JP3524005B2 - Optical pulse generator - Google Patents

Description

Description: 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. 2. Description of the Related Art Conventional optical pulse generation methods include a gain switching method of a semiconductor laser, 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 a short optical pulse from a single element by switching the excitation such as an injection current at a high speed by utilizing the high-speed response of a semiconductor laser. 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.).
l., "Generation of transform limited gain-switched
DFB-LD pulses <6ps with linear fiber compression
and spectral window ", Electronics Letters, vol.27,
pp.1981-1982, 1991). [0004] The mode-locking method is a method of generating a short optical pulse while keeping the phase between laser oscillation modes constant.
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 the optical pulse determined by the cavity length of the 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 using the non-linear 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 an optical pulse with a well-defined 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 depending on 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). 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. Further, when transmitting a high-output 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. FIG. 1 shows the basic configuration of an optical pulse generator according to the present invention. In the figure, an optical pulse generator according to the present invention includes 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 by the drive signal and outputs the light carrier. The driving unit 3 outputs a transmission signal of the light intensity modulator 2 as a drive signal for driving the light intensity modulator 2.
It has an amplitude of 2Vπ (Vπ is a half-wave voltage indicating a drive amplitude corresponding to the adjacent maximum transmittance and minimum transmittance) corresponding to the period, 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 such a 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 the case where the operating bias point of the drive signal is in mode A, and FIG.
Is the case of mode B. Here, a response of a sine wave is illustrated as a representative periodic function as a transmission characteristic of the light intensity modulator. When the drive signal of 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. 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, and 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. 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 changes the light branched into two optical waveguides 12a and 12b by a Y-branch waveguide 11 according to an applied voltage. In this configuration, the output light intensity is modulated by using an interference effect when the signals are combined in the Y-branch waveguide 13 by giving a phase difference. 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 up to twice (2Vπ) the half wavelength voltage Vπ of the MZ type optical intensity modulator 10, and the MZ type optical intensity modulator 10
Is 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 2 Vπ, and the optical carrier output from the semiconductor laser 15 is intensity-modulated. An optical pulse having a repetition frequency of 2f can be output from the intensity modulator. Note that the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform as shown in FIGS. (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 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 up to the half-wave 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 type light intensity modulator 1 is driven by driving signals having a frequency f and a driving amplitude Vπ and phases opposite to each other.
By performing 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 10. Note that the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform as shown in FIGS. The Mach-Zehnder interferometer type configuration can suppress chirping of an output optical pulse in principle, but further eliminates chirping by performing a push-pull operation with a driving signal having an opposite phase. be able to. FIG. 6 shows the spectrum of the output light pulse of the present embodiment. (a) shows the case where the device is operated in the mode A, and (b) shows the case where the device is operated in the 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, and modulating the output light intensity 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 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 (2Vπ), 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 optical 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 light intensity modulator 20 is driven by a driving signal having a frequency f and a driving amplitude of 2 Vπ.
Is driven to modulate the intensity of the optical carrier output from the semiconductor laser 15, so that the light of both the mode A and the mode B is output from the two output ports of the directional coupler type optical intensity modulator 20 at the repetition frequency 2 f. Pulses can be output complementarily. Note that the duty ratio of the light pulse can be set according to the bias voltage, the drive amplitude, and the drive waveform as shown in FIGS. 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. As described above, in the optical pulse generator of the present invention, the chirp characteristic of the optical pulse is controlled, and the pulse width of the output optical pulse is uniquely determined by the driving signal frequency and the bias operating point. Thus, it can be driven at any modulation frequency within the modulation band of the light intensity modulator. Further, it is possible to realize an optical pulse generator whose pulse width does not depend on the operating wavelength. 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 of 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. DESCRIPTION OF SYMBOLS 1 light source 2 light intensity modulator 3 drive means 4 bias control means 10 Mach-Zehnder interferometer type light intensity modulator (MZ type light intensity modulator) 11, 13 Y branch waveguides 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 Continuation of the front page (72) Inventor Hiroshi Toba 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (56) References JP-A-7-104223 (JP, A) JP-A-63-60432 (JP, A) JP-A-10-333106 (JP, A) JP-A-8-248366 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1 / 00-1/125

Claims (1)

(57) [Claim 1] A light source for outputting an optical carrier, a drive signal having a periodic response whose transmission characteristic is an even function around an operation bias point, and a drive signal Working via
A light intensity modulator that is set so that the transmittance is minimized at a point, and that modulates the intensity of the optical carrier with a driving signal; and the driving signal that drives the light intensity modulator, the transmission of the light intensity modulator. A driving unit having an amplitude corresponding to one cycle of the characteristic and outputting an electric signal of a frequency f, and having the same amplitude as a driving signal output from the driving unit;
A phase inversion that generates two drive signals whose phases are inverted from each other
And a bias control means for controlling an operation bias point of the drive signal of the light intensity modulator, and the two drive signals are input as the light intensity modulator.
Push-pull Machze with two electrodes
Repetition frequency 2 using a light interferometer type light intensity modulator
An optical pulse generator for outputting an optical pulse of f.