JP5521178B2 - Photodetector with time gate and multipoint measurement system using the same - Google Patents

Description

  The present invention relates to a photodetector with a time gate for measuring a faint ultrashort light pulse. The present invention also relates to a multipoint measurement system based on a time division multiplexing method using a photodetector with a time gate.

  In recent years, pulse lasers having time widths of femtoseconds and picoseconds are commercially available and used in many fields including laser processing, terahertz technology, and biophotonics. In these applications, it is important to know the time position and waveform information of the optical pulse, but the femtosecond and picosecond optical pulse widths are very wide, more than terahertz when converted to frequency, It is difficult to measure. If a high-speed streak camera is used, it is possible to directly measure optical pulses of about 200 femtoseconds or more. However, the device is special and expensive, and there is a problem in use such as the device being destroyed if the incident light quantity is incorrectly adjusted. Therefore, an indirect method is used to detect the light pulse, and generally an autocorrelation measurement is performed.

  FIG. 14 is a block diagram showing an example of a conventional optical pulse measuring device. This apparatus uses a Michelson interferometer and includes a pulsed laser light source LA, a beam splitter BS, two corner cube prisms P1 and P2, an optical lens LS, a photodetector PD, and the like.

  The light pulse to be measured from the pulse laser light source LA is branched by the beam splitter BS, a delay difference is given to the two light pulses while reciprocating the distance to the prisms P1 and P2, and interference synthesis is again performed by the beam splitter BS. The intensity of light is measured by the photodetector PD. When the reference arm is swept and the intensity of the signal is displayed as a function of the delay difference, an autocorrelation waveform reflecting the intensity spectrum of the optical pulse is drawn.

  On the other hand, in the field of optical measurement, knowing the time position of an optical pulse is important for obtaining distance information. The time width of a light pulse generated from a gain-modulated small semiconductor laser is several tens of picoseconds, and even if a high-speed photodetector and a high-speed sampling oscilloscope are used for direct measurement, the time resolution is not always sufficient. In addition, such high-speed measuring devices are generally very expensive.

  Unlike a large laser, the output intensity of a semiconductor laser is extremely low (for example, an average intensity of several tens of microwatts to several milliwatts, and a peak intensity of several tens of milliwatts or less). It is necessary to amplify to the extent that no distortion occurs.

Therefore, it is desired to develop a simple indirect measurement method that can detect a light pulse without using an optical amplifier for a light pulse with very low intensity.

  On the other hand, due to the growing social awareness of safety and security, there is a demand for multipoint measurement of temperature, strain, vibration, etc. in large structures, and it should be a system that can withstand long-term measurements comparable to the life of structures. It is requested. It is also assumed that the measurement environment is in harsh conditions such as exposure to radiation and electromagnetic waves and observation in water. Sensing devices that can operate under such conditions are optical fibers that are physically and chemically stable and have little secular change, and it is desired to construct a multipoint measurement system using an all-optical fiber system.

  At each measurement point in the multi-point measurement system, the physical quantity of the target is read from the fluctuations in the intensity and frequency of the reflected / transmitted light using the fiber Bragg grating, the optical fiber interferometer, or the optical fiber itself as a sensor element. Also, in order to realize multipoint measurement with a pair of light source and light receiver, the wavelength division multiplexing method in which the sensor element has wavelength dependency, and the time for separately measuring the signal light pulse from each sensor element There is a method of specifying a measurement point by adopting a division multiplexing method or an interferometer method and modulating the oscillation frequency of signal light or reference light.

JP-A-6-241929

K. Y. Song, Z. He, K. Hotate, "Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation domain analysis", Optics Letters, Vol. 31, No. 17, pp 2526-2528 (2006) Y. Mizuno, W. Zou, Z. He, K. Hotate, "Proposal of Brillouin optical correlation-domain reflectometry (BOCDR)", Optics Express, Vol. 16, No. 16, pp. 12148-12153 (2008)

  In optical interference measurement, it is necessary to prepare a reference arm in order to obtain time position information of an optical pulse. Therefore, when the signal arm length is long, a reference arm having an equivalent length must be prepared.

  Optical elements that enable observation of incident light pulses with an energy per light pulse of 1 femtojoule (fJ) are a class with high conversion efficiency. When this incident light pulse is made to correspond to, for example, a gain modulation pulse from a semiconductor laser, it becomes as follows.

Optical pulse width: 20 ps Repetition frequency: 1 GHz
Average power: 200 μW Peak intensity: 10 mW

  Since this corresponds to the total light energy output from the semiconductor laser, it is difficult to take out only a part of the light energy and perform the optical pulse measurement, indicating that at least an optical amplifier is required.

  In addition, in order to realize multi-point measurement using an all-optical fiber configuration, the wavelength-division multiplexing method is used to make the sensor element wavelength-dependent, and the reflected signal light from the sensor is wavelength-separated to measure the measurement position (measurement point ) Is identified. However, since it is difficult to set the wavelength separation width between the sensor elements necessary for separating the measurement points to be sufficiently small with respect to the use bandwidth of the optical amplifier, the upper limit of the number of measurement points is limited. Further, a special order product is required for the sensor element, and there is a problem that stable supply and stable operation are not always guaranteed.

  On the other hand, in the time division multiplex method using optical pulses, fluctuation information at the measurement point can be obtained by temporally separating and measuring the signal optical pulses from the sensor elements. However, in order to specify the measurement point with a high spatial resolution of several centimeters or less, the time width of the optical pulse must be set to about 200 ps or less. In this case, a high-speed light receiver is required for detecting the light pulse, and as a result, the measurement sensitivity is lowered.

  In the method based on the sensing principle of backward Rayleigh scattering, backward Brillouin scattering, and backward Raman scattering in an optical fiber, generally, a pulse light source is used, and a light pulse is reciprocated to a measurement point in the same manner as in the time division multiplexing method. Identify its location from time. Therefore, it is a low-sensitivity measurement that requires a high-speed light receiver. Further, the backscatter signal is a very weak signal, and its output intensity strongly depends on the incident light intensity, so that a burden on the light receiving system becomes large in order to realize stable measurement.

  In addition, the interferometer method can use a simple low-coherence light source and a high-sensitivity low-speed light receiver. However, a separate optical fiber for the reference signal must be prepared, and the signal processing for interference measurement is complicated. It becomes a problem to do.

  The object of the present invention is to measure an optical pulse with high temporal resolution without using an optical amplifier or preparing a long reference arm even for a weak optical pulse with low optical energy per pulse. It is to provide a photodetection device with a time gate.

  Another object of the present invention is to provide a simple configuration having high spatial resolution and high sensitivity characteristics by applying a photodetection device with a time gate in a time division multiplexing system which is one of the techniques for realizing a multipoint measurement system. Is to provide a multi-point measurement system.

In order to achieve the above object, a photodetection device with a time gate according to the present invention comprises:
A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for injecting a measured light pulse synchronized with the light pulse from the laser light source into the laser light source;
And a time delay adjusting means for adjusting a time difference between the optical pulse and the optical pulse to be measured.

In addition, the photodetection device with a time gate according to the present invention,
A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for branching a light pulse from a laser light source and injecting the branched light pulse again into the laser light source;
And a time delay adjusting means for adjusting a time difference between the optical pulse and the branched optical pulse.

In addition, the photodetection device with a time gate according to the present invention,
A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for injecting an optical pulse to be measured, which is synchronized with or substantially synchronized with an optical pulse from the laser light source, into the laser light source;
And a sweep circuit for sweeping the modulation frequency of the laser light source.

  In the present invention, the laser light source is preferably a semiconductor laser element.

  In the present invention, it is preferable to further include a variable attenuator for adjusting the intensity of the optical pulse injected into the laser light source.

  In the present invention, it is preferable to further include a polarization controller for aligning the polarization states of the optical pulse and the measured optical pulse.

  In the present invention, it is preferable to further include a frequency analyzer for analyzing the frequency component of the electrical signal from the photodetector.

  In the present invention, it is preferable to further include an electronic measuring instrument for measuring the time change of the electric signal from the photodetector.

  In the present invention, the time delay adjusting means preferably includes a movable mirror that can be displaced in the optical axis direction.

In addition, a multipoint measurement system according to the present invention includes the above-described photodetector with a time gate,
A plurality of light pulses reflected at a plurality of measurement points are used as injection light of the laser light source.

  According to the present invention, when the laser light source periodically generates light pulses, when the laser medium of the laser light source changes from loss to gain, there is a kind of time gate that responds sensitively to the injected light only for a specific time period. Arise. This time gate function substantially matches the optical pulse waveform generated from the laser light source. When the injection timing of the injection light is changed in the vicinity of the time gate, the output noise level changes sensitively according to the pulse intensity felt within the time gate width. Therefore, by measuring the change in the output noise level, the waveform information of the injected light can be obtained as a cross correlation waveform. As a result, even a weak injection light pulse can be detected with high time resolution.

  Further, according to the present invention, when the laser light source generates light pulses at a constant period, when a part of the output is injected into the laser light source, the injection light is obtained under the condition that the delay difference between the light pulses is near zero The output noise level rises with high sensitivity according to the intensity of the pulse. Therefore, when multiple optical pulses with the same period generated at different spatial positions are introduced into a single optical fiber via multiple optical couplers and formed as a signal optical pulse train for multipoint measurement, a time delay is scanned. By doing so, the optical information for each spatial position can be separated and measured.

It is a block diagram which shows 1st Embodiment of this invention. It is a graph which shows the noise intensity of spontaneous emission light at the time of return light incidence. It is a graph which shows the correlation (raw data) of the optical pulse spectrum, the time delay of a return optical pulse, and noise intensity, and the correlation which carried out signal processing, respectively. It is explanatory drawing which shows the principle of 1st Embodiment. It is a block diagram which shows 2nd Embodiment of this invention. It is explanatory drawing which shows the principle of 2nd Embodiment. It is a graph which shows the correlation (raw data) of the optical pulse spectrum, the time delay of external light pulse, and noise intensity, and the correlation which carried out signal processing, respectively. It is explanatory drawing which shows the influence of the spectral detuning between two laser light sources. It is a graph which shows an example of the measurement result of the intensity spectrum of a time gate. It is a block diagram which shows 3rd Embodiment of this invention. It is a block diagram which shows 4th Embodiment of this invention. It is a graph which shows the correlation (raw data) of the time delay and noise intensity of the external injection light pulse which synthesize | combined two light pulses. It is a block diagram which shows 5th Embodiment of this invention. It is a block diagram which shows an example of the measuring device of the conventional optical pulse width.

(First embodiment)
FIG. 1 is a configuration diagram showing a first embodiment of the present invention. The time-gated photodetector includes a laser light source LA, an isolator IS, optical couplers CP1 and CP2, an optical fiber FB, a photodetector PD, a variable attenuator ATT, a collimator lens CL, a movable mirror MR, and the like. Prepare.

  The laser light source LA is composed of a distributed feedback (DFB) semiconductor laser element or the like, and generates an optical pulse at a predetermined modulation frequency. In this embodiment, the optical pulse is emitted at an oscillation wavelength of 1550 nm and a modulation frequency of 1 GHz. It has occurred. The modulation frequency and modulation waveform of the laser light source LA can be arbitrarily set by the drive circuit DR. The light emitted from the laser light source LA passes through the optical fiber and enters the optical coupler CP1.

  The optical couplers CP1 and CP2 have a function of branching light incident from one direction in a plurality of directions and combining light incident from a plurality of directions in one direction. The optical coupler CP1 branches light from the laser light source LA in two directions with a predetermined branching ratio. In this embodiment, the one having a branching ratio of 99: 1 is used. The light branched at a branching ratio of 99/100 passes through the isolator IS and the optical fiber FB and enters the second optical coupler CP2. On the other hand, light branched at a branching ratio of 1/100 passes through another optical fiber FB and enters the variable attenuator ATT.

  The isolator IS is composed of a polarizing element, a Faraday rotator, etc., and has a function of attenuating return light to the laser light source LA. In this embodiment, an isolator IS having an attenuation factor of 50 dB (= 1 / 100,000) is used. doing.

  The optical coupler CP2 branches light from the isolator IS in two directions with a predetermined branching ratio. In this embodiment, the optical coupler CP2 has a branching ratio of 50:50. One branched light passes through the optical fiber and enters the photodetector PD, and the other branched light passes through another optical fiber FB and is supplied to the outside for use.

  The photodetector PD is composed of a photodiode or the like, and has a function of detecting the intensity of spontaneous emission light from the laser light source LA. In the present embodiment, the photodetector PD has a much slower response than the modulation frequency of 1 GHz. For example, a frequency band of 10 MHz is used. Therefore, the optical pulse waveform from the laser light source LA cannot be detected faithfully, and it is usually impossible to directly measure the optical pulse. However, according to the method of the present invention, it is possible to detect an optical pulse with high time resolution even with such a low-speed response photodetector.

  The variable attenuator ATT has a function of attenuating the intensity of light at a predetermined attenuation rate. In this embodiment, a variable attenuator ATT that can adjust the attenuation rate in a range of 1.5 to 50 dB is used. The light emitted from the variable attenuator ATT passes through the optical fiber and enters the collimating lens CL.

  The collimating lens CL has a function of converting incident light into parallel light. The parallel light from the collimating lens CL passes through the air and enters the movable mirror MR.

  The movable mirror MR is composed of a plane mirror or the like mounted on a movable stage that can be displaced in the optical axis direction, and has a function of adjusting the optical path length through which the optical pulse propagates. For example, when the movable mirror MR is moved backward by 15 cm, the optical path length increases by 30 cm in the reciprocation, and the time position of the optical pulse can be delayed by 1 ns. The light incident on the movable mirror MR is specularly reflected as it is and then incident on the collimator lens CL again, passes through the variable attenuator ATT → the optical coupler CP1, and enters the laser light source LA as return light. In this embodiment, by setting the variable attenuator ATT to an attenuation factor of 7 dB, an extremely weak light pulse attenuated by 54 dB from the output of the laser light source LA is used as return light.

  The electrical signal from the photodetector PD is subjected to frequency analysis using an electrical spectrum analyzer ESA. An oscilloscope may be used instead of the electric spectrum analyzer ESA.

  Further, an optical coupler may be added in the middle of the optical path through which the optical pulse passes to provide an optical spectrum analyzer for monitoring the wavelength or optical frequency of the branched light.

  FIG. 2 is a graph showing the noise intensity of spontaneously emitted light when returning light is incident. The vertical axis represents intensity (dBm), and the horizontal axis represents frequency (MHz). The lower graph shows the case where there is no return light. The upper graph shows the case where there is return light. Both graphs actually have a spike shape, but for the sake of easy understanding, the upper graph shows the upper envelope.

  When the position of the movable mirror MR is scanned along the optical axis, amplification of spontaneous emission light noise (ASE) is observed near a specific position. This phenomenon is due to the increase in the noise intensity of spontaneously emitted light as shown in the upper graph when the return light is incident at the timing of interaction with the stimulated emission of the laser light source LA. On the other hand, when the return light is not incident on the laser light source LA or is incident at a timing that does not interact with the stimulated emission of the laser light source LA, only the noise floor is observed as shown in the lower graph. .

  Therefore, with the measurement frequency of the electric spectrum analyzer ESA fixed at 4.5 MHz, the position of the movable mirror MR is swept, and the change in the noise intensity of the spontaneous emission light is examined while changing the time delay of the return light pulse.

  FIG. 3 is a graph showing the optical pulse spectrum, the correlation between the time delay of the return optical pulse and the noise intensity (raw data), and the correlation obtained by signal processing for easy observation. The lower the graph, the larger the modulation amplitude of the current that drives the laser light source LA. 3A to 3C, the vertical axis represents light intensity (linear, arbitrary unit), and the horizontal axis represents wavelength (μm). 3D to 3I, the vertical axis represents noise intensity (dBm), and the horizontal axis represents time delay (ps).

  3A to 3C, when the modulation amplitude is small, the spectral width of the optical pulse from the laser light source LA is relatively narrow, but as the modulation amplitude increases, the spectral width of the optical pulse becomes smaller. It turns out that it becomes wide.

  Next, looking at FIGS. 3D and 3G, when the position of the movable mirror MR is changed and the time delay of the return light pulse is changed, a waveform in which the noise intensity peaks at a specific position (full width at half maximum). 48.4 ps) is observed. At this time, a displacement of 0.15 mm of the movable mirror MR corresponds to a time delay of 1 ps of the return light pulse.

  3E and 3H, the noise change waveform (full width at half maximum of 34.7 ps) becomes steeper. In FIGS. 3F and i, the waveform of noise change (full width at half maximum of 28.4 ps) is obtained. Can be seen to be even steeper.

  Therefore, the spectrum of the optical pulse and the temporal waveform of the optical pulse are in a Fourier transform relationship with each other, suggesting that the waveform of such a noise change has a correlation with the waveform of the optical pulse.

  FIG. 4 is an explanatory diagram showing the principle of this embodiment. The laser light source LA generates optical pulses at a predetermined cycle (reciprocal of the modulation frequency). At this time, inside the laser medium of the laser light source LA, the carrier density varies with time due to sinusoidal modulation of the injected current. When the laser medium is not excited, the laser medium is an absorption medium. However, when excitation starts, the laser medium changes to a gain medium and stimulated emission starts. At this time, if the modulation frequency is set in the vicinity of the relaxation oscillation frequency of the laser light source LA, laser oscillation starts after the carrier density is accumulated to some extent beyond the threshold carrier density in the steady state. As a result, strong stimulated emission occurs, and the accumulated carrier density disappears in a short time. Along with this, the stimulated emission stops because the gain of the laser medium decreases. This operation is observed externally as the generation of a gain modulation pulse. Further, since the time width of the generated light pulse is faster than the modulation period of the injection current for driving the laser light source LA, it can be seen that this phenomenon is based on a nonlinear process.

  Therefore, when the external light pulse is incident on the laser medium during the period in which the light pulse is generated, the spontaneous emission light is efficiently amplified, and as a result, the noise intensity of the spontaneous emission light is considered to increase.

  By using an optical pulse generation period in which spontaneous emission light can be amplified in this way as a time gate, extremely short optical pulses can be detected with high temporal resolution.

The autocorrelation waveform is drawn by measuring the average intensity of the optical pulse component sensed within the time gate width, that is, the correlation between the time gate function and the return optical pulse waveform (occurrence of ASE noise) by delayed sweep.

(Second Embodiment)
FIG. 5 is a block diagram showing a second embodiment of the present invention. In this embodiment, an optical pulse generated by a laser light source DUT that is a device under measurement is measured. The laser light source DUT is composed of a distributed feedback (DFB) semiconductor laser element or the like, and is always operating as a light source for optical communication, for example. The laser light source DUT is driven by the drive circuit DR2 to generate an optical pulse, and the generated optical pulse is supplied to the outside through the isolator IS2, the optical coupler CP3, and the optical fiber FB.

  The isolator IS2 is composed of a polarizing element, a Faraday rotator, etc., and has a function of attenuating the return light to the laser light source DUT. In this embodiment, an isolator IS2 having an attenuation factor of 50 dB (= 1/100000) is used. doing. The optical coupler CP3 branches light from the isolator IS2 in two directions with a predetermined branching ratio. In this embodiment, the one having a branching ratio of 99: 1 is used. The light branched at a branching ratio of 99/100 passes through the optical fiber FB and is supplied to the outside, and is used, for example, for optical communication. On the other hand, the light branched at a branching ratio of 1/100 is used for optical pulse detection according to the present invention, passes through another optical fiber, and enters the optical circulator CC.

  The photodetector with a time gate according to the present embodiment includes a laser light source LA, optical couplers CP1 and CP2, an isolator IS1, an optical fiber FB, a photodetector PD, a variable attenuator ATT, and a polarization controller PC. And the above-described optical circulator CC, a collimating lens CL, a movable mirror MR, and the like.

  The laser light source LA is composed of a distributed feedback (DFB) semiconductor laser element or the like, and generates an optical pulse at a predetermined modulation frequency, and a laser light source having substantially the same oscillation wavelength as the laser light source DUT is selected. The drive circuit DR2 that drives the laser light source DUT outputs a synchronization signal SYNC that is synchronized with an external optical pulse, and the drive circuit DR1 drives the laser light source LA in synchronization with the synchronization signal SYNC. In the present embodiment, the laser light source LA generates an optical pulse with an oscillation wavelength of 1550 nm and a modulation frequency of 1 GHz. The light emitted from the laser light source LA passes through the optical fiber and enters the optical coupler CP1.

  The optical coupler CP1 branches light from the laser light source LA in two directions with a predetermined branching ratio. In this embodiment, the optical coupler CP1 has a branching ratio of 50:50. One branched light passes through the isolator IS1 and another optical fiber FB, and enters the second optical coupler CP2, and the other branched light passes through the polarization controller PC and the variable attenuator ATT and passes through the optical circulator CC. Is incident on. The branched light incident on the optical circulator CC is divided into clockwise and counterclockwise components. However, since both of them are greatly attenuated when passing through the optical circulator CC and the optical coupler CP3, the amount of light injected into the laser light source DUT is negligibly small. Will not affect.

  The optical coupler CP2 branches light from the optical coupler CP1 in two directions at a predetermined branching ratio. In this embodiment, the one having a branching ratio of 50:50 is used. One branch light passes through an optical fiber and enters the photodetector PD, and the other branch light passes through another optical fiber FB to monitor the wavelength or optical frequency of the light. Incident on OSA.

  The photodetector PD is composed of a photodiode or the like, and has a function of detecting the intensity of spontaneous emission light from the laser light source LA. In the present embodiment, the photodetector PD has a much slower response than the modulation frequency of 1 GHz. For example, a frequency band of 10 MHz is used.

  The electrical signal from the photodetector PD is subjected to frequency analysis using an electrical spectrum analyzer ESA. An oscilloscope may be used instead of the electric spectrum analyzer ESA.

  On the other hand, the optical circulator CC has a function of emitting light incident from one port to another port. Light from the laser light source DUT passes through the optical circulator CC and enters the collimating lens CL.

  The collimating lens CL has a function of converting incident light into parallel light. The parallel light from the collimating lens CL passes through the air and enters the movable mirror MR.

  The movable mirror MR is composed of a plane mirror or the like mounted on a movable stage that can be displaced in the optical axis direction, and has a function of adjusting the optical path length through which the optical pulse propagates. The light incident on the movable mirror MR is specularly reflected as it is, is incident on the collimator lens CL again, passes through the optical circulator CC, and enters the variable attenuator ATT.

  The variable attenuator ATT has a function of attenuating the intensity of light at a predetermined attenuation rate. In this embodiment, a variable attenuator ATT that can adjust the attenuation rate in a range of 1.5 to 50 dB is used. The light emitted from the variable attenuator ATT passes through the polarization controller PC, the optical fiber, and the optical coupler CP1, and enters the laser light source LA as externally injected light. The polarization controller PC has a function of adjusting the polarization so that the light pulses to be measured interact with high sensitivity in the laser light source LA. Since such external light is greatly attenuated in the middle, it becomes a light pulse with extremely weak intensity. For example, in this embodiment, the optical coupler CP3 provides an attenuation factor of 55 dB, which is 20 dB, and the variable attenuator ATT provides 35 dB.

  FIG. 6 is an explanatory diagram showing the principle of this embodiment. As in FIG. 4, the laser light source LA generates optical pulses at a predetermined period (reciprocal of the modulation frequency). At this time, inside the laser medium of the laser light source LA, the carrier density varies with time due to sinusoidal modulation of the injected current.

  When the laser medium changes to a gain medium and a gain modulation pulse is generated, if the external light pulse is incident on the laser medium, the spontaneous emission light is efficiently amplified, and as a result, the noise intensity of the spontaneous emission light increases. It is done.

  By using an optical pulse generation period in which spontaneous emission light can be amplified in this way as a time gate, extremely short optical pulses can be detected with high temporal resolution.

  The cross-correlation waveform is drawn by measuring the average intensity of the light pulse component sensed within the time gate width, that is, the cross-correlation (generation of ASE noise) between the time gate function and the external light pulse waveform by delay sweep.

  FIG. 7 is a graph showing the optical pulse spectrum, the correlation (raw data) between the time delay of the external optical pulse and the noise intensity, and the correlation obtained by signal processing for easy observation. The lower the graph, the larger the modulation amplitude of the current that drives the laser light source LA. 7A to 7C, the vertical axis represents light intensity (linear, arbitrary unit), and the horizontal axis represents wavelength (μm). 7D to 7F, the vertical axis represents noise intensity (dBm) and the horizontal axis represents time delay (ps). 7 (g) to (i), the vertical axis represents noise intensity (linear, nW) and the horizontal axis represents time delay (ps).

  7A to 7C, when the modulation amplitude is small, the spectrum width of the light pulse from the laser light source LA to be measured is relatively narrow, but the spectrum of the light pulse increases as the modulation amplitude increases. It can be seen that the width becomes wider.

  Next, looking at FIGS. 7D and 7G, when the position of the movable mirror MR is changed to change the time delay of the external light pulse, the waveform at which the noise intensity peaks at a specific position (full width at half maximum). 68.4 ps) is observed.

  7E and 7H, the noise change waveform (full width at half maximum of 56.3 ps) becomes steeper, and in FIGS. 7F and 7I, the waveform of noise change (full width at half maximum of 40.6 ps). Can be seen to be even steeper.

  FIG. 8 is an explanatory diagram showing the influence of spectral detuning between the two laser light sources DUT and LA. The laser light sources DUT and LA are independently temperature controlled using a Peltier element or the like, and can change the oscillation spectrum in accordance with the set temperature. As shown in FIG. 8A, the peak wavelength positions of the output spectra of the laser light source DUT and the laser light source LA are different, and the distance between the two can be arbitrarily set by temperature control.

  FIG. 8B is a graph showing changes in noise intensity due to spectral detuning. The vertical axis is the noise intensity (arbitrary unit), and the horizontal axis is the peak wavelength difference (nm). It can be seen that when the peak wavelengths of the two coincide, the noise intensity becomes maximum, and when the difference between the peak wavelengths increases, the noise intensity decreases. From this graph, it can be seen that the laser light source LA operates as a narrow-band light receiver.

  FIG. 9 is a result of performing the same measurement using the laser light source DUT as a narrow linewidth laser and obtaining an accurate time gate intensity spectrum, and the time gate bandwidth is estimated to be about 0.3 nm. . Since the intensity spectrum of the time gate substantially matches the optical pulse spectrum from the laser light source LA, it is confirmed that the time gate function is the same as the optical pulse waveform from the laser light source LA.

In this embodiment, the optical pulse generated by the laser light source LA is a Fourier transform limit pulse having almost no internal frequency chirp, and the intensity spectrum width of the optical pulse is the intensity spectrum width of the optical pulse generated by the laser light source DUT to be measured. If they are equal or wider than this, it is possible to estimate the time width of the optical pulse from the laser light source DUT by deconvolution processing the cross-correlation waveform obtained from the experiment.

(Third embodiment)
FIG. 10 is a block diagram showing a third embodiment of the present invention. Also in this embodiment, an optical pulse generated by a laser light source DUT that is a device under measurement is measured. The laser light source DUT is composed of a distributed feedback (DFB) semiconductor laser element or the like, and is always operating as a light source for optical communication, for example. The laser light source DUT is driven by the drive circuit DR2 to generate an optical pulse, and the generated optical pulse is supplied to the outside through the isolator IS2, the optical coupler CP2, and the optical fiber FB.

  The optical coupler CP2 branches light from the isolator IS in two directions with a predetermined branching ratio. In this embodiment, the one having a branching ratio of 99: 1 is used. The light branched at a branching ratio of 99/100 passes through the optical fiber FB and is supplied to the outside, and is used, for example, for optical communication. The light branched at a branching ratio of 1/100 is used for optical pulse detection according to the present invention, and another optical fiber FB, a variable attenuator ATT, another optical fiber FB, a polarization controller PC, and an optical coupler. It passes through CP1 and enters the laser light source LA.

  The incident light intensity from the laser light source DUT in this embodiment is attenuated by a total of 55 dB, which is 20 dB by the optical coupler CP2 and 35 dB by the variable attenuator ATT and the optical coupler CP1, and enters the laser light source LA as an external light pulse with extremely weak intensity. doing. Further, the polarization controller PC has a function of adjusting the polarization so that the measured light pulses interact with each other with high sensitivity in the laser light source LA.

  The photodetector with a time gate according to the present embodiment includes a laser light source LA, an optical coupler CP1, a polarization controller PC, a variable attenuator ATT, an optical fiber FB, an isolator IS1, a photodetector PD, and the like. Prepare.

  The laser light source LA is composed of a distributed feedback (DFB) semiconductor laser element or the like, and generates an optical pulse at a predetermined modulation frequency, and a laser light source having substantially the same oscillation wavelength as the laser light source DUT is selected. The laser light source LA is driven by the drive circuit DR1, and the modulation frequency is controlled by the sweep circuit SC. In the present embodiment, the laser light source LA sweeps in the range of an oscillation wavelength of 1550 nm and a center modulation frequency of 0.9 to 1.1 GHz, and generates an optical pulse having a time width of picoseconds. The light emitted from the laser light source LA passes through the optical fiber and enters the optical coupler CP1.

  The optical coupler CP1 branches light from the laser light source LA in two directions with a predetermined branching ratio. In this embodiment, the optical coupler CP1 has a branching ratio of 50:50. One branched light passes through the isolator IS1 and enters the photodetector PD. The other branched light passes through the variable attenuator ATT and the optical coupler CP2, and is attenuated so that the amount of light injected into the measured light source DUT can be sufficiently ignored.

  The photodetector PD is composed of a photodiode or the like, and has a function of detecting the intensity of spontaneous emission light from the laser light source LA. In the present embodiment, the photodetector PD has a much slower response than the modulation frequency of 1 GHz. For example, a frequency band of 10 MHz is used.

  The electric signal from the photodetector PD is observed as a change in noise intensity with time using an oscilloscope OSC. An electrical spectrum analyzer may be used instead of the oscilloscope OSC.

  In the first and second embodiments, the movable mirror MR is used to adjust the time difference between the return light pulse or the external light pulse. However, in this embodiment, the delay time is swept by sweeping the modulation frequency of the laser light source LA. The same function as adjustment is realized. According to the configuration for performing such electrical sweep, a moving mechanism such as the movable mirror MR is not necessary, so that the apparatus can be reduced in size and simplified.

  As described above, the experiment disclosed in this specification has succeeded in detecting a light pulse with respect to incident light under the following conditions.

Optical pulse width: 35 ps Repetition frequency: 1 GHz
Average power: 0.1 nW Peak intensity: 5 nW

This result means that the measurement sensitivity is improved by at least 30 dB compared to the conventional direct measurement method for light pulses. Therefore, the light pulse can be detected by using only a small part of the light pulse energy.

(Fourth embodiment)
FIG. 11 is a block diagram showing a fourth embodiment of the present invention. Here, a multipoint measurement system using a photodetection device with a time gate will be described. The laser light source LA1 is composed of a distributed feedback (DFB) semiconductor laser element or the like. The laser light source LA1 is driven by the drive circuit DR1 to generate an optical pulse. In the present embodiment, the drive circuit DR1 supplies a modulation current in which a sine wave current having a modulation frequency of 1 GHz is superimposed on a DC current 9.8 mA that matches the threshold current of the laser light source LA1, and the generated optical pulse is The time width is 35 ps at the center wavelength of 1550 nm. The optical pulse passes through the isolator IS1 and is supplied to the optical coupler CP1 and the optical coupler CP2 connected in cascade.

  The optical coupler CP1 branches light from the isolator IS1 in two directions at a predetermined branching ratio. In this embodiment, the optical coupler CP1 has a branching ratio of 50:50. One branched light is supplied to the optical coupler CP2, and is branched again with a 50:50 branching ratio. One of them is assumed to be further supplied to an optical coupler connected in cascade, but in this embodiment, it is released to the outside.

  The other branched lights of the optical couplers CP1 and CP2 enter the collimator lenses CL1 and CL2, respectively. The collimating lenses CL1 and CL2 have a function of converting incident light into parallel light. The parallel light from the collimating lenses CL1 and CL2 passes through the air and is reflected by the fixed mirrors MR1 and MR2, respectively, and again passes through the collimating lenses CL1 and CL2 and is introduced into the optical fiber.

  In this embodiment, the reflected light from the fixed mirrors MR1 and MR2 is processed as an optical fiber sensor signal that is reflected and returned from different spatial positions. That is, as an alternative to the optical couplers CP1 and CP2 and the fixed mirrors MR1 and MR2, an optical fiber sensor using an optical fiber interferometer, a fiber Bragg grating, or the like can be used, thereby simultaneously reflecting light reflected from a large number of spatial positions. It can be measured.

  Reflected light from different spatial positions again passes through the optical couplers CP1 and CP2, is branched by the optical coupler CP1, passes through the isolator IS2 and the circulator CC, and enters the collimating lens CL3. The parallel light from the collimating lens CL3 passes through the variable attenuator ATT in the air and enters the movable mirror MR3.

  The reflected light from the movable mirror MR3 again passes through the variable attenuator ATT in the air and is introduced into the optical fiber by the collimator lens CL3. Thereafter, the reflected light passes through the circulator CC, the polarization controller PC, and the optical coupler CP3 having a 50:50 branching ratio, and enters the laser light source LA2.

  In this embodiment, the average intensity of incident light from the laser light source LA1 is 113 μW immediately after the isolator IS1, and the average intensity of light injected into the laser light source LA2 is about 1 μW by adjusting the variable attenuator ATT.

  The photodetector with a time gate according to the present embodiment includes a laser light source LA2, an optical coupler CP3, a polarization controller PC, a variable attenuator ATT, an isolator IS3, a photodetector PD, and the like. The polarization controller PC has a function of adjusting the polarization so that the light pulses to be measured interact with high sensitivity in the laser light source LA2.

  The laser light source LA2 is composed of a distributed feedback (DFB) semiconductor laser element or the like, and generates an optical pulse at a predetermined modulation frequency. A laser light source having substantially the same oscillation wavelength as the laser light source LA1 is selected. The laser light source LA2 is driven by the drive circuit DR2, and is supplied with a modulation frequency synchronized with the drive circuit DR1. In the present embodiment, the laser light source LA2 sweeps at a modulation frequency of 1 GHz and generates an optical pulse having a center wavelength of 1550 nm and a time width of 35 ps. The light emitted from the laser light source LA2 passes through the optical fiber and enters the optical coupler CP3.

  The optical coupler CP3 branches the light from the laser light source LA2 in two directions at a predetermined branching ratio. In this embodiment, the optical coupler CP3 has a branching ratio of 50:50. One branched light passes through the isolator IS3 and enters the photodetector PD. The other branched light passes through the circulator CC and the isolator IS2, and is attenuated so that the amount of light returning to the laser light sources LA1 and LA2 can be sufficiently ignored.

  The photodetector PD is composed of a photodiode or the like, and has a function of detecting the intensity of spontaneous emission light from the laser light source LA2, and in this embodiment, has a remarkably low response compared to the modulation frequency of 1 GHz. For example, a frequency band of 10 MHz is used.

  The electrical signal from the photodetector PD is subjected to frequency analysis using an electrical spectrum analyzer ESA. An oscilloscope may be used instead of the electric spectrum analyzer.

  FIG. 12 is a graph showing the correlation (raw data) between the time delay of the injected light pulse and the noise intensity with the measurement frequency of the electric spectrum analyzer fixed at 3 MHz. The vertical axis represents noise intensity (dBm), and the horizontal axis represents time delay (ps).

  It can be seen that when the position of the movable mirror MR3 is changed to change the time delay of the injection light pulse, a waveform having a peak noise intensity is observed at a specific position. In this embodiment, two chevron waveforms are observed.

  By blocking the reflected light from the fixed mirrors MR1 and MR2 one by one, one of the two chevron waveforms disappears. From this, it can be seen that the chevron waveform on the left side of the figure is caused by the reflected light from the fixed mirror MR1, and the chevron waveform on the right side is caused by the reflected light from the fixed mirror MR2.

  In the graph, the time interval ΔT of the peak position of the mountain waveform is about 200 ps, which corresponds to being 6 cm away in terms of optical distance. For example, the distance interval ΔL between the peak positions in the present embodiment performed at a modulation frequency of 1 GHz follows the following equation.

    ΔL = W12−W11−30 cm × m (integer)

  Here, W12 is the reciprocal optical distance from the optical coupler CP1 to the fixed mirror MR2, and W11 is the reciprocal optical distance from the optical coupler CP1 to the fixed mirror MR1. The integer m is given so that the distance between the peak positions becomes a value from 0 to 30 cm. 30 cm is an optical distance corresponding to a modulation period of 1 ns.

  Thus, in this method, since the coherence of light is not used when detecting the time position of the light pulse, it is not necessary to prepare a long reference arm in the interferometer method. If the modulation frequency and the oscillation wavelength coincide with each other with respect to the light pulse emitted from another laser light source, the time delay of the laser light source LA2 is adjusted by adjusting the time delay within an optical distance of 30 cm corresponding to the modulation period. The injection light pulse is detected with high sensitivity as a noise signal amplified within the signal.

  In this embodiment, the two fixed mirrors MR1 and MR2 are regarded as reflection points of the optical fiber sensor, and the practicality of the two-point measurement is demonstrated. Therefore, a desired multipoint measurement system can be realized by connecting a large number of optical couplers in a subsequent stage of the optical coupler CP2 and installing fixed mirrors respectively.

  At that time, the branching ratio of each optical coupler is not constant, and may be set to be different, for example, 90:10 or 99: 1. The optical coupler closer to the laser light source LA1 has a smaller branching ratio. By configuring, the difference in the intensity of the reflected signal light from each optical fiber sensor becomes small, and the contrast between peak values of the chevron waveform in the output signal of FIG. 12 can be suppressed low.

  When the splitting ratio is small, the intensity of the light injected into the laser light source LA2 is low, and the signal-to-noise ratio of the output signal is reduced, either immediately after the isolator IS1 on the incident light side or the polarization on the light receiver side An optical amplifier can be inserted immediately after the controller PC and adjusted so as to have an appropriate injected light intensity.

  In the present embodiment, with respect to the modulation period of 1 ns, the reflected signal from one fixed mirror has a correlation width of about 100 ps in consideration of the bottom of the mountain-shaped waveform. Therefore, it can be seen that by adjusting the length of the optical fiber connecting to each measurement point, the reflected signals are arranged at equal intervals on the delay axis, and at least 10 different spatial positions can be measured.

  The upper limit of the number of measurement points possible with one laser light source LA1 and a pair of time-gated photodetectors (LA2, PD) depends on the modulation period, the relaxation oscillation frequency of the laser light source, and the excitation conditions for driving the laser light source. The number of measurement points can be increased in a low modulation period that depends on the delay distance.

  As described above, in the experiment disclosed in the present specification, the detection of the injection light pulse reflected from two different points has been successfully performed under the following conditions.

Optical pulse width: 35 ps Repetition frequency: 1 GHz
Average power of injection light to the laser light source for light reception: 1 μW Correlation width: 60 ps (18 mm)

  This result shows that in the measurement of a high-speed optical pulse having a time width in the picosecond range, a plurality of optical pulses can be separately measured by using only a small part of the optical pulse energy.

  Further, the optical distance to each measurement point can be obtained by changing the value of the modulation frequency for driving the laser light source LA1 by 4 to 5 points, detecting the peak position each time, and measuring the change in the delay distance. it can.

  For example, the distance from the optical coupler CP1 to the fixed mirror MR1 is D1, and is the distance to be measured. However, for simplicity, it is assumed that the gap between the collimating lens CL1 and the fixed mirror MR1 is connected by an optical fiber. Further, it is assumed that the noise peak due to the reflected light from the fixed mirror MR1 can be specified from the noise peak contrast and the like. Here, the distance from the laser light source LA1 to the optical coupler CP1 is D0, the distance from the optical coupler CP1 through the circulator CC to the collimating lens CL3 is D2, and the distance from the collimating lens CL3 to the movable mirror MR3 is D3. The distance from the collimating lens CL3 to the laser light source LA2 via the polarization controller PC and the optical coupler CP3 is D4.

The laser light sources LA1 and LA2 are both driven at the modulation frequency f1, and the delay position of the movable mirror MR3 is adjusted to determine the position D3 (1) where the noise peak is observed. At this time, the following relational expression is established using the modulation period T1 corresponding to the modulation frequency f1.
n × (D0 + 2 × D1 + D2 + D4) + 2 × D3 (1) = m1 × c × T1 (1)

Where c is the speed of light, n is the refractive index of the optical fiber, and m1 is an unknown integer. When the distances D0, D2, and D4 are known, the above equation is summarized for the distance D1 to be measured, and the following equation is obtained.
n × D1 = m1 × c × T1 / 2−D3 (1) −n × (D0 + D2 + D4) / 2 (2)

Next, when the same experiment is performed by changing the modulation frequency to f2, the following relational expression is obtained using the corresponding modulation period T2.
n * D1 = m2 * c * T2 / 2-D3 (2) -n * (D0 + D2 + D4) / 2 (3)

  Here, m2 is an unknown integer, and D3 (2) is the position of the movable mirror MR3 where a noise peak is observed at the modulation frequency f2.

Since the right sides of the above equations (2) and (3) are equal, the following equation holds.
c * (m1 * T1-m2 * T2) = 2 * (D3 (1) -D3 (2)) (4)

  The values of the integers m1 and m2 are not uniquely determined, but within the distance corresponding to the least common multiple of the optical distance (light speed × modulation cycle: c × T1, c × T2) corresponding to each modulation cycle, There is only one set of integers (m1, m2) that satisfies

Similarly, when an experiment in which the modulation frequency is changed to f3 (corresponding modulation period T3) is performed, the following relational expression is obtained.
c * (m1 * T1-m3 * T3) = 2 * (D3 (1) -D3 (3)) (5)

  Here, m3 is an unknown integer, and D3 (3) is the position of the movable mirror MR3 where a noise peak is observed at the modulation frequency f3. An integer set (m1, m2, m3) satisfying the above equations (4) and (5) is an optical distance corresponding to each modulation period (light speed × modulation period: c × T1, c × T2, c × T3). It exists as the only solution within a distance corresponding to the least common multiple.

  Therefore, by adding experimental results with varying modulation frequency values, the set of integers that satisfy the condition is the only solution for longer distances, and each integer value is determined virtually uniquely. Is done. Thus, the distance D1 to be measured can be obtained by substituting the obtained value of the integer m1 into the equation (2).

For example, by performing the above-described experiment under the following four modulation frequencies (light speed × modulation cycle), the range up to 6.068 km (= 31 × 29 × 27 × 25 cm) can be obtained up to the measurement point. The distance can be specified.
0.968 GHz (31 cm), 1.034 GHz (29 cm)
1.111 GHz (27 cm), 1.200 GHz (25 cm)

  Furthermore, the upper limit of the range for specifying the distance to the measurement point is expanded to 139.57 km by adding the result of an experiment performed under the modulation frequency (light speed × modulation period) of 1.304 GHz (23 cm). To do.

The method for specifying the distance to the measurement point by changing the above modulation frequency is not limited to the multipoint measurement system of the present embodiment, and the distance to the break point is measured in a general system connecting optical fibers. It can be applied to. In this case, the reflected light based on Fresnel reflection occurring at the breaking point is detected.

(Fifth embodiment)
FIG. 13 is a block diagram showing a fifth embodiment of the present invention. The overall configuration and operation principle of the multipoint measurement system is the same as in the fourth embodiment, but 1) the number of laser light sources used is changed from two to one, and 2) between the laser light sources LA1 and the isolator IS1. Is different in that an optical coupler CP0 is inserted.

  The light pulse from the laser light source LA1 driven in the same manner as in the fourth embodiment is branched by passing through the optical coupler CP0. The branching ratio of the optical coupler CP0 is assumed to be 50:50. One branched light passes through the subsequent isolator IS1 and enters the optical coupler CP1, and the signal light pulse is fed back to the optical coupler CP3 through the same process as in the fourth embodiment.

  The optical coupler CP3 is connected to the optical coupler CP0, and signal light pulses are injected into the laser light source LA1. The other branched light out of the output from the laser light source LA1 passes through the optical coupler CP3 and the isolator IS3 and is detected by the photodetector PD.

  As in the fourth embodiment, the position of the movable mirror MR3 is adjusted, and the signal light pulse is amplified by feedback injection so that it matches the time gate width (light pulse generation time zone) of the laser light source LA1. As a noise signal, the injection light pulse is detected with high sensitivity.

  Therefore, when a large number of reflected light pulses generated at different spatial positions are introduced into one optical fiber via a plurality of optical couplers and formed as a signal light pulse train for multipoint measurement, the same as in the fourth embodiment In addition, the optical information for each spatial position can be separately measured by scanning the time delay.

  Similarly to the fourth embodiment, by performing a series of experiments in which the modulation frequency of the laser light source LA1 is changed, the distance to the measurement point and the distance to the break point of the optical fiber can be specified.

  The present invention is extremely useful industrially in that a weak light pulse can be detected with high temporal resolution, and a multipoint measurement system having high spatial resolution and high sensitivity characteristics can be realized.

LA, DUT, LA1, LA2 Laser light source IS, IS1, IS2, IS3 Isolator CC Optical circulator CP0, CP1, CP2, CP3 Optical coupler FB Optical fiber PC Polarization controller ATT Variable attenuator CL, CL1, CL2, CL3 Collimating lens MR1 , MR2 fixed mirror MR, MR3 movable mirror DR, DR1, DR2 drive circuit SC sweep circuit PD photodetector ESA electrical spectrum analyzer OSA optical spectrum analyzer OSC oscilloscope

Claims (10)

A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for injecting a measured light pulse synchronized with the light pulse from the laser light source into the laser light source;
A photodetection device with a time gate, comprising: a time delay adjusting means for adjusting a time difference between the optical pulse and the measured optical pulse. A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for branching a light pulse from a laser light source and injecting the branched light pulse again into the laser light source;
A photodetection device with a time gate, comprising: a time delay adjusting means for adjusting a time difference between the optical pulse and the branched optical pulse. A laser light source that generates optical pulses at a predetermined modulation frequency;
A photodetector for detecting the intensity of spontaneously emitted light from the laser light source;
An optical coupler for injecting an optical pulse to be measured, which is synchronized with or substantially synchronized with an optical pulse from the laser light source, into the laser light source;
A photodetection device with a time gate, comprising: a sweep circuit for sweeping a modulation frequency of a laser light source.   The photodetection device with a time gate according to claim 1, wherein the laser light source is a semiconductor laser element.   The time-gate photodetection device according to any one of claims 1 to 3, further comprising a variable attenuator for adjusting the intensity of the light pulse injected into the laser light source.   The time-gate photodetection device according to any one of claims 1 to 3, further comprising a polarization controller for aligning the polarization states of the optical pulse and the measured optical pulse.   The time-gate photodetection device according to any one of claims 1 to 3, further comprising a frequency analyzer for analyzing a frequency component of an electric signal from the photodetector.   The photodetection device with a time gate according to any one of claims 1 to 3, further comprising an electronic measuring instrument for measuring a time change of an electric signal from the photodetector.   3. The photodetecting device with a time gate according to claim 1, wherein the time delay adjusting means includes a movable mirror that can be displaced in the optical axis direction. A photodetection device with a time gate according to any one of claims 1 to 9,
A multipoint measurement system using a plurality of light pulses reflected at a plurality of measurement points as injection light of a laser light source.