JP2005121938A - Diffraction grating with polarization control film, and diffraction optical device using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction grating with polarization control film which is high in utilization efficiency of light and low in polarization dependence in a wide wavelength region and at various incident angles, and to provide a diffraction optical device using the diffraction grating on proper conditions. <P>SOLUTION: The diffraction grating with polarization control film is a reflection type diffraction grating which has parallel grooves arranged at regular intervals on the surface of a planar base plate and has a reflective layer on the surface of the grooves. The groove side surfaces have at least two flat surface parts inclined to the surface of the base plate, a polarization control film 50 which is transparent at a prescribed wavelength λ and has a refractive index n is disposed on one side of the flat surface parts 14 and the average film thickness is in the range of 0.8 to 1.1 λ/4n. Further, the average film thickness of the film on the groove side surface on the other part is ≤0.1 λ/4n . <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a diffractive optical element and apparatus used in the fields of optical communication, analytical optics, and the like, and more particularly to polarization characteristic control of light utilization efficiency of a reflective diffraction grating.

  A diffractive optical element using the diffraction effect of light is widely applied as a spectroscopic element for analytical instruments and a multiplexing / demultiplexing element in the field of optical communication. A typical diffractive optical element is a diffraction grating in which a large number of parallel grooves are arranged at equal intervals on a substrate.

  The light utilization efficiency of the diffraction grating, that is, the diffraction efficiency, depends greatly on the relationship between the wavelength of light and the number of grooves per unit length, and is also affected by the polarization state of light, the incident angle, the shape of the grooves, etc. It is known.

  In particular, the diffraction efficiency changes strongly depending on the polarization state of the incident light under use conditions in which the groove pitch (groove interval) of the diffraction grating and the incident wavelength are substantially equal. In this polarization state, a state having an electric field amplitude in a direction parallel to the groove of the diffraction grating is referred to as TE polarization, and a state having an electrolytic amplitude in a direction perpendicular to the groove is referred to as TM polarization. In general, the wavelength and the groove pitch are substantially equal. Then, it is known that the diffraction efficiency of TE-polarized light is significantly reduced (see, for example, Non-Patent Document 1).

  In order to avoid such problems and obtain a relatively stable use efficiency of light without depending on the wavelength or polarization state of incident light, a diffraction grating having a wider groove pitch than the wavelength may be used. However, since the value of angular dispersion, which is the change in diffraction angle with respect to wavelength change, is proportional to the reciprocal of the groove pitch, that is, the number of grooves per unit length, there is a drawback that the angular dispersion becomes small in a diffraction grating with a wide groove pitch. .

Since the angular dispersion of the diffraction grating greatly affects the wavelength resolution, a diffraction grating having a large angular dispersion is desired. Recently, in order to meet the demand for miniaturization of optical devices, there are increasing designs for space-saving optical systems using diffraction gratings with a large number of grooves and large angular dispersion.
Furthermore, since the wavelength range to be used is also widened, there is a need for a diffraction grating that has low polarization dependence and high light utilization efficiency in a wide wavelength range.

  Conventionally, there are the following techniques regarding the diffractive optical element with respect to these problems. For low light utilization efficiency, for example, as disclosed in Patent Document 1, an attempt is made to optimize the groove shape. Further, a method of diffracting at a wavelength with high diffraction efficiency by providing a transparent material having a refractive index n on the groove and transmitting the incident wavelength λ as λ / n has been proposed (see Patent Document 2). . Further, there is a technique in which this technique is applied to a wide wavelength range as disclosed in Patent Document 3.

In general, a method of compensating for the polarization dependence of the diffractive optical element not only by the diffractive optical element itself but also in combination with another optical component such as a filter is often used.
JP 2002-224889 A JP-A-62-61002 JP-A-6-317705 Roger Petit, “Electromagnetic Theory of Gratings”, (USA), Springer-Verlag, 1980

  However, simply optimizing the shape and adjusting the apparent wavelength with the refractive index of the transparent material is not enough to improve the light utilization efficiency of the diffraction grating, and the polarization dependence cannot be reduced sufficiently. could not.

