JP3223938B2 - Short light pulse generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short light pulse generator for generating a short light pulse having a transform limit.

[0002]

2. Description of the Related Art Conventionally, a mode-locking method and a gain switch method have been known as methods for generating ultrafast short optical pulses from a semiconductor laser. However, the mode-locking method has a drawback that when used as a transmission pulse light source for optical pulse transmission, it does not conform to the required transmission speed because the repetition frequency of the generated optical pulse is determined by its resonator length. Was. Further, in the gain switch method, although the repetition frequency of the generated optical pulse can be set arbitrarily, in order to generate an optical pulse close to the transform limit (TL), it is necessary to remove the residual chirp at both ends of the gain switch pulse. (For example, K. Iwatsuki, et a)
l., "Generationof transform limited gain-switched D
FB-LDpulses <6ps with linear fiber-compression and
spectral windou ", Electron. Lett., 27, p. 1981 (1991) and the like).

Therefore, an optical pulse whose instantaneous wavelength is chirped linearly to the longer wavelength side at the center of the pulse is guided to an optical fiber or the like having a normal dispersion to perform pulse compression to generate a short optical pulse. By transforming the non-linear residual chirp at the inflection point portion of the optical pulse with an optical spectrum filter or the like, a transform-limited optical pulse is generated (for example, A. Takada, et al., "High-speed
picosecond optical pulse compression from gain-swi
ched 1.3μm distributed feedback-laser diode (DFB-L
D) through highly dispersive single-mode fiber ", IE
EE J. Lightwave Technol., Vol. LT-5, p. 1525-1533 (198
7) etc.).

On the other hand, a short light pulse generator as shown in FIG. 5 has been proposed (for example, M. Suzuki et al., "Appli"
cation of a 1 / 4-shifted DFB laser / electroabsorptio
n modulator monolithically integrated light source
to single-chip pulse generator with variable repe
tition rate ", IEEE Photon.Technol.Lett., p.1129 (199
2) etc.). This short optical pulse generator 1 has C
A semiconductor laser (LD) 2 for generating W light, an oscillator 3 for outputting a sinusoidal electric signal s having a relatively large amplitude, and a semiconductor laser 2 driven by the sinusoidal electric signal s output from the oscillator 3 And an absorption semiconductor (EA) modulator 4 for modulating the output CW light. In addition,
FIG. 6 shows an example of the nonlinear absorption characteristic of the absorption type semiconductor modulator 4.
Shown in The short light pulse generator 1 can generate a short light pulse p having a duty ratio of about 0.2 which is very close to the transform limit by utilizing the nonlinear absorption characteristics of the absorption type semiconductor modulator 4. .

[0005]

However, the above-described gain switching method has a drawback that the residual chirp at the tail portion of the optical pulse cannot be cut off and the pulse width is only about 5 ps. In the short optical pulse generator 1 described above, the drive voltage must be increased in order to obtain a duty ratio of 0.2 or less necessary for optical soliton transmission, but there is a restriction on the generation of a high-frequency drive voltage. In addition, the magnitude of the generated driving voltage is limited, and therefore, there is a problem that the generation of the optical soliton pulse becomes more difficult as the speed becomes higher. Note that the obtained pulse width can be narrowed in proportion to the driving frequency applied to the absorption type semiconductor modulator 4 if the driving frequency is increased. However, the condition is that short light pulses with a small duty ratio can be obtained by constant repetition. Is inappropriate because it does not fit.

In this short light pulse generator 1, C
Since the short light pulse is obtained by cutting out the W light with the optical gate by the absorption type semiconductor modulator 4, the peak light power that can be extracted in principle is limited, and a sufficient peak value necessary for optical soliton transmission is obtained. There was a drawback that it was difficult to obtain.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and requires a short light having a sufficiently large peak power, a small duty ratio, and a completely transform limit at the required ultra-high-speed repetition. An object of the present invention is to provide a short optical pulse generator capable of generating a pulse.

[0008]

In order to solve the above problems, the present invention employs the following short optical pulse generator.
In other words, a short optical pulse generator according to claim 1 is a semiconductor laser that generates a single longitudinal mode optical pulse, an oscillator that performs a gain switch operation on the semiconductor laser, and a multiplier that multiplies an output signal from the oscillator. An absorption type semiconductor modulator driven by an output signal from the multiplier to remove a nonlinear chirp component of an optical pulse output from the semiconductor laser; and a pulse width of an optical pulse output from the absorption type semiconductor modulator. And a normal dispersion optical medium for compressing the light.

According to a second aspect of the present invention, there is provided a short optical pulse generator according to the first aspect, wherein the semiconductor laser and the absorption type semiconductor modulator are integrated. .

[0010]

In the short light pulse generating device according to the first aspect of the present invention, the semiconductor laser performs a gain switch operation by an electric signal output from the oscillator, and generates a single longitudinal mode light pulse. Thus, it becomes possible to make an optical pulse having a large peak power incident on the absorption type semiconductor modulator, and a sufficient peak value necessary for optical soliton transmission can be obtained. Further, the absorption type semiconductor modulator is driven by the output signal from the multiplier to remove the nonlinear chirp component of the light pulse. This makes it possible to cut off the residual chirp at the bottom of the light pulse, to output a light pulse with a narrow pulse width, and to obtain a short light pulse with a small duty ratio by constant repetition. Further, the normal dispersion optical medium compresses the pulse width of the light pulse output from the absorption type semiconductor modulator, and generates a short light pulse which is completely transform-limited.

