JP2012049801A - Signal modulation device and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a signal modulation device and a method of the same to obtain an optical signal of high quality and multiple values.SOLUTION: A signal modulation device for modulating an electric two-value signal of N channels (N is an integer no less than 2 and no more than N) into an optical multiple-value signal of a single channel comprises N steps of modulation parts for modulating an electric signal into an optical signal. The modulation part of a first step modulates a first electric two-value signal into an optical signal to supply to a second step modulation part. The modulation parts from the second step on modulate an electric signal into an optical signal by an external modulation system. The modulator of the mth step (m is an integer no less than 2 and no more than N) modulates the mth electric two-value signal into an optical signal and outputs it, and also outputs the optical signal output by the modulation part of the Nth step as an optical multiple-value signal, with the optical signal output from the modulation part of the m-1th step as a light source.

Description

  The present invention relates to a signal modulation apparatus and method, and can be applied to, for example, a communication apparatus that performs high-speed communication using an optical fiber.

  In a modern optical fiber communication system, a code generally used for modulation when converting an electric signal to an optical signal is NRZ (Non Return to Zero) code on-off keying. This creates a two-stage state of light intensity “absent” and “present” corresponding to the “0” and “1” bits of the digital signal to be transmitted, and transmits it.

  On the other hand, by increasing the light intensity level more than two levels to obtain a multilevel signal (hereinafter referred to as “optical multilevel signal”), more information can be obtained without increasing the symbol rate. Is also transmitted (hereinafter referred to as “optical multilevel modulation method”).

  As one of conventional optical multilevel modulation methods, there is a method described in Non-Patent Document 1 for generating a multilevel signal in the state of an electrical signal and modulating the light intensity.

However, the method of generating a multi-value electric signal as in the technique described in Non-Patent Document 1 has the following problems. In the technology described in Non-Patent Document 1, when N binary signals are added to generate a 2 ^N- value multilevel electrical signal, reflection of high-frequency components due to impedance mismatch of the electronic circuit board Waveform distortion occurs. The technique described in Non-Patent Document 1 requires an electronic circuit having linear input / output characteristics that faithfully amplifies an analog waveform.

  Since the modulation speed of today's optical transmission apparatus is considerably increased to several tens of Gbps, it is necessary to increase the number of multi-values or to increase the modulation speed. When the speed is increased, the above two problems become particularly significant.

  Therefore, conventionally, as a method of generating a multi-value optical signal without handling a multi-value electric signal, there is a method of generating a multi-value signal in the optical region. Conventionally, as a method for generating a multilevel signal in the optical region, there are technologies described in Patent Documents 1 to 4.

In the technologies described in Patent Documents 1 to 3, N systems of binary intensity-modulated light are generated using N light sources, and they are combined with a level difference to generate 2 ^N- value light. Get a signal.

  In the technique described in Patent Document 4, the output from the same light source is branched into two on the waveguide, and is multiplexed after being modulated by two modulators capable of intensity modulation.

JP 63-005633 A JP 2009-124342 A JP 2002-333603 A Japanese Patent Laid-Open No. 1-223837

Shield Walkin et al, “Multilevel Signaling for Increasing the Reach of 10 Gb / s Lightwave Systems”, Journal of Lightwave Technology. 17, no. 11, pp. 2235-2248, 1999.

  However, in the technologies described in Patent Documents 1 and 2, interference does not occur if the wavelengths of light to be combined are sufficiently far apart, but then it becomes the same as wavelength division multiplexing (WDM) for transmission with different wavelengths, The merit of multi-value becomes less.

  In addition, in the technology described in Patent Document 3, there are high requirements in design and the like in order to prevent beat light from interfering with each other when the generated N systems of light are combined. It is difficult to obtain a quality multilevel optical signal, and even if it can be realized, the cost will be high.

  Furthermore, in order to realize the multilevel modulator of the technique described in Patent Document 4, it is necessary to satisfy the condition that the phase variation of the optical carrier does not occur before the branched optical signals are combined again. For this reason, the technology described in Patent Document 4 has high requirements such as optical path difference adjustment and temperature adjustment of the waveguide base with high accuracy, and a design that does not cause the phase variation of light, and a high-quality waveform with little distortion. Difficult to get.

  In view of the above-described problems of the prior art, a signal modulation apparatus and method capable of obtaining a high-quality multilevel optical signal are desired.

