JP2008089336A - Optical pulse generator and optical pulse tester - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pulse generator having a narrower pulse width than ever before and an optical pulse tester using such optical pulse generator. <P>SOLUTION: The optical pulse generators LSa, LSb generate optical pulses by passing a pulsed drive current Iop through a laser diode 5 using a predetermined drive means. The drive means passes a preliminary drive current Ipre, which falls below a level causing induced light emission, through the laser diode 5 prior to the drive current Iop. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical pulse generator and an optical pulse tester using the same.

As is well known, an optical pulse tester makes an optical pulse incident on an optical fiber to be tested, and detects backscattered light obtained from the optical fiber to detect the characteristics of the optical fiber (such as the position of a transmission loss or fault point). ) And is also called an optical fiber test device or OTDR (Opticai Time Domain Reflect Meter). The optical pulse tester uses an optical pulse generator that generates the optical pulse by driving a laser diode with a predetermined drive circuit. Details of such an optical pulse tester and optical pulse generator are disclosed, for example, in the following patent document.
JP 2000-28384 A Japanese Patent Laid-Open No. 6-20482

  Meanwhile, an important performance item of the optical pulse tester is spatial resolution. As is well known, this spatial resolution is the discrimination performance of the scattering occurrence position on the optical fiber (that is, the distance from the optical pulse tester on the optical fiber) based on the optical pulse tester, and generally the pulse width of the optical pulse. The narrower it is, the better. In the conventional optical pulse generator, the pulse width of the optical pulse is narrowed by using a high-speed semiconductor device or a peaking circuit in the drive circuit, but it is not sufficient.

  The present invention has been made in view of the above-described circumstances, and provides an optical pulse generator having a narrower pulse width than the prior art, and an optical pulse tester using such an optical pulse generator. It is intended to do.

In order to achieve the above object, in the present invention, as a first solution means for an optical pulse generator, an optical pulse is generated by applying a pulsed drive current to a laser diode using a predetermined drive means. It is an optical pulse generator, and the driving means employs means for energizing the laser diode prior to the driving current with a pre-driving current lower than a level that causes stimulated light emission.
As a second solving means related to the optical pulse generator, in the first means, the driving means includes a laser driving section for supplying the driving current to the laser diode based on a predetermined driving signal, and the driving signal. A drive signal generation unit that generates and supplies the laser drive unit with a preliminary drive current based on a predetermined preliminary drive signal, and generates the preliminary drive signal and generates the preliminary drive signal. A means is provided that includes a preliminary drive signal generation unit that supplies the drive unit.
As a third solving means relating to the optical pulse generator, in the first means, the driving means is configured to generate the driving current based on a predetermined driving signal and the preliminary driving current based on a predetermined preliminary driving signal. A laser driver for energizing each of the laser diodes, a drive signal generator for generating the drive signal and supplying it to the laser driver, and a preliminary drive for generating the preliminary drive signal and supplying it to the laser driver A means is provided that includes a signal generation unit.
Further, in the present invention, as a means for solving the optical pulse tester, the optical pulse generated by the optical pulse generator according to any one of the first to third is supplied to the optical fiber to be tested. A means of measuring the characteristics of the optical fiber based on the return light obtained from the optical fiber is adopted.

According to the optical pulse generator of the present invention, since the pre-driving current that is lower than the level causing the induced light emission by the driving means is applied to the laser diode prior to the driving current, the differential resistance of the laser diode is reduced by the pre-driving current. In this state, the laser diode emits light by the drive current. As a result, the pulse width of the light pulse emitted from the laser diode can be made narrower than in the prior art (in a state where the preliminary drive current is not applied), and the rise can be made steep.
According to the optical pulse tester using such an optical pulse generator, the optical pulse supplied to the optical fiber to be tested is narrowed, so that the spatial resolution can be improved. In addition, the spatial resolution can be further improved by the steep rise of the light pulse.

