Active Region Width Research Articles

High efficient net gain and low noise figure based vertical cavity semiconductor optical amplifiers for wavelength division multiplexing applications

Abstract This paper has demonstrated the high efficient net gain and low noise figure based vertical cavity semiconductor light amplifiers for wavelength division multiplexing applications. Previous study on the chip reflective gain variations versus SOA current under temperature effects is clarified. We have transferred light semiconductor amplifiers for wavelength multiplexing schemes applications. Amplifier output power is demonstrated with bias current and amplifier active layer region length variations based input signal power of 3, 6.5, 10 dBm, 1550 nm wavelength and optimum amplifier confinement factor of 0.45. The amplifier output noise variations are clarified against the bias current and amplifier active layer region width variations based input signal power of 3, 6.5, 10 dBm, 1550 nm wavelength and optimum amplifier confinement factor of 0.45. The output OSNR variations are studied clearly and deeply against the bias current and amplifier active layer region length variations based input signal power of 3, 6.5, 10 dBm, 1550 nm wavelength and optimum amplifier confinement factor of 0.45. The signal loss is demonstrated versus both active layer width/length and temperature based optimum 30 % gallium core dopant ratio and 28 % arsenide cladding dopant ratio at optimum input signal power of 10 dBm, 1550 nm wavelength and optimum amplifier confinement factor of 0.45. SOA amplifier output power can be enhance with the management of both bias current and input signal power and the reduction of active amplifier length. The amplifier output noise can be enhance with the management of both bias current and input signal power and the reduction of active amplifier width. Output OSNR system can be improved with the management of both bias current and input signal power and the reduction of active amplifier length. The SOA amplifier gain can be improved with the management of both bias current and input signal power and the reduction of active amplifier length. The SOA amplifier noise figure can be improved with the management of both bias current and input signal power and the reduction of active amplifier width. The signal loss can be controlled and managed by adjusting optimum 30 % gallium core dopant ratio and 28 % arsenide cladding dopant ratio, optimum active layer/with and the presence of room temperature.

Novel Compact Design and Investigation of a Super Wideband Millimeter Wave Antenna for Body-Centric Communications

This paper presents a novel design for a multiple band millimeter wave antenna with a wide active region in the extremely high frequency (EHF) range. The antenna's performance was tested at three evenly separated frequencies: 60 GHz within the V-band region, 80 GHz within the E-band region, and 100 GHz. Simulation exhibits satisfactory results in terms of gain and efficiency, although the efficiency falling tendency for higher frequency persists. As millimeter wave antennas have miniature-like dimensions and low penetration depth into human body layers, the performance of these antennas is less disturbed by the presence of a human body, making them ideal for body-centric wireless communication (BCWC) applications. Thus, a human body model was created virtually with the necessary property data. Simulations are repeated at the same frequencies as before, with the antenna kept close to the constructed human body model. The results were promising as the gains found increased radiation patterns and return loss curves remained almost identical, except some efficiencies that were considered. Some H-plane radiation patterns are changed by the presence of a human body. Although all three frequencies present satisfactory results, 60 GHz is found to be more balanced, but 100 GHz shows better gain and directivity. Multiple band operability makes this antenna suitable for various applications. Finally, a distance-based analysis was conducted to realize the in-depth characteristics of the antenna by placing the antenna at five different gaps from the human body. The result verifies the antenna’s category as suitable for body-centric communications.

