Abstract:
Optical power equalizer in a WDM optical communication system which can tune an optical power down to a desired extent and a variable optical attenuator for use therein, optical power equalizer including a variable optical attenuator for reducing a power of an optical signal to a given level, an optical coupler for detecting a portion of signal proportional to an output of the variable optical attenuator, and an optical power monitor for receiving an output of the optical coupler and generating an electrical control signal for controlling an output of the variable optical attenuator; and the variable optical attenuator including two asymmetric optical waveguides adjacent to each other to form a directional coupler, and a thermooptic electrode for varying the asymmetry of the asymmetric optical waveguides, thereby attenuating an optical power of an optical communication system.

Description:
BACKGROUND OF THE INVENTION 
     1 Field of the Invention 
     The present invention relates to an optical power equalizer in a wavelength division multiplexing(WDM) optical communication system, and more particularly, to an optical power equalizer in a WDM optical communication system which can tune an optical power down to a desired extent and a variable optical attenuator for use therein. 
     2. Background of the Related Art 
     Having overcome a level in which a light is merely taken as a transmission medium in place of electricity, currently the optical communication has come to a level in which an optical signal is converted into an electrical signal at a final reception terminal after amplifying the optical signal for a longer distance transmission or processing the optical signal while maintaining a form of the optical signal without converting the optical signal into an electrical signal. And, recently, the wavelength division multiplexing technique is rapidly developed and spread for increasing a transmission capacity, in which an individual light source is provided for each of wavelengths in an optical signal, multiplexed before transmission, demultiplexed at a reception terminal, split into the individual wavelengths, thereby receiving the optical signal. And, sometimes, an optical amplifier is provided in the middle of the transmission for a long distance transmission. An optical communication network is sometimes provided with an add-drop function in which portions of the optical signal are allocated to required places and optical signals from the places are added thereto. 
     FIG. 1 schematically illustrates a related art WDM add-drop system, in which an optical signal transmitted through an optical amplifier  10  is demultiplexed through a demultiplexer  11  into individual wavelengths λ 1 ,λ 2 ,- - -,λ n , add-drops are made for the individual wavelengths through optical switches  12 , multiplexed through a multiplexer  14 , amplified through an optical amplifier, and transmitted through one optical fiber. In this instance, as shown in FIG. 2A, the optical signal before the optical signal is multiplexed exhibits a non-uniform state between different wavelengths. Though there are many reasons which cause this non-uniform state, this non-uniform state typically comes from composite factors, such as differences of amplification gains between the wavelengths when the optical signal is amplified through the optical amplifier, differences in the wave splitting performances of the multiplexer  11  between the individual wavelengths, non-uniform switching characteristics of the optical switches  12 , which cause the non-uniform state between the individual wavelengths in the optical signal at an input terminal of the multiplexer  14  so serious that a signal characteristic is deteriorated too much unable to make a long distance communication. Consequently, it is required to make powers of the optical signals uniform before the multiplexing, for which the optical attenuators  13  as shown in FIG. 1 are provided, owing to which outputs of the individual wavelengths are made uniform as shown in FIG.  2 B. The optical attenuator, generally used currently with optical fibers handled manually, is not responsive to continuous signal changes since the system is fixed once adjusted initially. Therefore, a variable optical attenuator is required, which can be used continuously while monitoring an optical signal. Such a variable optical attenuator is disclosed, for example, in U.S. Pat. No. 4,644,145. Though the U.S. Pat No. 4,644,145 discloses variable optical attenuators used in optical waveguides for compensating variations of signals received at an optical receiver, with the object of the U.S. Pat. 4,644,145 lying on improvement of a receiver performance merely by making sizes of signals at a reception terminal uniform, it is very difficult to employ the optical attenuator in the WDM optical communication because the optical attenuator is one used before the optical communication system technology is matured, in which each of fast optical signals received in succession is attenuated at a fast speed for compensating time basis non-uniformity of the optical signals. That is, for modulation of the fast optical signals, though the variable optical attenuator in the U.S. Pat. No. 4,644,145 uses LiNbO 3  as a substrate for utilizing an electrooptical effect or GaAs, a semiconductor material, a device using LiNbO 3  as a substrate is expensive despite fast modulation is allowed, and not suitable for insertion in the middle of transmission for use, not in a few GHz range of fast modulation, but in the add-drop multiplexing system at a few kHz range of frequency because of a low coupling efficiency of the optical fiber and a device using GaAs, a semiconductor material, can not be put into practical use because of a further lower coupling efficiency and expensive material. And, the related art variable optical attenuator has a micrometer which uses an optical fiber for manual mechanical manipulation of the micrometer, or a micrometer fitted with a motor for electric control. However, the related art variable optical attenuator with a micrometer fitted with a motor, not only has many problems in actual application to the WDM optical communication system because it has a large power consumption, high driving voltage over 10V, relatively bulky, and a slow tuning speed of 50-1,400 msec, but also has a problem of high cost because the device is not suitable for mass production in view of the device structure. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an optical power equalizer in a WDM optical communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an optical power equalizer in a WDM optical communication system, in which a tuning for maintaining an output of an optical signal in an optical communication system constant is allowed. 