Embodiments of the present invention will be described below with reference to the drawings.
[First Embodiment]
FIG. 1 is a block diagram showing a functional configuration of the optical pulse tester according to the first embodiment. As shown in this figure, this optical pulse tester A includes a drive signal generation unit 1, a laser drive unit 2, a preliminary drive signal generation unit 3, a preliminary drive unit 4, a laser element 5, a directional coupler 6, and a light receiving unit. 7, an amplification unit 8, an A / D conversion unit 9, and a display unit 10. Of these components, the drive signal generation unit 1, the laser drive unit 2, the preliminary drive signal generation unit 3, the preliminary drive unit 4, and the laser element 5 constitute the optical pulse generator LSa in the present embodiment, and The drive signal generation unit 1, the laser drive unit 2, the preliminary drive signal generation unit 3 and the preliminary drive unit 4 constitute a drive unit in the first embodiment.

  The drive signal generation unit 1 is a pulse generation circuit that generates a pulse signal that specifies the light emission timing of an optical pulse and outputs the pulse signal to the laser drive unit 2 as a drive signal. The laser drive unit 2 is a circuit that drives the laser element 5 by turning on / off based on the drive signal. The preliminary drive signal generation unit 3 is a pulse generation circuit that generates a pulse signal as a preliminary drive signal in synchronization with the drive signal generation unit 1.

  Although details will be described later, the preliminary drive signal (pulse signal) generated by the preliminary drive signal generation unit 3 has a certain time difference from the drive signal (pulse signal) generated by the drive signal generation unit 1. The preliminary drive unit 4 is a circuit that drives the laser element 5 in cooperation with the laser drive unit 2 by turning on / off based on the preliminary drive signal. The laser element 5 is a laser diode. When a driving current is injected by the laser driving unit 2 and the preliminary driving unit 4, an optical pulse having a short pulse width is generated and emitted to the directional coupler 6.

  FIG. 2 is a circuit diagram showing details of the laser driver 2, the preliminary driver 4, and the laser element 5. As shown in FIG. As shown in this figure, the laser drive unit 2 is composed of a drive transistor 2a, a constant current source 2b, and a DC power source 2c, and the preliminary drive unit 4 is composed of a drive transistor 4a and a constant current source 4b.

  The drive transistor 2a is an NPN transistor whose emitter is grounded. The laser element 5 connected in series to the output side (collector terminal) and the DC power source 2c are connected to each other, and the drive side is connected to the input side (base terminal). A drive signal is input from the signal generator 1, and a constant current source 2b is connected to the emitter terminal.

  The constant current source 2b is provided to set the emitter current of the drive transistor 2a (collector current substantially equal to the emitter current) to a predetermined current value, which causes the laser element 5 to emit light. It is set to a sufficient level. The DC power source 2 c is for positively biasing the laser drive unit 2 and the output side of the drive transistor 2 a and the laser element 5 (laser diode), and the positive electrode is connected to the anode terminal of the laser element 5. The laser element 5 has a cathode terminal connected to the collector terminal of the drive transistor 2a.

  The drive transistor 4a is an NPN transistor whose emitter is grounded in the same manner as the drive transistor 2a of the laser drive unit 2, and the output side (collector terminal) is commonly connected to the output side (collector terminal) of the drive transistor 2a of the laser drive unit 2. Has been. A preliminary drive signal is input from the preliminary drive signal generator 3 to the input side (base terminal) of the drive transistor 4a, and a constant current source 4b is connected to the emitter terminal.

  The constant current source 4b is provided to set the emitter current of the drive transistor 4a (collector current substantially equal to the emitter current) to a predetermined current value. Note that the current value of the constant current source 4b is different from the current value of the constant current source 2b of the laser driving unit 2 described above, and is lower than a level necessary for causing the laser element 5 to emit light, for example, the laser element 5 The level is set slightly below the threshold current for transitioning from the OFF state to the ON state.

