MXPA00008153A - Fiber optic attenuators and attenuation systems - Google Patents

Abstract

Controllable fiber optic attenuators and attenuation systems (100) are disclosed for controllably extracting optical energy from a fiber optic (30), and therefore attenuating the optical signal being transmitted through the fiber optic (30). In one aspect, material is removed from a portion of the optical fiber (30), thereby exposing a surface through which optical energy can be extracted. A controllable material is formed over the surface for controllably extracting optical energy according to a changeable stimulus applied thereto, which affects the refractive index thereof. In an improved embodiment, a controllable material is formed over the exposed surface for controlling the amount of optical energy extracted from the fiber optic, and a bulk material is formed over the controllable material, into which the extracted optical energy is radiated.

Description

ATTENUATORS FOR OPTICAL FIBER AND ATTENUATION SYSTEMS FIELD OF THE INVENTION The present invention relates to controllable attenuators and attenuation systems for attenuating the optical energy transmitted through an optical fiber.

BACKGROUND OF THE INVENTION In fiber optic systems there is often a need for precise control of the optical signal levels that enter the various components of the system. This is particularly true for systems in the test and characterization stages of deployment. For example, a controllable optical attenuator can be used to characterize and optimize the optoelectronic response of high-speed photoreceptors, in which the sensing response capacity depends on the average optical energy incident on the photodiode. The majority of commercially available controllable fiber optic attenuators are currently based on thin film absorption filters. This requires breaking the fiber and placing the filters in line. The controllable attenuation is then achieved by mechanical means such as rotating or sliding the filter to change the optical path length within the absorbent material. This negatively affects the response speed of the device, the total mechanical stability, the loss of insertion of zero attenuation and the reflection of optical retreat. In general, fiber designs in fragments have numerous disadvantages such as high insertion loss, significant retraction reflection and large size. These factors can be minimized, although such corrective measures typically result in costs and / or aggregate size. What is needed are improved controllable fiber optic attenuators and attenuation systems that keep the fiber optic core intact and achieve controllable attenuation by controlling the loss of radiation from the fiber.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to controllable optical fiber attenuators (eg, variable optical attenuators "VOAs") and to attenuation systems, designed to operate in the conventional 1300 nm and 1550 nm telecommunications spectral window, or any other length of interest, especially those in which unimodal propagation occurs. The devices can be placed in networks or optical fiber systems by means of the simple fusion junction or by connection to attenuate the optical signal levels in a desired amount.

Controllable attenuation is achieved, for example, by thermally or electrically controlling the layers of controllable material. These devices can be used for controllable attenuation in optical fiber systems in the testing and characterization stage, or for active control during operational deployment. The side-polished fiber ("SPF") devices of the present invention are an improvement over fiber proposals in conventional segments due to the intrinsic continuity of the fiber. In a first embodiment of a controllable attenuator of the present invention, a fiber is mounted in a block and polished to within a close proximity (e.g., a few microns) of the core. A controllable base material with an approximately equal refractive index (with the effective fiber mode index) is applied to the polished surface. Adjusting the refractive index of the base material (for example, by the electro-optical or thermo-optical effect), results in a controllable amount of optical energy extracted from the optical fibers, thereby obtaining a controllable attenuation. Also described is an attenuation system, including a controllable attenuator in which a control circuit applies a changeable stimulus to the controllable material, in accordance with a stimulus of desired level, and / or a detected level stimulus received from a coupled detector circuit. to the optical fibers to detect a level of optical energy that is being transmitted therein.

