JPH03246439A - System for measuring dispersion characteristic of optical fiber - Google Patents

Abstract

PURPOSE:To measure the dispersion characteristic of an optical communication system inserted with an optical device of a narrow band in an optical fiber transmission system with a high resolving power and high accuracy at a faraway end by disposing a reference signal generating means on the signal receiving side of an optical fiber for measurement. CONSTITUTION:An output electric signal S4 corresponding to a wavelength change from an initial state is obtd. by a frequency measuring instrument 14 and an output electric signal S corresponding to a phase change from an initial state is obtd. by a phase comparator 6 when a control voltage value signal S1 output by a control voltage generator 14 is kept constant and the oscillation wavelength of a laser 2 for measurement is swept in this state. The dispersion characteristics of these signals are then measured via a control circuit 17. An oscillation wavelength detecting signal S3 of the laser 2 is also inputted to the circuit 17 by an optical wavelength meter 12. Since a reference electric signal S2 is not propagated in an optical fiber 4, the dispersion characteristics are measured even if the optical device of the narrow band is inserted into the optical communication system to be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical fiber dispersion characteristic measuring method for measuring the dispersion characteristic of an optical signal generated within an optical fiber.

[Prior Art] In optical fiber communication that uses optical fiber as a transmission path, the laser that is the light source has a spectrum spread in terms of frequency, and this, combined with the dispersion characteristics of the optical fiber, makes it difficult for the pulse waveform to be sent out correctly at the transmitting end. However, there is a problem in that waveform distortion occurs in the pulses received on the receiving side, which deteriorates the reception characteristics.

Therefore, when designing optical communication systems, it is essential to understand the dispersion characteristics of optical fibers.
Various dispersion measurement methods have been proposed so far.

Figure 2 is a block diagram of a conventional dispersion characteristic measurement method.
1 is an oscillator for supplying a modulated electric signal Sa for modulating the light emitted from the measurement laser with an external modulator;
2 is a DFB laser or DB that emits the measurement optical signal L1
A measurement laser which is an R laser, 4 a measurement optical fiber for measuring dispersion characteristics, 5 an optical receiver for charging and converting the intensity modulated light emitted from the measurement optical fiber 4, and 6 a demodulated electrical signal and a phase comparator for detecting the phase difference between the reference electric signal sb and the reference electric signal sb; An external modulator 9 modulates the intensity from the modulated electric signal Sa from the oscillator 1 without increasing the width. After passing through i8, an optical branching device for branching the measurement optical signal L2 branched from the optical fiber Fb into three at an appropriate ratio; 10 is a measurement laser 2;
a monitoring laser for measuring the oscillation wavelength of the measurement optical signal L1 output from the optical heterodyne detection, 11;
12 is a spectrum analyzer for monitoring the difference in oscillation wavelength between the measurement laser 2 and the monitor laser 10; 12 is a light for roughly measuring the oscillation wavelength of the measurement laser 2 and determining the oscillation wavelength of the monitor laser 10; Wavemeter, FC~~
Ff is an optical fiber, sb is a reference electrical signal, and SC is an intermediate frequency electrical signal.

Note that the monitor unit 20 (hereinafter referred to as
The monitor section 20 (simply referred to as a monitor section 20) is composed of a monitor laser 10, a light combiner 13, an optical receiver 5C, and a spectrum analyzer 11.

In the configuration shown in Fig. 2, the measurement laser 2 includes a DFB with excellent single longitudinal mode oscillation characteristics and a narrow spectral linewidth.
A laser or DBR laser is used. Furthermore, since the output light from the measurement laser 2 is modulated by the external modulator 8, the spectral line width of the light source is not affected in any way. Therefore, the influence of the spectrum broadening of the light source on the dispersion characteristics can be reduced.

