JP2005221913A - Optical incident apparatus for polarization maintaining optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical incident apparatus for a polarization maintaining optical fiber, the apparatus enabling linear polarization in the direction of birefringent main axis of the polarization maintaining optical fiber to be made incident on the polarization maintaining optical fiber without making additional adjustments, with respect to the light in which polarizing ellipticity and polarizing main axis direction are arbitrary. <P>SOLUTION: The optical incident apparatus for the polarization maintaining optical fiber is provided with: a first lens 3 that outputs, as a parallel luminous flux, the light emitted from a light source 1; a quarter-wave plate 8 and a polarizer 5 that successively pass the parallel luminous flux; and a second lens 6 that makes the outgoing light from the polarizer enter the polarization maintaining optical fiber, wherein the fast axis of the quarter-wave plate and the transmission axis of the polarizer mutually form a 45° angle, and the transmission axis of the polarizer is made parallel or orthogonal to the fast axis of the polarization maintaining optical fiber. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light incident device for a polarization maintaining optical fiber.

  Optical fibers are used for various optical measurements as well as communications. In general, optical fibers have a slight birefringence, so the polarization state of the light entering the optical fiber cycles from several meters to several tens of meters as it propagates through the optical fiber. As it changes. The degree of this polarization state change depends on the polarization state of the incident light, the relative angle between the polarization main axis of the incident light and the birefringence main axis of the optical fiber, and the environment where the optical fiber is placed, such as the temperature and stress state . In both communication and measurement, when only the power of the optical fiber output light is targeted, this polarization state change is not so much a problem, but in long distance communication, heterodyne detection, interference measurement, etc., the influence is great, It is considered a serious problem.

The polarization maintaining optical fiber is an optical fiber having a large birefringence of about 10 ⁻⁴ , and when linearly polarized light parallel to the birefringent main axis is incident, it does not depend on the length of the optical fiber at all, The polarization state in the optical fiber is always kept linearly polarized. Paying attention to this, polarization-maintaining optical fibers have already been used in the pumping light input section of optical fiber amplifiers that need to keep the polarization state constant, the introduction section from the light source to the interference measurement system, etc. New proposals have also been made (Patent Documents 1 to 3).

  As described above, in order to take advantage of the characteristics of the polarization maintaining optical fiber, it is necessary to make linearly polarized light incident parallel to the principal axis of birefringence. Patent Document 4 discloses a device in which a magnetic garnet is sandwiched between a semiconductor laser and a polarization-maintaining optical fiber, and the polarization direction of the light emitted from the semiconductor laser coincides with the polarization main axis of the polarization-maintaining optical fiber by adjusting the magnetic force of an electromagnet surrounding the semiconductor garnet. It is disclosed. However, the method cannot be used when the light source and the polarization maintaining optical fiber are not directly connected, such as when the light source and the polarization maintaining optical fiber are spatially separated or when it is desired to compare the characteristics by replacing the light source. Therefore, conventionally, an optical system as shown in FIG. 1 is used for this purpose.

  The light from the light source 1 may be extracted as a parallel light beam and may be incident on a polarization maintaining optical fiber disposed in the immediate vicinity of the light source 1, but in many cases, the light is extracted once by the optical fiber cord 2 and polarization maintaining light. Guided to near fiber 7. The divergent light emitted from the optical fiber cord 2 is converted into a parallel light beam by the first collimating lens 3. As the light source 1, a laser light source is usually used. The light immediately after being emitted from the laser light source is almost completely linearly polarized light. However, when passing through the optical fiber cord 2, the polarization state is disturbed and generally becomes elliptically polarized light. Therefore, the elliptically polarized light is converted into linearly polarized light using the polarizer 5 and then incident on the polarization maintaining optical fiber 7 using the second lens 6. At that time, the polarization maintaining optical fiber 7 or the polarizer 5 is rotated around the optical axis and adjusted so that the polarization main axis of the polarization maintaining optical fiber 7 and the transmission axis of the polarizer 5 coincide.

Assuming that the polarization main axis (major axis) direction of the incident light beam is ψ and the polarizer transmission axis direction is θ, the polarizer transmitted light power generally varies depending on θ as shown in FIG. In order for the parallel light beam to pass through the polarizer 5 efficiently, it is necessary to make the polarization main axis (long axis) of the parallel light beam coincide with the transmission axis of the polarizer 5 (that is, θ = φ). A half-wave plate 4 is inserted between the lens 3 and the polarizer 5. That is, when the main axis of the half-wave plate 4 is rotated by Θ, the polarization main axis of the parallel light beam is rotated by 2Θ, so that the transmission axis of the polarizer 5 and the polarization main axis (long axis) of the incident light are matched.
JP 2002-176217 A Japanese Patent Laid-Open No. 11-271028 JP 2001-127737 A JP-A-5-257106

  In the apparatus configuration of the conventional method, the order of the half-wave plate 4 and the polarizer 5 can be changed, and even if so, the result is exactly the same. In this case, the polarization main axis (long axis) of the light emitted from the first lens 3 and the transmission axis of the polarizer 5 are matched, but generally, the transmission axis of the polarizer 5 is unknown because the direction of the polarization main axis of the light emitted from the optical fiber cord 2 is unknown. It is necessary to adjust the direction for each state of the light source 1. In either case, it is indispensable to insert the half-wave plate 4 and adjust its axial direction.

