Abstract:
A method and apparatus for improving gain volume in semiconductor optical amplifiers is provided. A multi-mode interference coupler is used for expanding incoming optical signal. Thereafter, another multi-mode interference coupler combines the expanded optical signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to devices and methods used in fiber optics networks and more particularly, to semiconductor optical amplifiers.  
           [0003]    2. Background  
           [0004]    Semiconductor optical amplifiers (hereinafter referred as “optical amplifier” or “optical amplifiers”) are frequently used in fiber optics networks. FIG. 1A shows a top level block diagram of a fiber optics network  100 , which includes transmitter  101  that receives an electrical input (not shown) and converts it to an optical output  102  using a laser diode (not shown). Optical signal  102  is transmitted via optical fiber (not shown) to optical amplifier  103 . Optical amplifier  103 , described below in FIG. 1B, amplifies optical signal  102  and the amplified signal  102 ′ is transmitted to photodetector  105 , via filter  104 .  
           [0005]    [0005]FIG. 1B shows a top-level block diagram of a conventional optical amplifier  103 . Absorption layer (or active layer)  103 B is located between two semiconductor layers  103 A and  103 C. Optical signal  102  enters absorption layer  103 B and electric current  103 D is applied to amplifier  103 . Electrons due to current  103 D are stimulated to an excited state but move to a ground state due to optical signal  102 . Photons are generated when electrons lose energy by moving from the excited state to the ground state. The generated photons combine with the photons in optical signal  102  that caused the emission in the first place, and optical signal  102  is amplified to signal  102 ′.  
           [0006]    Typically, optical amplifier  103  is compatible with only single mode fibers. FIG. 1C shows the top view of optical amplifier  103  with a single mode waveguide  103 E that receives incoming optical signal  102  (FIG. 1A) through optical fiber  106 A. Waveguides are used by optical amplifiers to guide electromagnetic or optical light in a direction parallel to the waveguide axis. Optical signal  102  (FIG. 1C) travels through waveguide  103 E and emits an amplified signal  102 ′ to optical fiber  106 B.  
           [0007]    Typically, optical amplifier  103  must have high power saturation for efficiently amplifying optical signal  102 . Conventional optical amplifiers have drawbacks due to gain saturation because the gain in optical amplifier  103  is limited by available electrons injected in gain medium (absorption layer  103 B, FIG. 1B).  
           [0008]    One solution to the foregoing gain saturation problem is to forego the single mode property and expand the gain medium width. This is illustrated in FIG. 1D. Optical signal  102  enters single mode waveguide  103 E through fiber  106 A, passes through expanded gain region  107 , and exits through cylindrical lens  108  and lens  109  into optical fiber  106 B. FIG. 1E shows the side view of the FIG. 1D optical system. The optics shown in FIGS. 1D and 1E for improving gain volume are very complicated and increase the overall cost of amplifying optical signal  102 .  
           [0009]    Therefore, there is a need to improve power saturation in a photodetector without complex optics or increasing the overall cost of amplification.  
         SUMMARY OF THE INVENTION  
         [0010]    In one aspect of the present invention, the foregoing deficiencies are addressed by providing an apparatus for increasing gain volume in an optical amplifier without complicated and expensive optics. The apparatus includes an expansion region that receives and expands incoming optical signal. The expansion region is coupled to plural waveguides through which expanded incoming optical lights are reduced in intensity by a factor of N, with N being the number of single mode amplifer channels. So, the expanded optical signal can be amplified to its limit and then travel to a “combination” region. The combination region combines the expanded optical signal into a single beam. Both the expansion and combination regions operate as multi-mode interference (“MMI”) couplers.  
           [0011]    In another aspect of the present invention, the MMI couplers are less complicated and cheaper than the complex optics described above and shown in FIGS. 1D and 1E.  
           [0012]    This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1A as described above is a top-level block diagram of a conventional fiber optics network.  
         [0014]    [0014]FIG. 1B as described above is a block diagram of a conventional optical amplifier.  
         [0015]    [0015]FIG. 1C as described above is a top view of a conventional optical amplifier with a single mode waveguide.  
         [0016]    [0016]FIG. 1D as described above is a top view of a conventional optical amplifier with a flared gain region.  
         [0017]    [0017]FIG. 1E as described above is a side view of a conventional optical amplifier of FIG. 1D.  
         [0018]    [0018]FIG. 2A is a top view of an optical amplifier according to an embodiment of the present invention.  
         [0019]    [0019]FIG. 2B is a flow diagram according to an embodiment of the present invention.  
         [0020]    Features appearing in multiple figures with the same reference numeral are the same unless otherwise indicated. 
     