In an improved mode of the controllable attenuator of the present invention, the fiber is polished through its coating almost to the core, and a thin controllable material is placed between the fiber and a base coating material with a high index. The refractive index of the controllable material (approximately equal to that of the coating) is varied, which effectively varies the effective optical thickness (index x actual thickness) of the remaining coating. This controllable attenuator activated by coating ("CD") provides an almost spectrally flat optical attenuation in the wavelength ranges of interest, while maintaining all the intrinsic advantages of the SPF architecture. In addition, a design is described in which the typically used radio block that holds the fiber is removed, which allows the size of the device to be reduced so that it is not much larger than the fiber itself. In this sense, the present invention refers to, in its first embodiment, an attenuation system for attenuating the optical energy that is being transmitted through optical fiber. A controllable attenuator is adjusted with respect to a portion of the optical fiber having the material removed therefrom thereby exposing a surface thereof through which a portion of the optical fiber can be removed in a controllable manner. of optical energy. The attenuator includes a controllable material formed on the surface to controllably extract the optical energy in accordance with a changeable stimulus applied thereto which affects the refractive index thereof. A level detector circuit may be coupled to the optical fiber to detect a level of at least a portion of the optical energy that is being transmitted therein and provide a detected level stimulus to a control circuit, which is coupled with the controllable attenuator for applying the changeable stimulus to the controllable material thereof in accordance with the detected level stimulus received from the level detecting circuit. The changeable stimulus applied to the controllable material can be, for example, temperature (thermo-optical effect) or voltage (electro-optical effect). In a second, improved aspect, the present invention relates to a controllable attenuator activated by coating ("CD") to attenuate the optical energy transmitted through an optical fiber. The controllable attenuator is positioned with respect to a portion of the optical fiber having the material removed therefrom thereby exposing a surface thereof through which at least a portion of optical energy that is being transmitted in it can be extracted. The controllable attenuator includes a controllable material formed on the exposed surface to control an amount of optical energy extracted from the optical fiber in accordance with a changeable stimulus applied to the controllable material which affects the refractive index thereof. In addition, a layer of base material formed on the controllable material in which the extracted optical energy is irradiated is provided. In this embodiment, the controllable material has a controllable refractive index approximately equal to the refractive index of the coating, and the base material formed on the controllable material has a fixed refractive index greater than the effective index of the optical fiber. The attenuators for controllable fiber optics and attenuation systems of the present invention are valuable in any of the applications in which the control of transmission of optical energy in an optical fiber is required. The attenuators are especially useful in applications in which the attenuation spectral flattening is a concern. Due to the continuity of the fiber, this device presents the intrinsic benefits of low insertion loss, low recoil reflection (high return loss), polarization insensitivity, small size, low cost and mass production capacity.

BRIEF DESCRIPTION OF THE INVENTION The subject that is considered as the invention is particularly pointed out and claimed in a distinctive way in the final portion of the specification. However, the invention both in the organization and in the method of practice together with additional objects and advantages thereof, will be better understood by reference to the following detailed description of the preferred embodiments and the accompanying drawings in which: Figure 1a is a cross-sectional side view of a first embodiment of a controllable optical fiber attenuator in accordance with the present invention. Figure 1 b is a cross-sectional end view of the controllable attenuator of Figure 1a. Figures 2a-b are graphs (in percent, and decibels, respectively) showing the loss characterization against the refractive index of a base (e.g., liquid) coating for three example levels of side fiber grinding. Figure 3a is a detailed view of the material interfaces of the controllable attenuator of Figures 1a-b, and further shows an example profile of the optical energy mode that is being transmitted in the optical fiber. Figure 3b is a detail view of the material interfaces of a second activated mode with coating of a controllable optical fiber attenuator of the present invention. Figures 4a-b are respective graphs of spectral performance of two controllable attenuators of Figures 3a-b. Figure 5 is a graph of the resulting attenuation against the refractive index of the side attenuator fiber attenuator and shows the respective operating ranges of the controllable attenuators of Figures 3a-b.

Figure 6a is a cross-sectional side view of the second controllable attenuator activated by coating of Figure 3b. Figure 6b is a cross-sectional side view of an improvement for the controllable attenuator activated by coating of the present invention in which the coating is removed from the optical fiber without a radial mounting on a substrate block. Figure 7 is a functional block diagram of an exemplary attenuation system in accordance with the present invention; and Figure 8 is an example scheme of the attenuation system of Figure 7.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In accordance with the principles of the present invention, a first mode 100 of a controllable attenuator is shown in FIGS. 1 a-b, in which a unimodal optical fiber 30 (e.g., Corning SMF-28 for telecommunications) is polished on the sides through its coating 50 up to near its core 40, whereby an evanescent tail of the optical energy transmitted in the fiber is exposed through the surface 65. Typically, the thickness of the remaining coating is less than about 10 μm. The optical energy can be extracted from the core of the fiber by applying a base material 60 on the polished surface 65 of the fiber coating. The base material must have a refractive index slightly less than or approximately equal to that of the effective mode index of the nef fiber. This value is dependent on the core indices of the fiber and cladding, and on the dimensions of the core of the fiber, but in common form it is between the indices of the core and the cladding. The maximum optical energy is extracted from the fiber when the index of the base material is equal to the effective mode index of the fiber. In accordance with the present invention, and as discussed in more detail below, the base material can be formed from a material that is controllable, for example, its refractive index can be varied in accordance with a changeable stimulus applied thereto. . In the embodiment of Figure 1a, changes in temperature or voltage can be used, and a controllable heating element (or electrodes) 80 is provided, to provide a changeable temperature (or voltage) to the material 60 in accordance with a control stimulus. 105. Subsequently, the manufacture of the fiber portion with lateral polishing of the attenuator 100 and its characterization of subsequent loss are discussed first.; second, the alternate modes 100 'and 100"of a controllable attenuator, and finally, the implementation of an attenuation system that includes the controllable attenuator 100 (or 100' or 100"), in addition to other control subsystems.

