JP2003149613A - Optical modulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical modulation circuit capable of high quality signal transmission by compensating and reducing the influence of nonlinearity of an optical element being an external load of the optical modulation circuit and the influence of a nonlinear noise such as distortion that occurs in an optical modulation system. SOLUTION: This optical modulation circuit is provided with a duty rate adjustment circuit capable of adjusting the duty rate of an output signal, and is constituted so that it adjusts the duty rate by measuring a monitor of signal waveform modulated by the optical element or an error rate of a transmission signal, and feeding back the measured results to the duty rate adjustment circuit so as to be in the best state. Also, it is constituted so that the duty rate of the signal waveform is adjusted by inputting a single-phase signal to one input of a differential amplifier constituting an inputting part of the duty rate adjustment circuit for duty rate adjustment, applying the fed back signal to the other input, and supplying an output of the differential amplifier to a limiter amplifier.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has an optical load such as a laser light source such as a semiconductor laser diode, an electro-absorption type optical modulator, a lithium niobate optical modulator, and a light emitting diode as an external load. The present invention relates to an optical modulation circuit for driving these optical elements by inputting, and performing light intensity modulation thereby.

[0002]

2. Description of the Related Art A general example of a conventional optical modulation circuit is shown in FIG. In FIG. 4, the oscillation laser light output from the semiconductor laser diode (LD) 1 driven by the semiconductor laser diode drive circuit (LDDRV) 8 is intensity-modulated by the electroabsorption optical modulator (EA) 2 and the electric field The drive pulse voltage of the absorption type optical modulator (EA) 2 is supplied from the electroabsorption type optical modulator driving circuit (EADRV) 3.
The electro-absorption optical modulator (EA) 2 utilizes the fact that the absorption rate of light changes according to the applied voltage, and the continuous oscillation light of the semiconductor laser diode (LD) 1 is generated by the electro-absorption optical modulator (EA). ) Optically coupled so that it passes through 2,
Light intensity modulation is performed by changing the voltage applied to the electro-absorption optical modulator (EA) 2.

In the electroabsorption optical modulator drive circuit (EADRV) 3 shown in FIG. 4, a positive phase input signal terminal (DAT) is used.
A IN) 4 and the negative phase input signal terminal (DATA I
The input signal applied to NC) 4'is a buffer amplifier (DBU).
The FF) 5 amplifies the voltage pulse to have a predetermined amplitude, and the drive output circuit (DRV) 6 supplies a drive pulse current as a modulation signal to the electro-absorption optical modulator (EA) 2. The drive output circuit (DRV) 6 has a configuration of a differential amplifier composed of two transistors Q41 and Q42 which are emitter-coupled, and the emitter passes through a constant current circuit (CS41) 17 and a low potential side power source (Vee). 12
It is connected to the. The collectors of these transistors are connected to the high-potential side power source (Vcc) 11 via resistors R41 and R42, respectively. The output is taken from the collector of one transistor, and the collector of the other transistor is terminated with respect to the ground potential by a load resistor (RL) 16.

By the way, an optical element generally has a nonlinear input / output characteristic. For example, the reverse applied voltage emission output characteristic of the electro-absorption optical modulator has a nonlinear input / output characteristic as shown in FIG. 5A. Therefore, if the waveform shown in FIG. 5 by passing through the vessel
The waveform shown in (C) is obtained as an output signal that is laser light intensity modulated, and this output waveform is different from the waveform of the input signal applied to the input terminals 4 and 4'including the duty ratio. Will be. Alternatively, in the so-called direct modulation method in which the oscillation light is intensity-modulated by directly changing the current applied to the semiconductor laser diode to give a modulation signal, the time from the application of the current to the generation of carrier electrons in the laser is On the contrary, since there is a delay phenomenon in which the time from the reduction of the applied current to the disappearance of carrier electrons in the laser is different, there is a nonlinear phenomenon between the applied modulation signal and the intensity-modulated output laser light. Had a problem having a response. Therefore, in these cases, for example, even if an input signal with a duty ratio of 50% is given from the optical modulation circuit, the duty ratio of the optical signal output deviates from 50%. This problem has been a significant problem particularly when performing high-speed light modulation.