In addition, in the configuration shown in FIG. 2, the measurement optical signal L2 branched by the optical splitter 9 and the monitor optical signal LO from the monitoring laser 10 are combined by the optical combiner 13, and the beat signal components of both waves are combined. Create an intermediate frequency electrical signal SC using the optical receiver 5C.
When the wavelength change from the initial oscillation wavelength is observed on the spectrum analyzer 11 by photoelectrically converting the oscillation wavelength into the oscillation wavelength and observing it on the spectrum analyzer 11, the oscillation wavelength of the measurement laser 2 is gradually changed. at the same time,
The output signal S is measured to measure the dispersion characteristics with high resolution and i-accuracy. However, in the configuration shown in FIG. 2, since the reference electrical signal sb and the measurement optical signal L2 are measured through different transmission paths, even if the measurement optical fiber 4 expands or contracts due to a change in ambient temperature, etc. It is not possible to remove the effects of phase fluctuations in the demodulated electrical signal that occur in the demodulated electrical signal.

The third room addresses the above problems! The wavelength is approximately 1.0 mm, which is approximately the same as the zero dispersion wavelength of the optical fiber 4 for measurement.
The reference optical signal L3, which is the output light of the reference laser 2'' that oscillates at 3 μm, is propagated through the measurement optical fiber 4, and
A monitor section 20 using optical heterogain detection is arranged on the receiving side to enable far-end measurement. moreover,
The reference optical signal L3 and the measurement optical signal L1, which are the output lights of the reference laser 2# and the measurement laser 2', are converted into a reference optical signal L4 and a measurement optical signal L2, respectively, using separate external modulators 8a and 8b. is modulated into an optical demultiplexer 7° and an optical receiver 5a.
, 5b to demodulated electrical signals [)a and [)b, respectively. Note that FQ and Fh are optical fibers. Since the output light of the reference laser 2'' is close to the zero dispersion wavelength, it is almost unaffected by dispersion and can be easily connected to the measurement optical fiber 4.
Since the influence of the expansion and contraction of is equally affected by the output light from the measurement laser 2-, the measurement light is It is possible to remove the influence of expansion and contraction of the fiber 4 and measure the dispersion characteristics.

[Problem to be solved by the invention] By the way, in the conventional dispersion characteristic measurement method described above,
As the wavelength of the reference laser 2'', approximately 1.3 μm, which is the same as the zero dispersion wavelength of the measurement optical fiber 4, is used, and as the measurement laser 2', light in the wavelength band of 1.55 μm is normally used. In a system in which a narrowband optical device such as a semiconductor laser amplifier, an optical fiber laser amplifier, or an optical filter is inserted in the middle of the measurement fiber 4, the light with a wavelength of 1.3 μm sent out from the reference laser 2# is There was a problem that there was no separation, making measurement impossible.

The present invention was made in order to solve the problems of the prior art described above, and it is possible to perform measurements with high resolution and high precision even in an optical communication system in which a narrowband optical device is inserted in the middle of an optical fiber transmission system. The purpose of this invention is to provide an optical fiber dispersion characteristic measurement method that allows for the measurement of optical fiber dispersion characteristics.

[Means for Solving Problem 1] The above problem can be solved by using an optical fiber dispersion characteristic measurement method according to the present invention, which modulates the measurement signal light outputted from the measurement laser with an external modulator and converts it into the measurement optical fiber. After converting the measurement signal light incident on the measurement optical fiber and emitted from the output end of the measurement optical fiber into a measurement electrical signal by an optical receiver, the phase difference between the measurement electrical signal and the reference electrical signal and the measurement signal light are determined. In an optical fiber dispersion characteristic measurement method for measuring dispersion characteristics from changes in the oscillation wavelength of signal light, the method includes a reference signal generating means disposed on the optical receiver side and generating the reference electric signal, and the oscillation frequency is kept constant. inputting the measurement signal light into the measurement optical fiber, and comparing the measurement electrical signal converted by the optical receiver with the reference electrical signal generated from the reference signal generation means using a phase comparator. This is achieved by employing the above-described configuration means, in which the measurement signal light is initially set so that the phase differences are equal, and then the measurement signal light is swept to measure the dispersion characteristics of the measurement optical fiber.

