JP2010238890A - Semiconductor pulse laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor pulse laser capable of stably performing external control of the oscillation spectrum width and temporal waveform of an optical short pulse. <P>SOLUTION: This semiconductor pulse laser includes a gain region 13 having a gain with respect to a prescribed wavelength, and a first reflector region 11 optically coupled to the gain region 13, and a second reflector region 12. The first reflector region 11 has a first diffraction grating 21 to generate a first reflection spectrum, the second reflector region 12 has a second diffraction grating 22 to generate a second reflection spectrum, and the first diffraction grating 21 and the second diffraction grating 22 are structured so as to perform multiple mode oscillation in a wavelength range where the first reflection spectrum overlaps with the second reflection spectrum. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a semiconductor pulse laser, and more particularly to a monolithically integrated mode-locked semiconductor laser suitable for optical communication and optical information processing.

  Optical fiber communication technology is an important basic technology that supports the modern information society. Currently, the communication speed per channel of the trunk line system ranges from 10 to 40 Gb / s, and in the future, it is expected to realize ultrahigh-speed communication of 80 to 160 Gb / s or more. Various optical elements are required to realize such ultra-high-speed communication, but in particular, an optical clock pulse in which a short optical pulse is maintained at a constant frequency is indispensable, and is stable and free of jitter, that is, a signal on a time axis. Realization of an optical clock pulse generating element with less fluctuation is demanded.

  Known methods for generating an optical clock pulse include a method using direct modulation of a semiconductor laser, a method using a mode-locked laser, and a method using an external modulator. From the viewpoint of miniaturization of the network system and robustness against vibration, generation of an optical clock pulse by a semiconductor element is desirable, and a mode-synchronous semiconductor laser is desirable as a method capable of generating an ultrafast pulse.

  However, in a conventional mode-locked semiconductor laser, an element is fabricated by adjusting the response of a nonlinear medium such as an oscillation gain band or a saturable absorber, and the oscillation longitudinal modes determined by the effective resonator length are mutually synchronized. The short pulse light is generated. For this reason, the oscillation spectrum is defined at the time of production, and furthermore, the optical pulse time waveform that is related to the Fourier transform is uniquely determined. Therefore, the influence of chromatic dispersion, which is a problem for long-distance transmission, cannot be adjusted, and further, the time width of an RZ (Return to Zero) pulse suitable for the bit rate cannot be adjusted.

  Patent Document 1 describes control of the reflection wavelength width, oscillation frequency, and chirp of a mode-locked semiconductor laser. Control of the oscillation wavelength width is realized by changing the waveguide loss by injecting current into the optical waveguide region. The reflecting mirror has a constant reflection wavelength width of 2 nm when the reflectance is sufficiently high. Injecting current into the waveguide installed in front of the reflector to cause free carrier absorption causes waveguide loss. As a result, the effective reflectivity of the reflector decreases, so the end of the reflection band The reflectance of the portion is lowered, and the reflection wavelength width as the full width at half maximum can be reduced to 0.5 nm.

JP-A-4-2190

  FIG. 2 of Patent Document 1 describes a mechanism for adjusting the reflection wavelength width by injecting a current into the light guide layer in the reflection region, and the light short pulse time waveform can be adjusted. However, since the reflection wavelength width has a structure correlated with the waveguide loss, the light output cannot be adjusted stably if a constant pulse time waveform is obtained under a constant oscillation condition.

  That is, assuming a specific pulse time waveform, the reflection wavelength width related to the Fourier transform is uniquely determined, and it is necessary to adjust the waveguide loss by controlling the amount of current injected into the optical waveguide region. At the same time, since the amount of current injected into the active layer region for compensating the loss and maintaining oscillation is determined, the output light intensity has a specific value.

  Further, since the current in the optical waveguide region is fixed at a specific value, the oscillation phase cannot be adjusted, and the oscillation state of the resonator becomes unstable. Furthermore, since there is no mechanism for adjusting the center wavelength of the reflector independent of the adjustment of the reflection wavelength width, the oscillation wavelength cannot be finely adjusted to a desired wavelength in high-density wavelength division multiplexing optical communication.

  In view of the above-described problems, an object of the present invention is to provide a semiconductor pulse laser that can stably externally control the oscillation spectrum width and time waveform of an optical short pulse.

  The semiconductor pulse laser of the present invention includes a gain region having a gain with respect to a predetermined wavelength region, and a first reflecting mirror region and a second reflecting mirror region optically coupled to the gain region, and the first reflecting mirror. The region has a first diffraction grating that produces a first reflection spectrum, the second reflector region has a second diffraction grating that produces a second reflection spectrum, and the first reflection spectrum and the second reflection spectrum are The first diffraction grating and the second diffraction grating are configured to oscillate in a multimode in the overlapping wavelength range.

  In the semiconductor pulse laser of the present invention, the first diffraction grating and the second diffraction grating are configured so that multimode oscillation occurs in a wavelength range in which the first reflection spectrum and the second reflection spectrum overlap. Thereby, it is possible to stably externally control the oscillation spectrum width and the time waveform of the optical short pulse by shifting the wavelength of one reflection spectrum.

2 is a cross-sectional view in the resonator direction of the semiconductor pulse laser according to the first embodiment. FIG. 1 is a perspective view of a semiconductor pulse laser according to a first embodiment. FIG. 3 is a diagram showing a reflection wavelength spectrum of the semiconductor pulse laser according to the first embodiment. FIG. 3 is a diagram showing a reflection wavelength spectrum of the semiconductor pulse laser according to the first embodiment. FIG. 3 is a diagram showing a pulse waveform of the semiconductor pulse laser according to the first embodiment. It is a figure which shows the relationship between the oscillation mode number of the semiconductor pulse laser of Embodiment 1, and a time-bandwidth product. It is a figure which shows the reflection wavelength spectrum of the semiconductor pulse laser of Embodiment 2. FIG. FIG. 10 is a cross-sectional view in the resonator direction of the semiconductor pulse laser according to the fourth embodiment. FIG. 10 is a configuration diagram of a semiconductor pulse laser according to a fifth embodiment.

(Embodiment 1)
<Configuration>
FIG. 1 is a sectional view in the cavity direction of the semiconductor pulse laser according to the first embodiment of the present invention.

  The semiconductor pulse laser of the present embodiment includes an n-type InP substrate 51, an n-type InP cladding layer 52 formed thereon, an InGaAsP active layer 23 and an optical waveguide layer 24 formed thereon. . The semiconductor pulse laser according to the present embodiment includes a p-type InP cladding layer 55 formed on the active layer 23 and the optical waveguide layer 24, and a first diffraction grating formed on a part of the upper portion of the optical waveguide layer 24. 21 and the second diffraction grating 22, the p-type InGaAs contact layer 56 formed on the cladding layer 55, and the high resistance InP current confinement for confining the current formed on the side surfaces of the active layer 23 and the optical waveguide layer 24. Layer 57 (see FIG. 2).

  The semiconductor pulse laser includes a gain region 13 having a gain with respect to a predetermined wavelength region, a first reflecting mirror region 11 and a second reflecting mirror region 12 arranged so as to sandwich the gain region 13, and a waveguide. Region 14 is provided. The gain region 13 includes an active layer 23, an n-type cladding layer 52 corresponding to the active layer 23, a p-type cladding layer 55, and a substrate 51. The first reflector region 11 includes the first diffraction grating 21, the corresponding optical waveguide layer 24, the n-type cladding layer 52, the p-type cladding layer 55, and the substrate 51. The second reflector region 12 includes a second diffraction grating 22, an optical waveguide layer 24 corresponding thereto, an n-type cladding layer 52, a p-type cladding layer 55, and a substrate 51. The waveguide region 14 includes an optical waveguide layer 24, an n-type cladding layer 52, a p-type cladding layer 55, and a substrate 51 corresponding to a region where the diffraction gratings 21 and 22 and the active layer 23 are not formed. In the present embodiment, the first diffraction grating 21 and the second diffraction grating 22 are distributed Bragg reflectors having the same center wavelength.

  That is, the semiconductor pulse laser according to the first embodiment is characterized in that the first reflector region 11 and the second reflector region 12 are arranged so as to sandwich the gain region 13 therebetween. By adopting such a configuration, multimode oscillation is performed in a wavelength range where the first reflection spectrum of the first diffraction grating 21 and the second reflection spectrum of the second diffraction grating 22 overlap, and the wavelength of one reflection spectrum is shifted. Thus, the oscillation spectrum width can be controlled.

