JPH03180737A - Propagation loss measuring instrument for optical waveguide - Google Patents

Abstract

PURPOSE:To obtain an easy measuring instrument with high accuracy by providing a movable photodetector for measurement light emitted from the halfway of the optical waveguide and holding the liquid which has a higher refractive index than the optical waveguide therebetween and bringing the photodetector into contact with the surface of the optical waveguide. CONSTITUTION:The light receiving surface 27a of the photodetector 27 is provided in the vicinity of the surface of the optical waveguide 2' between incidence-side and projection-side prisms 24 and 25 in parallel to it and laser beam is coupled with the optical waveguide 2' through a prism. Then the light receiving surface 27a is coated with the liquid M with the high refractive index and brought into contact with the surface of the optical waveguide 2', and then a thin film of the liquid is formed between both; and the laser beam is emitted out forcibly and light-received partially by the light receiving surface 27a, and an ammeter 38 measures the light. Then the light receiving surface 27 is separated from the optical waveguide 2' and after surface remaining liquid on the optical waveguide 2' is wiped away, the light receiving surface is moved by a specific distance in an X-axis direction to take a measurement by the procedure. The movement distance is accurately measured by a measuring instrument 34. The output measuring operation by this photodetector 27 is performed at >=2 places on the surface of the optical waveguide 2' to enable the high-accuracy easy measurement.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a propagation loss measuring device for an optical waveguide.

(Prior Art) In an optical waveguide, as light propagates within the waveguide, the intensity of the propagating light gradually attenuates due to light absorption in the waveguide material and imperfections in the waveguide structure.

Propagation loss quantitatively represents the attenuation of propagating light propagating within the waveguide, and is defined as follows.

Here, Io is the intensity of the propagating light at an arbitrary position propagating within the waveguide, and IL is the intensity of the propagating light at a position a distance along the traveling direction of the propagating light from the arbitrary position. represent

Generally, when optically evaluating an optical waveguide, it is important to measure the propagation loss of the optical waveguide, and several measurement methods have been used to date.

The method for measuring the propagation loss of this optical waveguide includes, for example, the waveguide cutting method shown in FIG.

In this method, the measurement light is first end-coupled to one end surface of the optical waveguide 2' of the optical waveguide substrate 2 by a lens 1, and the measurement light propagating within the waveguide is directed to the outside from the other end surface. The amount of light propagating through the optical waveguide 2' is measured by guiding the emitted light to a photodetector 5 through a spatial filter 4 using a lens 3. Next, after cutting the optical waveguide substrate 2 at the position of line C-C in the figure to shorten the length, the amount of propagated light was measured again using the same method as described above, and the amount of light propagated before cutting was measured. The purpose is to find the propagation loss by comparing it with the amount of light.

Although this waveguide cutting method allows measurement with a simple configuration, it has difficulty reproducibility of the input/output coupling efficiency of propagating light, and is limited to waveguide materials for which good end faces can be obtained by cleaving etc. This has the serious drawback that the optical waveguide 2' is cut and destroyed.

Furthermore, in the prism movement method shown in FIG.
The light emitted to the outside is guided to a photodetector 5 via a spatial filter 4, and the amount of propagated light is measured. At this time, the entrance side prism 6 is fixed to the optical waveguide 2', and the output side prism 7 is moved along the traveling direction of the propagating light together with the photodetector 5, and the amount of propagating light is changed while changing the propagation distance. This is a method of determining propagation loss by measuring changes in

In this method, the output side prism 7 is connected to the optical waveguide 2-.
When moving the output side prism 7 from the optical waveguide 2-
Either separate it once, change its position, and bring it into contact with the optical waveguide 2- again, or impregnate the space between the optical waveguide 2- and the output side prism 7 with an appropriate liquid, and then move the output side prism 7 into the optical waveguide 2-.
There are two methods of sliding the surface of the optical waveguide substrate 2 and moving it continuously.
When the optical waveguide substrate 2 is easily deformed, even if the contact pressure of the output side prism 7 is constant, the state of warpage and deformation of the optical waveguide substrate 2 changes slightly depending on the contact position, so it is difficult to keep the input/output coupling efficiency constant. difficult.

In addition, the latter method requires a complicated prism moving mechanism and measurement technique because it is necessary to move the exit side prism 7 while maintaining a constant contact state.

Furthermore, the scattering detection method shown in FIG.
As the propagating light prism-coupled to the optical waveguide 2' propagates, the intensity of the scattered light is guided to the photodetector 5 along the traveling direction of the propagating light using the fiber probe 8, and is measured by the lock-in amplifier. Recorder 1 via logarithmic amplifier 8
This is a method of indirectly determining the propagation loss by outputting the signal to 1.

Although this method can measure scattered light with a high intensity (propagation loss of about 1 dB/cm or more), it is difficult to detect because the scattered light intensity becomes weak in the low-loss optical waveguide 2'. Also, the fiber probe 8 up to the photodetector 5
Since the intensity of the weak scattered light changes due to the bending of the optical fiber, it is not possible to improve the measurement accuracy.

The above conventional example is described in "Optical Integrated Circuits" (Publisher: Ohmsha, Author: Hiroshi Nishihara, Osaka University, et al.).

The present invention was made based on this background, and an object thereof is to provide an optical waveguide propagation loss measurement device that can easily and accurately measure the propagation loss of a low-loss optical waveguide. It is something to do.

(Means for Solving the Problems) In order to achieve this object, the present invention provides a light receiving element for receiving measurement light emitted from the middle of an optical waveguide in order to measure the propagation loss of the optical waveguide. Set up movably,
The light receiving element is brought into contact with the surface of the optical waveguide by sandwiching a liquid having a refractive index higher than that of the optical waveguide, and the propagation loss is measured.

(Function) According to the present invention, the measurement light emitted from the middle of the optical waveguide is directly detected by the light receiving element, the amount of measurement light is converted into an electrical signal, and the electric signal is sent to the measuring instrument via the signal line. This will measure propagation loss.

(Example) The present invention will be explained below with reference to an embodiment shown in the drawings.

In FIG. 1, S indicates a propagation loss measurement device for an optical waveguide, and 21 is a laser light source. The laser light from this laser light source 21 passes through a reflecting mirror 22 and a polarizer 23 and enters an incident side prism 24. It looks like this.

The entrance prism 24 is fixedly installed at one end of the optical waveguide substrate 2 using a micrometer head 24a. The micrometer head 24a is used to press the entrance side prism 24 against the optical waveguide substrate 2 uniformly and with a constant pressure. The laser light that has entered the incident side prism 24 is prism-coupled to the optical waveguide 2' of the optical waveguide substrate 2.

Further, at the other end of this optical waveguide substrate 2, an output side prism 25 is fixedly installed using a micrometer head 25a. The laser beam propagating through the optical waveguide 2' of the optical waveguide substrate 2 and reaching the output prism 25 is output from the output prism 25 and reaches the optical power meter 26. The function of the optical power meter 26 will be described later.

Near the surface of the optical waveguide 2' located between the incident side prism 24 and the output side prism 25, a light receiving element (for example, SPD; silicon photodiode) 27 has a light receiving surface 27a that is located on the surface of the optical waveguide 2'. It is placed parallel to the

The light receiving element 27 has a function of receiving a part of the propagating light emitted from the middle of the optical waveguide 2' as measurement light and converting it into an electric signal in order to measure the propagation loss of the optical waveguide 2'. This light receiving element 27 is attached to a spring 33 at the tip of a support rod 32 fixed to a moving table 31 of an X-Y stage 28.
Supported through. Here, the spring 33 is a weak spring with a spring constant of less than 0.05 kg/mm, and its expansion and contraction direction is orthogonal to the surface of the optical waveguide 2- and the light receiving surface 27a of the light receiving element 27. Furthermore, this light receiving element 2
7 is connected to an ammeter 38 by a signal line 37, so that its output can be read as an electrical signal proportional to the light intensity of the measuring light.

X - Y Stage 28 (7) X axis (arrow X in Figure 1)
The direction is parallel to the optical waveguide 2-. This X-Y
A length measuring device 34 is installed on the stage 28 to accurately determine the amount of movement of the moving table 31 in the X-axis direction.

In the figure, reference numeral 35 indicates an operating handle for moving the moving table 31 in the X-axis direction, and reference numeral 36 indicates an operating handle for moving the moving table 31 in the direction of the seven wheels (arrow Y in FIG. 1).

The measuring device S thus grooved is used as follows.