  Further, as shown in Patent Document 3, the configuration using both sides of the element in order to widen the wavelength band used has a low degree of freedom in the optical system and is difficult to handle.

  Furthermore, the method of compensating for the polarization dependence by adding components such as a filter has a problem that the loss of light increases and the utilization efficiency decreases, and the assembly becomes complicated due to an increase in the number of components. .

  The present invention has been made to solve these problems. In a wide wavelength range and various incident angles, the light utilization efficiency is increased and the polarization dependence of the characteristics is reduced regardless of the number of grooves of the diffraction grating. An object of the present invention is to provide a diffraction grating with a polarization control film necessary for the purpose, and to provide a diffractive optical apparatus using the diffraction grating under appropriate conditions.

  The diffraction grating with a polarization control film of the present invention is based on a structure in which N parallel grooves per 1 mm are arranged at equal intervals on the surface of a flat substrate, and the side surfaces of the grooves are inclined with respect to the substrate surface. It has a flat part. And it is a reflection type diffraction grating provided with the reflection layer on the surface. A polarization control film that is transparent with respect to a predetermined wavelength λ and has a refractive index n is provided on one of the parallel plane portions of each groove side surface of such a diffraction grating, and the average film thickness is 0.8λ / 4n. To 1.1λ / 4n. Further, the average film thickness of the film on the other side of the groove is 0.1λ / 4n or less.

  By using the diffraction grating with a polarization control film having this configuration under appropriate conditions, it is possible to provide a diffractive optical apparatus with little polarization dependency.

  The diffractive optical apparatus of the present invention comprises the above-described diffraction grating with a polarization control film, optical means for allowing a parallel light beam to be incident on the side surface of the groove provided with the polarization control film of the diffraction grating, and the light beam of the first-order diffracted light by the diffraction grating. And at least optical means for receiving light. Of the optical path lengths of the light rays in this light beam from an arbitrary plane perpendicular to the parallel light beam incident on one groove side surface of the diffraction grating to an arbitrary plane perpendicular to the light beam of zero-order diffracted light, The difference between the shortest optical path lengths is in the range of 0.825λ to 1.175λ.

  By setting the difference between the longest optical path length and the shortest optical path length in the light beam of the 0th-order diffracted light to be approximately one wavelength, that is, making the phase difference substantially equal to 2π, the condition that the 0th-order diffracted light does not exist is established. Therefore, the diffracted light is almost first-order diffracted light, and high first-order diffraction efficiency can be obtained.

In addition, when the incident angle of the parallel light beam incident on the diffraction grating with respect to the substrate surface is α (°), and the inclination angle of the groove side surface of the diffraction grating on which the parallel light beam is incident with respect to the substrate surface is θ (°),
α ≦ 90−θ
It is desirable to configure the diffractive optical apparatus so that the above relationship holds.
If this condition is not satisfied, the geometric optical zero-order diffracted light is directed toward the inside of the diffraction grating, and the optical path length cannot be defined.

Furthermore, when the angle between two planes forming the groove side surface of the diffraction grating is θ ′ (°),
θ ′ ≦ 90 + α−θ
It is desirable to configure the diffractive optical apparatus so that the above relationship holds.
By satisfying this condition, it is possible to avoid the incident light beam from entering the side surface of the groove where there is almost no polarization control film, and to optimize the optical path length difference.

Furthermore, when the 0th-order diffracted light beam is emitted from the polarization control film, it is desirable to emit all of the light beam from a side surface that is not parallel to the groove side surface of the polarization control film.
By satisfying this condition, the optical condition can be set so that the optical path length difference is optimized.

For diffraction orders m other than 0 and +1 in the above diffraction grating,
| Nmλ-sinα | ≧ 1
It is desirable to configure the diffractive optical apparatus so that the above condition is satisfied.
When this condition is satisfied, diffracted light of orders other than 0th order and + 1st order cannot be generated. Therefore, when each of the above conditions is satisfied, only + 1st order diffracted light is generated, and the diffraction efficiency is maximized. Can be increased to the limit.

