JPH10307079A - Apparatus for measuring wavelength dispersion of optical component - 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 inserted into a mode-locked ring laser ring which is formed of a light amplifier 11, of a wavelength-variable filter 12, of an intensity modulator 13 and of an optical delaying device 16 and in which light orbits. Then, while the modulation frequency of the intensity modulator 13 or the optical length of the optical delaying device 16 is being adjusted, a photodetector 3 which is connected to the ring and which detects a mode-locked oscillation is used, and an optimum optical length is decided. Then, the wavelength-variable filter 12 is operated, and the oscillation wavelength of a mode-locked ring laser is changed. At this time, the mode-locked oscillation is deviated from an optimum state. While the modulation frequency is being kept at a first value, the optical length of the optical delaying device 16 is adjusted, and a change portion which is required to detect the mode-locked oscillation is detected by itself. On the basis of the detected change portion 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 optical components.

[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, Δτ ≒ −Δf / f ² (4) The group delay time difference Δτ is calculated from the equation (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]

However, the above-mentioned prior art still has the following problems to be solved. That is, there is a problem in the accuracy of measuring the chromatic dispersion of the optical component to be measured. Factors that limit the chromatic dispersion measurement accuracy in the prior art include the following. In the measurement device, (1) frequency characteristics of each electric component, (2) variable range of wavelength, (3) measurement accuracy of frequency counter, (4) adjustment of each component, and the like. Hereinafter, these factors will be individually described.

First, the problem of the frequency characteristic of the electric component, which is the first factor, will be described. That is, there is a problem of an error due to frequency characteristics of the photodetector 3, the bandpass filter 4, the amplifier 5, and the amplitude limiter 6. The frequency characteristic that is a problem here is the dependence of the group delay of the electric signal on the frequency, and corresponds to the wavelength dispersion characteristic of the optical component. Specifically, in the measurement when the wavelength of the light source is changed, equation (2) does not hold, and f + Δf = 1 / (τ + Δτ + T + ΔT) (6) As shown in equation (6), the group delay time difference of the electric signal ΔT is generated, and apparently, the group delay time difference Δτ of light and the group delay time difference ΔT of electricity cannot be separated, which causes an error. When the optical component 2 to be measured is a long optical fiber, Δτ >> ΔT
Thus, the effect of ΔT can be ignored, but the effect of ΔT cannot be ignored when the chromatic dispersion of the optical component 2 to be measured is small.
In particular, in the ordinary band-pass filter 4, the frequency dependence of ΔT is large, and on the contrary, the filter whose frequency dependence is suppressed decreases the attenuation characteristic of the stop band, so that the stable operation as the oscillation loop may be impaired. There is a problem that there is.

Next, the problem of the variable range of the wavelength, which is the second factor, will be described. As is clear from equation (5),
The greater the difference Δλ between the two types of wavelengths, the higher the chromatic dispersion measurement accuracy. In the example of the prior art, the semiconductor laser 1 is usually used as a light source. The temperature of the semiconductor laser 1 is changed in order to obtain at least two types of output light having different wavelengths. The semiconductor laser 1 has the property that the wavelength of the output light changes when the temperature changes, but the variable range of the wavelength is about 3 to 5 nm.

The specific value of the chromatic dispersion measurement accuracy based on the variable range will be described later, but the expansion of the wavelength variable range leads to an improvement in the measurement accuracy. In addition, the narrowness of the variable wavelength range is becoming a problem from another viewpoint. That is, in the industrial field where measurement of chromatic dispersion is required, for example, in the design of an optical communication system, 50 to 100 nm is currently used.
It has become necessary to measure chromatic dispersion over a wavelength range of. Although there is a wavelength tunable laser capable of changing the output light over such a wavelength range, the price is extremely high and it is not common.

Further, the third factor, the measurement accuracy of the frequency counter, will be described. 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 chromatic dispersion measurement accuracy is approximately 0.5 psec / nm.
/ Km.

However, at present, 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.

Further, in practice, due to the influence of electric noise and the like, an operation such as averaging data measured over a long period of time is required to measure the frequency with such accuracy. Complications and long measurement times are inevitable.

