Mode Converter Research Articles

Thin-film lithium niobate polarization-independent modulators for mode and polarization multiplexing

We demonstrated a high-order mode modulator based on thin-film lithium niobate (TFLN). By introducing a mode converter and a polarization splitter-rotator (PSR), the modulator can achieve dual-mode modulation. Additionally, we designed a periodic capacitively loaded traveling-wave electrode on TFLN to enhance modulation efficiency and bandwidth. The modulator exhibits a polarization extinction ratio greater than 20 dB at a wavelength of 1550 nm, a modulation efficiency of V π LTE0=2.541V⋅cm and V π LTM0=2.570V⋅cm, and a 3 dB bandwidth exceeding 60 GHz. This not only meets the requirements for polarization-independent modulators but also supports on-chip multimode applications such as mode and polarization multiplexing.

Enhancing computational efficiency in topology-optimized mode converters via dynamic update rate strategies

In the big data era, mode division multiplexing, as a technology for extended channel capacity, demonstrates potential in enhancing parallel data processing capability. Consequently, developing a compact, high-performance mode converter through efficient design methods is an urgent requirement. However, traditional design methodologies for these converters face significant computational complexities and inefficiencies. Addressing this challenge, this paper introduces a novel topology optimization design method for mode converters employing a Dynamic Adjustment of Update Rate (DAUR). This approach markedly reduces computational overhead, accelerating the design process while ensuring high performance and compactness. As a proof-of-concept, an ultra-compact dual-mode converter was designed. The DAUR method demonstrated an 80% reduction in computational time compared to traditional methods, while maintaining a compact design (only 1.4 μm × 1.4 μm) and an insertion loss under 0.68 dB across a wavelength range of 1525 nm to 1575 nm. Meanwhile, simulated inter-mode crosstalk remained below − 24 dB across a 40 nm bandwidth. A comprehensive comparison with traditional inverse design algorithms is presented, demonstrating our method’s superior efficiency and effectiveness. Our findings suggest that DAUR not only streamlines the design process but also facilitates exploration into more complex micro-nano photonic structures with reduced resource investment.

Low-threshold CW eye-safe vortex generation from an intracavity pump-wave off-axis pumped OPO.

A low-threshold efficient setup for CW wavelength versatile vortex generation is proposed. The intracavity optical parametric oscillator is utilized for low-threshold operation. By using the pump-wave off-axis pumping for the signal-wave oscillator, high-order Hermite-Gaussian modes at the eye-safe region can be obtained, which can further generate a vortex via an astigmatism mode converter. The overall laser performance is additionally verified to be better by comparing it with the diode off-axis pumping method where the TEM1,0 pump wave was generated first for the signal-wave TEM1,0 mode oscillation. This method is expected to be further applied for more complicated structured light generation.

Modal phase-matching in thin-film lithium niobate waveguides for efficient generation of entangled photon pairs

Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device’s capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.

Broadband conversion between high-order angular modes based on double-sided exposure long-period fiber grating

Broadband high-order mode converters play a fundamental and crucial role in mode division multiplexing systems. Unfortunately, there have been no reports on achieving broadband mutual conversion between high-order modes using long-period fiber gratings (LPFGs). In this paper, based on the concept of “stepwise” progressive conversion (SPC), a double-sided exposure fabrication method of LPFGs to achieve broadband mutual conversion between high-order modes is proposed and demonstrated. Based on the proposed method, broadband mode conversion from LP11 to LP21, from LP21 to LP31 and from LP31 to LP41 with low insertion loss are achieved by utilizing low exposure power and shortened device lengths. The 10 dB bandwidths of the three converters are measured to be 80 nm, 110 nm, and 90 nm, respectively, and their insertion losses are all less than 0.2 dB. Theoretically, this method can achieve broadband conversion of even higher-order modes, providing a novel solution for the fabrication of stable broadband mode converters. More generally, such mode converters can convert between any two modes and are essential for building advanced MDM networks that require routing and mode switching.

Mode conversion in graded-index few-mode fiber via hollow cylindrical long-period fiber gratings.

