Abstract:
There is provided a light pulse generator. The light pulse generator includes: a laser diode; a voltage source that provides a bias voltage to the laser diode; a switching element that causes the laser diode to emit a light pulse by directly modulating the laser diode; and an auxiliary current circuit which starts to charge immediately after turn-on of the switching element and which starts to discharge after a forward current flows through the laser diode so as to provide a auxiliary current to the laser diode in the same direction as the forward current.

Description:
[0001]    This application claims priority from Japanese Patent Application No. 2009-189293, filed on Aug. 18, 2009, the entire contents of which are incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a light pulse generator and an optical time domain reflectometer using the light pulse generator. More specifically, the present disclosure relates to a light pulse generator which outputs light pulses having a steep rising edge and an optical time domain reflectometer using the light pulse generator. 
         [0004]    2. Related Art 
         [0005]    In optical communication systems which perform a data communication using an optical signal, it is important to monitor optical fibers for transmitting optical signals. An optical time domain reflectometer (hereinafter abbreviated as “OTDR”) is used in installation, maintenance or the like of optical fibers. 
         [0006]    The OTDR performs measurements relating to a disconnection, a loss or the like of an optical fiber to be measured by providing repetitive light pulses to the optical fiber from a measurement connector provided at the entrance/exit end of the OTDR and measuring levels and reception times of return light beams (reflection light beams, back scattering light beams, etc.) coming from the optical fiber. 
         [0007]    A light pulse generator is used in OTDRs as a light source for emitting light pulses to an optical fiber to be measured (see JP-A-2008-089336 and JP-A-2008-107319, for example). 
         [0008]      FIG. 9  is a circuit diagram showing the configuration of a related-art light pulse generator. As shown in  FIG. 9 , the light pulse generator includes: a laser diode  11 ; a transistor  12 ; a constant current source  13 ; a constant voltage source  14 ; and a modulation control signal source  15 . The light pulse generator emits light pulses. The laser diode  11 , the transistor  12 , the constant current source  13 , and the constant voltage source  14  form a closed loop. 
         [0009]    The laser diode (LD)  11  emits light pulses for measurement of an optical fiber. 
         [0010]    The transistor  12 , which is a switching element, is turned on/off (i.e., conduction between its collector and emitter is established/canceled) in response to a control signal that is provided to the base from the modulation control signal source  15 . The collector of the transistor  12  is connected to the cathode of the LD  11 . 
         [0011]    The constant current source  13 , one end of which is connected to the emitter of the transistor  12 , causes a flow of a constant emitter current while the transistor  12  is on ((emitter current)≅(collector current)). 
         [0012]    The constant voltage source  14 , the positive pole side of which is connected to the anode of the LD  11 , forward-biases the LD  11 . 
         [0013]    The modulation control signal source  15  provides, to the base of the transistor  12 , a modulation control signal for turning on/off the transistor  12 . 
         [0014]    The closed loop formed by the LD  11 , the transistor  12 , the constant current source  13 , and the constant voltage source  14  will be hereinafter referred to as a forward current loop. A current that flows through the LD  11  in the forward direction in the forward current loop will be hereinafter referred to as an LD forward current Id. 
         [0015]    The components  11 - 14  are mounted on a printed circuit board(s) or the like and electrically connected to each other by printed interconnections on the board, cables connecting the boards, etc. As a result, inductances L 1 -L 4  occur in the interconnections and cables connecting the components  11 - 14 . In other words, the series wiring inductances L 1 -L 4  exist in the forward current loop. 
         [0016]    The operation of the above laser pulse generator will be described below. 
         [0017]    The modulation control signal source  15  provides, to the base of the transistor  12 , a modulation control signal for turning on/off the transistor  12 . The transistor  12  is turned on when the level of the modulation control signal is changed from low to high, and the transistor  12  is kept on while the modulation control signal is kept at the high level. 
         [0018]    While the transistor  12  is on, the LD  11  is forward-biased by the constant voltage source  14  and the LD forward current Id (the constant current of the constant current source  13 ) flows through the LD  11 . The LD  11  emits laser light if the LD forward current Id is larger than its threshold current. 
         [0019]    On the other hand, the transistor  12  is turned off when the level of the modulation control signal supplied from the modulation control signal source  15  is changed from high to low, and the transistor  12  is kept off and the forward current loop is kept open while the modulation control signal is kept at the low level. In this state, the LD forward current Id is shut off and the LD  11  does not emit laser light. 