According to another aspect of the present invention, the semiconductor laser and the absorption type semiconductor modulator are integrated to reduce the coupling loss between the semiconductor laser and the absorption type semiconductor modulator. Thus, a small and stable short optical pulse generator can be realized.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the short light pulse generator according to the present invention will be described below. FIG. 1 is a schematic configuration diagram showing a short optical pulse generator according to one embodiment of the present invention. The short light pulse generator 11 includes a semiconductor laser 12 that generates a single longitudinal mode light pulse, an oscillator 3 that outputs a sinusoidal electric signal s to cause the semiconductor laser 12 to perform a gain switch operation, Output sine wave electric signal s
A multiplier 13 that outputs a high-frequency electric signal h that is an integer (N) times (multiplied) times the repetition frequency f, and is driven by the high-frequency electric signal h output from the multiplier 13 and output from the semiconductor laser 12. An absorption type semiconductor modulator 4 for eliminating a nonlinear chirp component of an optical pulse, and an optical fiber (normal dispersion optical medium) 14 having a normal dispersion for compressing a pulse width of an optical pulse output from the absorption type semiconductor modulator 4 It is configured. The optical fiber 14 is, for example, GT
An optical circuit having normal dispersion, such as an interferometer, can be used.

Next, the operation of the short light pulse generator 11 will be described with reference to FIG. First, the semiconductor laser 12 is operated as a gain switch by the sinusoidal electric signal s output from the oscillator 3 to generate a gain switch pulse (light pulse in a single longitudinal mode) having a high peak power (FIG. 7A). The chirp of the gain switch pulse at this time is shown in FIG. This gain switch pulse contains a non-linear chirp component.

Next, the gain switch pulse is guided to the absorption type semiconductor modulator 4, and the center of the gain switch pulse is cut off by the optical gate (FIG. 10 (c)) of the absorption type semiconductor modulator 4, and FIG. A light pulse having a narrow pulse width is generated as shown in FIG. As a result, of the chirp components of the gain switch pulse, non-linear chirp components at both ends of the pulse can be effectively deleted (FIG. 9E). In addition, the absorption type semiconductor modulator 4
Can be driven at an integer (N) times the repetition frequency f of the gain switch.
The duty ratio can be reduced as compared with.

Since the light pulse emitted from the absorption type semiconductor modulator 4 is chirped linearly throughout, the light pulse is guided to the optical fiber 14 to further reduce the pulse width (FIG. 1F). ), Generate short pulses that are completely transform-limited. The chirp of the short light pulse at this time is shown in FIG. The pulse width of the short light pulse is sufficiently narrower than the light pulse generated when the conventional short light pulse generator 1 or the semiconductor laser 12 is operated by a gain switch.

As described above, according to the short optical pulse generator 11 of the present embodiment, the semiconductor laser 12 that generates a single longitudinal mode optical pulse, the oscillator 3 that causes the semiconductor laser 12 to perform a gain switch operation, A multiplier 13 for outputting a multiplied high-frequency electric signal h, an absorption type semiconductor modulator 4 for eliminating a nonlinear chirp component of an optical pulse output from a semiconductor laser 12, and an optical fiber 14 for compressing a pulse width
, The peak power is sufficiently large, the pulse width is sufficiently narrow, the duty ratio is small, and a short optical pulse that is completely transform-limited can be generated, which is sufficient for optical soliton transmission. An optical pulse having a strong peak can be obtained.

FIG. 3 shows the short light pulse generator 11 described above.
FIG. 1 is a schematic configuration diagram showing a short light pulse generation device that embodies the above. The short optical pulse generator 21 includes a distributed feedback semiconductor laser (DFB-LD) 22, an oscillator 3, a multiplier 13, an optical isolator 23, a phase adjuster 24,
The oscillator 3 includes an absorption type semiconductor modulator 4 and an optical fiber 14, and a bias circuit 25 is provided on one output side of the oscillator 3.
A bias circuit 26 is connected to the output side of the phase adjuster 24.

In the short optical pulse generator 21, the chirped optical pulse from the distributed feedback semiconductor laser 22 gain-switched by the oscillator 3 and the bias circuit 25 is transmitted through the optical isolator 23 to the absorption semiconductor modulator 4.
Lead to. The absorption type semiconductor modulator 4 is driven through a bias 26 with a high frequency electric signal obtained by multiplying the sinusoidal electric signal output from the oscillator 3 by the multiplier 13. At this time, the time during which the optical gate of the absorption type semiconductor modulator 4 is opened is adjusted by adjusting the phase by the phase adjuster 24 so that only the central portion which is the linear chirp portion of the gain switch pulse is cut off.

The light pulse emitted from the absorption type semiconductor modulator 4 enters the optical fiber 14 to compensate for linear chirp. The optical pulse is compressed and chirped out while passing through the optical fiber 14, and is compressed to become a completely transform-limited short optical pulse.

