JPH08148749A - Optical clock generator - Google Patents

Abstract

PURPOSE: To provide a small optical clock generator which easily stabilizes frequency at low cost by providing a semiconductor laser which has a saturable absorption area that couples at least a part of optical power from the semiconductor laser which generates periodic light pulse by the direct modulation of exciting current. CONSTITUTION: A semiconductor laser 1 is driven by direct current of direct current A2 and the high frequency or periodic pulse signal from a signal generator 3 through a bias tee 4, and an input light pulse train 5 that allows less fluctuation is generated. The manually provided light pulse train 5 is coupled with a mode-locking semiconductor laser 8 by a collimation lens 6 and a focus lens 7. Though passive mode-locking is allowed for the mode-locking semiconductor laser 8 by direct current supply and reverse bias application to a saturable absorption area 11 by a bias power source 12 by direct current source B10 for a gain area 9, fluctuation of mode-locking operation is suppressed by incidence of the input light pulse train 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical clock generator for generating ultra-short repetitive ultra-short optical pulse trains useful for optical communication and optical measurement.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for a technique capable of generating an optical pulse train having a repetition frequency of several tens GHz or more as a basic technique for ultrahigh-speed optical communication and optical information processing. One of the optical oscillators that generate high-speed optical pulses is a semiconductor laser that operates in a mode-locked manner with a two-section or three-section structure. In this device, a reverse bias is applied to one section separated by an electrode to act as a saturable absorption region, so that a picosecond optical pulse train that is periodic in an operation mode called passive mode locking is generated.

Usually, in such an optical pulse train, the fluctuation of the pulse period is large, and therefore, in order to reduce the fluctuation, the output from the stabilized electric high frequency source having a frequency equal to the repetition of the optical pulse is emitted from the semiconductor laser. The method of adding to is taken. This technique is described, for example, in Journal of Quantum Electronics (IEEE J. Qua) by D. J. Derickson et al.
ntum Electron. ), Vol. 28, No. 10, p. 2186 to p. It is described in a paper published over 2202. FIG. 2 shows the configuration of a hybrid mode-locked semiconductor laser which is a conventional optical clock generator. In this configuration, the mode-locked semiconductor laser 8
Is a passive mode-locking operation by supplying a DC current from the DC power supply B10 to the gain region 9 and applying a reverse bias from the bias power supply 12 to the saturable absorption region 11, and further, in order to stabilize the operation, a bias is applied. A high frequency signal having the same repetition frequency as that of the output optical pulse train 14 is applied to the saturable absorption region 11 via the tee 4 by the signal generator 3.

[0004]

In optical communication applications, it is necessary to stabilize the accuracy of the optical pulse repetition frequency and the stability of operation as much as possible. In the above mode-locked semiconductor laser, an optical nonlinear phenomenon called passive mode-locking is stabilized by high frequency modulation.
Even with this method, it is not easy to achieve the stability required for optical communication. A high frequency of about 30 dBm must be input to the semiconductor laser for sufficient stabilization by electrical modulation. However, in order to use such a high-power ultra-high frequency, extremely special equipment and parts are required, which causes a problem that the apparatus becomes large and expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical clock generator which is small in size, inexpensive, and whose frequency can be easily stabilized, in which the above-mentioned drawbacks of the conventional optical clock generator are eliminated.

[0006]

An optical clock generator according to the present invention comprises a first semiconductor laser for generating a periodic optical pulse train by direct modulation of an excitation current, and at least a part of an optical output from the semiconductor laser. And a second semiconductor laser having a saturable absorption region. In addition, a first semiconductor laser that generates steady continuous light, a unit that intensity-modulates an optical output from the semiconductor laser to generate a periodic optical pulse train, and at least a part of the optical pulse train output are combined. Means to make
A second semiconductor laser having a saturable absorption region is provided. The second semiconductor laser is characterized by performing a self-pulsation operation or a passive mode-locking operation by a saturable absorption effect.

[0007]

In the present invention, the stabilized periodic optical pulse train is injected to stabilize the self-pulsation semiconductor laser and the passive mode-locking semiconductor laser by the injection locking operation. The stabilized optical pulse train can be obtained by directly modulating the excitation current of the semiconductor laser or by intensity-modulating the output of the continuously oscillating semiconductor laser with an optical intensity modulator. However, since these optical pulses usually have a time width of 10 ps or more, it is difficult to obtain an optical pulse train having a repetitive frequency of several tens GHz that is easily separated by optical multiplexing. Therefore, such an optical pulse train is
By generating the light at a frequency of about z and injecting the light into a semiconductor laser having a saturable absorption region, an optical pulse train whose time width is compressed is obtained. At this time, the operating pulse frequency of the semiconductor laser having the saturable absorption region is set so as to match the repetition frequency of the optical pulse train to be injected. Among semiconductor lasers that are optically injected, particularly in passive mode-locked semiconductor lasers, pulse compression is effectively generated by the circulation phenomenon of the optical pulse in the resonator. The optical pulse frequency from the semiconductor laser that is optically injected is stabilized in the same manner as the injected optical pulse train due to the center optical frequency of the injected light and the frequency pull-in phenomenon to the sideband in phase synchronization. That is, the optical non-linear phenomenon in the semiconductor laser having a saturable absorption effect can be efficiently stabilized not by electrical modulation but by an optical method by external light input.

