JP5273616B2 - Optical energy transmission device - Google Patents

Description

  The present invention is an optical energy transmission device that safely transmits high-density optical energy using an optical fiber, and in particular, immediately detects the occurrence of a fiber fuse that occurs when high-density optical energy is transmitted using an optical fiber. In addition, the present invention relates to an optical energy transmission device having a function of suppressing the progress of the fiber fuse and simultaneously stopping the leakage of the high-density optical energy to the outside of the optical fiber by immediately stopping the transmission of the optical energy.

  Due to the extension of repeatless distance and the increase in the number of multiplexing by wavelength division multiplexing (WDM) technology, the optical power propagating through the optical fiber tends to increase even in communication applications, and eventually the average power does not reach several W It is thought that there is not. There are also attempts to transmit power over optical fibers.

  When high-intensity light propagates through an optical fiber, it is known that an optical fiber destruction phenomenon called “fiber fuse” occurs due to fine dust or the like in the optical transmission path or the optical connector portion. Specifically, for example, the core of the optical fiber is ignited at a dust portion and the melting phenomenon proceeds toward the light source, and as shown in FIG. Average strength increases. The propagation speed of the fiber fuse phenomenon depends on the type of optical fiber and the light intensity, but is about several hundreds cm to several meters per second.

When fiber fuses suddenly occur as shown in FIG. 12, cavities are periodically formed in the optical fiber, and these cavities provide intensity modulation to the return light. For example, if the velocity of the fiber fuse is v (m / s) and the pitch of the cavity of the periodic structure generated in the fiber after the fuse is p (m), the time required to advance by one pitch is p / v. Become. Due to the progress of the fiber fuse in the light source direction, the return light reflected at the location where the fiber fuse occurs and propagates in the opposite direction is subjected to intensity modulation, and the basic repetition frequency f ₀ of the intensity modulation is f ₀ = v / p.

For example, in a fiber fuse experiment using a single mode optical fiber SMF-28 manufactured by Corning, Inc., when a 2.75 W laser beam is incident, the fuse traveling speed v is 0.46 m / s, and the pitch of the periodic structure The basic modulation frequency f ₀ obtained from the above equation is 31 μm and is 31 kHz. Such intensity modulation is thought to result in an increase in noise level.

  The destruction due to the fiber fuse proceeds unless the optical power density in the core falls below a certain value. The closer to the light source, the greater the optical power. Therefore, if the fiber fuse phenomenon is left unattended, the light source will eventually reach the light source, causing damage to the light source.

  As a means for preventing the progress of the fiber fuse phenomenon, it has been proposed to insert an optical fiber having a large core diameter in the middle (see Patent Document 1 (US Pat. No. 6640043)). Tapered portions for reducing the change in the core diameter are provided on both sides of the portion where the core diameter is increased to suppress a sudden change in the mode field diameter.

US Pat. No. 6640043 specification

"Observation of catastrophic self-propelled self-focusing in optical fibers," R. Kashyap and K. J. Blow, Electron. Lett., Vol. 24, no. 1, pp. 47-49, 1988. "Fiber Fuse Generation in Single-Mode Fiber-Optic Connectors", Yoshito Shuto, Shuichi Yanagi, Shuichiro Asakawa, Masaru Kobayashi, Member, and Ryo Nagase, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, Page 174-177, 2004. "Catastropic damag and accelerated ageing in bent fibers caused by high optical power," R.M.Percival, E.S.R.Sikora, and R. Wyatt, Electron. Lett., Vol 36, no. 5, 414-416 (2000) "Single mode tapers as fiber fuse damage circuit breakers," D.P. Hand and T.A.Birks, Electron.Lett. Vol. 25, No. 1, 33-34 (1989).

  In the optical energy transmission device using the method described in Patent Document 1, it is impossible to stop the progress of the fiber fuse in the portion of the optical fiber ahead of the optical fiber for cutting the fiber fuse. In addition, if the difference in core diameter between fiber fuse blocking optical fibers is large, mode conversion occurs, which adversely affects signal transmission characteristics in communication applications.

  An object of this invention is to show the optical energy transmission apparatus which detects a fiber fuse rapidly and can suppress progress of a fiber fuse if needed.

