FDMA-FSK star network with a tunable optical filter demultiplexer

An optical frequency-division-multiple-access (FDMA) star network is analyzed and demonstrated experimentally using two 45-Mb/s frequency-shift-keyed (FSK) laser channels at 1.5 μm. A tunable fiber Fabry-Perot (FFP) filter is used to select channels and convert FSK to intensity modulation for direct detection. The analysis predicts and experiment supports a minimum channel spacing of about six times bit rate B for a single FFP. These constraints are similar to those for more complex heterodyne demultiplexing. Estimates show that a network with 1000 users, independent of bit rate, is feasible with a tandem FFP. For B=1 Gb/s per channel the network capacity would be 1 Tb/s     

一种新的2FSK信号识别方法

在信号处理中常常因为各种原因导致获取的信号并不完整。残缺信号由于其信息量小、特征不明显等特点造成处理难度大或者无法识别等困难,是信号处理领域内研究较少的内容。文章在研究分析频移键控信号（FSK）特性的基础上提出一种用于残缺二进制频移键控信号（2FSK）识别的新方法。首先将残缺的2FSK信号识别出,然后两两提取相关特征,最后根据特征门限识别出完整的2FSK信号。计算机仿真实验证明了该算法在低信噪比下具有较好的性能。     

Computer simulation of FSK laser spectra and of FSK-to-ASKconversion

The performance of binary frequency shift keyed (FSK) modulated two-segment semiconductor injection lasers is simulated by generating a 64-b pseudorandom step-function bit pattern. This signal, superimposed on a DC bias, is used to drive a two-segment distributed feedback (DFB) laser. To obtain the optical laser spectrum, the input signal is passed through the laser's small-signal transfer function. The resulting spectrum is passed through a Fabry-Perot filter that rejects one of the two principal peaks of the wide-deviation optical spectrum and converts the FSK modulation to amplitude shift keying (ASK) modulation. This simulation determines how different system parameters influence the performance of a laser system used for signal transmission by means of FSK-to-ASK conversion     

Semiconductor laser FSK modulation and optical direct discrimination detection

FSK signals are generated in an AlGaAs double heterostructure laser by employing an electrical equalising circuit to compensate for its nonuniform FM modulation characteristics. A 10?9 bit error rate is achieved at a 100 Mbit/s data rate in the optical direct discrimination detection with a Michelson interferometer. The optical FSK discrimination detection performance is studied in terms of receiving signal power level and laser spectral linewidth.     

Characterization of an optical frequency-shift-keying transmitter based on carrier-suppressed phase modulation

We experimentally characterize an optical frequency-shift-keying transmitter based on optical carrier-suppressed phase modulation. Only one laser source is needed to generate an optical FSK signal. The demonstration of 10-Gb/s FSK signal generation and 50-km transmission verified the improved performance of the proposed transmitter, compared with the previous two-laser schemes. To further reduce the complexity of the transmitter, the phase modulator is omitted and a single MZM modulator is used for both optical carrier-suppression (OCS) and phase modulation. This simplified structure is verified by simulation, implying the feasibility that a FSK transmitter can be constructed with only one laser source and one modulator.     

Impact of laser linewidth on optical channel spacing requirements for multichannel FSK and ASK systems

The sensitivity penalty is evaluated for amplitude-shift-keyed and frequency-shift-keyed multichannel coherent systems that use lasers with linewidths which are a significant fraction of the bit rate. The study was conducted for both ASK and FSK systems using a single-filter receiver with nonsynchronous detection. For FSK systems, both NRZ (nonreturn-to-zero) and alternate mark inversion (AMI) signal formats were studied. The optical channel spacing is strongly determined by the laser linewidth. For example, with the FSK-NRZ data rate of 150 Mb/s, the optical channel spacing which gives 1-dB crosstalk penalty is 4 GHz when the intermediate frequency linewidth is 50 MHz (laser linewidth is 25 MHz), as opposed to 1.8 GHz when the linewidth is negligible.<>     

Frequency stabilisation of semiconductor laser using atomicabsorption line under direct FSK

The oscillation frequency of an AlGaAs laser diode is stabilised using the Rb-D2 absorption line (780 nm) under the direct frequency shift keying (FSK) condition. Only one side (mark or space) frequency signal is used to obtain the feedback signal for stabilisation in this system. The stability obtained is almost the same as that of the case without FSK     

FSK heterodyne transmission experiments at 560 Mbit/s and 1 Gbit/s

Optical frequency-shift-keying (FSK) signals are obtained from directly modulated distributed feedback (DFB) semiconductor lasers. Experimental studies of the direct frequency modulation (FM) characteristics of the DFB lasers show a nonuniform FM response due to the competing effects of thermal modulation of the laser active region and carrier density modulation. Equalization of the signal current to the laser is employed to produce a flat FM response from 30 kHz to 1 GHz. Optical FSK transmission and heterodyne detection experiments at 560-Mbit/s and 1-Gbit/s are conducted at a wavelength of 1497 nm. Receiver sensitivities of -39 dBm at 560 Mbit/s and -37 dBm at 1 Gbit/s are obtained. Transmission through 100 km of single-mode fiber at 1 Gbit/s is achieved with no degradation in receiver sensitivity.     

