Abstract:
Improved designs of optical devices for processing optical signals with own or more specified wavelengths are disclosed. According to embodiment, a filter mirror assembly appears an “L” shape and provides a filtering function as well as a reflecting function. The filter mirror assembly is so mounted that a rotation thereof will not alter the optical path the beam positions of signals resulted from the filter mirror assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related to U.S. application Ser. No. 10/046,445, filed Oct. 29, 2001, and entitled “Method for Bonding Two Optical Parts and Apparatus thereof,” which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is generally related to the area of optical communications. In particular, the invention is related to a method and apparatus for processing optical channel or channel band signals with specified wavelengths. 
   2. The Background of Related Art 
   The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths or channels in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networking. 
   There are many optical parts/devices used in the optical fiber communication networks. Optical tunable filter is one of the optical parts/devices widely used in many important fiber optical applications, such as, optical add/drop modules, optical cross connect systems and tunable receivers. An ideal filter is a device which can isolate an arbitrary spectral band with an arbitrary center wavelength over a broad spectral range. Accordingly, a tunable filter is known or desired to be able to transmit at any given wavelengths with some minor turning adjustments. 
   There are many ways of making a filter with tuning capability and, consequently, many types of tunable filters. These include those using fiber Bragg grating and tunable acoustical filter (TAOF), traditional interferometers such as Fabry-Perot, and liquid crystal filters. All have advantages and limitations and are ended up with a trade-off among the technical feasibility, the performance demands and costs. It is desirable to have tunable filters that have the advantages of simple structure, good performance, high reliability and low cost. 
   SUMMARY OF THE INVENTION 
   This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. 
   The present invention is related to designs of optical devices and methods for processing optical channel or channel band signals with arbitrarily specified wavelengths over a predefined spectral range. According to one aspect of the present invention, an optical filter, such as a thin film filter with a bandpass WDM filter coating on one side and an antireflection (AR) coating on the other side, both sides being substantially parallel with each other, is integrated with a high-reflective (HR) mirror. Specifically, the optical filter and the mirror are integrated in such a way that one side (e.g., the WDM filter coating) of the optical filter and the reflecting side of the mirror is perpendicular to each other, and such thus forming a right angle between the two surfaces. The integrated part is then so mounted that it can be rotated around a rotation axis positioned at the apex of the right angle, or which is the same an intersection of the WDM filter coating side of the optical filter and the reflecting side of the mirror. In general, the optical filter has a frequency response of a bandpass filter and the center bandpass frequency depends on an incident angle at which an incoming optical signal impinges upon the WDM filter coating side of the optical filter. As a result, the beam angles of the transmitted signal as well as the reflected optical signal are invariant to the rotation of the optical filter mirror assembly around said axis, and thus invariant to the incident angle of the incoming signal to the optical filter. By positioning the rotation axis at the intersection, not only the beam angle but the total position of the reflected beam will be invariant to the rotation of the filter-mirror assembly. Therefore, a wide range of wavelengths can be selected to transmit through the optical filter, and keep the reflected signal uninterrupted. 
   There are numerous benefits, features, and advantages in the present invention. One of them is a simple structure, good performance, high reliability and low cost in the tunable filters contemplated in the present invention. 
   Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  shows an optical device including an optical filter integrated with a mirror and three respective collimators according to one embodiment of the invention; 
       FIG. 2  shows that the integrated optical filter and the mirror have been rotated around a rotation axis from one position to another position; 
       FIG. 3A  shows a setting in which two optical parts (e.g. a collimator and an optical filter or a mirror) are being aligned before they are permanently bonded to a substrate to preserve an aligned optical path therebetween; 
       FIG. 3B  shows that a collimator is being positioned by wedges after it is aligned with an optical component or device; 
       FIG. 4A  shows a cross-section view of an optical part being positioned by two wedges and bonded to a substrate; and 
       FIG. 4B  illustrates a wedge having a cross-section being right triangular and being slid towards a cylindrical optical part. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will become obvious to those skilled in the art that the present invention may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the present invention. 
   Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
   Embodiments of the present invention are discussed herein with reference to  FIGS. 1–4B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
     FIG. 1  shows an optical device  100  according to one embodiment of the invention. In particular, the optical device  100  is capable of maintaining beam direction and angle of both transmission beam and reflection beam while rotating a filter mirror assembly  101  relatively to an incoming optical beam or signal (e.g., a multiplexed signal) with a plurality of wavelengths. As shown in the figure, the optical device  100  includes the filter mirror assembly  101  that appears an “L” shape and provides a filtering function as well as a reflecting function. According to one embodiment, the filter mirror assembly  101  includes an optical filter  102  and a mirror  104 . The optical filter  102  is so chosen that the frequency response thereof to an incoming signal depends on an incident angle of the incoming signal coming to its incident side  106 . The mirror is preferably of high reflection. According to another embodiment, the filter mirror assembly  101  includes a thin film filter that is substantially “L” shaped. In other words, one side of the thin film filter retains the filtering function and the other side of the thin film filter is coated with a high-reflection material to function as a mirror. Given the description herein, those skilled in the art may appreciate that there are alternative embodiments of the filter mirror assembly  101  that provides the similar functions contemplated in the present invention. 
   In general, the optical filter  102  has two sides, preferably, a bandpass WDM filter coating on one side and an antireflection (AR) coating on the other side with both side substantially parallel with each other. Depending on the use of the optical device  100 , either side can be an incident side to receive an optical signal. To facilitate the description of the present invention, it is assumed that the optical device  100  is used to drop or filter out a specific wavelength from an incoming multiplexed signal  107  as shown in  FIG. 1 . According to one embodiment of the present invention, the optical filter  102  is integrated with the mirror  104  in such a way that the incident side  106  of the optical filter  102  is perpendicular to the reflecting surface  108  of the mirror  104 . In operation, the incoming signal  107 , assumed to have wavelengths λ 1 , λ 2 , . . . , and λ N , is coupled from a collimator  110  to the optical filter  102 . According to a particular requirement, for example, only a signal with wavelength λ j (1≦j≦N) is to be transmitted through the optical filter  102  positioned at a particular position, for example, P 1 , at the same time, the remaining wavelengths in the signal  107  (i.e., the reflected signal  114 ) are reflected to the mirror  104  that further reflects the reflected signal  114  to a collimator  116 . 
   As a result, the collimator  110  couples in the incoming signal  107  with wavelengths λ 1 , λ 2 , . . . , and λ N , the collimator  112  outputs a transmitted signal  111  at a selected wavelength λ j  and the collimator  116  outputs the reflected signal  114  with all wavelengths except for λ j . 
   When there is a need to alter the selection of the transmitted wavelength λ j  to λ i , wherein 1≦i, j≦N and i≠j, the integrated optical filter  102  and the mirror  104  can be rotated accordingly to a new position, for example, P 2 . Referring now to  FIG. 2 , it shows that the integrated optical filter  102  and the mirror  104  have been rotated around the rotation axis  200  from a position P 1   202  to a new position P 2 ,  204 . Because the incident angle of the signal  107  is changed, only a signal with wavelength λ i  is transmitted through the optical filter  102  positioned at the present position, at the same time, the remaining wavelengths in the signal  108  are reflected to the mirror  104  that further reflects the reflected signal  114  to the collimator  116 . 
   It can be readily appreciated that the above description equally applied to the applications in which a signal at a specific wavelength (e.g., λ j ) is to be combined with an incoming signal by reversing the optical paths. A resultant newly combined or multiplexed signal will be output from collimator  110  via the mirror. 
