GeO2-doped Silica Research Articles

Efficient tunable cascaded Raman source with all-silica fibers based on 2-µm DSR pulse pumping.

We present an efficient tunable all-silica-fiber 2nd-order cascaded Raman pulse laser utilizing 2-µm dissipative-soliton-resonance (DSR) rectangular pulses for pumping and highly GeO2-doped silica fiber as Raman gain medium. When pumped at 1966.5 nm, the maximum 1st-order Raman optical conversion efficiency is up to 64.4% at 2153 nm, with 92.4% spectral purity and 0.39-W average power. The maximum 2nd-order Raman optical conversion efficiency is 19.3% at 2370 nm, with 39.2% spectral purity and 0.25-W average power. To our knowledge, these conversion efficiencies and spectral purities represent the highest levels achieved in a mid-infrared all-silica-fiber cascaded pulsed Raman laser. Additionally, by adjusting the central wavelength of the DSR seed pulse, the 2nd-order Raman light can be tuned within a range of 41 nm (2354∼2395 nm). Our system provides a simple and easy-to-implement solution for realizing efficient tunable cascaded pulsed Raman lasers in the 2.4-µm band.

Parabolic-Index Ring-Core Fiber Supporting High-Purity Orbital Angular Momentum Modes

We design a graded-index ring-core fiber with a GeO2-doped silica ring core and SiO2 cladding. This fiber structure can inhibit the effect of spin-orbit coupling to mitigate the power transfer among different modes and eventually enhance the orbital angular momentum (OAM) mode purity. By changing the high-index ring core from the step-index to parabolic graded-index profile, the purity of the OAM1,1 mode can be improved from 86.48% to 94.43%, up by 7.95%. The proposed fiber features a flexible structure, which can meet different requirements for mode order, effective mode area, etc. Simulation results illustrate that the parabolic-index ring-core fiber is promising in enhancing the OAM mode purity, which could potentially reduce the channel crosstalk in mode-division-multiplexed optical communication systems.

Six-Core GeO2-Doped Silica Microstructured Optical Fiber with Induced Chirality

This work presents a fabricated silica few-mode microstructured optical fiber (MOF) with a special six GeO2-doped core geometry, an outer diameter of 125 µm (that corresponds to conventional commercially available telecommunication optical fibers), and improved induced twisting up to 500 revolutions per 1 m (under a rotation speed of 1000 revolutions per meter with a drawing speed of ~2 m per minute). The article discusses some technological aspects and issues of manufacturing the above-described twisted MOFs with complicated structures and geometry as GeO2-doped silica supporting elements for them. We present results of some measurements performed for fabricated samples of chiral silica six-GeO2-doped-core few-mode MOFs with various orders of twisting and both step and graded refractive indexes of “cores”. These tests contain research on MOF geometrical parameters, attenuation, and measurements of the far-field laser beam profile.

Efficient Raman pulse fiber laser pumped by a dissipative soliton resonance pulse near 2 µm.

A high-efficiency Raman conversion from 1.987 µm to 2.177 µm is demonstrated experimentally in 45 m GeO2-doped silica fiber, adopting a dissipative soliton resonance (DSR) rectangular pulse as the pump. Over the entire spectral distribution, the spectral purity of the first-order Raman pulse is up to 96.8%, suggesting a nearly complete pump depletion before the onset of cascaded Raman shifts. The corresponding pump-to-Raman conversion efficiency of 67.4% is the highest up to date in this spectral region. Meanwhile, a large Raman pulse energy of 1.03 µJ was obtained at the repetition rate near MHz level, corresponding to 0.893 W average power. In the total output, the Raman-dominated spike has a Full Width Half Maximum (FWHM) of 1.18 ns far narrower than DSR's pulse duration of 10.25 ns. The results indicate that DSR is a promising candidate for developing efficient Raman nanosecond pulse fiber laser in mid-infrared (MIR) region.

High index core flat fiber surface plasmon resonance bio-sensor.

