Raman amplification excitation light source, light source device, method for driving a Raman amplification excitation light source, method for designing a Raman amplification excitation light source, Raman amplifier, and Raman amplification system

Abstract

To provide a light source, a light source device, a light source driving method, a Raman amplifier, and a Raman amplification system using the same, in which RIN and ripple are suppressed at the same time.SOLUTION: A light source includes a seed light source that outputs incoherent seed light having a predetermined band, and a booster amplifier which is a semiconductor optical amplifier that optically amplifies the seed light input from the first facet and outputs the amplified light from the second facet, and in the booster amplifier, nL which is the product of the refractive index n and the tip length L is set such that the relative intensity noise (RIN) and ripple are simultaneously suppressed in the amplified light.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a light source, a light source device, a method for driving a light source, a Raman amplifier, and a Raman amplification system. [Background technology]

[0002] Until now, erbium-doped fiber amplifiers (EDFAs) have been used in optical fiber communications to increase transmission distance and capacity. However, currently, it has become essential to utilize Raman amplification in addition to EDFAs and to effectively combine the two. Currently, the main type of Raman amplification used is back-pumped Raman amplification, in which excitation light is incident on the Raman amplification optical fiber so that it propagates in the opposite direction to the propagation direction of the signal light. However, for further speed increases (800 Gb / s), longer distances (1000 km transmission), and wider bandwidths (utilization of L and S bands) for the next generation, it is key to use a method called forward-pumped Raman amplification, in which excitation light is incident on the Raman amplification optical fiber so that it propagates in the same direction as the propagation direction of the signal light, simultaneously with back-pumped Raman amplification. This method is called bidirectional-pumped Raman amplification. It has been reported that while Raman gain flattening and broadbanding can be achieved with back-excited Raman amplification alone by using wavelength division multiplexing excitation, noise figure (NF) flattening cannot be achieved without using bidirectional excited Raman amplification (Non-Patent Documents 13, 14).

[0003] For forward excitation, an excitation source with low relative intensity noise (RIN) is required. The reason for this is explained below. RIN is an index that normalizes minute intensity fluctuation components of laser light by the total optical output. In Raman amplification, the lifetime of the excitation level that produces the gain is short (approximately a few fsec), so if there is intensity noise in the excitation light source, it will pass through the amplification process and become noise in the signal light. In EDFA, the lifetime of the excitation level is long (approximately 10 msec), so this risk was not present. Raman amplification has a much smaller gain per unit length compared to EDFA, but in forward-excited Raman amplification, the noise of the excitation light is gradually transferred to the signal light as the signal light propagates together through the optical fiber over long distances. This is called RIN transfer. In backward-excited Raman amplification, the signal light and excitation light are opposite each other, so the time during which the excitation light with a certain noise component intersects with the signal light is short, and the influence of the excitation light noise on the signal light is small. Also, since the noise of the excitation light is random, even if the signal light is affected, it is averaged out as it travels toward the other side. As can be seen from the above, forward-excited Raman amplification requires a low RIN transfer characteristic, and reducing this RIN transfer is particularly important in dispersion-shifted fibers (DSF) and non-zero dispersion-shifted fibers (NZDSF), where the group velocity difference between the signal light and the excitation light is small and the time spent transmitting them in parallel through the optical fiber is long (Non-Patent Documents 1, 3, 4). NZDSF is, for example, an optical fiber that conforms to the ITU-T G.655 standard of the International Telecommunication Union (ITU). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] U.S. Patent Publication No. 2014 / 0153083 [Patent Document 2] U.S. Patent No. 07190861 [Patent Document 3] U.S. Patent No. 07215836 [Patent Document 4] U.S. Patent No. 07236295 [Patent Document 5] U.S. Patent No. 07173757 [License 6] Patent No. 4069110 [License 7] U.S. Patent No. 10938175 [Non-licensed literature]

[0005] [Non-licensed Document 1] PELOUCH, Wayne S. Raman amplification: An enabling technology for long-haul coherent transmission systems. Journal of Lightwave Technology, 2015, 34.1: 6-19. [Non-licensed Document 2] YAMAMOTO, Y.; MACHIDA, S.; NILSSON, O. Amplitude squeezing in a pump-noise-suppressed laser oscillator. Physical Review A, 1986, 34.5: 4025. [Non-licensed Document 3] KEITA, Kafing, et al. Relative intensity noise transfer of large-bandwidth pump lasers in Raman fiber amplifiers. JOSA B, 2006, 23.12: 2479-2485. [Non-licensed Document 4] FLUDGER, CRS; HANDEREK, V.; MEARS, RJ Pump to signal RIN transfer in Raman fiber amplifiers. Journal of Lightwave Technology, 2001, 19.8: 1140. [Non-licensed Document 5] VAKHSHOORI, D., et al. Raman amplification using high-power incoherent semiconductor pump sources. In: Optical Fiber Communication Conference. Optical Society of America, 2003. p. PD47. [Non-Patent Document 6] 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. [Non-Patent Document 7] 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. [Non-Patent Document 8] 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. [Non-Patent Document 9] MORIMOTO, Masahito, et al. Co-propagating dual-order distributed Raman amplifier utilizing incoherent pumping. IEEE Photonics Technology Letters, 2017, 29.7: 567-570.

