JP2017216681A - Optical transmitter and drive adjustment method for optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter and a drive adjustment method for the optical transmitter, capable of improving accuracy of optical multi-value transmission using a pulse amplitude modulation system even in the case of performing APC control.SOLUTION: An optical transmitter 1 for optical multi-value transmission using a pulse amplitude modulation system comprises: a laser element 2; a light detection element 3 for detecting light emission intensity of the laser element 2; a memory 11 in which modulation characteristic data on the laser element 2 is stored; a control unit 12 for setting an upper limit value of the light emission intensity on the basis of a detection result by the light detection element 3; and an amplitude control unit 13 that, on the basis of the upper limit value and the modulation characteristic data, determines an amplitude range of an electric signal to be input to the laser element 2 and, on the basis of modulation characteristic data corresponding to the amplitude range, sets amplitude for each level of the electric signal so that light emission intensity difference between respective levels of an optical signal L1 to be output from the laser element 2 has a predetermined ratio.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission device and a drive adjustment method for the optical transmission device.

  Patent Document 1 below discloses an optical receiver that can automatically control an identification level and the like according to an input signal. This optical receiver includes a photodetector that converts an optical signal into an electrical signal, an identification level automatic control circuit to which the electrical signal is input, and a clock that extracts the clock signal of the electrical signal and supplies it to the identification level automatic control circuit And an extraction circuit.

JP 2002-141956 A

  In order to realize a high-speed and large-capacity optical communication system, an optical multilevel transmission technique using a pulse amplitude modulation (PAM) system has been studied. For example, when optical multilevel transmission is performed using a quaternary pulse amplitude modulation signal (PAM4 signal) that is classified into four levels depending on the signal intensity, the PAM4 signal is first converted into an electrical signal for input to the semiconductor laser element. . This electric signal has a waveform with an amplitude corresponding to the level defined by the PAM4 signal. Next, by inputting an electric signal to the semiconductor laser element, the semiconductor laser element outputs a modulated laser beam (optical signal). The optical receiver receives this optical signal, demodulates it into an electrical signal, and detects the level of the electrical signal, thereby implementing optical multilevel transmission using the PAM4 signal.

  By the way, the relationship between the amplitude of the electrical signal input to the semiconductor laser element and the light emission intensity of the optical signal (modulation characteristics of the semiconductor laser element) may exhibit nonlinearity depending on the amplitude of the input electrical signal. For this reason, depending on the amplitude of the electrical signal, a deviation may occur between the theoretical light emission intensity and the actual light emission intensity in the output optical signal. When this deviation occurs, the theoretical level and the actual level of the optical signal may be different. In this case, there is a problem that optical multilevel transmission is not accurately performed.

  In addition, depending on the optical communication system, an automatic power control circuit (APC circuit: Auto Power Control circuit) may be used to control the upper limit of the emission intensity of the semiconductor laser element. When such APC control is performed on the semiconductor laser element, the range (drive range) that the emission intensity of the semiconductor laser element can take changes, so the above problem becomes more prominent.

  An object of the present invention is to provide an optical transmission device and an optical transmission device drive adjustment method capable of improving the accuracy of optical multilevel transmission using a pulse amplitude modulation method even when APC control is performed. To do.

  An optical transmission device according to one aspect of the present invention is an optical transmission device for optical multilevel transmission using a pulse amplitude modulation method, and includes a laser element, a light detection element that detects light emission intensity of the laser element, and a laser. A memory for storing modulation characteristic data of the element, a control unit for setting an upper limit value of light emission intensity based on a detection result by the light detection element, and an amplitude of an electric signal input to the laser element from the upper limit value and the modulation characteristic data The amplitude of each level of the electrical signal is determined so that the difference in emission intensity between the levels of the optical signal output from the laser element becomes a predetermined ratio based on the modulation characteristic data corresponding to the amplitude range. An amplitude control unit for setting

  A drive adjustment method according to another aspect of the present invention is a drive adjustment method of an optical transmission device for optical multilevel transmission using a pulse amplitude modulation method, comprising: detecting a light emission intensity of a laser element; Corresponding to the amplitude range, the step of setting the upper limit value of the emission intensity based on the detection result of the intensity, the step of determining the amplitude range of the electric signal input to the laser element from the upper limit value and the modulation characteristic data of the laser element And setting the amplitude of each level of the electric signal so that the difference in emission intensity between the levels of the optical signal output from the laser element becomes a predetermined ratio based on the modulation characteristic data to be performed.

  ADVANTAGE OF THE INVENTION According to this invention, even when it is a case where APC control is implemented, the optical transmitter which can improve the precision of the optical multi-value transmission using a pulse amplitude modulation system, and the drive adjustment method of an optical transmitter can be provided.

FIG. 1 is a schematic configuration diagram illustrating an optical transmission apparatus according to an embodiment. FIG. 2A is a graph showing the modulation characteristics of the laser element on which the drive adjustment method according to the embodiment is performed. FIG. 2B is an eye pattern of an electric signal input to the laser element. FIG. 2C shows an eye pattern of an optical signal output from the laser element. FIG. 3 is a flowchart for explaining a drive adjustment method of the optical transmission apparatus according to the embodiment. FIG. 4A is a graph showing the modulation characteristics of a laser element that is not subjected to the drive adjustment method according to the embodiment. FIG. 4B is an eye pattern of an electric signal input to the laser element. FIG. 4C shows an eye pattern of an optical signal output from the laser element.

[Description of Embodiment of Present Invention]
First, the contents of the embodiment of the present invention will be listed and described.

  One embodiment of the present invention is an optical transmission device for optical multilevel transmission using a pulse amplitude modulation method, which includes a laser element, a light detection element that detects light emission intensity of the laser element, and a modulation characteristic of the laser element. A memory for storing data, a control unit for setting an upper limit value of light emission intensity based on a detection result by the light detection element, and an amplitude range of an electric signal input to the laser element from the upper limit value and the modulation characteristic data In addition, based on the modulation characteristic data corresponding to the amplitude range, the amplitude for setting the amplitude of each level of the electric signal so that the emission intensity difference between the levels of the optical signal output from the laser element becomes a predetermined ratio And a control unit.

  Another embodiment of the present invention is a drive adjustment method of an optical transmission device for optical multilevel transmission using a pulse amplitude modulation method, the step of detecting the emission intensity of a laser element, A step of setting an upper limit value of the emission intensity based on the detection result, a step of determining an amplitude range of an electric signal input to the laser element from the upper limit value and the modulation characteristic data of the laser element, and a modulation corresponding to the amplitude range Setting the amplitude of each level of the electrical signal based on the characteristic data so that the difference in emission intensity between the levels of the optical signal output from the laser element becomes a predetermined ratio. is there.

  In this optical transmitter and its drive adjustment method, after performing so-called APC control in which the upper limit value of the emission intensity of the laser element is set, the modulation characteristic data corresponding to the amplitude range of the electric signal input to the laser element is obtained. Based on this, the amplitude of each level of the electrical signal is set so that the difference in emission intensity between each level of the optical signal becomes a predetermined ratio. Thereby, even after the APC control is performed, it is possible to suppress a deviation between the theoretical light emission intensity and the actual light emission intensity in the output optical signal. Therefore, even when APC control is performed, the accuracy of optical multilevel transmission using the pulse amplitude modulation method can be improved.

[Details of the embodiment of the present invention]
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same reference numerals are used for the same elements or elements having the same functions, and redundant description is omitted.

  FIG. 1 is a schematic configuration diagram illustrating an optical transmission apparatus according to an embodiment. As shown in FIG. 1, the optical transmission device 1 is a direct modulation type device used for optical multilevel transmission using a pulse amplitude modulation method (PAM method). For this reason, the optical transmission device 1 converts the input signal into a pulse amplitude modulation signal (PAM signal) and outputs an optical signal based on the PAM signal. In the present embodiment, a quaternary pulse amplitude modulation signal (PAM4 signal) is used as the PAM signal. The optical transmission device 1 includes a laser element 2, a light detection element 3, an integrated circuit 4, and an external terminal 5.

  The laser element 2 is a semiconductor laser element. Although not shown, the laser element 2 is an electro-absorption modulator integrated laser element (EML) in which a modulator (electro-absorption modulator) is integrated in a laser unit. The laser element 2 outputs a modulated laser beam (optical signal L1) that travels forward and a laser beam L2 that travels backward. The optical signal L1 is a signal having an emission intensity corresponding to the level defined by the PAM4 signal output from the modulator provided in the preceding stage of the laser element 2. The laser beam L2 is continuous light output from a laser unit provided at the subsequent stage of the laser element 2.

  The light detection element 3 is optically coupled to the laser element 2 and detects the emission intensity of the laser element 2. In the present embodiment, the light detection element 3 is a photodiode. The light detecting element 3 detects the light emission intensity of the laser element 2 by receiving the laser light L2.

  The integrated circuit 4 is a chip that performs specified calculations and stores various data, and is electrically connected to the laser element 2, the light detection element 3, and the external terminal 5. The integrated circuit 4 may be composed of a single chip or a plurality of chips. The integrated circuit 4 includes a memory 11, a control unit 12, an amplitude control unit 13, a pulse amplitude modulation signal generation unit 14, and a drive circuit unit 15.

  Hereinafter, the memory 11, the control unit 12, the amplitude control unit 13, the pulse amplitude modulation signal generation unit 14, and the drive circuit unit 15 of the integrated circuit 4 will be described with reference to FIG. 2 as a specific example. FIG. 2A is a graph showing the modulation characteristic 21 of the laser element 2 on which a drive adjustment method described later is performed. In FIG. 2A, the horizontal axis represents the amplitude (unit V) of the electric signal, and the vertical axis represents the emission intensity of the laser element 2 (unit dBm, also referred to as the amplitude of the optical signal L1). 2B is an eye pattern of an electric signal input to the laser element 2, and FIG. 2C is an eye pattern of an optical signal L1 output from the laser element 2.

  The memory 11 is a ROM (Read Only Memory) in which various data are stored in advance at the time of shipment. The memory 11 stores modulation characteristic data of the laser element 2. The modulation characteristic data includes, for example, the amplitude of the electric signal input to the laser element 2 and the light emission intensity of the optical signal L1 output from the laser element 2 measured before shipping (manufacturing stage) of the optical transmitter 1. Is a data showing the relationship (modulation characteristic 21 shown in FIG. 2A).

  The control unit 12 sets an upper limit value of the light emission intensity of the laser element 2 based on the detection result by the light detection element 3. The control unit 12 receives the detection result by, for example, reading a current value output from the light detection element 3, and sets an upper limit value of the light emission intensity of the laser element 2. For example, as shown in FIG. 2A, the upper limit value of the emission intensity of the laser element 2 is set to −6.5 dBm. The laser element 2 may be a directly modulated semiconductor laser element (DML: directly modulated laser). When the laser element 2 is DML, the optical signal L1 and the laser light L2 are output as signal light having emission intensity corresponding to the level defined by the PAM4 signal. In this case, the control unit 12 receives the detection result by, for example, reading the maximum value of the current value output from the light detection element 3 and sets the upper limit value of the light emission intensity of the laser element 2.

  The amplitude control unit 13 determines the amplitude range of the electric signal input to the laser element 2 based on the set upper limit value and the modulation characteristic data of the laser element 2. The electric signal input to the laser element 2 is a multilevel signal generated by the drive circuit unit 15 and converted from the PAM4 signal. The amplitude range of the electric signal is a voltage range that the multi-level signal can have. For example, as illustrated in FIG. 2A, the amplitude control unit 13 sets the amplitude range of the electric signal input to the laser element 2 based on the upper limit value and the modulation characteristic 21 from −2.5 V to −0. Decide to 5V.

  In addition, the amplitude control unit 13 adjusts the emission intensity difference between the levels of the optical signal L1 output from the laser element 2 to a predetermined ratio based on the modulation characteristic data corresponding to the set amplitude range. Set the amplitude of each level of the electrical signal. For example, the amplitude control unit 13 acquires a range of emission intensity that the optical signal L1 can take based on the modulation characteristic data corresponding to the amplitude range. Next, the amplitude control unit 13 divides the acquired emission intensity range into three so as to become a predetermined ratio. At this time, four division points (corresponding to each level of the optical signal L1) for dividing the light emission intensity range into three are set. Then, four amplitudes of the electrical signal corresponding to the set four division points are acquired. The “predetermined ratio” is a ratio determined in advance at the manufacturing stage, for example, and is stored in the memory 11. Hereinafter, the amplitude of each level indicated by the electric signal set by the amplitude control unit 13 is also simply referred to as “amplitude information”.

  Here, an example of the function of the amplitude control unit 13 will be specifically described. First, the amplitude control unit 13 sets the emission intensity range of the optical signal L1 to −20 dBm to −6.5 dBm based on the modulation characteristic 21 corresponding to the determined amplitude range (−2.5 V to −0.5 V). (See FIG. 2A). Next, the amplitude controller 13 divides the set emission intensity range into three at a predetermined ratio. In the following drive adjustment method, the acquired emission intensity range is equally divided. For this reason, the amplitude control unit 13 sets a point where the light emission intensity of the optical signal L1 is −20 dBm in the modulation characteristic 21 as a division point indicating 0 level, and the light emission intensity of the optical signal L1 is −15. A point at 5 dBm is set as a dividing point indicating one level, and a point at which the emission intensity of the optical signal L1 is −11 dBm in the modulation characteristic 21 is set as a dividing point indicating two levels. A point at which the emission intensity is −6.5 dBm is set as a dividing point indicating three levels. As a result, the emission intensity difference B4 between the 0th level and the 1st level, the emission intensity difference B5 between the 1st level and the 2nd level, and the emission intensity difference B6 between the 2nd level and the 3rd level become −4.5 dBm, respectively (FIG. See also 2 (c)). And the amplitude control part 13 acquires the amplitude (-2.5V, -1.5V, -1V, and -0.5V) of each level of the electric signal corresponding to the dividing point which each shows 0-3 level. (See also FIG. 2 (b)).

  The pulse amplitude modulation signal generation unit 14 converts the signal input from the external terminal 5 into a signal suitable for the drive circuit unit 15. In the present embodiment, the pulse amplitude modulation signal generation unit 14 converts the modulation signal input from the external terminal 5 into a PAM4 signal.

  The drive circuit unit 15 generates and outputs an electrical signal for operating the laser element 2. For example, first, the PAM4 signal output from the pulse amplitude modulation signal generation unit 14 and the amplitude information of the electrical signal output from the amplitude control unit 13 are input to the drive circuit unit 15. The drive circuit unit 15 synthesizes the PAM4 signal and the amplitude information to generate an electric signal having an amplitude corresponding to the level defined by the PAM4 signal, and outputs the electric signal to the laser element 2. Accordingly, as described above, the laser element 2 outputs an optical signal having a light emission intensity corresponding to the PAM4 signal input to the drive circuit unit 15.

  Next, the drive adjustment method of the optical transmission device 1 according to the present embodiment will be described with reference to FIG. FIG. 3 is a flowchart for explaining a drive adjustment method of the optical transmission apparatus according to the embodiment.

  As shown in FIG. 3, as the first step, the light detection element 3 detects the light emission intensity of the laser element 2 (step S1). In step S1, the light detection element 3 detects the laser light L2 output from the laser element 2 to which an arbitrary electrical signal is input.

  Next, as a second step, an upper limit value of the light emission intensity of the laser element 2 is set (step S2). In step S2, the control unit 12 sets an upper limit value of the light emission intensity of the laser element 2 based on the detection result of the light emission intensity by the light detection element 3 (that is, the upper limit of the amplitude of the electric signal input to the laser element 2). By setting the value, so-called APC control for the laser element 2 is performed.

  Next, as a third step, the modulation characteristic data of the laser element 2 is read (step S3). In step S <b> 3, the control unit 12 reads the modulation characteristic data of the laser element 2 stored in the memory 11.

  Next, as a fourth step, the amplitude range of the electric signal input to the laser element 2 is determined based on the upper limit value of the emission intensity of the laser element 2 and the modulation characteristic data of the laser element 2 (step S4). In step S4, the amplitude range of the electric signal is determined by the amplitude control unit 13 as described above.

  Next, as a fifth step, based on the modulation characteristic data corresponding to the amplitude range, the electrical signal is set so that the difference in emission intensity between the levels of the optical signal L1 output from the laser element 2 becomes a predetermined ratio. The amplitude of each level is set (step S5). As described above, for example, the amplitude of each level of the electrical signal is set so that the difference in emission intensity between the levels of the optical signal L1 is equal (that is, the range of emission intensity is equally divided). By passing through these steps S1 to S5, the drive adjustment method of the optical transmission device 1 is completed.

  Next, the operation and effect of the optical transmission apparatus 1 in which the drive adjustment method according to the present embodiment is implemented will be described with reference to FIGS. 2 and 4. FIG. 4A is a graph showing the modulation characteristic 21 of the laser element 2 that has not been subjected to the drive adjustment method. In FIG. 4A, the horizontal axis indicates the amplitude (unit V) of the electric signal, and the vertical axis indicates the emission intensity (unit dBm) of the laser element 2. 4B is an eye pattern of an electric signal input to the laser element 2, and FIG. 4C is an eye pattern of an optical signal L1 output from the laser element 2.

  First, the modulation characteristic 21 of the laser element 2 will be described in detail with reference to FIGS. 2 (a) and 4 (a). As shown in FIGS. 2A and 4A, the modulation characteristic 21 of the laser element 2 has a region showing linearity and a region showing nonlinearity. Specifically, when the amplitude of the electrical signal is in the range of −1.5 V to −0.5 V, the modulation characteristic 21 is a linear region in which the amplitude of the electrical signal and the light emission intensity of the optical signal change proportionally. Yes. On the other hand, when the amplitude of the electric signal is in the range of −2.5 V to −1.5 V and in the range of −0.5 V to 0.5 V, the modulation characteristic 21 is proportional to the amplitude of the electric signal and the emission intensity of the optical signal. It is a non-linear region that does not change.

  For this reason, as shown in FIG. 4B and FIG. 4C, if the amplitude difference B1 to B3 of the 0 level to the 3 level of the electric signal is simply set evenly without performing the above drive adjustment method. The light emission intensity differences B4 to B6 of the 0th to 3rd levels of the optical signal L1 become uneven. As a result, a deviation occurs between the eye pattern of the electric signal input to the laser element 2 and the eye pattern of the optical signal L1 actually output from the laser element 2, and the theoretical light emission in the output optical signal. The intensity and actual light emission intensity may be different. Further, depending on the amplitude of the electric signal input to the laser element 2, the theoretical level and the actual level of the output optical signal may be different. For example, when an amplitude indicating one level in an electric signal is input to the laser element 2, the optical signal L1 actually indicates two levels even though the optical signal L1 theoretically indicates one level. Sometimes. Therefore, when the drive adjustment method according to the present embodiment is not performed, there is a problem that optical multilevel transmission is not accurately performed. In addition, when APC control is performed on the laser element 2, the upper limit value of the light emission intensity of the optical signal L1 varies, and the amplitude of the electrical signal needs to be changed accordingly. For this reason, the control of the amplitude of the electric signal and the like becomes complicated, and the problem that the optical multilevel transmission is not accurately performed becomes remarkable.

  On the other hand, when the drive adjustment method according to the present embodiment is performed, the upper limit of the emission intensity of the laser element 2 is set to −6.5 dBm by the APC control as described above, and is input to the laser element 2. The amplitude range of the electric signal is determined to be −2.5 V to −0.5 V (see FIGS. 2A to 2C). In the present embodiment, as described above, the amplitude of each level of the electrical signal is set so that the light emission intensity difference between the levels of the optical signal L1 output from the laser element 2 becomes equal. By carrying out the drive adjustment method for setting the amplitude of each level of the electric signal after the APC control is performed in this way, the theoretical light emission intensity and the actual light emission intensity in the optical signal L1 output from the laser element 2 are Can be suppressed. Therefore, according to this embodiment, even when APC control is performed, the accuracy of optical multilevel transmission using the pulse amplitude modulation method can be improved.

  The optical transmission device and the drive adjustment method thereof according to the present invention are not limited to the above-described embodiments, and various other modifications are possible. For example, the upper limit value of the light emission intensity of the laser element 2 by the APC control is not limited to −6.5 dBm, and may be an arbitrary value. Even in this case, the memory 11, the control unit 12, and the amplitude control unit 13 may cause the electric signal so that the difference in light emission intensity between the levels of the optical signal L1 output from the laser element 2 becomes a predetermined ratio. The amplitude of each level is set. Note that the difference in light emission intensity between the levels of the optical signal L1 only needs to correspond to a predetermined ratio that is determined in advance, and is not necessarily equal.

  Moreover, in the said embodiment, although the light detection element 3 has detected the light emission intensity of the laser beam L2, you may detect the light emission intensity of the optical signal L1. In this case, the light detection element 3 may detect a part of the optical signal L1 using, for example, a half mirror.

  In the above embodiment, the PAM4 signal is used as the PAM signal. However, an 8-valued PAM8 signal may be used, or a 16-valued PAM16 signal may be used.

  The drive adjustment method in the above embodiment may not be performed again after the upper limit value of the light emission intensity of the laser element 2 is once determined. Further, when the upper limit value of the emission intensity of the laser element 2 is changed, the drive adjustment method may be performed again.

  DESCRIPTION OF SYMBOLS 1 ... Optical transmission apparatus, 2 ... Laser element, 3 ... Photodetection element, 4 ... Integrated circuit, 5 ... External terminal, 11 ... Memory, 12 ... Control part, 13 ... Amplitude control part, 14 ... Pulse amplitude modulation signal generation part , 15... Drive circuit section.

Claims (2)

An optical transmitter for optical multilevel transmission using a pulse amplitude modulation method,
A laser element;
A light detecting element for detecting the light emission intensity of the laser element;
A memory storing modulation characteristic data of the laser element;
A control unit for setting an upper limit value of the light emission intensity based on a detection result by the light detection element;
An amplitude range of an electric signal input to the laser element is determined based on the upper limit value and the modulation characteristic data, and light output from the laser element based on the modulation characteristic data corresponding to the amplitude range An amplitude controller that sets the amplitude of each level of the electrical signal so that the difference in emission intensity between each level of the signal is a predetermined ratio;
An optical transmission device comprising: A drive adjustment method of an optical transmission device for optical multilevel transmission using a pulse amplitude modulation method,
Detecting the emission intensity of the laser element;
Setting an upper limit of the emission intensity based on the detection result of the emission intensity;
Determining an amplitude range of an electric signal input to the laser element based on the upper limit value and modulation characteristic data of the laser element;
Based on the modulation characteristic data corresponding to the amplitude range, the amplitude of each level of the electrical signal is set so that the difference in emission intensity between the levels of the optical signal output from the laser element becomes a predetermined ratio And steps to
A drive adjustment method comprising:

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