We demonstrate the realization of mode conversion using hollow cylindrical long-period fiber gratings (LPFGs) inscribed in graded-index few-mode fibers (FMFs) by a femtosecond laser. By precisely shaping the refractive index modulation into a hollow cylindrical structure, we enable efficient coupling from the fundamental mode to high-order modes (LP11, LP02, LP21, LP12, LP31, LP03, and LP22 modes). The method achieves high-efficiency and low-loss mode conversion across various mode groups, marking a first for graded-index FMFs with different grating periods. The influence of the hollow cylindrical LPFG radius on mode conversion efficiency and spectral characteristics is thoroughly investigated. Additionally, incorporating a linear polarizer allowed for distinguishing between LP modes within a mode group, enhancing the tunability of the mode converter. These mode conversion devices have significant potential for applications in mode gain equalization and mode scrambling for mode division multiplexing (MDM) systems.

Ultra-compact silicon nitride dual-polarization mode converter for visible light demonstrated on a thinned 320 nm silicon wafer

A dual-polarization higher-order mode converter is proposed for visible light, which operates within the (400–700) nm range, to enhance data capacity in on-chip visible light communication systems. The proposed structure is optimized at wavelengths of 410 nm, 560 nm, and 660 nm to cover the entire visible spectrum. These three optimized designs, in particular, have an intrinsic property wherein they can be optimized to simultaneously convert both polarized fundamental (TE0) and fundamental (TM0) modes to the second higher-order (TE2) and (TM2) modes, providing significant advantages for hybrid multiplexing systems like PDM-MDM and PDM-WDM schemes. The mode converters are constructed on a silicon nitride waveguide. This waveguide is etched and filled with silicon dioxide material to create two dielectric substrips, followed by an additional rectangle shape etched using the same process into the propagating waveguide. This alteration enhances insertion loss and diminishes crosstalk to the fundamental mode. The devices achieve a broad operating bandwidth of approximately 100 nm while maintaining a compact footprint of only 1µm×1.754µm for the entire device at the center wavelength of 560 nm. Upon optimizing the suggested structure, a TE0-to-TE2 mode converter with a modal conversion efficiency of 94% is designed at the wavelength of 410 nm. The insertion loss is 0.6025 dB, and the crosstalk with the transverse electric fundamental mode TE0 is 35 dB. The reported devices feature a straightforward structure with low insertion loss and minimal crosstalk.

Generation of sub-100 fs vortices from a Kerr-lens mode-locked Hermite-Gaussian Yb:CALGO oscillator.

Mode-locked oscillators with high-order transverse modes are excellent platforms for generating femtosecond optical vortices with high average power and good propagation stability. These have important applications in diverse fields such as optical communication, strong-field physics, and laser processing. So far, generating vortex pulses with ultrashort pulse duration remains a challenge. In this Letter, we report a Kerr-lens mode-locked Yb:CALGO laser oscillator delivering Hermite-Gaussian (HG) pulses with a pulse duration of 86 fs using a non-collinear pumping technique. 91 fs optical vortex pulses were generated by using a cylindrical-lens mode converter. To the best of our knowledge, this is the shortest pulse duration ever obtained from a diode pumped solid-state mode-locked oscillator with a pure high-order Hermite-Gaussian mode. The phase structures of the generated femtosecond vortices are characterized.

All-fiber tunable mode converter for a mini-two-arm Mach-Zehnder interferometer.

In this Letter, an all-fiber tunable mode converter for a mini-two-arm Mach-Zehnder interferometer (MTA-MZI) is proposed and realized for the first time to our knowledge. Employing an electric arc discharge technology, we couple a multi-mode fiber (MMF) and a single-mode fiber (SMF), resulting in an MZI characterized by centimeter-scale arms. The varied sensitivity of fiber modes to curvature makes the high-order modes of the MMF more prone to leakage when subjected to bending, leading to alterations in the output interference fringe pattern of the MZI. Through continuous monitoring of the interference fringes of the MZI, we effectively detect the mode properties within the MZI in real time. In addition, a verification experiment is conducted by introducing a peanut structure to excite further higher-order modes of light to enter the MZI in advance. Upon modifying the curvature of the MZI, these excited higher-order modes leak, causing the interference fringe pattern to revert to its initial state without a peanut structure. This experimental validation highlights the potential employment of the MTA-MZI as an all-fiber mode converter and paves the way for optical field mode conversion within an all-fiber framework.

