Abstract:
A variable optical attenuator has a mirror facing an input waveguide end and an output waveguide end. A lens is mounted in the optical path between the waveguide ends and the mirror. The waveguide ends, mirror and lens are mounted on thermally expansible elements, and the position of the mirror with the lens relative to the waveguide ends is linearly displaceable by differential thermal expansion.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention is concerned with variable optical signal attenuators, and more particularly with passive optical attenuators employed in fiber-optic transmission systems wherein the efficiency of transmission from an input port or waveguide to an output port or waveguide is controlled.  
         BACKGROUND OF THE INVENTION  
         [0002]    Attenuators, or attenuating couplers are well known and widely used in optical transmission systems. Various control mechanisms are employed to control the attenuation of optical signal transmitted between two or more waveguides. Simplicity, small size, reliability and resistance to wear are some of the requirements that the attenuating devices should meet. Several designs of attenuators, employing various types of actuators or control means have been proposed in the patent literature. It is known from U.S. Pat. No. 5,915,063 to Colbourne et al., to use an actuator with a thermally expansible member to control the angular position of a mirror, wherein heat applied to the expansible member will cause an elongation of the member. An asymmetric elongation of the actuator will in turn cause a controlled tilting of the mirror that reflects an optical signal beam between input and output waveguides.  
           [0003]    Lance et al, U.S. Pat. No. 4,516,827, proposes a variable attenuator having two optical fibers and a movably mounted reflective surface disposed therebetween.  
           [0004]    In a fiber optic system, there is sometimes a need for a passive attenuator, one that does not require outside energy or stimulus to effect a change in attenuation of an optical signal. For example, it may be desirable to compensate for a temperature-dependent power shift of a pump laser to maintain the power of the laser output beam at a relatively constant level. It may therefore be desirable to provide a compensation of this power variance by installing an attenuator in the path of the laser beam, the attenuator&#39;s performance designed to provide an attenuating effect as a function of temperature change, of an opposite effect to the laser power change due to the same temperature change. In other words, it is desirable to provide a passive optical attenuator for attenuating an optical signal as a function of ambient temperature, without controlled selective application of heat, pressure, electric voltage etc. to one of its elements.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, there is provided a variable optical attenuator comprising:  
           [0006]    an input port for emitting an optical signal beam,  
           [0007]    an output port for receiving an optical signal beam,  
           [0008]    a focusing means disposed in the optical path between the input port and the output port for focusing or defocusing the optical signal beam on the output port, and  
           [0009]    an actuator operable in response to ambient temperature for linearly displacing the focusing means relative to at least one of the input port and the output port to cause the focusing or defocusing of the optical signal beam on the output port.  
           [0010]    The displacement may be effected substantially in a linear manner along a linear axis, i.e. without changing the angular relationship of the focusing means and the input port or the output port.  
           [0011]    The actuator may be one operable by differential thermal expansion.  
           [0012]    In accordance with another aspect of the invention, there is provided an optical attenuator comprising:  
           [0013]    a reflective surface for receiving and reflecting an incident optical signal beam,  
           [0014]    an input port for emitting an incident optical beam towards the reflective surface,  
           [0015]    an output port for receiving the reflected optical beam upon reflection of the incident beam from the reflective surface, the reflective surface spaced from at least one of the input and output port, and  
           [0016]    a thermally expansible actuator for linearly displacing the reflective surface relative to at least one of the input port and the output port to change the direction of the reflected optical beam relative to the output port.  
           [0017]    The definition “reflective” encompasses “partly reflective”.  
           [0018]    The actuator may be one operable by differential thermal expansion.  
           [0019]    The attenuator may further comprise at least one focusing means disposed between the reflective surface and either the input port, the output port or both to facilitate the focusing of the reflected beam on the output port.  
           [0020]    The actuator may be a non-binary actuator operatively connected to the reflective surface for linear displacement of the reflective surface towards or away from at least one of the input port and the output port. Alternatively, the actuator may be operatively connected to one or both ports for the same purpose, i.e. for changing the spacing between the reflective surface and at least the output port. This change has the effect of axially changing the alignment, or shifting, of the reflected beam in parallel to its initial direction. The term “linear” means that essentially no angular displacement of the mirror or other elements of the attenuator is intended, in contrast to e.g. the device of U.S. Pat. No. 5,915,063. The reflective surface and the output port (and optionally also the input port) are brought towards or away from each other in a linear manner. This axial alignment has the effect of a change of transmission between the input port and the output port, a change that is promoted and precisely controlled by the provision of a lens between the reflective surface and the output port (and optionally also between the reflective surface and the input port). Because of the provision of the lens, the axial alignment translates into focusing or defocusing of the optical beam as will be explained below.  
