JPH043845B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for efficiently coupling light waves to an optical waveguide.

[Prior Art] In recent years, the use of integrated optical circuits has attracted great interest in optical communication systems, optical data processing, and various other systems that utilize light. The advantage of integrated optical circuits is that they can be made smaller and lower in cost. The use of optical waveguides can be considered as one of the devices for realizing the integration of optical circuit systems, but the thickness of optical waveguides is usually about the same as the wavelength of the light to be transmitted, so it is difficult to The reality is that it is quite difficult to efficiently introduce light into optical waveguides (in addition, there is an urgent need to develop systems that can process data at higher speeds; In order to achieve higher speeds, it is conceivable to utilize the interaction between waves (e.g. surface acoustic waves) excited at higher frequencies (in the GHz range) and the optical signals to be transmitted in integrated optical circuits. The difficulty in introducing light caused by the thinning of optical waveguides that accompanies this process is unimaginable.)

Conventionally, it is known to use internally reflecting prisms to couple light waves through the surface of an optical waveguide. Although such prismatic coupling has been found to be much more effective than the so-called Butt coupler, in which light is introduced through the end of the optical waveguide,
It is known that unless the thickness of the air gap layer interposed between the optical waveguide and the prism is stably maintained at intervals of 1/5 to 1/10 of the wavelength of the light wave, coupling will easily deteriorate. Furthermore, relatively speaking, prism couplers are difficult to miniaturize to some extent due to their dimensions. Reducing the dimensions of a coupler is of great significance for miniaturizing integrated circuits and lowering their cost, and for this purpose, optical couplers in which a volume hologram grating is provided within an optical waveguide are known. ing.
This is because light incident from the direction across the waveguide is reflected and diffracted by the grating within the optical waveguide, and by selecting an appropriate angle of incidence, the diffracted light is captured within the optical waveguide and is incident on the waveguide. Specific optical waveguide modes can be induced and coupled depending on the angle and periodicity of the grating.

Compared to prism coupling, these methods do not require an intervening air gap layer, so once they are formed within the optical waveguide using technology such as holography, there is no need for readjustment, and they can achieve the same coupling efficiency as prism coupling. Therefore, it can be said that it is superior to the prism method. However, it is always desirable to achieve even higher coupling efficiency in order to increase the efficiency of light utilization. In general, in a volume hologram formed in an optical waveguide, the grating layer cannot be thick enough, and therefore it is difficult to manufacture a diffraction grating coupler that satisfies Bragg diffraction conditions. As mentioned above, when interacting with light using high frequencies, higher frequencies (several to several tens of G) are used.
Hz), the thinner the optical waveguide must be, the thinner the optical waveguide must be, and the thickness of the optical waveguide must be less than 1μ. Therefore, when considering the volume hologram in the optical waveguide, it is generally difficult to produce a hologram that satisfies high coupling efficiency. To explain in more detail, the diffraction efficiency η of light by a volume hologram can generally be described as η=f(Δn, p, h), and the diffraction efficiency η is a function of Δn, p, h. Here, Δn is the refractive index difference that may occur within the hologram material, p is the pitch of the volume hologram grating, and h is the thickness of the hologram layer. The refractive index difference △n that can occur due to photolithography, electron beam writing, etc. is about 10 ^-3 to 10 ^-2 ,
Also, the pitch is about 1μm with photolithography, and 0.5μm with E13 exposure, and even with holography.
It is approximately 0.2 μm, and in order to increase the diffraction efficiency η as much as possible, it is most effective to increase the thickness of the hologram layer. Therefore, in the conventional method of installing a volume hologram grating inside an optical waveguide in order to couple light from the outside, the thickness of the optical waveguide (i.e., the volume hologram grating in this case) must be adjusted in order to efficiently introduce and couple light. Thickness of coating) to 5
On the other hand, the optical waveguide should be made as thin as possible, less than ~1 μm, in order to efficiently interact with the light waves propagating within the optical waveguide and other waves excited at high frequencies. We are faced with contradictions that must be suppressed and created.

[Problems to be Solved by the Invention] On the other hand, Japanese Patent Laid-Open No. 47-5745 proposes an optical coupler in which a volume hologram grating is formed on a waveguide. However, in such an optical coupler, since light is directly introduced from the outside, it is necessary to reduce the pitch of the grating in order to match the propagation constant of the air and the propagation constant of the waveguide. Furthermore, since the sensitive material used to create holograms has a limited resolution, attempting to create gratings with such a small pitch will reduce the diffraction efficiency and ultimately the coupling efficiency to the waveguide. It was hot.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art described above and to provide an optical coupler that is compact and has high coupling efficiency.

[Means for Solving the Problems] The above-mentioned object of the present invention is to provide an optical coupler in which a volume hologram is formed on a waveguide formed on one surface of a substrate. This is achieved by providing a reflective layer for reflecting light incident through the substrate and coupling it to the waveguide.

