Abstract:
A tunable optical filter is formed by the longitudinal alignment of two opposing end sections of single-mode optical fibers. On at least one of the end sections is a collimator fiber section which is formed from a section of a graded-index, multimode optical fiber which is an odd number of quarter pitches long. The collimator fiber section has an angled end surface which joined to the reciprocally angled end surface of the at least one single-mode optical fiber end section. Piezoelectric material controls the separation between the first and second single-mode optical fiber end sections and sets the wavelengths of optical signals carried through the first and second single-mode optical fiber end sections.

Description:
BACKGROUND OF THE INVENTION 
     This invention is related to etalon-type optical filters and, in particular, to tunable optical filters in which optical fiber ends are used to define the etalon cavity. 
     Optical filters which are tunable are highly desirable components for optical networks and for some optical systems, such as spectrometers, for example. Optical filters typically transmit light at particular wavelengths and block light at other wavelengths. An optical filter which is tunable allows the wavelengths to be selected. 
     Tunable optical filters can be quite varied in construction. For example, liquid crystal cells, thin film filters, ruled gratings, distributed Bragg reflectors (gratings), and fiber Bragg gratings, a type of distributed Bragg reflector, have been used as the tuning constituents in tunable optical filters. Of course, other parts are required to operate the tuning constituent in a tunable optical filter. The resulting optical performance, reliability, speed of operation, cost and size, among many parameters, of such tunable optical filters vary widely. 
     One conventional (and simple) structure for an optical filter is the etalon, also called a Fabry-Perot interferometer, in which two highly reflective, parallel surfaces form a resonating cavity for wavelength selection. To make the etalon tunable, the optical distance between the two reflecting mirrors is changed. One type of etalon, or Fabry-Perot interferometer, tunable optical filter uses the end surfaces of optical fibers as the reflecting surfaces of the etalon. However, the large numerical aperture (NA) of the optical fibers and resulting beam divergence cause a large insertion loss to the detriment of optical performance. One way of minimizing such losses is to use a concave surface at the end surface of the one of the optical fibers. Nonetheless, the insertion loss can still be lowered. These fiber-ended tunable optical filters also have significant sideband or side mode peaks in the transmission spectra which adversely affect the performance of the filters. Furthermore, current fiber etalon-type tunable optical filters remain expensive and their applications to real world problems are accordingly reduced. 
     The present invention is directed toward avoiding these problems, improving the optical performance of fiber etalon-type, tunable optical filters, and lowering their costs to expand their applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides for a tunable optical filter which has an end section of a first single-mode optical fiber having an end surface; an end section of a second single-mode optical fiber longitudinally aligned with the first single-mode optical fiber end section, the second single-mode optical fiber end section having an end surface facing the end surface of the first single-mode optical fiber end section; piezoelectric material controlling the separation between the first single-mode optical fiber end section and said second single-mode optical fiber end section and selecting wavelengths of optical signals carried through the end sections of the first and second single-mode optical fibers; and a collimator fiber section having a first end surface angled from a perpendicular plane to a longitudinal axis common to the at least one single-mode optical fiber end section and the collimator fiber section and joined to the end surface of the at least one single-mode optical fiber end section, the end surface of the at least one single-mode optical fiber end section reciprocally angled to the first end surface of said collimator fiber section. 
     The present invention further provides for the tunable optical filter with the collimator fiber section comprising a section of a multimode, graded index optical fiber which attached to at least one of the single-mode optical fiber end sections and is an odd number of quarter pitches long; the piezoelectric material comprising PMN-PT((1−x)Pb(Mg⅓Nb⅔)O 3−x —PbTiO 3 )); and that the tunable optical filter includes a package assembly to hold the end sections of the first and second single-mode optical fibers, the piezoelectric material; and the collimator fiber section, the package assembly further including a TEC (thermoelectric cooler) to maintain temperature in the packaging assembly for optimum optical performance by the tunable optical filter 
     Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a generalized representation of an etalon or Fabry-Perot interferometer. 
         FIG. 2A  shows a tunable optical filter with two optical fiber end sections, each with a collimator fiber section, according to one embodiment of the present invention;  FIG. 2B  shows a detailed view of a collimator fiber section in  FIG. 2A . 
