Double-clad Research Articles

Self-mode-locking and self-phase modulation in Tm3+-doped double clad fiber laser for pulse peak power enhancement and multi-wavelength generation

A dual frequency mode-locked Tm3+-doped double clad fiber laser (TDCFL) is proposed by combining self-mode-locking with self-phase modulation (SPM) effect. The SPM effect stimulates the 24th harmonic mode-locking operation. The unpumped gain fiber-based self-mode-locking effect initiates the fundamental mode-locked operation. This fundamental mode-locked pulse acts as a modulation envelope of the 24th harmonic one. In this situation, the maximum pulse peak power and the largest pulse energy improve greatly. Its maximum pulse peak power is at least 311 times higher than the fundamental mode-locked one. And the difference of pulse energy is ~7.8 times. Furthermore, this dual frequency mode-locked TDCFL generates multi-wavelength light under high pump power. These results could offer an exciting new prospect for obtaining simple, compact and high performance multi-wavelength mode-locked fiber lasers.

Dual sensing points Mach–Zehnder interferometer for refractive index and discrete liquid level sensing

An in-line all-fiber Mach–Zehnder interferometer (MZI) based on the combination of etched single mode fiber (ESMF) and double cladding fiber (DCF) is proposed for refractive index and discrete liquid level sensing. In the proposed structure, two sensing points are devised using two ESMF section of length. The two sections are separated by a refractive index insensitive region formed by DCF section. The sensor may be deployed to measure refractive index in single- or dual-point configuration. The experimental work revealed maximum refractive index (RI) sensitivity at − 37.2 nm/RIU for 1.305–1.335 RI range, with 1 mm compact sensor size. Additionally, the sensor performed as a two-point liquid level sensor with wavelength shift ranging between 0.84 nm and 1.44 nm. The proposed sensor displayed great potential to be deployed in liquid chemical storage to monitor liquid quality and level.

Tunable synchronized dual-wavelength pulsed operation in an Er/Yb double-clad fiber laser

We present a simple and efficient experimental approach to synchronized dual-wavelength pulsed operation in an all-fiber laser. Four different dual-wavelength emissions of synchronized picosecond pulses are generated through simple adjustments of a wave retarder. Autocorrelation traces exhibit modulation profiles corresponding to dual-wavelength emissions with wavelength separations ranging over one order of magnitude, from 0.96 to 9 nm. In all cases, it is observed stable dual-wavelength operation and perfect synchronization between both pulsed components, without requiring of intra-cavity dispersion management. For the first time, to the best of our knowledge, synchronous tunable dual-wavelength pulse emission in a single-cavity Er/Yb double clad ring fiber laser is experimentally demonstrated.

Method for calculating coupling coefficient in W-type multimode optical fibers

A simple method is proposed for determining the mode coupling coefficient D in W-type (double clad) multimode optical fibers. It only requires the observation of the far-field output pattern for one fiber length with the input light launched centrally along the fiber axis (θ 0= 0). For illustration, the coupling coefficient determined by this simple method for a W-type multimode glass optical fiber was used to calculate the coupling length L c at which the equilibrium mode distribution is achieved.

Few-Mode Gain-Flattening Filter Using LPFG in Weakly-Coupled Double-Cladding FMF

Recently, few-mode Erbium-doped fiber amplifiers (FM-EDFA) for mode-division multiplexing (MDM) transmission have attracted much interest. In this paper, we propose a novel few-mode gain-flattening filter (FM-GFF) based on long-period fiber gratings (LPFG) in double-cladding few-mode fiber (DC-FMF). Due to weak coupling among all the core modes and limited number of inner-cladding modes, the phase matching condition for a specific FM-LPFG can be satisfied only between one core mode and one inner-cladding mode each time and independent gain flattening can be realized for each core mode without influencing the other ones. Moreover, both DC-FMF and transmission FMF have the same core refractive index and radius, while the inner-cladding of DC-FMF and the cladding of transmission FMF have the same refractive index, which can effectively maintain the weakly-coupled condition for their connections. We show by simulation that the output gain spectra for all the 4 or 6 linearly-polarized (LP) core modes in two kinds of DC-FMFs can be independently flattened to less than 0.6-dB in the whole C-band by the cascading LPFGs in the DC-FMF. The proposed FM-GFF scheme is beneficial for the practical design of FM-EDFAs.

