Patent Publication Number: US-2004052475-A1

Title: Fiber collimator and method of manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a fiber collimator and a method of manufacturing the same. The fiber collimator includes a holder having a spacer provided therein, an aspherical lens, and a fiber pigtail. The aspherical lens and the fiber pigtail are separately fitly inserted into the holder to abut against and fix to two end surfaces of the spacer. Different machining tolerances of the spacer make finished fiber collimators have different optimal working distances. The finished fiber collimators may be then graded according to the working distances thereof.  
       [0002] In optical communication, it is often necessary to expand and collimate beams transmitted via an optical fiber, and let such expanded and collimated beams pass some functional elements before they are focused and coupled back to another optical fiber for subsequent transmission via the optical fiber. A fiber collimator is an optical element having the above-mentioned function. A fiber collimator system usually includes two mating fiber collimators. When a transmitted beam having a fixed beam divergence angle passes through a collimating lens of a first one of the two fiber collimators to be collimated and then moves across a working distance defined in the system, it is focused and coupled by the other fiber collimator back to anothe optical fiber. Various kinds of functional elements may be provided within the working distance. An optimal working distance for a fiber collimator system is a distance between two fiber collimators that allows a collimated beam between the two fiber collimators to maintain a required collimation while a lowest possible insertion loss is maintained for the system.  
       [0003]FIG. 1 shows a conventional fiber collimator  10  that includes a glass tube  11  having a smooth inner bore, a fiber pigtail  12  having an outer diameter the same as an inner diameter of the glass tube  11  for fixedly mounting in the glass tube  11  to locate an optical fiber  13  therein, and a graded-index lens (GRIN lens)  14  for collimating the beam transmitted via the optical fiber  13  or coupling the collimated beam back to another optical fiber. The glass tube  11  is enclosed with a stainless steel holder  15  to facilitate subsequent bonding, including laser welding, soldering, etc. To reduce the insertion loss of the fiber collimators during assembling of the fiber collimators, it usually needs to conduct a real-time adjustment and calibration of a position of the fiber pigtail  12  relative to the graded-index lens  14 , so that an output beam could be best collimated within the working distance. That is, the output beam may have a minimal beam divergence angle and a minimal deflection angle. The above-described conventional fiber collimator  10  using graded-index lens  14 , or the method for producing it, such as disclosed in U.S. Pat. No. 6,168,319 B1 entitled System and Method for Aligning Optical Fiber Collimators, has the following disadvantages in the application and the manufacturing process thereof:  
       [0004] 1. The graded-index lens requires highly difficult manufacturing techniques and could not be easily molded at reduced manufacturing cost.  
       [0005] 2. Once a graded-index lens having a particular length is selected, the working distance for the fiber collimator system using the graded-index lens is determined, too. Thus, in manufacturing a fiber collimator system, it is necessary to decide beforehand the working distance depending on the types of functional elements that are to be included in the system, and then decide the length of the graded-index lens. As a result, various lengths must be prepared for different graded-index lenses to increase a lot of troubles in production management of fiber collimator systems.  
       [0006] 3. To reduce the insertion loss of the fiber collimators during assembling of the fiber collimators, it usually needs to conduct a real-time adjustment and calibration of a position of the fiber pigtail  12  relative to the graded-index lens  14 , so that the output beam could be best collimated within the working distance. However, each time of optical adjustment and calibration involves alignment and adjustment of freedom of five axes X, Y, Z, θ, and Φ. The calibration is extremely complicate to increase the manufacturing cost of the fiber collimator.  
       [0007] 4. For an optical element that requires a long working distance, it is unable to maintain an insertion loss less than 0.15 dB for the graded-index type fiber collimator. Accordingly, the graded-index type fiber collimator has a reduced optical performance and is not suitable for optical elements having a long working distance from, for example, 100 mm to 140 mm, such as many multi-port optical devices, including, for example, optical circulator, optical interleaver, optical switch, etc. Thus, the graded-index type of fiber collimator is not qualified in terms of its performance.  
       [0008] An aspherical lens is functionally similar to the graded-index lens, and is able to convert a beam emitted from a point, such as from a fiber tip, within an effective focal length (EFL) f into a collimated beam. However, when an aspherical lens is directly used to replace the graded-index lens adopted in the conventional graded-index type fiber collimator, there are still many very complicate optical adjusting and calibrating operations involved in the manufacturing process of the fiber collimator. Up to date, there is not any fiber collimator using aspherical lens being found in the market to provide satisfactory performance. It is therefore tried by the inventor to develop an improved fiber collimator to eliminate the drawbacks existed in the conventional fiber collimators.  
       SUMMARY OF THE INVENTION  
       [0009] A primary object of the present invention is to provide a fiber collimator, and a method of manufacturing the fiber collimator.  
