Enhancement of optical receiver sensitivities by amplification of the carrier

The amplification before detection of the carrier of a modulated optical signal by a narrow-band quantum amplifier enhances the signal-to-noise ratio, particularly when the signal wave-front is distorted. A further improvement is obtained by using a combination of wide-band and narrow-band quantum amplifiers. The practical application of these schemes requires a degenerate regenerative ring-type amplifier capable of amplifying arbitrary transverse field configurations. Experiments show that such an amplifier with a gain of 24 dB and a bandwidth of 1 MHz is feasible. The incident beam axis can be displaced by as much as ten times the beam-waist radius without losing more than 4 dB in gain. Frequency modulation may be converted into amplitude modulation by the phase shift introduced in the carrier.     

Long-haul coherent optical fiber communication systems usingoptical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated     

Coherent-state optical amplifier: a proposal

A new kind of phase-insensitive optical amplifier is proposed whose output is in the coherent state (ideal laser light) if the input is in the coherent state. In-phase and quadrature modulations are preserved in absolute values. Both modulations can be tapped off. The amplifier employs conventional optical amplifiers, electrical feedback, and all-pass filters. Remarkably, these properties hold when either linear or nonlinear optical amplifiers are employed.<>     

Gain characteristics of coherent optical amplifiers using aMach-Zehnder interferometer with Kerr media

This paper investigates the gain characteristics of coherent optical amplifiers that amplify only one of two quadrature phase components in an input signal light according to the phase of pump light. It is constructed around a Mach-Zehnder interferometer with Kerr media. To heuristically obtain the design parameters of the coherent optical amplifier, small-signal analysis is adapted to the Mach-Zehnder interferometer with Kerr media. The theoretical results are then compared to experimental results and shown to agree well, which confirms the validity of this design approach. By utilizing low-loss and high-nonlinearity silica fiber as the Kerr medium and optimizing its length, a coherent optical amplifier is constructed that yields high-gain (up to 26 dB) operation     

Influence of nondegenerate four-wave mixing on coherenttransmission system using in-line semiconductor laser amplifiers

Three-channel coherent frequency-shift-keying (FSK) transmission experiments using in-line semiconductor laser amplifiers under the influence of nondegenerate four-wave mixing in laser amplifiers are discussed. The experiments show that the effect of four-wave mixing is reduced by applying modulation to the interfering carriers and enlarging the frequency separation of each carrier. However, transmission quality degradation becomes serious in multistage amplifier systems     

Lightwave systems with optical amplifiers

Fiber-optic communication systems using semiconductor laser amplifiers are investigated theoretically and experimentally. The noise and bit-error-rate characteristics of lightwave systems with optical amplifiers are calculated and the dependence of system performance on amplifier characteristics such as optical bandwidth, noise figure, gain, etc., is shown. Experimental results for both a 4-Gb/s optical preamplifier as well as coherent and direct detection systems with four inline amplifiers are presented     

Self-phase modulation and spectral broadening of optical pulses insemiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems     

Signal Design and Detection in Presence of Nonlinear Phase Noise

In optical fiber transmission systems using inline amplifiers, the interaction of a signal and amplifier noise through the Kerr effect leads to nonlinear phase noise that can impair the detection of phase-modulated signals. We present analytical expressions for the maximum-likelihood (ML) decision boundaries and symbol-error rate (SER) for phase-shift keying and differential phase-shift keying systems with coherent and differentially coherent detection, respectively. The ML decision boundaries are in the form thetas(r) = c₂r² + c₁r + c₀, where thetas and r are the phase and the amplitude of the received signal, respectively. Using the expressions for the SER, we show that the impact of phase error from carrier synchronization is small, particularly for transoceanic links. For modulation formats such as 16-quadrature amplitude modulation, we propose various transmitter and receiver phase rotation strategies such that the ML detection is well approximated by using straight-line decision boundaries. The problem of signal constellation design for optimal SER performance is also studied for a system with four signal points.     

Semiconductor laser optical amplifiers for multichannel coherentoptical transmission

The application of semiconductor laser optical amplifiers in multichannel coherent optical transmission systems is investigated. The amplifiers considered (λ=1.3 μm) exhibit a gain of 24 dB at a grain ripple <2 dB and a 3-dB bandwidth of about 4000 GHz. The characteristics of these amplifiers and transmission experiments with these amplifiers are described. The investigations concern noise accumulation in an amplifier chain, generation of echoes due to backward gain in cascaded amplifiers, crosstalk in multichannel transmission, and the effect of gain saturation due to spontaneous emission. A good fit is shown between the advantages of multichannel coherent optical transmission systems and the properties of semiconductor laser optical amplifiers, which are very promising for future long-haul optical transmission systems     

