Test Method for Measuring Bit Error Rate of Pulsed Transceivers in Presence of Narrowband Interferers

The popularity of pulse-based transceivers can be attributed to the high information rates that can be achieved in such systems compared to traditional narrowband systems. However, such systems are usually low-power and transmission efficiency is severely affected by the interference signals from other moderate to high-power narrowband transmitters. Hence, during manufacturing, pulse-based systems must be characterized and tested for bit error rate (BER) performance in the presence of narrowband interferers. Usually, at large interference levels, the BER is moderate (10^-4) to high (10^-2). However, at low interference levels when very few bits are in error, the BER is low (10^-6 - 10^-10 ) and testing for the BER becomes time consuming. We propose a new measurement technique employing sinusoidal pulses such that the BER value obtained is significantly large (10^-3 - 10^-4 ) even at low interference levels. BER values obtained using sinusoidal pulses are highly correlated to the actual BER values. Hence, the actual BER can be accurately estimated in a much smaller time without actually performing the standard test. This method was implemented in hardware using an Altera field-programmable gate-array development board. From the measurements, BER estimation error was less than 3%. In addition, significant reduction (up to 100 x) in test time was obtained using the proposed method.     

High-Bit-Rate Dense SS-WDM PON Using SOA-Based Noise Reduction With a Novel Balanced Detection

The major drawback of incoherent broadband sources (BBSs) is their inherent intensity noise. Semiconductor optical amplifiers (SOAs) can be exploited at the transmitter to mitigate this noise. Optical filtering at the receiver, however, leads to the return of most of suppressed noise. Wider filtering at the receiver is the best known strategy to maintain performance gains, at the price of reduced spectral efficiency due to the tradeoff between noise cleaning and adjacent channel crosstalk. We introduce a novel balanced receiver for wavelength division multiplexing (WDM) systems that maintains greater noise cleaning and leaves spectral efficiency unchanged. Unlike standard receivers, our balanced scheme does not filter the desired signal. In this paper, we first demonstrate that the newly proposed receiver is equivalent to standard WDM receivers when no SOA for noise cleaning is present at the transmitter. Although a 2.9-dB power penalty is incurred, network capacity is unchanged, i.e., bit error rate (BER) floors due to intensity noise are the same. When SOAs are employed to mitigate severe intensity noise, we show that our receiver outperforms the wide filtering strategy by two orders of magnitude. Dense WDM capacity is demonstrated up to 10 Gb/s using a thermal source, a saturated SOA, and the balanced detection scheme. A BER of 10^-6 is achieved at 10 Gb/s; further improvement is possible using low overhead forward error correction or a better SOA design. This demonstrates the ability of spectrum-sliced wavelength division multiplexing (SS-WDM) passive optical networks (PONs) to operate at 10 Gb/s at good spectral efficiency. Error performance better than 10^-9 is achieved up to 8 Gb/s with 30-GHz optical channel bandwidth and 100-GHz spacing.     

High-sensitivity FSK signal detection with an erbium-doped fiberpreamplifier and Fabry-Perot etalon demodulation

A report is presented on a 622-Mb/s optical transmission system with demodulation of alternate mark invert (AMI) encoded frequency shift keying (FSK) signals by a Fabry-Perot etalon. A 25-dB gain erbium-doped fiber preamplifier gave a receiver sensitivity of -40 dBm at 10^-9 BER, which was improved to -44 dBm with the addition of a narrowband optical filter. The theoretical sensitivity of the preamplified receiver and the effect of the etalon on the amplified spontaneous emission beat noise are discussed and related to experiment     

Noncoherent detection of coherent lightwave signals corrupted byphase noise

Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10 ^-9. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth     

Sensitivity of optically preamplified DPSK receivers withFabry-Perot filters

The error-rate performance of a DPSK lightwave receiver having an optical amplifier followed by a Fabry-Perot filter and delay-line demodulator is analyzed. Receivers with sampling and with integrate-and-dump threshold comparison are compared to the well-known result for a matched optical filter. The Fabry-Perot filter decreases the sensitivity at 10^-9 error-rate from 20 to 24.5 photons/b with optimum optical filter bandwidth and postdetection integration time     

Reach Extension of a DPSK Optical Link Using an Optical Filter Demodulator and MLSE Receiver

We have proposed a novel scheme based on chirped fiber Bragg grating (CFBG) as a narrowband optical filter demodulator and a maximum likelihood sequence estimation (MLSE) receiver to attain an enhanced unrepeatered reach in a nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) transmission link. An extended reach up to 500 km with only 2-dB optical signal-to-noise ratio penalty is found feasible for a 10.7-Gb/s NRZ-DPSK link with the CFBG-MLSE combination.      

