JP6031803B2 - Transmitter and communication system for optical multiplex communication - Google Patents

Description

  The present invention relates to a transmitter and a communication system for optical multiplex communication, and more particularly to a device that modulates light by changing a refractive index.

  As an optical multiplex communication system, one that controls a light source based on a drive current signal generated based on transmission data and outputs a visible light signal to a receiver has been proposed. Patent Document 1 describes a configuration in which a plurality of light sources are prepared using an optical branch circuit, and these light sources are respectively modulated in parallel by corresponding modulators.

JP-A-7-127303

  However, in the conventional optical multiplex communication system, it is necessary to prepare a plurality of light sources used for the transmitter. For this reason, there existed a problem that a structure became complicated and cost increased, for example. It is possible to branch light from a single light source as in Patent Document 1 and handle it like a plurality of light sources in a pseudo manner. However, in such a configuration, light supplied to each modulator is used. The strength of is reduced. Further, a branch circuit is required and the configuration becomes complicated.

  The present invention has been made to solve such a problem, and provides a transmitter and a communication system capable of transmitting a plurality of signals using a single light source in optical multiplex communication. Objective.

In order to solve the above-mentioned problem, a transmitter for optical multiplex communication according to the present invention is provided on a main light path, provided with a light source that emits light, a light guide unit that forms a main light path for guiding light, and a first light path. A first modulation unit that modulates light according to the first signal, and a second modulation that is provided downstream of the first modulation unit on the main optical path and modulates the light according to the second signal. The first modulation means is a refractive modulation means comprising a refractive index variable section, and the refractive index variable section changes the refractive index according to the voltage applied as the first signal, and the refractive modulation. The means branches a part of the light from the main optical path according to the change of the refractive index in the refractive index variable section, thereby modulating the light, and the light guide means and the first modulation means form the main optical path. A core and a clad disposed on the outer periphery of the main optical path, and the refractive index variable portion is the core or the core. The voltage varies within a predetermined range in response to the first signal, and when the voltage is within the range, the refractive index of the core is greater than the refractive index of the cladding, and the first modulation The means extends along the main optical path, and the first modulation means comprises a first bending portion that curves with a first curvature, and when the voltage is at the lower limit of the range, the light propagating in the main optical path is the first curve When total reflection occurs at the interface between the core and the cladding in the part, and the voltage is the upper limit of the range, a part of the light propagating through the main optical path is transmitted through the interface, thereby branching from the main optical path and guiding the light means comprises a second curved portion which is curved with a small second curvature than the first curvature, the core of the second curved portion and the first modulating means, curved in opposite directions to each other in the same plane .

  According to such a configuration, a plurality of modulation means are provided in the main optical path, and the modulation means positioned upstream branches a part of the light from the main optical path, thereby performing modulation. Further, the modulation means located downstream performs modulation using light that is not branched by the upstream modulation means.

The first modulation unit may change a voltage applied to the refractive index variable unit according to the first signal.
The second modulation means may be a refraction modulation means.
The light source may be a laser diode and the light guiding means may be an optical fiber .

  Further, the communication system according to the present invention is configured to demodulate the light received by the light detecting means, the light detecting means for receiving the light branched by the first modulating means, and the first signal. And a demodulating means for reproducing.

  According to the transmitter and the communication system of the present invention, the modulation means located downstream uses the light that has not been branched by the upstream modulation means, so that light is applied to a plurality of modulation means by a single light source. Can be supplied. For this reason, for example, the configuration of the light source is simplified and the cost can be reduced.

1 is a schematic diagram showing an outline of a configuration of an optical multiplex communication system according to Embodiment 1. FIG. It is an enlarged view explaining the structure relevant to the optical fiber part and modulation | alteration means of FIG. It is a graph showing the change of the refractive index with respect to the voltage applied to the core of FIG. It is sectional drawing which cut | disconnected the structure shown in FIG. 2 by the plane containing the axis | shaft of a core. FIG. 5 is a diagram for explaining how light is reflected and transmitted at different voltages in FIG. 4. It is a figure which shows the example of the relationship between the operation timing of each modulation means, and the pulse which generate | occur | produces in each modulation means. FIG. 11 is a diagram illustrating an example of time-sharing control in the second embodiment. FIG. 10 is a diagram illustrating an example of phase control in the third embodiment.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing an outline of a configuration of an optical multiplex communication system 1 according to Embodiment 1 of the present invention. The optical multiplex communication system 1 according to the present invention includes a transmitter 10 and receivers 21 to 23. The transmitter 10 includes an optical fiber unit 11, a plurality of modulation units (first modulation unit 31, second modulation unit 32, and third modulation unit 33) provided in the optical fiber unit 11, and these modulation units. Signal sources (first signal source 41, second signal source 42, and third signal source 43) for supplying signals to each of them, control means 50 for controlling each signal source, and light to the optical fiber section 11. And a light source 60 to be supplied.

