Abstract:
In an improved approach to mounting optical elements in a fiber optic device, the positioning and orientation of the element are more accurate and the temperature dependent variations of the position and orientation are reduced. The mount has a protruding tip contact region on a mounting surface. Adhesive is supplied between the optical element and the mounting surface, and the optical element is pressed into contact with the protruding contact tip region, substantially expelling the adhesive from between the optical element and the protruding contact tip region. The adhesive is cured at a temperature exceeding the predetermined temperature range. This permits the adhesive to pull the element on to the contact tip region throughout the operating temperature range. This also permits the optical element to contact the mounting surface so that the optical element&#39;s surface is oriented parallel to the mounting plane.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed generally to fiber optical devices, and more particularly to an approach for mounting optical elements used in the fiber optical devices.  
         BACKGROUND  
         [0002]    Optical fibers find many uses for directing beams of light between two points. Optical fibers have been developed to have low loss, low dispersion, polarization maintaining properties and can be incorporated into several different types of devices, such as amplifiers, filters, lasers and interferometers. As a result, optical fiber systems find widespread use, for example in optical communications.  
           [0003]    However, one of the important advantages of fiber optic beam transport, that of enclosing the optical beam to guide it between terminal points, is also a limitation. There are several optical components, important for use in fiber systems or in fiber system development, that are not implemented in a fiber-based form where the optical beam is guided in a waveguide. Instead, these optical components are implemented in a bulk form, and through which the light propagates freely. Examples of such components include, but are not limited to, filters, isolators, circulators, polarizers, switches and shutters. Consequently, the inclusion of a bulk component in an optical fiber system necessitates that the optical fiber system have a section where the beam path propagates freely in space, rather than being guided within a fiber.  
           [0004]    Free space propagation typically requires use of collimation units at the ends of the fibers to produce and receive collimated beams. In some units, the same focusing element is used to collimate the beams from two different fibers placed at different positions relative to the axis of the focusing optic. This produces collimated beams that propagate in non-parallel directions. The nonparallel propagation of the collimated beams introduces extra issues for aligning the components of the device, and may place some limits on making the device smaller in size.  
           [0005]    A fiber optical device typically includes a collimation unit at each end, to produce a collimated light beam in the region of free-space propagation. The collimation unit typically includes one or more lenses to collimate the light passing to or from a fiber. Bulk optical elements, such as filters, polarizers, and isolator units having birefringent elements and non-reciprocating elements, are disposed in the collimated light beam, or light beams to perform the desired function. The placement of these elements in the collimated light beams is important, since the position along the beam or angle relative to the beam may affect the operation of the device. There is a need, therefore, to ensure that the elements are mounted at the desired position and orientation, and that the desired position and orientation are maintained over a range of possible operating temperatures.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, there is a need for an improved approach to mounting optical elements in a fiber optic device that improves the positioning and orientation of the element and that reduces the temperature dependent variation of the position and orientation.  
           [0007]    One embodiment of the invention is directed to a method of mounting an optical element to a mount for use in a predetermined temperature range, where the mount has a protruding tip contact region on a mounting surface. The method includes providing adhesive between the optical element and the mounting surface, and pressing the optical element into contact with the protruding contact tip region thereby substantially expelling the adhesive from between the optical element and the protruding contact tip region. The adhesive is cured at a temperature exceeding the predetermined temperature range.  
           [0008]    Another embodiment of the invention is directed to an optical device for use in a predetermined temperature range. The device includes a mount having a first mounting surface provided with a protruding tip contact region, where the protruding tip contact region defines a mounting plane. An element to be mounted has a second mounting surface contacting the protruding tip contact region. Adhesive is attachingly disposed between portions of the first and second mounting surfaces not in mutual contact.  
           [0009]    Another embodiment of the invention is directed to a fiber optic device that includes a mount having a first mounting surface defining a first mounting plane. An optical element is adhesively surface mounted to the first mounting surface of the mount, a second mounting surface of the optical element contacts the first mounting surface of the mount so that the second mounting surface is parallel to the first mounting plane.  