The error performance of CD900-like cellular mobile radio systems

The symbol error performance of CD900-like digital cellular mobile radio systems over narrowband and urban wideband transmission channels was investigated. The basic performance is presented for Gaussian, flat-fading Rayleigh, and log-normal channels in the presence of selection and ratio combining space diversity schemes. For wideband channels having more than one resolvable fading path, a CD900-like system without diversity reception suffers from large residual symbol error probabilities P_R(≈10^-1). The introduction of adaptive correlation diversity (ACD) mitigates the effects of multipath, yielding a P_R of 6×10^-5. Although this P_R value is relatively low, the probability of symbol error (P_e) versus signal-to-noise ratio (SNR) is significantly poorer than for the Gaussian channel. By combining the ACD scheme with space diversity, the P_R is eliminated by P_e >10^-5, and the channel SNR is within 5 dB of the Gaussian channel performance when P_e is 10^-10     

Direct Detection of Direct Optically Filtered Millimeter-Wave Signals

Good performance of a direct optical microwave filter combined with direct detection consisting of a single superimposed grating used in reflection has been assessed through bit-error-rate (BER) measurement. In the passband with a 3-dB bandwidth of 2GHz, the BER is below 1.2times10^-11, showing nearly on-off behavior of the filter     

Fiber-optic millimeter-wave downlink system using 60 GHz-bandexternal modulation

In this paper, a fiber-optic millimeter-wave (mm-wave) downlink system using 60 GHz-band external modulation is investigated. We prepare the fiber-optic 60 GHz-band mm-wave downlink testbed. It consists of an optical modulation section with a mm-wave signal generator, an optical single sideband (SSB) filter, a standard single-mode fiber (SMF), an optical detection section with a 60 GHz-band radio transmitter and a 60 GHz-band radio receiver. To modulate the laser output with 60 GHz-band mm-wave signals directly, a specially designed electro-absorption modulator with high-efficiency at around 60 GHz is used. The use of this modulator makes the simpler system configuration possible. Using the downlink testbed, the 5 m-long free-space propagation of subcarrier multiplexed 156 Mb/s-DPSK 60 GHz-band mm-wave signals recovered by optical direct detection is successfully demonstrated. The transmission of the mm-wave signals over 85 km-long standard SMF is also successfully demonstrated, using an optical SSB filtering technique to overcome the fiber dispersion. The BER of 10^-9 is achievable at the optical received power of -7.0 dBm     

Ultrafast IR Laser Writing of Strong Bragg Gratings Through the Coating of High Ge-Doped Optical Fibers

Strong fiber Bragg gratings were written through the standard polymer coatings of "off-the-shelf" high numerical aperture single-mode optical fibers after a few seconds exposure with femtosecond pulse durations of infrared radiation through a phase mask. While writing through the acrylate coating, we obtained index modulations of up to 1.4 x 10^-3 and 7 x 10^-4 with and without H₂ -loading, respectively.     

High-Speed PDM-RZ-8QAM DWDM Transmission (160 $times$ 114 Gb/s) Over 640 km of Standard Single-Mode Fiber

Based on the practicability issue, using the standard single-mode fiber (SSMF) in the transmission line is important for high-speed optical transmission. Here, we propose and experimentally demonstrate a 640-km SSMF-based high-speed dense wavelength-division-multiplexing (160times114 Gb/s) transmission system in 25-GHz channel spacing employing polarization-division-multiplexing return-to-zero (RZ) eight quadrature amplitude modulation format and digital coherent detection. The filtering effects for RZ and nonreturn-to-zero formats have also been probed in this investigation. For all 160 channels (1530-1560 nm, cover the whole C-band), the bit-error rate is smaller than 2times10^-3 without Raman amplification, optical dispersion compensation, and any special fiber in this straight-line slink.     

Noise filtering with the nonlinear optical loop mirror

The operation of a nonlinear optical loop mirror as a passive and signal polarization independent noise filter, is investigated experimentally. Spontaneous emission not within the signal spectrum is rejected leaving a receiver noise limited solely by signal-spontaneous beat noise. The receiver power penalty caused by spontaneous emission (@BER=10^-9 (PRBS)) was improved from 3 dB to 0.4 dB by propagating a signal through the loop. A comparison between this nonlinear filtering: technique and a variable band-pass filter is also made     

Interferometric measurement of the linewidth enhancement factor ofa 1.55-μm strained multiquantum-well InGaAs/InGaAsP amplifier

