Raman amplifier and design method of Raman amplifier

Abstract

A Raman amplifier is provided with: a Raman amplification fiber that performs Raman amplification while transmitting incoherent light in which relative intensity noise (RIN) is suppressed; and an excitation light source that supplies excitation light to the Raman amplification fiber, in which [Delta] [lambda] [nm] is the full width at half maximum of the wavelength spectrum of the incoherent light, L [km] is the length of the Raman amplification fiber, and D [ps / nm / km] is the absolute value of the wavelength dispersion of the Raman amplification fiber at the center wavelength of the incoherent light. In the Raman-amplified incoherent light, the rotation angle frequency fc [Hz] at which suppression of RIN starts is estimated using the following mathematical formula (1). Fc = 1 / (D * [delta] [lambda] L)... (1)

Description

Technical Field

[0001] The invention relates to a Raman amplifier and a design method of the Raman amplifier. Background Art

[0002] So far, in optical fiber communication, the use of erbium-doped fiber amplifiers (EDFA) has achieved the expansion of transmission distance and transmission capacity. However, at present, it has become an indispensable technology to make use of not only EDFA but also Raman amplification to effectively combine the two. At present, the main Raman amplification used is backward-excited Raman amplification, which injects excitation light into the optical fiber used for Raman amplification so that it propagates in the direction opposite to the propagation direction of the signal light. However, in order to further increase the speed (800Gb / s), extend the distance (1000km transmission), and widen the bandwidth (L, S-band utilization) for the next generation, it is key to use the method called forward-excited Raman amplification and backward-excited Raman amplification at the same time, wherein the forward-excited Raman amplification injects excitation light into the optical fiber used for Raman amplification so that it propagates in the same direction as the propagation direction of the signal light. This method is called bidirectionally excited Raman amplification. In addition, there are reports that by using wavelength multiplexing excitation, flattening and broadbanding of Raman gain can be achieved using only backward-excitation Raman amplification, but flattening of the noise figure (NF) cannot be achieved without using bidirectional Raman amplification (non-patent documents 1 and 2).

[0003] In forward excitation, an excitation light source with small relative intensity noise (Relative Intensity Noise: RIN) is required. The reason for this is described below.

[0004] RIN is an index that normalizes the minute intensity fluctuation component of the laser with the total light output. Raman amplification is a phenomenon that occurs because the lifetime of the excited energy level that generates gain is short ( fsec), so if there is intensity noise in the excitation light source, it will directly become the noise of the signal light through the amplification process. In EDFA, due to the long lifetime of the excitation energy level, There is no such concern. Regarding Raman amplification, the gain per unit length is very small compared to EDFA, but in forward-excitation Raman amplification, since the signal light and the excitation light propagate together in the optical fiber over a long distance, the noise of the excitation light gradually transforms into the noise of the signal light. This is called RIN transfer. In backward-excitation Raman amplification, since the signal light is relative to the excitation light, the time for the excitation light with a certain noise component to cross the signal light is short, and the noise of the excitation light has less influence on the signal light. In addition, since the noise of the excitation light is random, even if the signal light is affected, it will be averaged in the process of relative advancement. From the above, it can be seen that in forward-excitation Raman amplification, the characteristic of low RIN transfer is required, especially in dispersion-shifted fiber (DSF) and non-zero dispersion-shifted fiber (NZDSF) where the group velocity difference between the signal light and the excitation light is small and the time for parallel transmission in the optical fiber is long, the reduction of this RIN transfer is very important (non-patent literature 3, 4, 5). NZDSF is, for example, an optical fiber based on the ITU-TG.655 standard of the International Telecommunication Union (ITU).

[0005] According to non-patent document 6, when amplifying the C-band and L-band together, in order to obtain a flat wavelength gain characteristic (hereinafter referred to as gain flatness) of less than 0.5 dB, it is necessary to multiplex about 6 Fabry-Perot (FP)-laser diode (LD) type excitation lasers with different wavelengths. This technology is called wavelength multiplexing excitation technology and is widely used. Later, C-band and L-band are sometimes combined and recorded as C+L-band.

[0006] On the other hand, the ASE (Amplified Spontaneous Emission) light from the semiconductor optical amplifier (SOA) has a power spectrum with a full width at half maximum (FWHM) of more than 25nm in a wide band. Therefore, compared with the FP-LD type pump laser, it is more advantageous to achieve gain flatness when the C+L-band is amplified at the same time. The results of simulating this situation are described below.

[0007] The simulation conditions are shown in Table 1 below. The so-called standard single-mode fiber (SMF) refers to an optical fiber that complies with the ITU-TG.652 standard. Conditions 1 and 3 are the cases where the ASE light of the SOA is used as the excitation light for Raman amplification. Condition 2 is the case where the laser from the FP-LD type laser is used as the excitation light for Raman amplification.

[0008] [Table 1]

[0009] (Table 1)

[0010]

[0011] Here, since the propagation loss of a standard single-mode optical fiber is about 0.2 dB / km, the loss of a standard single-mode optical fiber with a length of 40 km is 8 dB. The target gain for compensating this loss by Raman amplification is 8 dB. Here, in the usual transmission system design, the balance between the improvement of OSNR (Optical Signal Noise Ratio: optical signal-to-noise ratio) based on forward-excited Raman amplification and the suppression of nonlinear phenomena caused by the increase of optical power in the transmission optical fiber based on Raman amplification is considered, so that the forward-excited Raman amplification shares the compensation of optical fiber loss of 1 / 3 to 1 / 2 of the total length of the transmission system. For example, in the case of sharing 1 / 3, the total length of the transmission system becomes 120 km. This is the same distance as the widely used transmission system.

[0012] Fig.19 (a), (b), and (c) are graphs showing the Raman gain spectra obtained under conditions 1, 2, and 3, respectively. Fig.19 As shown in (a), under condition 1 of using ASE light of SOA, gain flatness in the entire C+L-band region can be achieved by excitation of only three different wavelengths. Fig.19 As shown in (b), under condition 2 using laser light from an FP-LD type laser, Fig.19 Compared with the case (a), the gain flatness in the entire C+L-band region is significantly deteriorated. This result is the same as the result shown in non-patent document 6. In addition, Patent Document 2 and Non-Patent Document 12 also disclose in detail that Raman amplification using broadband ASE light as excitation light is extremely beneficial to gain flattening.