In this short optical pulse generator 21 also,
As in the case of the short light pulse generator 11 described above, a short light pulse having a sufficiently large peak power, a sufficiently narrow pulse width, a small duty ratio, and a complete transform limit can be generated. An optical pulse having a sufficiently strong peak required for transmission can be obtained.

In this embodiment, the distributed feedback semiconductor laser 22 is used as a semiconductor laser for generating a single longitudinal mode light pulse. However, the semiconductor laser generates a single longitudinal mode light pulse. The same operation and effect can be obtained by using, for example, a Bragg reflection type semiconductor laser (DBR-LD).

FIG. 4 is a schematic configuration diagram showing a short optical pulse generator 31 according to another embodiment. The short light pulse generator 31 is different from the short light pulse generator 21 in that the distributed feedback semiconductor laser 22 and the absorption semiconductor modulator 4 are replaced by a distributed feedback semiconductor laser 32a and an absorption semiconductor modulator 32b.
Is replaced by an integrated optical device 32, and the other components are the same as those of the short optical pulse generator 21. Here, since the integrated optical device 32 is used, the optical isolator 23 is
2 on the exit side.

In this short optical pulse generator 31,
As in the case of the short light pulse generator 21 described above, a short light pulse having a sufficiently large peak power, a sufficiently narrow pulse width, a small duty ratio, and a complete transform limit can be generated. An optical pulse having a sufficiently strong peak required for transmission can be obtained. Moreover, since the optical device 32 in which the distributed feedback semiconductor laser 32a and the absorption semiconductor modulator 32b are integrated is used, the coupling loss between the distributed feedback semiconductor laser 32a and the absorption semiconductor modulator 32b is reduced. And a small and stable short optical pulse generator can be realized.

[0024]

As described above, according to the short optical pulse generator of the first aspect of the present invention, a semiconductor laser that generates a single longitudinal mode optical pulse and a gain switch operation of the semiconductor laser are performed. An oscillator, a multiplier that multiplies an output signal from the oscillator, and an absorption type semiconductor modulator driven by the output signal from the multiplier to remove a nonlinear chirp component of an optical pulse output from the semiconductor laser;
A normal dispersion optical medium for compressing the pulse width of the light pulse output from the absorption type semiconductor modulator, so that the peak power is sufficiently large, the pulse width is sufficiently narrow, the duty ratio is small, and the transformer is completely Form-limited short optical pulses can be generated, and an optical pulse having a sufficiently strong peak required for ultrahigh-speed long-distance optical transmission, particularly optical soliton transmission can be obtained.

According to the second aspect of the present invention, since the semiconductor laser and the absorption type semiconductor modulator are integrated, the coupling loss between the semiconductor laser and the absorption type semiconductor modulator can be reduced. A small and stable short optical pulse generator which can be reduced in size can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a short optical pulse generator according to one embodiment of the present invention.

FIG. 2 is a diagram showing light pulses and their chirp in a short light pulse generator according to one embodiment of the present invention.

FIG. 3 is a schematic configuration diagram showing a short optical pulse generation device embodying the present invention.

FIG. 4 is a schematic configuration diagram showing another short optical pulse generation device that embodies the present invention.

FIG. 5 is a schematic configuration diagram showing a conventional short optical pulse generator.

FIG. 6 is a characteristic diagram showing nonlinear absorption characteristics of the absorption type semiconductor modulator.

[Explanation of symbols]

 Reference Signs List 3 oscillator 4 absorption type semiconductor modulator 11 short optical pulse generator 12 semiconductor laser 13 multiplier 14 optical fiber (normal dispersion optical medium) 21 short optical pulse generator 22 distributed feedback semiconductor laser (DFB-LD) 23 optical isolator 24 Phase adjuster 25, 26 Bias circuit 31 Short optical pulse generator 32 Optical device 32a Distributed feedback semiconductor laser 32b Absorption semiconductor modulator s Sine wave electric signal h High frequency electric signal

──────────────────────────────────────────────────の Continuing on the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/26 10/28 (72) Inventor Masatoshi Saruwatari Nippon Telegraph and Telephone Corporation, 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo ( 56) References JP-A-6-326390 (JP, A) JP-A-6-281896 (JP, A) JP-A-6-177476 (JP, A) JP-A-5-283804 (JP, A) JP Hei 3-135090 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 G02F 1/015 H04B 10/04-10/28

Claims (2)

(57) [Claims] 1. A semiconductor laser for generating a single longitudinal mode light pulse, an oscillator for performing a gain switch operation on the semiconductor laser, a multiplier for multiplying an output signal from the oscillator, and an output signal from the multiplier An absorption semiconductor modulator driven by the semiconductor laser to eliminate a nonlinear chirp component of an optical pulse output from the semiconductor laser; and a normal dispersion optical medium that compresses a pulse width of the optical pulse output from the absorption semiconductor modulator. A short light pulse generator characterized by comprising: 2. The short optical pulse generator according to claim 1, wherein the semiconductor laser and the absorption type semiconductor modulator are integrated.