JPH01162392A - Laser diode driving system - Google Patents

Abstract

PURPOSE:To prevent the timing extraction of a receiver from deviating in phase by a method wherein a preliminary current smaller than a pulse current but larger than a bias current is made to flow before and after the pulse current flows. CONSTITUTION:The inverted output of a flip-flop 1 is inputted into a main pulse current generating circuit 100 through the intermediary of an NOR gate 2. The non-inverted output of the flip-flop 1 is inputted into a preliminary current generating circuit 103 through the intermediary of a delay circuit 16. The main pulse current is outputted from a main pulse generating circuit 3, and the preliminary current is outputted from the preliminary current generating circuit 103. The preliminary current is made to flow at a level lower than the main pulse current but higher than a bias current before and after the main pulse current flows. The main pulse current and the preliminary current are compounded to flow through a laser diode 4.

Description

[Detailed description of the invention] [Table of contents] Overview Industrial field of application Conventional technology (Figures 10 to 17) Problems to be solved by the invention Means for solving the problems (Figure 1) Function (Fig. 2) Embodiment (Figs. 3 to 9) Effects of the invention [Summary] The laser diode is directly modulated by setting the off mode with a bias current flowing and the on mode with a pulse current flowing. Regarding the laser diode drive method, it has a compact device configuration and has both waveform and spectrum, making long-distance and high-speed transmission possible, while preventing phase shifts in timing extraction at the receiver, and driving the laser diode. The purpose is to make it possible to use bipolar transistors as part of the semiconductor elements for the laser diode, making it cost-effective.
Preliminary currents are configured to flow at current levels smaller than this pulse current and larger than the bias current.

[Industrial Field of Application] The present invention relates to a laser diode drive method that directly modulates a laser diode by setting it in an off mode with a bias current flowing therethrough and into an on mode with a pulse current flowing therein.

In recent years, in the field of optical communications, devices with bit rates on the order of gigahertz (GHz) have been realized. On the other hand, efforts have been made to reduce loss in optical fibers in the 1.55 μm band, and a value of 0.2 dB/K m has been achieved. It is expected that such increases in capacity and long-distance transmission will continue in the future.

However, the biggest obstacle to promoting such increased capacity and long-distance transmission is the wavelength dispersion of optical signals caused by fibers.

Now, the spectrum width of the transmitting light source is Δλ (nm).

The fiber length is L (Km), and the fiber dispersion coefficient is m
(ps/Km/nm), the dispersion value Δt (ps) after L (Km) transmission is as follows.

Δt=m・ΔλparaL This is further normalized by the bit rate fb [N T
F (Normalized Time Fluctu)
ation) is as follows.

NTF=Δt-fb=m・Δλ-L-fb By the way, in the case of Fabry-Perot laser diode (F-P LD), the limit is Δλ cape 4 nm, NTF push 0.3, and L
The system is established when both conditions are satisfied: the loss during (Km) transmission does not exceed the level difference between the transmitter and the receiver, and the NTF during L (Km) transmission does not exceed 0.3.

In order to avoid the influence of chromatic dispersion, it is necessary to reduce the dispersion coefficient m of the fiber and to reduce the spectral width Δ of the transmitting light source.
It is effective to reduce λ; the former can be achieved by matching the zero dispersion wavelength of the fiber with the center wavelength of the light source, and the latter can be narrowed by using a DFB laser diode, for example.

However, the spectrum Δλ also depends on the modulation method of the laser diode, and even in the case of a DFB-laser diode,
Bit rate or transmission distance may be limited due to chirping, side mode oscillation, etc.

In general, directly modulating a laser diode degrades the spectrum. Therefore, the spectral width Δλ of the transmitting light source becomes equivalently large, making it susceptible to chromatic dispersion.

[Prior Art] Fig. 10 is a conventional laser diode drive circuit diagram when the laser diode is directly modulated. In Fig. 10, 1 indicates data input DATA and clock input CL.
2 is a NOR gate that receives the inverted output of flip-flop 1 and the clock CLK, 3' is a pulse generator, 4 is a laser diode,
A field effect transistor (FET) 5 drives the laser diode 4 by operating with a signal from the pulse generator 3'. And pulse generator 3
' and FET 5 constitute a pulse current generating circuit 100' that generates a pulse current for driving a laser diode.

Further, 6 is a bias generator that generates a signal for biasing the laser diode, and 7 and 8 are a low-pass filter and a comparator interposed between the non-inverting output terminal of the flip-flop 1 and the bias generator 6, respectively. Note that an output from a rear monitor 9 that monitors the light emission state of the laser diode 4 is fed back to the comparator 8 via a low-pass filter 10. A bipolar transistor 11 supplies a bias current to the laser diode 4 based on a signal from the bias generator 6.

