JP2006351571A - Mode-locked semiconductor laser and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize mode locking using a small gain current, and to generate high speed-optical pulses having narrow width in the time base of light-emitting pulses. <P>SOLUTION: A mode-locked semiconductor laser includes a gain region 24 for generating a stimulated emission light and a saturable absorption region 26 for partially absorbing the stimulation-emitted light generated in the region 24, in a series with respect to the longitudinal direction in an active layer 20. The gain region is constituted of 24 bulk semiconductor crystals, and the saturable absorption region 26 is constituted of a multiple-quantum-well structure. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mode-locked semiconductor laser and a manufacturing method thereof.

  One laser device that generates optical pulses is a passive mode-locked semiconductor laser.

  A conventional passive mode-locked semiconductor laser will be described with reference to FIG. FIG. 8 is a cross-sectional view schematically showing a conventional passive mode-locked semiconductor laser. The passive mode-locked semiconductor laser 110 has a stacked structure in which a lower cladding layer 112, an active layer 120, and an upper cladding layer 114 are stacked in this order. In the Fabry-Perot passive mode-locked semiconductor laser 110, a resonator is formed between end surfaces (also referred to as cleaved surfaces) 116a and 116b formed by cleavage at both ends in the longitudinal direction of the active layer 120.

  A common electrode 132 is provided on the lower surface 112 a of the lower cladding layer 112 continuously in the longitudinal direction of the passive mode-locking semiconductor laser 110. A gain region electrode 134 and a saturable absorption electrode 136 are provided on the upper surface 114 a of the upper cladding layer 114 so as to be separated in the longitudinal direction of the passive mode-locking semiconductor laser 110. A region of the active layer 120 corresponding to the gain region electrode 134, that is, a region between the gain region electrode 134 and the common electrode 132 is a region functioning as the gain region 124. In addition, a region of the active layer 120 corresponding to the saturable absorption region electrode 136, that is, a region between the saturable absorption region electrode 136 and the common electrode 132 is a region functioning as the saturable absorption region 126.

  A DC power supply 144 is connected to the gain region electrode 134, and a forward voltage is applied to the gain region 124 by the DC power supply 144. In the gain region 124, application of forward voltage generates multimode stimulated emission light that is stimulated emission light including a plurality of modes. The multimode stimulated emission light generated in the gain region 124 is amplified by a resonator formed between the two cleavage surfaces 116 a and 116 b of the active layer 120.

  A DC power source 146 is connected to the saturable absorption region electrode 136, and a reverse voltage is applied to the saturable absorption region 126 by the DC power source 146. In the saturable absorption region 126, a part of the multimode stimulated emission light amplified by the active layer 120 is absorbed by the application of the reverse voltage. Due to the absorption saturation action in the saturable absorption region 126, the phases of the multimode stimulated emission light are matched. As a result, a high-speed optical pulse with a short width on the time axis of the optical pulse is obtained.

  In the conventional passive mode-locked semiconductor laser described above, the active layer 120 is formed as a bulk semiconductor crystal uniformly grown on the lower cladding layer 112 (see, for example, Non-Patent Document 1).

  As another example of a conventional passive mode-locked semiconductor laser, the active layer 120 is formed as a multi-quantum well (MQW) structure grown uniformly on the lower cladding layer 112. Yes (for example, see Non-Patent Document 2).

  On the other hand, there are reports on the emission spectrum in the gain region 124 when the active layer 120 is a bulk semiconductor crystal and when the active layer 120 is an MQW structure (see, for example, Non-Patent Document 3).

  The analysis result will be briefly described with reference to FIGS. 9 and 10.

  FIG. 9 is a diagram showing the energy band of the active layer, showing the position in the height direction in the horizontal direction and taking the energy in the vertical direction. FIG. 9A shows an energy band when the active layer is a bulk semiconductor crystal, and FIG. 9B shows an energy band when the active layer has an MQW structure.

When the active layer shown in FIG. 9A is a bulk semiconductor crystal, the thickness T _bulk of the well layer 154 in which carriers are accumulated increases. Therefore, the energy distribution that the accumulated carriers can take becomes continuous and wide. On the other hand, when the active layer shown in FIG. 9B has an MQW structure, since the well layers 156 and the barrier layers 157 are alternately present, the thickness T _MQW of the well layer 156 is reduced. As a result, the energy distribution is discrete and narrow.

  FIG. 10 is a diagram showing an energy distribution during thermal equilibrium of electrons as carriers. On the horizontal axis, the electron energy E is normalized by kT. The vertical axis shows the distribution density n (E) corresponding to the energy of electrons.

A curve I in FIG. 10A shows an energy distribution at the time of thermal equilibrium in which electrons are excited when the active layer is a bulk semiconductor crystal. The bulk semiconductor crystal has a parabolic state density function ρ (E) (to (E-Eg) ^1.2 ) (indicated by a dotted line II in FIG. 10A).

  A curve III in FIG. 10B shows an energy distribution at the time of thermal equilibrium in which electrons are excited when the active layer has an MQW structure. The MQW structure has a step-like state density function ρ (E) (= constant) (indicated by a dotted line IV in FIG. 10B).

