Patent Publication Number: US-7212334-B2

Title: Optical signal-to-noise ratio improving device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 11/136,412, filed May 25, 2005 now U.S. Pat. No. 7,123,403. Application Ser. No. 11/136,412, filed May 25, 2005 is a U.S. continuation application filed under 35 USC 111(a) claiming benefit under 35 USC 120 and 365(c) of PCT application JP03/04775, filed on Apr. 15, 2003. The application is hereby incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention generally relates to an optical signal processing device and a method thereof, and particularly, to an optical signal processing device and a method thereof used in optical communication or optical measurements for improving a signal to noise (S/N) ratio of an optical signal. 
   TECHNICAL BACKGROUND 
     FIG. 1  and  FIG. 2  illustrate examples of optical signal processing devices used in optical communication or optical measurements for improving a signal to noise (S/N) ratio of an optical signal itself, as disclosed respectively by B. K. Nayar, et al., “Concatenated all-optical loop mirror switches”, Journal of Modern Optics, Vol. 40, No. 12, pp. 2327–2332, 1993, and by K. E. Stubkjaer, et al., “Wavelength conversion devices and techniques” Proceedings of 22nd European Conference on Optical Communication ECOC&#39;96, ThB. 2. 1, 1996. 
     FIG. 1  is a diagram illustrating a device using a non-linear loop mirror (NOLM). 
     FIG. 2  is a diagram illustrating a device using a wavelength converter. 
   In  FIG. 1 , an input optical signal having a wavelength of λs passes through an optical coupler  1  and is input to a non-linear optical fiber  2 . On the other hand, a continuum light beam having a wavelength of λprb is output from a laser  3 . This continuum light beam passes through a polarization controller  4 , and is input to an optical coupler  5 , in which the continuum light beam is divided into two probe light beams. One of the two probe light beams passes through the optical coupler  1  and is input to the non-linear optical fiber  2 , and the other probe light beam passes through a polarization controller  6  and is input to the non-linear optical fiber  2  from an opposite direction. In the non-linear optical fiber  2 , by cross-phase modulation, a phase-modulated light beam is generated which has the wavelength of λprb but includes a signal component of the wavelength of λs. This light beam passes through the polarization controller  6  and interferes with a light beam in an opposite direction in the optical coupler  5 , thereby, generating and outputting an intensity-modulated signal of the wavelength λprb with reduced noise. 
   In  FIG. 2 , a continuum light beam having a wavelength of λ 2  is output from a laser  10 , passes through a waveguide  11 , and is input to an optical coupler  12 . The input light beam is divided into two continuum light beams, and the two continuum light beams are respectively input to semiconductor optical amplifiers  13 ,  14 . On the other hand, an input optical signal having a wavelength of λ 1  is supplied from a waveguide  18 . This input signal passes through an optical coupler  15 , and is input to the semiconductor optical amplifier  13  in a direction opposite to the direction of the continuum light beam output from the semiconductor optical amplifier  13 . 
   The optical signals respectively output from the semiconductor optical amplifiers  13  and  14  are mixed in the optical coupler  16 , and a light beam is generated and output through a waveguide  17 , which has a wavelength of λ 2  and reduced noise due to interference and signal transcription caused by cross-phase modulation. 
   In addition, a method for improving an extinction ratio of an optical fiber is disclosed in Japanese Laid Open Patent Application No. 61-9606. 
   However, in the device using a non-linear loop mirror (NOLM) as illustrated in  FIG. 1 , because the non-linear optical fiber  2  is used, it is difficult to make the device compact. In addition, because it is required to set the power of the input light beams to be a sufficiently large value, it is difficult to reduce the power of the input light beams. 
   Further, in the device using a wavelength converter as illustrated in  FIG. 2 , when the frequency of the input signal is higher than 10 GHz, input/output performance of the device degrades, and it becomes difficult to output signals having the same wavelength obtained by converting the input signals, hence, such a device is not suitable for use of improving the S/N ratio of an optical signal. 
   DISCLOSURE OF THE INVENTION 
   A general object of the present invention is to provide an optical signal processing method and an optical signal processing device able to be made compact, reduce power consumption and able to improve a signal to noise ratio of an optical signal. 
   According to a first aspect of the present invention, there is provided an optical signal processing method including steps of dividing an input optical signal into a first polarization optical component and a second polarization optical component orthogonal to the first polarization optical component; supplying the first polarization optical component to a first gain device, in said first gain device, a gain of power of an input optical signal saturating at a first value; supplying the second polarization optical component to a second gain device, in said second gain device, a gain of power of an input optical signal saturating at a second value less than the first value; combining output light from the first gain device and output light from the second gain device; and outputting the combined optical signal through a polarization element. 
   According to a second aspect of the present invention, there is provided an optical signal processing device comprising one or more optical signal processing sections, wherein the optical signal processing section includes a divider that divides an input optical signal into a first polarization optical component and a second polarization optical component orthogonal to the first polarization optical component; a first gain device supplied with the first polarization optical component, in said first gain device, a gain of power of an input optical signal saturating at a first value; a second gain device supplied with the second polarization optical component, in said second gain device, a gain of power of an input optical signal saturating at a second value less than the first value; a combination unit that combines output light from the first gain device and output light from the second gain device; and a polarization unit supplied with an optical signal output from the polarized-light combination unit. 
   As an embodiment, the optical signal processing device further comprises a dividing device that divides an input optical signal into a first polarization optical component and a second polarization optical component orthogonal to the first polarization optical component, the first polarization optical component being supplied to one of the optical signal processing sections at a predetermined polarization angle, the second polarization optical component being supplied to another one of the optical signal processing sections at a predetermined polarization angle; and a combination unit that combines output light from the one optical signal processing section and output light from the another one optical signal processing section. 
   As an embodiment, the optical signal processing device further includes an optical amplification unit that amplifies an input optical signal, an optical signal output from the optical amplification unit being supplied to one of the optical signal processing sections; an optical detection unit that detects power of the input optical signal; and a gain control unit that controls a gain of the optical amplification unit according to the power of the input optical signal detected by the optical detection unit. 
   As an embodiment, the optical signal processing device further includes an optical amplification unit that amplifies an input optical signal, an optical signal output from the optical amplification unit being supplied to the one optical signal processing section and the other one optical signal processing section; an optical detection unit that detects power of the input optical signal; and a gain control unit that controls a gain of the optical amplification unit according to the power of the input optical signal detected by the optical detection unit. 
   As an embodiment, the optical signal processing device further includes a bias light source that outputs bias light to be combined with the optical signal output from the optical amplification unit, said combined light being supplied to the optical signal processing unit. 
   As an embodiment, the optical signal processing device further comprises a driving current control unit that calculates gains of a plurality of gain devices in the optical signal processing unit from a power level of the optical signal output from the optical amplification unit and respective power levels of optical signals output from the gain devices, and controls driving currents of the respective gain devices so that the gains of the gain devices become predetermined values. 
   As an embodiment, the optical signal processing device further comprises a continuum light generating unit that outputs a continuum laser beam; and a wavelength conversion unit that is supplied with an input optical signal and the continuum laser beam, and generates an output optical signal having a predetermined polarization state and including an intensity-modulated signal of the input optical signal at the wavelength of the continuum laser beam, an optical signal output from the wavelength conversion unit being supplied to one of the optical signal processing sections. 
   As an embodiment, the optical signal processing device further comprises a continuum light generating unit that outputs a continuum laser beam; and a wavelength conversion unit that is supplied with an input optical signal and the continuum laser beam, and generates an output optical signal having a predetermined polarization state and including an intensity-modulated signal of the input optical signal at the wavelength of the continuum laser beam, an optical signal output from the wavelength conversion unit being supplied to the one optical signal processing section and the another one optical signal processing section. 
   As an embodiment, the optical signal processing device further comprises an optical clock regeneration unit that regenerates an optical clock from an input optical signal; and a wavelength conversion unit that is supplied with an input optical signal and the regenerated optical clock, and generates an output optical signal having a predetermined polarization state and including an intensity-modulated signal of the input optical signal at the wavelength of the regenerated optical clock, an optical signal output from the wavelength conversion unit being supplied to one of the optical signal processing sections. 
   As an embodiment, the optical signal processing device further comprises an optical clock regeneration unit that regengerates an optical clock from an input optical signal; and a wavelength conversion unit that is supplied with an input optical signal and the regenerated optical clock, and generates an output optical signal having a predetermined polarization state and including an intensity-modulated signal of the input optical signal at the wavelength of the regenerated optical clock, an optical signal output from the wavelength conversion unit being supplied to the one optical signal processing section and the other one optical signal processing section. 
   As an embodiment, in the optical signal processing device, each of the first gain device and the second gain device is a semiconductor optical amplifier. 
   As an embodiment, in the optical signal processing device, each of the first gain device and the second gain device is a distributed feedback laser diode. 
   As an embodiment, in the optical signal processing device, each of the first gain device and the second gain device is a Raman amplifier. 
   According to the present invention, it is possible to provide an optical signal processing method and an optical signal processing device able to improve a signal to noise ratio of an optical signal, and the optical signal processing device can be made compact and has low power consumption. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a device using a non-linear loop mirror (NOLM); 
       FIG. 2  is a diagram illustrating a device using a wavelength converter; 
       FIG. 3  is a block diagram illustrating an optical signal processing device  20  according to a first embodiment of the present invention; 
       FIG. 4A  and  FIG. 4B  show input/output characteristics of the optical signal processing device  20  and gain characteristics of the SGD  26  and SGD  30 ; 
       FIG. 5  is a vector diagram illustrating a principle of the optical signal processing device  20 ; 
       FIG. 6  is a block diagram illustrating an optical signal processing device  40  according to a second embodiment of the present invention; 
       FIG. 7  is a block diagram illustrating an optical signal processing device  50  according to a third embodiment of the present invention; 
       FIG. 8  is a block diagram illustrating an optical signal processing device  60  according to a fourth embodiment of the present invention; 
       FIG. 9  is a block diagram illustrating an optical signal processing device  80  according to a fifth embodiment of the present invention; 
       FIG. 10  is a block diagram illustrating an optical signal processing device  90  according to a sixth embodiment of the present invention; and 
       FIG. 11  is a block diagram illustrating a configuration of a SGD formed from a Raman amplifier. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Below, embodiments of the present invention are explained with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 3  is a block diagram illustrating an optical signal processing device  20  according to a first embodiment of the present invention. 
   As illustrated in  FIG. 3 , in the optical signal processing device  20 , a linearly-polarized optical signal is output from an optical fiber  22 , and is input to a polarization element  24 . The polarization element  24  divides the input optical signal into two components having polarization directions perpendicular to each other, for example, one is along the X axis and the other is along the Y axis (not illustrated). The X component is input to a saturable gain device (SGD)  26 , and the Y component is reflected on a reflecting mirror  28  and is then input to an SGD  30 . 
     FIG. 4A  and  FIG. 4B  show input/output characteristics of the optical signal processing device  20  and gain characteristics of the SGD  26  and SGD  30 . 
   As indicated by a solid line Ix in  FIG. 4B , the SGD  26  has non-linear gain characteristics, specifically, when the power (Pin) of the X component of the input optical signal exceeds a value of Psx, the gain Gx of the SGD  26  saturates. 
   Returning to  FIG. 3 , the X component of the input optical signal amplified by the SGD  26  is reflected on a mirror  32 , and is input to a polarization element  34 . 
   On the other hand, as indicated by a solid line Iy in  FIG. 4B , the SGD  30  also has non-linear gain characteristics, specifically, when the power (Pin) of the Y component of the input optical signal exceeds a value of Psy, the gain Gy of the SGD  30  saturates. 
   Returning to  FIG. 3 , the Y component of the input optical signal amplified by the SGD  30  passes through an attenuator  36  and is input to the polarization element  34 . 
   The polarization element  34  combines the X component and the Y component. The output signal from the polarization element  34  is output to an optical fiber  39  through a polarization element  38 . 
     FIG. 5  is a vector diagram illustrating a principle of the optical signal processing device  20 . 
   As illustrated in  FIG. 5 , an input optical signal having a polarization direction inclined relative to the X axis and the Y axis is input to the optical signal processing device  20 . Because of dependence of gain on a polarization state, the polarization state changes after the optical signal transmits through the optical signal processing device  20 . If an upper limit of the gain of light polarized along the X axis is different from an upper limit of the gain of light polarized along the Y axis, the polarization state of the combination of the output light from the SGD  26  and the SGD  30  changes depending on the power of the light input to the SGD  26  and the SGD  30 . 
   The optical signal combined in the polarization element  34  enters into the polarization element  38 . The polarization element  38  is arranged such that the polarization element  38  blocks the output polarized light from the polarization element  34  when the optical signal input to the optical signal processing device  20  is at a “0” level, that is, the output polarized light from the polarization element  34  is orthogonal to the polarization state of the polarization element  38 . By such an arrangement, it is possible to reduce noise in the “0” level. In addition, by setting noise in a “1” level to be near an upper limit of the gain, it is possible to reduce noise in the “1” level. Hence, noise in both “0” level and “1” level can be reduced, and the S/N ratio of the optical signal can be improved. Returning to  FIG. 4A , a solid line I/O indicates the input-output characteristics of the optical signal processing device  20 . 
   Below, gain characteristics of the SGD  26  and SGD  30  are explained in detail. 
   First, assume the SGD  26  and SGD  30  are formed from semiconductor optical amplifiers (SOA) having gain saturation characteristics. The X axis is defined to be the direction parallel to a width direction of the active layer of the semiconductor optical amplifier, and the Y axis is defined to be the direction perpendicular to the X axis (the direction perpendicular to a plane of the active layer. 
   Light having an electrical component parallel to the X axis is incident on the semiconductor optical amplifier acting as the SGD  26 , and light having an electrical component parallel to the Y axis is incident on the semiconductor optical amplifier acting as the SGD  30 . 
   The linearly-polarized optical signal from the optical fiber  22  is indicated by Pin, and the polarization element  24  divides the input optical signal Pin into an X electric field component and a Y electric field component. The X component and the Y component are represented by Pinx and Piny, respectively. Pin, Pinx, and Piny satisfy the following relations:
 
