Abstract:
A delay demodulation device, which reduces chip size and polarization dependent frequency (PDf), is provided. The delay demodulation device comprises: an input waveguide, which receives DQPSK signals; a Y-shape waveguide, which splits the input waveguide; a first Mach-Zehnder interferometer; and a second Mach-Zehnder interferometer. Both end of two arm waveguides of first Mach-Zehnder interferometer and both ends of two arm waveguides of second Mach-Zehnder interferometer are angled toward the center portion of a Planar Lightwave Circuit (PLC). Because of the angle, the length of the two arm waveguides of the first Mach-Zehnder interferometer and the length of the two arm waveguides of the second Mach-Zehnder interferometer in Z-direction can be shortened, and input couplers and output couplers of the Mach-Zehnder interferometers in Z-direction can be shortened as well. The area occupied by the Mach-Zehnder interferometers is also reduced.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to delay demodulation devices used for optical fiber communication, and particularly, relates to delay demodulation devices equipped with Planar Lightwave Circuit (PLC) type Mach-Zehnder interferometers, which demodulate Differential Quadrature Phase Shift Keying (DQPSK) signals in Dense Wavelength Division Multiplexing (DWDM) transmission. 
       RELATED ARTS 
       [0002]    Recently, with the rapid growth in broadband networks, high speed optical transmission systems (toward transmission rate of 40 Gbps) are being investigated actively. However, when the transmission rate is increased, the time duration per 1 bit of optical signal is reduced and, because of the characteristics of an optical fiber, wave shape is deteriorated, and therefore the quality of a communication line is deteriorated. For 40 Gbps-class long distance transmission, transponders that transform an optical signal to an electrical signal and then transform the electrical signal back to an optical signal are needed in the transmission path. Therefore, it is difficult to create a high speed optical transmission system using existing optical fiber networks. 
         [0003]    Because of this issue, research and development has been done in Differential Quadrature Phase Shift Keying (DQPSK). DQPSK reduces deterioration of signal-wave profile by increasing the time duration per bit of an optical signal. 
         [0004]    DQPSK transmits four symbols of information as four corresponding optical phase shifts. In other words, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. 40 Gbps DQPSK transmission can transmit four times longer distance than conventional 40 Gbps transmission. Because of DQPSK, it is believed that construction of networks between large cities can be achieved using existing optical fiber networks. 
         [0005]    For example, conventional delay demodulation devices, which demodulate in receiving devices by using DQPSK signals or Differential Phase Shift Keying (DPSK) signals, are disclosed in Japanese Patent Application Laid-open 2007-60442, and in Japanese Patent Application Laid-open 2007-151026. 
         [0006]    Photo receiving circuits disclosed in Japanese Patent Application Laid-open 2007-60442 are equipped with Mach-Zehnder interferometers, which propagate return-to-zero (RZ) modulated DPSK signals through a pair of optical paths, which are equipped with a one-symbol delay element in one of the pair optical paths. 
         [0007]    Also, demodulation devices disclosed in Japanese Patent Application Laid-open 2007-151026 use Michelson interferometers to demodulate DPSK or DQPSK optical signals. 
         [0008]    In delay detection of 40 Gbps DQPSK transmission, two Mach-Zehnder interferometers and a Planar Lightwave Circuit (PLC)-type delay circuit (i.e. delay demodulation device), which demodulates DQPSK signals, are used. In the 40 Gbps DQPSK transmission, the permissible value of Polarization Dependent frequency (PDf) in the delay circuit is said to be less than 0.2 GHz. As a way to reduce the PDf, a half wave plate can be inserted in the Mach-Zehnder interferometers. However, it is difficult to lower the PDf to be less than 0.2 GHz by just inserting the half wave plate. Also, inserting the half wave plate causes yield ratio as well. Because of the above reasons, it is very difficult to manufacture delay demodulation circuits with small PFf (&lt;0.2 GHz) consistently. Also, size reduction and reduction in power consumption of optical fiber modules containing the demodulation devices are also desired. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention seeks to overcome the above-identified problems. The purpose of the present invention is to provide delay demodulation devices with reduced Polarization Dependent frequency and reduced chip size. 
