JP3575653B2 - Ultra-fast synchronous pulse light source - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ultrahigh-speed synchronous pulse light source that generates an optical pulse train having an ultrahigh repetition frequency that can be synchronized with an electric signal.
[0002]
[Prior art]
The passive mode-locked semiconductor laser can generate an optical pulse train having an extremely high repetition frequency by controlling the reverse bias of the saturable absorption region provided in the laser. As a method of using this passive mode-locked semiconductor laser to synchronize an optical pulse having a repetition frequency exceeding 100 GHz with an electric signal, a phase-locked loop has been studied (Reference: Buckman LA, et al., "Stabilization of". millimeter-wave frequencies from passive mode-locked semiconductor lasers using
an optoelectronic phase-locked loop ", IEEE J. Photonics Technology. Lett. 5, pp. 1137-1140, 1993).
[0003]
Here, an optical pulse train generated from a passively mode-locked semiconductor laser is detected by a photodiode, the phase of the detected signal is compared with that of an electric signal, and the reverse bias voltage in the saturable absorption region is minimized so that the obtained error signal is minimized. And the repetition frequency is controlled. In the phase locked loop, the error signal is superimposed on the reverse bias voltage through an optimum filter so as not to oscillate.
[0004]
[Problems to be solved by the invention]
In a conventional configuration using a phase-locked loop, since an optical pulse train generated by a passively mode-locked semiconductor laser is detected by a photodiode, the upper limit of the applicable repetition frequency is limited by the reaction time of the photodiode. At present, 50 GHz is the limit at the maximum.
[0005]
SUMMARY OF THE INVENTION It is an object of the present invention to provide an ultrafast synchronized pulse light source that can synchronize an optical pulse having a repetition frequency exceeding 100 GHz with an electric signal.
[0006]
[Means for Solving the Problems]
The ultrafast synchronous pulse light source of the present invention receives an optical pulse output from a passively mode-locked semiconductor laser with a photodiode after phase-modulating the optical pulse. The photodiode generates a beat signal between the modulation sidebands of the synchronous mode or between the modulation sideband and the adjacent synchronous mode, and forms a phase locked loop using the beat signal.
[0007]
The optical pulse having the repetition frequency F ₀ exhibits a synchronous mode with an interval F ₀ on the optical frequency spectrum as shown in FIG. When this optical pulse is phase-modulated, a number of modulation sidebands are generated around each synchronous mode at an interval of the modulation frequency Fm. The modulation sideband holds the phase information of the original synchronous mode, and accordingly, the phase information is also reflected on the beat between the modulation sidebands. Ideal photodiode band infinity, as shown in FIG. 3, the beat signal for each interval Fm around the mode locking signal frequency F ₀ on RF spectrum appears. Even an actual photodiode having a limited frequency band can detect a beat signal in a low frequency region. By using this beat signal and forming a phase-locked loop similar to the conventional one, it becomes possible to synchronize an optical pulse having a repetition frequency exceeding 100 GHz with an electric signal.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
(First Embodiment-Claims 1 and 2)
FIG. 1 shows a first embodiment of the ultrafast synchronous pulse light source of the present invention. This embodiment is applied to a pulse light source having a repetition frequency of 36 GHz.
In the figure, a passive mode-locked semiconductor laser 10, an isolator 11, an optical filter 12 as a light transmitting means, an optical coupler 13 for taking out an optical output, a LiNbO ₃ intensity modulator 14 as a light modulating means, a photodiode as a photoelectric converting means ( PD) 15, an amplifier 16, a mixer 17 as phase comparing means, a low-pass filter (LPF) 18 and an amplifier 19 as negative feedback means, and a saturable absorption region 101 of the passive mode-locked semiconductor laser 10 are connected in a loop. , Forming a phase locked loop. An RF spectrum analyzer 20 of an observation system is connected to the output branched by the optical coupler 13. The LiNbO ₃ intensity modulator 14 is driven by a 20 GHz modulation signal output from the electric oscillator 21. A 16 GHz sine wave signal is input from the electric oscillator 22 to the mixer 17.
