Abstract:
Techniques for designing optical devices that can be manufactured in volume are disclosed. An optical assembly is build individually and includes a first tube with one end being polished to a slanted angle and attached with a filter. Because of the slanted angle, a predefined angle of incidence can be defined. The assembly is then bonded to a substrate on which other components or more such assemblies are bonded to form an integrated piece. Depending on implementation, the first tube may include no other element, a lens, or a lens and a fiber pigtail that are encapsulated. In the case that the first tube includes no other element or only a lens, a second tube is provided to include a fiber pigtail and a lens, or simply a fiber pigtail. One of the advantages of having two tubes is the underlying mechanism providing a lateral adjustment between the two tubes.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is generally related to the area of optical devices. In particular, the present invention is related to optical wavelength multiplexing/demultiplexer or add/drop devices with new optical layouts and manufacturing processes. 
   2. The Background of Related Art 
   Optical add/drop and multiplexer/demultiplexer devices are optical components often used in optical systems and networks.  FIG. 1  shows a typical multiplexer/demultiplexer device  100  utilizing what is referred to as a free-space cascading structure. The input signals including wavelengths λ 1 , λ 2 , . . . λ K , . . . λ N  is coupled into a common collimator  102  of the device  100 . The beam of the input signals is collimated and then propagates in free space before impinging upon a first filter  104 . For example, the first filter transmits a wavelength λ 1  and reflects all others. As a result, a signal at wavelength λ 1  passes the filter  104  and coupled out via a collimator  106 . 
   The reflected signals going through a second filter  108  that transmits a signal at wavelength λ 2  and reflects all others. The signal at wavelength λ 2  passing through the second filter  108  is coupled out by a collimator  112 . The reflected signals from the second filter  108  are successively transmitted and reflected through the remaining filters and collimators. Subsequently, the signals at wavelengths λ 1 , λ 2 , . . . λ K , . . . λ N  are all separated through the multiplexer/demultiplexer device  100 . 
   It is, however, noted that the collimators and filters are bonded to a common substrate separately. Thus a filter and a collimator for the same channel (e.g., a particular wavelength) are isolated. The beam angle of incidence (AOI), which influences which wavelength can pass, is adjusted by rotating the filter. For 100 GHz DWDM, the central wavelength control accuracy is required to be within 0.03 nm, which means 0.1° of rotation with fixation accuracy assuming AOI=1.8°. Such accuracy is indeed a challenge for manufacturing such devices in volume. The prior art may work fine for CWDM as the margins of a central wavelength and the bandwidth thereof are relatively more tolerable, but it is difficult to extend such manufacturing process to DWDM devices. 
   Another significant disadvantage of the prior art devices is that they require coating surface perfectly perpendicular to the substrate.  FIG. 2A  shows a side view of two filters of  FIG. 1 , where both filters  116  and  118 , or their coating surfaces are perpendicular to the substrate  120 , requiring an incoming light beam is reflected back at the same height. If the sidewall of the filter  118  is not perpendicular to the substrate  120  as shown in  FIG. 2B , namely its angle Ψ is not 90°, then the reflected beam would point either up or down. Consequently, after multiple bounces between the filters as shown in  FIG. 1 , the beam may hit the edge of a filter, leading to performance degradation, or hit the substrate, leading to beam clipping loss. 
   Accordingly, there is a great need for multiplexing/demultiplexer or add/drop devices that can be efficiently manufactured in volume. 
   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 in this section as well as in the abstract and the title may be made to avoid obscuring the purpose of this section, the abstract and the title. Such simplifications or omissions are not intended to limit the scope of the present invention. 
   In general, the present invention pertains to improved designs of optical devices, particularly for adding or dropping a selected wavelength or a group of wavelengths as well as multiplexing a plurality of signals into a multiplexed signal or demultiplexing a multiplexed signal into several signals. For simplicity, a group of selected wavelengths or channels will be deemed or described as a selected wavelength hereinafter. According to one aspect of the present invention, an assembly is build individually. The assembly includes a first tube with two ends, one of the two ends being polished to a slanted angle and attached with an optical filter in accordance with a predefined angle of incidence such that the optical filter transmits a signal of a predefined wavelength. The assembly is then bonded to a substrate on which other components or such assemblies are bonded to form an integrated piece. 
   Depending on implementation, the first tube may include no other element, a lens, or a lens and a fiber pigtail that are encapsulated, both the lens and the fiber pigtail being coaxially aligned. In the case that the first tube includes no other element or only a lens, a second tube is provided to include a fiber pigtail and a lens, or simply a fiber pigtail. One of the advantages of having two tubes is the underlying mechanism providing a lateral adjustment between the two tubes. 
   The present invention may be implemented in many ways as a subsystem, a device or a method. According to one embodiment, the present invention is an optical apparatus. According to another embodiment, the present invention is a method for making such optical apparatus, the method comprises: building an assembly that includes a first tube with two ends, one of the two ends being polished to a slanted angle in accordance with a predefined angle of incidence; attaching an optical filter to the slanted end such that the optical filter transmits a signal of a predefined wavelength; and bonding the assembly and common collimator onto a substrate to form an integrated piece. 
