JP5402202B2 - Fiber Bragg grating device - Google Patents

Description

  The present invention relates to a fiber Bragg grating device, a subscriber terminal used in a phase code type optical code division multiplexing network provided with the fiber Bragg grating device as a phase encoder, and an optical network including the subscriber terminal. is there.

  In recent years, communication demand has been rapidly increasing due to the spread of the Internet and the like. In response to this increase in communication demand, high-speed and large-capacity optical networks using optical fibers are being developed. In such a high-speed and large-capacity optical network, an optical multiplex transmission technique for multiplexing a plurality of channels of optical signals and transmitting them through a single optical fiber transmission line is indispensable.

  As the optical multiplex transmission technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), and optical code division multiplexing (OCDM) are being studied. .

  Among these optical multiplex transmission technologies, OCDM has a feature that a plurality of channels can be set in the same time slot on the time axis, or a plurality of channels can be set at the same wavelength on the wavelength axis. So the multiplicity is high. In addition, OCDM is a technique of modulating (encoding) with a different code for each channel on the transmitting side and demodulating (decoding) with the same code as that of the transmitting side on the receiving side. Therefore, unless this code is known, It is not decrypted and is excellent in ensuring information security.

  As the OCDM, a wavelength hop / time spreading combination method (hereinafter referred to as a wavelength hop method) and a phase code encoding / decoding technique are known.

  The wavelength hop method is a method in which an optical pulse signal is branched into chip pulses having different wavelengths, and the arrangement order of the chip pulses on the time axis is used as a code. On the other hand, the phase code method is a method in which an optical pulse signal is branched into a plurality of chip pulses and a phase difference between the chip pulses is used as a code.

  As an encoder / decoder in the OCDM, one using a fiber Bragg grating (FBG) is known. An FBG is a device in which a grating is formed in the core of an optical fiber, and has a characteristic of reflecting light of a specific wavelength.

  In recent years, application of superstructure FBG (SSSFBG: Superstructured FBG) having a multipoint phase shift structure to an encoder / decoder of a phase code system OCDM has attracted attention.

  The SSFBG has a plurality of FBGs (unit FBGs) having the same configuration in the same optical fiber, and the interval between adjacent unit FBGs is set to “0” or a predetermined interval according to the constituent code. When encoding with a 15-bit aperiodic phase code, the phase of the chip pulse reflected by the 1st to 15th unit FBGs is, for example, “0, 0, 0, π, π, π, π, 0, The interval of the unit FBG is set so as to be “π, 0, π, π, 0, 0, π” (see, for example, Patent Document 1).

  In the phase code system OCDM, an optical pulse signal is spread on the time axis in accordance with the code of an encoder to form a chip pulse train. This chip pulse train is decoded into an original optical pulse signal by a decoder. In this case, the phase difference between the chip pulses of the chip pulse train formed by the encoder is a code.

  When the optical pulse signal is diffused into the chip pulse train, the encoder gives a phase difference between the chip pulses, and the decoder plays a role of canceling the phase difference between the chip pulses given by the encoder. When the same code is given, the structure of the encoder and the decoder is the same. In the following description, an encoder (encoder) and a decoder (decoder) are referred to as a phase encoder when they are not distinguished.

  When the codes given to the encoder and the decoder are the same, the waveform demodulated by the decoder becomes a waveform having a strong peak (autocorrelation waveform) by canceling the phase difference between the chip pulses given by the encoder. On the other hand, when the codes given to the encoder and the decoder are different, the waveform demodulated by the decoder is not canceled by the phase difference between the chip pulses given by the encoder, so that the waveform consisting of a plurality of small peaks (cross-correlation waveform) Become.

  In the phase code system OCDM, signal identification is performed based on the signal intensity ratio (contrast ratio) of the peaks of the autocorrelation waveform and the cross-correlation waveform.

  The Bragg reflection wavelength (hereinafter referred to as the operating wavelength) of the FBG constituting the encoder and decoder changes the effective refractive index and grating pitch of the FBG when the environmental temperature changes. As a result, the operating wavelength in the FBG changes.

