Phase-sensitive amplification (PSA) has unique properties, such as the quantum-limited noise figure of 0 dB and the phase clamping effect. This study investigates PSA characteristics when a chirped pulse is incident. The signal gain, the output waveform, and the noise figure for an optical pulse having been chirped through chromatic dispersion or self-phase modulation before amplification are analyzed. The results indicate that the amplification properties for a chirped pulse are different from those of a non-chirped pulse, such that the signal gain is small, the waveform is distorted, and the noise figure is degraded.

Range estimation based on time of arrival (TOA) is becoming increasingly important with the emergence of location-based applications and next-generation location-aware wireless sensor networks. For radar and positioning systems, chirp signals have primarily been used due to their inborn signal properties for decomposition. Recently, chirp signal has been selected as the baseline standard of ISO/IEC 24730-5 and IEEE 802.15.4a in 2.4GHz, organized for the development of a real-time accurate positioning system. When estimating the TOA of the received signals in multipath channel, the super-resolution algorithms, known as estimation of signal parameters via rotational invariance techniques (ESPRIT), multiple signal classification method (MUSIC) and matrix pencil (MP), are preferred due to their superiority in decomposing the received paths. For the super-resolution algorithm-based TOA estimation of chirp signals, the received chirp signals must be transformed into a sinusoidal form for the super-resolution algorithm. The conventional transformation, the de-chirping technique, changes the received chirp signals to sinusoids so that the super-resolution algorithms can estimate the TOA of the received chirp signals through a frequency estimation of the transformed sinusoids. In practice, the initial timing synchronizer at receiver tries to find the maximum energy point at which the received paths are overlapped maximally. At this time, the conventional de-chirping yields lossy transformed sinusoids for the first arrival path from the received samples synchronized to the maximum energy point. The first arrival path is not involved in the transformed sinusoids with the conventional transformation, leading to performance degradation. However, the proposed energy efficient time-frequency transformation achieves lossless transformation by using the extended reference chirp signals. The proposed transformation is incorporated with MUSIC-based TOA estimation. The effectiveness of the proposed transformation is analyzed and verified. The root mean squared error (RMSE) of the proposed transformation is compared with Cramer-Rao lower bound and those for the conventional algorithms such as super-resolution, ESPRIT and matrix pencil algorithm in multipath channel.

We theoretically analyze the performance of coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with a nonlinear optical thresholder. The coherent ultrashort light pulse CDMA is a promising system for an optical local area network (LAN) due to its advantages of asynchronous transmission, high information security, multiple access capability, and optical processing. The nonlinear optical thresholder is based on frequency chirping induced by self-phase modulation (SPM) in optical fiber, and discriminates an ultrashort pulse from multiple access interference (MAI) with picosecond duration. The numerical results show that the thermal noise caused in a photodetector dominates the bit error rate (BER). BER decreases as the fiber length in the nonlinear thresholder and the photocurrent difference in the photodetector increase. Using the nonlinear optical thresholder allows for the response time of the photodetector to be at least 100 times the duration of the ultrashort pulses. We also show that the optimum cut-off frequency at the nonlinear thresholder to achieve the minimum BER increases with fiber length, the total number of users, and the load resistance in the photodetector.

A simple system configuration was used to generate transform-limited optical pulses at 160 Gbit/s in the sub-picosecond range (625 fs). Pulse compression was achieved by broadening the spectrum using supercontinuum generation followed by a linear frequency chirping compensation.

Frequency chirping induced in an electorabsorption (EA) modulator can degrade transmission performance because of the chromatic dispersion of fiber. This letter studies the frequency chirping in an EA modulator from the viewpoint of the influence of the modulation bandwidth. Both simulations and experiments, in which fiber transmission was carried out applying modulation signals of different bandwidths to an EA modulator, show that a large bandwidth causes small degradation in the transmission performance. This result is attributed to the short chirping time that occurs when a large bandwidth signal is applied.

This paper proposes that a combination of pre-chirping and dispersion compensation is effective in suppressing the waveform distortion due to the self-phase modulation and the group-velocity dispersion in 10 Gb/s repeaterless transmission using 1.3-µm zero-dispersion single-mode fibers (SMF) operating at a wavelength of 1.55µm. The following results were obtained through simulation. 1) Setting the α-parameter of a LiNbO3 optical modulator negative (α1.0) gives a large tolerance of the launched power Pin. 2) For 90-km SMF transmission, the maximum Pin is obtained when the dispersion compensation ratio β is from 50% to 70%. 3) For the allowable β as a function of the transmission distance when a dispersion compensator is located in the receiver (post-compensation scheme), the lower limit of β is determined by the constant residual dispersion value, which agrees well with the dispersion tolerance without dispersion compensation. Our 90-km SMF transmission experiments using a LiNbO3 optical modulator and a dispersion compensating fiber (DCF) confirmed the simulation results regarding the optimum value of β and the large tolerance of the fiber launched power. Based on the above investigations, we achieved a 10-Gb/s repeaterless 140-km SMF transmission with α1.0 and post-compensation.