  The directional coupler 6 transmits the light pulse toward the optical fiber F to be tested, and emits return light incident from the optical fiber F toward the light receiving unit 7. The light receiving unit 7 photoelectrically converts the return light into an electric signal (light reception signal) and outputs it to the amplifier 8. The amplifying unit 8 amplifies the light reception signal with a predetermined amplification and outputs the amplified signal to the A / D conversion unit 9. The A / D conversion unit 9 receives the light reception signal (analog signal) input from the amplification unit 8. Is sampled at a predetermined time interval to convert it into a digital signal (light reception data) and output it to the display unit 10. That is, the light reception data is time series data indicating the intensity change of the return light.

  The display unit 10 performs predetermined signal processing on the received light data sequentially input as time series data from the A / D conversion unit 9 to convert it into display data, and displays a measurement screen based on the display data. As will be described later, this measurement screen shows the intensity change (time change) of the return light converted into the distance of the optical fiber F with the optical pulse tester A as a base point.

  Next, the operation of the optical pulse tester A configured as described above, particularly the operation of the optical pulse generator LSa, which is a feature of the optical pulse tester A, will be described.

FIG. 3 is a waveform diagram showing the waveform of each part in the optical pulse generator LSa. As shown in this figure, the drive signal output from the drive signal generation unit 1 to the laser drive unit 2 (drive transistor 2a) is a pulse signal having a predetermined pulse width Tw-iop. The preliminary drive signal output from the generation unit 3 to the drive transistor 4a of the preliminary drive unit 4 is a pulse signal that rises ahead of the drive signal by a time width Tw-pre.
The drive signal and the preliminary drive signal have different rising timings as described above, but the falling timing of the preliminary driving signal is set to coincide with the falling timing of the driving signal as shown in the figure.

  The drive transistor 2a of the laser drive unit 2 is turned off when the drive signal is at the L (low) level, and is turned on when the drive signal is at the H (high) level, thereby turning the laser element 5 on and off in the same manner. On the other hand, the drive transistor 4a of the preliminary drive unit 4 is turned off when the preliminary drive signal is at L (low) level, and is turned on when the preliminary drive signal is at H (high) level. Only in advance, transition from the OFF state to the ON state.

  As a result, the drive current as shown in the drive current waveform shown in the figure is supplied to the laser element 5 from the DC power source 2c. That is, a pulse-like light emission injection current Iop (injection current having a pulse width Tw-iop caused by a drive signal) at a level that causes stimulated light emission flows in the laser element 5 and is preliminarily preceded by the light emission injection current Iop. A minute pre-injection current Ipre (injection current lower than a level causing induced light emission) due to the drive signal flows. The light emission injection current Iop is a current value of a level necessary for causing light emission set by the constant current source 2b of the laser drive unit 2, while the preceding injection current Ipre is a constant value of the preliminary drive unit 4. The current value is lower than the level necessary for causing the light emission set by the current source 4b.

  Then, the optical pulse generated by the laser element 5 injects the preceding injection current Ipre into the laser element 5 in advance of the emission injection current Iop, so that the preceding injection current Ipre as shown in the illustrated optical pulse waveform. In the case of not injecting (conventional), the width becomes narrower and the rise becomes steeper than in the conventional case. In addition, when the pulse width Tw-iop is small, the drive current is not injected before the light pulse has risen in the past, but in this embodiment, the rise is steeper than in the conventional case, so that the light is lighter than in the conventional case as shown in the figure. The peak value of the pulse increases.

  As is well known, the differential resistance of a laser diode has non-linearity with respect to the drive current, and the laser diode is in an ON state (inductive light emission) from an OFF state (a state that emits spontaneous emission light but does not perform stimulated light emission). In the region of the drive current below the threshold value for transition to the light emission state), the resistance value greatly decreases as the drive current increases. On the other hand, if the drive current exceeds the threshold value, the resistance is changed even if the drive current changes. The change in value is relatively small.

  In the optical pulse generator LSa of the present optical pulse tester A, the preceding injection current Ipre is set to a level slightly lower than the threshold value for the laser element 5 (laser diode) to transition from the OFF state to the ON state. By supplying the preceding injection current Ipre to the laser element 5 prior to the emission injection current Iop, the emission injection current Iop is continuously injected into the laser element 5 in a state where the resistance value of the laser element 5 is reduced. Therefore, it is considered that the light pulse generated by the laser element 5 is narrowed and the rise is steep.

