JP2012132706A - Pulse light monitoring device and pulse light monitoring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulse light monitoring device and a pulse light monitoring method, capable of monitoring a state of time width of pulse light in real time.SOLUTION: A pulse light monitoring device comprises: a demultiplexer for demultiplexing laser pulse light which is a measurement target into two light beams; a first photodetector and a second photodetector for respectively receiving the demultiplexed two light beams; a signal computing processor; and an output apparatus. The first photodetector converts the energy of light received by using one-photon absorption into a first signal, and the second photodetector converts the energy of light received by using two-photon absorption into a second signal. The signal computing processor obtains a quotient obtained by dividing the second signal by the square of the first signal, and has a computing function indicating a state of time width of the pulse light by the quotient. A signal for indicating the state of the time width of the pulse light is outputted to the output apparatus to be monitored.

Description

  The present invention relates to a pulse light monitoring apparatus and a pulse light monitoring method capable of monitoring a time width state of laser pulse light substantially in real time. In particular, the present invention relates to a pulsed light monitoring device useful for a laser generator including an optical fiber.

  It is important to monitor the characteristics of laser pulse light when it is used. In particular, it is useful to be able to monitor the characteristics of laser pulse light in real time. Examples of the characteristics of the laser pulse light include the time width and peak power of the pulse light. In the laser pulse light, the peak power increases as the time width becomes shorter, and a nonlinear phenomenon is likely to occur in proportion to the square of the peak power.

  For example, in a two-photon microscope using a nonlinear phenomenon, a high-power pump laser with a femtosecond ultrashort pulse is used. When the light from this laser is guided by a fiber, in order to cause a nonlinear phenomenon at a predetermined position, there is a demand to quickly adjust the laser generation conditions so that the time width of the pulsed light becomes the shortest at that position. There is.

  It is also important to be able to monitor the peak power so that the time width of the pulsed light is always the shortest when the state of the oscillator of the laser generator changes or when the fiber is replaced. It is useful to be able to monitor the time width and peak power of pulsed light in real time when adjusting the laser generator and measuring dispersion. In particular, it is effective to combine a pulsed light monitoring device with a feedback mechanism of a laser generator.

Here, generation of pulsed light by mode synchronization will be briefly described as an example of laser light.
The spectrum of laser light is composed of a number of longitudinal modes, which are usually separated by a frequency corresponding to the reciprocal of the time that light travels through the laser oscillator. When the phase of each mode in a large number of longitudinal modes is completely irrelevant, the output of the laser beam shows irregular fluctuations in time. If the phases of the respective modes are aligned, that is, the mode is synchronized, pulse light having a short time width is emitted one after another at a predetermined time interval. At this time, the time width (Δt) of the pulsed light can be reduced to about the reciprocal (1 / Δν) of the spectral frequency width (Δν) of the laser light. For example, when the pulsed light has a Gaussian waveform, the product of the time width and the spectral width is Δt × Δν = 0.44.

Next, a method for measuring the time width of pulsed light will be briefly described.
The femtosecond order pulsed light has a very short time width, and even if a photodiode and an oscilloscope are used, the time width of the pulsed light is shorter than their time resolution, so it cannot be measured directly. Therefore, a nonlinear correlation method using a nonlinear optical element has been proposed. In this nonlinear correlation method, the pulsed light to be measured is divided into two, the optical path lengths are differentiated, one pulsed light is delayed, and this is incident on the second harmonic crystal (SHG crystal). And the overlap of two pulsed light is measured using the generated second harmonic. This measurement method uses a Michelson interferometer and can be implemented by the autocorrelator apparatus shown in FIG.

  In the autocorrelator apparatus 1000 shown in FIG. 10, the laser pulse light 600 to be measured is first divided into two by the beam splitter 301. One of the divided pulse lights passes through as it is and is reflected by the mirror 400 of the optical delay mechanism, and then the course thereof is changed to a right angle by the beam splitter 301 and enters the SHG crystal 700 which is a nonlinear optical element. The other of the divided pulse lights is changed in the direction of the right angle by the beam splitter 301, reflected by the fixed mirror 500, and then passes through the beam splitter 301 and is incident on the SHG crystal 700.

