Abstract:
An optical device may include first and second lasers generating first and second laser beams; and a photo detector detecting the first and second laser beams. The optical detector comprises a substrate, a first impurity layer on the substrate, an absorption layer on the first impurity layer and a second impurity layer on the absorption layer. The absorption layer generates a terahertz by a beating of the first and second laser beams and has a thickness of less than  0.2 μ m.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2012-0051117, filed on May 14, 2012, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    The present inventive concept herein relates to optical devices, and more particularly, to a photo detector and an optical device including the photo detector. 
         [0003]    A photo detector is a device that is essentially used in an optical communication. The photo detector converts light into an electric signal. The photo detector requires high responsivity, a high operation speed, a high power capability, etc. Responsivity represents how many photocurrents are generated with respect to input optical signal and may correspond to receiving sensitivity. An operation speed is standards for processing large amounts of data. A power capability is one of important factors for realizing a long-distance communication. An optical amplifier is disposed at every certain distance to compensate loss being generated in a long-distance communication. At this time, a high power input may enter a photo detector. Thus, the photo detector has to smoothly operate with respect to the high power input. 
         [0004]    A photo detector is being used in the field of photomixer. The photomixer has a similar function to a radio frequency mixer. For instance, if a radio frequency signal having frequencies f 1  and f 2  is input to the radio frequency mixer, the radio frequency mixer can generate radio a frequency component f 1 -f 2  (where, f 1 &gt;f 2 ). Similarly, if a laser beam having different frequencies from each other is input to a photomixer, the photomixer can output an electric signal having a frequency component corresponding to a difference of the input frequencies. That is, the photomixer can generate a high speed electric signal of terahertz wave using a laser beam. Thus, the photomixer can have high responsivity, a high operation speed and a high power capability. The high responsivity is an important factor as standards for increasing an efficiency of converting a laser beam into an electric signal. When the photomixer has a high operation frequency, it can output a high speed electric signal of terahertz wave. Also, the photomixer can operate at high speed in proportion to an input power of optical signal. The photo detector and the photomixer for optical communication have different uses but same performance is required for the photo detector and the photomixer. 
       SUMMARY 
       [0005]    Embodiments of the inventive concept provide an optical device. The optical device may include a laser generating first and second laser beams; and a photo detector detecting the first and second laser beams. The optical detector comprises a substrate, a first impurity layer on the substrate, an absorption layer on the first impurity layer and a second impurity layer on the absorption layer. The absorption layer generates a terahertz by a beating of the first and second laser beams and has a thickness of less than 0.2 μm. 
         [0006]    Embodiments of the inventive concept also provide a photo detector. The photo detector may include a substrate; a first impurity layer on the substrate; an absorption layer on the first impurity layer; and a second impurity layer on the absorption layer. The absorption layer has a thickness of less than 0.2 μm. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]    Preferred embodiments of the inventive concept will be described below in more detail with reference to the accompanying drawings. The embodiments of the inventive concept may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout. 
           [0008]      FIG. 1  is a cross sectional view illustrating an optical device in accordance with a first embodiment of the inventive concept. 
           [0009]      FIG. 2  is a perspective view illustrating a photo detector of  FIG. 1 . 
           [0010]      FIG. 3  is a graph showing a maximum allowed photocurrent relative to a thickness of absorption layer. 
           [0011]      FIG. 4  is a drawing illustrating an optical device in accordance with a second embodiment of the inventive concept. 
           [0012]      FIG. 5  is a drawing illustrating a beam width of first and second laser beams being transmitted from a lensed optical fiber. 
           [0013]      FIGS. 6 through 9  are drawings illustrating first and second laser beams having different beam widths from each other with respect to absorption layers having 0.3 μm, 0.2 μm, 0.1 μm and 0.05 μm respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0014]    Embodiments of inventive concepts will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like numbers refer to like elements throughout. 
         [0015]      FIG. 1  is a cross sectional view illustrating an optical device  100  in accordance with a first embodiment of the inventive concept. 
         [0016]    Referring to  FIG. 1 , the optical device  100  may include a first laser  12 , a second laser  14 , an optical waveguide  20 , a photo detector  30  and an output circuit  50 . The first and second lasers  12  and  14  can generate a first laser beam and a second laser beam respectively that have different wavelengths from each other. The first laser beam may be a main input optical signal and the second laser beam may be a sub input optical signal. For example, the first laser beam may have a wavelength of about 1550 nm and the second laser beam may have a wavelength of about 1551 to 1560 nm. 
         [0017]    The optical waveguide  20  may be disposed between the photo detector  30  and the first and second lasers  12  and  14 . The optical waveguide  20  may include a joint waveguide  22  connected to the photo detector  30 , a first branch waveguide  24  connected between the joint waveguide  22  and the first laser  12  and a second branch waveguide  26  connected between the joint waveguide  22  and second laser  14 . 
         [0018]    The photo detector  30  absorbs the first and second laser beams being transmitted through the optical waveguide  20  to output a terahertz wave by a beating of the first and second laser beams. That is, the photo detector  30  can generate a terahertz wave corresponding to a difference between wavelengths of the first and second laser beams. 
