JPH0531736B2 - - Google Patents

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 method for determining the zero-dispersion wavelength of an optical fiber that deviates from its theoretical value due to variations in fiber parameters of the optical fiber.

[Conventional technology]

The speed at which an optical signal propagates through an optical fiber varies depending on the wavelength of the optical signal. Therefore, since the pulse width of an optical pulse signal sent out from a light source with a wavelength spread spreads after propagating through an optical fiber,
The bandwidth must be narrowed, which limits the transmission speed of optical signals. For this reason, it is important to evaluate the wavelength dependence of the delay time of an optical fiber, that is, the chromatic dispersion caused by the fact that the group velocity differs depending on the wavelength of light, when designing an optical communication device.

Bandwidth limitation due to chromatic dispersion of an optical fiber has the least effect when the light source wavelength and the zero dispersion wavelength of the optical fiber match. This zero-dispersion wavelength can be theoretically determined from the fiber parameters (core diameter, material constants of the core material and cladding material, core dopant concentration, and refractive index distribution shape), but it is does not necessarily match this theoretical value due to variations in fiber parameters. Therefore, when designing an optical communication device, it is necessary to determine the zero-dispersion wavelength through actual measurements.

[Problem that the invention seeks to solve]

However, in the past, there was no method for directly and accurately measuring the zero dispersion wavelength of such an optical fiber.

In other words, the chromatic dispersion value in the vicinity of the zero-dispersion wavelength is determined using various chromatic dispersion measurement methods, the measured values are plotted with the wavelength as the horizontal axis, and then the zero-dispersion wavelength is determined by interpolation or extrapolation. Ta. However, chromatic dispersion measurement of optical fiber is
Since the smaller the dispersion value, the greater the influence of the error, there has been a strong desire to develop a method that can directly determine the zero-dispersion wavelength.

The present invention has been made in response to these demands, and it is a method to easily determine by actual measurement the zero dispersion wavelength of an optical fiber, which deviates from the theoretical value due to variations in fiber parameters that occur during the manufacturing of the optical fiber. An object of the present invention is to provide a method and apparatus for measuring zero dispersion wavelength of an optical fiber.

[Means for solving problems]

The zero-dispersion wavelength measurement method of the present invention is a method for measuring the zero-dispersion wavelength of an optical fiber to be measured in which the approximate value of the wavelength λ ₀ at which the chromatic dispersion value becomes zero is calculated theoretically. Input intensity-modulated incident light is input from one end, and the output light emitted from the other end of the optical fiber to be measured is converted into an electrical signal, and the center wavelength λ of the input light or the output light is set to the above wavelength. Apply wavelength modulation close to λ ₀ , change the center wavelength so that the primary spectrum of the phase modulation component of the electrical signal becomes minimum, and set the wavelength at the time when the spectrum disappears as the zero-dispersion wavelength of the optical fiber under test. shall be.

The zero-dispersion wavelength measuring device of the present invention receives a light source that emits light in a wide wavelength band, an oscillator, a power source, and the outputs of the oscillator and power source, and generates a modulation signal for intensity modulating the output light of the light source. and an intensity modulation circuit for driving the light source, means for making the output light of the light source enter the optical fiber to be measured, a photoelectric converter, and guiding the output light from the optical fiber to be measured to the photoelectric converter. a wavelength modulator inserted between the light source and the opto-electric converter to perform wavelength modulation on the output light; and an amplifier to amplify the electrical signal output from the opto-electric converter;
The wavelength modulator includes a synchronous detector that receives the output of the amplifier as an input, a means for transmitting the output of the oscillator to the synchronous detector, and a spectrum analyzer that observes the spectrum of the output of the synchronous detector. The modulation wavelength is variable, and this variable range is
It is characterized in that the center wavelength can be set within a range such that the primary spectrum of the phase modulation component observed by the spectrum analyzer becomes minimum.

[Effect]

In the present invention, an intensity-modulated and wavelength-modulated optical signal is input from one end of an optical fiber to be measured, and this propagating light is synchronously detected at an intensity modulation angular frequency ω _c .
Inputting this detection output into a spectrum analyzer, focusing on the spectrum component of this wavelength modulation angular frequency ω _n , and sweeping the center wavelength, when the center wavelength matches the zero dispersion wavelength λ _e , the wavelength modulation angle The spectrum of frequency ω _n disappears. This results in
The zero dispersion wavelength λ _e can be determined.

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.

〔Example〕

The wavelength dispersion value of an optical fiber can be calculated from the fiber parameters, and the optical wavelength at which this calculated value is equal to zero is called the "theoretical zero dispersion wavelength."

In a normal silica-based single mode optical fiber, if the cladding is pure quartz (125 μmφ), the core is germanium doped (10 μmφ), and the relative refractive index difference between the core and the cladding is approximately 0.3%, the zero dispersion wavelength is 1.3 μm. It is in the vicinity of The variation in the zero dispersion wavelength due to the fact that the core refractive index distribution shape slightly deviates from the ideal step shape is not very large, and is 1.30.
It can be assumed that the wavelength range is approximately 1.32 μm. The light source needs to have wide wavelength band characteristics that cover this wavelength range.

The present invention provides intensity modulation of modulation angular frequency ω _c and modulation angular frequency ω _n ,
Light that has undergone double simultaneous modulation of wavelength modulation with a center wavelength and a wavelength amplitude Δλ is made incident. The light that has undergone such modulation propagates through this optical fiber to be measured, and after exiting from the other end, is converted into an electrical signal proportional to the light intensity by a photoelectric converter. After this electrical signal is amplified to an appropriate level, it is synchronously detected at the intensity modulation angular frequency ω _c . When this detection output is input to a spectrum analyzer, a component of wavelength modulation angular frequency ω _n and a high frequency component twice that frequency are observed. Therefore, if we focus on the spectrum component of this wavelength modulation angular frequency ω _n and sweep the center wavelength, the spectrum of the wavelength modulation angular frequency ω _n disappears when the center wavelength matches the zero dispersion wavelength λ _e . Thereby, the zero dispersion wavelength λ _e can be determined.

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 an embodiment of the measuring device 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 incident on a wavelength modulator 5, and the output light is incident on 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 synchronous detector 11 which receives the output of an oscillator 2 via an amplifier 10.
and its output is further input to the spectrum analyzer 12.

The light source 1 is suitably a light emitting diode that emits light in a wide wavelength band. The light source 1 is driven by an intensity modulation circuit 4 that superimposes a sine wave signal from an oscillator 2 and a bias current from a current source 3. The intensity-modulated output light from the light source 1 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. After this electrical signal is amplified by an amplifier 10 with an appropriate gain, it is input to a synchronous detector 11 synchronized with the output of the oscillator 2 to remove the intensity modulation component, and a spectrum analyzer 12 performs spectrum observation. Since the component of the wavelength modulation angular frequency ω _n disappears when the center wavelength matches the zero dispersion wavelength λ _e , the zero dispersion wavelength λ _e is determined using this component.

There is no problem with the means for connecting the oscillator 2 and the synchronous detector 11 when the optical fiber 7 to be measured is in a wound state before installation, but after installation, the input end and the output end of the optical fiber 7 to be measured are connected. Since the ends are far apart, a transmission path is required. However, since the oscillation frequency of the oscillator 2 uses a frequency as low as the acoustic frequency, it is preferable to use a normal telephone line or an intervening pair of cables mounted on the optical fiber to be measured as the transmission line.

FIG. 2 is a schematic diagram showing an example of the configuration of the wavelength modulator 5. As shown in FIG. This determination method is a null method measurement by searching for the vanishing wavelength of the spectrum, so the wavelength amplitude Δλ
does not affect measurement accuracy, but if it is too small, it will be difficult to observe the spectrum, so it is necessary to know the approximate value.

In FIG. 2, the configuration is such that light diffracted by a diffraction grating 21 scans an output slit 23 (width 2W) in a sinusoidal manner by an 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 element 21 satisfies dθ/dλ=1/cosθ m/d (1). Here, θ is the diffraction angle (the angle between the emitted light and the normal line on the diffraction grating surface), m is the diffraction order, and d is the diffraction grating interval.

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

Sweeping of the center wavelength can be performed by rotating the diffraction grating. When a normal spectrometer is used as the diffraction grating system, the wavelength reading scale of this spectrometer indicates the center wavelength, and this center wavelength is the zero dispersion wavelength λ _e , making measurement extremely easy.

The measurement principle will be explained in detail below. The light emitted from the optical fiber is expressed as S ₀ (λ, t)∝1+cos[ω _c (t+τ)] ……(2) τ(k)=1/C(∂β /∂k) _k ……(3). However, S ₀ is the wavelength dispersion of the optical fiber, k
is the wave number and β is the propagation constant.