  In the diffraction grating of the present invention, by providing a polarization control film that adjusts the phase of TE-polarized zero-order light on the incident light side slope of the groove, the zero-order light is weakened and the first-order diffracted light is strengthened. Thereby, the diffraction efficiency difference with respect to both TE mode and TM mode polarized light can be reduced, and a very high diffraction efficiency can be maintained.

  The diffractive optical element targeted by the present invention is a diffraction grating in which a large number of parallel grooves are arranged at equal intervals on a substrate, and a reflection type diffraction provided with a reflective film for reflecting light in the incident wavelength range used on the surface. It is a lattice.

  An ideal form of the diffraction grating of the present invention is shown in FIG. The diffraction grating 10 has two groove side surfaces 14 and 16 which are inclined with respect to the surface 22 of the substrate 20, and a reflective film 40 is provided on the entire surface. Furthermore, the transparent body film 50 is provided only on the groove side surface 14 on the side irradiated with the incident light 100.

The transparent body film 50 provided on the reflective film 40 on the groove side surface 14 on the side where the incident light 100 of the diffraction grating is irradiated has a refractive index n and a film thickness t in a direction parallel to the incident light. , For operating wavelength λ
t = λ / 4n (1)
It is set to satisfy. This film behaves as an optical film having an optical film thickness of λ / 4 provided on the metal reflecting surface, and has the effect of a non-reflective film that cancels the 0th-order diffracted light that is reflected on the substrate surface of the diffraction grating. As a result, the 0th-order diffracted light is weakened, and the 1st-order diffracted light is relatively strengthened.

  FIG. 2 shows a parallel light beam 102 that can be incident on one groove side surface 14 of the diffraction grating at an incident angle α and a parallel light beam 104 in which the light beam is reflected as zero-order diffracted light. Here, the optical path length difference between the longest optical path 202 (represented by a solid line) and the shortest optical path 204 (represented by a broken line) from the plane I perpendicular to the incident parallel light beam 102 to the plane O perpendicular to the reflected parallel light beam 104. Is equal to the wavelength λ of the incident light.

  The optical path length in the light beam continuously changes between the longest optical path 202 and the shortest optical path 204. This means that the phase difference continuously changes from 0 to 2π from the shortest optical path 204 to the longest optical path 202. Therefore, when the entire light beam 104 of the 0th order diffracted light is integrated, it becomes 0, and the 0th order diffracted light exists. It is a condition that does not. Accordingly, the incident light travels toward the first-order diffracted light, and high first-order diffraction efficiency can be obtained.

  With such a configuration, an effect of canceling the TE-polarized zeroth-order diffracted light is obtained, and the intensity of the first-order diffracted light is increased. As a result, it is possible to obtain a diffraction grating with high light utilization efficiency and low polarization dependence.

In addition, in order for the 0th-order diffracted light to have a continuous phase change, it is necessary to consider the condition that the 0th-order diffracted light exists geometrically. Since the emission angle β ₀ of the _0th-order diffracted light is equal to −α, the direction of the light beam 104 of the 0th-order diffracted light is below the surface of the groove side surface 14 as shown in FIG. In this case, zero-order diffracted light cannot exist. Therefore, the incident angle and groove shape must be such that zero-order diffracted light exists.

The incident angle of the parallel light beam 102 incident on the diffraction grating with respect to the surface 24 parallel to the surface of the substrate is α (°), and the groove side surface 14 of the diffraction grating on which the parallel light beam 102 enters is inclined with respect to the surface 24 parallel to the substrate surface. When the angle is θ (°),
α ≦ 90−θ
If the optical system is designed so that the above relationship holds, the 0th-order diffracted light is emitted above the groove side surface 14.

