JPH11298416A - Light emitting element drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stable luminous intensity even if 'low' codes have been continued in a laser diode(LD) drive circuit of a light transmitter for optical digital transmission system. SOLUTION: In this LD drive circuit output light from a laser diode (1) is monitored (2), this monitor output is subjected to current/voltage conversion (4) and its level is detected (5). After applying a luminous intensity setting voltage so as to obtain a prescribed output light at the voltage level, input data are sampled and held by a sample-and-hold circuit (6), the result is fed back to a laser diode drive current circuit (3) so as to prevent reduction in a laser diode drive current due to consecution of low level of input data. Furthermore, a discharge resistor is connected in parallel with a hold capacitor in a peak detection circuit and effect of a malfunction due to noise is reduced by optimizing the time constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention drives a laser diode (hereinafter referred to as an LD), which is a light emitting element, for transmitting an optical signal in an optical transmission circuit for transmitting a digital signal in an optical communication system. The present invention relates to an LD drive circuit that controls a current flowing through an LD. In particular, the present invention relates to an LD drive circuit for burst transmission in which long "low" codes are continuous.

More specifically, the present invention is applied as an optical transmission circuit which is a key of optical transmission in an optical subscriber transmission system.

[0003]

2. Description of the Related Art An optical transmitter in a digital transmission system converts an electric signal into an optical signal. Generally, a laser diode (L) is used to convert an electric signal into an optical signal.
D) is used, and a light emission intensity corresponding to a current value flowing through the LD is obtained. The optical transmission circuit controls a current value for driving the LD to obtain a desired light emission intensity and modulates a signal to be transmitted.

FIG. 8 explains the principle of optical digital signal transmission using an LD, and shows the relationship in which an optical output signal intensity _PO is obtained by a signal current _IO which is an LD drive current. As shown in FIG. 8, the light output power, which is the light emission intensity of the LD, changes according to the flowing current. In the digital optical transmission system, the light emission intensity is kept constant, and the current ON / O
Digital signal “high” / “lo” by FF
The optical signal modulated as “w” is transmitted. For the transmitted modulated signal, as shown in FIG. 8, the drive current is digitally modulated (pulse-modulated) by the input signal, and the light corresponding to the light is modulated by the LD. Obtained as a blinking signal,
The optical output signal intensity P _O to the transmitted must be constant. This variation of the optical output signal intensity P _O of the transmitting side distortion causes and of the received waveform by follow-up delay of the gain control circuit becomes an amplitude variation of the received signal in the receiving circuit, the offset from the threshold of the decision circuit This is to cause identification errors. To the optical output signal intensity P _O is constant, APC (Automatic-Power Control) circuit is generally used. The APC circuit detects the light emission intensity of the LD using a monitoring photodiode (hereinafter referred to as MPD), converts it into an electric signal, and compensates for the intensity change △ P _Oに よ り by negative feedback to obtain a stable constant light emission intensity.

FIG. 9 shows a block diagram of a conventional LD drive circuit. In the figure, the laser light emitted from LD1 is MP
Is received by D2, taking out a part of the light current obtained as the output of MPD2 as a monitor current I _m, performs a current / voltage (I / V) signal level detection by the level detection circuit 5 after conversion, differential in the amplifier circuit 8 LD1 is supplied to the driving current control circuit 3 of LD1 as power control voltage V _p by the addition of emission intensity setting voltage V _ref so as to emit light at a predetermined light emission intensity. Here, in order to realize a high-speed response, a peak hold circuit is used for level detection, and it is necessary to set a small rising time constant of the peak detection circuit. When the signs are continuous, there is a problem that a level (decrease) error of the light emission intensity occurs due to the leak current. That is, in the conventional configuration, there is a problem that an attempt to obtain a high-speed rising characteristic of the APC required for the burst transmission or the like causes an APC level error due to a hold level error due to a leak current, which causes a level fluctuation of the LD light emission intensity.

[0006]

In the above-mentioned conventional LD driving circuit for digital light transmission, APC is used to keep the light output intensity constant. However, in burst transmission in which “low” codes continue for a long time, light emission level fluctuation occurs at the time of burst reception as described above in order to realize a high-speed response.

FIG. 10 shows the APC operation of the conventional circuit and its problems. In the horizontal axis represents the time axis of FIG, respectively ordinate the _{I m, V m, V p} , P O. That is, in the APC, detecting the emission intensity P _O of LD1 as a monitor current I _m corresponding to the emission intensity by MPD2. Converts the monitor current I _m into a voltage to detect the peak value of the output waveform as a level detection voltage V _m, the power control voltage corresponding to a predetermined emission intensity gives an offset voltage by adding the light emission intensity setting voltage V _ref and V _p, stable output is configured to be obtained by the feedback circuit for controlling a current for driving the LD1. In burst transmission, it is necessary to reduce the level hold capacity for peak detection and the response time constant in order to increase the APC response speed at the rising edge of data. in this case,
Long "low" when the code is continuous, charges accumulated in the hold capacitor as shown in FIG. 10 is lowered leaked peak detection voltage V _m. Therefore, this drops the power control voltage V _p which correspond, "low" emission intensity after encoding successive drops. The fluctuation ΔP _O of the light emission intensity is a problem because the signal level decreases on the receiver side that receives the optical signal via the transmission medium, causing a code error.