Finally, adjustment of each component, which is the fourth factor, will be described. For example, in the prior art apparatus shown in FIG. 7, a bandpass filter 4, an amplifier 5, and an amplitude limiter 6 are used. In this case, the operator must adjust the pass band, the amplification factor, and the amplitude limit value, which impairs the easiness of measurement, reproducibility, and objectivity. There is another example in which the radiation limiter 6 is not used. However, in this case, the band-pass filter 4 and the amplifier 5 are required, and another device such as a synthesizer is required. The problem has not been solved.

[0022]

Means for Solving the Problems In order to solve the above problems, the present invention employs the following constitution. Note that the reference numerals used in the embodiments are used. The gist of the present invention is to provide a mode-locked ring laser capable of selectively oscillating at at least two kinds of known wavelengths, and inserting the optical component to be measured into the ring of the mode-locked ring laser. An optical component chromatic dispersion measuring device for measuring the chromatic dispersion characteristics of an optical component, wherein the first means is connected to the ring and detects mode-locked oscillation.
5, second means 16 for adjusting the optical length of the mode-locked ring laser so that mode-locked oscillation occurs at a constant repetition frequency at any of the at least two known wavelengths, and Third means 15 and 14 for detecting a change in the length, and the measured object is measured based on the at least two known wavelengths and the change in the optical length detected by the third means. A chromatic dispersion measuring device for an optical component, which calculates a chromatic dispersion characteristic of the optical component.

That is, the optical length required for inserting the optical component under test 2 into the ring of the mode-locked ring laser provided with the wavelength selecting means and maintaining a constant pulse repetition frequency for mode-locked oscillation is required. The wavelength dispersion characteristic of the optical component 2 to be measured is obtained by measuring the change of the optical component 2.

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 based on the dispersion of the measured optical component. Expressing 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, assuming that the total dispersion amount is Da, the total dispersion amount is approximately expressed by Expression (7) using Expression (4). Da ≒ Δτ / Δλ = -Δf / (f ² × Δλ) (7)

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wavelength dispersion measuring apparatus for an optical component according to the present invention will be described below. The present invention uses a mode-locked ring laser for measuring the wavelength dispersion characteristics of an optical component. First, the concept of a ring laser and a mode-locked ring laser will be described, and then individual components will be described.

When a ring for returning the output of the optical amplifier to the input of the optical amplitude device is formed and the optical amplifier is driven to extract a part of the output, an optical output similar to that of a normal laser can be obtained. This is because light, such as noise, which is called spontaneous emission light emitted by the optical amplifier, is amplified to the output limit of the optical amplifier while traveling around the ring. The device configured in this manner is called a ring laser.

The oscillation wavelength of the above ring laser is fixed to a wavelength at which the amplification efficiency of the optical amplifier is highest. In order for the ring laser to have a wavelength selecting function, a wavelength selecting means may be provided in the ring. In this case, laser output light can be obtained at a wavelength at which the amplification efficiency is the highest as a total of the optical amplifier and the wavelength selection means. Generally, a wavelength variable optical filter is used as the wavelength selecting means. The device configured in this manner is also called a ring laser, but is strictly a wavelength variable ring laser.

An optical modulator or a non-linear optical medium is provided in the ring of the ring laser (including the variable wavelength ring laser), and a frequency at which light circulates around the ring (hereinafter referred to as a fundamental frequency) or a frequency that is an integral multiple thereof. , A pulse train light output is obtained. Such a device is called a mode-locked ring laser, and this oscillation state is called a mode-locked oscillation.

Here, the relationship between mode-locked oscillation and an optical pulse train will be described with reference to FIG. FIG. 2A shows an optical pulse train waveform in a time domain, in which the vertical axis represents light intensity and the horizontal axis represents time. FIG. 2B shows the Fourier transform of the optical pulse train, in which the vertical axis represents the power of light and the horizontal axis represents the optical frequency.

As shown in FIG. 2B, in the Fourier transform of the optical pulse train, light having a constant modulation frequency interval called a sideband (mode) is regularly arranged around a central optical frequency. It is shaped. Further, the phase of each mode keeps a constant relationship. Note that the modulation frequency matches the repetition frequency of the optical pulse train viewed in the time domain.