   One of the objects, features, advantages of the present invention is the easy adjustment of a beam angle of incidence. Another one of the advantages, features, or benefits in the present invention is that the requirement of a coating surface having to be perfectly perpendicular to the substrate is relaxed. 
   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 a typical multiplexer/demultiplexer device utilizing what is referred to as a free-space cascading structure; 
       FIG. 2A  shows a side view of two filters of  FIG. 1 ; 
       FIG. 2B  shows the sidewall of one of the two filters being not perpendicular to the substrate; 
       FIG. 3  shows one exemplary device according to one embodiment of the present invention; 
       FIG. 4  shows an exemplary configuration of a filter-collimator assembly that may be used in  FIG. 3 ; 
       FIG. 5  shows another exemplary configuration of a filter-collimator assembly that may be used in  FIG. 3 ; 
       FIG. 6A  and  FIG. 6B  show respectively two assemblies that each use two link tubes; 
       FIG. 7  shows an assembly with three link tubes; 
       FIG. 8A  shows a processes for the assembly of  FIG. 4 , which may be considered as two independent phases; 
       FIG. 8B  shows a building processes for the assembly of  FIG. 5 ; 
       FIG. 8C  shows one method, first building a regular collimator in tube  1 , then bonding a filter with tube  2 ; 
       FIG. 8D  shows another method of manufacturing an assembly; and 
       FIG. 9  shows an AOI design based on filter characteristics curve (CWL v.s. AOI). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The detailed description of the present invention is presented largely in terms of procedures, steps, logic blocks, processing, or other symbolic representations that directly or indirectly resemble the operations of optical devices or systems that can be used in optical networks. These descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. 
   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. 
   Referring now to the drawings, in which like numerals refer to like parts throughout the several views.  FIG. 3  shows one device  300  according to one embodiment of the present invention. One of the features in the device  300  is that there are individual assemblies, each integrating a filter and a collimator. As shown in  FIG. 3 , the device  300  includes four such assemblies  302 ,  304 ,  306  and  308 . For each channel, a filter and a collimator are bonded together, forming an assembly. More may be added depending on applications or the number of wavelengths or channels to be multiplexed or demultiplexed. 
   In operation, a multiplexed signal or light beam is coupled into the device  300  via a common collimator  310 . A collimated beam from the collimator  310  bounces in a zigzag fashion between the assemblies  302 ,  304 ,  306  and  308  to demultiplexe the multiplexed signal into individual channel signal. 
     FIG. 4  shows an exemplary configuration of a filter-collimator assembly  400  that may be used in  FIG. 3 . A filter  402  is attached to a tube  404  that encapsulates a collimator  406  and a fiber pigtail  408 . In one embodiment, a glass cylindrical tube encapsulates a fiber and a collimation lens, both coaxially aligned. A fiber pigtail is also inserted to face the collimation lens. A gap between the pigtail and the lens is adjusted so as to collimate properly a light beam from the fiber pigtail. The collimated beam then propagates to a filter attached to or bonded with the tube. Generally, the contact surface of the tube with the filter is well polished and at the right angle for a preferred beam AOI to the thin film filter. 
     FIG. 4  demonstrates one type of filter-collimator assembly, in which one link tube is used. The AOI is fixed and determined by the polished angle at the tube contact face. One of the advantages of this type is its inherent stability. It may be mostly suitable for applications without accurate wavelength requirement, such as CWDM. 
     FIG. 5  shows another exemplary configuration of a filter-collimator assembly  500  that may be used in  FIG. 3 . The assembly  500  features two link tubes  502  and  504 . The tube  502  holds a pigtail  506  while the tube  504  holds a lens  508  and a filter  510 . By laterally adjusting the offset between the tube  502  and the tube  504 , the beam exiting from the collimation lens can be tilted to a preferred angle. The contact surface of the tube  504  with the filter  510  is polished to a preferred angle. This polish angle, together with the laterally adjusting achieved via the tube  502 , determines the AOI to the filter. In one respect, a direct angle rotation in the prior arts can be done by an indirect offset adjustment. The angle tuning relation is: Δθ=Δx/F, where Δθ, Δx, and F represents an angle change, a lateral offset, and a lens focal length. For example, F=2.5 mm, then a 4 um lateral offset results in 0.1° AOI change, corresponding to 0.03 nm wavelength control accuracy. Practically, the lateral adjustment of 4 um is easier than direct rotation of 0.10. In operation, after all the adjustments are done, the tubes  502  and  504  are bonded together, for example, using a type of epoxy. 
   Referring now to  FIG. 6A  and  FIG. 6B , there show respectively two assemblies that each use two link tubes. As shown in  FIG. 6A , a gap between a pigtail  602  and a lens  604  is fully encapsulated in tube  1 . The pigtail  602 , the lens  604 , and tube  1  form a collimator except that the lens head is exposed or exceeds the length of the tube  1 . The tube  2  and the filter are integrated and can receive the exposed portion of the lens  604 . One of the advantages of this assembly is that the two tubes may rotate against each other. With rotating tube  2  around the mechanical axis of tube  1 , the normal of the filter also rotates but in a 3-dimension space before they are bonded together via the lens  604 .  FIG. 6A  and  FIG. 6B  show two special positions. For the position illustrated in  FIG. 6A , the filter normal is at the lowest, while for  FIG. 6B , the normal is at the highest. Consequently, the beam AOI: θ=α−β and θ=α+β, where α is the angle between the tube axis and the filter normal, and β is the angle between the tube axis and the beam exiting the lens. 
