Diffraction grating device and optical apparatus Number:7,385,761 from the United States Patent and Trademark Office (PTO) owispatent A diffraction grating device for splitting or coupling light beams permits the divergence of the light beams to be minimized easily. A first light beam is incident on a diffraction grating from the side thereof facing the inside of the device, and a second light beam is incident on the diffraction grating from the side thereof facing air. The diffraction grating transmits the first light beam by diffraction of the minus first order so that it travels in the reverse direction along the optical path of the second light beam before incidence, and transmits the second light beam by diffraction of the zero order. The second light beam, transmitted by diffraction of the zero order, spreads over a certain width of wavelengths, but does not diverge even after diffraction.<H Diffraction, apparatus, Diffraction, gra, 7385761.html, Patent, pto, 7385761

Title: Diffraction grating device and optical apparatus

Abstract: A diffraction grating device for splitting or coupling light beams permits the divergence of the light beams to be minimized easily. A first light beam is incident on a diffraction grating from the side thereof facing the inside of the device, and a second light beam is incident on the diffraction grating from the side thereof facing air. The diffraction grating transmits the first light beam by diffraction of the minus first order so that it travels in the reverse direction along the optical path of the second light beam before incidence, and transmits the second light beam by diffraction of the zero order. The second light beam, transmitted by diffraction of the zero order, spreads over a certain width of wavelengths, but does not diverge even after diffraction.

Patent Number: 7,385,761 Issued on 06/10/2008 to Ohmori, et al.

Inventors: Ohmori; Shigeto (Kawachinagano, JP), Sekine; Koujirou (Ibaraki, JP)
Assignee: Konica Minolta Holdings, Inc. (Tokyo, JP)

Appl. No.: 11/704,741

Filed: February 9, 2007

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
11091801	Mar., 2005	7199926

Foreign Application Priority Data


Nov 26, 2004 [JP]			2004-342485
Nov 26, 2004 [JP]			2004-342504
Nov 26, 2004 [JP]			2004-342526

Current U.S. Class: 359/569 ; 398/84

Field of Search: 359/566,569,571,572,575,576 398/84,87

References Cited [Referenced By]

U.S. Patent Documents


3108279	October 1963	Eisentraut
4079382	March 1978	Henry
6900939	May 2005	Hoshi et al.
7139127	November 2006	Arnold et al.

Foreign Patent Documents


2000-163791	Jun., 2000	JP

Primary Examiner: Amari; Alessandro
Attorney, Agent or Firm: Sidley Austin LLP

Parent Case Text

This application is a divisional application of application Ser. No. 11/091,801 filed Mar. 28, 2005, now U.S. Pat. No. 7,199,926, which is based on Japanese Patent Application Nos. 2004-342485, 2004-342504, and 2004-342526 filed on Nov. 26, 2004, the contents of which are hereby incorporated by reference.

Claims

What is claimed is:

1. A diffraction grating device that diffracts and reflects a light beam in a first band of wavelengths and that diffracts and reflects and thereby separates a plurality of light beams in a plurality of bands of wavelengths longer than the wavelengths of the first band, the plurality of light beams being incident from a direction in which the light beam in the first band of wavelengths is diffracted, wherein elevations and depressions on a diffraction grating have a first period in a first direction and a second period longer than the first period in a second direction perpendicular to the first direction, and wherein the following relationships are fulfilled: .lamda.1L<.lamda.1U<.lamda.2L<.lamda.2U<.lamda.3L<.lamda.3- U; n2<n1sin .theta.; .phi..noteq.0; 1/[n1(1-sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.].ltoreq..LAMBDA./.lamda.3U<.LAMBDA./.lamda.2L.ltoreq.1/[(n2.sup.- 2-n1.sup.2sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.]; and 1/[(n2.sup.2-n1.sup.2sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.].ltoreq..LAMBDA./.lamda.1U<.LAMBDA./.lamda.1L.ltoreq.2/[n1(1-sin- .sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.], where n1 represents a refractive index of a first medium present on a side of the diffraction grating that faces optical paths of the light beams; n2 represents a refractive index of a second medium present on a side of the diffraction grating opposite to the side thereof facing the optical paths of the light beams; .theta. represents an incidence angle at which a principal ray of the light beams is incident on the diffraction grating; .phi. represents an angle between a plane perpendicular to the diffraction grating and parallel to the first direction and an incidence plane of the principal ray of the light beams; .LAMBDA. represents the first period of the elevations and depressions on the diffraction grating; .lamda.1L represents a shortest wavelength of the first band of wavelengths; .lamda.1U represents a longest wavelength of the first band of wavelengths; .lamda.2L represents a shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of shortest wavelengths; .lamda.2U represents a longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of shortest wavelengths; .lamda.3L represents a shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of longest wavelengths; and .lamda.3U represents a longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, a band of longest wavelengths.

2. An optical apparatus comprising a first optical component that supplies a light beam in a first band of wavelengths and a second optical component that supplies a plurality of light beams in different bands of wavelengths longer than the wavelengths of the first band and that receives the light beam in the first band of wavelengths from the first optical component, wherein the optical apparatus comprises the diffraction grating device of claim 1, and uses the diffraction grating to diffract and reflect and thereby direct the light beam from the first optical component to the second optical component and to diffract and reflect and thereby separate the plurality of light beams from the second optical component.

3. The optical apparatus of claim 2, wherein the second optical component is an optical fiber.

4. The optical apparatus of claim 2, wherein the optical apparatus further comprises an optical component that condenses a light beam incident on or emerging from the diffraction grating.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffraction grating device designed to diffract light of different wavelengths, and to an optical apparatus that transmits and receives light of different wavelengths.

2. Description of Related Art

In optical communication, optical transport members such as optical fibers are used to achieve bi-directional transport of light of different wavelengths. In an optical apparatus used to transmit and receive light to perform such optical communication, i.e., in an optical apparatus that, on one hand, makes light carry signals and then transmits the light to an optical transport member and that, on the other hand, receives light from the optical transport member and then detects the signal carried by the light, it is necessary to share a single optical transport medium to handle both the transmitted and received light but to arrange in different positions a light emitter for transmitting light and a light receiver for receiving light. To achieve this, a splitting/coupling member for splitting and coupling light beams is arranged on an extension line from the optical transport member so that the optical path from the light emitter to the splitting/coupling member and the optical path from the splitting/coupling member to the light receiver are split from each other while the optical paths of those two light beams are coupled together (i.e., made coincident with each other) between the splitting/coupling member and the optical transport member.

To increase communication traffic, an optical transport member is often made to transport light of different wavelengths in the same direction. An optical apparatus of this type is provided with a plurality of light emitters or light receivers, and is further provided with either a plurality of splitting/coupling members or a single splitting/coupling member that has the capability of splitting light of different wavelengths fed from an optical transport member.

A splitting/coupling member is typically realized by the use of a multiple-layer film that reflects or transmits incident light according to wavelength. A multiple-layer film, however, has the disadvantages of requiring a complicated and time-consuming process for the production thereof and being expensive.

The splitting and coupling of light beams needs to be performed not only in an optical apparatus for optical communication but also in an optical recording/reproducing apparatus that uses light to achieve the recording and reading of information to and from a recording medium. Japanese Patent Application Laid-Open No. 2000-163791 proposes the use, as a splitting/coupling member, of a diffraction grating that diffracts incident light at different angles according to wavelength in the optical head of an optical recording/reproducing apparatus that uses light of different wavelengths.

A diffraction grating consists simply of elevations and depressions arranged periodically, and can therefore be produced by resin molding. Accordingly, a diffraction grating device provided with a diffraction grating has the advantage of being suitable for mass production and being inexpensive.

By exploiting the wavelength dependence of the diffraction angle offered by a diffraction grating, it is possible to spatially split a plurality of light beams having different wavelengths. To achieve significant splitting, however, the diffraction grating needs to have the elevations and depressions thereof formed with a small period. Moreover, since the light that is made incident on the diffraction grating to be diffracted thereby is spread within a certain width of wavelengths, even when a parallel light beam is made incident on the diffraction grating, the diffracted light beam inevitably becomes divergent. The divergence of the diffracted light beam is greater the wider the wavelength band of the incident light and the smaller the period of the diffraction grating.

In an apparatus for optical communication, if the diffracted light beam is divergent, part of the light to be transmitted may fail to enter the optical transport member, or part of the light emerging from the optical transport member may fail to enter the light receiver. This results in lower correctness in the signals transmitted and received. To prevent this, optical members for condensing light need to be arranged between the optical transport member and the splitting/coupling member and between the splitting/coupling member and the light receiver. This, however, has the disadvantage of making the apparatus larger.

In an optical recording/reproducing apparatus, if the diffracted light beam is divergent, the light cannot be converged in a very small area on a recording medium, resulting in a lower recording density, or part of the light reflected from the recording medium may fail to enter the light receiver, resulting in lower reading accuracy. To prevent this, the movable objective lens that is arranged between the splitting/coupling member and the recording medium needs to be made larger. This, however, has the disadvantages of making the apparatus larger and lowering the response speed of the objective lens and thus the processing speed of the apparatus.

The diffraction efficiency of a diffraction grating tends to be lower the smaller the period of the elevations and depressions thereof. One way of maintaining high diffraction efficiency while making the period of the elevations and depressions small is to adopt a Littrow arrangement, an arrangement in which the diffracted light beam is closer to the incident light beam than the normal to the diffraction grating at the incidence position. However, in an optical apparatus for optical communication, adopting the Littrow arrangement requires the optical transport member and the light receiver to be arranged spatially close together, making their arrangement difficult.

Moreover, making the period of the elevations and depressions of a diffraction grating smaller results in a greater difference between the diffraction efficiency for the polarization component that is p-polarized with respect to the diffraction grating and the diffraction efficiency for the polarization component that is s-polarized. In optical communication, it is customary to use linearly polarized light to transport signals, and therefore failing to take into consideration the polarization direction of light with respect to a diffraction grating results in lower intensity of the transmitted and received light, leading to lower correctness in the signals transmitted and received.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the various problems described above that are experienced with a diffraction grating device that is designed to diffract light of different wavelengths. More specifically, a first object of the present invention is to provide a diffraction grating device for splitting or coupling light beams that permits the divergence of the light beams to be minimized easily, to provide a diffraction grating device that offers high diffraction efficiency while simultaneously offering a great angle difference between the incident and diffracted light beams, and to provide a diffraction grating device that offers high diffraction efficiency regardless of the polarization direction of the incident light.

Another object of the present invention is to provide a high-performance optical apparatus that splits or couples a plurality of light beams having different wavelengths. More specifically, a second object of the present invention is to provide an optical apparatus that can minimize the loss of light, to provide an optical apparatus that permits easy arrangement of a component for supplying a light beam and a component for receiving the light beam, and to provide an optical apparatus that can minimize the lowering of the intensity of light.

To achieve the above objects, in one aspect of the present invention, in a diffraction grating device on which a first light beam having a first wavelength and a second light beam having a second wavelength longer than the first wavelength are made incident from different directions and that makes the first light beam emerge therefrom in the direction from which the second light beam is incident, the following relationships are fulfilled: n2.gtoreq.n1sin .theta.; .LAMBDA./.lamda.L.ltoreq.1/(n1+n1sin .theta.); and .LAMBDA./.lamda.S.gtoreq.1/(n1+n1sin .theta.)-0.04, where n1 represents the refractive index of the first medium present on that side of the diffraction grating on which the first light beam is incident; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof on which the first light beam is incident; .LAMBDA. represents the period of the elevations and depressions on the diffraction grating; .lamda.S represents the wavelength of the first light beam; .lamda.L represents the wavelength of the second light beam; and .theta. represents the incidence angle at which the first light beam is incident on the diffraction grating.

This diffraction grating device transmits, by diffraction, the first light beam having wavelength .lamda.S, and transmits, without diffraction, the second light beam having wavelength .lamda.L. Thus, this diffraction grating device does not introduce divergence into the second light beam.

To achieve the above objects, in another aspect of the present invention, in a diffraction grating device on which a first light beam having a first wavelength and a second light beam having a second wavelength longer than the first wavelength are made incident from different directions and that makes the first light beam emerge therefrom in the direction from which the second light beam is incident, the following relationships are fulfilled: n2<n1sin .theta.; .LAMBDA./.lamda.L.ltoreq.1/(n1+n1sin .theta.); and 1/(n1+n1sin .theta.).ltoreq..LAMBDA./.lamda.S.ltoreq.1/(n2+n1sin .theta.), where n1 represents the refractive index of the first medium present on that side of the diffraction grating on which the first light beam is incident; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof on which the first light beam is incident; .LAMBDA. represents the period of the elevations and depressions on the diffraction grating; .lamda.S represents the wavelength of the first light beam; .lamda.L represents the wavelength of the second light beam; and .theta. represents the incidence angle at which the first light beam is incident on the diffraction grating.

This diffraction grating device reflects, by diffraction, the first light beam having wavelength .lamda.S, and reflects, without diffraction, the second light beam having wavelength .lamda.L. Thus, this diffraction grating device does not introduce divergence into the second light beam.

To achieve the above objects, in another aspect of the present invention, in a diffraction grating device on which a first light beam having a first wavelength and a second light beam having a second wavelength longer than the first wavelength are made incident from different directions and that makes the first light beam emerge therefrom in the direction from which the second light beam is incident, the following relationships are fulfilled: n2<n1sin .theta.; 1/(n1+n1sin .theta.).ltoreq..LAMBDA./.lamda.L.ltoreq.1/(n2+n1sin .theta.); and 1/(n2+n1sin .theta.).ltoreq..LAMBDA./.lamda.S.ltoreq.2/(n1+n1sin .theta.), where n1 represents the refractive index of the first medium present on that side of the diffraction grating on which the first light beam is incident; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof on which the first light beam is incident; .LAMBDA. represents the period of the elevations and depressions on the diffraction grating; .lamda.S represents the wavelength of the first light beam; .lamda.L represents the wavelength of the second light beam; and .theta. represents the incidence angle at which the first light beam is incident on the diffraction grating.

This diffraction grating device reflects, by diffraction, the second light beam having wavelength .lamda.L, and reflects, without diffraction, the first light beam having wavelength .lamda.S. Thus, this diffraction grating device does not introduce divergence into the first light beam.

To achieve the above objects, in another aspect of the present invention, in a diffraction grating device on which a first light beam having a first wavelength and a second light beam having a second wavelength longer than the first wavelength are made incident from different directions, the diffraction grating device making the first light beam emerge therefrom in the direction from which the second light beam is incident, the following relationships are fulfilled: n2.gtoreq.n1sin .theta.; .LAMBDA./.lamda.L.ltoreq.1/(n2+n1sin .theta.); and 1/(n2+n1sin .theta.)-0.04<.LAMBDA./.lamda.S<1/(n2+n1sin .theta.)+0.02, where n1 represents the refractive index of the first medium present on that side of the diffraction grating on which the first light beam is incident; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof on which the first light beam is incident; .LAMBDA. represents the period of the elevations and depressions on the diffraction grating; .lamda.S represents the wavelength of the first light beam; .lamda.L represents the wavelength of the second light beam; and .theta. represents the incidence angle at which the first light beam is incident on the diffraction grating.

This diffraction grating device transmits, without diffraction, the second light beam having wavelength .lamda.L, and reflects, without diffraction, the first light beam having wavelength .lamda.S. Thus, this diffraction grating device does not introduce divergence into either of the first and second light beams.

In any of the diffraction grating devices described above, there may be further provided, separate from the surface on which the diffraction grating is formed, a surface capable of condensing light. This makes it possible to further reduce the divergence of the light beams, and even to make the light beams convergent.

The diffraction grating may be formed on a curved surface. This makes it possible to give the diffraction grating an optical power arising from refraction, and thus makes it possible to further reduce the divergence of the light beams after diffraction, and even to make the light beams convergent.

In that case, preferably, at a given point on the curved surface on which the diffraction grating is formed, the diffraction grating is projected onto the plane tangent thereto at that point, and the period .LAMBDA. of the elevations and depressions of the diffraction grating as observed on that plane and the incidence angle .theta. with respect to that plane are so chosen as to fulfill the relationships noted above.

Preferably, the elevations and depressions of the diffraction grating are given a substantially rectangular sectional shape as observed parallel to the direction of the period of the elevations and depressions. This makes it easy to design the diffraction grating, and makes it easy to produce the diffraction grating device by resin molding.

To achieve the above objects, according to another aspect of the present invention, an optical apparatus that splits or couples a plurality of light beams having different wavelengths is provided with one of the diffraction grating devices described above and uses the diffraction grating to split or couple the light beams. Thanks to the diffraction grating device being so designed as to reduce the divergence of the light beams after diffraction, it is possible to direct the light beams into a small area, and thereby to realize a diffraction grating device that operates with reduced loss of light.

Here, preferably, there is further provided a mechanism for varying the incidence angle at which a light beam is incident on the diffraction grating. With this construction, even in a case where the wavelength of light varies with temperature or the like, by varying the incidence angle, it is possible to make the diffracted light beam travel in a fixed direction.

There may be further provided an optical component that makes the light beam having the first wavelength incident on the diffraction grating and that receives the light beam having the second wavelength emerging from the diffraction grating. With this construction, the diffraction grating device requires only a single optical component through which to receive light of the first wavelength from the outside and through which to emit light of the second wavelength to the outside. An example of such an optical component is an optical fiber.

There may be further provided an optical component that condenses a light beam incident on or emerging from the diffraction grating. With this construction, it is possible to turn a light beam incident on the diffraction grating into a more closely parallel light beam, and to further reduce the divergence of the light beam emerging from the diffraction grating. Thus, it is possible to realize a diffraction grating device that operates with further reduced loss of light.