[0009]
As the passive mode-locked semiconductor laser 10, a multi-electrode laser in which a saturable absorption region 101 and a gain region 102 are integrally formed on a semiconductor laser waveguide is used. Such a passively mode-locked semiconductor laser 10 oscillates passively in a mode-locked manner due to nonlinear absorption characteristics in which the loss decreases with an increase in light intensity in the saturable absorption region 101, and has an optical pulse train having a pulse width of less than several picoseconds (full width at half maximum). Generate. This optical pulse train passes through an isolator 11 for preventing return light to the laser element, and further passes through an optical filter 12 that removes ASE beat noise due to extra modes and passes only optical frequency components involved in mode locking. , At the optical coupler 13. One of the optical pulse trains is input here as an optical output to the RF spectrum analyzer 20, and the other optical pulse train is input to the LiNbO ₃ intensity modulator 14 of the phase locked loop.
[0010]
The LiNbO ₃ intensity modulator 14 is driven at a modulation frequency of 20 GHz, generates primary modulation sidebands at 16 GHz intervals, and is converted into an electric signal by a photodiode 15. On the RF spectrum, a beat signal (16 GHz) between the primary modulation sideband and the adjacent synchronization mode and a beat signal (4 GHz) between individual primary modulation sidebands generated from the adjacent synchronization mode are used. Present. Here, a strong 16 GHz beat signal is used for the phase locked loop. The beat signal output from the photodiode 15 is input to the mixer 17 via the amplifier 16, and is compared in phase with a 16 GHz sine wave signal output from the electric oscillator 22. The obtained error signal passes through the low-pass filter 18, is amplified by the amplifier 19, and is superimposed on the reverse bias voltage applied to the saturable absorption region 101. By appropriately selecting the gain of the phase locked loop, the optical pulse train generated from the passively mode-locked semiconductor laser 10 can be synchronized with the electric oscillator (FIG. 1 (2)).
[0011]
(Second Embodiment-Claims 1 and 3)
FIG. 4 shows a second embodiment of the ultrafast synchronous pulse light source according to the present invention. This embodiment is applied to a pulse light source having a repetition frequency of 100 GHz.
In the figure, two-stage serially connected LiNbO ₃ intensity modulators 14-1 and 14-2 driven by a 24 GHz modulation signal output from an electric oscillator 23 are used as light modulation means, and a 4 GHz sine is synchronized. It is the same as the first embodiment shown in FIG. 1 except that an electric oscillator 24 for outputting a wave signal is used.
[0012]
In the LiNbO ₃ intensity modulator 14-2, the primary modulation sidebands at 48 GHz intervals are generated around each mode, so that the beat signal on the low frequency side appears at 4, 28, 52, 76 GHz. For the phase locked loop, a beat signal of the lowest 4 GHz, which is easily processed electrically, is used. After the mixer 17 obtains an error signal by comparing the phase with a 4 GHz sine wave signal, the process is the same as that of the first embodiment. The repetition frequency is a non-integer multiple of the modulation frequency. This is because the photodiode 15 cannot perform phase detection.
[0013]
(Third Embodiment-Claims 1 and 4)
FIG. 5 shows a third embodiment of the ultrafast synchronous pulse light source according to the present invention. This embodiment is applied to a pulse light source having a repetition frequency of 200 GHz.
In the figure, an optical frequency comb generator 31 driven by a 24 GHz modulation signal output from an electric oscillator 23 is used as an optical modulating means, and an electric oscillator 25 outputting a synchronized 8 GHz sine wave signal is used. , And is similar to the first embodiment shown in FIG.
