JPH05312676A - Measurement of mode field diameter of core enlarged optical fiber - Google Patents

Abstract

PURPOSE:To non-destructively measure a mode field diameter(MFD) in used wavelength of a core enlarged optical fiber by measuring transmission loss in other wavelength which is more sensitive to transmission loss change due to heating than the used wavelength of a heated optical fiber. CONSTITUTION:Heating time of a TEC fiber is changed and change in MFD in communication use wavelength of 1.3mum. 1.0mum is used as a second wavelength example, so that relation between heating time and transmission loss change at this wavelength is obtained to calculate relation between transmission loss change at a wavelength of 1.0mum and MFD at 1.3mum from relation between heating time and MFD at 1.03mum and relation between heating time and transmission loss change at wavelength of 1.0mum. Transmission loss at wavelength 1.03mum hardly changes as MFD largely changes, while transmission loss also largely varies as MFD changes at 1.03 p m, thereby obtaining high sensitivity. Therefore by measuring transmission loss change at wavelength of 1.0mum, MFD at 1.03mum which is used wavelength can be obtained accurately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a TEC fiber (TE
The present invention relates to a method capable of nondestructively measuring the mode field diameter of a TEC fiber for quantitative production of C: Thermally-diffused Expanded Core).

[0002]

2. Description of the Related Art In transmission of an optical cable for communication using an optical fiber, it is an important subject to reduce the loss at the connection point of the optical fiber. Particularly in the case of a single-mode fiber, a small loss of axis causes a large splice loss, so a low-loss splicing technique is desired. And as a technology to reduce splice loss, TEC fiber [S.Kawaka
mi et. al., "A method to realize fiber-embeded opti
cal devices ", OEC'89 172-173, (1988).] is expected.

The TEC fiber partially heats the optical fiber and diffuses the Ge dopant in the core portion of the optical fiber, thereby expressing a region in which light is guided. (Hereinafter, abbreviated as MFD) is an expanded core expansion fiber. Figure 1
A schematic sectional view of an example of a TEC fiber is shown in FIG. In the figure, reference numeral 1 indicates a normal core portion, and 2 indicates a core expansion portion. By cutting such a fiber at the maximum core enlarged portion as shown in FIG. (B) and applying it to the connecting portion so that this cut surface becomes the connecting surface, the main cause of the connection loss of the single mode fiber is It has been theoretically and experimentally confirmed that the loss due to a certain axis shift can be reduced.

[0004]

As a method of measuring the region in which the light of the TEC fiber is guided, that is, the MFD, conventionally, the optical fiber after heating is cut and the light emitted from the cross section of the optical fiber is cut. A method using an MFD measuring device for measuring MFD by measuring a distant solution has been performed. However, with the above measurement method, it is impossible to estimate the MFD before cutting the optical fiber,
In addition, it is impossible to further heat and process the cut optical fiber. On the other hand, TEC fibers are currently manufactured using an electric furnace or a gas burner. However, when manufacturing TEC fibers by such a method, it is difficult to accurately measure, control, and maintain the heating temperature. It is difficult to quantify the amount of change in MFD from only the heating conditions of temperature and temperature. Therefore, when manufacturing the TEC fiber,
There has been a demand for a method of monitoring and measuring the state of the TEC fiber on the way. The present invention has been made in view of the above circumstances, and provides a method capable of nondestructively measuring the MFD of a TEC fiber in the middle of heating.

[0005]

The method for measuring the MFD of a core-expanded optical fiber according to the present invention is the first method for measuring the core-expanded optical fiber.
A method of measuring MFD at a wavelength of, wherein a first relationship between the heating time of the optical fiber and the MFD at the first wavelength is obtained, and separately, the heating time of the optical fiber and the first A second relationship with a transmission loss at a second wavelength having a higher sensitivity to a change in transmission loss due to heating than a wavelength is obtained, and the first wavelength is based on the first relationship and the second relationship. Of the MFD and the transmission loss at the second wavelength in advance, and a step of measuring the transmission loss at the second wavelength of the core-expanded optical fiber to obtain the third relation. First from the relationship of 3
And a step of obtaining an MFD at the wavelength of 1.

[0006]

The method of the present invention, when the optical fiber is heated,
Although the change in transmission loss due to heating is small at the wavelength used for communication of the TEC fiber, it was found that at wavelengths other than this used wavelength, high sensitivity to change in transmission loss due to heating is obtained, and By measuring the transmission loss at the wavelength,
To obtain the FD.

[0007]

The present invention will be described in detail below. 2 to 4 are flowcharts showing a procedure for measuring MFD using the measuring method of the present invention. Here, the wavelength used for communication of the TEC fiber is the first wavelength in the present invention, and the MFD measurement at this wavelength will be described. First, as a pre-stage of producing a TEC fiber, the relationship between the fiber transmission loss due to heating and the MFD is obtained in advance by the procedure shown in FIGS. 2 and 3. That is, in the procedure as shown in FIG. 2, first, the heating time of the optical fiber and M
Find the relationship with FD. After heating the optical fiber for a certain period of time, it is cut at the center of the heating area. Then, the MFD at the used wavelength of the light emitted from the cut surface is measured using an MFD measuring device. Further, the heating time is changed and the same measurement is performed to obtain the first relationship between the heating time of the TEC fiber and the MFD at the used wavelength.

Separately from this, in the procedure as shown in FIG. 3, the heating time of the optical fiber and the transmission loss at a wavelength other than the used wavelength, which is highly sensitive to the change in transmission loss due to heating, are shown. Seek a relationship. First, with the light incident end and the light emitting end of the heated optical fiber fixed to the variable wavelength light source and power meter, measure the loss characteristics of the optical fiber before heating in a wide range of wavelengths (first step (a)), Next, this optical fiber is heated, and the loss characteristic of the obtained TEC fiber is measured in the same manner (second step (b)). After this,
The difference between the characteristics of the first and second steps is obtained to obtain the amount of change in transmission loss at each wavelength in a wide range. Further, the heating time is changed and the same measurement is performed. From these results, a wavelength (hereinafter, referred to as a second wavelength) that can obtain higher sensitivity than the used wavelength with respect to the change in transmission loss due to heating is arbitrarily selected. Then, the second relationship between the transmission loss at that wavelength and the heating time is obtained. Furthermore, a third relationship between the MFD at the used wavelength and the transmission loss at the second wavelength is obtained based on the first relationship and the second relationship. In the procedure shown in FIG. 3, when the suitable wavelength can be set as the second wavelength without investigating the change of the loss characteristic for a wide range of wavelengths, the first step and the first step In the second step, the measurement of the transmission loss change amount in a wide range of wavelengths can be omitted. Then, in this case, the transmission loss at the second wavelength may be measured to obtain the second relationship between the transmission loss at that wavelength and the heating time.

After such a previous step, in the TEC fiber manufacturing process, as shown in FIG. 4, the Ge-doped single mode optical fiber is heated with the light incident end and the light emitting end fixed. .. Then, the transmission loss at the second wavelength is measured, and the MFD at the used wavelength of the TEC fiber is obtained from the measurement result based on the third relationship obtained in the previous step. Then, heating is performed while repeating measurement and monitoring of the MFD in this manner, whereby a TEC fiber having an arbitrary MFD can be quantitatively manufactured.

(Embodiment) An embodiment of the method of the present invention is shown in FIGS.
8 will be described. The TEC fiber used here is M
Although a single-mode optical fiber having an FD of 9.6 μm, a relative refractive index difference of 0.3%, and a wavelength used for communication of 1.3 μm was heated, similar results were confirmed with other Ge-doped single-mode fibers. The heating time of the TEC fiber was changed, and the change in MFD at 1.3 μm was measured. The result is shown in FIG. The same result can be obtained by making the heating conditions (temperature) equal.

