We demonstrate record high energy of 2 mJ, with four nanosecond pulses a peak power of ⪆420 kW and average power of 660 W, in a fiber amplifier using a novel 26 μm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1kW. This is achieved at a diffraction limited beam quality of M2=1.14.
The importance of average power scaling of fiber lasers (FL) is well known. However, power scaling is strongly limited by factors such as thermal load, and non-linear effects. An alternative path for reaching high powers utilizes the stimulated Raman scattering mechanism, and harnesses its power and brightness enhancement potential to reach high average power, high brightness FL. kW scale Raman FLs have been demonstrated, however they are in core-pumping configurations, meaning that they require an a-priori existing brighter kW laser that acts as their pump modules. There have been only a few publications of Raman FLs where the generated signal has a higher brightness than the pump source at levels of ≥100W, the highest, being at 250W. Here we report a strictly all-fiber clad pumped Raman FL with a CW power output of 800 W with a conversion efficiency of 80%. To the best of our knowledge this is the highest power and highest efficiency Raman FL demonstrated in any configuration allowing brightness enhancement (i.e clad pumped or graded index fiber, excluding step-index core pumped), thus being the first kW-class Raman FL with brightness enhancement. This result was achieved with a specially designed triple-clad fiber (TCF). The core was 25 μm, 0.065 NA, and the inner cladding was 45 μm 0.22NA. The choice of the small inner clad allows obtaining sufficient Raman gain without requiring too long a fiber, as well as being compatible with the waist size of the pump source fiber. In addition this diameter complies with the inner-clad/core ratio which prevents generation of a 2nd Stokes laser beam. Two fiber Bragg gratings at 1120 nm written onto the TCF, were employed as the oscillator’s reflectors. The cavity was pumped by a lower beam-quality source with an M2 of ~8 at 1070 nm. The Raman signal generated in the core, at the first Stokes wavelength of 1120 nm, showed an improved beam-quality in relation to the pump.
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.
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.
We report on remote measurements of cross-wind and atmospheric turbulence, using a one-station scheme. As most
remote wind-sensing methods, our method is based on observing the drift of the scintillation pattern across the line of
sight. The scintillations are caused by naturally-occurring turbulence-induced refractive index irregularities in the
atmosphere, which drift at wind speed. Analyzing spatial-temporal cross-correlation function of the signals of two
elements in the array, it is possible to obtain the cross-wind speed. We use the zero-crossings technique for measuring
the cross-wind value, while the cross-wind direction is determined by comparing areas from both sides of the peak of the
cross-correlation function. Here we present results obtained using these techniques in comparison to independent
measurements of the anemometers. The experiments were performed along a uniform path over a flat beach parallel to
the Mediterranean Sea shore. Four white-screen diffusive targets were placed at distances of 300, 600, 850 and 1200m.
Five anemometers were placed along the laser beam path, one near each target and at the measurement station. Each
target was illuminated with a beam from a glass fiber pulsed infrared laser with a repetition rate of several thousand Hz,
and a sub-microsecond pulse-length, and output beam divergence of ~300 μrad. The receiver has an entrance aperture of
80mm, and the incoming radiation is focused onto an array of four 50×250um InGaAs detectors by a lens with
f=500mm. The results show good agreement. From the fluctuations of the signal on the detector array, our system also
measures the turbulence structure parameter Cn
2, using the angle-of arrival technique. The obtained results show
reasonable agreement with independent scintillometer measurements of Cn2, performed with a CW He-Ne laser in a
two-station setup with a detector at a distance of 60m from the laser.
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.
We have demonstrated the feasibility of cooling high power solid-state lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than sapphire's, the material conventionally used for this purpose. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. An output power of 200Watt was achieved from a single Nd:YVO4 slab in a zigzag configuration when pumped with 600Watt diodes at 808nm. The maximum output power previously reported in the literature with Nd:YVO4 using conventional cooling schemes is only about 100W. A 2.3x4x24mm3 slab was pumped from its broad side (4x24 mm2) through a 0.3mm thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. The output of a two-head configuration was 295Watt. By using a RTP Q-switch, 124Watt average power was obtained at 15kHz with a pulse width of 17nsec, pumping at 650Watt.
An additional larger head was developed to pump a Nd:YAG slab. The concept of the pumping and cooling is identical to the Nd:YVO4 laser head. An output power of 1000Watt was achieved from a single Nd:YAG slab when pumped with 2500Watt diodes at 808nm. The slab dimensions are 3×12×90mm3.
We have demonstrated the feasibility of cooling high power solid-stae lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than sapphire's. An output power of 200Watt was achieved froma single Nd:YVO4 slab in a zigzag configuration when pumped with 600Watt diodes at 808nm. The maximum output power reported in the literature with conventional cooling schemes is about 50W. A 2.3x4x24mm3 slab was pumped from its broad side (4x24 mm2) through a 0.3mm thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. By using a BBO Q-switch, 70Watt average power was obtained at 20kHz with a pulse width of 19msec and with a beam quality of 3 and 12 times diffraction limit in the zigzag and transverse directions respectively. The output of a two-head configuration was 295Watt.
We show a compact multi-pass amplifier, based on a single dual-rod laser-head whihc produces ultra-high gain. A double-pass produced a maximum small signal gain of 4x108. Another pass was permitted by including a specially designed Brillouin phase conjugate mirror (PCM). This enabled a total gain of 7.7x1010, which raised an input signal of 10pJ to 770mJ output signal. To the best of our knowledge this is the highest gain reported to date from any type of laser amplifier scheme. The amplification system is fairly simple in that it consists of only one dual-rod laser head and hence only a single power supply. We show that this system can be utilized for producing high-energy long temporally-smooth narrow linewidth pulses, as well as high power controllable, temporally-modulated pulses.
The angular field of view (FOV) of a Brillouin amplifier is shown to be dependent upon the spectral bandwidth of the interacting beams. Theory and experiment reveal that for single longitudinal mode pulses of 8.5 nsec duration interacting in CS2 the FOV is equal to +/- 18 degree(s) in a counterpropagating geometry. Proper design of the amplifier is required however, in order to take advantage of the full FOV afforded by the Brillouin interaction.
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