         FIG. 3A  shows a tunable optical filter with two optical fiber end sections, each with a collimator fiber section and one of the collimator fiber sections having a concave open end surface, according to one embodiment of the present invention;  FIG. 3B  shows a detailed view of the  FIG. 3A  collimator fiber section collimator having a concave open end surface. 
         FIG. 4  shows a tunable optical filter with two optical fiber end sections, each with an collimator fiber section with a concave open surface, according to an embodiment of the present invention. 
         FIG. 5A  shows a tunable optical filter with two optical fiber end sections, only one end section has a collimator fiber section, according to an embodiment of the present invention;  FIG. 5B  shows the details of the end surface of the  FIG. 5A  optical fiber end section without the collimator fiber section. 
         FIG. 6  shows a tunable optical filter assembly with two optical fiber end sections, only one end section has a collimator fiber section and that section has a concave open end surface. 
         FIG. 7A  shows another tunable optical filter assembly with two optical fiber end sections, only one section has a collimator fiber section. The end surface of the other end optical fiber end section is concave and is illustrated in detail in  FIG. 7B . 
         FIG. 8  shows another tunable optical filter assembly with two optical fiber end sections, only one section has a collimator fiber section. The open end surface of the collimator fiber section and the end surface of the other end optical fiber end section are both concave. 
         FIG. 9A  is an exploded view of a precision sleeve assembly which holds the two optical fiber end sections (and attached collimator fiber sections) in alignment;  FIG. 9B  shows the completed assembly. 
         FIG. 10  is a cross-sectional view of the packaged assembly of the tunable optical filter, according to an embodiment of the present invention. 
         FIG. 11A  is an exploded view of another precision sleeve assembly with one piezoelectric ring to tune the tunable optical filter;  FIG. 11B  shows the completed assembly. 
         FIG. 12A  is an exploded view of another precision sleeve assembly with two separated piezoelectric rings to tune the tunable optical filter;  FIG. 12B  shows the completed assembly. 
         FIGS. 13A-D  are plots of power in dB versus wavelength for different fiber etalon arrangements. 
     
    
    
     It should be noted that the same reference numerals are often used in different drawings to refer to elements or parts with identical or similar functions to better explain the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a simplified organization of an etalon. Two parallel, highly reflecting but partially transmitting, surfaces  14 , define a resonating cavity  11  in which incoming light is multiply reflected between the surfaces  14 . In this example light from the left is represented by an arrow and the multiple reflections are represented by a double-headed arrow. This arrangement is well-known in optics and is the basis of the Fabry-Perot interferometer. With monochromatic light, a series of rings  17  is formed on the focal plane around the optical axis of the etalon representing constructive and destructive interference patterns of the light. These are shown as concentric rings about a central axis of the etalon. The particular interference pattern, or more specifically, the details of the interference pattern, is determined by the parameters of the etalon, including L, the separation between the surfaces  14 , i.e., the length of the resonating cavity  11 , the index of refraction n of the medium in the resonating cavity  11 , and the wavelength λ of the monochromatic light, and even the reflectivity R of the reflecting surfaces  14 . Light with multiple wavelengths results in superimposed patterns of rings, each pattern for a wavelength. 
     Various arrangements have been used to adapt the device for different wavelengths of light. One classical arrangement is to vary the separation L to form a Fabry-Perot interferometer. The variation in one of the optical parameters allows the device to be “tuned” for one or more selected wavelengths. One such arrangement is to use the mirrored ends of optical fibers to define the resonating cavity of an etalon. The distance between the fiber ends is varied to select the filtered wavelength(s). A problem with this type of etalon-type tunable optical filter is that the optical fiber ends have a large numerical aperture (NA) and a large beam divergence which increase the insertion loss for the device. To counter this problem, some of the end surface(s) of the optical fiber(s) have been made concave. This ameliorates the problem to a certain extent, but there is still a considerable insertion loss. A second problem is that there are undesirable back reflection and side modes or side lobes in the transmission spectra, i.e., light is transmitted in the cladding, instead of the core, of the output optical fiber of the tunable optical filter with transmission peaks in desirable locations. 
