Refractive index modulation in photo-thermo-refractive fibers Eugeniu Rotari, Larissa Glebova, and Leonid Glebov College of Optics and Photonics/CREOL, University of Central Florida 4000 Central Florida Blvd. Orlando, FL 32816-2700

ABSTRACT Refractive index decrement was discovered in a fiber made from photo-thermo-refractive (PTR) glass. PTR glass is a fluorosilicate glass doped with cerium and silver which demonstrates refractive index change after UV exposure and thermal development due to precipitation of NaF nanocrystals in the irradiated areas. This glass is widely used for volume holographic optical elements recording. Photosensitivity in PTR optical fibers has been shown after exposure to radiation at 325 nm for about 1 J/cm2 followed by thermal development at 520°C. Refractive index difference between exposed and unexposed areas was about 1000 ppm. A Bragg mirror at 1088 nm was recorded in such fiber which showed narrow band reflection within 1 nm. (Key words: photo-thermo-refractive glass, fiber Bragg gratings, structural transformations)

1. INTRODUCTION Since the first fiber Bragg grating was reported in 1978 by Hill [1] these gratings became one of the main components in modern photonics which provide fine multiplexing and demultiplexing operations. However these gratings have some drawbacks caused by law thermal stability of image recorded in germanium doped silica. This is why search of new materials which can provide robust fiber Bragg grating is still important. New holographic material which enables recording of robust holographic elements was recently developed at University of Central Florida and used for a number of applications [2]. This is Photo Thermo Refractive (PTR) glass which is a fluorinated silicate glass doped with cerium and silver demonstrating refractive index change after UV exposure and thermal development due to the precipitation of nanocrystals in the irradiated areas. Thermal development magnifies refractive index modulation of the glass causing its outstanding photosensitivity [3]. The photosensitivity of PTR glass is due to the main dopants: cerium, silver and fluorine. Cerium in the glass matrix is represented in two ionized states: Ce 3+ and Ce 4+ with absorption bands at 305 and 250 nm correspondingly (Fig. 1A). Under UV irradiation Ce 3+ looses one electron which migrates along the glass matrix. This electron can be trapped by silver ion which is reduced to atomic state. During the thermal development atomic silver forms colloidal silver containing particle. Negative refractive index change is caused by formation of sodium fluoride nanoparticles around the silver centers during the high temperature development. The ultimate refractive index change reaches values up to 10-3. The developed material is transparent in the wide range from 500 to 2700 nm (Fig. 1B). Recorded image is stable and shows no degradation at the temperatures below 400 ºC. Thermal shift of the resonant wavelength of recorded Bragg gratings demonstrates linear behavior with thermal coefficient of 7 pm/K mostly due to thermal expansion of the material. There are two areas of significant absorption of the developed PTR glass (fig. 2). The first one covers near UV and short-wavelength visible region up to 500 nm. Optical losses in this region are mainly contributed by absorption of the glass matrix, cerium ions, colloidal silver and scattering by crystalline phase. On the other side of the spectrum in the near IR region there is absorption by “water” in the glass [4]. In-between, from 500 to 2700 nm the glass is transparent demonstrating losses less then 0.5 cm-1. This transparency window can be used for a large number of optical applications of the material. However no sensitivity was reported in fibers having composition similar to PTR glass.

Fiber Lasers II: Technology, Systems, and Applications, edited by L. N. Durvasula, Andrew J. W. Brown, Johan Nilsson, Proceedings of SPIE Vol. 5709 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.591217

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Fig. 1 Absorbtion spectra of virgin PTR glass (A) and of UV exposed and thermodeveloped PTR glass (B)

2. EXPERIMENTAL Glass melting was produced in an electrical furnace in fused silica or Pt crucibles at 1460ºC. Stirring was applied to homogenize the melt. Fiber specimens with diameters of 200 to 600 µm were drawn directly from the melt without cladding and cooled down to room temperature. Fiber samples were exposed to UV radiation at 325 nm for dosage of 0.9 J/cm2 at room temperature (Fig. 2). After exposure to one beam we have got a uniformly irradiated area in the fiber. Bragg gratings were recorded in the fibers when two beams were applied to a sample producing interference. UV laser beam was filtered, expanded and collimated using a spatial filter scheme. This beam was divided in two by optical beam splitter and crossed in the plane with the fiber sample producing interference. The periodicity of the resulting grating is dependent on the angles of the incident beams. Effective thickness of the recorded gratings in the fiber was 20 mm. Fibers with outside diameter of 200 to 700 microns were irradiated with the dosage in the maxima of 0.9 J/cm2. Thermal treatment was produced at temperatures ranged from 400 to 520°C for several hours.

Fig. 2.3.Schematics of recording of the fiber Bragg gratings RESULTS AND DISCUSSION by exposure to interference pattern of UV laser radiation.

