High Energy Lasers Activities at RRCAT PDF

Rajesh Kumar Patidar Email: [email protected] Lasing threshold – when gain (no. photons emitted in round trip) exceeds loss (number lost to absorption, through mirrors etc.). 2  Active medium or gain m...

Rajesh Kumar Patidar Email: [email protected] Lasing threshold – when gain (no. photons emitted in round trip) exceeds loss (number lost to absorption, through mirrors etc.). 2  Active medium or gain medium Laser output Pumping Essential elements: 1. Lasing (Active) medium - a collection of atoms, molecules, etc. , provides lasing transition, can be in Solid, liquid or gas 2. Pump source - puts energy into the laser medium for achieving population inversion – examples: another light source such as flashlamp or laser, electrical discharge, current, or chemical excitation etc. 3. Resonator cavity (Optical feedback )- provides a mechanism for the light to interact (possibly many times) with the laser medium → Stimulated emission 3 Why LASER radiation is so special Because of following properties Ø Highly directional → Low divergence and good focussibility) Ø High power or high intensity (CW vs pulsed) Ø Coherent, both in space as well as time Ø Can be highly monochromatic Ø CW lasers: Energy is continuously pumped- producing a continuous laser output. Ø Pulsed lasers- The output is in pulsed mode like flashes of light How to produce laser pulses 1. Free running: Depends on pump duration 2. Q-switching: nanosecond pulses 3. Mode-Locking : picosecond to femtoseconds pulses 4  Solid state lasers Widest class of laser systems, Most common now a days Lasing ion doped in crystalline host - Nd:YAG, Ti:sapphire Ion in glass -Nd:glass Fibre lasers – Er, Yb doped Semiconductor diode lasers Liquid lasers Solution of complex organic dyes, Rhodamine 6G (Wide tunability) Gas lasers: Usually electrically pumped He-Ne, CO2, Nitrogen, Excimer Wide range of wavelengths, uv (Excimer laser) to infrared (CO2) Low gain 6  High energy Nd: glass laser activity: MOPA based Old chain: 100J/0.5 ns to 1 ns, 1053 nm New archirecture based 150J laser Under development Final Target: Two beams of 500 J/3 ns High peak power ultrashort pulse activity: Ti: Sapphire , 800 nm, broadband gain medium PW laser: 25J /25 fs 150 TW, 3.75J, 25 fs High repetetion rate KHz fs laser system (mJ energy) Advanced Laser Development & Applications High power flashlamp pumped Nd: YAG laser for industrial application Diode pumped Nd: YAG laser Fiber Lasers LIGO activity Ultrafast spectroscopy KHz fs laser system (mJ energy), OPA baser tunable laser Pump probe spectroscopy at fs time scale Semiconductor laser development Diode lasers, Detectors Example: 808 nm diode for Nd: yag laser pumping Copper Vapor Laser Wavelengths: 510 nm (green) and 578 nm (yellow). Average laser power: 20 W to 100 W , Pulse repetition rates of 5 to 20 kHz Pulse width: ~ 40 ns. These laser are used to pump dye lasers for AVLIS Now a days frequecy double Nd: YAG lasers are used for this application CO2 lasers Wavelength: 10.6 um and 9.6 um Industrial applications, paper industry Laser applications based activities Laser additive manufacturing Fiber Bragg Grating: sensor development Biomedical applications Cold atom physics- Trapping of atoms , Gravimeter  Inertial Confinement Fusion (ICF) Laboratory Geophysics & Astrophysics Generation of intense thermal X-rays Nuclear substance simulation 12 Ø Size: nearly equal to three football fields; 192 beam Ø On, Dec 5, 2022 2.05 megajoules was fired at a target which resulted in the release of 3.15 megajoules (1.54 X gain): First demonstration of gain Ø On Feb 2024, NIF laser delivered 2.2 MJ of energy on the target and produced 5.2 MJ of fusion energy which is the highest fusion yield achieved till now. 13  What do we need? Maximum intensity on target Pulse Needed for the experiment energy E Ipeak = Increase the energy (E), Aτ Decrease the area of the focus (A). Decrease the duration (t), Beam Pulse area duration Maximum average power at the detector Needed to get useful results Pave = E r Signal is proportional to the number of Pulse Rep rate photons on the detector per energy integration time. 14 Ø Damage threshold of optical components Solution: Increase the beam size Ø No control over spatial and temporal quality of the laser, Multimode output in large aperture Ø Issues related with Amplified Spontaneous Emission (ASE) and parasitic lasing MOPA scheme provides scaling up of laser energy and peak power with better control over the laser parameters without optical damages. 