  Further, in order for the 0th-order diffracted light to have a continuous phase change, it is necessary to have a use condition in which the incident light beam 102 irradiates only one side surface of the groove and does not irradiate the opposite side surface. That is, as shown in FIG. 4, the arrangement in which the light ray 206 at the end of the incident light beam 102 is incident on the groove side surface 16 is not desirable.

In order to avoid this, assuming that the angle between two planes forming the groove side surface of the diffraction grating is θ ′ (°),
θ ′ ≦ 90 + α−θ
The optical system may be configured so that

  In addition, as shown in FIG. 2, a use condition is required in which all the light beams 104 of the 0th-order diffracted light are emitted from the side surface 52 of the transparent body coating 50. As shown in FIG. 5, it is not preferable that a part of the 0th-order diffracted light beam 104 is emitted from the surface 51 parallel to the groove side surface 14 of the transparent body coating 50.

Of course, it is desirable to use such a diffraction grating under conditions where only 0th-order and 1st-order diffracted light exists. Under the condition that second order or higher order diffracted light exists, a part of the incident light becomes higher order diffracted light, which is not desirable. In a diffraction grating in which N parallel grooves per 1 mm are arranged at equal intervals on a substrate, when light having a wavelength λ is incident at an incident angle α, the diffracted light for the diffraction order m is
sinα + sinβ _m = Nmλ (2)
Is obtained in the direction of the angle β _m such that

Therefore, the angle β ₀ of the _0th -order diffracted light (m = 0) is β ₀ = −α as described above.
The angle β ₁ of the _first -order diffracted light (m = + 1) is
β ₁ = −sin ⁻¹ (Nλ−sin α)
It is. Here, as for the sign of m, the order of the diffracted light generated on the incident light side with respect to the 0th order diffracted light is positive as shown in FIG.

For m other than 0 and +1, it is desirable that there is no β _m satisfying the expression (2). That is, for integers other than m = 0 or +1
| Sinβ _m | = | Nmλ−sinα | ≧ 1
It is desirable to satisfy the following conditions.

Next, the results obtained by calculating the characteristics of the diffraction grating having the above-described structure by electromagnetic wave analysis by the finite element method will be described.
In order to obtain the optical path length by expressing the incident light and the 0th-order diffracted light as light rays as described above, the following approximations and limitations on the groove shape and use conditions of the diffraction grating were performed.

  First, in the diffraction grating of the present invention, a transparent film having a refractive index n is provided as a polarization control film on the side surface of the groove where incident light is incident. However, the incident light is incident obliquely on the transparent film. The refraction of the optical path that occurs and the effective change in the refractive index are considered to be small in this case and are ignored. That is, as shown in FIG. 2, it is assumed that the optical path of the light incident on the transparent film 50 travels without being refracted in a medium having a refractive index n.

  When the number N of grooves is 900 / mm and 1100 / mm, when incident light having a wavelength λ in the range of 1270 nm to 1925 nm is incident at an incident angle α in the range of 44 ° to 60 °, Calculations were performed under conditions that would satisfy the conditions.

  In the calculation, the film thickness t of the transparent body film is made constant and the refractive index n of the film is adjusted so that the optical film thickness nt becomes equal to λ / 4 for each wavelength λ. To be satisfied. Of course, the refractive index may be kept constant and the film thickness may be adjusted. For comparison, the calculation is also performed for the case where there is no transparent film. In either case, the incident angle condition was set so that second-order or higher diffraction would not occur.

  The calculation results when N = 900 lines / mm and the incident angle α is 44 °, 52 °, 60 ° are shown in Tables 1 to 3 and FIGS. 6 to 8, N = 1100 lines / mm, α = 44 °, The calculation results in the case of 52 ° and 60 ° are shown in Tables 4 to 6 and FIGS.

  The optical system corresponding to FIG. 6 and Table 1 in which N = 900 lines / mm and the incident angle α is 44 ° is a so-called Littrow where the diffraction angle of the first-order diffracted light is substantially equal to the incident angle when the incident wavelength is 1550 nm. It is an optical system. This system is often used in the optical communication field.

