JPH0721598B2 - Optical fiber type nonlinear directional coupler - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical fiber type nonlinear directional coupler having a small number of constituent elements, a high operating speed, and a nonlinear optical input / output characteristic.

(Prior Art and its Problems) A conventional non-linear directional coupler is shown in FIG. 2 (A). (Reference 1D. Sarid and M. Sargert III, “Tunable
nonlinear directional coupler ", J.Opt.Soc.Am.voL7
2, p.835-838 (1982). ). This has a structure in which two waveguides 11 are formed in parallel on a substrate 12 and the evanescent waves (electric field components oozing into the clad portion) of the two waveguides 11 overlap each other.

In the structure shown in FIG. 2A, since the waveguide 11 is formed by implanting photoresist or diffused ions on the substrate 12, when the optical fiber is used for inputting / outputting light, a groove is cut on the substrate 12. The optical fiber and the waveguide 11 must be brought into contact with each other, and in order to suppress the optical loss due to the axis deviation of the optical fiber to 1 dB or less, processing technology with an accuracy of 0.5 μm or less on the substrate 12 is required. It became the above fault.
In addition, although LiTiO3, LiNbO3, etc. are used as the material of the substrate 12, the refractive index (LiNbO3
= 2.20, SiO2 = 1.46 causes Fresnel reflection (reflectance 4%) and butt splicing is performed. Therefore, the method of fixing optical fiber and its long-term reliability are different from fusion splicing of optical fibers. There was an inferior fault.

On the other hand, in order to eliminate these drawbacks, there is an optical fiber type polarization selective directional coupler 23 as shown in FIG. 2 (B) obtained by fusion-stretching two optical fibers. In this polarization-selective directional coupler 23, the optical power branching ratio to the two optical fibers is determined by the coupling constant and the coupling length determined by the overlap integral of the propagation modes propagating in the two cores. Since the birefringence is large, the refractive index cannot be changed by the input light intensity to change the branching ratio, and there is a drawback that a non-linear directional coupler cannot be realized. But,
Since the optical input / output terminals 21a, 22a, 21b, 22b are optical fibers, since they are connections of optical fibers, there is an advantage that the usual fusion splicing used in optical communication and the reinforcement method by the heat shrink tube can be used. there were.

Assuming a waveguide structure with a circular cross section in the conventional waveguide 11 of FIG. 2 (A), the output of a nonlinear directional coupler with a coupling length between the waveguides 11 of 2.5 cm and an interaction length of 1.8 m The calculation result of the waveform is shown in FIG. In the figure, the horizontal axis shows the standardized pulse width, and the vertical axis shows the standardized pulse intensity, which is shown by the solid line when an incident optical pulse with a peak value of 1.2 KW shown by the broken line is incident from one of the incident ends. As described above, the emitted light pulse becomes a light pulse train, and so-called optical breaking phenomenon (optical breaking,
Similar results are found in Ref. 2.R. Hoffe et al., Opt.Commun. 57,34.
(See (1986)), and it was impossible to extract a single optical pulse output.

(Object of the Invention) An object of the present invention is to obtain a single optical pulse having a narrower pulse width than an input optical pulse as an output, which cannot be obtained by a conventional nonlinear directional coupler. It can be easily connected to the optical system of the above through an optical fiber, and as a means of generating ultrashort optical pulses and waveform shaping,
An object of the present invention is to provide an all-fiber type nonlinear directional coupler applicable to a high-speed operation optical distributor having a picosecond to sub-picosecond response speed with a simple structure.

(Means for Solving the Problems) In order to achieve the above object, the present invention has a waveguide structure consisting of a core and a clad, and the core portion is birefringent, that is, a plane perpendicular to the propagation direction (x, One end of an optical fiber having a different refractive index in the (y-axis direction) is used as a light incident end and the other end is used as an emission end, and one end of an optical fiber type polarization selective directional coupler is connected to the emission end of the optical fiber. The two incident ends are connected so as to coincide with one of the main axes of the optical fiber, and light of an arbitrary polarization state is incident on the light incident end of the optical fiber, and one of the polarization selective directional couplers A non-linear optical output is obtained from the output end.

