JP2558344Y2 - Polarization crosstalk measurement system for polarization-maintaining optical fiber - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a polarization crosstalk measuring device for a polarization-maintaining optical fiber which improves the accuracy of crosstalk measurement.

[0002]

2. Description of the Related Art The construction of a conventional polarization crosstalk measuring apparatus will be described with reference to FIG. 2 is a constant current source, 2 is a laser diode (hereinafter, referred to as LD),
3 is a spherical lens, 4 and 6 are polarizers, 5 is a λ / 4 plate, and 7 and 1
2 is a servomotor, 8 and 10 are adapters, 9 is a polarization-maintaining optical fiber (hereinafter simply referred to as an optical fiber), 11
Denotes an analyzer, and 13 denotes an optical power meter. Adapter 8 and
The adapter 10 is, for example, a fiber adapter with a collimator.
Use a connector. Further , a photodiode may be used instead of the optical power meter 13.

In FIG. 2, an LD 2 driven by a constant current source 1
Is converted into a parallel beam by the spherical lens 3 and the polarizer 4
Is converted into linearly polarized light. The axis of the λ / 4 plate 5 is inclined by 45 ° with respect to the axis of the polarizer 4. Thereby, the light that has passed through the λ / 4 plate 5 becomes circularly polarized light. The servo motor 7 rotates the polarizer 6,
The light passing through the polarizer 6 is converted into linearly polarized light again. Polarizer 6
Since the light incident on the polarizer 6 is circularly polarized light, the level of linearly polarized light passing through the polarizer 6 is constant regardless of the rotation angle of the polarizer 6.

The linearly polarized light that has passed through the polarizer 6 enters the optical fiber 9 from the adapter 8, is converted into a parallel beam again by the adapter 10, and enters the analyzer 11. Analyzer 11
Is rotated by the servo motor 12, and the incident light is extracted as a linearly polarized light component at an arbitrary angle. The light level of the output of the analyzer 11 is measured by the optical power meter 13. The polarizer 6 and the analyzer 11 are alternately rotated to find the main axis of the optical fiber 9 so as to reduce the output light level, and the polarization crosstalk is measured. The polarization-maintaining optical fiber and the polarization crosstalk are described, for example, in a commentary article in the August 1981 issue of Nikkei Electronics. This article describes polarization crosstalk as the extinction ratio.

For example, if the output of the optical fiber 9 in the main axis direction is -5 dBm and the output of the optical fiber 9 in the direction perpendicular to the main axis is -36 dBm, the polarization crosstalk of the optical fiber 9 is -5 dBm- ( −36 dBm) = 31 dB.

It is desirable that the LD 2 has poor coherency (multi-mode). If the coherency is high (single mode), the polarization state of the output of the optical fiber 9 is determined by one mode, so that the phase difference is π / 2.
When the phase difference is an odd multiple of π, the light becomes circularly polarized light. When the phase difference is an integral multiple of π, the light becomes a linearly polarized light.

In the case of a multi-mode LD 2, an optical fiber 9
The polarization state of the output is the sum of several modes, so even if the output of one mode is linearly polarized light or circularly polarized light, it will be arbitrarily elliptically polarized light. The main axis can be detected without receiving the error.

[0008]

Next, the relationship between the wavelength and the output level of the LD 2 in FIG. 2 will be described with reference to FIG. FIG.
FIG. 4 is a characteristic diagram in the case where a Fabry-Perot LD is driven by a direct current to generate continuous light. The portion shown by the dotted line in FIG. 3 shows a state that changes with the passage of time. In FIG. 3, the level changes and is not stable. Since the polarization state of the output of the optical fiber 9 is the sum of the polarization states of the respective modes, when each mode changes, the polarization state also changes. Further, the measurement level fluctuates due to a difference in the form of the optical fiber 9 or a disturbance such as a side pressure, which causes an error in the crosstalk measurement.

This invention uses a high-frequency oscillator and a bias tee to apply a high-frequency modulation to the LD2 to suppress the level fluctuation of each mode, and to increase the number of modes, thereby deteriorating coherency and thereby affecting the influence of disturbance. And to improve the accuracy of crosstalk measurement.

[0010]

In order to achieve this object, in the present invention, the output light of the LD 2 is input to the polarizer 4 to be linearly polarized, and the output of the polarizer 4 is input to the λ / 4 plate 5. The output of the λ / 4 plate 5 is input to the polarizer 6 to be linearly polarized, and the output of the polarizer 6 is input to the adapter 8,
The output of the optical fiber 9 to which the output of the adapter 8 is input is input to the adapter 10, and the output of the adapter 10 is output to the analyzer 11.
The output of the constant current source 1 and the output of the high-frequency oscillator 14 are input to the bias tee 15 for the polarization crosstalk measuring device that inputs the output of the analyzer 11 to the optical power meter 13. The output drives LD2.