The bias generator 6 and the bipolar transistor 11 constitute a bias current generation circuit 101 that generates a bias current for driving a laser diode.

Note that in FIG. 10, R1 is a resistor for adjusting the output level of the transistor 11. Further, the output level of the FET 5 is adjusted by adjusting the gate-source voltage.

With such a configuration, assuming that the optical output-current characteristic (L-I characteristic) of this laser diode 4 is as shown in FIG. 11(a), the current flowing through the laser diode 4 is
In the case shown in FIG. 11(b), the optical output of the laser diode 4 is as shown in FIG. 11(c). In this example, the bias current is set smaller than the light emission threshold of the optical output. That is, in the bias current state, the laser diode 4
The laser diode 4 does not emit light, and the laser diode 4 starts emitting light only after a pulse current is supplied.

Furthermore, as shown in FIG. 13, it is also conceivable to set the bias current to be larger than the light emission threshold of the optical output. Therefore, in this case, even in the bias current state, the laser diode 4 emits light slightly, and when a pulse current is supplied, the laser diode 4 emits light even brighter. That is, in this example, assuming that the optical output-current characteristic (L-I characteristic) of the laser diode 4 is =7- as shown in FIG. 13(a), the current flowing through the laser diode 4 is
The optical output of the laser diode 4 is as shown in FIG. 3(b), and the optical output of the laser diode 4 is as shown in FIG. 13(c).

[Problems to be Solved by the Invention] However, such a conventional laser diode opening method has the following problems.

First, in the case shown in Fig. 11, as shown in Fig. 12,
Since the spectral characteristics are not sharp, the above-mentioned spectral deterioration is likely to occur, and a better spectrum can be obtained by setting the bias current above the threshold current as shown in Figure 13 (see Figure 14). .

However, in the case shown in FIG. 13, since light is emitted even in the portion where the signal is O, so-called extinction ratio deterioration occurs, resulting in a reduction in the reception level. Here, the extinction ratio is the maximum value of the optical output of the laser diode 4 as Pl, Px' [first
1.13 (see Figure (a))], and the minimum values are Po and Pa'
[See Figure 11.13 (a)], then 1010g
(P1/PO) 110 log(P □' / P
o').

That is, when the laser diode 4 is directly modulated, it is difficult to achieve both waveform and spectrum, which impedes long-distance transmission and high-speed transmission.

Therefore, as shown in FIG.
'' is split by a splitter 12, one of which is delayed by a delay device 13, the other is attenuated by an attenuator 14, and then added by an adder 15. right before,
A method of adding a prefix signal larger than the bias signal has also been proposed.

However, since this means is a device at the laboratory stage, for example, the delay device 13. Damping device 14. The adding device 15 and the like are bulky as a single item, which poses a problem in terms of practical application.

Therefore, it is conceivable to make an improvement to the one shown in FIG. 10, as shown in FIG. 16. That is, as shown in FIG. 16, the output of the NOR gate 2 is delayed by a delay circuit 16', and then input to the main pulse generator 3 using semiconductor elements such as FETs, and the output of the NOR gate 2 is delayed. input to the pre-pulse generator 17 using the same semiconductor element such as FET without causing
By adjusting the gate-source voltage of a field effect transistor (FET) 18 connected to the output terminal of the pre-pulse generator 17, a laser beam with the same waveform as that obtained by the device shown in FIG. 15 (see FIG. 17) can be generated. A diode drive current is supplied to the laser diode 4. That is, in this example, the optical output-current characteristic (L-I
If the characteristics) are as shown in Figure 17(a), the current flowing through the laser diode 4 will be as shown in Figure 17(b), and the optical output of the laser diode 4 will be as shown in Figure 17(c). Become. In addition, main pulse generator 3 and FET 5
These constitute a pulse current generation circuit 100 that generates a pulse current for driving a laser diode, and the pre-pulse generator 17 and FET 18 constitute a pre-pulse current generation circuit 1 that generates a pre-pulse current for driving a laser diode.
Configure 02.

However, although this type of laser diode drive system has a compact device configuration and also achieves both waveform and spectrum, enabling long-distance and high-speed transmission, it is prone to phase shifts in timing extraction at the receiver. , this may cause an error on the receiving side.