  The first aspect of the present invention is a signal modulation apparatus that modulates an N-channel (N is an integer of 2 or more) electrical binary signal into a 1-channel optical multilevel signal, and (1) modulates the electrical signal into an optical signal. (2) The first-stage modulation unit modulates the first electric binary signal into an optical signal, and supplies the optical signal to the second-stage modulation unit. The second and subsequent stages modulate the electrical signal into an optical signal by an external modulation method, and the mth stage (m is an integer of 2 or more and N or less) Using the optical signal output from the (m-1) th stage modulation unit as a light source, the mth electrical binary signal is modulated into an optical signal and output. (4) The Nth stage modulation unit outputs An optical signal to be output is output as the optical multilevel signal.

  The second aspect of the present invention is a signal modulation method for modulating an N-channel (N is an integer of 2 or more) electric binary signal into a single-channel optical multilevel signal. (1) N for modulating an electric signal into an optical signal A step of the first-stage modulation unit among the first-stage modulation units modulating the first electric binary signal into an optical signal and supplying the optical signal to the second-stage modulation unit; (2) The modulation units after the second stage modulate an electric signal into an optical signal by an external modulation method, and the modulation unit at the m-th stage (m is an integer of 2 or more and N or less) A step of modulating the m-th electric binary signal into an optical signal using the optical signal output from the first-stage modulation unit as a light source, and (3) the N-th stage modulation unit And a step of outputting an optical signal to be output as the optical multilevel signal.

  According to the present invention, a high-quality multilevel optical signal can be obtained.

It is a block diagram which shows the functional structure of the signal modulation apparatus which concerns on 1st Embodiment. It is explanatory drawing shown about the relationship between the value of an input electrical signal, and an output optical multilevel signal in the signal modulation apparatus which concerns on 1st Embodiment. It is explanatory drawing shown about the example of the extinction characteristic of the EA modulator which concerns on 1st Embodiment. It is the block diagram shown about the structural example of the signal demodulation apparatus which concerns on 1st Embodiment. It is explanatory drawing shown about the relationship between the electric signal input into the 1st stage EA modulator which concerns on 1st Embodiment, an extinction characteristic, and the power of an output optical signal. It is explanatory drawing shown about the relationship between the electric signal input into the 2nd stage EA modulator which concerns on 1st Embodiment, an extinction characteristic, and the power of an output optical signal. It is explanatory drawing shown about the relationship between the electric signal input into the 3rd step | paragraph EA modulator which concerns on 1st Embodiment, an extinction characteristic, and the power of an output optical signal. It is the timing chart shown about the example of the electrical binary signal input into the signal modulation apparatus which concerns on 1st Embodiment, and an output optical multilevel signal. It is a block diagram which shows the functional structure of the signal modulation apparatus which concerns on 2nd Embodiment.

(A) First Embodiment Hereinafter, a first embodiment of a signal modulation apparatus and method according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram illustrating a functional configuration of a signal modulation device 10 according to the first embodiment.

  The signal modulation device 10 is a device that modulates a digital electrical signal SE having a 3 channel (bit length) electrical binary signal into a 1 channel optical multilevel signal SO and outputs the modulated signal.

In the following, the values of each channel (each bit) constituting the digital electric signal SE are represented as B1, B2, and B3, respectively. Each value of B1 to B3 is represented by 0 level or 1 level. The values of the respective channels (each bit) constituting the digital electric signal SE _n (n is an integer equal to or greater than 1) at an arbitrary time are represented as B1 _n , B2 _n , and B3 _n . For example, the value of the next signal after B1 ₁ , B2 ₁ , B3 ₁ is expressed as B1 ₂ , B2 ₂ , B3 ₂ . In FIG. 1, as an example, the number of channels (bit length) of the digital electrical signal SE is set to 3, and an optical signal having a power corresponding to the value of the digital electrical signal SE (that is, a combination of the values of B1, B2, and B3) The multi-value signal SO is output, but the number of channels (bit length) constituting the digital electrical signal SE is not limited as long as it is 2 or more.

  FIG. 2 is an explanatory diagram showing the relationship between the values (B1 to B3) of each channel of the digital electrical signal SE and the optical multilevel signal SO.

In FIG. 2, for example, when the values of B1, B2, and B3 are 000 (B1 = B2 = B3 = 0), the signal modulation device 10 outputs an optical signal having a power of P ₀₀₀ as the optical multilevel signal SO. In the case of 001 (B1 = B2 = 0, B3 = 1), it indicates that an optical signal having the power of _P001 is output. Thus, in the signal modulation device 10, as shown in the example of FIG. 2, the power (P ₀₀₀ ,..., 111) corresponding to the respective values (000, 001, 010,. P ₀₀₁ , P ₀₁₀ ,..., P ₁₁₁ ) are output.

