Search

To achieve on-chip lasers for Si-photonics, monolithic integration using selective epitaxy is a favorable option due to the unique defect engineering and resultant bufferless structure. Among the intensively investigated selective epitaxy methods, lateral aspect ratio trapping generates III-V devices in the same plane as the Si layer enabling efficient coupling with Si waveguides. Here, we demonstrate telecom InGaAs/InP quantum well lasers selectively grown on commercial silicon-on-insulator substrates with an in-plane structure using a simple structural design and growth scheme without any regrowth steps. Leveraging on the lateral aspect ratio trapping technique, uniform quantum wells with high crystalline quality were obtained and shown by comprehensive material characterizations. Room temperature pulsed lasing was achieved on the fabricated micro-ring lasers with a low threshold of 20 μJ/cm2. The results present a crucial step towards electrically pumped lasers at telecom band as well as fully integrated Si-photonics. [ABSTRACT FROM AUTHOR]

We report a wavelength widely-tunable hybrid integrated external cavity laser (ECL) with synchronous tuning to achieve a large frequency chirp bandwidth and high chirp linearity. Si3N4 Vernier microring resonators (MRRs) in the ECL function as an optical bandpass filter to offer strong wavelength selectivity and fast frequency tunability. The wavelength tuning range is 56 nm, the maximum output optical power is 22.5 mW, the minimum intrinsic linewidth is 2.9 kHz, and the static frequency sweep bandwidth is 56 GHz. A frequency-modulated continuous-wave (FMCW) signal is generated under synchronous modulation with a chirp bandwidth of 7.68 GHz at a 1 kHz repetition rate, much larger than that produced by modulating the phase shifter or the Vernier filter alone. A chirp linearization method based on a non-uniform sampling algorithm and an iterative learning algorithm is employed to reduce the chirp nonlinearity to 1.02×10-7, allowing for a detection resolution of 2.47 cm. [ABSTRACT FROM AUTHOR]

Whispering gallery mode (WGM) microcavities can efficiently store and manipulate light with strong light confinement and long photon lifetime, while coupling light into and from WGMs is intrinsically hindered by their unique feature of rotational symmetry. Here, a scattering-assisted liquid crystal (LC) micropillar WGM laser is proposed. WGM lasing at the surface of the micropillar is obviously enhanced by fluorescence scattering in the core of the micropillar. Besides, weak scattering of LC molecules also builds efficient coupling channels between the laser modes and the axial transmission modes of the micropillar-based waveguide, providing an all-in-one liquid WGM laser with functions of self-seeding and self-guiding. Furthermore, based on the hysteresis characteristics of the electrically anchored LC molecules under the interaction of thermal force, an erasable read-write liquid memory device is proposed, paving the way for the application of logic-controllable WGM lasers in optical storage and optical control. [ABSTRACT FROM AUTHOR]

A miniaturepolymer Bragg grating (PBG) sensor is fabricated and employed for ultrasonic imaging of seismic physical models (SPMs). The sensing Bragg grating is inscribed into an ultraviolet (UV) glue polymer waveguide with a femtosecond laser. The uniform polymer waveguide is fabricated by sealing the UV glue into a capillary fibre through capillary effect. By using line-by-line inscription technique, the laser beam is scanned transversely to periodically modulate the refractive index of the polymer waveguide, leading to the formation of the PBGs with various grating lengths. The sensor response to ultrasonic waves are investigated experimentally. When compared to single-mode fiber Bragg grating and phase-shifted fiber Bragg grating, the PBG with the same reflectivity presents a higher response amplitude due to its lower Young’s modulus. Besides, the sensor has a good spectral stability when transferred from air to water due to the waterproof coating on the sensor end. Finally, the sensor is used to scan a large-scale 3D SPM and the structural features, such as fault, fluctuation, and depositional termination, can be distinctly reconstructed. The proposed PBG sensor provides a new technique with easy fabrication, high sensitivity, and good stability for high-fidelity ultrasonic imaging of seismicphysical models. [ABSTRACT FROM AUTHOR]

Lithium niobate (LiNbO3, LN) on insulator (LNOI) is a promising material platform for integrated photonics to realize high-capacity and high-speed information communication. Here, we developed the 515 nm femtosecond laser micro-machining technique with a 316 nm processing accuracy and demonstrated the fabrication of monolithically integrated waveguide-coupled micro-resonators with the quality factors (Q) of $1.8\times 10^{5}$ using 515 nm femtosecond laser ablation assisted ions beam etching (IBE) process. The chemical–mechanical polishing (CMP) is adopted to smoothing the etched edge of the hard mask patterned by femtosecond laser ablation. These fabrication techniques are compatible with high efficiency and high precision wafer-scale production, without counting on the UV/E-beam lithography technique, which is essential for LN photonic devices moving towards industrial applications. [ABSTRACT FROM AUTHOR]