  The light source 60 is, for example, a light emitting diode (LED), and emits light to enter the optical fiber unit 11. The optical fiber portion 11 functions as a light guiding unit that forms a main optical path L that guides incident light in the axial direction. The modulation means 31 to 33 are provided on the main optical path L in the order from upstream to downstream. That is, on the main optical path L, the modulation means 31 is provided on the most upstream side, the modulation means 32 is provided on the downstream side of the modulation means 31, and the modulation means 33 is provided on the downstream side of the modulation means 32. Further, the modulation means 31 to 33 modulate light in accordance with the input signals (first signal, second signal, and third signal), respectively.

  The light modulated by the modulation means 31 to 33 is branched from the main optical path L to the corresponding branched optical paths (first branched optical path L1, second branched optical path L2, and third branched optical path L3) (branching operation). Will be described later). The receivers 21 to 23 receive the light branched into the branched light paths L1 to L3 by the modulation units 31 to 33, respectively. For example, the receiver 21 includes first light detection means for receiving light propagating through the branch optical path L1. This light detection means can be constituted by a light receiving element using a known MOS or CCD. In addition, the receiver 21 includes first demodulating means for demodulating the received light and reproducing the first signal.

  The receiver 22 and the receiver 23 also have the same configuration as the receiver 21. With this configuration, the optical multiplex communication system 1 performs multiplex communication between the plurality of modulation units 31 to 33 and the plurality of receivers 21 to 23 using light from the single light source 60.

  FIG. 2 is an enlarged view illustrating a configuration related to the optical fiber unit 11 and the modulation means 31 to 33. FIG. 2 shows only the modulation means 31, but in this embodiment, the modulation means 32 and the modulation means 33 also have the same configuration as the modulation means 31.

  The optical fiber unit 11 has a configuration as a known optical fiber. For example, a core 101 forming the main optical path L and a clad 102 disposed on the outer periphery of the core 101 are provided. The modulation means 31 has a shape extending along the main optical path L, and includes a core 111 that forms the main optical path L, and a clad 112 that is disposed on the outer periphery of the core 111. The clad 112 exists so as to wrap the core 111.

  The core 111 of the modulation means 31 is made of a material that has transparency (that is, photoconductivity) and whose refractive index changes when a voltage is applied. That is, in the present embodiment, the core 111 is a refractive index variable unit, and the modulation unit 31 is a refractive modulation unit. As such a material, for example, PLZT can be used. Further, the cladding 112 of the modulation means 31 is made of a material having a refractive index lower than that of the core 111.

The clad 112 is provided with a pair of electrodes 140. FIG. 2 shows only one of these (for example, the anode), but the other electrode (for example, the cathode) is provided in the same configuration at a position opposite to the core 111 and the clad 112. The pair of electrodes 140 is connected to the signal source 41 and applies a voltage generated by the signal source 41 to the core 111 and the clad 112. As described above, the signal source 41 and the electrode 140 function as a voltage application unit that changes the voltage applied to the core 111 and the clad 112 according to the first signal. Along with this, the refractive indexes of the core 111 and the clad 112 change according to the applied voltage. Note that the amount of change in refractive index due to application of voltage differs between the core 111 and the clad 112. In the present embodiment, PLZT is used as the material of the core 111 as described above, and the refractive index change of the core 111 is larger than that of the clad 112.
In FIG. 2, the electrode 140 and the peripheral portion thereof are not provided with the clad 112 and the electrode 140 is in direct contact with the core 111, but the electrode 140 may be attached via the clad 112.

FIG. 3 is a graph showing a change in the refractive index n of the core 111 with respect to the voltage V applied to the core 111. The lower limit of the voltage (or the absolute value of the voltage) generated by the signal source 41 in response to the first input signal is V0 (for example, V0 = 0), and the upper limit is V1. That is, the voltage V applied to the core 111 fluctuates within the range of V0 ≦ V ≦ V1.
When V = V0, n = n0, and when V = V1, n = n1. When V0 <V <V1, n0 <n <n1. For example, n varies linearly with respect to V. Further, when the refractive index of the cladding 112 is n2, n> n2 is always satisfied when V0 ≦ V ≦ V1. (In this embodiment, n2 is always constant, but n2 may not be constant as long as n> n2 is satisfied in V0 ≦ V ≦ V1.)
Further, the core 101 and the clad 102 of the optical fiber portion 11 can be made of, for example, the same material as the core 111 and the clad 112 of the modulation means 31, respectively.

Returning to FIG. The modulation unit 31 includes a first bending portion 110. The first bending portion 110 is curved in a certain direction with a certain curvature 1 / R1 (first curvature) (in FIG. 2, the curvature 1 / R1 represents the curvature in the central axis of the core 111).
FIG. 4 is a cross-sectional view of the configuration shown in FIG. 2 cut by a plane including the axis of the core 111. At the boundary surface B between the core 111 and the cladding 112, only the outer edge B1 and the inner edge B2 appear.

  FIG. 5 is a diagram for explaining how light is reflected and transmitted according to different voltages in the structure of FIG. FIG. 5A shows a state where V = V0, and FIG. 5B shows a state where V = V1. The minimum value of the incident angle θ at which light propagating in the main optical path L is incident on the outer edge B1 is θmin (that is, all light propagating in the main optical path L is incident on the outer edge B1 with an incident angle θ equal to or greater than θmin). When V = V0, as shown in FIG. 3, the refractive index ratio n0 / n2 between the core 111 and the clad 112 is relatively large. For this reason, as shown in FIG. 5A, it is possible to design such that light incident from the main optical path L to the outer edge B1 at the incident angle θmin causes total reflection at the outer edge B1 and stays in the main optical path L without branching. It is. The same applies to the case where θ> θmin. Therefore, the light propagating in the main optical path L continues to propagate in the main optical path L without leaking to the outside.

  On the other hand, when V = V1, as shown in FIG. 3, the refractive index ratio n1 / n2 between the core 111 and the clad 112 is relatively small. For this reason, the critical angle of total reflection shifts to the wide angle side and becomes larger than θmin (note that θmin is the same value in the case of FIG. 5A and FIG. 5B). That is, as shown in FIG. 5B, a part of the light incident from the main optical path L to the outer edge B1 at the incident angle θmin is transmitted through the outer edge B1, thereby branching from the main optical path L to the branch optical path L1 and leaking. Can be designed to give out.

  As described above, the modulation unit 31 branches a part of the light from the main optical path L in accordance with the change of the refractive index n in the core 111, thereby modulating the light. For example, the presence of a pulse can be represented by branching light, and the absence of a pulse can be represented by not branching light.

Note that the curvature 1 / R1 (see FIG. 2) of the modulation means 31 is a characteristic of the main optical path L (variation with respect to the refractive index n and voltage V of the core 111, the refractive index n2 of the cladding 112, the minimum value θmin of the incident angle θ, Etc.) can be selected as an angle slightly larger than the critical curvature determined by the above (that is, slightly gentle curvature). Here, the “critical curvature” is the maximum curvature within a range where the light incident on the outer edge B1 at the incident angle θmin causes total reflection when the voltage V is the upper limit V1.
Further, the ratio of light branched from the main optical path L when V = V1 is determined by various conditions such as curvature and refractive index, and for example, about 30% of light can be branched.

  The above is the configuration of the modulation unit 31, but the modulation unit 32 and the modulation unit 33 also have the same configuration. Therefore, as shown in FIG. 1, the light that has not been branched by the modulation means 31 propagates through the main optical path L and reaches the modulation means 32, and a part of the light branches at the modulation means 32 and leaks outside. Further, the light that has not been branched by the modulation means 32 propagates through the main optical path L and reaches the modulation means 33, and a part of the light branches at the modulation means 33 and leaks outside. That is, the upstream modulation means (for example, the modulation means 31) constitutes a part of the main optical path L to the downstream modulation means (for example, the modulation means 32 and the modulation means 33). In this way, a plurality of signals can be extracted from the single light source 60.

  As described above, according to the transmitter 10 and the optical multiplex communication system 1 according to Embodiment 1 of the present invention, the modulation means located downstream using the light that is not branched by the upstream modulation means modulates. As a result, light can be supplied to a plurality of modulation means by a single light source 60. For this reason, for example, the configuration of the light source is simplified and the cost can be reduced.

  Further, as shown in FIG. 1, the optical fiber portion 11 includes a plurality of second bending portions 120. The second bending portion 120 is provided in the main optical path L at a position between the modulation means 31 and the modulation means 32, a position between the modulation means 32 and the modulation means 33, and a position downstream of the modulation means 33. The second bending portion 120 is curved at a constant curvature 1 / R2 (second curvature) in the same plane as the first bending portion 110 in the opposite direction to the first bending portion 110 (note that FIG. 1, the curvature 1 / R2 represents the curvature in the central axis of the optical fiber portion 11). In this embodiment, the curvature of the second bending portion 120 is gentler than the curvature of the first bending portion 110 (R1 <R2), but light propagating through the main optical path L leaks from the second bending portion 120 to the outside. As long as it does not come out, it may not satisfy this condition. That is, the curvature may be larger than the minimum curvature obtained from the refractive index difference between the core 111 and the clad 112. This is because the light propagating through the core 111 is totally reflected if the curvature is larger than the minimum curvature.

  Further, the second bending portion 120 is configured such that the branched optical paths L1 to L3 from the respective modulation means are parallel to each other. For example, the first bending portion 110 and the propagation direction of the light incident on the modulation means 31, the propagation direction of the light incident on the modulation means 32, and the propagation direction of the light incident on the modulation means 33 are all parallel. The 2nd bending part 120 is comprised, and also the modulation | alteration means 31-33 are all arrange | positioned in the same direction. According to such a configuration, since the receivers 21 to 23 can all be arranged in the same direction, the structure of the branched optical paths L1 to L3 and the arrangement of the receivers 21 to 23 can be simplified.

In the first embodiment described above, the following modifications can be made.
Since it is not necessary to maintain the main optical path L downstream of the most downstream modulation means (third modulation means 33), the most downstream modulation means may not be made of a material whose refractive index changes. . For example, it may be a modulation means for performing modulation by transmitting or shielding light according to an applied voltage. Further, when the third modulation means 33 is not provided, the second modulation means 32 may be similarly used as such a modulation means.

  The optical fiber part 11 may not include the second bending part 120. In this case, although the light branched from each modulation means is not parallel to each other as it is, light can be supplied to a plurality of modulation means by a single light source 60 as in the first embodiment.

Embodiment 2. FIG.
The second embodiment specifies the operation timing of each modulation means in the first embodiment.
FIG. 6 shows the operation timing of each modulation means and the pulse generated in each modulation means (ie, branching in each modulation means) when there is no restriction on the operation timing of each modulation means (an example of the first embodiment described above) The example of the relationship with the intensity | strength of light to do is shown. In this example, for convenience of explanation, each modulation means is shown to branch 50% of the light, but the light actually branched is about 30%, for example.

  FIG. 6A shows the intensity of light emitted from the light source 60, which is ideally constant. 6B to 6D show time widths and intensities of pulses generated by the first modulation unit 31, the second modulation unit 32, and the third modulation unit 33. FIG. This pulse corresponds to the light that each modulation means branches from the main optical path L. In this example, each pulse is an ideal rectangle. FIG. 6E shows the intensity of light that propagates downstream in the main optical path L from the most downstream modulation means (that is, the modulation means 33), that is, afterglow.

  As shown in FIG. 6 (b), the first modulation means 31 located on the most upstream side has a uniform pulse waveform and intensity, but as it goes downstream, as shown in FIG. 6 (c) and (d), The waveform and intensity of the pulse change under the influence of the upstream branch. For example, the pulse P1 and the pulse P2 overlap with each other in time, and 50% of the light in the main optical path L branches as the pulse P1 in the first modulation means 31, and therefore does not reach the second modulation means 32. The intensity of the pulse P2 of the second modulation means 32 decreases. Similarly, since the pulse P3 and the pulse P4 partially overlap with each other in time, the intensity of the downstream pulse P4 decreases in the overlapping portion, and the waveform is distorted.

  As described above, when there is no restriction on the operation timing of each modulation means, the light from the single light source 60 is taken out by the modulation means 31 to 33 arranged in series in the main optical path L, so that it is the latter stage. Accordingly, the variation of the light intensity in the main optical path L becomes large, and a design in consideration of the reliability of signal transmission is required.

On the other hand, in the second embodiment, the operation timing of each modulation means is controlled. For example, each modulation means is synchronized and controlled by time division.
FIG. 7 shows an example of time-sharing control in the second embodiment. Each modulation unit branches the light in the main optical path L at different timings according to the control of the control unit 50 (FIG. 1). As a time zone serving as a unit of control, a first time zone T1, a second time zone T2, and a third time zone T3 are provided and arranged in this order so as not to overlap each other.
Each time zone has an equal length to each other and is repeated with an equal period. In the present embodiment, the next first time zone T1 is arranged immediately after the third time zone T3.

  Each modulation means branches light only in the corresponding time zone. That is, the first modulation means 31 branches light only in the first time zone T1, the second modulation means 32 splits light only in the second time zone T2, and the third modulation means 33 The light is split only in the third time zone T3. Therefore, while a certain modulation means (for example, the second modulation means 32) branches a part of the light in the main optical path L, other modulation means (for example, the first modulation means 31 and the third modulation means 33). ) Will not split the light.

  As described above, in the second embodiment, the modulation means 31 to 33 are synchronized, and the signal transmission timing is time-division controlled. Therefore, the downstream modulation means (for example, the third modulation means 33) is not affected by the upstream modulation means (for example, the first modulation means 31 and the second modulation means 32), and FIGS. As shown in d), the quality of the signal from each modulation means can be maintained. That is, the optical signal transmitted from the modulation means 31 to 33 can be transmitted with the same quality regardless of whether it is upstream or downstream of the main optical path L.

  In the second embodiment described above, each time zone is continuous without a gap, but a margin time zone may be provided between each time zone. If control is performed so that none of the modulation means splits light during this margin time, it is possible to more reliably prevent the overlapping of adjacent time zones.

  In the second embodiment, the lengths of the time zones are equal to each other, but the lengths of the time zones may be different from each other. For example, the length of the time zone T1 may be controlled to be greater than the length of the time zone T2. In this way, signals of different rates can be transmitted in parallel.

Embodiment 3 FIG.
In the third embodiment, the operation timing of each modulation means is changed in the second embodiment.
FIG. 8 shows an example of phase control in the third embodiment. Each modulation unit branches the light in the main optical path L at different timings according to the control of the control unit 50 (FIG. 1). The cycle of control is T, and this cycle T is repeated. This period T is common to all the modulation means.

  Each modulation means branches light only at a time corresponding to a specific phase in the period T. That is, the first modulation means 31 branches light only at the time corresponding to the phase θ1 in the period T, the second modulation means 32 branches light only at the time corresponding to the phase θ2 in the period T, and the third The modulating means 33 branches the light only at the time corresponding to the phase θ3 in the period T. In FIG. 8, the temporal position of the pulse is determined based on the center of the pulse, but this may be determined based on the start time of the pulse. The width of the pulse is designed so that pulses of adjacent periods in the same period T (for example, two pulses corresponding to the phases θ1 and θ2) do not interfere with each other.

  Therefore, as in the second embodiment, while a certain modulation unit (for example, the second modulation unit 32) branches a part of the light in the main optical path L, another modulation unit (for example, the first modulation unit) is used. 31 and the third modulation means 33) do not split the light.

  As described above, in the third embodiment, the modulation means 31 to 33 are synchronized and the signal transmission timing is phase shift controlled. Therefore, as in the second embodiment, the downstream modulation means is not affected by the upstream modulation means, and the signal quality from each modulation means is maintained as shown in FIGS. 8B to 8D. Can do. That is, the optical signal transmitted from the modulation means 31 to 33 can be transmitted with the same quality regardless of whether it is upstream or downstream of the main optical path L.

  In the third embodiment, each modulation means generates a pulse only at a time corresponding to a specific phase in the period T, so that no pulse is continuously generated from one modulation means. For example, as apparent from FIGS. 8B to 8D, there is a time interval at least equal to the period T between pulses transmitted from one modulation means. Therefore, it is not necessary to increase the reception resolution on the light receiving side. For example, in the case shown in FIG. 8, it is necessary for the control means 50 on the transmission side to generate a maximum of three pulses in one cycle T, but the receiver 21, receiver 22 and receiver 23 on the reception side each have a cycle. It is only necessary to detect one pulse at the maximum at T.

  In the first to third embodiments, the light-emitting diode (LED) as the light source is, for example, a non-laser light-emitting diode, but may be a laser diode or another light source.

In the first to third embodiments, the core is the refractive index variable unit, and modulation is performed by changing the refractive index of the core according to the voltage. For example, in the example of FIG. 3, the refractive index n of the core decreases from n0 to n1 as the applied voltage V increases, while the refractive index of the cladding is constant at n2.
As a modification, the cladding may be a refractive index variable section, and modulation may be performed by changing the refractive index of the cladding according to the voltage. For example, PLZT is used as the cladding material so that the refractive index increases as the voltage increases, and the core material uses a material that has a higher refractive index than the cladding and a smaller refractive index variation than the cladding. Such a configuration can be realized.

  DESCRIPTION OF SYMBOLS 1 Optical multiplexing communication system, 10 Transmitter, 11 Optical fiber part, 21-23 Receiver, 31 1st modulation means (refractive modulation means), 32 2nd modulation means (refractive modulation means), 33 3rd modulation Means, 41 to 43 signal source, 50 control means, 60 light source, 101 core, 102 clad, 110 first curved portion, 111 core (refractive index variable portion), 112 clad, 120 second curved portion, 140 electrode, B boundary surface (B1 outer edge, B2 inner edge), L main optical path, L1 to L3 branching optical path, T1 first time zone, T2 second time zone, T3 third time zone, V voltage (V0 lower limit, V1 upper limit ), Θ1 phase (first phase time), θ2 phase (second phase time), and θ3 phase.

Claims (5)

A light source that emits light;
A light guiding means for forming a main optical path for guiding the light;
First modulation means provided on the main optical path and modulating the light in response to a first signal;
A second modulator provided on the main optical path downstream of the first modulator and modulating the light according to a second signal;
The first modulation means is a refractive modulation means including a refractive index variable unit,
The refractive index variable unit changes a refractive index according to a voltage applied as the first signal,
The refractive modulation means branches a part of the light from the main optical path according to a change in refractive index in the refractive index variable unit, thereby modulating the light ,
The light guide means and the first modulation means include a core that forms the main optical path, and a clad disposed on an outer periphery of the main optical path,
The refractive index variable part is configured as the core or the clad,
The voltage varies within a predetermined range in response to the first signal,
When the voltage is within the range, the refractive index of the core is greater than the refractive index of the cladding,
The first modulating means extends along the main optical path;
The first modulation means includes a first bending portion that bends with a first curvature,
When the voltage is the lower limit of the range, the light propagating in the main optical path causes total reflection at the boundary surface between the core and the clad in the first curved portion,
When the voltage is the upper limit of the range, a part of the light propagating through the main optical path is transmitted through the boundary surface, thereby branching from the main optical path,
The light guide means includes a second bending portion that curves with a second curvature smaller than the first curvature,
The second bending portion and the core of the first modulation means are bent in opposite directions in the same plane;
Transmitter for optical multiplex communication. It said first modulating means comprises the voltage applying unit for changing the voltage applied to the variable refractive index portion in response to the first signal, the transmitter of claim 1. It said second modulating means is the refraction modulating means, transmitter according to claim 1 or 2. The transmitter according to any one of claims 1 to 3 , wherein the light source is a light emitting diode, and the light guiding means is an optical fiber. The transmitter according to any one of claims 1 to 4 ,
Light detecting means for receiving the light branched by the first modulating means;
A communication system comprising: demodulation means for demodulating the light received by the light detection means to reproduce the first signal.

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