Therefore, in the optical energy transmission device of the present invention, a multi-cladding structure having two or more layers, a core, a multi-cladding optical fiber in which at least one clad surrounding the core serves as an optical path, and a core of the multi-cladding optical fiber are provided. detecting an incident light path which enters the transmission light to propagate, the light branching means for branching the return light propagating the multi-clad optical fiber, a signal indicating the fiber fuse from branched return light by the optical branching means A fiber fuse signal detector, and a control for receiving the detection output of the fiber fuse signal detector and controlling the transmission means of the transmission light so as to stop or suppress the incidence of the transmission light to the multi-clad optical fiber and means, comprising said return light of the transmission light introduced into said multi-clad optical fiber, the multi-clad The multi-clad optical fiber includes a first clad surrounding the core and a second clad surrounding the first clad, wherein the first clad is reflected light reflected by a fiber fuse generated in the fiber. The return light is collected and propagated in the first cladding by having a refractive index larger than that of the second cladding and smaller than that of the core, and the optical branching means transmits the return light propagating through the first cladding to the first cladding. Branch to the fiber fuse signal detector . This immediately detects the occurrence of a fiber fuse that occurs when high-density optical energy is transmitted using an optical fiber, and immediately stops the transmission of optical energy, thereby suppressing the progress of the fiber fuse and simultaneously increasing the density. An optical energy transmission device having a function of instantaneously stopping leakage of optical energy outside an optical fiber is realized.

  The optical branching means is, for example, a multimode coupler / brancher.

Further, the light branching means, for example, can be replaced by V grooves portion provided on said first cladding.

Monitor light for irradiating the fiber fuse portion and generating a signal indicating the fiber fuse as reflected light is incident on the multi-clad optical fiber through the optical branching means. As a result, the light branching means can be used for both the incidence of monitor light and the emission of return light.

  By immediately detecting the occurrence of a fiber fuse that occurs when high-density optical energy is transmitted using an optical fiber, and immediately stopping the transmission of optical energy, the progress of the fiber fuse is suppressed and simultaneously high-density optical energy is transmitted. Can be instantaneously stopped, and an optical energy transmission device for safely transmitting high-density optical energy using an optical fiber can be realized.

In the case of an optical fiber having a two-layer clad, it is a diagram showing a light component reflected or scattered at the tip of a traveling fiber fuse. It is a figure which shows the peripheral structural example of the optical branching means for injecting light into a multi clad optical fiber and extracting the return light by a fiber fuse. It is a figure which shows the peripheral structural example of the optical branching means for injecting light into a multi clad optical fiber and extracting the return light by a fiber fuse. It is a figure which shows the peripheral structural example of the optical branching means for injecting light into a multi clad optical fiber and extracting the return light by a fiber fuse. It is a figure which shows the peripheral structural example of the optical branching means for injecting light into a multi clad optical fiber and extracting the return light by a fiber fuse. It is a figure which shows the peripheral structural example of the optical branching means for injecting light into a multi clad optical fiber and extracting the return light by a fiber fuse. It is a schematic block diagram of the first embodiment of the present invention. It is a schematic diagram which shows the RF spectrum change of the return light by a fiber fuse. The optical fiber used is SMF-28, and the light power is about 2.8 W. It is a schematic block diagram of the second embodiment of the present invention. It is a schematic block diagram of the third embodiment of the present invention. It is a schematic block diagram of the fourth embodiment of the present invention. It is a schematic diagram which shows that the average intensity | strength of reflected light increases suddenly by generation | occurrence | production of a fiber fuse. It is a schematic block diagram of the fifth embodiment of the present invention. It is a schematic block diagram of 6th Example of this invention, and is an example which modulates a monitor light source directly. It is a schematic block diagram of 7th Example of this invention, and is an example which modulates the monochromatic light from a monitor light source with a modulator. It is a schematic block diagram of 8th Example of this invention. It is a block diagram which shows the example of the detection circuit which branches the output of a photodetector into 2 and compares in a different frequency band. It is a figure which shows that the peak group with the comb-shaped frequency distribution resulting from a fiber fuse is comprised with the several peak located in a line at equal intervals. It is a figure which shows the actual value of the advancing speed of fiber fuse, and incident light power.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  One of the features of the present invention is that a multi-clad optical fiber having a multi-clad structure having two or more layers of clads and having a core portion and a clad surrounding the core as optical paths is used.