In order to stabilize the lateral far-field profile of a broad area laser diode at high pump currents, the design of an asymmetric InGaAs/AlGaAs/GaAs laser heterostructure was developed. Lasers with an aperture formed by ten optically uncoupled ridge waveguides (each 6.5 μm wide) were developed on its basis. To maintain the single-mode operation of each ridge waveguide without coupling between emitters, a High Dense Microstripe Array (HDMSA, the pitch is 13.5 μm) and a Low Dense Microstripe Array (LDMSA, the pitch is 40 μm) were fabricated with a coupling parameter between adjacent lateral waveguides of 2·10−5 and 3·10−19, respectively. The maximum optical output power was 5 W (HDMSA) in CW mode and 25 W (HDMSA) and 26 W (LDMSA) in pulsed mode (130ns/1kHz). It is shown that in both CW and pulsed modes, lasers of both designs provide a stable Gaussian-like distribution in the lateral far field (LFF) with a FWHM of 8–11 degrees for HDMSA and 6.5–10 degrees for LDMSA. It was found that the LFF broadens with a pump current in the range of low currents and stabilizes in the range of high currents for both designs. Studies of the near field in both operating modes at any pump currents showed no array modes and the uncoupling mode is maintained for ridge waveguide arrays. [ABSTRACT FROM AUTHOR]

Structured light, in particular light possessing orbital angular momentum (OAM), has been actively studied in recent decades. Helical Bragg grating (HBG) is a reflecting optical element, which predicted to be able to convert the OAM of light in waveguides and fibers. However, the HBG has not been demonstrated experimentally yet. Here we report the first experimental manifestation of HBG created in the form of a waveguide with depressed-index cladding. Few-mode channel helical-depressed-cladding waveguides (HDCWs) have been written in a YAG crystal by a femtosecond laser beam. The HDCW pitches were comparable to near-IR wavelengths. The spectral and polarization characteristics of the transmitted and reflected light have been studied. It was shown that HDCWs behave like HBGs providing narrowband resonance coupling of the counter propagating modes, the OAM of the modes differing by the Bragg order or, what is the same, the topological charge of the helical cladding structure. Adjusting the HDCW parameters allows one to excite a reflected wave at the predetermined wavelength and with the predetermined OAM change with respect to the light coupled to the waveguide. Thus, for the first time, the HBGs suitable for the generation and filtration of the vortex light have been experimentally demonstrated. The experimental results obtained agree well with the coupled mode theory calculations. [ABSTRACT FROM AUTHOR]

A whispering-gallery-mode (WGM) resonator with an L-shaped microcavity for humidity sensing is proposed and demonstrated. The L-shaped microcavity is fabricated by using femtosecond laser micromachining, fusion discharge and chemical etching methods. By placing two microspheres in the microcavity, of which one is used as the whispering gallery mode resonant cavity, and the other plays the role of changing the optical path to help activating whispering gallery mode resonance. The device has a linear response to humidity with the linearity of 99.1% and the sensitivity of −0.23 dB/%RH, within the relative humidity range of 30%–70%, under the room temperature of 25 °C. The response time for the humidity increasing is as short as 240 ms, and the recovery time is 453 ms. The device is simple in fabrication, robust and has good operation stability. [ABSTRACT FROM AUTHOR]

This paper describes the design, fabrication, and record performance of a new class of ultra-wideband wavelength tuning, ultra-low noise semiconductor laser, the Integrated Coherent Tunable Laser (ICTL). The ICTL device is designed for, and fabricated in, a CMOS foundry based Silicon Photonics platform, utilizing heterogeneous integration of III-V material to create the integrated gain section of the laser–enabling high-volume mass-market manufacturing at low cost and with high reliability. The ICTL incorporates three or more ultra-low loss micro-ring resonators, with large ring size, in a Sagnac loop reflector geometry, creating exceptional laser reflector performance, plus an extended laser cavity length that enables highly-coherent output; ultra-low linewidth and phase noise. This paper describes record integrated laser performance; 118 nm wavelength tuning, covering S-, C- and L-bands, with Lorentzian linewidth <100 Hz, and with excellent relative intensity noise (RIN) of ≤ −155 dBc/Hz. The remarkable performance of the ICTL device, coupled with the high volume/low cost capability of the Silicon Photonics platform enables next-generation applications including ultra-wideband WDM transmission systems, fiber-optic and medical-wearable sensing systems, and automotive FMCW LiDAR systems utilizing wavelength scanning. [ABSTRACT FROM AUTHOR]

Compared to fiber-based equivalents, CMOS-compatible silicon-based nonlinear optical devices harness the superior features of lower operating powers, precise dispersion control and extensive transparency range. In this article, we propose a scheme of cascaded soliton compression on CMOS-compatible Silicon Nanophotonic Wire Waveguides in the mid-infrared region which includes the wavelength of 2.0 μm and 2.8 μm. Thanks to the cascaded pulse compression mechanism, we demonstrate the large compression factor on an integrated chip-scale waveguide. We can realize a high degree pulse compression that the pulse width decreases from 1 ps to 24.0 fs with a high compression factor of 41.7 and a low operating peak power of 1.56 W at 2.0 μm. In addition, even as the wavelength migrate to 2.8 μm, our scheme is equally effective. We achieve 31.9 compression factor of 1 ps chirp-free higher-order soliton pulse which can be compressed to 31.3 fs with a small initial peak power of 4.87 W. This research indicates the potential of dispersion engineering waveguides for mid-infrared low-power applications, such as ultrafast integrated lasers, chip-scale frequency-comb generation, minimally invasive skin surgeries and molecular spectroscopy. [ABSTRACT FROM AUTHOR]

