Broad Optical Bandwidth Research Articles

Controlled generation of harmonic states in mid-infrared quantum cascade laser frequency combs by external cavity optical feedback.

We demonstrate the implementation of external cavity optical feedback to improve coherence and promote generation of harmonic states by a mid-infrared quantum cascade laser frequency comb. In particular, we present a Vernier-like scheme to realize harmonic comb states that increase the repetition rate of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm-1 to 92 cm-1. Intermode beatnote and dual comb characterization indicate that the coherence of the comb has greatly improved for sub-optimal devices when the comb is operated in these harmonic states. This approach to control the generation of harmonic states and improve comb performance can be readily incorporated to various sensing systems and has great potential in spectroscopic measurements that require high repetition rates and/or broad optical bandwidth.

Development of a Mercury Bromide Birefringence Measurement System Based on Brewster's Angle.

Mercury bromide (Hg2Br2) has been used to develop acousto-optic tunable filters (AOTFs) because it has several advantages, including a high refractive index, a broad optical bandwidth, and a relatively high figure of merit. Therefore, the measurement of its birefringence is a highly important factor for ensuring AOTF quality. However, for single crystals, it is difficult (at the millimeter scale) to quantify the birefringence using an ellipsometer, as this equipment is only designed to conduct measurements on thin films. In this study, a simple birefringence measurement system for Hg2Br2 was developed based on Brewster's angle at the millimeter scale. The planar distributions of the Hg2Br2 crystal along the (100), (010), and (001) planes were used in the experiments. The developed measurement system can measure the reflected light intensity of the Hg2Br2 crystal depending on the incidence angles (rotations at 0.01125° steps) and can calculate the ordinary and extraordinary refractive indices and birefringence. The calculated birefringence of the Hg2Br2 crystal was 0.8548; this value exhibits an error of 0.64% compared with a value of 0.86 reported in the literature. The developed measurement system demonstrates the ability to be used to evaluate the quality of birefringent single crystals.

InP integrated optical frequency comb generator using an amplified recirculating loop.

A novel realisation of photonically integrated optical frequency comb generation is demonstrated on indium phosphide (InP) using a generic foundry platform. The architecture, based on the amplified recirculating loop technique, consists of cascaded electro-optic phase modulators embedded within a short waveguide loop. While an injected continuous wave laser signal is recirculated by the loop, the modulators are driven with a modulation frequency corresponding to the round-trip loop length frequency. This results in many phase coherent, evenly spaced optical comb lines being generated. The choice of InP as an integration platform allows immediate optical amplification of the modulated signal by embedded semiconductor optical amplifiers, enabling loop losses to be compensated and expanding the comb across broad optical bandwidths. This approach reduces the requirement for external, high-power optical amplifiers, improving the compactness and power efficiency of the full system. The system was modelled to identify off-resonance behaviour, outlining limits in matching both the modulation frequency and seed laser frequency to the round-trip loop frequency for optimal comb line generation to be achieved. The experimental device occupied a fraction of the 6 x 2 mm2 InP chip and operated at round-trip loop frequencies of 6.71 GHz to produce 59 comb lines within a 20 dB power envelope. All comb lines exhibited strong phase coherence as characterised by low composite phase noise measurements of -105 dBc/Hz at 100 kHz. A second device is also presented with a shorter loop length operating at ∼10 GHz which generated 57 comb lines. Both loop configurations included short waveguide phase shifters providing a degree of tunability of the free spectral range with a tuning range of 150 MHz for small injection currents of < 2.5 mA.

Dual-comb quartz-enhanced photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) using two optical combs is a new-born technique, offering appealing features, including broad optical bandwidths, high resolutions, fast acquisition speeds, and wavelength-independent photoacoustic detection, for chemical sensing. However, its further application to, e.g., trace detection, is jeopardized due to the fundamentally and technically limited sensitivity and specificity. Here, we take a different route to comb-enabled PAS with acoustically enhanced sensitivity and nonlinear spectral hole-burning defined resolution. We demonstrate dual-comb quartz-enhanced PAS with two near-infrared electro-optic combs and a quartz tuning fork. Comb-line-resolved multiplexed spectra are acquired for acetylene with a single-pass detection limit at the parts-per-billion level. The technique is further extended to the mid-infrared (for methane), enabling improved sensitivity. More importantly, we measure nonlinear dual-comb photoacoustic spectra for the 12C2H2 ν1 + ν3 band P(17) transition with sub-Doppler pressure-broadening dominated homogeneous linewidths (e.g., 45.8 MHz), hence opening up new opportunities for Doppler-free photoacoustic gas sensing.

