JPH0786627B2 - Optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention provides effective improvement of a nonlinear optical constant that causes an optical nonlinear phenomenon itself, and is a field requiring propagation of a nonlinear phenomenon over a long distance. And an optical waveguide such as a thin-film optical waveguide formed on an optical fiber or a substrate, which is particularly effective when it is desired to increase the interaction length of a nonlinear phenomenon.

[Conventional technology and problems]

In order to effectively enhance the nonlinear effect of light in an optical waveguide such as an optical fiber, a substance with a large nonlinear optical constant is used, the interaction length with a nonlinear optical medium is increased, and the beam diameter of light (optical and nonlinear optical It is effective to reduce the interaction cross section with the medium), increase the intensity of light incident on the nonlinear optical medium, and the like.

Conventionally, non-linear optical media include Ge, Si, GaAs, InSb, InAs, CdS, DdSe, HgCdT, which have large non-linear optical constants.
For example, a glass plate doped with a semiconductor such as e, CdSSe is a normal single-mode optical fiber with a small core diameter and long-distance waveguiding, and each high-power laser (YAG, Ar, Kr , Dye laser, etc.).
Recently, as a medium that simultaneously satisfies the above items (1) to ( ³⁾ , in the field of an optical active line represented by a fiber Raman laser, an optical fiber doped with ions such as Nd ³⁺ and Er ³⁺ has begun to be experimentally manufactured. However, though the nonlinear optical constant is large (even at wavelengths near the bandgap energy value, there are even x ⁽³⁾ to 10 ^-9 MKS), the optical loss is large and the interaction length cannot be increased. Can not. Regarding the optical fiber, for example, one with −0.2 dB / Km has been realized in the 1.5 μm band, which is advantageous, but it has an extremely small nonlinear optical constant of x ⁽³⁾ to 10 ³³ MKS. Nothing more. Therefore, the above-mentioned Nd ³⁺ , Er
It was devised to dope ³⁺ , but unfortunately,
These include the minimum loss wavelength range of optical fibers (1.3 μm, 1.5
There is a considerable absorption of the doped ions in the (μm band), which results in a large loss. Therefore, it is practically impossible to increase the interaction length and propagation length of a nonlinear phenomenon by using such an optical fiber.

An object of the present invention is to improve the above-mentioned conventional problems, to increase the nonlinear optical constant in a state without loss, and to improve the interaction length and the transmission plate length as compared with the conventional nonlinear optical medium. It is to realize an optical waveguide such as an optical fiber that can be significantly enlarged.

[Means for solving problems]

It is generally known that a relatively large non-linear optical constant can be obtained near the wavelength that resonates with the energy level.
The energy level referred to here means the transition level of atoms and ions, but in the case of semiconductors, it corresponds to the band gap. The exciton level and the impurity level also correspond to this.

However, when the transition between the energy levels contributes to absorption, it acts as a loss on the light intensity. Therefore, when it is desired to observe a non-linear effect over a long distance of 100 m to 10 Km or to obtain a large interaction length, it is inevitable that absorption should be made as small as possible.

Unlike the conventional one, the present invention brings about an increase in the nonlinear optical constant without increasing the loss due to linear absorption in the wavelength range of the light used. In particular, even in the minimum loss wavelength region (1.3 μm, 1.5 μm band) of the optical fiber, the above optical fiber can be realized as in the embodiment described later.

FIG. 1 shows a diagram of a two-photon process without a one-photon process. omega _1, the case where two photons of omega ₂ is almost resonance with omega becomes ωω _{1 +} ω ₂ to a 1-(a) figure, if a Omega2omega ₁ second 1-(b) Each is shown in the figure. In the semiconductor, the band gap Eg corresponds to hω. Note that the resonance state here means that light is absorbed because the energy of photons matches the band gap energy of a semiconductor, for example. In this figure, since there is no absorption level due to the one-photon process of ω ₁ and ω ₂ , there is no excess loss in the fiber,
The enhancement of the nonlinear optical constant by the two-photon process is realized.
When the exciton / impurity level is used as the level of this two-photon process, hω corresponds to this exciton / impurity level. FIG. 2 shows examples of band gaps and exciton levels.

Hereinafter, the present invention will be described in more detail with reference to Examples.
The present invention is not limited to these examples.

