Frequency Scanning Fibre Interferometer for Absolute Distance Measurements over a Large Target Area

Transcription

1 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 JA-6- Fquncy Scanning Fib ntfomt fo Absolut Distanc Masumnts ov a Lag Tagt Aa Yanmi Han, Amin Richold, Colin Py, Richad Bingham John Adam nstitut and Physics Dpatmnt, Univsity of Oxfod Th Lina Collid Alignmnt and Suvy (LiCAS) pojct aims to stablish th fnc suvy ntok of th LC (ntnational Lina Collid) by masuing th lativ positions of suvy fnc maks, locatd in a gula squnc along th tunnl alls of th LC. A fib distanc snso has bn dvlopd by th Oxfod LiCAS goup to masu th coodinats of th tunnl maks. t xploits th fquncy scanning intfomty (FS) tchniqu, supplying a fnc intfomt and multipl masumnt intfomts fom on shad fquncy scanning las souc. n th masumnt intfomt a fib tip ithout any optic is usd to launch and civ light to pfom absolut distanc masumnts ov a lag tagt aa. n this pap, calculations basd on th Gaussian fild distibution a psntd, hich pdict th fficincy of th flctd light fom th tunnl mak coupld back to th launching fib. Th high gain and lo nois photodtction unit is dscibd. Pliminay masumnt sults a potd. 1. NTRODUCTON n od to collid highly focusd bams ith dimnsions of th od of nanomts, th ntnational Lina Collid (LC) dmands vy pcis alignmnt of its componnts [1][]. Th quid accuacy is µm (5 µm) vtical (hoiontal) ov a 6m distanc, hich cannot b obtaind by convntional suvying tchniqus []. This ld to th dvlopmnt of th Rapid Tunnl Rfnc Suvyo (RTRS) by th Oxfod LiCAS goup in collaboation ith th DESY applid godsy goup. Th RTRS aims to stablish th fnc suvy ntok of th LC, it consists of 6 masumnt cas hich pfom masumnts btn nighbouing cas and th tunnl fnc maks, locatd in a gula squnc along th tunnl alls of th LC[3][4]. Figu 1: Snso configuation btn to masumnt units of th RTRS WEPO9 1

2 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 Figu 1 dpicts th snso configuation btn to masumnt cas of th RTRS, mo dtails of th RTRS masumnt pincipl can b found in [4] [5]. Among all ths masumnts, distancs btn th to masuing units along th tunnl all and th co-odinats of th tunnl maks in spct of th masuing ca a masud by fquncy scanning intfomty (FS) tchniqu Pincipl of th FS masumnt FS is an absolut distanc masumnt tchniqu dvlopd at Oxfod fo th ATLAS paticl dtcto, masuing th phas chang of an intfomt as th las fquncy is scannd [6]. Th must b a stabl fnc intfomt (R) ith a knon optical path diffnc (OPD) L, shaing th sam fquncy scanning las souc ith th masumnt intfomt (M) ith an unknon OPD D. Whn th fquncy of th las sps ov an intnal Δν, th inducd phas changs in th R and M a π ΔΦ LΔν (1) c and π ΔΘ DΔν () c spctivly. Povidd that both intfomt OPDs do not chang duing th fquncy scan, th atio of phas changs of th intfomts, q, can b xpssd by ΔΦ L q. (3) ΔΘ D Thn th unknon masumnt intfomt OPD can b calculatd using L D. (4) q Thus th basic aim in an FS masumnt is to find q fo ach intfomt in od to dtmin D. Fo a mo dtaild dsciption of th algoithm usd to xtact D s [7]. 1.. FS masumnt systm in LiCAS Th FS systm poposd fo LiCAS is shon schmatically in figu. Th ky dvic of th FS is a tunabl las. H a tlcom tunabl las (Agilnt 8164A) is xploitd, hich can tun continuously at spds of up to 4nm/s ov any 3nm sction in a avlngth ang fom 151nm to 164nm. Th maximum output po of th Agilnt 8164A las is 1.95mW. n od to cay out multipl lin masumnts, an Ebium Dopd Fib Amplifi (EDFA) (KPS- BT-C-1-Bo) is usd to amplify th light fom th las. t can amplify th las up to 1mW at an optical bandidth of nm, poviding a scalabl and cost ffctiv solution. Th tunabl las and EDFA a contolld via GPB, thus th avlngth bandidth, sping spd and output light po can b st motly. WEPO9

