Diode-pumped chirped pulse amplification to the joule level

The POLARIS project aims at the development of an all-diode-pumped, high-peak-power femtosecond laser system reaching the petawatt level. The laser amplifiers are based on pumping Yb³⁺-doped fluoride phosphate glass at 940 nm. Recently, stable operation of the first three amplifiers was achieved. Pulses with a bandwidth of 12 nm, which supports 135 fs pulses with energies up to 1.25 J, were generated in the third amplifier with a 12-pass configuration. The amplifier was pumped by 150 collimated high-power laser diodes. Successive polarization maintenance is used for a compact, stable, and convenient adjustable setup. Thermally induced abberations of lowest order were compensated by adaptive mirrors. PACS 42.55.Xi; 42.60.By; 42.70.Hj     

Ultra-high precision mid-IR spectrometer I: Design and analysis of an optical fiber pumped difference-frequency generation source

We report the results of a systematic characterization of various optical fiber pumping schemes for difference-frequency sources employing periodically poled LiNbO₃. Theoretical and practical considerations to achieve performance improvements for ultra-sensitive trace gas detection applications are discussed. As a result, we report a new mechanically robust and miniaturized mixing module employing a hybrid fiber with a plano-convex lens, reducing the size of previous DFG configurations by 65%. We also show pump, signal, and idler beam cross-sections before and after passing through a 100-m effective pathlength astigmatic Herriott multi-pass cell. PACS 42.55Wd; 42.60By; 42.60Jf; 42.62.Fi; 42.79.Nv     

Protection of phosphate glass by BK7 glass film in ion-exchange processing

Surface corrosion of phosphate glass during ion-exchange processing for optical waveguide fabrication is studied, and a protection method by using BK7 glass film is proposed and demonstrated in this paper. The protection coating processing is described and optical microscope and atomic force microscope observations are presented. Characteristics of the planar waveguide were measured also to show the protection coating would not introduce influence to waveguide formation in the phosphate glass. Experimental results show that the method can serve an effective protection, and will improve the performance of rare-earth-doped active phosphate glass optical waveguides.     

Generation of custom modes in a Nd:YAG laser with a semipassive bimorph adaptive mirror

Custom modes at a wavelength of 1064 nm were generated with a deformable mirror. The required surface deformations of the adaptive mirror were calculated with the Collins integral written in a matrix formalism. The appropriate size and shape of the actuators as well as the needed stroke were determined to ensure that the surface of the controllable mirror matches the phase front of the custom modes. A semipassive bimorph adaptive mirror with five concentric ring-shaped actuators and one defocus actuator was manufactured and characterised. The surface deformation was modelled with the response functions of the adaptive mirror in terms of an expansion with Zernike polynomials. In the experiments the Nd:YAG laser crystal was quasi-CW pumped to avoid thermally induced distortions of the phase front. The adaptive mirror allows to switch between a super-Gaussian mode, a doughnut mode, a Hermite-Gaussian fundamental beam, multi-mode operation or no oscillation in real time during laser operation. PACS 42.60.Jf; 42.60.Da; 42.60.By     

A new laser scheme for energy scaling with a composite six-mirror cavity

We have developed a novel and compact six-mirror cavity configuration to combine four multimode laser modules intracavity for energy scaling. Compared to the conventional 2×N channel scaling schemes, this close-ended six-mirror cavity is simpler, more stable and more practical. Diode-side-pumped rod laser geometry that is appropriate for beam combination is employed in the branch laser module. The characteristics of the output energy and the beam quality are investigated experimentally. With the six-mirror cavity, the output brightness is significantly improved compared to single F–P cavity laser. A single beam output exceeding 453 mJ with 165 μs duration is achieved, with the combination efficiency of 90.7%. PACS 42.60.Da; 42.60.Jf; 42.60.By     

1 J,1 Hz lamp-pumped high-gain Nd:phosphate glass laser amplifier

We present a lamp-pumped Nd: phosphate glass laser amplifier delivering up to 1 J of pulse energy at 1053 nm with a repetition rate of 1 Hz and an injected pulse energy of 2.5 mJ. The amplifier system employs a beamshaping module and a four-pass, lamp-pumped amplifier. The thermally induced wavefront distortion is mitigated and a uniform gain distribution is obtained by a four-lamp-pumped laser head in the amplifier. Thus,an excellent beam quality is obtained.     