           [0010]    The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 schematically illustrates a fiber optic communications system in which a tap is used to split off light for monitoring the optical signal on an optical fiber, according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 2 schematically illustrates an embodiment of a fiber optic tap monitor according to the present invention;  
         [0014]    [0014]FIG. 3 schematically illustrates an embodiment of a WDM device according to the present invention;  
         [0015]    [0015]FIG. 4 schematically illustrates a partial cross-section of a conventional fiber optic device;  
         [0016]    [0016]FIG. 5A schematically illustrates an exploded view of a dual fiber collimator according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 5B presents a cross-sectional view of the lens/filter mount illustrated in FIG. 5A;  
         [0018]    [0018]FIGS. 6A and 6B schematically illustrate a partial cross-section of a lens/filter mount, before and after mounting an optical element respectively, according to an embodiment of the present invention;  
         [0019]    [0019]FIG. 7A schematically illustrates a conventional mount showing problems arising from a varying thickness in an adhesive layer;  
         [0020]    [0020]FIG. 7B schematically illustrates an embodiment of the present invention showing that the optical element lies in the mounting plane; and  
         [0021]    [0021]FIG. 8 schematically illustrates another embodiment of a mount for mounting an optical element according to the present invention.  
         [0022]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
     
    
     DETAILED DESCRIPTION  
       [0023]    The present invention is applicable to fiber optic devices, and is believed to be particularly useful with fiber optic devices that include optical elements that are face-mounted within the device. Face-mounted elements include, for example, filters such as may be used in wavelength division multiplexed (WDM) devices or tap monitors, polarizers, birefringent plates, polarization rotators and the like.  
         [0024]    A WDM device is used to combine light at different wavelengths into a single optical signal or, in reverse, to separate different wavelength components of an optical signal. A fiber optic tap is important for extracting a fraction of the light propagating along a fiber so as to permit the optical signal to be monitored. Different types of monitors may be used, including a tap monitor and a channel monitor. In the tap monitor, the tapped fraction of the light is directed to a photodetector to measure the total power in the optical signal. A channel monitor is typically used in multiple channel communications systems, for example, dense wavelength division multiplexed (DWDM) systems. The channel monitor splits the tapped fraction of light into its separate channels and measures the amount of light in each channel individually. This permits the operator to determine whether the power in the multiple channel optical signal is evenly distributed among all optical channels.  
         [0025]    A schematic of an embodiment of an optical communications system  100  is presented in FIG. 1, showing how taps are employed to produce monitor signals. A DWDM transmitter  102  directs a DWDM signal having m 0  channels through a fiber communications link  104  to a DWDM receiver  106 .  
         [0026]    In this particular embodiment of DWDM transmitter  102 , a number of light sources  108   a - 108   m  generate light at different wavelengths, λ0, λ1 . . . λm 0 , corresponding to the different optical channels. The light output from the light sources  108   a - 108   m  is combined in a DWDM combiner unit  110 , or multiplexer (MUX) unit to produce a DWDM output  112  propagating along the fiber link  104 .  
         [0027]    Light sources  108   a - 108   m  may be modulated laser sources, or laser sources whose output is externally modulated, or the like. It will be appreciated that the DWDM transmitter  102  may be configured in many different ways to produce the DWDM output  112 . For example, the MUX unit  110  may include filter multiplexers and/or an interleaver to interleave the outputs from different multiplexers. Furthermore, the DWDM transmitter  102  may be equipped with any suitable number of light sources for generating the required number of optical channels. For example, there may be twenty, forty or eighty optical channels, or more. The DWDM transmitter  102  may also be redundantly equipped with additional light sources to replace failed light sources.  
         [0028]    Upon reaching the DWDM receiver  106 , the DWDM signal is passed through a demultiplexer unit (DMUX)  130 , which separates the multiplexed signal into individual channels that are directed to respective detectors  132   a - 132   m.    
