On-off Gain Research Articles

High gain distributed Raman amplifier for orbital angular momentum mode-division multiplexing and wavelength-division multiplexing in a 104-km ring-core fiber over the C + L band

Mode-division multiplexing (MDM) and wavelength-division multiplexing (WDM) are regarded as effective solutions for enhancing the communication capacity of fiber-optic transmission systems. To extend the transmission distance, a suitable amplification technique is required to amplify the signal light, with distributed Raman amplifier (DRA) offering a high-flat gain. In this paper, we experimentally demonstrate the transmission of 3 orbital angular momentum (OAM) mode-division multiplexing and 9 wavelength-division multiplexing ranging from 1530 nm to 1610 nm over a 104-km self-designed ring-core fiber (RCF) with the assistance of DRA. The homemade all-fiber mode selective couplers (MSCs) can (de)multiplex OAM modes (OAM01, OAM11, OAM21) efficiently in long-distance MDM + WDM fiber-optic transmission systems. The OAM-DRA provides high gain (with an on-off gain greater than 10 dB) as well as flat gain (with a differential wavelength gain of less than 0.88 dB and a differential mode gain of less than 0.28 dB) over the C + L band.

Research on CdSe/ZnS Quantum Dots-Doped Polymer Fibers and Their Gain Characteristics.

Polymer fibers are considered ideal transmission media for all-optical networks, but their high intrinsic loss significantly limits their practical use. Quantum dot-doped polymer fiber amplifiers are emerging as a promising solution to this issue and are becoming a significant focus of research in both academia and industry. Based on the properties of CdSe/ZnS quantum dots and PMMA material, this study experimentally explores three fabrication methods for CdSe/ZnS quantum dots-doped PMMA fibers: hollow fiber filling, melt-drawing, and melt extrusion. The advantages and disadvantages of each method and key issues in fiber fabrication are analyzed. Utilizing the CdSe/ZnS quantum dots-doped PMMA fibers that were fabricated, we theoretically analyzed the key factors affecting gain performance, including fiber length, quantum dots doping concentration, and signal light intensity. Under the conditions of 1.5 W power and 445 nm laser pumping, a maximum on-off gain of 16.2 dB was experimentally achieved at 635 nm. Additionally, using a white light LED as the signal source, a broadband on-off gain with a bandwidth exceeding 70 nm and a maximum gain of 12.4 dB was observed in the 580-650 nm range. This research will contribute to the development of quantum dots-doped fiber devices and broadband optical communication technology, providing more efficient solutions for future optical communication networks.

Gamma irradiation-induced enhancement of Pb-related near-infrared luminescence in PbS/Al co-doped silica fiber

The impact of Al co-doping and gamma irradiation (0.3-3 kGy) on the spectral properties of lead-related active centers (PACs) in PbS/Al co-doped silica fiber (PADF) has been studied. Distinct absorption peaks at 700 and 1030 nm, and photoluminescence at 1100 nm, originated from Al-related lead active center (PAC-Al). The luminescence intensity of PACs at 900 and 1150 nm increased with irradiation doses in PADF with 0.01 at% Al co-doping. Notably, after irradiation with a 0.5 kGy dose, over a 16 dB enhancement of luminescence from PAC-Al was observed in PADF doped with 0.1 at% Al under 980 nm pumping. The on-off gain of the PADF at 1100 nm increased ∼12 dB after irradiation with a dose of 0.3 kGy. It was found that PAC-Al was preferentially generated over PAC at 1150 nm in Al co-doped fibers under irradiation. The proposed underlying mechanism for photoluminescence enhancement is the irradiation-induced reduction of Pb ions and Al-related defects. PbS/Al co-doped silica fiber is a promising candidate for active fibers operating at ∼1100 nm.

Characterization of the optical gain at 1550 nm of erbium-oxalate single crystals

Erbium is well-recognized as a key element for optical amplification at the 1550 nm operation band. However, the limited solubility of this material in vitreous matrices sets a critical constraint to the achievable optical gain, which limits its applicability in photonic integrated platforms. One of the solutions to increase the concentration of erbium is to employ a crystalline structure instead of a glass. In this paper, we characterize samples of erbium and erbium-ytterbium oxalate single crystals synthesized using the gel diffusion method. X-ray diffraction spectra and thermogravimetric analyses reveal that the synthesis method indeed generated the expected compound, and the pump-and-probe experiments demonstrate an on-off gain coefficient of ≈ 6.5 dB/mm, making this material a high-potential candidate for the implementation of integrated optical amplifiers.

Broadband Amplification in the 2.6–2.9 μm Wavelength Range in High-Purity Er3+-Doped Zinc-Tellurite Fibers Pumped by Diode Lasers

In recent years, great progress has been made in the technology of high-purity and ultra-dry tellurite glasses, which has enabled the creation of high-purity single-mode tellurite fibers doped with rare-earth ions. This technology has made it possible to demonstrate laser generation in the range of about 2.7 μm in erbium-doped tungsten tellurite fibers. In this paper, we present an experimental study of broadband amplification in erbium-doped zinc-tellurite fibers. Zinc-tellurite glasses containing modifying components, such as Na2O, La2O3, Bi2O3, or rare-earth metal oxides, are known to have noticeably lower phonon energy than heavy metal-tellurite systems, namely, tungsten tellurite glasses, which leads to better lasing output. The on-off gain of 30- and 60-cm long zinc-tellurite fibers has been measured in a wide range of diode pump powers. It has been shown for the first time that the amplification band is essentially extended, with pump power reaching over 250 nm (2600–2850 nm) at a peak power of about 40 W for a 30-cm long fiber.

Ultralow noise C + L wideband WDM-IMDD transmission at 18 × 112 Gbps by using hybrid second-order distributed Raman and first-order lumped Raman amplification.

We experimentally investigated and demonstrated an ultralow noise hybrid amplifier that combines second-order distributed Raman amplifier (DRA) and first-order lumped Raman amplifier (LRA) in a cascaded approach. This approach allows for the reutilization of pump light from the LRA as the seed light in the second-order DRA, and simultaneous full-band dispersion compensation is realized by using dispersion compensation fiber in the LRA. This approach also supports broadband gain flattening based on the separated DRA and LRA configuration. The transmission application of the proposed amplifier was investigated using a set of 10 external cavity lasers (ECLs) in the C-band and 8 ECLs in the L-band. Ranging from 1531.12 nm to 1595.49 nm across C + L band, the proposed hybrid amplifier gives a maximum on-off gain of 27.2 dB and an average gain of 23.4 dB, with an extremely low effective noise figure (NF) of lower than -2.9 dB. Intensity modulation direct detection (IMDD) signal transmission is carried out at two different data rates across these 18 wavelengths in the C + L band: (1) 56 Gbps/λ PAM-4 signal; (2) 112 Gbps/λ PAM-4 signal. The results show that the error free transmissions are demonstrated over 101.6 km EX2000 fiber using both signals with 7% HD-FEC and 20% SD-FEC, respectively.

Raman amplification at 2.2 μm in silicon core fibers with prospects for extended mid-infrared source generation

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 μm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 μm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 μm) over a length of 6 cm, with losses as low as 0.2 dB cm−1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 μm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2–5 μm range.

Fiber Raman Amplifier Based on Improved Whale Algorithm

In this paper, an improved whale algorithm to optimize pump power and wavelengths of second-order fiber Raman amplifier (FRA) has been proposed. Adopted by the strategies of adaptive gradients and reverse learning, the normal whale algorithm is greatly improved. An optimized FRA is designed based on the six-wavelength backward pump structure. Compared by optimizing both wavelength and power with optimizing only power, the gain flatness curves of the two algorithms, the optimal combination scheme, and the corresponding noise characteristics are got. A 6-pumped FRA has been designed in which the error of on-off gain is 0.08% and the ripple is only 0.18dB.