[0013] However, if 8dB of Raman gain and gain flatness are to be achieved, it can be seen from Table 1 that the excitation light power on the short wavelength side (1425nm in Table 1) becomes much larger than the excitation light power on the long wavelength side. This is a phenomenon caused by using the signal light power on the short wavelength side to amplify the signal light power on the long wavelength side, which is called inter-channel stimulated Raman scattering (ISRS) and is particularly significant in broadband transmission such as C+L-band (Non-Patent Document 7). Specifically, under condition 1, the excitation light power on the short wavelength side (1425nm) required to obtain 8dB of Raman gain reaches 350mW.

[0014] On the other hand, the ASE light from the multi-stage connected SOA can reach a value of 200mW or more, but it is not easy to achieve more than 300mW. Therefore, as in condition 3 of Table 1, if the excitation light power of 1425nm is set to 200mW in reality, then Fig.19 As shown in (c), the gain does not reach the target 8dB.

[0015] Furthermore, in an actual system, due to the loss of various optical components used to connect the excitation light source and the transmission optical fiber, it is believed that the required excitation light power will further increase by more than 30%.

[0016] The FWHM of the currently widely used FP-LD type excitation laser is as small as about 1nm, so the total excitation power can be easily increased through wavelength multiplexing excitation technology. However, since the ASE light from the SOA is broadband, it is not suitable for the widely used wavelength multiplexing excitation. The FWHM of the ASE light from the SOA reaches about 25nm or more, for example, roughly equal to the C-band, that is, 30nm.

[0017] Here, as one of the means to increase the pumping power of the same wavelength, there is polarization multiplexing technology. By setting the output fiber of SOA as polarization-maintaining fiber, polarization multiplexing technology can be easily applied. However, in reality, the polarization multiplexing optical component itself has loss, so the pumping power that can be achieved by this method also has a limit. Therefore, in order to amplify the C+L-band using the broadband nature of ASE light, it is necessary to amplify the ASE light on the short wavelength side by some means and method.

[0018] Prior Art Literature

[0019] Patent Literature

[0020] Patent Document 1: Japanese Patent Application Publication No. 2016-212370

[0021] Patent Document 2: U.S. Patent No. 07215836

[0022] Non-patent literature

[0023] Non-patent literature 1: KADO, Soko, et al. Broadband flat-noise Raman amplifier using low-noise bidirectionally pumping sources. In: Proceeding 27th European Conference on Optical Communication (Cat. No. 01TH8551). IEEE, 2001.p.38-39.

[0024] Non-patent literature 2: EMORI, Yoshihiro; KADO, Soko; NAMIKI, Shu. Independent control of the gain and noise figure spectra of Raman amplifiers using bi-directional pumping. Furukawa Review, 2003, 23: 11-15.

[0025] Non-patent document 3: PELOUCH, Wayne S. Raman amplification: An enabling technology for long-haul coherent transmission systems. Journal of Lightwave Technology, 2015, 34.1: 6-19.

[0026] Non-patent document 4: KEITA, Kafing, et al. Relative intensity noise transfer of large-bandwidth pump lasers in Raman fiber amplifiers. JOSA B, 2006, 23.12: 2479-2485.

[0027] Non-patent literature 5: FLUDGER, CRS; HANDEREK, V.; MEARS, RJPump to signal RINtransfer in Raman fiber amplifiers. Journal of Lightwave Technology, 2001, 19.8: 1140.

[0028] Non-patent literature 6: NAMIKI, Shu; EMORI, Yoshihiro. Broadband Raman amplifiers design and practice. In: Optical Amplifiers and Their Applications. Optical Society of America, 2000. p. OMB2.

[0029] Non-Patent Document 7: SAAVEDRA, Gabriel, et al. Inter-channel stimulated Raman scattering and its impact in wideband transmission systems. In: 2018 Optical Fiber Communications Conference and Exposition (OFC). 2018. p. Th1C.3.

[0030] Non-Patent Document 8: YAMATOYA, T.; KOYAMA, F.; IGA, K. Noise suppression and intensity modulation using gain-saturated semiconductor optical amplifier. In: Optical Amplifiers and Their Applications. Optical Society of America, 2000. p. OMD12.

[0031] Non-Patent Document 9: KOYAMA, F. High power superluminescent diodes for multi-wavelength light sources. In: Conference Proceedings. LEOS′97. 10th Annual Meeting IEEE Lasers and Electro-Optics Society 1997 Annual Meeting. IEEE, 1997. p. 333-334.

[0032] Non-Patent Document 10: ZHAO, Mingshan; MORTHIER, Geert; BAETS, Roel. Analysis and optimization of intensity noise reduction in spectrum-sliced WDM systems using a saturated semiconductor optical amplifier. IEEE Photonics Technology Letters, 2002, 14.3: 390-392.

[0033] Non-patent literature 11: MORIMOTO, Masahito, et al. Co-propagating dual-order distributed Raman amplifier utilizing incoherent pumping. IEEE Photonics Technology Letters, 2017, 29.7: 567-570.

[0034] Non-patent literature 12: VAKHSHOORI, D., et a1. Raman amplification using high-powerincoherent semiconductor pump sources. In: Optical Fiber Communication Conference. Optical Society of America, 2003. p. PD47. Summary of the invention

[0035] -Problems that the invention aims to solve-

[0036] As a method of amplifying the ASE light of SOA, Raman amplification technology, especially concentrated constant type Raman amplification technology, can be considered. As disclosed in Patent Document 1, ASE light, which is incoherent light, remains incoherent light even after Raman amplification. Therefore, Raman amplification is effective as a method of amplifying the ASE light of SOA.

[0037] On the other hand, ASE light of multi-stage connected SOAs has low-frequency RIN suppressed (Non-Patent Documents 8, 9, 10). This is a very useful characteristic from the viewpoint of suppressing RIN shift, which is a problem of forward excited Raman amplification.

[0038] Therefore, it is preferable if Raman amplification of ASE light with suppressed RIN can be performed while maintaining the RIN suppression characteristic.

[0039] Here, it is known that the RIN-suppressed band (hereinafter sometimes referred to as the RIN suppression band) of the RIN-suppressed ASE light will be narrowed after propagating in the optical fiber (Non-patent Document 12). In Non-patent Document 12, it is pointed out that the narrowing of the RIN suppression band is caused by the wavelength dispersion of the optical fiber.

