Polarization dependent distortion in AM-SCM video transmissionsystems

The second-order intermodulation distortion product, namely composite second-order (CSO) distortion, of an AM-SCM video transmission system can be caused by the optical isolators, cascaded fiber amplifiers, or the fiber transmission cable employed. Moreover, the absolute distortion value depends on the polarization angle of light input to the optical isolators, cascaded fiber amplifiers, and fiber transmission line. The degraded distortion values are measured and their mechanisms are clarified     

A compensation method for dispersion-induced third-orderintermodulation distortion using an etalon

A method to compensate dispersion-induced third-order intermodulation distortion (IM3_CD) in subcarrier multiplexing (SCM) optical transmission systems is proposed. It is confirmed theoretically and experimentally that IM3_CD can be compensated by interference-induced third-order intermodulation distortion (IM3_IN). Simple analytic expressions of IM3_CD and IM3_IN are derived from the same expression of optical electric field which includes exact phase term taking into account the phase of laser chirp. From the theoretical analyses, we clarified the conditions of IM3_CD-compensation. IM3_IN can be easily generated by an etalon which is a kind of interferometer and a optical glass plate. More than 30 dB of IM3_CD -compensation was achieved, and IM3_CD-compensation was achieved regardless of the transmission distance by adjusting tilt angle of the etalon. Using this method, high quality signal transmission can be realized even though the wavelength of laser diode (LD) is not coincident with the fiber zero dispersion wavelength     

Reduction of intermodulation distortion of interferometric optical modulators through incoherent mixing of optical waves

A method to reduce intermodulation distortion by 30 to 40 dB in interferometric optical modulators through incoherent mixing of optical waves is proposed and analysed. Parameters for design optimisation and limitations in modulation frequency and transmission distance are given.<>     

Reduction of noise and distortion in amplifiers using adaptivecancellation

Application of adaptive cancellation techniques to feed-forward networks for amplifier linearization is discussed. Results of a test showing over 25 dB reduction in third order intermodulation products are presented     

Signal distortion and noise in AM-SCM transmission systemsemploying the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion <-62 dBc, composite triple beat (CTB) intermodulation distortion <-59 dBc and a carrier-to-noise ratio (CNR) >50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO <-59 dBc, a CTB <-57 dBc and a CNR >49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems     

Reduction of longitudinal distortion in ultrasonic holograms

A review is presented of a technique for processing a hologram to scale the image by the same factor in both the lateral and longitudinal axes. The application of this technique to reduce the longitudinal distortion in ultrasonic holograms is discussed, and the results and conclusions of the experiment are given.     

Reduction of nonlinear distortion in directly modulatedsemiconductor lasers by coherent light injection

A theoretical investigation of the second- and third-order intermodulation distortions (IMDs) in an injection-locked semiconductor laser under small-signal modulation is presented. The results show that a substantial reduction of the laser nonlinearity can be obtained, depending on both the injection level and frequency detuning between the master and slave lasers. The intensity modulation frequency response is also reported and shows that the injection-locked laser may also have a significantly improved behavior over the same free-running laser, revealed mostly in the reduced resonance peak and the broadening of the modulation bandwidth available     

Reduction of fiber chromatic dispersion effects in fiber-wireless and photonic time-stretching system using polymer modulators

We have investigated the general characteristics of the power penalty due to the fiber chromatic dispersion effects in both fiber-wireless and photonic time-stretching systems. Two different modulation schemes have been demonstrated to reduce this penalty using our novel polymer modulators incorporating a multimode interference (MMI) structure. A single-sideband (SSB) modulator configuration has almost completely eliminated this penalty without a bandwidth limit. A double-sideband (DSB) modulator configuration with an appropriate quadrature bias has also shown significant improvement in bandwidth limitations for a given fiber link length.     

Influence of stimulated Brillouin scattering on nonlinear distortion in SCM video transmission

It is shown analytically that nonlinear distortions are induced by stimulated Brillouin scattering in subcarrier multiplexed systems. They are expressed as a function of the additional loss of the transmission fibre which is induced by stimulated Brillouin scattering. Experimental results well support the theoretical conclusions.<>     

Intermodulation distortion in high dynamic range microwavefiber-optic links with linearized modulators

Linearization of integrated optic intensity modulators significantly reduces the two-tone intermodulation distortion. The resulting intermodulation distortion produced by these modulators then varies as the input power to the fifth-order link system, the overall intermodulation product is a combination of third-order and higher-order terms. The authors determine the dynamic range of a cascaded microwave network consisting of a preamplifier, a high-dynamic-range fiber-optic link with a highly linear modulator, and a postamplifier. An expression is found that relates the intermodulation power at the output to the relative suppression from the signal level. As an example, a hypothetical 10-GHz low-distortion fiber-optic link that has a dynamic range of 125 dB in a bandwidth of 1 Hz is cascaded with various preamplifiers, and it is shown that the dynamic range of the system is reduced by as much as 20 dB, depending on the third-order intercept of the amplifier     

Reduction of harmonic distortion and noise in a semiconductor optical amplifier using bias current feedback

The application of bias current feedback to a semiconductor optical amplifier is shown to improve considerably its noise and distortion performance in analogue applications. Improvements in harmonic distortion of up to 12.75dB, and in noise levels of up to 7.5dB, have been achieved.<>     

Reduction of semiconductor laser amplifier induced distortion andcrosstalk for WDM systems using light injection