  For example, in the case of an optical fiber having a two-layer clad, the refractive index thereof is core> first clad> second clad. In this case, as shown in FIG. 1, only a of the light components a, b, c, and d reflected or scattered at the tip of the traveling fiber fuse is guided by the core. Reflective components having a large NA (numerical aperture) such as b and c are collected by the inner cladding layer 1 having a large NA, and become return light directed in the direction opposite to the incident light.

  Therefore, the intensity of the return light (average intensity) propagating through the cladding becomes stronger after the fiber fuse is generated than before the fuse is generated. Therefore, the occurrence of fiber fuse can be detected from the change in the return light propagating through the first cladding.

  2 to 6 show the peripheral configuration of the light branching means for making light incident on the core of the multi-clad optical fiber and extracting the return light from the fiber fuse.

  In FIG. 2, a multimode coupler / branch is used as an optical branching unit, and an incident optical path through which a high-power laser signal light is incident as an incident laser beam is connected. To transmit. The light branching means branches the return light of the multi-clad optical fiber and outputs it to the fiber fuse signal detector. In this fiber fuse signal detector, light in a wavelength band including the signal light of the fiber fuse is extracted from the extracted light by an optical filter, and is converted into an electric signal by the photodetector.

  In FIG. 3, a double clad optical fiber doped with yttrium, erbium / yttrium, or the like so as to be capable of optical amplification is used. In this case, the excitation light for optical amplification is incident on the clad of the double clad optical fiber through the optical branching means.

  The optical branching unit is connected to an incident optical path for receiving high-power laser signal light, and transmits the incident light as transmission light to the multi-clad optical fiber. Further, the light branching means branches and takes out the return light of the multi-clad optical fiber. From the extracted light, light in a wavelength band including the signal light of the fiber fuse is extracted by an optical filter, and is converted into an electric signal by a photodetector.

  FIG. 4A is a diagram showing a part of an optical branching unit using a multimode coupler / branch, and shows an example in which a double clad optical fiber is used as a multiclad optical fiber. In this example, it arrange | positions so that the core of a multimode optical fiber may contact the clad near a core. At this time, one to six multimode optical fibers can be connected to one double clad optical fiber. 4 (b) and 4 (c) show two and six cases, respectively.

  Such an optical branching unit shown in FIG. 4 is often used in a fiber amplifier to perform multimode pumping. In addition, when applied to the example of FIG. 3, it is possible not only to input excitation light but also to extract a multimode component propagating through the cladding. In FIG. 3, there are two multimode ports, but any of two to six can be used. In the case of six, the cross section has a hexagonal shape as shown in FIG. When a plurality of ports are arranged, a return light extraction port and an excitation light input port can be prepared.

  FIG. 5 shows a double-clad optical fiber in which a V-groove intersecting the direction of the optical path of the double-clad optical fiber is provided as a light branching means in a part of the innermost clad in which return light propagates. By reflecting or scattering on the side surface of the V-groove, return light propagating through the cladding can be taken out of the double-clad optical fiber. The light extracted in this way is input to the photodetector through the condenser lens.

  In the configuration of FIG. 6, monitor light for detecting a fiber fuse is input using the V groove in FIG. 4. The monitor light enters the light from the multimode laser diode into the double clad optical fiber via the polarization beam splitter, the condenser lens, and the V groove. The return light from the fiber fuse is reflected or scattered by the V-groove, and enters the photodetector through the condenser lens and filter, thereby converting the intensity of the incident light into an electrical signal.

  An embodiment of an optical energy transmission device using the above optical branching means will be described in detail with reference to the drawings.

  FIG. 7 shows a schematic block diagram of the first embodiment of the present invention. The laser light source 10 outputs a pulse or CW (Continuous Wave) laser beam. Usually, the laser light output from the laser light source 10 is pulsed light or CW light in the case of signal transmission, and is CW light in the case of power transmission. An optical amplifier 12 composed of an erbium-doped optical fiber (EDFA) power-amplifies the output light from the laser light source 10. The power of the output laser beam from the optical amplifier 12 is on the order of several watts.

  The output laser light from the optical amplifier 12 enters the transmission optical fiber 16 via the optical circulator 14. The transmission optical fiber 16 is a double-clad optical fiber or a multi-clad optical fiber. The optical circulator 14 for extracting light input or reflected light to the transmission optical fiber 16 can use the light branching means shown in FIG. 2, FIG. 4, or FIG. In this case, the three ports A, B, and C of the optical circulator 14 are respectively a single mode optical input port, an output double clad optical fiber, and a return light for the optical branching means shown in FIG. 2, FIG. 4, or FIG. This corresponds to a multimode optical fiber. The optical circulator 14 is a known optical element that outputs light incident on the port A from the port B and outputs light incident on the port B from the port C. The output of the optical amplifier 12 is connected to the port A of the optical circulator 14, and one end of the optical fiber 16 is connected to the port B.

