Abstract:
An integrated bi-directional transceiver device for multiple wavelength optical signals that has a high level of wavelength isolation at the receivers of the device and low cross-talk of light between an external laser transmitter and the receivers. A WDM planar light wave circuit (PLC) assembly combines high spatial light confinement waveguide structures and a variable thickness dielectric wavelength selective filter (WSF) on the surface of the device to reflect a first wavelength signal and to pass a second wavelength signal. Embodiments of the invention include branching waveguide structures and folded path waveguide assemblies with multiple WSF&#39;s.

Description:
FIELD OF THE INVENTION 
   This invention relates to bi-directional planar light circuit (PLC) transceiver devices that enable analog and digital media communication over a single fiber. 
   BACKGROUND OF THE INVENTION 
   Optical transceiver devices are well known in the art and are often found in applications where a bi-directional data stream is employed for the transmission of both video, in one direction, and data, in two directions, over a single mode fiber optic cable. A 1300 nm emitter and a 1300 nm receiver are generally used for data communication and a 1550 nm receiver for video information. 
   Typically, these devices have been produced using conventional bulk optical elements such as lenses, WDM filters, and beam splitters, built into the device, to separate the analog and digital transmission wavelengths and to direct the signals back and forth from the emitter and receivers to the input optical fiber carrying the transmission signal. Recently, a new generation of devices has been demonstrated using planar light circuit (PLC) technology. 
   A major problem with conventional bulk optic transceiver devices is scattering of light from the edges and surfaces of the various components, lenses, WDM filters, and beam splitters, leading to cross-talk, i.e. light from the external 1300 nm emitter leaking into the photodetector receivers, and poor wavelength isolation due to insufficient blockage of selective light by the dielectric filters. In addition, in order to maintain a high level of wavelength isolation i.e. &gt;25 dB, for example, the overall light polarization requires that incident light be close to a normal incidence angle at the surface of the dielectric filters. Any incident light at a different angle is leaked through the filter. 
   To circumvent the cross-talk and high wavelength isolation problems associated with bulk optic transceiver devices, prior art Japan Patent Office Publication number 11-068705 discloses a two-way optical transmission and reception module based on a PLC chip, in which signals at first and second (1550 nanometer and 1310 nanometer) wavelengths are separated from an input light beam using a WDM filter assembly placed in a slot in the PLC chip. The dielectric multi-layer filter, inserted into the slot, is positioned at the tee of a branching waveguide structure to obtain the aforementioned purpose. Japan Patent Office Publication number 11-248977 also discloses transceiver devices with filters inserted into waveguide structures. 
   The use of waveguide structures significantly reduces light scattering because of the light confinement characteristics of waveguides, resulting in essentially no cross-talk of light between the emitter and the receivers of the device. In addition, the combination of waveguide structures and dielectric filters inserted in the waveguide enhances the wavelength isolation properties of the device. 
   A major manufacturing problem with prior art PLC based transceiver apparatus is the required filter thickness tolerance, on the order of micrometers, necessary for positioning the dielectric filter in a slot created directly in the optical path of the waveguides of the device. In order for the transmitted light signal to cross the filter, from one waveguide into another, without a significant loss off light at the coupling interface, the thickness of the slot, typically 20 micrometers, needs to be carefully controlled, as small deviations in thickness can result in significant power attenuation. Similarly, the thickness of the filter, typically 14 micrometers, needs to be carefully controlled in order to not exceed the thickness requirement. As such, the handling of thin filters creates manufacturing difficulties. 
   In addition, the fabrication of a waveguide channel trench designed to receive the filter requires a relatively large surface area chip, increasing the size and cost of the apparatus. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention comprises an improved bi-directional optical transceiver device for multiple wavelength signals that has a high level of wavelength isolation at the receivers of the device and low cross-talk of light between an external laser transmitter and the receivers. The device uses a combination of high spatial light confinement waveguide structures and a variable thickness dielectric wavelength selective filter (WSF) in a WDM planar light wave circuit (PLC) assembly. 
   Cross-talk in the device is minimized by using a specific waveguide design and wavelength isolation is enhanced by one or more dielectric interference filters placed on the surface of the PLC. In addition, the PLC can be produced using a material that absorbs light selectively so that the material itself acts as a WSF. 
