Higher Order Mode Losses Research Articles

Theoretical Study on Transverse Mode Instability in Raman Fiber Amplifiers Considering Mode Excitation.

Raman fiber lasers (RFLs), which are based on the stimulated Raman scattering effect, generate laser beams and offer distinct advantages such as flexibility in wavelength, low quantum defects, and absence from photo-darkening. However, as the power of the RFLs increases, heat generation emerges as a critical constraint on further power scaling. This escalating thermal load might result in transverse mode instability (TMI), thereby posing a significant challenge to the development of RFLs. In this work, a static model of the TMI effect in a high-power Raman fiber amplifier based on stimulated thermal Rayleigh scattering is established considering higher-order mode excitation. The variations of TMI threshold power with different seed power levels, fundamental mode purities, higher-order mode losses, and fiber lengths are investigated, while a TMI threshold formula with fundamental mode pumping is derived. This work will enrich the theoretical model of TMI and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI in the RFLs.

LMA fibers with increased higher-order mode loss for high average power, pulsed, diffraction-limited lasers

We demonstrate new, large-mode area (LMA) gain fibers with ∼25 µm mode-field diameter, and increased higher-order mode loss that enable diffraction limited, pulsed fiber lasers operating at high average power with high pulse energy. We achieved 1.6 mJ, ns pulses, with 1.2 kW average power and 370 kW peak power in one of the new Yb-doped gain fibers. In a second, higher absorption fiber, we demonstrate 2 mJ pulse energy with peak power of >420 kW at an average power of 660 W. To the best of our knowledge these are the highest demonstrated energies, powers and peak powers for any nanosecond diffraction-limited, all-fiber laser. The TMI thresholds of two of these fibers were measured to be 1.8 kW and 1 kW respectively.

Large Mode Area Single Mode Fiber with a Twisted Polygon-Shaped Core

We present numerical studies on twisted polygon-shaped core fibers for achieving single mode operation with a large mode area propagation in this paper. These large mode area fibers can achieve single mode operation due to twisting-induced high losses to higher order modes and coupling higher order modes with leaky modes. A tail-like profile will appear in higher order modes when they couple with leaky modes, and the tail-like profile also indicates that these modes carry angular momentum. Numerical simulations show that twisted octagon-shaped core fiber with a core diameter of 80 μm can obtain first higher order mode losses larger than 19 dB/m while maintaining fundamental mode losses at about 0.15 dB/m. In the case of twisted heptagon-shaped core, an 80 μm diameter fiber design exists with more than 17 dB/m first higher order modes propagation losses and less than 0.14 dB/m fundamental mode losses.

Large mode area double-layer all-solid anti-resonant fiber for high-power fiber lasers

A novel double-layer all-solid anti-resonant fiber (DL-AAF) is proposed to develop large mode area (LMA) fiber for high-power fiber laser applications. Single-mode property and mode field area scaling are investigated in simulations. Numerical results show the fiber can achieve strong suppression of higher order modes (HOMs) even with a large core size. With a core diameter of 80 μm, the loss ratio defined as the ratio of the lowest loss of HOMs to that of fundamental mode can exceed 3500, and the mode field area reaches 3030 μm2 at the wavelength of 1.064 μm. By further expanding the core size, the mode filed area can be enlarged to 7481 μm2 while the fiber is ensured to be single-mode. This is a significant enhancement in mode field area scale for the current works on LMA AAF.

Large-mode-area all-solid anti-resonant fiber with single-mode operation for high-power fiber lasers.

We investigate the feasibility of applying an anti-resonant guiding mechanism in an all-solid anti-resonant fiber (AS-ARF) to achieve a large mode area (LMA) and single mode for high-power fiber laser applications. A novel, to the best of our knowledge, AS-ARF with nonuniform rods is proposed to enhance the single-mode property and enlarge the mode area. The numerical results show that the core diameter can expand to 57, 80, and 100 µm at the wavelengths of 1.064, 1.55, and 2 µm, respectively. The loss ratio of the lowest loss of higher-order modes to the loss of the fundamental mode can exceed 1000, 550, and 860 at the wavelength of 1.064, 1.55, and 2 µm; thus, robust single-mode operation can be ensured. Besides, the fiber can also be adapted to bent condition under certain heat load. These indicate that the proposed AS-ARF with nonuniform rods is a great candidate as an LMA fiber for high-power fiber lasers.

