JP2010102045A - Mode synchronization semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mode synchronization semiconductor laser capable of outputting pulse laser light having low time jitter. <P>SOLUTION: The mode synchronization semiconductor laser includes, on a substrate made of a compound semiconductor: a whispering gallery mode resonator made of a compound semiconductor, having a circumferential part and configured such that light turns around on the inside of the circumferential part; a light amplification part made of a compound semiconductor, directly or indirectly connected to the whispering gallery mode resonator and having a light amplification gain; and an optical waveguide made of a compound semiconductor, directly or indirectly connected to the whispering gallery mode resonator and inputting/outputting light to/from the whispering gallery mode resonator. The whispering gallery mode resonator, the light amplification part and the optical waveguide are monolithically integrated and have a means of mode synchronization oscillating pulse laser light by mode synchronization. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a mode-locked semiconductor laser.

  In ultra-high speed optical communication, a laser is used as a signal light source, but as its characteristics, it is important that the repetition frequency is stable and the pulse width is narrow. In particular, in OTDM (Optical Time Division Multiplexing) transmission, when realizing a transmission capacity exceeding terabits, the pulse width of the signal light is 1 ps or less, and thus a short pulse light source with sufficiently smaller time jitter is required. ing.

  There is a mode-locked laser as an apparatus for generating an optical pulse train having a short pulse width and small time jitter. Mode synchronization is a phenomenon in which a short optical pulse is generated at a fixed repetition cycle timing when the phase between modes of the longitudinal mode of the oscillating laser light is not random but oscillates with the phases co-harmonized with each other. .

  Mode synchronization methods include forced mode synchronization, passive mode synchronization, hybrid mode synchronization, and reproduction mode synchronization. 12 and 13 are explanatory diagrams for explaining the principle of each mode synchronization method. As shown in FIG. 12A, forced mode synchronization is a driving in which an external resonator 503 is formed by an end face 501a of a semiconductor laser 501 and an external reflecting mirror 502, and is supplied from a power source 504 to an active layer 501b of the semiconductor laser 501. This is a mode in which mode synchronization is forcibly performed by using a current as a modulation current and setting the modulation frequency to a frequency c / 2L equal to the longitudinal mode interval of the external resonator 503. Here, c is the speed of light, and L is the resonator length of the external resonator 503. Reference numeral 505 denotes an optical pulse that reciprocates in the external resonator 503. The length of the active layer 501b of the semiconductor laser 501 is sufficiently shorter than the resonator length L of the external resonator 503.

  In addition, as shown in FIG. 12B, the passive mode synchronization means that the external resonator 507 is formed by the end face 506a of the semiconductor laser 506 and the external reflecting mirror 502 as in FIG. The drive current supplied from the semiconductor laser 506 to the active layer 506b of the semiconductor laser 506 is a constant current, and the saturable absorber 506c is disposed in a part of the external resonator 507. The saturable absorber is made of a material having such a characteristic that the absorption coefficient decreases as the incident light intensity increases. In FIG. 12B, a saturable absorber 506c formed in the semiconductor laser 506 is used as an example. When the saturable absorber 506c is present in the external resonator 507, the saturable absorber 506c is light that opens in synchronization with the passage of the optical pulse 509 and closes when the optical pulse 509 passes. Since it functions as a shutter, mode synchronization is performed as a result, and a time interval in which the optical pulse 509 reciprocates in the external resonator 507, that is, a c / 2L optical pulse train as a frequency is generated.

  The hybrid mode synchronization is a method using both forced mode synchronization and passive mode synchronization as shown in FIG. Specifically, a saturable absorber 506c made of a semiconductor whose transmittance increases when a reverse bias is applied while supplying a constant driving current to the active layer 506b of the semiconductor laser 506 is supplied from the power source 504 to the external resonator. An AC signal having a frequency c / 2L corresponding to the vertical mode interval of 507 is applied. Reference numeral 510 denotes an optical pulse that reciprocates in the external resonator 507.

  Further, as shown in FIG. 13, the reproduction mode synchronization means that laser light emitted from a semiconductor laser 601 and propagated through an optical fiber 602 is received by a photodetector 603 and converted into an electric signal, and this electric signal is converted into an electric signal. In this method, the signal is amplified by the electric amplifier 604, and a signal having a fundamental oscillation frequency or an integral multiple of the amplified electric signal is extracted from the amplified electric signal and superimposed on the driving current of the semiconductor laser 601. This method has an advantage that the resonator length of the resonator 606 (semiconductor laser-optical fiber-detector loop length) can be accurately matched to the repetition frequency of the optical pulse. Note that τ indicates the light delay time in the resonator 606. Further, such a reproduction mode synchronization type laser can be used as an electromagnetic wave oscillator by providing a coupler 607 after the electric filter 605 and outputting a signal of the above frequency as an RF (Radio Frequency) signal. A resonator that realizes such reproduction mode synchronization is sometimes called an optical-microwave oscillator (Optoelectronic Oscillator: OEO) (see, for example, Patent Documents 1 and 2 and Non-Patent Document 1).

  On the other hand, conventionally, microsphere, columnar and ring resonators called whispering gallery mode (WGM) resonators are known (see, for example, Patent Documents 2 and 3 and Non-Patent Documents 2 to 5). ). The resonance mode of the WGM resonator means, for example, in the case of a microsphere, an optical field that is confined in an inner region near the equator portion of the surface of the sphere and circulates due to total internal reflection inside the outer periphery. FIG. 14 is an explanatory diagram illustrating the configuration of the WGM resonator. The WGM resonator 701 shown in FIG. 14A is a microsphere, and the light 702 is confined and circulates in a region near the equator inside the outer periphery 701a. Further, the WGM resonator 703 shown in FIG. 14B has a cylindrical shape, and the light 704 is confined and circulates in the region inside the outer periphery 703a. Further, the WGM resonator 705 shown in FIG. 14C has a ring shape having an inner peripheral portion 705b, and the light 706 is confined around the outer periphery 705a.

