Fiber Communication Applications Research Articles

Normal-Incidence Germanium Photodetectors Integrated with Polymer Microlenses for Optical Fiber Communication Applications.

We present a novel photon-acid diffusion method to integrate polymer microlenses (MLs) on a four-channel, high-speed photo-receiver consisting of normal-incidence germanium (Ge) p-i-n photodiodes (PDs) fabricated on a 200 mm Si substrate. For a 29 µm diameter PD capped with a 54 µm diameter ML, its dark current, responsivity, 3 dB bandwidth (BW), and effective aperture size at -3 V bias and 850 nm wavelength are measured to be 138 nA, 0.6 A/W, 21.4 GHz, and 54 µm, respectively. The enlarged aperture size significantly decouples the tradeoff between aperture size and BW and enhances the optical fiber misalignment tolerance from ±5 µm to ±15 µm to ease the module packaging precision. The sensitivity of the photo-receiver is measured to be -9.2 dBm at 25.78 Gb/s with a bit error rate of 10-12 using non-return-to-zero (NRZ) transmission. Reliability tests are performed, and the results show that the fabricated Ge PDs integrated with polymer MLs pass the GR-468 reliability assurance standard. The demonstrated photo-receiver, a first of its kind to the best of our knowledge, features decent performance, high yield, high throughput, low cost, and compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes, and may be further applied to 400 Gb/s pulse-amplitude modulation four-level (PAM4) communication.

Research Progress on Carrier-Free Phase-Retrieval Receivers

In order to deal with the chromatic dispersion-induced power fading issue for short-reach direct-detection optical fiber communication applications, such as the ever-increasing data-center interconnections (DCIs), optical filed recovery is intensively being under investigation. To date, various direct detection schemes capable of optical field recovery have been proposed, including the Kramers–Kronig (KK) receiver, asymmetric self-coherence detection (ASCD) receiver, carrier-assisted differential detection receiver (CADD), Stokes vector receiver (SVR), and carrier-free phase-retrieval (CF-PR) receiver. Among those, the CF-PR receiver attracts lots of research attention because it can circumvent the requirement of a strong continuous-wave (CW) optical carrier for the beating with the signal. Generally, the CF-PR receiver consists of only two single-ended photodiodes (PDs) and one dispersive element, for the field recovery of the quadrature amplitude modulation (QAM) signals. Based on the theoretical and experimental studies reported so far, this paper reviews the latest progress of CF-PR receivers designed for high-speed optical short-reach transmission links.

Metafiber transforming arbitrarily structured light

Structured light has proven useful for numerous photonic applications. However, the current use of structured light in optical fiber science and technology is severely limited by mode mixing or by the lack of optical elements that can be integrated onto fiber end-faces for wavefront engineering, and hence generation of structured light is still handled outside the fiber via bulky optics in free space. We report a metafiber platform capable of creating arbitrarily structured light on the hybrid-order Poincaré sphere. Polymeric metasurfaces, with unleashed height degree of freedom and a greatly expanded 3D meta-atom library, were 3D laser nanoprinted and interfaced with polarization-maintaining single-mode fibers. Multiple metasurfaces were interfaced on the fiber end-faces, transforming the fiber output into different structured-light fields, including cylindrical vector beams, circularly polarized vortex beams, and arbitrary vector field. Our work provides a paradigm for advancing optical fiber science and technology towards fiber-integrated light shaping, which may find important applications in fiber communications, fiber lasers and sensors, endoscopic imaging, fiber lithography, and lab-on-fiber technology.

O-band tunable multiwavelength Brillouin-Raman fiber laser based on a wavelength-agile Raman pump.

