JPH04215489A - Short pulse light source device and short pulse light generation method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 and a short pulse light generation method for generating short optical pulses necessary for ultrahigh-speed optical communication using a mode-locking method.

0002

[Prior Art] As an optical device for generating short optical pulses by mode-locking, an integrated optical amplification region 1 and waveguide 2 as shown in Fig. 1 is the most basic and first realized optical device. Ta. The optical amplification region and the waveguide are arranged in series within a Fabry-Perot resonator formed by the crystal end face 3. In order to generate a short optical pulse, the period must be equal to the round trip time of the light generated in the optical amplification region 1 within the optical resonator, and the gain of the optical amplification region 1 must slightly exceed the threshold at its peak. A high frequency current is applied to the optical amplification gain. Under these operating conditions, each time the optical pulse passes through the amplification region 1, the center of the pulse will receive a greater gain than the two tails, so that the pulse will become sharper each time it is modulated. Furthermore, since the light intensity is strong near the peak of the pulse, the gain of the optical amplification region 1 decreases rapidly as the peak passes (gain saturation). Therefore, only the first half of the pulse is amplified, and the second half is also shaved off, making the pulse even sharper. It was reported by S. K. that a short pulse with a width of 4 ps was obtained using the mode-locked laser. Tucker elect
ronics Letters V. 25 p.
It was announced on 621.

FIG. 2 is a schematic diagram of a recently announced mode-locked laser. At the end of the waveguide 2 of the structure shown in FIG. 1, a saturable absorption region 4 is added. The saturable absorption region 4 is a laser structure to which a direct current below a threshold is applied, and the waveguide 5 is an active waveguide to which a direct current above the threshold is applied. In this device, in addition to the same pulse forming mechanism in the optical amplification region 1 as described above, the saturable absorption region 4
The pulse steepening effect also works. This is because when a pulse enters the saturable absorption region 4, the first half of the pulse is scraped off by the absorption of the saturable absorption region 4, whereas the region near the peak and the second half of the pulse are affected by the strong light near the peak. , the absorption is saturated and the pulse passes through as is. P. et al. shows that this structure provides a pulse width of 1.4 ps, which is shorter than the structure shown in FIG. A.
Applied Physics
Letters V. 56 p. It was announced on 111.

0004

By the way, the spectrum of laser light generated by a mode-locked laser consists of a large number of vertical modes, similar to the Fabry-Perot laser. In a Fabry-Perot laser, the phases of the vertical modes are scattered and not aligned, but when the phases are aligned, a short optical pulse is generated. In conventional mode-locked lasers, only a few vertical modes are aligned in phase to generate optical pulses. That is, theoretically, the pulse width of a mode-locked laser is
It can be made much shorter than the conventional one. Conventionally, the reason why the pulse width was wider than the theoretical value was because the steepening effect of the pulse was insufficient, and in order to shorten the pulse,
The pulse steepening effect due to gain saturation and saturable absorption must be strengthened.

