JP2005143037A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive optical transmitter capable of making a band of a transmission optical signal compact while precluding the possibility of deterioration in an extinction ratio. <P>SOLUTION: A low pass filter 5 applies band limitation to an NRZI signal 3 to produce a ternary signal 4, and a differential driver 6 generates an inverse signal 7 inverse to the ternary signal 4 and an in-phase signal 8 in phase to the ternary signal 4. A slicer 9 slices the inverse signal 7 at a reference level supplied from a reference voltage source 13 to provide a drive signal having a peak only at the maximum level of the inverse signal 7 to a semiconductor laser 1. Similarly a slicer 10 provides a drive signal having a peak only at the maximum level of the in-phase signal 8 to a semiconductor laser 2. Thus, the semiconductor laser 2 is stimulated to light emission at the highest level of the ternary signal 4 and the semiconductor laser 1 is stimulated at the lowest level of the ternary signal 4. Thus, the semiconductor lasers 1, 2 are subjected to intensity modulation at a band lower than a bit rate of the NRZI signal 3. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a direct modulation optical transmitter used in a wavelength division multiplexing optical communication system.

  In recent years, the speed of data signals in an optical transmission system has been remarkably increased, and accordingly, research on wavelength multiplexing technology has been actively conducted. In order to transmit a large amount of information at a low cost, it is advantageous to multiplex optical signals at high density. Therefore, it is desirable to make the bandwidth of the optical signal emitted from the optical transmitter as compact as possible.

  An optical duo-binary method has attracted attention as a method for reducing the bandwidth of an optical signal. In one of these methods, a binary differential NRZI (Non Return to Zero Inverted) signal pair is band-limited by a low-pass filter to generate a (0, 1, 2) ternary signal. There is a method in which the driving conditions of the light intensity modulator are aligned so that the output light intensity is minimized when one level is input and the light intensity is maximized when another level is input. By limiting the drive signal of the optical modulator to a band lower than the bit rate of the transmission signal, the output optical band of the optical modulator can be made compact.

  However, a light intensity modulator that is generally used includes LiNbO3, and requires a high drive signal level due to its characteristics, so that the burden on the driver circuit is large. When the bit rate of the data signal is increased, a higher speed response is required for the driver circuit, and therefore it is difficult to reduce the cost. In addition, the band of the phase modulation unit of the light intensity modulator may vary. In such a case, it is necessary to prepare a low-pass filter with an optimized cut-off frequency according to variations, which further increases costs. .

  On the other hand, as a technique to make the optical signal band compact by combining the direct modulation method of the laser with the data signal and the optical filter without using the optical modulator, the output light of the semiconductor laser in which the injection current is directly modulated with the data signal is used. A method of detecting a frequency modulation component with a narrow-band optical filter and converting it to an intensity modulation component has been considered. However, in this method, it is necessary to widen the frequency modulation band of the semiconductor laser to such an extent that high-speed modulation of several tens of Gb / s can be performed, which is difficult to realize. Further, where the frequency shift of the directly modulated light increases instantaneously, the extinction ratio of the optical signal may be reduced due to the group delay characteristic of the narrow band optical filter.

A related technique is disclosed in Patent Document 1. This document describes an optical transmission device that realizes a reduction in size and weight of an optical transmission device used in a high-speed optical transmission system by bifurcating an optical signal output from one light source and externally modulating each of the signals. It is disclosed.
JP 2001-27745 A (paragraph numbers [0025] to [0033], FIG. 1)

As described above, the existing optical transmission apparatus based on the optical duobinary system has a problem that the optical modulator is required to have severe characteristics when the transmission bit rate is increased, and thus the cost is inevitably increased. In addition, when the laser direct modulation method and the band suppression method using an optical filter are used in combination for reducing the device scale, there is a problem that the extinction ratio of the optical signal becomes small due to the group delay characteristic of the narrow band optical filter.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical transmitter capable of reducing the band of a transmission optical signal at a low cost without fear of deterioration of the extinction ratio.