An object of the present invention is to solve the above problems and to provide an LD drive circuit with a stable light emission output intensity.

[0009]

According to a first aspect of the present invention, there is provided an optical transmission circuit for converting an electric signal corresponding to a transmission code into an optical signal and transmitting the converted signal. / A light emitting element for performing light conversion, a light receiving element for monitoring the light emission intensity of the light emitting element and converting it to an electric signal, and a drive circuit for receiving the monitored electric signal and controlling a current for driving the light emitting element. A light emitting element driving circuit comprising: a sampler for sending, to the driving circuit, a result of sampling and holding an electric signal from the light receiving element whose emission intensity is monitored by the transmission code between the light receiving element and the driving circuit. & Hold circuit connected.

According to a second aspect of the present invention, a discharge element is connected to the sample and hold circuit in the circuit configuration of the light emitting element drive circuit of the first aspect.

By providing the sample-and-hold circuit in the APC loop in this manner, an excellent level holding function can be realized in a portion where the light emitting element drive current control voltage is missing, and the same code continuity resistance can be improved. it can.
That is, sampling is performed only when the data is “high”, and the voltage level for controlling the light emitting element driving current is detected. Therefore, when the sign is “low”, the level is held and “low”
The current flowing to the light emitting element after the code continuation can be constant. Further, according to the above circuit configuration, a stable power control signal can be held even after the “low” code continues, so that the light emission level fluctuation ΔP _O can be reduced. That is, with the above configuration, it is possible to provide an LD drive circuit that can output a stable light emission intensity even when “low” code continuation continues.

[0012]

FIG. 1 is a block diagram showing a configuration of an LD drive circuit according to the present invention. In the figure, a part of light emitted from a laser diode (LD) 1 as a light emitting element is received by a photodiode (MPD) 2 as a light receiving element for monitoring and converted into a current / voltage (I / V) as a monitor current _IO. It is supplied to the circuit 4. Here, the I / V-converted signal voltage is applied to the signal “1 (high)” or “0 (lo) by the level detection circuit 5.
performs level detection of w) ", obtaining a level detection voltage V _m. The level detected voltage V _m is obtained the sampled power control voltage V _p data signals input at the sample and hold circuit 6 as a sample signal, this voltage V _p is supplied to the drive current control circuit 3 to control the current level I _O of the input data so that the optical output signal intensity P _{O of the} LD 1 becomes constant.

FIG. 2 shows the operation of the present invention having the above configuration. Sampling the level detection voltage V _m of the monitor waveform data "high" signal reference numerals in FIG. But, "low" for by the symbol interval sampling pulse does not arrive "low"
The voltage level immediately before the code section is held.
Therefore, even if the level detected voltage V _m by leakage fluctuates level detection voltage V _m is "low" code continuity period, the held power control voltage V _p is maintained a certain level. Accordingly, "low" to "high" code after code continuity can provide a power control voltage V _p substantially equal to "low" code continuity before the optical output signal intensity.

FIG. 3 is a block diagram showing an example of a specific embodiment of the light emitting element drive circuit according to the first aspect of the present invention. FIG. 3 shows the basic configuration of the present invention shown in FIG. 1 more specifically. The level detection circuit 5 in FIG. 1 is composed of a peak detection circuit 7 and a differential amplifier circuit 8, and a data input circuit. Is the clock input (CLK In)
And a D-flip-flop circuit having
The digital transmission signal (Data In), which is an input signal, is reshaped into an NRZ code having a duty ratio of 1: 1. Here, the reason why the peak detection circuit 7 is used as the level detection circuit 5 is that the I / O signal is used in a continuous data transmission circuit having a constant mark rate.
Although an average value detection circuit that does not require a wide band is used in the V conversion circuit 4, when handling packet data, “lo
A peak value detection circuit is used because the w "sign becomes longer.
Further, the light emission intensity setting voltage voltage V in order to set a predetermined drive current I _O level which gives the optical output signal intensity P _O of LD1
_ref is added to the level detection voltage V _m ′ peak-detected via the differential amplifier circuit 8 to provide a necessary DC offset.