   Operationally, the angle tuning range is 2β, where the angle β is a characteristic angle for a regular collimator, often referred to as a pointing angle. It is determined by the pigtail and the lens polish angle at their joint as well as the pigtail fiber position, etc. Usually a pointing angle is around 0.5°, resulting in 1° AOI adjustment range, corresponding to about 1 nm wavelength range. It should be noted that 1° AOI change is obtained through rotating tube  2  by 360°. On average, the angular adjustment amplification factor is 360, which essentially makes the wavelength adjustment 360 times easier. 
   Referring now to  FIG. 7 , there shows an assembly  700  with three link tubes. Each of the tubes may be rotated individually. The beam AOI on the filter  702  can be adjusted either by a lateral offset between tube  1  and tube  2 , or rotation of tube  3  around tube  2 . 
     FIG. 4 ,  FIG. 5 ,  FIG. 6  and  FIG. 7  shows respectively some exemplary assemblies. Different assemblies may vary from their building processes.  FIG. 8A  shows the processes for the assembly of  FIG. 4 , which may be considered as two independent phases: regular collimator building and filter attachment. For the collimator building, first fixing a lens  802  in a link tube  804  with a certain amount of selected epoxy or other bonding means, then putting a pigtail  806  into the tube  804  from the flat side of the tube  804 , adjusting the position of pigtail  806 , especially the air gap between the pigtail  806  and the lens  802  to minimize the coupling loss from a reference collimator placed at the specified distance to the collimator under building, finally bonding the pigtail  806  with the tube  804  using the epoxy or other bonding means. Once the collimator building is done, bonding a filter  808  to the tube end at the lens side to form an assembly  810 . 
   The building processes for the assembly of  FIG. 5  is relatively more complicated than that of  FIG. 4 , and is shown in  FIG. 8B , first, bonding a pigtail  822  to a tube  824 , a lens  826  and a filter  828  to a tube  830 , adjusting the two tubes  824  and  830  for a lateral offset for a specified wavelength. The axial gap between the pigtail  822  and the lens  826  shall be kept for a minimum loss from a reference collimator to the collimator under building. The adjustment may be cyclic. Once the wavelength and loss fall into an acceptable range, bond the two tubes  824  and  830 . 
   As far as the building of the assembly in  FIG. 6A  or  FIG. 6B  is concerned, there are two methods according to one embodiment.  FIG. 8C  shows one method, first building a regular collimator in tube  1 , then bonding a filter with tube  2 . The tube  2  is placed to receive the extended portion of the lens beyond the tube  1  and rotated for a specified wavelength. After that, the tube  1  and tube  2  are boned with a type of epoxy. In this method, the collimator coupling loss is tuned with two collimation lenses. This works fine when the thin film filter is thin enough so that it can be treated as a flat plate, i.e., a radius of curvature is close to infinity, for example, in CWDM case. In reality, the actual coating, due to stress and tension, in the central part is slightly thicker than that in the outer part, resulting in a convex shape. Such a curvature is equivalent to a convex lens, which affects the beam propagation behavior, hence the coupling loss. Sometimes, it may be noted that there are more layers that a coating has, the more serious this effect may demonstrate. For 100 GHz DWDM filters, the lens effect may not be ignored. 
   To minimize such an effect,  FIG. 8D  shows another method of manufacturing an assembly. First a lens is bonded with tube  1 , then a filter is attached to tube  2 . Tube  2  is then placed to receive the extended portion of the lens from tube  1 , without fixation. After that a pigtail is inserted into tube  1 , following the standard collimator building processes to minimize the coupling loss and fix the pigtail, tube  2  is rotated to tune for a specified wavelength. Tube  1  is then bonded to tube  2 . 
   Once the filter-collimator assemblies have been built, they are mounted as needed to a common substrate as illustrated in  FIG. 2A . Since a wavelength has been pre-tuned, unlike the free-space WDM devices in prior art which need adjustment of both wavelength and loss on a substrate, only loss minimization is required. 
   In one embodiment, the devices contemplated in accordance with the present invention works very well for small filter incidence angle (AOI) ranging 0°˜60°. According to the interference filter theory, the passband shape (including bandwidth, ripple, steepness, etc) and the band central wavelength (CWL) are well formed. 
   For CWDM application, a certain range of CWL shift is manageable as the bandwidth is wide enough to tolerate. For DWDM case, the CWL shift is handled by two methods: (1) special coating design to obtain the right CWL; (2) special AOI design to shift current commercially available filters to an adjacent channel. In one embodiment, the AOI design is based on filter characteristics curve (CWL v.s. AOI) as demonstrated in  FIG. 9 . 
   While the present invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claim. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.