JP2000269594A - Integrated mode-locked semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode-locked semiconductor laser providing an accurate repetition film with less jitters. SOLUTION: In an integrated mode-locked semiconductor laser having an optical resonator 2, defined by an optical guide with an electric field absorptive modulation region 3 at a part, the optical guide is formed like a ring and has an optical isolator 6 for limiting the revolving direction of an optical pulse travelling in the ring optical guide. The isolator 6 is composed of a first optical guide through which a light in a first mode passes and a second optical guide for converting the light in the first mode into a light in a second mode, and the first and second modes are mutually perpendicular with respect to the oscillating direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to an integrated mode-locked semiconductor laser for generating an ultrashort optical pulse used in an ultrahigh-speed optical communication system such as a soliton optical transmission system.

[0002]

2. Description of the Related Art An integrated mode-locked semiconductor laser has been proposed as one of mode-locked semiconductor lasers for generating an ultrashort optical pulse used in an ultra-high-speed optical communication system or the like.
FIG. 5 is a sectional view showing the structure of a conventional integrated mode-locked semiconductor laser.

This integrated mode-locked semiconductor laser has a D
A waveguide type semiconductor laser having a BR structure, wherein an optical waveguide 53 is provided on a semiconductor substrate 51 with an active layer 52 interposed therebetween to form a feedback portion of an optical resonator. A diffraction grating 54 is provided. This optical resonator has an electro-absorption modulation region (hereinafter referred to as E
A modulation region 55) is provided, and a saturable absorption region 56 is provided near the laser light emission surface. In the saturable absorption region 56, light absorption in the region is increased according to the applied reverse bias voltage, and the phase between the laser oscillation longitudinal modes is synchronized by a self-absorption modulation effect. The EA modulation area 55 is for synchronizing the short pulse generation timing by the mode synchronization with an externally supplied signal clock, and the light absorption of the area increases according to the level of the supplied signal voltage. It is supposed to.

In the integrated mode-locked semiconductor laser described above, an electric signal (usually,
A sinusoidal voltage having a frequency corresponding to the reciprocal of the time during which the optical pulse makes one round trip in the optical resonator 53 is supplied. ) Is the weakest (light absorption is minimized) and the light pulse
By passing through the modulation region 55, the repetition frequency and phase of the generated optical pulse are synchronized with the frequency and phase of an externally supplied electric signal. By this hybrid mode locking operation, it is possible to obtain an optical pulse 57 having a small jitter and an accurate repetition frequency.

[0005]

In the case of the conventional integrated mode-locked semiconductor laser shown in FIG. 5, the light wave passes through the EA modulation region 55 twice during one round trip in the optical resonator. In the area 55, the light wave is simultaneously modulated in both the forward path and the return path to form a light pulse. If the phases of the optical pulses formed in these two paths are shifted, it becomes difficult to perform hybrid mode locking. By arranging the EA modulation region at the end of the feedback portion of the optical resonator, it is possible to minimize the phase difference between the optical pulses formed on the forward path and the return path. In the case of laser, due to its operating principle,
DBR region (diffraction grating 54) constituting the reflection mirror of the optical resonator
Region) must be placed at both ends of the resonator,
It is difficult to arrange the EA modulation region at the end of the optical resonator. As described above, the conventional integrated mode-locked semiconductor laser has a problem that the jitter of the output laser light increases because of the structure in which the hybrid mode-locking is not easily performed.

It is an object of the present invention to solve the above-mentioned problems and to provide an integrated mode-locked semiconductor laser having a small jitter and an accurate repetition frequency.

[0007]

To achieve the above object, the present invention provides an integrated mode-locked semiconductor laser in which an optical resonator is formed by an optical waveguide partially provided with an electro-absorption modulation region. The optical waveguide is formed in a ring shape, and the optical waveguide includes an optical pulse circling direction restricting means for restricting a circulating direction of an optical pulse traveling in the ring-shaped optical waveguide.