     Fiber End Sections Etalon-Type Arrangements 
     To address these problems, the present invention provides for different etalon-type arrangements of fiber end sections. At least one of the fiber end sections has a collimator fiber section, which as the name implies, helps collimate the beam from the fiber end section and focuses the beam entering the fiber end section. The collimator fiber section also reduces back reflection and side mode or side lobe amplitudes. 
     One embodiment of the present invention which overcomes, or substantially solves, these problems is illustrated in  FIG. 2A . The illustrated etalon arrangement has two optical fiber end sections  10   a  and  10   b  which face each other. The end sections  10   a  and  10   b  are the ends of single-mode optical fibers which have a core  13  of relatively small diameter, typically in a range around 9 μm, surrounded by a cladding  12  of 125 μm in diameter. Attached to each end section  10   a  and  10   b  are collimator fiber sections  11   a  and  11   b  respectively which are separated by a resonating cavity length L. The collimator fiber sections  11   a  and  11   b  are each formed from a section of a multimode optical fiber which has a core  16 , typically with a diameter of 50-62.5 μm, surrounded by a cladding  15  of 125 μm in diameter. It should be understood that the numbers given here are for purposes of better describing the subject tunable optical filter and should not be considered limiting. 
     The multimode optical fiber from which each of the collimator fiber sections  11   a ,  11   b  is formed is a graded index, multimode fiber and each section is an odd integer of a quarter-pitch long, i.e., N×¼P, N=1, 3, 5, 7 etc. The net effect is that if one assumes that light emerging from the core  13  of the single-mode optical fiber end section  10   a ,  10   b  is a point source, the light is bent by the graded index of the core  16  so that the light leaves the collimator fiber section  11   a ,  11   b  as collimated light. In the reverse direction, collimated light entering the core  16  of the collimator section  11   a ,  11   b  is bent by the graded index and focused on the core  13  of the attached single-mode optical fiber end section  10   a ,  10   b . Graded index, multimode fibers are readily available. For example, Corning, Inc. of Corning, N.Y., is a well-known manufacturer and supplier. The result is the collimation and focusing action of the collimator fiber sections  11   a ,  11   b  reduces the insertion loss of the resulting tunable optical filter. 
       FIG. 2B  illustrates the collimator fiber sections  11   a ,  11   b  in greater detail with an example collimator fiber section  11 . A flat open end surface  18  terminates one side of the collimator section  11  and an interior end surface  19  terminates the other side. The interior end surface  19  is cleaved at an angle θ from the plane perpendicular to the longitudinal axis  22  of the collimator fiber section  11 . The angle is exaggerated for purposes of explanation. As shown in  FIG. 2A , the end surface of the fiber end section  10   a ,  10   b  corresponding to the collimator fiber sections  11   a  and  11   b  is reciprocally slanted. The corresponding interior end surface  19  and end surface of the fiber end section are then fused together. The angled joining of the fiber end section  10   a ,  10   b  to the collimator fiber sections  11   a ,  11   b  reduces back reflection and side modes or side lobes in the transmission spectra of the tunable optical filter. In particular, the angled joining of the collimator fiber section on the output side of the resonating cavity helps to block back reflection and reduces side modes or lobes which is caused by light in the cladding  15  of the collimator fiber section by reflecting such light away. It has been found that angles with θ greater than 6° work better and a range of 8° to 15° is believed to be optimum. 
     The open end surface  18  of the collimator fiber section  11  is covered by a high reflectance layer  20  and an anti-reflection layer  21 . The high reflectance layer  20  formed from dielectric material covers the core portion and the anti-reflection layer  21  covers the cladding portion of the open end surface  18 . Deposition and photoresist masking techniques are used to create the layers  20  and  21 . The anti-reflection layer  21  minimizes undesirable back reflection and also reduces side mode transmission. 
