JPH06326390A - Optical pulse pattern generator - Google Patents

Abstract

PURPOSE:To provide an optical pulse pattern generator capable of generating an ultra-high speed optical pulse pattern having no jitter. CONSTITUTION:Ultra-high speed clock light, in which the positions of pulses are set at equal intervals and which has a wavelength lambda1 of a uniform intensity, is generated by an ultra-high speed clock light generating circuit 1 and after a modulation is performed on short optical pulse trains, which are comparatively low in speed, are small in a duty ratio and have a wavelength lambda2 (not equal to lambda1), by an optical pulse signal generating circuit 2, the pulse trains are multiplexed and an optical pulse signal to respond an ultra-high speed optical pulse pattern is generated. These of the ultra-high clock light and the optical pulse signal are enter into an optical switch 4, which utilizes a photo nonlinear effect, and the ultra-high speed clock light is switches by the optical pulse signal. Thereby, the clock light is directly modulated and an ultra-high speed optical pulse pattern having no jitter is generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse pattern generator capable of generating an ultrahigh-speed optical pulse pattern without jitter, which is required for evaluating an optical fiber transmission system of several tens Gb / s. .

[0002]

2. Description of the Related Art At present, there are a laser oscillation control method and a laser light control method as a method of generating a short optical pulse train using a semiconductor laser (LD). As a typical method of the former, in a very thin cell-like saturable absorber, CPM (Colliding) in which two optical pulses traveling in opposite directions collide with each other.
Pulse Mode-lockig) method, which has excellent pulse shortening effect and stability. Further, as the latter typical method, a beat generated by combining lights from two semiconductor lasers having different oscillation frequencies is solitonized by guiding it to an optical fiber having anomalous dispersion to obtain a short optical pulse train. There is a way. Both of these methods can generate a TL (Transform Limited) pulse suitable for optical pulse transmission.

[0003]

However, the short optical pulse train that can be generated by the above-mentioned method is extremely high speed, and the upper limit of the modulatable band of the external modulator currently in practical use is about 20 GHz. Considering the above, it is very difficult to directly modulate such an ultrafast optical pulse train, and there is a problem that an ultrafast optical pulse pattern cannot be generated. A method has also been proposed in which an ultrahigh-speed optical pulse pattern is generated by optically multiplexing a modulated relatively low-speed optical pulse pattern using an optical fiber delay line or the like (Electron Let
t., Vol.28, No.19, 1992, K. Iwatsuki, et al., p.182
1)), it is difficult to arrange the positions of each optical pulse at exactly equal intervals with this method, and it is impossible to obtain the optical pulse pattern without jitter required for evaluation of ultrahigh-speed optical transmission systems. There was a problem.

In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide an apparatus capable of generating an ultrafast optical pulse pattern without jitter.

[0005]

Means for Solving the Problems] For the present invention to achieve the above object, the ultrafast optical clock generation means for generating an ultrafast optical clock wavelength lambda _1, the short optical pulse train having a wavelength λ ₂ (≠ λ ₁₎ Optical pulse signal generating means for generating an optical pulse signal having a wavelength λ ₂ corresponding to an ultra-high-speed optical pulse pattern after optical modulation, and the repetition frequency and the optical frequency of the ultra-high-speed clock light. Phase synchronization means for synchronizing the average repetition frequency of the pulse signal, and the ultrahigh-speed clock light and the optical pulse signal are input, and the ultrahigh-speed clock light is switched by the optical pulse signal to obtain the ultrahigh-speed wavelength λ ₁ . An optical pulse pattern generator comprising an optical switch means for outputting an optical pulse pattern is proposed.

[0006]

According to the present invention, the ultra high speed clock light generating means generates the ultra high speed clock light having the wavelength λ ₁ , and the optical pulse signal generating means generates the wavelength λ ₂ (≠ λ ₁ corresponding to the ultra high speed optical pulse pattern. 2) optical pulse signals are generated, these are made incident on the optical switch means, and the ultrahigh-speed clock light is switched by the optical pulse signals to generate an ultrahigh-speed optical pulse pattern without jitter.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic structure of an optical pulse pattern generator of the present invention. In the figure, 1 is an ultrahigh speed clock light generating circuit, 2 is an optical pulse signal generating circuit, 3 is a phase comparator,
Reference numeral 4 is an optical switch.