[0008]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 schematically shows an embodiment of the configuration of an optical clock generator to which the present invention is applied.

The semiconductor laser 1 is driven via the bias tee 4 by the direct current by the direct current A2 and the high frequency or periodic pulse signal from the signal generator 3 to generate the input optical pulse train 5 with less fluctuation. To do. The input light pulse train 5 is coupled to the mode-locked semiconductor laser 8 by the collimation lens 6 and the focus lens 7. Although not shown, an isolator is arranged between the collimation lens 6 and the focus lens 7. The mode-locked semiconductor laser 8 performs a passive mode-locking operation by supplying a direct current from the DC power supply B10 to the gain region 9 and applying a reverse bias to the saturable absorption region 11 by the bias power supply 12, but the input having the same repetition frequency. The incidence of the light pulse train 5 suppresses the fluctuation of the mode-locking operation due to the synchronizing action on the light train. In this way, a stable optical synchronous output optical pulse train 13 with little fluctuation and narrow time width can be obtained.

In this embodiment, as the semiconductor laser 1, a distributed feedback laser having an InGaAs / InGaAsP multiple quantum well structure with a total length of about 0.5 mm is used. The high frequency frequency from the signal generator 3 was 10 GHz. The mode-locked semiconductor laser 8 has a total length of about 4 mm and is made of InGaA.
A two-electrode cleaved surface reflection type laser having an s / InGaAsP multiple quantum well structure was used. By operating these semiconductor lasers under appropriate conditions, an input optical pulse train 5 with a time width of about 20 ps can be obtained from the semiconductor laser 1, and the timing fluctuation can be input by coupling it to the mode-locked semiconductor laser 8 of about 100 mW. An optical synchronous output optical pulse train 13 having a time width of about 5 ps, which was suppressed to the same level as the optical pulse train 5, was obtained.

In the embodiment, the semiconductor laser having the InGaAs multiple quantum well structure is described in the embodiment, but it is not limited to a specific material system from the viewpoint of principle, and the resonator of the mode-locked semiconductor laser is not limited. Obviously, the reflector may be constructed by a distributed feedback structure instead of the cleaved surface. Further, a fiber may be used instead of the lens. Further, in the above embodiment, the case where the passive mode-locked semiconductor laser is used as the semiconductor laser having the saturable absorption region is shown, but the time width of the optical pulse is about 1.
If 0 ps is sufficient, a semiconductor laser that performs a self-pulsation operation can be used. Further, in the above embodiment, the semiconductor laser by the direct modulation of the current is described as the source of the stabilized input optical pulse, but in order to generate the input optical pulse, the light from the continuously operating semiconductor laser is used. The output may be intensity modulated by a light intensity modulator.

[0012]

As described above, according to the present invention, a sufficiently stabilized optical pulse in the picosecond region, which is important for future ultrahigh-speed optical communication applications, can be obtained by the configuration in which the load on the electrical control system is light. be able to.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an embodiment of an optical clock generator to which the present invention is applied.

FIG. 2 is a diagram showing a configuration of a conventional optical clock generator.

[Explanation of symbols]

 1 semiconductor laser 2 DC power supply A 3 signal generator 4 bias tee 5 input light pulse train 6 collimation lens 7 focus lens 8 mode-locking semiconductor laser 9 gain region 10 DC power supply B 11 saturable absorption region 12 bias power supply B 13 optical synchronization output light Pulse train 14 output optical pulse train

Claims (4)

[Claims] 1. A first semiconductor laser for generating a periodic optical pulse train by direct modulation of an excitation current, and means for coupling at least a part of optical output from the semiconductor laser.
An optical clock generator comprising a second semiconductor laser having a saturable absorption region. 2. A first semiconductor laser for generating a continuous continuous light, a means for intensity-modulating an optical output from the semiconductor laser to generate a periodic optical pulse train, and at least one of the optical pulse train outputs. An optical clock generator comprising means for coupling the parts and a second semiconductor laser having a saturable absorption region. 3. The optical clock generator according to claim 1, wherein the second semiconductor laser performs a self-pulsation operation by saturable absorption. 4. The optical clock generator according to claim 1 or 2, wherein said second semiconductor laser operates passively in a mode-locked manner by saturable absorption.