Optical Modulation Depth Research Articles

Q-switched Nd:YVO4 laser operating at 1064 nm with NiPS3 nanoflakes onto a silica metasurface as saturable absorbers.

In this work, we show that a metasurface can be used to improve the performance of the two-dimensional (2D) material saturable absorber in a Nd:YVO4 solid-state laser. To our knowledge, the hybrid saturable absorber was fabricated by spraying the NiPS3 nanoflakes onto a silica metasurface for the first time. It is shown that the optical absorption, modulation depth, saturation intensity, and ultrafast recovery time of the metasurface-NiPS3 saturable absorber exhibit better performance than the 2D material control device. In a proof-of-concept experiment, the Q-switched pulses with a pulse duration of 20.5 ns, repetition rate of 4.35 MHz, output power of 2.3 W, peak power of 30.61 W, and pulse energy of 0.63 μJ were experimentally demonstrated. These findings suggest that a hybrid saturable absorber is a promising candidate for developing pulsed laser and optical modulators.

Broadband saturable absorption responses of few-layered MoSe2 nanosheets for all-solid-state Q-switched lasers

Few-layered molybdenum diselenide (MoSe2) nanosheets were successfully fabricated by the ultrasound-assisted liquid phase exfoliation (UALPE) method. It exhibits broadband nonlinear optical (NLO) responses with optical modulation depths of 12.4% and 9.3% at 1.3 and 2 μm, respectively. Based on this, it was transferred onto a quartz substrate, acting as a saturable absorber (SA) to initiate solid-state pulsed lasers. The Q-switched Nd: GdVO4 laser with a minimum pulse duration of 366 ns and a repetition rate of 196 kHz was generated at around 1.3 μm. Meanwhile, the Q-switched Tm: YLF laser with a minimum pulse duration of 200 ns and a repetition rate of 43 kHz was obtained at around 2 μm. The experimental results indicate that few-layered MoSe2 nanosheets have a significant development prospect as a SA in optical modulation with a broad spectral band range at the near-infrared (NIR) band.

Realization of harmonic mode-locking in thulium-doped fiber laser using In2(SO4)3-based saturable absorber

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.

Gate‐Tunable Graphene Optical Modulator on Fiber Tip: Design and Demonstration

AbstractA fiber‐end‐face‐integrated electrically tunable optical modulator based on graphene field‐effect transistor and the fabrication technique are proposed and demonstrated. The all‐fiber device features a scheme of compactness and high integration when applied to fiber optical system, as well as a low insertion loss since spatial coupling of light is avoided. A reflecting structure is used and the electro‐optic modulator shows vivid modulation when gate voltage gets tuned, with maximum optical reflectance modulation depth over 1% in demonstration. To the best of the authors’ knowledge, it is the first time to build an all‐solid gate‐control modulator directly on fiber facet. The fabrication methods introduce workflows of silicon‐on‐insulator processing into the optical fiber end‐face micromachining domain which can be applied to more comprehensive configurations, broadening the path of fiber‐end‐face integrated device.

All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

AbstractLayered 2D materials display unique optical and electrical properties that can enable manipulation, propagation, and detection of electromagnetic waves over a broad spectral range, with a high level of control, offering the potential to activate different functionalities, by optical or electrical means, in a single chip. Here, a compact optoelectronic device behaving as an amplitude modulator, saturable absorber mirror (SA mirror), and frequency‐tuner is conceived at terahertz (THz) frequencies. It comprises a gate‐tunable single layer graphene (SLG), embedded in a quarter‐wave cavity, operating in the 1–5 THz range. The use of electrolyte ionic liquid gate ensures 40% optical amplitude modulation depth. Z‐scan self‐mixing interferometry reveals 60% reflectivity modulation, with ≈4.5 W cm−2 saturation intensity. By integrating the modulator/SA mirror with a heterogeneous THz quantum cascade laser frequency comb, in an external cavity configuration, fine‐tuning of the intermode beatnote frequency is also demonstrated. This opens intriguing perspectives for short pulse generation, phase‐locking, frequency tuning/chirping, phase modulation, and metrological referencing, inter alia.

