JP4447222B2 - Distributed feedback semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a distributed feedback semiconductor laser, and is particularly characterized in a configuration for stabilizing an oscillation range current value of a semiconductor laser used as a light source in a wavelength division multiplexing communication system or the like against changes in ambient temperature. The present invention relates to a distributed feedback semiconductor laser.
[0002]
[Prior art]
With the recent increase in speed and functionality of optical communication systems, semiconductor lasers with excellent wavelength stability are required as the light source. In particular, as a light source for wavelength multiplexing communication, a dynamic single mode is required. A distributed feedback semiconductor laser excellent in performance is used.
[0003]
In such a distributed feedback semiconductor laser, in order to realize stable single mode oscillation, a diffraction grating is provided in the optical waveguide in the laser resonator, and in particular, any of the ones near the center in the resonator. A stable single mode oscillation can be realized with a λ / 4 shift DFB semiconductor laser provided with a diffraction grating having a phase shift of λ / 4 (λ is a wavelength) at a position.
[0004]
In such a semiconductor laser, the oscillation threshold current generally increases as the ambient temperature rises, because the gain of the active layer decreases as the temperature rises.
[0005]
In addition, as a circuit for modulating laser light, it is currently necessary to design a complicated circuit taking into account the effect that the oscillation threshold of the semiconductor laser varies.
[0006]
In order to stabilize the oscillation threshold current value against such a change in ambient temperature, the laser light existing region is divided into a semiconductor layer having a negative refractive index temperature coefficient and a semiconductor layer having a positive refractive index temperature coefficient. It is proposed to combine them (for example, see Patent Document 1).
[0007]
[Patent Document 1]
JP 2000-223787 A
[Problems to be solved by the invention]
With the recent increase in optical communication capacity, the demand for optical fiber communication is increasing not only in the trunk line system but also in the subscriber line. However, semiconductor lasers used in the subscriber line are more peripheral than the trunk line system. It will be used under severe conditions with large temperature changes.
[0009]
For this reason, the oscillation threshold current value largely fluctuates due to the change in the ambient temperature. However, in order to simplify the modulation control circuit, a semiconductor laser whose temperature change in the threshold current value is smaller than the ambient temperature is required. .
[0010]
Therefore, it has been proposed to introduce a conductive medium having a negative refractive index change as described above as a semiconductor laser having a small variation in the threshold current value with respect to the ambient temperature. It is difficult to form a thick laminated structure including the current crystal growth technique.
[0011]
Accordingly, an object of the present invention is to stabilize the oscillation region current value of a semiconductor laser against changes in ambient temperature by a relatively easy process technique.
[0012]
[Means for Solving the Problems]
FIG. 1 is a block diagram showing the principle of the present invention. Means for solving the problems in the present invention will be described with reference to FIG.
In order to achieve the above object, in the distributed feedback semiconductor laser of the present invention, the diffraction grating 3 has a refractive index smaller than that of the first medium 1 and the first medium 1 and has a refractive index with respect to temperature. It is characterized by comprising the second medium 2 with a small increase rate.
[0013]
By forming the diffraction grating 3 by combining the first medium 1 and the second medium 2 having the above refractive index characteristic relationship, a semiconductor laser whose oscillation threshold current value is small with respect to the ambient temperature is configured. can do.
[0014]
Here, the following conditions are required for a semiconductor laser whose oscillation threshold current value does not depend on the ambient temperature.
Since the gain of the active layer decreases as the temperature rises, the current value for obtaining the oscillation threshold gain, that is, the threshold current value increases. However, since the decrease in the gain of the active layer with respect to the temperature rise cannot be avoided, the oscillation In order to keep the threshold current value stable, the oscillation threshold gain must decrease with increasing temperature.
[0015]
When the temperature rises, the gain obtained at a certain current decreases. However, in this structure, the threshold gain g _th decreases at the same time. Therefore, when the structure is ideally designed, the threshold current value I _th changes with the ambient temperature. Will be described below.
[0016]
The laser oscillation threshold current value I _th is approximately expressed by the following formula (1).