New synthetic-heterodyne demodulator for an optical fiberinterferometer

A new algorithm for modulating a PZT stack using a sinewave signal instead of a saw-tooth signal was developed in order to resolve the flyback problem in a PZT driver. An improved synthetic heterodyne demodulator circuit was designed and arranged to interrogate the output signal from an interferometric sensor with a sinewave modulation (phase-generated carrier). The depth of modulation, or a gain amplifier in the circuit design, was adjusted such that the standard heterodyne signal could be formed. The result is a conventional PM-modulated carrier that can be demodulated using standard techniques, followed by a lock-in amplifier or a phase meter, to recover the phase shift in the sensing information. Implementing it into a path-matching differential interferometer for the signal modulation validates this new method. As compared to the conventional technique, modulating the PZT stack in a saw-tooth signal, this technique supplies a wider modulated frequency and, therefore, a wider frequency response in a sensing system. Finally, the new synthetic heterodyne is applied to a differential optical fiber refractometer for measuring the refractive index change. A lock-in amplifier is chosen as the demodulator to extract the phase shift. Therefore, the resolution of this sensing system is 7.52×10^-7 refractive index unit for a 120-μm length of the sensing cavity     

A 280 mW, 0.07% THD+N class-D audio amplifier using a frequency-domain quantizer

Pulse density modulation (PDM) based class-D amplifiers can reduce non-linearity and tonal content due to carrier signal in pulse width modulation based amplifiers. However, their low-voltage analog implementations also require a linear loop filter and a quantizer. A PDM based class-D audio amplifier using a frequency-domain quantization is presented. The digital intensive frequency-domain approach achieves high linearity under low supply regimes. An analog comparator and a single-bit quantizer are replaced with a current controlled oscillator (ICO) based frequency discriminator. By using the ICO as a phase integrator, a third-order noise shaping is achieved using only two analog integrators. A single-loop, single-bit, class-D audio amplifier is presented with an H-bridge switching power stage, which is designed and fabricated on a 0.18???m CMOS process with 6 layers of metal achieving a total harmonic distortion plus noise (THD+N) of 0.065% and a peak power efficiency of 80% while driving a 4-?? loudspeaker load. The amplifier can deliver the output power of 280 mW.     

Measurement and analysis of phase noise generated fromsemiconductor optical amplifiers

The phase noise generated from traveling-wave semiconductor optical amplifiers is measured. It is found that the phase noise is strongly correlated with the intensity noise and has bandwidth of 600 MHz. Phase-noise formulas are derived for semiconductor optical amplifiers to explain such phase-noise characteristics. It is shown that the phase noise is induced by the carrier density fluctuation associated with the intensity noise and the spontaneous carrier recombination. For the coherent heterodyne DPSK system, the phase noise, instead of the intensity noise, is found to be the limiting factor on the number of repeater amplifiers     

On the noise properties of linear and nonlinear quantum-dot semiconductor optical amplifiers: the impact of inhomogeneously broadened gain and fast carrier dynamics

We present a detailed analytical model describing the noise properties of quantum-dot (QD) optical amplifiers operating in the linear and saturated regimes. We describe the dependence of the optical noise on the main physical parameters characterizing the QD gain medium as well as on operating conditions. The optical noise at the amplifier output shows a broad-band spectrum with an incoherent spectral hole due to the gain inhomogeneity. A coherent spectral dip stemming from noise-signal nonlinear interactions is superimposed on that broad-band spectrum. The broad-band incoherent component is also calculated using an approximate model which makes use of an equivalent inhomogeneous population inversion factor. The validity of the approximation is examined in detail. We also calculate the electrical relative intensity noise and observe a spectral hole corresponding to the spectral shape of the optical noise. The most important characteristics of the optical and electrical noise spectra are determined by the degree of inhomogeneous broadening and by the fast carrier dynamics of QD amplifiers. The fast dynamics causes a very wide noise spectral hole which has important potential consequences for detection of fast data and for all optical signal processing.     

Millimeter-wave generation and digital modulation in an InGaAs-InPheterojunction phototransistor: model and experimental characterizationof dynamics and noise

This paper describes the use of an InGaAs-InP photoheterojunction bipolar transistor (photo-HBT) for millimeter-wave generation and digital modulation. Optical mixing of two coherent signals generates the carrier, and a digital drive signal to the base is used for the modulation. We describe an advanced large signal model of the photo-HBT that takes into account distributed effects at high frequencies and all noise sources, including optical amplifier noise and noise correlations due to the high operation frequency and the nonlinear mixing processes. The model enables one to predict carrier-to-noise ratio dependence on frequency, optical power, and the transistor operating point. Frequency- and time-domain responses of the modulated millimeter- wave carrier and bit error rates are also calculated. Experiments at 10 and 45 GHz with modulation rates ranging between 50 Mb/s and 2.5 Gb/s were performed, and a superb fit to the calculated responses is found     

Transmission performance of chirp-controlled signal by usingsemiconductor optical amplifier

We examine the fiber transmission performance of the optical signal whose chirp is controlled by utilizing phase modulation in semiconductor optical amplifier (SOA) with both simulations and experiments. This chirp control technique converts a positive chirp created by electroabsorption (EA) modulator into negative chirp, which reduces the waveform degradation due to the chromatic dispersion in transmission over standard single-mode fiber (SMF). It also provides an optical gain that is sufficient to compensate the insertion loss of the EA modulator. We investigate how the chirp control is affected by the input power to the SOA and the carrier lifetime of the SOA. As the SOA input power increases, the negative chirp becomes large, while the waveform is largely distorted due to gain saturation. However, the waveform distortion at high SOA input powers can be shaped by using a frequency discriminator. The acceleration of the carrier lifetime also reduces the waveform distortion due to gain saturation. We demonstrate that the chirp control technique is effective even for a high bit rate optical signal up to 10 Gb/s, when the carrier lifetime is expedited by optical pumping     

Semiconductor laser diode optical amplifiers/gates in photonicpacket switching

In this paper, we will describe how semiconductor laser diode optical amplifiers/gates can be used in the photonic packet switching systems based on wavelength division multiplexed (WDM) techniques. First, we show that cross-gain modulation (XGM) can be suppressed when the device is used in the transparent condition of the waveguide material even when the input signal power exceeds +18 dBm. We then discuss an appropriate encoding for the optical signal. Experimental results show that high bit rate Manchester-encoding enables the use of semiconductor laser diode optical amplifiers/gates in the gain condition as well as the transparent condition. Finally, a new photonic packet receiver which utilizes a semiconductor laser diode optical amplifier as a packet power equalizer is proposed. This receiver accepts 17 dB power fluctuation at nanosecond speed for 10 Gb/s Manchester-encoded signal     

A remote supervisory system based on subcarrier overmodulation forsubmarine optical amplifier systems

A supervisory (SV) signal transmission scheme for long haul optical amplifier systems is described. The scheme is based on subcarrier overmodulation of the line signal and features simple configuration; due to the use of EDFA gain modulation, no additive optical components are required in a submarine repeater to transmit a response signal. This paper discusses the subcarrier modulation index and frequency used for the SV signaling, the most significant design parameters. The dependence of the signal-to-noise ratio (SNR) on the modulation index in SV signal transmission is analyzed to clarify the modulation index value required for reliable SV signaling. In addition, the line signal impairment caused by subcarrier overmodulation is also analyzed to show the system's in-service feasibility. The subcarrier frequencies used for command and response signal transmission are discussed from the experimental results on the frequency response of an optical amplifier system and EDFA gain modulation efficiency     

Optical coherent broad-band transmission for long-haul anddistribution systems using subcarrier multiplexing

Signal multiplexing techniques for coherent optical transmission are compared, and appropriate application for a coherent subcarrier multiplexing (SCM) system is discussed. Optical frequency modulation (FM) using direct modulation of a distributed-feedback laser diode (DFB-LD) and a heterodyne detection is shown to be feasible. A transmission system using a local laser in the transmitter is unaffected by polarization and is cost effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. This circuit can also effectively compensate for the frequency of instability of light sources. A theoretical simulation of a coherent SCM system showed that 100 channels of 30-MHz FM signal or 15 channels of 155-Mb/s signal can be distributed to 10000 subscribers using single-stage or double-stage optical amplifiers     

Analytical model of a semiconductor optical amplifier

The spatial dependence of the material gain is introduced in the model of a semiconductor optical amplifier. Analytical expressions of the profiles of the carrier density, spontaneous emission, and amplified fields are obtained for amplifiers with arbitrary facet reflectivities. The nonuniformity of the carrier density is demonstrated in the case of low facet reflectivities. The model predicts the output saturation power and gain ripple, with good agreement with experimental results in resonant and traveling-wave amplifiers. Very low-gain ripple measured in low facet reflectivities amplifiers is explained by the model. A comparison with the uniform gain model shows that important deviations can occur in the case of low facet reflectivities. It is also shown that with the currently achievable low facet reflectivities, the maximum available gain is limited by spontaneous emission     

Characteristics of optical phase conjugation of picosecond pulsesin semiconductor optical amplifiers

Nondegenerate four-wave mixing (FWM) of picosecond optical pulses in semiconductor optical amplifiers is investigated theoretically, taking into account various carrier processes, probe depletion, and cross-gain modulation effects. Analytical solutions for greatly simplified coupled-wave mixing equations are obtained from which conditions where probe depletion and cross-gain modulation effects cannot be neglected are found. On the other hand, the frequency detuning dependence of optimum input pump pulse energy and shaping of output conjugate pulses are observed from numerical simulations. In particular, calculations show that the frequency symmetry of conjugate and probe pulses with respect to the pump pulse does not hold for optical pulse FWM. Two-photon absorption and ultrafast nonlinear refraction effects are shown to decrease the FWM conversion efficiency