         [0010]    To solve the above issue, a Planar Lightwave Circuit (PLC)-type delay demodulation device, which demodulates Differential Quadrature Phase Shift Keying (DQPSK) signal, is invented. The delay demodulation device comprises: (i) an input waveguide, which receives the DQPSK signals; (ii) a Y-shape waveguide, which splits the input waveguide; and (iii) first and second Mach-Zehnder interferometers. The first Mach-Zehnder interferometer comprising: an input coupler, which is connected to one of the two waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths and connected between the input coupler and the output coupler. The second Mach-Zehnder interferometer comprising: an input coupler, which is connected to the other waveguide split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths and connected between the input coupler and the output coupler, wherein the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer cross within the same area. 
         [0011]    According to the construction above, the PLC can be made smaller, particularly because areas including the two arm waveguides in the first and second Mach-Zehnder interferometers are smaller, the PLC chip can be made smaller. 
         [0012]    By making the chip smaller, temperature distribution along the PLC surface evens out, and shifts in center wavelengths due to environment and temperature fluctuation are very small. Also, because of the smaller chip size, stress distribution within the chip, which causes birefringence, is reduced, and shifts in center wavelengths due to the environment and temperature fluctuation can be made very small. Therefore, there are practically no wavelength shifts due to environment and temperature fluctuation, and the delay demodulation devices with small initial PDf can be created. Furthermore, by reducing the chip size, optical fiber modules with delay demodulation devices can be smaller, and the power consumption of the modules can be reduced as well. 
         [0013]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer are placed in the same areas such that the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer cross each other. Because of the construction, the PLC chip can be reduced in size and in PDf. 
         [0014]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are placed on a PLC base plate such that the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are bilaterally-symmetric to each other. Because of the construction, the PLC chip can be further reduced in size and in PDf. 
         [0015]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a half-wave plate is inserted at the center portion of the two arm waveguides of the first Mach-Zehnder interferometer and at the center portion of the two arm waveguides of the second Mach-Zehnder interferometer. Because of the construction, the PLC chip can be reduced in PDf. 
         [0016]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the center portion of the two arm waveguides of the first Mach-Zehnder interferometer are parallel to and close to each other, and the center portion of the two arm waveguides of the second Mach-Zehnder interferometer are also parallel to and close to each other. Because of the construction, retardation by the half wave plate can be suppressed. 
         [0017]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, two individual ends of the output waveguides connected to the output coupler of the first Mach-Zehnder interferometer and two individual ends of the output waveguides connected to the output coupler of the second Mach-Zehnder interferometer are placed on the same side of a PLC chip. Because of the construction, both the PLC and the PLC chip can be further reduced in size. 
         [0018]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the PLC chip is approximately in square planar shape. 
         [0019]    According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, at least one heater is placed on at least one of the two arm waveguides of the first Mach-Zehnder interferometer, and at least one heater is placed on at least one of the two arm waveguides of the second Mach-Zehnder interferometer. 
         [0020]    According to the construction above, the PDf can be adjusted by using the heaters on either the first or the second Mach-Zehnder interferometer. After the adjustment, phase shift control (phase shift trimming) can be performed by using heaters on one of the two Mach-Zehnder interferometers to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer. 
         [0021]    The present invention can be provided to delay demodulation devices with reduced Polarization Dependent frequency and reduced chip size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken into connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
           [0023]      FIG. 1  is a plan view of a delay demodulation device in one embodiment of the present invention. 
           [0024]      FIG. 2  is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK). 
           [0025]      FIG. 3  is a cross-sectional drawing taken along line X-X in  FIG. 1 . 
           [0026]      FIG. 4  is a cross-sectional drawing taken along line Y-Y in  FIG. 1 . 
           [0027]      FIG. 5  is a graph showing a spectrum of the delay demodulation device disclosed in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Detailed description as follows; 
         [0029]    Construction of the delay demodulation device is shown in  FIG. 1  through  FIG. 5 . 
         [0030]      FIG. 1  is a plan view of a delay demodulation device in one embodiment of the present invention,  FIG. 2  is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK).  FIG. 3  is a cross-sectional drawing taken along line X-X in  FIG. 1 ,  FIG. 4  is a cross-sectional drawing taken along line Y-Y in  FIG. 1 .  FIG. 5  is a graph showing a spectrum of the delay demodulation device disclosed in  FIG. 1 . 