  In order to achieve the above object, according to one aspect of the present invention, a ternary signal having first to third levels is generated in order from the highest level by limiting the band of the transmission signal expressed by the NRZI code. Ternary signal generating means (for example, LFP5), first and second semiconductor lasers (for example, LD1 and LD2) having different output optical frequencies, and driving the first semiconductor laser when the first level appears. Drive means (for example, differential driver 6, slicers 9, 10 and reference voltage sources 13, 14) for driving the second semiconductor laser to emit light when the third level appears, and the first and second semiconductor lasers There is provided an optical transmission device comprising a multiplexer (for example, an optical coupler 11) that multiplexes output light and sends it to an optical transmission line.

  By taking such means, in the ternary signal represented by the (0, 1, 2) code, for example, the second semiconductor laser emits light when the 0 level appears, and the first semiconductor laser when the 2 level appears. Emits light. That is, each semiconductor laser is driven to blink by a binary drive signal. As a result, the output light of each semiconductor laser becomes intensity modulated light, and the modulation rate becomes lower than the bit rate of the transmission signal expressed by the NRZI code. That is, intensity-modulated light can be obtained without using an optical modulator, and therefore the driving power to be applied to the device can be reduced and the cost can be reduced. Further, by making the output optical frequency of each semiconductor laser different, the NRZ signal can be reproduced on the intensity modulated light receiving side. Thus, by generating a binary drive signal from a ternary signal and driving a plurality of semiconductor lasers respectively, a normal optical communication transceiver device can be used as it is, and cost reduction is promoted. it can. In addition, since the conversion from the frequency modulation component to the intensity modulation component by the optical filter becomes unnecessary, the possibility that the group delay characteristic of the filter adversely affects the extinction ratio can be eliminated.

  According to the present invention, it is possible to provide an optical transmission apparatus that can reduce the cost by eliminating the need for an optical intensity modulator and that can prevent the deterioration of the extinction ratio of the transmitted optical signal.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a functional block diagram showing a first embodiment of an optical transmission apparatus according to the present invention. In FIG. 1, a binary NRZI signal 3 is input to a low pass filter (LPF) 5. The NRZI signal 3 is generated by converting digital data expressed by, for example, an NRZ (Non Return to Zero) code by a precoder circuit (not shown). The NRZI signal is a signal whose sign is inverted when the NRZ data signal becomes 1 level. The low-pass filter 5 limits the band of the NRZI signal 3 so as to halve the frequency component that is approximately half the bit rate. As a result, the binary NRZI signal 3 is converted into a ternary signal 4 and then input to the differential driver 6. The differential driver 6 generates a reverse-phase signal 7 that is opposite in phase to the ternary signal 4 and an in-phase signal 8 that is in-phase with the ternary signal 4. The reverse phase signal 7 is input to the slicer 9, and the in-phase signal 8 is input to the slicer 10.

  The slicer 9 compares the level of the negative phase signal 7 with the reference value of the reference voltage source 13, and outputs a binary signal that becomes 1 only when the negative phase signal 7 is at the maximum level. This binary signal is appropriately amplified and supplied to the semiconductor laser (LD) 1 as a drive signal. The slicer 10 compares the level of the in-phase signal 8 with the reference value of the reference voltage source 14, and outputs a binary signal that becomes 1 only when the in-phase signal 8 is at the maximum level. This binary signal is appropriately amplified and supplied to the semiconductor laser 2 as a drive signal. Note that the output optical frequency of the semiconductor laser 1 is f2, and the output optical frequency of the semiconductor laser 2 is f4 different from f2.

  As a result, the semiconductor laser 1 emits light only when the highest level of the ternary signal 4 appears. The semiconductor laser 2 emits light only when the ternary signal 4 appears at the lowest level. That is, the intensity of each of the semiconductor lasers 1 and 2 is directly modulated by different binary signals. The output optical signals of the semiconductor lasers 1 and 2 are combined by the optical coupler 11 and transmitted to the optical receiver 12 through the optical fiber. The transmitted intensity-modulated light is photoelectrically converted by a photodetector (not shown) of the optical receiver 12 and received and processed.

  In the above configuration, between the output lights of the semiconductor lasers 1 and 2, the output light intensity of the other semiconductor laser is necessarily the minimum when the output light intensity of one of the semiconductor lasers is maximum. That is, the semiconductor lasers 1 and 2 are not turned on simultaneously. At this time, the output light intensity of the semiconductor laser that is off is preferably 0, but this is not always the case. In such a case, optical interference may occur between the optical signals output from the two semiconductor lasers 1 and 2.

