JP2714754B2 - Waveguide dispersion measurement method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is used for measuring optical fibers and other optical waveguides. The present invention is used for measuring an optical waveguide for optical communication. The present invention provides a waveguide-type light source for generating an optical pulse having a time width of picoseconds or less, a waveguide-type optical amplifier for amplifying the optical pulse, or a wavelength dispersion characteristic of a waveguide for transmitting the optical pulse. It is suitable as a method and an apparatus for measuring with high accuracy.

[0002] Here, "dispersion" refers to a phenomenon in which the propagation speed of light changes depending on the wavelength (or frequency) of the light propagating in the optical waveguide.

[0003]

2. Description of the Related Art In recent years, techniques for generating an optical pulse having a time width of picoseconds or less have been actively developed. As a result, when generating or transmitting an optical pulse with a short time width, the wavelength dispersion characteristics of the waveguide type component used for its generation, amplification or transmission, or the optical path that is an aggregate of these components, changes in the shape of the optical pulse. It has become clear that it has a significant effect.
For example, when a light pulse having a short time width passes through an optical path in which the wavelength dispersion characteristic changes rapidly, a phenomenon occurs that the waveform is significantly deformed. In addition, when an optical component whose wavelength dispersion characteristic changes rapidly is used, there is a problem that it is difficult to generate a light pulse having a short time width. In order to prevent this, it is necessary to minimize the chromatic dispersion characteristics described above, and there has been a strong demand for the development of a method for measuring the chromatic dispersion characteristics for controlling such chromatic dispersion characteristics.

What directly affects the deformation of an optical pulse having a short time width is the wavelength dispersion characteristic of a waveguide-type part or an optical path (hereinafter collectively referred to as an optical waveguide) which is an assembly of these parts. This is a part called group velocity dispersion. If the propagation speed (group velocity) of an optical signal differs for each wavelength of light when an optical pulse having a short time width passes through the optical waveguide, the time required for passing through the optical waveguide, the so-called group delay time, depends on the wavelength. Become like Here, for example, considering a case where an optical pulse passes through an optical waveguide whose group delay time is shorter as the wavelength becomes longer, the short wavelength component in the pulse is relatively delayed with respect to the long wavelength component after passing. As a result, the time width of the pulse is widened. This is an intuitive explanation of pulse deformation due to group velocity dispersion.

Optical angular frequency ω (2πc / λ between wavelength λ)
There is a relationship. (c is the speed of light in a vacuum) with the wavelength dispersion characteristic of the optical waveguide as φ (ω), the group delay time τ (ω) and the group velocity dispersion D (ω) are expressed as follows, respectively. .

Τ (ω) = dφ (ω) / dω (1) D (ω) = dτ (ω) / dω (2) For example, in an optical waveguide having a constant group velocity dispersion value D, the width T It is known that the pulse width after the Gaussian-type pulse has passed through this optical waveguide expands to T (1 + 4 (D / T ² ) ² ) ^1/2 . As is clear from this, in order to suppress the spread of the pulse, it is necessary to set D = 0, that is, to make the group velocity dispersion of the optical waveguide zero.

From the equation (2), if the group delay time is measured, the group velocity dispersion can be known by taking the derivative thereof. The group velocity dispersion measuring method based on this principle is as shown in FIG. Be composed. FIG. 7 is a block diagram showing a first conventional apparatus.

Next, an outline of the first conventional example will be described. In this conventional method, the group delay time of an optical waveguide is measured using an interferometer. In FIG.
A specific wavelength band is extracted by a variable optical filter 74 from a parallel light beam 1 that is a parallel light beam generated by a white light source, and a linearly polarized light component in a desired direction is extracted by a polarizer 2. This light beam enters a Michelson interferometer composed of the cube beam splitter 3, the fixed mirror 4, and the movable mirror 5. An optical waveguide 7 to be measured, which is an optical fiber, is inserted into one of the arms of the Michelson interferometer via coupling lenses 8 and 9. The intensity of light emitted from the interferometer is measured by the light detector 6. When the relative optical path length difference between the two arms of the Michelson interferometer is changed by the movement of the movable mirror 5, the output signal of the photodetector 6 shows a vibration caused by the light interference phenomenon. The value obtained by dividing the relative optical path length difference between the two arms of the interferometer by the speed of light is referred to as the delay time difference of the interferometer, and the delay time difference at which the amplitude of the vibration (interference signal) is maximized is obtained. Is observed together with the group delay time of the measured optical waveguide. By repeating the above measurement while changing the center wavelength of the variable optical filter 74 in this manner, the delay time for each wavelength is obtained.

Here, the problem of the first conventional example will be discussed. In the conventional waveguide dispersion measurement method described above,
The difference between the delay time at which the amplitude of the interference signal becomes the maximum and the group delay time to be correctly measured is that the transmission characteristic of the measured optical waveguide system T (ω) = | T (ω) | exp [iφ (ω) (I is an imaginary unit) only when the following conditions are satisfied. That is, with respect to the center wavelength (angular frequency) ω _{0 of} the variable optical filter and the transmission bandwidth Δ, | T (ω) |: constant (3) φ (ω) = φ (ω ₀ ) + (ω−ω ₀ ) Τ (ω ₀ ) (| ω−ω ₀ | <Δ) (4) Equation 3 requires that the absorption of the measured optical waveguide does not change in the bandwidth of the filter. The condition of the following formula (4) means that all phase changes of the optical waveguide under test within the bandwidth of the filter can be observed to be caused by the group delay time to be measured. In other words, it is required that higher-order terms (2 or more with respect to ω) are negligibly small. According to analytics, as long as the absorption characteristics | T (ω) | and the phase characteristics φ (ω) of the optical waveguide change continuously (which is generally satisfied in nature), the transmission band of the filter is If the width Δ is made sufficiently small, it is possible to always satisfy the above two conditions. However, it is not desirable to make the transmission bandwidth Δ smaller than necessary as described below.

In order to obtain a delay time at which the amplitude of the above-mentioned interference signal is maximized, it is advantageous to narrow the width of the envelope of the interference signal. However, the width of the envelope here is inversely proportional to the transmission bandwidth Δ of the filter. Therefore, from the viewpoint of measurement accuracy, it is necessary to increase the transmission bandwidth Δ. Thus, in the first conventional method, it is necessary to take a small value of Δ in order to maintain the theoretical validity of the measurement method, but it is desirable to take a large value of Δ in order to increase the measurement accuracy. Conflicting requirements for size will arise.

In the first prior art, the optimum transmission bandwidth Δ was determined as a compromise between the two requirements. However, the problem here is that the content of the measurement target τ ( ω ₀ ) (group delay time of the optical waveguide) itself. In other words, at a wavelength where the group delay time of the optical waveguide changes drastically, it is desirable that the transmission bandwidth of the filter be relatively narrow, but such optimization cannot be performed prior to the measurement. This is because whether or not the change in the group delay time of the optical waveguide at a certain wavelength is drastic becomes clear only through measurement. As a result, strict bandwidth optimization is impractical, and requires the measurement of all wavelengths with a slightly smaller bandwidth for safety while sacrificing some measurement accuracy.

As described above, in the first conventional method, the procedure of changing the setting of the bandwidth of the variable optical filter in accordance with the characteristics of the optical waveguide to be measured is inevitable. Sometimes, when there is a place where the change of the group delay time of the optical waveguide is more severe than expected, the interference signal does not show a single peak, and in this case, the group delay time corresponding to the maximum point of the amplitude is forcibly read as follows. However, an accurate group delay time at the wavelength cannot be obtained. In such a case, it is necessary to re-measure the bandwidth of the variable optical filter after setting it narrower. However, it is necessary to sweep the delay time of the interferometer for each measurement wavelength point, and the re-measurement as described above may be required.Therefore, this conventional technology lacks the speed of measurement. Absent. To further conclude, whenever the measurement is successful, the group delay time must be measured from the slightest shift on the envelope of the interfering signal with a wide width, compared to the width of the largest point that is difficult to determine. Become. For this reason, it was very difficult to obtain high accuracy.

The reason for the drawback of the first conventional method is that attention is paid only to the amplitude (envelope) of the observed interference signal, and the interference reflecting the chromatic dispersion characteristics of the optical waveguide to be measured more directly. It is known that the measurement principle discards without using the phase information of the signal. Based on this understanding, a method of measuring the chromatic dispersion of an element by measuring the phase of an interference signal is disclosed in, for example, Japanese Patent Laid-Open No. 2-13 / 1990.
No. 4543 (Japanese Patent Application No. 63-287566), “Method and Apparatus for Dispersion Measurement”. In this method, an element to be measured is inserted into one arm of an interferometer using white light, an interference waveform generated by changing a delay time difference is recorded, and phase information in a frequency domain obtained by performing a Fourier transform on the recorded waveform. From this, the wavelength dispersion of the device under test is determined. In particular, regarding an optical waveguide, Japanese Patent Application Laid-Open No. Hei 3-216530 (Japanese Patent Application No. 2-11813), “Method and Apparatus for Measuring Waveguide Dispersion”, and a white light interferometer disclosed in Japanese Patent Application Laid-Open No. 2-134543. A measuring instrument is disclosed in which an optical coupling optical system is provided in both arms to cancel out the wavelength dispersion of the optical coupling optical system for inputting and outputting light to and from the waveguide. FIG. 8 shows the configuration of the group velocity dispersion measurement method based on these principles. FIG.
FIG. 3 is a block diagram of a second conventional apparatus.

