JPH11136215A - Optical time division multiplex signal selection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical time division multiplex signal selection circuit at lower cost, without the need for an optical amplifier and a special configuration for realizing independence of polarization. SOLUTION: This circuit is provided with a phase shifter 2, that receives a clock signal and a control voltage and generates/outputs a timing signal for selecting an optical signal of a prescribed channel from an optical time division multiplex signal, a comb-like generator 3 that receives the timing signal and provides an output of a short pulse based on the signal, and an electric field absorption type modulator 4 whose application voltage versus transmission rate relation is steep and that selects and outputs the optical signal in a timing, when the short pulse is received from the optical time division multiplex signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical time division multiplex signal selection circuit for selecting an optical signal of a predetermined channel from an optical time division multiplex signal in an optical switching network.

[0002]

2. Description of the Related Art An optical switching network realizes a high-speed and high-bandwidth multimedia service utilizing optical technology. Switches using various multiplexing schemes have been studied to realize large-capacity services, and an optical time-division multiplexing (OTDM) type switch is one of them. This optical time division multiplexing is described in [1] A. Hiramatu, M. Tsukada, T. Matsunaga, and K. Yuki
matus, "Hyper-media photonic information network:
WDM-based solutions and prototype systems, "ISS'97
IS-02.20, Spt. 1997. [2] AJKeationg, M. Tsukada, and A. Hiramatsu, "All
-optical selector of 160-Mbit / s channels from a 5
1.2Gbit / s datastream, "Photonics in Switching, 97P
thA4, Stockholm, Apr. pp.80-83, 1997. [3] JGZhang, G. Picchi, "Self-synchronized all-op
tical time-division multiple-access broadcast netw
ork ", Electron. Lett., vol. 29, no. 21, 14th Oct. 19
93, pp.1871-1873. [4] RKBoncek, RPPrucnal, MFKrol, STJohns,
and JLStacy, "Five gibabit / second operation of a
50-channel optical time-division multiple-access
interconnect ", Opt.Eng., vol.31, no.11, Nov.1992,
pp.2442-2559.

An OTDM switch is, for example, 10 [Gbi
t / s] (= 10 [Mbit / s] × 10,000 channels) class circuit switching-based multicast service can be provided at low cost, and a selection circuit for selecting an optical signal of a predetermined channel from the optical time division multiplexed signal. Is the OTD
This is the most important technology for realizing the M switch. When a signal of a predetermined channel is selected from a high-speed optical time division multiplexed signal of several tens [Gbit / s], an optical control switch using an optical nonlinear effect is generally used. Up to now, many optically controlled switches have been proposed. As an example, FIG. 10 shows a configuration example of a wavelength conversion type optical switch using four-wave mixing in an optical fiber. 10, a clock signal for generating control light is input to a selection circuit 70, and a phase shifter 71 performs a phase shift for selecting a predetermined channel. On the basis of the signal from the phase shifter 71, a short pulse laser 72 for control light is used.
However, short-pulse light is output as control light. And
The control light and the optical time-division multiplexed signal light are input to the four-wave mixing switch 73, and the lights are superimposed in the multiplexer 74. The light from the multiplexer 74 is wavelength-converted by the optical fiber 75 by an optical nonlinear effect, and
At 6, the optical pulse of the channel corresponding to the control light is selected and output as the selected light. Such an optically controlled switch has an advantage in that the response time is sufficiently fast as compared with the control by an electronic circuit, so that ultrahigh-speed switching exceeding 100 [Gbit / s] is possible.