Biosensors employing photonics integrated circuits, and specifically those that rely on interferometric evanescent wave working principles, have outstanding performances due to the extreme sensitivity exhibited in one-step and direct assay, without the need of amplification. Within the interferometric configurations, the Bimodal Waveguide (BiMW) interferometric sensor stands out due to its demonstrated sensitivity for real-life applications and the simplicity of its design. To overcome the ambiguities that arise from the periodic nature of interferometric read-outs, a new all-optical modulation and the subsequent trigonometry-based algorithm have been proposed and applied to the BiMW biosensor. This new algorithm has been successfully employed for the selective identification and quantification of the external Spike (S) protein of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Our biosensing results from this simple, quick, and user-friendly method demonstrate high sensitivity and specificity and pave the way towards a point-of-care device for general use. [ABSTRACT FROM AUTHOR]

Modal absorptions in laser-like heterostructures containing InAs self-assembled quantum dots (QDs) and InGaAs quantum well-dots (QWDs) have been studied. The evaluation of photoresponse as a function of waveguide length has allowed us to determine per-layer modal absorptions of 69 and 13 cm-1 for the ground state optical transitions of QWDs and QDs, respectively. The values of the modal absorption can be used as a measure of the maximal (saturated) modal gain. To compare quantum heterostructures with different dimensionality we have introduced the layer gain constant, a parameter characterizing the light transmittance through the absorbing or gaining layer. We have shown that the QWD layer gain constant significantly exceeds quantum well and quantum dot ones. [ABSTRACT FROM AUTHOR]

The light-induced self-written (LISW) waveguide technique is a promising candidate for the practical realization of a passive alignment between telecommunication and silicon photonic devices. While silicon is transparent to wavelengths above 1100 nm, the one-photon polymerization wavelength range is limited below 1100 nm. In this study, LISW waveguide fabrication at telecommunication wavelengths was demonstrated using a single-photon photopolymerizing resin. The LISW waveguide fabrication threshold values are lower than that of the two-photon photopolymerization system by six to seven orders of magnitude. LISW waveguides are fabricated from a single-mode fiber, a vertical-cavity surface-emitting laser, and a silicon waveguide. Using this technique, fiber-to-chip self-coupling between silicon photonic devices at telecommunication wavelengths is demonstrated. The results demonstrate the potential of the LISW technique for optical self-coupling of telecommunication and silicon photonic devices. [ABSTRACT FROM AUTHOR]

An optical fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with long cavity length is demonstrated. The device is fabricated by using femtosecond laser to inscribe a waveguide structure in the optical fiber core, followed by discharging the waveguide area with a fusion splicer. The inner air-cavity structure is highly robust, and the relatively long cavity length corresponds to a small free spectral range, which makes it possible to implement accurate measurement. Such an inner air-cavity device fabricated in single mode fiber has good high temperature sustainability. The device is also “open” to the external environment when being fabricated in multimode fiber, thus supporting refractive index measurement. The proposed device is attractive in many optical fiber sensing applications. [ABSTRACT FROM AUTHOR]

A new methodology to completely characterize active multicore waveguides is proposed. The method is applied to two-core waveguides written in an Er3+/Yb3+ co-doped phosphate glass by femtosecond laser inscription. Determination of the core diameter and refractive index variation is based on asymmetric excitation of the dual core waveguide and measurements of the intensity output distribution. Moreover, a single core waveguide has also been written for characterization purposes and signal enhancement measurements carried out on it have allowed the determination of the glass active properties. Finally, enhancement measurements on the two-core waveguides for different pump powers and core-to-core separations have been performed, together with simulations of the experiments using the parameters determined in the characterization procedure. The good agreement between measurements and simulations supports the validity of the characterization method presented. The proposed characterization technique is suitable for any multicore symmetric structure regardless the fabrication method and the number of cores, allowing the benefits of multi-core fibre designs to be transferred to integrated active structures. [ABSTRACT FROM AUTHOR]

We present novel waveguide writing concepts in bulk PMMA. The writing relies on laser induced modification tracks that are completely surrounding a waveguide core. We found the optimal parameters to construct highly reproducible, single-mode waveguides with minimal propagation losses down to 0.6 dB  cm−1. Employing the best geometry, we demonstrate 2D and 3D Y-splitters that are the building blocks for creating complex optical networks such as sensors or lab-on-chip devices in polymer materials. [ABSTRACT FROM AUTHOR]

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C. [ABSTRACT FROM AUTHOR]

Nonlinear dynamics of mutually injected twin-microring semiconductor lasers are studied numerically and experimentally. For the twin-microring laser, a bridge waveguide directly connecting two identical microrings is utilized to realize mutual optical injection or mode coupling. The coupling coefficient is related to the waveguide parameter and influences the mutual injection process dramatically. By adjusting the frequency offset between two microrings, four-wave mixing, injection locking, periodic, and multi-period oscillation states are observed from the experimental lasing spectra. The plentiful nonlinear dynamical states are induced by the mode interaction between the two microrings and can be well explained by the rate equations with mutual optical injection. [ABSTRACT FROM AUTHOR]

In recent years, femtosecond laser direct writing technology has made great progress in integrated quantum photonic circuits due to its advantages of applicability to an extensive range of materials and true 3-D ability. However, a waveguide prepared via a femtosecond laser shows a non-circular cross section and high birefringence, which seriously restricts the development of polarization encoded quantum photonic integrated chips towards greater integration and higher functions. Here, we propose an amplitude-phase double shaping method to reduce waveguide birefringence, which involves using a cylindrical lens and slits. A waveguide prepared using this method shows a circularity cross-section that reaches up to 97.6% and a birefringence that is as low as 1.49 × 10−6. Based on such an ultralow birefringence waveguide, a polarization independent Hong-Ou-Mandel quantum interferometer was fabricated. Interference visibilities for identical photon pairs in different polarization states were found to be more than 95% with a standard deviation of 0.6%. The ultra-low birefringence single-mode waveguide and the polarization-independent interferometer realized using this approach will play an important role in large-scale polarization encoding integrated quantum photonic chips. [ABSTRACT FROM AUTHOR]

