Coherent Ultrashort Light Pulse Research Articles

Nonlinear XUV-optical transient grating spectroscopy at the Si L2,3–edge

Time-resolved transient grating (TG) spectroscopy facilitates detailed studies of electron dynamics and transport phenomena by means of a periodic excitation of matter with coherent ultrashort light pulses. Several current and next generation free-electron laser (FEL) facilities provide fully coherent pulses with few femtosecond pulse durations and extreme ultraviolet (XUV) photon energies. Thus, they allow for transient grating experiments with periodicities as small as tens of nanometers and with element specific photon energies. Here, we demonstrate the element specificity of XUV TG (X-TG) experiments by tuning the photon energy across the Si L2,3-edge of Si3N4. We observe a shortening of the signal decay when increasing the XUV photon energy above the absorption edge. The analysis of the wavelength dependent signal shows that the faster decay is driven by the increase in the charge carrier density. From the decay constants the interband Auger coefficient at elevated temperatures and high electron densities has been determined.

Internally coded time-hopping coherent ultra-short light pulse code division multiple access scheme with optical amplifier and its performance analysis using additive noise model

The internally coded time-hopping coherent ultra-short light pulse code division multiple access (CULP CDMA) scheme (recently introduced) with an optical amplifier is described and its performance in fibre-optic communication systems is analysed. In accordance with the important role of optical amplifiers in optical communication systems, a preamplifier at the input of the receiver is used in order to compensate the losses because of the spectral encoder, spectral decoder and optical fibre path. The authors evaluate the bit error rate of the system considering the effects of the multiple access interference, noise because of the optical amplifier and thermal noise using saddle point approximation, and compare the results with those of the conventional CULP CDMA system with and without an amplifier. The numerical results indicate a substantial improvement in the performance of the coded system in comparison with the uncoded one. In addition, the negative effect of amplifier noise in the proposed scheme is much less than that of the conventional CULP CDMA system.

Performance Analysis of Internally Coded Time-Hopping Coherent Ultrashort Light Pulse CDMA Scheme in Fiber-Optic Communication Systems

We consider an internally coded time-hopping coherent ultrashort light pulse code division multiple access scheme (TH-CULP CDMA) and analyze its performance in the fiber-optic communication systems. This system combines the TH and CULP CDMA techniques and exploits the advantages of both. In our method, each bit time interval is divided into Ns frames, and the spectral phase-coded pulse is transmitted in one of these frames. Two exclusive PN sequences are assigned to each user. One is added to a superorthogonal convolutional encoder output to select the transmission frame, and the other is used to encode the phase of the ultrashort pulse in the spectral domain. We evaluate the bit error rate (BER) of the system considering the effects of the multiple access interference (MAI) and thermal noise for both soft and hard decoders. The BER is evaluated using both the Chernoff bound and saddle point approximation. We compare the performance of our proposed system with that of the conventional CULP CDMA system. The numerical results indicate that for the same bit rate and processing gain, the performance of our proposed system (TH-CULP CDMA) is substantially better than that of the conventional CULP CDMA system. In addition, we observe that the hard decoder has a better performance at large number of users compared to the soft decoder

Improvement of Coherent Ultrashort Light Pulse CDMA Communication Systems with Distinct 4-Level m-Sequences

Optical fiber communications require multiple-access schemes to access a shared channel among multiple users. The coherent ultrashort light pulse code-division multiple-access (CDMA) system is one such scheme, and it also offers asynchronous-access communication. This system usually employs 2-level, i.e., binary, m-sequences as signature codes because of their low correlation. If the number of active users is greater than the length of the m-sequence, i.e., code length, distinct m-sequences are used. However, the distinct 2-level m-sequences do not exhibit low correlation, resulting in performance degradation. We therefore propose a coherent ultrashort light pulse CDMA communication system with distinct 4-level, i.e., quaternary, m-sequences to improve system performance when the number of users is greater than the code length. We created the 4-level m-sequences from 2-level m-sequences, and assess the correlation of the 4-level m-sequences. We also theoretically derive the bit error rate (BER) of the proposed system taking into account multiple-access interference (MAI), beat noise, amplified spontaneous emission (ASE), shot noise, and thermal noise. The numerical results show that BER for distinct 4-level m-sequences is more than an order of magnitude smaller than that for distinct 2-level m-sequences. BER is limited by MAI and beat noise when the power of the received signal is high, otherwise BER is limited by ASE, shot noise, and thermal noise.

Performance Analysis of Coherent Ultrashort Light Pulse CDMA Communication Systems with Nonlinear Optical Thresholder

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.