To achieve the above objects, according to another aspect of the present invention, in a diffraction grating device that diffracts and reflects a light beam in a first band of wavelengths and that diffracts and reflects and thereby separates a plurality of light beams in a plurality of bands of wavelengths longer than the wavelengths of the first band, the plurality of light beams being incident from the direction in which the light beam in the first band of wavelengths is diffracted, the elevations and depressions on the diffraction grating have a first period in a first direction and a second period longer than the first period in a second direction perpendicular to the first direction. Moreover, the following relationships are fulfilled: .lamda.1L<.lamda.1U<.lamda.2L<.lamda.2U<.lamda.3L<3U; n2<n1sin .theta.; .phi..noteq.0 1/[n1(1-sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.].ltoreq..LAMBDA./.lamda.3U<.LAMBDA./.lamda.2L.ltoreq.1/[(n2.sup.- 2-n1.sup.2sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.]; and 1/[(n2.sup.2-n1.sup.2sin.sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.].ltoreq..LAMBDA./.lamda.1U<.LAMBDA./.lamda.1L.ltoreq.2/[n1(1-sin- .sup.2 .theta.sin.sup.2 .phi.).sup.1/2+n1sin .theta.cos .phi.], where n1 represents the refractive index of the first medium present on that side of the diffraction grating that faces optical paths of the light beams; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof facing the optical paths of the light beams; .theta. represents the incidence angle at which the principal ray of the light beams is incident on the diffraction grating; .phi. represents the angle between the plane perpendicular to the diffraction grating and parallel to the first direction and the incidence plane of the principal ray of the light beams; .LAMBDA. represents the first period of the elevations and depressions on the diffraction grating; .lamda.1L represents the shortest wavelength of the first band of wavelengths; .lamda.1U represents the longest wavelength of the first band of wavelengths; .lamda.2L represents the shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of shortest wavelengths; .lamda.2U represents the longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of shortest wavelengths; .lamda.3L represents the shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of longest wavelengths; and .lamda.3U represents the longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of longest wavelengths.

In this diffraction grating device, the elevations and depressions of the diffraction grating have one period in the first direction and another period in the second direction, making it possible to produce diffraction also in the second direction. Thus, all the light beams can be made incident on the diffraction grating from directions inclined relative to the first direction so as to split, also in the second direction, the light beams in the plurality of bands of wavelengths longer than the wavelengths of the first band. This makes greater the angle difference between the incident light beam in the first band of wavelengths and the diffracted light beams in the plurality of bands of wavelengths longer than the wavelengths of the first band. In addition, fulfilling the relationships noted above permits the diffraction grating to reflect, without diffraction, the light beam in the first band of wavelengths and to reflect, while producing diffraction of the minus first order in them, the plurality of light beams in the bands of wavelengths longer than the wavelengths of the first band. As a result, the diffraction grating and the plurality of light beams in the bands of wavelengths longer than the wavelengths of the first band fulfill a relationship close to the Littrow arrangement, resulting in higher diffraction efficiency with those light beams.

To achieve the above objects, according to another aspect of the present invention, in a diffraction grating device that separates a plurality of light beams spread in different wavelength bands and overlapping with one another, the elevations and depressions of the diffraction grating have a first period in a first direction and a second period longer than the first period in a second direction perpendicular to the first direction. Moreover, the diffraction grating diffracts and reflects a light beam incident thereon in the same direction from which the light beam is incident with respect to the normal to the diffraction grating at the position at which the light beam is incident. Here, the angle between the plane perpendicular to the diffraction grating and parallel to the first direction and the incidence plane of the principal ray of the light beam incident on the diffraction grating is 0.5.degree. or more but 15.degree. or less.

This diffraction grating device fulfills a relationship close to the Littrow arrangement with the plurality of light beams spread in the different wavelength bands, resulting in high diffraction efficiency. Moreover, the elevations and depressions of the diffraction grating have one period in the first direction and another period in the second direction, and the light beams are made incident on the diffraction grating from directions inclined relative to the first direction. This makes it possible to split the light beams also in the second direction. This makes greater the angle difference between the incident light beams and the separated light beams, and makes greater the angle differences among the separated light beams.

In any of the diffraction grating devices described above, preferably, the elevations and depressions of the diffraction grating are given a substantially rectangular sectional shape as observed parallel to the direction of the period of the elevations and depressions. This makes it easy to design the diffraction grating, and makes it easy to produce the diffraction grating device by resin molding.

To achieve the above objects, according to another aspect of the present invention, in an optical apparatus provided with a first optical component that supplies a light beam in a first band of wavelengths and a second optical component that supplies a plurality of light beams in different bands of wavelengths longer than the wavelengths of the first band and that receives the light beam in the first band of wavelengths from the first optical component, the optical apparatus is further provided with the former diffraction grating devices, and uses the diffraction grating to diffract and reflect and thereby direct the light beam from the first optical component to the second optical component and to diffract and reflect and thereby separate the plurality of light beams from the second optical component.

In this optical apparatus, thanks to the design of the diffraction grating device, it is possible to efficiently direct the light beam from the first optical component to the second optical component, and to efficiently separate the plurality of light beams from the second optical component, while permitting the first and second optical components to be arranged in positions where they do not interfere with each other.

Here, the second optical component may be an optical fiber. This makes the diffraction grating device suitable for optical communication.

Advisably, there is further provided an optical component that condenses a light beam incident on or emerging from the diffraction grating. This makes it possible to reduce the divergence of the light beams, resulting in higher light use efficiency.

To achieve the above objects, according to another aspect of the present invention, in an optical apparatus provided with an optical component that supplies a plurality of light beams spread in different wavelength bands and overlapping with one another, the optical apparatus separating the plurality of light beams, the optical apparatus is further provided with the latter diffraction grating device, and uses the diffraction grating to separate the plurality of light beams. In this optical apparatus, thanks to the design of the diffraction grating device, it is possible to efficiently separate the light beams in the different wavelength bands, and in addition makes the handling of the separated light beams easy.

Here, the component that supplies the plurality of light beams may be an optical fiber. This makes the diffraction grating device suitable for optical communication.

Advisably, there is further provided an optical component that condenses a light beam incident on or emerging from the diffraction grating. This makes it possible to reduce the divergence of the light beams, resulting in higher light use efficiency.

To achieve the above objects, according to another aspect of the present invention, in a diffraction grating device that diffracts and reflects a light beam in a first band of wavelengths and that diffracts and reflects and thereby separates a plurality of light beams in a plurality of bands of wavelengths longer than the wavelengths of the first band, the plurality of light beams being incident from the direction in which the light beam in the first band of wavelengths is diffracted, the following relationships are fulfilled: .lamda.1L<.lamda.1U<.lamda.2L<.lamda.2U<.lamda.3L<.lamda.3- U; n2<n1sin .theta.; 1/(n1+n1sin .theta.).ltoreq..LAMBDA./.lamda.3U.ltoreq..LAMBDA./.lamda.2L.ltoreq.1/(n2- +n1sin .theta.); 1/(n2+n1sin .theta.).ltoreq..LAMBDA./.lamda.1U<.LAMBDA./.lamda.1L.ltoreq.2/(n1+n1s- in .theta.); and .LAMBDA./.lamda.3L<1/(2n1sin .theta.)<.LAMBDA./.lamda.2U, where n1 represents the refractive index of the first medium present on that side of the diffraction grating that faces optical paths; n2 represents the refractive index of the second medium present on that side of the diffraction grating opposite to the side thereof facing the optical paths; .theta. represents the incidence angle at which the principal ray of the light beams is incident on the diffraction grating; .LAMBDA. represents the period of the elevations and depressions on the diffraction grating; .lamda.1L represents the shortest wavelength of the first band of wavelengths; .lamda.1U represents the longest wavelength of the first band of wavelengths; .lamda.2L represents the shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of shortest wavelengths; .lamda.2U represents the longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of shortest wavelengths; .lamda.3L represents the shortest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of longest wavelengths; and .lamda.3U represents the longest wavelength of, of the plurality of bands of wavelengths longer than the wavelengths of the first band, the band of longest wavelengths.

Fulfilling the relationships noted above, this diffraction grating device offers high diffraction efficiency with all the light beams in the different wavelength bands, regardless of the polarization directions thereof.

Here, advisably, the period is the period that the elevations and depressions on the diffraction grating have in a first direction substantially parallel to the incidence plane of the principal ray of the incident light beams, and the elevations and depressions on the diffraction grating have another period in a second direction perpendicular to the first direction. With this construction, the light beams can be made incident on the diffraction grating from directions inclined relative to the first direction so as to produce diffraction also in the second direction. This makes greater the angle differences among the separated light beams.

Preferably, the following relationship is fulfilled: .LAMBDA..sup.2/.lamda.2L.sup.2.ltoreq..LAMBDA.y.sup.2/.lamda.2L.sup.2<- 1/{n1.sup.2[1-(sin .theta.-1.1.lamda.2L/(n1.LAMBDA.)).sup.2]} where .LAMBDA.y represents the period of the elevations and depressions on the diffraction grating in the second direction. Fulfilling this relationship helps reduce unnecessary diffraction, and helps increase diffraction efficiency.

Preferably, the elevations and depressions of the diffraction grating are given a substantially rectangular sectional shape as observed parallel to the direction of the period of the elevations and depressions. This makes it easy to design the diffraction grating, and makes it easy to produce the diffraction grating device by resin molding.

To achieve the above objects, according to another aspect of the present invention, in an optical apparatus provided with a first optical component that supplies a light beam in a first band of wavelengths and a second optical component that supplies a plurality of light beams in different bands of wavelengths longer than the wavelengths of the first band and that receives the light beam in the first band of wavelengths from the first optical component, the optical apparatus is provided with one of the diffraction grating devices described above, and uses the diffraction grating to diffract and reflect and thereby direct the light beam from the first optical component to the second optical component and to diffract and reflect and thereby separate the plurality of light beams from the second optical component.

In this optical apparatus, thanks to the design of the diffraction grating device, regardless of the polarization direction of the light beams, it is possible to efficiently direct the light beam from the first optical component to the second optical component, and to efficiently separate the light beams in the different wavelength bands from the second optical component.

The second optical component may be an optical fiber. This makes the diffraction grating device suitable for optical communication.

There may be further provided an optical component that condenses a light beam incident on or emerging from the diffraction grating. With this construction, it is possible to turn the light beams incident on the diffraction grating into a closely parallel light beam, and to reduce the divergence of the light beams emerging from the diffraction grating. This makes it possible to direct the light beams into a small area, and thereby to realize a diffraction grating device that operates with reduced loss of light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing the construction of the optical apparatus of a first embodiment of the invention;

FIG. 2 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of the first embodiment;

FIG. 3 is a diagram showing the relationship between the variation of the parameters of the diffraction grating and the variation of the diffraction efficiency in the optical apparatus of the first embodiment;

FIG. 4 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of a second embodiment of the invention;

FIG. 5 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of a third embodiment of the invention;

FIG. 6 is a diagram schematically showing the construction of the optical apparatus of a fourth embodiment of the invention;

FIG. 7 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of the fourth embodiment;

FIG. 8 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of a fifth embodiment of the invention;

FIG. 9 is a diagram showing the relationship between the variation of the parameters of the diffraction grating and the variation of the diffraction efficiency in the optical apparatus of the fifth embodiment;

FIG. 10 is a diagram schematically showing the construction of the optical apparatus of a sixth embodiment of the invention;

FIG. 11 is a diagram schematically showing the construction of the optical apparatus of a seventh embodiment of the invention;

FIG. 12 is a diagram schematically showing the construction of the optical apparatus of an eighth embodiment of the invention;

FIGS. 13A and 13B are a side view and a plan view, respectively, schematically showing the diffraction grating device used in the optical apparatus of a ninth embodiment of the invention;

FIG. 14 is a diagram schematically showing the construction of the optical apparatus of a tenth embodiment of the invention;

FIG. 15 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of the tenth embodiment;

FIG. 16 is a plan view schematically showing the diffraction grating used in the optical apparatus of an eleventh embodiment of the invention;

FIG. 17 is a perspective view schematically showing the relationship between the diffraction grating and the angles of the light beams in the eleventh embodiment;

FIG. 18 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of the eleventh embodiment; and

FIG. 19 is a diagram schematically showing the optical path in one practical example of the diffraction grating used in the optical apparatus of a twelfth embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

First Embodiment

FIG. 1 schematically shows the construction of the optical apparatus 1 of a first embodiment of the invention. The optical apparatus 1 is a transmitter/receiver apparatus for use in optical communication, and includes a light emitter 21, a light emission controller 22, an optical fiber 31, a light receiver 41, a signal detector 42, and a diffraction grating device 51.