[0005]

As described above, in the conventional optical modulation method, there is nonlinearity between the input signal waveform and the output optical signal waveform, and the resulting waveform distortion is jitter or the like during signal transmission. A time axis error occurs, which is one of the causes of code error during signal transmission. An object of the present invention is to compensate for and reduce the influence of non-linearity of an optical element which is an external load of the optical modulation circuit and the influence of non-linear noise such as distortion generated by the optical modulator circuit and the transmission device at the stage before the optical modulator circuit. It is to provide an optical modulation circuit that enables the above.

[0006]

In order to achieve the above object, the present invention discloses the following means. That is, in claim 1, the optical modulation system is, for example, a direct modulation system in which a modulation signal is input to a light source drive circuit such as a semiconductor laser, and the external input signal to be the modulation signal is a buffer amplifier. After that, the duty ratio of the signal waveform is controlled by the duty ratio adjusting circuit, and is applied to the above-mentioned light source drive circuit via the drive output circuit for driving the light source. The light thus modulated is monitored at either the transmitting side or the receiving side. When monitoring on the transmit side, a portion of the intensity-modulated light is monitored by a photodetector or other portion of the transmission system (eg, where the transmitter output is coupled to the transmission line). When monitoring at the receiving side, it is monitored by measuring the code error rate of the received waveform or signal transmitted. A control signal of the duty ratio adjusting circuit is generated based on the result of monitoring by any of these, and the generated control signal is fed back to the duty ratio adjusting circuit to adjust the duty ratio of the signal waveform. ing.

According to a second aspect of the present invention, a light source, such as a semiconductor laser diode, which continues to emit light by a light source drive circuit,
This is a case where the output laser light from the laser light source is composed of an optical element optically coupled to modulate the intensity. In the optical modulation circuit for driving this optical element,
The input signal from the outside, which is the modulation signal of the optical element, is applied to the above optical element through the buffer amplifier, the duty ratio of the signal waveform is controlled by the duty ratio adjustment circuit, and the drive output circuit that drives the optical element. It is configured to be. The light thus modulated is monitored at either the transmitting side or the receiving side. When monitoring on the transmit side, a portion of the intensity-modulated light is monitored by a photodetector or other portion of the transmission system (eg, where the transmitter output is coupled to the transmission line). When monitoring at the receiving side, it is monitored by measuring the code error rate of the received waveform or signal transmitted. A control signal of the duty ratio adjusting circuit is generated based on the result of monitoring by any of these, and the generated control signal is fed back to the duty ratio adjusting circuit to adjust the duty ratio of the signal waveform. ing.