Fabrication / characterization of fiber with lateral polishing Standard unimodal fibers have a core region 40 with a diameter of 8.3 μm of slightly elevated refractive index surrounded by a vaporized silica coating 50 of 125 ± 1 μm. The mode field diameter is 9.3 ± 0.5 μm at 1310 nm and 10.5 ± 0.5 μm at 1550 nm. The refractive index values provided by Corning for the SMF-28 fiber are:? = 1300 nm: nnúCieo = 1, 4541, nrev = 1, 4483? = 1550 nm: core = 1, 4505, nrev = 1, 4447 The small difference between the refractive indices of the core and the cladding combined with the small core size results in a unimodal propagation of optical energy with lengths above 1190 nm . Therefore, the fiber can be used in both spectral regions although it was designed for operation at 1310 nm in which the dispersion (combination of material and waveguide dispersion) is brought to a minimum and the attenuation is low (< 0.4 dB / km). The controllable fiber attenuator with lateral polishing of Figures 1a-b can be manufactured by burnishing and polishing techniques. The fiber is embedded in a block of vaporized silica substrate 20 containing a radio controlled groove. The material is carefully removed from the coating of the fiber 50 until the core 40 is reached. At this point, the evanescent field of the optical energy transmitted in the optical fiber can be accessed through the surface 65. The interaction length of the device can be controlled by the thickness of the remaining coating and the radius of curvature of the groove. Once the core of the fiber has been approached by the burnishing / polishing process, a multiple liquid drip procedure can be performed to characterize fiber loss with side grinding. This method involves placing a series of base coatings (eg, liquids, oils) of known refractive index on the polished surface of the fiber. This has the advantage that the interface between the oil and the side-polished fiber is always as good as the surface of the fiber and there is no need to treat the surface / oil interface in any special way. There is a set of Cargille refractive index liquids with refractive indexes and well-characterized dispersion curves. Therefore, an accurate loss / refractive index characterization of each fiber can be obtained with fabricated side grinding. Each liquid used in the measurements has an unspecified value, in which the subscript D indicates the wavelength of the sodium D line (? = 589 nm). Dispersion equations are available which allow the responses to be adjusted to the spectral region of interest, ie, 1300 nm or 1550 nm. Figures 2a-b show the transmission of optical energy in percent and decibels, respectively, against the refractive index response of the liquid for the three side-polished fibers which each have different remaining coating thicknesses (ie, 24% , 65% and 91% of polished coating levels). At liquid rates below the effective fiber mode index (nef), optical energy is not removed from the fiber. Near nTf, the transmission response drops abruptly and a strong extraction is observed. Above nef, the transmission of the fiber gradually approaches an adjusted level of attenuation. Prior to any coating removal, fiber guides light efficiently. When part of the coating is removed, there is a new coating which is composed of a small thickness of vaporized silica surrounded by air (n = 1). Since this mixed coating has an effective coating index lower than that of the core, the fiber can even operate efficiently as a waveguide. This is true for those coatings that have lower indexes of the effective fiber mode index, and therefore a 100% transmission of optical energy is presented. However, when the liquid index rises above net-, the fiber operates as a leaking waveguide and a base wave in the liquid is excited. Therefore, the energy escapes from the fiber within the region of interaction and some attenuation occurs. The coupling efficiency with the base wave is maximum when the liquid index is equal to the effective rate of the nef fiber. This efficiency is reduced when the liquid index is increased above nef, although a significant fraction of energy is still coupled outside the fiber. Transmission measurements can be made using Fabry-Perot diode lasers at 1300 nm and 1550 nm and a well-calibrated optical energy meter. Stronger attenuation numbers are observed for the same liquid index at 1550 nm as the evanescent penetration of the fiber mode field in the coating is greater than the longest wavelength. In accordance with the present invention, as discussed above, a base material 60 is applied on the exposed surface of the optical fiber with side grinding. The base material 60 is, for example, a controllable polymer (eg, electro-optical or thermo-optical) with a refractive index almost equal to that of the effective mode index of the fiber, and which exhibits a change in the refractive index proportional to a change in, for example, temperature or voltage. OPTI-CLAD® 145, available from Optical Polymer Research, Inc., is an example of one such polymer. Therefore, a controllable attenuator (100, FIG. 1a-b) is formed which is capable of extracting a controllable amount of optical energy from the fiber. Attenuation control is provided by the heating element (or electrodes) 80 controlled using a control stimulus 105. To achieve the maximum thermo-optical response capability, for example, the controllable attenuator is implemented to exploit the index response of the most sensitive characteristic refraction of the fiber with lateral polishing, determined as indicated above. This occurs when the refractive index of the base material is slightly lower than the effective index of the optical fiber (for example, nef = 1449), ie close to the vertical lines 99 drawn in the graphs of figures 2a -b.

