JPS61156232A - Method and device for optical sampling - Google Patents

Abstract

PURPOSE:To perform optical sampling at a high speed, by making input optical signals of prescribed polarization and pumped light from one end of a polarization maintaining optical fiber by multiplexing and extracting signal light polarized in the direction intersecting the polarizing direction of the input signal at right angles at the other end of the optical fiber. CONSTITUTION:Sufficiently weak input optical signals are made incident on a terminal 1 and adjustment is made so that the phase angle of the optical signals can coincide with one polarizing direction (direction of X-axis) of a polarization maintaining optical fiber 7 by means of a phase adjusting means 5. The optical signals are propagated through the optical fiber 7 and made incident on a polarizer optical fiber 8 in its minor axis direction (direction of y-axis). When pumped light is made incident from another terminal 2 under this state, the polarization maintaining characteristic declines and birefringence is produced in the fiber 7 and the optical signals propagated in the X-axis direction appear in the polarizing direction intersecting the X-axis direction (Y-axis direction also.The optical signals in the Y-axis direction are made incident on the polarizer optical fiber 8 in the major axis (x-axis direction) at a connecting point 9 and appear at a terminal 3 after they are propagated through the fiber 8. Therefore, input optical signals can be sampled at the timing of the pumped light.

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 an optical signal by utilizing the nonlinear optical effect of an optical fiber 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, in which signal light and pump light are combined and input 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 utilizing the fact that the birefringence characteristics of the polarizer change nonlinearly depending on 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. 2(a) is sampled at a desired timing to obtain optical pulses as shown by diagonal lines in FIG. 2(a). this is,
This can be done by opening and closing the path of the input optical signal at 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 makes the device more complex and requires higher precision machining to manufacture the device. 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 of generating birefringence in the optical fiber by inputting an input optical signal from one end of a polarization-maintaining optical fiber with a polarization that substantially coincides with the direction of one principal axis of the optical fiber. The present invention is characterized in that it includes a method of multiplexing and inputting a level pump light, and a method of extracting polarized light in a principal axis direction perpendicular to the one principal axis direction at the other end of the optical fiber.

The above extraction method is to
Connecting one ends of polarizer optical fibers having different propagation losses for polarized light in mutually orthogonal principal axes directions such that the long axis direction of the polarizer optical fibers substantially coincides with the orthogonal principal axis direction of the polarization maintaining optical fiber. However, it is desirable that the light be extracted as polarized light in the major axis direction appearing at the other end of the polarizer optical fiber.

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 multiplexing the output light of the 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 optical fiber is set to substantially coincide with one principal axis direction of the wave preserving optical fiber, and the extraction means is a means for extracting an optical signal polarized in a principal axis direction perpendicular to the one principal axis direction.

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 one principal axis direction of the polarization-maintaining optical fiber is It is preferable that the polarizer light/fiber be connected so as to substantially coincide with the minor axis direction of the fiber.

Preferably, the phase adjustment means is a quarter wave plate.

The above-mentioned expression "approximately corresponds" means to correspond 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. This optical signal passes through the polarization-maintaining optical fiber and appears at the other end of the polarization-maintaining optical fiber while maintaining the one polarization direction.

When a pump light with a considerably high power level is combined with this input optical signal and inputted from one end of the polarization-maintaining optical fiber, the optical power effect (K
err effect) causes birefringence, and an optical signal with a polarization direction perpendicular to the above polarization direction appears at the other end of the optical fiber, 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, the polarization direction of the human-powered optical signal is connected to the short axis direction of the polarizer optical fiber (high attenuation), and the polarization direction perpendicular to this polarization direction is connected to the long axis direction of the polarizer optical fiber (low attenuation ).

In this way, only when the pump light is present and birefringence is occurring, the polarizer optical fiber will be
That is, light enters the polarizer optical fiber in the polarization direction without attenuation, and this light appears at the other end of the polarizer 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. A human power optical signal is input to terminal 1. Pump light is input to terminal 2. 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 gold wave clock mirror 4. The output light of this mirror 4 is focused by a lens 6 and inputted to one end of a polarization maintaining optical fiber 7. This polarization-maintaining optical fiber 7 has different refractive indexes in the directions of optical principal axes that are orthogonal to each other, and has the property that light incident in one polarization direction appears unchanged at the other end in that polarization direction. The other end of this polarization maintaining optical fiber 7 is connected to one end of a polarizer optical fiber 8. The cross mark 9 in the figure 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.