Buried-Heterostructure Quantum Cascade Lasers Fabricated Using a Sacrificial Layer and a Two-Step Regrowth Process

In recent years, the technology of the quantum cascade laser (QCL) has drastically improved, as proven by room temperature continuous wave (CW) operation, high optical power, low dissipation devices, and accessible spectral range [1]. As a result, QCLs have drawn a great deal of interest for their various applications in trace-gas sensing, defense countermeasures, spectroscopy and free-space communications [2]. The performance of QCLs is primarily dependent on the design of the active region, usually gown by MBE (molecular beam epitaxy). Nevertheless, the laser performance are also influenced by the thermal properties and optical losses introduced by the fabrication technology, in particular in continuous-wave operation. In that respect, the buried heterostructure (BH) approach, which is a standard and reliable telecommunications laser manufacturing technique, offers unique possibilities to achieve effective thermal dissipation and lateral single modes. In standard BH designs, the process starts by the definition and etching of the ridge mesa using a dielectric mask. The latter (SiO2 or SiN) is also used as the masking material for the selective regrowth by metal-organic vapor phase epitaxy (MOVPE) of a thick insulating InP (Fe) layer over the entire laser structures. Unfortunately, the selective regrowth of InP:Fe is not a trivial process. Indeed, it is extremely difficult to achieve an ideal planarized regrowth, i.e, completely burying the laser ridge, while maintaining a planar surface. In addition to growth conditions, the characteristics of the regrown InP layer strongly depend on the sidewall profile of the ridges, as well as on the physical parameters of the dielectric mask (thickness, width…) [3]. The complexity of the etching and regrowth process further increases for thick layer structures, such as for long wave QCLs, whose ridges can be >10 µm deep. In effect, deep ridge etching generates big undercuts in the structures and consequently, highly non uniform active region widths. Moreover, the thick selective regrowth step, needed for planarization, tends to favor the appearance of crystal growth artifacts (crystallites, voids, “rabbit ears”…) in the vicinity of the ridges, which deteriorate the performance of the regrown structures. In some cases, additional photolithographic and back-etching steps might even be required to improve the planarity of the structure. Another method that allows overcoming the aforementioned limitations is to use a modified buried heterostructure concept, with two MOVPE regrowth steps [4]. In this case, the waveguide pattering and etching is constituted only by the active layers (<3 μm). Hence, the active region is laterally insulated and planarized using selective growth of a much thinner InP:Fe layer. The dielectric layer is subsequently removed and the optical cladding is grown over all the structure. An additional advantage of a thinner regrowth is the reduced amount of defects due to the selective growth of InP:Fe. However, the undercut formed by the overhanging dielectric mask (needed for the selective growth in first MOVPE step), and the tapered profile of the active region can result in a reduced material thickness close to the upper edges of the active region. In this case, the material thickness can be too thin and induce carrier tunneling through the interface between the active region and the lateral regrowth (insufficient electrical isolation). In this perspective, we report an optimized BH-QCL approach obtained by adding an InP layer and a sacrificial InGaAs cap layer on top of the MBE grown active region. As shown in the attached figures (FIB/SEM cross-section profiles), the lateral thickness of the insulating and planar regrowth is increased and reduces potential leakage paths. Using this approach, BH QCL regrowths were successfully carried out with reproducible and good epitaxial growth quality (smooth and uniform regrown layers, planar surfaces, no “rabbit-ears” and defect free interfaces) around the etched ridges, permitting the fabrication of BH-QCLs with good beam quality and hundreds of mW of optical power at room temperature. The growth and optimization of the BH process, as well as the fabrication and testing of the laser devices will be presented and compared to conventional double trench devices.

Power scaling and experimentally fitted model for broad area quantum cascade lasers in continuous wave operation

Experimental and model results for 15-stage broad area quantum cascade lasers (QCLs) are presented. Continuous wave (CW) power scaling from 1.62 to 2.34 W has been experimentally demonstrated for 3.15-mm long, high reflection-coated QCLs for an active region width increased from 10 to 20 μm. A semiempirical model for broad area devices operating in CW mode is presented. The model uses measured pulsed transparency current, injection efficiency, waveguide losses, and differential gain as input parameters. It also takes into account active region self-heating and sublinearity of pulsed power versus current laser characteristic. The model predicts that an 11% improvement in maximum CW power and increased wall-plug efficiency can be achieved from 3.15 mm×25 μm devices with 21 stages of the same design, but half doping in the active region. For a 16-stage design with a reduced stage thickness of 300 A, pulsed rollover current density of 6 kA/cm2, and InGaAs waveguide layers, an optical power increase of 41% is projected. Finally, the model projects that power level can be increased to ∼4.5 W from 3.15 mm×31 μm devices with the baseline configuration with T0 increased from 140 K for the present design to 250 K.

