JPH09102776A - Optical clock signal extraction circuit - Google Patents

Abstract

(57) Abstract: An ultra-high-speed optical transmission system using an optical time division multiplexing technique is provided, which has a simple optical clock extraction circuit without polarization dependency. SOLUTION: The time-division multiplexed / intensity-modulated signal light and the output light (unmodulated) of a clock light source 100 are multiplexed and input to an electroabsorption optical modulator 103. The electro-absorption modulator is driven by a sinusoidal electric signal, and the PLL control circuit 1 is arranged so that the intensity of the wavelength component of the signal light in the output light is maximized.
Phase synchronization control is performed using 06. [Effect] Since the clock light that has passed through the electro-absorption optical modulator 103 is simultaneously output with a short pulse, a short pulse light source for the clock becomes unnecessary. If a polarization-independent modulator is used, it will not be affected by changes in the polarization state of the input signal light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical clock signal extraction circuit, and more particularly to an optical clock signal extraction circuit used in optical time division multiplexing (OTDM) transmission which is one of optical signal transmission circuits.

[0002]

2. Description of the Related Art The transmission speed of signals in optical fiber transmission has been steadily increasing year by year, and now 10 Gbit / s optical transmission is about to be put into practical use. However, it is expected that it will be extremely difficult to cope with the conventional technology in ultra-high speed transmission of 40 Gbit / s due to the limitation of the band of the electric circuit. In recent years, studies have been conducted to realize a large-capacity optical transmission system of 40 to 200 Gbit / s without increasing the speed of an electric circuit by using the optical time division multiplexing technique. In this large-capacity optical transmission system, the optical time division multiplexing signal separation (DEMU) is used.
X) circuit is used to separate the received ultra-high-speed optical signal into low-speed optical signals for reception. For example, a high-speed optical switch can be used to cut out every fourth pulse from a 40 Gbit / s optical pulse train to convert it into a 10 Gbit / s optical signal. Since all such separation operations are performed in the light region, even if the operating speed of the electric circuit is about 10 Gbit / s
Large-capacity optical transmission of 40-100 Gbit / s is possible. Further, it is under study to realize an all-optical regenerative repeater by processing the optical signal multiplexed at a high bit rate as it is. This makes it possible to greatly extend the transmission distance in which light can be transmitted as it is.

In the optical time division multiplexing signal demultiplexing circuit, an optical clock signal whose phase is synchronized with the optical time division multiplexing signal is extracted as a timing signal for driving the optical time division multiplexing signal demultiplexing circuit and an optical regenerator. It becomes important to do. The clock extraction circuit was also used in the optical receiver of the conventional circuit (intensity modulation direct reception / NRZ system), but in the optical time division multiplexing circuit, the signal bit rate is several tens Gbit / s and it is difficult to realize it in an electric circuit. Therefore, an optical clock signal extraction circuit is required.

As an example of a conventionally known optical clock signal extraction circuit, for example, a method utilizing a nonlinear optical effect in a semiconductor optical amplifier or an optical fiber is being studied. For example, the 1994 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers B-907 (199
4th year), p. 505, shows an example in which an optical clock signal extraction circuit is realized by utilizing the four-wave mixing effect in a semiconductor optical amplifier.

FIG. 2 shows the configuration of an optical clock signal extraction circuit utilizing the above four-wave mixing effect. This circuit is an optical clock signal extraction circuit using a PLL circuit.
(Wavelength λs) and clock light c (wavelength λc)
Semiconductor optical amplifier 1 which functions as a phase comparator via 0
11 has been entered. The signal light a has a bit rate Rb.
Of the intensity-modulated light and the clock light c is a short optical pulse having a narrow pulse width of Rb / N (N is an integer). Four-wave mixing (FWM), which is a nonlinear effect, occurs in the semiconductor optical amplifier 111, and the four-wave mixing light b of both signals is generated only when the signal light a and the clock light c temporally overlap each other as shown in FIG. To do. The optical filter 112 is
Of the output light of the semiconductor optical amplifier 111, only the wavelength of the four-wave mixing light b is transmitted, is input to the photodetector 113, and is converted into an electric signal e. The electric signal e is supplied to the PLL control circuit 11
4. The sine wave signal g input to the frequency variable oscillator 115 and output from the frequency variable oscillator 115 drives the clock short pulse light source 116. The PLL control circuit 114 can always perform phase synchronization between the clock pulse c train and the signal pulse a train by outputting a control signal so that the output signal intensity of the photodetector 113 becomes maximum, for example. Therefore, if a part of the clock light c is branched by the optical coupler 117, the optical clock signal c phase-locked with the signal light a is obtained.
Can be obtained.