[Function] Since the present invention takes the above measures, the reference signal generating means is disposed on the optical receiver side and the reference signal is not propagated in the measurement optical fiber, so that the measurement optical fiber may be damaged due to changes in ambient temperature, etc. It is completely unaffected by the expansion and contraction of the optical fiber for measurement, or when a narrowband optical device such as a semiconductor laser amplifier, optical fiber laser amplifier, or optical filter is unavoidably inserted in the middle of the measurement optical fiber. The reference laser that generates a reference signal with a zero-dispersion wavelength different from the wavelength of the reference signal light can be omitted, resulting in higher resolution, higher precision, and
Achieve highly reliable dispersion measurement.

[Example] An embodiment of the invention will be described with reference to FIG.

Components that are the same as those of the conventional example are given the same reference numerals and explained in the following!
Omit the double. Note that the difference from the conventional configuration is that a reference signal generating means A that directly generates a reference signal of an electric signal is arranged on the receiving side of the measuring optical fiber 4 (the output side of the optical fiber 4 of the measuring signal light L2), The present invention is configured such that the dispersion characteristics can be measured by comparing the phases of the measuring optical signal L2 and the reference electric signal without propagating the reference signal through the measuring optical fiber 4. Below is the f:c next composition and the new composition!
The present invention will be described in detail, focusing on the following sections.

The demodulated electrical signal 100 demodulated by the optical receiver 5 is input to one input terminal of the phase comparator 6 .

On the other hand, the reference electric signal S2, which is the output of the voltage controlled oscillator 16 whose oscillation frequency is determined according to the II'm voltage value signal S1 from the control voltage generator 15 constituting the reference signal generating means A, which is a feature of the present invention, is , is input to another input terminal of the phase comparator 6.

Now, when starting a measurement, as an initial setting, the oscillation frequency of the measurement laser 2 is not swept but is oscillated at a certain fixed wavelength. At this time, the phase of the demodulated electrical signal is constant. In this state, the control voltage value signal S1 of the control voltage generator 15 is controlled by the adjustment electric signal S5 outputted from the control circuit 17 so that the output electric signal S of the phase comparator 6 shows a zero phase difference. The control voltage value signal S1 at which the phase difference of the phase comparator 6 is zero is transmitted to the voltage controlled oscillator 16.
, the output reference electrical signal S2 of the voltage controlled oscillator 16 becomes completely synchronized in phase with the demodulated electrical signal. Therefore, the reference electrical signal S2 outputted from the voltage controlled oscillator 16 in this state is completely equivalent to the reference electrical signal Sb in FIG. 2 of the conventional example or the demodulated electrical signal [)a in FIG. 3. can do.

Therefore, the control voltage value signal S output from the control voltage generator 15
1 is kept constant and the oscillation wavelength of the measurement laser 2 is swept in this state, the frequency measuring device 14 generates an output electrical signal S4 corresponding to the wavelength change from the initial state.
However, the phase comparator 6 can also obtain an output electrical signal S corresponding to a phase change from the initial state, and the dispersion characteristics of these signals can be measured via the control circuit 17. At the same time, the optical wavelength meter 12 also inputs the oscillation wavelength detection electric signal S3 of the measurement laser 2 to the control circuit 17.

As described above, since the present invention does not propagate the reference electrical signal S2 through the measurement optical fiber 4, it is possible to measure dispersion characteristics even if a narrowband optical device is inserted into the optical communication system to be measured. .

In addition to the conventional configuration shown in FIG.
Since the reference laser 2'' that generates the reference optical signal L3 of m) is not required, there is no need to adjust the external temperature of the two lasers with different oscillation characteristics, and high-resolution and high-precision dispersion measurement is possible. .

Furthermore, in the past, changes in the oscillation wavelength of the measurement laser 2' were observed on the spectrum analyzer 11, which necessitated manual measurement, which hindered practical application. By using a frequency measuring device 14 such as a frequency counter instead of the analyzer 11, a method is provided that allows accurate measurement and automatic measurement.

