Search

This letter proposes a polarization-tracking-free polarization division multiplexing (PDM) in an intermediate frequency over fiber mobile fronthaul system based on multiband direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) with adaptive modulation. The use of two orthogonally polarized optical carriers with a simple optical filter at each radio access unit is sufficient to perform passive demultiplexing of the corresponding multiband signals in PDM schemes without transmission efficiency loss in training signal, which is commonly found in conventional PDM demultiplexing schemes. We experimentally achieved a multiband OFDM signal with adaptive modulation, which maximizes signal capacity, in a PDM scheme with the data rate of 49.64 Gb/s, which doubles that of corresponding single polarization schemes, over a 25-km single mode fiber. We also achieved a 6-dB power margin which is sufficient for the wireless transmission part.

Performance of multicarrier code division multiple access (MC-CDMA) long reach passive optical network (LR-PON) implemented with colorless laser diode transmitter is evaluated and compared with the same system under typical orthogonal frequency division multiplexing (OFDM) format. The relative amount of the multicode interference (MCI) in the MC-CDMA PON can be efficiently suppressed by applying the minimum mean square error based MCI cancellation processing and be analyzed in term of the error vector magnitude deviation. Due to the double sideband nature of the directly modulated colorless laser diode, both OFDM and MC-CDMA formats suffer from severe power fading after fiber transmission. Partial subcarriers in the OFDM-PON fail to reach the specified forward error correction (FEC) threshold, causing a huge performance deviation among subcarriers and a 3% data loss after 75-km transmission. In contrast, a distance transparency transmitter design for subscribers ranging from back-to-back to 50 km is achieved in the MC-CDMA PON scheme, and the received performance among subcarriers are equalized under the FEC criterion even after 75-km transmission. Therefore, the MC-CDMA exhibits superior performance when comparing with the OFDM format in the directly modulated colorless laser diode based LR-PONs.

We propose and experimentally demonstrate an optical modulation format converter from a 25-Gbps quadrature phase-shift keying (QPSK) signal to two 12.5-Gbps binary phase-shift keying (BPSK) signals in a double-pass configuration, which is achieved by four-wave mixing in a single highly nonlinear fiber. The phase logic of the original QPSK signal is preserved on both of the converted BPSK signals. The clear constellation diagrams and the satisfactory bit-error-rate (BER) performance are successfully obtained both in numerical simulation and experimental demonstration. Moreover, the tolerance of phase mismatch among pumps and signals in the proposed format converter is also investigated.

We proposed and experimentally demonstrated an all-optical modulation format conversion mechanism by applying bidirectional cross-phase modulation in a single highly nonlinear fiber. In our experiment, four individual 12.5 Gb/s nonreturn to zero ON–OFF keying signals are successfully converted to a 50-Gb/s polarization division multiplexed return to zero quadrature-phase shift keying signal within a satisfactory power penalty. The performance and feasibility of the proposed converter are both numerically and experimentally investigated. Moreover, the timing-mismatch tolerance among all the signals in the proposed format converter is also analyzed.

In this letter, we propose and experimentally demonstrate an all-optical modulation format conversion scheme from amplitude shift keying (ASK) and quadrature phase shift keying (QPSK) to 8-ary quadrature amplitude modulation (8QAM), using four-wave mixing (FWM) effect in highly nonlinear fiber. In the FWM process, a 20-Gbits/s return-to-zero QPSK (RZ-QPSK) signal and a 10-Gbit/s ASK signal are utilized as probe light and pump light, respectively. A 30-Gbits/s RZ-8QAM signal, which is the idler light, is successfully obtained with optical signal-to-noise ratio larger than 30 dB and bit-error rate lower than 10-5.

By utilizing the cyclic filtering and free spectral range characteristics of an arrayed waveguide grating (AWG), we propose and experimentally demonstrate a novel 3-D ring-mesh sensing system with comprehensive self-healing function. The Bragg wavelengths of the employed fiber Bragg grating-based optical sensors are periodically assigned, with respect to the AWG. The proposed ring-mesh topology is constructed by 2 × 2 optical switches to link the ring-based subnets in a meshed architecture. Moreover, a simple and cost-effective signal sensing processor, provided by the AWG, is employed for classifying the sensing signals and monitoring the sensing elements. With the same constructional building block, the proposed sensing system can be easily scaled up which provides more self-healing paths in a self-reproduced manner. In addition, we also bring up a routing algorithm to seek for an optimal sensing path that has the least accumulated power loss to deal with any possible sensing point outage when the sensing system is scaling up.

Two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. To both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. In this paper, we demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps are achieved for averaging times from 1 s to 1 d. Moreover, the DPN data has less diurnal variation than that of the conventional TWSTFT. Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.

