Optical transmitter

An optical transmitter for converting an input data series into an optical multi-level signal and for outputting the same, includes an LUT in which data for executing optical multi-level modulation is stored and from which first modulation data and second modulation data are output based on the input data series. A DAC converts the first modulation data by D/A conversion to generate a first multi-level signal. A DAC converts the second modulation data by D/A conversion to generate a second multi-level signal. A dual-electrode MZ modulator includes a first phase modulator for modulating light from a light source in accordance with the first multi-level signal and a second phase modulator for modulating light from the light source in accordance with the second multi-level signal, and combines an optical signal from the first phase modulator and an optical signal from the second phase modulator to output the optical multi-level signal.

TECHNICAL FIELD

The present invention relates to an optical transmitter for converting an electrical signal into an optical signal and transmitting the optical signal.

BACKGROUND ART

In order to realize a large capacity of a WDM optical communication system, it is useful to increase a transmission rate per wavelength. When a symbol rate of symbols transmitted to an optical transmission line is increased without changing a modulation method, there is a problem in that wavelength dispersion tolerance of the optical transmission line reduces because an allowable residual dispersion amount is inversely proportional to the square of the symbol rate. It is necessary to execute electrical signal processing at a high rate, and hence there is a problem in that the cost of an analog electrical part increases.

Therefore, in recent years, researches for improving signal multiplicity per symbol without increasing the symbol rate so as to realize the large capacity of the system have been actively conducted.

Known examples of a method of improving the signal multiplicity include multi-level modulation methods such as a QPSK method of assigning two values (multiplicity is two) to each symbol to increase a transmission capacity two times, a 16-QAM method of assigning four values (multiplicity is four) to each symbol to increase the transmission capacity four times, and a 16-APSK method.

In general, when any of the multi-level modulation methods is executed, an I/Q modulator is used as an optical modulator. The I/Q modulator is modulator capable of independently generating orthogonal optical electric field components (I channel and Q channel) and has a special structure in which Mach-Zehnder (MZ) modulators are connected in parallel.

For example, when the QPSK modulation method is to be executed, a dual parallel MZ modulator (DPMZM) is used in which two MZ modulators are connected in parallel (see, for example, Patent Literature 1).

When the modulation by 16-QAM is executed, the DPMZM or a quad parallel MZ modulator (QPMZM) in which two DPMZMs are connected in parallel is used (see, for example, Non-Patent Literature 1).

Even when any of the modulators as described in Patent Literature 1 and Non-Patent Literature 1 is used, the number of MZ modulators increases, and hence there is a problem in that a cost and the number of bias control points increase.

Therefore, it is expected to use a dual-electrode MZ modulator (dual drive MZM (DDMZM)) in which two phase modulators are connected in parallel, so as to realize the multi-level modulation (see, for example, Patent Literature 2 and Non-patent Literature 2).

The dual-electrode MZ modulator is an optical part widely applied as a push-pull optical modulator to a normal optical transmitter-receiver, and hence a reduction in cost may be realized. In addition, a light insertion loss may be reduced because of the structure in which light passes through the MZ modulator only once.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, prior art has the following problem.

Conventional optical transmitters need to execute multi-level modulation processing for an input data series which is input at a high rate, and consequently have a problem in that the processing load is heavy.

The present invention has been made to solve the problem described above, and an object of the present invention is to provide an optical transmitter capable of lessening the processing load of multi-level modulation processing.

Solution to Problem

According to the present invention, an optical transmitter for converting an input data series which is an input electrical signal into an optical multi-level signal and for outputting the optical multi-level signal, includes: a look-up table in which data for executing optical multi-level modulation is stored and from which first modulation data and second modulation data are output based on the input data series; a first D/A converter which converts the first modulation data by D/A conversion to generate a first multi-level signal; a second D/A converter which converts the second modulation data by D/A conversion to generate a second multi-level signal; and a dual-electrode MZ modulator which includes a first phase modulator for modulating light from a light source in accordance with the first multi-level signal and a second phase modulator for modulating light from the light source in accordance with the second multi-level signal, and which combines an optical signal from the first phase modulator and an optical signal from the second phase modulator to output the optical multi-level signal.

Advantageous Effects of Invention

The optical transmitter according to the present invention includes a look-up table in which data for executing optical multi-level modulation is stored and from which first modulation data and second modulation data are output based on an input data series. This way, the first modulation data and the second modulation data are each output on a one-on-one basis with respect to the input data series.

The processing load of multi-level modulation processing can thus be lessened.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment of the present invention is described with reference to the attached drawings. In the respective drawings, the same or corresponding components are expressed for description by the same reference symbols.

