Abstract:
Improved designs of optical devices for processing optical signals with one 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 a rotation of the filter mirror assembly. To cancel or minimize a lateral shift introduced to a light beam going through an optical filter, an optical compensator is introduced and rotates oppositely whenever the optical filter rotates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. application Ser. No. 10/144,596, filed May 10, 2002, now U.S. Pat. No. 6,943,938 and entitled “Tunable Wavelength Filter with Invariant Transmission and Reflection Beam Angles,” 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, and more particularly, to optical add/drop devices and the method for making the same. 
   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. On the other hand, it is often needed to select a signal with a particular wavelength from a multiplexed signal with a group of wavelengths. This is advantageous in order to drop/add the same or different channel signals at various points within an optical network. Optical add/drop devices are often employed to add/drop one or more of these channel signals. 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 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, is integrated with a high-reflective (HR) mirror. Specifically, the optical filter and the mirror are integrated such that the mirror rotates accordingly when the optical filter rotates. The integrated part, also referred to as a filter mirror assembly, is then rotatably mounted around a rotation axis positioned at an intersection of the incident 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 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 filter mirror assembly around the 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 kept the reflected signal uninterrupted. 
   The present invention may be implemented as an apparatus and a method. According to one implementation, the present invention is an optical device comprising an optical filter having an incident side, a frequency response of the optical filter to an incoming signal depending on an incident angle of the incoming signal to the incident side; a mirror having a reflecting side and integrated with the optical filter to form an integrated part rotatably mounted on a rotation axis such that the mirror rotates accordingly when the optical filter is caused to rotate to a position in response to a selected wavelength; and a compensator configured to rotate oppositely with the optical filter to compensate a lateral shift in a light beam passing through the optical filter. 
   According to another implementation, the present invention is an optical device comprising a filter mirror assembly including an optical filter having an incident side, a frequency response of the optical filter to an incoming signal depending on an incident angle of the incoming signal to the incident side and a mirror having a reflecting side, the filter mirror assembly rotatably mounted on a rotation axis such that the mirror rotates accordingly when the optical filter is caused to rotate to a position to select a wavelength; a first collimator optically coupled to the optical filter; a second collimator optically coupled to the mirror; a third collimator; and an optical compensator optically coupled between the filter mirror assembly and the third collimator, wherein the compensator performs in accordance with the optical filter to cancel or minimize a lateral shift when a light beam goes through either one of the optical compensator and the optical filter. 
   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. 1A  shows an optical device including an optical filter integrated with a mirror to facilitate the understanding of the present invention; 
       FIG. 1B  shows characteristics of an exemplary optical filter; 
       FIG. 2  shows that a filter mirror assembly including an optical filter and a mirror has been rotated around a rotation axis from a position P 1  to a new position P 2 ; 
       FIG. 3A  shows an optical filter according to one embodiment of the present invention; 
       FIG. 3B  shows exemplary real tray tracing at three beam incident angles in which the reflected beam and the incident beam are kept the same position when the filter mirror assembly is rotated from 20 to 30 and to 40 degree; 
       FIG. 3C  shows a measurement of a lateral shift versus an incident angle onto the filter mirror assembly; 
       FIG. 3D  shows collective measurements of the coupling loss versus the lateral shift Δx for four given collimated beam sizes; 
       FIG. 3E  shows the T-channel (transmission) coupling loss as a function of the tilting angle of the filter mirror assembly, for a fixed 1.2 mm thickness substrate with an index of refraction value 1.5 at various beam waist radii; 
       FIG. 4  shows an exemplary mechanical structure that may be used to control the rotation of the filter mirror assemble as well as the compensator as shown in  FIG. 3A ; 
       FIG. 5A  shows a different configuration of the filter mirror assembly in which the angle between the thin film filter and the mirror is less than a right angle, for example 85 degree; and 
       FIG. 5B  shows an exemplary situation in which an optical compensator is not positioned perfectly systemic to a filter mirror assembly about a vertical bisector line, resulting in a two-degree offset in angle causes 0.1 dB insertion loss increase at 20° incident angle. 