Quartz enhanced photo acoustic spectroscopy (QEPAS) has gained a growing interest in recent years for gas sensing technology because of the high sensitivity provided by sharp resonant tuning forks (QTFs) exploited as piezoelectric sound wave detectors; and the modularity, compactness, and robustness of the sensors. The currently used experimental configurations of the QEPAS sensors still rely on free space optics, which are potentially subject to misalignment and require a relatively high space occupation. Here we propose the modelling and the design of a QEPAS sensor with the laser source and optical components for beam delivery bonded together and semi-integrated with the tuning fork. In particular, we propose a configuration in which an optical resonator placed between the prongs of the QTF is coupled with the laser source through a waveguide (OMRSI-QEPAS). For this setup design, COMSOL simulations provided pressure values of the acoustic wavefront comparable with the standard on-beam configuration employing acoustic resonator tubes. Therefore, the design here proposed aims to package the opto-acoustic core of a QEPAS sensor in a single module of few cubic centimeters to definitively address the misalignment issues and pave the way to a further level of miniaturization, integration and deployment for application like mobile and on-drone sensing. [ABSTRACT FROM AUTHOR]

We report on the design and fabrication of straight and S-curved waveguides in neodymium-doped yttrium aluminate (Nd:YAP) crystal by using direct femtosecond laser writing. These S-curved channel waveguides are based on the hexagonal optical-lattice-like and depressed cladding geometry. For each type waveguide, S-curves with three lateral offsets of 50 μm, 100 μm, and 150 μm are implemented. These waveguides are with good guiding properties and low bending losses. With S-curved waveguide as laser cavity, dual-wavelength, 31.6 GHz waveguide laser operating at 1064 nm and 1079 nm have been demonstrated based on MoS2 as a saturable absorber. This work paves the way to develop new on-chip ultrafast laser sources based on femtosecond laser inscribed S-curved waveguides. [ABSTRACT FROM AUTHOR]

Here in we present the fabrication and performance of waveguide tapers produced by femtosecond laser induced element redistribution in modified phosphate glasses. More particularly, it is demonstrated that by controlling the scan velocity during the writing process it is possible to adequately tune both the size of the modified area and the refractive index contrast to produce waveguides that can cope with mode field diameters in the range of 7–16 μm. In addition, we fabricated tapered structures through the induction of an acceleration in the laser scanning velocity, resulting in a device that can efficiently convert a wide range of mode fields. The fine control achieved over the index contrast in the range of 10−3 and 10−2 would allow the production of a wide variety of tapers that could potentially be used to couple numerous photonic devices. [ABSTRACT FROM AUTHOR]

A novel bending sensor based on an in-fiber Mach–Zehnder interferometer (MZI) has been fabricated by femtosecond laser. The MZI is formed by an off-axial straight waveguide inscribed in a no-core fiber spliced between two standard single mode fibers. The in-fiber MZI is formed due to the interference of a fundamental mode and a higher-order mode which excited in the no-core fiber. Analysis results of the structure agree well with the measured transmission spectra. Experimental results show that the device has low temperature crosstalk and high curvature sensitivity of 14.91 nm/m−1 and 12.86 nm/m−1 for the 0° and 180° orientations in the range of 0−3 m−1, respectively. We can distinguish the bending orientation of the device by the shift direction of the spectrum. The advantages of high sensitivity, low temperature crosstalk, compact structure, and easy fabrication make the device attractive for actual engineering bend sensing. [ABSTRACT FROM AUTHOR]

Microwave photonic systems are compelling for their ability to process signals at high frequencies and over extremely wide bandwidths as a basis for next generation communication and radar technologies. However, many applications also require narrow-band $(\sim\!\text{MHz})$ filtering operations that are challenging to be implemented using optical filtering techniques, as this requires reliable integration of ultra-high quality factor $(\sim\!10^8)$ optical resonators. One way to address this challenge is to utilize long-lived acoustic resonances, taking advantage of their narrow-band frequency response to filter microwave signals. In this article, we examine new strategies to harness a narrow-band acoustic response within a microwave-photonic signal processing platform through the use of light-sound coupling. Our signal processing scheme is based on a recently demonstrated photon-phonon emitter-receiver device, which transfers information between the optical and acoustic domains using Brillouin interactions, and produces narrow-band filtering of a microwave signal. To understand the best way to use this device technology, we study the properties of a microwave-photonic link using this filtering scheme. We theoretically analyze the noise characteristics of this microwave-photonic link, and explore the parameter space for the design and optimization of such systems. [ABSTRACT FROM AUTHOR]