Broadband 1-GHz mid-infrared frequency comb

Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm−1) spectral point spacing and a full bandwidth of >5 THz (>166 cm−1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources.

Mode division multiplexing on an InP membrane on silicon

A five-channel mode division multiplexed on-chip optical bus is designed and demonstrated on a submicron-thick InP membrane wafer bonded on a Si substrate. Dual-core adiabatic tapers are leveraged for realization of the mode (de)multiplexers. The optimized device shows low excess optical loss of maximum 0.14 dB compared to a reference waveguide and low cross talk of maximum −18.5 dB for all five channels over a broad optical bandwidth of 1510–1600 nm. High fabrication tolerance to width variations is also demonstrated, where a low excess loss of less than 1 dB and a low cross talk of less than −14 dB are maintained in the range of 1530–1585 nm, covering the C-band, when the width varies by up to 50 nm. The demonstrated results show an essential step toward a monolithic photonic layer on top of electronic chips for high-capacity on-chip optical interconnects.

2-[formula omitted] Ultra-broadband polarization beam splitter with tunable transmissions based on silicon-Ge[formula omitted]Sb[formula omitted]Se[formula omitted]Te[formula omitted] hybrid subwavelength gratings

Recently, integrated optical devices working at 2-μm wavelengths have attracted considerable attention. Among such devices, a polarization beam splitter is an essential building block for constructing the on-chip circuit. Here, we have proposed an ultra-broadband polarization beam splitter with tunable transmissions working at 2-μm waveband. Coupled-mode based directional coupler is utilized for the device design. Subwavelength grating structures provide efficient dispersion control, which endows broad optical bandwidths. In addition, a rear-bent coupler is cascaded at the output of the device to furtherly suppress the crosstalk. The silicon-Ge2Sb2Se4Te1 hybrid materials provide tunable transmissions. When the Ge2Sb2Se4Te1 is in the amorphous state, with high extinction ratios more than 20 dB, the bandwidths exceed 200 nm (1900∼2100 nm) and 130 nm (1925∼2055 nm) for TE0 and TM0, respectively. Furthermore, the insertion losses are less than 0.24 dB (1900∼2100 nm) and 0.56 dB (1925∼2055 nm) for TE0 and TM0, respectively. When being in the crystalline state, the device works as an absorber that fully attenuates the input energy. In different intermediate amorphous–crystalline​ hybrid states, it is also possible to achieve tunable transmissions.

High-sensitivity integrated SiN rib-waveguide long period grating refractometer

In this research, we demonstrate a high-sensitivity integrated silicon nitride long period grating (LPG) refractometer based on a rib waveguide with sinusoidally modulated width. While integrated LPG architectures typically achieve ultrahigh sensitivity only over a narrow optical bandwidth using a phase-matching turning-point optimization technique, our sensor exhibits a very high refractometric sensitivity that was designed to remain constant over a broad operational optical spectral bandwidth. The proposed design method relies on multi-modal dispersion tailoring that consists of homogenizing the spectral behaviors of both group and effective indices of the coupling modes. Experimental results are in agreement with numerical simulations, demonstrating not only a sensitivity reaching 11,500 nm/RIU but, more significantly, also that this sensitivity remains almost constant over a broad spectral range of at least 100 nm around 1550 nm. Additional advantages of the proposed sensor architecture encompass a low temperature sensitivity, down to −0.15 nm/K, and simplicity of the fabrication process. These results demonstrate the feasibility of chip-scale photonic integration to achieve both high sensitivity and large dynamic range of the proposed refractometer.

Ultra-rapid electro-optic sampling of octave-spanning mid-infrared waveforms.

We demonstrate ultra-rapid electro-optic sampling (EOS) of octave-spanning mid-infrared pulses centered at 9 μm, implemented by mechanically scanning a mirror with a sonotrode resonating at 19 kHz (forward and backward acquisition at 38 kHz). The instrument records the infrared waveform with a spectral intensity dynamic range of 1.6 × 105 for a single scan over a 1.6-ps delay range, acquired within 26 μs. The purely reflective nature of the delay scanning technique is compatible with broad optical bandwidths, short pulse durations (16 fs, centered at 1030 nm) and high average powers (Watt-level). Interferometric tracking of the sonotrode motion in combination with a predictor-corrector algorithm allows for delay-axis determination with down to single-digit attosecond precision. Ultra-rapid mid-infrared EOS will advance applications such as molecular fingerprinting of static samples as well as tracking of biological processes and chemical reactions and is likely to find new fields of application such as infrared-spectroscopic flow cytometry.