  Referring to FIG. 6, it can be seen that the effect of the present invention is remarkably obtained in this system. In the absence of the transparent film, the diffraction efficiency for TM polarized light is about 95%, whereas the diffraction efficiency for TE polarized light is less than 20%, and the polarization dependent loss is as large as about 7 dB.

  On the other hand, when a transparent body film is provided, the diffraction efficiency is about 90% for both TE-polarized light and TM-polarized light, and the PDL is 0.5 dB or less. This characteristic has little dependency on the wavelength, and it can be said that the effect of the present invention is remarkably exhibited.

  Of course, also in the incident angle away from this system and the number of grooves of the diffraction grating, as shown in FIGS. 7 to 11, the polarization dependence characteristics are greatly improved as compared with the case where there is no transparent film. When the incident angle α is set to 52 °, it is out of the Littrow optical system, but as shown in FIG. 7, the diffraction efficiency slightly decreases on the short wavelength side and the polarization dependence increases, but there is no transparent film. Compared to this, it shows good characteristics. Similarly, when the number of grooves is 1100 / mm, the effect of the present invention is also noticeable (see FIGS. 9 and 10).

From these simulation results, it was shown that a diffraction grating having sufficiently low polarization dependence characteristics can be provided by providing a transparent film, that is, a polarization control film.
Note that each of the above conditions is an ideal condition, and in fact, a sufficient effect may be obtained even if the conditions are slightly deviated from these conditions.

  FIG. 12 shows the diffraction grating efficiency for the TE mode when λ is changed from 1270 nm to 1880 nm with respect to the diffraction grating satisfying the expression (1) at the incident wavelength λ = 1550 nm. When λ is in the range of about 1200 to 1700 nm, a diffraction efficiency of 50% or more is obtained. In other words, the effect is maintained even when the incident wavelength is deviated in the range of about 0.8λ to 1.1λ.

In other words, the effect is maintained even when the film thickness t in the expression (1) deviates from λ / 4n by about the following range.
0.8λ / 4n ≦ t ≦ 1.1λ / 4n
Similarly, the case where t and λ are accurate and the refractive index n deviates from the relationship of the expression (1) can be considered similarly. In the case of TM polarized light, since it is hardly affected by the transparent film, the efficiency equal to or higher than that of TE polarized light is maintained.

  Moreover, if a material having a refractive index dispersion that just satisfies the above conditions in the operating wavelength range is used, an effect can be obtained not only in a single wavelength but also in a wide wavelength range.

The transparent film is desirably provided only on the side surface of the groove on the side where the incident light is irradiated. If the transparent body film is also attached to the side surface on the opposite side, the effect of the present invention is slightly reduced. For example, in order to realize the conditions used in the above calculation, the number of grooves N = 900 / mm, the angle formed between the groove side surface irradiated with incident light and the substrate is 32.7 degrees, and the two groove side surfaces are formed. When a TiO ₂ film having a thickness of t = 183 nm is formed on one side of the diffraction grating having an angle of 90 degrees by sputtering in the direction parallel to the incident angle, the film thickness adhering to the opposite side is about 10 nm. (About 0.06 t).

  In this way, the diffraction efficiency when TE polarized light is incident on the diffraction grating having the transparent film attached thereto at an incident angle α = 44 ° is calculated to be 83.8%, and the groove side on the side irradiated with incident light is calculated. Compared to the case where only the coating is present, the decrease in diffraction efficiency is estimated to be 2.1%, and the increase in PDL is estimated to be within 0.1 dB. There is almost no effect on TM polarized light. Therefore, if the film thickness of the transparent body film adhering to the opposite side surface is 1/10 or less, that is, 0.1λ / 4n or less compared to the side irradiated with incident light, sufficient effects can be obtained. I can say that.

  Even if the optical path difference between the shortest optical path and the longest optical path is deviated by about ± λ / 8 (± 0.175λ) from λ, the effect is small, and the effect of the present invention is maintained. That is, it may be in the range of 0.825λ to 1.175λ.

