JPH0385514A - Circularly polarized light generator - Google Patents

Abstract

PURPOSE:To stably obtain the light beam of circularly polarized light by providing a quarter wave plate which is disposed with 45 deg. deviation in the main axis bearing with respect to the polarization direction of the light beams of P polarized light component and S polarized light component and converts the light beam multiplexed by a polarized light multiplexing element to the circularly polarized light. CONSTITUTION:The positions of reflecting mirrors 18, 19 are so set that the optical path length of the light beam 15 of the S polarized light component is longer by the length corresponding to the coherence length of monochromatic light 13 than the optical path length of the light beam 14 of the P polarized light component. The light beam 20 which is made incident on the quarter wave plate 12 after the multiplexing is, therefore, preserved in the straight polarization state without becoming elliptically polarized light. The quarter wave plate 12 is disposed by deviating the main axis bearing thereof by 45 deg. with polarization direction of the light beam 14 of the P polarized light component and the light beam 15 of the S polarized light component so that the incident light beam 20 is converted to the light beam 21 of the circularly polarized light. The light beam of the complete circularly polarized light of always the specified output is easily obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a circularly polarized light generator capable of converting a circularly polarized light beam with a constant output even if the polarization state of incident light changes, and in particular to an optical fiber, etc. This is suitable for use as a light source when evaluating optical components.

<Prior art> A circularly polarized or non-polarized light beam is directly polarized by a polarizer.
When converting into a scratched light beam, it is possible to obtain a linearly polarized light beam with constant output no matter what direction the principal axis of the polarizer is in with respect to a circularly polarized or non-polarized light beam. . Therefore, by inputting a circularly polarized or non-polarized light beam into one end of a single-polarized optical fiber and examining the polarization state of the light beam exiting from the other end of the single-polarized optical fiber, it is possible to create a single-polarized optical fiber. It becomes possible to perform evaluations.

In this way, circularly polarized or unpolarized light beams are useful as reference light sources when evaluating optical components such as optical fibers. In this case, since there are various restrictions on using unpolarized natural light as a reference light source for evaluating optical components, generally a circularly polarized light beam is often used as a reference light source.

Conventionally, as a device for converting a light beam in an arbitrary polarization state into circularly polarized light, devices such as those shown in FIGS. 5 and 6 are known, for example.

The circularly polarized light generator, the concept of which is shown in FIG. With the quarter-wave plate 4 whose orientation is shifted by 45 degrees,
The light beam 3 is converted into a circularly polarized light beam 5.

A circularly polarized light generator, the concept of which is shown in FIG. 6, converts incident light 6 in an arbitrary polarization state into a circularly polarized light beam 8 by a phase compensation element 7. This phase compensation element 7 is a birefringent medium whose principal axis direction and phase delay can be electrically controlled by a controller 9.

<Problems to be Solved by the Invention> In the conventional circularly polarized light generator shown in FIG. 3 is the result of a change in the amount of light,
The output of the finally obtained circularly polarized light beam 5 fluctuates, and as it is, it is unreliable as a reference light source when evaluating optical components. Therefore, when using this circularly polarized light generator, it is necessary to take some measures to keep the polarization state of the incident light 1 constant.

On the other hand, in the conventional circularly polarized light generator shown in FIG. The operations involved are extremely troublesome.

Moreover, depending on the response delay of the phase compensation element 7 to the controller 9, etc., there is a possibility that the circularly polarized light beam 8 cannot be stably obtained.

<Means for Solving the Problems> A circularly polarized light generating device according to the first aspect of the present invention includes a polarization splitting element that splits a light beam from a light source into a P polarization component light beam and an S polarization component light beam; The P-polarized component light beam and the Sli! A polarization multiplexing element for recombining the optical component light beams, and a polarization multiplexing element interposed between the polarization multiplexing element and the polarization demultiplexing element to combine the optical component light beams again, and an optical path difference longer than the coherence length of the light beams to combine the polarized light beams. an optical path difference imparting means for providing the PIIa light component light beam and the S polarization component light beam between the multiplexing element and the polarization splitting element; and polarization of the P polarization component light beam and the S polarization component light beam. It is provided with a quarter-wave plate whose principal axis direction is shifted by 45 degrees with respect to the direction, and which converts the light beam multiplexed by the polarization multiplexing element into circularly polarized light.

