JPS5986010A - Light splitter - Google Patents

Abstract

PURPOSE:To split luminous flux at an equal intensity ratio regardless of the polarized state of the luminous flux by providing an equal-interval linear diffraction grating having a grating interval (d) and a luminous flux projector which projects homogeneous luminous flux with wavelength lambda on the grating surface of the diffraction grating at right angles, and performing control so that lambda<d<2lambda. CONSTITUTION:A diffraction grating unit 1 consists of a transparent substrate 1a and the equal-interval linear diffraction grating 1b formed on the substrate 1a in a specific shape. The grating 1b is composed of a relief grating formed in a corrugated shape and has the grating interval (d). The luminous flux projector 2 has, for example, a laser light source and homogeneous luminous flux 3 to be split is made incident to the surface of the grating 1 at right angles. The luminous flux 3 has wavelength lambda and is made incident to the grating 1b to obtain split pieces of luminous flux 4 and 5 at the exit side of the unit 1 while a part of the incident luminous flux is transmitted linearly to obtain luminous flux 6. At this time, the split intensity ratio is set to 1:1 accurately regardless of the wavelength and polarization direction of the incident luminous flux by letting lambda<d<2lambda. Further, the shape of the surface relief grating is specified to reduce light loss.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light splitting device that splits a monochromatic light beam into two light beams, and particularly to a light splitting device suitable for use in manufacturing a bolographic grating.

Conventionally, two types of beam splitters have been used to split light beams. That is, one of them is
One type has a metal thin film deposited on a transparent substrate, while the other type has a dielectric multilayer film deposited on a transparent substrate. Beam splitters using metal thin films use gold, silver, Inconel, etc. as the metal, and have wavelength characteristics as shown in FIG. As shown in Figure 1, in an unpolarized beam, the transmittance (T) and reflectance (R) are approximately the same over a wide wavelength range, so the beam obtained from the light source is divided into two beams of equal intensity. It shows that it is possible. however,
The transmittance and reflectance are relatively low at about 30%, indicating that a considerable amount of the light flux is absorbed by the metal film and is lost, resulting in a large optical loss.

In addition, in the case of polarized light flux, the intensity ratio of transmittance and reflectance is 1
:It will not be 1.

On the other hand, in a beam splitter using a dielectric R film, as mentioned above, unlike the case of a metal thin film, there is almost no absorption of light flux by the dielectric thin film, and therefore the beam splitter can be used without causing any optical loss. However, in this case, it is difficult to maintain an intensity ratio of 1:1 between the reflected light beam and the transmitted light beam over a wide wavelength range.However, as shown in FIG. For example, it is possible to set the intensity ratio of transmittance and reflectance to 1:1 at a specific wavelength (633 nm in Figure 2).However, in such a case, However, there are differences depending on the polarization state of the luminous flux, that is, it is possible to set T:R = 1:1 for an unpolarized luminous flux, but for an S-polarized luminous flux (polarized light perpendicular to the plane of incidence), Then, T: R=
0.35: 0.65, and in the case of P-polarized light (polarized light parallel to the plane of incidence), T: R= 0
,65: 0,35. Note that these values are values at the wavelength G 33 nm in FIG. 2.

A beam splitter using a dielectric thin film is complicated to manufacture because a multilayer dielectric with a high refractive index and a dielectric with a low refractive index are alternately stacked on a transparent substrate. In addition, when manufacturing a beam splitter using these metal thin films or dielectric thin films, it is necessary to precisely control the amount of metal or dielectric deposited in order to obtain a beam splitter with desired characteristics. The above difficulties exist.

The present invention was developed in view of the above points, and it is possible to accurately set the intensity ratio of two light beams divided over a wide wavelength range to 1:1 regardless of the polarization state of the light beam. It is an object of the present invention to provide a light splitting device that has low optical loss, can effectively utilize the amount of light, and is easy to manufacture. That is, the present invention provides a light splitting device that splits a monochromatic light beam into two light beams, including a linear diffraction grating having an evenly spaced linear diffraction grating having a grating interval d, and a monochromatic light beam having a wavelength λ on the grating plane of the diffraction grating. a beam emitter that vertically enters the beam, and the wavelength λ and the grating spacing d are such that λ<d
It is characterized by being controlled to the relationship of <2λ.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 3 shows one embodiment of the present invention, which includes a diffraction grating unit 1 and a light beam emitter 2 that makes a monochromatic light beam perpendicularly enter the diffraction grating unit 1.

