JPS5831860B2 - Optical fiber cutoff wavelength measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for detecting single mode operating conditions of a dielectric optical line in an optical transmission system, and particularly relates to an optical fiber suitable for measuring the structure of a dielectric optical line with a minute core diameter. This invention relates to a cutoff wavelength measuring device.

Conventionally, methods for investigating the single mode operating conditions of a dielectric optical path include (1) cutting the optical path into rings, observing the polished thin slices with a transmission interference optical microscope, and determining the refraction of the cross section of the optical path from the interference fringes; (ii) The light is vertically incident on a surface cut perpendicularly to the optical axis of the optical path, and the intensity distribution of the reflected light from the end surface is calculated from the incident point. After determining the refractive index distribution of the optical path cross section by some method, such as calculating the refractive index distribution from the distribution shape as in ([) above, G11) Inject light from one end of the optical path, observe the near-field image or far-field image at the other end, and change the wavelength of the light source to take advantage of the difference in images depending on the mode. There are methods such as finding it from the wavelength at which the image changes.

However, all of these methods are destructive measurements, and have the disadvantage that they cannot be used to measure the structure non-destructively and to correlate it with other measurements.

In view of the above points, the present invention has been made in order to solve such problems and eliminate such drawbacks.The purpose of the present invention is to provide a non-destructive and continuous high-speed optical path in the optical axis direction without cutting the optical path. An object of the present invention is to provide an optical fiber cutoff wavelength measuring device that can accurately measure the structural parameters of an optical line.

In order to achieve such an object, the present invention uses a wavelength tunable light source to measure the structure of a dielectric optical path, and measures the angular distribution of the intensity of scattered light from the optical path and the polarization state of the mode propagating through the optical path. By making the correspondence, TEol, TMol, HE2□ can be obtained from the change point of the polarization degree of scattered light with respect to wavelength
The cut-off wavelength (single mode condition) of the first higher-order mode group of each mode is measured non-destructively and with high precision.Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical fiber cutoff wavelength measuring device according to the present invention.

In the figure, 1 is a wavelength tunable light source, 2 is a light beam emitted from the wavelength tunable light source 1, 3 is a polarizer that makes the light beam 2 linearly polarized, 4 is a polarized beam from the polarizer 3, and 5 is a polarized beam 4.
These lenses constitute an enclosing part that linearly polarizes the light from the wavelength tunable light source and makes it enter the optical path to be measured (hereinafter abbreviated as optical path) 7.

Reference numeral 6 denotes an unnecessary mode removal container for removing unnecessary modes such as cladding modes excited by the optical path 7. This unnecessary mode removal container 6 is filled with a liquid whose refractive index is higher than that of the optical path 7. For example, 2゛lycerin,
Sedan oil etc. are used.

8 is a support pipe, and this support pipe 8 supports the photodetector 9 on both sides so that the optical axis of the optical path 7 does not move during measurement, and the support pipe 8 and the photodetector 9 are integrated. The structure is such that the scattered light intensity can be measured at any position on the optical path.

10 is a matching liquid container, and this matching liquid container 10
This is to prevent the light emitted from the optical path 7 from being reflected at the end, causing measurement errors, and is filled with a liquid whose refractive index is higher than that of the optical path 7, like the unnecessary mode removal container 6. .

A chopper 11 synchronizes incident light and scattered light, and is provided in the optical path of the light beam 2.

12 is a lock-in amplifier that receives the output of the chopper 11 and the output of the photodetector 9; 13 is an arithmetic processing unit that calculates the polarization state of the light propagating through the optical path 7 from the angular distribution of the light intensity;
Reference numeral 14 denotes a display section that displays the correspondence between the polarization state determined by the arithmetic processing section 13 and the wavelength of the wavelength tunable light source 1.

Fig. 2 is an explanatory diagram of the operation of Fig. 1, and shows an outline of the polarization state of the low-order mode of the dielectric optical path, where a shows the HE1□ mode, b shows the TEo1 mode, and C shows the TMo1 mode. , d indicate the first higher-order mode group of the HE2□ mode.

Next, the operation of the embodiment shown in FIG. 1 will be explained with reference to FIG. 2.

First, a light beam 2 emitted from a wavelength tunable light source 1 is passed through a polarizer 3, the light emitted from the polarizer 3 is made into linearly polarized light, this polarized beam 4 is focused by a lens 5, and is made to enter an optical path 7.

At this time, the light receiving part of the photodetector 9 is directed toward the optical axis of the optical path 7 in a direction perpendicular to the optical axis of the optical path 7, and the scattered light intensity P(θ) from the optical path 7 is measured.

Note that a rectangular slit is provided on the surface of this photodetector 9 in order to output an angular component.

Then, this photodetector 9 is rotated in a plane perpendicular to the optical axis of the optical path 7 so that the light receiving section always faces the optical axis of the optical path 7.

Here, the rotation angle of this photodetector 9 from a certain reference position is assumed to be θ.

On the other hand, since the scattered light is weak, the chopper 11 synchronizes the incident light and the scattered light and is interrupted in the optical path of the light beam 2 in order to measure accurately. Taking signals.

In addition, the optical signal from the photodetector 9 is also input to the lock-in amplifier 12, and after synchronous rectification, the electrical signal is amplified, and its output is
It corresponds to the scattered light intensity P(θ).

Then, the rotation angle θ of the photodetector 9 and the scattered light intensity P(/
The relationship 7) is entered into the arithmetic processing unit 13, and the polarization state of the light propagating within the optical path 7 is determined from the angular distribution of the scattered light intensity.

By the way, the modes propagating through the dielectric optical path differ depending on the structure of the optical path and the propagation wavelength.

