JP2009021520A - Optical amplification device and optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical amplification device where degradation of complex distortion characteristics is effectively reduced, and to provide an optical transmission system using the same. <P>SOLUTION: An optical amplification device which amplifies a light signal generated by direct modulation of semiconductor laser by a frequency multiplication image signal modulated by a predetermined modulation mode, comprises an optical input portion which receives the light signal, an excitation light source which generates an excitation light, a double cladding type optical amplification fiber where an optical amplification substance is added to a core portion, the light signal and the excited light are received, and the light signal is amplified by induction and release and a light output portion for outputting the amplified light signal. The optical amplification fiber is designed so that an absorption and length product expressed as a product of fiber length of the optical amplification fiber and a peak value at a predetermined wavelength band of absorption coefficient will be smaller than a folding point where a rate of reduction of composite distortion characteristics to a reduction in an absorption and length product drastically increases, in a curve showing the relation between the absorption and length product, and the composite distortion characteristics of the amplified light signal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical amplifying device for amplifying an optical signal generated by directly modulating a semiconductor laser with a frequency-multiplexed video signal modulated by a predetermined modulation method, such as a television signal, and an optical transmission system using the same. is there.

  Conventionally, in an optical video transmission system, one optical signal output from an optical transmitter is branched by an optical branching means such as an optical branching coupler and transmitted to a plurality of optical receivers. When the optical signal is branched in this way, the light intensity of each branched signal is reduced. Therefore, in the optical video transmission system, the optical signal is amplified using an optical amplification device. The optical amplifying device has an excitation light source and an amplification medium inside, and the excitation medium is excited by the excitation light output from the excitation light source, and the input optical signal is amplified and output by the excited amplification medium. Yes. A conventional optical amplifying apparatus for amplifying an optical signal having a wavelength of 1.55 μm uses a single mode erbium-doped fiber (EDF) in which a rare earth metal element erbium is added to the core as an amplifying medium. Was common. In addition, in the optical amplifying apparatus using the EDF as an amplifying medium, the output light intensity is generally limited to about +25 dBm in consideration of cost and heat radiation design.

  Here, in particular, in an optical video transmission system used in the broadcasting field such as CATV, one optical signal is branched into a larger number of optical signals in order to transmit an optical signal from one station to many subscriber houses. Need to be sent.

  In general, a single optical amplifier is targeted to support 128 subscribers. On the other hand, the input intensity of the optical signal to the receiving device that receives the optical signal must be −5 dBm or more. Therefore, considering the loss of the optical fiber that is the transmission line, the output light intensity of the optical amplifying device is required to be about +20 dBm. Further, in a larger scale system, a plurality of optical amplifying devices capable of outputting light with an intensity of about +20 dBm are required. In such a case, considering the installation space, cost, power consumption, etc. of the device, the optical output of one optical amplifying device is made as high as possible, and then the output is branched so that the light intensity becomes about +20 dBm. I am considering that.

  As described above, when a large-scale system is constructed with a conventional optical amplifying apparatus using an EDF as an amplifying medium, there is a problem that the installation space, cost, and power consumption of the apparatus increase.

In recent years, a multi-port type optical amplifying apparatus has been developed in order to solve the above problems (see Non-Patent Document 1). FIG. 10 is a block diagram showing a schematic configuration of a conventional multi-port optical amplifier. As shown in FIG. 10, the optical amplifying apparatus 500 includes an optical input port 51 such as an optical connector, an optical isolator 52, an amplification optical fiber 53, excitation light sources 541 and 542, an excitation light mixer 55, an optical light An isolator 56 and a light output unit 57 are provided. The optical output unit 57 includes 1 × 4 port optical distributors 57a to 57e such as optical couplers, and optical output ports 57f _{1 to} 57f ₁₆ such as optical connectors connected to the output sides of the optical distributors 57b to 57e. Is provided.

The operation of the optical amplification device 500 will be described. First, the optical input port 51 receives an optical signal having a wavelength of 1.55 μm, the optical signal input by the optical isolator 52 is guided to the amplification optical fiber 53, and the reflected return light propagates to the opposite side of the optical signal. Block the light. On the other hand, the excitation light sources 541 and 542 output excitation light having a wavelength of 800 to 1100 nm, and the excitation light mixer 55 guides the output excitation light to the amplification optical fiber 53. Here, the amplification optical fiber 53 is a double clad optical fiber in which erbium and yttrium are added to the core portion, receives an optical signal and pumping light, and amplifies the optical signal by stimulated emission. Next, the optical signal amplified by the excitation light mixer 55 and the optical isolator 56 is guided to the optical output unit 57. In the light output unit 57, the optical signal optical distributor 57a~57e was amplified 16 branches, optical output port _57f 1 _{~57f 16} respectively output the optical signals branched. For example, this optical amplifying apparatus 500 receives an optical signal having an intensity of about +10 dBm, amplifies it to a high intensity of about +34 dBm, branches the signal into 16 branches, and has an intensity of about +20 dBm from each of the optical output ports 57f _{1 to} 57f _16. The branched optical signal can be output.

