multilayer thin film coatings for reduced infrared loss in hollow glass waveguides
DESCRIPTION
ICO-22 Paper #2286190. 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 - PowerPoint PPT PresentationTRANSCRIPT
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