       [0010] The fiber collimator manufactured in the method of the present invention mainly includes a holder having a spacer provided therein, an aspherical lens, and a fiber pigtail. The spacer has a design thickness T (that is, a length of the spacer measured in an axial direction of the holder) taking a machining tolerance thereof into consideration to be equal to or larger than an effective focal length (EFL) f of the aspherical lens, with a difference between T and f no more than 30 μm. The fiber pigtail and the aspherical lens have an outer diameter the same as an inner diameter of the holder for separately inserting into two ends of the holder to abut against and fix to two end surfaces of the spacer. Since the spacer has a thickness varies with changes in its machining tolerance, a focus-out distance (Δd=d 1 −f, wherein d 1  is the actual distance from the fiber tip to the aspherical lens) of the fiber tip always randomly falls in a fixed range from 30 μm≧Δd≧0, and a finished fiber collimator always has an optimal working distance randomly falling in an enlarged range from 0 to 140 mm. The finished fiber collimator is screened and graded according to different working distances, so that users may easily select a desired fiber collimator depending on functional devices to be included in the optical transmission. A fiber collimator system established with the method of the present invention has a larger range of working distance from 0 to 140 mm and may therefore be applied to elements requiring long working distances while an insertion loss lower than 0.15 dB can be maintained for the collimator system to provide an enhanced optical performance. The present invention does not require complicate optical adjustment and calibration procedures and can therefore be manufactured with simplified process and at largely reduced cost. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein  
     [0012]FIG. 1 is a sectional view of a conventional fiber collimator;  
     [0013]FIG. 2 illustrates the optical properties of an aspherical lens;  
     [0014]FIG. 3 is a flowchart showing steps included in the method for manufacturing the fiber collimator of the present invention;  
     [0015]FIG. 4 is an exploded sectional view of a fiber collimator manufactured in the method of the present invention; and  
     [0016]FIG. 5 is an assembled sectional view of the fiber collimator of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0017] Please refer to FIG. 2 that shows the optical properties of an aspherical lens  20 . The aspherical lens  20  shown in FIG. 2 is an optimized aspherical lens having an extremely small aberration and an effective focal length f. When a Gaussian beam having a beam waist of ω 1  emitted from a fiber tip travels over a length d 1 , which is a distance from the fiber tip to the aspherical lens  20 , to pass through the aspherical lens  20  and be focused at d 2 , an optimal working distance for the current fiber collimator system is 2d 2 , and the beam now has a beam waist or a spot radius ω 2 . For a single-mode fiber, it is possible to derive a formula to reflect a relation among ω 2  and ω 1 , f, Δd (Δd=d 1 −f). From the above-mentioned optical properties, it can be known when an aspherical lens is used in place of the graded-index lens, it is possible to analyze various focusing conditions of the beam having passed through the aspherical lens  20  by way of changing the focus-out distance Δd of the fiber tip. The analysis results are shown as below:  
     [0018] Case 1: When d 1 =f, there is a Δd=0. In this case, the focus is located at a point away from the lens  20  by a distance of f, the collimated light has a maximal spot radius or spot size and a minimal beam divergence angle, and the optimal working distance is 2f.  
     [0019] Case 2: When d 1 &gt;f, there is a Δd&gt;0. That is, the fiber tip is moved toward a left side of FIG. 2. At this point, the focus gradually moves away from the lens  20  to locate beyond the distance f. That is, the optimal working distance gradually increases, the spot size gradually reduces, and the beam divergence angle gradually increases.  
     [0020] Case 3: When d 1 &gt;&gt;f, there is a Δd&gt;&gt;0. That is, the fiber tip is moved toward the left side of FIG. 2 to locate beyond a certain distance. At this point, the focus is located at a point between the lens  20  and the distance f, the spot size is very small, and the beam divergence angle is very big.  
     [0021] Case 4: When d 1 &lt;f, there is a Δd&lt;0. That is, the fiber tip is moved rightward toward the aspherical lens  20 . At this point, the collimated light does not focus but diverges directly, with a virtual focus thereof located in front of the aspherical lens  20 . That is, d 2 &lt;0. And, there is not an optimal working distance.  
     [0022] By substituting the optical properties of a single-mode fiber for the above formulas, it can be known further that when the focus-out distance Δd (Δd=d 1 −f) of the fiber tip is within the range from 5 to 60 μm, the optimal working distance available is within the range from 0 to 150 mm. At this point, the beam divergence angle is always smaller than 0.0025°.  
     [0023] The insertion loss of a fiber collimator system is resulted from misalignment and unmatched spot size between two Gaussian beams output from two collimators. Therefore, in the above-mentioned case 1 to case 3, given that the lens  20  has an aperture quite larger than a spot size of an input Gaussian beam, so long as the working distances of the two fiber collimators are adjusted to the optimal working distance 2d 2 , it is possible for the insertion loss of the collimators due to unmatched spot size to close to zero.  
     [0024] As shown in FIGS. 3, 4 and  5 , the present invention is designed according to the above-described principle by using the optical properties of an aspherical lens in a fiber collimator. More particularly, the present invention is designed according to the principle that the optimal working distance 2d 2  of a fiber collimator system can be controlled through effective control of the focus-out distance Δd of the fiber tip.  