  Here, a method for determining the characteristics of the conventional optical amplifying device will be described. FIG. 11 is a diagram illustrating the relationship between the absorption length product of an amplification optical fiber and the amplification gain at a wavelength of 1550 nm in an optical amplification device having a conventional configuration. The absorption length product is a value represented by the product of the length of the amplification optical fiber and the peak value of the absorption coefficient in a predetermined wavelength band. When erbium is added to the core portion, the peak value of the absorption coefficient in the vicinity of a wavelength of 1530 nm in the 1.53 μm wavelength band is generally used to represent the absorption length product. The peak value of the absorption coefficient is used as a scale indicating the erbium addition concentration. Moreover, the relationship shown in FIG. 11 is a case where a pumping light having a wavelength in the 800 to 1100 nm band and a light intensity of 8 W is used. As shown in FIG. 11, for example, when an optical signal having an intensity of about +10 dBm is desired to be amplified to about +34 dBm, the amplification gain needs to be about 24 dB. Therefore, the absorption length product of the amplification optical fiber to be used is about 400 dB. Set.

Mikiya Suzuki et al., “Development of high-power multi-port optical fiber amplifier” IEICE Tech. 107 No. 34 OCS2007-15 pp. 21-25

  By the way, in an optical video transmission system, generally, a semiconductor laser diode (LD) as a signal light source is directly intensity-modulated or output from a semiconductor LD by an analog video signal obtained by electrically frequency-multiplexing a plurality of subcarriers. An optical signal generated by externally modulating the CW light is used.

  However, the conventional double-clad type optical amplifying device has been designed to realize as high an optical output as possible. Therefore, when designing an optical amplifying device, for example, the design is performed mainly for high output and high efficiency, for example, by optimizing the absorption length product of the amplification optical fiber to be used with respect to the optical output. As a result, the optical nonlinearity of the amplification optical fiber increases, or the impurity concentration increases, so that the scattering of the optical signal by the impurities increases, the chromatic dispersion increases, and the inclination of the amplification gain spectrum further increases. There is a tendency to grow. Here, when using externally modulated light as an optical signal, since there is almost no wavelength fluctuation (chirp) of the signal light component, even if the optical signal is amplified using an optical amplifier that emphasizes high output, There is no degradation in the quality of the optical signal. However, in the case of using directly modulated light accompanied by chirping of the signal light component, there is a problem that the above-mentioned various trends affect the waveform quality of the optical signal in a complicated manner, resulting in significant deterioration of the composite distortion characteristics. there were.

  The present invention has been made in view of the above, and has newly clarified the relationship between the absorption length product and the composite strain characteristic, and optical amplification in which deterioration of the composite strain characteristic is effectively reduced. An object is to provide an apparatus and an optical transmission system using the same.

  In order to solve the above-described problems and achieve the object, an optical amplification apparatus according to the present invention amplifies an optical signal generated by directly modulating a semiconductor laser with a frequency multiplexed video signal modulated by a predetermined modulation method. An optical input device that receives the optical signal, an excitation light source that generates excitation light, and an optical amplifying substance added to the core portion, receives the optical signal and the excitation light, and A double-clad amplification optical fiber that amplifies a signal by stimulated emission; and an optical output unit that outputs the amplified optical signal, wherein the amplification optical fiber has a predetermined length and an absorption coefficient of the amplification optical fiber. Absorption length product represented by the product of the peak value in the wavelength band is a composite strain characteristic with respect to a decrease in the absorption length product in a curve indicating the relationship between the absorption length product and the composite distortion characteristic of the amplified optical signal. Decrease in Rate is characterized in that which is set to be smaller than the bending point increasing rapidly.

  In the optical amplifier according to the present invention as set forth in the invention described above, the amplified optical fiber is characterized in that erbium and yttrium are added to the core portion.

  Further, the optical amplification device according to the present invention is characterized in that, in the above invention, when the wavelength corresponding to the peak value of the absorption coefficient is near 1530 nm, the absorption length product of the amplification optical fiber is smaller than 350 dB. To do.