   

   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. 1A–5B . 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. 1A  shows an optical device  100  that may be used to facilitate the understanding of the present invention. 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 relatively to an incoming optical beam or signal (e.g., a multiplexed signal) with a plurality of wavelengths. The filter mirror assembly includes an optical filter  102  and a mirror  104 . 
   As shown in the figure, the filter mirror assembly appears an “L” shape and provides a filtering function as well as a reflecting function. As will be detailed more below, the angle between the optical filter  102  and the mirror  104  does not have to be a right angle (i.e., 90 degree). The drawing showing a 90 degree is for easy understanding only and shall not be understood as an implied limitation of the current invention. 
   According to one embodiment, 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  while the mirror is preferably of high reflection.  FIG. 1B  shows characteristics of an exemplary optical filter. A pass-through wavelength of the optical filter changes when the incident angle changes. For example, at an incident angle of 0 degree, the pass-through wavelength is 1550 nm while, at an incident angle of 27 degree, the pass-through wavelength is 1470 nm. 
   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 (selected) wavelength from an incoming multiplexed signal  107  as shown in  FIG. 1A . 
   In operation, the incoming signal or light beam  107 , assumed to have wavelengths λ 1 , (2, . . . , and (N, is coupled from a collimator  110  to the optical filter  102 . According to a particular requirement, for example, only a signal with wavelength (j (1≦j≦N) is to be transmitted through the optical filter  102  positioned at a particular position (angle), 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 (1, (2, . . . , and (N, the collimator  112  outputs a transmitted signal  111  at a selected wavelength (j and the collimator  116  outputs the reflected signal  114  with all wavelengths except for the selected wavelength (j. 
   When there is a need to alter the selection of the transmitted wavelength (j to (i, wherein 1≦i, j≦N and i≠j, the filter mirror assembly 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 a 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 . 
   According to one embodiment of the present invention,  FIG. 3A  shows an optical device  300  that may be readily understood if viewed in conjunction with  FIGS. 1A–2 . It is assumed that the device  300  is used to drop a selected wavelength λ j . Accordingly, the device  300  includes a filter mirror assembly  302  and a compensator  304  in addition to three collimators  306 ,  308  and  310  that are respectively labeled as input port collimator, express port collimator and transmission port. 
   In one embodiment, the filter mirror assembly  302  is similar to that in  FIG. 1A  and appears an “L” shape and provides a filtering function as well as a reflecting function. The filtering function is provided by, for example, a thin film filter  312  on top of a substrate  314 , and the reflecting function may be simply provided by a mirror  316 . As described above, when the filter mirror assembly  302  is controlled at a certain angle, only one selected wavelength λ j  in a light beam can pass through the thin film filter  312  and the substrate  314  to the transmission port collimator. The rest of the light beam with wavelengths other than the selected wavelength λ j  is reflected towards the mirror  316 . 
   The mirror  316  then redirects the beam to the direction that is parallel to the optical path of the input beam (or the input beam direction) to the express (or reflection) port  308 . One of the features of the device  300  is that the optical path of the reflected beam (or the reflected beam position) is always maintained as the same beam position of the incident beam while the filter mirror assembly  302  is controllably rotated around the rotation pivot  318 . When the angle in the filter mirror assemble  302  is other than 90 degrees, as detailed below, the incidental beam position and the reflected beam position remains unaltered, although not necessarily being parallel. In any case, it can be shown in  FIG. 3A  that the separation between the incident beam position and the reflected beam position is always  2 D, where D is the vertical distance between the rotation axis  318  to the incident beam position. Exemplary real tray tracing at three beam incident angles are shown schematically in  FIG. 3B  in which the reflected beam and the incident beam are kept the same position when the filter mirror assembly is rotated from 20 to 30 and to 40 degree. 