Multiwavelength Brillouin-Raman fiber laser (MBRFL) features broadband multiwavelength generation with flat-amplitude and high optical signal to noise ratio (OSNR), which has great potential in optical fiber communication applications. Till now, the spectral regions of MBRFLs are mostly concentrated at conventional C- and L-band and the tunability of MBRFL is limited by using the Raman pump with fixed wavelength. Here, by utilizing wavelength-agile random fiber laser which can emit tunable lasing at 1.2 µm band as the Raman pump, we experimentally demonstrate the tunable MBRFL in the O-band for the first time, to the best of our knowledge. At Raman and Brillouin pump powers of 920 mW and -3 dBm, respectively, up to 90 Stokes lines with 0.13 nm wavelength spacing and >13 dB OSNR can be obtained when the Raman and Brillouin pump wavelength are set at 1231 nm and 1300 nm, respectively. Moreover, by tuning the wavelength of Brillouin pump from 1295 nm to 1330 nm, tunable MBRFL can be achieved with similar multiwavelength generation bandwidth by simultaneously tuning the Raman pump wavelength, and the number of Stokes lines are beyond 85 across the tuning range. The bandwidth of the demonstrated O-band MBRFL is also the widest wavelength span ever reported for multiwavelength Brillouin fiber lasers at 1.3 µm band. Our work indicates that the use of wavelength-agile random fiber laser as Raman pump in MBRFL can provide an effective way to extend the spectral regions of MBRFL and also improve the tunability performance of MBRFL.

Three-core photonic crystal fiber for the in-line measurement of full-polarization states by multi-core polarization interference theory.

In this Letter, we demonstrate and experimentally verify the application of three-core photonic crystal fiber (3C-PCF) for the in-line detection of fully polarized states. We prove the response of 3C-PCF to full-polarization states under multi-core polarization interference through experiments. The sensitivity at 1472 nm is 0.0273 nm/rad, and the linear response is better than 98.9% (the optimal operating wavelength can be designed in the range of 1470 to 1570 nm). With the advantages of an all-fiber integrated system, robustness, and wide wavelength coverage, our design holds great promise for facilitating fiber-optic-integrated polarization meters for optical fiber communication and biomedical diagnostic applications.

Topologically-optimized on-chip metamaterials for ultra-short-range light focusing and mode-size conversion

Abstract The concept of metamaterials offers a flexible pathway to manipulate the macroscopic behavior of light by delicately designed microscopic subwavelength structures, which has been recently introduced to integrated photonics to create devices with ultra-compact footprint, excellent performance or versatile functionalities. However, the conventional design approach of metamaterials, including two separated steps of subwavelength structure design and the assembly of unit cells, often encounters challenges when facing extreme design targets. In this work, we propose a hierarchical inverse design approach by cascading a conventional unit-cell-based design with a holistic topology optimization. As a proof-of-concept, we demonstrate ultra-short-range light focusing and mode-size conversion enabled by on-chip meta-lenses. The shortening of tapering region pushes higher numerical aperture of on-chip lenses, leading to the violation of locally periodic approximation used in meta-lens design and thus poor device performance, which fortunately, can be well compensated by the follow-up holistic optimization step. We experimentally realize mode-size squeezing by almost 20 times in a tapering region as short as 8 μm and 5 μm with low insertion loss and broadband performance. The proposed design scheme provides practical guidelines to design metamaterials as flexible on-chip wavefront control and light routing devices for various applications in fiber communication, sensing and optical computing.

Design of IM-DD Optical Communication using Matlab

A modulation scheme where the intensity of an optical source is modulated by the RF or mm-wave signal. Demodulation is achieved through direct detection of the optical carrier and conversion using a photo detector is called IM-DD Systems. In these systems the multi-layer modulation (MLM) is used to design both asymmetric clipped optical OFDM (ACO-OFDM) and dc-biased optical OFDM (DCO-OFDM) as a function. In this systems the DCO OFDM is more spectrally efficient than non-DC biased systems because this have better tradeoff between spectral and power efficiency. In the direct detection system, the detection process is non-linear due to the photo current proportional to absolute square of electric field intensity due to this the signal is slow. The equalizer is comprised of one hidden layer which leads to super-fast of the signal .The channel capacity for this systems can be measured by the transmitting of the signals. This IM/DD is a cost-effective optical communication which finds wide applications in fiber communication, free-space optical communication, and indoor visible light communication. In these systems when the bandwidth of signal exceeds the modulation bandwidth of LED, multi path effect occurs. Therefore, we use ACO-OFDM to avoid this multi path distortions. OFDM is a suitable efficient cost-effective solution for GPON deployment with benefit of use of low-cost laser bandwidth in comparison with DCO-OFDM. At the same BER performance, in comparison with D-C ACO-OFDM at high bit rates transmission, DCO-OFDM promises to deliver higher throughput.