Improving the tolerances in high-performance silicon photonics modulators

A racetrack-shaped modulator, based on Double Injection method, has been recently presented able to provide multiple response shapes and exhibiting enhanced robustness to deviations in parameters. Fabrication process deviations that influence sensitive ingredients of ring-based modulators such as the ring's waveguide, couplers, and heat that is generated in Silicon Photonics devices due to carrier transport, can degrade the modulator extinction-ratio (ER) and optical modulation depth (OMD). The sensitivity of the, so-called, PIR20 configuration (Parameters-Insensitive Response with ER of 20 dB) to fabrication and temperature deviations is analyzed in detail. Comparison to other known modulators based on either a ring, MZI or Fabry-Pérot resonators, reveals that the PIR20 device is considerably less sensitive to deviations of these types.

Avalanche Photodiodes With Composite Charge-Layers for Low Dark Current, High-Speed, and High-Power Performance

In this work, a novel In_0.52Al_0.48As based top-illuminated avalanche photodiode (APD) is demonstrated. By combining the composite charge-layer design with a special p-side up etched mesa structure to zero the electric (E)-field at the periphery of this APD's multiplication (M-) layer, the edge breakdown phenomenon can be eliminated. This in turn leads to the simultaneous high-speed, high-saturation-power, high responsivity, and low-dark current performance characteristics of our APDs, which are essential for high-performance coherent receiver applications. The demonstrated device, with its simple top-illuminated structure exhibits a wide optical-to-electrical (O-E) bandwidth (21 GHz), high responsivity (5.5 A/W at 0.9 V_br), and saturation current as high as 8 mA with a large active diameter of 24 μm for easy optical alignment. Furthermore, the nonlinear driving of a wavelength sweeping laser in the self-heterodyne beating setup can generate an optical pulse train like waveform, providing an effective optical modulation depth of up to 158%, which leads to a maximum photo-generated RF power (at 10 GHz) from our APD as high as +5.5 dBm. The excellent performance of our demonstrated APDs opens up new possibilities for the next generation of coherent receivers.

Nonlinear optical response of inverse-designed integrated photonic devices.

Gradient-based optimization combined with the adjoint method has been demonstrated to be an efficient way to design a nano-structure with a vast number of degrees of freedom. However, most inverse-designed photonic devices are applied as linear photonic devices. Here, we demonstrate the nonlinear optical response in inverse-designed integrated splitters fabricated on a SiN platform. The splitting ratio is tunable under different incident powers. The thermo-optical effect can be used as an effective approach for adjusting the nonlinear optical response threshold and modulation depth of the device. These promising results indicate the great potential of inverse-designed photonic devices in nonlinear optics and optical communications.

Independent regulation of electrical properties of VO2 for low threshold voltage electro-optic switch applications

Vanadium dioxide has a unique insulator-metal phase transition characteristic near room temperature, meeting optical switches' requirements. However, due to its high resistance in the insulator state, the working voltage threshold of the VO2-based optical switch is also high. It is one of the obstacles for VO2-based optical switch applications. In this work, high-quality VO2 films were prepared by DC pulsed magnetron sputtering. By inserting a conductive ITO buffer layer, VO2 sheet resistance reduced about 2 orders, and the electrical phase change characteristics have almost disappeared. Still, the high optical modulation ability is hardly affected. The interesting results meet the requirements of a low threshold working voltage electro-optic switch. In the case of a 40 µm electrode gap size, the threshold voltage for VO2/ITO is only 1.4 V, which is much lower than 20 V of pure VO2 in the same condition, and the optical switch modulation depth at the wavelength of 1550 nm reaches 35%. For the switch with 5 µm electrode gap size, the threshold voltage is only 1.3 V, and the response times of "ON" and "OFF" are 0.24 ms and 2.1 ms, respectively. This result is valuable for the practical application of VO2-based optical switches.