  Therefore, as shown in FIG. 2, the difference between the output optical frequency f2 of the semiconductor laser 1 and the output optical frequency f4 of the semiconductor laser 2 is made larger than the reception band of the optical receiver 12. That is, the photodetector, front-end amplifier, and the like provided in the optical receiver 12 each have a band, but these bands are made sufficiently narrower than the difference between f2 and f4. By doing so, it is possible to prevent the optical receiver 12 from receiving the beat signal component between the light intensity modulation signals emitted from the semiconductor lasers 1 and 2. That is, only the sum component of the respective light intensity modulation signals emitted from the semiconductor lasers 1 and 2 is received by the optical receiver 12.

  In the above configuration, the band of the ternary signal 4 is naturally narrower than the band of the binary NRZI signal 3. Therefore, the bands of the anti-phase signal 7 and the in-phase signal 8 are both narrower than the band of the NRZI signal 3. Thus, even when the transmission bit rate frequency exceeds the upper limit of the modulation band of the semiconductor lasers 1 and 2, the semiconductor lasers 1 and 2 can be modulated and driven. In the present embodiment, this fact is utilized to drive the semiconductor lasers 1 and 2 in a band lower than the transmission bit rate frequency, while varying the output optical frequencies of the semiconductor lasers 1 and 2 to combine the output lights. By making waves, the total transmission bit rate frequency is satisfied.

  Normally, the characteristics of light intensity versus photocurrent of a photodetector used for optical communication are excellent in linearity. When the photodetector receives two optical signals having different wavelengths, crosstalk may occur between the signals when the input light intensity to the photodetector is high. This can also occur when the difference between the center frequencies of the two optical signals is made larger than the band of the photodetector and the beat component cannot be received.

  However, according to the present embodiment, when one semiconductor laser emits light, the light emission level of the other semiconductor laser becomes the lowest level, so that the influence of crosstalk can be almost eliminated.

  FIG. 3 is a functional block diagram showing the configuration of an existing optical transmission device for comparison. The optical transmission device of FIG. 3 obtains intensity-modulated light using an external modulator by a duobinary method. In FIG. 3, single mode light 101 emitted from a semiconductor laser 100 is intensity-modulated by a push-pull type light intensity modulator 102. On the other hand, the NRZI signal 105 as shown in FIG. 4A is band-limited by the low-pass filter 106 and ternarized. Thereby, the ternary signal shown in FIG. 4B is generated. The ternary signal is differentially amplified by the differential driver 107, and a negative phase signal 113 and an in-phase signal 114 are generated.

  The anti-phase signal 113 and the in-phase signal 114 are taken into the two optical phase modulation units of the optical intensity modulator 102. By adjusting the DC bias level of the light intensity modulator 102, the light intensity of the light intensity modulator 102 is minimized when the anti-phase signal 113 and the in-phase signal 114 are at an intermediate level, and the anti-phase signal 113 and the in-phase signal 113 When the signal 114 is at the maximum / minimum level, the light intensity of the light intensity modulator 102 can be maximized.

  The configuration of FIG. 3 generates a binary signal identifiable by an existing optical receiver as shown in FIG. 4C, even though the light intensity modulator 102 is driven by a ternary signal. Is done. In addition, since the band of the ternary signal is suppressed by the low-pass filter 106, the band of the modulator drive signal (the anti-phase signal 113 and the in-phase signal 114) is narrowed, and the spectrum width of the optical signal can be made compact. Is possible.

  However, since a general light intensity modulator using LiNbO3 requires a high output level in the driver circuit, it is difficult to reduce the cost when the bit rate of the transmission signal is increased.

