Abstract:
An optical filter assembly is provided which includes a first optical filter and a first counteracting ring. The first optical filter is of a filter material that transmits a selected transmission range within a wider range of wavelength of light, and reflects another selected reflection range of the wavelengths. The filter material has a refractory index, whereby heating of the first optical filter tends to cause an increase in refractory index with a corresponding increase in the transmission range in a first direction. The counteracting ring is attached to the first optical filter so that at least some of the light transmits through an aperture in the counteracting ring. Heating of the first counteracting ring causes enlargement of the first counteracting ring, which stretches the first optical filter. The selected transmission range is thereby at least partially stabilized.

Description:
BACKGROUND OF THE INVENTION 
     1). Field of the Invention 
     This invention relates to an optical filter, and in particular to a temperature compensated wavelength division multiplexing (WDM) optical filter. 
     2). Discussion of Related Art 
     Light filters are used for demultiplexing or multiplexing light in optical a fiber networks. A beam of light is transmitted from an input optical fiber through a light filter layer. The light filter layer is made of a material which transmits a selected range of wavelengths of the light and reflects another range of the wavelengths. It may be important to maintain a range of optical frequencies of transmitted light relatively stable for purposes of further processing of the light. There should be relatively little or no shift in selected transmitted wavelengths, in particular with an increase in temperature. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to an optical filter assembly comprising: 
     a first optical filter having a first and a second face, one of which is exposed to an incident beam of light, the first optical filter having a refractive index and being of a filter material that transmits a selected transmission range of wavelengths defining a center wavelength, and that reflects a reflection range of the wavelengths, a change in ambient temperature of the first optical filter causing a change in the refractive index with a corresponding change in the center wavelength of the transmission range; and 
     a first counteracting ring having a higher coefficient of thermal expansion than the filter material attached to the first or the second face of the first optical filter; 
     whereby a rise in the ambient temperature of the optical filter assembly results in a given length of the first counteracting ring to expand more than a same length of the first optical filter, which causes stretching of the first optical filter resulting in a reduction in thickness therein with a corresponding decrease in the center wavelength of the transmission range of wavelengths, thereby at least partially compensating for an increase in center wavelength caused by the rise in the ambient temperature of the optical filter assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described by way of example with reference to the accompanying drawings wherein: 
     FIG. 1 is a cross-sectional side view of the components of a light filter assembly according to an embodiment of the invention; 
     FIG. 2 is a graph illustrating how a counteracting ring of the components inFIG. 1 counteracts a shift in wavelength of filtered light due to an increase intemperature of an optial filter of the assembly; 
     FIG. 3 is a sectional side view of a light filter including the components of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 of the accompanying drawings illustrates components of a light filterassembly  30  according to an embodiment of the invention, including a transmissive glass substrate  32 , a light filter layer  34 , an epoxy layer  36 , and a metal counteracting ring  38 . 
     The light filter layer  34  is coated onto and thereby attached to the transmissive substrate  32 . A light beam  40  directed towards the light filter layer  34  is in part transmitted through the light filter layer  34  and the transmissive substrate  32  in the form of the transmitted beam  42 , and in part reflected from the light filter layer  34  in the form of the reflected beam  44 . The light beam  40  may have a wide range of wavelengths and the light filter layer  34  may be made of a material or materials that filter the light so that the transmitted beam  42  includes only light from a selected range of wavelengths within the wider range of wavelengths of the light beam  40 . The remainder of the light, i.e. the wavelengths not transmitted by the transmitted beam  42 , is reflected in the form of the reflected beam  44  so that the reflected beam  44  includes light having a reflected range of wavelengths not included in the transmitted beam  42 . 
     It may occur that the assembly  30  heats up due to operation or due to environmental conditions. Heating of the light falter layer  34  tends to cause an increase in a refractory index of the light filter material of the light filter layer  34 . An increase in a refractory index of the light filter layer, in turn, causes an increase in a center wavelength of tire wavelengths of the transmitted beam  42  according to the equation: 
     