  Furthermore, the semiconductor pulse laser according to the first embodiment includes a waveguide region 14 disposed between the first reflector region 11 and the second reflector region 12. Thereby, the resonator mode can be stabilized, and the time waveform of the oscillation light short pulse can be adjusted stably.

  The first diffraction grating 21 and the second diffraction grating 22 are distributed Bragg diffraction gratings. By using a distributed Bragg diffraction grating composed of only the same period as the diffraction grating for limiting the oscillation spectrum, a semiconductor pulse laser can be easily manufactured.

  The first reflector region 11, the gain region 13, the second reflector region 12, and the waveguide region 14 are electrically separated by separation grooves 37a, 37b, and 37c formed by etching.

  Further, the semiconductor pulse laser includes a first reflector region electrode 31, a second reflector region electrode 32, a gain region electrode 33, which are p-type electrodes formed on the p-type InGaAs contact layer 56 in each of the above-described regions. A waveguide region electrode 34 is provided. Of course, these electrodes are also provided so as to be separated from each other and electrically separated by the separation grooves 37a, 37b, and 37c.

  Further, the semiconductor pulse laser includes a back electrode 36 that is an n-type electrode formed on the back side of the n-type InP substrate 51, and includes anti-reflection coatings 38 on both end surfaces in order to reduce end surface reflection. Thus, the semiconductor pulse laser of this embodiment has a multi-electrode structure.

<Operation>
Next, the operation of the semiconductor pulse laser of this embodiment will be described.

  As described above, the semiconductor laser according to the present embodiment is divided into four regions in which the p-type electrodes 31, 32, 33, and 34 are electrically separated by the separation grooves 37a, 37b, and 37c. A current can be injected independently into each of the mirror region 11, the second reflecting mirror region 12, the gain region 13, and the waveguide region 14. That is, from the first reflector region electrode 31 to the first reflector region 11, from the second reflector region electrode 32 to the second reflector region 12, from the gain region electrode 33 to the gain region 13, and from the waveguide region electrode 34. Currents are independently injected into the waveguide regions 14. Among these, when a current is injected into the gain region 13, a strong excited state is generated in the active layer 23, and as a result, light emission occurs. The wavelength of light generated in the active layer 23 is regulated in the first reflecting mirror region 11 and the second reflecting mirror region 12, and multimode oscillation occurs.

<Spectral width adjustment>
FIG. 3 is a diagram showing a reflection spectrum of the semiconductor pulse laser in the first embodiment. Since the first diffraction grating 21 and the second diffraction grating 22 have the same center wavelength, in a state where no current is injected into both the first reflector region electrode 31 and the second reflector region electrode 32, The center wavelength of the reflection spectrum (first reflection spectrum) 101 of the first diffraction grating and the reflection spectrum (second reflection spectrum) 102 of the second diffraction grating 22 indicated by the dotted line coincide with each other, and the narrower reflection spectrum width overlaps the spectrum width. It becomes. When current is injected into the second diffraction grating 22 from the second reflector region electrode 32, the effective refractive index is lowered due to the plasma effect, and the second reflection spectrum 102 is shifted to the short wavelength side of, for example, 1.0 nm. The spectrum after the shift is shown as the second reflection spectrum 102 '(solid line). In FIG. 3, the height of the second reflection spectrums 102 and 102 ′ is set higher than that of the first reflection spectrum 101 for the purpose of easy viewing.

  Light that is in the wavelength range of the overlap width 103 of the first reflection spectrum 101 and the second reflection spectrum 102 ′ and is equal to or higher than the oscillation threshold level is oscillated.