Rydberg RF Receiver Operation to Track RF Signal Fading and Frequency Drift

Significant progress has been made in recent years demonstrating an all-optical high sensitivity Radio Frequency (RF) sensor exploiting the Electromagnetically Induced Transparency (EIT) effect in an atomic vapor. Rydberg RF sensors have previously been shown to exhibit excellent lab-based performance in measuring electric field properties. As a commercial network operator we wish to exploit these capabilities to produce a rugged high-sensitivity receiver for 5G signals, so we seek simple planning rules applicable to readily available low cost components. We present a simple analytic model for the operation of a sensitive Rydberg RF receiver to predict the optical modulation in response to an RF signal. We focus on maximizing the contrast in the optical response to a modulated RF signal rather than to a constant RF field. We show that the optical modulation depth can be maximized using an optimum coupling intensity. The optical modulation depth drops off rapidly as the RF source is detuned away from resonance. By detuning the coupling frequency to compensate for the RF detuning we demonstrate that the useful RF bandwidth of the receiver can be significantly extended. We measure this variation in optical response using a Rubidium vapor at a 15 GHz RF carrier frequency. Using readily available optical components we obtain good agreement with our model. This convenient analytic approach will simplify the design and operation of Rydberg RF receivers as sensitive components in future 5G mobile communications, particularly for the power-limited uplink from mobile device to base-station.

Generalized Analysis of Dual-Output Mach-Zehnder Modulator With Applications to Photonic-Assisted Instantaneous Frequency Measurement

We propose a general and exact analytical model forintensity modulation and direct detection (IM/DD) fiber optic links using dual-output dual-drive Mach-Zehnder modulator (DO-MZM). This model extends previously published results relying on the small-signal approximation and infinite extinction ratio at the modulator outputs. In our treatment the output optical powers are calculated through analytical expressions taking into account the modulator electronic driver and directional coupler characteristics. Closed-form expressions, explicitly displaying the dependence of the link output power on the directional coupler transmission coefficients, optical modulation depth, and bias point, are obtained for output optical power, frequency chirp, extinction ratio and static characteristic MZM curves. We also propose an equivalent interferometric representation of each DO-MZM output in terms of virtual dual-drive Mach-Zehnder modulators (DD-MZMs) and show that each modulator output has distinct characteristic curves. The model is applied to a photonic-assisted instantaneous frequency measurement (IFM) based on frequency-to-power mapping, in which an amplitude comparison function (ACF) is generated by the power fading functions. We have obtained a generalized ACF through which both the frequency and amplitude of the input RF signal can be determined by a search procedure.

MoxTa(1-x)Se2 ultrafast fiber laser

Ternary transition metal chalcogenides (TTMCs) are emerging as a promising branch of two-dimensional (2D) materials due to their special layered structure as well as wide-range tunable bandgap properties, which permit the implementation of novel optoelectronic characteristics. The manifestation of nonlinear optical properties of MoxTa(1-x)Se2 nanosheets and their application in ultrafast erbium-doped fiber (EDF) laser is reported in this work. The MoxTa(1-x)Se2 nanosheets were fabricated by employing the liquid phase exfoliation technology. In addition, the saturable absorption property of MoxTa(1-x)Se2-based saturable absorber (SA) is illustrated by obtaining a large optical modulation depth of 32.01 (0.76)%. Furthermore, an operational ultrashort pulse fiber laser was demonstrated. The laser has the ability to emit ultrashort pulses of 377 (0.3) fs at 1532 nm while a maximum output power of 10.52 mW was delivered by the employed within the EDF cavity MoxTa(1-x)Se2-based SA. Our experimental results provide concrete pieces of evidence that the MoxTa(1-x)Se2 nanosheets exhibit third-order optical nonlinearity and the significant potential in nonlinear optoelectronic devices.

In2S3-based saturable absorber for passively harmonic mode-locking in 2 μm region

There are few reports on the mode-locking operation of indium sulfide (In2S3) in fiber lasers. In this paper, we experimentally demonstrate a harmonic mode-locked fiber laser in 2-μm region by taking advantage of an In2S3-based saturable absorber (SA). The In2S3-based SA has nonlinear optical modulation depth of 20.8 % and saturation power of 0.14 kW at 1930 nm. The fundamentally mode-locked solitons at 1938.59 nm with the 3-dB bandwidth of 5.04 nm, pulse width of 1.16 ps, and pulse repetition rate of 41.73 MHz are generated. The maximum harmonic-resonance frequency is 459 MHz (11th harmonic), which is the highest operating frequency that has been demonstrated in In2S3-based mode-locked fiber lasers as far as we know. The spectral and temporal characteristics of the output pulses are investigated versus harmonic order. The results prove that In2S3 possesses the characteristics of mode-lockers and could be a promising optical switch for ultrafast pulses generation.

Heterointerface-Enhanced Ultrafast Optical Switching via Manipulating Metamaterial-Induced Transparency in a Hybrid Terahertz Graphene Metamaterial.

We have demonstrated the active manipulation of metamaterial-induced transparency (MIT) in a terahertz hybrid metamaterial with graphene overlayer under photoexcitation. It is found that the introduction of graphene can greatly modify the resonant dips and transparency window through the formed depolarization field around unequal-length double bars to weaken dipole resonances and their destructive interference. Transient control of MIT behaviors is determined by the photogenerated carrier dynamics, which influences the distributions of currents and electric fields in the resonant region to hinder the near-field coupling of two bright modes. Optical modulation depth is sensitive to bar spacing due to an anomalous increased double-bar coupling involving intracell and intercell interaction. Heterointerface formed by the added graphene with substrate could further enhance terahertz response via effective separation of the photoexcited carriers. Theoretical calculation based on the coupled Lorentz oscillator model reveals that the photoinduced terahertz response mainly originates from the coupling and damping in hybrid structures. Our findings could facilitate the development of graphene-based dynamical terahertz modulators and optoelectronic devices.

Optical control of fast and processive engineered myosins in vitro and in living cells.

Precision tools for spatiotemporal control of cytoskeletal motor function are needed to dissect fundamental biological processes ranging from intracellular transport to cell migration and division. Direct optical control of motor speed and direction is one promising approach, but it remains a challenge to engineer controllable motors with desirable properties such as the speed and processivity required for transport applications in living cells. Here, we develop engineered myosin motors that combine large optical modulation depths with high velocities, and create processive myosin motors with optically controllable directionality. We characterize the performance of the motors using in vitro motility assays, single-molecule tracking and live-cell imaging. Bidirectional processive motors move efficiently toward the tips of cellular protrusions in the presence of blue light, and can transport molecular cargo in cells. Robust gearshifting myosins will further enable programmable transport in contexts ranging from in vitro active matter reconstitutions to microfabricated systems that harness molecular propulsion.

High-Power and High-Responsivity Avalanche Photodiodes for Self-Heterodyne FMCW Lidar System Applications

In this work, we demonstrate the high-power and high-responsivity performance of the dual multiplication (M-) layers in In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</sub> As based avalanche photodiode (APD). The dual M-layer design in our APD structure effectively constrains the multiplication process to a thin high-field region rather than the whole thick M-layer. It thus minimizes the space charge effect (SCE) within and avoids increasing the tunneling dark current for the case of directly shrinking M-layer thickness in APD. Furthermore, by combining the specially designed mesa shape with this dual M-layer structure, the edge breakdown can be well suppressed. These benefits lead to an ultra-high gain-bandwidth product (450 GHz; 1 A/W at unit gain) and a high saturation current (>12 mA) can be simultaneously achieved in our device. By nonlinearly driving a wavelength sweeping laser in the self-heterodyne lidar setup, it can generate an optical pulse train-like waveform, providing an effective optical modulation depth of 200% to feed into our demonstrated APD at the receiver-end. Under such scheme, the photo-generated RF (1 GHz) power from our APD with a 6.3 A/W responsivity can be as high as +6.95 dBm at a high (7 mA) output photocurrent. Such high-power and high-responsivity characteristics of our APD can further improve the signal-to-noise (S/N) ratio and dynamic range performances in each pixel of the lidar image. A high-quality 3-dimensional (D) FMCW lidar image is constructed based on our APD, without the integration of any electrical amplifier at the receiver end.