Detecting the orbital angular momentum from speckle via convolutional neural network

The inhomogeneity of turbid medium disrupts the coherent vortex structure of the vortex beam and causes the formation of a speckle pattern. Here, we propose a new, flexible approach to measure the orbital angular momentum (OAM) of vortex beams through a turbid medium by a convolutional neural network. The proposed technique directly recognizes the speckle image and obtains the corresponding OAM mode information. The accuracies exceed 99% and 97% in simulations and experiments respectively. In addition, the relationship between the recognition accuracy and environmental noise level, is present. The results show great potential in fiber communication, biomedical imaging and astronomical application, etc.

2D materials-enabled optical modulators: From visible to terahertz spectral range

Two-dimensional (2D) materials with layered structures have a variety of exceptional electronic and optical attributes for potentially developing basic functions of light wave technology from light-emitting to -modulating and -sensing. Here, we present state-of-the-art 2D materials-enabled optical intensity modulators according to their operation spectral ranges, which are mainly determined by the optical bandgaps of the 2D materials. Leveraging rich electronic structures from different 2D materials and the governed unique light–matter interactions, the working mechanisms and device architectures for the enabled modulators at specific wavelength ranges are discussed. For instance, the tunable excitonic effect in monolayer transition metal dichalcogenides allows the modulation of visible light. Electro-absorptive and electro-refractive graphene modulators could be operated in the telecom-band relying on their linear dispersion of the massless Dirac fermions. The bendable electronic band edge of the narrow bandgap in few-layer black phosphorus promises the modulation of mid-infrared light via the quantum-confined Franz–Keldysh or Burstein–Moss shift effect. Electrically and magnetically tunable optical conductivity in graphene also supports the realizations of terahertz modulators. While these modulators were demonstrated as proof of concept devices, part of them have great potential for future realistic applications, as discussed with their wavelength coverage, modulation depth, insertion loss, dynamic response speed, etc. Specifically, benefiting from the well-developed technologies of photonic chips and optical fibers in telecom and datacom, the 2D materials-based modulators integrated on these photonic structures are expected to find applications in fiber and chip optical communications. The free-space mid-infrared and terahertz modulators based on 2D materials can expect application in chemical bond spectroscopy, free-space communications, and environment/health sensing.

Novel Eight-Core Double-Trench Fiber Displaying Low Loss and Supporting Five Linearly Polarized Modes

A novel eight-core double-trench fiber that supports five linearly polarized modes is proposed. The characteristics of the designed fiber are analyzed systematically using the finite element method. A step-pure silicon core and a ring refractive index trench are employed to effectively reduce loss. Furthermore, an internal trench is used to reduce dispersion and the differential mode group delay. The nonlinear coefficient and crosstalk are also reduced effectively by adjusting the fiber geometric parameters and material refractive index. The designed eight-core fiber has potential applications in high-capacity and high-quality fiber communication by mode division multiplexing.

A Comparison of Top Distributed Bragg Reflector for 1300 nm Vertical Cavity Semiconductor Optical Amplifiers Based on III–V Compound

In this work, the design of GaAs/AlGaAs distributed Bragg reflector (DBR) has been implemented for 1300 nm vertical cavity semiconductor optical amplifiers (VCSOAs) for optical fiber communication applications. The top DBR period and Al concentration are varied, the peak reflectivity of the DBR is increasing from 50% to 97.5% for 13 periods with increasing Al concentration, whereas the reflectivity bandwidth is increased to almost 190 nm. The relation between wavelength and incidence angle variation on DBR reflectivity is increasing with the incident angle (0°, 20°, 30°, and 50°), the resonant wavelength and bandwidth of the measured reflectance spectra shifts to shorter wavelength and wider bandwidth, respectively. In addition, a comparison between the linear, the graded, and the parabolic DBRs has been achieved with transfer matrix method using MATLAB software to show the influence of layer in DBRs and its effect on lasing wavelength. It is shown that using grading DBR mirror is much more beneficial compared to abrupt DBR, whereas it has lower reflectivity of almost 10% due to VCSOAs device which needs less number of top layers until prevent reaching lasing threshold.