Non-Patent Document 10

Non-Patent Document 11

Non-Patent Document 12

Non-Patent Document 13

[0006] Raman amplification has lower excitation efficiency compared to optical amplification using EDFA, so a high-power excitation light source is required to obtain sufficient gain. For this reason, Fabry-Perot (FP) type lasers with outputs of several hundred mW or more have been widely put into practical use, but FP type lasers have a large RIN transfer and are considered unsuitable for forward excitation (Non-Patent Documents 3, 4, 5).

[0007] In response to this, it is considered effective to use ASE (Amplified Spontaneous Emission) light from a semiconductor optical amplifier (SOA), which is incoherent light, as the excitation light source for forward-excited Raman amplification in order to suppress RIN transfer (Patent Documents 2, 3, Non-Patent Documents 3, 5).

[0008] For future ultra-high-speed, high-capacity transmission exceeding 800 Gbps, it is desirable that the Raman gain from forward excitation be, for example, 7 dB or more, or even 10 dB or more, in order to significantly improve the overall OSNR (Optical Signal to Noise Ratio) of the system. For this reason, there have been reports of achieving high output using a configuration in which cascaded SOAs are polarization-multiplexed and wavelength-multiplexed (Non-Patent Literature 5).

[0009] While small, low-cost SOA ASE light is certainly a promising candidate, it is known that ripple occurs due to light reflection from both ends of the SOA when attempting to achieve high output (Patent Documents 4, 5, 6).

[0010] According to Non-Patent Literature 3, when broadband incoherent light is used for forward-excited Raman amplification, RIN transfer is suppressed due to the averaging effect. Here, Non-Patent Literature 3 points out that, in order to obtain this averaging effect, there should be no numerous longitudinal modes (i.e., ripples) due to Fabry-Perot oscillation. Therefore, in order to take advantage of the benefits of incoherent light excitation in forward-excited Raman amplification, it is preferable that the incoherent light is high-power and has suppressed ripple.

[0011] In SOA, ripple occurs due to the formation of a Fabry-Perot resonator between the two end faces. Therefore, if the reflectivity at both end faces is zero, no ripple will occur. However, it is practically impossible to make the reflectivity at both end faces of an SOA zero. Thus, how to suppress ripple is an important issue. Conventionally, (1) anti-reflective coatings, (2) oblique waveguide structures, and (3) window structures have been used to suppress ripple. However, preferred excitation light sources for Raman amplification have high output power of several hundred mW or more, so even slight reflection can cause ripple. Also, ASE light has a wide spectral width of about several tens of nanometers, and it is not easy to achieve anti-reflective processing with low reflectivity over this wide spectral width. In response to this, for example, Patent Document 5 shows a method of providing a mirror behind the rear end face of the SOA to relax the requirements for the reflectivity of the anti-reflective coating end face.

[0012] On the other hand, it is known that low-frequency noise can be suppressed when a semiconductor laser is driven with a constant current in a drive current region sufficiently large above the oscillation threshold, i.e., in a saturated state (Non-Patent Document 2). Similarly, it is known that low-frequency RIN is suppressed in ASE light output from an SOA that is saturated with a large amount of incoherent light input (Patent Document 1, Non-Patent Documents 6, 7, 8).

[0013] Furthermore, according to Non-Patent Documents 3 and 4, RIN transfer is more likely to occur on the low-frequency side, while it is cut off on the high-frequency side. In other words, there is a cutoff frequency for RIN transfer. Therefore, the characteristic that low-frequency RIN is suppressed in the ASE light of a saturated SOA is extremely beneficial from the viewpoint of suppressing the degradation of transmission characteristics due to RIN transfer.

[0014] Figures 2 and 3 of Non-Patent Document 8 show that increasing the drive current (seed current, booster current) in a saturated SOA increases RIN suppression. However, increasing the drive current of the SOA increases both the optical output and the ripple. Furthermore, in actual Raman amplification systems, the Raman amplification gain is controlled by changing the excitation light source output. Therefore, there is a need for a method or means that can change the optical output of the SOA without affecting RIN suppression and keep the ripple small.

[0015] In summary, SOA's ASE light is a promising candidate as an incoherent excitation light source for Raman amplification from the standpoint of compactness and low cost, but it is desirable to have a source that simultaneously solves the following conflicting problems. (1) High output. (2) Ripple caused by SOA end-face reflection is suppressed over the wide operating and output ranges of the excitation light source. (3) RIN is suppressed over the wide operating and output range of the excitation light source. (4)Low power consumption.

[0016] To simultaneously address these challenges, a method has been proposed in which the ASE light of a relatively low-power SOA is second-order amplified using an FP-oscillating Raman amplification excitation laser (Patent Document 7, Non-Patent Documents 9, 10, 11, 12). This method is effective for long-distance, high-capacity Raman amplification transmission systems where higher gain is required. However, for relatively short-distance systems such as metro transmission systems, direct excitation using the ASE light of the SOA, which does not require second-order excitation, is desirable from the viewpoint of system simplification, cost reduction, and low power consumption.

[0017] The present invention has been made in view of the above, and aims to provide a light source, a light source device, a method for driving the light source, and a Raman amplifier and a Raman amplification system using the same, in which RIN and ripple are simultaneously suppressed. [Means for solving the problem]

[0018] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention comprises a seed light source that outputs incoherent seed light having a predetermined bandwidth, and a booster amplifier which is a semiconductor optical amplifier that optically amplifies the seed light input from a first end face and outputs the amplified light from a second end face, wherein the booster amplifier is a light source in which nL, which is the product of refractive index n and chip length L, is set so that relative intensity noise (RIN) and ripple are simultaneously suppressed in the amplified light.

