JPH1048094A - Bending loss evaluation method and device for optical fiber for amplification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily evaluate the bending loss, by selecting a prescribed number of light of wavelengths before and after the signal light, simultaneously emitting the light to an optical fiber for amplification, and evaluating the extent of loss on the basis of the power difference of the light after passing through the optical fiber. SOLUTION: In the evaluation of the extent of the bending loss, the light sources L1 , L2 are simultaneously lighted, the light from the light source L1 is chopped by a light chopper Cp1 , to be guided to a wave synthesizer WDM. The light from the light source L2 is attenuated by an optical attenuator ATT to the light-intensity almost agreed with that from the light source L1 , and chopped by a light chopper CP2 , to be guided to the wave synthesizer WDM. The light of which the wavelengths are synthetized by the wave synthesizer WDM, is simultaneously entered into an optical fiber for amplification DF, and the light of both wavelengths having passed through the optical fiber, is entered into a power meter PM. The power meter PM measures the power of the light of each wavelength, to determine the difference therebetween. The power difference is almost zero, when the bending loss is not generated, but the power difference is increased, when the loss is generated. Accordingly the extent of the bending loss can be evaluated on the basis of the power difference.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method and apparatus for evaluating a bending loss in an amplifying optical fiber for directly amplifying a signal light using a stimulated emission effect.

[0002]

2. Description of the Related Art In general, some optical amplifiers use an amplifying optical fiber for directly amplifying signal light by utilizing the stimulated emission effect as an amplifying element.

That is, this amplification optical fiber is one in which a core is doped with a rare earth element such as Er, Nd, etc., and the signal light is pumped by the pumping light so that the signal light is in an inverted distribution state. When it is incident,
This is directly amplified by the stimulated emission effect while propagating through the amplification optical fiber.

When an optical amplifier is constructed using such an amplification optical fiber, a certain length (several meters to several tens of meters) is required for the amplification optical fiber in order to obtain a required gain. Becomes Moreover, in this case, in order to prevent the apparatus from being enlarged unnecessarily, it is necessary to wind the amplification optical fiber around a reel or the like and store it compactly in a case.

[0005] Here, when the amplification optical fibers are wound around a reel, if the optical fibers are overlapped or uneven side pressure is applied, a loss due to so-called microbending or the like occurs, and the desired amplification occurs. Characteristics may not be obtained. Therefore, before shipping the product, it is necessary to evaluate in advance whether an excessive loss has occurred while the amplification optical fiber is wound on the reel.

As a method for evaluating the loss of an amplification optical fiber, a so-called cutback method is applied in the prior art.

[0007] The cut-back method, as shown in FIG. 3 (a), first, the coupling optical fiber CF ₀ at one end of the amplification optical fiber DF which is the reel winding like connected by fusion or the like, they optical fiber DF, a light source L ₀ and a power meter PM ₀ before and after the CF is arranged. Then, the light from the light source L ₀ through the coupling optical fibers CF is incident on the amplifying optical fiber DF, measuring the power P ₁ of the light from the exit end with a power meter PM _0.

Next, as shown in FIG. 1B, one end of the connection side of the amplification optical fiber DF with the coupling optical fiber CF is cut to remove the amplification optical fiber DF, and the cut end face is cut. placing the power meter PM ₀ to face. The light coupling optical fiber C from a light source L ₀
The light enters the power meter PM ₀ via F, and its power P ₂ is measured.

The difference Δ between the two powers P ₁ and P ₂ thus obtained is
The loss of the amplification optical fiber DF is determined from P (= P ₂ −P ₁ ). The reason for using the cutback method is that the coupling loss between the light incident end of the measured fiber and the light source is not clear.

[0010]

However, in the loss evaluation method using the conventional cutback method as described above, in determining the power difference ΔP for evaluating the loss,
The amplifying optical fiber DF after once cut, together with the reshaping such as by polishing the cut end surface, the work such as alignment is performed so that the optical axis coincides with the light emitting end and a power meter PM ₀ This necessitates a total of two measurements for the loss evaluation and a troublesome pre-process. Further, when spectral evaluation of the fiber is to be performed, the wavelength sweeping time of the spectroscope needs to be doubled.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to make it possible to more easily evaluate the degree of bending loss caused by microbending or the like of an amplification optical fiber as compared with the related art. And

[0012]

It is known that, when a bending loss is given to an amplification optical fiber by bending the optical fiber, light on the long wavelength side shows a remarkable increase in loss (for example, D.A.
Marcuse, "Curvature Loss Formula for Optical Fiber
s ", J. Opt. Soc. Am., 66, p. 216) In addition, the amplifying optical fiber has a wavelength region where absorption is large for a doped element in a discrete manner. It is also known that there are wavelength bands with low absorption (for example,
Yoshida et al., "Rare-earth element doped fibers and their applications", IEICE Technical Report OQE87-12 (1987)).

