Technical Field

The invention relates to the manufacture of a coated optical fiber, particularly to the manufacture of a coated fiber containing a fiber Bragg grating able to withstand higher temperatures than previously possible.

Background

A Bragg diffraction grating is a structure that has a periodic pattern of refractive index values. The values alternate from high and low optical refractive index values and the period of repetition is known as the grating period. It is well known to fabricate a fiber Bragg grating (FBG) within an optical fiber, in particular Bragg gratings may be formed by creating an interference pattern in the germanosilicate glass core of an optical fiber. Previous examples include those described in US 6272886 and US 5400422. The uses of Bragg gratings result from their ability to reflect a particular wavelength or "colour" of light. The colour that will be reflected by a grating is the colour whose wavelength exactly matches twice the effective grating period.

It is well known that the required interference pattern can be created by recombining two parts of the beam of an ultraviolet laser, although the first optical fiber Bragg gratings (FBG) were produced by end launching a portion of a laser beam back upon itself into the fiber, producing a standing wave interference pattern. In the bright sections of the interference pattern (where the forward- and backward-traveling waves reinforce each other), the laser light interacted with germanium sites in the fiber core and changed the local refractive index. At the dark sections of the interference pattern (where the two waves destructively interfere and cancel each other out), the refractive index remains unchanged.

Another technique is that of 'strip and recoat' fabrication in an off line process such as in US 6272886. Here gratings can be fabricated by applying optical radiation through the side of (e.g. normal to the length of) an optical fiber, after first removing the protective coating around the section of the fiber to leave a bare region to be treated with the radiation to fabricate the FBG.

The above-described techniques for producing optical fiber Bragg gratings are well established, but certain technical difficulties to date have prevented their use in large scale continuous or stepwise continuous production processes. For example, the 'end launch' method of writing or fabricating Bragg gratings in optical fibers does not allow any control, or any fine control, in the location of the grating within the fiber, the grating period or the grating angle. Also, for a strip and coat technique before writing the FBG, a significant production problem is removal of the coating which covers the section of the optical fiber to be treated with the laser or optical radiation. The thermal, mechanical or chemical means for, and the actions and operations of, stripping the coating from the bare fiber are time consuming and reduce the physical integrity of the fiber. This means that although here the individual properties of the FBG can be well controlled the written FBG are of limited strength and durability.

Thus far it has been difficult to produce optical fiber Bragg gratings in significant commercial quantities and there is a need for an improved optical fiber Bragg grating fabrication system.

Summary of the invention

In accordance with the present invention, as seen from a first aspect, there is provided a method of manufacturing a coated glass fiber containing at least one optical grating, the method comprising the steps of drawing a glass fiber from a drawing tower, providing a phase mask to generate an interferometric pattern, providing a laser, transversely exposing said glass fiber to light from said laser through said phase mask, writing an optical grating on said glass fiber with said transverse exposure, coating said glass fiber with a protective coating layer, wherein said protective coating layer is arranged to provide protection at high temperatures of up to around 300°C and preferably the coating is polyimide. By this technique the invention provides an on line manufacturing process with the result that the FBG strength and structure is not intrinsically degraded by the manufacturing process. A fiber and grating strength in excess of 250-300 kpsi can be achieved.

The coating of the fiber can usually protect the fiber from temperatures from 150 to 200°C (degrees C). The use of a coating or buffer material that is a polyimide material that can be cured with IR radiation advantageously avoids the use of a thermal oven and can extend this range to around 300°C. The coating process, for example with UV cured acrylate, often requires a curing stage at temperatures which would under other circumstances cause the FBG to anneal or become removed, resulting in a reduction in reflectivity. For Infrared (IR) cured Polyimide (PI) coatings though, as mentioned, they sometimes require high temperatures in the IR oven that could otherwise anneal or remove the FBG, but here don't and in addition currently require a two step coating procedure. In the present process and as described here this has been achieved both with writing of the FBG during the drawing process and the single step, on line application of an IR cured Polyimide. With the preferred embodiment below the material and FBG properties remain good up to 300 °C and beyond and the reflectivity of the FBG remains above 10%.

In an embodiment the transverse exposure comprises transversely exposing said glass fiber to a light pulse from said laser through said phase mask.

The advantageous technique of phase mask use here provides more reproducibility and flexibility than is currently available with interferometry based FBG writing. Interferometric FBG writing uses, for example, a Talbot interferometer. The phase- mask writing technique that the invention is proposing has improved reproducibility and flexibility over an interferometric-based system. In particular by using phase-mask based photo writing, the reproducibility of the FBG is improved with a drift <0.10nm +/- 0.01 nm between the wavelength of light reflected by neighbouring FBG, bringing the flexibility of the interferometric techniques alongside the reliable and reproducible features of phase mask use. In addition, FBGs written using an interferometry based system can suffer from limited spectral performance. Here, with the phase-mask based photo-writing FBG, the spectral quality of the FBG are improved with a side lobe suppression in excess of 15dB. Other spectral properties and characteristics such as chirp may also be achieved.