[0019] The tip length L may be 1 mm or more.

[0020] The tip length L may be 1.5 mm or more.

[0021] The tip length L may be 2 mm or more.

[0022] The seed light source and the booster amplifier may be driven with a drive current that simultaneously suppresses the relative intensity noise (RIN) and ripple in the amplified light.

[0023] The booster amplifier may be driven to operate in a gain saturation state.

[0024] The seed light source may be driven with a drive current that outputs seed light with a power close to the maximum power of the amplified light.

[0025] The seed light source may include at least one ASE (Amplified Spontaneous Emission) light source equipped with an SLD (Super Luminescent Diode), a semiconductor optical amplifier, and a rare-earth doped optical fiber.

[0026] The power of the amplified light output by the booster amplifier may be 100mW or more.

[0027] The end face reflectance of the first end face and the end face reflectance of the second end face of the booster amplifier are 10 -3 and 10 -5 It may also be within the range between [the specified values].

[0028] One aspect of the present invention is a light source device comprising the light source and a drive device for driving the light source.

[0029] One aspect of the present invention is a method for driving a light source, comprising: a seed light source that outputs incoherent seed light having a predetermined bandwidth; and a booster amplifier, which is a semiconductor optical amplifier that optically amplifies the seed light input from a first end face and outputs the amplified light from a second end face, wherein the booster amplifier has nL, which is the product of refractive index n and chip length L, set so that relative intensity noise (RIN) and ripple are simultaneously suppressed in the amplified light, and the method for driving a light source involves driving the seed light source and the booster amplifier with a drive current that simultaneously suppresses relative intensity noise (RIN) and ripple in the amplified light.

[0030] One aspect of the present invention is a Raman amplifier comprising the light source and a Raman amplification optical fiber to which the amplified light is input as excitation light.

[0031] One aspect of the present invention is a Raman amplification system comprising the light source and a Raman amplification optical fiber to which the amplified light is input as excitation light. [Effects of the Invention]

[0032] According to the present invention, a light source, a light source device, a method for driving the light source, and a Raman amplifier and a Raman amplification system using the same are provided, in which RIN and ripple are simultaneously suppressed. [Brief explanation of the drawing]

[0033] [Figure 1] Figure 1 is a schematic diagram of a light source device according to Embodiment 1. [Figure 2] Figure 2 shows a part of the light source module shown in Figure 1. [Figure 3] Figure 3 shows an example of the power spectrum of the output light from a booster amplifier. [Figure 4] Figure 4 shows an example of the output light power (Pf) relative to Ib. [Figure 5] Figure 5 shows an example of the RIN spectrum of the output light. [Figure 6] Figure 6 schematically shows the spectrum in which RIN is suppressed. [Figure 7] Figure 7 shows an example of the power spectrum of the output light when Is is set to 0 mA and Ib is varied. [Figure 8] Figure 8 shows examples of the power spectrum of the output light when Is is changed. [Figure 9] Figure 9 shows an example of the Is dependence of the output light power (Pf) on Ib. [Figure 10] Figure 10 shows examples of the RIN spectra of the output light when Is is changed. [Figure 11] Figure 11 is an explanatory diagram illustrating the relationship between RIN suppression and ripple suppression. [Figure 12] Figure 12 shows that ripple is suppressed when the round-trip frequency is within the RIN suppression band. [Figure 13] Figure 13 shows the center wavelength of the power spectrum of the output light from light source modules No. 11 to 31. [Figure 14] Figure 14 shows the ripple width with respect to Is for light source modules No. 11 to 31. [Figure 15] Figure 15 is a schematic diagram of a Raman amplifier according to Embodiment 2. [Figure 16] Figure 16 is a schematic diagram of the Raman amplification system according to Embodiment 3. [Modes for carrying out the invention]

[0034] Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the embodiments described below. Furthermore, in the drawings, the same parts are denoted by the same reference numerals as appropriate, and redundant descriptions are omitted as appropriate.

[0035] (Embodiment 1) Figure 1 is a schematic diagram of a light source device according to Embodiment 1. This light source device 100 comprises a light source module 10 and drive devices 101 and 102. The light source module 10 is an example of a light source.

[0036] The light source module 10 comprises a seed light source 11 which is an SOA, an optical isolator 12, a booster amplifier 13 which is an SOA, an optical isolator 14, and an output optical fiber 15. The seed light source 11, optical isolator 12, booster amplifier 13, and optical isolator 14 are optically cascaded in this order by optical fibers or optical elements.

[0037] The seed light source 11 outputs incoherent seed light L1 having a predetermined bandwidth. Incoherent light refers to light consisting of an uncorrelated collection of photons with a continuous spectrum, rather than a laser light source that oscillates in one or more discrete modes (longitudinal modes). The predetermined bandwidth is not particularly limited, but a broad bandwidth such as 25 nm or more as a wavelength bandwidth is preferred. The optical isolator 12 transmits the seed light L1 to the booster amplifier 13 and prevents the reflected light coming from the booster amplifier 13 from being input to the seed light source 11. The optical isolator 12 prevents or reduces instability in the operation of the seed light source 11 due to the input of reflected light.

