JPH08279788A - Optical transmitter - Google Patents

Abstract

PURPOSE: To enable high-speed transmission of data by eliminating the unstable state of an optical output from a laser diode. CONSTITUTION: Concerning the optical transmitter provided with an automatic output control function for monitoring the optical output power of a laser diode 1 while using a photodiode 5 and for performing control based on this result so as to suppress fluctuation in the optical output of the laser diode 1, the state of optical output power during the operation of the laser diode 1 is detected by the photodiode 5. The optical output power state is digitized and stored in a FIFO memory 16. When an input signal 3 for driving the laser diode 1 is interrupted, the optical output power information stored in the FIFO memory 16 is read out and while using this information, APC control is executed for a fixed time after the input signal 3 is impressed again.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter equipped with means for performing optical transmission using a laser diode as a laser source and monitoring the state of its optical output power, and more particularly,
The present invention relates to an optical transmitter suitable when a light-receiving element having a high-speed response is used for a power monitor of optical output.

[0002]

2. Description of the Related Art FIG. 3 is a perspective view showing a structural example of a laser diode (LD). The laser diode includes an active layer 21 made of gallium arsenide (GaAs) or the like.
P-type and N-type clad layers (AlG
aAs: gallium arsenide / aluminum) 22, an N-type (GaAs) substrate 23 disposed on one surface (lower side of the figure) of the clad layer 22 and functioning as a reflection layer, and the other surface of the clad layer 22 (see the figure). It is composed of a P-type cap layer 24 provided on the upper side). When a voltage is applied between a pair of electrodes (not shown) provided on the upper surface and the lower surface of the structure of FIG. 3 and a current 25 is made to flow, laser light 26 is generated from the light emitting region of the active layer 21.

FIG. 4 shows the optical output power P of the laser diode.
The relationship between o and the laser drive current I _LD is shown, and when the threshold current I _th of the laser drive current I _LD is exceeded, the optical output sharply increases and a high output laser beam is output. This threshold current I _th fluctuates with respect to temperature as shown in FIG. 4 (characteristics due to temperatures T ₁ and T ₂ ), so that the ambient temperature fluctuates when a constant laser drive current I _LD is driven. Then, the optical output power of the laser diode changes greatly.

As a means for solving this inconvenience, conventionally, a light receiving element such as a photodiode (PD) for monitoring the light output is installed in front of or behind the laser light of the laser diode. FIG. 5 shows a conventional drive circuit for a laser diode. A laser drive current switching circuit 2 is connected to the laser diode 1, and an input signal 3
It is driven by (drive signal). The laser drive current switching circuit 2 has an automatic output control circuit (APC circuit: A
Utomatic Power Control) 4 is connected, and the automatic output control circuit 4 is connected with a photodiode 5 for monitoring the laser diode 1. Further, a shutdown circuit 6 is connected to the automatic output control circuit 4, and the shutdown circuit 6 operates by the input signal 3.

Explaining the operation of the circuit of FIG. 5, the laser diode 1 is always supplied with a current having a predetermined threshold value from the laser drive current switching circuit 2, and the switching current is superposed on the current.
Is turned on / off to generate a laser beam having a shape in which a rectangular wave is continuously generated. The optical output power fluctuates due to changes in the external environment such as temperature. This fluctuation is performed by constantly monitoring the optical output power by the photodiode 5 for monitoring the optical output power. Based on the detection signal 8 from the photodiode 5, the automatic output control circuit 4 generates an APC signal 9, and the APC signal 9 causes the threshold current of the laser drive current switching circuit 2 to fluctuate so that the optical output power becomes constant. Control to become.

Further, in consideration of optical transmission of a signal in which a state with data and a state with no data are repeated in a burst signal in a long cycle, the automatic output control circuit 4 determines that a state without an optical signal continues for a long time. Since the threshold current is increased in the direction of strengthening the optical signal, normally, when there is no data for a long time, the shutdown circuit 6 interrupts the operation of the automatic output control circuit 4 (that is, the threshold value is fixed. To do).

Regarding this kind of technology, for example,
There is "Laser diode control method and circuit" in Japanese Patent Application Laid-Open No. 5-129692, and when the automatic output control means for controlling the laser lighting power supply is stopped as necessary, the output value of the laser diode lighting power supply is stopped. A means for latching the value immediately before is provided to prevent the variation of the light emission output due to the malfunction of the APC.