Here, it is assumed that P _{111 to} P ₀₀₀ > 0 [mw]. In addition, here, as an example, it is assumed that the difference between the values of P _{000 to} P ₁₁₁ is the same (equal intervals). For example, the difference between the value of P _{000 and} the value of P ₀₀₁ and the difference between the value of P _{001 and} the value of P ₀₁₀ have the same width.

  The signal modulator 10 includes a continuous wave laser diode (hereinafter referred to as “CW-LD”) 11, three electrical binary signal generators 12-1 to 12-3, and two delay elements 13 (13-1, 13−). 3) Three amplitude adjusting units 14 (14-1 to 14-3), three bias adjusting units 15 (15-1 to 15-3), and three ElectroAbsorption (hereinafter referred to as “EA”) modulators 16 (16-1 to 16-3).

  The electrical binary signal generators 12-1 to 12-3 generate the digital electrical signal SE, and for example, an existing pulse pattern generator can be used.

  Data (pulse pattern) of each channel (each bit) constituting the digital electrical signal SE is set in each of the electrical binary signal generators 12-1 to 12-3. The electrical binary signal generators 12-1 to 12-3 are each set with data set at a timing synchronized by a reference clock generator (not shown) (an existing clock generator can be applied). An electrical binary signal based on the pulse pattern is output. That is, in FIG. 1, the electrical binary signal generators 12-1 to 12-3 are predetermined binary signals having values (levels) B 1, B 2, and B 3 that are synchronized by the above-described reference clock generator. Are output at a bit rate BR [bps].

  In FIG. 1, the electrical binary signal generators 12-1 to 12-3 are provided in the signal modulator 10, but the electrical binary signal generator 12 is omitted from the signal modulator 10 and the external The digital electric signal SE may be supplied from Further, only the data (pulse pattern) of the digital electric signal SE generated by the electric binary signal generators 12-1 to 12-3 is supplied from the outside, and the electric 2 is generated based on the supplied data (pulse pattern). The value signal generators 12-1 to 12-3 may generate the digital electric signal SE.

  In the signal modulation device 10, each of the electrical binary signals output from the electrical binary signal generation units 12-1 to 12-3 is adjusted to a predetermined amplitude by the amplitude adjustment units 14-1 to 14-3, and The bias is adjusted to a predetermined bias by the bias adjusters 15-1 to 15-3 and input to the EA modulators 16-1 to 16-3. As the amplitude adjustment unit 14, for example, an existing circuit for performing amplitude adjustment (a variable gain amplifier, an attenuator, or the like) can be applied. In addition, the bias adjusting unit 15 can also use a circuit for adjusting the bias of an existing electric signal.

  Further, in the signal modulation device 10, the electrical binary signals output from the electrical binary signal generators 12-2 and 12-3 are delayed using the delay elements 13-2 and 13-3, respectively, and EA The signals are input to the modulators 16-2 and 16-3. On the other hand, the electrical binary signal output from the electrical binary signal generator 12-1 is input to the EA modulator 16-1 without being delayed. As the delay element 13, a phase shifter that delays an existing electrical signal for a predetermined time can be used.

  Hereinafter, the electrical binary signals input to the EA modulators 16-1 to 16-3 will be referred to as se1 to se3.

  The CW-LD 11 is a light source that emits continuous light having a wavelength λ. As CW-LD11, an existing laser diode can be applied.

  The signal modulation device 10 generates the optical multilevel signal SO by superimposing se1 to se3 on the continuous light emitted from the CW-LD11. As the EA modulator 16, an existing EA modulator (electroabsorption modulator) can be applied. The EA modulator 16 is not limited to the electroabsorption type, and may be replaced with another type as long as it is a device that modulates an electric signal into an optical signal by an external modulation method.

  The first-stage EA modulator 16-1 modulates the electrical binary signal se1 into an optical signal using the continuous light supplied from the CW-LD 11 as a light source, and supplies the optical signal to the EA modulator 16-2. The second-stage EA modulator 16-2 modulates the electrical binary signal se2 into an optical signal using the optical signal supplied from the EA modulator 16-1 as a light source, and supplies the optical signal to the EA modulator 16-3. is doing. The final stage (third stage) EA modulator 16-3 modulates the electrical binary signal se3 into an optical signal using the optical signal from the second stage EA modulator 16-2 as a light source, It is output as a value signal SO.

  That is, in the EA modulator 16 at the m-th stage (from the second stage, but excluding the final stage), the optical signal output from the EA modulator 16 at the (m-1) th stage is supplied as a light source. The optical signal output from the EA modulator 16 at the m-th stage (from the second stage, but excluding the final stage) is supplied as a light source to the next + 1-stage EA modulator 16. Thus, in the signal modulation device 10, the plurality of EA modulators 16 are connected.