Quantum Noise in a Fabry-Perot Interferometer Including the Influence of Diffraction Loss of Light

The DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is designed to detect gravitational waves at frequencies between 0.1 and 10 Hz. In this frequency band, one of the most important science targets is the detection of primordial gravitational waves. DECIGO plans to use a space interferometer with optical cavities to increase its sensitivity. For evaluating its sensitivity, diffraction of the laser light has to be adequately considered. There are two kinds of diffraction loss: leakage loss outside the mirror and higher-order mode loss. These effects are treated differently inside and outside of the Fabry-Perot (FP) cavity. We estimated them under the conditions that the FP cavity has a relatively high finesse and the higher-order modes do not resonate. As a result, we found that the effects can be represented as a reduction of the effective finesse of the cavity with regard to quantum noise. This result is useful for optimization of the design of DECIGO. This method is also applicable to any FP cavities with a relatively small beam cut and the finesse sufficiently higher than 1.

Impact of cladding elements on the loss performance of hollow-core anti-resonant fibers.

Understanding the impact of the cladding tube structure on the overall guiding performance is crucial for designing a single-mode, wide-band, and ultra low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly investigate on how the propagation loss is affected by the nested elements when their geometry is realistic (i.e., non-ideal). Interestingly, it was found that the size, rather than the shape, of the nested elements has a dominant role in the final loss performance of the regular nested HC-ARFs. We identify a unique 'V-shape' pattern for suppression of higher-order modes loss by optimizing free design parameters of the HC-ARF. We find that a 5-tube nested HC-ARF has wider transmission window and better single-mode operation than a 6-tube HC-ARF. We show that the propagation loss can be significantly improved by using anisotropic nested anti-resonant tubes elongated in the radial direction. Our simulations indicate that with this novel fiber design, a propagation loss as low as 0.11 dB/km at 1.55 μm can be achieved. Our results provide design insight toward fully exploiting a single-mode, wide-band, and ultra low-loss HC-ARF. In addition, the extraordinary optical properties of the proposed fiber can be beneficial for several applications such as future optical communication system, high energy light transport, extreme non-nonlinear optics and beyond.

Calculation of wave dispersion curves in multilayered composites using semi-analytical finite element method

The enormous potential of Guided waves have a significant use for non-destructive evaluation(NDE) and structural health monitoring(SHM). It is common to use different solution approaches to predict dispersion curves depending on the material type, isotropic, transversely isotropic, or orthotropic, of the medium in which the wave propagates, shape of the waveguide, etc. Inability to model waveguides of arbitrary cross-section by existing matrix methods has led to development of Semi-Analytical Finite Element(SAFE) Method. In this study, rapid and accurate prediction of the propagation characteristics for guided waves in a wide range of structures has been performed by implementation of SAFE method. Damped and undamped waveguides of different shapes can be modelled. Wave propagation characteristics in both isotropic and anisotropic composite structures can also be studied. To benchmark the solution, a case study using Transfer Matrix Method (TMM) for composite plate has been evaluated. It was found that the element discretizaton plays an important role in accurate prediction of modes in SAFE method, neglecting which leads to loss of higher order modes.

Bend-resistant side-leakage photonic crystal fiber with large-mode-area

A bend-resistant side-leakage photonic crystal fiber with large-mode-area (LMA) is proposed. Simulation results demonstrate that the robust single-mode operation is obtained thanks to an extremely high loss ratio (the minimum loss of high order modes to the loss of fundamental mode (FM)) at the wavelength of 2 μm while the bend radius is as small as 10 cm. Meanwhile, the FM suffers a low bend loss of 0.006 dB m−1 with an effective mode area exceeding 2000 μm2. The bend-resistant performance can be maintained over a broad range of wavelengths from 1.75 to 2.5 μm. Except for a wide waveband, a large tolerance for bend radius is also obtained. The fiber is able to operate under single-mode operation within a range of bend radius from 9 to 24 cm. LMA and single-mode operation under a small bend radius make this fiber have great potential in developing compact high power fiber lasers.

Optimal Design of Large Mode Area Photonic Crystal Fibers Using a Multiobjective Gray Wolf Optimization Technique

A new multiobjective optimization framework is presented for designing large mode area photonic crystal fibers (LMA-PCFs) with effective single-mode operation in the bent state. For optimizing the structure, we utilize the multiobjective gray wolf optimizer (MOGWO) to maximize the effective mode area (EMA) and the bending loss of higher order modes (HOMs), while minimizing the fundamental mode (FM) loss. The simulation results demonstrate that this framework enables us to improve the EMA by a factor of 1.26 and increase (decrease) the bending loss of the HOMs (the FM) by a factor of 7 (17), compared to the nonoptimal design. In addition, we investigate the dependence of the optical characteristics of the optimized LMA-PCFs on the wavelength and the bending radius. We found that some optimal structures are highly wavelength dependent and are not suitable for wideband applications. Furthermore, we found that the HOM loss is very sensitive to the bending radius. The proposed framework is comprehensive and can be employed to find a broad range of optimal designs for a wide range of applications.

