MXPA99008781A - Mach-zehnder optical fiber interferometer manufactured with asimetri couplers - Google Patents

Abstract

An asymmetric optical fiber Mach-Zehnder interferometer has first and second optical fibers connected to a first coupling row and a second coupling region. The first and second optical fibers. To form the asymmetry, the propagation constant in a portion of the first fiber optic in one of the regions differs from the propagation constant in a portion of the second optical fiber in such a coupling region. The asymmetric structure provides insulation of 30 dB over a longer distance of on

Description

MACH-ZEHNDER OPTICAL FIBER INTERFEROMETER MANUFACTURED WITH ASYMMETRICAL COUPLERS CROSS REFERENCE TO RELATED REQUESTS This application claims the benefit under 35 U.S.C. § 119 (e) of the Application of U.S. No. 60/101, 592, filed on September 24, 1998, the disclosure of which is incorporated herein by reference.

DECLARATION WITH RESPECT TO DEVELOPMENT OR RESEARCH SPONSORED FEDERALLY N / A BACKGROUND OF THE INVENTION A wavelength division multiplexer / demultiplexer can be fabricated from a fiber optic Mach-Zehnder interferometer (IMZ). The IMZ is manufactured from a pair of symmetrical couplers. The Bragg fiber grids are printed on the interference arms between the couplers. A Bragg fiber grating (RFB) is a change in the refractive index in the core of the fiber that reflects a selective wavelength over the fiber. In the operation of the IMZ to exclude or extract a wavelength, for example, as a demultiplexer, a signal carrying several channels or wavelengths? 1,? 2,? 3,? 4 and? 5 enters the first fiber of the first coupler. Other channel numbers, for example, 4 or 8, can enter the coupler. The signal is divided into the coupler to pass along both arms. The RFBs, which are identical, are resonant at a selected frequency, for example? 4. Therefore, in the RFBs, 4 is reflected, passes back through the first coupler, and is extracted from the second fiber of the first coupler. The remaining wavelengths,? 1,? 2,? 3 and? 5, pass through the second coupler and exit in the second fiber of the second coupler. In the operation to add or insert a wavelength, for example, as a multiplexer, a signal having the wavelength 4 4 is inserted on the first fiber in the second coupler. A signal of several wavelengths? 1,? 2,? 3 and? 5, enters the first fiber of the first coupler. As described above with respect to the demultiplexer,? 4 is reflected in the RFBs, then? 4 leaves the second fiber of the second coupler. Therefore, the output of the second coupler includes all wavelengths? 1,? 2,? 3,? 4 and? 5. In typical Mach-Zehnder interferometers manufactured from symmetrical couplers, 30 dB isolation is limited to distances of +/- 20 nm around the desired wavelength.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides an optical fiber Mach-Zehnder interferometer manufactured from asymmetric couplers, which allows 30 dB of isolation over a wavelength range greater than the desired wavelength. More particularly, the optical fiber Mach-Zehnder interferometer of the present invention provides first and second optical fibers connected to a first coupling region and a second coupling region. The optical fibers first and second optical fibers form two interference arms between the first and second coupling regions. To form the asymmetry, the propagation constant in a portion of the first optical fiber in one of the coupling regions, eg, the first coupling region, differs from the propagation constant in a portion of the second optical fiber in such a coupling region. The propagation constant in the first optical fiber and the propagation constant in the second optical fiber are chosen to provide 30 dB isolation in a pass port of the Mach-Zehnder interferometer over a distance of more than +/- 20 nm, and preferably more than +/- 60 nm, around a desired wavelength. The division ratio of the first coupling region is controlled to divide the power 50 percent at a desired wavelength.

DESCRIPTION OF THE DRAWINGS The description will be fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic illustration of a Mach-Zehnder interferometer according to the present invention; Figure 2 is a graph of the measured spectral response of a symmetric coupler of the state of the art; Figure 3 is a graph of the measured spectral response of the additional fiber in a Mach-Zehnder interferometer of the state of the art fabricated from symmetric couplers; Figure 4 is a graph of the measured spectral response of an asymmetric coupler used in the present invention; Figure 5 is a graph of the measured spectral response of a Mach-Zehnder interferometer according to the present invention; Figure 6 is a schematic illustration of a symmetric point of a Mach-Zehnder interferometer according to the present invention; Figures 7A-7E are graphs illustrating the operation of a symmetric point of the Mach-Zehnder interferometer according to the present invention; Figure 8 is a schematic illustration of a symmetric line of the Mach-Zehnder interferometer according to the present invention; and Figures 9A-9E are graphs illustrating the operation of a symmetric line of the Mach-Zehnder interferometer according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates an asymmetric Mach-Zehnder interferometer (IMZ) 10 according to the present invention. The IMZ comprises two coupling regions 12, 14 separated by the interference arms 16, 18. More particularly, in the polished embodiment, the IMZ is formed from the first and second optical fibers 20, 22 which are coupled in the first and second couplers 24, 26. For the purposes of the present invention, a coupler is a device that divides the incoming optical energy between two output fibers. The first and second fibers 20, 22 form the interference arms 46, 18 connecting the couplers 24, 26. A Bragg fiber grid (RFB) 25, 27 is printed on the core 28, 30 of each of the arms of interference 16, 18 between the two couplers 24, 26. In operation, on one side of the IMZ, one fiber, such as fiber 20, constitutes an inlet or insert port 32, and another fiber, fiber 22, constitutes an extraction port. or exclusion 34. On the other side of the IMZ, the fiber 20 constitutes a port of addition or insertion 36, and the fiber 22 constitutes a port of passage or exit 38.

