JPH098741A - Optical clock extracting circuit - Google Patents

Abstract

PURPOSE: To stably generate an optical clock by extracting a clock frequency component from an optical signal at an ultrahigh speed by converting an optical signal to an electric signal and efficiently driving a mode locked laser. CONSTITUTION: A mode locked laser 10 is constituted by arranging a gain medium 13 and a mode lock modulator 14 inside a Fabry-Pe'rot resonator composed of two confronting mirrors 11 and 12. The optical signal is converted to the electric signal by an optic/electric converter 20 and inputted through an impedance matching circuit 30 to the mode lock modulator 14 of the mode locked laser 10. Thus, the optical clock is generated at the frequency pulled into the clock frequency of the electric signal. Since the dependency of the optic/electric converter 20 onto the polarization and wavelength of incident light is reduced for such an optical clock extracting circuit, even when generating the optical clock, the dependency on the polarization and wavelength of incident light can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical clock extraction circuit for extracting and extracting an optical clock from an optical signal.

[0002]

2. Description of the Related Art FIG. 9 shows a configuration example of a conventional optical clock extraction circuit. In the figure, a cross phase modulator 81, an optical amplifier 82, an optical isolator 83, a wavelength filter 84, and an optical coupler 85 are connected through an optical fiber to form a ring type optical resonator. When the cross-phase modulator 81 enters an optical signal having a clock frequency f ₀ to the ring-type optical resonator, when the resonance frequency interval is an integral submultiple of 1 f _0, the optical coupler 85 is an optical clock having a frequency f ₀ Taken from. An optical clock extractor with this configuration was reported by Smith et al. ("All-optical clockrecover
y using a mode-locked laser ", Electronics Letters,
vol.28, pp.1814-1816 (1992)). In this optical clock extraction circuit, it is possible to extract a clock from a high-speed signal of 10 Gbit / s or more by using, for example, an ultrahigh-speed cross-phase modulator that uses a nonlinear optical effect in an optical fiber.

FIG. 10 shows another example of the configuration of a conventional optical clock extraction circuit. In the figure, the optical signal is an optical coupler 91.
Is incident on the mode-locked semiconductor laser 92 via
An optical clock corresponding to the optical signal is taken out via the optical coupler 91. An optical clock extraction circuit with this configuration has been reported by Ono et al. (“10 Gb / s optical clock extraction experiment using a monolithic mode-locked LD”, 1995).
IEICE General Conference B-1157).

[0004]

The conventional optical clock extraction circuit shown in FIG. 9 has the following problems. Since the resonator length is as long as several tens of meters or more, the ratio of the resonant frequency interval to the clock frequency of the optical signal becomes 100 times or more, making it difficult to keep the system stable. Since the resonator length is long, it takes time to oscillate a stable clock.

It is difficult to adjust the operating bandwidth. Strongly depends on the polarization of incident light. Strongly depends on the wavelength of incident light. At present, only a long optical fiber (several kilometers) can be used as an ultrahigh-speed cross-phase modulator, and the above-mentioned problems (1) to (4) are further increased.

In the conventional optical clock extraction circuit shown in FIG. 10, the problems of ,,,, are alleviated, but the problems of ,, remain. This is because the optical resonator characteristics of the mode-locked semiconductor laser generally have a strong polarization dependence, and the wavelength also has a strong wavelength dependence reflecting the resonator structure. The present invention solves the above problems and enables stable optical clock generation by extracting a clock frequency component from an ultrahigh-speed optical signal, capable of outputting a stable optical clock in a short time, and operating band An optical clock extraction circuit that can easily adjust the width, can reduce the dependency on the polarization of incident light, can reduce the dependency on the wavelength of incident light, and can be miniaturized by integration. The purpose is to do.

[0007]

The optical clock extraction circuit of the present invention comprises an opto-electric converter for converting a digital optical signal into an electric signal, an optical resonator, and a gain medium and a mode arranged in the optical resonator. A mode-locked laser composed of a mode-locked modulator and an impedance matching circuit that couples the output signal of the opto-electric converter to the mode-locked modulator of the mode-locked laser are provided. Outputs the corresponding optical clock.

Further, a frequency filter for limiting the tuning bandwidth of clock extraction is provided between the photoelectric converter and the impedance matching circuit. Further, an electric amplifier for amplifying the output of the photoelectric converter is provided between the photoelectric converter and the impedance matching circuit. In addition, an optical amplifier that amplifies a digital optical signal incident on the opto-electric converter is provided in the preceding stage of the opto-electric converter.

