JPH01232229A - Cutoff wavelength measuring method for single-mode optical waveguide - Google Patents

Abstract

PURPOSE:To realize the easy, nondestructive cutoff wavelength measuring method for the single-mode optical waveguide by putting one end of an optical fiber in contact with or nearby one end of the single-mode optical waveguide. CONSTITUTION:The white light from a light source 262 is made into monochromatic light 261, which is entered into the optical fiber 23 where the light can be propagated in two modes with the cutoff wavelength of an optical waveguide 21, and a high-order mode removal part 25 which is formed by winding the fiber nearby the incidence end around a column removes the high-order mode and the light in single mode is entered into the optical waveguide 21. Its output is photodetected completely by a multimode optical fiber 24 for photodetection which is put in contact while having its optical axis aligned and while photodetection parts 281-283 measure the output light, the optical axes of the fiber 23 and waveguide 21 are aligned with each other, so that the output light intensity of 0.7-1.7mum in wavelength is measured at prescribed intervals. Consequently, the loss value varies abruptly right below the cutoff wavelength lambdaCW of the waveguide 21 as shown in a figure (a) and the wavelength dependency of variation in the loss value is reduced on a shorter wavelength side than said wavelength. The cutoff wavelength of the waveguide 21 is measured without any destruction.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a non-destructive and simple method for measuring the transmission characteristics of a single mode optical waveguide.

(Prior art) The basic transmission characteristics of a single mode optical waveguide are as follows:
There is a loss value and a cutoff wavelength. Among these, the cutoff wavelength has almost no direct measurement method in the prior art, and is mainly determined by numerical calculation from the core dimensions of the optical waveguide and the relative refractive index difference between the core and the cladding. This calculation required an extremely large amount of calculation since the general structure of the optical waveguide is a three-dimensional rectangular shape. Furthermore, this calculation required measuring the relative refractive index of the core and cladding. In order to accurately measure the relative refractive index difference between the core and cladding of an optical waveguide, a method using interference microscopy has been conventionally used. This method requires cutting a thin sample from the optical waveguide, and has the disadvantage that it cannot be measured non-destructively.

On the other hand, the loss value of an optical waveguide has conventionally been determined by keeping the input light conditions to the optical waveguide constant, changing the length of the optical waveguide, and measuring the output light intensity from the optical waveguide. . This method requires constant input light conditions each time the length of the optical waveguide is changed, and the core diameter is several μm.
However, in the case of very small single-mode optical waveguides, this work requires a huge amount of time and has the disadvantage that it is necessary to cut the optical waveguide.

That is, in the prior art, methods for measuring the transmission characteristics of single mode optical waveguides are in a very immature state, and it can be said that there is no measurement method that is simple and non-destructive in the strict sense.

. (Problems to be Solved by the Invention) The present invention was made in view of the above-mentioned drawbacks, and utilizes the difference in propagation mode characteristics between an optical fiber and an optical waveguide.
An object of the present invention is to provide a simple and nondestructive method for measuring the cutoff wavelength of a single mode optical waveguide.

(Means for Solving the Problems) As shown in FIG. 2, the present invention considers a single mode optical waveguide connected to an optical fiber as a measurement sample, unlike the conventional technology in which only an optical waveguide was used as a measurement sample. It is very different.

The loss value of the optical fiber-attached waveguide shown in FIG. 2 is determined by three loss factors: optical fiber loss, optical waveguide loss, and connection loss between the optical fiber and the optical waveguide. Therefore, by setting only the fundamental mode (HE++ mode) as the mode that propagates in the optical fiber used, the connection loss between the optical fiber and the optical waveguide becomes extremely small, and the power distribution of the propagation mode of the optical waveguide is reflected. Become something.

Furthermore, the loss of the optical waveguide is a practical number dB/cn+
When the loss becomes as low as below, the main cause of loss in the optical waveguide is loss due to the irregular interface between the core and the cladding.

Therefore, the loss in the optical waveguide also becomes sensitive to the power distribution of the propagation mode in the optical waveguide.

In addition, optical fibers propagate only the fundamental mode, so unless a very large curvature is applied, the
The loss of the optical fiber is almost 0 dB even if the optical fiber is
It can be considered as

Therefore, the loss value of the optical fiber-equipped waveguide of the present invention reflects the change in the power distribution of the propagation mode of the optical waveguide, and when the propagation mode changes at that wavelength, such as at the cutoff wavelength. , the loss value of a waveguide with optical fiber shows a large change depending on the cutoff wavelength of the optical waveguide.