         [0019]    The output circuit  50  can receive a terahertz wave being generated from the photo detector  30 . The output circuit  50  may include a terahertz wave instrumentation device and/or an antenna. 
         [0020]      FIG. 2  is a perspective view illustrating a photo detector of  FIG. 1 . 
         [0021]    Referring to  FIG. 2 , the photo detector  30  may include a substrate  32 , a first impurity layer  34  on the substrate  32 , an absorption layer  36  on the first impurity layer  34  and a second impurity layer  38  on the absorption layer  36 . The substrate  32  may include silicon single crystal, indium phosphide InP or gallium arsenide GaAs. The first impurity layer  34 , the absorption layer  36  and the second impurity layer  38  may be a ridge waveguide  31  extending in a specific direction. The ridge waveguide  31  can protrude from the substrate  32 . 
         [0022]    The first impurity layer  34  may include indium phosphide InP doped with a first conductivity type. The first conductivity type may be an N type. First electrodes  40  may be disposed on the first impurity layer  34  of both sides of the absorption layer  36  and the second impurity layer  38 . The second impurity layer  38  may include indium phosphide InP doped with a second conductivity type opposite to the first conductivity type. The second conductivity type may be a P type. A second electrode  42  may be disposed on the second impurity layer  38 . 
         [0023]    The absorption layer  36  can absorb first and second laser beams being transmitted through the optical waveguide  20  to generate a terahertz wave. The absorption layer  36  may include intrinsic indium gallium arsenic (InGaAs). A characteristic of the photo detector  30  can be influenced by a thickness of the absorption layer  36 . The thickness of the absorption layer  36  can determine an operation speed and responsivity of the photo detector  30 . The thick absorption layer  36  absorbs a large quantity of laser beams to increase responsivity. The thin absorption layer  36  can increase an operation speed of the photo detector  30  due to a reduction of a distance between the first impurity layer  34  and the second impurity layer  38 . 
         [0024]      FIG. 3  is a graph showing a maximum allowed photocurrent relative to a thickness of absorption layer. A horizontal axis represents a reverse bias voltage applied between the first electrode  40  and the second electrode  42  and a vertical axis represents a maximum allowed optical current. 
         [0025]    Referring to  FIGS. 1 through 3 , when the absorption layer  36  is thin, it can generate a high maximum allowed optical current. For instance, the absorption layer  36  having a thickness of 0.1 μm can generate a maximum allowed optical current that increases to 0.08 A in proportion to a bias voltage of 5V or less. The bias voltage can applied from the output circuit  50 . 
         [0026]    The absorption layer  36  having a thickness of 0.5 μm can generate a maximum allowed optical current of 0.01 A or less. Thus, the absorption layer  36  can increase a maximum allowed input power in inverse proportion to its thickness. 
         [0027]    The output circuit  50  may have an internal resistance of about 100 Ω. The ridge waveguide  31  between the first electrode  40  and the second electrode  42  may have resistance and/or impedance in accordance with a bias voltage. The ridge waveguide  31  may have resistance and/or impedance being changed according to its area. The resistance and/or impedance of the ridge waveguide  31  can be determined by a thickness of each of the first and second impurity layers  34  and  38  and the absorption layer  36  and doping concentrations of the first and second impurity layers  34  and  38 . The photo detector  30  may be a travelling-wave photo detector. The travelling-wave photo detector matches resistance and/or impedance of the ridge waveguide  31  to internal resistance of the output circuit  50 . The travelling-wave photo detector can allow an error to the extent of about 50% between the resistance and/or impedance of the ridge waveguide  31  and the internal resistance of the output circuit  50 . 
         [0028]    Thus, since the optical device  100  in accordance with the first embodiment of the inventive concept includes a travelling-wave photo detector, it can show maximum performance. 
         [0029]      FIG. 4  is a drawing illustrating an optical device in accordance with a second embodiment of the inventive concept. 
         [0030]    Referring to  FIG. 4 , the optical device  100  may include a lensed optical fiber  28  which is disposed between the photo detector  30  and the first and second lasers  12  and  14  and focuses first and second laser beams on the photo detector  30 . The lensed optical fiber  28  may be arranged in the absorption layer  36 . 
         [0031]    As described above, the photo detector  30  can generate an electric signal of terahertz wave by a beating of the first and second laser beams. At this time, an operation speed of the photo detector  30  can be determined by transit time and a resistance capacitance (RC) effect of an electron and/or a hole generated from the absorption layer  36 . The transit time is elapsed time when an electron and/or a hole in the absorption layer  36  reaches the first impurity layer  34  and the second impurity layer  38 . The RC effect is a factor disturbing movement of electrons and/or holes. 