Ga-hybridization-mediated broadband optical amplification in Bi-activated photonic glass and fiber.

A Ga hybridization strategy is proposed for simultaneously enhancing the near-infrared activity and extending the bandwidth of Bi-activated photonic glass. Systematic studies on the near-infrared optical responses of Ga/Bi and Al/Bi co-doped silica glasses are performed. It is interesting to note that Ga/Bi co-doped glasses have a similar near-infrared emission center to Al/Bi co-doped glass, while the former is more effective in improving near-infrared activity. The different luminescence mechanisms of Ga/Bi and Al/Bi co-doped silica glasses are elucidated, and the corresponding microstructure-optical response relationship is discussed. In addition, the Ga/Bi co-doped silica optical fiber is successfully prepared, and the principal fiber amplifier device is fabricated. Furthermore, amplified spontaneous emission and broadband on-off gain are realized. The results suggest that Ga-hybridized Bi-activated photonic glass is a promising gain material for broadband fiber amplifiers.

Clustering engineering in tellurium-doped glass fiber for broadband optical amplification

Dense wavelength division multiplexing (DWDM) is recognized as one of the leading technologies for resolving the internet data traffic capacity issue. Although the successful development of rare-earth doped fiber amplifiers enables to achieve optical signal amplification in various telecommunication wavebands, their bandwidth is limited by the inherent sharp 4f electronic transition. Here, we propose the clustering engineering strategy in tellurium (Te)-doped glass fiber for broadband optical amplification. The correlation among the glass system, chemical state of Te and the optical response are systematically studied. The active Te-doped phosphate glass with intense and broadband luminescence is successfully achieved by control of the cluster state of the Te atoms. Subsequently, Te-activated phosphate fiber is successfully drawn and the optical response is transferred smoothly. Importantly, the amplified spontaneous emission and broadband optical amplification are achieved for the first time in Te-doped glass fiber. A small signal on-off gain covering an ultra-wide range of O + E + S + C + L bands is observed under excitation at 808 nm. Our results establish a critical step toward advancing Te-based photonic materials for novel broadband fiber amplifiers serviced in telecommunication system.

Crosstalk Noise in Bi-Directional Transmission on Multi-Core Fiber Link With Distributed Raman Amplifier

The crosstalk (XT) noise-to-signal ratio characteristics in a distributed Raman amplifier (DRA) used for bi-directional multi-core fiber (MCF) transmission are investigated. We propose a model for calculating XT noise by considering the noise contribution due to Rayleigh scattering. We verify the validity of our model both numerically and experimentally. Our model reveals that the XT noise-to-signal ratio ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) varies with the on-off gain. This feature is unique for bi-directional transmission, and it provides an on-off gain at which the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> can be minimized, although the corresponding net gain is less than 0 dB. We also show that the minimum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">XT</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> is relatively sensitive to the attenuation coefficient at both signal and pump wavelengths. Our model is beneficial for discussing the signal-to-noise ratio property in DRA-based MCF bi-directional transmission, and further study for hybrid DRA and discrete amplifier systems is expected.

High-order mode waveguide amplifier with high mode extinction ratio written in an Er3+-doped phosphate glass.

Inscription of fiber-compatible active waveguides in high-gain glass, followed by direct interconnection with few-mode fibers, is one of the most promising solutions for all-optical mode-division multiplexing. In this work, based on the femtosecond laser writing technique, we propose a general fabrication scheme for inscribing high-order mode waveguides in glass, by carefully tailoring the cross-section of the waveguides to match the mode intensity distribution via an improved multi-scan approach. Specifically, we design and fabricate two kinds of waveguides, namely, LP01-mode waveguide and LP11-mode waveguide in a highly Er3+-doped phosphate glass, enabling the insertion loss of the waveguides to be as low as 1.88 dB, and the mode extraction factor of the LP11-mode waveguide up to ∼24 dB. Importantly, we have successfully achieved optical amplification of the waveguides, with an on-off gain as high as 3.52 dB. This novel high-order mode waveguide amplifier has broad application prospects in monolithic on-chip integrated photonic light sources and optical interconnection with few-mode fiber and/or silicon-based waveguide.