The light emitter 21 emits a light beam LT to be transmitted. The light emission controller 22 controls the light emission by the light emitter 21 so as to make the light beam LT emitted by the light emitter 21 carry a signal to be transmitted. The light emitter 21 includes, though not illustrated, a laser diode and a condenser lens so as to emit a parallel light beam obtained by condensing with the condenser lens the light emitted by the laser diode.

The optical fiber 31 transmits to the outside the light beam LT, carrying the signal to be transmitted, from the light emitter 21. The optical fiber 31 also receives from the outside a light beam LR carrying a signal to be received.

The light receiver 41 receives the light beam LR received by the optical fiber 31, and outputs a signal that represents the amount of received light. The signal detector 42 detects from the output signal of the light receiver 41 the signal carried by the light beam LR. The light beam LT and the light beam LR are in different wavelength bands that are apart from each other. The wavelength of the light beam LT is shorter than that of the light beam LR.

The diffraction grating device 51 has a diffraction grating 52 (see FIG. 2) formed on the surface thereof so as to direct the light beam LT from the light emitter 21 to the optical fiber 31 and to direct the light beam LR from the optical fiber 31 to the light receiver 41.

Now, the design of the diffraction grating 52 will be described. Here, it is assumed that the period of the elevations and depressions of the diffraction grating 52 is .LAMBDA.; that the height difference between the elevations and depressions of the diffraction grating 52 is h; that, of the two media between which the diffraction grating 52 is sandwiched, the one present on the side thereof on which the light beam LT is incident has a refractive index of n1 and the other has a refractive index of n2; that the incidence angle at which the light beam is incident on the diffraction grating 52 is .theta.1; the emergence angle at which the light beam emerges from the diffraction grating 52 is .theta.2; that the center wavelength of the light beam LT having the shorter wavelength is .lamda.S; and that the center wavelength of the light beam LR having the longer wavelength is .lamda.L.

The diffraction grating 52 fulfills the relationships (A1) to (A3) below. n2.gtoreq.n1sin .theta.1 (A1) .LAMBDA./.lamda.L.ltoreq.1/(n1+n1sin .theta.1) (A2) .LAMBDA./.lamda.S>1/(n1+n1sin .theta.1) (A3).

Fulfilling these relationships, the diffraction grating 52 transmits, by diffraction of the minus first order, the light beam LT having the shorter wavelength, and transmits, by diffraction of the zero order, the light beam LR having the longer wavelength.

FIG. 2 schematically shows the optical path observed in one practical example. In this example, the center wavelength of the transmitted light beam LT is 1,310 nm, and the center wavelength of the received light beam LR is 1,490 nm; the light beam LT is made incident on the diffraction grating 52 from inside the diffraction grating device 51, and the light beam LR is made incident on the diffraction grating 52 from the air side thereof. The relevant parameters are listed in Table 1. Here, the incidence plane of the principal rays of the light beams LT and LR is parallel to the direction of the period of the diffraction grating 52.

TABLE-US-00001 TABLE 1 Diffraction Grating 52 Sectional Shape: Rectangular Elevation-Depression Period .LAMBDA.: 0.69 .mu.m Elevation-Depression Height Difference h: 1.39 .mu.m Elevation Width: 0.35 .mu.m Medium Refractive Index: 1.5 Light Beam LT Wavelength (.lamda.S): 1310 nm Period/Wavelength (.LAMBDA./.lamda.S): 0.53 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: -42.6.degree. S-Polarized Light Transmission Diffraction Efficiency: 0.72 Light Beam LR Wavelength (.lamda.L): 1490 nm Period/Wavelength (.LAMBDA./.lamda.L): 0.46 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: 35.3.degree. P-Polarized Light Transmissivity: 0.87 S-Polarized Light Transmissivity: 0.73 Mean Transmissivity: 0.8

In Table 1, the elevation width of the diffraction grating 52 denotes the width of each of the parts thereof that are elevated toward the side at which the light beam LT is incident (i.e., toward the inside of the diffraction grating device 51). Here, it should be noted that the values listed in Table 1 are those observed when, as opposed to in actual use in the optical apparatus 1, the light beams LT and LR are made incident from the same direction so as to be separated from each other. That is, in actual use in the optical apparatus 1, the incidence angle .theta.1 and the emergence angle .theta.2 of the light beam LT take the values of each other listed in Table 1.

FIG. 3 shows how the diffraction efficiency varies as the value of 1/(n1+n1sin .theta.1), appearing in formulae (A2) and (A3), varies in the practical example (n1=1 and .theta.1=60.degree.) described above. Here, the value of 1/(1+1sin 60.degree.) is 0.536. As will be understood from FIG. 3, the transmissivity of the light beam LR, which is transmitted by diffraction of the zero order, is increased by setting the center length .lamda.L thereof within the range defined by formula (A2), and the transmissivity of the light beam LT, which is transmitted by diffraction of the minus first order, is increased by setting the center length .lamda.S thereof within the range defined by formula (A3).

Since the divergence of the light beams after diffraction is proportional to the width of the wavelength band thereof, making the diffraction grating 52 transmit, without diffraction, the light beam LR having the longer wavelength as is the case with the diffraction grating device 51 used in the optical apparatus 1 of this embodiment is effective in preventing the divergence of the light beam LR. With this design, the entire light beam LR can be directed to the light receiver 41 without making the light receiver 41 large.

