JPH10307077A - Apparatus for measuring wavelength dispersion of optical component by using delaying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the wavelength dispersion of an optical component, to be measured, whose absolute value of a dispersion value is small with high accuracy by a method wherein the wavelength dispersion characteristic of the optical component is computed on the basis of a known wavelength and on the basis of the detected change portion of an optical length. SOLUTION: An optical component 2 to be measured is irradiated with a semiconductor laser 1, and a transmitted laser beam is photoelectrically converted by a photodetector 3 via an optical delaying device 11. Its electric signal is fed back to the semiconductor laser 1 via a filter 4, an amplifier 5, an amplitude limiter 6 and the like. At this time, a cutoff frequency, an amplification factor and a limiting amplitude are adjusted, and a closed circuit from the semiconductor laser 1 through the photodetector 3 to the semiconductor laser 1 oscillates a fundamental frequency. At this time, the optical length of the optical delaying device 11 is set in a proper position, and an oscillation frequency is measured by a frequency counter 10. Then, the temperature of the semiconductor laser 1 is changed, and an output wavelength is changed. At this time, since the oscillation frequency is changed, the optical delaying device 11 is adjusted so as to be kept at an initial fundamental frequency, and the change portion of the optical length is detected by itself. On the basis of the detected change portion of the optical length and on the basis of a known wavelength, the wavelength dispersion characteristic of the optical component 2 to be measured is computed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the measurement of chromatic dispersion characteristics of optical components performed at the stage of research, development and design of an optical communication system, and particularly to the accurate measurement of optical components having a small absolute value of dispersion value. The present invention relates to a chromatic dispersion measuring device for an optical component using a delay unit.

[0002]

2. Description of the Related Art The measurement of chromatic dispersion, which is a phenomenon in which the group velocity of light changes with frequency or wavelength, is required from the following points. First, when an optical pulse is represented by a bit 0 or 1, the width of the optical pulse is narrowed to improve the transmission capacity. However, when an optical pulse is passed through an optical fiber with chromatic dispersion, one part of the spectrum advances relatively early, and another part advances relatively late, resulting in the collapse of the pulse shape. Problems arise. The effect of this chromatic dispersion increases as the pulse width decreases.

[0003] In this case, it is possible to perform optical transmission by using an optical fiber having extremely small chromatic dispersion to reduce the influence of chromatic dispersion. Since use over a band is considered, it is necessary to accurately determine such a small chromatic dispersion. In addition, when such a narrow optical pulse is used over a wide wavelength band, not only the optical fiber but also the wavelength dispersion characteristics of various optical components such as lenses, optical amplifiers, and optical isolators existing on the transmission path. Since they cannot be ignored, it is necessary to measure their chromatic dispersion characteristics to understand the influence on the transmission path.

On the other hand, when compressing an optical pulse or when using a special pulse such as an optical soliton, a portion where chromatic dispersion exists is actively used, and it is necessary to know the chromatic dispersion characteristics. Is important.

[0005] An example of a conventional method of measuring the chromatic dispersion characteristics is disclosed in Japanese Patent Publication No. 2-33971. The details are described below. In this measurement method, light emitted from a light source is incident on a single-mode optical fiber, light emitted from the single-mode optical fiber is photoelectrically converted by a photoelectric converter, and an output from the photoelectric converter is used as a light source. A loop is formed so as to feed back the excitation current. Then, the chromatic dispersion of the single mode optical fiber is obtained from the change in the oscillation frequency of the loop when the wavelength of the light emitted from the light source is changed.

This measuring method will be described with reference to FIG.
That is, a semiconductor laser 1 having a narrow spectrum width as a light source
Is emitted to the measured optical component 2 and the light emitted from the measured optical component 2 is converted by the photodetector 3 into an electric signal proportional to the intensity of the light. The electric signal is amplified by an amplifier 5 via a band-pass filter 4 and applied to the semiconductor laser 1 via an amplitude limiter 6 and a capacitor 7. Then, a loop is formed by controlling the excitation current of the semiconductor laser 1 with this electric signal.

On the way, the light emitted from the semiconductor laser 1 is partially branched by a beam splitter (not shown), and the wavelength of the emitted light is measured by a wavelength meter 8. The semiconductor laser 1 is kept at a constant temperature by a constant temperature device (not shown), a DC bias current is supplied from a DC power supply 9, and the above-described amplified electric signal is superimposed on the DC bias current.

At this time, a loop in which light emitted from the semiconductor laser 1 becomes an electric signal and returns to the semiconductor laser 1 is a kind of oscillator, and a frequency corresponding to a cycle when the light and the electric signal circulate in this loop. Oscillation occurs at the fundamental frequency. The amplified electric signal is partially branched, and the oscillation frequency is measured by the frequency counter 10 or the like. In this technique, the oscillation frequency matches the fundamental frequency, and thus the fundamental frequency of the loop including the optical component 2 to be measured is measured.