Characterization of the optical gain in erbium-ytterbium-doped zinc and sodium-zinc phosphate glasses

Glasses doped with rare-earth elements represent a key-enabling amplification technology for fiber-based systems and integrated devices. Even if silica has been the preferred glass matrix for optical amplifiers, some other glasses have been proposed as an alternative. Phosphates, in particular, present a lower glass transition temperature and have a structure that envisages a capacity to host erbium and ytterbium dopants. In this paper, we characterize the gain of zinc and sodium zinc phosphate doped either only with erbium or with erbium and ytterbium. The experimental results reveal that, among the different considered configurations, the sodium zinc phosphate with erbium-ytterbium co-doping leads to an on-off gain coefficient as large as 40 dB/cm.

Suppression of Intensity Noises in Forward-pumped Raman Amplifier Utilizing Depolarizer for Multiple Pump Laser Sources

We investigate the random intensity noises that occur in forward-pumped distributed Raman amplifier systems. First, we show pump-to-signal intensity noise transfer characteristics, which strongly depend on the group velocities of pump light and signal light in optical fiber used as a gain medium. When signal light is in the C-band, dispersion shifted fiber (DSF) transfers much larger noise compared with standard single mode fiber. Next, we discuss the origin of the noise induced in pump light. We define the concept of synthesized polarization and show that fluctuation in the state of synthesized polarization (SOSP) can induce a large gain instability even if the relative intensity noise (RIN) of each pump laser source is negligible experimentally. Next, we propose a novel optical depolarizer for pump light. It can simultaneously depolarize pump light generated by multiple laser sources. Moreover, it can manage optical phases precisely to suppress the gain instability induced by the fluctuation in SOSP. Finally, we present a measurement of the Q factor of a 16-QAM signal (32 Gbaud) after 35-km transmission, with and without a forward-pumped Raman amplifier. Two pump laser sources with RINs of 134 to 126 dB/Hz were depolarized by the proposed depolarizer, and the generated on-off gain was 7 dB. Though amplified signal light was in the C-band and the gain medium was DSF, 16-QAM transmission specifications were successfully improved, and acceptable noise was observed for 5 hours operation.

Influence of liquid nitrogen cooling on the spectral performance of BAC-P in bismuth-doped phosphosilicate fibers under liquid nitrogen temperature

The first results of the liquid nitrogen cooling effect on the spectral properties of a phosphor-related bismuth active center (BAC-P) have been presented. It is found that the small-signal absorption bands of BACs are mildly affected at liquid nitrogen temperature (LNT, 77 K), while the unsaturable absorption level is lowered moderately. Meanwhile, noticeable shape changes of luminescence spectra and enhanced luminescence intensity (∼ twice) of BAC bands were revealed at LNT upon excitation at 830 or 980 nm, which is contributed by the notable growth of BAC-P. In addition, the influence of cooling on the on-off gain performance of BACs is also explored. Furthermore, the laser-induced effect on the luminescence of BAC-P upon cooling at LNT is also investigated, revealing a noticeable increase (∼10%) instead of photobleaching. These experimental results bring new insights into the temperature-related spectral properties of BACs and provide an effective way for tuning the luminescence scheme for Bi-doped fibers by cryogenics for the desired spectral range 1.26-1.36 μm.

Intelligent gain flattening in wavelength and space domain for FMF Raman amplification by machine learning based inverse design

We propose a machine learning based approach to design few-mode DRAs by using neural networks to optimize the pump wavelengths, powers and mode content in order to obtain flat gain spectrum with low mode-dependent gain (MDG). Based on the proposed intelligent inverse design method, amplification optimization for the random fiber laser based two-mode DRA can be achieved with gain flatness of 1.0 dB and MDG of 0.6 dB at 14.5 dB on-off gain level. For backward pumping four-mode DRA, gain flatness of 0.46 dB and MDG of 0.3 dB can be achieved at 12.5 dB on-off gain.

