JPS5831859B2 - Method and device for measuring propagation characteristics of optical waveguides - Google Patents

Description

Detailed Description of the Invention The present invention relates to a method and apparatus for measuring the propagation characteristics of an optical waveguide, and in particular to an optical waveguide suitable for measuring the propagation loss and mode conversion rate of a thin film optical waveguide formed on a dielectric substrate. The present invention relates to a method and apparatus for measuring propagation characteristics of.

Thin-film optical waveguides with a two-dimensional structure are attracting attention as a basic waveguide for optical integrated circuits.

When light propagates through this thin film waveguide, part of its energy is scattered due to refractive index fluctuations within the waveguide and surface non-uniformity.

Most of the scattered light is coupled with the radiation mode and scattered from the waveguide surface, but some of the light undergoes mode conversion, couples with other waveguide modes, and propagates inside the waveguide.

Conventional methods for measuring the propagation characteristics of an optical waveguide include a scattering method in which the scattered light is measured, and a transmission method in which transmitted light extracted from the optical waveguide using a prism coupler or the like is measured.

An example of measurement using the former scattering method is shown in FIG.

In this measurement, an optical coupler 1 is placed on an optical waveguide 4 formed on the surface of a dielectric substrate 2 to be measured, and receives incident light 6 and excites guided light 8 into the optical waveguide 4.
0, slits 12 and 14, and a scattered light detector 16 that detects scattered light passing through the slits.

In this measurement method, the slits 12 and 14 and the scattered light detector 16 are moved along the propagation direction of the guided light 8 (X direction in FIG. 1), and the scattered light intensity Is at each position X is measured. Determine the attenuation coefficient α of the guided light from the formula % formula % (1).

Here, A is a constant, and g is the guided light intensity at the position (x=0) excited by the input cobrism coupler 10.

The drawback of this method is that it is impossible in principle to measure the propagation characteristics by focusing on one mode.

That is, as described above, even if single mode guided light is excited into the optical waveguide, a part of it is converted into other modes during propagation.

Therefore, the detected scattered light intensity includes not only the scattered light caused by the guided light of the mode of interest, but also the scattered light caused by the guided light converted to another mode, but these two can be distinguished by the scattered light. Because you can't.

An example of measurement using the latter transmission method is shown in FIG.

In this measurement, in place of the slit and scattered light detector in the scattering method, an output cambrism coupler 20 is placed on the optical waveguide 4 and a transmitted light detector 24 receives the transmitted light 22 extracted by the output cambrism coupler 20. is placed.

In this measurement method, the output beam coupler 20 and the transmitted light detector 24 are connected in the propagation direction of the guided light 8 (
2), and measure the transmitted light intensity (2) at each position X to determine the attenuation coefficient α of the guided light from the relational expression % (2).

Here B is a constant.

The disadvantage of this method is that it is necessary to move the output coupler along the surface of the optical waveguide, so it cannot be measured unless the sample has a long propagation distance. Since the output beam coupler must be moved while keeping the optical path constant, the measurement operation is complicated and errors are likely to occur.

In addition, a common drawback of the two methods described above is that they both apply many measured values to equation (1) or (2), and then calculate the average propagation characteristics along the propagation optical path of the optical waveguide. Therefore, it is sometimes impossible to measure the propagation characteristics between two specific points.

The present invention has been made to solve the above-mentioned conventional drawbacks,
It is an object of the present invention to provide a method and apparatus for measuring the propagation characteristics of an optical waveguide, which has little measurement error and can measure the propagation characteristics between two specific points.

In the present invention, an incoming cobrism coupler and an outgoing cobrism coupler are arranged opposite to each other on the surface of an optical waveguide to be measured, and the coupling efficiency with the optical waveguide is approximately 100%. In a measurement method and apparatus for extracting guided light excited in a waveguide by an output beam coupler and determining the propagation characteristics of an optical waveguide from the intensity of the transmitted light, in addition to the transmitted light intensity, the incident light intensity and the incident light are measured. The incident angle or refraction angle when entering the Cabulism coupler, and the angle other than the synchronous angle when the incident angle is a synchronous angle of the light reflected on the coupling surface and output from the input Cabulism coupler during optical waveguide excitation. The propagation of the optical waveguide is determined by measuring the ratio of the intensity at the time of transmission and the angle of incidence or refraction when the transmitted light exits from the output coupler, and from these and the refractive index of the input coupler and output coupler. It is designed to find characteristics.