In the present invention, the asymmetric couplers, as well as the location of the fibers improves the spectral performance of the IMZ. In an asymmetrical coupler, the electric field in the coupling ports delays the electric field at the input port by a phase of p / 2. However, the same phase relationship is not retained for the asymmetric coupler. In an asymmetric coupler, the phase difference between the fields depends on the coupling resistance and the degree of asymmetry. In an asymmetric coupler, the constituent fibers have propagation constants in the coupling region such that the coupling ratio can be 100%. In the manufacture of the Mach-Zehnder interferometers, the splitting ratio of the constituent couplers is substantially 50%. The coupling ratio of an asymmetric coupler goes like the sine of the wavelength. The 50% point occurs in the quadrant of the response wavelength and is consequently sensitive to small changes in wavelength. Figure 2 illustrates a figure of the measured spectral response of a typical symmetric coupler. The excess loss of the coupler was approximately 0.07 dB. The nails are artifacts for measuring the mode of movement in the FP laser. From the graph, it can be seen that the variation of the wavelength in the region of interest (around the quadrant) is approximately 0.125% / nm. This variation of the wavelength is that which reduces the IMZ insulation to below 30 dB for distances greater than +/- 20 nm around the central wavelength. Figure 3 shows the measured spectral response of the addition fiber in an IMZ manufactured from typical symmetric couplers. From the graph, it can be seen that the 30 dB isolation can be reached in the addition fiber over a wavelength range of 40 nm, which is a narrow wavelength range. The limited distance of the insulation region of 30 dB comes from the sensitivity to the wavelength of the couplers used to make the IMZ.

In an asymmetric coupler, the propagation constants of the fibers are not identical in the coupling region. Thus, the maximum coupling ratio achievable is less than 100%. The division ratio of an asymmetric coupler also goes like the sine of the wavelength. However, the maximum division ratio can be controlled. Consequently, in an asymmetric coupler, the 50% point can substantially occur at the maximum of the response wavelength and is consequently less sensitive to changes in wavelength. Figure 4 is a graph of the measured spectral response of a typical asymmetric coupler. The excess loss of the coupler was approximately 0.09 dB. The nails are artifacts of measure! Movement mode in the FP laser. From the graph it can be seen that the variation in wavelength in the region of interest is approximately 0.03% / nm. This variation of reduced wavelength is what allows 30dB insulation in the pass-through fiber to be achieved over distances greater than +/- 60 nm around the central wavelength. This is substantially greater than the distance of +/- 20 nm achievable with the symmetric coupler discussed above. The most common method of fabrication of the coupler is the method of fused taper. For example, two fibers are stripped from their shirts by an appropriate length. The fibers are fastened together with the regions to be coupled maintained substantially together and in contact. The contact region is heated to a temperature that is approximately 1700 ° C until they melt together. The fibers can be heated with any suitable heating means, such as an electric arc, oxygen butane flame, or laser. While they are heated, the fibers also stretch or stretch to create a low or narrow waist or narrow region. The tapering causes the fiber cores to be closer together, increasing the interaction between the signals.