As explained above, the basic concept of the present invention is to measure the transmission characteristics by considering the optical fiber and the optical waveguide as one.

That is, the present invention brings one end of an optical fiber into contact with or approaches one end of a single-mode optical waveguide, and sets the propagation mode of the optical fiber to the fundamental mode (HE++ mode) near the cutoff wavelength of the optical waveguide, thereby forming an optical fiber. The wavelength dependence of the loss value of the optical waveguide and the optical waveguide is measured.

(Example) Hereinafter, the present invention will be described in detail based on Examples.

Figure 1 is a diagram for explaining the first embodiment of the present invention, in which 21 is an optical waveguide to be measured, which is a silica-based single mode optical waveguide fabricated by flame deposition and reactive ion etching. It is. The dimensions of the core part 22 are that the cross-sectional structure is a square of 8 x 8 μm, and the relative refractive index difference between the core and the cladding is 0.2.
%, and the cutoff wavelength is 1.08μ... 23
is a bimode optical fiber. In order to completely receive the output light from the optical waveguide 21, a multimode optical fiber 24 for light reception with a core diameter of 50 μm and a relative refractive index difference of 1% is brought into contact with the optical waveguide 21 with its optical axis aligned. Output light from the light-receiving multimode optical fiber 24 is converted into an electrical signal by a light receiver 281, and is input to a lock-in amplifier 283 via an amplifier 282. The white light from the halogen light source 262 is transferred to the spectroscope 2.
61, the light was made into monochromatic light, and then entered into an optical fiber 23 with a relative refractive index difference of 0.2%, a core diameter of 11 μm, a cutoff wavelength of 1.32 μm, and capable of propagating in two modes at the cutoff wavelength of the optical waveguide. . The optical fiber near the input end of the two-mode optical fiber 23 is removed by a higher-order mode removing section 25 which is wound around a cylinder with a diameter of 4 cm, and the optical fiber after passing through the higher-order mode removing section 25 is Changed the mission mode to basic mode only. 24. While measuring the output light from the optical waveguide with the light receiving section consisting of 281 to 283,
After aligning the optical axes of the mode optical fiber 23 and the optical waveguide,
The output light intensity for wavelengths of 0.7 μm to 1.7 μm is 0.01
Measurements were made in μm intervals.

Next, the output light intensity from the two-mode optical fiber 23 was measured to determine the loss characteristics of the optical waveguide with the optical fiber. The results are shown in FIG. As is clear from FIG. 3, the cutoff wavelength λ of the optical waveguide. The loss value changes rapidly just before , and the wavelength dependence of loss 4Ii change becomes smaller on the longer wavelength side than the cutoff wavelength.

Therefore, according to the present invention, the force and off wavelength of an optical waveguide can be measured non-destructively. In FIG. 3, a broken line shows a case where a refractive index matching agent is applied to the contact end surface of the optical fiber and the single mode optical waveguide to eliminate loss depending on minute interface disturbances at the connection end surface of the optical waveguide.

The results show that the cutoff wavelength can be determined more clearly by applying a refractive index matching agent.

The fact that a two-mode optical fiber was used in this example
This is to stabilize the fundamental mode propagating through the optical fiber at a wavelength of 0.7 μm to 1.7 μm, and using a single mode optical fiber does not change the essence of the present invention.

Moreover, from the same point of view, a multi-mode optical fiber may be used, and the essence of the present invention is to limit the mode propagated through the optical fiber to only the fundamental mode.

1 love! l! iλ FIG. 4 is a diagram illustrating a second embodiment of the present invention using wavelength modulation spectroscopy. The major difference from the first embodiment is that the wavelength dependence of the output light intensity is measured by It only takes a few times. In FIG. 4, an optical path modulator 462 is provided near the exit slit in a spectrometer 461, and the output light from the spectrometer 461 is modulated with a width of 50. This light is input into a single mode optical fiber 43 with a cutoff wavelength of 0.95 μm, and after passing through the optical waveguide 4I used in Example 1 and the multimode optical fiber 44 for light reception, the optical signal is received by a light receiver 481. It is converted into an electrical signal.

The electric signal from the light receiver 25481 is sent to the DC/AC separation circuit 4.
83, it is divided into a DC component and an AC component.