Here, at the connection point 9, one polarization direction (assuming the X-axis direction) of the polarization-maintaining optical fiber 7 is connected to the polarizer optical fiber 8.
Connect it so that it matches the short axis direction (y-axis direction) of □. That is, the polarized light in the one polarization direction (X-axis direction) of the polarization-maintaining optical fiber 7 is set so that it enters the short axis direction of the polarizer optical fiber 8 and is rapidly attenuated.

First, give a sufficiently weak input optical signal from terminal 1,
The phase angle of this input optical signal is adjusted so as to match the one polarization direction (X-axis direction) of the polarization-maintaining optical fiber 7. This optical signal propagates through the polarization-maintaining optical fiber 7 and enters the polarizer optical fiber 8 in the short axis direction (y-axis direction). This optical signal does not appear at terminal 3 because it attenuates rapidly.

When pump light enters from the terminal 2 in this state, in the polarization maintaining optical fiber 7, the polarization maintaining characteristic is destroyed due to the optical force effect and birefringence occurs, and the above one polarization direction (X-axis direction)
The optical signal that was propagating in the same direction also appears in the orthogonal polarization direction (Y-axis direction). This Y-axis direction optical signal enters the polarizer optical fiber 8 in the long axis direction (X-axis direction) at the connection point 9 .

In the polarizer optical fiber 8, since attenuation is small for polarized light in the long axis direction (X-axis direction), the light 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.

Since the two optical fibers 7 and 8 have linearity with respect to the amplitude of the optical signal passing through them, the optical signal appearing at the 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.

The wavelength relationship of each light will be explained in more detail. FIG. 3 is a loss characteristic diagram of the polarizer optical fiber 8 used in the above embodiment. 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. curve
is a loss characteristic curve in the major axis direction, that is, in the 4X axis direction. As can be seen from this figure, this polarizer optical fiber is
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 between wavelengths A and B.
By selecting , and setting the wavelength of the pump light to a wavelength longer than wavelength B, only the signal light is output to the output terminal 3,
The pump light will be attenuated and will not be 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. 5+al is a waveform photograph of the pump light supplied to terminal 2 observed with a synchroscope. Figure 5 (bl is a waveform photograph of the input optical signal input to terminal 1, also observed using a synchroscope. Figure 5 (C) shows the waveforms at output terminal 3 when this input optical signal and pump light are applied. This is a waveform photograph of the output light that appeared using a synchroscope.It is clear that the input optical signal is clearly sampled at the timing of the pump light.Figure 5+d) shows the sampled output light from terminal 3. This is a photograph of a waveform observed at a scale of 2 nS per division. That is, according to the test results of this example, sampling of an optical signal of approximately 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 an oscillation wavelength of 1.064 μm
Nd: Generated using a YAG laser.

This Nd:YAG laser was used with a mode rotor and a Q switch added in order to obtain high peak value pulsed light. The repetition frequencies of mode-locking and Q-switching are 100 MHz and 5001) z, respectively. The polarization maintaining optical fiber 7 has a core diameter of 4.4μ.
The relative refractive index difference Δ between the m1 core and the cladding is 0.26
%, and the length is 4.5 m. This polarization-maintaining optical fiber 7 becomes a single mode when the wavelength of the input optical signal is 0.84 μm.

In addition, the polarizer optical fiber 8 has a core diameter of 4.8 μm.
The relative refractive index difference Δ is 0.26%, and the length is 10 m.

At an input optical signal wavelength of 0.84 μm, its extinction ratio is approximately 20.
dB, and the loss in the y-axis and y-axis propagation modes with a pump light wavelength of 1.064 μm is 60 dB or more.

- The wavelengths of the optical signal and pump light described above do not necessarily have to be related to 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 in this case.

Furthermore, in the above embodiment, polarized light in orthogonal directions caused by birefringence of a polarization-maintaining optical fiber is detected using a polarizer optical fiber. It can also be detected by The present invention can also be practiced using this method.

