JP2697640B2 - Optical clock generator - Google Patents

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 an ultrahigh-speed repetition ultrashort optical pulse train 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 technology capable of generating an optical pulse train having a repetition frequency of several tens of GHz or more as a basic technology of ultrahigh-speed optical communication and optical information processing. As one of optical oscillators that generate high-speed optical pulses, there is a semiconductor laser that operates in a mode-locked manner with a two-section or three-section structure. In this device, a periodic picosecond optical pulse train is generated in an operation mode called passive mode locking by applying a reverse bias to one section separated by an electrode to act as a saturable absorption region.

Usually, in such an optical pulse train, the fluctuation of the pulse period is large, and in order to reduce the fluctuation, the output from a stabilized electric high-frequency generation source having the same frequency as the repetition of the optical pulse is applied to a semiconductor laser. Is taken. This technology is described, for example, by DJ Derrickson et al. In Journal of Quantum Electronics (IEEE J. Qua).
ntum Electron. Vol. 28, No. 10, p. 2186 to p. It is stated in a paper published over 2202. FIG. 2 shows a configuration of a hybrid mode-locked semiconductor laser which is a conventional optical clock generator. In this configuration, the mode-locked semiconductor laser 8
Performs a passive mode-locking operation by supplying a DC current to a gain region 9 by a DC power supply B10 and applying a reverse bias by a bias power supply 12 to a saturable absorption region 11; A high-frequency signal having the same repetition frequency as the output light 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 repetition frequency of optical pulses and the stability of operation as much as possible. In the above-described mode-locked semiconductor laser, the optical nonlinear phenomenon called passive mode-locking is stabilized by modulation at a high frequency.
Even with this method, it is not easy to achieve the stability required for optical communication. For sufficient stabilization by electric modulation, a high frequency of about 30 dBm must be input to the semiconductor laser. However, the use of such a high-output ultra-high frequency requires extremely special equipment and components, which causes a problem that the apparatus becomes large and expensive.

An object of the present invention is to provide an optical clock generator which is small, inexpensive and easy to stabilize the frequency, eliminating the above-mentioned disadvantages of the conventional optical clock generator.

[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. 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 a part of the output of the optical pulse train. Means to cause
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 saturable absorption.

[0007]

According to the present invention, a stabilized periodic optical pulse train is injected to stabilize a self-pulsation semiconductor laser or a passive mode-locked semiconductor laser by 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 the light 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 with a repetition frequency of several tens of GHz that is well separated by optical multiplexing. Therefore, such an optical pulse train is set to 10 GHz.
The light pulse is generated at a frequency of about z, and the light is injected into a semiconductor laser having a saturable absorption region to obtain an optical pulse train with a reduced time width. At this time, the operation 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 the semiconductor lasers to be light-injected, in particular, in a passive mode-locked semiconductor laser, pulse compression is effectively generated by a circulating phenomenon in a resonator of an optical pulse. Further, the reason why the light pulse frequency from the semiconductor laser to be light-injected is stabilized in the same manner as the light pulse train is due to the central light frequency of the injected light and the frequency pull-in phenomenon to the phase-locked side band. That is, the optical nonlinear phenomenon in the semiconductor laser having the saturable absorption function can be efficiently stabilized not by electric modulation but by an optical method using external light input.

[0008]

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

The semiconductor laser 1 is driven through a bias tee 4 by a DC current generated by a DC current A2 and a high frequency or periodic pulse signal from a signal generator 3, thereby generating an input optical pulse train 5 with little fluctuation. I do. This input light pulse train 5 is coupled to a mode-locked semiconductor laser 8 by a collimation lens 6 and a 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 to the gain region 9 by the direct-current power supply B10 and applying a reverse bias to the saturable absorption region 11 by the bias power supply 12; The incidence of the optical pulse train 5 suppresses the fluctuation of the mode-locking operation due to the synchronizing action thereon. In this way, a stable optical synchronous output optical pulse train 13 with little fluctuation and a narrow time width can be obtained.

In this embodiment, a distributed feedback laser having an InGaAs / InGaAsP multiple quantum well structure having a total length of about 0.5 mm is used as the semiconductor laser 1. The high frequency from the signal generator 3 was 10 GHz. The mode-locked semiconductor laser 8 has an InGaAs of about 4 mm in total length.
A two-electrode cleaved surface reflection 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 having a time width of about 20 ps is obtained from the semiconductor laser 1, and the timing fluctuation is input by coupling it to the mode-locked semiconductor laser 8 of about 100 mW. An optically synchronized output optical pulse train 13 having a time width of about 5 ps and suppressed to the same degree as the optical pulse train 5 was obtained.

In the embodiment, a semiconductor laser having an InGaAs multiple quantum well structure has been described in the embodiment. However, the semiconductor laser is not limited to a specific material system from a theoretical point of view. Obviously, the reflecting mirror may be constituted by a distributed feedback structure instead of a cleavage plane. Further, a fiber may be used instead of the lens. Further, in the above-described embodiment, the case where the passive mode-locked semiconductor laser is used as the semiconductor laser having the saturable absorption region has been described.
If 0 ps is sufficient, a semiconductor laser that performs a self-pulsation operation can be used. Further, in the above embodiment, the case where the semiconductor laser by the direct modulation of the current is used as the source of the stabilized input light pulse is described. However, in order to generate the input light pulse, the light from the continuously operating semiconductor laser is required. The output may be intensity modulated by a light intensity modulator.

[0012]

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

[Brief description of the 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]

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

Claims (4)

(57) [Claims] 1. A first semiconductor laser for generating a periodic optical pulse train by direct modulation of the excitation current, the second for generating a periodic optical pulse train Yu was itself a saturable absorption region
Semiconductor laser and light output from the first semiconductor laser.
Coupling at least a portion of the force to the second semiconductor laser;
Of the optical pulse train generated by the second semiconductor laser.
An optical clock generator comprising: means for suppressing fluctuations of the imaging . 2. A first semiconductor laser for generating a steady continuous light, means for generating a periodic optical pulse train of light output intensity modulated and from the semiconductor laser, have a saturable absorption region A second semiconductor laser that generates a periodic optical pulse train by itself, and an optical pulse from the generating means.
At least a portion of the laser output to the second semiconductor laser
Optical pulses generated by the second semiconductor laser when combined
Means for suppressing fluctuations in column timing . Wherein optical clock generator of claim 1, wherein that the self-pulsation operation by the second semiconductor laser saturable absorption. 4. The optical clock generator according to claim 1 or 2, wherein said second semiconductor laser is characterized in that a passive mode-locking operation by the saturable absorber action.