Pin=(Pinx 2 +Piny 2 ) 1/2 , and 1&lt;&lt;Piny/Pinx.
 
   Because the SGD  26  and SGD  30  are semiconductor optical amplifiers independent from each other, gains of the X component and the Y component can be set separately. The gains of the X component and the Y component are represented by Gx and Gy, respectively, and satisfy the following relations (1) and (2).
 
Gx= a (when Pinx&lt;Psx),
 
Gx= a *Psx/Pinx (when Pinx&gt;Psx)  (1)
 
Gy= b (when Piny&lt;Psy),
 
Gy= b *Psy/Piny (when Piny&gt;Psy)  (2)
 
   where Psx indicates a saturation input level of the X component, Psy indicates a saturation input level of the Y component, a is a constant indicating an unsaturated gain in the X direction, and b is a constant indicating an unsaturated gain in the Y direction. 
   The X component of the output light of the SGD  26 , which is indicated by Poutx, and the Y component of the output light of the SGD  30 , which is indicated by Pouty, are expressed by the following equations (3) and (4), respectively.
 
Poutx=Gx*Pinx  (3)
 
Pouty=Gy*Piny  (4)
 
   The polarization angle (θout) of the output light from the polarization element  34  is expressed by the following equation (5).
 
θout=tan −1 (Pouty/Poutx)  (5)
 
   Here, components of the input optical signal are respectively input to the SGD  26  and the SGD  30  by setting Pinx&lt;Piny. Because Gx and Gy can be set independently, if the Y component is set to reach gain saturation at relatively low input power, the value of θout can be expressed by the following equations (6a), (6b), and (6c).
 
θout=tan −1 [( b/a )*(Pouty/Poutx)]  (6a)
 
   (both Pinx and Piny are unsaturated, and θout is constant.)
 