However, in the circuit of FIG. 2, many optical parts such as a light source 116 for generating an optical pulse for a clock pulse and a non-linear device 111 for detecting the phase of the optical pulse are required, which makes the structure complicated. There was a point.
Moreover, since the four-wave mixing effect, which is a nonlinear optical effect, is used, there is a problem that the clock extraction operation becomes difficult depending on the polarization state of the input light. Further, a method of reducing the polarization dependence by a technique such as polarization diversity has been proposed, but there is a problem that the configuration becomes complicated. In addition to this, an optical clock extraction method using a mutual gain modulation in a semiconductor amplifier, a self-pulsation semiconductor laser, and a mode-locked laser has been proposed. These methods also have problems such as a complicated structure and a limited tuning range of the clock frequency.

It is an object of the present invention to provide an optical clock extraction circuit which does not require a pulse light source for generating an optical pulse for a clock pulse and has a simple structure. Another object of the present invention is to provide an optical clock signal extraction circuit which achieves the above object and which has no polarization dependence and has a wide tuning range of the clock frequency.

[0008]

In order to achieve the above object, in the optical clock signal extraction circuit of the present invention, two signals of the received signal light of bit rate Rb and the unmodulated light are input, and the repetition frequency Rb / N ( (N is an integer) An optical gate driven by an electric signal is provided, the optical gate is used as a phase comparator for the signal light and the electric signal to form a phase locked loop, and at the same time, the optical gate is used. And means for converting the unmodulated light into an optical clock signal having a repetition frequency Rb / N.

By using an electro-absorption optical modulator or a polarization independent optical gate as the optical gate, or by disposing a polarization scrambler at the signal light input section, the problem of polarization dependency is solved. Resolve.

To separate the signal light and the optical clock signal from the output of the optical gate, the wavelengths of the signal light and the unmodulated light are set to different wavelengths, and the wavelength difference between the signal light and the optical clock light is used. Separation is performed by a filter, or the signal light and the clock light are input to the optical gate in opposite directions.

Although the frequency of the optical clock signal extracted by the above configuration is Rb / N, an optical clock circuit having a bit rate higher than Rb / N is obtained by adding an optical multiplexing circuit to the optical output section. It may be configured as follows.

In the optical clock signal extraction circuit of the present invention, the optical gate is used as a phase comparator between the signal light and the electric signal applied to the optical gate, and at the same time, unmodulated light, that is, light that is a continuous wave is used to repeat Since the optical clock pulse is converted into the optical clock pulse having the frequency Rb / N, optical components such as a clock short pulse light source having a complicated configuration and a non-linear optical element are not required, and the optical clock signal extraction circuit can have a simple configuration.

Further, the wavelength of the signal light and the wavelength of the clock light are set to different wavelengths, and the signal light and the clock light are separated from the output light of the optical gate by utilizing the wavelength difference. Can be separated into Further, when the signal light and the clock light input to the optical gate enter the optical gate in opposite directions, the traveling directions are different from each other, and thus the signal light and the clock light have similar wavelengths. Both lights can be easily separated by using an optical coupler or an optical circulator.

In particular, in the electro-absorption optical modulator, since the light transmission characteristic changes non-linearly with respect to the applied voltage, a steep transmission characteristic is selected even by adding a sinusoidal electric signal, and the width synchronized with the signal light is selected. A narrow optical clock pulse or optical clock signal can be obtained. In the case of electroabsorption type optical modulator,
A large wavelength dependence is seen in the transmission characteristics, but if the wavelength difference between the clock light and the signal light is made sufficiently small, there is no problem of wavelength dependence. Also, even if the transmission characteristics at the signal light wavelength are slightly deteriorated, the quality of the output clock pulse is not significantly affected.Therefore, the wavelength of the clock light is set to the wavelength of the optimum transmission characteristics. There is no problem even if the wavelength is set to a wavelength band in which the transmission characteristics are slightly inferior.