Reconfigurable mode converter based on a Sb2Se3 phase change material and inverse design

In this paper, we combine the inverse design with a silicon-Sb2Se3 hybrid platform to design an on-chip mode converter that converts basic modes to higher-order modes. Firstly, we present a 1 × 2 mode converter with dimensions of 4.8 × 2.7 µm2 that enables TE0 mode input, TE0 or TE1 output in the C-band (1530 nm to 1565 nm) with an insertion loss (IL) of less than 0.8 dB and a crosstalk (CT) of less than -13 dB. Secondly, the device is extended to a 1 × 3 switchable three-mode converter. Using two controllable phase change regions as drivers, it can flexibly control the switching from TE0 mode input to three modes of TE0, TE1, or TE2 outputs, which enables mode switching and signal routing. The device can be switched between three modes and has broad application potential in broadband optical signal processing for mode division multiplexing systems, as well as optical interconnections. Finally, the device is extended to a 1 × 2 controllable (mode and power) beam splitter, which can control the power ratio between output modes. By modulating the crystallinity of Sb2Se3, the simulation achieves a multilevel switching of 36 levels (> 5-bit). These devices pave the way for high integration densities in future photonic chips.

Suppression of excess noise in a chiral optical mode converter without encircling an exceptional point

Suppression of excess noise in a chiral optical mode converter without encircling an exceptional point

Asymmetric bi-level dual-core mode converter for high-efficiency and polarization-insensitive O-band fiber-chip edge coupling: breaking the critical size limitation

Abstract Efficient coupling between optical fibers and on-chip photonic waveguides has long been a crucial issue for photonic chips used in various applications. Edge couplers (ECs) based on an inverse taper have seen widespread utilization due to their intrinsic broadband operation. However, it still remains a big challenge to realize polarization-insensitive low-loss ECs working at the O-band (1,260–1,360 nm), mainly due to the strong polarization dependence of the mode coupling/conversion and the difficulty to fabricate the taper tip with an ultra-small feature size. In this paper, a high-efficiency and polarization-insensitive O-band EC is proposed and demonstrated with great advantages that is fully compatible with the current 130-nm-node fabrication processes. By introducing an asymmetric bi-level dual-core mode converter, the fundamental mode confined in the thick core is evanescently coupled to that in the thin core, which has an expanded mode size matched well with the fiber and works well for both TE/TM-polarizations. Particularly, no bi-level junction in the propagation direction is introduced between the thick and thin waveguide sections, thereby breaking the critical limitation of ultra-small feature sizes. The calculated coupling loss is 0.44–0.56/0.48–0.61 dB across the O-band, while achieving 1-dB bandwidths exceeding 340/230 nm for the TE/TM-polarization modes. For the fabricated ECs, the peak coupling loss is ∼0.82 dB with a polarization dependent loss of ∼0.31 dB at the O-band when coupled to a fiber with a mode field diameter of 4 μm. It is expected that this coupling scheme promisingly provides a general solution even for other material platforms, e.g., lithium niobate, silicon nitride and so on.

Mode Conversion Trimming in Asymmetric Directional Couplers Enabled by Silicon Ion Implantation.

An on-chip asymmetric directional coupler (DC) can convert fundamental modes to higher-order modes and is one of the core components of mode-division multiplexing (MDM) technology. In this study, we propose that waveguides of the asymmetric DC can be trimmed by silicon ion implantation to tune the effective refractive index and facilitate mode conversion into higher-order modes. Through this method of tuning, transmission changes of up to 18 dB have been realized with one ion implantation step. In addition, adjusting the position of the ion implantation on the waveguide can provide a further degree of control over the transmission into the resulting mode. The results of this work present a promising new route for the development of high-efficiency, low-loss mode converters for integrated photonic platforms, and aim to facilitate the application of MDM technology in emerging photonic neuromorphic computing.

Low-loss and compact arbitrary-order silicon mode converter based on hybrid shape optimization

Low-loss and compact arbitrary-order silicon mode converter based on hybrid shape optimization