[0038] The booster amplifier 13 optically amplifies the input seed light L1 and outputs it as amplified light L2. The optical isolator 14 allows the amplified light L2 to pass through and be input to the output optical fiber 15, while also preventing light traveling from the output optical fiber 15 side from being input to the booster amplifier 13. The optical isolator 14 prevents or reduces instability in the operation of the booster amplifier 13 due to the input of reflected light.

[0039] The output optical fiber 15 is an optical fiber that guides the amplified light L2 to the outside of the light source module 10. The amplified light L2 is used, for example, as excitation light for Raman amplification.

[0040] The drive units 101 and 102 are known drive units for SOA. Drive unit 101 supplies a drive current C1 to the seed light source 11. Drive unit 102 supplies a drive current C2 to the booster amplifier 13.

[0041] Figure 2 shows a part of the light source module 10 shown in Figure 1. The booster amplifier 13 has a first end face 13a and a second end face 13b that face each other. The booster amplifier 13 receives seed light L1 from the first end face 13a and outputs amplified light L2 to the outside from the second end face 13b.

[0042] The first end face 13a and the second end face 13b are treated with reflection reduction processing such as AR (Ant-Reflection) coating. Alternatively, the first end face 13a and the second end face 13b may also be treated with reflection reduction processing by being inclined with respect to the optical axis of the optical amplification waveguide provided by the booster amplifier 13. Such a structure is also called an oblique waveguide structure.

[0043] If the end face reflectance of the first end face 13a is R1 and the end face reflectance of the second end face 13b is R2, then R1 and R2 are, for example, 10 -3 and 10 -5 It is the range between (R1 × R2). 1 / 2 For example, 10 -3 and 10 -5 It is within the range of 10. -3 and 10-5 The value within the range between this and that is, from the perspective of low reflectance, an example of a numerically range that is quite practical and feasible. Here, 10 -3 and 10 -5 The range between this and that means the range that includes 10 which is the upper limit -3 and 10 which is the lower limit -5 inclusive.

[0044] <Characteristics of Light Source Module 10> The characteristics of light source module 10 will be described. FIG. 3 is a diagram showing an example of the power spectrum of the output light (amplified light) of the booster amplifier. FIG. 3 shows the case where the drive current C1 supplied to the seed light source 11 is Is and the drive current C2 supplied to the booster amplifier 13 is Ib, with Is set to 50 mA and Ib set to 800 mA. The power spectrum of the output light in FIG. 3 generally has a Gaussian-like spectral shape, and its FWHM (Full Width Half Maximum) is about 25 nm to 30 nm.

[0045] FIG. 4 is a diagram showing an example of the power (Pf) of the output light (amplified light) from the booster amplifier with respect to Ib. In FIG. 4, Is is set to 50 mA. In the case of FIG. 4, it can be seen that when Ib is 100 mA, Pf is about 20 mW, and when Ib is 800 mA, Pf is about 120 mW. Note that FIGS. 3 and 4 are examples of light source modules fabricated using semiconductor optical amplifiers having substantially the same characteristics as the seed light source and the booster amplifier respectively.

[0046] Here, the inventors have discovered that in the light source module 10 as shown in FIG. 1, by suitably setting the product nL of the refractive index n and the chip length L of the booster amplifier 13, a state where both RIN and ripple are suppressed can be realized. The refractive index n of the booster amplifier 13 is the refractive index that acts on the seed light in the booster amplifier 13 until the seed light input to the booster amplifier 13 is output as amplified light, and is the refractive index of the active layer of the booster amplifier 13. Hereinafter, first, the suppression of RIN will be described, then the suppression of ripple will be described, and further, the relationship between the suppression of RIN and the suppression of ripple will be described.

[0047] <Suppression of RIN> FIG. 5 is a diagram showing an example of the RIN spectrum of the output light. Specifically, FIG. 5 shows the results of measuring the RIN of the output light from the light source modules No. 1 to 3 fabricated as examples of the light source module 10. The light source modules No. 1 to 3 were fabricated using semiconductor optical amplifiers having substantially the same characteristics as the seed light source and the booster amplifier, respectively, and have output characteristics equivalent to those shown in FIGS. 3 and 4. Also, for the light source modules No. 1 to 3, it was confirmed that the end face reflectance is in the range between (R1×R2) 1 / 2 is 10 -3 and 10 -5 and. Also, in FIG. 5, Ts indicates the case temperature (25°C) of the seed light source, and Tb indicates the case temperature (25°C) of the booster amplifier.

[0048] In FIG. 5, "only Ib" indicates the case where a drive current of 100 mA is supplied only to the booster amplifier and no drive current is supplied to the seed light source (that is, the supplied drive current is 0 mA). In this case, since no seed light source to be amplified is input to the booster amplifier, it operates as a mere ASE light source. On the other hand, "Is+Ib" indicates the case where a drive current of 100 mA is supplied to the booster amplifier and a drive current of 200 mA is supplied to the seed light source.

[0049] As can be seen from Figure 5, in the case of "only Ib," a relatively flat RIN spectrum with respect to frequency was obtained for all of the light source modules No. 1 to 3. This RIN is thought to originate from the ASE-ASE beat noise in the output light (ASE) output from the booster amplifier as an ASE light source.