   One of the features in the present invention is that the reflected signal always maintains the same beam position regardless how the incident angle to the optical filter  102  is changed, as long as the rotation of the filter mirror assembly is around the rotation axis  200  which is located at the intersection of the mirror and filter coating surfaces. Another one of the features in the present invention is that the transmitted beam&#39;s angle is also independent of the filter mirror assembly rotation when the two sides of the filter to be substantially parallel with each other. 
   To ensure a precise alignment between two or more optical elements, according to another embodiment, at least one of the three collimators  110 ,  112  and  116  are bonded to a substrate after it is aligned with a counter part.  FIG. 3A  shows a setting  300  in which two optical parts (e.g. a collimator and an optical or mirror)  302  and  304  are being aligned before the optical part  302  is permanently bonded to a substrate  306  to constitute a whole device or an integrated part of a device. In one exemplary alignment procedure, the optical part  302  is elevated a small distance  308  (i.e. gaps) from the substrate  306  so that adjustment of the optical part  302  can be performed with respect to the optical part or device  304 . Once the alignment of the two optical parts is done, the prior art method is to apply a kind of bonding agent, such as epoxy, to fill in the gaps between the aligned optical part and the substrate. Another prior art method is to fill in the gaps between the aligned optical part and the substrate with solder (or alloy), as a result, a single optical part or an integrated part of an optical device is formed. 
   In reality, however, it has been noticed that the filling material, either the bonding agent or the solder, can shrink when it is dried out or cool down, resulting in an undesirable alternation or disturbance to the positions of the originally aligned optical parts. According to one aspect of the present invention, the gaps illustrated in  FIG. 3A  is not to be filled with any agent, instead, two or more preformed wedges are used to hold up the originally aligned optical parts when a boding agent is applied. To bond the optical parts to the wedges, a small amount of a bonding agent (e.g. epoxy) is used but only applied to respective contacts between the optical parts and the wedges. Because the amount of the bonding agent is small and the wedges primarily position the optical parts, the alignment of the optical parts is preserved. In fact, the use of the wedges can sustain the alignment under very high environmental stresses (e.g. varying temperatures and vibrations). 
     FIG. 3B  shows that a collimator  320  is being positioned by wedges  322  after the collimator  320  has been aligned with an optical component or device  324  (collectively to represent one or more optical parts/devices). The gaps between the collimator  320  and the substrate  326  are created for aligning the collimator  320  with the device  324 . As shown in the figure, the wedges  322  are used to fill in the gaps and at the same time to hold up the positions of the collimator  320  to maintain the alignment. 
   Referring now to  FIG. 4A , there is shown a cross-section view of an optical part  406  being positioned by two wedges  402  and  404  and bonded to a substrate  408 . In an exemplary operation, the optical part  406  (e.g. a collimator) is first aligned with another optical part (not shown). To perform the alignment, one or both of the optical parts are slightly positioned away from the substrate  408  so that one or both of the optical parts can be adjusted appropriately to ensure that both of the optical parts are aligned with each other. Once the alignment is done, the positions of the optical parts shall be preserved. The two wedges  402  and  404  are respectively slid in from two different directions to hold up the positions of the optical parts when a boding agent is applied. According to one embodiment, a small amount of a bonding agent is applied to only the respective contacts between the contacting surfaces of the optical parts and the wedges. The wedges are then fastened to the substrate by a bonding means (e.g. adhesive, solder, or mechanic fixing). 
   According to one embodiment, the cross section  410  of the wedge  412  is shaped as a right triangle, shown in  FIG. 4B . The wedge can be slid in with the sliding face (formed by the hypotenuse of the right triangle) downward to avoid possible flipping over or up the already aligned optical parts. Other details of the bonding and/or aligning the respective collimators with the optical filter as well as the mirror can be found in co-pending U.S. application Ser. No. 10/046,445, entitled “Method for Bonding Two Optical Parts and Apparatus thereof”. 
   The present invention has been described in sufficient details with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. For example, the variable neutral density filter may be replaced by another device that can strengthen an optical signal. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.