Gain Linewidth Research Articles

Self-starting stable coherent mode-locking in a two-section laser

Coherent mode-locking (CML) uses self-induced transparency (SIT) soliton formation to achieve, in contrast to conventional schemes based on absorption saturation, the pulse durations below the limit allowed by the gain line width. Despite the great promise it is difficult to realize it experimentally because a complicated setup is required. In all previous theoretical considerations CML is believed to be non-self-starting. In this paper we show that if the cavity length is selected properly, a very stable (CML) regime can be realized in an elementary two-section ring-cavity geometry, and this regime is self-developing from the non-lasing state. The stability of the pulsed regime is the result of a dynamical stabilization mechanism arising due to finite-cavity-size effects.

Gain Compression and Linewidth Enhancement Factor in Mid-IR Quantum Cascade Lasers

We have observed and quantified the adiabatic and transient chirp in a directly modulated quantum cascade laser (QCL). Those wavelength tuning effects are well-characterized in diode lasers, and the rate equation model that successfully describe the diode laser behavior also provides an excellent fit for the QCL data. In this study, we have extracted the linewidth enhancement factor ( $\alpha _{H}$ ) from the transient chirp and the gain compression factor from the adiabatic chirp. We postulate that the extraction of the $\alpha _{H}$ from the transient chirp is valid for the QCL case, despite additional tuning effects in QCLs (e.g., voltage tuning) that are negligible in diode lasers. Also in the QCL the adiabatic chirp coefficient strongly increases with laser output power but still stays an order of magnitude below typical values known from diode lasers. We hypotesize possibility of the adiabatic chirp to be connected to the χ(3) nonlinearity (responsible for four-wave mixing that was recently attributed to the FM self-locking in QCLs), but the exact origin remains to be experimentally confirmed in future work.

Bandwidth reconfigurable microwave photonic filter based on stimulated Brillouin scattering

A bandwidth reconfigurable microwave photonic filter is proposed and numerically analyzed employing Brillouin gain spectrum narrowing and broadening. The stimulated Brillouin scattering (SBS) process is used to convert the phase modulation to intensity modulation to generate filter passband. Due to the fact that the passband is formed by mapping the Brillouin gain spectrum, bandwidth reconfiguration can be implemented by changing Brillouin gain linewidth. In this paper, both bandwidth reduction and increase are included in a single system and the details of gain spectrum narrowing and broadening are demonstrated. Theoretically, nearly 60% bandwidth reduction and hundreds times of bandwidth increase are achieved as compared to the case without gain spectrum process.

Structural and spectroscopic behavior of Er3+:Yb3+ co-doped lithium telluroborate glasses

A new series of Er3+:Yb3+ co-doped Lithium telluroborate glasses were prepared following the melt quenching technique. The structural analyzes were made through XRD, Raman, FTIR spectra to explore the different vibrations of borate and tellurite network. The absorption spectra have been used to determine the nature of the metal–ligand and further Band gap and Urbach's analysis have also been carried out. The oscillator strength value of the 2H11/2→4I15/2 hypersensitive transition is found to be higher and increases as the concentration of the RE ion increases which emphasis the asymmetry nature of the glasses. The magnitude of the JO intensity parameters follow the trend as Ω2>Ω4>Ω6 uniformly for all the prepared glasses. A bright green emission corresponding to the 2H11/2+4S3/2→ 4I15/2 transition and luminescence from 4I13/2→4I15/2 in eye safe region have also been observed. The radiative parameters such as radiative transition probability, stimulated emission cross-section, branching ratios, radiative lifetime, gain bandwidth and gain linewidth for the 4S3/2 and 4I13/2 level of the title glasses have also been determined. The absorption and emission cross-section corresponding to the 4I13/2 level has been calculated using McCumber theory. Lifetime measurements were made under 980nm excitation and the quantum efficiency were also calculated to evaluate the appropriateness of the host matrix for the fabrication of laser materials and broad band amplifiers.

