Abstract:
A planar lightwave circuit (PLC) of a PLC module is provided with many waveguides including a plurality of input (output) optical waveguide on a substrate. Junction faces between these input (output) optical waveguide and input (output) optical fibers are slant against the optical axis thereof. The PLC module has photodetectors opposed to corresponding junction faces disposed at the input-optical-fiber&#39;s-side (output-optical-waveguide&#39;s-side).

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Serial No. 60/191,458, filed Mar. 23, 2000, and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a planar lightwave circuit module comprising a planar lightwave circuit provided with a plurality of input ports or a plurality of output ports and having a function of monitoring the powers of multi-channel signal light beams coming incident on the plurality of input ports or emitting from the plurality of output ports. 
     2. Related Background Art 
     In a planar lightwave circuit (PLC) having a plurality of input ports or a plurality of output ports, e.g., a multi-channel optical variable attenuator, optical demultiplexer, or optical multiplexer, it becomes necessary to monitor the powers of the signal light beams of respective channels coming incident on the respective input ports or emitting from the respective output ports. Because it is necessary for adjusting the powers of the signal light beams of respective output channels in the optical demultiplexer or adjusting the characteristics of the multiplexed signal light in response to the power of the input signal light beams in the optical multiplexer. 
     In order to monitor the powers of the signal light beams of the respective channels, a photocoupler is provided to an optical waveguide connected to the input ports or output ports. Part of the signal light beam is branched by the photocoupler to another optical waveguide, and the power of the branched signal light beam is monitored by a photodetector. 
     For example, a multi-channel optical variable attenuator using a Mach-Zehnder type optical waveguide is a PLC having a plurality of input ports and a plurality of output ports. FIG. 11 is a view showing an arrangement corresponding to one channel of such a conventional multi-channel optical variable attenuator. This one-channel optical variable attenuator has a PLC on a substrate  1 , this PLC consists of an input optical waveguide  2 , first directional coupler  3 , first optical waveguide  4 , second optical waveguide  5 , second directional coupler  6 , output optical waveguide  7 , and monitoring optical waveguide  8 . And the attenuator has a heater  9  for adjusting the temperature of the first optical waveguide  4  and a photodetector  10  connecting the exit end of the monitoring optical waveguide  8 . The multi-channel optical variable attenuator is provided with such configured optical variable attenuators parallel aligned on the substrate  1 . 
     In this optical variable attenuator, a signal light beam input to the input optical waveguide  2  is branched by the directional coupler  3 . The branched signal light beams are input to the directional coupler  6  through the optical waveguides  4  and  5 , respectively. These signal light beams are output from the directional coupler  6  to the output optical waveguide  7  and monitoring optical waveguide  8  with a predetermined branching ratio. This branching ratio is adjusted by controlling the temperature of the optical waveguide  4  with the heater  9  so as to change the optical path length of the optical waveguide  4 . The power of the light beam emitting from the exit end of the monitoring optical waveguide  8  is detected by the photodetector  10 , and the temperature of the optical waveguide  4  is controlled by the heater  9 . Therefore, the ratio (Pout/Pin) of a power Pout of the signal light beam to be output to the output optical waveguide  7  to a power Pin of the signal light beam input to the input optical waveguide  2 , i.e., the optical attenuation amount, can be controlled. 
     In addition, as a PLC having a plurality of input ports or a plurality of output ports, an AWG (Arrayed Waveguide Grating) used as an optical multiplexer or optical demultiplexer is well known. For example, a reference “General Meeting of Year 1996 of The Institute of Electronics, Information and Communication Engineers, B-1183” describes an arrangement in which photodetectors are connected to ports, in an AWG serving as an optical demultiplexer, that output high-order diffracted light. In this AWG, the powers of the signal light beams of the respective wavelengths demultiplexed by the AWG are monitored on the basis of the detection results of the high-order diffracted light detected by the photodetectors. 
     SUMMARY OF THE INVENTION 
     In such conventional PLCS, monitoring optical waveguides must be separately provided in units of output ports. When multi-channel PLCs are integrated, the circuit configuration becomes complicated, and the module size increases. Although the monitoring signal light beam (demultiplexed light or high-order diffracted light) has characteristics which are different to those of the original exit signal light beam, the branching ratio may be fluctuated with the power of the original exit signal light beam. Therefor, the power of the original exit signal light beam cannot sometimes be monitored accurately. 
