A new microstructured optical fiber is demonstrated to detect acetone evaporation by observing the time response of the reflected signal at 1550nm. The sensor consists on a caterpillar-like fiber, with a transversal microfluidic channel created with a Focused Ion Beam technique, spliced to a single-mode fiber. Different stages were visible between the dipping and the evaporation of acetone and of a mixture of water and acetone. It was also possible to detect the presence of water vapor.
We developed a system to investigate resonant nonlinear optical interactions in acetylene molecules, confined in a hollow-core photonic crystal fiber (HC-PCF), using light injection through a low-loss splice from one end of the fiber, allowing us to work at low power. Electromagnetically induced transparency (EIT) was observed in the 1500 nm telecommunications window.
The optimized design of microstructured optical fibers (MOFs) for supercontinuum generation is investigated. The dependence of the group velocity dispersion and effective mode area on wavelength is analyzed. We consider first the case of a conventional MOF with a hexagonal pattern of air holes. In addition, we propose a new design, corresponding to a layered spiral microstructured optical fiber (LS-MOF). By changing appropriately its three parameters, it is possible to shift the zero dispersion wavelength to visible and near-infrared regions, as well as to achieve a very high nonlinearity near this wavelength. Supercontinuum generation is simulated in a LS-MOF with optimized design .
In this paper we present an experimental characterization of a highly nonlinear silica fiber. It includes the determination of the fiber effective nonlinear parameter using the nonlinear four-wave mixing process. From the experimental results, a value γ = 10.6 W−1 km−1 has been obtained for the nonlinear parameter in the co-polarized case, which was reduced to γ = 9.4 W−1 km−1 in the case of a decorrelated state of polarization between pump and signal waves. A threshold power of 17.8mW was found for the stimulated Brillouin scattering process. A Raman gain curve extending over a frequency range of 25 THz, with a peak value shifted by 13.1 THz from the pump frequency, was also measured.
In this paper we present results from the study of optical signal regeneration using Mamyshev type regenerator. We have performed the simulations and experimental characterization of regenerator by obtaining it`s transfer function and output optical signal to noise ratio measurements for two different filters - fixed and a tunable optical filter. Investigated regenerator setup consists of a high power erbium doped fiber amplifier, highly nonlinear fiber and a single stage optical filtering. Signal used for regeneration was an on-off keying return to zero code 40 Gbps pulse sequence. To find out optimum filter pass-band shift from signal`s central wavelength the regenerator`s transfer function was measured. Results show that highest output signal to noise ratio improvement for the fixed filter is at 0.6nm shift and amplifier output power set to 63 mW. While the tunable filter shift is 0.7nm at the 100 mW power level.
In this paper are present the design and characterization of a pH sensor using plastic optical fiber (POF) technology and a material produced by the sol-gel process with TEOS (tetraethyl orthosilicate) to immobilize universal indicator of pH (comprised of Thymol Blue, Methyl Red, Bromothymol Blue and Phenolphthalein) inside the silica matrix. This matrix is positioned between two extensions of plastic optical fiber tightly positioned at each side with both fibers aligned and sharing a common optical axis. This set will work as a pH sensor since the matrix embedded with indicator and in the presence of a solution (basic or acid solution) will change the optical transmittance properties. The optical source is a superluminescent white LED and the receiver is a photodiode having a good and linear responsivity in the visible spectrum. This pH sensitive matrix has large pores which allow the diffusion of the surrounding fluid molecules into the matrix and thus the close contact of these to the indicator molecules. This contact causes the change of color of the whole matrix allowing proper colorimetric detection by the photodiode. This variation of color associated with the detector wavelength linear response is the base of operation of the proposed device. This pH sensor presents many advantages over the standard and commercial pH meters namely, lightweight, portability and a low cost.