Design and experimental research of a high-precision wavelength controller for tunable fiber Fabry-Perot filters

A high-precision wavelength controller is presented in this paper. It is necessary to find out the difference between the central wavelength of a tunable fiber Fabry-Perot (FFP) filter and that of the input laser, while the wavelength controller operates at the states of wavelength-scanning and wavelength-locking modes. Firstly, a dynamic simulation model of tunable FFP filter is established, and the dynamic characteristic of tunable FFP filter modulated by an alternating current (AC) signal is simulated. Then the measuring time at wavelength-scanning mode compared with the theory time is discussed, and this time difference shows the difference between the central wavelength of a tunable FFP filter and that of the input laser. At last, the effects on wavelength-locking precision of time delays, including the time delay of opened-loop circuit, the time constant of the closed-loop circuit and the intrinsic hysteresis of piezoelectric (PZT) element, are analyzed. A wavelength controller of tunable FFP filter is designed and prepared. The experimental results at wavelength-locking mode show that a high locking precision is obtained.     

S/N and error rate evaluation for an optical FSK-heterodyne detection system using semiconductor lasers

Receiving performance is evaluated for an optical FSK-heterodyne detection system in which semiconductor lasers are used as both an FSK transmitter and an independent local oscillator. Noise and error rate are measured under feedback stabilization of IF signal frequency and electrical equalization of semiconductor laser FM modulation frequency characteristics. The minimum received signal power of -44 dBm, which is about 2 dB better than that in IM direct detection, is achieved at a 10^-9error rate for a 200 Mbit/s signal. Excess errors for FSK signals result from frequency broadening of the laser spectrum. Both AM and FM quantum noises in the lasers are primary factors which determine system performance. Error rate characteristics in an optical FSK direct detection system, in which a Michelson interferometer is employed as an optical frequency discriminator, are compared with the above results.     

Analysis of delay-and-multiply optical FSK receivers withline-coding and non-flat laser FM response

We present a theoretical analysis of the performance of coherent optical FSK systems when the driving laser signal is AMI or Manchester (biphase) line-coded to counteract the nonideal FM characteristic of the transmit laser diode, and the received signal is heterodyned and detected by a delay-and-multiply demodulator. The analysis takes into account IF filtering by assuming linear filtering of the noisy signal phase, accounting, for small linewidths, for laser phase noise in a straightforward and accurate manner. A simple equivalent baseband model of the system is derived for performance evaluation, applicable to both cases of large demodulator delay and when the delay tends to zero (ideal discriminator). Noise statistics include the clicks due to both signal and phase noise. The problem of performance evaluation is reduced to a classical intersymbol interference problem which is solved by mead of the method of Gauss quadrature rules. The analysis accurately predicts the effectiveness of AMI and Manchester line coding, depending on several system parameters such as linewidth, modulation index, IF bandwidth and laser FM response, and can be easily extended to other line coding techniques     

M-ary FSK Performance for Coherent Optical Communication Systems Using Semiconductor Lasers

This paper describes the design and performance of anMary frequency shift keyed (FSK) signaling and demodulation scheme for an optical communication system using semiconductor lasers and heterodyne detection. Frequency or phase noise in semiconductor lasers causes spectral spreading, producing a nonzero linewidth laser signal. This degrades communication performance when compared to a system using an ideal laser with zero linewidth. We present estimates of the bit error rate (BER) performance ofM-ary frequency shift keying (FSK) with noncoherent demodulation in the presence of white Gaussian frequency noise and additive channel noise. This is typical of an optical system using semiconductor lasers and heterodyne detection. Estimates use the union-Chernoff bound with a simplified channel model to predict the effects of frequency noise. Two effects of frequency noise are identified: signal attenuation or suppression, and crosstalk. These cause an offset in the BER curve from the BER in the absence of frequency noise, and an error rate floor, respectively. The error rate floor is lower than previously predicted. When performance is not crosstalk limited,M-ary FSK is found to perform better than binary FSK with the same system bandwidth constraints, as would be predicted if ideal lasers are used. Theoretical results are compared with Monte Carlo simulations of the system.     

