Abstract:
A semiconductor optical amplifier (SOA) module relies on Amplified Spontaneous Emission (ASE) from the first stage of an SOA in feedback-based regulation of the amplification factor. The ASE is deflected by an isolator in the input unit at a prescribed angle from a traveling path of the input optical signal and toward a photo-detector that detects the power of the ASE light. Regulation is performed by a controller that receives the detected power level from the photo-detector and also receives from another photo-detector a power level of the optical signal after amplification in the SOA.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “SEMICONDUCTOR OPTICAL AMPLIFIER MODULE,” filed in the Korean Intellectual Property Office on May 21, 2003 and assigned Serial No. 2003-32239, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor optical element, and more particularly to a semiconductor optical amplifier module for amplifying an optical signal entering the semiconductor optical element.  
           [0004]    2. Description of the Related Art  
           [0005]    In typical use are a variety of optical amplifiers for optical communication, one example being an optical fiber amplifier doped with any one of a several rare earth ions, such erbium ions and thulium ions. Such optical fiber amplifiers require that the rare earth ions be pumped to them with a pumping light so that the amplifier can amplify its own received optical signal.  
           [0006]    The semiconductor optical amplifier includes a plurality of layers deposited on a semiconductor substrate, i.e., an activation layer having a multi-quantum well, a waveguide layer serving as an I/O (Input/Output) medium, a clad layer surrounding the waveguide layer, an upper electrode layer, and a lower electrode layer.  
           [0007]    Too high an amplification factor for the semiconductor optical amplifier may harm other optical elements connected to the semiconductor optical amplifier and/or may deteriorate the amplifier&#39;s optical signal as evidenced by a low SNR (Signal-to-Noise Ratio). Such SNR characteristics indicate a ratio of signal power contained in an optical signal, present in either a general transmission/reception device (e.g., a receiver or an amplifier) or an optical communication system, to noise power. The SNR characteristics serve as an index for indicating the ratio of an optical signal to noise. The optical signal power is denoted by “S”, and the noise power is denoted by “N”.  
           [0008]    Maintaining a fixed ratio of entry optical signal to amplified optical signal, i.e., a fixed amplification factor, is very critical to the optimal performance of the semiconductor optical amplifier. A semiconductor optical amplifier module includes a light-receiving element for monitoring the power level of an I/O optical signal, and a controller for comparing a power level of the optical signal detected by the light-receiving element with a prescribed power level and for maintaining a prescribed, constant amplification gain.  
           [0009]    [0009]FIGS. 1 and 2 depict a conventional optical amplifier module. The conventional optical amplifier module includes a semiconductor optical amplifier (SOA)  110 , an input unit  140  containing a first detector  160 , an output unit  150  containing a second detector  170 , input and output optical fibers  120  and  130 , and a controller  180 .  
           [0010]    One end of the SOA  110  faces the input unit  140  while the other end faces the output unit  150 . the SOA  110  amplifies an optical signal  101  applied from the input optical fiber  120  to the input unit  140 , and outputs the amplified optical signal  103  to the output unit  150 .  
           [0011]    The input unit  140  includes a first collimating lens system  141  for collimating the optical signal  101  received from the input optical fiber  120 , a first glass window  142 , a first convergence lens system  144  for converging an optical signal collimated by the first glass window  142  at one end of the SOA  110 , a first isolator  143  disposed between the first glass window  12  and the first convergence lens system  144 , and a first detector  160  disposed between the first glass window  142  and the first isolator  143 . The input unit  140  serves as a signal combiner for converging onto the SOA  110  the optical signal  101  received from the input optical fiber  120 .  
           [0012]    The first collimating lens system  141  collimates the optical signal  101  received from the input optical fiber  120  therein. The first glass window  142  transmits an optical signal collimated at the first collimating lens system  141  to the first isolator  143 , and is disposed between the first collimating lens system  141  and the first detector  160 .  
           [0013]    The first isolator  143  transmits the optical signal it receives from the first glass  142  headed toward the first collimating lens system  144 , and cuts off an optical signal transmitted back from the first convergence lens system  144  toward the first detector  160 .  
           [0014]    The first convergence lens system  144  converges the optical signal generated by the first isolator  143  onto one end of the semiconductor optical amplifier  110 .  
           [0015]    The first detector  160 , disposed between the first glass window  142  and the first isolator  143 , includes a reflector  161  for partially reflecting the optical signal transferred from the first glass window  142  to the first isolator  143  perpendicular to a traveling path of the optical signal, and a first monitor photo-diode  162  for detecting a power level of the optical signal  102  reflected from the reflector  161 . The first detector  160  is adapted to monitor an amplification gain of the optical signal  103  amplified by the SOA  110 , and detects a power level of the optical signal  101  applied to the SOA  110 .  
