KR20010076611A - Opto-electronic Hybrid Apparatus and Method for Optical Clock Recovery - Google Patents

Abstract

PURPOSE: An opto-electronic mixing type optical clock reproducing device is provided, which extracts and reproduces an optical clock component to be a mode locking state in a fiber ring resonator for the outputs of a semiconductor optical amplifier. CONSTITUTION: A photodiode(11) receives an inputted optical signal and converts an electric signal. A semiconductor optical amplifier(12) generates an optical signal treated with an intensity modulation and makes a mode locking state in a fiber ring resonator(13). The fiber ring resonator(13) includes an optical isolator(15) for guiding the optical pulse for one direction, an optical band pass filter(16) for passing a partial bandwidth among the optical purse outputted from optical isolator(15), an optical coupler(17) and an optical delay line(18) for adjusting optical paths and transmitting the optical pulse passed through the optical band pass filter(16) to the semiconductor optical amplifier(12).

Description

Opto-electronic Hybrid Apparatus and Method for Optical Clock Recovery

The present invention relates to a technique for extracting and reproducing a clock component from an optical pulse stream of an optical transmission signal, and more specifically, to extract and reproduce a clock of an optical signal by an optical method. A photoelectric hybrid optical clock reproducing apparatus and method for reproducing and amplifying a waveform of an optical signal by using the same.

In general, the transmitted optical signal consists of a combination of optical pulses (optical pulse train) with intensity of 0 and 1. These optical pulse trains are degraded by various factors as they are transmitted, and the receiving side receives the deformed and distorted optical signal, that is, the optical pulse train. Optical clock reproduction technology is a technique for optically extracting clock components from such distorted optical pulse trains. Conventionally, the clock component is extracted and reproduced by an electric method, but this requires a complicated configuration such as optical to electrical or optical to optical conversion, and also a high-speed electronic device for processing the reproduced electrical pulses. Are required. However, as the optical transmission speed increased, the signal processing speed of the electronic device reached its limit, and an optical clock extraction and reproduction method was devised to solve this problem.

Conventional techniques for optically extracting and reproducing the clock component of an ultrafast optical signal include a fiber ring laser or a nonlinear optical loop mirror (NOLM), or a mode locked laser diode. A method of using a semiconductor optical device such as a mode locked laser diode) has been proposed.

1 is a block diagram illustrating an all-optical optical clock reproducing apparatus using a nonlinear optical modulator and an optical fiber ring laser according to a representative prior art. The transmitted optical signal (wavelength [lambda] 1) is multiplexed with the optical output (wavelength [lambda] 2) of the optical fiber ring laser via the WDM (1) and input to the nonlinear optical modulator (2). The nonlinear optical modulator 2 cross-modulates the optical output (wavelength λ2) of the optical fiber ring laser according to the pulse train of the transmitted optical signal (wavelength λ1) and modulates it into a pulse train having the same information. The optical signal (wavelength λ1) transmitted after passing through the nonlinear optical modulator 2 is output through the wavelength division multiplexer 3, and the cross-phase modulated optical signal (wavelength λ2) is input to the resonator of the optical fiber ring laser. do. Meanwhile, the optical fiber ring laser has an erbium-doped fiber amplifier (EDFA) composed of an erbium-doped fiber (7), an excitation light source (5), and a wavelength division multiplexer (6) as a gain medium in the resonator. An optical isolator (4) for advancing the signal in only one direction, a band-pass optical filter (8) for selecting the center wavelength of an optical signal having a clock component, and an optical delay for adjusting the length of the resonator (fiber) stretcher 9, an optical coupler 10 used to output an optical clock signal, and the like.

The nonlinear optical modulator 2 may be composed of an optical fiber such as dispersion shifted fiber, and pulses of the transmitted optical signal (wavelength λ1) using a physical phenomenon such as cross phase modulation. Optical signals (wavelength lambda 2 in this case) having different wavelengths are modulated according to the columns (information). At this time, in order to increase the interphase modulation efficiency, the wavelength λ 2 is selected to be symmetric about the zero dispersion wavelength, and is adjusted using the bandpass optical filter 8 of the optical fiber ring laser. The optical signal having a wavelength? 2 is generated by the optical fiber ring laser, and is locked in the resonator of the optical fiber ring laser after being synchronized with the optical signal (wavelength? 1) transmitted by being phase-modulated by the nonlinear optical modulator 2. Therefore, the optical signal λ2, which is the output of the optical fiber ring laser, is mutually phase-modulated by the nonlinear optical modulator 2 to be mode locked in the resonator of the optical fiber ring laser, and is synchronized with the bit rate of the transmitted input optical signal (wavelength λ1). The clock component can be obtained.

