High-precision confocal reflection measurement for two dimensional refractive index mapping of optical fibers

Transcription

1 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

2 Outline Motivation Measurement principle Instrument setup Results: stabilit calibration commercial fibers Improvements: lensed fibers & source Conclusion P. Raisin,

3 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, 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

4 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, 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), Meas. Sci. Technol. 13, P. Raisin,

5 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, 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,

6 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,

7 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,

8 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 step-width 0.45μm 0.22μm 0.04μm error time [min] -position [um] Pieo-stage has sufficient resolution P. Raisin,

9 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 refractive inde refractive inde (10-4 ) position [um] average deviation: time [h] average deviation of /h. P. Raisin,

10 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,

11 Results: Nufern LMA-GDF 30/250 Refractive inde map 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: cladding standard deviation: good agreement P. Raisin,

12 Results: HI-1060 SMF Refractive inde map 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: cladding standard deviation: higher than LMA P. Raisin,

13 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,

14 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, P. Raisin,

15 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 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, P. Raisin,

17 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,