An efficient integration scheme for a vertical cavity surface emission laser (VCSEL) on a silicon photonic circuit is introduced. A grating coupler fabricated on a tilted silicon membrane is used to couple the vertical emission of the VCSEL to the in-plane silicon waveguide with a high coupling efficiency and a low reflection back to the laser. Single-mode and single-polarization VCSELs at 1310nm wavelength band are flip-chip bonded on the silicon photonic chip. A coupling efficiency of -5.8dB is measured at the laser wavelength and a high optical power of 0.66mW is coupled to the silicon waveguide. Data transmission up to 10Gb/s with direct current modulation is realized using the integrated VCSEL. All the structures in the present coupling scheme are massively producible, and the hybrid integration of VCSELs relies on a standard high-accuracy flip-chip process with passive alignment. [ABSTRACT FROM AUTHOR]

We report on the first active surface Y-branch waveguide in the ∼2 μm spectral range. Depressed-cladding rectangular-cross-section surface waveguides with a splitting ratio of 1 × 2 are fabricated by femtosecond direct laser writing in a thulium (Tm3+) doped monoclinic double tungstate crystal. Confocal laser microscopy and μ-Raman spectroscopy reveal well preserved crystallinity of the waveguide core. Under high-brightness laser pumping at 0.8 μm, a simultaneous continuous-wave laser operation in both arms is achieved resulting in a total output power of 0.46 W at ∼1.84 μm with a slope efficiency of 40.6% and a laser threshold of 0.28 W. The laser output is linearly polarized and spatially multimode (TE12/TE22) with a power splitting ratio between arms of 52.1/47.9%). The waveguide propagation losses at 1.84 μm are ∼1.6 dB/cm and the loss from the Y-junction is 0.1 dB. The fabricated waveguides represent a route towards advanced photonic micro-structures such as a Mach–Zehnder interferometer for bio-sensing at ∼2 μm. [ABSTRACT FROM AUTHOR]

In this article, we present a comprehensive model for a CW Raman laser in a silicon-on-insulator (SOI) platform. We validate this model by comparing its output with experimental results from the literature. We then design a 2.232 μm Raman laser with an on-chip ring resonator cavity. Numerical analysis of laser characteristics, such as threshold power and output power, are conducted. The optimized values for the cavity length and power coupling ratios are determined taking into account fabrication variations. Finally, by designing a tunable directional coupler (DC) for the laser cavity, a robust design of Raman laser is presented. We show that the reduction of propagation loss down to 0.6 dB/cm for the silicon waveguides and free carriers removal, by the use of a p-i-n junction, will enable a Raman laser with a threshold power of 16 mW and a slope efficiency of 62%. [ABSTRACT FROM AUTHOR]

The experimental demonstration of vertical microring lasers requiring only one single epitaxial growth and two single-side lithographic steps is presented, in what is the simplest fabrication scheme for such devices published to date. The fabricated lasers show series resistance of around 20 Ω at forward bias, threshold currents at room temperature between 35 and 58 mA, and single-mode emission with a side-lobe suppression ratio higher than 30 dB. The measured optic output power level is of tens of microwatts. The approach allows the improvement of the optical features maintaining the simplicity of the manufacturing procedure. [ABSTRACT FROM AUTHOR]

Waveguide arrays are basic photonic elements in integrated optics. We propose a 3D hybrid-integrated waveguide array based on the femtosecond laser writing of lithium niobate (LiNbO3) crystals by engineering the layout of laser-induced tracks. The cross section of the structure is a cross-shaped geometry. The light propagation through this waveguide array shows polarization-dependent behaviors, which enables mode-coupling along either the vertical or the horizontal direction under excitation of linearly polarized light. This work opens new way to produce polarization-dependent optical devices in photonic systems by using femtosecond laser writing of LiNbO3. [ABSTRACT FROM AUTHOR]

An angular coupling configuration is shown to facilitate phase-matched second harmonic generation between the TE1 fundamental and TE4 second harmonic modes of a LiNbO3 planar waveguide. For an incident coupling angle of 10°, 2.6% coupling efficiency is achieved for the TE1 waveguide mode, thus permitting visible light generation at the wavelength of 402.4 nm through phase-matching to the TE4 second harmonic mode. For a 2.8 mm-long LiNbO3 planar waveguide excited at an energy of 1.8 nJ, the second harmonic generation conversion efficiency is 4.6 × 10−5. The ability of this angular coupling scheme to facilitate phase-matched frequency-conversion makes it ideal for use in optical computing, optical communications, and quantum computing. [ABSTRACT FROM AUTHOR]

We demonstrate a non-volatile optical power switch, fabricated by femtosecond laser inscription in a fused silica substrate, with switching operation based on microfluidics-controlled total internal reflection. The switch consists of crossed waveguides and a rectangular, high aspect ratio microfluidic channel, located at the waveguide crossing. The switching between total internal reflection and transmission at the channel wall is determined by the refractive index of the medium inside the channel. Femtosecond laser inscription allows for co-integration of low-loss optical waveguides and channels with smooth sidewalls and thus the fabrication of low insertion loss switches that are broadband and show low polarization dependent losses. The measured total internal reflection loss of the fabricated switch is about 1.5 dB at the wavelength 1550 nm. The loss due to transmission through the channel filled with refractive index matching liquid is about 0.5 dB. Detailed finite difference time domain and beam propagation method simulations of the switch's performance indicate that the losses can be further reduced by optimizing its geometry, together with further adjusting the inscription parameters. [ABSTRACT FROM AUTHOR]