  In the fiber fuse signal detector 100, first, the light receiving element 18 is connected to the port C of the optical circulator 14. That is, backward propagating light (that is, return light) that enters the port B of the optical circulator 14 from the optical fiber 16 is transferred to the light receiving element 18 by the optical circulator 14. The backward propagating light that enters the port B of the optical circulator 14 from the optical fiber 16 is various return lights that are generated when the output laser light of the optical amplifier 12 enters the optical fiber 16. And the reflected light generated at the end face of each part.

  The light receiving element 18 converts the backward propagating light from the optical fiber 16 into an electric signal, and the amplifier 20 amplifies the output electric signal of the light receiving element 18. The electric filter 22 extracts a noise component, for example, 10 kHz to 500 kHz, from the electric signal output from the amplifier 20. This is because when a fiber fuse is generated, an increase in intensity of 20 dB or more is detected in the 10 kHz to 500 kHz band in the case of CW transmission. As described above, when it is desired to detect the fuse from the periodic intensity modulation of the reflected light when the fuse is generated, the electric (RF) power meter 24 is indispensable, but the fuse is changed from the change in the average intensity of the reflected light. In the case of detection, the power meter 24 may be omitted.

FIG. 8 shows a display example when the output of the light receiving element 18 is input to the spectrum analyzer. It can be seen that the signal intensity when there is a fiber fuse is about 40 to 50 decibels larger than the signal intensity when there is no fiber fuse. In FIG. 8, fine fluctuations in the RF spectrum are observed, and this is a signal having a frequency that is an integral multiple of fc shown in FIG. 8. Further, as will be described below, a signal peak indicated by f _Doppler (= 2 nV / λ, where n is the refractive index of the optical fiber, v is the speed of the fuse, and λ is the wavelength of the light) is generated. This extraction band is an example, and in the case of communication use, it is only necessary to extract the level of the noise band excluding the band caused by the transmission signal. Extract it. Needless to say, the arrangement of the electric filter 22 and the amplifier 20 may be reversed.

  The power meter 24 measures the average power as the output level of the electric filter 22 and outputs a voltage value representative of the measured noise power. As the power meter 24, for example, a power sensor E9304A manufactured by Agilent, Inc. can be used. The comparison circuit 26 compares the output voltage of the power meter 24 with the threshold value Vth. When the fiber fuse phenomenon occurs in the optical fiber 16, the noise power output from the electrical filter 22 increases. In the experimental example, an increase of about 50 dB was detected as shown in FIG. The threshold value Vth may be set to a value slightly lower than the output noise power of the electric filter 22 after the occurrence of the fiber fuse. The comparison circuit 26 has a low (L) output when the fiber fuse is not occurring, and transitions to a high (H) output when the fiber fuse occurs.

  When the output of the comparison circuit 26 transitions from L to H, the warning circuit 28 constituting the control means 200 sends a stop signal to the laser light source 10 to stop the laser output. Thereby, the progress of the fiber fuse in the optical fiber 16 can be stopped immediately, and the situation of replacement due to burning of the long optical fiber 16 can be prevented in advance. The warning circuit 28 also outputs a warning sound from the speaker or outputs a warning signal for displaying a warning on the monitor screen.

  In the present embodiment, the warning circuit 28 outputs both the stop signal for the laser light source 10 and the warning signal for the administrator, but only the warning may be used.

  If the time margin from the occurrence of the fiber fuse to the cutoff of the laser incidence is T, the high frequency cutoff frequency of the electric filter 22 may be at most equivalent to 1 / T. When the high frequency cutoff frequency of the electric filter 22 is smaller than 1 / T, the speed at which the laser is blocked is determined by the response speed of the portion from the light receiving element 18 to the warning circuit 26.

  In this embodiment, when the fiber fuse is detected, the laser light source 10 is stopped. However, the output power of the laser light source 10 may be reduced to such an extent that the fiber fuse does not occur.