Next, the amount of change in transmission loss over a wide range of wavelengths when the TEC fiber was heated was measured. Further, this measurement was performed while changing the heating time. The result is shown in Figure 6.
It was shown to. From this result, it can be seen that the transmission loss is increased by increasing the MFD to be larger by increasing the heating time. At 1.3 μm, which is considered to be the operating wavelength of this optical fiber, the amount of increase in transmission loss due to heating is relatively small, and the amount of increase in other short wavelength side or long wavelength side is large. Is recognized. From this, it can be seen that high sensitivity to a change in transmission loss due to heating can be obtained by appropriately selecting a wavelength other than the used wavelength as the second wavelength. Therefore, using 1.0 μm as an example of the second wavelength, the relationship between the heating time and the transmission loss change amount at this wavelength was obtained from the result of FIG. 6, and is shown by the solid line in FIG. 7. In FIG. 7, for comparison, the relationship between the heating time and the amount of transmission loss change at the used wavelength of 1.3 μm is shown by a broken line.

Then, the heating time obtained in FIG.
From the relationship with the MFD at the wavelength of 1.0 μm and the relationship between the heating time and the variation of the transmission loss at the wavelength of 1.0 μm obtained in FIG.
The relationship with the MFD at 3 μm was determined and is shown by the solid line in FIG. For comparison, FIG. 8 shows the relationship between the transmission loss change amount at 1.3 μm and the MFD at 1.3 μm by a broken line. As a result, the correspondence between the transmission loss change amount at the wavelength of 1.0 μm and the MFD at the wavelength of 1.3 μm was obtained. Moreover, the transmission loss at the wavelength of 1.3 μm is MF.
Even if D greatly changes, it hardly changes,
At 1.0 μm, the transmission loss significantly changed with respect to the change in MFD, and high sensitivity was obtained. Therefore, wavelength 1.0
By measuring the amount of transmission loss change in μm,
The MFD at the used wavelength of 1.3 μm can be accurately obtained. In this example, 1.0 μm is used as an example of a wavelength other than the used wavelength, but the wavelength is not limited to this,
Any wavelength can be set as long as it has a higher sensitivity than the used wavelength to a change in transmission loss due to heating.
As an example of the measuring method of the present invention, M at the wavelength used
Although the FD measurement has been described, the present invention is not limited to this.
It is possible to measure the MFD at this target wavelength in the same manner by setting an appropriate wavelength as the target wavelength for measurement, that is, the first wavelength.

[0013]

As described above, according to the method of the present invention, it is possible to reduce the loss at the connection point of the optical fiber.
In the production of C fiber, the MFD at the used wavelength of the TEC fiber is accurately measured by measuring the transmission loss at another wavelength that is more sensitive to the transmission loss change due to heating than the used wavelength of the heated optical fiber. can do. Therefore, the MFD can be monitored and measured nondestructively, and the core-expanded optical fiber can be quantitatively manufactured.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of a TEC fiber, and FIG. 1 (B) is a perspective view showing a state during use.

FIG. 2 is a flowchart of a process before the MFD measurement of a TEC fiber.

FIG. 3 is a flow chart of a process before the MFD measurement of the TEC fiber.

FIG. 4 is a flow chart of MFD measurement at the stage of producing a TEC fiber.

FIG. 5 is a graph showing the relationship between the heating time of a TEC fiber and the change in MFD.

FIG. 6 is a graph showing the time when the heating time of the TEC fiber is changed,
It is a graph which shows the amount of transmission loss changes in a wide range of wavelengths.

FIG. 7 is a graph showing a relationship between a heating time of a TEC fiber and a transmission loss change amount.

FIG. 8 is a graph showing the relationship between the amount of transmission loss change of a TEC fiber and MFD.

[Explanation of symbols]

 1 Normal core part of TEC fiber 2 Expanded core part of TEC fiber

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Takeshi Nakajima 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A method for measuring a mode field diameter at a first wavelength of a core-expanded optical fiber in which a core of an optical fiber is partially heated and expanded, the heating time of the optical fiber and the first The first relationship with the mode field diameter at the wavelength is obtained, and separately from this, the heating time of the optical fiber and the transmission at the second wavelength having a higher sensitivity to the transmission loss change due to heating than the first wavelength. A second relationship with the loss is obtained, and a third relationship between the mode field diameter at the first wavelength and the transmission loss at the second wavelength is obtained based on the first relationship and the second relationship. It has a step of obtaining in advance and a step of measuring the transmission loss of the core-expanded optical fiber at the second wavelength and obtaining the mode field diameter at the first wavelength from the third relationship obtained in the step. Method of measuring the mode field diameter of the core expanded fiber characterized and.