Optical Confinement Factor Research Articles

Efficient carrier transport in GRIN-SCH transistor lasers

We report analytical and theoretical results of simulation of a graded-base, single quantum well (SQW) transistor laser (TL). Using an appropriate carrier transport model, device performances for different confinement structures are studies. Physical parameters including the diffusion constant and optical confinement factor are calculated, and the dependence of the optical response on both current level and structure design (e.g. base doping and cavity length) is investigated. Simulation results show that using graded index layers of AlξGa1 − ξAs (ξ: 0.1 → 0) in the left-hand side of the QW and AlξGa1 − ξAs (ξ: 0.05 → 0) in the right-hand side of the QW (instead of GaAs in the base region) increases the optical output power by a factor of 3, eliminates completely the resonance peak, and most interestingly increases optical bandwidth by ∼37% compared to the conventional (i.e. non-graded base) structure.

Effect of separate confinement hetero-structure layer on tunnel injection transistor laser-based transmitter for high-speed optical communication networks

Transistor Laser has already established as a promising candidate for high speed optical interconnects and optical telecommunication networks. The present work is focused on the performance of multiple quantum wells (MQW) based transistor laser by incorporating tunnel injection phenomenon and it’s performance by varying the width of the separate confinement heterostructure layer. The virtual state (VS) is assumed as a path for the movement of injected charge carriers from bulk to nanostructures. We have estimated the various characteristics such as base threshold current, light power output, and optical modulation response by considering various physical parameters such as diffusion time, gain compression factor, optical confinement factor, photon lifetime, tunneling time and diffusion length, as a function of separate confinement hetero-structure (SCH) layer thickness. It is found that with increasing thickness of the SCH layer, base threshold current reduces, the light output power increases, but the optical modulation response remains almost constant, which ensures the potentiality of the proposed structure for high speed opto-electronic transmitter with low power consumption.

Theoretical investigation of anti-index guiding inactively Q-switched two-section diode lasers

We present a new theoretical design for diode lasers which should be capable of generating sub-100-ps pulses with pulse energies of more than 10 nJ per 100-μm-contact width by active Q-switching. We show that the carrier-induced reduction of the refractive index in the active layer, i.e. the vertical anti-index guiding effect, results in a dependence of the optical confinement factor on the carrier density which can be exploited for a further enhancement of the pulse energy and a reduction of the pulse length.

Wide-Temperature-Range (25–80 °C) 53-Gbaud PAM4 (106-Gb/s) Operation of 1.3-μm Directly Modulated DFB Lasers for 10-km Transmission

A 1.3- μ m directly modulated distributed feedback laser was newly developed by adopting an asymmetric corrugation pitch modulated grating structure and an InGaAlAs multiquantum well active layer with both a high optical confinement factor and a high differential gain. A low threshold current and a large saturation current in light-current characteristics at high temperature were obtained and a high 3-dB bandwidth and a high relaxation oscillation frequency in electro-optical characteristics were also achieved over a wide temperature range. As a result, clear back-to-back eye openings were achieved for 26-Gbaud PAM4 (53-Gb/s) operation from 25 to 95 °C with large outer extinction ratio and sufficiently small transmission dispersion eye closure quaternary. Furthermore, 53-Gbaud PAM4 (106-Gb/s) operation from 25 to 80 °C was successfully demonstrated for both back-to-back and after 10-km single mode fiber transmission. These excellent characteristics make our DFB laser a promising cost-effective and compact transceiver with low power consumption for next-generation 400GbE applications.

InGaAs/InP multi-quantum-well nanowires with a lower optical leakage loss on v-groove-patterned SOI substrates.

