All-optical Intensity Modulation Research Articles

Toward Complete All-Optical Intensity Modulation of High-Harmonic Generation from Solids.

Optical modulation of high-harmonics generation in solids enables the detection of material properties, such as the band structure, and promising new applications, such as super-resolution imaging in semiconductors. Various recent studies have shown optical modulation of high-harmonics generation in solids, in particular, suppression of high-harmonics generation has been observed by synchronized or delayed multipulse sequences. Here we provide an overview of the underlying mechanisms attributed to this suppression and provide a perspective on the challenges and opportunities regarding these mechanisms. All-optical control of high-harmonic generation allows for femtosecond, and in the future possibly subfemtosecond, switching, which has numerous possible applications: These range from super-resolution microscopy to nanoscale controlled chemistry and highly tunable nonlinear light sources.

Ultrafast modulation of a high harmonic generation in a bulk ZnO single crystal.

Optical modulation of high harmonic generation (HHG) is of fundamental interest in science and technology, which can facilitate understanding of HHG generation mechanisms and expand the potential optoelectronic applications. However, the current established works have neither shown the advanced modulation performance nor provided a deep understanding of modulation mechanisms. In this work, taking wurtzite zinc oxide (ZnO) single crystal as a prototype, we have demonstrated an all-optical intensity modulation of high-order HHG with a response time of less than 0.2 ps and a depth of more than 95%, based on the pump-probe configuration with two different pumping wavelengths. Besides the achieved excellent modulation performance, we have also revealed that the modulation dynamics in ZnO single crystal highly depend on the excitation conditions. Specifically, the modulation dynamics with the near-bandgap or above-bandgap excitation are attributed to the non-equilibrium interband carrier relaxations, while for mid-gap excitation, the modulation dynamics are dominated by the nonlinear frequency mixing process. This work may enhance the current understanding of the HHG modulation mechanism and enlighten novel device designs.

Nanosecond-scale all-optical Ti3C2T x -MXene modulator

Inspired by recent advancements of graphene-based ultrafast photonic devices, Ti3C2T x -MXene, as a graphene-like two-dimensional layer-structure material, has gained extensive interest in nonlinear optical applications, especially in all-optical intensity modulation. Herein, we successfully fabricated an all-optical modulator based on Ti3C2T x -Mxene/polyvinyl-alcohol film with nanosecond-scale response time, modulation speed of 100 kHz, and modulation depth of 10%. Furthermore, the variation of fall edges of modulated signal pulse caused by thermo-optic effect under different pump power was also observed. Considering the ease of fabrication, low cost, and ease of integration, the proposed modulator may open the door for high-speed all-optical communications and signal processing.

All-optical modulator based on graphene-photonic crystal fiber

An all-optical modulator based on graphene-photonic crystal fiber (G-PCF) is demonstrated, where the G-PCF is fabricated by directly growing graphene onto inner hole walls of the PCF with atmospheric pressure chemical vapor deposition. A 1550 nm pump laser is used to induce all-optical intensity modulation of probe light. A G-PCF with a length of 2 cm achieves a significant modulation depth of 2.58 dB under a low pump power of 60 mW at wavelength of 895 nm. The flexible structure and all-optical control enable the G-PCF to have great potential in all-optical devices and photonics.

All-Optical Modulation Characteristics of a Microfiber Coupler Combined Sagnac Loop

In this paper, we theoretically and experimentally investigate the all-optical modulation characteristics of a microfiber coupler combined Sagnac loop (MCSL). We inject pump lights with different intensities into the MCSL and utilize the thermo-optical effect from the microfiber coupler to control the phase relationship between the counter-propagating beams inside the Sagnac loop. The spectral characteristics from the Sagnac loop can be controlled accordingly, and the rate of the frequency comb (interference characteristic wavelength) shift can be 1 pm/mW. In addition, at a specific wavelength (e.g., 1550 nm), the MCSL can be used as an intensity modulator as well as a reflectivity-tunable Sagnac reflector. The reflectivity variation of the Sagnac reflector can reach about 10% when the pump light is at milliwatt level. The intensity modulation depth can be 9.3% at 100 Hz when the intensity-modulated 980 nm pump light imposed on the MCSL is 5.5 mW. Based on the thermo-optical effect, the MCSL can achieve the all-optical intensity modulation and all optically tunable filtering functions. The device is expected to play an important role in the development of tunable, multi-wavelength, and compact fiber lasers.

