Gain Recovery Time Research Articles

Evaluation of transient gain-drop and following recovery property on microchannel plate: Comparison between two evaluation methods

It is widely recognized that the gain of the microchannel plate (MCP) drops temporally when a particle initiates an electron avalanche, and a large amount of charge is extracted from the channel. Electron multiplication was expected to deplete the charge from the microchannel wall, and this gain-drop phenomenon has been considered to last until the charges were replenished. The gain-recovery time has been measured and compared to the RC constant (i.e., a product of plate electrical resistance and capacitance) in several studies. In this research, the gain-recovery time constant of the same chevron MCP detector was evaluated using two methods (continuous irradiation method and double-pulse method). In the continuous irradiation method, the gain-recovery time constant is determined from a ratio of the MCP output current to the conduction current for continuous irradiation by using an analytical model. In the double-pulse method, the first pulse irradiates the MCP detector to cause the gain-drop, and the second pulse irradiates to monitor the gain. The gain-recovery time constant is determined by changing a time interval between the two pulses. The gain-recovery time constant obtained by the continuous irradiation method was over one order magnitude larger than the RC constant and those by the double-pulse method. This overestimation was attributed to the fact that the model formula used in the continuous irradiation method does not consider the spatial extension of the gain-drop. The gain-recovery time constant obtained by the continuous irradiation method was consistent with that obtained by the double-pulse method, modifying the model formula and assuming the electron multiplications for one photoelectron decrease the gains of the surrounding 20 channels to zero.

Gain recovery dynamics in active type-II semiconductor heterostructures

Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

Direct measurement of an ultrafast sub-bandwidth-limited signal in mid-infrared quantum cascade lasers

Frequency comb lasers with fast gain recovery times naturally favor the emission of frequency mod-ulated periodic signals, which are useful for multiple applications in spectroscopy and communications. Models and phase measurements predict an ultrashort strong intensity spike at the instantaneous frequency discontinuity of the cavity cycle. Here we experimentally study the ultrashort spike of fast-gain frequency modulated combs through direct upconversion sampling, and measure a width below that of a transform-limited pulse. Specifi-cally, we demonstrate a mid-infrared quantum cascade laser which inherently lases in a frequency modulated comb and produces a spike with full-width at half-maximum below 600 fs, which is below the Fourier-limit derived from the corresponding spectrum. We believe these ultrashort spikes can be highly beneficial for sub-bandwidth time domain measurements. Using mean-field theory based simulations, we confirm the occurrence of such features as well as further optimize our system for going even further below the Fourier-limit.

Enhanced Catalytic Palladium Embedded Inside Porous Silicon for Improved Hydrogen Gas Sensing

In this work, we reported on room temperature porous silicon (PS) and embedding PS using simple and economical techniques of electrochemical etching and thermal evaporation. The PS substrate was prepared using the technique of electrochemically etching the n-type Si (100) wafer at a constant current density of 10 mA/cm2 for 10 min under the illumination of incandescent white light. After PS formation, Ge pieces were thermally evaporated onto the two PS substrates in a vacuum condition. This was then followed by the deposition of the ZnO layer onto the Ge/PS substrate by the same method using commercial 99.9% pure ZnO powders. The three samples were identified as PS, Ge/PS and ZnO/Ge/PS samples, respectively. Pd finger contacts were deposited on the PS and embedding PS (Ge/PS and ZnO/Ge/PS) to form Pd on PS hydrogen sensors using RF magnetron sputtering. SEM and EDX suggested the presence of substantial Ge and ZnO inside the uniform circular pores for Ge/PS and ZnO/Ge/PS samples, respectively. Raman spectra showed that good crystalline Ge and ZnO nanostructures embedded inside the pores were obtained. For hydrogen sensing, Pd on ZnO/Ge/PS Schottky diode exhibited a dramatic change of current after exposure to H2 as compared to PS and Ge/PS devices. It is observed that the sensitivity increased exponentially with the hydrogen flow rate for all the sensors. The ZnO/Ge/PS showed more sensitivity towards H2 than that of PS and Ge/PS especially at high flow rate of H2 with higher current gain (69.11) and shorter response (180 s) and recovery times (30 s).

