Investigation of tapered silver / silver halide coated ...irfibers.rutgers.edu/resources/publications/2012/PSI821802.pdf · Investigation of tapered silver / silver halide coated

Investigation of tapered silver / silver halide coated hollow glass waveguides for the transmission of CO2 laser radiation

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

a Dept. of Material Science & Engineering, Rutgers University, Piscataway, NJ 08855 Saiko Kino b and Yuji Matsuura b

b Graduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan Jason M. Kriesel c

c Opto-Knowledge Systems, Inc., 19805 Hamilton Ave., Torrance, CA 90502-1341

ABSTRACT

The present study focuses on the theoretical and practical infrared radiation propagation properties of tapered silver / silver iodide coated Hollow Glass Waveguides (HGWs). Tapered HGWs with inner diameters ranging from 300 µm to 650 µm with a linear taper increasing at an approximate rate of 1.5 µm/cm were fabricated and optimized for low-loss transmission of CO2 laser radiation at of 10.6 µm. The theoretical losses in these tapered silver / silver iodide coated HGWs are calculated for light transmitted from the big to the small and vice versa. Theoretical calculations used in this study are based on ray-optics. Experimental loss measurements are likewise presented, along with the calculated and measured output beam divergence. The experimental bending losses of the tapered HGWs are studied and compared with those measured and for those for non-tapered, straight bore sizes from 300 to 700 µm. Experimental losses for tapered Ag/AgI HGWs ranged from 0.732 – 1.340 dB/m depending on configuration and bending radius. Keywords: Infrared fiber optics, hollow waveguides, tapered waveguides, ray tracing analysis, geometrical optics.

1. INTRODUCTION Silver (Ag) / silver iodide (AgI) coated HGWs have been successfully used for the low loss delivery of infrared radiation ranging from λ = 2 – 14 µm. Applications include laser surgery, photothermal imaging, and spectroscopy. The basic Ag / AgI HGW structure consists of a reflective Ag plus a dielectric AgI thin film deposited on the inner surface of a fused silica capillary all using a liquid-phase chemistry approach. The vast majority of Ag / AgI HGWs consist of silica capillary tubing whose bore diameter is constant along the entire length with bore sizes ranging from 200 – 1,000 µm. The ability to fabricate constant diameter low-loss Ag / AgI HGWs have led to their commercialization, particularly for medical systems requiring the delivery of high power CO2 laser radiation for surgical applications. In the past, Ag / AgI HGWs with different geometries have also been fabricated, most notably rectangular and square waveguides which maintain the polarization. Another Ag / AgI HGW structure involves a tapered design in which the bore diameter of the silica capillary varies along the HGW length. Both short and long tapered Ag / AgI HGWs have been shown to have interesting properties differing from those with a constant bore HGWs. [1] For comparison, the various geometries of various Ag / AgI HGWs which have been successfully fabricated to date are presented in Figure 1. * [email protected]; Phone 862-485-9289; irfibers.rutgers.edu

Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XII, edited by Israel Gannot, Proc. of SPIE Vol. 8218, 821802 · © 2012 SPIE

CCC code: 1605-7422/12/$18 · doi: 10.1117/12.912201

Proc. of SPIE Vol. 8218 821802-1

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Figure 1 – Representative diagrams of a) circular constant ID, b) circular tapered ID, and c) square HGWs

The HGWs used in this study were obtained from Polymicro Technologies, Inc. The guides had a circular cross section and they were tapered from approximately 650 µm to 300 µm over approximately 2 meters. Such tapered HGWs can be expected to have different properties than their constant circular bore. In particular, this study focuses on the incorporation of Ag/AgI thin film designs in tapered HGWs for low-loss transmission at λ = 10.6 µm.

