JP3142028B2 - Optical attenuator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an industrial field in which communication and sensing are performed by using signal transmission using an optical fiber. The present invention relates to an optical attenuator for reducing signal power.

[0002]

2. Description of the Related Art Conventionally, various types of optical attenuators for an optical fiber transmission system have been proposed and already manufactured and sold as products, but their operating principles can be roughly classified into two types. In the first type, an element component for attenuating optical power is inserted between an input fiber and an output fiber. In this case, a component for optically coupling the attenuating component and the input / output fiber is usually required.However, this optical coupling is assumed to be close to an ideal coupling so as to minimize coupling loss, and a predetermined Usually, the light attenuation is generated only by the attenuation element components.

FIG. 3 is a sectional view of an example of a typical optical attenuator having such a configuration, taken along a center plane passing through a fiber. In this example, an ND filter 12 is used as an attenuating element component, and short optical fibers 3 and 3 'are used for optical coupling between input / output fibers 1 and 2 connected to the outside and this ND filter. However, in many cases, a lens system is used as an optical coupling element component. In this example, the input / output fibers 1, 2 and the short fibers 3, 3 'are centered in the ferrules 4, 5, 6, 6', respectively, in order to enable mechanical attachment / detachment of the external input / output fibers. Retained, alignment of 1 and 3, 2 and 3 ', 3 and 3' is achieved by sleeves 8, 9 ', 9 which respectively mate with ferrules.
In this example, sleeve 8 does not belong to the optical attenuator, but to the external connector. Optical attenuator does not belong to that thing,
The sleeve 8, the input / output fibers 1, 2, and the ferrules 4, 5, which are components connected from the outside, are conceptually shown as being connected to the optical attenuator by broken lines in FIG.

The input / output fibers 1 and 2 and the short fiber 3 are connected by these ferrule-sleeving fitting mechanisms.
And the connection between the ND filter 12 and the short fibers 3 and 3 'is made low loss. Reference numeral 7 denotes a housing provided with a male (plug) on the side 3 and a female (receptacle) fastening mechanism on the side 3 ';
Is a male side coupling nut, and 10 is a coil spring for holding an internal structure such as a short fiber or an ND filter in a housing as a floating structure.

The second class does not use independent damping element parts,
An optical coupling system with an input / output fiber is intentionally made to have a large coupling loss, and a predetermined optical attenuation is obtained by itself. FIG. 4 is a cross-sectional view of a representative example of an optical attenuator having such a configuration, taken along a center plane passing through a fiber.
In this example, the short fibers 3 and 3 'for coupling are used between the external input fiber 1 and the external output fiber 2 as in the example shown in FIG. 3, but the short fibers face each other. A gap is provided between the coupling end faces so that coupling loss is intentionally generated. Other parts are the same as those in the example shown in FIG.

[0006]

However, the attenuating element component conventionally used in the first type of construction is not made especially in consideration of the matching with the propagation mode of the optical fiber, so that By inserting this between the input and output, a coupling loss necessarily occurs. This coupling loss is different from the inherent attenuating property of ND filters and the like. When input light passes through the attenuating element and recombines with the output fiber, it does not couple to the propagation mode, but to the radiation mode and the leakage mode. This is caused by coupling to the mode.

In the example shown in FIG. 3, since the ND filter 12 does not have a waveguide-like structure, the light beam emitted from the end face of the short fiber on the input side spreads according to the NA of the optical fiber during propagation of the ND filter. Diffuse at the corners,
At the time of incidence on the other short fiber, the beam diameter is larger than the mode field diameter of the incident short fiber, and in addition to the component coupled to the short fiber propagation mode, the component coupled to the radiation mode and the leaky mode Occurs. I inserted two infinite lenses in this part,
Even if a so-called collimator lens system is used, ND
It is the same that a greater or lesser coupling loss occurs due to a refraction effect on the filter surface, lens aberration, or an imaging error of the imaging system. Therefore, the amount of attenuation by such an optical attenuator is the sum of the amount of attenuation inherent to the attenuating element and the coupling loss caused by coupling to a mode other than the propagation mode of the output fiber (when the amount of attenuation is measured in dB). However, since the former is a value specific to an attenuation element component such as an ND filter, it is relatively easy to design and manufacture a predetermined value.