           [0016]    The output unit  150  is a combiner for collecting the optical signal  103  amplified by the SOA  110  in the output optical fiber  130  with minimum transfer loss. The output unit  150  includes a second convergence lens system  154  for collimating the optical signal  103  received from the SOA  110 , a second isolator  153 , a second convergence lens system  151  for converging the amplified optical signal  103  onto one end of the output optical fiber  130 , a second glass window  152  for transmitting the amplified optical signal  103  to the second convergence lens system  151 , and a second detector  170  disposed between the second isolator  153  and the second glass window  152 .  
           [0017]    The second detector  170  includes an beam splitter  172  for dividing a power level of the amplified optical signal  103  traveling from the second isolator  153  to the second glass window  152 , and a second monitor photo-diode  171  for detecting a power level of the optical signal  104  divided by the beam splitter  172 .  
           [0018]    The controller  180  receives the power level of the optical signal  102  from the first detector  160  and the power level of the power signal  104  from the second detector  170 , and compares the power level of the optical signal  102  applied to the SOA  110  with the power level of the optical signal  104  amplified by the SOA  110  to recognize an amplification gain of the SOA  110 . The controller  180  compares the power levels of the optical signals  102 ,  104  detected by the first and second detectors  160 ,  170 , and outputs a control signal to the SOA  110  to maintain a constant, prescribed amplification gain.  
           [0019]    The output optical fiber  130  outputs outside the SOA module a reception optical signal converging on one end of the fiber  130  by means of the second convergence lens system  151  of the output unit  150 .  
           [0020]    However, the conventional SOA module adapts a plurality of detectors each having either a high-priced power divider or a mirror to detect a power level of its own reception optical signal and a power level of an amplified optical signal, resulting in an increased number of fabrication steps and increased production costs. Therefore, the conventional SOA module decreases coupling efficiency between the SOA and an input unit along with coupling efficiency between the output unit and the output optical fiber, resulting in an increased noise factor of the SOA and a reduced saturation output power.  
         SUMMARY OF THE INVENTION  
         [0021]    Therefore, the present invention has been made in view of the above problems, and, in an aspect of the present invention, an SOA (Semiconductor Optical Amplifier) module monitors an amplification gain of an amplified optical signal without causing deterioration of the coupling efficiency.  
           [0022]    In accordance with the present invention, the above and other aspects can be accomplished by the provision of a semiconductor optical amplifier (SOA) module apparatus that includes a semiconductor optical amplifier (SOA) for amplifying an optical signal applied to its own first stage, outputting the amplified optical signal at its own second stage, and outputting an ASE (Amplified Spontaneous Emission) light at the first stage. The module further includes an input unit having a first isolator which transmits an input optical signal to the first stage of the SOA, controls the ASE light received from the first stage of the SOA to separate it from a traveling path of the input optical signal at a prescribed angle, and transmits the ASE light separated from the traveling path. A first monitor photo-diode with its own light-receiving surface oriented perpendicular to a traveling path of the ASE light emitted from the first isolator detects a power level of the ASE light. An output unit outputs the amplified optical signal received from the second stage of the SOA to the outside, and outputs a partially-uncoupled optical signal created therein for reception by a second monitor photo-diode for detecting an uncoupled optical signal emitted from the output unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which the same or similar features are annotated with identical or analogous numerals throughout the several views:  
         [0024]    [0024]FIG. 1 is a block diagram of detecting a power of an optical signal applied to a conventional SOA module;  
         [0025]    [0025]FIG. 2 is a block diagram of detecting a power of an optical signal outputted from a conventional SOA module;  
         [0026]    [0026]FIG. 3 is a block diagram of a SOA module in accordance with a first preferred embodiment of the present invention;  
         [0027]    [0027]FIG. 4 is a block diagram of a SOA module in accordance with a second preferred embodiment of the present invention;  
         [0028]    [0028]FIG. 5 a  is a graph illustrating a relationship between a power level of an optical signal applied to a SOA shown in FIG. 2 and a power level of an amplified spontaneous emission (ASE) light created at the first stage of the SOA; and  
         [0029]    [0029]FIG. 5 b  is a graph illustrating a relationship between a power level of an uncoupled optical signal emitted from an output unit of the SOA and a power level of an output signal amplified by the SOA. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the following description, detailed description of known functions and configurations incorporated herein will be omitted for clarity of presentation.  
         [0031]    [0031]FIG. 3 is a block diagram depicting, by way of non-limitative example, an SOA module in accordance with a first preferred embodiment of the present invention. The SOA module includes an input optical fiber  220 , an output optical fiber  230 , a SOA  210  for amplifying a received optical signal, an input unit  240  for transmitting an optical signal  201  received from the input optical fiber  220  to one end of the SOA  210 , a first monitor photo-diode  260 , an output unit  250  for converging an optical signal  203  amplified by the SOA  210  onto one end of the output optical fiber  230 , a second monitor photo-diode  270 , and a controller  280  for controlling an amplification gain of the optical signal  203  amplified by the SOA  210 .  
