Vertically integrated As_2S_3 ring resonator on LiNbO_3

A vertically integrated As2S3 ring resonator side-coupled to Ti:LiNbO3 waveguide was designed and fabricated. We have demonstrated an ultra-high Q value of 3.5×105 and low propagation loss of 1.2 dB/cm over a 1.7 cm race-track ring resonator.

We demonstrate ring and racetrack resonators with Qs of 3.8 to 7.5 million and 100 MHz bandwidth racetrack resonator filters, implemented in a thick silicon-on-insulator foundry platform that features a 3 µm thick device layer. We show that special racetrack resonators (with weakly guiding straight sections that transition to strongly confining bends) implemented in this platform can be preferable to rings for applications such as integrated microwave-photonic signal processing that require filters with sub-GHz bandwidth, tens of GHz of free spectral range (FSR), and a compact footprint for dense system-on-chip integration. We demonstrate ring resonators with 7.5×106 intrinsic Q, but limited FSR of 5.1 GHz and a taxing footprint of 21mm2 due to a large 2.6 mm bend-loss-limited radius. In comparison, we demonstrate two racetrack resonator designs with intrinsic Qs of 3.8×106 and 4.3×106, larger respective FSRs of 11.6 GHz and 7.9 GHz, and less than 1/20th the area of the ring resonator. Using racetrack resonators, we implemented a four-channel, 100 MHz wide passband filter bank with 4.2 to 5.4 dB insertion loss to drop ports.

Second harmonic generation (SHG) by quasi-phase matching (QPM) in LiNbO3 waveguides has recently attracted a great deal of attention. Using periodically domain inverted grating on LiNbO3, normalized conversion efficiency of order of 20 ~ 80%/W.cm2 have been achieved.1-4 However, the phasematching bandwidth length product is rather small for QPM waveguides, thus imposes a very critical limit on the diode pump sources. Temperature tuning of the QPM wavelength in LiNbO3 waveguide have been demonstrated,3 a tuning rate which is comparable to the diode laser temperature tuning can be achieved. However, in order to obtain stable SHG output, a fast responding fine tuning scheme is required for the electronic feedback stabilization system. In this paper, we demonstrate the wavelength tuning and modulation of the quasi-phase matched SHG signal using the electro-optic (EO) effect in LiNbO3 waveguide.

Numerous areas, both in science and industry level, have benefited in recent years from the application of ultrafast mode-locked lasers. The introduction of a saturable absorber (SA) in the cavity is a convenient and simple way to achieve passive self-starting mode-locking in fibre lasers. The applicability of such passively mode-locked ultrafast fibre sources is mainly limited by the achievable energy and peak power, the latter usually going up to only a few kW in the femtosecond pulse range. In our current study we rely on a novel InN-based SA, recently presented and studied in [1,2], to set-up an ultrafast, high peak power ultrafast passively mode-locked fibre laser with operation wavelength at 1.56 μm in telecommunication C-band. The fibre laser is implemented as a ring resonator. It is based on a commercial EDFA as gain medium and the aforementioned SA [1], that consists of InN grown by molecular beam epitaxy on a GaN-on-sapphire template, used in reflection configuration as SESAM, which is located in free space and connected to the ring through an optical circulator. The outstanding properties of this material are here further validated, especially its tolerance to extremely high fluences (>1TW/cm2). The cavity length is increased introducing a 1-km standard single mode fibre (SSMF). Note that no polarization control is needed in the system and, as shown in [2], the basic ring configuration around 40-m-long can achieve stable mode-locking with sub-250 fs pulses and a repetition rate around 5 MHz. However, if polarization control is introduced, it is possible to tune the temporal and spectral widths of the pulses, from 150 fs to 320 fs. With a 1 km-long cavity, fundamental mode-locking is easily achievable increasing cavity losses with a VOA. Since symmetric Gaussian pulse spectra are generated, very similar to those delivered in the basic ring configuration, it is possible to assume that all the energy is contained in the pulse spectrum, without significant contribution of continuous white noise. This results in pulses with a temporal width at FWHM of 239 fs and a spectral width of 25.4 nm. Since the repetition rate is 196.4 kHz and the average power is 30.5 mW, pulses with peak power of 650.5 kW and pulse energy of 155.3 nJ are produced. If losses in the cavity are gradually reduced, mode-locking to higher harmonics is obtained in cavities up to 6-km-long, proving the excellent stability of the system. This allows a broad choice of high peak powers and repetition rates. To summarise, for the first time to our knowledge, it has been experimentally demonstrated a polarization-independent passive ultrafast harmonic mode-locked ring fibre laser operating at 1.56 μm achieved with a simple low-cost set-up and the use of an InN-based SESAM, which in combination with a 1-km-long SSMF cavity, delivers femtosecond pulses with a peak power of 650.5 kW, the highest obtained to date, to our knowledge, in this kind of lasers.

