Photonic band gap fibers using a jacket with a depressed softening temperature

Abstract

The present invention is generally directed to a photonic bad gap fiber and/or fiber preform with a central structured region comprising a first non-silica based glass and a jacket comprising a second non-silica based glass surrounding the central structured region, where the Littleton softening temperature of the second glass is at least one but no more than ten degrees Celsius lower than the Littleton softening temperature of the first glass, or where the base ten logarithm of the glass viscosity in poise of the second glass is at least 0.01 but no more than 2 lower than the base ten logarithm of the glass viscosity in poise of the first glass at a fiber draw temperature. Also disclosed is a method of making a photonic bad gap fiber and/or fiber preform.

Claims

1. A photonic band gap fiber preform, comprising: a central structured region comprising a first non-silica based glass, wherein the first glass has a Littleton softening temperature; and a jacket comprising a second non-silica based glass, wherein the second glass comprises a different composition than the first glass, wherein the jacket surrounds the central structured region, and wherein the second glass has a Littleton softening temperature; wherein the Littleton softening temperature of the second glass is at least one but no more than ten degrees Celsius lower than the Littleton softening temperature of the first glass; and wherein the second glass fills any voids between the central structured region and the jacket.

2. The fiber preform of claim 1, wherein the first glass and second glass are individually selected from the group consisting of chalcogenide glass, chalcohalide glass, oxide glass, silicate glass, germanate glass, phosphate glass, borate glass, gallate glass, tellurite glass, and halide glass.

3. A photonic band gap fiber preform, comprising: a central structured region comprising a first non-silica based glass, wherein the first glass has a glass viscosity at a fiber draw temperature; and a jacket comprising a second non-silica based glass, wherein the second glass comprises a different composition than the first glass, wherein the jacket surrounds the central structured region, and wherein the second glass has a glass viscosity at the fiber draw temperature; wherein the base ten logarithm of the glass viscosity in poise of the second glass is at least 0.01 but no more than 2 lower than the base ten logarithm of the glass viscosity in poise of the first glass at the fiber draw temperature; and wherein the second glass fills any voids between the central structured region and the jacket.

4. The fiber preform of claim 3, wherein the first glass and second glass are individually selected from the group consisting of chalcogenide glass, chalcohalide glass, oxide glass, silicate glass, germanate glass, phosphate glass, borate glass, gallate glass, tellurite glass, and halide glass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of a cross-section of a PBG fiber where R is the core radius, is the hole spacing (periodicity), a is the air hole radius, and fill is the air to solid (glass) ratio and represented by the following equation:

(2) $\mathrm{Fill}=\frac{a^2}{{}^2\frac{\sqrt{3}}{2}}.$

(3) FIG. 2 is a schematic of a PBG fiber preform comprising a structured central region 200, a jacket tube 210, and interstitial voids 220.

(4) FIG. 3 is a viscosity profile of two glass compositions having a difference in softening temperature (T.sub.Soft) of 1 to 10 C.

(5) FIG. 4 is a viscosity/temperature profile of two glass compositions with different viscosities at a common fiber draw temperature (T.sub.Draw) such that the difference of the base ten logarithm of the viscosities in poise is about 0.4, which is in the range of 0.01 to 2.

(6) FIG. 5A shows a PBG fiber preform with interstitial voids 220 between the central structured region 200 and the jacket tube 210 as well as internal interstitial gaps 230 within the central structured region 200. FIG. 5B shows a PBG fiber preform that was collapsed using a jacket 210 with a 5 lower softening temperature and has a void-free interface between the central structured region 200 and the jacket tube 210 and no internal interstitial gaps within the central structured region 200.