However, since the latter coupling loss is governed by the distribution ratio to various modes at the time of fiber recombination, it is determined by the recombination conditions of transmitted light, and is not an amount that can be controlled as an intrinsic amount of element parts. The conditions for recombination of the transmitted light depend on many factors such as the structural parameters such as the NA and mode field diameter of the input and output fibers, the optical power distribution to various modes of the input fiber, the wavelength of the input light, and the relative position of both fibers. It changes depending on the input mode excitation conditions during use, the environmental temperature, the connection conditions of the output fiber to the optical attenuator, and the like, as well as being difficult to adjust to a predetermined value during manufacturing. For this reason, it is difficult to stably maintain a constant attenuation, and it is desirable that the recombination loss after passing through such an attenuation element component is suppressed as much as possible, and the attenuation is determined by an attenuation rate specific to the element component.

[0009] In a product which is often used and has a connector fastening mechanism at both ends and is formed in a compact size, an unstable factor is further added. In such products, as shown in the example of FIG. 3, there are many configurations in which short fibers 3 and 3 ′ are used for optical coupling on the input and output sides. It is inserted in order to stabilize the optical coupling conditions between the input / output fibers 1 and 2 and the attenuating element component 12 which are attached and detached. The coupling between the fiber and the attenuating element component is performed between a non-detachable short fiber, thereby suppressing the above-described fluctuation in the coupling loss.

The mechanical attachment / detachment is performed between the short fiber and the input / output fiber. For the coupling between optical fibers that have modal consistency with each other, optical connector parts that are relatively stable in mechanical attachment and detachment have already been obtained.
This is used. However, in a short fiber, the leaky mode and the radiation mode also propagate through the cladding, and the optical power that should have been lost without much loss reaches the other end.

In the configuration as shown in FIG. 3, the length of the short fiber is about 10 mm or so, and loss of the mode propagating in the cladding can hardly be expected. If the coupling between the output short fiber and the external output fiber is ideal, the propagation mode of the short fiber core will be the propagation mode of the output fiber core, and the light in the cladding will leak or leak in the cladding of the output fiber. Portions that couple to the radiation mode and do not couple to the core-propagating mode when entering the short fiber, undergo attenuation in the normally long output fiber cladding and are eventually lost.

However, the coupling between the short fiber and the output fiber is generally not ideal, and some mode conversion occurs at this coupling portion. In this case, only the part where the short core mode is converted to the output side leakage or radiation mode and becomes a loss is noticed by the connector,
In fact, the opposite may occur, that is, the short-side leakage or radiation mode may enter the output-side propagation mode, and the apparently lost light in the cladding may appear again as output without being lost. Since the coupling between the short fiber and the output fiber is made mechanically detachable, it is difficult to keep the coupling condition strictly constant, and it is difficult to avoid the mode conversion coefficient from changing each time the coupling is detached. As a result, the output power also fluctuates, and it is not easy to obtain a stable attenuation.

The situation is further exacerbated by the second configuration. In this configuration, there is no part determined by the attenuation amount inherent to the component parts, and all the attenuation amount is given only by the coupling loss of the coupling system inserted between the input and output fibers. Since the coupling loss itself, which is inherently unstable and should be suppressed as much as possible, is actively used, the amount of attenuation is greatly affected by the change in the factors that determine the coupling loss as described above, and the stable It is increasingly difficult to maintain a high attenuation. In particular, when a short fiber is used as the coupling element, most of the power lost when coupling to the short fiber on the output side enters the leaky mode or the radiation mode of the short fiber, and propagates through the cladding to a considerable extent as described above. Reaches the other end without loss.

For example, in the example shown in FIG. 4, the length of the gap between the short fibers 3 and 3 'required to obtain the attenuation from several dB to about 20 dB as required in actual applications is as follows. The single-mode fiber is at most 1 mm or less, the far field diameter of the light beam emitted from one short fiber at the other short fiber end face is only about 100 μm, and as long as a normal 125 μm diameter fiber is used, all It enters into the fiber. Of these, the component entering the cladding hardly attenuates in the short fiber of about 10 mm as described above, and appears at the other end of the short fiber. If the set attenuation of the attenuator is large (for example, 10 dB or more), the portion that has undergone the original attenuation, that is, the energy that has entered the short fiber propagation mode may even be smaller than the portion that propagates through the cladding.

