This invention relates generally to a method and system for use in optical fiber technology. More particularly, this invention relates to a method and system for manufacturing an improved wavelength division multiplexed coupler.
In optical fiber technology, wavelength division multiplexed (WDM) couplers are used to combine or separate optical signals having different wavelengths. As the WDM couples are being more broadly applied in the telecommunications, data communications and CATV industries, the fiber optic component industry is now confronted with increasing requirements for WDM couplers with higher level of performance and reliability as well as lower cost.
The performance and reliability of the WDM couplers depend heavily on their design and packaging technologies. Currently, two major kinds of design and packaging technologies are being widely employed in manufacturing the WDM couplers and each kind has its own advantages and disadvantages. In applying a first kind of technology for designing and packaging the WDM couplers, all optical parts are bonded together by applying epoxy bonding. The applications of this first type of WDM couplers show potential reliability risk of epoxy bonding in long-term operation.
FIG. 1A shows the structure of a typical WDM coupler manufactured according to the first kind of design and packaging technology based on epoxy bonding. The WDM coupler includes a dual fiber pigtail 25, a GRIN lens 35, a WDM filter 40, a GRIN lens 50, and a single fiber pigtail 60. In a typical manufacturing process, the GRIN lens 35, the WDM filter 40 and the GRIN lens 50 are first fixed together by applying a heat-curing epoxy 45. The relative position of the GRIN lens 35 to the fiber pigtail 25 is adjusted to achieve a lowest transmission loss from the input fiber 15 to the output fiber 20 for optical signals having reflection wavelengths. Then the dual fiber pigtail 25 is fixed to the GRIN lens 35 by applying a heat-curing epoxy 30. Then the relative position of the GRIN lens 50 to the fiber pigtail 60 is adjusted to achieve a lowest transmission loss from the input fiber 15 to the output fiber 65 for optical signals having transmission wavelengths. And then, the single fiber pigtail 60 is fixed to the GRIN lens 50 by applying a heat-curing epoxy 55. The conventional method and system provides the WDM couplers with good performance and reliability suitable for many types of applications. However, the WDM couplers manufactured according to the conventional method and system have a risk of failure when they are applied in high power optical transmission systems. In general, the heat-curing epoxies inevitably spread over all the optical paths in the WDM couplers. More specifically, the heat-curing epoxies 30, 45 and 55 spread over the optical paths between the dual fiber pigtail 25 and the GRIN lens 35, between the GRIN lenses 35, 50 and the WDM filter 40 and between the GRIN lens 50 and the single fiber pigtail 60, respectively. Under long-term operation, the epoxies 30, 45 and 55 when exposed to the transmitted optical signals may gradually become degraded and susceptible to damages and thus lead to unreliable performance after continuously absorbing the optical signal energy. In the typical WDM coupler, the diameter of the optical signal beam is changing from about 10 m at the epoxy 30 to about 450 m at the epoxy 45 to about 10 m at the epoxy 55. Thus, the optical signal power densities at the epoxies 30 and 55 are about 2500 times higher than that at the epoxy 45. Therefore, the risk for high optical power damage is significantly higher at the epoxies 30 and 55 than at the epoxy 45. The difficulties are specially pronounced for transmission of optical signals of high power. Because of the heat absorption problem, many optical system designers and operators now prefer or even demand to have all optical paths of the WDM couplers epoxy-free. Due to the significantly high power density and thus reliability risk, as the first step toward all epoxy-free optical paths, the optical system designers and operators now require not to use any epoxy on the optical paths between the GRIN lenses and the fiber pigtails. However, by applying the conventional WDM method and system, this epoxy-free optical path requirement can not be easily achieved. Thus, further development of reliable fiber optic components with high level of performance and reliability is limited by these difficulties.
In a pending patent application as shown by FIG. 1B, entitled xe2x80x9cImproved Wavelength Division Multiplexed Coupler xe2x80x9d, filed recently by the present inventor, improvements are achieved for the reliability of the WDM couplers in long-term high-power operation. In the pending application, epoxies are prevented to spread over or diffused into the optical paths between the GRIN lenses 110xe2x80x2160xe2x80x2 and the fiber pigtails 135xe2x80x2175xe2x80x2 by employing several holding tubes 120xe2x80x2130xe2x80x2 and 165xe2x80x2. As a result, the optical paths between the GRIN lenses and the fiber pigtails are epoxy-free. The improved WDM couplers have significantly reduced risk of high optical power damage. Therefore, the improved WDM couplers can be employed in fiber optic components for broadened applications with being much less limited by the reliability problems of the WDM couplers as that encountered in the prior art. Since production costs have been being an important factor in practical implementation of fiber optic technologies, it is highly desirable that production costs would be as low as possible. However, in the pending application, a special single fiber pigtail 175xe2x80x2 formed by cutting off one of two fibers of a high-concentricity dual fiber pigtail as that shown by FIG. 1B, is employed. The purpose of the use of the special single fiber pigtail is to obtain the same optical signal outgoing orientation of the single fiber collimator as that of the dual fiber collimator. A difficult arises when the optical signal outgoing orientation of the single fiber collimator is different from that of the dual fiber collimator. Specifically, when aligning the outgoing orientation to achieve a lowest transmission loss, a poor contact between the end surfaces of the holding tubes 120xe2x80x2 and 165xe2x80x2 will be configured because the holding tubes 120xe2x80x2 and 165xe2x80x2 must be slightly slanted to adjust for the difference of outgoing orientation angles. Thus the reliability of epoxy bonding between the single and dual collimators is degraded. A specially configured single fiber pigtail is used to resolve this difficulty by cutting off one of the two fibers to form a single fiber pigtail to achieve the same outgoing orientation as a dual fiber pigtail. However, as the high-concentricity dual fiber pigtails are much more expensive compared to standard single fiber pigtails, a high cost is paid for providing this improvement for the WDM couplers. Thus, further development of the WDM couplers must be engaged to lower the improvement cost when it is still limited by these difficulties.
