JP2000201109A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the drive current of a light emitting element with high accuracy via an APC operation even when the burst transmission is carried out by prividing a light emitting output control which generates a control signal based on the light receiving signal of a monitoring photodetector. SOLUTION: A transmitting light emitting element drive circuit 4 makes every transmitting laser diode 1 emit light according to the data signal that is inputted to a modulation circuit 2. A monitoring light emitting element drive circuit 4a makes a monitoring laser diode 1a emit light according to the clock signal that is inputted to a modulation circuit 2a. A light emitting output control (APC) circuit 6 generates a modulation current control signal and a bias current control signal based on the light receiving signal of a photodiode 5 serving as a monitoring photodetector so as to set the mean value of light emitting output of the diode 1a at a prescribed level. Then the modulation current control signal is supplied to the circuits 2 and 2a and the bias current control signal is supplied to the bias circuits 3 and 3a, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for transmitting a plurality of data in parallel via an optical fiber, and more particularly to an optical transmitter suitable for transmitting a plurality of data in a burst.

[0002]

2. Description of the Related Art An optical transmitter converts input electric signal data into a change in light emission intensity of a light emitting element, and transmits light generated by the light emitting element through an optical fiber. Here, it is known that the optical output of a semiconductor laser which is one of the light emitting elements fluctuates due to a change in ambient temperature, a change in the temperature of the semiconductor laser, or the like. Therefore, in order to obtain a constant optical output even if a temperature change occurs, the optical transmitter detects the optical output of the semiconductor laser with a light receiving element for monitoring, for example, a photodiode, and based on the detection signal, the semiconductor laser. In many cases, an automatic output control (hereinafter, sometimes referred to as APC) circuit for controlling the drive current (bias current and modulation current) is provided.

In recent years, there has been a strong demand for optically transmitting a plurality of data in parallel. In such parallel optical transmission, the data signal line is often used as a bus line, and therefore, burst transmission (transmission in which the mark ratio of data is undefined) is often performed.

[0004] However, a method of controlling the light output by detecting the average value of the light output, which is a general APC method, cannot cope with such burst transmission. There is also a method called a peak-bottom detection type APC in which a high level and a low level of an optical output are peak-held and an optical output is controlled.
It is difficult to realize an operation at a speed of Gbps or higher and cannot cope with data having a long low level. Therefore, in general, the temperature characteristics of the semiconductor laser are compensated by detecting the temperature of the semiconductor laser using a thermistor or the like.

[0005]

However, simply detecting the temperature of the semiconductor laser and controlling the light emission output of the semiconductor laser cannot accurately reproduce the temperature characteristics of the semiconductor laser. Value current,
The bias current and the modulation current cannot be controlled accurately with respect to the change in the luminous efficiency. Therefore, the following problem occurs. (1) The optical output of a semiconductor laser fluctuates with temperature. The fluctuation of the optical output causes the rise skew and the fall skew to deteriorate when DC detection of the optical signal is performed. (2) The emission delay time of the semiconductor laser varies with temperature. As a result, the temperature dependence of the output skew occurs, and D-
The timing margin when retiming is performed by the FF is reduced, and high-speed operation becomes difficult. (3) In order to realize a high-speed operation, it is necessary to control the bias current near the threshold current. However, it is difficult to achieve this at the same time as securing the extinction ratio of the semiconductor laser, and the extinction ratio may decrease. In this case, the noise margin is reduced, and the reception sensitivity is reduced.

An object of the present invention is to provide an optical transmitter capable of controlling a driving current of a light emitting element with high accuracy by an APC operation even when performing burst transmission.

[0007]

To achieve the above object, the present invention provides an optical transmitter for transmitting a plurality of data in parallel via an optical fiber, comprising: a plurality of transmission light emitting elements; Light emitting element driving circuits for causing the light emitting elements to emit light in accordance with the corresponding data signals, a monitor light emitting element provided in the vicinity of the plurality of light emitting elements, and a monitor for emitting the monitor light emitting element Light-emitting element drive circuit, a monitor light-receiving element for receiving light generated by the monitor light-emitting element, and a control signal for controlling the light-emitting output of the monitor light-emitting element to a predetermined value based on the light-receiving signal of the monitor light-receiving element. And a light emission output control circuit for outputting the control signal to a plurality of light emitting element driving circuits for transmission and a plurality of light emitting element driving circuits for monitoring.

In the present invention configured as described above,
The light generated by the monitor light emitting element is received by the monitor light receiving element, and a control signal generated based on this light receiving signal is output to the monitor light emitting element drive circuit to set the light emission output of the monitor light emitting element to a predetermined value. At the same time, the control signal is also output to a plurality of transmission light emitting element drive circuits to adjust the light emission output of the plurality of transmission light emitting elements. Therefore, even in the case of performing the burst transmission by the parallel link, the driving current of the plurality of light emitting elements can be controlled with high accuracy by the APC operation.
The light output of the plurality of light emitting elements can be made substantially constant.

In the above optical transmitter, preferably, the plurality of transmission light emitting elements and the monitoring light emitting element are integrated as one light emitting element array. Thereby, the temperature characteristics of the plurality of transmission light emitting elements and the monitoring light emitting elements become uniform. Further, these light emitting elements can be easily mounted on the chip.

Preferably, the monitor light emitting element driving circuit causes the monitor light emitting element to emit light in accordance with a clock signal extracted and reproduced from an externally input clock signal or data signal. Thereby, noise resistance is secured, and the APC operation by detecting the average value of the light output of the light emitting element can be performed efficiently.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram illustrating a configuration of the optical transmitter according to the present embodiment. In the figure, the optical transmitter 100
Is for transmitting a plurality of (n) data in parallel via an optical fiber (not shown).

The optical transmitter 100 includes a laser diode array 11 composed of n laser diodes 1 as transmission light emitting elements and a laser diode 1a as one monitor light emitting element, a modulation circuit 2 and a bias circuit 3. A plurality of transmission light-emitting element driving circuits 4 for causing each transmission laser diode 1 to emit light in accordance with a data signal input to the modulation circuit 2, a modulation circuit 2a and a bias circuit 3a
And a monitor light emitting element drive circuit 4a for emitting a monitor laser diode 1a in response to a clock signal input to the modulation circuit 2a; and a laser diode 1a provided near the monitor laser diode 1a. 5 serving as a monitor light receiving element for receiving the light generated by the light emitting device, and a light emission output control for controlling each transmission light emitting element driving circuit 4 and the monitoring light emitting element driving circuit 4a based on the light receiving signal of the photodiode 5. Circuit (APC
Circuit 6).

Here, the laser diode array 11
A tape fiber in which n optical fibers (described above) are bundled is connected via an optical connector. Then, light is emitted from the transmission laser diode 1 toward the end face of the optical fiber. The photodiode 5 monitors the back light leaking from the back end of the monitor laser diode 1a (the side coated with the high-reflectance reflective film).

FIG. 2 is a configuration diagram showing the details of the transmission light emitting element drive circuit 4. As shown in FIG. In FIG. 1, a modulation circuit 2 has field effect transistors (hereinafter, referred to as FETs) 21 and 22 forming a differential pair, and a data signal is input to the gates of the FETs 21 and 22. Note that the inverted gate of the data signal input to the FET 21 is input to the gate of the FET 22. The laser diode 1 is connected between the drain of the FET 22 and the power supply voltage terminal Vcc. An FET 23 is connected between the common source of the FETs 21 and 22 and the ground terminal GND.
A modulation current control signal (voltage signal) described later from the APC circuit 6 is applied to the gate of T23, and a modulation current Im corresponding to the signal is set.

The bias circuit 3 has an FET 31 connected between the drain of the FET 22 and the ground terminal GND. A bias current control signal (voltage signal) described later from the APC circuit 6 is applied to the gate of the FET 31. , A bias current Ib corresponding to the signal is set.

In such a transmission light emitting element driving circuit 4, the FETs 21 and 22 are changed according to the change of the input data signal.
Perform a switching operation, and when the FET 22 is on, the drive current I flowing through the laser diode 1 is the sum of the modulation current Im and the bias current Ib, and the laser diode 1 emits light with a light intensity corresponding to the drive current I. Also, FE
When T22 is off, the drive current I flowing through the laser diode 1 becomes the bias current Ib, and the laser diode 1 is extinguished.

Although not shown, the monitor light-emitting element driving circuit 4a is composed of the same modulation circuit 2a as the modulation circuit 2 and the same bias circuit 3a as the bias circuit 3, and the FETs 21 and 22 of the modulation circuit 2a include: For example, 100 kHz
Is input. Here, the reason why the clock signal is input is to ensure the noise resistance and to facilitate the APC operation by detecting the average value of the optical output of the laser diode 1a. As the clock signal, an external input clock may be used,
A clock extracted and reproduced from any one of the n data signals may be used.

The APC circuit 6 converts the light receiving signal (current signal) of the photodiode 5 into a voltage signal, and modulates the modulation current control signal and the bias current so that the average value of the light emission output of the monitoring laser diode 1a becomes a predetermined value. A control signal is generated, and a modulation current control signal is supplied to each of the modulation circuits 2 and 2a.
And 3a to control the light intensity of each transmission laser diode 1 and monitor laser diode 1a. Details of the light output control operation by the APC circuit 6 will be described below.

FIG. 3 is a diagram showing a driving current-light output characteristic of the laser diode 1a used for light output control by the APC circuit 6. In FIG. In the figure, the dotted line shows the characteristic at normal temperature, and the solid line shows the characteristic when the temperature rises from that state by a predetermined temperature. As can be seen from this characteristic diagram, the luminous efficiency decreases as the ambient temperature increases. Therefore, in order to keep the light output P of the laser diode 1a constant, it is necessary to increase the drive current I of the laser diode 1a.

For example, in order to set the light output to a value having a low level and a high level like Pc in the figure (the light output average value is P0), the bias current Ib is set to this value at a normal temperature as shown by a dotted line Q. It is necessary to set the characteristic threshold current to Ib1 and the modulation current Im to Im1. On the other hand, at the time of high temperature, as shown by the solid line R, it is necessary to set the bias current Ib to Ib2 which is the threshold current of this characteristic and the modulation current Im to Im2.

The APC circuit 6 detects the light emission intensity of the monitoring laser diode 1a from the light receiving signal of the photodiode 5, and based on the detection result, the monitoring light emitting element 1a determines a predetermined light emission element 1a by using the driving current-light output characteristic. The monitor light-emitting element drive circuit 4a and each transmission light-emitting element drive circuit 4 are controlled so as to have a value. That is, when the light emission intensity of the monitoring laser diode 1a is reduced due to a change in the ambient temperature or the like, it is determined that the light emission intensity of each transmission laser diode 1 is also reduced, and the modulation current control for increasing the modulation current Im is performed. A signal is output to each of the modulation circuits 2 and 2a, and a bias current control signal for increasing the bias current Ib is output to each of the bias circuits 3 and 3a. On the other hand, when the emission intensity of the monitoring laser diode 1a increases, it is determined that the emission intensity of each transmission laser diode 1 has also increased due to a change in ambient temperature or the like, and the modulation current I
The modulation current control signal for decreasing m
And 2a, and a bias current control signal for reducing the bias current Ib.
And 3a.

As described above, in the present embodiment, the monitoring laser diode 1a is provided separately from the transmission laser diode 1, and the transmission laser diode 1a is detected by detecting the emission intensity of the monitoring laser diode 1a. 1, the APC operation is performed by grasping the change in the light emission intensity. Therefore, even when performing burst transmission in which data having a long low level is supplied to each transmission laser diode 1, each transmission Current I of laser diode 1
m and the bias current Ib can be accurately controlled. Therefore, the problem that occurs when the APC operation is not performed can be solved.

That is, since the fluctuation of the light output of each laser diode 1 due to the temperature change is reduced, the light output is changed to DC.
Rise skew and fall skew that occur when detecting are reduced. Further, since the fluctuation of the light emission delay time of each laser diode 1 due to the temperature change is also reduced, the temperature dependency of the output skew is reduced, the timing margin when retiming is performed by the D-FF is increased, and the high-speed operation is performed. Will be able to do it. Furthermore, the laser diode 1
Since the extinction ratio can be increased, the noise margin increases and the receiving sensitivity improves.

Further, since the n transmission laser diodes 1 and the monitoring laser diode 1a are integrated as one laser diode array 11, the temperature characteristics of the laser diodes 1 and 1a become uniform, thereby The light output of each laser diode 1 can be accurately controlled. Further, the laser diodes 1 and 1a can be easily mounted on the chip.

A second embodiment of the present invention will be described with reference to FIG. In the drawing, the same or equivalent members as those of the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 4 is a block diagram showing the optical transmitter according to the present embodiment. In the figure, an optical transmitter 100A has n
A laser diode array 11A including a plurality of transmission laser diodes 1 is provided, and a monitoring laser diode 1a is arranged at a position adjacent to the laser diode array 11A. As described above, by placing the monitoring laser diode 1a adjacent to the laser diode array 11A, the temperature characteristic of the monitoring laser diode 1a and the temperature characteristic of the transmission laser diode 1 become almost the same. At this time, the laser diode array 11A
It is preferable that the laser diode and the monitor laser diode 1a are thermally coupled to each other so as to be at the same temperature regardless of the environmental temperature. The monitoring laser diode 1a is arranged so that the output light is emitted in the direction opposite to the fiber end face, and is directly incident on the photodiode 5. Other configurations are the same as those of the first embodiment.

In this embodiment, similarly to the first embodiment, even when burst transmission is performed, the drive current I of each transmission laser diode 1 can be accurately controlled by the APC operation.

In this embodiment, the transmission laser diode 1 and the monitoring laser diode 1a are separately mounted.
The light generated by the monitor laser diode 1a in the laser diode array 11 as in the first embodiment can be directly incident on the photodiode 5. No need.

In the embodiment described above,
Although the modulation circuit 2a modulates according to the clock signal, the modulation is not particularly limited to the clock signal, and may be performed according to predetermined dummy data or the like. In addition, a constant current may be supplied to the monitoring laser diode 1a. In this case, the FET 2 of the modulation circuit 2a
3 is set to one of the gate width of the FET 23 of the modulation circuit 2.
By setting / 2, it is possible to make the consumption currents of the monitor laser diode 1a and the transmission dither diode 1 substantially the same.

[0031]

According to the present invention, even when burst transmission is performed by a parallel link, the driving current of a plurality of light emitting elements can be controlled with high accuracy by the APC operation. This makes it possible to reduce the fluctuation of the light emission intensity of each light emitting element and the fluctuation of the light emission delay time of each light emitting element due to the temperature change.

[Brief description of the drawings]

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

FIG. 2 is a configuration diagram showing details of a transmission laser diode drive circuit shown in FIG. 1;

FIG. 3 is a diagram showing a drive current-light output characteristic of a monitor laser diode used for light output control by the APC circuit shown in FIG. 1;

FIG. 4 is a configuration diagram illustrating an optical transmitter according to a second embodiment of the present invention.

[Explanation of symbols]

1 ... transmission laser diode (transmission light emitting element), 1a
… Monitoring laser diode (light emitting element for monitoring),
2, 2a: modulation circuit, 3, 3a: bias circuit, 4: light-emitting element driving circuit for transmission, 4a: light-emitting element driving circuit for monitoring, 5: photodiode (light-receiving element for monitoring), 6 ...
APC circuit (optical output control circuit), 11, 11A: laser diode array, 100, 100A: optical transmitter.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04J 14/08

Claims (3)

[Claims] 1. An optical transmitter for transmitting a plurality of data in parallel via an optical fiber, comprising: a plurality of transmission light-emitting elements; and a plurality of light-emitting elements for causing the plurality of transmission light-emitting elements to emit light in accordance with respective corresponding data signals. A light emitting element driving circuit for transmission, a light emitting element for monitoring provided near the light emitting element for transmission, a light emitting element driving circuit for emitting light from the light emitting element for monitoring, and a light emitting element generated by the light emitting element for monitoring. A monitoring light-receiving element for receiving light; and a control signal for generating an emission output of the monitor light-emitting element to a predetermined value based on a light-receiving signal of the monitoring light-receiving element. An optical transmitter, comprising: a light emitting element driving circuit for monitoring; and a light emitting output control circuit for outputting light to the monitoring light emitting element driving circuit. 2. The optical transmitter according to claim 1, wherein said plurality of transmission light emitting elements and said monitoring light emitting element are integrated as one light emitting element array. 3. The monitor light emitting element driving circuit according to claim 1, wherein the monitor light emitting element drive circuit emits light in accordance with an externally input clock signal or a clock signal extracted and reproduced from the data signal. 2. The optical transmitter according to 2.