Abstract:
An optical pulse testing apparatus incorporating an optical pulse generator composed of low cost components. The optical pulse testing apparatus comprises: a ring optical path including an optical fiber  30  with a rare earth element added to; an excitation light source  32  which enters excitation optical pulses into the optical fiber  30 ; an optical branching filter  38  for branching the circulating optical pulses circulating through the ring optical path to emit output optical pulses; and a photodetector  40  for detecting the circulating optical pulses circulating through the ring optical path to obtain signals indicative of a light intensity and a generation timing of the circulating optical pulses. Thus, the optical pulse generator, and the optical pulse testing apparatus and method using the optical pulse generator require no expensive optical parts and complicated device control.

Description:
This application claims the benefit of foreign filing priority under 35 U.S.C. 109(e) based on Japanese Patent Application No. 2000-313691, filed Oct. 13, 2000. 
   FIELD OF THE INVENTION 
   The present invention relates to an optical pulse generator which is usable as a pulse light source of an optical pulse testing apparatus, and an optical pulse testing apparatus using the optical pulse generator and an optical pulse testing method. 
   BACKGROUND ART 
   Broken points of optical fiber cables, distributions of transmission losses, coupling losses, etc. can be measured by an optical pulse testing apparatus (OTDR; Optical Time Domain Reflectometer). 
   The structure of a conventional optical pulse testing apparatus will be explained with reference to  FIG. 8 .  FIG. 8  is a block diagram of the structure of the optical pulse testing apparatus. 
   The conventional optical pulse testing apparatus includes a semiconductor laser  100  as a means for generating optical pulses as a probe. The semiconductor laser  100  is connected to a pulse generating circuit  102  which operates so that the semiconductor laser  100  outputs optical pulses at a predetermined cycle. The semiconductor laser  100  has the output connected to an optical fiber-to-be-measured  104  via a directional coupler  108  and an input/output connector  106 . The directional coupler  108  is connected to a photodetector  110  which detects returning light reflected from the optical fiber  104 . The photodetector  110  is connected to an amplifier  112  which amplifies electric signals from the photodetector  110 . The amplifier  112  is connected to an A-D converter  114  which converts electric signals amplified by the amplifier  112  to digital signals. The A-D converter  114  is connected to a computer circuit  116  which computes digital signals supplied by the A-D converter  114 . The computer circuit  116  includes a display  118  which indicates measured results given by the computer circuit  116 . The A-D converter  114  and the computer circuit  116  are connected to a timing circuit  120  to be controlled by the timing circuit  120 . 
   Then, the principle of the measuring of the optical pulse testing apparatus will be explained with reference to  FIG. 8 . 
   First, the timing circuit  120  inputs trigger signals to the pulse generating circuit  102 . In response to the trigger signals, the pulse generating circuit  102  generates pulse currents for driving the semiconductor laser  100 . The semiconductor laser  100  is controlled by the pulse currents from the pulse generating circuit  102  to output optical pulses of a predetermined pulse width and a predetermined cycle. Optical pulses emitted by the semiconductor laser  100  are supplied to the optical fiber-to-be-measured  104  connected to the input/output connector  106  via the directional coupler  108 . 
   Optical pulses incident on the optical fiber-to-be-measured  104  propagate in the optical fiber-to-be-measured  104 . Reflected light due to mismatching of the transmission paths at the coupling points of the optical fiber-to-be-measured  104 , back scattering light, such as Rayleigh scattering light caused by trivial a small amount of disuniformity in the optical fiber-to-be-measured  104  returns to the optical pulse testing apparatus. A time from the incidence of an optical pulse on the optical fiber-to-be-measured  104  till the return of the optical pulse to the optical pulse testing apparatus is proportional to a distance from the end of the optical fiber-to-be-measured  104  into which the optical pulse was introduced to a reflection point or a scattering point of the optical fiber  104 . 
   The light which has returned from the input/output connector  106  to the optical pulse testing apparatus is detected by the photodetector  110  via the directional coupler  108  and converted to electric signals. The converted electric signal is amplified by the amplifier  112 , then converted to digital signals by the A-D converter  114 , based on signals from the timing circuit  120 , and synchronously added at each cycle of the pulses by the computing circuit  116 . Light propagating through an optical fiber exponentially attenuates. Therefore, the added signals are logarithmically transformed by the computer circuit  116 . The measured results are presented on the display  118  in distances of the optical fiber-to-be-measured  104  proportional to periods of time on the horizontal axis and intensities of the reflected light or the scattering light on the vertical axis. Broken points of an optical fiber and loss distributions in the optical fiber can be thus measured. 