Furthermore, in the one shown in FIG. 16, the main pulse generator 3 and the pre-pulse generator 17 share a common signal [
Since the laser diode 4 is driven based on the RZ (Return to Zero) signal, a field effect transistor (FET) suitable for high-speed operation must be used as the semiconductor element for driving the laser diode 4, which is disadvantageous in terms of cost. There is also the problem of becoming

The present invention aims to solve these problems, and has a compact device configuration, and also achieves both waveform and spectrum, enabling long-distance transmission and high-speed transmission, while improving the phase of timing extraction in the receiver. To provide a laser diode driving system which is less likely to be misaligned and which is advantageous in terms of cost by allowing the use of a bipolar transistor in part as a semiconductor element for driving the laser diode.

[Means for solving problems] FIG. 1 shows a block diagram of the principle of the present invention.

In FIG. 1, 1 is a flip-flop that receives data input DATA and clock CLK, 2 is a NOR gate that receives the inverted output of flip-flop 1 and a clock, 3 is a main pulse generator, 4 is a laser diode, and 5 is a main This is a field effect transistor (FET) that operates with a signal from the pulse generator 3 to drive the laser diode 4. The main pulse generator 3 and the FET 5 constitute a main pulse current generating circuit 100 that generates a main pulse current 1 for driving a laser diode based on the inverted output (RZ multiplied signal) of the flip-flop 1.

Further, 6 is a bias generator that generates a signal for biasing the laser diode, and 7 and 8 are a low-pass filter and a comparator interposed between the non-inverting output terminal of the flip-flop 1 and the bias generator 6, respectively. Note that an output from a rear monitor 9 that monitors the light emission state of the laser diode 4 is fed back to the comparator 8 via a low-pass filter 10. A bipolar transistor 11 supplies a signal from the bias generator 6 to the laser diode 4. The bias generator 6 and the bipolar transistor 11 constitute a bias current generation circuit 101 that generates a bias current IB for the laser diode.

In addition, 16 is the non-inverting output of flip-flop 1 [N R
Z (Non Return to Zero) signal]
19 is a preliminary signal generator;
20A is a bipolar transistor operated by a signal from the preliminary signal generator 19. The preliminary signal generator 19 and the bipolar transistor 20A constitute a preliminary current generation circuit 103 that generates a preliminary current IA for driving a laser diode.

Also, as can be seen from the configuration in Figure 1, the FET 5 and the bipolar transistor 11.20A are connected in parallel with each other, and the main pulse current IM + bias current I
B and the preliminary current IA are combined and supplied to the laser diode 4.

In addition, in FIG. 1, R1 and R2 are transistors 11.20
This is a resistor for adjusting the output level of A.

Further, the output level of the FET 5 is adjusted by adjusting the gate-source voltage.

[Function] With such a configuration, assuming that the optical output-current characteristic (L-I characteristic) of this laser diode 4 is as shown in FIG. As shown in b), before and after the main pulse current IN, a preliminary current IA having a current level smaller than the main pulse current IA but larger than the bias current IB is caused to flow, so the optical output of the laser diode 4 is The result will be as shown in Figure 2 (Q). That is, the bias current IB is set to be smaller than the light emission threshold of the optical output, and the preliminary current IA is set to be slightly larger than the threshold. Therefore, in the bias current state,
The laser diode 4 does not emit light, and when the preliminary current IA is supplied, the laser diode 4 emits light slightly, and then the main pulse current 1. When supplied, the laser diode 4 emits bright light.

At this time, the preliminary current IA appears before and after the main pulse current, so the output pulse waveform becomes symmetrical.
This makes it difficult for a phase shift in timing extraction in the receiver to occur.

Furthermore, since the preliminary current generation circuit 103 is driven based on the NRZ signal, the preliminary current IA is supplied to the laser diode 4.
A bipolar transistor, which operates at a slower speed and is lower in cost than an FET, can be used as a semiconductor element for supplying the power.

=15- [Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 3 is a block diagram showing an embodiment of the present invention. In FIG. 3, 1 is a flip-flop that receives data input DATA and clock CLK, and 2 is a flip-flop that receives the inverted output of flip-flop 1 and a clock. NOR gate,
3 is a main pulse generator, 4 is a laser diode, and 5 is an FET as an output section that operates with a signal from the main pulse generator 3 to drive the laser diode 4.
It is. And this main pulse generator 3 and F
ET5 constitutes a main pulse current generating circuit 100 that generates a main pulse current for driving the laser diode based on the inverted output (RZ multiplied signal) of the flip-flop 1.

Here, the main pulse generator 3 has an FET 3A that receives a signal from the NOR gate 2 via a capacitor C1, as shown in FIG. In addition, C2 is F
A capacitor connecting ET3A and FET5, R3 is a resistor, and Do is a diode.