Next, when the temperature maintained by the thermostat is changed by the temperature controller, the wavelength of the light emitted by the semiconductor laser 1 changes, and the wavelength is measured. Here, if the optical component 2 to be measured has wavelength dispersion, that is, the property that the group velocity varies depending on the wavelength of light passing through, the optical distance of the wave measuring optical component 2 (= physical length × refractive index or less, optical length ) Changes, so the fundamental frequency also changes. In this technique, a changed fundamental frequency is measured by measuring a changed oscillation frequency.

Assuming that a fundamental frequency for a certain wavelength λ is f, the fundamental frequency f is theoretically expressed by the following equation. f = 1 / (τ + T) (1) where τ is a group delay time for light to pass through the measured optical component 2, and T is a portion of the loop other than the measured optical component 2 where light and electric signals are transmitted. This is the group delay time that passes.

Next, assuming that the fundamental frequency changes from f to f + Δf when the wavelength is changed from λ to λ + Δλ, and expressed in the same manner as in equation (1), the following is obtained. f + Δf = 1 / (τ + Δτ + T) (2) Here, Δτ is referred to as a group delay time difference, which represents how much time the light of wavelength λ + Δλ passes through the measured optical component 2 with respect to the light of wavelength λ. Quantity.

When the operation of (2)-(1) is performed, Δf ≒ −Δτ / (τ + T) ² = −Δτ × f ² (3) Therefore, by measuring f and Δf, the group delay time difference Δτ is calculated from Δτ ≒ −Δf / f ² (4).

The chromatic dispersion D is obtained by differentiating the group delay time difference Δτ per unit length with respect to wavelength. Assuming that the physical length of the measured optical component 2 is L, the chromatic dispersion D is approximately: D 近似 Δτ / (L × Δλ) = − Δf / (f ² × L × Δλ) (5) expressed. From the above equation, the chromatic dispersion D of the measured optical component 2 is calculated by measuring f, Δf, L and Δλ.

[0014]

One of the problems of the prior art described above is the problem of errors due to the frequency characteristics of the photodetector 3, the bandpass filter 4, the amplifier 5, and the amplitude limiter 6. . The frequency characteristic that matters here is the dependence of the group delay of the electric signal on the frequency,
This corresponds to chromatic dispersion in an optical component. Specifically, in the measurement when the wavelength of the light source was changed (2)
The equation does not hold, and f + Δf = 1 / (τ + Δτ + T + ΔT) (6) As shown in equation (6), a group delay time difference ΔT of the electric signal is generated,
The optical group delay time difference Δτ and the electric group delay time difference ΔT cannot be separated. When the measured optical component 2 is a long optical fiber or the like, Δτ >> ΔT and the effect of ΔT can be ignored. However, when the chromatic dispersion of the measured optical component 2 is originally small, the effect of ΔT cannot be ignored. In particular, in the ordinary band-pass filter 4, the frequency dependence of the group delay is large, and if the frequency dependence is to be suppressed, a trade-off in which the characteristics of the filter deteriorate is inevitable.

In the measuring method described in the prior art,
Theoretically, the oscillation frequency may oscillate even if it is an integral multiple of the equation (1). In such a case, a measurement error occurs. Therefore, the oscillation is performed at the fundamental frequency f by a filter.
However, in the actual measurement, the group delay time τ of the light varies depending on the optical component 2 to be measured.
Since f also changes according to equation (1), it is necessary to change the center frequency of the filter 4 accordingly. When f changes, ΔT occurs as described above, which causes an error.
When τ changes in this way, the measurement has been complicated and has errors.

Another problem of the prior art is measurement accuracy of chromatic dispersion. This will be described with specific numerical values. Now, when an optical fiber having a length of 100 m is considered as the optical component 2 to be measured, the group delay time τ of light in the equation (1) is about 500 ns. On the other hand, since the group delay time T of electricity is generally about 10 ns, f is about 2 MHz, which is substantially dependent on τ. The wavelength variable range of the semiconductor laser 1 is about 5 nm. on the other hand,
With currently available frequency counters, the measurement accuracy of Δf is about 1 Hz. By substituting these values into equation (5), the measurement accuracy of chromatic dispersion is approximately 0.4 psec / nm.
/ Km.

At present, however, a fiber having extremely small chromatic dispersion such as a zero-dispersion fiber has been developed.
In many cases, a measurement accuracy of 1 psec / nm / km or less is required. Therefore, in the prior art, the accuracy of the measurement of the chromatic dispersion of the optical fiber tends to be insufficient.

[0018]

Means for Solving the Problems The above-mentioned problems of the prior art have been solved in the present invention by the following means. Note that the reference numerals used in the embodiments are used. The gist of the present invention is to pass light emitted from a light source capable of selecting at least two kinds of known light wavelengths to an optical component under measurement 2 and convert the passed light into an electric signal by a photoelectric converter, In the chromatic dispersion measuring device of the optical component having a closed circuit that generates the oscillation frequency corresponding to the optical wavelength by returning the electric signal to the excitation current of the light source, the oscillation frequency corresponding to the optical wavelength is set to Means for varying the optical length from the light source to the photoelectric converter so as to keep the oscillation frequency constant when generated;
3 or means 15 for varying the electrical length from the photoelectric converter to the light source, and means 1 for detecting a change in the optical length
Means 15 for detecting a change in 1, 3, or electrical length, based on the at least two known wavelengths and the detected change in optical length, or based on the at least two known wavelengths. A wavelength dispersion measuring apparatus for an optical component using a delay device, wherein a wavelength dispersion characteristic of the optical component to be measured is calculated based on the wavelength of the optical component and the detected change in the electrical length.