These lines 99 therefore describe, in general, the theoretical operating range of the first mode of a controllable fiber attenuator with lateral polishing.

Alternate modes of controllable attenuator. One aspect of the controllable attenuator mode 100 discussed above is that the level of attenuation may vary with the wavelength, which may cause design problems for multiple wavelength transmission systems. In accordance with the present invention, a controllable fiber attenuator with improved surface-activated side-by-side ("CD") polishing is described which improves the spectral performance while preserving all intrinsic performance intensities of fiber devices with side grinding. non-invasive Figures 3a-b respectively show in detail the material interfaces of the controllable attenuator with base coating 100 discussed above, and the controllable attenuator attenuated by improved coating 100 'of the present invention. Referring to Figure 3a, the controllable attenuator 100 includes a fiber core 40, a remaining portion of coating 50 (thickness Cn-like, eg, less than about 10 μm) having an exposed surface 65 thereof. through which the optical energy is drawn to the controllable base material 60. A profile of mode 90 is also shown that approximates the amount of optical energy present in the layers of material, including evanescent glue 91 (whose penetration in layer 60 it can be controlled as indicated above). The controllable deactivator activated by coating 100 'of Figure 3b also includes a fiber core 40', but the remaining portion 50 'of the coating (thickness for example less than about 2 μm) is a very thin layer and a thin film (eg example, smaller thickness of about 10 μm) of controllable material 60 'is placed on the coating 50'. A base material 70 'is placed on the layer 60' and is a high index material. The evanescent tail 91 'of the mode profile 90' penetrates through the exposed surface 65 'towards the elevated index layer 70' at a depth determined by the effective optical thickness (index x actual thickness) of controllable material 60 ', the which has an index approximately equal to that of the coating. This effective optical thickness of the layer 60 '(index x actual thickness) is controlled by varying the refractive index thereof in accordance with the techniques discussed above, for example, by thermo-optical or electro-optical effect. The most significant differences between the 100 'activated coating mode and the controllable 100 basis material mode are: (i) most of the fiber coating is initially removed (on the polished side) and replaced with a thin layer of coating with equalized but controllable index of material 60 '(a thermo-optical polymer having an index of about 1447 at 1300 nm) and (ii) the base coating 70' is of a higher index, for example silicon, with an index of approximately 3.5. As shown in the graphs of Figures 4a-b, which in respective form represent the spectral performance of the controllable attenuator modes 100 and 100 ', these improvements result in a better spectral uniformity. The reasons for this spectral uniformity can be understood by referring to the respective operating ranges 99 and 99 'of the attenuation graph of Figure 5. The attenuation of a fiber with side-grinding is a sensitive function of both: (i) the remaining coating thickness and (ii) the index of any of the coating materials. In the first controllable attenuator mode 100, a significant portion of the evanescent tail of the fiber mode profile propagates within the remaining coating. Therefore to achieve significant attenuation, the fiber with side polishing is coated with a base material 60 which has a refractive index that is close to the effective fiber mode index nef. By adjusting the index of the base material, an attenuation transfer function is produced which follows the sharpest edge of the attenuation response curve, that is, close to the vertical line 99. However, because this edge is very sharp , the amount of attenuation is very sensitive to variations of the fiber mode profile. Therefore, effects such as dispersion (changes in refractive index versus wavelength), can result in a wavelength-dependent performance. Another effect, perhaps more significant, is presented simply because the fiber mode itself is greater at long wavelengths. This results in an increased evanescent penetration towards the coating, and therefore a greater attenuation. The controllable attenuator mode activated by coating 100 'eliminates these effects because its operation is based on a completely different transfer function. As shown to the right hand side of FIG. 5, the coating activated proposal is adjusted to the effective optical thickness of the remaining coating using the controllable coating layer with equal index 60 ', and therefore changes the amount of evanescent glue 91'. towards the base material 70 'which has a fixed high index. Therefore the attenuation is much less sensitive to variations in the refractive index of the base material when that index is well above nef. Therefore, the coated device operates along vertical line 99 'to the right side of FIG. 5. This has been shown to produce attenuation levels that are almost independent of wavelength (FIG. 4b), and therefore improves the spectral uniformity of the device. The insensitivity of the index of the base material 70 'also implies that for a given amount of remaining fiber lining (which determines the amount of attenuation at high rates for a given interaction length), the variation of the index of the base material 70' (for example through the thermo-optical effect) will not significantly alter the amount of attenuation. Therefore, the response of a device as such without a controllable coating layer could be very small. The solution to this apparent dead spot was found by noting that the amount of attenuation (with a high index base material) is very sensitive to the amount of remaining fiber coating thickness; that is, the more coating that is removed, the greater the attenuation will be (as shown by the curves to the right side of Figure 5). Thus, if an SPF-based device is produced which operates along the transfer function 99 'to the right-hand side of FIG. 5, then both a highly responsive capability of the device and a high spectral flattening can be obtained. . The controllable attenuator 100 'activated by coating achieves these results. In the controllable attenuator activated by 100 'coating, almost all of the original fiber (silica) coating is removed (typically by polishing, although chemical pickling is possible). This would normally result in a coating coupler with more than 99% high index (&-20 dB). However, the removed coating is replaced with a thin film of controllable material 60 '(equal in thickness to the evanescent penetration depth) which has a similar environmental refractive index (matched fiber coating). In addition, the refractive index of this material is much more responsive to an applied signal (for example thermo-optical: heat, or electro-optical: voltage), than that of the original silica coating. Above this thin film, a high index base material 70 'is applied to preserve the spectral flattening, as discussed above. Under ambient conditions, a device with very low attenuation results. However, by applying a changeable stimulus to the replacement lining layer 60 '"which raises its index (to that of the index effectively), the evanescent penetration can be varied by way of this lining layer". replacement '60', and therefore the depth of its penetration towards the base coating of high index 70 ', effecting controllable attenuation. Eliminating the stimulus reduces the refractive index of the replacement coating layer 60 ', which restores transmission with low loss. Any variation induced in the refractive index of the base material 70 'is very small due to the intrinsic insensitivity of the device to this parameter. Therefore, the controllable attenuator activated by coating 100 'simultaneously achieves high spectral responsiveness and flattening, as well as the characteristics of low insertion loss, low back reflection, small size and low loss of devices based on SPF, of which all make this modality quite attractive. Figure 4b shows cross-sectional side views of two potential modes (100 'and 100") of controllable attenuators activated by coating.The embodiment of Figure 4a, discussed generally above, is a design based on the block of typical SPF radio slot, in which the radius of the fiber, after polishing it, results in a flat surface 65 'through which the optical energy can be extracted.Figure 4b our design without alternative block 100" which is manufactured by removing the material to produce a radial surface 65", through which the controllable material 60" and the material 70"are shaped so as to fit, up to an outer diameter of the fiber. remains (Cn-th thickness "of less than about 2μm) Removal of the SPF block in the design 100" allows the size of the device to be reduced, so that this is not much larger than the fi same bra. Those skilled in the art will recognize that the embodiment 100 discussed above can also be manufactured using this design without block.