The prior art as shown in FIG. 1 has unstable interphase modulation characteristics of a nonlinear optical modulator, requires fine wavelength adjustment for efficient interphase modulation, and optical fiber ring lasers are sensitive to external environments such as temperature and vibration, and thus wavelength stability. The characteristics of are unstable. In addition, control circuits are required to compensate for the unstable characteristics, and therefore the configuration becomes very complicated.

As described above, nonlinear fiber loop mirrors or fiber ring lasers are very sensitive to external environments such as temperature and vibration, and are highly unstable to control them because they are greatly affected by polarization. A large number of components, such as additional optical components and controllers such as a polarization controller and a temperature control circuit, are required. Therefore, there is a problem that the configuration is complicated and the characteristics of the reproduced optical clock are unstable. In addition, the method using a semiconductor optical device such as a mode-locked laser diode is simpler than the all-optical method, but requires an additional component for controlling polarization characteristics, and an environmental component for controlling the environmental characteristics of these components. There is a disadvantage that a device is required and the device technology is not yet mature.

Accordingly, the present invention has been made to solve the above problems of the prior art, and comprises a semiconductor optical amplifier (SOA) and a fiber ring resonator including the same, and is generally used Opto-optoelectronics extracts and reproduces optical clock components by driving semiconductor optical amplifiers with the output of low-cost photodiodes (PDs), allowing the outputs of semiconductor optical amplifiers to be mode locked in an optical fiber ring resonator. It is an object of the present invention to provide a hybrid optical clock reproducing apparatus and method.

In addition, the present invention overcomes the problems of optical clock characteristic instability and structural difficulty of the conventional all optical optical clock reproducing method, and provides a more economical and efficient optical clock reproducing apparatus and method. It is to provide.

1 is a block diagram of an all-optical clock reproducing apparatus using a nonlinear optical modulator and an optical fiber ring resonator according to the prior art;

2 is an embodiment of a photoelectric hybrid optical clock reproducing apparatus using a photodetector, a semiconductor optical amplifier, and an optical fiber ring resonator according to one embodiment of the present invention;

3 is a graph illustrating clock components extracted and reproduced using the photoelectric hybrid optical clock reproducing apparatus according to the present invention.

※ Explanation of code about main part of drawing ※

WDM: wavelength division multiple device

NOM: Nonlinear Light Dimmer

EDFA: Erbium-doped Fiber Optic Amplifier

Er: Orbium

F: optical filter

I: Photoisolator

FS: Photo Delay

FC: Optocouplers

In order to achieve the above object, the photoelectric hybrid optical clock reproducing apparatus according to the present invention includes a photodetector for converting an optical pulse of an optical transmission signal into an electrical pulse, and a semiconductor optical amplifier for outputting an intensity pulse modulated according to the electrical pulse. And a ring resonator including the semiconductor optical amplifier and mode-locking the intensity modulated light pulse.

Preferably, the ring resonator comprises: an optical isolator for propagating the intensity modulated light pulse output from the semiconductor optical amplifier in only one direction; and a band transmission for transmitting only a wavelength of a predetermined band among the light pulses passing through the optical isolator. And an optical delayer for adjusting an optical path of the optical signal and transmitting an optical pulse passed through the band-pass optical filter to a semiconductor optical amplifier, wherein a part of the optical pulse transmitted through the band-pass optical filter is included. It further comprises an optical coupler for separating to the outside and transferring the rest to the optical delay.

In addition, the photoelectric hybrid optical clock reproduction method according to the present invention, the first step of converting the optical transmission signal into an electrical light pulse, the second step of converting the electrical light pulse into an intensity modulated light pulse, and the intensity modulation And a third step of mode locking the pulsed light pulses.