Effect of Nested Elements on Avoided Crossing between the Higher-Order Core Modes and the Air-Capillary Modes in Hollow-Core Antiresonant Optical Fibers

Optimal suppression of higher-order modes (HOMs) in hollow-core antiresonant fibers comprising a single ring of thin-walled capillaries was previously studied, and can be achieved when the condition on the capillary-to-core diameter ratio is satisfied (d/D≈0.68). Here we report on the conditions for maximizing the leakage losses of HOMs in hollow-core nested antiresonant node-less fibers, while preserving low confinement loss for the fundamental mode. Using an analytical model based on coupled capillary waveguides, as well as full-vector finite element modeling, we show that optimal d/D value leading to high leakage losses of HOMs, is strongly correlated to the size of nested capillaries. We also show that extremely high value of degree of HOM suppression (∼1200) at the resonant coupling is almost unchanged on a wide range of nested capillary diameter dNested values. These results therefore suggest the possibility of designing antiresonant fibers with nested elements, which show optimal guiding performances in terms of the HOM loss compared to that of the fundamental mode, for clearly defined paired values of the ratios dNested/d and d/D. These can also tend towards a single-mode behavior only when the dimensionless parameter dNested/d is less than 0.30, with identical wall thicknesses for all of the capillaries.

Thermally induced mode loss evolution in the coiled ytterbium doped large mode area fiber

We propose a model to calculate the thermally induced mode loss evolution in the coiled ytterbium doped large mode area (LMA) fiber. The mode loss evolution in the coiled conventional step index LMA 20/400 fiber is investigated. Meanwhile, a model of fiber amplifier considering thermally induced mode loss evolution is established. The higher order mode (HOM) suppression between a co-pumping scheme and counter-pumping scheme under the heat load are compared. The simulation shows that the HOM loss decreases quasi-exponentially with the heat load and the bending radius of the ytterbium doped fiber (YDF) should be optimized according to the heat load to achieve effectively single mode operation. Besides, the counter-pumping fiber amplifier shows much better HOM suppression than the co-pumping fiber amplifier. The results in this paper will provide guidance in the design of novel ytterbium doped LMA fiber and the optimization of the high power single mode fiber amplifier.

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes.

We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Λ parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some ∼100 higher than for the LP01 mode, with LP01 losses <0.2 dB/m in the near-infrared and a spectral flatness ∼1 dB over a >110 THz bandwidth.

Optimization for designing the waveguide of 980 nm AlGaAs/InGaAs semiconductor laser

In order to raise the laser power and improve the far-field characteristics, a new method for designing the waveguide of 980nm AlGaAs/InGaAs semiconductor laser is demonstrated. Theory analysis shows the smaller the confinement factor is, the greater laser power will be. The confinement factors and near field profiles for various optical modes are simulated. The main factors influenced outpower are the fundamental and second order modes. To suppress the mode competition, the position of quantum well was optimized, common symmetric waveguide is replaced by the asymmetric one based on the distribution of various modes in optical field. The simulated results show threshold current (Ith) of the optimized structure is decreased from 230mA to 191mA, while the corresponding differential efficiency (ηd) is increased from 0.92W/A to 1.05W/A. Additional, the vertical divergence angle is reduced from 33.4° to 25.0°, compared to the common one. The designed device structure is grown by solid source molecular beam epitaxy (MBE) system. The fabricated asymmetric waveguide 980nm laser diode with 100μm strip width and 1000μm cavity length has threshold current 210mA, slope efficiency of 0.85W/A. The method for designing the waveguide structure can efficiently increase the loss of high order modes, lower the threshold current, improve the output power and far-field characteristics, which is applied to any kind of edge emitting lasers or laser bars.

Measurement of higher-order mode propagation losses in effectively single mode fibers

We present a direct and nondestructive measurement of the propagation loss of higher-order modes (HOMs) in effectively single-mode fibers. Lossy HOMs are excited by applying local stress at various points alongside a straight single mode fiber. The change of the HOM power as a function of the propagation distance is recorded at the fiber end by performing a modal decomposition with a correlation filter. The results for the HOM propagation loss are compared to simulations yielding very good agreement.