JP-T-2005-522887 JP-A-8-101411 JP-T-2004-503816 X.S.Yao and L.Maleki, “Optoelectronic microwave oscillator”, J. Opt. Soc. Am. B, vol.13, no.8, pp.1725-1735, Aug. 1996. A.B.Matsko et al., “Active mode locking with whispering-gallery Modes”, J. Opt. Soc. Am. B, Vol.20, No.11, pp.2292-2296 (2003) M.L.Gorodetsky et al., “Ultimate Q of optical microsphere resonators”, Opt. Lett. 21, pp.453-455 (1996) D. Rabus et al., “High-Q channel-dropping filters using ring resonators with integrated SOAs”, IEEE Photon. Technol. Lett. 14, pp.1442-1444 (2002) S.H.Kim et al., “Two-dimensional photonic crystal hexagonal waveguide ring laser”, Applied Physics Letter Vol.81 pp.2499-2501 (2002)

  By the way, the time jitter of the optical pulse, which is an important characteristic of the mode-locked laser, largely depends mainly on the Q value of the laser resonator of the mode-locked laser. Here, the Q value is expressed by Expression (1).

_Where f _osc is the optical pulse repetition frequency, _ng is the group refractive index in the laser resonator, L _opt is the resonator length, c is the speed of light, δ is the loss constant of the laser resonator, and τ _delay is the laser resonator This is the light delay time. The time jitter of the optical pulse of the mode-locked laser is smaller as the Q value is higher, that is, the characteristics are improved.

However, the conventional typical mode-locked semiconductor laser having a repetition frequency of 40 Gb / s has a small Q value because its resonator length _Lopt is as short as about 1.1 mm. For this reason, the time jitter of this semiconductor laser is about 0.2 ps when the repetition frequency is 40 GHz. On the other hand, as described above, when OTDM transmission with a transmission capacity of Tb / s or more is realized, the pulse width of the signal light is 1 ps or less. Therefore, when a conventional mode-locked semiconductor laser is used, a semiconductor is used. Since the time jitter of the laser and the pulse width of the signal light have the same size, it causes an increase in the error rate, which is not preferable. Therefore, a semiconductor laser having a smaller time jitter has been demanded as a light source for OTDM transmission of Tb / s or more.

  The present invention has been made in view of the above, and an object of the present invention is to provide a mode-locked semiconductor laser capable of outputting pulsed laser light with small time jitter.

  In order to solve the above-described problems and achieve the object, a mode-locked semiconductor laser according to the present invention has a whispering gallery mode resonator having an outer peripheral portion on a substrate made of a compound semiconductor, and an inner side of the outer peripheral portion. Whispering gallery mode resonator made of compound semiconductor configured to circulate light, and directly or indirectly connected to the whispering gallery mode resonator, an optical amplification unit made of compound semiconductor having optical amplification gain, and An optical waveguide made of a compound semiconductor that is directly or indirectly connected to the whispering gallery mode resonator and inputs / outputs light to / from the whispering gallery mode resonator, and the whispering gallery mode resonator and the optical amplification unit, The optical waveguide is monolithically integrated and has mode-locking means for oscillating pulsed laser light by mode-locking.

  Further, in the mode-locked semiconductor laser according to the present invention, the whispering gallery mode resonator includes an active layer having an optical amplification gain on a substrate made of a compound semiconductor, and has an outer peripheral portion, and light is emitted inside the outer peripheral portion. A compound semiconductor whispering gallery mode resonator configured to circulate, connected directly or indirectly to the whispering gallery mode resonator, and made of compound semiconductor that inputs and outputs light to the whispering gallery mode resonator These optical waveguides and whispering gallery mode resonators are monolithically integrated and have mode-locking means for oscillating pulsed laser light by mode-locking.

  In the mode-locked semiconductor laser according to the present invention, in the above-described invention, the mode-locking means includes a compound semiconductor light detection unit that receives a part of the pulsed laser light and converts it into an electrical signal, and the light. A laser that is connected to a detection unit and driven by applying a fundamental oscillation frequency component or a harmonic component of the fundamental oscillation frequency determined by the delay time of the pulsed laser light included in the electrical signal via an electrode structure And a light modulation unit that modulates light.

  In the mode-locked semiconductor laser according to the present invention, in the above-described invention, the mode-locking means includes a compound semiconductor light detection unit that receives a part of the pulsed laser light and converts it into an electrical signal, and the light. Connected to a detector and modulates the injection current of the pulse laser via the electrode structure with a fundamental oscillation frequency component or a harmonic component of the fundamental oscillation frequency determined by the delay time of the pulse laser light included in the electrical signal It is characterized by doing.

  In the mode-locked semiconductor laser according to the present invention as set forth in the invention described above, the light modulation section is made of a saturable absorber.

  The mode-locked semiconductor laser according to the present invention is characterized in that, in the above-mentioned invention, the optical modulation section is composed of a Mach-Zehnder modulator.

  In the mode-locked semiconductor laser according to the present invention, in the above invention, the whispering gallery mode resonator including the active layer has a high mesa shape, and at least on the surface of the active layer on the side surface of the high mesa shape, A protective layer having a band gap energy larger than that of the active layer is formed.

  In the mode-locked semiconductor laser according to the present invention, in the above invention, the whispering gallery mode resonator has a circular outer periphery, and the optical waveguide inputs and outputs light along a tangential direction of the outer periphery. It is arranged so that it may be arranged.

  The mode-locked semiconductor laser according to the present invention is the above-described invention, wherein the whispering gallery mode resonator has a two-dimensional refractive index periodic structure in a plane perpendicular to the semiconductor lamination direction. In FIG. 2, a line defect portion to which light is guided is provided, and the line defect portion has a loop shape.

  The mode-locked semiconductor laser according to the present invention further includes an output terminal for outputting the fundamental oscillation frequency component or the harmonic component included in the electrical signal in the above invention, and can extract electromagnetic waves to the outside. It is characterized by that.

  According to the present invention, by introducing the WGM resonator to a part of the laser resonator of the mode-locked semiconductor laser, the photon lifetime in the laser resonator can be extended, thereby increasing the Q value of the laser resonator. Therefore, it is possible to realize a mode-locked semiconductor laser that can output pulsed laser light with small time jitter.

  Embodiments of a mode-locked semiconductor laser according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. In the drawings, the same components are denoted by the same reference numerals.

(Embodiment 1)
FIG. 1 is a schematic plan view of a mode-locked semiconductor laser according to Embodiment 1 of the present invention. As shown in FIG. 1, this mode-locked semiconductor laser 100 includes a WGM resonator 2, optical waveguides 3 to 6, and optical modulation as mode-locking means each made of a compound semiconductor on a substrate 1 made of a compound semiconductor. This is a monolithically integrated saturable absorber 7. The WGM resonator 2, the optical waveguides 3 to 6, and the saturable absorber 7 have a high mesa shape, and are surrounded by polyimide 8 that is an insulating material.

