JP3434705B2 - Injection-locked laser oscillator and optical communication system using the oscillator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection-locked laser oscillator for generating an ultra-short repetitive ultrashort optical pulse train useful for optical communication and optical information processing. Furthermore, the present invention relates to an optical communication system using such an injection locking type laser oscillator as a laser amplifier of a repeater.

[0002]

2. Description of the Related Art In recent years, in order to construct an ultrahigh-speed optical communication system, there is an increasing demand for an all-optical optical signal processing technique that is not subject to speed limitation by an electronic circuit. As its main technique, there is an all-optical clock extraction that generates an optical clock pulse train synchronized with the optical signal pulse train .
As an attempt to realize all-optical clock extraction, the following techniques have been conventionally proposed.

(1) Optical timing detection circuit (Japanese Patent Laid-Open No. 6-13
(See Japanese Patent Publication No. 981): This optical timing detection circuit uses an all-optical clock using injection locking to a one-chip passive mode-locking semiconductor laser device having a cavity length that is synchronized with the fundamental repetition frequency of an input optical signal. It is designed to perform extraction. Its concrete configuration is shown in FIG.

Referring to FIG. 3, this optical timing detection circuit is a one-chip type passive mode-locking semiconductor laser device 101 having a saturable absorption region 111 and a gain region 112.
And the fiber coupling lens 103, the optical isolator 104, and the element coupling lens 105 provided for inputting an optical signal pulse train 118 from the outside to the passive mode-locking semiconductor laser device 101, and the passive mode-locking semiconductor laser device 101. It has a collimation lens 106, an optical isolator 107, and a fiber coupling lens 108 that are provided to output an optical pulse (optical clock pulse train 119) to the outside.

The resonator length of the passive mode-locking semiconductor laser device 101 of the optical timing detection circuit is configured to be synchronized with the basic repetition frequency of the optical signal pulse train 118. As a result, the optical signal pulse train 118 is input to the passive mode-locking semiconductor laser device 101 having a fundamental mode-locking frequency that is almost the same as the fundamental repetition frequency, and the passive mode-locking semiconductor laser device 101 shifts to the optical signal pulse train 118. A synchronized optical clock pulse train 119 is obtained. With this optical timing detection circuit, it is possible to generate an ultrashort repeating optical pulse train of 10 GHz or higher, for example.

(2) All-optical clock extraction (R. Ludwing et a
l., "40 Gbit / s demultiplexing experiment with 10 G
Hz all-optical clock recovery using a modelocked s
emiconductor laser, "Electron. Let., vol.32, No.4,
pp.327-329, February 1996): This all-optical clock extraction uses a mode-locked semiconductor laser device with variable cavity length. The specific structure is shown in FIG.

The all-optical clock extraction configuration shown in FIG. 4 has a one-chip mode-locking semiconductor laser device 201 having a saturable absorption region 211 and a gain region 212, and an external resonator between the semiconductor laser device 201. , A collimation lens 214 provided between the semiconductor laser element 201 and the total reflection mirror 215 for optically coupling the two, and an optical signal pulse train 218 from the outside are mode-locked. It has a circulator 209, a fiber coupling lens 203, and a device coupling lens 205 provided for inputting to the semiconductor laser device 201. Gain region 2 of mode-locked semiconductor laser device 201
An antireflection film 213 is provided on the end surface on the 12th side.

The total reflection mirror 215 is constructed so as to be movable along the optical axis of the mode-locking semiconductor laser device 201.
By moving the total reflection mirror 215, the resonator length (mode cycle frequency) can be changed. The circulator 209 is
The optical signal pulse train 218 is applied to the mode-locked semiconductor laser device 2
01, and the optical clock pulse train 219 generated from the mode-locked semiconductor laser device 201 is separated and output.

In the above all-optical clock extraction configuration, the resonator length is set so as to obtain a fundamental mode-locking frequency that is substantially the same as that of the optical signal pulse train 218 having a desired fundamental repetition frequency. As a result, it is possible to eliminate the deviation between the basic repetition frequency of the optical signal pulse train 218 and the basic mode-locking frequency of the mode-locked semiconductor laser device 201, and the optical clock pulse train 21 output from the circulator 209.
9 is synchronized with the optical signal pulse train 218.

[0010]

However, the conventional techniques described above have the following problems.