Ab initio investigation of lasing thresholds in photonic molecules

We investigate lasing thresholds in a representative photonic molecule composed of two coupled active cylinders of slightly different radii. Specifically, we use the recently formulated steady-state ab initio laser theory (SALT) to assess the effect of the underlying gain transition on lasing frequencies and thresholds. We find that the order in which modes lase can be modified by choosing suitable combinations of the gain center frequency and linewidth, a result that cannot be obtained using the conventional approach of quasi-bound modes. The impact of the gain transition center on the lasing frequencies, the frequency pulling effect, is also quantified.

Modeling and evaluating the performance of Brillouin distributed optical fiber sensors

A thorough analysis of the key factors impacting on the performance of Brillouin distributed optical fiber sensors is presented. An analytical expression is derived to estimate the error on the determination of the Brillouin peak gain frequency, based for the first time on real experimental conditions. This expression is experimentally validated, and describes how this frequency uncertainty depends on measurement parameters, such as Brillouin gain linewidth, frequency scanning step and signal-to-noise ratio. Based on the model leading to this expression and considering the limitations imposed by nonlinear effects and pump depletion, a figure-of-merit is proposed to fairly compare the performance of Brillouin distributed sensing systems. This figure-of-merit offers to the research community and to potential users the possibility to evaluate with an objective metric the real performance gain resulting from any proposed configuration.

Enhanced gain and narrow linewidth of an optical cavity by the Doppler effect in a four-level atomic system

A scheme for high gain and narrow linewidth of an optical cavity with a four-level atomic system is proposed by the Doppler effect via active Raman gain (ARG) process. Atomic motion leads to Doppler frequency shift which induces constructive interference for the linear susceptibility. The enhanced normal dispersion greatly narrows the cavity linewidth, and the amplified gain gives rise to a high cavity transmission. Simulation results show that the cavity linewidth based on ARG is about one order of magnitude narrower than that based on electromagnetically-induced transparency under the same conditions, and the cavity transmission intensity could be enhanced by nearly 30 times.

Study on highly stable single-frequency Brillouin fiber laser

Single-frequency Brillouin fiber lasers (SF BFLs) have been attracting extensive interests because of their compact- ness, stableness and their characteristics of linewidth narrowing and noise suppression. They have important potential applications in many fields, such as fiber sensing systems, high-resolution spectroscopy and microwave photonics, and so on. In this paper, we demonstrated the SF BFL using the tunable laser source (TLS) as the Brillouin pump. When the cavity length is cut short to 10.3 m and the FSR is 20.1 MHz being comparable to the Brillouin gain linewidth, the BFL begins to operate in single-longitudinal-mode. The laser has the out power conversion efficiency of 42.5% and intensity fluctuation bellow 4%. The SF BFL’s signal to noise ratio (SNR) is 10 dB greater than the TLS. Also, the laser has good performance for suppressing the noise and the linewidth of which is measured to be below 5 kHz.

Tunneling-induced high gain and narrow linewidth of a cavity with an asymmetric quantum-well system

A scheme is proposed for obtaining high gain and narrow linewidth of a cavity with an asymmetric quantum-well system. Due to resonant tunneling, destructive interference for linear absorption leads to a tunneling-induced transparency window which compresses the cavity linewidth; moreover, constructive interference for cross-nonlinear susceptibility occurs, which introduces high gain and large dispersion, and the cavity linewidth is much compressed. In the latter case, the intensity of cavity transmission could be enhanced one order of magnitude larger than that of input field, and its linewidth could be one-seventieth of the empty cavity.

