JP3237384B2 - Method and apparatus for measuring chromatic dispersion of optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring chromatic dispersion of an optical device applicable to a long optical fiber.

[0002]

2. Description of the Related Art In an optical fiber communication system, an optical signal undergoes waveform distortion due to chromatic dispersion of an optical fiber during propagation and is degraded. Also, in ultrafast all-optical switching, the switching speed is limited by the chromatic dispersion of the optical device used. Therefore, chromatic dispersion of an optical device including an optical fiber is the most important parameter when designing an optical communication system, an all-optical switching circuit, or the like. There are three main types of optical device chromatic dispersion measurement methods: interferometry, pulse measurement, and phase shift measurement [LEONARD G. C
OHEN: "Comparision of Single-Mode Fiber Dispersion
Measurement Techniques ", JOURANL OF LIGHTWAVE TEC
HNOLOGY, VOL.LT-3, No.5, OCTOBER 1985, pp 958-966
reference〕. In the interferometry, a Mach-Zehnder or Michelson interferometer is configured using a CW white light source such as a halogen lamp with a sufficiently short coherence length as a light source, and the measurement is affected by the dispersion of the reference light and the optical device to be measured. The chromatic dispersion is calculated by calculating the relative delay time from the optical path difference between the two interference arms when the contrast of interference with light is maximized. In the pulse method, a group delay time in which a light pulse is generated in a device to be measured is measured as a time delay amount of a light pulse using a wavelength variable pulse light source or a white pulse light source, and wavelength dispersion is calculated. A wavelength-tunable mode-locked laser or the like is used as a wavelength-tunable pulse light source, and a white pulse light source has a wavelength range of 100 when a single-mode fiber is excited by ultrashort pulse light.
Super continuum light and high-order stimulated Raman scattered light generated over nm are used. In the phase shift method, the chromatic dispersion is calculated by measuring a relative delay time as a phase shift amount of an optical envelope using sine wave modulated light of an LED or LD. According to this method, the measurement can be performed with a time accuracy equal to or higher than that of the pulse method without being limited by the light source and the light receiving system in the electric band.

[0003]

However, in the interferometry, it is necessary to make the optical path lengths of the interference light and the measurement light coincide with each other, which is completely unsuitable for measurement of an optical fiber having a length of the order of km or more. Met.

Next, the pulse method has an advantage that a light source covering a wide wavelength range can be used.
The accuracy of the directly measured delay time is limited by the bandwidth of the O / E converter, and a wide-band measuring instrument such as a sunproscope or a streak camera is required. The accuracy of the measuring time is currently at most about 10 picoseconds. . In addition, the phase shift method has an advantage that it is not subject to the band limitation of the electric system,
Conventionally, LEDs and LDs have been used as light sources, and thus have the disadvantage that it is difficult to cover a wide wavelength range with a single light source. As described above, in the related art, it is difficult to measure the chromatic dispersion of an optical device including an optical fiber having a length of the order of km or more with high accuracy over a wide wavelength range without being limited by an electric system. .

In order to solve the above-mentioned problems, the present invention provides a method and an apparatus having both the advantages of the pulse method and the phase shift method, that is, the advantages of the phase shift method which is not subject to the band limitation of the electric system and the wide wavelength range. Provided is a method and apparatus for measuring chromatic dispersion of an optical device, which has the advantage of a pulse method in which a light source to be covered can be used.

[0006]

According to the present invention, the dispersion characteristic is measured by measuring a delay time caused by a repetitive optical pulse train passing through a device under test as a phase shift amount of one Fourier sine wave component of an optical pulse envelope. Ask. FIG. 1 shows a spectrum diagram for explaining this principle on the frequency axis. 1-1 represents the spectrum of an optical pulse train having a carrier (optical) angular frequency ω ₀ and a repetitive angular frequency Ω, and its envelope corresponds to the spectrum of a single optical pulse. 1-2 is 1
-1 is an electric signal obtained by envelope detection, 1-3 is 1-2
One of the filtered Fourier sine wave components (here, a fundamental frequency component). In the case of the pulse method, it is necessary to detect many harmonic components by wide-band O / E conversion in order to improve the measurement time accuracy. However, this apparatus filters only the fundamental frequency component or only one harmonic component that is repeated. To measure the phase difference before and after the device under test.