[0011]

Next, the configuration of the polarization crosstalk measuring apparatus according to the present invention will be described with reference to FIG. 1 is a high-frequency oscillator, 15 is a bias tee, and others are the same as those in FIG. That is, FIG. 1 shows the high-frequency oscillator 1 shown in FIG.
4 and a bias tee 15 are added. Bias tee 15 is constituted by a circuit as shown in FIG. 4, a high frequency onset
The output of the vibrator 14 is superimposed on the output of the constant current source 1. Bahia
The stee 15 is, for example, 33150A manufactured by Hewlett-Packard Company. The output of the bias tee 15 is as shown in FIG. V in FIG. 5 is the voltage of the constant current source 1, and m is the output waveform of the high-frequency oscillator 14. For example,
The output frequency of the high-frequency oscillator 14 is a 100 MHz sine wave, and the peak output is 1V. 30mA from constant current source 1
Of current.

Next, the relationship between the wavelength and the output level of the LD 2 in FIG. 1 will be described with reference to FIG. In FIG. 6, there is no portion shown by the dotted line in FIG. That is, FIG.
Becomes more stable than Also, the number of modes increases.

Next, the operation of FIG. 1 will be described with reference to FIG.
FIG. 7 is a partially enlarged view of FIG. 6, and the emission spectrum of the LD 2 subjected to high-frequency modulation by the high-frequency oscillator 14 oscillates in a stable multi-mode as shown in FIG. When the laser oscillates in the five modes a to e, each mode is incident on the optical fiber 9 by the polarizer 6 in a linearly polarized state. Each of the incident modes is given a phase difference by the optical fiber 9, and thus has an arbitrary polarization state as shown in FIG. The sum of the polarization states in each mode is the polarization state of the output of the optical fiber 9 as shown in FIG. This polarization state is stable because the level fluctuation of each mode is stable. Also, since there are many modes, coherency deteriorates,
The polarization crosstalk of the optical fiber 9 can be measured without being affected by disturbance. FIG. 9 shows the sum of the waveforms of FIG.

[0014]

According to this invention, the output of the constant current source and the output of the high-frequency oscillator are used as the input of the bias tee, and the output of the bias tee is supplied to the LD, so that the level of each mode of the LD does not fluctuate. Polarization crosstalk can be measured in a stable state.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a polarization crosstalk measuring device according to the present invention.

FIG. 2 is a configuration diagram of a conventional polarization crosstalk measuring device.

FIG. 3 is a graph showing wavelength and output level characteristics of the LD 2 of FIG. 2;

FIG. 4 is a configuration diagram of a bias tee 15 ;

FIG. 5 is an output waveform diagram of the bias tee 15 ;

FIG. 6 is a graph showing wavelength and output level characteristics of the LD 2 of FIG. 1;

FIG. 7 is a partially enlarged view of FIG. 6;

FIG. 8 is a polarization state diagram of the polarizer 6;

FIG. 9 shows a waveform obtained by adding the waveforms of FIG. 8;

[Explanation of symbols]

 Reference Signs List 1 constant current source 2 LD (laser diode) 3 spherical lens 4 polarizer 5 λ / 4 plate 6 polarizer 7 servo motor 8 adapter (fiber adapter with collimator) 9 optical fiber (polarization preserving optical fiber) 10 adapter 11 analyzer 12 Servo motor 13 Optical power meter 14 High frequency oscillator 15 Bias tee

Claims (1)

(57) [Scope of request for utility model registration] 1. A spherical lens (3) for converting the output light of a laser diode (2) into a parallel beam; a first polarizer (4) for linearly polarizing the output of the spherical lens (3); Λ inclined by 45 ° with respect to the axis of the polarizer (4) to make the output of the first polarizer (4) circularly polarized
And a second polarizer (6) which receives the outputs of the plate (5) and the λ / 4 plate (5) as inputs, is rotated by a first servomotor (7), and converts the circularly polarized light into linearly polarized light. A first adapter (8) receiving the output of the second polarizer (6) as an input, and a second adapter receiving the output of the polarization-maintaining optical fiber (9) connected to the first adapter (8) . and 2 of the adapter (10), the output of the second adapter (10) as an input, analyzer which are rotated by the second servo motor (12)
(11) and an optical power meter using the output of the analyzer (11) as input
The output of the constant current source (1) and the output of the high-frequency oscillator (14) are used as the input of the bias tee (15) for the polarization crosstalk measuring device of the polarization-maintaining optical fiber composed of (13), and the bias tee is A polarization crosstalk measuring device for a polarization-maintaining optical fiber, characterized in that a laser diode (2) is driven with the output of (15).