Precision of two-way satellite time and frequency transfer (TWSTFT) highly depends on the residual nonreciprocity delays; one of them is caused by the Sagnac effect. In this paper, we employ the satellite ephemeris data to compute the precise Sagnac effect and discuss the possible additive effect related to the Sagnac effect. We also describe a method to calculate the diurnal correction based on the time derivative of the Sagnac effect. Through the analysis of the link between the National Institute of Information and Communications Technology in Japan and the Telecommunication Laboratories in Taiwan, we demonstrate that the correction was useful to improve the TWSTFT data. The time deviation at an 8-h averaging time is reduced from 346 to 156 ps after the correction.

We analytically and experimentally demonstrate a linearly field-modulated, direct-detected virtual single-sideband orthogonal frequency-division multiplexing (VSSB-OFDM) system that employs a tunable frequency gap and an iterative detection technique. The VSSB-OFDM that uses no frequency gap, which is referred to as the gapless VSSB-OFDM, is proposed as a spectrally efficient format. Compared with the intensity-modulated SSB-OFDM, the gapless VSSB-OFDM saves half the electrical bandwidth (BW), and exhibits better receiving sensitivity and more robust tolerance against fiber chromatic dispersion (CD). Furthermore, by incorporating a tunable frequency gap between the optical carrier and the OFDM data sideband, the calculating burden of the iterative detection is greatly alleviated and the system performance can be flexibly improved within moderate iterations. The width of the optimum frequency gap is found to be ${\sim} {\hbox {0.35}}$ sideband BW, which is reached by trading the levels of signal–signal beat interference and the residual image beat interference. Such a gapped VSSB-OFDM system requires fewer iterations to extract the desired data from the interfered signal and exhibits greater robustness against the carrier-to-signal-power ratio (CSPR) variation, compared with the gapless VSSB-OFDM. In this paper, the analytical model of the proposed gapped VSSB-OFDM system will be addressed. In addition, we also successfully conduct a gapped VSSB-OFDM signal transmission over 1600 km of uncompensated standard single-mode fiber (SSMF) with only ${\sim} {\hbox {3}}$ dB optical SNR (OSNR) penalty, and obtain a significant OSNR sensitivity improvement of ${\sim} {\hbox {8}}$ dB, compared with the gapless VSSB-OFDM, after such a 1600-km fiber link.

In this paper, we provide a numerical bit error rate (BER) estimation approach for direct-detected orthogonal frequency division multiplexing (OFDM) signals in the presence of optical preamplified receivers. The individual BER of each subcarrier is first computed by considering their electrical signal-to-noise ratio (SNR), and then the ensemble BER is derived simply by taking the average of all the subcarriers' BERs. The calculated BER is verified by the conventional error-counting approach with high precision and is still accurate with higher quadratic-amplitude modulation (QAM) formats, even under the influences of the optical filtering and polarization mode dispersion (PMD) effects. Based on our simulation approach, the required extra power budget for 16- and 64-QAM relative to 4-QAM format are found to be ~3.8 and 8.2 dB, respectively, at a BER of 10-9. Furthermore, we use this approach to compare the receiving sensitivities and PMD tolerances for the previous proposed gapped and interleaved radio-frequency (RF)-tone-assisted OFDM systems. The results show that the gapped OFDM has a better sensitivity while the interleaved OFDM has a more PMD-tolerable capability.

This work demonstrates how electrical driving voltage of a Mach–Zehnder modulator and the bandwidth of electrical low-pass filters (LPFs) affect optical duobinary signals. The fact that a 100% driving voltage does not necessarily yield optimal results in duobinary systems is established both numerically and experimentally. Various filter bandwidths correspond to respective optimum driving voltages. In a 10 Gbps duobinary system, compared with the conventional condition of using a 2.5 GHz electrical LPF, a 2.5 dB sensitivity improvement is obtained by employing a 3 GHz LPF with an 88% driving voltage of fully driving. © 2007 Published by Elsevier Inc.

A periodic add/drop system in a fiber ring network was investigated using a reconfigurable optical add/drop multiplexer (ROADM) in a re-circulating fiber loop. After seven cascaded add/drop nodes at every 150 km along the transmission, at bit error ratio (BER) equals to 10 −9 and data rate of 10 Gbps, we observed a 2.5 dB power penalty for the passing through channels with 1050 km transmission distance, and 0.3 dB sensitivity penalty variation for the periodic add/drop channels at every 150 km, respectively.

We measure the system impacts due to the amplitude of group-delay (GD) ripple in single and cascaded chirped fiber Bragg gratings (CFBGs). Signals with smaller pulse width result in smaller performance variation at the same data rate. A 65-ps peak-to-peak GD ripple induces 0.9, 1.7, and 2.7 dB maximum penalties for 10, 20, and 40-Gb/s, respectively. We also find that cascading gratings with random ripple causes much less degradation than cascading gratings with the same ripple profile.