The measuring method according to the present invention will be explained below.

Generally, in the measurement by the transmission method shown in FIG. 3, the incident light is output from the output coupler after the intensity is reduced due to several reasons.

The causes of this decrease are as follows.

(a) Fresnel reflection when entering the input kabrism coupler (b) Loss during guided light excitation to the optical waveguide by the input kabrism coupler (due to coupling efficiency not being 1) ←→ Loss during propagation in the optical waveguide (Due to radiation, mode conversion, etc.) (2) Loss when the guided light is extracted from the optical waveguide by the output coupler (E) Fresnel reflection when the guided light is output from the output coupler ), 2), and 5) can be evaluated as follows.

(1) Evaluation of the amount of decrease according to (a) and (e) The light transmittance T at the boundary between the prism and the air is given by the following formula.

Here, nl and n2 are the air and the incoming fog in the case of (a), the outburst and the refractive index of the air in the case of (e), and α1 is the angle of incidence of the incident light on the prism in the case of (a),
In the case of (e), it is the incident angle of the transmitted light into the air, and P is a constant having the following value.

■ (When the guided light is in TE mode) ””nt/.

2 (waveguide optical power (TM-E l'(7) (!:ki)) Therefore, the refractive index n2i ynlO of the input and output coupler, the incident angle α1i to the input and output coupler, and the output and output coupler. By measuring the incident angle α1o into the air when the light is output from the coupler, the corresponding light transmittance Ti
and T.

From this, the amount of decrease due to (a) and (e) can be found.

In the above description, α1 is both an incident angle, but it is clear from Snell's law that if the refractive index is known, one or both may be a refraction angle.

(i+) (b) Evaluation of the amount of reduction by excited into the wave path.

Here, the angle of incidence at this time is called a synchronous angle.

When the intensity of the light reflected at the coupling surface is 1 when the angle is synchronous, and when the angle is not synchronous, the intensity of the light reflected from the coupler is 12, then the coupling efficiency between the input cobrism coupler and the optical waveguide is η is given by the following formula.

Therefore, ■1. ■If you measure 2, you can find the coupling efficiency η,
From this, the amount of decrease due to (0) can be found.

(iii) Evaluation of the amount of reduction due to (2) The loss when the guided light is extracted by the output coupler is that the space between the optical waveguide surface and the output coupler is filled with matching waves with a higher refractive index than the optical waveguide. Therefore, it can be reduced to zero.

Therefore, the amount of decrease due to this) can be ignored.

As is clear from the above, the incident light intensity ■.

and transmitted light intensity ■3, (a), (0), (e)
By determining the amount of decrease due to the cause (c), it is possible to know the change in characteristics due to the cause (c), that is, the propagation characteristics of the optical waveguide.

For example, the propagation loss of guided light can be found from the following equation.

Here, ■3□ is the intensity of transmitted light in the same mode as the guided light excited by the incident cobrism coupler, and Tol is the transmittance determined from the incident angle or refraction angle of the transmitted light into the air.

Moreover, the mode conversion rate of guided light can be found from the following equation.

Here, I32 is the intensity of transmitted light in a mode different from the mode of the guided light excited by the incident cobrism coupler, and T02 is the transmittance determined from the incident angle or refraction angle of the transmitted light into the air.

In measurement using the transmission method, since the synchronous angle differs depending on the mode, the transmitted light in the same mode as the guided light and the transmitted light in a different mode from the guided light are spatially separated and emitted from the output coupler. .

Therefore, by changing the position of the transmitted light detector, it is possible to simultaneously determine propagation loss and mode conversion.

An embodiment of the measuring device of the present invention will be described below with reference to FIG.

