Optical semiconductor module, optical semiconductor unit, and raman amplifier

Abstract

This optical semiconductor module comprises: a semiconductor optical amplifier capable of optically amplifying input light that has been input; an optical fiber to which output light output from the semiconductor optical amplifier is coupled; and a package in which the semiconductor optical amplifier is accommodated and to which the optical fiber is attached. The output light is coupled to the optical fiber without passing through an optical isolator. The optical semiconductor module may be further provided with a lens that is accommodated in the package and couples the output light to the optical fiber.

Description

Optical semiconductor module, optical semiconductor unit and Raman amplifier

[0001] The present invention relates to an optical semiconductor module, an optical semiconductor unit and a Raman amplifier.

[0002] In recent years, the increase in data traffic has become a problem due to the rapid popularization of the Internet and the rapid increase in connections between in-house LANs. Accordingly, to solve this problem, WDM systems (wavelength division multiplexing transmission systems) have achieved remarkable development and become widespread. In a WDM system, by carrying a plurality of signals on lights of different wavelengths respectively, large-capacity transmission as much as 100 times that of the conventional art is realized with a single optical fiber. Particularly, in a WDM system, optical amplification by an optical fiber amplifier such as an erbium-doped optical fiber amplifier (hereinafter referred to as EDFA) or a Raman amplifier is essential, and this optical amplification enables wideband long-distance transmission. Here, an EDFA is an optical fiber amplifier that applies the principle that when pumping light having a wavelength of 1480 nm or 980 nm is input from a pumping laser into a special optical fiber doped with erbium, which is a rare earth element (hereinafter referred to as EDF), light in the 1550 nm wavelength band, which is transmission signal light input simultaneously, is amplified in the EDF.

[0003] Furthermore, as a usage mode of EDFA, when amplifying signal light in the middle of a transmission optical fiber laid on the seabed, a so-called remote pump type has been proposed, in which a pumping laser is arranged on land, and pumping light emitted from the pumping laser is made incident to the EDF via the transmission optical fiber. In a remote pump type EDFA, arranging the pumping laser on land enables easy maintenance and replacement of the pumping laser.

[0004] On the other hand, Raman amplifiers are distributed optical fiber amplifiers that use ordinary optical fibers as the gain medium, without requiring special optical fibers such as erbium-doped optical fibers, as is the case with EDFAs. Because Raman amplifiers have a broadband and flat gain, they have the advantage of achieving a broadband transmission range compared to conventional EDFA-based WDM transmission systems. However, because the amplification gain of Raman amplifiers is lower than that of EDFAs, the excitation lasers used in them are required to have higher output characteristics than those of EDFAs.

[0005] Therefore, in order to improve the stability of WDM systems and reduce the number of relays, stable and high optical output capability is required for the excitation laser. Various semiconductor laser elements with structures such as embedded hetero (BH) structures are used as excitation lasers, and currently, for the reasons mentioned above, the development of high-power semiconductor laser elements is particularly active.

[0006] Furthermore, in recent long-distance, high-capacity, ultra-high-speed communication systems using digital coherent communication methods, as transmission speeds increase, especially in transmission systems of 800 Gbps or higher, the degradation of the signal-to-noise ratio (OSNR) of the optical signal after transmission leads to a reduction in transmission distance, which is a challenge. As a countermeasure, the importance of fiber Raman amplifiers is increasing. In particular, the forward-pumped Raman method propagates the excitation light for Raman amplification in the same direction as the signal light, making it an effective technology for reducing OSNR degradation. The introduction of this technology is expected to extend the transmission distance even during high-speed transmission.

[0007] For forward-pumped Raman amplifiers, it has been reported that low-noise (low RIN) excitation light sources with a spectral width of about 30 nm are effective in suppressing RIN transfer, where noise from the excitation light affects the signal light after transmission (Patent Documents 1, 2; Non-Patent Documents 1-3).

[0008] U.S. Patent No. 7,190,861, Japanese Patent Publication No. 2003-84807

[0009] VAKHSHOORI, D., et al., “Raman amplification using high-power incoherent semiconductor pump sources.” Optical Fiber Communication Conference. Optical Society of America, 2003. p. PD47.S. Takasaka et. Al., “Quasi-Constant Signal Power Transmission with Low Signal RIN by DRA with Incoherent-Forward and Coherent-Backward Pumps” OFC 2023, W2A.11.M. Morimoto et. Al., “Co-Propagating Dual-Order Distributed Raman Amplifier Utilizing Incoherent Pumping” IEEE Photonics Technology Letters, Vol. 29, No 7, April 1, 2017.

[0010] However, when it comes to the practical application of the aforementioned broad spectral width forward-pumped Raman amplifier excitation light sources, a challenge arises: such light sources have lower power-to-optical conversion efficiency than semiconductor lasers. This results in a problem of low Raman gain when applied as an excitation light source for a Raman amplifier. Furthermore, in order to obtain a Raman gain of 7 dB or more, it is essential to achieve high output power of 200 mW or more using polarization synthesis or wavelength division multiplexing techniques. However, reducing the number of components in the Raman amplifier excitation light source unit and lowering the cost is a challenge for practical application.

[0011] The present invention has been made in view of the above, and aims to provide an optical semiconductor module, an optical semiconductor unit, and a Raman amplifier with improved power-to-optical conversion efficiency.

[0012] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is an optical semiconductor module comprising: a semiconductor optical amplifier capable of optically amplifying input light; an optical fiber to which output light output from the semiconductor optical amplifier is coupled; and a package that houses the semiconductor optical amplifier and to which the optical fiber is attached, wherein the output light is coupled to the optical fiber without passing through an optical isolator.

[0013] The optical semiconductor module may further include a lens housed in the package that couples the output light to the optical fiber.

[0014] The optical semiconductor module may further include a light source that outputs the input light.

[0015] The optical semiconductor module may further include an optical isolator located between the semiconductor optical amplifier and the light source.

[0016] The optical semiconductor module may further include a lens that couples the input light output from the light source to the semiconductor optical amplifier.

[0017] The optical semiconductor module may further include an in-line type optical isolator provided on the optical fiber outside the package.

[0018] One aspect of the present invention is an optical semiconductor module comprising two optical semiconductor modules and a polarization combiner that polarization combines the output light output from the two optical semiconductor modules.

[0019] The optical semiconductor unit may further include two or more sets of the two optical semiconductor modules that output the output light of different wavelengths, and a wavelength multiplexer that wavelength multiplexes the output light polarized and combined by the polarization combiner.

[0020] One aspect of the present invention is an optical semiconductor module comprising two or more optical semiconductor modules that output output light of different wavelengths, and a wavelength multiplexer that wavelength multiplexes the output light output from the two or more optical semiconductor modules.

[0021] The optical semiconductor unit may further include two sets of the two or more optical semiconductor modules, and a polarization combiner that polarizes the output light wavelength-multiplexed by the wavelength multiplexer.

[0022] The optical semiconductor unit may further include an in-line optical isolator provided in an optical fiber that propagates the polarized output light or the wavelength-multiplexed output light.

[0023] One aspect of the present invention comprises an optical semiconductor unit and a Raman amplification optical fiber to which the output light output from the optical semiconductor unit is input, wherein the Raman amplification optical fiber is a Raman amplifier that uses the output light as excitation light to Raman amplify the input signal light.

[0024] The Raman amplifier may be configured as a forward-excitation type.

[0025] The Raman amplifier may be configured as a back-excited type.