InGaAs/InP multi-quantum-well nanowires were directly grown on the v-groove-patterned SOI substrate by metal organic chemical vapor deposition. The surface morphology of the nanowires, the thickness of the quantum wells, and the photoluminescence spectra were characterized by scanning electron microscope, transmission electron microscopy, and micro-photoluminescence, respectively. We found in the experiments that the work of removing part of top Si on both sides of the nanowire to further reduce the optical leakage loss could be completed perfectly without complicated processes, such as a lithography process. Numerical simulations showed that the III-V nanowire was able to support an extraordinarily stable optical guided mode with a lower optical leakage loss of 0.21 cm-1 when etching away part of top Si on both sides of the nanowire, and the optical confinement factor of the multi-quantum-well active region was about 8.8%. This approach opens up a way for monolithic photonic integration of III-V compound semiconductors on Si to occur.

Transverse mode competition in narrow-ridge diode lasers

The influence of waveguide parameters (thicknesses and contrasts in both transverse and lateral directions) on optical mode compositions in narrow-ridge lasers is numerically investigated. The proposed numerical model explains our experimental results on the drastic difference in optical mode compositions in broad-area and narrow-ridge lasers processed from the same wafer based on a broadened GaAs/AlGaAs transverse-waveguide heterostructure. It is shown that in the broadened transverse waveguides the fundamental mode tends to have a much lower 2D optical confinement factor than high-order modes. We suppose that waveguides possessing no more than two transverse eigenmodes would provide more opportunities for designing narrow-ridge lasers with robust single-mode emission.

Threshold Current Density of Al0.1Ga0.9N/GaN Triple Quantum Well Laser

Semiconductor laser is used in processing many issues related to the scientific, military, medical, industrial and agricultural fields due to its unique properties such as coherence and high strength where GaN-based components are the most efficient in this field. Current technological developments mention to the strong connection of GaN with sustainable electronic and optoelectronic devices which have high-efficiency. The threshold current density of Al0.1Ga0.9N/GaN triple quantum well laser structure was investigated to determine best values of the parameters affecting the threshold current density that are well width, average thickness of active region, cavity length, reflectivity of cavity mirrors and optical confinement factor. The optimum value of the threshold current density is 2670 A/cm2 was obtained when the well width (w= 2.5 nm), reflectivity of cavity mirrors (R1=0.75, R2=0.9), cavity length (L=2mm), average thickness of active region (d= 11.5 nm), and optical confinement factor ( Γ= 0.034) at room temperature.

Monolithic Waveguide-Based Linear Photodetector Array for Use as Ultracompact Spectrometer

We propose a monolithic spectral waveguide photodetector (SWGPD) concept based on a waveguide with a chemical gradient of the semiconductor alloy forming the absorbing medium that allows for spectrally resolved detection. The chemical gradient correlates the absorption of different wavelengths with the position on the waveguide. The generated charge carriers are collected by a linear array of photodetectors along the waveguide. As model system and proof of concept (Mg,Zn)O alloys in the wurtzite phase (Zn-rich) are considered that allow photon detection in the near ultraviolet spectral region. Our concept allows for high overall detection of (in principle) all photons coupled into the waveguide. The spectral range and resolution are determined by the chemical gradient, the spectral broadening of the absorption edges, and the optical confinement factor. At the same time, the photodetectors can provide high-time resolution, limited by the photodetector geometry.

Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD

GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al0.21Ga0.79N two quantum wells (QW) type active region on Al0.08Ga0.92N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved.

All-Electric Laser Beam Control Based on a Quantum-Confined Heterostructure with an Integrated Distributed Bragg Grating

The characteristics of semiconductor chip modulators providing the angular deflection of a laser beam due to all-electric modulation of the optical characteristics of a semiconductor heterostructure are theoretically studied. Designs for quantum-confined semiconductor waveguide heterostructures with a distributed Bragg reflector integrated on the heterostructure surface are developed. The design of a waveguide structure including 20 periods of coupled asymmetric quantum wells, which provide an optical confinement factor of the waveguide mode of 20%, and an optimized doping profile allowing a uniform electric-field distribution to be maintained in the quantum-well region in the entire range of operating voltages are proposed. In this waveguide structure, the change in the refractive index due to the quantum-confined Stark effect reaches 0.086 when the control signal changes from 0 to 6 V. It is shown that the spatial (angular) scanning of the laser beam emitted from the surface of a distributed Bragg mirror with a divergence of less than 0.1° in the range of 1°–2° is feasible for the proposed waveguide structure design.