The asymmetry can be achieved by tapering, etching or polishing previously one of the fibers coupled in the coupling region, or by tilting the coupling region in the plane of the fiber. The asymmetry can also be achieved by melting one of the fibers to a third tube or glass rod in the coupling region. The tube or glass rod that reaches the fiber propagation constant is attached to the coupling region. A combination of these techniques can also be used. The degree of asymmetry and resistance to coupling is controlled in such a way that the maximum splitting ratio of the coupler is substantially 50%. Figure 5 is a measured response of the fiber added to an IMZ manufactured using asymmetric couplers. The peaks are measurement artifacts. From the graph, it can be seen that 30 dB isolation can be achieved in the addition, or exclusion, fiber over a wavelength range of 120 nm, which is substantially greater than the 40 nm range achievable by the asymmetric IMZ of the state of the art described above. After the two couplers were formed, the Bragg fiber grids are printed on the fiber cores in the two interference arms. Bragg fiber grids can be printed in any known manner. For example, using a photosensitive fiber, a phase mask of silica having slots there recorded with the desired periodicity is placed near the fiber. The fiber and mask are illuminated with a source of ultraviolet (UV) light, such as a laser beam. The phase mask diffracts the light coming from the UV light source, creating multiple interference beams. The UV light passes through the photosensitive fiber, altering the distribution of the refractive index and forming an RFB. In another technique to form the RFB, a UV laser beam travels along the length of the fiber and turns on and off while traveling. Other techniques can be used to record the RFB, such as interference holograms, interference laser beams, or amplitude masks. Two modalities are preferred in the present invention, a symmetric point IMZ and a symmetric line IMZ. Both modalities can be used to make the IMZ and both can be used in conjunction with the Bragg fiber grids. Referring to Figure 6, the symmetric point IMZ is one that is substantially symmetric around the point 40 in the middle of the arms of the interferometer. The fiber 20 in the coupling region 12 has a first propagation constant, preferably, the propagation constant of a standard fiber. The fiber 22 in the coupling region 12 is modified to have a different propagation constant. The fiber 22 in the coupling region 14 has the first or standard propagation constant. The fiber 20 in the coupling region 14 is modified to have a different propagation constant. The operation of the symmetric point IMZ is described as follows, with reference to Figures 7A-7E (the couplers are described in terms of transfer matrices and all typical variables are given with typical values): ?? to: = 7.48? a: = 03.665 ll: = 10000 12: = 10000 ?? b: = ?? a? b: =? a ál: = li-12? a- ^? 1 B-? i Ca (?): =? t •? l = 0 ßo (?): = 2-- -1.458 Cb (?): = p ?? a Fa ?? b Fb Ca (1.5) = 1.174 cos (Ca (?)) i-fa-i-sin (Ca (?)) i-Fa-sin (Ca (?)) Leg4: = E?: = Ma (?): = iFasin (Ca (?) ) cos (Ca (?)) - fa-isin (Ca (?)) cos (Cb (?)) - f b-i'SÍn (Cb (?)) iFbsin (Cb (?)) Leg2: = Mb ( ?): = Fbsin (Cb (?)) Cos (Cb (?)) - t-fb-i-sin (Cb (?)) OPa (?): = (| Ma (?) IPLeg21) 2 OPb < ? ): = (| b (?) -IPLeg21) 2 OPaC 1.5) = 0.49, ißo. { ?) ll 0 Mf (?): = ¡• Po (?) - 12 OP4 (?): = (| Mb (?) Mf (?) -Ma (?) IP ee4 |) 2 Il ??): = 101og (l- OP4 (?)) Referring to Figure 8, a symmetric line IMZ is one that is symmetrically around a line 42 through the interference arms. One fiber, such as the fiber 20, has a first propagation constant, preferably the propagation constant of a standard fiber, in both, the coupling region 12 and the coupling region 14. The other fiber, the fiber 22, is modified to having a propagation constant different in both, to the coupling region 12 and the coupling region 14. The operation of the symmetric line IMZ is described as follows, with reference to Figures 9A-9E (the couplers are described in terms of matrices transfer and all the variables are given with typical values): ?? a: = 7.48? a. '= 03.665 11: = 10000 12: = 10000.32265 ?? b: = ?? a? B: =? A? 1: = U-I2 Ca (i.5) = 1.174 cos (Ca (?)) - r-fa-isin (Ca (?)) i-Fasin (Ca (?)) roí ri Ma (?): = Leg4 GP: = iFasin (Ca (?)) cos (Ca (?)) - faisin (Ca (?)) LU L cos (Cb (?)) -? - fbisin (Cb (?)) i-Fbsin (Cb (?)) Mb (?): = i-Fbsin (Cb (?)) cos (Cb (?)) - fbisin (Cb (?)) ^ 2: = [o] OPa (?): = (| Ma (?) IP-Leg2 |) 2 OPb (?): = (| Mb (?) IPLeg2¡) 2 OPa (1.5) = 0.49 i-Po (?) 42 OP4 (?): = (| Mb (?) - Mf (?) - Ma (?) - IP-Leg4 |) 2 H): = 101og (1 - OP4 (?)) In the equations above and in Figures 7A-7E and Figures 9A-9E, the notation follows that of AW Snyder and J.D. Love in Optical Waveguide Theory, published by Chapman and Hall. Note that in the symmetric line mode, the path lengths are not equal, indicated by I2: = 10000.32265 and? I1 = - 0.323. As mentioned above, a number of alternate embodiments can be assembled for the present invention. For example, the coupler can be manufactured using polished or D-shaped fiber couplers. Again, it can be achieved by tapering, etching or polishing and one of the fibers in the coupling region, or by tilting the plane coupling region. of fiber. You can also use a combination of techniques. The fibers used to make the necessary couplers do not necessarily melt together as in the case of a polished block coupler.

