JPS6218900B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is suitable for use in optical communications. TECHNICAL FIELD The present invention relates to a method and apparatus for sampling optical signals using nonlinear optical effects of optical fibers without using an electrical system.

〔overview〕

The present invention is a method for obtaining a sampling pulse with an amplitude proportional to the amplitude of an input optical signal, and the method includes combining a signal light and a pump light and inputting the signal light from one end of a polarization-maintaining optical fiber. An optical sampling output is obtained at the other end of the polarizer optical fiber by nonlinearly changing the birefringence characteristics of the polarizer according to the level of the incident optical signal.

[Conventional technology]

The optical sampling method is a method in which the input optical signal shown in FIG. 2a is sampled at a desired timing to obtain optical pulses as shown by diagonal lines in FIG. 2b. This can be done by opening and closing the input optical signal path at the desired timing. As a conventional high-speed optical sampling method, a method is known in which a Matsuhatsu Enda type optical waveguide modulator is disposed in an optical signal path, and a high frequency electric field is applied to the modulation to open and close the optical signal path.

[Problem that the invention seeks to solve]

In this method, it is necessary to provide an electrode on the modulator, and there is a limit to speeding up the response characteristics due to the stray capacitance of the electrode. In order to improve this, efforts have been made to make the electrode structure comb-shaped, but this has made the device more complex, and the precision of the machining required to manufacture the device has increased. It has the disadvantage of being expensive.

The present invention improves on this, and aims to provide an optical sampling method and device that has a simple structure and can provide high-speed response.

[Means for solving problems]

The first aspect of the present invention is a method, which combines an input optical signal in a certain polarization state from one end of a polarization-maintaining optical fiber and a high-level pump light that causes birefringence into the optical fiber. and a method of extracting polarized light in the principal axis direction orthogonal to the polarization direction that appears when the input optical signal is not combined with the pump light at the other end of the optical fiber.

The above extraction method involves attaching one end of a polarizer optical fiber whose propagation loss differs for polarized light in the directions of the principal axes that are perpendicular to each other to the other end of the polarization-maintaining optical fiber, and attaching it to the other end of the polarization-maintaining optical fiber in the short axis direction (attenuation is The light appearing at the other end of the polarizer optical fiber is sampled. It is desirable that the signal be extracted as a signal.

In addition, in the above optical sampling method, the wavelength of the input optical signal is set to a region where the short axis polarization loss of the polarizer optical fiber is large and the long axis polarization loss is small, and the wavelength of the pump light is set to the region where the loss of the short axis polarization of the polarizer optical fiber is small. It is desirable to set it in a region where both short-axis polarized light loss and long-axis polarized light loss are large.

The second aspect of the present invention is an apparatus, comprising: a phase adjustment means for adjusting the polarization direction of an input optical signal; a means for combining the output light of this means with a pump light synchronized with the sampling timing; The phase adjustment means includes a polarization-maintaining optical fiber into which the output light of the means enters at one end, and an extraction means provided at the other end of the polarization-maintaining optical fiber. The output from the extraction means is set to be minimum when the beams are not combined, and the extraction means changes the polarization direction due to the optical Kerr effect caused by the pump light in the polarization preserving optical fiber. It is characterized in that it is a means for extracting components of an optical signal.

The extraction means includes a polarizer optical fiber connected to the other end of the polarization-maintaining optical fiber, and the polarization-maintaining optical fiber and the polarizer optical fiber are configured to operate when the input optical signal is not combined with the pump light. It is desirable that the polarization-maintaining optical fiber is connected so that the long axis direction of the light emitted from the polarization-maintaining optical fiber substantially coincides with the axial direction of the polarizer optical fiber in which attenuation is large.

The phase adjustment means is preferably a quarter wavelength plate.

The expression "approximately match" as mentioned above means match to such an extent that a substantial effect can be obtained.

The above sampling method or device can also be considered as a modulation method or device in which the input optical signal is a signal wave and the pump light is a carrier wave.

