• S M Oak

Articles written in Pramana – Journal of Physics

• Dual wavelength operation in diode-end-pumped hybrid vanadate laser

Dual wavelength operation at 1062.8 nm and 1064.1 nm in a diode-pumped hybrid laser comprising of Nd3+-doped birefringent YVO4 and GdVO4 crystals is demon-strated. A detailed characterization of the laser is performed under CW and pulsed operation. Under Q-switching, 4 W of average power at 5 kHz repetition rate is obtained with 32 ns FWHM pulse duration corresponding to 25 kW of the peak power. The intensity and the polarization of the individual spectral components can be easily controlled by changing the relative gain and the relative orientation of the two crystals. The resulting pulsed dual-wavelength laser has the potential to be used as a source for generating terahertz radiation.

• 101 W of average green beam from diode-side-pumped Nd:YAG/LBO-based system in a relay imaged cavity

Studies on intracavity frequency doubling of acousto-optically Q-switched Nd:YAG rod laser using 18 mm long type-II phase-matched LBO crystal in a relay-imaged cavity is reported. A single pump head comprised of Nd:YAG rod, diffusive reflectors and linear array laser diode bars is used. 101 W of average green power at a total diode pumping power of 700 W is obtained corresponding to 14.4% optical-to-optical conversion efficiency. The pulse repetition rate is 30 kHz with an individual pulse duration of 200 ns.

• Efficient and high-power green beam generation by frequency doubling of acousto-optic Q-switched diode-side pumped Nd:YAG rod laser in a coupled cavity

A 52-W green laser at 532 nm by extra-cavity second-harmonic generation in a coupled-cavity configuration is demonstrated. The fundamental laser is a diode-side-pumped acousto-optic (AO) Q-switched Nd:YAG rod laser producing 84 W of average power at 1064 nm at 8 kHz repetition rate. Type-II phase-matched polished KTP crystal is used as the nonlinear crystal for second-harmonic generation. The individual green pulse width is 50 ns and the fundamental to second harmonic conversion efficiency is 61.8%.

• Nonlinear optical studies in semiconductor-doped glasses under femtosecond pulse excitation

Nonlinear optical studies in semiconductor-doped glasses (SDGs) are performed under femtosecond laser pulse excitation. Z-scan experiments with 800 nm wave- length pulses are used to excite SDG samples in the resonance and non-resonance regimes. Schott colour glass filter OG 515 shows stronger two-photon absorption than GG 420 and both the samples exhibit positive nonlinearity. However, in resonantly excited RG 850 the intensity-dependent Z-scan shows transition from saturable to reverse saturable absorption behaviour with the increase in intensity.

• Development of underwater laser cutting technique for steel and zircaloy for nuclear applications

In nuclear field, underwater cutting and welding technique is required for post-irradiation examination, maintenance, decommissioning and to reduce storage space of irradiated materials like used zircaloy pressure tubes etc., of nuclear power plants. We have developed underwater cutting technique for 4.2 mm thick zircaloy pressure tubes and up to 6 mm thick steel using fibre-coupled 250 W average power pulsed Nd:YAG laser. This underwater cutting technique will be highly useful in various nuclear applications as well as in dismantling/repair of ship and pipe lines in water.

• Green laser photocoagulator for the treatment of diabetic retinopathy developed at Raja Ramanna Centre for Advanced Technology

An all-solid-state green laser photocoagulator at 532 nm with output power varying from 100 mW to 1 W in a step of 10 mW and exposure time varying from 50 ms to 1000 ms in a step of 10 ms is developed for the treatment of diabetic retinopathy. The output power stability is better than $\pm 1.5%$ with a nearly diffraction-limited beam quality. The system includes various safety and operational features like internal power monitoring system, safety interlock, emergency switch-off, graphical LCD display with table-top touch mode portable control panel, smart delivery device selection, aiming laser beam with controllable intensity, foot switch, patient records, service mode etc. The system has successfully passed the clinical trials and is being used on patients.

• Erbium–ytterbium fibre laser emitting more than 13W of power in 1.55 𝜇m region

We report the work on erbium:ytterbium-doped double clad fibre laser (EYDFL), that is pumped at 976 nm. The maximum output power generated is 13.6Win 1550 nm region with a slope efficiency of about 21%. To the best of our knowledge, this is the highest power reported from an EYDFL, that uses commercially available off-the-shelf large mode area Er:Yb-doped double-clad fibre.

