Broad Band Mid-IR Transmitting Single Mode Fibers (SMFs) and Integrated Optical Circuits (IOCs) - Spatial Filters for the ESA Spatial Filters for the ESA DARWIN Project DARWIN Project Abraham Katzir Tel Aviv University, Tel Aviv, ISRAEL

The TPF and the Darwin projects The TPF and the Darwin projects Nulling interferometry Nulling interferometry Spatial & modal filtering Spatial & modal filtering Single mode fiber as a modal filter Single mode fiber as a modal filter Silver halide material and fibers Silver halide material and fibers Single mode silver halide fiber Single mode silver halide fiber Measurements & results Measurements & results Micro-structured fibers Micro-structured fibers Single mode flat waveguide (for Integrated Optics Circuits) Single mode flat waveguide (for Integrated Optics Circuits) Conclusions Conclusions Summary Summary Lecture Outline

Performing atmosphere spectroscopy in the 8-20 μ m mid-IR spectral range for planets near stars. Indications for the presence of life? Target: A star “masks” the radiation from a neighboring planet Problem: DARWIN and TPF projects DARWIN and TPF projects Nulling interferometry Solution:

Selecting the operating region 4µm - 20µm NullingInterferometry TPF and DARWIN basic idea

Darwin - Alain Leger, Paris Pierre Kern, Grenoble TPF – Peter Lawson, Alex Ksendzov JPL TPF – Peter Lawson, Alex Ksendzov JPL Collaboration & Funding

Result: Phase deviations caused by: Wave front (phase) deviations A. Dust B. Telescope imperfections C. Telescope pupil Destroying the interference pattern Proposed Solutions: A. Spatial filtering (Pinhole) B. Modal filtering (Single mode fibers or waveguides) B. Modal filtering (Single mode fibers or waveguides )

d 2ρ2ρ z0z0 Reflectingsurfaces Modal filtering using Single Mode Fibers

IR Transmitting Single Mode Fibers Beam splitter Fold Mirror F old Mirrors Compensation Plate (  phase shift) Space Telescope Spatial Filter for the Nulling Interferometer Spatial Filter for the Nulling Interferometer IR Detector

Theoretical evaluation of the modal filtering by a step index single mode fiber * *O. Wallner et. al.

Step index fiber configuration a b Theoretical model: b → ∞ b → ∞ Real fibers: b - finite b - finite

Single Mode Conditions Single mode condition (LP 01 ) V<2.405 Waveguide parameter - Number of modes -  Small difference between indices of refraction  Small core diameter

*Theoretical evaluation of *Theoretical evaluation of the minimal filter length - z 0 Modal filtering is length dependent !! *O. Wallner et. al.

A= P LP 0 1 (z 0 ) / P LM (z 0 ) For modal filtering:  A= 10 6  Filter losses ~ 1-2 dB/m Definition: Attenuation Factor – Model 2ρ2ρ z0z0 Theoretical Estimates - O. Wallner et. al.

IR Transmitting Materials Wavelength [  m] Silica Glasses Sapphire Fluoride Glasses Silver Halide Crystals Chalcogenide Glasses Most Suitable

Candidates for Single mode fibers (Other than Silver Halides) Chalcogenides* glasses seems to have the most promising performance the most promising performance * Proc. SPIE 5905, 447, 2005 * J. Opt. Adv. Mat. 4, 665, 2002 Developed by the University of Rennes France Under DARWIN contract Fluorides Chalcogenides

Silver Halide Crystals and Fibers at Tel Aviv University (TAU)

Silver Halides Crystals - Optical Properties - Transmission Range AgCl AgBr 0.4 to 25  m 0.45 to 35  m

Crystal Growing System

cm AgClBr Crystals Typical Dimensions

Heaters Crystal Upper & Lower Plates Fiber Rod Die Press Extrusion of a Silver Halide Fiber

Polycrystalline Structure – Typical Grain Size ~ 1µm Silver Halide Unclad Fibers – Properties

Transmission Range & Loss Coefficient* Silver Halide Unclad Fibers – Properties Rayleigh Gans scattering λ ≈D scat ; I scat α λ 2 * Measured by FTIR * Measured by FTIR

* Measured at TAU where x – the molar fraction of chlorine in the compound. * Measured at TAU where x – the molar fraction of chlorine in the compound. Silver Halides Crystals - Optical Properties - Refractive Indices of AgCl x Br 1-x Solid Solutions *