2. THEORETICAL PROPERTIES OF AG / AGI TAPERED HGWS 2.1 Theoretical Ray Propagation in Tapered HGWs

The functional differences of tapered diameter versus constant bore diameter circular HGWs can be most simply explained by accounting for light propagation through the hollow core using a ray optics approach. In constant diameter HGWs, the angle of incidence of a propagating ray on reflection from the inner HGW surface is equal to the angle of reflection as long as surface irregularities arising from surface roughness can be neglected. In the case of tapered HGWs the incident angle is not equal to the reflection angle. This concept is sketched in Figure 2 for light rays propagating along tapers with increasing and decreasing bore sizes. [1,2,3]

Figure 2 – Representative ray propagation in a) increasing ID and b) decreasing ID tapered HGWs

The unusual properties of HGWs arise from this important difference relative to constant bore circular HGWs. Furthermore, the loss and beam divergence depends on whether the bore is increasing or decreasing along the path of propagation, the slope of the taper, and whether the tapered HGW is straight or bent. Regardless of the slope of the taper and the initial angle of propagation of incident light, the angle of reflection is greater than the angle of incidence for a ray travelling in an HGW whose bore increases along the path of propagation. Figure 2 (a) represents such light ray propagating in the hollow core of a tapered HGW with increasing bore size. As a result, a ray travelling through such a HGW experiences fewer reflections relative to a constant bore HGW and it also exhibits low output beam divergence.

b)

a)

a)

b)

c)



Tapered

660 m

-f330 tm

_-*__ 0.4 dB 2.8 deg-f

Non-Tapered Loss Beam Divergence

660 mm

0.1 dB 1.Odeg

330 tm 1.8 m

+ 1.0dB 1.5deg

However, if the bore of the taper decreases along the path of propagation, then the angle of reflection of the incident ray is less than the angle of incidence. [2,3] Figure 2(b) shows light propagation through the hollow core of a tapered HGW for this case. Furthermore in this case the ray travelling through such a HGW experiences greater reflections compared to a constant bore HGW and it exhibits high output beam divergence. The differing geometry presented by tapered HGWs gives rise to their unusual light propagation properties. [1]

2.2 Geometrical Ray Optics Analysis of Tapered HGWs

Theoretical loss calculations for constant bore HGWs can be carried out using either geometrical (ray) optics or wave optics methods. Such analytical analysis can be used to account for a variety of parameters influencing optical attenuation in HGWs ranging from HGW bore size to deposited film structure and wavelength of operation. In the case of tapered HGWs, such analytical methods cannot be as easily implemented as the change in diameter along the HGW length poses a complication. [4] Theoretical loss calculations in tapered HGWs can, however, be derived using numerical methods involving a ray optics approach, in which the propagating beam is approximated by rays propagating down the HGW. The angle of propagation depends on the input beam mode and the coupling conditions. Using numerical analysis methods, the losses for Ag/AgI coated tapered HGWs, 1.8 m in length with a linear taper from 330 μm to 660 μm at a rate of +1.8 μm/cm and optimized for λ = 10.6 µm, were calculated for a number of different configurations. Such analysis was performed using both the 330 µm as well as the 660 μm ends as inputs. Input Gaussian beams with waist diameters of 158 μm (53% of bore size) and 265 μm (41% of bore size) were assumed for the simulations using the 330 μm and 660 μm end as input, respectively. Simulations were performed using OptiCAD software incorporating 10,000 simultaneous propagating rays corresponding to the Gaussian input. This 3-D modeling included propagation of skew rays in addition to meridional rays. Output beam divergences for the various configurations were also calculated. The calculated losses and beam divergences for constant bore and tapered HGWs under no applied bend as determined through this analysis are shown in Figure 3.

Figure 3 – Calculated straight losses and beam divergences for constant bore and tapered HGWs (L = 1.8 m)

It should be noted that straight losses for tapered HGWs in which the 330 µm end was used as input are not presented due to the inherent difficulties in modeling such a configuration. The difficulty in developing a consistent and reliable model for this particular case is based on the fact that the propagating beam (and corresponding rays) diverges along the HGW length as the bore size increases. Since the input beam diverges at a lower rate than the bore size increases, calculations involving the propagation of light for this particular case are often unreliable and relatively inaccurate when compared to measured losses. From the results derived from the computational models, the straight losses in tapered Ag/AgI HGWs for a 660 μm bore input end can be expected to be intermediate to those achieved in constant bore 660 µm and 330 μm Ag/AgI HGWs. Furthermore, for the case of the big end as input, the output beam divergence can be expected to be rather small, as would be logical following the representative analysis of ray propagation in HGWs presented in Figure 2(b).

Simulations were likewise performed for the cases in which HGWs with both constant and tapered bores are bent with a 16-cm radius. The bending configurations for these calculations are shown in Figure 4. For these simulations the initial 80-cm segment of the HGW was held straight and the last 100 cm segment had a curvature, 1/R, of 6.25 m-1. The coupling conditions were the same as for the cases in which the straight losses were calculated.



Non-Tapered Loss

1.2 dB

3.5dB

Tapered Loss

2.0 dB

1.3 dB

Figure 4 – Calculated bending losses for constant bore and tapered HGWs (L = 1.8 m)

From the simulation results, it can be seen that the losses for tapered HGWs under bending are intermediate between the losses measured using constant bore HGWs. Furthermore, the calculated losses obtained when using the 330 µm input end are considerably lower than for the case in which the 660 μm end is used as input. This can be explained by the fact that less reflections occur for the former configuration. The calculated losses for the tapered HGW using a 330 µm end as input are close to those measured for a constant 660 μm ID HGW, thus allowing for an HGW with low-loss that is still capable of maintaining important properties of small bore HGWs such as attenuation of higher order modes resulting in the propagation of low order modes, particularly at shorter wavelengths.

3. FABRICATION TAPERED AG / AGI HGWS 3.1 Deposition of Improved Ag Thin Films

The initial step in the fabrication of any Ag/AgI coated HGWs, including tapered Ag/AgI HGWs, involves the deposition of a high purity, low-surface roughness reflective silver film of adequate thickness on the inner surface of the silica capillary. To achieve this, the silver film is deposited via conventional Dynamic Liquid Phase Deposition (DLPD) techniques in which precursor solutions are continuously pumped through the silica capillary by a peristaltic pump at a fixed fluid flow speed which depends on the dimensionality, particularly the bore, of the HGW sample. [1] The methodology commonly implemented for fabricating constant bore HGWs has likewise been adapted, with some modifications, for the fabrication of tapered HGWs. Recent studies focusing on improving the quality of such deposited Ag films have allowed for significant reduction in transmission losses through the incorporation of an advanced Ag plating procedure. This procedure involved the use of an acidic 1.55 mM tin (II) chloride dihydrate solution prior to the Ag deposition process which serves as a sensitizer for the inner capillary silica surface, thus reducing the necessary Ag plating procedure time and corresponding increased surface roughness. After the sensitization procedure, the Ag thin film is deposited by simultaneously pumping a caustic 14.36 mM complexed silver (I) ion solution at pH ≈ 9.5 and a 3.10 mM dextrose reducing solution through the HGW. The fabrication configurations used in the sensitization and subsequent Ag plating procedures are presented in Figures 5(a) and 5(b), respectively.



Figure 5 – Configuration for a) sensitization and b) Ag plating procedures

The Ag film thickness should be minimized so as to reduce surface roughness related scattering losses while at the same time being of adequate thickness to be able to form a dielectric AgI thin film of desired thickness during the subsequent iodization procedure. In practice, adequate Ag film thicknesses for the fabrication of Ag/AgI HGWs with optimal response around λ = 10.6 µm have been consistently deposited using this fabrication technique with Ag coating times of 22 – 25 minutes.

