Data Acquisition and Analysis ideas for THz Time Domain Spectroscopy - PowerPoint PPT Presentation

Data Acquisition and Analysis ideas for THz Time Domain Spectroscopy Plasma Diagnostics Frascati 27 March 2012

Extrem Extremel ely broa roadband nd (10 (100GHz GHz-30THz THz) ) singl ngle- e-cy cycl cle e THz THz pulses es are re routi routinely nely generated generated wi with th fem femtosecond tosecond laser laser and and used used for for ti time- e- domain s spectroscopy ( (TDS). The wide frequency range, 200 GHz, 8.8 THz, cove vers a large range of plasma plasma conditions onditions in in a a Tokamak. Tokamak. By By illuminating illuminating the the plasma plasma with with a THz THz bea eam ph phase, se, intensi ntensity, ty, pol polari rizati tion on and nd frequency requency of of th the reflected o or t transmitted b beam c can b be d detected. Th The e tech techni nique que has s th the e potenti potential to to rea realise se a com compl pletel etely y new new concep concept t ins nstru trument ent allow owing ng multi tiple e mea easurem rements ents with th one one single single diagnostic diagnostic. . Reflectom Reflectometry, etry, Interferom nterferometry, etry, Polari Polarimetry, etry, ECE ECE Supr Suprat athe hermal mal Emissio mission. . A A collabo llaborat atio ion on this his subj subject be betwe ween ENEA ENEA Fra Frasca cati ti and nd th the e Ph Photoni otonics cs grou group of of Ph Physics cs Dep Dept. t. of of Oxf Oxford ord Unive versity i is i in p progress. Data acquisition Data acquisition and and processing processing will will be be a a key key issue, issue, with with many any innova vative ve a aspects w which w will b be i introduced i in t this p presentation.

Schematic of the THz-TDS setup (adapted from James Lloyd-Hughes PhD thesis, Oxford) • The 790nm is divided in REFERENCE and PUMP • The PUMP generates a THz pulse from the GaAs Photoconductive Optical Switch • Amplitude & Phase of THz Electric field E(t)= E(t)=E 0 e i φ (t) (t) are detected with a ZnTe Electro-Optical detector, enabled by the REFERENCE beam • The THz spectrum is obtained by FFT of the THz pulse E(t)

Numbers Ti:Sapphire laser centred at a wavelength of 790 nm (IR) Pump Laser: Verdi V5 continuous wave, diode-pumped Nd:YVO solid state laser Coherent. 4 W beam at 532 nm (gr green) DURATION OF LASER PULSE: 10 f fs = = 1 1/100 p ps LASER (and THz) REPETITION RATE: 75 M MHz corresponding to ∼ 13.3 ns average POWER: power 500 m mW, 7 nJ pulse energy [P=E/dt = 7 nJ / 13.3 ns ∼ 500 mW] Power within the 10 fs pulse P = E/dt = 7nJ /10 fs = 700 K KW THZ PULSE length: 5 p ps (i.e. laser pulse 1/100 ps: a delta on THz pulse timescale) POWER emitted by THz Switch: ∼ 0.5 mW (average) (conversion efficiency 1/1000, pulse energy 7 pJ)

Data Reduction Thomson, S Sir G George. The I Inspiration o of S Science, O OUP ( (1961) Simplicity: f freedom f from c complexity a and i intricacy ( (it w works i in Science, b but a also i in F Fashion, A Architecture, M Music....etc) Sim Simplic plicit ity y does n not m mean u unsophisticated a and e easy, i instead i it is t the u ultimate g goal o of d development a and p progress. Our c case: 5 5ps p pulse, 1 1ps s structure, 0 0.5 p ps s sampling: required a acquisition f frequency > 2 > 2000 G GHZ! A c clear N NO G GO. I Instead, b by s scanning t the p path d difference between R REF a and P PUMP o over c c*5 p ps = = 1 1.5 m mm i in 1 1ms, w we c can sample t the T THz p pulse w with 1 10 p pts: 10 K KHz

Accuracy v vs. P Precision ( ( Bevi vington, M McGraw H Hill 1 1969 ) ) Ac Accuracy: a measure of how closely the experimental results agree with the true value. Requires calibration against a known standard. Pr Preci ecision on: a measure of experimental reproducibility for multiple measurements. Usually described by the standard deviation or confidence interval. error: precise, but not accurate measurements. Systematic e Bull’s Eye cartoon: The Accuracy is the distance of the darts spot from the centre, the Precision is the area covered by the scatter of the hits.

Towards a diagnostic....1 The v very s short p pulses ( (5 p ps s spaced b by 1 13 n ns) w will b be a averaged t to achieve t the r required S Signal t to N Noise r ratio, w which i improves better t than √ N, d due t to t the s synchronous h heterodyning ( (“lock- in”) o operation. T The r relevant t timescale i is n not t the p pulse duration, b but t the t time r required t to “ “map” i it, i i.e. t the p path difference s scanning t time. Ex Example: e: 1ms s scan, av averaging 1 10 t to 1 100 s scans will b be compatible w with m most f fast p plasma p phenomena. ( (The s scan i is achievable w with b beating o of a a p pair o of l linked T Ti:Sapphire femtosecond l lasers, w with a a f fixed r repetition r rate d difference). Good u understanding o of t the p physics o of t the u ultrafast p processes i is required b before e even d designing t the a acquisition a and a analysis system t to a avoid o overflow o of d data.

Towards a diagnostic....2 field E(t)= E(t)=E 0 e i φ (t) (t) of a In T TDS t the e electric f a p pulse ( (rather t than i its intensity) i is r recorded v vs. i its a arrival t time, t then A Amplitude a and Phase s spectrum a are c calculated b by F Fourier a analysis : : A( ν ) = A( = F FFT(E(t)) Φ ( ν )= A ATAN[FFT(sin)/FFT(cos)] The s subsequent p plasma i information a are e extracted a accordingly to t the c chosen i interaction. I Interferometry f for d density measurements w will b be a a s straightforward a application. Real-time a availaibility c can b be d designed o on t the b basis o of t the minium t time r required t to a achieve a a s suitable S SNR. I It c can b be assumed t that p processing t time w will b be c comparably n negligible.

.......in summary THz-TDS will...... �  Introduce the cutting edge THz-TDS technology in the Tokamak environment  Offer High sensitivity, due to the background rejection potential of the THz TDS technology  Use Sources & detectors intrinsically broadband, wider than a Fourier Spectrometers and without beamsplitter restrictions.  Be a solid-state device, operating at room temperature  Lead to a completely new concept instrument, allowing multiple measurement with one single device with potential for real time measurement of fluctuations, charge density and conductivity, with unprecedented resolution. Data A Acquisition a and A Analysis w will b be a a c crucial i issue to a achieve ve t the f full d diagnostic p potential.