JP2823094B2 - Short pulse light source device and method of generating short pulse light - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short-pulse light source device for generating short optical pulses required for ultra-high-speed optical communication by a mode-lock method and a method of generating short-pulse light.

[0002]

2. Description of the Related Art As an optical element for generating a short optical pulse by mode-locking, an element in which an optical amplification region 1 and a waveguide 2 as shown in FIG. 1 are integrated is the most basic and realized first. Was. The optical amplification region and the waveguide are arranged in series in a Fabry-Perot resonator constituted by the crystal end face 3. In order to generate a short light pulse, the light generated in the optical amplification region 1 has a period equal to the round trip time in the optical resonator, and the gain of the optical amplification region 1 slightly exceeds the threshold value at the peak. A high frequency current is applied to the optical gain.
Under such operating conditions, each time an optical pulse passes through the amplification region 1, the center of the pulse will receive a gain greater than both tails, and thus will become sharper each time the pulse is modulated. Further, since the light intensity is high near the peak of the pulse, the gain of the optical amplification region 1 sharply decreases by passing the peak (gain saturation). Accordingly, only the first half of the pulse is amplified and the second half is removed, so that the pulse becomes sharper. The fact that a short pulse with a width of 4 ps was obtained by the mode-locked laser was confirmed by Tucker is Elect
ronics Letters V. 25 p. 621
Has announced.

FIG. 2 is a schematic diagram of a recently announced mode-locked laser. A saturable absorption region 4 is added to the end of the waveguide 2 having the structure shown in FIG. The saturable absorption region 4 has a laser structure to which a DC current equal to or less than a threshold is applied, and the waveguide 5 is an active waveguide to which a DC current equal to or more than the threshold is applied. In this device, in addition to the same pulse forming mechanism of the optical amplification region 1 as described above, the saturable absorption region 4
The steepening action of the pulse also works. This is because when a pulse is incident on the saturable absorption region 4, the first half of the pulse is shaved off by absorption in the saturable absorption region 4, whereas in the vicinity of the peak and the second half of the pulse, strong light near the peak is generated. Because the absorption is saturated and the pulse passes as it is. The fact that this structure provides a pulse width of 1.4 ps, which is shorter than the structure shown in FIG. A.
Morton et al., Applied Physics
Letters V. 56 p. It was announced on 111.

[0004]

By the way, the spectrum of the laser light generated by the mode-locked laser is composed of many vertical modes, like the Fabry-Perot laser. In the Fabry-Perot laser, the phases of the vertical modes are not uniform and not aligned, but when the phases are aligned, short optical pulses are generated. In a conventional mode-locked laser, the phases are aligned only between a few vertical modes, and light pulses are generated. That is, theoretically, the pulse width of the mode-locked laser can be made sufficiently shorter than that of the conventional one. Conventionally, the pulse width was wider than the theoretical value because the steepening action of the pulse was insufficient.To shorten the pulse, the steepening action by gain saturation and saturable absorption had to be strengthened. No.

The present invention provides a short pulse light source device and a short pulse light source device capable of enhancing the pulse steepening action in an optical amplification region and a saturable absorption region and obtaining light having a shorter pulse width than a conventional semiconductor mode-locked laser. The purpose is to provide a method for generating pulsed light.

[0006]