[0026] The Raman amplifier may be configured as a bidirectional excitation type.

[0027] According to the present invention, the effect of improving the power-to-light conversion efficiency is achieved.

[0028] Figure 1 is a schematic configuration diagram of an optical semiconductor module according to Embodiment 1. Figure 2 is a schematic cross-sectional view of the semiconductor optical amplifier shown in Figure 1. Figure 3 is a diagram showing an example of the correspondence between the semiconductor layer and energy bands shown in Figure 2. Figure 4 is a schematic configuration diagram of an optical semiconductor module according to a comparative form. Figure 5A is a diagram showing the output characteristics of the light source in the optical semiconductor module according to Embodiment 1 and the comparative form, as a single unit. Figure 5B is a diagram showing the output characteristics of the semiconductor optical amplifier in the optical semiconductor module according to Embodiment 1 and the comparative form, as a single unit. Figure 6A is a diagram showing the output characteristics of the optical semiconductor module according to Embodiment 1 and the comparative form. Figure 6B is a diagram showing the output characteristics of the optical semiconductor module according to Embodiment 1 and the comparative form. Figure 7 is a schematic configuration diagram of an optical semiconductor module according to a modified example of Embodiment 1. Figure 8 is a diagram showing the output characteristics of the optical semiconductor module according to Embodiment 1, the modified example, and the comparative form. Figure 9 is a diagram showing the spectrum of the output light of the optical semiconductor module according to Embodiment 1, the modified example, and the comparative form. Figure 10 is a diagram showing the wavelength shift of the spectrum of the output light of the optical semiconductor module according to the modified example. Figure 11 is a schematic configuration diagram of an optical semiconductor module according to Embodiment 2. Figure 12A is a diagram showing the output characteristics of an optical semiconductor module according to Embodiment 2 and a comparative embodiment. Figure 12B is a diagram showing the output characteristics of an optical semiconductor module according to Embodiment 2 and a comparative embodiment. Figure 13 is a schematic configuration diagram of an optical semiconductor unit according to Embodiment 3. Figure 14 is a diagram showing the output characteristics of an optical semiconductor unit according to Embodiment 3. Figure 15 is a schematic configuration diagram of an optical semiconductor unit according to Embodiment 4. Figure 16 is a schematic configuration diagram of an optical semiconductor unit according to Embodiment 5. Figure 17 is a schematic configuration diagram of an optical semiconductor unit according to Embodiment 6. Figure 18 is a schematic configuration diagram of a Raman amplifier according to Embodiment 7. Figure 19 is a schematic configuration diagram of a Raman amplifier according to Embodiment 8. Figure 20 is a schematic configuration diagram of a Raman amplifier according to Embodiment 9.

[0029] Embodiments will be described below with reference to the drawings. However, this embodiment does not limit the present invention. Furthermore, in the drawings, the same or corresponding elements are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. It should also be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from reality. Even between drawings, there may be parts where the dimensional relationships and ratios differ.

[0030] (Embodiment 1) [Overall Configuration of Optical Semiconductor Module] Figure 1 is a schematic configuration diagram of an optical semiconductor module according to Embodiment 1. The optical semiconductor module 100 comprises a package 1, a lens holder 2, a ferrule 3, a thermoelectric cooling element (TEC) 4, a light source 5, a lens 6, an optical isolator 7, a lens 8, a semiconductor optical amplifier (SOA) 9, lenses 10 and 11, and an optical fiber 12.

[0031] Package 1 is a known 14-pin butterfly package and comprises a main body 1a and a window 1b. The main body 1a houses the TEC 4, light source 5, lens 6, optical isolator 7, lens 8, SOA 9, and lens 10. The window 1b protrudes from the main body 1a and has a transparent window. In Figure 1, Package 1 is shown with the lid removed for illustrative purposes.

[0032] The lens holder 2 is attached to the window portion 1b and holds the lens 11. The ferrule 3 is attached to the lens holder 2 and the optical fiber 12 is inserted and fixed into it. In other words, the optical fiber 12 is attached to the package 1.

[0033] TEC4 is mounted on the bottom of package 1. TEC4 cools the light source 5 and SOA9 by an externally supplied current via pins. The components mounted on TEC4, such as the light source 5 and SOA9, are mounted on submounts made of a highly thermally conductive material such as AlN before being mounted on TEC4.

[0034] In this embodiment, the light source 5 is a semiconductor optical amplifier, which outputs spontaneous emission light (ASE light) in response to an externally supplied current through a pin. Hereafter, in this embodiment, the ASE light may be referred to as input light. The ASE light has, for example, a peak wavelength of 1510 nm, but is not particularly limited.

[0035] Lens 6 collimates the input light from the light source 5 and outputs it to the optical isolator 7. The optical isolator 7 is located between the SOA 9 and the light source 5, allowing the input light to pass through while blocking light traveling in the opposite direction to the input light. This suppresses temporal instability in the characteristics of the light source 5. Lens 8 focuses the input light that has passed through the optical isolator 7 and couples it to the SOA 9. Lenses 6 and 8 are examples of lenses that couple the input light output from the light source to the SOA. It is preferable that lenses 6 and 8 have an anti-reflective coating on their surfaces.

[0036] SOA9 is configured to optically amplify the input light. SOA9 uses an externally supplied current via a pin to optically amplify the input light and output it as output light. Preferred configurations for light source 5 and SOA9 will be described in detail later.

[0037] Lens 10 collimates the output light emitted from SOA9 and outputs it to lens 11. At this time, the output light passes through the window of window portion 1b. Then, lens 11 focuses the output light emitted from lens 10 and couples it to the end face 12a of the optical fiber 12. The end face 12a is approximately perpendicular to the optical axis of the optical fiber 12. Lenses 10 and 11 are examples of lenses that couple output light to an optical fiber. It is preferable that lenses 10 and 11 have an anti-reflective coating on their surfaces.

[0038] The optical fiber 12 propagates the coupled output light. The optical fiber 12 is, for example, a polarization-maintaining optical fiber. In this case, the polarization state of the output light is stable. However, the optical fiber 12 may also be a standard single-mode optical fiber used as a transmission line in optical fiber communication.

[0039] [Example of Configuration of Light Source and SOA] Next, a preferred example of the configuration of the light source 5 and the SOA 9 will be described. FIG. 2 is a schematic cross-sectional view of the SOA 9. Note that the light source 5 may also have the same configuration as the SOA 9.

[0040] The SOA 9 includes a substrate 9a, a lower cladding layer 9b, a lower optical confinement layer 9c, an active layer 9d, an upper optical confinement layer 9e, a lower current blocking layer 9f, an upper current blocking layer 9g, an upper cladding layer 9h, a contact layer 9i, an upper electrode 9j, and a lower electrode 9k.

[0041] The substrate 9a and the lower cladding layer 9b are made of n-type InP. The lower optical confinement layer 9c is made of, for example, InGaAsP, and constitutes a SCH (Separate Confinement Heterostructure). The active layer 9d is made of, for example, InGaAsP, and has an MQW (Multi Quantum Well) structure as described later. The upper optical confinement layer 9e is made of, for example, InGaAsP, and constitutes the SCH structure.

[0042] A part of the lower cladding layer 9b, the lower optical confinement layer 9c, the active layer 9d, and the upper optical confinement layer 9e form a striped mesa structure. Both sides of the striped mesa structure (in the left-right direction of the drawing) are buried by the lower current blocking layer 9f made of p-type InP and the upper current blocking layer 9g made of n-type InP.

