Broad Band Mid-IR Broad Band Mid-IR Transmitting Transmitting

Single Mode Fibers (SMFs) Single Mode Fibers (SMFs) and Integrated Optical and Integrated Optical

Circuits (IOCs) - Circuits (IOCs) - Spatial Filters for the ESASpatial Filters for the ESA

DARWIN ProjectDARWIN ProjectAbraham KatzirAbraham Katzir

Tel Aviv University, Tel Aviv, Tel Aviv University, Tel Aviv, ISRAEL ISRAEL

www.tau.ac.il/~applphyskatzir@post.tau.ac.ilkatzir@post.tau.ac.il

• The TPF and the Darwin projectsThe TPF and the Darwin projects• Nulling interferometryNulling interferometry• Spatial & modal filteringSpatial & modal filtering• Single mode fiber as a modal filterSingle mode fiber as a modal filter• Silver halide material and fibersSilver halide material and fibers• Single mode silver halide fiber Single mode silver halide fiber • Measurements & resultsMeasurements & results• Micro-structured fibersMicro-structured fibers• Single mode flat waveguide (for Integrated Single mode flat waveguide (for Integrated

Optics Circuits)Optics Circuits)• ConclusionsConclusions• SummarySummary

Lecture OutlineLecture Outline

Performing Performing atmosphere atmosphere spectroscopy in the 8-spectroscopy in the 8-2020μμm mid-IR spectral m mid-IR spectral range for planets near range for planets near stars. stars. Indications for the Indications for the presence of life?presence of life?

TargeTargett::

A star “masks” the A star “masks” the radiation from a radiation from a neighboring planetneighboring planet

ProblemProblem::

DARWIN and TPF DARWIN and TPF projectsprojects

Nulling interferometryNulling interferometrySolutiSolution:on:

Selecting Selecting the the

operating operating region region

4µm - 20µm4µm - 20µm

NullingNullingInterferomInterferom

etryetry

TPF and DARWIN basic ideaTPF and DARWIN basic idea

Darwin - Alain Leger, ParisDarwin - Alain Leger, Paris Pierre Kern, GrenoblePierre Kern, Grenoble

TPF – Peter Lawson, Alex Ksendzov JPLTPF – Peter Lawson, Alex Ksendzov JPL

Collaboration & FundingCollaboration & Funding

ResulResult:t:

Phase deviations caused by:Phase deviations caused by:

Wave front (phase) deviationsWave front (phase) deviations

A. DustA. Dust

B. Telescope imperfectionsB. Telescope imperfections

C. Telescope pupilC. Telescope pupil

Destroying the interference patternDestroying the interference pattern

Proposed Solutions:Proposed Solutions:

A. Spatial filtering (Pinhole)A. Spatial filtering (Pinhole)

B. Modal filtering (Single mode fibers B. Modal filtering (Single mode fibers or waveguidesor waveguides ))

d

2ρ

z0

ReflectingReflectingsurfacessurfaces

Modal filteringModal filtering using Single Mode using Single Mode FibersFibers

IR TransmittingSingle Mode Fibers

Beam splitter

Fold Mirror

Fold Mirrors

Compensation Plate

( phase shift)

Space Telescope

Spatial Filter for the Nulling Interferometer

Spatial Filter for the Nulling Interferometer

IRDetector

Theoretical evaluation Theoretical evaluation of of

the modal filtering by the modal filtering by a step a step

index single mode index single mode fiber fiber **

*O. Wallner *O. Wallner et. al.et. al.

Step index fiber configurationStep index fiber configuration

r

n

1n

2n a

b13n

Theoretical model:Theoretical model:

b → b → ∞∞

Real fibers:Real fibers:

b - finiteb - finite

Single Mode Conditions Single Mode Conditions

2122

21

0

nnλ

a2πV

2

VN

2

Single mode condition (LP01)V<2.405

Waveguide parameter Waveguide parameter - -

Number of modesNumber of modes --

Small difference between indices of refractionSmall difference between indices of refraction Small coreSmall core diameter diameter

*Theoretical evaluation of *Theoretical evaluation of

the minimal filter length the minimal filter length -- z z00

Modal filtering Modal filtering

is length dependent !!is length dependent !!