         [0029]    The fiber link  104  may include one or more fiber amplifier units  114 , for example rare earth-doped fiber amplifiers, Raman fiber amplifiers or a combination of rare earth-doped and Raman fiber amplifiers. The fiber link  104  may include one or more DWDM channel monitors  126  for monitoring the power in each of the channels propagating along the link  104 . Typically, a fraction of the light propagating along the fiber link  104  is coupled out by a tap coupler  124  and directed to the DWDM channel monitor  126 . The fiber link  104  may also include one or more gain flattening filters  140 , typically positioned after an amplifier unit  114 , to make the power spectrum of different channels flat. The channel monitor  126  may optionally direct channel power profile information to the gain flattening filter  140 . The gain flattening filter  140  may, in response to the information received from the channel monitor  126 , alter the amount of attenuation of different channels in order to maintain a flat channel power profile, or a channel power profile having a desired profile.  
         [0030]    The fiber link  104  may include one or more optical add/drop multiplexers (OADM)  116  for, for example, directing one or more channels to a local loop. In the particular embodiment illustrated, the OADM  116  drops the ith channel, operating at wavelength λi, and directs it to the local loop  118 . The local loop  118  also directs information back to the OADM  116  for propagating along the fiber link  104  to the DWDM receiver  106 . In the illustrated embodiment, the information added at the OADM  116  from the local loop  118  is contained in the ith channel at λi. It will be appreciated that the information directed from the local loop  118  to the OADM  116  need not be at the same wavelength as the information directed to the local loop  118  from the OADM  116 . Furthermore, it will be appreciated that the OADM  116  may direct more than one channel to, and may receive more than one channel from, the local loop  118 .  
         [0031]    The amount of light being added to the fiber link  104  from the local loop  118  may be monitored and controlled so that the optical power added in the channel at λi is at approximately the same level as the power in the other channels λ0 to λi-1, and λi+1 to λm 0 . The light from the local loop  118  may be passed through a power level controller  142  (plc) that controls the level of power in the channels being added in the OADM  116 . The power level controller  142  may include a variable attenuator to reduce power and/or an amplifier to increase power. A tap  144  extracts a fraction of the light and passes the extracted light to the monitor  146  that detects the power of the light being added in the OADM  116 . The monitor  146  directs a signal to the power level controller  142  which adjusts the power of the light upwards or downwards, depending on the signal received from the monitor  146 , so as to set the power of the light being added at the OADM  116  to be approximately the same as that of the other channels.  
         [0032]    A tap  150  and monitor  152  may also be positioned to monitor the DWDM signal  112  emitted by the transmitter  102 . The monitor  152  may feed back a control signal to the transmitter  102  to control the level of the DWDM signal  112 , based upon the power level detected by the monitor  152 .  
         [0033]    One type of tap monitor  200  is schematically illustrated in FIG. 2, and is described in greater detail in U.S. Pat. No. 09/999,533. The tap monitor  200  includes a dual fiber collimator  201  having a first lens  202  and dual-fiber ferrule  204 . Two fibers  206  and  208  are held in the ferrule  204 , with their ends  206   a  and  208   a  positioned at a distance from the lens  202  equal to about the focal length of the lens  202 . The ferrule end  204   a,  and the fiber ends  206   a  and  208   a  may be polished at a small angle, approximately 1°-8° or so, to prevent reflections feeding to other elements.  
         [0034]    The lens  202  may be a GRIN lens or may be a lens having a curved refracting surface. For example, the lens  202  may be an aspheric lens. The lens  202  may be formed from glass, an optically transmitting polymer, or other suitable transmitting material. The focal length of the lens  202  is typically in the range 1 mm-5 mm, although it may lie outside this range.  
         [0035]    In the illustrated embodiment, a first light beam  210 , from the first fiber  206 , passes through the lens  202  and is collimated. However, since the beam  210  is not positioned on the lens axis  212 , the collimated beam  214  propagates at an angle, θ1, to the axis  212 . The value of θ1 is typically in the range 1.5°-2.5°, although it is not restricted to this range. The collimated beam  214  is incident on the filter  216 , which has a reflective coating on its front surface  216   a.  The reflectivity of the reflective coating is typically high, and may be in the range 90%-99.9%, so that only a small fraction (0.1%-10%) of the power in the beam  214  is transmitted through the filter  216 . The light  218  reflected by the filter  216  is directed to the first lens  202  which focuses the beam  220  to the second fiber  208 . The filter  216  may also transmit a portion of a particular wavelength band or an individual optical channel, to permit monitoring of that wavelength band or individual channel.  