Enhanced performance of all-optical half-subtracter based on cross-gain modulation (XGM) in semiconductor optical amplifier (SOA) by accelerating its gain recovery dynamics

In this article, the acceleration attained in gain recovery dynamics of travelling-wave-type semiconductor optical amplifier (SOA) at the expense of structural optimization is illustrated via numerical simulations. A pump–probe scheme has been utilized in order to study the outcomes of optimization of SOA operational and structural parameters on its effective gain recovery time ( $${\tau _\mathrm{e}}$$ ). A set of optimized SOA parameters are formulated from gain recovery dynamics studies after keeping practical implementation considerations in vision. Further, the impacts of altering SOA structural and operational parameters such as injection current (I), amplifier length (L), active region width (w), active region thickness (t) and optical confinement factor ( $${\varGamma } $$ ) on gain recovery time improvement achieved are further investigated on the performance of a cross-gain modulation (XGM) in SOA-based all-optical half-subtracter in terms of two designated performance metrics: quality factor (Q-factor) and extinction ratio (ER). It has been revealed that reduced gain recovery time-optimized SOAs-based all-optical half-subtracter arranged in a co-propagating manner exhibits improved Q-factor and ER (dB) performance at high bit rates of operation ( $$\le $$ 80 Gbps).

Output pulse width of mode‐locked ring laser based on Er3+‐doped fibre amplifier and semiconductor optical amplifier

The time width and the spectral width of the output pulse of ring laser based on Er3+-doped fibre amplifier and semiconductor optical amplifier have been investigated theoretically. Influences of Er3+-doped concentration, active region width, input current, the length of the cavity imposed on the spectral bandwidth of the output pulse have been analysed and discussed. By using the analyses and conclusions in this study, the authors can control the pulse width by adjusting the related parameters and optimise the structure of the mode-locked laser system.

Low-threshold-current-density AlGaN-cladding-free m-plane InGaN/GaN laser diodes

We demonstrate AlGaN-cladding-free m-plane InGaN/GaN laser diodes with threshold current densities that are comparable to state-of-the-art c-plane InGaN/GaN laser diodes. Thick InGaN waveguiding layers and a relatively wide active region with three 8 nm quantum wells were used to provide adequate refractive index contrast with the GaN cladding layers, thus eliminating the need for AlGaN cladding layers. Despite the large active region volume, lasing was achieved at a threshold current density of 1.54 kA/cm2, suggesting that the realization of even lower threshold current densities should be possible by reducing the number of quantum wells in the active region.

High-Speed (60 GHz) and Low-Voltage-Driving Electroabsorption Modulator Using Two-Consecutive-Steps Selective-Undercut-Wet-Etching Waveguide

Based on a novel structure of waveguide, a broadband electroabsorption modulator (EAM) with low driving voltage and high extinction ratio has been demonstrated in this letter. The waveguide of InGaAsP-InP p-i-n layer structure is fabricated by two consecutive steps of selective undercut-wet-etching: 1)HCl : H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> PO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> on p-InP (p- layer), and 2)H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> PO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> : H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> : H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O on InGaAsP (active region), showing a wide ridge with a narrow undercut active region. Low capacitance and low cladding impedance can thus be simultaneously attained in such waveguides, leading to low microwave loss and high-speed electrooptical (EO) response. A ridge as wide as 8 mum with a 3-mum- wide active region and a 450-nm gap height in the undercut portion has been fabricated. A 350- mum -long waveguide of EAM is designed, revealing a high extinction ratio of > 30 dB (D.C.) and a modulation efficiency of > 20 dB/V (D.C.) with polarization-insensitive operation at a wavelength of 1550 nm. As high as 60 GHz of a 3-dB bandwidth is measured in the high-speed EO conversion. Calculations by an equivalent circuit model are quite fitted with the measurement, revealing that broadband performance is mainly attributed to the low microwave propagation loss in such waveguides.

Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning

A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB) and the other one is related to the Risken-Nummedal-Graham-Haken instability predicted in the 1960s. A model that can account for coherent phenomena, a saturable absorber, and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with a narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have a weaker saturable absorber, do not exhibit a Rabi splitting and their multimode regime is governed by SHB. This experimental phenomenology is well-explained by our theoretical model. The temperature dependence of the multimode regime is also presented.

Analysis of interface recombination and self-absorption effect on the performance of QWIP–HBT–LED integrated device

In this paper, the effect of interface recombination and self-absorption within the light emitting diode (LED) active region on the efficiency of QWIP–HBT–LED integrated device is considered. This device is composed of a quantum well infrared photodetector (QWIP), a heterojunction bipolar transistor (HBT) and an LED. The evaluation is based on solving the continuity equation describing the carrier diffusion within the LED active region. Analytical expression describing the effect of self-absorption and surface recombination on the LED quantum efficiency is derived. In addition, the active region width and all interested device parameters are involved. It is observed that the quantum conversion efficiency of the device under consideration is degraded by the self-absorption and interface recombination within the recombined region of the LED. Also, the quantum conversion efficiency of the device is increased with the increase of the LED active region as long as the recombination velocity is above a specified value, while it is decreased with the increase of the LED active region as long as the recombination velocity is below this specified value.

IBIC characterisation of novel detectors for single atom doping of quantum computer devices

Single ion implantation and online detection is highly desirable for the emerging application, in which single 31P ions need to be inserted in prefabricated silicon cells to construct solid-state quantum bits (qubits). In order to fabricate qubit arrays, we have developed novel detectors that employ detector electrodes adjacent to the prefabricated cells that can detect single keV ion strikes appropriate for the fabrication of shallow phosphorus arrays. The method utilises a high purity silicon substrate with very high resistivity, a thin SiO 2 surface layer, nanometer masks for the lateral positioning single phosphorus implantation, biased electrodes applied to the surface of the silicon and sensitive electronics that can detect the charge transient from single keV ion strikes. A TCAD (Technology Computer Aided Design) software package was applied in the optimisation of the device design and simulation of the detector performance. Here we show the characterisation of these detectors using ion beam induced charge (IBIC) with a focused 2 MeV He ions in a nuclear microprobe. The IBIC imaging method in a nuclear microprobe allowed us to measure the dead-layer thickness of the detector structure (required to be very thin for successful detection of keV ions), and the spatial distribution of the charge collection efficiency around the entire region of the detector. We show that our detectors have near 100% charge collection efficiency for MeV ions, extremely thin dead-layer thickness (about 7 nm) and a wide active region extending laterally from the electrodes (10–20 μm) where qubit arrays can be constructed. We demonstrate that the device can be successfully applied in the detection of keV ionisation energy from single events of keV X-rays and keV 31P ions.

Low-damage etched/regrown interface of strain-compensated GaInAsP/InP quantum-wire laser fabricated by CH4/H2 dry etching and regrowth

GaInAsP/InP strain-compensated five-layered quantum-wire lasers with the wire width of 23 nm were fabricated by electron-beam lithography, CH4/H2-reactive ion etching and organometallic vapor-phase-epitaxial regrowth, and the quality of the etched/regrown interface was evaluated from the spontaneous emission efficiency dependence on the active region width in comparison with that of unetched quantum-well lasers. As a result, the product of the surface recombination velocity and the carrier lifetime at the etched/regrown interface was estimated to be less than 3 nm at room temperature. Finally, no noticeable degradation in the spontaneous emission efficiency of this quantum-wire laser was observed within measurement temperature from 25 °C to 85 °C.