We investigate numerically the temporal and spectral characteristics of fixed-shape pulses, resulting from pulsating,
erupting and creeping soliton solutions of a generalized complex Ginzburg-Landau equation (CGLE), which includes the
third-order dispersion, intrapulse Raman scattering, and self-steepening effects. In general, the resulting fixed-shape
solutions are asymmetric and chirped pulses. The interaction between such fixed-shape pulses is also investigated, and
we show that a stable propagation is achieved, except when the pulses have an oscillating tail.
The supercontinum generation has been achieved mainly by two different approaches, namely, with femtosecond
intense pulses or using a continuous wave laser or larger pulses centered on the anomalous dispersion region. In
order to improve temporal coherence, it has been suggested the introduction of a pulse seed or the propagation
of both a large pulse pump and a small weaker continuous wave to control the soliton fission. Here we propose
supercontinuum generation using a hybrid input, we pump with a continuous laser and copropagate a picosecond
signal. We compare the bandwidth of the supercontinuum using only the continuous pump or the hybrid setup.
Simulations of the generalized Schrodinger equation, using an adequate input-noise model to reproduce the
spectrum of the continuous signal, are performed in order to investigate the supercontinuum generation in the
optical communication window under different dispersion regimes.
In this work we develop an analysis of polarization control schemes suitable for quantum key distribution systems.
Both time division multiplexing and wavelength division multiplexing based schemes are considered. A model
for the optimization of the temporal separation between reference pulses and polarization encoded photons
is presented. The model accounts for the reference pulse shape, the single photon detector gate width, and
the respective temporal separation between them. The theoretical results are validated through experimental
measurements. These results can be used to optimize the performance of polarization control schemes and
therefore to optimize the polarization encoded quantum key distribution systems.
We perform an accurate numerical modelling of both tapered and microstructured fibers using a finite element numerical
approach. We compute the propagation modes, i.e., distribution of the electric field over the cross section, effective
modal area and dispersion characteristics. The dependence of the dispersion properties on wavelength are calculated by
solving the eigenvalue equation for fiber modes and using Sellmeier equation. The optical properties of tapered fibers are
shown to be very sensitive to the core size. The zero dispersion wavelength can be shifted to the visible range and, in
some cases, two zero dispersion wavelengths are observed. The dependence of the dispersion properties and of the
effective mode area of a microstructured fiber with hexagonal symmetry on the air-hole diameter and the hole-to-hole
spacing is also investigated.
We investigate numerically the dynamics of pulsating, erupting and creeping soliton solutions of a generalized complex
Ginzburg-Landau equation (CGLE), including the third-order dispersion (TOD), intrapulse Raman scattering (IRS) and
self-steepening (SST) effects. We show that these higher-order effects (HOEs) can have a dramatic impact on the
dynamics of the above mentioned CGLE solitons. For small values of the HOEs, the periodic behavior of some of these
pulses is eliminated and they are transformed into fixed-shape solitons. However, a rather different behaviour is observed
by increasing the magnitude of the HOEs. Some particular interesting cases are discussed concerning the combined
action of the three HOEs.
In this paper we investigate the interaction between solitary-plain pulses (SP) of the quintic CGLE modified,
which describes the soliton behaviour in the presence of spectral filtering, linear and nonlinear gains, and selffrequency
shift (intrapulse Raman scattering). In particular, we look for a clear understanding of fundamental
properties of the bound sates, especially as concerns their stability. We use the interaction plane (distancephase
difference) to analyze the dynamics of the two soliton system. We have found stable BS's of plain
pulses when the phase difference between them is π / 2 + ▵, and ▵ is a quantity that depends on a selffrequency
Nonlinear effects in optical fibers impose different limitations on the communications link, and an understanding of such
effects is almost a prerequisite for actual lightwave-system designers. On the other hand, they offer a variety of
possibilities for all-optical signal processing, amplification and regeneration. The nonlinear effects are enhanced
dramatically and new phenomena are observed in the so called photonic crystal fibers. In this paper we review the effects
- both detrimental and potentially beneficial - of optical nonlinearities in conventional and in photonic crystal fibers.