       FIGS. 3A and 3B  show another embodiment of the present invention. In this fiber etalon arrangement the fiber end section  10   a  and collimator fiber section  11   a  are the same as previously described in  FIG. 2A . But instead of the collimator fiber section  11   a , a collimator fiber section  51   b  having a concave open end surface  18  is attached to the fiber end section  10   b . Shown in greater detail in  FIG. 3B , the section  51   b  is formed from a graded index, multimode optical fiber and is N×¼P long, where N is an odd integer, for collimation and focusing functions as described previously. Its interior end surface  19  is angled as described previously, but its open end surface  18  is concave. Specifically, the open surface  18  of the collimator fiber section  51   b  is formed by polishing with a convex polishing surface to create a concave end surface  52 . Then a layer  51  of anti-reflection material is deposited over the cladding portion and a high reflectance, dielectric material layer  54  is deposited over the core portion of the end surface  52  with photoresist masking techniques. A concave surface  53  is created in the high reflectance layer  54  by polishing with the convex polishing surface to create a concave end surface  53 . The concavity of the end surfaces  52  and  53  is set by empirically adjusting the softness of the polishing surface and controlling the pressure of the polishing surface upon the polished surface  52 ,  53 . Gravity may be used for pressure control by selecting the weight of a polishing fixture which holds the polishing surface tool against the polished surface below. 
     The resulting concavity of the collimator fiber section  51   b  is useful in minimizing beam divergence if the action of the quarter-pitch (or odd multiple integer of a quarter-pitch) of the collimator fiber section is not sufficient. 
     Another arrangement of the fiber etalon according to the present invention is illustrated in  FIG. 4 . In this embodiment the left collimator fiber section  11   a  is replaced with a collimator fiber section  51   a  which has a concave open end surface  18  as shown in  FIG. 3B . The open end surfaces of both collimator fiber sections  51   a ,  51   b  are concave. 
       FIG. 5A  illustrates another embodiment of the present invention in which one of the collimator fiber sections  11  of the  FIG. 2A  arrangement is not used. That is, the left optical fiber end section  10   a  is attached to the collimator fiber section  11   a , but the right optical fiber end section  10   b  does not have a collimator fiber section  11 . The end surface  118  of the single-mode fiber end section  10   b  is flat and perpendicular to the longitudinal axis of the section  100   b , as illustrated in  FIG. 5B . The core  13  and most of the cladding  12  surrounding the core  13  at the end surface  118  is covered by a high-reflectance layer  124  and the cladding  12  near the edge of the end surface  118  is covered by an anti-reflection layer  121 . This is done in the same manner as described with respect to  FIG. 2B . It should be noted that output side of the fiber etalon-type arrangement is the left fiber end section  10   a  since it carries the angled splice to the collimator fiber section  11   a . Another embodiment of the present invention is shown in  FIG. 6 . Here the collimator fiber section  11   a  of  FIG. 3A  is replaced by the collimator fiber section  51   a  which has a concave end surface. 
       FIG. 7A  illustrates another embodiment of the present invention. But instead of the flat end surface  118  of the right fiber end section  10   b  of  FIGS. 5A and 5B , the section  10   b  of  FIG. 7B  has a concave end surface  68 . As shown in  FIG. 7B , the end surface  68  is formed by first being polished with a convex polishing surface to create a concave end surface  62 . Then a layer  61  of anti-reflection material is deposited over the cladding portion and a high reflectance, dielectric material layer  64  is deposited over the core portion of the end surface  62 . A concave surface  63  is created in the high reflectance layer  64  by polishing with the convex polishing surface to create a concave end surface  63 . 
       FIG. 8  shows still another arrangement of optical fiber end sections  10   a  and  10   b . The left optical fiber end section  10   a  is attached to a collimator fiber section  51   a  with a concave end surface  18  (see  FIG. 3B ). The right optical fiber end section  10   b  terminates with a concave end surface  68  (see  FIG. 7B ). Again it should be noted that some of the described arrangements have a collimator fiber section on only one single-mode optical fiber end section. Hence the output side of the fiber etalon-type arrangements of  FIG. 6  (and  FIGS. 7A and 8 ) is the left fiber end section  10   a  since that section has the angled splice of the collimator fiber section  51   a  (and  11   a  and  51   a ). 