  FIG. 9 shows a schematic block diagram of the second embodiment of the present invention. In the embodiment shown in FIG. 7, when the occurrence of the fiber fuse is detected, the warning circuit 28 stops the output of the laser light source 10, but as shown in FIG. 9, the warning circuit 28 of the control means 200 is replaced by the optical amplifier 12. May be stopped. For example, the pump light input of the optical amplifier 12 is stopped. In this case, the optical amplifier 12 functions as an attenuator. The optical fiber 16 that is a double-clad optical fiber or a multi-clad optical fiber is irradiated with laser light having a power that does not generate a fiber fuse, and the fiber fuse of the optical fiber 16 is the same as in the first embodiment. Progress can be stopped immediately. The optical circulator 14 for extracting light input or reflected light to the transmission optical fiber 16 can use the light branching means shown in FIG. 2, FIG. 4, or FIG. As described above, when it is desired to detect the fuse from the periodic intensity modulation of the reflected light when the fuse is generated, the electric (RF) power meter 24 is indispensable, but the fuse is changed from the change in the average intensity of the reflected light. In the case of detection, the power meter 24 may be omitted.

  Further, the warning circuit 28 of the control means 200 may reduce the amplification factor of the optical amplifier 12 to such an extent that the fiber fuse does not occur without completely stopping the amplification operation of the optical amplifier 12.

  FIG. 10 shows a schematic block diagram of the third embodiment of the present invention. As shown in FIG. 10, an optical circuit breaker 30 is provided between the laser light source 10 and the optical amplifier 12 or between the optical amplifier 12 and the optical circulator 14, and light is generated when a fiber fuse is generated by the output of the warning circuit 28. The breaker 30 may block light. As schematically shown in FIG. 10, the optical circuit breaker 30 is composed of a normally closed optical switch, and is opened by a cutoff control signal from the warning circuit 28, thereby blocking the incidence of the optical signal on the optical fiber 16. The optical circulator 14 uses the optical branching means shown in FIG. 2, FIG. 4, or FIG. 5, and the three ports A, B, and C of the optical circulator 14 are respectively shown in FIG. 5 corresponds to the single mode optical input port, the output double clad optical fiber, and the return multimode optical fiber.

  Instead of the optical circuit breaker 30, a variable optical attenuator may be arranged so that the attenuation rate of the variable attenuator is increased in accordance with detection of the fiber fuse to reduce the laser power input to the optical fiber 16. Good. Normally, the configuration in which the optical attenuator is removed from the optical path and the optical attenuator is inserted into the optical path in response to the detection of the fiber fuse performs the same operation as the variable optical attenuator although the response is slow.

  In each of the above-described embodiments, when an end face with high reflectivity is generated due to the optical fiber 16 being bent or the like, there is a possibility that high intensity return light may enter the light receiving element 18 and destroy the light receiving element 18. . Therefore, an embodiment in which the possibility of such destruction of the light receiving element 18 is reduced will be described next. FIG. 11 shows a schematic block diagram of the embodiment.

  The laser 110 outputs a pulse of several W order or CW laser light. The wavelength of this laser beam is assumed to be a signal wavelength λp. Normally, the laser light output from the laser 110 is pulsed light in the case of pulse modulation, as in the case of the laser light source 10, and is CW light in the case of phase modulation or power transmission. An optical amplifier 112 made of an erbium-doped optical fiber amplifies the output light of the laser 110.

  On the other hand, the monitor laser light generator 130 generates monitor light or monitor laser light having a wavelength λm (monitor wavelength) different from the wavelength λp. The optical combiner 132 combines the output light of the optical amplifier 112 with the monitor laser light from the monitor laser light generator 130 and makes the combined light enter the port A of the optical circulator 114. The optical circulator 14 uses the optical branching means shown in FIG. 2, FIG. 4, or FIG. 5, and the three ports A, B, C of the optical circulator 14 are respectively shown in FIG. 5 corresponds to the single mode light input port, the output double clad optical fiber, and the return light multimode optical fiber.

  A transmission optical fiber 116 is connected to the port B of the optical circulator 114, and an optical filter 134 that transmits light of the monitor wavelength λm and blocks light of the signal wavelength λp is connected to the port C.

  The optical circulator 114 supplies the multiplexed light incident on the port A from the optical multiplexer 132 to the transmission optical fiber 116 from the port B, and transmits the backward propagation light incident on the port B from the transmission optical fiber 116 from the port C. This is supplied to the optical filter 134.

