Multilayer thin film coatings for reduced infrared loss in hollow glass waveguides

Carlos M. Bledt*a, Daniel V. Kopp a, and James A. Harrington a

a Dept. of Material Science & Engineering

Rutgers, the State University of New Jersey

Jason M. Kriesel b

b Opto-Knowledge Systems, Inc.

19805 Hamilton Ave., Torrance CA 90502

August 19, 2011

ICO-22 Paper #2286190

Background on Hollow Glass Waveguides

• Hollow Glass Waveguides (HGWs) are used in the low loss broadband transmission of infrared radiation ranging from λ = 2 – 16 μm

• HGWs function due to enhanced reflection of incident IR radiation

• Structure of HGWs:– Silica capillary tubing substrate– Silver (Ag) film ~200 nm thick– Dielectric film(s) such as AgI, CdS, PbS, PS– Current research on multilayer structures

Silica Wall

Dielectric Film

Polyimide Coating

Silver Film

• Inherent loss dependence in HGWs:– Proportional to 1/a3 (a is bore radius)– Proportional to 1/R (R is bending radius)– Decreases with increasing number of layers

Advantages of Hollow Glass Waveguides

• Advantages of HGWs include:– High laser damage threshold– Broadband IR transmission– Customizability of optical response– No end reflection losses– Low production costs– Low losses at IR wavelengths– Small beam divergence– Single-mode radiation delivery– Higher order-mode filtering

• Applications of HGWs include:– Surgical laser delivery– IR chemical & gas sensing– Thermal imaging– IR spectroscopy

Theoretical Loss in HGWs

• Losses in HGWs depend on:– Propagating modes– Dielectric thin film materials– Thickness of deposited films– Quality and roughness of films

• Losses calculated using both wave and ray optics theories

α = power attenuation coefficient

a = HGW inner diameter size

R = power reflection coefficient

θ = angle of propagating ray

• Power reflection coefficient, R(θ), depends on incident angle as well as on film materials & structure

𝟐𝜶 (𝜽 )= 𝟏−𝑹 (𝜽 )𝟐𝒂𝒄𝒐𝒕 (𝜽 )

Ray optics equation for calculating loss

• HE11 mode is lowest loss mode in dielectric coated HGWs

• Loss contribution due to surface roughness (scattering)

Ag/AgI HGW

Experimental Approach

• Research objectives:– Theoretically determine the loss of HGWs incorporating dual layer structures– Develop a deposition kinetics model for the growth of AgI, CdS, and PbS films– Optimize dual layer HGWs incorporating secondary PS films for Tmax @ λ = 10.6 μm

– Fabricate dual layer HGWs and determine efficacy in reducing loss

• Experimental Approach– 700 μm ID HGWs used in study– Film growth kinetics determined– Growth kinetics used to determine optimal AgI,

CdS, and PbS film thicknesses– Separate study of PS film thickness as a

function of solution concentration– Optimization of films for dual dielectric structure

with secondary PS film– Comparison of measured losses for dual layer

HGWs with theory and single layer HGWs

nL Film (PS)

nH Film (AgI, CdS, PbS)

Silver Film

n

Index profile

Fabrication Methodology

• Films deposited in HGWs from precursor solutions via dynamic liquid phase deposition (DLPD)

• The DLPD process:– Peristaltic pumps used to flow precursor solutions through HGW– Constant flow of precursor solutions allows for deposition of films

Ag, AgI, CdS, and PbS Film Deposition System Configuration

Precursor Solution #1

Precursor Solution #2

HGW

 

  

 

  

 

 

 

  

 

Waste

Peristaltic Pump

x.xx rpm

Polystyrene Solution

HGW

Peristaltic Pump

Micro-Bore Tubing

 

 

 

 x.xx rpm

 

 

 

• No depletion of solution concentrations in DLPD

• DLPD setup depends on precursor solution viscosity

• DLPD process used for Ag, AgI, CdS, PbS, and PS thin films in HGWs

PS Film Deposition System Configuration

FTIR Analysis of Single Dielectric Thin Films

• FTIR analysis from λ = 2 – 15 μm used to determine mid-IR spectral response of HGWs as a function of dielectric film deposition time

• Spectral response shifts to longer wavelengths with increase in dielectric film thickness

