The fabrication of optical filters whose reflection/transmission response is spatially-graded has been the object of numerous research studies over the past decades given their applications in areas including multi- and hyperspectral imaging, structural colouring and even holographic encryption. In this context, the key enabling feature is the ability to tailor the thickness profile of at least one layer of the optical coating multilayer stack. To-date, this 3-dimensional structuration has been achieved either at the deposition stage or as an additional post-deposition process step. In the former case, the technique relies on the shaping of the material deposition flux thanks to the insertion of a (moving) mask inside the evaporation or sputtering machine. As such, the method is usually limited to the implementation of centimetre-scale variations. A contrario, to reach sub-millimeter-scale features, the preferred approach is based on postdeposition layer structuration, which is performed using grayscale lithography in the form of multi-(mask-)level optical lithography, or using e-beam or laser lithography. All these approaches are nevertheless relatively complex since they involve either multiple steps or need a very precise calibration of the exposition curve. In this paper, we report that the evaporation through re-usable shadow masks can be used to create optical filters whose spatial variations can be controlled with a ~70-µm-resolution. Using metal-mirror Fabry-Pérot interferometer structures as representative optical filters, we demonstrate the ability to adjust the resonance wavelength, the filter bandwidth and extinction ratio, and the coupling strength and splitting in cascaded resonators.
Guided Mode Resonant Filters (GMRFs) have long been studied as a support surface for nonlinear optical interactions due to their intrinsically high Q-factor. However, their operation relies on a non-localized and large-area guided mode that limits the achievable power density and requires complex phase-matching approaches. Conversely, photonic crystal nano-cavities have shown promising results due to both their high-Q factors to enhance the pump field and their localized nature that allows phase-matching-free implementation and high power density excitation. However, their intrinsic small size restricts the supported input power and hinders the coupling efficiency of the pump into the mode. In this paper, we report the first experimental demonstration of continuous-wave second harmonic generation in a Cavity-Resonator Integrated Grating Filter (CRIGF). This intermediary device, which can be described as a cavity-enhanced finite-size GMRF or, equivalently, as a low-index-contrast photonic crystal micro-cavity, will be shown to offer a practical route to nonlinear interactions with viable power (<20 mW) and excitation conditions (surface excitation with a ~10-μm-waist spot size). In practice, the devices under study make use of a lithium-niobate on insulator (LNOI) waveguide with a nanostructured silicon nitride upper cladding as a pragmatic way to implement a high second-order nonlinearity platform with established processing technology. The already-demonstrated versatility of the CRIGF design (demonstrated at wavelengths of 850 nm using S3iN4/SiO2 platform, 4.5 μm with the GaAs/AlGaAs technology and, here, at 1.55 μm with the LiNbO3 platform) coupled to the electro-optical tuning afforded by lithium niobate system makes this approach extremely promising for pixelated non-linear systems.
Numerical studies on Kerr frequency comb generation with vertically-coupled whispering-gallery-mode (WGM) Si3N4 resonators are presented. These resonators include a frequency-dependent access coupler and are characterized by a free spectral range (FSR) of 220 GHz. We present numerical simulations based on the Ikeda map that allows implementation of complex-valued frequency-dependent and non-reciprocal access coupler transfer matrix in the simulation of Kerr comb in the cavities modelled by Arlotti et al. We use a Runge-Kutta 4 Interaction picture (RK4IP) method with adaptive step-size control as developed by Balac et al. to circumvent the numerical burden added by this modelling approach and successfully simulate Kerr comb generation using an approach that accurately models any optical cavity that can be considered as spatially one-dimensional regardless of its quality factor, finesse or dispersive properties which comes in useful in this study when access coupling properties degrade the resonator quality factor.
