JP3224081B2 - Apparatus and method for measuring connection tolerance of multimode optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system that includes a laser diode (LD) light source having high coherence, uses a multimode optical fiber as a transmission medium, and transmits a high-speed optical signal over a short distance of several km or less. The present invention relates to evaluation of a connection portion between optical fibers in a local high-speed optical LAN and regulation of connector accuracy.

[0002]

2. Description of the Related Art FIG. 6 is a conceptual diagram showing a speckle pattern generated when a multi-mode optical fiber is excited by an LD having a high coherence and a speckle noise generated when a positional shift occurs in a connection portion between optical fibers. .

In FIG. 1, reference numeral 4a denotes an end face of a first multi-mode optical fiber, 4b denotes an end face of a light-emitting core of a first multi-mode optical fiber, and 6a denotes a second multi-mode optical fiber which is displaced from the first optical fiber. An end face of the optical fiber, 6b is an end face of a second multi-mode optical fiber core for receiving light emitted from the first multi-mode optical fiber, and 17 is a speckle pattern. Here, in order to explain speckle noise, both fiber end faces are described with their axes shifted. The speckle pattern is “uneven intensity” that appears on the end face of a fiber when a short-distance multimode optical fiber is excited by light having high coherence. This unevenness in strength, ie, speckle pattern 17
Varies spatially in the optical fiber core 4b with time. At time t1 shown in FIG. 6 as (a) in the figure, the light intensity is concentrated in the central region in the core 4b, but in FIG.
At time t2 shown as, the state is concentrated in the upper region of the core. Due to this phenomenon, as shown in FIG. 6, if there is a positional shift between the fibers, the light intensity transmitted to the second optical fiber fluctuates.

A conventional method for evaluating the tolerance of connection between multimode optical fibers will be described with reference to FIG. FIG. 7 schematically shows the apparatus, in which 4 is a first multi-mode optical fiber, 5 is a fine moving table, 6 is a second multi-mode optical fiber, 1
3 is a laser diode, 14 is a 10 km long dummy fiber, 15 is an averager, and 16 is an optical power meter.

In order to avoid power fluctuation due to speckle noise during tolerance measurement, a dummy fiber 1
4 to reduce the coherency of the light source by transmitting a sufficiently long distance compared to the coherence length of the LD transmission light, and to reduce the fluctuation of the speckle by the vibration given by the averager 15. By performing the conversion process, the specified excitation state is realized at the emission end face of the first multimode optical fiber 4 while avoiding speckle noise. After performing such processing, the received light power transmitted through the first multi-mode optical fiber 4 and the second multi-mode optical fiber 6 is monitored by the optical power meter 16, and an experiment and evaluation of connection tolerance are performed. Do.

[0006]

In recent years, it has been active to introduce in-building high-speed optical LANs using LDs capable of high-speed modulation and multimode optical fibers as transmission media into buildings, campuses, and offices. It is being considered.
In the optical communication transmission system studied so far, long-distance transmission, such as between cities, was used.In tolerance evaluation, the transmission distance in the optical fiber was extended by using a dummy fiber to reduce the coherency of the light source, and Although the tolerance evaluation method with reduced noise was realistic, in the case of short-distance transmission such as on-premises high-speed optical LAN, a transmission length of several tens of kilometers was set for each terminal. The tolerance evaluation method obtained by the conventional measurement method is impractical because of the inability to install the optical device and the bulk type optical component such as the averager 15. Therefore, when the connector accuracy of a multimode optical fiber specified based on the tolerance measurement result by the conventional measurement method is applied to a short-distance transmission system, even if the misalignment is sufficiently smaller than that of the core system, the transmission quality can be improved. This has led to the need for new connector tolerance standards and evaluation methods.

An object of the present invention is to provide a highly practical evaluation of various multimode optical fiber connection tolerances in a short distance in consideration of speckle noise generated by a light source having high coherency such as a laser diode (LD). It is an object of the present invention to provide a multimode optical fiber connection tolerance measuring apparatus and a measuring method which can specify connector fitting accuracy in various multimode optical fibers.