[0014]
The optical frequency comb generator 31 has a configuration in which a LiNbO ₃ phase modulator is included in a Fabry-Perot resonator whose FSR is the same as the modulation frequency of 24 GHz. As a result, a large number of modulation sidebands at 24 GHz intervals are generated around each mode. Therefore, detectable beat signals are generated at 8, 32, 56, 80, 104, 128, 152 and 176 GHz. As in the second embodiment, a beat signal of the lowest 8 GHz, which is easily processed electrically, is used for the phase locked loop. After the mixer 17 obtains an error signal by comparing the phase with an 8 GHz sine wave signal, the process is the same as that of the first embodiment.
[0015]
(Fourth embodiment-Claim 5)
FIG. 6 shows a fourth embodiment of the ultrafast synchronous pulse light source according to the present invention. In this embodiment, the optical frequency comb generator 31 used in the third embodiment is replaced with a semiconductor laser, and is applied to a pulse light source having a repetition frequency of 100 GHz.
In the figure, a semiconductor laser 33 is arranged at a position of an optical frequency comb generator 31 via a circulator 32. The drive current of the semiconductor laser 33 is modulated by a 6 GHz electric signal output from the electric oscillator 26. In the semiconductor laser 33, the carrier density is thereby modulated, the refractive index is further modulated, and light injected from the outside undergoes phase modulation. Although the upper limit of the modulation frequency is limited by the response of the carrier, a large number of combs can be generated since the degree of modulation can be increased.
[0016]
Here, since the modulation frequency is 6 GHz, the beat signal that can be detected appears every 6 GHz from 4 GHz to 94 GHz. Among them, the beat signal of 4 GHz is the easiest to process, but since the intensity decreases as the distance from the carrier increases, the beat signal of the maximum frequency that can be easily processed by an electric circuit is used. Here, the upper limit of the electric circuit is set to 20 GHz, and a beat signal of 16 GHz is used for the phase locked loop. After the mixer 17 obtains an error signal by comparing the phase with the 16 GHz sine wave signal, the process is the same as that of the first embodiment.
[0017]
It should be noted that the semiconductor laser 33 may be replaced with a multi-electrode laser having a saturable absorption region and a gain region, and a refractive index change may be realized by modulating a reverse bias current applied to the saturable absorption region. The gain region is used to compensate for loss due to passing through the saturable absorption region. Since the response of the refractive index by reverse bias modulation is faster than by current injection, high frequency modulation can be realized.
[0018]
(Fifth Embodiment-Claim 6)
This embodiment relates to a configuration for improving the signal-to-noise ratio in the third embodiment or the fourth embodiment in which a large number of beat signals are generated by the optical frequency comb generator 31 or the semiconductor laser 33.
Energy is distributed to each sideband by generating a number of modulation sidebands by phase modulation. Due to this energy distribution, the strength of the beat signal deteriorates, and the quality of the error signal of the phase locked loop deteriorates. For this reason, when using the beat between higher-order sidebands, it is necessary to maximize the energy distributed to the sideband that gives the beat signal of interest. This can be achieved by optimizing the maximum frequency shift. That is, since the intensity of each modulation sideband is given by the square of the Bessel function (J _n [Δφ]) ² with the maximum phase shift Δφ of the phase modulation as an argument, by appropriately selecting Δφ, It is possible to maximize the beat signal. For example, as shown in FIG. 7, when Δφ is set near the zero point of the first-order Bessel function, the intensity of the secondary sideband becomes almost maximum. That is, the optimum value in the case of using the secondary sideband can be adjusted by adjusting the amplitude of the phase modulation to the zero point of the first-order Bessel function.
[0019]
(Sixth Embodiment-Claim 7)
The present embodiment relates to a configuration for optimizing a phase locked loop for controlling a repetition frequency of a passively mode-locked semiconductor laser by obtaining an error signal from a beat signal and minimizing phase noise of an optical pulse train.