In the optical timing detection circuit shown in FIG. 3,
The repetition frequency of the optical clock pulse train 119 is determined by the resonator length of the passive mode-locking semiconductor laser device 101, and it is difficult to control the repetition frequency strictly at present. Therefore, in order to extract the optical clock in the optical timing detection circuit shown in FIG.
The fundamental repetition frequency of 18 and the cavity length (fundamental mode-locking frequency) of the passive mode-locking semiconductor laser device 101 must be matched. However, due to manufacturing variations in the element length of the passive mode-locking semiconductor laser element 101, in reality, a deviation occurs between the basic repetition frequency of the optical signal pulse train 118 and the basic mode-locking frequency of the passive mode-locking semiconductor laser element, There is a problem that the optical clock cannot be extracted. Furthermore, the passive mode-locking semiconductor laser device 101 of this optical timing detection circuit has a problem that the wavelength of the optical clock pulse train that can be generated cannot be arbitrarily selected.

In the configuration of all-optical clock extraction shown in FIG. 4, even if there is a deviation between the fundamental repetition frequency of the optical signal pulse train and the fundamental mode-locking frequency of the mode-locked semiconductor laser device as described above, By changing the length, both frequencies can be matched. However, since the optical signal pulse train 218 and the optical extraction clock pulse train 219 are structured to pass through the same path,
The circulator 209 is required as a light separating element.
At present, the use of the circulator tends to induce the external resonator effect due to the returning light, and the light output extraction efficiency of the light extraction clock pulse train 219 also deteriorates.

An object of the present invention is to provide an injection-locked laser oscillator in which the fundamental repetition frequency of the injected optical signal pulse train can be matched with the mode-locking frequency, and which does not require the use of a circulator. .

Another object of the present invention is to provide an optical communication system using such an injection locking type laser oscillator as a laser amplifier of a repeater.

[0015]

In order to achieve the above object, an injection locking type laser oscillator of the present invention has a saturable absorption region and a gain region, and the end face on the saturable absorption region side is provided with external light. External resonance between a semiconductor laser element having an input surface into which an optical signal pulse train from a signal oscillation source is injected and an end surface on the gain region side as an output surface, and an end face on the saturable absorption region side of the semiconductor laser device. It is characterized by having a semi-transmissive mirror that is configured to be movable so that the resonator length of the external resonator can be varied.

The injection locked laser oscillator of the present invention
Has a saturable absorption region and a gain region,
Optical signal pulse from an external optical signal source on the end face on the region side
The row is injected as the input surface, and the end surface on the gain region side is output.
Semiconductor laser device as a force surface, and the semiconductor laser device
An external resonator is formed between the end face on the saturable absorption region side of
It can be moved so that the resonator length of the external resonator can be changed.
The semi-transparent mirror that is configured to
The optical clock pulse synchronized with the optical signal pulse train
And a wavelength selection element that selectively sets the wavelength of the column
It is characterized by

In the above case, the wavelength is selected by the wavelength selection element.
A part of the wavelength component of the optical clock pulse
You may have the external optical filter cut out selectively .

Further, a collimation lens for optically coupling the semiconductor laser element and the semi-transparent mirror is provided between the semiconductor laser element and the semi-transparent mirror, and the wavelength selection element is provided between the collimation lens and the semi-transparent mirror. The configuration may be different.

Further, the wavelength selection element may be a dispersive element.

An optical communication system of the present invention is characterized in that the injection locking type laser oscillator described in any one of the above is used as a laser amplifier of a repeater.

(Operation) In the present invention as described above, the resonator length can be changed by moving the semi-transparent mirror forming the external resonator, so that the external optical signal oscillation source injected into the semiconductor laser device can be used. It is possible to match the fundamental repetition frequency of the optical signal pulse train of 1 and the mode-locking frequency of the external resonator.

Further, in the present invention, since the light transmitted through the semi-transparent mirror of the external resonator becomes the output light, the circulator as in the prior art is not necessary.

Further, since the wavelength of the optical clock pulse train oscillated from the external resonator can be set arbitrarily by the wavelength selection element, for example, an optical clock pulse train having a center wavelength different from that of the optical signal pulse train can be oscillated. it can.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a schematic configuration of an injection locking type laser oscillator according to an embodiment of the present invention. This injection-locked laser oscillator has a saturable absorption region 1 to which a DC voltage source 12 is connected.
1 has a gain region 13 to which the DC power supply 14 is connected,
A tandem electrode type semiconductor laser device 1 and a reflecting mirror 15 forming an external resonator between the semiconductor laser device 1 and the semiconductor laser device 1.
And a collimation lens 16 provided between the semiconductor laser device 1 and the reflecting mirror 15 for optically coupling the two, and a wavelength selection element provided between the collimation lens 16 and the reflecting mirror 15. 17 and.