Attenuation system or systems using controllable attenuators An example of attenuation system 500 using the controllable attenuator 100 (or 100 'or 100") is shown in Figure 7. The attenuation system 500 includes a controllable attenuator 100 (or 100). 'or 100'), a control circuit 300, and an optional level detector circuit 200. The control circuit 300 supplies the control stimulus 105 to the controllable attenuator 100 to change the changeable stimulus (temperature or voltage) and therefore the refractive index of the controllable material thereof. The control circuit 300 receives as an optional input a stimulus of desired level 305 coming, for example, from a user, and adjusts the control stimulus 105 as a function thereof. The control circuit 300 may also receive an optional detected level stimulus from the level detector circuit 200. This detected level stimulus may be, for example, a ratio of measured levels of optical energy both before and after the attenuation thereof. by the attenuator 100. By comparing these detected level stimuli with the desired level stimulus, the control circuit 300 can vary the value of the control stimulus 105 until the stimulus of the desired level fed and the stimulus of the detected level are equalized. The example attenuation system 500 is shown in an exemplary schematic form in Figure 8. The controllable attenuator 100 is preceded and followed by 1% fiber couplers (separators 210, 230) which intercept a small fraction of the optical energy that propagates in the fiber. The decoupled light is carried to characterized photodetectors (220, 240) and the generated photocurrents are analyzed by a ratio meter 250. The comparator circuit 310 receives the detected level stimulus output from the ratio meter and / or a desired level stimulus. 305 (from a user) and transmits a signal to the temperature controller 320. The temperature controller provides the control stimulus 105 to the controllable attenuator 100 to change the changeable stimulus (temperature or voltage) and hence the rate of refraction of the controllable material thereof. In this way, the optical attenuation level (photocurrent ratio) is directly compared to a calibrated attenuation adjustment signal 305 (system user input) until these are equalized. This feedback loop controls the attenuation made by the controllable attenuator and therefore ensures accurate performance. The present invention also extends to the methods for forming and using the controllable attenuators and attenuation system described, and also to methods for attenuation, discussed above. Those skilled in the art will also recognize that the present invention extends to i) fixed point attenuators in which, under controlled environmental conditions, the layers of controllable material are designed with a predetermined refractive index so as to obtain an attenuation of predetermined fixed level, thereby denying the need for a changeable stimulus applied to the controllable material, and ii) the adaptive attenuation at which a fixed attenuation level is desired, and the changeable stimulus is applied in an adaptable form to the controllable material as a function of changing environmental conditions which unintentionally affect the refractive index of the controllable material. Although the invention has been shown in a particular manner and described with reference to preferred embodiments thereof, those skilled in the art will understand that various changes can be made in form and detail therein without departing from the scope and scope of the invention.

