WO2000033490A1

WO2000033490A1 - Optical transceiver module

Info

Publication number: WO2000033490A1
Application number: PCT/JP1998/005384
Authority: WO
Inventors: Kazuyuki Mori; Tamotsu Akashi
Original assignee: Fujitsu Limited
Priority date: 1998-11-30
Filing date: 1998-11-30
Publication date: 2000-06-08

Abstract

An optical transceiver module which can suppress electrical crosstalk from the transmission side to the reception side. A low-pass filter composed of a resistor (14) and a capacitive element (16) removes the high-frequency components of the driving current of a laser diode (10) on the transmission side.

Description

Description Optical transceiver module Technical field

The present invention relates to crosstalk suppression in an optical transceiver module, and more particularly to a downlink transmission bit rate in an ATM-PON (Passive Optical Ne two rk) described in ITU-T Recommendation G.983. The present invention relates to suppression of electrical crosstalk between transmission and reception in an optical transceiver module suitable for asymmetric transmission of 622 MbZ s and 156 Mb / s upstream transmission. Background art

In an optical transceiver module in which an optical transmission module and an optical reception module are housed in a single case, a part of the electric signal on the transmission side leaks to the reception side, and electrical crosstalk becomes a problem. There is. In a conventional optical transceiver module, as a method of suppressing the electrical shortage between transmission and reception, a shield structure is employed in which the receiving module is shielded with metal. In some cases, a transmitter / receiver separation module is used instead of a transceiver structure.

One problem with the conventional method of suppressing electrical crosstalk between transmission and reception is that when a metal shield is used, a process of attaching the metal shield occurs during the assembly process. However, there were many problems such as the need for man-hours for forming holes for holes, and the difficulty of physically reducing the size.

In addition, there is a problem that the number of parts at the time of assembling is increased when the transmission / reception separation module is used, and the cost is increased. Disclosure of the invention

An object of the present invention is to provide an inexpensive optical transceiver module capable of suppressing electric crosstalk.

According to the present invention, a light emitting element, a transmitting circuit for driving the light emitting element, a light receiving element, a receiving circuit connected to the light receiving element, and a driving current for the light emitting element are provided between the light emitting element and the transmitting circuit. A low-pass filter that removes high-frequency components of the light-emitting element within a range where the light output from the light-emitting element satisfies a predetermined rule and suppresses crosstalk from the transmission circuit to the reception circuit. An optical transceiver module characterized by the following is provided. BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a schematic diagram illustrating electrical crosstalk in an optical transceiver module.

FIG. 2 is an equivalent circuit diagram of FIG.

FIG. 3 is a circuit diagram of the optical transceiver module according to the first embodiment of the present invention.

Figures 4A, 4B, 5A and 5B are diagrams illustrating the reduction of crosstalk noise by extending the signal rise / fall time.

Figures 6A, 6B and 6C are waveform diagrams illustrating the delay of the optical signal at the beginning of the burst and the improvement.

FIG. 7 is a circuit diagram of an optical transceiver module according to a second embodiment of the present invention.

FIG. 8 is a circuit diagram of a modification of the circuit of FIG.

9A, 9B and 9C are waveform diagrams illustrating the improvement of the delay at the head of the burst by the circuit of FIG.

Figure 10 shows the I-L characteristics of the laser diode at high and low temperatures. It is a graph.

FIG. 11 is a principle block diagram of an optical transceiver module according to a third embodiment of the present invention.

FIG. 12 is a graph showing the relationship between the reverse bias voltage of the diode and the capacitance.

FIG. 13 is a diagram showing a change in voltage at points A to C in FIG. 12 at the time of low drive power and at the time of high drive power.

FIG. 14 is a more detailed circuit diagram of FIG.

FIG. 15 is a diagram similar to FIG. 13 in which a change in the potential at point C due to a change in data is considered.

FIG. 16 is a circuit diagram of a modification of the circuit of FIG.

FIG. 17 is a waveform chart for explaining the operation of the circuit of FIG.

FIG. 18 is a diagram similar to FIG. 15 in the circuit of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Fig. 1 is a schematic diagram illustrating electrical crosstalk in an optical transceiver module. LD and PD indicate a laser diode and a photo diode for optical transmission and optical reception, respectively. I indicates the drive current of the LD, and R in is the input impedance of the receiving circuit. Since electric crosstalk is capacitive, it is indicated by capacitance Cx.

The equivalent circuit of Fig. 1 is shown in Fig. 2. The LD is current-biased to near the light emission threshold, and the equivalent resistance (differential resistance) at that time is indicated by Rd. The impedance of PD is so high that it is omitted.

From Figure 2

I = I I

1

I _LD Rd = I (R in +).

j ω C x Therefore, from these,

IRd

I-i-2 π fC _x IRd… (1)

Rin + Rd +

j ω C:

Is obtained. Therefore, the crosstalk transfer function X (f) = IxRin / I from the driving current I on the transmitting side to the crosstalk voltage Rin on the receiving side is

X (f)-} 2π fCxRinRd… (2)

Equation (2) shows that mainly high-frequency components of the components of the drive current I cause crosstalk, and the higher the frequency, the greater the effect of crosstalk.

In the optical transceiver module, as the bit rate on the transmission side increases, the high-frequency component in the transmission signal increases, and as a result, crosstalk increases as described above. On the other hand, on the receiving side, the higher the bit rate, the wider the receiving band needs to be.Therefore, it is more difficult to avoid the effect of crosstalk by limiting the band on the receiving side as the bit rate increases. It becomes bad. In other words, in optical transceiver modules, the higher the bit rate, the more serious the problem of electrical crosstalk becomes <£>.

FIG. 3 is a circuit diagram of the optical transceiver module according to the first embodiment of the present invention. Between the laser diode (LD) 10 for optical transmission and the transmission circuit 12, a low-pass consisting of a resistor 14 connected in series with the transmission circuit 12 and a capacitor 16 connected in parallel with the transmission circuit 12 A filter 18 is provided. The low-pass filter 18 removes high-frequency components by extending the rise time and fall time of the drive current of the LD 10, and thereby removes the high-frequency component to the receiving side including the photodiode (PD) 20 and the receiving circuit 22. No electricity Reduces mechanical cross talk.

Fig. 4A schematically shows the waveform of the LD drive current in the time domain when the low-pass filter 18 is not provided, and Fig. 5A shows the LD drive current when the low-pass filter 18 is provided. 5 schematically shows a waveform in the time domain. In the figure, T. Indicates the pulse width, and tr1 and tr2 indicate the rise and fall times of the pulse. Figures 4B and 5B show the spectral envelop in the frequency domain when this pulse is continuous, along with a graph of the crosstalk transfer function. Since the crosstalk noise is the product of the LD drive current and the crosstalk transfer function, the amount of crosstalk can be expressed by the shaded area in Figs. 4B and 5B. As is evident from the figure, extending the rise and fall times of the signal pulse with the low-pass filter 18 removes the high-frequency components, thereby reducing the crosstalk noise. I understand. It is desirable that the low-pass filter 18 be disposed near the transmitting circuit 12.

On the other hand, if the rise-Z fall time is extremely extended, the mask specification applied to the output waveform of the transmitted light will not be satisfied. Therefore, the constant of the low-pass filter 18 must be set so that the light output from the LD 10 satisfies a predetermined mask specification.

According to the method of the present invention, when the bit rate of the receiving side is higher than that of the transmitting side, such as the subscriber side of the ATM-P0N system, the transmitting side removes high-frequency components that cause crosstalk. Since the bit rate on the transmitting side is lower than that on the receiving side, even if high-frequency components that affect reception are removed on the transmitting side, it is possible to satisfy the specifications of the optical output waveform on the transmitting side

In a burst transmission such as ATM-P0, do not emit light during this time because the transmission of other subscribers is not interrupted during the time when no signal is emitted. Is necessary. However, it was necessary to satisfy the mask specification from the first bit of the burst, and to prevent this, it was necessary to use a light emitting element with a low threshold. If the present invention introduces a low-pass filter by RC, for example, a new problem occurs in that the light emission of the first bit is delayed due to a pattern effect or the like.

That is, when a rectangular wave driving current as shown by the solid line in Fig. 6A is given, the first pulse starts from the state where there is no voltage change in the capacitance of LD 10 and capacitor 16. Therefore, as shown by the solid line in FIG. 6B, the rise of the voltage is delayed by the time required for the charge. Therefore, the light emission of the LD is delayed as shown by the solid line in FIG. 6C, and the mask specification cannot be satisfied.