         [0031]    Delay demodulation device  1  shown in  FIG. 1  is a Planar Lightwave Circuit (PLC)-type delay demodulation device, which demodulates Differential Quadrature Phase Shift Keying (DQPSK) signals. Delay demodulation device  1  is, for example, a 40 Gbps DQPSK delay demodulation device used in a 40 Gbps DQPSK optical transmission system shown in  FIG. 2 . 
         [0032]    In the optical transmission system, DQPSK signals are transmitted from an optical transmitter  40  to an optical fiber transmission line  54 . In DQPSK signals, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. DQPSK signals transmitted from the optical fiber transmission line  54  to an optical receiver  50  are converted to optical signals with four different intensities by the delay demodulation device  1 , and furthermore, the optical signals are converted to electric signals by balanced receivers  51  and  52 . In a receiving electric circuit  53 , various processes such as decryption process are performed. 
         [0033]    Delay demodulation device  1  shown in  FIG. 1  comprises an input waveguide  2 , which receives DQPSK signals; a Y-shape waveguide  3 , which splits the input waveguide  2 ; a first Mach-Zehnder interferometer  4 ; and a second Mach-Zehnder interferometer  5 . 
         [0034]    The first Mach-Zehnder interferometer  4  comprises: an input coupler  6  connected to one of the two waveguides  14 ,  15  (in  FIG. 1 , it is waveguide  14 ), which are split by the Y-shape waveguide  3 ; an output coupler  7  connected to output ends of two output waveguides  21 ,  22 ; and two arm waveguides  8 ,  9 , which are connected between the both couplers  6 ,  7 . The two arm waveguides  8 ,  9  are different in lengths. Similarly, the second Mach-Zehnder interferometer  5  comprises: an input coupler  10  connected to the other waveguides (in  FIG. 1 , it is waveguide  15 ) of the two waveguides  14 ,  15 , which are split by the Y-shape waveguide  3 ; an output coupler  11 , which is connected to output ends of two output waveguides  23 ,  24 ; and two arm waveguides  12 ,  13 , which are connected between the both couplers  10 ,  11 . The two arm waveguides  12 ,  13  are different in lengths. 
         [0035]    The input couplers  6 ,  10  and the output couplers  7 ,  11  are 2 inputs×2 outputs-type, 3 dB couplers (50% directional couplers). One end of the input coupler  6  of the first Mach-Zehnder interferometer  4  is connected to the waveguide  14 , and one end of the input coupler  10  of the second Mach-Zehnder interferometer  5  is connected to the waveguide  15 . 
         [0036]    Also, the two output ends (a through port and a cross port) of the output coupler  7  of the first Mach-Zehnder interferometer  4  are connected to the first and second output waveguides  21 ,  22 , respectively. In a similar fashion, the two output ends (a through port and a cross port) of the output coupler  11  of the second Mach-Zehnder interferometer  5  are connected to the third and fourth output waveguides  23 ,  24 , respectively. 
         [0037]    Also, in the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4 , there is a difference in length of the waveguides ΔL to make phase shift of the DQPSK signal in one end (i.e. the arm waveguide  8 ) delay by π radians against the other (i.e. the arm waveguide  9 ). In a similar fashion, in the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5 , there is a difference in length of the waveguides ΔL to make the phase shift of the DQPSK signal in one end (i.e. arm waveguide  12 ) delay by π radians against the other (i.e. arm waveguide  13 ). 
         [0038]    Characteristics of the delay demodulation device  1 , which relate to an embodiment of the present invention, is that on a PLC  1 A, the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4 , and the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  are overlapped in the same areas. 
         [0039]    In one of the embodiments of the present invention, as an example of overlapping, the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  are placed on the same areas of the PLC  1 A such that the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  cross each other four times. 