That is, in the conventional technique, when the wavelength of light is changed, the optical length changes according to the change in the wavelength,
This changes the fundamental frequency of the closed circuit. The amount of change in the fundamental frequency is measured to determine chromatic dispersion.

On the other hand, the present invention is characterized in that there is provided means for keeping the oscillation frequency oscillated in a closed circuit constant when the wavelength of the light source is changed. That is, the chromatic dispersion can be measured by the means for keeping the oscillation frequency constant detecting another change amount required to keep the oscillation frequency constant.

As means for keeping the oscillation frequency constant,
There are two types, a method of varying the optical length of the portion from the light source to the photoelectric converter, and a method of varying the electrical length of the portion from the photoelectric converter to the light source.

When the measured optical component 2 is composed of a plurality of types of optical components such as an optical isolator, the definition of the chromatic dispersion, which is the amount per unit length, is defined by the optical component 2 to be measured. Expressing dispersion characteristics is sometimes inconvenient. Therefore, in this specification, the wavelength derivative of the group delay time difference of the entire optical component is defined and used as the total dispersion. That is, the total dispersion is D
If a is used, it is approximately expressed by equation (7) using equation (4). Da ≒ Δτ / Δλ = -Δf / (f ² × Δλ) (7)

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a chromatic dispersion measuring apparatus for optical parts using a delay unit according to the present invention will be described below.
First, a feature of the present invention, that is, a means for varying the optical length from the light source to the photoelectric converter so as to keep the oscillation frequency constant will be described.

As a first means for varying the optical length from the light source to the photoelectric converter, a means for delaying light can be considered.
As means for delaying the light, an optical delay device is generally used. FIG. 3 shows an example of the structure of the optical delay unit. FIG. 3A is an operation principle diagram of an optical delay device that delays light using a corner cube mirror, and FIG. 3B is a schematic configuration diagram of the optical delay device.

In this case, there is a method in which a corner cube mirror is mounted on a linear movement stage and driven. The corner cube mirror is formed by bonding three mirrors at right angles and has a property that incident light and outgoing light are always parallel. When this corner cube mirror is moved in parallel with the optical axis, the emitted light returns to the same place, and the distance of the light changes, so that the amount of delay of the light can be controlled. An advantage of using the corner cube mirror is that chromatic dispersion can be ignored because light propagates in space, and high-precision measurement can be performed without performing calibration described later.

Examples of the realization of the optical delay device include a device using a prism, a device using a temperature expansion and contraction of an optical waveguide in addition to a device using a corner cube mirror, and any of them can be used in the present invention. . Each of the prism and the optical waveguide has its own wavelength dispersion.
It is necessary to calibrate these chromatic dispersions when measuring the chromatic dispersions with high accuracy. The method of calibration will be described later.

In order to optically couple the light source and the optical delay unit, or the optical delay unit and the photoelectric converter, a method of spatially coupling using a lens, a mirror or the like has been desired in the prior art. In the above, a method of coupling using an optical fiber is also possible. When a coupling method using an optical fiber is used, the chromatic dispersion of the optical fiber causes an error in the measurement, but the prior art could not calibrate it. On the other hand, in the present invention, the influence of the chromatic dispersion of the coupling optical fiber can be calibrated by a calibration method described later. The method of spatially coupling using lenses, mirrors, etc., has the disadvantages that it takes time to assemble and adjust the measuring device and is susceptible to vibration and temperature changes.
The coupling method using an optical fiber hardly causes such a disadvantage.

A second method for varying the optical length from the light source to the photoelectric converter (the photoelectric converter is the same as the photodetector 3 described in the related art) so as to keep the oscillation frequency constant.
The light detector 3 is a photoelectric converter that receives the light that has passed through the measured optical component 2 instead of delaying the light.
Can be considered. For this purpose, the photodetector 3, which is a photoelectric converter, may be installed on a linear moving stage, and the moving direction of the stage may be parallel to the optical axis. By moving the linear movement stage, the same effect as providing a delay can be obtained. Also in this case, since the light that has passed through the optical component 2 to be measured propagates in space, chromatic dispersion can be ignored and high-precision measurement can be performed.