15 Master Oscillator: Provides seed pulse of desired quality (Temporal shape and width, Wavelength, spectral bandwidth, spatial distribution) Power Amplifiers: Amplify the seed pulse to desired energy level 16  Nd: glass Laser Chain NEW project MOPA based laser system Energy : 500 J/ beam Energy : 100 J Pulse duration : 3 ns Pulse duration : 0.5 – 1.5 ns Multipass architecture, Fiber front end Beam dia. :  100 mm Wavelength : 1053 nm Desired properties of gain medium for HEL: Ø Higher stimulated emission cross-section Ø Higher saturation energy Ø Longer upper laser level life time Ø High energy storage capacity Ø Should be manufactured in large size rods and discs Disadvantage of glass: Poor thermal conductivity→ Single shot operation  Ø Fixed pulse duration, shape & spatial profile: governed by the oscillator Ø Single pass linear amplifier: More number of amplifcation stage and poor efficiency Q-switched ns Oscillator as master oscillator Limited control on laser pulse width Energy = 10s of mJ, & Duration = 100s of ps to few ns No control over temporal shape of the laser pulse 18 1. All Fiber Optic Front End System (FOFES) serving as Master Oscillator 2. Multipass, High gain diode pumped pre-amplfier 3. Large aperture Multipass Nd glass disc based power amplfier FOFES advantages over Q-switched oscillator Ø Various parameters, e.g. pulse width, temporal profile, repetition rate, pulse energy can be varied. Ø System is alignment free, ease of integration for various fiber optic components, stable and rugged Why nuclear fusion 1. Large amount of fuel Deuterium: from seawater, Tritium: breeding from lithium Enough readily available lithium for 1000’s of years. 2. Green energy 3. Limited, controllable radioactive waste Challanges is achieving nuclear fusion: Coulomb energy barrier must be overcome , proportional to Z1*Z2 Isotopes of hydrogen have smallest coulomb barrier For D-T reaction : Z1, Z2 = 1 and r =10fm, Kinetic energy required ~ 100 keV Lawson criteria: Breakeven For D-T reaction at 10 keV nτ =1014 cm-3.sec For D-D reaction at 10 keV nτ =1015 cm-3.sec  Ions compressed into a pellet and heated so quickly that fusion ignites before inertia of ions is overcome Achieve enough number of fusion reactions in the inertial confinement time. Since the reaction rate is proportional to n2, so ICF approach requires operation at higher density, and smaller confinement time. Laser Plasma Experiments -What do we need? Maximum intensity on target Needed for the experiment Pulse energy Increase the energy (E), E Ipeak = Decrease the area of the focus (A). A t Decrease the duration (t), Beam Pulse area duration 1 mJ/1ns : 1 MW peak power 1 mJ/100 fs: 10 GW peak power Maximum average power at the detector Signal is proportional to the number of Pave = E r photons on the detector per integration time. Pulse Rep rate energy Needed to get useful results 22  Femtosecond lasers Main Characteristic Ø Ultrashort pulse Ø Ultrahigh light intensity (~ 10 20 W/cm2 ) Ø Ultrabroad bandwidth (coherent) ( ∆ν = k/∆τ ) Applications 1. Ultrashort pulse Nonlinear optics Optical communication Ultrafast spectroscopy ( Pump /Probe spectroscopy), Femtochemistry Nanosurgery Micromachining, 2. High Intensity Laser induced plasma: X ray, gamma ray, THz radiation Electron, proton, ion acceleration 3. High coherent pulse train Multi photon excitation spectroscopy, Precise measurement of light frequency Terahertz time resolved spectroscopy  Ultrashort pulse: the time – bandwidth