Multiplexer and demultiplexer for single mode optical fiber communication links

Abstract

An optical multiplexer and demultiplexer for dense wavelength division multiplexed (“DWDM”) fiber optic communication systems is disclosed.. As a multiplexer, the device functions to spatially combine the optical signals from several laser sources (each of which is a different wavelength) and launch the spatially combined laser beams into a single optical fiber. As a demultiplexer, the device functions to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber. In either embodiment, the device includes both bulk optic and integrated optic components. The spatial separation or spatial combination of laser beams of different wavelength is achieved with the use of bulk diffraction gratings. Also, bulk optical components are used to collimate and shape (or steer) the free space propagating laser beams to enable efficient coupling of light into single mode optical fibers, or integrated optic waveguides, and to reduce optical cross talk. Polarizing beamsplitters orient the polarization direction of the light to enable maximum diffraction efficiency by the gratings and to reduce the polarization dependent loss. Further, the end faces of optical fibers and integrated optic waveguides are angle polished to reduce back reflection and thereby reduce noise caused by feedback to the laser source. Preferably, the diffraction grating and focusing optics are specified to permit multiplexing and demultiplexing of laser wavelengths separated by 0.4 nanometers (nm) in the 1550 nm wavelength band. The preferred field of view of the optics permit multiplexing and demultiplexing of up to 32-48 wavelength channels separated by 0.4 nanometers in the 1550 nm wavelength band. Although examples of performance are provided for the 1550 nm optical wavelength band, the device components can be designed for use at other wavelength bands, e.g., the optical fiber low absorption loss band at λ˜1310 nm.

What is claimed is:  
     
         1 . An optical fiber transmission apparatus, the apparatus comprising: 
 a. a plurality of laser sources for generating optical beams;    b. a first optical transmission fiber;    c. multiplexer means for spatially combining the optical beams from the laser sources, each of which is a different wavelength, and launching the spatially combined optical beams into the optical transmission fiber to form a wavelength division multiplexed optical signal, wherein the multiplexer means includes: 
 i. a diffraction grating;  
 ii. means for shaping the optical beams; and  
 iii. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized;  
   d. a plurality of second optical fibers; and    e. demultiplexer means for spatially separating the different wavelengths from the optical transmission fiber and launching each of the different wavelengths into separate second optical fibers, wherein the demultiplexer means includes: 
 i. a diffraction grating;  
 ii. means for shaping the optical beams; and  
 iii. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized.  
   
     
     
         2 . The apparatus of  claim 1 , wherein the first optical transmission fiber is a single mode optical fiber communication link.  
     
     
         3 . The apparatus of  claim 1 , wherein the means for adjusting the polarization direction of the optical beams includes means for rotating the polarization direction of either the p polarized beam or the s polarized beam.  
     
     
         4 . The apparatus of  claim 3 , wherein the means for rotating the polarization direction includes a polarizing beamsplitter and a half-wave plate.  
     
     
         5 . The apparatus of  claim 4 , wherein the polarizing beamsplitter and half-wave plate are a monolithic structure comprised of: 
 a. a right angle prism;    b. a beam displacement prism; and    c. a half-wave plate.    
     
     
         6 . The apparatus of  claim 1 , wherein the means for shaping the optical signals includes a prism.  
     
     
         7 . A bi-directional optical apparatus, of the type which is used in connection with optical beams generated by a plurality of laser sources and which is carried by optical fibers, the apparatus comprising: 
 a. a diffraction grating;    b. means for shaping the optical beams; and    c. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized;    
     
     
         8 . A bi-directional optical apparatus, of the type which is used in connection with optical signals generated by a plurality of laser sources and which is carried by optical fibers, the apparatus comprising: 
 a. an optical fiber;    b. multiplexer means for spatially combining the optical signals from several laser sources, each of which is a different wavelength, and launching the spatially combined optical signals into a single optical fiber to form a wavelength division multiplexed optical signal; and    c. demultiplexer means for spatially separating the different wavelengths from the single optical fiber carrying wavelength division multiplexed optical signals and launching each of the different wavelengths into a separate optical fiber, wherein the multiplexer and demultiplexer means are comprised of identical components.    
     
     
         9 . The apparatus of  claim 8 , wherein the optical fiber is a single mode optical fiber communication link.  
     