Next, an outline of the second conventional example will be described. In FIG. 8, a linearly polarized light component in a desired direction is extracted by a polarizer 2 from a parallel light beam 1 which is a parallel light beam generated by a white light source. This light beam enters a Michelson interferometer composed of the cube beam splitter 3, the fixed mirror 4, and the movable mirror 5. An optical waveguide 7 to be measured, which is an optical fiber, is inserted into one of the arms of the Michelson interferometer via coupling lenses 8 and 9. At the same time, in order to remove the influence of the dispersion characteristics of the optical elements other than the optical waveguide 7 to be measured as much as possible, correction lenses 10 and 11 for canceling the dispersion of the coupling lenses 8 and 9 are provided on the arm of the interferometer including the movable mirror 5. Has been inserted. The intensity of light emitted from the interferometer is measured by the light detector 6. When the relative optical path length difference between the two arms of the Michelson interferometer is changed by the movement of the movable mirror 5, the output signal of the photodetector 6 shows a vibration caused by the light interference phenomenon. This signal is recorded one by one in the waveform storage device 12, and then the stored signal is subjected to Fourier analysis by the computer 13. Each optical frequency component obtained as a result of the Fourier analysis, that is, the phase of the Fourier component gives the wavelength dispersion characteristic of the optical waveguide 7 to be measured.

Let S ₁ (τ) be the output voltage of the photodetector 6 recorded in the waveform storage device 12 as a function of the delay time τ obtained by dividing the relative optical path length difference by the light speed. In the measurement interferometer including the cube beam splitter 3, the fine movement mirror 4, and the movable mirror 5, when the phase imbalance between the arms due to the dispersion characteristics of the optical elements other than the measured optical waveguide 7 can be ignored. , And the Fourier transform of the signal S ₁ (τ) is represented by F [S ₁ (τ)] = T (ω) U (ω) (5) Here, F represents a Fourier transform, and T
(Ω) is the transmission characteristic of the optical waveguide to be measured,
(Ω) is the light spectrum of the parallel light flux 1.

The light spectrum is always a positive real number. Therefore, the phase as a complex number in equation (5) is always T
(Ω), that is, only the phase characteristic φ (ω) of the optical waveguide is reflected. That is, arg (F [S ₁ (τ)] = φ (ω) (6) From the phase φ (ω) obtained here, the group delay time is obtained by the equation (1). Equation (1) is a basic equation representing the principle of the waveguide dispersion measuring method according to the second conventional method.

In the second conventional method, the disadvantages of the first conventional method described above are eliminated. That is, the (5)
As a result of extracting only the phase resulting from the Fourier transform of the equation in the equation (6), there is no principle that the absorption characteristic | T (ω) | of the optical waveguide to be measured is mixed in the measurement of the dispersion characteristic.
Therefore, the conventional method can be applied to the measurement of an optical waveguide exhibiting absorption (or gain as negative absorption) such as a semiconductor optical amplifier or an optical fiber type optical amplifier without any problem.
In addition, from the signal obtained by only one interferometer sweep,
Since chromatic dispersion characteristics over all wavelength ranges included in the used white light source are required at once, quick measurement can be realized.

[0018]

However, the second conventional method is based on the premise that the optical path length of the optical waveguide to be measured is kept constant during the measurement. If the optical path length fluctuates, no matter how highly accurate the relative optical path length difference of the measuring interferometer is calibrated, the collected interference signal is Fourier-transformed and expressed by equation (5). Results are not obtained. This is because the fluctuation of the optical path length is equivalent to the error of the relative optical path length difference.

The speed of light is now written as c, and the relative light of the interferometer is
Assuming that the interference signal is collected at every interval of the path length difference cΔτ,
According to the well-known sampling theorem, the result of Fourier analysis is decomposed into Fourier components from ω = 0 to ω = π / Δτ. Each frequency that appeared here
Upper end of number ω_NYQSquares the well-known Nyquist frequency
It is expressed in the form of frequency. This upper end ω_NYQ Is white
Short wavelength end λ of light included in colored light_LThe optical angular frequency ω corresponding to_L= 2πc / λ_L Greater than ω_NYQ> Ω_L is necessary. This is due to the Fourier transform of discrete data.
Aliasing, the source
Signal contains frequency components exceeding the Nyquist frequency
Then, the frequency component is folded back to the Nyquist frequency
This is to prevent the phenomenon of overlapping with the lower part. this
When the condition is converted into the optical path length difference cΔτ, the condition cΔτ <λ_L/ 2 (7) is derived. According to Equation 7, for example, the short wavelength end of white light
If it is 800 nm, it is at least finer than 400 nm.
It is necessary to perform measurement in increments of the optical path length difference
I understand. For this purpose, at least a few tens of nm accuracy is required.
It is essential to calibrate the differential optical path length.

As described above, since the fluctuation of the optical path length of the optical waveguide to be measured is equivalent to the error of the relative optical path length difference, it is necessary that the fluctuation of the optical path length is at most about several tens nm.

For example, the reciprocating optical path length of a semiconductor laser having an element length of about 300 μm is about 2 nm, and the element temperature is 1 nm.
A change in the optical path length of 150 nm accompanies the change in ° C. Therefore, in this case, a temperature control on the order of 0.1 ° C. is necessary in order to suppress the fluctuation of the optical path length to within a range allowed by the second conventional method. Since this degree of temperature control can be easily achieved by the current technology, the dispersion of a semiconductor laser optical amplifier having an element length of 300 μm is actually measured using the second conventional waveguide dispersion measurement method ( IEEE
IEEE Journal of Quantum El
ectronics ", WQE-27, 1280-1287, page 1
991).

The allowable range of the temperature fluctuation is inversely proportional to the optical path length of the optical waveguide to be measured. Therefore, even with the same semiconductor laser, if the typical element length is about 3 nm as in a monolithic mode-locked semiconductor laser, the required temperature control is on the order of 0.01 ° C. Such a level of temperature control is expensive and therefore it is not easy to measure the cavity dispersion of such a long semiconductor laser using this conventional method.

The above semiconductor laser is an all solid state device,
It belongs to the class in which the optical path length fluctuation of the resonator is small. On the other hand, a normal optical fiber has luck, and the bending causes an optical path length fluctuation. Empirically, for a 20 cm long optical fiber that is freely suspended with both ends supported, the optical path length variation due to touching the suspended portion extends to 5 μm or more. This variation in the optical path length exceeds the value permitted by the second conventional resonator dispersion measurement method by two digits or more. Even when the entire length of the optical fiber is laid down on the optical surface plate more carefully, the micro-bending is always randomized by the flow of air or the vibration from the environment passing through the supporting parts and the surface plate, and as a result, 1 μm A slight change in the optical path length cannot be avoided. Therefore, it is practically difficult to measure the chromatic dispersion of an optical fiber using the second conventional method.

As described above, the second conventional method imposes a condition on the optical waveguide to be measured that the fluctuation of the optical path length is small,
The measurement object to which it can be applied practically is practically limited to semiconductor elements having a short element length.

The present invention has been made under such a background, and an object of the present invention is to solve these problems and to provide a versatile waveguide dispersion measuring method. In particular, it is an object of the present invention to provide a waveguide dispersion measuring method which is applicable to an optical waveguide exhibiting absorption (or gain) and which is not affected by an optical path length variation of a measured optical waveguide.

An object of the present invention is to provide a method and an apparatus for controlling the optical path length of an optical waveguide to be measured to be constant and accurately measuring the dispersion thereof. SUMMARY OF THE INVENTION It is an object of the present invention to provide a dispersion measuring method and apparatus capable of removing the influence of the optical path length of a measured optical waveguide from measurement. SUMMARY OF THE INVENTION It is an object of the present invention to provide a dispersion measuring method and apparatus capable of automatically removing the influence of the optical path length of a measured optical waveguide.

[0027]

SUMMARY OF THE INVENTION A first aspect of the present invention is as follows.
This is a waveguide dispersion measuring device for measuring the dispersion of the optical waveguide to be measured inserted into the arm provided with the reflection mirror of the Michelson interferometer.

Here, a feature of the present invention is that a system for detecting a change in the optical path length including the optical waveguide to be measured is separately provided, and the optical effective length of the optical path length is changed so as not to change. There is a means for automatically changing the mechanical length of the optical path length.

It is preferable that the system for detecting the change in the optical path length is another Michelson interferometer having the arms provided with the reflecting mirrors of the Michelson interferometer and configured in common with each other.

That is, according to the present invention, there is provided a half mirror (3) for splitting a first parallel light beam (1) having a wide continuous spectrum into first and second two light beams, and a first mirror transmitted through the half mirror. The first reflecting mirror (4) that reflects a light beam
A second reflecting mirror (5) for reflecting a second light beam reflected on the half mirror, and a light reflected on the first reflecting mirror and a light reflected on the second reflecting mirror on the half mirror. And a first photodetector (6) for detecting interference light generated by multiplexing the optical waveguide to be measured, wherein an optical waveguide to be measured (7) is inserted between the half mirror and the first reflecting mirror. A waveguide dispersion measuring device.

Here, a feature of the present invention is that a second parallel light beam (1) parallel to the first parallel light beam (1) is provided.
4), a third light beam transmitted through the half mirror and a fourth light beam reflected by the half mirror are generated using the half mirror (3) in common. Means for coupling to measurement optical waveguide (8, 9)
And a third reflecting mirror (17) that reflects the fourth light beam
A second light detector for detecting interference light generated by combining the light reflected by the first reflecting mirror (4) and the light reflected by the third reflecting mirror on the half mirror (19)
And an adjusting means for automatically adjusting the optical path length of the waveguide into which the optical waveguide to be measured is inserted so that the output of the second photodetector is constant.

The first parallel light flux (1) and the second parallel light flux (14) are configured to generate output light of one light source (51) by passing through respective propagation paths having different delay times. You can also. Thereby, mutual interference in the commonly used half mirror can be reduced. The adjusting means may include a means for changing the position of the first reflecting mirror (4), or may include an optical path length adjusting device (61) inserted in series with the measured optical waveguide (7). It is good also as composition containing.

Further, a waveform storage device (12) for recording an optical level with respect to an optical path length of an optical path reflected by the second reflecting mirror (5) is connected to an output of the first photodetector (6). It is desirable.

According to a second aspect of the present invention, an optical waveguide to be measured is inserted into an arm provided with one of the two reflecting mirrors of the Michelson interferometer, and the light reflected by the two reflecting mirrors is reflected. This is a method of measuring waveguide dispersion by measuring interference.

Here, the feature of the present invention is that a system for detecting a change in the optical path length including the optical waveguide to be measured is separately provided, and the optical effective length of the optical path length is changed so as not to change. This is to automatically change the mechanical length of the optical path length. Waveguide dispersion can be measured by detecting the interference of two or more light sources having different wavelengths, or by changing the optical path length of light incident on a wide continuous spectrum light source and reflected by the other reflector. The waveguide dispersion can be measured by detecting a change in interference caused by the change and performing a Fourier analysis on the detection result.

That is, according to the present invention, a first parallel light beam (1) having a wide continuous spectrum is split into a first light beam and a second light beam by a half mirror (3), and the first parallel light beam transmitted through this half mirror is transmitted. The light flux is reflected by the first reflecting mirror (4), the second light flux reflected by the half mirror is reflected by the second reflecting mirror (5), and the light reflected by the first reflecting mirror and the second An interference light generated by multiplexing the light reflected by the reflecting mirror on the half mirror to a first photodetector (6)
The optical waveguide to be measured (7) is inserted between the half mirror and the first reflecting mirror, and the position of the second reflecting mirror (5) is changed while the first light flux and This is a waveguide dispersion measurement method by detecting a change in interference caused by the second light beam and performing Fourier analysis on the change in the interference.

Here, a feature of the present invention is that a second parallel light beam (1) parallel to the first parallel light beam (1) is provided.
4), a third light beam transmitted through the half mirror and a fourth light beam reflected by the half mirror are generated using the half mirror (3) in common. The fourth light flux is coupled to the measuring optical waveguide (7), reflected by the third reflecting mirror (17), and reflected by the first reflecting mirror (4) and reflected by the third reflecting mirror. The second light detector (19) detects an interference light generated by multiplexing the separated lights on the half mirror, and controls the measured light guide so that the output of the second light detector becomes constant. The wave path (7) lies in adjusting the inserted optical path length.

[0038]

In addition to the Michelson interferometer for measuring dispersion, a system for detecting a change in the optical path length including the optical waveguide to be measured is provided, and a system is provided so that the optical effective length of the optical path length does not change. The mechanical length of the optical path is automatically changed to stabilize the measurement system.

An automatic control system is constructed by using another Michelson interferometer in which a system for detecting a change in the optical path length is configured so that the arms provided with the reflecting mirrors of the Michelson interferometer are commonly used. be able to.

The above-mentioned configuration is either a method of measuring dispersion using light having a plurality of different wavelengths as light source light, or a method of measuring dispersion by Fourier analysis using light having a wide continuous spectrum as light source light. Can also be used.

The two parallel light beams having a broad continuous spectrum are shared by the beam splitter and the optical waveguide to be measured, the light coupling means therefor, and the end mirror on the side of the optical path where the optical waveguide to be measured is installed. Simultaneously incident on two Michelson interferometers, and converts the intensity of the interference light generated by the first Michelson interferometer (hereinafter referred to as “measurement Michelson interferometer”) into a voltage value by a photodetector for measurement. . A fine movement device is attached to the end mirror or the light coupling means. At this time, the fine adjustment device is driven so that the intensity of the interference light generated by the second Michelson interferometer (hereinafter, referred to as “auxiliary Michelson interferometer”) is kept constant, and the measurement Michelson interferometer is used. The output voltage value of the photodetector is temporarily and sequentially stored every time the relative optical path length difference of the auxiliary Michelson interferometer changes relative to the relative optical path length difference, and the stored data is Fourier-transformed. The wavelength dispersion characteristics of the optical waveguide to be measured are measured from the phase information in the frequency domain obtained as a result. This allows absorption (or gain)
Can be applied, and the dispersion of the waveguide can be measured for a general purpose without being affected by the fluctuation of the optical path length of the measured optical waveguide.

The principle of determining the chromatic dispersion of a measured optical waveguide from a Fourier transform of an interference waveform by inputting a parallel light beam to a Michelson interferometer including the measured optical waveguide in one optical path is described in the above-mentioned first embodiment. It is the same as the second prior art. However, it is different from the above-described second conventional technique in that an accurate interference waveform can be obtained even when the optical path length of the optical waveguide to be measured fluctuates. Therefore, this point will be further described.

The inventor of the present application examined what caused the above-mentioned problem in the second conventional measurement technique. As a result, in order to determine the chromatic dispersion of the measured optical waveguide from the Fourier transform of the interference waveform, it is necessary to collect the interference waveform for the optical path length difference based on the optical path length of the measured optical waveguide for one wavelength. Nevertheless, it has been found that in the conventional method, the interference waveform is sampled for the absolute optical path length difference. That is, if the optical path length of the optical waveguide to be measured fluctuates, the optical path length difference of the interference waveform must be changed accordingly, but this is not performed in the prior art.

Here, the inventor of the present application constantly monitors the optical path length of the optical waveguide to be measured, and automatically corrects the optical path length difference relating to the sampling of the interference waveform according to the fluctuation of the optical path length of the optical waveguide to be measured. The present invention has been reached. In order to monitor the optical path length of the optical waveguide to be measured, an interferometer (correction Michelson interferometer) separate from an interferometer (measurement Michelson interferometer) for collecting an interference waveform is added. Also, for automatic correction of the optical path length difference, the two interferometers share the beam splitter and the coupling system of light to one end mirror or the optical waveguide to be measured, and the optical path length fluctuation of the laser cavity to be measured Following this, a configuration is used in which the end mirror or the light coupling system is moved by the fine movement device.

[0045]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a view showing a waveguide dispersion measuring apparatus according to a first embodiment of the present invention, and shows a basic configuration for implementing the present invention.

The present invention is a waveguide dispersion measuring apparatus for measuring the dispersion of a measured optical waveguide 7 inserted in an arm provided with a movable mirror 5 as a reflecting mirror of a Michelson interferometer.

Here, a feature of the present invention is that a system for detecting a change in the optical path length including the optical waveguide 7 to be measured is separately provided so that the optical effective length of the optical path length does not change. There is a means for automatically changing the mechanical length of the optical path length. A system for detecting the change in the optical path length is another Michelson interferometer having the arms common to each other.

That is, the apparatus according to the first embodiment of the present invention comprises a cube beam splitter 3 including a half mirror for splitting a parallel light beam 1 having a wide continuous spectrum into two light beams,
Fine movement mirror 4 that reflects the light beam transmitted through this half mirror
And a movable mirror 5 that reflects the light beam reflected by the half mirror, and an interference light 80 generated by combining the light reflected by the fine movement mirror 4 and the light reflected by the movable mirror 5 on the cube beam splitter 3. This is a waveguide dispersion measuring device including a photodetector 6 for detection and an optical waveguide 7 to be measured inserted between the cube beam splitter 3 and the fine mirror 4.

Here, the feature of the present invention is that a parallel light beam 14 parallel to the parallel light beam 1 is provided, and the cube beam splitter 3 is commonly used to reflect the light beam transmitted through the half mirror and to the half mirror. A reflecting mirror 15, a semi-transmissive mirror 16, and a means for coupling the light transmitted through the half mirror to the optical waveguide 7 to be measured.
The coupling lenses 8 and 9, the fixed mirror 17 that reflects the fourth light beam, and interference generated by combining the light reflected by the fine movement mirror 4 and the light reflected by the fixed mirror 17 on the cube beam splitter 3. A light detector 19 for detecting the light 81; and an adjusting means for automatically adjusting the optical path length of the waveguide into which the optical waveguide 7 to be measured is inserted so that the output of the light detector 19 is constant. It is provided with a feedback circuit 20 and a fine movement device 21.

Next, the operation of the first embodiment of the present invention will be described. A cubic beam splitter 3 as a first Michelson interferometer (measurement Michelson interferometer) for splitting a parallel light beam into two light beams, propagating them to different light paths, and then combining and interfering again, and a fine mirror 4 and movable mirror 5
The provided, comprising a coupling lens 8, 9 for coupling the measuring Mai Ke Luzon interferometer of the fine mirror 4 side the measured optical waveguide 7 in the optical path of and light on it, the two optical paths of the measuring Michelson interferometer When the relative optical path length difference is sequentially changed, the interference light 80 emitted from the measurement Michelson interferometer
Detector 6 and waveform storage device 12 as measuring means for measuring the intensity of light in accordance with the change in the relative optical path length difference.
And a calculator 13 as arithmetic means for calculating chromatic dispersion characteristics from phase information in the frequency domain obtained by Fourier transforming the measured light intensity waveform.

The second Michelson having a fixed mirror 17 which shares the cube beam splitter 3 and the fine movement mirror 4, the optical waveguide 7 to be measured, and the coupling lenses 8 and 9 for coupling the light with the measurement Michelson interferometer and has a fixed mirror 17 A reflecting mirror 15 and a semi-transmissive mirror 16 for providing an interferometer (a Michelson interferometer for correction) and guiding light to an optical waveguide 7 to be measured and coupling lenses 8 and 9 around the optical waveguide 7 to be measured in one optical path of the interferometer for correction. Interference light 8 emitted from the correction Michelson interferometer
1 is provided with a photodetector 19, a feedback circuit 20, and a fine movement device 21 as feedback means for adjusting the position of the fine movement mirror 4 so that the intensity of 1 is constant, and the movable mirror 5 of the Michelson interferometer for measurement is The position is formed movably based on the position of the fixed mirror 17 in the correction Michelson interferometer.

In order to reduce the phase imbalance between the two optical paths of the measurement Michelson interferometer due to the dispersion characteristics of the optical elements other than the optical waveguide to be measured, the movable mirror of the measurement Michelson interferometer is used. Correction lenses 10 and 11 having the same optical properties as the coupling lenses 8 and 9 and a correction plate 18 having the same optical properties as the semi-transparent mirror 16 are inserted into the optical path on the fifth side. The fixed mirror 17 is fixed during measurement on one optical waveguide, but can be moved when measuring optical waveguides having different optical path lengths.

Parallel light flux 1 having a wide continuous spectrum
Is split by the cube beam splitter 3 into two light beams. One of the split light beams passes through the optical waveguide 7 to be measured via the coupling lens 8 and
After that, the light beam is again converted into a parallel light beam, and is reflected by the fine mirror 4 and follows the same return path as the outward path. The other of the split light beams passes through the correction plate 18 and the correction lenses 10 and 11, is reflected by the movable mirror 5, and follows the same return path as the outward path.
As a result, these two light beams re-enter the cube beam splitter 3 and are combined there. This combined light is incident on the photodetector 6, and the light intensity waveform is measured.

On the other hand, a parallel light beam 14, which is a parallel light beam having a wide continuous spectrum, enters the correction Michelson interferometer and is split into two by the cube beam splitter 3. Here, one of the two split light beams is a reflecting mirror 1
5. Coaxially coupled to one of the split light beams of the parallel light beam 1 by the semi-transparent mirror 16, passed through the optical waveguide 7 to be measured via the coupling lens 8, and converted again into a parallel light beam by the coupling lens 9. Thereafter, the light is reflected by the fine mirror 4 and follows the same return path as the outward path. The other of the split light beams is reflected by the fixed mirror 17 and follows the same return path as the outward path. As a result, these two light beams enter the cube beam splitter 3 and are combined there. This multiplexed light enters the photodetector 19.

Here, the Michelson interferometer for correction is installed to monitor the optical path length of the optical waveguide 7 to be measured. The output signal of the photodetector 19 attached to the correction Michelson interferometer is fed back by the feedback circuit 20 to the fine movement device 21 holding the fine movement mirror 4. As a result, for the optical path length variation of ΔT of the optical waveguide 7 to be measured, a change in the optical path length due to the displacement of the fine mirror by ΔT occurs, and the relative optical path length difference of the correction Michelson interferometer always becomes constant. It changes following the optical path length of the wave path 7. At the same time, the relative optical path length difference of the measurement Michelson interferometer is also shifted according to the optical path length of the optical waveguide 7 to be measured. In this state, the movable mirror 5 of the measurement Michelson interferometer is moved to change the optical path length difference, and the output signal of the photodetector 6 is stored in the waveform storage device 12 one by one.

Thereafter, the signal waveform recorded in the waveform storage device 12 is subjected to Fourier analysis by the computer 13. The phase at each angular frequency of the light obtained as a result of this analysis, that is, the phase of the Fourier component, gives the phase characteristic φ (ω) of the measured optical waveguide 7. The phase characteristic φ thus obtained
From (ω), the group delay time τ (ω) of the optical waveguide 7 to be measured is obtained by the equation (1).

The above operation is performed when the phase imbalance caused by the dispersion characteristics of the optical elements other than the optical waveguide 7 to be measured between the arms of the measurement Michelson interferometer can be neglected, that is, the expression (5) And Equation (6) is assumed to hold. If this phase imbalance cannot be neglected, the above equation (4) representing the Fourier transform of the interference signal S ₁ (τ) is as follows: F [S ₁ (τ)] = T (ω) t _bias (ω) U (ω) ) (8) is changed. As before, F is the Fourier transform, S ₁ (τ)
Is the interference signal, T (ω) is the transmission characteristic of the measured optical waveguide 7,
U (ω) is an optical spectrum of the parallel light flux 1 and a complex value function t _bias (ω) representing phase imbalance is added here. As a result, the phase of t _bias (ω), that is, the phase imbalance between the arms of the measurement Michelson interferometer, is mixed in the phase as a complex number in Expression (8), and the phase of T (ω) is mixed. That is, the phase characteristic φ (ω) of the measured optical waveguide 7 cannot be separated and taken out.

This problem is caused by performing two types of interference signal measurement depending on the presence or absence of the optical waveguide 7 to be measured, and determining the chromatic dispersion of the optical waveguide 7 to be measured based on the difference in phase information in the frequency domain obtained by Fourier transforming each. Ask. More specifically, the problem is solved by taking the ratio of the Fourier transforms. The ratio of the Fourier transform of the interference signal S ₁ (τ) when the measured optical waveguide 7 is inserted to the Fourier transform of the interference signal S ₀ (τ) when the measured optical waveguide 7 is removed is , F [S ₁ (τ)] / F [S ₀ (τ)] = T (ω) (9) Here, the phase imbalance t appearing in equation (8)
_Bias (ω) and U (ω) are completely eliminated together with the light spectrum of the parallel light beam 1. Therefore, the phase of the ratio of the equation (9) gives the phase characteristic φ (ω) of the optical waveguide 7 to be measured. From this phase characteristic φ (ω), the group delay time τ (ω) of the optical waveguide 7 to be measured is obtained by the equation (1).

FIG. 2 is a diagram showing the operation of the fine moving mirror 4.
In the present invention, the relative optical path length difference of the measurement Michelson interferometer related to the sampling of the interference waveform is automatically determined in accordance with the change in the optical path length of the optical waveguide to be measured monitored by the correction Michelson interferometer. The configuration to correct is essentially important. This function is realized by the fine mirror 4 shared by the two Michelson interferometers. This operation will be described.

The graphs in FIGS. 2A and 2C show changes in the output signal of the photodetector 19 associated with the interferometer for correction with respect to the optical path length variation ΔT of the optical waveguide 7 to be measured. FIG.
The graphs of (b) and (d) show the output signal of the photodetector 6 attached to the measuring Michelson interferometer at this time. In the graphs of FIGS. 2B and 2D, the horizontal axis is
The delay time τ obtained by dividing the optical path length difference by the Michelson interferometer for measurement by the light speed. FIGS. 2A and 2B show output signals when the feedback circuit 20 is stopped, that is, when the fine mirror 4 is stationary. FIGS. 2C and 2D show output signals when the feedback circuit 20 is operated and the fine movement mirror 4 is displaced.

When the feedback circuit 20 is stopped, when the optical path length variation ΔT of the measured optical waveguide 7 occurs, the output signal of the photodetector 19 becomes a sinusoidal waveform shown in the graph of FIG. Change. At the same time, the output signal of the photodetector 6 as a function of the delay time τ, ie the interference signal,
The translation is performed as shown in the graph of FIG. Here, in the graph of FIG. 2B, a solid line represents an interference signal when the optical path length variation ΔT is zero, and a broken line represents an interference signal when a positive optical path length variation ΔT occurs. As described above, the interference signal moves rightward with the optical path length variation ΔT.

The optical path length variation ΔT of the measured optical waveguide 7 is an unpredictable random phenomenon. During the operation of the waveguide dispersion measuring apparatus, the interference signal is sampled in time series while changing the delay time. During this sampling, if the interference signal undergoes a random translation at each moment due to the optical path length variation ΔT, the collected interference signal is compressed at a certain point with respect to the delay time and expanded at another point. . As a result, only a distorted interference signal is obtained, and even if this signal is Fourier-transformed, the chromatic dispersion of the measured optical waveguide 7 cannot be obtained. This is exactly the problem to be solved by the present invention.

In the embodiment of the present invention, fine adjustment is performed by the feedback circuit 20 so that the output signal voltage of the photodetector 19 is fixed to the value indicated by the black circle in the graph of FIG. The device 21 is driven to displace the fine mirror 4.
Specifically, the feedback circuit 20 obtains a difference between the set reference voltage value and the output signal voltage of the photodetector 19, and supplies the difference to the fine movement device 21 through the integration circuit and the amplifier. At this time, when the output signal voltage of the photodetector 19 is smaller than the reference voltage value, the fine movement device 21 is driven in the direction in which the fine movement mirror 4 moves forward.

This reference voltage value is desirably set near the average value of the sinusoidal output signal of the photodetector 19.
This is because the optical path length variation ΔT of the optical waveguide 7 to be measured
This is because the change in the output signal of the photodetector 19 is largest, in other words, the feedback sensitivity of the feedback circuit 20 is maximized. When the reference voltage value is set near the maximum or minimum value of the output signal of the photodetector 19, the feedback sensitivity becomes close to zero, and the desired feedback operation cannot be achieved.

When the feedback circuit 20 is operating, the output signal of the photodetector 19 is shown in the graph of FIG. 2C even if the optical path length variation ΔT of the measured optical waveguide 7 occurs. Fixed to the reference voltage value. In this case, in the interference signal as a function of the delay time τ, as shown in the graph of FIG. 2D, the translation caused by the optical path length variation ΔT of the measured optical waveguide 7 disappears. This is because the relative optical path length difference of the measurement Michelson interferometer is
This is because the displacement is automatically changed by the amount corresponding to the optical path length variation ΔT of the optical waveguide 7 to be measured.

As a result of no parallel movement of the interference signal caused by the optical path length variation ΔT of the optical waveguide 7 to be measured, even if the optical path length variation ΔT occurs randomly during the time-series sampling of the interference signal,
The interference signal is not subject to translation. As a result, a distortion-free interference signal is obtained, which can be Fourier-transformed to determine the chromatic dispersion of the measured optical waveguide.

As described in the equation (7), in order to obtain the chromatic dispersion by the Fourier transform of the interference signal, the relative optical path length difference of the measurement Michelson interferometer must be at least several tens of nm. Calibration is required. This relative optical path length difference is changed by the difference between the optical path length difference of the measurement Michelson interferometer and the optical path length difference of the correction Michelson interferometer, that is, the portion intentionally changed by the movable mirror 5 and the fine movement mirror 4. It can be observed as the sum with the part to be made. The method of making the latter change in the optical path length difference follow the optical path length variation of the measured optical waveguide 7 with high accuracy has been described above. Subsequently, the former method of calibrating the change in the optical path length difference will be described. For this calibration, in the configuration shown in FIG. 1, it is necessary to measure the displacement of the movable mirror 5 of the measurement Michelson interferometer with respect to the fixed mirror 17 of the correction Michelson interferometer with high accuracy. . In order to realize highly accurate movable mirror displacement measurement, the following methods are conceivable.

The first method is to use a two-frequency He-Ne stabilized laser which has already been widely used.
In this method, since the position resolution of 5 to 10 nm has already been achieved, the application of this technique can realize the movable mirror displacement measurement with the accuracy required by the present invention. However, the two-frequency He-Ne stabilized laser required for this measurement method is extremely expensive as compared with a normal He-Ne laser, and the second method described below is more economically advantageous.

The second method is to use a linearly polarized monochromatic laser light source, for example, an ordinary He-Ne laser, as the length reference light source. A third interferometer having the fixed mirror 17 and the movable mirror 5 as end mirrors is formed, and a linearly polarized monochromatic laser light source is incident on the third interferometer. The linearly polarized light is converted into circularly polarized light by one arm of the interferometer, and the generated interference light is separated and measured. As a result, two interference signals having a phase difference of 90 degrees from each other are obtained. By using these two interference signals, a length measurement resolution of 1/50 or more of the wavelength of the reference light source can be easily achieved.

In addition, a high resolution can be obtained even if a single interference signal is processed by a phase locked loop (PLL). However, this method requires higher uniformity of the interferometer sweep speed than other methods.

As a method of measuring the displacement of the movable mirror 5 with respect to the fixed mirror 17, a method other than these three methods can be used. For example, the present invention can be similarly implemented by using a method of observing a change in Moire fringes between transmission diffraction gratings attached to respective mirrors.

In the configuration shown in FIG. 1, the measurement Michelson interferometer and the correction Michelson interferometer have their arms on the side of the fine-adjustment mirror 4 converging. The part is reflected by the semi-transparent mirror 16 on the return path and reaches the photodetector 19 of the Michelson interferometer for correction.
In addition, a parallel beam 1 for irradiating the correction Michelson interferometer
A part of the light 4 passes through the semi-transparent mirror 16 on the return path and reaches the photodetector 6 of the measurement Michelson interferometer. In this way, mixing of the two parallel beams 1, 14 occurs on the photodetectors 6, 19, where the two parallel beams 1, 14 can interfere. That is, the presence of mutual interference may have a detrimental effect on the method.

Assume that interference between the parallel light beam 14 and the mixed parallel light beam 1 simultaneously occurs on the photodetector 19 of the Michelson interferometer for correction, in addition to the interference of the parallel light beam 14 itself. . In this case, the change in the output signal of the photodetector 19 depends not only on the optical path length variation ΔT of the measured optical waveguide 7 but also on the relative optical path length variation between the parallel light flux 14 and the parallel light flux 1. Usually two parallel light beams 14, 1
After emitting the light source, the light propagates along a long optical path of 1 m or more before reaching the waveguide dispersion measuring apparatus in FIG. 1, and the relative optical path length variation between them extends over 10 μm or more. Therefore, the portion of the output signal of the photodetector 19 caused by the interference between the two parallel light beams 14 and 1 changes extremely randomly, and as a result of the superposition of this random signal, the output signal of the photodetector 19 becomes It becomes impossible to monitor the optical path length variation ΔT of the optical waveguide 7 to be measured.

On the other hand, on the photodetector 6 of the measurement Michelson interferometer, in addition to the interference of the parallel light beam 1 itself, the parallel light beam 1
When the interference between the collimated light flux 14 and the mixed light beams occurs simultaneously, an interference signal on which a random signal is superimposed is obtained, and the obtained signal is subjected to Fourier transform, and is expressed by Expression (5). No result. Eventually, interference between the two parallel light beams 1 and 14 hinders the functions of both the measuring Michelson interferometer and the correcting Michelson interferometer.

Here, if the two parallel light beams 1 and 14 having a wide continuous spectrum are obtained from independent light sources, specifically, individual lamps, they do not interfere with each other, so the above problem is raised. Does not occur. However, it is of course more economical to obtain two parallel light beams from one light source. A method for obtaining two parallel light beams from one light source in a manner that does not hinder the operation of the present waveguide dispersion measuring apparatus will be described below.

As one method, there is a method of using linearly polarized light beams orthogonal to each other as parallel light beams 1 and 14, which are two parallel light beams. In this case, a polarization beam splitter that transmits the polarized light of the parallel light flux 1 that is a parallel light flux and reflects the polarized light of the parallel light flux 14 that is a parallel light flux is used as the semi-transmissive mirror 16. In this way, a part of the parallel light beam 1 is reflected by the semi-transparent mirror 16 on the return path and does not reach the photodetector 19 of the Michelson interferometer for correction, and a part of the parallel light beam 14 is not reflected on the return path. Thus, the light does not reach the photodetector 6 of the Michelson interferometer for measurement. That is, it is possible to prevent the parallel light beams 1 and 14 that are the above-described parallel light beams from being mixed on the photodetectors 6 and 19.

In the method using the orthogonally polarized light, the measured optical waveguide 7 propagates both orthogonally polarized light components,
In addition, it is effective only when it has a polarization maintaining property. It is not unusual for propagation loss and coupling efficiency to depend on polarization in a semiconductor optical waveguide, and it is not easy to propagate both orthogonal polarization components in such a waveguide.
In addition, a perfect circular optical fiber generally does not have a polarization maintaining property. That is, the polarization of the propagating light is disturbed by slight twisting or bending. Therefore, the method using the orthogonally polarized light is difficult to use when the semiconductor optical waveguide or the perfect circular optical fiber is used as the optical waveguide 7 to be measured, and the method using the orthogonally polarized light is not sufficiently versatile.

As a second more general method, the delay time position where the mutual interference between the parallel light beams 1 and 14 occurs is determined by:
There is a method of intentionally shifting from the delay time range related to the measurement. Hereinafter, this method will be described with reference to FIG. FIG. 3 is a diagram illustrating the influence of mutual coherence of light sources.

The graph of FIG. 3A shows the measured optical waveguide 7.
Represents a change 31 in the output signal of the photodetector 6 of the measurement Michelson interferometer when no is inserted. Here the horizontal axis is
It represents the delay time τ obtained by dividing the optical path length difference of the measurement Michelson interferometer by the speed of light. The graph of FIG. 3B shows a change 35 of the output signal of the photodetector 6 with respect to the delay time τ when the optical waveguide 7 to be measured is inserted.

Now, when the measured optical waveguide 7 is not inserted, if the delay time is virtually swept substantially, FIG.
As shown in the graph, interference signals are observed at two points on the delay time axis. One of them is a component 32 caused only by the parallel light flux 1, and the other is a component 32 due to the parallel light flux 1 and the parallel light flux 14.
Is a component 33 due to mutual interference. When the delay time interval 34 at which these interference signals appear is written as X, the following holds: X = (d + L ₂ −L ₁ ) / c (10) Here, after d is each of the parallel light beam 1 and the parallel light beam 14 incident to the waveguide dispersion measuring apparatus, the optical path length difference between the optical path traced until it reaches the measured optical waveguide 7, L ₁
The optical path length leading to the present waveguide dispersion measuring apparatus of the parallel light beam 1 of light source, L ₂ is an optical path length leading to the present waveguide dispersion measuring apparatus from the light source parallel light flux 14. In the configuration of FIG. 1, d is equal to the distance between the reflection point on the reflection mirror 15 and the reflection point on the semi-transmission mirror 16 under an approximation that ignores the refractive indices of the polarizer 2 and the semi-transmission mirror 16. If the delay time interval X is larger than the width of the interference signal component 32 or 33, it can be observed that self-interference of the parallel light beam 14 and mutual interference between the parallel light beams 14, 1 do not occur simultaneously, The problems due to the mutual interference mentioned above are avoided.

Similarly, when the optical waveguide 7 to be measured is inserted and the delay time is virtually swept substantially, FIG.
A signal as shown in the graph is observed. One of them is a component 37 caused by only the parallel light beam 1, and the other is a component 33 due to mutual interference between the parallel light beam 1 and the parallel light beam 14. The delay time at which these interference signals appear is delayed by a delay time displacement 36 from the delay time at which the corresponding interference signal appears in the above graph. Here, the delay time displacement is equal to a value obtained by dividing the optical path length of the measured optical waveguide 7 by the light speed.
The delay time interval 39 at which these interfering signals appear is unchanged and equals X as described above. If the delay time interval X is larger than the width of the interference signal component 37 or 38, it can be observed that self-interference of the parallel light beam 14 and mutual interference between the parallel light beams 14, 1 do not occur simultaneously, The above-described mutual interference avoids the problem.

In comparison with the width of the component 32 or 33 of the interference signal and the width of the component 37 or 38, the latter is generally larger. Therefore, if the delay time interval X is set to be larger than the latter width, the problem caused by mutual interference can be avoided both in the case where the measured optical waveguide 7 is inserted and in the case where it is not inserted. As described below, the measurement of the interference signal is performed over a delay time difference range that completely includes the component 37 of the interference signal. Therefore, the above condition can be rephrased as setting the delay time interval X to be larger than the delay time difference range related to the measurement.

When illuminated by the expression (10), the delay time interval X can be easily set large by increasing the difference L ₂ −L ₁ in the optical path length from the light source between the parallel light flux 14 and the parallel light flux 1. . Usually, since the two parallel light beams 1 and 14 are guided from the light source to the present waveguide dispersion measuring apparatus by an optical fiber, making a difference in the optical path length can be realized by simply using optical fibers of different lengths. as a result,
A method for easily, universally and inexpensively avoiding the problem caused by mutual interference has been found.

FIG. 4 is a block diagram showing a waveguide dispersion measuring apparatus according to a second embodiment of the present invention, and shows a specific configuration for implementing the present invention. In this specific example,
The measurement Michelson interferometer includes a cube beam splitter 3, a fine mirror 4, and a movable mirror 5. The correction Michelson interferometer includes a cube beam splitter 3, a fine mirror 4, and a fixed mirror 17.

The two parallel light beams respectively incident on the measurement Michelson interferometer and the correction Michelson interferometer are supplied from one light source 51, and the second method described above is used to avoid mutual interference. Used. That is,
The white light emitted to one surface side of the light source 51 is
After being incident on the optical fiber 53 through the optical fiber 2 and exiting from the optical fiber 53, the light is converted into a parallel light beam by the coupling lens 54 and incident on the Michelson interferometer for measurement. On the other hand, the light source 5
The white light emitted to the other surface side of 1 is incident on the optical fiber 56 via the coupling lens 55, and after exiting the optical fiber 56, is converted into a parallel light beam by the coupling lens 57 and enters the correction Michelson interferometer. . Here, a difference is intentionally provided between the lengths of the two optical fibers 53 and 56. In the embodiment of the present invention, an optical fiber having a length difference of 0.5 m is used, and the delay time interval X of 2.4 ns or more is obtained from the equation (10).
Have gained. The delay time difference change range required for measurement is at most several tens of ps, and therefore, the delay time interval X is a value sufficient to avoid the problem due to mutual interference.

The fine movement device 21 includes a piezoelectric element (PZ).
T) is used. The feedback circuit 20 drives the piezoelectric element so that the output signal of the photodetector 19 that receives the light emitted from the correction Michelson interferometer becomes equal to the set reference voltage value.

In the embodiment of the present invention, when measuring the chromatic dispersion of the near-infrared optical waveguide in the wavelength band of 0.8 to 1.8 μm, a light source 51 receives a light emitted from a halogen lamp and a measurement Michelson interferometer. It is preferable to use a germanium photodetector as the photodetector 6 and the photodetector 19 that receives the light emitted from the correction Michelson interferometer. In order to satisfy the above equation (7) at the short wavelength end, 800 nm, and to prevent the aliasing phenomenon, the output signal of the photodetector 6, that is, the interference signal, is measured at intervals of the optical path length difference of less than 400 nm. Just do it.

As a length measuring method for calibrating the optical path length difference increment with high accuracy, the second method described above is employed in this example. The third Michelson interferometer for measuring the displacement of the movable mirror includes the beam splitter 43, the fixed mirror 17, and the movable mirror 5. Linearly polarized light from the monochromatic laser light source 42 enters the third Michelson interferometer. The monochromatic laser light source 42 has a wavelength 63
A He-Ne laser oscillating at 2.8 nm is used. The laser light from the monochromatic laser light source 42 is linearly polarized in the direction of 45 degrees on the paper. The reflecting mirror 44 is one of the arms of the third Michelson interferometer, in this example, the arm on the movable mirror side, and bends the path of light from the monochromatic laser light source 42 in a direction directly incident on the movable mirror 5. The light from the monochromatic laser light source 42 passes through the １ wavelength plate 45 by one arm of the third Michelson interferometer, in this example, the arm on the fixed mirror side, and is reflected by the fixed mirror 17 again. The light passes through the 波長 wavelength plate 45 in the opposite direction. As a result of this reciprocating passage, an effect equivalent to passing through a quarter-wave plate is produced, and linearly polarized light is converted into circularly polarized light.

The interference light having a wavelength of 632.8 nm emitted from the third Michelson interferometer enters the polarization beam splitter 47 via the reflection mirror 46, and is converted into a polarization component perpendicular to the paper surface and a component parallel to the paper surface. Separated and individual photodetectors 4
The light intensity of each component is converted into a voltage value by 8 and 49.
The two interference voltage signals having a phase difference of 90 degrees are input to the trigger generator 50. Trigger generator 50
From these two voltage signals, the optical path length difference is 632.8 nm.
, Ie, 316.4 nm, one voltage pulse is generated as a trigger signal. With this trigger signal, the waveform storage device 12 sequentially stores the output voltage value of the photodetector 6 at the time when the voltage pulse is generated. The voltage signal time series, that is, the interference signal, stored in the waveform storage device 12 is read out by the computer 13 and subjected to Fourier transform.

The interference signal S ₀ (τ) when the optical waveguide 7 to be measured is removed is obtained by always placing the movable mirror 5 and the fixed mirror 17 at the same position. On the other hand, the interference signal S ₁ (τ) when the optical waveguide 7 to be measured is inserted.
Since the optical path length difference between both the measurement Michelson interferometer and the correction Michelson interferometer appears near the optical path length of the optical waveguide 7 to be measured, the movable mirror 5 It is necessary to find a suitable position for placing the fixed mirror 17.

In the case of a short optical waveguide such as a semiconductor laser, a movable mirror in which the optical path length difference becomes zero as a position where the interference signal S ₀ (τ) appears in a state where the optical waveguide 7 to be measured is removed. 5 and the position of the fixed mirror 17 are found, and subsequently, the optical waveguide 7 to be measured is inserted. Then, the movable mirror 5 and the fixed mirror 17 are retracted from the positions to obtain the interference signal S.
_It is effective to find the position where ₁ (τ) appears. The same effect can be obtained even if the fine movement mirror 4 is moved forward instead of moving these mirrors backward. During the operation of finding the position of the fixed mirror 17, it is desirable to stop the operation of the feedback circuit 20. This is because when the feedback circuit 20 is operating, the fixed mirror 1
This is because it is difficult to observe the interference signal with the movement of the microscopic mirror 7 or 4.

In the case of an optical waveguide having a relatively long optical path length exceeding several centimeters, this method is not practical because the movement range of these mirror movement mechanisms is limited. Therefore, for example, the following method is used. That is, the optical path length of the measured optical waveguide is calculated in advance from the length of the optical waveguide and the refractive index data, and the movable mirror 5 and the fixed mirror 17 are retracted by the calculated optical path length from the position where the optical path length difference becomes zero. And set it temporarily. Next, move the movable mirror 5 around the position.
Is moved back and forth, the output voltage value of the photodetector 6 of the measurement Michelson interferometer is observed to find the position where the interference signal S ₁ (τ) appears. Similarly, the output voltage value of the photodetector 19 of the correction Michelson interferometer is observed while moving the position of the fixed mirror 17 around the temporary installation position, and the interference signal S
Find the position where ₁ (τ) appears. Here, the same effect can be obtained even if the fine movement mirror 4 is moved back and forth after the temporary installation of the movable mirror 5 and the fixed mirror 17. As in the above case, it is desirable to stop the feedback circuit 20 during the operation of finding the position of the fixed mirror 17. In the case of the erbium-doped optical fiber of this measurement example,
With this method, the position where the interference signal S ₁ (τ) appears can be easily found. That is, the movable mirror 5 and the fixed mirror 17 are temporarily installed at a position corresponding to the optical path length of 7.4 cm calculated from the fiber length of 5 cm and the approximate refractive index of 1.48, and 2 mm around the temporary installation position. Within the range, the position where the interference signal S ₁ (τ) appears was found.

The overall operation of the embodiment shown in FIG. 4 will be described in more detail. First, after finding the position of the fixed mirror 17 where the interference signal S ₁ (τ) appears, the reference voltage value of the feedback circuit 20 is set near the average of the output voltage values of the photodetector 19, and the feedback operation, that is, Then, the drive of the fine movement device (piezoelectric element) 21 mounted on the fine movement mirror 4 is started.

Next, the movable mirror 5 is advanced from the position where the interference signal S ₁ (τ) is observed to a position where the signal is sufficiently eliminated. Here, the storage of the waveform storage device 12 is erased, and the data write position is reset to the start address of the waveform storage device 12. Next, when the movable mirror 5 is slowly retracted,
Each time the optical path length difference changes by 316.4 nm, a trigger voltage signal is supplied to the waveform storage device 12, and the output voltage signal value of the photodetector 6 is stored in the waveform storage device 12.

Here, “slowly” means that the trigger generator 50
At a sweep speed within a range that can be followed by the analog-to-digital conversion and write operation of the waveform storage device 12 with respect to the repetition of the trigger signal generated from. For example, when the analog-to-digital conversion and writing speed of the waveform storage device 12 is 20 kHz, the maximum possible interferometer optical path length difference changing speed is 20,000 (/sec)×316.4 (nm) = 6.328.
(Mm / sec), and the maximum possible moving speed of the movable mirror 5 is 3.164 mm / sec, which is half of this. Here, the reason for halving is that the light turns back on the surface of the movable mirror 5, so that the optical path length changes twice as much as the moving amount of the movable mirror 5. The change range of the delay time difference required for the measurement is about twice the change amount of the group delay time of the optical waveguide 7 to be measured in the wavelength range of the white light incident on the Michelson interferometer for measurement.

FIG. 5 is a graph showing the results of measuring the chromatic dispersion of the erbium-doped optical fiber. For example, in the erbium-doped optical fiber having a length of 5 cm illustrated in FIG. 5, the total change in the group delay time is at most about 0.25 picoseconds within the wavelength range of 1.2 to 1.8 μm. The change range of the delay time difference is about 0.5 picoseconds.

In actual measurement, sweeping was performed over a delay time difference change range of 1.1 picoseconds with a margin. When this delay time difference change range is converted to an optical path length difference change range, 0.1
When this range is swept with the maximum possible moving speed of the movable mirror 310, the time required for signal measurement is only about 0.05 seconds. Even if the sweep is delayed with a margin, the signal measurement is completed within 0.5 seconds. A known mechanism can be applied as a moving mechanism for this sweep.
In this example, it is driven by an inexpensive DC motor. A linear stage with a steel ball guide was used. The number of data points collected at this time was 1024, and the calculation of the Fourier transform of this data could be executed in 1 second or less using a general-purpose 32-bit personal computer as the computer 513.

In this measurement example, in addition to the measurement of the above interference signal S ₁ (τ), the measurement of the interference signal S ₀ (τ) when the optical waveguide 7 to be measured was removed was performed. The procedure for measuring the interference signal S ₀ (τ) is based on the movable mirror 5 and the fixed mirror 17.
Is the same as the above-described measurement procedure of the interference signal S ₁ (τ), except that the interference signal S ₁ (τ) is set forward by an amount corresponding to the measured optical waveguide 7. The time required for signal measurement and the calculation time for Fourier transform of data are the same as above.

According to the equation (9), the interference signal S
The ratio between the Fourier transform of ₁ (τ) and the Fourier transform of the interference signal S ₀ (τ) is calculated, and the phase characteristic φ of the optical waveguide 7 to be measured is calculated.
(Ω) was determined. From this phase characteristic φ (ω), the (1)
The group delay time τ (ω) of the measured optical waveguide 7 was calculated by the equation. The calculation of the ratio and the calculation and display of the phase were completed within one second using the above-described calculator.

FIG. 5 shows an example of the wavelength dispersion characteristics thus obtained.
Shown in The erbium-doped fiber used as the optical waveguide 7 to be measured is an active optical waveguide used for amplifying light, and has an absorption in a wavelength range of 1.45 to 1.55 μm. FIG. 5 clearly shows the dispersion characteristic accompanying the absorption characteristic, and shows that measurement with high wavelength resolution is realized by the waveguide dispersion measurement method. In contrast, for example, with the first conventional dispersion measurement method,
Exactly because of this absorption characteristic, such a sample could not be measured.

In the above measurement, the time required for signal measurement and calculation is about 3 seconds in total. to this,
Even if 2 minutes are added as the time required for moving the movable mirror 5 and the fixed mirror 17 forward and re-installing, the total time required for the measurement does not exceed 3 minutes. Thus, a quick resonator chromatic dispersion measurement was realized.

FIG. 6 is a block diagram showing a waveguide dispersion measuring apparatus according to a third embodiment of the present invention, and shows another specific configuration for implementing the present invention.

In this embodiment, the measurement Michelson interferometer includes the cube beam splitter 3, the fixed mirror 68, and the movable mirror 5. The correction Michelson interferometer includes the cube beam splitter 3, the fixed mirror 68, and the fixed mirror 17. In the embodiment of FIG.
Is used to automatically shift the relative optical path difference of the measurement Michelson interferometer by an amount corresponding to the optical path length variation ΔT of the optical waveguide 7 to be measured. The incident end 7 and the coupling lens 8 are fixed to the optical coupling portion support plate 69, and the optical coupling portion support plate 69 is displaced by an amount corresponding to the optical path length variation ΔT of the optical waveguide 7 to be measured. For this,
The fine movement device 61 driven by the feedback circuit 20 is mounted on the optical coupling unit support plate 69.

The other end of the optical waveguide 7 to be measured is fixed mirror 68
, And reflects the light propagating through the optical waveguide 7 to be measured without leaving the space. This configuration is different from the configuration in which light is once output to the space shown in the embodiment of FIG.
There is a feature that the loss can be reduced, and for example, the present invention can be applied to an optical waveguide having an end surface treatment that is hardly buckled, such as an optical fiber having a ferrule.

In the present embodiment, a method for avoiding mutual interference between two parallel light beams supplied from one light source 51,
The length measuring method for calibrating the optical path length difference step with high accuracy, and the entire operation are the same as those in the second embodiment described above.

Using the optical waveguide dispersion measuring apparatus of this embodiment, an erbium-doped fiber having optical connectors at both ends was measured as the optical waveguide 7 to be measured, and a result equivalent to that shown in FIG. 5 was obtained. .

[0107]

As described above, the waveguide dispersion measuring method of the present invention can be applied to an optical waveguide exhibiting absorption (or gain), and is not affected by the fluctuation of the optical path length of the measured optical waveguide. In general, waveguide dispersion can be measured.

According to the present invention, it is possible to provide a method and an apparatus for controlling the optical path length of an optical waveguide to be measured to be constant and accurately measuring the dispersion thereof. According to the present invention,
The effect of the optical path length of the measured optical waveguide can be removed from the measurement. According to the present invention, the influence of the optical path length of the measured optical waveguide can be automatically eliminated.

The present invention can be used for a test after manufacturing an optical fiber or a waveguide, a test for developing an ultrashort optical pulse laser using such a waveguide type optical component, and an adjustment after the manufacturing. A great effect can be obtained.

The present invention provides a waveguide type light source for generating an optical pulse having a time width of picoseconds or less, a waveguide type optical amplifier for amplifying the optical pulse, or a wavelength of a waveguide for transmitting the optical pulse. Utilization for measuring dispersion characteristics with high accuracy has a great industrial effect.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the operation of a fine movement mirror.

FIG. 3 is a diagram showing the influence of mutual coherence of light sources.

FIG. 4 is a block diagram of a device according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a result of measuring chromatic dispersion of an erbium-doped optical fiber.

FIG. 6 is a block diagram of a device according to a third embodiment of the present invention.

FIG. 7 is a block diagram of a first conventional apparatus.

FIG. 8 is a block diagram of a second conventional apparatus.

[Explanation of symbols]

 1, 14 parallel light flux 2 polarizer 3 cube beam splitter 4 fine movement mirror 5 movable mirror 6, 19, 48, 49 photodetector 7 optical waveguide to be measured 8, 9, 52, 54, 55, 57 coupling lens 10, 11 correction Lens 12 Waveform storage device 13 Computer 15, 44, 46 Reflecting mirror 16 Semi-transparent mirror 17, 68 Fixed mirror 18 Correction plate 20 Feedback circuit 21, 61 Fine movement device 31, 35 Change 32, 33, 37, 38 Component 34, 39 Delay time Interval 36 Delay time displacement 42 Monochromatic laser light source 43 Beam splitter 45 1/8 wavelength plate 47 Polarizing beam splitter 50 Trigger generator 51 Light source 53, 56 Optical fiber 69 Optical coupling unit support plate 74 Variable optical filter 80, 81 Interference light

Claims (10)

(57) [Claims] 1. A waveguide dispersion measuring apparatus for measuring the dispersion of an optical waveguide to be measured inserted into an arm provided with a reflecting mirror of a Michelson interferometer , wherein the arms provided on the Michelson interferometer are commonly used.
A separate system for detecting a change in the optical path length including the waveguide to be measured, which is composed of another Michelson interferometer, is provided separately, and the optical path length of the waveguide to be measured is constantly monitored.
Means for automatically correcting the optical path length difference relating to the collection of the interference waveform so that the optical effective length does not change in accordance with the fluctuation of the optical path length of the constant waveguide. Characteristic waveguide dispersion measuring device. 2. A half mirror (3) for splitting a first parallel light beam (1) having a wide continuous spectrum into first and second two light beams, and a first light beam transmitted through the half mirror is reflected. A first reflecting mirror (4), a second reflecting mirror (5) reflecting a second light beam reflected on the half mirror, a light reflected on the first reflecting mirror, and a second reflecting mirror A first photodetector (6) for detecting interference light generated by combining light reflected on the half mirror with the light reflected between the half mirror and the first reflecting mirror. In the waveguide dispersion measuring device into which the measurement optical waveguide (7) is inserted, a second parallel light beam (1) parallel to the first parallel light beam (1) is provided.
4), a third light beam transmitted through the half mirror and a fourth light beam reflected by the half mirror are generated using the half mirror (3) in common. Means (8, 9, 15, 16) for coupling to the measuring optical waveguide, a third reflecting mirror (17) for reflecting the fourth light beam, and light reflected on the first reflecting mirror (4). A second light detector (19) for detecting interference light generated by multiplexing the light reflected by the third reflecting mirror on the half mirror, and an output of the second light detector. A waveguide dispersion measuring apparatus, comprising an adjusting means for automatically adjusting the optical path length of the waveguide into which the optical waveguide to be measured is inserted so that the measured optical waveguide becomes constant. 3. The first parallel light flux (1) and the second parallel light flux (14) are generated by passing output light of one light source (51) through propagation paths having different delay times. The waveguide dispersion measuring apparatus according to claim 2 . 4. The waveguide dispersion measuring apparatus according to claim 2 , wherein said adjusting means includes means for changing a position of said first reflecting mirror. 5. The waveguide dispersion measuring apparatus according to claim 2 , wherein said adjusting means includes an optical path length adjusting device (61) inserted in series with said optical waveguide to be measured (7). 6. A waveform storage device (12) for recording an optical level with respect to an optical path length of an optical path reflected by the second reflecting mirror (5) is connected to an output of the first photodetector (6). waveguide dispersion measuring apparatus according to any one of claims 2 to 5. 7. An optical waveguide to be measured is inserted into an arm provided with one of the two reflecting mirrors of the Michelson interferometer, and the interference is measured by measuring the interference of the light reflected by the two reflecting mirrors. In the method for measuring the wave dispersion, the measured waveguide of the Michelson interferometer is inserted.
Arms provided with reflectors are common to each other
Separately provided, the measured waveguide the system for detecting a change in optical path length of the measurement waveguide made of another Michelson interferometer
The optical path length of the waveguide to be measured is constantly monitored, and the optical path length of the measured waveguide is changed.
Depending on the movement, such that a change in the optical effective length does not occur, interference
A waveguide dispersion measurement method, which automatically corrects an optical path length difference related to sampling of an interference waveform . 8. The waveguide dispersion measuring method according to claim 7, wherein waveguide dispersion is measured by detecting interference of two or more light sources having different wavelengths. 9. A light source having a broad continuous spectrum is incident,
8. The waveguide dispersion measuring method according to claim 7, wherein the optical path length reflected by the other reflecting mirror is changed, a change in interference caused by the change is detected, and the detection result is subjected to Fourier analysis to measure the waveguide dispersion. . 10. A first parallel light beam (1) having a wide continuous spectrum is split into a first light beam and a second light beam by a half mirror (3), and the first light beam transmitted through the half mirror is converted into a first light beam. The light reflected by one of the reflecting mirrors (4), the second light beam reflected by the half mirror is reflected by the second reflecting mirror (5), the light reflected by the first reflecting mirror and the second reflecting mirror The first light detector (6) detects interference light generated by multiplexing the light reflected on the half mirror on the half mirror, and detects a light guide to be measured between the half mirror and the first reflection mirror. A wave path (7) is inserted, a change in interference caused by the first light beam and the second light beam is detected while changing the position of the second reflecting mirror (5), and Fourier analysis is performed on the change in interference. In the waveguide dispersion measurement method by performing, the first Yukimitsutaba (1) parallel to the second parallel light beam (1
4), a third light beam transmitted through the half mirror and a fourth light beam reflected by the half mirror are generated using the half mirror (3) in common. The fourth light flux is coupled to the measuring optical waveguide (7), reflected by the third reflecting mirror (17), and reflected by the first reflecting mirror (4) and reflected by the third reflecting mirror. The second light detector (19) detects an interference light generated by multiplexing the divided lights on the half mirror, and controls the light transmission of the measured light so that the output of the second light detector becomes constant. A waveguide dispersion measuring method, comprising adjusting an optical path length in which a wave path (7) is inserted.