[0004]

However, in order to realize the above-described light control type switch, a control light pulse of relatively high power is required to cause a nonlinear phenomenon of light. Therefore, it is usually difficult to perform sufficient switching with the optical power output from the semiconductor laser, and an optical amplifier is required after the semiconductor laser. Therefore, the selection circuit becomes very expensive. In addition, the optical nonlinear phenomenon generally has polarization dependence at the time of switching due to its generation process. That is, in order to perform sufficient switching, it is necessary to control the polarization state of the control pulse with respect to the input pulse. A method for eliminating polarization dependence is also described in M. Saruwatari, "Highspeed optical signal processing.
for communication systems ", IEICE Trans. Communic
ations, vol.E78-B, no.5, May 1995, pp.635-643, but the system becomes complicated and the selection circuit becomes very expensive. Therefore, a selection circuit capable of high-speed switching by electric control without using a light control type switch is desired. The present invention has been made in view of such circumstances, and does not require an optical amplifier or a special configuration for making polarization independent, and enables high-speed switching by electric control, thereby enabling inexpensive optical time division multiplexing. It is an object to provide a signal selection circuit.

[0005]

To achieve the above object, according to the present invention, a clock signal and a control signal are inputted, and the clock signal and the control signal are supplied from the clock signal. A timing generation circuit for generating and outputting a timing signal for selecting an optical signal of a predetermined channel from an optical time division multiplexed signal in which a signal having a bit rate corresponding to the frequency of the signal is multiplexed, and a timing from the timing generation circuit A comb generator that inputs a signal and outputs a short pulse based on the timing signal; and a short pulse from the optical time-division multiplexed signal and the comb generator as an input signal. The switching is performed in a time shorter than the pulse width of the short pulse, and the optical signal of the predetermined channel is converted from the optical time division multiplexed signal. An optical time division multiplex signal selection circuit is characterized in that a of the electro-absorption modulator for the selection and output. Further, the invention according to claim 2 is the same as the invention according to claim 1.
3. The optical time-division multiplexing signal selection circuit according to item 1, wherein the timing generation circuit is a phase shifter that can adjust the phase of the clock signal by 360 degrees by the control signal.

[0006] Further, the invention according to claim 3 is based on claim 1.
3. The optical time-division multiplexing signal selection circuit according to 2, wherein the timing generation circuit is a bias tee that inputs a control voltage as the control signal and superimposes the control voltage on a voltage signal related to the clock signal. I have.
According to a fourth aspect of the present invention, in the optical time division multiplexing signal selection circuit according to the third aspect, the optical time division multiplexing selection circuit further includes a delay circuit, and the delay circuit includes the clock signal and the clock signal. A setting signal for delay setting is input, a clock signal obtained by delaying the clock signal by the setting signal is output, and the bias tee receives a clock signal delayed by the delay circuit. .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical time division multiplexing signal selection circuit according to an embodiment of the present invention will be described with reference to the drawings.

(First Embodiment) FIG. 1 is a configuration diagram of an optical time division multiplexed signal selection circuit according to a first embodiment of the present invention. As shown in FIG. 1, the optical time division multiplexed signal selection circuit 1 includes a phase shifter 2 as a timing generation circuit, a comb generator 3, and an electro-absorption modulator (hereinafter referred to as “EA (Electro-Abso)”.
rption) modulator 4). In FIG. 1, a thin line represents an electric signal, and a thick line represents an optical signal. The optical time division multiplex signal processed by the optical time division multiplex signal selection circuit 1 is Bi
It is assumed that a signal of [bit / s] is obtained by multiplexing a signal of Bo [Bit / s] for N channels (Bi = Bo × N). Here, the phase shifter 2 has a frequency Bo [Hz] corresponding to the bit rate of each channel of the optical time division multiplexed signal.
Clock signal (sine wave) and a control voltage are input. The phase shifter 2 generates a timing signal for selecting an optical signal of a predetermined channel from the optical time-division multiplexed signal by a control voltage from the input clock signal by changing the phase of the clock signal, and outputs the timing signal. .
It is desirable that the phase shifter be such that the phase of the clock signal changes linearly by a DC voltage input as a control voltage and is variable by 360 degrees. This is because if the clock phase changes linearly, the phase control by this phase shifter becomes easy, and if it is variable by 360 degrees, all the channels can be selected.