Simulations of Pump Absorption in Tandem-Pumped Octagon Double-Clad Fibers

The cladding pumping within a double-clad fiber structure is an effective technique to convert high-power multimode beam into high-power, almost diffraction limited beam. Since the pump efficiency is limited by the presence of higher skew-ray modes, geometrical perturbations are used to scramble the modes and to achieve a higher overlap of the electromagnetic field with the doped core. In this paper, the combined effect of fiber coiling and twisting is investigated in the double-clad fiber structure with the octagonal shape of an inner cladding. Electromagnetic field propagation through fibers with different cross-section areas, bending radii and twisting rates is numerically simulated using finite element beam propagation method. Holmium-doped double-clad fiber pumped in tandem configuration at wavelength 1950 nm is selected as an example. Nontrivial dependence of pump absorption on bending radius with the presence of local maxima is revealed. The observed impact of fiber coiling and twisting increases with the size of the cross-section area of the inner cladding.

Multiple mode-locked regimes of an Er/Yb double-clad fiber laser based on NPR

In this paper, we experimentally demonstrate generation of diverse structures of robust optical pulses obtained with a ring cavity passively mode-locked fiber laser based on a double-clad erbium-ytterbium doped fiber, and operating at net-anomalous cavity dispersion. A half-wave and a quarter-wave retarder plates allow adjustable saturable absorbing action by the nonlinear polarization rotation technique. Pulsed laser operation in quasi- continuous wave, molecules of solitons, and single-wavelength noise-like pulses (NLPs) regimes is obtained by properly adjusting the wave retarders. In the soliton and NLPs operating regimes, the central laser wavelength lies in the 1540 nm region. The obtained NLPs exhibit narrow peak coherence of ∼231 fs.

Mode coupling in large-mode-area double-cladding chirped and tilted fiber Bragg gratings

Recently, chirped and tilted fiber Bragg gratings (CTFBGs) have received great attention because they can realize suppression of stimulated Raman scattering (SRS) in high-power fiber lasers. In this study, the possible coupling between the core modes and cladding modes in CTFBGs inscribed in large-mode-area double-cladding fibers is investigated for the first time. Theoretical results show that the coupling between the LP11 mode and cladding modes would destroy the transmission spectra envelope only considering the coupling of LP01 for single-mode CTFBGs, which will degenerate the SRS suppression performance. This was confirmed experimentally by measuring the spectral response under different mode excitations. A reliable method is demonstrated to ease the LP11-excitation-induced spectral deterioration by choosing an appropriate chirp rate for the inscription of CTFBGs, which is useful for improving the Raman suppression effect of large-mode-area double-cladding CTFBGs in high-power fiber lasers.

Efficient approach to designing low OH diffusion in the thermally shaped double-clad fibers

Hydroxyl groups (OH) penetrate the surface of the fiber preforms as a result of preform processing in the oxy-hydrogen flame. Designing double-clad fibers (DCFs) that have a complex noncircular inner-cladding shape with concave and convex edges are possible by shaping their optical preforms thermally with a CO2 laser. Unlike the mechanical technique, the thermal shaping via CO2 laser has paved the way for fabricating DCFs with various inner-cladding geometries that are not limited to the flat edge. However, due to the thermo-physical effects of the CO2 laser on the preform surface, the OH groups are still present in the drawn fibers as they were diffused in the preform while shaping. The OH groups cause attenuation on certain wavelengths that lay rather close to the commonly pumping wavelengths in DCFs. In this work, we have overcome this issue by approaching low OH diffusion in DCFs drawn of thermally shaped optical preforms. This approach was based on treating the preforms with hydrofluoric (HF) acid prior the CO2 laser shaping process in order to remove a thin layer soaked with OH groups. The OH penetration depth was estimated based on the optical attenuation measurement at water peak to be about ~600 µm from preform surface. By etching just 200 µm of the preform surface followed by laser shaping with ablation depth of 300 µm can reduce the attenuation attributed to OH groups by 43%. Surface scattering of the inner-clad shaped DCFs drawn of the etched preforms was shown to be low compared to the DCFs drawn of ground preforms. The measurements were carried out in multimode fibers drawn from pure silica Heraeus F300 rods and coated with low index material.

Broadband mid-infrared amplified spontaneous emission from Er/Dy co-doped fluoride fiber with a simple diode-pumped configuration

Er3+/Dy3+ co-doped double-clad ZBLAN optical fiber has been used to obtain amplified spontaneous emission (ASE) broadband light sources cladding-pumped by 980-nm multimode laser diode (LD) sources. It has been demonstrated that mid-infrared broadband emission extending from 2515 to 3735 nm was obtained by energy transfer between Er3+ and Dy3+. We experimentally investigated the optimum design of Er3+/Dy3+ co-doped ZBLAN fiber in terms of ion concentration, fiber length, pumping configuration, and pumping power. The ASE output power was more than 2.5 mW when the LD pump power was set at 5 W. To assess its potential for gas sensing applications, the fabricated ASE light source was used to successfully detect methane gas with concentrations at 1% and 5%. The simple and stable construction of our ASE light source is suitable for practical purposes.