*O. Wallner *O. Wallner et. al.et. al.

A= PA= PLPLP0 1 0 1 (z(z00))

/ /

PPLM LM (z(z00))For modal filteringFor modal filtering::

A= 10A= 1066 Filter losses Filter losses ~ ~ 1-2 1-2 dB/mdB/m

DefinitionDefinition::

Attenuation Factor – Model Attenuation Factor – Model

2ρ

z0

Theoretical Estimates - O. Theoretical Estimates - O.

Wallner et. al.Wallner et. al.

IR Transmitting MaterialsIR Transmitting Materials

0.10.1 11 1100WavelengthWavelength[ [ mm]]

Silica Glasses

Sapphire

Fluoride Glasses

Silver Halide CrystalsSilver Halide Crystals

Chalcogenide Glasses

Most Most SuitaSuitableble

Candidates for Single mode fibersCandidates for Single mode fibers((Other than Silver HalidesOther than Silver Halides))

Chalcogenides* glasses seems to haveChalcogenides* glasses seems to have the most promising performancethe most promising performance

* Proc. SPIE * Proc. SPIE 5905 5905, 447, 2005, 447, 2005* J. Opt. Adv. Mat. 4, 665, 2002* J. Opt. Adv. Mat. 4, 665, 2002

Developed by the Developed by the University of Rennes FranceUniversity of Rennes France

Under DARWIN contract Under DARWIN contract

FluoridesFluoridesChalcogenidesChalcogenides

Silver HalideSilver Halide

Crystals and Crystals and

FibersFibers

at Tel Aviv University at Tel Aviv University

(TAU)(TAU)

Silver Halides CrystalsSilver Halides Crystals - -Optical PropertiesOptical Properties- -

Transmission RangeTransmission Range

AgClAgCl

AgBrAgBr

0.4 to 250.4 to 25mm

0.45 to 350.45 to 35mm

Crystal Growing SystemCrystal Growing System

cm

AgClBr CrystalsAgClBr Crystals

Typical DimensionsTypical Dimensions

Heaters

Crystal

Upper & LowerPlates

Fiber

Rod

Die

Press

Extrusion of a Silver Halide FiberExtrusion of a Silver Halide Fiber

Polycrystalline Structure – Typical Grain Polycrystalline Structure – Typical Grain Size Size ~~ 1µm 1µm

Silver Halide Unclad Fibers – Properties Silver Halide Unclad Fibers – Properties

Transmission Range & Loss Coefficient*Transmission Range & Loss Coefficient*

Silver Halide Unclad Fibers – Properties Silver Halide Unclad Fibers – Properties

Rayleigh Gans scattering Rayleigh Gans scattering λλ≈D≈Dscatscat ; I ; Iscat scat αα λλ22 * Measured by * Measured by

FTIRFTIR

* Measured at TAU where x – the molar fraction of * Measured at TAU where x – the molar fraction of chlorine in the compound.chlorine in the compound.

Silver Halides CrystalsSilver Halides Crystals - -Optical PropertiesOptical Properties- -

Refractive Indices of AgClRefractive Indices of AgClxxBrBr1-x1-x Solid Solutions * Solid Solutions *

Summary of silver halide fiber Summary of silver halide fiber parametersparameters

Spectral rangeSpectral range

2 - 25 μm2 - 25 μm

Optical losses at 10 μmOptical losses at 10 μm

uncladunclad

0.2 dB/m0.2 dB/m (or 95%* per meter)  

core/cladcore/clad

~~1 dB/m1 dB/m (or 93%* per meter)  

core diametercore diameter

uncladunclad

0.7 - 0.9 mm0.7 - 0.9 mm

core/cladcore/clad

0.3 - 0.6 mm0.3 - 0.6 mm  LengthLength

2 - 10 m2 - 10 m  

Field of view Field of view ~ 45º~ 45º

Flexible, Non toxic, Non-hygroscopic, BiocompatibleFlexible, Non toxic, Non-hygroscopic, Biocompatible