3 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 Tunabl Las FST O8 O8 C1 PD BS M1 EDFA... M R T8... T1 O1 O1 L1 R-R M Photodtction and Signal Conditioning FS Wokstation Fast ADC GPB Figu : Th schmatic diagam of th laboatoy FS systm fo LiCAS. FST: fib splitt t; O1, O1 O8, O8 : 16 output channls of th FST; T1 T8: 8 fdback channls of th FST; R, fnc intfomt; C1: collimation optics; BS: bam splitt; PD: Photodiod; M1, M, flcting mios; M: masumnt intfomt; L1: collimation lns; R1: to-flcto. Th output light of th EDFA is snt to a fib splitt t (FST) hich splits th light into many output channls, alloing simultanous lngth masumnts to b mad by compaison ith th sam fnc intfomt R. Singl mod fib Coning SMF-8 is usd h fo light dlivy. ntfnc signals fom th R and Ms a fd to a FS okstation fo analysis Rquimnts fo th xtnal shot lin masumnt As shon in figu 1, th masumnt of th x, y, co-odinats of th tunnl maks in spct of th masuing ca is pfomd by a st of 6 FS distanc masumnts in th fom of a con. Th combination of th 6 lins can ov-dtmin th 3D co-odinats. Th tunnl maks a sph mountd to-flctos of 1 inch cla aptu. Th distanc btn th ca and th to-flcto is 4mm~5mm, and th to-flcto oams in a 5 5 5mm 3 cub, as shon in figu 3. At a masumnt distanc of 4mm, th to-flcto can b about 33mm off-axis. Thn th cucial quimnt fo th xtnal FS is to cov a lag aa of about 7mm diamt. WEPO9 3

4 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 FS launch optics Vtical cnt lin of cas 5 Rtoflcto Eo cub fo position of to-flcto (5 5 5 mm 3 ) F A B E C 4mm D Ointation of hxagonal aay of xtnal FS lins, viing toads th to-flcto Figu 3: Optical aangmnt of th xtnal shot FS lins (six p ca) Possibl solutions fo th xtnal FS a listd in tabl 1. Tabl 1: Possibl solutions fo th xtnal FS lins Solution Mthod Chaactistic 1 Us a collimatd bam of ~7mm diamt Lag optics St a collimatd bam of a small diamt High tun po, automatic moving pats 3 Us th bam dictly fom th fib Simpl but lo tun po Th solution 1 nds a collimato of at last 7mm cla aptu, and th hol optical systm ill b vy bulky. Th cost to dsign and manufactu such a big si collimato ith no chomatic abation is vy high, but it is vy dsiabl fo th FS masumnt. Fo th solution, an automatic positioning mchanical systm is quid. This ill duc th masumnt spd hich dfinitly nds to b vy fast. Th xtnal lin masumnt nds accuat knoldg of th position of launch optics, and thn this solution is also xcludd on pincipl. Thus a fib tip ithout any optic as chosn fo th xtnal shot lin masumnt du to its simplicity, lo cost and vanishing chomatic abation. Th disadvantag of this solution is that th po of th flctd light coupld WEPO9 4

5 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 back into th fib ill b vy small. n this pap, calculation and xpimntal sults a potd, th high gain and lo nois photo-dtction unit is also dscibd.. CALCULATONS.1. Output optical fild distibution fom singl mod fib SMF O θ R Sampling sufac S Figu 4: Output optical fild distibution fom singl mod fib n od to kno th lvl of th flctd light fom th tunnl mak coupld back to th launching fib, th calculation mthod basd on Gaussian fild distibution as usd. t bgan ith th optical fild distibution of th light fom a singl mod fib (SMF) (s Figu 4). At a distanc fom th fib nd, th adius of th 1/ intnsity contou of th las bcoms + H psnts th adius of th las aist. λ 1 π Whn th distanc is much lag than th Rayligh ang, λ π Th divgnt angl of th las bam, θ, can b xpssd by 1. (5). (6) λ π. (7) On th fib nd th light intnsity at th cntal point is ; it bcoms o at th distanc. Th output po P can b calculatd by WEPO9 5

6 6 d d d d P π π π π. (8) Th intgal 1 a dx x x a. (9) Thus 1 1 P π π. (1) So that. (11) Thn th light intnsity at any point on th sampling sufac S can b xpssd by R R, (1) h R is th off-axis distanc of th point... Fib coupling fficincy Figu 5: Coupling a plan av light into a SMF Fo a plan av nomal incidnt light (shon in Figu 5(a)), th ffctiv collcting aa of a singl mod fib, A f, is θ (a) Nomal incidnc. (b) Obliqu incidnc. 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 WEPO9

