We have developed a high power single-mode (SM) monolithic fiber laser at 1018 nm, producing 230 W CW, with an M2 of 1.17 and light to light efficiency of 75%. To the best of our knowledge this is the highest power described in the open literature from a SM fiber laser at this wavelength. Careful simulations were employed which take into account the various wavelength dependent parameters such as the fiber absorption and emission as obtained from the fiber manufacturers, and the cavity mirrors’ reflection, in addition to the fiber geometrical parameters. It was found that the major obstacle for increasing the power at 1018nm is the self-generation of amplified spontaneous emission at wavelengths of 1030-1040nm. If the laser is not designed properly these undesired wavelengths dominate the output spectrum.
A monolithic coherent combiner scheme for combining multiple fiber lasers based on a photonic crystal fiber is described. Beam propagation method (BPM) simulations show that the beam combiner efficiency can reach 96% for a 4×1 combiner, 94% for an 8×1 combiner, and 91% for a 16×1 combiner, provided the fiber lasers are phase matched. In addition, a 2×1 intensity polarization combiner is proposed and simulated through full vectorial BPM, yielding a combining efficiency of 95%. This concept can lead to a rugged and efficient combiner for multiple fiber lasers.
We demonstrate intra-fiber couplers performance that is close to complete brightness preservation up to 3kW. Furthermore, when mutually coherent sources were used, the same couplers were able to achieve brightness enhancement with almost no beam quality (BQ) deterioration. The couplers are based on an adiabatic, all-fiber, mode coupling device preserving the lowest spatial mode orders. Brightness levels that approach the theoretical limits were achieved by compressing the participating modes into a tight cross section. Incoherent combination is shown for 2×1, 3×1 and 7×1 combined elements. Additionally, we present a solution for preserving the beam propagation factor of the coupler by using a specialty engineered core delivery fiber. The fabricated components are fully fiber- integrated, hence, without free-space limitations. An overall transmission of <90% was obtained, while the coupler-delivery connection is responsible for less than 0.5% loss. Consequently, relatively low temperatures were observed in the combiner package. Alternatively, utilizing two mutually coherent sources, a quadratic brightness factor improvement was demonstrated. The scheme does not require polarization preserving fibers, and achieved rugged 'in-phase' mode-locking. This allows for a significantly simplified scheme, compared to common coherent combining methods. Prospect on future trends relating to nonlinearities and thermal load management are discussed.
Beam combining with tapered array of single mode fibers is presented. A mutual incoherent CW power
launch into the entries introduces intensity addition of which brightness can be utmost conserved. The best
limit for the combined output beam quality is given by the brightness conservation law. An experiment
where three and seven input CW signals are combined supports this claim. The respective M2 values were
close to the best predicted limit, thus it is found to have good agreement with the brightness limit. The
power transfer is higher than 90%.
KEYWORDS: Multimode fibers, Fiber lasers, Near field optics, Waveguides, Optical fibers, Near field, Birefringence, Light sources, Step index fibers, Polarization
Beam quality analysis of mutual off-centered fibers is presented. The relevant scenario is where a single
mode input fiber is injecting optical radiation into a few-mode output fiber. Both are step-index, single clad
fibers. Assuming the source's coherence time is short enough, the major finding is that the output beam
quality is the highest when the injecting fiber is at some offset with respect to the center of the receiving
core. A Theoretical analysis and an experiment are in good agreement with each other. These results may
have practical consequences in beam-coupling schemes and in applications emphasizing the importance of
beam quality.
In this paper we present for the first time to our knowledge an efficient and rugged light source in the visible, based on a
gain switched Yb3+ doped fiber laser source, frequency doubled by a non-critically phase matched LiB3O5 crystal. The
simple setup proves to be robust and durable against back reflections, which in turn remove the requirement for optical
isolators along most of the system.
Gain switched fiber lasers typically produce long pulses with low peak power, which are not optimal for frequency
conversion applications. However, as opposed to MOPA laser configurations, based on a semiconductor laser diode as a
seed source, the narrow spectral line width and chirp free operation of gain switched lasers render them suitable for
efficient frequency doubling.