FIG. 3 shows a schematic diagram of a device according to claim 1 of the present invention. A saturable absorption region 4, a first waveguide 6, an optical amplification region 1, and a second waveguide 7 are integrated in series in an optical resonator formed in a semiconductor crystal. The saturable absorption region 4 is in contact with the light reflection surface 8 or the diffraction grating for light reflection, and the first waveguide 6 and the light amplification region 1 and the second Are arranged in series in this order, and the second waveguide 7 is in contact with the other light reflection surface 9 of the optical resonator or the diffraction grating for light reflection. In this element, in order to obtain a short pulse by applying a high-frequency current to the light amplification region 1, the lengths of the waveguides on both sides are selected so that the light amplification region 1 is located exactly at the center of the device. In order to obtain a short pulse by this structure, as in the method of generating a short pulse light according to claim 3 of the present invention, the period of half the round trip time of the light generated in the optical amplification region 1 in the optical resonator is required. Or a high-frequency current having a period equal to an integral number of the period is superimposed on a DC current as necessary and applied to the optical amplification region 1 (hybrid mode synchronization). As a result, the light pulse is amplified twice before leaving the saturable absorption region 4 and entering the saturable absorption region 4 again, so that a strong light pulse is obtained. Therefore, a saturable absorber having strong absorption can be used. In general, a saturable absorber having a higher absorption has a higher degree of saturation of the absorption, and thus a shorter pulse is easily obtained. Also, twice while the pulse goes around this element,
Receiving a steep pulse due to gain saturation in the optical amplification region 1 also contributes to shortening the pulse. Claim 1 of the present invention
In order to use the device described in a state where a DC current is applied to the optical amplification region 1 (passive mode locking), the optical amplification region 1
The value of the current injected into the optical amplification region or the position of the optical amplification region is determined by determining whether the pulse passing through the optical amplification region 1 is the right or left reflection surface 9,8.
It is selected so that the gain saturated by the passage of the first pulse can be sufficiently recovered before being reflected by and returning. With such a structure, the pulse is amplified twice while circulating in the element once, so that a stronger pulse than in the absence of the second waveguide 7 is generated.
And passive mode locking is efficiently achieved. Further, the pulse abruptly caused by gain saturation in the optical amplification region 1 twice while the pulse makes one round of this element also contributes to the shortening of the pulse. As in claim 2 of the present invention,
If the reflection surface 8 in contact with the saturable absorber is made to be a high reflection surface by coating it with high reflection, interference occurs between the incident wave and the reflection while the pulse is being reflected. A diffraction grating is transiently formed in the saturable absorber to enhance the saturable absorption effect and shorten the pulse. The point that the above structure is different from the conventional mode-locked laser is that
The present invention is characterized in that the reflection surface 8 in contact with the saturable absorber is made to have a high reflectance by a high reflection film coating. The operation method of the device according to claim 3 of the present invention and claim 4 according to the present invention is as follows.
Specific to this element. When a conventional mode-locked laser is driven by a high-frequency current, the period of a pulse or one integer fraction of the cycle of the element may be set as the period of the high-frequency current. The cycle must be one of Also, in the case of driving with a direct current, in the conventional type, it is sufficient that the gain is recovered during one round of the pulse, but in the short pulse light generation method of the present invention, the gain is recovered twice during the round of the pulse. To

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first, second and third embodiments of the present invention will be described below with reference to the drawings.

(1) First Embodiment FIG. 4 shows an embodiment when mode locking is performed by injection of high-frequency current. The element length is 5 mm. Light amplification region 1 located at the center of the device
Is the graded index InGaAs / InGa
It has an AsP quantum well laser structure, and the length of the light amplification region 1 is 300 μm. The thickness of the InGaAs well layer of the quantum well 10 is 10 nm, the thickness of the InGaAsP (absorption short wavelength 1.2 μm) barrier layer is 4 nm, and the number of layers is 12 and 11, respectively. The cladding layer 11 is made of InP, and the uppermost part of the epi layer is an InGaAs cap layer 12. The optical waveguides 6 and 7 are active waveguides, and apply a DC current slightly higher than the threshold to the above-described laser structure. The saturable absorber region 4 is operated as a saturable absorber by applying a reverse bias to the same laser structure. The length of the saturable absorption region 4 is 25 μm. Electrical isolation between the regions is achieved by removing the cap layer 12 between the regions by etching. In the first embodiment, a repetition frequency of 8 GHz and a width of 1 ps are applied to the optical amplification region 1 by applying a high-frequency current of 16 GHz.
Is obtained. This value of 16 GHz corresponds to a half cycle of the round trip time of light in the optical resonator.

(2) Second embodiment. The reflection surface 8 where the saturable region of the mode-locked laser shown in the first embodiment is in contact with the saturable region is coated with a high reflection coating of 99% reflectivity by SiO2. A diffraction grating is formed transiently and the pulse width is 1p
s or less.