First, laser light is coupled to the optical waveguide 2- by the entrance prism 24.

Next, after coating the light receiving surface 27a of the light receiving element 27 with a liquid M having a higher refractive index than the optical waveguide 2- such as silicone oil or Chinese oil, the X-Y stage 28 is operated to allow the laser light to propagate. A light receiving element 27 is placed on the surface of the optical waveguide 2-.
The light-receiving surfaces 27a of the two are brought into slight contact with each other.

The force that presses the light receiving element 27 against the optical waveguide 2- is exerted by the spring 3.
3 should start to shrink slightly.

At this time, if the amount of contraction of the spring 33 is less than 1 mm, the force with which the light receiving element 27 presses the optical waveguide substrate 2 is 0.05 kgf.
The liquid M applied in advance to the light-receiving surface 27a of the light-receiving element 27 will not deform the optical waveguide substrate 2 and change the prism coupling efficiency of the laser light. When the light-receiving surface 27a, which has the function of forcibly emitting the laser light propagating through the optical waveguide 2-, comes into contact with the light-receiving surface 27a, a thin film of liquid M is formed between the light-receiving surface 27a and the optical waveguide 2'. As a result, the laser beam is forcibly emitted to the outside, a part of which is received by the light-receiving surface 27a of the light-receiving element 27 as measurement light, and an electric signal (hereinafter referred to as , this electrical signal is abbreviated as "output of the light receiving element") is measured. After this measurement, the operation handle 36 is operated to separate the light-receiving element 27 from the surface of the optical waveguide 2-.
After wiping off the liquid M remaining on the surface, operate the operating handle 35 to move the light receiving element 27 to another X-axis position a predetermined distance away from the previously measured X-axis position, and then repeat the measurement. After applying liquid M to the light-receiving surface 27a of the light-receiving element 27, a measurement similar to that described above is performed on this light-receiving element 27.
The moving distance in the X-axis direction is accurately measured by the length measuring device 34.

In order to determine the propagation loss of the optical waveguide 2-, the laser beam propagates through the process of bringing the light-receiving surface 27a of the light-receiving element 27 into contact with the surface of the optical waveguide 2- and measuring the output of the light-receiving element 27, as described above. It suffices to perform this at at least two different positions on the surface of the optical waveguide 2-. In this case, the output of the light receiving element 27 at a position close to the incident side prism 24 is
If the output of the light receiving element 27 at a position close to the output side prism 25 is AL%, and the distance between these two positions is L, then the propagation loss of the optical waveguide 2- can be determined as follows.

Further, the above-mentioned measurements may be performed at three or more different positions on the surface of the optical waveguide 2- through which the laser beam propagates, and when measurements are performed at three or more positions, the optical waveguide substrate 2-
The propagation loss of the optical waveguide 2' is log+
It can be determined from the slope of the graph of sAz vs. 9. Note that here, 9 is the moving distance of the light receiving element 27 with respect to an arbitrary origin in the X-axis direction, and Al is the output of the light receiving element 27 measured at the moving distance 9 as described above.

FIGS. 3 to 6 are graphical representations of the measurement results obtained in this manner.

In this measurement, as shown in FIG. 2, the origin is set at a position 8 mm away from the entrance prism 24 and the exit prism 25, and the distance in the X-axis direction from this origin toward the exit prism 25 is set to 9. . In FIGS. 3 to 6, this distance 2 is taken as the horizontal axis of the graph, and the output A of the light receiving element 27 at a distance Q is normalized by the output All of the light receiving element 27 at the origin (Q=O). The resulting normalized output A.

(=Az/Ao) is plotted on the vertical axis using a logarithmic scale. In these embodiments, the entrance side prism 2
Measurements were made at 15 different locations on the surface of the optical waveguide 2- through which the laser light propagates between the prism 4 and the output side prism 25.
The propagation loss is determined from the slope of the straight line that is most suitable for the plot points of the measured data at 15 locations on the logarithmic graph described above.

Figure 3 shows the optical waveguide 2' formed by treating soda glass in molten KNO3 (potassium nitrate) at 400''C for 4 hours.
The measurement results of Sample 1 in which .

In the case of this sample 1, the propagation loss is 0. It is 55 dB/cm.

FIG. 4 shows the measurement results of sample 2, which was treated in the same manner as the sample shown in FIG. 3, but with a treatment time of 8 hours.

