Slow-light based tunable delay and narrowband comb filtering at 2 µm.

Development of coherent sources, wideband thulium-doped fiber amplifiers, and fiber components has opened up the wavelength region around 2000 nm for optical communications, sensing, and medical surgery. However, several key functionalities that are critical to enable these applications are not yet well developed and, therefore, need attention. Here, we present demonstration of two critical signal processing tasks viz: (1) tunable delay and (2) tunable narrowband filter, which are important for enabling optical communications using wavelengths around 2000 nm. We exploit stimulated Brillouin scattering (SBS) in a 100 m long SM1950 fiber to report the first demonstration of slow-light based tunable delay around 2000 nm. For a 35 ns input pulse, we tune the delay up to a maximum of 18.8 ns, which corresponds to a relative delay of ∼0.5, by varying the pump power to achieve a maximum gain of 12.6 dB. For the 1550 nm wavelength regime, it has been shown that the Brillouin slow-light typically results in a delay of 1 ns/dB, which is less than the 1.67 ns/dB obtained at 2000 nm. For the same gain, the large delay at 2 µm compared to 1.55 µm results from the narrow gain bandwidth (∼22 MHz) of the SBS process in this wavelength regime. Using the narrow gain bandwidth of SBS, we present a proof-of-concept experiment to demonstrate filtering of individual comb lines in a comb with frequency spacing <50 MHz. Demonstration of tunable narrowband filter and delay enables optical signal processing and line-by-line control of comb lines at 2 µm.

On-chip high sensitivity laser frequency sensing with Brillouin mutually-modulated cross-gain modulation.

We report the first demonstration of a photonic-chip laser frequency sensor using Brillouin mutually-modulated cross-gain modulation (MMXGM). A large sensitivity (~9.5 mrad/kHz) of the modulation phase shift to probe carrier frequency is demonstrated at a modulation frequency of 50 kHz using Brillouin MMXGM in a ~7 cm long chalcogenide rib waveguide.

Si3N4 ring resonator-based microwave photonic notch filter with an ultrahigh peak rejection.

We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si3N4 optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter.

Photonic-chip-based all-optical ultra-wideband pulse generation via XPM and birefringence in a chalcogenide waveguide.

We report a photonic-chip-based scheme for all-optical ultra-wideband (UWB) pulse generation using a novel all-optical differentiator that exploits cross-phase modulation and birefringence in an As2S3 chalcogenide rib waveguide. Polarity-switchable UWB monocycles and doublets were simultaneously obtained with single optical carrier operation. Moreover, transmission over 40-km fiber of the generated UWB doublets is demonstrated with good dispersion tolerance. These results indicate that the proposed approach has potential applications in multi-shape, multi-modulation and long-distance UWB-over-fiber communication systems.

Frequency agile microwave photonic notch filter with anomalously high stopband rejection.

We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (>60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This performance is enabled by a new concept of sideband amplitude and phase controls using an electro-optic modulator and an optical filter. This concept enables energy efficient operation in active MWP notch filters, and opens up a pathway toward enabling low-power nanophotonic devices as high-performance RF filters.

Observation of Brillouin dynamic grating in a photonic chip.

We report demonstration of a Brillouin dynamic grating (BDG) in a photonic chip. A BDG was characterized in a 6.5 cm long chalcogenide (As(2)S(3)) rib waveguide using CW pumps in x polarization and read using a CW probe in y polarization. The measured reflectivity, on-off ratio, and 3 dB bandwidth (f(3 dB)) for the BDG were 0.4%, ~28 dB, and ~6 GHz, respectively.

Narrow linewidth Brillouin laser based on chalcogenide photonic chip.

We present, to the best of our knowledge, the first demonstration of a narrow linewidth, waveguide-based Brillouin laser that is enabled by large Brillouin gain of a chalcogenide chip. The waveguides are equipped with vertical tapers for low-loss coupling. Due to optical feedback for the Stokes wave, the lasing threshold is reduced to 360 mW, which is five times lower than the calculated single-pass Brillouin threshold for the same waveguide. The slope efficiency of the laser is found to be 30%, and the linewidth of 100 kHz is measured using a self-heterodyne method.

Design for broadband on-chip isolator using Stimulated Brillouin Scattering in dispersion-engineered chalcogenide waveguides.