The modification of the Ag coating procedure for tapered HGWs relative to constant bore HGWs involved the alteration of the fluid flow speed, which is controlled by the peristaltic pump speed and has a pronounced effect on the integrity, homogeneity, and overall quality and uniformity of DLPD deposited thin films. For the case of constant bore HGWs, the fluid flow speed must be sufficiently high to ensure the deposition of high quality films along the entire HGW length while not being excessively high so as to minimize fabrication expenses as faster fluid flow speeds require larger amounts of precursor solutions. In practice, a fluid flow speed of approximately 75 cm/s has been shown to be optimal in attaining this balance for HGWs with constant bore sizes ranging from 250 – 1,000 µm. In the case of tapered HGWs, however, it is important to select the proper fluid flow speed as the diameter spans a broad range of sizes along the sample length. The tapered HGWs used in this study were found to have a nearly linear bore slope from 300 – 650 µm along a length of 2 m and all DLPD procedures were carried out with the fluid flow in the direction of decreasing bore. As a result of the non-constant bore, the peristaltic pump rate was adjusted accordingly so as to obtain the fastest possible flow speed through the tapered HGW while minimizing the occurrence of pressure related problems due to the combination of high fluid flow rates and small bore sizes. The optimal fluid flow rate for the fabrication of tapered HGWs with the aforementioned dimensionality was determined to be approximately 30 – 35 cm/s.

3.2 Fabrication of AgI Dielectric Thin Films

Following the Ag thin film plating procedure, an AgI dielectric thin film of appropriate thickness is fabricated as a result of the subtractive partial iodization of the pre-deposited Ag film. This is achieved by pumping a39.4 mM iodine in cyclohexane solution cooled to 12 °C. The dielectric AgI film thickness depends on the iodization procedure time and can be accurately modeled by the mth root law dependence given in Equation 1. [5] √ 1

Where m is an experimentally derived constant and k is a constant which depends on the iodine solution concentration. The optimal AgI thin film layer thickness for low-loss transmission at λ = 8.0 – 10.5 µm is approximately d = 0.69 – 0.90 µm, corresponding to iodization procedure times of t = 180 – 380 sec as determined by Equation 1. [5] In this particular study, an iodization procedure time of 180 seconds was implemented at a fluid flow speed of approximately 30 – 35 cm/s as for the Ag thin film deposition procedure.

a) b)



4. ANALYSIS OF EXPERIMENTAL RESULTS 4.1 IR Spectral Analysis of Ag/AgI Coated Tapered HGWs

Fourier-Transform Infrared (FTIR) spectroscopy was carried out on the initial and final 10 cm segments of the Ag/AgI tapered HGW sample to determine the optical response including the AgI dielectric film quality, uniformity, and thickness. FTIR analysis was performed using a Bruker Tensor 37 FTIR spectrometer in conjunction with a Teledyne-Judson MCT/A cryogenic detector. The result spectra for both segments are given in Figure 6.

Figure 6 – IR spectra of input and output ends of Ag/AgI tapered HGW

From the IR spectra, it is evident that despite some apparent deviation in AgI film thickness along the sample length as determined by the slight difference in the two spectra, the optical response is nearly identical along the entire length, suggesting that the current fabrication method can indeed be successfully implemented in fabricating high-film quality, uniform AgI dielectric films in tapered HGWs. The AgI film thickness was calculated from the wavelength position of the first interference peak and the refractive index of AgI at that wavelength and found to be approximately 0.58 µm, which is slightly less than the optimal film thickness for the desired wavelength range. Still, the optical response determined by the spectra show minimal absorption at the target wavelength of λ = 10.6 µm. 4.2 Attenuation Measurements of Ag/AgI Coated Tapered HGWs

Attenuation measurements of the Ag/AgI coated tapered HGW was carried out using a 15 W CW Synrad CO2 gas laser operating at λ = 10.6 µm. Straight and bending measurements were taken using two different setup configurations, with the first using the 300 µm bore end as input and the second using the 650 µm bore end as input. For the case of the 300 µm bore input, a ZnSe lens with a focal length of 7.62 cm was used to efficiently couple the laser beam into the HGW. Bending measurements involved holding the initial 90 cm of the sample straight while subjecting the next 100 cm of the sample to an applied bend of a fixed radius and holding the final 10 cm of the sample straight. In this manner, bending measurements were taken at bending radii of 2.5, 2.0, 1.5, 1.0, 0.75, 0.50 and 0.25 m. The corresponding measured losses are shown in Figure 7.