  That is, the semiconductor pulse laser according to the first embodiment includes a gain region 13 having a gain with respect to a predetermined wavelength region, and a first reflector region 11 and a second reflector region 12 that are optically coupled to the gain region 13. The first reflector region 11 includes a first diffraction grating 21 that generates a first reflection spectrum, and the second reflector region 12 includes a second diffraction grating 22 that generates a second reflection spectrum. And the first diffraction grating 21 and the second diffraction grating 22 are configured so that multimode oscillation occurs in a wavelength range in which the first reflection spectrum and the second reflection spectrum overlap. Thereby, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

  The first reflection spectrum and the second reflection spectrum have the same center wavelength when no current is injected into the first and second diffraction gratings 21 and 22. Therefore, the narrower spectral width becomes the overlapping width, and the oscillation spectral width is determined. In addition, if the wavelength of one of the reflection spectra is shifted, the overlap width changes, so that the oscillation spectrum width can be adjusted.

  In the present embodiment, the first diffraction grating 21 and the second diffraction grating 22 are formed so that the overlap width 103 includes at least two oscillation longitudinal modes 104, preferably 3-10. For example, when generating 40 Gb / s short pulse light, the wavelength interval between the longitudinal modes needs to be about 0.32 nm, and the overlap width 103 of the reflection spectrum including the ten oscillation longitudinal modes 104 is approximately 3 .5 nm. Therefore, the lengths of the regions of the diffraction gratings 21 and 22 and the coupling coefficient with respect to the optical waveguide layer 24 are adjusted so that the wavelength widths of the first reflection spectrum 101 and the second reflection spectrum 102 are 3.5 nm or more, respectively. . For example, when the coupling coefficient is ˜300 / cm and the diffraction grating length is ˜150 μm, the spectral wavelength width of the diffraction grating is about 5 nm.

  Now, the width of the first reflection spectrum 101 is set to 4.0 nm, the width of the second reflection spectrum 102 is set to 5.0 nm, and a current is injected from the second reflector region electrode 32 into the second diffraction grating 22 to thereby generate the second reflection spectrum. If 102 is shifted to the short wavelength side by 1.0 nm, the overlap width 103 of the first reflection spectrum 101 and the second reflection spectrum 102 ′ is 3.5 nm, and about 10 longitudinal modes oscillate. By adjusting the amount of current injected into one of the diffraction gratings in this way, the overlap width 103 of the reflection spectrum can be adjusted. It is known that the wavelength that can be shifted by injecting a current into the distributed Bragg reflector is generally about 5 nm. In this embodiment, the overlap width 103 is adjusted in the range of 0 nm to 4 nm.

  The length of the gain region 13, the length of the waveguide region 14, and the amount of current to each gain region electrode 33 and waveguide region electrode 34 are set so that a plurality of oscillating longitudinal modes are phase-synchronized. When adjusted appropriately, mode-locked short pulse light oscillates. It is known that the envelope of the oscillation light spectrum 105 (see FIG. 4) and the time waveform 107 of the optical pulse (see FIG. 5) are in a Fourier transform relationship with each other in a phase-synchronized state. 4 shows the case where the length of the first reflector region 11 is 150 μm, the length of the second reflector region 12 is 150 μm, the length of the gain region is 500 μm, and the length of the waveguide region 14 is 300 μm. FIG. 5 shows an oscillation spectrum, and FIG. 5 shows a time waveform under the above conditions. By adjusting the overlap width 103 of the reflection spectrum in FIG. 3, in other words, by adjusting the shift amount of the second reflection spectrum 102 ′, the width of the envelope 105 of the oscillation light spectrum can be adjusted. The time waveform 107 can be adjusted.

<Number of oscillation longitudinal modes>
In the above description, the number of oscillation longitudinal modes 104 is preferably 3 to 10, and the reason will be described. Although pulse oscillation occurs when there are two longitudinal modes, it is generally a sinusoidal pulse due to the Fourier transform relationship between the spectrum and the time waveform, and is not suitable for optical communication except in special cases. On the other hand, in the design of the mode-locked semiconductor laser, each longitudinal mode is strongly influenced by the dispersion of the gain medium, but the laser oscillation is described by a non-linear multimode equation, and it is very difficult to analyze each correlation. Therefore, an analytical solution of more than four multimodes has not been obtained, but it has been suggested that it is difficult to completely eliminate the phase difference between modes in a semiconductor laser. That is, as the number of modes increases, the phase difference between the modes accumulates, resulting in a mode-synchronized state that deviates from the complete Fourier transform relationship.