         [0020]    As described above, the LD forward current Id of the LD  11  is caused to flow or shut off by tuning on or off the transistor  12 . The LD  11  is caused to emit light pulses by directly intensity-modulating in the LD  11 . 
         [0021]      FIG. 10  is a graph showing a laser light emission characteristic of the LD  11 , wherein the horizontal axis represents the LD forward current Id and the vertical axis represents the laser output optical power. The laser output power increases as the LD forward current Id increases after it exceeds the threshold current. 
         [0022]    To cause the LD  11  to emit a light pulse, it is necessary to cause a pulse-shaped LD forward current that is proportional to an optical power of laser light to flow through the LD  11 . It is possible, by using electronic components on the market, to cause the modulation control signal source  15  to generate a control signal having a pulse width of several nanoseconds and provide it to the transistor  12  and to cause the transistor  12  to be turned on/off in response to such a modulation control signal. 
         [0023]    On the other hand, the printed circuit board on which the components  11 - 14  are mounted and the components  11 - 14  themselves have the inductances L 1 -L 4  as shown in  FIG. 9 . 
         [0024]    An inductance L′ in the forward current loop and the LD forward current Id which flows through the forward current loop while the transistor  12  is on are given by the following Equations (1) and (2), respectively. 
         [0000]        L′=L 1 +L 2 +L 3 +L 4  (1)
 
         [0000]        Id=T on·( E 1 −Vf )/ L′   (2)
 
         [0025]    In Equations (1) and (2), L′ is a combined wiring inductance of the inductances L 1 -L 4 . Ton is the elapsed time from turn-on of the transistor  12 , E 1  is the voltage of the constant voltage source  14 , and Vf is the voltage between the two terminals of the LD  11  (i.e., the forward voltage of the LD  11 ). 
         [0026]    Therefore, ΔId/ΔTon is restricted by the forward current loop during a period from turn-on of the transistor  12  to a start of laser light emission of the LD  11 . That is, it is difficult to generate a laser light pulse having a steep rising edge using a large current. 
         [0027]    Further, it is difficult to remove the inductances of the components  11 - 15  and the printed circuit board on which they are mounted, and hence shortening of the width of a light pulse becomes more difficult as the necessary LD forward current Id increases (i.e., as the necessary output optical power of the LD  11  increases). 
       SUMMARY 
       [0028]    Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above. 
         [0029]    Accordingly, it is an illustrative aspect of the present invention to provide a light pulse generator capable of outputting a light pulse having a steep rising edge and an optical time domain reflectometer using such a light pulse generator. 
         [0030]    According to one or more illustrative aspects of the present invention, there is provided a light pulse generator. The light pulse generator includes: a laser diode; a voltage source that provides a bias voltage to the laser diode; a switching element that causes the laser diode to emit a light pulse by directly modulating the laser diode; and an auxiliary current circuit which starts to charge immediately after turn-on of the switching element and which starts to discharge after a forward current flows through the laser diode so as to provide a auxiliary current to the laser diode in the same direction as the forward current. 
         [0031]    According to one or more illustrative aspects of the present invention, there is provided an optical time domain reflectometer which provides a light pulse to an optical fiber and measures a characteristic of the optical fiber based on return light of the light pulse, coming from the optical fiber. The reflectometer includes: the light pulse generator. 
         [0032]    Other aspects of the invention will be apparent from the following description, the drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is a circuit diagram showing the configuration of a light pulse generator according to an embodiment of the present invention; 
           [0034]      FIG. 2  shows waveforms of a forward voltage and output optical power of a laser diode of the light pulse generator shown in  FIG. 1 ; 
           [0035]      FIGS. 3A and 3B  are circuit diagrams showing how capacitors (auxiliary current circuit) are charged and discharged in the light pulse generator shown in  FIG. 1 ; 
           [0036]      FIG. 4  shows laser light pulse waveforms of the circuit of  FIG. 1  and the circuit of  FIG. 9 ; 
           [0037]      FIG. 5  is a circuit diagram illustrating low-voltage driving of the laser diode in the light pulse generator shown in  FIG. 1 ; 
           [0038]      FIG. 6  schematically shows relationships between various voltages and currents in the light pulse generator shown in  FIG. 1  in the case of low-voltage driving of the laser diode; 
           [0039]      FIGS. 7A and 7B  show laser light pulse waveforms of the light pulse generator of  FIG. 9  and the light pulse generator of  FIG. 1 , respectively, in the case of low-voltage driving of the laser diode; 
           [0040]      FIG. 8  is a block diagram showing the configuration of an optical time domain reflectometer using the light pulse generator of  FIG. 1 , according to another embodiment of the invention; 
           [0041]      FIG. 9  is a circuit diagram showing the configuration of a related-art light pulse generator; and 
           [0042]      FIG. 10  is a graph showing a laser light emission characteristic of a laser diode. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0043]    Embodiments of the present invention will be now described in detail with reference to the drawings. 