Expanding the delay time τ around the zero dispersion wavelength (λ _e = 2π/k ₀ ), we get τ(k) = L/C [(∂β/∂k) _k0 + (k−k ₀ ) (∂ ² β／∂
k ² ) _k0 + (k−k ₀ ) ² /2! (∂ ³ β／∂k ³ ) _k0 +…〕……(4
) ∴τ(k) L/C [τ ₀ −(λ _e −λ) ² /2(dS/dλ)]...
(5) becomes. Here, to derive equation (5), the chromatic dispersion S, that is, S=2π/Cλ(∂ ² β/∂k ² ) _k ……(6) is calculated as follows at the wavelength λ _e (=2π/k ₀ ). Assume that it is zero,
Higher-order terms after the fourth order in equation (4) were ignored.

In the present invention, the wavelength of light received by the photoelectric converter 9 is changed sinusoidally around the center wavelength with an amplitude of Δλ, so λ in equation (5) is λ=+Δλcosω _n t ...( 7), and if we set dS/dλ=S' in equation (2), then S ₀ (λ, t)1 +cos[ω _c t'+S'Δλ ² ω _c L/4cos2ω _n t +S'Δλ( λe−λ)ω _c L/2cosω _n t] ...(8). When the signal expressed by equation (8) is synchronously detected at the intensity modulation angular frequency ω _c , a spectral component of the wavelength modulation angular frequency ω _n and its second harmonic component are obtained.
It can be seen from equation (8) that when the center wavelength and the zero dispersion wavelength λ _e match, the primary spectrum of ω _n disappears.

Therefore, 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.

FIG. 3 is a block diagram showing an embodiment in this case. Here, a case is shown in which a telephone line is used as a transmission line means for transmitting the output of the oscillator 2 to the synchronous detector 11, and a transmitter 13 that transmits the output of the oscillator 2 to the telephone line, and a transmitter 13 that transmits the transmitted signal. Receiver 1 that receives and connects to synchronous detector 11
4 is displayed.

〔Effect of the invention〕

The zero-dispersion wavelength method of the present invention enables highly accurate measurement by the zero-position method, and since the reading wavelength of the spectrometer becomes the zero-dispersion wavelength as it is, it is an extremely simple direct measurement method.

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

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the measuring device of the present invention. FIG. 2 is a schematic diagram showing an example of the configuration of a wavelength modulator. FIG. 3 is a block diagram showing another embodiment of the measuring device of the present invention. DESCRIPTION OF SYMBOLS 1... 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... Photoelectric converter, 10... Amplifier, 11... Synchronous detector, 12... Spectrum analyzer, 13... Transmitter, 14... Receiver, 21... Diffraction grating, 22... Optical deflection element, 23... Output slit.

Claims (1)

[Claims] 1. In a method for measuring the zero dispersion wavelength of an optical fiber to be measured, in which the approximate value of the wavelength λ ₀ at which the chromatic dispersion value becomes zero is calculated theoretically, from one end of the optical fiber to be measured. , inputting intensity-modulated incident light, converting the outgoing light emitted from the other end of the optical fiber to be measured into an electrical signal, and making the incident light or the outgoing light have a center wavelength close to the wavelength λ ₀ . An optical fiber that undergoes wavelength modulation, changes the center wavelength so that the primary spectrum of the phase modulation component of the electrical signal becomes minimum, and sets the wavelength at the time when the spectrum disappears as the zero-dispersion wavelength of the optical fiber to be measured. Zero dispersion wavelength measurement method. 2. The zero-dispersion wavelength measurement method for an optical fiber according to claim 1, wherein wavelength modulation of the center wavelength is applied to the incident light of the optical fiber to be measured. 3. The zero-dispersion wavelength measurement method for an optical fiber according to claim 1, wherein wavelength modulation of the center wavelength is applied to the output light of the optical fiber to be measured. 4. A light source that emits light in a wide wavelength band, an oscillator and a power source, and an intensity modulator that uses the outputs of the oscillator and power source as input, generates a modulation signal for intensity modulating the output light of the light source, and drives the light source. a circuit, means for introducing the output light of the light source into the optical fiber to be measured, a photoelectric converter, a means for guiding the output light of the optical fiber to be measured to the photoelectric converter, and A wavelength modulator that is inserted between the converter and performs wavelength modulation on the output light, an amplifier that amplifies the electrical signal output from the opto-electrical converter, and a synchronous detector that receives the output of this amplifier as input. a means for transmitting the output of the oscillator to the synchronous detector; and a spectrum analyzer for observing the spectrum of the output of the synchronous detector; the wavelength modulator has a variable modulation wavelength; A zero-dispersion wavelength measuring device for an optical fiber, characterized in that the center wavelength can be set within a range such that the primary spectrum of the phase modulation component observed by the spectrum analyzer becomes minimum.