Patent Description:
Draw Tower Gratings are produced using a process that combines the drawing of the optical fiber with the writing of the grating. The production process has as an input a glass pre-form. After heating the pre-form, the pulling and formation of the fiber will be initiated. Further in the production process, the fiber crosses the optical axis of a laser and the interferometer that create a periodical interference pattern in order to write the grating. When the grating has been written the fiber is coated by entering a coating reservoir, followed by a-curing step of the coating. Finally the location of the FBG is marked automatically and the fiber is reeled onto a drum. This process of simultaneously drawing the fiber, writing the grating and coating the fiber directly after the grating inscription, results in high strength grating chains. As such the commonly used stripping and recoating process of standard FBGs is not necessary and the pristine fiber strength is maintained during the DTG manufacturing process. Although good fibers can be obtained, the stability of the fiber drawing process may influence the accuracy by which the gratings are written. Consequently, there is still room for improving the process.

Documents reflecting the state of the art are the following:.

It is an object of the present invention to provide accurate methods and system for producing fibers with an inscribed grating array.

It is an advantage of embodiments of the present invention that efficient methods and systems are provided for producing fibers with an inscribed grating array.

It is an advantage of embodiments of the present invention that methods and systems are provided that allow to produce optical fibers with well-defined fiber Bragg grating array configurations, e.g. in terms of distances between the fiber Bragg gratings and the wavelengths for which the fiber Bragg gratings are to be used.

The object is obtained by embodiments of the present invention.

The present invention is as set out in the claims.

It is to be noted that where reference is made to a controller, this encompasses a solution where there is a single controller that handles one or more parts of a laser of a writing system, a marking system, a shutter and/or a fiber length and/or drawing detecting means. The controller also may have other functions, known to the person skilled in the art. Alternatively, where reference is made to a controller this also encompasses the situation wherein a plurality of controllers is used for controlling one or a number of tasks, and whereby the controller refers to the plurality of controllers.

It is an advantage of embodiments of the present invention that for inscribing the grating array, the actual fiber length and drawing speed can be taken into account, based on information obtained using the fiber length and/or drawing detecting means. In this way, extremely accurate grating array configurations can be inscribed in the fiber. It is an advantage of embodiments of the present invention that accurate inscription of fiber Bragg gratings can be performed resulting in accurate spatial position of the fiber Bragg gratings and accurate specific wavelengths for which the fiber Bragg gratings are to be used.

It is an advantage of embodiments of the present invention that the time flow of the inscription process is controlled by the information from the fiber length and/or drawing detecting means.

It is an advantage of some embodiments of the present invention that operation can be performed at a high laser repetition rate, by selecting only those pulses of interest to be used for the inscription process.

<FIG> illustrates a schematic overview of a system for drawing a fiber with inscribed grating array, according to an embodiment of the present invention.

Where in embodiments of the present invention reference is made to the drawings speed or the fiber drawing speed or the speed of the drawing process or the speed of the fiber, reference is made to the speed at which the fiber is drawn. If this speed is not as predetermined, this influences the correctness of the positions of the fiber Bragg gratings introduced and possibly also the correctness of the wavelengths of the fiber Bragg gratings introduced. In embodiments of the present invention, variations of the speed are taken into account.

However, it is understood that embodiments of the invention may be practiced without these specific details insofar as these embodiments are within the scope of the claims.

In a first aspect, the present invention relates to a system for producing an optical fiber with inscribed gratings in accordance with claim <NUM>.

The gratings may be a fiber Bragg grating array.