  Further, as described above, it is not preferable that the direction of the light beam of the 0th-order diffracted light as shown in FIG. 3 is lower than the surface of the groove side surface. However, when the incident angles α = 60 ° and θ = 32.7 ° shown in FIGS. 8 and 11, α> 90−θ, and the direction of the light beam of the 0th-order diffracted light is lower than the surface of the groove side surface. Become. In this case, the direction of the 0th-order diffracted light passes through the lower side of the reflective film by a depth of about 0.02λ from the reflective film surface.

  As shown in FIG. 8 or FIG. 11, the diffraction efficiency in this case tends to decrease on the short wavelength side, and the polarization dependence slightly increases. However, on the long wavelength side, a value comparable to that obtained when the 0th-order diffracted light passes through the transparent film is obtained.

  However, when incident light is incident at a shallow angle, that is, at a large incident angle, the characteristics deteriorate rapidly. For comparison, FIGS. 13 and 14 and Tables 7 and 8 show diffraction efficiencies when incident light is incident on a diffraction grating having N = 900 grooves and 1100 grooves / mm at α = 72 °. In this case, the direction of the 0th-order diffracted light is about 0.04λ from the reflective film surface. From this figure, it can be seen that in this case the diffraction efficiency is significantly reduced.

  Therefore, regarding the direction of the 0th-order diffracted light, when the 0th-order diffracted light passes through the depth of 0.04λ from the reflecting film surface, it can be handled equivalently to the case where the 0th-order diffracted light passes through the transparent film. Can be said.

  Incident light irradiation on the side opposite to the side irradiated with incident light is affected even when a light beam equivalent to 1/20 of the width of the incident light beam perpendicular to the incident direction is irradiated on the opposite side surface. Is small and the effects of the present invention can be obtained.

  Further, as shown in FIG. 5 described above, it is not preferable that a part of the light beam of the 0th-order diffracted light is emitted from the surface parallel to the groove side surface of the transparent body coating. However, the system in which the characteristics of FIG. 6 are obtained, that is, the number of grooves N = 900 / mm, the angle formed by the groove side surface to which the incident light is irradiated and the substrate is 32.7 degrees, and the angle formed by the two groove side surfaces is 90 degrees. When a transparent coating film with n = 2.2 is formed on the one side slope of the diffraction grating having a thickness of t = 183 nm, a portion of the light beam corresponding to 0.025 of the light flux width in the 0th-order diffraction direction Is emitted from the upper surface of the transparent film.

  However, even in this case, the effect of the present invention is maintained, and even if a light beam corresponding to 1/20 of the width of the 0th-order diffracted light beam perpendicular to the direction of the 0th-order diffracted light is emitted from the upper surface of the transparent film. It can be said that the effect of the present invention can be obtained.

  Examples of the diffraction grating of the present invention will be described in detail below. First, a method for manufacturing a diffraction grating will be described.

The produced diffraction grating is a diffraction grating 10 in which grooves having a sawtooth cross-sectional shape as shown in FIG. 1 are arranged in parallel at a density of 900 lines / mm. The design shape of the groove in this embodiment is such that the angle formed by the groove side surface 14 on the incident light side and the substrate surface 22 is 32.7 degrees, and the angle formed by the two groove side surfaces 14 and 16 (groove apex angle: θ ′). It was 90 degrees.
The diffraction grating may be manufactured by an arbitrary method, but in this embodiment, a manufacturing method by transfer molding capable of mass-producing a good microstructure at low cost is adopted as an example.

  First, a silicon alkoxide and a sol solution mainly composed of an acid aqueous solution are applied onto the quartz glass substrate 20, and when the obtained gel film is in a soft state, the mold is pressed in a vacuum and kept at about 60 ° C. The gel film was cured. As the material for forming the groove shape, other materials may be used as long as they can withstand the temperature at the time of forming the transparent body coating.