  As described above, due to the occurrence of the fiber fuse, the return light of the wavelength λp is intensity-modulated, and as a result, the noise level seen in the frequency domain increases. The return light of the monitor light having the monitor wavelength λm is similarly intensity-modulated, and the noise level seen in the frequency domain increases. Therefore, the occurrence of the fiber fuse can be detected by monitoring the return light of the monitor light instead of the return light of the wavelength λp as in the first to third embodiments.

  The optical filter 134 extracts only the component of the monitor wavelength λm from the output light from the port C of the optical circulator 114 and supplies it to the light receiving element 118. Since the monitor light is not high power, even if an end face with high reflectivity appears due to cutting of the optical fiber 116 or the like, return light having a high-intensity signal wavelength λp does not enter the light receiving element 118, and the light receiving element 118 can be protected.

  The light receiving element 118 converts the backward propagation light having the monitor wavelength λm from the optical filter 134 into an electric signal, and the amplifier 120 amplifies the electric signal output from the light receiving element 118. The electric filter 122 has the same filter characteristics as the electric filter 22 and extracts a noise component from the output electric signal of the amplifier 120. Also in this embodiment, the arrangement of the electric filter 122 and the amplifier 120 may be reversed. Similarly to the power meter 24, the power meter 124 measures the average power of the output of the electric filter 122 and outputs a voltage value representing the measured value.

  The comparison circuit 126 compares the output voltage value (noise power) of the power meter 124 with the threshold value Vth. When the fiber fuse phenomenon occurs in the transmission optical fiber 116, the output power of the electric filter 122 increases. The comparison circuit 126 has a low (L) output when the fiber fuse is not generated, and transitions to a high (H) output when the fiber fuse occurs.

  When the output of the comparison circuit 126 transitions from L to H, the warning circuit 128 of the control means 200 sends a stop signal to the laser 110 to stop the laser output. Thereby, the progress of the fiber fuse in the optical fiber 116 can be stopped immediately, and the situation of replacing the long optical fiber 116 can be prevented in advance. The warning circuit 128 also outputs a warning sound from the speaker or outputs a warning signal for displaying a warning on the monitor screen. The warning circuit 128 outputs both a stop signal for the laser 110 and a warning signal for the administrator, but only the warning may be used.

  The warning circuit 128 also controls the optical circuit breaker installed between the optical amplifier 112 and the optical circulator 114 even when the amplification operation of the optical amplifier 112 is stopped as in the second embodiment, and the signal to the optical fiber 116 is controlled. The incidence of laser light having a wavelength λp may be blocked.

  Although an embodiment has been described in which high-power laser light is stopped or blocked at various stages between the laser light source 10 or 110 and the transmission optical fiber 16 or 116, or the incident power thereof is reduced. Or 110 itself is most reliable. This is because the possibility of a fiber fuse occurring in the optical transmission path located between the laser light source 10 or 110 and the transmission optical fiber 16 or 116 cannot be excluded. When the laser light source 10 or 110 cannot be stopped, it is preferable to block the laser light or reduce the power at a location close to the laser light source 10 or 110.

  The present invention is also effective in the case where the optical fiber 16 or 116 or a part thereof is a Raman amplification medium and the excitation light of the Raman amplification of the output laser light of the laser light source 10 or 110 is used. That is, the present invention can be applied to an optical system in which a fiber fuse can occur in an optical fiber that is a Raman amplification medium or a transmission optical fiber ahead of it by high-power Raman pumping light.

  Obviously, the above invention is not limited to a particular laser wavelength. That is, although 1.55 μm is exemplified as the laser light source wavelength, other types of laser light having different wavelengths, for example, Yb laser light having a wavelength of 1.05 μm may be used.

  In the optical energy transmission device 1 shown in FIG. 13, the laser light from the laser light source 10 enters the optical fiber 16. At the time of the incidence, the monitor light from the monitor light source 8 is combined with the laser light through the optical path 9 by using, for example, a wavelength division multiplexing (WDM) coupler 6 as an optical branching unit. When the return light from the optical fiber 16 passes through the wavelength division multiplexing coupler 6, the monitor light is branched to the optical path 9 by the wavelength division multiplexing coupler 6.

  By making this branching different in the wavelengths of the laser light and the monitor light and combining the optical paths in accordance with the characteristics of the WDM coupler 6, the monitor light contained in the return light is almost completely separated. Can do.