[Effect]

The polarization direction of a weak input optical signal is adjusted to one polarization direction, and the signal is made to enter one end of a polarization-maintaining optical fiber. When a pump light with a fairly high power level is combined with this input optical signal and input from one end of the polarization-maintaining optical fiber, birefringence occurs due to the optical Kerr effect of the polarization-maintaining optical fiber. At the other end of the optical fiber, an optical signal appears in a polarization direction perpendicular to the polarization direction, depending on the presence or absence of this pump light. This optical signal is a signal obtained by sampling the input optical signal at the timing of the pump light.

In order to extract optical signals in orthogonal polarization directions, a polarizer optical fiber is connected to the output end of the polarization maintaining optical fiber. In this connection method, when the input optical signal is not combined with the pump light, the polarization direction of the output light from the polarization-maintaining optical fiber is connected to the short axis direction (axis direction with large attenuation) of the polarizer optical fiber. , the polarization direction perpendicular to this polarization direction is connected to the long axis direction of the polarizer optical fiber (the axial direction with small attenuation). In this way, only when the pump light is present and birefringence is occurring, light enters the polarizer optical fiber in its long axis direction, that is, in the direction of polarization with no attenuation of the polarizer optical fiber. It will appear at the other end of the optical fiber.

Furthermore, if the wavelength of this pump light is selected to be in a long wavelength range in which the polarized light in the long axis direction of the polarizer optical fiber is also attenuated, the pump light will not appear at the other end of the polarizer optical fiber. Therefore, a pulse obtained by sampling the input optical signal is sent to the output end of the polarizer optical fiber in synchronization with the timing of the pump light.

〔Example〕

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. An input optical signal enters terminal 1 . terminal 2
Pump light is incident on the The timing of the pump light is the sampling timing, and the signal has a considerably higher power level than the input optical signal. A sampled output optical signal is sent to the terminal 3. The input optical signal at the terminal 1 passes through the phase adjustment means 5 and is combined with the pump light by the combining dichroic mirror 4. The output light of this mirror 4 is condensed by a lens 6, and is then passed through a polarization-maintaining optical fiber 7.
Enter at one end of. This polarization maintaining optical fiber 7
have different refractive indices in the directions of the optical principal axes that are perpendicular to each other. The other end of this polarization maintaining optical fiber 7 is connected to one end of a polarizer optical fiber 8 . Cross mark 9 in the diagram
represents the connection point. The other end of the polarizer optical fiber 8 is the output terminal 3 described above.

The polarizer optical fiber 8 has a property that the propagation loss of polarized light in directions of optical principal axes orthogonal to each other is different. The phase adjustment means 5 is, for example, a quarter-wave plate here.

Now, if we consider the case where a linearly polarized input optical signal is input so that it almost coincides with the principal axis of the polarization-maintaining optical fiber 7, this optical signal is also maintained as linearly polarized light in the direction of the principal axis at the connection point 9 and is emitted. The polarization preserving optical fiber 7 may be connected so that one polarization direction (assumed to be the X-axis direction) coincides with the short axis direction (y-axis direction) of the polarizer optical fiber 8.
In this way, the polarized light in the one polarization direction (X-axis direction) of the polarization-maintaining optical fiber 7 enters the short axis direction of the polarizer optical fiber 8 and is rapidly attenuated.

When pump light enters from terminal 2 in this state,
In the polarization-maintaining optical fiber 7, new birefringence occurs due to the optical Kerr effect, and an optical signal with a component in a direction perpendicular to the polarization direction when the pump light is not present appears at the connection point 9. This optical signal is connected to the connection point 9
The light enters the polarizer optical fiber 8 in the long axis direction (the axial direction where attenuation is small), propagates through the polarizer optical fiber 8, and appears at the terminal 3. That is, the input optical signal appears at the terminal 3 only at the timing when the pump light is present at the terminal 2. In the two optical fibers 7 and 8,
Since there is linearity with respect to the amplitude of the optical signal passing through, the optical signal appearing at terminal 3 is a signal proportional to the amplitude of the input optical signal, and as a result, the input optical signal is sampled at the timing of the pump light. It turns out.