[Example] FIG. 1 is a diagram showing an example of the present invention. In the optical coupler of the present invention, the reflective volume hologram 33 placed on the surface of the waveguide 11 opposite to the light incident surface and in contact with the layer 12 having a lower refractive index than the optical waveguide allows Coherent light is coupled into the optical waveguide. This reflective volume hologram is
Transmissive volume hologram and metal reflective layer 43 thereon
Consisting of The incident light passes through the optical waveguide 11 and the low refractive index layer 1.
After passing through the optical waveguide 2, the reflected light is reflected and diffracted by the grating 14 and the metal reflective layer 43, and a part of the reflected light is captured in the optical waveguide with one or more orders of diffraction depending on the angle of incidence of the incident light. Ru. The light thus captured has a constant propagation constant within the longitudinal direction of the optical waveguide and induces a particular optical waveguide mode, depending in particular on the angle of incidence of the incident light and the periodicity of the grating. and join.

In FIG. 1, 16 is a light source such as a helium or neon laser that has a coherent output light of 0.6328 μm. Of course, any other light source can be used, and the specific light sources described here are merely provided for purposes of illustration. Lens system 1 to focus the light if necessary
5 may be used. An optical waveguide 11 formed of any suitable transparent optical waveguide material such as alumina (Al ₂ O ₃ ) is mounted on a glass substrate 10 suitable for the wavelength of the light to be coupled. The refractive index of the optical waveguide 11 and the substrate 10 is n ₂
It is desirable that the selection be made to satisfy the relationship ＞n ₃ ＞n ₁ . However, here, n ₂ is the refractive index of the optical waveguide 11, n ₃ is the refractive index of the substrate 10, and n ₁ is the refractive index of the outside world (air).

A transparent volume hologram 13 of the device shown in FIG. 1 is mounted on an optical waveguide 11 formed on a substrate 10 via a low refractive index layer 12. The refractive index of this low refractive index layer 12 is lower than that of the waveguide 11 and the hologram 13. The transmission volume hologram 13 is the second
As shown in the figure, by the holographic method,
For example, a plane wave object beam 18 and a plane wave reference beam 19 are placed in PVK (polyvinyl carbazole) 23.
is made incident through the prism 20. In this case, a grating 14 as a refractive index distribution is formed. PVK (polyvinylcarbazole) 23, which is a sensitive material for holograms, becomes a coatable gel by adding the same solution of CI ₄ to a chlorobenzene solution of PVK. After forming Al ₂ O ₃ by evaporation or sputtering to a thickness of 0.5 μm, and further evaporating the low refractive index layer 12 to a thickness of 0.1 μm, PVK as a hologram material was formed using a spinner at 1000 rpm and 10 seconds. The film thickness was set in a range of 1.5 μm to 10.0 μm, but in the case of this example, a thickness of 5 to 10 μm was most preferable in order to increase the diffraction efficiency from the created hologram grating. The performance of PVK as a hologram sensitive material is in the spectral sensitivity range ~540nm,
Sensitivity >10mJ/ ^cm2 (λ=0.488μm), maximum diffraction efficiency
The transmittance was 83.1%, the transmittance was 85.1%, and the resolution was >3400 leie/mm. Between the prism 20 and PKV 23 is a medium 21 (for example, α
- iodonaphthalene) may be used as a matching material. Originally, matching is done by setting the refractive index of the photosensitive material 13, the glass substrate 10, the prism 20, and the absorption plate 3 to be equal, and also by setting the refractive index of the photosensitive material 13 and the prism 2
0. An immersion liquid having the same refractive index is provided at the boundary between the absorption plates 3 so as not to cause reflection and scattering at the boundary. Here, we will create a volume hologram.
A 0.488 μm argon laser was used.

Although PVK was used as the medium as the hologram material when forming the hologram in Figure 2, it is of course possible to use other hologram materials, and the specific examples described here are merely for illustrating one example. This is just a demonstration of PVK as a holographic material. As another specific example of a volume type hologram coupler, LiNbO ₃ (lithium niobate) is used as the substrate 10, and MgF ₂ (lithium niobate) is formed on the optical waveguide 11 formed by Ti diffusion thereon. In particular, As ₂ S ₃ (arsenic trisulfide) may be used as the hologram material via the low refractive index layer 12 of magnesium. In this case, as an optical waveguide
It is known that an optical waveguide formed by thermal diffusion of a Ti film deposited on LiNbO ₃ has an optical propagation loss of about 0.5 dB/cm, which is significantly smaller than that of an Al ₂ O ₃ optical waveguide. It is more effective for the purpose.

