Patent Publication Number: US-2003223763-A1

Title: Optical transmitter and optical communication system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an optical transmitter that includes a light source driven by a direct intensity modulation method for emitting light, and an optical communication system that includes the optical transmitter.  
       [0003] 2. Description of the Background Art  
       [0004] In an optical communication system, signal light output from an optical transmitter propagates through an optical fiber transmission line and is received at an optical receiver. The optical transmitter includes a light source, e.g. a laser diode, and outputs pulsed light as signal light by means of an external intensity modulation method or direct intensity modulation method. In the external intensity modulation method, continuous oscillation light is emitted from a laser diode, and this oscillation light is modulated by the external modulator and is output. On the other hand, in the direct intensity modulation method, the driving current that has been intensity-modulated is supplied to the laser diode and the oscillation light intensity-modulated according to the driving current is output from the laser diode. The direct intensity modulation method is advantageous in view of simple composition and low cost as compared with the external intensity modulation method.  
       [0005]FIGS. 9A and 9B are diagrams for explaining the direct intensity modulation method. FIG. 9A shows the time variation of the driving current supplied to the laser diode and FIG. 9B shows the time variation of the output power of the laser diode. The value of the driving current supplied to the laser diode is set to be greater than or smaller than a given bias value. According to this, the output power of the laser diode becomes power value “a” or power value “(a+b)”; however, an overshoot occurs at the onset of power. The extinction ratio of this output light is represented by 10log 10  (b/a) and can be adjusted by the bias value of the driving current.  
       [0006]FIGS. 10A to  10 C are diagrams for explanation in the case where the extinction ratio is large in the direct intensity modulation method. FIG. 10A shows the time variation of the output power of the laser diode, FIG. 10B shows the frequency fluctuation of light emitted by the laser diode and FIG. 10C shows the time variation of the power of the light which has reached an optical receiver. When the extinction ratio is high as shown in FIG. 10A, the oscillation frequency changes considerably greatly as shown in FIG. 10B. The change of the oscillation frequency at the onset of the pulsed light is called “chirp”. If a chirp occurs, the waveform of the light which has reached an optical receiver is greatly distorted and the quality of the light transmission degrades due to the optical fiber transmission line having chromatic dispersion as shown in FIG. 10C.  
       [0007]FIGS. 11A and 11B are diagrams for explaining the case in which the extinction ratio is small in the direct intensity modulation method. FIG. 11A shows the time variation of output power of the laser diode, and FIG. 11B shows the frequency fluctuation of light emitted from the laser diode. If the extinction ratio is small as shown in FIG. 11A, the change of the oscillation frequency is small as shown in FIG. 11B at the onset of the output pulse, and hence the degree of the deterioration of signal waveform due to the chromatic dispersion of an optical fiber transmission line is small. However, when the extinction ratio is small, the reception sensitivity at the optical receiver is poor.  
       [0008] Thus, if the extinction ratio is high, the light transmission quality degrades due to the chromatic dispersion of the optical fiber transmission line, and conversely when the extinction ratio is low, the reception sensitivity is inferior. That is, the improvement of light transmission quality and that of the reception sensitivity have a trade-off relationship.  
       SUMMARY OF THE INVENTION  
       [0009] The object of the present invention is to provide an optical transmitter in which both light transmission quality and reception sensitivity are improved, and an optical communication system that is equipped with such optical transmitter.  
       [0010] In order to achieve this object, the present invention provides an optical transmitter which comprises (1) a light source driven by the direct intensity modulation method and (2) an intensity modulator having an input/output characteristic such that the ratio R (R=P out /P in , where P in  is the power of input light, and P out  is the power of output light) increases according to the increase of the power P in  of input light, and in which light input from the light source is output via the intensity modulator.  
       [0011] Also, the present invention provides an optical communication system which comprises such optical transmitter and in which signal light output from the optical transmitter propagates through an optical fiber transmission line.  
       [0012] The intensity modulator may be such that the amount of light absorption decreases as the power P in  of input light increases, and when the power P in  of input light is less than the threshold value, all input light is absorbed. When the power P in  of input light exceeds the threshold, the amount of the input light corresponding to the threshold value may be absorbed. For example, the intensity modulator may preferably include a saturable absorber.  