     The present invention has been made in order to solve the problems described above, and has as its object to provide a PLC module which can accurately monitor the power of signal light beams of the respective channels with a simple arrangement. 
     In order to solve the above-mentioned problems, the PLC module according to the present invention comprises (1) a PLC provided with many waveguides including a plurality of output optical waveguides on a substrate, wherein output end faces of the output optical waveguides are slant against the optical axis thereof; (2) an output optical fiber array including output optical fibers each coupled to corresponding output optical waveguide; and (3) photodetector array including photodetectors each located on the surface of the PLC and opposed to the corresponding junction between the output optical waveguide and output optical fiber. 
     Alternatively, the PLC module according to the present invention may comprise (1) a PLC provided with many waveguides including a plurality of input optical waveguides on a substrate, wherein input end faces of the input optical waveguides are slant against the optical axis thereof; (2) an optical fiber array including input optical fibers each coupled to the corresponding input optical waveguide; and (3) a photodetector array including photodetectors each located on the input optical fiber array and opposed to the corresponding junction between the input optical waveguide and input optical fiber. 
     In these PLC modules, a junction face between the input optical fiber and the input optical waveguide or between the output optical fiber and the output optical waveguide is slant against their optical axes, so that a part of signal light is reflected at this junction face. The reflected light is detected by a corresponding photodetector arranged in the photodetector array which is opposed to the junction face. As a consequence, power of the signal light can be accurately measured with a simple arrangement. 
     An angle between the junction face and the optical axis thereof is preferable within a range of 45 to 70 degrees, whereby the photodetectors can be located whereby the photodetectors can be displaced from a point directly above the junction faces, thus achieving a simple arrangement. Further, a distance between the junction face and the photodetector can be enlarged by providing the surface of the optical waveguide with a light-transmitting resin layer and arranging the photodetector on the resin layer. Therefore, the photodetector can be displaced, to an extent of the enlarged distance, toward the center of the substrate of the PLC from the point directly above the junction face, thus preferably achieving a simple arrangement. 
     A reflecting film may be disposed at the junction of the waveguides and the optical fibers, and the reflecting film is preferably a dielectric multi-layer film. Therefore, the reflected ratio at the junction can be kept constant and power of the signal light can be accurately measured with a simple arrangement. 
     The present invention can be suitably applied to a PLC comprising a multi-channel optical variable attenuator, an optical demultiplexer or an optical multiplexer. For example, in the application to the multi-channel optical variable atttenuator, power of light of each channel can be monitored so as to control power or attenuation of light emitted from each channel. Further, in the application to the optical demultiplexer or the optical multiplexer, light of each demultiplexed or mupltiplexed wavelenth can be monitored. It is natural that the present invention can be applied to a system comprising the multi-channel optical variable attenuator, the optical demultiplexer and the optical multiplexer. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a PLC module of the first embodiment according to the present invention; 
     FIG. 2 is an exploded perspective view showing a portion in vicinity of an output port of the PLC module shown in FIG. 1; 
     FIGS. 3A and 3B are cross section views showing modified embodiments wherein the connection end face of the output port in the PLC module shown in FIG. 1 is provided with a reflecting film; 
     FIG. 4 is a cross section view showing another arrangement of the optical detector in the PLC module shown in FIG. 1; 
     FIG. 5 is a plan view of a PLC module of the second embodiment according to the present invention; 
     FIG. 6 is a sectional view taken near the input ports of the PLC module shown in FIG. 5; 
     FIGS. 7 to  10  are respective plan views of PLC modules of the third to sixth embodiment according to the present invention; and 
     FIG. 11 is a view showing an arrangement corresponding to one channel of a conventional multi-channel optical variable attenuator using a Mach-Zehnder type optical waveguide. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. 
     (First Embodiment) 
     FIG. 1 is a plan view of a PLC module of the first embodiment according to the present invention. 