         [0036]    The light  222  transmitted through the filter  216  passes to a photodetector unit  224 , which detects the power of the transmitted beam  222 . The photodetector unit  224  may be a photodiode, or other type of light detecting device. Where the photodetector unit  224  is based on a semiconductor material, the band gap of the semiconductor is advantageously arranged to be less than the energy of the photons being detected. For example, the light entering the tap monitor  200  may be an optical communications signal having a wavelength in the range 1300-1650 nm. Accordingly, the photodetector unit  224  may be based on a semiconducting material that absorbs light in this wavelength range, for example indium gallium arsenide and the like.  
         [0037]    In this particular embodiment, the filter  216  is wedged at an angle, for example around 5°, so that refraction of the transmitted beam  222  by the filter  216  directs the beam  222  along a direction parallel to the optical axis  212  of the first lens  202 , towards the photodetector  224 . The DFC  201  is aligned within a housing  230 , with its axis  212  substantially parallel to the axis of the housing  230 . Therefore, the transmitted beam  222  propagates largely parallel to the housing  230 . The use of a wedged element to produce a light beam propagating parallel to the axis from a dual fiber collimator is discussed further in U.S. patent application Ser. No. 09/999,891, entitled “DUAL FIBER COLLIMATOR ASSEMBLY POINTING CONTROL”, filed on Oct. 31, 2001 and incorporated herein by reference. Typically, the first surface  216   a  of the filter has the reflective coating while the second surface  216   b  has an antireflection coating.  
         [0038]    In an example of a device as illustrated in FIG. 2, the fibers  206  and  208  have a diameter of around 125 μm and are set in the dual-fiber ferrule  204  at a center-to-center spacing of 125 μm. The lens  202  is aspherical, having a focal length in the range 1.5-2.5 mm, and so θ1 has a value of approximately 1.5°-2.5°. The filter  216  may be based on a substrate formed of glass, such as BK7 or B270 glass, and have a wedge angle of around 4.8°. It is to be understood that the values for the various components provided in this paragraph are provided for illustrative purposes only, and are not intended to limit the invention in any way. For example, the wedge angle of the filter  216  depends on the angle of incidence on the filter face  316   a  and the refractive index of the filter glass substrate, and may range from 2°-5° or more. Although the illustrated embodiment includes a wedged filter, the invention is not restricted to the use of wedged filters that parallelize the transmitted light with the lens axis. The light passed through the filter may propagate in a direction non-parallel to the axis  212 .  
         [0039]    Another type of filter-based device, a WDM  300 , is schematically illustrated in FIG. 3. A dual-fiber collimator  301  includes a first lens  302  and a dual-fiber ferrule  304 . The first lens  302  is mounted on a lens/filter mount  317 . Two fibers  306  and  308  are held in the ferrule  304 , with their ends  306   a  and  308   a  positioned at a distance from the lens  302  equal to about the focal length of the lens  302 . The ferrule end  304   a,  and the fiber ends  306   a  and  308   a  may be polished at a small angle to prevent reflections feeding to other elements.  
         [0040]    A first light beam  310 , from the first fiber  306 , passes through the lens  302  and is collimated. However, since the beam  310  is not positioned on the lens axis  312 , the collimated beam  314  propagates at an angle, θ1, to the axis  312 . For typical systems, the value of θ1 may be around 2°, depending on such factors as the focal length of the lens  302  and the separation between the two fibers  306  and  308 .  
         [0041]    The collimated beam  314  is incident on the filter  316 , which is mounted on the lens/filter mount  317 . The filter  316  reflects a portion of the beam  314  as a reflected beam  318 , and transmits the remainder of the beam  314  as a transmitted beam  322 . The reflected beam  318  is reflected to the first lens  302  which focuses the beam  320  to the second fiber  308 .  