In such a case, the amount of light coming out of the short fiber clad by coupling to the output fiber propagation mode by the mode conversion at the connector coupling part with the output fiber only passes through the original core. Not negligible for the amount coming out. In particular, when the fiber used is a single mode fiber, the number of modes is only two degenerate fundamental modes as propagation modes, whereas there are innumerable leakage modes or radiation modes.
It is almost impossible to configure the attenuator to couple only to the short fiber propagation mode, and relatively large light energy will be coupled to the leakage or radiation mode. The contribution of the component via the short fiber cladding is larger than that of a multimode fiber in which the balance with the number of radiation modes is not so deviated.

For example, when the attenuation due to the coupling loss is set to 15 dB, the optical power that propagates through the core of the short fiber and emerges in the propagation mode of the output fiber is [(−15 dB) − (Coupling loss at the connector part)], but the coupling loss at the normal connector part can be suppressed to 1 dB or less even in the case of a single mode fiber or a fluctuation due to mechanical attachment / detachment. Therefore, this value is -15 dB to -16 dB. It will be near.

On the other hand, the optical power that enters the short fiber clad and propagates through the clad with some loss, is mode-converted at the connector portion, and comes out in the propagation mode of the output fiber, is expressed by [(short fiber (Coupling coefficient to cladding mode) − (attenuation rate of cladding mode in short fiber) + (conversion coefficient to output fiber propagation mode)], but in a typical example of a single mode fiber, the first term Is -3 dB, second
The term is 6 dB, and the third term varies greatly depending on the coupling conditions of the connector, and varies from about −30 dB to about −10 dB due to attachment and detachment.

That is, even if the internal coupling loss is kept constant at 15 dB by a precise component arrangement and a fixing method that is stable against environmental changes, the internal fiber is transmitted through the short fiber cladding to the output fiber propagation mode at the connector. When the coupling amount is large, it is -19 dB and cannot be ignored. Moreover, since this amount is greatly affected by fluctuations in the connector portion such as mechanical attachment / detachment, the resulting attenuation becomes unstable.

In particular, DFB which has recently been used
For an optical signal source with high coherence, such as a laser diode, the optical component that propagates through the short fiber core and enters the propagation mode of the output fiber and the output fiber that propagates through the short fiber cladding and enters the connector section Since the light component coupled to the propagation mode keeps the coherence, the power around -16 dB and the power fluctuating from -19 dB to tens of dB interfere with each other. The power that is superimposed and fluctuates greatly due to the phase difference. The phase difference between these different modes can easily change by π or more due to changes in the wavelength of light and changes in thermal expansion and refractive index due to changes in environmental temperature. The amount becomes very unstable.

As described above, in the conventional optical fiber attenuator, the attenuation rate cannot be completely controlled only by the attenuation rate specific to the attenuation element component, and the relative position and input of the optical coupling component cannot be controlled. The drawback that internal fiber coupling loss, which is influenced by many factors such as fiber excitation conditions and the wavelength of input light, and is susceptible to fluctuations due to environmental temperature and mechanical detachment, contributes to the overall attenuation factor to a considerable extent. was there.

Therefore, an object of the present invention is to overcome the above-mentioned drawbacks of the prior art, and to minimize the internal optical fiber coupling loss, and to attenuate the fluctuation of environmental temperature and the mechanical attachment / detachment of input / output fibers. An object of the present invention is to provide an optical fiber attenuator having a stable rate.

[0022]

According to the present invention, there is provided an optical fiber attenuator comprising: an optical fiber made of an attenuating medium as an attenuating element used in an optical fiber attenuator; A predetermined amount of attenuation was obtained while maintaining the matching of the waveguide mode with the external input / output fiber, the internal coupling loss was suppressed as much as possible, and the amount of attenuation was stabilized.

[0023]

In the optical attenuator configured as described above,
An optical fiber made of an attenuating material acts as an internal attenuating element to generate attenuation.
It is determined by the attenuation rate of the material itself (the distribution of the attenuation rate in the cross section of the fiber when the material is different between the core and the cladding) and the mode distribution in the fiber.

In the former case, stable control can be easily performed by controlling characteristics inherent to the element components, such as the doping amount of the attenuating medium and the length of the fiber. The latter depends on the conditions under which this attenuating fiber is excited by the externally connected input fiber,
It is possible to design and manufacture an attenuating fiber as a waveguide structure compatible with input and output fibers,
Therefore, it is possible to achieve the coupling in which the mode conversion is very unlikely to occur at the coupling portion with the input / output fiber.