The volume hologram grating created in this manner forms an angle with respect to the normal 4 perpendicular to the optical waveguide 11 and the substrate 10 in FIG. 1, and has a period of pitch p. In FIG. 1, the substrate 10 is made of BK7 glass (n ₃ = 1.52), the optical waveguide 11 is made of Al ₂ O ₃ (n ₂ = 1.63) deposited to a thickness of 0.5 μm, and the low refractive index layer 12 is made of MgF ₂ (n ₄ = 1.38), using PVK (n=1.64) as the hologram material 13, a grating 14 with respect to the normal line 4 in order to input a 0.6328 μm helium neon laser from the direction of the normal line 4 and couple and capture the light to the optical waveguide 11. Find the pitch p that satisfies the tilt angle and the bragging condition. From the theory of optical coupling, the angle y at which the light beam is incident on the interface between the hologram 13 and the low refractive index layer 12 is expressed as y=
If the angle is set to 74.6688°, the light with the wavelength λ diffracted from the hologram will be coupled to the optical waveguide 11. At this time,
There is a relationship between the refractive index n ₅ of the hologram medium, the pitch p, and the tilt angle of the grating 14 with respect to the normal 4 as follows: λ/n ₅ = 2p sin∴p = λ/2n ₅ sin, = y/2 =37.3344゜, _n5 =1.64,
By substituting λ=0.6328 μm, we obtain a value of p=0.3181 μm. In this embodiment, as shown in FIG. 2, if a volume hologram is formed by the holographic method, a desired hologram grating coupler can be obtained, and the obtained holographic grating coupler is a light guide. Any thickness can be selected without depending on the thickness of the wave path 11, and therefore it can be formed to a thickness of several μm to several tens of μm, preferably 5 μm to 10 μm, and a high diffraction efficiency η can be achieved. It is.

The hologram material was As ₂ S ₃ (n ₅ = 2.52), the low refractive index layer 12 was MgF ₂ (n ₄ = 1.38), and the substrate 10 and optical waveguide 11 were LiNbO ₃ (n ₃ =
2.20) and Ti-diffused LiNbO ₃ (n ₂ = 2.22), the inclination angle with normal 4 =
Obtain the values 30.7856, y=61.5712, p=0.2453 μm.

The novelty of the present invention can be better understood by comparing the conventional internal reflection prism coupler shown in FIG. 3 with the volume hologram grating coupler according to the present invention shown in FIG. That is, the air gap layer in FIG. 4 corresponds to the low refractive index layer 12 in FIG. Therefore, the conditions for maintaining the air gap layers that occur when using a conventional prism coupler at a constant interval can also be achieved by arbitrarily selecting the thickness of the low refractive index layer 12 by vapor deposition when creating the coupler. Once created, the thickness of the gap layer can be fixed over the entire bonding surface. This provides a novel coupler that does not require any adjustments that would be unthinkable with conventional prism couplers.

On the other hand, the end face 27 of the prism coupler 7 shown in FIG. Even when using a hologram, the optical waveguide 11 spanning the hologram grating coupler 13 and the low refractive index layer 12 shown in FIG.
It is desirable to make the end face as perpendicular as possible. To do this, the PVK hologram material and the MgF ₂ low index layer are removed by plasma etching in O ₂ or CF ₄ or a mixture thereof using a suitable mask. Further, when As ₂ S ₃ is used as the hologram material, a conventional chemical etching method can also be applied. In this case, first, a photoresist layer is placed on the hologram material 13 of As ₂ S ₃ using a spinner or the like, exposed to light, and ordinary chemical etching is performed. After forming the end face 17 perpendicular to the optical waveguide 11 on the volume hologram 13 and the low refractive index layer 12 in this manner, the photoresist layer is chemically removed to obtain the end face 17 as shown in FIG. As a result, the coupling efficiency in the optical couplers of these embodiments is increased.

[Effects of the Invention] As explained above, the present invention provides a conventional optical coupler with a reflective layer on top of the volume hologram to couple light to the waveguide via the substrate. This method requires only a large pitch of coupling, and has the effect of improving coupling efficiency.

In addition, since the light is incident from the substrate side, when introducing convergent light, the beam diameter at the back side of the substrate is larger than the beam diameter at the part where it enters the hologram, and scratches, dust, etc. on the back side of the substrate It also has the effect of having a small effect on

Furthermore, the reflective layer can prevent the light beam that has not been diffracted by the hologram from passing through to the side opposite to the substrate and becoming stray light.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating an optical coupler according to an embodiment of the present invention, FIG. 2 is a diagram illustrating the creation of a volume hologram grating, and FIG. 3 is a diagram illustrating a conventional prism coupler. In the figure, 10... substrate, 11... optical waveguide, 12
...Low refractive index layer, 13...Hologram material, 14...
... Hologram grating, 15 ... Condensing lens, 16 ... Light source, 23 ... Metal reflective layer, 18 ...
...object light, 19...reference light, 20...prism,
21...Immersion liquid, 17...End face, 7...Prism, 8...Air gap layer.

Claims (1)

[Scope of Claims] 1. In an optical coupler in which a volume hologram is formed on a waveguide formed on one surface of a substrate, light incident on the volume hologram via the substrate is reflected. An optical coupler characterized in that a reflective layer is provided for coupling the waveguide to the waveguide.