(Operation) According to the present invention, in the birefringent optical fiber, the birefringence (optical Kerr effect) generated by the input light and the intrinsic birefringence of the low birefringent optical fiber compete with each other, and the y-axis The non-linear conversion of the optical power occurs from the propagation mode of the above to the propagation mode of the x-axis. The converted x-axis propagation mode and the remaining y-axis propagation modes are incident on the incident end of the polarization-selective directional coupler, and these are separated by the polarization-selective directional coupler, and x is propagated on the exit end. The axial propagation mode and the y-axis propagation mode are output at the exit end, respectively.

(Embodiment) FIG. 1 is a view for explaining a first embodiment of the present invention,
Reference numeral 3 denotes a polarization-selective directional coupler similar to that shown in FIG. 2 (B), and one entrance end 1a thereof and an exit end 4b of the low birefringence optical fiber 4 have their fast axes (hereinafter y And the slow axis (hereinafter referred to as the x axis) are connected together. Further, in FIG. 1, arrows in circles indicate polarization states at respective positions. 2a is the other incident end, and 1b and 2b
Is an emitting end, and 4a is a light incident end.

The operation of the all-fiber nonlinear directional coupler of the present invention is as follows. The linearly polarized input light is made incident from the light incident end 4a of the low birefringence optical fiber 4 such that the y-axis polarization plane of the optical fiber 4 becomes substantially parallel. In the low birefringence optical fiber 4, a competition between the birefringence (optical Kerr effect) generated by the input light and the inherent birefringence of the low birefringence optical fiber 4 occurs, and the propagation mode of the y axis changes to the x axis. A non-linear conversion of optical power to the propagating mode occurs. The converted x-axis propagation mode Ex and the remaining y-axis propagation mode Ey are incident on the incident end 1a of the polarization-selective directional coupler 3, and these are separated by the polarization-selective directional coupler 3. The x-axis propagation mode Ex is output to the emission end 2b, and the y-axis propagation mode Ey is output to the emission end 1b. Depending on the design of the extension coupling portion length of the polarization selective directional coupler 3, the polarized waves to be emitted may be switched. The waveform changes due to these operations are as shown in FIG. 5 which will be described in detail later, and they are non-linearly operated with respect to the input waveform, and the output waveform (Ex. ) Will be emitted.

In the present embodiment, when the low birefringence optical fiber 4 and the polarization selective directional coupler 3 are connected, the x axis and the y axis of the main axis are made to coincide with each other. The x axis and the y axis may be aligned. At this time, the polarized waves to be emitted are switched and the x-axis propagation mode Ex is emitted to the emitting end 1b. Although the simplest linearly polarized light is incident on the main axis of the low birefringence optical fiber as the incident condition of the input light, it is more general even if the polarization direction and the main axis are inclined. It is applicable even when elliptically polarized light is incident and the principal axis of the ellipse and the principal axis of the fiber are inclined. These cases can be realized by appropriately selecting a fiber length other than the peak value of the Stokes parameter S1 according to the tilt angle or the like, as shown in FIG. 4 described later in detail.

In order to efficiently excite the nonlinear effect with a low excitation input, it is desirable to use an optical fiber with low birefringence, or to use a material with a large nonlinear susceptibility, such as LiNbO3, P
LZT, YIG, GaAs, InP, NiO2-B2O3-Na2O, As2S3, PMMA, MNA,
Polydiacetylene, diacetylene, PCH1083, etc. have a large nonlinear susceptibility compared to SiO2 and can be used with low excitation input.

Next, let x be the electric field in the low birefringence optical fiber, and x
Quantitatively shows the non-linear coupling of the electric field in the y-axis. Electric field ▲ ▼
Is expressed by the following equation.

here, Are the unit vectors of the slow axis and the fast axis, Ex and Ey are the electric field components in that direction, r and l are the unit vectors in the clockwise and counterclockwise rotation display, Is. Since the orthogonal mode of this low birefringence optical fiber is used as the waveguide mode, the two waveguides shown in FIG. 2 (A) are not required, and the nonlinear operation can be performed with one fiber. . Here, the change in the polarization state in this fiber, that is, the change in the optical energy between the two guided modes is evaluated using the Stokes parameter S = (S1, S2, S3). (Reference 3 B. Daino, G. Gregori, and S. Wabn
itz “New all-optical devices based on third-orde
r nonlinearity of birefringent fibers ", Qpt.Lett.vo
l 11, P. 42-44 (1986). ) The change of ▲ ▼ in the Z direction is given by the following equation in Poincare sphere display.