θout=tan −1 [( b/a )*(Psy/Pinx)]  (6b)
 
   (Pinx is unsaturated, and θout decreases along with increase of Pinx.)
 
θout=tan −1 [( b/a )*(Psy/Psx)]  (6c)
 
   (both Pinx and Piny are saturated, and θout is constant.) 
   Here, Piny/Pinx is set to be a constant sufficiently greater than 1. 
   The polarization element  38  is arranged at the output end of the polarization element  34 , and is set so as to allow transmission of light having a polarization angle perpendicular to the polarization angle (θout=θ 0 ) of the light output from the polarization element  34  when both Pinx and Piny are unsaturated. 
   The output of the light transmitting the polarization element  38  Poutp is expressed by the following equation (7).
 
Poutp=[(Poutx) 2 +(Pouty) 2 ]] 1/2 *cos(π/2−θ 0 +θout)  (7)
 
where, θ 0  is the value of θout when both Pinx and Piny are unsaturated.
 
   The equation (7) can be reduced to be the following equations (8a), (8b), and (8c).
 
Poutp=0  (8a)
 
(both Pinx and Piny are unsaturated, cos(π/2)=0)
 
Poutp=[(Poutx) 2 +(Psy) 2 ]] 1/2  *cos{π/2−θ 0 +tan −1 [(b/a)*(Psy/Pinx)]}  (8b)
 
   (Pinx is unsaturated)
 
Poutp=[(Psx) 2 +(Psy) 2 ]] 1/2 *cos{π/2−θ 0 +tan −1 [( b/a ) *(Psy/Psx)]}  (8c)
 
   (both Pinx and Piny are saturated) 
   In this way, it is possible to reduce noise in both “0” level and “1” level to an optical level. 
   Second Embodiment 
     FIG. 6  is a block diagram illustrating an optical signal processing device  40  according to a second embodiment of the present invention. 
   The optical signal processing device  20  in  FIG. 3  suffers from polarization dependence in principle. The optical signal processing device  40  of the present embodiment is devised to solve this problem. 
   As illustrated in  FIG. 6 , in the optical signal processing device  40 , a waveguide  41  supplies an input optical signal, the input optical signal is supplied to a polarization divider  42 , and the polarization divider  42  divides the input optical signal into two linearly-polarized light components having polarization directions perpendicular to each other. The two linearly-polarized light beams are input to optical processing sections  44  and  46 , respectively, with their polarization angles being appropriately set. The optical processing sections  44  and  46  have the same structure as the optical signal processing device  20  in  FIG. 3 . 
   The linearly-polarized light beams respectively output from the optical processing sections  44  and  46  are supplied to a polarization combiner  48  with the two linearly-polarized light beams being orthogonal to each other. The polarization combiner  48  combines the two linearly-polarized light beams, and the combined light beam is output through a waveguide  49 . 
   The optical signal processing device  40  configured in this way is operable for input optical signals of most polarization states. 
   Third Embodiment 
     FIG. 7  is a block diagram illustrating an optical signal processing device  50  according to a third embodiment of the present invention. 
   As illustrated in  FIG. 7 , a waveguide  51  supplies an input optical signal to the optical signal processing device  50 . The input optical signal is divided into two optical signals at an optical coupler  52 , and the two optical signals are supplied to an optical amplifier  54  and an optical detector  56 , respectively. 
   The optical detector  56  detects an average value Pav of the power of the input optical signal, and sends the result to a controller  58 . The controller  58  controls the gain of the optical amplifier  54  according to the average power Pav of the input optical signal. 
   When the average power Pav of the input optical signal is equal to a basic value Pset, the controller  58  sets the gain Gamp of the optical amplifier  54  to be a basic value Gset. 
   The power Posp of the optical signal supplied to an optical processing section  62  is expressed by the following equation,
 
Posp=Pav*Gamp* T, 
 
   Where, T indicates a transmission rate between the optical amplifier  54  and the optical processing section  62 . 
   If the upper limit of an optical signal input to the optical processing section  62  is represented by Ph, and the lower limit is represented by P 1 , when Posp&lt;Ph, the controller  58  sets the gain Gamp of the optical amplifier  54  in a region described by the following relation (9a).
 