FIG. 4 is a circuit diagram showing the sample-and-hold circuit 6 shown in FIGS. 1 and 3 in more detail, and comprises a sampling switch 10 and a hold capacitor 11. FIG. 5 is a waveform chart for explaining the operation of the circuit of the present invention shown in FIG. FIG. 5A shows the digital modulation signal (Data In) shown in FIGS. 3 and 4, and FIG.
Is a monitor current I _m of the light LD1 emits light by the digital modulation signal, the monitor current I _m voltage digital modulated signal peak detection obtained by sampling at V _m '
And shows the relationship between the power control voltage V _p which is the output voltage of the sample-and-hold circuit 6, FIG. 5
(C) shows the state of the optical output signal intensity P _O of the LD 1 driven by the LD drive current I _O thus obtained. That is, the input data (Data
The In) by multiplying the modulation of the digital signal to the current for the LD driving, obtaining light emission output P _O drives the LD1 in intensity modulated signal shown in Figure 5 (a). This light emission output P _O
Is monitored by a monitoring photodiode (MPD) 2, a monitor current _Im is obtained, and after current-voltage conversion, the peak value is detected to detect the optical output signal intensity. Further, in order to set the light output signal intensity _Po to a predetermined value, a light emission intensity setting voltage _Vref is applied via a differential amplifier 8 to obtain a DC offset voltage. This peak detection voltage V _m '
By using the transmission data sequence shown in FIG. 5A as a sampling signal, sampling and holding the monitor output, the peak detection voltage V _m ′ corresponding to the emission intensity during the “high” signal period is acquired, and “low” This value is held during the code period. Therefore, even if the peak detection voltage V _m ′ leaks during the “low” code period in which no charge is accumulated, the peak value detected by the sample and hold circuit 6 is maintained, so that the “low” code is kept for a long time. in continuous current corresponding to the optical output signal intensity P _O detected even immediately after it is possible to drive the LD1.

FIG. 6 is a block diagram showing a specific embodiment of the LD drive circuit according to the present invention. Symbols 12 and 13 in the figure are discharge resistors. FIG. 7 shows a specific operation explanatory diagram of the embodiment shown in FIG. In FIG. 7, the level shown by the broken line is the peak detection voltage when there is no discharge resistor, and the broken line shown by the solid line below is the peak detection voltage when there is a discharge resistor.

If the discharge time constant of the peak hold is extremely long in the circuit diagrams shown in FIGS. 1 and 4 as a circuit relating to claim 1, the noise level is held against a level change due to noise such as surge. I will. Power supply O
When the power is turned on again after the FF, there is a problem that the electric charge immediately before the power is turned off is held and light cannot be emitted at a predetermined set potential. On the other hand, as shown in FIG. 6, by connecting the discharge resistor 12 in parallel with the hold capacitor 11 and grounding the peak detection circuit 7 with the discharge resistor 13 to provide a discharge path for the electric charge, signal transmission is achieved. At the same time as the high holding ability for the case where the "low" code is continuous,
The response to the fluctuation of the drive voltage can be improved. In particular, the present method is effective for eliminating the influence of the above-described residual charge when the power is turned on.

[0018]

As described above, by using the light emitting element drive circuit according to the present invention, the LD drive circuit of the optical transmitter for an optical digital transmission system can provide stable light emission intensity even when the "low" code is continuous. It is possible to obtain. That is, by applying the LD drive circuit according to the present invention, the optical output intensity of the optical transmitter performing the optical digital transmission is stabilized, and the pattern dependence of the amplitude deterioration in the receiving circuit can be reduced. Since the deterioration of the reception sensitivity is improved in this way, the level difference between transmission and reception can be increased, and the transmission distance can be lengthened. In particular, ATM (Asynchronous Transfer M
ode) and PDS (Passive Double Star) transmission system.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of a light emitting element drive circuit according to the present invention.

FIG. 2 is a waveform chart illustrating the operation principle of the LD drive circuit according to the present invention.

FIG. 3 is a block diagram showing a basic configuration of the present invention corresponding to claim 1;

FIG. 4 is a circuit diagram specifically showing the contents of a sample and hold circuit in the circuit configuration of FIG. 3;

FIG. 5 is a waveform chart for explaining the operation of the circuit shown in FIG. 4;
(A) is an input data waveform diagram, (b) is a waveform diagram showing a relationship between a monitor current, a peak detection voltage, and a power control voltage, and (c) is a waveform diagram showing an optical output.

FIG. 6 is a circuit diagram showing an embodiment corresponding to claim 2;

FIG. 7 is a waveform chart for explaining an operation corresponding to claim 2;

FIG. 8 is a waveform chart showing the operation principle of the LD drive circuit.

FIG. 9 is a block diagram of a conventional LD drive circuit.

FIG. 10 is a waveform chart for explaining a problem of the conventional circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser diode (LD) 2 ... Monitor photodiode (MPD) 3 ... LD drive current control circuit 4 ... Current / voltage (I / V) conversion circuit 5 ... Level detection circuit 6 ... Sample & hold circuit 7 ... Peak detection Circuit 8 ... Differential amplifier circuit 9 ... D-flip-flop 10 ... Sampling switch 11 ... Hold capacity 12 ... Discharge resistor 13 ... Discharge resistor

Claims (2)

[Claims] An optical transmission circuit for converting an electric signal corresponding to a transmission code into an optical signal and transmitting the optical signal, a light emitting element for performing electric / optical conversion by current control, and a light emitting intensity of the light emitting element. A light emitting element driving circuit comprising: a light receiving element that monitors and converts the signal into an electric signal; and a drive circuit that receives the monitored electric signal as input and controls a current that drives the light emitting element. A light-emitting element drive circuit, wherein a sample-and-hold circuit for sending the result of sampling and holding an electric signal from the light-receiving element whose emission intensity is monitored by the transmission code and holding the result to the drive circuit is connected to the drive circuit. . 2. The light emitting element driving circuit according to claim 1, wherein a discharging element is connected to the sample and hold circuit.