Tailoring a sub-diffraction optical focus via a straightforward interferometric approach

An approach for yielding light focuses below Abbe’s diffraction limit in Gaussian beams is presented. The method uses only standard passive optical elements as lenses, filters and mirrors and consists of a Michelson interferometric setup, where one of the light branches is modified in amplitude and/or phase. The focus narrowing process is carried out at the focal plane of a spherical lens by the interference of altered and unaltered light branches. The main focus features, namely, the focus intensity and size as well as the sidelobe intensity, are adjusted by varying two external parameters in a controllable manner under the conditions of pure destructive interference. Narrowing of the diffraction limit close to 40% with reduced intensity sidelobes (10%) is achieved. Due to the use of only lenses and mirrors, the approach does work with laser beams within a broad optical bandwidth ranging from infrared to ultraviolet in continuum regime as well as in ultra-short pulse regime. The method can also be implemented for high-power lasers and temporal domains. The focus-narrowing process emerges as a natural mechanism to the light interference, bringing a fresh perspective to applications from a few controllable degrees of freedom. The good performance of the sub-diffraction optical focus and the simplicity of the experimental setup promote new opportunities in fields ranging from optical manipulation of particles at sub-wavelength scale to optical writing and super-resolution microscopy.

Comparison Study of Multi-Slot Designs in Epsilon-Near-Zero Waveguide-Based Electro-Optical Modulators

We present a systematical comparative research on the modulation performance and the optimization of the multi-slot waveguide modulator at the telecom wavelength of 1.55 μm. It is found that the existence of epsilon-near-zero indium tin oxide slots in modulators can enhance the optical confinement, thus yielding a high extinction ratio (ER) and low insertion loss. Among the designed four types of slot modulators, the single-slot modulator exhibits the widest modulation bandwidth of 78.75 GHz and lowest energy consumption of 1.15 pJ/bit, while the dual-slot one shows the moderate performance. For the more-slot designs, the tri-slot has the advantages of a maximum figure of merit of ~106, and the quadri-slot one has the highest ER of 1.38 dB/μm. By integrating the multi-slot modulator on the silicon waveguide, the quadri-slot modulator waveguide exhibits broad optical bandwidth of 83 nm from 1479 to 1560 nm and large modulation depth (~18.3 dB). The performance of four types of slot modulators can be further improved from different aspects by the optimization of their geometric parameters. The results of this work could be useful in the design and selection of high performance on-chip modulators for optical communications and ultrafast data processing.

Surface acoustic microwave photonic filters in standard silicon-on-insulator

The processing of analog microwave-frequency signals using optical means becomes increasingly important as part of advanced cellular networks. Chip-level integration of microwave photonic filters, particularly in silicon, is considered necessary for their large-scale deployment. Discrete-time, delay-and-sum filters are widely used to select narrow spectral bands out of broad optical bandwidths. However, the long delays that are required to obtain narrowband filters are difficult to accommodate in integrated optic waveguide paths. In this work, we report discrete-time, integrated microwave photonic filters on standard silicon-on-insulator. Long delays are realized through the conversion of incoming radio-frequency modulation to the form of slow-moving surface acoustic waves. Conversion relies on thermo-elastic expansion of metallic gratings and does not involve piezoelectricity. Information is recovered in the optical domain via photoelastic modulation of probe light in a resonator waveguide. The resonator is patterned to support multiple delayed modulation events. Filters having up to 12 taps are demonstrated, with 175 ns-long delays and passbands as narrow as 5 MHz. The magnitude and radio-frequency phase of each filter tap are designed arbitrarily, independent of those of all others. The coherent summation of delayed waveform replicas is free of environmental phase drifts. Surface acoustic wave modulation of a compact, defect grating waveguide is demonstrated as well. Surface acoustic wave devices can significantly extend the signal-processing capabilities of silicon photonics.