[Effects of the Invention] Thus, the present invention can be achieved by arranging and initializing the reference signal generating means A that directly generates the electrical reference signal S2 on the output side of the measuring signal light, that is, on the receiving side of the measuring optical fiber 4. , can measure the dispersion characteristics of an optical communication system in which an optical device with narrowband characteristics is inserted into an optical fiber transmission system at the far end with high resolution and precision, and is used as a reference that is easily fluctuated due to external temperature changes, etc. Laser 2# can be eliminated.

By configuring the reference signal generating means A by the control voltage generator 15 and the voltage controlled oscillator 16, it is possible to generate a simple and highly accurate reference electric signal S2.

By using a frequency measuring device 14 such as a frequency counter as a means for measuring changes in the oscillation wavelength of the measurement laser 2, accurate and automatic measurement becomes possible.

Therefore, the present invention can be widely applied to the design of dispersion compensation circuits for coherent optical communication, and can also be applied to optical communication systems in which a narrowband optical device is inserted in the middle of an optical fiber for measurement. The effect is extremely large.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an optical fiber dispersion characteristic measurement method as an example of implementing the present invention, Fig. 2 is a block diagram of a conventional optical fiber dispersion characteristic measurement method, and Fig. 3 is the effect of expansion and contraction of the optical fiber for measurement. 1 is a block diagram of a conventional optical fiber dispersion characteristic measurement method that does not undergo 1...Oscillator 2.2-...Measurement laser 2#...Reference laser 4...Measurement optical fiber 5.5a, 5b, 5c...Optical receiver 6...Phase comparison Device 7... Optical splitter 8.8a, 8b... External converter [5 9... Optical splitter 10... Monitoring laser 11... Spectrum analyzer 12... Optical wavelength meter 13 ... Optical combiner 14 ... Frequency measuring device 15 - ... Control voltage generator 16 ... Voltage control oscillator 17 ... Control circuit 20 ... Monitor part A of optical heterodyne detection ... Reference signal generation Means, Da, Db... Demodulated electrical signal SC... Intermediate frequency electrical signal fO... Frequency Fa-Fh, F', F"... Optical fiber LO... Monitor optical signal Ll, L2. ...Measurement optical signal L3.L4...Reference optical signal...Beat signal Sa...Variable TATi signal Sb, 82...Reference electrical signal S, S4...Output electrical signal Sl...・Control voltage value signal S3...Oscillation wavelength detection electrical signal Sb...Adjustment electrical signal

Claims (1)

[Scope of Claims] 1. The measurement signal light output from the measurement laser is modulated by an external modulator and enters the measurement optical fiber, and the measurement signal light is emitted from the output end of the measurement optical fiber. In an optical fiber dispersion characteristic measurement method in which a signal light is converted into a measurement electrical signal by an optical receiver, and then the dispersion characteristics are measured from the phase difference between the measurement electrical signal and a reference electrical signal and the change in the oscillation wavelength of the measurement signal light. , comprising a reference signal generating means disposed on the optical receiver side to generate the reference electrical signal, the measurement signal light having a constant oscillation frequency is incident on the measurement optical fiber, and the measurement signal light is The measurement electrical signal converted by the receiver and the reference electrical signal generated from the reference signal generation means are compared with a phase comparator to initialize the phase difference so that they are equal, and then the measurement signal light is An optical fiber dispersion characteristic measuring method, characterized in that the dispersion characteristic of the optical fiber for measurement is measured by sweeping the dispersion characteristic of the measuring optical fiber. 2. Claim 1, wherein the reference signal generating means is comprised of a control voltage generator and a voltage controlled oscillator.
Optical fiber dispersion characteristics measurement method described. 3. The optical fiber dispersion characteristic measuring method according to claim 1, wherein the means for measuring the change in the oscillation wavelength of the measurement signal light is a frequency measuring device.