  As described above, according to the conventional method, depending on the state of light that is desired to be incident on the polarization-maintaining optical fiber, there is a problem that rotation adjustment in the principal axis direction of the half-wave plate is indispensable every time light enters. It was.

  In order to solve this problem, the present invention does not depend on the polarization state of light to be incident at all, that is, without any additional adjustment for light whose polarization ellipticity and polarization main axis direction are arbitrary. It is an object of the present invention to provide a light incidence device for a polarization maintaining optical fiber that allows linearly polarized light in the birefringent principal axis direction of the optical fiber to be incident on the polarization maintaining optical fiber.

As a result of diligent research, the present inventors have found that a quarter wave plate and a pair of polarizers whose main axes form an angle of 45 ° with each other are inserted between the light source and the polarization maintaining optical fiber. It was found that the above-mentioned object can be achieved by providing an apparatus with one lens each for taking out light from a light source and condensing light onto a polarization-maintaining optical fiber. . That is, the present invention is as follows.
(Invention 1) The 1st lens which outputs the light radiate | emitted from the light source as a parallel light beam, the quarter wavelength plate and polarizer which pass the said parallel light beam one by one, and the polarization | polarized-light maintenance of the emitted light from this polarizer A second lens incident on the optical fiber, the fast axis of the quarter-wave plate and the transmission axis of the polarizer forming an angle of 45 ° with each other, and the transmission axis of the polarizer A light incidence device for a polarization-maintaining optical fiber, characterized by being parallel or orthogonal to the fast axis of the polarization-maintaining optical fiber.
(Invention 2) The invention also has a 1/4 wavelength plate / polarizer integrated rotation mechanism that holds the 1/4 wavelength plate and the polarizer integrally and is rotatable around the optical axis. A light incident device for the polarization-maintaining optical fiber.
(Invention 3) The polarization maintaining optical fiber according to claim 1 or 2, further comprising a polarization maintaining optical fiber rotating mechanism that allows a fast axis of the polarization maintaining optical fiber to rotate around the optical axis. Light incident device.

  In the conventional method, each time the optical fiber cord connecting the light source or the light source and the collimating lens is changed, it is necessary to adjust the polarization main axis of the half-wave plate. Since it is not affected at all by the rotation of the polarization main axis of the light incident on the first lens due to torsion or the like, there is an excellent effect that once it is assembled, no further adjustment is necessary.

  FIG. 3 is a schematic diagram showing an example of an optical system according to an embodiment of the present invention. The light from the light source 1 is extracted by the optical fiber cord 2 and guided to the vicinity of the polarization maintaining optical fiber 7. The divergent light emitted from the optical fiber cord 2 is converted into a parallel light beam by the first collimating lens 3. The roles of the optical fiber cord 2 and the first lens 3 so far are exactly the same as in the conventional method, and the parallel light beam obtained by the first lens 3 is generally elliptically polarized, and the principal axis direction is unknown.

  The quarter-wave plate 8 and the polarizer 5 inserted between the first lens 3 and the second lens 6 described later are adjusted in advance so that the principal axis directions thereof form an angle of 45 ° with each other. Are rotatable around a common optical axis. It should be emphasized that this main axis angle adjustment need only be performed once when the apparatus is assembled, and that there is no need to readjust each time the light source 1 is replaced or the polarization maintaining fiber 7 to which light is incident. .

  A more detailed explanation is given below. The quarter-wave plate 8 has two axes called a fast axis and a slow axis in a plane perpendicular to its optical axis (that is, the direction in which the light passes), and these axes are perpendicular to each other. The polarizer 5 also has one transmission axis that transmits only light having a specific electric field oscillation direction in a plane orthogonal to the optical axis. If the fast axis of the quarter wave plate 8 and the transmission axis of the polarizer 5 are arranged at an angle of 45 ° and the optical axes of these two optical elements are aligned, the light enters from the quarter wave plate 8 side. For light in any polarization state, the power of the light emitted from the polarizer 5 remains constant regardless of the direction of the polarization main axis.