  The WGM resonator 2 is formed in a ring shape and has a circular outer peripheral portion. The optical waveguides 3 and 5 are linearly formed substantially parallel to each other, and are respectively connected to the WGM resonator 2 along the tangential direction of the WGM resonator 2. The saturable absorber 7 is formed in a straight line and is interposed between the optical waveguides 3 and 4. The optical waveguide 4 is formed in a straight line, and its end face 4a is coated with HR (High Reflection) so that the reflectivity becomes, for example, about 99% by a dielectric or the like. The optical waveguide 5 is coated so that its end surface 5a has a transmittance of about 10% with a dielectric or the like. The optical waveguide 6 is formed in a straight line and is disposed on the same line as the optical waveguide 5. The end face 6a of the optical waveguide 6 is formed as an oblique end face with an angle of 7 °, and the other end face 6b is AR (Anti-Reflection) coated so that the reflectance becomes extremely small. Further, an electrode 9 a is formed on the WGM resonator 2, and an electrode 9 b is formed on the saturable absorber 7. Lead wires 10a and 10b are connected to the electrodes 9a and 9b, respectively.

  The mesa widths of the WGM resonator 2, the optical waveguides 3 to 6, and the saturable absorber 7 are all 2 μm. The diameter of the WGM resonator 2 is 1.2 mm, and the size of the electrode 9a is 200 μm × 1.5 mm in terms of (vertical length) × (horizontal length) on the paper surface. The length of the saturable absorber 7 is 75 μm, and the size of the electrode 9b is 75 μm × 75 μm. The total length of the optical waveguide 4, the saturable absorber 7, and the optical waveguide 3 is 1000 μm. The length of the optical waveguide 5 is 800 μm. Therefore, the size of the mode-locked semiconductor laser 100 is 2 mm × 2 mm, and it is integrated in a very small size.

  Next, the cross-sectional structure of this mode-locked semiconductor laser 100 will be described. FIG. 2 is a cross-sectional view taken along line AA ′ in FIG. As shown in FIG. 2, the WGM resonator 2 is an active element, and a buffer layer 12, an active layer 13, an upper clad layer 14, a contact layer 15, on a substrate 1 on which an n-side electrode 11 is formed on the back surface. A p-side electrode 16 is sequentially laminated. The buffer layer 12 has a protruding portion 12a, and the portion from the protruding portion 12a to the contact layer 15 has a high mesa shape. Further, the side surface of the high mesa shape and the surface of the buffer layer 12 are passivated by a protective layer 17. The substrate 1 and the buffer layer 12 are both made of n-InP. The active layer 13 is made of InGaAsP, and a light guide layer having a refractive index larger than that of the buffer layer 12 and the upper clad layer 14 is provided on both sides of the six multi-quantum well layers (MQW). It has a separate confinement heterostructure (SCH) structure. The upper cladding layer 14 is made of p-InP. The contact layer 15 is made of InGaAsP. The protective layer 17 is made of a material having a band gap energy larger than that of the active layer 13, and is made of InP, for example. The height from the lower surface of the protrusion 12a to the upper surface of the contact layer 15 is about 3.6 μm. The saturable absorber 7 which is another active element also has the same laminated structure as the WGM resonator 2.

  On the other hand, FIG. 3 is a sectional view taken along line BB ′ in FIG. As shown in FIG. 3, the optical waveguide 5 is a passive element, and a buffer layer 12, a core layer 18, an upper clad layer 19, and a SiNx film 20 are sequentially formed on a substrate 1 having an n-side electrode 11 formed on the back surface. It is configured by stacking. The buffer layer 12 has a protruding portion 12a, and the protruding portion 12a to the SiNx film 20 have a high mesa shape. Further, the side surface of the high mesa shape and the surface of the buffer layer 12 are passivated by a protective layer 17. The core layer 18 is made of InGaAsP. The upper cladding layer 19 is made of p-InP. The height from the lower surface of the protruding portion 12a to the upper surface of the upper cladding layer 19 is about 3.1 μm. Further, the optical waveguides 3, 4 and 6 which are other passive elements also have the same laminated structure as the optical waveguide 5.

  Next, a case where the mode-locked semiconductor laser 100 is operated in a hybrid mode-locked manner will be described. First, when a DC voltage is applied between the n-side electrode 11 and the p-side electrode 16 via the lead wire 10a and the electrode 9a and a driving current is passed, current is injected into the active layer 13 of the WGM resonator 2 and activated. Layer 13 emits light in the 1.55 μm band. This light circulates inside the outer periphery of the WGM resonator 2 in the active layer 13 in the WGM mode, and is input to and output from the optical waveguides 3 and 5, the core layer 18 of the optical waveguides 3 to 5, and the saturable absorber. 7 active layer 13 is guided.

  Here, the WGM resonator 2, the optical waveguides 3 to 5, and the saturable absorber 7 constitute a laser resonator having end faces 4a and 5a as end faces. Further, the active layer 13 of the WGM resonator 2 functions as an optical amplification unit having an optical amplification gain. As a result, the light generated in the active layer 13 of the WGM resonator 2 is oscillated by the laser resonator. As described above, the mesa widths of the WGM resonator 2, the optical waveguides 3 to 6, and the saturable absorber 7 are all 2 μm, and the active layer 13 of the WGM resonator 2 and the saturable absorber 7. And the thickness of the core layer 18 of the optical waveguides 3 to 6 are both 0.3 μm, and the thickness of the protruding portion 12a of the buffer layer 12 is 0.5 μm. Therefore, the optical waveguide serving as a laser resonator satisfies the condition of a single transverse mode at a wavelength of 1.55 μm band, and the laser light that oscillates is in a single transverse mode. Further, since the protective layer 17 having a band gap energy larger than the band gap energy of the active layer 13 is formed on the side surface of the active layer 13, surface recombination on the side surface of the active layer 13 is prevented, and the light emission efficiency. Is prevented.