  On the other hand, according to this embodiment, the NRZI signal 3 is band-limited by the low-pass filter 5, and the ternary signal 4 is generated based on the same principle as the duobinary method. Then, the differential driver 6 generates a reverse-phase signal 7 that is opposite in phase to the ternary signal 4 and an in-phase signal 8 that is in-phase with the ternary signal 4. Up to this point, it is the same as the existing optical transmission device (FIG. 3). However, in this embodiment, the semiconductor lasers 1 and 2 that emit light at different frequencies f2 and f4 are provided, the semiconductor laser 1 is driven to emit light only at the maximum level of the reverse phase signal 7, and the semiconductor only at the maximum level of the in-phase signal 8. The laser 2 is driven to emit light. As a result, the intensity of the semiconductor lasers 1 and 2 can be modulated in a band lower than the bit rate of the NRZI signal 3.

  Further, by combining the output lights of the semiconductor lasers 1 and 2 by the optical coupler 11 and sending them to the optical fiber, although the wavelength lights f2 and f4 are mixed, the intensity modulation having the same waveform as in FIG. Intensity-modulated light with a compact band can be obtained without losing light, that is, the amount of information of the NRZI signal 3.

  That is, according to the present embodiment, the intensity-modulated light having a compact band can be generated by directly modulating the semiconductor lasers 1 and 2. Therefore, the light intensity modulator can be dispensed with. In addition, since the drive signal levels of the semiconductor lasers 1 and 2 can be much lower than the driver signal level for driving the light intensity modulator, the drive power can be reduced, and the cost can be reduced. Furthermore, since the operation of converting the frequency modulation into the intensity modulation using the narrow band optical filter is not required, there is no problem such as deterioration of the extinction ratio of the optical signal due to the group delay characteristic of the narrow band optical filter. For these reasons, it becomes possible to provide an optical transmission device capable of reducing the band of the transmission optical signal at a low cost without fear of deterioration of the extinction ratio.

[Second Embodiment]
FIG. 5 is a functional block diagram showing a second embodiment of the optical transmission apparatus according to the present invention. In FIG. 5, parts common to FIG. 1 are given the same reference numerals, and only different parts will be described here.

  FIG. 5 is characterized in that a variable delay device 20 is provided between the slicer 9 and the semiconductor laser 1. That is, the drive signal output from the slicer 9 is supplied to the semiconductor laser 1 after being delayed by a predetermined amount by the variable delay device 20. The variable delay device 20 is realized as a variable delay line, for example, and has a delay amount set in accordance with the output optical frequency difference between the semiconductor lasers 1 and 2 (here, f1 to f2). The drive signal to be given a phase difference.

  In this embodiment, since the oscillation center frequencies of the two semiconductor lasers 1 and 2 are different (f1, f2), when the output lights of the semiconductor lasers 1 and 2 are combined and sent to the optical fiber, they are sent to the receiving end of each output light. The arrival time differs depending on the chromatic dispersion characteristics of the optical fiber. Therefore, the waveform of the electrical signal output from the photodetector is greatly disturbed because the two optical signals are added with their timings shifted.

  Therefore, in this embodiment, the variable delay device 20 is inserted into the drive signal supply path of the semiconductor laser on one side, and according to the fiber length from the optical transmitter to the receiving end and the difference in oscillation frequency between the two semiconductor lasers 1 and 2. The delay length is variably adjusted. This makes it possible to match the arrival timing of the optical signal at the receiving end. This is particularly effective when the waveform change due to the fiber chromatic dispersion characteristic is small.

  In a situation where the transmission distance to the receiving end (fiber length) is long and waveform deterioration due to chromatic dispersion cannot be ignored, a dispersion compensator, a dispersion compensating fiber, or the like may be inserted in the transmission path. In this case, the variable delay device 20 is not necessary in principle, but in reality, there is a possibility that the timing is shifted between the two optical signals due to variations in the response speeds of the semiconductor lasers 1 and 2. In order to cope with this, the timing between the two optical signals may be adjusted by appropriately setting the line length of the variable delay device 20.

  In summary, since the semiconductor lasers 1 and 2 are directly modulated by the differential binary signal, if the output light intensity of one semiconductor laser is at 1 level, the other is always at 0 level. However, when fiber transmission is performed, a time difference between the two semiconductor laser output optical signals reaching the optical receiver occurs due to the chromatic dispersion characteristics of the fiber, and intersymbol interference occurs in the optical receiver output. Therefore, in this embodiment, the variable delay device 20 is provided in the drive circuit unit on one side of the optical transmission device so as to compensate for the time difference caused by the transmission length of the fiber and the oscillation frequency difference of the semiconductor lasers 1 and 2. Thereby, in addition to obtaining the same effect as that of the first embodiment, it is possible to prevent the deterioration of the transmission waveform even when the transmission path length is long. Further, when the waveform disturbance due to fiber dispersion is not large, it is not necessary to provide a dispersion compensating fiber in the transmission line, and it is possible to promote cost reduction of the system.

