AlGaAs Waveguides Research Articles

Modal gain characteristics of a two-section InGaAs/GaAs double quantum well passively mode-locked laser with asymmetric waveguide

Monolithic two-section InGaAs/GaAs double quantum well (DQW) passively mode-locked lasers (MLLs) with asymmetric waveguide, consisting of the layers of p-doped AlGaAs waveguide and no-doped InGaAsP waveguide, emitting at ~ 1.06 μm, with a fundamental repetition rate at ~ 19.56 GHz have been demonstrated. Modal gain characteristics, such as a gain bandwidth and a gain peak wavelength of the MLL, as a function of the saturable absorber (SA) bias voltage (Va) as well as the injection current of gain section (Ig), were investigated by the Hakki-Paoli method. With the increase of Va, the lasing wavelength and net modal gain peak of the MLL both exhibited red-shifts to longer wavelength significantly, while the modal gain bandwidth was narrowed. Both the net modal gain bandwidth and gain peak of the MLL followed a polynomial distribution versus the reverse bias at the absorber section. In addition, for the first time, it was found that Va had an obvious effect on the modal gain characteristics of the MLL.

Improvement of the current – voltage performance of broadened asymmetric waveguide InGaAs/AlGaAs/GaAs semiconductor lasers (λ = 940 – 980 nm)

The purpose of this work is to improve the current – voltage (I – V) performance of semiconductor lasers based on broadened asymmetric waveguide InGaAs/AlGaAs/GaAs separate-confinement double heterostructures. We analyse the effect of AlGaAs waveguide layer composition on the output characteristics of the lasers and demonstrate that the decrease in the series resistance of the lasers and the threshold voltage of their I – V characteristic as a result of a decrease in the percentage of AlAs in the waveguide layers shifts the drop in the differential quantum efficiency of the lasers to higher pump currents, despite the decrease in the energy depth of the quantum wells in the active region.

Geometry-dependent two-photon absorption followed by free-carrier absorption in AlGaAs waveguides

Nonlinear absorption can limit the efficiency of nonlinear optical devices. However, it can also be exploited for optical limiting or switching applications. Thus, characterization of nonlinear absorption in photonic devices is imperative for designing useful devices. This work uses the nonlinear transmittance technique to measure the two-photon absorption coefficients ( α 2 ) of AlGaAs waveguides in strip-loaded, nanowire, and half-core geometries in the wavelength range from 1480 to 1560 nm. The highest α 2 values of 2.4, 2.3, and 1.1 c m / G W were measured at 1480 nm for 0.8-µm-wide half-core, 0.6-µm-wide nanowire, and 0.9-µm-wide strip-loaded waveguides, respectively, with α 2 decreasing with increasing wavelength. The free-carrier absorption cross section was also estimated from the nonlinear transmittance data to be around 2.2 × 10 − 16 c m 2 for all three geometries. Our results contribute to a better understanding of the nonlinear absorption in heterostructure waveguides of different cross-sectional geometries. We discuss how the electric field distribution in the different layers of a heterostructure can lead to geometry-dependent effective two-photon absorption coefficients. More specifically, we pinpoint the third-order nonlinear confinement factor as a design parameter to estimate the strength of the effective nonlinear absorption, in addition to tailoring the bandgap energy by varying the material composition.

Highly coherent and multi-octave mid-infrared supercontinuum generations in a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths.

In this paper, a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56, and 8.89 µm. The nonlinearity coefficient of the proposed reverse-strip AlGaAs waveguide is calculated as 2.09W-1m-1 at wavelength 4.9 µm. The effects of pump pulse parameters, waveguide length, and noise coefficient on the nonlinear dynamics of supercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulse with a wavelength of 4.9 µm, peak power of 900 W, and duration of 100 fs is launched into the proposed waveguide and propagated after a 3 mm length, highly coherent and multi-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 µm (more than 2.7 octaves, at -40dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAs waveguide is introduced. Our research results have important applications in MIR photonics, MIR spectroscopy, optical precision measurement, and more.