Hereinafter, a "photoelectric hybrid optical clock reproducing apparatus and method" according to an embodiment of the present invention will be described with reference to the accompanying drawings.

2 is an embodiment of a photoelectric hybrid optical clock reproducing apparatus using a semiconductor optical amplifier and an optical fiber ring resonator according to an embodiment of the present invention. The photoelectric hybrid optical clock reproducing apparatus of the present invention generates an optical signal subjected to intensity modulation and an optical detector 11 for receiving an input optical signal and converting it into an electrical signal, which is then used as a fiber ring resonator. And an optical fiber ring resonator 13 including a semiconductor optical amplifier 12 and a semiconductor optical amplifier 12 for mode locking in 13.

The photodetector 11 is a low-cost photodetector that is generally used, the optical pulse train of the transmitted optical signal is converted into an electrical signal, that is, an electrical pulse train by the photodetector 11. The electrical pulse train, which is the output of the photodetector 11, is driven by a semiconductor optical amplifier 12 in the optical fiber ring resonator 13 and converted into an intensity modulated optical signal, that is, an optical pulse train.

The optical fiber ring resonator 13 generates a semiconductor signal which has a constant gain in a certain wavelength band, which is used to generate an intensity modulated optical signal and mode lock it in the resonator, a general optical fiber 14, an optical signal Optical isolator 15 for advancing only in one direction, an optical band pass filter 16 for transmitting only a wavelength of a predetermined band, and an optical coupler 17 for separating a part of an optical signal And an optical delay line 18 for adjusting the length of the optical path, and mode-lock the intensity modulated optical signal, which is the output of the semiconductor optical amplifier 12.

The optical pulse train of the optical signal input to the input port a of FIG. 2 is converted into an electric pulse train by the general photodetector 11. The electrical signal generated by the photodetector 11, that is, the electric pulse train, drives the semiconductor optical amplifier 12 of the optical fiber ring resonator 13, and the semiconductor optical amplifier 12 is generated by the photodetector 11. Generate an optical signal intensity modulated in accordance with the electrical pulse train, that is, an optical pulse train. The pulse train of optical signals generated by the semiconductor optical amplifier 12 has gain over a wide wavelength range. An optical signal intensity-modulated by the photodetector 11 and the semiconductor optical amplifier 12 is input to the general optical fiber 14 and passes through an optical isolator 15 for advancing the optical signal in only one direction. Pass 16. The band-pass optical filter 16 limits the optical signal of the semiconductor optical amplifier over a wide wavelength band to a certain wavelength so as to efficiently lock the mode in the optical fiber ring resonator 13. The optical signal passing through the band-pass optical filter 16 is output by the optical coupler 17 to a part of the optical signal to the output port b, and the remaining optical signal is returned to the optical delay unit 18 through the general optical fiber 14. It proceeds and is mode locked in the optical fiber ring resonator 13. The optical delay unit 18 mode-locks an optical signal, that is, an optical pulse train, which proceeds in the optical fiber ring resonator 13 composed of the semiconductor optical amplifier 12, the general optical fiber 14, and the optical components 15 to 17. It adjusts the light path so that it can.

Mode locking of the optical signal in the optical fiber ring resonator 13 is related to the length of the resonator and the modulation frequency of the optical modulator. Resonating modes existing in the ring resonator 13 having the optical path L exist at intervals of c / L, and these resonant modes have different phases and amplitudes, respectively. At this time, when the modulator is present in the resonator, and the modulator modulates the optical signal at a frequency corresponding to the interval of the resonance mode, the modulator has a constant phase and mode lock occurs. The optical path L of the resonator (= nl, n is the refractive index of the optical fiber, l is the optical fiber, in order to mode-lock the intensity modulated optical signal generated in the semiconductor optical amplifier 12, that is, the optical pulse train in the optical fiber ring resonator 13 The actual length of the ring resonator 13 should be adjusted to match the phase of the optical clock. If the light speed is c [m / s] and the transmission speed of the optical signal is f [Hz], the optical path L of the resonator should be L = Nc / f (N is an integer), which is the length of the optical fiber 14. Using the optical delay unit 18 and accurately adjusted. That is, the optical path of the optical fiber ring resonator 13 is a time interval of the intensity pulsed optical signal generated by the semiconductor optical amplifier 12, that is, the time when the optical pulse train travels the optical fiber ring resonator 13 once. To be an integer multiple of. Therefore, the optical pulse train proceeding in the optical fiber ring resonator 13 is phase locked (resonated) every time the optical fiber ring resonator 13 goes through, and the mode is locked. The mode pulsed optical pulse train coincides exactly with the clock component of the optical pulse train transmitted, thereby extracting and reproducing the optical clock.