The ultra-high-speed clock light generation circuit 1 generates TL ultra-high-speed clock light of wavelength λ _{1 in} which the positions of the pulses are evenly spaced and the intensity is uniform. Specifically, a method of introducing a beat obtained by combining lights from two semiconductor lasers having different oscillation frequencies into an anomalous dispersion optical fiber and shortening the optical pulse by adiabatic compression (IEEE J. Quantum Electron , Vo
l.27, No.10, 1991, Pavel V. Mamyshev, et al., p.234
7), CPM semiconductor laser (CPM-LD) (Ele
ctron Lett., Vol.25, No.10, 1989, RSTucker, et a
l., p.621, or CLEO'91, CWK3, 1991, YKChen, et a
l., p.304) to generate ultra-high-speed clock light.

The optical pulse signal generation circuit 2 comprises a short optical pulse train generation circuit 2a, a modulator 2b and a pulse pattern generator 2c.
And an optical multiplexing circuit 2d. First, the short light pulse train generation circuit 2a generates a short light pulse train of a wavelength λ ₂ (≠ λ ₁ ) having a relatively low speed and a small duty ratio. Specifically, it occurs when the LD is gain-switched or mode-locked at a relatively low speed. Next, the short optical pulse train is incident on a modulator 2b driven by an electric signal from a pulse pattern generator 2c, modulated, and then optically multiplexed by an optical multiplexing circuit 2d to generate an optical pulse signal. . Specifically, optical multiplexing is performed by using a delay line formed of an optical waveguide or by using a polarization maintaining fiber to give a group delay.

At this time, the obtained optical pulse signal corresponds to an ultrafast optical pulse pattern to be generated.
The intensity of each pulse and its position are not constant and cannot be used as they are. Therefore, the optical pulse signal is guided to the optical switch 4 together with the ultra high speed clock light generated by the ultra high speed clock light generation circuit 1, and the ultra high speed clock light is switched by the optical pulse signal for modulation.

The optical switch 4 used in the present invention needs to have the following two characteristics.

First, the relationship between the input light intensity of the optical pulse signal and the output light intensity of the ultrahigh speed clock light after switching has a non-linear characteristic as shown in FIG. As a result, the output light intensity of the ultrahigh-speed clock light after switching hardly changes with respect to the change of the input light intensity of the optical pulse signal.

Secondly, it has a switching window wider than the pulse width of the ultra high speed clock light which is the pulse to be switched. This is because the ultrafast clock light of wavelength λ ₁ and the wavelength λ ₁
By providing a group velocity difference with the optical pulse signal of ₂ , the time delay is generated as they propagate through the optical switch 4 as shown in FIGS. 3 (a), (b), and (c). (Walk-o
ff) By making the optical pulse signal relatively pass over the ultra-high speed clock light, it functions as a switching window wider than the pulse width of the ultra-high speed clock light. In addition, walk-off
When the time is longer than the pulse width of the ultrahigh-speed clock light, the shape of the window becomes almost rectangular (Fig. 3 (d)). As a result, even if the optical pulse signal has jitter, it is absorbed and
As shown in (e), the jitter is suppressed in the ultra-high-speed clock light after switching.
-737, Kamino et al., Or Nonlinear Fiber Optics,
1989, Academic Press, GPAgrawal, p.180).

Such an optical switch is a non-linear Sagnac Interferometer Swic
h: NSIS) or optical car shutter. On this occasion,
The first feature is satisfied by the fact that the input and output characteristics change in a sinusoidal manner because their operating principle uses optical interference, and the second feature is the nonlinear medium of the optical switch. by choosing the dispersion of certain dispersion shifted fiber appropriately, is satisfied by generating the walk-off between the ultrafast optical clock wavelength lambda ₂ of the optical pulse signal having the wavelength lambda _1.

The phase comparator 3 serves as a phase synchronizing means for synchronizing the repetition frequency of the ultra-high speed clock light and the average repetition frequency of the optical pulse signal.