FIG. 6 shows the result of experimental evaluation of the deviation from the Fourier transform limit from the number of modes included in the mode-locked oscillation spectrum and the time pulse waveform in a specific semiconductor structure having chromatic dispersion. FIG. 6 shows the relationship between the number of oscillation longitudinal modes on the horizontal axis and the time / bandwidth product on the vertical axis. The lower limit of the time / bandwidth product is physically determined depending on the waveform, and is known to be 0.315 for the sech ² type. This state is called the Fourier transform limit. As shown in FIG. 6, as the number of modes increases, it suddenly deviates from the Fourier transform limit, and the mode synchronization state is canceled when the number of modes exceeds 10.

  When the center wavelength of the oscillation spectrum 105 (see FIG. 4) is slightly different from the wavelength desired in the optical communication system, the current injected into the first reflector region electrode 31 and the second reflector region electrode 32 is By adjusting simultaneously, it is possible to adjust to a desired wavelength.

<Effect>
The semiconductor pulse laser according to the first embodiment includes a gain region 13 having a gain with respect to a predetermined wavelength region, a first reflector region 11 and a second reflector region 12 optically coupled to the gain region 13, The first reflector region 11 has a first diffraction grating 21 that produces a first reflection spectrum, and the second reflector region 12 has a second diffraction grating 22 that produces a second reflection spectrum. The first diffraction grating 21 and the second diffraction grating 22 are configured such that multimode oscillation occurs in a wavelength range in which the first reflection spectrum and the second reflection spectrum overlap. Thereby, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

  The semiconductor pulse laser according to the first embodiment is characterized in that the first reflecting mirror region 11 and the second reflecting mirror region 12 are arranged so as to sandwich the gain region 13 therebetween. By adopting such a configuration, multimode oscillation is performed in a wavelength range where the first reflection spectrum of the first diffraction grating 21 and the second reflection spectrum of the second diffraction grating 22 overlap, and the wavelength of one reflection spectrum is shifted. Thus, the oscillation spectrum width can be controlled.

  Furthermore, the semiconductor pulse laser according to the first embodiment includes a waveguide region 14 disposed between the first reflector region 11 and the second reflector region 12. Thereby, the resonator mode can be stabilized, and the time waveform of the oscillation light short pulse can be adjusted stably.

  The first reflection spectrum and the second reflection spectrum have the same center wavelength when no current is injected into the first and second diffraction gratings 21 and 22. Therefore, the narrower spectral width becomes the overlapping width, and the oscillation spectral width is determined. In addition, if the wavelength of one of the reflection spectra is shifted, the overlap width changes, so that the oscillation spectrum width can be adjusted.

  The first diffraction grating 21 and the second diffraction grating 22 are distributed Bragg diffraction gratings. By using a distributed Bragg diffraction grating composed of only the same period as the diffraction grating for limiting the oscillation spectrum, a semiconductor pulse laser can be easily manufactured.

(Embodiment 2)
<Configuration>
In the first embodiment, the center wavelength of the reflection spectrum 101 of the first diffraction grating is set to be the same as the center wavelength of the reflection spectrum 102 of the second diffraction grating, but in the second embodiment, the center wavelength is set to be the same. Are set differently. The diffraction grating is formed by electron beam exposure, but the center wavelength can be controlled by changing the exposure pitch. FIG. 7 is a diagram showing a reflection spectrum of the semiconductor pulse laser in the second embodiment. The first diffraction grating and the second diffraction grating are formed so that the second reflection spectrum 102 is on the longer wavelength side of the first reflection spectrum 101 and has the same spectral width as the first reflection spectrum 101.

  The other configuration of the semiconductor pulse laser according to the second embodiment is the same as that of the first embodiment, and thus the description thereof is omitted.

<Operation>
As in the first embodiment, current is injected into the second reflector region electrode 32, and the second reflection spectrum 102 is shifted to the short wavelength side by 1.0 nm by the plasma effect. Thereby, the overlap width 103 of the first reflection spectrum 101 and the second reflection spectrum 102 ′ is about 1.5 nm. In the present embodiment as well, the overlapping width 103 of the reflection spectrum can be adjusted in the range of 0 nm to 4 nm as in the first embodiment.

