Fiber-based laser systems enable high output power in combination with diffraction limited beam quality. Their output power is generally limited by the onset of nonlinear effects. The chirally coupled core (CCC) fiber provides a large mode field diameter while also suppressing higher-order-modes. This is needed to further increase a laser’s output power and maintaining single-mode operation. However, the integration of specialty fibers in an all-fiber laser setup is in most cases not possible because suitable fiber components are not available. We report on the development of a cladding light stripper and a signal-pump combiner with integrated 34/250-µm CCC fibers which allow for the development of spliceless all-fiber amplifier systems. The cladding light stripper is manufactured by structuring the CCC-fiber’s cladding using a CO2-laser to interrupt pump light guiding within the cladding. The cladding light stripper enables a stripping efficiency of 19 dB and was tested up to a stripped optical power of 100W, which is sufficient to enable kW-class amplifier systems. The signal-pump combiner relies on a side-pumped design with four pump input fibers. Its characterization reveals a pump-to-signal fiber coupling efficiency of 90% and a signal-to-pump isolation of 30 dB. Component stability was tested at a pump input power of 500W. An S2 -measurement confirmed that the spatial mode content of the signal light propagating through the CCC-fiber-based signal-pump combiner remains unaffected. Furthermore, a signal-pump combiner was subjected to temperature cycles between -5 and 85 °C over a time period of <1000 h and showed no degradation.
Current research focuses on very-large-mode-area fibers (core diameters of 34 μm and above) and all-fiber laser systems to deliver high output power with linear polarization, low noise properties, narrow-band linewidth and high fundamental mode content. All-fiber systems have the advantage of low-maintenance and alignment-free operation. Because power scaling in fiber-based amplifiers is limited by non-linear effects like stimulated Brillouin scattering, very-large-mode-area fibers such as specialty fiber designs like the chirally coupled core (CCC) fiber are investigated. This fiber type offers a signal core of 34 μm or larger while also providing a near single-mode output beam quality. It thereby enables further power scaling in systems that are limited by such nonlinear effects. However, efficient components such as signal-pump-combiners (SPC) with this fiber type need to be developed. The SPC couples the required pump light of multiple high power laser diodes into the gain fiber of the laser system. We report on the development of a SPC with an integrated 34=250 μm feed-through CCCfiber with a pump-to-signal fiber coupling efficiency of 90% and three input pump fibers with a signal-to-pump isolation of 30 dB. The device is tested with an input power of up to 380 W. In addition, different experiments for monolithic implementation of the CCC-fiber type into systems that rely on standard polarization-maintaining (PM) fibers are conducted. We show the polarization maintaining behavior (polarization extinction ratio (PER) < 19 dB over several hours) of the fiber by imprinting externally induced birefringence on the fiber. Experiments with all-fiber setups using the CCC-fiber and a step-index PM-fiber show a PER of < 15 dB with reduced long-term stability.
Lasers are essential tools for a wide variety of materials processing applications. The speed, quality, and process window are determined in part by the laser beam properties, including size, shape, and divergence. Most laser sources have fixed beam characteristics, resulting in processing and material limitations and nonoptimized performance. nLIGHT has developed a fiber-laser product line that provides rapid tunability of the beam characteristics directly from the delivery fiber using a novel, all-fiber mechanism. The broad range of beam sizes and shapes and real-time programmability allow adjustments on-the-fly and optimization of each process step using a single laser source, enabling development of versatile tools that provide optimum performance for a range of processing needs. We describe the underlying technology, performance, and beam characteristics and show results for the largest industrial laser applications, including metal cutting, welding, and additive manufacturing.
Specialty fibers such as chirally-coupled-core fibers (3C®-fiber) show a high potential for further power scaling of single-frequency fiber amplifiers. Especially, the application of gravitational wave detectors requires a high optical output power at low noise characteristics. The output power of fiber-based single-frequency amplifiers is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged and 3C®-fibers have been specifically designed to enable single-mode operation with a large mode area core. This fiber type consists of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which allows a high-purity modal content in the core. The development of compact all-fiber lasers in conjunction with specialty fibers combines the advantages of both techniques. For the first time, we demonstrate a spliceless all-fiber amplifier, where all optical components are directly integrated in a single Yb3+-doped 3C®-fiber. Such a spliceless laser design allows a compact and robust architecture using specialty fibers, while maintaining excellent beam properties. At an output power of 336 W, a fundamental mode content of 90.4% was demonstrated. This work emphasizes the suitability of 3C®-fibers in high-power laser and amplifier systems and the potential as laser sources for the next generation of gravitational wave detectors.