  In the optical amplifier according to the present invention, the intensity of the amplified optical signal output from the amplified optical fiber is +25 dBm or more.

  In the optical amplifying device according to the present invention as set forth in the invention described above, the optical input unit includes input optical branching means for branching and outputting the optical signal, and includes a plurality of the amplified optical fibers. Is characterized by amplifying each optical signal branched by the input light branching means.

  In the optical amplifier according to the present invention as set forth in the invention described above, the optical output unit includes output optical branching means for branching and outputting the amplified optical signal.

  An optical transmission system according to the present invention includes an optical transmission device that outputs an optical signal generated by directly modulating a semiconductor laser with a frequency-multiplexed video signal modulated by a predetermined modulation method, and the optical transmission device outputs An optical amplifying device according to any one of the above inventions for amplifying an optical signal to be transmitted, an optical receiving device for receiving an amplified optical signal output from the optical amplifying device, the optical transmitting device, and the optical amplifying device A transmission-side optical transmission path to be connected, and a reception-side optical transmission path to connect the optical amplification device and the optical reception device are provided.

  According to the present invention, the rate of reduction of the composite strain characteristic with respect to the decrease of the absorption length product in the curve showing the relationship between the absorption product length of the amplification optical fiber and the composite strain characteristic of the amplified optical signal. Is set to be a value smaller than the inflection point at which it rapidly increases, so that it is possible to realize an optical amplifying device in which the deterioration of the composite distortion characteristics of the optical signal to be amplified is effectively reduced.

  Hereinafter, embodiments of an optical amplifying apparatus and an optical transmission system according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 is a block diagram showing a schematic configuration of an optical amplifying device 100 according to Embodiment 1 of the present invention. As shown in FIG. 1, this optical amplifying device 100 includes an optical input port 11 such as an optical connector as an optical input unit, an optical isolator 12, an amplification optical fiber 13, an excitation light source 14, and an excitation light mixer 15. And an optical isolator 16 and an optical output unit 17. The optical output unit 17 includes a 1 × 2 port optical distributor 17a such as an optical coupler, 1 × 4 port optical distributors 17b and 17c, and optical connectors connected to output sides of the optical distributors 17b and 17c. Optical output ports 17d _{1 to} 17d ₈ .

  FIG. 2 is a schematic diagram showing a refractive index profile corresponding to the cross section of the amplification optical fiber shown in FIG. As shown in FIG. 2, the amplification optical fiber 13 includes a core portion 13a, a first cladding portion 13b, and a second cladding portion 13c, and the refractive index profile P indicates that the refractive index is high, The core portion 13a, the first cladding portion 13b, and the second cladding portion 13c are arranged in this order, and an optical signal propagates through the core portion 13a in a single mode, and excitation light passes through the core portion 13a and the first cladding portion 13b in a multimode. Designed to propagate in. The core portion 13a and the first cladding portion 13b are made of, for example, quartz glass, and the second cladding portion 13c is made of, for example, resin. Moreover, erbium and yttrium are added to the core portion 13a. On the other hand, the excitation light source 14 is a multimode semiconductor LD that generates excitation light having a wavelength of 800 to 1100 nm and a light intensity of 8 W.

  Next, an operation when the optical amplifying apparatus 100 amplifies an optical signal will be described. As an optical signal, an optical signal having a wavelength of 1550 nm generated by directly modulating a semiconductor LD with an AM-VSB signal composed of a sine wave of 40 carriers having a frequency of 91.25 MHz to 343.25 MHz is used. First, the optical input port 11 receives an input of an optical signal, and the optical isolator 12 guides the input optical signal to the amplification optical fiber 13 and blocks light such as reflected return light propagating to the opposite side of the optical signal. . On the other hand, the excitation light source 14 outputs excitation light, and the excitation light mixer 15 guides the output excitation light to the amplification optical fiber 13.

  The amplification optical fiber 13 propagates the pumping light input from the pumping light mixer 15 side in the multimode in the core portion 13a and the first cladding portion 13b. The excitation light is absorbed by yttrium while propagating through the amplification optical fiber 13, and yttrium is indirectly excited by erbium. At the same time, the amplification optical fiber 13 propagates the optical signal input from the optical isolator 12 side in a single mode in the core portion 13a. As a result, the optical signal is amplified by stimulated emission of excited erbium while propagating through the amplification optical fiber 13. As described above, the double-clad amplification optical fiber 13 has a structure for propagating pump light in a multimode, so that a multimode semiconductor LD having a watt-class optical output can be used. A light output of a watt level can be easily realized as compared with the type of EDF.

Next, the optical signal amplified by the pumping light mixer 15 and the optical isolator 16 is guided to the optical output unit 17. In the light output unit 17, the optical signal optical distributor 17a~17c was amplified 8 branches, optical output port _17d 1 _{~17d 8} outputs the optical signals branched respectively. For example, if the intensity of the optical signal input to the amplification optical fiber 13 is +8 dBm and the intensity of the optical signal output from the amplification optical fiber 13 is +31 dBm, the intensity of each optical signal output from the optical output ports 17d _{1 to} 17d ₈ is Considering the optical loss caused by the excitation light mixer 15, the optical isolator 16, the optical distributors 17a to 17c, etc., it is about +20 dBm. Since the intensity of the optical signal output from the amplification optical fiber 13 is +25 dBm or more, cost reduction and easy heat radiation design that cannot be obtained with a conventional optical amplification apparatus using a single mode type EDF as an amplification medium. Such effects are obtained.

  Here, in the optical amplifying apparatus 100, since the absorption length product of the amplifying optical fiber 13 is set to 200 dB, the degradation of the second order composite distortion (CSO) characteristic of the amplified optical signal is reduced. It is effectively reduced. This will be specifically described below.

FIG. 3 shows the relationship between the absorption length product and the CSO value of an optical signal when an optical signal is amplified using an amplification optical fiber having a different absorption length product in an optical amplification apparatus having the same configuration as FIG. FIG. In FIG. 3, the peak value near the wavelength of 1530 nm in the 1.53 μm wavelength band is used as the absorption coefficient used for calculating the absorption length product. Further, the optical signal is a light output port 17d ₁ ~17d ₈ selects the arbitrary optical signal from an optical signal to be output. The CSO value is a value in a signal having a frequency of 91.25 MHz. The intensity of the optical signal input to the amplification optical fiber is +8 dBm, and the intensity of the optical signal output from the amplification optical fiber is +31 dBm.

  As shown in FIG. 3, the CSO value of the optical signal gradually decreases as the absorption length product decreases in the region where the absorption length product is larger than 350 dB, but when the absorption length product becomes smaller than 350 dB. It is decreasing rapidly, that is, the rate of decrease in CSO with respect to the decrease in absorption length product is increasing rapidly.

  Here, in the optical amplifying apparatus 100 according to the first embodiment, the absorption stripe length product of the amplification optical fiber 13 is set to 200 dB, and the inflection point at which the reduction rate of CSO rapidly increases with respect to the reduction of the absorption stripe length product. Therefore, the degradation of the CSO value of the amplified optical signal is effectively reduced. Since the peak value of the absorption coefficient in the vicinity of the wavelength 1530 nm of the amplification optical fiber 13 is, for example, 20 to 40 dB / m, in order to set the absorption length product to 200 dB, the length of the amplification optical fiber 13 is set to 5 to 10 m. do it.

  This inflection point is a result of complicated factors such as optical nonlinearity of the amplification optical fiber, impurity scattering of the optical signal, wavelength dispersion, and inclination of the amplification gain spectrum, which become more prominent as the absorption length product increases. , Is considered to occur.

  Next, the CSO characteristics of the optical amplifying apparatus 100 when the intensity of the input optical signal is changed will be described. FIG. 4 is a diagram illustrating a relationship between the optical signal input intensity when the intensity of the input optical signal is changed and the CSO value of the optical signal amplified by the optical amplifying apparatus 100. In FIG. 4, for comparison, the CSO value of the input optical signal itself and the CSO value in the conventional optical amplifying apparatus in which the absorption length product of the amplification optical fiber is 400 dB are shown together. In any of the optical amplification devices, the intensity of the pumping light is adjusted so that the intensity of the optical signal output from the amplification optical fiber is +31 dBm. As shown in FIG. 4, the lower the input intensity of the optical signal, the more the deterioration of the CSO value is reduced, and the optical amplifying apparatus 100 has a lower CSO value of about 8 to 10 dB than the conventional one. Is confirmed.

  By the way, in general, in an optical transmission system that transmits an AM-VSB signal, the CSO value of the optical signal that is finally received by the optical receiver at the subscriber's home is −55 dBc or less in terms of transmission quality. It is desirable. Here, since the CSO value of the optical signal is also deteriorated by the chromatic dispersion characteristic of the optical fiber during transmission through the optical fiber that is the optical transmission line, the CSO value of the amplified optical signal in the optical amplifying apparatus is −60 dBc. The following characteristics are generally desired.