The following embodiment has a configuration that includes an S/P conversion unit1and a P/S conversion unit4. However, the S/P conversion unit1and the P/S conversion unit4are unnecessary in the case where processing of an LUT2is executed at the same rate as that of an input data series.

First Embodiment

FIG. 1is a block configuration diagram illustrating an optical transmitter according to a first embodiment of the present invention.

InFIG. 1, the optical transmitter includes the S/P (serial/parallel) conversion unit1, the look-up table2, a look-up table control unit3, the P/S (parallel/serial) conversion unit4, two D/A converters,5aand5b(a first D/A converter and a second D/A converter), two electrical amplifiers,6aand6b, a light source7, and a dual-electrode MZ modulator8.

The dual-electrode MZ modulator8is constituted of two phase modulators,8aand8b.

In the following description, the look-up table2is referred to as LUT2. The D/A converters (digital/analog converters)5aand5bare referred to as DACs5aand5b.

Respective functions of the components of the optical transmitter are described below.

The S/P conversion unit1develops in parallel an input data series that has been input to the optical transmitter, in keeping with the processing rate of the LUT2. The LUT2has as many tables as the number of parallel streams developed by the S/P conversion unit1, and outputs set values of the DACs5aand5b(first modulation data and second modulation data) that are associated with the input data series developed in parallel. The LUT2stores data for accomplishing optical multi-level modulation, and the stored data can be reset to new data. The LUT control unit3(look-up table control means) changes stored data of the LUT2as the need arises. The processing of changing stored data which is executed by the LUT control unit3is described later.

The P/S conversion unit4performs P/S conversion on an output from the LUT2to generate set values of the DACs5aand5bthat have a data rate suited to the output data update rate of the DACs5aand5b, and outputs the set values to the DACs5aand5b, respectively. The DACs5aand5bperform D/A conversion on the set values output from the P/S conversion unit4to respectively generate a first multi-level signal and a second multi-level signal. The electrical amplifiers6aand6bamplify the first and second multi-level signals from the DACs5aand5bto a voltage amplitude necessary for optical modulation, thereby respectively generating a first drive electrical signal V1(t) and a second drive electrical signal V2(t).

The dual-electrode MZ modulator8drives the phase modulators8aand8bwith the first and second drive electrical signals V1(t) and V2(t), which are from the electrical amplifiers6aand6b, to modulate continuous-wave (CW) light from the light source7, and outputs a modulated optical signal.

Data for accomplishing optical multi-level modulation is stored as stored data of the LUT2, and hence an optical multi-level signal is obtained as the modulated optical signal output from the dual-electrode MZ modulator8.

In order to output set values of the DACs5aand5bthat are suited to the signal multiplicity (the number of signals mapped onto one symbol) of the optical multi-level signal, the table configuration of the LUT2has an address size equal to or larger than the signal multiplicity of the optical multi-level signal, and a bit width equal to or larger than the quantifying bit number of the DACs5aand5b. This table configuration allows desired set values of the DACs5aand5bto be read by referring to data with a bit string of an input data series as the address.

Optical multi-level modulation is described below taking as an example a case in which modulation by 16-QAM is executed with the use of the dual-electrode MZ modulator8(DDMZM).

In modulation by 16-QAM, one symbol of an optical signal is assigned 4-bit data. Through this operation of turning a signal into a multi-level signal, an optical signal that has a symbol rate of, for example, 10 Gsymbol/s can be used to transfer information of 40 Gb/s. An example of the relation between a 4-bit input data set and the arrangement of signal constellation points on a complex plane is illustrated inFIG. 2.

A transfer function of the dual-electrode MZ modulator8is expressed by the following Expression (1) based on the display format of polar coordinates. In Expression (1), Vnrepresents the half-wave voltage of the dual-electrode MZ modulator8.

The first and second drive electrical signals V1(t) and V2(t) that are necessary to generate the signal constellation points ofFIG. 2are expressed by the following Expression (2) through inverse transform of Expression (1).

The DACs5aand5bare used to actually output voltages necessary for the generation of the signal constellation points expressed by Expression (2). The output data update rate of the DACs5aand5bis commonly set to a rate twice the transmission symbol rate or higher.

FIG. 3illustrates the relation between the resolution (bit number) of the DACs5aand5band a penalty, which is expressed by the average value of the amounts of deviation from an optimum signal constellation point.

InFIG. 3, white circles represent calculation results with respect to the dual-electrode MZ modulator8(DDMZM) and black circles represent calculation results with respect to the dual parallel MZ modulator (DPMZM) described above.

It is understood fromFIG. 3that the DACs5aand5bthat have a resolution of 6 bits or higher are suitable for the use of the dual-electrode MZ modulator8(DDMZM), when taking into consideration a range where the penalty is negligible.

