This paper investigates the detailed impact of all three graded-index-fiber (GIF) parameters on the balance between DMD and crosstalk. We report an optimized 4-LP-mode GIF offering LP01, LP11, LP21, and LP02, with a minimum |Δneff| = 0.6 × 10−3, maximum |DMD| = 5.4 ns/km, while minimum |Aeff| = 80 μm2 and bending loss (BL) of the highest order mode is 0.005 dB/turn (much lower than 10 dB/turn) at a 10 mm bend radius, for core-radius a = 7 to 9 μm, Δn = 0.014 to 0.016, and α = 2 to 4. In this study, we successfully addressed the challenge of degeneracy between LP21 and LP02 in FM-GIF, which has been difficult to overcome. To the best of the authors’ knowledge, this is the lowest reported DMD value (≈ 5.4 ns/km) achieved in such a weakly-coupled (|Δneff| = 0.6 × 10−3) 4-LP-GI-FMF.
In this work we demonstrate spectrometer based mid-infrared (MIR) optical coherence tomography (OCT) at 4 µm but with an increase of state-of-the-art imaging speed by at least 10 times. The improvement is based on exploiting a chirped periodically poled lithium niobate crystal. We show more than 3 kHz line rate OCT imaging. With this significant increase in imaging speed we hope to expand the efficacy of mid-infrared OCT.
We demonstrate a 70 mol % GeO2 doped fiber for mid-infrared supercontinuum generation. Experiments ensure a highest output power for a broadest spectrum from 1000nm to 3000nm from this fiber, while being pumped by a broadband 4 stage Erbium fiber based MOPA. Our investigations reveal the unexploited potential of Germania doped fiber for mid-infrared supercontinuum generation. To the best of our knowledge, this is the record power, for an ultrabroadband, all-fiberized, and compact device size supercontinuum light source based on Silica and Germania fiber, ever demonstrated to the date.
We present a mid-infrared spectral-domain optical coherence tomography system operating at 4.1 μm central wavelength with a high axial resolution of 8.6 μm enabled by more than 1 μm bandwidth from 3.58-4.63 μm produced by a mid-infrared supercontinuum laser. The system produces 2D cross-sectional images in real-time enabled the high-brightness of the supercontinuum source in combination with broadband upconversion of the signal to the range 820-865 nm, where a standard 800 nm array spectrometer can be used for fast detection. We discuss the potential applications within nondestructive testing in highly scattering materials and within biomedical imaging for achieving the in-vivo optical biopsy.
Concatenating fibers to extend supercontinuum generation beyond the multiphonon absorption edge of silica to access longer wavelengths in the mid infrared region has received much attention due to the various molecular fingerprints that exist in this region. Thulium doped (Tm) fiber has been used as an intermediate fiber to get above the multiphonon absorption edge of silica through absorption (900 and 1600 nm) and emission processes between (1800 – 2100 nm) and (2200 - 2600 nm). The advantages of the Tm-doped fiber is not only limited to generating SC above the multiphonon edge of silica but also reducing the high peak power which easily damages the facet of soft glass fibers whose transmission window extends well into the longer mid infrared region. The mechanisms governing the generation of SC in active fibers is quite different from passive fibers as there is interplay of the nonlinear processes that are commonly observed in passive fibers as well as energy transition contributions by the dopants in the fiber.
SC generation and its application are hindered by pulse-to-pulse fluctuations as the generation is initiated by noise seeded processes especially in the long pumped regime (> 1ps). In this work we have experimentally studied the pulse fluctuations in thulium gain fiber whose SC spans 1550 – 2700 nm at three different repetition rates (10 KHz , 100 KHz, 200 KHz). We have illustrated the relative intensity noise across the whole spectrum most importantly within the absorption and emission regions of the Tm-doped fiber.
We demonstrate a 74 mol % GeO2 doped fiber for mid-infrared supercontinuum generation. Experiments ensure a highest output power for a broadest spectrum from 700nm to 3200nm from this fiber, while being pumped by a broadband 4 stage Erbium fiber based MOPA. The effect of repetition rate of pump source and length of Germania-doped fiber has also been investigated.
Further, Germania doped fiber has been pumped by conventional Silica based photonic crystal fiber supercontinuum source. At low power, a considerable broadening of 200-300nm was observed. Further broadening of spectrum was limited due to limited power of pump source. Our investigations reveal the unexploited potential of Germania doped fiber for mid-infrared supercontinuum generation. This measurement ensures a possibility of Germania based photonic crystal fiber or a step-index fiber supercontinuum source for high power ultra-broad band emission being pumped a 1060nm or a 1550nm laser source. To the best of our knowledge, this is the record power, ultra-broadband, and all-fiberized SC light source based on Silica and Germania fiber ever demonstrated to the date.
We demonstrate a 60μm core diameter single-trench Yb free Er-La-Al doped fiber having 0.038 ultra-low-NA, using conventional MCVD process in conjunction with solution doping process. Numerical simulations ensure an effective single mode, the effective area varies from 1,820μm2 to 1,960μm2 for different thicknesses of trenches and resonant rings. This fiber has been fabricated with conventional fabrication process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Moreover, all solid structure ensures easy cleaving and splicing. Experimental measurements demonstrate a robust effective single mode operation. Furthermore, this fiber in 4%-4% laser cavity shows a record efficiency of 46% with respect to absorbed power.
This paper reviews our recent work on novel large-mode area fibers for high power lasers and amplifiers. An ultra-low- NA fiber and single-trench fiber have been proposed for mode area scaling of the fundamental mode. In case of single-trench fiber design, resonant coupling of the higher order modes has been exploited to achieve effective single mode operation in fiber with large effective mode area. Our proposed fiber designs are easy to fabricate using conventional low-loss fiber fabrication techniques, and moreover, being all solid structure, they ensure easy cleaving and splicing. A monolithic and compact high power fiber laser/amplifier device with a good output beam quality can be achieved using Single-trench fiber design.
We propose a novel fiber design single-trench fiber (STF) for large mode area fiber laser and amplifier. Fiber offers cylindrical symmetry and also offer higher refractive index of core compared to cladding. This avoids the need of stack and draw process and refractive index compensation of core doped with index raising rare earth and co-dopants ions, which are an indispensable condition in most of other fiber designs. That is why, this fiber design can be fabricated with conventional modified chemical vapour deposition process in conjunction with solution doping process, which can dramatically reduce the fabrication cost, hence suitable for mass production. Fiber offers very high loss (>10dB/m) and low power fraction in core (<50%) to the higher order modes for low loss of fundamental mode (<0.1dB/m) and high power fraction in core (>80%) thanks to the resonant coupling between core and ring modes. We fabricated a preform for 30μm core STF using MCVD process in conjunction with solution doping process in a single step, without using any micro-structuration and pixilation of core. Experiments ensure the robust single mode behaviour irrespective of launching condition of input beam.
Furthermore, this fiber used in three stages MOPA provides 80kW peak pulses with repetition rate of 500 kHz, average power up to 10W, with M2 ~ 1.14. Moreover, all solid structure ensures easy cleaving and splicing. In nutshell, an ultra-low cost, monolithic, compact, and an effective single mode fiber amplifier device can be achieved using single-trench fiber.
SiC nanocrystals of size 4×30×40nm have been fabricated on Si (100)3° by a low cost spin-on technique in which Htermination
produces smaller crystallites but larger crystallite density at the substrate surface steps. Low temperature
photoluminescence and atomic force microscopy (AFM) confirms the formation of quantum dashes. It has been observed
that the spin speed and H-termination plays a crucial role in the formation of quantized structures on Si.
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