Abstract:
Optical interleavers and de-interleavers that can be used to combine and separate optical signals are described. These devices are based on Mach-Zehnder interferometers having one or two Gires-Toumois mirrors. The optical designs disclosed herein include several key improvements over prior arts. These improvements of interleavers enabled lower cross talk and more accurate channel center frequencies. The new designs also improve on the stability of the devices and substantially reduce the manufacturing costs. With the usage of multi-fiber collimators, a pair of interleaver and de-interleaver is fabricated using the same optical block thereby further reducing the manufacturing costs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of optical communication and more particularly to interleaver and de-interleaver devices used in dense wavelength division multiplexing (DWDM) applications. 
     2. Background Art 
     Optical communication has been an active area of development and is crucial to the enhancement of several key technological advancements, e.g., Internet and related new technologies. An important technology that enabled a higher data transmission rate is the dense wavelength division multiplexing (DWDM) technology. In many DWDM applications, there is a need to filter a group of signal channels such that these channels can be further separated, redirected to a different direction, or a fraction of these channels be dropped and added. In certain applications, interleavers and de-interleavers are used to bridge technologies with different channel spacing, enabling the usage of more economical solutions associated with larger channel spacing. In FIG. 1, an interleaver design ( 100 ) based upon a Gires-Toumois (GT) mirror ( 166 ,  168 ) and a Michelson interferometer is displayed. These interleavers separate a composite input optical signal ( 102 ) into two complementary signals in which the odd data channels are branched into one output ( 112 ) and the even channels are directed back into the input ( 102 ). In an interleaver application, the frequency space is divided into two groups of pass bands, one for output  1  and the other for output  2 . Dingel and Izutsu described this prior art interleaver in a publication (Optics Letters, Jul. 15, 1998, vol 23, pages 1099-1101) and later in a U.S. Patent (U.S. Pat. No. 6,304,689 B1, Oct. 16, 2001). These documents are therefore incorporated herein by reference as relevant background materials. Another improved prior art interleaver is illustrated in FIG.  2 . In this device, the input signal ( 202 ) is coupled to a 50% non-polarizing cubic beam splitter ( 220 ) trough a collimating lens such as a graded index lens (GRIN) lens ( 208 ). A GT mirror ( 266 ,  268 ) and a regular mirror ( 248 ) are used to form the interferometer. The odd channels return to one output fiber ( 212 ) through another lens ( 218 ) whereas the even channels return to the input fiber ( 202 ) through a lens ( 208 ). This type of interleavers and related devices has been disclosed in recent U.S. Patents (U.S. Pat No. 6,169,626 issued Jan. 2, 2001, and U.S. Pat. No. 6,275,322 issued Aug. 14, 2001). These patents are also incorporated herein by reference as relevant background materials. In FIG. 3, another prior art interleaver ( 300 ) based on a polarization beam splitter (PBS) and two GT mirrors is displayed. This prior art device has been disclosed recently in U.S. Pat. No. 6,169,604 issued on Jan. 2, 2001 to Cao and U.S. Pat. No. 6,310,690 B1 issued on Oct. 30, 2001 to Cao and Mao. These patents are therefore incorporated herein by reference as relevant background materials. In this prior art device, the input signal ( 302 ) is coupled to a PBS ( 320 ) through a collimating lens ( 308 ). The two arms of the device are two interferometers, one for each of the polarization components. For each interferometer, a polarization and phase-modified GT mirror ( 332 - 348 ) is used as two mirrors of the interferometer. The phases and Free Spectra Ranges (FSR) of the GT mirrors are modified/adjusted using waveplates  332  and  334 . The relative phases of the two paths of each of the interferometers are adjusted by changing the orientations and thickness of the waveplates  332  and  334 . Both interferometers are adjusted such that the odd channels return to one output fiber ( 304 ) through lens ( 308 ) whereas the even channels return to the other fiber ( 312 ) through another lens ( 318 ). 
     Another related prior art of a GT mirror has been disclosed in a pending U.S. patent application Ser. No. 09/796,565 filed on Mar. 2, 2001 by Qian. This patent is also incorporated herein by reference as relevant background material. As illustrated in FIG. 4, The GT mirror consists of optically contacted front and rear windows, with a precision, temperature insensitive spacer. The sealed cavity is filled with optical medium at certain density such that precise center frequency may be achieved Another cavity may be added in front of the GT such that the relative phase of the GT may be adjusted. 