FIG. 4illustrates an example of the first and second drive electrical signals V1(t) and V2(t) that are necessary to realize the signal constellation point arrangement ofFIG. 2and stored data of the LUT2that is stored when the DACs5ato5bhave a resolution of 6 bits.

InFIG. 4, the values of the first and second drive electrical signals V1(t) and V2(t) are the values of voltages applied to the dual-electrode MZ modulator8that are expressed with the half-wave voltage of the dual-electrode MZ modulator8as a reference.

The LUT2uses a 4-bit data set as an address input and uses, as an output, 6-bit stored data that is associated with the address, thereby obtaining 6-bit data to be set to the DACs5aand5b.

The 4-bit data set is also associated with the signal constellation points ofFIG. 2. Therefore, by driving the dual-electrode MZ modulator8with outputs from the DACs5aand5bwhich correspond to 6-bit data obtained as outputs of the LUT2, signal constellation points suited to modulation by 16-QAM are realized as an optical multi-level signal.

When the output data update rate of the DACs5aand5bis set to a rate n times the symbol rate of the optical multi-level signal in this example, optimum data can be set for each scheduled update of output data of the DACs5aand5C by preparing n or more sets of stored data of the LUT2illustrated inFIG. 4.

The first and second drive electrical signals V1(t) and V2(t) and stored data of the LUT2are not limited to the values ofFIG. 4, and can take other values as long as the same signal constellation point arrangement is realized with that combination of values.FIG. 5illustrates another example of the first and second drive electrical signals V1(t) and V2(t) and stored data of the LUT2that is stored when the DACs5aand5bhave a resolution of 6 bits.

FIG. 6illustrates the relation between a drive amplitude fluctuation (amplitude deviation) and a penalty when the first and second drive electrical signals V1(t) and V2(t) that conform to the table settings ofFIG. 4are used.

InFIG. 6, white circles represent calculation results with respect to the dual-electrode MZ modulator8(DDMZM) and black circles represent calculation results with respect to the DPMZM.

It is understood fromFIG. 6that the tolerance for a deviation from optimum settings of the drive electrical signals is smaller when the dual-electrode MZ modulator8(DDMZM) is used than when the DPMZM is used.

FIG. 7illustrates the relation between a 4-bit input data set and the arrangement of signal constellation points on a complex plane when the drive amplitude of the drive electrical signals is reduced from an optimum value (to 0.9 times).FIG. 8illustrates the relation between a 4-bit input data set and the arrangement of signal constellation points on a complex plane when the drive amplitude of the drive electrical signals is increased from the optimum value (to 1.1 times).

InFIGS. 7 and 8, white circles represent a signal constellation point arrangement that corresponds to the optimum drive amplitude, and black circles represent a signal constellation point arrangement that is observed when the drive amplitude deviates from the optimum value.

It is understood fromFIGS. 7 and 8that a deviation of the drive amplitude greatly affects the signal quality.

When the optical transmitter is actually driven, an optimum drive amplitude of drive electrical signals to be applied to the dual-electrode MZ modulator8does not match stored data of the LUT2in some cases even when the stored data of the LUT2illustrated inFIG. 4is optimum settings. Possible factors for this include the incompleteness of outputs of the DACs5aand5b, individual differences in gain and in the amplitude of output signals between the electrical amplifiers6aand6b, and individual differences in the half-wave voltage of the dual-electrode MZ modulator8.

The deterioration of signal quality can therefore be reduced by measuring in advance a deviation of the drive voltage due to the characteristics of the DACs5aand5b, the characteristics of the electrical amplifiers6aand6b, the necessary drive voltages of the phase modulators8aand8b, which constitute the dual-electrode MZ modulator8, and other factors, and correcting stored data of the LUT2in a manner that reduces the measured deviation of the drive voltage.

Another method of reducing the deterioration of signal quality is to keep the amplitude of the drive electrical signals optimum by changing stored data of the LUT2as the need arises with the use of the LUT control unit3. In this case, a deviation of the drive voltage due to the characteristics of the DACs5aand5b, the characteristics of the electrical amplifiers6aand6b, the necessary drive voltages of the phase modulators8aand8b, which constitute the dual-electrode MZ modulator8, and other factors is detected as needed and stored data of the LUT2is changed in accordance with the amount of deviation.

In this manner, stored data of the LUT2can be set for each individual optical transmitter to suit the performance of the components after the optical transmitter is built, and the deterioration of signal quality is prevented.

Methods that can be used to optimize stored data of the LUT2include one in which an optical multi-level signal output from the dual-electrode MZ modulator8is observed and an adjustment is made to obtain optimum signal constellation points, one in which multiplication by reverse characteristics of a series obtained from a response to a known training pattern is performed, and one in which the error rate of received data in an opposed optical receiver is minimized.