Ultrafast Buildup Dynamics of Terahertz Pulse Generation in Mode-Locked Quantum Cascade Lasers

Ultrashort terahertz pulse generation is essential for a range of proven terahertz applications, from time-resolved spectroscopy of fundamental excitations to nondestructive testing and imaging. Recently, it has been shown that semiconductor-based terahertz quantum cascade lasers (QCLs) can be used to generate pulses as short as a few picoseconds through active mode locking. However, further progress for subpicosecond and high peak power pulse generation is hampered by poor knowledge on how the electric field actually forms in these lasers. Here, we theoretically and experimentally show the amplitude- and phase-resolved buildup of pulse generation through active mode locking, from initiation of pulse generation to the nanosecond steady state. The experimental results, using an ultrafast coherent seeding technique to probe the laser from femtosecond to nanosecond time scales, are in full agreement with the theoretical calculations based on a theoretical model using multimode reduced rate equations. In particular, we show that the electric field buildup to achieve short pulse operation is extremely fast, requiring only a few photon round trips, owing to the ultrafast gain dynamics of the lasers. Further, this shows a gain recovery time of the order of a few picoseconds, an order of magnitude smaller than the photon round-trip time, highlighting that terahertz QCLs are categorically class-A lasers. This demonstration marks an important formulism for future progress towards exploring the ultrafast pulse generation buildup dynamics of these complex semiconductor lasers.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

Active Modelocking Improvement of MIR-QCL by Integrating Graphene as a Saturable Absorber

The active mode-locking technique of quantum cascade lasers is highly affected by the bias current level and the magnitude of the modulated signal because of the spatial hole burning effect. The laser generates a stable pulse train near the threshold current and for a high modulation magnitude. We show in this paper an improvement of these conditions with an integration of a single-layer graphene which works as a saturable absorber. The light-matter interaction in the laser active region is described by the two-level Maxwell-Bloch equations and in graphene layer by the Maxwell-Ampere equation and Maxwell-Bloch equations. These system equations are solved using the Finite-Difference Time-Domain (FDTD) method which has the main advantage of not involving any approximation. The weakly coupled splitting scheme is adopted for the time discretization of Maxwell and Bloch equations. The graphene saturable absorption is modelled by a saturable conductivity in Maxwell-Ampere equation and with the dipole moment and carrier density in Maxwell-Bloch equations. For a QCL structure with a gain recovery time of 50 ps, simulation results show better stability of the mode-locking over a broad range of injection current and for smaller magnitude of the modulated signal. QCL structure with gain recovery time close to 1 ps reveals the generation of two pulses per roundtrip linked to spatial hole burning. The carrier grating strength can be reduced by changing the boundary conditions of the cavity such as with a high reflectivity mirror in front of graphene layer.

Characterization of Silicon Photomultipliers (SiPMs) for Space Exploration

Silicon Photomultipliers (SiPMs) are the devices, which along with scintillation detectors can be used for the radiation measurements. We are developing instruments using scintillation detector and SiPM arrays for the measurements of high-energy X-rays and charged particles. These instruments are being developed for the future space exploration programs. Here, we report the work carried out for the characterization of SiPM and its response in the hardened environments. Results from two different experiments are presented. In the first experiment: electrical characterizations of SiPMs from two manufacturers i.e. SensL (now Onsemi) and Ketek have been carried out. Three different pixel sizes 20 μm, 35 μm and 50 μm of MicroC series from SensL and 50 μm pixel size of PM3350 series of Ketek have been tested. In order to see ‘device – to - device’ variations, three SiPMs of each pixel size are used in the experiment i.e. total twelve SiPMs have been characterized for the breakdown voltage, quenching resistor and their temperature dependences. Variations in the gain and microcell recovery time due to temperature are also estimated. In the second experiment: one of the SiPM models (SensL make 35 μm pixel size) has been subjected to various environmental tests mimicking the space conditions. Matching of pre and post I-V measurements for each tests confirms that performance of the device is not degraded and hence selected model of SiPM can be explored for the future space instrumentation.

Field-resolved high-order sub-cycle nonlinearities in a terahertz semiconductor laser

The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.

Execution of all-optical Boolean OR logic using carrier reservoir semiconductor optical amplifier-assisted delayed interferometer

It is known that the conventional bulk semiconductor optical amplifier (SOA) faces the problem of the slow gain recovery time, which limits its application as a nonlinear element at higher data rates. Therefore, our goal is to find an alternative that works at higher rates with acceptable performance. In this paper, we employ, for the first time to our knowledge, a carrier reservoir SOA (CR-SOA) followed by a delayed interferometer (DI) to execute all-optically the Boolean OR operation at a data rate of 100 Gb/s. A performance comparison between the CR-SOA- and the conventional bulk SOA-based DI is made by studying the variation of the quality factor (Q-factor) against various key operating parameters, which include the transition time from the carrier reservoir to the active region, the population inversion factor, the injection current, the equivalent pseudorandom binary sequence length, the alpha-factor, the optical confinement factor, the operating data rate, the input pulse energy, and the continuous wave power in the presence of amplified spontaneous emission noise. The obtained results demonstrate that owing to the CR-SOA faster gain and phase recovery, the CR-SOA-DI is more suitable for realizing the OR logic gate at 100 Gb/s as it manages to achieve a more than acceptable Q-factor value of 9, compared to the unacceptable value of 3 when using the conventional SOA-DI.

Harmonic injection locking of high-power mid-infrared quantum cascade lasers

High-power, high-speed quantum cascade lasers (QCLs) with stable emission in the mid-infrared regime are of great importance for applications in metrology, telecommunication, and fundamental tests of physics. Owing to the intersubband transition, the unique ultrafast gain recovery time of the QCL with picosecond dynamics is expected to overcome the modulation limit of classical semiconductor lasers and bring a revolution for the next generation of ultrahigh-speed optical communication. Therefore, harmonic injection locking, offering the possibility to fast modulate and greatly stabilize the laser emission beyond the rate limited by cavity length, is inherently adapted to QCLs. In this work, we demonstrate for the first time the harmonic injection locking of a mid-infrared QCL with an output power over 1 W in continuous-wave operation at 288 K. Compared with an unlocked laser, the intermode spacing fluctuation of an injection-locked QCL can be considerably reduced by a factor above 1 × 10 3 , which permits the realization of an ultrastable mid-infrared semiconductor laser with high phase coherence and frequency purity. Despite temperature change, this fluctuation can be still stabilized to hertz level by a microwave modulation up to ∼ 18 GHz . These results open up the prospect of the applications of mid-infrared QCL technology for frequency comb engineering, metrology, and the next-generation ultrahigh-speed telecommunication. It may also stimulate new schemes for exploring ultrafast mid-infrared pulse generation in QCLs.

Compensation of SOA-induced nonlinear phase distortions by optical phase conjugation

To answer the question: "Is optical phase conjugation (OPC) capable of compensating nonlinear distortions caused by not only Kerr effect of optical fibre, but also the carrier dynamics of semiconductor optical amplifiers (SOAs)?", we investigate the effectiveness of OPC-based nonlinear compensation for SOAs amplifying a few-channel WDM signal modulated with m-QAM. We use a pair of SOAs with an OPC stage sandwiched between the two so that the combination works as a low-distortion amplifier. Symbol-period longer than the gain recovery time is chosen in our experiments to avoid bit-pattern effects introduced by the SOA. We amplify a 12Gbaud, 16QAM modulated three-channel WDM signal with this technique in the back-to-back configuration which remarkably outperforms a single SOA in the nonlinear regime of operation with an average Q2 improvement better than 4 dB for an output power of 4 dBm. We further demonstrate the practical advantage of the low distortion higher output power capability of the SOA shown in the back-to-back result by carrying out a transmission of the amplified signal through a 160-km fibre, where relatively high launch power is desirable. We also study the case of 64QAM signals and show that approximately a 3 dB Q2 factor improvement can be obtained over single SOA, while without nonlinear phase distortion compensation, the demodulation is nearly impracticable.