FIG. 7 is a circuit diagram of an optical transceiver module according to a second embodiment of the present invention that solves this problem. The same components as those in FIG. 3 are denoted by the same reference numerals, and the circuit on the receiving side is omitted.

A constant voltage is applied to the base terminal of the transistor 24, and plays a role of constantly flowing a constant minute current to the LD 10, as shown by the broken line in FIG. 6A. As a result, even when there is no signal, the capacitance of the LD 10 and the capacitor 16 are charged with the voltage, and the voltage rises with the rise of the signal current as shown by the broken line in FIG. As shown by the broken line, the light emission of LD 10 is not delayed.

In the case of an ATM-P0N subscriber unit, light from multiple subscriber units is multiplexed by an optical power brass and reaches the optical line terminal. Therefore, even if weak light generated by the bias current when there is no signal is multiplexed by the optical power blur, the bias current must be very small so as not to affect the receiving sensitivity of the optical line terminal. Must.

FIG. 8 shows a modification of the circuit of FIG. In the circuit of Figure 8, the transformer Rather than always flowing a constant bias current, the register 24 follows the control voltage 28 and, as indicated by the dashed line in FIG. 9A, a period beginning at the end prior to the burst transmission and ending at the end of the burst. At, a bias current is supplied to LD.

This also eliminates the delay in the rise of the LD voltage as shown by the broken line in FIG. 9B, and the delay in the emission of LD 10 as shown by the broken line in FIG. 9C. Is lost. Also, since the bias current is turned off at the latest until the end of the burst, even if it is applied to the ATM-P0N subscriber equipment, weak light from a large number of subscriber equipment due to the bias current will be transmitted to the station. There is no adverse effect on the receiving sensitivity of the side device.

Fig. 10 shows the I-L characteristics of the glue. As shown in FIG. 10, in general, the higher the temperature of an LD, the higher the current threshold value and the lower the luminous efficiency. Therefore, in order to always obtain a constant optical power, the magnitude of the drive current is controlled by monitoring the optical output or the temperature of the LD. As a result, when the LD temperature rises, the drive current I increases accordingly. On the other hand, as is clear from equation (1), the cross current I _x increases with an increase in the LD drive current I.

FIG. 11 is a principle block diagram on the transmitting side of the optical transceiver module according to the third embodiment of the present invention in consideration of the above points. In FIG. 11, as described above, the power controller 30 generates a control voltage such that the optical output from the LD 10 becomes constant according to the output of the temperature sensor or the optical monitor, and sends the control voltage to the LD drive circuit 12. give. The comparator 32 compares the output of the personal controller 30 with the reference voltages V and _e Pl, and provides the comparison result to the diode 34 node. The diode 34 is provided in place of the capacitor 16 in FIG. 3 as a capacitance element whose value changes according to the change of the reverse bias voltage.

FIG. 12 shows the relationship between the reverse bias voltage of the diode 34 and the capacitance. Ma FIG. 13 shows the voltages at A to C in FIG. As shown in FIG. 13, when the output of the node controller 30 (point A) indicates a low driving current with small crosstalk, the output voltage of the comparator 32 (point B) becomes a low value. becomes, the reverse bias voltage between the BC becomes V _2. At this time Daio - volume of de 34, as shown in FIG. 12, the removal of high frequency components is not carried out by the low-pass full I filter consisting of a lower value C _2, and the resistors 14 and Daio de 34. When the temperature of the LD rises and the output of the power controller 30 (point A) indicates a high drive current with large crosstalk, the output voltage of the comparator 32 (point B) increases, and the reverse bias between BC and The voltage is V ,. At this time, the capacitance of the diode 34 becomes a higher value C, (FIG. 12), and the high-frequency component is removed by the low-pass filter, and the crosstalk is suppressed.

In other words, the low-pass filter 36 composed of the resistor 14 and the diode 34 in FIG. 11 has variable pass characteristics, and when the LD drive current is large and the crosstalk is large, the high frequency of the drive current is high. The component has a pass characteristic that suppresses crosstalk by removing the component. When the LD drive current is small and the crosstalk is small, the pass characteristic does not remove the high-frequency component of the drive current.