CdSe nanocrystals in novel phosphate glass matrix

The CdSe nanoparticles have been prepared in the novel glass matrix P₂O₅–Na₂O–ZnO–Li₂O. The prepared nanoparticles and glass matrix are characterized by differential thermal analysis, X-rays diffraction, UV–vis optical absorption, and infrared spectroscopy. X-rays diffraction and optical absorption show that both of the annealing temperature and time play an important role in nanoparticles’ growth. The well-annealed and high-temperature-annealed samples suggest Zn atoms to substitute Cd atoms to form an additional ZnSe–CdSe system. Infrared spectroscopy confirms the decrease of the Zn-content in the host glass due to growing of nanoparticles, causing long phosphate chains within.     

Radiophotoluminescence properties of Ag-doped phosphate glasses

The radiophotoluminescence (RPL) of Ag-doped phosphate glass has been used for personal dosimetry for a long time. In this study, we investigated the optical properties of Ag-doped various phosphate glasses. It is revealed that alkali metals in the phosphate are required to achieve the RPL, and that the alkali metals content has an influence on the RPL properties. Also, the ESR signal corresponding to the valence change of Ag was observed only for the phosphate glasses containing alkali metals. These results indicate that the RPL occurs based on the change in Ag valence state, which is consistent with commercial phosphate glasses used for dosimetry. In addition, the decay-time constants of the RPL were on the order of microseconds. Furthermore, initialization of the RPL was observed after heating at 473 K, which indicates that these glasses can be used repeatedly.     

Self-mixing laser Doppler velocimetry with a dual-polarization Yb:Er glass laser

Self-mixing laser Doppler velocimetry using a dual-polarization Yb:Er-doped phosphate glass laser is investigated. The two orthogonally polarized eigenstates emitted simultaneously by the laser oscillator are employed to perform a heterodyne detection, allowing a complete characterization of the lateral sidebands generated via the optical feedback around the beating peak. Two different experimental set-ups have been implemented: (i) direct feedback on one of the two modes or (ii) crosstalk between the two modes via a non-reciprocal Faraday element. The results are analysed and applications to velocity measurements are discussed. Finally, some considerations of the noise correlated in phase and in opposite phase on the two polarization states are also discussed. PACS 42.55.Xi; 42.60.Rn; 42.81.Pa     

Experimental investigation of enhancing the subsurface damage threshold of Nd-doped phosphate glass

Experimental investigation was performed with a 1064-nm, 10-ns Nd:YAG laser to determine the effects of the surface hydrogen acid etching on laser damage, compared with damage of conventionally polished surface. The investigation was helpful for us to understand the negative effects of Nd-doped phosphate glass surface and subsurface damage (SSD) on laser induced damage threshold (LIDT). A set of samples was polished, and then chemically etched in a cool buffered 10%HF 20%H2SO4 solution at different times. Another set of samples was ground and etched in the hot-buffered solution, and then polished. All the samples were irradiated with Nd: YAG laser and characterized by optical microscopy. Results of LIDT were obtained according to International standard ISO/DIS 11254-1.2. Chemical treatment can remove the contaminants in the polished re-deposition layer and the SSD for improving the laser damage resistance of Nd-doped phosphate glass surfaces. The method of using hot solution was more effective than that ofusing cool solution.     