         [0032]    The input optical fiber  220  transmits an optical signal to be amplified to the SOA module, and the output optical fiber  230  outputs the optical signal  203  amplified by the SOA  210  to the outside of the SOA module.  
         [0033]    The SOA  210  amplifies an applied optical signal  201  at its own first stage, and outputs the amplified optical signal  203  at its second stage. A lower clad layer, an activation layer, and an upper clad layer are sequentially deposited on a semiconductor substrate of the SOA  210 . A window layer for restricting a current applied to the activation layer may be deposited on both ends of a ridge stripe disposed at the center of the resultant layer on which the lower clad layer, the activation layer, and the upper clad layer are sequentially deposited. A cap layer may be deposited on the uppermost layer of the SOA. If the input light passes through the activation layer of the SOA  210 , the output light of the SOA  210  is amplified by an amplification gain of the activation layer.  
         [0034]    The SOA  210  has a configuration similar to that of a semiconductor laser device. However, in contrast to the semiconductor laser device, the SOA  210  deposits an antireflective coating layer on both ends of a cleaved region in such a way that a traveling-wave-type SOA is formed. The SOA  210  outputs the ASE light  202  created therein while amplifying an optical signal through one of its ends serving to receive the optical signal to be amplified.  
         [0035]    The input unit  240  includes a first isolator  243 , a first collimating lens system  241  for collimating an input optical signal  201 , a first glass window  242  for transmitting the collimated optical signal to the first isolator  243 , and a first convergence lens system  244  disposed between the first isolator  243  and the SOA  210 . The input unit  240  serves as a combiner for coupling the optical signal  201  received from the input optical fiber  220  with one end of the SOA  210 .  
         [0036]    The first collimating lens system  241  faces one end of the input optical fiber  220 , and collimates the optical signal  201 . The first glass window  242  is disposed between the first collimating lens system  241  and the first isolator  243 , and transmits the optical signal collimated at the first collimating lens system  241  to the first isolator  243 .  
         [0037]    The first convergence lens system  244 , disposed between the first isolator  243  and the SOA  210 , converges the optical signal received from the first isolator  243  onto a first stage of the SOA  210 , and outputs the ASE light  202  emitted from the first stage of the SOA  210  to the first isolator  243 .  
         [0038]    The first isolator  243  transmits the optical signal received from the first glass window  242  to the first convergence lens system  244 , controls the ASE light  202  received from the SOA  210  to separate it from a traveling path of the optical signal collimated at the first collimating lens system  241  at a prescribed angle to the traveling path, and transmits the ASE light  202  separated from the traveling path. An isolator independent of a polarized light may be adapted as such a first isolator  243 , and is made of a birefringence material.  
         [0039]    The first monitor photo-diode  260  is arranged at one end of the input unit  240  to detect a power level of the ASE light  202  received from the first isolator  243 , and outputs the detected power level of the ASE light  202  to the controller  280 . For this purpose, the first photo diode  260  is arranged to allow its activation layer (not shown) to be perpendicular to the traveling path of the ASE light  202 .  
         [0040]    The output unit  250  serves as a combiner for converging the optical signal  203  amplified by the SOA  210  onto one end of the output optical fiber  230 . The output unit  250  includes a second collimating lens system  254  for collimating the optical signal  203  amplified by the SOA  210 , a second isolator  253  for transmitting the optical signal collimated at the second collimating lens system  254 , a second convergence lens system  251  for converging the optical signal  203  amplified by the SOA  210  onto one end of the output optical fiber  230 , and a second glass window  252  disposed between the second isolator  253  and the second convergence lens system  251 .  
         [0041]    The second collimating lens system  254  faces a second stage of the SOA  210 , and collimates the optical signal  203  amplified by the SOA  210 .  
         [0042]    The second isolator  253  transmits the optical signal collimated at the second collimating lens system  254 , controls a partially-uncoupled optical signal  204  to separate it from a traveling path of the optical signal collimated at the second collimating lens system  254  at a prescribed angle to the traveling path, and transmits the uncoupled optical signal  204  separated from the traveling path. The uncoupled optical signal  204  is emitted at a prescribed angle while traveling through the second isolator  253 , the optical signal having escaped from the traveling path of the collimated optical signal. An isolator independent of a polarized light may be adapted as such a second isolator  253 .  
         [0043]    The second glass window  252  is disposed between the second isolator  253  and the second convergence lens system  251 , and transmits the collimated optical signal received from the second isolator  253  to the second convergence lens system  251 . The second convergence lens system  251  is disposed between the second glass window  252  and one end of the output optical fiber  230 , and converges the optical signal received from the second glass window  252  onto one end of the output optical fiber  230 .  
