Abstract:
This specification discloses a power polarization beam combiner and its applications in fiber communications. The power polarization beam combiner uses the photonic band gap formed in a photonic crystal to produce a left-hand material with a negative refractive index and high dispersion rate. Using such properties of the photonic crystal, several beams with different wavelengths and polarizations are combined and output to a common port.

Description:
This Non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 092137207 filed in Taiwan on Dec. 26, 2003, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a power polarization beam combiner used in fiber network systems and, in particular, to a power polarization beam combiner made of highly dispersive material with a negative refractive index and its applications in fiber network systems. 
     2. Related Art 
     With the increase of local network systems in metropolitan areas, one is forced to increase the number of wavelengths in order to transmit a huge amount of information. Therefore, the CWDM transceiver and CWDM receiver become important. As the transmission distance gets longer, a higher optical transmission power is imperative. In particular, the use of the EDFA and the Raman amplifier requires a good power combiner. It is mainly because the laser power is insufficient for long-distance transmissions. Thus, a set of EDFA or the Raman amplifier is often installed every 40 km to enhance the laser power. Nonetheless, as the fiber distance gets longer, the number of laser amplifiers also increases. This inevitably increases the cost for fiber equipment and maintenance. 
     Moreover, conventional amplifiers, it is either EDFA or Raman amplifier, mainly combine beams of different polarizations within the transmissible wavelength range to increase the transmitted optical signal power. This results in huge sizes for the conventional power combiners. 
     The U.S. Pat. No. 6,188,819 discloses a wavelength division multiplexing (WDM) device designed using a photonic crystal with a normal refractive index. Although it is successful in reducing the device size, it cannot combine beams of different polarizations. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the invention provides power polarization beam combiner and shows its applications in fiber communications. The design of a WDM with a negative refractive index can effectively reduce the device size to achieve the goal of compact integration. At the same time, beams of perpendicular polarizations in several wavelengths can be combined into a common port for output. 
     The disclosed power polarization beam combiner utilizes a highly dispersive device, such as a photonic crystal. The photonic band gap thus formed produces left-hand and highly dispersive materials with a negative refractive index. Such properties combine the power of beams of several wavelengths and polarizations into a common output port. 
     Based upon this idea, the highly dispersive device is used in the power polarization beam combiner to couple optical signals in different wavelengths and polarizations. It can be used to increase the laser power for the laser amplifier in multi-wavelength fiber communications. Used in optical transmitting modules and receiving modules, the invention can minimize the device size to achieve the goal of compact integration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  shows the energy bands of a photonic crystal; 
         FIG. 2  shows the Brillion zone of a photon; 
         FIGS. 3A ,  3 B, and  3 C are schematic views of the disclosed power polarization beam combiner; 
         FIG. 4  is a schematic view of the disclosed MDW fiber network system; 
         FIG. 5A  is a schematic view of the disclosed optical transmitting module; and 
         FIG. 5B  is a schematic view of the disclosed optical receiving module. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention primarily uses the negative refractive index property of a highly dispersive device to couple optical signals in different wavelengths and polarizations. In the following, we use a photonic crystal as an example to explain the spirit of the invention. 
       FIG. 1  shows the band gap of a photonic crystal. Each mode has two polarizations, the transverse electric field (TE) and the transverse magnetic field (TM). In the fundamental mode (its TE and TM being shown by the two curves at the bottom of the plot), the material looks the same as ordinary dielectric materials. The difference in the refractive indices of the two modes is tiny. However, with appropriate selections, one can obtain big refractive index difference in the TE and TM modes by exciting photons above the second band. As described in “Theory of light propagation in strongly modulated photonic band gap: Refraction-like behavior in the vicinity of the photonic band gap” (M. Notomi, Physical Review B), we know that as the operating frequency gets closer to the Brillouin Zone  11  (the point Γ, M, and K in  FIG. 2  corresponding to the horizontal coordinates Γ in  FIG. 1 ) of the photonic crystal  10 , its energy direction and phase velocity will be parallel to each other. Therefore, in practice, one only needs to determine the direction of the wave number in order to figure out the energy direction. According to the paper, the slope of the band determines whether the photonic crystal is a right-material or a left-material and which band has a positive or negative refractive index. Consequently, one can readily obtain a photonic crystal with a negative refractive index through careful band selection. 
