Revital Feldman

Passive Q-switching in Nd:YAG/Cr4+:YAG monolithic microchip laser

Abstract Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting between Nd:YAG and Cr,Ca:YAG crystal wafers followed by prolonged heating at elevated temperatures. Heating of the composite devices under reducing and/or oxidizing environments allowed to control the Cr4+ ion concentration in the Cr,Ca:YAG, thus affecting its absorption saturation behavior. Optical absorption saturation measurements on partially reduced Cr,Ca:YAG crystal were performed. Residual absorption of the saturable absorber at 1064 run results from the Cr4+ ion excited-state absorption. Laser damage threshold at the gain/absorber interface of the composite device, 14.7 J/cm2, is higher than at the entrance face. The device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited short (∼5 ns), high repetition-rate pulsing.

Depolarization induced by pump edge effects in high average power laser rods

Non-radially symmetric residual birefringence in laser rods due to pump edge effects is analyzed both theoretically and experimentally. For cubic crystals such as yttrium aluminum garnet (YAG), this depolarization has a unique sixfold symmetry that is closely connected to the crystals cubic symmetry. While this depolarization is small compared to that observed with linear or circular polarizations, it is measurable and important when using radial or azimuthal polarizations in rods generating heat powers in excess of 70 W/cm. A simple criterion was defined in order to help estimate the amount of depolarization in a high-power laser rod system.

Single-mode 230 W output power 1018 nm fiber laser and ASE competition suppression

We present a high-power single-mode (SM) monolithic fiber laser centered 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 taking into account the various wavelength-dependent parameters, such as the doped fiber absorption, emission, saturation effects, and the cavity mirrors’ reflection, in addition to the fiber geometrical parameters. Parameters that were found to be most critical for suppressing the amplified spontaneous emission at higher wavelengths were the fiber length and the extinction ratio of the fiber Bragg grating reflectivity between 1018 nm and above 1030 nm.

Single mode 1018nm fiber laser with power of 230W

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.

Effect of intracavity diffusion-bonded optical elements on laser performance

Diffusion-bonded surfaces inside a cavity usually have little effect on a laser beam when oriented perpendicular to the beam direction. They may have a severe effect on lasing efficiency and mode structure when oriented parallel to the beam. Characterization of the interface between diffusion bonded elements and the understanding of its effect on lasing is, Therefore, important. Isolating the effect of the bonding interface from effects such as an index difference between the bulks, we used bonded BK7 slabs as the bonded element. We measured the reflections arising from the bonding interface, and then put the slabs as a passive element inside a hemispherical resonator. It was found that even for similar materials, with no refraction index difference, significant reflections occur at the bonding interface for oblique angles of incidence. When put intracavity parallel to beam axis the bonded slabs caused a significant power loss and forced higher order lasing modes. The power loss depended on both, the transmission through the interface and the size of the interface cross section relative to beam diameter.

Chromium ion valence transformations in Cr, Ca:YAG used for passive Q-switching

Composite laser devices of passively Q-switched Nd:YAG were prepared by optical contacting of a Ca,Cr:YAG crystal onto a Nd:YAG wafer, followed by thermal treatments at elevated temperatures. Thermal treatments of the composite devices under reducing and/or oxidizing environments enabled to control the Cr4+ ion concentration, which affects its absorption saturation behavior. Optical absorption saturation measurements on partialy reduced Ca,Cr:YAG crystal revealed that residual absorption of the saturable absorber at 1064 nm results from excited-state absorption. Laser damage measurements at the gain/absorber interface of the monolithic device showed that the damage threshold at the gain/absorber interface of the composite device is higher than at the entrance interface. The high optical quality device thus obtained was end-pumped by a fiber-optic-coupled diode laser, and exhibited a high repetition-rate oscillations of short pulses, with fair beam quality.

Use of polycrystalline Nd:YAG rods to achieve pure radially or azimuthally polarized beams from high-average-power lasers.

We report maintenance of perfect radial-polarization purity in high-power, cw pump chambers using strengthened, polycrystalline Nd:YAG laser rods. Although the cubic symmetry of single-crystal rods caused threefold symmetric birefringence due to shear stresses at the ends of the pumped zone, polycrystalline rods macroscopically behaved as isotropic material and enabled polarization preservation. Elimination of this source of depolarization prevents the major source of bifocusing aberrations in a chain of amplifiers.

Maintaining radial-polarization and beam-quality in muli-kW rod-based lasers through the use of polycrystalline Nd:YAG rods

MOPA configurations rely on initial beam parameters set by the oscillator (beam quality, polarization) that must be preserved during amplification. The main factors that degrade beam quality in rod based lasers are: birefringence induced bifocusing, thermal spherical aberrations, and pump-nonuniformity induced azimuthal aberrations. Thermally induced bifocusing was totally bypassed using cylindrically (radially or azimuthally) polarized beams [1]. Cylindrical polarizations are, however space variant, so attention must be paid to factors that degrade such polarizations. Main factors are: a non-concentrically aligned beam and pumped rods axes; and non-radially symmetric optical perturbations, such as azimuthal aberrations, non-radially symmetric absorption/amplification, and non-radially symmetric birefringence. Our STAR pump chambers were designed to side-pump rods while producing perfect radially-symmetric pump distributions and demonstrated low azimuthal aberrations. Spherical aberrations were corrected for each STAR by using specially designed wave-plates produced by Asphericon. Maximum optical pump power per STAR was 7kW (at 806nm) and short-cavity output-power was 3.1kW. Figure 1 presents the measured wave-front deformations in radially-polarized beam after single passes through each pump chamber that we tested, and the beam quality degradation calculated based on the measured WFs. The total WF distortion accumulated from all of the amplifiers and the predicted beam quality degradation appears on the right. The remaining azimuthal aberrations resulted from variations between diode arrays can be corrected using a single free-form phase-plate.

Six-fold residual birefringence in radially-polarized high-power Nd:YAG laser rods

Radial polarization is used in high-power solid-state lasers in order to overcome the thermally induced bifocusing that is the primary optical aberration when using the rod geometry [1]. Maintaining radial polarization purity is crucial for beam-quality preservation in high-power amplifiers. Working at a very high heat load of 1500W (100W/cm) in a Nd:YAG rod-based laser amplifier with radially-symmetric pumping, a six-fold deviation from radial symmetry was measured in the polarization of a probe beam after propagation through the amplifier (fig. 1(a)). Such deviation was found to originate in the laser rod itself, and was not directly connected to pump profile or the probe beam, which were measured to be radially symmetric to a high degree. The YAG rod axis was grown to be parallel to the [111] cubic crystal axis. In such growth geometry, the cubic unit-cells projection on the cross section of the rod is in the shape of a hexagon. Therefore, a six-fold pattern is intrinsic in this geometry.