           [0021]    The actuator may be an actuator for controlling, in a non-binary manner, a linear spacing between the reflective surface and the output port. The term “thermally expansible actuator” denotes that the actuator comprises a thermally expansible element or a number of thermally expansible elements.  
           [0022]    A first waveguide and a second waveguide may be connected to the input port and the output port respectively. In one embodiment of the invention, both waveguides are disposed proximate to each other to reduce the angle of incidence on the reflective surface. In another embodiment of the invention, the waveguides are disposed at distal ends of the attenuator with focusing means in the optical path between the waveguide ends.  
           [0023]    The device may comprise a base or a guide, a reflective element mounted on the base, an input waveguide end mounted on the base to launch an input optical signal beam towards the reflective surface, an output waveguide end mounted on the base to receive a reflected optical signal beam upon reflection of the input signal beam from said surface, the reflective surface spaced from the input waveguide end and the output waveguide end in a predetermined angular relationship thereto. A thermal actuator having a thermally expansible element, may also be disposed on or in the base or guide for controlling a distance between the reflective element and at least one of the input and output waveguide ends upon a change of temperature of the thermally expansible element. A lens may be disposed between the reflected element and the output waveguide and spaced from the output waveguide by approximately a focal distance of the lens such that when the actuator is activated, the distance between the reflective surface and the output waveguide end changes thereby bringing the reflected beam, via the lens, into focus on the output waveguide or out of focus.  
           [0024]    The focusing means may be a lens, e.g. a GRIN lens. It may be mounted either in a proximity to the waveguide ends or in a proximity to the mirror/reflective element. At least one of the spacing, either between the lens and the output port or between the lens and the mirror, is controlled by the operation of the actuator to obtain the attenuating effect.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention and its embodiments will be explained in more detail by way of the following description to be taken in conjunction with the drawings in which  
         [0026]    [0026]FIG. 1 is a schematic cross-sectional view of an embodiment of the present invention,  
         [0027]    [0027]FIG. 2 and FIG. 3 are schematic representations of the operation of the embodiment of FIG. 1,  
         [0028]    [0028]FIG. 4 is a schematic representation of another embodiment of the invention,  
         [0029]    [0029]FIG. 5 is a schematic representation of the operation of the embodiment of FIG. 4, and  
         [0030]    [0030]FIG. 6 is a schematic representation of still another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    As illustrated in FIG. 1, an exemplary attenuator of the invention has a sleeve  10  made of a high CTE material e.g. steel. A stack  12 , made of a low CTE material, e.g. Kovar, is welded or glued to the sleeve  10  and extends axially within the sleeve. Mounted on the stack  12  are a mirror  14  and a lens  16 . The lens  16  is mounted on a spacer  18 , also made of Kovar.  
         [0032]    Facing the lens and axially mounted in the sleeve  10  is a double fiber glass tube  20  with two optical fibers  22 ,  24  mounted in the tube such that the polished ends of the fibers are ground flat with the face  26  of the tube  20 . In order to minimize back reflection, the face  26  is ground at a small angle, about 60, to the vertical as seen in exaggeration in FIG. 1.  
         [0033]    The optical axis of the sleeve, lens and fiber tube is indicated as OA.  
         [0034]    The coefficients of thermal expansion (CTE) of the materials used are shown in the enclosed table:  
                             TABLE 1                           Component materials and CTE.                    Coefficient of thermal       Component   Material   Expansion (PPM/° C.)               Centerpiece sleeve   steel   18       Focus   (air gap)   —       Mirror stack   KOVAR    5       Spacer   KOVAR    5                  
 
         [0035]    The GTE difference between steel and Kovar will cause a differential expansion of the materials of the device, particularly the sleeve and the stack, and as a result, a difference in the linear distance between the lens  16  and the ends of the fibers  22 ,  24 . The linear displacement is clearly different than the angular displacement of the prior art Colbourne U.S. Pat. No. 5,915,063.  
         [0036]    [0036]FIG. 2 and FIG. 3 show the focusing of the optical beam at two temperatures. For the higher temperature (FIG. 2), the distance L 2  between the lens  16  and the output fiber end  28  is such that the beam launched from the input fiber  24  passes through the lens  16 , reflects from the mirror  14 , passes again through the lens and focuses on the tip  28  of the output fiber. Thus, the optical alignment is at the optimum and the attenuation of the input signal is relatively low.  
         [0037]    When the temperature changes, the resulting differential thermal expansion/contraction of the actuator causes misalignment of some elements of the attenuator and consequently a signal attenuation.  