  The optical pulse generated by the optical pulse generator LSa in this way enters the directional coupler 6 and enters the optical fiber F through the directional coupler 6. As a result, scattered light is sequentially generated in the optical fiber F at the propagation position of the optical pulse, and the backscattered light scattered in the direction opposite to the propagation direction of the optical pulse propagates in the optical fiber F. Then, it enters the directional coupler 6 as return light. In addition, in a position where a reflection element such as a connector exists in the optical fiber F, reflected light of an optical pulse is generated, and the reflected light also enters the directional coupler 6 as return light.

  Such return light is received by the light receiving unit 7 via the directional coupler 6 and converted into a light reception signal (electrical signal) by the light receiving unit 7. The light reception signal is amplified by the amplification unit 8, converted into light reception data (time-series data) by the A / D conversion unit 9, and input to the display unit 10. Then, the display unit 10 generates and displays a measurement screen as shown in FIG. 4 based on the received light data.

  In this measurement screen, the overall inclination of the light intensity indicates the transmission loss of the optical fiber F, and the sudden change in the light intensity at the distance La indicates the occurrence of reflection. Spatial resolution indicates the discrimination position of such reflection, that is, the distance La, but in this optical pulse tester A, the optical pulse generator LSa generates an optical pulse that is narrower than before. Therefore, such spatial resolution can be improved as compared with the conventional optical pulse tester. Also, the spatial resolution can be further improved by the steep rise of the light pulse.

In the laser element 5 (laser diode), ASE light (spontaneously emitted light) having an intensity corresponding to the relationship between the injection current value corresponding to the emission threshold value of the laser element 5 and the injection current generated by the preliminary drive signal is generated. To do. Since the intensity of the return light incident from the optical fiber F is weak, the optical pulse tester increases the dynamic range between the peak level and the noise level of the optical pulse in order to improve the measurement accuracy. Although it is necessary to improve the N ratio, the ASE light reduces the S / N ratio.
However, in this embodiment, since the fall timing of the preliminary drive signal coincides with the fall timing of the drive signal, ASE light is not generated after the output of the optical pulse, and thus the S / N ratio is lowered. There is nothing.

  In addition, since the optical pulse tester is often driven by a battery, it is required to reduce power consumption. However, by making the falling timing of the preliminary driving signal coincide with the falling timing of the driving signal, the extra time by the preliminary driving signal can be reduced. Power consumption can be suppressed, so that the battery can last longer.

[Second Embodiment]
Next, an optical pulse tester according to the second embodiment will be described.
FIG. 5 is a block diagram showing a functional configuration of the optical pulse tester B according to the second embodiment. This optical pulse tester B is different from the optical pulse generator LSa of the first embodiment only in the configuration of the optical pulse generator LSb as shown in the figure. In FIG. 5, the same components as those of the optical pulse tester A according to the first embodiment are denoted by the same reference numerals.

  That is, the optical pulse generator LSb in the optical pulse tester B drives the laser drive unit 2 with both the drive signal output from the drive signal generation unit 1 and the preliminary drive signal output from the preliminary drive signal generation unit 3B. The drive transistor 2a is driven by a composite drive signal that is applied to the base terminal of the transistor 2a, that is, a drive signal and a preliminary drive signal. Note that the drive signal generation unit 1, the laser drive unit 2, and the preliminary drive signal generation unit 3B constitute drive means in the second embodiment.

Here, the preliminary drive signal output from the preliminary drive signal generation unit 3 of the first embodiment described above is a pulse signal having a level sufficient to turn on / off the drive transistor 4a. As shown in FIG. 6, the preliminary drive signal generator 3B in FIG. 6 operates a pulse signal having a level smaller than that of the preliminary drive signal of the preliminary drive signal generator 3, that is, the drive transistor 4a in the active region. A combined drive signal obtained by combining the preliminary drive signal and the drive signal is a stepped pulse signal as shown in FIG.
In the second embodiment as well, the falling timing of the drive signal and the preliminary drive signal is set to be the same in order to prevent the generation of ASE light after the output of the optical pulse.