A multiwavelength fiber ring laser that is based on an S-band erbium-doped fiber amplifier (EDFA) and a semiconductor optical amplifier (SOA) is developed. An optical switch is used to switch the multiwavelength fiber laser between S-band and L-band. This fiber laser can stably lase seven wavelengths in the S-band or 28 wavelengths in the L-band. Additionally, the lasing wavelengths with a signal-to-noise ratio of over 33 dB and a wavelength spacing of 100 GHz are demonstrated experimentally. The average powers of the lasing wavelength in the S-band and the L-band are −7.53 and −12.15 dBm, respectively.

This investigation presents and demonstrates the feasibility of a novel fiber Bragg grating (FBG) sensor system using a fiber ring laser with a hybrid amplifier. The hybrid amplifier comprises an erbium-doped waveguide amplifier (EDWA) and a semiconductor optical amplifier (SOA). Experimental results demonstrate that the hybrid amplifier has a higher amplifier spontaneous emission power and gain spectrum. Moreover, this novel fiber ring laser provides a stable multiwavelength output with an optical signal-to-noise ratio over 50 dB, even when FBGs are located at a 25 km remote sensing position. The primary feature of the proposed fiber laser is that it facilitates a long-distance or a large-scale sensor system.

By using a widely wavelength lockable Fabry-Perot laser diode (FPLD) with low facet reflectance under intensity modulation and direct detection, a multicarrier code division multiple access (MC-CDMA) enhanced long-reach passive optical network (LR-PON) architecture is demonstrated to perform the dense-spectral-utilized transmission with orthogonal frequency division multiplexing (OFDM) format. Enlarging the power injected into the low-facet-reflectance FPLD enables the broadening of operational wavelength margins at a cost of slightly increased bit-error rate (BER). After LR-PON transmission, the signal-to-noise ratio among subcarriers strongly correlates with the frequency chirp of the wavelength-locked FPLD, which affects the allowable wavelength locking range. By substituting MC-CDMA for the typical OFDM format and using the minimum-mean-squared-error-based multicode interference cancellation processing, the BER degradation is minimized, accompanied by a 16% extension on the lockable wavelength margin achieved under 3 dBm injection. Such a widely wavelength lockable FPLD-based MC-CDMA enhances the LR-PON with a power penalty reduction of 1 dB as compared to the typical OFDM LR-PON after 60 km transmission.

This study investigates an all-optical scheme based on period-one (P1) nonlinear dynamics of semiconductor lasers, which regenerates the microwave carrier of an orthogonal frequency division multiplexing radio-over-fiber (OFDM-RoF) signal and uses it as a microwave local oscillator for coherent detection. Through the injection locking established between the OFDM-RoF signal and the P1 dynamics, frequency synchronization with highly preserved phase quality is inherently achieved between the recovered microwave carrier and the microwave carrier of the OFDM-RoF signal. A bit-error ratio down to 1.9×10sup-9/supis achieved accordingly using the proposed scheme for coherent detection of a 32-GHz OFDM-RoF signal carrying 4 Gb/s 16-quadrature amplitude modulation data. No electronic microwave generators or electronic phase-locked loops are thus required. The proposed system can be operated up to at least 100 GHz and can be self-adapted to certain changes in the operating microwave frequency.

We conceptually propose and experimentally demonstrate a novel multifunction all-optical packet switch architecture with less active optical components and simpler traffic control by adopting a periodic wavelength arrangement between wideband fiber Bragg gratings and an arrayed waveguide grating in the optical buffering unit and output ports, respectively. Three functionalities of switch, including first-in-first-out multiplexing, switching, and add-drop are successfully exhibited by the proposed architecture with negligible power penalties.

By strictly assigning and periodically arranging the Bragg wavelengths of fiber Bragg gratings and composing three 2 × 2 optical switches to construct the links and replace the nodes, respectively, we theoretically propose and experimentally demonstrate a novel three-dimensional mesh-based sensing system with comprehensive self-healing functionality. Moreover, a simple and cost-effective sensing signal processor for the classification of sensing signals and the monitoring of sensing elements is also proposed by utilizing the cyclic filtering and free spectral range characteristics of an arrayed waveguide grating.

We investigate the lower sensitivity bound, in terms of optical signal-to-noise ratio (OSNR), for direct-detected orthogonal frequency-division-multiplexing (DD-OFDM) systems in the presence of optically preamplified receivers. Using the spectrally matched optical filter, which is composed of two parallel filters for the carrier and sideband and has a passband matching to the signal spectrum, the ultimate sensitivity of DD-OFDM is found theoretically to approach that of coherent optical OFDM. We also investigate the effects of the filter's orders and bandwidths to the improved OSNR, and found that these effects could be effectively mitigated via the use of an optical amplifier for the carrier.