     [0025] Please refer to FIG. 4. The fiber collimator of the present invention mainly includes an holder  30  having a spacer  31  provided in an inner bore thereof, an aspherical lens  40 , and a fiber pigtail  51 . The fiber collimator of the present invention is manufactured through the following steps:  
     [0026] Preparing a holder  30  having a spacer  31  fixed in an inner bore thereof; a machining tolerance for the spacer  31  during manufacturing thereof is taken into consideration, so that the spacer  31  has a design thickness T equal to or larger than the effective focal length (EFL) f of an aspherical lens to be used, and a portion of the thickness T that exceeds the EFL f is preferably controlled to be less than 30 μm;  
     [0027] Providing an optical fiber  50  having a fiber pigtail  51  attached to a fiber tip thereof; the fiber pigtail  51  has an outer diameter the same as an inner diameter of the holder  30  for inserting into the holder  30  to abut against a first end surface  32  of the spacer  31  and be fixed thereto;  
     [0028] Providing an aspherical lens  40  having an outer diameter the same as the inner diameter of the holder  30  for inserting into the holder  30  to abut against a second end surface  33  of the spacer  31  and be fixed thereto; and  
     [0029] Examining the effective focal length f of the aspherical lens using light wavelength.  
     [0030] In the fiber collimator obtained from the above steps, since the thickness T of the spacer  31  varies with changes in the machining tolerance of the spacer  31 , it is possible for the focus-out distance (Δd=d 1 −f, wherein d 1  is the distance from the fiber tip  52  to the aspherical lens  40 ) of the fiber tip  52  to be always controlled within a fixed range of 30 μm≧Δd≧0. This enables control of the optimal working distance for each fiber collimator to be always within the range from 0 mm to 140 mm. Therefore, different working distances for all finished fiber collimators can be detected using appropriate optical instruments. The range of working distance from 0 mm to 140 mm may then divided into several grades with, for example, every 20 mm as a grade, and the produced fiber collimators may be screened and graded based on these grades to be easily selected for use with various optical elements requiring different working distances.  
     [0031] The aspherical lens  40  may be a molding aspherical glass lens of approximating to zero aberration. That is, the aberration of the aspherical lens  40  approximates to zero (&lt;0.025λ at λ=0.6328 μm) after being compensated with an aspherical high-order coefficient. The holder  30  may be made of a stainless steel material or a glass material, and the spacer  31  may be integrally formed with the holder  30 . Since the thickness T of the spacer  31  is controlled to be equal to or larger than the EFL f of the aspherical lens  40 , and the portion of the thickness T larger-than the EFL f is preferable smaller than 30 μm, the value of T may be designed to be T=(f+15 μm)±15 μm, given the machining tolerance is within the range from 5 to 15 μm.  
     [0032] The fiber pigtail  51  and the aspherical lens  40  may be fixed to the first and the second end surfaces  32 ,  33 , respectively, of the spacer  31  with, for example, UV glue. Moreover, the holder  30  is provided at a lengthwise surface within the length of the spacer  31  with an air vent  34  of predetermined dimensions, so that the holder  30  is maintained at an internal pressure similar to an ambient pressure to provide an enhanced reliability of environmental factors.  
     [0033] In brief, in the fiber collimator system of the present invention, the working distance may have an enlarged range from 0 mm to 140 mm through effective control of the spacer thickness T, and the insertion loss may be maintained at less than 0.15 dB. Moreover, the present invention does not require optical adjustment and calibration in the process of actual assembling thereof. It needs only to be screened and graded based on different working distances resulted from different machining tolerances of the spacer when the present invention is subjected to examination of optical beam profile at a rear stage of the production. Therefore, the present invention may be manufactured at largely reduced cost without the need of conducting technically difficult and time and labor consuming optical adjustment and calibration. On the other hand, the present invention provides upgraded optical performance and may be applied to optical elements requiring long working distance, such as multi-port optical devices, including optical circulator, optical interleaver, optical switch, etc.  
     [0034] The present invention provides at least the following advantages:  
     [0035] 1. It does not require complicate optical adjusting and calibrating procedures. In view that changes in the optimal working distance may be obtained from different machining tolerances, the present invention needs only to be tested for its optical performance, including the insertion loss, the reflection loss, and the optimal working distance, after a passive alignment procedure thereof.  
     [0036] 2. In order to obtain the optimal optical properties at different working distances, including the minimal beam divergence angle, the lowest insertion loss, the minimal deflection angle, and the lowest reflected light, it is necessary in the conventional graded-index lens technique to conduct real-time adjustment and calibration of the position of the fiber pigtail relative to the graded-index lens. However, in the present invention that uses an aspherical lens, such complicate adjustment and calibration are not necessary because an error between the relative position of the fiber pigtail and the aspherical lens results in only changes in the optimal working distance, but not any increase of non-compensated insertion loss.  
     [0037] 3. The present invention employs the property of machining and does not require real-time adjustment and calibration to manufacture fiber collimators of different working distances.  
     [0038] The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications in the described embodiment can be carried out without departing from the scope and the spirit of the invention as defined by the appended claims.