         [0042]    The transmitted beam  322  passes through the filter  316  to a single fiber collimator unit (SFC)  330 . The SFC  330  includes a lens  332  and a fiber  334  that is held in the single fiber ferrule  336 . When used in conjunction with the DFC  301  and the filter  316 , the transmitted beam  322  is focused by the lens  332  into the third fiber  334  as beam  324 . In this embodiment, the third fiber  334  is disposed on the axis  338  of the lens  332 , and the SFC  330  is oriented so that the beam  322  from the DFC  301  is parallel to the axis  338 . The ferrule end  336   a  and the fiber end  334   a  may be polished at a small angle to prevent reflections feeding back to other elements.  
         [0043]    The filter  316  may have a multilayer dielectric filter coating, typically on the first surface  316   a,  with the second surface  316   b  having an anti-reflection coating. The filter  316  may transmit a fraction of the light incident from the first fiber  306  to the third fiber  334 . For example, where the light  310  contains light in multiple optical channels at different channel wavelengths, the filter  316  may transmit light in only one or a small number of optical channels, reflecting the remaining light to the second fiber  308 . The filter  316  may also be wedged so that the light  322  that is passed through the filter from the first fiber  302  propagates in a direction parallel to the lens axis  312 .  
         [0044]    A filter-based WDM device  300  may be useful for adding or dropping channels in a multi-channel optical communications system. The device may also be used for combining light at light at different wavelengths into a single output, or for separating light at different wavelengths into different outputs.  
         [0045]    In many situations, it is important for fiber optic devices, including taps and WDMs, that various characteristics such as insertion loss, return loss, etc. be as independent of temperature as possible. In many conventional fiber optic devices, the filter is glued to a holder that positions the filter relative to the collimating lenses. A cross-section of part of a conventional fiber device  400  is illustrated in FIG. 4, which shows a mount  402  having a recess  404  for mounting an optical element, such as a filter, lens, polarizer, birefringent plate, or any other type of bulk optical element that may be used in a fiber optic device.  
         [0046]    A lip  408  in the recess provides a flat surface  410  against which the element  406  may be glued. However, a layer of glue  414 , generally of indeterminate thickness, remains between the flat surface  410  and the element  406  due to capillary action, even after the element  406  has been pressed against the flat surface  410 . The absolute thickness of the layer of glue  414  is typically not well controlled and may vary from assembly to assembly. Furthermore, the thickness of the glue layer  414  may vary around the element  406  so that the orientation of the element relative to the axis  416  is not well controlled. Consequently, even if the mount  402  is fabricated with extremely small tolerances on its mounting faces, the uncertainty in the thickness of the glue layer  414  results in an uncertainty in the orientation of the element  406 , and so the orientation of the mount  402  may have to be adjusted when inserting it into a collimator unit.  
         [0047]    Various factors may affect the thermal stability of the device  400 . For example, where the layer of glue  414  is thicker on one side of the mount  402  than the other, any thermal expansion or contraction may result in a tilting of the element  406 . Also, if the glue  414  is not extremely homogeneous, for example, due to incomplete mixing of the different glue components, different regions of the glue layer  414  may manifest different temperature-dependent thicknesses, which also leads to tilting of the element  406 . Several characteristics of the device  400 , such as return loss and insertion loss, may be critically dependent on the tilt of the element, for example where the element  406  is a filter, and, consequently, may change with temperature. For example, a tilt of one side of a filter through 0.01° may lead to a change in the insertion loss of as much as 0.01 dB.  
         [0048]    It is often advantageous to reduce the temperature dependence of the device characteristics. It is also often advantageous to ensure that the element  406  is mounted with an orientation relative to the mount  402  that is as precise as possible. An exploded view of an embodiment of a DFC  500  that has characteristics with reduced temperature dependence is schematically illustrated in FIG. 5A. The fibers  506  and  508  are mounted within the dual fiber ferrule  502 . The lens  504  may be provided with a flat surface  505  for mounting against a corresponding surface  507  of the lens/element mount  510  using an adhesive. A ferrule sleeve  512  may be attached to the outside of the ferrule  502  and the ferrule inserted in the mount  510 , with the sleeve face  520  against the ferrule-mounting face  522  at the end of the mount  510 . The sleeve  512  is mounted at a distance from the end of the ferrule  502  that ensures that the fibers  506  and  508  are correctly spaced from the lens  504 .  