In addition to this, Japanese Patent Laid-Open No. 5-129691 discloses a technique for speeding up the rise of a laser diode. Furthermore, even if variations occur in the light conversion element for monitoring due to ambient temperature, etc., the output of the light conversion element can be output based on the temperature characteristic information that is grasped in advance so that the light emission amount of the light emitting element can be accurately monitored. JP-A-6-90046 for correction
There is JP-A-6-132592 for easily and surely preventing an overcurrent of the semiconductor laser that occurs when the semiconductor laser returns from the disconnection state to the original conduction state.

Further, even if an abnormality such as breakage occurs in the components of the drive circuit of the laser diode, the laser diode is prevented from extremely increasing the light emission power.
Japanese Laid-Open Patent Publication No. 164036 discloses a method of shortening the time for which an excessive current flows through a light emitting element so that transmission can be carried out over a long distance with an average small current, and the degree of quantization efficiency of a semiconductor laser. And the light output can be stabilized regardless of the threshold value.
No. 5, etc. are available.

[0010]

However, in the above-mentioned prior art, when the state without data is changed to the state with data, the threshold current is set to the optimum value and the optical output power is stabilized. Although it takes a certain time to reach the stable state, several bits of data are sacrificed as shown in FIG. As described above, the time delay is caused by the response speed of the automatic output control circuit. Since the time until the light output stabilizes is constant, the faster the data, the more data that will be lost before the light output stabilizes. Therefore, when trying to transmit high-speed data in a format such as a burst signal, it becomes difficult with the conventional configuration.

The present invention eliminates the unstable state of the optical output,
It is an object of the present invention to provide an optical transmitter that enables high speed transmission of data.

[0012]

To achieve the above object, the present invention provides a laser diode, driving means for driving the laser diode based on a drive signal, and optical output power of the laser diode. In the optical transmission device including a light receiving element for controlling the output of the laser diode based on the monitor result, the light receiving element operates as the laser diode. Storage means for storing the detected optical output power value as optical output power information, and read the optical output power information stored in the storage means when the drive signal is cut off, and use this optical output power information. And a control means for operating the output control means until a fixed time after the drive signal is applied again. .

A FIFO memory can be used as the storage means.

[0014]

According to the above means, while the laser diode is being driven, the information of the optical output power detected by the light receiving element for monitoring is successively stored for a certain time by the storage means, and the drive signal (input signal) is obtained. When the optical output power information is read from the storage means, the APC is operated based on this information, and the drive signal is re-input, the optical output power by the laser diode becomes stable as in the normal operation. The status is output. As a result, the unstable state (unstable time) of the optical output power of the laser diode is eliminated, and the speed of optical signal transmission can be increased.

Further, since the FIFO memory used as the storage means rewrites certain data (information) in the input order,
The optical output power information immediately before the drive signal is always stored, the operation of the laser diode can be restarted by using this information, and the laser diode can be driven in a state where the drive conditions are approximate.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. 1 is a block diagram showing an embodiment of an optical transmitter according to the present invention. The optical transmitter 10 according to the present invention includes a comparator 11 to which the output voltage of the photodiode 5 (light receiving element) is applied, a comparator 12 that compares the output signal of the comparator 11 with a reference voltage 13, and an output signal of the shutdown circuit 6. A control circuit 14 (which forms control means together with the shutdown circuit 6) that operates based on the above, and an A / D converter 1 that converts an analog signal output from the comparator 11 into a digital signal.
5. FIFO (first-in / first-out) memory 16 (storing means) for storing digital digital output from the A / D converter 15, D / for converting digital data output from the FIFO memory 16 into an analog signal A converter 17, comparator 12
It is composed of a transistor 18 driven by an output signal (which constitutes the APC circuit portion together with the comparator 11) and a resistor 19 for changing the output current of the photodiode 5 into a voltage value.

The transistor 18 keeps the output of the comparator 12 until the voltage value set by the resistor 19 is reached.
A threshold current is made to flow from the collector to the emitter side, and the laser diode 1 operates as an APC that makes the optical output power constant. Further, the control circuit 14 instructs the FIFO memory 16 to read the output of the A / D converter 15 during signal transmission. Then, when the input signal 3 is interrupted, it is instructed to output the stored value. Also,
When the input signal 3 starts to be transmitted again, the FIFO memory 16 is instructed to continue outputting for a certain period of time, stop the output after that, and start reading.

Next, the operation of the embodiment having the above configuration will be described with reference to FIG. As shown in FIG. 2, the input signal 3 becomes a continuous rectangular wave when there is a signal and disappears when there is no signal. While the input signal 3 is being applied to the laser drive current switching circuit 2, its optical output power is detected by the photodiode 5, and based on this result, the optical transmitter 10 keeps the optical output power of the laser diode 1 constant. To work. In this process, the output voltage of the comparator 11 is the A / D converter 1
It is converted into a digital value in 5, and this digital voltage is
It is stored in the O memory 16.