Here, k is the Boltzmann constant, T is the absolute temperature, Eg is the band gap energy of the bulk semiconductor crystal, and E ₁ is the lowest quantum level energy in the MQW structure.

  As shown in FIG. 10A, when the active layer is a bulk semiconductor crystal, the width (half width) of the energy distribution is about 1.8. On the other hand, as shown in FIG. 10B, when the active layer has an MQW structure, the width (half width) of the energy distribution is about 0.7. Thus, when the active layer 120 is made of a bulk semiconductor crystal, it can be seen that the width of the energy distribution becomes wider than when the MQW structure is used.

  Since the width of the energy distribution of the emission spectrum due to electron recombination corresponds to the width of the energy distribution of electrons, the bandwidth of the emission wavelength is wider in the bulk semiconductor crystal and narrower in the MQW active layer.

  Next, absorption saturation in the saturable absorption region 126 will be described (for example, see Non-Patent Document 3). In the saturable absorption region 126, when the light intensity increases, the density of excitons excited by the light also increases. When the exciton density is increased and two or more excitons exist in the space occupied by one exciton determined by the Bohr radius, the exciton state is destroyed by the interaction between the excitons. The As a result, the light absorption peak due to excitons disappears and absorption saturation occurs. Therefore, in order for absorption saturation to occur, the saturable absorption region 126 needs to be in a state where two or more excitons must exist in the space occupied by one exciton.

In the case where the saturable absorption region 126 is formed of a bulk semiconductor crystal, the thickness T _bulk of the well layer 154 that confines excitons is thick, so that a space for receiving excitons is wide, and therefore, to cause absorption saturation. Requires high intensity light.

On the other hand, when the saturable absorption region 126 is formed in the MQW structure, the thickness T _MQW of the well layer 156 that confines the exciton is thin, so that the space for receiving the exciton is narrow, resulting in absorption saturation. The intensity of light required for the reduction is low. That is, when the saturable absorption region 126 has the MQW structure, absorption saturation is more likely to occur than when the bulk semiconductor crystal is used.
M.M. Kwakernaak, Roland Schreieck, Andreas Neiger Heinz Jackel, Emilio Gini Werner Vogt, "Spectral Phase Measurement of Mode-Locked Diode Laser Pulses by Beating Sidebands Generated by Electrooptical Mixing", IEEE Photonics Technology Letters Vol. 12, no. 12, pp. 1677-1679, 2000 Hiroshi Ogawa, Shin Arahira, Yukio Kato, Daisuke Kunimatsu “Ultrafast Mode-Locked Semiconductor Laser”, IEICE Transactions, Vol. J84-C, no. 1, pp. 1-10, January 2001 Edited by Japan Society of Applied Physics, “Basics of Semiconductor Lasers” pp. 164-171, Ohmsha, 1987

  In a mode-locked semiconductor laser, an optical spectrum waveform indicating an energy distribution and an optical pulse waveform indicating a time distribution are in a Fourier transform relationship with each other.

  Therefore, a passive mode-locked semiconductor laser including the active layer 120 formed as a bulk semiconductor crystal is advantageous for shortening the pulse because the energy distribution width of its emission spectrum is wide. On the other hand, since absorption saturation is unlikely to occur in a bulk semiconductor crystal, this passive mode-locked semiconductor laser requires a large gain current to realize mode locking.

  Further, in a passive mode-locked semiconductor laser including an active layer formed as an MQW structure, absorption saturation is likely to occur, so that mode locking can be realized with a small gain current. However, since the energy distribution width of the emission spectrum is narrow, this passive mode-locked semiconductor laser is disadvantageous for shortening the pulse.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to generate a high-speed optical pulse that realizes mode locking with a small gain current and has a narrow width on the time axis of the light emission pulse. It is an object of the present invention to provide a mode-locked semiconductor laser and a manufacturing method thereof.

  In order to achieve the above-described object, a mode-locked semiconductor laser according to the present invention includes a gain region for generating stimulated emission light and a saturable absorption for partially absorbing stimulated emission light generated in the gain region. And an active layer including the region in series with respect to the longitudinal direction. The gain region is constituted by a bulk semiconductor crystal, and the saturable absorption region is constituted by a multiple quantum well structure.

  In implementing the mode-locked semiconductor laser described above, preferably, the active layer further includes a passive waveguide region, and the gain region, the saturable absorption region, and the passive waveguide region are arranged in series in the longitudinal direction of the active layer. It is good to have.

  The active layer further includes a passive waveguide region, and includes a gain region and a saturable absorption region between the passive waveguide region and one end face with respect to the longitudinal direction of the active layer. A Bragg grating may be provided.

  In implementing the mode-locked semiconductor laser described above, it is preferable to provide a plurality of gain regions in the longitudinal direction of the active layer.