  The return light on the optical path 9 is branched to the photodetector 2 by the branching device 7 such as an optical coupler or a circulator in the fiber fuse signal detector 100.

  The output of the photodetector 2 is amplified as necessary and filtered by an electric filter 3 to extract a signal specific to the optical fiber fuse. As described in the first embodiment, since this signal is in a characteristic frequency band, it can be selected by a frequency filter. The output of the electric filter 3 is measured by the electric power measuring device 4 and the measurement result is determined. The comparison means used for this determination depends on the internal circuit of the electric power measuring instrument 4, but may be an analog signal comparator or a logic circuit that makes a digital signal and performs numerical determination. The output of the laser light source 10 is controlled through the control means 200 based on the determination result in the electric power measuring device 4. This control is to stop or suppress the output of the laser light source 10 when an optical fiber fuse is generated.

  In the optical energy transmission device 1 shown in FIG. 14, unlike the case of the fifth embodiment, the monitor light uses not the monochromatic light itself but the light whose spectrum is dispersed. Such monitor light can be easily obtained by modulating monochromatic light with a signal from the modulation signal generator 31. In FIG. 14, for example, a semiconductor laser is used as the light source of the monitor light. In general, it is known that a semiconductor laser can be modulated with a supplied current. In order to suppress Brillouin scattering, this modulation cycle may be performed at a frequency between several tens of MHz and several hundreds of MHz, for example. This modulation can suppress Brillouin scattering. On the other hand, since the modulation band does not overlap with the modulation band specific to the fiber fuse, the detection of the fiber fuse is not adversely affected.

  Further, in the optical energy transmission device 1 shown in FIG. 15, the modulated monitor light is used as in the above case, but unlike the case of the sixth embodiment, the light source of the monitor light is not directly modulated but is reduced. The monochromatic light from the noise monitor light source is modulated by the signal from the modulation signal generator 32 using the modulator 34.

  The optical energy transmission device 1 shown in FIG. 16 is an example in which the laser light from the optical amplifier 12 is used instead of the laser light from the laser light source 10 in FIG. When the fiber fuse detector 100 detects the occurrence of the fiber fuse, a signal is emitted to stop the amplification of the optical amplifier 12 or reduce the amplification factor. Further, instead of controlling the amplification factor, the input light to the optical amplifier 12 may be blocked using an optical switch or the like.

  Looking at the graph when there is a fiber fuse in FIG. 8, there is a peak group having a comb-shaped frequency distribution due to the fiber fuse up to a little less than 0.6 MHz, and the traveling speed of the fiber fuse is about 0.87 MHz. It can be seen that there is a peak due to the Doppler effect. This Doppler frequency is generated from the beats of reflected light from a progressive fuse and spurious reflected light (spurious, reflected light from an optical fiber connector or the like). This experimental value of 0.87 MHz is the same value as the theoretical value (2 nv / λ) of the Doppler frequency when the refractive index n = 1.5, the wavelength λ = 1.55 μm, and v = 0.45 m / s. . Therefore, as shown in FIG. 17, the output of the photodetector 2 is branched into two, and one of them is extracted from the frequency band A or a part thereof in which the peak group having a comb-shaped frequency distribution due to the fiber fuse is remarkable. And an electrical filter 3-2 that extracts the frequency band B between the frequency band A and the peak due to the Doppler effect or a part thereof. The frequency band B is, for example, 0.6 to 0.85 MHz. The electrical filter 3-1 may be a band pass filter at the center of the Doppler frequency, and the electrical filter 3-2 may have a similar bandwidth with the center slightly shifted so as not to include the Doppler frequency component. Moreover, each intensity | strength is measured with each power measuring device 4-1, 4-2. In this case, when the bandwidths of the frequency bands A and B are different, they are also converted into values corresponding to the respective frequency power densities. The outputs of the power measuring devices 4-1 and 4-2 are determined by a determination device using, for example, the differential amplifier 5. As a matter of course, it can be converted into a digital signal and a numerical determination can be performed by a logic circuit.

  Each element peak of the peak group is a harmonic of a 31 kHz signal, as shown in FIG. For this reason, it turns out that the detection sensitivity can be improved by requiring a comb filter for selectively extracting each peak in the electric filter 3-1. Further, since the frequency of the comb filter can be easily made variable, it is easy to use a frequency variable comb filter in the electric filter 3-1.