Next, the relationship among the powers of the input optical signal, pump light, and transmitted output light will be quantitatively explained. The output optical power P when the input optical signal is transmitted through the optical fiber is Pt - Ps 5in (φ/2) sin (2θ) when the input optical signal input power is P5 and the pump optical power is Pp.
----------(1) θ-jan-' (P % /
2 t') ・−−−−−−−−−−+3
It becomes 1. Here, the subscripts x and y above represent the components in the X-axis and y-axis directions, respectively, φ is the amount of phase change depending on the optical intensity caused by the pump light power, and θ is the incidence of the input optical signal and the optical principal axis of the fiber. represents an angle. Also L
is the length of the polarization-maintaining optical fiber, λ1 is the signal light wavelength, and χ is the nonlinear coefficient.

From the above equation (1), the transmitted output optical power Pt is maximum when φ=π, and when the pump light is incident at 45° to the optical principal axis of the polarization-maintaining optical fiber 7, φ=O. The transmitted signal light power becomes zero. This φ=π can be realized by appropriately selecting the pump light power for a fixed fiber length, as shown in equation (2). Also, from equation (1), by setting the incident angle of the signal light to the optical principal axis of the optical fiber 7 to be θ#π/4, the transmitted light P can be maximized.

Next, polarization-maintaining optical fibers in practical use have a slight inherent birefringence even when a weak optical signal is input, and the amount of phase shift between orthogonal polarizations caused by this is compensated for and We will explain how to convert the polarization into linearly polarized light that coincides with the y-axis of the polarizer fiber at the connection point. In principle, this can be achieved by generating a phase shift amount of opposite sign to the amount of phase shift that occurs. In addition to the method using the wave plate shown in FIG. 1, there is a method shown in FIG. 6.

That is, by applying tension, bending, pressure, or twisting to the position 15 just before the connection point,
By applying stress to the local optical fiber 7, the inherent birefringence of the optical fiber is changed to obtain an effect equivalent to that of a wave plate. The method shown in FIG. 7 is a method in which another polarization-maintaining light beam 16 is connected to the output end of the polarizer optical fiber 8 to compensate for the phase shift.

〔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 force effect mentioned above is about 1 picosecond, and in principle it is possible to increase the speed to this extent.

Since the device of the present invention is composed of 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 a 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. FIGS. 6 and 7 are block diagrams showing a method for compensating for phase shifts. DESCRIPTION OF SYMBOLS 1... A terminal into which an input optical signal is incident, 2... A terminal into which a pump light is incident, 3... A terminal into which a sampled optical signal is sent out, 4... A dichroic mirror for multiplexing,
5... Phase adjustment means, 6... Lens, 7... Polarization maintaining optical fiber, 8... Polarizer optical fiber, 9...
connection point.

Claims (6)

[Claims] (1) From one end of a polarization-maintaining optical fiber, an input optical signal with a polarization that almost coincides with the direction of one principal axis of this optical fiber is combined with a high-level pump light that causes birefringence in this optical fiber. and a method of extracting polarized light in a principal axis direction perpendicular to the one principal axis direction at the other end of the 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 one end of a polarizer optical fiber whose long axis direction is above the other end of a polarization-maintaining optical fiber. The method of claim 1 is a method of connecting polarization-maintaining optical fibers so as to substantially coincide with the orthogonal principal axes thereof, and extracting the polarized light in the long axis direction appearing at the other end of the polarizer optical fiber. The optical sampling method described in section 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. , an optical sampling method characterized in that the wavelength of the pump light is set in a region where both short axis polarization loss and long axis polarization loss of the polarizer optical fiber are large. (4) a phase adjustment means for adjusting the polarization direction of the input optical signal; a means for combining the output light of this means with a pump light synchronized with the sampling timing; and the output light of this means is incident on one end. It comprises a polarization-maintaining optical fiber and an extraction means provided at the other end of the polarization-maintaining optical fiber, and the phase adjustment means is configured to adjust the polarization of the output light in the direction of one principal axis of the polarization-maintaining optical fiber. An optical sampling device, wherein the extraction means is a means for extracting an optical signal polarized in a principal axis direction perpendicular to the one principal axis direction. (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:
The optical sampling device according to claim 3, wherein the polarization maintaining optical fiber is connected such that one principal axis direction substantially coincides with the short axis direction of the polarizer optical fiber. (6) The optical sampling device according to claim (3), wherein the phase adjustment means is a quarter wavelength plate.