The system according to embodiments of the present invention may be referred to as the draw tower grating process, whereby the inscription of the grating array, e.g. fiber Bragg grating array, occurs during the drawing process. In some embodiments, the inscription occurs prior to application of the fiber coating. Alternatively, methods and systems according to embodiments of the present invention also may relate to inscription of gratings that occurs through the fiber coating. One known technique to obtain such inscription through the fiber coating is by implementing a femtosecond laser. The array of gratings may be an array of identical gratings, but also may be an array of different gratings, whereby both the grating design as well as the distance between the gratings may vary. The system according to embodiments of the present invention comprises a fiber drawing apparatus for drawing an optical fiber. Such a fiber drawing apparatus typically may comprise a material infeed system for feeding material from which the fiber is to be drawn into the system. Such material also may be referred to as the preform. The system also typically may comprise a heating means for heating the preform material. It may comprise a means for monitoring the diameter of the fiber, or more particularly of the fiber drawn from the preform material, prior to the fiber being inscribed. The system also comprises a writing system comprising a coherent radiation source, e.g. a laser, and a radiation modulating means, configured for inscribing a grating array in the optical fiber during the drawing process of the optical fiber and prior to the optical fiber being coated.

The system furthermore comprises a controller for controlling the writing system. According to the present invention, the fiber drawing apparatus also comprises a fiber length and/or drawing detecting means for controlling the position of the fiber or the drawing speed of the fiber during the drawing process. Such a fiber length and/or drawing detecting means may for example measure the position or rotation speed of a capstan, although embodiments are not limited thereto. The fiber length and/or drawing detecting means may for example be positioned downstream the position where the grating array is inscribed and where the coating is applied. The system furthermore is configured for capturing information from the fiber length and/or drawing detecting means, the information being representative for properties or a status of the drawing process and/or for determining the actual length of the fiber. Properties or a status of the drawing process may include for example a drawing speed. The system furthermore comprises controlling the writing system, e.g. the coherent radiation source and/or the radiation modulating means, based on the captured information.

By way of illustration, embodiments of the present invention not being restricted thereto, an exemplary draw tower grating system will further be discussed with reference to <FIG>, illustrating standard and optional features of such a system.

<FIG> illustrates a draw tower grating system <NUM> for drawing a fiber with inscribed grating, e.g. fiber Bragg grating. The system <NUM> comprises a fiber drawing apparatus <NUM>. The exemplary system comprises a material feed <NUM> configured for feeding material, also referred to as the preform. The preform input may be selected so as to have a predetermined diameter or to be within a predetermined diameter range, such that in combination with the appropriate heating process, a fiber of the appropriate dimensions can be produced. The preform, shown as item <NUM>, typically may be transferred to a heating means <NUM> for heating the preform such that from the heated material a fiber can be drawn. The heating means <NUM> may be an oven. The system furthermore typically may comprise a system for monitoring <NUM> the diameter of the fiber prior to the fiber being inscribed. The system for monitoring <NUM> may for example allow to adjust the heating and or drawing parameters, based on the monitored fiber diameter. The system for monitoring <NUM> may comprise a detector for detecting the diameter of the fiber. The detectors may be optical detectors, although embodiments are not limited thereto.

Downstream the system for monitoring <NUM>, the fiber being drawn in some embodiments may be first inscribed. Therefore a writing system is used, which will be discussed in more detail later. After inscription, typically a protective coating is applied. The fiber drawing apparatus <NUM> therefore furthermore typically may comprise a coating means <NUM> for coating the fiber after it has been inscribed. The coating means <NUM> therefore is positioned downstream the area where the inscription occurs.

Alternatively in some embodiments the fiber coating may be applied first and inscription of the fiber grating may be applied afterwards, i.e. through the fiber coating. The latter may for example be performed using a femto-laser system, as known by the person skilled in the art.

The fiber drawing apparatus furthermore may comprise a curing means <NUM> for curing the coating of the fiber. Such a curing means <NUM> may be a UV curing means or a heater for curing via heating.

According embodiments of the present invention, the system comprises a fiber length and/or drawing detecting means <NUM> for detecting a fiber length, and/or a drawing speed, or another characteristic or status of the drawing process. The fiber length and/or drawing detecting means <NUM> may for example be a rotating means where the drawn fiber is guided in a non-slipping mode. In some embodiments the fiber length and/or drawing detecting means <NUM> may comprise a rotation counting means for detecting how many rotations or fractions thereof are performed by the rotating means. The rotating means may be a capstan. Information of the length of the fiber may thus be based on a number of revolutions of the rotation means or capstan.