A variational approach with an arbitrary ansatz is used to derive the governing equations for the characteristic parameters
of dispersion-managed solitons. The Gaussian pulses are considered as a particular case. Moreover, the adiabatic
evolution equations of the dispersion-managed pulse parameters under perturbations are derived, considering an arbitrary
pulse profile. The theory is applied to the case of Gaussian pulses under different types of perturbations, such as the
amplifier noise, nonlinear interaction between pulses, and polarization-mode dispersion.
A comparative study among the Jones matrix analytical models with high-order PMD is presented. The models that make use of an exponential expansion arrested up to second order or consider the dispersion vector as a Taylor series expansion do not give good results in the approximation of high-order PMD effects, because of the nonlimited behavior with respect to frequency of the modulus of their dispersion vectors. On the other hand, the analytical model which describes the dispersion vector as rotating on a circumference in the Stokes space is found to be the most accurate. Moreover, it can be used to obtain an analytical expression of the pulse broadening, which is often chosen as a quality-system parameter.
A variational approach with an arbitrary ansatz is used to derive the governing equations for the characteristic parameters of dispersion-managed solitons. The Gaussian pulses are considered as a particular case. The possibility of soliton propagation when the average dispersion is zero or normal is examined. Both polarization preserving fibers and birefringent fibers are considered.
An exact analytical solution is derived for the variance of the timing jitter of a dispersion-managed
soliton in the presence of synchronous amplitude modulators and filters. We show that, for a given
position of these control elements, a total suppression of the timing jitter is possible, which will permit
unlimited error free transmission distances of DM solitons. However, if only the modulator or the filter
is used, the asymptotic behavior of the timing jitter shows a linear dependence with distance, which is in
contrast with the cubic dependence in the uncontrolled case.
A variational approach with an arbitrary ansatz is used to derive the governing equations for the characteristic parameters of dispersion-managed solitons. Both Gaussian and super-Gaussian pulse profiles are considered as particular cases. The fundamental dynamics of DM pulses are characterized by their pulse width and frequency chirp. The possibility of soliton propagation when the average dispersion is zero or normal is examined.
In this paper we developed a simplified numerical averaging algorithm in order to obtain exactly periodic solutions of the non-linear Schrodinger equation (NLSE) with periodically varying coefficients. We calculate the pulse shape of the true dispersion-managed soliton, and show its long term stable propagation. This simplified model constitutes a variant of the original method much easier to program.
A variational method with an arbitrary ansatz is used to reduce the governing equation in the case of a periodic dispersion-managed fiber system to a coupled set of nonlinear ordinary differential equations. The phase-plane dynamics of the reduced system and the main characteristics of the dispersion managed pulses, namely the possibility of propagation when the average dispersion is zero or normal, are examined.
Dispersion-managed soliton transmission control using either synchronous amplitude modulators or narrow-band filters is examined. Exact analytical solutions are derived for the variance of the timing jitter in both cases. We show that a complete suppresion of the timing jitter is in general possible by a conventient choice of the strength and the relative position of the modulator or of the guiding filter in the dispersion map. The asymptotic behavior of the timing jitter shows a linear dependence with distance in both cases, which is in contrast wiht the cubic dependence in the uncontrolled case.
We consider the formation and stability characteristics of bound states in the complex Ginzburg-Landau equation. Using the perturbation theory, we derive a dynamical system describing the interaction between two weakly overlapping pulses. Two types of bound states were found, which correposnd to fixed points of this system. One of them is unstable, while the other corresponds to practically stable stationary points of the dynamical system governing the interaction. Our numerical results indeed confirm the existence of stable bound sttes of two solitons when thephase difference between them is plus or minus π/2. This happens when we consider the interaction of both two standard plain pulses and of two composite pulses. We find that two-composite pulses bound states have zero velocity, which is contrast with the behavior of the bound states formed by plain pulses. The existence of stable bound states with zero velocity formed by multiple composite pulses is also demonstrated.
