We present an efficient way to remove unwanted Amplified Stimulated Emission (ASE) in high-power fiber lasers and amplifiers using intracavity Chirped Tilted Fiber Bragg Grating (CTFBG) filters. The grating is written with tilted fringes so that the unwanted ASE is reflected into the fiber cladding where it is no longer amplified. Depending on the desired emission wavelength and active fiber, one or several filters are spliced within the active fiber to suppress ASE before it reaches a detrimental power. Numerical simulations clearly show that adding the filters allows amplification in configurations that would just be impossible due to the onset of ASE. The filter bandwidth and extinction, and the maximum allowed active fiber length between each filter are also computed depending on the core/cladding diameter ratio of the active fiber used and the targeted emission wavelength. As an example, a fiber laser at 1018 nm is assembled in a 20/400μm core/cladding diameter ytterbium fiber that is cladding pumped at 976 nm. Two CTFBGs with 20 dB attenuation from 1025 nm to 1070 nm are spliced within the 6-meter-long ytterbium fiber. 432 W of laser emission at 1018 nm is efficiently achieved at 77% slope efficiency with respect to the absorbed pump power. The extinction between the 1018 nm signal and the ASE is greater than 50 dB. Removing the ASE filters from the cavity clearly leads to only self-pulsation of the ASE between 1030 nm and 1050 nm, no generation of 1018 nm light was possible. The measured thermal slope of the filters shows scalability above the kW level. Demonstration at 1908 nm with a 25/400 core/cladding diameter thulium doped fiber is also done. Tests were done to inscribe the CTFBG directly in an ytterbium fiber for simpler implementation and avoid additional splicing.
We present recent developments regarding fiber Bragg gratings for kilowatt-level fiber lasers. First, we show that writing grating reflectors through the fiber coating using an ultrafast laser improves reliability and enables higher pump power handling. The use of ultrafast laser technology also offers more options to produce gratings in larger core fibers. Finally, we show that Raman suppression gratings are a good solution for SRS mitigation with their large (<20 dB) rejection over 15 nm and low reflectivity at Raman wavelengths, and negligible insertion loss at the laser wavelength.
Double-clad fibers (DCF) found in kilowatt-class fiber lasers typically have a second cladding made of fluoroacrylate. At high power, thermal damage or accelerated aging of this material becomes an issue. The operating temperature of the fluoroacrylate coating is found to be highly sensitive to the numerical aperture (NA) distribution of the pump light flowing through the fiber. Characterization of this effect with an optical loss measurement is impractical as this loss remains typically very low. Measurement of the coating temperature for a given input power and far-field distribution is much more sensitive. Furthermore, it directly gives the parameters that are key to the design of a high-power fiber laser. A system for the measurement of the thermal slope of DCF fibers and high-power fiber components has been built and tested. This system allows varying the input power and the source NA under high power with a unique splice to the device being tested. To achieve this, different types of fiber-coupled pump diodes are spliced to the inputs of a pump combiner. Fiber tapers are used to fine tune the sources’ NA. By turning on different diodes, the NA of the injected pump light can be varied. The thermal slope for a given NA can then be measured with a thermal camera and a power meter. Measurements show differing thermal slopes of DCF measured before and after a damp heat tests. These thermal slope variations are stronger when operating at a high numerical aperture.
We present a compact four-laser source based on low-noise, high-bandwidth Pound-Drever-Hall method and optical phase-locked loops for sensing narrow spectral features. Four semiconductor external cavity lasers in butterfly packages are mounted on a shared electronics control board and all other optical functions are integrated on a single silicon photonics chip. This high performance source is compact, automated, robust, operates over a wide temperature range and remains locked for days. A laser to resonance frequency noise of 0.25 Hz/rt-Hz is demonstrated.
A growing number of applications are calling for compact laser sources operating in the mid-infrared spectral region. A review of our recent work on monolithic fiber lasers (FL) based either on the use of rare-earth fluoride fibers or on Raman gain in both fluoride and chalcogenide glass fibers is presented. Accordingly, an erbium-doped double clad fluoride glass all-FL operating in the vicinity of 3 μm is shown. In addition, we present recent results on the first demonstrations of both fluoride and chalcogenide Raman fiber lasers operating at 2.23 and 3.34 μm, respectively. It is shown that based on this approach, monolithic FLs could be developed to cover the whole 2 to 4 μm spectral band.
A growing number of applications are calling for compact laser sources operating in the mid-infrared spectral region. A review of our recent work on monolithic fiber lasers based either on the use of rare-earth fluoride fibers or on Raman gain in both fluoride and chalcogenide glass fibers is presented. Accordingly, an erbium-doped double clad fluoride glass all-fiber laser operating in the vicinity of 3 μm is shown. In addition, we present recent results on the first demonstrations of both fluoride and chalcogenide Raman fiber lasers operating at 2.23 μm and 3.34 μm, respectively. It is shown that based on this approach, monolithic fiber lasers could be developed to cover the whole 2-4 μm spectral band.
A highly efficient and high power Raman fiber laser was developed based on the use of broadband Fiber Bragg gratings
as optical couplers. The broadening of the Stokes signal is analyzed in both cases where the laser emission is restricted
or not by the FBGs bandwidth. Since broadband fiber Bragg gratings are involved, the effects of cladding mode losses
have to be considered. In order to reduce overall losses in the cavity, an optimal cavity configuration has been
determined based on the physical orientation of the fiber Bragg gratings.
The thulium-doped ZBLAN fiber laser converts infrared radiation in the range 1100-1160 nm to visible radiation around 480 nm through an upconversion pumping process. A brief description of the laser through a rate equation analysis modeling is presented. A compact experimental setup using a diode-pumped Yb3+ fiber laser
at 1108 nm is presented. Single transverse mode blue emission at 482 nm with a maximal output power of 96 mW was obtained. A slight increase of the laser threshold due to photoinduced losses from 103 mW to 240 mW was observed, which is significantly less than previously reported values. Further optimization of the laser
efficiency is proposed using Raman fiber laser pumping at 1125 and 1160 nm. Numerical simulations are used to calculate the expected increase in blue output power.
re present a model for a 455 nm thulium-doped ZBLAN fiber laser co-pumped at 645, 684 and 1064 nm. This model
calculates the blue laser output power using a rate equation analysis and takes into account the cross-relaxation and
cooperative upconversion processes. We calculate the optimal pump wavelength for the case of a single visible pump
wavelength, and then show the benefits of adding the second visible pump wavelength at 684 nm for a low 1064 nm pump
power (120 mW). At high infrared pump powers (≥600 mW), we then show that adding the second pump does not increase
the efficiency of the laser. These results are explained by studying the population densities of the energy levels of the
thulium ions for each pumping configuration.
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