Apparatus and method of measuring polarization mode dispersion, and recording medium

An apparatus is provided which is capable of measuring the polarization mode dispersion of an objective without changing a wavelength of an angle frequency of light incident upon the objective. The apparatus comprises a variable wavelength light source 10 generating an incident light, a light modulator 54 modulating the incident light on the basis of the frequency f of the signal for modulation oscillated from an oscillator 52 and outputting the modulated light, a polarization controller 20 polarizing the modulated light, changing a polarizing condition so that an modulated light polarizing is passed through the axes having a minimum and a maximum propagation group velocity of the light in DUT 30, and outputting the polarized light for incidence, a phase comparator 64 measuring the phase difference &phgr; between a phase &phgr;s of the transmitted light which the polarized light for incidence is transmitted through DUT 30 and a phase &phgr;r of the signal for modulation and a polarization mode dispersion measuring unit 66 calculating the polarization mode dispersion of DUT 30 from the phase difference &phgr;. The apparatus is capable of measuring of the polarization mode dispersion without changing the wavelength of the incident light and if the wavelength of the incident light is changed, it is capable of measuring the wavelength dependent characteristic of the polarization mode dispersion.

DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, the preferred embodiments of the present invention will be described referring to the attached drawings. FIG. 1 is a block diagram showing the configuration of a polarization mode dispersion measuring apparatus according to an embodiment of the invention. The polarization mode dispersion measuring apparatus according to an embodiment of the invention measures the polarization mode dispersion of the DUT (objective; Device Under Test) 30 such as an optical fiber. The polarization mode dispersion measuring apparatus according to an embodiment of the invention is provided with a variable wavelength light source 10 , a polarization controller 20 , an oscillator 52 , a light modulator 54 , a photoelectric converter 62 , a phase comparator 64 and a polarization mode dispersion measuring unit 66 . The variable wavelength light source 10 generates an incident light. A wavelength &lgr; of the incident light can be swept by the variable wavelength light source 10 . The oscillator 52 generates an electric signal for modulation having a predetermined frequency f and supplies it to the light modulator 54 . A phase of the electric signal for modulation is expressed by &phgr;r. The light modulator 54 modulates the variable wavelength light to the frequency f. The light modulator 54 has a Lithium-Naiobate (LN). The incident light is modulated to a modulated light by the light modulator 54 . And, the light modulator does not need to have LN, if it is able to modulate a variable wavelength light. For example, it can be an EA (Electro Absorption) modulator. The modulated light outputted from the light modulator 54 is supplied to the polarization controller 20 . The polarization controller 20 functions as a polarizing means which the modulated light is polarized. The polarization controller 20 changes the polarizing condition of the modulated light. And the polarizing condition stands for a kind of the polarized light (linearly-polarized light, elliptically-polarized light, etc), a direction of the polarized light, etc. The polarization controller 20 has a polarizer 22 , a {fraction (1/4)} wavelength plate 24 , a {fraction (1/2)} wavelength plate 26 . The polarizer 22 linearly-polarizes the modulated light. The {fraction (1/4)} wavelength plate circularly or elliptically polarizes the linearly polarized light outputted from the polarizer 22 . For example, as shown in FIG. 2 ( a ), it is assumed that the linearly polarized light 100 outputted from the polarizer 22 inclines from a main axis x1 of the {fraction (1/4)} wavelength plate 24 by 30 degrees. Then, a x1 component of the linearly polarized light 100 becomes {square root}{square root over ( )}3rsin&thgr; (r is an predetermined integer, &thgr; is a function of time) and a y1 component becomes rsin&thgr; (r is an predetermined integer, &thgr; is a function of time). Furthermore, the phases of x1 component and y1 component of the linearly polarized light 100 are equal. Herein, x1 component (refers to X1) of a ray generated by the x1 component of the linearly polarized light 100 after transmitting the {fraction (1/4)} wavelength plate 24 becomes {square root}{square root over ( )}3rsin&thgr; (r is an predetermined integer, &thgr; is a function of time) and the y1 component (refers to Y1) becomes rcos&thgr; (r is an predetermined integer, &thgr; is a function of time). According to a characteristic of the {fraction (1/4)} wavelength plate 24 , the phases of X1 and Y1 are displaced by &pgr;/2 each other, therefore, if the phase of X1 is sin&thgr;, the phase of Y1 becomes cos&thgr;. Accordingly, as shown in FIG. 2 ( b ), the linearly polarized light becomes an elliptically polarized light by passing through the {fraction (1/4)} wavelength plate 24 . Furthermore, if the linearly polarized light 100 outputted from the polarizer 22 inclines from the main axis x1 of the {fraction (1/4)} wavelength plate 24 by 45 degrees, it becomes a circularly polarized light. The {fraction (1/2)} wavelength plate 26 functions as a rotary polarizer because it modulates the linearly polarized light having an azimuth (angle) of &bgr; into the linearly polarized light having an azimuth (angle) of −&bgr; with respect to the main axis of the {fraction (1/2)} wavelength plate 26 . Accordingly, as shown in FIG. 2 ( c ), by rotating the {fraction (1/2)} wavelength plate 26 , an output of {fraction (1/4)} wavelength plate 24 is rotated. In the DUT 30 , an axis that the propagation group velocity of the light is a minimum is x2, an axis that the propagation group velocity of the light is a maximum is y2. Hereinafter, the axes x2, y2 may be referred as the main axes of DUT 30 . As shown in FIG. 2 ( d ), x2 and y2 are at right angles each other and are displaced from the main axes x1, y1 of the {fraction (1/4)} wavelength plate 24 by the predetermined angle &PSgr;. Accordingly, by half-rotating the {fraction (1/2)} wavelength plate 26 , an oblique of a major (minor) axis of the circularly polarized light can be varied from 0 degree to 360 degree. Accordingly, the major axis or the minor axis of the circularly polarized light passes through the main axes x2, y2 of DUT 30 . Also, if the oblique of the main (minor) axis of the circularly polarized light is varied from 0 degree to 360 degree, the polarized lights are generated in all polarizing planes, this is called a random polarized light in this specification. However, without limit to the random polarized lights, it is enough if the polarized light is generated so that the main axes x2, y2 of DUT 30 are passed. Furthermore, the light passing through the {fraction (1/2)} wavelength plate 26 is the polarized light for incidence outputted from the polarization controller 20 . The polarized light for incidence is supplied to the DUT 30 . The polarized light for incidence transmits the DUT 30 . The light transmitting through the DUT 30 is called a transmitted light. The photoelectric converter 62 photoelectric converts the transmitted light and outputs it. In the case of photoelectric converting the transmitted light, for example, the photoelectric converter 62 takes out a part of major axis of the elliptically polarized light and photoelectric converts it. The phase comparator 64 measures a phase difference &phgr; between the phase &phgr;s of an photoelectric converted signal of the transmitted light and the phase &phgr;r of an electric signal for modulation. Namely, &phgr;&equals;&phgr;s−&phgr;r. The polarization mode dispersion measuring unit 66 calculates a maximum value (&phgr;max ) of &phgr; and a minimum value (&phgr;min) of &phgr; from the output of the phase comparator 64 . The &phgr;max and &phgr;min correspond to the main axes x2, y2 of DUT 30 , respectively. The group delay time difference of the light between the main axes x2, y2 of DUT 30 becomes to the polarization mode dispersion. Accordingly, the polarization mode dispersion measuring unit 66 calculates the polarization mode dispersion from &phgr;max, &phgr;min and frequency f of the electric signal for modulation. For example, the polarization mode dispersion is defined as a value which the difference between the maximum value of &phgr; and the minimum value of &phgr; is divided by 2&pgr;f, namely, (&phgr;max−&phgr;min)/2&pgr;f. And the polarization mode dispersion measuring unit 66 records the polarization mode dispersion corresponding to a wavelength &lgr; of the variable wavelength light source 10 , as shown in FIG. 3 . That is, it records the polarization mode dispersion t0 at wavelength &lgr;0, the polarization mode dispersion t1 at wavelength &lgr;1, . . . ,the polarization mode dispersion tn at wavelength &lgr;n. In the embodiment of the invention, even though the wavelength of the incident light, which the variable wavelength light source 10 generates, is fixed, the polarization mode dispersion can be obtained. However, by recording the polarization mode dispersion as the polarization mode dispersion t0 at the wavelength &lgr;0, the polarization mode dispersion t1 at the wavelength &lgr;1, . . . , the polarization mode dispersion tn at the