Light amplification relay system employing remote excitation

Abstract

This light amplification relay system comprises an optical transmitter that outputs signal light, an optical fiber that transmits the signal light output from the optical transmitter, an optical receiver that receives return signal light, an optical fiber for optical amplification that is disposed at a part of the optical fiber in the longitudinal direction and to which at least erbium is added, and an excitation light source that remotely excites the optical fiber for optical amplification, wherein: the wavelength of the signal light is within the amplification wavelength band of the optical fiber for optical amplification; and the wavelength of the excitation light that remotely excites the optical fiber for optical amplification is on the longer wavelength side of 1.49 μm and on the shorter wavelength side of the signal light wavelength within the amplification band of the optical fiber for optical amplification.

Description

Optical amplifier repeater system using remote pumping

[0001] The present invention relates to the field of optical fiber, remote pumping.

[0002] <Wavelength Dependence of Optical Fiber Transmission Loss> The transmission loss of optical fibers, which are widely used today as a communications medium and whose main material is silica glass, varies depending on the wavelength of the light being transmitted. The wavelength at which loss is minimal is the 1.55 μm band, which is currently used for communications. Research and development trends regarding the reduction of loss in optical fibers are shown, for example, in Figs. 2 to 4 of Non-Patent Document 3. The wavelength with the lowest loss and suitable for long-distance transmission continues to be around 1550 nm, and the further away from that wavelength the greater the loss coefficient, making it disadvantageous for long-distance transmission.

[0003] <Erbium-doped optical amplifiers> Optical amplifiers that use optical fibers doped with rare-earth elements as their amplification medium are widely used today. In long-distance optical transmission, the 1.55 μm band, which is the wavelength band with the lowest loss for optical fibers made of silica glass, is used as the signal light, and optical fibers doped with erbium (Er), a rare-earth element that can amplify this wavelength band using laser (LASER: Light Amplification by Stimulated Emission of Radiation), are widely used as the amplification medium.

[0004] Light of 0.98 μm or 1.48 μm is used to pump erbium-doped fiber (EDF). This pump light excites erbium elements, and when signal light in the 1.55 μm band is incident on it, the signal light is output with increased power due to stimulated emission. In other words, the power of the pump light is transferred to the signal light via the erbium element, resulting in amplification.

[0005] Semiconductor lasers (laser diodes: LDs) are commonly used as light sources for 0.98 μm and 1.48 μm pumping light. A semiconductor laser is an element that uses current energy to pump an optical semiconductor material, causing laser oscillation in an optical resonator and outputting light. Typically, an EDF and a pumping light source are packaged together and sold commercially as a single optical amplifier.

[0006] <Remote Pumping Method> When transmitting optical signals over long distances through optical fiber cables, the optical signal is attenuated due to transmission loss in the optical fiber, limiting the communication distance. For this reason, an optical amplifier repeater is required, which inserts an optical amplifier in the middle to compensate for the loss.

[0007] In a typical optical amplifier configuration, a light source of pumping light for pumping the EDF is placed in close proximity to the EDF. A typical example of a pumping light source is a semiconductor laser. This semiconductor laser is driven by commercial power.

[0008] However, it is often necessary to install optical amplifier repeaters in places where commercial power is difficult to obtain, such as in cables laid under the sea, and one solution to this problem is a technique called remote pumping, in which pump light is also sent to the repeater through optical fiber. This is a well-known technique that is widely used, as explained in, for example, Non-Patent Documents 1 and 2.

[0009] In the case of remote pumping, the pumping wavelength of 1.48 μm is used for EDFA (Er-doped Fiber Amplifier), which allows long-distance transmission in communication optical fibers. 0.98 μm is more susceptible to attenuation than 1.48 μm, making long-distance transmission difficult, and is therefore not used for remote pumping.

[0010] Although 1.48 μm pump light also attenuates over long distances, remote pumping techniques transmit pump light at high enough power to compensate for this attenuation and send it to repeaters in remote locations. It is empirically known that a minimum pump light power of 5 mW to 10 mW (7 dBm to 10 dBm) is required for an EDF to achieve practical gain. This is also explained in Non-Patent Document 1 (p. 42, first column). In a typical configuration of conventional remote pumping, approximately 1 W (+30 dBm) of pump light is transmitted to pump an EDF located approximately 100 km away. The typical transmission loss of 1.48 μm in telecommunication optical fiber is approximately 20 dB to 23 dB, so the power reaching the repeater is +7 dBm to +10 dBm, making pumping possible. However, it is difficult to transmit sufficient pump light power to an EDF located further away.

[0011] As explained in Figure 2.2-2 of Non-Patent Document 2, the pump light for remote pumping may be wavelength-multiplexed with the signal light at the terminal station and sent over the same fiber, or the signal light and the pump wavelength may be transmitted over separate fibers from the terminal station to the optical amplifier and then combined just before the EDF in the remote pumping light amplifier. If the pump light is sent over the same fiber, a phenomenon called stimulated Raman scattering occurs in the optical fiber, and part of the energy of the pump light is transferred to the signal light, i.e., an optical amplification phenomenon occurs. This phenomenon can be actively utilized, but when it is necessary to deliver the pump light over a long distance with as little loss as possible (the problem that this invention aims to solve), it is preferable to send it over a separate fiber because energy loss is undesirable.