DETAILED DESCRIPTION OF THE INVENTION

(7) According to the present invention, a structured photonic band gap fiber and/or fiber preform uses at least two different compositions of non-silica based specialty glass in the same fiber and/or fiber preform to reduce or eliminate the interstitial voids in the structured fiber preform and/or the fiber. As shown in FIG. 2, the structured central region 200 of the fiber preform comprises a specialty non-silica based glass whose composition is chosen such that it has the desired optical properties for band gap guidance at the wavelength of interest. The central structured region 200 of the fiber preform has open holes that run the length of the fiber preform in predetermined positions. The structured central region 200 is surrounded by a jacket 210 comprising a different composition of a non-silica based glass than the structured central region 200. The jacket 210 may be a jacket tube. The jacket 210 has a single open hole which runs the length of the fiber preform and can be round or some other shape (e.g., hexagonal) which more closely matches the outer shape of the structured central region 200. The jacket 210 is comprised of a glass similar to that of the structured region 200, except that its composition differs slightly so as to yield either (a) a Littleton softening temperature that is at least 1 C. but not more than 10 C. lower than the Littleton softening temperature of the glass of the structured region 200 (see FIG. 3); (b) a glass viscosity at a fiber draw temperature that is lower than the glass viscosity of the glass of the structured region 200 and the base ten logarithm of the glass viscosity in poises differs by at least 0.01 but no more than 2 (see FIG. 4); or both (a) and (b). Between the structured central region 200 and the jacket 210 are interstitial voids 220. These interstitial voids 220 lead to significant problems when the fiber preform is drawn into a fiber.

(8) Before fiber drawing, the assembled fiber preform may or may not be collapsed in a furnace in a controlled atmosphere or under vacuum at a temperature corresponding to a glass viscosity in the range of about 10.sup.8 to 10.sup.14 poises, with or without the assistance of gas pressure applied to the intended holes, and/or vacuum applied to the interstitial voids. Irrespective of whether the assembled fiber preform undergoes collapse, it is stretched on a fiber draw tower at a temperature corresponding to a glass viscosity in the range of about 10.sup.4 to 10.sup.7.5 poises, into a fiber with considerably smaller dimensions than the fiber preform.

(9) FIG. 5A and FIG. 5B show two structured chalcogenide glass HC-PBG fiber preforms. FIG. 5A highlights what happens to a structured fiber preform that has the same glass for both the structured region and the jacket. Interstitial voids 220 are clearly evident between the central structured region 200 and the jacket tube 210. Additionally, there are internal interstitial gaps 230 within the central structured region 200. These interstitial voids 220 and internal interstitial gaps 230 can lead to significant problems during fiber drawing. FIG. 5B highlights what happens when the jacket tube 210 has a lower softening temperature than the glass comprising the structured region 200. The interstitial voids and internal interstitial gaps are not present, which means that the fiber can be drawn without interstitial defects.

(10) The fiber preform in FIG. 5B was inserted into a tight-fitting heat-shrinkable Teflon sleeve and heated to a temperature of 170 C. in a vacuum of approximately 510.sup.5 Torr, such that the glass of the jacket tube flowed into and filled the interstitial voids between the central region and the jacket tube. The difference between the Littleton softening temperatures for the glass of the jacket tube (181 C.) and the glass of the central region (186 C.) was 5 C. The difference between the Littleton softening temperatures for the glass of the jacket tube (263 C.) and the glass of the central region (268 C.) was 5 C.

(11) The present invention pertains to HC-PBG fibers made from non-silica based specialty glasses such as chalcogenide glasses including sulfides, selenides, tellurides and their mixtures, as well as chalcohalide glasses and other oxide glasses, including specialty silicates, germanates, phosphates, borates, gallates, tellurites, and their mixtures. It is also possible to apply this methodology to halide glasses such as fluorides. Fabrication of the HC-PBG fiber preforms using the tube stacking technique is only one example of fabricating these micro structured fiber preforms and the central structured region of the fiber preforms. Other techniques such as extrusion, templating, laser machining, chemical etching or mechanical drilling of glass, any combination of these, and other glass forming and shaping techniques may be used to fabricate the HC-PBG fiber preforms or the central structured region of the fiber preforms or any portion thereof. Additionally, if the tube stacking technique is used, any shape of tube may be used.

(12) The method of reducing interstitial voids in a structured fiber preform by using a jacket tube with a depressed softening temperature may also be applied to photonic crystal fibers in which there is a solid core surrounded by an array of holes. Furthermore, it is not limited to the type of structure shown in FIG. 1, but can also be used for more complex structures.

(13) The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles a, an, the, or said, are not to be construed as limiting the element to the singular.