[0008]

According to a first aspect of the present invention, there is provided an optical signal having a narrow line width and a single spectrum wavelength modulated by a pulse waveform generator having an arbitrary frequency. The generated light source and a fine moving table that can arbitrarily change the relative positions of the connection end faces facing each other at one end of each of the first and second multimode optical fibers of the at least two multimode optical fibers to be measured. A variable optical attenuator provided between the other end of the first multimode optical fiber and one end of the single mode optical fiber; and an isolator provided between the other end of the single mode optical fiber and the light source. Receiving an output signal from the other end of the second multimode optical fiber, measuring and displaying a signal waveform,
Provided is a multimode optical fiber connection tolerance measuring device, comprising a transmission waveform measuring device for measuring a signal-to-noise characteristic. According to a second aspect of the present invention, the connection end face between the first multimode optical fiber and the second multimode optical fiber is set so that the output signal from the second multimode optical fiber has the maximum amplitude by the transmission waveform measuring device. The relative positional relationship is adjusted by a fine adjustment table, and the maximum position is set to 0 for the relative position shift between the first and second multimode optical fibers, and it is necessary to obtain a constant signal-to-noise ratio at this time. A first measurement procedure for obtaining an average light receiving level, and thereafter, a fine adjustment table is used to arbitrarily shift the relative positions between the multi-mode optical fibers to obtain the constant signal-to-noise ratio for each amount of shift. And a second measurement procedure for obtaining an average received light level required for each of the positional deviation amounts and an increase in the positional deviation amount and the average received light level increase amount based on the required average received light level when there is no positional deviation. From relationship, constant And a third measurement procedure to determine the connection misalignment acceptable for equalizing the received light level increment, providing the connection tolerance measurement method of the multi-mode optical fiber. According to the present invention, it is possible to measure the positional deviation tolerance between optical fibers in consideration of the effect of speckle noise generated when a signal is transmitted from a fiber with a light source having high coherency. This makes it possible to evaluate practically high tolerance at an optical fiber connection part in a transmission system using a short-distance multimode optical fiber.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below, but the present invention is not limited to these examples.

FIG. 1 is a view for explaining a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a DFB (Ditributed Feed Back) laser having a 0.1 nm half-width single spectrum,
2 is a single mode fiber, 3 is a variable optical attenuator, 4 is a first multimode optical fiber having a length of 1 m (for example, 80 μm
Core type step index type fiber), 5 is a fine moving table, 6 is a second multimode optical fiber having a length of 1 m, 7 is a broadband optical / electrical signal converter, 8 is an oscilloscope, 9 is D
FB-LD modulation signal transmission coaxial cable, 10 is a trigger signal transmission coaxial cable, 11 is a pulse waveform generator,
12 is an isolator.

When a high-frequency signal (for example, 1 Gbps, NRZ signal) generated by the pulse waveform generator 11 is sent to the DFB laser 1 via the coaxial cable 9, the DFB laser 1
The modulated optical signal is emitted from The emitted optical signal passes through an isolator (for example, 30 dB return loss) 12, is optically coupled into the single mode fiber 2, and transmitted, and is transmitted through the variable optical attenuator 3 to the first multimode optical fiber 4. Is transmitted. The output end face of the first multimode optical fiber 4 and the input end face of the second multimode optical fiber 6 have a relative positional relationship. The second multimode optical fiber 6 receives and transmits the signal light from the first optical fiber 4. The signal light transmitted through the second multi-mode optical fiber 6 is converted into an electric signal by the broadband optical / electrical signal converter 7 and then received by the oscilloscope 8 to observe the waveform.

At this time, the first optical fiber 4 and the second optical fiber 6 are aligned with the fine moving table 5 so that the oscilloscope 8 maximizes the waveform amplitude. It is assumed that the relative displacement between the first optical fiber and the second optical fiber is 0 μm (the displacement is 0 μm).
m).

After the setting, the input optical power is changed by adjusting the variable optical attenuator 3 to obtain the relationship between the signal-to-noise ratio and the average light receiving level. FIG. 2 shows an example of the result.

Next, an arbitrary displacement amount between the optical fibers is obtained.

For example, the relationship between the signal-to-noise ratio and the average light receiving level is changed in the direction perpendicular to the optical axis (light propagation direction) by 5 μm at the time of setting each position shift amount in the same manner as described above. Ask. FIG. 3 shows an example of the result when the position is shifted by 5 μm in the direction perpendicular to the optical axis, and FIG. 4 shows an example of the result when the position is shifted by 10 μm in the direction perpendicular to the optical axis.

3 and 4, as the amount of displacement increases,
It can be seen that the variation of the signal-to-noise ratio occurs at a high average received light level.

This is due to an increase in speckle noise due to the displacement.

From these results, at each offset,
It can be seen that the average received light level needs to be increased to obtain a constant signal-to-noise ratio.

This result means not only a decrease in the received light power transmitted to the second optical fiber due to the displacement, but also a substantial decrease in the received light power caused by speckle noise. Would mean.