   Typically, in prior art devices, light passing through an internal filter is directed to a small single-mode waveguide. In the PLC of the invention however, light passing through the WSF is directly interfaced to a detector that is five to ten times larger than the aperture of a single mode waveguide, making the design more tolerant to beam expansion in the free-space path through the filter. 
   By placing one or more WSF&#39;s on the surface of the PLC, a waveguide channel trench for each filter is not required, and thicker filters can be used to reduce chip manufacturing costs. In addition, a dichroic or interference WSF can be advantageously placed on an edge of the PLC chip using a direct deposition process with a signal detector placed directly over the WSF. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art transceiver device; 
       FIG. 2  is an illustrative diagram of a PLC transceiver device in accordance with the principles of this invention; 
       FIGS. 3(   a ) and  3 ( b ) are schematic diagrams of an input signal assembly of the invention; 
       FIG. 4  is a schematic diagram of a V-groove assembly of the invention; 
       FIGS. 5(   a ) and  5 ( b ) are illustrative views of a PLC/WSF embodiment of the invention; 
       FIGS. 6(   a ) and  6 ( b ) are illustrative views of another embodiment of a PLC/WSF of the invention; 
       FIG. 7  is an illustrative view of a PLC of the invention with a folded optical beam path; 
       FIG. 8  is an illustrative view of a PLC of the invention having a multiple folded beam path configuration; and 
       FIG. 9  is an illustrative view of another embodiment of the invention with a multiple folded beam path configuration. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A typical prior art data and video transmission transceiver apparatus is illustrated in  FIG. 1 , where bi-directional wavelength optical communication signals can be sent and received simultaneously. In such apparatus, an optical fiber  10  transmitting upstream and downstream 1310 nm and 1550 nm signals is interfaced to the bi-directional transceiver device where a wavelength division multiplexer (WDM) filter assembly  11 , placed in a slot  12 , separates the incoming 1310 nm and 1550 nm signals and distributes them between a 1550 nm photodiode detector  13  and a 1310 nm photodiode detector  14 . Simultaneously, a 1310 nm laser diode  15  interfaced to the bi-directional transceiver device transmits 1310 nm signals upstream into the fiber. Clear separation of signals in the apparatus is a problem because of cross-talk between the 1310 nm signal from the built-in laser diode leaking into the 1310 nm and 1550 nm photodetectors, and because of poor wavelength isolation at each of the 1310 nm and 1550 nm photodetectors. 
   As stated above, manufacturing of these devices requires extremely narrow spatial tolerances (on the order of six microns) to machine the slot in the optical path of the PLC chip and to maintain the thickness dimension of the WDM filter. If the spatial and thickness tolerance requirements are not precisely met, the devices will not achieve performance specifications. 
   In one embodiment, of the invention, as shown in  FIG. 2 , a branching waveguide structure  27  is defined in a PLC chip  23 . 
   One branch of the waveguide structure  27  is coupled to an external photodiode detector  24  for detecting λ 1  wavelength signals. The other branch of the waveguide is coupled to an external laser diode  25 . 
   A WSF  22  on the surface of the chip  23  is adapted to reflect a band of signals centered around a λ 2  wavelength and to pass a band of signals centered around a λ 1  wavelength. 
   In operation, the input signal source  21  that includes light signals at both λ 1  and λ 2  wavelengths is coupled to the PLC so that the input signal light beam is directed toward the WSF  22  at a forty-five degree angle, for example, causing reflection of the λ 2  signal to an external detector  20 . Accordingly, only λ 1  signals enter the waveguide structure. 
   The detector  24  at one branch of the waveguide thus detects clear λ 1  signals with high extinction wavelength isolation from the λ 2  signal. Similarly, the detector  20  also exhibits high extinction wavelength isolation from λ 1  signals from the laser diode  25  and from incoming λ 1  signals in the fiber. 
   Coupling of the input signal light beam to the PLC chip can be conveniently accomplished by using a commercially available V-groove device for an optical fiber as shown in  FIGS. 3 and 4 . Other devices can also be used for coupling an input signal light beam including a fiber within a ferrule or a free-standing angle polished fiber. 