  According to another preferred embodiment of the mode-locked semiconductor laser of the present invention, a gain region for generating stimulated emission light and a modulation region for modulating the stimulated emission light generated in the gain region are provided. An active layer is included. The gain region is composed of a bulk semiconductor crystal, and the modulation region is composed of a multiple quantum well structure.

  The method for manufacturing a mode-locked semiconductor laser according to the present invention includes the following steps. First, an MQW active layer having a multiple quantum well structure is uniformly grown on the lower cladding layer. Next, a selection mask is formed on the MQW active layer so as to cover a portion of the MQW active layer in the first region corresponding to the saturable absorption region and to expose a portion of the MQW active layer in the second region corresponding to the gain region. Form. Next, the MQW active layer portion in the second region is removed by etching using a selection mask. Next, using a selection mask, a bulk semiconductor crystal is grown on the lower cladding layer of the second region to form a gain region.

  According to another embodiment of the method for manufacturing a mode-locked semiconductor laser of the present invention, the following steps are provided. First, an MQW active layer having a multiple quantum well structure is uniformly grown on the lower cladding layer. Next, on the MQW active layer, the MQW active layer portion of the first region corresponding to the saturable absorption region is covered, and the second region corresponding to the gain region and the third region corresponding to the passive waveguide region are covered. A first selection mask that exposes a portion of the MQW active layer is formed. Next, the MQW active layer portions in the second region and the third region are removed by etching using the first selection mask. Next, a bulk semiconductor crystal is grown on the lower cladding layer of the second region and the third region using the first selection mask. Next, a second selection mask is formed on the bulk semiconductor crystal so as to cover the bulk semiconductor crystal portion in the second region and expose the bulk semiconductor crystal portion in the third region. Next, the bulk semiconductor crystal portion in the third region is removed by etching using the first and second selection masks. Next, a bulk semiconductor crystal is grown on the lower cladding layer of the third region to form a passive waveguide region.

  According to the mode-locked semiconductor laser of the present invention, the gain region of the active layer is made into a bulk semiconductor crystal that is advantageous for the generation of high-speed optical pulses, and the saturable absorption region of the active layer is subjected to multiple quantum wells (MQW : Multi-Quantum Well) structure, it is possible to generate a high-speed optical pulse with a narrow pulse width on the time axis and to realize a small gain current.

  Further, by providing a passive waveguide region in the active layer, it is possible to adjust the frequency, such as reducing the repetition frequency. Furthermore, when a Bragg grating is formed in the passive waveguide region, it is possible to resonate light having a wavelength determined according to the period of the Bragg grating, that is, it is possible to control the oscillation wavelength of the mode-locked semiconductor laser.

  Furthermore, if the active layer has a plurality of gain regions, the optical pulse generated in each gain region propagates in the resonator, so the repetition frequency can be set to a frequency multiplied by the number of gain regions. it can.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the composition (material) and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiment.

(Configuration of Mode-locked Semiconductor Laser of First Embodiment)
A mode-locked semiconductor laser according to the first embodiment will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining a configuration example of the mode-locked semiconductor laser according to the first embodiment, and is shown by a cut in a longitudinal section perpendicular to the upper surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  A mode-locked semiconductor laser (hereinafter sometimes simply referred to as a semiconductor laser) 10 has a stacked structure in which a lower cladding layer 12, an active layer 20, and an upper cladding layer 14 are stacked in this order. In the Fabry-Perot type mode-locked semiconductor laser 10, a resonator is formed in the active layer 20 between end faces (also referred to as cleaved faces) 16a and 16b formed by cleavage at both ends in the longitudinal direction.

  Here, the refractive index of the active layer 20 is set to be larger than the refractive indexes of the lower cladding layer 12 and the upper cladding layer 14. For this reason, leakage of light from the active layer 20 to the lower cladding layer 12 and the upper cladding layer 14 is prevented.

  The lower cladding layer 12 is made of, for example, N type InP, and the upper cladding layer 14 is made of, for example, P type InP.

  The active layer 20 includes a gain region 24 and a saturable absorption region 26 arranged in series along the longitudinal direction. The gain region 24 is a region for generating stimulated emission light, and the saturable absorption region 26 is a region for partially absorbing the stimulated emission light generated in the gain region 24.

Here, the gain region 24 is doped with, for example, In _X Ga _1-X As _Y P _1-Y (where X and Y satisfy 0 ≦ X ≦ 1 and 0 ≦ Y ≦ 1 respectively). It is not formed as a bulk semiconductor crystal having a film thickness of about 10 nm to 100 nm. In the following _{description, In X Ga 1-X As} Y P 1-Y a represents a InGaAsP, selection of the composition ratio of InGaAsP shall be performed by appropriately selecting the values of X and Y. The composition ratio of InGaAsP in the gain region 24 is selected so as to obtain a desired band gap wavelength, for example, a band gap wavelength of 1.55 μm.

  The saturable absorption region 26 is formed as a multiple quantum well (MQW) structure in which InGaAsP having a thickness of 10 nm or less per layer is stacked. The composition ratio of InGaAsP in the saturable absorption region 26 is set to be equal to or slightly larger than the band gap wavelength of the gain region 24 as is conventionally known. Here, in order to enhance the light absorption effect in the saturable absorption region 26, as is conventionally known, the band gap wavelength of the saturable absorption region 26 is slightly smaller than the band gap wavelength of the gain region 24, for example, It is preferable that the thickness is set to be about 0.01 to 0.09 μm.

  A common electrode 32 is provided on the lower surface 12 a of the lower cladding layer 12 continuously in the longitudinal direction of the semiconductor laser 10. The shared electrode 32 is connected to a reference potential, for example, a ground potential. A gain region electrode 34 and a saturable absorption region electrode 36 are provided on the upper surface 14 a of the upper cladding layer 14 so as to be separated in the longitudinal direction of the semiconductor laser 10. The gain region electrode 34 is provided in the upper surface region immediately above the gain region 24. The saturable absorption region electrode 36 is provided in the upper surface region corresponding to the saturable absorption region 26.

  A DC power supply 44 is connected to the gain region electrode 34, and a forward voltage is applied to the gain region 24 by the DC power supply 44. In the gain region 24, multimode stimulated emission light, which is stimulated emission light including a plurality of modes, is generated by applying a forward voltage. Multimode stimulated emission light generated in the gain region 24 is amplified by a resonator configured in the active layer 20.

  A DC power supply 46 is connected to the saturable absorption region electrode 36, and a reverse voltage is applied to the saturable absorption region 26 by the DC power supply 46. In the saturable absorption region 26, a part of the multimode stimulated emission light is absorbed by application of a reverse voltage. Due to the absorption saturation action in the saturable absorption region 26, the phases of the multimode stimulated emission light are matched. As a result, a high-speed optical pulse with a short width on the time axis of the optical pulse is obtained.

(Method for Manufacturing Mode-Locked Semiconductor Laser of First Embodiment)
With reference to FIG. 2, a method of manufacturing the mode-locked semiconductor laser according to the first embodiment will be described. FIG. 2 is a process diagram for explaining the method of manufacturing the mode-locked semiconductor laser according to the first embodiment. 2 (A) to 2 (D), as in FIG. 1, are shown by cuts in a vertical section perpendicular to the top surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  First, an MQW active layer 60 having an MQW structure is uniformly grown on the lower cladding layer 12. The MQW active layer 60 may be grown by any suitable known method such as metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) (Molecular Beam Epitaxy). FIG. 2 (A)).

  Next, an insulating film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film is formed on the MQW active layer 60 by any publicly known method, for example, a CVD method. Thereafter, the insulating film is processed by photolithography and etching to cover the portion of the MQW active layer 60 in the first region 76 corresponding to the saturable absorption region 26 and in the MQW of the second region 74 corresponding to the gain region 24. A selection mask 70 that exposes a portion of the active layer 60 is formed (FIG. 2B).

  Next, the portion of the MQW active layer 60 in the second region 74 is removed by wet etching using the selection mask 70. This wet etching is performed using, for example, perhydrosulfuric acid in which sulfuric acid, hydrogen peroxide solution, and water are mixed at a mass ratio of sulfuric acid: hydrogen peroxide solution: water = 3: 1: 1. The remaining portion of the MQW active layer 60 in the first region 76 becomes the saturable absorption region 26 by the wet etching. Note that dry etching, for example, reactive ion etching using a mixed gas of chlorine and argon as an etching gas may be performed instead of wet etching (FIG. 2C).

  Next, using the selection mask 70 used in wet etching as it is, a bulk semiconductor crystal is grown on the lower cladding layer 12 of the second region 74 to form the gain region 24. The bulk semiconductor crystal may be grown by any suitable known method such as MOVPE method or MBE method (FIG. 2D).

  As described in Non-Patent Document 2, the time width of the optical pulse depends on the length of the saturable absorption region 26. When the length of the saturable absorption region 26 is increased, the time width of the optical pulse is increased. Will spread. Further, when the length of the saturable absorption region 26 is shortened, the saturation absorption action is reduced, and mode synchronization is not appropriately performed, and the time width of the optical pulse is also widened. Therefore, the length of the saturable absorption region 26 needs to be controlled with high accuracy. Here, since the selection mask 70 is formed by using a photolithography method, the saturable absorption region 26 can be formed with extremely high precision.

  Next, after removing the selection mask 70, the upper cladding layer 14 is grown on the active layer 20, and the gain region electrode 34 and the saturable absorption region electrode 36 are formed on the upper surface 14 a of the upper cladding layer 14. By forming the common electrode 32 on the lower surface 12a of the lower cladding layer 12, the mode-locked semiconductor laser according to the first embodiment described with reference to FIG. 1 is formed. The formation of the upper cladding layer 14, the gain region electrode 34, the saturable absorption region electrode 36, and the common electrode 32 may be performed by a conventionally known method, and the description thereof is omitted here.

(Operation of Mode-locked Semiconductor Laser of First Embodiment)
The operation of the mode-locked semiconductor laser according to the first embodiment will be described with reference to FIG. A direct current voltage in the direction from the upper cladding layer 14 to the lower cladding layer 12, that is, the forward direction, is applied between the gain region electrode 34 and the common electrode 32. Further, a DC voltage in the direction from the lower cladding layer 12 to the upper cladding layer 14, that is, the reverse direction is applied between the saturable absorption region electrode 36 and the common electrode 32.

  By applying a forward DC voltage between the gain region electrode 34 and the common electrode 32, carriers are injected into the gain region 24 of the active layer 20, and multimode which is stimulated emission light including a plurality of modes. Stimulated emission light is generated. The multimode stimulated emission light is amplified by a resonator formed between the cleavage surfaces 16 a and 16 b of the active layer 20.

  A part of the multimode stimulated emission light resonating in the active layer 20 is absorbed when passing through the saturable absorption region 26, and the phase is matched and becomes pulsed by the absorption saturation in the saturable absorption region 26. . That is, the semiconductor laser of the first embodiment functions as a passive mode-locked semiconductor laser. The light pulsed in the saturable absorption region 26 is emitted from at least one of the cleavage planes 16a and 16b.

  According to the mode-locked semiconductor laser of the first embodiment, since the gain region of the active layer is a bulk semiconductor crystal that is advantageous for generating high-speed optical pulses, the gain region is on the time axis as compared with the case where the gain region has an MQW structure. It is possible to generate a high-speed optical pulse with a narrow pulse width. Further, since the saturable absorption region has an MQW structure in which absorption saturation is likely to occur, a small gain current can be realized.

  Here, an example has been described in which the active layer 20 includes an InGaAsP bulk semiconductor crystal and an MQW structure, but the material of the active layer 20 is not limited to InGaAsP, and for example, an AlGaAs-based semiconductor may be used. good.

(Configuration of Mode-locked Semiconductor Laser of Second Embodiment)
A mode-locked semiconductor laser according to the second embodiment will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining a configuration example of the mode-locked semiconductor laser according to the second embodiment, which is shown by a cut in a longitudinal section perpendicular to the upper surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  The mode-locked semiconductor laser 10a of the second embodiment is that the active layer 20a is further provided with a passive waveguide region 28. The mode-locked semiconductor laser 10 of the first embodiment described with reference to FIG. Is different. In the mode-locked semiconductor laser 10a of the second embodiment, a gain region 24a, a passive waveguide region 28, and a saturable absorption region 26a are sequentially provided in the active layer 20a along the longitudinal direction of the mode-locked semiconductor laser 10a. . Since the configuration is the same as that of the mode-locked semiconductor laser 10 of the first embodiment except that the passive waveguide region 28 is provided, redundant description is omitted.

  The passive waveguide region 28 may be formed of either a bulk semiconductor crystal or an MQW structure. Here, the passive waveguide region 28 will be described as being provided with an InGaAsP bulk semiconductor crystal. The passive waveguide region 28 is added to increase the resonator length, that is, the length of the mode-locked semiconductor laser 10 in the longitudinal direction when the repetition frequency is lowered. The composition ratio of InGaAsP in the passive waveguide region 28 is such that the band gap wavelength is either the gain region 24a or the saturable absorption region 26a in order to suppress absorption of stimulated emission light in the passive waveguide region 28. Is set to a smaller value.

  As is well known, the refractive index is different between InGaAsPs having different bandgap wavelengths. For this reason, when the band gap wavelength of the passive waveguide region 28 is set to a wavelength significantly different from 1.55 μm, the passive waveguide region 28 and the portion other than the passive waveguide region 28 of the active layer 20a, that is, the gain region. Reflection of light due to the difference in refractive index occurs at the boundary between 24a and the saturable absorption region 26a. As a result, noise may be generated, and mode synchronization may not occur. On the other hand, when the band gap wavelength of the passive waveguide region 28 is set to a value close to 1.55 μm, the absorption of light in the passive waveguide region 28 increases, which may cause a loss.

  Therefore, when the band gap wavelength of the gain region 24a is 1.55 μm, the band gap wavelength of the passive waveguide region 28 is in the range of 1.0 to 1.5 μm, preferably in the range of 1.2 to 1.3 μm. It is good to be.

(Method for Manufacturing Mode-Locked Semiconductor Laser of Second Embodiment)
With reference to FIG. 4, a method of manufacturing the mode-locked semiconductor laser according to the second embodiment will be described. FIG. 4 is a process diagram for explaining the method of manufacturing the mode-locked semiconductor laser according to the second embodiment. 4A to 4E, as in FIG. 3, are shown by cuts in a longitudinal section perpendicular to the top surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  First, an MQW active layer 60 having an MQW structure is uniformly grown on the lower cladding layer 12. The growth of the MQW active layer 60 may be performed by any suitable known method such as the MOVPE method or the MBE method (FIG. 4A).

  Next, an insulating film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film is formed on the MQW active layer 60 by any publicly known method, for example, a CVD method. Thereafter, the insulating film is processed by photolithography and etching to cover the MQW active layer 60 portion of the first region 76a corresponding to the saturable absorption region 26a, and to correspond to the gain region 24a and the passive waveguide region 28, respectively. A first selection mask 71 that exposes the MQW active layer 60 in the second and third regions 74a and 78 is formed (FIG. 4B).

  Next, portions of the MQW active layer 60 in the second region 74 a and the third region 78 are removed by wet etching using the first selection mask 71. This wet etching is performed using, for example, the above-described perhydrosulfuric acid as an etchant. The remaining portion of the MQW active layer 60 in the first region 76a becomes the saturable absorption region 26a by the wet etching. As in the first embodiment, dry etching may be performed instead of wet etching (FIG. 4C).

  Next, the bulk semiconductor crystal 62 is grown on the lower cladding layer 12 of the second region 74 a and the third region 78 using the first selection mask 71 used in the wet etching as it is. The bulk semiconductor crystal 62 may be grown by any suitable known method such as MOVPE method or MBE method. Thereafter, a second selection mask 72 is formed on the bulk semiconductor crystal 62 so as to cover the portion of the bulk semiconductor crystal 62 in the second region 74 a and expose the portion of the bulk semiconductor crystal 62 corresponding to the third region 78. The second selection mask 72 can be formed in the same manner as the first selection mask (FIG. 4D).

  Next, the portion of the bulk semiconductor crystal 62 in the third region 78 is removed by wet etching using the first and second selection masks 71 and 72. This wet etching is performed using, for example, the above-described perhydrosulfuric acid as an etchant. By this wet etching, the portion of the second region 74a of the remaining bulk semiconductor crystal 62 becomes the gain region 24a. Note that dry etching may be performed instead of wet etching as in the removal of the MQW active layer 60. Thereafter, a bulk semiconductor crystal is grown on the lower cladding layer 12 of the third region 78 to form a passive waveguide region 28. The active region 20a is configured by the gain region 24a, the saturable absorption region 26a, and the passive waveguide region 28 (FIG. 4E).

  As described in Non-Patent Document 2, the time width of the optical pulse depends on the length of the saturable absorption region. As the length of the saturable absorption region increases, the time width of the optical pulse increases. End up. Further, when the length of the saturable absorption region is shortened, the saturated absorption action is reduced, and mode synchronization is not appropriately performed, and the time width of the optical pulse is also increased. Therefore, the length of the saturable absorption region needs to be controlled with high accuracy. Here, since the first and second selection masks 71 and 72 are formed using a photolithography method, they can be formed with extremely high precision.

  Next, after removing the first and second selection masks 71 and 72, the upper cladding layer 14 is grown on the active layer 20a, and the gain region electrode 34 and the saturable absorption are formed on the upper surface 14a of the upper cladding layer 14. By forming the region electrode 36 and forming the common electrode on the lower surface 12a of the lower cladding layer 12, the mode-locked semiconductor laser according to the second embodiment described with reference to FIG. 3 is formed. The formation of the upper cladding layer 14, the gain region electrode 34, the saturable absorption region electrode 36, and the common electrode 32 may be performed by a conventionally known method, and the description thereof is omitted here.

  According to the mode-locked semiconductor laser of the second embodiment, since the passive waveguide region 28 is added to the active layer 20a, the repetition frequency is lowered in addition to the effect obtained by the mode-locked semiconductor laser of the first embodiment. It is possible to adjust the frequency such as.

  Here, an example is shown in which a gain region 24a, a passive waveguide region 28, and a saturable absorption region 26a are sequentially provided in the active layer 20a along the longitudinal direction of the mode-locking semiconductor laser 10a. The arrangement of the passive waveguide region 28 and the saturable absorption region 26a is not limited to this order, and the same effect can be obtained regardless of the order.

(Mode-locked semiconductor laser of the third embodiment)
A mode-locked semiconductor laser according to the third embodiment will be described with reference to FIG. FIG. 5 is a schematic diagram for explaining a configuration example of the mode-locked semiconductor laser according to the third embodiment, and is shown by a cut in a longitudinal section perpendicular to the upper surface of the passive mode-locked semiconductor laser and along the longitudinal direction thereof.

  In addition, the description which overlaps with the mode synchronous semiconductor laser of 2nd Embodiment demonstrated with reference to FIG. 3 is abbreviate | omitted. The mode-locked semiconductor laser according to the third embodiment includes a gain region 24b, a saturable absorption region 26b, and a passive waveguide region 29 in this order between the end faces 16a and 16b of the active layer 20b. The difference from the mode-locked semiconductor laser of the second embodiment is that the grating 80 is provided.

  The Bragg grating 80 is formed in the surface region on the upper cladding layer side of the passive waveguide region 29. The Bragg grating 80 is provided with a uniform period (lattice interval). The passive waveguide region 29 reflects light with a Bragg wavelength determined by the period of the Bragg grating 80, that is, has wavelength selectivity. For this reason, a resonator is formed between the one end face 16a and the passive waveguide region 29, and a mode-locking operation occurs with respect to light having a Bragg wavelength.

  In manufacturing the mode-locked semiconductor laser according to the third embodiment, the steps until the active layer 20b including the gain region 24b, the saturable absorption region 26b, and the passive waveguide region 29 are configured will be described with reference to FIG. This is performed in the same manner as the mode-locked semiconductor laser of the second embodiment.

  The formation of the Bragg grating 80 is performed, for example, by any suitable known photolithography and etching after the bulk semiconductor crystal is formed in the region corresponding to the passive waveguide region 29. In this case, the Bragg grating 80 is formed as an uneven structure in the passive waveguide region 29.

  The example in which the active region 20b is sequentially provided with the gain region 24b, the saturable absorption region 26b, and the passive waveguide region 29 along the longitudinal direction of the mode-locked semiconductor laser 10b has been described with reference to FIG. The arrangement of the gain region 24b, the saturable absorption region 26b, and the passive waveguide region 29 is not limited to this order. Between the one cleavage plane 16a and the passive waveguide region 29, the longitudinal direction of the mode-locked semiconductor laser 10 is provided. If the gain region 24b and the saturable absorption region 26b are formed, the same effect can be obtained regardless of the order.

  According to the mode-locked semiconductor laser of the third embodiment, since the Bragg grating is formed in the passive waveguide region 29, in addition to the effect of the mode-locked semiconductor laser of the second embodiment, the oscillation of the mode-locked semiconductor laser is further performed. The effect that the wavelength can be controlled is obtained.

(Mode-locked semiconductor laser of the fourth embodiment)
A mode-locked semiconductor laser according to the fourth embodiment will be described with reference to FIG. FIG. 6 is a schematic diagram for explaining a configuration example of the mode-locked semiconductor laser according to the fourth embodiment, and is shown by a cut in a longitudinal section perpendicular to the upper surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  Here, the description overlapping the mode-locked semiconductor laser of the second embodiment described with reference to FIG. 3 is omitted. The mode-locked semiconductor laser 10c according to the fourth embodiment has a saturable absorption region 26c disposed in the center of the resonator composed of the cleaved surfaces 16a and 16b, and includes a plurality of gain regions in the active layer 20c on both sides of the saturable absorption region 26c. For example, it differs from the mode-locked semiconductor laser of the second embodiment in that it includes two gain regions, a first gain region 25a and a second gain region 25b.

  According to the mode-locked semiconductor laser of the fourth embodiment, an optical pulse is generated from each of the first gain region 25a and the second gain region 25b, and the optical pulse enters the saturable absorption region 26c from the left and right of the saturable absorption region 26c. Since it is incident, two optical pulses propagate in the resonator, and the repetition frequency can be doubled.

  Here, an example is shown in which the saturable absorption region 26c is arranged in the center of the resonator, but the position of the saturable absorption region is not limited to this. For example, the repetition frequency can be tripled by setting the distance from the two cleavage surfaces 16a, 16b to the saturable absorption region 26c to 1: 2. Thus, by arranging the saturable absorption region inside the resonator instead of the resonator end and providing the gain regions on both sides thereof, the repetition frequency can be set to a multiplied frequency.

  Although an example applied to a mode-locked semiconductor laser including the passive waveguide regions 28c and 28d has been described here, the present invention is not limited to this example. The passive waveguide region may not be provided, or a Bragg grating may be provided in the passive waveguide region to provide wavelength selectivity.

(Mode-locked semiconductor laser of the fifth embodiment)
A mode-locked semiconductor laser according to the fifth embodiment will be described with reference to FIG. FIG. 7 is a schematic diagram for explaining a configuration example of the mode-locked semiconductor laser according to the fifth embodiment, and is shown by a cut in a longitudinal section perpendicular to the upper surface of the mode-locked semiconductor laser and along the longitudinal direction thereof.

  Here, the description overlapping the mode-locked semiconductor laser of the first embodiment described with reference to FIG. 1 is omitted.

  The active layer 20d includes a gain region 24d and a modulation region 27, which are sequentially arranged along the longitudinal direction. The gain region 24d is a region for generating stimulated emission light, and the modulation region 27 is a region for modulating the stimulated emission light generated in the gain region 24.

  A gain region electrode 34 and a modulation region electrode 37 are provided on the upper surface 14a of the upper cladding layer 14 so as to be separated in the longitudinal direction of the semiconductor laser 10d. The gain region electrode 34 is provided in an upper surface region immediately above the gain region 24d. The modulation region electrode 37 is provided in the upper surface region immediately above the modulation region 27.

  The mode-locked semiconductor laser according to the first embodiment described with reference to FIG. 1 is a passive type that generates saturated absorption of an optical pulse by applying a reverse voltage to the saturable absorption region 26. Functions as a mode-locked semiconductor laser.

  In contrast, in the mode-locked semiconductor laser of the fifth embodiment, an oscillator 47 is connected to the modulation region electrode 37. By using this oscillator 47, mode synchronization can be performed by performing frequency modulation at an oscillation frequency that matches an integer multiple of the repetition frequency of the resonator formed in the active layer 20d, and the mode synchronization of the fifth embodiment is achieved. The semiconductor laser functions as a so-called active mode-locked semiconductor laser.

  According to the mode-locked semiconductor laser of the fifth embodiment, the same effect as that of the passive mode-locked semiconductor laser of the first embodiment can be obtained even with the active mode semiconductor laser.

It is the schematic for demonstrating the structural example of the mode synchronous semiconductor laser of 1st Embodiment. It is process drawing for demonstrating the manufacturing method of the mode synchronous semiconductor laser of 1st Embodiment. It is the schematic for demonstrating the structural example of the mode-locking semiconductor laser of 2nd Embodiment. It is process drawing for demonstrating the manufacturing method of the mode synchronous semiconductor laser of 2nd Embodiment. It is the schematic for demonstrating the structural example of the mode-locking semiconductor laser of 3rd Embodiment. It is the schematic for demonstrating the structural example of the mode-locking semiconductor laser of 4th Embodiment. It is the schematic for demonstrating the structural example of the mode-locking semiconductor laser of 5th Embodiment. It is the schematic for demonstrating the structural example of the conventional mode-locking semiconductor laser. It is a figure which shows the energy band of an active layer. It is a figure which shows the energy distribution at the time of the thermal equilibrium of an electron.

Explanation of symbols

10, 10a, 10b, 10c, 10d mode-locked semiconductor laser
12, 112 Lower cladding layer 12a, 112a Lower cladding layer 14, 114 Upper cladding layer 14a, 114a Upper cladding layer upper surface 16a, 16b, 116a, 116b End face (cleavage surface)
20, 20a, 20b, 20c, 20d, 120 Active layer 24, 24a, 24b, 24d, 124 Gain region 25a First gain region 25b Second gain region 26, 26a, 26b, 26c, 126 Saturable absorption region 27 Modulation region 28, 28c, 28d, 29 Passive waveguide region 32, 132 Common electrode 34, 134 Gain region electrode 34a First gain region electrode 34b Second gain region electrode 36, 136 Saturable absorption region electrode 44, 44a, 44b, 46, 144, 146 DC power supply 47 Oscillator 60 MQW active layer 62 Bulk semiconductor crystal 70 Selection mask 71 First selection mask 72 Second selection mask 74, 74a Second region 76, 76a First region 78 Third region 80 Bragg Lattice 110 Passive mode-locked semiconductor laser 154, 156 Well layer 157 Bar A layer

Claims (7)

An active layer including a gain region for generating stimulated emission light and a saturable absorption region for partially absorbing the stimulated emission light generated in the gain region in series with respect to the longitudinal direction. ,
A mode-locked semiconductor laser, wherein the gain region is constituted by a bulk semiconductor crystal, and the saturable absorption region is constituted by a multiple quantum well structure. The active layer further includes a passive waveguide region,
2. The mode-locked semiconductor laser according to claim 1, wherein the gain region, the saturable absorption region, and the passive waveguide region are arranged in series in a longitudinal direction of the active layer. The active layer further includes a passive waveguide region,
The gain region and the saturable absorption region are disposed between the passive waveguide region and one end face with respect to the longitudinal direction of the active layer,
2. The mode-locked semiconductor laser according to claim 1, wherein a Bragg grating is provided in the passive waveguide region. The mode-locked semiconductor laser according to claim 1, wherein a plurality of the gain regions are provided in a longitudinal direction of the active layer. An active layer including a gain region for generating stimulated emission light and a modulation region for modulating the stimulated emission light generated in the gain region;
A mode-locked semiconductor laser, wherein the gain region is formed of a bulk semiconductor crystal, and the modulation region is formed of a multiple quantum well structure. A step of uniformly growing an MQW active layer having a multiple quantum well structure on the lower cladding layer;
A selection mask is formed on the MQW active layer so as to cover a portion of the MQW active layer in the first region corresponding to the saturable absorption region and to expose a portion of the MQW active layer in the second region corresponding to the gain region. Forming, and
Removing a portion of the MQW active layer in the second region by etching using the selection mask;
Forming a gain region by growing a bulk semiconductor crystal on the lower cladding layer in the second region using the selection mask. A step of uniformly growing an MQW active layer having a multiple quantum well structure on the lower cladding layer;
A portion of the MQW active layer in the first region corresponding to the saturable absorption region is covered on the MQW active layer, and the second region corresponding to the gain region and the third region corresponding to the passive waveguide region are covered. Forming a first selective mask exposing a portion of the MQW active layer;
Removing portions of the MQW active layer in the second region and the third region by etching using the first selection mask;
Using the first selection mask to grow a bulk semiconductor crystal on the lower cladding layer in the second region and the third region;
Forming a second selection mask on the bulk semiconductor crystal so as to cover the bulk semiconductor crystal portion of the second region and to expose the bulk semiconductor crystal portion of the third region;
Removing the bulk semiconductor crystal portion of the third region by etching using the first and second selection masks;
And a step of growing a bulk semiconductor crystal on the lower cladding layer in the third region to form a passive waveguide region.

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