We study the optical pulse dynamics in a transmission system with periodic variation of dispersion, using the variational approach (VA). The dependence of soliton parameters on dispersion map strength is examined. We find that there is a critical map strength above which finite energy solitons can propagate at zero and normal average dispersion. The existence of two branches of soliton solutions in the normal dispersion regime for different levels of the pulse energy is observed.
The soliton propagation and interaction characteristics in the presence of spectral filtering, linear and nonlinear gain are investigated. Using a perturbation approach, it is shown that the nonlinear gain has a significant impact on the soliton interaction when the adjacent solitons have different phases or amplitudes. In a system with purely nonlinear gain, for which arbitrary amplitude solitons can propagate, we find that the phase difference varies continuously and the solitons oscillate only slightly around their initial time separation. Concerning the quintic Ginzburg-Laudau equation and taking into account the soliton chirping, we find the existence of two types of bound states. One of them is unstable, while the other corresponds to practically stable stationary points of the dynamical system governing the interaction. These findings are in accordance with the numerical results obtained by ourselves as well as by other authors.
In this paper we examine the dynamics of a pulse in fibers with periodic dispersion on the basis of the variational approach. Using this approach a set of two coupled differential equations for the evolution ofthe pulse width and chirp is obtained and numerically solved. We consider two different symmetric dispersion maps. The main difference is found to be on the dependence ofthe pulse energy on the map strength. It is shown that a dispersion —managed (DM) soliton can be supported in an optical fiber even when the average dispersion is in the normal regime. Our results show that the variational approach can be a useful and effective analytical method for optimization of soliton transmission systems with variable dispersion.
We review the main properties and the potential applications of photorefractive solitons. Several different types of photorefractive solitons are referred, but a special attention is paid to one type: the photorefractive screening soliton. Both bright and dark screening solitons, together with the corresponding induced waveguides, are identified and discussed. The main characteristics of soliton interactions and the more recent topic of incoherent solitons are also reviewed.
We investigate, both analytically and numerically, the effectiveness of the nonlinear gain to suppress the background instability in bandwidth-limited soliton transmission. Different types of analytical solutions of the complex Ginzburg-Landau equation (CGLE), namely solutions with fixed amplitude and solutions with arbitrary amplitude, are discussed. The conditions for the stable pulse propagation are defined within the domain of validity of the soliton perturbation theory. The CGLE is solved numerically assuming various input waveforms with different phase profiles, amplitudes and durations. Relatively stable pulse propagation can be achieved over long distances by the use of suitable combination of linear and nonlinear gains. For the cubic CGLE, truly stable propagation of arbitrary amplitude solitons can be achieved in a system with purely nonlinear gain. A new soliton compression effect is demonstrated both for fixed- amplitude and for arbitrary-amplitude solitons. This compression can be particularly significant when the system parameters are chosen near the singularity of the fixed- amplitude solution.
We examine the influence of the third-order filter contribution on soliton propagation in a system with sliding-frequency guiding filters. The soliton loss in a system with up-sliding than it is in a system with down- sliding. We derive also an analytical expression for the variance of the timing jitter of a soliton transmission system using sliding-frequency guiding filters, taking into account the third-order filter term. The variance of timing jitter is significantly increased by the sliding action. As a consequence of the third-order filter contribution, the timing jitter is lower in a system with down-sliding than it is in a system with up-sliding at the same sliding rate.
The stability of soliton propagation in a system with spectral filtering, linear and nonlinear gain is numerically investigated. Different types of analytical solutions of the cubic complex Ginzburg-Landau equation, namely solutions with fixed amplitude and solutions with arbitrary amplitude, are presented. Then, the evolution equation is solved numerically assuming various input waveforms. Our results show that it will be possible to achieve relatively stable pulse propagation over long distances by the use of suitable combination of linear and nonlinear gains. However, truly stable propagation of arbitrary amplitude solitons can be achieved only in a system with purely nonlinear gain. A new soliton compression effect is demonstrated both for fixed- amplitude and arbitrary-amplitude solitons.