P1/(Pav* T )&lt;Gamp&lt;Ph/(Pav* T )  (9a)
 
   When Posp&gt;Ph, the controller  58  sets the gain Gamp of the optical amplifier  54  in a region described by the following relation (9b).
 
(Pav* T )/Ph&lt;Gamp&lt;(Pav* T )/P1  (9b)
 
   The optical signal amplified by the optical amplifier  54  is input to an optical band-pass-filter (BPF)  60 , the optical band-pass-filter  60  removes an ASE (Amplified Spontaneous Emission) noise component generated in the optical amplifier  54 , and the resulting optical signal is input to the optical processing section  62 . 
   The optical processing section  62  has the same structure as the optical signal processing device  20  in  FIG. 3  or the optical signal processing device  40  in  FIG. 6 . The optical signal from the optical processing section  62  is output through a waveguide  65 . 
   It should be noted that the band-pass-filter  60  is not always required, but may be provided only when necessary. 
   The optical signal processing device  20  in  FIG. 3  or the optical signal processing device  40  in  FIG. 6  can effectively reduce noise in binary digital signals related to power of input optical signals in a specified region with the upper limit being Ph and the lower limit being P 1 . Therefore, by controlling the power of the optical signal input to the optical processing section  62 , it is possible to increase the range of the power of the input optical signal. 
   Fourth Embodiment 
     FIG. 8  is a block diagram illustrating an optical signal processing device  70  according to a fourth embodiment of the present invention. In  FIG. 8 , the same reference numbers are assigned to the same elements as those in  FIG. 7 . 
   As illustrated in  FIG. 8 , a waveguide  51  supplies an input optical signal to the optical signal processing device  70 . The input optical signal is divided into two optical signals at an optical coupler  52 , and the two optical signals are supplied to an optical amplifier  54  and an optical detector  56 , respectively. 
   The optical detector  56  detects an average value Pav of the power of the input optical signal, and sends the result to a controller  72 . The controller  72 , the same as the controller  58  in  FIG. 7 , controls the gain of the optical amplifier  54  according to the average power Pav of the input optical signal. This control operation is the same as that in the previous embodiment, and overlapping descriptions are omitted. 
   The optical signal amplified by the optical amplifier  54  is input to an optical processing section  74 . The optical processing section  74  has substantially the same structure as the optical signal processing device  20  in  FIG. 3  or the optical signal processing device  40  in  FIG. 6 , except that the optical processing section  74  further has a temperature control unit and a SGD degradation compensation unit (for example, a unit for driving current of each SGD) for temperature control and SGD degradation compensation under control of the controller  72 . 
   A bias laser  76 , while being controlled by the controller  72 , emits a laser beam, the bias laser beam is combined with an optical signal output from the optical amplifier  54  at an optical coupler  77 , and the resulting signal is input to the optical processing section  74 . The power of output optical signals from each SGD in the optical processing section  74  is detected by an output monitor  78 , and is sent to the controller  72 . The optical signal from the optical processing section  74  is output through a waveguide  79 . 
   The temperature control unit of the optical processing section  74  detects the temperature of each SGD, and the detected temperature is sent to the controller  72 . The controller  72  controls the temperature control unit of the optical processing section  74  so that the temperature of each SGD is within ±5° C. relative to a preset temperature. 
   Degradation of a SGD is basically expressed by reduction of the gain. For this purpose, the controller  72  calculates the gain of each SGD (indicated by Gsgd(i)) from the detected value Pom of each SGD given by the output monitor  78 , and the power of the optical signal input to the optical processing section  74  (indicated by Posp, and Posp=Pav*Gamp*T) by using the following equation (10).
 