Effects of an undoped-InGaN waveguide on the optical confinement and carrier dynamics of InGaN laser diodes

In this paper, we have analyzed the performance of laser diodes (LDs) with an undoped InxGa1−xN waveguide in association with variable In mole fractions varying from 0% to 6%. The InGaN waveguide significantly affected optical confinement and carrier dynamics in the LD. Laser output power, internal quantum efficiency (IQE), quantum well (QW) gain and optical confinement factor (OCF) are analyzed in correlation with In content in the waveguide. The OCF for TE1 mode at x = 0 is 0.0079, and it increases with x. The OCF for x = 0.06 is 0.0109. Quantum states in the QW reduced with In content, which reduced QW gain. However, improved OCF consequently improved the overall gain of the device. Peak IQE for x = 0 is ~0.28, which increased to ~0.683 for x = 0.06. Laser output power, slope efficiency and IQE improved with increasing In content. The threshold current density for x = 0 is 3.911 kA cm−2, which drops to 1.806 kA cm−2 for x = 0.06. Auger recombination, Shockley-Read-Hall recombination and electron leakage are reduced for high In content in the waveguide. Auger carrier loss remains nearly constant above the threshold, hence efficiency droop above the threshold current density is attributed primarily to leakage current density. The efficiency droop for x = 0 is 25.59%, which increased to 36.66% for x = 0.06 at 25 kA cm−2 current density. However, the IQE for x = 0 at 25 kA cm−2 is 0.202, which increased to 0.436 for x = 0.06 at 25 kA cm−2. Slope efficiency increases with In content; slope efficiency for x = 0.06 improves by 0.2126 W A−1. Laser output power is improved by 1.91 W at 25 kA cm−2 for x = 0.06.

Enhancing the performance of GaN based LDs by using low In content InGaN instead of GaN as lower waveguide layer

Performance of InGaN based blue laser diodes (LDs) with different In content of (In)GaN lower waveguide (LWG) layers are investigated by simulation and experimental methods. It is found from the simulation results that the threshold current decreases and slope efficiency increases when the In content of InGaN LWG layer increases from 0% to 6%. However, when the In content is too high, the optical confinement factor of MQWs decreases, recombination rate in LWG layer increases and the surface morphology becomes roughness, all of these result in the deterioration of the performance of LDs with high In content InGaN LWG layer, the output power decreases from 97 to 83 mW at the injection current of 100 mA. It agrees well with the experiment results that the performance of LDs can be improved by using a low In content InGaN instead of GaN as lower waveguide layer.

Highly efficient power extraction in terahertz quantum cascade laser via a grating coupler

We demonstrate the highly efficient power extraction from terahertz quantum cascade laser (THz-QCL) via a grating coupler (GC). The GC, formed in the top metallization of a Fabry-Perot (FP) THz-QCL with a metal-metal (MM) waveguide, diffracts the THz wave into the free space when it oscillates between the two cleaved facets. The radiation loss can be tailored in a wide range by changing the structure of the GC. The asymmetric positioning of the GC enables near-unidirectional emission, the enlarged emission surface reduces the beam divergence, and the MM waveguide keeps the optical confinement factor near unity. Experimental results confirm that, when the operating frequency is near 2.7 THz, the laser with a GC shows superior performances than the conventional FP lasers in terms of output power and operation temperature.

