JPH08179248A - Manufacture of polarized light modulator - Google Patents

Abstract

PURPOSE: To adjust the main-axis direction of a polarized wave maintaining fiber which is led in the input terminal of the polarized light modulator in a short time by monitoring an interference signal included in the output light of the modulator. CONSTITUTION: Polarized light signals 101 and 102 which are different in wavelength are made incident on two orthogonal main axes of the polarized light maintaining fiber 2. The light from this polarized light maintaining fiber is made incident on the polarized light modulator 1, which modulates the signals with a driving pulse signal 103. The two modulated lights are supplied to an analyzer 5 to cut components in one polarization direction, an optical detector 6 detects them to generate an electric signal, and a spectrum analyzer 7 monitors the interference signal 104 generated owing to the frequency difference between the two lights. The angle of rotation of a rotary mechanism 5, the main axis direction of the analyzer, and the amplitude of the driving pulse signal are controlled so that the interference signal becomes minimum, and when the interference signal becomes minimum, the polarized light maintaining fiber 2 and polarized light modulator 1 are fixed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to manufacture of a polarization modulator used in optical fiber communication and the like, and more particularly to adjustment of a main axis direction of a polarization maintaining fiber introduced into an input end of a polarization modulator.

[0002]

2. Description of the Related Art In optical fiber communication systems for transmitting high-speed digital signals, polarization modulation is often an important technique. For example, in a transmission system that uses an erbium-doped optical fiber amplifier (hereinafter referred to as an EDFA) as a repeater, the EDFA has a gain and a loss existing in a polarization state, so that the reception end is accompanied by a change in the polarization state in the transmission line. There is a problem that the state of signal light in the field changes and stable transmission cannot be realized. As a method of avoiding this, it has been proposed to modulate the polarization of the transmitted light at high speed and average the polarization in the transmission path (called polarization scrambling).

A polarization modulation-direct detection method has been proposed as an effective communication method for avoiding the non-linear optical effect which is a problem in a transmission system using the EDFA as a repeater. In this method, since the binary digital signal is assigned to two polarization states, the polarization modulation means is an important technique in the transmitter.

As a simple and compact means for realizing this polarization modulation, there is a method using a LiNbO ₃ polarization modulator. FIG. 1 shows the principle of this type of polarization modulator. The linearly polarized light signal 100 from the light source is incident on the polarization modulator 1 at 45 ° with respect to the X-axis and Y-axis waveguide principal axes shown by the alternate long and short dash lines, and is incident in the two waveguide principal axis directions. Divide into 1: 1. In the polarization modulator, since the phase change amount of the drive pulse signal 103 with respect to the applied voltage is different between the two axes (it does not change in the X-axis direction but changes in the Y-axis direction), the modulator exit corresponding to the applied voltage. The phase relationship between the two lights changes at, and as a result, the polarization is modulated by 90 ° as shown by selecting the applied voltage.

In the manufacture of this polarization modulator, it is an important condition to ensure the performance that the plane of incidence polarization, that is, the principal axis direction of the knitting wave preserving fiber and the direction of incidence of the polarization modulator are aligned and connected. Therefore, the angle adjustment of the knitting wave storage fiber introducing portion is performed by visual observation through a microscope.

[0006]

In the polarization scrambling or polarization modulation-direct detection method, polarization modulation must be performed between two orthogonal polarization states. For this purpose, the incident angle of 45 ° of the incident light to the modulator must be accurately adjusted, and a deviation from 45 ° leads to a large deterioration in performance. For example, when the angle deviates from 45 ° by 1 °, the degree of polarization indicating the degree of polarization scrambling deteriorates by about 3.5%.

However, the current visual angle adjustments are: It is difficult to determine the correct principal axis direction of the polarization modulator.・ Because the axes are connected at 45 ° instead of being aligned,
There is a problem that a reference line must be provided separately, and it is difficult to expect high-precision coupling.

Furthermore, when the polarization modulator has different losses in the two axial directions, the incident angle that gives the orthogonal polarization modulation state is not 45 °. Therefore, in this case, the polarization modulation state cannot be obtained even if the incident angle is adjusted to 45 °. Further, there is a problem that it is difficult to accurately determine the angle to be incident in this case.