Semiconductor laser amplifiers cause both distortion and crosstalk in WDM systems due to gain saturation. A method using light injection is experimentally demonstrated to reduce distortion and crosstalk. In a two-channel experiment with a data rate of 2.5 Gbit/s, it is found that light injection can eliminate the BER floor due to distortion and crosstalk     

High-accurate fault location technology using FSK-ASK probebackscattering reflectometry in optical amplifier submarine transmissionsystems

This paper proposes a new modulation format for optical time domain reflectometry (OTDR) to eliminate optical surge and improve OTDR performance in optical amplifier submarine transmission systems. The modulation format, FSK-ASK, uses a short high-power probe pulse and a long dummy pulse. Thanks to the slow gain dynamics of erbium-doped fiber amplifiers, both pulses experience an identical gain, equal to the loss of a span, so that the probe pulse maintains its high power and does not develop into an optical surge. Fault location experiments verify a theoretical prediction that FSK-ASK improves the signal-to-noise ratio (SNR) of OTDR by an amount as large as the power ratio of the probe to dummy signal. They also confirm the elimination of the optical surge caused by conventional OTDR using a solitary probe pulse. An FSK-ASK OTDR is applied in a commercial submarine amplifier transmission system which has a total transmission length of 890 km and a repeater spacing of 90 km. These field trial results show that subtle fiber anomalies can be located, with a spatial resolution of 1 km, along the entire length of the amplifier transmission system from a terminal end     

On the use of tapered linearly chirped gratings asdispersion-induced distortion equalizers in SCM systems

We report on the use of tapered linearly chirped fiber gratings to equalize dispersion-induced distortion in long distance/broad-band subcarrier multiplexed (SCM) systems. Different apodization functions for the coupling factor of the fiber grating have been considered and their equalizing capability of fiber-induced distortion in a subcarrier system have been compared. The results of the theoretical analysis show that both harmonic and intermodulation distortions are significantly reduced when the equalizer parameters have been properly designed. Higher reduction of the nonlinear distortion is obtained when the length of the filter increases, even though a saturation effect on this reduction figure appears when the equalizer length is about four or five times its minimum value to achieve equalization, which is derived theoretically based on the SCM optical system parameters. The performance of a Tanh-type tapered linearly chirped grating equalizer on a realistic broadband SCM system is also considered. The results reveal that the dispersion-induced distortion can be neglected when the equalizer is employed     

Reduction of intermodulation distortion in microwave activebandpass filters-theory and experiments

This paper examines, both theoretically and experimentally, the dependency of the third-order intermodulation (IM) distortion and power saturation upon circuit and device parameters of an active bandpass filter using negative-resistance compensation. Nonlinear analysis is performed by means of the Volterra series formulation. We show that the IM distortion can be reduced by several orders of magnitude with suitable choice of external gate-source and feedback capacitance values. Measured performances of some 900-MHz experimental MESFET bandpass filters are presented     

Calculation of intermodulation distortion levels in f.e.t. mixers and modulators

The intermodulation spectrum produced by a junction gate f.e.t. mixer or modulator may be predicted in `class-A? operation by suitably combining recently reported analytical procedures. Extension to discontinuous operation under high-level drive has been found to give satisfactory accuracy.     

微机与8031组成数据采集系统中的多线程处理

对微机与8031单片机组成的数据采集系统中的上位机使用多线程处理方式,使上位机可以并行处理数据.给出了系统的硬件实现、多线程流程和多线程实现的关键部分的实现.     

Delta-sigma modulators using frequency-modulated intermediatevalues

This paper describes a new first- and second-order delta-sigma modulator concept where the first integrator is extracted and implemented by a frequency modulator with the modulating signal as the input. The result is a simple delta-sigma modulator with no need for digital-to-analog converters, allowing straightforward multibit quantization. Without the frequency modulator, the circuit becomes a frequency-to-digital converter with delta-sigma noise shaping. An experimental first- and second-order modulator has been implemented in a 1.2-μm standard digital CMOS process and the results confirm the theory. For the first-order modulator an input signal amplitude of 150 mV resulted in a signal-to-quantization noise ratio (SQNR) of ≈115 dB at 2 MHz sampling frequency and signal bandwidth of 500 Hz     

Reduction of noise in AVHRR channel 3 data with minimum distortion

The channel 3 data of the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA series of weather satellites (NOAA 6-12) are contaminated by instrumentation noise. The signal to noise ratio (S/N) varies considerably from image to image and the between sensor variation in S/N can be large. The characteristics of the channel noise in the image data are examined using Fourier techniques. A Wiener filtering technique is developed to reduce the noise in the channel 3 image data. The noise and signal power spectra for the Wiener filter are estimated from the channel 3 and channel 4 AVHRR data in a manner which makes the filter adaptive to observed variations in the noise power spectra. Thus, the degree of filtering is dependent upon the level of noise in the original data and the filter is adaptive to variations in noise characteristics. Use of the filtered data to improve image segmentation, labeling in cloud screening algorithms for AVHRR data, and multichannel sea surface temperature (MCSST) estimates is demonstrated. Examples also show that the method can be used with success in land applications. The Wiener filtering model is compared with alternate filtering methods and is shown to be superior in all applications tested     

Reduction in harmonic distortion through use of aperiodic timing in frequency demodulation

It is demonstrated, theoretically and experimentally, that aperiodic timing based on the FM signal, as opposed to periodic timing based on a reference carrier, can lead to a significant reduction in harmonic distortion in an FM demodulator based on an exclusive OR gate and a sample and hold structure. A 20-dB improvement in harmonic distortion can be readily achieved.