  In particular, in the phase code system OCDM, if the operating wavelength of the FBG that constitutes the encoder on the transmitting side and the operating wavelength of the FBG that constitutes the decoder on the receiving side differ by several pm or more, decoding is not performed on the receiving side. Therefore, it is necessary to perform control such that the difference in operating wavelength between the FBG constituting the transmitting encoder and the FBG constituting the receiving decoder is less than several pm. Therefore, stabilization is performed so that the operating wavelength of the FBG constituting the encoder and decoder fluctuates by only a few pm even when the environmental temperature changes, or at least one operating wavelength of the FBG constituting the encoder or decoder is adjusted as needed. There is a need to.

  As a means of adjusting the operating wavelength of the FBG, the operating wavelength of the FBG is stabilized by using a thermo module configured with a Peltier element and controlling the temperature of the FBG to be constant without depending on the environmental temperature. There is an FBG device (see, for example, Patent Document 1).

  On the other hand, as a means of stabilizing the operating wavelength of the FBG regardless of the environmental temperature, the FBG was fixed to a base material having a negative thermal expansion coefficient, and the amount of wavelength fluctuation accompanying the environmental temperature change was changed by the thermal expansion and contraction of the base material. There is a non-thermosensitive optical element that compensates by the amount of FBG wavelength variation caused by stress (see, for example, Patent Document 2).

  There is also a temperature-compensating optical element in which a cantilever is constituted by two or more types of metals having different thermal expansion coefficients, and the amount of FBG wavelength variation due to environmental temperature changes is compensated by the amount of FBG wavelength variation due to stress (for example, And Patent Document 3).

JP 2005-173246 A Special Table 2000-503415 Special table 2003-526812 gazette

  However, the FBG device disclosed in the above-mentioned Patent Document 1 requires electric power for controlling the operating wavelength of the FBG. In addition, in the non-thermosensitive optical element disclosed in Patent Document 2 described above, it is necessary to procure a special material having a negative thermal expansion coefficient. In addition, the structure of the temperature-compensated optical element disclosed in Patent Document 3 is complicated.

  Furthermore, in the non-thermosensitive optical element and the temperature compensation optical element, it is necessary to design carefully in order to sufficiently increase the wavelength stability accuracy, so that the manufacturing cost increases.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to have a simple structure without using a special material, and to further operate the operating wavelength of the FBG accompanying a change in environmental temperature. variation is small, and to provide a slow fiber Bragg grating equipment.

In order to achieve the above-described object, the fiber Bragg grating device of the present invention includes an optical fiber having a fiber Bragg grating portion and a cylindrical mounting tube to which the optical fiber is fixed. Here, the fiber Bragg grating portion is vacuum-sealed in the mounting tube by a sealing material having flexibility even after curing at both ends of the mounting tube .

According to a preferred embodiment of the fiber Bragg grating device and the subscriber terminal of the present invention, the mounting tube may have a thermal expansion coefficient of 2 × 10 ⁻⁶ / K at the maximum. The material of the mounting tube may be any one of Invar, Super Invar, and glass ceramic.

  Further, according to a preferred embodiment of the fiber Bragg grating device and the subscriber terminal of the present invention, the vacuum sealing may be performed by a sealing material having flexibility even after curing. The sealing material is preferably silicon gel.

According to the fiber Bragg grating equipment of the present invention, the fiber Bragg grating portion or SSFBG have been vacuum-sealed in the mounting tube. As a result, the temperature change in the FBG region can be moderated with respect to the environmental temperature change with a simple configuration without using a special material or a complicated configuration.

It is a schematic block diagram of a fiber Bragg grating apparatus. 1 is a schematic configuration diagram of an optical network using a phase code type OCDM. FIG. It is a figure which shows the relationship between environmental temperature and FBG temperature.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(Fiber Bragg grating device)
With reference to FIG. 1, an embodiment of the fiber Bragg grating device of the present invention will be described. FIG. 1 is a schematic configuration diagram of a fiber Bragg grating device, which is shown by a cut section. Note that hatching indicating a cross section is partially omitted.

  The fiber Bragg grating (FBG) device 10 includes an optical fiber 20 having a fiber Bragg grating (FBG) portion 22 and a mounting tube 30 to which the optical fiber 20 is fixed. Here, the FBG portion 22 is vacuum-sealed in the mounting tube 30.