7 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 A f π, (13) h is th mod-fild adius of th fib[8]. Th collcting aa is tic th aa nclosd by th MFD (mod-fild diamt). f th incidnt light is obliqu (shon in Figu 5(b)), th ffctiv collcting aa A ( θ ) f π, (14) h θ is th light incidnt angl, and θ is th divgnt angl of th las bam fom th fib (s quation 7). f th incidnt light is mittd fom th oth SMF (shon in Figu 6), and is much lag than Rayligh ang, th light at any point on S can b tatd as plan av. Emitting fib θ θ θ R off Rciving fib Figu 6: Coupling light fom a SMF to a SMF f th collcting fib is at off-axis distanc R off, th po collctd by it can b calculatd as P c off π R off π. (15) Whn th light is tansmittd and collctd by th fibs ith idntical mod filds,, thn P Thus th coupling fficincy c 4 Roff π. (16) WEPO9 7

8 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 h Pc C P tg Roff 4 R off, (17) θ. (18) Substituting quation (6), (7) and (18) into quation (17), th coupling fficincy can b xpssd by C R off π π λ. λ. (19).3. Coupling fficincy vsus th position of to-flcto Emitting fib mio imag in R-R Emitting and collcting fib R-R R off θ R off Figu 7: Coupling light flctd fom a to-flcto n ou st-up, th mitting fib also civs th light aft it has bn flctd by th to-flcto. Th civing fib can b gadd as bing locatd at th mio location of th mitting fib, s figu 7. Th distanc btn th light souc and th collcting fib bcoms, and th off-axis distanc of th collcting fib bcoms R off. Thn th coupling fficincy vsus th position of to-flcto is WEPO9 8

9 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, π C λ () π λ.. Roff π 4 λ Roff π 4 λ, () Wh is th distanc btn th fib tip and th to-flcto, and R off is th off-axis distanc of th toflcto. At 155nm, th mod-fild adius of SMF-8 fib,, is 5.μm. Whn is 4mm, th adius of th light spot is 4mm (adius of th 1/ intnsity contou, calculatd by quation (6)). This mts th quimnt of th xtnal FS systm povidd that th tun light can b liably dtctd in th nti fild.. Rsults of th fib coupling fficincy vsus th position of to-flcto (calculatd by quation ()) a shon in figu 8 (a) and (b). Whn th to-flcto is at 4mm, th coupling fficincy of th flctd light fom th to-flcto back to th launching fib ill fall fom 1-8 to 1-9 as th off-axis distanc of th to-flcto gos fom to 35mm. Thn th tun light is 16pW ~ 1pW hn th output po of th fib is 1mW. (a) WEPO9 9

10 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, x 1-8 Th distanc btn th fib tip and R-R is 4mm Coupling fficincy Off-axis distanc of R-R(mm) 3. PHOTO-DETECTON UNT (b) Figu 8: Fib coupling fficincy vsus th position of R-R Whn th fquncy of th las scans, th phas of a M ith OPD D is π ( t) Dν ( t) c Θ, (1) h ν(t) psnts th las fquncy as a function of tim and can b xpssd as ν t) ν + & ν t (. () H ν is th las fquncy at th stat point. ν& is th las fquncy scanning spd. n ou application, th maximum las fquncy scanning spd is usd. t is 5TH/s fo th Agilnt 8164A las. Thn th fquncy of th intfnc signal is D f i ν&. (3) C Th distanc btn th ca and th to-flcto is 4mm ~ 5mm, thus D fo th shot lin FS is 8mm~1mm. Thn th fquncy of th intfnc signal is about 13.4kH ~16.7kH. n od to dtct th vy ak intfnc signal, a photodiod/amplifi unit as dsignd to giv snsitiv dtction of small fluctuations in intnsity in th 1kH to 5kH fquncy ang, as shon in figu 9. Th dtcto as an ngaas PN photodiod, Pkin-Elm typ C3617, ith an ST optical fib conncto. Th photodiod as WEPO9 1