In FIG. 1, since the number of channels (bit length) of the digital electrical signal SE is 3 in the signal modulation device 10, three stages of EA modulators 16-1 to 16-3 are arranged. When a digital signal having the number of channels (bit length) is modulated, it is necessary to connect the EA modulators 16 corresponding to the number of channels (bit length) in the same manner as in FIG. Further, when the number of channels (bit length) of the digital electrical signal SE is 2, the configuration related to the third-stage EA modulator 16-3 is omitted, and the configuration up to the EA modulator 16-2 is adopted. You may do it. For example, when the number of channels (bit length) of the digital electrical signal SE is m, m stages of EA modulators 16-1 to 16-m are connected. Each of the second and subsequent EA modulators 16 is provided with a corresponding electric binary signal generator 12, delay element 13, amplitude adjuster 14, and bias adjuster 15. Further, in FIG. 1, since the number of channels of the digital electric signal SE (bit length) is 3, the optical multilevel signal SO is the optical signal of 2 ^ternary (= 8 values), the channel of the digital electric signal SE When the number (bit length) is m, the optical multilevel signal SO is a 2 ^m- value optical signal.

In the following, as an example, se1 input to the EA modulator 16-1, amplitude in _V 1 ~V _0, it is assumed that a signal offset level _{V b1.} In addition, here, it is assumed that se1 is a signal that fluctuates in the range of V _b1 + V _{1 to} V _b1 −V ₀ . In this case, se1 when the value of B1 is 1 shows the value of _V b1 + _{V 1,} when the value of B1 is zero, se1 denote the value of _V b1 -V _0.

In the following, EA modulator 16-2, the amplitude is _{V 1 / 2~V 0/2,} ( assumed to be _{V b2 = V b1 -V 1/} 2) offset level _V b2 signal Shall be entered. Like the se1 se2 _is assumed to be a signal that varies in a range of _{V b2 + V 1 / 2~V b2} -V 0/2. In this case, se2 when the value of B2 is 1 shows the values of _V b2 _{+ V} 1/2, when the value of B2 is 0, se2 indicates the value of _V b2 _-V 0/2 Shall.

Furthermore, in the following, EA modulator 16-3, the amplitude is _{V 1 / 4~V 0/4,} ( assumed to be _{V b3 = V b2 -V 1/} 4) offset level _V b3 signal Shall be entered. Similar to SE1 and SE2 se3 _is assumed to be a signal that varies in a range of _{V b3 + V 1 / 4~V b3} -V 0/4. In this case, se3 when the value of B3 is 1 shows the values of _V b3 _{+ V} 1/4, if the value of B3 is zero, se3 indicates the value of _V b3 _-V 0/4 Shall.

In this way, each EA modulator 16 is supplied with an electrical binary signal having different amplitudes. Specifically, when an electrical binary signal supplied to the EA modulator 16-m of the m-th stage and sem, amplitude sem ^{_{is, (V 1/2 m-}} 1 ~V 0/2 m-1) It becomes. Further, the _{V bm = V bm-1 -V} 1/2 m-1 when the offset level of the sem and _{V bm.}

Note that V ₁ , V _0, and V _b1 are desirably set to values according to the extinction characteristics of the EA modulators 16-1 to 16-3. Here, description will be made assuming that the extinction characteristics of the EA modulators 16-1 to 16-3 are all the same.

  FIG. 3 is an explanatory diagram showing an example of the extinction characteristic of the EA modulator.

  In FIG. 3, the horizontal axis represents the voltage (Inverse bias) of the electric signal input to the EA modulator, and the vertical axis represents the power (Extension Power) extinguished by the EA modulator. In general, the EA modulator has extinction characteristics having both a non-linear region and a linear region as shown in FIG. Therefore, it is desirable that the EA modulator 16 be operated using this linear region.

For example, when the extinction characteristic of the EA modulator 16 is as shown in FIG. 3, the signal values of se1 to se3 are within this linear region (for example, between 1 [V] and 2 [V]). It is desirable to set the values of V ₁ , V ₀ and V _b1 (for example, V ₁ = V ₀ = 0.5 [V], V _b1 = 1.5 [V]).

  Next, an outline of signal processing by the EA modulators 16-1 to 16-3 will be described.

  As described above, in the signal modulation device 10, the EA modulators 16-1 to 16-3 are used to superimpose the se1 to se3 on the continuous light emitted by the CW-LD 11, thereby generating the optical multilevel signal SO. Is generated.