For example, the interferometer may be made of one or more photosensitive fibers. Additionally, the coupler can be composed of more than two fibers. For example, the 1x3 coupler can be manufactured in which one or more of the fibers are photosensitive and the remaining fibers are photoinsensitive. Moreover, the division ratio can be different from 50%. For example, the maximum division ratio of the asymmetric couplers can be 40%. The IMZ can be composed of a symmetric and an asymmetric one. The division ratio and the maximum division ratio of both couplers are not necessarily the same. Additionally, the IMZ interference arms need to be not necessarily balanced. Consequently, the structure can be used for wavelength division multiplexers, with or without Bragg fiber grids. The IMZ can be constructed of three or more couplers, in which one or more of the couplers are asymmetric. In another modality, the IMZ interference arms are different. By controlling the difference, the output response wavelength could be controlled. The invention will not be limited by what has been particularly shown and described, except for the appended claims.

Claims (20)

1. CLAIMS 1. A fiber optic Mach-Zehnder interferometer comprising: first and second optical fibers connected to the first coupling region and the second coupling region, the first and second optical fibers additionally forming two interference arms between the first region coupling and the second coupling region; wherein a propagation constant in a portion of the first optical fiber in the first coupling region is different from a propagation constant in a portion of the second optical fiber in the first coupling region.
2. 2. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the first and second coupling regions comprise first and second taper, fused couplers.
3. 3. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein a splitting ratio of the first coupling region is controlled to divide the 50 percent power into a desired length.
4. 4. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the propagation constant in the first optical fiber and the propagation constant in the second optical fiber are chosen to provide 30 dB isolation in a port of the Mach-Zehnder interferometer over a distance of more than +/- 20 nm around a desired wavelength.
5. 5. The asymmetric optical fiber Mach-Zehnder interferometer of claim 4, wherein the propagation constant in the first optical fiber and the propagation constant in the second optical fiber are chosen to provide 30 dB insulation in an optical fiber hole. step of the Mach-Zehnder interferometer over a distance of more than +/- 60 nm around the desired wavelength.
6. 6. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein a propagation constant in the first optical fiber in the second coupling region is different from a propagation constant in a portion of the second optical fiber in the second coupling region.
7. 7. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the portion of the first optical fiber in the first coupling region has a first propagation constant; and the portion of the second optical fiber in the first coupling region has a propagation constant different from the first propagation constant.
8. 8. The asymmetric optical fiber Mach-Zehnder interferometer of claim 7, wherein: a further portion of the first optical fiber in the second coupling portion has the first propagation constant; and an additional portion of the second optical fiber in the second coupling region has a propagation constant different from the first propagation constant.
9. 9. The asymmetric optical fiber Mach-Zehnder interferometer of claim 7, wherein: a further portion of the first optical fiber in the second coupling region has a propagation constant different from the first propagation constant; and an additional portion of the second optical fiber in the second coupling region has the first propagation constant.
10. 10. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the propagation constant in the portion of the second optical fiber is provided by a tapered, taped, or pre-polished section of the second optical fiber.
11. 11. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the first and second optical fibers in the first coupling region are inclined in a fiber plane to provide different propagation constants.
12. 12. The asymmetric optical fiber Mach-Zehnder nterferometer of claim 1, wherein a Bragg fiber grid is printed on each of the interference arms.
13. 13. The asymmetric optical fiber Mach-Zehnder nterferometer of claim 1, wherein the first and second optical fibers comprise polished optical fibers.
14. 14. The asymmetric optical fiber Mach-Zehnder nterferometer of claim 1, wherein the first and second optical fibers comprise optical fibers having a D-shaped cross-section.
15. 15. The fiber optic Mach-Zehnder interferometer asymmetric of claim 1, further comprising a third optical fiber coupled to the first and second optical fibers in one of the first and second coupling regions.
16. 16. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein at least one of the first and second optical fibers comprises a photosensitive fiber.
17. 17. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the first and second optical fibers are coupled in a third coupling region.
18. 18. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the two interference arms are balanced.
19. 19. The asymmetric optical fiber Mach-Zehnder interferometer of claim 1, wherein the two interference arms are not balanced.
20. 20. A multiplexer / demultiplexer comprising the asymmetric optical fiber Mach-Zehnder interferometer of claim 1.