         [0044]    Specifically, the output unit  250  converges the optical signal  203  amplified by the SOA  210  onto one end of the output optical fiber  230 , and outputs a partially-uncoupled optical signal  204 , which, due to reflection or dispersion, escapes from a traveling path of the amplified optical signal  203  toward the output optical fiber.  
         [0045]    The second monitor photo-diode  270  detects the uncoupled optical signal  204  created from the second isolator  253  of the output unit  250 , and outputs a power level of the uncoupled optical signal  204  to the controller  280 . An activation layer (not shown) of the second monitor photo-diode  270  is arranged to be perpendicular to a traveling path of the uncoupled optical signal  204 . Specifically, due to reflection or dispersion to create the uncoupled optical signal, its path is diverted away from the output optical fiber  230  and toward the second monitor photo-diode  270  which has been disposed to receive the signal.  
         [0046]    The controller  280  compares a power level of the ASE light  202  detected by the first monitor photo-diode  260  with a power level of the uncoupled optical signal  204  detected by the second monitor photo-diode  270 , and calculates an amplified gain of the optical signal  203  amplified by the SOA  210 . The controller  280  compares a real amplification gain of the SOA  210  with a prescribed amplification gain to be maintained at the SOA  210 , and outputs a control signal for allowing the SOA  210  to constantly maintain a prescribed stable amplification gain to the SOA  210 .  
         [0047]    [0047]FIG. 5 a  depicts a graph illustrating, by way of example and based on exemplary experimental data, a relationship between a power level of an optical signal applied to the SOA  210  shown in FIG. 2 and a power level of an amplified spontaneous emission (ASE) light created from a first stage of the SOA  210 . The X-axis (Pin) denotes the power level of the optical signal  201  applied to the SOA  210 , the left Y-axis (Pout) denotes a power level of the optical signal  203  amplified by the SOA  210 , and the right Y-axis (MPDin) denotes a power level of the ASE light  202  detected by the first monitor photo-diode  260  disposed at one side of the input unit  240 . As seen from the graph, the power level of the ASE light  202  detected by the first monitor photo-diode  260  is inverse-proportional to the power level of the optical signal  201  applied to the SOA  210 .  
         [0048]    The box indicated by dotted lines in FIG. 4 a  denotes an effective detection range  400  for detecting a power level of the optical signal  201  applied to the input unit  240  upon receiving a power level of the ASE light  202  detected by the first monitor photo-diode  260 . The effective detection range  400  denotes a prescribed zone wherein the power level of the ASE light  202  detected by the first monitor photo-diode  260  is inversely proportional to the power level of the optical signal  201  applied to the input unit  240 . The amplified optical signal readings outside the range  400  represent power levels higher than inverse proportionality would suggest. The effective detection range  400  is therefore confined to 0.0˜0.6 mW, as shown.  
         [0049]    [0049]FIG. 5 b  depicts a graph illustrating a relationship between a power level of an uncoupled optical signal emitted from the output unit of the SOA  210  shown in FIG. 2 and a power level of an output signal amplified by the SOA  210 . The X-axis (Pin) denotes a power level of the optical signal  201  applied to the SOA  210 , the left Y-axis (Pout) denotes a power level of the optical signal  203  amplified by the SOA  210 , and the right Y-axis (MPDout) denotes a power level of the uncoupled optical signal  204  detected by the second monitor photo-diode  270  disposed at one side of the output unit  250 . As can be seen from the graph, the power level of a partially-uncoupled optical signal  204  created from the output unit  250  varies linearly with the power level of the optical signal  203  amplified by the SOA  210 .  
         [0050]    [0050]FIG. 4 is a block diagram showing a possible embodiment for an SOA module in accordance with a second preferred embodiment of the present invention. As in the first embodiment, the output unit  250  creates a partially-uncoupled signal  204  by diverting the optical signal  203  from its traveling path and at a prescribed angle toward the second photo-detector  270 . This second embodiment differs from the first embodiment, however, in that it is the second glass window, rather than the second isolator, which partially uncouples the optical signal  204 , and in that the separation in the second embodiment is by means of reflection. Accordingly, in the second embodiment, the amplified optical signal  303  is transmitted by the second isolator  353  to the second glass window  352 , and it is the second glass window  352  that creates the partially-uncoupled signal  204 . As apparent from the above description, the SOA module according to the present invention detects a power level of a reflection or uncoupled optical signal created from either an isolator or a prescribed module such as a glass window, such that it is not affected by the coupling efficiency of I/O optical signals, and at the same time detects an amplification gain of an optical signal amplified by the SOA, resulting in a minimal noise factor and a minimal saturation output power.  
         [0051]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.