       [0013] The intensity modulator may output input light after optical amplification thereof, and therefore it may be such that the gain of optical amplification may be increased according to the increase of the power P in  of input light, and when the power P in  of input light is less than the threshold value, it may optically amplify input light and output it. For example, the intensity modulator may comprise: (1) an optical coupler including a first port, second port, third port and fourth port such that light input to the first port is divided so as to be output to the second port and third port, and light input to the second port is output to the first port and fourth port, and light input to the third port is output to the first port and fourth port; (2) an optical amplifier which is provided in the optical path between the second port and third port and used for optically amplifying and outputting input light, and (3) an optical fiber which is provided on either of the optical path between the optical amplifier and the second port of the optical coupler and the optical path between the optical amplifier and the third port of the optical coupler.  
       [0014] In such case, the ratio (n NL /A eff ) of nonlinear refractive index n NL  of the optical fiber to the effective core area A eff  may be equal to or more than 1×10 −9 W −1  in the wavelength of light emitted from the laser diode.  
       [0015] It is desirable that the maximum chirp at the onset of the pulsed light emitted from the laser diode that is pulse-driven be equal to or less than 10 GHz. Preferably, the extinction ratio of the pulsed light is equal to or less than 8 and equal to or more than 4. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1A shows the composition of an optical transmitter  10  according to one embodiment of the present invention. FIGS. 1B and 1C are graphs showing an example of the characteristics of the input/output of the intensity modulator  13  that is a component part of the optical transmitter  10 .  
     [0017]FIGS. 2A and 2B are graphs showing the second example of the input/output characteristics of the intensity modulator  13 .  
     [0018]FIGS. 3A and 3B are graphs showing the third example of the input/output characteristics of the intensity modulator  13 .  
     [0019]FIGS. 4A and 4B are graphs showing the fourth example of the input/output characteristics of the intensity modulator  13 .  
     [0020]FIG. 5 is a graph showing the fifth example of the input/output characteristic of the intensity modulator  13 .  
     [0021]FIG. 6 is a graph showing the sixth example of the input/output characteristic of the intensity modulator  13 .  
     [0022]FIG. 7A is a schematic diagram illustrating an exemplary structure of the intensity modulator  13 . FIG. 7B shows the input/output characteristic of the intensity modulator  13 .  
     [0023]FIG. 8 is a schematic diagram showing an example of an optical communication system  1  according to the present invention.  
     [0024]FIGS. 9A and 9B are graphs showing the relationship between a driving current and output light in the direct intensity modulation method.  
     [0025]FIGS. 10A to  10 C are diagrams for explaining examples where the extinction ratio is large in the direct intensity modulation method.  
     [0026]FIGS. 11A and 11B are diagrams for explaining examples where the extinction ratio is small in the direct intensity modulation method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0027] Embodiments of the present invention are explained below by referring to the accompanying drawings. In the drawings, the same number refers to the same part to avoid duplicate explanation. The ratios of the dimensions in the drawings do not necessarily coincide with the explanation.  
     [0028] First, an embodiment of the optical transmitter according to the present invention is explained. FIGS. 1A to  1 C are diagrams for explaining the optical transmitter  10  according to one embodiment of the present invention. FIG. 1A shows the composition of the optical transmitter  10 , and FIG. 1B shows the time variation of the power P in  of light emitted from a laser diode  12  and input to the intensity modulator  13 . FIG. 1C shows the time variation of the power P out  of light output from the intensity modulator  13 .  
     [0029] As shown in FIG. 1A, the optical transmitter  10  is equipped with a driver  11 , the laser diode  12  as a light source, and the intensity modulator  13 . The driver  11  outputs an intensity-modulated driving current and supplies the driving current to the laser diode  12 . The laser diode  12  driven by the driving current supplied from the driver  11  emits intensity-modulated laser light. That is, the laser diode  12  is driven by the direct intensity modulation method and emits laser light. The DFB laser or the Fabry-Perot type laser diode can be used as the laser diode.  
     [0030] The intensity modulator  13 , to which intensity-modulated light having a power P in  emitted from the laser diode  12  is input and from which the intensity-modulated light having a power P out  is output, has an input/output characteristic such that the greater the power P in  of input light, the greater the ratio R is. The intensity modulator  13  outputs, according to such input/output characteristic, the light that has been emitted from the laser diode  12 .  