[0008] In the above case, the light pulse circling direction limiting means may include a first optical waveguide through which light of the first waveguide mode of the first and second waveguide modes whose vibration directions are orthogonal to each other passes. And a second optical waveguide for converting the light of the first waveguide mode into the light of the second waveguide mode, wherein the optical isolator means is formed in the ring-shaped optical waveguide. Good.

A plurality of the electro-absorption modulation regions are arranged at equal intervals in the ring-shaped optical waveguide, and the light pulse circling direction limiting means includes a plurality of electro-absorption modulation regions, and an electric field provided to these electro-absorption modulation regions. Supply means for applying a signal, wherein the supply means is configured such that a phase of an electric signal applied to each of the electroabsorption modulation regions is synchronized with a phase of an optical pulse orbiting the ring-shaped optical waveguide. May be used.

In any one of the integrated mode-locked semiconductor lasers described above, the ring-shaped optical waveguide is a quadrangular optical waveguide in which the waveguide is bent at a right angle. A configuration in which a mirror having an angle of 45 ° is provided may be employed.

Further, the ring-shaped optical waveguide may be constituted by two Y-branch optical waveguides.

In one of the above-mentioned integrated mode-locked semiconductor lasers, a gain region for amplifying an optical pulse circulating in the optical waveguide is provided in the ring-shaped optical waveguide.
A saturable absorption region for achieving phase synchronization between laser oscillation longitudinal modes and a diffraction grating region for selecting a laser oscillation wavelength may be formed.

(Operation) In a conventional integrated mode-locked semiconductor laser using a waveguide type optical resonator having a DBR structure, the modulation in the electroabsorption modulation region is performed on a light wave reciprocating in the resonator. And the optical pulse generation timing on the return path are different. For this reason, the hybrid mode synchronization is hardly applied. On the other hand, in the present invention as described above, the optical resonator is constituted by the ring-shaped optical waveguide, and the modulation in the electroabsorption modulation region is performed on the light wave circulating in the optical resonator. Therefore, the generation timing of the optical pulse formed by the modulation is the same. Therefore, in the present invention, unlike the conventional case, optical pulses having different phases are not formed in the optical resonator.

Further, in the present invention, the circulating direction of the optical pulse circulating in the ring-shaped optical waveguide is restricted to either the clockwise direction or the counterclockwise direction by the optical pulse circling direction restricting means. Therefore, in the electro-absorption modulation region, the modulation is performed only on the light wave circling in one direction. Therefore, in the present invention, oscillation occurs only in the resonance mode in one of the rotation directions.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings.

(Embodiment 1) FIGS. 1A and 1B show the structure of an integrated mode-locked semiconductor laser according to a first embodiment of the present invention, wherein FIG. 1A is a plan view and FIG. 1B is XX 'of FIG. It is sectional drawing.

This integrated mode-locked semiconductor laser has the following characteristics:
This is a structure in which a ring-shaped optical resonator 2 is formed on a semiconductor substrate 1 of nP or the like. The optical resonator 2 includes a semiconductor waveguide layer having a function of guiding and amplifying light on the semiconductor substrate 1 (a structure in which an active layer is sandwiched between optical waveguides from above and below) by, for example, MOVPE.
(Metal Organic Vapor Phase Epitaxy), and then the semiconductor waveguide layer is processed into a ring-shaped ridge waveguide by dry etching. The feedback is obtained by forming a closed loop for the light wave by the ring-shaped optical waveguide.

An EA modulation region 3, a gain region 4, an SA region 5, an optical isolator 6, and a diffraction grating 8 are sequentially arranged in a clockwise direction in a ring-shaped optical waveguide constituting the optical resonator 2. A Y-branch optical waveguide 7 is provided between the optical isolator 6 and the diffraction grating region 8 for extracting an optical pulse orbiting the optical waveguide.