Furthermore, if a polarization independent optical gate is used as the optical gate, it becomes possible to realize a clock extraction circuit having no polarization dependency. In particular, even in the case of the electroabsorption optical modulator, a bulk type element that does not use the multiple quantum well (MQW) structure can obtain transmission characteristics with little polarization dependence.
If the polarization dependence is about several dB, the operating speed of the phase locked loop is sufficiently faster than the polarization fluctuation speed (for example, 2
It is possible to solve the problem of polarization dependence. Even when an optical gate having a larger polarization dependence is used, the polarization dependence problem can be completely solved by disposing the polarization scrambler in the signal light input section. On this occasion,
By setting the polarization scrambling speed sufficiently faster than the speed of polarization fluctuation and the operating speed of the phase locked loop (for example, twice or more), the effects of polarization fluctuation and polarization scrambler on the phase locked loop are affected. Can be removed. Further, even when the operating speed of the electric circuit is lower than the required frequency of the clock signal, the frequency of the clock light can be multiplied by disposing the optical multiplexing circuit in the clock light output section.

[0016]

1 is a block diagram showing a first embodiment of an optical clock signal extraction circuit according to the present invention. FIG. 4 shows signal waveforms or characteristics of the main part of FIG. The signal light a (wavelength λs) intensity-modulated at the bit rate Rb and the unmodulated clock light c0 (wavelength λc) that is a continuous wave output from the clock light source 101 are added to the optical coupler 102 and combined. , To the optical gate 103. The optical gate 103 outputs an electric signal h of a repetition frequency (Rb / N + Δf; N is an integer) output from the variable frequency oscillator 107.
Has been added. The electrical signal h is used as the optical gate 103.
The one that can obtain a sharp transmission window periodically in synchronism with is used. For example, when an electro-absorption optical modulator is used, when a sine wave electric signal with an amplitude of several volts having a negative bias level is applied as shown in FIG. A steep transmittance like 3) is obtained.
Therefore, when the unmodulated clock light c0 that is a continuous wave passes through the optical gate 103, a short pulse c is generated as shown in FIG.
Converted to 0 '. Further, since the signal light a is intensity-modulated as shown in FIG. 4 (1), only the pulse of the portion synchronized with the electric signal passes through the optical gate 103, and the output light as shown in FIG. a'is obtained.

At this time, when there is a deviation between the pulse position of the signal light a in FIG. 4 (1) and the peak position of the sine wave in FIG. 4 (2), the signal light a'transmitted through the optical gate 103 (see FIG. 4 ( 4)) strength decreases. Therefore, only the wavelength λs of the signal light a ′ demultiplexed by the optical wavelength demultiplexer 104 is extracted, and its intensity is detected by the photodetector 105, and the electrical signal i is obtained. Electric signal i is P
The pulse position of the signal light a and the phase of the sine wave signal h are constantly adjusted by adjusting the phase of the electric signal added to the LL circuit 106 and obtained from the variable frequency oscillator 107 so that the output of the photodetector 105 is maximized. .. are synchronized, Δf = 0, and the PLL operation is achieved. That is, the optical gate 10
Since the transmission characteristic (rate) of 3 is phase-locked with the pulse of the signal light a, when the unmodulated continuous light is passed through the optical gate 103 having the transmission characteristic that is phase-locked with the pulse of the signal light a, the input signal light a An optical clock pulse train co 'having a repetition period Rb / N that is phase-synchronized with the pulse of the pulse is obtained.

In practice, it is necessary to detect whether the phase of the two is shifted to the front or back. Therefore, for example, the optical gate 103 is based on the case where the shoulder portion of the signal light pulse overlaps the peak position of the sine wave signal. It suffices to operate the PLL so that the average intensity of the signal light passing through is always constant. Alternatively, as a more complicated method, the output signal of the variable frequency oscillator 107 is phase-modulated in advance with a low frequency sine wave signal of a constant frequency, and the constant frequency component in the output signal of the photodetector 105 is synchronously detected. However, the PLL control circuit 106 may be operated so that the intensity of this component becomes zero.
In this example, it is possible to always obtain a clock signal with a correct phase even when the optical gate 103 has polarization dependency.