The observation result of an optical multi-level signal that is usually used is a signal series expressed by a Cartesian coordinate system of an I-channel component (real component) and a Q-channel component (imaginary component) as the arrangement of signal constellation points on a complex plane. When the dual-electrode MZ modulator8is used, because the drive electrical signals and the optical multi-level signal have the relation expressed by the above Expression (1), a deviation from an ideal state of the I-channel component and the Q-channel component of the optical multi-level signal does not correspond to the drive electrical signals on a one-on-one basis, unlike when the DPMZM is used.

Therefore, in the case where the reverse characteristics described above are extracted in a system that uses the dual-electrode MZ modulator8, arithmetic operation performed needs to take into account the transfer functions of the dual-electrode MZ modulator8which are expressed by the above Expressions (1) and (2).

FIG. 9is an explanatory diagram illustrating an example of how stored data of the LUT2is set when the drive amplitude of the drive electrical signals is reduced from an optimum value.

The premise ofFIG. 9is that the DACs5aand5bhave a resolution of 7 bits and that the LUT2in an initial state stores data ofFIG. 4which is equivalent to a resolution of 6 bits.

When the voltage amplitude in the initial state is 0.9 times the optimum value and a compensation for turning the voltage amplitude to 1.1 times the optimum value (=an increase by 1/0.9 times) is to be executed, the drive electrical signals are easily compensated by changing the stored data of the LUT2to values in the two right-hand side columns ofFIG. 9. Modulation by 16-QAM can thus be executed under optimum drive conditions.

As illustrated inFIG. 9, the bit width of stored data of the LUT2and the bit width of the DACs5aand5bdesirably have a size obtained by adding a compensation bit to a bit number necessary for a modulation method to be implemented. For instance, when a bit width necessary to execute modulation by 16-QAM is estimated as 6 bits based on the penalty ofFIG. 3, the stored data of the LUT2is set to 7 bits, and by using the DACs5aand5bthat have a resolution of 7 bits, the compensation processing ofFIG. 9can be executed.

The description given next is about a memory capacity that the LUT2has when, for example, information of 40 Gb/s is transferred through modulation by 16-QAM.

Here, the symbol rate of an optical multi-level signal to be generated is 10 Gsymbol/s, the resolution of the DACs5aand5bis 7 bits, the data width of stored data of the LUT2is 7 bits, the rate of scheduled update of outputs of the DACs5aand5bis twice the symbol rate of the optical multi-level signal (double oversampling), and an internal operation of the S/P conversion unit1is 64-bit parallel computing.

The LUT2in this case needs to have only a memory capacity of 14 Kbits (=64×16×7×2) for stored data of the first drive electrical signal V1(t) and stored data of the second drive electrical signal V2(t) each.

The optical transmitter according to the first embodiment of the present invention includes a look-up table in which data for executing optical multi-level modulation is stored and from which first modulation data and second modulation data are output based on an input data series. This way, the first modulation data and the second modulation data are each output on a one-on-one basis with respect to an input data series.

The processing load of multi-level modulation processing can thus be lessened.

This optical modulator also includes look-up table control means which changes stored data of the look-up table as the need arises.

Stored data of the look-up table can therefore be set for each individual optical transmitter to suit the performance of the components after the optical transmitter is built, and the deterioration of signal quality is prevented.

The first embodiment takes as an example a compensation of the drive amplitude of the dual-electrode MZ modulator8inFIG. 8. However, the present invention where data setting using the LUT2is executed can handle linear compensation and non-linear compensation both.

The first embodiment describes a case in which the additional bit is 1 bit inFIG. 8. However, the present invention is not limited thereto and a bit width suited to what compensation is to be made may be added arbitrarily.

The first embodiment describes data compensation executed at the time when signal constellation points are realized. However, common linear compensation and common non-linear compensation which use n-times oversampling data can be executed by having n table values associated with scheduled updates of outputs of the DACs5aand5bfor the same signal constellation point.

The first embodiment takes into account only the characteristics of the components of the optical transmitter in a compensation of stored data of the LUT2, but the present invention is not limited thereto. When stored data of the LUT2is compensated by additionally taking into account the characteristics such as (signal error rate) of an optical receiver which is opposed to the optical transmitter, there is more chance of accomplishing optimum operation. The characteristics of the optical receiver can be fed back through a communication path of the opposite direction, through information transfer with the use of monitoring control light or a public network, or the like.

The use of the LUT2allows signal arrangement (mapping) of an input data series to a multi-level signal and a compensation of the drive electrical signals to be executed simultaneously. The circuit size can therefore be reduced.

REFERENCE SIGNS LIST