   In the above-described optical pulse testing apparatus, the means for generating pulses as the probe can be provided by a pulse excitation variable wavelength ring laser in place of the semiconductor laser  100 . 
   An example of structure in such a pulse excitation variable wavelength ring laser is shown in  FIG. 9 . As shown in  FIG. 9 , the pulse excitation variable wavelength ring laser includes an optical fiber  122  doped with rare earth element (s) for photoamplification. 
   The optical fiber  122  has an excitation light source  124  which is disposed via an optical multiplexer  126  for exciting the optical fiber  122 . The excitation light source  124  is connected to a light source driving circuit  134  to drive the excitation light source  124  at selected excitation intensities, selected time intervals and selected repetition frequencies. 
   In the variable wavelength ring laser of the structure shown in  FIG. 9 , after the excitation light source  124  is turned on by the light source driving circuit  134 , the excitation of the optical fiber  122  is started; laser outputs are not obtained until a period of time in which the laser oscillation starts, and becomes CW (continuous wave) outputs of the CW excitation ring laser after a plurality of optical pulse series are temporarily generated during the transient period.  FIG. 10A  shows outputs of the variable wavelength ring laser whose excitation time of the excitation light source  124  is shortened and is optimized to produce single pulse outputs. 
   Structures of such variable wavelength ring laser which can produce single pulse outputs have been proposed. One example of the conventional variable wavelength ring lasers which can easily produce single pulse outputs will be explained with reference to  FIG. 11 . 
   The variable wavelength ring laser shown in  FIG. 11  includes an optical switch  136  disposed between the optical branching filter  130  and the isolator  132  in addition to the structure of the variable wavelength ring laser shown in  FIG. 9 . The optical switch  136  is connected to an optical switch controller  138  for controlling the optical switch  136 . 
   The optical switch  136  is turned on for a short period of time by the optical switch controller  138  and can generate optical pulses of high output power corresponding to a length of the optical fiber  122  and a concentration level of rare earth element(s) added (doped) to the optical fiber  122 . Single pulses as the outputs are taken out by the optical branching filter  130 . The details are described in Japanese Patent Laid-Open Publication No. Hei 5-21880 (1993). 
   However, parameters, such as a peak intensity, a half-value width, a delay time, etc. of a single pulse output generated by the conventional pulse excitation variable wavelength ring laser shown in  FIG. 9  are determined by a concentration ratio of rare earth element(s) added to the optical fiber  122 , a length of the optical fiber  122 , an excitation intensity of the excitation light source  124 , a repetition frequency of an excitation pulse, an excitation pulse width, a branch ratio of the optical branching filter  130 , an oscillation wavelength determined by the variable wavelength filter  128  and an overall length of the ring laser constituted by the above noted optical parts. Accordingly, an excitation intensity and an excitation pulse width of the excitation light source  124  must be controlled corresponding to intended wavelength, output and a repetition frequency, and a timing control of the optical pulse testing apparatus must be adjusted. 
   On the other hand, the use of the variable wavelength ring laser shown in  FIG. 11  to simplify the timing control of the optical pulse testing apparatus requires the expensive optical switch  136  for generating optical pulses. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an optical pulse generator which can be formed without using expensive optical parts and requires no complicated device control, and an optical pulse testing apparatus using the optical pulse generator and an optical pulse testing method. 
   In the above-described optical pulse generator, it is possible that the optical fiber excitation means stops supplying the excitation optical pulse to the optical fiber when the optical pulse detecting means detects the signal indicating a required light intensity or a required peak output of the circulating optical pulse. 
   The above-described object is also achieved by an optical pulse testing apparatus comprising: an optical pulse generator including a ring optical path with a rare earth element added to, an optical fiber excitation means for supplying an excitation optical pulse to the optical fiber for exciting the optical fiber, and an optical pulse branching means for branching the circulating optical pulse circulating through the ring optical path to emit a probe optical pulse, the optical pulse generator supplying the probe optical pulse to an optical fiber-to-be-measured; a reflected light detecting means for detecting a reflected light of the probe optical pulse that has been reflected while propagating through the optical fiber-to-be-measured; and an analyzing means for analyzing a light transmission state of the optical fiber-to-be-measured based on a detection result of the reflected light detection means; the optical pulse generator further comprising an optical pulse detecting means for detecting the circulating optical pulse which is circulating through the ring optical path to obtain a signal indicative of light intensity and generation timing of the circulating optical pulse; and the optical pulse testing apparatus further comprising a control means for controlling the timing of the analyzing means starting the analysis, based on a detected result of the optical pulse detecting means. 