Second Embodiment

The optical apparatus 2 of this embodiment, too, is for use in optical communication, and has a construction similar to that of the optical apparatus 1 shown in FIG. 1. Specifically, the optical apparatus 2 includes a light emitter 21, a light emission controller 22, an optical fiber 31, a light receiver 41, a signal detector 42, and a diffraction grating device 51.

Now, the design of the diffraction grating 52 formed on the diffraction grating device 51 in the optical apparatus 2 will be described. Here, as in the first embodiment, it is assumed that the period of the elevations and depressions of the diffraction grating 52 is .LAMBDA.; that the height difference between the elevations and depressions of the diffraction grating 52 is h; that, of the two media between which the diffraction grating 52 is sandwiched, the one present on the side thereof on which the light beam LT is incident has a refractive index of n1 and the other has a refractive index of n2; that the incidence angle at which the light beam is incident on the diffraction grating 52 is .theta.1; the emergence angle at which the light beam emerges from the diffraction grating 52 is .theta.2; that the center wavelength of the light beam LT having the shorter wavelength is .lamda.S; and that the center wavelength of the light beam LR having the longer wavelength is .lamda.L.

The diffraction grating 52 fulfills the relationships (B1) to (B3) below. n2<n1sin .theta. (B1) .LAMBDA./.lamda.L.ltoreq.1/(n1+n1sin .theta.1) (B2) 1/(n1+n1sin .theta.1).ltoreq..LAMBDA./.lamda.S.ltoreq.1/(n2+n1sin .theta.1) (B3)

Fulfilling these relationships, the diffraction grating 52 reflects, by diffraction of the minus first order, the light beam LT having the shorter wavelength, and reflects (regularly reflects), by diffraction of the zero order, the light beam LR having the longer wavelength.

FIG. 4 schematically shows the optical path observed in one practical example. In this example, the center wavelength of the transmitted light beam LT is 1,310 nm, and the center wavelength of the received light beam LR is 1,490 nm; the light beams LT and LR are made incident on the diffraction grating 52 from inside the diffraction grating device 51. The relevant parameters are listed in Table 2. Here, the incidence plane of the principal rays of the light beams LT and LR is parallel to the direction of the period of the diffraction grating 52.

TABLE-US-00002 TABLE 2 Diffraction Grating 52 Sectional Shape: Rectangular Elevation-Depression Period .LAMBDA.: 0.585 .mu.m Elevation-Depression Height Difference h: 0.42 .mu.m Elevation Width: 0.293 .mu.m Medium Refractive Index: 1.5 Light Beam LT Wavelength (.lamda.S): 1310 nm Period/Wavelength (.LAMBDA./.lamda.S): 0.45 Incidence Angle .theta.1: 45.degree. Emergence Angle .theta.2: -51.8.degree. S-Polarized Light Transmission Diffraction Efficiency: 0.85 Light Beam LR Wavelength (.lamda.L): 1490 nm Period/Wavelength (.LAMBDA./.lamda.L): 0.39 Incidence Angle .theta.1: 45.degree. Emergence Angle .theta.2: 45.degree. P-Polarized Light Reflectivity: 0.89 S-Polarized Light Reflectivity: 0.86 Mean Reflectivity: 0.875

In Table 2, the elevation width of the diffraction grating 52 denotes the width of each of the parts thereof that are elevated toward the side at which the light beams LT and LR are incident (i.e., toward the inside of the diffraction grating device 51). Here, it should be noted that the values listed in Table 1 are those observed when, as opposed to in actual use in the optical apparatus 2, the light beams LT and LR are made incident from the same direction so as to be separated from each other. That is, in actual use in the optical apparatus 2, the incidence angle .theta.1 and the emergence angle .theta.2 of the light beam LT take the values of each other listed in Table 2.

The reflectivity of the light beam LR, which is reflected by diffraction of the zero order, is increased by setting the center length .lamda.L thereof within the range defined by formula (B2), and the reflectivity of the light beam LT, which is reflected by diffraction of the minus first order, is increased by setting the center length .lamda.S thereof within the range defined by formula (B3). Here, the value of 1/(1.5+1.5sin 45.degree.) is 0.391, and the value of 1/(1+1.5sin 45.degree.) is 0.485.

Since the divergence of the light beams after diffraction is proportional to the width of the wavelength band thereof, making the diffraction grating 52 reflect, without diffraction, the light beam LR having the longer wavelength as is the case with the diffraction grating device 51 used in the optical apparatus 2 of this embodiment is effective in preventing the divergence of the light beam LR. With this design, the entire light beam LR can be directed to the light receiver 41 without making the light receiver 41 large.

Third Embodiment

The optical apparatus 3 of this embodiment, too, is for use in optical communication, and has a construction similar to that of the optical apparatus 1 shown in FIG. 1. Specifically, the optical apparatus 3 includes a light emitter 21, a light emission controller 22, an optical fiber 31, a light receiver 41, a signal detector 42, and a diffraction grating device 51.

Now, the design of the diffraction grating 52 formed on the diffraction grating device 51 in the optical apparatus 3 will be described. Here, as in the first embodiment, it is assumed that the period of the elevations and depressions of the diffraction grating 52 is .LAMBDA.; that the height difference between the elevations and depressions of the diffraction grating 52 is h; that, of the two media between which the diffraction grating 52 is sandwiched, the one present on the side thereof on which the light beam LT is incident has a refractive index of n1 and the other has a refractive index of n2; that the incidence angle at which the light beam is incident on the diffraction grating 52 is .theta.1; the emergence angle at which the light beam emerges from the diffraction grating 52 is .theta.2; that the center wavelength of the light beam LT having the shorter wavelength is .lamda.S; and that the center wavelength of the light beam LR having the longer wavelength is .lamda.L.

The diffraction grating 52 fulfills the relationships (C1) to (C3) below. n2<n1sin .theta.1 (C1) 1/(n1+n1sin .theta.1).ltoreq..LAMBDA./.lamda.L.ltoreq.1/(n2+n1sin .theta.1) (C2) 1/(n2+n1sin .theta.1).ltoreq..LAMBDA./.lamda.S.ltoreq.2/(n1+n1sin .theta.1) (C3)

Fulfilling these relationships, the diffraction grating 52 reflects, by diffraction of the minus first order, the light beam LR having the longer wavelength, and reflects (regularly reflects), by diffraction of the zero order, the light beam LT having the shorter wavelength.