Since the waveform in the time domain and its Fourier transform (including phase information) have a one-to-one relationship, in the optical frequency domain, many modes are generated around a certain central optical frequency, and If the phase between the modes is fixed so as to maintain a constant relationship, the light becomes an optical pulse train when viewed in the time domain. This is mode-lock oscillation. In order to realize mode-lock oscillation, light may be modulated at the fundamental frequency or a frequency that is an integral multiple of the fundamental frequency.

Mode-locked ring lasers are further roughly classified into active mode-locked ring lasers and passive mode-locked ring lasers. The difference between active and passive types is
The difference is whether an optical modulator or a non-linear optical medium is used as an apparatus for modulation, and details will be described later.

In the ring laser itself, in principle, light traveling in the ring can exist both clockwise and counterclockwise at the same time. However, in practice, it is usually limited so that only one direction of light exists, due to the characteristics of the optical component used.

A mode-locked ring laser is generally widely used as a means for obtaining an optical pulse train. However, in the present invention, the mode-locked ring laser is used as a device for knowing the fundamental frequency. Although a normal mode-locked ring laser does not necessarily need a wavelength selecting means, in the present invention, since it is necessary to selectively oscillate at at least two kinds of wavelengths, a wavelength selecting means therefor is an essential component. It is.

In the present invention, the optical component to be measured needs to be inserted into the ring of the mode-locked ring laser. Further, in the present invention, means for changing the optical length of the ring must be inserted into the ring so as to keep the repetition frequency of the mode-locked oscillation constant.

Hereinafter, each device constituting the mode-locked ring laser will be described with reference to FIG. Optical amplifier 11
Is a device for amplifying the amplitude from about 10 times to 1000 times while maintaining the same wavelength and phase of the input light. In the current state of the art, the optical amplifier 11 is roughly classified into two types: a fiber amplifier and a semiconductor amplifier.

The fiber amplifier is obtained by adding a rare earth element to an ordinary optical fiber, and has a function of amplifying the amplitude of input light by supplying light of a specific wavelength called excitation light. The amplification factor depends on the intensity of the excitation light.
It is also possible to obtain an amplification factor of 00 or more. The wavelength band of the input light at which the amplifying action can be obtained is generally about 50 nm, though it depends on the rare earth element to be added.

On the other hand, the semiconductor amplifier is formed by depositing a non-reflection film on both end surfaces of a normal semiconductor laser, and has an effect of amplifying the amplitude of input light by supplying an excitation current. Although the amplification factor depends on the excitation current, it is generally 10 to 50.
It is about twice. However, the band in which the amplification effect can be obtained is generally about 100 nm, which is wider than that of the fiber amplifier.

In the present invention, as the optical amplifier 11 of the mode-locked ring laser, not only a fiber amplifier and a semiconductor amplifier but also any optical amplifier having the function of amplifying light can be used.

In the present invention, a wavelength λ, a wavelength λ + Δλ changed from this wavelength λ, and an optical delay amount Δτ required for mode-lock oscillation at the same modulation frequency with respect to the wavelength λ.
Then, a method of obtaining the chromatic dispersion from is adopted. Therefore, it is necessary for the mode-locked ring laser to oscillate selectively at at least two known wavelengths.

As this wavelength selecting means, a wavelength tunable optical filter 12 is usually used. An optical filter is a device that allows only light in a specific wavelength band to pass therethrough and does not allow other light to be absorbed or reflected and passed. As a passband selection mechanism,
There are a type in which a measurer manually moves a dial and the like, and a type in which a dial is electrically controlled by a personal computer or the like. Bandwidth is 1-3 nm
The degree is appropriate. By using this tunable optical filter 12 in combination with the optical amplifier 11, light of a desired wavelength can be obtained.

For the purpose of measurement of the present invention, it is only necessary to selectively set at least two types of wavelengths. Therefore, not only the wavelength tunable optical filter 12 but also at least two types of optical filters having a fixed pass band are prepared (passed). It is assumed that the bands are different from each other), and even if a means for switching between them is prepared, it can be used as a wavelength selecting means. In FIG. 1, a wavelength tunable optical filter is used as a wavelength selecting means.