According to a third aspect of the present invention, the duty ratio adjusting circuit for adjusting the duty ratio is a differential amplifier which is emitter-coupled and has a constant current source arranged between the emitter coupling section and the low potential side power source. It has an input section and a limiter amplifier arranged on the output side. That is, at the common node, the respective emitter terminals are used as first electrode terminals, and these are connected between the first transistor and the second transistor commonly connected to each other and the common node and the lowest potential. A differential amplifier having a constant current power supply is arranged on the input side. Here, the second electrode terminal of the first transistor corresponds to the base electrode to which an input signal is input and is connected to the signal input terminal, and the second electrode terminal of the second transistor is connected to the base electrode which is the second electrode terminal. Is fed back with a control signal based on the above monitoring result, and the control signal is to superimpose the control signal on a reference potential centered on an intermediate potential between the high level and the low level of the input signal. Is gained by. The third electrode terminals of the first and second transistors correspond to the collector terminals, are connected to the subsequent limiter amplifier input terminals, and the limiter amplifier limits the output amplitude to adjust the duty ratio. The signal whose duty ratio has been adjusted is supplied to the optical modulator via the subsequent optical modulation circuit. The above is a description of the case where the differential amplifier is composed of bipolar transistors. However, when the field effect transistor is used, the first electrode terminal is the source electrode, the second electrode terminal is the gate electrode, and the second electrode terminal is the gate electrode. The three electrodes correspond to the drain electrodes, respectively.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. (Embodiment 1) FIG. 1 is a block diagram showing an example of the basic configuration of an optical modulation circuit according to the present invention. In FIG. 1, an electroabsorption optical modulator driving circuit (EADRV) 3 is a buffer amplifier (DBUFF) 5, a duty ratio adjusting circuit (DU).
TY) 7 and drive output circuit (DRV) 6. The output of the drive output circuit (DRV) 6 drives an electroabsorption type optical modulator (EA) 2 which is optically coupled to a semiconductor laser (LD) 1 driven by a semiconductor laser drive circuit (LDDRV) 8. . That is, according to the present invention, the buffer amplifier (DBUFF) 5 and the drive output circuit (DR) are different from the conventional drive circuit shown in FIG.
V) 6 and a duty ratio adjusting circuit (DUTY) 7 is inserted, and the optical output signal waveform of the electro-absorption optical modulator (EA) 2 driven by the drive output circuit (DRV) 6 is monitored. Observation such as measurement of the transmission waveform or the code error rate after optical transmission is performed, and after processing the observation result, it is fed back to the duty ratio adjustment circuit (DUTY) 7 and modulated by the electro-absorption optical modulator (EA) 2. By adjusting and controlling the duty ratio of the optical output signal waveform so that the signal transmission is in the best state, the electro-absorption optical modulator (EA) 2
It is possible to compensate for the non-linearity of. here,
As the feedback signal, as in the example shown in FIG. 1, the modulated optical output from the optical modulator is detected by the photodetector element (S) for monitoring.
E) A method of detecting by 30 and processing this detection output by a control signal processing circuit (PROC) 31, a signal at another place in the transmitter system (for example, a coupling portion between a transmission line and a transmitter). Method or the error rate of the signal transmitted on the receiving side is measured, this error rate is converted into a voltage value and fed back to the duty ratio adjusting circuit (DUTY) 7 to minimize the error rate. As described above, there are various methods such as a method of controlling the potential of the base terminal 22.

The structure having the light source and the modulator for intensity-modulating the light emitted from the light source has been described above. However, in the case of the direct modulation method for modulating the driving current or driving voltage of the light source itself, the driving is performed. The output of the output circuit (DRV) 6 is supplied to the light source drive circuit (LDDRV) 8. (Second Embodiment) FIG. 2 shows a circuit configuration of the second embodiment. The second embodiment clearly shows the configuration of the duty ratio adjusting circuit (DUTY) 7 in the first embodiment, and the duty ratio adjusting circuit (DUTY) 7 includes a differential amplifier and a limiter amplifier (which constitute an input section). LIM) 10. FIG. 2 shows the duty ratio adjusting circuit (DUTY) described above.
Electroabsorption optical modulator drive circuit (EADRV) 3 including 7
1 shows a circuit configuration of a single-phase signal input terminal (DAT
The signal input from A IN) 9 is applied to a buffer amplifier (DBUFF) 5 having a single-phase input / output terminal. The single-phase output signal thus obtained is input to the duty ratio adjusting circuit (DUTY) 7. Duty ratio adjustment circuit (D
The input part of UTY) 7 is a differential amplifier composed of transistors Q21 and Q22, the emitters of these two transistors are coupled to each other, and these emitters are connected to the low potential side via constant current source (CS21) 13. It is connected to the. The output part of the duty ratio adjusting circuit (DUTY) 7 is a limiter amplifier (LIM) 10.

Here, the base terminal 21 of one of the differential pair transistors (Q21 in FIG. 2) of this differential amplifier.
The output of the buffer amplifier (DBUFF) 5 is input to the base terminal 2 of the other transistor (Q22 in FIG. 2).
A control voltage (CV) 32 for adjusting the duty ratio is applied to 2. As the control voltage (CV) 32, a feedback voltage obtained by using any one of the various methods described in (Example 1) for a reference potential centered on the intermediate potential between the high level and the low level of the input signal. A signal that is changed from the above reference potential is used by superimposing.
A bias voltage is applied to the output waveforms from the collector terminals of the differential pair transistors Q21 and Q22. The output given this bias voltage is used as a limiter amplifier (LIM) in the subsequent stage.
The limiter amplifier (LI) is input to the input terminals 23 and 24 of the amplifier 10 to limit the output amplitude.
The output waveform of M) 10 may be one in which the duty ratio is changed as compared with the input signal applied to the input terminal (DATA IN) 9. This limiter amplifier (LIM)
The output of 10 is input to the drive output circuit (DRV) 6, amplified, and then supplied to the electro-absorption optical modulator (EA) 2. The output terminal on the opposite phase side of the drive output circuit (DRV) 6 is terminated by the load resistor (RL) 16 as in the case of FIG.

FIG. 3 illustrates the operating principle of the second embodiment shown in FIG. That is, the input terminal (DATA
IN) 9 is applied with an input signal having the waveform shown in FIG. 3A, the duty ratio adjusting circuit (DUTY) 7
Of the transistor Q21 of the differential amplifier that constitutes the input circuit of the buffer amplifier (D
The same rectangular pulse signal as the input signal is input as the output of the BUFF) 5. Further, a reference potential is applied to the base terminal 22 of the other transistor Q22, and this reference potential is applied to FIG.
When changes are made as shown in (a), (b), and (c) in (A), the difference between the input signals at the base terminals 21 and 22 of the two transistors is amplified, so that the output signal of the differential amplifier is increased. At the input terminals 23 and 24 of the limiter amplifier (LIM) 10, the average potential of the output waveform of the differential amplifier rises and falls according to the reference potential as shown in FIG. 3 (B).

When this output signal is further applied to the limiter amplifier circuit (LIM) 10 at the subsequent stage and the output amplitude of the differential amplifier shown in FIG. 3 (B) is limited, the output of the limiter amplifier circuit (LIM) 10 is output. As for the signal, a waveform with a changed duty ratio is obtained as shown in FIG. This output is input to the drive output circuit (DRV) 6 at the subsequent stage, and the drive output circuit (DR) having a drive voltage waveform with a desired duty ratio is supplied.
The output signal of V) 6 can be supplied to the electro-absorption optical modulator EA.

A result of monitor reception of the oscillation output light of the semiconductor laser diode (LD) 1 optically modulated by the electro-absorption optical modulator (EA) 2 and demodulation, or a measurement result of a reception error rate after optical transmission. Is fed back to the duty ratio adjusting circuit (DUTY) 7 and the transistor Q2
By controlling the reference potential applied to the base terminal 22 of No. 2, the duty ratio of the drive voltage signal supplied to the electro-absorption optical modulator (EA) 2 can be adjusted and compensated so as to have a desired optical output waveform. it can.

In the above embodiments, the case where the bipolar transistor is used as the active element has been described, but this is also applicable to the field effect transistor, and the present embodiment also applies to the light modulation element. Although the electroabsorption optical modulator has been described in the form, it goes without saying that this is also applicable to other types of optical modulators such as an electro-optical type optical modulator (EO modulator).
Further, as the light source, various laser light sources such as a semiconductor laser and a gas laser can be used.

[0016]

As described above, in the optical modulation circuit according to the present invention, 1. A duty ratio adjusting circuit for controlling the duty ratio of the output signal is provided inside the optical modulation circuit,
2. The duty ratio adjusting circuit has a simple configuration by combining a single-phase input of a differential amplifier and a limiter amplifier, and has a simple circuit configuration for controlling the duty ratio by a simple operation of controlling the reference potential. It became possible to do in.

[Brief description of drawings]

FIG. 1 is a circuit block diagram according to a first embodiment.

FIG. 2 is a circuit diagram according to a second embodiment.

FIG. 3 is a signal waveform diagram at each terminal of the duty ratio adjusting circuit in the second embodiment, where (A) is an input signal waveform and (B) is a signal waveform diagram.
Is the output waveform of the differential amplifier, and (C) is the output waveform of the limiter amplifier.

FIG. 4 is a circuit block diagram of an electroabsorption type optical modulator drive circuit showing an example of a conventional optical modulation circuit.

5A and 5B show input / output characteristics of the electro-absorption optical modulator, where FIG. 5A is an optical output characteristic diagram with respect to a reverse applied voltage emission output characteristic, FIG. 5B is an input signal waveform diagram, and FIG. Signal waveform diagram.

[Explanation of symbols]

1: Semiconductor laser diode (LD) 2: Electroabsorption type optical modulator (EA) 3: Electroabsorption optical modulator drive circuit (EADRV) 4: Positive phase signal input terminal (DATA IN) 4 ': Reverse-phase signal input terminal (DATA INC) 5: Buffer amplifier (DBUFF) 6: Drive output circuit (DRV) 7: Duty ratio adjustment circuit (DUTY) 8: Semiconductor laser diode drive circuit (LDDRV) 9: Input terminal (DATA IN) 10: Limiter amplifier (LIM) 11: High potential side power supply (Vcc) 12: Low-potential power supply (Vee) 13, 17: constant current source (CS41) 16: Load resistance (RL) 21, 22: Base terminal of transistor 23, 24: Limiter amplifier input terminals 30: Photodetector (SE) 31: Control signal processing circuit (PROC) 32: Control voltage (CE)

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Makoto Nakamura             12-12 Dogenzaka, Shibuya-ku, Tokyo N             Titi Electronics Co., Ltd. F-term (reference) 2H079 AA02 BA01 BA03 CA05 DA02                       DA16 FA01 FA02 FA03                 5F041 BB03 BB10 BB13 FF14                 5F073 BA01 EA12 EA29 GA12 GA38                 5J001 AA01 BB00 BB25 CC00 DD06                 5J050 AA01 BB20 CC12 DD03 EE02                       EE17 EE28 EE31 FF06 FF08

Claims (3)

[Claims] 1. A light source and a light source driving circuit for directly modulating the light emission intensity of the light source, a buffer amplifier for an input signal to be a modulation signal, and a duty ratio adjusting circuit for adjusting a duty ratio of an output signal waveform of the buffer amplifier. A drive output circuit that drives the light source drive circuit according to the output of the duty ratio adjustment circuit, and monitors the modulated light of the light source at either the transmission side or the reception side, and responds to this monitoring result. And a signal for controlling the duty ratio adjusting circuit is fed back to the duty ratio adjusting circuit. 2. An optical element for intensity-modulating output light from a light source, a drive circuit therefor, a buffer amplifier for an input signal to be a modulation signal, and a duty ratio adjusting circuit for adjusting a duty ratio of an output signal waveform of the buffer amplifier. And a drive output circuit that drives the optical element according to the output of the duty ratio adjusting circuit, and monitors the light modulated through the optical element on either the transmitting side or the receiving side, An optical modulation circuit, wherein a signal for controlling the duty ratio adjusting circuit is fed back to the duty ratio adjusting circuit according to the monitoring result. 3. A duty ratio adjusting circuit for adjusting the duty ratio comprises: a first transistor and a second transistor whose first electrode terminals are commonly connected to each other at a common node; and the common node and a low transistor. A differential amplifier having a constant current power supply connected between the power supply on the potential side, and a second electrode terminal of the first transistor is connected to a signal input terminal to which an input signal is input, A control signal based on the monitoring result is input to the second electrode terminal of the second transistor, and the control signal is a reference centered on an intermediate potential between the high level and the low level of the input signal. The third electrode terminals of the first and second transistors, which are superposed on the potential, are connected to a limiter amplifier, and the output amplitude is limited by the limiter amplifier, and the subsequent drive output is limited. Optical modulation circuit according to claim 1 or claim 2, wherein it is a configuration that is connected to the circuit.