In this embodiment, an in-cabulism coupler 10 and an out-cabulism coupler 20 are placed opposite to the surface of an optical waveguide 4 to be measured.
, a rotary stage 30 on which the optical waveguide 4, the input beam coupler 10, and the output beam coupler 20 are placed, and a semi-transparent plate 32 placed in the input optical path. A reflected light detector 34 measures the intensity of the light reflected by the semi-transparent plate, and an incident light detector 3 measures the intensity of the light incident on the incident cobrism coupler 10.
6, and a transmitted light detector 24 that measures the intensity of the transmitted light emitted from the output beam coupler 20.

The input cabulism coupler 10 and the output cabulism coupler 2
0 is a right-angled prism having the same shape and the same refractive index, and a matching liquid 38 is filled between the surfaces of the output beam coupler 20 and the optical waveguide 4, so that the coupling efficiency between the two becomes approximately 100%. .

A scale plate 40 is provided around the rotation stage 30, and the angle of incidence or angle of refraction can be read from the rotation angle of the scale plate 40.

Further, the surface of the rotary stage 30 is parallel to the plane of incidence of the incident light 6, and the optical waveguide surface and the like are placed on the rotary stage 30 so as to be perpendicular to the rotary stage 30.

Furthermore, the rotation axis of the rotation stage 30 is perpendicular to the stage surface and coincides with a reflection point 42 of the incident light on the optical waveguide surface (located near the edge of the input cubism coupler).

The measurement method using this device will be described below.

First, the incident light detector 36 is placed in the optical path of the incident light 6 such as a laser beam, and the intensity of the incident light (2) is measured.

Measure. Next, the incident light detector 36 is removed, and the incident angle is set to a state where the optical waveguide 4 is excited (synchronous angle).

At this time, the incident angle α11 to the incoming cobrism coupler and the incident angle α1o from the outgoing cobrism coupler to the air are determined by the rotating stage 30, and the intensity I31 or I3□ of the transmitted light 22 is determined by the transmitted light detector 24 and the optical waveguide 4. When exciting, the reflected light 46 is reflected by the coupling surface 44, totally reflected within the incoming cobrism coupler 10, and emitted antiparallel to the incident light 6. The intensity 1 of the reflected light 46 is transmitted through the semi-transparent plate 32 to the reflected light detector. Measured according to 34.

Next, the incident angle is set to a state in which the optical waveguide 4 is not excited (other than the synchronous angle), and the intensity (2) of the reflected light 746 at this time is measured by the reflected light detector 34 in the same manner as described above.

Incidentally, the deviation of the incident angle other than this synchronous angle from the incident angle at the synchronous angle depends on the strength of the coupling between the coupling surfaces, but generally a deviation of about 30' is sufficient.

From the above, the incident light intensity ■.

, intensity of transmitted light ■3, or ■32, angle of incidence to the input cubism coupler αllv angle of incidence from the output cubism coupler to the air α1o1 Intensity of reflected light at synchronous angle and at times other than synchronous angle■ By measuring the above, the propagation loss or mode conversion rate can be determined from the above equation (5) or (6) from these and the refractive index of the prism.

In this example, since the incoming cobrism coupler and the outgoing cobrism coupler have the same refractive index, the refraction angle of transmitted light in the same mode as the guided light is equal to the incident angle α11 to the incoming cobrism coupler. .

Therefore, Ti=To1, and when only the propagation loss is determined, there is no need to measure the angle of incidence α1o into the air or the angle of refraction at the time of emission.

As described above, according to the measuring method and apparatus of the present invention, in the transmission method, in addition to the transmitted light intensity, the incident light intensity and the incident angle or refraction angle when the incident light enters the incident cobrism coupler are measured. , the ratio of the intensity of the light reflected by the coupling surface during excitation of the optical waveguide and emitted from the incoming cobrism coupler when the incident angle is a synchronous angle and when the incident angle is a non-synchronous angle, and the ratio of the intensity of the transmitted light to the output cobrism coupler. The incident angle or refraction angle at the time of output from the optical waveguide is measured, and the propagation characteristics of the optical waveguide are determined from these and the refractive indices of the input and output couplers, so the output coupler can be moved. This eliminates the need for a small measurement sample, and eliminates measurement errors caused by movement.

Also, since it is possible in principle to measure the propagation characteristics between two specific points,
The loss of light due to relatively large scattering centers in the propagation path can be measured independently.