   In the above-described optical pulse testing apparatus, it is possible that the optical fiber excitation means stops supplying the excitation optical pulse to the optical fiber when the optical pulse detecting means detects the signal indicating a required light intensity or a required peak output of the circulating optical pulse. 
   In the above-described optical pulse generator, it is possible that the optical fiber has a total length which permits at least a required pulse width of the output optical pulses, and the generator further comprises: an optical switch for controlling a pulse width of the output optical pulse; and an optical switch control means for controlling the operation of the optical switch, based on a detection result of the optical pulse detecting means, to make a pulse width of the output optical pulse a required width. 
   The above-described object is also achieved by an optical pulse testing apparatus comprising: an optical pulse generator including a ring optical path with a rare earth element added to, an optical fiber excitation means for entering an excitation optical pulse into the optical fiber for exciting the optical fiber, and an optical pulse branching means for branching the circulating optical pulse circulating through the ring optical path, so as to emit a probe optical pulse, the optical pulse generator entering the probe optical pulse into an optical fiber-to-be-measured; a reflected light detecting means for detecting a reflected light of the probe optical pulse entered into the optical fiber-to-be-measured, which (the probe optical pulse) has been reflected while propagating through the optical fiber-to-be-measured and exited at the end where the probe optical pulse was entered into the optical fiber-to-be-measured; and an analyzing means for analyzing a light transmission state of the optical fiber-to-be-measured, based on a detection result of the reflected light detection means, the optical pulse generator further comprising an optical pulse detecting means for detecting the circulating optical pulse which is circulating through the ring optical path, so as to obtain a signal indicative of light intensity and generation timing of the circulating optical pulse, and the optical pulse testing apparatus further comprising a control means for controlling the timing of the analyzing means starting the analysis, based on a detected result of the optical pulse detecting means. 
   In the above-described optical pulse testing apparatus, it is possible that the optical fiber excitation means stops entering the excitation optical pulse into the optical fiber when the optical pulse detecting means detects a required light intensity or a required peak output of the circulating optical pulse. 
   In the above-described optical pulse testing apparatus, it is possible that the apparatus further comprises: an optical switch for controlling a pulse width of the probe optical pulse; and an optical switch control means for synchronizing the operation of the optical switch and the emission of the excitation optical pulse by the optical fiber excitation means. 
   In the above-described optical pulse testing apparatus, it is possible that the optical fiber of the ring optical path has a total length which permits the probe optical pulse to have at least a required pulse width, and the apparatus further comprises: an optical switch for controlling a pulse width of the probe optical pulses; and a switch control means for controlling the operation of the optical switch to make a pulse width of the probe optical pulse is a required width, based on a detected result of the optical pulse detecting means. 
   The above-described object is also achieved by an optical pulse testing method comprising the steps of: supplying a probe optical pulse to an optical fiber-to-be-measured; detecting a reflected light of the probe optical pulse supplied to the optical fiber-to-be-measured that has been reflected while propagating through the optical fiber-to-be-measured; and analyzing a light transmission state of the optical fiber-to-be-measured based on a detection result of the reflected light, the excitation optical pulse being entered into a ring optical path including an optical fiber with a rare earth element added to excite the optical fiber, and the circulating optical pulse which is circulating through the ring optical path being branched as the probe optical pulse; the circulating optical pulse circulating through the ring optical path being detected; and a timing of starting the analysis of the light transmission state of the optical fiber-to-be-measured being controlled based on a detection result of the circulating optical pulse. 
   In the above-described optical pulse testing method, it is possible that the entry of the excitation optical pulse is stopped after a required light intensity or a peak output of the circulating optical pulse is detected. 
   In the above-described optical pulse testing method, it is possible that the control of a pulse width of the probe optical pulse and the entry of the excitation optical pulse are synchronized with each other. 
   In the above-described optical pulse testing method, it is possible that the optical fiber of the ring optical path has a total length which permits the probe optical pulse to have at least a required pulse width, and a pulse width of the probe optical pulse is controlled based on a detection result of the circulating optical pulse. 