  As shown in FIG. 4, even in the conventional optical amplifying device having a large absorption length product, the CSO value can be lowered by reducing the input intensity of the optical signal. However, if the optical signal input intensity is excessively reduced, noise characteristics generated in the optical amplifying apparatus increase, and the necessary gain increases, so that power consumption increases and the input intensity of the optical signal to the optical amplifying apparatus is strict. Since adjustment must be performed, adjustment is complicated, which is not preferable.

  On the other hand, in the optical amplifying apparatus 100 according to the first embodiment, since the absorption length product of the amplification optical fiber is set to a suitable value, the CSO value is −60 dBc even if the optical signal input intensity is increased to about +15 dBm. The following values are preferable.

  As described above, the optical amplifying apparatus 100 according to the first embodiment effectively reduces the degradation of CSO characteristics when amplifying an optical signal having a wavelength of 1550 nm modulated by an AM-VSB signal.

  In the first embodiment, an optical signal having a wavelength of 1550 nm modulated by an AM-VSB signal is used. However, the optical signal is not limited to this. For example, the optical signal may be obtained by wavelength multiplexing an optical signal having a wavelength of 1552 nm and an optical signal having a wavelength of 1559 nm. FIG. 5 is a diagram illustrating an example of frequency-multiplexed signal assignment when using wavelength-multiplexed optical signals. The horizontal axis indicates the signal frequency, and the vertical axis indicates the signal level. As shown in FIG. 5, it is assumed that an optical signal with a wavelength of 1552 nm is obtained by directly modulating a semiconductor LD that outputs light with a wavelength of 1552 nm by an image signal such as an FM signal in the BS-IF band and the CS-IF band. The optical signal is obtained by externally modulating light having a wavelength of 1559 nm output from the semiconductor LD with a video signal such as AM-VSB, QAM, or OFDM signal in the VHF band and the UHF band. Even when the optical amplifying apparatus 100 according to the first embodiment amplifies a wavelength-multiplexed optical signal including a directly-modulated optical signal and an externally-modulated optical signal that has been assigned a frequency-multiplexed signal as shown in FIG. Degradation of the CSO characteristics of the optical signal is effectively reduced.

  In FIG. 5, FM signals in the BS-IF band and CS-IF band are directly used for modulation, and video signals such as VHF band and UHF band AM-VSB, QAM, or OFDM signals are externally modulated. Although the frequency multiplexed signal is assigned to be used, this assignment is not limited to this, and for example, the reverse assignment may be used. That is, the present invention can also be applied to a case where the semiconductor LD is directly modulated by a frequency multiplexed signal modulated by various modulation methods including VSB-AM, FM, QAM, OFDM signals, and further PSK signals.

(Embodiment 2)
Next, an optical amplifying device according to Embodiment 2 of the present invention will be described. In the optical amplifying device according to the second embodiment, the optical input unit includes an input optical branching unit that branches and outputs an optical signal, and includes a plurality of amplified optical fibers, and each of the amplified optical fibers includes an input optical branch. Each optical signal branched by the means is amplified.

FIG. 6 is a block diagram showing a schematic configuration of an optical amplifying apparatus 200 according to Embodiment 2 of the present invention. As shown in FIG. 6, the optical amplifying apparatus 200 includes an optical input unit 21 having an optical input port 21a and a 1 × 2 port optical distributor 21b as input optical branching means, and further includes an optical distributor. An optical isolator 221, an amplification optical fiber 231, an excitation light source 241, an excitation light mixer 251, an optical isolator 261, and an optical output unit 271 are connected to one output port of 21b. On the other hand, an optical isolator 222, an amplification optical fiber 232, an excitation light source 242, an excitation light mixer 252, an optical isolator 262, and an optical output unit 272 are connected to the other output port of the optical distributor 21b. ing. The optical output units 271 and 272 are provided on the output sides of the 1 × 2 port optical distributors 271a and 272a, the 1 × 4 port optical distributors 271b and 272b, and the optical distributors 271b, 271c, 272b, and 272c, respectively. Connected optical output ports 271d _{1 to} 271d ₈ and 272d _{1 to} 272d ₈ .

