Abstract:
An apparatus capable of reducing a waveform distortion of outgoing light when light of an optical wavelength in a certain specific narrow range is incident upon optical fiber comprises optical source  10  for supplying the incident light to optical fiber line  110 , waveform monitor  42  for measuring a waveform distortion of the transmitted light and adjusting unit  44  for adjusting an output of the incident light so that the measured waveform distortion falls within a predetermined range. By adjusting the output of the incident light, a S/N ration is lowered. Since the noise exists within a relatively wide range of wavelength, the rang of wavelength of the incident light is widened. Therefore, it is possible to reduce the waveform distortion of the outgoing light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an apparatus for measuring a wavelength dispersion characteristic of DUT (Device Under Test) such as a combination of an optical fiber and an optical amplifier. 
     2. Description of the Related Art 
     It may cause a great loss of light when light is transferred to a long distance only through an optical fiber. Therefore, the loss is prevented by using an optical fiber line combined with an optical amplifier (EDFA) which amplifies an optical signal therein. The optical amplifier passes light to a unilateral direction. The optical fiber line means a combination of an optical fiber and an optical amplifier. 
     FIG. 7 shows a construction of a measuring system for measuring a wavelength dispersion characteristic of an optical fiber line. An optical fiber line  110  is formed by a combination of an optical fiber  112  and an optical amplifier  114 . The optical fiber line  110  passes light into a right direction. A variable wavelength light source  12  generates light by changing a wavelength. The light is modulated to a frequency of a power supply for modulation  14  by an optical modulator  15 , and then is incident upon the optical fiber line  110 . The light transmitting the optical fiber line  110  is converted into an electric signal by photoelectric converter  22 , and a phase thereof is compared with that of an electric signal generated from the power supply for modulation  14  by a phase comparator  24 . That is, a phase difference is calculated. It is possible to obtain a group delay or a wavelength dispersion of the optical fiber line  110  from the phase difference. 
     In addition, generally, light with the condition of WDM (wavelength division multiplying) is inputted into the optical fiber line  110 . On the premise of this, the optical fiber line  110  is designed to maintain a quality of transferred waveform of the optical fiber line  110  by incidence of light, such as 16 waves and 40 waves and the like, which has a wavelength magnification in accordance with a design upon the optical fiber line  110 . Therefore, each optical amplifier  114  sets an automatic gain feedback to maintain a fixed level of an outgoing light. 
     SUMMARY OF INVENTION 
     At this point, as shown in FIG. 8, there is a case that an optical wavelength in a certain specific narrow range is incident upon the optical fiber line  110 . FIG.  8 ( a ) shows a relationship between wavelength λ and power P, and FIG.  8 ( b ) shows a relationship between time t and the power P. As shown in FIG.  8 ( a ), the power is increased at a certain wavelength λ 0 . In this case, as shown in FIG.  8 ( b ), there is no distortion in a waveform. 
     FIG. 9 shows an outgoing light in case that the light of an optical wavelength in a certain specific narrow range is incident upon the optical fiber line  110 . FIG.  9 ( a ) shows a relationship between wavelength λ and power P, and FIG.  9 ( b ) shows a relationship between time t and power P. As shown in FIG.  9 ( b ), a distortion occurs in an output waveform. That means a time jitter of signal. This is derived from the following reasons. 
     That is, when the optical wavelength in certain specified narrow range is incident upon the optical fiber line  110 , an amplifier gain of the optical fiber line  110  is changed over a range estimated at the time of designing. Accordingly, the light of a certain wavelength λ 0  is propagated with a strong power that is over the level estimated at the time of designing. When the powerful light is propagated the optical fiber line  110 , the optical fiber line  110  is put into a non-linear region. Due to the non-linearity of the optical fiber line  110 , an index of refraction of the optical fiber line  110  is varied with the lapse of time according to the optical power, the distortion of the output waveform is caused. 
     As the output waveform is distorted, an error is caused when detecting a phase difference from the phase comparator  24 . Accordingly, the group delay or the wavelength dispersion of the optical fiber line  110  which are obtained from the phase difference may generate the error. 