We have examined the main constraints on the design of single-channel, high-capacity soliton communication systems. In the average soliton regime, the Gordon-Haus timing jitter limits the bit rate to less than 7 Gb/s for transoceanic fiber links, while for shorter transmission distances the main limitation is fiber perturbations arising from discrete in-line amplification. The use of dispersion-decreasing fibers and of periodic optical phase conjugation for jitter control can increase significantly the capacity of soliton communication systems. We found that the third-order dispersion assumes a prominent role in this case. Reducing this effect sufficiently, the main constraint becomes the soliton-soliton interactions and bit rates superior to 110 Gb/s can be achieved for amplifier spacings less than 80 km. For higher amplifier spacings the bit rate is limited by the Raman induced timing jitter.
We examine the main constraints on the design of single- channel, high-bit rate soliton communication systems. In the average soliton regime, the main limitation for transoceanic fiber links is the Gordon-Haus jitter, while for shorter transmission distances it arises from the discrete in-line amplification. The use of dispersion-decreasing fibers and of periodic optical phase conjugation can increase the system capacity.
This paper summarizes the research activities of the optical communications group at University of Aveiro and Institute of Telecommunications-Aveiro pole. Several activities like clock recovery systems, both electrical and all optical, electrical equalizers for very high bit rate DST systems, post-detection filters for multigigabit optical receivers, soliton systems, simulation work on WDM, DST, EDFA and short pulse generation for high bit rate systems are presented.
We have examined the main constraints on the design of single-channel, high-capacity soliton communication systems. In the average soliton regime, the Gordon-Haus timing jitter limits the bit rate to about 5 Gb/s for transoceanic fiber links, while for shorter transmission distances the main limitation is fiber perturbations arising from discrete in-line amplification. The use of dispersion-decreasing fibers and of periodic optical phase conjugation for jitter control can increase significantly the capacity of soliton communication systems. We found that the third-order dispersion assumes a prominent role in this case. Reducing this effect sufficiently, the main constraint becomes the soliton-soliton interactions and bit rates superior to 110 Gb/s can be achieved for amplifier spacings less than 80 km. For higher amplifier spacings the bit rate is limited by the Raman induced timing jitter.
Soliton propagation in a system with linear and nonlinear amplifiers and spectral filtering is explored. We discuss different types of solutions of the cubic and the quintic complex Ginzburg-Landau equation (CGLE), namely solutions with fixed amplitude and solutions with arbitrary amplitude. The conditions to achieve a stable soliton propagation are analyzed within the domain of validity of the soliton perturbation theory. We obtain also a boundary for the region in the parameter space at which stable pulselike solutions of the quintic CGLE exist. In addition, an expression for the minimum value of the peak amplitude of these solutions is found, which depends uniquely on the quotient between the linear excess gain and the quintic saturating gain term.
An analytical expression was obtained for the timing jitter of ultrashort solitons, taking into account the group velocity dispersion, the Raman effect and the third-order dispersion. This expression can also be useful to evaluate the timing jitter of a communication system using dispersion-decreasing fibers. It is shown that the higher- order effects increase the timing jitter beyond that of the Gordon-Haus value for solitons shorter than ~ 10 ps. Dispersion-decreasing fibers show a greater variation of the jitter control distance with the amplifier spacing, in comparison with the constant-dispersion fibers.
We derive an analytical expression for the variance of the timing jitter of a soliton transmission system using sliding-frequency guiding filters, taking into account the third-order filter term. We show that the variance of timing jitter is significantly increased by the sliding action. As a consequence of the third-order filter contribution, the timing jitter is lower in a system with down-sliding than it is in a system with up-sliding at the same sliding rate. Consequently, in what concerns the timing jitter, it is advantageous to use down-sliding.
