JP2000089176A - Optical pulse generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pulse generator in which a driving is made with an arbitrary modulation frequency within a modulation band, the chirping characteristic of an optical pulse is controlled, the pulse width is made independent against an operating wavelength and carrier suppressed optical pulses are generated. SOLUTION: The generator is provided with a light source 1, which outputs optical carrier, an optical intensity modulator 2, in which a transmissive characteristic has a periodic response of an even function centered around an operating bias point against driving signals and the optical carrier is intensity modulated by the driving signals, a driving means 3 which outputs electric signals having an amplitude 2Vπ corresponding to two periods of the transmissive characteristic of the modulator 2 and a frequency f, and a bias controlling means 4 which controls the operating bias point of the driving signals of the modulator 2. Thus, the modulator 2 outputs optical pulses each having a repetitive frequency 2f.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generator for generating high-speed optical pulses used for optical communication, optical measurement, and the like.

[0002]

2. Description of the Related Art Conventional optical pulse generation methods include a semiconductor laser gain switching method, a mode locking method, a method using an electroabsorption type semiconductor modulator (EA modulator), and various other methods.

The gain switching method is a method of generating short optical pulses from a single element by utilizing the high-speed response of a semiconductor laser and switching the excitation such as an injection current at a high speed. Any modulation frequency can be easily selected. However, there is a disadvantage that the optical spectrum has a large chirp characteristic and the pulse waveform depends on the individual characteristics of the semiconductor laser. Therefore, in order to obtain a transform-limited optical pulse, a method of externally providing a grating for performing chirp compensation has been proposed (Reference 1: K. Iwatsuki et al.,
"Generation of transform limited gain-switched DFB
-LD pulses <6ps with linear fiber compression and
spectral window ", Electronics Letters, vol.27, p
1981-1982, 1991).

[0004] The mode-locking method is a method of generating a short optical pulse with a constant phase between laser oscillation modes.
The optical spectrum is controlled, and a transform-limited optical pulse can be easily generated. However, there is a disadvantage that the modulation frequency is limited by the physical length of the laser resonator. Therefore, a method has been proposed in which the range of the repetition frequency of an optical pulse determined by the cavity length of a mode-locked laser is expanded by using a chirp grating (Reference 2: K. Sato et al., "F.
requency Range Extension of actively mode-locked l
asers integrated with electroabsorption modulators
using chirped grating ", J. of selected topics in
quantum electronics, vol.3, no.2, pp.250-255, 199
7)

In the method using the EA modulator, a short optical pulse is generated by utilizing the nonlinear characteristic of the pass characteristic of the EA modulator. An arbitrary modulation frequency can be selected in the modulation band of the EA modulator. It is possible to generate a light pulse with a good transform limit. But,
Since the non-linear characteristic of the pass characteristic of the EA modulator has wavelength dependence, it is necessary to adjust the pulse width according to the wavelength (Ref. 3: M. Suzuki et al., "New apprication of s").
inusoidal driven InGaAsP electroabsorptionmodulato
r to in-line optical gate with ASE noise reduction
effect, J. Lightwave Technol., vol.10, pp.1912-191
8, 1992).

[0006]

As described above, according to the conventional optical pulse generation method, the optical pulse has a chirp characteristic,
Each has its own problems, such as the limitation on the modulation frequency and the dependence of the pulse width on the operating wavelength.

When transmitting a high-power optical pulse through an optical fiber, the optical pulse signal with suppressed carriers is more susceptible to backward scattering due to stimulated Brillouin scattering than an ordinary optical pulse having a carrier. It is known to be difficult. However, the conventional optical pulse generation method cannot generate an optical pulse with suppressed carriers without an additional circuit.

The present invention provides an optical pulse generator which can be driven at an arbitrary modulation frequency within a modulation band, can control the chirp characteristic of an optical pulse, and has a pulse width independent of an operating wavelength. The purpose is to: Another object of the present invention is to provide an optical pulse generator capable of generating an optical pulse with suppressed carriers without using an additional circuit.

