JPH01179919A - Nonlinear optical element - Google Patents

Abstract

PURPOSE:To enable good operation with low power by providing reflecting mirrors consisting of multilayered dielectric films which form an optical resonator with a nonlinear crystal in-between and further providing transparent electrodes on both side faces of the reflecting mirror. CONSTITUTION:The reflecting mirrors 12, 13 consisting of the multilayered dielectric films forming the optical resonator in common use as insulating layers are provided on both side faces of the nonlinear medium 11 having photoconductivity and electrooptic effect to input light 17 and further the output light has a differential gain characteristics or history characteristic with respect to the input light from the side faces of the transparent electrodes 14, 15 provided on both side faces on the outside of the reflecting mirrors. Namely, this element consists of the mechanism to impress a high voltage to the transparent electrodes from the outside, to generate induced refractive indices in the nonlinear crystal and to decrease the induced refractive indices according to the intensity of the input light thereafter. The good operation to the sufficiently low intensity of the input light is thereby enabled.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to nonlinear optical elements, and is particularly applicable to optical modulators, optical memories, and optical switches in various fields such as optical computers and optical communications that use light as an information medium. The present invention relates to a nonlinear optical element suitable for use as an optical functional element such as an optical amplifier, an optical threshold element, or an optical logic operation device.

(Prior Art) Various types of nonlinear optical elements-t have been proposed as elements that produce stable states of two different optical outputs for the same optical input intensity.

FIG. 2 is a schematic diagram of a conventional nonlinear optical element. In the figure, 21 is a nonlinear medium exhibiting nonlinearity in absorption, dispersion, or both with respect to the incident light intensity, and 22 and 23 are the predetermined transmittances forming an optical resonator containing the nonlinear medium 21 therein. It is a reflecting mirror with

In the nonlinear optical element shown in the figure, when incident light from one side passes through a reflecting mirror 22 and enters into a nonlinear medium 21, the incident light is absorbed by the nonlinear medium 21, dispersion, etc.
3, some of them are transmitted, others are reflected, and return to the nonlinear medium 23 again.

At this time, if parameters such as the phase difference of the light during the round trip within the optical resonator and the reflectance of the reflecting mirror are appropriately set, nonlinearity will appear in the human output characteristics of the light.

For example, as shown in FIG. 3, the intensity of light incident on the nonlinear optical element 21 is . When it gradually increases from 0 to a certain point, a phenomenon appears in which the transmitted light intensity It suddenly increases.

This characteristic is generally called a differential gain characteristic.

In addition, depending on the settings of the parameters, for example, the fourth
As shown in the figure, it is also possible to provide a so-called hysteresis characteristic in which the characteristics when the incident light intensity I0 is increased and the characteristics when it is decreased are different. Note that the hysteresis characteristic is sometimes called non-optical bistability.

Nonlinear optical elements having such characteristics can be widely used as functional elements that use light as a medium, such as memories, switches, amplification, logic operations, optical control, and the like.

The operating mechanism of nonlinear media can be roughly divided into two types: one in which the level of electrons changes due to the incidence of light and the refractive index or absorption coefficient changes, and the other in which the refractive index or absorption coefficient changes due to the incidence of light. Heat is generated due to this, and the refractive index or absorption coefficient changes. Those that utilize thermal effects tend to operate at a slightly lower power than those that utilize electronic effects, but they have problems such as slow operating speed and lack of stability.

On the other hand, those that utilize electron effects are capable of high-speed operation, but have a problem in that the operating power is large, and changes in refractive index, absorption coefficient, etc. are small, so the width of nonlinear operation is small.

In general, the biggest problem with nonlinear optical elements, which include a nonlinear medium in an optical resonator and use a reflective mirror in the optical resonator for optical feedback, is that the operating power is large. There is.

For example, Zn5c, which is said to be capable of optically bistable operation at room temperature and relatively low power, and GaAlAs.
Even in this case, a power density of several hundred W/c12 or more is required.

If the operating power is large, the stability of the nonlinear optical element will deteriorate, and the generation of thermally excited electrons will increase lateral diffusion, which will deteriorate the element resolution when processing two-dimensional information in parallel, for example. . Furthermore, the relaxation time becomes longer and the device operation cycle becomes shorter. Furthermore, when working with other elements with lower operating power, it is necessary to attenuate the operating power of the other elements, resulting in various problems such as energy loss and heat generation in the entire system.

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the problem of extremely large operating power in conventional elements, and to provide a nonlinear optical element with a simple configuration that can operate well with low power. shall be.

(Means for solving the problem) A reflective mirror made of a dielectric multilayer film that forms an optical resonator that also serves as an insulating layer on both sides of a nonlinear medium that has photoconductivity and electro-optic effects for input light. Further, transparent electrodes are provided on both outer sides of the reflecting mirror for applying an electric field to the nonlinear medium, and output light has differential gain characteristics or hysteresis characteristics with respect to input light from the sides of the transparent electrodes. That's true.

(Embodiment) FIG. 1 is a schematic diagram of an embodiment of the present invention. In the figure, 10 is a nonlinear optical element, 11 is a nonlinear medium having photoconductive and electro-optic effects, and 12 and 13 are dielectric multilayer films that also serve as an insulating film or contain an insulating film inside and form an optical resonator. They are reflecting mirrors and are provided on both sides of the nonlinear medium 11. 14 and 15 are transparent electrodes, and 16 is an external power source.

In this embodiment, with respect to the input intensity of the incident light 17, the output intensity of the transmitted light 18 and the reflecting mirror 19 produces a differential gain characteristic or a history characteristic.

Next, we will explain the process of deriving the operating mechanism of the nonlinear optical element 10 according to the present example. Change in amount (b) This will be explained separately in two processes: the incident light intensity-output light intensity (transmitted and reflected light intensity) characteristics caused by the change in the phase delay amount.

First, the motion analysis model of the nonlinear optical element in the process (a) will be explained with reference to FIG.

As the nonlinear medium 11 having a thickness D, an electro-optic crystal having a large photoconductivity and electro-optic coefficient is used. For example B50
(Bi1251020), BTO (Bi12 Ti0
20).

BGO(lli, □GeO2,), BaTi0. ,,
SBN ((Ba, 5r)Nb20.).

GaAs, Zn5e, CdTe, LiNbO3, etc. are used (hereinafter, 11 is also referred to as electro-optic crystal). A dielectric multilayer reflector in which layers with a high refractive index and layers with a low refractive index are alternately laminated with the electro-optic crystal 11 sandwiched therebetween. 12.1
3 is formed.

In this embodiment, the reflecting mirrors 12 and 13 also serve as an insulating film or include an insulating film therein. Furthermore, reflector 1
Transparent electrodes 14.15 are provided on both outer sides of 2.13.

In this embodiment, the reflectance, transmittance, light absorption, etc. of the entire structure including the transparent electrodes 14 and 15 and the dielectric multilayer film reflector are appropriately set.

As the dielectric multilayer film, for example, MgF2 (n=1.38
), S i 02 (n-1,46), A 1
20:+(n-1,62), MgO(ni,7
5), T h 02 (n-1, 8), S io
(n-1,7~2.0), Zr02(n-2,
1) , Ce02(n-2,2), Ti0z(n=
2.2 to 2.7) etc. can be applied. In addition, the transparent electrode film may be organic or inorganic, for example, S not
(n-1,9), I n2 03 (n-2,
0).

ITO (synthesis of Sn02 and In,03), etc. can be applied.

FIG. 6 shows the electro-optic crystal 11 and the optical fiber fif' which also serves as an insulating film by applying a voltage from the external Ti source 16 in FIG.
It shows the electric field applied to the reflecting mirrors 12 and 13 forming +3. The figure shows a state in which no light is incident, and in this case, an external electric field is directly applied to the electro-optic crystal 11.

Next, as shown in FIG. 7, when incident light 61 enters, carriers 62 excited in that region move due to the external electric field, are trapped at the interface with the reflecting mirrors 12 and 13, and become space charges. Therefore, within the electro-optic crystal 11, the space charge becomes an internal electric field that acts in a direction that cancels out the electric field applied from the outside. As described above, the electric field in the electro-optic crystal 11 is determined according to the light intensity in the electro-optic crystal 11. The electric field within the electro-optic crystal 11 at this time is as follows.

Carrier density N (cm-') caused by optical excitation in the crystal
・5-1) is quantum efficiency η, incident photon energy hν
, the light intensity within the crystal tc (訃c+o-''), and the absorption coefficient α(cl'), it can be expressed as follows.

For the purpose of the present invention, a sufficient amount of the light incident on the electro-optic crystal 11 needs to be emitted as transmitted or/and reflected light. For this purpose, the thickness of the electro-optic crystal 11 is usually considered to be thin compared to the absorption length of light, and the generated carriers are separated without loss and trapped on both surfaces of the electro-optic crystal 11, and the distribution of carriers is considered to be only on the surface. is reasonable.

Then, as shown in Figure 7, the voltage v0 across the optical bistable element is
Electro-optic crystal 11 when light is incident with
The carrier density distribution at the interface between the reflector 12 and 13 is σ. = 00, σ−2 −00 However, a o (c◆cm−”)= e f NdL
--------H(2)e: Unit charge (C). As shown in FIG. 7, the potentials in the three regions A, B, and C are set to φ8. φb, φ. Then, the coefficients A, B, and C are the initial potentials of each layer, and the coefficients Ao, B
, , C0 is the electric field in each layer.

First, from the above condition that the potential is continuous, on the other hand, from the boundary condition of the electric field, the following equation is obtained.

Each coefficient is determined by solving equations (4) and (5).

The required value now is the electric field B0 applied to the electro-optic crystal.

Electric field B. If E is, then Equations (4) and (5) are obtained.

As described above, the electric field inside the crystal can be determined from the externally applied voltage of the optical bistable element and the internal light intensity of the electro-optic crystal 11 using equations (1), (2), and (6).

Next, we will discuss how the refractive index of the electro-optic crystal 11 changes depending on the electric field.

The state of refractive index change differs depending on the electro-optic crystal 11, but here, the BSO (fish school 23) and Ga
As, CdTe (fish school 43m) will be explained. All of the above crystals are optically isotropic crystals. Therefore, it does not exhibit birefringence in its natural state. However, by applying an electric field, it becomes birefringent.

In this embodiment, the direction in which the light travels and the direction of the electric field are the same as shown in FIG. 8, which is a so-called vertical operation. By applying an electric field, the coordinate system is rotated by 145° with respect to the original crystal axes x and y, centering on the two axes, as shown in Figure 8, and new positions are created on the A principal axis appears.This can be expressed as the principal axis coordinate system (x', y
'.

2), and if Δn is the refractive index change with respect to polarized light in the x' and y' axis directions at that time, the electro-optic coefficient γ41 is used.

Here, nd is the refractive index of the crystal in its natural state.

In the case of linearly polarized light with a polarization plane on the x' axis in Figure 8,
As the light intensity IC inside the optical cavity changes, the refractive index changes to nd+
It changes from Δn(ao−0) to nd. When the plane of polarization is on the y-axis, conversely, from nd-Δn (ooJ) to n
It changes up to d.

Regarding other polarization components, from nd-Δn to nd+Δn
A change in refractive index occurs within a range of

Next, the process (b) mentioned above will be explained using FIG.

As shown in FIG. 5, the refractive index of each layer constituting the reflecting mirror 12, 13 is determined from the incident light 17 side as follows: no+nI, n2
+ "", film thickness is d, , d2. -(nm), the wavelength of the incident light is λ(nm), and the intensity of the incident light is 1°(
cm-2), and the transmitted light intensity is 11 (Cff1-
2), the reflected light intensity is 1. (Cl1l-2), the internal light intensity within the electro-optic crystal ll is I C (W-cm-"). Also, the reflection coefficient between each layer is r! + 2 + "
”, transmission coefficient tl.

Let it be t2.

Using the above parameters, the intensity transmittance and intensity reflectance of the so-called Fabry Bellows resonator having the configuration shown in FIG. 5 are determined. According to the matrix method using Fresnel coefficients, the amplitude reflectance R' and amplitude transmittance T' of the entire nonlinear optical element are expressed as follows.

However, CIl and C21 are calculated as follows.

Transmission side multilayer reflector 1.j In the above, ji+ is the phase difference of each layer Further, α is the absorption coefficient of the electro-optic crystal.

Therefore, from equations (8) and (9), the intensity reflectance R1 and the intensity transmittance T are expressed as follows.

The refractive index (n9) of the electro-optic crystal included in C, , C2, in equations (12) and (13) changes with the internal light intensity IC as explained in the process (a) above, so I
It is a function of C. That is, n9: nd + Δ n (I c ) =
......-(14).

Next, from the feedback conditions on the transmission side and reflection side, the internal light intensity IC and the reflected light intensity 1. ,,The relationship between the transmitted light intensity lt is shown in FIG. 5, where the thickness d9 of the electro-optic crystal 11 is represented by D, It =A・■c...
......(15) ■, =■. -B-IC...
・・・・−(16) However, ・・・・・・・−−−−−(+7) ・・・・・・・・・−(ta)

RB in the formulas (+7) and (+8) is the intensity reflectance of the multilayer mirror on the transmission side, and is determined as follows.

First, if we express the electric field coefficient as in equation (10), then
Using the coefficients of equation (19), it becomes.

Internal light intensity from equations (13), (15), (+2) and (16), respectively! 6, the relationship between the incident light intensity I0 and the transmitted light intensity 1t and the relationship between the incident light intensity I0 and the reflected light intensity 11 are obtained. The obtained relationship is given by a transcendental equation, and an incident-transmitted light intensity characteristic as shown in FIG. 10 is analytically obtained. Differential gain characteristics and history characteristics can be obtained by setting parameters. In the history characteristic, a state with a negative slope actually indicates a stable state, either upward or downward, as shown by the arrow in the figure.

Similarly, the incident-reflected light intensity as shown in FIG. 11 can also be obtained.

Next, a first embodiment of the production of a nonlinear optical element according to the present invention will be described. Approximately 500 μm thick with crystal orientation <001> plane
mgl'2/Z on both sides of the BSO board in order of proximity to the BSO.
rO2/ MgF2 / ZrO2/ MgF2 /
A film of ZrO2/MgI+2/ZrO2 was formed to serve as a reflective mirror for a dielectric multilayer film, and a Fabry-Bello type optical resonator was formed with BSO sandwiched therebetween.

The thickness of each film is designed to match the wavelength of the Ar laser beam (λ=514.5 nm) used for the operation of this device, so that the optical thickness n−d is 1080 nm.

The entire film also serves as an insulating film.

Further, ITOM was formed as a transparent electrode on the outside. IT
A sorting wire is welded to a part of the O electrode 1, and a D of 5000V is applied.
C voltage was applied between I T O'ii fj.

Change the polarization direction of the Ar laser beam from the <100> direction to -45
The output light intensity of reflection and transmission was measured with the pulse width fixed at 1 m5ec and the light intensity gradually increasing from 0.

Next, the input light intensity was conversely reduced, and the reflected and transmitted light intensities were measured in the same manner. The results at this time are shown in Figures 12 and 13.
As shown in the figure. The parameters in the figure indicate the initial phase amount and the phase difference from the resonant state of the 7-abbrew resonator. It can be seen that the input and output light intensity characteristics exhibit differential gain characteristics and bistable characteristics depending on the initial phase setting.

Next, as a second embodiment, the film structure of the reflecting mirror of the dielectric multilayer film was different on the incident side and the transmission side. Constituent film material is ZrO
2, MgF2, and ITO, but the film thickness was changed.

The film structure on the incident side is MgF2 (1130
), ZrO2 (745 people), MgF2 (11
30 people), Zr02 (745 people), MgF2 (
1130 people), Zr02 (745 people).

Mgh (1130 people>), ITO (745 people), and the membrane configuration on the permeation side is MgF2 (750 people) in order from 850 faces.
), ZrO□ (495 people), MgFz (7
50 people), ZrO, (495 people).

MgF2 (750 people), Zr02 (495 people),
MgF2 (750 people) and ITO (495 people).

The input/output characteristics (transmitted and reflected light intensity) using the initial phase amount as a parameter are shown in FIGS. 14 and 15. By selecting the reflectance of the reflecting mirrors constituting the co-excavator, a nonlinear optical element with even greater nonlinear movement than in the first embodiment can be obtained. In this way, by appropriately setting the structure of the dielectric multilayer film, the initial phase amount, etc., it is possible to obtain nonlinear optical elements having various input/output characteristics.

Next, as a third embodiment of the present invention, a GaAs crystal plate having one side polished to a mirror surface was used, and a portion of the reflective surface was etched to form a mirror surface. i+'ii with mirror surfaces on both sides
M had dimensions of 5ai+5 mm and a thickness of about 300 μm. Subsequently, four alternating layers of SiO□ and TiO2 were formed on both sides, and ITOllSl was formed on the outermost surface to form an optical resonator. A tonic line was applied to the ITO transparent electrode, and a DC voltage of 3000 V was applied to form a device.

Note that the operable wavelength is near-infrared light of about 0.8 to 1.1 μm. By adjusting the film configuration of the dielectric multilayer film and the initial phase amount of the optical resonator, the first. Nonlinear human output characteristics similar to those of the second example were obtained.

(Effects of the Invention) The nonlinear optical element of the present invention uses the photoconductive effect and the electro-optic effect as operating principles. A dielectric multilayer reflective mirror forming an optical resonator is provided with a nonlinear crystal sandwiched therebetween, and transparent electrodes are further provided on both sides of the reflective mirror. A high voltage is then externally applied to the transparent electrode to create an induced refractive index in the nonlinear crystal. It is characterized by a mechanism in which the induced refractive index is then reduced according to the input light intensity.

With this configuration, an excellent nonlinear optical element that can operate with sufficiently low input light intensity, has a high degree of integration, and has high operational stability has been achieved.

In addition, by forming the reflective mirror that forms the optical resonator with a dielectric multilayer film, a nonlinear optical element with less optical loss due to absorption and also serving as an insulating film can be achieved with a simplified overall configuration. It has features such as being able to

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment of the nonlinear optical element of the present invention,
FIG. 2 is a schematic diagram of a conventional nonlinear optical element, and FIG. FIG. 4 is an explanatory diagram of the input/output characteristics of a conventional nonlinear optical element, FIG. 5 is an explanatory diagram of the operation analysis model of FIG. Explanatory diagram, 1st
Figure 0 is an explanatory diagram of the input-transmitted light characteristics of the nonlinear optical element of the present invention, Figure 11 is an explanatory diagram of the human power-reflected light characteristic of the nonlinear optical element of the present invention, and Figures 12 and 13 are diagrams of the input-transmitted light characteristic of the nonlinear optical element of the present invention. FIGS. 14 and 15 are explanatory diagrams of the input/output characteristics of the first embodiment of the nonlinear optical element, and FIGS. 15A and 15B are explanatory diagrams of the input/output characteristics of the second embodiment of the nonlinear optical element of the present invention. In the figure, 11 is a nonlinear medium, 12.13 is a dielectric multilayer film reflector, 14.15 is a transparent electrode, 16 is an external power source, and 17
．． 61 is incident light, 18 is transmitted light, 19 is reflected light, and 62 is excited carrier. Patent applicant Canon Co., Ltd. Representative Yuki Takanashi M”
'i, , Figure 1 Figure 2 Figure 3 The mouth is first strong. Figure 11 Man's Endurance Arm 12th Illustration 77 L Foam/l (w/cm") 13th Illustration Thickness R4 Light 7415 degrees (w/cm")
14 Kasumi appetizer: 5 degrees of arc (w/cmλ) 15th flash 0.00, CI4, O8O, 120, 1G
O, 200, 240, 28 entry 1 t? Arc degree (
w/cmL) Procedure Neho Sho 11 (Voluntary) April 21, 1988 Director General of the Patent Office 1, Indication of the case Patent Application No. 3543 of 1988 2, Title of invention Nonlinear optical element 3, Person making correction case Relationship with Patent applicant address 3-30-2 Shimomaruko, Ota-ku, Tokyo Name (100
) Canon Co., Ltd. Representative Nozo Kaku
Part 4. Agent's residence: 301 Jiyugaoka, Belheim, 2-17-3 Okusawa, Setagaya-ku, Tokyo 158 (Telephone: +8-5614) 6. Contents of amendment (1) The description is amended as follows. 3 8 Non-optical bistability Optical bistability ゝ-
/page Line Wrong Correct 6 5
Reflector Reflected light 6 16 Figure 5
Figure 19 12 Optical bistable nonlinear optics 11 3
~4 Optical bistable nonlinear optics 12 13
:y@y' axis 13 9 t
2 t2... to -1 to -11 178 Delete "analytically"

Claims (1)

[Claims] A reflecting mirror made of a dielectric multilayer film that forms an optical resonator that also serves as an insulating layer is provided on both sides of a nonlinear medium that has photoconductivity and electro-optic effects for input light, and furthermore, both sides of the outside of the reflecting mirror are provided. A nonlinear optical element, characterized in that a transparent electrode is provided on a surface for applying an electric field to the nonlinear medium, and output light has differential gain characteristics or hysteresis characteristics with respect to input light from the side surface of the transparent electrode.