     This PLC module has eight Mach-Zehnder type optical variable attenuators parallel aligned on a common substrate  1 , and has a function of an 8-channel optical variable attenuator. Each optical variable attenuator (n=1 to 8) consists of an input optical waveguide  2   n , a first directional coupler  3   n , a first optical waveguide  4   n , a second optical waveguide  5   n , a second directional coupler  6   n , an output optical waveguide  7   n , and heater  9   n . The first and second optical waveguides  4   n  and  5   n  are parallel connected between the first and second directional coupler  3   n  and  6   n  which are inserted between input and output optical waveguide  2   n  and  7   n  in series. The heater  9   n  is disposed on the first optical waveguide  4   n . 
     This PLC module also has a photodetector array  11  having arranged eight photodetectors  11   1  to  11   8 , an input optical fiber array  12  having arranged eight input optical fibers  12   1  to  12   8 , and output optical fiber array  13  having arranged eight output optical fibers  13   1  to  13   8 . Further, a control circuit  40  is connected to photodetectors  11   1  to  11   8  and heaters  9   1  to  9   8 . (Details of connected signal lines are omitted in the drawings.) 
     FIG. 2 is a perspective view showing a portion (output port portion) in vicinity of a junction portion between output optical waveguide  7   n  and output optical fiber  13   n  of one channel in the PLC module. As shown in FIG. 2, the output optical waveguide  7   n  of the PLC has core  7 A and cladding  7 B formed on the substrate  1 . The output optical fiber  13   n  also has core  13 A and cladding  13 B. The output optical fiber  13   n  is accommodated in V-groove  131  formed in base  130  of optical fiber array  13 . The output optical fiber  13   n  is secured in respective V-groove  131  by pushing down the plate  132  toward the V-grooves  131 . Here, the end face of output optical waveguide  7   n  and the end face of output optical fiber  13   n  are polished to form a certain slant angle θ with their respective optical axes. Then, the end faces of output optical waveguide  7   n  and output optical fiber  13   n  are jointed in such a manner that the optical axes of output optical waveguide  7   n  and output optical fiber  13   n  lie on one straight line. The angle θ between the end face and the optical axis of output optical waveguide  7   n  as well as output optical fiber  13   n  is preferably within a range of 70 to 45 degrees. 
     In addition, the n-th photodetector  11   n  in photodetector array  11  is disposed above output optical waveguide  7   n  and at a position very close to the junction face between output optical waveguide  7   n  and output optical fiber  13   n . The photodetector  11   n  is fixed on output optical waveguide  7   n  via resin layer  110 . Due to the setting of the foregoing angle θ, a part of light propagated from output optical waveguide  7   n  is reflected by the junction face between output optical waveguide  7   n  and output optical fiber  13   n . The photodetector  11   n  needs to be located at a position capable of receiving the light thus reflected. As the photodetector  11   n , for example, a photodiode is suitably used. The reflectance of the exit light beam at the junction face is preferably about 3% to 20%. 
     The PLC module of this embodiment operates in the following manner. A signal light beam input to the input optical waveguide  2   n  from the input optical fiber  12   n  is branched by the directional coupler  3   n . The branched signal light beams are input to the directional couplers  6   n  through the optical waveguides  4   n  and  5   n , respectively. The composite signal light beam is output from the directional coupler  6   n  to the output optical waveguides  7   n . Here, the power of the composite signal light beam is adjusted by the controlling the optical path length of the optical waveguides  4   n  by way of changing the temperature with heater  9   n . The signal light beams output to the output optical waveguides  7   n  are reflected by the junction face (output end faces of the output optical waveguides  7   n ) at a constant reflectance toward the photodetectors  11   n , and their powers are detected by the photodetectors  11   n . The signal light beams transmitted through the end faces of the output optical waveguides  7   n  come incident on the output optical fibers  13   n . 
     The control circuit  40  controls the powers of the signal light beams that have reached and detected by the photodetectors  11   n  by adjusting the temperatures, i.e., the optical path lengths, of the optical waveguides  4   n  with the heaters  9   n . Therefore, the ratio (Pout/Pin) of a power Pout of the signal light beam to be output to the output optical waveguide  7   n  to a power Pin of a signal light beam input to the input optical waveguide  2   n , i.e., the optical attenuation amount, can be controlled, or the power Pout of the signal light beam to be output to the output optical waveguide  7   n  can be controlled. Also, the powers of the signal light beams to be respectively output from the output optical waveguides  7   n  to the output optical fibers  13   n  (n=1 to 8) can be set equal to each other. 