[0043] The upper cladding layer 9h is made of p-type InP, and is formed so as to cover the upper optical confinement layer 9e and the upper current blocking layer 9g.

[0044] The contact layer 9i is made of, for example, p-type GaInAsP, and forms ohmic contact with the upper electrode 9j. The upper electrode 9j contains, for example, titanium, platinum, gold, or the like.

[0045] The lower electrode 9k is provided so as to form ohmic contact with the substrate 9a. The lower electrode 9k contains, for example, gold, nickel, or the like.

[0046] Figure 3 shows an example of the correspondence between the semiconductor layer and energy band shown in Figure 2. The active layer 9d is formed by alternating heterojunctions of well layers 9da and barrier layers 9db, and in this example, it has an MQW structure with 5 wells. The lower photo-confinement layer 9c and upper photo-confinement layer 9e, located between the active layer 9d and the lower cladding layer 9b and upper cladding layer 9h, are formed symmetrically in composition and thickness with respect to the active layer 9d, and each consists of two semiconductor layers.

[0047] An impurity-doped region is formed in at least one well layer 9da and the adjacent barrier layer 9db of the active layer 9d by doping with impurities. This impurity-doped region is not limited to a pair of adjacent well layers 9da and barrier layers 9db, but may include many more logarithmic well layers 9da and barrier layers 9db, or the entire active layer 9d may be an impurity-doped region. When an impurity-doped region is formed throughout the active layer 9d, the series resistance and thermal resistance of the device become smaller, which improves the heat dissipation of the device and increases the optical output, making it preferable.

[0048] n-type impurities are preferred for forming this impurity-doped region, and specifically, one or more of S, Se, and Si are used. The doping concentration of these n-type impurities is 1 × 10⁻⁶ 17 ~3 x 10 18 cm -3 It is preferable to set it to 1 × 10. 17 cm -3 If the value becomes even lower, the doping effect described above will not be obtained, and the increase in light output will be insufficient, and 3 × 10 18 cm -3As the intensity increases, the crystallinity of the active layer 9d deteriorates, the non-luminescent component increases, hindering high-light-output operation and making it difficult to ensure reliability. Furthermore, doping with n-type impurities can suppress the diffusion of p-type impurities, such as Zn, into the active layer 9d, thereby reducing the degradation of light output due to Zn's light absorption. Additionally, doping with n-type impurities can increase the pseudo-Fermi level of the active layer, resulting in a wider gain spectrum than when doping is not performed.

[0049] Furthermore, the impurities used to form the impurity-doped region may be p-type impurities. Examples of p-type impurities in this case include one or more of Be, Mg, and Zn.

[0050] Furthermore, the thicknesses of the lower optical confinement layer 9c and the upper optical confinement layer 9e may be 20 to 50 nm, respectively. If the thicknesses of the lower optical confinement layer 9c and the upper optical confinement layer 9e are made thinner than 20 nm, the optical confinement coefficient decreases, and the mode gain that contributes to optical amplification decreases. As a result, the number of carriers that do not contribute to optical amplification increases, so the optical saturation output of the SOA9 decreases, making it unsuitable for high-power operation. Also, if the thicknesses of the lower optical confinement layer 9c and the upper optical confinement layer 9e are made thicker than 50 nm, the DC resistance component of the device increases, and saturation of the optical output due to thermal saturation may occur. In addition, in the embedded heterostructure, kinks may occur in the optical output-current characteristics and the drive voltage-drive current characteristics.

[0051] The lower photoconfinement layer 9c and the upper photoconfinement layer 9e form heterojunctions with the outermost barrier layer 9db located on the active layer 9d, and also with the adjacent cladding layer (lower cladding layer 9b or upper cladding layer 9h). In this case, the bandgap energy of the photoconfinement layer is determined by the difference between the bandgap energy of the outermost well layer 9da constituting the active layer 9d and the adjacent outermost barrier layer 9db, and the bandgap of the cladding layer. Specifically, as shown in Figure 3, the layers constituting the photoconfinement layer have larger bandgaps the closer they are to the cladding layer. It is preferable that the difference between the bandgap energy E1 of the barrier layer and the bandgap energy E2 of the cladding layer is 90 meV or more.

[0052] Furthermore, in this embodiment, the lower optical confinement layer 9c and the upper optical confinement layer 9e are each composed of two semiconductor layers 9ca, 9cb or 9ea, 9eb. In this case, for example, when the band gap energy of the lower cladding layer 9b at the junction end between the lower optical confinement layer 9c and the lower cladding layer 9b is connected to the band gap energy of the barrier layer 9db at the junction between the well layer 9da, which is located closest to the lower optical confinement layer 9c in the active layer 9d, and the band gap energies of the semiconductor layers 9ca and 9cb (points P1, P2) lie on the envelope L1, and it is preferable that the semiconductor layers 9ca and 9cb are formed such that the envelope L1 is linear or a parabolic shape that is convex upwards or downwards (hereinafter, this envelope L1 is referred to as the band gap energy line). In this case, the lower optical confinement layer 9c has a linear GRIN (Graded INDEX)-SCH structure. Furthermore, it is preferable that the upper light confinement layer 9e also has a linear GRIN-SCH structure.

[0053] Furthermore, the lower optical confinement layer 9c and the upper optical confinement layer 9e are not limited to a two-layer structure, and a larger number of layers is preferable. However, increasing the number of layers necessitates compositional control for each layer, and if even one of these layers deviates from the lattice matching conditions, it can lead to crystal degradation due to crystal defects, making it difficult to achieve high optical output operation and high reliability. In addition, increasing the number of layers increases the number of growth operations, which poses a problem from a mass production standpoint. For these reasons, it is generally preferable to set the number of layers to around 5 to 6. Even in this case, it is preferable that each of the multiple semiconductor material layers constituting the optical confinement layer is formed with a composition in which its bandgap energy lies above the bandgap energy line described above.

[0054] Furthermore, in the active layer 9d, it is preferable that the well layer 9da has a compressive strain of 0.5% or more relative to the substrate 9a. When a strain compensation structure is not used, the upper limit is preferably around 1.5%. In this embodiment, the light source 5 and SOA 9 output light with the same TE mode polarization, but as long as the polarization states of both are the same, a strain quantum well structure with tensile strain is acceptable.

[0055] Furthermore, in the active layer 9d, the well layers 9da may all have the same thickness, or they may have different thicknesses to broaden the wavelength spectral width of the emission. In addition, to achieve high optical gain, it is preferable that the number of wells be 4 to 8 and the product of the number of wells and the well thickness be in the range of 30 to 75 nm. In this embodiment, for example, the number of wells × well thickness is 6 × 7 nm = 42 nm.

[0056] Alternatively, an asymmetric waveguide structure may be formed in which the electric field distribution is distributed toward the substrate by interposing an electric field control layer, which has a refractive index higher than that of the lower cladding layer 9b and lower than that of the barrier layer 9db, at a certain distance from the lower optical confinement layer 9c of the lower cladding layer 9b. In this case, intervalence band absorption in the device can be suppressed, making it effective for higher power operation. Note that if the light source or SOA substrate is a p-type semiconductor substrate, the electric field control layer is provided on the side opposite to the substrate relative to the active layer.