MEMS‐enabled Ultralow Power Consumption Programmable Arbitrary Order Mode Switch

AbstractMode‐division‐multiplexing (MDM) recently attracted much interest due to its ability to enhance the capacity for optical information communication. To meet growing data transmission demands and enable flexible on‐chip mode manipulation, programmable optical mode converter switches are essential. This work introduces a micro‐electromechanical‐system (MEMS)‐enabled programmable arbitrary order mode switch, capitalizing on the benefits of MEMS technology such as low power consumption, robust modulation capabilities, and seamless large‐scale integration. Employing newly developed MEMS‐tunable mode converter (MTMC) units, the system achieves high conversion efficiencies from the fundamental mode to arbitrary higher‐order modes and supports programmable power distribution among different modes by cascading multiple MTMCs. As a proof‐of‐concept, a programmable TE0–TE0/1/2 mode switch operates in the 3.64 to 3.71 µm wavelength range, achieving maximum crosstalk values of –9.56 and –9.88 dB for TE0 to TE1 and TE0 to TE2 mode conversion, respectively, and a remarkably low power consumption of 67 pW. Furthermore, a programmable mode multiplexer is developed by simultaneously actuating two MEMS actuators to modulate phase matching conditions, enabling arbitrary power splitting ratios among these modes. The demonstrated scalability and flexibility of the proposed system highlight its potential as a foundational component for MDM systems.

Generating broadband cylindrical vector modes based on polarization-dependent acoustically induced fiber gratings using the dispersion turning point

Cylindrical vector beams (CVBs) with special polarization distribution have been extensively investigated due to the unique ways of interacting with matter. Although several configurations have been developed to generate CVBs, such as Q-plates and subwavelength gratings, the bandwidth of a single CVB is inherently narrow due to the phase geometry, which would limit its application for femtosecond lasers. Here, a broadband CVB mode converter based on an acoustically induced fiber grating (AIFG) and a tuning method of dispersion turning point (DTP) is demonstrated both theoretically and experimentally with the 3-dB bandwidth of 125 nm, which is more than 10 times that of conventional AIFGs. Not only can the DTP wavelength be tuned from the original 1500 nm to 1650 nm by thinning the fiber, but also the stable generation of a single broadband HE21odd/even mode can be controllably implemented by adjusting the polarization state of the incident light, owing to the larger beat length difference between HE21 and other CV modes. Additionally, the femtosecond CVBs and orbital angular momentum (OAM) modes are successfully generated and amplified by combining the broadband AIFG with a figure-9 mode-locked fiber laser. Meanwhile, it is verified by simulation that the choice of broadband CV mode and the tunability of DTP wavelength can be realized by designing ring-core fibers with different structures, which can furthermore improve the flexibility of generating high purity CVBs. This study provides a highly controllable technique for the generation of broadband CVBs and OAMs paving the way for high-capacity CVBs communication.

Monolithically integrated mode converter from terahertz substrateless silicon guide to conductive slotline

Although substrateless micro-scale silicon waveguides are a useful and versatile platform for terahertz waves, the fact that modal fields occupy the volume of the core limits the potential to dynamically manipulate guided waves. To address this, we introduce an aperiodic lattice structure to enable the monolithic co-integration of a Vivaldi antenna-like mode converter with a substrateless silicon waveguide. This broadband transition is experimentally confirmed to exhibit ∼2.5 dB average loss for two couplers, from 220 GHz to 330 GHz, and enables a photoexcited variable attenuator as proof-of-concept demonstration. This is an important enabling step to incorporate general-purpose dynamic reconfigurability, sensing, and modulation functionality into terahertz-range silicon-based integrated circuits, which are currently limited to primarily all-passive structures.

Ultrahigh-channel-count OAM mode conversion utilizing a hybrid few-mode fiber configuration.

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

Helical Long-Period Fiber Grating-Based Band- Selectable and Bandwidth-Enhanced Flat-Top Filter and Its Application to Wideband OAM Mode Converter

Helical Long-Period Fiber Grating-Based Band- Selectable and Bandwidth-Enhanced Flat-Top Filter and Its Application to Wideband OAM Mode Converter

Polarization-Insensitive Lithium Niobate-on-Insulator Interferometer.

The key components of a polarization-independent electro-optic (EO) interferometer, including the beam splitter, mode converter, and directional coupler, are designed based on a lithium niobate (LN) platform on an integrated insulator to ensure high extinction ratios. By elaborately designing the geometric structure of the multimode interference (MMI) coupler, beam splitting of equal proportions and directional coupling of higher-order modes are realized. The most prominent characteristic of the proposed interferometer is polarization insensitivity, which is realized by converting TM polarization into TE polarization using a mode converter, providing conditions for the study of light with different polarizations. At 1550 nm, the visibility of the interferometer is 97.59% and 98.16% for TE and TM, respectively, demonstrating the high performance of the proposed LN polarization-independent interferometer.