Figure 7 – Loss as a function of curvature for taper with increasing ID size

The attenuation as a function of curvature in tapered Ag/AgI HGWs with increasing ID size can be seen to follow the general 1/R dependence of constant bore HGWs, with a slight anomaly at small bending radii for which the loss decreases to a minimum. This can be attributed to the propagation of whispering gallery modes at such low curvatures, which result in fewer reflections of the propagating ray with the HGW surface and thus lower transmission losses. It is suggested that further analysis be carried out to test this particular hypothesis. Furthermore, the losses for this tapered configuration (~0.74 dB/m straight) are indeed lower than for constant bore 300 µm ID HGWs (~1.1 dB/m straight) but higher than for typical values of constant bore 700 µm ID HGWs (~0.15 dB/m straight).

For the case of the 650 µm ID input, a ZnSe lens with a focal length of 12.70 cm was used to efficiently couple the laser beam into the HGW. Bending measurements involved holding the initial 80 cm of the sample straight while subjecting the next 100 cm of the sample to an applied bend of a fixed radius and holding the final 10 cm of the sample straight. In this manner, bending measurements were taken at bending radii of 2.5, 2.0, 1.5, 1.0, 0.75, 0.50 and 0.25 m. The corresponding measured losses are presented in Figure 8.

Figure 8 – Loss as a function of curvature for taper with decreasing ID size

The attenuation as a function of curvature for the case when the larger 650 µm ID end was used as input and the HGW ID was decreasing along the direction of propagation was seen to follow a considerably different trend. In this case, the expected 1/R loss dependence was not as definite and the loss was seen to increase with increasing curvature for all cases. The loss was seen to increase much faster with increasing curvature relative to the case where the ID size was decreasing along the direction of propagation. Furthermore, both straight and bending losses were found to be considerably higher than for the case when the ID size is decreasing along the direction of propagation as could be expected from theory and is predicted through the computational simulations. The losses for this tapered configuration were likewise found to be lower (~0.80 dB/m straight) than for constant bore 300 µm ID HGWs (~1.1 dB/m straight) but higher than for typical values of constant bore 700 µm ID HGWs (~0.15 dB/m straight).



Table 1 has been provided to provide a comparison between the theoretical and experimental loss values for several configurations involving tapered Ag/AgI coated HGWs.

Table 1 – Comparison of theoretical and experimental attenuation values

Sample Configuration Theoretical Loss Experimental Loss

Straight ~650 → 300 µm ID 0.22 dB/m 0.80 dB/m

Bent (R = 16 cm) ~300 → 650 µm ID 0.72 dB/m 0.96 dB/m

Bent (R = 16 cm) ~650 → 300 µm ID 1.11 dB/m 1.34 dB/m

Experimental loss measurements were seen to deviate to different degrees from those obtained through computational analysis. In all cases, computationally determined losses were considerably lower than experimentally determined values, as is often the case as computational calculations often do not fully account for other loss mechanisms such as surface roughness and deviation from optimal values. 4.3 Divergence Measurements of Ag/AgI Coated Tapered HGWs

Output beam divergence measurements were performed so as to allow for an experimental comparison to the divergence values calculated through the theoretical ray optics analysis of tapered Ag/AgI HGWs. The output beam divergence was determined by measuring the full width at half maximum (FWHM) of the output spatial power density distribution at different distances from the end of the HGW sample. Spatial power density distribution measurements were taken using a Spiricon Pyrocam I with a 124 × 124 pixel (1 mm × 1mm) pyroelectric array. This method was used to determine the beam divergence when the HGW sample was held straight and the input end is either 300 µm or 650 µm. The corresponding normalized 2-D cross-sectional spatial power density distributions for both configurations at distances of 8, 10, and 12 cm from the HGW output are given in Figure 9.