The feature of photo writing while drawing on -line avoids the previous issues with the lengthy step procedures of strip and coat and so overall addresses the speed and cost of the system. The coating provides a protected package chemically resistant and capable of withstanding high temperatures up to around 300 °C and beyond.

Preferably, the method of manufacturing includes the laser arranged to provide pulses of UV radiation in the range of nanoseconds. Other options could include an Infrared (IR) laser emitting Femto-second pulses. As mentioned above the optical grating is preferably a fiber Bragg grating. Preferably, the method further comprises the step of curing the polyimide with an Infrared (IR) source, preferably curing at a temperature between 400 °C and 600 °C. Preferably, the method further comprises the step of coating the glass fiber with at least one further layer of polyimide. In the preferred embodiment the method further comprises the step of curing each of said further one or more layers of polyimide with an Infrared (IR) source. The PI coating is provided to advantageously protect the fiber in order that there can be continuous operation up to 300 °C over and above the currently available 150-200 °C. The coating and curing process is optimised such that the optical grating, such as a FBG is tolerant of the thermal annealing such that a reflectivity of 10% is achieved. In addition the benefits of the phase mask on line photo writing technique is that once written the FBG arrays produced have very low drift to other wavelengths of reflection. Other benefits of less processing of the fiber are an increase in strength allowing the production of endless arrays without splices and the potential for applications in the medical field.

In the preferred embodiment the method further comprises marking the coated glass fiber with at least one ring marker and further comprises the step of locating said ring marker in a pre-determined position. The ring marker acts a reference for further processing or post processing steps. Preferably, the method includes the step of monitoring the diameter of the drawn glass fiber, and more preferably it further comprises the step of monitoring the diameter of the coated glass fiber. A monitoring step is useful for measuring any change in production output quality and can assist with manufacturing quality control. In an embodiment it may also be preferred to write a plurality of optical gratings into the glass fiber.

In accordance with the present invention, as seen from a second aspect, there is provided a coated glass fiber, manufactured using the method as set out above, comprising: a continuously drawn glass fiber, and at least one optical grating written into the continuously drawn glass fiber, the glass fiber coated with at least one polyimide layer over substantially its entire surface. In a preferred embodiment the continuously drawn glass fiber is continuously drawn from a drawing tower. Preferably the drawing speed is between 5m/min and 150m/min. Preferably, the optical grating is a fiber Bragg grating and preferably, the fiber Bragg grating has a reflectivity of at least 10% at any single wavelength of light. In the preferred embodiment described below the glass fiber has an operating temperature above 150 °C, more preferably above 200 °C and preferably has a plurality of optical gratings written into the glass fiber.

In accordance with a still further aspect of the present invention, there is provided apparatus for manufacturing a coated glass fiber as hereinbefore described, comprising a fiber drawing tower, including tensioning apparatus and furnace apparatus; further comprising phase mask application apparatus including a laser providing pulsed electromagnetic radiation, a coating station, a curing station and product spool storage.

Brief description of the drawings

Figure 1 is a schematic view of apparatus for carrying out a method of fabrication in accordance with an embodiment of the present invention as seen from a first aspect;

Figure 2 is a schematic illustration in detail of a part of an apparatus such as Figure 1 , for carrying out a prior art method of fabrication of a fiber Bragg grating; Figure 3 is a schematic illustration in detail of a part of the apparatus and method for carrying out fabrication of a fiber Bragg grating in accordance with an embodiment of the present invention as seen from a first aspect;

Figure 4 is a schematic illustration in detail of a further part of the apparatus of Figure 1 , for carrying out a method of drawing an optical fiber in accordance with an embodiment of the present invention as seen from a first aspect; and

Figure 5 is a flow diagram of a method of fabrication as shown in Figure 1 , and in accordance with a further aspect of the present invention.

Detailed description

With reference to figures 1 , 3 and 4 of the drawings, there is illustrated apparatus 1 for drawing an optical fiber with the on line addition of the writing of a FBG. The apparatus is illustrated with all the components in place in Figure 1 and is set out within a drawing tower 10, comprising a preform feeder 12 for accommodating a preform cylinder block 14, a furnace 16 located downstream of the preform feeder, diameter monitoring apparatus 18 and a photo writing system 20. The existing photo writing system 20' for writing FBG is described in further detail below along with the photo writing system of the invention. Further downstream within the drawing tower the apparatus includes coating station 30, further diameter monitoring apparatus 40 and a take up unit 50 in the line of fabrication. The fabrication apparatus of the preferred embodiment features the step wise location of these pieces but in alternative embodiments an alternative arrangement of components can be considered in order to prepare the same on line drawn optical fiber with FBG.

Turning firstly to the detail of the FBG writing system 20' known up until now, with components that can be seen in some detail in Figure 2, with reference to block 20'. A strip function 22 as is known is located at the input end of the writing system 20, a UV laser 24 and phase mask 25 are provided downstream of the window stripping apparatus 22 and a coating applicator 26 follows in line after the phase mask portion 25. The FBG shown here as being processed in steps Fi to F ₄ is written with a transverse pattern (perpendicular to the waveguide).