Thus, when the displacement amount is 0 μm, the constant signal-to-noise ratio (here, the bit error rate = 10 ⁻¹⁰⁾
Is assumed. ),
FIG. 5 shows a graph of the relationship between the coupling loss including the speckle noise and the amount of displacement. In FIG.
The relationship between the displacement amount and the coupling loss in the conventional method is also described. As shown in the figure, the tolerance for the positional deviation when the level down of 1 dB is allowed is ± 13 μm in the conventional measurement method.
m, whereas in the measurement method according to the present invention, it is ± 7 μm due to speckle noise.

As a result, it can be seen that the positional deviation tolerance is stricter by about half as compared with the conventional tolerance measuring method. From this result, it can be seen that this measurement method including speckle noise is effective for evaluating the tolerance of the connection portion.

[0022]

As described above, according to the present invention, a short-distance multimode optical fiber transmission system using a laser having high coherency as a light source (for example, a high-speed optical LAN in a premises).
In the optical fiber connection section described above, it is possible to perform a practically useful connection tolerance evaluation including speckle noise, and to make use of this evaluation result in the precision specification of an optical connector used for connection between optical fibers.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a measurement system according to an embodiment of the present invention.

FIG. 2 shows a positional deviation amount of 0 μm measured by a measurement system.
Diagram showing the relationship between the average received light level and the signal-to-noise ratio at the time

FIG. 3 shows the amount of displacement = 5 μm measured by a measuring system.
Diagram showing the relationship between the average received light level and the signal-to-noise ratio at the time

FIG. 4 shows the amount of displacement = 10 μ measured by a measurement system.
Diagram showing the relationship between the average received light level at m and the signal-to-noise ratio

FIG. 5 is a diagram illustrating a relationship between a positional shift amount and a coupling loss in consideration of speckle noise, measured by a measurement system.

FIG. 6 is an explanatory diagram of speckle noise.

FIG. 7 is an explanatory diagram of a measurement system according to the related art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DFB (Distributed Feed Back) laser, 2 ... Single mode fiber, 3 ... Variable optical attenuator, 4 ... 1st multimode optical fiber, 4a ... 1st multimode optical fiber end face, 4b ... 1st multimode End face of light emitting core of mode optical fiber, 5: fine adjustment table, 6: second multimode optical fiber, 6a
... second position shifted with respect to first multimode optical fiber
The end face of the multimode optical fiber, 6b ... the end face of the core of the second multimode optical fiber which receives the light emitted from the first multimode optical fiber core, 7 ... the broadband optical / electric signal converter,
8 oscilloscope, 9 coaxial cable for transmitting a modulation signal, 10 coaxial cable for transmitting a trigger signal, 11 pulse generator, 12 isolator,

────────────────────────────────────────────────── ─── Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01M 11/00-11/08 G02B 6/24-6/42 G02B 26/00-26/08 JICST file ( JOIS)

Claims (2)

(57) [Claims] 1. A light source for generating an optical signal having a single line width of a narrow line width modulated by a pulse waveform generator having an arbitrary frequency, and a light source of at least two multimode optical fibers to be measured. A fine moving table that can arbitrarily change the relative positions of the connection end faces facing each other on one end side of the first and second multimode optical fibers; and the other end of the first multimode optical fiber and the single mode optical fiber. A variable optical attenuator provided between one end and an isolator provided between the other end of the single mode optical fiber and a light source; and an output signal from the other end of the second multimode optical fiber. And a transmission waveform measuring device for measuring and displaying a signal waveform and measuring a signal-to-noise characteristic. 2. A transmission waveform measuring apparatus, wherein an output signal from a second multimode optical fiber has a maximum amplitude.
The relative positional relationship between the connection end faces of the first multi-mode optical fiber and the second multi-mode optical fiber is aligned using a fine adjustment table, and the maximum position is determined as the relative positional deviation between the first and second multi-mode optical fibers. Is set to 0, a first measurement procedure for obtaining an average light receiving level required to obtain a constant signal-to-noise ratio at this time, and thereafter, the relative position is arbitrarily shifted between the multimode optical fibers by a fine adjustment table. A second measurement procedure for obtaining an average light receiving level required to obtain the constant signal-to-noise ratio for each position shift amount; and a required average light reception when there is no position shift for each position shift amount. A third measurement procedure for determining a connection position deviation that is allowable for a constant average received light level increase amount from a relationship between the positional shift amount increase and the average received light level increase amount based on the level. Multimodal Connection tolerance measurement method mode optical fiber.