The method of the present invention will be described on a time axis. A waveform f (t, z; ω ₀ ) when an optical pulse train having a repetition period T (carrier angular frequency ω ₀ ) propagates through a device under test having a propagation constant β (ω) by a distance z is as follows. Is done.

(Equation 5) Here, F ₀ (ω) represents the Fourier transform of the single light pulse envelope f ₀ (t), and Ω = 2π / T. The conventional phase shift method corresponds to the case where F ₀ (nΩ) ≠ 0 only for n = 0 and ± 1. Assuming that f ₀ (t) is a real function and nΩ is sufficiently smaller than ω ₀ , f (t, z; ω ₀ ) is further obtained by superimposing the sine wave cos nΩt.

(Equation 6) I can write Here, φ (ω) is a real and odd function of the argument of F ₀ (ω). β ₁ (ω ₀ ) represents the derivative of β (ω) at ω = ω ₀ , which is the group delay amount of the device under test.
The chromatic dispersion D (λ) is

(Equation 7) (C is the speed of light in a vacuum). Group delay β ₁ (ω
₀ ) is the measurement of the n-th harmonic component (cos nΩt) contained in the envelope of the optical pulse train f (t, z; ω ₀ ) using a narrow-band measuring instrument such as a lock-in amplifier or a vector voltmeter. Phase delay amount θ _d = nΩβ ₁ (ω) before and after the device (length L)
₀ ) It is determined by measuring L. The phase delay amount θ _d increases in proportion to the harmonic order n. For this reason, unlike the conventional phase shift method using only the fundamental wave, in the method of the present invention in which a short light pulse is detected and a harmonic component can be used, F ₀ (nΩ) is sufficiently large in a range of n. Greater n
By measuring the second harmonic component, it is possible to easily increase the sensitivity of the phase measurement without increasing the repetition frequency of the white pulse and the device length to be measured.

[0008]

The operation of the first and third aspects will be described with reference to FIG. Reference numerals 3-1 and 3-2 denote optical pulses of wavelength λ corresponding to the reference light and the output of the device under measurement, respectively, and the time difference t _d (λ)
= T _out (λ) -t _in (λ) corresponds to the group delay amount β ₁ (2πc / λ) of the device under test. Reference numerals 3-3 and 3-4 denote one Fourier sine wave components of the envelopes of the light pulses 3-1 and 3-2, which are fundamental frequency components (1 / T) here. The phase difference θ _d (λ) of the sine waves 3-3 and 3-4 is measured using a phase difference detecting means such as a vector voltmeter. Generally, when detecting the n-th harmonic, θ _d (λ) and t _d (λ) are connected by the relation θ _d (λ) / (2πn) = t _d (λ) / T. Therefore,

## EQU8 ## t _d = β ₁ (2πc / λ) L = Tθ _d (λ) /
(2πn) (4) Substituting this into equation (3)

(Equation 9) Is obtained.

The second invention of the present application corresponding to claims 2 and 4 will be described with reference to FIG. 4-1 is a time-resolved spectral image of a white pulse incident on the device to be measured, 4-2 is a time-resolved spectral image of a white pulse emitted from the device to be measured, and these are relative times of each wavelength component included in the white pulse. Represent a relationship. Reference numerals 4-3 and 4-4 denote light pulses of wavelengths λ ₁ and λ, respectively, obtained by filtering the incident light 4-1 of the device under test with a band-pass filter or the like. Reference numerals 4-5 and 4-6 denote the device under test, respectively. Are light pulses of wavelengths λ ₂ and λ filtered from the outgoing light 4-2. The second invention is different from the first invention in that the delay time of the outgoing light pulse with respect to the incident light pulse (reference light) of the device under measurement is not directly compared with the time-resolved spectral images 4-3, 4-4. The relative delay times Δt _in (λ) and Δt _out (λ) of the wavelength components of the white pulse light obtained from 4-5 and 4-6 are measured, and the delay time due to the device under test is measured.