By realizing such coupling, the excitation condition of the attenuating fiber can be controlled stably, and as a result, the attenuation of the attenuator can be stabilized. In other words, since the function of attenuation and the function of optical coupling between input and output fibers are realized as an integral element in the form of an attenuating fiber, lens systems and ND filters other than the attenuating fiber Additional components such as short fibers inserted only for fiber coupling can be minimized.

Therefore, the internal fiber coupling loss, which is inevitably caused by the insertion of these components which are not compatible with the input / output fiber and the waveguide structure, is minimized, and the attenuation rate is unstable. It is possible to reduce the inviting factors. Further, as described in claim 2, when the attenuation rate distribution of the attenuating fiber is set to be higher in the cladding than in the core,
This fiber acts as a kind of mode filter, in which the attenuation factor for the mode propagating through the cladding is higher than that for the propagation mode of the core.

As mentioned in the previous paragraph, it is possible to minimize the internal coupling loss by using an attenuating fiber as the attenuating element component, but it is nevertheless possible to connect the external input / output fiber with the fiber. It is inevitable that some coupling loss occurs at the two coupling portions. In particular, when mechanical attachment / detachment at this joint is necessary, the current optical connector technology level is 0.1 mm for single mode fiber. The loss of several dB is inevitable. As described above, this loss enters a mode almost transmitted through the clad. In this way, the amount of the component propagating in the clad fluctuates depending on the excitation conditions of the input fiber and the connection conditions of the connector section, and the rate of recombining with the propagation mode of the output fiber by mode conversion also fluctuates. It will contribute as a stable factor.

Therefore, if a fiber having a high cladding attenuation factor is used as in the second aspect, an unstable component transmitted through the cladding is removed by the action of a mode filter, and a predetermined attenuation factor at the core is obtained. The degree of attenuation can be controlled stably. Such a configuration is very effective when the optical attenuator is to be realized in a small size, because components propagating in the cladding can be effectively removed even with a short fiber.

With such a configuration, a configuration similar to the above-described second type, which actively utilizes the fiber coupling loss (different from the attenuation caused by the attenuation fiber) inside the attenuator, is used together with the attenuation fiber. It is also possible to do. As described above, the configuration that considers the matching of the waveguide structure with the input and output fibers using the attenuating fiber is affected by the internal coupling loss fluctuation and the mode conversion as compared with the multimode fiber for the above-mentioned reason. It is needless to say that this is particularly effective in an easy single mode fiber.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of an embodiment according to the present invention, taken along a center plane passing through a fiber. In this figure, reference numerals 1 and 2 denote input / output fibers connected to the outside, and reference numeral 13 denotes an attenuating fiber in which the core and the cladding are formed of an attenuating medium, and the attenuation factor of the cladding is higher than that of the core. is there.

FIG. 3 shows that the input and output fibers and the attenuating fiber are centered and held by ferrules 4, 5 and 6, respectively, and their alignment is realized by fitting with sleeves 8 and 9. This is the same as the above example. The internal structure is held in a housing 7 as a floating structure by a coil spring 10, and this housing is also provided with a connector fastening mechanism including a coupling nut 11.

The attenuating fiber gives a predetermined amount of attenuation to light propagating in the core and further attenuates the light propagating in the cladding. The parameters are equivalent to the input and output fibers. The coupling between the input and output fibers and the attenuating fiber can be achieved with the connector technology already established, with the lowest coupling loss levels currently available. The propagation of the light component in the cladding, which is a problem inherent in the short fiber, is effectively removed by the function of the attenuating fiber as a mode filter.

Therefore, in this embodiment, the effective attenuation as a whole is almost completely controlled only by the attenuation factor for the core propagation mode of the attenuating fiber, and the connector coupling loss added thereto can be obtained at present. Minimized by using the best connector technology. Even if some coupling to the mode in the cladding occurs at the connector, the component suffers a large loss in the cladding of the attenuating fiber and does not contribute to the output light. Therefore, in this embodiment, the fluctuation factor caused by the minimum mode conversion in the connector section can be eliminated.

FIG. 2 is a cross-sectional view of another embodiment according to the present invention, taken along a center plane passing through a fiber. This embodiment is for an application that does not require mechanical attachment / detachment with a connector, and the shape of the input and output sides is such that the optical fiber can be freely processed in a certain length.
This is a so-called pigtail structure at both ends. Reference numerals 1 and 2 denote input / output fiber pigtails, and reference numeral 13 denotes an attenuating fiber having an attenuation rate distribution similar to that of the example of FIG. The joints 1 and 2 and 3 are permanently connected by the already established fiber optic fusion technique and are thus integrated with better mode conversion and less coupling loss than with connectors. .

Thus, the occurrence of modes propagating in the cladding is minimized, but even its minimal components are suppressed by losses in the cladding of the attenuating fiber and are almost completely eliminated. Is done. Therefore, even if an external input / output fiber is connected in close proximity to the attenuator (that is, by shortening the input / output pigtail) by fusion or connector connection, a mode that propagates through the clad generated at the connection point is obtained. The influence does not make the attenuation amount unstable. The fusion splice point between the input / output pigtail and the attenuating fiber is housed in a protective tube 14 and covered by a suitable sheath tube 15 and is mechanically protected, as is usually done in fusion.

[0036]

As described above, according to the present invention, by using an attenuating fiber compatible with an external input / output fiber as a waveguide structure as an attenuating element component in an attenuator for an optical fiber, a predetermined fiber can be obtained. Since the attenuation element component and the coupling element component are realized as one component at the same time, the attenuation amount is extremely small while obtaining the attenuation amount at the same time, so that the attenuation amount can be easily and stably achieved to a predetermined value. There is. Since the attenuating element component has a structure compatible with general optical fibers, the already established connector or fusion technology can be used as it is for the connection with the input and output fibers, and stable connection can be easily performed. realizable.

[0037] Costly parts such as additional components such as lens systems and precise assembly between their damping element components can be eliminated. Since the mode conversion at the fiber connection is minimized, the output should not be unstable even for highly coherent optical signal sources that have recently been used. Can be. This effect is remarkable especially when a medium with higher damping than the core is used for the cladding, and even if a connection technique such as a connector with somewhat poor performance (that is, a cheaper connector) is used for the input / output unit, the damping is reduced. This has the effect of eliminating instability due to interference, which occurs when a plurality of modes generated inside the device are recombined with the propagation mode of the output fiber.

The above-described effects are particularly remarkable in a single-mode fiber attenuator in which it is difficult to realize coupling with low loss and is easily affected by mode conversion.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an embodiment of an optical fiber attenuator according to the present invention, taken along a center plane passing through a fiber.

FIG. 2 is a cross-sectional view of an embodiment of an optical fiber attenuator according to the present invention, without a fastening mechanism using a connector, taken along a center plane passing through the fiber.

FIG. 3 is a cross-sectional view of an embodiment of an optical fiber attenuator according to a conventional method, taken along a center plane passing through the fiber.

FIG. 4 is a cross-sectional view of another embodiment of the optical fiber attenuator according to the conventional method, taken along the center plane passing through the fiber.

[Explanation of symbols]

1, connected to the optical fiber attenuator from outside
Input / output fiber 4,5 Ferrule holding external input / output fiber 7 Housing 8 Sleeve 9 used for fastening to external input / output fiber 9 Sleeve 10 Coil spring 11 Coupling nut 13 Damping fiber 14 Reinforcement pipe 15 Coated tube

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-313102 (JP, A) JP-A-3-25403 (JP, A) JP-A-57-200009 (JP, A) 94612 (JP, U) JP-B 61-19001 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6/00

Claims (1)

(57) [Claims] 1. An optical signal input from an optical fiber,
Light attenuation that is attenuated by a fixed amount and output to another optical fiber
In vessels, one end is fusion spliced to the input fiber of the optical signal, the other end
Is fused to the optical signal output fiber,
The lad has a medium that is attenuating at the wavelength of the optical signal.
And the damping of the cladding is greater than the damping of the core.
A highly attenuating fiber; and the attenuation of the attenuating fiber and the input fiber.
End connected to a decaying fiber and said decaying fiber
And the output fiber connected to the attenuating fiber
End pigtails comprising a protective tube for accommodating an end to be sealed therein and a covering tube covering the protective tube.
Optical attenuator having a metal structure.