Strain-Insensitive Simultaneous Measurement of Bending and Temperature Using Long-Period Fiber Grating Inscribed on Double-Clad Fiber with CO₂ Laser.

Here we report an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using a long-period fiber grating (LPFG) inscribed on doubleclad fiber (DCF) with a CO₂ laser at ˜10.6 μm. The LPFG inscribed on DCF, referred to as a DC-LPFG, was fabricated by scanning CO₂ laser pulses on an unjacketed DCF with a specific period. Due to co-directional mode coupling, the fabricated DC-LPFG has discrete attenuation bands widely distributed over hundreds of nanometers. Among these wavelength-dependent loss dips, adjacent two dips with different resonance wavelengths were selected as sensor indicators for the measurement of bending and temperature. For these two indicator dips designated as dips A and B, their bending and temperature responses were investigated in a curvature range of 4.90 to 21.91 m-1 and a temperature range of 30 to 110 °C. With increasing bending applied to the DC-LPFG at room temperature, dips A and B showed different blue shifts. The bending sensitivities of dips A and B were measured to be approximately -0.77 and 0.51 nm/m-1, respectively. Unlike the bending response, they showed red shifts of different amounts with increasing ambient temperature, while the sensor head (i.e., the DC-LPFG) remained straight without any applied bending. The temperature sensitivities of dips A and B were measured to be ˜0.094 and ˜0.078 nm/°C, respectively. Owing to their linear and independent responses to bending and temperature, bending and temperature changes applied to the DC-LPFG could be simultaneously estimated from the measured wavelength shifts of the two indicator dips using their pre-determined bending and temperature sensitivities. Moreover, in a strain range of 0 to 2200 με (step: 200 με), strain-induced spectral variations of dips A and B were also measured, and the strain sensitivities of dips A and B were evaluated as approximately -0.028 and -0.013 pm/με, respectively. These strain-induced wavelength shifts were so small that they had little effect on the measurement results of bending and temperature. Thus, it is concluded that the fabricated DC-LPFG can be employed as a cost-effective sensor head for strain-insensitive separate measurement of bending and temperature.

이중 클래딩 광섬유에 새겨진 위상 천이 투과형 격자를 이용한 비틀림 및 온도 분리 측정 센서

In this paper, we propose an optical fiber sensor capable of separately measuring twist and temperature by incorporating a phase-shifted transmission grating written on double-clad fiber(DCF) with a CO₂ laser. The sensor head exhibited two separate attenuation dips in the resonance wavelength band, and these dips were selected as indicator dips denoted by LD and RD. The torsion and temperature responses of both dips were investigated in a twist angle range from -300° to 300° and a temperature range from 30 to 100°C. When a twist angle increased from 0° to 300°, LD and RD showed red shifts. In contrast, when a twist angle decreased from 0° to -300°, they showed blue shifts. In terms of the temperature response, LD and RD showed red shifts as the ambient temperature increased. As a result, it is confirmed that the torsion and temperature sensitivities of LD and RD are different and the two dips show linear behavior when the two variables (i.e., twist and temperature) are applied to the sensor head, so that changes in torsion and temperature can be separately estimated using the sensitivity coefficient matrix.

Multicolor fiber-optic two-photon endomicroscopy for brain imaging.

Visualizing activity patterns of distinct cell types during complex behaviors is essential to understand complex neural networks. It remains challenging to excite multiple fluorophores simultaneously so that different types of neurons can be imaged. In this Letter, we report a multicolor fiber-optic two-photon endomicroscopy platform in which two pulses from a Ti:sapphire laser and an optical parametric oscillator were synchronized and delivered through a single customized double-clad fiber to excite multiple chromophores. A third virtual wavelength could also be generated by spatial-temporal overlapping of the two pulses. The performance of the fiber-optic multicolor two-photon endomicroscope was demonstrated by in vivo imaging of a mouse cerebral cortex with "Brainbow" labeling.