T _d (λ) = Δt _out (λ) −Δt _in (λ) (6) FIG. 4 shows the relationship between these light pulses and the Fourier sine wave component of the envelope. 5-1, 5-2, 5-3
5-4 are the light pulses 4-3, 4-4, 4-5 and 5-5 in FIG.
4-5, 5-5, 5-6, 5-7, 5
Reference numeral -8 denotes a fundamental Fourier sine wave component of the envelope of the light pulses 5-1 to 5-2. Here, from the phase difference Δθ _in (λ) between the sine waves 5-5 and 5-6, Δt
_in (λ) is derived and the phase difference Δθ between the sine waves 5-7 and 5-8
Δt _out (λ) is derived from the _{out (λ).} Here, Δθ _in (λ) / 2π = Δt _in (λ) / T, Δθ _out
(Λ) / 2π = Δt _out (λ) / T. Therefore,

[Equation 11] From equations (3), (4) and (6)

(Equation 12) Is obtained.

[0010]

FIG. 5 shows an example of a white pulse light source used in the present invention. 6-1 is an excitation short light pulse light source, and 6-2 is an excitation optical fiber. The short light pulse emitted from the excitation short light pulse light source 6-1 generates a white pulse caused by stimulated Raman scattering or a four-wave mixing process with the propagation of the excitation optical fiber 6-2. When the energy conversion from the short light pulse to the white pulse is incomplete, the excitation short light pulse component remains in the white pulse as shown by a dotted line. FIG. 6 shows another example of a white pulse light source provided with a band elimination optical filter for attenuating a residual excitation pulse component at a stage subsequent to the excitation optical fiber. 7-1 is an excitation short light pulse light source, 7-2 is an excitation optical fiber, and 7-3 is a band elimination optical filter. By attenuating the residual excitation pulse component, unnecessary nonlinear effects newly generated in the device under test can be suppressed.

An embodiment of the method for measuring chromatic dispersion of an optical device according to the present invention will be described below. Claims 1 and 3 (first invention)
FIG. 7 shows an embodiment corresponding to FIG. Reference numeral 8-1 denotes a white pulse light source, and reference numeral 8-2 denotes a tunable optical bandpass filter, which constitutes a tunable pulse light source (see Japanese Patent Application No. 4-344876, "Waveband Broadband Pulsed Light Generator"). 8-3 is an optical splitter that splits the output of the wavelength tunable bandpass filter 8-2 into the next incident light to the device under test and reference light, 8-4 is the device under test, and 8-5 and 8-6 are O / E converter, 8-
Reference numerals 7 and 8-8 denote RF bandpass filters that pass only the fundamental frequency 1 / T of the white pulse or its harmonics. 8-9
Is a phase difference detecting means such as a vector voltmeter. The reference light and the output light of the device under test are output from different O / E converters 8-5,
The envelope signals are detected by 8-6. These two envelope signals are applied to respective RF bandpass filters 8.
The repeated fundamental frequency component or its harmonic component is extracted by -7 and 8-8, and the phase difference θ _d (λ) is measured to obtain the delay time t by the device under test.
Calculate _d (λ). Each pair of the O / E converter and the RF bandpass filter 8-5, 8-7 and 8-6, 8-8 can be replaced by a narrow band light receiver having sensitivity only to a desired sine wave component.