Specifically, for example, the value of B1, B2, B3 is the case was 111, the power of the optical signal output from the EA modulator 16-1 is equivalent to _{P 100,} the EA modulator 16 the power of the optical signal output from the 2 corresponds to _{P 110,} the power of the optical signal output from the EA modulator 16-3 (optical multilevel signal SO) is _{P 111.}

In the following, the power of the optical signal output from the EA modulator 16-1 is represented by either P ₀ (when B1 = 0) or P ₁ (when B1 = 1). The power of the optical signal output from the EA modulator 16-2 is P ₀₀ (when B1 = 0 and B2 = 0), P ₀₁ (when B1 = 0 and B2 = 1), P ₁₀ (B1). = 1, B2 = 0) or P ₁₁ (B1 = 1, B2 = 1). The power of the optical signal output from the EA modulator 16-3 is expressed in the same format as the optical multilevel signal SO.

  Next, the function of the delay element 13 will be described.

  As described above, the signal modulation device 10 modulates (superimposes) the binary electric signals se1 to se3 stepwise on the continuous light output from the CW-LD 11 using the EA modulators 16-1 to 16-3. is doing. However, if electric signals are output from the electric binary signal generators 12-1 to 12-3 and modulation of the signals is started by the EA modulators 16-1 to 16-3, for example, EA modulation is performed. Before the signal is modulated by the modulator 16-1, the modulation is performed by the EA modulators 16-2 and 16-3 at the subsequent stage. Therefore, in the signal modulation device 10, the delay elements 13-2 and 13-3 are provided to adjust the timing at which the electric signal is input to the second and subsequent EA modulators 16-2 and 16-3.

If there is no delay element 13, for example, the value of B1, B2, B3 is, when the transition to the 000 111, the power of the optical signal output from the EA modulator 16-1 is changed from _{P 0} to _{P 1} before, the power of the optical signal output from the most downstream EA modulator 16-3 is the result in a transition from _{P 000} to _{P 001.} Such a shift in timing may cause problems such as distortion of the waveform of the optical multilevel signal SO. Therefore, in the signal modulation device 10, delay elements 13-2 and 13-3 are provided, and the states of the EA modulators 16-1 to 16-3 are adjusted so as to be superimposed in order from the front stage to the rear stage. In other words, the delay time set in the delay element 13 is set to a larger delay time as it becomes later.

Although the delay time set for each of the delay elements 13-2 and 13-3 is not limited, for example, an appropriate time may be obtained and set by experiment or simulation. For example, the delay time set in the delay element 13-2 is the time required from the time when the optical signal is modulated by the EA modulator 16-1 to the time when the optical signal is modulated by the EA modulator 16-2. Equal time (hereinafter referred to as “t ₁ ”) may be applied. Similarly, for the delay element 13-3, a time (hereinafter referred to as the time required for the optical signal to be modulated by the EA modulators 16-1 and 16-2 and the optical signal to be modulated by the EA modulator 16-3) (Referred to as “t ₂ ”). Further, the delay time set for each of the delay elements 13-2 and 13-3 may be set to a value that minimizes the distortion of the waveform of the optical multilevel signal SO through experiments and simulations.

Further, for example, when the delay time t ₁ set for the second-stage delay element 13-2 is obtained, the delay time t ₂ set for the third-stage delay element 13-3 is set to 2 × t _1. You may make it do. In this way, only the delay time t ₁ set in the second stage is acquired by experiment or simulation, and for example, the delay time t _m set in the delay element 13 in the m-th stage (after the third stage) is (m−1). ) × t ₁ may be set.

  Next, a configuration for demodulating the optical multilevel signal SO output from the signal modulation device 10 into a digital electric signal in the same format as the original digital electric signal SE will be described.

  FIG. 4 is a block diagram illustrating a configuration example of the signal demodulator 30.

  For example, when the signal modulation device 10 is disposed in the communication device on the optical signal transmission side, the signal demodulation device 30 is disposed in the communication device on the reception side.

  The signal demodulator 30 receives the optical multilevel signal SO and converts it into an electrical signal, and the multilevel electrical signal given from the light receiver 31 has the same format as the original digital electrical signal SE. An AD converter 32 for converting into a digital electric signal.

  An existing converter that performs photoelectric conversion can be applied to the light receiving unit 31. Further, as the AD converter 32, a converter that converts an existing analog electric signal into a digital electric signal can be applied.

In the AD converter 32, the value of the input optical multilevel signal SO is any one of P ₀₀₀ , P ₀₀₁ , P ₀₁₀ ,..., P ₁₁₁ according to the output voltage of the signal output from the light receiving unit 31. The digital electric signal corresponding to the determination result is output.