I _th = (dwL / η _i ) × {g _th / [Γβ (T)] + J _g (T)} (1)
However,
d: Active layer thickness (μm)
w: active layer width (cm)
L: Resonator length (cm)
η _i : Internal differential quantum efficiency g _th : Threshold gain (cm ⁻¹ )
Γ: optical confinement coefficient β (T): differential gain coefficient (cm · μm / A)
J _g (T): Current density at which the medium becomes transparent (A / cm ² · μm)
It becomes.
[0017]
It is known that the differential gain coefficient β (T) in this equation (1) decreases with increasing temperature, and the current density J _g (T) at which the medium becomes transparent increases with increasing temperature. However, this can be empirically expressed by the following equations (2) and (3).
β (T) = 1032exp (−T / T ₀ ) (2)
J _g (T) = 0.098exp (T / T ₀ ) (3)
[0018]
Here, in order to make I _th of the equation (1) constant or small with respect to the temperature change with respect to the temperature rise, from the equations (1) to (3),
X (T) = g _th / [Γβ (T)] + J _g (T) (4)
Must be constant with respect to the temperature T, or the amount of change must be kept small.
[0019]
For this purpose, it is necessary to reduce the threshold gain g _th with respect to the temperature rise.
That is, by increasing κL with respect to the temperature T, the threshold gain g _th can be decreased, whereby the threshold current value I _th can be kept constant, or the variation can be reduced compared to the normal structure. Can do.
[0020]
However, the diffraction grating 3 made of a semiconductor having two different refractive indexes having the properties of the first medium 1 increases the refractive index of both semiconductors at the same rate as the temperature rises. There is little change.
[0021]
On the other hand, when the diffraction grating 3 is composed of the first medium 1 and the second medium 2 having different refractive index characteristics as described above, the first medium 1 and the second medium 2 increase as the temperature rises. The difference in refractive index with the medium 2 increases.
Therefore, as the temperature rises, the coupling coefficient of the diffraction grating 3 increases. As a result, the product κL of the coupling coefficient κ and the resonator length L increases as the temperature rises. The gain g _th decreases.
[0022]
In this case, as the semiconductor having the characteristics of the first medium 1 whose refractive index increases as the temperature rises, there are GaAs, AlGaAs mixed crystal, InP, InGaAs mixed crystal, InGaAsP mixed crystal, etc., and materials constituting the semiconductor laser Of the majority.
[0023]
On the other hand, examples of the semiconductor having the characteristics of the second medium 2 having the property of increasing the refractive index with respect to the temperature change smaller or lower than those of the above semiconductor include HgCdTe mixed crystal, GaInAsBi mixed crystal, and GaInPBi mixed crystal. , TlInGaP mixed crystal, TlInGaAs mixed crystal and the like.
However, these materials have a higher refractive index than the semiconductors having the characteristics of the first medium 1 listed above.
As a non-conductive dielectric material having the characteristics of the second medium 2, it is known that benzocyclobutene (BCB) has a refractive index that decreases with increasing temperature.
[0024]
In addition, in the configuration of the diffraction grating 3, in order to further stabilize against changes in the ambient temperature, the first medium 1 is made of a material whose refractive index increases with respect to the temperature, and the second medium 2. Is preferably made of a material whose refractive index decreases with temperature.
[0025]
As an element structure, a diffraction grating 3 composed of a first medium 1 made of a conductive medium provided with a groove 4 and a second medium 2 made of a non-conductive medium provided so as to bury at least the groove 4 is active. A conductive medium having the same refractive index characteristic as that of the first medium 1 is provided on both sides of the second medium 2 constituting the diffraction grating 3 and located on the mesa structure including the layers, and the first medium The electrode may be provided on a conductive medium having the same refractive index characteristic as 1.
[0026]
Alternatively, the diffraction grating 3 composed of the first medium 1 made of a conductive medium provided with the grooves 4 and the second medium 2 made of a nonconductive medium provided so as to at least fill the grooves 4 includes an active layer. The first medium which is located on the mesa structure and is completely embedded with the first medium 1 on both sides of the second medium 2 constituting the diffraction grating 3 and also embedded on both sides of the second medium 2 An electrode may be provided above 1.
Note that the electrode in this case may be provided on the first medium 1 via a contact semiconductor layer.
[0027]
Further, the diffraction grating 3 composed of the first medium 1 made of a conductive medium provided with the groove 4 and the second medium 2 made of a non-conductive medium provided so as to fill at least the groove 4, on the active layer It may be provided so as to be located on both sides of the ridge structure provided in the.
[0028]
The second medium 2 is preferably benzocyclobutene (BCB), polystyrene, polymethyl methacrylate (PMMA), or lithium fluoride (LiF).
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Here, the distributed feedback semiconductor laser according to the first embodiment of the present invention will be described with reference to FIGS.
2A and 2B, FIG. 2A is a front view seen from the optical axis direction of the distributed feedback semiconductor laser according to the first embodiment of the present invention, and FIG. It is a schematic sectional drawing of the diffraction grating along the dashed-dotted line which connects AA 'in 2 (a).
[0030]
First, an n-type InP clad layer 12 having a thickness of, for example, 200 nm and an i-type InGaAsPSCH layer having a thickness of, for example, 50 nm are formed on the n-type InP substrate 11 by metal organic chemical vapor deposition (MOCVD). 13. An i-type InGaAsP active layer 14 having a thickness of, for example, 20 nm, an i-type InGaAsPSCH layer 15 having a thickness of, for example, 50 nm, and a p-type InP cladding layer 16 having a thickness of, for example, 200 nm are sequentially deposited. Let
[0031]
Then, thickness of, for example, after depositing a SiO ₂ film of 0.3 [mu] m, width by etching, for example, to form a 1.5μm of SiO ₂ film pattern (not shown), the SiO ₂ Mesa etching is performed until the n-type InP substrate 11 is reached using the film pattern as a mask.
[0032]
Next, a p-type InP buried layer 17 and an n-type InP current blocking layer 18 having a thickness of, for example, 100 nm are sequentially grown by MOCVD using the SiO ₂ film pattern as it is as a selective growth mask to bury the mesa side surface. .
Next, after removing the SiO ₂ film pattern, the p-type InP layer 19 is again deposited to a thickness of, for example, 200 nm using the MOCVD method until the surface becomes flat.
[0033]
Next, a resist is applied, and a resist pattern (not shown) is formed by exposing and developing a 120 nm line-and-space diffraction grating pattern with a width of, for example, 10 μm and a period of 240 nm at the center of the laser, Using this resist pattern as a mask, the exposed portion of the p-type InP layer 19 is etched to a depth of, for example, 10 nm, thereby forming the groove 20.
In this case, the phase shift region 21 is provided near the center of the groove 20.
[0034]
Next, after removing the resist pattern, the groove forming portion is covered with a SiO ₂ pattern (not shown). Using this SiO ₂ pattern as a selective growth mask, the p-type having a thickness of, for example, 50 nm is again formed by MOCVD. An InGaAs contact layer 22 is deposited.
[0035]
Next, after removing the SiO ₂ pattern provided in the groove forming portion, the p-type InP layer 19 and the BCB layer are coated with benzocyclobutene (BCB) to a thickness of, for example, 200 nm. Next, a p-side electrode 25 is provided on the p-type InGaAs contact layer 22, and an n-side electrode 26 is formed on the back surface of the n-type InP substrate 11.
Finally, after cleaving to a resonator length of 200 μm, the antireflection film 27 is formed on the end face perpendicular to the optical axis, thereby completing the basic configuration of the distributed feedback semiconductor laser according to the first embodiment of the present invention. .
[0036]
Since the distributed feedback semiconductor laser according to the first embodiment of the present invention uses the BCB which is an insulator, a structure in which a current flows from both sides of the BCB is employed.
Note that the use of BCB for the cladding layer itself is described in a non-patent document (Japan Journal of Applied Physics, vol. 41, p. L249-251, 2002).
[0037]
3. FIG. 3 FIG. 3 shows a calculation result regarding the temperature dependence of κL of the distributed feedback semiconductor laser of the first embodiment.
InP constituting the diffraction grating 24 increases at a change rate of about 3 × 10 ⁻⁴ [K ⁻¹ ] with respect to the temperature rise, but BCB (refractive index at 300 ° C. is 1.545) has a change rate with respect to the temperature rise. Is approximately −3.0 × 10 ⁻⁵ [K ⁻¹ ], and decreases with increasing temperature. As the temperature increases, the difference in refractive index between the two increases, thereby increasing the coupling coefficient κ, so that the product κL is Rise with temperature.
[0038]
FIG. 4 shows the calculation result of the temperature change of the threshold current value I _th of the distributed feedback semiconductor laser of the first embodiment described above. The diffraction grating is formed only by the InP-based semiconductor. compared with the conventional example, it can be seen that the amount of change in the threshold current value I _th is small.
[0039]
In the first embodiment of the present invention, since the diffraction grating portion is wider than the mesa width of the active layer portion, the electric field distribution of light does not reach the p-side electrode portion. The p-side electrode does not adversely affect the laser characteristics.
[0040]
However, if the diffraction grating forming portion is too wide, there is a problem that the area of the p-type InGaAs contact layer is reduced and the element resistance is increased.
Conversely, if the p-type InGaAs contact layer portion is widened to reduce the device resistance, there arises a problem that the electric field distribution of light is applied to the p-type InGaAs contact layer and the laser characteristics deteriorate.
[0041]
Next, a distributed feedback semiconductor laser according to a second embodiment of the present invention will be described with reference to FIG. 5, except that the width of the diffraction grating and the configuration in the vicinity thereof are different. Since this is the same as the embodiment, only the differences will be described.
FIG. 5A is a front view seen from the optical axis direction of the distributed feedback semiconductor laser according to the second embodiment of the present invention, and FIG. It is a schematic sectional drawing of the diffraction grating along the dashed-dotted line which connects AA 'in 5 (a).
[0042]
Just like the manufacturing process of the distributed feedback semiconductor laser of the first embodiment, the p-type InP buried layer 17 and the n-type InP current blocking layer 18 are provided on both sides of the stripe-shaped mesa including the i-type InGaAsP active layer. Then, the SiO ₂ film pattern is removed, and the p-type InP layer 19 is again deposited to a thickness of, for example, 300 nm by using the MOCVD method.
[0043]
Next, a resist is applied, and a resist pattern (not shown) is formed by exposing and developing a 120 nm line-and-space diffraction grating pattern with a width of, for example, 10 μm and a period of 240 nm at the center of the laser, Using this resist pattern as a mask, the exposed portion of the p-type InP layer 19 is etched to a depth of, for example, 10 nm, thereby forming the groove 20.
In this case as well, the phase shift region 21 is provided near the center of the groove 20.
[0044]
Next, after removing the resist pattern, the groove forming portion is covered with a SiO ₂ pattern (not shown), and using this SiO ₂ pattern as a selective growth mask, the thickness is again p-type, for example, 150 nm by MOCVD. An InP layer 28 and a p-type InGaAs contact layer 22 having a thickness of, for example, 50 nm are sequentially deposited.
[0045]
Next, after removing the SiO ₂ pattern provided in the groove forming portion, BCB is coated on the groove forming portion to a thickness of, for example, 200 nm, and the diffraction consisting of the p-type InP layer 19 and the BCB layer 23 is performed. A lattice 24 is formed, and then a p-side electrode 25 is provided on the p-type InGaAs contact layer 22 and an n-side electrode 26 is formed on the back surface of the n-type InP substrate 11.
Finally, after cleaving to a resonator length of 200 μm, the antireflection film 27 is formed on the end face perpendicular to the optical axis, thereby completing the basic configuration of the distributed feedback semiconductor laser according to the second embodiment of the present invention. .
[0046]
In the second embodiment of the present invention, the diffraction grating portion is narrowed to reduce the element resistance. However, since the p-type InP layers are formed thick on both sides of the diffraction grating formation portion, The electric field distribution is not applied to the p-side electrode, and the laser characteristics are not deteriorated.
[0047]
Next, a distributed feedback semiconductor laser according to a third embodiment of the present invention will be described with reference to FIG.
FIG. 6A is a front view of the distributed feedback semiconductor laser according to the third embodiment of the present invention as seen from the optical axis direction, and FIG. FIG. 6A is a schematic cross-sectional view of the diffraction grating along the alternate long and short dash line connecting AA ′ in FIG.
[0048]
First, using an MOCVD method on an n-type InP substrate 31, an n-type InP clad layer 32 having a thickness of, for example, 200 nm, an i-type InGaAsPSCH layer 33 having a thickness of, for example, 50 nm, and a thickness of, for example, 20-nm i-type InGaAsP active layer 34, thickness of, for example, 50-nm i-type InGaAsPSCH layer 35, 200-nm thickness of p-type InP cladding layer 36, thickness of, eg, 5.2 μm, p-type InP layer 37 and a p-type InGaAs contact layer 38 having a thickness of 50 nm, for example, are sequentially deposited.
[0049]
Then, thickness of, for example, after depositing a SiO ₂ film of 0.3 [mu] m, width by etching, for example, to form a 3.0μm of SiO ₂ film pattern (not shown), the SiO ₂ Using the film pattern as a mask, the p-type InP layer 37 is etched to a depth of 5 μm to form a ridge 39.
[0050]
Next, after removing the SiO ₂ film pattern, a resist is applied, a 120 nm line-and-space diffraction grating pattern is exposed on both sides of the ridge 39 at a cycle of 240 nm, and developed to form a resist pattern (not shown). Then, using this resist pattern as a mask, the exposed portion of the p-type InP layer 37 is etched to a depth of, for example, 10 nm to form the groove 40.
Also in this case, the phase shift region 41 is provided in the vicinity of the center of the groove 40.
[0051]
Next, after removing the resist pattern, the BCB is coated on the groove forming portion to a thickness of, for example, 200 nm to form the diffraction grating 43 including the p-type InP layer 37 and the BCB layer 42, and then The p-side electrode 44 is provided on the p-type InGaAs contact layer 38, and the n-side electrode 45 is formed on the back surface of the n-type InP substrate 31.
Finally, after cleaving to a resonator length of 200 μm, an antireflection film 46 is formed on the end face perpendicular to the optical axis, thereby completing the basic configuration of the distributed feedback semiconductor laser according to the third embodiment of the present invention. .
[0052]
In the third embodiment, since the ridge structure is adopted, there is an advantage that it can be manufactured with less process technology as compared with the buried heterostructure.
The electric field distribution of light spreads on both sides of the ridge and is influenced by the diffraction grating, so that a distributed feedback semiconductor laser is obtained.
[0053]
Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations and conditions described in the embodiments, and various modifications can be made.
For example, in each of the above embodiments, BCB is taken as a material whose refractive index decreases with respect to temperature. However, the material is not limited to BCB, but is an organic material such as polystyrene or PMMA, or lithium fluoride. Alternatively, the dielectric may be used instead.
[0054]
In each of the above embodiments, the active layer is formed of an InGaAsP bulk layer. However, an MQW (multiple quantum well structure) active layer may be used.
[0055]
In each of the above embodiments, since a light source for optical communication is assumed, a semiconductor laser is configured with an InGaAsP / InP system. However, the semiconductor laser is not limited to an InGaAsP / InP system, but an AlGaAs / GaAs system or the like. Other compound semiconductors may be used.
[0056]
In the third embodiment, the diffraction grating groove is formed after the ridge is formed. However, as in the second embodiment, the diffraction grating groove is formed. Then, the ridge may be selectively grown.
[0057]
In the first and second embodiments, the p-type InGaAs contact layer or the p-type InP layer is selectively grown after the diffraction grating groove is formed. Similarly to the embodiment, after the entire layer structure is grown, a recess reaching the p-type InP layer 19 may be formed, and a groove for the diffraction grating may be formed on the surface of the recess.
[0059]
【The invention's effect】
According to the present invention, the diffraction grating includes a first medium such as InP and a second medium such as BCB that has a refractive index smaller than that of the first medium and that has a smaller increase in refractive index with respect to temperature. Therefore, it is possible to obtain a semiconductor laser having a stable oscillation threshold current value I _th with respect to changes in the ambient temperature, thereby simplifying the circuit configuration of the modulation control circuit, and consequently This greatly contributes to the cost reduction of the communication system.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a basic configuration of the present invention.
FIG. 2 is a configuration explanatory diagram of a distributed feedback semiconductor laser according to a first embodiment of the present invention.
FIG. 3 is an explanatory diagram of the temperature dependence of κL in the first embodiment of the present invention.
FIG. 4 is an explanatory diagram of temperature dependence of an oscillation threshold current value according to the first embodiment of the present invention.
FIG. 5 is a diagram illustrating the configuration of a distributed feedback semiconductor laser according to a second embodiment of the present invention.
FIG. 6 is a diagram illustrating the configuration of a distributed feedback semiconductor laser according to a third embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st medium 2 2nd medium 3 Diffraction grating 4 Groove 11 N-type InP substrate 12 n-type InP clad layer 13 i-type InGaAsPSCH layer 14 i-type InGaAsP active layer 15 i-type InGaAsPSCH layer 16 p-type InP clad layer 17 p N-type InP buried layer 18 n-type InP current blocking layer 19 p-type InP layer 20 groove 21 phase shift region 22 p-type InGaAs contact layer 23 BCB layer 24 diffraction grating 25 p-side electrode 26 n-side electrode 27 antireflection film 28 p-type InP layer 31 n-type InP substrate 32 n-type InP clad layer 33 i-type InGaAsPSCH layer 34 i-type InGaAsP active layer 35 i-type InGaAsPSCH layer 36 p-type InP clad layer 37 p-type InP layer 38 p-type InGaAs contact layer 39 ridge 40 groove 41 Phase shift region 42 BCB layer 43 Diffraction grating 44 p-side electrode 45 n-side electrode 46 antireflection film

Claims (7)

A first medium made of a conductive medium provided with a groove and a non-conductive medium provided so as to at least fill the groove, and has a refractive index smaller than that of the first medium and with respect to a rise in temperature. The diffraction grating composed of the second medium having a small increase rate is positioned on the mesa structure including the active layer, and has the same refraction as the first medium on both sides of the second medium constituting the diffraction grating. A distributed feedback semiconductor laser comprising a conductive medium having a refractive index characteristic and an electrode provided on a conductive medium having the same refractive index characteristic as that of the first medium. A first medium made of a conductive medium provided with a groove and a non-conductive medium provided so as to at least fill the groove, and has a refractive index smaller than that of the first medium and with respect to a rise in temperature. A diffraction grating composed of a second medium having a small increase rate is positioned on a mesa structure including an active layer, and both sides of the second medium constituting the diffraction grating are completely covered by the first medium. A distributed feedback semiconductor laser characterized in that it is embedded and an electrode is provided above the first medium embedded on both sides of the second medium. A first medium made of a conductive medium provided with a groove and a non-conductive medium provided so as to at least fill the groove, and has a refractive index smaller than that of the first medium and with respect to a rise in temperature. A distributed feedback semiconductor laser, characterized in that diffraction gratings composed of a second medium with a small increase rate are positioned on both sides of a ridge structure provided on an active layer. A first medium made of a conductive medium provided with a groove and having a refractive index that increases with an increase in temperature , and a non-conductive medium provided so as to bury at least the groove and refracted from the first medium. A diffraction grating composed of a second medium having a small refractive index and a refractive index decreasing with increasing temperature is positioned on the mesa structure including the active layer, and is located on both sides of the second medium constituting the diffraction grating. A distributed feedback type characterized in that a conductive medium having the same refractive index characteristic as that of the first medium is provided and an electrode is provided on the conductive medium having the same refractive index characteristic as that of the first medium. Semiconductor laser. A first medium made of a conductive medium provided with a groove and having a refractive index that increases with an increase in temperature , and a non-conductive medium provided so as to bury at least the groove and refracted from the first medium. A diffraction grating composed of a second medium having a small refractive index and a refractive index decreasing with increasing temperature is positioned on the mesa structure including the active layer, and is located on both sides of the second medium constituting the diffraction grating. However, the distributed feedback semiconductor laser is characterized in that an electrode is provided above the first medium that is completely embedded in the first medium and that embeds both sides of the second medium. A first medium made of a conductive medium provided with a groove and having a refractive index that increases with an increase in temperature , and a non-conductive medium provided so as to bury at least the groove and refracted from the first medium. A distributed feedback semiconductor laser characterized in that a diffraction grating composed of a second medium having a small refractive index and a refractive index decreasing with increasing temperature is located on both sides of a ridge structure provided on the active layer .   7. The distributed feedback semiconductor laser according to claim 4, wherein the second medium is made of any one of benzocyclobutene, polystyrene, polymethyl methacrylate, or lithium fluoride. .

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