The present invention has been made in view of such a fact. In order to solve the above-mentioned problems, a method for evaluating a bending loss of an amplification optical fiber according to the first aspect of the present invention uses a stimulated emission effect. For the amplification optical fiber that directly amplifies the signal light by using, so as to sandwich the wavelength of the signal light,
Two lights included in the wavelength region where the light absorption before and after the light is small are selected, each of the selected lights is simultaneously input into the amplification optical fiber, and the power of each light passing through the amplification optical fiber is respectively determined. Measurement is made and the degree of bending loss is evaluated from the difference between the two powers.

Further, the apparatus for evaluating bending loss of an optical fiber for amplification according to the second aspect of the present invention generates light included in a wavelength range where light absorption is small before and after the signal light so as to sandwich the wavelength of the signal light. Two light sources, and an incident unit for simultaneously inputting light from each of these light sources to the amplification optical fiber,
Photometric means is provided for measuring the power of each light passing through the amplification optical fiber and obtaining the degree of bending loss from the difference between the two powers.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In this embodiment, a case will be described in which the loss of a silica-based amplification optical fiber doped with Er (erbium) in a core is evaluated.

First, the principle of the method of the present invention will be described.

FIG. 1 shows the results of investigation of the spectral loss characteristics of the Er-doped amplification optical fiber of the present embodiment.

That is, FIG. 1 shows the result of examining the loss when white light is spectrally scanned and wavelength scanning is performed, and monochromatic light of each wavelength is incident on the amplification optical fiber. As can be seen from FIG. Centering around each wavelength of 0.98 μm and 1.55 μm
There are places where the absorption specific to Er is large. As the signal light to be amplified by the amplifying optical fiber, light in the 1.55 μm band where a silica-based minimum loss is obtained is usually used, and thus overlaps with a wavelength band having a large absorption. In addition, there is a wavelength region where absorption is small between wavelength regions where absorption is large. That is, 1.55 μm which is the wavelength of the signal light
Focusing on the wavelength, 1.07 to 1.2 on the short wavelength side.
1 μm, and 1.72 μm or more on the long wavelength side. In particular, on the short wavelength side, absorption is extremely low around 1.19 μm.

On the other hand, in an amplification optical fiber having a spectral loss characteristic as shown in FIG. 1, if a loss is generated by intentionally bending the light, light on the long wavelength side rather than on the short wavelength side causes a remarkable increase in loss. Shown in

Therefore, as light incident on the amplifying optical fiber, 1.07 to 1.21 μm (preferably 1.1) having less absorption on the shorter wavelength side than the signal light wavelength λ ₀ = 1.55 μm.
9 .mu.m), or 1.72μm on the long wavelength side (preferably, the wavelength lambda ₁ of 1.75 [mu] m) in consideration of the sensitivity or the like of the light source or the detector, selects the lambda ₂ light, each of these were selected Wavelength λ ₁ ,
λ ₂ light is simultaneously incident on the amplification optical fiber, and the power P of the light of each wavelength λ ₁ and λ ₂ that has passed through the amplification optical fiber.
₁ and P ₂ are measured, and the difference ΔP between the two powers P ₁ and P ₂ is measured.
When (= P ₁ −P ₂ ) is obtained, if there is an increase in loss due to microbending or the like, ΔP also increases at the same time, and the degree of bending loss can be evaluated from this.

FIG. 2 is a block diagram showing a bending loss evaluation apparatus for carrying out the method of the present invention.

[0022] In the figure, DF wavelength lambda ₁ in the amplification optical fiber of a silica-based doped with Er in the core, L ₁ is than the wavelength lambda ₀ of the signal light (= 1.55 .mu.m) to the short wavelength side ( = 1.
A first light source L ₂ for generating light of 19 μm) is a _second light source for generating light of wavelength λ ₂ (= 1.75 μm) which is longer than the wavelength λ _{0 of the} signal light, and both light sources L ₁ as the L _2, SLD (super luminescent photodiode) is applied to generate e.g. incoherent steady light.

The ATT is an optical attenuator for adjusting the light from the two light sources L ₁ and L ₂ so as to make the incident intensity on the amplifying optical fiber DF equal. If such an optical attenuator ATT is provided. The measurement is easier, but can be omitted.

[0024] CP _1, CP ₂ in the optical chopper for chopping with at different frequencies of light from the light sources L _1, L _2, in this embodiment, the chopping frequency f ₁ with respect to the light from the first light source L ₁ is 520Hz, the chopping frequency f ₂ with respect to light from the second light source L ₂ are respectively set to 270 Hz. Chopping may be a method of directly modulating the light emitting element itself. Further, the chopping frequency is not limited to this frequency.

WDM is a multiplexer as an incidence means for simultaneously inputting the light from each of the light sources L ₁ and L ₂ to the amplification optical fiber DF, and CF is the light from the multiplexer WDM. Optical fiber for coupling.

Reference numeral PM denotes a power meter as photometric means for measuring the power of light from each of the light sources L ₁ and L ₂ having passed through the amplification optical fiber DF.
Is configured to include a light receiving element such as a photomultiplier or a photodiode, a lock-in amplifier, a recorder, and the like (all not shown).

In this configuration, the amplification optical fiber DF
When evaluating the degree of bending loss of each light source L ₁ ,
At the same time to light the L _2. The light of the wavelength λ ₁ (= 1.19 μm) from the first light source L ₁ is chopped by the optical chopper CP ₁ to a chopping frequency f ₁ = 520 Hz, and is guided to the multiplexer WDM. On the other hand, the light of the wavelength λ ₂ (= 1.75 μm) from the second light source L ₂ is attenuated by the optical attenuator ATT so as to substantially match the light intensity from the first light source L ₁ , and furthermore, the light chopper CP ₂ by chopping frequency f _{2 = 2}
It is chopped to 70 Hz and guided to the multiplexer WDM. Therefore, both wavelengths λ ₁ and λ ₂ multiplexed by the multiplexer WDM
Are simultaneously incident on the amplification optical fiber DF via the coupling optical fiber CF.

Each wavelength λ that has passed through the amplification optical fiber DF
_Since the lights ₁ and λ ₂ are incident on the power meter PM, the powers P ₁ and P ₂ of the lights having the wavelengths λ ₁ and λ ₂ are measured by the power meter PM. Then, the difference ΔP (= P ₁ −) between the two powers
P ₂ ) is obtained.

Here, when bending loss due to microbending or the like hardly occurs in the amplification optical fiber DF, the difference ΔP between the two powers is substantially zero, as can be seen from the spectral loss characteristics shown in FIG. . On the other hand, when bending loss occurs, the loss of light of wavelength λ ₁ on the short wavelength side increases little while the loss of light of wavelength λ ₂ on the long wavelength side increases. its power P ₂ is reduced, resulting, [Delta] P becomes large. Therefore, the degree of bending loss can be evaluated from ΔP. By pre-evaluating ΔP in a state where no bending loss occurs, it is possible to detect the occurrence of the bending loss more accurately and with high sensitivity.

In the above embodiment, the case of evaluating the loss of the silica-based amplification optical fiber DF doped with Er in the core has been described. However, the present invention is not limited to this, and Nd, Yb, etc. The present invention can also be applied to an amplification optical fiber doped with or alone.

Further, as in the above embodiment, the low wavelength region of light absorption before and after sandwiching the wavelength lambda ₀ of the signal light (wavelength:
It is preferable to evaluate the degree of bending loss using λ ₁ , λ ₂ ) because accurate results can be obtained except for the influence of drift of the measurement system, etc., but bending loss mainly occurs on the long wavelength side From the light on the longer wavelength side than the wavelength λ _{0 of the} signal light (wavelength:
By measuring only the power of λ ₂ ) with a power meter, the degree of loss can be evaluated.

Furthermore, in this embodiment, the optical choppers CP ₁ and CP ₂ are provided so that the power of two wavelengths λ ₁ and λ ₂ can be simultaneously checked on the power meter PM side. By providing an optical shutter instead of the choppers CP ₁ and CP ₂ , the power of the light of each wavelength λ ₁ and λ ₂ can be measured in a time-division manner.

[0033]

According to the present invention, in order to measure the loss of the amplification optical fiber as in the conventional cutback method,
Since it is not necessary to cut the amplification optical fiber once and then perform the measurement again, the labor of the measurement is greatly reduced.
Therefore, the degree of bending loss due to micro-bending of the amplification optical fiber can be more easily evaluated than before.

[Brief description of the drawings]

FIG. 1 is a diagram showing a spectral loss characteristic of an amplification optical fiber doped with Er.

FIG. 2 is a configuration diagram of an apparatus for evaluating bending loss of an amplification optical fiber according to an embodiment of the present invention.

FIG. 3 is a diagram for explaining a procedure for measuring a loss of an amplification optical fiber by a conventional cutback method.

[Explanation of symbols]

DF ... amplification optical fiber, L _1, L ₂ ... light source, ATT ... optical attenuator, CP _1, CP ₂ ... optical chopper, WDM ... multiplexer
(Incident means), PM ... power meter (photometric means).

Claims (2)

[Claims] 1. An amplifying optical fiber for directly amplifying a signal light by utilizing the stimulated emission effect, wherein light included in a wavelength range where light absorption before and after the signal light is small is sandwiched so as to sandwich the wavelength of the signal light. One of these is selected, and each of these lights is simultaneously incident on the amplification optical fiber, and the power of each light passing through the amplification optical fiber is measured, and the degree of bending loss is evaluated from the difference between the two powers. A method for evaluating bending loss of an amplification optical fiber, characterized in that: 2. An apparatus for evaluating bending loss of an amplifying optical fiber for directly amplifying a signal light by using a stimulated emission effect, wherein the light before and after the signal light is interposed so as to sandwich the wavelength of the signal light. Two light sources that generate light contained in a wavelength region with little absorption; an incident means for simultaneously inputting light from each of these light sources to the amplification optical fiber; and a light source for each light that has passed through the amplification optical fiber. An optical fiber bending loss evaluation device, comprising: photometric means for measuring powers and obtaining a degree of bending loss from a difference between the two powers.