We present the development of an all-fiber side-fused signal-pump-combiner based on an integrated 3C R feedthrough fiber. This specialty fiber uses a 34 μm single-mode core and shows great potential to enable further output power scaling while maintaining high beam quality. The side-fusing technique has the advantage of an uninterrupted signal core and can be used in co- and counter-pumped fiber lasers and amplifiers. The signal-pump-combiner was operated up to an input power of 600W from four pump fibers and coupling efficiencies of 79% were achieved. The component was additionally investigated by computer tomography imaging, which revealed that the cladding structure of this specialty fiber prevented the required level of glass fusion of the 3C R fiber with the pump fibers. The investigation will help to further increase the pump coupling efficiency of the signal-pump-combiners. This represents the first step of developing all-fiber and high power capable laser systems based on the 3C R fiber.
The output power of fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In addition to a high output power, long-term stable and less complex laser systems are required. It has been shown that all-fiber amplifier systems can be a suitable option to avoid power scaling problems of single-frequency solid-state lasers with injection locking. Chirally-coupled-core (3C®) fibers have been specifically designed to enable single-mode operation with a large mode area core to overcome these limitations. 3C®-fibers consist of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. For this purpose, the all-solid design of 3C®-fibers allows a manufacturing process of fiber-based components. We present various optical components based on 3C®-fibers for the realization of a single-frequency all-fiber amplifier. These amplifiers typically consist of a mode field adapter (MFA), cladding light stripper (CLS) and pump combiner (PC) to minimize the excitation of higher order modes, remove residual pump light and optimize the coupling efficiency of the pump light in the 3C®-fibers. The components have been specifically designed for the first time with 3C®-fibers and tested according to their performance. As a first prototype, a robust and monolithic fiber amplifier with an ytterbiumdoped 3C®-fiber in combination with commercially available standard fibers was developed. Overall, the fiber amplifier achieves an optical output power of 165W in a linearly polarized TEM00-mode. This work emphasizes the high potential of amplifiers based on 3C®-fibers as laser sources for the next generation of gravitational wave detectors and demonstrates that compact and robust amplifiers can be realized using 3C®-fibers.
The output power of fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged. Chirally-coupled-core (3C®) fibers have been specifically designed to enable single-mode operation with a large mode area core. 3C®-fibers consist of a step-index fiber structure, whose signal core is additionally chirally surrounded by one ore more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which enables a high-purity modal content in the core. We present a robust and monolithic fiber amplifier based on an ytterbium-doped 3C®-fiber in combination with commercially available standard fibers. For the realization of such a monolithic system, a mode field adapter (MFA) was designed and installed between a standard polarization-maintaining fiber and an active 3C®-fiber for the first time. The MFA was tested regarding the guided modal content by means of a S2-system. Overall, the fiber amplifier achieves a polarization extinction ratio of 17.6 dB and an optical output power of 100.1W in a linearly polarized TEM00-mode. To our knowledge, the fundamental mode content of 98.9% is the highest TEM00-mode content of fiber amplifiers reported at this power level. This work emphasizes the high potential of fiber amplifiers based on 3C®-fibers as laser sources for the next generation of gravitational wave detectors and demonstrates that compact and robust amplifiers can be realized using 3C®-fibers.
We report detailed characterization results of Yb-doped Chirally-Coupled-Core (3C) fibers fabricated with Direct Nanoparticle Deposition (DND) technique. Two types of 3C fibers with core/clad geometries of 34/250μm and 55/400μm and another 25/250μm conventional large-mode-area (LMA) fiber are measured and the results are compared in terms of modal content, transmission spectrum, etc. A picosecond fiber amplifier is built based on 55/400μm 3C fiber, showing robust single-mode operation with peak power >1MW with no sign of stimulated Raman scattering (SRS).
We demonstrate flexible performance in a fiber MOPA system based on nLIGHT’s PFL seed laser platform and chirally coupled core (3C®) fiber. The 33μm core, 27μm MFD 3C fiber used in these demonstrations is fabricated in volume at nLIGHT’s Finland facility. A variety of pulse formats are amplified to nonlinearity-limited peak power <300kW, including single pulses in the 50ps to 1ns regime at a variety of repetition rates from 10’s of kHz to MHz. Beam quality in these 3C based MOPAs is exceptional with M2<1.15 and circularity <95% at all power levels. Beam pointing often evident in other LMA fiber technologies due to higher order mode content is minimal in these fiber MOPAs. Burst mode operation of the seed laser system using flexible burst packet repetition rates (10’s of kHz to several MHz) and adjustable pulse-to-pulse spacing within bursts (<10ns to 100ns) is demonstrated and amplified in the same 3C fibers. Bursts of up to ten 50ps pulses amplified to total energies exceeding 160μJ are demonstrated at 200kHz burst repetition rate and 32W average power at high efficiency (74% slope). Bursts of up to five 500ps pulses are also amplified to up to 360μJ total energy. In both cases, the varying degree of pulse saturation win a burst and mitigation paths are reviewed.
