Search Results (18)

We propose and experimentally demonstrate an Integrated Multiband-Mode Multiplexing Photonic Lantern (IM3PL) that enables the selective excitation of high-order modes and stable modal preservation across multiple wavelength bands. As a proof-of-concept configuration, the IM3PL integrates a custom-designed input fiber array composed of three 980 nm single-mode fibers (SMFs) and two few-mode fibers (FMFs) operating at 1310 nm and 1550 nm, respectively. Simulations verify that 980 nm input signals can selectively excite $L{P}_{01}$, $L{P}_{11a}$, and $L{P}_{11b}$ modes at the FMF output, while the modal integrity of high-order linear polarized modes is preserved at 1310 nm and 1550 nm. The fabricated IM3PL device is experimentally validated via near-field pattern measurements, confirming the selective excitation at 980 nm and low-loss, mode-preserving transmission at the signal bands. This work offers a scalable and reconfigurable solution for multiband high-order-mode multiplexing, with promising applications in mode-division multiplexed fiber communication systems and multiband high-mode fiber lasers. Full article

The optical spectrum resource in the C-band has been used up due to dense wavelength division multiplexing (DWDM). Because of devices’ compatibility with both the C-band and the L-band, the L-band is a good choice for further capacity expansion. Meanwhile, the mode division multiplexing (MDM) method has been applied to increase the number of channels. However, the few-mode erbium-doped fiber amplifier must be redesigned to overcome the power differences among channels. In this work, a few-mode erbium-doped fiber (FM-EDF) is optimized and manufactured. Then, an in-line gain-equalized L-band FM-EDFA is constructed. The experimental results show that the FM-EDFA works well in the wavelength range between 1575 nm and 1610 nm. The minimum differential modal gain (DMG) is 0.54 dB, and the maximum modal gain is 22.22 dB. Due to the excellent performance of the L-band FM-EDFA, a DSP-free transmission scheme in the L-band is demonstrated. The bit error rates (BERs) of each channel are below 1 × 10⁻⁵ with a DSP-free receiver. Full article

Mode division multiplexing (MDM) techniques provide significant enhancement of the capacity of optical intensity modulation and direct detection (IM/DD) short-reach communication systems, like the datacenter interconnection scenarios. While the introduction of multiple modes leads to mode coupling that will extremely deteriorate the received signals, two approaches have been explored to address this issue: one involves the application of all-link weakly coupled components to suppress modal crosstalk, while the other utilizes optical multiple-input–multiple-output (MIMO) equalizers based on optical devices for signal decoupling. However, pure digital signal processing (DSP)-based schemes for mode decoupling in IM/DD MDM systems with strong mode coupling remain unexplored. In this paper, we propose to use a frequency-domain MIMO equalizer for compensating the modal interference in the strong-coupling linear-polarized (LP) MDM IM/DD system. The signal recovery capability of the proposed method is verified through numerical simulation. Finally, we successfully experimentally demonstrate the transmission of on–off-key (OOK) signals in a six-LP-mode strong-coupling MDM IM/DD system over a 10 km few-mode fiber, employing a pair of strong-coupling mode multiplexers/demultiplexers. The experimental results indicate that, with the frequency-domain MIMO equalizer, OOK signals from all modes can be recovered with an 11% hard-decision forward error correction threshold of 8.3 × ${10}^{-3}$. The proposed method facilitated by flexible DSP software offers an alternative for short-reach communication systems and has the potential to advance the practical application of MDM techniques in the future. Full article

Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP₀₁ mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP₀₁ and LP₁₁ modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems. Full article