[Example 1] As a nonlinear optical medium having a relatively large x ⁽³⁾ , semiconductor fine particles are doped into the core of an optical fiber. This is because the absorption loss is considerably large at the resonance wavelength in the one-photon process. (For this reason, as a typical example, 60% transmission is obtained with a length of 2 to 3 mm.) FIG. 2 shows an example of band gaps and exciton levels of semiconductors and metal oxides. . These show transmissivity in the low loss wavelength range (1.0 to 1.6 μm band) of the optical fiber, but they correspond to ω which is ωω ₁ + ω ₂ .
It has a band gap or exciton level in the 5 to 0.8 μm band. Therefore, in the low-loss wavelength region of the optical fiber, the nonlinear optical constant is enhanced by the two-photon process without being absorbed as a loss by the one-photon process.

Specifically, there is CdS _x Se _1-x which is a mixed crystal of CdS and CdSe. This is a semiconductor in which the band gap value of CdS _x Se _1-x can be set to an arbitrary energy value between the band gaps of CdS and CdSe by adjusting the composition ratio (x) of S and Se. X ⁽³⁾ in the one-photon process of transmission is 10 ^-16 to 10 ^-17 KMS
Reach When this is doped into an optical fiber, x ⁽³⁾ can be enhanced by the two-photon process in the low loss wavelength range (0.8 μm to 1.5 μm band) without the loss due to the one-photon process.

[Example 2] Although it is possible to set the energy level used in the two-photon process to the exciton level or the impurity level, an example of the exciton level is shown here. As mentioned above, as a nonlinear optical medium,
If these are used in a state of almost resonating with the exciton absorption peak of the one-photon process, a large absorption loss is caused. By setting the wavelength of the light so that the excitons are non-resonant in the one-photon process and are almost resonant in the two-photon process, a medium having a large nonlinear optical constant without absorption loss can be realized. This is because x ⁽³⁾ is enhanced by resonance in the two-photon process. For example, as shown in Fig. 2, CdS and CdSe are 2.54 eV, 1.82
near eV, CdS _x Se _1-x near 2.04 to 2.09 eV,
Each has a peak of exciton absorption. 1.3 for CdS
A μm YAG laser and a 0.77 μm semiconductor laser,
The energy sum of two photons is as shown in Fig. 1- (a).
Since it can be set so as to be near the peak of this exciton absorption, the above-mentioned condition can be satisfied. Also, CdS, CdS
In e, CuCl, and CuBr, two excitons are bound to each other to form an exciton molecule, so that a large nonlinear optical constant is obtained. This means that even if one photon is non-resonant with the exciton absorption peak, it is possible to create a state in which two photons almost resonate with the level of the exciton molecule, as shown in Fig. 1- (b). , Because the non-linear constant is increased. Since a large electron orbit exists in this exciton molecule, a giant oscillator effect occurs and a particularly large nonlinear optical constant is obtained. To take advantage of this effect in the low loss wavelength region of the fiber, GaA
Use of s, InP, CdTe, CdSe, etc. is effective.

It is known that when the size of the crystal to be doped is about 100 Å or less (fine particles), the binding energy of electrons increases due to the quantum size effect, and the peak of exciton absorption can be observed even at room temperature. A high-quality nonlinear optical medium can be realized by utilizing the transition. Further, by controlling the size of the doped particles, the energy value of exciton absorption can be controlled.

〔The invention's effect〕

As described above, according to the present invention, in the wavelength range of the desired light, especially 1.3 μm which is the minimum loss wavelength range of the optical fiber,
An optical fiber with a large non-linear effect can be realized without excessive loss in the 1.5 μm band. Therefore, the present invention is a field requiring propagation of long-distance non-linear optical phenomena such as soliton laser, soliton transmission, optical pulse compression, nondestructive measurement of photon number, and photon number state distortion-free transmission, and lengthening interaction length. It will make a great contribution to the necessary fields and the fields where the loss has a great influence on the deterioration of characteristics.

[Brief description of drawings]

1- (a) is a diagram of a two-photon process of ω ₁ ≠ ω ₂ and ω ω ₁ + ω ₂ , and FIG. 1- (b) is ω ₁ = ω ₂ and ω ₁ = ω ₂ .
2ω ₁ diagram, FIG. 2 is a diagram showing a band gap of a semiconductor, a metal oxide, and an exciton level.

Claims (1)

[Claims] 1. At least in the core of an optical waveguide, one-photon absorption does not occur at the wavelength of light used, and multiphoton resonance of two or more photons occurs near the band gap energy value or near the exciton absorption energy value. An optical waveguide characterized by being doped with fine particles of such a semiconductor or metal oxide.