  In the second embodiment, the overlap width 103 of the reflection spectrum is small when the current injection amount is small, and the overlap width 103 is large when the current injection amount is large. When oscillating a short pulse laser, in order to shorten the pulse width, it is required to widen the spectrum width from the relationship of Fourier transform. However, when the spectrum width is widened to increase the number of oscillation longitudinal modes, phase synchronization between longitudinal modes may hardly occur due to the influence of dispersion as shown in FIG. Therefore, it is desirable that the number of oscillation longitudinal modes is about three from the viewpoint of stable phase synchronization, and it is necessary to appropriately reduce the overlap width 103 of the reflection spectrum.

  In the distributed Bragg reflector, it is known that a loss due to free carrier absorption occurs at the same time that the refractive index decreases due to the plasma effect as the amount of current injection increases. In this embodiment, it is designed to have a sufficient reflectance, but it goes without saying that a small amount of current to be injected is preferable in order to avoid unnecessary loss. In other words, phase synchronization can be easily caused by setting the overlapping width 103 of the reflection spectrum to be small with a small amount of injected current. In the present embodiment, the overlap width 103 of the reflection spectrum is increased as the amount of injected current increases.

  That is, the semiconductor pulse laser according to the second embodiment is characterized in that the first reflection spectrum and the second reflection spectrum have different center wavelengths when no current is injected into the first and second diffraction gratings 21 and 22. If the center wavelength of one reflection spectrum is shifted by about the spectrum width of the other reflection spectrum when no current is injected, the overlap width 103 can be appropriately narrowed by a small amount of current injection with little loss due to free carrier absorption. And stable phase synchronization in the oscillation longitudinal mode can be obtained.

<Effect>
The semiconductor pulse laser according to the second embodiment has the following effects as described above. That is, the semiconductor pulse laser according to the second embodiment is characterized in that the first reflection spectrum and the second reflection spectrum have different center wavelengths when no current is injected into the first and second diffraction gratings 21 and 22. If the center wavelength of one reflection spectrum is shifted by about the spectrum width of the other reflection spectrum when no current is injected, the overlap width 103 can be appropriately narrowed by a small amount of current injection with little loss due to free carrier absorption. And stable phase synchronization in the oscillation longitudinal mode can be obtained.

(Embodiment 3)
<Configuration>
In the first and second embodiments, distributed Bragg diffraction grating reflectors are used as the first diffraction grating 21 and the second diffraction grating 22. However, in the third embodiment, one or both of the first diffraction grating 21 and the second diffraction grating 22 is a chirped diffraction grating reflector. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

<Chirp diffraction grating>
The distributed Bragg reflector is manufactured by repeating a grating having a single period, and the reflection spectrum width and the reflectance are adjusted by adjusting the coupling efficiency between the length and the waveguide layer. In the distributed Bragg reflector, the product of the length of the diffraction grating and the coupling coefficient is increased in order to increase the reflection spectrum width, and as a result, the reflectivity is high and the oscillation threshold is reduced.

  For example, a chirped diffraction grating for producing a diffraction grating by sequentially changing the period of the grating is used to obtain a similar wide reflection spectrum width. In the third embodiment, this is used as a reflecting mirror. The chirped diffraction grating can broaden the reflection spectrum corresponding to the adjustment width of the period, and has an effect of adjusting the reflectance according to the number of repetitions of the diffraction grating of each period.

  Therefore, by using a chirped diffraction grating as a reflecting mirror, it is possible to arbitrarily design a desired spectral width and appropriately reduce the reflectance. For example, when a multiple phase shift diffraction grating, which is one method for realizing a chirped grating, is employed, the coupling coefficient is set to 40 / cm, and a group of diffraction gratings between phase shifts is formed in about 20 cycles. A reflecting mirror having a reflectance reduced to about 0.5 can be obtained. Thereby, although the oscillation threshold value increases, the light output can be increased.

  That is, the semiconductor pulse laser according to the third embodiment is characterized in that the first diffraction grating 21 and the second diffraction grating 22 are chirped diffraction gratings. By using a chirped diffraction grating whose period changes as the diffraction grating that limits the oscillation spectrum, it is possible to obtain a semiconductor pulse laser with a high degree of freedom in designing optical output and time waveform.

<Effect>
The semiconductor pulse laser according to the third embodiment has the following effects as described above. That is, the semiconductor pulse laser according to the third embodiment is characterized in that the first diffraction grating 21 and the second diffraction grating 22 are chirped diffraction gratings. By using a chirped diffraction grating whose period changes as the diffraction grating that limits the oscillation spectrum, it is possible to obtain a semiconductor pulse laser with a high degree of freedom in designing optical output and time waveform.

(Embodiment 4)
<Configuration>
In the first embodiment, the waveguide region 14 is provided between the gain region 13 and the second reflector region, but the waveguide region 14 may not be provided. As shown in FIG. 8, the semiconductor pulse laser of the fourth embodiment is obtained by removing the waveguide region 14 in the first embodiment. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

<Operation>
Since the operation of this embodiment is substantially the same as that of Embodiment 1, the description thereof is omitted.

<Effect>
Also in the present embodiment, since the spectrum width of the oscillation light is determined by the overlapping width of the reflection spectrum of the first diffraction grating 21 and the reflection spectrum of the second diffraction grating 22, the wavelength of one reflection spectrum is shifted to obtain the light. It is possible to externally control the time waveform by adjusting the oscillation spectrum width of the short pulse.

(Embodiment 5)
<Configuration>
FIG. 9 is a top view of the semiconductor pulse laser according to the fifth embodiment. In the semiconductor pulse laser of the fifth embodiment, the gain region 13, the first reflector region 11, the second reflector region 12, the first reflector region 11 and the second reflector region 12 are optically integrated with the gain region 13. In addition to the multimode interference coupler 41 to be coupled, the saturable absorption region 15 is provided, and the waveguide region 14 is preferably further provided.

  The configurations of the gain region 13, the first reflector region 11, the second reflector region 12, and the waveguide region 14 are the same as those in the first embodiment. The gain region 13 includes the active layer 23, and the first reflector region 11 includes The first diffraction grating 21 is provided, and the second reflecting mirror region 12 is provided with a second diffraction grating 22.

  The saturable absorption region 15 is provided on the light emission side of the semiconductor pulse laser from the gain region 13, and the first reflecting mirror region 11 and the second reflecting mirror region 12 are opposite to the gain region 13 with respect to the saturable absorption region 15. Are arranged in parallel to each other and are optically coupled to the gain region 13 independently via the multimode interference coupler 41. Further, the waveguide region 14 is provided between the multimode interference coupler 41 and the first reflector region 11 and the second reflector region 12. In the present embodiment, each of the first diffraction grating 21 and the second diffraction grating 22 is a distributed Bragg reflector having the same center wavelength.

  That is, the semiconductor pulse laser according to the fifth embodiment further includes a saturable absorption region 15 provided on the light emission side of the gain region and having saturable absorption characteristics with respect to a predetermined wavelength region, and includes the first reflector region 11. The second reflector region 12 is arranged in parallel to the gain region 13 on the opposite side of the saturable absorption region 15. Thereby, in the mode-locked pulse laser having the saturable absorption region 15, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

  The semiconductor pulse laser according to the fifth embodiment further includes a waveguide region 14 provided between the first reflector region 11 and the second reflector region 12. By including the waveguide region 14 that stabilizes the resonator mode, the time waveform of the oscillation light short pulse can be stably adjusted in the mode-locked pulse laser having the saturable absorption region 15.

  Further, the semiconductor pulse laser of the fifth embodiment further includes a multimode interference coupler 41 that optically couples the first and second reflector regions 11 and 12 and the gain region 13. Thereby, in the mode-locked pulse laser having the saturable absorption region 15, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

<Operation>
The saturable absorption region 15 takes out an optical pulse outside the resonator when the internal light intensity exceeds the saturation level. By releasing energy to the outside, passive mode-locked oscillation is realized in which light emission stops below the saturation level.