Alternatively or in addition thereto a speed of the fiber drawing may be determined and the fiber length and/or drawing detecting means may comprise a speed detection means for determining a speed of the fiber drawing. Examples of speed detection means may for example be a Doppler detector, although embodiments are not limited thereto.

The information that may be obtained thus may be a length of the fiber or information being equivalent therewith such as positions on the fiber, a speed of the fiber, a status of the drawing process such as the drawing process being within predetermined operating conditions, other characteristics of the drawing process, such as for example the weight of the preform. Such information may thus be captured by the fiber length and/or drawing detecting means and transferred to a controller <NUM> controlling one or more of the writing process, a marking system, etc. Alternatively, the fiber length and/or drawing detecting means may also function itself as a controller <NUM> for controlling the writing process, a marking system, etc. The fiber drawing apparatus typically also may comprise a winding system for winding the fiber so it can be stored or transported.

Besides the fiber drawing apparatus <NUM>, the system also comprises a writing system <NUM> for writing a grating in the fiber.

The writing system <NUM> comprises a coherent radiation source <NUM>, such as for example a laser, and typically may comprise a shutter unit <NUM> for controlling whether or not a laser pulse is used for writing the grating and a radiation modulating means <NUM>. The laser used may be any suitable laser such as for example a KrF-Excimer laser. The laser may be setup on top of the tower, so vibrations of the draw tower do not exert influence ion the laser beam positioning and stability. The other components such as the shutter unit <NUM> and the radiation modulating means <NUM> or parts thereof such as the interferometer itself may be fixed at the tower structure. The shutter may be a mechanical shutter, such as for example a computer controlled mechanical shutter, for controlling the number of laser pulses that is used for the FBG inscription process.

As indicated above the system typically also comprises a controller <NUM>. Such a controller <NUM> may be configured, e.g. programmed, for controlling the writing system <NUM>, the fiber length and/or drawing detecting means <NUM>, the marking system <NUM>, etc. The controller <NUM> may be a single controller combining different controlling tasks or it may be a combination of different sub-controllers each performing one or more functions or being dedicated to a specific function. With respect to the writing system <NUM>, the controller <NUM> may be configured, e.g. programmed, for controlling the coherent radiation source <NUM>, e.g. a repetition rate, for controlling a shutter unit <NUM> and/or for controlling the mirrors and/or translation stages of the radiation modulating means <NUM>, e.g. when the radiation modulating means <NUM> is an interferometer such as a Talbot interferometer. The controlling may be such that the mirror rotations and the movement of the translation stages is synchronized, although embodiments are not limited thereto.

The radiation modulating means <NUM> may for example be an interferometer, such as for example a Talbot interferometer, although embodiments of the present invention are not limited thereto.

In a particular embodiment, the radiation modulating means is a Talbot interferometer, whereby the mirrors and/or the translation stage of the Talbot interferometer are continuously in motion during the drawing process. The latter results in the advantage that no sudden changes in movement of the Talbot interferometer components are required when the Talbot interferometer needs to be adjusted for writing at another Bragg wavelength, thus resulting in a more fluent and accurate process. The continuous motion may be a rotation of the mirrors of the Talbot interferometer and/or movement over the translation stage.

The system also comprises a marking system <NUM> for providing marking on the drawn fiber. The marking system <NUM> may be a mechanical or optical marking means, providing for example grooves, ink indications, lasered features, etc. for indicating a position or length of the drawn fiber, a position of the next fiber grating in the grating array, etc. As indicated above, the marking system <NUM> may use information from the fiber length and/or drawing detecting means <NUM>, thus using accurate fiber length, fiber speed or other parameter information.

In a second aspect, the present invention relates to a method for producing an optical fiber with an inscribed grating array in accordance with claim <NUM>.

The method comprises drawing an optical fiber.

The method also comprises inscribing a grating array in the optical fiber during the drawing process of the optical fiber. According to embodiments of the present invention, the method also is characterized in that, during said drawing, the method comprises capturing information regarding the length of the fiber and/or the drawing speed and/or the drawing process and controlling the writing based on the captured information. Further method steps may correspond with the functionality of the features of the system as described in the first aspect.

