Abstract:
A light concentrator for an optical antenna gradually narrows from the light receiving end to the end in contact with a light detector, and has a refractive index that gradually increases from the first to the second end, to afford a greater acceptance angle for the incoming optical signal. The increase may occur in stages of corresponding layers of the light concentrator, the layers being arranged in order of increase in refractive index from the first end to the second end.

Description:
CLAIM OF PRIORITY 
   This application claims priority under 35 U.S.C. § 119 to an application entitled “Optical Antenna and Wireless Optical System Using the Same,” filed in the Korean Intellectual Property Office on Jan. 28, 2004 and assigned Serial No. 2004-5515, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an optical antenna, and in particular, to an optical antenna that includes an optical concentrator. 
   2. Description of the Related Art 
   A typical optical antenna includes a light concentrator in the form of a hemispherical lens. A distinctive shortcoming of this light concentrator is the limited acceptance angle range in which an optical signal is receivable. 
   To overcome the problem, Warwick University, U.K. proposed a trumpet-shaped light concentrator. The optical antenna is applicable to infrared wireless optical communication systems, a variety of signal processes, analog or digital systems, and portable phones. As one simple example, the optical antenna can be implemented in a wireless remote controller for home use. 
     FIG. 1  illustrates a conventional optical antenna  100  including a trumpet-shaped light concentrator  110 , and an optical detector  120  for detecting an optical signal incident through the light concentrator  110 . 
   Like a trumpet, the light concentrator  110  becomes narrow in width, starting from a first end  110   a  at which optical signals are incident to a second end  110   b  contacting the optical detector  120 . The light concentrator  110  totally internally reflects to the optical detector  120  the optical signal incident at the first end  10   a  within an acceptance angle. The acceptance angle refers to an angle over which the optical detector  120  accepts an optical signal incident on the light concentrator  110 . The totally internally reflected optical signal from the light concentrator  110  is applied to the input of the optical detector  120 . 
   The optical detector  120  can be a waveguide photodiode (PD). A refraction index matching layer or a band pass filter can be inserted between the light concentrator  110  and the optical detector  120 . 
     FIG. 2  illustrates an operational characteristic of the optical antenna depicted in  FIG. 1 . An optical signal incident at a larger angle than an acceptance angle is shown. Referring to  FIG. 2 , the input optical signal is radiated outside the optical concentrator  110  after total reflection. 
   In the above optical antenna, while the trumpet-shaped light concentrator offers a wider acceptance angle than the hemispherical lens, any incoming optical signal beyond the acceptance angle is re-reflected outside. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an optical antenna structure that offers a wide acceptance angle range. 
   The above object is achieved by providing a light concentrator, an optical antenna using the same, and a wireless optical system using the antenna. A light concentrator has a first end for receiving an external optical signal and a second end in contact with an optical detector for detecting the optical signal. The concentrator has a refractive index and a width. The width decreases, and the refractive index increases, from the first end to the second end. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a side view of a conventional optical antenna structure; 
       FIG. 2  is the side view of  FIG. 1  that illustrates an operational characteristic of the optical antenna; 
       FIG. 3  is a side view of an optical antenna including a light concentrator according to an embodiment of the present invention; 
       FIG. 4  is a conceptual diagram demonstrating light refraction in the embodiment of  FIG. 3 . 
       FIG. 5  is a graph illustrating the total reflection characteristics of the optical antenna illustrated in  FIG. 3 ; 
       FIG. 6  is a graph illustrating the reflection characteristic of an optical signal between refractive index layers in the light concentrator illustrated in  FIGS. 3 and 4 ; and 
       FIG. 7  is a block diagram of a wireless optical system according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention are described below with reference to the accompanying drawings. In the following description, well-known functions or constructions are omitted for clarity of presentation. 
     FIG. 3  presents, by way of illustrative and non-limitative example, an optical antenna  200  that includes an optical detector  220  for detecting an optical signal and a light concentrator  210  whose width becomes narrow, starting from a first end  210   a  to a second end  210   b , in accordance with the present invention. The first end  210   a  receives the optical signal and the second end  210   b  is in contact with the optical detector  220 . An active layer (not shown) for detecting the optical signal may include a waveguide PD in contact with the second end  210   b  of the light concentrator  210 . 
   The light concentrator  210  has, as seen in  FIG. 3 , a plurality of refractive index layers  211 - 1  to  211 -n having refractive indexes that increase, starting from the first end  210   a  to the second end  210   b . An optical signal incident on the light concentrator  210  is therefore refracted gradually via the refractive index layers  211 - 1  to  211 -n as the signal travels toward the optical detector  220 . Specifically, light traveling through the air, for example, is refracted upon entering the layer  211 - 1 . That refracted light is again refracted upon entering layer  211 - 2 , and so on. 
     FIG. 5  demonstrates the total reflection characteristics of the optical antenna illustrated in  FIG. 3 . Total reflection is a phenomenon in which as light travels from a first medium  410  (n 1 ) to a second medium  420  (n 2 ), the light is reflected at the boundary surface  401  between the first and second mediums  410 ,  420  without intensity loss. An axis perpendicular to the boundary surface  401  is called a normal line. The light is incident on the boundary surface  401  at an angle of i or i′, from normal, and reflected from the boundary surface  401  at a reflection angle or refraction angle of r or r′, from normal. 
   Total reflection occurs when light travels from a dense medium having a high refractive index to a less dense medium having a low refractive index, or when the light is incident on the boundary surface  401  at a larger angle than the threshold angle i′. When, as seen in  FIG. 5 , the angle of incidence is smaller than the threshold angle, the light is partially reflected and partially refracted. Light refracted from the sides of the concentrator  210  does not reach the photodiode  220  and thus amounts to optical loss. Advantageously, the wide acceptance angle afforded by the present invention avoids optical loss that is characteristic of the prior art. 
   The threshold angle i′ is that incident angle at which refraction at the boundary surface  401  causes the light to then travel along the boundary surface. If the light is incident at an angle larger than the threshold angle, it is totally reflected. 
   The light concentrator  210 , since it progressively becomes narrower, starting from the first end  210   a  to the second end  210   b , delivers more of the entering light incident on the sides of the concentrator  210  at an incidence angle exceeding the threshold angle. This causes more of the optical signal to be totally reflected and to thus arrive at the optical detector  220  with minimal loss. 
     FIG. 6  is a graph illustrating the difference in refractive index between refractive index layers in the light concentrator illustrated in  FIGS. 3 and 4 . As shown in  FIG. 6 , optical signal  303  reaches the boundary surface  301  which separates the first and second mediums  310 ,  320  having respectively different refractive indexes. In particular, the first medium  310  has a refractive index of n 1  and the second medium  320  has a refractive index of n 2 . The optical signal travels in the first medium  310  at an angle of θ 1  to a normal line  302  perpendicular to the boundary surface  301  and is refracted into the second medium  320  at an angle of θ 2  from normal. 
   According to the Snell&#39;s law, θ 1 , θ 2 , and the refractive indexes of the first and second mediums  310  and  320  relate as follows: 
   