Low-operating-energy directly modulated lasers for short-distance optical interconnects

We review recent developments in directly modulated lasers (DMLs) with low operating energy for datacom and computercom applications. Key issues are their operating energy and the cost for employing them in these applications. To decrease the operating energy, it is important to reduce the active volume of the laser while maintaining the cavity Q-factor or photon lifetime in the cavity. Therefore, how to achieve high-reflectivity mirrors has been the main challenge in reducing the operating energy. In terms of the required output power from the lasers, the required input power into the photodetector and the transmission distance determine the lower limit of laser active volume. Therefore, the operating energy and output power are in a trade-off relationship. In designing the lasers, the cavity volume, quantum well number, and optical confinement factor are critical parameters. For reducing the cost, it is important to fabricate a large-scale photonic integrated circuit (PIC) comprising DMLs, an optical multiplexer, and monitor photodetectors because the lower assembly cost reduces the overall cost. In this context, silicon (Si) photonics technology plays a key role in fabricating large-scale PICs with low cost, and heterogeneous integration of DMLs and Si photonics devices has attracted much attention. We will describe fabrication technologies for heterogeneous integration and experimental results for DMLs on a Si substrate.

Design of a 1.55 µm nanoscale laser array on v-groove patterned Si substrates

A 1.55 µm nanoscale laser array is proposed and designed on v-groove patterned Si (0 0 1) substrates. The laser array is designed by taking full advantage of the periodic groove structures of the patterned substrates. The periodic groove structures separated by a SiO2 mask can effectively reduce threading dislocation density in the active region and provide a strong lateral optical confinement for the laser array structure. The optical mode distributions of the single laser are investigated in detail, and the optical confinement factor is calculated to optimize the structure parameters of the laser array. An optimal value of 9.3% for the optical confinement factor is obtained. The analysis of threshold conditions shows that the laser array can achieve lasing with a cavity length of about 1 mm for an internal loss of 40 cm−1.

Estimation of Rayleigh scattering loss in a double-clad photonic crystal fiber

In this paper the effect of photonic crystal fiber’s structure parameters on Rayleigh scattering was investigated. Rayleigh scattering loss (RSL) has been numerically estimated by average Rayleigh scattering coefficient based on the empirical relations for \(V\) and \(W\) parameters of double-clad photonic crystal fibers (DC PCFs). The dependence of RSL on the two structural parameters—the air hole diameter and the hole pitch was demonstrated. We have shown that RSL depends on the index profiles because of the different optical power confinement factors in every layer of DC PCF. Using these results, the RSL can be optimized by adjusting the fiber parameters—air-hole diameter as well as the air-hole pitch.

Performance analysis of tin-incorporated group-IV alloy based transistor laser

Steady state analysis of GeSn alloy based single quantum well mid-infrared transistor lasers (TL) is reported in this paper. A theoretical model for the device is developed by considering the active Ge0.85Sn0.15 well in the base of the transistor. The Model includes the effect of strain on the material gain and free carrier absorption in the active layer and optical confinement factor in various layers of the device. Laser rate equation and continuity equation for bulk minority carrier with 2-D carrier distribution in the active layer are solved to obtain the threshold base current density and other steady-state parameters of the device like photon density, current gain, etc. Results show that Sn composition in the active GeSn layer and some device parameters play important roles in the performance of the device. An estimated threshold base current density of 2.6 kA/cm2 with the current gain of 2722 is obtained for a particular composition of Sn into SiGeSn/GeSn alloy in the base.

Design of active region for GaAsP/AlGaAs tensile strain quantum well laser diodes near 800 nm wavelength