Thereafter, the mold was released from the gel film at atmospheric pressure, and heat treatment was performed at 350 ° C. to obtain a diffraction grating 10 composed of a molded body 30 having a predetermined uneven shape.
An aluminum (Al) film was formed as a reflective film 40 on the surface of the formed concavo-convex shaped molded body 30 by a sputtering method. The reflective film 40 can be formed not only by sputtering but also by vacuum evaporation.

  Al is desirable as a reflective film because it has a high reflectance in a wide wavelength range and has good adhesion to a molding material and a transparent film. Further, the reflective film may have a laminated structure of a plurality of materials in addition to a single layer film such as gold (Au) as long as the material has a high reflectance in the used wavelength range. The film thickness of the reflective film may be any film thickness that does not transmit light in the used wavelength region when incident on the groove slope at the incident angle used. A film thickness of about 50 nm to 300 nm is desirable because sufficient reflectivity can be obtained without impairing the underlying groove shape.

  The diffraction efficiency of the reflection type diffraction grating thus produced was measured. The measurement was carried out with a wavelength of 1550 nm and an angle between the incident light and the first-order diffracted light of 4 ° (incident angle 46.2 °, first-order diffraction angle 42.2 °). The TM mode first-order diffraction efficiency was 96.4%, the TE-mode diffraction efficiency was 24.2%, and the polarization-dependent loss was 6.0 dB.

Next, a transparent film 50 having a phase adjustment effect for canceling the TE mode zero-order diffracted light is provided on the side surface of the groove where the reflective film is formed on the side irradiated with the incident light 100. Titanium oxide (TiO ₂ ) was used as a material for the transparent film. The film was formed in such a design that the film thickness in the direction parallel to the incident angle was 183 nm, the film thickness was λ / 4, and the phase difference of the 0th-order light was 2π.

  It is desirable that the transparent coating material be as transparent as possible in the wavelength range of use in order not only to satisfy the usage conditions of the present invention but also to suppress loss due to absorption.

  The transparent film was formed by sputtering. In order to form a film only on one side of the slope, the target and the side of the diffraction grating that is irradiated with the incident light are placed so that the side of the target is facing directly, and the material is selected only on the side of the groove on one side using the concavo-convex structure. The film was formed so as to adhere.

  The diffraction efficiency of this diffraction grating was measured. The measurement was performed at a wavelength of 1550 nm as described above, and the angle between the incident light and the first-order diffracted light was 4 °. As a result of measurement, the diffraction efficiency of TM mode is 58.2%, the diffraction efficiency of TE mode is 53.9%, and the polarization-dependent loss is 0.3 dB. By providing a transparent film, the polarization-dependent characteristics of the diffraction grating are greatly improved. It was possible to improve. That is, it was confirmed that the transparent film functions as a polarization control film.

  The reason why the efficiency is lower than the simulation is as follows. The simulation was performed on an ideally shaped diffraction grating as shown in FIG. 1A, whereas the actually produced diffraction grating has a shape as shown in FIG. The molded body 32 is slightly dull in the uneven shape. This is due to blunting of the shape of the mold itself and blunting in the molding process. In addition to this, it is considered that the efficiency has decreased compared to the ideal state due to the refractive index shift of the transparent body film 50 and the like.

  Even in such a case, as shown in FIG. 1B, the above-mentioned concept can be applied if a part of the side surface of the groove is a flat surface. Moreover, although the groove shape of the said Example consisted of two side surfaces, it is not restricted to this. The case where the cross section in which the top part of the groove is a plane is trapezoidal or the bottom part of the groove is a flat surface may be used. Also in this case, the present invention can be applied by regarding two surfaces inclined with respect to the substrate surface as the groove side surfaces.

  Further, the film thickness of the transparent body film is desirably uniform, but may be actually distributed. In such a case, the average film thickness over the planar portion of the groove side surface can be regarded as the film thickness t.