Internal optical loss and internal quantum efficiency of a high-power GaAs laser operating in the CW mode

We continue the comparative studies of high-power AlGaAs/InGaAs/GaAs semiconductor lasers with different waveguide designs. In this work, measurements were carried out for a continuous-wave mode of operation: light–current and current–voltage characteristics, threshold characteristics and their temperature dependences and crystal temperature depending on the pump current. The data obtained made it possible to reconstruct the dependences of internal optical losses and internal quantum efficiency using calculations. It was shown that the main factor affecting the power characteristics in the continuous mode of operation is the temperature stability of the laser and its relationship with internal optical losses and internal quantum efficiency. Thus, lasers based on a heterostructure with a doped GaAs waveguide with characteristic temperatures T 0 and T 1 of 120 K and 170 K, respectively, demonstrated an increase in internal optical losses to 2 cm−1 and a drop in the quantum efficiency to 90%, which led to saturation of the output power by high pump currents. Lasers based on the heterostructure with an AlGaAs waveguide, which are characterized by higher temperature stability (T 0 = 161 K and T 1 = 280 K), had lower internal optical losses and a higher quantum efficiency. This made it possible to obtain optical power 1 W higher than that for lasers with GaAs waveguides.

Polarization-insensitive reverse-ridge AlGaAs waveguide for the mid-infrared supercontinuum generation

In this paper, a polarization-insensitive reverse-ridge AlGaAs waveguide is designed for the supercontinuum generation. In the considered wavelength range from 3.4 to 8 μm, the dispersion profiles, nonlinear coefficients, and effective mode field areas for the quasi-TE and quasi-TM modes coincide well. The simulation results show that when pump pulse with wavelength of 4.2 μm, peak power of 4.8 kW, and width of 90 fs is launched into the anomalous dispersion region of the quasi-TE and quasi-TM modes of the 3.4-mm-long waveguide, the two generated SCs overlap well. For the quasi-TE and quasi-TM modes, the SCs generated extend from 2.17 to 8.53 μm and 2.23 to 8.61 μm, respectively, spanning more than 1.95 octaves. The proposed reverse-ridge AlGaAs waveguide structure is expected to provide a possible solution for alleviating the undesired polarization effect related to the nonlinear dynamics.

Difference-frequency generation in an AlGaAs Bragg-reflection waveguide using an on-chip electrically-pumped quantum dot laser

Nonlinear frequency conversion is ubiquitous in laser engineering and quantum information technology. A long-standing goal in photonics is to integrate on-chip semiconductor laser sources with nonlinear optical components. Engineering waveguide lasers with spectra that phase-match to nonlinear processes on the same device is a formidable challenge. Here, we demonstrate difference-frequency generation in an AlGaAs Bragg reflection waveguide which incorporates the gain medium for the pump laser in its core. We include quantum dot layers in the waveguide that generate electrically driven laser light at ∼790 nm, and engineer the structure to facilitate nonlinear processes at this wavelength. We perform difference-frequency generation between 1540 nm and 1630 nm using the on-chip laser, which is enabled by the broad modal phase-matching of the AlGaAs waveguide, and measure normalized conversion efficiencies up to ()% W−1 cm−2. Our work demonstrates a pathway towards devices that utilize on-chip active elements and strong optical nonlinearities to enable highly integrated photonic systems-on-chip.

Tunable four-wave mixing in AlGaAs waveguides of three different geometries

The AlGaAs material platform has been intensively used to develop nonlinear photonic devices on-a-chip, thanks to its superior nonlinear optical properties. We propose a new AlGaAs waveguide geometry, called half-core etched, which represents a compromise between two previously studied geometries, namely the nanowire and strip-loaded waveguides, combining their best qualities. We performed tunable four-wave mixing (FWM) experiments in all three of these geometries in the telecommunications C-band (wavelengths around 1550nm), with a pulsed pump beam and a continuous-wave (CW) signal beam. The maximum FWM peak efficiencies achieved in the nanowire, strip-loaded and half-core geometries were about −5dB, −8dB and −9dB, respectively. These values are among the highest reported in AlGaAs waveguides. The signal-to-idler conversion ranges were also remarkable: 161nm for the strip-loaded and half-core waveguides and 152nm for the nanowire. Based on our findings, we conclude that the half-core geometry is an alternative approach to the nanowire geometry, which has been earlier deemed the most efficient geometry, to perform wavelength conversion in the spectral region above the half-bandgap. Moreover, we show that the half-core geometry exhibits fewer issues associated with multiphoton absorption than the nanowire geometry.

Second-Harmonic Generation in Suspended AlGaAs Waveguides: A Comparative Study.

Due to adjustable modal birefringence, suspended AlGaAs optical waveguides with submicron transverse sections can support phase-matched frequency mixing in the whole material transparency range, even close to the material bandgap, by tuning the width-to-height ratio. Furthermore, their single-pass conversion efficiency is potentially huge, thanks to the extreme confinement of the interacting modes in the highly nonlinear and high-refractive-index core, with scattering losses lower than in selectively oxidized or quasi-phase-matched AlGaAs waveguides. Here we compare the performances of two types of suspended waveguides made of this material, designed for second-harmonic generation (SHG) in the telecom range: (a) a nanowire suspended in air by lateral tethers and (b) an ultrathin nanorib, made of a strip lying on a suspended membrane of the same material. Both devices have been fabricated from a 123 nm thick AlGaAs epitaxial layer and tested in terms of SHG efficiency, injection and propagation losses. Our results point out that the nanorib waveguide, which benefits from a far better mechanical robustness, performs comparably to the fully suspended nanowire and is well-suited for liquid sensing applications.

Polarization-resolved strong light–matter coupling in planar GaAs/AlGaAs waveguides

We study the influence of optical selection rules and polarization splittings on properties of exciton polaritons in a planar AlGaAs waveguide containing embedded GaAs quantum wells. We demonstrate that transverse electric and transverse magnetic modes couple differently with light- and heavy-hole quantum well excitons, which leads to distinct polarization splittings of the resulting polariton modes. The experimental data are in good agreement with modeling based on theoretical data for the optical selection rules for quantum well excitons.

On-chip III-V monolithic integration of heralded single photon sources and beamsplitters

We demonstrate a monolithic III-V photonic circuit combining a heralded single photon source with a beamsplitter, at room temperature and telecom wavelength. Pulsed parametric down-conversion in an AlGaAs waveguide generates counterpropagating photons, one of which is used to herald the injection of its twin into the beamsplitter. We use this configuration to implement an integrated Hanbury-Brown and Twiss experiment, yielding a heralded second-order correlation gher(2)(0)=0.10±0.02 that confirms single-photon operation. The demonstrated generation and manipulation of quantum states on a single III-V semiconductor chip opens promising avenues towards real-world applications in quantum information.

Directionally induced quasi-phase matching in homogeneous AlGaAs waveguides.

We report on the experimental observation of quasi-phase matching in a homogeneous waveguide. By fabricating a monolithic snake-shaped suspended AlGaAs nanowire on a (001) GaAs wafer, we demonstrate the unraveled version of a χ(2) whispering-gallery-mode microdisk, obtaining second-harmonic generation in the optical telecom wavelength range. With a radius of curvature of 50μm and four spatial oscillations along the (110) average direction, a splitting of the second-harmonic spectrum occurs around the phase-matching wavelength of the corresponding straight waveguide. This splitting, which increases as the radius of curvature decreases, provides a useful degree of freedom for the design of small-footprint nonlinear photonic devices on-chip.

Design of zigzag folded inversion-stacked AlGaAs waveguides for ultra-compact wavelength converters.

We have proposed and designed zigzag-folded U-shaped waveguides to realize highly efficient wavelength conversion with an extremely small footprint. Extreme high efficiencies are achieved with a combination of modal phase matching and 4¯ quasi phase matching in inversion-stacked AlGaAs/Alox waveguides. Numerical simulations reveal that the conversion efficiency of second harmonic generation pumped at 1.55 μm as high as 12000 %W-1 can be achieved in an 8.0-mm-long AlGaAs/Alox waveguide that is folded up into a small domain of 0.8×0.6 mm2 area. A phase matching signal bandwidth of difference frequency generation, 29 nm, covers 83 % of the telecommunication C band.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.