  A part of the pulsed light incident on the SHG crystal 700 is wavelength-converted to generate a second harmonic. Then, the fundamental wave of the pulsed light is removed by the filter 800, and only the second harmonic enters the photodetector 900. From this photodetector, an output corresponding to the power of pulsed light is obtained.

  The power of the second harmonic generated by the nonlinear crystal is proportional to the degree of overlap of the two pulse lights, that is, the difference in optical path length (delay distance). When two pulse lights that are delayed by providing a difference in optical path length are overlapped, the power of the pulse light increases as the overlap increases. When these two pulse lights are incident on the SHG crystal, a second harmonic corresponding to the power is generated. As a result, the relationship between the delay distance and the power of the second harmonic is required.

  Therefore, if the delay distance is divided by the speed of light, the relationship between time and the power of the second harmonic is obtained. Therefore, the time width of the pulsed light can be estimated by measuring the power of the second harmonic as a function of the optical path length difference. Further, the peak power of the pulsed light can be obtained from the estimated time width of the pulsed light, repetition frequency, energy per pulse, and average power of the pulsed light.

  Here, since the optical delay mechanism in the apparatus shown in FIG. 10 has a structure for mechanically moving the mirror, it takes time to move the mirror. For this reason, the measurement of the time width and peak power of the pulsed light by this nonlinear correlation method requires at least a time on the order of seconds.

  Regarding the measurement of the time width of the pulsed light, the present inventors have proposed an “autocorrelator” in Japanese Patent Laid-Open No. 11-173921. In this autocorrelator, an avalanche photodiode is provided as a light detector to increase measurement sensitivity.

Japanese Patent Application Laid-Open No. 2010-093243 discloses an “optical peak power detection device” characterized in that the average intensity of laser light and the power of laser light subjected to wavelength conversion are measured simultaneously and individually. Yes.
In this “optical peak power detection device”, the second signal, which is the second harmonic detection signal, reflects the average power and peak power of the pulse, and the first signal reflects the average power of the pulse. In order to emphasize the change in peak power, calculations such as the second signal being calculated by the first signal are performed (see paragraph [0027] and the like of the publication).

Japanese Patent Laid-Open No. 11-173921 JP 2010-093243 A

  In the nonlinear correlation method described above, it takes at least seconds time to measure the pulse width and peak power, so it is impossible to monitor the pulse width and peak power in real time. Met.

  In addition, the “optical peak power detection device” disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2010-093243 describes that “the average intensity of laser light and the power of laser light that has passed through or reflected by a nonlinear optical element are simultaneously and “Measured individually”. Specifically, the nonlinear optical element is a harmonic generation optical element (nonlinear optical crystal) or a nonlinear absorption element. In the harmonic generation optical element, wavelength conversion is performed by passing laser light. Further, in the embodiment, a nonlinear optical crystal is used.

  In installing the nonlinear optical crystal, unless the polarization direction, angle, or temperature in the crystal is adjusted to an optimum state, the second harmonic is not efficiently generated and a signal cannot be obtained from the photodetector. For this reason, there is a problem that the adjustment takes time. If the angle or temperature of the nonlinear optical crystal is slightly shifted, the conversion efficiency is greatly changed, and the measurement accuracy is significantly deteriorated. In addition, it is difficult to simplify the configuration of the measurement system.

Furthermore, in the above-mentioned Japanese Patent Application Laid-Open No. 2010-093243, no specific mention is made regarding the measurement of the time width of the pulsed light.
Further, if the time width of pulsed light is monitored in real time using a nonlinear optical crystal, there are the following problems.

  For example, in a mode-locked fiber laser or the like, the wavelength and polarization direction of a short pulse laser in the fiber are constantly changing. If the wavelength changes or the polarization direction changes while monitoring the time width of the pulsed light, those values will change greatly. Therefore, it has been difficult to monitor the time width of the pulsed light in real time using the nonlinear optical crystal.

  In the autocorrelator disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 11-173921, the light interfered by the Michelson interferometer is converted into an electric signal by a photodetector using two-photon absorption.

  An object of the present invention is to provide a pulsed light monitoring device and a pulsed light monitoring method capable of monitoring the time width state of pulsed light in real time with a simple configuration without requiring a nonlinear optical element or a Michelson interferometer. Is to provide.

  As a result of intensive studies, the present inventor has found that the above-described object can be achieved without using the second harmonic by a non-linear optical element or interference light by a Michelson interferometer. That is, according to the present invention, the laser pulse light to be measured is demultiplexed into two lights, the light detection using two-photon absorption and the light detection using one-photon absorption are performed, and the obtained signals are respectively obtained. By performing the arithmetic processing, the time width state of the pulsed light can be monitored in real time.

The monitoring device for pulsed light according to the present invention comprises:
A demultiplexer that demultiplexes the laser pulse light to be measured into two lights;
A first photodetector and a second photodetector, each receiving two demultiplexed lights;
A signal processor,
An output device,
The first photodetector converts the energy of light received using one-photon absorption into a first signal, and the second photodetector detects the energy of light received using two-photon absorption as a second. Converted into a signal,
The signal arithmetic processing unit obtains a quotient obtained by dividing the second signal by the square of the first signal, and has an arithmetic function that represents a time width state of the pulsed light from the quotient.
A signal representing the obtained time width state of the pulsed light is output to the output device and monitored.

  The state of the time width of the pulsed light is a change in the time width of the pulsed light.

The state of the time width of the pulsed light is the time width of the pulsed light,
The signal arithmetic processor may monitor the time width of the pulsed light from the relational expression of the time width of the pulsed light and the quotient obtained in advance.

  Further, the signal arithmetic processor may monitor the time width of the pulsed light according to the relational expression calibrated using the time width of the pulsed light measured in advance.

  A condensing lens may be provided between the duplexer and the second photodetector.

  Furthermore, a condensing lens may be provided between the duplexer and the first photodetector.

  The input intensity ratio of the pulsed light in the first photodetector and the two photodetectors may be in the range of 1: 9 to 1: 999.

  A variable loss device may be provided between the duplexer and the first photodetector.

The first photodetector is indium gallium arsenide / photodiode,
The second photodetector may be an avalanche photodiode using silicon.

  The second photodetector may be a laser diode capable of oscillating light having a wavelength shorter than the wavelength of the laser pulse light to be measured.

  The duplexer is an optical fiber coupler.

  The duplexer may be a beam splitter.

Also, the method for monitoring pulsed light according to the present invention includes:
A step of splitting the laser pulse light to be measured into two lights;
In the first photodetector and the second photodetector that receive the two demultiplexed lights, respectively, the first photodetector converts the energy of the received light into a first signal using one-photon absorption. And the second photodetector converts the energy of light received using two-photon absorption into a second signal;
An arithmetic processing step for obtaining a quotient obtained by dividing the second signal by the square of the first signal, and representing a time width state of the pulsed light from the quotient;
An output step of outputting a signal of the time width state of the pulsed light to the output device and monitoring it.

  In this specification, being able to monitor substantially in real time means being able to monitor in the order of microseconds to milliseconds, and the specific measurement time is being 10 milliseconds or less. The state of the time width of the pulsed light refers to the time width of the pulsed light itself or a change in the time width.

  The pulse light monitoring device and the pulse light monitoring method according to the present invention have the configuration as described above, and only the detected signal is processed, so that the time width state of the pulse light, that is, the change in time width or the time width itself. Can be accurately monitored. Further, since a Michelson interferometer or the like is not required and no mechanically moving structure is provided, the time width state of the pulsed light can be monitored substantially in real time.

  In addition, the “optical peak power detection device” disclosed in Japanese Patent Application Laid-Open No. 2010-093243 requires a nonlinear optical element. However, the pulsed light monitoring apparatus and the pulsed light monitoring method according to the present invention do not require a nonlinear optical element. For this reason, adjustment of the nonlinear optical element is unnecessary, which is preferable.

