JPS6161535A - Optical transmitter - Google Patents

Abstract

PURPOSE:To prevent generation of background light due to naturally irradiated light from a laser diode by adding a switch means operated synchronously with a transmission request signal in an optical transmitter transmitting intermittently data. CONSTITUTION:Transmission data 2 is applied to a transistor (TR)16 synchronously with a transmission request signal 15. The TRs 16, 17 form a current changeover switch. When the request signal 15 reaches a threshold voltage 18 of the TR17, an output current of a current amplifier 5 is outputted to the laser diode 4 as a bias current 6 via the TR16. On the other hand, when the request signal 15 reaches a voltage 18 or below, the output current of the amplifier 5 is outputted to a load 19 via the TR17. Through the operation above, no current flows to the diode 4 at all at the non-transmission system, then no background optical output light due to naturally irradiated light is caused.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical transmitter that transmits data intermittently.

[Prior art]

FIG. 1 shows a configuration diagram of a conventional optical transmitter. In Figure 1, (1) is a modulator, (2) is transmission data, (3) is a modulation current, (4) is a laser diode, (5) is a current amplifier, (6) is a bias current, (7) is an optical signal, (8)
is the power supply.

(9) is a light receiving element, α1 is a capacitor constituting an average circuit, α power is a signal current, U is a reference current source, (+3 is the output current of the reference current source @).

FIG. 2 is a waveform diagram of each part of a conventional optical transmitter.

In Figure 2, the α force is background light.

A modulator (1) outputs a modulation current (3) to a laser diode (4) in response to intermittently generated transmission data (2).

A current amplifier (5) outputs a bias current (6) to a laser diode (4). The laser diode (4) is
An optical signal (7) is output using a bias current (6) and a modulation current (3). A light receiving element (9) reverse biased by a power source (8) receives a part of the two optical signals (7) and converts it into an electric current.

The output current of the light receiving element (9) is averaged by a capacitor that constitutes an averaging circuit, and becomes a signal current aυ.

The capacitor is set to average over a period sufficiently longer than the data transmission cycle, so the signal voltage
The flow aIJ does not vary within a period of time approximately equal to the data transmission cycle. The current amplifier (5) outputs the output current a of the reference current source a3.
The current obtained by subtracting the signal current (Iυ) from 3 is amplified and outputted to the laser diode (4) as a bias current (6).

The current amplifier (5), the laser diode (4) and the photodetector (9) constitute a negative feedback loop, and the current amplifier 1 is connected to the device (5).
) can make the signal current aυ of the light-receiving element (9) equal to the output current a9 of the reference current source Q3 regardless of variations in the characteristics of the laser diode (4) due to temperature variations or the like. Signal current a of light receiving element (9)
υ is directly proportional to the average of the 'yc signal (7), and when the signal current riυ is controlled, the optical signal (7) is also controlled.

In the waveform diagram of each part of the optical transmitter shown in FIG.

The optical signal (7) includes a background light α calendar. this is.

This is due to spontaneous emission light output from the laser diode (4) due to the bias current (6). The magnitude of the background light I is approximately 5% of the peak value of the + signal (7).

When multiplex communication is performed by connecting multiple optical transmitting/receiving devices using passive optical circuit components such as a star coupler or T-coupler, from the optical transmitting device in a non-transmitting state.

When background light is output, it interferes with optical signals transmitted from other optical transmitters, degrading one communication characteristic.

For example, if the background light (+4) is about 5 inches from the peak of the optical signal (7), the total background light output from the 20 optical transmitters will be about the same as the optical signal peak value, which will significantly affect the communication characteristics. deteriorate.

As described above, when performing multiplex communication in a transmission system in which a plurality of optical transmitting/receiving devices are connected by passive optical circuit components using a conventional optical transmitting device, there is a drawback that the communication characteristics are significantly deteriorated.

[Summary of the invention]

In the present invention, the transmission data that is generated intermittently is input to the optical transmitter in synchronization with the transmission request signal, and the transmission request signal switches the path of the bias current flowing to the laser diode. This prevents the generation of background light by preventing current from flowing through the area.

In the transmission state, the purpose is to control the optical signal output to a specified value.

[Embodiments of the invention]

An embodiment of an optical transmitter according to the present invention will be described below with reference to one drawing. FIG. 3 is a configuration diagram of an optical transmitter according to the second invention. In fig.

(I!9 is a transmission request signal, (l[9, aη is a transistor, α power is a switching threshold voltage, α9 is a load.

FIG. 4 is a waveform diagram of each part of the optical transmitter according to the present invention.

FIG. 5 shows another embodiment of the optical transmitter according to the present invention.

In FIG. 5, (5a) and (6b) are bias currents before and after integration, respectively, and (7a) and (7b) are optical signals for bias currents (sa) and (5b).

The operation of the modulator (1) is the same as that of a conventional optical transmitter. Transmission data (2) is input to the base of transistor aQ in synchronization with transmission request signal αS. Transistor α
Q and the transistor aη constitute a current selector switch, and when the transmission request signal a9 exceeds the switching threshold voltage inputted to the base of the transistor aη, the transistor (14G is turned on, the transistor αη is turned off, and the current amplifier ( The output current of 5) is output as a bias current (6) to the laser diode (4) via the transistor (11G).

When the switching threshold value αυ or lower, the transistor αQ is turned off and the transistor η is turned on, and the output current of the current amplifier (5) is outputted to the load (lj) via the transistor 0η.The above operation causes no transmission. In state a, no current flows through the laser diode (4), so no background output source due to spontaneous emission light is generated.

In the current selector switch composed of transistors ttn and tt7, one of the transistors is always on.

The other transistor is in the OFF state and 1! Since the load impedance seen from the output of the current amplifier (H+) does not change, the current path switching operation by the transmission request signal α9 does not affect the operation of the current amplifier (5). The impedance looking into the common emitter of the switch is low.

Therefore, the output current of the current amplifier (5) is I'i! which is the same as before. , can be obtained. Transistor αe, (
The current selector switch configured with I is capable of high-speed switching operation, and outputs the same bias current (6) to the laser diode (4) immediately after starting data transmission as at the end of the previous data transmission. Therefore, a normal optical signal (7) can be obtained from the first bit of the transmission data.

- constant modulation current (3a) when using the code (4);

Even under the condition of the bias current (6a), the amplitude of the optical signal (7a) immediately after the start of data transmission is larger than that immediately before the end of data transmission. In this case, as shown in Figure 5, a transistor Q and a laser diode (4
) is provided with a current integrator including a capacitor (2) that integrates the current, and a bias current (6b) with a large response time constant is provided.
By outputting the signal to the laser diode (4), it is possible to obtain a single optical output optical signal (7b) with the thermal sag effect compensated for.

The optical output thermal sag effect is thought to be mainly caused by an increase in the oscillation threshold current of the laser diode as the junction temperature increases due to the applied current. In this case, one-dimensional output (7a
) is expressed by the following formula.

From P-η' tfio-δ0 τ1
) 11J Here, P is the peak value of the optical signal (7), η is the source/current conversion efficiency of the laser diode (4), E is the peak 1 axis of the modulation signal (3), and E is the bias current (6a) , It.

is the oscillation threshold of the laser diode (4) in a steady state, and δ and τ1 are the oscillation threshold change and the response time constant due to the rise in junction temperature.

In Figure 5, the resistor ■, r2D and capacitor (
When the bias current is integrated by the current integrator consisting of (a), the bias current (6b) is expressed by the following equation.

-3"B= From. + (IBa) - (Bo) (1-62)
[21Here, 9co is the bias current in steady state.

Here, R and R2 are the resistance values of the resistance wire and Qυ, respectively.

Here, C is the capacitance value of the capacitor +22.

From equation tll~(4), if R4, R2, and Ct- are set so that (R-)-R)C-τ, (6), then the optical signal (7b) during data transmission is as follows.

P-η(Im"IBO:l-tho)", which makes it possible to eliminate optical output fluctuations due to the thermal sag effect.

In the above description, the output current of the reference current source Ii2 corresponds to the mark rate and duty of the transmission data.

Although the case where the present invention does not depend on the sending period, sending time, etc. has been described, the same effect can be obtained when using a reference current source provided with means for controlling the current value in proportion to the average value of the signal voltage of the transmitted data. in this case.

The same effect as conventional devices of this type can be obtained in that stable operation can be obtained even with variations in the above-mentioned parameters such as the mark rate of transmission data.

〔Effect of the invention〕

As described above, in the optical transmitter according to the second aspect of the present invention, since no optical signal is output from the optical transmitter when data is not being transmitted, the effect is that there is no interference with the communication of other stations connected to the same transmission path. be. Also.

Another advantage is that it can compensate for the Motoda thermal nag effect of the laser diode.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional optical transmitter, FIG. 2 is a waveform diagram of each part of the conventional optical transmitter, FIG. 3 is a block diagram of an optical transmitter according to the present invention, and FIG. 4 is a block diagram of the optical transmitter according to the present invention. FIG. 5 is a waveform diagram of each part of the optical transmitter according to the present invention; FIG. 5 is a schematic diagram of another embodiment of the optical transmitter according to the present invention; FIG. It is a waveform diagram of each part of an example. In the figure, (1j is a modulator, (4) is a laser diode, (1j is a modulator, (4) is a laser diode, (
5) is the current amplifier, α[ is the capacitor that constitutes the average circuit, (the first floor is the basic SEA signal source, αG, [17] is the transistor that constitutes the changeover switch, (Li3 is the load, (a), Qυ is The resistors (c) constitute the current integrator. Identical or corresponding parts in the two figures are indicated by the same reference numerals. Applicant Hirobe Yoda, Director of the Agency of Industrial Science and Technology (Figure 1) 2 Figure 3 Figure 4

Claims (2)

[Claims] (1) In an optical transmitter that uses a laser diode to output an optical signal corresponding to transmission data that occurs intermittently, a transmission request signal and transmission data synchronized with the transmission request signal are input, and a modulation current corresponding to the transmission data is generated. a reference signal source, a light-receiving element that receives a portion of the laser diode output light, an averaging circuit that averages the output of the light-receiving element over a period longer than the transmission request signal cycle; An amplifier that outputs a current according to the difference between the output of the reference signal source and the average output of the light receiving element, and a changeover switch that changes the path so that the output current of the amplifier flows to the laser diode when the transmission request signal is significant. An optical transmitter. (2) In an optical transmitter that uses a laser diode to output an optical signal corresponding to transmission data that occurs intermittently, a transmission request signal and transmission data synchronized with the transmission request signal are input, and a modulation current corresponding to the transmission data is generated. a reference signal source, a light-receiving element that receives a portion of the laser diode output light, an averaging circuit that averages the output of the light-receiving element over a period longer than the transmission request signal cycle; an amplifier that outputs a current according to the difference between the output of the reference signal source and the average output of the light receiving element; and a changeover switch that switches the path so that the output current of the amplifier flows to the laser diode when the transmission request signal is significant. , and a current integrator provided between the changeover switch and the laser diode.