Here, ▲ ▼ is the rotational angular velocity of the Poincare sphere about the three axes (S1, S2, S3 axes). Stokes parameters are
S1 ＝ | Ex | ² − | Ey | ² , S2 ＝ Ex ・ Ey ^＊ ＋ Ex ^＊・ Ey, S3 ＝ i (Ex
^{· Ey * -Ex * · Ey)} , S 2 0 = S 2 1 + S 2 2 + S 2 3 = | Ex | 2 + | Ey |
Given in ² . * Represents a complex conjugate. Angular velocity ▲ ▼
Includes linear and nonlinear birefringence effects, as shown in the following equation, due to the change in refractive index caused by the incident light intensity.

Here, Ω1 is the angular velocity due to the birefringence peculiar to the low birefringence optical fiber, λ is the wavelength, and x is the susceptibility of the waveguide. In the calculation example, assuming a silica fiber, λ = 1.06 μm, x = 2.5
× 10 ⁻⁹ , and the birefringence of the low birefringence optical fiber was set to 10 ⁻⁶ .
FIG. 4 shows changes in the Stokes parameter S1 in the propagation direction when 1.2 KW CW linearly polarized light is incident with a low birefringence optical fiber shifted by 0.2 ° from the y-axis. It is standardized by the total light intensity S0. The Stokes parameter S1 represents the intensity difference between the Ex component and the Ey component. When S1 = 1, the Ex component, and S1 = −1,
It shows that only Ey component exists. It can be seen from Fig. 4 that the optical power shifts from the Ey component to the Ex component at fiber lengths of about 1.8m, 5.5m, and 9.3m. Therefore, at least 1.
After a low birefringence optical fiber with a length of 8 m, an optical fiber type polarization selective directional coupler (Ref. 4.I.Yokohama, k.Okamot)
o, J.Noda, “Fiber optic polarising beam splitter em
poloying birefringent−fiber coupler, "Electron.Let
t.voL, 21, pp.415-416 (1985). ) Are connected, the Ex and Ey components can be separated and taken out from the respective emission ends 2b and 1b. At this time, if the principal axis of the low birefringence optical fiber and the principal axis of the fiber at the input end of the fiber-type polarization-selective directional coupler match, the Ex and Ey components are detected. The optical output of | ² + | Ey sin θ | ² is obtained. From this equation, it can be seen that the optical output becomes almost | Ex | with a slight deviation of θ and is hardly influenced by θ, and that the waveform can be changed to some extent by selecting θ.

Fig. 5 is the result of calculating the output waveform when a Gaussian optical pulse shown by the dotted line with a peak output of 1.2 KW is injected into a 1.8 m long low birefringence optical fiber under the same conditions as in Fig. 4. Indicates. However, the analysis using the Stokes vector describes the change in the longitudinal direction of the polarization state in the birefringent fiber when continuous light is incident, and it is necessary to obtain the pulse response characteristics in the polarization state when the optical pulse is directly incident. I can't. Here, the change in the polarization state is calculated from the light intensity at each point on the time axis of the incident pulse, and the output light intensity obtained by dividing on the output side on the time axis is combined to change the polarization state of the light pulse. Is calculated. The output light intensity is standardized by the input light intensity, and a transmitted light output of about 65% is obtained. From the above numerical calculation results, it can be seen that an all-fiber type nonlinear directional coupler with a simple structure and high performance can be realized. There is a light intensity at which the Stokes vector does not change when the incident light intensity becomes strong, and the intensity is the incident light intensity at which the Ω ₁ + 2πxS ₁ S ₀ / 3λ component of equation (3) becomes zero. For example, the intrinsic birefringence of a birefringent fiber is 10 ^-6.
, And the and the condition S ₁ -1, the value becomes 1.28KW, of greater strength than this light, for example light intensity 1.3kW
When such light is incident, the single pulse extraction which is the object of the present invention cannot be performed.

Further, FIG. 6 shows changes in the propagation direction of the Stokes parameter when a birefringence distribution is provided in the longitudinal direction. Here, the change in the propagation direction of birefringence is (1,064−0.15 × sin (π
z) × 10 ⁻⁶ . Compared with FIG. 3, the Stokes parameter S1 is significantly changed, and the conversion from the y-axis to the x-axis mode can be changed to fiber lengths of 2.5 m, 3.6 m, and 4.6 m.