During the last six years, intense research on single-mode rare-earth doped fiber lasers and amplifiers has led to the development of a range of active devices, in particular the Er3+-doped fiber amplifiers with application for optical telecommunications at 1.5 to 1.6 µm. Integrated optics may offer an alternative way for realizing such single-mode waveguide active devices with potential advantages over fibers,such as a more compact size, the feasibility of Q-switching, frequency doubling and the possibility of processing many devices on a single substrate. It also offers possibilities for new monolithic laser or amplifier components combining the medium’s gain and the large range of integrated optical functions already demonstrated. It may include coupled cavity, Q-switched, mode-locked or frequency doubled lasers and amplified optical integrated circuits with no insertion losses. The possibility of ion-doping of standard integrated-optic waveguides has recently allowed the demonstration of Nd- and Er-doped single-mode channel waveguide lasers in single crystal Ti:LiNbO3 waveguides[1'3] and various glass waveguides[4-7]. Waveguide lasers have a number of useful applications including amplification of high speed modulated signals, optical generation and manipulation of microwave signals(e.g.,injection locking for phased-array antennas), all-optical repeaters for long distance communication, optical interconnection and coherent communication. However,due to the intrinsic limitations of LiNb03 and glass, the above mentioned applications are strictly limited. First,glass waveguides are not electrooptic, therefore, an active device cannot be made using these substrates. Second, LiNb03 waveguides have significant walk-off between the refractive indices of microwave and optical waves. Consequently, the modulation speed of the EO modulator, which serves as the input signal generator for the waveguide amplifier, is limited to -40 GHz. Third, the waveguide fabrication methods on LiNbO3 and glass substrates are not universal. They are not transferable to other substrates. For instance,Si and GaAs are the most frequently used substrates for optoelectronics. LiNbO3 and glass waveguide lasers

In this paper, the SWAP gate is realized using the electro-optic (EO) phenomenon in the Mach-Zehnder Interferometer (MZI). In the structuring of the SWAP gate, two MZI is being used, and output is obtained from the bar port of both MZI. The SWAP gate output is obtained from MATLAB by using the mathematical description of MZI. The proposed layout of the SWAP gate is designed using titanium diffused lithium niobate (Ti:LiNb03) waveguide and simulated using the beam propagation method (BPM) at 1.33 μm wavelength.

Evaluation of low index contrast in lithium niobate waveguides at telecom wavelengths

We report on the characterization of single-mode and multimode proton-exchanged periodically segmented waveguides in lithium niobate. Variation of effective indices of different modes for different annealing times as a function of duty cycle and period of segmentation is presented. It is shown that the proton-exchanged periodically segmented waveguide can be modeled by an equivalent z-invariant depth-independent graded-index waveguide with a Gaussian index distribution. Although the depth of the equivalent waveguide is independent of duty cycle, the peak refractive-index change increases almost linearly with the duty cycle and saturates at large duty cycles. The results should benefit the design of linear and nonlinear devices that are based on segmented waveguides.

We present our analysis and optimization of slot ring resonators by investigating the slotted coupler properties in order to achieve high Q ring resonators. Firstly, different ring resonators loaded by conventional straight bus ring resonator couplers with different gaps were fabricated with a cavity length of 828 µm and a bending radius of 50 µm. They were measured with an average Q-factor around 85,000 and proved to present large intrinsic coupler losses. We then proposed a new symmetric racetrack ring resonator coupler to reduce coupler loss thanks to a smoothed transition in coupling region between the coupled supermodes and achieved Q factor above 120,000 with total intrinsic loss around 2.2 dB/cm in a 50 µm radius ring resonator with total cavity length 326 µm by this approach. These optimized slot ring resonators could bring a new progress towards low loss silicon slot waveguiding structures for hybrid Si photonics including on-chip nonlinear optics and sensing in future.

Optical ring/racetrack resonators have the sufficient flexibility to realise many functions in a single device, from filters/multiplexers, to modulators, to switches. The use of Silicon-On-Insulator (SOI) material, coupled with Ultra Large Scale Integration (ULSI) processing techniques, may allow the cost of these devices to become economically advantageous over current components. This paper describes our recent work in developing polarisation independent ring resonators, and subsequent, work on increasing the limited free spectral range and full width half maximum of the resonance. There are two key components that comprise a polarisation-independent racetrack resonator: a polarisation-independent rib waveguide and a polarisation-independent directional coupler. Polarisation independence is achieved in the waveguides when the geometrical design ensures that both polarisation modes propagate with the same effective index. We report on such devices together with polarisation independent couplers, which are achieved by allowing different inter multiples of the coupling length for the TE and the TM modes. By combining these components, the resulting device is a polarisation independent ring resonators. These devices have been thermally modulated by means of a modulated visible laser and alternatively via small heaters fabricated on the waveguides. We have also modelled ring resonator modulators via carrier injection and depletion. Subsequently we have improved the device characteristics by employing smaller bend radii to increase the free spectral range by a factor of 5, and by cascading racetracks to improve the full width half maxima of the resonance by almost 40%. Experimental results are reported for most of the above characteristics. We will further investigate the opportunities for increasing the FSR whilst retaining polarisation independence, the possibility of retaining polarisation independence whilst utilising the properties of the ring resonator to form improved modulators.