Note that the output level of the FET 5 is adjusted by adjusting the gate-source voltage.

Further, in FIG. 3, 6 is a bias generator that generates a signal for biasing the laser diode, and 7 and 8 are low-pass filters interposed between the non-inverting output terminal of the flip-flop 1 and the bias generator 6, respectively. It is a comparator. Note that an output from a rear monitor 9 that monitors the light emission state of the laser diode 4 is fed back to the comparator 8 via a low-pass filter 10. A bipolar transistor 11 supplies a bias signal from the bias generator 6 to the laser diode 4. The bias generator 6 and the bipolar transistor 11 constitute a bias current generation circuit 101 that generates a bias current for a laser diode.

Here, as shown in FIG. 5, the bias generator 6 converts the signal from the comparator 8 into a Zener diode Z.
It has a bipolar transistor 6A that receives the signal via D. Note that R1, R4° and R5 are resistors, and R1 is a resistor for adjusting the output level of the transistor 11°.

Further, in FIG. 3, 16 is a delay circuit that delays the non-inverted output (NRZ signal) of the flip-flop 1, 19 is a preliminary signal generator that generates a preliminary signal for driving the laser diode, and 20 is a preliminary signal generator 17. This is an output circuit that operates on signals. A preliminary signal generator 19 and an output circuit 20 generate a preliminary current generation circuit that generates a preliminary current for driving a laser diode, which is set to a current level smaller than the main pulse current and larger than the bias current, before and after the main pulse current. 1
Configure 03.

Here, as shown in FIG. 6, the preliminary signal generator 19 of the preliminary current generation circuit 103 receives a signal from the delay circuit 16 and connects a bipolar transistor 19A.
, 19B. Further, the output circuit 20 of the preliminary current generation circuit 103 includes two bipolar transistors 2 having a current switch configuration.
0A.

I have 20B. In addition, in this FIG. 6, R6
, R7, and R8 are resistors, and R2 is a transistor 20.
This is a resistor for adjusting the output level of A and 20B.

As can be seen from the configurations in Figures 3 to 6, the FET
5 and the bipolar transistor 11.20A are mutually connected in parallel, and the main pulse current IM+bias current IB and preliminary current IA are combined and supplied to the laser diode 4.

With the above configuration, if data DATA is inputted to the flip-flop 1 as shown in FIG. 7(a), the lock CLK is also inputted to the flip-flop 1 as shown in FIG. 7(b). Therefore, the inverted output of the flip-flop 1 becomes an NRZ signal as shown in FIG. 7(c), and the non-inverted output of the flip-flop 1 becomes an RZ multiplied signal as shown in FIG. 7(e). Further, the output of the NOR gate 2 is as shown in FIG. 7(d).

The non-inverting output of flip-flop 1 is output from delay circuit 1.
6 to the preliminary current generating circuit 103, which outputs a preliminary current IA as shown in FIG. 7(f).

In addition, the NOR gate output is the main pulse current generation circuit 1
00, this main pulse current generating circuit 1
From 00, the main pulse current 1.00 as shown in FIG. 7(g). is output.

Furthermore, since the bias current generating circuit 101 outputs the required bias current IB [see FIG. A current will flow [see Figure 7(h)]. If we focus on this composite current, we can see that before and after the main pulse current IM, there is a preliminary current level (.
-20- Stream IA is flowing.

Therefore, if the optical output-current characteristic (L-I characteristic) of the laser diode 4 is as shown in FIG. , before and after the main pulse current H, a preliminary current IA having a current level smaller than the main pulse current IM and larger than the bias current IB is applied, so the optical output of the laser diode 4 is as shown in FIG. 2(c). It becomes like this.

That is, the bias current IB is set to be smaller than the light emission threshold of the optical output, and the preliminary current IA is set to be slightly larger than the threshold. Therefore, in the bias current state, the laser diode 4 does not emit light, and when the preliminary current IA is supplied, the laser diode 4 emits light slightly, and when the main pulse current IM is further supplied, the laser diode 4 emits bright light. It has become.

As a result, in addition to obtaining a good spectrum with a compact device configuration (see Figure 8), as shown in Figure 9,
As can be seen from the characteristics shown in (ma), there is almost no deterioration due to extinction ratio deterioration or wavelength dispersion of the optical fiber. This enables long-distance and high-speed transmission by achieving both waveform and spectrum.