For example, in an autocorrelator, in the optical system, it is not only necessary to accurately adjust the optical axis during assembly, but also the accuracy of the optical axis must be maintained during operation, which makes the apparatus very expensive. It was. In addition, a detection apparatus using a nonlinear optical element requires an adjustment mechanism for the nonlinear optical element.
On the other hand, in the pulse light monitoring apparatus according to the present invention, if adjustment is made so that the pulse light accurately enters the photodetector at the time of assembly, no special adjustment is required during operation. Also, the required accuracy is not high compared to the autocorrelator. Furthermore, there may be few components. Therefore, a pulsed light monitoring device can be established at low cost.

When the monitoring device for pulsed light according to the present invention is incorporated into a laser generator, the state of the time width of the generated pulsed light can be monitored in real time, so that the laser generation conditions can be optimized quickly.
Further, since the pulsed light monitoring apparatus according to the present invention can be made compact, it can also be applied to field use.

It is a schematic block diagram of the monitoring apparatus of the pulsed light which is one Embodiment of this invention. It is a figure which shows the mode of condensing in a 2nd photodetector (SiAPD). It is a figure which shows the mode of condensing in a 2nd photodetector (LD). It is a figure which shows the mode of condensing in a 1st photodetector. It is the graph which showed the relationship between the input power of pulsed light, and the output value of a photodetector. It is a graph which shows the relationship between the time width | variety and pulse power of pulsed light with respect to fixed input power in two photon absorption. In 2 photon absorption, it is a graph which shows the change of the peak power at the time of changing the time width of pulsed light with a dispersion variable device. It is a graph which shows the relationship between (P2 / P1 ² ) and time width. It is a schematic block diagram of the monitoring apparatus of the pulsed light which is another embodiment of this invention. It is a schematic block diagram of an autocorrelator.

  The pulse light monitoring apparatus and pulse light monitoring method according to the present invention monitors the state of the time width of pulse light as follows.

  First, after the pulsed light is demultiplexed into two lights, light detection is performed using one-photon absorption and two-photon absorption, respectively. In light detection using one-photon absorption, the output value is proportional to the power of the incident pulsed light in a linear function, and the output value reflects the average power of the pulsed light. On the other hand, in light detection using two-photon absorption, the output value is proportional to the power of the incident pulsed light in a quadratic function, and the output value reflects the peak power of the pulsed light. (See FIG. 5).

  In the present invention, since light detection is performed using two-photon absorption, it is necessary to make the second light detector enter pulsed light that causes two-photon absorption. For this reason, it is preferable that as much of the pulsed light that has been split into the two lights is incident on the second photodetector. For this purpose, the light is incident on the two photodetectors at a predetermined intensity ratio. Further, in the second photodetector using two-photon absorption, it is preferable to narrow the pulsed light with a condensing lens to increase the power density.

  In order to accurately monitor the state of the pulsed light, the difference between the output value (P2) by light detection using two-photon absorption and the output value (P1) by light detection using one-photon absorption is expressed as follows: It is preferable to make it as large as possible.

  Furthermore, in the pulse light monitoring apparatus and the pulse light monitoring method according to the present invention, it is also one of the features that a non-linear optical element or a Michelson interferometer is not used.

Here, the measurement principle in the pulse light monitoring apparatus and the pulse light monitoring method according to the present invention will be described.
In order to accurately monitor the state of the time width of the pulsed light, it is necessary to accurately detect the measurement values reflecting the average power and peak power in the pulsed light. Further, when the pulsed light is ultrashort pulsed light, it is not easy to monitor the state of the time width in real time as described above.

  Therefore, in the pulsed light monitoring apparatus and the pulsed light monitoring method according to the present invention, first, the measurement values reflecting the average power and peak power in the pulsed light are separately detected. For this purpose, the laser pulse light to be measured is input to a demultiplexer and is demultiplexed into two lights. Then, a measured value reflecting the average power is detected by the first photodetector using one-photon absorption in one of the divided pulse lights, and two-photon absorption is used in the other pulsed light. The measured value reflecting the peak power is detected by the second photodetector.