Heterogeneous integration of III-V lasers on Silicon on Insulator (SOI) is crucial for next-generation optical interconnects. However, reducing the footprint of the couplers between III-V laser sources and silicon chips has been a challenge for dense photonic integrated circuit (PIC). Here, we propose a slot coupler and a bridge-SWG coupler with coupling lengths of only 5.5 and 5 μm respectively, to heterogeneously integrate III-V lasers on silicon chips. For such compact coupling structures, they also show high coupling efficiencies. At the wavelength of 1550 nm, the coupling efficiencies of the fundamental TE mode can reach 93.7% and 95.5% for the slot coupler and the bridge-SWG coupler, respectively. Moreover, these coupling structures possess excellent fabrication tolerance. In addition, we optimize the taper adiabatic coupler with a total coupling length of only 4 μm. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate all-optical flip-flop and inverter operations using two adjacent lasing wavelengths in a quantum-well microring laser, and that the state is switched between the flip-flop and inverter operations by changing the phase in the outer cavity composed of busline waveguides. We also propose multimodal rate equations for a microring laser taking into account clockwise and counter-clockwise light propagations. By comparing experimental results and simulated results based on the rate equations, the principle of the switching between the flip-flop and inverter operations is clarified. [ABSTRACT FROM AUTHOR]

We introduce a new III-V-on-Silicon (Si) heterogeneous integration platform, where the III-V material is bonded to the back of a processed Si photonic wafer. This “Back-Side-on-Buried Oxide” (BSoBOX) process is fully compatible with active, multilayer Si photonics platforms. This article describes the process flow and reports on O-band hybrid distributed feedback (DFB) lasers of various grating periods fabricated on this platform. A comprehensive set of measurements show that the quarter-wave shifted DFB lasers have comparable performance to front-side platforms. Single-mode emission with a side mode suppression ratio around 50 dB was measured between 20  $^\circ$ C – 60  $^\circ$ C. The DFB lasers had threshold currents as low as 32 mA and produced output powers in the Si waveguide from a single-end of about 15 mW at 170 mA before the devices began to mode hop. Output powers of $\sim$ 20 mW were measured before the onset of thermal roll-off and operation up to 80  $^\circ$ C was achieved. The characteristic temperatures and thermal impedance of the lasers were evaluated and future improvements are discussed. [ABSTRACT FROM AUTHOR]

Membrane buried-heterostructure III-V/Si distributed feedback (DFB) lasers with a stopband-modulated cavity on a Si substrate have been developed. The membrane III-V layers with 230-nm thickness enable us to construct an optical supermode with a 220-nm-thick Si waveguide that is used in standard Si photonics platform. We employ a uniform grating and Si waveguide, in which Si waveguide width is modulated to control the center wavelength of the stopband. The cavity can be designed by controlling the modulation width and modulation length of Si waveguide. Therefore, it is easy to engineer and fabricate the laser cavity compared with the cavity using λ/4-phase shift grating. Output light from the cavity is coupled to Si waveguide through InP inverse taper waveguide, and then coupled to SiOx waveguide through Si inverse taper waveguide, which provides the 2-dB fiber coupling loss. We have demonstrated single-mode lasing by using Si waveguide, where its width is increased 80 nm at the center of the cavity. The threshold current and maximum fiber output power are 3 mA and 4 mW, respectively. By extending the active region length to 1 mm, 17-mW fiber coupled output power is obtained. High-temperature operation up to 130 °C is also obtained with a 1-mW fiber output power. [ABSTRACT FROM AUTHOR]

A depressed cladding waveguide composed of 36 parallel tracks is inscribed in high purity 70TeO2-22WO3-8Bi2O3 glass by femtosecond laser operating at 1030 nm. The Sellmeier formula for the refractive index of the investigated glass is obtained for the range of 1170–2350 nm. The module of the negative refractive index change in a track is found to increase with increasing wavelength in the range of 640–2000 nm. The waveguide shows single mode behavior in the wavelength range of 1700–2100 nm with mode field diameter (MFD) of 20 μm. The propagation loss is predominantly due to scattering in the waveguide. Spectrum broadening as large as 503 nm was demonstrated in a 14 mm long waveguide pumped by femtosecond pulses at 1950 nm. In the low input pulse energy region, the spectrum broadening is well described by generalized nonlinear Shrödinger equation (GNLSE) taking into account Kerr and the Raman nonlinearities under the assumption of constant MFD along the waveguide. Under higher energy pumping, an extra broadening was observed that is attributed to the focusing effect of Kerr lens in the waveguide mode. [ABSTRACT FROM AUTHOR]

We present a novel method to fabricate planar photonic devices for high power applications using modified chemical vapor deposition (MCVD) technology. The MCVD process, usually applied to optic fibers, is used here to deposit a layer of Ge doped silica on a fused silica slab. The doped layer was then heat-treated by a CO2 laser in order to achieve high optical quality and increase its damage threshold. Finally, direct laser lithography, followed by deep dry etching, was used to shape straight and curved large-core air-cladded waveguides with smooth sidewalls. The straight waveguides were shown to have an excellent propagation loss, 0.33 dB/cm, and the curved waveguide had a propagation loss of 1 dB/cm. [ABSTRACT FROM AUTHOR]