The comb generator 3 receives the output of the phase shifter 2 and outputs a short pulse based on the input signal. Here, the comb generator 3 generally outputs a short pulse when the voltage falls below a certain value when the sine wave rises and falls. Therefore, when the sine wave whose phase has been changed by the phase shifter 2 is input to the comb generator 3, the timing at which a short pulse is output follows the timing at which the phase is changed. Therefore, in the phase shifter 2, the phase shift is performed by the control voltage so that the timing at which the short pulse is output from the comb generator 3 becomes the timing of channel selection. The pulse width of the short pulse generated here is, for example, on the order of several hundred [ps], and generates an electric short pulse having a wideband frequency characteristic from a low-speed signal serving as an input signal.
It can be said.

The EA modulator 4 receives a Bi [bit / s] optical time division multiplexed signal and a short pulse from the comb generator 3 as input signals, and outputs a desired optical signal from the optical time division multiplexed signal at a short pulse timing. Make a selection. Here, the EA converter 4 has a steep relationship between the applied voltage and the transmittance. The reason will be described with reference to FIG. FIG. 2 is a diagram illustrating a relationship between an applied voltage and a transmittance in an electro-absorption modulator and a switching window with respect to an input pulse. FIG. 2A is a diagram showing characteristics of the applied voltage versus the transmittance of the EA modulator 4. FIG. 4B is a diagram illustrating a waveform of an input pulse serving as an applied voltage, and FIG. 4C is a diagram illustrating transmitted light power in the characteristic of FIG. Note that FIG.
The scales of the time axes of (b) and (c) are the same.

FIG. 2A shows that the relationship between the applied voltage and the transmittance changes sharply around the point B.
Then, as shown in FIG. 2 (a), when a pulse having a point B where the transmittance changes sharply and the peak voltage is almost a peak voltage is input as an applied voltage, the transmission power of light as shown in FIG. Obtainable. In the pulse shown in FIG. 2B, the pulse width at a position where the voltage is half of the peak voltage is about 100 [ps]. On the other hand, in the light transmission power shown in FIG. 2C, the peak is about -9 [dBm]. Here, the position where the intensity becomes half of this peak is about -3 [dBm] (= 10 log10 (1 /
2)), it becomes -12 [dBm].
The time width (switching window) at this time is about 50 [ps]. As described above, by using the EA modulator 4 in which the relationship between the applied voltage and the transmittance changes sharply, the switching can be performed in a time shorter than the width of the applied electric pulse. Therefore, high-speed switching becomes possible.

As described above, in the optical time-division multiplexed signal selection circuit 1 shown in FIG. 1, the use of the comb generator 3 enables generation of an electric short pulse having a wide-band frequency characteristic from the input low-speed signal. By using the EA modulator 4 having a steep transmittance relationship, a switching window having a shorter time than the applied electric short pulse can be provided, and high-speed switching can be performed. Therefore,
Switching can be performed by an electric circuit, and an inexpensive optical time division multiplexed signal selection circuit can be provided. In addition, by using a phase shifter 2 that can change the phase by 360 degrees as a timing generation circuit that generates and outputs a timing signal for selecting an optical signal of a predetermined channel from an input clock according to a control signal, the From the time division multiplexed signal, the optical pulse signals (Bo
[Bit / s]) can be selected.

(Second Embodiment) In the first embodiment, the phase shifter 2 is used as a timing generation circuit for generating and outputting a timing signal for selecting an optical signal of a predetermined channel. In the second embodiment, a bias tee is used as the timing generation circuit. FIG.
Optical time division multiplex selection circuit 1 according to second embodiment of the present invention
FIG. In the figure, parts corresponding to the respective parts in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In addition,
The optical time-division multiplex signal processed by the optical time-division multiplex signal selection circuit 1 of the present embodiment is the same as that of the first embodiment.
It is assumed that a signal of [bit / s] is a signal obtained by multiplexing a signal of Bo [Bit / s] on N channels (Bi = Bo × N). The radio frequency port of Bias Tee 5 is Bo
A clock signal (sine wave) of [Hz] is input, and a control voltage is input from a DC port. And bias tee 5
Outputs a sine wave in which a DC component as a control voltage is superimposed on this clock signal.