In the SA region 5, the light absorption in the region is increased in accordance with the applied reverse bias voltage, and the SA region 5 is synchronized with the phase between the oscillation longitudinal modes of the laser by the self-absorption modulation effect. Take it. The EA modulation area 3 is for synchronizing the short pulse generation timing by the mode synchronization with a signal clock supplied from the outside, and is configured so that a sine wave voltage 9 is applied. Accordingly, the light absorption of the region increases. The wiring for supplying the sine wave voltage 9 can be formed on the semiconductor substrate 1 by a microstrip line.

The gain region 4 is a region for amplifying light traveling in the optical waveguide. The diffraction grating 8 is for selecting the wavelength of the oscillating light pulse, and is provided with a quarter wavelength phase shift region at the center thereof, so that only the light of the Bragg wavelength corresponding to the period of the diffraction grating can be obtained. It has the function of a transmission wavelength filter that selectively passes.

Normally, in a ring-shaped optical resonator, clockwise and counterclockwise traveling waves are formed, and these traveling waves have independent resonance modes. However, in order to perform ideal mode locking, It is necessary to oscillate in only one of the resonance modes. In this embodiment, one of the resonance modes can be selected by the optical isolator 6. Hereinafter, the configuration of the optical isolator 6 will be briefly described.

The optical waveguide is TE mode light or TM light.
It is possible to have a structure that allows passage of either mode light, and by depositing a magnetic material on the waveguide portion, TE-TM mode conversion by the magneto-optical effect can be performed. In the present embodiment, an optical isolator is configured using this fact. That is, the optical isolator 6
A first optical waveguide configured to pass either TE-mode light or TM-mode light, and a second optical waveguide that performs TE-TM mode conversion on the light passing through the first optical waveguide. And an optical waveguide (a magnetic material deposited on the waveguide portion).

As an example, in FIG. 1A, as the optical isolator 6, a TE mode optical waveguide as a first optical waveguide and a TE-mode optical waveguide as a second optical waveguide are arranged in a clockwise direction.
The operation of the optical isolator 6 when the TM mode conversion optical waveguides are provided in order will be described. In the light traveling in the clockwise direction, first, only the TE mode light passes through the TE mode optical waveguide, and the TE mode light is converted into the TM mode light by the TE-TM mode conversion optical waveguide. .
On the other hand, light traveling in the counterclockwise direction is converted into TM mode light by the TE-TM mode conversion optical waveguide, but the TM mode light does not pass through the TE mode optical waveguide. In this manner, the direction in which the optical pulse circulates around the ring-shaped optical resonator 2 is restricted.

The most significant feature of the integrated mode-locked semiconductor laser according to this embodiment is that the optical resonator 2 is formed in a ring shape and controls the direction in which the optical pulse circulates in the optical resonator 2. That is, the optical isolator 6 is provided. According to this configuration, the light wave circulating in the optical resonator 2 passes through the EA modulation region 3 arranged in the optical resonator 2 only once each time. Therefore, assuming that the sine wave voltage 9 having a frequency corresponding to the reciprocal of the time during which the optical pulse circulates in the optical resonator 2 (the time required for one round) is supplied to the EA modulation region 3, the sine wave voltage 9 There is only one position where the voltage is lowest at a time corresponding to one period of the optical resonator 2.

Next, the operation of this embodiment will be described. The basic operation method is the same as that of the conventional integrated mode-locked semiconductor laser. First, a reverse bias voltage (not shown) is applied to the SA region 5, and a forward current (not shown) is injected into the gain region 4 to set a state where passive mode locking occurs. Next, when a sine wave voltage 9 corresponding to the circulating frequency of the light pulse is applied to the EA modulation region 3, a hybrid mode locking operation is obtained. By this hybrid synchronous operation, an optical pulse 10 with a small jitter and an accurate repetition frequency is extracted from the Y-branch optical waveguide 7.