[0050] In contrast, in the case of "Is+Ib," it can be seen that RIN is suppressed in all of the light source modules No. 1 to 3, from a specific frequency fc down to lower frequencies. In the case of "Is+Ib," the seed light (ASE light) output by the seed light source is input to the booster amplifier, increasing the number of photons inside, and it is thought that the booster amplifier is operating in a gain saturation state. What is noteworthy here is that in Figure 5, even though Ib is a relatively small current of 100mA and the power of the output light from the booster amplifier is relatively low, the RIN suppression state is already achieved. This point will be explained in detail later.

[0051] Figure 6 schematically shows the spectrum in which RIN is suppressed. Here, the output light from a light source such as a light source module in this embodiment is the ASE light of SOA, so its RIN should be determined by the magnitude of the ASE-ASE beat noise. In Figure 6, the level of line 210 is the level of the ASE-ASE beat noise.

[0052] The power spectral width of typical SOA ASE light is several tens of nanometers, which translates to several THz in terms of frequency. Since the measurement bandwidth of RIN is sufficiently small, several tens of GHz, RIN is calculated using the following equation (1) (Non-Patent Literature 15). RIN = 0.66 / Δν ASE [Hz -1 ] ··· (1) Here, 0.66 is the coefficient when the power spectrum of ASE light is Gaussian, and Δν ASE This is the FWHM of the power spectrum in question.

[0053] For example, the power spectrum of the output light shown in Figure 3 is Gaussian in type, and its FWHM is approximately 30 nm. Using equation (1) above, RIN can be calculated to be approximately -127 dB / Hz. This value is in general agreement with the result shown in Figure 5.

[0054] In contrast, the RIN of an SOA operating in a gain-saturated state is suppressed in the lower frequency region 211, which is also called the corner frequency 213, below fc. For example, RIN suppression of about 10 dB to 20 dB from the level of line 210 has been reported (Patent Document 1, Non-Patent Documents 6, 8). Line 212 is the level of shot noise.

[0055] In this specification, "RIN is suppressed" means that it is suppressed by 10 dB or more compared to the RIN calculated by formula (1) above. The degree of RIN suppression may be even less than 16 dB, or even less than 20 dB.

[0056] <Suppression of Ripple> Next, we will explain ripple suppression. Specifically, we measured the power spectrum of the output light from a light source module (referred to as light source module No. 4) with output characteristics equivalent to light source module No. 1, while varying the driving conditions of the seed light source and booster amplifier.

[0057] Figure 7 shows an example of the power spectrum of the output light when Is is set to 0 mA and Ib is varied. Figure 8 shows an example of the power spectrum of the output light when Is is varied.

[0058] In Figure 7, ripple occurs in both cases where Ib is 200mA and 800mA. In particular, when Ib is 200mA, ripple occurs despite the output light power being relatively small, approximately 18mW. Furthermore, ripple was particularly likely to occur when Is was set to 0mA. As mentioned above, this ripple is caused by light reflection from both ends of the SOA, and the end-face reflectivity of the booster amplifier is (R1 × R2) 1 / 2 10 -3and 10 -5 Ripple is occurring even within a sufficiently small range between [specific parameters]. The fact that ripple occurs despite such low drive current (low power) and low reflectivity demonstrates how difficult it is to suppress ripple in Raman amplification excitation light sources where the optical output can reach several hundred mW in some cases. Furthermore, the ripple contained components of multiple different periods.

[0059] In contrast, as shown in Figure 8, when Ib is fixed at 800mA and Is is increased to 25mA, the ripple is suppressed, and when Is is 50mA, it is suppressed even further. However, when Is is further increased to 400mA, the ripple increases again.

[0060] As shown in Figure 8, when seed light is input to the booster amplifier, a gain saturation state is reached, and there is a region where ripple is suppressed. This indicates that ripple can be suppressed by controlling the power of the seed light input to the booster amplifier, even without lowering the end-face reflectivity of the booster amplifier to a degree that suppresses ripple. However, since the ASE light spectrum of SOA is broad, ranging from a few nanometers to tens of nanometers, it is not easy to apply a process (such as anti-reflective treatment) to reduce the end-face reflectivity to cover such a wide bandwidth. Therefore, the technique of suppressing ripple by controlling the power of the seed light is a technique that can alleviate the difficulties in setting the end-face reflectivity of the booster amplifier and in fabrication, and is an extremely effective technique in practical use.

[0061] Ripple suppression is preferable as it reduces the ripple size. The magnitude of the ripple is indicated by the maximum value of the ripple width (peak-to-bottom) appearing on the power spectrum at a given wavelength (for example, around 1510 nm). In this case, it is preferable to suppress the ripple so that, for example, the ripple width peak-to-bottom is 5 dB or less, 3 dB or less, 1 dB or less, or 0.5 dB or less.

[0062] Therefore, it is preferable to drive the seed light source and the booster amplifier with drive currents (Is, Ib) that suppress both RIN and ripple simultaneously in the amplified light when driving the light source module as a light source.

[0063] <Is Dependence of RIN Spectrum> Next, the Is dependence will be described for the No. 4 light source module whose ripple characteristics are shown in FIGS. 7 and 8.