The DC component enters the photoreceiver power supply 482 as a control signal, and by adjusting the sensitivity of the photoreceiver, it is made to have a constant DC intensity in the measurement wavelength range.

The alternating current component enters a lock-in amplifier 484, and the wavelength modulation component is detected. As a result, lock-in amplifier 484
The value at will give the wavelength change rate of the loss value to be measured.

FIG. 5 shows the measurement results according to Example 2.

In FIG. 5, λr is the cutoff wavelength of the optical fiber,
■. represents the DC component of the current signal, and Δr represents the AC component of the current signal.

As is clear from FIG. 5, the cutoff wavelength λ of the optical waveguide. The output changes just before. The wavelength characteristic shown in FIG. 5 corresponds to the wavelength differentiation of the loss value, and is a method that can accurately measure loss characteristics with wavelength changes. Note that 0 in Figure 5
．． The change in the vicinity of 9 μm is due to the cutoff wavelength of the single mode optical fiber used.

(Effects of the Invention) As explained above, according to the cutoff wavelength measuring method of the present invention that considers an optical fiber and an optical waveguide as one, it is possible to nondestructively and easily measure the cutoff wavelength of a single mode optical waveguide. Can measure wavelength. Furthermore, the relative refractive index difference of the optical waveguide can be calculated from the measurement of the cutoff wavelength, and the present invention has the advantage of being a non-destructive method for measuring the relative refractive index difference of the optical waveguide.

Furthermore, the splice loss between the optical fiber and the optical waveguide can be calculated from the structure of the optical fiber used and the structure of the optical waveguide, and by subtracting the splice loss from the loss value shown in the broken line shown in Figure 3, it is possible to calculate the splice loss between the optical fiber and the optical waveguide. Another advantage is that the loss value can be determined, and the present invention can be applied to a method for nondestructively measuring the loss characteristics of an optical waveguide.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a measurement system for the cutoff wavelength of an optical waveguide according to Example 1 according to the present invention. FIG. 2 is a measurement sample consisting of an optical fiber and a connected single-mode optical waveguide to be measured by the method of the present invention. 3 is a diagram showing the measurement results of loss characteristics according to Example 1 of the present invention. FIG. 4 is a diagram showing a measurement system for the cutoff wavelength of an optical waveguide according to Example 2 of the present invention. The figure is a diagram showing measurement results according to Example 2 of the present invention. 21... Optical waveguide 22... Core part 23.
...Two-mode optical fiber 24...Multi-mode optical fiber for light reception 25...Higher-order mode removal section 261 of optical fiber...
Spectrometer 262...Halogen light source 263...
・Spectrometer drive unit 27... Optical chopper 281...
Light receiver 282...Amplifier 283...Lock-in amplifier 29...Control and data processing section 41...Optical waveguide 42...Core section 43...
... Single mode optical fiber 44 ... Multimode optical fiber for light reception 461 ... Spectrometer 462 ... Optical path modulator 481 ... Light receiver 482 ... Power source for light receiver 483 ... DC/AC Separation circuit 484...Lock-in amplifier Fig. 2 Fig. 3

Claims (1)

[Claims] 1. One end of an optical fiber is brought into contact with or close to one end of a single mode optical waveguide, and the propagation mode of the optical fiber is changed to the fundamental mode (HE_1_1) near the cutoff wavelength of the optical waveguide.
A method for measuring the cutoff wavelength of a single mode optical waveguide, characterized by measuring the wavelength dependence of a loss value of an optical fiber and an optical waveguide as a single unit. 2. As the optical fiber, use an optical fiber in which two modes propagate at the cutoff wavelength of the optical waveguide, give a large curvature near the input end of the optical fiber by winding, etc., and use a cutback method to create a large curvature. The single mode optical waveguide according to claim 1, characterized in that the propagation mode of the optical fiber after passing through a given portion is set to only the fundamental mode, and the loss value of the optical fiber and the optical waveguide is measured. Cutoff wavelength measurement method. 3. The method for measuring the cutoff wavelength of a single mode optical waveguide according to claim 1, which comprises applying a refractive index matching agent to the contact end surface between the optical fiber and the single mode optical waveguide. 4. The method for measuring the cutoff wavelength of a single mode optical waveguide according to claim 1, wherein wavelength modulation spectroscopy is used as a method for measuring the wavelength dependence of the loss value.