The operation when the optical amplifying apparatus 200 amplifies the optical signal is the same as that of the optical amplifying apparatus 100 according to the first embodiment described above, but in the optical amplifying apparatus 200, the optical input port 21a receives an optical signal. The optical distributor 21b branches the optical signal into two, and the amplification optical fibers 231 and 232 amplify the branched optical signals, respectively. Thereafter, in the optical output unit 271, the optical distributors 271a to 271c branch the optical signal amplified by the amplification optical fiber 231, and the optical output ports 271d _{1 to} 271d ₈ output the branched optical signals, respectively. To do. On the other hand, in the optical output unit 272, the optical distributors 272a to 272c branch the optical signal amplified by the amplification optical fiber 232 into eight branches, and the optical output ports 272d _{1 to} 272d ₈ output the branched optical signals, respectively. To do. For example, assuming that the intensity of each optical signal input to the amplification optical fibers 231 and 232 is +8 dBm and the intensity of each optical signal output from the amplification optical fibers 231 and 232 is +31 dBm, the optical output ports 271d _{1 to} 271d ₈ , 272d ₁ to The intensity of each optical signal output by the 272d ₈ is about +20 dBm in consideration of optical loss caused by the excitation light mixers 251, 252, the optical isolators 261, 262, the optical distributors 271a-271c, 272a-272c, and the like.

  Here, in the optical amplifying apparatus 200, as in the optical amplifying apparatus 100, the absorption length products of the amplification optical fibers 231 and 232 are both 200 dB, and the rate of decrease in CSO with respect to the decrease in the absorption length is abrupt. It is set to a value smaller than 350 dB, which is an increasing bending point. Therefore, as in the case of the first embodiment, the optical amplifying apparatus 200 generates the semiconductor LD by directly modulating the semiconductor LD with an AM-VSB signal composed of a sine wave of 40 carriers with a frequency ranging from 91.25 MHz to 343.25 MHz. When an optical signal having a wavelength of 1550 nm is amplified, deterioration of the CSO characteristics of the amplified optical signal is effectively reduced.

  Furthermore, since the optical amplifying apparatus 200 splits the input optical signal into two, the intensity of the optical signal when input to each of the amplification optical fibers 231 and 232 is 3 dB higher than the intensity of the optical signal input to the optical input port 21a. Lower. Here, as described above, the smaller the input intensity of the optical signal, the more the degradation of the CSO value is reduced. Therefore, also in the optical amplifying apparatus 200, the degradation of the CSO value is further reduced.

FIG. 7 is a diagram showing the relationship between the optical signal input intensity when the intensity of the input optical signal is changed and the CSO value of the optical signal amplified by the optical amplifying apparatus 200. The optical signal in FIG. 7 is obtained by selecting an arbitrary optical signal from the optical signals output from the optical output ports 271d _{1 to} 271d ₈ and 272d _{1 to} 272d ₈ . The CSO value is a value in a signal having a frequency of 91.25 MHz. Further, in FIG. 7, similarly to FIG. 4, the CSO value of the input optical signal itself and the CSO value in the conventional optical amplifying apparatus in which the absorption length product of the amplification optical fiber is 400 dB are shown together. . In any of the optical amplification devices, the intensity of the pumping light is adjusted so that the intensity of the optical signal output from the amplification optical fiber is +31 dBm. As shown in FIG. 7, in the optical amplifying apparatus 200, it is confirmed that the CSO value can be further reduced as compared with the conventional optical amplifying apparatus and the optical amplifying apparatus 100.

  As described above, in the optical amplifying apparatus 200 according to the second embodiment, the degradation of CSO characteristics when amplifying an optical signal having a wavelength of 1550 nm modulated by an AM-VSB signal is effectively reduced.

(Modification)
In Embodiment 2 described above, the absorption strip length product of each of the amplification optical fibers 231 and 232 and the number of branches of the amplified optical signal in the optical output units 271 and 272 are the same. The number may be different.

  FIG. 8 is a block diagram showing a schematic configuration of an optical amplifying apparatus 300 according to a modification of the second embodiment of the present invention. This optical amplifying device 300 is the same as the optical amplifying device 200 according to Embodiment 2, except that the amplifying optical fiber 232 is replaced with an amplifying optical fiber 332 and the light output unit 272 is replaced with a light output unit 372.

The optical output unit 372 includes 1 × 2 port optical distributors 372a to 372c and optical output ports 372d _{1 to} 372d ₄ connected to the output sides of the optical distributors 372b and 372c. Since the number of branches of the optical output unit 372 is 4, which is half of that of the optical output unit 271, when the intensity of each optical signal output from the optical output ports 372 d _{1 to} 372 d ₄ is about +20 dBm, the optical output unit 372 outputs from the amplification optical fiber 332. The intensity of the optical signal to be transmitted can be +28 dBm, which is 3 dB lower than in the case of the optical amplifier 200. Therefore, in the optical amplifying apparatus 300, the concentration length product of the amplification optical fiber 332 can be lower than 200 dB, and the CSO characteristic of the optical signal to be amplified can be further reduced.