[Third Embodiment]
FIG. 6 is a functional block diagram showing a third embodiment of the optical transmission apparatus according to the present invention. In FIG. 6, parts common to FIG. 1 are given the same reference numerals, and only different parts will be described here.

  In the third embodiment, a mode capable of improving the use efficiency of the optical band by making the occupied optical band of the optical signal appropriate is disclosed. In FIG. 6, the optical transmitters shown in FIG. 1 or FIG. 5 are prepared for a plurality of channels (for two channels in FIG. 6), and the intensity-modulated light output from the optical transmitter of each channel is multiplexed by the optical coupler 11. And send it to the optical fiber. At the receiving end, the multiplexed light is separated by the optical filter 35, and each separated light is received by the optical receivers 36-1 and 36-2 provided for each channel. The output optical frequencies of the semiconductor lasers 31, 32, 33, and 34 of the optical transmitter are assumed to be f1, f3, f2, and f4, respectively.

  FIG. 7 is a schematic diagram showing the arrangement of the output optical frequencies f1, f3, f2, and f4 of the semiconductor lasers 31, 32, 33, and 34 in FIG. In FIG. 7, the frequencies f1 and f3 (without hatching) of the semiconductor lasers 31 and 32 corresponding to the channel 1 are alternately arranged with the frequencies f2 and f4 (with hatching) of the semiconductor lasers 33 and 34 corresponding to the channel 2. The That is, the two intensity-modulated light spectra (f1, f3) from one optical transmission device are arranged alternately with the two intensity-modulated light spectra (f2, f4) from the other optical transmission device.

  On the other hand, as the optical filter 35 at the receiving end, a device such as a Mach-Zehnder interferometer or an optical interleaver whose transmittance with respect to the optical frequency changes periodically is used. In this way, the optical signals that are multiplexed alternately can be separated for each channel, and it is possible to suppress the inter-channel crosstalk and achieve good reception characteristics.

  In FIG. 6, the number of channels is 2, but this embodiment is effective even when the number of channels is increased. Further, as the optical filter 35 at the receiving end, an optical filter having a different cycle of transmittance variation is connected, and only the two optical spectra of the desired channel are guided to the photodetectors in the receivers 36-1 and 36-2. This makes it possible to further increase the wavelength multiplexing density in the optical fiber.

  In summary, in this embodiment, two signal components having different wavelengths are transmitted from the two semiconductor lasers provided in the optical transmission apparatuses of the respective channels, thereby widening the frequency interval of the signal spectrum. Therefore, wavelength multiplex transmission with improved frequency utilization efficiency can be realized by alternately arranging signal spectra of optical signals of adjacent channels.

[Fourth Embodiment]
FIG. 8 is a functional block diagram showing a third embodiment of the optical transmission apparatus according to the present invention. In FIG. 8, parts common to FIG. 1 are given the same reference numerals, and only different parts will be described here.

  In FIG. 8, the negative phase signal 7 and the positive phase signal 8 output from the differential driver 6 are directly input to the semiconductor lasers 1 and 2, respectively. On the other hand, DC bias sources 43 and 44 are connected to the semiconductor lasers 1 and 2, respectively. The DC bias source 43 applies a bias current to the semiconductor laser 1 in order to make the intermediate level of the reverse phase signal 7 coincide with the emission threshold value of the semiconductor laser 1. The DC bias source 44 applies a bias current to the semiconductor laser 2 in order to make the intermediate level of the positive phase signal 8 coincide with the emission threshold value of the semiconductor laser 2.

  In the above configuration, the threshold value of the two semiconductor lasers 1 and 2 is used to convert the ternary signal 42 output from the low-pass filter 5 into a binary signal. Thereby, the slicers 9 and 10 and the reference voltage sources 13 and 14 can be omitted.