FIG. 14 is a circuit diagram showing the circuit of FIG. 11 in more detail. In FIG. 14, the increase or decrease of the optical output due to the temperature change of the LD is detected by the temperature sensor 38 such as a thermistor for detecting the temperature of the LD or the photo diode 40 for detecting the optical output itself. The voltage is converted into a voltage by the voltage converter 42. Part controller 30 is a controller. It is composed of Laywei 44 and 46 controllers. The comparator 44 compares the output of the current-Z voltage converter 42 with the reference voltage V _{rep 2} and outputs a comparison signal. The controller 46 generates a control voltage to keep the optical output of the LD 10 constant based on the comparison result of the comparator 44. Fig. 15 is similar to Fig. 13 and takes into account the change in the voltage at point C due to the change over time. According to FIG. 15, the potential of the point C also changes due to the change in the data, and the reverse bias voltage (voltage between BC) of the diode 34 changes accordingly. However, the voltage at point C becomes lower during the period in which data exists and LD10 emits light and crosstalk occurs, so the reverse bias voltage becomes V, at which crosstalk can be suppressed. FIG. 16 is a modification of the circuit of FIG. FIG. 17 shows waveforms at points C to G in FIG. In FIG. 16, the data signal (D) is delayed by delay elements 48 and 50 and supplied to the LD drive circuit 12. The reference value supplied to the comparator 32 is obtained by taking NAND of a data signal that is delayed by a delay time longer than the delay time of the delay elements 48 and 50 (E) and a data signal that is not delayed (F). Generated (G). Therefore, as shown in FIG. 17, the potential at the point G is maintained at the high potential during the period when the potential at the point C changes. By changing the reference value of the comparator 32 in this manner, as shown in FIG. 18, the reverse bias voltage of the diode becomes smaller at the change point of the signal where the high frequency component is large and the crosstalk is increased. The crosstalk can be effectively suppressed.

Claims

The scope of the claims

1. a light emitting element;

A transmission circuit for driving the light emitting element,

A light receiving element,

A receiving circuit connected to the light receiving element,

Provided between the light emitting element and the transmission circuit, removes the high frequency component of the driving current of the light emitting element within a range where the light output from the light emitting element satisfies a predetermined regulation, and transmits the signal from the transmission circuit to the reception circuit. An optical transceiver module comprising: a low-pass filter for suppressing crosstalk.

2. The optical transceiver module according to claim 1, wherein the receiving side performs asymmetric transmission at a higher bit rate than the transmitting side.

3. The low-pass filter is

A resistance element connected in series to the transmission circuit,

2. The optical transceiver module according to claim 1, further comprising a capacitance element connected in parallel to the transmission circuit.

4. The light according to claim 3, further comprising a micro-bias circuit for supplying a micro-bias current to the light-emitting element to charge the light-emitting element and the capacitive element, thereby preventing a light emission delay at the head of the burst. Transceiver module.

5. A bias current is applied to the light-emitting element to charge the light-emitting element and the capacitor element at the beginning of the burst and before the end of the burst, but before the end of the burst, to prevent the delay of light emission at the beginning of the burst. 4. The optical transceiver module according to claim 3, further comprising a pre-bias circuit.

6. The pass characteristic of the low-pass filter is variable; 2. The optical transceiver module according to claim 1, wherein a transmission characteristic is changed according to a drive current of the optical transceiver.

7. The low-pass filter is

A resistance element connected in series to the transmission circuit,

A diode connected in parallel with the transmitting circuit,

By comparing the control voltage of the driving current of the light emitting element with the reference voltage, when the driving current is larger than a predetermined value, the reverse bias voltage of the diode is reduced to increase the capacitance of the diode. 7. The optical transceiver module according to claim 6, further comprising a comparator that causes the low-pass filter to remove a high-frequency component of the current of the light-emitting element.

8. The low-pass film includes:

A resistance element connected in series to the transmission circuit,

A diode connected in parallel with the transmitting circuit,

The control voltage of the driving current of the light emitting element is compared with the reference voltage, and when the control voltage is higher than the reference voltage, the reverse bias voltage of the diode is reduced to increase the capacitance of the diode. In addition, the low-pass filter further includes a comparator that removes the high-frequency component of the current of the light-emitting element, and a reference voltage control circuit that raises the reference voltage during the period in which the transmission data changes to ensure that the high-frequency component is removed. The optical transceiver module according to claim 6, wherein