The soliton propagation and interaction characteristics in the presence of spectral filtering, linear and nonlinear gain are investigated. We show that, as the linear gain vanishes, the steady-state soliton amplitude and inverse duration diverge. The nonlinear gain is shown to have a significant impact on the soliton interaction when the adjacent solitons have different phases or amplitudes. In these cases, a (pi) phase difference between adjacent pulses is induced when the nonlinear gain only partially replaces the linear gain. As the linear gain vanishes, the phase difference varies continuously and the solitons oscillate only slightly around their initial time separation.
The steady-state soliton parameters are studied both for anomalous and normal dispersion regimes of a fiber laser, taking into account the effects of fiber dispersion and nonlinearity, linear gain and gain dispersion, nonlinear gain and two-photon absorption. It is shown that both the soliton chirp and width are significantly larger for normal than for anomalous dispersion. The existence of singularities for the soliton width and amplitude are revealed for both dispersion regimes. In the case of anomalous dispersion, the soliton perturbation theory and the phase-plane formalism are used to investigate the stability of the steady-state solution.
Pulse propagation in the presence of spectral filtering, two-photon absorption, linear and nonlinear gain is investigated both for anomalous and normal dispersion regime. We analyze the steady-state pulse propagation in such a system and we observe that both the soliton chirp and width are significantly larger for normal than for anomalous dispersion. An unexpected singularity of the soliton amplitude and its width is observed in the anomalous dispersion regime when the net linear gain vanishes. We also show that arbitrary-amplitude solitons can propagate in this dispersion regime for a given relation between spectral filtering, nonlinear gain and two-photon absorption.
The stable propagation of ultrashort optical solitons in an ultralong active fiber is investigated. We show that the soliton self-frequency shift is very sensitive to the nonresonance of the carrier frequency and that the linearly frequency dependent gain with positive slope is the effective component of gain spectrum for the compensation of this effect. Furthermore, we demonstrate that the inclusion of nonlinear gain that contains terms proportional tot he second and fourth power of the amplitude can provide stable propagation of ultrashort optical solitons.
The propagation of a nonresonant optical soliton in the presence of spectral filtering, linear, and nonlinear gain is investigated. By keeping the filtering fixed, we show that the steady-state soliton amplitude diverges as the linear gain vanishes. We also show that the nonlinear gain only reduces but does not suppress the instability due to the background linear waves. This instability can be suppressed by considering the saturation of the nonlinear gain, while it is accentuated by the detuning of the soliton from the gain peak frequency.
The small-signal response of a (lambda) /4-shifted multiple-quantum well (MQW) distributed feedback (DFB) laser is investigated. The gain spectrum shows two major peaks located at the lowest heavy-hole and light-hole transitions. Therefore, the MQW DFB laser is treated as a three-level system. In the low-power range the heavy-hole transition dominates, while in the high-power range there is a mixing of the heavy-hole and the light-hole transitions. It is shown that the dynamic response of the MQW DFB laser is significantly affected by the contribution due to the light-hole transition. Its dependence on the number of quantum wells and on the coupling coefficient of the grating ((kappa) ) is also discussed.
A review of the spectral and modulation characteristics of tunable DBR lasers is presented. The dependence of the linewidth and FM response on the nonuniform injection currents to the active sections and on the tuning currents to the passive sections of a four section DBR LD is investigated and a tradeoff between them is found. Some guidelines for the structure optimization are suggested.
The equations describing the stimulated Brillouin scattering process in optical fibers with distributed gain, including an equation for the second-order Stokes wave, are numerically solved. An analytical solution for the case of no net gain or loss is also presented. We note that the presence of distributed gain along the fiber leads to a non-monotonous evolution of the signal power and to ready generation of multiple Stokes orders. The influence of a copropagating or counterpropagating signal at the first or second-order Stokes wavelength, respectively, is also examined.