[0009]

FIG. 1 shows a basic configuration of an optical pulse generator according to the present invention. In the figure, the optical pulse generator of the present invention comprises a light source 1, a light intensity modulator 2, a driving unit 3, and a bias control unit 4. Light source 1
Outputs an optical carrier. The light intensity modulator 2 has a periodic response whose transmission characteristic is an even function around the operation bias point with respect to the drive signal, and intensity-modulates the optical carrier with the drive signal and outputs the light carrier. The driving means 3 outputs the driving signal for driving the light intensity modulator 2 as the transmission signal of the light intensity modulator 2
It has an amplitude of 2Vπ (Vπ is a half-wavelength voltage indicating a driving amplitude corresponding to the adjacent maximum transmittance and minimum transmittance) corresponding to the cycle, and outputs an electric signal of frequency f. The bias control means 4 controls an operation bias point of a drive signal of the light intensity modulator 2. With this configuration, the light intensity modulator 2 outputs a light pulse having a repetition frequency of 2f.

An optical pulse generator according to the present invention has a structure as shown in FIG.
In the configuration (1), the transmittance of the light intensity modulator is set to be maximum at the operation bias point of the drive signal (mode A), or the transmittance of the light intensity modulator is minimum at the operation bias point of the drive signal. It is preferable that the configuration (mode B) is set so that

The principle of operation of the optical pulse generator according to the present invention will be described below with reference to FIG. FIG. 2A shows a case where the operation bias point of the drive signal is in mode A, and FIG.
Is the case of mode B. Here, as a transmission characteristic of the light intensity modulator, a response of a sine wave is illustrated as a representative periodic function. When the drive signal of the frequency f is a sine wave,
When the light intensity modulator is driven in mode A, the duty ratio of the generated light pulse becomes 1/3 and the repetition frequency becomes 2
f. Further, when the light intensity modulator is driven in mode B, the duty ratio of the generated light pulse becomes 2/3,
The repetition frequency becomes 2f.

The duty ratio of the light pulse is determined only by the amplitude of the drive signal and the operating bias point, and does not depend on other factors such as the physical length of the element. When the input waveform of the drive signal is a rectangular wave, the duty ratio of the light pulse in mode A shown in FIG. 3A is further reduced, and the duty ratio of the light pulse in mode B shown in FIG. Becomes even larger.

As the light intensity modulator used in the light pulse generator of the present invention, a Mach-Zehnder interferometer type light intensity modulator or a directional coupler type light intensity modulator can be used.

Further, the apparatus has a phase inverting circuit for generating two driving signals having the same amplitude and inverted phases from each other from the driving signals output from the driving means. As a light intensity modulator, two complementary driving signals are provided. A configuration using a Mach-Zehnder interferometer-type light intensity modulator having two input electrodes and performing a push-pull operation may be employed.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment: Claim 1,
2, 3, 4) FIG. 4 shows the first embodiment of the optical pulse generator of the present invention.
An embodiment will be described.

In the figure, a Mach-Zehnder interferometer type light intensity modulator (MZ type light intensity modulator) 10 is provided in accordance with a voltage applied to light branched into two optical waveguides 12a and 12b by a Y branch waveguide 11. In this configuration, the output light intensity is modulated by using the interference effect when the signals are combined in the Y-branch waveguide 13. As shown in FIGS. 2 and 3, the transmittance of the MZ type light intensity modulator 10 periodically changes in accordance with the voltage applied to the electrodes.

The optical carrier output from the semiconductor laser (LD) 15 is input to the MZ type light intensity modulator 10.
The signal of the frequency f output from the oscillator 16 is amplified by the amplifier 17 to a drive amplitude twice as large as the half-wave voltage Vπ (2Vπ) of the MZ light intensity modulator 10 (2Vπ).
Are applied to the electrodes. The bias control circuit 18 inputs a part of the output light of the MZ type optical intensity modulator 10 via the optical coupler 14 and sets the operation bias point of the MZ type optical intensity modulator 10 to the mode A or the mode A shown in FIGS. Set to any of mode B.