    
     DETAILED DESCRIPTION  
       [0021]    In one aspect of the present invention, a system is provided that expands incoming optical signal using multi mode interference couplers; thereafter the expanded or split, optical signal travels through a waveguide and is combined by a combining device that also operates as a multi mode interference coupler. The expansion and combining of incoming optical signal is performed without complex and expensive optical systems.  
         [0022]    Referring in detail to FIG. 2A, is an optical amplifier  200  according to an embodiment of the present invention that utilizes multi-mode interference (“MMI”) couplers to expand and combine optical signals. Optical fiber  106 A is coupled with a MMI Expander  201 , which in turn is coupled to plural single mode waveguides  202 . Waveguides  202  are coupled to a MMI combiner  203  that receives amplified optical signal from waveguides  202  and combines them into a single beam (not shown) that is transmitted through optical fiber  106 B.  
         [0023]    Input optical beam  102  enters Expander  201  and expands. Expander  201  operates as a MMI coupler between optical fiber  106 A and waveguides  202 . The expanded optical beam (not shown) propagates through Expander  201  and forms in-phase, as well as out of phase interference pattern. The condition for in-phase resonance is given as follows:  
         [ L=Neff W   2   /Nλ]   
         [0024]    where Neff is the effective refractive index of the MMI region, L is the length of Expander  201 , N is the number of output waveguides, W is the width of MMI Expander  201  and λ is the wavelength of the input optical beam. When the MMI is designed with a length L and a width W that satisfies equation (1), the split light entering waveguides  202  are in phase. Equation (1) also applies to MMI in the opposite direction so the same design can work as a combiner when the multiple inputs are in phase. In addition, the insertion loss, i.e., the cumulative optical power in waveguides  202  as a fraction of incoming power in waveguide  106 A, is near 100 percent. This means that the penalty the use the MMI for both splitting and combining optical power is relatively low.  
         [0025]    In another aspect of the present invention, a process is provided such that incoming optical signal is expanded using a MMI based expansion region; the expanded optical signal then travels through plural single mode waveguides; and thereafter, combined by a MMI based combiner.  
         [0026]    Referring to FIG. 2B, a flow diagram is provided for improving gain in an optical amplifier without using complex optics, comprising the steps of: receiving incident light; expanding the incident light using an MMI expander; and then combining the expanded optical signal using a MMI combiner.  
         [0027]    Turning in detail to FIG. 2B, in step S 201 , incoming optical signal is received by the optical fiber. FIG. 2A shows incoming optical beam  102  received by optical fiber  106 A.  
         [0028]    In step S 202 , input light is expanded. As shown in FIG. 2A, optical beam  102  enters MMI expander  201  and is expanded based upon multimode interference principles. Input optical beam  102  diffracts and expands as it enters Expander  201 , and forms in-phase and out of phase interference patterns as it propagates through Expander  201  and through plural waveguides  202 . The condition for in-phase resonance is given by:  
         
       L=Neff W 
       2 
       /Nλ 
     
         [0029]    where Neff is the effective refractive index of the MMI.  
         [0030]    In step S 203 , the expanded optical beam is combined. According to FIG. 2B, a MMI combiner  203  combines the expanded light (not shown) using multi-mode interference coupling principles. The combined light leaves optical amplifier  200  via optical fiber  106 B.  
         [0031]    Another aspect of the present invention is that power saturation is improved and MMI couplers are less complicated and cheaper than conventional optics described above.  
         [0032]    While the present invention is described above with respect to what is currently consider its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.