Abstract:
A tuning element tunes an optical component by adjusting the length of an optical fiber used within the optical component. Changes in length of a piezoelectric element are amplified by a surround structure, and the surround is provided with optical fiber fixing portions. The piezoelectric element is provided with an opening for receiving a fiber, such that a portion of a fiber can pass transversely through the opening, the length of the portion being adjustable by controlling the length of the piezoelectric element. The piezoelectric actuators have a fast response time and provide reliable operation.

Description:
FIELD OF THE INVENTION 
     This invention relates to tuning of optical fiber components, and in particular provides a tuning element as well as an optical component provided with such a tuning element. 
     BACKGROUND OF THE INVENTION 
     Various optical fiber components are known for use within optical fiber communication systems, and in particular for overcoming various distortions which arise as an optical signal is transmitted along a fiber span. For example, the use of Bragg reflection gratings is known for causing wavelength-dependent delays, which compensate for dispersion effects. 
     Many optical materials exhibit different responses to optical signals of different wavelengths. Chromatic dispersion, often simply referred to as “dispersion”, is one well-known resulting phenomenon, in which the index of the refraction of a medium is dependent on the wavelength of an optical wave. Dispersion can cause optical waves of different wavelengths to travel at different speeds in a given medium, since the speed of light is dependent on the index of refraction. The dispersion of optical materials in general relates nonlinearly to the wavelength. 
     In many applications, an optical signal is composed of spectral components of different wavelengths. For example, a single-frequency optical carrier may be modulated in order to impose information on the carrier. Such modulation generates modulation sidebands at different frequencies from the carrier frequency. Also, optical pulses, which are widely used in optical data processing and communication applications, contain spectral components in a certain spectral range. The dispersion effect may cause adverse effects on the signal due to the different delays on the different spectral components. 
     Dispersion in particular presents obstacles to increasing system data rates and transmission distances without signal repeaters in either single-channel or wavelength-division-multiplexed (“WDM”) fiber communication systems. Data transmission rates of tens of Gbit/s may be needed in order to meet the increasing demand in the marketplace. Dispersion can be accumulated over distance to induce pulse broadening or spread. Two adjacent pulses in a pulse train thus may overlap with each other at a high data rate due to dispersion. Such pulse overlapping can often cause errors in data transmission. 
     There have been various proposals for overcoming the dispersion effect, including use of a Bragg grating. Such gratings are known both with linearly chirped (i.e. varying) grating periods and with non-linearly chirped grating periods, in order to achieve the desired spectral response along the length of the optical carrier (fiber or waveguide). 
     A spectral component in an optical signal having the Bragg wavelength is reflected back from a Bragg grating. Other spectral components are transmitted through the grating. The Bragg wavelengths at different positions in the fiber grating are differentiated by the chirping of the grating period, so that the Bragg wavelength of the fiber grating changes with the position. As the grating period increases or decreases along a direction in the fiber grating, the Bragg wavelength increases or decreases accordingly. Therefore, different spectral components in an optical signal are reflected back at different locations and have different delays. Such wavelength-dependent delays can be used to negate the accumulated dispersion in a fiber or waveguide link. 
     To use a chirped Bragg grating, an optical circulator is typically used to couple the input optical signal to the grating and to route the reflected signal. An optional optical isolator or anti-reflection termination may be placed at the other end of the grating to reject any optical feedback signal. 
     To provide a tuneable fiber grating, it is necessary to alter the chirp of the grating. Known methods for providing tuning of Bragg gratings within optical components rely upon mechanical, thermal or thermo-mechanical arrangements. These techniques have relatively slow response times, high power dissipation and may also suffer from reliability problems. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a tuning element for tuning an optical component by adjusting the length of an optical fiber used within the optical component, the tuning element comprising; 
     an elongate piezoelectric element, the length of the piezoelectric element being controllable by applied control signals; and 
     a surround around the piezoelectric element and contacting the ends of the piezoelectric element, wherein the surround has a width, transverse to the longitudinal axis of the piezoelectric element, which varies when the length of the piezoelectric element is varied thereby amplifying changes in length of the piezoelectric element, 
     wherein opposite sides of the surround are provided with an optical fiber fixing portion, and the piezoelectric element is provided with an opening for receiving a fiber, such that a portion of a fiber can pass transversely through the opening, the length of the portion being adjustable by controlling the length of the piezoelectric element. 
     The arrangement of the invention uses piezoelectric actuators, which have a fast response time and provide reliable operation. The use of a surround around a piezoelectric element, so that changes in length of the element are converted into changes in width of the surround, results in amplification of the change in dimension of the piezoelectric element. 
     Each optical fiber fixing portion may comprise a tube through which the fiber can pass, with one end of the tube contacting the surround and the other end of the tube being connected to the fiber. This enables the length of the fiber being stretched by the tuning element to be different to the width of the surround. Thus, the tuning element can be designed simply to provide the required change in length of the fiber, and the optical fiber fixing portion enable this change in length to be provided to the desired length of fiber. 