[0040] However, the quantitative relationship between the RIN suppression band and the wavelength dispersion of the optical fiber is not shown in Non-Patent Document 12. Furthermore, it is not clear how the RIN suppression band and the RIN itself will change when the ASE light is Raman amplified.

[0041] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Raman amplifier having a controlled RIN suppression band and a method for designing the Raman amplifier.

[0042] -Methods to solve the problem-

[0043] In order to solve the above-mentioned problems and achieve the purpose, one method of the present invention is a Raman amplifier, which comprises: a Raman amplification optical fiber that performs Raman amplification while transmitting incoherent light with suppressed relative intensity noise (RIN); and an excitation light source that supplies excitation light to the Raman amplification optical fiber. If the half-width at half maximum of the wavelength spectrum of the incoherent light is set to Δλ [nm], the length of the Raman amplification optical fiber is set to L [km], and the absolute value of the wavelength dispersion of the Raman amplification optical fiber at the center wavelength of the incoherent light is set to D [ps / nm / km], then in the incoherent light that has undergone Raman amplification, the corner frequency (corner frequency) fc [Hz] at which the suppression of RIN begins is estimated using the following mathematical formula (1).

[0044] fc=1 / (D·Δλ·L)…(1)

[0045] A combination of the Δλ, L, and D may be provided such that the fc becomes 10 GHz or more.

[0046] A combination of the Δλ, L, and D may be provided such that the fc becomes 1 GHz or more.

[0047] A combination of the Δλ, L, and D may be provided such that the fc becomes 500 MHz or more.

[0048] A combination of the Δλ, L, and D may be provided such that the fc becomes 100 MHz or more.

[0049] Alternatively, the incoherent light may be output from an incoherent light source including a plurality of semiconductor optical amplifiers connected in stages.

[0050] Alternatively, the incoherent light source may be driven so that relative intensity noise (RIN) and ripple are suppressed simultaneously in the incoherent light.

[0051] One embodiment of the present invention is a method for designing a Raman amplifier, the Raman amplifier comprising: a Raman amplification optical fiber that performs Raman amplification while transmitting incoherent light with suppressed relative intensity noise (RIN); and an excitation light source that supplies excitation light to the Raman amplification optical fiber. If the full width at half maximum of the wavelength spectrum of the incoherent light is set to Δλ [nm], the length of the Raman amplification optical fiber is set to L [km], and the absolute value of the wavelength dispersion of the Raman amplification optical fiber at the center wavelength of the incoherent light is set to D [ps / nm / km], then in the incoherent light that has undergone Raman amplification, the corner frequency fc [Hz] at which suppression of RIN begins is estimated using the following mathematical formula (1).

[0052] fc=1 / (D·Δλ·L)…(1)

[0053] -Effects of the Invention-

[0054] According to the present invention, a Raman amplifier having a controlled RIN suppression band and a method for designing the Raman amplifier can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 is a schematic diagram of a light source device having an incoherent light source.

[0056] Figure 2 It is shown Figure 1 A diagram of a portion of a light source module is shown.

[0057] Figure 3 FIG. 1 is a diagram showing an example of a power spectrum of output light from a power amplifier.

[0058] Figure 4 This is a diagram showing an example of the power (Pf) of the output light relative to Ib.

[0059] Figure 5 It is a diagram showing an example of the RIN spectrum of output light.

[0060] Figure 6 It is a diagram schematically showing a spectrum in which RIN is suppressed.

[0061] Figure 7 This is a diagram showing an example of the power spectrum of output light when Is is set to 0 mA and Ib is changed.

[0062] Figure 8 It is a diagram showing an example of the power spectrum of output light when Is is changed.

[0063] Fig. 9 This is a schematic diagram of the Raman amplifier according to the first embodiment.

[0064] Fig.10This is a schematic diagram of a Raman amplifier according to the second embodiment.

[0065] Fig.11 This is a diagram schematically showing RIN spectra of incoherent, non-amplified light and Raman amplified light in the Raman amplifier according to the first and second embodiments.

[0066] Fig.12 This is a diagram for explaining the narrowing of the RIN suppression band.

[0067] Fig.13 It is a diagram showing the RIN spectra of measurement examples 500, 501, and 600.

[0068] Fig.14 It is a diagram showing the RIN spectra of measurement examples 510, 510-2, 610, and 610-2.

[0069] Fig.15 It is a diagram showing the RIN spectrum of measurement examples 510 and 530.

[0070] Fig.16 It is a diagram showing the RIN spectrum of measurement examples 521-2 and 521Rb-2.

[0071] Fig.17 It is a diagram showing the RIN spectra of the measurement examples 520Rf, 520Rb, and 610.

[0072] Fig.18 This is a schematic diagram of a Raman amplification system including the Raman amplifier according to the first embodiment.

[0073] Fig.19 is a diagram showing a Raman gain spectrum. DETAILED DESCRIPTION

[0074] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In addition, the present invention is not limited to the embodiments described below. Furthermore, in the description of the accompanying drawings, the same reference numerals are appropriately given to the same parts, and repeated descriptions are appropriately omitted.

[0075] The inventors of the present invention have found that when incoherent light with suppressed RIN propagates in an optical fiber, the corner frequency fc [Hz] at which RIN suppression starts shifts to the low frequency side according to the following mathematical formula (1), thereby narrowing the RIN suppression band.

[0076] fc=1 / (D·Δλ·L)…(1)

[0077] Here, Δλ[nm] is the half-maximum full width of the wavelength spectrum of the incoherent light, L[km] is the length of the optical fiber, and D[ps / nm / km] is the absolute value of the wavelength dispersion of the optical fiber at the center wavelength of the incoherent light. Furthermore, it was found that when the incoherent light with suppressed RIN is Raman amplified in the optical fiber, the corner frequency fc is also simply expressed by mathematical formula (1).

[0078] Therefore, it was found that if the corner frequency fc is estimated using the mathematical formula (1) to design or manufacture a Raman amplifier, a Raman amplifier having a preferred RIN suppression band in a desired frequency band can be realized.

[0079] Hereinafter, first, the structure of an incoherent light source and the suppression of RIN and ripple in the incoherent light source will be described, and then the structure of a Raman amplifier and its characteristics will be described.