FIG. 4 shows a first embodiment of the present invention. In the figure, 11 is an oscillator, 12 is a pulse pattern generator, and 13, 14 and 15 are distributed feedback semiconductor lasers (hereinafter, DFB-LD). 16) is a modulator, 17 and 1
Reference numeral 8 is a PIN photodiode (hereinafter referred to as PIN-PD), 19 is an optical delay line, 20 and 21 are erbium-doped optical fiber amplifiers (hereinafter referred to as EDFA), 22 is a loop mirror, and 23 is an optical filter. , 24 is a distributed optical amplifying fiber designed to be doped with erbium at a concentration lower than usual and gradually amplified, and 25 is an amplifier, 2
Reference numerals 6 and 27 are filters, 28 is a multiplier, 29 is a phase comparator, and 31, 32, 33, 34 and 35 are optical fiber couplers. The operation of this embodiment will be described below.

The DFB-LD 13 gain-switched by the sine wave signal from the oscillator 11 generates a short optical pulse train with a relatively low speed and a small duty ratio. The optical pulse train enters a modulator 16 driven by an electric signal from a pulse pattern generator 12, is modulated, and then enters an optical fiber coupler 31 to generate an optical pulse signal 101 and reference light for phase synchronization (contents). Is the same as the optical pulse signal 101). The optical pulse signal 101 is optically multiplexed by the optical delay line 19 to become an ultrahigh-speed optical pulse signal, and
Amplified by FA20.

On the other hand, the DFB-LDs 14 and 15 generate CW light having slightly different oscillation wavelengths.
The two CW lights are incident on the optical fiber coupler 32 and are combined to generate a beat. The beat is amplified by the EDFA 21, guided to the distributed optical amplifying fiber 24, solitonized under adiabatic conditions, and then incident on the optical fiber coupler 33. The same as the high-speed clock light 102).
Here, instead of the distributed optical amplification fiber 24, a method using a dispersion shift fiber whose dispersion gradually decreases in the longitudinal direction, or a method of connecting fibers having different dispersion values in multiple stages via EDFAs is the same. The effect of is obtained.

Further, in order to synchronize the phase of the optical pulse signal 101 and the ultra high speed clock light 102, the reference light of the optical pulse signal 101 is converted into an electrical reference signal 103 by the PIN-PD 17 and the ultra high speed clock light 102 is also converted. 102 reference light is P
It is converted into an electrical reference signal 104 by the IN-PD 18. The reference signal 103 is amplified by the amplifier 25, passes through the filter 26, is multiplexed by 2 ^{n by the} multiplier 28, further passes through the filter 27, and is input to the phase comparator 29 together with the reference signal 104. At this time, the obtained error signal is one of DFB-LD 14 or 15,
Here, it is fed back to 15 injection currents or a temperature control circuit (not shown) to form a phase locked loop (PLL).

The repetition frequency of the optical pulse signal 101, for example, f ₁ and the repetition frequency of the ultrahigh-speed clock light 102, for example, f ₂ are not arbitrary and are limited to f ₂ = 2 ⁿ × f ₁ .

The optical pulse signal 101 and the ultra high speed clock light 102 are guided to a loop mirror 22 which is one of optical switches, and the ultra high speed clock light 102 is modulated by switching. Specifically, the ultra high speed clock light 102 is divided into two by the optical fiber coupler 34, and the loop mirror 2
2 propagate in opposite directions. The optical pulse signal 101 is guided to the loop mirror 22 by the optical fiber coupler 35,
Cross phase modulation (XPM) via the optical Kerr effect is applied to the ultrafast clock light 102 propagating in the same direction. As a result, a phase difference is generated between the ultra high speed clock lights 102 propagating in opposite directions, and if the intensity of the optical pulse signal 101 is adjusted so that the phase difference becomes π, the ultra high speed clock light 102 will be completely Switched.

Jitter and intensity noise can be suppressed by utilizing the walk-off between the optical pulse signal and the ultrahigh-speed clock light and the nonlinear characteristics of the loop mirror as described above. Also, an optical car shutter can be used as the optical switch instead of the loop mirror.

The optical pulse signal 101 is removed from the output light of the loop mirror 22 by the optical filter 23, and an encoded ultrafast short optical pulse train, that is, an optical pulse pattern is obtained.