  

Single Mode Fibers (SMFs)Single Mode Fibers (SMFs)

- Basic “theoretical” demands -- Basic “theoretical” demands -

212

2

2

10

2nn

aV

B. Small coreB. Small core

A. Small difference between indices of refraction A. Small difference between indices of refraction

≤ ≤ 2.4052.405

Predicted Region for Single Predicted Region for Single Mode Operation @ 10.6Mode Operation @ 10.6mm

AgClBr single mode fibers AgClBr single mode fibers applicable for nulling applicable for nulling

interferometer missioninterferometer mission

Silver halide Silver halide AgClAgClxxBrBr1-x1-x Single Single Mode Fiber (SMF) Mode Fiber (SMF)

configurationconfiguration

r

n1n

2na

bxx

x+x+0.020.02

60µm>2a>50µm60µm>2a>50µm 2b=900µm2b=900µm

Improvement of the core-clad interface:Improvement of the core-clad interface:- Reducing the roughness- Reducing the roughness- Reducing the impurities- Reducing the impurities

Solving the problem of cracksSolving the problem of cracks

Small core = Extrusion process:Small core = Extrusion process:

Small Small ΔΔn = Homogeneous crystals:n = Homogeneous crystals:

Reduction of core diameter to 2a Reduction of core diameter to 2a ~~ 60 - 3060 - 30mm

Reduction of Reduction of n=nn=n11-n-n2 2 to to nn ~~ 0.005 0.005

Silver Halide SMFSilver Halide SMF

- Practical demands for single mode operation -- Practical demands for single mode operation -

Crystal Homogeneity:Crystal Homogeneity:

Crystal Growing Crystal Growing

Crystal Composition Crystal Composition

MeasurementsMeasurements

The Composition as a Function of The Composition as a Function of Position in Various Cross Sections Position in Various Cross Sections

Along a Vertical LineAlong a Vertical LineFOR EXAMPLEFOR EXAMPLE

Nominal composition: 83% BrNominal composition: 83% Br

83.5 ± 0.883.5 ± 0.8

84.084.0

84.084.084.084.0

84.584.5

84.584.5

83.083.0

82.582.5

82.582.5

Lower Lower layerlayer

181

181

4141

5252

6565

1010

66

[mm][mm]

Reduction of Core DiameterReduction of Core Diameter

αα [dB/m][dB/m] = 0.5 = 0.5 (2a=350µm)(2a=350µm), 1, 1((140µm)140µm), 4 - 5, 4 - 5(6(60µm)0µm)

Measurements at Measurements at =10.6=10.6mm

Smooth Smooth Interface;Interface;

Round (±5%) and homogeneous Round (±5%) and homogeneous corescores

60 60 m core m core fiberfiber

900 900 mm

MM5050 MM500500

60 60 mm

Core :Core :AgClAgCl4040BrBr6060

Clad :Clad : AgClAgCl9595BrBr55

RRzz~~200-250nm200-250nm ( ( Former RFormer Rzz~~1 to 2µm1 to 2µm))

IR IR

Problem: Clad modes Problem: Clad modes interfere interfere

with core radiation with core radiation

Output end of the Step Index (SI) core-clad Output end of the Step Index (SI) core-clad silver halide fiber of length silver halide fiber of length L=50 cm and core L=50 cm and core

diameter 2a = 60diameter 2a = 60mm Significant total energy in the cladSignificant total energy in the clad

Removal of Removal of Clad ModesClad Modes

Goal:Goal:

Attenuation of clad modes Attenuation of clad modes 40dB 40dB

Method:Method:Adding an absorbing layer onAdding an absorbing layer on the external surface of the fiberthe external surface of the fiber

Clad mode attenuation by Clad mode attenuation by Application of an Application of an absorbing layerabsorbing layer

r

n1n

2na

bAbsorbing layerAbsorbing layer

Output end of a coated SIOutput end of a coated SIcore-clad silver halide core-clad silver halide

fiber (comment: fiber (comment: photograph overexposed)photograph overexposed)