     There are several areas of improvements of prior art devices  100  through  300 . For instance, the use of a Michelson interferometer with one output returning to the same direction in  100  and  200  requires the use of an optical circulator in the optical “circuit” in order to physically separate the output from the input. Another area of improvements is in the temperature stability of the devices. Device  100  is not based on a balanced design and will require temperature stabilization whereas devices  200  and  300  uses thin glass plates/wave plates for fine adjustments of the interferometers and these thin plates introduces reliability issues such as the use of epoxy and certain temperature related drifts. There is a need therefore for improved art such that more stable and reliable interleaver devices can be fabricated. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a group of new optical designs of interleavers and de-interleavers. These new designs are based on Mach-Zehnder interferometers with one or two GT mirrors. The GT mirrors used in these designs have tunable/adjustable FSR and phases. Tuning and adjustment of FSR and phases of these improved devices are accomplished by changing the densities of the optical medium in air-spaced cavities. Another embodiment of the present invention utilizes multi-fiber pigtails and collimators in interleaver and de-interleaver devices. This embodiment enabled device integration such that two or more interleavers can be constructed using the same optical block. Another usage of interleaver and de-interleaver devices with multi-fiber pigtails/collimators is in the area of multi-channel add/drop applications. In this case, each interleaver functions as a pair of interleaver and de-interleaver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of preferred embodiments when taken in conjunction with the following drawings in which: 
     FIG. 1 shows a prior art interleaver consists of a Michelson interferometer with a GT mirror and a regular mirror; 
     FIG. 2 illustrates another prior art interleaver based on a Michelson interferometer with a GT mirror and a regular mirror; 
     FIG. 3 displays an optical interleaver formed with a PBS and two waveplate-modified GT mirrors; 
     FIG. 4 displays a prior art GT mirror consists of a coated window, a mirror and a precision spacer; 
     FIG. 5 illustrates an optical interleaver consists of a Mach-Zehnder interferometer with a GT mirror and a regular mirror in accordance with an embodiment of the present invention; 
     FIG. 6 illustrates an alternate optical interleaver consists of a Mach-Zehnder interferometer with a GT mirror and a regular mirror in accordance with an embodiment of the present invention; 
     FIG. 7 displays an optical interleaver consists of a Mach-Zehnder interferometer with two GT mirrors in accordance with an embodiment of the present invention; 
     FIG. 8 shows the input and output light beam directions in an optical add-drop interleaver where the odd channels can be dropped and added in accordance with an embodiment of the present invention; 
     FIG. 9 illustrates the input and output light beam directions in a pair of optical add-drop interleavers using the same optical block where the odd channels can be dropped and added in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention discloses a set of new designs of optical interleaver and de-interleaver. These new designs share several key improvements over prior art devices. The first class of improvements is related to the method and arrangement to tune the devices to ITU frequency grid and relative phases of the two arms of an interferometer. The second group of improvements is based on the usage of multi-fiber pigtails and collimators. 
     One preferred embodiment of the present invention is illustrated in FIG.  5 . One input ( 502 ) and one output ( 504 ) are coupled to the device through a dual fiber pigtail ( 506 ) and a collimating lens ( 508 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 508 ) are typically AR-coated to reduce the insertion loss of the device. 
     A second input ( 512 ) and second output ( 514 ) are coupled to the device through a second dual fiber pigtail ( 516 ) and a second collimating lens ( 518 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 518 ) are typically AR-coated to reduce the insertion loss of the device. 
     Center to the main optical block is a broadband beam splitter ( 520 ), the beam splitter consists of two pieces of optical glasses with an interface ( 522 ) at the diagonal of the cube. The interface has a polarization insensitive coating with 50% reflection and 50% transmission. The entrance and the exit surfaces of the central optical block are covered with two glass wedges ( 524 ). Normally the outside surface of the wedges is AR-coated to reduce the insertion loss of the device. 
