Large Mode Area Research Articles

TMI and polarization static energy transfer in Yb-doped low-NA PM fibers.

In this work, we conduct experimental investigations of transverse mode instabilities (TMI) in a large mode area ultra-low numerical aperture polarization maintaining fiber amplifier. This fiber is few mode in the slow-axis (conventional operation mode), but single mode in the fast-axis. We test the stability of the output beam by changing the input polarization angle and systematically investigate the transverse mode instability threshold in the two principal polarization axes. The lowest TMI threshold at 300 W was found when the input polarization angle was aligned parallel to the slow-axis. Detuning the input polarization angle from the slow-axis led to increased TMI thresholds. For input polarization angle of 90° (parallel to the fast-axis), the output signal was stable up to 475 W and further scaling was limited by the available pump power. However, for fast-axis operation a lower polarization ratio compared to slow-axis operation was observed as well as an unexpected static energy transfer from the fast-axis into the slow-axis above 400 W.

Performance analysis of photonic crystal fiber structure based on different highly nonlinear materials

Generally, fiber with a large mode area is desired for fiber laser and beam delivery applications. But the use of these fiber types is restricted to linear optical applications due to the lower value of the nonlinear coefficient. This paper investigates a single material-based photonic crystal fiber (PCF) with improved optical characteristics, applicable for linear and nonlinear optics. The light propagation in the structure is explored by applying finite element solver-based COMSOL multiphysics software. Five nonlinear optical materials are utilized as background material to get the optimized material with high performance PCF. Using this PCF, the improved optical properties of PCF such as moderate effective mode area of 115.85 [Formula: see text], chromatic dispersion, confinement loss of 10[Formula: see text] dB/m, the nonlinear coefficient of 543W[Formula: see text]Km[Formula: see text], splicing loss of 2.013 dB, and numerical aperture are numerically analyzed. This single-mode PCF also shows the low bending loss for the bending radius of 7[Formula: see text]mm. The amended PCF structure is found to be suitable for high-power fiber laser, supercontinuum generation, wavelength conversion, and slow-light generation.

Co-Existence of Watt-Level Dissipative Solitons and Synchronous Dual-Wavelength Mode-Locked Pulses in Yb Fiber Laser

We present a high-average-power laser configuration to simultaneously generate both dissipative solitons (DSs) and synchronous dual-wavelength mode-locked (ML) pulses using large-mode-area (LMA) fibers in combination with a birefringence-based chirped-pulse spectral filter. DSs with average power as high as 5.8 W and pulse width as short as 178 fs are obtained at the output port. Meanwhile, up to 1.5 W average power in synchronous dual-wavelength ML pulsed operation is also obtained at the rejected port with central wavelengths located at 1029.7 nm and 1046.1 nm respectively. To our knowledge, observation of such behavior with both DSs and synchronous dual-wavelength pulse generation simultaneously has not been yet realized in watt-level fiber lasers.

Er/Yb co-doped 345-W all-fiber laser at 1535 nm using hybrid fiber.

The 1.5-µm fiber laser is widely used in the fields of laser lidar, remote sensing, and gas monitoring because of its advantages of being eye-safe and exhibiting low atmospheric transmission loss. However, due to the ∼1-µm amplified spontaneous emission (ASE) of the Er/Yb co-doped fiber (EYDF), it is difficult to improve the laser power. Here, we simulated the effect of the Er3+ concentration and the seed power on ∼1-µm ASE, and fabricated a large mode area EYDF by the modified chemical vapor deposition process. Additionally, a piece of ytterbium-doped fiber was introduced into the master oscillator power amplifier (MOPA) configuration to absorb the generated ∼1-µm ASE simultaneously. Experimental results show that an output power of 345 W with a slope efficiency of 43% at 1535 nm is obtained in an all-fiber configuration, profiting from effective suppression of ∼ 1-µm ASE. To the best of our knowledge, this is the highest output power available with an Er/Yb co-doped fiber from an all-fiber MOPA configuration.