Further, this injection locked laser oscillator is
An optical fiber 2, a fiber coupling lens 3, an optical isolator 4 and an element coupling lens 5 are provided as means for inputting an optical signal pulse train 18 from an external optical signal oscillation source to the semiconductor laser device 1, and are generated by the external resonator. An optical isolator 6, a fiber coupling lens 7 and an optical fiber 8 are provided as means for outputting the optical pulse (optical clock pulse train 19) to the outside.

The semiconductor laser device 1 comprises a saturable absorption region 1
InGaAs with two polarized electrodes of 1 and gain region 13
/ InGaAsP multiple quantum well structure. A reverse bias is applied to the saturable absorption region 11 of the semiconductor laser device 1 by the DC voltage source 12, and a forward current is injected to the gain region 13 by the DC power supply 14. An antireflection film 10 is provided on the end face of the semiconductor laser device 1 on the gain region 13 side, and the Fabry-Perot type external resonator is formed by the end face of the saturable absorption region 11 of the semiconductor laser device 1 and the reflecting mirror 15. Are configured.

The reflecting mirror 15 is a collimation lens 16
It is a semi-transparent mirror (half mirror) that transmits a part of the collimated parallel light flux, and is movable along the optical axes of the semiconductor laser element 1 and the wavelength selection element 17. That is, the reflecting mirror 15 is movable so that the resonator length of the external resonator formed by the end face of the saturable absorption region 11 of the semiconductor laser device 1 can be changed.

The optical isolators 4 and 6 prevent the external resonator effect due to the returning light of the optical clock pulse and also prevent the influence of the light from the next stage. The wavelength selection element 17 is composed of a dispersive element such as a prism or a diffraction grating, and can arbitrarily set the wavelength of light oscillated from the external resonator by adjusting the angle formed with the optical axis of the external resonator. You can

The physical mechanism of all-optical clock extraction is based on modulation by light absorption in the saturable absorption region of the mode-locked semiconductor laser device by an injection pulse. In this embodiment,
A light signal pulse is injected from the end face on the saturable absorption region 11 side to effectively cause absorption modulation of the saturable absorption region 11. On the other hand, amplification of the optical signal pulse due to stimulated emission in the gain region 13 of the semiconductor laser device 1 can be suppressed.

In the injection locked laser oscillator configured as described above, the optical signal pulse train 18 oscillated from the external optical signal oscillation source is guided by the optical fiber 2, and the fiber coupling lens 3, the optical isolator 4, and the element coupling are provided. The light is incident on the end face of the semiconductor laser device 1 on the saturable absorption region 11 side via the lens 5. Here, the light (optical signal pulse train 18) incident on the end face of the semiconductor laser device 1 on the saturable absorption region 11 side is
It has a light intensity sufficient to saturate the absorption in the saturable absorption region 11.

The saturable absorption region 11 of the semiconductor laser device 1
When the optical signal pulse train 18 is incident on the end face on the side, an optical clock pulse train whose frequency and phase are synchronized is generated in the optical signal pulse train 18 in the external resonator composed of the semiconductor laser device 1 and the reflecting mirror 15. Then, the optical clock pulse train passes through the reflecting mirror 15 of the external resonator, and the transmitted optical clock pulse train (optical clock pulse train 19) is output as an optical output through the optical isolator 6, the fiber coupling lens 7 and the optical fiber 8. Taken out.

In the injection-locked laser oscillator according to this embodiment, the wavelength selection element 1 provided inside the external resonator is used.
The wavelength of the optical clock pulse train 19 can be set to an arbitrary wavelength by adjusting the angle formed by 7 with the optical axis of the external resonator. Thereby, the optical signal pulse train 18 which is the input light and the optical clock pulse train 19 which is the output light
The center wavelengths of and can be different.