The wavelength relationship of each light will be explained in more detail. Third
The figure is a loss characteristic diagram of the polarizer optical fiber 8 used in the above example. The horizontal axis shows wavelength, and the vertical axis shows loss. The curve y is a loss characteristic curve in the minor axis direction, that is, the y-axis direction. The curve x is a loss characteristic curve in the major axis direction, that is, the x-axis direction. As can be seen from this figure, in this polarizer optical fiber, as the wavelength increases in the short axis direction, the loss rapidly increases at wavelength A. Further, as the wavelength increases in the long axis direction, there is a characteristic that the loss rapidly increases at a considerably long wavelength B. Therefore, the wavelength λs of the input optical signal is selected between wavelengths A and B, and the wavelength of the pump light is set to wavelength B.
By setting a longer wavelength, output terminal 3
Only the signal light is output, and the pump light is attenuated and not output.

FIG. 4 is a cross-sectional structural diagram of the polarizer optical fiber used in the above embodiment.

Next, the test results of the above examples will be explained. FIG. 5a is a waveform photograph of the pump light supplied to terminal 2 observed with a synchroscope. FIG. 5b is a waveform photograph of the input optical signal input to terminal 1, which was also observed using a synchroscope. Figure 5c
is a waveform photograph obtained by observing the output light appearing at the output terminal 3 with a synchroscope when this input optical signal and pump light are applied. It can be clearly seen that the input optical signal is clearly sampled at the timing of the pump light. FIG. 5d is a waveform photograph of the sampled output light from terminal 3 observed at a scale of 1 division 2nS. That is, according to the test results of this example, sampling of an optical signal of about 10 nS was performed.

The detailed data of the above-mentioned example device are as follows. The input optical signal was generated using a semiconductor laser with a center wavelength of 0.84 μm. Pump light has oscillation wavelength
Generated using a 1.064 μm Nd:YAG laser. This Nd:YAG laser was equipped with a mode lock and a Q switch in order to obtain pulsed light with a high peak value. The repetition frequencies of the mode lock and Q-switch are 100 MHz and 500 Hz, respectively. The polarization-maintaining optical fiber 7 has a core diameter of 4.4 μm, a relative refractive index difference Δ between the core and the cladding of 0.26%, and a length of 4.5 m.
This polarization maintaining optical fiber 7 is connected to the wavelength of the input optical signal.
At 0.84 μm, it becomes a single mode.

Also, the core diameter of the polarizer optical fiber 8 is 4.8
μm, the relative refractive index difference Δ is 0.26%, and the length is 10 m. When the wavelength of the input optical signal is 0.84 μm, its extinction ratio is about 20 dB, and the loss in the x-axis and y-axis propagation modes when the pump light wavelength is 1.064 μm is 60 dB or more.

Regarding the wavelengths of the optical signal and pump light described above, it is not necessary to necessarily set the relationship with the characteristics of the polarizer optical fiber as described above. For example, even if the wavelength of the input optical signal and the wavelength of the pump light both appear at the output terminal 3, if they can be separated using a separately provided filter, the present invention can also be practiced.

Furthermore, in the above embodiment, polarized light in the orthogonal direction caused by birefringence of a polarization-maintaining optical fiber is detected using a polarizer optical fiber.
Detection can also be done using other methods such as using polarizers and filters. The present invention can also be carried out using this method.

The above example shows an example in which linearly polarized light is incident on a polarization-maintaining optical fiber by aligning it with the main axis, but in general, an optical signal is directed in a fixed direction at the exit of a polarization-maintaining optical fiber, that is, at the connection point with a polarizer optical fiber. It is sufficient to emit it as linearly polarized light. This state is achieved by adjusting the phase adjustment means 5 of FIG. 1 while detecting the polarization state at the output end of the polarization-maintaining optical fiber. The effects of the present invention can be obtained by connecting polarizer optical fibers so that this polarization direction is incident on the short axis of the polarizer optical fiber.

Furthermore, the light emitted from the polarization-maintaining optical fiber at the connection point 9 does not need to be completely linearly polarized;
It is sufficient that the light is at least elliptically polarized and that its long axis direction is incident on the short axis direction of the polarizer optical fiber. In this case, the component in the short axis direction of the elliptically polarized light passes through the polarizer optical fiber even when no pump light is present, forming the background, so the sampling S/N ratio decreases. become.

In a polarization-maintaining optical fiber used in practical use, there is fluctuation in the refractive index in the longitudinal direction, so even when linearly polarized light is incident on the principal axis, orthogonal polarization components are generated, resulting in elliptically polarized light. In order to improve the S/N ratio, it is necessary to return this to linearly polarized light, and for this purpose, in principle, a phase shift amount of the opposite sign to that due to the fluctuation of the refractive index is generated.