Regenerated Fiber Bragg Gratings in Large Mode Area Fibers for High-Temperature Sensing

Regenerated fiber Bragg gratings (RFBGs) have been created in large mode area (LMA) fibers, and their key properties for their application as high-temperature sensors were experimentally characterized for the first time. UV written Type-I seed gratings in standard single-mode fibers (SMF-28) and in low numerical aperture LMA fibers with 125 μm and 250 μm cladding diameters were regenerated, and their wavelength drift characteristics at high temperatures, their temperature sensitivities, and their mechanical strengths were tested and compared with each other. We found that low dopant concentrations in the fiber cores led to lower wavelength drift rates during and after regeneration. A temperature calibration from room temperature to 800 °C showed that LMA fibers have generally similar but slightly lower temperature sensitivities than SMF-28 fibers. The tensile strength tests showed that LMA fibers with 250 μm cladding diameter have similar tensile fracture stresses as SMF-28 fibers but four times larger tensile force tolerances because of their twofold fiber diameter. Our research shows that using LMA fibers for RFBGs offers significant advantages for high-temperature sensing than their counterparts in standard fibers, which are also applicable to other silica fiber-based high-temperature sensors.

Optimized segmented cladding fiber for extreme large mode area using latin hypercube sampling

In this paper a new design of octo-wing segmented cladding fiber (SCF) with a uniform core containing a ring and a non-uniform double cladding was proposed and optimized. The Latin hypercube sampling was applied to generate design cases as the input values for estimation of the objective functions as the output variables. Three physical parameters of the core assisted ring: thickness, radius, and refractive index difference were selected as the design variables. Twenty different design cases were numerically simulated by finite element method. To optimize the effective mode area (EMA) as the objective function, response surface methodology was applied. The fiber offers the single mode operation with a confinement loss ratio of over 100 between the fundamental mode (FM) and the higher order mode (HOM). Operating at the wavelength of 1.550μm, the EMA is significantly enlarged to about 884μm2. We have investigated the properties of the proposed optimized ring assisted SCF (RA-SCF) in terms of confinement losses of FM and first HOM considering a perfect matched layer. The proposed RA-SCF provides the potential applications in compact high power fiber lasers.

Designing hybrid-guidance large mode area fiber for high-power lasers

All-solid large-mode-area antiresonant fibers possess unique characteristics of broadband, low-loss transmission of fundamental mode with effective single mode operation. Hinging on the principle, we propose a unique hybrid guidance mechanism, combining antiresonant guidance with total internal reflection-based guidance to design bend-tolerant, ultra large mode field area (>3000 µm2) fibers for high-power laser applications. Low-index rods are strategically interspersed into the antiresonant cladding to enhance the modal confinement in the fiber, resulting in lower confinement loss of the fundamental mode and its increased tolerance to bending and to the refractive index dip caused by non-uniform doping of active ions in the fiber core. We show that total internal reflection guidance greatly helps confine the fundamental mode without masking the characteristic features of antiresonant guidance, like effective single mode operation over broad spectral bandwidth. We present a detailed theoretical analysis to design a bendable, effectively single-moded passive and active hybrid-guidance antiresonant fibers, optimized to work in the 1 µm wavelength band. The design principles are universal and can be extended to shift the operating wavelength to the 2 µm range as well.

All-Glass Single-Mode Leakage Channel Microstructured Optical Fibers with Large Mode Area and Low Bending Loss

The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode.

Non-Zero Dispersion-Shifted Ring-Core Fiber Supporting 60 OAM Modes

Orbital angular momentum (OAM) modes have becoming more promising to further achieve higher spectral efficiency and data capacity in space-division-multiplexing (SDM) fiber communication systems. In this work, we design and optimize a graded-index non-zero dispersion-shifted ring-core fiber (GI-NZDSRF) to support OAM modes in fiber-based transmission system. A record high number of 60 OAM modes in the C band with <10 ps/nm/km chromatic dispersion (CD) is obtained. A low differential mode delay (DMD) (<5.11 ps/m) and a large effective mode area (>1418 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of OAM modes are achieved by the designed fiber. Furthermore, the effects of fiber parameters on OAM modes are also investigated, including the index shape, doping concentration, central location, and half width of the high index region.

