JPH01233419A - Optical filter using diffraction grating - Google Patents

Abstract

PURPOSE:To obtain the optical filter which has high diffraction efficiency over a wide wavelength range irrelevantly to the polarization state of incident light by arranging 1st and 2nd reflecting mirrors so that each reflected light beam is incident at right angles to a 2nd polarizer. CONSTITUTION:Incident light is split by a 1st polarizer 11a into a P and an S linear polarized light beam, which are incident on 1st and 2nd diffraction grating 12a and 12b as S waves; and the diffracted light beams from the 1st and 2nd diffraction gratings 12a and 12b are reflected by 1st and 2nd reflecting mirrors 13a and 13b, made incident on the 2nd polarizer 11b orthogonally and then polarized and multiplexed, and projected as one light beam. In this case, the angles of diffraction of the diffraction gratings depend upon the wavelength, so only light with specific wavelength passes through the optical filter. The light beams incident on the diffraction gratings are all S waves, so the optical fiber which has the high diffraction grating and wide wavelength range without reference to the polarization states is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical filter using a diffraction grating.

(Prior Art) Conventionally, optical filters that utilize light separation using a diffraction grating have been widely used. FIG. 2 is a conceptual diagram of light separation in a diffraction grating. When white light 23 is incident on the groove surface 22 of the diffraction grating 2, the diffraction angle 24 changes depending on the wavelength of the light.
is obtained. Optical filters utilize this property to pass specific wavelengths.

(Problems to be Solved by the Invention) However, the diffraction grating has polarization dependence on incident light as shown in FIGS. 3 and 4. In FIGS. 3 and 4, the vertical axis is the diffraction efficiency, and the horizontal axis is the optical wavelength.
) is polarized light parallel to the incident plane of the diffraction grating (hereinafter referred to as P wave), and (b) is polarized light perpendicular to the incident plane of the diffraction grating (hereinafter referred to as P wave).
(referred to as S wave), (c) is the case of unpolarized light. Still (d) shows the case of aluminum reflection. From Figures 3 and 4, we can see that the maximum diffraction efficiency of P waves is 95 cm, and its wavelength range is about 50 nm, and that the maximum diffraction efficiency of S waves is approximately 100 cm, and the diffraction efficiency is approximately 50 nm. The wavelength range of 95% or more is 500 nm or more, the wavelength at which the P-wave diffraction efficiency peaks is different from the wavelength at which the S-wave diffraction efficiency peaks, and the diffraction efficiency of unpolarized light is Maximum 9
It is clear that the number of optical filters remains at about 3,000 cm, but for this reason, conventional optical filters have the following problems.

(1) If the diffraction grating is used at a wavelength where the diffraction efficiency of P and S waves is the same, that is, at the wavelength corresponding to point P in Figure 3, the same diffraction efficiency will be maintained regardless of the polarization state of the incident light. However, strictly speaking, the wavelength at this time is at one point, and even if a ±2 inch variation in light intensity is allowed, only a range of 20 to 30 nm can be obtained. For this reason, if the same diffraction efficiency is required at other wavelengths outside the above range regardless of the polarization state of the incident light, diffraction such that the diffraction efficiency of P waves and S waves are the same at that wavelength is required. I had to prepare a new grid.

(2) If a diffraction grating is used at a wavelength where the diffraction efficiency of P waves and S waves is different, the diffraction efficiency will vary depending on the polarization state of the incident light, and the light intensity of the diffracted light will change. For example, linearly polarized light (such as a radar diode) is transmitted through a normal single-mode optical fiber (
When propagating through SMF (SMF), linear polarization is maintained, but the state of polarization easily changes due to changes in the installation state of the optical fiber or vibrations applied to the optical fiber. For this reason, when linearly polarized light propagated through a single-mode optical fiber is used as the incident light, the light intensity of the diffracted light tends to fluctuate. In addition, considering the worst diffraction efficiency, the above 20 to 3
Outside the wavelength range of 0 nm, it is 90% or less.

The present invention eliminates the above-mentioned problem that the wavelength band is very narrow and the diffraction efficiency fluctuates depending on the polarization state of the incident light, and achieves high diffraction efficiency over a wide wavelength band regardless of the polarization state of the incident light. An object of the present invention is to provide an optical filter having the following characteristics.

(Means for Solving the Problems) 1. The present invention provides a first polarizer that polarizes and separates incident light into linearly polarized light of P waves and S waves, and each of the P waves.

The first wave is arranged so that each S wave is incident as an S wave.
and a second diffraction grating, first and second reflecting mirrors that respectively reflect the diffracted lights from the first and second diffraction gratings, and polarizing the reflected lights from the first and second reflecting mirrors. a second polarizer for wave combining, and the first and second reflecting mirrors are arranged so that each of the reflected lights enters the second polarizer orthogonally. It is an optical filter using a grating.