In the above-described integrated mode-locked semiconductor laser according to the present embodiment, the diameter of the optical resonator 2 is determined by the repetition frequency of the optical pulse. Here, the repetition frequency of the optical pulse corresponds to the reciprocal of the time during which the optical pulse makes one round in the ring-shaped optical resonator 2. For example, 10GH
In order to obtain a repetition frequency of z, the rotation time of an optical pulse that makes one round in the ring-shaped optical resonator 2 is 100 ps. Therefore, the diameter of the ring-shaped optical resonator 2 may be set to about 2.74 mm.

The positional relationship among the gain region 4, the SA region 5, the optical isolator 6, and the diffraction grating region 8 arranged in the ring-shaped optical resonator 2 is not particularly limited, and may be variously arranged depending on the design. be able to.

Further, the shape of the ring-shaped optical resonator 2 is not limited to a circular shape, but may be any shape as long as it is a ring shape.

(Embodiment 2) FIGS. 2A and 2B are views showing an integrated mode-locked semiconductor laser according to a second embodiment of the present invention, wherein FIG. 2A is a plan view, and FIG. 2B is YY 'of FIG. FIG.

The optical resonator of this integrated mode-locked semiconductor laser has no optical isolator and has a plurality of EA modulation regions 3.
The configuration is almost the same as that of the first embodiment except that A to 3C are provided. In FIG. 2, the same components are denoted by the same reference numerals.

The EA modulation regions 3A to 3C are arranged at equal intervals in a ring-shaped optical waveguide constituting the optical resonator 2. Here, the diffraction grating region 8 is formed in the ring-shaped optical resonator 2.
An EA modulation area 3A is arranged between the gain area 4 and the gain area 4, an EA modulation area 3B is arranged between the gain area 4 and the SA area 5, and an EA modulation is formed between the SA area 5 and the Y branch waveguide 7. The area 3B is arranged. These EA modulation areas 3A to 3A
C are respectively connected to the delay line 12,
The sine wave voltage 9 is supplied via the.

The delay line 12 can be formed on the semiconductor substrate 1 using, for example, a microstrip line. Such a microstrip line is formed, for example, by depositing a metal such as copper, aluminum, and gold on the semiconductor substrate 1 and processing the deposited metal thin film into a line having a width of about several millimeters. be able to.

When the three EA modulation regions 3A to 3C are arranged at equal intervals in the optical resonator 2 as in the present embodiment, the phase of the sine wave voltage 9 applied to each of the EA modulation regions 3A to 3C is ring-shaped. It is necessary to match the phase velocity of the optical pulse circulating in the optical resonator 2. For example, if the repetition frequency is 10G
When obtaining an optical pulse of 1 Hz, the time required for the optical pulse to make one revolution in the optical resonator 2 is 100 ps. Therefore, the delay amount of the sine wave voltage 9 between each of the EA modulation regions 3A to 3C is approximately 33 ps. Is set to

Next, the operation of the integrated mode-locked semiconductor laser of this embodiment will be described. Here, as an example, 10G
A case in which a sine wave voltage 9 of Hz is applied to each of the EA modulation regions 3A to 3C will be described.

A sine wave voltage 9 is applied to each of the EA modulation areas 3A to 3C.
Is applied, a rotating electric field is generated as if rotating at a speed of 10 GHz in the clockwise direction along the ring-shaped optical resonator 2. The phases of the optical pulses circling clockwise in the inside are synchronized to perform mode locking.

On the other hand, the light pulse circulating in the ring-shaped optical resonator 2 in the counterclockwise direction is not synchronized with the phase of the electric field rotating in the clockwise direction, and thus gradually attenuates. Eventually, since the optical pulse circulating in the counterclockwise direction disappears, only the optical pulse circulating in the clockwise direction exists in the optical resonator 2.

Therefore, in the present embodiment, unlike the first embodiment, it is possible to limit the circling direction of an optical pulse without providing an optical isolator in the optical resonator 2.