High energy multi-mode nanosecond pulsed fiber amplifier based on the 940 nm LD pumping with effective suppression of the ASE and the SRS effect

We demonstrated, for the first time, an amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS) free multi-mode nanosecond pulsed fiber amplifier based on the 940 nm LD pumping. The maximum pulse energy reached 15.3 mJ at the average output power of 306 W using a 100/400 Yb-doped extremely large mode area fiber. A near top-hat beam with M2 = 10.4 was achieved. This is the highest pulse energy generated from the 940 nm LD pumped multi-mode nanosecond pulsed fiber amplifier. In addition, comparative experiments based on the conventional 915 nm LD pumping and the 976 nm LD pumping were conducted. The results showed that the 915 nm LD pumping lead to intensive forward 1030 nm ASE, and the 976 nm LD pumping was limited by strong backward ASE power. These confirmed that the 940 nm LD pumping provided an effective method to suppress the ASE and the SRS effect in the high energy multi-mode nanosecond pulsed fiber amplifier.

Single-frequency 45-mJ pulses from a MOPA system using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier.

We demonstrate a master oscillator power amplifier (MOPA) system that emits single-frequency high-energy optical pulses at 1540 nm using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier. A double under-cladding and a 50-µm-thick core structure are employed for the planar waveguide amplifier to increase the output energy without degrading the beam quality. A pulse energy of 45.2 mJ with a peak power of 27 kW is generated at a pulse repetition rate of 150 Hz with a pulse duration of 1.7 µs. Moreover, the beam quality factor M2 of the output beam is 1.84 at the highest pulse energy thanks to its waveguide structure.

All-glass leakage channel microstructured optical fibers with large mode area and low bending loss

The present paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with large mode area and low bending loss. These MOFs comprise two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters and different distances between them. Numerical analysis of the properties of these MOFs is performed using the finite element method. The leakage losses for fundamental and higher order modes in the spectral range from 0.70 μm to 1.65 μm for straight and bended MOFs is calculated. It is shown that the proposed MOF design makes it possible to obtain a single-mode regime in the spectral range from 0.98 μm to 1.14 μm with a bending radius up to 0.08 m, while the leakage loss for a bended MOF at a wavelength of 1.05 μm is 0.052 dB/m. The measured loss of manufactured MOF with a core diameter of 22.5 μm at a bending radius of 0.1 m was less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It has been demonstrated that segments of this MOF with a length of more than 5 m are single-mode.

A large effective mode area photonic crystal fiber supporting 134 OAM modes

A large effective mode area photonic crystal fiber supporting 134 OAM modes

Watt-level deep-UV subnanosecond laser system based on Nd-doped fiber at 229 nm.

We report an efficient deep-UV master-oscillator power amplifier (MOPA) laser system at 229 nm that generates 350 ps pulses at 2 MHz repetition rate with an average power of 1.2 W. The use of a polarization-maintaining large mode area neodymium-doped fiber operating on the 4F3/2→4I9/2 transition allows high-power laser emission of up to 28 W near 915 nm in the sub-nanosecond regime with low spectral broadening. Two nonlinear frequency conversion stages (LBO + BBO crystals) in a single-pass configuration directly convert the IR laser emission to deep UV. This laser demonstrates the great potential of Nd3+-doped fiber lasers to produce high-power deep-UV emission.

Large mode-area all-solid anti-resonant fiber based on chalcogenide glass for mid-infrared transmission.

A large mode-area chalcogenide all-solid anti-resonant fiber has been designed and successfully prepared for the first time. The numerical results show that the high-order mode extinction ratio of the designed fiber can reach 6000, and the maximum mode-area is 1500 um2. The fiber possesses a calculated low bending loss of less than 10-2 dB/m as the bending radius is larger than 15 cm. In addition, there is a low normal dispersion of -3 ps/nm/km at 5 μm, which is beneficial for the transmission of high power mid-infrared laser. Finally, a completely structured all-solid fiber was prepared by the precision drilling and two-stage rod-in-tube methods. The fabricated fibers transmit in the mid-infrared spectral range from 4.5 to 7.5 μm with the lowest loss of 7 dB/m @ 4.8 μm. Modeling suggests that the theoretical loss of the optimized structure is consistent with that of the prepared structure in the long wavelength band.

Confined-doped fiber enabled kilowatt-level all-fiber laser with 1.28 GHz linewidth.