The dynamic response of a multiple-quantum well (MQW) laser is investigated assuming that it can possess multiple levels. The results reveal that the lowest heavy-hole and the lowest light-hole transitions are dominant. In the low-power range the heavy-hole transition dominates, while in the high-power range there is a mixing of the heavy-hole and the light- hole transitions. In the last case the frequency response is improved and it can be correctly predicted only treating the MQW as a three-level system.
FIRST is a RACE project where 5 main European telecoms operators, 4 equipment manufacturers and one university have joined up to define and test in a field trial in Portugal a cost effective Optical Access Network. The main design target has been a system which gives cost effective provision of wideband services for small and medium business customers. The system however, incorporates provision of telephone, ISDN and analog and digital video for residential customers as well. Technologies have been chosen with the objective of providing a simple, robust and flexible system where initial deployment costs are low and closely related to the service take up. The paper describes the main technical features of the system and network applications which shows how the system may be introduced in network planning. The system is based on Passive Optical Network technology where video is distributed in the 1550 nm window and telecoms services transmitted at 1300 nm in full duplex mode. The telecoms system provides high capacity, flexibility in loop length and robustness towards outside plant performance. The Subcarrier Multiple Access (SCMA) method is used for upstream transmission of bi-directional telecoms services. SCMA has advantages compared to the Time Division Multiple Access technology used in other systems. Bandwidth/cost tradeoff is better and the lower requirements to the outside plant increases the overall cost benefit. Optical beat noise due to overlapping of laser spectra which may be a problem for this technology has been addressed with success through the use of a suitable modulation and control technique. This technology is further validated in the field trial. The video system provides cost effective long distance transmission on standard fiber with externally modulated lasers and cascaded amplifiers. Coexistence of analog and digital video on one fiber with different modulation schemes i.e. BPSK, QPSK and 64 QAM have been validated. Total life cycle cost evaluations based on availability data, maintenance requirements and expectations for service development have been made. The field trial will be running for two years.
A theoretical model is presented to describe the spectral and modulation characteristics of tunable DBR lasers with two active and two passive sections, by taking into account the actual frequency dependence of the Bragg reflector, the nonlinear gain compression effect, the possibility of laser operation away from gain maximum, and the distribution of the spontaneous emission along the laser cavity. General expressions are derived for the intensity and frequency modulation response, frequency noise spectrum, spectral linewidth, effective linewidth enhancement factor, and spontaneous emission rate. The dependence of the linewidth and of the FM efficiency on the nonuniform injection currents to the active sections and on the tuning currents to the passive sections is particularly investigated.
A theoretical analysis is performed of the frequency noise, relative intensity moise and frequency chirp of tunable two-section and three-section DBR lasers (LDs), taking into account the nonlinear gain compression effect. For a two-section DBR LD the frequency and intensity noise is shown to depend significantly on the tuning current, particularly at low frequencies (below the GHz range) and for low and moderate values of the grating coupling coefficient. For high values of this coefficient the tuning dependence is generally negligible when the tuning is performed only through the DBR section, but it becomes significant when using the phase control current in a three-section device. Except in the vicinity of a mode jump, the frequency chirp of a DBR LD is shown to have a little dependence on the tuning current. The possibility to measure the effective linewidth enhancement factor from the high-frequency behaviour of the chirp is suggested.
The static tuning and the small-signal modulation characteristics of multielectrode DBR lasers are analysed. A theoretical model is presented to describe in particular the performance of a three electrode DBR laser with two electrodes in the active region and another electrode in the passive DBR region. Several direct modulation schemes are considered and nonlinear gain is taken into account. A nearly flat FM response over several GHz is observed by a nonuniform current injection and modulation of the lower carrier density section in the active region. A perfect cancellation of spurious intensity modulation together with the removal of the FM resonance peak seems to be possible by push-pull modulation of the active section with an appropriate current splitting ratio. However this situation does not provide generally ideal FM characteristics because of the thermal-effects. Tuning without distortion of the FM characteristics is an attractive feature of this device.