The present invention provides a short pulse light source device and a short pulse light source device that can strengthen the pulse steepening effect in the optical amplification region and the saturable absorption region and obtain light with 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 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 are located behind the saturable absorption region 4.
and the second waveguide 7 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. do. In order to obtain a short pulse by applying a high-frequency current to the optical amplification region 1 in this device, the lengths of the waveguides on both sides are selected so that the optical amplification region 1 is located exactly at the center of the device. In order to obtain short pulses with this structure, as in the short pulse light generation method shown in claim 3 of the present invention, the period is half the round trip time within the optical resonator of the light generated in the optical amplification region 1. A high-frequency current with a period of 1 or an integer fraction of the period is applied to the optical amplification region 1 with DC current superimposed as necessary (hybrid mode locking). by this,
Since the optical pulse is amplified twice before it leaves the saturable absorption region 4 and enters the saturable absorption region 4 again, a strong optical pulse is obtained. Therefore, a saturable absorber with strong absorption can be used. Generally, the stronger the saturable absorber, the stronger the degree of absorption saturation, and therefore the easier it is to obtain a short pulse. Furthermore, the fact that the pulse is steepened twice due to gain saturation in the optical amplification region 1 while the pulse goes around this element also contributes to shortening the pulse. In order to use the device according to claim 1 of the present invention in a state where a direct current is applied to the optical amplification region 1 (passive mode locking), the value of the current injected into the optical amplification region 1 or the position of the optical amplification region , optical amplification region 1
The gain is selected so that the gain saturated by the passage of the first pulse can be sufficiently recovered before the pulse that has passed is reflected by the reflecting surfaces 9 and 8 at the right or left end and returns. With this structure, the pulse is amplified twice while it goes around the element once, so a stronger pulse enters the saturable absorption region 4 than when the second waveguide 7 is not provided. , passive mode locking is efficiently achieved. Furthermore, the fact that the pulse is steepened twice due to gain saturation in the optical amplification region 1 while the pulse goes around this element also contributes to shortening the pulse. As claimed in claim 2 of the present invention, when the reflective surface 8 in contact with the saturable absorber is coated with a high reflective material to make it a highly reflective surface, interference occurs between the incident wave and the reflected wave while the pulse is being reflected. occurs, and the interference pattern generated at this time transiently forms a diffraction grating within the saturable absorber, strengthening the saturable absorption effect and shortening the pulse. The above structure differs from conventional mode-locked lasers in the following points:
Claim 1 of the present invention is that waveguides are provided on both sides of the optical amplification region 1, and Claim 2 of the present invention is that the reflective surface 8 in contact with the saturable absorber is coated with a highly reflective film. The point is to increase the reflectance by coating. Claim 3 of the present invention
The method of operation of the device according to claim 4 of the present invention is unique to this device. When driving a conventional mode-locked laser with a high-frequency current, the period of the high-frequency current can be set to the time it takes for a pulse to go around the device or an integer fraction thereof, but in the device of the present invention, the period of the high-frequency current is set to half the circuit time or an integer fraction thereof. must be one period. Furthermore, when driving with direct current, in the conventional type, it is sufficient that the gain is recovered during one cycle of the pulse, but in the short pulse light generation method of the present invention, the gain is recovered twice during one cycle of the pulse. Make it.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment, a second embodiment, and a third embodiment of the present invention will be described below with reference to the drawings.

(1) First embodiment. FIG. 4 shows an implementation diagram when mode locking is applied by high-frequency current injection. The element length is 5 mm. The optical amplification region 1 located at the center of the element is made of graded index InGaAs/InGaA.
It has an sP quantum well laser structure, and the length of the optical amplification region 1 is 300 μm. The thickness of the InGaAs well layer of the quantum well 10 is 10 nm, and the thickness of InGaAsP (absorption short wavelength 1
．． (2 μm) The thickness of the 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 top of the epitaxial layer is an InGaAs cap layer 12. The optical waveguides 6, 7 are active waveguides and apply a direct current slightly above the threshold to the laser structure described above. The saturable absorption region 4 is made to operate 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 each region is achieved by etching away the cap layer 12 between the regions. In the first example,
By applying a high frequency current of 16 GHz to the optical amplification region 1, pulses with a repetition frequency of 8 GHz and a width of 1 ps are obtained. This value of 16 GHz corresponds to half the period of the round trip time of light within the optical resonator.

(2) Second embodiment. When a high reflection coating of SiO2 with a reflectance of 99% is applied to the reflecting surface 8 in contact with the saturable region of the mode-locked laser shown in the first embodiment, the interference of the reflected waves themselves causes the inside of the saturable absorber to A diffraction grating is formed transiently, and the pulse width is 1p.
It became less than s.