[0057] Such electric field control layers may be, for example, a GaInAsP layer with a thickness of about 5 μm and a composition wavelength of about 0.95 to 1 μm, or a multilayer structure composed of a GaInAsP layer and an InP layer to realize an optically equivalent GaInAsP layer, or a pseudo-GaInAsP layer composed of GaInAsP and an InP layer in which quantum size effects are exhibited. In other words, the constituent material of the electric field control layer is, for example, an AlGaInAsP-based material.

[0058] This SOA9 employs an embedded heterostructure to achieve a nearly circular beam profile and high coupling efficiency to optical fibers, but a ridge structure or SAS structure would also be acceptable. The far-field image of the SOA9 in the vertical and horizontal directions is, for example, 27.4° × 22.7°, with an aspect ratio of approximately 1.2.

[0059] Furthermore, while the element length of the SOA9 is, for example, 1.8 mm, it is not limited to this.

[0060] Furthermore, SOA9 can be fabricated using crystal growth equipment such as MOCVD and MBE, dielectric film deposition equipment, lithography equipment, semiconductor etching equipment, and electrode formation equipment. When fabricating SOA9, it is effective to deposit a low-reflectivity film consisting of a dielectric layer of 0.2% or less on both end faces to reduce the end-face reflectivity and suppress laser oscillation. In addition, the waveguide (active layer 9d) near both end faces of the element may be made into an oblique waveguide such that it is at an angle of 3° or more with respect to the central axis of the element, or a window structure may be adopted. Alternatively, a combination of these structures may be used. Furthermore, the waveguide structure is not limited to a linear shape; tapered or multimode interference waveguides (MMI) or a combination of a linear and tapered structure are acceptable as long as a single transverse mode can be maintained. In addition, the waveguide structure may be equipped with, for example, a mode-field converter or a spot-size converter on the side where the output light is output, in order to improve the coupling efficiency with other elements or optical fibers.

[0061] [Operation of the Optical Semiconductor Module] Next, the operation of the optical semiconductor module 100 will be described. The light source 5 outputs ASE light as input light. The lens 6 collimates the input light from the light source 5 and outputs it to the optical isolator 7. The optical isolator 7 allows the input light to pass through. The lens 8 focuses the input light that has passed through the optical isolator 7 and couples it to the SOA 9.

[0062] SOA9 amplifies the input light and outputs it as output light. Lens 10 collimates the output light output from SOA9 and outputs it to lens 11. The output light output from lens 10 is focused and coupled to the end face 12a of optical fiber 12. Optical fiber 12 propagates the coupled output light. The output light thus output is ASE light amplified by SOA9, and the optical semiconductor module 100 functions as an ASE light source.

[0063] Here, the output light from SOA9 is coupled to the optical fiber 12 without passing through the optical isolator. This suppresses the effects of insertion loss in the optical isolator, resulting in higher output power and improved power-to-optical conversion efficiency, as well as cost reduction by reducing the number of components.

[0064] [Differences in Characteristics between Embodiment 1 and Comparative Embodiment] Next, an example (first example) of the differences in characteristics between the optical semiconductor module 100 according to Embodiment 1 and the optical semiconductor module 100a according to the comparative embodiment will be described. Figure 4 is a schematic configuration diagram of the optical semiconductor module according to the comparative embodiment. The optical semiconductor module 100a differs from the optical semiconductor module 100 of Embodiment 1 in that an optical isolator 13 is provided between lens 10 and lens 11. The optical isolator 13 allows the output light output from lens 10 to pass through, while blocking light traveling in the opposite direction to the output light. The insertion loss of the optical isolator 13 is 0.27 dB.

[0065] Figures 5A and 5B show the individual output characteristics of the light source 5 and SOA9 in the optical semiconductor modules 100 and 100a, respectively. Figure 5A shows the characteristics of the light source 5, and Figure 5B shows the characteristics of the SOA9. The horizontal axis represents the drive current, and the vertical axis represents the output optical power.

[0066] In Figure 5A, line L11 shows the characteristics of the light source 5 used in the optical semiconductor module 100, and line L21 shows the characteristics of the light source 5 used in the optical semiconductor module 100a. In Figure 5B, line L12 shows the characteristics of the SOA9 used in the optical semiconductor module 100, and line L22 shows the characteristics of the SOA9 used in the optical semiconductor module 100a. As can be seen from Figures 5A and 5B, the light source 5 and SOA9 had equivalent characteristics individually. That is, both had a clearing current of approximately 50 mA and an optical power of 135 mW to 150 mW at a drive current of 1000 mA.

[0067] Figures 6A and 6B show the output characteristics of the optical semiconductor modules 100 and 100a. Figure 6A shows the case where the drive current supplied to the light source 5 is zero. The horizontal axis represents the drive current of SOA9, and the vertical axis represents the optical power output from the optical fiber 12.

[0068] In Figure 6A, line L13 represents the characteristics of the optical semiconductor module 100, and line L23 represents the characteristics of the optical semiconductor module 100a. In addition, in Figure 6A, line L23A represents the characteristics of line L23 plus 0.27 dB, which is equivalent to the insertion loss of the optical isolator 13.

[0069] As can be seen from Figure 6A, up to a drive current of 397 mA and an optical power of 48.3 mW, the optical semiconductor modules 100 and 100a exhibited equivalent characteristics. At drive currents higher than this, the optical power of optical semiconductor module 100 was lower than that of optical semiconductor module 100a.

[0070] However, when the drive current was 1236 mA or higher, the optical power of the optical semiconductor module 100 became greater than the optical power of the optical semiconductor module 100a. Furthermore, regarding the maximum optical power (maximum optical output) and the saturation optical output current, which is the drive current at which the maximum optical output occurs, the optical semiconductor module 100 had values ​​of 115.4 mW and 1301 mA, respectively, while the optical semiconductor module 100a had values ​​of 112 mW and 1139 mA. This confirmed that the optical semiconductor module 100 had a maximum optical output that was about 3% higher and a saturation optical output current that was about 14% higher. It was also confirmed that the maximum optical output at line L23A was 119.4 mW, which was about 3% higher than that of the optical semiconductor module 100, but the saturation optical output current was smaller than that of the optical semiconductor module 100.

[0071] Next, Figure 6B shows the case where the drive current supplied to the light source 5 is 100 mA. The horizontal axis represents the drive current of SOA9, and the vertical axis represents the optical power output from the optical fiber 12.

[0072] In Figure 6B, line L14 represents the characteristics of the optical semiconductor module 100, and line L24 represents the characteristics of the optical semiconductor module 100a. Also in Figure 6B, line L24A represents the characteristics obtained by adding 0.27 dB, which is equivalent to the insertion loss of the optical isolator 13, to line L24.

[0073] In the case of Figure 6B, the input light from the light source 5 is optically amplified and output by SOA9. As can be seen from Figure 6B, the maximum optical output and saturated optical output current of optical semiconductor module 100 were 212.3 mW and 1256 mA, respectively, while the maximum optical output and saturated optical output current of optical semiconductor module 100a were 189 mW and 1188 mA, respectively. In other words, it was confirmed that optical semiconductor module 100 has a maximum optical output that is about 12% higher and a saturated optical output current that is about 6% higher. Furthermore, the maximum optical output at line L24A was 201 mW, and it was confirmed that optical semiconductor module 100 has a maximum optical output that is about 6% higher.

[0074] In other words, it was confirmed that the optical semiconductor module 100 can achieve an improvement in maximum optical output that exceeds the result when the insertion loss of the optical isolator 13 is taken into account.

[0075] The reason for this is thought to be, for example, the reflection of output light that occurs at the end face 12a when the output light is coupled from lens 11 to optical fiber 12. If the refractive index n1 of the core in optical fiber 12 is 1.5, then because the end face 12a is flat, the reflectance R (assuming normal incidence) is R = [(1 - n1) / (1 + n1)] 2 = 4%. Therefore, it is thought that the difference in increasing the output power of the optical output characteristics arises depending on whether the end face 12a becomes a reflection point and the reflected light is input to SOA9 or blocked by the optical isolator 13. In other words, in the optical semiconductor module 100 without the optical isolator 13, the reflected light propagating in the opposite direction to the output light from SOA9 is optically amplified within SOA9 and output from the element end face closer to the light source 5. In this case, it is thought that the carrier consumption within SOA9 increases due to the optical amplification of the reflected light, so the effective injected carriers are reduced and the power of the ASE light generated in SOA9 is reduced. Furthermore, in the optical semiconductor module 100, since carrier consumption increases due to the optical amplification of the reflected light, gain saturation in SOA9 is suppressed, and it is thought that the saturated optical output current and maximum optical output are larger than in the case of optical semiconductor module 100a.

[0076] As described above, the optical semiconductor module 100 according to Embodiment 1 achieves higher output optical output characteristics and a corresponding improvement in power-to-optical conversion efficiency, as well as cost reduction through a reduction in the number of components. Furthermore, the optical semiconductor module 100 has high optical output power, maximum optical output, and saturation optical output current for large drive currents, and reactive power due to heat is reduced, resulting in a more reliable high-output ASE light source. In addition, the optical alignment process when incorporating the optical isolator 13 is eliminated, resulting in shorter lead times during manufacturing and improved yield due to reduced coupling variations through process reduction. Moreover, by not including the optical isolator 13, the optical semiconductor module 100 has the advantage of providing more space within the package 1, allowing for a wider design margin to realize an element with excellent heat dissipation characteristics by increasing the element length of the SOA9.

[0077] (Modified Version of Embodiment 1) Figure 7 is a schematic diagram of an optical semiconductor module according to a modified version of Embodiment 1. The optical semiconductor module 100b has a configuration in which the optical isolator 7 is removed from the optical semiconductor module 100 according to Embodiment 1.

[0078] Even with such an optical semiconductor module 100b, the same effects as the optical semiconductor module 100 according to Embodiment 1 can be obtained.

[0079] [Differences in Characteristics between Embodiment 1 and Modified and Comparative Embodiments of Embodiment 1] Next, an example (second example) of the differences in characteristics between the optical semiconductor module 100 according to Embodiment 1, the optical semiconductor module 100b according to the modified version, and the optical semiconductor module 100a according to the comparative embodiment will be described. In the first example described above, the element lengths of the light source 5 and SOA9 are equal, but in this second example, the lengths of the light source 5 and SOA9 are in a ratio of 1:4. The sum of the lengths of the two elements is approximately 4.4 mm to 4.6 mm.

[0080] In this second example, the reflectivity of both end faces of the SOA9 is 0.4% or less. Furthermore, the SOA9 has a structure that suppresses ripple caused by longitudinal modes due to the element length by tilting the output light exit end face at 6 to 7 degrees with respect to the longitudinal direction of the element. On the other hand, the light source 5 has a straight waveguide structure and an end face reflectivity of 95% or more on the rear end face side, and an oblique waveguide structure and an end face reflectivity of 0.4% or less on the exit end face side. With a light source 5 of this structure, high output can be obtained with a lower current than when an oblique waveguide structure and low reflectivity are adopted on both end faces.

[0081] Furthermore, the optical fiber 12 in this second example is a polarization-maintaining optical fiber. The SOA9 end face of the optical fiber 12 is flat, and its reflectivity is approximately 4%.

[0082] In this second example, the drive current of light source 5 may be referred to as the seed current, and the drive current of SOA9 may be referred to as the booster current.

[0083] Figure 8 shows the output characteristics of optical semiconductor modules according to Embodiment 1, a modified example, and a comparative example. The horizontal axis represents the booster current. The seed current is 150 mA. In the case of the modified example (line L35), a 29% higher output was obtained compared to the comparative example (line L25), and an 8% higher output was obtained compared to Embodiment 1 (line L15).

[0084] Figure 9 shows the spectrum of the output light of an optical semiconductor module according to Embodiment 1, a modified example, and a comparative example. The seed current is 150 mA and the booster current is 1000 mA. In Embodiment 1 (line L16), the spectral width was narrower than in the comparative example (line L26), and in the modified example (line L36), the spectral width was narrower than in Embodiment 1.

[0085] In Embodiment 1, the reflected light from the end face of the optical fiber 12 returns to the SOA9, where it is amplified within the SOA9 to become ASE light. This promotes carrier consumption within the SOA9, resulting in a narrower spectral width. In the modified example, the ASE light amplified within the SOA9 is input to the light source 5, where it is amplified further, reflected from the rear end face, and then input back into the SOA9 as seed light. This further promotes carrier consumption within the SOA9, resulting in an even narrower spectral width.

[0086] In Raman amplifiers, in order to keep the gain flatness below 1 dB, excitation light from multiple excitation sources is sometimes combined by wavelength synthesis. When an excitation source with a broad spectrum of about 30 nm, as in the comparative configuration, is used, insertion loss problems may occur due to the limitation of the wavelength bandwidth of the wavelength multiplexer (WDM coupler). In contrast, in Embodiment 1 and its modified form, the spectral width is relatively narrow, so it is possible to reduce the insertion loss during wavelength combination, and thus reduce the reactive power of the excitation source constituting the Raman amplifier.

[0087] Figure 10 shows the wavelength shift of the output light spectrum of a modified optical semiconductor module. The horizontal axis represents the drive current, which is the seed current. The vertical axis represents the wavelength shift when the booster current is increased from 150 mA to 1000 mA. It was confirmed that when the seed current is 200 mA or higher, the wavelength shift can be suppressed to about 30% or less compared to when the seed current is 150 mA or lower.

[0088] In excitation light sources for Raman amplifiers, wavelength shift can lead to variations in the shape of the Raman gain spectrum, potentially affecting the flatness of the gain. In contrast, the modified design reduces the wavelength shift to a small value of approximately 2 nm or less by controlling the seed current, thereby mitigating this effect.

[0089] (Embodiment 2) Figure 11 is a schematic diagram of the optical semiconductor module according to Embodiment 2. The optical semiconductor module 100A has a configuration in which an in-line type optical isolator 14 is provided in the middle of the optical fiber 12 of the optical semiconductor module 100 according to Embodiment 1 and outside the package 1. Here, the in-line type optical isolator 14 is an optical isolator that has pigtail fibers provided at both ends and can be inserted in the middle of the optical fiber.

[0090] Similar to the optical semiconductor module 100, the optical semiconductor module 100A can achieve effects such as increased optical output characteristics, improved power-to-optical conversion efficiency, high reliability, improved yield, and wider design margins. Furthermore, because the optical semiconductor module 100A is equipped with an optical isolator 14, even if unintended light travels through the optical fiber 12 from outside the optical semiconductor module 100A, for example from the transmission system to which the optical semiconductor module 100A is applied, it is blocked by the optical isolator 14 and does not reach the SOA9. As a result, the temporal instability of the SOA9 characteristics is suppressed.

[0091] Figures 12A and 12B show the output characteristics of the optical semiconductor modules 100A and 100a. Figure 12A shows the case where the drive current supplied to the light source 5 is 100mA. In Figures 12A and 12B, the insertion loss of the optical isolator 14 is 0.36dB. The horizontal axis represents the drive current of SOA9, and the vertical axis represents the optical power output from the optical fiber 12.

[0092] In Figure 12A, line L34 shows the characteristics of optical semiconductor module 100A, and line L24 shows the characteristics of optical semiconductor module 100a, similar to Figure 6B. As shown in Figure 12A, optical semiconductor module 100A has higher optical output power for a large drive current, higher maximum optical output, and higher saturation optical output current compared to optical semiconductor module 100a, and reactive power due to heat is reduced, thus realizing a more reliable high-output ASE light source.

[0093] Figure 12B shows the spectrum of light output from the optical fiber 12. Line L35 represents the characteristics of optical semiconductor module 100A, and line L25 represents the characteristics of optical semiconductor module 100a. In both optical semiconductor modules, the spectral width is approximately 30 nm, making it possible to realize a light source suitable for the forward-excited Raman amplification method.

[0094] In the cases of Figures 12A and 12B, the insertion loss of the optical isolator 14 is 0.36 dB. However, using an optical isolator 14 with an insertion loss lower than 0.36 dB is preferable because it further improves the output characteristics of the optical semiconductor module 100A.

[0095] (Embodiment 3) Figure 13 is a schematic diagram of the optical semiconductor unit according to Embodiment 3. The optical semiconductor unit 1000 comprises two optical semiconductor modules 100 shown in Figure 1, a polarization combiner (PBC) 1001, an optical fiber 1002, and an optical isolator 1003.

[0096] The two optical semiconductor modules 100 output output light from the optical fiber 12 with mutually orthogonal polarization states. The PBC 1001 polarizes and combines the output light from the two optical semiconductor modules 100 and outputs it to the optical fiber 1002. The optical fiber 1002 propagates the polarized and combined output light. The optical fiber 1002 is a polarization-maintaining optical fiber, but is not limited to this. The optical isolator 1003 is an in-line type optical isolator and is provided in the middle of the optical fiber 1002. The optical isolator 1003 is an example of an in-line type optical isolator provided in the optical fiber that propagates the polarized and combined output light. As a variation of the two optical semiconductor modules 100, there is no problem with arranging the components of the two optical semiconductor modules 100 in a single package equipped with two output optical fibers. This configuration provides the effect of saving space. Furthermore, there is no problem with a configuration in which the components of two optical semiconductor modules 100 are arranged in a single package and focused into a single optical fiber using a spatial coupling system with a lens.

[0097] In the optical semiconductor unit 1000 configured as described above, the effects obtained in the optical semiconductor module 100 are realized, and even higher optical output characteristics are achieved through polarization synthesis. Furthermore, since it is not necessary to provide an optical isolator 1003 for each optical semiconductor module 100, the number of components used can be reduced.

[0098] Figure 14 shows the output characteristics of the optical semiconductor unit 1000. In Figure 14, the insertion loss of the PBC 1001 is 0.45 dB, and the insertion loss of the optical isolator 1003 is 0.36 dB. The drive current supplied to the light source 5 is 100 mA. The horizontal axis represents the sum of the drive currents of the two SOA9s, and the vertical axis represents the optical power output from the optical fiber 1002. As can be seen from Figure 14, the optical semiconductor unit 1000 has a maximum optical output of 300 mW or more, achieving even higher optical output characteristics than a single optical semiconductor module 100.

[0099] Furthermore, when the optical semiconductor unit 1000 is used as an excitation light source in a Raman amplifier, the optical fiber 1002 is often fusion-spliced ​​to other standard single-mode optical fibers. In this case, if the optical fiber 1002 is a standard single-mode optical fiber, the connection loss in the fusion splicing can be reduced.

[0100] (Embodiment 4) Figure 15 is a schematic diagram of the optical semiconductor unit according to Embodiment 4. The optical semiconductor unit 1000A includes the optical semiconductor module 100 shown in Figure 1, the optical semiconductor module 100B, the WDM coupler 1004, the optical fiber 1002, and the optical isolator 1003.

[0101] Optical semiconductor module 100B differs from optical semiconductor module 100 in that it is configured to output output light from the optical fiber 12 that has a different wavelength from the output light output from optical semiconductor module 100. Optical semiconductor modules 100 and 100B are an example of two or more optical semiconductor modules that output output light of different wavelengths from each other.

[0102] The WDM coupler 1004 wavelength-multiplexes the output light of different wavelengths from the optical semiconductor modules 100 and 100B and outputs it to the optical fiber 1002. The WDM coupler 1004 is an example of a wavelength-multiplexer. The optical fiber 1002 and optical isolator 1003 are the same as those shown in Figure 13, so their explanation is omitted. The optical isolator 1003 is an example of an in-line type optical isolator provided in the optical fiber that propagates the wavelength-multiplexed output light.

[0103] In the optical semiconductor unit 1000A configured as described above, the effects obtained with the optical semiconductor module 100 are realized, and even higher optical output characteristics are achieved through wavelength division multiplexing. Furthermore, since it is not necessary to provide an optical isolator 1003 for each optical semiconductor module 100 and 100B, the number of components used can be reduced. In addition, since the optical semiconductor unit 1000A undergoes wavelength division multiplexing within the package, as disclosed in Non-Patent Document 1, for example, compared to a configuration affected by wavelength variations between chips, the wavelength characteristics of each optical semiconductor module 100 and 100B can be confirmed and selected before assembly. As a result, high wavelength accuracy can be achieved, and the yield during wavelength division multiplexing can be increased. Such an optical semiconductor unit 1000A is suitable as an excitation light source for wavelength division multiplexing Raman amplification.

[0104] (Embodiment 5) Figure 16 is a schematic diagram of the optical semiconductor unit according to Embodiment 5. The optical semiconductor unit 1000B comprises two optical semiconductor modules 100, two optical semiconductor modules 100B, two optical semiconductor modules 100C, three PBCs 1001, three optical fibers 1002, an optical isolator 1003, a WDM coupler 1005, and an optical fiber 1006.

[0105] The optical semiconductor module 100C differs from the optical semiconductor modules 100 and 100B in that it is configured to output output light from the optical fiber 12 that has a different wavelength from the output light output from the optical semiconductor modules 100 and 100B. The two optical semiconductor modules 100, 100B, and 100C are an example of a configuration in which there are two or more pairs (specifically three pairs) of optical semiconductor modules that output output light of different wavelengths from each other.

[0106] Each of the three PBCs 1001 polarizes and combines the output light from two optical semiconductor modules 100, two optical semiconductor modules 100B, and two optical semiconductor modules 100C, each with mutually orthogonal polarization states, and outputs it to each of the optical fibers 1002. Each optical fiber 1002 propagates the polarized and combined output light. The optical fibers 1002 are polarization-maintaining optical fibers, but are not limited to this. The WDM coupler 1005 wavelength-multiplexes the polarized and combined output light of the three optical fibers 1002, each with different wavelengths, and outputs it to the optical fiber 1006. The optical fiber 1006 propagates the polarized and wavelength-multiplexed output light. The optical fiber 1006 are polarization-maintaining optical fibers, but are not limited to this. The optical isolator 1003 is an in-line type optical isolator and is provided in the middle of the optical fiber 1006.

[0107] In the optical semiconductor unit 1000B configured as described above, the effects obtained in the optical semiconductor module 100 are realized, and even higher optical output characteristics are achieved through polarization synthesis and wavelength division multiplexing. Furthermore, since only one optical isolator 1003 is required for six optical semiconductor modules 100, 100B, and 100C, the number of components used can be reduced more effectively.

[0108] (Embodiment 6) Figure 17 is a schematic diagram of the optical semiconductor unit according to Embodiment 6. The optical semiconductor unit 1000C comprises two optical semiconductor modules 100, two optical semiconductor modules 100B, two optical semiconductor modules 100C, two WDM couplers 1005, two optical fibers 1002, an optical isolator 1003, a PBC 1001, and an optical fiber 1006.

[0109] The optical semiconductor modules 100, 100B, and 100C are examples of two or more optical semiconductor modules that output light of different wavelengths. The optical semiconductor unit 1000C comprises two sets of optical semiconductor modules 100, 100B, and 100C.

[0110] Each of the two WDM couplers 1005 wavelength-multiplexes the output light from the three optical semiconductor modules 100, 100B, and 100C and outputs it to the optical fiber 1002. The two optical fibers 1002 each propagate the wavelength-multiplexed output light. The PBC 1001 polarization-combines the wavelength-multiplexed output light from the two optical fibers 1002, which have mutually orthogonal polarization states, and outputs it to the optical fiber 1006. The optical fiber 1006 propagates the wavelength-multiplexed and polarization-combined output light.

[0111] In the optical semiconductor unit 1000C configured as described above, the effects obtained in the optical semiconductor module 100 are realized, and even higher optical output characteristics are achieved through polarization synthesis and wavelength division multiplexing. Furthermore, since only one optical isolator 1003 is required for the six optical semiconductor modules 100, 100B, and 100C, the number of components used can be reduced more effectively.

[0112] (Embodiment 7) Figure 18 is a schematic diagram of a Raman amplifier according to Embodiment 7. The Raman amplifier 10000 comprises an optical semiconductor unit 1000C according to Embodiment 6, a Raman amplification optical fiber 10001, WDM couplers 10002 and 10003, a secondary excitation optical unit 10004, an optical isolator 10005, a WDM coupler 10006, a secondary excitation optical unit 10007, and an optical isolator 10008.

[0113] The Raman-amplifying optical fiber 10001 is an optical fiber that, when an excitation light is input, generates a Raman gain with a peak wavelength approximately 100 nm longer than the wavelength of the excitation light. When an optical signal with a wavelength within the Raman gain wavelength band is input while Raman gain is being generated, the Raman-amplifying optical fiber 10001 Raman-amplifies the signal light and outputs it. The Raman-amplifying optical fiber 10001 is, for example, an optical fiber that constitutes a transmission line in optical fiber communication.

[0114] The WDM coupler 10002 combines the excitation light PL1 (described later) and the optical signal LS1 that has propagated through the transmission line and outputs it to the Raman amplified optical fiber 10001.

[0115] The WDM coupler 10003 outputs optical signal LS2, which is an optical signal obtained by Raman amplification of optical signal LS1, to the transmission line, and also outputs excitation light PL2, which will be described later, to the Raman amplified optical fiber 10001.

[0116] The secondary excitation light unit 10004 outputs secondary excitation light. The secondary excitation light unit 10004 is configured to include, for example, a semiconductor laser element and an ASE light source. The semiconductor laser element may be configured to include, for example, a semiconductor laser element whose laser oscillation wavelength is stabilized by a fiber Bragg grating (FBG).

[0117] The optical isolator 10005 allows the secondary excitation light output from the secondary excitation light unit 10004 to pass through, while blocking light traveling in the opposite direction to the secondary excitation light.

[0118] The WDM coupler 10006 wavelength-multiplexes the excitation light, which is the output light from the optical semiconductor unit 1000C, and the secondary excitation light, which is the output light from the secondary excitation light unit 10004, and outputs it as excitation light PL1.

[0119] The secondary excitation light unit 10007 outputs secondary excitation light. The secondary excitation light unit 10007 is configured similarly to the secondary excitation light unit 10004. The optical isolator 10005 passes the secondary excitation light output from the secondary excitation light unit 10004 through to output excitation light PL2, and blocks light traveling in the opposite direction to the secondary excitation light.

[0120] Next, the operation of the Raman amplifier 10000 will be explained. First, the Raman amplification optical fiber 10001 receives the excitation light PL1 and PL2 output from the WDM couplers 10002 and 10003. As a result, the Raman amplification optical fiber 10001 is in a state where it can Raman amplify the optical signal LS1 using the output light (excitation light) from the optical semiconductor unit 1000C contained in the input excitation light PL1. In this state, when the optical signal LS1 is input, the Raman amplification optical fiber 10001 Raman amplifies the optical signal LS1 and outputs it as the optical signal LS2.

[0121] Furthermore, the Raman amplifier 10000 is configured as a forward-excited type because the optical signal LS1 and the excitation light that Raman-amplifies it propagate in the same direction.

[0122] Furthermore, the secondary excitation light and excitation light PL2 included in excitation light PL1 are set to wavelengths that allow the output light (excitation light) from the optical semiconductor unit 1000C to be Raman amplified by the Raman amplification optical fiber 10001.

[0123] The Raman amplifier 10000 configured as described above achieves the effects obtained from the optical semiconductor module 100 and the optical semiconductor unit 1000C.

[0124] In particular, the Raman amplifier 10000 uses wavelength-multiplexed output light from the optical semiconductor unit 1000C as excitation light, thus enabling the acquisition of a wider wavelength range of Raman gain. For example, if the optical semiconductor modules 100, 100B, and 100C of the optical semiconductor unit 1000C are configured to output light with peak wavelengths of 1425 nm, 1461 nm, and 1495 nm, respectively, and a spectral width of 25 nm or more, then even with a relatively small number of wavelengths of three, a relatively flat Raman gain spectrum can be achieved from the C band (1530-1565 nm) to the L band (1565-1610 nm), which is the most commonly used wavelength for communication. In contrast, if a more coherent excitation light source, such as a semiconductor laser element whose laser oscillation wavelength is stabilized by an FBG, is used instead of the optical semiconductor modules 100, 100B, and 100C, then, for example, only 4 to 9 wavelengths are required to achieve a relatively flat Raman gain spectrum from the C band to the L band.

[0125] Furthermore, in the Raman amplifier 10000, the excitation light is amplified using secondary excitation light, which allows the Raman amplification gain received by the optical signal LS1 due to the excitation light to be increased.

[0126] Furthermore, since the Raman amplifier 10000 is configured as a forward-excited type, it can suppress RIN transfer of excitation light to the optical signal and SBS (stimulated Bruyen scattering), which is a fiber nonlinear effect, thereby suppressing degradation of the optical signal due to OSNR degradation.

[0127] (Embodiment 8) Figure 19 is a schematic diagram of a Raman amplifier according to Embodiment 8. Raman amplifier 10000A has a configuration in which the portion consisting of the secondary excitation light unit 10007 and the optical isolator 10008 is connected to the WDM coupler 10002, and the portion consisting of the optical semiconductor unit 1000C, the secondary excitation light unit 10004, the optical isolator 10005, and the WDM coupler 10006 is connected to the WDM coupler 10003.

[0128] In this type of Raman amplifier 10000A, the optical signal LS1 and the excitation light that Raman-amplifies it propagate in opposite directions, so it is configured as a back-excitation type.

[0129] In the Raman amplifier 10000A configured as described above, the effects obtained from the optical semiconductor module 100 and the optical semiconductor unit 1000C, as well as the effect of obtaining a Raman gain over a wider wavelength band and the effect of increasing the gain of the Raman amplification received by the optical signal LS1, are realized, similar to the Raman amplifier 10000.

[0130] (Embodiment 9) Figure 20 is a schematic diagram of a Raman amplifier according to Embodiment 9. The Raman amplifier 10000B has a configuration in which a portion consisting of an optical semiconductor unit 1000C, a secondary excitation light unit 10004, an optical isolator 10005, and a WDM coupler 10006 is connected to a WDM coupler 10003.

[0131] Such a Raman amplifier 10000B is configured as a bidirectional excitation type.

[0132] In the Raman amplifier 10000B configured as described above, the same effects as the Raman amplifier 10000 are achieved, and an even higher Raman gain is realized due to the dual back excitation type.

[0133] In the above embodiment, the light source 5 and the SOA9 may be the same element, but they may be different elements. For example, it is preferable from the viewpoint of improving heat dissipation and saturation gain if the element length of the SOA9 is longer than the element length of the light source 5.

[0134] Furthermore, the arrangement order of lens 6, optical isolator 7, and lens 8 in the optical semiconductor module 100 is not limited to this. For example, the positions of lens 6 and optical isolator 7 may be swapped. Alternatively, an optical fiber with a focusing function may be used instead of lens 10. In addition, the end face of optical fiber 12 may be beveled. Alternatively, optical fiber 12 may be a lensed fiber with a lens-shaped end face. In this case, for example, lens 11 may be omitted. Lensed fibers can be, for example, wedge-shaped, biconical, or pencil-shaped, and the type is appropriately selected depending on the mode field of the output light from SOA9.

[0135] Furthermore, while the optical semiconductor module 100 is, for example, a high-power, wide-spectrum ASE light source, various other forms of optical semiconductor modules can be configured in this embodiment. For example, SOA9 may be a semiconductor optical amplifier employing a tapered structure or an MMI structure. Also, the light source 5 may be replaced with a Fabry-Perot laser or a single-mode laser, or with an optical functional integrated element such as a modulator-integrated laser or a tunable laser that integrates multiple functions on the same substrate.

[0136] Furthermore, for example, in the optical semiconductor module 100, the light source 5, lenses 6 and 8, and optical isolator 7 may be omitted. In this case, the SOA 9 has no input light and functions as an ASE light source.

[0137] Furthermore, for example, in the optical semiconductor module 100, a bandpass filter may be provided between the light source 5 and the SOA9. In this case, the spectral width of the output light output from the optical fiber 12 will be narrower than the spectral width of the input light from the light source 5, depending on the transmission bandwidth of the bandpass filter.

[0138] Furthermore, the light source 5 and the SOA9 may be optically connected by a semiconductor optical waveguide. In this case, the light source 5 and the SOA9 may be integrated into a single semiconductor element.

[0139] Furthermore, SOA9 may be composed of multiple SOAs connected in a multi-stage configuration.

[0140] Furthermore, the constituent materials of the light source 5 and SOA9 may be GaAs-based semiconductor materials. Examples of constituent materials include AlGaAs and AlGaInP. In this case, the light source 5 and SOA9 may have a semiconductor stacked structure formed on a substrate made of GaAs. In addition, the active layers of the light source 5 and SOA9 may have a quantum dot structure that is suitable from the viewpoint of operation at high temperatures.

[0141] 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.

[0142] This invention can be used in optical semiconductor modules.

[0143] 1: Package 1a: Main body 1b: Window 2: Lens holder 3: Ferrule 4: Thermoelectric cooling element (TEC) 5: Light source 6, 8, 10, 11: Lens 7, 13, 14, 1003, 10005, 10008: Optical isolator 9: Semiconductor optical amplifier (SOA) 9a: Substrate 9b: Lower cladding layer 9c: Lower optical confinement layer 9ca, 9cb, 9ea, 9eb: Semiconductor layer 9d: Active layer 9da: Well layer 9db: Barrier layer 9e: Upper optical confinement layer 9f: Lower current blocking layer 9g: Upper current blocking layer 9h: Upper cladding layer 9i: Contact layer 9j: Upper electrode 9k: Lower electrode 12, 1002, 1006: Optical fiber 12a: End face 100, 100a, 100b, 100A, 100B, 100C: Optical semiconductor module 1000, 1000A, 1000B, 1000C: Optical semiconductor unit 1001: Polarization combiner (PBC) 1004, 1005, 10002, 10003, 10006: WDM coupler 10000, 10000A, 10000B: Raman amplifier 10001: Raman amplified optical fiber 10004, 10007: Secondary excitation optical unit L1: Envelope L11, L12, L13, L14, L21, L22, L23, L23A, L24, L24A, L25, L34, L35: Line LS1, LS2: Optical signal P1, P2: Point PL1, PL2: Excitation light

Claims

1. An optical semiconductor module comprising: a semiconductor optical amplifier capable of optically amplifying input light; an optical fiber to which the output light output from the semiconductor optical amplifier is coupled; and a package to which the semiconductor optical amplifier is housed and the optical fiber is attached, wherein the output light is coupled to the optical fiber without passing through an optical isolator.

2. The optical semiconductor module according to claim 1, further comprising a lens housed in the package for coupling the output light to the optical fiber.

3. The optical semiconductor module according to claim 1, further comprising a light source that outputs the input light.

4. The optical semiconductor module according to claim 3, wherein the light source is a semiconductor optical amplifier.

5. The optical semiconductor module according to claim 3 or 4, further comprising an optical isolator located between the semiconductor optical amplifier and the light source.

6. The optical semiconductor module according to claim 3 or 4, further comprising a lens for coupling the input light output from the light source to the semiconductor optical amplifier.

7. The optical semiconductor module according to claim 1, further comprising an in-line type optical isolator provided on the optical fiber outside the package.

8. An optical semiconductor module according to claim 1, comprising: two optical semiconductor modules; and a polarization combiner for polarization combining the output light output from the two optical semiconductor modules.

9. The optical semiconductor unit according to claim 8, further comprising two or more sets of the two optical semiconductor modules that output output light of different wavelengths, and a wavelength multiplexer that wavelength multiplexes the output light polarized and combined by the polarization combiner.

10. An optical semiconductor module according to claim 1, comprising: two or more optical semiconductor modules that output output light of different wavelengths from each other; and a wavelength multiplexer that wavelength multiplexes the output light output from the two or more optical semiconductor modules.

11. The optical semiconductor unit according to claim 10, further comprising two sets of the two or more optical semiconductor modules, and a polarization combiner for polarization combining the output light wavelength-multiplexed by the wavelength multiplexer.

12. The optical semiconductor unit according to any one of claims 8 to 11, further comprising an in-line optical isolator provided in an optical fiber that propagates the polarization-combined output light or the wavelength-multiplexed output light.

13. A Raman amplifier comprising: an optical semiconductor unit according to any one of claims 8 to 11; and a Raman amplification optical fiber into which the output light output from the optical semiconductor unit is input, wherein the Raman amplification optical fiber uses the output light as excitation light to Raman amplify the input signal light.

14. The Raman amplifier according to claim 13, configured as a forward-excitation type.

15. The Raman amplifier according to claim 13, configured as a back-excited type.

16. The Raman amplifier according to claim 13, configured as a bidirectional excitation type.

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