Next, when the input signal 3 is interrupted, the shutdown circuit 6 operates and the application of the reference voltage 13 to the comparator 12 is cut off. Since one input signal of the comparator 12 becomes "0", no output voltage is generated in the comparator 12 and the transistor 18 stops operating.
Further, since the laser drive current switching circuit 2 becomes inoperative due to the disconnection of the input signal 3, the optical output power of the laser diode 1 becomes "0".

At the same time when the input signal 3 is stopped (interrupted), the shutdown circuit 6 stops operating and the control circuit 14 operates. The control circuit 14 issues a command to the FIFO memory 16 to start reading the voltage value (optical output power information) stored in the FIFO memory 16 as shown in FIG. In this state, when the application of the input signal 3 is restarted, the shutdown circuit 6 operates so as to apply the reference voltage 13 to the comparator 12, and the comparator 12 restarts the operation. As a result, the output of the FIFO memory 16 is converted into an analog voltage by the D / A converter 17, and then applied to the comparator 12 via the comparator 11, and an output voltage (pseudo monitor output) is generated from the comparator 12, and this The output voltage causes the transistor 18 to operate,
A threshold current is applied to the laser diode 1. That is, the transistor 18 operates using the output voltage of the comparator 12 as a base bias, and functions to flow a current according to the bias voltage to the laser diode 1. As a result, the laser diode 1 can generate the optical output power in a stable state from the beginning.

By the way, if the reading from the FIFO memory 16 is continued forever, the monitoring function of the photodiode 5 will not work. Therefore, the control circuit 14 waits for a certain time (for example, the time until the optical output power is stabilized in the prior art) from the time when the input signal 3 is generated again, and then the FIFO memory 16 is started.
Send the command to stop the output from. The FIFO memory 16 does not stop the writing from the A / D converter 15, but the writing operation is continuously performed.
At this point in time, since the photodiode 5 has output, even if the signal from the D / A converter 17 disappears, the comparator 11 still outputs, and the comparator 12 can be operated based on this output. Therefore, after that, the APC operation is continued until the signal is disconnected.

In the above embodiment, the light output power of the laser diode is monitored by using the photodiode, but the present invention is not limited to this, and another light receiving element is used instead of the photodiode. A device such as a phototransistor can also be used.

[0023]

As is apparent from the above, according to the present invention, the storage means for storing the detected optical output power value detected by the light receiving element during the operation of the laser diode as the optical output power information and the drive signal are disconnected. The optical output power information stored in the storage means is read by being struck, and the automatic output control means is operated until a certain time after the time when the drive signal is reapplied using the optical output power information. Since the configuration is provided, the unstable state (unstable time) of the optical output power of the laser diode is eliminated, and the signal transmission speed can be increased.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram showing operation waveforms of various parts of the embodiment of FIG.

FIG. 3 is a perspective view showing a structural example of a laser diode.

FIG. 4 is a characteristic diagram showing the relationship between the optical output power P _O of the laser diode and the laser drive current I _LD .

FIG. 5 is a block diagram showing a conventional drive circuit for a laser diode.

FIG. 6 is an explanatory diagram showing the occurrence of an unstable state of the optical output waveform after the burst signal is interrupted by the laser diode.

[Explanation of symbols]

 1 Laser Diode 2 Laser Drive Current Switching Circuit 3 Input Signal 5 Photodiode 6 Shutdown Circuit 10 Optical Transmitter 11, 12 Comparator 13 Reference Voltage 14 Control Circuit 15 A / D Converter 16 FIFO Memory 17 D / A Converter 18 Transistor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 10/26

Claims (2)

[Claims] 1. A laser diode, drive means for driving the laser diode based on a drive signal, a light receiving element for monitoring the optical output power of the laser diode, and a laser diode for the laser diode based on the monitor result. In an optical transmitter including an output control unit that performs control for suppressing fluctuations in optical output, the optical output power detection value detected by the light receiving element during operation of the laser diode is stored as optical output power information. Storage means, and reading out the optical output power information stored in the storage means when the drive signal is cut off, and using the optical output power information to output the output until a certain time after the time when the drive signal is applied again. An optical transmitter comprising: a control unit that operates the control unit. 2. The optical transmitter according to claim 1, wherein the storage unit is a FIFO memory.