Figure 9 – Spatial output power density distributions for a) 650 µm ID and b) 300 µm ID output

It is evident that the beam divergence is much smaller when the input end is small and the output end is large than conversely. This phenomenon is in accordance with the theoretical propagation of light rays through a tapered HGW and the computational simulations performed via OptiCAD analysis. By taking into account the distance along the optic axis between the HGW output and the Spiricon input as well as the FWHM of the near-Gaussian beam profiles at each of the individual distances, the beam divergence can be approximated. Using this methodology, the full angle beam divergence with the 300 µm bore end as input and the 650 µm bore end as output was determined to be approximately 0.9 – 1.0 deg. When using the 650 µm bore end as input and the 300 µm end as output on the other hand, the full angle beam divergence was determined to be approximately 2.3 – 2.5 deg, which is close to the calculated value for this particular configuration of 2.8 deg.

a) b)



5. SUMMARY AND CONCLUSIONS The present study has been successful in developing a further understanding of tapered Ag/AgI HGWs ranging from theoretical numerical analysis to improved fabrication methods and the experimental analysis of their atypical optical properties. Accurate modeling of the transmissive properties of tapered Ag/AgI HGWs was achieved through ray optics based numerical simulations carried out in OptiCAD and allowed for theoretical values of both loss and divergence to be derived so as to have a theoretical model to which the experimental results could be compared. Through improved fabrication methods and experimental determination of optimal parameters for key fabrication variables, Ag/AgI HGWs with near linear taper slopes ranging from 300 – 650 µm in bore size over 2 meter lengths have been successfully fabricated in the laboratory. Subsequent characterization has concluded that high quality AgI thin films of adequate thicknesses and uniformity have been deposited in tapered HGWs. Optical attenuation measurements have shown a bending dependency intermediate relative to 700 µm and 300 µm constant bore Ag/AgI HGWs as would be expected from the computational analysis. Furthermore, experimental losses have been shown to be considerably different depending on whether the 300 µm end or the 650 µm end of the tapered HGW is used as input. Divergence measurements have likewise shown a considerable difference between the two cases with a substantially larger beam divergence when using the 300 µm end as output than when using the 650 µm bore size end as output. While the experimentally determined values do indeed follow the same general trends as the computational model, some deviation is present, with generally lower losses obtained through theoretical analysis than through experimental analysis.

6. ACKNOWLEDGEMENTS This work is based on research supported by the Department of Energy under Award Number DE-SC0001466. The authors would furthermore like to thank SPIE for its support of travel expenses to Photonics West 2012 through its annual scholarship in Optics & Photonics.

REFERENCES

[1] Harrington, J. A., [Infrared Fiber Optics and Their Applications], SPIE Press, Bellingham, WA, (2004). [2] Gibson, D. J. and Harrington, J. A., “Gradually tapered hollow glass waveguides for the transmission of CO2 laser

radiation,” Applied Optics, 43(11), 2231 – 2235 (2004). [3] Gibson, D. J. and Harrington, J. A., "Tapered and noncircular hollow glass waveguides", Proc. SPIE 3596, 8 (1999). [4] Miyagi, M. and Kawakami, S., "Design theory of dielectric-coated circular metallic waveguides for infrared

transmission,“ IEEE Journal of Lightwave Technology. LT-2, 116-126 (1984). [5] Matsuura, K., Matsuura, Y., and Harrington, J. A., “Evaluation of gold, silver, and dielectric-coated hollow glass

waveguides,” Optical Engineering, 35(12), 3418 – 3421 (1996).



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Investigation of tapered silver / silver halide coated ...irfibers.rutgers.edu/resources/publications/2012/PSI821802.pdf · Investigation of tapered silver / silver halide coated