  On the other hand, simultaneously with the laser oscillation, a DC voltage as a reverse bias and a predetermined synthesizer generated between the n-side electrode 11 and the p-side electrode of the saturable absorber 7 via the lead wire 10b and the electrode 9b. An RF voltage signal having a frequency is applied. Then, hybrid mode locking is realized in the laser resonator by the optical shutter effect of the saturable absorber 7. As a result, pulsed laser light is output from the end face 5 a of the optical waveguide 5. This pulsed laser light is incident on the end face 6 a of the optical waveguide 6 and is output to the outside of the mode-locked semiconductor laser 100 from the other end face 6 b. Since the end face 6a is an oblique end face, the light reflected by the end face 6a is prevented from returning to the optical waveguide 5.

  In this mode-locked semiconductor laser 100, since the WGM resonator 2 is introduced into a part of the laser resonator, the time jitter of the output pulse laser beam is extremely small while being small. That is, as described above, the time jitter of the optical pulse of the mode-locked laser decreases as the Q value increases. On the other hand, since the WGM resonator 2 circulates in the WGM mode inside the outer periphery of the WGM resonator 2, the Q value becomes extremely high, so that the time jitter becomes extremely small. Note that the Q value of the WGM resonator is mainly determined by scattering due to unevenness of the surface, absorption loss, volume, and the like. The relationship between the Q value, loss, and diameter of the WGM resonator is described in Non-Patent Document 3. The WGM resonator 2 according to the first embodiment is a ring-type WGM resonator using an InP-based semiconductor material. In this case, when the diameter is 1.2 mm, the Q value is about 130,000. And extremely large (see Non-Patent Document 4).

When the photon lifetime of the WGM resonator 2 is τ _{WGM, delay} , τ _{WGM, delay} is given by the following equation (2).

Q _WGM is the Q value of the WGM resonator 2, and f _opt is the frequency of light. Using equation (2), for light with a wavelength of 1.55 μm, when the Q value is about 130,000, the photon lifetime τ _{WGM, delay} is about 100 ps.

On the other hand, if the frequency of the RF signal to be applied to the saturable absorber 7 is f _rep , f _rep is given by the following equation (3).

Note that N is an integer, and L _opt is the total resonator length of portions other than the WGM resonator 2, that is, the optical waveguides 3 to 6 and the saturable absorber 7. When Expression (3) is used, in the case of the mode-locked semiconductor laser 100 according to the first embodiment, it is necessary to set f _rep to an integral multiple of 4.19 GHz.

  Next, the performance comparison between this mode-locked semiconductor laser 100 and a conventional semiconductor laser is performed. The Q value of this mode-locked semiconductor laser 100 is shown in equation (4).

Here, δ is a loss constant of the laser resonator. That is, the Q value is proportional to the photon lifetime τ _{WGM, delay} . Since the photon lifetimes of the portions of the optical waveguides 3 to 6 and the saturable absorber 7 are 19.2 ps, the total photon lifetime of the laser resonator of the mode-locked semiconductor laser 100 is expressed by the coefficient 2 in Equation (4). Taking this into account, 19.2 ps × 2 + 100 ps × 2 = 238 ps. On the other hand, a conventional semiconductor laser having a resonator length of 1.1 mm has a photon lifetime of 23.4 ps. Therefore, if the loss constant is the same, it can be seen that the Q-semiconductor laser 100 has a Q value that is about one digit higher than that of the conventional semiconductor laser.

  On the other hand, in the mode-locked laser, the time jitter Δτ and the Q value of the pulsed light approximately satisfy the relationship expressed by the following equation (5).

  Note that τ is the optical pulse repetition time interval. As shown in the equation (5), it can be seen that the time jitter Δτ and the Q value are in an inversely proportional relationship. Therefore, it can be seen that this mode-locked semiconductor laser 100 has a time jitter that is about an order of magnitude smaller than that of the conventional semiconductor laser.

  As described above, the mode-locked semiconductor laser 100 according to the first embodiment can output pulsed laser light with extremely small time jitter as compared with the conventional semiconductor laser, and is a light source for OTDM transmission of Tb / s or more. It becomes suitable as.

  In the above description, the mode-locked semiconductor laser 100 according to the first embodiment has been described as operating in the hybrid mode-locked mode. However, the mode-locked semiconductor laser 100 can be operated in a passive mode-locked manner by not applying a voltage to the saturable absorber 7. Even if the mode-locked semiconductor laser 100 is operated by the passive mode-locking method, the mode-locked semiconductor laser 100 can output pulsed laser light with extremely small time jitter.

  In addition, the mode-locked semiconductor laser 100 according to the first embodiment can be operated in a forced mode-locking system by replacing the saturable absorber 7 serving as a modulator with a modulator such as a Mach-Zehnder modulator.

(Embodiment 2)
Next, a mode-locked semiconductor laser according to Embodiment 2 of the present invention will be described. The mode-locked semiconductor laser according to the second embodiment is operated by the reproduction mode synchronization method.

  FIG. 4 is a schematic plan view of the mode-locked semiconductor laser according to the second embodiment. As shown in FIG. 4, the mode-locked semiconductor laser 200 is the same as the mode-locked semiconductor laser 100 shown in FIG. 1, except that the optical waveguide 6 is deleted, the optical waveguide 5 is replaced with the optical waveguide 22, and the electrode 9b is replaced with the electrode 9e. Furthermore, the optical waveguide 21, the electrodes 9c and 9d, the lead wire 10c, and the light detection unit 23 as a part of the reproduction mode synchronization means are added. Further, the end face 4a of the optical waveguide 4 is coated so as to have a transmittance of about 10%.

  The optical waveguides 21 and 22 and the light detection unit 23 have a high mesa shape. The optical waveguide 22 is linearly formed substantially parallel to the optical waveguide 3, and is connected to the WGM resonator 2 along the tangential direction of the WGM resonator 2. The light detection unit 23 is formed in a straight line and is connected to the optical waveguide 22. The optical waveguide 21 is formed in a straight line and is disposed on the same line as the optical waveguide 4. Further, the end face 21a of the optical waveguide 21 is formed as an oblique end face having an angle of 7 °, and the other end face 21b is AR-coated. The electrode 9 e is formed in a region near the WGM resonator 2 on the saturable absorber 7. The electrode 9 c is formed in a region near the WGM resonator 2 on the light detection unit 23. The electrode 9d extends from a region far from the WGM resonator 2 on the saturable absorber 7 to a region far from the WGM resonator 2 on the light detection unit 23 and further extends to the end of the substrate 1 on the right side of the paper. Is formed. Also, lead wires 10b and 10c are connected to the electrodes 9e and 9c, respectively.