11 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 opatd ith a bias of appoximatly 4V, into a 1Mohm load sisto. This allod fo an optical input po up to 5nW bfo th bias voltag as unaccptably ducd. Th signal as buffd ith a souc follo using a N4416 JFET opating at 1.5mA, ith a gain of.8 and a 3 ohm output impdanc. Th signal fom th souc follo as amplifid by to ac-coupld op-amp stags (Maxim MAX418), to bing th input stag signal up to a convnint lvl fo subsqunt acquisition, and to coct and stict th ovall fquncy spons. +1V +8V +8V 1K C3617 1M N K MAX418 n 47K MAX418 1u 1u 1K 1u 1M 1K5 u 5K1 1R 1K 1K 1K 4n7 5K1 1R 1K 1u 1K 1K u 1u 41n V Figu 9: Cicuit diagam of th photo-dtction unit 4. EXPERMENTAL RESULTS AP Fib Tip 4 o R-R Figu 1: St-up fo th lag aa accptanc studis n th shot lin FS, th light flctd fom th R-R intfs ith th light flctd fom th fib tip. Fo a flat polishd nd fib, aound 4% of th light is flctd back at th nd. Whn th output light po is 1mW, th flctd light fom th flat nd tip is about 4µW, much ov th 5nW maximum optical input po of th photodtction unit. A fib ith angl polishd (AP) nd is usd h to fom th shot am of th M du to its lo back flction (high tun loss > 6dB). Th bam fom th AP fib dviats fom th mchanical axis about 4 o, as shon in Figu 1. WEPO9 11

12 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, Sis1 Sis Sis3 Masud OPD (mm) Tansvs displacmnt of R-R (mm) Figu 11: Masumnt sults n th cunt st-up, a Michlson gomty intfomt is usd as th fnc intfomt, ith a path lngth diffnc of appoximat 4.7m. Light fom an output channl of th fib splitt t is dlivd to th intfomt via a m long fib. An ngaas photodiod dtcts th fnc signal. Th to-flcto as placd on a comput contolld lina stag fo tansvs motion in th lag aa accptanc study. n th xpimnt, th stag movd in stps of 5mm ov a ang of 9mm. At ach stop, data of R and M signals acquid, and thn th OPD of th M as calculatd. Figu 11 shos 3 sis of th OPD masumnt sults, indicating that th singl mod fib tip ithout any optic can cov a lag masumnt aa of about 75mm diamt. gnoing th th ightmost points of th gn sis, th standad dviation of th constuctd OPD vais btn.6µm and 6µm. 5. CONCLUSONS Basd on th FS tchniqu, a fib snso suitabl fo masuing th co-odinats of th tunnl maks insid th LC tunnl has bn dvlopd. n th masumnt intfomt a fib tip ithout any optic is usd to launch and civ light to pfom absolut distanc masumnts. Calculations indicat that th fib coupling fficincy of th flctd light fom th tunnl mak back to th launching fib vais fom 1-8 to 1-9. A high gain and lo nois photo-dtction unit as dsignd to dtct th vy ak signals. Expimntal sults sho that at ~4mm distanc this fib intfomt can cov a lag masumnt aa of 75mm diamt. WEPO9 1

13 9th ntnational Wokshop on Acclato Alignmnt, Sptmb 6-9, 6 Rfncs [1] G. Glak t al., Simulation of th pfomanc of th LiCAS tain, Pocdings of th 8th ntnational Wokshop on Acclato Alignmnt, CERN, Sitland/Fanc, Octob 4. [] A. Richold t al., Th LiCAS-RTRS A Suvy Systm fo th LC, Pocdings of EPAC6, Edinbugh, Jun 6, [3] A. Mita t al., Masumnt of th LiCAS systms, Pocdings of th 8th ntnational Wokshop on Acclato Alignmnt, CERN, Sitland/Fanc, Octob 4. [4] A. Richold t al., Th LiCAS-RTRS -A Rapid and Cost Efficint Suvy Systm fo th LC, Pocdings of th 9th ntnational Wokshop on Acclato Alignmnt, SLAC, USA, Sptmb 6. [5] G. Glak t al., Simulation of th LiCAS Suvy Systm fo th LC, Pocdings of th 9th ntnational Wokshop on Acclato Alignmnt, SLAC, USA, Sptmb 6. [6] P. Co t al., Fquncy scanning intfomty in ATLAS: mot, multipl, simultanous and pcis distanc masumnts in a hostil nvionmnt, Mas. Sci. Tchnol. 15 (4) [7] J. Gn, An Automatd Las Mtology & Alignmnt Systm fo th ntnational Lina Collid, to b publishd soon. [8] R. Bingham, Analysis of dfocus xpimnt, Octob 4. WEPO9 13