VCSELs became dominant laser sources in many short optical link applications such as datacenter, active cables, etc. Actual standards and commercialized VCSEL are providing 25 Gb/s data rates, but new solutions are expected to settle the next device generation enabling 100 Gb/s. Directly modulated VCSEL have been extensively studied and improved to reach bandwidths in the range of 26-32 GHz [Chalmers, TU Berlin], however at the price of increased applied current and thus reduced device lifetime. Furthermore, the relaxation oscillation limit still subsists with this solution. Thus, splitting the emission and the modulation functions as done with DFB lasers is a very promising alternative [TI-Tech, TU Berlin]. Here, we study the vertical integration of an ElectroAbsorption Modulator (EAM) within a VCSEL, where the output light of the VCSEL is modulated through the EAM section. In our original design, we finely optimized the EAM design to maximize the modulation depth by implementing perturbative Quantum Confined Stark Effect (QCSE) calculations, while designing the vertical integration of the EAM without penalty on the VCSEL static performances. We will present the different fabricated vertical structures, as well as the experimental electrical and optical static measurements for those configurations demonstrating a very good agreement with the reflectivity and absorption simulations obtained for both the VCSEL and the EAM-VCSEL structures. Finally, to reach very high frequency modulation we studied the BCB electrical properties up to 110 GHz and investigated coplanar and microstrip lines access to decrease both the parasitic capacitance and the influence of the substrate.
Over the last two decades, integrated whispering-gallery-mode resonators have been increasingly used as the basic building blocks for selective filters, high-sensitivity sensors, and as nonlinear converters. In the latter two cases, optimum performance is achieved when the intra-cavity power or the resonance feature contrast are maximum. For devices with transversely singlemode resonator and access waveguides, the above-mentioned conditions are obtained when the system is critically coupled i.e. when the coupler power transfer rate corresponds to the single-pass intra-cavity loss. Designing coupled resonators for which critical-coupling is maintained over a large spectral range is therefore attractive to facilitate sensing or nonlinear frequency conversion.
In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si3N4/SiO2 racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.
In this article, we apply the coupled-mode theory to vertically-coupled micro-disk resonators presenting an asymmetric distribution of refractive index and a multilayer separation region between the two waveguide cores, resulting in an effective propagation constant phase-mismatch in the coupling region. We introduce a criterion which, given the coupler overall permittivity distribution, clarifies how to best choose the individual decomposition index profiles among the various possible solutions. Following our recent experimental demonstration we subsequently exploit the derived decomposition to evaluate the theoretical transmission characteristics of an AlGaAs/AlOx-based structure as function of wavelength and as function of the position of the resonator relative to the access waveguide.We show that the resonant dips of the intensity transmission, spaced by the cavity FSR, are modulated by an envelop which governs the coupling regime of the resonator-waveguide system.
Integrated whispering-gallery mode resonators are attractive devices which have found applications as selective filters, low-threshold lasers, high-speed modulators, high-sensitivity sensors and even as nonlinear converters. Their performance is governed by the level of detrimental (scattering, bulk, bending) loss incurred and the usable loss represented by the coupling rate between the resonator and its access waveguide. Practically, the latter parameter can be more accurately controlled when the resonator lies above the access waveguide, in other words, when the device uses a vertical integration scheme. So far, when using such an integration technique, the process involved a rather technically challenging step being either a planarization or a substrate transfer step. In this presentation, we propose and demonstrate an alternative method to fabricate vertically-coupled whispering-gallery mode resonators on III-V semiconductor epitaxial structures which has the benefit of being planarization-free and performed as single-side top-down process. The approach relies on a selective lateral thermal oxidation of aluminum-rich AlGaAs layers to define the buried access waveguide and enhance the vertical confinement of the whispering-gallery mode into the resonator. As a first experimental proof-of-principle of this approach, 75 µm-diameter micro-disk devices exhibiting quality factor reaching ~4500 have been successfully made.
The ability to control the temporal output from a femtosecond laser can enable the same laser to be used for multiple
functions, for example, the laser used in an optical tweezers system could be used as a constant-intensity source to trap a
biological cell and then be temporarily switched to mode-locked operation to effect photoporation. Here, we report the
rapid switching of a Cr4+:forsterite laser between mode-locked and unmode-locked continuous wave (CW) regimes via
the optical pumping of an intracavity SESAM element. Mode-locking of the laser was initiated by an intracavity
quantum well (GaInAsN) SESAM having an anti-resonant design (ΔR~0.3%, λPL~1310nm) that yielded transform-limited
89fs pulses centered around 1296nm with a repetition rate of 162MHz at an average power of 64mW. Upon
excitation of the SESAM with 600mW of extra-cavity power from an 808nm semiconductor diode laser, switching could
be induced between the unmode-locked and mode-locked regimes. Transitions free of Q-switching or relaxation
oscillations were observed with <200μs switching times for both for the initiation and cessation of mode-locking.
