Pavel Cerny

Highly efficient picosecond Raman generators based on the BaWO/sub 4/ crystal in the near infrared, visible, and ultraviolet

The stimulated Raman scattering process in a BaWO/sub 4/ crystal was employed to frequency downshift the first, second, and third harmonics of a Nd:YAG actively-passively mode-locked laser system. Single-pass, double-pass, and external cavity configurations were investigated for this purpose. In each experimental arrangement, the Stokes radiation properties were characterized regarding energy, beam profile, spectrum, and temporal development. The peak pump-to-first-Stokes conversion efficiency was measured to be 55% in the near infrared and 85% in the visible spectral region. The BaWO/sub 4/ picosecond Raman gain at a pump wavelength of 355 nm was measured to be 38 cm/GW, and a 15% conversion efficiency was achieved. A study of the Raman-cavity output beam profile development as a function of pumping energy was conducted. This work shows that the BaWO/sub 4/ crystal can be used in picosecond solid-state laser systems as an efficient frequency converter with a wide range of pump radiation wavelengths.

Efficient picosecond Raman lasers on BaWO4 and KGd(WO4)2 tungstate crystals emitting in 1.15 to 1.18um spectral region

Detailed study of Stimulated Raman scattering (SRS) of 1.06- micrometers , 50-ps pulses in BaWO4 and KGW crystals was performed. Single-pass, double-pass and Raman cavity arrangements were compared regarding threshold intensity, conversion efficiency and pulse structure. Single-pass conversion efficiencies reached 25 percent for both BaWO4 and KGW. Double passing of the pump beam slightly improved the efficiency to 35 percent. Placing the BaWO4 crystal into a cavity and optimizing the performance by varying the output coupler spectral characteristics and the resonator length, 55 percent pump-to-first-Stokes efficiency has been obtained. The maximum of the Raman laser output energy was 3 mJ. The output consisted of 2-4 picosecond pulses at the 1180 nm wavelength. Dependence of the BaWO4 Raman laser output beam structure on the pumping energy was studied. The concentric ring profile, typical for transient SRS, was homogenized using a cyclic olefin polymer coated silver hollow glass waveguide. A similar study was performed with KGW crystal, for which the first Stokes wavelengths are 1159 nm or 1177 nm, depending on the sample orientation. Due to lower gain value, Raman laser energy reached 2 mJ and maximum efficiency was 30 percent. Based on our comparative study, a new BaWO4 crystal is highly suitable for utilization in the near IR picosecond Raman lasers.

Delivery of high-energy radiation in midinfrared spectral region by hollow waveguides

Due to increasing number of requirements dealing with the application of a high energy mid-infrared radiation in various branches of medicine (cardiology, dentistry, dermatology, urology, gastroenterology), an enough flexible and lossless delivery system is required. For a transport of this high energy pulses in a mid-infrared region special cyclic olefin polymer-coated silver (COP/Ag) hollow glass waveguides were prepared and tested. A length of the waveguides was 0.5 m and inner diameter 1 mm. As a radiation source, an Er:YAG laser was used. The system generated the energy up to 2.16 J or 2.35 J (in dependence on a repetition rate used - 3 Hz or 4 Hz, respectively). The length of transmitted pulses was measured to be from 110 up to 550 usec in dependence on output energy used. The output radiation was coupled into the COP/Ag waveguide and a throughput and losses values were measured in dependence to input radiation parameters. The transmission obtained was 91%. The maximum delivered energy was dependent on a damage threshold of the waveguide. It was found that the damage threshold is dependent on the repetition rate which shows the dependences on the heat dissipated in the waveguide wall. The value of the damage was 1.7 J and 1.5 J for 3 Hz and 4 Hz repetition rate, respectively. The safe delivered power reached the value of 5 W. The characteristics obtained make this specially constructed COP/Ag hollow glass waveguide promising for the delivery of high-energy laser pulses in medicine and also in other applications.

Polymer-coated silver hollow glass waveguides for delivery of first and second harmonic Nd:YAG laser picosecond pulses

Cyclic olefin polymer-coated silver hollow glass waveguides with inner diameters of 540 micrometers , 700 micrometers , and 1000 micrometers were used for transmission of single pulses in picosecond region generated by a high-power Nd:YAG laser system (wavelengths 1.06 micrometers -first (fundamental) harmonic and 0.53 micrometers -second harmonic). Maximum first and second harmonic intensities delivered by 1000 micrometers inner diameter waveguide were 185 GW/cm2 and 48 GW/cm2, respectively. For fundamental radiation guided by the waveguide with the inner diameters 540 micrometers , and 700 micrometers the maximum measured transmitted intensities were 397 GW/cm2 and 331 GW/cm2, respectively. Temporal development with picosecond resolution, spatial profile, and divergence of delivered laser pulses were determined for each waveguide.

High-power laser radiation delivery

Special hollow glass waveguides were developed for delivery of high power infrared laser radiation. Maximum peak power up to 1.5 GW and mean power up to 5 W were delivered without any breakdown. For the medical application, the sealed version of the hollow glass waveguide was used.