  FIG. 9 is a diagram showing the relationship between the current-output light intensity characteristics of the semiconductor lasers 1 and 2 of FIG. 8 and the DC bias level. As shown in FIG. 9, by setting the DC bias level so that the central level of the ternary signal 4 matches the emission threshold of the semiconductor laser, the highest level (or the lowest level) of the ternary signal is obtained. Only the semiconductor laser can be driven to emit light.

  As is well known, the injection current of a semiconductor laser has a threshold characteristic. That is, the semiconductor laser emits weak light in the LED mode in the driving current range below the threshold, and the semiconductor laser oscillates in the driving current range above the threshold. Therefore, in this embodiment, the ternary signal 4 is such that the ternary signal 4 exceeds the threshold value of the semiconductor lasers 1 and 2 at the +1 level, and the ternary signal 4 is sufficiently smaller than the threshold value at other levels. The amplitude of 4 and the DC bias level of the injection current of the semiconductor lasers 1 and 2 are set.

  When the semiconductor laser is driven as shown in the present embodiment, high-order harmonic components may be generated in the optical signal spectrum due to nonlinearity of the semiconductor laser light intensity with respect to the injected current. Since such harmonic components are unnecessary for signal transmission, an optical filter is installed at the receiving end to remove the harmonic components, thereby suppressing the influence on the reception characteristics due to beats between the harmonic components. Can do.

[Fifth Embodiment]
FIG. 10 is a functional block diagram showing a fifth embodiment of the optical transmission apparatus according to the present invention. In FIG. 10, the same reference numerals are given to the portions common to FIG. 1, and only the different portions will be described here.

  The present invention can also be applied to the transmission of multi-level signals, and FIG. 10 shows an implementation example thereof. In FIG. 10, the quinary signal 51 is broken down into four binary signals 52-1 to 52-4 by signal processing. That is, the n-value signal can be decomposed into (n-1) binary signals.

  The plurality of semiconductor lasers 53-1 to 53-4 have different output optical frequencies, and are directly modulated by the binarized signals 52-1 to 52-4, respectively, and output intensity-modulated light. Each intensity-modulated light is multiplexed by the optical coupler 11 and sent to the optical fiber. The oscillation frequency between the semiconductor lasers 53-1 to 53-4 is set so that the difference between the oscillation frequencies is sufficiently larger than the band of the photodetector 54. The intensity-modulated light reaching the receiving end via the optical fiber is photoelectrically converted by the photodetector 54, and the multilevel signal 56 is received and reproduced.

  FIG. 11 is a functional block diagram showing the configuration of the signal processing unit 55 in FIG. In FIG. 5, the quinary signal 51 is branched into four and input to the slicers 55-1 to 55-4, respectively. The slicers 55-1 to 55-4 are connected to reference voltage sources that generate different reference voltages, respectively, and generate and output binary drive signals having peaks only when the quinary signal 51 is at a specific level. Each drive signal is supplied to the semiconductor lasers 53-1 to 53-4, and the semiconductor lasers 53-1 to 53-4 are turned on / off to generate intensity-modulated light.

  FIG. 12 is a diagram showing the relationship between the drive signal applied to each of the semiconductor lasers 53-1 to 53-4 in FIG. The quinary signal 51 takes one of a first level (highest level) to a fifth level (lowest level) as time passes.

  The slicer 55-1 generates the drive signal 52-1 that becomes the +1 level only when the quinary signal 51 takes the first level, and applies the drive signal 52-1 to the semiconductor laser 53-1. The slicer 55-2 generates a drive signal 52-2 that becomes +1 level only when the quinary signal 51 takes a level equal to or higher than the second level, and applies this drive signal 52-2 to the semiconductor laser 53-2. To do. The slicer 55-3 generates a drive signal 52-3 that becomes +1 level only when the quinary signal 51 takes a level equal to or higher than the third level, and applies this drive signal 52-3 to the semiconductor laser 53-3. To do. The slicer 55-4 generates a drive signal 52-4 that becomes +1 level only when the quinary signal 51 takes a level equal to or higher than the fourth level, and applies this drive signal 52-4 to the semiconductor laser 53-4. To do.