FIG. 1 is a schematic configuration diagram of a monitoring apparatus for pulsed light according to the present invention.
The peak power monitoring device 100 includes a branching filter 30, a first photodetector 10, a second photodetector 20, an arithmetic processor 40, and an output device 50.

  The duplexer 30 is configured such that the pulsed light 60 input from the input port 32 is demultiplexed into two lights. The two demultiplexed lights are output from the output ports 33 and 34, respectively. The duplexer 30 shown in FIG. 1 is configured by, for example, an optical fiber coupler. In the duplexer, the pulsed light may be demultiplexed into at least two lights and may be demultiplexed into three or more lights. However, since the amount of pulse light for monitoring is often limited, a duplexer that demultiplexes into two lights is preferable.

  The first photodetector 10 receives the pulsed light from the output port 34 and converts the energy of the received pulsed light into the first signal 11. In the example illustrated in FIG. 1, a lens 12 for condensing light and a filter 13 for cutting light in a predetermined wavelength region are provided on the incident side of the first photodetector 10.

  The second photodetector 20 receives the pulsed light from the output port 33 and converts the energy of the received pulsed light into the second signal 21. In the example shown in FIG. 1, a lens 22 for condensing light and a filter 23 for cutting light in a predetermined wavelength range are provided on the incident side of the second photodetector 20.

The signal processor 40 obtains a quotient (P2 / P1 ² ) obtained by dividing the second signal value P2 from the second photodetector by the square of the first signal value P1 from the first photodetector. . Further, an arithmetic processing indicating the state of the time width of the pulsed light is performed from the quotient, and the signal 41 subjected to the arithmetic processing is output to the output device 50 to monitor the state of the time width of the pulsed light.

From the quotient (P2 / P1 ² ), the simplest process for representing the state of the time width of the pulsed light is to display the quotient itself in time series and monitor the change in the time width of the pulsed light.

The signal arithmetic processor 40 further stores a relational expression (Y = aX + b) of the time width of the pulse light and the quotient (P2 / P1 ² ), which has been obtained in advance, as a process representing the state of the time width of the pulse light. It is preferable to have a processing function of converting the time width of the pulsed light from the value of the quotient (P2 / P1 ² ) using this relational expression and outputting it.

  In addition, the signal arithmetic processor preferably has a processing function for calibrating the relational expression using the time width of the pulsed light measured in advance. Specifically, the slope a and the constant term b of the relational expression (Y = aX + b) are calibrated. When the time width of the pulsed light is converted using the calibrated relational expression, the time width can be obtained more accurately. The signal calculation process will be described later in detail.

  The output device 50 is a display device 51 that displays the time width state of the pulsed light, or a signal output terminal (not shown) that outputs a processing signal 41 that represents the time width state of the pulsed light. The display 51 is a liquid crystal display panel or the like that displays the state of the time width of the pulsed light, specifically a numerical value of the time width and a change in the time width in a graph. For example, in the laser generator, the signal output terminal outputs a processing signal 41 representing the state of the time width of the pulsed light in order to optimize the generation conditions.

  Furthermore, in addition to the signal output terminal, the output device 50 may have a signal input terminal that receives a generation condition signal from the laser generator. With such a configuration, it is possible to display the time width of the pulsed light on the display 51 together with the laser generation conditions.

  Furthermore, in the pulsed light monitoring apparatus according to the present invention, monitoring may be performed in a state where the average power and the peak power are in a predetermined relationship in order to accurately detect the ratio of the average power and the peak power in the pulsed light.

  As described above, the second photodetector uses two-photon absorption to monitor the measurement value reflecting the peak power of the pulsed light, and the output value is a quadratic function with respect to the peak power of the pulsed light. Are proportional. In light detection using two-photon absorption, more efficient detection can be realized as the power density of the received pulsed light is higher.

