mode-locked laser can effectively broaden the spectral bandwidth, instead of the fiber nonlinearity, which can then support a high-repetition-rate "linear dissipative soliton" pulse shaping in a very compact design.

Pilot-symbols-aided cycle slip mitigation for DP-16QAM optical communication systems.

A pilot-symbols-aided phase unwrapping (PAPU), which utilizes the time-division multiplexed pilot symbols that are transmitted with data, is proposed to do cycle slip detection and correction with the carrier phase estimation (CPE). Numerical simulations for 10 Gbaud dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) systems show that the block averaging quadrature phase-shift keying (QPSK) partitioning with PAPU greatly eliminates the performance degradation caused by cycle slips, maintains a low CS probability with less influence of filter length, and achieves a bit-error-rate (BER) performance below soft-decision forward error correction (FEC) limit 2 × 10⁻² at 15 dB optical signal-to-noise ratio with only 1.56% overhead and 6 MHz combined laser linewidth.

Intermodulation distortion suppression for intensity-modulated analog fiber-optic link incorporating optical carrier band processing.

An intermodulation distortion suppression method based on the optical carrier band processing is demonstrated. A systematic analysis of the main optical spectrum contributors for the third-order intermodulation distortion in the nonlinear system is presented. Theoretical analysis shows that the third-order intermodulation distortion terms can cancel each other if a proper phase shifting is imposed to the optical carrier band. We experimentally demonstrate the approach with a two-tone test and a suppression of about 33 dB in the third-order intermodulation distortion is obtained. Experimental results show that an overall fundamental to third-order intermodulation distortion ratio of up to 64 dB is achieved and the link dynamic range is improved by 14.7 dB, compared with the conventional link without the proposed optical carrier band processing.

Closed-form path loss model of non-line-of-sight ultraviolet single-scatter propagation.

Non-line-of-sight ultraviolet propagation models have been developed for both coplanar and noncoplanar geometries. Based on an exact integral-form single-scatter model, this Letter proposes an approximate closed-form model for tractable analysis applicable to noncoplanar geometries with a narrow transmitter beam or receiver field of view. Numerical results on path loss are presented for various system geometries. These results are verified with the integral-form model and a previous approximate model, showing our model agrees well with the former and outperforms the latter.

Stable radio-frequency delivery by λ dispersion-induced optical tunable delay.

We propose and demonstrate a novel stable radio frequency (RF) delivery system based on a radio-over-fiber link. The proposed scheme acts as a long phase-locking loop where an optical tunable delay line is involved to compensate dynamically for the time-delay variation that arises from fiber-link fluctuation. An optical carrier with variable wavelength under fiber-link dispersion results in the desired tunable delay. The tunable range is in proportion to the length of the fiber link, so a large phase-error correction capacity under long-distance delivery can be realized. The large as well as fine optical-delay tunability is experimentally demonstrated, and the RF reference of 2.42 GHz is transferred for 54 km where a time jitter compression factor of 588 is achieved.

Non-line-of-sight ultraviolet single-scatter propagation model in random turbulent medium.

Non-line-of-sight (NLOS) ultraviolet communication (UVC) uses the atmosphere as a propagation medium. In previous literature, various scatter propagation models have been derived based on the premise that atmospheric turbulence was ignored and the atmosphere was considered as a turbid medium, also called random scatterers. In this Letter, a NLOS single-scatter propagation model is proposed to describe the singly scattered radiation in a turbulent medium, also called a random continuum, such as the clear atmosphere. The model is established based on the relationship between the scattered power and the characteristics of the random turbulent medium. The scattering cross section is further investigated in terms of different correlation distances and wavelengths. The received power dependence for NLOS UVC is also analyzed for different factors, including refractive-index structure parameter and transceiver range.

Stable radio frequency phase delivery by rapid and endless post error cancellation.

We propose and demonstrate a phase stabilization method for transfer and downconvert radio frequency (RF) signal from remote antenna to center station via a radio-over-fiber (ROF) link. Different from previous phase-locking-loop-based schemes, we post-correct any phase fluctuation by mixing during the downconversion process at the center station. A rapid and endless operation is predicted. The ROF technique transfers the received RF signal directly, which will reduce the electronic complexity at the antenna end. The proposed scheme is experimentally demonstrated, with a phase fluctuation compression factor of about 200. The theory and performance are also discussed.

Complexity-reduced digital predistortion for subcarrier multiplexed radio over fiber systems transmitting sparse multi-band RF signals.

A novel multi-band digital predistortion (DPD) technique is proposed to linearize the subcarrier multiplexed radio-over-fiber (SCM-RoF) system transmitting sparse multi-band RF signal with large blank spectra between the constituent RF bands. DPD performs on the baseband signal of each individual RF band before up-conversion and RF combination. By disregarding the blank spectra, the processing bandwidth of the proposed DPD technique is greatly reduced, which is only determined by the baseband signal bandwidth of each individual RF band, rather than the entire bandwidth of the combined multi-band RF signal. Experimental demonstration is performed in a directly modulated SCM-RoF system transmitting two 64QAM modulated OFDM signals on 2.4GHz band and 3.6GHz band. Results show that the adjacent channel power (ACP) is suppressed by 15dB leading to significant improvement of the EVM performances of the signals on both of the two bands.

Nonlinear signal-noise interactions in dispersion managed coherent PM-QPSK systems in the presence of PMD.

Considering the polarization mode dispersion(PMD), the transmission penalty induced by nonlinear signal-noise interactions (NSNI) between the amplified spontaneous emission noise (ASE) and the information signal is investigated numerically for 40(100)G dispersion-managed(DM) polarization-multiplexed quadrature phase-shift keying (PM-QPSK) systems. We show that for single-channel PM-QPSK systems, PMD is helpful to reduce the NSNI-induced penalty. For multi-channel PM-QPSK system, however, the NSNI-induced nonlinear penalty is significantly enhanced by PMD, especially at low bit-rate. Our results show that due to the NSNI, the reduction of allowed input power that gives 1-dB Q penalty after 1600-km nonlinear transmission will increase from 1dB without PMD to 3.7dB with PMD for 42.8-Gbit/s coherent return-to-zero (RZ)-PM-QPSK systems.