[0080] <An example of the structure of an incoherent light source>

[0081] Figure 1 1 is a schematic diagram of a light source device including an incoherent light source. The light source device 100 includes a light source module 10 and driving devices 101 and 102. The light source module 10 is an example of an incoherent light source.

[0082] The light source module 10 includes a seed light source 11 as an SOA, an optical isolator 12, a power amplifier 13 as an SOA, an optical isolator 14, and an output optical fiber 15. The seed light source 11, the optical isolator 12, the power amplifier 13, and the optical isolator 14 are optically cascade-connected in this order through optical fibers, optical elements, etc. That is, the light source module 10 includes a multi-stage connected SOA.

[0083] The seed light source 11 outputs incoherent seed light L1 having a given frequency band. Here, the seed light L1 is ASE light of SOA. The so-called incoherent light is not a laser source oscillating in a single or multiple discrete modes (longitudinal modes), but refers to light composed of a collection of unrelated photons with a continuous spectrum. The given frequency band is a wide band, for example, a wavelength of 25 nm or more. The optical isolator 12 transmits the seed light L1 and inputs it into the power amplifier 13, and prevents the return light coming from the side of the power amplifier 13 from being input into the seed light source 11. The optical isolator 12 prevents or reduces the instability of the operation of the seed light source 11 due to the input of the return light.

[0084] The power amplifier 13 optically amplifies the input seed light L1 and outputs it as incoherent light L2. The optical isolator 14 transmits the incoherent light L2 and inputs it to the output optical fiber 15, and blocks the light coming from the output optical fiber 15 from being input to the power amplifier 13. The optical isolator 14 prevents or reduces the instability of the operation of the power amplifier 13 caused by the input of the return light.

[0085] The output optical fiber 15 is an optical fiber that guides the incoherent light L2 to the outside of the light source module 10 .

[0086] The driving devices 101 and 102 are known driving devices for SOA. The driving device 101 supplies a driving current C1 to the seed light source 11 . The driving device 102 supplies a driving current C2 to the power amplifier 13 .

[0087] Figure 2 It is shown Figure 1 FIG. 1 is a diagram showing a portion of the light source module 10. The power amplifier 13 has a first end surface 13a and a second end surface 13b facing each other. The power amplifier 13 receives the seed light L1 from the first end surface 13a and outputs the incoherent light L2 to the outside from the second end surface 13b.

[0088] The first end face 13a and the second end face 13b are subjected to a reflection reduction treatment such as AR (Ant-Reflection) coating. In addition, the first end face 13a and the second end face 13b may be subjected to a reflection reduction treatment by being tilted relative to the optical axis of the optical amplification waveguide provided in the power amplifier 13. Such a structure is also called a tilted waveguide structure.

[0089] The end surface reflectivity of the first end surface 13a is defined as R1, and the end surface reflectivity of the second end surface 13b is defined as R2. Then, R1 and R2 are, for example, 10 -3 with 10 -5 Or, (R1×R2) 1 / 2 For example, 10 -3 with 10 -5 In the low reflectivity point of view, 10 -3 with 10 -5 The range of values ​​between is an example of a numerical range that is sufficiently practical and implementable. -3 with 10 -5 The range between refers to the range with 10 as the upper limit. -3 and 10 as the lower limit -5 range.

[0090] <Characteristics of the light source module 10 >

[0091] The characteristics of the light source module 10 will be described. Figure 3 FIG. 1 is a diagram showing an example of a power spectrum of output light (incoherent light) of a power amplifier. Figure 3 The figure shows a case where Is is set to 50 mA and Ib is set to 800 mA when the drive current C1 supplied to the seed light source 11 is set to Is and the drive current C2 supplied to the power amplifier 13 is set to Ib. Figure 3 The power spectrum of the output light has a roughly Gaussian spectrum shape, with a central wavelength (measured using the RMS method) of 1445 nm and a FWHM of approximately 28 nm.

[0092] Figure 4 1 is a diagram showing an example of the power (Pf) of the output light (incoherent light) from the power amplifier relative to Ib. Figure 4 In the example, set Is to 50mA. Figure 4 In the case of , it can be seen that when Ib is 1000mA, Pf is about 200mW. In addition, Figure 3 , Figure 4 This is an example of a light source module manufactured by using semiconductor optical amplifiers having substantially the same characteristics as a seed light source and a power amplifier.

[0093] Here, the inventors of the present invention found that Figure 1 The light source module 10 shown can realize a state in which both RIN and ripple are suppressed. Hereinafter, suppression of RIN will be described first, suppression of ripple will be described next, and further, the relationship between suppression of RIN and suppression of ripple will be described.

[0094] <Inhibition of RIN>

[0095] Figure 5 is a diagram showing an example of the RIN spectrum of output light. Specifically, Figure 5 The results of measuring the RIN of the output lights from No. 1 to No. 3 light source modules manufactured as the light source module 10 are shown. In addition, semiconductor optical amplifiers having substantially the same characteristics are used as seed light sources and power amplifiers, respectively, to manufacture the No. 1 to No. 3 light source modules, which have the same characteristics as those of the light source modules 10. Figure 3 , Figure 4 In addition, for the light source modules No. 1 to No. 3, regarding the end face reflectivity, it was confirmed that (R1×R2) 1 / 2 For 10 -3 with 10 -5 In addition, Figure 5 In FIG. 5 , Ts represents the shell temperature of the seed light source (25° C.), and Tb represents the shell temperature of the power amplifier (25° C.).

[0096] exist Figure 5In the above, “only Ib” means that only a driving current of 100mA is supplied to the power amplifier, and no driving current is supplied to the seed light source (that is, the driving current supplied is 0mA). In this case, the seed light source to be amplified is not input to the power amplifier, so it acts as a simple ASE light source. On the other hand, “Is+Ib” means that a driving current of 100mA is supplied to the power amplifier, and a driving current of 200mA is supplied to the seed light source.

[0097] according to Figure 5 It can be seen that in the case of "only Ib", a relatively flat RIN spectrum with respect to frequency can be obtained in any of the light source modules No. 1 to No. 3. This RIN is considered to be derived from the ASE-ASE beat noise in the output light (ASE) output from the power amplifier as the ASE light source.