Statistical analysis of coherent ultrashort light pulse CDMA with multiple optical amplifiers using additive noise model

This paper describes a study of the performance of various configurations for placing multiple optical amplifiers in a typical coherent ultrashort light pulse code-division multiple access (CULP-CDMA) communication system using the additive noise model. For this study, a comprehensive performance analysis was developed that takes into account multiple-access noise, noise due to optical amplifiers, and thermal noise using the saddle-point approximation technique. Prior to obtaining the overall system performance, the input/output statistical models for different elements of the system such as encoders/decoders, star coupler, and optical amplifiers were obtained. Performance comparisons between an ideal and lossless quantum-limited case and a typical CULP-CDMA with various losses exhibit more than 30 dB more power requirement to obtain the same bit-error rate (BER). Considering the saturation effect of optical amplifiers, this paper discusses an algorithm for amplifiers' gain setting in various stages of the network in order to overcome the nonlinear effects on signal modulation in optical amplifiers. Finally, using this algorithm, various configurations of multiple optical amplifiers in CULP-CDMA are discussed and the rules for the required optimum number of amplifiers are shown with their corresponding optimum locations to be implemented along the CULP-CDMA system.

Coherent Ultrashort Light Pulse CDMA Communication Systems with Multipulse Pulse Position Modulation

We propose coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with multipulse pulse position modulation (MPPM). Coherent ultrashort light pulse CDMA is an asynchronous multiplexing technique for high-speed fiber-optic communications. MPPM is introduced to outperform both pulse position modulation (PPM) and on-off keying (OOK), when the optical bandwidths for these three modulation methods are the same. We theoretically derive the bit error rate (BER) of the proposed system. The performance characteristics are discussed with respect to a number of system parameters, and are compared with those of PPM and OOK. Our results show that MPPM is able to achieve a lower BER than that of both PPM and OOK when the bit rates for these methods are the same. We also find that there exists the optimum number of pulses in an MPPM signal block, which is inversely proportional to both the bit rate and the total number of multiple access interference (MAI) users.

Performance comparison of coherent ultrashort light pulse and incoherent broad-band CDMA systems

We compare the theoretical bit-error-rate (BER) performance limits of coherent ultrashort light pulse and incoherent broad-band code-division multiple access (CDMA) communication systems. Our comparison highlights the fundamental differences in the scaling of BER and throughput with important system parameters. The results indicate that for the same optical bandwidth and BER, an ultrashort pulse CDMA with a code length of 512 can in principle yield a substantial throughput advantage over the incoherent broad-band systems.

Limits to the encoding and bounds on the performance of coherent ultrashort light pulse code-division multiple-access

The authors obtain lower and upper bounds on the peak intensity of a spectrally phase encoded ultrashort light pulse as a function of the length of spectral phase encoding. It is shown that using random phase codes for spectral encoding of ultrashort light pulses in a coherent ultrashort light pulse code-division multiple-access (CDMA) system is almost optimal. Furthermore, simple upper bounds and an asymptotic approximation on the bit error rate (BER) for a wide class of ultrashort light pulse CDMA systems are obtained. It is shown that, in spite of being simple, these upper bounds can be used to evaluate the performance of a coherent ultrashort light pulse CDMA system. >

Coherent ultrashort light pulse code-division multiple access communication systems

A new technique for encoding and decoding of coherent ultrashort light pulses is analyzed. In particular, the temporal and statistical behavior of pseudonoise bursts generated by spectral phase coding of ultrashort optical pulses is discussed. the analysis is motivated by recent experiments that demonstrate high-resolution spectral phase coding of picosecond and femtosecond pulses and suggest the possibility of ultrahigh speed code-division multiple-access (CDMA) communications using this technique. The evolution of coherent ultrashort pulses into low intensity pseudonoise bursts as a function of the degree of phase coding is traced. The results are utilized to analyze the performance of a proposed CDMA optical communications system based upon encoding and decoding of ultrashort light pulses. The bit error rate (BER) is derived as a function of data rate, number of users, and receiver threshold, and the performance characteristics are discussed for a variety of system parameters. It is found that performance improves greatly with increasing code length. >

A novel method of measuring coherent ultrashort light pulses using diffraction by a grating

We propose a novel method of measuring coherent ultrashort light pulses in a single shot by monitoring the ratio of the energy of a pulse, diffracted by a grating, to the energy of the incident pulse. In the case of a gaussian pulse we present numerical calculations which allow easy determination of the pulse width. The proposed method is extremely versatile with respect to wavelength regions and pulse duration.