         [0032]    In a low frequency band of about 100 GHz or less, a general optical communication does not have speed restrictions of an electron and/or a hole by transit time and an RC effect. The transit time and the RC effect may become important factors with respect to a speed of an electron and/or a hole in an optical communication of a terahertz wave band. When the photo detector  30  detects a laser beam signal of a frequency band of 40 GHz, the absorption layer  36  may have a thickness of about 0.5 μm. At this time, a transit limited bandwidth may be about 62 GHz. When the photo detector  30  is used as a photomixer of terahertz frequency band and/or a terahertz wave generator, it has a low operation speed by a limit of transit time. Thus, a thickness of the absorption layer  36  has to be small. When the absorption layer  36  is thin, capacitance may become large. If a size of the photo detector  30  is small, capacitance may be reduced. 
         [0033]    To overcome this restriction of operating frequency, we can use a travelling-wave scheme, which is not under RC limitation. We can overcome RC limitation with usage of travelling-wave scheme. A basic of travelling-wave scheme is an matching characteristic impedance of photo detector to external circuits. 
         [0034]    The lensed optical fiber  28  is not coupled to the photo detector  30  and may be spaced apart from the photo detector  30 . When the first and second laser beams having a similar size are transmitted to the lensed optical fiber  28  and the photo detector  30 , coupling loss may be reduced. Generally, when the lensed optical fiber  28  and the absorption layer  36  have a similar thickness, a coupling efficiency may be high. However, when the absorption layer  36  becomes thick, an operation speed for generation of terahertz wave may be slowed down. Thus, a coupling efficiency can be checked by comparing beam widths in the lensed optical fiber  28  and the ridge waveguide  31 . 
         [0035]      FIG. 5  is a drawing illustrating a beam width of first and second laser beams being transmitted from a lensed optical fiber  28 . 
         [0036]    Referring to  FIG. 5 , the lensed optical fiber  28  can output first and second laser beams  16  having a beam width of about 3 to 3.5 μm. The beam width can be defined by a vertical line width of the first and second laser beams  16 . The first and second laser beams  16  in the lensed optical fiber  28  can be simulated. At this time, the first and second laser beams  16  may have the same beam width in the lensed optical fiber  28 . 
         [0037]      FIGS. 6 through 9  are drawings illustrating first and second laser beams having different beam widths from each other with respect to absorption layers having 0.3 μm, 0.2 μm, 0.1 μm and 0.05 μm respectively. A horizontal axis and a vertical axis represent a horizontal distance and a vertical distance respectively. The beam width can be represented by a vertical distance of the first and second laser beams  16 . 
         [0038]    Referring to  FIGS. 2 ,  4  to  9 , when the absorption layer  36  has a thickness of less than 0.2 μm, the ridge waveguide  31  can absorb the first and second laser beams  16  having a beam width of more than 0.5 times that of the lensed optical fiber  28 . The first and second laser beams  16  in the ridge waveguide  31  can be simulated as a beam width being changed according to a thickness of the absorption layer  36 . For instance, if the first and second laser beams  16  having a beam width of about 3 to 3.5 μm from the lensed optical fiber  28  enter the photo detector  30 , the ridge waveguide  31  having the absorption layer  36  of a thickness of about 0.3 μm can absorb the first and second laser beams  16  as a beam width of about 1 μm ( FIG. 6 ). At this time, the ridge waveguide  31  can absorb the first and second laser beams  16  having a beam width of less than 0.5 times that of the lensed optical fiber  28 . Thus, the absorption layer  36  having a thickness of more than 0.2 μm can generate coupling loss of the first and second laser beams  16 . 
         [0039]    When the absorption layer  36  has a thickness of about 0.1 μm, it can absorb the first and second laser beams  16  having a beam width of about 3 μm similar to that in the lensed optical fiber  28  ( FIG. 8 ). At this time, a coupling efficiency of the ridge waveguide  31  and the lensed optical fiber  28  may be maximally heightened. The absorption layer  36  of a thickness of about 0.05 μm can absorb the first and second laser beams  16  having a beam width of about 4 μm ( FIG. 9 ). The absorption layer  36  may ideally have a thickness of about 0.005 μm. 
         [0040]    Thus, the optical device  100  in accordance with the second embodiment of the inventive concept may have a high coupling efficiency with respect to the absorption layer  36  having a thickness of less than 0.2 μm. Although not illustrated in the drawing, the optical device  100  in accordance with the second embodiment may further include the output circuit  50  connected to the photo detector  30 . Also, the photo detector  30  may be a travelling-wave type photo detector matching the ridge waveguide  31  of resistance and/or impedance to an internal resistance of the output circuit  50 . 
         [0041]    An optical device in accordance with some embodiments of the inventive concept may include first and second lasers, an optical waveguide connected to the first and second lasers and a photo detector which is connected to the optical waveguide and has an absorption layer having a thickness of less than 0.2 μm. The photo detector can generate a terahertz wave by a beating of first and second laser beams generated from the first and second lasers. When the absorption layer has a thickness of 0.1 μm, it can generate a maximum allowed photocurrent of about 0.08 A. When the absorption layer has a thickness of 0.5 μm, it can generate a maximum allowed photocurrent of less than 0.01 A. Thus, the absorption layer can increase a maximum allowed input power in inverse proportion to its thickness. 
         [0042]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the inventive concept. Thus, to the maximum extent allowed by law, the scope of the inventive concept is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.