       
         Δλ c   αΔn/n   
       
     
     where 
     λ c  is the center wavelength and 
     n is the refractive index. 
     FIG. 2 illustrates what tends to happen to a center wavelength of the transmitted beam  42  with an increase in temperature. Temperature is given on a horizontal axis in degrees celcius (° C.) and a center wavelength of the transmitted beam  42  is given on a vertical axis in nanometers (nm). The center wavelength of the transmitted beam  42  is plotted along the line  66 . The line  66  therefore illustrates what tends to happen to the center wavelength of the transmitted beam  42  due to an increase in temperature and a corresponding increase in the refractory index, in the absence of any effects by the counteracting ring  38 . The center wavelength of the transmitted beam  42  is about 1549.235 nm when the light filter layer  34  and the transmissive substrate  32  are at 10° C. The center wavelength tends to increase substantially linearly with an increase in temperature so that the center wavelength of the transmitted beam  42  tends to be about 1549.38 nm at 85° C. There is thus an increase of 0.15 nm in 75° C., or a linear increase of about 2 pm/°C. The tendency for the center wavelength to shift along the line  66  with an increase in temperature is counteracted by the counteracting ring  38  so that an actual change in the center wavelength is along the line  68 . 
     Referring again to FIG. 1, the epoxy layer  36  is located between a surface of the light filter layer  34  and the counteracting ring  38  so as to attach the counteracting ring  38  to the light filter layer  34 . Heating of the counteracting ring  38  causes expansion thereof in a direction  70 . The counteracting ring  38  has a coefficient of thermal expansion of for example about 17 ppm/°C. which is more than a coefficient of thermal expansion of the light filter layer  34  and more than a coefficient of thermal expansion of the transmissive substrate  32 . Because of the higher coefficient of thermal expansion of the counteracting ring  38 , a given length of the counteracting ring  38  expands more than a given length of the light filter layer  34 , and a given length of the counteracting ring  38  expands more than a given length of the transmissive substrate  32 . The counteracting ring  38  tends to stretch the light filter layer  34 , and the transmissive substrate  32  in a direction  72  which is in a plane of the light filter layer  34 . In another embodiment another counteracting ring may be used, depending on requirement, having a coefficient of thermal expansion between 13 ppm/°C. and 19 ppm/°C. 
     Stretching of the light filter layer  34  in the direction  72  by the counteracting ring  38  tends to cause a reduction in thickness of the light filter layer in a direction  74 . An increase in thickness of the light filter layer causes an increase in a center wavelength of the wavelengths of the transmitted beam  42  according to the equation: 
     
       
         Δλ c   αΔd/d   
       
     
     where 
     λ c  is the center wavelength and 
     d is the thickness 
     In combination therefore, a total change in the center wavelength is expressed as 
     
       
         Δλ c   αΔn/n+Δd/d.   
       
     
     Heating causes an increase in the refractory index n and a decrease in the thickness d. By correctly selecting the material of the counteracting ring, any positive change in Δn/n can be counteracted by a negative change in Δd/d so that Δλ c  remains substantially zero. 
     It can also be said that heating of the light filter layer  34  tends to cause a shift in a center wavelength of the transmission range in one direction, and that the effect of the counteracting ring  38  is to tend to cause a shift in the transmission range in an opposite direction so that the selected transmission range remains substantially stable. 
     Referring again to FIG. 2 it can be seen from line  68  that the center wavelength of the selected transmission range of the transmitted beam  42  is about 1549.38 nm at 10° C. and about the same at 85° C. The selected transmission range falls slightly between these extremes to about 1549.37 nm, thus only about 10 pm. 
     FIG. 3 illustrates a light filter  100 , and two sets of the components  30 A and  30 B shown in FIG.  2 . In addition, the light filter  100  includes a tubular support structure  102 , tubed glass capillaries  104  and  106 , an input optical fiber  108 , a transmission optical fiber  110 , at a reflection optical fiber  112 , a metal holder  114 , and two lenses  116  and  118 . The input optical fiber  108  and the reflection optical fiber  112  are inserted into the glass capillary  104  and terminate at an air gap near the lens  116 . The glass capillary  104  is rigidly secured to the tubular support structure  102 . The lens  116  is secured to the glass capillary  104  and one set of the components  30 A is attached to the lens  116 . The set of component  30 A is thus secured to and within the tubular support structure  102 . 
     The transmission optical fiber  110  is located within the glass capillary  106  and terminates at an air gap near the lens  118 . The lens  118  is attached to the glass capillary  106  which, in turn, is secured to the tubular support structure  102 . The lens  118  is thereby unmovably secured to and within in the tubular support structure  102 . 
     The counteracting ring  38  of the components  30 B is secured within and to the metal holder  114 . The metal holder  114  is made of the same material as the counteracting ring  38 . The metal holder  114  is initially movably secured to the tubular support structure  102 . A device (not shown) pivots the metal holder  114 , and therefore also the components  30 B, relative to the tubular support structure  102  to a required degree, whereafter the metal holder  114  is secured to the tubular support structure. 
     In use, light is transmitted through the input optical fiber into the lens  116 . The lens  116  focuses the light and also causes a change in direction in the light so that there is an incidence angle between light transmitted through the input optical fiber  108  and light being transmitted through the lens  116 . Some of the light is reflected by the light filter layer  34  of the components  30 A and, due to the angle, is transmitted back through the lens  116  to the reflection optical fiber  112 . The angle thus allows for the light to reach the reflection optical fiber  112 . No shift in a center wavelength of transmitted light would occur should the light strike the light filter layer  34  of the component  30 A at right angles (at a given temperature). However, the angle also causes a shift in the selected transmission range of light being transmitted through the component  30 A. The orientation of the component  30 B allows for correction in the shift of the selected transmission range. By pivoting the metal holder  114 , the light filter layer  34  and the transmissive substrate  32  of the components  30 B are also pivoted. These components are pivoted to a degree which ensures that light striking the transmissive substrate  32  of the components  30 B is at a correction angle relative to a surface of the transmissive substrate  32 , which ensures that there is a return shift in the wavelengths of the transmitted light. 
     The light then enters the lens  118  and travels into an end of the transmission optical fiber  110  whereafter the light is transmitted therethrough. Any shift in wavelengths of a transmitted beam due to spacing between a center line of the input optical fiber  108  and the reflection optical fiber  112  and a consequent angle at which light strikes the components  30 A is thus corrected by the orientation of the components  30 B so that a desired range of wavelengths of the light transmits through the transmission optical fiber  110  is maintained. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.