Diode-pumped passively Q-switched nanosecond Raman laser on BaWO4 crystal converter

All-solid-state diode-pumped intracavity Raman laser was realized. The laser was based on three-mirror linear cavity with triangular Brewster-angle-cut Nd:YAG slab crystal pumped by 300 W quasi CW diode Q-switched by Cr4+:YAG saturable absorber operated at 1064 nm. The BaWO4 crystal (6×6×33 mm) was used as a Raman converter. After the Raman laser optimization for the first Stokes (1180 nm) the reproducible conversion efficiency was about 40%. The corresponding output energy and pulse duration were 1.46 mJ and 3.5 ns, respectively. The highest Raman laser output energy was reached 2.3 mJ (efficiency 55%). The second and third Stokes with first anti-Stokes lines were also detected at the laser output.

Picosecond stimulated Raman scattering in new crystals Nd:SrWO 4 and SrMoO 4

New Nd:SrWO4 and SrMoO4 crystals (45 mm and 33 mm of legnth, respectively) were investigated as Raman frequency converters of 50 picosecond Nd:YAG pulses and compared with already previously measured BaWO4 and KGd(WO4)2(KGW) tungstate crystals (33 mm and 40 mm of length, respectively). A forward SRS action was achieved in both new crystals. During the experiment the threshold energy of stimulated Raman process, generated wavelengths, and conversion efficiencies were measured. Single-pass first Stokes conversion efficiencies reached 25% and 21% for Nd:SrWO4 and SrMoO4 crystals, respectively. These values were compared with the maximal first Stokes efficiencies of previously studied crystals measured in the same experimental setup yielding: BaWO4 (η = 25% and KGW (η = 22%). As concerned the threshold - it was comparable for BaWO4, Nd:SrWO4, and SrMoO4. The SRS effect in KGW crystal started for ~25% higher pump energy. Our study shows that new Nd:SrWO4 and SrMoO4 crystals are other promising materials for picosecond Raman generation.

Picosecond stimulated raman scattering in BaWO4 crystal with close to quantum limit efficiency

Stimulated Raman scattering of 35-ps pulses in BaWO4 crystal was studied. The second harmonics of a Nd:YAG double-mode-locked laser system was used as a pumping radiation source. The first Stokes conversion efficiency reached 38% in singlepass and 85% in double-pass setup. The second Stokes was generated with 20% (singlepass) and 50% (double-pass) conversion efficiency. Temporal profiles measurement of pump and first Stokes radiation with picosecond resolution was performed. The obtained conversion efficiency data were compared with a plane-wave numerical model. BaWO4 crystal can be considered an efficient Raman-active material for utilization in picosecond solid- state laser systems.

Properties of transient and steady-state stimulated Raman scattering in KGd(WO4)2 and BaWO4 tungstate crystals

BaWO4 and KGd(WO4)2 (KGW) tungstate crystals (33 mm and 40 mm of length, respectively) were investigated as Raman frequency converters of picosecond and nanosecond second-harmonic Nd:YAG pulses. During the experiment the threshold energy of stimulated Raman (SR) process, generated wavelengths, and energy for nanosecond and picosecond pump pulses with both nonlinear crystals were measured. For BaWO4 crystal, the threshold pumping intensity was measured to be 530 MW/cm2 for psec pumping and 200 MW/cm2 for nsec temporal region. The corresponding Raman gain values were 14.3 cm/GW (picosecond pump) and 38 cm/GW (nanosecond gain). For KGW crystal the threshold intensity values 530 MW/cm2 for psec and 340 MW/cm2 for nsec were measured with corresponding Raman gain values of 11.8 cm/GW (psec) and 18.6 cm/GW (nsec). The Stokes components up to the third order in both psec and nsec regions were detected. Temporal length measurements of pump and Stokes pulses in both crystals revealed pulse shortening by a factor of approximately 2 during the SR process. Due to a high value of Raman gain of a new BaWO4 crystal under both nsec and psec pumping, this crystal can be considered as a unique candidate for utilization in solid-state Raman laser systems.

Mid-infrared lasers for transmyocardial laser revascularization

Transmyocardial laser revascularization was fount to be a new emerging technique for the treatment of end-stage coronary artery diseases. In our study, a comparison has been made of mid-IR solid-state laser radiation interaction with a myocardium tissue. For this purpose, the Tm:YAG, CTH:YAG and Er:YAG laser systems were designed, constructed and used for the interaction experiments. The ablation coefficients were measured to be 3 mm/J for Tm:YAG; 0.03 mm/J for CTH:YAG, and 10.5 mm/J for Er:YAG interacting laser radiation. From the histological examination follows that the channels ablated by Er:YAG laser radiation are without any thermal damage of the surrounding tissue and with the minimal mechanical injury, therefore Er:YAG laser could be considered as a good candidate for the TMLR.