  The mesa widths of the optical waveguides 21 and 22 and the light detection unit 23 are all 2 μm. The length of the light detection unit 23 is 100 μm. The size of the electrodes 9c and 9e is 45 μm × 75 μm. The size of the electrode 9d is 30 μm × 2 mm. The length of the optical waveguide 22 is 800 μm. Therefore, the mode-locked semiconductor laser 200 has a size of 5 mm × 5 mm and is integrated in a very small size.

  Note that the light detection unit 23 is an active element, has the same cross-sectional structure as the WGM resonator 2 shown in FIG. 2, and functions as a photodiode. The optical waveguides 21 and 22 are passive elements and have the same cross-sectional structure as the optical waveguide 5 shown in FIG.

  Next, a case where the mode-locked semiconductor laser 200 is operated in a reproduction mode synchronization method will be described. First, when a DC voltage is applied between the n-side electrode and the p-side electrode of the WGM resonator 2 via the lead wire 10a and the electrode 9a and a drive current is passed, current is injected into the active layer of the WGM resonator 2. The active layer emits light in the 1.55 μm band. Here, the WGM resonator 2 and the optical waveguides 3, 4 and 22 constitute a laser resonator having the WGM resonator 2 as a ring resonator and the end face 4a and the end face 22a as end faces of the resonator. The active layer 13 of the WGM resonator 2 has an optical amplification gain. As a result, the light generated in the active layer 13 of the WGM resonator 2 is oscillated by the laser resonator. Note that all optical waveguides serving as laser resonators satisfy the condition of a single transverse mode at a wavelength of 1.55 μm band, and the laser light that oscillates is in a single transverse mode. A part of the laser light that circulates around the WGM resonator 2 is output to the optical waveguide 22.

  On the other hand, simultaneously with the laser oscillation, a DC voltage as a reverse bias is applied between the n-side electrode and the p-side electrode of the light detection unit 23 via the lead wire 10c and the electrode 9c. Then, the light detection unit 23 receives the laser light from the end face of the optical waveguide 22 and converts the received light into an electrical signal. This electric signal includes a fundamental oscillation frequency determined by the delay time of the pulsed laser beam and a harmonic modulation signal having a frequency that is an integral multiple of the fundamental oscillation frequency. The electric signal including the modulation signal is conveyed to the saturable absorber 7 via the electrode 9d.

  On the other hand, a DC voltage as a reverse bias is applied to the saturable absorber 7 between the n-side electrode and the p-side electrode via the lead wire 10b and the electrode 9e. An electrical signal including a signal is applied via the electrode 9d. As a result, the saturable absorber 7 functions as an optical shutter that is driven at the frequency of the modulation signal, so that reproduction mode synchronization is realized in the laser resonator. As a result, pulsed laser light is output from the end face 4 a of the optical waveguide 4. This pulsed laser light is incident on the end face 21 a of the optical waveguide 21 and is output to the outside of the mode-locked semiconductor laser 200 from the other end face 21 b.

  Since this mode-locked semiconductor laser 200 uses a reproduction mode-locking method, unlike the forced mode-locking or hybrid mode-locking, there is no need to supply an RF signal from the outside using an oscillator, and it is generated by self-excited oscillation. The optical output is modulated by an electrical signal. At this time, the time jitter of the electric signal generated by the self-excited oscillation is about two digits better than the electric oscillator based on the conventional crystal oscillator. That is, in the conventional oscillator, for example, a vibrator is oscillated at 10 MHz, and an electric signal having a high frequency is generated by multiplying the oscillator, so that time jitter is deteriorated. On the other hand, since the mode-locked semiconductor laser 200 oscillates directly at a high frequency without multiplying a low-frequency signal, the time jitter is small.

  Therefore, in this mode-locked semiconductor laser 200, the time jitter is reduced by the high Q value of the WGM resonator 2, and the effect of reducing the time jitter by self-excited oscillation can be obtained. Can output light.

By the way, this mode-locked semiconductor laser 200 is a ring type laser. Therefore, _assuming that the oscillation frequency is f _osc , f _osc is expressed by the following equation when the light delay time is sufficiently smaller than the electrical delay time. It is given by (6).

  In addition, each variable is the same as that of Formula (3). That is, in comparison with Expression (3) which is a case of a linear laser, Expression (6) has a form in which the denominator coefficient 2 is removed.

  Further, the mode-locked semiconductor laser 200 can output an electric signal including a fundamental oscillation frequency and its harmonic component as an RF output from the electrode 9d. Therefore, this mode-locked semiconductor laser 200 can also be used as an electromagnetic wave oscillator. Since this mode-locked semiconductor laser 200 uses an optical delay path that is less susceptible to the influence of electromagnetic noise in a part of the resonator than a conventional electromagnetic wave oscillator, it generates a high-purity electrical signal with small time jitter. Can do.

  As described above, the mode-locked semiconductor laser 200 is also excellent as an electromagnetic wave oscillator, and therefore can be used as a frequency standard by combining with atoms such as Cs and Rb. Such a high-purity standard device can be applied to GPS (Global Positioning System) and the like.

(Embodiment 3)
Next, a mode-locked semiconductor laser according to Embodiment 3 of the present invention will be described. The mode-locked semiconductor laser according to the third embodiment is operated by the reproduction mode-locked method as in the second embodiment. However, the WGM resonator is a passive element, and the semiconductor laser section as an optical amplifier section is provided. The difference is that it is provided separately and used as an electromagnetic wave oscillator.

  FIG. 5 is a schematic plan view of the mode-locked semiconductor laser according to the third embodiment. As shown in FIG. 5, the mode-locked semiconductor laser 300 is the same as the mode-locked semiconductor laser 200 shown in FIG. 4, except that the optical waveguide 21, the polyimide 8, the electrode 9a, and the lead wire 10a are deleted, and the WGM resonator 2 is made WGM resonant. The optical waveguide 4 is replaced with the optical waveguide 31, and the semiconductor laser 32, the polyimide 33, the electrode 9f, and the lead wire 10f are added.