Balance of physical effects causing stationary operation of Fourier domain mode-locked lasers

We present a detailed analysis of the optical field dynamics in a Fourier domain mode-locked (FDML) laser. We employ a numerical simulation based on the FDML evolution equation, describing the propagation of the optical light field. The temporal evolution of the instantaneous power spectrum at different points in the laser cavity is investigated. The results are carefully validated against experimental data, yielding good agreement. Deeper insight is gained into the role of the physical effects governing FDML dynamics, such as gain recovery and linewidth enhancement in the semiconductor optical amplifier, dispersion and self-phase modulation in the optical fiber, and the sweep filter action.

Detrimental effect of self-phase modulation on the performance of Brillouin distributed fiber sensors

We show that the spectral broadening of the pump pulse through self-phase modulation in a time-domain distributed Brillouin sensor has a considerably detrimental effect in the measurement, especially in the case of long distances and high-resolution pulses. Using 30 ns pump pulses with peak power of 276 mW, self-phase modulation leads to a doubling of the effective gain linewidth after some 20 km, which is equivalent to a contrast loss of 2dB in the measurement. The impact is higher for shorter pulses (higher resolution). The theoretical modeling is fully confirmed by experimental results.

Coherence of resonant-tunneling transport in terahertz quantum-cascade lasers

We develop simple density-matrix models to describe the role of coherence in resonant-tunneling (RT) transport of quantum-cascade lasers (QCLs). Specifically, we investigate the effects of coherent coupling between the lasing levels with other levels on the transport properties and gain spectra. In the first part of the paper, we use a three-level density-matrix model to obtain useful analytical expressions for current transport through the injector barrier in a QCL. An expression for the slope discontinuity in the current-voltage characteristics at the lasing threshold is derived. This value is shown to be a direct measure of the population inversion at threshold and contradicts the previously held belief of it being indicative of ratio of the laser level lifetimes. In the second part of the paper, we use density matrices to compute the gain spectrum for a resonant-phonon terahertz QCL design. The large anticrossing of the doublet of lower radiative levels is reflected in a broad gain linewidth due to a coherent RT assisted depopulation process. At certain bias conditions, the gain spectrum exhibits double peaks which is supported by experimental observations.

Temperature dependent gain characteristics in GaN-based vertical-cavity surface-emitting lasers

Temperature dependent gain characteristics and linewidth enhancement factor (alpha-factor) of vertical-cavity surface-emitting lasers with InGaN/GaN multiple quantum wells were studied by measuring the photoluminescence spectra below the threshold condition and analyzed by using the Hakki-Paoli method. The optical gain and differential gain showed a more rapid increase as a function of the injected carriers as temperature decreased. The alpha-factor for the lasing mode was estimated as 2.8 at room temperature and decreased to a value as low as 0.6 at 80 K.

Strained InGaAsP multi-quantum-well structures for InP-based wide linewidth and polarization-insensitive semiconductor optical amplifiers

We have calculated the band structure of 1.55μm InGaAsP/InGaAsP multi-quantum-well structures using Lüttinger–Kohn Hamiltonian taking into account the strain in the quantum wells (QWs) and barriers, and the confinement in the quantum wells. Using the calculated dispersion curves and oscillator strength between the different interband transitions, we have determined the optical gain in TE and TM mode and the spontaneous amplified emission as a function of injected carrier density in devices composed of quantum wells with different thicknesses. We find that an optical gain linewidth larger than 130nm with a TE/TM polarization dependence lower than 1dB can be obtained using a three-quantum-well In0.53Ga0.47As0.96P0.04/InGaAsP active layer with quantum well thicknesses of 10, 14 and 19nm.