The optical pulse signal can be removed by the optical filter 23 in the wa in the optical switch.
This is because the center wavelengths of the optical pulse signal and the ultrafast clock light are set to be different because lk-off is used.

FIG. 5 shows a second embodiment of the present invention, in which a frequency divider is used instead of the multiplier in the first embodiment. That is, in the figure, 41 is a frequency divider, which is inserted between the PIN-PD 18 and the phase comparator 29.

In the above structure, the reference light of the optical pulse signal 101 is converted into the electrical reference signal 103 by the PIN-PD 17, amplified by the amplifier 25, passed through the filter 26, and then input to the phase comparator 29. . On the other hand, the reference light of the ultrahigh-speed clock light 102 is converted into an electrical reference signal 104 by the PIN-PD 18, and the frequency divider 41 divides it by 1/2 ^n.
After frequency division, it is input to the phase comparator 29. On this occasion,
The obtained error signal is fed back to the injection current of either one of the DFB-LDs 14 or 15 or the temperature control circuit to configure a phase locked loop (PLL). In addition,
Other configurations and operations are similar to those of the first embodiment.

FIGS. 6 and 7 show third and fourth embodiments of the present invention. Here, a grating and a Fabry-Perot semiconductor laser are used instead of the DFB-LD 13 in the first and second embodiments, respectively. The example used is shown below.
That is, in the figure, 42 is a grating and 43 is a Fabry-Perot semiconductor laser (hereinafter referred to as FP-LD).

In the above structure, the signal from the oscillator 11 is sent to the FP-LD 43, and the pulse pattern generator 12 is used.
The data signal from is supplied to the modulator 16. FP-L
The pulse generated in D43 is mode-locked by the grating 42, enters the modulator 16, is modulated, and then enters the optical fiber coupler 31. The optical pulse signal 101 and the reference light for phase synchronization (content is the optical pulse signal). Same as 101). The optical pulse signal 101 is the optical delay line 1
It is optically multiplexed by 9 to form an ultrahigh-speed optical pulse signal, which is further amplified by the EDFA 20. The other configurations and operations are similar to those of the first and second embodiments.

The present embodiment is characterized in that the mode-locking method is adopted for generating the optical pulse signal. Further, in the present embodiment, the semiconductor active mode-locking method is used for generation, but a passive mode-locking semiconductor laser may be used.

FIGS. 8, 9, 10, and 11 show the fifth, sixth, seventh, and eighth embodiments of the present invention, in which the first, second, third, and eighth embodiments, respectively. In the fourth embodiment, the DFB-LDs 14 and 15 and the distributed amplification type optical fiber 2
An example in which an active CPM semiconductor laser is used instead of No. 4 will be shown. That is, in the figure, reference numeral 44 is an active CPM semiconductor laser (hereinafter referred to as an active CPM-LD).

In the above configuration, the active CPM-LD4
After being amplified by the EDFA 21, the ultra-high-speed clock light generated by the laser beam No. 4 is incident on the optical fiber coupler 33 and separated into the ultra-high-speed clock light 102 and the reference light (the contents are the same as the ultra-high-speed clock light 102). Regarding the phase synchronization, as in the case of the first to fourth embodiments, the reference light of the optical pulse signal 101 is converted into an electrical reference signal 103 by the PIN-PD 17, and the reference light of the ultra-high speed clock light 102 is converted. To the electrical reference signal 104 by the PIN-PD18.
To the injection current control circuit (not shown) of the active CPM-LD 44, and the error signal obtained by inputting these signals into the phase comparator 29 is fed back to form a phase locked loop circuit (PLL).

This embodiment is characterized in that a pulse train obtained by a mode-locking method using an active CPM-LD is used to generate an ultrahigh-speed clock light.

FIGS. 12, 13, 14, and 15 show the ninth, tenth, eleventh, and twelfth embodiments of the present invention, in which the first, second, third, and fourth embodiments, respectively. An example in which a passive CPM semiconductor laser and a voltage controlled oscillator are used instead of the DFB-LDs 14 and 15 and the distributed amplification optical fiber 24 in the fourth embodiment will be described. That is, in the figure, 45 is a passive CPM semiconductor laser (hereinafter, referred to as a passive CPM-LD).
Called. ) And 46 are voltage controlled oscillators (VCOs).