Fluence homogenization of a 240 J-diode-laser pump system for a multi-pass solid state laser amplifier

Within the POLARIS project multi-pass amplification to the 10 joule level for a femtosecond CPA laser system has been achieved by diode-pumping an Yb³⁺ doped fluoride-phosphate glass disk with a total energy of 240 J. The collimated light of 40 diode stacks is focussed onto a circular area of the glass disk with a diameter of 18 mm. A two-sided ring shaped setup is used for axially symmetric pumping. We developed a computer aided optimization routine for the positioning of single pump foci to gain a smooth homogeneously distributed, top-hat shaped pump profile. The homogeneity of the pump light distribution was improved substantially by this new method.PACS 42.55.Xi; 42.60.By; 42.60.Jf     

Electron paramagnetic resonance and optical absorption spectra of Cr ions in cadmium phosphate glass

EPR and optical absorption investigations of chromium doped cadmium phosphate glass are carried out. Crystal field, spin-Hamiltonian and bonding parameters are evaluated. From the results and analyses of the EPR and optical studies, the site symmetry of Cr³⁺ ion in the glass is ascribed to a distorted octahedron. The bonding parameters suggest the ionic nature of Cr³⁺ ion with the ligands.     

On the design of electrically pumped vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs) yield an excellent beam quality in conjunction with a scalable output power. This paper presents a detailed numerical analysis of electrically pumped VECSEL (EP-VECSEL) structures. Electrical pumping is a key element for compact laser devices. We consider the optical loss, current confinement, and device resistance. The main focus of our investigation is on the achievement of an adequate radial carrier distribution for fundamental transverse mode operation. It will be shown that a trade off between the conflicting optical and electrical optimization has to be found and we derive an optimized design resulting in guidelines for the design of EP-VECSELs which are compatible with passive mode locking. PACS 42.55.Px; 42.60.By; 85.30.De     

Influence of self-focusing effect on femtosecond collinear nondegenerate optical Kerr measurements

We performed the femtosecond nondegenerate optical Kerr shutter (OKS) measurements in a 10-mm-thick optical glass. The time-resolved OKS signals showed an obvious asymmetry and a large broadening with respect to that for thin samples. The time-resolved OKS signals in samples with different thicknesses showed that, the temporal broadening of the OKS signals was attributed to the group velocity mismatch between pump and probe beams of different wavelengths. By imaging the filament induced inside the glass, we attributed the asymmetry of the time-resolved OKS signals to the focusing position change of pump beam caused by self-focusing and re-focusing effect.     

Experimental investigation on laser-induced surface damage threshold of Nd-doped phosphate glass

Nd-doped phosphate glass is the dominant amplifier material used in solid state high average power laser systems.Surface imperfection and subsurface damage(SSD)of the glass,resulting from the optical fabrica-tion process,limit the increment of laser system energy output.Thus,it is important to enhance the surface damage threshold of Nd-doped phosphate glass surface.The influence of abrasive size,polishing powder,grinding mode,and chemical treatment on the laser-induced damage threshold(LIDT)of Nd-doped phos-phate glass surface is investigated.Results show that the LIDT is affected little by different polishing powders and grinding modes.The LIDT correlates with the abrasive size,which produced different depths of SSD.A suitable acid etching treatment can remove the imperfection and the SSD for improving the LIDT of Nd-doped phosphate glass surface.The combination of several effective techniques and methods,which are low-cost and practical,should be useful to enhance the LIDT of Nd-doped phosphate glass surface.     

Optical properties in Ag+-doped phosphate glass irradiated with X-rays and α-particles

The objective of this study is to investigate the emission mechanism of radiophotoluminescence (RPL) in the Ag⁺-doped phosphate glass (glass dosimeter), which is now used as individual radiation dosimeter, because the emission mechanism of RPL in glass dosimeter has been not fully understood. We have investigated the assignments and characteristics of the X-ray induced colour centres in the Ag⁺-doped phosphate glass up to now (Miyamoto et al., 2010). Optical properties such as optical absorption spectra related with X-ray and α-particles irradiation were measured for commercially available glass dosimeter.In this study optical properties such as optical absorption spectrum as a function of X-rays and α-particles irradiation were measured for commercially available glass dosimeter. Comparison of the RPL in Ag⁺-doped phosphate glass irradiated with X-rays and α-particles is discussed.     