Furthermore, it has excellent effects such as being able to measure average propagation loss, mode conversion rate, and scattering loss due to large scattering centers, which are the basic qualities of an optical waveguide, with the same measuring device.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view showing a method for measuring the propagation characteristics of an optical waveguide using a conventional scattering method, Fig. 2 is a cross-sectional view showing a method for measuring the propagation characteristics of an optical waveguide using a conventional transmission method, and Fig. 3 is a cross-sectional view showing a method for measuring the propagation characteristics of an optical waveguide using a conventional transmission method. FIG. 3 is a cross-sectional view showing a force method and apparatus for measuring the propagation characteristics of an optical waveguide. In the figures, the same reference numerals indicate the same or equivalent parts, 2 is a dielectric substrate, 4 is an optical waveguide, 6 is incident light, 8 is waveguide light, 10
is an input cabulism coupler, 20 is an output cabulism coupler, 2
2 is a transmitted light, 24 is a transmitted light detector, 30 is a rotation stage, 32 is a semi-transparent plate, 34 is a reflected light detector, 36 is an incident light detector, 38 is a matching liquid, 44 is a coupling surface, 46 is a reflection It is light.

Claims (1)

[Claims] 1. An incoming cobrism coupler and an outcobrism coupler whose coupling efficiency with the optical waveguide is approximately 100% are placed opposite to the surface of an optical waveguide to be measured, and the incoming cobrism is In a measurement method in which guided light excited into an optical waveguide by a coupler is extracted by an output coupler and the propagation characteristics of the optical waveguide are determined from the transmitted light intensity, in addition to the transmitted light intensity, the incident light intensity, The incident angle or refraction angle when the incident light enters the input cobrism coupler is synchronized with the incident angle of the light that is reflected on the coupling surface during optical waveguide excitation and exits from the input coburism coupler when the incident angle is a synchronous angle. Measure the ratio of the intensity at a time other than the eggplant angle and the incident angle or refraction angle when the transmitted light exits the output cobrism coupler, and calculate the propagation from these and the refractive index of the input cobrism coupler and the output cobrism coupler. A method for measuring propagation characteristics of an optical waveguide, characterized by determining the characteristics. 2 Among the transmitted light taken out from the optical waveguide by the output coburism coupler, the transmitted light intensity of the same mode as the guided light excited by the input coburism coupler and the incident angle or refraction angle at the time of output are determined. 2. A method for measuring propagation characteristics of an optical waveguide according to claim 1, which comprises measuring and determining propagation loss from the measurement. 3. Of the transmitted light taken out from the optical waveguide by the output coburism coupler, the intensity of the transmitted light in a mode different from the mode of the guided light excited by the input coburism coupler, and the incident angle or refraction angle at the time of output. A method for measuring the propagation characteristics of an optical waveguide according to claim 1, which comprises measuring the propagation characteristics of an optical waveguide and determining the mode conversion rate therefrom. 4. An input cobrism coupler with a known refractive index placed on the surface of the optical waveguide, and an output cobrism coupler with a known refractive index placed opposite to the input cobrism coupler so that the coupling efficiency with the optical waveguide is approximately 100%. a rotary stage on which the optical waveguide, the incoming cobrism coupler, and the outgoing cobrism coupler are fixed, and one of the repulsed lights reflected at the coupling surface of the incoming cobrism coupler and the leading waveguide and emitted from the incoming cobrism coupler. a reflected light detector that measures the intensity of the light reflected by the semitransparent plate; an incident light detector that measures the intensity of the light incident on the incoming cobrism coupler; An apparatus for measuring propagation characteristics of an optical waveguide, including a transmitted light detector that measures the intensity of transmitted light emitted from a coupler. 5. Claim 4, wherein the transmitted light detector is arranged at a position where it receives transmitted light in the same mode as the guided light excited by the input coburism coupler, and is adapted to determine propagation loss. A device for measuring the propagation characteristics of optical waveguides. 6. Claim 4, wherein the transmitted light detector is arranged at a position to receive transmitted light of a mode different from the mode of the guided light excited by the input coburism coupler, and is configured to determine a mode conversion rate. An apparatus for measuring propagation characteristics of the optical waveguide described above.