     The interleaver consists of two mirrors attached to the central beam splitter. One of the mirrors is a GT-mirror with a partial reflective surface (R˜14%) ( 566 ) and a high reflective surface (R˜100%) ( 568 ). The GT mirror follows the prior art of Qian as disclosed in a pending U.S. patent application Ser. No. 09/796,565 filed on Mar. 2, 2001 by Qian. The GT mirror consists of a front window ( 560 ) and a back window ( 564 ) and a precision spacer ( 562 ) made of low expansion glass such as ULE® of Coming or Zerodur® of Schott. The requirement of the interleaver, the incident angle of the beam, and the optical thickness of the optical coatings determine the thickness of the spacer. For an interleaver used to separate input signal with 50 GHz channel spacing to two outputs with 100 GHz channel spacing, the thickness is about 3 mm. The front window, the air spacer and the back window are attached together through optical contacting process to forms a hermetically sealed cavity. There is a small channel ( 567 ) on the back window that is used to change the density of optical medium inside of the cavity. Once the desired optical density is achieved, the cavity is sealed off with another piece of glass ( 569 ). 
     The other mirror is a regular mirror with reflectivity close to 100%. An air spaced cavity is constructed with a front window ( 540 ), a precision spacer ( 542 ), and a back window ( 544 ). The precision spacer ( 542 ) is made of low expansion glass such as ULE® of Corning or Zerodur® of Schott. The requirement of the interleaver, the incident angle of the beam, and the optical thickness of the optical coatings determine the thickness of the spacer. For an interleaver used to separate input signal with 50 GHz channel spacing to two outputs with 100 GHz channel spacing, the thickness of the spacer is about 1.5 mm. The front window, the air spacer and the back window are attached together through optical contacting process to form a hermetically sealed cavity. There is a small channel ( 567 ) on the back window that is used to change the density of optical medium inside of the cavity. Once the desired optical density is achieved, the cavity may be sealed off with another piece of glass (not shown). The inside surface of the front window normally has an anti-reflective coating ( 546 ). 
     All the interfaces within the interleaver device may be attached together through optical contacting process. Such a process is frequently used to achieve strong and hermetically sealed interface bonding. 
     Another preferred embodiment of the present invention is illustrated in FIG.  6 . One input ( 602 ) and one output ( 604 ) are coupled to the device through a dual fiber pigtail ( 606 ) and a collimating lens ( 608 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 608 ) are typically AR-coated to reduce the insertion loss of the device. 
     A second input ( 612 ) and second output ( 614 ) are coupled to the device through a second dual fiber pigtail ( 616 ) and a second collimating lens ( 618 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 618 ) are typically AR-coated to reduce the insertion loss of the device. 
     Center to the main optical block is a broadband beam splitter ( 620 ), the beam splitter consists of two pieces of optical glasses with an interface ( 622 ) at the diagonal of the cube. The interface has a polarization insensitive coating with 50% reflection and 50% transmission. The entrance and the exit surfaces of the central optical block are covered with two glass wedges ( 624 ). Normally the outside surface of the wedges is AR-coated to reduce the insertion loss of the device. 
     The interleaver consists of two mirrors attached to the central beam splitter. One of the mirrors is a GT-mirror with a partial reflective surface (R˜14%) ( 646 ) and a high reflective surface (R˜100%) ( 648 ). The GT mirror follows the prior art of Qian as disclosed in a pending U.S. patent application Ser. No. 09/796,565 filed on Mar. 2, 2001 by Qian. The GT mirror consists of a front window ( 640 ) and a back window ( 644 ) and a precision spacer ( 642 ) made of low expansion glass such as ULE® of Corning or Zerodur® of Schott. The requirement of the interleaver, the incident angle of the beam, and the optical thickness of the optical coatings determine the thickness of the spacer. For an interleaver used to separate input signal with 50 GHz channel spacing to two outputs with 100 GHz channel spacing, the thickness is about 3 mm. The front window, the air spacer and the back window are attached together through optical contacting process to form a hermetically sealed cavity. There is a small channel ( 647 ) on the back window that is used to change the density of optical medium inside of the cavity. Once the desired optical density is achieved, the cavity is sealed off with another piece of glass ( 649 ). 
     In order to adjust the phase of the GT mirror, another air cavity is constructed through the use of a spacer ( 634 ). There is a small channel ( 632 ) used to access this cavity such that the density of the optical medium inside may be changed. The cavity may be sealed off with another piece of glass (not shown). In order to reduce the insertion loss of the device, the surface of the front window ( 640 ) and the surface of the beam splitter inside of the air cavity is normally AR coated. The thickness of the spacer ( 632 ) is not critical and is in the range of 1 to 10 mm. The spacer is preferably made of low expansion glass such as ULE® of Corning or Zerodur® of Schott. The GT mirror, the spacer ( 632 ), and the beam splitter are attached together through optical contacting process to forms a hermetically sealed cavity. 