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Drawing of the sample fibers was done from the melt of PTR glass at the temperature over 1200 C. At this temperature some volatile elements like fluorine may eliminate and change the properties of the drawn fibers compared to the initial glass. Therefore prior to any further experiments photosensitivity of the PTR-fibers was measured. The fibers were irradiated by lateral irradiation with He-Cd laser (4 mW, λ=325 nm) with the diameter of the beam 1.1 mm at a dosage of 0.9 J/cm2 and developed by the standard procedure. The change of refractive index was detected using a shearing interferometer (Fig. 3). Previously this setup was proposed for photosensitometric measurements of refractive index change in the flat PTR glass samples [5]. Measurement of the refractive index in a fiber is complicated by the small size and the round profile of the surface. In order to overcome this problem the fiber is submerged into a cell filled with immersion liquid with matched refractive index. The cell itself was built of two fused silica flats 50x50x10 mm with surface quality better then λ/8. The first flat serves as front transparent window. The second one has a dielectric reflecting coating which reflects the probe beam. The studied sample is fixed horizontally in the 2 mm space between the quartz flats. Parallel beam from He-Ne laser (633 nm) is filtered using a spatial filter (Fig. 3, components 1, 2, 3). Collimated beam is launched onto the front window of the liquid cell at an angle within 5-10 degrees. A part of the light is reflected from the front window due to Fresnel reflection. The rest of the light traverses through the volume between the flats and reflects from the back flat. Both beams reflected from the front and back plates are overlapped in the plane of a CCD camera creating interference pattern on it’s screen. The CCD camera is connected to a personal computer equipped with a frame grabber. The angle between the flats can be adjusted gradually, varying period and orientation of the interference fringes.

Fig. 3 Schematics of shearing interferometer for measurement of rerfractive index change in fibers. 1 - focusing lens, 2 – pinhole, 3 – collimating lens, 4 - fused silica plate, 5 - fiber submerged into matching liquid, 6 - fused silica plate with reflecting coating, 7 – objective, 8 - CCD camera

The method allows registration of refractive index variations in the range from 5 to 50 ppm for samples with different thickness from 1 mm to 100 microns correspondingly. The spatial resolution of the setup is 30 to 50 microns depending on the used magnification. The refractive index change can be seen at the minimum of the fringes crossing the fiber. In this case the area with the modified refractive index can be found as a spot corresponding to the phase shift within irradiated area (Fig. 4A). In

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order to estimate the value of refractive index change the fringes should be oriented vertically. In this case the phase shift in the irradiated area will contribute in the distance between the fringes (Fig. 4B).

Fig. 4 Far field interference pattern of UV irradiated and developed PTR fiber submerged into matching liquid. The interference fringes are oriented horizontally along the fiber (A) and when the fringes are oriented perpendicularly to the fiber.

The period of the fringes should be adjusted in the way when the one fringe is corresponds to unirradiated area and the following one to the center of the irradiated spot. Refractive index change can be calculated using formula:

∆n =

λ × P1

2D × P2

, where:

∆ n - refractive index change, P1 - distance between two fringes in the irradiated area, P2 - distance between two fringes in the unirradiated area, λ - wavelength of the probe beam (633 nm for He-Ne laser), D - diameter of the fiber. Refractive index change was calculated from the obtained interferograms. The photosensitivity of the fibers was 1.0 ppm×cm2/mJ and was close to the photosensitivity of the initial PTR glass. The maximum refractive index increment was about 10-3.

The recorded fiber Bragg gratings were detected by diffraction of He-Ne laser beam (633 nm, 1 mW). At the certain angles a flash of diffracted light could be seen on the screen in the shape of ellipsoid opposite to the transmitted beam (Fig. 5)

Fig. 5 Schematics (A) and photograph (B) of diffraction from a Bragg grating recorded in PTR fiber.

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Fiber Bragg gratings were tested in transmission mode using a tunable neodymium laser with spectral resolution of 0.1 nm and operating range from 1080 to 1100 nm. The narrowed and collimated beam of ~ 0.5 mm in diameter was launched into the fiber. The power of the transmitted light was measured using a silicon photodetector at the different resonant wavelengths. The grating was recorded in a PTR fiber 200 microns thick with the period of 2176 nm. It can be seen that there is a minimum in transmission around 1088 nm which corresponds to the designed resonance wavelength of the grating. Relatively large width of the line (~ 1 nm at half-minimum) can be explained by the large number of modes in the fiber without cladding, as well as by overmodulation of the grating due to high refractive index change and effective length of the grating (~ 20 mm along its axis).

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Fig.6 Normalized transmission spectrum of a PTR fiber Bragg grating.

4. CONCLUSIONS Fibers drawn from photo-thermo-refractive glass show refractive index decrement after UV exposure followed by thermal development. Fiber Bragg gratings were recorded in PTR fiber and 1 nm width notch filter was demonstrated.

5. ACKNOWLEDGEMENTS The work has been supported by DARPA/SHEDS contract # HR-01-1041-0004 . The authors thank V. Smirnov for useful discussions and help in hologram recording.

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6. REFERENCES 1. 2. 3.

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K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, "Photosensitivity in optical fiber waveguides: application to reflection filter fabrication", Appl. Phys. Lett., 32, 647649, 1978 L.B. Glebov. Photosensitive glass for phase hologram recording. Glastech.Ber.Glass Sci. Technol., 71C (1998) 8590. M. Efimov, L.B. Glebov, L.N. Glebova, K.C. Richardson, and V.I. Smirnov. High-Efficiency Bragg Gratings in Photothermorefractive Glass. Appl. Optics, Optical Technology and Biomedical Optics (OT&BO), 38 (1999) 619627.182 L. Glebov, E. Boulos. Absorption spectra of iron and water in silicate glasses. Proc. of International Commission on Glass Conference “Glass in the New Millenium.” Amsterdam (2000) S4-2. Oleg M. Efimov, Leonid B. Glebov, Hervé P. Andre, Measurement of the Induced Refractive Index in a Photothermorefractive Glass by a Liquid-Cell Shearing Interferometer, Applied Optics, Volume 41, Issue 10, 18641871, April 2002

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