A controller is described for controlling a fiber drawing process for grating array inscribed fibers, the controller being configured, e.g. programmed, for capturing information regarding the length of the fiber and/or the drawing speed and/or the drawing process and controlling the writing based on the captured information. The controller has the functionality as described in the first aspect.

A system for producing an optical fiber with an inscribed grating array is also described in which the radiation modulating means may be Talbot interferometer and in which the mirrors and/or the translation stage of the Talbot interferometer are continuously in motion during the drawing process. The latter results in the advantage that no sudden changes in movement of the Talbot interferometer components are required when the Talbot interferometer needs to be adjusted for writing at another Bragg wavelength, thus resulting in a more fluent and accurate process. The continuous motion, e.g. rotation, of the mirrors of the Talbot interferometer and/or the movement of the translation stage may correspond with or be a substantial part of the adjustment of the writing process as function of information obtained from the fiber length and drawing detecting means, the continuous motion may be induced partly of the need for adjusting the Bragg wavelength for describing a grating at another Bragg wavelength.

The continuous motion may be a rotation of the mirrors of the Talbot interferometer and/or movement over the translation stage. Further features and characteristics may be as described above.

A system for producing an optical fiber with an inscribed grating or grating array is also described which is configured for inscribing a grating using a pulse train that is generated such that the pulses in the pulse train lie sufficiently close together such that the fiber can be considered static with respect to the pulse train. The different pulses of the pulse train thus advantageously are positioned sufficiently close together such that only one grating will be created by the pulse train.

The time difference between two pulses of the pulse train, for example the difference between the first pulse and the last pulse of the pulse train or for example the difference between two subsequent pulses of the pulse train, may be such that the fiber has moved less than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, of the distance between subsequent two fringes of the grating written. In embodiments wherein the grating is such that the distance between fringes of the grating is not constant (e.g. for chirped gratings), the time difference between two pulses of the pulse train, for example between the first pulse and the last pulse of the pulse train or for example between two subsequent pulses of the pulse train, may be such that the fiber has moved less than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, preferably no more than <NUM>%, of the largest distance between two subsequent fringes of the grating written.

It is an advantage that the total energy used to inscribe the grating can be increased, while the energy density of the individual pulses of the pulse train can be limited compared to the situation where this energy would be in a single pulse. In this way, the total energy used to write a grating can be increased while the energy density can be restricted such that the inscribed grating may have a higher reflectivity and remains a type I grating, rather than a type II grating, i.e. than a damage written grating.

The pulse train may exist of a plurality of separated pulses, although depending on the delay induced between the pulses of the pulse train, the different pulses of the pulse train also may overlap and result in a stretched pulse, rather than in a pulse train. Also in this case, the total energy used to write a grating can be increased while the energy density can be restricted such that no type II grating is induced.

Claim 1:
A system (<NUM>) for producing an optical fiber with inscribed gratings, the system (<NUM>) comprising
- a fiber drawing apparatus (<NUM>) arranged for drawing an optical fiber,
- a writing system (<NUM>) comprising
o a coherent radiation source (<NUM>) being a laser for generating irradiation pulses, the laser having a laser parameter being a laser repetition rate at which the irradiation pulses are generated by the laser, and
o a radiation modulating means (<NUM>), configured for inscribing gratings in the optical fiber during the drawing process of the optical fiber,
- a controller (<NUM>) configured for controlling the driving of the writing system (<NUM>),
wherein the fiber drawing apparatus (<NUM>) comprises drawing detecting means (<NUM>) for determining the fiber length and/or fiber drawing speed and/or a fiber drawing parameter during the drawing process, and
wherein
the controller is being configured (<NUM>) for capturing information from the drawing detecting means (<NUM>), and the controller is being configured (<NUM>) for tuning, based on the information from the drawing detecting means (<NUM>), the laser to a particular laser repetition rate during the drawing of the optical fiber, thereby controlling the grating positions and tuning the spacing between the gratings of a grating array.