High-precision confocal reflection measurement for two dimensional refractive inde mapping of optical fibers Philippe Raisin a, Jonas Scheuner a, Valerio Romano a,b, Manuel Rser a a Institute of Applied Phsics, Universit of Bern, Sidlerstr. 5, CH-3012 Bern, Switerland b Bern Universit of Applied Sciences, Pestaloistr. 20, CH-3400 Burgdorf, Switerland 1 Conference 9507 Session 6: Testing and Characterisation Methods Applied to Special Fibre Tpes
Outline Motivation Measurement principle Instrument setup Results: stabilit calibration commercial fibers Improvements: lensed fibers & source Conclusion P. Raisin, 16.4.2015 2
Motivation: Wh refractive inde mapping? light guiding properties NA, SM >MM core-cladding inde difference: from 10-2 down to <10-3 for LMA fibers desired instrument precision: <10-4 Step-inde fiber new specialt optical fibers need for 2D maps instead of bulk measurements qualit control leakage channel fiber transmitted near-field method (TNF) refracted near-field method (RNF) more recentl: phase measurement techniques (scanning along fiber) P. Raisin, 16.4.2015 Drawing tower at Universit of Bern challenges mapping techniques PM photonic crstal fiber (PCF) 3 Immersion of fiber in indematching oil (air hole structures) heav modifications necessar for Si-based waveguide structures
Measurement principle: reflected power Idea: Reflectivit R of fiber end-face: connected to refractive inde via Fresnel-coefficients (vertical incidence) detector reflected beam fiber source incident beam sample mount sample on multi-ais stage and raster scan surface for 2D-profile Weidel, E. & Eickhoff, W. (1975). Measuring method for the refractive inde profile of optical glass fibers, Optical and Quantum Electronics 7.2, 109-113 Youk, Y & Kim, D.Y (2002). Thightl focused epimicroscope technique for submicrometer-resolved highl sensitive refractive inde measurement of an optical waveguide. Applied optics, 46(15), 2949-53 Meas. Sci. Technol. 13, 695 699 P. Raisin, 16.4.2015 4
Signal optimiation: FOCON detector reflected beam source fiber optic confocal microscop (FOCON) Glass, M., Dabbs, T.P. (1992). Fiber-optic confocal microscope, Applied Optics 31.16, 3050-5 fiber core blocks unwanted reflections incident beam refl. power mismatch sample matching beam waist with sample surface aial () scan at each point necessar Advantages minimal fiber preparation (cleave) straightforward & in-fiber setup Challenges Semi-quantitative: calibration samples needed confocal setup: pinhole/core sensitive to mechanical shocks/drifts P. Raisin, 16.4.2015 5
Instrument setup: Overview 980nm SLED variable optical attenuator wideband coupler 2% 98% port 1 port 3 port 2 circulator fied fiber collimator CCD Splitter Source Detection Confocal/Scanning Unit microscope objective pieo-stage with sample fiber-core inside collimator acts as pinhole theoretical spot-sie on fiber: 1.8μm (NA=0.65, 40) pieo-stage with travel-range of 44±2μm removable 50:50 Splitter with CCD for sample adjustment dedicated LabVIEW-Software CCD Image (PM-LMA fiber) spot core scan area PM rods P. Raisin, 16.4.2015 6
Instrument setup: Source 980nm SLED variable optical attenuator wideband coupler 2% 98% port 1 port 3 port 2 circulator fied fiber collimator CCD Splitter Source Detection Confocal/Scanning Unit microscope objective pieo-stage with sample SLED spectrum low-coherence source FWHM = 27nm 4mW optical power monitoring with Si-Photodiode to cancel instabilities P. Raisin, 16.4.2015 7
Instrument setup: Confocal Unit 980nm SLED variable optical attenuator wideband coupler 2% 98% port 1 port 3 port 2 circulator fied fiber collimator CCD Splitter Source Detection Confocal/Scanning Unit microscope objective pieo-stage with sample refractive inde Focusing scan (X) 8.34μm ma. reflected power to sample surface refractive inde (10-3 ) Step widths for focus scans +1.447 step-width 0.45μm 0.22μm 0.04μm error 2.5 10 4 1.4 10 4 4.8 10 5 time [min] -position [um] Pieo-stage has sufficient resolution P. Raisin, 16.4.2015 8