On the other hand, there is also a means for completely changing the idea and varying the electrical length from the photoelectric converter to the light source so as to keep the oscillation frequency constant. If the electrical length is made variable and a change in the electrical length is detected, the same effect as when the optical length is changed can be obtained. The obtained change in the electrical length can be handled in exactly the same manner as the change in the optical length. However, in this specification, the electric length is defined as the propagation time taken in a certain electric circuit multiplied by the speed of light. This definition will be described in the section of the embodiment using the electric length varying means.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a specific embodiment of an apparatus for measuring chromatic dispersion of an optical component using a delay unit according to the present invention will be described. (First Embodiment) FIG. 1 is a schematic configuration diagram of a chromatic dispersion measuring apparatus for an optical component using a delay unit according to a first embodiment of the present invention. In the first embodiment, the optical length from the light source to the photoelectric converter is made variable, and the optical delay unit 11 is used as means for changing the optical length.

The light source may be any light source that can be intensity-modulated by an external electric signal and that can set the wavelength to at least two known wavelengths. Therefore, it is also possible to prepare a laser light source in which at least two wavelengths are fixed to different values, and to selectively use them during measurement. Further, since the semiconductor laser 1 has a property that the wavelength of the output light changes when the device temperature is changed, and is small and inexpensive, a simple wavelength dispersion measuring device using the semiconductor laser 1 is realized. can do. In this embodiment, the temperature of the semiconductor laser 1 is controlled in a thermostat (not shown), and the wavelength of the output light can be changed by changing the set temperature.

The measured optical component 2 to be measured includes:
In principle, anything can be measured as long as light passes through it. In particular, the present invention is particularly advantageous in measuring chromatic dispersion of a device having a small chromatic dispersion or a device having a small actual length or actual size. Specific examples include a dispersion-shifted optical fiber, an optical fiber of 100 m or less, a lens, an optical amplifier, an optical modulator, an optical filter, and a polarizer.

The light detector 3, which is a photoelectric converter, converts the received light into an electric signal proportional to the intensity of the light and outputs the electric signal. As described above, the optical delay unit 11 may use the corner cube mirror shown in FIG. 3, but the insertion position is generally behind the optical component 2 to be measured.
However, there is no problem even in front of the optical component 2 to be measured.

As will be described later, what is required for measuring the chromatic dispersion is a change in the optical length of the optical delay device 11 with respect to an arbitrarily set reference position, and the optical length itself is not required. However, if a means for detecting an absolute optical length is provided, a change in the optical length can be easily obtained based on the means. Hereinafter, assuming that there is no means for detecting the optical length, a description will be given of a means for detecting a change in the optical length.

Various means can be considered as means for detecting the change in the optical length. Usually, the means is provided as a function of the optical delay unit 11. For example, if an optical delay device 11 that can set the optical length and its change from a dedicated controller or a personal computer is used, changing the set value is equivalent to detecting the change in the optical length. .

In the optical delay device 11 having means for detecting the amount of movement of the linear moving stage on which the corner cube mirror is installed, it is assumed that light reciprocates in the optical delay device. Therefore, the amount of change in the optical length can be obtained by doubling the amount of movement of the stage.

If the optical delay unit 11 is not provided with a function for detecting a change in the optical length as its function, the amount of movement of the stage on which the corner cube mirror is installed may be detected by some means. If the stage is driven by a stepping motor, the number of electric pulses sent to the stepping motor to move the stage is counted, and the amount of movement of the stage can be known from the number of pulses and the gear ratio of the stage. it can. Still another method is to provide a reference point on an extension of the movement axis of the stage and measure the distance between the reference point and the stage using a micrometer or the like. By doubling the moving amount of the stage obtained by these methods, a change in the optical length can be detected.

In order to strictly determine the optical length and its change, it is necessary to multiply twice the moving amount of the stage by the refractive index of air. However, since the index of refraction of air is about 1.0003, the error caused by air does not usually exceed 0.03% even if the index of refraction of air is ignored.

As an example of the configuration of the optical delay unit 11, a method using temperature expansion and contraction of the waveguide can be considered. Again,
Some means for detecting a change in the optical length includes an optical delay unit 11.
In general, it is provided as a function. If such means is not provided, the change in the actual length may be detected and multiplied by the refractive index of the waveguide medium.

Further, as a means for changing the optical length, the light detector 3
May be used. In this case, light from the optical component to be measured is beamed by a lens into a space and emitted into a space, which is received by a photodetector. In order to detect the change in the optical length, the amount of movement of the photodetector 3 may be detected as in the case of the optical delay device. That is, a method of counting the number of electric pulses sent to the stepping motor, a method of measuring a distance between a reference point provided outside and the photodetector 3, and the like can be considered.

The filter 4 is a band pass filter.
As described above, the fundamental frequency f is set so that the device does not oscillate at a frequency (harmonic) that is an integral multiple of the fundamental frequency f.
It functions as a means for removing other components. However, in the present invention, even if the embodiment apparatus oscillates at a high frequency, the chromatic dispersion calculation method described later is not affected. Therefore, S / N due to the inclusion of many harmonic components
In an application in which a decrease in the ratio is allowed, the filter 4 is not essential in the embodiment device.

The amplifier 5 receives the signal from the photodetector 3 and amplifies the intensity of the input signal to provide an appropriate signal strength to the amplitude limiter 6 and the capacitor 7 at the subsequent stage, and outputs the signal. I do.

The frequency counter 10 receives the signal amplified by the amplifier 5 and counts the oscillation frequency.
Actually, in FIG. 1, a duplexer such as a beam splitter is placed between the amplifier 5 and the amplitude limiter 6 to extract a frequency signal in a closed circuit (not shown).

The amplitude limiter 6 receives the amplified signal, attenuates a signal having an excessive amplitude, and outputs a signal whose amplitude is limited to a certain value or less. The capacitor 7 is
Receiving the signal from the amplitude limiter 6, removing the DC component of the signal and outputting only the AC component, the DC bias current from the DC power supply 9 for the semiconductor laser 1 is applied in the opposite direction to the signal transmission direction, That is, it is prevented from flowing in the direction of the amplitude limiter 6.

The DC power supply 9 generates a DC bias current and sends the DC bias current to the semiconductor laser 1. Hereinafter, the measurement procedure in the first embodiment will be described. First, a DC bias current is supplied from the DC power supply 9 to the semiconductor laser 1 so that the semiconductor laser 1 emits laser light. The wavelength of the laser beam is set to an arbitrary one (hereinafter referred to as λ) of at least two types of wavelengths of the semiconductor laser 1. This laser light is guided to the measured optical component 2, and the laser light that has passed through the measured optical component 2 is guided to the optical delay device 11. The laser beam that has passed through the optical delay unit 11 is converted into a photodetector 3 that is a photoelectric converter.
Lead to. The electric signal output from the photodetector 3, which is a photoelectric converter, is fed back to the semiconductor laser 1 via a filter 4, an amplifier 5, an amplitude limiter 6, and a capacitor 7.

The path of light from the semiconductor laser 1 to the photodetector 3 as a photoelectric converter and the path of an electric signal from the photodetector 3 as a photoelectric converter to the semiconductor laser 1 together constitute a kind of closed circuit. Can be considered. Where filter 4
When the cutoff frequency, the amplification factor of the amplifier 5 and the limit amplitude of the amplitude limiter 6 are adjusted to appropriate values, the closed circuit oscillates. The oscillation frequency is a fundamental frequency, that is, a frequency corresponding to a cycle in which light and an electric signal make one round in a closed circuit. The oscillation frequency is measured by the frequency counter 10 with the optical length of the optical delay unit 11 set at an appropriate position.
Let f be the measured oscillation frequency.

Next, the temperature of the semiconductor laser 1 is changed by operating a constant temperature device (not shown) to change the output wavelength of the semiconductor laser 1. The output wavelength at this time is λ + Δλ. At this time, if the optical length of the optical delay unit 11 is not changed, the oscillation frequency changes according to the same principle as in the related art. In the present embodiment, the optical length of the optical delay unit 11 is adjusted so that the oscillation frequency is maintained at the first fundamental frequency f. A change in the optical length necessary to keep the oscillation frequency at the fundamental frequency f is detected by a detection unit such as the optical delay unit 11. The amount of change in the optical length detected in this manner is ΔL.

At this time, the relation between the group delay time difference Δτ and the oscillation frequency corresponding to the equation (2) is given by the equation (8). f = 1 / (τ + Δτ + ΔL / c + T) (8) where c is the speed of light (299792458 m / s). From equations (1) and (8), the group delay time difference Δτ is
It is derived by equation (9).

Δτ = −ΔL / c (9) Accordingly, the wavelength fraction D in the present embodiment is expressed by the following equation (10), corresponding to the above-described equation (5).

D ≒ Δτ / (L × Δλ) = − ΔL / (c × L × Δλ) (10) From the above equation, the chromatic dispersion D is calculated by measuring f, ΔL, L and Δλ. .

Further, corresponding to the above equation (7), the total amount of dispersion Da of the measured optical component 2 is expressed by the equation (11). Da ≒ Δτ / Δλ = −ΔL / (c × Δλ) (11) By performing the above measurement, the problem of the error due to the frequency characteristic of the electric component described in the problem of the prior art can be solved. It does not occur in the embodiment. However, the optical delay unit 11
When a material or a device having a wavelength dispersion characteristic such as a prism is used as a component of the optical component, or when an optical fiber having a wavelength dispersion characteristic is used for coupling optical components, an error occurs due to the wavelength dispersion characteristic. In this embodiment, such an error can be calibrated and suppressed by the method described below.

The group delay time difference Δτ obtained by the equation (9) is the sum of the group delay time difference Δτ _M caused by only the measured optical component 2 and the group delay time difference Δτ _L caused by the optical components other than the measured optical component 2. It is believed that there is. Therefore, the input end and the output end of the optical component 2 to be measured are combined to form a chromatic dispersion measuring apparatus excluding the optical component 2 to be measured, and the same measurement as described above is performed. Also at this time, the optical length of the optical delay unit 11 is adjusted so that the oscillation frequency maintains the same frequency f as when the measured optical component 2 is measured.

At this time, the value obtained by the expression (9) is a group delay time difference Δτ _L between optical components other than the optical component 2 to be measured.
Therefore, by subtracting this Δτ _L from the group delay time difference Δτ obtained when the measured optical component 2 is actually inserted, the group delay time difference Δτ _{M of} only the measured optical component 2 is obtained. The obtained group delay time difference Δτ _M and
From the equation (10), the chromatic dispersion D of only the measured optical component 2 is obtained. The above is the calibration method.

When the incident end and the output end of the optical component 2 to be measured are similarly short-circuited in the prior art, the repetition frequency becomes higher than at the time of the original measurement due to the shortened overall optical length. Except when there is no frequency characteristic at all, it is not possible to calibrate the chromatic dispersion characteristics other than the optical component 2 to be measured.

By the way, as described in the section of the prior art, the chromatic dispersion is mathematically represented by the derivative of the group delay time difference with respect to the wavelength. There is also a method of measuring chromatic dispersion such that this differential amount can be obtained directly, and that method may be used,
In this embodiment, what is directly measured is the group delay time difference. The method described so far is based on an approximate expression in which the derivative of the group delay time difference with respect to the wavelength is replaced with the difference,
In addition, it is a method of obtaining chromatic dispersion only from measurements at two different wavelengths.

However, if the number of wavelengths is set to more than two and the group delay time difference between the respective wavelengths is measured, the approximation accuracy is improved. For example, three types of wavelengths λ
At −Δλ, λ and λ + Δλ, it is assumed that the changes in the optical length of the optical delay unit 11 required to maintain the oscillation frequency at the constant value f are measured as −ΔL ₁ , 0, and ΔL ₂ , respectively. If the three sets of measured values are approximated by a quadratic function, the chromatic dispersion D of the measured optical component 2 at the wavelength λ is calculated by the equation (12).

D ≒ − (ΔL ₁ + ΔL ₂ ) / (2 × c × L × Δλ) (12) Further, the total amount of dispersion Da of the optical component 2 to be measured is described above in (11).
It is calculated by equation (13) corresponding to the equation.

Da ≒ − (ΔL ₁ + ΔL ₂ ) / (2 × c × Δλ) (13) Expression (10) or (11) when there are two sets of measurements,
Equations (12) or (13) when there are three sets of measurements, and the corresponding equations when there are more measurements,
The means for calculating the chromatic dispersion D or the total amount of dispersion Da may be a method in which the measurer calculates after the measurement, but it is naturally possible to incorporate this calculation procedure into the measuring device.

Further, the operation of adjusting the optical length of the optical delay unit 11 so as to keep the oscillation frequency measured by the frequency counter 10 constant may be performed manually by a measurer, but may be incorporated in a measuring device. Of course it is possible.

(Second Embodiment) FIG. 2 is a schematic configuration diagram of a chromatic dispersion measuring apparatus for an optical component using a delay unit according to a second embodiment of the present invention.

The basic configuration of the second embodiment is the same as that of the first embodiment shown in FIG. 1 except that an oscillator 12 and a frequency phase comparator 1 are used instead of the frequency counter 10.
3, and a frequency / phase difference display 14 which receives the signal from the frequency / phase comparator 13 and displays the frequency and phase difference between the signal of the closed circuit and the signal from the oscillator 12. The oscillator 12, the frequency-phase comparator 13 and the frequency-phase difference display 14 are not special, but may be commercially available.

The measurement procedure in this embodiment is as follows.
First, oscillation is performed in the same manner as in the first embodiment. At this time, the oscillation frequency of the oscillator 12 is adjusted so that the frequency and the phase difference displayed on the frequency / phase difference display 14 become zero. Next, when the wavelength of the semiconductor laser 1 is changed by Δλ, the group delay time difference Δτ is changed by the chromatic dispersion, the frequency and phase of the fundamental frequency f are changed, and the frequency and phase difference are displayed on the frequency / phase difference display 14. Therefore, if the optical delay unit 11 is adjusted so that it becomes zero again, the delay amount becomes the group delay time difference Δτ. Here, the frequency phase comparator is the same as that used in a general phase locked loop circuit.

Next, using the second embodiment shown in FIG. 2, a description will be given of a measurement result when a wavelength distribution of an optical fiber having a length of 100 m is measured as an example. In this embodiment, the factor that limits the measurement accuracy is considered to be the measurement accuracy for the change in the optical length. Assuming that the measurement accuracy of the change in optical length is 1 μm and the wavelength variable range of the semiconductor laser 1 is 5 nm, the measurement accuracy of chromatic dispersion is approximately 0.0007 ps.
It is about c / nm / km.

On the other hand, in the prior art, the theoretical measurement accuracy under similar conditions was about 0.5 psec / nm / km as described in the section of the subject. The measurement accuracy of the second embodiment is improved by about two orders of magnitude over the prior art.

As described above, the signal of the closed circuit and the oscillator 1 are generated by using the frequency phase comparator 13 and the frequency phase difference display 14.
By comparing the frequency and the phase difference with the signal from the optical component 2, the chromatic dispersion of the optical component 2 can be measured with higher accuracy.

(Third Embodiment) FIG. 4 is a schematic configuration diagram of a chromatic dispersion measuring apparatus for an optical component using a delay unit according to a third embodiment of the present invention.

In the third embodiment, the electrical length from the photodetector 3 as a photoelectric converter to the semiconductor laser 1 as a light source is variable. That is, the configuration of the third embodiment is almost the same as the configuration of the first embodiment of FIG.
The difference is that there is no optical delay unit 11 in the first embodiment, and instead, a means for varying the electrical length is inserted between the photodetector 3 as the photoelectric converter and the semiconductor laser 1 as the light source. An electric delay unit 15 is employed as a means for changing the electric length.

As the electric delay unit 15, a linear electric delay unit shown in FIG. 5 and a folded electric delay unit shown in FIG. 6 are put to practical use. FIG. 5A is an axial cross-sectional view of a linear electric delay device, and FIG.
FIG. 4 is a radial cross-sectional view when cut along a line A-A ′. The linear electric delay device includes a cylindrical fixed portion 18 and a movable portion 17 which is automatically inserted into and removed from the fixed portion 18, and is used to input an electric signal to the left end of the movable portion 17. An input end 16 is formed, and an output end 19 for outputting an electric signal is formed at a right end of the fixed portion 18. The outer conductor of the movable part 17 and the outer conductor 21 of the fixed part 18 are in contact with each other, and the space between the outer conductor 21 and the inner conductor 20 in the fixed part 18 is filled with an insulator 22.

By adjusting the length of the movable portion 17 to be inserted into the fixed portion 18, the contact length at which the internal conductor 20 of the movable portion 17 contacts the internal conductor 20 of the fixed portion 18 is changed, and The electrical length between the terminal 16 and the output terminal 19 is changed.

FIG. 6A is an axial cross-sectional view of the folded electric delay device, and FIG. 6B is a sectional view taken along line A in FIG. 6A.
It is sectional drawing when cut | disconnected in the position of -A 'line. In this folded electric delay device, an input terminal 23 for inputting an electric signal and an output terminal 24 for outputting an electric signal.
Are attached to the same side of the housing constituting the outer conductor 25. Then, a pair of internal conductors 26 connected to the input terminal 23 and the output terminal 24 are housed in the housing.
In addition, a short-circuit plate 27 for electrically connecting the pair of internal conductors 26 is provided. Note that the other end of the pair of internal conductors 26 is attached to the opposite surface of the housing on the opposite side of the housing as an insulator 28 on the opposite surface of the housing constituting the outer conductor 25.

The electrical length between the input terminal 23 and the output terminal 24 is changed by moving the short-circuit plate 27 in the direction of the arrow in the figure. The installation position of the electric delay unit 15 shown in FIGS. 5 and 6 may be any position in the electric signal path shown by the solid thin line in FIG.

By the way, as described in the embodiment, in this specification, the electric length is defined as the propagation time of an electric signal multiplied by the speed of light. This definition is for treating the propagation time of an electric signal in the same way as the optical length.

Since the propagation time of an electric signal is obtained by dividing the physical length of an electric line by the propagation speed of the electric signal, the electric length is calculated by dividing the physical length of the electric line by the speed of light and the propagation speed of the electric signal. It is equivalent to a value obtained by multiplying a speed ratio (hereinafter, referred to as a conversion coefficient). This conversion coefficient depends on the shape of the electric line and the dielectric constant of the insulator between the outer conductor and the inner conductor. In this specification, this conversion coefficient is treated as a known quantity. If the conversion coefficient is not known, the group delay characteristic with respect to the physical length can be separately measured using a network analyzer or the like, and the conversion coefficient can be obtained by calculation.

The means for detecting the change in the electrical length is as follows:
It is the same as the means for detecting the change in the optical length, and various means are conceivable. For example, if an electric delay device 15 that can set the electric length and the change thereof from a dedicated controller, a personal computer, or the like, is used, Changing the set value is synonymous with detecting a change in the electrical length.

If the electric delay unit 15 is not provided with a means for detecting a change in the electric length, the moving amount of the movable part 17 in FIG. 5 or the moving amount of the short-circuit plate 27 in FIG. 6 may be detected by some means. For example, if these are driven by a stepping motor, by counting the number of electric pulses sent to the stepping motor,
The movement amount of the stage can be known from the number of pulses and the gear ratio of the stage. Still another method is to provide a reference point on an extension of the movement axis of the stage and measure the distance between the reference point and the stage using a micrometer or the like. The amount of change in the electrical length can be detected by multiplying the amount of movement of the stage obtained by these methods by a conversion factor to the electrical length. Of course, in the folded type shown in FIG. 6, the amount of movement of the short-circuit plate 27 must be doubled and multiplied by the conversion coefficient.