UV-visible and infrared absorption spectra of gamma irradiated CuO-doped lithium phosphate, lead phosphate and zinc phosphate glasses: A comparative study

Undoped and CuO-doped lithium phosphate, lead phosphate and zinc phosphate glasses were prepared. UV-visible and infrared absorption spectra of the prepared samples were measured before and after successive gamma irradiation. Experimental optical spectra of the undoped samples reveal strong UV absorption bands, which are attributed to the presence of trace iron impurities in both the lithium and zinc phosphate glasses while the lead phosphate glass exhibits broad UV bands due to combined absorption of trace iron impurities and divalent lead ions. The CuO-doped glasses reveal an extra broad visible band due to Cu²⁺ ions in octahedral coordination. The effects of gamma irradiation have been analyzed for both the sharing of all constituent components including trace iron impurities. Infrared absorption spectra of the prepared samples were investigated by the KBr disk technique. The FTIR spectra reveal main characteristic absorption bands due to different phosphate groups. The IR spectra are observed to be slightly affected by the increase of CuO in the doping level (0.2-3%) indicating the stability of the main network units.     

Near-diffraction-limited segmented broad area diode laser based on off-axis spectral beam combining

The beam quality of a 500-μm-wide broad area diode laser with five active segments has been improved beyond the beam quality of the individual segments. The principle of this new laser system is based on off-axis feedback in combination with spectral beam combining. By using a double-feedback scheme we are able to improve the beam quality of the laser by a factor of 23 from M²=55 for the free-running diode laser to M²=2.4 for the laser with feedback at a drive current of 2.2 A. The improved M² value is a factor of 3.4 below M²=8.2 for a single free-running segment. This is the first time that the beam quality of a segmented broad area diode laser has been improved beyond the beam quality of the individual segments. PACS 42.55.Px; 42.60.Da; 42.60.Jf     

Nd-doped phosphate glass microstructured optical fiber laser

We experimentally demonstrated a single-mode laser at 1056 nm with Nd-doped phosphate glass microstructured optical fiber (MOF), which was fabricated with conventional stack-and-draw method. The laser action was observed from a Fabry-Perot cavity formed by placing two dichroic mirrors of ∼100 and 85% reflectivity, to the two end facets of MOF. Pumped by CW laser diodes (LDs) at 808 nm, the MOF laser yielded a maximum output power of 8.5 mW and a slope efficiency of 2%.     

Optical spectroscopic investigations on silver doped sodium phosphate glass

This research investigates the optical spectroscopic characteristics of silver-doped phosphate antibacterial glasses with chemical compositions 42P₂O₅–42ZnO–(16?x) Na₂O, (where x = 0, 0.4, 0.8, 1.2, 1.6 Ag₂O). The glass samples were prepared by conventional melting quenching techniques. The structures of all the homogenous prepared glasses were studied by XRD, and UV–Vis spectroscopy. The glass formability was tested and the amorphous nature was approved using XRD-technique. Archimedes method has been employed to measure the density of the samples hence, the molar volume was calculated. The molar volume and density shows discontinuity of measurements under effect of increasing Ag₂O concentration. The optical spectroscopic analyses for the obtained glass samples has been investigated over the whole range (190–2500 nm) for studying the effect of bandpass absorption glass filter, its color peak center and UV cut-off. Transmittance of some glass samples showed cut-off for UV and short visible wavelengths in some glass samples, so these samples composition can be considered as long-pass edge filters and from band stop ranges started from 190 to 515 nm and increase by increasing doped silver concentration. The optical energy gap decreases by increasing the Ag₂O concentration from 4.43 to 3.61 eV. The refractive index and extinction coefficient and some optical properties are studied and the results indicate clearly that there is no high remarkable change with changing wavelength. The refractive index is found to be increased by increasing the Ag₂O content. The wavelength dependence of extinction coefficient and the dielectric constants exhibit higher values for higher Ag₂O contents.