JP2641737B2 - Optical multipoint signal multiplex transmission equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical-applied multipoint multiplex transmission apparatus, and more particularly to an apparatus for improving the stability of transmission.

<< Conventional Technology >> Multipoint signal multiplexing by modulation frequency division based on the principle of phase modulation of light using the photoelastic effect of an optical fiber using a Mach-Zehnder interferometer as a kind of applied technology of optical fiber A device has been proposed.

FIG. 5 shows an example of a multipoint signal multiplexer based on this principle.

The multiplexing device shown in FIG. 1 uses, for example, a semiconductor laser as a light source 1 and a pair of a reference light fiber 2 and a modulation signal fiber 3 constituted by a single mode optical fiber.
And a light receiving element 4.

The signal fiber 3 is coupled at both ends to the reference light fiber 3 via the optical fiber couplers 5,5. With this configuration, the light wave incident from the light source 1 is branched by the front coupler 5. After passing through each of the fibers 2 and 3, they are joined again at the rear coupler 5.

The signal fiber 3 is provided with two modulators 6 and 7 for modulating a light wave passing therethrough at intervals.

In the device having such a basic configuration, two light paths (reference light fiber) having different light waves emitted from the light source 1 are used.
2. After passing through the modulation signal fiber 3), they are merged, so that an interference wave is observed at the light receiving element 4, and the interference wave is transmitted with a modulation signal.

Now, the output of the light source 1 is P ₀ , the input amplitude of the modulator 6 is A ₁ , the angular frequency is ω ₁ , the input amplitude of the modulator 7 is A ₂ , and the angular frequency is ω ₂ . Then, the output P obtained by the light receiving element 4 is expressed by the following equation.

P = P ₀ [1 + sin {A ₁ sin (ω ₁ t + φ ₁ ) + A ₂ sin (ω ₂ + φ ₂ ) + θ}] In this equation, θ is the above-described device and is different from the light wave 2
This is a factor of a phase variation due to a temperature change or the like occurring between the two reference light fibers 2 and the modulation signal fiber 3 because they pass through the two.

By the way, if such a phase fluctuation is left as it is, the operating point (optical bias) of the modulation applied by each of the modulators 6 and 7 becomes extremely unstable.

Therefore, conventionally, such a phase fluctuation has been compensated for using a compensating device.

FIG. 5 shows an example of the phase fluctuation compensator 8 used in the conventional transmission apparatus. In this example, the phase fluctuation compensator 8 is installed on the downstream side of the modulator 7 to compensate for the phase fluctuation. Table 8
Is feedback-controlled by the differential amplifier 9 for the output signal of the light receiving element 4.

However, such means for compensating for phase variations include:
There were technical issues described below.

<< Problems to be Solved by the Invention >> That is, as the above-mentioned phase fluctuation compensator 8, those which apply stress to the modulation signal fiber 3, for example, load applying means by PZT, thermal stress applying means by heating, etc. are proposed. However, these compensating means compensate for the phase fluctuation due to the shortage of the dynamic range of the control when the optical fiber that is the transmission path becomes long and the amount of phase fluctuation to be compensated increases. There was a problem that it became impossible.

Further, in the configuration in which the phase fluctuation compensator 8 is feedback-controlled, there is a problem that it is not possible to respond to a phase fluctuation at a relatively high speed, so that it cannot quickly follow the phase fluctuation.

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem, and an object of the present invention is to provide an optical-applied multipoint signal multiplex that can ensure stable transmission without compensating for phase fluctuation. It is an object of the present invention to provide a simplified transmission device.

<< Means for Solving the Problems >> In order to achieve the above object, the present invention provides a polarizing beam splitter that splits a light wave emitted from a laser light source into two light waves, a first light wave and a second light wave. A pair of Faraday rotators that receive the first and second light waves from the beam splitter and rotate the polarization axis by 45 degrees for transmission, and the first and second light waves rotated by the Faraday rotator A polarization-maintaining optical fiber into which these light waves are incident from both ends in a state where the respective polarization axes and one of the intrinsic polarization axes coincide with each other, and any one from the axial center position of this polarization-maintaining optical fiber A plurality of modulators provided at positions deviated to one side, and the first and second light waves pass through the polarization-maintaining optical fiber and then pass through the Faraday rotator again. The polarization axis is rotated by 45 degrees, and a quarter-wave plate that is incident while being merged in the same direction via the polarizing beam splitter, and an analyzer that receives the output light of the quarter-wave plate, And a light receiving element that receives the output light of the analyzer and converts it into an electric signal.

<< Operation >> According to the multiplex transmission apparatus having the above-described configuration, the first and second light waves emitted from the laser light source and branched by the polarization beam splitter are incident from both ends of the polarization-maintaining optical fiber and are respectively applied to the other ends. Since these two light waves pass through the same side, these two light waves pass through exactly the same optical path, so that the amount of phase fluctuation is the same and the compensation thereof is not necessary.