For example, as shown in FIG. 2 (a), the optical signal pulse train 18 having the center wavelength λ ₁ is
By injecting the external cavity semiconductor laser element 1 of which to adjust the wavelength selection element 17 to perform the mode-locking operation at different center wavelengths lambda ₂ and the center wavelength lambda _1, the optical clock pulse train 19 having a central wavelength lambda ₂ Can be obtained.
Further, as shown in FIG. 2B, a part of the wavelength component of the optical clock pulse train 19 of the central wavelength λ ₂ is selectively cut out by an external optical filter (not shown), and the optical clock pulse train of the central wavelength λ ₃ is extracted. Can also be obtained. In either case, it is possible to obtain the optical clock pulse train 19 synchronized with the optical signal pulse train 18 and having a different center wavelength, and this can be utilized for the next stage optical signal process such as four-wave mixing.

In the injection locked laser oscillator described above, the light output can be efficiently extracted by appropriately selecting the reflectance of the reflecting mirror 15. Such an injection locking type laser oscillator can be used as a laser amplifier of a repeater of an optical communication system.

[0036]

According to the present invention configured as described above, the resonator length of the external resonator is changed by changing the distance between the end surface of the semiconductor laser device on the saturable absorption region side and the reflecting mirror. Therefore, the mode-locked frequency in the external resonator can be changed over a wide range, and a highly reliable injection-locked laser oscillator can be provided.

Furthermore, since the injection locked laser oscillator of the present invention does not use a circulator, it is more excellent in preventing the external resonator effect due to the returning light than the conventional one, and the optical clock pulse train can be used. This has the effect of increasing the output extraction efficiency.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an injection locking laser oscillator according to an embodiment of the present invention.

2A is a diagram showing spectra of an optical signal pulse and an optical clock pulse in the injection locked laser oscillator shown in FIG. 1, and FIG. 2B is a wavelength of the optical clock pulse shown in FIG. It is a figure which shows the spectrum of the optical clock pulse which cut out a part of component by the external optical filter.

FIG. 3 is a schematic configuration diagram of an optical timing detection circuit disclosed in JP-A-613981.

FIG. 4 is a diagram showing a configuration of conventional all-optical clock extraction capable of varying the resonator length.

[Explanation of symbols]

1 Semiconductor laser device 2 optical fiber 3,7 Fiber coupling lens 4,6 optical isolator 5 element coupling lens 10 Antireflection film 11 Saturable absorption region 12 DC voltage source 13 Gain area 14 DC power supply 15 Reflector 16 Collimation lens 17 Wavelength selection element 18 Optical signal pulse train 19 Optical clock pulse train

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H04B 10/142 10/152 (56) References JP-A-9-191147 (JP, A) JP-A-8-321663 (JP, A) Japanese Unexamined Patent Publication No. 3-114280 (JP, A) IEEE Journal of Q Quantun Electronics, 27 [4], pp. 1048-1060 IEEE Journal of Q Quantun Electronics, 28 [10], pp. 2186-2200 Proceedings of the 1997 IEICE Electronic Society Conference 1, p.p. 317-318

Claims (5)

(57) [Claims] 1. A saturable absorption region and a gain region, wherein the end face on the saturable absorption region side is an input face into which an optical signal pulse train from an external optical signal oscillation source is injected, and the end face on the gain region side. An external resonator is formed between a semiconductor laser device having an output surface and an end face on the saturable absorption region side of the semiconductor laser device, and is configured to be movable so that the resonator length of the external resonator can be changed. Injection-locked laser, comprising: a semitransparent mirror, and a wavelength selection element that selectively sets the wavelength of an optical clock pulse train that is synchronized with the optical signal pulse train and that is transmitted through the semitransparent mirror of the external resonator. Oscillator. 2. The injection locked laser oscillator according to claim 1 , further comprising an external optical filter that selectively cuts out a part of a wavelength component of the optical clock pulse that is selectively set by the wavelength selection element. A characteristic injection-locked laser oscillator. 3. The injection locked laser oscillator according to claim 1 , wherein a collimation lens for optically coupling the semiconductor laser element and the semitransparent mirror is provided between the semiconductor laser element and the semitransparent mirror. An injection locking laser oscillator, wherein the wavelength selection element is provided between the semitransparent mirror and the semitransparent mirror. 4. The injection locked laser oscillator according to claim 1 , wherein the wavelength selection element is a dispersive element. 5. The optical communication system characterized by using an injection-locked laser oscillator according to any one of claims 1 to 4, as a laser amplifier of the repeater.