Impact of fiber outer boundaries on leaky mode losses in leakage channel fibers

In a leakage channel fiber, the desired fundamental mode (FM) has negligible waveguide loss. Higher-order modes (HOM) are designed to have much higher waveguide losses so that they are practically eliminated during propagation. Coherent reflection at the fiber outer boundary can lead to additional confinement especially for highly leaky HOM, leading to lower HOM losses than what are predicted by conventional FEM mode solver considering infinite cladding. In this work, we conducted, for the first time, careful measurements of HOM losses in two leakage channel fibers (LCF) with circular and rounded hexagonal boundary shapes respectively. Impact on HOM losses from coiling, fiber boundary shapes and coating indexes were studied in comparison to simulations. This work, for the first time, demonstrates the limit of the simulation method commonly used in the large-mode-area fiber designs and the need for an improved approach. More importantly, this work also demonstrates that a deviation from circular fiber outer shape may be an effective method to mitigate HOM loss reduction from coherent reflection from fiber outer boundary, even in double-clad fibers, with HOM losses in excess of 20 dB/m measured in the hexagonal LCF with ~50 µm core diameter while keeping FM loss negligible.

Cord identification technique for ultra-low bending loss fibers using higher order modes of visible light

We propose a cord identification technique for ultra-low bending loss fiber using higher order modes of visible light. With this kind of fiber, bending losses are greatly reduced and it is difficult to obtain sufficient leaked light with a conventional macro-bending technique. The bending loss of higher order modes is several orders larger than that of fundamental modes. Higher order modes can exist at shorter wavelengths and their guiding loss is small when the fiber is not tightly bent. As a result, higher order modes are suitable for cord identification purposes with ultra-low bending loss fiber. We determined that the LP21 and LP02 modes at 650nm (red) and the LP31 mode at 532nm (green) are the most effective for cord identification purposes. We employed an offset launch technique to excite higher order modes, and achieved a sensitivity improvement of more than 14dB. By using our method, a cord can be identified by red or green light even with the naked eye.

Micro-bending based optical band-pass filter and its application in S-band Thulium-doped fiber amplifier

A new approach for filtering an optical band-pass in optical amplifier is proposed using a macro bending. The proposed filter leverages the bending loss of higher order modes at shorter wavelengths. At longer wavelengths, the filter increases fiber's bending loss as the fundamental mode 'tail' is leak out from the cladding. The combination of wavelength dependent loss at longer and shorter wavelength gives rise to the optical band-pass filter characteristic inside the fiber. The simulated spectral response of the filter is found to be in good agreement with the experimental results. Subsequently, the proposed optical band-pass filter is applied in Thulium-doped fiber amplifiers (TDFA) system for gain and noise figure enhancements. The filter functions to suppress both the amplified spontaneous emission (ASE) at 800 nm and 1800 nm wavelength regions and thus improves both gain and noise figure performances in S-band region. By bending of the gain medium, gain and noise figure of the TDFA are improved by about 2 dB and 0.5 dB respectively, within a wavelength region from 1440 and 1500 nm when the 1050 nm pump power is fixed at 250 mW.

Loss Analysis of High Order Modes in Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

A new technique for analyzing the loss of high order modes in photonic crystal VCSEL is reported. The technique proposed is a semi-empirical approach that uses comparison to experimental data. For that purpose, photonic crystal VCSEL devices of single mode and multi mode are fabricated and measured for device characteristics comparison. The fabricated devices are modeled using finite difference frequency domain technique for the purpose of investigating the photonic crystal guided modes and loss analysis. In order to verify the findings, full three dimensional analyses are performed based on the fabricated single mode and multi mode devices structures using common finite difference time domain technique. The results show close agreement between the computed and experiment findings, justifying the used of the proposed semi-empirical technique for analyzing the loss of high order modes in photonic crystal VCSEL. Results for guided modes and near- and far-field are also presented for the photonic crystal VCSEL.

Bend performance-enhanced photonic crystal fibers with anisotropic numerical aperture

Numerical calculations predict that particular birefringent photonic crystal fiber designs exhibit slightly better performance in a coiled configuration than non-birefringent step-index fiber designs with respect to higher order mode suppression for the realization of large mode area effectively single transverse mode fibers. The passive losses of the fundamental and first few higher order modes of a birefringent photonic crystal fiber design with a 41 microm diameter core incorporating stress applying parts (SAP) were calculated using an integrated electromechanical finite element method. Minimum higher order mode losses of up to 5.5 dB/m were predicted for fundamental mode losses of only 0.0014 dB/m. The bend performance of this PCF design was predicted to be relatively insensitive to manufacturing tolerances with respect to air hole size and device assembly tolerances with respect to coiling diameter based on the calculated dependence of the mode losses on these parameters. The positions and refractive index of the SAP render the numerical aperture of the core anisotropic allowing further tailoring of the bend performance by adjusting the angle between the coiling plane of the fiber and the orientation of the SAP within the cladding. Fundamental and higher-order mode losses are calculated for step-index fiber (SIF) designs with a 40 microm diameter core for comparison. The step-index fiber designs were predicted to exhibit slightly inferior bend loss mode discrimination and higher sensitivity to packaging configuration compared to the photonic crystal fiber designs presented.