  The configuration of the FBG unit 22, for example, the number of FBGs formed in the FBG unit 22, the length of each FBG, the lattice spacing in each FBG, and the like are suitably designed according to the application. Here, as a configuration example of the FBG unit 22, a case where the SBGBG used in the phase code type OCDM is provided in the FBG unit 22 will be described.

  The SSFBG has a multi-point phase shift structure including a plurality of unit FBGs 24 that are refractive index modulation regions in the same optical fiber 20. As the optical fiber 20, for example, a single mode optical fiber in which germanium or the like is added to the core to enhance ultraviolet sensitivity can be used.

  The length of the unit FBG 24 and the diffraction grating interval are all formed equally. When the SSFBG is used as an encoder (encoder) or a decoder (decoder) in OCDM communication of the phase code system, the SSFBG is configured to include a number of units FBGs 24 equal to the code length M.

  When an optical pulse signal is input to the SSFBG, a part of the optical pulse signal is reflected by each unit FBG 24 and partly transmitted. As a result, M chip pulses are output from the SSFBG input side. In the SSFBG, the interval between the adjacent unit FBGs 24 is set to “0” or a predetermined interval according to the constituting code. When encoding with a 15-bit aperiodic phase code, the phase of the chip pulse reflected by the 1st to 15th unit FBGs 24 is, for example, “0, 0, 0, π, π, π, π, 0, The interval of the unit FBG 24 is set so as to be “π, 0, π, π, 0, 0, π”.

  The mounting tube 30 has a cylindrical shape with an outer diameter of 5 mm and an inner diameter of 3 mm, for example. The length of the mounting tube 30 may be a length that sufficiently includes the FBG portion 22 in which the SSFBG is formed. For example, when the length of the FBG portion 22 is 85 mm, the length of the mounting tube 30 may be about 100 mm.

When the mounting tube 30 expands and contracts with a change in the environmental temperature, the stress applied to the FBG unit 22 may change, causing fluctuations in the operating wavelength. Therefore, the mounting tube 30 is preferably made of a material having a low coefficient of thermal expansion (CTE: Coefficient of Thermal Expansion). That is, the CTE of the mounting tube 30 is preferably 2 × 10 ⁻⁶ / K at the maximum. As the material of the mounting tube 30, for example, any one of invar having a CTE of about 2 × 10 ⁻⁶ / K, super invar and glass ceramic having a CTE of about an order of magnitude or less than the CTE of the invar Can be used.

  The vacuum sealing of the FBG portion 22 is performed by passing the optical fiber 20 through the mounting tube 30 so that the FBG portion 22 is sufficiently contained, and then sealing with the sealing material 40 at both ends of the mounting tube 30. ing.

  The sealing material 40 may be any material that is suitable for hermetic sealing for maintaining the vacuum in the mounting tube 30 after sealing, and any suitable material can be used.

  For example, when the mounting tube 30 is made of Invar, the mounting tube 30 slightly expands and contracts with changes in environmental temperature. When the expansion / contraction of the mounting tube 30 is transmitted to the FBG unit 22 as it is, the stress applied to the FBG unit 22 changes, which causes a variation in the operating wavelength. In this case, it is preferable to use silicon gel (for example, SE9186 (product name: manufactured by Toray Dow Corning Co., Ltd.)) as a material having flexibility even after the sealing material 40 is cured.

  Thus, if the sealing material 40 has flexibility, even if the mounting tube slightly expands and contracts due to fluctuations in environmental temperature, the mounting tube 40 expands and contracts due to the flexibility of the sealing material 40. Is not transmitted to the FBG unit 22. For this reason, the fluctuation | variation of the operating wavelength accompanying the fluctuation | variation of the stress applied to the FBG part 22 does not generate | occur | produce.

(Optical network)
With reference to FIG. 2, an optical network using a phase code type OCDM will be described.

  The optical network 100 includes one station-side device 300 and a plurality of subscriber terminals 200-1 to 200-n (n is an integer of 2 or more) connected to the station-side device 300 through an optical fiber network. The

  The subscriber terminal 200 includes a transmission / reception unit 210 and a code processing unit 220. The transmitter / receiver 210 is provided with a transmitter 212 and a receiver 214. The code processing unit 220 is provided with an encoder 222 and a decoder 224. As the encoder 222 and the decoder 224, the FBG device 10 having the SSFBG described with reference to FIG. 1 is used.