A novel side coupling technique between two multimode high NA fibers is described. The
technique is used to efficiently pump fiber lasers and amplifiers by low brightness fiber coupled
pump diodes. With the presented technique, identical multimode fibers with 0.46NA and core
diameters extending from 125&mgr;m to 400&mgr;m, can be coupled together, and provide pump coupling
efficiency of >90%. Direct coupling to a rare-earth doped fiber is possible. In this configuration
one fiber is used as the pump guiding fiber and the second fiber is the rare-earth doped double
clad fiber. By utilizing the presented pump coupling technique, highly efficient, rugged and low
cost short pulse and CW all-fiber lasers were implemented, with average output power extending to 300W and peak power of 600kW.
An analytic model is developed for evaluating the extractable energy from high energy pulsed Ytterbium (Yb) doped and Ytterbium:Erbium (Yb:Er) co-doped fiber amplifiers and lasers. The energy extraction capabilities under the limitation of spurious lasing, due to amplified spontaneous emission (ASE), are mapped for various numerical apertures, single and multi transverse mode evolution and operating wavelengths. The calculation results of the analytic model show good match with experimental results carried out for various Yb doped double clad fiber amplifiers. The model provides an accurate assessment for the maximum pulse energy that can be extracted from a given fiber. In addition, for a specific pump power, the model can be used to determine the minimum repetition rate and optimal length, under which the laser source can be operated before spurious lasing occurs.
A marked improvement is observed in the performance of pulsed microwave excited slab carbon-dioxide laser when the gas is permitted to expand following a discharge pulse. The gas pressure changes following each discharge pulse indicate a heating followed by expansion and contraction stages. A sequence of over ten reciprocating cycles is observed after each pulse. Two modes of operation (open versus closed) were investigated. The gas motion in and out of the discharge space in the open structure, leads to a faster overall cooling than for fully confined gas. Pressure changes driving the reciprocating motion of the activated gas are monitored. Maximum average output power of 20 W with 10% efficiency and about 1 KW peak power were obtained from the open structure at gas pressure of 210 mmHg. The absence of the expansion cooling mechanism in the closed structure caused about 40% reduction in the output power and efficiency. The results of the present research suggest a new architecture for pulsed lasers that provides a gas cooling markedly faster than the conductive cooling of a stagnant gas in the same geometry.
Transverse electric atmospheric (TEA), or multi atmospheric (TEMA) lasers deliver intense short laser pulses of considerable energies. Recurrent high repetition rate pulse trains afford substantial average power levels. In a high rep-rate operation the gas flows across the cavity and is externally cooled to maintain a reasonably low temperature. The gas flow gear and heat exchanger are bulky and costly. In this work we present a repetitively pulsed TEA or TEMA laser that combines energy and peak power features in an individual pulse with the substantial average power levels of a pulse train in a thin layer of gas. Excess heat is disposed of, by conduction through the gas, to cooled enclosing walls. The gas does not flow. The method applies to vibrational transition molecular lasers in the infrared, where elevated temperatures are deleterious to the laser operation. The gist of the method draws on the law that heat conductivity in gases does not depend on their pressure. The fact lends unique operational flexibility and compactness, desirable for industrial and research purposes.
Strip-line molecular lasers with a narrow gas sheath confined by bare metal electrodes are sensitive to chemical processes that influence the gas composition during the RF discharge. Both CO2 and CO gas mixtures in a stationary gas in the discharge reach a steady state composition, and an associated asymptotic laser performance is attained. However, the process of CO2 dissociation has the opposite effect on the gas composition than CO + O recombination or 2CO ?CO2 + C disproportionation reaction. Therefore, materials like gold, that catalyze the CO2 formation, exhibit complementary behavior with respect to power decline in sealed off CO2 versus CO laser operations. We describe the operation of a RF excited strip-line CO gas laser at a cooling temperature of 0°C. The laser power and efficiency are critically dependent on the purity of the constituent gases and on the nature of the electrode metal. Changes in the laser power and efficiency during the discharge are correlated with measurements of changes in the gas constitution monitored mass spectrometrically. A solid powder (definitely not carbon soot) which has a deleterious effect on the laser performance was observed. Following Bhaumik et al, mercury vapor was introduced into the gas mixture in a conventional DC excited Pyrex tube. An improvement of better than 40% in the power and efficiency as well as changes in the spectral distribution of a CO laser were observed.
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