Passive mode locking of inhomogeneously broadened lasers

Passive mode locking of inhomogeneously broadened lasers is studied in three typical regimes: pure self-amplitude modulation (SAM) mode locking, soliton mode locking, and mode locking in the presence of self-phase modulation (SPM) and positive group-delay dispersion (GDD). When the mode-locking strength becomes weak and/or the GDD is large, the lockable spectral width reduces and mode locking becomes unstable, which is a common and unique feature for broadband inhomogeneously broadened lasers. Mode locking by the pure SAM is essentially a phase-locking process. The soliton mode locking best resists the impact of insufficient gain filtering, while mode locking in the presence of strong SPM and positive GDD is the weakest, and a certain amount of gain filtering (narrowing) is necessary for stable mode locking. The scaling relations of mode-locking characteristics with the gain linewidth, GDD, and nonlinearities, and the influence of the degree of inhomogeneity of the gain medium are examined. A linearized saturable absorber may not be able to mode lock a strongly inhomogeneously broadened laser, and a minimum unsaturated absorber loss is required for stable mode locking in most situations.

Controlling Chaos in a Semiconductor Laser via Weak Optical PositiveFeedback and Modulating Amplitude

Numerical analysis of weak optical positive feedback (OPF)controlling chaos is studied in a semiconductor laser. The physical model ofcontrolling chaos produced via modulating the current of semiconductor laseris presented under the condition of OPF. We find the physical mechanism thatthe nonlinear gain coefficient and linewidth enhancement factor of the laserare affected by OPF so that the dynamical behaviour of the system can beefficiently controlled. Chaos is controlled into a single-periodic state, adual-periodic state, a tri-periodic state, a quadr-periodic state, apenta-periodic state, and the laser emitting powers are increased by OPF insimulations. Lastly, another chaos-control method with modulating theamplitude of the feedback light is presented and numerically simulated tocontrol chaotic laser into multi-periodic states.

Stimulated Brillouin scattering in single-mode tellurite glass fiber

Stimulated Brillouin scattering properties in a single-mode tellurite glass fiber were studied using a cw laser with an operating wavelength of 1.54 mum. The Brillouin frequency shift v(B) and the gain linewidth Deltav(B) were 7.882 GHz and 23.6 MHz, respectively. A Brillouin gain coefficient g(B) in the range of 1.47 x 10(-10)-2.16 x 10(-10) m/W was measured. The higher gain coefficient of the tellurite fiber, together with its relatively low loss compared with other non-silica fibers, makes it a suitable candidate for realizing efficient, all-optical, slow-light devices.

An accurate high-resolution technique for distributed sensing based on frequency-domain Brillouin scattering

A novel high-resolution temperature/strain profile reconstruction approach, based on frequency-domain stimulated Brillouin scattering (SBS) measurements in single-mode optical fibers, is proposed and experimentally demonstrated. The proposed technique is based on an accurate modeling of the SBS interaction in the frequency domain, where even the modulation of the acoustic wave involved in the scattering process is taken into account. The reconstruction algorithm determines the unknown profile by minimizing a cost function representing the error between the measured and the model data. Experimental results show that the proposed technique allows accurate temperature/strain profile reconstructions, even when dealing with modulation frequencies rising above the natural Brillouin gain linewidth. This permits us to achieve a spatial resolution below 1m.

Self-consistent theory of the gain linewidth for quantum-cascade lasers

The linewidth in intersubband transitions can be significantly reduced below the sum of the lifetime broadening for the involved states, if the scattering environment is similar for both states. This is studied within a nonequilibrium Green function approach here. We find that the effect is of particular relevance for a recent, relatively low doped, THz quantum cascade laser.

Observation of strong stimulated Brillouin scattering in single-mode As2Se3 chalcogenide fiber

Strong stimulated Brillouin scattering in single-mode As2Se3 chalcogenide fiber is observed using a cw laser at 1.55 microm wavelength region. Brillouin threshold for a 5-m-long fiber is as small as 85 mW. The Brillouin frequency shift vB and the gain linewidth DeltavB are 7.95 GHz and 13.2 MHz, respectively, measured with heterodyne detection and an RF spectrum analyzer. A Brillouin gain coefficient gB of 6.0 x 10(-9) m/W, about 134 times larger than that of fused silica fiber, is obtained for As2Se3 single mode fiber from measurements of Brillouin threshold power and the gain linewidth.