Tunable optical filter 1 as wavelength selecting means
In No. 2, means for knowing the center wavelength of the set pass band is usually provided, and in the example described later, the wavelength obtained by this means is used as the oscillation wavelength of mode-locked oscillation. If the wavelength tunable optical filter 12 is not provided with a means for knowing the center wavelength of the pass band, or in order to know the oscillation wavelength with higher accuracy, the output light is partially branched and a wavelength meter (not shown) is used. The oscillation wavelength may be measured.

An optical modulator is a device for changing the intensity, frequency or phase of light passing therethrough, and is distinguished from an intensity modulator, a frequency modulator and a phase modulator, respectively. In order to construct a mode-locked ring laser, an intensity modulator and a phase modulator are particularly useful. In FIG. 1, an intensity modulator 13 is used as an optical modulator.

A mode-locked ring laser using the intensity modulator 13 or the phase modulator is called an active mode-locked ring laser. An active mode-locked ring laser requires an external signal source that supplies a modulation signal to the modulator. What is required of the signal source is that a sine wave signal is output and its frequency is variable. Note that such items are not particularly special, and a general signal generator can be used. Mode locked oscillation occurs when the frequency of the sine wave signal is the fundamental frequency of the ring or an integer multiple thereof.

In an active mode-locked ring laser, the frequency of the modulation signal supplied to the modulator (hereinafter, referred to as modulation frequency) is determined by the basic frequency of the ring and an integer multiple thereof. There is no difference in the subsequent handling. However, in general, the higher the modulation frequency, the narrower the pulse width of the optical pulse generated by the mode-locked oscillation, and as a result, the resolution in the chromatic dispersion measurement is improved.

A mode-locked ring laser using a non-linear optical medium instead of an optical modulator is called a passive mode-locked ring laser. A nonlinear optical medium refers to a medium whose characteristics such as absorption and refractive index change depending on the intensity of light passing therethrough. When an optical pulse passes through such a medium, intensity modulation or phase modulation is automatically generated according to the pulse waveform, so that an effect similar to that of an optical modulator in an active mode-locked ring laser can be obtained.

Since a light pulse modulates itself in the passive mode-locked ring laser, an external modulation signal required in the active mode-locked ring laser is unnecessary in the passive mode-locked ring laser. In this case, the repetition frequency of the optical pulse train automatically matches the fundamental frequency of the ring.

As the nonlinear optical medium, a saturable absorber is most generally used. A saturable absorber refers to a substance or a device that exhibits a property that when the intensity of incident light is increased to some extent, the absorptance is reduced (the absorption is saturated) and the transmittance is increased. For example,
By suppressing the excitation current of the optical semiconductor amplifier to a current value at or below which an amplifying action can be obtained, it can be used as a saturable absorber. Consider when a light pulse passes through a saturable absorber.
For a while after the light pulse is incident, the absorption is large, and the intensity of the light passing therethrough is small. Next, as the light pulse approaches the maximum intensity, the absorption saturates and the intensity of the light passing through increases sharply. When the light pulse turns from maximum intensity to decrease, the opposite effect occurs, and the intensity of the passing light decreases sharply.

Accordingly, the light pulse that has passed as a result is a narrow pulse that is narrower than before the light pulse. This has the same effect as when the intensity modulator 13 is used as an optical modulator in an active mode-locked ring laser. The specific configuration of the device using the passive mode-locked ring laser will be described later.

Next, a second means for adjusting the optical length of the mode-locked ring laser (hereinafter referred to as an optical length variable means)
Will be described. An optical delay device is generally used as the optical length varying means. FIG. 3 shows an example of the structure of the optical delay unit. FIG.
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 driven by being mounted on a linear movement stage. 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, besides the one using a corner cube mirror, the one using a prism and the one using the expansion and contraction of the temperature of an optical waveguide, all of which 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.

The optical component 2 to be measured inserted into the ring of the mode-locked ring laser can measure in principle any component that allows light to pass through.
In particular, the present invention has an advantage in the measurement of chromatic dispersion of fundamentally small actual length or actual size. Specifically, a dispersion-shifted optical fiber, a dispersion-flattened optical fiber, and the like can be given.

The optical amplifier 11 described above, the wavelength tunable optical filter 12 as a wavelength selecting means, the intensity modulator 13 as an optical modulator or a non-linear optical medium, and the optical delay device as an optical length variable means. The optical fiber may be used for coupling so as to form a ring that circulates, or may be spatially coupled using a lens, a mirror, or the like.

The arrangement including the optical component 2 to be inserted into the ring can be arranged in an arbitrary order in principle. However, in practice, an optimal arrangement may exist depending on the characteristics of each component. For example, since spontaneous emission light is generally added to the output of the optical amplifier 11, it is desirable to connect a tunable optical filter 12, which is a wavelength selecting means, immediately after the optical amplifier 11. Immediately after the wavelength tunable optical filter 12 serving as the wavelength selecting means, the optical component 2 to be measured is compared with the intensity modulator 13 (or a non-linear optical medium) serving as an optical modulator unless otherwise specified. It is desirable to connect the one with the larger maximum allowable light input.

As described above, the mode-locked ring laser capable of selectively oscillating at least two kinds of known wavelengths is provided, and the optical component 2 to be measured is inserted into the ring of the mode-locked ring laser. doing. By connecting the first means for detecting mode-locked oscillation and the third means for detecting a change in optical length changed by the optical length variable means to this configuration, an apparatus for measuring chromatic dispersion characteristics is provided. Complete. However, when a passive mode-locked ring laser is used as the mode-locked ring laser, a means for examining the pulse repetition frequency is necessary as described later.

The description will be continued with reference to FIG. FIG. 1 shows an example in which an active mode-locked ring laser is used as a mode-locked ring laser. As described above, the wavelength tunable optical filter 12 is used as the wavelength selecting means. Further, an intensity modulator 13 is used as an optical modulator.

Hereinafter, the first means for detecting mode-lock oscillation will be described. Usually, the photodetector 3 and the oscilloscope 15 connected to it are used. However, there are various means for this, and it is not necessarily limited to the following example.

The first means for detecting the mode-locked oscillation includes an optical amplifier 11, a tunable optical filter 12 as a wavelength selecting means, an optical component 2 to be measured and an intensity modulator 13 as an optical modulator or a non-linear optical medium. And a saturable absorber. Then, a part of the light in the ring is extracted and guided to the photodetector 3. In order to extract a part of the light from the ring, an optical coupler can be used when the light is coupled by an optical fiber, and a half mirror or a beam splitter can be used for the spatial coupling (not shown). The output signal of the photodetector 3 is observed with a normal oscilloscope 15 or the like. When the mode-lock oscillation is performed, the output signal is a pulse signal corresponding to the optical pulse train, so that it can be easily determined.

There are two types of oscilloscopes 15 that require an external trigger signal and those that do not. In the active mode-locked ring laser, since the signal generator 14 exists as an external signal source, this signal can be used as an external trigger signal. In the example of FIG. 1, an oscilloscope 15 of a type that requires an external trigger signal is used.

As another means for detecting mode-lock oscillation, instead of using the photodetector 3 and the oscilloscope 15, a streak camera may be used to directly observe whether or not an optical pulse train is generated (see FIG. Not shown).

Next, the optical delay unit 16 which is an optical length variable means
The means for detecting the change in the optical length changed by the above will be described. As will be described later, what is required for measuring the chromatic dispersion is a change in the optical length with respect to an arbitrarily set reference position of the optical delay device 16, 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, but the means is normally provided as a function of the optical delay unit 16. For example, if the optical delay 16 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. .

Further, in the optical delay device 16 in which means for detecting the amount of movement of the linear moving stage provided with the corner cube mirror is provided, 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 16 does not have a function of detecting a change in the optical length as a function thereof, the movement amount 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 precisely 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 device 16, a system using temperature expansion and contraction of a waveguide can be considered. Again,
Some means for detecting the change in the optical length includes an optical delay unit 16.
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.

Here, the meaning of adjusting the optical length of the mode-locked ring laser so that mode-locked oscillation occurs at a constant pulse repetition frequency in the active mode-locked ring laser will be described. I do.

In an active mode-locked ring racer, the pulse repetition frequency always coincides with the modulation frequency of the optical modulator. Therefore, to keep the pulse repetition frequency constant in the active mode-locked ring laser means to keep the modulation frequency of the optical modulator constant. Here, a case is considered where the wavelength is changed while the modulation frequency of the optical modulator is kept constant in the active mode-locked ring laser that is performing mode-lock oscillation. If the ring laser has chromatic dispersion, the optical length of the ring laser slightly changes, and thus the fundamental frequency of the ring laser changes. Therefore, mode-lock oscillation does not occur, or even if it occurs, it is not in an optimal state.

Therefore, in the active mode-locked ring laser, what is stated in the claims is, specifically, that even if the wavelength is changed while the modulation frequency of the optical modulator is kept at a constant value, the mode-locked This means that the optical length of the optical delay device, which is the optical length variable means, is adjusted so that the mode locked oscillation is continuously detected by the first means for detecting oscillation.

Hereinafter, the measurement procedure will be described. First, mode-lock oscillation is generated at the first wavelength. For this purpose, the first means for detecting mode-locked oscillation is used to determine the optimum modulation frequency or optical length while adjusting one or both of the modulation frequency of the optical modulator and the optical length of the optical delay unit. I do. At this time, the adjustment of the modulation frequency of the optical modulator and the adjustment of the optical length of the optical delay unit theoretically have the same effect, but generally, as described later in the effects of the invention, the optical delay A higher resolution can be obtained by adjusting the optical length of the vessel. The optical length of the optical delay device when the mode-locked oscillation is obtained is set to [0] in order to use it as a reference amount for subsequent measurements.

Next, the wavelength tunable optical filter 12 as the wavelength selecting means is operated to change the oscillation wavelength of the mode-locked ring laser. The output wavelength at this time is λ + Δλ. At this time, unless the chromatic dispersion of the entire mode-locked ring laser including the optical component 2 to be measured is 0, the mode-locked oscillation deviates from the optimum state due to the change in the oscillation wavelength.

In the present invention, while maintaining the modulation frequency of the optical modulator at the initial value, the optical delay device 1 as the optical length varying means is used.
Adjust the optical length of 6. A change in the optical length necessary for detecting the optimal mode-locked oscillation by the first means for detecting the mode-locked oscillation is detected by a detecting means such as the optical delay unit 16. The amount of change in the optical length detected in this manner is defined as ΔL.

At this time, the relationship between the group delay time difference Δτ and the oscillation frequency corresponding to equation (2) is given by 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 (9)
It is derived by the formula.

Δτ = −ΔL / c (9) Accordingly, the wavelength fraction D in the present embodiment is represented 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 ΔL, L and Δλ. In addition, corresponding to the above-described equation (7), the total amount of dispersion Da of the measured optical component 2 is expressed by the following equation (11).

Da ≒ Δτ / Δλ = −ΔL / (c × Δλ) (11) By performing the above-described measurement, the problem of the error due to the frequency characteristic of the electric component described in the problem of the prior art is obtained. Does not occur in this embodiment. However, the optical delay 16
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 16 is adjusted so that the oscillation frequency maintains the same frequency f as when the measured optical component 2 is measured.

The value obtained by the expression (9) at this time is the group delay time difference Δτ _L between the 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.

[0082]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a specific embodiment of the chromatic dispersion measuring apparatus for optical components according to the present invention will be described. (First Embodiment) FIG. 4 shows a first embodiment for measuring the fundamental frequency with higher accuracy than the measuring apparatus of FIG.

In the first embodiment, as shown in FIG. 4, the output signal of the photodetector 3 is partially branched and guided to the power meter 18 via the low-pass filter 17. The pass band of the low-pass filter 17 is set so as to pass approximately several tens of kHz, which is a frequency called a relaxation oscillation frequency of the optical amplifier 11. It is known that the low-frequency power measured by the power meter 18 is minimized at the time of perfect mode-locked oscillation (Takara et al., Stabilization of mode-locked Er-doped fiber laser by suppression of relaxation oscillation frequency component). , 1995 IEICE General Conference,
B-1156). If the modulation frequency is adjusted so as to achieve a completely mode-locked oscillation state, the change in the optical length can be easily measured with an accuracy of 1 μm. The other configuration of FIG. 4 is completely the same as that of the measuring apparatus of FIG.

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 the embodiment of the present invention, it is the group delay time difference that is directly measured. The method described so far is a method of obtaining chromatic dispersion by an approximation formula in which the derivative of the group delay time difference with respect to the wavelength is replaced by the difference, and only by measurement at two different wavelengths.

However, if the number of wavelengths at which the group delay time difference (or the change in the optical length) is measured is made larger than two types, and the change in the optical length corresponding to each wavelength is measured, the approximation accuracy is improved. improves. 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 16 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 16 which is an optical length varying means so as to maintain the mode-locked oscillation in an optimum state may be performed manually by a measurer. It is of course possible to incorporate them into

(Second Embodiment) FIG. 5 shows a second embodiment of a wavelength dispersion measuring apparatus for optical components using a passive mode-locked ring laser.
This is an example.

In the second embodiment, a saturable absorber 19 is used as a nonlinear optical medium. When the oscilloscope 15 is used as the first means for detecting the mode locked oscillation, an oscilloscope 15 of a type that does not require an external trigger signal must be used because an external trigger signal cannot be used. In addition, the optical length varying means and the means for detecting a change in the optical length changed by the optical length varying means include:
This is the same as the first embodiment shown in FIG. 4 using the active mode-locked laser described above.

The meaning of adjusting the optical length of the mode-locked ring laser so that mode-locked oscillation is generated at a constant pulse repetition frequency in the passive mode-locked ring laser will be described.

In a passive mode-locked ring laser, the pulse repetition frequency changes according to the change in the optical length of the ring laser. In a passive mode-locked ring laser, if the optical length of the mode-locked ring laser slightly changes from the state of optimal mode-locked oscillation, pulse repetition is maintained while the state of optimal mode-locked oscillation is maintained. The frequency changes in response to the change in the optical length.

Therefore, in the passive mode-locked ring laser, what is stated in the claims is, specifically, an optical length variable so as to cancel the change in the repetition frequency of the pulse resulting from the change in the wavelength. This means that the optical length of the optical delay device is adjusted.

For this reason, in the second embodiment using the passive mode-locked ring laser, it is necessary to add a means for measuring the repetition frequency of the output light pulse. In order to measure the repetition frequency of the output optical pulse, the optical pulse train is converted into an electric signal by the photodetector 3, then divided into two, and the waveform is observed with the oscilloscope 15 or the like. What is necessary is just to measure the repetition frequency of a pulse train. That is, when the wavelength is changed, the optical length of the optical delay unit 16 as the optical length varying means is adjusted so that the repetition frequency of the electric pulse train measured by the frequency counter 20 or the like is maintained at a constant value. Hereinafter, the method of calculating chromatic dispersion is the same as that described in the embodiment and the first example.

(Third Embodiment) FIG. 6 shows a third embodiment of a chromatic dispersion measuring apparatus for optical parts using a passive mode-locked ring laser.
This is an example.

The basic structure of the third embodiment is the same as that of the second embodiment shown in FIG. 5, except that an oscillator 21 and a frequency phase comparator 22 are used instead of the frequency counter 20.
Further, a frequency / phase difference display 23 for receiving a signal from the frequency / phase comparator 22 and displaying a frequency and a phase difference between a signal of the closed circuit and a signal from the oscillator 21 is provided. Note that the oscillator 21, the frequency / phase comparator 22, and the frequency / phase difference display 23 are not special ones, and may be commercially available.

The measuring procedure in the third embodiment is as follows. First, similarly to the second embodiment, mode-lock oscillation is performed. At this time, the oscillation frequency of the oscillator 21 is adjusted so that the frequency and the phase difference displayed on the frequency / phase difference display 23 become zero. Next, when the wavelength is changed, the repetition frequency of the pulse is changed by the wavelength dispersion, and the frequency and phase difference are displayed on the frequency / phase difference display 23. Therefore, the optical length of the optical delay unit 16 is adjusted so that the number of rounds and the phase difference become zero again. Hereinafter, the method of calculating chromatic dispersion is the same as that described in the embodiment and the first example.

In the second and third embodiments, the operation of adjusting the optical length of the optical delay unit 16 so as to keep the pulse repetition frequency constant may be performed manually by a measurer. It is of course possible to incorporate them into

[0099]

In the chromatic dispersion measuring apparatus for optical parts according to the present invention, since the ring is made up of only light, the light detector 3, the band-pass filter 4, the amplifier 5 and the oscillator in the prior art are used. An error does not occur due to the frequency response characteristic of the electric component in the radiation limiter 6, particularly the dependence of the group delay on the frequency.

In the present invention, optical components such as a wavelength tunable optical filter 12 and an intensity modulator 13 are added to the prior art, and even if these have wavelength dispersion characteristics that cannot be ignored in measurement. However, in that case, if the procedure described in the calibration section is followed, strict chromatic dispersion of only the optical component 2 to be measured can be obtained. On the other hand, it is very difficult to avoid the problem of the frequency response characteristic of the electric signal in the related art.

Next, according to the present invention, the use of the optical amplifier 11 has an effect that the variable range of the wavelength is wider than that of the prior art. As described in the description of the optical amplifier 11, when combined with an appropriate optical filter, 50 to 1
A variable range of wavelength of 00 nm is obtained. This characteristic can be said to be a sufficiently variable range in the design of an optical communication system and the like.

Further, as is apparent from the equation (9), the larger the variable range of the wavelength, the smaller the chromatic dispersion can be measured. Therefore, it can be said that the resolution of the chromatic dispersion is increased in the present invention. Furthermore, the optical power incident on the optical component under test 2 can be changed by adjusting the gain of the optical amplifier 11, so that the measurement can be performed in a form closer to actual use conditions.

Next, the modulation frequency of the mode-locked ring laser is extremely sensitive to the fundamental frequency of the light circulation.
If the above means is used, the variation ΔL of the optical length becomes 1
It is easy to measure with an accuracy of μm. The measurement accuracy of chromatic dispersion corresponding to this accuracy is as follows: when an optical fiber having a length of 100 m is considered as the optical component 2 to be measured, the wavelength change amount Δλ
Is 5 nm, which is the same as the conventional technology, and 0.007 psec /
nm / km. This accuracy is nearly two orders of magnitude above the theoretical limits of the prior art.

Further, the present invention has another effect due to the system in which the entire ring portion is circulated by light. That is, the adjustment of each component of the electric unit, which is a problem of the related art, is not required, and the easiness of measurement and the objectivity (reproducibility) are increased.

Further, it is naturally possible to measure the group delay characteristic. That is, by measuring the change amount ΔL of the optical length at at least two different wavelengths, the group delay time difference Δτ is calculated from Expression (9). The effect of the group delay measurement on the conventional technology can be pointed out in the same manner as the effect of the present invention on the chromatic dispersion measurement on the conventional technology.

[Brief description of the drawings]

FIG. 1 is a diagram showing an apparatus for measuring chromatic dispersion of an optical component according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining the relationship between mode locked oscillation and an optical pulse train.

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 for an optical component according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a schematic configuration of a chromatic dispersion measuring apparatus for an optical component according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a schematic configuration of an apparatus for measuring chromatic dispersion of an optical component according to a third embodiment of the present invention.

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

[Explanation of symbols]

 2 Optical Component Under Measurement 3 Optical Detector 11 Optical Delay Device 12 Wavelength Variable Optical Filter 13 Intensity Modulator 14 Signal Generator 15 Oscilloscope 16 Optical Delay Device 17 Low Pass Filter 18 Power Meter 19 Saturable Absorber 20 Frequency Counter 21 Oscillator 22 frequency phase comparator 23 frequency phase difference display

 ──────────────────────────────────────────────────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. A mode-locked ring laser capable of selectively oscillating at least two kinds of known wavelengths, wherein the optical component to be measured is inserted into a ring of the mode-locked ring laser so that the optical component is measured. A chromatic dispersion measuring device for an optical component for measuring chromatic dispersion characteristics, comprising: a first means (3, 15) connected to the ring for detecting mode-locked oscillation; Second means (16) for adjusting the optical length of the mode-locked ring laser so that mode-locked oscillation occurs at a constant repetition frequency at any of known wavelengths, and detecting a change in the optical length. Third means (15, 1
4) calculating a wavelength dispersion characteristic of the measured optical component based on the at least two known wavelengths and a change in the optical length detected by the third means. Chromatic dispersion measuring device for optical components.