Therefore, in the second photodetector, in order to increase the power density on the light receiving surface, the incident pulse light may be narrowed down to a range of several μm ² . For this purpose, a condensing lens is preferably provided. FIG. 2 shows the state of light collection by the second photodetector. The pulsed light from the demultiplexer is emitted from the end of the fiber that is the output port 33, and is converted into parallel light by the collimating lens 24. Then, the light is condensed on the light receiving surface of the second photodetector 20 by the condensing lens 22. The condensing lens preferably has a numerical aperture of 0.1 or more. The condensing lens may be a spherical lens, a combined spherical lens, an aspherical lens, or a GRIN lens.

  Further, in the second photodetector, in order to increase the power density of the light receiving surface, it is preferable that the incident pulsed light has a certain level of power. Specifically, depending on the photodetector used, for example, in the case of an avalanche photodiode, it is preferably 0.01 to 10 mW.

  In addition, the second photodetector may include a filter 23. This filter preferably has a characteristic of cutting light having a wavelength shorter than a predetermined wavelength so that only one photon of light is transmitted.

  As the second photodetector using the two-photon absorption, an avalanche photodiode (hereinafter also referred to as SiAPD) or a silicon photodiode can be employed. In SiAPD, since light detection is performed using a non-linear effect, the conversion speed from an optical signal to an electrical signal is relatively fast. Therefore, even for ultra-short pulse light, the change can be accurately monitored.

  The SiAPD is preferably made of a material that has no sensitivity to one-photon absorption and is highly sensitive to two-photon absorption. A specific example of the material is Si crystal. Since the SiAPD using Si crystal increases sensitivity by utilizing a phenomenon called avalanche multiplication, it can be suitably used in the peak power monitoring device according to the present invention. Further, the SiAPD may be appropriately biased.

  Further, a waveguide type semiconductor such as a laser diode (LD) that can oscillate at a wavelength shorter than the wavelength of the pulsed light to be measured can be employed as the second photodetector. This semiconductor is preferable because its band gap is wider than the energy level corresponding to the wavelength of the pulsed light to be measured, and is not sensitive to one-photon absorption and sensitive to two-photon absorption.

  FIG. 3 shows how light is collected when a waveguide type semiconductor 202 is used as the second photodetector. The other symbols are the same as those in FIG. In such a waveguide type semiconductor, incident light is concentrated in the waveguide due to its structure. The light that has entered the waveguide is preferable because it is propagated through total reflection repeatedly in the waveguide, is incident on the light receiving surface in a condensed state, and two-photon absorption is efficiently realized.

  Next, in the first photodetector, a measurement value reflecting the average power of the pulsed light is detected by using one-photon absorption, and the output value is proportional to the average power of the pulsed light in a linear function. is doing. Therefore, it is preferable that the pulsed light is incident on the light receiving surface of the photodetector without leakage. FIG. 4 shows the state of light collection by the first photodetector. The pulsed light from the demultiplexer is emitted from the end of the fiber that is the output port 34, and is condensed by the lens 12 so as to be incident on the entire light receiving surface of the first photodetector 10. In addition, before the 1st photodetector 10, it is good to provide the filter 13 which has the characteristic which cuts the light of a wavelength shorter than a predetermined wavelength as needed.

  The power of the incident pulsed light is preferably at least 1 μW, although it depends on the characteristics of the photodetector used. As the first photodetector, indium gallium arsenide / photodiode (InGaAsPD) can be used. Other examples include an infrared photomultiplier, a germanium photodiode, and a PbS photoconductive element.

  FIG. 5 is a graph conceptually showing the relationship between the input power of pulsed light and the output power of the photodetector. As described above, in the second photodetector using two-photon absorption, the output power is proportional to the square of the input power of the pulsed light. On the other hand, in the first photodetector using one-photon absorption, the output power is directly proportional to the input power of the pulsed light. In this figure, the noise level is ignored for simplicity.