Turning next to the detail of the FBG writing system 20, with components that can be seen in some detail in Figure 3, with reference to block 20 and as the preferred embodiment of the present invention whereby the grating is written on the fiber before the coating is applied and without the need for stripping. F represents the fiber at the input end of the writing system 20, a UV laser 24 and phase mask 25 are provided downstream and a coating applicator 26 follows in line after the phase mask portion 25. In this embodiment F is bare fiber without coating. With this arrangement the writing system can be implemented on and within optical fiber on being drawn through the tower 10, as set out in processed fiber portions as shown in steps Fio to F13. The FBG illustrated here is written with a transverse pattern (perpendicular to the waveguide), although a tilted or other FBG could be used as well.

A coating apparatus, shown broadly as 30 is provided as a component used for covering and strengthening the fiber product created. In particular at the region over the recently fabricated FBG.

Finally, we turn to the drawing in Figure 4 to show the remaining features of the drawing tower 10 in detail. Downstream of the one or more coating applicators 26 are located ovens or heaters 27 to fix, or cure, the protective Polyimide (PI) coating such that the coating is applied and cured in a single draw or pass. The coating is applied on all sides as a protective jacket, and with a ring coating. There may be multiple coating stages for fabrication of a robust fiber portion.

The curing temperature required will depend on the coating product used and should be above the Tg of the coating material in order to provide sufficient resistance to high temperature when protecting the fiber. This temperature should typically be in excess of 400° C. In typical operation in the drawing tower illustrated in Figures 1 and 3 the curing temperature set for the ovens can be as high as 600° C, as the fiber is moving through the drawing tower so the effective temperature seen both by the coating and by the FBG will be less.

In order to effectively cure the PI coating whilst efficiently drawing and producing quality fiber the period of time that the fiber and the FBG spend within the draw tower should be limited. This is achieved with a minimum drawing speed of around 5m/min. Other speeds may be used for other coatings or other fiber requirements. The speed of 5m/min has been found in the preferred embodiment to provide good results. A drawing speed that is much faster does not provide sufficient time for the coating portion to be sufficiently cured. Coatings that are not fully cured result in a fiber with poor performance properties with increasing temperature. This leads to an upper limit on drawing speed to, in the region of 150m/min, the figure assumes a single layer and potentially multiple curing ovens without point applicators.

The change in tension during the drawing should be minimal in order to avoid drift in FBG central wavelength.

The further diameter monitoring apparatus 40 is followed by a ring marking unit 45 for accurate post processing location of the FBG. A capstan 48 acts to spool the processed fiber onto the take up unit 50.

In operation, as set out in the diagram at Figure 4, also with reference to Figure 2, the operation of the on line fiber drawing with FBG fabrication is as follows. The fiber F, which may have a protective coating, or may be without coating as from the drawing tower and preform, is heated by a furnace 16, and drawn into the desired diameter of fiber, F. The diameter monitor 18 upstream of the FBG writing apparatus is arranged to detect and monitor a desired diameter and range of diameter. The fiber that will form FBG fiber is labelled Fn and a phase mask 25 is used together with a UV laser pulse 24 to write the required FBG into the core as Fi2 and the uncoated, unprotected fiber F13 then moves further downwards in the drawing tower where one or more coating processes are executed to create the fiber Fu. Further coating layers may be applied, and diameter monitored again at apparatus 40, before a ring 45 is applied to act as an identification and location marker for the FBG. The coating is PI and is a commercially available product from HD microsystem or MicroQuartz. The coating application is likely to be with the glass fiber going through a coating cup then the coating is cured in an IR or heat releasing oven, arranged in and around the drawing tower to generate 3 to 4 layers of PI protective coating. The coating allows operation at higher temperatures than have previously been available and the fibers could be capable of surviving >200° C. Other materials are typically UV cured and are both limited to 150° C and the UV curing process can have the tendency to anneal the FBG which can change its characteristics and reduce the FBG reflectivity.

Finally, the product fiber Fu is complete and can be described as fiber fabricated in a combination drawing and on line FBG application process where the FBG is written with a single pulse phase mask photo writing technique. A capstan 48 and a spool take up unit 50 then roll and store the fabricated fiber ready for use or transport.

The advantages of the writing technique and process described here are the result of the combined apparatus. An extremely stable photo-writing system has been developed that is capable of repeatably and reliably writing an FBG in the fiber with a single pulse, whilst being flexible enough to provide a rapid change of FBG pattern during the optical fiber drawing. The writing technique is integrated into the frame of the draw tower and is synchronized and works with the optical fiber drawing and protective coating and ring marking for FBG identification/positioning.

The key features achieved are as follows;

Chirped Option Yes

Attenuation <3dB/km @ 1550nm

FBG writing per second 100 per second

Mechanical Strength >250-300kpsi

Various modifications may be made to the described embodiment without departing from the scope of the present invention. The structure and orientation of the apparatus may be of an alternative design and shaping, there may be one or more fibers and one or more drawing towers. The apparatus may comprise any suitable material or construction. The laser pulses may be of an appropriate strength and wavelength as suits the fabrication of the FBG, for example IR laser Femto-Second pulses may be suitable. The number of coating layers may be varied, along with the number of location rings.