JP2528686B2 - Optical pulse separation circuit and optical pulse multiplexing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used for an optical multiplex communication device. In particular, the present invention relates to an all-optical type optical pulse multiplexing circuit and an optical pulse demultiplexing circuit that time-division multiplex or demultiplex a signal optical pulse train using an optical pulse as a control signal.

[Conventional technology]

 FIG. 7 shows the configuration of a conventional optical pulse separation circuit.

The optical coupler 1 includes a multiplexed signal light pulse train having a bit rate of 2f ₀ [bit / sec] and a repetition period f ₀ [pulse / second].
Second] and the control light pulse train of
Incident on. The optical Kerr element 2 is composed of a medium exhibiting the optical Kerr effect, and changes the polarization state of the signal light pulse in the portion where the signal light pulse and the control light pulse overlap on the time axis. Therefore, in the optical pulse train that has passed through the optical Kerr device 2, the gate optical pulse train component is removed by the optical coupler 4 and then passed through the polarization beam splitter 3, and two signal lights of f ₀ [bit / sec] are obtained. A pulse train is obtained.

In this conventional example, since no electric signal is used, the optical pulse can be separated at high speed without being limited by the response speed of the electric system. Further, the optical pulse can be multiplexed in a time division manner by utilizing the polarization switching effect by the optical Kerr effect.

[Problems to be solved by the invention]

However, in the conventional optical pulse separation circuit, one circuit can separate only two series of optical pulse trains, and in order to separate into N = 2 ⁿ (n is a positive integer) series of optical pulse trains, N−1
Separate circuits are required. Furthermore, in order to obtain a control light pulse, N which generates a light pulse at n kinds of timings is used.
-1 light source is required. Moreover, it is necessary to adjust the intensity of each control light pulse, and also to adjust the timing between the control light pulse and the signal light pulse. Therefore, when trying to separate a large number of sequences of signal light pulse trains by the conventional technique, the circuit configuration and its control circuit become complicated, and there are problems in terms of circuit miniaturization, stabilization, and cost reduction. This problem also applies to the optical pulse multiplex circuit that uses the optical Kerr effect. Further, since the pulse width of the control light pulse needs to be wider than the pulse width of the signal light pulse, there is a drawback that the requirements for the light source are high.

An object of the present invention is to solve the above problems and to provide an optical pulse demultiplexing circuit capable of demultiplexing N-multiplexed signal pulse trains into N series by a single control optical pulse train.

[Means for solving problems]

In the optical pulse separation circuit of the present invention, a plurality of optical elements for changing the polarization state of the signal light pulse depending on the intensity of incident light and a plurality of polarization beam splitters for separating the signal light pulse in a specific polarization state are connected in cascade alternately. The optical element is characterized by having a length such that a difference in group delay amount between the signal light pulse and the control light pulse is substantially equal to an interval between incident signal light pulses.

The optical element is preferably composed of a medium exhibiting the Kerr effect. In this case, the respective optical elements are arranged such that their principal axes are inclined by about 45 ° and about 0 ° with respect to the polarization directions of the incident signal light pulse and the control light pulse, respectively. The optical element is configured to rotate the polarization direction of the signal light pulse by 90 ° when the control light pulse is incident, and the main axis of the polarization beam splitter is tilted at 45 ° with respect to the main axis of the optical element. Are placed.

Further, the optical pulse multiplexing circuit of the present invention is a reverse of the input-output relationship in the above-mentioned optical pulse demultiplexing circuit, a polarization coupler for multiplexing signal light pulses having different polarization states,
In the optical pulse multiplex circuit including an optical element that changes the polarization state of a signal light pulse that is overlapped with the output light of the polarization coupler and the control light pulse and is incident, the optical element and the polarization coupling A plurality of units are alternately connected in cascade, and the optical element has a length such that a difference in group delay amount between the signal light pulse and the control light pulse is substantially equal to an interval of incident signal light pulses. Is characterized by.

[Work]

The optical pulse demultiplexing circuit and the optical pulse multiplexing circuit of the present invention utilize the group delay difference due to the wavelength difference between the signal light pulse and the control light pulse. In other words, the length of each optical element is set so that the control optical pulse overlaps the next signal optical pulse in the next optical element by utilizing the timing difference between the two optical pulses while passing through the optical element. Keep it. Accordingly, in the case of the optical pulse separation circuit, the signal light pulse train of f ₀ [bit / sec] can be sequentially separated from the signal light pulse train of Nf ₀ [bit / sec] every time it passes through the optical element. . The operation is reversed in the case of the optical pulse multiplexing circuit.

Therefore, it is possible to collectively separate or multiplex N series f ₀ [bit / second] signal light pulse trains by using a single series of control light pulses having a repetition period f ₀ [pulses / second]. In addition, it is not necessary to separate or multiplex each half by the multistage structure, and it is possible to directly separate or multiplex.

Further, since the group delay difference between the signal light pulse and the control light pulse is used, these two light pulses are not made to enter the optical element at the same time, but are made to enter at the timing when they overlap each other in the medium of the optical element. If the intensity of the control light pulse is appropriate, it is possible to change the signal light pulse to a desired polarization state only in a partial area in the optical element. Therefore, even if there is a synchronization jitter between the signal light pulse and the control light pulse, the signal light pulse can be accurately separated.

Moreover, since two optical pulses are overlapped in the medium of the optical element, it is not necessary to make the pulse width of the control optical pulse wider than the pulse width of the signal optical pulse as in the conventional case. Light pulses can also be used. In that case, the control light pulse sweeps one signal light pulse at the speed of the group delay difference, and the polarization state of the overlapping portions gradually changes. As a result, the area on the time axis where the polarization state is changed (optical car shutter window)
The shape of can be made substantially rectangular, and the crosstalk characteristic can be improved. When a mode-locked pulse or other light pulse having a constant pulse energy is used as the control light pulse, the peak value of the optical Kerr shutter waveform remains unchanged even if the peak value fluctuates, and the crosstalk characteristic deteriorates. There is no such thing.

〔Example〕

FIG. 1 is a block diagram of an optical pulse separation circuit according to the first embodiment of the present invention.

In the circuit of this embodiment, the optical Kerr elements 2-1 to 2- (N-), into which the signal light pulse and the control light pulse are incident, and which change the polarization state of the signal light pulse overlapping the control light pulse.
1) and polarization beam splitters 3-1 to 3- (N-1) for separating signal light pulses in a specific polarization state from the output light of the optical Kerr elements 2-1 to 2- (N-1). . The optical Kerr elements 2-1 to 2- (N-1) and the polarization beam splitters 3-1 to 3- (N-1) are alternately connected in cascade, and the optical Kerr elements 2-1 to 2- (N-1) are connected. ) Is configured such that the difference in group delay amount between the signal light pulse and the control light pulse is substantially equal to the interval between the incident signal light pulses.

The optical Kerr elements 2-1 to 2- (N-1) include a medium exhibiting the optical Kerr effect, and their main axes are approximately 45 ° and approximately 0 ° with respect to the polarization directions of the incident signal light pulse and the control light pulse, respectively. It is arranged at an angle. Optical Kerr element 2-
1-2- (N-1) is a configuration for rotating the polarization direction of the signal light pulse by 90 ° when the control light pulse and the signal light pulse overlap in the medium, and the polarization beam splitter 3- The main axes of 1 to 3-(N-1) are arranged at an angle of 45 ° with respect to the main axis of the optical Kerr medium.

The optical pulse separation circuit further includes an optical coupler 1 that combines a signal light pulse train and a control light pulse train and supplies them to the optical Kerr element 2-1, and a polarization beam splitter 3- (N-1).
And an optical coupler 4 that separates the control light pulse and the signal light pulse from the output.

Here, the bit rate of the signal light pulse train is Nf ₀ [bit / sec], the wavelength is λ ₁ , the repetition period of the control light pulse train is f ₀ [pulse / sec], and the wavelength is λ ₂ . In addition, the lengths of the media of the optical Kerr elements 2-1 to 2- (N-1) are respectively set.
l ₀ .

FIG. 2 shows the wavelength dependence of the intensity transmittance for each polarization component of the fiber type polarization beam splitter. (A) shows the intensity transmittance of the first arm of the polarization beam splitter, and (b) shows the intensity transmittance of the second arm.

The polarization beam splitter has wavelength dependence in intensity transmission characteristics for each polarization component. Therefore, the wavelength of the signal light pulse is made to match the wavelength at which the intensity transmittance in the two arms of the polarization beam splitter is different from the parallel polarization and the orthogonal polarization, and the wavelength at which all the light is coupled to only one arm. To match the wavelength of the control light pulse. This allows
100% transmission of the control light pulse independent of its polarization,
Moreover, it is possible to separate only the signal light pulse whose polarization direction has changed.

FIG. 3 shows the relationship between the incident signal light pulse train and the separated signal light pulse.

The positional relationship on the time axis of the control light pulse with respect to the signal light pulse changes in the optical Kerr elements 2-1 to 2- (N-1) due to the influence of the group delay difference τ. If the control light pulse and the signal light pulse overlap while this positional relationship changes, the polarization direction of the signal light pulse rotates 90 ° due to the optical Kerr effect. Here, the area in which the positional relationship changes is referred to as the “window of the optical car shutter”. The waveform is called "optical car shutter waveform". In FIG. 3, the optical Kerr shutter waveform is shown by a broken line.

The window of the optical car shutter sets the polarization direction of the signal light pulse to 90 degrees.
This is an area that can be rotated by °. The width of the window of the optical Kerr shutter is the group delay amount difference τl ₀ between the signal light pulse and the control light pulse. Here, since the positional relationship on the time axis between the control light pulse and the signal light pulse changes, even if the pulse width of the control light pulse is narrower than the pulse width of the signal light pulse, the entire signal light pulse of the time elapses. The polarization direction can be rotated. The narrower the pulse width of the control light pulse, the closer the optical Kerr shutter waveform can be made to a rectangular shape, and the crosstalk characteristic can be improved.

Further, since the control light pulse is superposed on the signal light pulse by the group delay difference τ, the power of the signal light pulse appearing in the window of the optical Kerr shutter should be such that the length of the Kerr medium is approximately L _s = 2t ₀ / τ or more. If it becomes saturated. However, t ₀ is the pulse width of the control light pulse. At this time, if the power of the control light pulse is appropriately selected, the polarization direction of the signal light pulse appearing in the window of the optical Kerr shutter can be completely rotated.

Therefore, the length l ₀ of the Kerr medium of the optical Kerr elements 2-1 to 2- (N-1) is set to l ₀ = 1 / (τ · Nf ₀ ) and l ₀ > L _s. If so, the windows of the optical Kerr shutters in the respective optical Kerr elements 2-1 to 2- (N-1) shift at the signal light pulse intervals, and the polarization directions of the signal light pulses can be sequentially rotated. . Therefore, from the polarization beam splitters 3-1 to 3- (N-1), f
A signal light pulse train of ₀ [bit / sec] is obtained.

FIG. 4 shows how the signal light pulse is separated by the light pulse separation circuit. Here, for simplicity, the case of N = 4 is shown. The signal light pulse train of 4f ₀ [bit / sec] is
The optical coupler 1 multiplexes the control optical pulse train having the repeating period f ₀ , and the optical Kerr elements 2-1 to 2-3 and the polarization beam splitters 3-1 to 3-3 split the control optical pulse train into four optical pulse trains.

FIG. 5 shows a circuit configuration diagram of an optical pulse multiplexing circuit according to the second embodiment of the present invention.

This optical pulse multiplexing circuit includes polarization couplers 5-1 to 5- (N-1) that combine signal optical pulses having different polarization states,
The optical Kerr elements 2-1 to 2-1 which change the polarization state of the signal light pulse which the output light of the polarization couplers 5-1 to 5- (N-1) and the control light pulse enter and which overlaps with the control light pulse.
2- (N-1). Optical Kerr elements 2-1 to 2-
(N-1) and polarization couplers 5-1 to 5- (N-1)
Are alternately connected in cascade, and the optical Kerr elements 2-1 to 2-1
2- (N-1) is configured to have a length such that the group delay difference between the signal light pulse and the control light pulse is substantially equal to the interval between the incident signal light pulses.

Further, this optical pulse multiplexing circuit combines the signal light pulse train and the control light pulse train and supplies them to the optical Kerr element 2-1 via the polarization coupler 5-1 and the optical Kerr element 2-. The optical coupler 4 for separating the control light pulse and the signal light pulse from the output of the optical coupler 3 is provided.

For the polarization couplers 5-1 to 5- (N-1), the polarization beam splitter in the optical pulse separation circuit is used with the input / output relationship reversed.

FIG. 6 shows how the signal light pulses are time division multiplexed. Here, as in the example of FIG. 4, an example of N = 4 is shown.

With this configuration, the signal light pulse train f ₀ [bit / s] is time-division multiplexed into a signal pulse train 4f ₀ [bit / sec].

〔The invention's effect〕

As described above, the optical pulse demultiplexing circuit and the optical pulse multiplexing circuit according to the present invention can demultiplex and multiplex the signal optical pulse by the control optical pulse of one system, and can simplify the circuit configuration. Moreover, there is an effect that the region where the polarization state of the optical signal pulse changes can be accurately defined and the crosstalk characteristic can be improved.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of an optical pulse separation circuit according to a first embodiment of the present invention. FIG. 2 is a diagram showing the wavelength dependence of the intensity transmittance for each polarization component of the fiber type polarization beam splitter. FIG. 3 is a diagram showing a relationship between an incident signal light pulse train and a separated signal light pulse. FIG. 4 is a diagram showing how the signal light pulse is separated by the light pulse separation circuit. FIG. 5 is a circuit configuration diagram of an optical pulse multiplexing circuit according to a second embodiment of the present invention. FIG. 6 is a diagram showing how signal light pulses are multiplexed by a light pulse multiplexing circuit. FIG. 7 is a diagram showing a configuration of a conventional optical pulse separation circuit. 1, 4 ... Optical coupler, 2, 2-1 to 2- (N-1) ...
Optical Kerr element, 3, 3-1 to 3- (N-1) ... Polarizing beam splitter, 5-1 to 5- (N-1) ... Polarizing coupler.

Claims (5)

(57) [Claims] 1. A signal light pulse and a control light pulse are incident,
In the optical pulse separation circuit including an optical element that changes the polarization state of the signal light pulse that overlaps with the control light pulse, and a polarization beam splitter that separates the signal light pulse of a specific polarization state from the output light of the optical element, A plurality of optical elements and the polarization beam splitters are alternately connected in cascade, and the optical element has a length such that the difference in group delay between the signal light pulse and the control light pulse is substantially equal to the interval of the incident signal light pulse. An optical pulse demultiplexing circuit characterized by being constructed. 2. The optical pulse demultiplexing circuit according to claim 1, wherein the optical element includes a medium exhibiting an optical Kerr effect. 3. The optical element has a principal axis of approximately 45 with respect to the polarization directions of the incident signal light pulse and the control light pulse, respectively.
3. The optical pulse demultiplexing circuit according to claim 2, wherein the optical pulse demultiplexing circuit is arranged at an angle of .degree. 4. The optical element is configured to rotate the polarization direction of the signal light pulse by 90 ° when the control light pulse and the signal light pulse overlap each other in the medium, and the polarization beam splitter has its main axis. The optical pulse separation circuit according to claim 3, wherein the optical pulse separation circuit is arranged at an angle of 45 ° with respect to the main axis of the optical element. 5. A polarization coupler that combines signal light pulses having different polarization states, and a polarization state of the signal light pulse that is overlapped with the control light pulse upon incidence of the output light of the polarization coupler and the control light pulse. In the optical pulse multiplexing circuit including an optical element for changing, a plurality of the optical elements and the polarization couplers are alternately connected in cascade, and the optical element is a group between the signal light pulse and the control light pulse. An optical pulse multiplexing circuit having a length such that the difference in delay amount is approximately equal to the interval between incident signal light pulses.