[0012] <Optical Fiber Sensing> Optical fibers are widely used as a communication medium, but they are also used as sensors. For example, applying sound waves or vibrations to an optical fiber modulates the light passing through it. By detecting the reflected or transmitted light, environmental information such as sound and vibrations can be detected and measured remotely. One representative example of such a measuring device is a distributed acoustic sensing (DAS). DAS detects environmental information from backscattered light propagating through an optical fiber. DAS is a type of optical time domain reflectometry (OTDR). The measuring device part of a DAS is called an interrogator. An interrogator is an interrogator, and its role is to illuminate each point on the optical fiber (the sensor) with light to obtain environmental information sensed at each point. By analyzing Rayleigh backscattered light, a DAS can measure vibrations and strains at each point through which a probe pulse passes. Other known devices include a BOTDR, which analyzes Brillouin backscattered light.

[0013] US Patent No. 7595865 JP 09-113941 JP 09-129956 JP 07-181529

[0014] Development of a Submarine Remotely Pumped Optical Amplifier System Installed Between Okinawa Island and Miyako Island, Tomoyoshi Kataoka et al., NTT Technical Journal, June 2005; Optical Submarine Cable (Book), 2.2.2 Transmission Distance Extension Technology for Repeaterless Transmission Systems, Optical Submarine Cable Writing Committee (This section was written by Hidenori Taga), May 2010; Low-Loss Optical Fiber, Yoshinori Yamamoto, Laser Research, Vol. 40, No. 6, June 2012

[0015] We would like to enable optical fiber sensing and transmission by exciting EDFs located at a great distance from the terminal station and optically amplifying and repeating them, which was difficult with the conventional remote excitation method. A great distance is, for example, a distance significantly exceeding 100 km.

[0016] Conventional remote pumping uses a pumping light source in the 1.48 μm band, and in order for an EDF to achieve a practical gain of about 15 dB or more, it generally requires pumping light in the 1.48 μm band of at least about 10 mW (10 dBm).

[0017] The output of a 1.48 μm band semiconductor laser is about 0.5 W, and even when wavelength multiplexed and polarization combined, the output remains at about 1 W to 1.5 W (+30 dBm to 32 dBm). If the distance from the light source to the EDF is 100 km using a typical optical fiber, the attenuation of the 1.48 μm band will be about 22 dB, so the pumping light power reaching the repeater will be about +10 dBm, the minimum pumping power required by the EDF. For this reason, the conventional remote pumping configuration, which pumps from one direction, has a maximum distance of about 100 km.

[0018] The present invention has been made in consideration of the above circumstances, and has as its object to enable more pumping power to be sent to a remotely located EDF, thereby increasing the distance between the pumping light source of a remotely pumped EDFA and the EDF, and increasing the output power of the remotely pumped EDFA.

[0019] The optical amplification relay system using remote pumping of the present invention comprises an optical transmitter that outputs signal light, an optical fiber that transmits the signal light output from the optical transmitter, an optical receiver that receives return signal light, an optical fiber for optical amplification (EDF) that is arranged in a portion of the longitudinal direction of the optical fiber and is doped with at least erbium, and a pumping light source that remotely pumps the optical fiber for optical amplification, wherein the wavelength of the signal light is within the amplification wavelength band of the optical fiber for optical amplification, and the wavelength of the pumping light that remotely pumps the optical fiber for optical amplification is longer than 1.49 μm and shorter than the wavelength of the signal light within the amplification band of the optical fiber for optical amplification.

[0020] According to the present invention, it is possible to extend the distance from the pumping light source to the EDFA and increase the output power of the remotely pumped EDFA.

[0021] FIG. 1 is a configuration diagram of a first embodiment. FIG. 2 is an explanatory diagram of the wavelength dependence of optical fiber loss and the wavelength allocation of the present invention. FIG. 3 is a comparative explanatory diagram of OTDR traces when an optical amplifier repeater is used and when it is not used. FIG. 4 is an example of the gain wavelength dependence of an EDFA pumped with power after 180 km transmission. FIG. 5 is a configuration diagram of a second embodiment. FIG. 6 is a configuration diagram of a third embodiment. FIG. 7 is a configuration diagram of a modified example of the third embodiment. FIG. 8 is a configuration diagram of a fourth embodiment. FIG. 9 is a configuration diagram of a fifth embodiment. FIG. 10 is an explanatory diagram of the wavelength characteristics of absorption and fluorescence of an EDF. FIG. 11 is an explanatory diagram of changes in EDFA gain when the pump wavelength is shifted to the longer wavelength side. FIG. 12 is an explanatory diagram of an example of a 1.53 μm band pump light source. FIG. 13 is an explanatory diagram of an example of a 1.53 μm band pump light source.

[0022] The embodiment is characterized in that the wavelength of light for remotely pumping the EDF is longer than 1.49 μm and shorter than the signal wavelength within the amplification band of the EDFA.

[0023] For example, the wavelength of the signal light is 1560 nm, and the pump light wavelength is 1529 nm to 1532 nm. Hereinafter, pump light in this wavelength band will be referred to as 1.53 μm-band pump light. Also, for example, the wavelength of the signal light is 1550 nm, and the pump light wavelength is 1510 nm. Hereinafter, pump light in this wavelength band will be referred to as 1.51 μm-band pump light. In the present invention, these 1.51 μm-band and 1.53 μm-band pump light are applied to the remote pumping system.

[0024] An example of the absorption and fluorescence spectra of an EDF is shown in Figure 10. The solid line shows the absorption spectrum, and the dashed line shows the fluorescence spectrum. As such, 1.48 μm band excitation is an excitation method that utilizes the spread of levels within the same level, and amplification can be achieved if the wavelength of the excitation light is on the shorter wavelength side (higher energy side) than the wavelength of the signal light (light that undergoes amplification). The 1.48 μm band was selected as the optimal excitation wavelength for this level, and the 1.51 μm and 1.53 μm bands have not been used for the following reasons.