product Ultrashort pulses are usually Fourier transform-limited pulses Dw·Dt ≈ Constant Large spectral bandwidth for shorter duration Dl ≈ l2 /(cDt) Dl ≈ 21 nm for 100 fs pulses with l0 = 800 nm Important challenges and limitations for amplification Ø Maximum compressible energy : limitation from beam size(ultimately grating size) Ø Compressed pulse duration: Gain - bandwidth limitation Ø Material Dispersion Ø Prepulse contrast Ø Average power: Repetition rate. 24 Chirped pulse amplification: Route to high intensity lasers Concept given by Prof. G Morou and D. Strickland in 1985 They got 2018 Physics Nobel Prize for this along with Prof A. Ashkin (For optical tweezers). Basic Principle Oscillator Stretcher Amplifiers Compressor (nJ, fs) (nJ, ns) (mJ-J, ns) (mJ-J, fs) Why not start with a ns pulse, amplify it and then compress?? Application: Laser plasma wakefield acceleration, laser ion acceleration, t Higher harmonic generation, attosecond pulse generation 25  CPA based laser systems@ RRCAT Indigenously Developed 1 TW Nd: glass Laser system 40 TW Nd: glass Laser system Energy : 1J Energy : 24 J Pulse duration : 1ps Pulse duration : 600 fs Beam size : ~ 25 mm Beam size : ~ 75 mm Commercial State of the art systems 10 TW Ti: sa Laser system 150 TW Ti: Sa Laser system Energy : 450mJ Energy : 3.75J Pulse duration : 45 fs Pulse duration : 25 fs Beam dia. : ~50 mm Beam dia. : ~ 100 mm 1 PW Ti:Sa Laser system Energy : 25J Pulse duration : 25 fs Beam dia. : ~ 200 mm  Solid state lasers: Nd:YAG laser development Ø Lasing wavelength 1064 nm Ø Pumped by Flashlamp ( Kr filled) or Laser diodes @ 808 nm Ø can be operated in both CW and pulsed mode Ø Robust, Remote beam delivery through fibers Different Nd:YAG laser systems developed at RRCAT Ø Flashlamp pumped high average power (0.25 -1KW), high peak power (5-20kW), 2-40 ms pulse duration, 1-100 Hz repetetion rate Ø Diode pumped CW laser, 1 kW average power Ø Flashlamp pumped 7J/10 ns laser system Ø Frequency doubled ( Green, 532 nm ), KHz repetetion rate, diode pumped, nanosecond laser : These are replacing CVL for laser isotope separation  Solid state lasers: Nd:YAG laser development Nd: YAG lasers are extensively used for Laser Cutting, Welding applications in Indian nuclear power program  Laser Cutting Advantages over conventional cutting Remote beam delivery through fibers Less material loss focussed laser beam Less heat affected zone→ Less thermal distortions Laser Cutting  Fiber Lasers Advantages: 1. Alignment free, Compact footprint 2. Higher efficiency, lower cooling requirements 3. High power using multi-port laser Applications: Most versatile tool for laser material processing applications Ø Rock and concrete drilling for gas and oil well exploration, Ø Deep penetration welding in the range of ~10-30 mm Ø Cutting of thick components (~100 mm) for shipyard industry, and reactor decommissioning Ø Laser additive manufacturing Ø Directed energy weapons in defense applications.  Fiber Lasers Yb-doped double-clad fiber as gain medium having a core dia of 20 µm and an inner-clad dia of 400 µm 500W CW 1080 nm fiber laser Different fiber laser system developed by RRCAT Ø 1 kW Yb doped cw fiber Laser (1080 nm) for material processing Ø 100 W Thulium doped CW fiber laser(1940 nm) in eye safe regime for surgical applications Ø 50 W Er doped cw fiber laser (1550 nm) for sensing application Ø 4 W mode locked 160 fs Yb laser system Different dopants  Semiconductor (Diode) Lasers Advantages of diode lasers Ø Large range of spectrum (variety of materials) 0.3 - 3um Ø High conversion efficiency Ø Wide range of power Ø Smaller size, robust, long life  Gas Lasers Copper Vapor Laser Wavelengths: 510 nm (green) and 578 nm (yellow). Average laser power: 20 W to 100 W Pulse repetition rates of 5 to 20 kHz Pulse width: ~ 40 ns. These laser are used to pump dye lasers for AVLIS Now a days frequecy double Nd: YAG lasers are used for this application Excimer lasers Wavelength: 100 – 300nm Photolithography, material processing Main