Due to tremendous design flexibility and ease of light control capability, the photonic crystal fiber offers efficient, flexible, and miniaturized plasmonic biosensors with attractive features. In this work, a high index (GeO2 doped silica) core flat fiber is proposed and analyzed for RI sensing ranging from 1.53 to 1.60. A rectangular analyte channel is created on top of a flat fiber to better handle the liquid analyte. To introduce the plasmonic effect, TiO2 and gold are deposited to the analyte channel. The sensing performance is carried out for two operating wavelengths, as two peaks are obtained for each analyte. The second operating wavelength shows better sensing performance than the first one. However, the proposed sensor offers average wavelength sensitivity of 5000 nm/RIU with a sensor resolution of 2×10-05 RIU. In addition, the proposed sensor shows identical linearity, which is quite rare in prior sensors. Moreover, the proposed flat sensor provides outstanding detection accuracy of 0.01nm-1, detection limit of 79.28 nm, signal to noise ratio of -4.1497dB, and figure of merit of 50RIU-1. Owing to outstanding sensing performance and a unique detection range, this sensor can be effectively used in biological and organic analyte sensing applications.

Limits of broadband fiber optic parametric devices due to stimulated Brillouin scattering

We experimentally find a practical stimulated Brillouin scattering (SBS) threshold for broadband high-performance fiber optical parametric devices relying on dispersion-stable GeO2-doped silica highly nonlinear fibers. We demonstrate that SBS limits the nonlinear phase shift in such fibers to ~0.3 rad per pump unless the SBS is mitigated in some way. We consequently derive corresponding limits on signal gain and conversion efficiency and find the required SBS mitigation factor for a range of fiber optic parametric devices’ applications. Finally, we examine the level of SBS mitigation using air gaps and fiber tapers for implementation in polarization-insensitive fiber optic parametric devices employing bidirectional loops. We observe that an air gap or fiber taper are not very efficient for SBS mitigation as they provided an increase in SBS threshold up to 0.7 dB attributed primarily to their excess loss.

Analysis of a Single Solid Core Flat Fiber Plasmonic Refractive Index Sensor

In this article, a single solid core flat fiber (SSCFF) refractive index sensor based on surface plasmon resonance (SPR) is proposed and analyzed numerically using the finite element method (FEM). The proposed flat fiber consists of a single array of five circular holes. Among them the central hole is made of GeO2-doped silica which is forming the core. Other holes are filled with air and situated symmetrically on both sides of the central solid core. The upper flat surface of the fiber is coated with a thin plasmonic gold layer which is protected by an active titanium dioxide layers. Analyte is situated on top of these layers. The wavelength interrogation technique is applied to study the coupling characteristics between the core-guided mode and the surface plasmon mode as well as for the refractive index measurement. Numerical analysis results show that this sensor is able to detect high refractive index analytes from 1.49 to 1.54 with a good linear response. Additionally, the dependence of surface plasmonic resonance wavelength on analyte refractive index is studied. The maximum wavelength sensitivity of this sensor is found to be 4782 nm/RIU with a high resolution of 2.09 × 10−5 RIU. The effects of different structural parameters on loss spectrum are studied in detail to optimize this SSCFF structure. In comparison to traditional PCF, this SSCFF structure is fabrication complexity free as well as a suitable candidate for developing portable devices and high refractive index analyte sensors, particularly chemical and protein sensors.

In-band pumping of Tm:LiYF4 channel waveguide: a power scaling strategy for ∼2 μm waveguide lasers.

We report on a novel power scaling strategy for thulium waveguide (WG) lasers relying on in-band pumping by high-brightness Raman fiber lasers (RFLs) and the use of liquid-phase-epitaxy-grown fluoride crystalline thin films for better thermal management. Thulium channel WGs are produced by microstructuring the Tm3+:LiYF4/LiYF4 epitaxies via diamond-saw dicing. They are pumped by a RFL based on an erbium master oscillator power amplifier and a GeO2-doped silica fiber and emit polarized output at 1679nm. A CW in-band-pumped (H63→F43) Tm3+:LiYF4 WG laser generates up to 2.05W of a linearly polarized single-transverse-mode output at 1881nm with a slope efficiency of 78.3% and a laser threshold of only 12mW (versus the absorbed pump power).