  In the diffractive optical apparatus using the reflective diffraction grating of the present invention under the above-mentioned conditions of use, as an optical means for entering parallel light at a predetermined incident angle α into the diffraction grating, for example, emitted from an optical fiber or a light emitting element It is possible to use an optical system that converts the divergent light to be converted into parallel light using a collimating lens system. On the other hand, as the optical means for receiving the first-order diffracted light, an optical system that collects the parallel light emitted from the diffraction grating and enters the optical fiber or the light receiving element can be used.

It is a cross-sectional schematic diagram of the diffraction grating which is embodiment of this invention. It is a figure which shows the light ray of the conditions from which the phase difference of 0th-order diffracted light will be 2 (pi). It is a figure which shows a light ray in case 0th-order diffracted light comes under a diffraction grating reflective surface. It is a light ray schematic diagram in case incident light irradiates the side surface on the opposite side to the groove | channel side of the incident side of a groove | channel. It is a light ray schematic diagram in case 0th order diffracted light radiate | emits from the upper surface of a transparent body film. It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 900 / mm, incident angle 44 degrees). It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 900 / mm, incident angle 52 degrees). It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 900 / mm, incident angle 60 degrees). It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 1100 / mm, incident angle 44 degrees). It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 1100 / mm, incident angle 52 degrees). It is a figure which shows the calculation result of the diffraction efficiency of the diffraction grating of this invention (the number of grooves 1100 / mm, incident angle 60 degrees). It is a figure which shows the calculation result of the wavelength dependence of the diffraction efficiency in the diffraction grating of this invention. It is a figure which shows the comparative example of the diffraction efficiency of a diffraction grating (the number of grooves 900 / mm, incident angle 72 degrees). It is a figure which shows the comparative example of the diffraction efficiency of a diffraction grating (the number of grooves 1100 / mm, incident angle 72 degrees).

Explanation of symbols

10 Diffraction grating 20 Glass substrate 30 Molded body 40 Reflective film 50 Transparent body coating

Claims (6)

N parallel grooves per 1 mm are arranged at equal intervals on the surface of the flat substrate, the side surfaces of the grooves have at least two plane portions inclined with respect to the substrate surface, and the grooves are provided. In a reflective diffraction grating having a reflective layer on the surface of a substrate,
A polarization control film that is transparent with respect to a predetermined wavelength λ and has a refractive index n is formed on one of the plane portions parallel to each other on the side surfaces of the grooves, and the average film thickness is in the range of 0.8λ / 4n to 1.1λ / 4n. A diffraction grating with a polarization control film, wherein the average film thickness of the film on the side surface of the groove other than the above is 0.1λ / 4n or less. 2. A diffraction grating with a polarization control film according to claim 1, optical means for making a parallel light beam incident on a side surface of a groove provided with the polarization control film of the diffraction grating, and optical means for receiving a light beam of primary diffraction light by the diffraction grating; In a diffractive optical apparatus comprising at least
The longest optical path length and the shortest optical path among the optical path lengths of the light beams in the light beam from an arbitrary plane perpendicular to the parallel light beam incident on the diffraction grating to an arbitrary plane perpendicular to the light beam of the 0th-order diffracted light. A diffractive optical apparatus, wherein a difference from the length is in a range of 0.825λ to 1.175λ. When the incident angle of the parallel light beam incident on the diffraction grating with respect to the substrate surface is α (°), and the inclination angle of the groove side surface of the diffraction grating on which the parallel light beam is incident with respect to the substrate surface is θ (°),
α ≦ 90−θ
The diffractive optical apparatus according to claim 2, wherein the relationship is established. When the angle between two planes forming the groove side surface of the diffraction grating is θ ′ (°),
θ ′ ≦ 90 + α−θ
The diffractive optical apparatus according to claim 2, wherein the relationship is established.   3. The diffractive optical system according to claim 2, wherein when the zero-order diffracted light beam is emitted from the polarization control film, all of the light beam is emitted from a side surface that is not parallel to the groove side surface of the polarization control film. apparatus. For diffraction orders m other than 0 and +1 in the diffraction grating, | Nmλ−sinα | ≧ 1
The diffractive optical apparatus according to claim 2, wherein the following condition is satisfied.

Images