As single-mode-fiber transmission systems are reaching their capacity limits, spatial-division multiplexing (SDM) techniques have been investigated to increase the per-fiber capacity. However, the compatibility with current single-mode transponders severely hinders the near-term deployment of SDM systems. In this paper, we experimentally propose two real-time SDM transmission schemes using commercial single-mode 400 G dual-polarized 16 quadrature amplitude modulation equipment. In the first experiment, 60 km weakly coupled single-mode 7-core fiber with a pair of fan-in and fan-out devices are adopted. In the second experiment, the fiber link consists of 60 km/150 km weakly coupled few-mode fiber (FMF) and low-modal-crosstalk mode multiplexers, in which only non-degenerate LP₀₁ and LP₀₂ modes are utilized. In order to investigate the effect of splice on SDM fiber links, 20-roll, 3 km multicore fibers (MCFs) and FMFs are spliced and tested in the experiments. The bit error rates of all SDM experiments are all below 4.75 × 10⁻² forward-error-correction threshold of the 400 G transponders. The experimental results prove that the near-term deployment of SDM systems could be accelerated by utilizing weakly coupled MCFs or non-degenerate modes of weakly coupled FMFs which are compatible with commercial single-mode transponders without any software or hardware modifications. Full article

Mode division multiplexing (MDM) is currently one of the most attractive multiplexing techniques in optical communications, as it allows for an increase in the number of channels available for data transmission. Optical modal converters are one of the main devices used in this technique. Therefore, the characterization and improvement of these devices are of great current interest. In this work, we present a dataset of 49,736 near-field intensity images of a modal converter based on a long-period fiber grating (LPFG) written on a few-mode fiber (FMF). This characterization was performed experimentally at various wavelengths, polarizations, and temperature conditions when the device converted from ${\mathrm{LP}}_{01}$ mode to ${\mathrm{LP}}_{11}$ mode. The results show that the modal converter can be tuned by adjusting these parameters, and that its operation is optimal under specific circumstances which have a great impact on its performance. Additionally, the potential application of the database is validated in this work. A modal decomposition technique based on the particle swarm algorithm (PSO) was employed as a tool for determining the most effective combinations of modal weights and relative phases from the spatial distributions collected in the dataset. The proposed dataset can open up new opportunities for researchers working on image segmentation, detection, and classification problems related to MDM technology. In addition, we implement novel artificial intelligence techniques that can help in finding the optimal operating conditions for this type of device. Full article

We herein present a weakly-coupled double bow-tie multi-ring elliptical core multi-mode fiber (DBT-MREC-MMF) supporting 22 eigenmodes for mode division multiplexing across the C+L+U band. The proposed fiber introduces a multi-ring elliptical core, bow-tie air holes, and bow-tie stress-applying areas to effectively split adjacent eigenmodes. By utilizing the finite element method (FEM), we accordingly optimized the fiber to support the 22 modes under the weakly-coupled condition. We evaluated the impact of fiber parameters on the minimum effective refractive index difference (min Δn_eff) between adjacent eigenmodes, model birefringence (B_m), and bending loss at a wavelength of 1550 nm. Additionally, broadband performance metrics, such as effective modal index (n_eff), effective index difference (Δn_eff), effective mode area (A_eff), differential mode delay (DMD), and chromatic dispersion (D), were comprehensively studied over the entire C+L+U band, ranging from 1530 to 1675 nm. The proposed fiber is capable of supporting 22 completely separated eigenmodes with a min Δn_eff between adjacent eigenmodes larger than 3.089 × 10⁻⁴ over the entire C+L+U band. The proposed DBT-MREC-MMF holds great potential for use in short-haul communication systems that require MDM to improve transmission capacity and expand bandwidth. Full article