Claims (47)

1. NOVELTY OF THE INVENTION CLAIMS 1. - A controllable attenuator for attenuating the optical energy transmitted through an optical fiber, the controllable attenuator being arranged with respect to a portion of the optical fiber having a lateral surface thereof through which it can be extracted at least a part of said optical energy transmitted, including the controllable attenuator: a controllable material formed on the surface to control an amount of optical energy extracted from said optical fiber in accordance with a changeable stimulus applied to the controllable material which affects the rate of refraction thereof; and a base material, formed on the controllable material, in which the extracted optical energy is irradiated. 2.- The controllable attenuator in accordance with the claim 1, further characterized in that the controllable material has a controllable refractive index that is approximately equal to the refractive index of the optical fiber coating; and the base material formed on the controllable material has a fixed refractive index greater than the effective refractive index of the optical fiber. 3. The controllable attenuator according to claim 1, further characterized in that the changeable stimulus applied to the controllable material comprises temperature or voltage. 4. - An attenuation system, comprising: the controllable attenuator according to claim 1; and a control circuit coupled to the controllable attenuator to control a changeable stimulus value applied to the controllable material. 5. The attenuation system according to claim 4 further comprising: a level circuit for providing a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber to said circuit of control; and further characterized in that the control circuit controls the value of the changeable stimulus applied to the controllable material in accordance with the feedback stimulus. 6. The attenuation system according to claim 5, further characterized in that the level circuit comprises at least one detector for detecting a level of at least a portion of the optical energy transmitted in the optical fiber, providing the circuit of level the feedback stimulus in accordance with said detected level. 7. A method for forming a controllable attenuator for attenuating the optical energy transmitted through an optical fiber, comprising: providing a portion of the optical fiber having a lateral surface thereof through which it can be extracted by at least a portion of said transmitted optical energy; forming a controllable material on the surface to control an amount of optical energy extracted from the optical fiber in accordance with changes in the refractive index induced therein by a changeable stimulus to be applied to it; forming a base material on the controllable material in which the extracted optical energy is radiated. 8. The method according to claim 7, further characterized in that: the controllable material is formed to have a controllable refractive index approximately equal to the refractive index of the optical fiber coating; and the base material formed on the controllable material is formed to have a fixed refractive index greater than the effective refractive index of the optical fiber. 9. The method according to claim 7, further characterized in that the changeable stimulus to be applied to the controllable material comprises temperature or voltage. 10. A method for forming an attenuation system, comprising: forming a controllable attenuator in accordance with the method of claim 7; and providing a control circuit coupled to the controllable attenuator to control a changeable stimulus value to be applied to the controllable attenuator. 11. The method according to claim 10, further comprising: providing a coupled level circuit to provide a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber to said control circuit; further characterized in that the control circuit is formed to control the value of the changeable stimulus to be applied to the controllable material in accordance with the feedback stimulus. 12. The method according to claim 11, further characterized in that the level circuit comprises at least one detector for detecting a level of at least a portion of the optical energy transmitted in the optical fiber, providing the level circuit the feedback stimulus in accordance with said detected level. 13. A method for controllably attenuating the optical energy transmitted through a portion of an optical fiber having a lateral surface through which an evanescent tail of an optical mode field transmitted through the fiber is exposed. optics, the method comprising: extracting the optical energy from the evanescent tail of the optical mode field using a base material, placed on said portion of the optical fiber, into which the evanescent tail penetrates; the use of a controllable material placed between the base material and a core of the optical fiber to vary a depth of penetration of the evanescent glue into the base material, including varying the refractive index of the controllable material. 14. The method according to claim 13, further characterized in that most of the coating is removed between an outer radius thereof and the core of the portion of the optical fiber, and wherein: said use of a controllable material includes varying an effective optical thickness thereof varying the refractive index thereof, whereby the penetration depth of the vanishing tail in the base material is varied. 15. The method according to claim 13, further characterized in that: the controllable material has a controllable refractive index that is approximately equal to the refractive index of the coating; and the base material has a fixed refractive index greater than the effective refractive index of the optical fiber. 16. The method according to claim 13, further characterized in that said use of a controllable material includes: varying the refractive index of the controllable material by changing a temperature or voltage stimulus applied thereto. 