         [0040]    In other words, as shown in  FIG. 1 , the arm waveguide  8  of the first Mach-Zehnder interferometer  4  crosses with the arm waveguide  12  of the second Mach-Zehnder interferometer  5  at a crossover point  61 , with the arm waveguide  13  of the second Mach-Zehnder interferometer  5  at a crossover point  62 , with the arm waveguide  13  at a crossover point  63 , and with the arm waveguide  12  at a crossover point  64 . 
         [0041]    The arm waveguide  9  of the first Mach-Zehnder interferometer  4  crosses with the arm waveguide  12  of the second Mach-Zehnder interferometer  5  at a crossover point  65 , with the arm waveguide  13  of the second Mach-Zehnder interferometer  5  at a crossover point  66 , with the arm waveguide  13  at a crossover point  67 , and with the arm waveguide  12  at a crossover point  68 . 
         [0042]    In a similar fashion, the arm waveguide  12  of the second Mach-Zehnder interferometer  5  crosses with the arm waveguide  9  of the first Mach-Zehnder interferometer  5  at the crossover point  65 , with the arm waveguide  8  of the first Mach-Zehnder interferometer  5  at the crossover point  61 , with the arm waveguide  8  at the crossover point  64 , and with the arm waveguide  9  at the crossover point  68 . 
         [0043]    Then, the arm waveguide  13  of the second Mach-Zehnder interferometer  5  crosses with the arm waveguide  9  of the first Mach-Zehnder interferometer  5  at the crossover point  66 , with the arm waveguide  8  of the first Mach-Zehnder interferometer  5  at the crossover point  62 , with the arm waveguide  8  at the crossover point  63 , and with the arm waveguide  9  at the crossover point  67 . 
         [0044]    In each of the crossover point  61 ˜ 68 , where two arm waveguides cross, the optical signal after the cross over point propagates in the same arm waveguide, which the signal was propagated before. For example, at the crossover point  61 , where two arm waveguides  8 ,  12  cross, the optical signal propagates in the arm waveguide  8  continues to propagate in the arm waveguide  8  after the crossover point  61 . In a similar way, the optical signal propagates in the arm waveguide  12  continues to propagate in the arm waveguide  12  after the crossover point  61   
         [0045]    PLC  1 A shown in  FIG. 1  has the input waveguide  2 , the Y-shape waveguide  3 , the first Mach-Zehnder interferometer  4 , the second Mach-Zehnder interferometer  5 , and the four output waveguides  21 ˜ 24  made all from silica glass. The delay demodulation device  1  comprising the PLC  1 A is manufactured as follow. 
         [0046]    With flame hydrolysis deposition (FHD), silica material (SiO 2 -type glass particles), which makes a lower cladding layer and a core layer, is deposited on a PLC base plate  30  (such as a silica base plate) as shown in  FIG. 3 . Then, a glass coating made by the deposition is fused (and becomes transparent) by adding heat. Later, desired waveguides are created by photo lithography and reactive ion etching, and a upper cladding is created with FHD method. In  FIG. 3 , on the PLC base plate  30 , a cladding layer is created by the lower cladding layer and the upper cladding layer, and the arm waveguides  8 ,  12  are created as the core layer inside of the cladding layer  31 . The PLC base plate  30  is approximately in square planar shape as shown in  FIG. 1 . 
         [0047]    In the delay demodulation devices related to the present invention, the first Mach-Zehnder interferometer  4  and the second Mach-Zehnder interferometer  5  are placed symmetrical to each other on the PLC base plate  30 . 
         [0048]    Also, to reduce PDf, a half wave plate  47  is inserted at the center portion of the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4  and at the center portion of the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  of the delay demodulation device  1 . As shown in  FIG. 4 , a groove  49  is created on the cladding layer  31  to insert the half wave plate  47 . The groove  49  is tilted by 8° to make the half-wave plate  47  tilt by 8° as shown in  FIG. 4 , to prevent loss due to reflections by the half-wave plate  47 . 
         [0049]    Also, as shown in  FIG. 1 , in the delay demodulation device  1 , the center portion of the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4  are parallel to and close to each other, and the center portion of the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  are also parallel to and close to each other. 
         [0050]    The other characteristics of the delay demodulation device  1  are as follows. As shown in  FIG. 1 , ends of the input waveguide  2 , the two output waveguides  21 ,  23 , and the other two output waveguides  23 ,  24  appear on the same side  1   a  of the PLC chip  1 B, which is approximately square planar in shape. In other words, the ends of the waveguides  2  and the four output waveguides  21 ˜ 24  appear on the same side  1   a  (one of four sides) of the PLC chip  1 B, and are close to each other. 
         [0051]    Also, in the delay demodulation device  1 , heaters are placed on the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4 , and the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5 . 
         [0052]    In an embodiment of the present invention, as an example, heaters A, C are placed on the both sides of the center portion of the arm waveguide  8 ; and heaters B, D are placed on the both sides of the center portion of the arm waveguide  9 . In a similar way, heaters E, G are placed on the both sides of the center portion of the arm waveguide  12 , and heaters F, H are placed on the both sides of the center portion of the arm waveguide  13 . Heaters A˜H are placed above the corresponding arm waveguide, and the heaters are Tantalum compound thin film heaters made by a weld slag, which are placed onto the upper cladding (the cladding layer  31  in  FIG. 3 ). In  FIG. 3 , the heaters A, E placed above the cladding layer  31  of the arm waveguides  8 ,  12  are shown. 
         [0053]    Also, in the delay demodulation device  1 , the output ends of output waveguides  21 ,  22  are output ports (a first output port and a second output port), which output signals  1 ,  2  wherein the phase of one output signal is shifted by π radians with respect to the other (see  FIG. 5 ). In a similar way, the output ends of output waveguides  23 ,  24  are output ports (a third output port and a fourth output port), which output signals  3  and  4  wherein the phase of one output signal is shifted by π radians with respect to the other (see  FIG. 5 ). 
         [0054]    In the delay demodulation device  1 , DQPSK signal transmitted from the optical fiber transmission line  54  to the optical receiver  50  splits by the Y-shape waveguide  3 , and the split DQPSK signals propagate to the two arm waveguides  8  (wherein the lengths are different) of the first Mach-Zehnder interferometer. The Mach-Zehnder interferometer  4  shifts the phase of the DQPSK signal transmitted in one arm waveguide  8  by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide  9 . Similarly, the second Mach-Zehnder interferometer  5  shifts the phase of the DQPSK signal transmitted in one arm waveguide  12  by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide  13 . 
         [0055]    The delay demodulation device  1  adjusts PDf, for example, by using the heaters A, C or the heaters B, D of the Mach-Zehnder interferometer  4 . After the adjustment, the delay demodulation device  1  performs phase shift control (or phase shift trimming) to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer, for example, by using the heaters A and C. 
       EMBODIMENTS 
       [0056]    Delay demodulation device  1  has a PLC  1 A on a silica base plate  30  shown in  FIG. 3 . The PLC  1 A comprises: an input waveguide  2 ; a Y-shape waveguide  3 ; Mach-Zehnder interferometers  4 ,  5 ; and output waveguides  21 ˜ 24 , wherein all of the components are made from silica glass. To create the demodulation device  1  FHD method, photo lithography, and reactive ion etching are used. 
         [0057]    In the manufactured delay demodulation device  1 , the difference in the refractive indexes between the cladding layer and the core layer (specific refractive index difference Δ) is 1.5%, and the size of the circuit (i.e. PLC chip  1 B) is relatively small (i.e. 19 mm by 16 mm). Its free spectral range (FSR) is 20 GHz. The PDf is adjusted by using heaters on one of the two Mach-Zehnder interferometers  4 ,  5 . After the adjustment, phase shift control (or phase shift trimming) is performed by using heaters on one of the two Mach-Zehnder interferometers  4 ,  5  to shift the phases of one Mach-Zehnder interferometer by π/2 radians with respect to the phase of the other Mach-Zehnder interferometer. 
         [0058]    To create a packaging, a fiber array comprising four optical fibers in a line is connected to one side  1   a  of the PLC chip  1 B. The side  1   a  has the ends (i.e. output ports) of output waveguides  21 ˜ 24 , which output optical signals to the outputs  1 ˜ 4 , respectively. Also, as a temperature control device, a peltier element and a thermostat are used. Then, an optical fiber module having the delay demodulation device  1  is manufactured. 