Instrument setup: Stabilit variable optical attenuator 2% port 3 fied fiber collimator CCD microscope objective 980nm SLED wideband coupler 98% port 1 port 2 circulator Splitter Source Detection Confocal/Scanning Unit pieo-stage with sample Repeated single-line scan on commercial sample Refocusing on-point for T=6h +1.447 refractive inde 8 10 3 refractive inde (10-4 ) 7 10 4 -position [um] average deviation: 1.1 10 4 time [h] average deviation of 1.85 10 4 /h. P. Raisin, 16.4.2015 9
Results: Calibration Calibration curve Sample map refractive inde n=1.75 n=1.45 -position [um] detector signal [a.u.] detector signal [a.u] -position [um] Five 1 polished commerciall available samples hard to find calibration samples! questionable use for inde differences <10 3 P. Raisin, 16.4.2015 10
Results: Nufern LMA-GDF 30/250 Refractive inde map +1.449 Line Profile -displacement [um] clad. PM rod core dust refractive inde refractive inde (10-3 ) cladding core -displacement [um] -position [um] averaged inde difference: (1.410 ± 0.007) 10 3 theoretical inde difference: 1.325 10 3 cladding standard deviation: 2.3 10 4 good agreement P. Raisin, 16.4.2015 11
Results: HI-1060 SMF Refractive inde map +1.457 Line Profile -displacement [um] core cladding refractive inde refractive inde (10-3 ) cladding core -displacement [um] -position [um] averaged inde difference: (6.75 ± 0.05) 10 3 theoretical inde difference: 6.74 10 3 cladding standard deviation: 7 10 4 3 higher than LMA P. Raisin, 16.4.2015 12
Results: M31L02 graded inde fiber Refractive inde map Line Profile -displacement [um] cladding core refractive inde refractive inde cladding core 2 nd degree polnomial fit -displacement [um] -position [um] NA=0.275 core Ø62.5 µm P. Raisin, 16.4.2015 13
Towards all-in fiber setup: lensed fibers 980nm SLED variable optical attenuator wideband coupler 2% 98% port 1 port 3 port 2 circulator fied fiber collimator CCD Splitter Source Detection Confocal/Scanning Unit microscope objective pieo-stage with sample lensed fiber (commercial) sample fiber lensed fiber tip Working distance 15±2μm Spot-sie 3.0±0.5μm lensed fiber collimator and microscope objective combined full in-fiber setup Park, Y., Seong, N.H, Youk, Y & Kim, D.Y (2002). Simple scanning fibreoptic confocal microscop for the refractive inde profile measurement of an optical fibres, Meas. Sci. Technol. 13, 695 699 P. Raisin, 16.4.2015 14
Lensed-fibers: preliminar results Calibration HI-1060 SM-fiber map refractive inde -position [um] cladding detector signal [a.u.] detector signal [a.u.] Compare lensed fiber to working setup Impact of surface topolog? core -position [um] strong inhomogeneities artefacts? P. Raisin, 16.4.2015 15
Outlook: Intrinsicall stable source 980nm SLED variable optical attenuator wideband coupler 2% 98% port 1 port 3 port 2 circulator fied fiber collimator CCD Splitter Source Detection Confocal/Scanning Unit microscope objective pieo-stage with sample Yb-ASE source variable optical attenuator Idea: highl stable low power Yb-ASE source + dedicated detectors no need for source monitoring Source Monteiro, F., Guerreiro, T., Sanguinetti, B & Zbinden, H. (2013). Intrinsicall stable light source at telecom wavelengths, Appl. Phs. Lett. 103, 051109 P. Raisin, 16.4.2015 16
Conclusions Averaged inde-step of commercial fibers <10-4 Unreliable absolute refractive inde measurements (errors >10-3 ) More calibration samples needed High cladding standard deviations: Topolog? Goal: reliabl measure in the range of 10-5 P. Raisin, 16.4.2015 17