An optical filter having a free spectrum range that is an integral multiple of the mode frequency interval of the mode-locked laser and that generates a harmonic clock is provided after the mode-locked laser. The optical clock extraction circuit of the present invention is
The mode-locked laser is a mode-locked semiconductor laser integrated on a semiconductor substrate.

The optical clock extraction circuit of the present invention is constructed by integrating the respective constituent elements on a semiconductor substrate.

[0011]

The optical clock extraction circuit of the present invention converts an optical signal into an electric signal and efficiently drives a mode-locked laser to generate an optical clock at a frequency drawn into the clock component of the electric signal. You can

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a first embodiment of the optical clock extraction circuit of the present invention.
The structure of an Example is shown. In the figure, a mode-locked laser 10 is constructed by arranging a gain medium 13 and a mode-lock modulator 14 in a Fabry-Perot resonator composed of two mirrors 11 and 12 facing each other. The optical signal is converted into an electric signal by the photoelectric converter 20 and input to the mode-lock modulator 14 of the mode-lock type laser 10 via the impedance matching circuit 30. The photoelectric converter 2
For 0, a photodiode, an avalanche photodiode, an MSM photodetector, or the like can be used. Further, an optical filter that determines the oscillation frequency band may be inserted in the Fabry-Perot resonator.

FIG. 2 shows another configuration example of the mode-locked laser 10 in the first embodiment. In the figure, a mode-locked laser 10 is configured by arranging a gain medium 13 and a mode-lock modulator 14 in a ring resonator composed of three or more mirrors, optical fibers, or optical waveguides. Further, an optical branching means 15 for extracting an optical clock is arranged in the ring resonator, and an optical isolator 16 for determining the circulation direction of light and an optical filter 17 for determining the oscillation frequency band are inserted as necessary.

FIG. 3 shows electrical coupling when a mode-locked semiconductor laser is used as the mode-locked laser 10. In the figure, a mode-locked semiconductor laser is monolithically integrated into a waveguide on a semiconductor substrate and is shown by an equivalent circuit in which diodes corresponding to a gain region and a modulation region are coupled via a resistor. The impedance matching circuit 30 includes capacitors 31, 32, a coil 33, and a resistor 34.
It has a function of efficiently transmitting a high frequency signal input from the device to the mode-locked semiconductor laser and applying a bias voltage V _B. Since the resistor 34 has a high impedance in the modulation region of the mode-locked semiconductor laser, a resistor having a low resistance of tens of Ω is usually used.

It is assumed that an optical signal coded at a clock frequency f ₀ is input to the optical clock extraction circuit of the present invention. If the optical signal is intensity-modulated by RZ (return zero), this RZ signal has a strong frequency component at f ₀ . When the optical signal is intensity-modulated by NRZ (non return zero), the clock frequency component can be generated using the Mach-Zehnder interferometer shown in FIG. When the NRZ signal is incident on the Mach-Zehnder interferometer, it is branched into two paths. The optical path difference of each path is set to 1/2 or less of the time of 1 bit and the phase difference is set to a length corresponding to an odd multiple of π. When the lights propagating through the respective paths are combined, they interfere with each other and disappear from the waveguide as a divergence mode. That is, a new optical pulse is generated corresponding to the edge of the original pulse, and a clock component is generated.

When the optical signal is phase-modulated, it is incident on the optical clock extraction circuit of the present invention after being converted into intensity modulation by a dispersion medium or the like. The optical signal incident on the optical clock extraction circuit of the present invention is converted into an electric signal by the photoelectric converter 20. This electric signal is transmitted to the impedance matching circuit 30.
And is efficiently applied to the mode-locked modulator 14 of the mode-locked laser 10. Mode-locked laser 1
0 is the mode frequency interval f corresponding to the resonator structure.
_When the mode-lock modulator 14 is modulated with a signal of _0MD , the respective oscillation light modes are combined. At this time, the light emitted from the mode-locked laser 10 becomes a short pulse train whose repetition frequency is equal to the modulation frequency.

The mode frequency interval f _0MD of the resonator is f _0MD = c / (2 · N _eff , where L is the resonator length of the laser, N _eff is the effective refractive index in the resonator, and c is the speed of light.・ L). Here, when the clock frequency f ₀ and the mode frequency interval f _0MD of the resonator match, optical mode coupling is performed with the weakest signal, but when the signal is strong, f ₀ and f
_Even when _0MD does not match and is close to each other, it is pulled in by the signal applied to the mode-lock modulator and an optical clock of f ₀ is generated. If the applied signal strength is sufficient, the pull-in operation occurs within the range of | f ₀ −f _0MD | / f ₀ <0.003.