         [0059]      FIG. 5  shows the results of the optical characteristics of the 40 Gbps DQPSK delay demodulation device  1  (i.e. PLC type demodulation Mach-Zehnder interferometers delay circuit for DQPSK signal). Insertion loss of less than 6 dB and extremely low PDf (less than 0.1 GHz) are achieved. 
         [0060]    According to the embodiment presented above, the following advantages can be achieved. 
         [0061]    In the delay demodulation device  1 , the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4 , and the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  are overlapped in the same areas within the PLC  1 A. More specifically, the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  are placed on the same areas of the PLC  1 A such that the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  cross each other four times. Because of such construction, the PLC  1 A can be made smaller. In particular, because areas including the two arm waveguides  8 ,  9  in the first Mach-Zehnder interferometer  4  and two arm waveguides  12 ,  13  in the second Mach-Zehnder interferometer  5  are smaller, the chip (i.e. PLC chip)  1 B can be made smaller as well. 
         [0062]    By making the PLC chip  1 B smaller, temperature distribution within the PLC surface  1 A improves and shifts in the center wavelengths due to environment and temperature fluctuation can be made very small. 
         [0063]    Also, by making the PLC chip smaller, stress distribution within the PLC chip  1 B, which causes birefringence, is reduced and shifts in the center wavelengths due to the environment and temperature fluctuation can be made very small. Therefore, there is little or no wavelength shift due to the environment and temperature fluctuation, and the delay demodulation devices with small initial PDf can be made. 
         [0064]    By making the PLC chip smaller, the optical fiber modules with delay demodulation devices can be made smaller, and power consumption can be reduced. 
         [0065]    Because the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  are placed within the same area of the PLC  1 A (the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  cross each other four times), the PLC chip  1 B can be made smaller and achieve low PDf. 
         [0066]    Because the first Mach-Zehnder interferometer  4  and the second Mach-Zehnder interferometer  5  are placed symmetrically on the PLC base plate  30 , the PLC chip  1 B can be further reduced in size and in PDf. 
         [0067]    Because the half wave plate  47  is inserted at the center of the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4  and at the center of the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  of the delay demodulation device  1 , the PDf can be reduced. 
         [0068]    The center portions of the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4  are placed in parallel and close to each other. The center portions of the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  are placed in parallel and close to each other. Because of the construction, retardation by the half wave plate  47  can be suppressed. 
         [0069]    Because the ends of the input waveguide  2  and four output waveguides  21 ˜ 24  face on the same side  1   a  of the PLC chip  1 B, the PLC chip  1 B can be further reduced in size. 
         [0070]    Because the heaters A˜H are placed on the arm waveguides of the first and second Mach-Zehnder interferometers  4 ,  5 , the PDf can be adjusted by using the heaters on either the first or the second Mach-Zehnder interferometer. After the adjustment, phase shift control (phase shift trimming) can be performed by using heaters on one of the two Mach-Zehnder interferometers  4 ,  5  to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer. 
         [0071]    In the above embodiment, because the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  cross each other four times, there are some transmission losses at crossover points  61 ˜ 68 . However, the total transmission loss is relatively small (i.e. 0.1˜0.2 dB). 
         [0072]    Also, in the above embodiment, the arm waveguides  8 ,  9  and the arm waveguides  12 ,  13  cross each other four times. However, the present invention can be applied to delay demodulation devices, which cross two arm waveguides of a first Mach-Zehnder interferometer and two arm waveguides of a second Mach-Zehnder interferometer twice. 
         [0073]    Also, in the above embodiment, as a preferred embodiment, the center portions of the two arm waveguides  8 ,  9  of the first Mach-Zehnder interferometer  4 , and the two arm waveguides  12 ,  13  of the second Mach-Zehnder interferometer  5  are placed adjacent to each other. However, without depending of such construction of the embodiment, the present invention can be applied to delay demodulation devices, whose center portions of two arm waveguides of a first Mach-Zehnder interferometer, and the center portions of two arm waveguides of a second Mach-Zehnder interferometer can be placed apart from and parallel to each other. 
         [0074]    The present invention is not limited to the above described embodiments and various and modifications may be possible without departing from the scope of the present invention.