         [0038]    In the embodiment of FIG. 2 and FIG. 3, the distance L 1  does not change significantly when the temperature changes because of the low CTE of the spacer  18 . It should be noted that the distance L 1  is selected to collimate the input beam on the mirror. In contrast, the lens-to-fiber distance L 2  changes as the sleeve  10  and the stack  12  change length. Since the focal distance of the lens does not vary significantly with temperature, the change of distance L 2  will result in a de-focusing of the beam on the output fiber  22  and hence a greater attenuation.  
         [0039]    In the embodiment of FIG. 4, a double fiber tube  20  and a lens  16  are mounted in a sleeve  30  that is fixedly attached to a base  32  via a post  34 . A spherical concave mirror  36  is fixed to the base  32  via a post  38 . The post  34  and the sleeve  30  are made of a low CTE material, while the post  38  is made of a high CTE material. Also, the length of the post  38  is significantly greater than the length of the post  34 . In a specific ambient temperature, the device may be calibrated so that an optical beam launched from one of the fibers  22  passes through the lens  16 , reflects from the mirror  36  and passes again through the lens  16  to become focused on the end of the other fiber  24 . When the ambient temperature changes, the difference in thermal expansion of the posts  34  and  38  will cause the mirror to become vertically displaced, as shown with the arrow, relative to the lens and the fiber tube. As a result, optical beam launched from the fiber  22  will impinge on another spot of the mirror and because of the curvature of the mirror, the reflected beam will be defocused from the end of the fiber  24 .  
         [0040]    The focusing/defocusing effect will be achieved with a non-planar mirror of another curvature, e.g. parabolic, ellipsoidal etc.  
         [0041]    It will also be noted that the provision of a lens is not an absolute necessity for the attenuator to work. The lens or lenses serve to increase the coupling efficiency of the attenuator.  
         [0042]    In an alternative embodiment of the invention illustrated schematically in FIG. 5, the actuator is composed of a post  40  that supports the double fiber tube  20  and a post  42  that supports a lens  44 . Differential thermal expansion of the actuator  40 ,  42  and consequently a differential displacement of the lens relative to the output fiber end and input fiber end i.e. the input port and the output port, respectively, causes a shift of an optical beam on its path from the input port through the lens and back as reflected from the mirror. Consequently, focusing or defocusing of the beam (or in other words, a change of coupling efficiency) will take place as a result of a change in ambient temperature.  
         [0043]    It will be noted that in all the embodiments illustrated herein, the attenuator is a passive one as the actuator is devoid of an active control member or element e.g. a piezoelectric element that requires a voltage to operate. It is of course possible to modify the attenuator of the invention to provide an active actuator e.g. a piezoelectric, magnetostrictive actuator etc.  
         [0044]    It will be recognized that, where space saving is of primary concern, the sleeve-mounted embodiment of FIG. 1 is typically more compact than the embodiment of FIG. 5, but both embodiments are within the scope of the invention.  
         [0045]    As shown in FIGS.  1 - 5 , the input port and output port are disposed on one side of the lens, and a displacement of the lens and/or the mirror disposed on the other side of the lens affects the focal distance between the lens and one or both ports. This is a so-called “folded” space saving arrangement. However, it is also conceivable to place the lens in the optical path and between the input port and the output port, eliminating the mirror. Either of the ports and the lens may be mounted to a base via a separate support member having a dissimilar CTE. The support members, functioning as actuator, may be designed for relative axial displacement as in FIG. 1 or for a transverse displacement as in FIGS. 4 and 5. A change in ambient temperature would cause a linear displacement of the lens relative to either port amounting to a focusing or defocusing of the optical beam on the output port and a resulting change in attenuation.  
         [0046]    Various modifications of the embodiments illustrated and described hereinabove are possible. For instance, the materials of the sleeve and stack may be interchanged resulting in an opposite displacement of the mirror vs. the fibers with a change in temperature. Accordingly, the device may be calibrated such that the minimum attenuation occurs at the room temperature and rises with increasing ambient temperature, or that the minimum attenuation occurs at a preselected elevated temperature and increases as the ambient temperature decreases.  
         [0047]    Where a focusing means is recited, it denotes at least one lens or an equivalent element. Specifically, a lens may be associated separately with input port and output port. Fibers with expanded mode field (TEC fibers) may be useful to focus the optical beam for the purposes of the invention. In the embodiments described and illustrated herein, a single lens is provided for economic and space-saving reasons.  
         [0048]    As will be recognized by those skilled in the art, the device requires calibration, so that a predetermined attenuation is produced at specific temperatures. The calibration procedure does not require further explanation.