     As described above, in the PLC module according to this embodiment, monitoring optical waveguides need not be separately provided in units of output ports. Even when multi-channel PLCs are integrated, the circuit configuration is simple, and the module size is small. Since the reflectance at the junction face has almost constant value, the power of the exit light beam can be accurately monitored. 
     Here, the reflectance is preferably, as described above, about 3% to 20%. In order to keep a desirable state of the reflection characteristic, reflecting film  14  is preferably disposed on the end face of output optical waveguide  7   n , as shown in FIG. 3A, in which (and other drawings that will be referred to hereinafter) the portions having no relation to the explanation are omitted. Reflecting film  14  is preferably a dielectric multi-layer film that can be easily manufactured corresponding to desired reflection characteristic. Alternatively, reflecting film  14 ′ may also be disposed on the end face of output optical fiber  13   n , as shown in FIG.  3 B. Reflecting film  14 ′ is also preferably a dielectric multi-layer film. 
     When photodetector  11   n  is placed directly on cladding  7 B of output optical waveguide  7   n , as shown in FIG. 2, a light-receiving portion of photodetetor  11   n  needs to be arranged close to the junction face, and therefore it may come into contact with output optical fiber  13   n  when placing and fixing photodetector  11   n , thus causing deterioration in the connection efficiency between output optical waveguide  7   n  and output optical fiber  13   n . 
     Therefore, in the embodiment shown in FIG. 4, transparent resin layer  111  is provided on cladding  7 B of output optical waveguide  7   n , and photodetector  11   n  is placed on resin layer  111 . As a consequence, the light-receiving portion of photodetector  11   n  is isolated from the junction face between output optical waveguide  7   n  and output optical fiber  13   n , thus attaining a simpler arrangement of photodetector  11   n  while effectively preventing the connection efficiency from lowering. Transparent resin layer  111  has preferably the same refractive index as cladding  7 B. 
     (Second Embodiment) 
     FIG. 5 is a plan view of a PLC module of the second embodiment according to the present invention. 
     This PLC module has an 8-channel optical variable attenuator, having a similar arrangement to that of the first embodiment, as a PLC. This PLC module also has two photodetector arrays  10  and  11  each having arranged eight photodetectors  10   1  to  10   8  or  11   1  to  11   8 , an input optical fiber array  12  having arranged eight input optical fibers  12   1  to  12   8 , and an output optical fiber array  13  having arranged eight output optical fibers  13   1  to  13   8 . The controlling circuit  40  is connected to photodetectors  10   1  to  10   8  and  11   1  to  11   8  and heaters  9   1  to  9   8 . 
     This PLC module has a different input ports arrangement from the PLC module of the first embodiment. FIG. 6 is a sectional view taken near the input ports of the PLC module of the second embodiment. The structure shown as FIG. 4 is similar to the structure in vicinity of the output port of the PLC module in the first embodiment shown in FIG.  2 . Specifically, the input optical waveguide  2   n  of the PLC has core  2 A and cladding  2 B formed on the substrate  1 . The input optical fiber  12   n  also has core  12 A and cladding  12 B. The input optical fiber  12   n  is accommodated in V-groove  121  formed in base  120  of optical fiber array  12 . The input optical fiber  12   n  is secured in respective V-groove  121  by pushing down the transparent plate  122  toward the V-grooves  121 . Here, the end face of input optical waveguide  2   n  and the end face of input optical fiber  12   n  are polished to form a certain slant angle θ′ with their respective optical axes. Then, the end faces of input optical waveguide  2   n  and input optical fiber  12   n  are jointed in such a manner that the optical axes of input optical waveguide  2   n  and input optical fiber  12   n  lie on one straight line. The angle θ′ between the end face and the optical axis of input optical waveguide  2   n  as well as input optical fiber  12   n  is preferably within a range of 70 to 45 degrees. 
     The n-th photodetector  10   n  in photodetector array  10  is disposed above input optical fiber  12   n  and at a position very close to the junction face between input optical waveguide  2   n  and input optical fiber  12   n . The photodetector  10   n  is fixed on input optical fiber  12   n  via resin layer  100 . Due to the setting of the foregoing angle θ′, a part of light propagated from input optical fiber  12   n  is reflected by the junction face between input optical fiber  12   n  and input optical waveguide  2   n . The photodetector  10   n  needs to be located at a position capable of receiving the light thus reflected. As the photodetector  10   n , for example, a photodiode is suitably used. The reflectance of the exit light beam at the junction face is preferably about 3% to 20%. For achieving this reflectance value the reflecting film is preferably disposed on the end face of input optical fiber  12   n  or waveguide  2   n , as shown in FIG. 3A or  3 B. The reflecting film may be a dielectric multi-layer film. 