Therefore, the HE1□ mode, which is the fundamental mode, is not blocked and propagates at any wavelength, and the polarization state of its electric field is linearly polarized as shown in FIG. 12a.

On the other hand, TEol and TMol forming the first higher mode group
.

HM2. Each mode is blocked, and light above a certain wavelength does not propagate, and its electric field component is shown in Figure 2, c.

As shown in d, both are axially symmetrical.

Therefore, when each mode constituting the first higher-order mode group propagates through the optical path 7, the internal polarization state is reflected in the intensity of scattered light outside the optical path.

Here, in the case of HE1□ mode, which is the basic mode shown in FIG. 2a, p (@oc cos2(θ + θ.

), and TEol, TM shown in Figure 2, c and d.
ol, P in the case of the first higher mode group of HE2□ mode
(/ll'l is a constant number.

However, θ0 is a constant that changes depending on how the reference position of the rotation angle θ of the photodetector 9 is taken.

Therefore, it becomes 1 in case of mode, TEol, TMo
1, HE2□ is O in the case of the first higher mode group of each mode.

However, Iw, , Iben77 are the values when the scattering intensity is the maximum value and the minimum value, respectively.

In addition, the reference mode HE1□ mode and TEol, T
When the first higher-order modes of each mode propagate simultaneously, the degree of polarization Pd depends on its excited state,
It takes a value between O and 1.

Since the degree of polarization of the scattered light from the optical path 7 is in such a situation, the arithmetic processing unit 13 detects the peak position from the angular distribution of the scattered light, calculates the degree of polarization Pd, and calculates the degree of polarization Pd.
- Display the Y coordinate of the Y recorder on the display section 14.

Then, the wavelength λ of the wavelength tunable light source 1 is changed, and the cutoff wavelength at the measurement point is determined from the wavelength at which the degree of polarization Pd changes.

Here, the wavelength range in which the optical path is used is usually assumed beforehand when manufacturing the optical path, so for example, in the case of an optical path in the 0.85 μm band, a dye laser pumped by an N2 laser can be used as the wavelength tunable light source 1. Further, as the photodetector 9, a 5i-PIN diode can be used.

In this way, the structural parameters of the optical path can be measured non-destructively and continuously in the optical axis direction with high accuracy without cutting the optical path.

FIG. 3 is a block diagram showing an extracted main part of another embodiment of the present invention, and shows another embodiment of the part related to scattered light intensity measurement.

In the figure, 7 is an optical path, and 9a and 9b are first and second optical paths.
The photodetectors 15a and 15b are rectangular slits.

The two photodetectors 9a and 9b are arranged in a plane perpendicular to the optical axis of the optical path 7, so as to be perpendicular to the optical axis of the optical path 7.

In the device configured in this way, under the single mode condition, as described above, Pθoc cos2(θ+θ.

), there is a maximum value and a minimum value every 90 degrees, so by rotating the first and second photodetectors 9a and 9b around the optical axis, the output from the photodetectors will be the maximum value. The position of the photodetector in the plane perpendicular to the optical axis is fixed at the position where the minimum value is obtained.

Then, the optical path 7 or the first and second photodetectors 9a
, 9b are relatively moved to obtain information in the length direction, the measurement time can be further shortened.

At this time, since two photodetectors are used, it is necessary to calibrate the sensitivity in advance.

As explained above, the present invention uses a wavelength tunable light source to measure the structure of a dielectric optical path, and by correlating the angular distribution of the intensity of scattered light from the optical path with the polarization state of the mode propagating through the optical path, TEol from the change point of the degree of polarization of scattered light with respect to wavelength.

Since the cutoff wavelength of the first higher-order mode group of each TMo□ and HE21 mode is measured, the structural parameters of the optical path can be determined non-destructively and continuously in the optical axis direction with high accuracy without cutting the optical path. It has the advantage of being able to perform measurements.

It is also extremely effective in that it can be used to measure the structure of a dielectric optical path in a non-destructive manner and to correlate it with other measurements.

As described above, the present invention has great effects compared to conventional devices of this type, and can be used as an optical fiber cutoff wavelength measurement device for detecting the single mode operating condition of a dielectric optical line in an optical transmission system. It is unique.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing one embodiment of the optical fiber cutoff wavelength measuring device according to the present invention, Fig. 2 is an explanatory diagram of the operation of Fig. 1, and Fig. 3 extracts the main part of another embodiment of the present invention. FIG. 1... wavelength variable light source, 2... light beam, 3... polarizer, 4... polarized beam,
5... Lens, 6... Unnecessary mode removal container, 7... Light path, 8... Support pipe, 9, 9a, 9b... ...Photodetector, 10...
... Matching liquid container, 11 ... Chopper, 12 ... Lock-in amplifier, 13 ...
- Arithmetic processing unit, 14...Display unit, 15a, 15
b... Rectangular slit.

Claims (1)

[Claims] 1 An optical fiber cutoff wavelength measurement device that measures the cutoff wavelength of the first higher-order mode group of an optical fiber from the point of change of the degree of polarization of scattered light with respect to the wavelength, and is a device that linearly polarizes light from a wavelength tunable light source. an incidence part for making the light incident on the measurement optical path;
means for measuring the angular distribution of scattered light intensity in a plane perpendicular to the optical axis of the optical path at any point on the optical path to be measured; and light propagating through the optical path to be measured based on the angular distribution of the scattered light intensity. An optical fiber cutoff wavelength measuring device comprising: an arithmetic processing section for determining the polarization state of the arithmetic processing section; and a section for displaying the correspondence between the polarization state obtained by the arithmetic processing section and the wavelength of the wavelength tunable light source.