Further, the mode-locked laser 10 has a f ₀
It is possible to generate an optical clock having a frequency (2f ₀ , 3f ₀ , ...) Of an integral multiple of f ₀ even for an applied signal having a frequency (2f ₀ , 3f ₀ , ...) Of an integral multiple of. That is, the mode frequency interval f _0MD of the resonator is approximately an integer fraction of the clock frequency f _0.
The optical clock extraction circuit of f ₀ can be configured by setting such that

In the present embodiment, the photoelectric converter 20 generally has little dependence on the polarization and the wavelength, so that the dependence on the polarization and the wavelength of the incident light can be reduced even in the optical clock extraction circuit. In particular, when a mode-locked semiconductor laser is used, the resonator length is short and f ₀ has a frequency several times higher than f _0MD at most, so that a stable optical clock can be output. Also, a stable optical clock can be output in a short time. Further, the mode-locked semiconductor laser can be driven at an ultrahigh speed of 10 GHz or more, and an ultrahigh-speed optical clock can be easily generated.

FIG. 5 shows the configuration of the second embodiment of the optical clock extraction circuit of the present invention. The feature of this embodiment is that the photoelectric converter 20 and the impedance matching circuit 3 in the first embodiment are different.
Between 0 and 0, the electric amplifier 41 and the frequency filter 42
Is in place. Since the electric amplifier 41 and the frequency filter 42 have independent functions,
It is not always necessary to place both at the same time.

Even if the optical signal incident on the opto-electric converter 20 is weak and the level of the electric signal output is small,
An optical clock can be extracted by driving the mode-locked laser 10 after being amplified by the electric amplifier 41. Further, by disposing the frequency filter 42, the tuning bandwidth of clock extraction can be controlled. Further, even if the input signal from the opto-electric converter 20 contains a frequency component other than f ₀ , it is possible to generate a stable clock by removing it with the frequency filter 42. Further, even if the optical clock has a high speed, the required frequency bandwidth is narrow, so that it can be easily constructed.

FIG. 6 shows the configuration of the third embodiment of the optical clock extraction circuit of the present invention. The feature of this embodiment is that an optical amplifier 43 is arranged in the front stage of the photoelectric converter 20 in the first embodiment, and an optical filter 44 is arranged in the rear stage of the mode-locked laser 10.
Is in place. Since the optical amplifier 43 and the optical filter 44 have independent functions, they do not necessarily have to be arranged at the same time.

Even if the optical signal is weak, the optical clock can be extracted by being amplified by the optical amplifier 43 and entering the photoelectric converter 20. The optical filter 44 has a function of generating a harmonic clock of f ₀ from a signal of the clock frequency f ₀ . For example, although NRZ signal is a frequency component of f _0/2, but can not extract the clock is unchanged since the frequency components there is little f _0, the optical filter 44
It becomes possible by using. Hereinafter, the optical filter 44
The function of will be described with reference to FIG. Here, f
And _0/2 = f _0MD.

FIGS. 7A and 7C show a mode-locked laser 1
The optical spectrum and time waveform of 0 output are shown. Mode spacing is f _0/2, the frequency band of the envelope of the optical spectrum is f _B. The optical spectrum and the time waveform have a Fourier conjugate relationship, and the time waveform of (c) corresponds to the optical spectrum of (a). Here, for example, the optical mode on the optical spectrum is thinned by the optical filter 44 having the free spectral range f ₀ ,
When the optical spectrum shown in FIG. 7 (b) is generated, the corresponding time waveform is as shown in FIG. 7 (d). That is, it is possible to generate an optical clock having a frequency double that of the optical clock output from the mode-locked laser 10. Similarly, if the free spectrum range of the optical filter 44 is changed to change the thinning-out method of the optical mode, it is possible to generate a harmonic clock of triple or more.

FIG. 8 shows an implementation example of the optical clock extraction circuit of the present invention. In the figure, on the InP semiconductor substrate 51,
A pin photodiode 21, which serves as an optoelectric converter 20, an impedance matching circuit 30, and a mode-locked laser 1.
0, the optical filter 44 is formed. Such a structure in which the constituent elements are integrated on the InP semiconductor substrate 51 can be easily realized by a known technique. It should be noted that the impedance matching circuit 30 is characterized by operating at a very high speed with InP type materials.

