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 investigate the measurement of bend losses in few-mode large-mode-area (LMA) fibers. The influence of the light source spectral characteristics, modal power content and cladding light on the measurement accuracy and precision is studied experimentally. Monte-Carlo simulations are performed to understand the distribution of the variations. This study provides practical guidelines for bend loss measurements.
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 present an experimental study on mode coupling characteristics of few-mode large-mode-area (LMA) fibers, which are widely used in high power fiber lasers. The modal power allocation is measured by modal decomposition of the nearfield intensity profile of the output beam. Cut-back measurements are carried out with commonly-used fibers with different fiber geometries. The evolution of the modal power content due to mode coupling is presented. The influence of the fiber geometry on mode coupling is discussed.
We report an experimental study on mode coupling in various large-diameter multi-mode silica optical fibers. The
evolution of the far-field angular power distribution is experimentally measured, and the mode coupling characteristics are studied on a variety of fibers with diverse parameters, including core/cladding diameter (50-400μm/125-480μm), length (few to hundreds of meters), NA (0.15-0.46), etc. The influences of fiber geometry, bending are discussed. This study could provide practical guidance in designing power delivery fibers for high-power diode, solid-state and fiber lasers to preserve the input brightness and beam quality.
By confining the Yb doping within a smaller radius in the center of the core of the few-mode large-mode-area fiber, the
fundamental mode, which overlaps better with the Yb ions, sees higher gain than the higher order modes, and dominates
the output. Hence, improved beam quality can be achieved. A confined-doped fiber with 41/395μm core/cladding
diameters is fabricated by Direct Nanoparticle Deposition (DND) process. The fiber is characterized in an amplified
spontaneous emission (ASE) source setup. Near-diffraction-limited beam quality (M2~1.3) is experimentally
demonstrated.
Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the
conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO2
doped with GeO2 was coated with layers of Yb2O3 and Al2O3 prior to sintering, using the ALD method. ALD is a surface
controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity
and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of
320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses,
lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating
Yb-doped fibers with high concentration of dopants.
We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.
By using a modal interference method, photodarkening-induced refractive index change in ytterbium doped fibers is
measured with high accuracy. This refractive index change is positive at the ytterbium lasing wavelengths near 1080nm,
and it gradually approaches a saturated level over time. It is found that the refractive index change is linearly
proportional to the photodarkening-induced excess loss at an arbitrary probe wavelength in the visible band. The
maximum refractive index change in our experiment is close to 1×10-5. The origin and influence of the photodarkening-induced
refractive index change on fiber lasers is discussed.
Power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities.
Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a
step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion
confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of
the fibers. Both fibers exhibited a mode field diameter comparable to a 40μm step index fiber with 0.07 NA. The fibers
were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam
quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality
enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to
the fibers.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.