With such a configuration, the MZ-type light intensity modulator 10 is driven by a drive signal having a frequency f and a drive amplitude of 2Vπ, and the optical carrier output from the semiconductor laser 15 is intensity-modulated, thereby obtaining the MZ-type light. An optical pulse having a repetition frequency of 2f can be output from the intensity modulator. As shown in FIGS. 2 and 3, the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform.

(Second Embodiment: Claims 1, 2, 3, 3)
5) FIG. 5 shows a second embodiment of the optical pulse generator according to the present invention. In the figure, an optical carrier output from a semiconductor laser (LD) 15 is input to an MZ type light intensity modulator 10. The signal of the frequency f output from the oscillator 16 is input to the phase inverting circuit 19, and two drive signals having the same amplitude and inverted phases are generated. These two drive signals are input to amplifiers 17a and 17b, respectively, and the drive amplitude is amplified to a half-wavelength voltage Vπ of the MZ light intensity modulator 10 and applied to each electrode of the MZ light intensity modulator 10.
The bias control circuit 18 inputs a part of the output light of the MZ type optical intensity modulator 10 via the optical coupler 14 and sets the operation bias point of the MZ type optical intensity modulator 10 to the mode A or the mode A shown in FIGS. Set to any of mode B.

With such a configuration, the MZ light intensity modulator 1 is driven by driving signals having a frequency f and a driving amplitude Vπ and having phases opposite to each other.
By performing a push-pull operation on 0 and intensity-modulating the optical carrier output from the semiconductor laser 15, an optical pulse having a repetition frequency of 2f can be output from the MZ type optical intensity modulator. As shown in FIGS. 2 and 3, the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform. In addition, the configuration of the Mach-Zehnder interferometer type can suppress chirping of the output optical pulse in principle, but further eliminates chirping by performing push-pull operation with a driving signal of opposite phase. be able to.

FIG. 6 shows the spectrum of the output light pulse of the present embodiment. (a) shows the case of operating in mode A, and (b) shows the case of operating in mode B. Operate in mode B where the output light intensity is minimized at the drive bias point,
By generating light intensity modulated signals having the same light intensity and inverted phases, carrier components (indicated by arrows in the figure) in the signal spectrum can be suppressed.

(Third Embodiment: Claims 1, 2, 3,
6) FIG. 7 shows a third embodiment of the optical pulse generator according to the present invention. In the figure, a directional coupler type light intensity modulator 2
Numeral 0 denotes a configuration using an optical coupler 21 having two inputs and two outputs, wherein the output light intensity is modulated by utilizing an interference effect between coupled optical waveguides accompanying phase modulation according to an applied voltage.
As shown in FIGS. 2 and 3, the transmittance of the directional coupler type light intensity modulator 20 periodically changes in accordance with the voltage applied to the electrodes.

The optical carrier output from the semiconductor laser (LD) 15 is input to one input port of the directional coupler type optical intensity modulator 20. The signal of the frequency f output from the oscillator 16 is amplified by the amplifier 17 up to twice the half wavelength voltage Vπ (2Vπ) of the directional coupler type optical intensity modulator 20, and the directional coupler type optical intensity modulation is performed. Applied to the electrodes of the vessel 20. The bias control circuit 18 converts a part of one output light of the directional coupler type light intensity modulator 20 into the optical coupler 14.
And the operating bias point of the directional coupler type optical intensity modulator 20 is set to the mode A or the mode B shown in FIGS.
Set to one of

With such a configuration, the directional coupler type optical intensity modulator 20 is driven by a driving signal having a frequency f and a driving amplitude of 2 Vπ.
Is driven, and the intensity of the optical carrier output from the semiconductor laser 15 is modulated, so that both the mode A and the mode B light are output from the two output ports of the directional coupler type optical intensity modulator 20 at the repetition frequency 2f. Pulses can be output complementarily. As shown in FIGS. 2 and 3, the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform.