FIG. 8 shows an embodiment based on claims 2 and 4 (second invention). 9-1 is a white pulse light source, 9-
Reference numeral 2 denotes a device to be measured, 9-3 denotes an optical splitter, 9-4 denotes an optical bandpass filter having a fixed passing wavelength, and 9-5 denotes a wavelength variable optical bandpass filter. 9-6 and 9-7 are O
The / E converters 9-8 and 9-9 are RF bandpass filters that pass only the fundamental frequency 1 / T of the white pulse or its harmonics. 9-10 is a phase difference detecting means. Each pair of the O / E converter and the RF band-pass filter 9-6, 9-8 and 9-7, 9-9 can be replaced by a narrow band light receiver having sensitivity only to a desired sine wave component. The white pulse train output from the device under test is branched at 9-3, and the wavelength-phase characteristic Δθ _out (λ) with respect to the wavelength-delay time characteristic Δt _out (λ) of 3-2 shown in FIG. 3 is measured. This Δ
The delay time t _d (λ) is calculated by subtracting the measurement result Δθ _in (λ) without the device under test from θ _out (λ) to obtain θ _d (λ). This embodiment does not particularly provide an optical path for the reference light as in the first invention, and uses a part of the output of the device under measurement as a reference light pulse, thereby enabling stable measurement against disturbances such as temperature and vibration. And For this reason, this configuration is an embodiment particularly suitable for far-end measurement of the laid fiber.

In both the first and second inventions, θ _d (λ) can be obtained without increasing the repetition frequency of white pulse light or the device length to be measured by measuring the phase of higher harmonic components. Can be increased, and high-resolution phase measurement can be easily performed.

When the frequency of the harmonic component to be measured is too high to be measured by a lock-in amplifier or the like, it can be converted to a measurable intermediate frequency by using the heterodyne method. FIG. 9 shows an embodiment in which this is applied to the second invention. 10-
1 is a white pulse light source having a repetition frequency f, 10-2 is a device to be measured, 10-3 is an optical splitter, 10-4 is an optical bandpass filter having a fixed passing wavelength, and 10-5 is a variable wavelength optical bandpass. Filter. 10-6,10-7
Is an O / E converter, 10-8 and 10-9 are RF bandpass filters, 10-10 is a local oscillator having a frequency f _L ,
11 and 10-12 are frequency mixers, and 9-10 is a phase difference detecting means. In this case, the frequency mixers 10-11,
10-12 signals from harmonics of the repetition frequency of the white pulse light (frequency nf) local oscillator is input to the signal of the frequency nf-f _L is obtained from the output. The phase information of the harmonic is stored even if it is converted into an intermediate frequency signal.

[0015]

As described above, the present invention provides a method and an apparatus having the advantages of the pulse method and the phase shift method. Therefore, it is not necessary to use an expensive measuring instrument such as a sun proscope or a streak camera, and the chromatic dispersion of an optical device using a white pulse light source that covers a wide wavelength range with a simple configuration can be measured. Measurement of a higher-order sine wave component of the detected light pulse makes it possible to easily perform high-sensitivity phase measurement. Furthermore, the use of the method of the second invention provides a measuring means which is resistant to disturbance and can be applied to the laid fiber.

[Brief description of the drawings]

FIG. 1 is a frequency spectrum diagram for explaining the principle of the present invention.

FIG. 2 is a time chart for explaining the principle of the first invention of the present application.

FIG. 3 is a wavelength versus delay time characteristic diagram for explaining the principle of the second invention of the present application.

FIG. 4 is a time chart for explaining the principle of the second invention of the present application.

FIG. 5 is a diagram showing an example of a light source used in the present invention.

FIG. 6 is a diagram showing another example of the light source used in the present invention.

FIG. 7 is a block diagram showing an embodiment of the first invention of the present application.

FIG. 8 is a block diagram showing an embodiment of the second invention of the present application.

FIG. 9 is a block diagram showing a modification of the embodiment of FIG.

[Explanation of symbols]

1-1 Spectrum of optical pulse train 1-2 Electric signal (fundamental wave component) 1-3 Fourier sine wave component 3-1 One wavelength component of white pulse before input to device under test 3-2 Output from device under test One of the later white pulses
Wavelength component 3-3 Fourier sine wave component of 3-1 3-4 Fourier sine wave component of 3-2 4-1 Time-resolved spectral image of white light incident on device under measurement 4-2 Time resolution of white light emitted from device under measurement Spectral image 4-3 Light pulse of wavelength λ ₁ 4-4 Light pulse of wavelength λ 4-5 Light pulse of wavelength λ ₂ 4-6 Light pulse of wavelength λ 5-1 5-2-3 4 Optical pulse 5-5, 5-6, 5-7, 5-8 Basic Fourier sine wave component of envelope 6-1, 7-1 Excited short optical pulse light source 6-2, 7-2 Optical fiber for excitation 7 -3 Band elimination optical filter 8-1, 9-1, 10-1 White pulse light source 8-2 Wavelength variable optical band pass filter 8-3, 9-3, 10-3 Optical splitter 8-4, 9-2 , 10-2 Device under test 8-5, 8-6, 9-6, 9-7, 10-6, 10-7
O / E converter 8-7, 8-8, 9-8, 9-9, 10-8, 10-9
RF bandpass filter 8-9, 9-10, 10-13 Phase difference measuring means 9-4, 9-5, 10-4, 10-5 Optical bandpass filter 10-10, 10-11 Frequency mixer 10-12 Local oscillator