Summary of silver halide fiber parameters Spectral range μm Optical losses at 10 μm unclad 0.2 dB/m unclad 0.2 dB/m (or 95% * per meter) core/clad ~1 dB/m core/clad ~1 dB/m (or 93% * per meter) core diameter unclad mm core/clad mm Length m Length m Field of view ~ 45º Flexible, Non toxic, Non-hygroscopic, Biocompatible

Single Mode Fibers (SMFs) - Basic “theoretical” demands - B. Small core A. Small difference between indices of refraction ≤ 2.405

Predicted Region for Single Mode 10.6  m Predicted Region for Single Mode 10.6  m AgClBr single mode fibers applicable for nulling interferometer mission AgClBr single mode fibers applicable for nulling interferometer mission

Silver halide AgCl x Br 1-x Single Mode Fiber (SMF) configuration a b x x µm>2a>50µm2b=900µm

Improvement of the core-clad interface: - Reducing the roughness - Reducing the impurities  Solving the problem of cracks  Small core = Extrusion process: Small Δ n = Homogeneous crystals: Reduction of core diameter to 2a ~  m  Reduction of  n=n 1 -n 2 to  n ~  Silver Halide SMF - Practical demands for single mode operation -

Crystal Homogeneity: Crystal Growing Crystal Composition Measurements

The Composition as a Function of Position in Various Cross Sections Along a Vertical Line FOR EXAMPLE Nominal composition: 83% Br 83.5 ± Lower layer [mm]

Reduction of Core Diameter α [dB/m] = 0.5 (2a=350µm), 1 (140µm), (60µm) Measurements at =10.6  m Smooth Interface; Round (±5%) and homogeneous cores    60  m core fiber 900  m M  50 M   m Core : AgCl 40 Br 60 Clad : AgCl 95 Br 5 AgCl 95 Br 5 R z ~ nm (Former R z ~ 1 to 2µm) (Former R z ~ 1 to 2µm)

IR Problem: Clad modes interfere with core radiation Output end of the Step Index (SI) core-clad silver halide fiber of length L=50 cm and core diameter 2a = 60  m Significant total energy in the clad

Removal of Clad Modes Goal: Attenuation of clad modes  40dB Method: Adding an absorbing layer on the external surface of the fiber the external surface of the fiber

Clad mode attenuation by Application of an absorbing layer a b Absorbing layer Output end of a coated SI core-clad silver halide fiber (comment: photograph overexposed) Core diameter = 60  m IR 900  m

Optical Properties of Silver Halide Single Mode Fibers

SMF With Core Diameter = 50 µm - Typical Losses   dB/m   dB/m “Smooth” far field pattern “Smooth” far field pattern Composition: Core: AgCl 0.3 Br 0.7 Inner clad:AgCl 0.32 Br 0.68 Inner clad: AgCl 0.32 Br 0.68 Far field distribution (L=50cm)

Radialfar field distribution Radial far field distribution Typical far field pattern of a 50µm core Silver halide SMF, L=50cm V# =2.1033

CO 2 laser Demonstration of modal filtering SMF L=50 µm Lens Silicon windows Spiricon IR camera

Microstructured Optical Fibers J. C. Flanagan et al. Microstructured fibers are potentially better suited for modal filtering than step index (SI) fibers Main claim:

3 August 2015Applied Physics Group41 A schematic drawing of a configuration of a TIR - PCF Photonic Crystal Fibers - PCFs Transmission via Total Internal Reflection - TIR C B D n1n1 n2n2 n 2 <n 1

3 August 2015Applied Physics Group42 A Thermal Image of a CO 2 Laser Beam Transmitted through a large core PCF Laser CO 2 PCF Thermal Camera Input Output Beam confined to the core area

Flat Waveguide Flat Waveguide Y coupled waveguides will be the basis of integrated optical circuits > 20  m core thickness  x ~ 5%

* Radiation was coupled directly to the flat guide, using a F= 36cm lens (D=2.54cm). Thermal image of the output end of the waveguide The input end was illuminated by a CO 2 Laser radiation* Single Mode Flat Waveguide

We have developed a new crystal growing technique ensuring composition homogeneityof about ±1% We have developed a new crystal growing technique ensuring composition homogeneity of about ±1% Discussion We have developed an absorbing coating that is useful for stripping of cladding modes. We established special extrusion conditions needed for the extrusion of core-clad fibers of extremely small cores.    We have developed and fabricated fibers having small core and small  n that exhibit Single Mode properties.  The extrusion process has been improved 

We have developed a new single mode flat waveguide which can be used for fabrication of integrated optical circuit. Discussion  We have developed microstcutured fiber and demonstrated transmission through its core. 

Conclusions