2. The present invention comprises a polarizer that polarizes and separates incident light into linearly polarized light of P waves and S waves, a prism that makes the traveling directions of the P waves and S waves parallel, and a prism that separates the P waves and the S waves. An A wavelength plate that unifies P waves or S waves, and the two linearly polarized lights that unify P waves or S waves are provided as S waves, and the diffracted lights are polarized and synthesized by the polarizer. This is an optical filter using a diffraction grating, characterized in that it consists of a diffraction grating.

(Function) 1. The present invention uses a first polarizer to convert incident light into 6p waves and S
Polarization separation of the wave into linearly polarized light, and the first wave as an S wave.
and said first and second diffraction gratings incident on a second diffraction grating.
The diffracted lights from the diffraction gratings are reflected by the first and second reflecting mirrors, respectively, and are orthogonally incident on the second polarizer. There, the polarizations are combined and emitted as a negative light beam.

Since the diffraction angle of the diffraction grating depends on the wavelength, only light of a predetermined wavelength will pass through this optical filter.

Furthermore, since all S waves are incident on the diffraction grating, it is possible to obtain an optical filter with high diffraction efficiency and a wide wavelength band regardless of the polarization state.

2. The present invention uses a polarizer to separate the incident light into linearly polarized P waves and S waves, a prism to make the traveling directions parallel, and a difference wavelength plate to unify the incident light into either S waves or P waves. and enters the diffraction grating as an S wave. The diffracted light from the diffraction grating travels in the opposite direction along the light path of the light incident on the diffraction grating, and is output as 91 light beams by the polarizer.

Although the same effects as the above-mentioned invention are obtained, the light ray locus is planar and there is only one polarizer and one diffraction grating, so the configuration is simpler.

(Embodiment) FIG. 1 is a configuration diagram of an optical filter using a diffraction grating showing a first embodiment of the present invention. In the same figure, 11a, llb
is a polarizing beam splitter (polarizer) that separates one beam into two beams or combines two beams into one beam, 12m.
, 12b is a diffraction grating for separating light using diffraction, and 13a.

Reference numeral 13b is a reflecting mirror that reflects light and changes the traveling direction of the light.

Incident light 14 from the outside is separated by a polarizing beam splitter lla.
The polarization separated light 15a becomes a P wave and the polarization separation light 15b becomes an S wave with respect to the incident surface. The polarization separated light 15a is
The diffracted light 16a from the J grating 12a, which is incident on the diffraction grating 12a installed so that the polarization separated light 15a becomes an S wave with respect to the incident plane, is reflected by the reflecting mirror 13a and reaches the polarized beam sliver llb. do. Although the diffraction angle for the diffracted light differs depending on its wavelength, in this embodiment it is set so that only the diffracted light 16a having a predetermined wavelength can reach the polarizing beam splitter via the reflecting mirror 13a. This makes it possible to realize the effect of a filter. On the other hand, the polarization separated light 15b is transmitted to a diffraction grating 1 installed so that the polarization separated light 15b becomes an S wave with respect to the incident surface.
2b. The diffracted light 16b from the diffraction grating 12b is reflected by the reflecting mirror 13b, and is reflected by the polarizing beam slit.
Reach b. Here, as in the case of the diffracted light 16a, a reflecting mirror 13b is installed so that only the diffracted light 16b can reach the polarizing beam splitter Ilb. The reflecting mirror 1
3m and 13b are arranged so that the reflected diffracted lights 16a and 16b intersect at right angles at one point of the polarizing beam splitter Ilb, so the diffracted lights 16a and 16b reaching the polarizing beam splitter Ilb are polarized and combined. , - is emitted as a beam.

As explained above, according to this embodiment, the incident light is separated into P waves and S waves, and both of them are made to enter the diffraction grating as S waves, so an optical filter with low loss and a wide wavelength band can be used. It can be realized.

FIG. 2 is a configuration diagram of an optical filter using a diffraction grating showing a second embodiment of the present invention. In the figure, numeral 5 is a polarizing beam sinter (polarizer), 52 is a triangular prism, 53 is a difference wavelength plate, and 54 is a diffraction grating.

Incident light 55 from the outside is polarized and separated by the polarizing beam sinter 51.
The polarization separated light 56a becomes a P wave and the polarization separation light 56b becomes an S wave with respect to the incident surface. The polarization separated light 56a is converted into linearly polarized light 57a from the 6p wave to the S wave by the difference wavelength plate 53.
and enters the diffraction grating 54. On the other hand, the polarization separated light 56b is passed through the triangular prism 52 to the polarization separated light 56t.
It is totally reflected to travel parallel to the same direction as s, and the diffraction grating 5
4. The diffraction grating 54 has an incident linearly polarized light 57
m and the polarization-separated light totally reflected by the triangular norism 52 are installed so that they both become S waves with respect to the incident surface, achieving high diffraction efficiency and a wide wavelength band. The diffracted light 57b from the diffraction grating 54 is converted again into a P wave 58a by the difference wavelength plate 53, and enters the polarizing beam splitter 5. The diffracted light 58b from the diffraction grating 54 is totally reflected by the triangular prism 52 and enters the polarizing beam sinter 51. Here, the function of a filter is obtained by utilizing the fact that the diffraction angle in the diffraction grating 54 differs depending on the wavelength. The diffracted light 5 which is the P wave incident on the polarized beam splinter 5
8a and the diffracted light 58b, which is an S wave, are polarized and combined to form a single light beam 59 and are emitted. Note that since the emitted light beam 59 has a different optical path from the incident light 55, the angle of incidence of the light on the polarizing beam sinter 51 and the difference wavelength plate 53 may deviate somewhat from the ideal value of 90°. However, since the extinction ratios of the polarizing beam splinter 51 and the wavelength-coupling plate 53 are both 25 dB (loss 0.01 dB) or more,
It is fully permissible if the incident angle is within 90'±5°. The second embodiment uses an A wavelength plate and a triangular prism to diffract two linearly polarized lights with one diffraction grating, and performs polarization separation and polarization synthesis in one.
However, as in the first embodiment, since the input to the diffraction grating is the S wave, it is possible to realize an optical filter with low loss and a wide wavelength band. In addition, the polarizing beam splitter and the diffraction grating are
In addition, since the light ray locus can be made planar, an optical filter with a simple configuration can be obtained.