(A-2) Operation of the First Embodiment Next, the operation (signal modulation method of the embodiment) of the signal modulation device of the first embodiment having the above configuration will be described.

  First, operations related to each of the EA modulators 16-1 to 16-3 will be described.

  The electrical binary signal (B1 signal) output from the electrical binary signal generator 12-1 is adjusted by the amplitude adjuster 14-1 and the bias adjuster 15-1, and is sent to the EA modulator 16-1 by the se1. Is entered as

  Then, in the EA modulator 16-1, se1 is modulated into an optical signal using the continuous light given from the CW-LD 11 as a light source, and is given to the EA modulator 16-2.

  FIG. 5 is an explanatory diagram showing the relationship between se1 input to the EA modulator 16-1, the extinction characteristic of the EA modulator 16-1, and the power of the optical signal output from the EA modulator 16-1. It is.

  In FIG. 5, the extinction characteristic of the EA modulator 16-1 is described as having the characteristic indicated by L10.

As shown in FIG. 5, the value of se1 input to the EA modulator 16-1 is V _b1 −V ₀ (when B1 = 0) or V _b1 + V ₁ (when B1 = 1). It has become.

In the EA modulator 16-1, an optical signal having a power of P ₀ or P ₁ is output based on the input value of se1 and the extinction characteristic indicated by L10.

For example, in the EA modulator 16-1, when V _b1 −V ₀ (when B1 = 0) is input as se1, the power of the output optical signal is P ₀ due to the extinction characteristic indicated by L10. Further, in the EA modulator 16-1, the (case of _{B1 = 1) V b1 + V} 1 is input as se1, the extinction characteristic as shown in L10, the power of the output optical signal becomes _{P 1.}

  Next, the operation of the EA modulator 16-2 will be described.

  The electrical signal (B2 signal) output from the electrical binary signal generator 12-2 is delayed for a predetermined time by the delay element 13-2, and further adjusted by the amplitude adjuster 14-2 and the bias adjuster 15-2. And input to the EA modulator 16-2 as se2.

  In the EA modulator 16-2, se2 is modulated into an optical signal using the optical signal provided from the EA modulator 16-1 as a light source, and is provided to the EA modulator 16-3.

  FIG. 6 shows the relationship of the extinction characteristic between se2 input to the EA modulator 16-2, the extinction characteristic of the EA modulator 16-2, and the power of the optical signal output from the EA modulator 16-2. FIG.

In FIG. 6, the extinction characteristic of the EA modulator 16-2 when the power of the optical signal input from the EA modulator 16-1 is P ₀ is described as being represented by L20. Further, the extinction characteristics of the EA modulator 16-2 when 6, the power of the optical signal inputted from the EA modulator 16-1 was _{P 1} is described as represented by L21.

Then, as shown in FIG. 6, the value of se2 input to the EA modulator 16-2 _(in the case of _{B2 = 0) V b2 -V 0} /2 _{or, V b2 + V 1/2} (B2 = 1 In the case of).

In the EA modulator 16-2, based on the power value of the optical signal input from the EA modulator 16-1, the value of se2 input, and the extinction characteristic indicated by L20 or L21, P ₀₀ , P An optical signal having a power of any one of ₀₁ , P ₁₀ and P ₁₁ is output.

For example, in the EA modulator 16-2, EA modulator values of the power of the optical signal input from 16-1 _{P 0,} (when the B2 = 1) the value of se2 input is _V b2 _{+ V} 1/2 Suppose that it was. In this case, the EA modulator _16-2, are applied extinction characteristic of L20 (in the case of _{B2 = 1) V b2 + V} 1/2, the power of the optical signal output is _{P 01.}

  Next, the operation of the EA modulator 16-3 will be described.

  The electrical signal (B3 signal) output from the electrical binary signal generator 12-3 is delayed for a predetermined time by the delay element 13-3, and further adjusted by the amplitude adjuster 14-3 and the bias adjuster 15-3. And input to the EA modulator 16-3 as se3.

  In the EA modulator 16-3, se3 is modulated into an optical signal using the optical signal supplied from the EA modulator 16-2 as a light source, and output as an optical multilevel signal SO.

  FIG. 7 shows the relationship between the extinction characteristics of se3 input to the EA modulator 16-3, the extinction characteristics of the EA modulator 16-3, and the power of the optical signal output from the EA modulator 16-3. FIG.