   According to the present invention, in the optical pulse testing method comprising the steps of: supplying a probe optical pulse an optical fiber-to-be-measured; detecting reflected light of the probe optical pulse entered into the optical fiber-to-be-measured that has been reflected while propagating through the optical fiber-to-be-measured; and analyzing a light transmission state of the optical fiber-to-be-measured, based on a detection result of the reflected light, an excitation optical pulse is supplied to a ring optical path including an optical fiber with a rare earth element added to excite the optical fiber, and the circulating optical pulse which is circulating through the ring optical path is branched as the probe optical pulse; the circulating optical pulse circulating through the ring optical path is detected; and a timing of starting the analysis of a light transmission state of the optical fiber-to-be-measured is controlled based on a detection result of the circulating optical pulse. Thus, the optical pulse generator can be constituted without using expensive optical parts, and the light transmission state of the optical fiber-to-be-measured can be measured without requiring complicated device control. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a structure of the optical pulse testing apparatus using the optical pulse generator according to a first embodiment of the present invention. 
       FIG. 2  is a block diagram of the structure of the optical pulse generator according to the first embodiment of the present invention. 
       FIGS. 3(   a )– 3 ( f ) are timing charts showing the control operations of the optical pulse testing apparatus using the optical pulse generator according to the first embodiment of the present invention. 
       FIG. 4  is a block diagram of a structure of the optical pulse generator according to a second embodiment of the present invention. 
       FIGS. 5(   a )– 5 ( f ) are timing charts showing the control operations of the optical pulse testing apparatus using the optical pulse generator according to a second embodiment of the present invention. 
       FIG. 6  is a block diagram of a structure of the optical pulse generator according to a third embodiment of the present invention. 
       FIG. 7  is a block diagram of a structure of the optical pulse generator according to a fourth embodiment of the present invention. 
       FIG. 8  is a block diagram of the structure of the conventional optical pulse testing apparatus. 
       FIG. 9  is a block diagram of the structure of the conventional ring laser. 
       FIGS. 10A–10B  are graphs showing an example of outputs of the conventional ring laser. 
       FIG. 11  is a bock diagram of the structure of conventional ring laser which can generate single pulse outputs. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
   The optical pulse generator according to a first embodiment of the present invention, and the optical pulse testing apparatus using the same and the optical pulse testing method will be explained with reference to  FIGS. 1 to 3 .  FIG. 2  is a block diagram of a structure of the optical pulse generator.  FIGS. 3(   a )– 3 ( f ) are timing charts showing the control operations of the optical pulse testing apparatus. 
   First, the structure of the optical pulse testing apparatus according to the present embodiment will be explained with reference to  FIG. 1 . 
   The optical pulse generator  10  used as a pulse light source of the optical pulse testing apparatus is connected to a timing circuit  12  which drives the optical pulse generator  10 . An optical fiber-to-be-measured  14  is connected to the output of the optical pulse generator  10  via a directional coupler  18  and an input/output connector  16 . The directional coupler  18  is connected to a photodetector  20  which detects light which has been reflected in the optical fiber  14  and returned to the optical pulse testing apparatus. The photodetector  20  is connected to an amplifier  22  which amplifies electric signals from the photodetector  20 . The amplifier  22  is connected to an A-D converter  24  which converts electric signals amplified by the amplifier  22  to digital signals. The A-D converter  24  is connected to a computer circuit  26  which computes digital signals given by received from the A-D converter  24 . The computer circuit  26  has a display which displays measured results obtained by the computer circuit  26 . A timing circuit  12  is connected to the A-D converter  24  and the computer circuit  26  to control the operation timings of the A-D converter  24  and the computer circuit  26 . 
   Next, the structure of the optical pulse generator  10  mainly characterizing the optical pulse testing apparatus according to the present embodiment will be explained with reference to  FIG. 2 . 
   The optical pulse generator  10  according to the present embodiment has a ring optical fiber  30  with rare earth element(s) added for photoamplification. The optical fiber  30  is connected to an excitation light source  32  for exciting the optical fiber  30  via an optical multiplexer  34 . The excitation light source  32  is connected to a light source driving circuit  46  which drives the excitation light source  32 , based on signals from the timing circuit  12  of the optical pulse testing apparatus. 
   The optical fiber  30  includes an isolator  44  for determining a direction of light circulating through the optical fiber (ring optical path)  30 , a variable wavelength filter  36  for determining an oscillation wavelength, and an optical branching filter  38  which emits optical pulses as outputs of the optical pulse generator. 
   The optical fiber  30  further includes an optical branching filter  42  which branches light circulating through the optical fiber  30 . The optical branching filter  42  is connected to a photodetector  40  which detects light branched by the optical branching filter  42 . The photodetector  40  is connected to the timing circuit  12  of the optical pulse testing apparatus. 