[0025] Reason 1): Reduced signal bandwidth. EDFAs are often used to simultaneously amplify wavelength-multiplexed signal light, particularly in backbone optical communication networks. These wavelength-multiplexed signals typically range from 1530 nm to 1562 nm, and EDFAs must have sufficient gain across this wide wavelength range. To achieve this, 1.48 μm pumping is required. Figure 11 shows the gain profile of a certain EDF pumped at a relatively low pumping power of +8 dBm. While 1.48 μm pumping provides sufficient gain near 1532 nm, the gain around 1532 nm drops sharply as the pumping wavelength increases beyond 1.49 μm. This makes them unsuitable for amplifying the aforementioned wide signal bandwidth. Furthermore, since the 1.53 μm band is part of the EDFA's amplification wavelength range, using it as the pumping wavelength clearly reduces the available signal bandwidth. However, for applications that do not require a wide amplification bandwidth, the reduced amplifiable bandwidth is not an immediate drawback.

[0026] Reason 2) Energy conversion efficiency is poor. This is because as the pumping wavelength approaches the 1.53 μm band, which is the absorption peak wavelength of EDF, from the 1.48 μm band, not only absorption but also stimulated emission becomes active. If pumping is possible at the 1.48 μm or 0.98 μm band, there is no advantage to pumping at wavelengths longer than the 1.49 μm band.

[0027] However, in cases where remote pumping of an EDF over a very long distance is required, pumping light wavelengths shorter than the 1.49 μm band have lower optical fiber loss than the 1.48 μm band, so they have the advantage of being able to pump EDFs over long distances that are attenuated by the 1.48 μm band and cannot reach. Over such long distances, the benefits outweigh the drawbacks.

[0028] A high-power, stable, and economical light source in such a wavelength band is also important for practical use. Such an excitation light source will be described in the embodiments.

[0029] <First embodiment> A first embodiment of an optical fiber sensing system will be described with reference to Figure 1. The first embodiment is an example in which the signal light is probe pulse light and backscattered light of OTDR-type optical fiber sensing, and its wavelength is 1.56 µm. Several OTDR-type optical fiber sensing methods are known, a representative one being DAS. DAS can measure environmental information such as vibration, strain, and temperature at each point through which the probe pulse light passes by analyzing Rayleigh backscattered light. Other known methods include BOTDR, which analyzes Brillouin backscattered light.

[0030] FIG. 1 shows a terminal station 10, optical amplifier repeaters 11 and 12, and cables 21, 22, and 23 connected to them. These cables include optical fibers 40, 41, 42, and 43. The terminal station 10 includes a DAS interrogator 30 that outputs signal light to the optical fiber 40 and receives the return signal light. The optical fiber 40 transmits probe pulse light L1 and backscattered light L2 returning from various locations. As the probe pulse light L1 passes through various locations on the optical fiber 40 and backscattered light L2 is generated and begins to propagate in the reverse direction, the backscattered light L2 is affected by environmental information, such as sound and vibration, experienced by the optical fiber 40. The DAS interrogator 30 receives and analyzes the backscattered light L2 to detect and measure environmental information at various locations on the optical fiber 40. In other words, the optical fiber 40 serves both as a transmission path and a sensor element. Since the optical fiber core 40 is housed in the structure of the cables 21, 22, and 23, it can be said that the cables 21, 22, and 23 act as sensor elements. The cables 21, 22, and 23 are laid so as to optimally sense the environmental information that the user wishes to collect. Figure 1 shows the situation in which the cables 21, 22, and 23 sense vibrations as an example of the environmental information they sense.

[0031] The terminal station 10 includes a first pumping light source 31 that outputs pumping light to an optical fiber core 41, a second pumping light source 32 that outputs pumping light to an optical fiber core 42, and a third pumping light source 33 that outputs pumping light to an optical fiber core 43. The first pumping light source 31 serves as a pumping light source for the optical amplifier repeater 11. The second pumping light source 32 and the third pumping light source 33 serve as pumping light sources for the optical amplifier repeater 12. The optical amplifier repeater 12 includes an EDF 51, WDM (Wavelength Division Multiplexing) filters 52 and 53, and BPFs (Band Pass Filters) 54 and 55. The BPFs 54 and 55 are BPFs that pass signal light and remove ASE (Amplified Spontaneous Emission), and a BPF with a signal light wavelength of 1.56 μm is abbreviated as a "1.56 μm BPF." A WDM filter is a three-port filter element that multiplexes or demultiplexes wavelength bands A and B with low loss, and a WDM filter that multiplexes or demultiplexes the 1.53 μm band and the 1.56 μm band is abbreviated as "1.53 / 1.56WDM."

[0032] In Figure 1, two optical amplifier repeaters 11 and 12 driven by remote pumping are installed, for example, 90 km apart. The optical amplifier repeater 11 is located 90 km away from the terminal station 10, and pumping light output from a first pumping light source 31 is transmitted 90 km to the optical amplifier repeater 11, obtaining energy for optical amplification. This optical amplifier repeater 11 is located within 100 km of the terminal station 10 and can be realized using known remote pumping technology, so detailed description thereof will be omitted. A typical pumping wavelength is in the 1.48 μm band. While Figure 1 shows an example in which the optical fiber core 40 through which the optical fiber sensing signal light is transmitted and the optical fiber core 41 through which the pumping light from the first pumping light source 31 is transmitted are separate, they may be wavelength-multiplexed and transmitted on the same core, utilizing the effect of distributed Raman amplification.