issue : Poisonous gases – chlorine, fluorine! CO2 lasers Wavelength: 10.6 um and 9.6 um Industrial applications, paper industry  Laser Additive Manufacturing (LAM) What is LAM ISO/ASTM 52900 defines Five criteria for a process to be called “Additive Manufacturing” a) Process of joining of materials b) Starting from 3 D model data c) Layer by layer build up approach d) Not subtractive manufacturing methodologies e) Not formative manufacturing methodologies When laser is used as a heat source, it is called LASER ADDITIVE MANUFACTURING Advantages of LAM 1. Complexity for Free 2. Simplification of Part Fixturing 3. Mass Customization 4. Integrated Components 5. Material Design Freedom 6. Logistic Freedom 7. Reduced Wastage  Two types of LAM machines developed by RRCAT Directed Energy Deposition LAM Specification Ø Laser: 2 kW Fiber laser Ø Job manipulation: 5 axis (Siemens 810 D controller) Ø Working volume: 250 x 250 x 250 mm3 Ø Powder feed rate: 2- 20 g/min Ø No of powder feeder: 2 Ø Wire feeding( 1- 2 mm dia) : 2- 20 m/min Ø Purity in Glove box: 20 ppm O2 ; 30 ppm H20 Poweder Bed Fusion LAM Specification Ø Laser : 500 W Ø Beam manipulation: Scanlab Galvanoscanner Ø Beam Diameter: 500 microns Ø Scan Speed: 0.02- 10 m/s Ø Working volume: 250 x 250 x 250 mm 3 Ø Layer thickness: 25- 250 microns Thanks & Best wishes Recall: CW vs pulsed laser Ø CW lasers: Energy is continuously pumped- producing a continuous laser output. Ø Pulsed lasers- The output is in pulsed mode like flashes of light CW Pulsed How to produce laser pulses 1. Free running: Depends on pump duration 2. Q-switching: nanosecond pulses 3. Mode-Locking : picosecond to femtoseconds pulses 41 Free running laser Pulsed pumping of active medium The laser pulse duration depends on the pump duration 42 Q-Switching Ø Q–quality factor of laser resonator is ratio of energy stored in the cavity to the energy loss in one cycle. Ø High Q– Low losses & Low Q - High losses Ø Q-switching: Sudden switching of the cavity Q from low value to a high value. 43 Q-Switching Pockels cell based Q switching: HR Mirror Polarizer P2 Pockels cell PR Mirror Active medium with pumping QWP 44 Mode Locking Ø Mode locking: allow the intense spike to grow in the cavity Ø How to suppress the random background 45 Mode Locking Ø How to create intensity dependent loss Ø Two methods: Saturable absorber, Kerr lens 46 Kerr lens Mode Locking 47  Nuckolls’ Pulse d2 R /dt2 = - P. 4 π R2 / 4 π R02 ρ0 ΔR0 where M = 4 π R02 ρ0 ΔR0 (The mass is assumed to be in a spherical annular shell form and same before and after compression) -ve sign because compression. R = R0 (I-Xt)4-3γ P = P0 (I-Xt) –(4-3 γ)3γ. where X = {1/3(4-3 γ)(1- γ)}1/2 ( P0 / ρ0 ΔR0 ) 1/2 highest pressure is required when t= 1/X=t0 Pressure Perfect implosion through series of shocks Time t0 Experimental set-up The shock luminosity with Shadowgraphs at two times respect to fiducial. t1 -1.5ns t2 -3.5ns Particle velocity & shock velocity The shock and particle velocity are related in the form Us  a  b *U p where ‘a’ and ‘b’ are the material constants. a (X106 cm/s) b 0.55 1.36 Experimental 0.538 1.30 J. Appl. Phys., Vol. 93, No. 1, Jan 2003  EOS of material using two frame shadowgraphy with Bi-prism in step target Schematic for step target The ratio of pressures of the incident wave from the Generation of two pump beams aluminum foil exit surface and transmitted compression wave into the step layer is  u step  P2  P1  2  pAl   up  Experimental Set-up Shadow grams at different times EOS of Gold EOS of Mixed (Gold/Copper) The results are compared with The EOS of mixed-z alloy of Au/Cu coated on aluminum (Al) reference-foil was measured. The composition of the thin film is: Au, Cu mixed with 87.8% and 12.2% by weight literature, the intercepts is respectively. slightly lower than the reported, where as slope is observed close 100  to literature value.   Au % Wt     Cu % Wt  Au      Cu   Paramet Experi LASL ers mental data Us  a  b *U p a (x 106 0.21 0.31 us  c  s* up cm/s) b 1.47 1.51