Mode Converter Research Articles

Full higher-order Poincaré sphere beam mode-locked fiber laser

In this paper, the controlled generation of full higher-order Poincaré sphere (HOPS) beams from a mode-locked fiber laser was introduced and demonstrated. In the cavity, two mirror-placed spatial mode converters, consisted of the vortex waveplate, quarter waveplate and half waveplate, were applied to achieve the mutual conversion of the LP01 mode and HOPS beams. Then, the full HOPS beams mode-locked fiber laser was established to obtain the circularly polarized vortex beams, cylindrical vector beams (CVBs), and elliptically polarized CVBs, with pulse width of 390 ps, repetition frequency of 14.7 MHz, central wavelength of 1043.35 nm and 3 dB bandwidth of 0.53 nm. The results may be potentially important for the generation of full HOPS beam from a mode-locked fiber laser.

Robust optical mode converter based on topological waveguide arrays

Abstract Optical mode converters are essential for enhancing the capacity of optical communication systems. However, fabrication errors restrict the further improvement of conventional mode converters. To address this challenge, we have designed an on-chip TE0–TE1 mode converter based on topologically protected waveguide arrays. The simulation results demonstrate that the converter exhibits a mode coupling efficiency of 93.5% near 1550 nm and can tolerate a relative fabrication error of 30%. Our design approach can be extended to enhance the robustness for other integrated photonic devices, beneficial for future development of optical network systems.

Accelerating finite-difference frequency-domain simulations of inverse designed structures in nanophotonics using deep learning

The inverse design of nanophotonic devices is becoming increasingly relevant for the development of complex photonic integrated circuits. Electromagnetic first-order simulations contribute to the overwhelming computational cost of the optimization routines in established inverse design algorithms, requiring more efficient methods for enabling improved and more complex design process flows. Here we present such a method to predict the electromagnetic field distribution for pixel-discrete planar inverse designed structures using deep learning. Our model is able to infer accurate predictions used to initialize a conventional finite-difference frequency-domain algorithm and thus lowers the average time required for simulating the electromagnetic response of nanophotonic device layouts by up to 53% in iterative design process flows. We demonstrate the applicability of our deep learning method for the inverse design of photonic integrated powersplitters and mode converters, and we highlight the possibility of exploiting previous learning results in subsequent design tasks of novel functionalities via fine-tuning reduced data sets, thus improving computational speed further.

Multiparameter sensor based on long-period grating in few-mode ring-core fiber for vector curvature and torsion measurement

With the rapid development of optical fiber sensor in physical measurement, it is of practical significant to realize multi-parameter measurement. In this work, we proposed and demonstrated a multiparameter sensor for curvature, torsion and temperature. The sensor is formed from CO2 laser-inscribed long-period grating (LPG) in few-mode ring-core fiber (RCF), which can convert fundamental mode to LP11 mode. One-side inscription enhances asymmetric index modulation in fiber, resulting in directionally sensitive to bend by intensity interrogation. The curvature sensitivities in various bending directions perform linearity in curvature range of 5–9.466 m−1. Curvature sensitivities are related to the bending directions. The intensity sensitivity achieves − 0.584 dB/m−1, 0.879 dB/m−1, and − 0.735 dB/m−1, respectively. And torsion sensitivity from -6.98 rad/m to 6.98 rad/m displays linearity with −0.554 dB/(rad/m). The measured influence of temperature can be negligible. The proposed RCF-LPG could not only be used as high-efficiency mode converter in mode division multiplexing systems but also be potential in few-mode fiber based multiparameter sensing fields.