Distributed Reflector Laser with Wirelike Active Regions for Asymmetric Output Property

A distributed reflector (DR) laser with two different wirelike active region widths was realized for the first time. As a result, a threshold current as low as 1.8 mA, an output power ratio of the front to the rear facet of 8 and a submode suppression ratio of 33 dB at 1.2 times the threshold were obtained under a room-temperature continuous-wave condition.

Very low threshold current density operation of 1.5 [micro sign]m DFB lasers with wire-like active regions

Very low threshold 1.5 µm-wavelength GaInAsP/InP distributed feedback lasers consisting of deeply etched double quantum-well wire-like active regions have been successfully realised by CH4/H2 reactive ion etching and metal organic vapour phase epitaxial regrowth. A threshold current density Jth of as low as 94 A/cm2 was achieved for an active region width of 115 nm and cavity length of 600 µm.

Analysis of waveguide properties of organic semiconductor lasers

The wave-guiding properties of organic semiconductor lasers operating at 630 nm have been analysed. Calculations for gain-guided lasers show that single lateral mode operation can be supported in structures having active region widths of under 6 µm, albeit with confinement highly dependent on active layer thickness. Ridge-guided structures are investigated as a means of providing single-mode operation with stable lateral confinement at high-gain values. It is shown that ridge heights of about 0.03 µm are sufficient to provide lateral confinement of about 0.5.

Sidewall Recombination Velocity in GaInAsP/InP Quantum-Well Lasers with Wire-like Active Region Fabricated by Wet-Chemical Etching and Organo-Metallic Vapor-Phase-Epitaxial Regrowth

We estimated the nonradiative recombination velocity at the sidewall of GaInAsP/InP quantum-well lasers with narrow wire-like active region, which had been fabricated by wet-chemical etching and two-step organo-metallic vapor-phase-epitaxial (OMVPE) regrowth, from the active region width dependence of spontaneous emission efficiency under current injection. It was shown theoretically that this estimation method of the nonradiative recombination velocity is more accurate than a conventional method using threshold current dependence on the width of an active region, and was confirmed experimentally. The sidewall recombination velocity of 5 stacked multiple-quantum-well and a single-quantum-well structures fabricated by wet-chemical etching and OMVPE regrowth method was estimated to be 1.3 ×103 cm/s and 2.0 ×103 cm/s, respectively, at room temperature, while it was estimated to be very small (less than 10 cm/s) at low temperatures (T = 90 K).

Analysis of the active layer in SI GaAs Schottky diodes

The behavior of the active region width W of semi-insulating gallium arsenide Schottky diodes versus reverse biasing has been investigated by optical beam induced current and surface potential techniques. It has been found that at low applied voltages, W follows the square root law peculiar to a Schottky barrier while, for a bias higher than 20 V, the active layer increases linearly with the voltage applied. To go deeper into this matter, the spatial distribution of the electric field has been analyzed in a wide range of bias voltages and it has been observed that at high voltages a plateau occurs, followed by a linear decrease down to a quasi-zero value. In terms of space charge distribution this means that there is a box-shaped space charge region moving towards the ohmic contact at increasing bias.

The Structure and Properties of Solar Active Regions and Quiet‐Sun Areas Observed in Soft X‐Rays withYohkoh/SXT and in the Extreme‐Ultraviolet with SERTS