We demonstrate a robust, compact, low-cost, pulsed, linearly polarized, 1064 nm, Yb:fiber laser system capable of generating ~100 kW peak power pulses and >17 W average power at repetition rates of 80 – 285 kHz. The system employs a configurable microchip seed laser that provides nanosecond (~1.0 – 1.5 ns) pulse durations. The seed pulses are amplified in an all-fiber, polarization maintaining, large mode area (LMA) fiber amplifier optimized for high peak power operation. The LMA Yb:fiber amplifier enables near diffraction limited beam quality at 100 kW peak power. The seed laser, fiber amplifier, and beam delivery optics are packaged into an air-cooled laser head of 152×330×87 mm3 with pump power provided from a separate air-cooled laser controller. Due to the high peak power, high beam quality, spectral purity, and linearly polarized nature of the output beam, the laser is readily frequency doubled to 532 nm. Average 532 nm powers up to 7 W and peak powers exceeding 40 kW have been demonstrated. Potential for scaling to higher peak and average powers in both the green and infrared (IR) will be discussed. This laser system has been field tested and demonstrated in numerous materials processing applications in both the IR and green, including scribing and marking. We discuss recent results that demonstrate success in processing a diverse array of representative industrial samples.
We have demonstrated a pulsed 1064 nm PM Yb:fiber laser system incorporating a seed source with a tunable pulse repetition rate and pulse duration and a multistage fiber amplifier, ending in a large core (>650 μm2 mode field area), tapered fiber amplifier. The amplifier chain is all-fiber, with the exception of the final amplifier’s pump combiner, allowing robust, compact packaging. The air-cooled laser system is rated for >60 W of average power and beam quality of M2 < 1.3 at repetition rates below 100 kHz to 10’s of MHz, with pulses discretely tunable over a range spanning 50 ps to greater than 1.5 ns. Maximum pulse energies, limited by the onset of self phase modulation and stimulated Raman scattering, are greater than 12.5 μJ at 50 ps and 375 μJ at 1.5 ns , corresponding to >250 kW peak power across the pulse tuning range. We present frequency conversion to 532 nm with efficiency greater than 70% and conversion to UV via frequency tripling, with initial feasibility experiments showing >30% UV conversion efficiency. Application results of the laser in scribing, thin film removal and micro-machining will be discussed.
This paper demonstrates the value of D-type optical fibers (D-fibers) in a variety of sensing applications. The principal
advantage of the D-fiber is that it allows for interaction with light traveling in the core of an optical fiber with materials
or structures placed in contact with the fiber. This permits stimulus sensitive materials to be placed on the D-fiber to
interact with the light in the core of the fiber. The presentation shows that this feature of D-fibers can be used to create
alternatives to sensors formed in standard optical fibers for measuring temperature, strain, and shape change. In addition,
D-fiber sensors have been fabricated to measure chemical concentrations, and electric fields.
The Surface Relief Fiber Bragg Grating (SR-FBG) is a viable alternative to the thermocouple for high temperature measurements in industry. To fabricate the SR-FBG we etch a grating into the flat surface of an elliptical-core D-fiber. At high temperature (1000 °C) the optical fiber becomes brittle. To overcome brittleness we thread the fiber through a preheated 0.020 inch diameter stainless steel tube. We insert the small tube into a larger one with a diameter of 0.125 inches. The smaller tube rests on ceramic inserts to prevent contact with the large tube. The end of the D-fiber is fitted with a standard fiber optic connecter. With this packaging scheme we conduct a series of test at high temperature. The sensor is robust with no power loss or Bragg wavelength shift, even after heating for 24 consecutive hours.
We recently reported on a new fiber Bragg grating etched into the flat surface of a D-fiber and its potential use as a high temperature sensor. Since then we have investigated more in depth many of the characteristics that are unique due to the surface relief nature of the grating. In this paper we show that a surface relief fiber Bragg grating exhibits some significant advantages when compared to standard fiber Bragg gratings including: high temperature operation, polarization selectivity, and the ability for multi axis strain sensing. We also show the uniqueness of these gratings for bend sensing with two degrees of freedom.
In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber
Bragg grating (FBG) strain and temperature sensors for use in high temperature environments - where there is a
presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a
stable reflectivity response of FBGs and their survival at elevated temperatures, we are using surface relief fiber Bragg
gratings (SR-FBG). These gratings, instead of being written in the core of a photosensitive or hydrogen-loaded fiber,
are formed by introducing a periodic surface relief - through photolithographic and etching processes - in the cladding
above the core. Samples of SR-FBGs were successfully encapsulated and mounted onto metal shims. The packaged
sensors displayed a linear response with temperature and a sensitivity factor of 11pm/°C.
We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface relief fiber Bragg gratings (SR-FBGs) are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. In this work we describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors. We show that SR-FBGs resist much higher temperatures than standard FBGs by demonstrating their operation up to 1100 degrees Celsius.
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