         [0049]    An optical element  516 , for example a filter, or the like, is mounted to a mounting surface  518  of the mount  510  using an adhesive. The element  516  may have a circular cross-section, but may also have a non-circular cross-section. The illustrated example of filter  516  has a rectangular or square cross-section, which is conveniently fabricated from slicing and dicing a large sheet. An expanded view of a cross-section of the mount  510  is illustrated in FIG. 5B, showing the lens and element mounting surfaces  507  and  518 .  
         [0050]    A cross-section through part of the mount  510  is schematically illustrated in FIG. 6A. The element mounting surface  518  may be provided in a recess  520 , although this is not a requirement. The element mounting surface  518  includes a raised portion  522  and may also include a well  524  on one or both sides of the raised portion  522 . In the illustrated embodiment, a well  524  is provided on one side of the raised portion  522 . The raised portion  522  presents a tip  526  for contacting the element  516 , rather than a flat surface. The element  516  is shown close to the mounting surface  518 , with adhesive  528  disposed between the element  516  and the mounting surface  518 , prior to mounting.  
         [0051]    As the element  516  is forced towards the mounting surface  518 , the adhesive  528  is expelled from the region between the tip  526  and the element  516  until the element  516  contacts the tip  526 . The expelled adhesive  528  flows away from the tip  526 , down one or both sides of the raised portion  522 , and may flow to the well  524 . The well  524  need not be filled with expelled adhesive  528 . Since the tip  526  has a very small area, it is possible to overcome capillary action and expel the adhesive  528  entirely from between the tip  526  and the element  516 , so that the element  516  contacts the tip  526 , as illustrated in FIG. 6B. The lens/filter mount  510 , filter  516  and adhesive  528  are raised in temperature, preferably to a temperature higher than the expected operating temperature of the resulting fiber optic device. The adhesive  528  is then cured at the high temperature.  
         [0052]    After curing, the assembly  530  comprising the mount  510 , element  516  and adhesive  528  is allowed to cool. The adhesive  528  cools under tension. The adhesive  528  has a higher thermal expansion coefficient than the mount  510 . As long as the operating temperature of the assembly  530  is less than the cure temperature, the adhesive  528  remains in tension, pulling the element  516  toward the mounting surface  518 . Since the element  516  is in actual contact with the mounting surface  518  at the contact tips  526 , the element  516  does not move relative to the mount  510  as the temperature changes within the operating range. Consequently, when the operating temperature of the assembly  530  varies, the element  516  does not tilt with respect to the mount  510 , thus reducing the temperature dependence of the device&#39;s operating characteristics. For example, where the assembly  530  is employed in a tap monitor, the temperature dependence of the coupling and insertion losses may be reduced as a result of the mounting technique just described.  
         [0053]    One example of a suitable adhesive  528  is type 353 NDT produced by Epotek Corp., Billerica, Mass. This is a two-part epoxy that is cured thermally. Furthermore, the type 353 NDT epoxy is thixotropic, which reduces the ability of the adhesive to flow even under elevated temperatures. Thus, the adhesive does not flow along the surface of the filter  516  while curing. Other types of adhesive that cure at elevated temperatures may also be used.  
         [0054]    In one particular embodiment, the mount  510  was manufactured from a martensitic, Se-doped stainless steel, type 182. The mount  510  was mounted in a jig and the mixed epoxy was applied to the mounting surface  518 . The element  516 , in the form of a multilayer dielectric filter formed on a substrate of B270 glass and presenting a face approximately 1.5 mm×1.5 mm to the mount  510 , was forced against the mounting surface  518  with a force of 1 N, and the jig assembly was inserted into an oven for curing at 120° C. for 30 mins.  