     This PLC module operates in similar manner to the first embodiment. A signal light beam propagated through the input optical fiber  12   n  is partly reflected by the junction face between the input optical waveguide  2   n  at constant ratio. Thus reflected light beam passes through the plate  122  and reaches the photodetector  10   n , and its power is detected by the photodetector  10   n . The signal light beam transmitted through the junction face and passes through the input optical waveguide  2   n . 
     According to this embodiment, the powers of the input and output signal light beams can be accurately monitered. Therefore, the ratio (Pout/Pin) of a power Pout of the signal light beam to be output to the output optical waveguide  7   n  to a power Pin of the signal light beam input to the input optical waveguide  2   n , i.e., the optical attenuation amount, can be controlled, or the power Pout of the signal light beam to be output to the output optical waveguide  7   n  can be controlled. Also, the powers of the signal light beams to be respectively output from the output optical waveguides  7   n  to the output optical fibers  13   n  (n=1 to 8) can be set equal to each other. 
     As described above, in the PLC module according to this embodiment has same advantages as the first embodiment. 
     The photodetector  10   n  may be disposed below transparent plate  122 , namely in contact with cladding  12 B of optical fiber  12   n , and may also be disposed with an interposed transparent resin layer, similar to the structure shown in FIG.  4 . 
     (Third Embodiment) 
     FIG. 7 is a plan view of the PLC module of the third embodiment according to the present invention. 
     This PLC module has an AWG, serving as an optical demultiplexer, as a PLC. This PLC consists of an input optical waveguide  2 , a slab waveguide  20 , an array waveguide portion  21 , a slab waveguide  22 , and output optical waveguides  7   1  to  7   N  on a substrate  1 , and has one input port and N output ports. This PLC module further comprises of a photodetector array  11  having N arrayed photodetectors  11   1  to  11   N , one input optical fiber  12 , and an output optical fiber array  13  having N optical fibers  13   1  to  13   N . 
     The slab waveguide  20  diffracts a signal light beam input from the input optical waveguide  2 , and guides it to come incident on a plurality of optical waveguides constituting the array waveguide portion  21 . The slab waveguide  22  diffracts signal light beams input from the array waveguide portion  21 , and guides them to come incident on the output optical waveguides  7   1  to  7   N . The array waveguide portion  21  formed between the slab waveguides  20  and  22  is comprised of the plurality of optical waveguides. These plurality of optical waveguides have optical path lengths that are different from each other by a predetermined value, to phase-shift the light beams being guided in them. The sectional arrangement near the output ports of this PLC module is identical to that shown in FIG.  2 . 
     This PLC module operates in the following manner. A signal light beam input from the input optical fiber  12  to the input optical waveguide  2  is guided in the input optical waveguide  2 , and is input to the slab waveguide  20 . The signal light beam input to the slab waveguide  20  is guided toward the array waveguide portion  21  while being diffracted. The respective wavelength components of the signal light beam input to the array waveguide portion  21  are guided through all the plurality of optical waveguides of the array waveguide portion  21 , and are input to the slab waveguide  22 . The respective wavelength components of the signal light beam are guided toward the output optical waveguides  7   1  to  7   N  while being diffracted in the slab waveguide  22 . 
     Since the plurality of optical waveguides of the array waveguide portion  21  have optical lengths that are different from each other by the predetermined value, the signal light beams guided in them are phase-shifted in accordance with their wavelengths. When these signal light beams are guided through the array waveguide portion  21  and slab waveguide  22  and come incident on the output optical waveguides  7   n , signal light beams having a wavelength λ n  are strengthened by each other, while signal light beams having another wavelength λ m  are canceled by each other (n, m=1 to N and n≠m). Therefore, the signal light beams having the wavelength kn are demultiplexed and output to the output optical waveguides  7   n . 