In this manuscript, a narrow linewidth fiber amplifier based on confined-doped fiber is established, and the power scaling and beam quality maintaining capabilities of this amplifier are investigated. Benefitted from the large mode area of the confined-doped fiber and precisely controlling the Yb-doped region in the fiber core, the stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects are effectively balanced. As a result, a 1007 W signal laser with just 1.28 GHz linewidth is obtained by combining the advantages of confined-doped fiber, near-rectangular spectral injection, and 915 nm pump manner. As far as we know, this result is the first beyond kilowatt-level demonstration of all-fiber lasers with GHz-level linewidth, which could provide a well reference for simultaneously controlling spectral linewidth, suppressing the SBS and TMI effects in high-power, narrow-linewidth fiber lasers.

M2 factor for evaluating fiber lasers from large mode area few-mode fibers

Evaluating the laser quality accurately is one of the most important and fundamental physical issues for laser sources, and the beam quality of lasers from the large mode area few-mode fibers have been haunted by the presence of high order mode for many years. This paper presents a modification to the M2 factor, which can be used to evaluate the mode content of fiber lasers accurately and efficiently, no matter whether the fiber modes are superposited coherently or incoherently. By mathematical derivation, the origin of the influence of relative phase on the M2 factor has been determined mathematically. A modification to the second moment of the beam intensity profile has been proposed, which eliminates the impact of uncontrollable relative phase on the second moment, and subsequently restores the one-to-one mapping between mode content and M2 factor even for coherent superposition cases. Also presented are the results of numerical simulations, which support the validity of the modified M2 factor to evaluate the mode content of the high power fiber lasers. With modified M2 factor being less than 1.1, the power fraction of LP11 mode content is unique and determined to be less than 3%.

An all-fiber high-power picosecond amplifier with Yb-doped polarization-maintaining tapered fiber

A high-power picosecond all-fiber master oscillator power amplifier (MOPA) was demonstrated based on the Yb-doped polarization-maintaining (PM) tapered double-clad fiber (T-DCF). An all-fiberized pumping unit instead of the free space pumping structure was adopted in the MOPA system to provide a compact robust solution for high-performance amplifiers. The total average output power of 180.3 W was achieved with a slope efficiency of 80.8% by taking advantages of the large fiber mode area and the suppression of nonlinear effects in the T-DCF. The peak power was calculated to be 27.7 kW at the pulse width of 260.8 ps and the repetition rate of 24.91 MHz. The polarization extinction ratio and beam quality (M 2 factor) were measured to be ∼10 dB (91%) and ∼12, respectively.

Er-Yb fibers based on aluminophosphorosilicate glass

The paper presents results of studying properties of Er-Yb fibers based on aluminophosphorosilicate glass matrix. The dependence of the pump-to-signal conversion efficiency on the glass composition is revealed. Optimal core compositions, which makes it possible to provide a short amplifier length (no more than 1 meter) and a small core-clad refractive index difference (below 0.0035), has been determined. It opens up the possibility of creating single-mode Er-Yb fibers with a large mode area and a high threshold of nonlinear effects.

Femtosecond Yb-doped tapered fiber pulse amplifiers with peak power of over hundred megawatts.

Ultrafast fiber lasers combining high peak power and excellent beam quality in the 1-µm wavelength range have been explored to applications in industry, medicine and fundamental science. Here, we report generation of a high-energy sub 300 fs polarization maintaining fiber chirped pulse amplification (CPA) system by using a Yb-doped large mode area tapered polarization maintaining (PM) optical fiber with the core/cladding diameters of 35/250 µm at the thin end and 56/400 µm at the thick end. The taper fiber design features a confined core for selective gain amplification and multi-layer cladding for enhanced suppression of higher order modes. In this regime, we have demonstrated 266 fs pulse amplification with peak power of up to 132 MW at a repetition rate of 2 MHz and high beam quality with measured M2 value of 1.1∼1.3. To the best of our knowledge, it is the highest peak power reported in such tapered Yb-doped fiber (T-YDF) amplifier in the femtosecond regime. This work indicates the great potential of the T-YDF to realize further power scaling, high laser efficiency, and excellent beam quality in high-power femtosecond fiber lasers.