Enhanced continuous-wave four-wave mixing efficiency in nonlinear AlGaAs waveguides.

Enhancements of the continuous-wave four-wave mixing conversion efficiency and bandwidth are accomplished through the application of plasma-assisted photoresist reflow to reduce the sidewall roughness of sub-square-micron-modal area waveguides. Nonlinear AlGaAs optical waveguides with a propagation loss of 0.56 dB/cm demonstrate continuous-wave four-wave mixing conversion efficiency of -7.8 dB. Narrow waveguides that are fabricated with engineered processing produce waveguides with uncoated sidewalls and anti-reflection coatings that show group velocity dispersion of +0.22 ps²/m. Waveguides that are 5-mm long demonstrate broadband four-wave mixing conversion efficiencies with a half-width 3-dB bandwidth of 63.8-nm.

High-visibility two-photon interference of frequency-time entangled photons generated in a quasi-phase-matched AlGaAs waveguide.

We demonstrate experimentally the frequency-time entanglement of photon pairs produced in a CW-pumped quasi-phased-matched AlGaAs superlattice waveguide. A visibility of 96.0±0.7% without background subtraction has been achieved, which corresponds to the violation of the Bell inequality by 52 standard deviations.

Efficient continuous-wave four-wave mixing in bandgap-engineered AlGaAs waveguides.

We present a side-by-side comparison of the nonlinear behavior of four passive AlGaAs ridge waveguides where the bandgap energy of the core layers ranges from 1.60 to 1.79eV. By engineering the bandgap to suppress two-photon absorption, minimizing the linear loss, and minimizing the mode area, we achieve efficient wavelength conversion in the C-band via partially degenerate four-wave mixing with a continuous-wave pump. The observed conversion efficiency [Idler(OUT)/Signal(IN)=-6.8 dB] is among the highest reported in passive semiconductor or glass waveguides.

Nonlinear properties of AlGaAs waveguides in continuous wave operation regime

Aluminum Gallium Arsenide (AlGaAs) is an attractive platform for the development of integrated optical circuits for all-optical signal processing thanks to its large nonlinear coefficients in the 1.55-μm telecommunication spectral region. In this paper we discuss the results of the nonlinear continuous-wave optical characterization of AlGaAs waveguides at a wavelength of 1.55 μm. We also report the highest value ever reported in the literature for the real part of the nonlinear coefficient in this material (Re(γ) ≈521 W(-1)m(-1)).

Quantum Dot parametric source

We present the modeling of an active nonlinear source designed for spontaneous parametric down-conversion in an AlGaAs waveguide. Electrically pumped InAs Quantum Dots embedded in the waveguide core provide the gain necessary for the lasing of a second-order pump mode at about 980nm, which in turns generates two cross-polarized fundamental signal and idler modes with wavelengths around 1960nm. We numerically evaluate the source efficiency and highlight the advantages provided by our choice of active medium for the fabrication of the device.

Polarization-Insensitive Wavelength Conversion by FWM of 100-GHz-Spaced DWDM 4$\,\times\,$10 Gb/s RZ-BPSK Signals in a Birefringent Nonlinear AlGaAs Waveguide

Polarization-insensitive (PI) wavelength conversion of 100-GHz-spaced wavelength-division multiplexed 4 × 10 Gb/s return-to-zero binary phase-shift keyed (RZ-BPSK) signals was successfully demonstrated utilizing four-wave mixing for the first time in a passive, birefringent nonlinear AlGaAs waveguide. In addition to a pump launch angle adjustment, a minimum pump-probe detuning was also required to mitigate polarization-induced power fluctuations in the probe. The PI-wavelength-converted signals all demonstrated a pre-amplified receiver sensitivity penalty of less than ≈ 2 dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> bit-error-rate, relative to their respective baseline RZ-BPSK signals.