Core diameter = 60Core diameter = 60mm

IRIR

900 900 mm

Optical Optical

Properties of Properties of

Silver Halide Silver Halide

Single Mode Single Mode

Fibers Fibers

SMF With Core Diameter = SMF With Core Diameter = 5050µmµm

- Typical Losses- Typical Losses 15-20 dB/m15-20 dB/m

“ “Smooth” far field Smooth” far field patternpattern

Composition:Composition:

Core:Core: AgClAgCl0.30.3BrBr0.70.7

Inner clad:Inner clad: AgClAgCl0.320.32BrBr0.680.68

Far field Far field distributiondistribution

))L=50cmL=50cm((

RadialRadial far field far field distributiondistribution

Typical far field pattern of a 50µm coreTypical far field pattern of a 50µm coreSilver halide SMF, L=50cmSilver halide SMF, L=50cm

V# =2.1033V# =2.1033

CO2 laser

Demonstration of modal filteringDemonstration of modal filtering

SMF SMF L=50L=50µmµm

LensLensSilicon Silicon windowswindows

SpiricoSpiriconnIR IR

cameracamera

Microstructured Optical FibersMicrostructured Optical FibersJ. C. Flanagan J. C. Flanagan et al.et al.

Microstructured fibers are Microstructured fibers are potentially better suited potentially better suited for modal filtering than for modal filtering than step index (SI) step index (SI) fibersfibers

Main claim:Main claim:

April 19, 2023 Applied Physics Group 41

A schematic drawing of a configuration of a TIR - PCF

Photonic Crystal Fibers - PCFsPhotonic Crystal Fibers - PCFs

Transmission via Total Internal Transmission via Total Internal Reflection - TIRReflection - TIR

C

B

D

n1

n2

n2<n1

April 19, 2023 Applied Physics Group 42

A Thermal Image of a COA Thermal Image of a CO22 Laser Beam Laser Beam

Transmitted through a large core PCFTransmitted through a large core PCF Laser CO2

PCF

Thermal

Camera

Input

Output

Beam confined to the core Beam confined to the core areaarea

nnnnV coclco 222 22

Flat Flat WaveguideWaveguideY coupled waveguides Y coupled waveguides will be thewill be thebasis of integrated basis of integrated optical circuitsoptical circuits

> 20> 20mm core thickness core thicknessxx~~5%5%

* Radiation was coupled directly to the flat guide, using a F= 36cm * Radiation was coupled directly to the flat guide, using a F= 36cm lens (D=2.54cm).lens (D=2.54cm).

Thermal image of the output end of the Thermal image of the output end of the waveguide waveguide

The input end was illuminated by a COThe input end was illuminated by a CO22 Laser radiation*Laser radiation*

Single Mode Flat WaveguideSingle Mode Flat Waveguide

We have developed a new crystal growing We have developed a new crystal growing technique ensuring composition technique ensuring composition

homogeneityhomogeneity of about ±1%of about ±1%

DiscussionDiscussion

We have developed an absorbing coating We have developed an absorbing coating that is useful for stripping of cladding modes.that is useful for stripping of cladding modes.

We established special extrusion conditions We established special extrusion conditions needed for the extrusion of core-clad fibers of needed for the extrusion of core-clad fibers of extremely small cores.extremely small cores.

We have developed and fabricated fibers We have developed and fabricated fibers having small core and small having small core and small n that exhibit n that exhibit Single Mode properties.Single Mode properties.

The extrusion process has been improved The extrusion process has been improved

We have developed a new single mode flat We have developed a new single mode flat waveguide which can be used for fabrication waveguide which can be used for fabrication of integrated optical circuit. of integrated optical circuit.

DiscussionDiscussion

We have developed microstcutured fiber and We have developed microstcutured fiber and demonstrated transmission through its core. demonstrated transmission through its core.

ConclusionsConclusions