     
       
         
           
             
               
                 
                   
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       1 
                     
                   
                   
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       2 
                     
                   
                 
                 = 
                 
                   
                     n 
                     2 
                   
                   
                     n 
                     1 
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   Because the light concentrator  210  becomes gradually narrower, starting from the first end  210   a  to the second end  210   b , and includes the refractive index layers  211 - 1  to  211 -n having refractive indexes that respectively increase, starting from the first end  210   a  to the second end  210   b , an incoming signal is successively refracted into near perpendicularity with respect to the incident or “boundary” surface of the optical detector  220 . 
   The degree to which the optical detector  220  detects the optical signal effectively is expressed as 
                       A   eff   bare     ⁡     (   ψ   )       =     Acos   ⁢           ⁢     (   ψ   )         ,     0   &lt;   ψ   &lt;     π   /   2               (   2   )                     A   eff   bare     ⁡     (   ψ   )       =   0     ,     ψ   &gt;     π   /   2               (   3   )               
where
 
             A   eff   bare     ⁡     (   ψ   )           
denotes effective area and constitutes a measure of effective efficiency for optical signal detection in the optical detector  220 . ψ denotes the incident angle to the boundary surface of the optical detector  220 .
 
   Referring to equations (2) and (3), as the incidence of the optical signal on the active layer of the optical detector  220  approaches perpendicularity, the effective area increases. 
   By contrast, at the other extreme, if the incident angle is 90 degrees, i.e., the optical signal is incident in parallel to the active layer of the optical detector  220 , the effective area is 0 according to equation (3) and thus the effective efficiency of the optical detector  220  is lowest. 
   It is thus concluded that the optical detector  220  achieves a maximum effective efficiency when the incident angle of the optical signal is 90 degrees to the boundary surface. 
   Referring to  FIG. 6  and equations (1), (2) and (3), ψ is θ 1  or θ 2  when the optical signal travels in the first or second medium  310  or  320 . The refractive indices of the layers  211 - 1  to  211 -n increase with nearness to the second end  210   b . Thus, the optical signal incident on the light concentrator  210  is refracted at successively decreasing angles to the normal line. Accordingly, since the angle of the optical signal to the boundary surface between the optical detector  220  and the light concentrator  210  gradually increases, i.e., gradually decreasing the refraction angle of the optical signal to the normal line, the effective efficiency of the optical detector  220  is maximized. 
     FIG. 7  is a block diagram of an exemplary wireless optical system according to another embodiment of the present invention. The wireless optical system comprises an optical transmitter  510  for transmitting an optical signal and an optical receiver  520  for detecting the optical signal. 
   The optical transmitter  510  includes a light source  511  for generating the optical signal, and a spreader  512  and a lens unit  513  for spreading the optical signal. The optical signal generated from the light source  511  is spread by the spreader  512  and the lens unit  513  and output to the optical receiver  520 . 
   The optical receiver  520  includes an optical detector  523  for detecting the optical signal, a light concentrator  521 , and a filter  522  between the light concentrator  521  and the optical detector  523 . Thus, the optical receiver  520  functions as an optical antenna for detecting the optical signal received from the optical transmitter  510 . 
   The light concentrator  521  decreases in width, starting from the first end, at which the optical signal is received, to the second end, in contact with the optical detector  523 , and increases in refractive index from the first end to the second end. 
   The filter  522  between the light concentrator  521  and the optical detector  523  passes to the optical detector  523  only an optical signal at a predetermined wavelength. 
   The wireless optical system having the above-described configuration can be used in wireless infrared communication systems and various wireless optical communication applications. 
   In accordance with the present invention as described above, a light concentrator in an optical antenna narrows from a first, light entry, end to a second end, and has a refractive index that increases from the first to the second end. Thus, the optical antenna offers a wider acceptance angle for the optical signal input at the first end of the light concentrator. 
   While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.