JPS61105439A - Method for measuring wavelength dispersion of optical fiber - Google Patents

Abstract

PURPOSE:To make possible simple measurement with high accuracy by making incident the light signal subjected to intensity modulation and wavelength modulation on an optical fiber to be measured from one end thereof and measuring the max. phase shift or max. frequency shift of the phase modulation component of the exit light from the other end. CONSTITUTION:A measuring instrument is constituted of a light source 1 consisting of an LED, an intensity modulating circuit 4, a wavelength modulator 5, lenses 6, 8, the optical fiber 7 to be measured, a photoelectric converter 9 and a spectrum analyzer 11, etc. After the length L of the fiber 7 is measured, the light subjected to the simultaneous double modulations, i.e., the intensity modulation of a modulating angle frequency omegac and wavelength modulation of a modulating angle frequency omegam, central wavelength lambda and wavelength amplitude DELTAlambda is made incident on said fiber from one end thereof. The max. phase shift beta of the phase modulation component of the exit light from the other end is measured and the absolute value ¦D¦ of the wavelength dispersion value is calculated by the equation ¦D¦=beta/L.DELTAlambda.omegac, by which the easy measurement of the wavelength dispersion of the fiber 7 over the wide wavelength region with high accuracy is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for measuring chromatic dispersion, which is one of the basic characteristics of optical fibers. In particular, the present invention relates to a chromatic dispersion measuring method that allows intensity-modulated and wavelength-modulated optical signals to be incident on an optical fiber to be measured, thereby easily measuring the chromatic dispersion of the optical fiber to be measured.

[Conventional technology]

The speed at which an optical signal propagates through an optical fiber varies depending on the wavelength of the optical signal. Therefore, the pulse width of an optical pulse signal transmitted from a light source with a wavelength difference widens after propagating through an optical fiber, so the bandwidth must be narrowed and the transmission speed of the optical signal is limited. For this reason, it is very important to evaluate the wavelength dependence of the delay time of an optical fiber, that is, the chromatic dispersion caused by the difference in group velocity depending on the wavelength of light, in designing optical communication equipment.

In this way, chromatic dispersion means that the group velocity of an optical signal propagating within an optical fiber differs depending on the wavelength of the light, and is a quantity that determines the band of the optical fiber. The group delay time τ per unit fiber length that the light with wavelengths λ1 and λ1 receives is given by the chromatic dispersion value D (ps−
nm-'-ko+-'), and τ=D·1λ, -λb l Cps/)rn+).

Conventional methods for measuring wavelength dispersion of optical fibers include ■pulse method, ■interference method, ■difference method, and ■phase difference method.

The pulse method is a method in which optical pulses with different wavelengths are input into an optical fiber to be measured 9, and the delay time difference (arrival time difference) of each optical pulse is measured to calculate chromatic dispersion.

The optical interference method is a method in which the optical path length of light propagating through an optical fiber changes by changing the wavelength of the light source, and the clarity of the interference fringes is measured to determine wavelength dispersion.

The differential method is a method of calculating chromatic dispersion by measuring the change in phase of sinusoidally intensity-modulated light with respect to the light source wavelength after propagation through an optical fiber.

The phase difference method is a method in which lights of different wavelengths are modulated at the same frequency, made incident on an optical fiber to be measured, and the delay phase difference of the modulated signal received during propagation through the optical fiber is measured to calculate chromatic dispersion.

[Problem that the invention seeks to solve]

However, such conventional wavelength dispersion measurement methods have the following problems.

The pulse method has problems such as its measurement accuracy is limited to 50 ps or less due to the optical pulse width, jitter and band of the measurement electrical system, and it requires a wideband transmitting/receiving circuit to generate optical pulses.゛Interferometry has the advantage of not requiring high-speed circuits, but interference fringes disappear if the optical path difference between the air-propagating light, which serves as the phase reference, and the optical fiber-propagating light exceeds the coherence length. Therefore, it is unsuitable for long optical fibers installed. In addition, a special experimental environment such as a seismic isolation table for interference experiments is required, and it is not something that can be easily measured in a field environment.

The difference method and the phase difference method have the disadvantage that large-scale light source devices such as a wavelength tunable laser device and an optical modulator are required. Another drawback is that the wavelength control technology for laser equipment has not been sufficiently established, and in the case of optical fibers after installation, there is the problem that reference signal light must be separately transmitted between the input and output ends of the optical fibers. There is a point.

The present invention has been made by focusing on such conventional problems, and provides a novel chromatic dispersion measurement method that can easily and highly accurately measure the chromatic dispersion of an optical fiber over a wide wavelength range. The purpose is to

[Means for solving problems]

The present invention measures the length of an optical fiber to be measured, makes incident light intensity modulated with a modulation angular frequency ωゎ enter from one end of the optical fiber to be measured, and The output light emitted from the end is converted into an electrical signal, and the input light or the output light is subjected to wavelength modulation with a variable eye angle frequency ωc, a wavelength modulation amplitude Δλ, and a center wavelength X, and the phase modulation component of the electrical signal is The maximum phase shift β is measured, and the absolute value |D| of the chromatic dispersion value is determined from this value.

The maximum frequency shift Δf of the phase modulation component of the electrical signal is measured, and the chromatic dispersion value is determined from this value.

[For production]

In the present invention, an intensity-modulated and wavelength-modulated optical signal is input from one end of an optical fiber to be measured.

Since the sine wave signal of intensity modulation angular frequency ωm is subjected to phase modulation of wavelength modulation angular frequency ωm, this propagating light can be determined by measuring the maximum phase shift or maximum frequency shift of this phase modulation. The variance value can be calculated.

The present invention can be practiced even if the wavelength modulator is installed on the output end side of the optical fiber to be measured.

Furthermore, in order to measure the maximum phase shift or the maximum frequency shift and calculate the chromatic dispersion value, it is necessary to input the data into an arithmetic processing device.

〔Example〕

The present invention first measures the length L of the optical fiber to be measured, and from one end of the optical fiber to be measured, the intensity modulation of the modulation angular frequency ωゎ, the modulation angular frequency ω1, the center wavelength X, and the wavelength of the wavelength amplitude Δλ is performed. Inject light that has undergone double simultaneous modulation. The light that has undergone such modulation propagates through this optical fiber to be measured, and after exiting from the other end, is converted by a photodetector into an electrical signal proportional to the light intensity. This electrical signal is a sine wave with an intensity modulation angular frequency ωm when there is no dispersion in the optical fiber to be measured, but when a finite chromatic dispersion exists at the center wavelength X, the intensity modulation angular frequency ωc The sinusoidal signal of is subjected to phase modulation at wavelength modulation angular frequency ωc.

Maximum phase deviation (modulation index) β that represents the degree of this phase modulation
For, β=|D| ・L・Δλ・ωm −−−−−−
−(Since there is a relationship of 11, from equation (1), chromatic dispersion |D
| can be found.

Here, an optical fiber to be measured with a length L) 20 us is intensity-modulated at a modulation frequency (ωc/2π) 100 MHz, and at the same time a wavelength amplitude (Δλ) 100 rv and a modulation frequency (
ωm/2π) The absolute value of chromatic dispersion (|D|) at the center wavelength X is 2X10-"(ps#++w/1u
l), the electrical signal obtained by square law detection of the light emitted after propagating through the optical fiber under test has a phase modulation spectrum with a modulation index (β) of 2.51 centered on the intensity modulation angular frequency (ωC). is observed. These spectra are
It is composed of higher-order component spectra of integral multiples of 10 kHz around a 0-fire component of 100 MHz. When observed with a spectrum analyzer, a spectrum as shown in FIG. 3 is obtained. Alternatively, the behavior of the spectrum may be observed by gradually increasing the wavelength amplitude (Δλ) from zero. In this case, when the modulation index (β) reaches the wavelength amplitude (Δλ) that satisfies 2.405, the zero component becomes zero, so use equation (1) to find the chromatic dispersion |D| be able to.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a measuring device according to an embodiment of the present invention. In FIG. 1, a light source 1 is connected to an intensity modulation circuit 4 which connects an oscillator 2 and a current source 3. In FIG. The output light from this light source 1 is input to a wavelength modulator 5, and the output light is input to an optical fiber 7 to be measured via a lens 6. The light emitted from the optical fiber 7 to be measured is input to a photoelectric converter 9 via a lens 8, and its output is input to a spectrum analyzer 11 via an amplifier IO.

The light source 1 is suitably a light emitting diode that emits light in a wide wavelength band. A light source 1 receives a sinusoidal signal from an oscillator 2 and a current source 3.
It is driven by an intensity modulation circuit 4 that superimposes a bias current from The intensity-modulated output light from the light source I enters the wavelength modulator 5 and undergoes wavelength modulation. The wavelength modulator 5 can be realized by a normal spectrometer equipped with an optical deflector. The output light from the wavelength modulator 5 propagates through the optical fiber 7 to be measured and is converted by the opto-electrical converter 9 into an electrical signal proportional to the optical signal. This electrical signal is amplified by an amplifier 10 with an appropriate gain and then input to a spectrum analyzer 11.
The degree of phase modulation that the intensity modulation signal receives due to the wavelength dispersion characteristics of the optical fiber 7 to be measured is measured using the maximum phase shift β (modulation index) as a parameter. Based on this value, the chromatic dispersion value ID+ is calculated from equation (11).The chromatic dispersion value |D| at this time is.

In other words, the phase modulated signal can also be regarded as a frequency modulated signal. Maximum frequency deviation Δr in this case
satisfies the relationship Δf=β・ω1/2π −・・−(3), so even if the maximum frequency deviation Δf is measured, (
The chromatic dispersion value |D| can be calculated from equations 2) and 3. The wavelength dispersion value at this time is as follows.

Here, in order to obtain the maximum phase shift β or the maximum frequency shift Δf, a method is used in which the input is input to an arithmetic processing device connected to the spectrum analyzer 1, and then the chromatic dispersion value is calculated.

FIG. 2 is a schematic diagram showing an example of the configuration of the wavelength modulator 5,
Determination of the wavelength amplitude Δλ will be explained below.

In FIG. 2, the light diffracted by the diffraction grating 21 is configured to scan the output slot 23 (width 2W) in a sinusoidal manner by the optical deflection element 22. At this time, the distance between the optical deflection element 22 and the output slit 23 is assumed to be h. The wavelength change Δλ corresponding to the diffraction angle change Δθ of the diffraction grating 21 satisfies ([λ CO3a d. Here, θ is the diffraction angle (the angle between the emitted light and the normal on the diffraction grating surface), and m is The diffraction order, d, is the diffraction grating spacing.

For example, m = 1, d = 1/600 (mm
), 2 W=100(-), h=50 (cm)
and cosθ=1/6, the wavelength fluctuation amplitude Δλ of the light emitted from the exit slit is 10 (, +111
1). In an actual spectrometer, an imaging system is inserted between the diffraction grating 21 and the exit slit 23, but in this embodiment, it is omitted because there is no woody effect.

Further, even if the wavelength modulator 5 is installed on the output end side of the optical fiber 7 to be measured, the present invention can be implemented in the same manner.

Furthermore, if the zero dispersion wavelength of the optical fiber 7 to be measured is within the wavelength modulation region, an error will occur in the measurement results, so it is desirable to perform wavelength modulation taking this into account.

The length of the optical fiber to be measured can be measured using a tape measure, but it is more accurate to use the optical length by injecting a light pulse from one end and measuring its propagation time. It is.

〔Effect of the invention〕

Since the chromatic dispersion measuring method of the present invention does not require measuring the extremely small time difference between two optical signals, a light source that emits light in a wide wavelength band can be used.

Therefore, it is possible to accurately measure the chromatic dispersion value in a wide wavelength range.

Additionally, if either the intensity modulation frequency or the wavelength amplitude is variable, zero-level measurements can be made by adjusting a specific modulation component to zero, making it possible to measure even higher brightness. can do.

Therefore, even in a measurement environment where the input end and output end of an optical fiber are far apart, that is, even in a transmission line optical fiber that has been installed on site, it is possible to easily and accurately measure chromatic dispersion. This has an excellent effect of enabling more efficient operation in forming a communication system.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a measuring device according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the configuration of a wavelength modulator. Figure 3 is a diagram showing an example of the observed phase modulation spectrum (β
=2.51). l... light source, 2... oscillator, 3... current source, 4...
...Intensity modulation circuit, 5...Wavelength modulator, 6.8...
Lens, 7... Optical fiber to be measured (single mode optical fiber), 9... Optoelectric converter, 10... Amplifier, 1
DESCRIPTION OF SYMBOLS 1... Spectrum analyzer, 21... Diffraction grating, 22... Light deflection element, 23... Output slit.

Claims (4)

[Claims] (1) Measure the length L of the optical fiber to be measured, enter the incident light that has been intensity-modulated at the modulation angular frequency ω_c from one end of the optical fiber to be measured, and enter the incident light from the other end of the optical fiber to be measured. Convert the emitted light to an electrical signal, perform wavelength modulation on the incident light or the emitted light with a modulation angular frequency ω_m and a wavelength modulation amplitude Δλ, and measure the maximum phase shift β of the phase modulation component of the electrical signal. Then, from this value, the absolute value |D| of the chromatic dispersion value is determined as |D|=β/L・Δλ・ω_c. (2) Measure the maximum frequency deviation Δf of the phase modulation component of the electrical signal, and calculate the chromatic dispersion value D from this value as D=2π・Δf/L・Δλ・ω_c・ω_m ) The method for measuring chromatic dispersion of an optical fiber as described in item 1. (3) A method for measuring chromatic dispersion of an optical fiber according to claim (1) or (2) of applying wavelength modulation of modulation angular frequency ω_m and wavelength modulation amplitude Δλ to the incident light of the optical fiber to be measured. . (4) Chromatic dispersion of the optical fiber i according to claim 1 or 2, which imparts wavelength modulation of modulation angular frequency ω_m and wavelength modulation amplitude Δλ to the output light of the optical fiber to be measured. Measuring method.