     
         10 . The apparatus of  claim 8 , wherein the multiplexer and demultiplexer means include: 
 a. means for collimating the light exiting the optical fiber end face;    b. means for splitting the plurality of optical wavelength signals into two beams;    c. means for rotating the polarization direction of either the p polarized beam or the s polarized beam;    d. means for expanding the diameter of the collimated beams in the direction of the polarization;    e. means for diffracting each of the different wavelength into a different angular direction relative to a defined direction;    f. means for reducing the expanded diameter of the collimated beams in the direction of the polarization;    g. means for recombining the two optical beams at each wavelength into a single beam for each wavelength signal, and wherein the recombined beam for each wavelength has two mutually perpendicular polarization components and is propagating in a different angular direction relative to an optic axis;    h. means for focusing the wavelength signals to a different spatial location along a line in the focal plane of the focusing means; and    i. means for receiving the focused signals and launching the individual signals into separate optical fibers.    
     
     
         11 . The apparatus of  claim 10 , wherein the means for expanding the diameter of the beam in the direction of the polarization is a prism.  
     
     
         12 . The apparatus of  claim 10 , wherein the means for diffracting the beam is a diffraction grating.  
     
     
         13 . The apparatus of  claim 12 , wherein the diffraction grating is a holographic grating.  
     
     
         14 . The apparatus of  claim 10 , wherein the means for receiving the focused signals is an optical waveguide.  
     
     
         15 . The apparatus of  claim 10 , wherein the means for splitting the collimated light beam into two beams which are polarized in two mutually perpendicular directions, is a glass prism assembly with a multi-layer dielectric polarizing beamsplitter coating.  
     
     
         16 . The apparatus of  claim 10 , wherein the means for rotating the polarization of the beams is a half-wave plate.  
     
     
         17 . The apparatus of  claim 8 , wherein the multiplexer and demultiplexer means include: 
 a. means for collimating the light exiting the optical fiber end face;    b. means for splitting the plurality of optical wavelength signals into two beams;    c. means for rotating the polarization direction of either the p polarized beam or the s polarized beam;    d. means for fine tuning the propagation direction of the collimated beams;    e. means for diffracting each of the different wavelength into a different angular direction relative to a defined direction;    f. means for recombining the two optical beams at each wavelength into a single beam for each wavelength signal, and wherein the recombined beam for each wavelength has two mutually perpendicular polarization components and is propagating in a different angular direction relative to an optic axis;    g. means for focusing the wavelength signals to a different spatial location along a line in the focal plane of the focusing means; and    h. means for receiving the focused signals and launching the individual signals into separate optical fibers.    
     
     
         18 . The apparatus of  claim 17 , wherein the means for fine tuning the propagation direction of the beam is a prism.  
     
     
         19 . The apparatus of  claim 17 , wherein the means for diffracting the beam is a diffraction grating.  
     
     
         20 . The apparatus of  claim 19 , wherein the diffraction grating is a holographic grating.  
     
     
         21 . The apparatus of  claim 17 , wherein the means for receiving the focused signals is an optical waveguide.  
     
     
         22 . The apparatus of  claim 17 , wherein the means for splitting the collimated light beam into two beams which are polarized in two mutually perpendicular directions, is a glass prism assembly with a multi-layer dielectric polarizing beamsplitter coating.  
     
     
         23 . The apparatus of  claim 17 , wherein the means for rotating the polarization of the beams is a half-wave plate.  
     
     
         24 . A bi-directional device comprising: 
 a. a diffraction grating;    b. means for shaping the optical beams; and    c. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized,    d. wherein the device may be used as both: 
 i. a multiplexer to spatially combine the optical beams from several laser sources, each of which is a different wavelength and launch the spatially combined laser beams into a single optical fiber;  
 ii. a demultiplexer to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber.  
   
     
     
         25 . The apparatus of  claim 24 , wherein the components comprising the multiplexer and the demultiplexer are identical and are used in connection with a single mode optical fiber communication link.  
     
     
         26 . The apparatus of  claim 24 , wherein the device is used in a single mode optical fiber communication link environment utilizing dense wavelength division multiplexing (DWDM) in which the wavelengths of the laser sources are separated by integer multiples of 0.4 nm.  
     
     
         27 . A method of demultiplexing a plurality of light signals carried by an optical fiber comprising the steps of: 
 a. collimating the light exiting an optical fiber end face;    b. splitting the plurality of optical wavelength signals into two beams;    c. rotating the polarization of either the p polarized or s polarized beam;    d. expanding the diameter of the collimated beams in the direction of the polarization;    e. diffracting each of the different wavelength beams into a different angular direction relative to a defined direction;    f. reducing the expanded diameter of the collimated beams in the direction of the polarization;    g. recombining the two optical beams at each wavelength into a single beam for each wavelength signal, and wherein the recombined beam for each wavelength has two mutually perpendicular polarization components and is propagating in a different angular direction relative to an optic axis;    h. focusing the optical wavelength signals to a different spatial location along a line in the focal plane of the focusing means; and    i. receiving the focused signals and launching the individual signals into separate optical fibers.    
     