     Other object of the present invention is to provide an optical power equalizer in a WDM optical communication system, which can provide a stable operation and has a low material cost and a simple fabrication process suitable for mass production; and a variable optical attenuator for use therein. 
     Another object of the present invention is to provide an optical power equalizer in a WDM optical communication system, which is operative with easy and has a low insertion loss; and a variable optical attenuator for use therein. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the optical power equalizer in a WDM optical communication system includes a variable optical attenuator for reducing a power of an optical signal to a given level, an optical coupler for detecting a portion of signal proportional to an output of the variable optical attenuator, and an optical power monitor for receiving an output of the optical coupler and generating an electrical control signal for controlling an output of the variable optical attenuator. 
     In another aspect of the present invention, there is provided a variable optical attenuator including two asymmetric optical waveguides adjacent to each other to form a directional coupler, and a thermooptic electrode for varying the asymmetry of the asymmetric optical waveguides, thereby attenuating an optical power of an optical communication system. 
     The optical power equalizer in a WDM optical communication system of the present invention not only allows to maintain a performance of an optical signal at an optimal state because the optical power can be always monitored and regulated at a level, but also is applicable to an optical attenuator for a WDM optical communication which requires no fast speed operation, which has stable operation and low cost and allows mass production owing to a simple fabrication process because a polymer thin film, which has a high thermooptic effect compared to silica used currently and is low cost, is used as an optical waveguide thin film in the optical attenuator. 
     And, in a case when two asymmetric optical waveguides are used in the optical attenuator, fine adjustment of the optical attenuator is available, and the optical attenuator has a low power consumption and driving voltage because the polymer thin film formed on a surface of the optical attenuator has a thermooptic effect 10 time greater than silica, reduces an insertion loss because the polymer is easy to adjust a refractive index by synthesizing and has a high coupling efficiency to an optical fiber, and allows a continuous use of the optical attenuator while monitoring the optical signal because an optical power reduction in the reference(channel) optical waveguide can be adjusted using a current flowing through the thermooptic electrode depending on which optical power of the reference optical waveguide is changed. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 schematically illustrates a related art WDM add-drop system; 
     FIGS. 2A and 2B respectively illustrate powers of optical signals for wavelengths before being multiplexed and in a case an optical attenuator is used in a related art add-drop WDM device; 
     FIG. 3 illustrates a block diagram showing an optical power equalizer in accordance with a preferred embodiment of the present invention; 
     FIG. 4 illustrates a cut-off type optical attenuator for use in an optical power equalizer of the present invention; 
     FIG. 5 illustrates a Mach-Zehnder interferometer type optical attenuator for use in an optical power equalizer of the present invention; 
     FIGS.  6 ˜ 8  respectively illustrate an asymmetric directional coupler type optical attenuator for use in an optical power equalizer of the present invention; 
     FIGS.  9 A˜ 9 C each illustrates an example of a beam propagation and an optical attenuation characteristic of an optical attenuator having a directional coupler of two symmetric optical waveguides; 
     FIGS.  10 A˜ 10 C illustrate graphs of the optical powers in FIGS.  9 A˜ 9 C each shown with respect to a propagation direction; 
     FIGS.  11 A˜ 11 C each illustrates an example of a beam propagation and an optical attenuation characteristic of an optical attenuator having a directional coupler of two asymmetric optical waveguides; and, 
     FIGS.  12 A˜ 12 C illustrate graphs of the optical powers in FIGS.  11 A˜ 11 C each shown with respect to a propagation direction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 3 illustrates a block diagram showing an optical power equalizer in accordance with a preferred embodiment of the present invention, and FIGS.  4 ˜ 8  illustrate sections showing various embodiments of the variable optical attenuators in an optical power equalizer of the present invention. 