We propose a scheme for on-chip isolation in chalcogenide (As2S3) rib waveguides, in which Stimulated Brillouin Scattering is used to induce non-reciprocal mode conversion within a multi-moded waveguide. The design exploits the idea that a chalcogenide rib buried in a silica matrix acts as waveguide for both light and sound, and can also be designed to be multi-moded for both optical and acoustic waves. The enhanced opto-acoustic coupling allows significant isolation (> 20 dB) within a chip-scale (cm-long) device (< 10 cm). We also show that the bandwidth of this device can be dramatically increased by tuning the dispersion of the waveguide to match the group velocity between optical modes: we find that 20 dB isolation can be extended over a bandwidth of 25 nm.

Multi-wavelength gratings formed via cascaded stimulated Brillouin scattering.

We present the experimental observation of multi-wavelength fiber Bragg gratings in As2Se3 fiber. The gratings are internally written via two-photon absorption of 1550 nm pump light and its first and second order Stokes waves generated by cascaded stimulated Brillouin scattering (SBS). We demonstrate a parameter regime that allows for 4 dB grating enhancement by suppression of SBS.

Photonic chip based tunable and reconfigurable narrowband microwave photonic filter using stimulated Brillouin scattering.

We report the first demonstration of a photonic chip based dynamically reconfigurable, widely tunable, narrow pass-band, high Q microwave photonic filter (MPF). We exploit stimulated Brillouin scattering (SBS) in a 6.5 cm long chalcogenide (As2S3) photonic chip to demonstrate a MPF that exhibited a high quality factor of ~520 and narrow bandwidth and was dynamically reconfigurable and widely tunable. It maintained a stable 3 dB bandwidth of 23 ± 2MHz and amplitude of 20 ± 2 dB over a large frequency tuning range of 2-12 GHz. By tailoring the pump spectrum, we reconfigured the 3 dB bandwidth of the MPF from ~20 MHz to ~40 MHz and tuned the shape factor from 3.5 to 2 resulting in a nearly flat-topped filter profile. This demonstration represents a significant advance in integrated microwave photonics with potential applications in on-chip microwave signal processing for RADAR and analogue communications.

Photonic-chip-based tunable slow and fast light via stimulated Brillouin scattering.

We report the first (to our knowledge) demonstration of photonic chip based tunable slow and fast light via stimulated Brillouin scattering. Slow, fast, and negative group velocities were observed in a 7 cm long chalcogenide (As(2)S(3)) rib waveguide with a group index change ranging from ~-44 to +130, which results in a maximum delay of ~23 ns at a relatively low gain of ~23 dB. Demonstration of large tunable delays in a chip scale device opens up applications such as frequency sensing and true-time delay for a phased array antenna, where integration and delays ~10 ns are highly desirable.

Highly-efficient, octave spanning soliton self-frequency shift using a specialized photonic crystal fiber with low OH loss.

We report the first demonstration of efficient, octave spanning soliton self-frequency shift. In order to achieve this we used a photonic crystal fiber with reduced OH absorption and widely spaced zero-dispersion wavelengths. To our knowledge, this is the largest reported frequency span for a tunable, fiber-based source. In addition, we observe the generation of light above 2 µm directly from a Ti:Sapphire laser in the form of Cerenkov emission by the soliton when the red-shift saturates at the edge of the anomalous dispersion region.

On-chip stimulated Brillouin scattering.

We demonstrate on-chip stimulated Brillouin scattering (SBS) in an As2S3 chalcogenide rib waveguide. SBS was characterized in a cm long waveguide with a cross-section 4 µm x 850 nm using the backscattered signal and pump-probe technique. The measured Brillouin shift and its full-width at half-maximum (FWHM) linewidth were ~7.7 GHz and 34 MHz, respectively. Probe vs. pump power measurements at the Brillouin shift were used to obtain the gain coefficient from an exponential fit. The Brillouin gain coefficient obtained was 0.715 x 10(-9) m/W. A probe gain of 16 dB was obtained for a CW pump power of ~300 mW.

Ultrahigh-bandwidth, on-chip all-optical pulse erasure using the χ(3) process in a nonlinear chalcogenide waveguide.

We demonstrate on-chip all-optical pulse erasure based on four-wave mixing and cross-phase modulation in a dispersion engineered chalcogenide (As(2)S(3)) rib waveguide. We achieve an erasure efficiency of ~15 dB for picosecond pulses in good agreement with numerical simulations using the nonlinear Schrödinger equation. The combined effect of the high instantaneous optical nonlinearity (γ = 9900 (W km)(-1)) and small group-velocity dispersion (D = 29 ps/nm km), which reduces pulse walk-off, will enable all-optical pulse erasure for ultrafast signal processing.

Photonic chip-based all-optical XOR gate for 40 and 160 Gbit/s DPSK signals.