     
         28 . The method of  claim 27 , wherein the polarization of both beams is perpendicular to the grooves on a diffraction grating.  
     
     
         29 . An optical device, comprising a polarizing beamsplitter; and a half wave plate, wherein an incoming beam of any arbitrary optical polarization state is converted into an optical beam which is linearly polarized in any specified direction.  
     
     
         30 . An optical fiber transmission apparatus, the apparatus comprising: 
 a. a plurality of laser sources for generating optical beams;    b. a first optical transmission fiber;    c. multiplexer means for spatially combining the optical beams from the laser sources, each of which is a different wavelength, and launching the spatially combined optical beams into the optical transmission fiber to form a wavelength division multiplexed optical signal, wherein the multiplexer means includes: 
 i. a diffraction grating;  
 ii. means for-fine tuning the propagation direction of the optical beams; and  
 iii. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized;  
   d. a plurality of second optical fibers; and    e. demultiplexer means for spatially separating the different wavelengths from the optical transmission fiber and launching each of the different wavelengths into separate second optical fibers, wherein the demultiplexer means includes: 
 i. a diffraction grating;  
 ii. means for fine tuning the propagation direction of the optical beams; and  
 iii. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized.  
   
     
     
         31 . The apparatus of  claim 30 , wherein the first optical transmission fiber is a single mode optical fiber communication link.  
     
     
         32 . The apparatus of  claim 30 , wherein the means for adjusting the polarization direction of the optical beams includes means for rotating the polarization direction of either the p polarized beam or the s polarized beam.  
     
     
         33 . The apparatus of  claim 32 , wherein the means for rotating the polarization direction includes a polarizing beamsplitter and a half-wave plate.  
     
     
         34 . The apparatus of  claim 33 , wherein the polarizing beamsplitter and half-wave plate are a monolithic structure comprised of: 
 a. a right angle prism;    b. a beam displacement prism; and    c. a half-wave plate.    
     
     
         35 . The apparatus of  claim 30 , wherein the means for steering the propagation direction of the optical signals includes a prism.  
     
     
         36 . A bi-directional optical apparatus, of the type which is used in connection with optical beams generated by a plurality of laser sources and which is carried by optical fibers, the apparatus comprising: 
 a. a diffraction grating;    b. means for fine tuning the propagation direction of the optical beams; and    c. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized;    
     
     
         37 . A bi-directional device comprising: 
 a. a diffraction grating;    b. means for fine tuning the propagation direction of the optical beams; and    c. means for adjusting the polarization direction of the optical beams, whereby the diffraction grating efficiency is improved and the polarization dependent loss is minimized,    d. wherein the device may be used as both: 
 i. a multiplexer to spatially combine the optical beams from several laser sources, each of which is a different wavelength and launch the spatially combined laser beams into a single optical fiber;  
 ii. a demultiplexer to spatially separate the different wavelengths of a wavelength division multiplexed optical link and launch each of the different wavelengths into a different optical fiber.  
   
     
     
         38 . The apparatus of  claim 37 , wherein the components comprising the multiplexer and the demultiplexer are identical and are used in connection with a single mode optical fiber communication link.  
     
     
         39 . The apparatus of  claim 37 , wherein the device is used in a single mode optical fiber communication link environment utilizing dense wavelength division multiplexing (DWDM) in which the wavelengths of the laser sources are separated by integer multiples of 0.4 nm.  
     
     
         40 . A method of demultiplexing a plurality of light signals carried by an optical fiber comprising the steps of: 
 a. collimating the light exiting an optical fiber end face;    b. splitting the plurality of optical wavelength signals into two beams;    c. rotating the polarization of either the p polarized or s polarized beam;    d. diffracting each of the different wavelength beams into a different angular direction relative to a defined direction;    e. recombining the two optical beams at each wavelength into a single beam for each wavelength signal, and wherein the recombined beam for each wavelength has two mutually perpendicular polarization components and is propagating in a different angular direction relative to an optic axis;    f. focusing the optical wavelength signals to a different spatial location along a line in the focal plane of the focusing means; and    g. receiving the focused signals and launching the individual signals into separate optical fibers.    
     
     
         41 . The method of claim  40 , wherein the polarization of both beams is perpendicular to the grooves on a diffraction grating.