   The optical pulse generator  10  according to the present embodiment is based on the principle of the pulse excitation variable wavelength ring laser and emits optical pulses as a probe for measuring broken points of the optical fiber-to-be-measured  14  and loss distributions of coupling losses, etc. in the optical fiber-to-be-measured  14 . The emission of optical pulses from the optical pulse generator  10  is performed in response to pulse output commands from the timing circuit  12 . 
   Next, the operation of the optical pulse generator  10  as the pulse light source of the optical pulse testing apparatus will be detailed. 
   The light source driving circuit  46  of the optical pulse generator  10  drives the excitation light source  32  in response to pulse output commands from the timing circuit  12  of the optical pulse testing apparatus and emits optical pulses from the excitation light source  32 . The optical pulses emitted by the excitation light source  32  are provided to the optical multiplexer  34 . It is preferable to store information regarding combinations of excitation intensities and excitation pulse widths of single pulse outputs in an external memory (not shown), and, based on the information, the excitation light source  32  is driven. 
   The optical pulses emitted from the excitation light source  32  enter the optical fiber  30  via the optical multiplexer  34  and circulate through the optical fiber  30  to be amplified. A direction of the circulation of the optical pulses through the optical fiber  30  is determined by the isolator  44 . 
   The variable wavelength optical filter  36  determines an oscillation wavelength of the optical pulse generator  10  as a ring laser and selectively passes a required wavelength component of the optical pulses circulating the optical fiber  30 . 
   The optical pulses emitted by the excitation light source  32  circulate through the optical fiber  30 , whereby the optical pulse generator  10  starts oscillation. At this time, a part of the optical pulses circulating through the optical fiber  30  is branched by the optical branching filter  38  to output the optical pulses from the optical pulse generator  10 . Here, even when a pulse output command from the timing circuit  12  has arrived at the optical pulse generator  10 , the emission of the optical pulses from the optical pulse generator  10  is delayed by a delay time depending on a repetition frequency, a wavelength and a temperature. 
   Similarly, a part of the optical pulses circulating through the optical fiber  30  is branched to the optical branching filter  42  to be supplied to the photodetector  40 . The photodetector  40  detects the optical pulses circulating through the optical fiber  30  of the optical pulse generator  10 . Detected signals of these optical pulses are received by the timing circuit  12  to be used for controlling the A-D converter  24  and the computer circuit  26 . 
   As described above, the optical pulse generator  10  according to the present embodiment is characterized in that the detected optical pulse signals are transmitted to the timing circuit  12  of the optical pulse testing apparatus as control signals indicating timings of the emission of the output optical pulses from the optical branching filter  38 . 
   The output optical pulses emitted by the optical branching filter  38  of the optical pulse generator  10  are entered into the optical fiber-to-be-measured  14  connected to the input/output connector  16  via the directional coupler  18  of the optical pulse testing apparatus. A part of the optical pulses entered into the optical fiber-to-be-measured  14  is reflected at broken points, etc. in the optical fiber-to-be-measured  14  and returns to the optical pulse testing apparatus via the input/output connector  16 . The reflected light returning to the optical pulse testing apparatus is detected by the photodetector  20  via the directional coupler  18 . Based on the results of the detection, light transmitting states of the optical fiber-to-be-measured  14  are measured. 
   The operation of the optical pulse testing apparatus according to the present embodiment will be explained with reference to  FIGS. 1 and 3(   a )– 3 ( f ). 
   The timing circuit  12  outputs timing signals to the A-D converter  24  and the computer circuit  26  based on the detected optical pulse signals received from the photodetector  40  of the optical pulse generator  10 . Therefore, even when a delay time is present between an optical pulse output command to the light source driving circuit  46  and the output of optical pulses from the optical pulse generator  10 , optical signals of returning light reflected in the optical fiber-to-be-measured  14  can be added synchronously at each cycle thereof.  FIGS. 3(   a )– 3 ( f ) are timing charts showing the control timings and waveforms by such the timing circuit  12 . 
   First, the timing circuit  12  outputs an optical pulse output command shown in  FIG. 3(   a ) to the light source driving circuit  46 . Thus, the excitation light source  32  is driven at the timing shown in  FIG. 3(   b ), and at the timing shown in  FIG. 3(   c ), an output optical pulse is generated by the optical pulse generator  10 . Subsequently, based on a detected optical pulse signal shown in  FIG. 3(   d ) from the photodetector  40 , the timing circuit  12  supplies a timing signal shown in  FIG. 3(   e ) to the A-D converter  24  to drive the A-D converter  24 . 