3 is a result of experiments with the photoelectric hybrid optical clock reproducing apparatus as described above, and shows the pulse train of the optical signal to be input and the reproduced optical clock. In each graph, the upper pulse is a bit pattern of an input optical signal pulse string, and the lower pulse is a reproduced optical clock component. 3 a) to h) show the reproduced optical clock components for the case where the bit patterns are 11111111, 01111111, 00111111, 00011111, 00001111, 00000111, 00000011 and 00000001, respectively. At this time, the input optical signal is in the form of RZ (return to zero), the transmission rate is 309.9Mb / s. According to the present invention, the frequency of the reproduced clock coincides with the clock of the input optical signal.

While the invention has been described above based on the preferred embodiments thereof, these embodiments are intended to illustrate rather than limit the invention. It will be apparent to those skilled in the art that various changes, modifications, or adjustments to the above embodiments can be made without departing from the spirit of the invention. Therefore, the protection scope of the present invention will be limited only by the appended claims, and should be construed as including all such changes, modifications or adjustments.

As described above, according to the present invention, the optical transmission signal to be reproduced by using a low-cost photodetector is converted into an electrical signal, and the optical signal is generated by intensity-modulating the semiconductor optical amplifier using the electrical signal of the low-cost photodetector to generate an optical signal, and the intensity modulation. By mode-locking the optical signal of the semiconductor optical amplifier using the optical fiber ring resonator, it is possible to implement an optical clock reproducing apparatus having a simple configuration, a very stable characteristic of the reproduced clock, and a low cost.

In addition, the optical fiber ring resonator used in the present invention is relatively insensitive to external environmental characteristics such as a bandpass optical filter, an optical isolator, and a optical delay, and has a small polarization effect, so that the characteristics are very stable, and thus a small amount of additional control circuit is not required. By using the optical components of the optical clock reproducing apparatus of simple, economical and stable characteristics can be implemented. Therefore, according to the present invention, it is possible to improve the difficulty of the configuration of the conventional all-optical optical clock reproducing method and the instability of the reproduced clock, to use a small amount of optical parts, to minimize the control function, and to provide economical and efficient optical. A clock reproducing method can be provided.

Claims (6)

An apparatus for extracting and reproducing an optical clock of an optical transmission signal, A photodetector for converting an optical pulse of the optical transmission signal into an electrical pulse; A semiconductor optical amplifier for outputting an optical pulse intensity-modulated according to the electrical pulse; And a ring resonator including the semiconductor optical amplifier and mode locked the intensity modulated light pulse. The method of claim 1, wherein the ring resonator, An optical isolator for propagating the intensity-modulated optical pulse output from the semiconductor optical amplifier in only one direction; A band-pass optical filter for transmitting only a wavelength of a predetermined band among the optical pulses passing through the optical isolator, and And an optical delay unit for adjusting an optical path of the optical signal and transmitting an optical pulse passing through the bandpass optical filter to a semiconductor optical amplifier. The photoelectric hybrid optical clock reproducing apparatus according to claim 2, further comprising an optical coupler for separating a part of the light pulse transmitted through the band-pass optical filter to the outside and transferring the remaining part to the optical delay unit. The photoelectric hybrid optical clock reproducing apparatus according to claim 1, wherein the photodetector is a photodiode. The photoelectric hybrid optical clock reproducing apparatus according to any one of claims 1 to 4, wherein the time that the optical pulse travels one optical fiber ring resonator is an integer multiple of the time interval of the optical pulse train. In the method for extracting and reproducing the optical clock of the optical transmission signal, Converting the optical transmission signal into an electrical optical pulse; A second step of converting the electrical light pulse into an intensity modulated light pulse, and And a third step of mode locking the intensity modulated light pulses.