         [0044]      FIG. 1  is a circuit diagram showing the configuration of a light pulse generator according to an embodiment of the invention. The same components as those in  FIG. 9  will be given the same reference symbols as the latter and will not be described in detail. 
         [0045]    As shown in  FIG. 1 , capacitors C 1 -C 4  are provided in parallel with the LD  11 . Although in  FIG. 1  the four capacitors C 1 -C 4  are provided in parallel with the LD  11 , the number of such capacitors may be any number, that is, may be either one or a plural number. 
         [0046]    The capacitors C 1 -C 4  correspond to the term “auxiliary current circuit”. 
         [0047]    The operation of the above-configured light pulse generator will be described below.  FIG. 2  shows waveforms of a forward voltage and output optical power of the LD  11 . The horizontal axes represent time, and the vertical axes represent the forward voltage (bias voltage; Vf in  FIG. 1 ) provided to the LD  11  and the output optical power of the LD  11 . 
         [0048]    Firstly, a description will be made of a transient characteristic (a transient period shown in  FIG. 2 ) of the forward voltage of the LD  11  when it starts light emission. 
         [0049]    The modulation control signal source  15  provides, to the transistor  12 , a modulation control signal for turning on and off the transistor  12 . When the modulation control signal is at the low level, the transistor  12  is off and hence the LD  11  is in the unbiased state (no bias voltage is supplied from the constant voltage source  14 ). When the modulation control signal is changed from the low level to the high level, the transistor  12  is turned on. 
         [0050]    The LD  11  starts to be driven from the unbiased state, and the differential resistance (initial differential resistance) of the LD  11  is very large immediately after the turn-on of the transistor  12  (time t 0  (see FIG.  2 )), that is, immediately after the application of a bias voltage. Therefore, almost no LD forward current Id flows through the LD  11 . As a result, immediately after the turn-on of the transistor  12  (in the period when the differential resistance is large), a large voltage drop occurs between the two terminals of the LD  11 . The term “large” is a result of comparison with a voltage drop between the two terminals of the LD  11  in a steady state (a steady-state period is shown in  FIG. 2 ). 
         [0051]    After a lapse of a prescribed time from the turn-on of the transistor  12 , a current starts flowing through the LD  11  and the resistance of the LD  11  decreases rapidly. The LD forward current Id increases quickly and exceeds the threshold current, whereupon the LD  11  starts to emit laser light. In the light emission steady state, the voltage between the two terminals of the LD  12  (forward voltage Vf) has a constant value. The LD  11  is forward-biased by the constant voltage source  14  and a constant LD forward current Id of the constant current source  13  flows through the LD  11 . The LD  11  continues to emit laser light as long as the LD forward current Id is larger than the threshold current. 
         [0052]    Next, the operation of the capacitors C 1 -C 4  (auxiliary current circuit) will be described.  FIGS. 3A and 3B  are circuit diagrams showing how the capacitors C 1 -C 4  are charged and discharged. In  FIGS. 3A and 3B , the capacitors C 1 -C 4  are together represented by a capacitor C′. 
         [0053]    As described above, in the activation transient period, the LD  11  exhibits a light emission forward voltage transient characteristic. Therefore, while the transistor  12  which is connected in series with the LD  11  is on and the differential resistance (initial differential resistance) of the LD  11  is large (a period T 1  in  FIG. 2 ), the capacitor C′ which is connected in parallel with the LD  11  is charged to produce the forward voltage Vf. No laser light is emitted during this period. 
         [0054]    When the LD forward current Id starts flowing through the LD  11 , because of a second-phase of the light emission forward voltage transient characteristic of the LD  11  (a forward voltage attenuation characteristic with a rapid decrease of the resistance of the LD  11 ; a period T 2  shown in  FIG. 2 ), the forward voltage Vf becomes lower than a peak value in the period T 1 . The capacitor C′ is discharged to cause an additional current Ia to flow through the LD  11 . That is, in addition to the LD forward current Id, the auxiliary current Ia coming from the capacitor C′ flows through the LD  11 . 
         [0055]    The inductance of the current path (called an auxiliary current path) consisting of the LD  11  and the capacitor C′ is smaller than the series inductance L′ of the forward current loop. As in the light pulse generator of  FIG. 9 , the LD forward current Id flowing through the forward current loop is affected by the series inductance L′ and hence affected by ΔId/ΔTon (immediately after activation). On the other hand, since the inductance of the auxiliary current path consisting of the LD  11  and the capacitor C′ is smaller than the series inductance L′, ΔIa/ΔTon is larger than ΔId/ΔTon (immediately after activation). 
         [0056]    The LD  11  starts laser light emission when the auxiliary current Ia coming from the capacitor C′ exceeds the threshold current of the LD  11 . More strictly, the LD  11  emits laser light when the auxiliary current Ia plus the LD forward current Id exceed the threshold current. However, since as described above ΔIa/ΔTon is larger than ΔId/ΔTon immediately after activation, almost only the auxiliary current Ia contributes to the start of laser light emission. 
         [0057]    Furthermore, as shown in  FIG. 10 , the output optical power of the LD  11  increases in proportion to the forward current flowing through the LD  11 . Therefore, laser light having a steep rising edge is emitted during the period T 2  (when the auxiliary current Ia flows through the auxiliary current path) of the transient period (see  FIG. 2 ). 
         [0058]    A steady state is established upon completion of the discharge of the capacitor C′, and only a constant LD forward current Id flows through the forward current loop as in the related-art light pulse generator shown in  FIG. 9  (a period T 3  shown in  FIG. 2 ). 
         [0059]    When the modulation control signal which is supplied from the modulation control signal source  15  is changed from the high level to the low level, the transistor  12  is turned off and the forward current loop is opened. As a result, the LD forward current Id is shut off and the laser light emission from the LD  11  is stopped. 
         [0060]      FIG. 4  shows laser light pulse waveforms of the case with the capacitor C′ (circuit of  FIG. 1 ) and the case without the capacitor C′ (circuit of  FIG. 9 ). As can be seen from  FIG. 4 , the rising edge of the light pulse is steeper in the case with the capacitor C′ than in the case without the capacitor C′. The optical power level is increased by the auxiliary current Ia coming from the capacitor C′. Each of the waveforms of  FIG. 4  is of a short light pulse and has no steady-state period T 3 . 
         [0061]    As described above, since the auxiliary current path whose inductance is smaller than the series inductance L′ of the forward current loop is formed by connecting the capacitors C 1 -C 4  parallel with the LD  11 , a light pulse having a steep rising edge can be emitted even if the current is increased to obtain a large optical power. 
         [0062]    When the transistor  12  is turned on and a forward bias voltage starts to be applied to the LD  11  from the constant voltage source  14 , the capacitor C′ which is provided parallel with the LD  11  starts to be charged (period T 1 ) because of the light emission forward voltage transient characteristic of the LD  11 . When a forward current starts flowing through the LD  11 , the capacitor C′ starts to be discharged and causes an additional current to flow through the LD  11  in the same direction as the forward current (period T 2 ). The auxiliary current coming from the capacitor C′ which is parallel with the LD  11  is not affected by the inductances between the LD  11 , the transistor  12 , the constant current source  13 , and the constant voltage source  14 . Therefore, a current having a steep rising edge can flow through the LD  11 , as a result of which a light pulse having a steep rising edge can be emitted. 
         [0063]    Next, a description will be made of the voltage level of the constant voltage source  14  of the light pulse generator. As described above, the LD forward current Id which flows through the forward current loop is affected by the series inductance L′ and hence by ΔId/ΔTon (immediately after activation). 