 G   sgd ( i )= A ( i )*Pom( i )/Pav*Gamp* T   (10)
 
   where, the index i indicates the number of the SGDs. 
   The controller  72  controls the driving current Id(i) of each SGD in the optical processing section  74  so that G sgd (i) is close to preset gains of each SGD, indicated by G sgd-set (i). 
   Specifically, when G sgd (i)&lt;G sgd-set (i), the controller  72  increases Id(i), and when G sgd (i)&gt;G sgd-set (i) the controller  72  maintains the Id(i) to be unchanged. 
   Basically, the bias laser has two effects. First, the bias laser biases the optical input to the SGDs so that saturation occurs easily even when the power of the input light is low. Second, although the input light consumes carriers inside the SGDs to acquire gains, the driving current Id(i) and the bias laser induce regeneration of the consumed carriers, namely, the bias laser helps improve response speed of the SGDs. 
   For this reason, if the control operation for maintaining G sgd (i) of each SGD to be constant by control of the driving current Id(i) is not sufficient, the power of the bias laser (denoted as Pbias) is controlled. Specifically, when G sgd (i)&lt;G sgd-set (i), Pbias is increased, and when G sgd (i)&gt;G sgd-set (i), Pbias is maintained to be unchanged so as not to influence the response speed. 
   Fifth Embodiment 
     FIG. 9  is a block diagram illustrating an optical signal processing device  80  according to a fifth embodiment of the present invention. 
   As illustrated in  FIG. 9 , a waveguide  81  supplies an input optical signal to a wavelength converter  82  of the optical signal processing device  80 . For example, the wavelength converter  82  is a cross-phase modulation wavelength converter which employs a change of refractive index induced by a change of electron density in a gain saturation region of a semiconductor optical amplifier and has small polarization dependence. A laser  83  supplies a continuum laser beam to the wavelength converter  82 . The wavelength of the continuum laser beam is the conversion destination wavelength of the wavelength converter  82 . 
   Hence, an intensity-modulated signal of the input optical signal is obtainable from the wavelength converter  82 , which is at the wavelength of the continuum laser beam and is converted to a predetermined polarization state. The thus obtained optical signal is input to an optical band-pass-filter (BPF)  84 , the optical band-pass-filter  84  removes the input optical signal component, and transmits the resulting optical signal to the optical processing section  86 . 
   The optical processing section  86  has the same structure as the optical signal processing device  20  in  FIG. 3  or the optical signal processing device  40  in  FIG. 6 . The optical signal from the optical processing section  86  is input to an optical band-pass-filter (BPF)  88 , the optical band-pass-filter  88  removes an ASE (Amplified Spontaneous Emission) noise component, and the resulting optical signal is output through a waveguide  89 . 
   Due to the dependence on the polarization state of the input optical signal in the previously described optical signal processing devices  20  and  40 , possibly, the optical signal processing devices  20  and  40  cannot adequately suppress noise. In contrast, in the optical signal processing device  80  of the present embodiment, the dependence on the polarization state is removed, and noise can be adequately suppressed. 
   Sixth Embodiment 
     FIG. 10  is a block diagram illustrating an optical signal processing device  90  according to a sixth embodiment of the present invention. In  FIG. 10 , the same reference numbers are assigned to the same elements as those in  FIG. 9 . 
   As illustrated in  FIG. 10 , a waveguide  91  supplies an input optical signal to the optical signal processing device  90 . The input optical signal is divided into two optical signals at an optical coupler  92 , and the two optical signals are supplied to a wavelength converter  94  and an optical clock regenerator  96 , respectively. 
   The optical clock generator  96  generates an optical clock signal, which is a continuum light beam having a wavelength of λc, from the input light having a wavelength of λs. 
   For example, the wavelength converter  94  is a cross-phase modulation wavelength converter which employs a change of refractive index induced by a change of electron density in a gain saturation region of a semiconductor optical amplifier and has small polarization dependence. The optical clock regenerator  96  supplies the optical clock signal having a wavelength of λc to the wavelength converter  94  as bias light. 
   The wavelength converter  94  performs cross-phase modulation on the input optical signal and the optical clock signal with the optical clock signal as bias light. The wavelength converter  94  adjusts the level of the input optical signal and the driving current of a built-in semiconductor optical amplifier so that the wavelength converter  94  performs the conversion efficiently only in a time interval when the optical clock pulses overlap with the optical signal pulses. Hence, by reproducing timing information of the optical input signal, an optical signal free from jitter is obtainable. 