In FIG. 7, the extinction characteristics in the EA modulator 16-3 when the power of the optical signal input from the EA modulator 16-2 is P ₀₀ , P ₀₁ , P ₁₀ , P ₁₁ are L30 and L31, respectively. , L32, and L33.

Then, as shown in FIG. 7, the value of se3 input to the EA modulator 16-3 _(in the case of _{B3 = 0) V b3 -V 0} /4 _{or, V b3 + V 1/4} (B3 = 1 In the case of).

In the EA modulator 16-3, based on the power value of the optical signal input from the EA modulator 16-2, the value of se3 input, and the extinction characteristic shown in any of L30 to L33, P An optical signal having any power of _{000 to} P ₁₁₁ is output.

For example, in the EA modulator 16-3, the optical signal power value _{P 10} of the input from the EA modulator 16-2 (in the case of B3 = 1) the value of se3 input is _V b2 _{+ V} 1/4 Suppose that it was. In this case, the EA modulator _16-3, are applied extinction characteristic of L32 (in the case of _{B3 = 1) V b2 + V} 1/4, the power of the optical signal output is _{P 101.}

  Next, the operation of the entire signal modulation apparatus 10 will be described.

  FIG. 8 is a timing chart showing an example of the contents of the digital electrical signal SE (B1, B2, B3) input to the signal modulation device 10 and the optical multilevel signal SO.

  In FIG. 8, based on the digital electric signal SE (B1, B2, B3), the waveforms output from the electric binary signal generators 12-1 to 12-3 and the EA modulators 16-1 to 16- 3 shows the waveform of the optical multilevel signal SO output as a result of modulation by 3. In FIG. 8, the operation times of the EA modulators 16-1 to 16-3 are not shown.

  In FIG. 8, the timings of T1 to T6 are synchronized with a reference clock generator (not shown) described above, and the electrical binary signal generators 12-1 to 12-1 are synchronized with the timings of T1 to T6. The voltage value output from 12-3 transitions.

  In FIG. 8, the power of the optical multilevel signal SO output in response to the transition of the signals output from the electrical binary signal generators 12-1 to 12-3 is transitioned.

For example, between timings T2 and T3, the electrical binary signal generators 12-1 to 12-3 output electrical signals based on B1 = 1, B2 = 1, and B3 = 1, respectively. power value signal SO has a _{P 111.}

(A-3) Effects of First Embodiment According to the first embodiment, the following effects can be achieved.

(A-3-1) In the signal modulation device 10, the EA modulators 16 are connected for the number of channels (bit length) of the digital electrical signal SE, and the electrical 2 based on the digital electrical signal SE is connected to each EA modulator 16. A value signal is being input. The second and subsequent EA modulators 16 superimpose all the electrical binary signals by modulating the input electrical binary signal using the optical signal output from the preceding EA modulator 16 as a light source. The generated optical multilevel signal SO is generated. As a result, the signal modulation device 10 can generate a multi-level modulated optical signal from one light source without handling a multi-level electrical signal that has been difficult to generate without waveform distortion in the past.

(A-3-2) In the signal modulation device 10, the connected EA modulators 16-1 to 16-3 only modulate se <b> 1 to se <b> 3 into optical signals, respectively. Unlike the technology, it does not adopt a method of multiplexing a plurality of optical signals.

  For example, the technique described in Patent Document 4 requires a design that performs optical path difference adjustment and temperature adjustment of a waveguide substrate with high accuracy and does not cause light phase fluctuations. In the technique described in Patent Document 3, in order to suppress the generation of beat noise associated with the multiplexing of optical signals, the wavelength output from the optical modulation signal generating means must be a different value or the polarization must be orthogonal. There is a restriction that it must be done (see paragraph 0017 of Patent Document 3). Therefore, in the technique described in Patent Document 3, each of the optical modulation signal generating means generates binary amplitude-modulated optical signals with different wavelengths or polarizations based on the input electric signals ( (See FIG. 1, FIG. 2, FIG. 6, FIG. 7, FIG. 9, and paragraph 0036 of Patent Document 3).

  That is, in the signal modulation device 10 of the first embodiment, as in the conventional technologies described in Patent Documents 1 to 4, a configuration for multiplexing a plurality of optical signals is not employed, and an optical multilevel signal is generated. Since high-level design and control are not required, a high-quality optical multilevel signal with less distortion can be easily generated, and can be constructed at a lower cost than in the prior art.

(B) Second Embodiment Hereinafter, a second embodiment of a signal modulation apparatus and method according to the present invention will be described in detail with reference to the drawings.

  FIG. 9 is a block diagram illustrating a functional configuration of the signal modulation device 10A according to the second embodiment.