[0064] FIG. 9 is a diagram showing an example of the Is dependence of the power (Pf) of the output light with respect to Ib. Comparing the curve at Is = 0 mA in FIG. 9 with FIG. 7, it can be seen that at Ib = 200 mA, although Pf is as small as about 18 mW, which is about 1 / 3 of the maximum output of about 60 mW, ripple occurs as in FIG. 7. Also, at Ib = 800 mA, although Pf is close to the maximum output, it can be seen that the width of the ripple is 10 dB or more from peak to bottom as in FIG. 7.

[0065] On the other hand, FIG. 10 is a diagram showing an example of the RIN spectrum of the output light when Is is changed. Note that Ib was fixed at 1000 mA. As can be seen from FIG. 10, the larger Is is, the more RIN is suppressed. This is considered to be because the degree of gain saturation of the booster amplifier becomes higher as Is becomes larger.

[0066] In FIG. 10, based on the above formula (1), when calculating the level (RIN ASE-ASE beat ) of the ASE-ASE beat noise, it was -127 dB / Hz. Therefore, when the line of the graph at Is = 40 mA was extended to the high-frequency side like a broken line and the corner frequency fc was examined from the intersection of the extended line and the level (broken line) of RIN ASE-ASE beat , it was about 30 GHz. When fc was examined using the lines of the graphs for other values of Is in the same way, it was about 30 GHz. From this, it was confirmed that in the No. 4 light source module, RIN is suppressed at least from about 30 GHz to the low-frequency side.

[0067] <Relationship between RIN suppression and ripple suppression> The inventor of the present invention considered the relationship between RIN suppression and ripple suppression as follows. FIG. 11 is an explanatory diagram of the relationship between RIN suppression and ripple suppression. In FIG. 11, a chip of the SOA that constitutes the booster amplifier 13 is shown. The chip length of the SOA is L. The seed light with power P0 input from the first end face of the SOA is amplified while propagating through the SOA, receives a gain G, and becomes amplified light with power GP0, and reaches the second end face after the elapse of time ΔT. Then, a part of the amplified light is reflected at the second end face, and the first reflected light with power ΔP1 propagates toward the first end face. The first reflected light with power ΔP1 is amplified while propagating through the SOA and reaches the first end face after the elapse of time ΔT. Then, a part of the first reflected light is reflected at the first end face and propagates further toward the second end face as the second reflected light with power ΔP2. By such partial reflection and propagation of light at the end faces, fluctuations in the optical power due to reflection occur. This fluctuation becomes a factor of ripple.

[0068] The time for the fluctuation of the optical power to reciprocate in the SOA that constitutes the booster amplifier 13 is τ RT (Round Trip Time). This τ RT When expressed in terms of frequency, it is f RT = 1 / τ RT Hereinafter, f RT may be described as the round trip frequency.

[0069] τ RT is defined by the following formula (2) based on the length L of the chip of the SOA and the refractive index n. Also, formula (3) is derived from formula (2). τ RT =(2nL / c) ··· (2) f RT = 1 / τ RT =(c / 2nL) ··· (3)

[0070] Figure 12 shows that ripple is suppressed when the round-trip frequency is within the RIN suppression band. The inventors have identified the round-trip frequency f as the fluctuation frequency. RT We found that if the fluctuation is within the RIN suppression frequency band (RIN suppression area), the fluctuation is suppressed and the generation of ripple is reduced. As can be seen from equations (2) and (3), f RT This depends on the refractive index n and the tip length L. Therefore, it is preferable that the booster amplifier 13 has nL, which is the product of the refractive index n and the tip length L, set so that RIN and ripple are simultaneously suppressed in the amplified light.

[0071] Here, in the light source module No. 4, we can assume that the chip length L is 1.8 mm and the refractive index n is 3.5. In this case, f RT This is 23.8 GHz, which is within the suppression frequency band of RIN, i.e., a frequency lower than fc, as shown in Figure 10. In other words, nL = 3.5 × 1.8 = 6.3 is a preferred example of nL.

[0072] Also, if the refractive index n is 3.5, the tip length L and f RT Table 1 shows an example of the relationship. The tip length L is preferably 1 mm or more, more preferably 1.5 mm or more, and even more preferably 2 mm or more. Note that the refractive index n depends on the oscillation wavelength and the composition ratio of the active layer, so Table 1 is just one example.

[0073] [Table 1]

[0074] The RIN suppression frequency band is determined by the Ib and saturation state of the booster amplifier 13 (Non-Patent Literature 8). The refractive index of the booster amplifier 13 depends on the wavelength of the seed light and the composition ratio of the active layer, but is generally in the range of 3.2 to 3.6. The specific preferred chip length L is determined by the RIN suppression frequency band and the refractive index of the booster amplifier 13. From the viewpoint of ripple suppression, there is no upper limit to the value of L, but considering the internal loss of the SOA chip and the overall size of the light source module 10, it is preferable to have a length of about 5 mm or less.

[0075] Thus, ripple is suppressed by the RIN suppression phenomenon, but it is a given that the reflectivity of the first end face 13a and the second end face 13b of the booster amplifier 13 is low enough that the ripple suppression effect is obtained by the RIN suppression. The reflectivity of the first and second end faces in the light source modules No. 1 to 4 described above is (R1 × R2) 1 / 2 10 -3 and 10 -5 The range is roughly between [values], and is processed to be sufficiently low for practical purposes. Nevertheless, Raman amplification excitation sources generally require large outputs of several hundred mW or more, and ripple can easily occur. However, as in this disclosure, if the round-trip frequency derived from nL, which is the product of the chip length L and the refractive index n, is placed within the RIN suppression frequency band, ripple can be suppressed even within the range of widely used end-face reflectances.