Coupled derivative nonlinear Schrödinger III equation: Darboux transformation and higher-order rogue waves in a two-mode nonlinear fiber

The coupled derivative nonlinear Schrödinger III (cDNLSIII) equation describes pulse propagations in a two-mode nonlinear fiber. In this paper, we construct the N-fold generalized Darboux transformation (DT) with the limit approach, from which the general higher-order rogue wave solutions of the cDNLSIII equation are obtained. These solutions, divided into three categories, namely, rogue waves and rogue waves on multi-soliton and multi-breather backgrounds, have applications in nonlinear fiber communication. Furthermore, we present some interesting interactions between rogue waves and solitons or breathers and analyze the features of each type.

Research on the dispersion characteristics of silica-based ring-core photonic crystal fiber used to transmit orbital angular momentum modes

In this paper, based on the silica-based ring-core photonic crystal fiber (PCF) used to transmit orbital angular momentum (OAM) modes, the effects of central air hole and first-layer cladding air holes on the dispersion of ring-core PCF are analyzed. As a result, it is found that the sizes of air holes have a regular regulation effect on the dispersion of PCF. Furthermore, a ring-core PCF which can support 44 OAM modes in communication band is proposed. The characteristics of the designed ring-core PCF, such as number of supported modes, dispersion, nonlinear coefficient and confinement loss, are systematically analyzed by using the finite element method (FEM). In summary, the conclusion is of guiding significance for the design of silica-based ring-core PCF structures, and the designed ring-core PCF has potential applications in the high-capacity fiber communication.

A design of nested photonic crystal fiber with low nonlinear and flat dispersion supporting 30+50 OAM modes

In this paper, a novel nested photonic crystal fiber (PCF) with two guided modes regions is proposed, which can support 80 orbital angular momentum (OAM) modes over C wavelength band. The characteristics of designed nested PCF are systematically analyzed by using the finite element method (FEM). The results show that the isolation is kept above 40dB, the crosstalk between different guided modes regions can be neglected, and the dispersion is relatively flat, the nonlinear coefficient is lower than 2.65km−1W−1. Additionally, the confinement loss and effective mode field area of the designed nested PCF are also analyzed. In summary, the designed nested PCF has potential applications in the high capacity fiber communication of mode division multiplexing (MDM).

A novel dual guided modes regions photonic crystal fiber with low crosstalk supporting 56 OAM modes and 4 LP modes

In this paper, a novel photonic crystal fiber (PCF) with dual guided modes regions is proposed, which can support 56 orbital angular momentum (OAM) modes and 4 linear polarization (LP) modes over S + C + L wavelength bands. The introduction of a low refractive index ring enables separation of the guided modes regions and makes the designed PCF easier to fabricate. The characteristics of designed PCF are systematically analyzed by using the finite element method (FEM). The results show that the crosstalk between channels can be ignored and the confinement loss is lower than 5 × 10−8dB/m. Furthermore, dispersion and purity are also analyzed. In summary, the designed PCF has potential applications in the high capacity fiber communication of space division multiplexing (SDM).

Near-infrared-II photodetection realized by introducing organic-inorganic charge-transfer-complex photosensitive material into pentacene phototransistor

Abstract Organic phototransistor (OPT) is a promising photodetector with substantial attention due to its advantages such as low cost, light weight, flexibility and ease of large area preparation. Although the second near-infrared (NIR-II) window (1.0–1.7 μm) has important applications in fiber communication, the organic infrared photosensitive materials are scarce in this waveband, which greatly limits the application of organic photodetectors in near-infrared bands. In this work, a charge-transfer-complex (CTC) film with significant hyperchromic effect in the infrared range of 1000–1400 nm was prepared by co-evaporation of metal oxide WO3 and organic molecule m-MTDATA. A high performance near-infrared OPT was fabricated by combining this CTC photosensitive layer and pentacene (PENT) high-mobility channel layer, the photoresponsivity R of which has surpassed the recent reported level of 1.1 μm band inorganic infrared detector. The physical mechanism underlying the operation of CTC-PENT-OPT was clarified and the thickness of the CTC photosensitive film in this device was optimized. This new type of organic-inorganic CTC infrared-sensitive material will offer a new choice for the future study on organic infrared detectors.