We demonstrate a photonic chip-based all-optical exclusive-OR (XOR) gate for phase-encoded optical signals via four-wave mixing in a highly nonlinear, dispersion-engineered chalcogenide (As2S3) planar waveguide. We achieve error-free, XOR operation for 40 Gbit/s differential phase shift keying (DPSK) optical signals with no power penalty. The effectiveness and broad bandwidth operation of our approach is highlighted by implementing an XOR gate for 160 Gbit/s DPSK signals.

Soliton self-frequency shift performance in As(2)S(3) waveguides.

The soliton self-frequency shift in As(2)S(3) is investigated theoretically. Differences in the shapes of the Raman spectra in silica and As(2)S(3) are predicted to lead to variations of less than 25% in the frequency shift rate of a fundamental soliton. Detailed simulation of the propagation of a low peak power pulse in a chalcogenide ridge waveguide shows the concepts of Raman soliton behaviour in silica to be transferrable to As(2)S(3). Thus we predict the effectiveness of the soliton self-frequency shift in contributing to wide bandwidth generation in low-power supercontinua at mid-infrared wavelengths in this highly nonlinear chalcogenide, as well as other nonlinear processing applications such as digital quantization for optical analogue to digital conversion.

Kinetics of diuron under aerobic condition and residue analysis in sugarcane under subtropical field conditions.

The phenylureas group includes persistent herbicides which are major pollutants to soil and water. Dissipation kinetics of diuron in different soils under sugarcane field conditions was investigated. Diuron was extracted with acetone and florisil solid phase extraction clean-up and characterized by high-performance liquid chromatography-UV. Diuron persisted for more than 100 days and dissipation followed monophasic first-order kinetics. Persistence was more in sandy loam compared to silty clay loam soil. Half-life of diuron in silty clay loam soil was 22.57 and 32.37 days and in sandy loam was 28.35 and 43.93 days at 2 and 4 kg ha-1applications, respectively. Average recovery in soil, bagasse, leaf-straw and juice ranged from 75.95% to 84.20%, 80.15% to 89.35%, 77.46% to 86.19% and 81.88% to 92.68%, respectively. The quantitation limits for soil, bagasse, leaf-straw and juice were 0.01, 0.03, 0.04 µg g-1 and 0.008 µg mL-1, respectively. Application of diuron inhibited growth of soil microbes initially but they recovered later. At harvest, diuron residues were below maximum residue limits in all samples. The study revealed that under subtropical conditions, diuron is safe for use in weed management and would not pose any residual/environmental problem and that sugarcane crop could be used safe for human/animal consumption.

Optimal pump profile designs for broadband SBS slow-light systems.

We describe a methodology for designing the optimal gain profiles for gain-based, tunable, broadband, slow-light pulse delay devices based on stimulated Brillouin scattering. Optimal gain profiles are obtained under system constraints such as distortion, total pump power, and maximum gain. The delay performance of three candidate systems: Gaussian noise pump broadened (GNPB), optimal gain-only, and optimal gain+absorption are studied using Gaussian and super-Gaussian pulses. For the same pulse bandwidth, we find that the optimal gain+absorption medium improves the delay performance by 2.1 times the GNPB medium delay and 1.3 times the optimal gain-only medium delay for Gaussian pulses. For the super-Gaussian pulses the optimal gain-only medium provides a fractional pulse delay 1.8 times the GNPB medium delay.

Improved slow-light delay performance of a broadband stimulated Brillouin scattering system using fiber Bragg gratings.

We present a technique for improving the pulse-delay performance of a stimulated Brillouin scattering (SBS) based broadband slow-light system by combining it with fiber Bragg gratings (FBG). We optimize the physical device parameters of three systems: (1) broadband SBS, (2) broadband SBS + a single FBG, and (3) broadband SBS + a double FBG for maximizing the delay performance. The optimization is performed under distortion and system resource constraints for a range of bit rates from 0.5 to 8.5 Gbps. We find that an optimized broadband SBS + a double FBG system improves the fractional delay 1.8 times that of the broadband SBS system at an optimum bit rate of 3 Gbps. Also, pump power consumption is reduced by 15% as compared to the broadband SBS system at the same bit rate.

Limitations of self-phase-modulation-based tunable delay system for all-optical buffer design.

The distortion, noise, and bit-delay performance of a self-phase-modulation-based tunable delay system are analyzed. The pulse amplification required for achieving large spectral broadening results in large amplifier noise. We quantify the resulting delay versus signal-to-noise ratio trade-off. We demonstrate that for high bit rates it is difficult to achieve both large bit delay and good data fidelity. We find that for a given bit rate, reducing the duty cycle improves the fractional bit delay. For a duty cycle of 16%, a maximum bit delay of 15 bits is achieved.