Further, the circularly polarized light generating device according to the second aspect of the present invention includes a polarization splitting element that splits the light beam from the light source into a PIl light component light beam and an S polarization component light beam, and the P polarization component light beam. a polarization multiplexing element for recombining the S-polarized component light beam; and a polarization multiplexing element interposed between the polarization multiplexing element and the polarization demultiplexing element to create an optical path difference longer than the coherence length of the light beam. an optical path difference imparting means for imparting an optical path difference to the pH light component light beam and the Sll light component light beam between the polarization multiplexing element and the polarization splitting element; and reflective optics constituting a part of the optical path difference imparting means. a polarization control element interposed between the system and the polarization multiplexing element to return the change in the polarization state caused by the reflective optical system to the original state and guide it to the polarization multiplexing element; a quarter-wave plate whose principal axis direction is shifted by 45 degrees with respect to the polarization direction of the Sll light component light beam and which converts the light beam multiplexed by the polarization multiplexing element into circularly polarized light; It is something that

Effect〉 The light beam from the light source is split into a pH light component light beam and an Sll light component light beam by a polarization demultiplexing element, and is again split into a P polarization component light beam and an S polarization component light beam by a polarization multiplexing element. are combined. When the polarization state of the light beam from the light source changes, only the ratio of the amount of light between the pH light component light beam and the S polarization component light beam changes correspondingly, and the sum of these light amounts is always constant. be. In addition, since the optical path difference between the pH light component light beam and the Sll light component light beam between the polarization demultiplexing element and the polarization multiplexing element is set to be longer than the coherence distance of the light beam from the light source, The light beam after the wave remains linearly polarized without becoming elliptically polarized. Therefore, the light beam that passes through the quarter-wave plate always becomes circularly polarized light with a constant output.

On the other hand, when the P-polarized component light beam or the Sll light component light beam is reflected by the reflective optical system that constitutes a part of the optical path difference providing means, the polarization state of these components may change. Therefore, the polarization control element returns the light to its original polarization state, and the P [light component light beam and the Sll light component light beam] are combined by the polarization multiplexing element with respect to the principal axis direction of the quarter-wave plate. Make sure that the polarization direction is always shifted by 45 degrees.

<Embodiment> As shown in FIG. 1, which schematically shows the structure of an embodiment of the circularly polarized light generating device according to the present invention, a light source is provided between a monochromatic light source 11 and its quarter-wave plate 12. 11 toward the quarter-wave plate 12 into a PIi light component light beam 14 and an S-polarized light component light beam 15; Pj
A polarization beam splitter 17 which is a polarization multiplexing element that combines the Ia light component light beam 14 and the S polarization component light beam 15.
are arranged in a straight line. The polarizing beam splitters 16 and 17 according to this embodiment have a P-polarized light beam 14.
The S-polarization component light beam 15, whose polarization direction is orthogonal to the S-polarization component light beam 15, is transmitted as is, and the S-polarization component light beam 15 is reflected.
Of course, it is also possible to adopt one that allows the polarized component light beam 15 to pass through as it is.

The S-polarized component light beam 15 reflected by the polarizing beam splitter 16 is guided toward the polarizing beam splitter 17 by two reflecting mirrors 18 and 19 constituting the optical path difference providing means, and is reflected by the polarizing beam splitter 17. It is then combined into a PIi light component light beam 14 that passes through this. In this case, S from the polarizing beam splitter 16 to the polarizing beam splitter 17 via the reflecting mirror 18
The reflecting mirror is arranged so that the optical path length of the polarized component light beam 15 is longer than the optical path length of the P polarized component light beam 14 between the polarized beam splitters 16 and 17, which corresponds to the coherence length of the monochromatic light 13. The position is set at 18°19. As a result, the multiplexed light beam 20 that enters the quarter-wave plate 12 does not become elliptically polarized light but maintains its linearly polarized state.

In addition, the sm light component light beam 15 is formed by the reflecting mirrors 18 and 19.
It is preferable to set the principal axis direction of the S-polarized component light beam 15 parallel to the reflecting surface of the reflecting mirror 18, 19 so that the principal axis direction of the S-polarized light beam 15 does not change. However, if such a configuration cannot be adopted, a polarization control element is interposed just before the polarization beam splitter 17 (between the polarization beam splitter 17 and the reflection [1119]) to change the principal axis direction of the S-polarization component light beam 15. It is best to return to the original state immediately after demultiplexing.

The principal axis direction of the quarter-wave plate 12 is shifted by 45 degrees with respect to the polarization direction of each of the p-polarized light beam 14 and the s-polarized light beam 15, so that the quarter-wavelength plate 12 A light beam 20 incident on the plate 12 is converted into a circularly polarized light beam 21.

Next, in order to confirm whether the circularly polarized light beam obtained by the device of the present invention is completely circularly polarized, a quarter-wave plate 23 is set using an optical power meter 22 shown in FIG. The absolute level of the amount of transmitted circularly polarized light beam 24 was detected.

Specifically, a laser diode that oscillates a multimode laser beam with a wavelength of 0.78 μm is used as the light source 25,
This is made to enter the polarization beam splitter 28 as a parallel light beam 27 by the collimating lens 26, and the Pj polarization component light beam 29 is transmitted as it is and is input to the polarization beam splitter 30, while the S polarization component light beam 31 is split into two reflected beams. Polarizing beam splitter 30 via 32.33
I tried to make it inject into .

The optical path difference between the p-polarized light beam 29 and the s-polarized light beam 31 between these polarized beam splitters 28 and 30 is 3.
The distance was set to 0 cm, which was about 10 cm longer than the coherence distance of the multimode laser beam from the light source 25, which is about 20 cm.