Heterogeneous microgrids: Centralized control strategy with distributed grid-forming converters

Advanced microgrid (MG) is a likely model for reaching the goal of 100% renewable grid. A complete advanced MG control must steer the power flow in grid-connected mode; regulate voltage/frequency in islanded mode; and perform power sharing among distributed energy resources (DERs) in both modes. Power-based control (PBC) is a well-known centralized control capable of achieving these three targets. A serious disadvantage of usual MG structures using PBC approach is the existence of a single centralized grid-forming converter to form the islanded MG. Centralized grid-forming converter is a single point of failure, which reduces the capability of the MG expansion, and it is an expensive element for the MG. This paper proposes a MG control strategy using an improved centralized control together with a droop-based power-loop in distributed voltage-controlled mode (VCM) converters to allow the MG to operate in both modes, without a centralized grid-forming converter and critical islanding detection. Moreover, the new approach considers single- and three-phase DERs, controlled in VCM or current-controlled mode, and DERs with self-imposed limits, characterizing a heterogeneous MG. Real-time hardware-in-the-loop setup is used to test the proposed control strategy in several operating conditions considering ten heterogeneous DERs. Results show grid power flow control, power sharing, unbalance compensation, voltage/frequency restoration, and smooth transitioning between operating modes without critical islanding detection.

Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP01-LP11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields.

Magneto-optical fiber-based orbital angular momentum mode converters.

Orbital angular momentum (OAM) mode division multiplexing (MDM) systems can support large-capacity and high-speed rate information transmission, in which the OAM mode conversion devices play an important role. In this paper, the mode conversion principle of magneto-optical fiber-based long-period grating (MOF-LPG) is analyzed for further developing new magneto-optical (MO) OAM mode converters, including three types of C P 01 to O A M ±1,1, O A M ±1,1 to O A M ±2,1, and O A M ±1,1 to C P 02. It is shown that the magnetic tunability of the mode converters through the propagation constants of the eigenmodes is useful for compensating for process errors and increasing the operating wavelength range. The implementation of MOF-LPGs is also discussed from the aspect of the prospective experiments.

Switchable dual-wavelength mode-locked cylindrical vector beam fiber laser based on unpumped EDF saturable absorber and nonlinear polarization rotation

A switchable single- and dual-wavelength mode-locked pulse laser with cylindrical vector beam (CVB) based on an unpumped EDF saturable absorber and a nonlinear polarization rotation (NPR) hybrid mode-locked mechanism has been proposed and experimentally demonstrated. An unpumped EDF saturable absorber is utilized to generate a mode-locked operation that generates high-order laser pulses. Utilizing a hybrid mode-locked mechanism, the signal-to-noise ratio (SNR) and operating stability of the laser are improved compared to the mode-locked operation with a single unpumped EDF saturable absorber. Through adjusting the polarization controller, single- and dual-wavelength stable mode-locked pulses can be easily realized. The single-wavelength mode-locked pulse operates with central wavelengths of 1550.16 nm and 1549.80 nm, repetition frequencies of 6.33 MHz and 6.35 MHz, corresponding to pulse intervals of 158 ns and 157 ns. Further, the long-period grating (LPFG) is used as a mode converter to achieve pulse output with a purity exceeding 95 % in the TM01 and TE01 modes at different operating wavelengths. The proposed fiber laser with a simple structure has provided a flexible pulsed CVB laser source for laser processing, time division, and mode division multiplexing applications.

Wide-bandwidth mode converter based on a dual-hollow-core anti-resonant fiber

This paper proposes a broadband mode converter utilizing a dual-hollow-core anti-resonant fiber (DHC-ARF) for the first time, in which the mode index matching between LP01 and LP11 modes is achieved by adjusting the core diameter size of the hollow-core anti-resonant fiber (HC-ARF). Notably, the air gap can become the main channel for the energy exchange between the two cores by adjusting the air gap size between the two cores of the DHC-ARF. The coupling length of the mode converter is greatly reduced, and a compact and broadband mode converter is realized. Simulation results demonstrate that the proposed mode converter achieves a coupling efficiency (CE) of 92.79 % at a wavelength of 1.55 μm at a length of 10.7 mm. The bandwidth of 330 nm at a CE greater than 90 % covers the E + S + C + L + U bands. The proposed mode converter exhibits a large core diameter compatible with hollow-core fiber communication and is expected to be applied for high-speed and high-capacity hollow-core mode-division-multiplexing fiber-optic communication. The device design methodology presented in this paper can also provide technical references for developing higher-order mode converters, such as LP01 mode to LP02 mode and LP01 mode to LP21 mode, based on DHC-ARF.

Asymmetric dual-core liquid crystal channel-based tunable mode converter

In this work, a higher order-to-fundamental mode converter is reported and analyzed based on an asymmetric dual channel waveguide (ADC-WG) on silicon. In the reported structure, one of the two waveguides is infiltrated with nematic liquid crystal (NLC) material to add temperature tunability while the other one is a solid BK7 waveguide. The modal characteristics are obtained using the full vectorial finite difference method (FVFDM). In addition, the structural parameters and optical characteristics of the employed materials are investigated to achieve good wavelength selectivity with a short device length (LD). Thus, a compact mode converter that can work at different wavelengths including the telecommunication wavelength i.e., 1.55 μm with LD ~ 482.31 μm and a low crosstalk of − 19.86 dB is presented. To prove the thermal tunability of the suggested mode converter, its operation is tested through a temperature range between 20 and 35 °C and the results show that the mode conversion process is achieved at each temperature with different phase matching wavelengths (λPMW) but with quite similar coupling length (LC). The proposed device can therefore be effectively utilized in integrated photonic circuits.

Study on the Generation of 1.9 μm Mode Superposition Conversion Laser by Double-End Off-Axis Pumping

In this paper, the Laguerre–Gaussian (LG) mode superposition is obtained by using the technology of double-end off-axis pumping Tm:YLF crystal, and the LG mode superposition is achieved by combining the extra-cavity conversion method. The impact of changing the off-axis distance on the order of Hermite–Gaussian (HG) mode and the topological charge of LG mode is studied. The results show that when the off-axis distance of the pump source at both ends is tuned, when the off-axis distance is in the range of 260 μm~845 μm, the single-ended 0~10 order HG mode can be obtained. Subsequently, the mode converter is placed to obtain the LG mode beam, and the double-end simultaneously pumps the crystal to obtain the superimposed LG mode. The tuning off-axis quantity changes the topological charge number. When P = 0, l1=l2, the superimposed LG mode is a single-ring spot, and the vortex beam center’s dark hollow area increases with the topological charge number. When P = 0, l1=−l2, the superimposed LG mode is a petal-like spot. The number of petals differs from the topological charges of two opposite numbers. Finally, in the case of changing the topological charge number of the double-ended LG mode, the output of the vortex array structured beams of the tuning mode order 1.9 μm Tm:YLF is completed in the case of conversion and superposition.

Numerical analysis of on-chip acousto-optic modulators for visible wavelengths.

On-chip acousto-optic modulators that operate at an optical wavelength of 780nm and a microwave frequency of 6.835GHz are proposed. The modulators are based on a lithium-niobate-on-sapphire platform and efficiently excite surface acoustic waves and exhibit strong interactions with tightly confined optical modes in waveguides. In particular, a high-efficiency phase modulator and single-sideband mode converter are designed. We found that for both microwave and optical wavelengths below 1µm, the interactions at the cross-sections of photonic waveguides are sensitive to the waveguide width and are significantly different from those in previous studies. Our designed devices have small footprints and high efficiencies, making them suitable for controlling rubidium atoms and realizing hybrid photonic-atomic chips. Furthermore, our devices have the potential to extend the acousto-optic modulators to other visible wavelengths for other atom transitions and for visible light applications, including imaging and sensing.

Ultra-broadband multi-channel orbital angular momentum mode generator using a helical long-period grating pair

We demonstrate an ultra-broadband multi-channel orbital angular momentum (OAM) mode generator using a helical long-period grating (HLPG) pair in a few-mode fiber. The numbers of the channel of the OAM mode generator can be flexibly adjusted by designing the cascaded length of the HLPG pair. When the cascaded length is 50 cm, the maximum channel number of OAM mode generator reaches 46, and the mode convert efficiencies of the channels are more than 90 %, which is, to the best of our knowledge, the highest number of channels among OAM mode converters. By reducing grating length and enhancing the refractive index modulation depth, the 3 dB bandwidth of the HLPG is significantly increased, and thus the wavelength range of the HLPG pair can be expanded. The maximum wavelength ranges of the channels with coupling efficiency greater than 90 % are measured to be 379 nm experimentally. By adjusting the spiral direction of the helical refractive index modulation, we can achieve multi-channel ± 1 order OAM mode conversion for each channel. The multi-channel OAM mode generator holds substantial potential for applications in mode division multiplexing systems, wavelength filters, and fiber sensing.