Pulse pump–quasi-CW probe technique to measure the gain recovery time of an erbium-doped fiber amplifier

A two-wavelength pump–probe technique to measure the gain recovery time of an erbium-doped fiber amplifier is reported. In the measurement, pulse fields and quasi-continuous-wave fields generated from a single-distributed-feedback laser array are respectively used as the pump and probe light. To distinguish the probe field from spontaneous emissions from the amplifier, a circulator and fiber Bragg grating are incorporated in the experimental setup.

Observation and evaluation of dynamic population gratings formed by two‐wave mixing in an erbium‐doped fiber amplifier

AbstractDynamic population gratings formed by two‐wave mixing in an erbium‐doped fiber amplifier are reported. One of the counter‐propagating fields is intensity‐modulated to distinguish reflection from the grating from amplified transmission fields. The modulation‐frequency dependence of the reflection is studied under single‐wavelength and two‐wavelength input conditions to separate reflection from transient gain. The results are analyzed by measuring the gain saturation and recovery times using self‐probe and pump–probe techniques.

Performance enhancement of an all-optical XOR gate using quantum-dot based reflective semiconductor optical amplifiers in a folded Mach-Zehnder interferometer

The feasibility of implementing an All-Optical (AO) XOR gate for 320 Gb/s data pulses using Quantum Dot (QD) based Reflective Semiconductor Optical Amplifiers (RSOAs) in a Folded Mach–Zehnder Interferometer (FMZI) is investigated using detailed numerical models. Detailed performance comparisons are made between the conventional QD-SOA based MZI XOR and the specific scheme. The influence of various parameters such as SOA length, input optical powers, rear-facet reflectivity, and bias current on the XOR gates output quality are investigated. The results indicate that the gain recovery time in the QD-RSOA can be reduced to approximately 50% of its value in the QD-SOA, which can help avoid restoring sophisticated gain recovery acceleration techniques. It is shown that at low input powers and high bit rates the MZI XOR fails to perform the logic operation, while the FMZI XOR can deliver a good performance at data rates as high as 320 Gb/s with reducing the power consumption of the structure. The simulations demonstrate an about 5 dB (1 dB) improvement on the XOR output extinction ratio (amplitude modulation) for 320 Gb/s input bit sequence. The combination of the ultrafast gain dynamics of QD materials with the inherent advantages of RSOA devices and using the former in the latter inside the high-performance FMZI can be utilized to realize power-efficient ultrafast signal processing and switching functions while being cascadable and scalable for constructing more complex AO circuits.

Quantum dot semiconductor optical amplifier: investigation of ultra-fast cross gain modulation in the presence of a second excited state

In this paper, a theoretical model is proposed to investigate and analyze an ultra-fast cross gain modulation (XGM) in quantum dot semiconductor optical amplifiers (QDSOAs). The proposed model is based on a set of carrier’s rate equations, propagation equations for pump and probe signals and phase equations. To investigate the XGM mechanism, nonreturn-to-zero (NRZ) pulse trains with various bitrates are supposed to propagate as the input pump signals through the QDSOA’s active layer. The probe signal is assumed to be a continuous wave (CW) signal. Furthermore, optical gain of quantum dots (QDs) is calculated using a density matrix approach. For the first time, to the best of our knowledge, a second excited state is considered in the band diagram of QDs, to obtain the ultra-fast XGM in QDSOAs. Therefore, in the presented model, the rate equations are written for the carriers in the ground state, first and second excited states (ES2), continuum state and the wetting layer. In the presence of ES2, gain recovery time and gain saturation is reduced and, therefore, ultra-high bitrate pulse trains can be modulated with negligible pattern effects and wave distortion. The effect of the injection current density increment and carrier’s relaxation lifetime decrement are illustrated to show the improvement in the XGM mechanism in the QDSOA. It is shown theoretically that ultra-high bitrate XGM up to 450 Gbps is possible due to the reduced gain recovery time in the presence of ES2. It is demonstrated that at the same pump power for higher injection current densities and lower carrier relaxation lifetimes, the pattern effect and wave distortion has almost vanished. Although, in this case, the extinction ratio (ER) is decreased. It is shown that to recover the ER, the input pump power should be increased. Furthermore, based on the results in the presence of ES2, the ER is decreased by 0.5 dB.