[0098] In contrast, in the case of "Is+Ib", it can be seen that in any of the light source modules No. 1 to No. 3, RIN is suppressed from a specific frequency (corner frequency fc) to the low frequency side. In addition, in the case of "Is+Ib", since the seed light (ASE light) output by the seed light source is input into the power amplifier, the number of photons inside increases, and it is believed that the power amplifier operates in a gain saturation state. Here, it is worth noting that in Figure 5 In the driving state, although the current Ib is relatively small, such as 100 mA, and the power of the output light from the power amplifier is relatively small, it has become a state of RIN suppression.

[0099] Figure 6 It is a diagram schematically showing a spectrum in which RIN is suppressed.

[0100] Here, since the output light from the light source such as the light source module in the embodiment is the ASE light of the SOA, the RIN should be determined by the magnitude of the ASE-ASE inter-beat noise. Figure 6 In FIG. 2 , the level of line 210 is the level of the ASE-ASE inter-beat noise.

[0101] The power spectrum width of ASE light of a common SOA is several tens of nm, which is several THz when expressed in frequency. Since the measurement band of RIN is sufficiently small, which is several tens of GHz, RIN is calculated by the following mathematical formula (Fiber optic test and measurement / edited by Dennis Derickson. Upper Saddle River, 1998.).

[0102] RIN=0.66 / Δ ASE [Hz-1 ]

[0103] Here, 0.66 is the coefficient when the power spectrum of ASE light is Gaussian, and Δv ASE is the FWHM of the power spectrum.

[0104] For example, Figure 3 The power spectrum of the output light shown is Gaussian, and its FWHM is about 30nm. Therefore, if the above mathematical formula (1) is used, the RIN is calculated to be about -127dB / Hz. This value is similar to Figure 5 The results shown are largely consistent.

[0105] In contrast, the RIN of the SOA operating in the gain saturation state is suppressed in the low frequency region 211 below the corner frequency 213. For example, there is a report that the RIN is suppressed by about 10 dB to 20 dB from the level of line 210 (Non-patent Documents 8, 9, 10). In addition, line 212 is the level of short noise.

[0106] In this specification, the term "RIN is suppressed" means that the RIN is suppressed by 10 dB or more compared to the RIN calculated by the above formula. The degree of suppression of RIN may be further reduced to 16 dB or less, and further reduced to 20 dB or less.

[0107] <Suppression of ripples>

[0108] Specifically, the power spectrum of output light from a light source module (referred to as light source module No. 4) having the same output characteristics as light source module No. 1 was measured while changing the driving conditions of the seed light source and the power amplifier.

[0109] Figure 7 This is a diagram showing an example of the power spectrum of output light when Is is set to 0 mA and Ib is changed. Figure 8 1 is a diagram showing an example of the power spectrum of output light when Is is changed.

[0110] exist Figure 7 In the figure, ripples are generated in both cases where Ib is 200mA and 800mA. In particular, when Ib is 200mA, ripples are generated even though the output light power is relatively small, about 18mW. In addition, when Is is set to 0mA, ripples are particularly likely to be generated. As mentioned above, the ripples are caused by the reflection of light on the two end faces of the SOA. However, although the end face reflectivity of the power amplifier is (R1×R2) 1 / 2 For 10 -3 with 10 -5Even in a sufficiently small range such as the range between 1 and 2, ripples are generated. Even with such low drive current (low power) and low reflectivity, ripples are generated, which can be said to be an example of how difficult it is to suppress ripples in the excitation light source for Raman amplification, whose light output can reach several hundred mW depending on the situation. In addition, the ripples contain components of different multiple cycles.

[0111] In contrast, Figure 8 As shown in the figure, if Ib is fixed at 800mA and Is is increased to 25mA, the ripple is suppressed, and is further suppressed when Is is 50mA. However, if Is is further increased to 400mA, the ripple increases again.

[0112] like Figure 8 As shown, if the seed light is input to the power amplifier, the gain becomes saturated and there is a region where the ripples are suppressed. This means that even if the end face reflectivity of the power amplifier is not reduced to the extent that the ripples are suppressed, the ripples can be suppressed by controlling the power of the seed light input to the power amplifier. In addition, since the ASE light spectrum of the SOA is relatively wide, ranging from a few nm to tens of nm, it is not easy to implement a process to reduce the end face reflectivity (such as a reflection-free process) to cover such a wide frequency band. Therefore, the technology of suppressing ripples by controlling the power of the seed light is a technology that can alleviate the setting conditions and manufacturing difficulties of the end face reflectivity of the power amplifier, and is a very effective technology in practical use.

[0113] In the suppression of ripples, the smaller the ripples, the better. The size of the ripples is represented by the maximum value of the width (peak to bottom) of the ripples appearing on the power spectrum at a given wavelength (e.g., around 1510 nm). In this case, it is preferable to suppress the ripples, for example, so that the width of the ripples from the peak to the bottom is less than 5 dB, less than 3 dB, less than 1 dB, or less than 0.5 dB.

[0114] Therefore, as a method of driving a light source module as a light source, it is preferable to drive the seed light source and the power amplifier by a driving current (Is, Ib) that suppresses both RIN and ripple in incoherent light.

[0115] <Structure of Raman amplifier>

[0116] Fig. 9 Schematic diagram of a Raman amplifier according to Embodiment 1. The Raman amplifier 1000A is a so-called backward pumping type, and includes a Raman amplification optical fiber 300 , an excitation light source 400 , and a WDM (Wavelength Division Multiplexing) coupler 500 .

[0117] The light source device 100 outputs the incoherent light L2 with suppressed RIN from the output optical fiber 15 to the Raman amplification optical fiber 300. In order to supply the excitation light L3 for Raman amplification to the Raman amplification optical fiber 300, the excitation light source 400 outputs the excitation light L3 to the WDM coupler 500. In order to Raman amplify the incoherent light L2, the excitation light L3 has a wavelength that is about 100 nm shorter than the wavelength of the incoherent light L2. The excitation light source 400 includes, for example, an FP-LD type excitation laser with a fiber Bragg grating.