Numerical Analysis of CW Raman Amplifier in Silicon-on-Insulator Nano-Waveguides

Numerical analysis of continuous wave (CW) on-off optical Raman gain in Silicon-On-insulator (SOI) nano-waveguides has been investigated. Two types of waveguide structures rib and strip waveguides have been incorporated. The waveguide structures have designed to be 220 nm in height. Three different widths of (350, 450, 1000) nm were studied. At telecommunication wavelength of 1550 nm, Raman amplification was calculated to be about 65 dB in rib SOI waveguide. The obtained Raman amplification is the heist reported value at relatively low pump power. High Raman gain amplification in SOI nano-waveguides presents an important step towards integrated on-chip optoelectronic devices.

Ultra-wideband and flat-gain optical properties of the PbS quantum dots-doped silica fiber.

We investigate the microstructural characteristics and optical properties of PbS quantum dots-doped silica fiber (PQDF), prepared by atomic layer deposition (ALD) doping technique. The fiber exhibits ultra-wideband luminescence and flat-gain with 3 dB bandwidth of 300 nm. The on-off gain and net gain can reach to 7.1-15.0 dB and 6.0-9.2 dB at 1050-1350 nm, respectively. The results of high-resolution transmission electron microscopy (HRTEM) further reveal the effects of PbS QDs doping in PQDF. The broadband luminescence spectrum originating from three active centers (1086, 1179, and 1304 nm), can be attributed to the dimension effect of PbS QDs (3.7, 4.0, and 4.3 nm, respectively). Moreover, the calculation results indicate that the multi-sized PbS QDs concentrated at 3.65-4.45 nm make the 3 dB gain bandwidth increase, which is six times wider than that of traditional erbium-doped fiber (EDF). Therefore, this type of PQDF is a promising gain medium for optical amplifiers and broadband light sources.

Electron beam irradiation and thermal-induced effects on the spectral properties of BAC-Al in Bi/Er codoped aluminosilicate fibers

The effect of electron irradiation (5-140 kGy) on the spectral properties of bismuth active centers (BACs) in Bi/Er codoped aluminosilicate fibers (BEDFs) has been studied. It is revealed that the near-infrared (NIR) emitting BACs exhibit high radiation resistance, evidenced by the negligible change in the saturable absorption and on-off gain of BACs. In contrast, the radiation-induced absorption (RIA) increases sharply with the increasing radiation dose. The contribution of each of the defects to the RIA level has been analyzed, and it is found to be mainly linked to the generation of Al-OHC in the Al2O3-SiO2 host. Furthermore, the thermal annealing on the irradiated BEDFs has also been investigated, revealing the higher thermal stability of BACs due to the thermal bleaching of Al-OHC. This is the first time that the BACs show good resistance to the strong electron irradiation.

PbSe quantum-dot-doped broadband fiber amplifier based on sodium-aluminum-borosilicate-silicate glass

A PbSe quantum-dot doped fiber amplifier (QDFA) based on the sodium-aluminum-borosilicate-silicate glass in near-infrared (NIR) waveband is realized experimentally for the first time. The quantum-dot doped fiber (QDF) with embedded particle size of 4.08–5.88 nm is prepared by optimizing the melting annealing heat-treatment condition. The prepared QDF has strong and stable photoluminescence (PL) emission in NIR region. The QDFA comprises the QDF, wavelength division multiplexer, isolator, and pump laser. It is observed that the broadband signal in QDFA is amplified in the wavelength-region of 1260–1380 nm depending on the size of QDs. The on-off gain is 16.4 dB at the central wavelength, and the 3-dB bandwidth reaches 80 nm when the inputting signal power is −17 dBm. An excitation threshold and gain saturation are also observed in the experiment. This glass-based QDFA demonstrates a novel approach to extend the current waveband of erbium-doped fiber amplifiers, with industrial applications in the future.