  This remarkable characteristic is disclosed in Japanese Patent No. 1287596 (Japanese Patent Publication No. 55-12563) entitled “Polarization plane rotating device” by the present inventor when the incident light is linearly polarized light. Furthermore, for the general theory of this principle and the experimental verification results, for example, “Constant polarization conversion device composed of passive optical elements” by the present inventor (Technical Report OFFT 2001-3, 2001, Optical Fiber Application Technology Research Group, IEICE) May) and “Polarization state fixer composed of passive optical devices” (JOSA A, Vol. 20, No. 2, pp. 342-346, 2003). According to these papers, even if the incident light to the quarter-wave plate is not limited to linearly polarized light but arbitrary elliptically polarized light, the light emitted from the polarizer becomes linearly polarized light regardless of the rotation of the polarization main axis, It has been clarified in both theory and experiment that the polarization direction and power are always kept constant.

  The present invention takes advantage of this property. That is, as described above, the parallel light beam guided from the light source 1 by the optical fiber cord 2 and emitted from the first lens 3 is usually elliptically polarized light, and its polarization principal axis direction is unknown. Further, due to the twist of the optical fiber cord 2 or the like, temporal fluctuations also occur in the polarization main axis direction. However, due to the action of the ¼ wavelength plate 8 and the polarizer 5 described above, the light emitted from the polarizer 5 always has a constant power, and naturally, linearly polarized light having the polarization main axis in the transmission axis direction of the polarizer 5 is emitted. Is done.

  The light emitted from the polarizer 5 is condensed on the core of the polarization maintaining optical fiber 7 by the second lens 6. At that time, either the fast axis or the slow axis of the polarization maintaining optical fiber 7 is matched with the transmission axis of the polarizer 5. In order to make this matching process easily feasible, in the preferred embodiment of the present invention, the quarter wavelength plate 8 and the polarizer 5 are held together and can be rotated around the optical axis. A plate / polarizer integrated rotation mechanism 9 was provided. Instead of or in addition to this, a polarization maintaining optical fiber rotating mechanism (not shown) that allows the fast axis of the polarization maintaining optical fiber 7 to rotate around the optical axis may be provided. With such a polarization-maintaining optical fiber rotating mechanism, the ¼ wavelength plate 8 and the polarizer 5 remain fixed, and the process of matching is also performed by rotating the polarization main axis of the wavelength-maintaining optical fiber 7. Can do.

The process itself for matching the polarizer transmission axis with the fast axis or the slow axis of the polarization maintaining optical fiber is the same as that in the conventional method, and is performed, for example, as follows. As shown in FIG. 4, an analyzer 10 whose transmission axis direction θ is rotatable at the exit end of the polarization-maintaining optical fiber 7 (which is exactly the same as the polarizer 5 as an element, but is called an analyzer depending on how it is used). ) And the optical power transmitted therethrough is measured by the light receiver 11, the received light power P (φ) changes depending on the analyzer transmission axis direction φ as shown in FIG. Assuming that θ is fixed and the minimum value of the received light power when φ is changed is Pmin (θ), Pmin (θ) changes with θ as shown in FIG. Therefore, the polarizer transmission axis θ is rotated to fix the polarizer at an angular position θ ₀ that minimizes Pmin (θ).

  The present invention can be used in the fields of optical fiber communication and optical measurement.

It is a schematic diagram which shows the optical system of the light-injection apparatus to the polarization maintaining optical fiber by a conventional method. It is a characteristic view which shows the polarizer transmission axis direction dependence of polarizer transmission power. It is a schematic diagram which shows the optical system of an example of embodiment of this invention. It is explanatory drawing which shows the matching method of the polarizer transmission axis and the polarization main axis of a polarization maintaining optical fiber. FIG. 6 is a characteristic diagram showing dependence of analyzer transmission power on analyzer transmission axis rotation angle φ. FIG. 6 is a characteristic diagram showing dependency of analyzer transmission minimum power on a polarizer transmission axis θ.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Optical fiber cord 3 1st lens 4 1/2 wavelength plate 5 Polarizer 6 2nd lens 7 Polarization-maintaining optical fiber 8 1/4 wavelength plate 9 1/4 wavelength plate and polarizer integrated rotation mechanism 10 Analyzer 11 Receiver

Claims (3)

  A first lens that outputs light emitted from a light source as a parallel light beam, a quarter-wave plate and a polarizer that sequentially pass the parallel light beam, and light emitted from the polarizer are incident on a polarization-maintaining optical fiber A fast axis of the quarter-wave plate and a transmission axis of the polarizer are at an angle of 45 °, and the transmission axis of the polarizer is the polarization-maintaining optical fiber. A light incident device for a polarization-maintaining optical fiber, characterized in that it is parallel or orthogonal to the fast axis.   2. The polarization maintaining device according to claim 1, further comprising a 1/4 wavelength plate / polarizer integrated rotation mechanism that integrally holds the 1/4 wavelength plate and the polarizer and is rotatable around an optical axis. Light incident device to optical fiber.   3. A light-injecting device for a polarization-maintaining optical fiber according to claim 1, further comprising a polarization-maintaining optical fiber rotating mechanism capable of rotating a fast axis of the polarization-maintaining optical fiber around the optical axis. .

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