   It may be observed in  FIG. 1A  or  FIG. 3  that, as the light beam at the selected wavelength λ j  passes the filter mirror assembly  302 , there is a certain lateral shift, noted as Δx, of the transmitted light with respect to the incident beam position. Such lateral shift Δx, observable in  FIG. 1A  or  FIG. 3A , is largely caused by the difference between the two media. The magnitude of the lateral shift Δx depends on the combination of the thickness of the thin film filter  312  and the index of the substrate  314 . A measurement of such lateral shift versus an incident angle onto the filter mirror assembly  302  is shown in  FIG. 3C . 
   In operation, such lateral shift Δx, depending on the magnitude thereof, ultimately affects the transmission of the light beam at the selected wavelength λ j , thus introducing a coupling loss. Measurements of the coupling loss versus the lateral shift Δx for four given collimated beam size on the transmission port coupling loss are collectively shown in  FIG. 3D . Combining these two effects together, the T-channel (transmission) coupling loss as a function of the tilting angle of the filter mirror assembly, for a fixed 1.2 mm thickness substrate with an index of refraction value  1 . 5 , at various beam waist radii are plotted in  FIG. 3E . 
   One of the important features in the present invention is the introduction of the compensator  304  in the device  300 . The compensator  304  is made as identical as possible to the substrate  314  such that the lateral shift Δx can be cancelled or minimized when the light beam at the selected wavelength λ j  passes the compensator  304 . In operation, the shifted light beam enters the compensator  304  and is shifted in a direction opposite to that of the substrate  314 , thus resulting in a cancellation of the lateral shift Δx or at least a minimization of the lateral shift Δx. As a result, the coupling loss is significantly reduced. 
   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 the collimator  306 . 
     FIG. 4  shows an exemplary mechanical structure  400  that may be used to control the rotation of the filter mirror assemble as well as the compensator as shown in  FIG. 3A . The mechanical structure  400  includes four rigid arms  402 ,  404 ,  406  and  408  connected at its ends to form a frame, wherein both of the arms  406  and  408  are attached a filter  410  and a compensator  412 . The arm  408  is also extended to include a mirror  414  and thus provides an exemplary filter mirror assembly. In operation, to drop or filter out a selected wavelength, the filter mirror assembly is caused to rotate to a position where the spectral response of the filter  410  falls on the selected wavelength. As the arm  414  rotates, because of the framing of the mechanical structure  400 , the compensator  412  moves oppositely with the filter  410 . 
   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  410  is changed, as long as the rotation of the filter mirror assembly is around the rotation axis which is located at the intersection of the mirror and filter coating surface of the filter. 
     FIG. 5A  shows a different configuration of the filter mirror assembly in which the angle between the thin film filter and the mirror is less than a right angle, for example 85 degree. It shows by ray tracing that as long as the filter mirror assembly rotates about the rotation pivot, the incident light path and the reflection light path will remain unaltered. It can be appreciated that the filter mirror assembly of  FIG. 5A  can be still supported by the mechanical structure  400  of  FIG. 4 . 
   There are some results observed. For the reflection port, for example,  308  of  FIG. 3 , the angle between the filter surface and the reflection mirror surface does not have to be 90° and the incident and reflection optical paths can still maintain unchanged while rotating the filter mirror assembly. This feature has been demonstrated in  FIG. 5A  in which the reflection port collimator is no longer parallel to the input port collimator while the light coupling is kept at optimal condition. 
   For the transmission port, for example,  310  of  FIG. 3A , if the compensator  304  is not positioned perfectly systemic to the filter mirror assembly about a vertical bisector line  320 , the situation is modeled and shown in  FIG. 5B , a two-degree offset in angle causes 0.1 dB insertion loss increase at 20° incident angle. In operation and mechanically, such angle offset can easily be controlled under 2 degree. 
   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. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.