  The WGM resonator 30, the optical waveguide 31, and the semiconductor laser unit 32 have a high mesa shape. The WGM resonator 30 is formed in a ring shape like the WGM resonator 2 and has a circular outer peripheral portion. Further, the diameter and mesa width are the same as those of the WGM resonator 2, but unlike the WGM resonator 2, it is a passive element and has the same cross-sectional structure as the optical waveguide 5 shown in FIG. Further, the optical waveguide 31 is formed so as to be connected to the saturable absorber 7. Further, the semiconductor laser part 32 is formed so as to be connected to the optical waveguide 31. In addition, the polyimide 33 is formed so as to embed the saturable absorber 7, the light detector 23, and the semiconductor laser part 32. The electrode 9f is formed on the semiconductor laser section 32. A lead wire 10f is connected to the electrode 9f.

  The mesa width of the optical waveguide 31 is 2 μm. The size of the semiconductor laser unit 32 is 300 μm × 2 μm. The size of the electrode 9f is 300 μm × 75 μm.

  The semiconductor laser unit 32 is a distributed feedback (DFB) laser and has the same cross-sectional structure as the WGM resonator 2 shown in FIG. A lattice is formed. The optical waveguide 31 is a passive element and has the same cross-sectional structure as the optical waveguide 5 shown in FIG.

  Next, the case where the mode-locked semiconductor laser 300 is operated in the reproduction mode-locked mode will be described. First, when a DC voltage is applied between the n-side electrode and the p-side electrode of the semiconductor laser part 32 via the lead wire 10f and the electrode 9f and a driving current is passed, current is injected into the active layer of the semiconductor laser part 32. The active layer emits light in the 1.55 μm band. The emitted light is converted into an electrical signal by the photodetector 23 and output as an RF signal. The RF signal generated at this time is generated from a ring type resonator including the saturable absorber 7, the optical waveguides 3 and 22, the WGM resonator 30, the light detector 23, and the electrode 9d.

  On the other hand, like the mode-locked semiconductor laser 200, the light detection unit 23 receives the laser light from the end face of the optical waveguide 22, and uses the received light as the fundamental oscillation frequency determined by the delay time of the pulse laser light and its harmonics. To an electric signal including the modulated signal. This electric signal is conveyed to the saturable absorber 7 via the electrode 9d.

  On the other hand, the saturable absorber 7 is applied with a DC voltage as a reverse bias and functions as an optical shutter that is driven at the frequency of the modulation signal, similarly to the mode-locked semiconductor laser 200, thereby realizing reproduction mode synchronization. Is done. Here, in the case of this mode-locked semiconductor laser 300, the output terminal of the pulse laser beam is not particularly provided, but an electric signal including the fundamental oscillation frequency and its harmonic component is output as an RF output from the electrode 9d. Can do. Therefore, this mode-locked semiconductor laser 300 can be used as an electromagnetic wave oscillator that generates a high-purity electric signal with small time jitter.

  Since this mode-locked semiconductor laser 300 is also a ring type laser, its oscillation frequency is given by the above-described equation (6).

(Production method)
Next, a method for manufacturing the mode-locked semiconductor laser according to the first to third embodiments will be described. FIG. 6 is a schematic diagram for explaining a method of manufacturing mode-locked semiconductor lasers 100 to 300 according to the first to third embodiments.

  First, as shown in FIG. 6A, an MOCVD (Metal Organic Chemical Vapor Deposition) crystal growth apparatus is used to form a buffer layer 12, an active layer 13, and p-InP on a substrate 1 at a growth temperature of 600 ° C. The cladding layer 35 is successively crystal-grown.

  However, when the mode-locked semiconductor laser 300 according to the third embodiment is manufactured, a diffraction grating is formed in the cladding layer 35 only in a region where the semiconductor laser portion 32 is formed. A method for manufacturing a diffraction grating is described in, for example, Japanese Patent Application Laid-Open No. 2002-305350.

  Next, as shown in FIG. 6 (b), the active elements (WGM resonator 2, saturable absorber 7, light detector 23, semiconductor laser 32, etc., light amplifier, light detector) A mask M1 made of SiNx is formed in the region SA to be formed, and the cladding layer 35 and the active layer 13 in the region SP that is the other passive portion are removed by etching. Next, as shown in FIG. 6C, the core layer 18 of the optical waveguide made of InGaAsP and the clad layer 36 made of p-InP are formed by butt joint growth in the region SP removed by etching. Thereafter, as shown in FIG. 6D, the cladding layer 37 and the contact layer 15 made of p-InP are crystal-grown in both the region SA and the SP passive region. Next, as shown in FIG. 6E, the contact layer 15 is removed by etching only in the region SP.

  Thereafter, a high mesa shape having a width of 2 μm is formed by dry etching using the SiNx film as a mask. In this dry etching, etching is performed to a depth that penetrates the cladding layer 37, the cladding layer 35 and the cladding layer 36, the active layer 13, and the core layer 18 and reaches a part of the upper side of the buffer layer 12. Therefore, immediately after the etching, the side surface of the active layer 13 is exposed.

Next, the SiNx film used for the mesa etching is directly used as a crystal growth mask, and the protective layer 17 is grown on the side of the high mesa-shaped mesa. The protective layer 17 has a thickness of 0.1 μm at the side wall of the active layer 13. When the conductivity of the protective layer 17 is p-type and the material of the protective layer 17 is InP, the doping amount of a p-type dopant such as Zn is about 5 × 10 ¹⁷ cm ⁻³ .

  FIG. 7 is a cross-sectional view taken along the line CC ′ of FIG. 6E, and FIG. 8 is a cross-sectional view taken along the line DD ′ of FIG. 7 and 8, reference numeral 20 indicates a SiNx film. FIGS. 7 and 8 correspond to FIGS. 2 and 3, respectively. The cladding layers 35 and 37 constitute the upper cladding layer 14, and the cladding layers 36 and 37 constitute the upper cladding layer 19.

  Thereafter, an electrode structure is formed for each active element using positive polyimide as an insulating material. FIG. 9 is a schematic diagram for explaining an electrode forming method. In addition, FIG. 9 has shown the part of the AA 'line cross section of FIG. 1 as an example.

  First, as shown in FIG. 9A, after applying a positive resist R1, cueing of the mesa top is performed by RIE (Reactive Ion Etching) using oxygen plasma. Next, as shown in FIG. 9B, after applying a negative resist R2, photolithography is performed to open a window W1 having a width of about 4 μm in the mesa portion. Next, as shown in FIG. 9C, the SiNx film 20 is removed by RIE, and the p-side electrode 16 having an Au / AuZn structure is deposited. Thereafter, as shown in FIG. 9D, the p-side electrode 16 is formed into a desired shape by lift-off.