(Embodiment 3) FIG. 3 is a plan view of an integrated mode-locked semiconductor laser according to a third embodiment of the present invention.

This integrated mode-locked semiconductor laser has substantially the same configuration as that of the second embodiment except that the shape of the optical resonator is different. 3, the same components are denoted by the same reference numerals.

In the second embodiment, a circular ring-shaped optical resonator is used.
A ring-shaped optical resonator 14 using a mirror is used. The optical resonator 14 is a quadrangular ring-shaped optical resonator in which mirrors 13a to 13d are arranged at four corners at an angle of 45 °. This optical resonator 14 can be formed as follows. First, as in the first embodiment, an optical waveguide layer is formed on the semiconductor substrate 1 by, for example, the MOPVE method. Thereafter, by performing dry etching capable of vertical etching, a rectangular optical waveguide having a 45-degree mirror is formed, and the optical resonator 14 is obtained. In the optical resonator 14, the optical pulse goes around the optical resonator 14 while being reflected by the mirrors 13a to 13d.

The essential difference between the present embodiment using the optical resonator 14 as described above and the second embodiment is as follows.

In the case of a circular ring resonator structure, as the radius of curvature of the circle decreases, the resonator loss increases, and laser oscillation becomes difficult. For example, when a ring-shaped optical resonator having a high mesa ridge structure is used, the radius of curvature of the optical resonator is limited to about 0.3 mm, and the repetition frequency is limited to about 40 GHz. On the other hand, when a ring-shaped optical resonator having a 45-degree mirror is used as in the present embodiment, there is no restriction due to the radius of curvature of the optical resonator. it can.

In this embodiment, as in the second embodiment, the sine wave voltage 9 is supplied to each of the EA modulation regions 3A to 3B while being delayed through the delay line 12, so that the circling direction of the optical pulse is restricted. The extraction of the optical pulse 10 can be performed by providing the Y-branch waveguide 7 in the optical resonator 14 in the same manner as in the first and second embodiments. Further, one of the four mirrors arranged at the four corners of the optical resonator 14 is formed so that its reflectance is smaller than 100%, so that the optical pulse 10 transmitted through the mirror is extracted. You may do so.

(Embodiment 4) FIG. 4 is a plan view showing the structure of an integrated mode-locked semiconductor laser according to a fourth embodiment of the present invention.

This integrated mode-locked semiconductor laser has substantially the same structure as that of the second embodiment except that the structure of the waveguide constituting the optical resonator is different.
3, the same components are denoted by the same reference numerals.

The ring-shaped optical resonator 15 used in this embodiment has a structure in which a ring-shaped optical waveguide is partially formed by combining two Y-branch optical waveguides 16a and 16b. The diffraction grating 8 is formed at the end A on the Y-branch optical waveguide 16a side, and the optical pulse 10 is extracted from the end B on the Y-branch optical waveguide 16b side.

In this embodiment, as in the second and third embodiments, the sine wave voltage 9 is supplied to each of the EA modulation regions 3A to 3B while being delayed through the delay line 12, so that the circling direction of the optical pulse is restricted. Is done. Therefore, an optical pulse traveling in the optical resonator 15 passes through the optical waveguide D and Y
The light reflected at the end face of the end A of the branch optical waveguide 16a travels toward the end B of the Y-branch optical waveguide 16b through the optical waveguide C. The light reflected at the end face of the portion B goes around the optical waveguide D again toward the end A of the Y-branch optical waveguide 16a. Then, the optical pulse 10 is extracted from the end B on the Y-branch optical waveguide 16b side.

In the above-described second to fourth embodiments, the gain regions 4, SA arranged in the ring-shaped optical resonator
The positional relationship between the region 5 and the diffraction grating region 8 is not particularly limited.
It can be arranged at any position on the ring-shaped optical resonator 2.