In addition, in FIG. 9, 0. The characteristics indicated by * are the characteristics related to the one shown in FIG. 11 (first conventional example), the characteristics indicated by ◯ indicate the case of a short optical fiber, and the characteristics indicated by ･ indicate the case of a long optical fiber. Similarly, the characteristics indicated by Δ and Mu are those related to the one shown in FIG. 13 (second conventional example), the characteristics indicated by Δ are for a short optical fiber, and the characteristics indicated by Mu are for a long optical fiber. shows. Further, the characteristics indicated by squares are related to this embodiment as described above, and the characteristics indicated by squares are for a short optical fiber, and the characteristics indicated by squares are for a long optical fiber.

By the way, since the preliminary current IA appears before and after the main pulse current, the output pulse waveform becomes symmetrical,
As a result, a phase shift in timing extraction in the receiver is less likely to occur, and reception errors are less likely to occur.

Further, the preliminary current generation circuit 103 generates the NRZ signal [Fig.
e)], a bipolar transistor, which operates at low speed and is low in cost, can be used as a semiconductor element for supplying the preliminary current IA to the laser diode 4, as shown in FIG.

Note that even if the preliminary current generation circuit 103 is configured with FETs,
There is no problem, and in this case, the configuration will be almost the same as the circuit shown in FIG. 4.

Furthermore, if high-speed transmission is not required, the main pulse current generating circuit 100 may be constructed of bipolar transistors. In this case, the main pulse current generating circuit 100 becomes almost the same as the circuit shown in FIG.

[Effects of the Invention] As detailed above, according to the laser diode drive method of the present invention, it is possible to achieve both long-distance transmission and high-speed transmission with a compact device configuration and with a compatible waveform and spectrum. One advantage of this is that it is less likely to cause a phase shift in timing extraction at the receiver.

Furthermore, since a bipolar transistor can be used as a part of the semiconductor element for driving the laser diode, there is also an advantage in terms of cost.

[Brief explanation of the drawing]

Fig. 1 is a principle block diagram of the present invention, Fig. 2 is an explanatory diagram of the operation of the present invention, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 is a main pulse of an embodiment of the present invention. FIG. 5 is an electric circuit diagram of a bias current generation circuit according to an embodiment of the present invention; FIG. 6 is an electric circuit diagram of a preliminary current generation circuit according to an embodiment of the present invention; Figures (a) to (h) are time charts for explaining one embodiment of the present invention, Figure 8 is a spectrum characteristic diagram of one embodiment of the present invention, and Figure 9 is an error rate characteristic of the present invention and the conventional method. A graph showing a comparison with that of the example, Fig. 10 is a block diagram of the conventional example, Fig. 11 is an action explanatory diagram of the conventional example, Fig. 12 is a spectrum characteristic diagram of the conventional example, and Fig. 13 is another conventional example. Fig. 14 is a spectrum characteristic diagram of another conventional example, Fig. 15 is a block diagram of yet another conventional example, Fig. 16 is a block diagram of the one devised in the process of creating the present invention, Fig. 17 The figure is an explanatory view of the operation of the one shown in FIG. 16. In the figure, 1 is a flip-flop, 2 is a NOR gate, 3 is a main pulse generator, 4 is a laser diode, 5 is a FET, 6 is a bias generator, 7 is a low-pass filter, 8 is a comparator, 9 is a rear monitor, 10 is a low-pass Filter, 11 is a bipolar transistor, 16 is a delay circuit, 19 is a preliminary signal generator, 2o is an output circuit, 20A, 20B are bipolar transistors, 100 is a main pulse current generation circuit, 101 is a bias current generation circuit, 103 is a preliminary current generation It is a circuit. (G) (C) (b) Samarai 1 + J/1 action Kichimen Meizu I footwork example Suho 7 drum special/l life chart Figure 12 (a) (b) Monk/l ina [rairai Fig. 13 A conventional illustration of Suhe Gutov A special/1/raw drawing Fig. 14

Claims (4)

[Claims] (1) In a laser diode drive method that directly modulates the laser diode (4) by setting it in off mode with a bias current flowing and turning it into on mode with a pulse current flowing, this pulse is applied before and after the pulse current. A laser diode driving method characterized by flowing preliminary currents each having a current level smaller than the current and larger than the bias current. (2) First and second semiconductor elements (5), (20A) are connected in parallel to the laser diode (4), and the pulse current is supplied through the first semiconductor element (5). The laser diode driving method according to claim 1, wherein the preliminary current is supplied through the second semiconductor element (20A). (3) The laser diode driving method according to claim 2, wherein the pulse current is generated based on an RZ pulse signal, and the preliminary current is generated based on an NRZ pulse signal. (4) The first semiconductor element is a field effect transistor (5
), and the second semiconductor element is a bipolar transistor (20A
) The laser diode drive system according to claim 3, comprising: