JPH0656351B2 - Device for measuring mode field diameter of single mode optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a far-field image of a single-mode optical fiber.
The present invention relates to a measuring device for measuring a pattern field (FFP) to obtain a mode field diameter, in particular, aiming at high speed and high accuracy of the measuring operation and downsizing of the device.

[Conventional technology]

FIG. 4 shows a conventional single mode optical fiber field diameter measuring device of this kind. Here, 1 is a light source, and 2 is a measured fiber for measurement. Reference numeral 3 denotes a light receiver composed of a photoelectric conversion element (light receiving element) such as Ge-APD, which is scanned by an unillustrated scanning mechanism in an arc shape with a radius R centering on the end O of the measured fiber 2. Reference numeral 4 is an amplification unit including a lock-in amplifier for amplifying the signal from the light receiver 3 and an A / D converter for A / D conversion. Reference numeral 5 is a computer for controlling the scanning mechanism to scan the light receiver 3 and calculating the mode field diameter of the single mode optical fiber based on the output of the amplification section 4. Reference numeral 6 is a drive signal for the light source 1 and the amplification section 4. It is a signal generator to be generated.

FIG. 5 shows an example of the FFP measured by such an apparatus, and the measurement data is A / D converted by the amplifier 4 and read by the computer 5. And computer 5
Calculates the mode field diameter 2W based on the following equation (1),
Output.

However, λ is the wavelength of the light source 1.

In the configuration shown in FIG. 4, the arc-shaped scanning radius (that is, the distance between one end of the fiber 2 and the light receiver 3) R by which the light receiver 3 is scanned has been empirically set to about 80 mm (telecommunication research). Research and Practical Use Report Vol.35, No.7, 1986, No. 736
page).

FIG. 6 shows the relationship between the radius R and the measured value of the mode field. Here, as the light receiver 3, an InGaAs-PIN diode having a light receiving surface with a nominal diameter of 80 μm was used. What is indicated by a dot in the figure is a measured value, and when R ≧ 50 mm, the measured value is saturated. Therefore, R = 80 in the measuring device shown in FIG.
The adoption of mm is reasonable.

On the other hand, the solid line in the figure is the Fresnel diffraction integral, that is,(However, in equation (2) ∫_r∫ Is the polar coordinate display
In-plane integration, a is receiver radius, J₀Is the Oth-order Bessel function,
ρ is the measured fiber radius coordinate value, k = 2π / λ
It ) Shows the calculated value. In the calculation, the mode field
Considering a Gaussian field with a diameter of 10 μm, R ≧ 10 in the calculation
Saturated in mm. The cause of the difference between this experimental value and the calculated value
As a result, the effective light receiving area of the light receiving element is generally the nominal value.
It turned out to be larger.

FIG. 7 shows a measurement example of the light-receiving surface sensitivity distribution. Since the light receiving element has sensitivity even outside the nominal light receiving surface, when it is used as the light receiving device 3, it becomes an equivalent light receiving device having a large light receiving area. Therefore, it is considered that the measurement error of F (θ) becomes large unless the radius R is made larger.

[Problems to be solved by the invention]

Therefore, in the configuration shown in FIG. 4, there is a problem that the radius R is increased and a larger optical system is required to reduce the error. In addition, the configuration shown in FIG. 4 uses a lock-in amplifier to secure a good signal-to-noise ratio (S / N ratio) as the radius R increases. For this reason, there is a problem that it takes time for the measured values to stabilize, and about 10 minutes is required for each measurement. In addition, the optical sensitivities of the parts other than the nominal diameter of the photodetector are not usually specified, and it is necessary to select and test the surface sensitivity distribution of the photodetector to compensate the error related to R. There was also.

An object of the present invention is to reduce the error factor depending on the surface sensitivity distribution of the light receiving element and to reduce the distance R between the end face of the fiber to be measured and the sensor to reduce the size and speed of the mode field diameter. An object of the present invention is to provide a mode field diameter measuring device for a single mode optical fiber that can perform measurement.

[Means for solving problems]

Therefore, the present invention comprises a sensor that receives light emitted from one end of a single-mode optical fiber, and scans the sensor with respect to one end to measure a far-field image of the single-mode optical fiber. In a single-mode fiber mode-field diameter measuring device for measuring a mode-field diameter of a single-mode optical fiber, a sensor is used to measure a measuring optical fiber whose at least one end is scanned with respect to one end of the single-mode optical fiber. And a light receiving member formed of a photoelectric conversion element coupled to the other end of the optical fiber for use.

[Action]

In the present invention, the far-field image (FFP) is measured through an optical fiber for measurement instead of directly using the light receiving member as in the conventional case. As a result, the light receiving surface sensitivity distribution characteristic of the photoelectric conversion element is ensured, and the R value can be shortened to a fraction of the conventional value.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail and specifically with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. Here, for each unit that can be configured in the same manner as in FIG. 4, corresponding portions are given the same reference numerals, and description thereof will be omitted. In the device according to the present example, the light emitted from the end portion O of the measured fiber 2 is not directly received by the light receiver 3 as in the conventional example shown in FIG. 4, but is received through the measurement optical fiber 7. Is done. For this purpose, one end of the measuring optical fiber 7 is fixed by a jig 7'to a scanning mechanism for performing an arcuate scanning so that the end face is scanned in an arcuate shape with respect to the optical axis of the measuring fiber 2. In addition, the other end surface of the measurement optical fiber 7 is connected to the light receiver 3
Is optically coupled to. Then, the output of the photodetector is amplified by the amplifier 8 and input to the computer 5 via the digital voltmeter 9 to measure the FFP or calculate the mode field.

FIG. 2 shows the relationship between the radius R and the measurement error of the mode field diameter when the measurement is performed using the apparatus shown in FIG. In this experiment, a measuring optical fiber 7 having a core diameter of 80 μm and Δn = 1.1% was used. Also, in the figure, the light-receiving surface diameter is 80
The data according to the conventional example in which the photodetector 3 having a μm InGaAS-PIN diode is provided and light is directly received from the measured fiber 2 is also shown for reference.

As is apparent from FIG. 2, the measured value (broken line) obtained by the configuration shown in FIG. 1 and the calculated value (solid line) by Fresnel diffraction integration are relatively in agreement (D = 80 μm in the same figure).
φSI curve), mode field diameter measurement value is R ≧ about 5m
saturated at m.

As a result, compared to the conventional example, there is no problem even if R is set to about 1/10, which contributes to the speeding up and downsizing of the device. Further, compared with the case where a general light receiving element is directly used, there is an advantage that the work of checking the sensitivity distribution of the light receiving surface is unnecessary and the measurement can be performed with high accuracy. When the radius R becomes 1/10, the signal light power density becomes 100 times, and a system configuration with an extremely good S / N ratio becomes possible. Of course, the lock-in amplifier used in the conventional example is unnecessary, and when measured with R = 10 mm, the measurement time is about 1
That's 10 minutes faster.

The measurement optical fiber 7 is preferably a step-type fiber having a good incident angle sensitivity distribution, and a larger numerical aperture thereof is also desirable.

FIG. 3 shows a second embodiment of the present invention. In this embodiment, a pulse motor, a servomotor or the like is used as a drive source instead of the arcuate scanning mechanism, and the end face of the measuring optical fiber 7 is linear. A scanning mechanism 10 for scanning in a circular pattern is provided. Then, in this example, FFP is corrected by the following equation (3).

F (θ) = F ′ (θ) · [(R ² + d ² ) / R ² ] · (1 / cosθ) (3) where F ′ (θ) is the measured value and d is the measured fiber 2
Is the distance between the optical axis of and the optical axis of the measuring fiber 7,
The [(R ² + d ² ) / R ² ] term is the ray density inverse square law correction term, (1 / cos
The θ) term is an angle correction term for the light receiving end surface of the measuring optical fiber.

According to the configuration of this example, since the circular arc scanning is not performed, the configuration is extremely simple and easy, and there is an advantage that the device can be configured at low cost. The distance R between the fiber end faces is 10
Since a scanning width of about 10 mm is sufficient, it is possible to use a small scanning mechanism 10 whose movable range is about ± 2 mm when the maximum divergence angle of the measured light is 0.2 rad. Can contribute to realization. Therefore, the other inspection items (loss,
Combined with a measuring device (such as dispersion), there is an advantage that it is suitable for building a small and high-speed automatic inspection system.

〔The invention's effect〕

As described above, according to the present invention, the FFP measurement that does not depend on the light receiving surface sensitivity distribution of the light receiving element that constitutes the light receiving device becomes possible, and the single mode optical fiber that is compact and enables high-speed and high-accuracy measurement There is an advantage that the mode field measuring device can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a mode field diameter measuring apparatus for a single mode optical fiber of the present invention. FIG. 2 is a dependence of a measured value of the mode field diameter on the scanning radius R according to the embodiment shown in FIG. FIG. 3 is a block diagram showing a second embodiment of the present invention, FIG. 4 is a block diagram showing the configuration of a conventional mode field diameter measuring device for a single mode optical fiber, Fig. 5 is a diagram showing an example of a far-field image (FFP) of a single mode optical fiber, and Fig. 6 shows measured and calculated values of the mode field diameter of the single mode optical fiber according to the conventional example shown in Fig. 4. FIG. 7 is an explanatory diagram showing an example of a light receiving surface sensitivity distribution of a light receiving element used as a light receiving device. 1 ... Light source, 2 ... Fiber to be measured, 3 ... Receiver, 4 ... Amplifying section, 5 ... Computer, 6 ... Signal generator, 7 ... Measurement optical fiber, 8 ... Amplifier, 9 ...... Digital voltmeter, 10 ・ ・ ・ Linear scanning mechanism.

Claims (4)

[Claims] 1. A sensor for receiving light emitted from one end of a single mode optical fiber, wherein the sensor is scanned with respect to the one end to measure a far-field image of the single mode optical fiber. A mode field diameter measuring device for a single mode optical fiber for measuring a mode field diameter of the single mode optical fiber, wherein the sensor measures at least one end with respect to the one end of the single mode optical fiber. Optical fiber,
A mode field diameter measuring device for a single mode optical fiber, comprising a light receiving member composed of a photoelectric conversion element coupled to the other end of the measuring optical fiber. 2. The single mode optical fiber mode field diameter measuring device according to claim 1, wherein the measuring optical fiber has a graded refractive index distribution. Device for measuring mode field diameter of mode optical fiber. 3. A mode field diameter measuring device for a single mode optical fiber according to claim 1 or 2, wherein the one end of the measuring optical fiber is connected to the one end of the single mode optical fiber. A mode field diameter measuring device for a single mode optical fiber, wherein the far field image is measured by scanning in an arc shape with one end as a center. 4. The mode field diameter measuring device for a single mode optical fiber according to claim 1 or 2, wherein the optical axis of the one end of the measuring optical fiber is the single mode optical fiber. Of the one end of the single mode optical fiber is measured while measuring the spatial power distribution while keeping the optical axis parallel to the one end of the single mode optical fiber. A single mode characterized in that the far-field pattern is obtained by correcting the optical power distribution by the elevation angle with respect to the optical axis of the center of the one end of the measuring optical fiber viewed from the center of the one end of Optical fiber mode field diameter measuring device.