The diffraction grating unit 1 is composed of a transparent substrate 1a made of glass or the like, and equally spaced linear diffraction gratings 1b formed on the transparent substrate 1a in a predetermined shape using photoresist or the like. In the embodiment shown in FIG. 3, the diffraction grating 11) is constituted by a relief grating formed in a waveform, with a grating interval d and a grating height 11. have. The light beam emitter 2 is used for, for example, a laser light source, and makes the monochromatic light beam 3 to be divided perpendicularly enter the surface of the diffraction grating 1b. The incident light beam 3 has a wavelength λ, is diffracted by entering the diffraction grating 1b, and split light beams 4 and 5 are formed at the exit side of the diffraction grating unit 1, and a part of the incident light beam is is transmitted straight as it is to form a light beam 6.

In this state, between the wavelength 2 of the incident light beam 3, the grating spacing d of the diffraction grating 1b, and the diffraction angle On of the divided light beams 4 and 5 (well, the following equation holds) do.

sin θ□=ll+・λ, −′cl
(1) Note that m is an integer and represents the order of diffraction.

Under the conditions shown in FIG. 3, the diffraction grating T.

The incident angle of the light beams 4 and 5 diffracted and split by 0 is 0.
Therefore, if the shape of the surface relief convex part forming the diffraction grating is symmetrical with respect to its center, the intensities of the light beams 4 and 5 will be exactly 1:1. By the way, in order to effectively split the incident light beam 3, it is preferable that the number of light beams to be diffracted is small. Therefore, only two light beams whose diffraction order corresponds to ±1 are diffracted (it is preferable to set the order of a). C
1. From the above equation (1), the following condition is derived.

s:nO-/! /d < 1 (
2) n and sinθ −2·λ/d〉1 n (3) By combining the above equations (2) and (3), the following relationship is obtained.

λ 〈 d ku 2 ・λ
(4) As is clear from the relationship in the above equation, the grating spacing d of the diffraction grating can be set to a length between the wavelength λ of the incident light and twice the wavelength 2λ. I understand.

Incidentally, when the diffracted light beams 4 and 5 formed by the incident light beam 3 being diffracted by the diffraction grating 1b pass through the transparent substrate 1a and emerge from the interface 12 with air, a part of the light beam is internally reflected. There is a possibility that losses may occur. In such a case, by forming an antireflection film on the interface 12, it is possible to prevent the light intensity of the diffracted light beams 4 and 5 from decreasing. It goes without saying that various known techniques such as a dielectric multilayer double diffraction grating can be used as such an antireflection film.

FIG. 4 shows another embodiment of the diffraction grating unit 1 that can be used in the present invention. The diffraction grating 1b- has an uneven shape, has a grating interval d, and has a convex width C and a convex height h. Note that the convex portions C are arranged at equal intervals with a grid interval d. By using the beam emitter 2 together with the diffraction grating unit 1 shown in FIG. 4, it is possible to construct the light splitting device of the present invention.

As the diffraction gratings 1b and ib=, in addition to surface relief gratings, there are phase gratings based on a change in refractive index, but both surface relief gratings and phase gratings can be manufactured using known holography technology, and the present invention Of course, it can be used. Incidentally, in the case of a surface relief grating, it is also possible to manufacture it by using a known "marking" technique.

Figure 5 shows the incident luminous flux intensity of the divided luminous flux when the luminous flux is divided using the sinusoidal relief grating shown in Fig. 3 and the rectangular relief grating shown in Fig. 4. It shows the wavelength dependence on the wavelength as a percentage. Incidentally, when the sinusoidal relief grating shown in FIG. 3 is used, the grating spacing d = 0. hm, and the grating depth I
I = 0.45 pm, while when using the rectangular relief grating shown in Figure 4, the convex width C = 0.3++ m, the grating spacing d = 0.8 lobes, This is a case where the grating depth l+ is set to 0.45 pm. In addition, the incident light beam in this case is 5 (iA). Also, the intensity ratio of the two divided diffracted light beams is 1:1.As is clear from the graph in Figure 5. In the case of a rectangular relief grating, the intensity of the divided diffracted beam exceeds 340%, but in the case of a sinusoidal relief grating, the intensity of the divided diffracted beam is 27 to 3.
This shows a relatively low value of 9%. This shows that it is necessary to take the shape of the diffraction grating into consideration in order to effectively split the incident light beam.

FIG. 6 shows how the division strength changes when the grating depth h and the convex width C are changed in the rectangular relief grating shown in FIG. In this case, the grating interval d = 0.75 μm, and the wavelength of the incident light beam λ =
633 nn, and S-polarized light is used as the incident light beam. As is clear from the graph in Figure 6, in order to effectively divide the incident luminous flux and use it for some purpose (
However, there are cases where a division ratio of 4 or a luminous intensity of about 40% or more is required. Therefore, in order to perform effective light splitting using a rectangular relief grating, it is necessary that the grating spacing d, the convex width C, and the grating depth h satisfy the following relationship. .