• Establish deposition kinetics to deposit films of desired thickness

CdS Thin FilmsAgI Thin Films

PbS Thin Films

Deposition Kinetics of Dielectric Thin Films

• Film thickness of dielectric films can be calculated from FTIR spectra

AgI Thin Films

CdS Thin Films

For a single dielectric layer

λp = 1st interference peak position

nd = dielectric film refractive index

df = dielectric film thickness

𝒅𝒇=𝝀𝒑

𝟒√𝒏𝒅𝟐−𝟏

• df as a function of deposition time determined for AgI, CdS, PbS

Growth Rate: 3.00 nm/sec

Growth Rate: 1.59 nm/min

PbS Thin Films

Growth Rate: 3.13 nm/min

Polystyrene Thin Films

• Polystyrene (PS) can be used as low index thin film in HGWs

• Polystyrene thin films can be deposited via DLPD process

• Advantages of PS dielectric thin films– Low refractive index material (n = 1.58 @ λ = 10.6 μm) – Ability to be deposited from aqueous solution– Inexpensive and non-toxic material– Ability to deposit uniform thin films

• Control of PS film thickness:– Solution concentration– Deposition pump rate– Volume of solution– Drying/curing process– Deposition time independent

Practical Design of Multilayer HGWs

• Considerations in the practical design of multilayer HGWs:– Correct compound film thickness for Tmax @ desired λ range

– Careful design of individual layer thicknesses using kinetics studies– Individual films must be mechanically, thermally, and optically compatible– Higher refractive index contrast of films yields lowest losses

For a multilayer dielectric stack

λi = ith layer 1st interference peak contribution

ni = ith dielectric film refractive index

dc = composite dielectric film thickness

𝒅𝒄=∑𝒊

𝒋 𝝀𝒊

𝟒 √𝒏𝒊𝟐−𝟏

Dielectric Material

Deposition Time

Dielectric Film Thickness (µm)

AgI 55 sec 0.40

CdS 330 min 0.41

PbS 55 min 0.23

• Desired secondary PS film thickness: ~ 0.15 μm (3 wt % PS sol. used)

• First interference peak of dual layer HGW designed @ λ = 4 μm ± 10%

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/AgI N/A 0.183 0.43

Ag/AgI/PS 0.52 0.097 0.11

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/AgI N/A 0.183 0.43

Ag/AgI/PS

Polystyrene n = 1.58

Silver Iodide n = 2.10

Silver Film

Silver Iodide n = 2.10

Silver Film

Silver Iodide / Polystyrene Coated HGWs

• Waveguide loss reduced by a factor of 1.88

• Polystyrene film was of good uniformity along sample length

Diagram representation of Ag/AgI HGW before and after PS addition

Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/CdS N/A 0.259 0.42

Ag/CdS/PS 0.67 0.131 0.16

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/CdS N/A 0.259 0.42

Ag/CdS/PS

Polystyrene n = 1.58

Cadmium Sulfide n = 2.27

Silver Film

Cadmium Sulfide n = 2.27

Silver Film

Cadmium Sulfide / Polystyrene Coated HGWs

• Waveguide loss reduced by a factor of 1.97

• Polystyrene film was of good uniformity along sample length

Diagram representation of Ag/CdS HGW before and after PS addition

Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/PbS N/A 0.409 0.22

Ag/PbS/PS 2.42 0.194 0.16

HGW Structure

Δn(|nL-nH|)

Loss (dB/m)

Dielectric Film Thickness (µm)

Ag/PbS N/A 0.409 0.22

Ag/PbS/PS

Polystyrene n = 1.58

Lead Sulfide n = 4.00

Silver Film

Lead Sulfide n = 4.00

Silver Film

Lead Sulfide / Polystyrene Coated HGWs

• Waveguide loss reduced by a factor of 2.11

• Polystyrene film was of decent uniformity along sample length

Diagram representation of Ag/PbS HGW before and after PS addition

Shift in spectra seen after deposition of PS film shows successful deposition of dual dielectric layer stack

Conclusion

• Successful fabrication of dual layer HGWs incorporating PS thin films

• Deposited films via DLPD process exhibited:– Good uniformity & IR spectral response shift– Reduced loss with addition of PS film– Chemical & mechanical structural stability– Lower losses with > film index contrast

n2 Film

n1 FilmSilver Film

n

Index profile

• Future research:– Incorporation of novel materials such as

ZnS & ZnSe as dielectric thin films– Fabrication of HGW structures with larger

number of alternating index films– Possibility of photonic bandgap structure

with larger number of alternating layers– Multilayer dielectric designs in gradually

tapered HGWs