As an active region, the tensile strain GaAs1-xPx quantum well plays an important role in the high power semiconductor laser diode with a wavelength of about 800 nm. Accompanied with the improved stability due to the Al-free active region, the GaAs1-xPx quantum well laser also shows a high level of catastrophic optical mirror damage because of the non-absorbing window at the facet, which is formed automatically by the relaxation of the tensile strain GaAs1-xPx material. On the other side, the GaAs1-xPx quantum well laser can provide a transverse magnetic (TM) polarized light source which is important for many solid state laser systems. However, the energy band structure of the tensile strain GaAs1-xPx quantum well is more complicated than that of the compressed or lattice matched quantum well. Although the light hole band is on the top of the heavy hole band for the bulk tensile strain GaAs1-xPx material, the situation may be different from the tensile strain GaAs1-xPx quantum well, in which the first light hole subband lh1 can be either on the top of the first heavy hole subband hh1 or reversed, that will cause the laser to generate either TM or transverse electric (TE) polarized light according to the well structure. So it is meaningful to optimize the tensile strain GaAs1-xPx quantum well structure based on the analysis of the energy band structure. Firstly, according to the 6×6 Luttinger-Kohn theory, the energy band structure of the tensile strain GaAs1-xPx quantum well is calculated by the finite difference method. The relationship between the interband transition energy and the well structure parameters is established. It is found that the well composition x and the well width should increase simultaneously, in order to fix the first subband transition wavelength at about 800 nm. Special attention is paid to the 808 nm quantum well, the valence structures of different well widths are calculated, the detailed analysis of the envelope function shows that the top valence subband is lh1 for wider well width, while it is changed to hh1 for narrower well width. Meanwhile, both the TE and the TM momentum matrix element are calculated as a function of the transverse wave vector for the subband transition from c1 to lh1, lh2, hh1 and hh2, respectively. Further, the threshold optical gains of different well widths are simulated for 808 nm laser diode with the tensile strain GaAs1-xPx quantum well as an active region, the wider well width benefits the TM mode, while the narrower one is favor of TE mode. Finally, according to the threshold carrier density, the relationship between the threshold current density and the well width is analyzed for 808 nm laser diode by considering both the spontaneous and the Auger recombination, an optimum combination of the well width and the well composition exists. For wider well width, the threshold current density will be higher because of the high energy subband carrier filling effect. For narrower well width, the decrease of the optical confinement factor will lead to the increase of threshold current density.

Полностью электрическое управление разверткой лазерного луча на основе квантово-размерной гетероструктуры с интегрированным распределенным брэгговским зеркалом

AbstractThe characteristics of semiconductor chip modulators providing the angular deflection of a laser beam due to all-electric modulation of the optical characteristics of a semiconductor heterostructure are theoretically studied. Designs for quantum-confined semiconductor waveguide heterostructures with a distributed Bragg reflector integrated on the heterostructure surface are developed. The design of a waveguide structure including 20 periods of coupled asymmetric quantum wells, which provide an optical confinement factor of the waveguide mode of 20%, and an optimized doping profile allowing a uniform electric-field distribution to be maintained in the quantum-well region in the entire range of operating voltages are proposed. In this waveguide structure, the change in the refractive index due to the quantum-confined Stark effect reaches 0.086 when the control signal changes from 0 to 6 V. It is shown that the spatial (angular) scanning of the laser beam emitted from the surface of a distributed Bragg mirror with a divergence of less than 0.1° in the range of 1°–2° is feasible for the proposed waveguide structure design.

Active material, optical mode and cavity impact on nanoscale electro-optic modulation performance

AbstractElectro-optic modulation is a key function in optical data communication and possible future optical compute engines. The performance of modulators intricately depends on the interaction between the actively modulated material and the propagating waveguide mode. While a variety of high-performance modulators have been demonstrated, no comprehensive picture of what factors are most responsible for high performance has emerged so far. Here we report the first systematic and comprehensive analytical and computational investigation for high-performance compact on-chip electro-optic modulators by considering emerging active materials, model considerations and cavity feedback at the nanoscale. We discover that the delicate interplay between the material characteristics and the optical mode properties plays a key role in defining the modulator performance. Based on physical tradeoffs between index modulation, loss, optical confinement factors and slow-light effects, we find that there exist combinations of bias, material and optical mode that yield efficient phase or amplitude modulation with acceptable insertion loss. Furthermore, we show how material properties in the epsilon near zero regime enable reduction of length by as much as by 15 times. Lastly, we introduce and apply a cavity-based electro-optic modulator figure of merit, Δλ/Δα, relating obtainable resonance tuning via phase shifting relative to the incurred losses due to the fundamental Kramers-Kronig relations suggesting optimized device operating regions with optimized modulation-to-loss tradeoffs. This work paves the way for a holistic design rule of electro-optic modulators for high-density on-chip integration.