  Alternatively, it is also known that active mode-locked oscillation is realized by applying a high-frequency signal from the outside to the saturable absorption region 15. The optical spectrum waveform at this time is first defined by the semiconductor quantum structures of the active layer 23 and the saturable absorber layer 25, and secondly limited by the reflection spectra of the first reflector region 11 and the second reflector region 12. The

  Conventionally, there is one reflecting mirror, and after the fabrication of the semiconductor quantum structure, since the range of adjustment is extremely limited, it has been difficult to externally adjust the oscillation spectrum waveform. However, in the semiconductor pulse laser of the fifth embodiment, the reflection spectra of the first diffraction grating 21 and the second diffraction grating 22 can be adjusted, and laser oscillation occurs corresponding to the overlap width 103 of the reflection spectra. It is possible to adjust the oscillation spectrum waveform to a desired width. That is, it is possible to adjust the optical pulse time waveform in the Fourier transform relationship.

<Effect>
The semiconductor pulse laser according to the fifth embodiment has the following effects as described above. That is, the semiconductor pulse laser according to the fifth embodiment further includes a saturable absorption region 15 provided on the light emission side of the gain region and having saturable absorption characteristics with respect to a predetermined wavelength region, and includes the first reflector region 11. The second reflector region 12 is arranged in parallel to the gain region 13 on the opposite side of the saturable absorption region 15. Thereby, in the mode-locked pulse laser having the saturable absorption region 15, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

  The semiconductor pulse laser according to the fifth embodiment further includes a waveguide region 14 provided between the first reflector region 11 and the second reflector region 12. By including the waveguide region 14 that stabilizes the resonator mode, the time waveform of the oscillation light short pulse can be stably adjusted in the mode-locked pulse laser having the saturable absorption region 15.

  Further, the semiconductor pulse laser of the fifth embodiment further includes a multimode interference coupler 41 that optically couples the first and second reflector regions 11 and 12 and the gain region 13. Thereby, in the mode-locked pulse laser having the saturable absorption region 15, the oscillation spectrum width can be adjusted by external control, and the time waveform of the oscillation light short pulse can be adjusted.

<Modification>
In the above description, the first diffraction grating 21 and the second diffraction grating are distributed Bragg reflectors having the same center wavelength. However, the distributed Bragg reflectors having different center wavelengths are set as in the second embodiment. It is also good. Alternatively, either or both of the first diffraction grating 21 and the second diffraction grating may be a chirped diffraction grating reflecting mirror as in the third embodiment.

  11 First reflector region, 12 Second reflector region, 13 Gain region, 14 Waveguide region, 15 Saturable absorption region, 21 First diffraction grating, 22 Second diffraction grating

Claims (10)

A gain region having a gain for a predetermined wavelength region;
A first reflector region and a second reflector region optically coupled to the gain region;
The first reflector region has a first diffraction grating that produces a first reflection spectrum;
The second reflector region has a second diffraction grating that produces a second reflection spectrum;
The semiconductor pulse laser, wherein the first diffraction grating and the second diffraction grating are configured so that multimode oscillation occurs in a wavelength range in which the first reflection spectrum and the second reflection spectrum overlap.   2. The semiconductor pulse laser according to claim 1, wherein the first reflector region and the second reflector region are disposed so as to sandwich the gain region.   The semiconductor pulse laser according to claim 2, further comprising a waveguide region disposed between the first reflecting mirror region and the second reflecting mirror region. A saturable absorption region provided on the light exit side of the gain region, and having a saturable absorption characteristic for a predetermined wavelength range;
2. The semiconductor pulse laser according to claim 1, wherein the first mirror region and the second mirror region are arranged in parallel on the opposite side of the saturable absorption region with respect to the gain region.   The semiconductor pulse laser according to claim 4, further comprising a waveguide region provided between the first reflecting mirror region and the second reflecting mirror region.   6. The semiconductor pulse laser according to claim 4, further comprising a multimode interference coupler that optically couples the first and second reflector regions and the gain region.   The first reflection spectrum and the second reflection spectrum have the same center wavelength when no current is injected into the first and second diffraction gratings, according to any one of claims 1 to 6. Semiconductor pulse laser.   The semiconductor pulse according to any one of claims 1 to 6, wherein the first reflection spectrum and the second reflection spectrum have different center wavelengths when no current is injected into the first and second diffraction gratings. laser.   The semiconductor pulse laser according to claim 1, wherein the first diffraction grating and the second diffraction grating are distributed Bragg diffraction gratings.   The semiconductor pulse laser according to claim 1, wherein the first diffraction grating and the second diffraction grating are chirped diffraction gratings.

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