Based on arsenic tri-sulfide films on titanium-diffused lithium niobate, we designed a hybrid optical waveguide for efficient mid-infrared emission by phase-matched difference frequency generation (DFG). The hybrid waveguide structure possesses a low-index magnesium fluoride buffer layer sandwiched between two high-index As(2)S(3) slabs, so that pump and signal waves are tightly confined by titanium-diffused waveguide while the DFG output idler wave at mid-infrared is confined by the whole hybrid waveguide structure. On a 1 mm-long hybrid waveguide pumped at 50 mW powers, a normalized power conversion efficiency of 20.52%W(-1)cm(-2) was theoretically predicted, which is the highest record for mid-infrared DFG waveguides based on lithium niobate crystal, to the best of our knowledge. Using a tunable near-infrared pump laser at 1.38-1.47 µm or a tunable signal laser at 1.95-2.15 µm, a broad mid-infrared tuning range from 4.0 µm to 4.9 µm can be achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths.

This Letter reports on the quadratic electro-optic effect of polymers, observed in a silicon slot-waveguide at low voltages. We demonstrate that in narrow slots, the electro-optic response with respect to refractive index change is strong enough for on-chip wavelength tuning and intensity modulation using voltages as low as 1V. A silicon slot-waveguide embedded by a nonlinear optical polymer, consisting of the dye Disperse Red 1 in poly(methyl methacrylate), serves as the phase shifter in a racetrack ring resonator. As deduced from the experimental data, the third-order susceptibility of the utilized electro-optic polymer is about 2·10-19 m2/V2. The demonstrated low-voltage operation and inherently thermal stability show the potential for silicon-organic hybrid devices using the quadratic electro-optic effect.

Plasmonic chemical and biological sensors offer significant advantages such as really compact sizes and extremely high sensitivity. Biosensors based on plasmonic waveguides and resonators are some of the most attractive candidates for mobile and wearable devices. However, high losses in the metal and complicated schemes for practical implementation make it challenging to find the optimal configuration of a compact plasmon biosensor. Here, we propose a novel plasmonic refractive index sensor based on a metal strip waveguide placed under a waveguide-based racetrack ring resonator made of the same metal. This scheme guarantees effective coupling between the waveguide and resonator and low loss light transmittance through the long-range waveguide. The proposed device can be easily fabricated (e.g., using optical lithography) and integrated with materials like graphene oxide for providing adsorption of the biomolecules on the sensitive part of the optical elements. To analyze the properties of the designed sensing system, we performed numerical simulations along with some analytical estimations. There is one other interesting general feature of this sensing scheme that is worth pointing out before looking at its details. The sensitivity of the considered device can be significantly increased by surrounding the resonator with media of slightly different refractive indices, which allows sensitivity to reach a value of more than 1 μm per refractive index unit.

In this work, CMOS compatible hybrid insulator metal insulator (HIMI) planar plasmonic waveguide based components such as, racetrack ring resonator, y-splitter, coupler and dual parabolic profile nano antenna have been proposed and investigated at 1.55 μm operating wavelength for the first time. The proposed components are analyzed at various parametric variations using CST microwave studio. The proposed racetrack ring resonator offers 153.88 nm free spectral range, 0.3232 nm FWHM and 46.57 dB extinction ratio at 0.40 μm radius and 0.49 μm separation length. Y-splitter divides equal power of 46.85% in both output arms and coupler couples till 86.5% power in coupled arm. The dual parabolic profile nano antenna offers a gain, bandwidth and efficiency of 7.99 dB, 203 THz and 84.56 % respectively. Using all the components, HIMI plasmonic integrated circuits have also been designed and investigated for on chip applications. Proposed components work very efficiently in chip scale integrated circuit. This investigation provides groundwork for future chip scale applications.

Subwavelength metallic structures capable of creating strongly localized electromagnetic field with enormous enhancement under illumination/excitation are designed for direct radio frequency (RF) imaging. Three dimensional finite element method models are applied to investigate the electromagnetic field concentrations of two types of split ring resonators. Under appropriate linearly polarized illumination, a highly confined field located at the gap of the ring resonator is found due to strong scattering resonance. The numerical studies show that field enhancement as high as 6,800 is achieved for a planar D-shaped split ring resonator. The enhancement can be further increased through shrinking the gaps size or the ring width. Crescent shaped split ring resonator is designed for broadband application. It provides an enhanced bandwidth which is 1.15 times of the resonant frequency. The concentrated electromagnetic field facilitates nonlinear processes that find lots of applications. Optimized RF concentrators integrated with electro-optic modulators are demonstrated to directly modulate optical carrier. The combination of RF concentrator and EO modulator could enable a focal plane RF imager array that allows direct RF imaging, and significantly decrease RF aperture size and weight. Additional benefits include enhanced functionality such as inherent polarimetric imaging capability.