[0009]

According to the method of manufacturing a polarization modulator of the present invention, the main axis direction of the polarization maintaining fiber introduced into the input end of the polarization modulator is made to coincide with the incident direction of the polarization modulator. After that, in the method for manufacturing a polarization modulator in the step of fixing the polarization maintaining fiber to the polarization modulator, first and second polarizations having different wavelengths from the tip of the polarization maintaining fiber and polarization directions orthogonal to each other. Means for inputting signals by polarization multiplexing; means for applying a drive pulse signal for obtaining a modulation degree of 90 ° to the polarization modulator temporarily connected to the polarization maintaining fiber; A rotation mechanism provided at a temporary connection portion with the polarization modulator, means for cutting output light in one polarization direction from an output end of the polarization modulator, and converting the output light into an electric signal. The first signal included in the signal And an interference wave display means for extracting and displaying the first interference signal generated by the frequency difference between the second polarized signal and the rotation mechanism so that the level of the first interference signal is minimized. The rotation angle of the polarization maintaining fiber is adjusted and fixed.

Further, a means for mixing the output light of one polarization direction cut out from the output end of the polarization modulator with a third polarization signal having a frequency different from that of both the first and second polarization signals is used. A second interference signal, a second polarization signal, and a third polarization signal generated by a frequency difference between the first polarization signal and the third polarization signal displayed on the interference signal display means.
And the third interference signal generated due to the frequency difference from the polarized light signal is polarized at a predetermined level or more, and the rotation angle of the polarization maintaining fiber is fixed so that the level of the first interference signal is minimized. You can also do it.

[0011]

The concept of the present invention is shown in FIG. The polarization maintaining fiber 2 connected to the incident side of the polarization modulator 1 guides the incident linearly polarized light polarization signals 101 and 102 to the polarization modulator. A rotating mechanism 4 for rotating the fiber is attached to the fiber side at the introduction part of the polarization maintaining fiber 2 so that the angle of the incident polarization plane can be adjusted. Two polarization signals 101 and 102 having slightly different wavelengths are incident on the two main axes of the polarization maintaining fiber 2.

The polarization modulator 1 has a rectangular drive pulse signal 1
03 is applied, and two signals are polarization-modulated by this signal. The light emitted from the polarization modulator has one polarization direction cut out by the analyzer 5 attached to the output end. The analyzer 5 has a rotating mechanism and is capable of adjusting the polarization direction to be cut out. A photodetector 6 is attached to this detection output, and converts the interference signal of the output light into an electric signal. The level of the interference signal 104 is displayed by the spectrum analyzer 7 following the photodetector.

Next, the angle adjusting method will be described with reference to FIGS. The two polarization signals 101 and 102 that enter the polarization modulator 1 from the polarization maintaining fiber 2 enter at an optimum angle, and the amplitude of the drive pulse signal to the polarization modulator 1 passes through the two axes of the polarization modulator. The phase difference of the light
When the voltage is changed, the two polarization signals at the output end of the polarization modulator are switched while maintaining the orthogonal linear polarization state in the orthogonal relationship, as shown in FIGS. 3 (a) and 3 (b), respectively. To do.

Here, when the axis of the analyzer 7 attached to the exit end of the polarization modulator is in the direction A shown in FIGS. 3 (a) and 3 (b), the change in the optical power incident on the photodetector is observed. Is as shown in FIG. That is, the polarization signals 101 and 102 are alternately output with the same power according to the drive pulse signal. As a result, since only one of the two incident lights appears in the photodetector, no interference signal is generated due to the frequency difference between the two polarization signals.

On the other hand, when the incident angle of the polarization maintaining fiber 2 is deviated from the optimum direction A to B, at the exit end of the polarization modulator 1, for example, as shown in FIGS. In addition, the polarization modulator switches between polarization states that are out of phase with each other due to the phase difference π of light. As a result, only one of the lights cannot be cut out, no matter how the direction of the analyzer axis at the exit end is set. Therefore, the powers of the two polarization signals incident on the photodetector are as shown in, for example, FIG. 4C, and since there are two polarization signals at the same time, an interference signal generated by this frequency difference appears in the spectrum analyzer.