Simple oscillators-based readout circuit for low-power biomedical implant system

This paper reports a wireless sensor readout circuit for continuous physiological parameters monitoring including a potentiostat, a data generation unit and a frequency-shift-keying (FSK) modulator unit with the low drop-out (LDO) regulator for biomedical implant system. The potentiostat can generate an output potential of 0.7?V for the data generation unit. The data generation unit is designed based on a relaxation oscillator scheme and can be used to sense a current signal from any amperometric biomedical sensor and convert the signal to a square waveform in which the frequency of the square wave signal is proportional to the sensor current. FSK modulation scheme has been selected for wireless transmission. Designed with a very simple ring oscillator, this modulator integrates the modulation functionality into the oscillator itself by using the data signal to control the oscillation frequency. The prototype circuits have been fabricated in a 0.35???m bulk complementary metal-oxide semiconductor (CMOS) process. Working with a regulated 1.8?V supply, the potentiostat consumes only 2???A of current while the data generation unit can generate around 15.7?kHz output frequency with an input current of 1???A. The FSK modulator consumes a total current of around 19???A for a carrier frequency around 1?MHz. An off-chip demodulator is constructed to demodulate the data signal from the FSK modulator and the demodulated signal has less than 1.6?% variation of frequency.     

Wideband frequency-shift keying with a spectrally bistable cleaved-coupled-cavity semiconductor laser

By operating a cleaved-coupled-cavity (C3) laser in a spectrally bistable regime with large hysteresis we demonstrate a new kind of high-speed FSK transmitter. This scheme can tolerate large fluctuations in modulation signal amplitude and show a detection limited switching time of 1 ns, indicating the possibility of high-speed high-reliability FSK systems.     

Combined frequency and amplitude modulation for the STARNET WDMcomputer communication network

STARNET is an experimental wavelength-division multiplexed computer communication network that makes use of combined modulation of a single transmitter laser to implement a virtual-ring control sub-network and a reconfigurable high-speed sub-network over a passive star architecture. In this letter, we demonstrate a new technique of combined modulation that works by direct frequency-shift keyed (FSK) modulation and external amplitude-shift keyed (ASK) modulation of a semiconductor laser. By adjusting the amplitude modulation depth to 0.8, both the 125 Mb/s ASK receiver and the 1.244 Gb/s FSK receiver operate with equal bit-error ratios less than 10^-9 for a received signal power of -24 dBm     

The effect of spurious intensity modulation in semiconductor diode lasers on the performance of optical heterodyne frequency shift-keyed communications systems

Analytical expressions are derived for the bit-error rate (BER) of anM-ary frequency shift-keyed (FSK), heterodyne, optical communication system with noncoherent demodulation in the presence of spurious intensity modulation (SIM) and frequency noise. The SIM degradation of an FSK system, implemented with semiconductor diode lasers, is estimated for lasers with zero and nonzero linewidths and will be discussed for a distributed feedback laser operating at 1.5μm and a channeled substrate planer laser operating at 0.83 μm. The SIM power penalty is typically less than 1 dB, but can exceed 1 dB for 2-, 4-, and 8-ary FSK at data rates above 1 Gbit/s.     

Performance of linecoded optical heterodyne FSK systems withnonuniform laser FM response

A simple theoretical analysis for evaluating the performance of linecoded optical frequency shift keying (FSK) systems is presented. It accounts for the combined effects of laser phase noise, receiver noise, and nonuniform BM response of distributed feedback lasers. A close form expression for the random frequency noise due to the combined effect of laser nonuniform FM response and phase noise is developed. The analysis is carried out for three different linecoding schemes, i.e., alternate mark inversion, Miller code or delay modulation, and Manchester coding, to investigate the efficacy of the line coding schemes in counteracting the effect of nonflat FM response. Theoretical and simulation results show that the sensitivities of linecoded FSK systems are within 0.7 and 0.4 dB for single-branch and dual-branch detection, respectively, at a bit error probability of 10^-9 relative to the random non-return to zero FSK with flat FM response     

FM and FSK response of tunable two-electrode DFB lasers and theirperformance with noncoherent detection

Two-electrode distributed-feedback (DFB) lasers show promise for combining high speed and frequency tunability for frequency-division-multiplexed-frequency-shift-keyed (FDM-FSK) networks. The FM and FSK responses of such lasers have been measured up to modulation frequencies of ~1 GHz. Using these lasers in a noncoherent detection system in which a fiber Fabry-Perot tunable optical filter converts an FSK signal into an amplitude-shift-keyed (ASK) format, a 10 ^-9 BER was observed up to 1 Gb/s. Nonuniform FM response and consequent tone broadening of the optical FSK spectra can lead to system power penalties due to optical-filtering effects. Thus, for a given FM response, the behavior of these lasers in FSK optical systems can be projected     

Simulation of optical heterodyne single-filter FSK system with line coding

The simulation results for an optical heterodyne single-filter FSK system are presented when two different line-coding plans, viz. alternate-mark-inversion (AMI) and delay modulation (DM), for the laser-driving signal are used to overcome the adverse effect of nonuniformity in the FM response of the transmitting laser.<>     

Coherent FSK transmitter using a negative feedback stabilised semiconductor laser

An electrical negative frequency feedback scheme is applied to a semiconductor laser to improve its direct frequency modulation performance in an FSK transmitter. An error signal caused by imperfect FM response is extracted through heterodyne discrimination detection with a stable master laser and fed back to the FM semiconductor laser.