         [0064]    On the other hand, where a battery is used as a power source for driving the LD  11 , the LD forward current Id is also prone to be affected by the saturation of the transistor  12 . This will be described below. Immediately after turn-on of the transistor  12  (in the period when its differential resistance is large), a large voltage drop occurs between the two terminals of the LD  11 . Where a commercial power line (100 V), for example, is used, it is easy to apply tens of volts (e.g., 30 to 50 V). On the other hand, in the case of battery driving (power source voltage: several volts to a little more than 10 V), the application voltage is at most several volts (e.g., 5 V). 
         [0065]    Therefore, when a large voltage drop occurs between the two terminals of the LD  12 , the voltage applied between the emitter and the collector of the transistor  12  is lower and the transistor  12  is saturated longer in the case of battery driving than in the case of using a commercial power line. That is, generation of a laser light pulse having a steep rising edge becomes more difficult as the voltage of the constant voltage source  14  becomes lower. 
         [0066]    In the related art light pulse generator of  FIG. 9  which is not provided with the auxiliary current circuit (capacitor C′), in the case of battery driving in which the application voltage is lower than in the case of using a commercial power line, immediately after power-on of the transistor  12 , the transistor  12  is saturated because of the forward voltage transient characteristic of the LD  12 . As a result, a rectangular-wave-shaped pulse current cannot flow through the LD  11 . 
         [0067]    On the other hand, where the auxiliary current circuit (capacitor C′) is provided parallel with the LD  11 , a current flows through the capacitor C′ bypassing the LD  11  in the light emission transient period (the period T 1  in  FIG. 2 ) of the LD  11 . As a result, the saturation time of the transistor  12  can be shortened and the waveform quality of a light pulse can be improved (i.e., the rising edge can be made steeper) even in the case of battery driving or the like. 
         [0068]    A more detailed description will be made with reference to  FIGS. 5-7 . 
         [0069]      FIG. 5  is a circuit diagram illustrating low-voltage driving of the LD  12  in the light pulse generator. To simplify the description, the inductances L 1 -L 4 , the constant current source  13 , and the modulation control signal source  15  are omitted in  FIG. 5 . The voltage of the constant voltage source  14  is represented by E and the minus-side potential of the constant voltage source  14  is assumed to be 0 V. The voltage between the minus-side of the constant voltage source  14  and the emitter of the transistor  12  is represented by Vr, and the emitter current (in the direction from the transistor  12  to the minus-side of the constant voltage source  14 ) is represented by Ir. The forward voltage Vf, the auxiliary current Ia, and the LD forward current Id are defined in the same manners as in  FIGS. 3A and 3B . 
         [0070]      FIG. 6  schematically shows relationships between the forward voltage Vf, the emitter voltage Vr, the auxiliary current Ia, the current (Id+Ia) flowing through the LD  11 , and the emitter current Ir. The horizontal axis represents time. 
         [0071]      FIGS. 7A and 7B  show actually measured light pulse waveforms of a case without the auxiliary current circuit (light pulse generator of  FIG. 9 ) and a case with the auxiliary current circuit (light pulse generator of  FIG. 1 ), respectively. 
         [0072]    As seen from  FIGS. 7A and 7B , the influence of the saturation of the transistor  12  can also be reduced and a light pulse having a steep rising edge can be emitted, because the auxiliary current Ia flows through the capacitor C′ bypassing the LD  11  (along the path shown in  FIG. 3A ) in the transient period T 1  (from the turn-on of the transistor  12  to the light emission of the LD  11 ) and then the auxiliary current Ia flows through the LD  11  in the same direction as the forward current while the capacitor C′ is discharged (in the transient period T 2 ). 
       Second Embodiment 
       [0073]      FIG. 8  is a block diagram showing the configuration of an OTDR which uses the light pulse generator of  FIG. 1 . In  FIG. 8 , a measurement subject optical fiber F 1  is a line for transmitting an optical signal. 
         [0074]    The OTDR  100  has, at the entrance/exit end, a measurement connector CN to which the measurement subject optical fiber F 1  is connected. Light pulses are input to the measurement subject optical fiber F 1  from the measurement connector CN. Return light beams (reflection light beams or back scattering light beams) of the light pulses that are input to the measurement subject optical fiber F 1  are input to the OTDR  100  via the measurement connector CN. 
         [0075]    The OTDR  100  is equipped with the light pulse generator  10  of  FIG. 1 , a directional coupler  20 , a light receiving unit  30 , a sampling unit  40 , a signal processor  50 , and a display unit  60 . The light pulse generator  10  inputs light pulses to the measurement subject optical fiber F 1  via the directional coupler  20  and the measurement connector CN according to an instruction from the signal processor  50 . 