  The transmitter 212 includes, for example, a pulse light source and an optical modulator. The pulse light source generates an optical pulse train in which optical pulses are arranged at regular regular intervals. The optical modulator generates an optical pulse upstream signal (indicated by an arrow S213 in the figure) reflecting a binary digital electric pulse signal as transmission data obtained by optically modulating the optical pulse train. Thus, the transmitter 212 generates the optical pulse uplink signal S213 corresponding to the transmission data.

  The optical pulse upstream signal S 213 generated by the transmitter 212 is sent to the encoder 222 via the circulator 232. The encoder 222 spreads each optical pulse of the optical pulse uplink signal S213 on the time axis according to the code given for each encoder 222, and indicates an encoded uplink signal as a chip pulse train (indicated by an arrow S223 in the figure). ) Is generated. The encoded uplink signal S223 is sent to the station side device 300.

  An encoded downlink signal (indicated by an arrow S301 in the figure) received from the station side device 300 is sent to the decoder 224 via the circulator 234. The decoder 224 is configured similarly to the encoder 222. When the same code is given to the decoder 224 and the encoder 222, the structures of the decoder 224 and the encoder 222 are the same.

  When decoding with the same code as the code at the time of encoding, the waveform obtained by decoding with the decoder 224 has a strong peak because the phase difference between the chip pulses given by the encoder 222 is canceled. (Autocorrelation waveform). On the other hand, in the case of decoding with a code different from the code at the time of encoding, the waveform obtained by decoding with the decoder 224 has a plurality of small sizes because the phase difference between chip pulses given by the encoder 222 is not canceled. It becomes a waveform consisting of peaks (cross-correlation waveform).

  An optical pulse downlink signal (indicated by an arrow S225 in the figure) obtained by decoding by the decoder is sent to the receiver 214 via the circulator 234.

  The receiver 214 includes, for example, a photoelectric converter, and generates reception data from the optical pulse downlink signal S225.

  The station-side device 300 includes transmission / reception units 310-1 to 310-n and code processing units 320-1 to 320-n corresponding to the number of subscriber terminals 200 constituting the optical network 100. The transmission / reception units 310-1 to 310-n and the code processing units 320-1 to 320-n included in the station-side device 300 are configured in the same manner as the transmission / reception unit 210 and the code processing unit 220 included in each subscriber terminal 200.

  Here, the encoder and the decoder included in the station-side device 300 are configured to be able to adjust the operating wavelength, for example, as in the conventional configuration disclosed in Patent Document 1. The station side device 300 includes a control unit 340 including a wavelength adjusting unit 344. The wavelength adjusting unit 344 adjusts the operating wavelength of the encoders and decoders of the code processing units 320-1 to 320-n.

  At this time, each of the subscriber terminals 200-1 to 200-n may include any suitable temperature detection means 240. This temperature detection means 240 measures the ambient temperature around which the encoder 222 and the decoder 224 are installed, and sends the ambient temperature information to the station side device 300 via the optical fiber network. As the temperature detecting means 240, any suitable one such as a thermistor, a thermocouple, a platinum thermal resistor, or the like can be used. The environment temperature information may be sent to the station side device 300 via a communication line (not shown) different from the optical fiber network.

  The station-side apparatus 300 includes a temperature monitoring unit 342 that monitors information on the environmental temperature sent from the temperature detection unit 240, and the surroundings of the encoders and decoders of the subscriber terminals 200-1 to 200-n monitored by the temperature monitoring unit 342. Using the temperature information, the wavelength adjusting unit 344 adjusts the operating wavelengths of the encoders and decoders of the code processing units 320-1 to 320-n corresponding to the subscriber terminals.

(Operation when environmental temperature changes)
With reference to FIG. 3, the operation of the FBG device when the temperature of the environment where the phase encoder is installed changes will be described. FIG. 3 is a diagram showing the relationship between the environmental temperature and the FBG temperature (the temperature of the FBG portion). In FIG. 3, the horizontal axis indicates time (unit: minutes) and the vertical axis indicates temperature (unit: ° C.). FIG. 3 shows the environmental temperature (I), the temperature (II) of the FBG part when the FBG part is vacuum-sealed, and the temperature (III) of the FBG part when the FBG part is not vacuum-sealed. Yes. Here, the temperature of the FBG portion is a result obtained by heat conduction analysis.