     As shown in  FIG. 3A , the power polarization beam combiner is a highly dispersive device. For example, the photonic crystal  10  contains several circular vent holes  101  disposed in a periodic hexagonal pattern. Its refractive index satisfy the condition |n|≦7. In particular, the optical signals TE 1  and TE 2  are optical signals with the same polarization in the E direction. The optical signals TM 1  and TM 2  are optical signals with the same polarization in the M direction. Moreover, TE 1  and TM 1  have the same wavelengths; TE 2  and TM 2  have the same wavelengths. 
     Using the negative refractive index, optical signals TE 1 , TE 2  (or TM 1 , TM 2 ) of different wavelengths are coupled into a single output port  20 . Likewise, optical signals TE 1 , TM 1  (or TE 2 , TM 2 ) are coupled into the output port  20 . Moreover, one can design an incident surface  102  to have different angles using wedge objects, polishing, or etching, in order for optical signals TE 1 , TM 1 , TE 2 , and TM 2  to enter the photonic crystal  10  in the parallel direction (see  FIG. 3B ). Therefore, the disclosed power polarization combiner can be used to couple optical signals of different wavelengths and polarizations. As shown in  FIG. 3C , optical signals TE 1 , TE 2 , TE 3  . . . TEN (or TM 1 , TM 2 , TM 3  . . . TMN) can be combined into a single output port  20  too. The optical signals TE 1 , TM 1  (or TE 2 , TM 2  or TE 3 , TM 3  or TEN TMN) can also be combined into the output port  20 . 
     When using the above-mentioned power polarization combiner in fiber communications, as shown in  FIG. 4 , the system contains an optical receiving module  40 , an optical transmitting module  50 , a fiber  32 , and a plurality of laser amplifiers  60 . The optical receiving module  40  and the optical transmitting module  50  are connected to both ends of the fiber  32  for transmitting optical signals. The laser amplifiers  60  enhance the power of laser inside the fiber  32 . Each laser amplifier  60  consists of two isolators  61 ,  62  on both ends and a filter  63  to prevent reverse transmission of the optical signals. They are coupled by two couplers  67 ,  68 . Two erbium-doped fibers (EDF)  64 ,  65  and a dispersion compensation fiber (DCF)  66  are employed to enhance the power and compensate for signal decays. It further uses the above-mentioned power polarization beam coupler as the pump source of the EDF  67 ,  68 . For example, suppose there are only five pump sources available. With beams of different polarizations, the disclosed laser amplifier  60  can combine 10 different optical signals to increase the power by about a factor of two. 
     On the other hand, the invention uses the idea of negative refractive index on the optical transmitting module  50 . As shown in  FIG. 5A , it contains several light emitters  51 , a superprism  52 , a waveguide  53 , and a spot size converter  54 . The light emitters  51  receive an external signal, modulate it and emit an optical signal into the superprism  52 . The superprism  52  can couple several optical signals of different wavelengths and polarizations into the waveguide  53 . The optical signal is thus guided into the spot size converter  54 , entering the fiber  32 . Likewise, the superprism  52  has the above-mentioned photonic crystal design so that the volume of the optical transmitting module  50  can be compactly integrated. The waveguide  53  can be a photonic crystal waveguide to further minimize the size. 
     As shown in  FIG. 5B , the optical receiving module  40  contains several light receivers  41 , a superprism  42 , a waveguide  43 , and a spot size converter  44 . The optical signal enters the spot size converter  44  from the fiber  32 . Guided by the waveguide  43 , the optical signal is split by the superprism  42  into respective light receivers  41 . Likewise, the superprism  42  has the above-mentioned photonic crystal design so that the volume of the optical transmitting module  40  can be compactly integrated. The waveguide  43  can be a photonic crystal waveguide to further minimize the size. 
     Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.