Since the optical clock extraction circuit of the present invention can be integrated on a semiconductor substrate and miniaturized as in the present embodiment, temperature control and the like are easy, so that a stable optical clock can be generated. You can

[0027]

As described above, in the optical clock extraction circuit of the present invention, the polarization of the photoelectric converter with respect to the incident light and the dependence on the wavelength are small, and therefore the polarization of the incident light with respect to the generation of the optical clock is also small. And the dependence on wavelength can be reduced.

Since the optical clock extraction circuit of the present invention can be constructed in a small size, the ratio of the resonance frequency interval and the clock frequency of the optical signal is small, and the atmosphere can be easily controlled, so that a stable optical clock can be generated. You can Also, a stable optical clock can be output in a short time. Further, by arranging an optical amplifier or an electric amplifier before and after the opto-electric converter, the mode-locked laser can be driven to generate an optical clock even for a weak optical signal. Further, by arranging a frequency filter after the opto-electric converter, the tuning bandwidth of clock extraction can be controlled, and unnecessary frequency components can be removed to enable stable clock generation. An optical filter arranged at the output stage of the mode-locked laser can be used to generate a harmonic clock of the clock of the input optical signal.

Further, the optical clock extraction circuit of the present invention can be easily integrated on a semiconductor substrate and can be made into a small module, so that it can be easily handled and mounted on other devices. You can

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of an optical clock extraction circuit of the present invention.

FIG. 2 is a mode-locked laser 10 according to the first embodiment.
The block diagram which shows the other structural example.

FIG. 3 is a diagram showing electrical coupling when a mode-locked semiconductor laser is used.

FIG. 4 is a diagram showing a method of generating an optical signal having a clock frequency component from an NRZ signal.

FIG. 5 is a block diagram showing the configuration of a second embodiment of the optical clock extraction circuit of the present invention.

FIG. 6 is a block diagram showing the configuration of a third embodiment of the optical clock extraction circuit of the present invention.

FIG. 7 is a diagram illustrating the function of an optical filter 44 according to a third embodiment.

FIG. 8 is a diagram showing an implementation example of an optical clock extraction circuit of the present invention.

FIG. 9 is a block diagram showing a configuration example of a conventional optical clock extraction circuit.

FIG. 10 is a block diagram showing another configuration example of a conventional optical clock extraction circuit.

[Explanation of symbols]

 10 mode-locked laser 11, 12 mirror 13 gain medium 14 mode-lock modulator 15 optical branching means 16 optical isolator 17 optical filter 20 photoelectric converter 30 impedance matching circuit 31, 32 capacitor 33 coil 34 resistor 41 electrical amplifier 42 frequency Filter 43 Optical amplifier 44 Optical filter 51 InP Semiconductor substrate

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H04B 10/00 10/02 (72) Inventor Kazuo Hagimoto 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (7)

[Claims] 1. A bit rate of f ₀ [bit / s] or Nf
_In an optical clock extraction circuit for inputting a digital optical signal of ₀ [bit / s] (N is an integer) into an optical resonator having a resonance frequency interval f ₀ [Hz] and outputting an optical clock, the digital optical signal is converted into an electrical signal. A mode-locked laser composed of an optical-electrical converter for converting into an optical resonator, the optical resonator, a gain medium and a mode-locked modulator arranged in the optical resonator, and an output signal of the optical-electrical converter. And an impedance matching circuit that couples to the mode-locked modulator of the mode-locked laser. 2. The optical clock extraction circuit according to claim 1, further comprising a frequency filter between the photoelectric converter and the impedance matching circuit, the frequency filter limiting a tuning bandwidth of the clock extraction. Clock extraction circuit. 3. The optical clock extraction circuit according to claim 1, further comprising an electric amplifier for amplifying an output of the opto-electric converter between the opto-electric converter and the impedance matching circuit. Extraction circuit. 4. The optical clock extraction circuit according to claim 1, wherein an optical amplifier for amplifying a digital optical signal incident on the opto-electric converter is provided in the preceding stage of the opto-electric converter. Extraction circuit. 5. The optical clock extraction circuit according to claim 1, wherein a free spectrum range that is an integral multiple of a mode frequency interval of the mode-locked laser is provided in a subsequent stage of the mode-locked laser, and a harmonic clock is generated. An optical clock extraction circuit comprising an optical filter. 6. The optical clock extraction circuit according to claim 1, wherein the mode-locked laser is a mode-locked semiconductor laser integrated on a semiconductor substrate. 7. The optical clock extraction circuit according to claim 1, wherein the constituent elements are integrated on a semiconductor substrate.