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-61-105439 (JP, A) JP-A-56-103343 (JP, A) JP-A-2-227627 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01M 11/00-11/02

Claims (5)

(57) [Claims] 1. An optical pulse train having a center wavelength λ and a repetition period T is branched into two, one of which is a time-delayed reference light, and the other is incident on an optical device to be measured having an element length L, and the envelope of the reference pulse train is obtained. The phase delay θ _d (λ) [rad] of the n-th harmonic component of the envelope of the output optical pulse train of the device under test with respect to the n-th harmonic component of the line is measured as a function of the wavelength λ. ] A wavelength dispersion D of the optical device, wherein the wavelength dispersion D is obtained from the following equation. Wherein for n-th harmonic component of the envelope of the repetition period T white pulse train fixed center wavelength lambda ₁ of the optical pulse train included in the, n harmonics of the envelope of the optical pulse train any center wavelength lambda Phase delay Δθ _in (λ) [r
ad] as a function of the wavelength λ, the white pulse train is incident on an optical device under test having an element length L, and the envelope of the light pulse train having a fixed center wavelength λ ₂ included in the emitted light thereof. The phase delay amount Δθ _out (λ) [rad] of the n-th harmonic component of the envelope of the optical pulse train having an arbitrary center wavelength λ with respect to the n-th harmonic component is measured as a function of the wavelength λ. A wavelength dispersion D of the optical device, wherein the wavelength dispersion D is obtained from the following equation. 3. A wavelength-variable pulse light source having a repetition period T, an output of the wavelength-variable pulse light source, a part of which is used as a time-delayed reference light, and the other part of which is a device under test having an element length L. An optical branching means which is incident on the target light, and an envelope of an optical pulse train having a central wavelength λ included in the output light of the device under test with respect to an n-th harmonic component of an envelope of the optical pulse train having the central wavelength λ included in the reference light. Is measured as a function of the wavelength λ, and the phase delay amount θ _d (λ) [rad] of the n-th harmonic component of Chromatic dispersion measuring apparatus for an optical device, comprising: a measuring means for determining a chromatic dispersion D from the following. Wherein for n-th harmonic component of the envelope of the repetition period T white pulse train fixed center wavelength lambda ₁ of the optical pulse train included in the, n harmonics of the envelope of the optical pulse train any center wavelength lambda Phase delay Δθ _in (λ) [r
ad] as a function of the wavelength λ, the white pulse train is incident on an optical device to be measured having an element length L, and the envelope of the light pulse train having a fixed center wavelength λ ₂ included in the emitted light is obtained. The phase delay amount Δθ _out (λ) [rad] of the n-th harmonic component of the envelope of the optical pulse train having an arbitrary center wavelength λ with respect to the n-th harmonic component is measured as a function of the wavelength λ. Measuring means for obtaining a chromatic dispersion D from the light source, wherein the white pulse train is generated from a light source means composed of at least one set of a short pulse light source for excitation and an optical fiber for excitation. Wavelength dispersion measuring device for optical devices. 5. A means for removing a wavelength component of the short pulse light for excitation from the pulse train of white light before entering the optical device to be measured, and removing the wavelength component of the pulse train of white light before entering the optical device to be measured. The chromatic dispersion measuring apparatus for an optical device according to claim 4, further comprising at least one of means for attenuating the intensity.