The first specific method is to adjust the phase adjustment means 5 shown in FIG. 1, and the second method is to adjust the phase adjustment means 5 shown in FIG.
There is a method of locally applying stress to the optical fiber 7 by applying tension, bending, pressure, or twisting to the position of , changing the birefringence inherent in the optical fiber and obtaining the same effect as a wavelength plate. .

〔Effect of the invention〕

As described above, according to the present invention, high-speed optical sampling can be performed with a simple structure.
Since the method and apparatus of the present invention do not use an electric field, there is no factor that inhibits speeding up due to electrode capacitance. The response limit of the optical Kerr effect mentioned above is about 1 picosecond,
In principle, it is possible to increase the speed to this extent.
Since the device of the present invention is constituted by an optical system mainly composed of optical fibers, its structure is small, lightweight, and simple, and can be constructed at low cost.

In the future, it is thought that various devices for converting analog optical signals into digital optical signals will be used in optical communication devices, and the present invention is expected to have great effects when implemented in these devices.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram of optical sampling. Figure 3 is a characteristic diagram of the polarizer optical fiber. FIG. 4 is a cross-sectional structural diagram of a polarizer optical fiber. FIG. 5 is an oscilloscope waveform photograph showing the test results of the device according to the present invention. FIG. 6 is a block diagram showing a method of compensating for phase shift. 1...Terminal where input optical signal is incident, 2...Terminal where pump light is incident, 3...Terminal where sampled optical signal is sent out, 4...Dichroic mirror for multiplexing, 5...Phase adjustment Means, 6... Lens, 7... Polarization maintaining optical fiber, 8... Polarizer optical fiber, 9... Connection point.

Claims (1)

[Claims] 1. A method of multiplexing and inputting an input optical signal in a certain polarization state and a high-level pump light that causes birefringence into the polarization-maintaining optical fiber from one end of the polarization-maintaining optical fiber. and a method for extracting polarized light in a principal axis direction orthogonal to the polarization direction that appears when the input optical signal is not combined with the pump light at the other end of the polarization-maintaining optical fiber. 2. The extraction method is to attach one end of a polarizer optical fiber whose propagation loss differs for polarized light in directions of principal axes that are orthogonal to each other to the other end of a polarization-maintaining optical fiber, and connect the polarizer optical fiber with the axial direction where the attenuation is greater. In this method, the input photon is connected so as to almost match the long axis direction of the output light when it is not combined with the pump light, and the light appearing at the other end of the polarizer optical fiber is extracted as a sampled signal. An optical sampling method according to claim 1. 3. In the optical sampling method according to claim 2, the wavelength of the input optical signal is set to a region where the loss of short-axis polarized light of the polarizer optical fiber is large and the loss of long-axis polarized light is small, and the pump light is An optical sampling method characterized in that the wavelength is set in a region where both short-axis polarized light loss and long-axis polarized light loss of the polarizer optical fiber are large. 4 Phase adjustment means for adjusting the polarization direction of an input optical signal; means for multiplexing the output light of this means with pump light synchronized with the sampling timing; and polarized light at one end of which the output light of this means is incident The phase adjustment means comprises a polarization preserving optical fiber and an extraction means provided at the other end of the polarization preserving optical fiber, and the phase adjustment means adjusts the output from the extraction means when the input optical signal is not combined with the pump light. and the extracting means is a means for extracting a component of an optical signal whose polarization direction has changed due to the optical Kerr effect caused by the pump light in the polarization-maintaining optical fiber. Optical sampling device. 5. The extraction means includes a polarizer optical fiber connected to the other end of the polarization-maintaining optical fiber, and the polarization-maintaining optical fiber and the polarizer optical fiber are such that the input optical signal is not combined with the pump light. Claim 3, wherein the polarization-maintaining optical fiber is connected such that the long axis direction of the light emitted from the polarization-maintaining optical fiber substantially coincides with the axial direction of the polarizer optical fiber with high attenuation. Optical sampling device. 6. The optical sampling device according to claim 3, wherein the phase adjustment means is a quarter wavelength plate.