  Next, as shown in FIG. 9E, positive type polyimide 8 is applied and exposed to the cleavage site. Next, as shown in FIG. 9F, photolithography of the polyimide 8 is performed and cured at 150 ° C. for 30 minutes, and then further cured at 350 ° C. for 60 minutes. Next, as shown in FIG. 9G, an electrode 9a having a Ti / Pt / Au structure is formed by lift-off. Finally, as shown in FIG. 9H, the back surface of the substrate 1 is polished, and an n-side electrode 11 having an AuGeNi / Au structure is formed by vapor deposition. Thereafter, the predetermined portion is cleaved to complete the mode-locked semiconductor laser.

(Embodiment 4)
Next, a mode-locked semiconductor laser according to Embodiment 4 of the present invention will be described. The mode-locked semiconductor laser according to the fourth embodiment is used as an electromagnetic wave oscillator as in the third embodiment, except that the WGM resonator is formed as a line defect in the photonic crystal.

  FIG. 10 is a schematic plan view of a mode-locked semiconductor laser according to the fourth embodiment. As shown in FIG. 10, the mode-locked semiconductor laser 400 is the same as the mode-locked semiconductor laser 300 shown in FIG. , 47.

  The photonic crystal portion 40 has a structure in which a buffer layer, a core layer, an upper clad layer, and a SiNx film are sequentially stacked on the substrate 1 as in the other passive elements described above. Furthermore, air holes 41 are arranged in a triangular lattice pattern in the photonic crystal portion 40. This air hole 41 is formed deeper than at least the lower surface of the core layer. As a result, the air hole 41 forms a periodic structure with a two-dimensional refractive index in a plane perpendicular to the semiconductor lamination direction, and constitutes a photonic crystal.

  Further, in the photonic crystal formed by the air circular hole 41, hexagonal ring-shaped line defect WGM resonators 42 and linear line-defect optical waveguides 43, as line defect portions to guide light, 44 is formed. The line defect optical waveguides 43 and 44 are formed so as to be connected to the line defect WGM resonator 42, respectively. Further, a high-mesa optical waveguide 45 is formed so as to connect the line defect optical waveguide 43 and the optical waveguide 47. Similarly, a high mesa optical waveguide 46 is formed so as to connect the line defect optical waveguide 44 and the optical waveguide 48.

In the photonic crystal unit 40, the line defect WGM resonator 42 functions as a WGM resonator, and the line defect optical waveguides 43 and 44 function as optical waveguides that input and output light to and from the line defect WGM resonator 42. Reference numeral 45 indicates a diameter a _WGM of the line defect WGM resonator 42. Here, a _WGM is 1.2 mm.

  Hereinafter, the structural parameters of the photonic crystal part 40 will be described. FIG. 11 is a schematic plan view of the photonic crystal part 40. In FIG. 11, reference numerals 49, 50 and 51 denote the waveguide widths defined by the radius r of the air circular hole 41, the lattice constant a of the triangular lattice, and the distance between the centers of the air circular holes 41 located on both sides of the line defect part. W is shown. Here, a is set to 775 nm, the radius r is set to 0.29a, the hole depth is set to 0.58a, and W is set to 1.08a.

  The operation principle of the mode-locked semiconductor laser 400 is the same as that of the mode-locked semiconductor laser 300, and can be used as an oscillator that generates a high-purity electric signal with small time jitter. In particular, this mode-locked semiconductor laser 400 uses a photonic crystal line defect waveguide as a WGM resonator and has the structure parameters as described above. Therefore, due to the low group velocity effect in the line defect waveguide, Furthermore, time jitter is greatly reduced.

That is, for example, as described in the paper by Kiyota et al. “Laser and optical amplifier using low group velocity effect of photonic crystal line defect waveguide”, Furukawa Electric Time Report No. 118 (issued in July 2006), In a line defect waveguide formed in a photonic crystal, as the normalized wave number k / (2π / a) of guided light approaches 0.5, which is the end of the Brillouin zone, the group refractive index _ng Become very large. For example, in the photonic crystal part 40, when a is set to 775 nm and the above structural parameters are set, a numerical value of about 100 is obtained as the group refractive index _{ng at} a wavelength of 1.55 μm. Therefore, although the diameter of the line defect WGM resonator 42 is 1.2 mm, which is the same as that of the WGM resonator 2 of the first embodiment, the delay time is 31 times that of the WGM resonator 2, that is, 3. It can be 1 ns. Accordingly, the Q value of the WGM portion can also be set to 31 times, and a Q value of about 270 times that of the conventional semiconductor laser can be obtained. That is, the mode-locked semiconductor laser 400 is a mode-locked semiconductor laser that realizes a time jitter of about 1/270 of the conventional semiconductor laser.

  The photonic crystal part 40 can be manufactured as follows. That is, first, in the state of FIG. 6C, a SiN film is formed in the region SP as an etching mask. Next, the circular hole pattern of the photonic crystal formed using electron beam lithography and including the line defect portion is transferred to the SIN film. Thereafter, the semiconductor under the circular hole pattern is etched using ICP-RIE (Inductive Coupled Plasma-RIE) to form air circular holes 41. In addition, about the other part of the mode synchronous semiconductor laser 400, it can manufacture by the method similar to the mode synchronous semiconductor lasers 100-300.

  The mode-locked semiconductor lasers 300 and 400 according to the third and fourth embodiments can be used as a normal mode-locked semiconductor laser that outputs pulsed laser light if an optical waveguide for optical output is provided separately.

  In the fourth embodiment, the photonic crystal unit 40 is a passive element, but the WGM of the mode-locked semiconductor lasers 100 and 200 according to the first and second embodiments is used by replacing the core layer with an active layer as an active element. It can also be used in place of the resonator 2.

  In the fourth embodiment, the photonic crystal unit 40 is a passive element, but the WGM of the mode-locked semiconductor lasers 100 and 200 according to the first and second embodiments is used by replacing the core layer with an active layer as an active element. It can also be used in place of the resonator 2.

  In each of the above embodiments, the WGM resonator 2 and the semiconductor laser unit 32 perform optical amplification by current injection. However, a separate excitation laser may be provided and optical amplification may be performed by optical excitation instead of current injection. Good.

