GARY PICKRELL, ANBO WANG, BRIAN SCOTT, ZHIHAO YU

D E P A R T M E N T O F M A T E R I A L S S C I E N C E & C E N T E R F O R P H O T O N I C S T E C H N O L O G Y

V I R G I N I A T E C H , B L A C K S B U R G , VA 2 4 0 6 1P I C K R E L L @ V T . E D U

Novel Optical Fibers for High Temperature

In-Situ Miniaturized Gas Sensors (DE-FC26-05NT42441)

NETL CrossCutting Review Meeting

Motivation

Gas Species Sensor for harsh environments Coal Gasification products Advanced power generation

Gas species determination Process optimization Process fault/Failure identification

Scope of Work

Scope: The objective of the proposed program is to develop novel

modified fiber-based materials for miniaturized optical sensors for in-situ detection of a wide range of gaseous species at high temperatures and in harsh environments.

Background: Technical Approach

Transmission of light is not 100% Loss from impurities Loss from evanescent field Loss due to direct absorption

The Evanescent field is a portion of the EM field that extends beyond the core/cladding boundary

The interaction between the evanescent field or injected light and a gas provides an absorption spectra

This spectra is specific to the gas

I

RR-CoreR-Core

Evanescent

field

1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570-40

-35

-30

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Wavelength (nm)

Tran

smis

sion

(dB

)

Sample 550Y 700-50

Transmission in airTransmission with acetylene blow on

Background: Technical Approach

Design is fabricated by A spinodally phase separating

cladding glass Produces a 3-D interconnected

cladding Porosity has a radial

orientation Porosity connects the fiber

surface to the core

core

Radial oriented pores

Porous cladding

Solid core

Background: Porous Glass

Porous glass are made from phase separable glass compositions Article is made desired

shape Article is annealed Annealing induces phase

separation so two interconnected phases are present

One of the phases is removed

Porous skeleton remains with a 3-D interconnected porosity

Background: Prior Work

Two part structure Cladding glass- Porous glass

Core glass- solid borosilicate glass

Hollow core Pore size range

10-30 nm avg. Range tunable 4 nm to 100 nm

Sapphire Photonic Crystal Sapphire bundle based

optical structure

Background: Prior Work

Sensing ~ 1 second response time Stable up 500° C

1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570-40

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-5

0

WAVELENGTH(nm)

TRA

NS

MIS

SIO

N(d

B)

FIBER TRANSMISSION

1520 1525 1530 1535 1540 1545-32

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WAVELENGTH(nm)

TRA

NS

MIS

SIO

N(d

B)

ABSORPTION PEAKS OF ACETYLENE

Lead in Fiber Lead out Fiber

Fiber Stage

Gas outletPorous clad optical fiber

Current Tasks

Sapphire Photonic Crystal Fiber (SPCF) gas sensor Fabrication Sapphire bonding Testing

Porous Glass Optical Fiber Gas Sensor Sensor integration in optical system Transmission Loss reduction Gas sensing sensitivity and resolution characterization Field test prototype

Long wavelength laser system development

Center Gap Sensor

Center gap between highly polished sapphire fibers enables measurement of gas absorption peaks

Center fibers and gap held in place using six surrounding fibers in hexagonal formation

Center Gap Sensor Construction

Sensor is assembled using tapering funnels and platinum wire

Sensor Bonding- Center Gap Design

Bonding colloid is applied to areas outside of the gap area Fired at 1600°C for 12-72 hours to encourage diffusion

bonding

Colloidal particles infiltrate fibers, forming strong bond

Sapphire bundle gas sensor

• Diameter of the outer sapphire fibers is 70µm and center sapphire fiber is 50µm

• Air cavity = 10mm

Sapphire bundle with different gas concentration

3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6

x 104

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Nor

mal

ized

Vol

tage

Gas absorption in sapphire bundle

100%50%25%10%

• Wavelength at 1530.4nm

3.55 3.555 3.56 3.565 3.57 3.575 3.58 3.585 3.59 3.595

x 104

0.55

0.6

0.65

0.7

0.75

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0.95

1

Nor

mal

ized

Vol

tage

Gas absorption in sapphire bundle

100%50%25%10%

New sapphire bundle with FC connector

• Sapphire bundle sensor with (a) FC connectors and (b) splicing points (Both air cavity=1cm)

• Transmitted power of the sapphire bundle with FC connector has two orders of magnitude stronger than one with splicing point

• Sapphire bundle with FC connector has less modal noise

Long Wavelength SystemWhy the mid-infrared?

4 4.1 4.2 4.3 4.40

1

2

3

4x 10‐18

Wavelength (m)

Line

stre

ngth

 (cm

2 cm

‐1)

Optical Spectrum of CO2

The mid-infrared, or mid-IR, is a portion of the electromagnetic spectrum spanning wavelengths between 3-15μm

The Long Wavelength System operates between 4-4.42μm

This range falls inside a transparent atmospheric window where light transmits well

This is also where chemical compounds exhibit fundamental vibrational modes

Optical spectrum measurements are used to observe the effects of these modes to both uniquely identify & quantify chemical compounds, e.g. CO2, CO, N2O, SO3

Direct absorption detection technique

Main advantage is simplicity

Light passes through a gas cell that contains the chemical for detection

Light interacts with the chemical as it traverses a porous silica or sapphire waveguide

Light intensity values are recorded for different wavelengths

These values form an absorption spectrum from which the gas is both identifiable chemically and quantifiable in terms of concentration

Laser DetectorGas

Cell WaveguideChemical for Detection

PC

CO25.3%

Signal ProcessingIdentifying and quantifying gases

The detection technique provides an absorption spectrum that appears as multiple dips across a slowly varying background

To understand the significance of these dips requires some signal processing that assigns numbers to their magnitudes & wavelength positions

Mathematical techniques such as correlation are used to relate these numbers to the identity of a specific gas

Measurements of unknown concentrations can be determined by comparison versus data of known concentrations

CO25.3%

Porous Glass Sensor Joining Techniques

Trial methods Fusion Splicing CO2 laser bonding Ceramic adhesive

Trial evaluation Transmission Bond stability

Best method CO2 laser bonding

Before bonding

After Bonding

Porous Glass Sensor

Transmission improvement Gold coating sensor interior Increased boundary

reflection Reduced loss

Nano- Porous Glass capillary tube wall

Sub micron Gold coating layer

Signal Improvement Methods

Gold plating of inner surface of glass prior to leeching.

Porous gold coating obtained on surface.

High variation, so many samples needed to obtain best results.

Plated using commercially available electroless solutions

Alternative Structures

Ordered tube arrays using vycor to obtain Photonic Crystal Fiber Structure.

Tubes stacked from pulled fibers using machined mold

Pulled using mini draw tower.

Very sensitive to heat profile of furnace.

Potential Packaging Design Two

potential designs

Provide protection to the fragile nature of the glass.

Future work

Testing of SPCF gas Optical properties Gas sensing capability

Continuing evaluation of LW system Long wavelength system gas sensing demonstration

Sensitivity measurement of SPCF and porous glass sensors

Construction of porous glass gas sensor prototype Porous glass photonic crystal fiber prototype

Thank you

Questions?