     Precision Sleeve Assembly to Hold Fiber Etalon Arrangement and Package Assembly 
     The fiber end sections  10   a ,  10   b  and their corresponding collimator fiber sections  11   a ,  11   b  of the fiber etalon arrangements described above are held in alignment by a precision sleeve assembly shown in  FIGS. 9A and 9B  which provides for proper guidance for the optical fiber end sections and their collimator fiber sections, if any.  FIG. 9A  shows the assembly in an exploded view. The fiber ferrule subassemblies  31   a ,  31   b  respectively hold the fiber end sections  10   a ,  10   b  and the collimator fiber sections  11   a ,  11   b . An alumina sleeve  33  having a lengthwise cut fits firmly over the internal ferrules  37   a ,  37   b  of the fiber ferrule subassemblies  31   a ,  31   b . Metal tubes  32   a ,  32   b  fit over the shoulders  38   a ,  38   b  of the fiber ferrule subassemblies  31   a ,  31   b  and over parts of a first metal holder  34  and second metal holder  36 . The completed sleeve assembly is shown in  FIG. 3B . Precision sleeve assemblies and their parts are available from Seikon Giken Co. of Matsudo City, Chiba, Japan. 
     Two piezoelectric disk rings  35   a ,  35   b , also shown in a frontal view with central openings, are mounted between the first and second holders  34  and  36 . Depending upon the voltages across the disks, the piezoelectric disks  35   a ,  35   b  expand and contract along the longitudinal axis of the sleeve assembly to drive fiber ferrule subassemblies  31   a ,  31   b  apart or together. The length L of the resonance cavity of the etalon is thus set or “tuned.” Electric leads which carry voltages to the disks  35   a ,  35   b  are not shown in the drawings. Though the two disk rings  35   a ,  35   b  are shown as assembled together, the two rings  35   a ,  35   b  are electrically driven separately to obtain the maximum displacement per volt. Furthermore, it is preferable that the rings  35   a ,  35   b  comprise comprises PMN-PT((1−x)Pb(Mg⅓Nb⅔)O 3−x —PbTiO 3 )). Compared to other piezoelectric materials, such as PZT and PLZT, PMN-PT has a greater displacement per volt and faster tuning speeds. 
     Finally, the completed sleeve assembly  30  is mounted into a package assembly  40  shown in  FIG. 10 . The package assembly has a base  41  with a top  42  and sides  43  which enclose and protect the sleeve assembly  30 . To ensure temperature stability the base portion of the holder  34  is mounted on the top of a TEC (thermoelectric cooler)  44 ) which in turn is mounted on the base  41 . In response to a temperature-monitoring sensor (not shown), the TEC  44  maintains the sleeve assembly  30  and the enclosed etalon in a temperature range for optimum optical performance by the tunable optical filter. The package assembly  40  is miniaturized, not more than 4.5 cm long×1.8 cm wide×1.4 cm high. Installation is easy, yet optical performance is high. 
       FIGS. 11A ,  11 B and  12 A,  12 B illustrate alternative precision sleeve assemblies.  FIG. 11A  shows an exploded view of a precision sleeve assembly with only one piezo-electric disk ring  35 ;  FIG. 11   b  shows the completed sleeve assembly.  FIG. 12A  shows an exploded view of a precision sleeve assembly with only two piezo-electric disk rings  35 A,  35 B which are separated;  FIG. 12   b  shows the completed sleeve assembly. 
     Some Empirical Results of the Tunable Optical Filters 
       FIG. 13A  illustrates a plot of power in dB versus wavelength for a fiber etalon arrangement illustrated by  FIG. 6 . The plot shows the performance of the arrangement with a free spectral range (FSR) of 102 nm and a cavity length L of 11 μm.  FIG. 13B  illustrates a plot of power versus wavelength for a  FIG. 6  fiber etalon arrangement. With an FSR of 95 nm, an insertion loss of less than 5 dB was found and a side-lobe suppression ratio (SSR) of 30 dB was found.  FIG. 13C  illustrates a plot of power versus wavelength for a  FIG. 8  fiber etalon arrangement. The plot shows the performance of the arrangement with an (FSR) of 62 nm and a cavity length L of 19 μm. Likewise,  FIG. 13D  shows the performance of a  FIG. 8  fiber etalon arrangement with FSR about 120 nm and an SSR of about 28 dB. 
     Hence the present invention provides a fiber etalon-type tunable optical filter which has high optical performance and is relative cheap compared to similar tunable optical filters and is miniaturized for easy installation. 
     This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.