[0118] The WDM coupler 500 outputs the excitation light L3 to the Raman amplification optical fiber 300. The Raman amplification optical fiber 300 performs Raman amplification on the incoherent light L2 while propagating the incoherent light L2. The WDM coupler 500 outputs the Raman amplified light L10, wherein the Raman amplified light L10 is the light that has been Raman amplified on the incoherent light L2. In addition, when the excitation light L3 is not input and the incoherent light L2 is not Raman amplified, the WDM coupler 500 outputs the non-amplified light L11, wherein the non-amplified light L11 is the light that has not been Raman amplified on the incoherent light L2 and has only propagated in the Raman amplification optical fiber 300.

[0119] Fig.10 Schematic diagram of a Raman amplifier according to Embodiment 2. The Raman amplifier 1000B is a so-called forward excitation type, and similarly to the Raman amplifier 1000A, includes a Raman amplification fiber 300 , an excitation light source 400 , and a WDM coupler 500 .

[0120] The light source device 100 outputs the incoherent light L2 from the output optical fiber 15 to the WDM coupler 500. In order to supply the excitation light L3 for Raman amplification to the Raman amplifier optical fiber 300, the excitation light source 400 outputs the excitation light L3 to the WDM coupler 500. The WDM coupler 500 outputs the incoherent light L2 and the excitation light L3 to the Raman amplifier optical fiber 300. While propagating the incoherent light L2, the Raman amplifier optical fiber 300 Raman amplifies the incoherent light L2 and outputs the Raman amplified light L10. In addition, when the excitation light L3 is not input and the incoherent light L2 is not Raman amplified, the Raman amplifier optical fiber 300 outputs the non-amplified light L11, wherein the non-amplified light L11 is the light after the incoherent light L2 is not Raman amplified and is only propagated in the Raman amplifier optical fiber 300.

[0121] Fig.11 The diagram schematically shows the RIN spectra of the incoherent light L2 , the non-amplified light L11 , and the Raman amplified light L10 in the Raman amplifiers 1000A and 1000B according to the first and second embodiments.

[0122] exist Fig.11In FIG. 2 , the level of line 220 is the level of the ASE-ASE beat noise, and the level of line 222 is the level of the short noise.

[0123] The corner frequency 223 is the corner frequency of the incoherent light L2 before propagating in the Raman amplification optical fiber 300. Fig.11 The RIN of the incoherent light L2 is suppressed in the low frequency region 221 below the corner frequency 223.

[0124] According to the inventors' intensive research, the corner frequency of the non-amplified light L11 as the incoherent light L2 propagating through the Raman amplification fiber 300 is shifted to the low frequency side like the corner frequency 223. The corner frequency fc is represented by the following mathematical formula (1).

[0125] fc=1 / (D·Δλ·L)…(1)

[0126] Δλ[nm] is the half-maximum full width of the wavelength spectrum of the incoherent light L2, L[km] is the length of the Raman amplifier fiber 300, and D[ps / nm / km] is the absolute value of the wavelength dispersion of the Raman amplifier fiber 300 at the center wavelength of the incoherent light L2.

[0127] Therefore, the RIN of the non-amplified light L11 is suppressed in the low-frequency region 221 a lower than the corner frequency 223 a , and the RIN suppression band is narrowed.

[0128] Furthermore, according to the in-depth study of the present inventors, the corner frequency of the Raman amplified light L10, which is the light propagated in the Raman amplification optical fiber 300 as the incoherent light L2 and Raman amplified, is expressed by the mathematical formula (1) as the corner frequency 223b, similarly to the case of the non-amplified light L11. Therefore, the RIN of the non-amplified light L11 is suppressed in the low-frequency region 221b lower than the corner frequency 223b, but the RIN suppression band is not narrowed by Raman amplification. In addition, the degree of suppression of RIN is reduced by Raman amplification, but even so, it is still lower than the level of the ASE-ASE inter-beat noise of the line 220.

[0129] In addition, the corner frequency fc measured in practice may not always be consistent with the value obtained from the mathematical formula (1) due to errors, etc., but even so, it is still roughly consistent with the value obtained from the mathematical formula (1), so it can be said that the frequency at which the suppression of RIN starts can be estimated by the mathematical formula (1). In addition, since the error is likely to become large when the length of the Raman amplifier fiber 300 is short, when the absolute value of the wavelength dispersion of the Raman amplifier fiber 300 is small, and when the wavelength dispersion characteristics of the Raman amplifier fiber 300 deviate in the long side direction, it is necessary to pay attention to the measurement and application of the measured value. In addition, when the full width at half maximum of the wavelength spectrum of the incoherent light L2 is wide, the dispersion slope of the Raman amplifier fiber 300 becomes a factor of error, so it is necessary to pay attention.

[0130] Fig.12 This is a diagram illustrating the narrowing of the RIN suppression band after propagation in the Raman amplifier fiber 300. When the incoherent light L2 composed of many photons with different wavelengths and having a central wavelength of λ0 and a FWHM of Δλ propagates in the Raman amplifier fiber 300 of length L[km], if the absolute value of the wavelength dispersion of the Raman amplifier fiber 300 is set to D[ps / nm / km], the propagation time of each photon of the non-amplified light L11 (or the Raman amplified light L10) after propagation has a time width of about Δτ=D·Δλ·L. Therefore, if the observation time is longer than Δτ, the temporal fluctuation caused by the propagation cannot be observed, and if the observation time is shorter than Δτ, the fluctuation can be observed. For this reason, if the corner frequency fc is defined by 1 / ΔT=fc, f_c can be set as an indicator representing the boundary of the RIN suppression band.

[0131] <Corner frequency measurement results: Non-amplified light>

[0132] Next, the measurement results of the corner frequency in the combination of various incoherent light and Raman amplification fiber are described. Table 2 shows the combination of ASE light of SOA as incoherent light and Raman amplification fiber, and the value of the corner frequency fc calculated based on L, D, and Δλ. However, in the measurement example of Table 2, the structure of backward excitation is the same as that of embodiment 1, but the RIN spectrum of non-amplified light without Raman amplification is also measured.

[0133] In Table 2, HNLF refers to High NonLinearity Fiber. The number of light sources refers to the number of light source modules. When the number of light sources is 2, the ASE light from the SOA of the two light source modules is polarization combined. The wavelength dispersion D is an absolute value.