(3) Third embodiment. In FIG. 4, optical amplification region 1
Mode locking can be applied by injecting a direct current instead of a high-frequency current. The element length is 5 mm. The optical amplification region 1 differs from the first embodiment in that the midpoint of the optical amplification region is located at 3/5 of the total device length from the reflecting surface 8 in contact with the saturable absorption region 4. The optical amplification region 1 is made of graded index InGaAs/InGaAsP.
It is a quantum well laser, and the length of the amplification region is 300μ.
It is m. The thickness of the InGaAs well layer is 10 nm, and the thickness of the InGaAs well layer is 10 nm.
GaAsP (absorption short wavelength 1.2 μm) barrier layer thickness 4n
m, and the number of layers is 12 layers and 11 layers, respectively. The cladding layer 11 is InP, and the top layer of the epitaxial layer is InGaA.
s cap layer 12. That is, the epitaxial layer structure is the same as in the first embodiment. The optical waveguides 6, 7 are active waveguides and apply a direct current slightly above the threshold to the laser structure described above. The saturable absorption region 4 is made to operate 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 each region is achieved by etching away the cap layer 12 between the regions. Third
In the example, by adjusting the direct current applied to the optical amplification region 1, pulses with a repetition frequency of 8 GHz were obtained.

Although active waveguides are used in the above embodiments, similar effects can be obtained with mixed crystal waveguides formed by mixing the laser structure or passive waveguides formed by a combination of etching and regrowth. . In addition, the resonator is not a Fabry-Perot resonator using crystal end faces, but a diffraction grating 1 as shown in Figure 5.
3 can also be used.

0012

As explained above, the mode-locked laser according to the present invention can be used in both hybrid and passive mode-locked modes, and in either case, short pulses are achieved by the mechanism described below. In the mode-locked laser according to claim 1 of the present invention, the optical pulse is amplified twice before it exits the saturable absorption region and enters the saturable absorption region again, so that a strong optical pulse can be obtained. Therefore, a saturable absorber with strong absorption can be used. Generally, the stronger the saturable absorber, the stronger the degree of absorption saturation, and therefore the easier it is to obtain a short pulse. Also, while the pulse goes around this element, the pulse is steepened twice due to gain saturation in the amplification region. Due to the above two effects, the pulse can be made shorter than that of conventional mode-locked lasers. Furthermore, as in claim 2 of the present invention, when the reflective surface in contact with the saturable absorber is made a highly reflective surface, interference occurs between the incident wave and the reflected wave while the pulse is being reflected.
The interference pattern generated at this time transiently forms a diffraction grating within the saturable absorber, achieving collision mode locking and further shortening the pulse.

[Brief explanation of the drawing]

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

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

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

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

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

[Explanation of symbols]

1 Optical 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 the saturable absorption region 9 Light reflection in contact with the second waveguide Surface 10 InGaAs/InGaAsP quantum well 1
1 InP cladding 12 InGaAs cap layer 13 diffraction grating

Claims (4)

[Claims] Claim 1: A saturable absorption region, a first waveguide, an optical amplification region, and a second waveguide are integrated in series in an optical resonator formed in a semiconductor crystal. The saturable absorption region is in contact with one of the light reflection surfaces or the diffraction grating for light reflection, and the first waveguide, the light amplification region, and the second waveguide are located behind the saturable absorption region. A semiconductor laser device characterized in that the second waveguide is arranged in series in this order and is in contact with the other light reflecting surface of the optical resonator or a diffraction grating for light reflection. 2. The semiconductor laser device according to claim 1, wherein the light reflecting surface in contact with the saturable absorption region is coated with a highly reflective film. 3. An optical amplification region is disposed in the center of the semiconductor laser device, and a high-frequency current having a period of half the round trip time of light generated in the optical amplification region within the optical resonator or a period of one integer fraction of the period is provided. 3. The method of generating short pulse light according to claim 1, wherein mode-locking is achieved by applying a high-frequency current of 1 to the optical amplification region, superimposing a direct current as necessary. 4. The method for generating short pulse light according to claim 1 or 2, characterized in that a direct current is applied to the amplification region.