  Since the output power of the second photodetector is proportional to the square of the power of the pulsed light, in order to accurately monitor the state of the time width of the pulsed light, the output power of the second photodetector is Try to make it as large as possible. For this purpose, it is preferable to make the first detector as large as possible in the range exceeding the noise level and as large as possible in the second photodetector. Thus, the difference between the output power of the second photodetector and the output power of the first detector is made as large as possible.

Therefore, specifically, the input intensity ratio of the pulsed light in the first photodetector and the two photodetectors is preferably in the range of 1: 9 to 1: 999.
For example, when the branching filter is configured with an optical fiber coupler, a variable loss device may be disposed in front of the first photodetector in order to obtain such an intensity ratio.
Further, when the branching filter is configured by a beam splitter, in order to obtain such an intensity ratio, a configuration in which a variable loss device is disposed in front of the first photodetector may be adopted. When the beam splitter is a half mirror type, the transmittance of the half mirror may be adjusted, or the beam splitter may be configured by a reflectivity variable partial reflection mirror.

  With such a configuration, in the pulse light monitoring apparatus and the pulse light monitoring method, the first photodetector and the second photodetector are in a state having appropriate sensitivity efficiency with respect to the input, respectively. be able to.

  In the above description, the power of the pulsed light incident on the first photodetector is limited by arranging a variable loss device in front of the first photodetector. By the way, when the repetition frequency of pulsed light is small, the power of the pulsed light incident on the second photodetector may be too large. In this case, a variable loss device may be arranged in front of the second photodetector.

  In general, there is a relationship in which the peak power of the pulsed light increases as the time width of the pulsed light becomes shorter. FIG. 6 shows the relationship between the time width and the peak power when the power of the incident pulse light is constant and the time width of the pulse light is changed by the variable dispersion device in the photodetector using two-photon absorption. It is a graph. From the graph, it can be seen that the peak power rapidly increases as the time width of the pulsed light becomes shorter.

  Similarly, in a photodetector using two-photon absorption, a graph of the relationship between the variable time of the time width of pulsed light and the peak power is shown in FIG. From the graph, it can be seen that the peak power increases rapidly when the time width of the pulsed light is around a specific time.

Specifically, in order to monitor the time width state of the pulsed light, the following may be performed.
First, the relationship between the input power of the incident pulsed light, the average power, and the peak power is examined in advance.

  The input power of the incident pulsed light can be known from the setting of the laser generator, the demultiplexing ratio in the demultiplexer, the loss ratio of the variable loss device, and the like. The repetition frequency of the pulsed light can be measured by observing the signal of the photodetector with an oscilloscope.

  The average power of the pulsed light can be obtained by a one-photon photodetector standardized by a power meter. The time width of the pulsed light can be accurately measured by an autocorrelator. The peak power of the pulsed light can be calculated by dividing the output of the one-photon photodetector by the product of the time width measured by the autocorrelator and the repetition frequency.

  Therefore, in a laser generator in which the repetition frequency, the average power, and the time width are known in advance, a relational expression between the input power and the peak power of the pulsed light is obtained while maintaining a state where the peak power is maximized. Note that the peak power of the pulsed light is maximized and the time width of the pulsed light is minimized.

  Here, if the time width of the pulsed light increases, the peak power decreases, but if the relation between these is known, the amount of decrease in the peak power can be accurately determined from the extent of the time width of the pulsed light. Can be estimated well.

  By dividing the average power (P1) of the pulsed light measured by the power meter by the repetition frequency measured by the oscilloscope, the energy per pulse is obtained. By dividing the energy per pulse by the time width measured by the autocorrelator, the peak power (P2) of the pulsed light can be obtained.

Then, a relational expression between the time width measured by the autocorrelator and the value of the quotient (P2 / P1 ² ) obtained by dividing the calculated peak power (P2) by the square of the average power (P1) is obtained. Note that the autocorrelator takes time, but the time width of the pulsed light can be accurately measured.