Super- and Anti-Principal-Modes in Multimode Waveguides

We introduce a new type of states for light in multimode waveguides featuring strongly enhanced or reduced spectral correlations. Based on the experimentally measured multi-spectral transmission matrix of a multimode fiber, we generate a set of states that outperform the established "principal modes" in terms of the spectral stability of their output spatial field profiles. Inverting this concept also allows us to create states with a minimal spectral correlation width, whose output profiles are considerably more sensitive to a frequency change than typical input wavefronts. The resulting "super-" and "anti-principal" modes are made orthogonal to each other even in the presence of mode-dependent loss. By decomposing them in the principal mode basis, we show that the super-principal modes are formed via interference of principal modes with closeby delay times, whereas the anti-principal modes are a superposition of principal modes with the most different delay times available in the fiber. Such novel states are expected to have broad applications in fiber communication, imaging, and spectroscopy.

Optical receiver sensitivity enhancement by single- and dual-band fiber optical parametric amplifier.

A semi-classical model is proposed theoretically and demonstrated experimentally on the optical receiver sensitivity enhancement by single-band (signal or idler) and dual-band (signal and idler) fiber optical parametric amplifier (FOPA). The sensitivity enhancement by single-band is determined by the gain of FOPA and the transmission loss of signal and idler, and it can be further improved by up to 3-dB using amplified signal and phase-conjugated idler together at dual-band configuration. The theoretical results are experimentally verified in both fiber communication and biomedical imaging applications. This detection sensitivity enhancement scheme can be potentially applied in the scenarios where ultrafast broadband signal at low-power level is being handled.

Propagation properties of vortex beams in a ring photonic crystal fiber

In the last decade, the vortex beams have received lots of attention for their orbital angular momentum.When they are applied to optical fiber communication field,the data channels will increase and information propagation speed will be effectively improved. Recently, researchers have shown the capabilities of long length stably propagation, nonlinear frequency conversion and mode division multiplexing of vortex modes in a ring fiber. Due to the photonic crystal fiber(PCF) having very flexible design degrees of freedom, it will enable a wide range of propagation properties. In this paper, a SiO2 air-hole ring PCF is proposed for separation and propagation of optical vortex modes.By using COMSOL Multiphysics software,the vortex modes(TE01, HE21± and TM01) are simulated and calculated. The differences in effective refractive index between them are 4.59×10-4 and 3.62×10-4 respectively. One can analyze the propagation properties of vortex beams in the ring PCF by changing the size of first layer air hole radius and air hole pitch. When the incident light wavelength of TE01 mode ranges from 1650 nm to 1950 nm, this ring PCF can achieve a total dispersion variation between 44.18 to 45.83 ps·nm-1·km-1, which is tend to be flat. When incident light wavelength is 1550 nm, the nonlinear coefficient of TE01 mode vortex light is 1.37 W-1·km-1. Due to the fact that long wavelength light is easier to leakage through the cladding than the short wavelength light, the confinement loss increases with the wavelength. When incident light wavelength is 2000 nm, there is still an eight-orders-of-magnitude of the low confinement loss. Theoretically, flat dispersion and low loss vortex beams in this fiber can be beneficial to propagating stably, and the vortex modes lay the foundation for long distance propagation in the optical fiber. In the future, this ring PCF will be used in optical fiber communication field and applications in aspects such as continuous spectrum research, which can make it have immense advantage over traditional fibers.

High‐power random distributed feedback fiber laser: From science to application

A fiber laser based on random distributed feedback has attracted increasing attention in recent years, as it has become an important photonic device and has found wide applications in fiber communications or sensing. In this article, recent advances in high‐power random distributed feedback fiber laser are reviewed, including the theoretical analyses, experimental approaches, discussion on the practical applications and outlook. It is found that a random distributed feedback fiber laser can not only act as an information photonics device, but also has the feasibility for high‐efficiency/high‐power generation, which makes it competitive with conventional high‐power laser sources. In addition, high‐power random distributed feedback fiber laser has been successfully applied for midinfrared lasing, frequency doubling to the visible and high‐quality imaging. It is believed that the high‐power random distributed feedback fiber laser could become a promising light source with simple and economic configurations. image