[0076] Next, we will describe the ripple characteristics of light source modules with output characteristics equivalent to those of light source module No. 4 (referred to as light source modules No. 11 to 31). Figure 13 shows the center wavelength of the power spectrum of the output light of light source modules No. 11 to 31. The center wavelength was measured using the RMS method.

[0077] In light source modules No. 11 to 31, the chip length of the booster amplifier is 1.8 mm in all cases. However, as shown in Figure 13, the peak wavelength of light source modules No. 25 and 26 is longer than that of the other light source modules. This is because the refractive index of the booster amplifier in light source modules No. 25 and 26 is relatively low.

[0078] Figure 14 shows the ripple width with respect to Is for light source modules No. 11 to 31. Note that Ib is fixed to a predetermined value. As shown in Figure 14, the ripple width is larger for light source modules No. 25 and 26 compared to the other light source modules. The reason for this is thought to be that although the oscillation wavelength of light source modules No. 25 and 26 is longer than that of the other light source modules, the refractive index of InP-based semiconductor materials is negatively correlated with wavelength, so the nL of the booster amplifier becomes smaller compared to the nL in the other light source modules.

[0079] As described above, the light source device and light source module according to the embodiment are suitable as excitation light sources for Raman amplification, especially for forward excitation, because RIN and ripple are simultaneously suppressed in the amplified light by suitably setting nL, which is the product of the refractive index n and the tip length L of the booster amplifier. They are particularly excellent in suppressing RIN and suppressing RIN transfer.

[0080] (Embodiment 2) Figure 15 is a schematic diagram of a Raman amplifier according to Embodiment 2. The Raman amplifier 1000 is configured as a forward-excited, concentrated optical amplifier equipped with the light source module 10 of Embodiment 1 as an excitation light source. The Raman amplifier 1000 comprises a light source device 100 equipped with the light source module 10, a signal optical input section 1001, an optical multiplexer 1002, a Raman amplification optical fiber 1003 such as a highly nonlinear optical fiber, and a Raman amplified optical output section 1004.

[0081] The signal light input unit 1001 receives the signal light L11. The optical multiplexer 1002 combines the signal light L11 and the amplified light L2, which is output from the output optical fiber 15 of the light source module 10 as excitation light, and inputs it to the Raman amplification optical fiber 1003. The Raman amplification optical fiber 1003 uses the amplified light L2 as excitation light to perform Raman amplification of the signal light L11. The Raman amplification light output unit 1004 outputs Raman amplification light L12, which is the signal light L11 that has been Raman amplified.

[0082] The amplified light L2 from the light source module 10 is set to a wavelength that allows the signal light L11 to be Raman amplified by the Raman amplification optical fiber 1003.

[0083] The Raman amplifier 1000 is particularly excellent in its ability to suppress RIN and RIN transfer.

[0084] (Embodiment 3) Figure 16 is a schematic diagram of a Raman amplification system according to Embodiment 3. The Raman amplification system 2000 is configured as a forward-excited distributed optical amplification system equipped with the light source module 10 of Embodiment 1 as an excitation light source. The Raman amplification system 2000 comprises a light source device 200 equipped with the light source module 10, a signal optical input section 2001, Raman amplification optical fibers 2003 such as standard single-mode fibers, dispersion-shifted fibers, and non-zero dispersion-shifted fibers, and a Raman amplification optical output section 2004. A standard single-mode fiber is, for example, an optical fiber conforming to the ITU-T G.652 standard.

[0085] The light source device 200 has a configuration that adds an optical multiplexer 201 to the light source device 100. Furthermore, as the Raman amplification optical fiber 2003, for example, an optical fiber used for optical communication laid in the field can be used.

[0086] The signal light input unit 2001 receives the signal light L21. The optical multiplexer 201 combines the signal light L11 and the amplified light L2, which is output from the output optical fiber 15 of the light source module 10 as excitation light, and inputs it to the Raman amplification optical fiber 2003. The Raman amplification optical fiber 2003 uses the amplified light L2 as excitation light to perform Raman amplification of the signal light L21. The Raman amplification light output unit 2004 outputs Raman amplification light L22, which is the signal light L21 that has been Raman amplified.

[0087] The amplified light L2 from the light source module 10 is set to a wavelength that allows the signal light L21 to be Raman amplified by the Raman amplification optical fiber 2003.

[0088] The Raman amplification system 2000 is particularly excellent in its ability to suppress RIN and RIN transfer.

[0089] Although the Raman amplifier and Raman amplification system in the above embodiment are configured as forward-excited types, the embodiments of the present invention are not limited thereto, and may be configured as backward-excited or bidirectional-excited types.

[0090] Furthermore, the light source module and light source device of the above embodiment are not limited to excitation light sources for Raman amplification, but can be widely used as light sources in which RIN and ripple are simultaneously suppressed.

[0091] Furthermore, in the above embodiment, the seed light is ASE light, but it may also be incoherent light such as spontaneous emission (SE).