微纳光纤耦合器光致热引起的全光强度调控特性

微纳光纤耦合器光致热引起的全光强度调控特性

Investigation on an all-optical intensity modulator based on an optical microfiber coupler

An all-optical intensity modulator based on an optical microfiber coupler (OMC) is presented. The modulator works at 1550 nm wavelength and is modulated directly by heating the coupling region with 980 nm pump light injected through the coupling port of the OMC. The OMC is controlled to have at least a 30 mm long coupling region with diameter smaller than 8 μm, and the uniform waist region diameter is about 3 μm. This is helpful to ensure the optical modulation function based on the light induced thermal effect in the coupling region, while pump light is injected. The modulation response is measured to show good linearity when the 980 nm pump light has a lower intensity (with power below 2.5 mW), which proves that the OMC acts as an all-optical modulator. The bandwidth of the modulator can be at 0.2–50 kHz with the average power of the intensity-modulated pump light about 2 mW, which can be further improved by optimizing the design of the coupler. The demonstrated modulator may have potential value for the application in an all-optical integration system.

All-optical intensity modulation based on graphene-coated microfibre waveguides

We investigate graphene-covered microfibre (GCM) waveguides, and analyse the microfibres’ evanescent field distributions in different diameters and lengths by numerically simulation. According to the simulation results, we designed a graphene-based all-optical modulator using 980 nm and Amplified Spontaneous Emission (ASE) lasers, employing the microfibre’s evanescent field induced light–graphene interaction. We studied the modulation effect that is influenced by the microfibre’s diameter, number of graphene layers, and effective graphene length. Compared to a single graphene layer of shorter length, the double graphene layer with longer length presents stronger absorption and higher modulation depth. Using a 2-μm diameter microfibre covered by ∼0.3 cm double graphene sheets, we achieved a modulation depth of 8.45 dB. This modulator features ease of fabrication, low cost, and a controllable modulation depth.

All-Optical Intensity Modulator by Polarization-Dependent Graphene-Microfiber Waveguide

We demonstrated all-optical intensity modulator based on polarization-dependent graphene-covered microfiber (GMF) waveguide. By controlling the polarization mode of incident light, a greatly adjustable enhanced interaction between the propagating light and the graphene can be obtained via the evanescent field of the microfiber. By employing 980-nm pump light and 1550-nm signal light in continuous wave, the strong light–graphene interaction enables a maximum modulation depth of ∼20.86 dB, and by pumping 980 nm wave pulses, we obtained the temporal response characteristics of signal light with modulation rate of 5.13 kHz. This all-optical intensity modulator is compatible with optical fiber systems, and features with ease of fabrication, and steerable high modulation depth, which show potential in graphene's applications such as all-optical switching and all-optical communications.

Incoherent optical modulation of graphene based on an in-line fiber Mach-Zehnder interferometer.

We investigated the incoherent optical modulation of graphene based on an in-line fiber Mach-Zehnder interferometer fabricated by offset splicing of a D-shaped fiber and single-mode fibers. The D-shaped flat surface was coated with a single-layer graphene whose refractive index can be modulated by an external laser. With a backward pump-probe setup, we found that the incoherent optical modulation occurred independently of the wavelength difference between the pump and probe lasers, supporting the theoretical analysis that the incoherent optical modulation is mainly contributed by the carrier-phonon process of graphene. Further, both the decrease of the pump wavelength and the increase of the pump power would deepen the modulation in an exponential trend. The experimental results are beneficial to understanding the incoherent optical modulation process in graphene. The proposed interferometer covered by graphene can also be used as a compact all-optical intensity modulator.

Temporal response of optical intensity modulation based on side-mode injection locked single-mode Fabry–Pérot laser diode

In this paper, we have experimentally demonstrated time domain characteristics of all-optical intensity modulation using cross-gain modulation induced by side-mode injection locking in a single-mode Fabry–Pérot laser diode (SMFP-LD). Both bistable hysteresis and injection locking behaviors are observed and discussed for different wavelength detuning between the injection beam and the selected side mode, which is the direct physical mechanism to successfully perform optical intensity modulation in the laser chip. Both the modulation depth of the injection signal and wavelength detuning have very important influences on the optical characteristics, including the modulation depth, and rise and fall time of the output signal at the dominant mode. The obtained modulation depth in the output signal can reach as high as ∼25 dB when a pulsed injection beam with a time duration of 8 ns is introduced into the laser chip under conditions of a bias current of 12 mA and an operating temperature of 16 ° C.

All-optical intensity modulation of near infrared light in a liquid crystal channel waveguide

We demonstrate a nonlinear optical channel waveguide made of E7 nematic liquid crystal infiltrated in a silica on silicon groove. Near infrared light at the wavelength of 1560 nm fiber coupled to the core of the liquid crystal waveguide was optically modulated by an optical beam with power below 25 mW by exploiting the optical Freedericks transition. By modeling the optical molecular reorientation in the nematic liquid crystal confined in a waveguiding geometry we are able to reproduce the experimental results.

Cascaded phase shift and intensity modulation in aperiodic quasi-phase-matched gratings

I propose a method for enhancing the cascaded phase shift of the fundamental wave associated with second-harmonic generation in aperiodic quasi-phase matched (QPM) gratings. The position and length of one of the domains were varied to maximize the cascaded phase shift. A maximum enhancement of 84% in the cascaded phase shift relative to a perfectly periodic QPM grating was obtained at a moderate fundamental input intensity without initial seeding of the second-harmonic wave. All-optical intensity modulation is also proposed with another optimized aperiodic QPM grating.

All-optical fiber signal processing and regeneration for soliton communications

Ultrafast all-optical signal processing techniques are expected to play a major role in future ultrafast single-carrier soliton systems, because they remove the electronics bottleneck. In this paper, two all-optical devices, the nonlinear optical loop mirror (NOLM) and the Kerr fiber modulator (KFM), are used to achieve major functions related to high bit rate soliton links. At the interface with existing networks, conversions from data at the nonreturn-to-zero (NRZ) format to return-to-zero (RZ) and soliton data, and vice-versa are required. These two conversions are demonstrated through NOLMs, and their limitations investigated. However, the main part of this paper is devoted to in-line soliton regeneration through synchronous modulation. Synchronous modulation requires both clock recovery and in-line optical modulation. In the following, all-optical approaches for these two functions are considered separately, before being associated in a true all-optical regenerator. All-optical clock recovery techniques are first reviewed. An experimental implementation of one of these techniques is described. On the other hand, all-optical modulation can be done either with intensity or phase modulators. We initially proposed the NOLM as all-optical intensity modulator. We analyze it theoretically, both from the component and the system application viewpoints. A modified configuration of the NOLM, having two optical controls, removes some limitations pertaining to the single-control configuration, yielding even higher performance. The other all-optical synchronous modulator considered here is the KFM, which is a pure phase modulator. Its potential is demonstrated in a 20-Gb/s soliton transmission experiment, when driven by an optoelectronic optical clock generation device. Issues specific to the implementation of both types of all-optical fiber-based modulators are discussed. Finally, a true all-optical synchronous regenerator, combining all-optical clock recovery circuit and KFM, is tested in an actual soliton transmission experiment at 20 Gb/s.