     The other mirror is a regular mirror with reflectivity close to 100%. An air spaced cavity is constructed with a front window ( 660 ), a precision spacer ( 662 ), and a back window ( 664 ). The spacer ( 662 ) is made of low expansion glass such as ULE® of Corning and Zerodur® of Schott. The requirement of the interleaver, the incident angle of the beam, and the optical thickness of the optical coatings determine the thickness of the spacer. For an interleaver used to separate input signal with 50 GHz channel spacing to two outputs with 100 GHz channel spacing, the thickness of the spacer is about 1.5 mm plus that of the thickness of the spacer ( 632 ) in front of the GT mirror. The front window, the air spacer and the back window are attached together through optical contacting process to forms a hermetically sealed cavity. There is a small channel ( 667 ) on the back window that is used to change the density of optical medium inside the cavity. Once the desired optical density is achieved, the cavity may be sealed off with another piece of glass (not shown). The inside surface of the front window normally has an anti-reflective coating ( 666 ). 
     All the interfaces within the interleaver device may be attached together through optical contacting process. Such a process is frequently used to achieve strong and hermetically sealed interface bonding. 
     An additional preferred embodiment of the present invention is illustrated in FIG.  7 . One input ( 702 ) and one output ( 704 ) are coupled to the device through a dual fiber pigtail ( 706 ) and a collimating lens ( 708 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 708 ) are typically AR-coated to reduce the insertion loss of the device. 
     A second input ( 712 ) and second output ( 714 ) are coupled to the device through a second dual fiber pigtail ( 716 ) and a second collimating lens ( 718 ). The fiber pigtail has a polished and AR-coated output surface. This output surface is not perpendicular to the direction of the fibers and is set to have a few degrees of angle from the normal in order to reduce the return loss of the device. The surfaces of the lens ( 718 ) are typically AR-coated to reduce the insertion loss of the device. 
     Center to the main optical block is a broadband beam splitter ( 720 ), the beam splitter consists of two pieces of optical glasses with an interface ( 722 ) at the diagonal of the cube. The interface has a polarization insensitive coating with 50% reflection and 50% transmission. The entrance and the exit surfaces of the central optical block are covered with two glass wedges ( 724 ). Normally the outside surface of the wedges is AR-coated to reduce the insertion loss of the device. 
     The interleaver consists of two GT mirrors attached to the central beam splitter. Both GT mirrors have a partial reflective surface (80%&gt;R&gt;5%) ( 746 ,  766 ) and a high reflective surface (R˜100%) ( 748 ,  768 ). These GT mirrors follow the prior art of Qian as disclosed in a pending U.S. patent application Ser. No. 09/796,565 filed on Mar. 2, 2001 by Qian. The GT mirrors consist of a front window ( 740 ,  760 ) and a back window ( 744 ,  764 ) and a precision spacer ( 742 ,  762 ) made of low expansion glass such as ULE® of Corning or Zerodur® of Schott. The requirement of the interleaver, the incident angle of the beam, and the optical thickness of the optical coatings determine the thickness of the spacer. For an interleaver used to separate input signal with 50 GHz channel spacing to two outputs with 100 GHz channel spacing, the thickness is about 1.5 mm. The front window, the air spacer and the back window are attached together through optical contacting process to form a hermetically sealed cavity. For each of the GT mirrors, there is a small channel ( 747 ,  767 ) on the back window that is used to change the density of optical medium inside the cavity. Once the desired optical density is achieved, the cavity is sealed off with another piece of glass ( 749 ,  769 ). 
     In order to adjust the phase of the GT mirrors, two air cavities are constructed through the use of two spacers ( 734 ,  754 ). There is a small channel ( 732 ) used to access one of the cavities such that the density of the optical medium inside may be changed. The cavity may be sealed off with another piece of glass (not shown). In order to reduce the insertion loss of the device, the surfaces of front windows ( 740 ,  760 ) and the surfaces of the beam splitter inside of the air cavities are normally AR coated. The thickness of the two spacers is identical and ( 734 ,  754 ) is not critical to the performance of the device. Typically the thickness is in the range of 1 to 10 mm. The spacer is preferably made of low expansion glass such as ULE® of Corning or Zerodur® of Schott. The GT mirrors, the spacers ( 734 ,  754 ), and the beam splitter are attached together in accordance with the present invention, as illustrated in FIG. 7, through optical contacting process to forms a hermetically sealed cavity. 