We demonstrate an equalization technique to simultaneously compensate for the fiber chromatic dispersion and the distortions induced by the in-phase/quadrature-phase (I/Q) imbalances and bias deviation in the Mach-Zehnder modulators (MZMs). With this technique, the requirement for the monitoring circuits at the transmitter could be relaxed or even eliminated, and thus simplifies the system design and reduces the system cost. The system exhibits a good performance with our proposal over a broad range of imbalances and bias deviation in the MZM. With deliberately introducing I/Q imbalances and bias deviation, we have observed a 2- and 4-dB optical signal-to-noise ratio improvement, for the back-to-back and 800 km of uncompensated standard single-mode fiber transmission, respectively, with our proposed scheme.

The SONET OC-192 receiving performance of In0.53Ga0.47As p-i-n photodiode grown on linearly graded metamorphic InxGa1-xP buffered GaAs substrate is reported. With a low-cost TO-46 package, such a device exhibits a frequency bandwidth up to 8 GHz, a bit-error rate (BER) of 10 -9 at 10 Gb/s, a sensitivity of -17.8 dBm, and a noise equivalent power of 3.4times10-15 W/Hz1/2 owing to its ultralow dark current of 3.6times10-7 A/cm2 . Eye diagram analysis at 10 Gb/s without transimpedance amplification reveals a statistically distributed Q-factor of 8.21, corresponding to a minimum BER of 1.1times10-16 at receiving power of -6 dBm

This investigation presents a novel 50-GHz interleaved bidirectional transmission system with eight wavelengths that uses four-port interleavers in a bidirectional recirculating loop. This bidirectional configuration shares optical components in the fiber network and an interleaver is utilized to enable unidirectional amplification in an erbium-doped fiber amplifier to block noise associated with Rayleigh backscattering. After bidirectional transmission through 500-km LEAF fibers in the recirculating loop, power penalties of less than 2.5 dB were achieved at 10 Gb/s for all channels

This investigation proposes and experimentally demonstrates an optical code-division multiple-access (OCDMA) light source using a directly modulated Fabry-Pe/spl acute/rot laser diode in an external injection scheme. This light source has an optical sidemode suppression ratio of over 27.5 dB and a pulsewidth of around 55 ps. Such a narrow pulsewidth has great potential for a two-dimensional OCDMA system. Additionally, the bit-error-rate performance with a 2/sup 31/-1 pseudorandom bit sequence indicates the feasibility of this light source in a 2.488-Gb/s system application.

A cost-effective method, based on the injection-locking technique of a compact self-seeded Fabry-Peacuterot laser diode with a 10-mm-long embedded fiber Bragg grating cavity, is proposed and experimentally compared with the two-mode injection-locked scheme for all-optical 2R regeneration at 10 Gb/s. Placing the presented all-optical 2R regenerator after a 50-km fiber significantly improves the transmission performance of a signal over 50- and 100-km fiber links. The power penalties at BER=10-9 are similar to those of the two-mode injection-locked scheme

A novel bidirectional transmission system is proposed and experimentally demonstrated using a four-port interleaver to enable unidirectional amplification. Following bidirectional transmission through 210 km of standard single-mode fibers, sensitivity variations of less than 0.2 dB, with a bit-error rate level of 10/sup -9/ at 10 Gb/s, were observed between bidirectional and unidirectional transmissions.

This letter proposes and experimentally demonstrates a cost-effective phase-modulation duobinary modulation scheme for improving self-phase modulation tolerance using a single modulator. Successful transmission over 230 km of standard single-mode fiber was achieved.

A multipurpose network which integrates the functionalities of single radio channel and baseband data distribution has been proposed recently. In this letter, we further propose and compare two possible combination schemes which combine multiple radio channel and single digital baseband data in electrical and optical domains. Linearity of multiple radio channels and performance of digital baseband data are studied in this externally modulated hybrid analog-digital link. It is found that signals combined in the optical domain, while with higher cost, will outperform that in the electrical domain on the carrier-to-noise and distortion ratio. In addition, the baseband data performance is also improved with optical combining scheme since the optimum required modulation index is lower.

A 50-GHz channel spaced dispersion-compensated interleaver pair for a metro add-drop network application was demonstrated using a recirculating loop. After five cascaded nodes (ten interleavers), a 2.5-dB sensitivity differential at bit-error-rate level around 10/sup -9/ was observed between the compensated and uncompensated pairs within a /spl plusmn/10-GHz detuning frequency window.