  Here, the initial state of the environmental temperature is 25 ° C. First, the temperature is changed from 25 ° C. to 35 ° C. at a temperature change rate of 1 ° C./min. Next, after maintaining the environmental temperature at 35 ° C. for 10 minutes, the temperature is changed from 35 ° C. to 25 ° C. at a temperature change rate of 1 ° C./min. Next, the environmental temperature is maintained at 25 ° C. for 10 minutes.

  When the temperature of the environment where the FBG device 10 used as the phase encoder is installed changes, the temperature of the region exposed to the outside air such as the surface of the mounting tube 30 changes according to the change of the environmental temperature. At this time, if the inside of the mounting tube 30 is vacuum, the change in environmental temperature is hardly transmitted to the FBG portion 22 due to the vacuum heat insulating effect. The influence of the heat conduction of the optical fiber 20 itself is inevitable, but since the heat conduction coefficient of about 1.4 W / mK can be used as the optical fiber 20, the heat conduction is moderate. Become.

  The temperature (III) of the FBG portion when the FBG portion is not vacuum sealed is substantially equal to the environmental temperature (I), whereas the temperature of the FBG portion when the FBG portion is vacuum sealed ( II) changes gradually with respect to the environmental temperature (I). For example, it is assumed that the environmental temperature of 25 ° C. at time 0 minutes rose to 33 ° C. at a temperature change rate of 1 ° C./min in 8 minutes. At this time, the temperature (II) of the FBG portion that vacuum seals the FBG portion is about 26 ° C., and changes about 1 ° C.

  In order to simplify the explanation, it is assumed that if the wavelength variation rate accompanying the temperature change of the FBG section is 10 pm / ° C. and the operating wavelength of the encoder and the decoder to which the same code is assigned differs by 10 pm, decoding cannot be performed on the receiving side. . In this case, if the temperature of the FBG section changes by 1 ° C., decoding cannot be performed on the receiving side.

  For example, when the environmental temperature changes as shown in (I) of FIG. 3, in the phase encoder in which the FBG portion is not vacuum-sealed, the operating wavelength needs to be adjusted by the local device every minute. . In contrast, in a phase encoder that is vacuum-sealed, the wavelength may be adjusted every 6 to 8 minutes. In this way, the frequency of performing wavelength adjustment can be reduced.

  In the above-described embodiment, the example in which the FBG device is used as a phase encoder in OCDM has been described. However, the present invention is not limited to this. The same effect can be obtained by applying the FBG device to a wavelength filter device or the like in WDM.

10 Fiber Bragg Grating (FBG) Device 20 Optical Fiber 22 Fiber Bragg Grating (FBG) Unit 24 Unit FBG
DESCRIPTION OF SYMBOLS 30 Mounting tube 40 Sealing material 100 Optical network 200 Subscriber terminal 210, 310 Transmission / reception part 212 Transmitter 214 Receiver 220, 320 Code processing part 222 Encoder 224 Decoder 232, 234 Circulator 240 Temperature detection means 300 Station side apparatus 340 Control part 342 Temperature monitoring means 344 Wavelength adjusting means

Claims (6)

An optical fiber having a fiber Bragg grating portion;
A cylindrical mounting tube to which the optical fiber is fixed;
The fiber Bragg grating part is vacuum-sealed in the mounting tube ,
The fiber Bragg grating device , wherein the vacuum sealing is performed by a sealing material having flexibility even after curing at both ends of the mounting tube . 2. The fiber Bragg grating device according to claim 1, wherein the vacuum sealing is performed by sealing end surfaces of both ends of the mounting tube with the sealing material.   The vacuum sealing is made so that expansion and contraction of the mounting tube is not transmitted to the fiber Bragg grating portion by the flexibility of the sealing material by the sealing material having flexibility even after curing at both ends of the mounting tube. Have
The fiber Bragg grating device according to claim 1. Fiber Bragg grating device according to any one of claims 1-3, characterized in that the thermal expansion coefficient of the mounting tube is a 2 × 10 -6 ^{/ K} at most. The fiber Bragg grating device according to any one of claims 1 to 3, wherein a material of the mounting tube is any one of Invar, Super Invar, and glass ceramic. The sealing material is a fiber Bragg grating device according to any one of claims 1 to 5, characterized in that a silicone gel.

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