17. The method according to claim 13, further comprising: providing a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber to said control circuit; further characterized in that said use of a controllable material includes varying the refractive index of the controllable material in accordance with the feedback stimulus. 18. The method according to claim 17, further characterized in that said providing a feedback stimulus comprises: detecting a level of at least a portion of the optical energy transmitted in the optical fiber, and providing the feedback feedback stimulus. with said detected level. 19. - An attenuation system comprising: a controllable attenuator, arranged with respect to a portion of an optical fiber, the portion of the optical fiber having a lateral surface thereof through which at least a portion of an optical fiber can be extracted; the optical energy, including the attenuator, a controllable material formed on the surface to controllably extract the optical energy in accordance with a changeable stimulus applied to the controllable material which affects the refractive index thereof; a level circuit for providing a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber; and a control circuit led with the controllable attenuator to control a quantity of the changeable stimulus applied to the controllable material in accordance with the feedback stimulus. 20. The attenuation system according to claim 19, further characterized in that the changeable stimulus comprises temperature or voltage, and wherein the control circuit provides a control stimulus to change the temperature or voltage of the controllable material. 21. The attenuation system according to claim 20, further characterized in that the controllable attenuator includes a control element, which has an input to receive the control stimulus from said control circuit, and arranged with respect to the controllable material to change the temperature, or voltage, and therefore the refractive index thereof in accordance with the control stimulus. 22. The attenuation system according to claim 19, further characterized in that the level circuit includes: a first detector circuit for detecting an amount of optical energy transmitted in said optical fiber before any extraction thereof by said controllable attenuator; a second detector circuit for detecting an amount of optical energy transmitted in said optical fiber following any extraction thereof by means of said controllable attenuator; and a circuit for determining a level of optical energy extracted from said optical fiber in accordance with the detected amounts of optical energy before and after the extraction thereof by means of said controllable attenuator, and providing the feedback stimulus to the circuit of control based on said level of extracted optical energy. 23. The attenuation system according to claim 22, further characterized in that the control circuit includes a comparison circuit for comparing the feedback stimulus and a desired level stimulus applied thereto, and, based on any difference between the same, change the value of the control stimulus provided to the controllable attenuator. 24. The attenuation system according to claim 23, further characterized in that the changeable stimulus applied to the controllable material comprises temperature or voltage, and wherein the control circuit provides a control stimulus to said controllable attenuator to change the temperature or voltage applied to said controllable material. 25. The attenuation system according to claim 24, further characterized in that the controllable attenuator includes a control element, which has an input to receive said control stimulus from said control circuit, and which is arranged with respect to to the controllable material to change the temperature, or voltage, and therefore the refractive index thereof in accordance with the control stimulus. 26. The attenuation system according to claim 19, further characterized in that: the controllable attenuator includes a base material formed on the controllable material in which the extracted optical energy is radiated. 27. The attenuation system according to claim 26, further characterized in that: the controllable material has a controllable refractive index that is approximately equal to the refractive index of the fiber optic coating; and the base material formed on the controllable material has a fixed refractive index greater than the effective refractive index of the optical fiber. 28. The attenuation system according to claim 29, further characterized in that said control circuit controls the amount of the changeable stimulus applied to the controllable material also in accordance with a stimulus of desired level. 29. The attenuation system according to claim 19, further characterized in that the level circuit comprises at least one detector for detecting a level of at least a portion of the optical energy transmitted in the optical fiber, providing the circuit of level the feedback stimulus in accordance with said detected level. 30. A method for forming an attenuation system for attenuating optical energy transmitted through an optical fiber, comprising: providing a portion of the optical fiber having a lateral surface thereof through which it can be extracted a part of the transmitted optical energy; forming a controllable material on the surface to control an amount of optical energy extracted from the optical fiber in accordance with a changeable stimulus applied thereto which affects the refractive index thereof; providing a level circuit for providing a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber; and providing a control circuit for controlling a quantity of the changeable stimulus applied to the controllable material in accordance with the feedback stimulus. 