In this composite drive signal, the pre-injection current Ipre (injection current below the level necessary for causing light emission) shown in the waveform diagram of FIG. 3 is set by the waveform portion S1 corresponding to the level of the preliminary drive signal, and the drive The light emission injection current Iop (injection current of the pulse width Tw-iop caused by the drive signal) is set by the waveform portion S2 corresponding to the total level of the signal and the preliminary drive signal.
Therefore, according to the second embodiment, the optical pulse generated by the laser element 5 can be made narrower than before and the rise can be made steeper than before as in the first embodiment. Can do.

The present invention is not limited to the above-described embodiments. For example, the following modifications can be considered.
(1) In the above embodiments, the fall timing of the drive signal and the fall timing of the preliminary drive signal are the same timing. However, the drive signal may fall at a timing before the drive signal falls. good.
In addition, when the above-described decrease in the S / N ratio of the return light due to the ASE light can be ignored (for example, when the optical pulse generator is applied to an application other than the optical pulse tester), the preliminary drive is performed. The signal may be constantly set to H (high) level.
Furthermore, the preliminary drive signal may fall when a predetermined time elapses after the drive signal falls.
(2) In the above embodiments, the optical pulse generator is applied to an optical pulse tester. However, the optical pulse generator according to the present invention can be applied to various applications other than the optical pulse tester. it can.

It is a block diagram which shows the function structure of the optical pulse tester A concerning 1st Embodiment of this invention. It is a circuit diagram which shows the detailed structure of the optical pulse generator LSa in 1st Embodiment of this invention. It is a wave form diagram which shows each part waveform in the optical pulse generator LSa of 1st Embodiment of this invention. It is a schematic diagram of the measurement screen in 1st Embodiment of this invention. It is a block diagram which shows the function structure of the optical pulse tester B concerning 2nd Embodiment of this invention. It is a wave form diagram which shows each part waveform in the optical pulse generator LSb of 2nd Embodiment of this invention.

Explanation of symbols

  A, B ... Optical pulse tester, LSa, LSb ... Optical pulse generator, 1 ... Drive signal generator, 2 ... Laser drive unit, 3, 3B ... Preliminary drive signal generator, 4 ... Preliminary drive unit, 5 ... Laser Element: 6 ... Directional coupler, 7 ... Light receiving unit, 8 ... Amplifying unit, 9 ... A / D conversion unit, 10 ... Display unit, F ... Optical fiber

Claims (4)

An optical pulse generator that generates an optical pulse by energizing a pulsed drive current to a laser diode using a predetermined driving means,
The optical pulse generator, wherein the driving means supplies a pre-driving current lower than a level causing induced light emission to the laser diode prior to the driving current. The driving means includes
A laser drive unit for supplying the drive current to the laser diode based on a predetermined drive signal;
A drive signal generator that generates the drive signal and supplies the drive signal to the laser driver;
A pre-driving unit for energizing a pre-driving current based on a predetermined pre-driving signal to the laser diode;
The optical pulse generator according to claim 1, further comprising: a preliminary drive signal generation unit that generates the preliminary drive signal and supplies the preliminary drive signal to the preliminary drive unit. The driving means includes
A laser drive unit that supplies the drive current to the laser diode based on a predetermined drive signal and the preliminary drive current to the laser diode based on a predetermined preliminary drive signal;
A drive signal generator that generates the drive signal and supplies the drive signal to the laser driver;
The optical pulse generator according to claim 1, further comprising: a preliminary drive signal generation unit that generates the preliminary drive signal and supplies the preliminary drive signal to the laser driving unit.   An optical pulse generated by the optical pulse generator according to any one of claims 1 to 3 is supplied to an optical fiber to be tested, and the characteristics of the optical fiber are measured based on return light obtained from the optical fiber. An optical pulse tester characterized by

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