The linewidth enhancement factor of an InGaAs/InGaAsP strained multiquantum well optical amplifier was measured interferometrically. It varied from 3 to 18 over the wavelength range from 1500 to 1600 nm with injection currents varying from one to four times the lasing threshold of the uncoated device. A rate equation model gave differential gain and refractive index change per carrier, respectively, in the range 0.3 to 2.5×10^-15 cm² and -5 to -8×10^-20 cm³     

High-speed DPSK coherent systems in the presence of chromaticdispersion and Kerr effect

The error probability for a single-channel coherent optical differential phase-shift keying (DPSK) transmission system based on repeaterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is obtained by using a numerical solution of the nonlinear Shrodinger equation and the characteristic function method. The results show that the selection of an optimized IF filter bandwidth is crucial to obtaining the best system performance. When chromatic dispersion dominates, the best performance is achieved in the normal dispersion region whereas when the Kerr effect has the most limiting effect on system performances, the lowest error probability is attained in the anomalous regime. The maximum link length is limited by the presence of Kerr effect, independently of the amount of transmitted optical power, to be shorter than a threshold length if an error probability of 10^-9 is to be achieved     

Measurement of mode couplings and extinction ratios inpolarization-maintaining fibers

Polarization mode couplings in the axial direction are evaluated for polarization-maintaining fibers using optical heterodyne detection. To verify the validity of this approach for fibers with various coupling constants, the method is applied to three fibers with modal birefringence values of 3.0×10^-4, 1.1×10^-4 , and 1.5×10^-4, respectively. The coupling constants in the 1.7×10^-4 m^-1 to 6.4×10^-7 m^-1 range are evaluated with a length resolution of 1 m. The extinction ratios are obtained from the coupling constants averaged over the fiber lengths. These values are in good agreement with the values measured directly from power ratios between the orthogonally polarized modes     

LOCNET-an experimental communications system using optical routing

Experiences gained with the installation and operation of an experimental communication system are reported. The system uses distributed switching in its narrow-band part, integrating ISDN services and high-speed data communication, and a star coupler as a central device. LiNbO₃ optical switches behind the star coupler prevent line tapping. In the wideband subsystem, optical routing is used for videophone. The central device is a space-division-switching unit consisting of cascaded 4×4 LiNbO₃ switching matrices. Both the narrowband and wideband parts of the system have been operated with bit error rates less than 10^-9     

Millimeter wave narrowband optical fiber links using externalcavity semiconductor lasers

We present the theoretical analysis and the experimental implementation of a narrowband millimeter wave optical fiber communication system using an external cavity semiconductor laser. We derive analytic expressions and present experimental data for the modulation response, relative intensity noise, carrier-to-noise ratio, and harmonic distortion for a semiconductor laser in an external cavity operating as a transmitter in the millimeter wave frequency range. We present a system implementation of this capable of transmitting 40-Mbt/s digital data at a 35-GHz subcarrier frequency with bit-error rates below 10^-9 over a 6.3-km-long optical fiber link     

Chromatic Dispersion Compensation of OOK Signals With MLSE Utilizing Diverse VSB Filtering

We present diverse vestigial-sideband-filtering maximum-likelihood sequence estimation (DVSB-MLSE), a modification of MLSE, for residual chromatic dispersion (CD) compensation in nonreturn-to-zero (NRZ) on–off keying (OOK) systems. While VSB filtering can considerably increase residual CD tolerance of MLSE receivers, the performance of MLSE can be further enhanced by diverse reception, through which the MLSE module jointly equalizes samples from upper and lower VSB filtered signals. Simulation results show that compared to conventional MLSE, DVSB-MLSE increases the residual CD tolerance at 3-dB optical signal-to-noise ratio penalty by 110% in a 42.7-Gb/s NRZ-OOK system.      

Design and Application of Compact and Highly Tolerant Polarization-Independent Waveguides

In this paper, the design, fabrication, and application of a highly tolerant polarization-independent optical-waveguide structure suited for operation in the third communication window is presented. The waveguide structure has been optimized toward minimized sensitivity to technological tolerances and low fabrication complexity. The tolerance analysis has been based on the typical processing tolerances of the widely applied silicon-oxynitride technology, being plusmn3times10 ^-4 in refractive index, plusmn1% in thickness, and plusmn0.1 mum in channel width. The optimized waveguide design fulfills the criterion of a channel birefringence within 5times10^-5, including processing tolerance. It also enables a fiber-to-chip coupling loss below 1 dB/facet and is suited for the realization of low-loss bends with a radius down to 600 mum. Based on this waveguide design, a passband-flattened optical wavelength filter with 50-GHz free spectral range has been realized and tested. The measured TE-TM shift of 0.03 nm confirms the polarization dependence of the optical waveguides being as low as 3times10^-5     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold