JP2005274375A - Polarization dispersion measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of measuring inexpensively and easily polarization dispersion in a single mode optical fiber and various kinds of optical devices imparting an important influence on a transmission characteristic of a light speed transmission system. <P>SOLUTION: In this method of measuring the polarization dispersion concerned in the present invention, a means for measuring the polarization dispersion, using an RF power of a modulated component as a function of a relative polarization direction of a measured optical medium to an intermediate polarization element, when a circular polarization light or rotating linear polarization wave is made to get incident into a measured object using a semiconductor laser and a sine wave amplification modulator in a light source part, and when an output thereof is detected by an RF power meter via the intermediate polarization element with a double refraction set in a prescribed value. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polarization dispersion measuring method. More specifically, the present invention relates to a polarization mode dispersion measuring method capable of easily measuring the polarization mode dispersion of a single mode optical fiber and various WDM optical devices that have an important influence on the transmission characteristics of a high-speed optical communication system.

  Currently, 10Gb / s is the mainstream transmission speed for trunk-line optical communication transmission systems. However, in order to cope with the future traffic increase due to the rapid spread of the Internet, the practical use of high-speed transmission systems of 40Gb / s or higher is also under consideration. Has been. In such a high-speed optical transmission system, the polarization dispersion generated by a slight asymmetry of the core of a single mode optical fiber (hereinafter abbreviated as SMF), which is a transmission path, as the transmission distance increases causes the bit error rate to deteriorate. That is a problem. This polarization dispersion is also caused by optical components used in the transmission line. In particular, in the case of the former SMF, polarization dispersion varies randomly due to distortion of the optical fiber due to a change in the temperature of the transmission line, and so compensation has become a major issue. Therefore, when constructing a transmission system, it is necessary to measure beforehand the polarization dispersion of the optical cable and to know in advance whether the compensation is necessary. Also, with the development of optical fiber amplifier and WDM optical transmission technology, since many optical components are used in the transmission line, it is stipulated that the polarization dispersion of individual optical components is also measured in advance at the factory shipment stage.

  As described in detail in Non-Patent Document 1, there are two types of polarization dispersion measurement methods: frequency domain and time domain. In the case of SMF, interferometry, which is a time domain measurement method, is mainly used. On the other hand, ellipsometry and fixed analyzer methods, which are frequency domain measurement methods, are used for optical components. In April 2000 ITU-TSG15, the Jones matrix method and the Poincare sphere method, which are one type of ellipsometry, were certified as reference measurements. On the other hand, as an alternative reference measurement method, a polarization state method, which is a kind of ellipsometry, and an interference method, which is a time domain measurement method, are defined.

  In the interferometry, a broadband light source is usually used, and only average polarization dispersion of the spectrum of the light source can be measured. On the other hand, in the ellipsometry, on the incident side, a tunable laser and a polarization control device that generates four types of polarization states are required. On the other hand, an ellipsometer (polarimeter) is necessary on the light receiving side, and the price of the measurement system is usually as high as 20 million yen or more. The fixed analyzer method, which is an alternative reference measurement method, is generally expensive because it requires a device for controlling the wavelength and polarization state of the light source on the incident side and an optical spectrum analyzer at the light receiving unit. There are other methods for measuring polarization dispersion, such as the pulse method, where the light source is driven with current pulses and the difference in the arrival time of the pulses between the polarization modes is observed at the light receiving unit. There is a modulation wave phase measurement method that measures the phase difference of the modulated signal by amplitude modulation and polarization mode, but a network analyzer or a selection level meter is required to measure the phase difference of amplitude modulation wave in orthogonal polarization mode, and the incident side In addition, since an apparatus for controlling the polarization is required, there is a problem that it is expensive.

Published by Namihira, "DWDM optical measurement technology", Chapter 8, Optronics, Inc., March 10, 2001.

Henrik Sunnard et al., “Outcome Properties in PMD Compensated Transmission Systems”, Proc. 27th ECOC (ECOC'01), Tu. A. 3.1, Sept. 30-Oct. 4, 2001, Amsterdam. (This is a report by Chalmers University of the previous ECOC-01-Tu.A.3.1 Sweden.) K. Ogaki, M .; Nakada, Y .; Nagano, “Fluctuation Differences in the Principal States of Polarization in Aial and Burred Cables”, KDDI, OFC 2003, MF13, March. 23-28, 2003, Atlanta, USA.

  This patent provides an inexpensive method for measuring polarization dispersion of an optical transmission medium such as an optical cable for optical communication or an optical component in order to solve the above problems.

In order to achieve the above object, the polarization dispersion measuring method according to the present invention uses a semiconductor laser and a sine wave amplitude modulation device as a light source unit to make a circularly polarized wave or a rotating linearly polarized wave incident on the object to be measured and to output the output. When detecting with an RF power meter via an intermediate polarization element whose refraction is set to a predetermined value, the polarization dispersion is measured using the RF power of the modulation component as a function of the relative polarization direction of the measured optical medium and the intermediate polarization element. Adopted means.

  As described above, according to the polarization dispersion measuring method of the present invention, it is not necessary to use a complicated polarization generator for the light source unit, and it is not necessary to use a network analyzer or an optical spectrum analyzer for the light receiving unit. It is possible to provide a method that can easily measure the polarization dispersion of the light at low cost.

  An embodiment of the polarization dispersion measuring method of the present invention will be described with reference to FIG. The light emitted from the DFB laser 1 is optically modulated in the amplitude modulator 2 by the sine wave signal output from the sine wave signal generator 3. The wavelength of the light source is 1550 nm. The modulation frequency was 10 GHz. As the amplitude modulator 2, an external modulator having a Mach-Zehnder interference system formed on the electro-optic crystal LiNbO3 was used. The output of the modulator is collimated by a lens and is incident on the optical medium to be measured, in this case the SM optical fiber 5, via the polarization controller 4. In this case, the polarization controller of No. 4 used a circular polarization optical system composed of a polarizer and a quarter wave plate. The fact that the light source output is circularly polarized means that, in principle, the two intrinsic polarization modes of the measured medium are excited with equal amplitude. The output of the medium to be measured 5 is incident on a polarization-maintaining optical fiber (PMF) 7 via a polarization controller 6, led to an RF (high frequency) power meter via a light receiver 8, and then led to a signal processing circuit 10. The RF power meter used was a commercially available high-frequency power meter with a filter through which components near the modulation frequency pass.

  The polarization control device 6 is a type in which the direction of the half-wave plate is electrically rotated, and the polarization plane of the outgoing light of the optical cable is rotated, and the intrinsic polarization orientation of the polarization-maintaining optical fiber 7 and the intrinsic polarization of the optical cable to be measured The relative orientation (hereinafter referred to as α) with the orientation of the mode (PSP) is changed. The same effect can be obtained by omitting the polarization controller 6 and rotating the polarization-maintaining optical fiber 7. The medium 5 to be measured is wound around a bobbin with an SM fiber cable and has a length of about 50 km. As the birefringent medium, rutile or YVO4 (Yttrium Vanadate) can be used in addition to the polarization-maintaining optical fiber.

  If the modulation frequency f is 10 GHz, the period of the modulation wave in the medium having the refractive index n is expressed by the following equation.

  When f = 10 GHz and n = 1.5, Λ is about 2 cm. Here, the length d of the polarization-maintaining optical fiber 7 is set so as to be defined by the modulation frequency and the following relational expression.

  Here, since L is 3 mm at a wavelength of 1550 nm, d = 10 m. Equation 2 means that when the amplitude-modulated light passes through the wave polarization preserving optical fiber, the intrinsic polarization mode is given a phase difference of 90 degrees or a polarization dispersion of 25 ps, which is a quarter of a modulation period of 100 ps. To do.

  Here, the average value of the polarization dispersion of the optical cable to be measured is τ (ps). Therefore, the amplitude-modulated wave transmitted through the optical cable has a phase difference φ expressed by the following equation between the intrinsic polarization modes PSP.

  Since the amplitude-modulated wave is now incident as circularly polarized light in the light source unit, the PSP of the optical cable is incident with the same amplitude and an optical phase difference of 90 degrees as described above. When considering the phase of the baseband amplitude-modulated wave, the optical carrier phase difference of 90 degrees may be ignored. In such a case, the baseband modulation signal amplitude P at the output end of the optical cable is expressed by the following equation.

  If there is no polarization dispersion in the optical cable, φ = 0 and it can be seen that P is not attenuated by polarization dispersion. When φ = π, that is, when the polarization dispersion is 50 ps, the amplitude of the baseband modulation component becomes zero.

  Since the polarization-preserving optical fiber 7 having a polarization dispersion of π / 2 is used in the light receiving unit, the output (the amplitude of the baseband modulation signal) P of the output is the PSP of the optical cable and the polarization-preserving light. The result of some calculations as a function of the intrinsic polarization orientation difference α of the fiber 7 is expressed below.

Here, consider a method of measuring φ of the optical cable to be measured, that is, polarization dispersion τ. Α can be obtained by measuring the baseband signal amplitude P with α changed from Equation 5.
P in the case of α = 0 and α = π / 2 is expressed as follows.

  Here, W is defined by Equation 8.

  From Equation 6, Equation 7, and Equation 8, φ is obtained by the following equation.

  In order to obtain φ by Expression 8, it is necessary to specify the direction of α = 0. That is, it is necessary to obtain the PSP direction of the optical cable to be measured. When α = 0, the polarization dispersion φ of the optical cable to be measured matches the Fast axis and Slow axis of the polarization-maintaining optical fiber for measurement, and P is minimized because the polarization dispersion is added. That is, this case is a case where α = 0. As a result of the measurement, φ = 2 degrees was obtained. From Equation 3, φ was measured to be 0.56 ps.

  Another embodiment will be described. Since the circularly polarized light source described above may lose its circular polarization at the lead portion to the optical fiber to be measured, consider the case where the polarization controller 4 is a device that can rotate linearly polarized light. In this case, the polarization direction was rotated at a constant speed faster than the speed at which the relative polarization direction between the measured optical fiber 5 and the intermediate birefringent medium 7 was changed. Let β be the polarization direction of the light source and the relative polarization direction of the PSP of the optical cable 5.

  If the polarization controller 6 is driven on the receiving side while changing β, α is changed, and the output P of the RF power meter is monitored, it changes with time as a function of α and β. Since β changes faster than α, P is maximized when β = 0 and α = 0 or 90 degrees. It can be seen that when α = 0, P is minimized when β = 45 degrees, and P at that time is given by Equation 6. It can also be seen that P is minimum when α = 90 degrees, and P at that time is given by Equation 7.

It is the basic composition figure showing one example of the polarization dispersion measuring method of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1: Semiconductor laser 2 Amplitude modulator 3 Sine wave signal generator 4 Polarization controller 5 Optical medium to be measured 6 Polarization control element 7 Birefringence optical medium 8 Light receiver 9 RF power meter 10 Signal processing circuit

Claims (3)

  An optical part capable of entering light whose amplitude is modulated with a constant transmission bit rate or periodic electrical signal into a measured optical medium in a specific polarization state, and having an output having an intermediate birefringence determined by a modulation frequency. In an optical system that receives light through a birefringent medium, the amplitude of the fundamental frequency component of the modulation signal is determined as a function of the relative azimuth of the intrinsic polarization axis of the measurement medium and the intrinsic polarization axis of the intermediate birefringent medium. A polarization dispersion measurement method for measuring polarization dispersion of an optical medium.   2. A polarization dispersion measuring method in which the birefringence of the intermediate birefringent medium used in the polarization dispersion measuring method according to claim 1 is such that the birefringence gives a phase difference of 90 degrees between the baseband modulation signals of the intrinsic polarization modes. . 2. The polarization dispersion measuring apparatus according to claim 1, wherein the output from the light source is circularly polarized light or linearly polarized light whose polarization direction can be rotated.

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