   Based on the above-described timing signal from the timing circuit  12 , the A-D converter  24  starts to convert the photosignals detected by the photodetector  20  and amplified by the amplifier  22  to digital signals and store the data.  FIG. 3(   f ) shows the measured waveform portion of the data stored by the A-D converter  24 . 
   The computer circuit  26  performs synchronous addition of measured data received from the A-D converter  24  to compute measured result, such as broken points, distributions of transmission losses, etc. of the optical fiber-to-be-measured  14 . The measured results obtained by the computer circuit  26  can be presented on the display  28 . 
   The optical pulse testing apparatus according to the present embodiment is characterized in that the photodetector  40  for detecting optical pulses circulating through the optical fiber of the optical pulse generator  10  is provided, and based on the detected optical pulse signals of the photodetector  40 , the timing circuit  12  controls the A-D converter  24  and the computing circuit  26 . 
   Next, the optical pulse testing method according to the present embodiment will be explained with reference to  FIGS. 1 to 3 . 
   First, an optical fiber  14  to be measured about transmission states of light, such as transmission losses, broken points, etc. is connected to the input/output connector  16 . 
   Then, the timing circuit  12  supplies a pulse output command shown in  FIG. 3(   a ) to the light source driving circuit  46  of the optical pulse generator  10 . 
   The excitation light source  32  is driven at the timing shown in  FIG. 3(   b ) by the light source driving circuit  46  which has received the pulse output command from the timing circuit  12 . Optical pulses generated by the excitation light source  32  are entered into the optical fiber  30  via the optical multiplexer  34 . 
   The optical pulses entered into the optical fiber  30  circulate through the optical fiber  30  to be thereby amplified. The optical pulses being amplified by circulating through the optical fiber  30  have the wavelength component determined to be a required value by the variable wavelength filter  36 . 
   Thus, the optical pulses circulate through the optical fiber  30  of the optical pulse generator  10 , whereby the laser oscillation starts, and optical pulse outputs are produced from the optical branching filter  38  at the timing shown in  FIG. 3(   c ). The emission of the optical pulses from the optical pulse generator  10  is delayed with respect to the pulse output command from the timing circuit  12  shown in  FIG. 3(   a ) by a delay time depending on a repetition frequency, a wavelength and a temperature. 
   Concurrently, the optical pulses circulating through the optical pulse generator  10  are detected by the photodetector  40  via the optical branching filter  42 . The detected signals from the photodetector  40  are shown in  FIG. 3(   d ). The detected optical pulse signals are transmitted to the timing circuit  12 . Based on the detected optical pulse signals, the timing circuit  12  controls the operation timings of the A-D converter  24  and the computer circuit  26 . 
   A part of the optical pulses supplied to the optical fiber-to-be-measured  14  is reflected at broken points, etc. in the optical fiber-to-be-measured  14  and returned to the optical pulse testing apparatus via the input/output connector  16 . 
   The optical pulses thus emitted by the optical branching filter  38  of the optical pulse generator  10  are entered into the optical fiber-to-be-measured  14  connected to the input/output connector  16  via the directional coupler  18 . 
   A part of the photpulses entered into the optical fiber-to-be-measured  14  is reflected on broken points, etc. in the optical fiber-to-be-measured  14  and returns to the optical pulse testing apparatus again via the input/output connector  16 . 
   The reflected light returning from the optical fiber-to-be-measured  14  is separated from the optical pulses emitted from the optical pulse generator  10  by the directional coupler  18 , and detected by the photodetector  20 . 
   Subsequently, the detected optical pulse signals from the photodetector  20  are amplified by the amplifier  22 . 
   In the meantime, the timing circuit  12  transmits timing signals shown in  FIG. 3(   e ) to the A-D converter  24  based on the detected optical pulse signals from the photodetector  40  shown in  FIG. 3(   d ). 
   Based on the timing signals from the timing circuit  12  described above, the A-D converter  24  starts to convert the detected optical pulse signals from the photodetector  20  to digital signals as shown in  FIG. 3(   f ) and store the digital data. Thus, even there is a delay time between a pulse output command from the timing circuit  12  to the light source driving circuit  46  and the emission of an optical pulse from the optical pulse generator  10 , the optical pulses reflected in the optical fiber-to-be-measured  14  and returned to the optical pulse testing apparatus can be synchronously added at each one cycle of the pulses by the computing circuit  26 . 
   Concurrently, the synchronous addition of the measured data stored in the A-D converter  24  is performed by the computer circuit  26  to compute measured results regarding the states of the light transmission, such as broken points, transmission losses, etc. of the optical fiber-to-be-measured  14 . The computed results obtained by the computer circuit  26  are presented on the display  28 . Thus, the measurement of the states of light transmission in the optical fiber by the optical pulse testing apparatus is finished. 
   As described above, according to the present embodiment, optical pulse generation in the optical pulse generator is detected, and based on the detection result, the optical pulse testing apparatus is controlled, which permits the optical pulse generator and optical pulse testing apparatus can be constituted without using expensive optical parts, and without complicated device control, states of light transmission of optical fibers can be measured. 
   Second Embodiment 
   The optical pulse generator, and the optical pulse testing apparatus and method according to a second embodiment of the present invention will be explained with reference to  FIGS. 4 and 5(   a )– 5 ( f ).  FIG. 4  is a schematic diagram showing a structure of the optical pulse generator according to the present embodiment.  FIGS. 5(   a )– 5 ( f ) are timing charts of showing the control timings and waveforms of the optical pulse testing apparatus. The same members of the present embodiment as those of the optical pulse generator and the optical pulse testing apparatus according to the first embodiment are represented by the same reference numbers to simplify their explanation. 
   The structure of the optical pulse generator shown in  FIG. 4  according to the present embodiment is substantially the same as that of the first embodiment, and the structure of the optical pulse testing apparatus is also the same. The optical pulse generator  10  according to the present embodiment is characterized in that a photodetector  40  is connected to a timing circuit  12  and a light source driving circuit  46 . Thus, the signals of light circulating through an optical fiber  30 , which have been detected by the photodetector  40 , are supplied to both the timing circuit  12  and the light source driving circuit  46 . 
   The operation of the optical pulse generator and the optical pulse testing apparatus according to the present embodiment will be explained below. 
   First, the timing circuit  12  of the optical pulse testing apparatus supplies a pulse output command shown in  FIG. 5(   a ) to the light source driving circuit  46  to start driving the excitation light source  32 . 
   The excitation light source  32  driven by the light source driving circuit  46  emits excitation light, and the excitation light is supplied to the optical fiber  30  via the optical multiplexer  34  and circulates through the optical fiber  30  as in the first embodiment to start the oscillation of the laser. 
   During this time, the photodetector  40  detects via the optical branching filter  42  a part of the light circulating through the optical fiber  30  to constantly monitor signal intensities of the light circulating the through the ring laser (ring optical path). When a required pulse intensity or a maximum peak as shown in  FIG. 5(   d ) is monitored, the drive of the excitation light source  32  is stopped. This makes it unnecessary to control the excitation light source  32  corresponding to a parameter of output pulses, such as an excitation pulse width or others, and a required single pulse output as shown in  FIG. 5(   c ) can be easily obtained from the optical pulse generator  10 . 
   By using the thus-obtained single pulses, the measurement of states of light transmission through the optical fiber-to-be-measured  14  can be performed in the same way as in the first embodiment. 
   As described above, according to the present embodiment, the optical pulse generation by the optical pulse generator is detected, and based on the result of the detection, the control of the optical pulse testing apparatus is performed. Thus, the optical pulse generator and the optical pulse testing apparatus can be constituted without using expensive optical parts and can perform the measurement of light transmission states of optical fibers without requiring complicated device control. 
   Third Embodiment 
   The optical pulse generator, and the optical pulse testing apparatus and method according to a third embodiment of the present invention will be explained with reference to  FIG. 6 .  FIG. 6  is a schematic diagram showing a structure of the optical pulse generator according to the present embodiment. The same members of the present embodiment as those of the optical pulse generator and the optical pulse testing apparatus according to the first embodiment are represented by the same reference numbers to simplify their explanation. 
   Optical pulses given by the optical pulse generator according to the first embodiment have output peaks which can be controlled by excitation intensities of the excitation light source  32 , and output pulse frequencies which can be controlled by excitation pulse repetition frequencies of the excitation light source  32 . However, an output pulse width is substantially determined by the optical parts constituting the optical pulse generator  10 . 
   The optical pulse generator according to the present embodiment can control the output pulse width as well. 
   The structure of the optical pulse generator according to the present embodiment as shown in  FIG. 6  is substantially the same as that of the first embodiment, and the structure of the optical pulse testing apparatus is the same as that of the first embodiment. The present embodiment further includes an optical switch  48  disposed on the output side of the optical branching filter  38  of the optical pulse generator  10 . The optical switch  48  is connected to an optical switch controller  50 . The optical switch controller  50  is connected to a timing circuit  12 . 