An advanced CMOS-compatible 300- $\text{mm}$ -wafer silicon-photonics platform is introduced that consists of a silicon layer with eight doping masks, two silicon-nitride layers, three metal and via layers, a dicing trench for smooth edge-coupled facets, and a gain-film trench that enables interaction between the gain material and waveguide layers. The platform was used to demonstrate an electrically-steerable integrated optical phased array powered by an on-chip erbium-doped laser. Lasing with a single-mode output, $30\,\text{dB}$ side-mode-suppression ratio, and $\text{40 mW}$ lasing threshold was shown, and one-dimensional beam steering with a $\text{0.85}^\circ \times \text{0.20}^\circ$ full-width at half-maximum and $\text{30}^\circ /\text{W}$ electrical steering efficiency was demonstrated. This system represents the first demonstration of a rare-earth-doped laser monolithically-integrated with an active CMOS-compatible silicon-on-insulator photonics system. [ABSTRACT FROM AUTHOR]

We introduce a method of inscribing in-fiber devices using a femtosecond laser that is applicable to crucial components, such as cladding waveguides (CWGs), cladding Mach–Zehnder interferometers (MZIs), embedded waveguide Bragg gratings (WBGs), and waveguide Fabry–Perot cavities using the same key femtosecond laser parameters, via an “inscribe and step,” plane-by-plane approach, applied as necessary on two orthogonal axes. This leads to femtosecond laser-inscribed cladding waveguides and ultra-compact MZIs that can support functional, integrated fiber Bragg gratings; the unique sensing characteristics of the filters are maintained and provide complementary measure and information. This ensures a single inscription process, offering reliability and repetition in component manufacture, as the basic conditions to inscribe the here-demonstrated elements are common. We characterize CWG-WBG devices for axial strain, bend, and response to refractive index. The MZI-WBG is exposed to temperature and humidity excursions, confirming that the unique sensor responses are maintained for this compact, compound sensor. The MZI exhibits response to external refractive index, a large, negative wavelength response with temperature and high sensitivity to humidity, whereas the MZI-located WBG displays a similar sensitivity to conventional core-based Bragg grating sensors to temperature and no response to relative humidity. We consider that this research is an important step in the development of compact, smart optical fiber sensors. [ABSTRACT FROM AUTHOR]

We report on a one-step fabrication method of horizontal dual-line waveguides supporting TE guidance in LiTaO3 crystal utilizing multiple refocusing mechanisms of the femtosecond laser. Through the nonlinear process, multiple foci can be formed spontaneously with the interval of a few micrometers along the laser propagation direction. The number and the separation of focal spots, even the entire length of laser-induced tracks, could be modulated by varying the laser parameters. Taking advantage of this spontaneous phenomenon, we further implemented the multi-scan technique to write two parallel damage lines along transverse direction, realizing stress-induced waveguides by horizontal light confinement. Moreover, using this method, one-step two-channel guidance could also be demonstrated by adjusting the pulse energy. The fabrication details and waveguiding performances were studied by both experimental and computational methods. This paper opens the alternative way to construct complex integrated platforms in LiTaO3 crystal by using femtosecond laser writing. [ABSTRACT FROM AUTHOR]

We report on the fabrication of depressed cladding waveguides inscribed by direct femtosecond laser pulses irradiation in transparent nc-yttria stabilized zirconia (YSZ) polycrystalline ceramics. The waveguides consisted of two different circular structures with diameters of 30 and 100 $\mu \text{m}$ , and different lengths. Optical characterization at 633  $\text{}$ and 1568 nm was performed. The obtained modal profiles show dependence on the structure and size of the waveguides. We estimate a laser-induced refractive index contrast of nearly 10 $^{-2}$ , larger than can be explained by induced residual stresses alone. We conjecture that laser interactions lead to localized physical damage, i.e., microvoids cracks and possibly a decreased degree of crystallinity in the laser processed zones, which is supported by Raman measurements. Our results suggest a promising potential use of nc-YSZ in photonic bio-applications. [ABSTRACT FROM AUTHOR]

SiO $_{2}$ –HfO $_{2}$ rib waveguides were fabricated using the UV-photolithography method followed by wet chemical etching technique. Graphene oxide (GO) integrated SiO $_{2}$ –HfO $_{2}$ rib waveguide works as an on-chip TE-pass waveguide polarizer and also allows controlled intensity tuning of the selective state of polarization when irradiated using a 532 nm ns laser. The tuning of the TE polarization intensity was confirmed by performing the experiments at three different pump fluences viz., 0.2, 0.3, and 0.6 mJ/cm $^{2}$ , wherein at low pump fluence, TE polarization intensity showed more controlled absorption. The results indicate that GO integrated glass waveguides can act as efficient polarization dependent power splitters for integrated optic applications. [ABSTRACT FROM AUTHOR]

In this paper, we present a theoretical and experimental analysis of mid-infrared (4.7 μm) quantum cascade laser waveguides with different taper waveguides geometries: linear, convex, and concave tapers. We have calculated and measured brightness of the optical beam and maximal power density on the laser front mirror for different widths and lengths of the taper. Convex geometry shows the best performance and the highest powers were obtained. [ABSTRACT FROM AUTHOR]

Optical waveguides directly written in fused silica using a femtosecond laser were characterized from 350 to 1750 nm to gain insight on the waveguide's loss mechanisms and their dependence on processing parameters, such as pulse energy, scan velocity, and annealing temperature. Two major loss mechanisms were identified. In the range of parameters tested, high pulse energy was seen to improve coupling losses at long wavelengths, while high scan velocity has a negative effect in both Rayleigh scattering and coupling losses at long wavelengths. Thermal annealing of the waveguides demonstrated an improvement of the Rayleigh scattering at a cost of higher coupling losses at long wavelengths. Wavelength independent Mie scattering was also observed, evolving negatively with pulse energy. A minimum Rayleigh scattering coefficient of ≍0.5 dB⋅cm−1⋅μm4 (≍0.08 dB⋅cm−1⋅μm4 for thermally treated waveguides) together with a Mie scattering coefficient of ≍0.2–0.65 dB/cm are reported. [ABSTRACT FROM AUTHOR]