Bit-loaded quadrature amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM) encoding and photonic-crystal-engineered multi-mode vertical-cavity surface-emitting lasers (MM-VCSELs) transmission performance are analyzed. Two different surface photonic-crystal designs are used to configure the core and cladding regions of MM-VCSELs, producing continuous-wave and digital-encoding outputs. These outputs are combined with the end-face-flattened OM5 multi-mode fiber (MMF) for 100 m short-reach transmission. The photonic-crystal (PhC) structure exhibits a spatial mode-filtering ability, supporting few or single-mode outputs from the MM-VCSEL. This helps reduce the modal dispersion during OM5-MMF transmission of the encoded data. Comparing the original MM-VCSEL with two different surface-photonic-crystal-configured MM-VCSELs, the allowable bit-loaded QAM-OFDM data rate can be increased from 60.7 (for the VCSEL without the PhC structure) to 85/65 Gbit/s (for the PhC VCSELs with 2-layer PhC structures in the cladding layer and the ones with a 1-layer PhC structure in the core layer and 2-layer PhC structures in the cladding layer, respectively) under back-to-back (BtB) encoding and enable the 100 m OM5-MMF transmission to increase from 58.5 (for the VCSEL without the PhC structure) to 81.2/64.6 Gbit/s (for the PhC VCSELs with 2-layer PhC structures in the cladding layer and the ones with a 1-layer PhC structure in the core layer and 2-layer PhC structures in the cladding layer), respectively. Furthermore, by comparing the 7°-titled and 0°-normalized vertical coupling conditions, it can be observed that the purely normalized vertical coupling can collect more output power, resulting in an improved signal-to-noise ratio. This significantly increases the allowable error-free data rate from 85 to 98.9 Gbit/s in the BtB case and from 81.2 Gbit/s to 95.3 Gbit/s in the 100 m OM5-MMF case. Full article

Due to the popularity of different high bandwidth applications, it is becoming increasingly difficult to satisfy the huge data capacity requirements, since the traditional electrical interconnects suffer significantly from limited bandwidth and huge power consumption. Silicon photonics (SiPh) is one of the important technologies for increasing interconnect capacity and decreasing power consumption. Mode-division multiplexing (MDM) allows signals to be transmitted simultaneously, at different modes, in a single waveguide. Wavelength-division multiplexing (WDM), non-orthogonal multiple access (NOMA) and orthogonal-frequency-division multiplexing (OFDM) can also be utilized to further increase the optical interconnect capacity. In SiPh integrated circuits, waveguide bends are usually inevitable. However, for an MDM system with a multimode bus waveguide, the modal fields will become asymmetric when the waveguide bend is sharp. This will introduce inter-mode coupling and inter-mode crosstalk. One simple approach to achieve sharp bends in multimode bus waveguide is to use a Euler curve. Although it has been reported in the literature that sharp bends based on a Euler curve allow high performance and low inter-mode crosstalk multimode transmissions, we discover, by simulation and experiment, that the transmission performance between two Euler bends is length dependent, particularly when the bends are sharp. We investigate the length dependency of the straight multimode bus waveguide between two Euler bends. High transmission performance can be achieved by a proper design of the waveguide length, width, and bend radius. By using the optimized MDM bus waveguide length with sharp Euler bends, proof-of-concept NOMA-OFDM experimental transmissions, supporting two MDM modes and two NOMA users, are performed. Full article

We propose a mode switch based on hybrid-core vertical directional couplers with an embedded graphene electrode to realize the switching function with low power consumption. We designed the device with Norland Optical Adhesive (NOA) material as the guide wave cores and epoxy polymer material as cladding to achieve a thermo-optic switching for the E₁₁, E₂₁ and E₁₂ modes, where monolayer graphene served as electrode heaters. The device, with a length of 21 mm, had extinction ratios (ERs) of 20.5 dB, 10.4 dB and 15.7 dB for the E₂₁, E₁₂ and E₁₁ modes, respectively, over the C-band. The power consumptions of three electric heaters were reduced to only 3.19 mW, 3.09 mW and 2.97 mW, respectively, and the response times were less than 495 µs, 486 µs and 498 µs. Additionally, we applied such a device into a mode division multiplexing (MDM) transmission system to achieve an application of gain equalization of few-mode amplification among guided modes. The differential modal gain (DMG) could be optimized from 5.39 dB to 0.92 dB over the C-band, together with the characteristic of polarization insensitivity. The proposed mode switch can be further developed to switch or manipulate the attenuation of the arbitrary guided mode arising in the few-mode waveguide. Full article