  For example, when the quinary signal 51 is at the first level, all of the drive signals 52-1 to 52-4 become +1, and all the semiconductor lasers 53-1 to 53-4 are turned on. Conversely, when the quinary signal 51 is at the fourth level, only the drive signal 52-4 becomes +1 and only the semiconductor laser 53-4 is turned on. By generating such drive signals, intensity-modulated light that represents all states of the quinary signal 51 can be generated by the four semiconductor lasers 53-1 to 53-4.

  More generally, in the present embodiment, an n-value signal having levels 1 to n (n is a natural number of 3 or more) is generated from digital data to be transmitted, and the n-value signal is used to generate the n-value signal. When the value signal is equal to or higher than the mth (1 ≦ m ≦ n−1) level, an mth (1 ≦ m ≦ n−1) driving signal having a +1 level is generated. These first to (n-1) th drive signals are individually applied to the first to (n-1) th semiconductor lasers having different output optical frequencies, and the output light of each semiconductor laser is multiplexed. Then, it is sent out to the optical transmission line.

  Thus, as the number of multilevel signals generated from transmission data increases, the number of semiconductor lasers increases, contrary to the increase in the number of semiconductor lasers, one third or four minutes of the bit rate frequency of the data signal. Thus, the modulation band of the semiconductor laser can be reduced. Therefore, it is possible to realize ultra high density optical transmission at a lower cost. Incidentally, according to this embodiment, the modulation band of each of the semiconductor lasers 53-1 to 53-4 can be reduced to one fifth of the bit rate frequency of the data signal.

  Note that a timing shift between optical signals caused by the chromatic dispersion characteristics of the fiber can be corrected by providing a variable delay device 20 as shown in FIG.

  The present invention is not limited to the above embodiment. For example, instead of the semiconductor lasers 1 and 2, a semiconductor laser module in which a semiconductor laser and an electroabsorption optical modulator (EA modulator) are integrated may be used. In this case, the EA modulator may be driven by the outputs of the slicers 9 and 10.

  In each of the above embodiments, the drive signals of the semiconductor lasers 1 and 2 are generated by using the slicers 9 and 10 and comparing with the reference value. Alternatively, a logic circuit can be used. That is, it is preferable to use a logic circuit that returns one level only when the level of the ternary signal exceeds the voltage value of the reference voltage source. Alternatively, a flip-flop (FF) circuit or the like can be used. When the FF circuit is used, the FF output returns level 1 only when the level of the ternary signal is (+1), and level 0 is returned for other levels (0, −1). It is preferable to set the threshold level of FF to.

  Furthermore, the present invention can be embodied by modifying the constituent elements without departing from the spirit of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is a functional block diagram showing a first embodiment of an optical transmission apparatus according to the present invention. FIG. 2 is a schematic diagram showing a relationship between an output light frequency f2 of the semiconductor laser 1 of FIG. 1 and an output light frequency f4 of the semiconductor laser 2. The functional block diagram which shows the structure of the existing optical transmitter for a comparison. The schematic diagram which shows each waveform of the NRZI signal concerning a duobinary system, a ternary signal, and intensity modulation output light. The functional block diagram which shows 2nd Embodiment of the optical transmitter concerning this invention. The functional block diagram which shows 3rd Embodiment of the optical transmitter concerning this invention. FIG. 7 is a schematic diagram showing an arrangement of output optical frequencies f1, f3, f2, and f4 of the semiconductor lasers 31, 32, 33, and 34 in FIG. The functional block diagram which shows 4th Embodiment of the optical transmitter concerning this invention. The figure which shows the relationship between the electric current-output light intensity characteristic of the semiconductor lasers 1 and 2 of FIG. The functional block diagram which shows 5th Embodiment of the optical transmitter concerning this invention. The functional block diagram which shows the structure of the signal processing part 55 of FIG. FIG. 12 is a diagram showing a relationship between drive signals and quinary signals 51 applied to the semiconductor lasers 53-1 to 53-4 in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 2 ... Semiconductor laser, 3 ... NRZI signal, 4 ... Tri-valued signal, 5 ... Low pass filter, 6 ... Differential driver, 7 ... Reverse phase signal, 8 ... In-phase signal, 9, 10 ... Slicer, 11 ... Optical coupler, 12 ... optical receiver, 13, 14 ... reference voltage source, 20 ... variable delay device, 31-34 ... semiconductor laser, 35 ... optical filter, 36-1, 36-2 ... optical receiver, 43, 44 ... DC bias source, 53-1 to 53-4 ... Semiconductor laser, 54 ... Photo detector, 55 ... Signal processing unit, 55-1 to 55-4 ... Slicer