The MZ type optical intensity modulator 10 shown in the first embodiment and the second embodiment has a one-input one-output configuration because it uses a Y-branch waveguide.
By replacing the optical coupler with two inputs and two outputs, like the directional coupler type optical intensity modulator 20 of the third embodiment,
Both mode A and mode B light pulses can be output complementarily.

[0026]

As described above, in the optical pulse generator of the present invention, the chirp characteristic of the optical pulse is controlled, the pulse width of the output optical pulse is uniquely determined by the drive signal frequency and the bias operating point, It can be driven at any modulation frequency within the modulation band of the intensity modulator. Further, an optical pulse generator whose pulse width does not depend on the operating wavelength can be realized.

In particular, the optical pulse generator according to claim 5 can generate an optical pulse with suppressed carriers without using an additional circuit.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an optical pulse generator according to the present invention.

FIG. 2 is a view for explaining the operation principle (sine wave input) of the optical pulse generator of the present invention.

FIG. 3 is a view for explaining the operation principle (rectangular wave input) of the optical pulse generator of the present invention.

FIG. 4 is a block diagram showing a first embodiment of the optical pulse generator of the present invention.

FIG. 5 is a block diagram showing a second embodiment of the optical pulse generator according to the present invention.

FIG. 6 is a diagram illustrating a spectrum of an output light pulse according to the second embodiment.

FIG. 7 is a block diagram showing a third embodiment of the optical pulse generator according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Light source 2 Light intensity modulator 3 Driving means 4 Bias control means 10 Mach-Zehnder interferometer type light intensity modulator (MZ type light intensity modulator) 11, 13 Y branch waveguide 12a, 12b Optical waveguide 14 Optical coupler 15 Semiconductor laser (LD) 16 Oscillator 17, 17a, 17b Amplifier 18 Bias control circuit 19 Phase inversion circuit 20 Directional coupler type light intensity modulator 21 Optical coupler

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Akira Hirano, Inventor: Nippon Telegraph and Telephone Corporation 3-9-1, Nishishinjuku, Shinjuku-ku, Tokyo (72) Inventor: Hiroshi Toba 3-9-1, Nishishinjuku, Shinjuku-ku, Tokyo No. Nippon Telegraph and Telephone Corporation F-term (reference) 2H079 AA02 AA12 BA01 CA04 CA11 EA04 EA05 FA01 FA02 HA14 KA18 KA19

Claims (6)

[Claims] A light source that outputs an optical carrier; and a light intensity that has a periodic response of an even function in transmission characteristics around an operating bias point with respect to a drive signal, and that intensity-modulates the optical carrier with the drive signal. A modulator; a driving unit that outputs an electric signal having a frequency f as an amplitude corresponding to two periods of transmission characteristics of the light intensity modulator as the driving signal for driving the light intensity modulator; An optical pulse generator, comprising: bias control means for controlling an operation bias point of a drive signal of an intensity modulator, wherein the optical intensity modulator outputs an optical pulse having a repetition frequency of 2f. 2. The optical pulse generator according to claim 1, wherein the optical pulse is configured so that the transmittance of the optical intensity modulator is maximized at an operation bias point of the drive signal. Generator. 3. The optical pulse generator according to claim 1, wherein the optical pulse is set so that the transmittance of the optical intensity modulator is minimized at an operation bias point of the drive signal. Generator. 4. The optical pulse generator according to claim 1, wherein a light intensity modulator of a Mach-Zehnder interferometer type is used as the light intensity modulator. Optical pulse generator. 5. The optical pulse generator according to claim 1, wherein two drive signals having the same amplitude and inverted phases are generated from the drive signals output from the drive means. An input circuit for inputting the two drive signals as a light intensity modulator;
An optical pulse generator comprising a Mach-Zehnder interferometer type light intensity modulator having two electrodes and performing a push-pull operation. 6. The optical pulse generator according to claim 1, wherein a directional coupler type light intensity modulator is used as the light intensity modulator. Light pulse generator.