FIG. 5 schematically shows the optical path observed in one practical example. In this example, the center wavelength of the transmitted light beam LT is 1,310 nm, and the center wavelength of the received light beam LR is 1,490 nm; the light beams LT and LR are made incident on the diffraction grating 52 from inside the diffraction grating device 51. The relevant parameters are listed in Table 3-1. Here, the incidence plane of the principal rays of the light beams LT and LR is parallel to the direction of the period of the diffraction grating 52.

TABLE-US-00003 TABLE 3-1 Diffraction Grating 52 Sectional Shape: Rectangular Elevation-Depression Period .LAMBDA.: 0.6 .mu.m Elevation-Depression Height Difference h: 0.645 .mu.m Elevation Width: 0.3 .mu.m Medium Refractive Index: 1.5 Light Beam LT Wavelength (.lamda.S): 1310 nm Period/Wavelength (.LAMBDA./.lamda.S): 0.46 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: 60.degree. Reflectivity: 0.81 (-1.83 dB) Light Beam LR Wavelength (.lamda.L): 1490 nm Period/Wavelength (.LAMBDA./.lamda.L): 0.40 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: -52.1.degree. P-Polarized Light Reflection Diffraction Efficiency: 0.83 (-1.63 dB) S-Polarized Light Reflection Diffraction Efficiency: 0.87 (-1.20 dB) Mean Reflection Diffraction Efficiency: 0.85 (-1.41 dB)

In Table 3-1, the elevation width of the diffraction grating 52 denotes the width of each of the parts thereof that are elevated toward the side at which the light beams LT and LR are incident (i.e., toward the inside of the diffraction grating device 51). In Table 3-1 are also listed the dB equivalent values of the reflectivity and the reflection efficiency.

The reflectivity of the light beam LR, which is reflected by diffraction of the minus first order, is increased by setting the center length .lamda.L thereof within the range defined by formula (C2), and the reflectivity of the light beam LT, which is reflected by diffraction of the zero order, is increased by setting the center length .lamda.S thereof within the range defined by formula (C3). Here, the value of 1/(1.5 +1.5sin 60.degree.) is 0.357, the value of 1/(1+1.5sin 60.degree.) is 0.434, and the value of 2/(1.5+1.5sin 60.degree.) is 0.715.

When the wavelength bands of the light beams LR and LT have the same width, the light beam LT having the shorter wavelength diverges less than the light beam RT after diffraction. However, even the light beam LT having the shorter wavelength, as the width of the wavelength band thereof increases, diverges more after diffraction. This makes it difficult to make the entire light beam LT enter the optical fiber 31. In the diffraction grating device 51 used in the optical apparatus 3 of this embodiment, however, the diffraction grating 52 produces diffraction of the zero order, i.e., no diffraction, in the light beam LT. This prevents the light beam LT from diverging, and makes it easy to make the entire light beam LT enter the optical fiber 31, of which the diameter is as small as of the order of .mu.m.

The parameters related to the light beam LT as observed when the wavelength band of the light beam LT has a width of .+-.50 nm around wavelength .lamda.S are listed in Tables 3-2 and 3-3. The parameters related to the light beam LR as observed when the wavelength band of the light beam LR has a width of .+-.10 nm around wavelength .lamda.L are listed in Tables 3-4 and 3-5. The parameters other than those listed in these tables are the same as in Table 3-1.

TABLE-US-00004 TABLE 3-2 Light Beam LT Shortest Wavelength (.lamda.S - 50): 1260 nm Period/Wavelength (.LAMBDA./(.lamda.S - 50)): 0.48 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: 60.degree. Reflectivity: 0.85 (-1.43 dB)

TABLE-US-00005 TABLE 3-3 Light Beam LT Longest Wavelength (.lamda.S + 50): 1360 nm Period/Wavelength (.LAMBDA./(.lamda.S + 50)): 0.44 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: 60.degree. Reflectivity: 0.78 (-2.11 dB)

TABLE-US-00006 TABLE 3-4 Light Beam LR Shortest Wavelength (.lamda.L - 10): 1480 nm Period/Wavelength (.LAMBDA./(.lamda.L - 10)): 0.41 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: -51.1.degree. P-Polarized Light Reflection Diffraction Efficiency: 0.82 (-1.76 dB) S-Polarized Light Reflection Diffraction Efficiency: 0.81 (-1.80 dB) Mean Reflection Diffraction Efficiency: 0.81 (-1.78 dB)

TABLE-US-00007 TABLE 3-5 Light Beam LR Longest Wavelength (.lamda.L + 10): 1500 nm Period/Wavelength (.LAMBDA./(.lamda.L + 10)): 0.40 Incidence Angle .theta.1: 60.degree. Emergence Angle .theta.2: -53.2.degree. P-Polarized Light Reflection Diffraction Efficiency: 0.83 (-1.62 dB) S-Polarized Light Reflection Diffraction Efficiency: 0.91 (-0.79 dB) Mean Reflection Diffraction Efficiency: 0.87 (-1.20 dB)

The diffraction grating 52 does not produce diffraction in the light beam LT, and thus does not cause any variation in reflection angle even at the shortest or longest wavelength of the wavelength band thereof. Moreover, as will be clearly understood from Tables 3-2 and 3-3, high reflectivity is obtained even at the shortest and longest wavelengths.

Fourth Embodiment

FIG. 6 schematically shows the construction of the optical apparatus 4 of a fourth embodiment of the invention. This optical apparatus 4, too, is, like the optical apparatuses 1 to 3 of the first to third embodiments, a transmitter/receiver apparatus, but, unlike them, receives two light beams LR1 and LR2 in different wavelength bands via an optical fiber 31. Accordingly, the optical apparatus 4 includes, in addition to a light emitter 21, a light emission controller 22, an optical fiber 31, a light receiver 41,