     All the interfaces within the interleaver device may be attached together through optical contacting process. Such process is frequently used to achieve strong and hermetically sealed interface bonding. 
     The present invention also discloses a general improvement over prior art interleavers as illustrated in FIG.  8  and FIG.  9 . In FIG. 8, an add-drop interleaver device is illustrated. The add-drop interleaver consists of an interleaver optical block, two dual fiber collimators (or equivalently, 4 single fiber collimators), and the necessary packaging of the interleaver. The central block may be one of the embodiments disclosed in the preceding paragraphs ( 500 ,  600 , or  700 ), or alternately, one of the prior art central blocks (e.g.,  100 ,  200 , or  300 ). The four fiber add-drop device works in the following way: The input signal ( 820 ) with a certain channel spacing (e.g., 50 GHz) is separated into two outputs ( 830  and  832 ) with twice of the channel spacing (i.e., 100 GHz) with the reflection ( 830 ) carrying the even channels and the transmission ( 832 ) carrying the odd channels. This input ( 820 ) in combination with the two outputs ( 830 ,  832 ) function as a prior art interleaver. By symmetry, the other input signal ( 822 ) with a certain channel spacing (e.g., 50 GHz), is separated into two outputs ( 832  and  830 ) with twice of the channel spacing (i.e., 100 GHz) with the reflection ( 832 ) carrying the even channels and the transmission ( 830 ) carrying the odd channels. This input ( 822 ) in combination with the two outputs ( 832 ,  830 ) also function as a prior art interleaver. 
     The combined four-fiber device as illustrated in FIG. 8 has unique functions. One of the functions is to interchange half of the signal channels (the odd channels) between the two inputs ( 820 ,  822 ). The other application is a group add-drop application with  820  as the signal input,  832  as the drop,  822  as the add input, and  830  as the output. In this case, half of the signal channels (the odd channels) are dropped to  832 . The signals associated with even channels from a different source can be added through  822 . 
     The functions of the four-fiber device disclosed here can be duplicated if four fiber pigtails/collimators are used instead of two fiber pigtails/collimators. The directions of optical signals are indicated in FIG. 9 for such an eight-fiber device. The add-drop interleaver consists of an interleaver optical block, two four-fiber collimators (or equivalently, 8 single fiber collimators), and the necessary packaging of the interleaver. The central block may be one of the embodiments disclosed in the preceding paragraphs ( 500 ,  600 , or  700 ), or alternately, one of the prior art central blocks (e.g.,  100 ,  200 , or  300 ). The eight-fiber add-drop device works in the following way: The input signal ( 920 ) with a certain channel spacing (e.g., 50 GHz) is separated into two outputs ( 930  and  932 ) with twice of the channel spacing (i.e., 100 GHz) with the reflection ( 930 ) carrying the even channels and the transmission ( 932 ) carrying the odd channels. This input ( 920 ) in combination with the two outputs ( 930 ,  932 ) function as a prior art interleaver. By symmetry, the other input signal ( 922 ) with a certain channel spacing (e.g., 50 GHz), is separated into two outputs ( 932  and  930 ) with twice of the channel spacing (i.e., 100 GHz) with the reflection ( 932 ) carrying the even channels and the transmission ( 930 ) carrying the odd channels. This input ( 922 ) in combination with the two outputs ( 932 ,  930 ) also function as a prior art interleaver. 
     The combined eight-fiber device as illustrated in FIG. 9 has unique functions. One of the functions is to interchange half of the signal channels (the odd channels) between the two inputs ( 920 ,  922 ). The other application is a group add-drop application with  920  as the signal input,  932  as the drop,  922  as the add input, and  930  as the output. In this case, half of the signal channels (the odd channels) are dropped to  932 . The signals associated with even channels from a different source can be added through  922 . 
     By symmetry, inputs  940 ,  942  and outputs  950  and  952  form another add-drop interleaver that functions identical with that of the add-drop interleaver formed by  920 ,  930 ,  922  and  932  as described in the proceeding paragraphs. 
     It will be apparent to those with ordinary skill of the art that many variations and modifications can be made to these interleaver devices disclosed herein without departing form the spirit and scope of the present invention. It is therefore intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.