In order to give a distribution in the longitudinal direction, a plurality of holding plates 5 are discontinuously distributed along the axial direction of the optical fiber 4 as shown in FIG. 7, or evenly distributed, or as shown in FIG. Similarly, attach electrodes to distribute electrostatic or high-frequency electric field,
Partial birefringence is added. In FIG. 7, the optical fiber 4 including the holding plate 5 is surrounded by the heat shrinkable tube 6,
Birefringence is applied by utilizing the contraction force of the heat-shrinkable tube 6. Further, instead of the holding plate 5, a piezoelectric material is sandwiched over the entire length of the low birefringence optical fiber 4, and the birefringence of the low birefringence optical fiber or the elliptic core optical fiber is perturbed by utilizing stress or electric vibration of the piezoelectric material. Or by inducing birefringence in a state where there is no birefringence like a true-core optical fiber,
Non-linear operation can be performed.

(Effects of the Invention) As described above, the optical fiber type nonlinear directional coupler according to the present invention is composed of all optical fibers, and has an advantage that the structure is simple and connection with the optical fibers is easy. There is. Further, as can be seen from the numerical calculation result, there is an advantage that the incident light pulse can be shaped and extracted as an extremely short pulse. Further, if a material having a high susceptibility x is used, the intensity of input light can be reduced, and it can be used in the fields of optical communication and spectroscopy.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical fiber type nonlinear directional coupler showing an embodiment of the present invention, FIG. 2 (A) is a block diagram of a conventional nonlinear directional coupler, and FIG. 2 (B) is a conventional block diagram. Fig. 3 is a block diagram of the polarization selective directional coupler of Fig. 3, Fig. 3 is a diagram showing the optical pulse input / output characteristics of a conventional nonlinear directional coupler, and Fig. 4 is a change of the Stokes parameter S1 in the longitudinal direction (z direction). FIG. 5 is a diagram showing a calculation example of the operating characteristics of this embodiment with respect to a Gaussian input waveform, and FIG. 6 is a low birefringence optical fiber of −0.15.
FIG. 7 is a diagram showing changes in the Stokes parameter S1 when a distribution of sin (πz) birefringence is added, and FIG. 7 is a configuration diagram showing another embodiment of the present invention. 1a, 2a …… Injection end, 1b, 2b …… Exit end, 3 …… Polarization selective directional coupler, 4 …… Low birefringence optical fiber, 5 …… Holding plate, 6 …… Heat shrink tube .

Claims (5)

[Claims] 1. An optical fiber having a waveguide structure composed of a core and a clad, and having a core portion having birefringence, that is, having a different refractive index in a plane (x, y axis direction) perpendicular to the propagation direction One end of the optical fiber type polarization-selective directional coupler is formed at the output end of the optical fiber so as to coincide with one of the main axes of the optical fiber. All of the features of the present invention are characterized in that the optical fiber is connected, light of an arbitrary polarization state is made incident on the light incident end of the optical fiber, and a nonlinear optical output is obtained from one exit end of the polarization selective directional coupler. Fiber type nonlinear directional coupler. 2. An electrode in which an electrostatic field or a high frequency electric field is uniformly or discontinuously distributed in the propagation direction of the optical fiber is provided as a birefringence control unit of the optical fiber having birefringence. An optical fiber type nonlinear directional coupler according to claim 1. 3. A piezoelectric material for applying a static pressure perpendicular to the propagation direction of the optical fiber is used as a birefringence control unit of the optical fiber having birefringence. Fiber optic nonlinear directional coupler. 4. A piezoelectric material which applies a static pressure perpendicularly to the propagation direction of the optical fiber is used as a birefringence control part of the optical fiber having birefringence. Fiber optic nonlinear directional coupler. 5. An optical fiber comprising LiNbO3, PLZT, YIG, GaAs, In
Any one of P, SiO2, NiO2-B2O3-Na2O, As2S3, PMMA, MNA, polydiacetylene, diacetylene, PCH1083 is used, and any one of claims 1 to 4 is used. The optical fiber type nonlinear directional coupler according to item 1.