     Referring to FIG. 3, the optical power equalizer in accordance with a preferred embodiment of the present invention includes an optical waveguide  20 , a variable optical attenuator  21  for attenuating a power of an optical signal to a given level, an optical coupler  22  for separating a portion of an output of the optical attenuator  21 , and an optical power monitor  23  for receiving an output of the optical coupler  22  and generating an electrical control signal which is used as a signal for controlling an output level of the optical attenuator  21 . 
     Various embodiments of the optical attenuator  21  are shown in FIGS.  4 ˜ 8 . 
     FIG. 4 illustrates a cut-off type optical attenuator which is one embodiment of the optical attenuator  21  for use in an optical power equalizer of the present invention, including a single mode optical waveguide  20  having a substrate and polymer thin film formed on a surface of the substrate and a thermooptic electrode  31  on the optical waveguide  20 . If there is no current flowing through the thermooptic electrode  31 , a waveguide mode propagates without loss, and, if there is a current flowing through the thermooptic electrode  31 , an output of the optical attenuator  21  is reduced because a heat generated in the thermooptic electrode  31  due to the current flow is transferred to the underlying optical waveguide  20 , to raise a temperature of the optical waveguide  20  with a consequential reduction of a refractive index of the optical waveguide  20  due to a thermooptic effect of the polymer, that gradually converts the optical waveguide  20  into a cut off mode, with an increased propagation loss. Therefore, as explained, if a power of the optical signal transmitted through the optical waveguide  20  is greater than a desired value, to detect a greater power of the optical signal at the optical coupler  22 , the power of the optical signal transmitted through the optical waveguide  20  in the optical attenuator  20  is reduced because a current control signal corresponding to the greater power of the optical signal is generated in the optical power monitor  23 , and applied to the thermooptic electrode  31  in the optical attenuator. Thus, since the power of the optical attenuator  21  is controlled according to the current flowing through the thermooptic electrode  31 , the optical power is always kept the same level. 
     FIG. 5 schematically illustrates a section of a Mach-Zehnder interferometer type optical attenuator, another embodiment of the present invention, for use in an optical power equalizer of the present invention, including a first optical waveguide  32  and a second optical waveguide  33  connected in parallel and a thermooptic electrode  34  formed on a surface of the second optical waveguide  33 . In the aforementioned optical attenuator  21 , if there is no current flowing through the thermooptic electrode  34 , output power is not charged as it is because a constructive interference is occurred when both optical waves are met since phases of optical waves passing through the optical waveguides  32  and  33  are the same. However, if there is a current flowing through the thermooptic electrode  34  in response to an electrical control signal applied from the optical power monitor  23 , with a heat generated therein in proportion to the current, which causes a reduction of a refractive index of the optical waveguide  33  under the thermooptic electrode  34  coming from a thermooptic effect of the polymer formed on a surface of the optical waveguide  33 , there is a difference of optical paths between the first and second optical waveguides  32  and  33 . Accordingly, when the optical waves are met after the optical waves travel through the first and second optical waveguides  32  and  33 , since a difference of the phases causes an destructive interference along with a reduction of the power, as explained before, the optical power of the optical attenuator  21  can be kept at the same level in response to the electrical control signal from the optical power monitor  23 . 