31. The method according to claim 30, further characterized in that the changeable stimulus applied to the controllable material comprises temperature or voltage, and wherein the control circuit is formed to provide a control stimulus to change the temperature or voltage of the material controllable. 32. The method according to claim 31, further comprising: providing a control element, having an input to receive the control stimulus from the control circuit, and which is arranged with respect to the controllable material to change the temperature, or voltage, and therefore the refractive index thereof in accordance with said control stimulus. 33. The method according to claim 30, further comprising: forming a base material on the controllable material in which the extracted optical energy is radiated. 34.- The method according to claim 33, further characterized in that: the controllable material is formed to have a controllable refractive index that is approximately equal to the refractive index of the optical fiber coating; and the base material formed on the controllable material is formed to have a fixed refractive index greater than the effective refractive index of the optical fiber. The method according to claim 30, further characterized in that the level circuit includes: a first detector circuit for detecting an amount of optical energy transmitted in said optical fiber before any extraction thereof by means of said controllable attenuator; a second detector circuit for detecting an amount of optical energy transmitted in said optical fiber following any extraction thereof by means of said controllable attenuator; and a circuit for determining an optical energy level extracted from said optical fiber in accordance with the detected amounts of optical energy before and after the extraction thereof by means of said controllable attenuator, and providing the feedback stimulus for the control circuit based on said level of extracted optical energy. 36. The method according to claim 35, further characterized in that said providing the control circuit further comprises providing a comparison circuit for comparing the feedback stimulus and a desired level stimulus applied thereto., and, based on any difference between them, change the value of the changeable stimulus. 37. The method according to claim 30, further characterized in that said control circuit further controls the amount of the changeable stimulus applied to the controllable material also in accordance with a stimulus of desired level. 38.- The method according to claim 30, further characterized in that the level circuit comprises at least one detector for detecting a level of at least a portion of the optical energy transmitted in the optical fiber, providing the level circuit the feedback stimulus in accordance with said detected level. 39. - A method for controllably attenuating the optical energy transmitted through an optical fiber, a portion of the optical fiber having a lateral surface through which at least a part of the optical energy can be extracted, comprising: using an attenuator having a controllable material formed on the surface to controllably extract the optical energy in accordance with a changeable stimulus applied to the controllable material which affects the refractive index thereof; providing a feedback stimulus having a value related to at least a portion of the optical energy transmitted in the optical fiber; and controlling a quantity of the changeable stimulus applied to the controllable material in accordance with the feedback stimulus. The method according to claim 39, further characterized in that the changeable stimulus comprises temperature or voltage. The method according to claim 40, further characterized in that said control includes: using a control element, arranged with respect to the controllable material to change the temperature, or voltage, and therefore the refractive index thereof. 42. The method according to claim 39, further characterized in that said use of an attenuator includes: using a base material on the controllable material in which the extracted optical energy is radiated. 43. - The method according to claim 42, further characterized in that a portion of the coating that houses a core is removed from the portion of the optical fiber, and wherein: the controllable material has a controllable refractive index that is approximately equal to the index refraction of the coating; and the base material formed on the controllable material has a fixed refractive index greater than the effective refractive index of the optical fiber. The method according to claim 39, further characterized in that said providing a feedback stimulus includes: detecting an amount of optical energy transmitted in the optical fiber before any extraction thereof through the surface; detecting an amount of optical energy transmitted in said optical fiber after any extraction thereof through the surface; and determining a level of optical energy extracted from the optical fiber in accordance with the detected amounts of optical energy before and after the extraction of the same across the surface, and providing the feedback stimulus based on said level of energy. extracted optical energy. 45. The method according to claim 44, further characterized in that said control includes comparing the feedback stimulus and a stimulus of desired level, and, based on any difference between them, change the value of the exchangeable stimulus provided by the attenuator. 46. - The method according to claim 39, further characterized in that said control comprises controlling the amount of the changeable stimulus applied to the controllable material also in accordance with a stimulus of desired level. 47. The method according to claim 39, further characterized in that said providing a feedback stimulus comprises: detecting a level of at least a portion of the optical energy transmitted in the optical fiber, and providing the feedback stimulus in accordance with said detected level.