The beat signal includes a phase noise component corresponding to the repetition frequency fluctuation and an intensity noise component corresponding to the intensity fluctuation of the optical pulse train. Therefore, even if the error signal is created by simply dropping the beat signal to the baseband, the noise may increase due to the intensity noise component, and conversely, the optical pulse train may become unstable. In order to prevent this, for example, a beat signal of a higher-order mode synchronization signal having an integral multiple of 2 or more of the repetition frequency is used in the phase locked loop. At this time, in the higher-order mode-locked signal, the phase noise component increases in proportion to the square of the order, and the intensity noise component becomes relatively small, so that an error signal can be created purely from the phase noise component. Become. By using such an error signal, even if the gain of the phase locked loop is sufficiently increased, the loop does not oscillate, and the optical pulse train can be stabilized with high accuracy.
[0020]
【The invention's effect】
As described above, the ultrahigh-speed synchronous pulse light source of the present invention can generate an optical pulse train that can be synchronized with an electric signal and has a repetition frequency exceeding 100 GHz.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of an ultrafast synchronous pulse light source according to the present invention.
FIG. 2 is a diagram showing an optical frequency spectrum.
FIG. 3 is a view showing an RF spectrum.
FIG. 4 is a diagram showing a second embodiment of the ultrafast synchronous pulse light source according to the present invention.
FIG. 5 is a diagram showing a third embodiment of the ultrafast synchronous pulse light source according to the present invention.
FIG. 6 is a diagram showing a fourth embodiment of the ultrafast synchronous pulse light source according to the present invention.
FIG. 7 is a diagram showing a Bessel function of the first kind.
[Explanation of symbols]
Reference Signs List 10 passive mode-locked semiconductor laser 11 isolator 12 optical filter 13 optical coupler 14 LiNbO ₃ intensity modulator 15 photodiode (PD)
16 Amplifier 17 Mixer 18 Low-pass filter (LPF)
19 amplifier 10 RF spectrum analyzer 21 electric oscillator (20 GHz)
22 Electric oscillator (16 GHz)
23 Electric oscillator (24GHz)
24 Electric oscillator (4GHz)
25 Electric oscillator (8 GHz)
26 Electric oscillator (6GHz)
31 optical frequency comb generator 32 circulator 33 semiconductor laser 101 saturable absorption region 102 gain region

Claims (7)

A passive mode-locked semiconductor laser that generates an optical pulse train at a first repetition frequency;
Light transmission means for transmitting only the optical frequency components involved in mode locking from the optical output of the passive mode-locked semiconductor laser,
Branching means for branching the output of the light transmitting means and taking out one as a light output;
Light modulation means for optically modulating the other light branched by the branching means at a second frequency, photoelectric conversion means for converting the light output of the light modulation means into an electric signal,
Phase comparing means for comparing the phase of the beat signal between the modulation sidebands of the synchronous mode or between the modulation sideband and the adjacent synchronous mode with the third frequency, which is output from the photoelectric conversion means,
An ultrafast synchronous pulse light source, comprising: negative feedback means for superimposing an error signal output from the phase comparing means on a reverse bias voltage of the passive mode-locked semiconductor laser. 2. The ultrafast synchronous pulse light source according to claim 1, wherein the light modulation means is a light intensity modulator. 2. The ultrahigh-speed synchronous pulse light source according to claim 1, wherein the light modulating means is a light intensity modulator connected in multiple stages in series. 2. The ultrafast synchronous pulse light source according to claim 1, wherein the optical modulation means is an optical frequency comb generator. The ultrafast synchronous pulse light source according to claim 4, wherein the optical frequency comb generator is a semiconductor laser having a gain region and an electric modulation means. The ultrahigh-speed synchronous pulse light source according to claim 4 or 5, wherein the modulation amplitude of the light modulation means is set to a value at which the intensity of the beat signal output from the photoelectric conversion means is maximized. 2. The ultrahigh-speed synchronous pulse light source according to claim 1, wherein the beat signal output from the photoelectric conversion means is based on a high-order mode synchronous signal.

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