     FIG. 6 illustrates a directional coupler type optical attenuator, another embodiment of the optical attenuator of the present invention, for use in an optical power equalizer of the present invention, including one channel optical waveguide  35 , another optical waveguide  37  fitted parallel to the channel optical waveguide  35 , and a thermooptic electrode  36  formed to cover both the channel optical waveguide  35  and the optical waveguide  37 . The optical attenuator  21  has refractive indices between the electrode and respective optical waveguides and a directional coupling coefficient, both set to make the power in a state no current is applied to the electrode  36  is the maximum. When a current is caused to flow to the thermooptic electrode  22 , a refractive index of a portion under the thermooptic electrode  22  is reduced in correspondence to the current, which changes an optical coupling characteristic, allowing to obtain an optical attenuation. The optical attenuator  21  of this embodiment transmits an optical wave in a waveguide mode provided to the channel optical waveguide  35  when no current is applied to the thermooptic electrode  36 . However, if a current is caused to flow to the thermooptic electrode  36  as the electric control signal is applied to the electrode  36  from the optical power monitor  23 , refractive indices of the optical waveguides  35  and  37  and a refractive index between the optical waveguides  35  and  37  are reduced, with consequential changes of the directional coupling coefficient and a propagation constant of the waveguide mode, the directional coupling characteristic is changed. That is, if the power of the light from the optical attenuator  21  is great, the current detected from the optical coupler  22  is great in proportion to the power of the light. When an electric control signal corresponding to the current is applied to the optical attenuator  21  from the optical power monitor  23 , because the refractive index is reduced due to a thermooptic effect of the polymer formed on a surface of the optical attenuator to reduce an output from the optical attenuator  21 , eventually, a level of the optical signal from the optical attenuator  21  is kept constant regardless of the power of the optical signal traveling through the optical waveguide  20 . 
     FIGS.  9 A˜ 9 C each illustrates a beam propagation through a directional coupler of two symmetric optical waveguides ‘a’ and ‘b’ according to a current provided to a thermooptic electrode, FIGS.  10 A˜ 10 C illustrate graphs of the optical powers in left side optical waveguides ‘a’ in FIGS.  9 A˜ 9 C each shown with respect to a propagation direction obtained by simulation, FIGS.  11 A˜ 11 C each illustrates a beam propagation through a directional coupler of two asymmetric optical waveguides ‘c’ and ‘d’ in an optical attenuator, and FIGS.  12 A˜ 12 C illustrate graphs of the optical powers in left side optical waveguides ‘c’ in FIGS.  11 A˜ 11 C each shown with respect to a propagation direction obtained by simulation. In all of the cases of the aforementioned drawings, input/output is made using the left side optical waveguide ‘a’ or ‘c’ . A last measurement in the propagation direction is the output, an output from the optical attenuator; FIG. 9A shows that the last output is 100%(a monitor value a.u=1.0), FIG. 9B shows that the last output is approx. 60%(a monitor value a.u=0.6), and FIG. 9C shows that the last output is 0%(a monitor value a.u=0), and FIGS. 11A,  11 B, and  11 C show that the last outputs are 100%, approx. 60%, and approx. 20%, respectively. Therefore, as shown in FIGS.  9 A˜ 9 C, since 100% output at the maximum and 0% output at the minimum are obtainable in cases of symmetric directional couplers, though a long modulation length is obtainable, fine adjustment of power is difficult when a relatively identical thermooptic effect is used. However, as shown in FIGS.  11 A˜ 11 C, since 100% output at the maximum and 20% output at the minimum are obtainable in cases of asymmetric directional couplers, since a modulation length is short, fine adjustment of power is easy. Accordingly, the optical attenuator of the present invention that employs asymmetric optical waveguides can make a fine adjustment of an optical attenuation with easy. 
     FIGS. 7A and 7B respectively illustrate another embodiment of directional coupler type optical attenuator and a graph showing an optical attenuation characteristic in terms of coupling length versus output Pout of the directional coupler type optical attenuator. 