Further, the first and second light waves that have passed through the polarization maintaining fiber, have their polarization axes rotated by 45 degrees by the Faraday rotator, and have been merged again by the polarization beam splitter are in a state where their polarization axes are orthogonal to each other. Therefore, it becomes possible to use a quarter-wave plate, and these interference waves whose phases have been changed by 90 degrees by the quarter-wave plate are output to the light receiving element, thereby reducing the optical bias of the modulator. Obtainable.

Embodiments Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 and FIG. 2 show an embodiment of an optical applied multipoint signal multiplexing transmission apparatus according to the present invention.

The transmission apparatus shown in FIG. 1 uses a loop-type interferometer using a polarization maintaining optical fiber 10 as a basic configuration.

A Faraday rotator 12, a polarization beam splitter 14, and a laser light source 16 are installed in this order on the optical axis of an end face 10a on one end side of the polarization maintaining optical fiber 10 from the end face side of the optical fiber 10.

The optical axis of the end face 10b on the other end side of the polarization-maintaining optical fiber 10 is set so as to be orthogonal to the optical axis of the end face 10a on the one end side.On this optical axis, another Faraday rotator 18, A polarization beam splitter 14, a quarter-wave plate 20, an analyzer 22, and a light receiving element 24 are sequentially installed.

The applied magnetic fields of the Faraday rotators 12 and 18 are set such that the polarization axes are rotated by 45 degrees in the counterclockwise rotation direction when viewed from the incident light side, and are emitted.

Also, the length L in the axial direction of the polarization maintaining optical fiber 10 is described.
(The optical fiber 10
One end l ₁ and l ₂ positions) in the two modulators 26 and 28 are installed from the.

In the apparatus having the above configuration, when the laser light source 16 emits a light wave at a linear polarization direction of 45 degrees, the first light wave A (second light wave A) is emitted.
The second light wave B (shown by a dotted line in FIG. 2) and the second light wave B (shown by a dotted line in FIG. 2) reach the light receiving element 24 as shown in FIG.

The light wave emitted from the laser light source 16 in the state having the vertical and horizontal polarization axes is transmitted and reflected by the polarization beam splitter 14, thereby forming the first light wave A having the vertical polarization axis and the horizontal light direction. Into a second lightwave B having a polarization axis of

Then, the branched first light wave A and second light wave B
Have a polarization axis of 4 by the Faraday rotators 12 and 18, respectively.
Polarization-maintaining optical fiber 1 rotated 5 degrees counterclockwise
The light is incident on both end faces 10a and 10b of 0.

Here, one of the intrinsic polarization axes of the polarization maintaining optical fiber 10 is made to coincide with the polarization axes of the rotated first light wave A and second light wave B.

The first light wave A and the second light wave B incident on the polarization maintaining optical fiber 10 pass on the same unique polarization axis toward the other end, respectively, and the first light wave A and the second light wave B The optical path lengths of the light wave A and the second light wave B are exactly the same, and the phase fluctuation compensator, which is indispensable in a conventional device of this type, becomes unnecessary.

The first and second light waves A and B have a polarization plane optical fiber 10.
When the signal passes through the inside, a modulation signal is added by the modulators 26 and 28, respectively.

The first light wave A and the second light wave B emitted from the other end surface of the polarization maintaining optical fiber 10 are again transmitted to the Faraday rotator 1.
The polarization axes of the first lightwave A and the second lightwave B are shifted in the left and right directions by 45 degrees at 2,18, respectively. The light enters the polarization beam splitter.

The first light wave A and the second light wave B that enter in such a state of the polarization axis are transmitted and reflected by the polarization beam splitter 14, merged again, and emitted.

First light wave A emitted from polarization beam splitter 14
Since the polarization axes of the light wave B and the second light wave B are orthogonal to each other, it is possible to apply a 90-degree optical bias using the quarter-wave plate 20 and change the phase.

Then, the first light wave A and the second light wave B biased by the quarter-wave plate 20 are incident on the analyzer 22 whose optical axis is inclined by 45 degrees with respect to these polarization axes. Interference is caused, and the interference light is converted into an electric signal by the light receiving element 24 and output.

Now, the incident light E0x, which enters the polarizing beam splitter 14,
Assuming that E0y is E0x = E0y = ae ^iωot (ω ₀ = angular frequency of light wave, a = amplitude of light wave) and the modulator 26 performs phase modulation of amplitude b and angular frequency ω ₁ , the light reaches the analyzer 22. The first light wave A is The second light wave B is (Β: propagation constant in the fiber 10, l ′ = L−1).