[0033] On the other hand, the optical amplifier repeater 12 is located 180 km away from the terminal station 10, and the pumping light output from the second pumping light source 32 and the third pumping light source 33 is transmitted 180 km through optical fiber cores 42 and 43 to the optical amplifier repeater 12, where it obtains energy for optical amplification. Conventional pumping light in the 1.48 μm band has large transmission loss and does not allow sufficient pumping power to reach the EDF. For this reason, this embodiment uses pumping light in the 1.53 μm band.

[0034] The first embodiment is an example in which the optical amplifier repeater 12 is configured as a single-core bidirectional type. In the case of OTDR-type optical fiber sensing, the probe pulse light L1 and the backscattered light L2 propagate in opposite directions through a single optical fiber, so the optical amplifier repeater 12 is also compatible with a single-core bidirectional type. However, a single-core bidirectional type optical amplifier cannot differentiate the amplification degree between the light traveling to the right and the light traveling to the left in the figure, and the output levels cannot be adjusted independently of each other.

[0035] Here, the output power of the second pumping light source 32 and the third pumping light source 33 is +37 dBm each. The distance from the terminal station 10 to the optical amplifier repeater 12 is 180 km, and the transmission loss in the 1.53 μm band is 30 dB. The power reaching the optical amplifier repeater 12 is attenuated to +7 dBm. This pumping light is transmitted through two optical fiber cores 42 and 43 and input to the EDF 51 via WDM filters 52 and 53 from the front and rear of the EDF 51, respectively, to amplify the 1.56 μm band signal light. Since the EDFA output light also contains ASE light, which can cause various problems, BPFs 54 and 55 are placed at the output of the EDFA to extract only the signal light band and remove the ASE light before outputting the signal light to the next transmission path and sensor, cables 22 and 23.

[0036] The wavelength allocation and the improvement effect of 1.53 μm remote pumping will be explained using Figure 2. Currently, commercially available low-loss optical fibers for long-distance transmission achieve a loss of approximately 0.16 dB / km in the 1.55 μm band, which is the signal wavelength band. In contrast, the transmission loss in the 1.48 μm band, which is the typical pump wavelength for EDFAs and is a wavelength band capable of long-distance transmission with transverse single-mode optical fibers, is approximately 0.19 dB / km, even for optical fibers considered to be relatively low-loss. In contrast, the transmission loss in the 1.53 μm band, which is the pump light used in this embodiment, is approximately 0.165 dB / km, which is not much different from the 1.55 μm band. After 180 km transmission, the pump light power in the 1.53 μm band is approximately 4.5 dB higher than that in the 1.48 μm band. In addition, the 1.53 μm band can be pumped by a transverse multimode LD and amplified by an EYDFA (Er / Yb co-doped fiber amplifier), which is capable of high-power output that is difficult to achieve with a semiconductor laser output from a transverse single-mode optical fiber, making it possible to economically prepare a stable, high-power pumping light source. The EYDFA is a type of EDFA, and details will be discussed later.

[0037] <Explanation of the effect of an amplified repeater in an OTDR measuring instrument> The effect of optically amplified repeaters on optical fiber transmission line loss compensation in optical fiber sensing will be explained using Figure 3. Figure 3 shows an OTDR trace obtained using the OTDR function of the DAS, with the horizontal axis representing distance and the vertical axis representing the attenuation of reflected return light. Without an optically amplified repeater, this measuring instrument's detection limit for returned light is just over 20 dB, and the upper limit of the measurement range when using this optical fiber is approximately 100 km.

[0038] When optical amplifier repeaters 11 and 12 of this embodiment are installed at points 90 km and 180 km apart, the optical amplifiers compensate for the 90 km of loss before transmitting the signal to the next transmission section. This shows that a 270 km range is now measurable. This diagram simply indicates that the measurable range of a typical OTDR, which measures loss in optical fiber transmission lines, has been extended, but does not directly mean that the measurable range of a DAS has been expanded. However, in reality, if the DAS has sufficient processing performance, there is a strong correlation between the distance at which the returned light becomes so weak that it is difficult to detect in OTDR measurements and the measurement data becomes dominated by noise, and the similar distance in DAS measurements. In fact, in this case, vibrations applied to the optical fiber cable were detected up to 270 km away.

[0039] In an OTDR type measuring instrument, the interval between light pulses is limited by the round-trip time of light, i.e., the distance to the far end. The longer the measurement distance, the longer the interval between light pulses must be. In this embodiment, the far end is 270 km away, so the repetition frequency of the light pulses is limited to approximately 370 Hz or less.

[0040] In an optical amplifier using remote pumping, the output level of a repeater amplifier at a remote location can be adjusted within a small range by increasing or decreasing the pumping light power sent from the terminal station 10. However, since the optical amplifier in this embodiment is a single-fiber bidirectional type, it is difficult to adjust the saturation output power independently for each direction of light propagation. However, by changing the ratio between the forward pumping light power and the backward pumping light power, it is possible to adjust the saturation output power slightly differently depending on the direction.

[0041] Figure 4 shows an example of the wavelength dependence of gain when pumping an EDF using 1.53 μm pump light transmitted 180 km in the first embodiment. The pump light transmission power from the terminal station 10 was +37 dBm, and after 180 km transmission it was +7.3 dBm. The EDF length was set so that the gain in the 1.56 μm band was maximized at this pump light power. As shown above, with 1.53 μm pumping, the wavelength range in which gain can be obtained narrows toward the long wavelength side, so it is desirable to set the signal light wavelength toward the long wavelength side of the general EDFA amplification wavelength band. This is because the pump wavelength of 1.53 μm is close to the signal wavelength, resulting in poorer energy conversion efficiency with 1.53 μm pumping compared to 1.48 μm pumping.

[0042] In this embodiment, an example is shown in which 1.56 μm is used as the signal light wavelength, but it can be set anywhere within the EDFA amplification wavelength band other than the 1.53 μm pumping wavelength band. However, as shown in an example in Figure 4, 1.56 μm tends to be the wavelength at which gain is easiest to obtain. Another advantage of setting the signal light to the 1.56 μm band is that, as shown in Figure 2, this is the wavelength at which the transmission loss of the optical fiber is almost minimal.

[0043] <Modification of Wavelength Band of Second Embodiment> A second embodiment will be described with reference to FIG. 5. This is a modification of the first embodiment, using a different wavelength band, in which a pump light wavelength of 1.51 μm and a signal light wavelength of 1.55 μm are used. In the configuration example of the first embodiment shown in FIG. 1, it was difficult to set the signal light wavelength shorter than 1.56 μm. This was because the pump wavelength was 1.53 μm, which resulted in a decrease in amplification efficiency and a deterioration in noise figure for the 1.55 μm signal wavelength. The 1.53 μm band is an excellent pump wavelength with lower transmission loss than the 1.48 μm pump wavelength used in conventional remote pumping. However, because it is a pump wavelength within the same EDFA amplification wavelength band, it has the disadvantage of restricting the signal wavelength band to longer wavelengths than 1.56 μm.

[0044] The second embodiment shown in Figure 5 eliminates the restriction of using an EDFA as a pumping light source, thereby enabling the amplification of signal light in the 1.55 μm band. To achieve this, a pumping wavelength in the 1.51 μm band, which has rarely been used in the past, is used. A candidate for a high-power pumping light source in this pumping wavelength band is the cascaded fiber Raman laser, which has recently become commercially available as a high-power light source. Hereinafter, this will be abbreviated as FRL. The fiber Raman laser will be described in more detail later, but its features include high power output and the ability to design the oscillation wavelength almost freely.

[0045] The configuration of the second embodiment shown in Figure 5 is the same as the configuration of the first embodiment shown in Figure 1 except for the pumping light source, pumping wavelength, and signal light wavelength, and therefore will not be described again. Note that, in accordance with the wavelength differences, the BPFs 54 and 55 are referred to as "1.55 μm BPFs," and the WDM filters 52 and 53 that multiplex and demultiplex the 1.51 μm and 1.55 μm bands are referred to as "1.51 / 1.55 ​​WDM." The wavelength allocation and the improvement effect of 1.51 μm remote pumping will be explained using Figure 2. The transmission loss of a conventional 1.48 μm pump wavelength band is approximately 0.19 dB / km, even in optical fibers with relatively low loss. In contrast, the transmission loss of the 1.51 μm band used as pump light in the configuration example shown in Figure 5 is low, at approximately 0.173 dB / km. After 180 km transmission, the pumping light power of the 1.51 μm band is approximately 3.1 dB higher than that of the 1.48 μm band.

[0046] Figure 4 shows an example of the wavelength dependence of gain when pumping an EDF using 1.51 μm band pump light transmitted 180 km in the second embodiment. The pump light transmission power from the terminal station 10 was +37 dBm, and after 180 km transmission it was +5.9 dBm. The EDF length was set so that the gain in the 1.55 μm band was the highest at this pump light power. Compared to 1.53 μm pumping, 1.51 μm pumping has a higher gain on the short wavelength side, and it can be seen that even though the pump light power is lower, a higher gain can be obtained in the 1.55 μm band than 1.53 μm pumping.

[0047] In the second embodiment, the pump wavelength was described as 1.51 μm. However, the pump wavelength may be selected to have a smaller transmission loss in the optical fiber longer than the 1.49 μm band. Figure 4 also shows an example of the wavelength dependence of gain when pumping an EDF using commonly used 1.48 μm pump light transmitted over 180 km. The pump light transmission power from the terminal station 10 was +37 dBm, and after 180 km transmission it was +2.8 dBm. The EDF length was set so that the gain in the 1.55 μm band was maximized with this pump light power, but the pump power was too weak, resulting in almost no gain. As shown, as the pump wavelength approaches the commonly used 1.48 μm, the transmission loss in the optical fiber increases, the delivered pump power decreases, and EDFA gain is no longer obtainable. Figure 4 shows that pumping in a wavelength band with low optical fiber transmission loss is advantageous when long-distance pumping is required.

[0048] <Third embodiment> A third embodiment will be described using Figure 6. The third embodiment is a modification of the first embodiment. The optical amplifier repeater 11 shown schematically in Figure 1 can be realized using conventional technology, so it will not be shown in the subsequent explanatory diagrams. In other words, it can be thought of as if the cable 21 and optical amplifier repeater 11 in Figure 1 have been omitted from Figure 6. The optical amplifier repeater 12 in Figure 6 is located, for example, 180 km away from the terminal station 10.

[0049] The terminal station 10 is provided with a second pumping light source 32, a third pumping light source 33, a fourth pumping light source 34, and a fifth pumping light source 35 as pumping light sources for the optical amplifier repeater 12. Pumping light from these second, third, fourth, and fifth pumping light sources (32, 33, 34, 35) is output to optical fiber cores 42, 43, 44, 45.

