INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering

 
The wavelengths of the Er:YAG and the Er:YSGG laser operating at 2.94 µm (3400 cm^-1) and 2.79 µm (3580 cm^-1) are very close to the very intense and broad absorption maximum of tissue water at 3 µm. As a consequence, the threshold energy density for tissue ablation is minimal at these wavelengths and the erbium laser tissue interaction is so strong that the incident laser energy is absorbed in a very small volume. The thermal damage in the adjacent tissue is therefore almost negligible. As water is ubiquitous in biological environments (Tab. 1) these IR lasers would be important tools for medical treatments if there were suitable transmission systems. 

Liquid-core lightguides (Fig. 1), which are being developed in our group, are a highly promising alternative to conventional solid-core fibers or to hollow waveguides, because they are very variable in diameter (< 0.5 - 10.0 mm), highly flexible (even with large diameters) in combination with a low bend-ing strength, mechanically sta-ble, regenerable, and inexpen-sive. 

 

Fig. 1:  Components of a liquid-filled lightguide. 

The properties of our wave-guides can be varied in depend-ence of the cladding material or the core liquid. They have inner diameters of 0.5 and 1.6 mm for the used teflon FEP tubes, 260 and 550 µm for the fused-silica capillaries, respectively. Bending radii down to 10 mm bending radius and smaller are obtainable. The achieved transmissions are 50 - 60 % for 1 m (2 - 3 dB/m) in all cases. Even if the liquid lightguides have been developed for erbium wavelengths it is possible to make them usable in the range between the near UV and the NIR by using a CCl₄/C₂Cl₄ mixture as the core liquid. Some important characteristics of our lightguides are summarized in Tab. 2. 
 
 
 

Cladding	Core	Damage threshold   [J·cm-2]	Output energy density   [J·cm-2]	Limiting   bending radius a)   [mm]	Optical window   [nm]
FEP	CCl4	20	10	20	350 - 3000
Fused silica	CCl4	27	14	20	350 - 500   1200 - 3000
Fused silica	CCl4 / C2Cl4	52	21	20	350 - 3000
FEP	circulating   CCl4	> 350	80	20	350 - 3000

             Tab. 2:  Some actual data of our liquid-filled lightguides. Damage thresholds and output energy densities are 
                          for 2.94 µm (free-running  Er:YAG laser). 
                                  ^a) Lowest radius with negligible bending losses 

 

References:
[1]	M. Haisch, "Untersuchungen über Lichtleiter für die Übertragung hoher Laserenergien: Ray-Tracing-Rechnungen, - Lichtleiter mit flüssigem Kern," MPQ-Report 160 (1991).
[2]	R. Jung, "Flüssigkeitslichtwellenleiter für gepulste Laser im IR," Diploma thesis, Heinrich-Heine-Universität Düsseldorf, Germany, (1996).  You can download this paper here: Diplom_Jung (1391K). (The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.)
[3]	S. Klein, "Optische Eigenschaften von Flüssigkeitslichtwellenleitern im nahen IR," Diploma thesis, Heinrich-Heine-Universität Düsseldorf, Germany, (1997).  You can download this paper here: Diplom_Klein (795K). (The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.)
[4]	S. Diemer, W. Fuß, M. Haisch, J. Meister, and P. Hering, "Liquid lightguides for 2.94 µm," Proc. SPIE 2396, 88-94 (1995).
[5]	J. Meister, R. Jung, S. Diemer, M. Haisch, W. Fuß, and P. Hering, "Advances in the development of liquid core waveguides for IR applications," Proc. SPIE 2677, 120-126 (1996).
[6]	J. Meister, S. Diemer, R. Jung, S. Klein, W. Fuß, and P. Hering, "Liquid core fused silica capillary lightguides for applications in the UV/VIS and NIR spectral range," Proc. SPIE 2977, (1997).
[7]	S. Klein, J. Meister, S. Diemer, R. Jung, W. Fuß, and P. Hering, "High power laser waveguide with a circulating liquid core for IR applications," Proc. SPIE 2977, (1997).
[8]	J. Meister, "Flüssigkeitslichtleiter für gepulste Hochleistungslasersysteme im infraroten Spektralbereich", Dissertation, Heinrich-Heine-Universität Düsseldorf, Germany, (1998).  You can download this paper here: Diss_Meister (3219K). (The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.)
[9]	J. Bongartz, "Flüssigkeitsgefüllte Lichtwellenleiter für medizinische Anwendungen", Diploma thesis, Heinrich-Heine-Universität Düsseldorf, Germany, (1998).  You can download this paper here: Diplom_Bongartz (3369K). (The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.)
Cooperation:	W. Fuß, Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany.