JP2007082193A - Optical transmission system, optical transmitter, and optical receiving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that an increase of number of light emitting elements is effective for improving transmission rate of multi-valued optical transmission, however in a system in which optical outputs from a plurality of light emitting elements are added, a deterioration of transmission quality by common mode noise exists notably since common mode noise contained in each optical output is added. <P>SOLUTION: Data is assigned to the difference of two optical outputs and transmitted. Concretely, in an optical transmitter 102, data Dt which should be transmitted is converted to multi-valued optical signals OSm1 and OSm2, and the converted multi-valued optical signals are transmitted to an optical receiving device 103. The optical receiving device 103 reconstructs the data Dt which is previously assigned to the difference of electrical signals ESr1 and ESr2 which were converted from the multi-valued optical signals OSm1 and OSm2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission system, an optical transmitter, and an optical receiver. More specifically, the present invention relates to an optical transmission system that multi-value encodes data to be transmitted and transmits multi-value encoded data, and an optical transmission device and an optical reception device that constitute the system. is there.

  In the optical communication field, as a technique for increasing the transmission speed, TDM (time division multiplexing) for multiplexing transmission data along a time axis and WDM (wavelength division multiplexing) for multiplexing along a wavelength axis are generally used. Are known.

  As a technique for improving frequency utilization efficiency and increasing the transmission speed, multilevel optical transmission is known in which amplitude and phase are subjected to multilevel encoding and a multilevel encoded signal is transmitted. In particular, the method of assigning a multi-level code to the amplitude level of a transmission signal by modulating the intensity of light emitted from a light source has a configuration of a transmitting device and a receiving device as compared with a method of assigning a multi-level code to a phase. There is an advantage in that it can be simplified.

  Conventionally, as a method of assigning a multi-level code to an amplitude level, there are a method using a single light emitting element (for example, refer to Patent Document 1) and a method using a plurality of light emitting elements (for example, refer to Patent Document 2). Are known. Hereinafter, both methods will be briefly described.

  FIG. 11 is a diagram for explaining a multilevel optical signal transmission method using a single light emitting element.

  Referring to FIG. 11, transmission / reception circuit 760 includes a D / A conversion circuit 700, a light emitting element 710, an optical fiber 720, a light receiving element 730, an amplifier 740, and an A / D conversion circuit 750.

  First, on the transmission side, the D / A conversion circuit 700 converts a combination of digital signals input from a plurality of channels into a multi-valued logic level, and generates an analog signal corresponding to the converted logic level. The light emitting element 710 receives the analog signal output from the D / A conversion circuit 700 and performs electro-optical conversion on the received analog signal. The light emitting element 710 emits an optical signal obtained by electro-optical conversion to the optical fiber 720.

  Next, on the receiving side, the light receiving element 730 receives an optical signal emitted from the optical fiber 720 and generates an electrical signal corresponding to the intensity of the received optical signal. The amplifier 740 amplifies the electrical signal output from the light receiving element 730 and outputs the amplified electrical signal to the A / D conversion circuit 750. The A / D conversion circuit 750 A / D converts the electrical signal output from the amplifier 740 into a digital signal, and restores a plurality of digital signals.

  FIG. 12 is a diagram for explaining a multilevel optical signal transmission method using a plurality of light emitting elements.

  Referring to FIG. 12, the optical transmission device 860 includes a drive circuit 800 that receives binary signals from four channels, and light emitting elements 811 to 814 assigned to each of the four channels.

The drive circuit 800 separately drives the light emitting elements 811 to 814 so that each of the light emitting elements 811 to 814 is turned on or off corresponding to each of the received binary signals. In addition, the drive circuit 800 drives the light emitting elements 811 to 814 so that the respective light emission intensities are different from each other. Therefore, the optical transmission device 860 can output 16 types of multilevel optical signals having different light intensities corresponding to the 16 types of combinations of turning on and off of the light emitting elements 811 to 814.
JP-A-8-79186 JP 2004-112235 A

  However, each of the conventional multilevel optical signal transmission techniques has the following problems.

  First, when a single light-emitting element is used to generate a multilevel optical signal, the number of multilevel levels that can be actually set is limited, and thus there is a limit to increasing the transmission speed. is there. The multi-level design method will be described in detail below.

  FIG. 13 is a diagram showing current-light intensity characteristics of the semiconductor laser.

  As shown in FIG. 13, the output light intensity of the semiconductor laser is linear in the region A where the amount of supplied current is a certain value or less, but in the region B where the amount of supplied current exceeds a certain value. Non-linear. The reason why the output light intensity of the semiconductor laser exhibits nonlinearity in the region B is that the output of the semiconductor laser is saturated in the region B.

  Therefore, as a first condition, in order to change the intensity of the optical signal following the change in the waveform of the electric signal supplied to the light emitting element, a plurality of light intensity levels in a range smaller than the maximum light intensity P5 in the region A. It is necessary to set P0 to P4.

Furthermore, it is necessary to consider the characteristics of the optical receiver as the second condition. In general, as the optical signal intensity received by the optical receiver increases, the noise generated in the optical receiver increases. Therefore, in order to reduce the influence of noise on the optical signal and to easily identify the optical signal level, the level design is performed so that the difference between two adjacent levels increases as the intensity of the optical signal increases. It is preferable. For example, as shown in FIG. 13, the level is designed so that the output light intensities P0 to P5 satisfy the following inequality relationship.
P5-P4>P4-P3>...> P1-P0

  However, even if the level design is performed so as to satisfy the first condition and the second condition, the light output intensity is within the light intensity range in which linearity is ensured (region A in FIG. 13). Multiple multi-value levels are concentrated in the lower range.

  When the light-emitting element is a semiconductor laser or LED, the frequency band is narrowed as the bias current supplied to the light-emitting element decreases. Therefore, when the light-emitting element is driven with a low bias current, the frequency of the light source The band is insufficient compared to the frequency band of the input signal. As a result, interference between multi-levels due to waveform distortion (eye occlusion) occurs, leading to degradation of transmission quality.

  Therefore, when the multi-level optical signal is generated by a single light emitting element, the number of levels that can be set in a range where the light output intensity is linear is actually limited.

  Second, when a plurality of light emitting elements are used to generate a multilevel optical signal, there is a problem that transmission quality deteriorates due to the influence of in-phase noise.

  Specifically, for example, in the optical transmission apparatus shown in FIG. 12, when noise occurs, the generated noise may be included as in-phase noise in the output optical signal from each light emitting element. On the other hand, the optical receiver collectively receives the optical signals output from the respective light emitting elements and outputs an electrical signal corresponding to the intensity of the received optical signal. As a result, the optical receiving apparatus photoelectrically converts the optical signal to which the in-phase noise included in the output light of each light emitting element is added, so that the influence of the in-phase noise on the optical signal is increased, leading to deterioration in transmission quality.

  Therefore, the present invention can improve the transmission speed by increasing the number of multi-level levels and can reduce the degradation of transmission quality due to in-phase noise, and the optical transmission that constitutes the system. An object is to provide a device and an optical receiver.

  The first aspect of the present invention is directed to an optical transmission system that multi-value encodes data to be transmitted and transmits multi-value encoded data. The optical transmission system includes an optical transmission device and an optical reception device.

  More specifically, the optical transmission device includes a drive unit that converts data to be transmitted into a first multi-level code and a second multi-level code that are assigned in advance to data to be transmitted with each difference. A first light emitting unit that outputs a first multi-level optical signal having an intensity corresponding to the first multi-level code; and a second multi-level optical signal having an intensity corresponding to the second multi-level code. And a second light emitting unit.

  On the other hand, the optical receiver receives a first multi-valued optical signal, converts a first multi-valued optical signal into a first electrical signal, and a second multi-valued optical signal. A second light receiving unit that receives light and converts the second multi-level optical signal into a second electrical signal; and a decoding unit that decodes data pre-assigned to the difference between the first and second electrical signals. Including.

  According to such a configuration, the optical transmission device converts data to be transmitted into a combination of the first multi-level code and the second multi-level code, and each of the first and second multi-level codes. Corresponding first and second multilevel optical signals are output. Therefore, in the optical transmission system according to the present invention, the number of levels of the multilevel optical signal can be set larger than that of the multilevel optical transmission system having only a single light source, and therefore the transmission speed is increased. can do.

  Further, the data to be transmitted is predetermined so as to correspond to the difference between the first and second multi-level codes, and the optical receiving apparatus uses two multi-level optical signals to decode the transmitted data. The difference between the two electrical signals converted from the signal is calculated. As a result, even if in-phase noise is generated in the drive unit or the transmission path, the in-phase noise included in the two multilevel optical signals is canceled out, so that transmission quality is improved.

  The first light emitting unit modulates the intensity of the first multilevel signal at a light emission intensity level of M value (where M is an integer of 3 or more), and the second light emitting unit has N value (where N is The second multi-value signal may be intensity-modulated at a light emission intensity level of an integer of 3 or more, and the decoding unit may detect a difference between the amplitude of the first electric signal and the amplitude of the second electric signal. .

  According to such a configuration, encoding of (M × N) at the maximum can be realized by appropriately setting the values of M and N.

  The first multi-level code is an M-value code in which one code takes M values (where M is an integer of 3 or more), and the M values are selected from M values. The two differences are preferably set so as to be different for each combination of two arbitrary values.

  According to such a configuration, since the first multi-level code is set to a different value, the encoding efficiency is improved.

  Further, the second multi-level code is an N-value code in which one code takes N values (where N is an integer of 3 or more), and the N values are selected from the N values. More preferably, the two differences are set so as to be different for each combination of two arbitrary values.

  According to such a configuration, since the second multi-level code is set to a different value, the encoding efficiency is improved.

  Alternatively, the first multi-level code is an M-value code in which one code takes M values (where M is an integer of 3 or more), and the second multi-level code has N codes. (Where N is an integer greater than or equal to 3), M values and N values are one positive value selected from M values and N values. May be set so that the difference from one positive value selected from is unique.

  According to such a configuration, since the difference between the first multi-level code and the second multi-level code becomes a unique value, the encoding efficiency can be maximized.

  The first multi-level optical signal and the second multi-level optical signal are multiplexed, and the receiving apparatus separates the first multi-level optical signal and the second multi-level optical signal from each other. A separation unit may be further included.

  According to such a configuration, it is not necessary to transmit the first multilevel optical signal and the second multilevel optical signal separately, so that the degree of freedom in the configuration of the optical transmission system is improved.

  In addition, the first light emitting unit emits light having a first wavelength, the second light emitting unit emits light having a second wavelength different from the first wavelength, and the separating unit includes first light A first wavelength filter disposed so as to cover the light receiving portion and transmitting light of the first wavelength; and a second wavelength disposed so as to cover the second light receiving portion and transmitting light of the second wavelength. A filter may be included.

  According to such a configuration, it is possible to separate the first multilevel optical signal and the second multilevel optical signal. In addition, disturbance light other than the first multilevel optical signal and the second multilevel optical signal can be removed.

  In addition, at least one of the first wavelength and the second wavelength may be included in the visible light region.

  According to such a configuration, visible light emitted from the optical transmission device can be used as guide light for adjusting the optical axis. Further, it is possible to realize an optical transmission system that has both a function as a communication device and a function as a lighting fixture.

  The first light emitting unit emits light having a first polarization plane, the second light emitting unit emits light having a second polarization plane orthogonal to the first polarization plane, The separation unit is disposed so as to cover the first light receiving unit, and is disposed so as to cover the first polarizing filter that transmits light having the first polarization plane and the second light receiving unit, and the second polarization A second polarizing filter that transmits light having a surface may be included.

  According to such a configuration, it is possible to separate the first multilevel optical signal and the second multilevel optical signal. In addition, it is possible to reduce the influence of disturbance light and reflected light other than the first multilevel optical signal and the second multilevel optical signal that cause noise.

  Each of the first light emitting unit and the second light emitting unit may include either a semiconductor laser or a light emitting diode.

  According to such a configuration, it is possible to realize high-speed modulation of several tens of Mbps to several Gbps.

  The driving unit may drive the first light emitting unit and the second light emitting unit at different modulation speeds.

  According to such a configuration, one of the light emitting units can be configured using a light emitting element that has a low modulation speed but is inexpensive, and the manufacturing cost of the optical transmission system can be reduced.

  The second aspect is directed to an optical transmission device that multi-value encodes data to be transmitted and transmits the multi-value encoded data. The optical transmission apparatus includes: a drive unit that converts data to be transmitted into a first multi-level code and a second multi-level code that are assigned in advance to data to be transmitted with each difference; A first light emitting unit that outputs a first multilevel optical signal having an intensity corresponding to a value code, and a second light emitting unit that outputs a second multilevel optical signal having an intensity corresponding to a second multilevel code With.

  According to such a configuration, the optical transmission device can set a larger number of levels of the multilevel optical signal than the multilevel optical transmission system having only a single light source, thereby increasing the transmission speed. can do.

  The third aspect is directed to an optical receiver that receives multi-level encoded data. The optical receiver receives a first multi-value optical signal, receives a first multi-value optical signal, a first light-receiving unit that converts the first multi-value optical signal into a first electrical signal, and a second multi-value optical signal. And a second light receiving unit that converts the second multi-level optical signal into a second electrical signal, and a decoding unit that decodes data pre-assigned to the difference between the first and second electrical signals. .

  According to such a configuration, it is possible to cancel the common-mode noise generated by the common-mode noise in the drive unit and the transmission path, and improve the transmission quality.

  According to the present invention, since the data to be transmitted is associated with the difference between the two multilevel optical signals in the optical transmitter, the data is obtained by obtaining the difference between the two multilevel optical signals in the optical receiver. Decrypted. Therefore, it is possible to realize an optical transmission system that can increase the transmission speed and improve the S / N ratio by canceling in-phase noise at the time of data decoding.

(Embodiment 1)
FIG. 1 is a functional block diagram illustrating a schematic configuration of the optical transmission system according to Embodiment 1 of the present invention, and FIG. 2 is a diagram illustrating an example of a code conversion table held by the drive unit illustrated in FIG. is there.

  The optical transmission system 101 includes an optical transmission device 102 and an optical reception device 103. The optical transmitter 102 converts the data Dt to be transmitted into two systems of multilevel optical signals OSm1 and OSm2, and outputs the two converted optical signals. The optical receiver 103 decodes the data Dt based on the difference in intensity between the two multilevel optical signals OSm1 and OSm2 received through the transmission medium 190. The transmission medium 190 is, for example, an optical fiber, an optical waveguide, or free space.

  The optical transmission device 102 includes a drive unit 100, a first light emitting unit 131, and a second light emitting unit 132. The drive unit 100 includes a multi-value signal generation unit 110 and drivers 121 and 122. The oscillation wavelength of the first light emitting unit 131 is different from the oscillation wavelength of the second light emitting unit 132.

  On the other hand, the optical receiving device 103 includes a first light receiving unit 141, a second light receiving unit 142, and a decoding unit 170. The first light receiving unit 141 includes a first wavelength filter 181, a first light receiving element 151, and an amplifier 161, and the second light receiving unit 142 includes a second wavelength filter 182 and a second light receiving unit. An element 152 and an amplifier 162 are included. The first wavelength filter 181 transmits light having the oscillation wavelength of the first light emitting unit 131, and the second wavelength filter 182 transmits light having the oscillation wavelength of the second light emitting unit 132.

  Hereinafter, further details of each of the above components will be described along the flow of data transmission in the optical transmission system 101. In the following, for ease of explanation, the first multi-level code Dmc1 and the second multi-level code Dmc2 represent the light emission intensities of the first light-emitting unit 131 and the second light-emitting unit 132, respectively. .

  First, multilevel optical signal transmission processing by the optical transmission apparatus 102 will be described.

  The multilevel signal generation unit 110 of the drive unit 100 receives the data Dt to be transmitted, and converts the received data Dt into the first multilevel optical signal OSm1 and the second multilevel optical signal OSm2 according to a predetermined code conversion table. Convert to

  Referring to FIG. 2, the code conversion table held by drive unit 100 includes a first multi-level code Dmc1 output to driver 121 and a second output output to driver 122 for each predetermined unit of data. A combination with the multilevel code Dmc2 is defined in advance. In particular, the first multi-level code Dmc1 and the second multi-level code Dmc2 are determined such that the difference between them uniquely corresponds to the data. Specifically, as shown in FIG. 2, each of the 3-bit input data strings is different from each other and equal to the difference value between the first multi-level code Dmc1 and the second multi-level code Dmc2. Each of the codes is assigned.

  Here, it is assumed that data to be transmitted is serial data. The multilevel signal generation unit 110 converts data to be transmitted into a conversion code in units of three consecutive bits. Then, the multilevel signal generation unit 110 includes a first multilevel code Dmc1 (any one of a0, a1, and a2) determined in advance so that each difference becomes equal to the conversion code, and a second multilevel code. Dmc2 (any one of b0, b1, and b2) is generated.

  As an example, it is assumed that an input data string “010” is input to the transmission device 2 as a part of the data Dt to be transmitted. The multi-level signal generation unit 110, according to the code conversion table shown in FIG. 2, the first multi-level code a0 pre-assigned to the conversion code (a0-b2) corresponding to the input data string “010”, 2 multi-level code b2.

  The multilevel signal generation unit 110 outputs the generated first multilevel code Dmc1 to the driver 121 and outputs the generated second multilevel code Dmc2 to the driver 122.

  The data Dt to be transmitted does not necessarily need to be serial data, and may be parallel data.

  Next, the driver 121 generates a first drive signal ESd1 for driving the first light emitting unit 131 based on the first multilevel code Dmc1 output from the multilevel signal generator 110. Similarly, the driver 122 generates a second drive signal ESd2 for driving the second light emitting unit 132 based on the second multilevel code Dmc2 output from the multilevel signal generation unit 110.

  Next, the first light emitting unit 131 performs electro-optical conversion on the first drive signal ESd1 output from the driver 121, and has a first multilevel optical signal OSm1 having an intensity corresponding to the first multilevel code Dmc1. Is output. Similarly, the second light emitting unit 132 performs electro-optical conversion on the second drive signal ESd2 output from the driver 122 and has a second multilevel optical signal having an intensity corresponding to the second multilevel code Dmc2. OSm2 is output. The first multilevel optical signal OSm1 and the second multilevel optical signal OSm2 are transmitted to the optical receiver 103 through the transmission medium 190.

  Next, multilevel optical signal reception processing in the optical receiver 103 will be described.

  The first light receiving unit 141 receives the first multilevel optical signal OSm1 and converts the received first multilevel optical signal OSm1 into a first electrical signal ESr1. More specifically, in the first light receiving unit 141, the first wavelength filter 181 is a first multivalue having the oscillation wavelength of the first light emitting unit 131 among the optical signals transmitted through the transmission medium 190. Only the optical signal OSm1 is transmitted, and the second multilevel optical signal Osm2 is removed by absorption or reflection. The first light receiving element 151 photoelectrically converts the light transmitted through the first wavelength filter 181. Then, the amplifier 161 amplifies the output signal from the first light receiving element 151 and outputs the amplified signal to the decoding unit 170 as the first electric signal ESr1.

  Similarly, in the second light receiving unit 142, the second light receiving element 152 photoelectrically converts the second multilevel optical signal OSm2 that has passed through the second wavelength filter 182, and then the amplifier 162 The output signal of the light receiving element 152 is amplified, and the amplified signal is output to the decoding unit 170 as the second electric signal ESr2.

  The decoding unit 170 obtains the amplitude level difference between the first electric signal ESr1 and the second electric signal ESr2. The obtained amplitude level difference corresponds to the conversion code (FIG. 2) assigned to the input data string. The decoding unit 170 decodes the data Dt assigned to the obtained amplitude level difference and polarity according to a code conversion table prepared in advance, and outputs the decoded data Dt.

  Here, the level design method of the first multilevel optical signal OSm1 and the second multilevel optical signal OSm2 will be described in detail.

  FIG. 3 is a matrix diagram showing combinations of the level of the first multilevel optical signal and the level of the second multilevel optical signal.

  FIG. 3 shows a difference value obtained by subtracting each of the levels b0 to b2 that can be taken by the second multilevel optical signal OSm2 from each of the levels a0 to a2 that can be taken by the first multilevel optical signal OSm1. Yes. That is, when the difference value is positive, it indicates that the output level of the first electrical signal ESr1 detected by the optical receiver 103 is higher than the output level of the second electrical signal ESr2. Conversely, when the difference value is negative, it indicates that the output level of the first electrical signal ESr1 is smaller than the output level of the second electrical signal ESr2. Further, when the difference value is “0”, it indicates that the output level of the first received signal and the output level of the second received signal are substantially equal. Further, the absolute value “1” of the difference indicates a minimum value that can be discriminated by the decoding unit 170 of the optical receiving apparatus 103.

As shown in FIG. 3, the first multilevel optical signal levels a0, a1 and a2 are set so as to satisfy the following formula (1).
a0: a1: a2 = 1: 6: 8 (1)

Similarly, the second multilevel optical signal levels b0, b1, and b2 are set so as to satisfy the following formula (2).
b0: b1: b2 = 1: 4: 5 (2)

Furthermore, the first multilevel optical signal levels a0, a1, and a2 are set such that the difference between any two values is different for each combination of two values. That is, a0 to a2 further satisfy the following mathematical formula (3).
(A1-a0) :( a2-a0) :( a2-a1) = 5: 7: 2 (3)

Similarly, the second multilevel optical signal levels b0, b1, and b2 are set such that the difference between any two values is different for each combination of two values. That is, b0 to b2 further satisfy the following mathematical formula (4).
(B1-b0) :( b2-b0) :( b2-b1) = 3: 4: 1 (4)

  In the present embodiment, the first light emitting unit 131 outputs a quaternary multilevel optical signal (0, a0, a1, a2). The same applies to the second light emitting unit 132. However, in the optical receiving device, when the level of the multilevel optical signal is 0 (the light emitting element is in the non-lighted state), there is no data to be transmitted and the multilevel optical signal output from the optical transmitting device is A case where light cannot be received due to a failure is assumed. Therefore, as shown in FIG. 3, the zero levels of the first and second multilevel optical signals are not assigned for data transmission.

  When the first multilevel code Dmc1 and the second multilevel code Dmc2 are designed as described above, an arbitrary multilevel level represented by the first multilevel optical signal OSm1 and the second multilevel optical signal OSm2 are represented. The difference from an arbitrary multilevel level is unique as shown in FIG. Accordingly, when the first light emitting unit 131 and the second light emitting unit 132 output optical signals of M value and N value, respectively, a multi-value signal of {(M−1) × (N−1)} value at maximum. (Where M and N are integers greater than or equal to 3).

  Further, since the decoding unit 170 decodes the data Dt according to the difference in output level between the first electric signal ESr1 and the second electric signal ESr2, it is caused by fluctuations in the power supply voltage in the driving unit 100, jumping noise, or the like. It is possible to remove common mode noise.

  Furthermore, a method for synchronizing the optical transmission apparatus 102 and the optical reception apparatus 103 according to the present embodiment will be described.

  FIG. 4 is a diagram illustrating an example of a signal output from the optical transmission device illustrated in FIG. 1. 4 shows the waveform of the signal output from the first light emitting unit 131, and the lower part of FIG. 4 shows the waveform of the signal output from the second light emitting unit 132. Yes.

  In order to synchronize with the optical receiver 103 when the transmission of the multi-level optical signal is started, the optical transmitter 102 generates an alternating signal in which the emission intensity alternately repeats 0 and a predetermined level for a predetermined synchronization period. Output.

  In particular, in the present embodiment, the first light emitting unit 131 and the second light emitting unit 132 change the amplitude of the synchronization signal stepwise. For example, as shown in the upper part of FIG. 4, the first light emitting unit 131 first outputs an alternating signal with an amplitude of a0 and then outputs an alternating signal with an amplitude of a1 in the synchronization period, Finally, an alternating signal having an amplitude a2 is output. Similarly, the second light emitting unit 132 also outputs a synchronization signal whose amplitude changes stepwise (lower stage in FIG. 4).

  On the other hand, the optical receiver 103 synchronizes with the optical transmitter 102 based on the detected synchronization signal. Further, in the optical receiving apparatus 103, the amplifiers 161 and 162 determine the amplification factor based on the maximum amplitude level of the received synchronization signal.

  As described above, the optical transmission system 101 according to the present embodiment includes two light emitting units, that is, the first light emitting unit 131 and the second light emitting unit 132. Therefore, compared to an optical transmission system having a single light emitting element, the number of levels of the multilevel optical signal can be increased, and the transmission speed can be increased.

  In addition, in the optical transmission system 101 according to the present embodiment, a difference between two multilevel codes is assigned to data to be transmitted. Therefore, in order for the decoding unit 170 to decode the data, the difference between the first electric signal ESr1 and the second electric signal ESr2 can be obtained, so that the common-mode noise can be canceled and the S / N ratio can be improved. Connected. In particular, when the M-value first multilevel code Dmc1 and the N-value second multilevel code Dmc2 are designed as shown in FIG. 3, the maximum {(M−1) × (N−1 )} Data can be transmitted using multiple multilevel signals (where M and N are integers of 3 or more).

  In the first embodiment, the driving unit 100 includes the multi-value signal generating unit 110 and the drivers 121 and 122. However, the driving unit 100 determines that the data Dt to be transmitted is different from each other. What is necessary is just to be comprised so that it may convert into the 1st multi-value code | cord | chord Dmc1 and 2nd multi-value code | cord | chord Dmc2 previously allocated to the said data to be transmitted.

  In the first embodiment, each of arbitrary binary differences (a2-a1, a1-a0, a2-a1) selected from the first multi-level code Dmc1 and the second multi-level code Dmc2 are used. The first multi-level code Dmc1 and the second multi-level code Dmc2 are set so that each of the selected binary differences (b2-b1, b1-b0, b2-b1) are all different. It is preferable.

  Further, when the two multilevel optical signals OSm1 and OSm2 are equal in intensity, the first light receiving element 151 and the second light receiving element are set so that the levels of the first electric signal ESr1 and the second electric signal ESr2 are equal. More preferably, the photoelectric conversion efficiencies of the elements 152 are substantially equal. In addition, when one emission intensity level of the multi-level optical signal is reduced due to an external factor or the like, the first light receiving unit 141 and the second light receiving unit 141 and the second light receiving unit 141 and the second light receiving unit 141 may be compensated by gain control of the amplifiers 161 and 162. The light receiving unit 142 may further include an AGC (automatic gain control device).

  Furthermore, the synchronization signal output when the optical transmission apparatus 102 starts transmission of the multilevel optical signal is not limited to the example of FIG. For example, the first light emitting unit 131 and the second light emitting unit 132 may output a binary (0 and constant level) alternating signal.

  Furthermore, in Embodiment 1, a value obtained by subtracting the second multilevel code Dmc2 from the first multilevel code Dmc1 is assigned to the data Dt to be transmitted. A value obtained by subtracting the first multi-level code Dmc1 from the multi-level code Dmc2 may be assigned.

  Furthermore, in the first embodiment, the first light emitting unit 131 and the second light emitting unit 132 output the light having different oscillation wavelengths from each other, instead of outputting light having different oscillation wavelengths. May output linearly polarized light having polarization planes orthogonal to each other. In this case, the first light receiving unit 141 includes a first polarizing filter that transmits light of a polarization plane that the output light of the first light emitting unit 131 has, and the second light receiving unit 142 is a second light emitting unit. It is only necessary to include a second polarizing filter that transmits light of a polarization plane included in the output light 132.

(Embodiment 2)
The configuration of the multilevel optical transmission system according to the present embodiment is the same as that according to the first embodiment. However, the multilevel optical transmission system according to the present embodiment is characterized by a method for setting a bias current for driving the light emitting unit. Below, it demonstrates centering on the characteristic point of this Embodiment.

  FIG. 5 is a diagram showing the relationship between the bias current supplied to the first light emitting unit and the light emission intensity level of the first light emitting unit, and FIG. 6 shows the bias current supplied to the second light emitting unit. It is a figure which shows the relationship between and the light emission intensity level of a 2nd light emission part. Further, FIG. 7 is a diagram showing the relationship between the frequency response band of the first light emitting unit and the bias current, and FIG. 8 is a diagram showing the relationship between the frequency response band of the second light emitting unit and the bias current. is there.

  Referring to FIGS. 5 and 6, the emission intensity levels of the first light emitting unit 131 and the second light emitting unit 132 are defined by the supplied bias current. Here, the bias currents when the light emission intensities a0, a1, and a2 of the first light emitting unit 131 are Ia0, Ia1, and Ia2, respectively, and the light emission intensities b0, b1, and b2 of the second light emitting unit 132 are obtained. The bias currents are Ib0, Ib1 and Ib2, respectively.

Referring to FIGS. 7 and 8, generally, a semiconductor laser or a light emitting diode has a characteristic that a frequency response band becomes wider as the bias current IB increases. Therefore, in the present embodiment, the bias current Ia0 corresponding to the light emission intensity a0 of the first light emitting unit 131 is set so as to satisfy the condition of the following equation (5). Here, fa0 is the frequency response band of the first light emitting unit 131 when the bias current is Ia0, and fs1 is the frequency band of the modulation signal output by the first light emitting unit 131.
fa0 ≧ fs1 × 0.7 (5)

Similarly, the bias current Ib0 corresponding to the light emission intensity b0 of the second light emitting unit 132 is set so as to satisfy the condition of the following formula (6). Here, fb0 is the frequency response band of the second light emitting unit 132 when the bias current is Ib0, and fs2 is the frequency band of the modulation signal output by the second light emitting unit 132.
fb0 ≧ fs2 × 0.7 (6)

  According to the bias setting described above, it is possible to compensate for the frequency response bands of the first light emitting unit 131 and the second light emitting unit 132. Therefore, deterioration of transmission quality due to insufficient modulation band of the light emitting element is suppressed. In addition, the waveform distortion of the first multi-level optical signal and the second multi-level optical signal and the eye occlusion that occurs remarkably at the minimum light emission intensity level are improved, and therefore the influence of intersymbol interference is reduced.

  The modulation speed of the first multilevel optical signal and the modulation speed of the second multilevel optical signal may be different from each other. When the modulation speed differs between the first multilevel optical signal and the second multilevel optical signal, the modulation speed of the first multilevel optical signal is K times the modulation speed of the second multilevel optical signal, or 1 / K times (where K is a natural number).

(Embodiment 3)
FIG. 9 is a diagram showing a schematic configuration of the multilevel optical transmission apparatus according to Embodiment 3 of the present invention.

  The optical transmission device 302 according to the present embodiment is realized as a lighting fixture or a part thereof, and is used in a state of being attached to the ceiling 340, for example. Although the optical receiver is not shown in the drawing, it may have the same function as that according to the first embodiment. For example, the optical receiver (not shown) is disposed below the optical transmitter 302 in a state where it is connected to or incorporated in a terminal device such as a portable computer. The reception device receives data transmitted from the optical transmission device 302 using light emitted from the optical transmission device 302.

  The optical transmission device 302 includes a multi-value signal generation unit 310, a driving unit 300 including drivers 321 and 322, a first light emitting unit 331, and a second light emitting unit 332. The first light emitting unit 331 includes a plurality of light emitting elements that output visible light having a light emission intensity level of M values in accordance with a drive signal output from the driver 321. The second light emitting unit 332 includes a semiconductor laser that outputs infrared light having an N-level light emission intensity level in accordance with a drive signal output from the driver 322.

  FIG. 10 is a diagram illustrating an example of a signal output from the optical transmission device illustrated in FIG. 9.

  In general, a semiconductor laser can modulate a signal at a higher speed than an LED. Therefore, in the optical transmission device 302 according to the present embodiment, the drive unit 300 includes a first light emitting unit 331 and a second light emitting unit. The unit 332 is driven at a different modulation speed. For example, as illustrated in FIG. 10, the driving unit 300 drives the second light emitting unit 332 at a modulation speed four times that of the first light emitting unit 331. Therefore, the second light emitting unit 332 outputs the multilevel optical signal four times while the first light emitting unit 331 transmits the multilevel optical signal once.

  In particular, in this embodiment mode, the positional relationship between the first light emitting unit 331 and the second light emitting unit 332 is such that the irradiation region 333 of the infrared light output from the second light emitting unit 332 is the first. It is set to be included in the irradiation region 334 of visible light output from the light emitting unit 331. Specifically, as illustrated in FIG. 9, the first light emitting unit 331 and the second light emitting unit 332 have a central axis of the first light emitting unit 331 and an optical axis of the second light emitting unit 332. What is necessary is just to arrange | position so that it may correspond.

  The reason why the first light emitting unit 331 and the second light emitting unit 332 are arranged in this way is as follows.

  In order to receive data from the optical transmission apparatus 302 according to the present embodiment, the user receives both the first multilevel optical signal (visible light) and the second multilevel optical signal (infrared light). It is necessary to arrange the optical receiver at a position where light can be received. However, it is not easy for the user to visually determine whether or not the optical receiver receives the second multi-valued light that is infrared light. Therefore, the arrangement of the first light emitting unit 331 and the second light emitting unit 332 is adjusted as described above so that the second multilevel light signal functions as guide light that roughly indicates the irradiation position. .

  According to such a configuration, in order to receive two multilevel optical signals, the user simply arranges the optical receiver within the visible light irradiation range below the optical transmitter 302. Therefore, convenience for the user is improved.

  As described above, the optical transmission device 302 according to the present embodiment can achieve the same effects as those according to the first embodiment. Furthermore, according to the present embodiment, it is possible to realize a transmission device having an illumination function and a guide function for optical axis adjustment in addition to the data transmission function.

  In the present embodiment, the second light emitting unit 332 outputs infrared light, but may output visible light instead of infrared light.

  In the present embodiment, the light irradiation range by the second light emitting unit 332 is smaller than the light irradiation range by the first light emitting unit 331, but the light irradiation range by the second light emitting unit 332 is It may be more than the light irradiation range by the first light emitting unit 331.

  Since the present invention can reduce received signal noise and intersymbol interference due to waveform distortion in a multilevel optical transmission system, it is useful, for example, as a configuration for improving the transmission quality of an optical transmission apparatus.

1 is a functional block diagram showing a schematic configuration of an optical transmission system according to Embodiment 1 of the present invention. The figure which shows an example of the code conversion table which the drive part shown by FIG. 1 hold | maintains Matrix diagram showing a combination of the level of the first multilevel optical signal and the level of the second multilevel optical signal It is a figure which shows an example of the signal which the optical transmitter shown by FIG. 1 transmits. The figure which shows the relationship between the bias current supplied to a 1st light emission part, and the light emission intensity level of a 1st light emission part. The figure which shows the relationship between the bias current supplied to a 2nd light emission part, and the light emission intensity level of a 2nd light emission part. The figure which shows the relationship between the frequency response band of a 1st light emitting element, and bias current. The figure which shows the relationship between the frequency response band of a 2nd light emitting element, and bias current. The figure which shows schematic structure of the multi-value optical transmitter which concerns on Embodiment 3 of this invention. The figure which shows an example of the signal which the optical transmitter shown by FIG. 9 outputs The figure for demonstrating the multi-value optical signal transmission system using a single light emitting element The figure for demonstrating the multi-value optical signal transmission system using a several light emitting element It is a figure which shows the electric current-light intensity characteristic of a semiconductor laser.

Explanation of symbols

100, 300 Drive unit 101 Optical transmission system 102, 302 Optical transmission device 103 Optical reception device 110, 310 Multilevel signal generation unit 121, 122, 321, 322 Driver 131, 331 First light emitting unit 132, 332 Second light emission Unit 141 first light receiving unit 142 second light receiving unit 151 first light receiving element 152 second light receiving elements 161 and 162 amplifier 170 decoding unit 181 first wavelength filter 182 second wavelength filter 190 transmission medium

Claims (13)

An optical transmission system that multi-value encodes data to be transmitted and transmits multi-value encoded data,
A driver that converts the data to be transmitted into a first multi-level code and a second multi-level code in which each difference is pre-assigned to the data to be transmitted;
A first light emitting unit that outputs a first multilevel optical signal having an intensity corresponding to the first multilevel code;
A second light emitting unit that outputs a second multilevel optical signal having an intensity corresponding to the second multilevel code;
An optical transmitter including:
A first light-receiving unit that receives the first multi-valued optical signal and converts the first multi-valued optical signal into a first electrical signal;
A second light-receiving unit that receives the second multi-valued optical signal and converts the second multi-valued optical signal into a second electrical signal;
A decoding unit for decoding data pre-assigned to the difference between the first and second electrical signals;
An optical transmission system comprising: an optical receiver including: The first light emitting unit modulates the intensity of the first multilevel signal at a light emission intensity level of M value (where M is an integer of 3 or more),
The second light emitting unit modulates the intensity of the second multilevel signal at an emission intensity level of N values (where N is an integer of 3 or more),
The optical transmission system according to claim 1, wherein the decoding unit detects a difference between an amplitude of the first electric signal and an amplitude of the second electric signal. The first multi-level code is an M-value code in which one code takes M values (where M is an integer of 3 or more),
The M values are set such that two differences selected from the M values are different for each combination of two arbitrary values. Optical transmission system. The second multi-level code is an N-value code in which one code takes N values (where N is an integer of 3 or more),
The N values are set such that two differences selected from the N values are different for each combination of two arbitrary values. Optical transmission system. The first multi-level code is an M-value code in which one code takes M values (where M is an integer of 3 or more),
The second multi-level code is an N-value code in which one code takes N values (where N is an integer of 3 or more),
The M values and the N values are unique in the difference between one positive value selected from the M values and one positive value selected from the N values. The optical transmission system according to claim 2, wherein the optical transmission system is set as follows. The first multilevel optical signal and the second multilevel optical signal are multiplexed,
The optical transmission system according to claim 1, wherein the reception device further includes a separation unit that separates the first multi-level optical signal and the second multi-level optical signal from each other. The first light emitting unit emits light of a first wavelength,
The second light emitting unit emits light having a second wavelength different from the first wavelength,
The separation unit is
A first wavelength filter disposed so as to cover the first light receiving portion and transmitting the light of the first wavelength;
The optical transmission system according to claim 6, further comprising: a second wavelength filter disposed so as to cover the second light receiving unit and transmitting the light of the second wavelength.   The optical transmission system according to claim 7, wherein at least one of the first wavelength and the second wavelength is included in a visible light region. The first light emitting unit emits light having a first polarization plane;
The second light emitting unit emits light having a second polarization plane orthogonal to the first polarization plane;
The separation unit is
A first polarizing filter disposed so as to cover the first light receiving portion and transmitting light having the first polarization plane;
The optical transmission system according to claim 6, further comprising: a second polarizing filter that is disposed so as to cover the second light receiving unit and transmits light having the second polarization plane.   2. The optical transmission system according to claim 1, wherein each of the first light emitting unit and the second light emitting unit includes one of a semiconductor laser and a light emitting diode.   The optical transmission system according to claim 1, wherein the driving unit drives the first light emitting unit and the second light emitting unit at different modulation speeds. An optical transmitter that multi-value encodes data to be transmitted and transmits multi-value encoded data,
A driver that converts the data to be transmitted into a first multi-level code and a second multi-level code in which each difference is pre-assigned to the data to be transmitted;
A first light emitting unit that outputs a first multilevel optical signal having an intensity corresponding to the first multilevel code;
An optical transmitter comprising: a second light-emitting unit that outputs a second multi-level optical signal having an intensity corresponding to the second multi-level code. An optical receiver that receives multi-level encoded data,
A first light-receiving unit that receives a first multi-value optical signal and converts the first multi-value optical signal into a first electric signal;
A second light-receiving unit that receives a second multi-value optical signal and converts the second multi-value optical signal into a second electric signal;
An optical receiving device comprising: a decoding unit that decodes data pre-assigned to a difference between the first and second electric signals.

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