The multiple-input and multiple-output (MIMO) technology is a promising area of research to cope up with the demands of higher data rates and capacity. In the optical communication domain, the combination of space-division multiplexing (SDM) with higher-order modulation (HOM) formats over an optical MIMO system actively addresses these challenges. By allowing multi-level signaling with limited increment in the transmitter’s complexity, a jointly designed pre- and post-equalization (PPE) for an optical MIMO system with a multi-mode fiber (MMF) link is proposed. Cost-effectiveness of the system is incorporated by utilizing intensity modulation/direct detection (IM/DD) with HOM formats such as pulse-amplitude modulation (PAM) schemes. With the aid of a numerical optimization algorithm, the proposed joint-PPE filter coefficients are optimized with respect to the MMF channel and the transmit power constraint. In contrast to existing research on the single-mode fiber (SMF) based optical systems, the effectiveness of the proposed joint-PPE filter is analyzed on an MMF link, which is considerably degraded by the modal dispersion. In the analyzed experimental scenario, the proposed joint-PPE scheme confirms to be beneficial as compared to the post-equalization only (PE-only) in terms of bit-error rate (BER) performance. Furthermore, the required average received optical power to reach a BER ${10}^{-4}$ by the joint-PPE scheme is improved by 2 dB with comparison to the minimum mean-squared error (MMSE) PE-only. Full article

Modal decomposition of light is essential to study its propagation properties in waveguides and photonic devices. Modal analysis can be carried out by implementing a computer-generated hologram acting as a match filter in a spatial light modulator. In this work, a series of aspects to be taken into account in order to get the most out of this method are presented, aiming to provide useful operational procedures. First of all, a method for filter size adjustment based on the standard fiber LP-mode symmetry is presented. The influence of the mode normalization in the complex amplitude encoding-inherent noise is then investigated. Finally, a robust method to measure the phase difference between modes is proposed. These procedures are tested by wavefront reconstruction in a conventional few-mode fiber. Full article

We propose and theoretically demonstrate an optically induced long-period gratings (OLPGs) based core switching technique for multi-core fibers (MCFs). This switching technique is fully reconfigurable in terms of signal wavelength and input–output fiber cores. The switching window and its efficiency can be adjusted by changing the peak power and the shape of the pump pulses, respectively. Switching efficiencies of 76% were found. Furthermore, we also show that the switched signal can be reshaped by tuning the shape of the pump pulses. Full article

We propose a new terahertz fiber based on a circular-core photonic crystal fiber (PCF) structure to support high-performance orbital angular momentum (OAM) modes transmission. The modal characteristics of the proposed terahertz fiber were thoroughly analyzed to vary the parameters of air holes radius and the annular thickness by the full-vector finite element method (FEM). The optimal parameters are selected to realize the stable transmission of five-order OAM mode with high mode quality, low confinement loss and wide bandwidth. The mode purity is in excess of 91%, and the confinement loss is less than 10⁻⁷ dB/m over the 0.2 THz to 0.55 THz band. Furthermore, the design of this PCF is relatively simple and flexible, since it consists only of circular air holes. Due to its excellent transmission characteristics, the proposed OAM fiber has a potential application in terahertz mode division multiplexing (MDM) communication system. Full article

Few-mode erbium-doped fiber amplifiers (FM-EDFAs) are one of the most important optical subsystems for successful space division multiplexed transmission systems. In this paper, we propose a new FM-EDFA designed to achieve significantly reduced differential modal gain (DMG) via strong mode coupling. Using a new numerical model based on a fiber transfer matrix, the DMGs of FM-EDFAs are systematically investigated and two different types of six-mode fiber amplifiers are analyzed, as exemplar demonstrations. In a uniformly doped step-index fiber, the DMG can be reduced from 9.3 to 1.1 dB (i.e., 8.2 dB reduction) and further reduced to 0.5 dB in a dual-layer doping structure. Full article