In the present embodiment, by using the optical gate 103 which has no polarization dependence of the transmission characteristics, the light which does not depend on the polarization state of the signal light and is completely polarization independent in principle. Clock extraction can be realized. Also, the optical gate 103
For example, a Fabry-Perot type optical switch, a cascaded Mach-Zehnder type optical modulator, or the like may be used as an optical gate having a steep transmission characteristic. Further, if the variable frequency oscillator 107 having a steep pulse output is used, it becomes possible to use an ordinary Mach-Zehnder type optical gate. If necessary, the optical gate 1
It is possible to insert an optical amplifier such as an optical fiber amplifier at any position such as before and after 03. The optical wavelength demultiplexer 104 can also be realized by combining an optical coupler and an optical filter or the like.

FIG. 5 shows the configuration of the second embodiment of the optical clock signal extraction circuit according to the present invention. This embodiment is the first
Optical gate 1 for clock light c0 and signal light a in the embodiment
The incident direction in 03 is the opposite direction. The principle of optical clock signal extraction is the same as in the embodiment of FIG. When a lumped-constant type element is used as the optical gate 103, such a configuration is possible because the switching characteristics do not depend on the incident direction. In this case, the signal light a and the clock light c0 ′ can be separated without using the optical wavelength filter 104 of FIG. In addition, the clock light c0 and the signal light a
It is not necessary to have a specific relationship between the wavelengths of the two, and both wavelengths can be made equal.

When the optical gate 103 has a polarization dependency, the transmission characteristics of the optical gate change due to changes in the polarization states of the clock light c0 and the signal light a, and the output pulse intensity and the clock light c0 change. Polarization state may change or jitter may occur in the output clock pulse. In this case, the problem of polarization fluctuation of the clock light can be solved by connecting the clock light source 101, the optical coupler 102, and the optical gate 103 using a polarization maintaining fiber or a polarization maintaining coupler. When the signal light a is transmitted through a long-distance fiber, it has fluctuations in the polarization plane due to fluctuations in the polarization axis in the optical fiber. However, when the polarization dependence of the optical gate 103 is several dB or less, if the time constant of the PLL control circuit is set sufficiently faster than the polarization fluctuation speed, stable clock extraction without being affected by the polarization will be performed. Will be able to do.

FIG. 6 shows the configuration of the third embodiment of the optical clock signal extraction circuit according to the present invention. In the present embodiment, when the transmission characteristic of the optical gate 103 has a large polarization dependency,
The configuration is such that a sufficient signal light output can be obtained regardless of the polarization state of the signal light a. That is, this is an example in which the polarization scrambler 121 is inserted in the signal light input section in the circuit of FIG. 1 to eliminate the polarization dependence. Portions substantially the same as those in the configuration of FIG. By setting the polarization scrambling speed sufficiently higher than the control speed of the PLL control circuit, it is possible to always stably extract the clock signal without depending on the polarization state of the input light.

FIG. 7 shows the configuration of the fourth embodiment of the optical clock signal extraction circuit according to the present invention. The present embodiment has a configuration in which an optical multiplexing circuit 120 is added to the output unit of the clock light of FIG. Portions substantially the same as those in the configuration of FIG. The repetition frequency (Rb of the clock light obtained by the optical clock signal extraction circuit of FIG.
If / N) is less than the desired value, the optical multiplexer circuit 120 can increase the repetition rate. Such an optical multiplexing circuit can be configured with a combination of a polarization maintaining optical fiber coupler and an optical fiber delay line, a PLC type optical waveguide, or the like.