Periods of mode-locked operation of custom duration could be produced by appropriate control of the SESAM pump
diode enabling the generation of bursts of pulses as short as 400μs. Switching was confirmed to originate from local
pump-induced heating of the SESAM by observing the laser going through identical regime switching when the chip
temperature of the 'unpumped' SESAM was raised by ~20°C.
We report the performance and characteristics of a GaInNAs-based tunable fibre vertical-cavity surface-emitting laser
emitting at a wavelength of ~1200nm which may find application as a source in gasoline octane detection. The device,
which is pumped through the fibre by 810nm diodes, can emit more than 0.3mW of output power and is tunable over
17nm around 1197nm.
We report the first use of a Semiconductor Disk Laser (SDL) as a pump source for ~2μm-emitting Tm3+ (,Ho3+)-doped
dielectric lasers. The ~1213nm GaInNAs/GaAs SDL produces >1W of CW output power, a maximum power transfer net
slope efficiency of 18.5%, and a full width half maximum wavelength tuning range of ~24nm. Free-running operation of
a Tm3+-doped tellurite glass laser under 1213nm SDL pumping generated up to 60mW output power with 22.4% slope
efficiency. Wavelength tunable output is also obtained from 1845 to 2043nm. Improved performance with output powers
of ~200mW and a slope efficiency of ~35% are achieved by replacing the Tm3+-doped glass with a Tm3+-doped KYW
active medium. Emission of a Tm3+,Ho3+-codoped tellurite glass laser is also reported with maximum output power of
~12mW and a ~7% slope efficiency. Finally, preliminary investigations of 1213nm-pumping of a Tm3+,Ho3+-codoped
silica fibre laser lead to 36mW output power with ~19.3% slope efficiency.
Semiconductor Disk Lasers (SDLs) are compact lasers suitable for watt to multi-watt direct generation in the 670-
2350nm waveband and frequency-doubled operation in the ultraviolet and visible regions. This is, however, critically
dependent on the thermal management strategy used as, in this type of laser, the pump is absorbed over micrometer
lengths and the gain and loss are temperature sensitive. In this paper, we compare the two heat dissipation techniques that
have been successfully deployed to-date: the "thin device" approach where the semiconductor active mirror is bonded
onto a heatsink and its substrate subsequently removed, and the "heatspreader" technique where a high thermal
conductivity platelet is directly bonded onto the active part of the unprocessed epilayer. We show that for SDLs emitting
at 1060nm with pump spots of ~80µm diameter, the heatspreader approach outperforms the thin-device alternative, with
the best results being obtained with a diamond heatspreader. Indeed, the thermal resistances are measured to be 4.9, 10.4
and 13.0 K/W for diamond-bonded, SiC-bonded and flip-chip devices respectively. It is also observed, as expected, that
the thermal management strategy indirectly affects the optimum output coupling and thus the overall performance of
these lasers.
A detailed study of the high-power pulsed operation of C-band optically-pumped GaInNAsSb vertical cavity surface emitting lasers is reported.
The devices employ a resonant periodic gain structure grown by molecular beam epitaxy on a GaAs substrate with a 31-pair GaAs/AlAs bottom
distributed Bragg reflector and a 4-λ,
GaAs-based resonant cavity containing 10 GaInNAsSb quantum wells distributed among the 7 antinodes of the electric field.
A dual-pump-band SiO2/TiO2 dielectric top mirror allows efficient optical pumping via low reflectivities at 808nm and 1064nm
while providing very high reflectivity at the 1.55μm target emission wavelength. The laser characteristics were evaluated using both a Q-switched Nd:YAG
1064nm pump and a 20W-peak 180ns-pulsed 850nm diode laser. The importance of the gain-cavity detuning was evident from time-dependent spectral
measurements of laser material
subjected to post-growth annealing at different temperatures between 725 and 775°C. The highest annealing temperature produces the largest blue shift of the
gain peak relative to the cavity resonance, resulting in the best power transfer characteristics as well as reduced temperature sensitivity.
High thermal conductivity intra-cavity crystalline heatspreaders are used to control the pump-induced temperature increase limiting the power scaling of vertical external-cavity surface-emitting lasers (VECSELs). Pump-power-limited output of greater than 0.4 W was achieved from a GaAs-based VECSEL at room temperature with the use of a silicon carbide heatspreader bonded to the surface of the gain element and 0.5 W by water-cooling the system to 7.5°C.
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