  Note that the intensity of the optical signal output from the optical output port can be appropriately determined according to the configuration of the optical transmission system to which the optical amplifying device is applied. For example, when the optical loss budget of the optical transmission system is small and the optical signal intensity per output port may be small, the absorption length product of the amplification optical fiber can be set small.

  Further, the number of branches of the optical signal in the optical output unit may be appropriately determined according to the setting of the absorption length product of the amplification optical fiber. For example, according to the present invention, the absorption length product of the amplification optical fiber is set so that the composite distortion characteristic can be effectively reduced, and one output is obtained according to the intensity of the amplified optical signal that can be realized in the set absorption length product. The number of branches of the optical signal is determined so that the intensity of the optical signal per port satisfies a desired value.

(Embodiment 3)
Next, an optical transmission system according to Embodiment 3 of the present invention will be described. The optical transmission system according to the present embodiment is an optical video transmission system that distributes an optical signal having a wavelength of 1550 nm generated by directly modulating a semiconductor LD with an AM-VSB signal, which is a video signal, to subscribers' homes.

FIG. 9 is a block diagram showing a schematic configuration of an optical transmission system 400 according to Embodiment 3 of the present invention. As illustrated in FIG. 9, the optical transmission system 400 connects the optical transmission device 41, the optical amplification device 100 according to the first embodiment, the optical reception device 44, and the optical transmission device 41 and the optical amplification device 100. A transmission-side optical fiber transmission line 42, and a reception-side optical fiber transmission line 43 that connects the optical output port 17 d ₈ of the optical amplification device 100 and the optical reception device 44. In addition, another optical receiving device is connected to each of the other optical output ports 17d _{1 to} 17d ₇ of the optical amplifying device 100 via another optical transmission path.

  Here, the positional relationship between the optical transmission device 41 and the optical amplification device 100 is not particularly limited, and the optical transmission device 41 and the optical amplification device 100 may be installed, for example, in the same or adjacent location of the transmission station. The optical amplifying device 100 may be installed at a location away from the optical transmission device 41, for example, at a relay station.

The optical transmission device 41 outputs an optical signal having a wavelength of 1550 nm generated by directly modulating the semiconductor LD 41a with the AM-VSB signal S. The optical fiber transmission line 42 transmits the optical signal output from the optical transmission device 41 and guides it to the optical amplification device 100. Optical amplifying device 100, an optical signal inputted from the optical fiber transmission line 42 amplified and branched, and outputs an optical signal from the optical output port _17d 1 _{~17d 8.} The optical fiber transmission line 43 transmits the optical signal output from the optical output port 17 d ₈ and guides it to the optical receiver 44.

  The optical receiver 44 is installed in the subscriber's house and receives the transmitted optical signal. Here, since the optical transmission system 400 uses the optical amplifying apparatus 100 in which the degradation of the composite distortion characteristics when a multiplexed analog signal is amplified is effectively reduced, the optical reception apparatus 44 uses the composite distortion characteristics. It is possible to receive an optical signal including a high-quality video signal with reduced degradation of the image.

  In the above embodiment, it has been explained that the deterioration of the CSO characteristic of the amplified optical signal is effectively reduced. However, in the optical amplifying device according to the present invention, the third-order complex distortion (CTB) : Composite Triple beat) Other compound distortions such as composites can also effectively reduce the deterioration of their characteristics.

  Further, in the above embodiment, the case of using a double clad amplification optical fiber in which erbium and yttrium is added to the core portion has been described, but lanthanum is added instead of yttrium, or thulium is added to the core portion, A rare earth element such as praseodymium, yttrium, terbium, or neodymium, or another substance having an amplification effect similar to that of the rare earth element may be added. Further, the glass constituting the core part and the first cladding part is not limited to a quartz type, and may be a tellurite type or a fluoride type. In this case, the wavelength of the optical signal, the wavelength of the excitation light, the wavelength band including the peak value of the absorption coefficient when defining the absorption strip product, and the absorption strip product are determined by the characteristics of the additive and glass, respectively. . In the above embodiment, the backward excitation method is used as the excitation method. However, the excitation method is not limited to this method, and forward excitation, bidirectional excitation, or a combination of these excitation methods as appropriate. Good.