• LOPUT Laser: A novel concept to realize single longitudinal mode laser

We propose a novel type of cavity design to generate single longitudinal mode laser known as LOPUT cavity. LOPUT cavity stands for linear orthogonally polarized modes resulting in unidirectional travelling wave cavity. The technique can be applied to both isotropic as well as anisotropic gain mediums. In the present paper, we applied the technique to anisotropic gain medium such as a-cut Nd:YVO4. Using the LOPUT cavity, we demonstrated nearly 2 W of single longitudinal mode laser with nearly diffraction-limited spatial profile. Linewidth measurement using a custom-made Fabry Perot interferometer revealed instrument-limited linewidth of ∼5 MHz at 1064 nm.

• Thermal birefringence-compensated linear intracavity frequency doubled Nd:YAG rod laser with 73 ns pulse duration and 160Wgreen output power

In a thermally birefringence-compensated linear cavity configuration, ∼160 W of average green power by intracavity frequency doubling of AO Q-switched Nd:YAG/LBO-based laser is demonstrated. The corresponding optical to optical conversion efficiency is estimated to be ∼12.7%. The pulse repetition rate is 20 kHz with the individual pulse duration of 73 ns. The beam quality parameter is measured to be 18.

• Efficient yellow beam generation by intracavity sum frequency mixing in DPSS Nd:YVO4 laser

We present our studies on dual wavelength operation using a single Nd:YVO4 crystal and its intracavity sum frequency generation by considering the influence of the thermal lensing effect on the performance of the laser. A KTP crystal cut for type-II phase matching was used for intracavity sum frequency generation in the cavity at an appropriate location for efficient and stable yellow output power. More than 550 mW of stable CW yellow-orange beam at 593.5 nm with beam quality parameter ($M^2$) ∼ 4.3 was obtained. The total pump to yellow beam conversion efficiency was estimated to be 3.83%.

• Efficient delivery of 60 J pulse energy of long pulse Nd:YAG laser through 200 𝜇m core diameter optical fibre

Most of today’s industrial Nd:YAG lasers use fibre-optic beam delivery. In such lasers, fibre core diameter is an important consideration in deploying a beam delivery system. Using a smaller core diameter fibre allows higher irradiances at focus position, less degradation of beam quality, and a larger stand-off distance. In this work, we have put efforts to efficiently deliver the laser output of ‘ceramic reflector’-based long pulse Nd:YAG laser through a 200 𝜇m core diameter optical fibre and successfully delivered up to 60 J of pulse energy with 90% transmission efficiency, using a GRADIUM (axial gradient) plano-convex lens to sharply focus down the beam on the end face of the optical fibre and fibre end faces have been cleaved to achieve higher surface damage thresholds.

• Laser shock peening of steam turbine blade for enhanced service life

Fretting-fatigue is an important factor influencing service life of turbine blades. The present paper describes laser shock peening of potential crack nucleation site in the root region of steam turbine blade for its enhanced service life. The experimental study, performed with an in-house developed 2.5 J/7 ns Nd:YAG laser demonstrated that laser peening introduced a residual surface compressive stress of −260 to −390 MPa. Case depth of laser peened surface layer was found to be more than 900 𝜇m.

• Coherent oscillations of holes in GaAs0.86P0.14/Al0.7Ga0.3As surface quantum well

We show that in a GaAs0.86P0.14/Al0.7Ga0.3As near-surface quantum well, there is coherent oscillation of holes observed in time-resolved reflectivity signal when the top barrier of the quantum well is sufficiently thin. The quantum well states interact with the surface states under the influence of the surface electric field. The time period of the observed oscillation is 120±10 fs.

• CO2 laser-inscribed low-cost, shortest-period long-period fibre grating in B–Ge co-doped fibre for high-sensitivity strain measurement

We have developed high sensitivity long-period fibre gratings (LPGs) in B–Ge codoped fibre for strain sensing application. These LPGs are shortest grating period (180 𝜇m) LPGs inscribed in B–Ge co-doped fibre using CO2 laser-based grating inscription set-up. Strain sensitivity of 1.77 dB/mε has been obtained for attenuation band corresponding to the turnaround point mode. TAP operation of LPG facilitates intensity-based detection using simple optical power meter instead of wavelength-based detection.

• Linearly polarized intracavity passive Q-switched Yb-doped photonic crystal fibre laser

In this paper we report linearly polarized high average power passive Q-switched ytterbium-doped photonic crystal fibre laser with a Cr4+:YAG crystal as a saturable absorber. An average output power of 9.4 W with pulse duration of 64 ns and pulse repetition rate of 57.4 kHz with a slope efficiency of 52% was achieved. Measured polarization extinction ratio (PER) of the Q-switched laser output was 10.5 dB.

• # Pramana – Journal of Physics

Volume 96, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019