   With reference to  FIG. 3(   a ), the end face  32  of a V-groove device  31  holding an input signal fiber  30  is cut at a forty-five degree angle, and is polished with the polished edge abutted against the PLC chip to reflect signals to an external detector  33 . The detector  33  can be positioned if desired, along the top surface of the V-groove device as shown. 
   With reference to  FIG. 3(   b ), an input signal fiber  35  is shown placed inside a ferrule structure  37  to couple the fiber to a PLC chip. The ferrule can be made of glass, silicon, or a near-infrared transparent material. 
   With reference to  FIG. 4 , a V-groove device is shown where an input signal fiber  43  is captured in a V-groove shaped substrate  41  using an adhesive coating  42  to adhere a glass cover layer  40  over the fiber to the substrate  41 . The cover layer  40  is transparent to a selected wavelength. The V-groove itself can be defined in a silicon or glass substrate, as is well known in the art. 
   Another embodiment of the invention is shown in  FIGS. 5(   a ) and  5 ( b ) where an input signal optical fiber  50 , is set in a V-groove block structure  51 , and is interfaced to a PLC  52  that has an edge surface  58  cut at a forty-five degree angle. 
   A WSF  56 , configured to reflect signals at a first wavelength λ 1  and to pass signals at a second wavelength λ 2 , is positioned on the cut edge surface  58  of the PLC so that the first wavelength signal is reflected at a forty-five degree angle. 
   As shown in  FIGS. 5(   a ) and  5 ( b ), an external λ 1  detector photodiode  55 , is placed adjacent to an edge of the PLC to receive signals reflected by the WSF. 
   A waveguide assembly  57  directs the λ 2  signals from the input signal optical fiber that are passed through the WSF to an external photodiode detector  54  and directs λ 2  signals from an external laser diode signal source  53  through the WSF to the input signal optical fiber. 
   A dual path branching waveguide assembly is illustrated, although a single path waveguide assembly can be also be used. In addition, an external laser diode signal source is illustrated but it is understood that other types of signal sources can be used including, for example, an LED. 
   The PLC chip can be fabricated using an intrinsic material, designed to absorb specific wavelength signals. The combination of absorptive chip material and the WSF provides additional filtering so that any undesired signals that may not be reflected by the WSF can be effectively absorbed by the PLC chip material to prevent cross-talk. If the absorptive material is not present, a second WSF can be used to reflect unwanted signals from reaching the λ 2  detector. 
   With reference to  FIGS. 6(   a ) and  6 ( b ), another embodiment of the invention has an input signal optical fiber  60 , set in a V-groove block or ferrule structure  61 , interfaced to a PLC  62 . 
   A WSF  66 , configured to reflect signals at a first wavelength λ 1  and to pass signals at a second wavelength λ 2 , is positioned along the vertical axis of the PLC at a forty-five degree angle. An external λ 1  detector photodiode  65  is also positioned along the vertical axis of the PLC to receive the signals reflected by the WSF, as shown in  FIG. 6(   b ). 
   A PLC waveguide assembly  67  directs the λ 2  signals that are passed through the WSF to an external photodiode detector  64 , placed on an edge surface of the PLC. 
   The waveguide assembly also directs signals from an external λ 2  laser diode source  63  through the WSF to the input signal optical fiber. 
   A dual path branching waveguide assembly is illustrated, although a single path waveguide assembly can be also be used. In addition, an external laser diode signal source is illustrated but it is understood that other types of signal sources can be used including, for example, an LED. 
   As stated above, the PLC chip can be fabricated using an intrinsic material, designed to absorb specific wavelength signals, to prevent cross-talk. If the absorptive material is not present, a second WSF can be used to prevent unwanted signals from reaching the λ 2  detector. 
   In another embodiment of the invention, a folded beam path configuration that can reduce the size of a PLC chip is shown in  FIG. 7  with an input signal optical fiber  70  set in a V-groove block or ferrule structure  71  and interfaced to a PLC  72 . 