The extinction ratios of the polarizing beam splitter 28.30 for the P-polarized light beam 29 and the S-polarized light beam 31 were both 35 dB or more.

In addition, as the reflecting mirrors 32 and 33, although there is a change in the principal axis direction due to reflection, there is no change in the polarization state to elliptically polarized light, and the P polarization component light beam 29 and the 5I polarization component light beam 31 are combined. Between the polarizing beam splitter 30 and the reflecting mirror 33, an all-needle single-wavelength plate is installed as a polarization control element in order to correct changes in the principal axis direction caused by the reflection of the S-polarized component light beam 31 by the reflecting mirrors 32 and 33. 34 was inserted.

On the other hand, in order to guide the circularly polarized light beam 24 transmitted through the quarter-wave plate 23 to the optical power meter 22, a condensing lens 35 is interposed between them, and the condensing lens 35 and the quarter-wave plate A polarizer 36 is arranged between the one-wavelength plate 23 and the polarizer 3
While rotating the optical power meter 6, the absolute level of the amount of received light beam incident on the optical power meter 22 was measured.

The results are shown in FIG. According to this, the absolute level of the amount of received light is always constant regardless of the rotation of the polarizer 36,
The amount of variation was less than 0.01 dB, which is the measurement resolution of the optical power meter 22 used, and it was confirmed that the light beam 24 transmitted through the quarter-wave plate 23 was completely circularly polarized.

Next, in order to confirm that the absolute level of the amount of received light beam incident on the optical power meter 22 does not change due to changes in the polarization state of the multimode laser beam from the light source 25, the polarizer 36 was fixed. While rotating the light source 25 in this state, the absolute level of the amount of received light beam incident on the optical power meter 22 was measured.

The results are shown in FIG. According to this, regardless of the rotation of the light source 25, the absolute level of the light quantity of the light beam is exactly the same as in the case of FIG. 3, and the polarization state of the parallel light beam 27 entering the polarizing beam splitter 28 changes. too,
It was confirmed that the absolute level of the amount of circularly polarized light beam 24 passing through the quarter-wave plate 23 did not change. Note that the extinction ratio of the multimode laser beam from the light source 25 in this case was 20 dB or more.

<Effects of the Invention> According to the circularly polarized light generator of the present invention, even if the polarization state of the light beam from the light source changes, a completely circularly polarized light beam with a constant output can always be obtained without performing any mu. For,
When this is used as a light source when evaluating optical components, it is possible to give highly reliable evaluations to the optical components.

[Brief explanation of drawings]

Fig. 1 is a conceptual diagram of an embodiment of the circularly polarized light generator according to the present invention, Fig. 2 is a conceptual diagram of another embodiment of the circularly polarized light generator according to the present invention, and Fig. 3 is a conceptual diagram of an embodiment of the circularly polarized light generator according to the present invention. Fig. 4 is a graph showing the absolute level of the amount of light received by the optical power meter when the light source is rotated, and Figs. 5 and 6 are graphs showing the absolute level of the amount of light received by the optical power meter when the light source is rotated. It is a conceptual diagram showing an example of a generator. Also, in the figure, 11.25 is a light source, 12.23 is a quarter-wave plate, 14.29 is a P-polarized component light beam, 15.
31 is an S-polarized light beam, 16, 17, 28, and 30 are polarizing beam splitters, 18, 19, 32, and 33 are reflecting mirrors, 21° and 24 are circularly polarized light beams, and 34 is an all-needle single-wavelength plate. be.

Claims (2)

[Claims] (1) a polarization splitting element that splits a light beam from a light source into a P-polarized component light beam and an S-polarized component light beam;
a polarization multiplexing element for recombining the polarization component light beam and the S-polarization component light beam; an optical path difference providing means for providing a long optical path difference to the P polarization component light beam and the S polarization component light beam between the polarization multiplexing element and the polarization splitting element; a quarter-wave plate whose principal axis direction is shifted by 45 degrees with respect to the polarization direction of the S-polarized component light beam, and which converts the light beam multiplexed by the polarization multiplexing element into circularly polarized light. Circularly polarized light generator. (2) a polarization splitting element that splits a light beam from a light source into a P polarization component light beam and an S polarization component light beam;
a polarization multiplexing element for recombining the polarization component light beam and the S-polarization component light beam; an optical path difference providing means for providing a long optical path difference to the P polarization component light beam and the S polarization component light beam between the polarization multiplexing element and the polarization splitting element; and a part of the optical path difference provision means. a polarization control element that is interposed between the reflective optical system constituting the P and the polarization multiplexing element to return the change in the polarization state caused by the reflective optical system to the original state and guide it to the polarization multiplexer; A quarter whose principal axis direction is shifted by 45 degrees with respect to the polarization direction of the polarized component light beam and the S-polarized component light beam, and which converts the light beam multiplexed by the polarization multiplexing element into circularly polarized light. A circularly polarized light generator equipped with a wavelength plate.