   The optical signal output from the wavelength converter  94  is input to the optical band-pass-filter (BPF)  84 , the optical band-pass-filter  84  removes the input optical signal component, and transmits the resulting optical signal to the optical processing section  86 . 
   The optical processing section  86  has the same structure as the optical signal processing device  20  in  FIG. 3  or the optical signal processing device  40  in  FIG. 6 . The optical signal from the optical processing section  86  is input to the optical band-pass-filter (BPF)  88 , the optical band-pass-filter  88  removes an ASE (Amplified Spontaneous Emission) noise component, and the resulting optical signal is output through a waveguide  89 . 
   Concerning the optical clock regenerator  96 , for example, reference can be made to H. Kurita, et al., IEICE Trans. Electon., Vol. E81-C, No. 2, pp. 129 (1998). 
   Seventh Embodiment 
   The SGD  26  and SGD  30  illustrated in  FIG. 3  may be a distributed feedback semiconductor laser diode (DFB-LD) having gain saturation characteristics, a semiconductor amplifier (SOA), or a Raman amplifier. 
   Concerning the DFB-LD, for example, reference can be made to Taketa et al., Proc. of the 1999 communications society conference of IEICE , B-10-61 (1999). Concerning the SOA, for example, reference can be made to K. Inoue, IEEE, Photo. Tech. Lett. Vol. 8, pp. 458 (1996). 
     FIG. 11  is a block diagram illustrating a configuration of a SGD formed from a Raman amplifier. 
   As illustrated in  FIG. 11 , an input optical signal and an excitation light beam propagating in the same direction as the input optical signal are input to a polarization maintaining optical fiber (PMF)  106  through an optical coupler  102 . For example, the input optical signal is set to be in a polarization state parallel to one axis of the polarization maintaining optical fiber (PMF)  106 . The Raman amplifier formed from the polarization maintaining optical fiber (PMF)  106  functions as a gain device maintained in a certain polarization state, and generally, the Raman amplifier is used as a SGD having gain saturation characteristics. 
   Because the length of the polarization maintaining optical fiber (PMF)  106  is of the order of, for example, a kilometer, walk-off may happen due to wavelength dispersion between the optical signal and the excitation light, and timing shift may arise in the gain saturation characteristics of each bit. In order to prevent this phenomenon, an optical circulator  108 , a mirror  109  used in the signal wavelength band, a mirror  110  used in the excitation wavelength band, and a mirror  111  are provided between the two polarization maintaining optical fibers  106 , and these elements form a compensation optical system which compensates for the timing shift between the optical signal and the excitation light. Due to this, the SGD in  FIG. 11  is operable even at a high bit rate. 
   While the invention has been described with reference to preferred embodiments, the invention is not limited to these embodiments, but numerous modifications could be made thereto without departing from the basic concept and scope described in the claims. 
   In the above embodiments, because a SOA, a DFB-LD or another is used as the SGD, the power of the input optical signal inducing gain saturation can be set relatively freely relative to device design; hence, it is possible to induce gain saturation at input power lower than 10 mW, and allow the device to operate with an improved extinction ratio. In contrast, in a device using a non-linear loop mirror (NOLM) as in the related art, usually, peak input power near 100 mW is required, and it is necessary to mount an optical fiber as long as a few hundreds meters to a few kilometers. 
   That is, the optical signal processing device of the present invention can be made to have small size and low power consumption compared to the NOLM device. 
   In addition, compared to a device using wavelength converter having different wavelengths at input and output ends, the optical signal processing device of the present invention allows input and output of optical signals having the same wavelength, and the optical signal to noise ratio can be improved. As a result, the optical signal processing device of the present invention can be used without being limited by network configurations, and has high usability. 
   Further, in the present invention, even in the case in which the output power corresponding to a “0” level (space) is not “0” in a single device having gain saturation characteristics used as a SGD, with gains of two unsaturated SGDs being adjusted to be equal to each other, by utilizing polarization properties, theoretically, it is possible to set the output power to be “0”. In doing so, it is possible to effectively and greatly improve an extinction ratio regardless of the performance of the gain saturation device in use.