Light-enhanced incoherence of electronic transport in quantum cascade lasers

Since their invention in the middle of the 1990s, quantum cascade lasers (QCLs) attract increasing theoretical interest stimulated by their widening applications. One of the key theoretical issues is the optimization of electronic transport which in most of these devices is governed by the injection barrier of QCL heterostructure. In the paper, the nonequilibrium Green’s function formalism is used to study electronic transition through the injection barrier as a function of laser field in the cavity; for the increasing field, a crossover is observed from the strong coupling regime, in which electronic transport through the barrier is coherent, to the weak coupling regime, in which electronic transport gets incoherent. This crossover is characterized by gain recovery time, τrec, which takes sub-picosecond values for mid-IR QCLs operating at room temperature. This time is also important for the performance of devices under steady-state conditions; the maximum output power is obtained when the figure of merit, FOM = (g(0)/gth − 1)/gcτrec [g(0) is the linear response gain, gth is the threshold gain needed to compensate all losses, gc is the gain cross-section], reaches maximum. It is shown that the use of this optimization criterion can result in the structures essentially different from those which can be obtained when the optimized quantity is the linear response gain, g(0).

Twofold gain enhancement by elongation of QDs in polarization preserving QD-SOAs

The impact of quantum dot (QD) elongation on key parameters of QD-based semiconductor optical amplifiers (SOAs) is investigated using a combination of 8-band -theory including strain and piezoelectricity up to second order and a rate equation model describing the population of QD ground, excited and wetting layer states. By considering columnar QDs of selected aspect ratios, we show that chip gain and saturation gain can be enhanced by up to +3.6 dB via an increased elongation of the individual QDs while retaining polarization preserving amplification and gain recovery times below 700 fs. Our results enable the optimization of polarization preserving QD-SOA devices which combine ultrafast gain recovery with high gain and low power consumption.

Gain dynamics in a heterogeneous terahertz quantum cascade laser

The gain recovery time of a heterogeneous active region terahertz quantum cascade laser is studied by terahertz-pump-terahertz-probe spectroscopy. The investigated active region, which is based on a bound-to-continuum optical transition with an optical phonon assisted extraction, exhibits a gain recovery time in the range of 34–50 ps dependent on the operation condition of the laser. The recovery time gets shorter for stronger pumping of the laser while the recovery dynamics slows down with increasing operation temperature. These results indicate the important role of the intracavity light intensity for the fast gain recovery.

Modeling the effects of interband and intraband transitions on phase and gain stabilities of quantum dot semiconductor optical amplifiers

In this paper, the effects of interband and intraband transitions on the gain and phase stabilities in quantum dot semiconductor optical amplifier (QD-SOA) are investigated both temporally and spectrally employing electrical and optical pumping schemes. For this purpose, the carrier rate equations in different energy states coupled to the traveling wave optical field equation have been numerically solved to derive the dynamical behavior of QD-SOA. Our results show that the gain and phase response can be stabled under optical pumping (OP) scheme because the role of the interband and intraband transitions on the dynamics of QD-SOA is reduced. This behavior leads to high-speed pattern effect-free cross-phase modulation (XPM) in QD-SOA. It is found that optically pumped QD-SOA can have high performance in phase based applications. Moreover, it is shown that under OP scheme although the QD-SOA has lower gain value and slower gain recovery time, the ultrafast cross-gain modulation (XGM) without pattern effect is possible and the phase is recovered within a shorter time compared to EP scheme. The behavior arises from the different capacity of the carrier reservoir for pumping schemes.