We analyze the small- and large-signal response of a photonic crystal Fano laser. Conventional current modulation, as well as modulation of the laser via the mirror, is investigated using a numerical approach as well as linear small-signal analysis. The results show that the amplitude modulation bandwidth of one of the laser ports (the through-port) and the frequency modulation bandwidth of another port (the cross-port) extend to the THz region. Large-signal simulations of the laser response to a 500 Gb/s pseudo-random bit sequence modulation of the nanocavity are in good agreement with the predictions of the small-signal analysis. Finally, it is investigated how the design of the Fano mirror, in particular, the quality-factor (Q) of the nanocavity, affects the modulation properties. [ABSTRACT FROM AUTHOR]

Semiconductor lasers that use high-order surface gratings formed with etched slots have simple fabrication. Such gratings however increase loss and have a small free-spectral range that can increase threshold, lower slope efficiency, and limit SMSR. In this paper, we look at the design of a grating and use 2, 3, and 4 period structures within a 37 ${\text{th}}$ order grating. At fixed temperature, a single period device should work best while temperature tuning requires a multiperiod device. We found that the two period grating structure works best among the multiperiod devices. A follow-on study with the scattering matrix model shows that the slope efficiency which is quite low in this study (about 0.1 mW/mA at best) can be improved to a value of upto 0.17 mW/mA by reducing the number of slots used to form the grating from the present design that has 24 slots to 18. [ABSTRACT FROM AUTHOR]

The emergence of the fifth generation wireless networks (5G) and the Internet of Things drives the research on radio access networks. Future wireless networks aim to deliver a dramatic increase in bandwidth to an ever increasing amount of connected devices. To realize such a vision, advanced optical transceivers that link the base-station and the antennas will be required. In this paper, we present the first demonstration of an integrated silicon photonic radio-over-fiber transmitter and receiver, consisting of a directly modulated III-V-on-silicon distributed feedback laser and a Ge-on-Si waveguide photodiode cointegrated with a linear SiGe Bipolar CMOS (BiCMOS) transimpedance amplifier, respectively. Transmission of a high-spectral-efficiency long-term evolution 20 MHz bandwidth 120 Mbit/s 64 quadrature amplitude modulation (64-QAM) orthogonal frequency division multiplexing signal on a 3.5 and 5 GHz carrier over 5 km of standard single mode fiber is demonstrated. We further demonstrate the transmission of 16 Gbps 16-QAM data on a 20 GHz carrier over a 5 km link with a received rms error vector magnitude of 7%. [ABSTRACT FROM AUTHOR]

The optoelectronic oscillators (OEOs) based on a 1.55-μm direct-modulated AlGaInAs/InP square microcavity laser without optical injection are demonstrated for narrow-linewidth photonic microwave signal generation. The square microcavity laser with a side length of 16 μm under an injection current of 14 mA is used in the experiment. High-quality photonic microwave signals with 3-dB linewidths around 30 Hz are obtained by the first harmonics of the single-loop and dual-loop OEOs. Single-sideband phase noise at 10 kHz frequency offset is as low as −116 dBc/Hz for the microwave signal. With the dual-loop OEO, the side-mode suppression ratios up to 55 dB are obtained for the first harmonics, and tunable microwave signals from 1.85 to 10.24 GHz are realized with narrow linewidths and low-phase noises. [ABSTRACT FROM AUTHOR]

A parity-time symmetric vertical Bragg reflection waveguide structure is proposed and analyzed for realizing single-mode operation of a laser diode. Impact of the key design parameters on laser performance are investigated for the purpose of achieving high side-mode suppression ratio and high injection slope efficiency. Our studies indicate that an electrically pumped edge-emitting single-mode laser is possible with proper design. Such lasers exhibit a number of highly desirable and in some cases unique characteristics, such as high-modulation bandwidth, high transverse gain, and high confinement of optical field in the low-effective index Fabry–Perot cavity. [ABSTRACT FROM AUTHOR]

We report the first demonstration of gain-switched optical pulses generated by continuous-wave 1.3 μm InAs/GaAs quantum dot (QD) broad-area lasers directly grown on silicon. The shortest observed pulses have typical durations between 175 and 200 ps with peak output powers of up to 66 mW. By varying the drive current pulsewidth and amplitude systematically, we find that the peak optical power is maximized through sufficiently long high-amplitude drive pulses, whereas shorter drive pulses with high amplitudes yield the narrowest achievable pulses. A three-level rate equation travelling-wave model is used for the simulation of our results in order to gain a first insight into the underlying physics and the laser parameters responsible for the observed behavior. The simulations indicate that a limited gain from the InAs QDs and a very high gain compression factor are the main factors contributing to the increased pulsewidth. As the optical spectra of the tested broad-area QD laser give a clear evidence of multitransverse-mode operation, the laser's dynamic response could be additionally limited by transversal variations of the gain, carrier density, and photon density over the 50 μm wide laser waveguide. [ABSTRACT FROM AUTHOR]