In the above structure, the passive CPM-LD4
5 and the ultra-high-speed clock light generated by the VCO 46 is E
After being amplified by the DFA 21, it is incident on the optical fiber coupler 33 and is separated into the ultra-high speed clock light 102 and the reference light (the contents are the same as the ultra-high speed clock light 102). Regarding the phase synchronization, as in the case of the first to fourth embodiments, the reference light of the optical pulse signal 101 is converted into the electrical reference signal 103 by the PIN-PD 17, and the reference light of the ultra-high speed clock light 102 is converted. Is converted into an electrical reference signal 104 by the PIN-PD 18, and an error signal obtained by inputting these into the phase comparator 29 is fed back to the VCO 46 as a reverse bias voltage signal, so that the phase synchronization circuit (PLL) is activated. Constitute.

This embodiment is characterized in that a pulse train obtained by a mode-locking method using a passive CPM-LD is used to generate an ultrahigh-speed clock light.

[0036]

As described above, according to the present invention, ultra-high-speed clock light in which the positions of each pulse are equally spaced and uniform in intensity, and a short optical pulse train having a relatively low speed and a small duty ratio are modulated. Since the optical pulse signal generated by multiplexing as described above is incident on the optical switch and the ultrahigh-speed clock light is directly modulated, an ultrahigh-speed optical pulse pattern without jitter can be generated.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the relationship between the light intensity of an optical pulse signal in an optical switch and the light intensity of ultrafast clock light after switching.

FIG. 3 is a diagram showing how an ultrafast clock light is switched by an optical pulse signal.

FIG. 4 is a configuration diagram showing a first embodiment of the present invention.

FIG. 5 is a configuration diagram showing a second embodiment of the present invention.

FIG. 6 is a configuration diagram showing a third embodiment of the present invention.

FIG. 7 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 10 is a block diagram showing a seventh embodiment of the present invention.

FIG. 11 is a configuration diagram showing an eighth embodiment of the present invention.

FIG. 12 is a configuration diagram showing a ninth embodiment of the present invention.

FIG. 13 is a configuration diagram showing a tenth embodiment of the present invention.

FIG. 14 is a configuration diagram showing an eleventh embodiment of the present invention.

FIG. 15 is a configuration diagram showing a twelfth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ultra high-speed clock light generation circuit, 2 ... Optical pulse signal generation circuit, 3 ... Phase comparator, 4 ... Optical switch, 11 ... Oscillator, 12 ... Pulse pattern generator, 13-15 ... Distributed feedback semiconductor laser, 16 … Modulator, 17, 18… PIN
Photodiode, 19 ... Optical delay line, 20, 21 ... Erbium-doped optical fiber amplifier, 22 ... Loop mirror, 2
3 ... Optical filter, 24 ... Distributed optical amplification fiber, 25
... Amplifier, 26, 27 ... Filter, 28 ... Multiplier, 2
9 ... Phase comparator, 31-35 ... Optical fiber coupler, 4
1 ... Frequency divider, 42 ... Grating, 43 ... Fabry-Perot semiconductor laser, 44 ... Active CPM semiconductor laser,
45 ... Passive CPM semiconductor laser, 46 ... Voltage controlled oscillator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masatoshi Saruwatari 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] _1. An ultra-high-speed clock light generating means for generating an ultra-high-speed clock light of wavelength λ _{1 and} a short optical pulse train of wavelength λ ₂ (≠ λ ₁ ) are generated, modulated, and then optically multiplexed. Optical pulse signal generating means for generating an optical pulse signal having a wavelength λ ₂ corresponding to an ultrafast optical pulse pattern, and phase synchronization for synchronizing the repetition frequency of the ultrafast clock light and the average repetition frequency of the optical pulse signal Means and an optical switch means for inputting the ultra-high-speed clock light and the optical pulse signal and switching the ultra-high-speed clock light with the optical pulse signal to output an ultra-high-speed optical pulse pattern of wavelength λ _1. Characteristic optical pulse pattern generator.