         [0055]    It will be appreciated that other optical elements, and not only an optical filter, may be mounted in a similar manner. For example, the lens  504  may be mounted to the mount  510  in the same way in order to reduce movement of the lens due to changing temperature.  
         [0056]    A useful figure of merit to describe thermal effects is the temperature dependent loss over the range −20° C. −75°C. in other words how much the loss of the device changes between −20° C. and 75° C. Conventional tap monitors typically have a temperature dependent loss in the range 0.1 dB-0.15 dB. A tap monitor of the design illustrated in FIG. 2 was fabricated with the lens and filter mounted as illustrated in FIG. 6B. The temperature dependent loss of that device was measured to be 0.04 dB, significantly lower than other devices.  
         [0057]    Another important advantage of the present invention over conventional approaches to face mounting is illustrated with respect to FIG. 7. In conventional approaches, for example as illustrated in FIG. 4, the thin layer of adhesive between the element and the mount may vary in thickness at different parts of the mount. Consequently, the element may not be oriented correctly relative to the mounting surface, irrespective of the accuracy of the mounting surface relative to the axis of the mount. Therefore, the transmission and reflection spectra of the device may not be as designed, and may differ from part to part. In the approach described herein, the optical element is mounted in contact with the mount, and so the accuracy of the orientation of the face relative to the optical axis is determined by the accuracy of reproducing the mounting face of the mount, and not the uniformity of the adhesive layer.  
         [0058]    [0058]FIG. 7 schematically illustrates a face mount  700  for a filter  702  and a lens  704 . The lens  704  is illustrated as a GRIN lens, although the invention is not so restricted, and the lens  704  may be any other suitable type of lens, for example an aspheric lens. A filter mounting plane  710  is defined by the mounting surface  712  upon which the filter  702  is mounted. Likewise, a lens mounting plane  720  is defined by the mounting surface  722  upon which the lens  704  is mounted. The lens  702  need not be face mounted and may be edge mounted to the inside surface  724  of the mount  700 .  
         [0059]    Using the mounting technique described above, the filter  702  contacts the filter mounting surface  712  and is pulled towards the filter mounting surface by the adhesive  714 . Consequently, the filter surface  702   a  lies parallel and immediately adjacent to the mounting plane  710 , and so the angle of the filter surface  702   a  relative to the axis  716  is well-controlled.  
         [0060]    The mounting surfaces  518  and  712  were shown to be cylindrically symmetric. However, the mounting surfaces  518  and  712  need not be uniform, for example due to manufacturing tolerances. One example of non-uniformity is that the height of the tip  526  above the well  524  may vary tangentially around the mount  510 . In such a case, the filter  516  may not contact the entire tip region  526  all the way around the mount  510 . The filter  516  does, however, contact at least three points of the raised portion  522  around the mount  510 , which provides sufficient filter/tip contact to prevent the filter  516  from moving relative to the mount  510  under conditions of changing temperature. An advantage of this approach to surface mounting optical elements is that, since the element contacts the mounting surface, the surface of the element lies in the plane defined buy the mounting surface. Accordingly, the precision with which the element&#39;s surface is oriented is dependent on the precision of manufacturing the mounting surface.  
         [0061]    A mount  810  having another type of mounting surface  818  is schematically illustrated in FIG. 8. Although the aperture  820  is circular, raised portions  822 , having tips  826 , are positioned at various points around the mounting surface  818 , rather than being provided as a ring. The three highest tips  826  define a mounting plane on which the surface of the element rests. More raised portions  822  may be provided on the surface  818 . A well  824  may extend as a ring around the mount  810 , or individual wells (not illustrated) may be provided close to each raised portion  822 .  
         [0062]    It should be understood that the mount may have a different shape, and need not be cylindrically symmetric. A cylindrical symmetry is useful because the mount can be readily manufactured by turning. Other geometries may be used, for example, the mount may have a square or rectangular cross-section.  
         [0063]    As noted above, the present invention is applicable to fiber optic devices and is believed to be particularly useful in fiber optic devices that use one or more surface-mounted elements. It will be appreciated that the invention is not restricted to mounting filters, but may be used for mounting any surface-mounted optical element. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification.