Optical pulse generator and optical pulse testing instrument and method

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.

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 toFIG. 8.FIG. 8is a block diagram of the structure of the optical pulse testing apparatus.

The conventional optical pulse testing apparatus includes a semiconductor laser100as a means for generating optical pulses as a probe. The semiconductor laser100is connected to a pulse generating circuit102which operates so that the semiconductor laser100outputs optical pulses at a predetermined cycle. The semiconductor laser100has the output connected to an optical fiber-to-be-measured104via a directional coupler108and an input/output connector106. The directional coupler108is connected to a photodetector110which detects returning light reflected from the optical fiber104. The photodetector110is connected to an amplifier112which amplifies electric signals from the photodetector110. The amplifier112is connected to an A-D converter114which converts electric signals amplified by the amplifier112to digital signals. The A-D converter114is connected to a computer circuit116which computes digital signals supplied by the A-D converter114. The computer circuit116includes a display118which indicates measured results given by the computer circuit116. The A-D converter114and the computer circuit116are connected to a timing circuit120to be controlled by the timing circuit120.

Then, the principle of the measuring of the optical pulse testing apparatus will be explained with reference toFIG. 8.

First, the timing circuit120inputs trigger signals to the pulse generating circuit102. In response to the trigger signals, the pulse generating circuit102generates pulse currents for driving the semiconductor laser100. The semiconductor laser100is controlled by the pulse currents from the pulse generating circuit102to output optical pulses of a predetermined pulse width and a predetermined cycle. Optical pulses emitted by the semiconductor laser100are supplied to the optical fiber-to-be-measured104connected to the input/output connector106via the directional coupler108.

Optical pulses incident on the optical fiber-to-be-measured104propagate in the optical fiber-to-be-measured104. Reflected light due to mismatching of the transmission paths at the coupling points of the optical fiber-to-be-measured104, back scattering light, such as Rayleigh scattering light caused by trivial a small amount of disuniformity in the optical fiber-to-be-measured104returns to the optical pulse testing apparatus. A time from the incidence of an optical pulse on the optical fiber-to-be-measured104till 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-measured104into which the optical pulse was introduced to a reflection point or a scattering point of the optical fiber104.

The light which has returned from the input/output connector106to the optical pulse testing apparatus is detected by the photodetector110via the directional coupler108and converted to electric signals. The converted electric signal is amplified by the amplifier112, then converted to digital signals by the A-D converter114, based on signals from the timing circuit120, and synchronously added at each cycle of the pulses by the computing circuit116. Light propagating through an optical fiber exponentially attenuates. Therefore, the added signals are logarithmically transformed by the computer circuit116. The measured results are presented on the display118in distances of the optical fiber-to-be-measured104proportional 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 laser100.

An example of structure in such a pulse excitation variable wavelength ring laser is shown inFIG. 9. As shown inFIG. 9, the pulse excitation variable wavelength ring laser includes an optical fiber122doped with rare earth element (s) for photoamplification.

The optical fiber122has an excitation light source124which is disposed via an optical multiplexer126for exciting the optical fiber122. The excitation light source124is connected to a light source driving circuit134to drive the excitation light source124at selected excitation intensities, selected time intervals and selected repetition frequencies.

In the variable wavelength ring laser of the structure shown inFIG. 9, after the excitation light source124is turned on by the light source driving circuit134, the excitation of the optical fiber122is 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. 10Ashows outputs of the variable wavelength ring laser whose excitation time of the excitation light source124is 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 toFIG. 11.

The variable wavelength ring laser shown inFIG. 11includes an optical switch136disposed between the optical branching filter130and the isolator132in addition to the structure of the variable wavelength ring laser shown inFIG. 9. The optical switch136is connected to an optical switch controller138for controlling the optical switch136.

The optical switch136is turned on for a short period of time by the optical switch controller138and can generate optical pulses of high output power corresponding to a length of the optical fiber122and a concentration level of rare earth element(s) added (doped) to the optical fiber122. Single pulses as the outputs are taken out by the optical branching filter130. 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 inFIG. 9are determined by a concentration ratio of rare earth element(s) added to the optical fiber122, a length of the optical fiber122, an excitation intensity of the excitation light source124, a repetition frequency of an excitation pulse, an excitation pulse width, a branch ratio of the optical branching filter130, an oscillation wavelength determined by the variable wavelength filter128and 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 source124must 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 inFIG. 11to simplify the timing control of the optical pulse testing apparatus requires the expensive optical switch136for 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.

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 toFIGS. 1 to 3.FIG. 2is 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 toFIG. 1.

The optical pulse generator10used as a pulse light source of the optical pulse testing apparatus is connected to a timing circuit12which drives the optical pulse generator10. An optical fiber-to-be-measured14is connected to the output of the optical pulse generator10via a directional coupler18and an input/output connector16. The directional coupler18is connected to a photodetector20which detects light which has been reflected in the optical fiber14and returned to the optical pulse testing apparatus. The photodetector20is connected to an amplifier22which amplifies electric signals from the photodetector20. The amplifier22is connected to an A-D converter24which converts electric signals amplified by the amplifier22to digital signals. The A-D converter24is connected to a computer circuit26which computes digital signals given by received from the A-D converter24. The computer circuit26has a display which displays measured results obtained by the computer circuit26. A timing circuit12is connected to the A-D converter24and the computer circuit26to control the operation timings of the A-D converter24and the computer circuit26.

Next, the structure of the optical pulse generator10mainly characterizing the optical pulse testing apparatus according to the present embodiment will be explained with reference toFIG. 2.

The optical pulse generator10according to the present embodiment has a ring optical fiber30with rare earth element(s) added for photoamplification. The optical fiber30is connected to an excitation light source32for exciting the optical fiber30via an optical multiplexer34. The excitation light source32is connected to a light source driving circuit46which drives the excitation light source32, based on signals from the timing circuit12of the optical pulse testing apparatus.

The optical fiber30includes an isolator44for determining a direction of light circulating through the optical fiber (ring optical path)30, a variable wavelength filter36for determining an oscillation wavelength, and an optical branching filter38which emits optical pulses as outputs of the optical pulse generator.

The optical fiber30further includes an optical branching filter42which branches light circulating through the optical fiber30. The optical branching filter42is connected to a photodetector40which detects light branched by the optical branching filter42. The photodetector40is connected to the timing circuit12of the optical pulse testing apparatus.

The optical pulse generator10according 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-measured14and loss distributions of coupling losses, etc. in the optical fiber-to-be-measured14. The emission of optical pulses from the optical pulse generator10is performed in response to pulse output commands from the timing circuit12.

Next, the operation of the optical pulse generator10as the pulse light source of the optical pulse testing apparatus will be detailed.

The light source driving circuit46of the optical pulse generator10drives the excitation light source32in response to pulse output commands from the timing circuit12of the optical pulse testing apparatus and emits optical pulses from the excitation light source32. The optical pulses emitted by the excitation light source32are provided to the optical multiplexer34. 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 source32is driven.

The optical pulses emitted from the excitation light source32enter the optical fiber30via the optical multiplexer34and circulate through the optical fiber30to be amplified. A direction of the circulation of the optical pulses through the optical fiber30is determined by the isolator44.

The variable wavelength optical filter36determines an oscillation wavelength of the optical pulse generator10as a ring laser and selectively passes a required wavelength component of the optical pulses circulating the optical fiber30.

The optical pulses emitted by the excitation light source32circulate through the optical fiber30, whereby the optical pulse generator10starts oscillation. At this time, a part of the optical pulses circulating through the optical fiber30is branched by the optical branching filter38to output the optical pulses from the optical pulse generator10. Here, even when a pulse output command from the timing circuit12has arrived at the optical pulse generator10, the emission of the optical pulses from the optical pulse generator10is 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 fiber30is branched to the optical branching filter42to be supplied to the photodetector40. The photodetector40detects the optical pulses circulating through the optical fiber30of the optical pulse generator10. Detected signals of these optical pulses are received by the timing circuit12to be used for controlling the A-D converter24and the computer circuit26.

As described above, the optical pulse generator10according to the present embodiment is characterized in that the detected optical pulse signals are transmitted to the timing circuit12of the optical pulse testing apparatus as control signals indicating timings of the emission of the output optical pulses from the optical branching filter38.

The output optical pulses emitted by the optical branching filter38of the optical pulse generator10are entered into the optical fiber-to-be-measured14connected to the input/output connector16via the directional coupler18of the optical pulse testing apparatus. A part of the optical pulses entered into the optical fiber-to-be-measured14is reflected at broken points, etc. in the optical fiber-to-be-measured14and returns to the optical pulse testing apparatus via the input/output connector16. The reflected light returning to the optical pulse testing apparatus is detected by the photodetector20via the directional coupler18. Based on the results of the detection, light transmitting states of the optical fiber-to-be-measured14are measured.

The operation of the optical pulse testing apparatus according to the present embodiment will be explained with reference toFIGS. 1 and 3(a)–3(f).

The timing circuit12outputs timing signals to the A-D converter24and the computer circuit26based on the detected optical pulse signals received from the photodetector40of the optical pulse generator10. Therefore, even when a delay time is present between an optical pulse output command to the light source driving circuit46and the output of optical pulses from the optical pulse generator10, optical signals of returning light reflected in the optical fiber-to-be-measured14can 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 circuit12.

First, the timing circuit12outputs an optical pulse output command shown inFIG. 3(a) to the light source driving circuit46. Thus, the excitation light source32is driven at the timing shown inFIG. 3(b), and at the timing shown inFIG. 3(c), an output optical pulse is generated by the optical pulse generator10. Subsequently, based on a detected optical pulse signal shown inFIG. 3(d) from the photodetector40, the timing circuit12supplies a timing signal shown inFIG. 3(e) to the A-D converter24to drive the A-D converter24.

Based on the above-described timing signal from the timing circuit12, the A-D converter24starts to convert the photosignals detected by the photodetector20and amplified by the amplifier22to digital signals and store the data.FIG. 3(f) shows the measured waveform portion of the data stored by the A-D converter24.

The computer circuit26performs synchronous addition of measured data received from the A-D converter24to compute measured result, such as broken points, distributions of transmission losses, etc. of the optical fiber-to-be-measured14. The measured results obtained by the computer circuit26can be presented on the display28.

The optical pulse testing apparatus according to the present embodiment is characterized in that the photodetector40for detecting optical pulses circulating through the optical fiber of the optical pulse generator10is provided, and based on the detected optical pulse signals of the photodetector40, the timing circuit12controls the A-D converter24and the computing circuit26.

Next, the optical pulse testing method according to the present embodiment will be explained with reference toFIGS. 1 to 3.

First, an optical fiber14to be measured about transmission states of light, such as transmission losses, broken points, etc. is connected to the input/output connector16.

Then, the timing circuit12supplies a pulse output command shown inFIG. 3(a) to the light source driving circuit46of the optical pulse generator10.

The excitation light source32is driven at the timing shown inFIG. 3(b) by the light source driving circuit46which has received the pulse output command from the timing circuit12. Optical pulses generated by the excitation light source32are entered into the optical fiber30via the optical multiplexer34.

The optical pulses entered into the optical fiber30circulate through the optical fiber30to be thereby amplified. The optical pulses being amplified by circulating through the optical fiber30have the wavelength component determined to be a required value by the variable wavelength filter36.

Thus, the optical pulses circulate through the optical fiber30of the optical pulse generator10, whereby the laser oscillation starts, and optical pulse outputs are produced from the optical branching filter38at the timing shown inFIG. 3(c). The emission of the optical pulses from the optical pulse generator10is delayed with respect to the pulse output command from the timing circuit12shown inFIG. 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 generator10are detected by the photodetector40via the optical branching filter42. The detected signals from the photodetector40are shown inFIG. 3(d). The detected optical pulse signals are transmitted to the timing circuit12. Based on the detected optical pulse signals, the timing circuit12controls the operation timings of the A-D converter24and the computer circuit26.

A part of the optical pulses supplied to the optical fiber-to-be-measured14is reflected at broken points, etc. in the optical fiber-to-be-measured14and returned to the optical pulse testing apparatus via the input/output connector16.

The optical pulses thus emitted by the optical branching filter38of the optical pulse generator10are entered into the optical fiber-to-be-measured14connected to the input/output connector16via the directional coupler18.

A part of the photpulses entered into the optical fiber-to-be-measured14is reflected on broken points, etc. in the optical fiber-to-be-measured14and returns to the optical pulse testing apparatus again via the input/output connector16.

The reflected light returning from the optical fiber-to-be-measured14is separated from the optical pulses emitted from the optical pulse generator10by the directional coupler18, and detected by the photodetector20.

Subsequently, the detected optical pulse signals from the photodetector20are amplified by the amplifier22.

In the meantime, the timing circuit12transmits timing signals shown inFIG. 3(e) to the A-D converter24based on the detected optical pulse signals from the photodetector40shown inFIG. 3(d).

Based on the timing signals from the timing circuit12described above, the A-D converter24starts to convert the detected optical pulse signals from the photodetector20to digital signals as shown inFIG. 3(f) and store the digital data. Thus, even there is a delay time between a pulse output command from the timing circuit12to the light source driving circuit46and the emission of an optical pulse from the optical pulse generator10, the optical pulses reflected in the optical fiber-to-be-measured14and returned to the optical pulse testing apparatus can be synchronously added at each one cycle of the pulses by the computing circuit26.

Concurrently, the synchronous addition of the measured data stored in the A-D converter24is performed by the computer circuit26to compute measured results regarding the states of the light transmission, such as broken points, transmission losses, etc. of the optical fiber-to-be-measured14. The computed results obtained by the computer circuit26are presented on the display28. 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 toFIGS. 4 and 5(a)–5(f).FIG. 4is 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 inFIG. 4according 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 generator10according to the present embodiment is characterized in that a photodetector40is connected to a timing circuit12and a light source driving circuit46. Thus, the signals of light circulating through an optical fiber30, which have been detected by the photodetector40, are supplied to both the timing circuit12and the light source driving circuit46.

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 circuit12of the optical pulse testing apparatus supplies a pulse output command shown inFIG. 5(a) to the light source driving circuit46to start driving the excitation light source32.

The excitation light source32driven by the light source driving circuit46emits excitation light, and the excitation light is supplied to the optical fiber30via the optical multiplexer34and circulates through the optical fiber30as in the first embodiment to start the oscillation of the laser.

During this time, the photodetector40detects via the optical branching filter42a part of the light circulating through the optical fiber30to 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 inFIG. 5(d) is monitored, the drive of the excitation light source32is stopped. This makes it unnecessary to control the excitation light source32corresponding to a parameter of output pulses, such as an excitation pulse width or others, and a required single pulse output as shown inFIG. 5(c) can be easily obtained from the optical pulse generator10.

By using the thus-obtained single pulses, the measurement of states of light transmission through the optical fiber-to-be-measured14can 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 toFIG. 6.FIG. 6is 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 source32, and output pulse frequencies which can be controlled by excitation pulse repetition frequencies of the excitation light source32. However, an output pulse width is substantially determined by the optical parts constituting the optical pulse generator10.

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 inFIG. 6is 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 switch48disposed on the output side of the optical branching filter38of the optical pulse generator10. The optical switch48is connected to an optical switch controller50. The optical switch controller50is connected to a timing circuit12.

The optical pulse generator10has the above-described constitution, and operation timings of the optical switch48and the light source driving circuit46are synchronized, whereby the output pulse width of the optical pulse generator10can be controlled.

By using optical pulses having output pulse widths controlled, light transmission states of an optical fiber-to-be-measured14can 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 toFIG. 7.FIG. 7is 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 controller50as 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 inFIG. 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 fiber30of the optical pulse generator10has an optical fiber52so as to generate optical pulses of a required wide pulse width. Because of the optical fiber52, a time during which light circulates through the optical fiber30is extended by a time during which the light circulates through the optical fiber52, whereby increasing the pulse width of the optical pulses. A length of the optical fiber52is changed to adjust a pulse width of the optical pulses which can be emitted by the optical pulse generator.

Furthermore, a photodetector40detects intensities of the optical pulses circulating through the optical fiber30and the optical fiber52via an optical branching filter42. Based on the result of the detection, a timing circuit12of the optical pulse testing apparatus controls the optical switch controller50so that when a required optical pulse is detected, the optical switch48is 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 switch48to the actual operation of the optical switch48, it is preferable to use the optical fiber52of a length considering the delay time.

As described above, in the present embodiment, the optical switch48is 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 circuit46for driving the excitation light source32with the optical switch48.

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 photodetector40are 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.