Design of bimetal-coated photonic crystal fiber filter based on surface plasmon resonance

A novel the dual channel bimetal-coated photonic crystal fiber (PCF) single-polarization filter with a high birefringence structure based on surface plasmon resonance (SPR) is proposed and investigated by the finite element method (FEM). The advantages of the filter include high unwanted loss, good extinction ratio characteristics, flexibility in regulating the filtering range, large crosstalk value, as well as broad optical bandwidth. Specifically, the orthogonal two polarized modes can be operated simultaneously at communication wavelengths, and the unwanted losses are up to 591.5 dB/cm and 555.1 dB/cm at 1300 nm, 1570 nm. For a fiber length L of 1000 µm, the crosstalk (CT) values are −492.8 dB and 447.5 dB at 1300 nm and 1570 nm, respectively, corresponding to available optical bandwidths of 258.5 nm and 521.3 nm. The filtering range of the single-polarization filter can be adjusted by varying structural parameters. X-polarized mode filtering range is 1300–1530 nm, while the filtering range of y-polarized mode is 1570–1830 nm.

Integrated nano-optomechanical displacement sensor with ultrawide optical bandwidth

Optical read-out of motion is widely used in sensing applications. Recent developments in micro- and nano-optomechanical systems have given rise to on-chip mechanical sensing platforms, potentially leading to compact and integrated optical motion sensors. However, these systems typically exploit narrow spectral resonances and therefore require tuneable lasers with narrow linewidth and low spectral noise, which makes the integration of the read-out extremely challenging. Here, we report a step towards the practical application of nanomechanical sensors, by presenting a sensor with ultrawide (∼80 nm) optical bandwidth. It is based on a nanomechanical, three-dimensional directional coupler with integrated dual-channel waveguide photodiodes, and displays small displacement imprecision of only 45 fm/Hz1/2 as well as large dynamic range (>30 nm). The broad optical bandwidth releases the need for a tuneable laser and the on-chip photocurrent read-out replaces the external detector, opening the way to fully-integrated nanomechanical sensors.

High-Speed Plasmonic-Silicon Modulator Driven by Epsilon-Near-zero Conductive Oxide

Transparent conductive oxides (TCOs) such as indium-tin oxide (ITO) have attracted increasing interests in integrated photonics and silicon photonics, owing to their large plasma dispersion and epsilon-near-zero (ENZ) effect. The nonlinear change of refractive index induced by free carrier modulation leads to a large electro-optic modulation with ultra-compact device footprint and unprecedented energy efficiency. However, high-speed modulation result is rare, mainly due to the lack of high-speed device design and fabrication quality. In this article, we characterize the fundamental electro-optic modulation structure consisting of Au/ITO/oxide/p-Si capacitor, showing that the property of ITO is greatly affected by the process condition. We also report an 8-μm-long hybrid plasmonic-silicon modulator driven by an ENZ ITO capacitor, achieving 100fJ/bit energy efficiency, 3.5 GHz modulation bandwidth, and 4.5 Gb/s data rate. The electro-absorption modulator covers a broad optical bandwidth from 1515 to 1580 nm wavelength. For future development of such modulators, we point out that by replacing ITO with higher mobility TCO materials, we can achieve 40 GHz modulation bandwidth and 0.4 fJ/bit energy efficiency using a 3-μm-long device.

LED as laboratory test source for astronomical intensity interferometry.

The challenge of astronomical intensity interferometry is to detect the small photon-bunching signals of distant sources with a broad optical bandwidth. We have built a Hanbury Brown-Twiss-like laboratory intensity interferometer with a focus on a relatively broad bandwidth (1nm FWHM optical filter) and high photon rates (up to 10MHz) per channel compared to typical (non-astronomical) intensity interferometry applications. As a light source we use a green LED to simulate starlight. The LED has proven to be a compact high-power source of stochastic light with a special advantage of a small emission area, which favours spatial coherence. Using single-photon correlations, we detect a bunching signal in the second-order correlation function with a coherence time of <1ps and an amplitude of <4⋅10-4 and describe signal and background quantitatively for a 40 hours measurement. In this paper we show our setup, present the correlation measurements and compare them to theoretical expectations.

Sensitivity Comparison of Refractive Index Transducer Optical Fiber Based on Surface Plasmon Resonance Using Ag, Cu, and Bimetallic Ag-Cu Layer.