Note that in the second embodiment shown in FIG.
3 is inserted on the optical path side of the polarization separation light 56b, the diffraction grating 5
A similar effect can be obtained by appropriately arranging 4.

(Effects of the Invention) As described above in detail, according to the present invention, the following effects can be obtained.

(1) The diffraction efficiency of the diffraction grating for S-waves is higher than that for P-waves and is almost 100%, while the diffraction efficiency for unpolarized light is at most 93%. Even considering the loss of the polarizing beam splitter (P wave: ≦2%, S wave: <1%) and the loss of the reflecting mirror (1%), the unpolarized light is incident on the diffraction grating. It is possible to obtain a filter with lower loss than conventional optical filters. Similarly, an optical filter using a wavelength plate or a triangular prism can also achieve low loss.

(2) The diffraction grating has high diffraction efficiency for S waves (≧
Since the wavelength band (95%) is 500 nm or more, extremely high diffraction efficiency can be obtained over a wide wavelength range. In conventional optical filters that input unpolarized light into a diffraction grating, the wavelength band in which the diffraction efficiency is always over 90% is between 20 and 30.
It is only nm. That is, according to the present invention, an optical filter with a wide wavelength band can be obtained.

(3) The wavelength at which the intensity of the diffracted light of the diffraction grating is constant regardless of the polarization state of the incident light is only at one specific point, and even if the light intensity fluctuation of ±2 inches is allowed, the wavelength is 20 to 30%. Only a wavelength range of nm can be obtained.

According to the present invention, since all incident light is incident on the diffraction grating in the form of S waves, the intensity of the polarization-synthesized diffracted light can be made constant over a wide wavelength range, regardless of the polarization state of the incident light. Can be done. For example, stable diffracted light intensity can be obtained even for the output light of a single mode fiber laser diode module whose polarization state tends to change, and the insertion loss of the optical filter is constant regardless of the polarization state. be able to.

In addition, by using a bar wavelength plate and a triangular prism, it is possible to reduce the number of polarizing beam splitters and diffraction gratings to one each, and to make the ray locus planar, resulting in an optical filter with a simple configuration. be able to.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the first embodiment of the present invention, FIG. 2 is a conceptual diagram of optical demultiplexing of a diffraction grating, and FIGS. 3 and 4 show polarization dependence of diffraction efficiency of a diffraction grating. FIG. 5 is a block diagram showing a second embodiment of the present invention. Ia, Ib, 51...Polarizing beam splitter, 12
a, 12 b, 54...-diffraction grating, 13
a, 13b...Reflecting mirror, 52...Triangular prism, 53...Difference wavelength plate. Patent Applicant: Oki Electric Industry Co., Ltd. 11a, lfb: Tajiko Vinimusuriwa 7 (Fu 1 Yugi) 12α, +2 b: Diffraction linkage 4 I3α, +3b: Sorimura Shibori Agricultural Praise L's Furustain (>l'!< T structure'
-L'A@1 Figure #lte4's Kokai Nitan phase ■S concave Figure 2

Claims (1)

[Claims] 1. A first polarizer that polarizes incident light into linearly polarized light of P waves and S waves, and arranged so that each of the P waves and S waves enters as S waves. first and second diffraction gratings, first and second reflecting mirrors that respectively reflect diffracted light from the first and second diffraction gratings, and reflection from the first and second reflecting mirrors. and a second polarizer that polarizes and synthesizes light, and the first and second reflecting mirrors are arranged so that each of the reflected lights enters the second polarizer orthogonally. An optical filter using a diffraction grating. 2. A polarizer that polarizes incident light into linearly polarized light of P waves and S waves; a prism that makes the traveling directions of the P waves and S waves parallel; and a prism that converts the P waves and S waves into P waves or S waves. a 1/2 wavelength plate that unifies the waves, and a diffraction grating that is provided so that the two linearly polarized lights that are unified into the P wave or the S wave are incident as S waves, and the diffracted lights are polarized and synthesized by the polarizer. An optical filter using a diffraction grating, characterized by comprising: