JP2565964B2 - Optical medium for nonlinear optical devices - Google Patents

Description

The present invention relates to a non-linear optical medium applicable to optical switches, optical memories, optical signal arithmetic processing devices, etc., which will be used in the future in optical information processing and optical communication systems. It is about.

[Conventional technology and problems]

The nonlinear optical effect means that when light is incident on a substance, the electric polarization P of the substance is represented by the general formula P = x ⁽¹⁾ E + x ⁽²⁾ E ² + x ⁽³⁾ E ³
It can be written as + ... (II), but it refers to the effect exhibited by the second and subsequent terms (x ⁽ⁱ⁾ is the i-th order electric susceptibility, and E is the electric field intensity of light). In particular, the second harmonic generation (SHG) according to the second term and the third harmonic generation (THG) according to the third term are well known as wavelength conversion effects, but the third term also depends on the light intensity. It is important to give a change in optical constants, for example, a nonlinear refractive index effect or a nonlinear absorption effect. For example, the nonlinear refractive index effect is that the refractive index n of a substance changes in proportion to the incident light intensity, and is described by n = n ₀ + n ₂ I (n ₀ is a constant, n ₂ is a nonlinear refractive index coefficient). It is.) When a medium exhibiting this effect is combined with another optical element such as an optical resonator, a polarizer, or a reflecting mirror, it can be used in optical information processing or optical communication systems such as an optical bistable element, an optical switch, or a phase conjugate wave generator. It is possible to realize important devices that will be used in the future.

As a conventional example of a non-linear optical device using a medium exhibiting such a non-linear refractive index effect, an actual example of an optical bistable element is described below.
It is shown below with reference to the drawings.

In FIG. 5, reference numeral 11 is an optical material having a nonlinear refractive index (nonlinear refractive index medium), and 52 and 52 'are nonlinear refractive index medium 11
It is a dielectric vapor deposition mirror with a reflectance of about 90% coated on both sides of. In this configuration, when the resonance condition is satisfied by slightly changing the wavelength of the human power light, the output light Pt has the characteristics as shown in FIGS. 1C and 1D with respect to the input light intensity Pi ( For the principle of operation, refer to the document Applied Physics Letter (Appl.Phys.Lett.) Vol.35,451 (1976). These correspond to limiter operation and bistable operation, respectively, and can be applied to waveform shaping of input optical pulse, optical switch, optical signal memory, optical logic operation in optical communication and optical information processing systems. Is.

By the way, in this type of non-linear optical device, as a characteristic, a usable input light wavelength and input light intensity, and a response time of 3 which can follow the intensity change of the optical signal are used.
One value is important. For example, the nonlinear optical medium in FIG.
In the example using a superlattice crystal prepared by alternately and repeatedly growing semiconductor thin films of GaAs and GaAlAs as 11, the refractive index depends on the light intensity because the excitons are excited by light absorption in the crystal. (The absorption nonlinear effect) is the operating principle, the coefficient of the third term of the formula (II) is large (that is, the efficiency of the third-order nonlinearity is high), and the input light intensity required for the operation is 5 ×. It is excellent in that it can be as small as 10 ⁴ W / cm ^2, but the usable input light wavelength is limited to an extremely narrow range near the exciton absorption spectrum, and the response time is determined by the exciton lifetime. And 3 × 10 ^-8 se
There was a problem that it could not be used for optical signal processing faster than c.

Further, in another conventional example in which a glass cell filled with carbon disulfide (CS ₂ ) which is a nonlinear optical liquid is used as the nonlinear optical medium, and an external mirror is used instead of the dielectric vapor deposition mirrors 52 and 52 ′, Since the operating principle is that the refractive index depends on the light intensity due to the rotational arrangement of molecules according to the optical electric field (molecular rotation nonlinear effect), the usable input light wavelength is in a wide range from visible to near infrared region. Although it is excellent in that point, the efficiency of the third-order nonlinearity is not so high, the input light intensity required for operation is as large as 10 ⁸ W / cm ² , and the response time is determined by the rotational relaxation time of the molecule. There is a problem that it cannot be used for optical signal processing faster than 10 ^-11 to 10 ^-12 sec.

As is clear from the above, the performance of the nonlinear optical device is almost determined by the characteristics of the nonlinear optical material.
Therefore, it is eager to develop a material that has a wide usable wavelength range, high third-order nonlinear efficiency, low input light intensity necessary for operation, and short response time, and active research is being conducted toward that end. It is the current situation.

Among the materials exhibiting the third-order nonlinear effect, organic nonlinear optical materials having π-electron conjugation such as benzene ring and double or triple bond have recently attracted particular attention. For example, polydiacetylene bis- (paratoluene sulfonate) (abbreviation PT
In S), the third-order effect constant x ⁽³⁾ is x ⁽³⁾ = 1 × 10 ^-10 esu
(Converted to a nonlinear refractive index coefficient, n ₂ = 2 × 10 ⁻¹² (W / c
m ² ) ^-1 . ), Which is two orders of magnitude greater than the CS ₂ liquid described above. Furthermore, since the mechanism of this non-linear effect is not due to absorption or interaction with molecules or crystal lattices but is purely due to electronic polarization, it is possible to follow the intensity change of the optical signal. Response time 10
It has an extremely high speed of ^-14 sec, and has the excellent characteristics that the usable input light wavelength is in a wide range from around 0.65 μm to over 2.0 μm.

However, polydiacetylene such as the above PTS,
In general, since it is a crystalline polymer, when it is used as a medium of an optical device, it is necessary that the entire medium is a single crystal. If an amorphous part exists in a part of the medium, or if the whole is in a polycrystalline state and is an aggregate of minute single crystals, the scattering loss of incident light will increase immediately. ,Useless. Therefore, in manufacturing an optical device using polydiacetylene, techniques such as growing large crystals or thin-film crystallization according to the purpose are indispensable, and even when these crystals are obtained, the crystals are cut. It is necessary to establish a processing technique for polishing and polishing the surface. These peripheral technologies for polydiacetylene have not yet been solved. Therefore, none of the non-linear optical devices using polydiacetylene has been realized yet.

The present invention has been made in view of the above circumstances, and an object thereof is an optical switch including a conventional optical bistable element,
Problems in non-linear optical devices using non-linear refractive index such as phase conjugate wave generators, that is, limited wavelength used, slow response, high operating input light intensity, or inability to easily obtain desired optical medium It is intended to eliminate defects such as, and has a wide wavelength range to be used.
It is an object of the present invention to provide a non-linear optical medium which has a high-speed response and which can be used in a practical high-performance non-linear optical device in which the operating input light intensity is small.

[Means for solving problems]

In order to achieve the above object, the present invention has the following configurations. That is, the present invention is a nonlinear optical device composed of an optical medium having a nonlinear refractive index and an optical element such as an optical resonator, a polarizer, or a reflecting mirror. It is characterized by using the indicated poly- (paraphenylene vinylene).

Poly- (paraphenylene vinylene (abbreviated as PPV) is a conductive polymer material with an amorphous structure. As described later, it can be synthesized by a method involving a soluble polymer intermediate, so it has excellent moldability. The present inventors were able to clarify for the first time that the third-order nonlinear effect of PPV has an extremely large value, and is easy to handle and chemically stable. Therefore, it was confirmed that the nonlinear refractive index coefficient was large, and by using this material as a nonlinear optical medium, nonlinear optic devices such as optical bistable elements, optical switches, phase conjugate wave generators, etc. could be constructed and put to practical use. This is the most important point of the present invention.

FIGS. 4 (a) and 4 (b) show measured data of an experiment conducted to evaluate the efficiency of the third-order effect of PPV. The third-order effect of an organic material is generally evaluated by measuring the intensity of third harmonic light (THG light). Figure 4 (a) shows a PPV film with a uniform thickness (in this case, the thickness is 0.69μ).
m), a laser pulse light having a wavelength of about 2.0 μm is incident thereon,
This is a result of measuring the emitted THG light intensity while rotating the sample around an axis perpendicular to the laser light incident direction. This pattern is called a maker fringe. If a material whose efficiency is already known (using quartz glass in this case) is measured with the same observation system and compared with the peak intensity of THG, the efficiency of the third order effect (The details of the measurement principle can be found in the literature, Electronics Letters (Electron. Lett.) Vol 23, No. 11, 595 (1987)). FIG. 4 (b) shows the obtained results by the third-order electrical susceptibility x ⁽³⁾ , which is plotted against the incident light wavelength. The value of x ⁽³⁾ is 5-8 × 10 ^-12 esu,
It was confirmed that the value was about an order of magnitude smaller than that of S. From this measured value, the nonlinear refractive index is n ₂ = 1 to 1.5.
It is calculated as × 10 ^-13 (W / cm ² ) ^-1 .

The absorption spectrum of PPV is about 20 μm.
It was measured with a spectrophotometer using the film. As a result, it was found that the non-absorption wavelength range was as wide as 0.57 to 2.2 μm. In other words, this material has a usable wavelength range of PTS
Even better and better. The wavelength range is 0.57 μm-1.
Although the third-order effect efficiency of PPV at 8 μm has not been measured, considering that the mechanism of the nonlinear effect is due to pure electronic polarization as in PTS, x ⁽³⁾ or n of PPV is also observed in these wavelength regions. It is easily inferred that ₂ has a value almost equal to the value in Fig. 4 (b).

Furthermore, from the same idea, the response time of this material is estimated to be about 10 ^-14 sec, similar to PTS, and it has sufficient high speed.

Hereinafter, the features of the nonlinear optical device using poly- (paraphenylene vinylene) according to the present invention will be described in detail with reference to examples.

Example 1 FIG. 1 (a) is a diagram for explaining an example of the nonlinear optical device of the present invention, in which the nonlinear refractive index medium 11 used in the present invention is used.
Is a PPV film manufactured by the method described below. In the device of the present invention, external optical resonance in which dielectric multilayer mirrors 12 and 12 'that reflect about 90% of input light and transmit the rest are arranged opposite to each other It is a vessel.

The method for producing the PPV film will be described. First, para-xylene bis (diethyl sulfonium bromide) was dissolved in water, 10% alcohol in which caustic soda was dissolved was added as an alkaline solution, and the mixture was stirred and polymerized. Next, this was formed into a film on a quartz glass plate by a casting method and air-dried. Finally, this is 300 ℃, 2
A PPV film with excellent optical quality was obtained by heating in an inert gas for a period of time. FIG. 1B shows a reaction scheme in the last heat treatment process. Like this
A film-like optical medium having a size of about 10 × 10 × 1 mm ³ was obtained.

In order to operate the device shown in FIG. 1 (a), the resonance condition is changed by slightly changing the input light wavelength or by slightly changing the resonator length (mirror interval) as in the case of FIG. Adjust it. In the case of the present embodiment, 1.064 μm light from the Nd ³⁺ -YAG laser was used, so the adjustment depended on the method of changing the cavity length. Between the input light intensity Pi and the output light intensity Pt, the limiter operation and the bistable operation as shown in FIGS. 1C and 1D were obtained.

The minimum input light intensity (Pi ^min ) required for operation is analytically Pi ^min (Kλ) / (n ₂ l) (where λ is the wavelength of light, l is the optical medium length, and K (~ 0.00
1) is given by the reflectance of the mirror and the coefficient determined by the cavity length adjustment). In this embodiment, λ = 1.064 μm and l = 1 mm, so Pi ^min = 1 × 10 ⁷ W / cm ² is obtained. .

When a semiconductor laser with an effective output of 300 mW (pulse oscillation) and an oscillation wavelength of 0.83 μm is used as the light source, if the beam diameter is narrowed down to 1 μm, the light intensity is calculated to be 3.8 × 10 ⁷ W / cm ^2, and the nonlinear optical at this wavelength is calculated. It is sufficiently larger than the Pi ^min value of the device. In fact, this nonlinear optical device was operable even when a semiconductor laser was used as a light source.

It has already been mentioned that the response time τ of this material (PPV) is estimated to be about 10 ^-14 sec. However, the reaction time of the device is determined by the medium response time τ and the photon lifetime tp in the resonator.
Is determined by the larger value of tp = −lop / (ClnR) (where lop is the optical length of the resonator, C is the speed of light, and R is the reflectance of the mirror), and tp is 6 × 10 ⁻ I got ¹¹ sec,
Since tp> t, this value becomes the response time of the device. In this example, it was confirmed that the response time was shorter than 10 ⁻¹⁰ sec.

Example 2 FIG. 2 is a diagram for explaining an example of the optically controlled optical switch. In the figure, reference numerals 21 and 21 'designate an orthogonal polarizer system composed of two polarizers arranged so that the polarization axes thereof are direct rays, and 11 is the medium crystal obtained in the first embodiment. In this configuration, linearly polarized light that has passed through the orthogonal polarizer 21 only while the gate pulse Pg is incident receives rotation of the polarization angle due to the change in the refractive index of the nonlinear refractive index medium 11, and passes through the orthogonal polarizer 21 '. To do. That is, the incident light is optically switched by the gate light.

Also in this device, the usable wavelength range, the response time, and the value of the operating input light intensity (gate pulse light) are important, but it is clear from Example 1 that the usable wavelength range and the response time are extremely excellent. It goes without saying.

The required gate pulse light intensity (Pπ / 2) is analytically given by Pπ _{/ 2} = λ / (2 · n ₂ l), and in this embodiment, λ = 1.064 μm and l = 1 mm, so Pπ _{/ 2} = 5 × 10 ⁹ W / cm ² was calculated. Actually, even at a gate pulse light intensity lower than this, the transmittance T follows the formula of T = sin ² {(Pi / Pπ _{/ 2} ) × π / 2}, so that the semiconductor laser light with a narrowed beam diameter can be used. Also found to be operational.

Third Embodiment FIG. 3 is a diagram illustrating an embodiment of a phase conjugate wave generator. In the figure, reference numerals 31 and 31 'are semi-transmissive mirrors, 32 is a total reflection mirror,
Reference numeral 11 represents the optical medium obtained in Example 1. This configuration is an optical configuration called degenerate four-wave mixing. When three light waves A ₁ , A ₂ (in the opposite direction to A ₁ ) and Ap (oblique incidence) enter a medium having a nonlinear refractive index, Ap , A fourth light wave (Ac) in which only the spatial phase term is conjugate is generated. This phase conjugate wave has attracted attention as an effective means for image correction and real-time holography in image information processing technology (for application, refer to the document O plus (O plus
E) March issue, p.73 (1982)).

Also in this example, high-speed response and low operation input light intensity of the device were confirmed.

〔The invention's effect〕

The poly- (paraphenylene vinylene) according to the present invention is
It has the characteristics of extremely high response speed and large nonlinear refractive index. Therefore, a film of poly- (paraphenylene vinylene) can be applied as a medium of a nonlinear optical device which is important in future optical information processing or optical communication fields such as an optical bistable element, an optical switch, or a phase conjugate wave generator. As a result, compared with conventional devices using nonlinear optical liquids and semiconductor superlattice crystals, it is possible to make them far superior in terms of wavelength dependence, response speed, and input light intensity required for operation. It has a practical value in that the semiconductor laser can be moved as a light source.

[Brief description of drawings]

FIGS. 1A to 1D show a first embodiment of a nonlinear optical device using an optical medium according to the present invention. FIG. 1A is a configuration diagram of an optical bistable element, and FIG. Non-linear optical medium Poly-
Reaction schemes for synthesizing (paraphenylene vinylene), (c) and (d) show operating characteristics, (c) shows limiter operation, (d) shows bistable operation, and arrow in the figure Shows the pathway that shows the characteristics of Pt when Pi increases and decreases. FIG. 2 shows a second embodiment of a nonlinear optical device using an optical medium according to the present invention, and is a configuration diagram of an optically controlled optical switch. FIG. 3 shows a third embodiment of a nonlinear optical device using an optical medium according to the present invention, and is a configuration diagram of a phase conjugate wave generator. FIG. 4 (a) is a diagram showing a THG intensity pattern of poly- (paraphenylene vinylene) according to the present invention, and FIG. 4 (b) is a spectrum of x ⁽³⁾ value of the poly- (paraphenylene vinylene). FIG. FIG. 5 is a configuration diagram showing an example of a conventional nonlinear optical device (optical bistable element). 11 …… Nonlinear refractive index medium, 12,12 ′ …… (External) Dielectric multilayer vapor deposition film mirror, Pi …… Input light, Pt …… Output light, 21,2
1 '... Polarizer (constituting orthogonal polarizer system), Pg ...
… Gate pulse light, 31,31 ′ …… Semi-transmissive mirror, 32 …… Total reflection mirror, 52,52 ′ …… Dielectric vapor deposition mirror (Reflectance about 90
%).

Continued on the front page (72) Inventor Takashi Kurihara 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Shogo Saito 4-1-118 Yakuin, Chuo-ku, Fukuoka City, Fukuoka Prefecture (72 ) Inventor Tetsuo Tsutsui 8-66, Koyogaoka-Higashi, Kasuga-shi, Fukuoka (72) Inventor Shizuto 1-2-1, Shirakibara, Onojo-shi, Fukuoka 202-Corporate Hamada

Claims (1)

(57) [Claims] 1. An optical medium for a nonlinear optical device, which is poly- (paraphenylene vinylene) represented by the following formula (I).