1 is a block diagram illustrating a schematic configuration of an optical amplifying device according to a first embodiment of the present invention. It is the schematic which shows the refractive index profile corresponding to the cross section of the amplification optical fiber shown in FIG. FIG. 2 is a diagram illustrating a relationship between an absorption length product and a CSO value of an optical signal when an optical signal is amplified using an amplification optical fiber having a different absorption length product in the optical amplifying apparatus having the same configuration as in FIG. 1. . It is a figure which shows the relationship between the optical signal input intensity at the time of changing the intensity | strength of the optical signal to input, and the CSO value of the optical signal amplified by the optical amplifier of FIG. It is a figure which shows an example of allocation of the frequency multiplex signal in the case of using the wavelength-multiplexed optical signal. It is a block diagram which shows schematic structure of the optical amplifier which concerns on Embodiment 2 of this invention. It is a figure which shows the relationship between the optical signal input intensity at the time of changing the intensity | strength of the optical signal to input, and the CSO value of the optical signal amplified by the optical amplifier of FIG. It is a block diagram which shows schematic structure of the optical amplifier which concerns on the modification of Embodiment 2 of this invention. It is a block diagram which shows schematic structure of the optical transmission system which concerns on Embodiment 3 of this invention. It is a block diagram which shows schematic structure of the conventional multiport-type optical amplifier. It is a figure which shows the relationship between the absorption length product of an amplification optical fiber, and the amplification gain in wavelength 1550nm in the optical amplification apparatus of the conventional structure. Note that a value near a wavelength of 1530 nm is used as the peak value of the absorption coefficient.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11, 21a Optical input port 12, 16, 221, 222, 261, 262 Optical isolator 13, 231, 232, 332 Amplification optical fiber 13a Core part 13b First clad part 13c Second clad part 14, 241, 242 Excitation light source 15 , 251 and 252 the excitation light mixer 17,271,272,372 light output section _{17d 1 ~17d 8, 271d 1 ~271d} 8, 272d 1 ~272d 8, 372d 1 ~371d 4 optical output port 17a to 17c, 21b, 271a to 271c, 272a to 272c, 372a to 372c Optical distributor 21 Optical input unit 41 Optical transmitter 41a Semiconductor LD
42, 43 Optical fiber transmission line 44 Optical receiver 100-300 Optical amplifier 400 Optical transmission system

Claims (7)

An optical amplifying device for amplifying an optical signal generated by directly modulating a semiconductor laser with a frequency multiplexed video signal modulated by a predetermined modulation method,
An optical input unit for receiving the optical signal;
An excitation light source that generates excitation light;
An optical amplifying substance is added to the core, receives the optical signal and the excitation light, and amplifies the optical signal by stimulated emission;
An optical output unit for outputting the amplified optical signal;
The amplification optical fiber has an absorption length product represented by a product of the length of the amplification optical fiber and the peak value of the absorption coefficient in a predetermined wavelength band, and the absorption length product and the amplified optical signal. In the curve showing the relationship with the composite strain characteristic, the reduction rate of the composite strain characteristic with respect to the decrease in the absorption strip length product is set to be a value smaller than the inflection point at which it rapidly increases. Optical amplification device.   The optical amplifying apparatus according to claim 1, wherein erbium and yttrium are added to the core of the amplification optical fiber.   The optical amplification device according to claim 2, wherein when the wavelength corresponding to the peak value of the absorption coefficient is in the vicinity of 1530 nm, the absorption length product of the amplification optical fiber is smaller than 350 dB.   The optical amplification device according to any one of claims 1 to 3, wherein the intensity of the amplified optical signal output from the amplification optical fiber is +25 dBm or more. The optical input unit includes input optical branching means for branching and outputting the optical signal,
5. The optical amplifying device according to claim 1, comprising a plurality of the amplifying optical fibers, wherein each of the amplifying optical fibers amplifies each optical signal branched by the input light branching unit. .   The optical amplifying apparatus according to claim 1, wherein the optical output unit includes an output optical branching unit that branches and outputs the amplified optical signal. An optical transmitter that outputs an optical signal generated by directly modulating a semiconductor laser with a frequency multiplexed video signal modulated by a predetermined modulation method;
The optical amplification device according to any one of claims 1 to 6, which amplifies an optical signal output from the optical transmission device;
An optical receiver for receiving the amplified optical signal output by the optical amplifier;
A transmission side optical transmission line connecting the optical transmission device and the optical amplification device;
A reception-side optical transmission line that connects the optical amplification device and the optical reception device;
An optical transmission system comprising:

Images