Although three EA modulation regions are arranged at equal intervals, the present invention is not limited to this configuration. By arranging a plurality of EA modulation regions in the optical resonator, a desired mode can be obtained. Synchronization can be obtained.

Further, the shape of the ring-shaped optical resonator is not limited to the illustrated shape, but may be any shape as long as it is ring-shaped.

[0051]

As described above, according to the present invention,
Since optical pulses having different phases are not formed in the optical resonator, hybrid mode locking can be reliably applied, and jitter of oscillation light cannot be realized conventionally.
It can be reduced to 5 ps or less. Therefore, an integrated mode-locked semiconductor laser with an accurate repetition frequency can be realized. Further, an ultra-high-speed optical communication system can be realized by using the integrated mode-locked semiconductor laser.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing a structure of an integrated mode-locked semiconductor laser according to a first embodiment of the present invention, wherein FIG. 1A is a plan view and FIG.

FIGS. 2A and 2B are views showing the structure of an integrated mode-locked semiconductor laser according to a second embodiment of the present invention, wherein FIG. 2A is a plan view and FIG.

FIG. 3 is a plan view showing the structure of an integrated mode-locked semiconductor laser according to a third embodiment of the present invention.

FIG. 4 is a plan view showing a structure of an integrated mode-locked semiconductor laser according to a fourth embodiment of the present invention.

FIG. 5 is a side sectional view showing a structure of a conventional integrated mode-locked semiconductor laser.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Optical resonator 3 Electroabsorption modulation area 4 Gain area 5 Supersaturation absorption area 6 Optical isolator 7 Y branch optical waveguide 8 Diffraction grating 9 Sine wave voltage 10 Optical pulse

Claims (6)

[Claims] 1. An integrated mode-locked semiconductor laser in which an optical resonator is formed by an optical waveguide partially provided with an electroabsorption modulation region, wherein the optical waveguide is formed in a ring shape, and the ring-shaped optical waveguide is formed. An integrated mode-locked semiconductor laser comprising an optical pulse circling direction restricting means for restricting a circulating direction of an optical pulse traveling in a wave path. 2. The integrated mode-locked semiconductor laser according to claim 1, wherein said optical pulse circling direction limiting means includes a first waveguide of a first waveguide mode and a second waveguide mode of which oscillation directions are orthogonal to each other. A first optical waveguide through which wave mode light passes; and a second optical waveguide which converts the first waveguide mode light into the second waveguide mode light. An integrated mode-locked semiconductor laser characterized by optical isolator means formed in a wave path. 3. The integrated mode-locked semiconductor laser according to claim 1, wherein a plurality of said electro-absorption modulation regions are arranged at equal intervals in said ring-shaped optical waveguide; And a supply unit for applying an electric signal to the electroabsorption modulation region, wherein the supply unit applies the electric signal to each of the electroabsorption modulation regions in a phase of the ring-shaped optical waveguide. An integrated mode-locked semiconductor laser, wherein the integrated mode-locked semiconductor laser is configured to synchronize with a phase of an optical pulse circulating around the laser. 4. The integrated mode-locked semiconductor laser according to claim 1, wherein the ring-shaped optical waveguide is a rectangular optical waveguide whose waveguide is bent at a right angle. An integrated mode-locked semiconductor laser, wherein a mirror having an angle of 45 ° with respect to the waveguide is provided at each corner. 5. The integrated mode-locked semiconductor laser according to claim 1, wherein said ring-shaped optical waveguide is constituted by two Y-branch optical waveguides. Integrated mode-locked semiconductor laser. 6. The integrated mode-locked semiconductor laser according to claim 1, wherein said ring-shaped optical waveguide is used to amplify an optical pulse circulating in said optical waveguide. An integrated mode-locked semiconductor laser comprising a gain region, a saturable absorption region for phase synchronization between longitudinal modes of laser oscillation, and a diffraction grating region for selecting a laser oscillation wavelength.