[0092] Furthermore, 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), SOA, and ASE light source equipped with a rare-earth doped optical fiber. Such an SLD, SOA, and ASE light source is suitable as an incoherent light source.

[0093] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of Symbols]

[0094] 10: Light source module 11: Seed light source 12, 14: Optical isolators 13: Booster Amplifier 13a: First end surface 13b: Second end surface 15: Output optical fiber 100, 200: Light source device 101, 102: Drive unit 1000: Raman amplifier 1001, 2001: Signal optical input section 1002, 201: Optical multiplexer 1003, 2003: Optical fiber for Raman amplification 1004, 2004: Raman amplified light output section 2000: Raman amplification system C1, C2: Drive current L1: Seed light L11, L21: Signal light L12, L22: Raman amplified light L2: Amplified light

Claims

1. A seed light source that outputs incoherent seed light having a predetermined bandwidth, A booster amplifier, which is a semiconductor optical amplifier that optically amplifies the seed light input from the first end face and outputs the amplified light from the second end face, Equipped with, The booster amplifier has an end face reflectivity of 10 such that the first and second end faces do not cause a large number of longitudinal modes, i.e., ripples, to disappear due to Fabry-Perot oscillation between the first and second end faces. －３ and 10 －５ The product nL, which is the refractive index n and the tip length L, is set to a range between and and is driven to operate in a gain saturation state, such that the relative intensity noise (RIN) in the amplified light is suppressed to -147 dB / Hz or less and the ripple magnitude is suppressed to 1 dB or less, and the nL is set such that the round-trip frequency c / 2nL (where c is the speed of light) of the fluctuations in optical power caused by the reflection of light from the first or second end face of the booster amplifier falls within the RIN suppression frequency band. Excitation light source for Raman amplification.

2. The chip length L is 1 mm or more. The Raman amplification excitation light source according to claim 1.

3. The aforementioned chip length L is 1.5 mm or more. The Raman amplification excitation light source according to claim 1.

4. The chip length L is 2 mm or more. The Raman amplification excitation light source according to claim 1.

5. The seed light source is driven with a drive current that outputs seed light with a power such that the power of the amplified light approaches its maximum. The Raman amplification excitation light source according to any one of claims 1 to 4.

6. The seed light source includes at least one Amplified Spontaneous Emission (ASE) light source comprising an SLD (Super Luminescent Diode), a semiconductor optical amplifier, and a rare-earth doped optical fiber. The excitation light source for Raman amplification according to any one of claims 1 to 5.

7. The power of the amplified light output by the booster amplifier is 100 mW or more. The excitation light source for Raman amplification according to any one of claims 1 to 6.

8. A Raman amplification excitation light source according to any one of claims 1 to 7, A drive device for driving the Raman amplification excitation light source, A light source device equipped with the following features.

9. A seed light source that outputs incoherent seed light having a predetermined bandwidth, A booster amplifier, which is a semiconductor optical amplifier that optically amplifies the seed light input from the first end face and outputs the amplified light from the second end face, A method for driving a Raman amplification excitation light source comprising: The booster amplifier has an end face reflectivity of 10 such that the first and second end faces do not cause a large number of longitudinal modes, i.e., ripples, to disappear due to Fabry-Perot oscillation between the first and second end faces. －３ and 10 －５ The product of the refractive index n and the tip length L, nL, is set to be within the range between the above, and the relative intensity noise (RIN) in the amplified light is suppressed to be -147 dB / Hz or less, and the ripple magnitude is suppressed to be 1 dB or less, and nL is set so that the round-trip frequency c / 2nL (where c is the speed of light) of the fluctuations in optical power caused by the reflection of light from the first or second end face of the booster amplifier falls within the RIN suppression frequency band. The booster amplifier is driven to operate in a gain saturation state. The seed light source and the booster amplifier are driven with a drive current that simultaneously suppresses the relative intensity noise (RIN) and ripple magnitude in the amplified light. A method for driving an excitation light source for Raman amplification.

10. A Raman amplification excitation light source according to any one of claims 1 to 7, A Raman amplification optical fiber into which the amplified light is input as excitation light, A Raman amplifier equipped with [a specific feature / feature].

11. A Raman amplification excitation light source according to any one of claims 1 to 7, A Raman amplification optical fiber into which the amplified light is input as excitation light, A Raman amplification system equipped with [the necessary components].

12. A seed light source that outputs incoherent seed light having a predetermined bandwidth, A booster amplifier, which is a semiconductor optical amplifier that optically amplifies the seed light input from the first end face and outputs the amplified light from the second end face, A method for designing a Raman amplification excitation light source comprising, wherein the booster amplifier is driven to operate in a gain-saturated state, Regarding the booster amplifier, the first and second end faces have an end face reflectivity of 10 such that the numerous longitudinal modes, i.e., ripples, caused by Fabry-Perot oscillation between the first and second end faces are not lost. －３ and 10 －５ The product of the refractive index n and the tip length L, nL, is set to be within the range between -147 dB / Hz and -147 dB / Hz, and the relative intensity noise (RIN) in the amplified light is suppressed to be 1 dB or less, and the ripple magnitude is suppressed to be 1 dB or less, and nL is set so that the round-trip frequency in the booster amplifier of the fluctuation in optical power caused by the reflection of light from the first or second end face of the booster amplifier falls within the RIN suppression frequency band. Design method for excitation light sources for Raman amplification.

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