     The signal light beam having the wavelength λ n  and output to the optical waveguide  7   n  is reflected by the junction face. (end face of the optical waveguide  7   n ) at a predetermined reflectance. The beam thus reflected reaches the photodetector  11   n , and its power is detected by the photodetector  11   n . The signal light beam transmitted through the junction face comes incident on the output optical fiber  13   n . More specifically, by detecting the power of the beam thus reflected, the power of the signal light beam of each wavelength to be output from the output optical waveguide  7   n  to the output optical fiber  13   n  can be monitored accurately. 
     As described above, in the PLC module according to this embodiment has same advantages as the first and second embodiment. 
     (Fourth Embodiment) 
     FIG. 8 is a plan view of the PLC module of the fourth embodiment according to the present invention. 
     This PLC module has an AWG, serving as an optical multiplexer, as a PLC. This PLC consists of input optical waveguides  2   1  to  2   N , a slab waveguide  20 , an array waveguide portion  21 , a slab waveguide  22 , and an output optical waveguide  7  on a substrate  1 , and has N input ports and one output port. This PLC module further comprises of a photodetector array  10  having N arrayed photodetectors  10   1  to  10   N , an input optical fiber array  12  having N arrayed input optical fiber  12   1  to  12   N , and one output optical fiber  13 . 
     The slab waveguide  20  diffracts a signal light beam having a wavelength λ n  and input from the input optical waveguide  2   n , and guides it to come incident on a plurality of optical waveguides constituting the array waveguide portion  21 . The slab waveguide  22  diffracts signal light beams input from the array waveguide portion  21 , and guides them to come incident on the output optical waveguide  7 . The array waveguide portion  21  formed between the slab waveguides  20  and  22  is comprised of the plurality of optical waveguides. These plurality of optical waveguides have optical path lengths that are different from each other by a predetermined value, to phase-shift the light beams being guided in them. The sectional arrangement near the input ports of this PLC module is identical to that shown in FIG.  6 . 
     This PLC module operates in the following manner. A signal light beam having a wavelength λ n  emitted from the corresponding input optical fiber  12   n  is partly reflected by the junction face between the input optical waveguide  2   n  at a predetermined reflectance. The beam thus reflected reaches a photodetector  10   n , and its power is detected by the photodetector  10   n . By detecting the power of the thus reflected beam, the power of the signal light beam of each wavelength to be input from the input optical waveguide  12   n  to the input optical fiber  2   n  can be monitored accurately. 
     A signal light beam transmitted through the end face of the input optical waveguide  2   n  is guided in the input optical waveguide  2   n , and is input to the slab waveguide  20 . The signal light beam input to the slab waveguide  20  is guided toward the array waveguide portion  21  while being diffracted. The respective wavelength components of the signal light beam input to the array waveguide portion  21  are guided through all the plurality of optical waveguides of the array waveguide portion  21 , and are input to the slab waveguide  22 . The respective wavelength components of the signal light beam are guided toward the output optical waveguide  7  while being diffracted in the slab waveguide  22 . 
     In this case, since the plurality of optical waveguides of the array waveguide portion  21  have optical lengths that are different from each other by the predetermined value, the signal light beams guided in them are phase-shifted in accordance with their wavelengths. When these signal light beams are guided through the array waveguide portion  21  and slab waveguide  22  and come incident on the output optical waveguide  7 , signal light beams of the respective wavelengths are multiplexed, and the resultant multiplexed signal light beam is output to the output optical waveguide  7 . The signal light beam output to the output optical waveguide  7  comes incident on the output optical fiber  13 . 
     As described above, the PLC module according to this embodiment has same advantages as the first to third embodiments. 
     (Fifth Embodiment) 
     FIG. 9 is a plan view of a PLC module of the fifth embodiment according to the present invention. The PLC of this PLC module comprises of an AWG as serving as an optical demultiplexer of the third embodiment, and a multi-channel optical variable attenuator provided to the output of the AWG of the first embodiment. 
     An output-side slab waveguide  22  of the AWG and directional couplers  3   n  of the multi-channel optical variable attenuator are connected to each other via optical waveguides  30   n  (n=1 to 8). The sectional arrangement near the output ports of this PLC module is identical to that shown in FIG.  2 . In this embodiment, the PLC module also has a detection circuit  40  and control circuit  41 . The detection circuit  40  receives output electrical signals from photodetectors  11   1  to  11   8  in the photodetector array  11 . The control circuit  41  controls the temperatures of heaters  9   1  to  9   8 . 