   A WSF  76 , configured to pass signals at a first wavelength λ 1  and to reflect signals at a second wavelength λ 2  is placed along a first edge surface of the PLC. An external detector photodiode  74  is positioned adjacent to said first edge surface to receive the λ 1  signals passed through the WSF. 
   A bi-directional waveguide assembly  77  directs the λ 2  signals reflected by the WSF to an external photodiode detector  75  positioned adjacent to a second edge surface of the PLC and directs the signals from an external laser diode  73  to the WSF for reflection to the λ 2  detector. 
   A dual path branching waveguide assembly is illustrated, although a single path waveguide assembly can be also be used. In addition, an external laser diode signal source is illustrated but it is understood that other types of signal sources can be used including, for example, an LED. 
   In another embodiment of the invention, a multiple wavelength folded beam path configuration is shown in  FIG. 8  where part of the beam is transmitted and part is reflected. Multiple folds can be configured, without cross-talk between waveguides, when the waveguide crossing angle is optimally ninety degrees. 
   With reference to  FIG. 8 , an input signal optical fiber  80 , set in a V-groove block or ferrule structure  81 , is interfaced to a PLC  82 . 
   A first PLC waveguide assembly  87  directs multiple wavelength signals λ 1 , λ 2 , and λ 3  from an optical fiber  80  to a first WSF  86 . The WSF  86  is placed on an edge surface of the PLC and is configured to pass λ 1  signals and to reflect λ 2  signals. An external photodiode detector  85  is positioned adjacent to the first WSF  86  to receive the λ 1  signals. 
   A second PLC waveguide assembly  88  directs the λ 2  signals, reflected by the first WSF  86 , to a second WSF  90  placed on an edge surface of the PLC. The second WSF is configured to pass λ 2  signals. 
   An external photodiode detector  84  is positioned adjacent to the second WSF  90  to receive the λ 2  signals directed by the second PLC waveguide assembly  88 . 
   A third PLC waveguide assembly  89  directs λ 3  signals from an external laser diode  83  to the second WSF  90  where the signal is reflected into the second PLC waveguide assembly  88  and subsequently into the first PLC waveguide assembly  87  where it is directed to the optical fiber  80 . 
   An external laser diode signal source is illustrated but it is understood that other types of signal sources can be used including, for example, an LED. 
   In another embodiment of the invention, a multiple-folded beam path configuration is shown in  FIG. 9 . 
   With reference to  FIG. 9 , an input signal optical fiber  91 , set in a V-groove block structure  92 , is interfaced to a PLC  93 . 
   A first PLC waveguide assembly  94  directs the multiple wavelength signals λ 1 , λ 2 , and λ 3  from the input signal optical fiber  91  to a first WSF  95 . The first WSF  95  is placed on a first edge surface of the PLC and is configured to pass λ 1  signals and to reflect λ 2  signals. An external photodiode detector  96  is positioned adjacent to the first WSF  95  to receive the λ 1  signals. 
   A second PLC waveguide assembly  97  directs the λ 2  signals reflected by the first WSF  95  to a second WSF  98 ; placed on a second edge of the PLC and configured to pass λ 2  signals. 
   An external photodiode detector  99  is positioned adjacent to the second WSF  98  to receive the λ 2  signals directed by the second PLC waveguide assembly  97 . 
   A third PLC waveguide assembly  100  directs λ 3  signals from an external laser diode  101  to the second WSF  98  where the signal is reflected into the second PLC waveguide assembly  97  and subsequently into the first PLC waveguide assembly  94  where it is directed to the optical fiber  91 . 
   An external laser diode signal source is illustrated but it is understood that other types of signal sources can be used including, for example, an LED. 
   Various waveguide structures in the PLC of the invention are shown but it is understood that other types of waveguide structures, including tapered waveguides, can be used to direct light from external signal sources and to distribute light signals within the PLC assembly, to improve PLC light coupling efficiency. 
   Although the various features of novelty that characterize the invention have been described in terms of certain preferred embodiments, other embodiments will become apparent to those of ordinary skill in the art, in view of the disclosure herein. Accordingly, the present invention is not limited by the recitation of the preferred embodiments, but is instead intended to be defined solely by reference to the appended claims.