  The signal modulation device 10A according to the second embodiment includes the CW-LD 11, the amplitude adjustment unit 14-1, the bias adjustment unit 15-1, and the EA modulator 16-1 according to the first embodiment, the LD driver 17 and the direct adjustment. This is different from the first embodiment in that the modulation LD 18 is replaced.

  The direct modulation LD 18 is a direct modulation type signal modulator that modulates an electrical signal into an optical signal by applying the electrical signal to a semiconductor laser that is a light source. As the direct modulation LD 18, an existing direct modulation signal modulator can be applied.

  The LD driver 17 is a driver that directly drives the modulation LD 18 based on the electrical signal output from the electrical binary signal generator 12-1. As the LD driver 17, a driver used for an existing direct modulation type signal modulator can be applied.

  In the first embodiment, the electrical signal output from the electrical binary signal generator 12-1 is modulated into an optical signal using the EA modulator. In the second embodiment, the EA modulator Instead, a direct modulation signal modulator (direct modulation LD18) is used.

  Since the direct modulation type signal modulator is less expensive than the EA modulator, the signal modulation device 10A of the second embodiment can be constructed at a lower cost than the first embodiment.

  Further, in the second embodiment, since the direct modulation signal modulator is used in the first stage instead of the EA modulator, the EA modulator can be reduced by one stage compared to the first embodiment. An optical multilevel signal SO having a higher extinction ratio can be obtained.

(C) Other Embodiments The present invention is not limited to the above-described embodiments, and may include modified embodiments as exemplified below.

(C-1) In each of the above embodiments, the electrical binary signals (se2, se3) input to the second and subsequent EA modulators are subjected to delay processing by the delay elements. May be omitted. The delay element is inserted to suppress distortion or the like generated in the optical multilevel signal SO. However, when the delay element is not required to be accurate to the optical multilevel signal SO (for example, optical For example, the delay element can be omitted in the case where the frequency of the multilevel signal SO is low.

(C-2) In each of the above embodiments, an amplitude adjustment unit and a bias adjustment unit are provided to adjust a signal output from the electrical binary signal generation unit. However, when the amplitude or bias of the signal output from the electrical binary signal generator is adjusted to a predetermined value from the beginning, the amplitude adjuster and the bias adjuster may be omitted.

  DESCRIPTION OF SYMBOLS 10 ... Signal modulation apparatus, 11 ... CW-LD, 12, 12-1 to 12-3 ... Electric binary signal generation part, 13, 13-2, 13-3 ... Delay element, 14, 14-1 to 14- 3... Amplitude adjustment unit, 15, 15-1 to 15-3... Bias adjustment unit, 16-1 to 16-3... EA modulator.

Claims (5)

In a signal modulation device that modulates an N-channel (N is an integer of 2 or more) electrical binary signal into a single-channel optical multilevel signal,
N stages of modulation units for modulating electrical signals into optical signals are provided,
The first-stage modulation unit modulates the first electric binary signal into an optical signal, and supplies the optical signal to the second-stage modulation unit.
The modulation units in the second and subsequent stages modulate an electrical signal into an optical signal by an external modulation method, and the modulation sections in the m-th stage (m is an integer of 2 or more and N or less) Using the optical signal output from the first-stage modulation unit as a light source, the mth electrical binary signal is modulated into an optical signal and output,
An optical signal output from the N-th stage modulation unit is output as the optical multilevel signal.   2. The signal modulation device according to claim 1, further comprising signal adjustment means for adjusting the amplitude of each of the electric binary signals to be different. In the signal adjustment means, when the amplitude of the first electric binary signal supplied to the first stage modulation unit is V, the mth electric supply to be supplied to the mth stage modulation unit. The signal modulation device according to claim 2, wherein the amplitude of the binary signal is V / 2 ^m .   The signal modulation device according to claim 1, wherein the modulation unit at the first stage modulates the first electric binary signal into an optical signal by a direct modulation method. In a signal modulation method for modulating an N-channel (N is an integer of 2 or more) electrical binary signal into a single-channel optical multilevel signal,
Of the N-stage modulation units that modulate the electric signal into the optical signal, the first-stage modulation unit modulates the first electric binary signal into the optical signal, and the second-stage modulation unit. Supplying to
The modulation units after the second stage modulate an electric signal into an optical signal by an external modulation method, and the modulation unit at the m-th stage (m is an integer of 2 or more and N or less) Using the optical signal output from the first-stage modulation unit as a light source, modulating the m-th electric binary signal into an optical signal, and outputting the optical signal;
And a step of outputting the optical signal output from the N-th stage modulation unit as the optical multilevel signal.

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