     [0031] That is, in the optical transmitter  10 , light emitted from the laser diode  12  driven by the driver  11  by the direct intensity modulation method is output in the state in which the extinction ratio is increased according to the input/output characteristic of the intensity modulator  13 . Furthermore, even if the extinction ratio of light emitted from the laser diode  12  is small, the extinction ratio of light output from the intensity modulator  13  increases. Also, since the extinction ratio of light emitted from the laser diode  12  may be small, the degree of chirp decreases. Therefore, in an optical communication system which transmits signal light output from the optical transmitter  10 , the deterioration of light transmission quality due to the chromatic dispersion of an optical fiber transmission line is restrained, and the deterioration of the reception sensitivity is also restrained. Thus, this optical communication system can improve both the light transmission quality and the reception sensitivity.  
     [0032] It is desirable that the maximum chirp at the onset of pulsed light output from the laser diode  12  be 10 GHz or less. In such case, the waveform distortion of light due to chirp can be reduced sufficiently. Preferably, the extinction ratio of light emitted from the pulse-driven laser diode  12  is equal to or less than 8. In such case, the waveform distortion due to chirp can be reduced sufficiently. Also, it is desirable that the extinction ratio of the pulsed light emitted from the pulse-driven laser diode  12  be equal to or more than 4. In such case, the waveform distortion at a lower level due to nonlinear optical phenomenon can be restrained.  
     [0033] Next, the input/output characteristics of the intensity modulator  13  contained in the optical transmitter  10  according to the present embodiment will be further explained. FIGS. 2A to  6  are diagrams for explaining the input/output characteristics of the intensity modulator  13 . FIGS. 2A to  4 B show the input/output characteristics of the intensity modulator  13  in the case where the intensity modulator  13  has no optical amplification feature. FIGS. 2A, 3A, and  4 A show the time variation of the power P in  of input light, and the time variation of the absorption power P A =P in −P out , respectively. FIGS. 2B, 3B, and  4 B show the input/output characteristics (P in  vs. R, or P in  vs. P out ). The input/output characteristics of the intensity modulator  13  shown in each of FIGS. 5 and 6 are those in the case where the intensity modulator  13  has the function to optically amplify input light, and each figure shows the time variation of the power P in  of input light and that of the power P out  of output light.  
     [0034] The input/output characteristics of the intensity modulator  13  as shown in FIGS. 2A and 2B are such that the greater the power P in  of input light, the smaller the absorption power P A  (the hatching parts) is, and the greater the ratio R is. The input/output characteristics of the intensity modulator  13  as shown in FIGS. 3A and 3B are such that when the power P in  of input light is equal to or less than the threshold value PT, all of the input light is absorbed, and when the power P in  of input light exceeds the threshold value, the input light is absorbed to the extent corresponding to the threshold value. That is, only when the power P in  of input light is greater than the threshold value, the power P out  of output light which is the difference between the input light power and the threshold value is obtained. The input/output characteristics of the intensity modulator  13  as shown in FIGS. 4A and 4B are such that the greater the power P in  of input light, the greater the absorption power P A  (the hatching parts) is, whereas the ratio R is great. Among the input/output characteristics shown in FIGS. 2A to  4 B, that shown in FIG. 2A exhibits the greatest improvement effect of the extinction ratio, and that shown in FIG. 3A exhibits the second greatest improvement.  
     [0035] The input/output characteristic of the intensity modulator  13  as shown in FIG. 5 is such that the greater the power P in  of input light, the greater the gain of optical amplification is. The input/output characteristic of the intensity modulator  13  as shown in FIG. 6 is such that when the power P in  of input light is equal to or less than the threshold value P T , a part of the input light is absorbed, and when the power P in  of input light exceeds the threshold value, input light is output after it has been optically amplified. In the cases of the input/output characteristics as shown in FIGS. 5 and 6, the intensity modulator  13  has an optical amplification feature, and enables the long-haul transmission. Moreover, in the case of the input/output characteristic shown in FIG. 6, a part of input light is absorbed when the power P in  of input light is equal to or less than the threshold value, therefore the excessive optical amplification is restrained and the noise factor improves.  
     [0036] The composition of the intensity modulator  13  contained in the optical transmitter  10  according to the present embodiment will be further explained. In the case of the intensity modulator  13  having no optical amplification feature, the composition preferably includes a saturable absorber. The saturable absorber consists of a substance such as Cr 4+ :YAG crystal, for example, which absorbs input light when the input light power is small, and the absorption percentage decreases when the input light power is greater. When a saturable absorber is used in the intensity modulator  13 , it is possible to achieve downsizing by integrating the laser diode  12  and the intensity modulator  13 .  
     [0037] Also, the composition shown in FIGS. 7A and 7B may preferably be used for the intensity modulator  13  having an optical amplification feature. FIGS. 7A and 7B schematically show an example of the intensity modulator  13 . FIG. 7A shows the composition of the intensity modulator  13  and FIG. 7B shows its input/output characteristic. The intensity modulator  13  shown in FIG. 7A includes an optical coupler  131 , optical amplifier  132  and optical fiber  133 . The optical coupler  131  has a first port P 1 , second port P 2 , third port P 3  and fourth port P 4 . This optical coupler  131  branches light input to the first port P 1  for outputting into the second port P 2  and third port P 3 . It branches light input to the second port P 2  so as to output into the first port P 1  and fourth port P 4 . It also branches light input to the third port P 3  into the first port P 1  and fourth port P 4 .  
     [0038] The optical amplifier  132  and optical fiber  133  are provided on the optical path between the second port P 2  and third port P 3 . The optical amplifier  132  optically amplifies and outputs input light. Erbium-Doped Fiber Amplifier is preferably used for this. It is desirable that the optical fiber  133  have high nonlinearity and that the ratio (n NL /A eff ) of nonlinear refractive index n NL  to effective core area A eff  be equal to or more than 1×10 −9 W −1  in the wavelength of light emitted from the laser diode  12 .  
     [0039] In this intensity modulator  13 , light emitted from the laser diode  12  is input to the first port P 1 , and branched by the optical coupler  131  so as to be output from the second port P 2  and third port P 3 . The light output from the second port P 2  propagates through the optical fiber  133  after it has been amplified optically by the optical amplifier  132 , and enters the third port P 3 . On the other hand, the light output from the third port P 3  is amplified optically by the optical amplifier  132  after it has propagated through the optical fiber  133  and enters the second port P 2 . Apart of light that has entered the second port P 2  and third port P 3  is output from the fourth port P 4  as output light from the intensity modulator  13 .  
     [0040] As compared with light propagating counterclockwise through the optical fiber  133  before passing through the optical amplifier  132 , light propagating clockwise to enter the optical fiber  133  after passing through the second optical amplifier  132  has greater power. Consequently, it tends to suffer from self phase modulation in the optical fiber  133  having high nonlinearity, thereby causing different phase variation. Therefore, the ratio R (=P out /P in ) between the power P out  of light output from the fourth port P 4  and the power P in  of input light fluctuates repeatedly as shown in FIG. 7B with respect to the increase of the power P in  of light that is emitted from the laser diode  12  and enters the first port P 1 . In the range from the power P in  of input light where the ratio R is minimal to the power P in  of input light where the ratio R becomes maximal in FIG. 7B, the greater the power P in  of input light, the greater the gain of optical amplification is, and hence the greater the ratio R becomes.  
     [0041] An embodiment of the optical communication system according to the present invention will be described. FIG. 8 is a schematic diagram showing the structure of an optical communication system  1  according to the present embodiment. The optical communication system  1  shown in FIG. 8 is equipped with an optical transmitter  10 , an optical receiver  20  and an optical fiber transmission line  30 . The optical transmitter  10  is the above-described embodiment of the present invention and has the driver  11 , laser diode  12 , and intensity modulator  13 . The optical fiber transmission line  30  is provided between the optical transmitter  10  and the optical receiver  20 , and transmits signal light emitted from the optical transmitter  10  to the optical receiver  20 . The optical receiver  20  receives signal light which has propagated through the optical fiber transmission line  30 .  
     [0042] In the optical communication system  1 , light emitted from the laser diode  12  driven by the driver  11  by the direct intensity modulation method is modulated based on the input/output characteristics of the intensity modulator  13  so that the extinction ratio increases, and is discharged therefrom as signal light into the optical fiber transmission line  30 . The signal light propagates through the optical fiber transmission line  30  and is received by the optical receiver  20 . In this optical communication system  1 , despite a small extinction ratio of light emitted from the laser diode  12 , the extinction ratio of signal light output from the intensity modulator  13  increases. Also, since a small extinction ratio of light emitted from the laser diode  12  may be acceptable, the degree of the chirp of signal light decreases. Therefore, in the optical communication system  1  which transmits signal light discharged from this optical transmitter  10 , the deterioration of the light transmission quality due to the chromatic dispersion of the optical fiber transmission line  30  can be restrained, as well as the degradation of the reception sensitivity at the optical receiver  20 . This optical communication system  1  is improved in terms of both light transmission quality and reception sensitivity.  
     [0043] The entire disclosure of Japanese Patent Application No. 2002-154144 filed on May 28, 2002 including the specification, claims drawings and summary are incorporated herein by reference in its entirety.