wavelength &lgr;n, the wavelength dependent characteristic of the polarization mode dispersion can be measured. Next, the operation according to an embodiment of the invention will be described using a flow chart of FIG. 4 . First, the wavelength &lgr; of the incident light generated from the variable wavelength light source 10 is defined as the lower limit (S 10 ). And, if the wavelength &lgr; of the incident light is not reached to the upper limit (S 12 , No), the {fraction (1/2)} wavelength plate 26 is located at the predetermined initial angle (S 14 ). And, if the rotation of {fraction (1/2)} wavelength plate 26 is not finished (S 16 , No), the {fraction (1/2)} wavelength plate 26 is rotated (S 18 ). At this time, the polarizer 22 and {fraction (1/4)} wavelength plate 24 are fixed at the predetermined angle. And, it measures the phase difference &phgr; between the phase &phgr;s of transmitted light after photoelectric converting and the phase &phgr;r of the electric signal for modulation (S 20 ) and records the phase difference &phgr; in the polarization mode dispersion measuring unit 66 (S 22 ). Herein, if the rotation of the {fraction (1/2)} wavelength plate 26 is finished (S 16 , Yes), the polarization mode dispersion measuring unit 66 measures the polarization mode dispersion from the maximum value &phgr;max and the minimum value &phgr;min of the phase difference &phgr; and the frequency f of the electric signal for modulation (S 24 ). And, the polarization mode dispersion measuring unit 66 records the polarization mode dispersion tn corresponding to the wavelength &lgr;n of the incident light generated from the variable wavelength light source 10 (S 26 ). Herein, even though the wavelength &lgr;n of the incident light generated from the variable wavelength light source 10 is fixed, the polarization mode dispersion tn can be measured. However, in order to measure the wavelength dependent characteristic of the polarization mode dispersion, the variable wavelength light source 10 increase the wavelength &lgr;n of the incident light (S 28 ), and then it return to determination (S 12 ) as to whether the wavelength (&lgr;) of the incident light is reached to the highest limit. If the wavelength &lgr; of the incident light is reached to the highest limit (S 12 , Yes), the operation is finished. According to the embodiment of the invention, the phase of the transmitted light in which the polarized light for incidence transmits the DUT 30 is effected by the polarization mode dispersion. Accordingly, the phase &phgr;s of the transmitted light generates the phase &phgr;r of the signal for modulation and the phase difference &phgr; as much as it is affected by the polarization mode dispersion. Also, the polarized light for incidence is made to pass through the axis x2 where the propagation group velocity of the light is minimum and the axis y2 where the propagation group velocity of the light is maximum in the DUT 30 . Accordingly, the phase comparator 64 measures the phase difference &phgr; between the phase &phgr;s of the transmitted light and the phase &phgr;r of the signal for modulation. The polarization mode dispersion measure unit 66 calculates the group delay time difference of the light in the axis x2 having the minimum propagation group velocity and the axis y2 having the maximum propagation group velocity of the light. Thereby, the polarization mode dispersion of DUT 30 can be measured without changing the wavelength of the incident light. Furthermore, if the wavelength of the incident light is varied, the wavelength dependent characteristic of the polarization mode dispersion can be measured. And, because the phase difference &phgr; is a function of the polarization mode dispersion, even though the polarized light for incidence is not passed through the main axes x2, y2, it is theoretically possible to calculate the polarization mode dispersion from the phase difference &phgr;. Also, the above embodiment according to the invention is executed as follow. In a media reading device of computer comprising a CPU, a hard disk, media (flopy 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. &lsqb;Effect of the Invention&rsqb; According to the invention, the phase of the transmitted light in which the polarized light for incidence is transmitted through the objective is effected by the polarization mode dispersion. Accordingly, the difference between the phase of the transmitted light and the phase of the signal for modulation is generated as much as it is affected by the polarization mode dispersion. Accordingly, the phase difference measuring means measures the phase difference between the phase of the transmitted light and the phase of the signal for modulation, thereby it is possible to measure the polarization mode dispersion of the objective.