Claims (7)

Multi-value signal generating means for generating n-value signals having first to n-th (n is a natural number of 3 or more) levels from digital data to be transmitted;
First to (n-1) light-emitting elements that are driven to blink by binary drive signals given individually and have different output optical frequencies;
A binary signal having a first value and a second value, and the n-th signal takes the first value when the n-value signal is at a level equal to or higher than the m-th (1 ≦ m ≦ n−1) level. m ≦ n−1) driving signal generating means, and driving signal generating means for individually supplying the first to (n−1) th driving signals to the first to (n−1) light emitting elements,
An optical transmitter comprising: a multiplexer that multiplexes the output lights of the first to (n-1) th light emitting elements and sends them to an optical transmission line. Ternary signal generating means for generating a ternary signal having first to third levels in order from a high level by limiting a band of a transmission signal expressed by an NRZI (Non Return to Zero Inverted) code;
First and second semiconductor lasers having different output optical frequencies;
Drive means for driving the first semiconductor laser to emit light when the first level appears, and for driving the second semiconductor laser to emit light when the third level appears;
An optical transmitter comprising: a multiplexer that multiplexes the output lights of the first and second semiconductor lasers and sends them to an optical transmission line. When an optical receiver that receives light transmitted from the multiplexer is connected to the optical transmission path,
The optical transmission device according to claim 2, wherein an output optical frequency difference between the first and second semiconductor lasers is larger than a reception band of the optical reception device. The first and second semiconductor lasers are each driven to blink by a drive signal given individually,
The driving means includes
A differential driver that generates a reverse-phase signal and an in-phase signal of the ternary signal;
First slice means for slicing the negative phase signal based on a reference value to generate a binary first drive signal that exhibits a peak when the third level appears, and for supplying the first drive signal to the first semiconductor laser When,
Second slicing means for slicing the in-phase signal based on a reference value to generate a binary second drive signal that exhibits a peak when the first level appears, and for supplying the second drive signal to the second semiconductor laser The optical transmission device according to claim 2, further comprising: The first and second semiconductor lasers are each driven to blink by a drive signal given individually,
The driving means includes
A first drive signal having a phase opposite to that of the ternary signal and a second drive signal having the same phase as the ternary signal are generated, the first drive signal is provided to the first semiconductor laser, and the second drive signal is generated. A differential driver for supplying to the second semiconductor laser;
A first bias source for applying a bias signal to the first semiconductor laser so as to make an intermediate level of the first drive signal coincide with an emission threshold value of the first semiconductor laser;
3. A second bias source for applying a bias signal to the second semiconductor laser so as to make an intermediate level of the second drive signal coincide with an emission threshold value of the second semiconductor laser. Optical transmitter. 6. The method according to claim 4, further comprising phase control means for varying a phase difference between the first and second drive signals in accordance with a difference between output optical frequencies of the first and second semiconductor lasers. An optical transmitter according to claim 1. A plurality of optical transmitters for outputting first and second modulated lights having different frequencies from each other;
A multiplexer that multiplexes the first and second modulated lights output from all the optical transmitters and sends them to an optical transmission line;
Each of the plurality of optical transmitters includes:
Ternary signal generating means for generating a ternary signal having first to third levels in order from a high level by limiting a band of a transmission signal expressed by an NRZI (Non Return to Zero Inverted) code;
A first semiconductor laser that outputs the first modulated light;
A first semiconductor laser that outputs the second modulated light;
Driving means for driving the first semiconductor laser to emit light when the first level appears, and driving the second semiconductor laser to emit light when the third level appears,
The frequency of the said 1st and 2nd modulated light differs across all the said optical transmission parts, The optical transmitter characterized by the above-mentioned.

Images