[0050] 6 has a configuration in which two stages of the optical amplifier repeater 12 of the first embodiment are connected in series. That is, the optical amplifier repeater 12 has a first stage configuration consisting of an EDF 51 and WDM filters 52 and 53 and a second stage configuration consisting of an EDF 61 and WDM filters 62 and 63 connected in series. Furthermore, BPFs 54, 55, and 56 are provided at the input and output stages, respectively. Pumping light from the second pumping light source 32 and the third pumping light source 33 is supplied to the EDFs via the WDM filters 52 and 53, and pumping light from the fourth pumping light source 34 and the fifth pumping light source 35 is supplied to the EDFs via the WDM filters 62 and 63.

[0051] When the gain or output power is insufficient in a single stage, this can be compensated for by configuring multiple stages in this way. The only way to supply multiple transverse single-mode pumping lights transmitted through separate cores to an EDFA is to input them to the forward pumping and backward pumping ports. Therefore, even if the pumping light power per core becomes insufficient due to transmission attenuation, there is no way to combine them to add up the pumping power. Therefore, by configuring multiple stages, the gain and output power that are insufficient in a single stage can be increased.

[0052] Generally, if multiple pumping lights are made to have different wavelengths or orthogonal polarizations, they can be supplied to one EDF using wavelength multiplexing or polarization synthesis combining means. However, if this is difficult, a method can be considered in which the EDFA is configured in multiple stages as shown in FIG. 6 to increase the number of input ports for forward pumping and backward pumping.

[0053] To polarization-multiplex two beams of light at a remote location, the transmission path between them must be constructed using polarization-maintaining optical fiber, which is difficult. Wavelength multiplexing at a remote location is possible, and this is also a modification. However, the pumping light wavelength range in which the effects of the present invention can be obtained is not very wide, so it must be selected carefully. For example, 1528 nm to 1530 nm and 1531 nm to 1533 nm are examples.

[0054] <Method for adjusting the level in one direction in a single-fiber bidirectional optical amplifier> In the single-fiber bidirectional optical amplifier repeater 12 described in the first and second embodiments, it is not possible to make the amplification degree different for the light traveling to the right and the light traveling to the left in the figure, but by adding the components surrounded by dotted lines in Fig. 7, it is possible to attenuate only the light traveling in one direction or pass it through a BPF. In the example of Fig. 7, only the probe pulse light L1 is attenuated by the optical attenuator 65.

[0055] The circulator 64 is an element that rotates the output direction in the direction of the arrow in the symbol, so that light entering from port a is output to port b, light entering from port b is output to port c, and light entering from port c is output to port a. The probe pulse light L1 entering from port a is output to port b by the circulator 64, attenuated by the optical attenuator 65, reflected by the FRM (Faraday Rotate Mirror) 66, attenuated again by the optical attenuator 65, enters the circulator 64 from port b, and is output from port c. Because the light passes through the optical attenuator 65 twice in this way, it is necessary to use an attenuator with an attenuation amount half the desired value. On the other hand, the backscattered light L2 entering the circulator 64 from port c is output from port a, does not pass through the optical attenuator 65, and is not attenuated.

[0056] The FRM 66 is a mirror element that rotates the polarization state of incident light by 90 degrees before outputting it. This has the effect of canceling out the polarization dependency of elements on the path that the light passes through twice, on the way there and back, preventing any adverse effects. This embodiment can be realized using a normal mirror, but it is preferable to use the FRM 66. Because this level adjustment section has loss, it is preferable to place it in a section sandwiched between the EDFAs in the preceding and following stages. If a section with loss is placed on the input side of the EDFA, it will directly degrade the NF (noise figure), so it is preferable to amplify the signal once and then place a section with loss.

[0057] <Fourth embodiment> A fourth embodiment will be described with reference to Fig. 8. The fourth embodiment also shows a modification of the first embodiment, and illustration and description of the configuration related to the optical amplifier repeater 11 will be omitted. The distance between the terminal station 10 and the optical amplifier repeater 12 is, for example, 180 km.

[0058] In the optical amplifier repeater 12, the optical signal is divided into directions using circulators 75 and 76, and each signal is amplified, and then the signals are combined by circulators 76 and 75 and sent out to the cable 23 and cable 22. That is, the optical amplifier repeater 12 has a configuration in which light travels to the right in the figure using EDF 71, WDM filter 72, and BPF 57, and a configuration in which light travels to the left in the figure using EDF 81, WDM filter 82, and BPF 58.

[0059] The terminal station 10 is provided with a second pumping light source 32 and a third pumping light source 33 as pumping light sources for the optical amplifier repeater 12. The pumping light from the second pumping light source 32 is output to an optical fiber core 42 and supplied to an EDF 81 via a WDM filter 82. The pumping light from the third pumping light source 33 is output to an optical fiber core 43 and supplied to an EDF 71 via a WDM filter 72.

[0060] In the single-fiber bidirectional optical amplifier repeater 12 described in the first embodiment, it is not possible to make the amplification factor different for light traveling to the right and light traveling to the left in the figure, but if the optical amplifier repeater 12 has separate optical amplifiers for each direction as shown in the fourth embodiment in Figure 8, it has the advantage of being able to make the gain different for each direction or to make the characteristics of the BPFs (57, 58) different for each direction. In addition to being able to make the amplification factor different for each direction from the time of manufacture, there is also the advantage that even after installation, the output power can be adjusted independently for each direction by adjusting the pump light power sent from the terminal station 10.

[0061] However, in the configuration of the optical amplifier repeater 12 in FIG. 8, the number of optical amplifiers is doubled, so that the pumping light power is also required to be approximately doubled, and the scale of the remote pumping light source in the terminal station 10 is nearly doubled.

[0062] Fifth Embodiment A fifth embodiment will be described with reference to Fig. 9. The fifth embodiment is a modification of the fourth embodiment.

[0063] The terminal station 10 is equipped with an optical transceiver 39. The signal transmitted is the signal from the optical transceiver 39 that transmits information. In optical fiber communications, single-core bidirectional transmission is not widely used because it has no advantage other than reducing the number of cores required and is cumbersome to handle. It is common to use one core for each direction, so optical fiber cores 40A and 40B are used. The optical amplifier repeater 12 differs from that shown in Figure 8 in that it does not include circulators 75 and 76. It is equipped with an EDF 71, a WDM filter 72, and a BPF 57 for amplifying light traveling to the right of the figure, and an EDF 81, a WDM filter 82, and a BPF 58 for amplifying light traveling to the left of the figure.

[0064] In this case, a one-way optical amplifier is used, and an optical isolator is provided to prevent the effects of reflected light, which can cause various problems in communication. This configuration is a common configuration for optical transmission systems using remote pumping, but the feature of this embodiment is that it uses the 1.51 μm band as the pump wavelength, which allows for lower loss and longer distance transmission than the 1.48 μm band, making it possible to send pump power to amplifiers that are further away.

[0065] As described above, the optical fiber amplifier may be configured in any suitable combination of forward pumping, backward pumping, forward and backward pumping, unidirectional amplification, bidirectional amplification, etc., as an embodiment of the present invention.

[0066] <Regarding the pumping light source> The present invention relates to remote pumping and uses a pumping wavelength different from conventional ones, so the availability of pumping light sources is important for practical use. A light source that has high power of several watts to approximately 10 watts, yet is stable, reliable for the long term, and economical is required. Here, a specific configuration example will be described.

[0067] The most widely used pumping light source for EDFA today is a semiconductor laser module that outputs light of 0.98 μm or 1.48 μm through a transverse single-mode optical fiber. However, the output power of these modules is limited to approximately 0.5 W per module, which is insufficient for use as a remote pumping light source, which requires higher power.

[0068] Therefore, a configuration can be considered in which light of a desired wavelength, even if its power is weak, is obtained as seed light, and this light is amplified by an optical amplifier to be used as a pump light source. There are only a limited number of optical amplifiers that can amplify the wavelength band from 1.49 μm to 1.53 μm to high power, as used in the embodiments of the present invention. One is the EYDFA (EDFA), and the other is the FRL (cascade fiber Raman laser).

[0069] 12 shows an example of a 1.53 μm pump light source using an EDFA. First, the ASE (Amplified Spontaneous Emission) light from the EDFA 91 is filtered by a BPF 92 to a wavelength range of 1529 nm to 1532 nm to produce seed light, which is then amplified by an EDFA 93. The light is then split into four by a 4-way coupler 94, and each split is input to four EYDFAs 95, producing four high-power pump light beams, each with 5 W (+37 dBm).

[0070] The reason why ASE light is used as the seed light for the pump light is that, as explained on page 56 of Non-Patent Document 2, light with low coherency and spectral purity is desirable in order to minimize nonlinear phenomena in optical fibers, particularly Stimulated Brillouin Scattering (SBS). Similarly, the wavelength width is set to about 3 nm, to prevent undesired nonlinear phenomena in the optical fiber due to an increase in spectral purity caused by narrowing the wavelength width too much. Incidentally, a configuration in which ASE light from an optical amplifier is used as pump light for the optical amplifier is well known, as disclosed in Patent Document 3, for example.

[0071] The features of the EYDFA95 are explained below. Today, in most optical amplifiers, the fundamental pump light source is a semiconductor laser. The output of one semiconductor laser is about 0.5 W in the case of a transverse single-mode output that can be transmitted through an optical fiber transmission line. Therefore, in order to bundle the light from multiple semiconductor lasers and input it into a single optical fiber, an output of about 1 W to 1.5 W has been achieved by wavelength multiplexing with a slight wavelength shift or polarization multiplexing, but it has been difficult to achieve a light source with a higher power than this.

[0072] In contrast, today, by using multiple high-power semiconductor lasers with transverse multimode output of several watts and combining their output light with a technology called double-clad pumping, it is possible to obtain high-power transverse single-mode light. By adding ytterbium (Yb) to the EDF in addition to erbium, the ytterbium absorbs the pump light in the 900-1100 nm band, which is easily obtained from high-power light sources, and then transfers that energy to the erbium, enabling efficient pumping. High-power EDFAs (EYDFAs) with output powers of several tens of watts are now commercially available. By inputting seed light in the 1.53 μm band into this high-power optical amplifier, high-power 1.53 μm band pump light can be obtained.

[0073] 13 shows a configuration in which, contrary to the configuration shown in FIG. 12, when one EYDFA 96 is capable of outputting the optical power of two beams, the beam is split into two and transmitted separately. The ASE light from the EDFA 91 is filtered by a BPF 92 to fall within the wavelength range of 1528 nm to 1532 nm, and this light is used as seed light, which is amplified by an EDFA 93. The EYDFA 96 then produces the optical power of two beams, which is then split into two beams by a two-branch coupler 97.

[0074] Since there is a risk in transmitting power of more than a few watts with ordinary optical fibers for communication, it is preferable to transmit power in parallel over multiple cores. In this case, as shown in Figure 13, branching the output of a single high-power EYDFA 96 to generate pump light output reduces the number of parts, which may be more economical.

[0075] Another high-power light source for wavelengths longer than 1.49 μm and shorter than 1.53 μm is the cascade fiber Raman laser (FRL), as disclosed in Patent Document 4. When strong pump light is injected into an optical fiber, stimulated Raman scattering in the optical fiber creates an optical amplifier (fiber Raman amplifier). Attaching a mirror (fiber bragg grating (FBG)) that selectively reflects a specific wavelength band within the Raman amplification gain band to the input / output section of this amplification optical fiber creates an optical resonator, which serves as a laser light source for that wavelength. The peak of stimulated Raman scattering gain in a quartz fiber occurs at an optical frequency on the low-frequency side of approximately 13 THz. In other words, the laser light source is approximately 13 THz longer than the pump light. By performing this frequency conversion in multiple stages, wavelength conversion to almost any wavelength band can be achieved. In this case, too, a light source in the 900 nm to 1100 nm band, which is a readily available high-power light source, is used as the initial pump light source for Raman amplification.

[0076] In the case of an FRL light source, if one unit can output the pumping light power equivalent to the input of multiple cores, it may be possible to configure it more economically by branching the output into multiple parts as shown in Figure 13 rather than arranging multiple FRLs with medium output power.

[0077] Variations in the configuration of the excitation light source are possible, and they may be combined as appropriate as elements of the present invention.

[0078] <Modifications not shown> The above-mentioned Patent Document 2 also addresses the same problem as the present invention and discloses a solution in which a multimode fiber (transverse multimode fiber) is used as the transmission fiber for pumping light. This has the effect of increasing the cross-sectional area of ​​the optical fiber and reducing the power density in order to avoid several undesirable phenomena that occur when high-power pumping light is input into a transverse single-mode fiber. In fact, transverse multimode fibers are commonly used for transmitting very high-power light, such as for laser welding.

[0079] However, transverse multimode fiber has the drawback of having a larger transmission loss per distance than transverse single-mode fiber, and there is no technology for connecting transverse multimode fiber to transverse single-mode fiber with low loss, so it is not currently used as a long-distance transmission medium for pump light. However, if these obstacles are removed in the future, the present invention may be combined with transverse multimode fiber.

[0080] <Other Examples of Pumping Light Sources> For signal light in the 1.55 μm band, the absorption wavelength band of this level of the EDF continues from around 1.45 μm to 1.53 μm as shown in Figure 10, so pumping can be done anywhere in that band. However, the transmission loss of the optical fiber must be smaller the closer to 1.55 μm, and a high-output light source must be economically available.

[0081] Currently, there are no rare-earth doped optical fiber amplifiers capable of outputting high enough power to be used as a remote pumping light source in the 1.49 μm to 1.53 μm wavelength range, except for fiber Raman lasers. For example, gain-shifted thulium-doped fiber amplifiers (TDFAs) using thulium (Tm) are a future candidate. However, because silica glass cannot be used as the glass base material and a fluoride fiber base material is required, their commercial use has been limited due to reliability and other factors. However, if higher output power becomes possible, they may be used.

[0082] 10 terminal station 11, 12 optical amplifier repeater 21, 22, 23 cable 30 DAS interrogator 31 first pumping light source 32 second pumping light source 33 third pumping light source 34 fourth pumping light source 35 fifth pumping light source 39 optical transmitter / receiver 51, 61, 71, 81 EDF 52, 53, 62, 63, 72, 82 WDM filter 54, 55, 56, 57, 58 BPF 75, 76 circulator

Claims

an optical transmitter that outputs a signal light; an optical fiber for transmitting signal light output from an optical transmitter; an optical receiver for receiving the return signal light; an optical fiber for optical amplification, which is disposed in a portion of the optical fiber in the longitudinal direction and is doped with at least erbium; a pumping light source that remotely pumps the optical fiber for optical amplification; Equipped with the wavelength of the signal light is within the amplification wavelength band of the optical fiber for optical amplification, the wavelength of the pumping light for remotely pumping the optical fiber for optical amplification is longer than 1.49 μm and shorter than the wavelength of the signal light within the amplification band of the optical fiber for optical amplification; Optical amplifier repeater system using remote pumping. 　 The signal light wavelength is set in the 1.55 μm to 1.56 μm band, and the pump light wavelength is set to 1.53 μm.

2. An optical amplifier repeater system using remote pumping according to claim 1. 　 The signal light wavelength is set in the 1.53 μm to 1.56 μm band, and the pump light wavelength is set in the 1.49 μm to 1.52 μm band.

2. An optical amplifier repeater system using remote pumping according to claim 1. 　 the optical fiber for optical amplification that is remotely pumped by the pumping light source is a single-core bidirectional optical amplifier in which signal light and return signal light are transmitted in opposite directions and amplified, the remotely pumped optical amplifiers are connected in two stages in series, Between the stages of the optical amplifier, An optical circulator that splits and combines light according to the direction of travel, a functional element which is an optical attenuation element or an optical filter; mirrors that reflect light back onto its original path.

2. An optical amplifier repeater system using remote pumping according to claim 1. 　 the optical transmitter is an optical transmitter that outputs sensing probe light, the optical receiver is an optical receiver that receives backscattered return light of the sensing probe light propagating through the optical fiber to detect distant environmental information; 5. An optical amplifier repeater system using remote pumping according to claim 1.

Images