We observed two solar active regions (NOAA regions 7563 and 7565), quiet-Sun areas, and a coronal hole region simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Yohkoh Soft X-ray Telescope (SXT) on 1993 August 17. SERTS provided spatially resolved active region and quiet-Sun slit spectra in the 280 to 420 A wavelength range, and images in the lines of He II λ303.8, Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λλ335.4 and 360.8 SXT provided images through multiple broadband filters in both the full-frame imaging mode and the partial-frame imaging mode. The SERTS images in Fe XV (log Tmax = 6.33, where Tmax is the temperature which maximizes the fractional ion abundance in the available ionization equilibrium calculations, i.e., the formation temperature) and Fe XVI (log Tmax = 6.43) exhibit remarkable morphological similarity to the SXT images. Whereas the Fe XV and XVI images outline the loop structures seen with SXT, the cooler He II (log Tmax = 4.67) and Mg IX (log Tmax = 5.98) images outline loop footpoints. In addition, the Mg IX emission outlines other structures not necessarily associated with the hot loops; these may be cool (T 1 × 106 K) loops. From the spatially resolved slit spectra, we obtained emission-line profiles for lines of He II λ303.8, Mg IX λ368.1, Fe XIII λ348.2, Si XI λ303.3, Fe XIV λ334.2, Fe XV λ284.1, and Fe XVI λ335.4 for each spatial position. Based upon the spatial variations of the line intensities, active region 7563 systematically narrows when viewed with successively hotter lines, and appears narrowest in the broadband soft X-ray emission. The active region width (full width at half-maximum intensity) diminishes linearly with log Tmax; the linear fit yields an extrapolated effective log Tmax of 6.51 ± 0.01 for the X-ray emission. The most intense, central core straddles the magnetic neutral line. Active region and quiet-Sun one-dimensional temperature scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines, and from coregistered SXT filter ratios. The highest plasma temperatures were measured in the most intense, central core of region 7563. The temperatures derived from Fe XVI λ335.4/Fe XV λ284.1 and Fe XVI λ335.4/Fe XIV λ334.2 vary significantly (based upon the measurement uncertainties) but not greatly (factors of less than 1.5) across the slit. The average log T values derived from the above two ratios for region 7563 are 6.39 ± 0.04 and 6.32 ± 0.02, respectively. Somewhat larger systematic variations were obtained from all available SXT filter ratios. The average active region log T values derived from the SXT AlMgMn/thin Al, thick Al/thin Al, and thick Al/AlMgMn filter ratios are 6.33 ± 0.03, 6.45 ± 0.02, and 6.49 ± 0.03, respectively. Active region and quiet-Sun one-dimensional density scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines of Fe XIII and Fe XIV. The derived densities show neither systematic nor significant variations along the slit in either the active region or the quiet-Sun, despite the fact that the intensities themselves vary substantially. This indicates that the product of the volume filling factor and the path length (fΔl) must be greater by factors of 3-5 in the active region core than in the outskirts. Furthermore, the derived active region densities are ~2 times the quiet-Sun densities. This density difference is adequate to explain the factor of ~4 intensity difference in Fe XII and Fe XIII between the active and quiet areas, but it is not adequate to explain the factor of ~8 intensity difference in Fe XIV between the active and quiet areas. We attribute the latter to a greater fΔl in the active regions. Statistically significant Doppler shifts are not detected in region 7563 or in the quiet-Sun with any of the EUV lines.

Structural and carrier density dependence of carrier lifetime in InGaAs/GaAs multiple-quantum-well lasers

The carrier lifetime in undoped and p-doped mesa and ridge waveguide In0.35Ga0.65As/GaAs multiple-quantum-well lasers is extracted from the frequency response of the spontaneous emission. The radiative recombination coefficient is found to be the same for mesa and ridge waveguide lasers, and is nearly independent of the doping level. For ridge waveguide lasers, a simple method is proposed to obtain the lateral broadening of the active region due to carrier diffusion. When the corrected active region width is considered, the threshold carrier densities for both undoped and p-doped lasers are independent of the lateral structure and cavity width. Further, the surface recombination velocity in mesa lasers is determined.

An investigation of electric field behaviour in semi-insulating GaAs using current pulses

The transient current technique has been used to examine current pulses in semi-insulating (SI) GaAs and comparisons have been made with calculated current pulse shapes obtained by analysing both theoretical and measured electric field shapes. This is the first time that this technique for examining such bulk behaviour has been used with SI GaAs. The results show that the form of the weighting field required to reproduce the experimental current pulse shapes in this case is not the expected 1/d, where d is the active region width, but rather is closer to , where E is the electric field across the detector due to the applied bias and is the bias necessary for the active region to extend fully across the detector.