[0134] [Table 2]

[0135]

[0136] Fig.13 500, 501, and 600. The measurement example 600 refers to the ASE light (equivalent to Figure 1 The RIN spectrum of the incoherent light L2 in . Fig.13 In the measurement example 500, the corner frequency of the RIN spectrum of the incoherent light (non-amplified light) after propagating through the Raman amplification optical fiber is f 500 In measurement example 501, f 501 , the calculated values ​​in Table 2 are shown by dotted lines, but are substantially consistent with the corner frequencies obtained by measurement.

[0137] Next, when ASE lights of different wavelengths propagate in the same Raman amplifier fiber, if the spectra of these ASE lights do not overlap, the half-maximum full widths of these wavelength spectra can be added together to be considered as the half-maximum full width of the total wavelength spectrum. In addition, when Raman amplifier fibers of different wavelength dispersions are connected to form a Raman amplifier fiber, the product of the wavelength dispersion and the length of the Raman amplifier fiber is accumulated. Therefore, for example, when two ASE lights of non-overlapping spectra (the respective half-maximum full widths are set to Δλ1 and Δλ2) propagate in a Raman amplifier fiber connected to a first Raman amplifier fiber and a second Raman amplifier fiber of different wavelength dispersions and lengths, the corner frequency is expressed by the following mathematical formula (2). D1 and L1 are the wavelength dispersion and length of the first Raman amplifier fiber, respectively, and D2 and L2 are the wavelength dispersion and length of the second Raman amplifier fiber, respectively.

[0138] fc=1 / [(Δλ1+Δλ2)·(D1·L1+D2·L2)]…(2)

[0139] Fig.14 The diagram shows the RIN spectra of the measurement examples 510, 510-2, 610, and 610-2. The measurement examples 610 and 610-2 refer to the ASE light (equivalent to the ASE light) before propagation in the Raman amplification fiber (NZDSF). Figure 1 The RIN spectrum of the incoherent light L2 in . Fig.14 In the measurement example 510, the corner frequency is f 510 , in Measurement Example 510-2, as f 510-2 , showing the calculated values ​​in Table 2, but are roughly consistent with the corner frequencies obtained by measurement. In particular, in measurement example 510-2, the corner frequency calculated by mathematical formula (2) is roughly consistent with the measurement results.

[0140] Table 3 shows the combination of ASE light of SOA and Raman amplification fiber, and the value of the corner frequency fc calculated from L, D, and Δλ. In the measurement example of Table 3, as the Raman amplification fiber, a Raman amplification fiber in which two Raman amplification fibers (optical fibers 1 and 2) having different lengths and wavelength dispersions are connected in this order is used.

[0141] [Table 3]

[0142]

[0143] Fig.15 510 and 530. Fig.14 The same situation is shown. Fig.15 In the measurement example 510, the corner frequency is f 510 , in Measurement Example 530, as f 530 , which shows the calculated values ​​of Table 2 or Table 3, is substantially consistent with the corner frequency obtained by measurement. In particular, in measurement example 530, the corner frequency calculated by mathematical formula (2) is substantially consistent with the measurement result.

[0144] <Corner frequency measurement results: Raman amplified light>

[0145] Next, the measurement results of the corner frequency of the RIN spectrum of the Raman amplified light are described. In order to Raman amplify the ASE light while maintaining the superiority of the ASE light with suppressed RIN, it is important to determine whether the RIN suppression band of the ASE light is preserved during the Raman amplification process.

[0146] Fig.16 Graphs showing the RIN spectra of measurement examples 521-2 and 521Rb-2. Measurement example 521-2 is the RIN spectrum of the non-amplified light in the Raman amplification optical fiber shown in Table 2, and measurement example 521Rb-2 is the RIN spectrum when Raman amplification is performed using the backward excitation structure of embodiment 2. Fig.16 In the measurement example 521-2, the corner frequency is f 521-2 , in Measurement Example 521Rb-2, as f 521Rb-2 The calculated values ​​in Table 2 are shown, but they are roughly consistent with the corner frequencies obtained by measurement. Fig.16 As shown, it was found that when Raman amplification is performed on ASE light with suppressed RIN, although the RIN of the Raman amplified light increases, the corner frequency maintains the RIN suppression band according to the mathematical formula (1).

[0147] Table 4 shows the combination of ASE light and Raman amplification fiber of SOA, and the value of the corner frequency fc calculated from L, D, and Δλ. In the measurement examples 520Rf and 520Rb, the ASE light and the Raman amplification fiber are the same, but in the measurement example 520Rf, Raman amplification is performed by the forward excitation structure as in the embodiment 2, and in the measurement example 520Rb, Raman amplification is performed by the backward excitation structure as in the embodiment 1.

[0148] [Table 4]

[0149]

[0150] Fig.17 520Rf, 520Rb, and RIN spectra of measurement examples 610. The measurement example 610 refers to ASE light (equivalent to Figure 1 The RIN spectrum of the incoherent light L2 in the Fig.17 It can be seen that no difference in the corner frequency is observed depending on the excitation direction. However, it is found that the RIN characteristics of the backward excitation are relatively stable.

[0151] As described above, if the full width at half maximum of the wavelength spectrum of the incoherent light is set to Δλ [nm], the length of the Raman amplification fiber is set to L [km], and the absolute value of the wavelength dispersion of the Raman amplification fiber at the center wavelength of the incoherent light is set to D [ps / nm / km], then in the incoherent light that has undergone Raman amplification, the corner frequency fc [Hz] at which the suppression of RIN starts is calculated according to the following mathematical formula (1), thereby being able to estimate and control the RIN suppression band of the incoherent light that has undergone Raman amplification, and obtaining incoherent light that has undergone Raman amplification while maintaining the superiority of RIN suppression.

[0152] fc=1 / (D·Δλ·L)…(1)

[0153] As shown in Non-Patent Documents 4 and 5, considering that the RIN shift can reach about 100 MHz, it is preferable to set the combination of Δλ, L, and D such that fc is 100 MHz or more. Furthermore, it is preferable to set the combination of Δλ, L, and D such that fc is 500 MHz or more, 1 GHz or more, or 10 GHz or more.

[0154] <Raman Amplification System>

[0155] Fig.18 Schematic diagram of a Raman amplification system including the Raman amplifier according to Embodiment 1. The Raman amplification system 10000 includes an excitation light source 10001 , a WDM coupler 10002 , and a transmission optical fiber 10003 .