Scaling power, bandwidth, and efficiency of mid-infrared supercontinuum source based on a GeO2-doped silica fiber

We demonstrate a supercontinuum source with a 20 dB bandwidth from ∼1 to ∼3 μm with output power exceeding 6 W based on a GeO2-doped silica fiber. This is the highest output power reported for a 3 μm supercontinuum source based on germania-doped silica fiber in an all-fiberized and compact size device. We further demonstrate a spectrum spanning from ∼1.7 to ∼3.4 μm (∼10 dB bandwidth from ∼1.8 to ∼3.2 μm) at a low power of tens of milliwatts with more than 50% power fraction above 2400 nm, which makes this source suitable for several applications where a broadband source at low power is required to avoid damage of the samples. Our investigations reveal the unexploited potential of germania-doped fiber for mid-infrared supercontinuum generation and surpass the current state-of-the-art results.

Under-Coupling Whispering Gallery Mode Resonator Applied to Resonant Micro-Optic Gyroscope.

As an important sensing element, the whispering gallery mode resonator (WGMR) parameters seriously affect the resonant micro-optic gyroscope (RMOG) performance. This work proposes an under-coupling resonator to improve the resonator’s Q value and to optimize the coupling coefficient to maximize the RMOG’s sensitivity. GeO2-doped silica waveguide-type resonators with different coupling coefficients were simulated, designed, fabricated and tested. An under-coupling ring resonator with a quality factor of 10 million is reported. The RMOG system was built based on this resonator and the scale factor was tested on a uniaxial high-precision rotating platform. Experimental results show that this resonator could improve the RMOG sensitivity by five times.

Random distributed feedback fiber laser at 2.1 μm.

We demonstrate a random distributed feedback fiber laser at 2.1μm. A high-power pulsed Tm-doped fiber laser operating at 1.94μm with a temporal duty ratio of 30% was employed as a pump laser to increase the equivalent incident pump power. A piece of 150m highly GeO2-doped silica fiber that provides a strong Raman gain and random distributed feedbacks was used to act as the gain medium. The maximum output power reached 0.5W with the optical efficiency of 9%, which could be further improved by more pump power and optimized fiber length. To the best of our knowledge, this is the first demonstration of random distributed feedback fiber laser at 2μm band based on Raman gain.

Raman scattering enhancement characteristic of NbCl5- and Nb2O5-doped silica fibers

Two kinds of Nb-doped silica fibers, an NbCl5-doped fiber and an Nb2O5-doped fiber, are fabricated and characterized in this Letter. First, the refractive index profiles of both fibers are obtained, and then their Raman spectra are measured with 785 nm exciting light. The Nb-doped fibers’ Raman spectra are compared with a conventional GeO2-doped single-mode silica fiber that is prepared with the same method and under the same conditions. As a result, the Raman gain coefficients of the Nb-doped silica fiber core are obtained. The experimental results show that Nb2O5 doping can enhance the Raman scattering intensity of the optical fibers.

Over an octave cascaded Raman scattering in short highly germanium-doped silica fiber

In this paper, we report on cascaded Raman scattering (RS) in a highly germanium-doped silica fiber (HGDF) pumped by a picosecond pulsed master oscillator power amplifier (MOPA) system at 1064 nm in the normal dispersion regime. Benefited by the higher Raman gain of germanium (GeO2) than silica in the core, the length of the HGDF is only several meters. The broadest output spectrum comprises of 10 orders Raman stokes waves and eventually evolves into a supercontinuum (SC) spanning from 1000 to beyond 2100 nm with an output average power up to Watt scale. To the best of our knowledge, this is the first time to obtain such a broad cascaded RS spectrum in a short length of GeO2-doped step index silica fiber. We also numerically investigate the propagation of picosecond pulses in this HGDF based on the generalized nonlinear Schrödinger equation (GNLSE) which includes most of the dispersive and nonlinear effects, and the simulation results are in fairly good agreement with our experiments. It is believed that the numerical approach adopted in this paper is very beneficial for designing customized cascaded Raman fiber lasers before experimental implementations.