     Moreover, it may be possible to minimize the distortion of the output waveform by adjusting a gain characteristic of the optical fiber line  110 , however, it is difficult to adjust the gain characteristic. 
     Therefore, it is an object of the present invention to provide an apparatus for reducing a waveform distortion of an outgoing light in case that light of an optical wavelength in a certain specific narrow range is incident upon DUT. 
     In an optical characteristic measuring apparatus so constructed when incident light of a wavelength in a narrow range is supplied to the light transmitting objective, a distortion in the waveform of a transmitted light is caused. However, by adjusting the output of the incident light, a SIN ratio (signal to noise ratio) is lowered. The noise is of a relatively wide range of wavelength. Accordingly, if the S/N ratio is lowered properly, the incident light of a wavelength in the relatively wide range can be supplied to the objective. Therefore, it is possible to reduce a waveform distortion of the outgoing light. 
     In addition, there is an optical fiber, or a combination of an optical fiber and an optical amplifier for use as the light transmitting objective. 
     In accordance with the present invention, the incident light supplying unit includes a variable wavelength light source for generating light of a variable wavelength, and the optical output adjusting unit adjusts an output of the variable wavelength light source. 
     In accordance with an aspect of the present invention, the incident light supplying unit includes an optical modulating unit for modulating a light, and the optical output adjusting unit adjusts an amplitude of an output of the optical modulating unit. 
     In accordance with an aspect of the present invention, the incident light supplying unit includes an optical attenuating unit for attenuating the light, and the optical output adjusting unit adjusts an attenuating ratio of the optical attenuating unit. 
     According to the present invention an optical characteristic measuring apparatus for measuring a characteristic of a light transmitting objective includes: an incident light supplying unit for supplying incident light to the objective a waveform distortion measuring unit for measuring a waveform distortion of the light transmitted from the objective; a multi-wavelength light adding unit for adding multi-wavelength light of a plurality of wavelengths to the incident light; and a multi-wavelength light adjusting unit for adjusting an output of the multi-wavelength light so that the waveform distortion measured by the waveform distortion measuring unit falls within a predetermined range. 
     Further, in an optical characteristic measuring apparatus so constructed, when incident light of a wavelength in a narrow range is supplied to the objective, a distortion in a waveform of the transmitted light is caused. However, by adjusting the output of the multi-wavelength light of the plurality of wavelengths, being added to the incident light, the incident light of a wavelength in a wide range can be supplied to the objective. Therefore, it is possible to reduce the waveform distortion of the outgoing light. 
     In accordance with an aspect of the present invention, the multi-wavelength light is noise light. 
     In accordance with an aspect of the present invention, the multi-wavelength light adding unit is an operational amplifier, an input of which is not given, and the multi-wavelength light adjusting unit is a light attenuating unit for attenuating the output of the operational amplifier and changing an attenuating ratio in accordance with the waveform distortion. 
     In accordance with an aspect of the present invention, the waveform distortion is kept at minimum. 
     In accordance with an aspect of the present invention, the incident light supplying unit includes: a variable wavelength light source for generating light of a variable wavelength; a power supply of a modulating frequency use to modulate the variable wavelength light; and an optical modulating unit for modulating the variable wavelength light into the modulating frequency. The apparatus further includes: a photoelectric converting unit for photoelectrically converting the transmitted light a phase comparing unit for measuring a phase difference between an output of the photoelectric converting unit and an output of the power supply and a characteristic calculating unit for obtaining a group delay or a wavelength dispersion of the objective from the phase difference. 
     According to the present invention an optical characteristic measuring method for measuring a characteristic of a light transmitting objective includes: supplying an incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; and adjusting an output of the incident light so that the waveform distortion measured in the waveform distortion measuring step falls within a predetermined range. 
     According to the present invention an optical characteristic measuring method for measuring a characteristic of a light transmitting objective includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; adding multi-wavelength light having combined therein a plurality of wavelengths to the incident light; and adjusting an output of the multi-wavelength light so that the waveform distortion measured in the waveform distortion measuring step falls within a predetermined range. 
     According to the present invention a computer-readable medium having a program of instructions for execution by a computer to perform an optical characteristic measuring process for measuring a characteristic of a light transmitting objective is provided. The optical characteristic measuring process includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; and adjusting an output of the incident light so that the waveform distortion measured in the waveform distortion measuring processing falls within a predetermined range. 
     According to the present invention, a computer-readable medium having a program of instructions for execution by a computer to perform an optical characteristic measuring process far measuring a characteristic of a light transmitting objective is provided. The optical characteristic measuring process includes: supplying incident light to the objective; measuring a waveform distortion of the light transmitted from the objective; adding multi-wavelength light having combined therein a plurality of wavelengths to the incident light; and adjusting an output of the multi-wavelength light so that the waveform distortion measured in the waveform distortion measuring processing falls within a predetermined range. 
     Here, FIG. 2 ( d ) shows a relationship between the optical output and the waveform distortion. When the output power of the incident light  3  is set on P 0 , the waveform distortion becomes a minimum value Smin. The waveform distortion becomes large when the output power of the incident light  3  is either over or under the P 0 . Accordingly, if the optical output adjusting unit  44  adjusts the ouput of the incident light generated from the optical source system  10  and sets to P 0 , the waveform distortion can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a construction of an optical characteristic measuring apparatus according to the first embodiment of the invention. 
     FIG. 2 shows principles for suppressing a waveform distortion of a transmitted light within a predetermined range. 
     FIG. 3 is a flow chart showing an operation of the first embodiment of the invention. 
     FIG. 4 is a block diagram showing a construction of an optical characteristic measuring apparatus according to the second embodiment of the invention. 
     FIG. 5 shows principles for suppressing a waveform distortion of a transmitted light within a predetermined range. 
     FIG. 6 is a flow chart showing an operation of the second embodiment of the invention. 
     FIG. 7 is a block diagram showing a construction of a conventional measuring system for measuring a wavelength dispersion characteristic of an optical fiber line. 
     FIG. 8 is a waveform diagram of an incident light showing a case that an optical wavelength in a certain specific narrow range is incident upon an optical fiber line  110 . 
     FIG. 9 is a waveform diagram of an outgoing light showing a case that an optical wavelength in a certain specific narrow range is incident upon an optical fiber line  110 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described referring to the attached drawings. 
     &lt;The First Embodiment&gt; 
     FIG. 1 is a block diagram showing a construction of an optical characteristic measuring apparatus according to the first embodiment of the present invention. The optical characteristic measuring apparatus of the first embodiment has an optical source system  10  connected to an end of an optical fiber line  110 , a characteristic measuring system  20  connected to the other end of the optical fiber line  110 , and an optical output adjusting system  40  for adjusting an output of the optical source system  10  in accordance with a waveform distortion of a transmitted light. 
     The optical fiber line  110  has an optical fiber  112 , and an optical amplifier  114  connected on the way of the optical fiber  112  and amplifying the light. The optical fiber line  110  passes light in the right direction. 
     The optical source system  10  comprises a variable wavelength light source  12 , a power supply for modulation  14 , an optical modulator  15 , and an optical attenuator  16 . The variable wavelength light source  12  generates a variable wavelength light which a waveform is varied. A wavelength λx of the variable wavelength light can be swept by the variable wavelength light source  12 . The optical modulator  15  modulates the variable wavelength light to a frequency f. The optical modulator  15  has Lithium-Niopate (LN). However, the optical modulator does not need to have LN if it can modulate the light. The optical attenuator  16  attenuates the variable wavelength light and then supplies it to the optical fiber line  110 . 
     An incident light supplied to the optical fiber line  110  transmits the optical fiber line  110 . The light transmitting the optical fiber line  110  is referred to as a transmitted light. 
     The characteristic measuring system  20  includes a photoelectric converter  22 , a phase comparator  24 , and a characteristic calculating unit  28 . 
     The photoelectric converter  22  converts the transmitted light into an electric signal. The phase comparator  24  measures a phase difference between an output of the photoelectric converter  22  and an output of the power supply for modulation  14 . The characteristic calculating unit  28  calculates a group delay characteristic or a wavelength dispersion characteristic of the optical fiber line  110 , on the basis of the phase measured from the phase comparator  24 . The group delay characteristic can be calculated from a relationship between a phase measured from the phase comparator  24  and a modulated frequency f. The wavelength dispersion characteristic can be obtained by differentiating the group delay characteristic with the wavelength. 
     The optical output adjusting system  40  has a waveform monitor  42  and an optical output adjusting unit  44 . The waveform monitor  42  measures a relationship between an output of the transmitted light and the waveform distortion of the transmitted light from the output of the photoelectric converter  22 . The optical output adjusting unit  44  adjusts an output of the incident light generated from the optical source system  10 , by controlling the optical source system  10 . More particularly, it adjusts at least one factor in the variable wavelength light source  12 , the optical modulator  15 , or the optical attenuator  16 . 
     Namely, the optical output adjusting unit  44  adjusts the output power of the variable wavelength light source  12 . Or, it adjusts an amplitude of an output of the optical modulator  22 . Or, it adjusts an attenuating ratio in the optical attenuator  16 . By means of such adjustment, the optical output adjusting unit  44  enables the waveform distortion measured by the waveform monitor  42  to be placed within a predetermined range. It is preferable for the optical output adjusting unit  44  to minimize the waveform distortion (jitter) measured by the waveform monitor  42 . 
     The optical output adjusting unit  44  adjusts the output of the incident light generated from the optical source system  10  and enables for the waveform distortion (jitter) of the transmitted light to be placed within a predetermined range. The principle will be described referring to FIG.  2 . In addition, the waveform distortion (jitter) means a time jitter of a signal. 
     FIG.  2 ( a ) shows the waveform of the incident light. The incident light  30  has wavelength λ 0  which a range of waveform is narrower and power is greater than that of noise  32 . If it is incident upon the optical fiber line  110  as it is, the waveform of the transmitted light is distorted. Thus, as shown in FIG.  2 ( b ), it lowers an output power of the incident light  30  and a difference between the output power of the incident light  30  and an output power of the noise  32  is reduced. That is, it lowers a S/N ratio. Then, an imaginary incident light  34  is deemed to be incident upon the optical fiber line  110 . Since the imaginary incident light  34  has a wide range of wavelength, the waveform distortion of the transmitted light is minimized. At this time, the output power of the incident light  30  is set on P 0 . That is, as shown in FIG.  2 ( c ), when the output power of the incident light  30  is smaller than the P 0 , it is dominated by the noise  32  and thereby the waveform distortion of the transmitted light becomes large. 
     Here, FIG.  2 ( d ) shows a relationship between the optical output and the waveform distortion. When the output power of the incident light  30  is set on P 0 , the waveform distortion becomes a minimum value Smin. The waveform distortion becomes large when the output power of the incident light  30  is either over or under the P 0 . Accordingly, if the optical output adjusting unit  34  adjusts the output of the incident light generated from the optical source system  10  and sets to P 0 , the waveform distortion can be reduced. 
     Next, an operation of the first embodiment of the present invention will be explained referring to a flow chart in FIG.  3 . First, the variable wavelength light source  12  generates light by changing a wavelength. The light is modulated into the frequency f of the power supply for modulation  14  through the optical modulator  15 , and then is incident upon the optical fiber line  110 . The light transmitting the optical fiber line  110  is converted into the electric signal by the photoelectric converter  22 , and then a phase thereof is compared with that of the electric signal generated from the power supply for modulation  14  by the phase comparator  24 . That is, a phase difference is calculated. The group delay or the wavelength dispersion of the optical fiber line  110  can be obtained by the characteristic calculating unit  28 , using the phase difference value. 
     Here, the waveform monitor  42  measures a relationship between the output of the transmitted light and the waveform distortion of the transmitted light with reference to the output of the photoelectric converter  22 . The optical output adjusting unit  44  measures whether the waveform distortion is the minimum or not (S 12 ). It is practical to determine whether or not the waveform distortion is the minimum by recording the waveform distortion corresponding to an output power of the incident light. Or, it is practical to determine whether a value differentiated by the output power of the incident light is 0 or not. If the waveform distortion is not the minimum (S 12 , No), the output of the incident light is adjusted by the optical power adjusting unit (S 14 ). And then, returns back to the determination (S 12 ) for determining whether or not the waveform distortion is the minimum. By the contrary, if the waveform distortion reaches the minimum (S 12 , Yes), the optical output adjusting system  40  terminates the adjustment of the output of the incident light. 
     According to the first embodiment, even when the wavelength range of the incident light is narrow, by lowering the S/N ratio properly, the light having a wide wavelength range can be incident upon imaginarily and thereby a wavelength distortion of the transmitted light can be reduced. 
     &lt;The Second Embodiment&gt; 
     An optical characteristic measuring apparatus according to the second embodiment is similar to that of the first embodiment, except that a multi-wavelength light adding system  50  which adds a multi-wavelength light having combined a plurality of wavelengths is provided for an incident light, instead of the optical output adjusting system  40  in the first embodiment. 
     FIG. 4 is a block diagram showing an outline of a construction of an optical characteristic measuring apparatus according to the second embodiment of the invention. The optical characteristic measuring apparatus of the second embodiment has an optical source system  10  connected to an end of an optical fiber line  110 , a characteristic measuring system  20  connected to the other end of the optical fiber line  110 , and a multi-wavelength light adding system  50  for adding the multi-wavelength light on the output from the optical source system  10  on the basis of the waveform distortion of the transmitted light. Hereinafter, a part identical to the first embodiment, the same numerical reference numeral will be given and the explanation thereof will be omitted. 
     The optical source system  10  comprises a variable wavelength light source  12 , a power supply for modulation  14 , an optical modulator  15 , an optical attenuator  16  and a frequency mixer  19 . The frequency mixer  19  adds an output from the multi-wavelength light adding system  50  to light outputted from the optical attenuator  16 . An output from the frequency mixer  19  is supplied to the optical fiber line  110  as an incident light. 
     The incident light supplied to the optical fiber line  110  transmits the optical fiber line  110 . The light transmitting the optical fiber line  110  is referred to as a transmitted light. 
     The characteristic measuring system  20  includes a photoelectric converter  22 , a phase comparator  24 , and a characteristic calculating unit  28 . The construction of the characteristic measuring system  20  is identical to that of the first embodiment. 
     The multi-wavelength light adding system  50  has a waveform monitor  52  and a multi-wavelength light adjusting unit  54 . The waveform monitor  52  measures a relationship between an output of the transmitted light and a waveform distortion of the transmitted light from an output of the photoelectric converter  22 . The multi-wavelength light adjusting unit  54  generates a multi-wavelength light, adjusts an output power of the multi-wavelength light on the basis of the measuring result from the waveform monitor  52 , and then supplies to the frequency mixer  19 . In addition, the multi-wavelength light means a light having combined plurality of wavelengths. For example, the multi-wavelength light is a noise such as ASE (spontaneous emission light). 
     The multi-wavelength light adjusting unit  54  has an operational amplifier  54   a  and an optical attenuator  54   b . An input is not given to the operational amplifier  54   a . The light outputted from the operational amplifier  54   a  is a noise such as ASE (spontaneous emission light). The optical attenuator  54   b  attenuates the light outputted from the operational amplifier  54   a  in order to enter the waveform distortion of the transmitted light transmitting the optical fiber line  110  to a predetermined range. It is preferable for the optical attenuator  54   b  to set the waveform distortion at the minimum which the waveform monitor  52  has been measured. 
     The optical attenuator  54   b  attenuates the light outputted from the operational amplifier  54   a , thus it enables the waveform distortion (jitter) of the transmitted light to be placed within a predetermined range. The principle will be described referring to FIG.  5 . In addition, the waveform distortion means a time jitter of a signal. 
     FIG.  5 ( a ) shows the waveform of the incident light. The incident light  30  has wavelength λ 0  which a range of waveform is narrower and power is greater than that of a noise  32 . If it is incident upon the optical fiber line  110 , the waveform of the transmitted light is distorted. Thus, as shown in FIG.  5 ( b ), a supplemental noise  33  produced by attenuating the light outputted from the operational amplifier  54   a  properly by the optical attenuator  54   b  is added to the incident light and lowers the S/N ratio. Then, as shown in FIG.  5 ( c ), the supplemental noise  33  is added to the noise  32  and this noise is referred to as an imaginary noise  36 . Accordingly, an imaginary incident light  34  is deemed to be incident upon the optical fiber line  110 . Since the imaginary incident light  34  has a wide range of wavelength, the waveform distortion of the transmitted light is minimized. At this time, the output power of the supplemental noise is set on N 0 . In addition, when the supplemental noise is too large, over N 0 , the incident light  30  is dominated by the imaginary noise  36  and thereby the waveform distortion of the transmitted light becomes large. 
     Here, FIG.  5 ( d ) shows a relationship between the optical output and the waveform distortion. When the output power of the supplemental noise  33  is set on N 0 , the waveform distortion becomes a minimum value Smin. The waveform distortion becomes large when the output power of the supplemental noise  33  is either over or under the N 0 . Accordingly, if the optical attenuator  54   b  attenuates the light outputted from the operational amplifier  54   a  and sets the output power to N 0 , the waveform distortion can be reduced. 
     Next, an operation of the second embodiment of the present invention will be explained referring to a flow chart in FIG.  6 . First, the variable wavelength light source  12  generates light by changing a wavelength. The light is modulated into the frequency f of the power supply for modulation  14  through the optical modulator  15 , and then is incident upon the optical fiber line  110 . The light transmitting the optical fiber line  110  is converted into an electric signal by the photoelectric converter  22 , and then a phase thereof is compared with that of the electric signal generated from the power supply for modulation  14  by the phase comparator  24 . That is, a phase difference is calculated. The group delay or the wavelength dispersion of the optical fiber line  110  can be obtained by the characteristic calculating unit  28 , using the phase difference value. 
     Here, the waveform monitor  52  measures a relationship between the output of the transmitted light and the waveform distortion of the transmitted light with reference to the output of the photoelectric converter  22 . The multi-wavelength light adjusting unit  54  measures whether the waveform distortion is the minimum or not (S 12 ). It is practical to determine whether or not the waveform distortion is the minimum by recording the waveform distortion corresponding to an output power of the supplemental noise. or it is practical to determine whether a value differentiated by the output power of the supplemental noise is 0 or not. If waveform distortion is not the minimum (S 12 , No), the output of the supplemental noise is adjusted by the multi-wavelength light adjusting unit  54  (S 14 ). And then, the process returns back to the determination (S 12 ) for determining whether or not the waveform distortion is the minimum. By the contrary, if the waveform distortion reaches the minimum (S 12 , Yes), the multi-wavelength light adjusting unit  54  terminates the adjustment of the output of the supplemental noise. 
     According to the second embodiment, even when the wavelength range of the incident light is narrow, by adding the supplemental noise and lowering the S/N ratio, the light having wide wavelength range can be incident upon imaginarily and thereby the wavelength distortion of the transmitted light can be reduced. 
     Also, the above embodiments according to the invention are executed as follow. In a media reading device of computer comprising a CPU, a hard disk, media (floppy disk, CD-ROM, etc) reading device, it is read the media recording a program embodying each component of the above-mentioned, and is installed to the hard disk. By the above method, the above function can be executed. 
     According to the present invention, by adjusting the output of the incident light or adding the multi-wavelength light such as noise and like to the incident light, a ratio of incident light and noise, that is, a S/N ratio (signal to noise ratio) is lowered. The noise is existed within a range of wide wavelength relatively. Accordingly, if the S/N ratio is lowered properly, the incident light of which the wavelength is in the wide range can be supplied to the objective. Therefore, it is possible to reduce the waveform distortion of an outgoing light.