Spectrally broadband electro-optic modulation with nanoelectromechanical string resonators.

In this paper, we present a shutter-based electro-optical modulator made of two parallel nanoelectromechanical silicon nitride string resonators. These strings are covered with electrically connected gold electrodes and actuated either by Lorentz or electrostatic forces. The in-plane string vibrations modulate the width of the gap between the strings. The gold electrodes on both sides of the gap act as a mobile mirror that modulate the laser light that is focused in the middle of this gap. These electro-optical modulators can achieve an optical modulation depth of almost 100% for a driving voltage lower than 1 mV at a frequency of 314 kHz. The frequency range is determined by the string resonance frequency, which can take values of the order of a few hundred kilohertz to several megahertz. The strings are driven in the strongly nonlinear regime, which allows a frequency tuning of several kilohertz without significant effect on the optical modulation depth.

2D Ductile Transition Metal Chalcogenides (TMCs): Novel High‐Performance Ag2S Nanosheets for Ultrafast Photonics

AbstractAs a member of the 2D transition metal chalcogenides (TMCs), 2D Ag2S with large absorption coefficient, broadband optical response, extraordinary metal‐like ductility, high plastic deformation strain, unique layered structure, and very low solubility has offered a platform for achieving tremendous success in various fields. Great achievements have been made in exploring the linear optical, mechanical, and photoelectric properties of 2D Ag2S nanosheets. However, the nonlinear optical properties of 2D Ag2S nanosheets and their applications in ultrafast photonics have not been studied. The saturable absorption property of 2D Ag2S nanosheets is experimentally illustrated at the telecommunication band with a large optical modulation depth of 15.5% and a low saturable intensity of 250.4 MW cm−2. Moreover, the 229th harmonic soliton molecule with repetition rate of 1.1 GHz based on 2D Ag2S saturable absorber is realized at the telecommunication band for the first time to the authors' knowledge. Thankfully, 2D Ag2S nanosheets display excellent nonlinear optical properties suggesting that this may open a new door for advanced photonic devices based on 2D Ag2S nanosheets.

Comparison Study of Gold Nanorod and Nanoparticle Monolayer Enhanced Optical Terahertz Modulators

Nanostructure modified semiconductors show enormous potential for the optical terahertz (THz) spatial modulation due to their enhanced modulation depth along with their inherent broad bandwidth, small insertion loss, and fast modulation speed. As a result, it is necessary to explore the mechanism of the performance enhancement by the interface modification. Here, the self-assembled monolayer of gold nanorods and nanoparticles was coated on silicon to enhance the optical modulation depth of THz wave. Comparison experiment and the finite-difference time-domain simulation have been performed between the self-assembled monolayer of gold nanorods and nanoparticles to understand the mechanism for improving the light modulation of THz wave. We found that the modulation depth and the speed of THz wave were mainly determined by the light resonance frequency of gold nanoparticles/nanorods and the area with enhanced light intensity on silicon surface. Our results provide a guide for the light-controlled THz modulator based on the nanostructure modified semiconductor.

Silicon photonic switch-based optical equalization for mitigating pulsewidth distortion.

Optical transmitters typically require electrical pre-amplification using driver amplifiers to optimize the optical modulation depth. To enhance the detection sensitivity and optimize the overall link budget, equalization is required to compensate for undesired signal distortion induced by the transmitter. In this paper, we propose and demonstrate a novel optical equalization scheme using a silicon photonic micro-ring resonator (MRR)-based switching circuit for mitigating driver-amplifier-induced pulsewidth distortion. The switching circuit simultaneously functions as a spatial optical switch as well as a two-stage optical bandpass filter for optical equalization. The experimental results indicate a 4.5-dB detection sensitivity enhancement at a data rate of 12.5 Gbits/s. The proposed approach is robust to different levels of pulsewidth distortion without additional signal processing, and has possibilities to support higher data rates by adjusting the MRR parameters.