FIG. 8 shows the configuration of an optical time division signal demultiplexing (optical DEMUX) circuit using the optical clock extraction circuit according to the present invention. In the figure, the received optical signal is the optical coupler 1
22 5 optical lines 125-1, 125-2, 125-
3, 125-4 and 125-5 are branched, of which the optical line 125-1 is connected to the optical clock extraction circuit composed of the components 101 to 107, and the remaining four optical lines 125-
2, 125-3, 125-4 and 125-5 are light control optical switches 123-1, 123-2 and 123-3, respectively.
And 123-4 via the optical receivers 124-1 and 124-.
2, 124-3 and 124-4. On the other hand, the extracted optical clock signals output from the optical demultiplexer 104 are the optical control optical switches 123-1, 123-2, 123.
-3 and 123-4. The optical control optical switch 123 has a function of transmitting signal light only when an optical clock signal is input, and functions as an optical AND circuit. By shifting the phase of the optical clock supplied to each optical control optical switch from each other by 1 bit,
The function of the optical DEMUX can be realized. As the above-mentioned optical control optical switch, an optical switch utilizing an optical nonlinear effect such as an optical loop mirror using an optical fiber or a semiconductor optical amplifier can be used. Further, although the present embodiment shows the case where the number of branches is 4 and the number of clock extracting circuits is one, the number of branches and the number of clock extracting circuits are not limited to the numbers used in this example and can be realized.

FIG. 9 shows the configuration of an all-optical regenerator (optical 3R) using an optical clock extraction circuit according to the present invention. In the present embodiment, the input optical signal is branched into two by the optical coupler 122, one is connected to the optical control optical switch 123, and the other is connected to the optical clock extraction circuit composed of the components 101 to 107. . The repetition frequency of the optical clock output from the optical wavelength demultiplexer 104 is the transmission bit rate R
Since the frequency is divided into 1 / N of b, it is supplied to the optical control optical switch 123 after the repetition frequency is multiplied by N by the optical multiplexing circuit 120. There is no problem even if the optical multiplexing circuit 120 is removed or the multiplication number is changed as necessary. In the optical control optical switch 123, the clock light is transmitted only when the signal light is input, so that the waveform
Perform timing playback. Also, not limited to optical time division transmission,
It can also be used for applications such as clock reproduction in soliton reproduction in soliton transmission.

[0026]

By using the optical gate for two purposes, that is, the phase comparator for the signal light and the electric signal, and the short pulse conversion of the clock light, the number of optical parts is reduced and the structure is simplified. There is an effect. Further, the PLL configuration has the advantages that the tuning range of the clock frequency can be wide and stable operation is possible. Moreover, since unmodulated light is used as the light source of the clock light, there is an effect that the selection of the clock light source becomes easy.

Further, by setting the wavelength λs of the signal light and the wavelength λc of the clock light to be different from each other, the signal light and the clock light can be easily separated from the output light of the optical gate. Alternatively, the same effect can be obtained even if the signal light and the clock light are oriented in opposite directions.

Further, in the electro-absorption optical modulator, since the light transmission characteristic changes non-linearly with respect to the applied voltage, a steep transmission characteristic can be obtained even by adding a sinusoidal electric signal, and the present configuration can be realized. It has the effect of making it easier.

Furthermore, by using a polarization-independent optical gate as the optical gate, there is an effect that a clock extraction circuit having no polarization dependency can be realized. This effect can be obtained by using, for example, a bulk electroabsorption optical modulator. Even when using an optical gate with a large polarization dependence,
By disposing the polarization scrambler in the signal light input section, there is an effect that the polarization dependence can be solved.

Further, by arranging the optical multiplexing circuit in the clock light output section, there is an effect that the frequency of the clock light can be multiplied and a clock light pulse having a desired high repetition frequency can be obtained. That is, there is an effect that the bit rate of the signal light can be increased as compared with the frequency of the electric signal.

Further, when the time division multiplexing optical signal demultiplexing circuit is constructed by using the clock extracting circuit according to the present invention, there is an advantage that the configuration is simplified and the demultiplexing circuit having a simple configuration can be constructed at low cost. Similarly, when the optical regenerator is configured, there is an advantage that the configuration is simplified.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of an optical clock signal extraction circuit according to the present invention.

FIG. 2 is a configuration diagram of a conventional optical clock extraction circuit.

FIG. 3 is an explanatory diagram of an operation of a conventional optical clock extraction circuit.

FIG. 4 is an explanatory diagram of an operation of the optical clock signal extraction circuit of FIG.

FIG. 5 is a configuration diagram of a second embodiment of an optical clock signal extraction circuit according to the present invention.

FIG. 6 is a configuration diagram of a third embodiment of an optical clock signal extraction circuit according to the present invention.

FIG. 7 is a configuration diagram of a fourth embodiment of an optical clock signal extraction circuit according to the present invention.

FIG. 8 is a configuration diagram of an optical time division signal demultiplexing circuit using an optical clock extraction circuit according to the present invention.

FIG. 9 is a block diagram of an all-optical regenerative repeater using an optical clock extraction circuit according to the present invention.

[Explanation of symbols]

101 ... Clock light source, 102 ... Optical coupler, 103 ... Optical gate, 104 ... Optical wavelength demultiplexer, 105 ... Photodetector, 106 ... PL
L control circuit, 107 ... Frequency variable oscillator, 110 ... Optical coupler, 111 ... Semiconductor optical amplifier, 112 ... Optical filter, 113 ...
Photodetector, 114 ... PLL control circuit, 115 ... Frequency variable oscillator, 116 ... Clock short pulse light source, 117 ... Optical coupler, 120 ... Optical multiplexing circuit, 121 ... Polarization Wave scrambler, 122 ・
..Optical coupler, 123 ... Optical control optical switch (optical AND circuit), 124 ... Optical receiver, 125 ... Optical line.

Claims (11)

[Claims] 1. A non-modulated light source and a repetition frequency Rb / N (N
Is an integer), the signal light intensity-modulated at the bit rate Rb and the light from the unmodulated light source are input,
An optical gate driven by an electric signal of the electric signal source, for performing a phase comparison between the signal light and the electric signal, a control circuit for controlling the frequency of the electric signal source by the signal light passing through the optical gate, An optical clock signal extraction circuit having a separating means for separating the light from the unmodulated light source that has passed through the optical gate as an optical clock signal having a repetition frequency Rb / N. 2. The optical clock signal extraction circuit according to claim 1, wherein the wavelengths of the signal light and the unmodulated light are different, and the separating means outputs the signal light and the unmodulated light from the output light of the optical gate. An optical clock signal extraction circuit, characterized in that it is means for separating the optical clock signals by a wavelength difference. 3. An unmodulated light source and a repetition frequency Rb / N (N
Is an integer), the signal light intensity-modulated at the bit rate Rb and the light from the non-modulated light source are input in directions opposite to each other, and are driven by the electric signal of the electric signal source. An optical gate that performs phase comparison between light and the electric signal, a control circuit that controls the frequency of the electric signal source by the signal light that has passed through the optical gate, and is arranged on the signal light input side of the optical gate. And an optical coupler for separating the light from the unmodulated light source that has passed through the optical gate into an optical clock signal having a repetitive frequency Rb / N. 4. The optical clock signal extraction circuit according to claim 1, wherein the optical gate is an electroabsorption optical modulator. 5. The optical clock signal extraction circuit according to claim 1, wherein the optical gate is a polarization independent optical gate. 6. An optical clock signal extraction circuit according to claim 1, wherein a polarization scrambler is arranged at the input part of the signal light. 7. The optical clock signal extraction circuit according to claim 1, wherein an optical multiplexing circuit is arranged in the clock optical output section. 8. A time division multiplexing optical signal demultiplexing receiver using the optical clock extraction circuit according to any one of claims 1 to 6. 9. An optical coupler for branching a signal light intensity-modulated at a bit rate Rb into a plurality of optical lines, and an optical coupler coupled to one of the plurality of optical lines. Optical clock extraction circuit, and an optical control optical switch coupled to each of a plurality of other optical lines and driven by the optical clock extracted by the optical clock signal extraction circuit, and outputs of the optical control optical switch. A time division multiplexing optical signal demultiplexing receiver having a plurality of optical receivers connected to the side. 10. A time-division multiplexed optical signal regenerator using the optical clock extraction circuit according to claim 1. Description: 11. An optical clock extracting circuit according to claim 1, wherein the optical coupler for branching the signal light intensity-modulated at the bit rate Rb is coupled to one of the branched optical lines. And an optical control optical switch that is coupled to another optical line and is driven by the optical clock extracted by the optical clock signal extraction circuit to reproduce the signal light. Repeater.