     This PLC module operates in the following manner. A multi-wavelength signal light beam input from an input optical fiber  12  to an input optical waveguide  2  is demultiplexed into signal light beams of wavelengths λ n  by the optical demultiplexing function of the AWG including a slab waveguide  20 , an array waveguide portion  21 , and the slab waveguide  22 , and the signal light beams having the wavelengths λ n  are output to the optical wavelengths  30   n . The signal light beams having the wavelengths λ n  are attenuated by the optical variable attenuator including the directional couplers  3   n , optical waveguides  4   n , optical waveguides  5   n , directional couplers  6   n , and the heaters  9   n . The attenuated signal light beams are output to output optical waveguides  7   n . 
     The signal light beams output to the optical waveguides  7   n  are reflected by the junction faces between the output optical fibers  13   n  at a predetermined reflectance. The beams thus reflected are guided toward the corresponding photodetectors  11   n . The signal light beams transmitted through the junction faces between the output optical fibers  13   n  pass through corresponding output optical fibers  13   n . 
     The photodetectors  11   n  output the electrical signals in response to the powers of the incident beams and the detection circuit  40  detected these powers from these electrical signal. The control circuit  41  controls the optical path lengths of the optical waveguides  4   n  by heating them with heaters  9   n . Therefore, the powers of the signal light beams having the wavelength λ n  and output to the output optical waveguides  7   n  can be feedback-controlled. Also, the powers of the signal light beams having the wavelengths λ n  and to be output from the output optical waveguides  7   n  to the output optical fibers  13   n  (n=1 to 8) can be set equal to each other. 
     As described above, the PLC module of this embodiment has same advantages as any aforementioned embodiments. 
     (Sixth Embodiment) 
     FIG. 10 is a plan view of the PLC module of the sixth embodiment according to the present invention. The PLC of this PLC module comprises of a multi-channel optical variable attenuator of the first embodiment and an AWG, connected to the output of the multi-channel optical variable attenuator and serving as an optical multiplexer of a fourth embodiment. 
     Directional couplers  6   n  of the multi-channel optical variable attenuator and an input-side slab waveguide  20  of the AWG are connected to each other through optical waveguides  31   n  (n=1 to 8). The sectional arrangement near the input ports of the PLC module according to this embodiment is identical to that shown in FIG.  6 . In this embodiment, the PLC module also has a detection circuit  40  and control circuit  41 . The detection circuit  40  receives electrical signals output from photodetectors  10   1  to  10   n  in the photodetector array  10 . The control circuit  41  controls the temperatures of heaters  9   1  to  9   8 . 
     This PLC module operates in the following manner. Signal light beams having a wavelength λ n  pass through input optical fibers  12   n  are reflected by the junction faces between input optical waveguides  2   n  at a predetermined reflectance. The beams thus reflected are guided toward the photodetectors  10   n . The photodetectors  10   n  output the electrical signals in response to the powers of the incident beams and the detection circuit  40  detected these powers from these electrical signal. The signal light beams transmitted through the junction faces pass through the input optical waveguides  2   n  and are attenuated by the optical variable attenuator including directional couplers  3   n , optical waveguides  4   n , optical waveguides  5   n , the directional couplers  6   n , and the heaters  9   n . The attenuated signal light beams are output to the optical waveguides  31   n . The signal light beams having the wavelength λ n  are multiplexed by the photomultiplexing function of the AWG including the slab waveguide  20 , an array waveguide portion  21 , and a slab waveguide  22 . The resultant multiplexed signal light beam is output to an output optical fiber  13  through an output optical waveguide  7 . 
     If the insertion loss of each wavelength λ n  between the input optical fibers  12   n  and the output optical fiber  13  is known, the power of the output signal light component having the wavelength λ n  can be feedforward-controlled by controlling the optical path lengths of the optical waveguides  4   n  based on the detected power of corresponding reflected beam. For example, the powers of components in the multiplexed beam output to the output optical fiber  13  can be set equal to each other. 
     As described above, the PLC module according to this embodiment has same advantages as the first to third embodiments. 
     The present invention is not limited to the above embodiments, but can be modified in various manners. For example, the PLC is not limited to a multi-channel optical variable attenuator or AWG, but can be any other optical circuit.