     The tuning element may comprise two elongate piezoelectric elements arranged side by side and parallel to each other, both surrounded by the surround. In this case, the separation between the two piezoelectric elements will be selected in dependence upon the length of the fiber to be stretched by the tuning element. 
     Each optical fiber fixing portion may be provided for fixing at least two optical fibers. Thus, in an optical component which uses a number of fibers, for example a number of Bragg gratings, which are to be tuned in the same manner, this can be achieved with a single tuning element. 
     The two optical fiber fixing portions may additionally be arranged such that the length of the two fibers are different, so that different tuning is provided for the two different fibers. 
     The use of two piezoelectric elements also enables independent control of the tuning of two fibers using the single tuning element. To achieve this, one fiber is connected to one side of the surround and to one of the piezoelectric elements, and the other fiber is connected to the other side of the surround and the other piezoelectric element. Each piezoelectric element results in a change in shape of one side of the surround, and this has an effect only on one of the optical fibers in the tuning element. 
     To enable easy assembly of the tuning element, the optical fiber fixing portion may comprise a ferrule for attachment to the optical fiber, the ferrules having a narrow portion for engagement in an opening in the surround, and wider portions on each side of the narrow portion which prevent movement of the ferrule through the opening. The ferrule can then be attached to the surround by sliding the narrow portion into a slot in the surround. In addition, the opening in the piezoelectric element may also comprise a slot, so that the optical fiber, with ferrules attached, may simply be slotted into the tuning element during assembly. 
     The invention also provides a tuneable optical component comprising an optical fiber and a tuning element of the invention. 
     As described above, the fiber may be fixed to opposite sides of the surround, or alternatively the fiber may be fixed in one position to the surround, and in another position to the piezoelectric element, particularly in the case where two piezoelectric elements are provided and the tuning element is for tuning two fibers simultaneously and independently. 
     Typically, a Bragg grating is written into the fiber, and the tuning operation then results in a change in the chirp of the Bragg grating. This Bragg grating may form part of a dispersion compensator or a tuneable fiber filter. The tuned fiber may also contain a laser cavity defined by mirror or grating reflectors. 
     The invention also provides a method of tuning an optical component using the tuning element of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Examples of the invention will now be described in detail with reference to the accompanying drawings in which: 
     FIG. 1 shows a first example of tuning element according to the invention; 
     FIG. 2 shows a second example of tuning element of the invention; 
     FIG. 3 shows a third example of tuning element of the invention; 
     FIG. 4 shows a fourth example of tuning element of the invention; 
     FIG. 5 shows a fifth example of tuning element of the invention; 
     FIG. 6 shows one possible ferrule design for use in the tuning element; 
     FIG. 7 illustrates schematically the components of a dispersion compensator; and 
     FIG. 8 shows two further possible uses of the tuneable component. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a tuning element  10  of the invention for adjusting the length of an optical fiber  12 . The tuning element  10  has a piezoelectric element  14  arranged as an elongate bar. The length of the element  14  is controllable by applying control signals to a terminal  16 . 
     A surround  18  is provided around the element  14  and contacts the ends  20  of the piezoelectric element. The surround  18  is formed from a material which is deformable in such a way that the length of the surround remains substantially constant. Thus, changes in the length of the piezoelectric element  14  result in a change in shape of the surround, giving rise to a different width W. To achieve this, the material of the surround  18  is selected, in particular, with appropriate bending modulus and elastic modulus. 
     The surround  18  may simply contact the ends  20  of the element  14 , and remain in place by the natural bias of the surround  18  towards a shape in which the ends  22  of the surround are closer together. Alternatively, the surround  18  may be fixed to the ends  20  of the piezoelectric element  14 . 
     Opposite sides  24  of the surround  18  are provided with an optical fiber fixing portion  26 , so that the fiber  12  extends across the width of the tuning element  10 . 
     In the example shown in FIG. 1, the optical fiber fixing portion  26  comprises a tube through which the fiber passes. The fiber is connected to the tube  26  at locations  28  and is not supported between these points. Thus, the length of a portion of the fiber  12  between the fixing points  28  is to be controlled by the tuning element  10 . 
     The fiber  12  passes through the sides  24  of the surround  18  and also through an opening  30  in the center of the piezoelectric element  14 . 
     The tuning element converts changes in length of the piezoelectric element  14  into changes in width W of the surround  18 . These width changes in turn result in movement of the optical fiber fixing portions  26 , thereby changing the length of the fiber portion between the fixing points  28 . This enables the size of the tuning element  10  to be selected simply as a function of the change in length required for the fiber, rather than the length of the fiber itself. 
     Typically, the fiber  12  includes a Bragg grating in the portion between the fixing points  28 , and the length of the Bragg grating may typically be of the order of 10 cm, whereas the length of the piezoelectric element may be of the order of 5 cm. 