The measuring method is almost the same as in the first embodiment. Although the optical length is adjusted in the first and second embodiments in order to keep the oscillation frequency at a constant value, the third embodiment is different only in that the electrical length is adjusted. Assuming that the change in the electrical length detected by the above-described detecting means is ΔL, the chromatic dispersion D is obtained by the above-described equation (10), and the total dispersion Da of the optical component 2 to be measured is obtained by the above-described equation (11). Can be obtained.

As described above, the means for varying the optical length from the semiconductor laser 1 as the light source to the photodetector 3 as the photoelectric converter is used as the first embodiment, and the frequency-phase comparator 13, the oscillator 12, and the oscillator 12 are used as the second embodiment. Although the means using the frequency / phase difference display 14 and the means for varying the electrical length from the photoelectric converter to the light source have been described as the third embodiment, the present invention is not limited to the first to third embodiments. It is not limited. That is, in the third embodiment, the frequency counter 10
Of course, it is also possible to use the frequency-phase comparator 13, the oscillator 12, and the frequency-phase difference display 14 in place of.

[0078]

According to the chromatic dispersion measuring apparatus for optical parts using the delay device of the present invention, the oscillation frequency is fixed at a constant value by adjusting the optical length or electric length of the closed circuit. The problem of the frequency characteristic of the electrical feedback section, which has been a problem, has been solved. That is, in the present invention, the frequency of the electric circuit is the same even when the oscillation wavelength is changed. Therefore, only the change in the optical length due to chromatic dispersion can be extracted without being affected by the group delay characteristic of the electric circuit in the measurement. As a result, the chromatic dispersion measurement accuracy for the measured optical component 2 can be greatly improved.

When the frequency phase comparator 13 is used as a means for confirming that the oscillation frequencies are the same, the phase difference can be detected up to about 0.01 degree even with the general frequency phase comparator 13. It can measure with higher accuracy than the method using No.10.

In the measuring method described in the prior art, a measurement error occurs, so that the filter 4 oscillates at the fundamental frequency f. However, in actual measurement, the group delay time τ is Since the fundamental frequency f varies according to the equation (1) when τ varies, the center frequency of the filter 4 needs to be changed in accordance with the variation, and the measurement is complicated. When f changes, the group delay time difference Δ
Since T also changes, the measurement results of a plurality of optical components 2 to be measured having different τ (for example, the same type of optical fiber having only a different length) could not be strictly compared.

On the other hand, in the present invention, since the filter 4 can be omitted, it is easy to measure a plurality of optical components 2 to be measured having different group delay times τ, and an automatic measuring device can be used. It is. In addition, the above-described error does not occur in the measurement result, and strict comparison of the measurement results is possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a chromatic dispersion measuring device of an optical component using a delay unit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a schematic configuration of a chromatic dispersion measuring apparatus for an optical component using a delay unit according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating an operation principle and a schematic configuration of an optical delay device using a corner cube mirror;

FIG. 4 is a diagram showing a schematic configuration of a chromatic dispersion measuring apparatus of an optical component using a delay unit according to a third embodiment of the present invention.

FIG. 5 is a diagram showing a schematic configuration of a linear electric delay device.

FIG. 6 is a diagram showing a schematic configuration of a folded-type electric delay device.

FIG. 7 is a diagram showing a schematic configuration of a conventional wavelength dispersion measuring device for optical components.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Optical component to be measured 3 Optical detector 4 Filter 5 Amplifier 6 Amplitude limiter 7 Capacitor 8 Wavelength meter 9 DC power supply 10 Frequency counter 11 Optical delay unit 12 Oscillator 13 Frequency phase comparator 14 Frequency phase difference display 15 Electricity Delay device 16 Input terminal 17 Moving part 18 Fixed part 19 Output terminal 20 Internal conductor 21 External conductor 23 Input terminal 14 Output terminal 27 Short-circuiting plate

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Takashi Saito, Inventor 5--10-27 Minamiazabu, Minato-ku, Tokyo Inside Anritsu Corporation (72) Inventor Hidehiko Takara 3-9-1, Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Satoru Kawanishi 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Masatoshi Saruwatari 3-192 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. An optical device under test allows light emitted from a light source capable of selecting at least two types of known light wavelengths to pass therethrough, and the passed light is converted into an electric signal by a photoelectric converter. In a chromatic dispersion measuring apparatus for an optical component having a closed circuit for generating an oscillation frequency corresponding to the optical wavelength by returning a signal to an excitation current of the light source, generating an oscillation frequency corresponding to the optical wavelength. Means (11, 3) for varying the optical length from the light source to the photoelectric converter so as to keep the oscillation frequency constant.
Or, a means (15) for varying the electrical length from the photoelectric converter to the light source, and a means (11) for detecting a change in the optical length or a means (15) for detecting a change in the electrical length. Based on the at least two known wavelengths and the detected change in optical length, or based on the at least two wavelengths.
An apparatus for measuring the chromatic dispersion of an optical component using a delay device, wherein a chromatic dispersion characteristic of the optical component to be measured is calculated based on the known wavelength of the type and the detected change in the electrical length.