Abstract:
The present invention discloses a semiconductor optical amplifier module with a monitoring device. The amplifier module includes a housing having a window on both sides of the opposite walls for forming a path of a first optical fiber and a second optical fiber, respectively; a semiconductor optical amplifier mounted in the housing for amplifying input optical signals and outputting the amplified optical signals; a first supporter for supporting the first optical fiber; a second supporter for supporting the second optical fiber; and a first optical detector for detecting non-coupled light, which is generated at the end of the first optical fiber.

Description:
CLAIM OF PRIORITY 
     This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application entitled, “Semiconductor Optical Amplifier Module with Monitoring Device” filed in the Korean Industrial Property Office on Oct. 12, 2001 and there duly assigned Serial No. 2001-62883. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention The present invention relates to optical communications devices and, more particularly, to an optical semiconductor module for optical amplification application. 
     2. Description of the Related Art 
     In general, an optical amplifier for use in optical communications is mainly divided into an optical-fiber amplifier and a semiconductor optical amplifier (SOA). An example of the optical-fiber amplifiers is erbium-doped optical-fiber amplifier (EDFA), in which a light source is pumped into an erbium-doped optical fiber so that the inputted optical signals are amplified through stimulated emission by the erbium element. In contrast, a commonly used semiconductor optical amplifier consists of layered structures formed on a semiconductor substrate in sequence, including an active layer with a multiple (or single) quantum well structure for a fiber amplification, a waveguide layer operable as a broadcast media of the inputted optical signals, a clad layer that encompasses the waveguide layer to confine the inputted optical signals therein, an upper electrode layer, and a lower electrode layer. It is well known that the semiconductor optical amplifier is more advantageous in that the level of the current applied to the upper electrode layer can be adjusted as occasion demands. 
     However, it is important to maintain a constant ratio between the inputted optical signal power and the outputted optical signal power (or an optical amplification ratio) in the semiconductor optical fiber. That is to say, if the optical amplification ratio of the semiconductor optical amplifier exceeds a threshold value, it can have a bad influence on the operation of other optical elements that are connected to the semiconductor optical amplifier. In addition, if the optical-amplification ratio of the semiconductor optical amplifier is smaller than the threshold value, it can deteriorate the characteristics of the outputted optical signals, such as the signal-to-noise ratio. As a result, a monitoring device is typically employed to ensure that optical characteristics are not affected by a discrepancy in the optical-amplification ratio. 
     FIG. 1 shows a monitoring device of a semiconductor optical amplifier according to the related art. As shown in FIG. 1, the monitoring device includes a semiconductor optical amplifier  110 , a bean splitter  129 , an optical detector  130 , an analog/digital converter (ADC)  140 , a bias circuit  150 , a digital/analog converter (DAC)  160 , and a controller  170 . 
     The semiconductor optical amplifier  110  amplifies the inputted optical signals within a designated optical-amplification ratio, and outputs the amplified optical signals. 
     In operation, the beam splitter  120  splits a portion of the optical signals corresponding to x % of the total optical power outputted from the semiconductor optical amplifier  110  (hereinafter referred to as an optical signal sample), then outputs the optical signal sample to the optical detector  130 . The other optical signal outputs corresponding to (100−x) % power are passed through the beam splitter  120 . Meanwhile, the optical detector  130  converts the inputted optical-signal sample from the beam splitter  120  to electric signals and outputs the corresponding electric signals. The analog/digital converter  140  converts the output signals from the optical detector  130  to corresponding digital signals and outputs the converted digital signals to the controller  170 . The controller  170  determines the power of the amplified optical signal that is outputted from the semiconductor optical amplifier  110  based on the digital signal outputs from the converter  140 . In particular, the controller  170  obtains a difference between the amplified optical signal&#39;s power and a predetermined power threshold level, then adjusts the current level that is applied to the semiconductor optical amplifier  110  to adjust the optical amplification ratio of the semiconductor optical amplifier  110 . Accordingly, the controller  170  outputs a control signal indicative of the adjusted current level to the digital/analog converter  160 , which then converts the control signal to an analog signal and outputs the converted analog signal to the bias circuit  150 . Finally, the bias circuit  150  applies the current responsive to the control signal to the semiconductor optical amplifier  110  to adjust the gain of the semiconductor optical amplifier  110 . This change in the gain causes the optical amplification ratio of the semiconductor optical amplifier  110  to change. Accordingly, the controller  170  can control the optical-amplification ratio of the semiconductor optical amplifier  110  to maintain at a specific level. 
     There are some drawbacks in the prior art monitoring device in that it requires extra components, such as a beam splitter  120  for monitoring the outputted optical signal&#39;s power. As a result, it increases manufacturing costs. In addition, the semiconductor optical amplifier  110 , the beam splitter  120 , and the optical detector  130  must be positioned precisely in an array, and further require other devices (not shown) for the same application. Furthermore, the maximum output power of the semiconductor optical amplifier  110  suffers as a portion of the outputted optical signals are extracted during the monitoring operation. 
     In summary, the conventional structure of the semiconductor optical amplifier module that is mounted with the semiconductor optical amplifier and the monitoring device in a housing has drawbacks associated with high manufacturing cost, low integration due to the deployment of the beam splitter and additional optical components, and a loss in the maximum output power. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems, and provides additional advantages by providing a semiconductor optical amplifier module with a monitoring device that has a low manufacturing cost, a high integration capability, and a maximum output power. 
     According to an aspect of the invention, there is provided a semiconductor optical amplifier with a monitoring device, which includes: a housing having a window on both sides of opposite walls for forming a path of a first optical fiber and a second optical fiber, respectively; a semiconductor optical amplifier fixated in the housing for amplifying the inputted optical signals and outputting the amplified optical signals; a first supporter for supporting the first optical fiber; and, a second supporter for supporting the second optical fiber. 
     According to another aspect of the invention, the semiconductor optical amplifier further includes a first optical detector that is arrayed in such a way to detect the non-coupled light generated in the line of the first optical fiber. 
     The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead is placed upon illustrating the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the method and apparatus of the present invention is available by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 shows a diagram explaining a monitoring device of a semiconductor optical amplifier according to a related art; 
     FIG. 2 illustrates a schematic sectional view showing a semiconductor optical amplifier with a monitoring device according to a preferred embodiment of the present invention; and, 
     FIG. 3 schematically illustrates the monitoring process of the semiconductor optical amplifier depicted in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     FIG. 2 is a schematic sectional view of a semiconductor optical amplifier with a monitoring device in accordance with a preferred embodiment of the present invention. As shown in FIG. 1, the semiconductor optical amplifier module  200  includes a box-shaped housing  210 , a thermoelectric cooling device  260 , a substrate  270 , a submount  280 , a semiconductor optical amplifier  290 , a first supporter  300  and  310 , a second supporter  320  and  330 , a first optical detector  340 , and a second optical detector  350 . 
     The housing  210  has the size of 20 mm×7 mm×7 mm and includes a plurality of electrodes (not shown) for receiving current and exchanging signals from an outer source. Formed on one sidewall of the housing  210  is a first window  220  for forming a pathway for a first optical fiber through which optical signals are inputted. On the other opposing side wall of the housing  210  is a second window  240  for forming a pathway for a second optical fiber  250  through which the amplified optical signals are outputted. Preferably, the first and the second windows  220  and  240  have a diameter of about 3 mm, respectively. Also, a preferable material for use for the first and the second optical fibers  230  and  250  is a single mode fiber tapered with a V-shaped edge. 
     The thermoelectric cooling device  270  has the size of 20 mm×7 mm×7 mm and is fixated on the bottom of the housing  210 . Typically, the thermoelectric cooling device  270  is operated using the current supplied from the outer source. A major function of the thermoelectric cooling device  270  is to maintain a constant temperature in the semiconductor optical amplifier  290 . The substrate  270  is fixably mounted on the upper surface of the thermoelectric cooling device  260  using a matrix made of InAg having a melting point of 130° C. Also, the substrate  270  can be made of kovar to facilitate laser welding. The submount  280  is fixably mounted on the upper surface of the substrate  270  using a matrix made of AuSn having a melting point of 280° C. The submount  280  is aligned in a straight line that connects the first window  220  to the second window  240 . 
     The semiconductor optical amplifier  290  having a first cut-end and a second cut-end is fixably mounted on the upper side of the submount  280  using the same matrix material used for the submount, AuSn, having a melting point of 280° C., and amplifies the optical signals inputted through the first cut-end and outputs the amplified signals through the second cut-end. As a bi-directional optical device, the semiconductor optical amplifier  290  can amplify the optical signals inputted through the second cut-end and outputs the amplified signals through the first cut-end. The semiconductor optical amplifier  290  is formed by an approximately 1 mm-sized semiconductor substrate (not shown), which is made of a layered structure of an active layer having a multiple (or single) quantum well structure for a fiber amplification; a waveguide layer as a broadcast media for the inputted optical signals; a clad layer that encompasses the waveguide layer to confine the inputted optical signals; an upper electrode layer for receiving current from the outside, and a lower electrode layer. Note that the gain of the semiconductor optical amplifier  290  can be selectively changed by adjusting the current level that is supplied to the upper electrode layer from an external source. 
     The first supporter  300  and  310 , including a first ferrule  310  and a first submodule  300 , are mounted on the upper surface of the substrate  270 . The first submodule  300  is preferably made of the same material as the substrate  270  and aligned along a straight line that connects the first cut end of the semiconductor optical amplifier  290  to the first window  220 . The first ferrule  310  has a diameter of about 2.7 mm and is extended through the first submodule  300  using a laser welding technique. The first optical fiber  230  is inserted into a cylinder-shaped hole (not shown) mounted on the first ferrule  310 . Here, one end of the first optical fiber  230  is disposed in the opposite end of the first cut-end of the semiconductor optical amplifier  290 . 
     The first optical detector  340  is fixated on the upper surface of the substrate  270 , interposed between the submount  280  and the first submodule  300 . To detect the non-coupled light generated at the end of the first optical fiber  230 , the first optical detector  340  is disposed in a straight line that connects the first cut end of the submount  280  with one end of the first optical fiber  230 . A preferable material for the first optical detector  340  is a photodiode chip. Moreover, the first optical detector  340  is fixated on the upper surface of the substrate  270  using a matrix made of AuSn with a melting point at 280° C. The first optical detector  340  is operative to detect the non-coupled light, such as scattered light or bounce light that are usually generated by the incident optical signals on the linear cut-end of the first optical fiber  230 , after the optical signals is first amplified by the semiconductor optical amplifier  290  and outputted through the first cut-end of the semiconductor optical amplifier  290 . Another function of the first optical detector  340  is to output a first electric signal indicative of the power level of the non-coupled light. 
     The second supporter  320  and  330  includes a second ferrule  330  and a second submodule  320 , and is fixated on the upper side of the substrate  270 . 
     With a continued reference to FIG. 2, the second submodule  320  is made of the same material with the substrate  270  and positioned along a straight line connecting the second cut-end of the semiconductor optical amplifier  290  and the second window  240 . Similar to the first ferrule  310 , the second ferrule  330  has a diameter of about 2.7 mm and extended through the second submodule  320  using a laser welding technique. The second optical fiber  250  is inserted into a cylinder-shaped hole (not shown) mounted on the second ferrule  320 . Note that one end of the second optical fiber  250  is disposed to be opposite the second cut-end of the semiconductor optical amplifier  290 . 
     Again, similar to the first optical detector  340 , the second optical detector  350  is fixated on the upper surface of the substrate  270 , interposed between the submount  280  and the second submodule  320 . To detect the non-coupled light generated at one end of the second optical fiber  250 , the second optical detector  350  is positioned along a straight line that connects the second cut-end of the submount  280  to one end of the second optical fiber  250 . A preferable material for the second optical detector  350  is a photodiode chip. The second optical detector  350  is mounted on the upper surface of the substrate  270  using a matrix made of AuSn having a melting point of 280° C. The second optical detector  350  is operative to detect the non-coupled light, such as scattered light or bounce light that are usually generated by the incidental optical signals on one end of the second optical fiber  250 , after the optical signals are first amplified by the semiconductor optical amplifier  290  and outputted through the second cut-end of the semiconductor optical amplifier  290 . Another function of the second optical detector  350  is to output a second electric signal indicative of the power level of the non-coupled light. 