     Referring to FIG. 7A, the optical attenuator  21  includes a directional coupler with a coupling length ‘L’ having a reference optical waveguide  38  and an asymmetric optical waveguide  39  disposed in parallel to the reference optical waveguide  38  with a propagation constant different from the reference optical waveguide  38  and a thermooptic electrode  40  formed between the optical waveguides  38  and  39 . A width of the optical waveguide  39  in the directional coupler is made different from the reference optical waveguide  38 , to make an effective refractive index of the optical waveguide  39  slightly different from the reference optical waveguide  38 . Though the optical attenuator of this embodiment makes no optical attenuation if no current is applied to the thermooptic electrode  40 , if a current is applied to the thermooptic electrode  40 , the optical attenuator makes an optical attenuation down to a minimum power as a refractive index between the optical waveguides  38  and  39  is reduced due to a thermooptic effect of the polymer, which in turn reduces a power from the output terminal on the optical attenuator  40 , that in turn increases the current to enhance the thermooptic effect of the polymer. A size of the minimum power is dependent on an extent of asymmetry of the initial optical waveguides  38  and  39  at which no current is applied to the thermooptic electrode  40 . FIG. 7B illustrates an optical power P 1  when no current is applied to the thermooptic electrode  40  and an optical power P 2  when a current is applied to the thermooptic electrode  40  versus coupling length ‘L’. In order to obtain an optical power with a required modulation depth, the coupling length should be adjusted, appropriately. The optical attenuator of this embodiment is suitable for making an exact adjustment of a small sized optical attenuation while a comparatively small optical attenuation is made. 
     FIGS.  8 A˜ 8 C respectively illustrate another embodiment of a directional coupler type optical attenuator and a graph showing an optical attenuation characteristic of the directional coupler type optical attenuator in terms of coupling length ‘L’ versus output Pout thereof. 
     Referring to FIG. 8A, the another embodiment optical attenuator includes a directional coupler having a reference optical waveguide  41  and an asymmetric optical waveguide  42  disposed parallel to the reference optical waveguide  41  with a propagation constant set different from the reference optical waveguide  41  and a thermooptic electrode  43  formed on the asymmetric optical waveguide  42 . The optical attenuator has a coupling length corresponding to a length ‘L’ of the optical waveguide  42 . FIG. 8B illustrates an optical power P 3  when no current is applied to the thermooptic electrode  43  and an optical power P 4  when a current is applied to the thermooptic electrode  43  versus coupling length ‘L’. Though the aforementioned variable optical attenuator makes almost no optical attenuation when no current is applied to the thermooptic electrode  43 , if a current is applied to the thermooptic electrode  43 , the variable optical attenuator makes an optical attenuation down to a minimum power because the refractive index between the optical waveguides is reduced due to a thermooptic effect of the polymer, to reduce an output on an output terminal according to a coupling length ‘L’, which in turn increases the current, that in turn enhances the thermooptic effect of the polymer. Therefore, if the coupling length ‘L’ is selected appropriately, 100% power K 0  at the maximum and 20% K 1  at the minimum are obtainable, and a modulation depth, which is a variation of the optical attenuation, is dependent on an extent of asymmetry of the asymmetric optical waveguides and the coupling depth, i.e., a size of the minimum power can be set depending on the extent of asymmetry. As the extent of asymmetry becomes the greater, with a weaker optical coupling and a smaller optical attenuation, the minimum power becomes the greater, and vice versa. Thus, the extent of asymmetry of the asymmetric optical waveguides according to a coupling length adjusts an extent of optical attenuation. And, a size of the minimum power in this embodiment variable optical attenuator is dependent on the extent of asymmetry of the initial optical waveguides at which no current is applied to the thermooptic electrode  43 . And, this embodiment variable optical attenuator is suitable for making a fine adjustment on a small sized optical attenuation while making a comparatively great optical attenuation. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the optical power equalizer in a WDM optical communication system and a variable optical attenuator for use therein of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.