     FIG. 2 shows a second embodiment of tuning element in which two piezoelectric elements are provided  14 A,  14 B. The shape of the surround  18  between the elements  14 A,  14 B is fixed by plates  40  (which are optional and may not be required), so that the change in shape of the surround  18  manifests itself as deformation only of the sides  24 . This provides a more mechanically stable design. Furthermore, the centers of the two piezoelectric elements  14 A,  14 B are stationary during operation of the tuning element. The use of metal spacers at the centers of the elements therefore provide suitable mounting points for the tuning element. In the example shown in FIG. 2, the optical fiber  12  is fixed to opposite sides of the surround  18 , again at fixing points  28 . The spacing between the two piezoelectric elements  14 A,  14 B can in this case be designed in dependence upon the length of the fiber portion, thereby avoiding the need for the fixing portions  26  shown in FIG.  1 . 
     FIG. 2 also shows that each piezoelectric element  14  nay comprise two sections  42 ,  44  on either side of a metal spacer  46  which provides the through-hole  30  for the fiber  12 . 
     The tuning element of the invention may be used for controlling the length of two or more optical fibers, simultaneously. 
     In the example shown in FIG. 3, two fibers  12 A,  12 B pass through the piezoelectric elements  14 A,  14 B. Control of the tuning element can be arranged to provide different stretching of the two fibers  12 A,  12 B. In the example of FIG. 3, optical fiber connecting portions  26  are shown in which the connection points of the two fibers are angled, so that the length of the portion of each fiber is different. The connection points of the fibers are again indicated as numeral  28 , and FIG. 3 shows schematically how the portion of fiber  12 A can be defined to have a greater length than the portion of the fiber  12 B. Whilst this arrangement enables the stretching of one fiber to be scaled with respect to the stretching of the other fiber, it does not provide independent control of the fiber lengths. 
     FIG. 4 shows an arrangement in which independent control of the length of two fibers can be provided. One fiber  12 A is connected to one of the piezoelectric elements  14 A and to one side  24 A of the surround, whereas the other fiber  12 B is connected to the other piezoelectric element  14 B and to the other side  24 B of the surround. 
     Operation of one piezoelectric element only results in the change in shape in one side of the surround  18 . As a result, it is possible to provide independent length control of the two fibers. 
     FIG. 5 shows an arrangement in which a single piezoelectric element  14  provides control of the length of two fibers  12 A,  12 B in opposite senses. One fiber  12 A is connected to one side of the surround  18  and the other fiber  12 B is connected to the opposite side of the surround  18 . The two fibers are fixed at locations  29  remote from the tuning element. The positions of the locations  29  may provide different lengths of tuned fiber. 
     The connections  28  of the fibers may be achieved by any known technique. For example, a metallized fiber may simply be soldered to a metal component, or any fiber may be fixed within a passageway using an epoxy adhesive. 
     To simplify assembly of the tuning element  10 , the fiber or fibers may pass through slots rather than openings. Thus, the opening  30  in the or each piezoelectric element  14  may be defined as a slot so that the fiber does not need to be threaded through the piezoelectric element. 
     Similarly, the openings in the surround  18  may also be defined as slots to simplify assembly. 
     FIG. 6 shows one possible arrangement for providing a point of connection of a fiber to the surround  18  or piezoelectric element  14 , as required. A ferrule  50  is mounted around the fiber  12  and is fixed to the fiber, for example using epoxy adhesive. The ferrule  50  has a narrow central portion  52  and two wider end portions  54 . When the narrow portion  52  is inserted into a slot  56  provided in the surround  18  or the piezoelectric element  14 , the wider portions  54  prevent lateral movement of the fiber. 
     The tuning element of the invention is particularly suitable for adjusting the length of a fiber carrying a Bragg grating. There are numerous optical components in which tuneable Bragg gratings may be required. For example, FIG. 7 shows a tuneable dispersion compensator comprising an input  60 , and a circulator  62  which routes the input to a tuneable Bragg grating  64 . The Bragg grating  64  reflects signals at different points along its length as a function of wavelength, and these reflected signals are routed to an output  66  of the device. An isolator  68  may also be provided to prevent reflection of unwanted signals at the output of the Bragg grating  64 . 
     FIG. 8 shows a tuneable fiber laser  70  comprising a grating structure  72  which defines a laser cavity. The fiber laser is a DFB (distributed feedback) fiber laser, and the grating structure  72  may for example comprise two offset and phase shifted gratings. These provide wavelength-selective reflectors at each end of a laser cavity, The grating structure  72  is mounted within the tuning arrangement of the invention. FIG. 8 also shows a filter  74  in which an input  76  is routed to the tuneable Bragg grating  78  by a circulator  80 . The Bragg grating  78  is tuned to allow the passage of most wavelengths with low attenuation, but reflects the wavelengths to which the filter is tuned, which are in turn routed by the circulator  80  to port  82 . 
     Various modifications will be apparent to those skilled in the art.