Q, <c/' d ≦ 0.5 (5
)0,45, <l+/d<
0.75(6) FIG. 7 shows the wavelength dependence of the split intensity when a rectangular relief grating satisfying the above equations (4), (5), and (6) is used. In this case, the lattice spacing d = 0.80
pm, grating depth II = 0.40 prn, and convex width C = 0.30 μm. FIG. 7 shows not only the case of non-polarized light but also the divided intensities in the case of P-polarized light and S-polarized light. As is clear from Fig. 7, the wavelength dependence of the split intensity differs somewhat depending on the polarization direction;
It can be seen that in the wavelength region of 00 to 800 nm, the split intensity is about 40%, which is a uniform value.

FIG. 8 shows an example of an apparatus for manufacturing a holographic grating using the light splitting apparatus of the present invention. In FIG. 8, a monochromatic light beam is emitted from a laser light source 20, and the light beam passes through an objective lens 21 and a pinhole 22 to become a diverging light beam 'having a substantially Gaussian distribution. Next, the diverging light beam passes through the collimator 1 through the lens 23 and the distribution correction filter 24 having an inverse Gaussian distribution transmittance, thereby forming a parallel light beam having a uniform distribution. This parallel light beam having a uniform distribution corresponds to the incident light beam 3 in the present invention. Therefore, this parallel light beam is perpendicularly incident on the diffraction grating 25 of the light splitting device of the present invention, and as a result, the diffracted light beams 30 and 3
It is divided into 1. On the other hand, the transmission hole 32 that linearly passes through the diffraction grating 25 is covered by a screen 26. The diffracted light beams 30 and 31 are reflected by the respective reflecting mirrors 27 and 28 and are incident on the hologram recording member 29 to produce a desired holographic grating.

As explained in detail above, according to the present invention, the split intensity ratio is exactly 1=1 regardless of the wavelength of the incident light beam and its polarization direction.
It is possible to remove the divided two luminous fluxes set to . Furthermore, the device of the present invention has a simple structure and can be manufactured relatively easily.

Furthermore, by using the diffraction grating as a rectangular surface relief grating having predetermined dimensions, it becomes possible to reduce the optical loss of the incident light beam and to effectively utilize the amount of light.

Although the specific configuration of the present invention has been explained in detail above,
The present invention should not be limited to these specific examples,
Of course, various modifications can be made without departing from the technical scope of the invention.

[Brief explanation of the drawing]

Figure 1 is a graph showing the wavelength characteristics of a beam splitter made of a metal thin film, Figure 2 is a graph showing the wavelength characteristics of a beam splitter made of a dielectric WI film, and Figure 3 is a graph showing the wavelength characteristics of a beam splitter made of a dielectric WI film.
The figure is an explanatory diagram showing one practical example of the present invention, Fig. 4 is an explanatory diagram showing another practical example of the diffraction grating unit 1-1 that can be used in the present invention, and Fig. 5 is an explanatory diagram showing another practical example of the diffraction grating unit 1-1 that can be used in the present invention. FIG. 6 is a graph showing wavelength dependent characteristics of a grating and a rectangular diffraction grating. FIG. 7 is a graph showing the division strength characteristics of the shape relief grating, and is based on the conditional expression (4) described in the specification.
), (5), and (6). Figure 8 is a graph showing the wavelength-dependent characteristics of splitting intensity in a rectangular relief grating that satisfies (5) and (6). FIG. (Explanation of symbols) 1: Diffraction grating unit l~ 1a: Transparent substrate 1b, Ib-: Diffraction grating 2: Luminous flux emitter 3: Incident luminous flux (wavelength:, ;!> 4.5: Split (diffraction) luminous flux 6: Transmitted light flux C: Convex width d: Grid spacing 11: Grid height Patent applicant Ri Co., Ltd.] - Agent
Person Masaaki Hashi ε1°
, l 11 400 600 '80
0 wavelength (n m) No. 21.1 Wavelength (n, m) No. 31-.1 No. 411 No. 51"l '<'r'; 6 m, 1.
1, ,71, θi length λ (n m) Fig. 8

Claims (1)

[Scope of Claims] 1. In a light splitting device that splits a monochromatic light beam into two light beams, there is provided a diffraction grating unit equipped with equally spaced linear diffraction gratings having a grating interval d, and a wavelength λ on the grating plane of the diffraction grating. and a luminous flux emitter that vertically enters a monochromatic luminous flux,
A light splitting device characterized in that the wavelength λ and the grating spacing d are controlled to have a relationship of λ<d<2λ. 2. The light splitting device according to item 1 above, wherein the diffraction grating is of a phase type or a surface relief type. 3. In the above item 1, the diffraction grating is of a surface relief type, and the surface relief grating has a substantially rectangular shape, and the width of the convex part of the rectangular shape is C, and the depth of the convex part is h. In this case, 0 < C76≦ 0.5 and 0.45 < h/d
A light splitting device characterized by satisfying the relationship of <0.75.