  In each of the above embodiments, the active layer 13 has an MQW-SCH structure, but may be a bulk layer. In each of the above embodiments, an InP-based semiconductor material is used as the compound semiconductor, but other semiconductor materials such as a GaAs-based material may be used, and there is no particular limitation.

  Further, the shape of each WGM resonator is not limited to a circular or hexagonal ring shape, but may be an ellipse or another polygonal ring shape or a cylindrical shape.

A compact WGM resonator is manufactured using microspheres having a diameter of about 10 to 10 ³ μm. Since the size of such a resonator is much larger than the wavelength of light, the loss of light due to the finite curvature of the WGM resonator can be reduced. As a result, a high Q value can be achieved using such a resonator. In such a WGM resonator, the main causes of light loss are light absorption by a dielectric material constituting the WGM resonator and light scattering due to unevenness of the sphere (for example, irregularities on the surface of the sphere). Certain microspheres with dimensions less than 1 mm have been demonstrated to exhibit very high Q values for light waves, with Q values exceeding 109 for quartz microspheres. Thus, when light energy is coupled to the WGM mode, it can travel at or near the equator of the sphere with a long delay time.

1 is a schematic plan view of a mode-locked semiconductor laser according to a first embodiment. It is the sectional view on the AA 'line in FIG. It is the BB 'sectional view taken on the line in FIG. 6 is a schematic plan view of a mode-locked semiconductor laser according to Embodiment 2. FIG. 6 is a schematic plan view of a mode-locked semiconductor laser according to Embodiment 3. FIG. It is a schematic diagram explaining the manufacturing method of the mode synchronous semiconductor laser which concerns on Embodiment 1-3. It is CC 'sectional view taken on the line of FIG.6 (e). It is the DD 'line sectional view of Drawing 6 (e). It is a schematic diagram explaining the electrode formation method. FIG. 6 is a schematic plan view of a mode-locked semiconductor laser according to a fourth embodiment. 3 is a schematic plan view of a photonic crystal unit 40. FIG. It is explanatory drawing explaining the principle of the system of a mode synchronization. It is explanatory drawing explaining the principle of the system of a mode synchronization. It is explanatory drawing explaining the structure of a WGM resonator exemplarily.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2, 30 WGM resonator 3-6, 21, 22, 31, 45-48 Optical waveguide 4a-6a, 6b, 21a, 21b, 22a End surface 7 Saturable absorption part 8, 33 Polyimide 9a-9f Electrode 10a- 10f Lead wire 11 n-side electrode 12 buffer layer 12a protrusion 13 active layer 14, 19 upper cladding layer 15 contact layer 16 p-side electrode 17 protective layer 18 core layer 20 SiNx film 23 photodetection unit 31 optical waveguide 32 semiconductor laser unit 35 ˜37 Clad layer 40 Photonic crystal part 41 Air hole 42 Line defect WGM resonator 43, 44 Line defect optical waveguide 49 Radius 50 Lattice constant 51 Waveguide width 100 to 400 Mode-locked semiconductor laser M1 Mask R1, R2 Resist SA, SP area W1 window

Claims (10)

On a substrate made of a compound semiconductor,
A whispering gallery mode resonator made of a compound semiconductor configured so that the whispering gallery mode resonator has an outer peripheral portion and light circulates inside the outer peripheral portion;
An optical amplifying unit made of a compound semiconductor connected directly or indirectly to the whispering gallery mode resonator and having an optical amplification gain;
An optical waveguide made of a compound semiconductor that is directly or indirectly connected to the whispering gallery mode resonator and inputs / outputs light to / from the whispering gallery mode resonator, and the whispering gallery mode resonator and the optical amplification unit And a mode-locked semiconductor laser having a mode-locking means for oscillating pulsed laser light by mode-locking. On a substrate made of a compound semiconductor,
The whispering gallery mode resonator has an active layer having an optical amplification gain and has an outer peripheral portion, and has a whispering gallery mode resonator made of a compound semiconductor configured to circulate light inside the outer peripheral portion,
The whispering gallery mode resonator is directly or indirectly connected to the whispering gallery mode resonator, and a compound semiconductor optical waveguide for inputting / outputting light to / from the whispering gallery mode resonator and the whispering gallery mode resonator are monolithically integrated. A mode-locked semiconductor laser having mode-locking means for oscillating laser light. The mode synchronization means includes:
A photodetection unit made of a compound semiconductor that receives a part of the pulse laser beam and converts it into an electrical signal;
Connected to the photodetection unit and driven by applying a fundamental oscillation frequency component or a harmonic component of the fundamental oscillation frequency determined by the delay time of the pulsed laser light included in the electrical signal through the electrode structure The mode-locked semiconductor laser according to claim 1, further comprising: a light modulation unit that modulates the laser beam to be emitted. The mode synchronization means includes:
A photodetection unit made of a compound semiconductor that receives a part of the pulse laser beam and converts it into an electrical signal;
An injection current of the pulse laser connected to the photodetection unit via an electrode structure with a fundamental oscillation frequency component or a harmonic component of the fundamental oscillation frequency determined by the delay time of the pulse laser light included in the electrical signal The mode-locked semiconductor laser according to claim 1, wherein the mode-locked semiconductor laser is modulated.   The mode-locked semiconductor laser according to claim 3, wherein the light modulator is made of a saturable absorber.   The mode-locked semiconductor laser according to claim 3, wherein the light modulation unit includes a Mach-Zehnder modulator.   The whispering gallery mode resonator including the active layer has a high mesa shape, and a protective layer having a band gap energy larger than the band gap energy of the active layer on at least the surface of the active layer on the side surface of the high mesa shape. The mode-locked semiconductor laser according to claim 1, wherein the mode-locked semiconductor laser is formed.   The outer periphery of the whispering gallery mode resonator is circular, and the optical waveguide is disposed so as to input and output light along a tangential direction of the outer periphery. The mode-locked semiconductor laser according to claim 1.   The whispering gallery mode resonator is formed by providing a line defect portion to guide light in a photonic crystal that forms a periodic structure having a two-dimensional refractive index in a plane perpendicular to the semiconductor lamination direction. The mode-locked semiconductor laser according to claim 1, wherein the line defect portion has a loop shape.   Furthermore, it has an output terminal which outputs the component of the fundamental oscillation frequency or the component of the harmonic contained in the electrical signal, and can extract electromagnetic waves to the outside. Mode-locked semiconductor laser for electromagnetic wave oscillators.

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