         [0076]    The directional coupler  20  inputs light coming from the light pulse generator  10  to the measurement subject optical fiber F 1  via the measurement connector CN, and supplies the light receiving unit  30  with return light coming from the measurement subject optical fiber F 1  via the measurement connector CN. 
         [0077]    The light receiving unit  30 , which is an avalanche photodiode, for example, outputs a photocurrent corresponding to optical power of the return light. 
         [0078]    The sampling unit  40  converts an electrical signal (photocurrent) supplied from the light receiving unit  30  into a voltage and samples it. The signal processor  50  causes the light pulse generator  10  to emit light pulses. The signal processor  50  causes the sampling unit  40  to perform sampling, and performs computation on a resulting electrical signal. The display unit  60  displays a processing result of the signal processor  50 . 
         [0079]    The operation of the above-configured OTDR  100  will be described below. 
         [0080]    The signal processor  50  sets a light pulse width (i.e., an on-time of the transistor  12 ) in the modulation control signal source  15  of the light pulse generator  10  in advance. A timing generator (not shown) of the signal processor  50  sends timing signals to the modulation control signal source  15  at prescribed intervals. The modulation control signal source  15  turns on the transistor  12  in synchronism with the timing signals and thereby causes the LD  11  to emit light pulses. The light pulses emitted from the LD  11  are provided to the measurement subject optical fiber F 1  via the directional coupler  20  and the measurement connector CN. 
         [0081]    Rayleigh scattering occurs inside the measurement subject optical fiber F 1 , and part of scattering light goes in the direction that is reverse to the traveling direction of the input light pulse and returns to the OTDR  100  as return light. Fresnel reflection light that is generated at a connecting point or a breaking point of the measurement subject optical fiber F 1  also returns to the OTDR  100 . 
         [0082]    The return light coming from the measurement subject optical fiber F 1  enters the light receiving unit  30  via the measurement connector CN and the directional coupler  20 . The light receiving unit  30  converts the received return light into an electrical signal (photocurrent) corresponding to optical power of the return light, and outputs the electrical signal to the sampling unit  40 . 
         [0083]    An I-V conversion circuit (not shown) of the sampling unit  40  converts the photocurrent supplied from the light receiving unit  30  into a voltage, and a multi-stage amplifier (not shown) of the sampling unit  40  amplifies the voltage. Then, an A-D conversion circuit (not shown) of the sampling unit  40  converts the analog electrical signal into a digital signal using, as a temporal reference, a timing signal supplied from the signal processor  50 , and supplies the digital signal to the signal processor  50 . 
         [0084]    The signal processor  50  determines a time from the emission of the light pulse from the LD  11  to the detection of the return light by the light receiving unit  30  based on the output timing of the timing signal and the digital signal supplied from the sampling unit  40 . The signal processor  50  thus measures distances in the measurement optical fiber F 1  and levels of return light optical signals. Measurement results are displayed on the display unit  60  in such a manner that the horizontal axis represents the distance and the vertical axis represents the return light optical signal level. 
         [0085]    Since the signal level of return light is very low, noise is reduced by inputting a light pulse repeatedly to the measurement subject optical fiber F 1  and averaging plural measurement values with the signal processor  50 . 
         [0086]    Since as described above the measurement subject optical fiber F 1  is tested by generating light pulses using the light pulse generator shown in  FIG. 1 , the dynamic range, the distance resolution, etc. can be increased and the measurement subject optical fiber F 1  can be measured and tested accurately. 
         [0087]    The present invention is not limited to the above embodiments and may be embodied in the following manners. 
         [0088]    Although in the second embodiment the light pulse generator shown in  FIG. 1  is used in the OTDR  100 , it can be used in any measuring instruments which output a light pulse(s). 
         [0089]    Although the four parallel capacitors C 1 -C 4  are provided, a light pulse can be emitted faster (it can be given a steeper rising edge) by increasing the number of capacitors because the current that one capacitor can produce by discharge has an upper limit. However, the number of capacitors should be determined taking into consideration the circuit scale, the cost, the output optical power of the LD  11 , the performance and characteristics of the LD  11 , and other factors. 
         [0090]    While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.