  FIG. 19 shows measured values of the fiber fuse traveling speed and the incident light power. This is for DCF (dispersion compensating optical fiber), DSF (dispersion shifted single mode optical fiber), and SMF (general purpose single mode optical fiber), and the arrow indicates the power threshold value at which each optical fiber fuse occurs. When the light intensity is lowered below this threshold, the generation of fuses is interrupted. As the intensity of the incident laser light increases, the traveling speed increases. From this, the frequency position of the peak due to the Doppler effect can be easily estimated, so that the frequency band B can be determined.

  In the case of a double-clad optical fiber, the intensity of laser light is highest in the core portion, so it is considered that a fiber fuse is generated in the core portion. Therefore, the relationship between the traveling speed of the fiber fuse and the input power is the same as that shown in FIG. 19 and depends on the diameter of the core of the double clad fiber. Accordingly, any core having the same core diameter as any core of SMF, DCF or DSF fiber can be derived from FIG. Further, even when the core diameter is different, it can be easily assumed from FIG.

  When the optical energy transmission device of the present invention is actually used, it is desirable to perform an operation test after installation. In order to intentionally generate a fiber fuse, it is easy to put a liquid containing fine particles, such as correction fluid (stationery), on the end face of the optical fiber with several watts of light coming out of the end face. Can occur. An optical fiber having a length of several meters is sufficient for this test, so that after the test, the broken portion can be cut out and returned to the original state again.

  However, the fiber fuse is a destructive phenomenon. For example, in a transmission line for optical communication, the communication path is destroyed, so it is practically difficult to actually perform an operation test by raising the fiber fuse. For this reason, a pseudo fiber fuse signal is transmitted from the side opposite to the laser beam incident side of the optical fiber 16, and the optical energy transmission device actually detects this, thereby performing an operation test without damaging the optical fiber. can do. Since such a pseudo fiber fuse signal may be similar to the signal when the fiber fuse of FIG. 8 is present, the generation thereof is easy.

DESCRIPTION OF SYMBOLS 1 Optical energy transmission apparatus 2 Photo detector 3, 3-1, 3-2 Electric filter 4, 4-1, 4-2 Power measuring device 5 Differential amplifier 6 Wavelength division multiplexing coupler 7 Branch device 8 Monitor light source 9 Optical path 10 Laser light source 11 Incident optical path 12 Optical amplifier 14 Optical circulator 16 Multi-clad optical fiber 18 Light receiving element 20 Amplifier 22 Electric filter 24 Power meter 26 Comparison circuit 28 Warning circuit 30 Optical circuit breaker 31, 32 Modulation signal generator 34 Modulator 100 Fiber fuse signal detector 110 Laser 112 Optical amplifier 114 Optical circulator 116 Transmission optical fiber 118 Light receiving element 120 Amplifier 122 Electric filter 124 Power meter 126 Comparison circuit 128 Warning circuit 130 Monitor laser light generator 132 Optical multiplexer 134 Optical filter 200 Control Means

Claims (4)

A multi-clad optical fiber having a multi-clad structure of two or more layers, a core, and at least one of the clads surrounding the core serving as an optical path;
An incident optical path for transmitting the transmission light so as to propagate through the core of the multi-clad optical fiber;
Optical branching means for branching the return light propagating through the multi-clad optical fiber;
A fiber fuse signal detector for detecting a signal indicative of the fiber fuse from branched return light by the optical splitting means,
Control means for receiving the detection output of the fiber fuse signal detector and controlling the transmission means of the transmission light so as to stop or suppress the incidence of the transmission light to the multi-clad optical fiber;
A, a,
The return light is the reflected light reflected by the fiber fuse generated in the multi-clad optical fiber of the transmission light introduced into the multi-clad optical fiber,
The multi-clad optical fiber is
A first cladding surrounding the core and a second cladding surrounding the first cladding;
The first clad is larger than the second clad and has a refractive index smaller than that of the core so that the return light is collected and propagated to the first clad;
The light branching means is
An optical energy transmission device, wherein return light propagating through the first cladding is branched toward the fiber fuse signal detector . 2. The optical energy transmission device according to claim 1 , wherein the optical branching means is a multimode coupler / brancher. Said optical branching means, an optical energy transmission device according to claim 1, characterized in that the V-shaped groove provided on the first cladding. Through the light branching means, a monitor light for generating a signal indicating the fiber fuse, optical energy transmission according to any one of claims 2 or 3, characterized in that incident on the multi-clad optical fiber apparatus.

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