[0156] The excitation light source 10001 includes: a light source device 100A that outputs ASE light L2A of SOA with a central wavelength of λ1; a light source device 100B that outputs ASE light L2B of SOA with a central wavelength of λ2; a light source device 100C that outputs ASE light L2C of SOA with a central wavelength of λ3; and a Raman amplifier 1000A that performs Raman amplification on the ASE light L2A and outputs it as Raman amplified light L10A. Here, λ1<λ2<λ3. For example, as shown in Table 1, λ1, λ2, and λ3 are 1425nm, 1455nm, and 1495nm.

[0157] The light source devices 100A, 100B, and 100C all have Figure 1 The Raman amplifier 1000A has the same structure as the light source device 100 shown in FIG. Fig. 9 The structure shown.

[0158] The light source device 20000 outputs the wavelength multiplexed signal light L30 of C+L band to the WDM coupler 10002. The WDM coupler 10002 outputs the wavelength multiplexed signal light L30 and the wavelength multiplexed excitation light L20 which is the light wavelength multiplexed by Raman amplified light L10A, ASE light L2B and ASE light L2C to the transmission optical fiber 10003.

[0159] The transmission optical fiber 10003 is, for example, an SMF having a length of 40 km. The transmission optical fiber 10003 supplies the wavelength multiplexing pumping light L20 to perform Raman amplification on the wavelength multiplexing signal light L30, thereby obtaining the amplified wavelength multiplexing signal light L31.

[0160] If used Fig.19 As explained above, in order to achieve high gain flatness in the entire C+L-band region, it is necessary to amplify the ASE light on the short wavelength side by some means or method. Fig.18 In the Raman amplification system 10000, since the short-wavelength ASE light L2A is amplified by the Raman amplifier 1000A, for example, Fig.19 As shown in (a) of FIG. 1 , high gain flatness can be achieved in the entire C+L-band region.

[0161] Furthermore, in the above-described embodiment, the seed light in the light source module is ASE light, but it may be incoherent light such as spontaneous emission (SE).

[0162] In the above embodiment, the seed light source is a semiconductor optical amplifier, but it may also include at least one of an SLD (Super Luminescent Diode), an SOA, and an ASE light source having a rare earth doped optical fiber. Such SLD, SOA, and ASE light sources are preferred as incoherent light sources.

[0163] In addition, the present invention is not limited to the above-mentioned embodiments. The structure formed by appropriately combining the above-mentioned various structural elements is also included in the present invention. In addition, those skilled in the art can easily derive other effects and variations. Therefore, the wider aspects of the present invention are not limited to the above-mentioned embodiments, and various changes can be made.

[0164] -Description of Reference Numerals-

[0165] 10: Light source module

[0166] 11: Seed light source

[0167] 12, 14: Optical isolator

[0168] 13: Power Amplifier

[0169] 13a: 1st end face

[0170] 13b: 2nd end face

[0171] 15: Output fiber

[0172] 100, 100A, 100B, 100C: Light source device

[0173] 101, 102: Driving device

[0174] 210, 212, 220, 222: Line

[0175] 211, 221, 221a, 221b: Low frequency area

[0176] 213, 223, 223a, 223b: Corner frequency

[0177] 300: Raman amplification fiber

[0178] 400: Excitation light source

[0179] 500: WDM coupler

[0180] 1000A, 1000B: Raman amplifier

[0181] 10000: Raman amplification system

[0182] 10001: Excitation light source

[0183] 10002: WDM coupler

[0184] 10003: Optical fiber for transmission

[0185] 20000: Light source device

[0186] C1, C2: driving current

[0187] L1: Seed Light

[0188] L2: Incoherent light

[0189] L2A, L2B, L2C: ASE light

[0190] L3: Excitation light

[0191] L10, L10A: Raman amplification

[0192] L11: Non-amplified light

[0193] L20: wavelength multiplexing excitation light

[0194] L30: wavelength multiplexing signal light

[0195] L31: Amplified wavelength multiplexed signal light.

Claims

1. A Raman amplifier, characterized in that: have: Raman amplification optical fiber performs Raman amplification while transmitting incoherent light having suppressed relative intensity noise (RIN); and an excitation light source for supplying excitation light to the Raman amplification optical fiber, If the full width at half maximum of the wavelength spectrum of the incoherent light is set to Δλ [nm], the length of the Raman amplification fiber is set to L [km], and the absolute value of the wavelength dispersion of the Raman amplification fiber at the center wavelength of the incoherent light is set to D [ps / nm / km], then the corner frequency fc [Hz] at which the suppression of RIN starts in the incoherent light subjected to Raman amplification is estimated using the following mathematical formula (1): fc=1 / (D.Δλ.L)…(1).

2. The Raman amplifier according to claim 1, characterized in that: The Raman amplifier has a combination of the Δλ, L, and D such that the fc becomes 10 GHz or more.

3. The Raman amplifier according to claim 1, characterized in that: The Raman amplifier has a combination of the Δλ, L, and D such that the fc becomes 1 GHz or more.

4. The Raman amplifier according to claim 1, characterized in that: The Raman amplifier has a combination of the Δλ, L, and D such that the fc becomes 500 MHz or more.

5. The Raman amplifier according to claim 1, characterized in that: The Raman amplifier has a combination of the Δλ, L, and D such that the fc becomes 100 MHz or more.

6. The Raman amplifier according to any one of claims 1 to 5, characterized in that: The incoherent light is output from an incoherent light source, wherein the incoherent light source includes semiconductor optical amplifiers connected in multiple stages.

7. The Raman amplifier according to claim 6, characterized in that: The incoherent light source is driven so that relative intensity noise (RIN) and ripple are suppressed simultaneously in the incoherent light.

8. A method for designing a Raman amplifier, characterized in that: The Raman amplifier has: Raman amplification optical fiber performs Raman amplification while transmitting incoherent light having suppressed relative intensity noise (RIN); and an excitation light source for supplying excitation light to the Raman amplification optical fiber, If the full width at half maximum of the wavelength spectrum of the incoherent light is set to Δλ [nm], the length of the Raman amplification fiber is set to L [km], and the wavelength dispersion of the Raman amplification fiber at the center wavelength of the incoherent light is set to D [ps / nm / km], then in the incoherent light subjected to Raman amplification, the corner frequency fc [Hz] at which the suppression of RIN starts is estimated using the following mathematical formula (1): fc=1 / (D·Δλ·I)…(1).

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