As described in the first embodiment, the comb generator 3 generally generates
When the voltage falls below a certain level, a short pulse is output. Therefore, by controlling the DC component of the sine wave input to the comb generator 3 by the bias tee 5, the timing of the short pulse output from the comb generator 3 changes. FIG. 4 shows this state. In FIG. 4, waveforms indicated by reference numerals 21, 22, and 23 are input waves input to the comb generator 6. And reference numeral 21,
In the input waves 22 and 23, the DC components superimposed by the bias tee 5 have 0 [V], + ΔV [V], and −, respectively.
ΔV [V]. Here, assuming that the voltage serving as a threshold for generating a short pulse is −Vth [V] indicated by a dotted line, the input wave 21 outputs a short pulse at the timing indicated by reference numeral 25, and In the case of 23, short pulses are output at timings indicated by reference numerals 26 and 27, respectively. Thus, the control voltage to the bias tee 5 is-△
By changing the range from V [V] to + △ V [V], the timing of generating a short pulse in the comb generator 3 can be changed within a range of △ τ.

However, in the configuration shown in FIG. 3, the bias tee 5 only superimposes a DC voltage on the voltage waveform of the clock, and the comb generator 3 can be used only at the rise and fall of the input sine wave. Degrees, ie
You cannot select all channels. Therefore, bias tee 5
Before the control, a delay circuit for delaying the clock signal is provided, and control is performed in consideration of the delay time of the delay circuit and the channel for selecting the control voltage of the bias tee 5, so that all channels can be selected. FIG. 5 shows an example of this delay circuit. In FIG. 5, a delay circuit 6 includes a circuit 61 for delaying a clock waveform by 120 degrees, a circuit 62 for delaying 240 degrees, and 0 °, 120 °,
Select one of the clock waveforms delayed by 40 degrees
It is composed of a 3 × 1 switch for outputting. Then, by inputting the signal from the delay circuit 6 to the bias tee 5, all channels can be selected. In FIG. 5, the phase to be delayed is 0 degree, 120 degrees, and 240 degrees, and the step width of the phase is 120 degrees. It should be set accordingly.

As described above, by using the comb generator 5 as the timing generation circuit, the circuit configuration is simpler than the circuit configuration shown in the first embodiment.
It is possible to select a predetermined range of channels. Further, by further providing a delay circuit 6 in the configuration of FIG. 4 and controlling the delay circuit 6 and the bias tee 5 in consideration of the channel as shown in FIG. 5, it is possible to select all the channels. Become.

(Experimental Example) Next, an experimental example for confirming the basic operation of the optical time-division signal selection circuit 1 shown in the second embodiment will be described below. FIG. 6 is a diagram showing the configuration of the device used for the experiment. In FIG. 6, a transmitter 31 that outputs a clock of 6.4 [GHz] is synchronized with a transmitter 41 that outputs a clock of 100 [MHz]. First, a circuit for generating an optical time-division multiplexed signal sequence will be described. The short pulse laser 32 outputs a short optical pulse having a repetition period of 6.4 [GHz] using a clock signal of 6.4 [GHz] as an input signal. On the other hand, the pulse pattern generator 33 generates a 6.4 [Gbit / s] pulse pattern in which a signal of 100 [Mbit / s] is multiplexed for 64 channels. Thus, the modulator 34 modulates the optical pulse train from the short pulse laser 32 with the pulse pattern from the pulse pattern generator 33. In a portion indicated by reference numeral 35 of this signal, a signal of 12.8 [Gbit / s] is generated by using an optical delay line. This corresponds to a signal multiplexed for 128 channels. The output light from here is optically amplified by the optical amplifier 36. This optical amplifier is ASE (Am
An optical band-pass filter 37 is provided to remove the ASE because it also outputs the optical noise described as “Plified Spontaneous Emittion”. The optical bandpass filter 37
The wavelength of the optical time division multiplexed signal sequence output from
In [ns], the pulse width is about 15 [ps]. Also,
Since the EA modulator 4 used in this experiment has polarization dependence, it further includes a polarization controller 3
After passing through 8, the signal is input to the input terminal C1 of the optical time division multiplexed signal selection circuit 1.