Utilizing the above phenomenon, the rotation angle of the polarization maintaining fiber is adjusted so as to minimize the level of the interference signal appearing on the spectrum analyzer, thereby optimizing the polarization maintaining fiber and the polarization modulator. It is possible to obtain various bond angles. Further, minimizing the level of the interference signal can be used for adjusting the optimum value of the applied voltage of the drive pulse signal and the axial direction of the analyzer.

[0017]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 5 is a block diagram of the first embodiment of the present invention. The polarization maintaining fiber 2 is installed on an XYZ stage 8 having a rotating mechanism 4 that rotates in the fiber axis direction. The polarization signals 101 and 102 from the two laser emission sources 16 and 17 used for measurement are polarization-multiplexed by the polarization beam splitter 17, and then enter the two principal axes of the polarization maintaining fiber 2.

A rectangular drive pulse signal 103 is applied to the polarization modulator 1. A single mode optical fiber 9, a polarization controller 10, and a polarization beam splitter 13 are connected to the emission end of the polarization modulator 1. The polarization controller 10 and the polarization beam splitter 13 cut out an optical output in one polarization direction from the output end of the polarization modulator and branch it, and correspond to the analyzer 5 in FIG.

The first output light of the polarization beam splitter 13 is detected by the wide band photodetector 6 and becomes an electric signal. Two light interference signals 104 included in this detection signal
Are displayed by the spectrum analyzer 7. On the other hand, the second output light of the polarization beam splitter 13 becomes an electric signal by the photodetector 14 and is level-monitored.

Next, a method of adjusting the polarization preserving fiber 2 in the principal axis direction will be described. First, the control unit 12 sets XY so that the output level of the photodetector 14 is maximized according to an instruction from the operator.
The Z stage 8 is controlled so that the polarization maintaining fiber 2 and the input end waveguide of the polarization modulator 1 are aligned with each other.

Next, the polarization preserving fiber 2 is adjusted in the main axis direction. The amplitude of the drive pulse signal 103 is set to a predetermined reference value in advance, and the angle adjustment of the polarization controller 10 is also set to a predetermined reference value. And put. Control unit 1 according to an operator's instruction
From 1, the control signal is sent to the rotating mechanism 4 and adjusted so that the level of the interference signal 104 displayed on the spectrum analyzer is minimized. Next, the control unit 11 sends a control signal to the polarization modulator 1 and the polarization controller 10, and the interference signal 104
The level of is adjusted to the minimum.

Next, the control signal is again sent from the control unit 11 to the rotating mechanism 4 and readjusted so that the level of the interference signal is minimized. Thereafter, the same adjustment operation is repeated in the adjustment direction of the minimum level to obtain the final minimum value of the interference signal, and the polarization maintaining fiber and the polarization modulator are fixed at this position.

At this time, the amplitude value of the drive pulse signal and the angle of the polarization controller are left as data and are used when the polarization modulator is attached to the apparatus. The control units 11 and 12 automatically perform the above adjusting operation in a short time according to the instruction of the operator.

However, in the embodiment of FIG. 5, when the polarization maintaining fiber 2 is rotatable in a wide range, as shown in FIG. 6, the incident polarization directions of the polarization signals 101 and 102 are as shown, and the phase modulator. 1 axis, the principal axis direction C of the analyzer at the output end
Since only one light beam will pass through even when all are aligned, the interference signal 10 is minimized and there is a risk of misadjustment. In order to avoid this, it is sufficient to limit the rotatable angular range of the incident polarization maintaining fiber to a small value in the vicinity of 45 °, but the preparation process becomes troublesome.

The second embodiment shown in FIG. 7 solves this problem. FIG. 7 is a block diagram showing the second embodiment. The difference from FIG. 5 is that a polarization-maintaining optical fiber coupler 6 is inserted between the polarization beam splitter 13 and the photodetector 6, and the polarization signals 101 and 102 are input to the input fiber of the optical fiber coupler 6. The wavelength of which is different from that of any of the light
This is a point at which the optical signal from the laser light emitting source 19 is incident and mixed with the output light of the polarization beam splitter 17.

The mixed output light is detected by the photodetector 6 and converted into an electric signal. The polarization signal 101,
102 interference signal 104 (hereinafter referred to as f ₁ component), interference signal 105 between polarization signal 101 and the third optical signal (hereinafter f ₁ )
₂ is referred to as a component), monitoring the interference signal 106 with the polarization signal 102 and the third optical signal (hereinafter referred to as f ₃ component) with Spectra analyzer 7. In the case of the state of FIG. 6, either f ₂ component or f ₃ becomes 0. Therefore, in the control, the f ₁ component should be minimized without setting the f ₂ f ₃ component to 0. , Controls the main axis direction of the polarization maintaining fiber. As a result, it is possible to avoid falling into the state of FIG.

[0027]

As described above, according to the method of manufacturing the polarization modulator of the present invention, the adjustment of the direction of the principal axis of the polarization maintaining fiber introduced at the input side of the polarization modulator is performed by adjusting the level of the interference signal of the modulator output light. And the amplitude of the modulator drive pulse related to this adjustment and the complicated adjustment of the axial output direction of the modulator output light by simultaneously monitoring the level of the interfering signal. This has the effect of simplifying the overall work and enabling it to be done quickly and systematically.

[Brief description of drawings]

FIG. 1 is a principle diagram of a polarization modulator.

FIG. 2 is a conceptual diagram of an example of the present invention.

FIG. 3A is a polarization signal 1 at a normal incident angle in FIG.
01 is the polarization state, (b) is the polarization state of the polarization signal 102,
(C) is a characteristic diagram showing a power fluctuation state of modulated output light.

4 (a) is a polarization state of a polarization signal 101 and FIG. 4 (b) is a polarization state of a polarization signal 102 at different incident angles in FIG.
(C) It is a characteristic view which shows the power fluctuation state of a modulator output light, respectively.

FIG. 5 is a block diagram of a first embodiment of the present invention.

FIG. 6 is a principle diagram of a second embodiment of the present invention.

FIG. 7 is a block diagram of a second embodiment of the present invention.

[Explanation of symbols]

 1 polarization modulator 2 polarization maintaining fiber 4 rotation mechanism 5 analyzer 6 photodetector 7 spectrum analyzer 8 XYZ stage 9 optical fiber 10 polarization controller 11, 12 control unit 13, 17 polarization beam splitter 14 photodetector 15, 16, 19 Laser emission source 18 Optical fiber coupler

Claims (4)

[Claims] 1. The polarization maintaining fiber is fixed to the polarization modulator after the main axis direction of the polarization maintaining fiber introduced to the input end of the polarization modulator and the incident direction of the polarization modulator are matched. In the method for manufacturing a polarization modulator in the step of, the means for inputting polarization-multiplexed first and second polarization signals having different wavelengths and having mutually orthogonal polarization directions from the tip of the polarization-maintaining fiber; Means for applying a drive pulse signal for obtaining a modulation degree of 90 ° to the polarization modulator tentatively connected to the wave preserving fiber, and rotation provided at a tentative connection portion of the polarization preserving fiber with the polarization modulator. Mechanism, means for cutting out output light in one polarization direction from the output end of the polarization modulator, frequency of the first and second polarization signals contained in the electric signal, the output light being converted into an electric signal First caused by the difference Interference wave display means for extracting and displaying an interfering signal and adjusting and fixing the rotation angle of the polarization maintaining fiber by the rotating mechanism so that the level of the first interference signal is minimized. A method for manufacturing a polarization modulator, comprising: 2. A means for mixing output light in one polarization direction cut out from the output end of the polarization modulator with a third polarization signal having a frequency different from both of the first and second polarization signals is used. Of the second interference signal, the second polarization signal, and the third polarization signal generated by the frequency difference between the first polarization signal and the third polarization signal displayed on the interference signal display means. The rotation angle of the polarization maintaining fiber is adjusted and fixed so that the third interference signal generated by the frequency difference is above a predetermined level and the level of the first interference signal is minimized. The method for manufacturing a polarization modulator according to claim 1. 3. The polarization modulator according to claim 1, wherein the optimum amplitude is determined by adjusting the amplitude of the drive pulse signal so that the level of the first interference signal is minimized. Manufacturing method. 4. The optimum direction is determined by adjusting the direction in which the output light of one polarization direction is cut out from the output end of the polarization modulator so that the first interference signal is minimized. 2. The method for manufacturing a polarization modulator according to 2.