   The optical pulse generator  10  has the above-described constitution, and operation timings of the optical switch  48  and the light source driving circuit  46  are synchronized, whereby the output pulse width of the optical pulse generator  10  can be controlled. 
   By using optical pulses having output pulse widths controlled, light transmission states of an optical fiber-to-be-measured  14  can be measured by the optical pulse testing apparatus. 
   As described above, according to the present embodiment, the optical pulse generation by the optical pulse generator is detected, and based on the result of the detection, the control of the optical pulse testing apparatus is performed. Thus, the optical pulse generator and the optical pulse testing apparatus can be constituted without using expensive optical parts and can perform the measurement of light transmission states of optical fibers without requiring complicated device control. 
   Fourth Embodiment 
   The optical pulse generator, and the optical pulse testing apparatus and method according to a fourth embodiment of the present invention will be explained with reference to  FIG. 7 .  FIG. 7  is a schematic diagram showing a structure of the optical pulse generator according to the present embodiment. The same members of the present embodiment as those of the optical pulse generator and the optical pulse testing apparatus according to the third embodiment are represented by the same reference numbers to simplify their explanation. 
   In the optical pulse generator according to the third embodiment, an excitation pulse width is changed by a wavelength or a repetition frequency so as to output a single pulse from the optical pulse generator. Accordingly, control signals for controlling the optical switch controller  50  as well must be changed. It is impossible to generate optical pulses of a pulse width larger than a pulse width determined by parameters of the optical parts constituting the optical pulse generator. The present embodiment solves this problem. 
   As shown in  FIG. 7 , the structure of the optical pulse generator according to the present embodiment is substantially the same as that of the third embodiment. In the present embodiment, furthermore, an optical fiber  30  of the optical pulse generator  10  has an optical fiber  52  so as to generate optical pulses of a required wide pulse width. Because of the optical fiber  52 , a time during which light circulates through the optical fiber  30  is extended by a time during which the light circulates through the optical fiber  52 , whereby increasing the pulse width of the optical pulses. A length of the optical fiber  52  is changed to adjust a pulse width of the optical pulses which can be emitted by the optical pulse generator. 
   Furthermore, a photodetector  40  detects intensities of the optical pulses circulating through the optical fiber  30  and the optical fiber  52  via an optical branching filter  42 . Based on the result of the detection, a timing circuit  12  of the optical pulse testing apparatus controls the optical switch controller  50  so that when a required optical pulse is detected, the optical switch  48  is turned on for a time interval equal to the required pulse width. At this time, when a delay time is present from the input of the control signal to the optical switch  48  to the actual operation of the optical switch  48 , it is preferable to use the optical fiber  52  of a length considering the delay time. 
   As described above, in the present embodiment, the optical switch  48  is changed over when a required optical pulse has been obtained. Accordingly, it is not necessary to perform a complicated operation to synchronize the light source driving circuit  46  for driving the excitation light source  32  with the optical switch  48 . 
   As described above, according to the present embodiment, the optical pulse generation by the optical pulse generator is detected, and based on the result of the detection, the control of the optical pulse testing apparatus is performed. Thus, the optical pulse generator and the optical pulse testing apparatus can be constituted without using expensive optical parts and can perform the measurement of the light transmission states of optical fibers without requiring complicated device control. 
   Modified Embodiments 
   The present invention is not limited to the above-described embodiments and can cover other various modifications. 
   For example, in the above-described embodiments, the optical pulse generator according to the present invention is used as the pulse light source of the optical pulse testing apparatus, but the optical pulse generator according to the present invention is not limited to the application to the pulse light source of the optical pulse testing apparatus. It is possible that in a device using the optical pulse generator according to the present invention as the pulse light source, the signals detected by the photodetector  40  are used, as required, as timing signals for controlling the device. 
   In the optical pulse generator, and the optical pulse testing apparatus and method according to the present invention, excitation optical pulses for exciting an optical fiber included in a ring optical path are supplied to the optical fiber; and when the optical pulses circulating through the ring optical path are branched to output the optical pulses, the optical pulses circulating through the ring optical path are detected to control the operation of the optical pulse testing apparatus based on the result of the detection. The optical pulse generator can be constituted without using expensive optical parts and requires no complicated device control, and is useful in the optical pulse testing apparatus and method.