In the case of this sample 2, the propagation loss is 0.55 dB/cm,
It is similar to that in FIG.

Both FIGS. 5 and 6 show optical waveguides formed by thinning Corning 7059 on a quartz substrate by sputtering, but these are separate samples.

In sample 3 in FIG. 5, 3. There is a propagation loss of 3 dB/c+s, and sample 4 in FIG. 6 has a propagation loss of 3. It can be seen that the propagation loss is 7 dB/c.

In the measuring device S of this embodiment, the optical power meter 26 receives the laser beam emitted from the output side prism 25 and detects the light intensity.
By using the detected value of No. 6 as a correction parameter in the following correction calculation, it is possible to obtain an accurate propagation loss.

That is, two different positions flI of the light receiving element 27,
Regarding the detection value of the optical power meter 26 at Q2, when the light receiving surface 27a of the light receiving element 27 is not in contact with the surface of the optical waveguide 2', P11'Pz2' is assumed, and when the light receiving surface 27a of the light receiving element 27 is not in contact with the surface of the optical waveguide 2', When in contact with the surface, P7+
, Pz2, and the outputs of the light receiving element 27 at two different positions Q1 and Q2 are Az+, Al2
Then, the light receiving surface 27a of the light receiving element 27 is connected to the optical waveguide 2-
The emission rate at which the laser beam is emitted to the outside at the position where it contacts is as follows.Here, Rjl represents the emission rate at position Q1, and R,2 represents the emission rate at position Q2.Therefore, these The propagation loss is determined using the output ratio as follows.

In this embodiment, since the intensity of the laser beam emitted from the output side prism 25 is detected and monitored by the optical power meter 26, the output of the light receiving element 27 is corrected using this formula,
A more accurate propagation loss can be obtained. This means that slight changes in the contact pressure or contact state that causes the light receiving surface 27a of the light receiving element 27 to come into contact with the surface of the optical waveguide 2- can be completely corrected.

(Effects of the Invention) As explained above, the present invention provides a movable light receiving element for receiving measurement light emitted from the middle of an optical waveguide in order to measure the propagation loss of the optical waveguide. Since the propagation loss is measured by bringing the light into contact with the surface of the optical waveguide, the measurement light emitted from the middle of the optical waveguide is directly transmitted to the light receiving element while keeping the coupling efficiency of the propagating light to the optical waveguide constant. This means that the signal can be detected, converted into an electrical signal, and sent to the measuring instrument via the signal line, thus avoiding the change in light intensity associated with conventional transmission to the measuring instrument using optical fiber, and increasing the Accurate measurements can be made. In addition, by bringing the light receiving element into contact with the optical waveguide across a liquid that has a higher refractive index than the optical waveguide, the propagating light is forcibly emitted to the outside with high accuracy even when the propagation loss of the optical waveguide is low. can be measured.

Furthermore, there is no need to cut the optical waveguide, and the measurement can be carried out in a short time without using a complicated prism moving mechanism or sophisticated measurement techniques, making the measurement easy.

[Brief explanation of drawings]

Fig. 1 is an overall configuration diagram of an apparatus according to an embodiment of the present invention, Fig. 2 is a partial explanatory diagram of the apparatus, Fig. 3 is a graph showing the measurement results of sample 1, and Fig. 4 is a graph showing the measurement results of sample 2. 5 is a graph showing the measurement results of sample 3, and FIG. 6 is a graph showing the measurement results of sample 4.
Graphs showing the measurement results, Figures 7 to 9 relate to the conventional technology, Figure 7 is an illustration of the waveguide cutting method, Figure 8 is an illustration of the prism moving method, and Figure 9 is the scattered light detection method. FIG. S: Measuring device, M: High refractive index liquid 2; Optical waveguide substrate, 2': Optical waveguide 21; Laser light source 24: Incident side prism, 25; Output side prism 26; Optical power meter, 27; Light receiving element

Claims (1)

[Claims] In order to measure the propagation loss of an optical waveguide, a light-receiving element for receiving measurement light emitted from the middle of the optical waveguide is movably provided, and the light-receiving element is mounted on the surface of the optical waveguide at a higher height than the optical waveguide. An optical waveguide propagation loss measuring device characterized by measuring propagation loss by sandwiching and contacting a liquid having a refractive index.