(3) Third embodiment. In FIG. 4, the optical amplification region 1
The mode locking can be performed by injecting a DC current instead of a high-frequency current. The element length is 5 mm. The light amplification region 1 is different from the first embodiment in that the midpoint of the light amplification region is located at ３ of the total element length from the reflection surface 8 in contact with the saturable absorption region 4. The optical amplification region 1 has a graded index of InGaAs / InGaAsP.
It is a quantum well laser and the length of the amplification region is 300 μm
It is. The thickness of the InGaAs well layer is 10 nm,
aAsP (absorption short wavelength 1.2 μm) barrier layer thickness 4 nm
And the number of layers is 12 and 11, respectively. The cladding layer 11 is made of InP, and the top of the epi layer is made of InGaAs.
The cap layer 12 is formed. That is, the epilayer structure is the first
This is the same as the embodiment. The optical waveguides 6 and 7 are active waveguides, and apply a DC current slightly higher than the threshold to the above-described laser structure. The saturable absorber region 4 is operated as a saturable absorber by applying a reverse bias to the same laser structure. The length of the saturable absorption region 4 is 25 μm.
Electrical isolation between the regions is achieved by removing the cap layer 12 between the regions by etching. Third
In the example, a pulse with a repetition frequency of 8 GHz was obtained by adjusting the DC current applied to the optical amplification region 1.

In the above embodiment, an active waveguide is used. However, a similar effect can be obtained with a mixed crystal waveguide in which the laser structure is mixed or a passive waveguide formed by combining etching and regrowth. . Also, the resonator is not a Fabry-Perot resonator using a crystal end face, but as shown in FIG.
3 can be used.

[0012]

As described above, the mode-locked laser according to the present invention can be used in both the hybrid mode and the passive mode-locked mode. In any case, a short pulse is achieved by the mechanism described below. In the mode-locked laser according to claim 1 of the present invention,
Since the light pulse is amplified twice before exiting the saturable absorption region and re-entering the saturable absorption region, a strong light pulse is obtained. Therefore, a saturable absorber having strong absorption can be used. In general, a saturable absorber having a higher absorption has a higher degree of saturation of the absorption, and thus a shorter pulse is easily obtained. In addition, the pulse undergoes sharpening due to gain saturation in the amplification region twice during one round of the element. With the above two effects, a shorter pulse can be achieved as compared with the conventional mode-locked laser. Further, when the reflection surface in contact with the saturable absorber is made a high reflection surface as in claim 2 of the present invention, interference occurs between the incident wave and the reflection while the pulse is being reflected. Due to the interference pattern, a diffraction grating is formed transiently in the saturable absorber, thereby achieving collision mode locking and further shortening the pulse.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a first mode-locked laser.

FIG. 2 is a schematic diagram of a first mode-locked laser.

FIG. 3 is a schematic diagram of a mode-locked laser according to the present invention.

FIG. 4 is an implementation diagram of short pulse generation according to the present invention.

FIG. 5 is a schematic view of a mode-locked laser using a diffraction grating according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light amplification region 2 waveguide 3 crystal end face 4 saturable absorption region 5 active waveguide 6 first waveguide 7 second waveguide 8 light reflection surface in contact with saturable absorption region 9 light reflection in contact with second waveguide Surface 10 InGaAs / InGaAsP quantum well 11 InP clad 12 InGaAs cap layer 13 Diffraction grating

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-97889 (JP, A) JP-A-58-202581 (JP, A) JP-A-1-168086 (JP, A) JP-A-4- 2190 (JP, A) JP 61-509995 (JP, A) Electron. Lett. 25 [10] (1989) 621-622 Appl. Phys. Lett. 56 [2] (1990) 111-113 Appl. Phys. Lett. 57 [8] (1990) 759-761 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18 H01S 3/096 H01S 3/103 H01S 3/133

Claims (4)

(57) [Claims] 1. An optical resonator formed in a semiconductor crystal, comprising:
A saturable absorption region, a first waveguide, an optical amplification region, and a second waveguide are integrated in series, and are in contact with one light reflection surface of an optical resonator or a diffraction grating for light reflection. There is a saturable absorption region, and after the saturable absorption region, the first waveguide, the optical amplification region, and the second waveguide are arranged in series in this order, and the second waveguide is an optical resonator. A semiconductor laser device which is in contact with another light reflection surface or a diffraction grating for light reflection. 2. The semiconductor laser device according to claim 1, wherein the light reflection surface in contact with the saturable absorption region is coated with a high reflection film. 3. A high-frequency current having a cycle of half the reciprocating time of light generated in the optical amplification area in the optical resonator or a cycle of an integral number of the cycle, wherein the optical amplification area is arranged at the center of the semiconductor laser device. 3. The method for generating short pulse light according to claim 1, wherein the high-frequency current is superimposed with a direct current as necessary and applied to the optical amplification region to achieve mode locking. 4. The method for generating short pulse light according to claim 1, wherein a direct current is applied to the amplification region.