Novel electro-optical modulator utilizing GeO2-doped silica waveguide

In order to achieve a modulator with broad bandwidth and perfect impedance match, a novel electro-optical modulator based on GeO2-doped silica waveguides on silicon substrate is designed. The finite element model of the whole electro-optical modulator is established by means of ANSYS. With the finite element method analysis, the performance of the novel modulator is predicted. The simulation reveals that the designed modulator operates with a product of 3 dB optical bandwidth and modulating length of 226.59 GHz·cm, and a characteristic impedance of 51.6 Ω at 1 550 nm wavelength. Moreover, the calculated electrical reflected power of coplanar waveguide electrode is below −20 dB in the frequency ranging from 45 MHz to 65 GHz. Therefore, the designed modulator has wide modulation bandwidth and perfect impedance match.

UV laser induced transmission change of pure and doped silica glass

Pure and F, GeO2-doped silica glass cut from fiber preforms prepared by plasma assisted chemical vapor deposition(PCVD) were investigated by ultraviolet absorption spectroscopy. The ultraviolet absorption characteristics of these glasses were also studied after UV laser irradiation and heating treatment. It was found that absorption band near 240 nm assigned to GODC was found both in GeO2-doped and F-GeO2 co-doped silica glass, but absorption intensity of the latter was lower than that of the former. It’s because F can react with GODC and GeE’ simultaneously and reduce their concentration. After irradiation, UV absorption change of F-GeO2 co-doped silica glass was weaker than that of GeO2-doped silica glass,it thus can be concluded that introduction of F could depress the UV absorption of GeO2-doped silica core effectively.

Modeling of Radiation Heat Transfer in the Drawing of an Optical Fiber With Multilayer Structure

A numerical model is developed to study the radiative heat transfer in a furnace for optical fiber drawing with a core-cladding structure in the fiber. The focus is on the effect of the difference in composition and thus the radiation properties in the two regions on radiative transport. The zonal method is applied to calculate the radiative heat transfer within the neck-down region of the preform. The radiative heat transfer between the preform and the furnace is computed by an enclosure analysis. A parallel computational scheme for determining the direct exchange areas is also studied. The radiation model is verified by comparisons with benchmark problems. Numerical results for a pure silica preform, a GeO2-doped silica core with a pure silica cladding preform, and a pure silica core with a B2O3-doped silica cladding preform are presented. Radiation properties for these are obtained from the literatures and a three-band model is developed to represent the values. It is found that radiative heat flux on the surface of the preform is strongly affected by the differences in the absorption coefficient due to doping. However, changes of about 1% in the refractive index have only a small effect on radiative heat transfer. The basic approach is outlined in order to form the basis for simulating optical fiber drawing processes, which typically involve fibers and preforms with a core and a cladding. Furthermore, the approach can apply to estimate the multi-layer fiber drawing, which is of interest in the fabrication of specialty fibers that have been finding uses in a variety of practical applications. The model can be extended to other similar processes, which involve multiple regions with different radiation properties. The main interest in this study is on the approximate representation of radiation properties and on the modeling of the transport process.

Enhancement of stimulated Brillouin scattering of higher-order acoustic modes in single-mode optical fiber

Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.

Near-infrared continuum generation of femtosecond and picosecond pulses in doped optical fibers

We report broad continuum generation in GeO2-doped silica fibers induced by femtosecond and picosecond pulses generated by novel high-power laser sources. Over 0.5 W of average power in a broad-band picosecond continuum is generated in the near-infrared part of the spectrum from 800 to 1500 nm.

An Electro-Optic Modulator Based on GeO2-Doped SilicaRidge Waveguides with Thermal Poling

A Mach-Zehnder electro-optic modulator is designed and fabricatedbased on upper-clad GeO2-doped silica ridge waveguideswith thermal poling. The electro-optic coefficient obtained is about0.05 pm/V and is polarization-insensitive. An extinction ratio of over17 dB is achieved. The transmission loss of the modulator for the TEmode is 2-3 dB higher than that for the TM mode after the poling.

Origin of Excess Loss in Single-Mode Optical Fibers with High GeO2-Doped Silica Core

The problem of excess loss in single-mode germanosilicate optical fibers with GeO2content 20 to 35 mol% is experimentally studied. Excess loss in such fibers is due to the drawing-induced defects, UV absorption tail and scattering. It is sensitive to drawing conditions and to codoping with F and P. Codoping with P results in additional loss presumably caused by P-related drawing-induced defects. Codoping with F diminishes the excess loss presumably by smoothing the inhomogeneous distribution of GeO2in SiO2–GeO2glass.