On the other hand, a sine wave of 100 [MHz] is input from the input terminal C 3 as a clock to the optical time division multiplexed signal selection circuit 1 in this experiment, and is amplified by the amplifier 42. The amplified signal is input to the bias tee 5, and the control voltage input from the input terminal C4 is superimposed thereon,
An electrical short pulse is output by the comb generator 3.
This signal is supplied to the EA through an attenuator 43, an inverting output unit 44, and a bias tee 45 for adjusting the bias of the EA modulator 4.
An optical signal of a predetermined channel is selected from the input optical time division multiplexed signal and input to the modulator 4 and output. The selected optical signal is output from the output terminal C2, and is converted from the optical signal into an electric signal by the receiver 39 for bit error rate measurement or the like. In the optical time division multiplexed signal selection circuit 1 of FIG. 6, in addition to the configuration shown in FIG. 3, an amplifier 42, an attenuator 43, an inverting output unit 44, and a bias tee 45 for adjusting the bias of the EA modulator 4 are also used. However, this is a circuit required from the four characteristics of the bias tee 5, the comb generator 3, and the EA modulator used for the experiment this time.

Next, FIGS. 7 to 9 show experimental results. FIG. 7 is a diagram showing the relationship between the control voltage applied to the bias tee 5 and the received light power of the selected optical signal. In the figure, the solid line is a value when a signal of 100 [Mbit / s] is selected from the optical time division multiplexed signal of 12.8 [Gbi / s]. A bit error rate of 10 ^-9 is obtained at -44 [dB]
m], there are 37 channels from the figure. This is about 30% of all channels (= 37/128 × 10
0). As described above, the reason why all the channels cannot be selected is that the bias tee 5 only superimposes the DC voltage on the voltage waveform of the clock, and the comb generator 3 operates only at the falling edge of the input sine wave. Because it is available, 360 degrees, ie, not all channels can be selected. For reference, FIG.
100 [M] from an optical time division multiplexed signal of 4 [Gbit / s]
[bit / s] is indicated by a broken line.
In the circuit configuration for this experiment, -44 [d
In order to select all the channels so as to satisfy [Bm], it is expected that the delay circuit 6 (phase step width of 120 degrees) in FIG. You.

FIG. 8 is a diagram showing the relationship between the peak voltage of the electric pulse of the comb generator and the tuning range with respect to the control voltage for the bias tee. In the figure,
The horizontal axis is the control voltage (unit [V]) applied to the bias tee 5, and the left vertical axis is the voltage value (peak voltage) (unit (V)) of the electric pulse output from the comb generator 3. And the right vertical axis is the tuning range (unit [ns]). Focusing on the tuning range,
As shown by the dotted line in the figure, it can be seen that the tuning can be performed substantially linearly at the center. It can also be seen that a total tuning range of 3.5 [ns] can be obtained with a change width of the control voltage to the bias tee 5 of 2.5 V (-1.5 V to +1.0 V). Note that the period of 100 [MHz] is 10 [n].
s], the optical time-division multiplexed signal selection circuit 1 of FIG. 6 can select a maximum of 44 channels (= 128 × 3.5 / 10) ignoring the bit error rate, which is consistent with the result of FIG. ing.

FIG. 9 is a diagram showing power penalty and bit error rate characteristics for each channel. In FIG.
The diagram on the left shows the power penalty for each channel, and the diagram on the right shows the bit error rate. From FIG.
The best channel, denoted by, has a power penalty of 0.3
[DB] and 1.1 [dB] even in the worst channel indicated by reference numeral 52, indicating that a good selection result is obtained.

[0022]

As described above, according to the optical time division multiplexed signal selection circuit of the present invention, the following effects can be obtained. According to the first aspect of the present invention, there is provided a timing generation circuit which receives a clock signal and a control signal, and generates and outputs a timing signal for selecting an optical signal of a predetermined channel from an optical time division multiplexed signal. A comb generator that inputs a signal and outputs a short pulse based on this signal, and selects and outputs an optical signal from an optical time division multiplexed signal at a short pulse timing with a steep relationship between applied voltage and transmittance. And an electro-absorption modulator.
As described above, an electric short pulse having a wide band frequency characteristic can be generated from a low-speed signal input by using the comb generator, and the electric short pulse is applied by using an electro-absorption modulator having a steep relationship between applied voltage and transmittance. A switching window having a shorter time than that of the electric short pulse can be used, and as a result, high-speed switching can be performed. Therefore, switching can be performed by an electric circuit, and an inexpensive optical time division multiplexed signal selection circuit can be provided.

According to the second aspect of the present invention, the timing generation circuit is a phase shifter capable of adjusting the phase of the clock signal by 360 degrees. This makes it possible to select the optical pulse signals of all the channels from the optical time division multiplexed signal. According to the third aspect of the present invention, the timing generation circuit is a bias tee for superimposing a control voltage on a clock signal.
As a result, it is possible to select a predetermined range of channels with a simpler circuit configuration than when a phase shifter is used.
According to the fourth aspect of the present invention, a delay circuit for outputting a clock signal obtained by delaying a clock signal by a setting signal is provided, and this output is used as an input signal to a bias tee. As a result, even when the bias tee is used as the timing generation circuit, all channels can be selected.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical time division multiplexed signal selection circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an applied voltage and a transmittance in an electroabsorption modulator and a switching window with respect to an input pulse.

FIG. 3 is a configuration diagram of an optical time division multiplexed signal selection circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a control voltage and a short pulse output timing in a comb generator.

FIG. 5 is a diagram illustrating a configuration example of a delay circuit for selecting all channels in a second embodiment.

FIG. 6 is a diagram showing a configuration of a device used in an experiment.

FIG. 7 is a diagram illustrating a relationship between a control voltage for a bias tee and a received light power of a selected optical signal.

FIG. 8 is a diagram showing a relationship between a control voltage for a bias tee, a beak voltage of an electric pulse generated by a comb generator, and a tuning range.

FIG. 9 is a diagram showing a power penalty and a bit error rate characteristic for each channel.

FIG. 10 is a configuration diagram of a conventional example of a selection circuit.

[Explanation of symbols]

 Reference Signs List 1 optical time division multiplexed signal selection circuit 2 phase shifter (timing generation circuit) 3 comb generator 4 electroabsorption modulator 5 bias tee (timing generation circuit) 6 delay circuit

Claims (4)

[Claims] 1. A clock signal and a control signal are inputted,
Timing for generating and outputting a timing signal for selecting an optical signal of a predetermined channel from an optical time division multiplexed signal in which a signal of a bit rate corresponding to the frequency of the clock signal is multiplexed by the control signal from the clock signal. A generation circuit, a timing signal from the timing generation circuit is input, and a comb generator that outputs a short pulse based on the timing signal; and the optical time division multiplexed signal and the short pulse from the comb generator are input signals, An electroabsorption type in which the relationship between the applied voltage and the transmittance is steep, switching is performed in a time shorter than the pulse width of the short pulse, and the optical signal of the predetermined channel is selected and output from the optical time division multiplexed signal. An optical time division multiplexed signal selection circuit, comprising: a modulator. 2. The timing generation circuit according to claim 1, wherein the phase shifter is capable of adjusting the phase of the clock signal by 360 degrees by the control signal.
3. The optical time division multiplex signal selection circuit according to 1. 3. The timing generation circuit according to claim 1, wherein the timing generation circuit is a bias tee that inputs a control voltage as the control signal and superimposes the control voltage on a voltage signal related to the clock signal. Optical time division multiplex signal selection circuit. 4. The optical time division multiplexing / selecting circuit further includes a delay circuit, wherein the delay circuit inputs the clock signal and a setting signal for delay setting, and delays the clock signal by the setting signal. The optical time-division multiplexed signal selection circuit according to claim 3, wherein the clock signal output by the delay circuit is input as the bias tee.