Therefore, a need still exists in the art of design and manufacturing of the WDM couplers to provide improved material compositions, device structure, and manufacturing processes to overcome the difficulties discussed above. Specifically, a technique to provide the WDM couplers with epoxy-free optical paths between the GRIN lenses and the fiber pigtails at lower cost is required.
It is therefore an object of the present invention to provide an improved design and process for fabricating a WDM coupler with improved reliability at lower cost. While the epoxies are prevented to spread over or diffused into the optical paths between the GRIN lenses and the fiber pigtails, the low-cost standard single fiber pigtails are used to replace the expensive special single fiber pigtails. Therefore, the aforementioned difficulties and limitations in the pending application can be overcome.
Specifically, it is an object of the present invention to provide a design and process to fix all optical parts of the WDM couplers together by applying heat-curing epoxies. While several holding tubes are used between the GRIN lenses and the fiber pigtails to prevent the heat-curing epoxies from spreading over or diffused to the optical paths between the GRIN lenses and the fiber pigtails, the low-cost standard single fiber pigtails are employed. A requirement to implement the more expensive special single fiber pigtails is therefore eliminated. As a result, according to the new method and system of the WDM couplers of this invention, the costs are reduced while the optical paths between the GRIN lenses and the fiber pigtails are epoxy-free. The WDM couplers produced according to the presently improved design and process have significantly reduced risk of high optical power damage as well as lower cost. Therefore, the WDM couplers of this invention can be employed in fiber optic components for broadened applications with being much less limited by the reliability and cost problems of the WDM couplers as those encountered in the prior arts.
Briefly, in a preferred embodiment, the present invention discloses a WDM coupler. The WDM coupler includes a WDM filter attached to a first GRIN lens by applying a first heat-curing epoxy. The WDM coupler further includes a first holding tube for holding the first GRIN lens. The first GRIN lens is inserted and fixed in the first holding tube by applying a second heat-curing epoxy. The WDM coupler further includes a second holding tube holding a dual fiber pigtail. The dual fiber pigtail is disposed at a first optimal position from the first GRIN lens to achieve a lowest reflection loss with the first and second holding tubes being in contact with each other. The dual fiber pigtail and the first and second holding tubes are fixed together by applying a third heat-curing epoxy. The WDM coupler further includes a second GRIN lens inserted and fixed into a third holding tube by applying a fourth heat-curing epoxy. The WDM coupler further includes a fourth holding tube holding a standard single fiber pigtail. To achieve a lowest transmission loss, the single fiber pigtail is disposed at a second optimal position from the second GRIN lens while the first GRIN lens is disposed at a third optimal position from the second GRIN lens. With the first, third and fourth holding tubes being in contact with each other, a fifth heat-curing epoxy is applied to fix the third and fourth holding tubes together and a sixth heat-curing epoxy is applied to fix the first and third holding tubes together.
The present invention further discloses a method for fabricating a WDM coupler. The method includes the steps of: a) attaching a WDM filter to a first GRIN lens by applying a first heat-curing epoxy; b) inserting and fixing the first GRIN lens with the WDM filter into a first holding tube having a length slightly longer than the combined length of the first GRIN lens and the WDM filter by applying a second heat-curing epoxy; c) inserting a dual fiber pigtail into a second holding tube then adjusting a relative position between the dual fiber pigtail and the first GRIN lens on an alignment stage to achieve a lowest reflection loss; d) sliding the second holding tube along the dual fiber pigtail without moving the dual fiber pigtail until the first and second holding tubes are in contact then fixing the first and second holding tubes and the dual fiber pigtail together by applying a third heat-curing epoxy; e) inserting and fixing a second GRIN lens into a third holding tube having a length slightly longer than that of the second GRIN lens by applying a fourth heat-curing epoxy; f) inserting a single fiber pigtail into a fourth holding tube; g) mounting the first holding tube with the first GRIN lens and the WDM filter, the third holding tube with the second GRIN lens and the fourth holding tube with the single fiber pigtail on an alignment stage then adjusting relative positions of the first GRIN lens to the second GRIN lens and the single fiber pigtail to the second GRIN lens until a lowest transmission loss is achieved with the first, third and fourth holding tubes being in contact with each other; and h) fixing the third and fourth holding tubes together by applying a fifth heat-curing epoxy and then fixing the first and third holding tubes together by applying a sixth heat-curing epoxy.