Multi-wavelength Er–Yb-doped fibre ring laser using a double-pass Mach–Zehnder interferometer with a Sagnac interferometer

In this paper, the performance of a switchable multi-wavelength Er–Yb-doped fibre ring laser incorporating a comb filter is described. The comb filter was developed by using the double-pass Mach–Zehnder interferometer with a Sagnac interferometer (DMZI–SI) arranged in series, a 10-m-long PANDA-type polarisation-maintaining optical fibre, and polarisation controllers (PCs). The laser output can be switched from 1536 to 1548 nm (C-band) via careful adjustment of the PCs. A side-mode separation ratio, greater than 28 dB, was obtained to facilitate stable single-, double-, quadruple-, and quintuple-wavelength emissions, with spectral power and wavelength fluctuations of 0.47–1.06 dB and 0.01–0.06 nm, respectively. The measurements were performed for 60 min at 25 °C. The proposed comb-filter configuration facilitates the double-clad fibre ring laser to realise switchable multi-wavelength emission. Experimental results reveal that the proposed laser demonstrates high stability and significantly improved performance compared to the existing DMZI–SI filter-based systems. This indicates their potential for use in applications involving optical sensing, communication systems, and microwave generation.

All-fiber pulsed laser generation of 25.5 mJ and solution of parasitic oscillation.

A Q-switched, high-energy pulsed master oscillator power amplifier fiber laser utilizing the lab-built 100/400 µm double-cladding Yb-doped fiber is demonstrated. After two-stage amplification, the pulse energy was boosted to 25.5 mJ, for an average power of 510 W at a repetition of 20 kHz, yielding a slope efficiency of approximately 72.8%; the pulse duration was approximately 140 ns, and corresponding peak power was 182.1 kW. What is more, the limitation of further promotion of pulse energy was proposed: the threshold-like parasitic oscillation, which was determined by the injecting power, repetition, and fiber length, was the main restriction on power scaling in ultra-high-energy systems. Efficient solutions were proposed to suppress the parasitic oscillation by experimentally studies.

Corrections to: Experimental and Numerical Demonstration on the Gain and Power Saturation of Yb-Doped Double-Clad Fiber Amplifiers in Multiwavelength CW and Ultrafast Pulsed Operation

Presents corrections to the above named paper.

Strain-Insensitive Simultaneous Measurement of Bending and Temperature Based on Cascaded Long-Period Fiber Gratings Inscribed on Double-Clad Fiber

We demonstrated an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using cascaded long-period fiber gratings (LPFG’s) inscribed on double-clad fiber with a CO2 laser at $10.6~\mu \text{m}$ . In the interference fringe spectrum of the fabricated cascaded LPFG’s, we selected three transmission dips as sensor indicators for the simultaneous measurement of bending and temperature. The measured bending sensitivities of the three indicator dips were −427.63, −911.29, and 233.33 pm/ $\text{m}^{-1}$ , and their measured temperature sensitivities were 60.49, 56.44, and 74.61 pm/°C. Owing to their linear and uncorrelated responses to bending and temperature, we could simultaneously estimate applied bending and temperature with two of the three indicators by converting the measured wavelength shifts of the two selected dips with their pre-determined bending and temperature sensitivities. In particular, our sensor showed a very low strain sensitivity less than 0.05 pm/ $\mu \varepsilon $ , 25 times less than that of a conventional LPFG, which can significantly reduce strain-induced measurement uncertainty. With two optimal indicator dips chosen considering the measured perturbation responses of the three indicators, we carried out the simultaneous measurements of bending and temperature for arbitrary sixteen points of applied bending and temperature. The standard deviations of the measured bending and temperature values were $\sim 0.059\,\,\text{m}^{-1}$ and ~0.85 °C, respectively.

Simulation of the Effect of Relative Refractive Index for Light Transmission Through Double Cladding Step Index Optical Fibre

This work establishes computational analysis of relative refractive index property for light transmission via fiber optics using MATLAB simulation. Optical fiber is a dielectric wave guide which transmits signal with a low attenuation and dispersion at higher bandwidth or data rate. Recent advances in fiber optics introduced a largely new dimension into optical technology with the effect of improving transmission through the application of cladding system. Cladding in fiber communication is importantly applied to ease light transmission in the fiber-core interface by total internal reflection at the boundary between two light propagation region of a typical fiber. Optical fiber communication is a well recognized aspect of optical light transmission where clad fibers of different refractive indices recount for a measurable Numerical aperture and Acceptance angle in fiber networking and communication technology. This has lead to the efficient and effective optical transmission due to the adequate emergence and utilization of optical properties and parameters in optical fiber applications. A double cladding was set for the fiber and a maximum relative refractive index of 2.000% is obtained for an acceptance angle and numerical aperture range of 59°-64° and 0.859-0.900 respectively. This implies fast light transmission, high bandwidth and small bending sensitivity for fiber networking with applications in medicine and telecommunication compared to single cladding fiber.

基于粗锥结构级联LPFG的双包层光纤多参量传感器

基于粗锥结构级联LPFG的双包层光纤多参量传感器

基于粗锥结构级联LPFG的双包层光纤多参量传感器

基于粗锥结构级联LPFG的双包层光纤多参量传感器