A single-mode optical fiber sensor uses surface plasmon resonance (SPR) with a bimetallic silver–copper (Ag–Cu) coating compared to a single layer of Ag and Cu itself. Bimetallic Ag–Cu sensors are constructed by simple fabrication on a side-polished optical fiber, followed by an electron beam evaporation of Ag and Cu films. For this investigation, the thickness of the single Ag layer was set to 30 nm and the single Cu layer was set to 30 nm; whereas for the bimetallic combined Ag–Cu layer the thickness of Ag was 7 nm and Cu 23 nm. The sensor performance was analyzed and compared experimentally and numerically using the COMSOL Multiphysics. A white light source was used with a broad optical bandwidth to provide a range of wavelengths to the optical fiber. The characteristics of the thin layers of Ag, Cu, and Ag–Cu as alcohol sensors were evaluated. We found that Cu was the most sensitive metallic layer compared to the Ag and the bimetallic Ag–Cu layers. For a 100% alcohol concentration, Cu showed a sensitivity of 425 nm/RIU followed by the bimetallic Ag–Cu layer with 108.33 nm/RIU, whereas the Ag layer was not detected. Interestingly, sensitivity reached saturation beyond the 20 nm thick layer of Ag. This shows that the Cu and the bimetallic Ag–Cu layers are suitable for an alcohol-based optical sensor.

Stable multi-megahertz circular-ranging optical coherence tomography at 1.3 µm.

In Fourier-domain optical coherence tomography (OCT), the finite bandwidth of the acquisition electronics constrains the depth range and speed of the system. Circular-ranging (CR) OCT methods use optical-domain compression to surpass this limit. However, the CR-OCT system architectures of prior reports were limited by poor stability and were confined to the 1.55 µm wavelength range. In this work, we describe a novel CR-OCT architecture that is free from these limitations. To ensure stable operation, temperature sensitive optical modules within the system were replaced; the kilometer-length fiber spools used in the stretched-pulse mode-locked (SPML) laser was eliminated in favor of a single 10 meter, continuously chirped fiber Bragg grating, and the interferometer's passive optical quadrature demodulation circuit was replaced by an active technique using a lithium niobate phase modulator. For improved imaging penetration in biological tissues, the system operating wavelength was shifted to a center wavelength of 1.29 µm by leveraging the wavelength flexibility intrinsic to CFBG-based dispersive fibers. These improvements were achieved while maintaining a broad (100 nm) optical bandwidth, a long 4 cm imaging range, and a high 7.6 MHz A-line rate. By enhancing stability, simplifying overall system design, and operating at 1.3 µm, this CR-OCT architecture will allow a broader exploration of CR-OCT in both medical and non-medical applications.

Silicon photonic wavelength cross-connect with integrated MEMS switching

We report on monolithically integrated wavelength cross-connects (WXCs) on an enhanced silicon photonic platform with integrated micro-electro-mechanical-system (MEMS) actuators. An 8 × 8 WXC with 8 wavelength channels comprising 16 echelle gratings and 512 silicon photonic MEMS switches is integrated on a 9.7 mm × 6.7 mm silicon chip. The WXC inherits the fundamental advantages of silicon photonic MEMS space switches, including low loss, broad optical bandwidth, large fabrication tolerance, and simple digital control. The WXC exhibits a low crosstalk of −30 dB, a submicrosecond switching time of 0.7 µs, and on-chip optical insertion losses varying from 8.8 dB to 16.4 dB. To our knowledge, it is the largest channel capacity (64 channels = 8 ports × 8 wavelengths) integrated WXC ever reported.

A Dual-Slot Electro-Optic Modulator Based on an Epsilon-Near-Zero Oxide

A dual-slot silicon electro-optic modulator based on indium tin oxide is proposed with high modulation efficiency, low loss, and broadband operation leveraging the epsilon-near-zero (ENZ) effect. The design is verified by numerical simulation with Lumerical Solutions. The active modulation region consists of two slots for enhancing the overlap between the optical mode and the ENZ region to improve the performance of the electro-absorption modulator. By combining the confinement of the dual-slot waveguide and the ENZ effect, we can obtain a high modulation efficiency of 1.44 dB/ $\mu$ m at the wavelength of 1.55 $\mu$ m. The modulator with a modulation region length of only 10 $\mu$ m demonstrates a low insertion loss and a high extinction ratio, with the corresponding values of 0.37 and 14.4 dB, respectively. Through the optimized design, the efficient coupling can be achieved between the silicon waveguide and the dual-slot waveguide with the coupling loss of only 0.12 dB. Moreover, the modulator can operate in a wide spectrum, covering the S- , C- , and L -band. The dual-slot electro-optic modulator thus provides various merits including high modulation efficiency, ultracompact footprint, broad optical bandwidth, and complementary metal–oxide–semiconductor compatibility.