Few-Mode Fiber-Based Long-Period Fiber Gratings: A Review

Long-period fiber gratings (LPFGs) are efficient ways to achieve high-order core mode conversion and vortex mode conversion in few-mode fibers (FMFs), which have the benefits of flexible structure, high integration, low insertion loss, and strong wavelength selectivity. With the rapid development of mode division multiplexing (MDM) optical communications, the FMF-LPFGs could have promising applications in the field of MDM optical communications and optical sensors. In this paper, we briefly introduce the principle of optical coupling, theoretical analysis, fabrication techniques, and applications of the FMF-LPFGs in optical communications and optical sensors. The CO2-laser writing technique and arc discharge technique are widely used for the fabrication of FMF-LPFGs and FMF helical long-period gratings (HLPGs). The hydrogen-oxygen flame technique has the advantage on the fabrication of FMF-HLPGs. The mechanical micro-bending and acoustic induction techniques are flexible and effective methods in fundamental researches. The femtosecond laser technique has the advantages for the fabrication of parallel FMF-LPFGs and micro-structure FMF-LPFGs. The mode converters based on the FMF-LPFGs have been demonstrated using different inscription techniques. The FMF-LPFG converters have the advantages such as temperature insensitivity and wavelength tunability. The broadband converters have been demonstrated based on the phase-shift FMF-LPFG, cascaded FMF-LPFGs, and FMF-LPFG operating at dispersion turning point. The FMF-LPFGs and FMF-HLPGs are an excellent option to generate all-fiber orbital angular momentum (OAM) modes in different specialty FMFs, especially the FMF-HLPGs can excite different-order OAM modes directly. As the novel fiber component, the sensing application of FMF-LPFGs and FMF-HLPGs is also briefly introduced.

Compact mode converter on SOI based on a polygonal subwavelength grating structure.

In this Letter, we design and experimentally demonstrate compact mode converters with a lightning-like and arrow-like polygonal subwavelength grating (SWG) structure on a silicon-on-insulator (SOI) platform, which can convert the TE0 mode to the TE1 and TE2 modes, respectively. The footprints of the proposed TE0-1 and TE0-2 mode converters are only 4.44 × 1.3 and 5.89 × 1.8 µm2, respectively. The experimental results show the mode converters have a low insertion loss (<1 dB) and a broad bandwidth (>50 nm). The measured cross talks of the TE0-1 and TE0-2 mode converters are -7.2 dB and -10.3 dB, respectively. In addition, the proposed mode converters with the SWG structure have the advantage in fabrication, since only a one-step full-etching process is required.

Validation and characterization of algorithms and software for photonics inverse design

In this work, we present a reproducible suite of test problems for large-scale optimization (“inverse design” and “topology optimization”) in photonics, where the prevalence of irregular, non-intuitive geometries can otherwise make it challenging to be confident that new algorithms and software are functioning as claimed. We include test problems that exercise a wide array of physical and mathematical features—far-field metalenses, 2d and 3d mode converters, resonant emission and focusing, and dispersion/eigenvalue engineering—and introduce an a posteriori lengthscale metric for comparing designs produced by disparate algorithms. For each problem, we incorporate cross-checks against multiple independent software packages and algorithms, and reproducible designs and their validations scripts are included. We believe that this suite should make it much easier to develop, validate, and gain trust in future inverse-design approaches and software.

Multifunction integrated lithium niobate photonic chip for photon pairs generation and manipulation.

We report on a unique photonic quantum source chip highly integrating four-stage photonic elements in a lithium niobate (LN) waveguide circuit platform, where an aperiodically poled LN (APPLN) electro-optic (EO) polarization mode converter (PMC) is sandwiched between two identical type-0 PPLN spontaneous parametric down-converters (SPDCs), followed by an EO phase controller (PC). These core nonlinear optic and EO building blocks on the chip are systematically characterized stage by stage to show its high performance as an integrated quantum source. The APPLN EO PMC, optimally constructed by a genetic algorithm, is characterized to have a broad bandwidth (>13 nm), benefiting an efficient control of broadband type-0 SPDC photon pairs featuring a short correlation time. We demonstrate an efficient conversion of the |VV› photon-pair state generated from the first PPLN SPDC stage to the |HH› state through the APPLN EO PMC stage over its operating bandwidth, a broadband or broadly tunable polarization-entangled state can thus be possibly produced via the superposition of the |VV› state generated from the other PPLN SPDC on the third stage of the chip. Such a state can be further manipulated into two of the Bell states if the relative phases between the two polarization states can be properly modulated through the EO PC on the fourth stage of the chip. Such a multifunction integrated quantum photonic source chip can be of high value to developing a compact, efficient, and high-speed quantum information processor.

Inverse design of compact nonvolatile reconfigurable silicon photonic devices with phase-change materials

Abstract In the development of silicon photonics, the continued downsizing of photonic integrated circuits will further increase the integration density, which augments the functionality of photonic chips. Compared with the traditional design method, inverse design presents a novel approach for achieving compact photonic devices. However, achieving compact, reconfigurable photonic devices with the inverse design that employs the traditional modulation method exemplified by the thermo-optic effect poses a significant challenge due to the weak modulation capability. Low-loss phase change materials (PCMs) exemplified by Sb2Se3 are a promising candidate for solving this problem benefiting from their high refractive index contrast. In this work, we first developed a robust inverse design method to realize reconfigurable silicon and phase-change materials hybrid photonic devices including mode converter and optical switch. The mode converter exhibits a broadband operation of &gt;100 nm. The optical switch shows an extinction ratio of &gt;25 dB and a multilevel switching of 41 (&gt;5 bits) by simply changing the crystallinity of Sb2Se3. Here, we experimentally demonstrated a Sb2Se3/Si hybrid integrated optical switch for the first time, wherein routing can be switched by the phase transition of the whole Sb2Se3. Our work provides an effective solution for the design of photonic devices that is insensitive to fabrication errors, thereby paving the way for high integration density in future photonic chips.

Digital metamaterials with nanoscale silicon/Sb2Se3 pixels for reconfigurable integrated mode converters

Waveguide-integrated reconfigurable nanophotonic devices are of importance for on-chip all-optical connection and flexible photonic circuits. In this paper, we propose inversely designed reconfigurable TE0-to-TE1 and TE0-to-TE2 mode converters based on digital metamaterials consisting of nanoscale silicon and non-loss phase change material Sb2Se3 pixels. The design strategy combines finite element method, density method and method of moving asymptotes. Only 73 and 56 optimization iterations are taken to obtain the optimal digital metamaterials for targeted mode conversions. The footprints of ultra-compact functional regions are 0.92 and 1.04 square center wavelengths, respectively. TE0-to-TE1 and TE0-to-TE2 mode conversions with high transmission efficiency and output mode purity are realized with crystalline Sb2Se3, while TE0 mode directly pass through with amorphous Sb2Se3. Meanwhile, by cascading these two mode converters, TE1-to-TE2 mode conversion is achieved as well. Moreover, the robustness of these designs is demonstrated by introducing random wrong pixels.

Ultracompact silicon on-chip polarization controller

On-chip polarization controllers are extremely important for various optical systems. In this paper, a compact and robust silicon-based on-chip polarization controller is proposed and demonstrated by integrating a special polarization converter and phase shifters. The special polarization converter consists of a 1×1 Mach–Zehnder interferometer with two polarization-dependent mode converters at the input/output ends. When light with an arbitrary state of polarization (SOP) is launched into the chip, the TE0 and TM0 modes are simultaneously excited. The polarization extinction ratio (PER) and the phase difference for the TE0/TM0 modes are tuned by controlling the first phase shifter, the polarization converter, and the second phase shifter. As a result, one can reconstruct the light SOP at the output port. The fabricated polarization controller, as compact as ∼150 μm×700 μm, exhibits an excess loss of less than 1 dB and a record PER range of &gt;54 dB for arbitrary input light beams in the wavelength range of 1530–1620 nm.