Therefore, the output P obtained by the analyzer 22, P = 1/2 and [E1x + E1y] ^{2 =} a ^2/2 [1 + sin {2b sin (βω1 / 2ω0 (L -2l1)) sin (ω1t + βω1 / 2ω0L)} ] Become.

Here, when a ^2/2 = Po and to 2b sin (βω1 / 2ω0 (L -2l1)) «π / 2, {2b sin (βω1 / 2ω0 (L-2l1)) sin (ω1t + βω1 / 2ω0L)} ≪π / 2. Accordingly, sin ｛2b sin (βω1 / 2ω0 (L−211)) sin (ω1t + βω1 / 2ω0L)｝ = 2b sin (βω1 / 2ω0 (L−211)) sin (ω1t + βω1 / 2ω0L), and P = Po becomes _{{1 + ρsin (ω t t} + βω1 / 2ω0L)} ····· (I) ρ = 2b sin (βω1 / 2ω0 (L-2l1).

The first term in the above formula (I) is a constant value, and the second term is a modulated output value. The amplitude (relative value) | ρ | of the output, the modulation frequency ω1 and the modulation position (L-2l1) Is shown in FIG.

As is clear from the figure, when the relationship between the modulation frequency ω1 and the modulation position (L−211) becomes an integral multiple of π / 2, the sensitivity of the modulation output becomes the best, and each modulation is performed so as to satisfy this condition. It is desirable to set the frequency and position of the devices 26, 28.

When no 90 ° optical bias is applied by the quarter-wave plate 20, E1y of the second light wave B becomes And the output P becomes P = a ^2/2 [1 + cos {2b sin (βω1 / 2ω0 (L -2l1)) sin (ω1t + βω1 / 2ω0L)} ]. Here, the same approximation as above is applied. That is, if 2b sin (βω1 / 2ω0 (L−211)) lπ / 2, then 2b sin (βω1 / 2ω0 (L−211)) sin (ω1t + βω1 / 2ω0L) ≪π / 2. Therefore, cos {2b sin (βω1 / 2ω0 (L−211)) sin (ω1t + βω1 / 2ω0L)} = 1 Therefore, P = Po (1 + 1) = 2Po, and almost no modulation output is obtained.

However, in the present invention, by applying a 90-degree optical bias between the first light path A and the second light wave B by the quarter-wave plate 20, these combined light waves are converted into circularly polarized light. ,
As shown in FIG. 4, the operating points of the modulators 26 and 28 are moved from the position P of the linearly polarized light to the point Q of the linear part, whereby the output signal is amplified and increased, and stable transmission is achieved. The state is secured.

The installation positions l ₁ and l ₂ of the modulators 26 and 28 are
Since the measurement can be performed by separating the frequency of the modulated signal received at 24, the device of the present invention can be used as a multipoint sensor.

<< Effects of the Invention >> As described in the above embodiments, in the optical applied multipoint signal multiplexing transmission apparatus according to the present invention, even if the phase fluctuation compensator which is conventionally required in this type of apparatus is not provided, the temperature can be increased. An excellent effect is obtained that the influence of the phase fluctuation due to the change or the like can be reliably eliminated and a relatively large transmission output signal can be stably obtained.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing one embodiment of the apparatus of the present invention, and FIG.
FIG. 3 is an explanatory view showing a change in the state of the polarization axis in the optical system of the apparatus, FIG. 3 is an explanatory view showing the relationship between the output of the modulator, frequency and position, and FIG. FIG. 5 is an overall configuration diagram of a conventional apparatus. 10 ... Polarization maintaining optical fiber 12,18 ... Faraday rotator 14 ... Polarizing beam splitter 16 ... Laser light source 20 ... Quarter wavelength plate 22 ... Analyzer 24 ... Light receiving element 26,28 ... Modulator A: First light wave B: Second light wave

Claims (1)

(57) [Claims] 1. A polarization beam splitter for splitting a light wave emitted from a laser light source into two, a first light wave and a second light wave, and receiving the first light wave and the second light wave from the polarization light beam splitter, respectively. A pair of Faraday rotators for rotating and transmitting the axes by 45 degrees, and the respective polarization axes of the first and second light waves rotated by the Faraday rotator coincide with one of the intrinsic polarization axes. A polarization-maintaining optical fiber into which these light waves are incident from both ends in the state, and a plurality of modulators provided at positions deviated to one side from the axial center position of the polarization-maintaining optical fiber, After the first and second light waves pass through the polarization-maintaining optical fiber, they pass through the Faraday rotator again to rotate the polarization axis by 45 degrees, and the polarization beam splitter A quarter-wave plate which is incident in a state of being merged in the same direction through the light-emitting device, an analyzer for receiving the output light of the quarter-wave plate, and a light receiving device for receiving the output light of the analyzer and converting it into an electric signal. An optical-applied multipoint multiplex transmission device comprising: an element;