     FIG. 3 is a schematic view explaining a monitoring procedure of the semiconductor optical amplifier module  200  shown in FIG.  2 . As depicted in FIG. 3, there are a semiconductor optical amplifier module  200 , a first analog/digital converter  440 , a second analog/digital converter  470 , a digital/analog converter  510 , a bias circuit  530 , and a controller  490 . The semiconductor optical amplifier module  200  is a bi-directional optical device operative to amplify and output both the forward direction optical signal  410  and the reverse direction optical signal  420 . 
     During the forward amplification process, the semiconductor optical amplifier module  200  outputs a first electric signal  460  indicative of the power level for the non-coupled light in respond to the forward optical signal  410  that has been inputted through the first optical fiber  230 . The second analog/digital converter  470  converts the first electric signal  460  outputted from the semiconductor optical amplifier module  200  to a corresponding first digital signal  480 , then outputs the digital signal  480  to the controller  490 . Thereafter, the controller  490  determines the power level of the amplified forward optical signal  410  that has been outputted from the semiconductor optical amplifier module  200 . More specifically, the controller  490  determines a difference between the pre-designated power and the power level of the amplified forward optical signal  410 , then adjusts the level of the current applying to the semiconductor optical amplifier module  200  based on the power difference in order to maintain the optical amplification ratio of the semiconductor optical amplifier module  200  at a pre-designated optical-amplification ratio. As such, the controller  490  outputs a control signal  500  indicative of the required change in the current level to achieve the constant amplification ratio. 
     Meanwhile, the digital/analog converter  510  converts the control signal  500  to an analog signal  520  and outputs the analog signal  520  to the bias circuit  530 . The bias circuit changes the optical amplification ratio of the semiconductor optical amplifier module  200  by applying the current according to the analog signal  520  received from the DAC  510 . In summary, the controller  490  monitors the optical-amplification ratio of the semiconductor optical amplifier module  200  to control the optical-amplification ratio against the forward optical signal  410  to be in accord with the pre-designated value. 
     During the reverse amplification process, the semiconductor optical amplifier module  200  outputs a second electric signal  430  indicative of the power level of the non-coupled light in respond to the reverse optical signal  430  that has been inputted through the second optical fiber  250 . The first analog/digital converter  440  converts the second electric signal  430  inputted from the semiconductor optical amplifier module  200  to a corresponding second digital signal  450  and outputs the digital signal to the controller  490 . 
     Based on the second digital signal  450 , the controller  490  determines the power level of the amplified reverse optical signal  410  that has been outputted from the semiconductor optical amplifier module  200 . More specifically, the controller  490  determines the difference between the pre-designated power level and the power level of the amplified reverse optical signal  420 , then adjusts the level of the current that is applied to the semiconductor optical amplifier module  200  to maintain the optical amplification ratio of the semiconductor optical amplifier module  200  at a pre-designated amplification ratio. Accordingly, the controller  490  outputs a control signal  500 . 
     The digital/analog converter  510  converts the control signal  500  to an analog signal  520  and outputs the analog signal  820  to the bias circuit  530 . Then, the bias circuit  530  changes the optical-amplification ratio of the semiconductor optical amplifier module  200  by applying the current according to the analog signal  520 . As such, the controller  490  monitors the optical-amplification ratio of the semiconductor optical amplifier module  200  to control the optical amplification ratio against the forward optical signal  410  to be at a pre-designated value. 
     To conclude, the semiconductor optical amplifier module with the monitoring device according to the present invention detects the power level of the non-coupled light that is generated at one end of the first/second optical fiber using the photodiode chip, then determines the power level of the amplified forward or reverse optical signal. As a result, the inventive semiconductor optical amplifier module does not require an expensive beam splitter or a monitoring device that requires a precise disposition. Besides, as the monitoring procedure has no influence on the outputted optical signals, the semiconductor optical amplifier module of the present invention can be manufactured at a very low cost without compromising the maximum output power. 
     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 the scope of the invention as defined by the appended claims.