Here, when the quotient (P2 / P1 ² ) changes, the time width of the pulsed light changes while maintaining this relational expression. As a specific example, when the average power (P1) is 2 mW and the repetition frequency is 50 MHz, if one pulse energy is 40 pJ and the time width is 0.3 ps, the peak power (P2) is 0.13 kW. FIG. 8 is a double logarithmic graph with (P2 / P1 ² ) as the vertical axis and the time width of the pulsed light as the horizontal axis.

As is clear from FIG. 8, (P2 / P1 ² ) and the time width of the pulsed light show a linear relationship, and a linear function relational expression (Y = aX + b) is obtained. Here, when the quotient (P2 / P1 ² ) changes, the time width of the pulsed light changes on a straight line represented by the relational expression of this linear function.

Here, a method for obtaining the relational expression will be described. While appropriately changing the generation conditions with a certain laser generator, the pulse light monitoring device of the present invention measures ((P2 / P1 ² ) = X) and the time width (Y) of the pulsed light with an autocorrelator. Substituting the multiple sets of measurement values (X, Y) thus obtained into the relational expression (Y = aX + b), the slope a and the constant term b are obtained. As a result, the relational expression (Y = aX + b) can be determined.

When the relational expression thus obtained is solved for X, an expression (X = (Y−b) / a) for obtaining the time width is obtained. Specifically, the time width of the pulsed light can be calculated by substituting (P2 / P1 ² ) measured in the equation for obtaining this time width.
Thus, if the relational expression is calibrated using the time width of the pulsed light measured in advance, the time width of the pulsed light can be accurately calculated and monitored.

The signal calculation processing described above can be realized by software. This software may be programmed including the following steps.
(S1) The slope a and the constant term b of the relational expression (Y = aX + b) obtained in advance are stored as constants.
(S2) The A / D conversion is performed on the output signals from the second photodetector and the first detector, the digital signal is captured, and stored as a variable.
(S3) (P2 / P1 ² ) is calculated, and is substituted into an equation (X = (Y−b) / a) for determining the time width, thereby calculating the time width.
(S4) The calculated time width value is output to the output device.
Thereafter, (S2) to (S4) are sequentially repeated.

  In the above, an example in which signal calculation processing is executed by software has been described. However, the present invention is not limited to this, and the same signal calculation processing can be performed by hardware.

  In the above description, since the slope a and the constant term b of the relational expression (Y = aX + b) are obtained using numerical values accurately measured by the autocorrelator, the time width of the pulsed light is accurately monitored. be able to. However, if only the change in the time width of the pulsed light is seen, it is possible to monitor without doing this.

  In the “optical peak power detection device” disclosed in Japanese Patent Application Laid-Open No. 2010-093243, wavelength conversion is performed by allowing a nonlinear optical element to pass or reflect a laser beam. Then, a calculation is performed to divide the second signal, which is the second harmonic detection signal subjected to wavelength conversion, by the first signal, which is the detection signal of the fundamental beam. Furthermore, there is no description of using a photodetector using two-photon absorption as the photodetector.

In contrast, the pulsed light monitoring apparatus according to the present invention does not use a nonlinear optical element. An arithmetic process is performed to divide the second signal using two-photon absorption by the square of the first signal using one-photon absorption.
As described above, the laser light (pulse light) detected by the photodetector is different between the “light peak power detection device” disclosed in Japanese Patent Application Laid-Open No. 2010-093243 and the pulse light monitoring device according to the present invention. The arithmetic processing is also different.

  In the arithmetic processor of the pulsed light monitoring apparatus according to the present invention, the time of the pulsed light is obtained by dividing the output value (P2) of the second photodetector by the square of the output value (P1) of the first photodetector. The width status is monitored. As described above, in the pulse light monitoring apparatus and the pulse light monitoring method according to the present invention, the state of the time width of the pulse light can be monitored and mechanically moved only by performing the arithmetic processing of the electrical signal. No structure is required.

  For example, in a conventional autocorrelator, the time width of pulsed light requires at least a time on the order of seconds. However, when the pulsed light monitoring apparatus according to the present invention is used, monitoring can be performed in a time on the order of microseconds, and monitoring can be performed substantially in real time.

  1 is an example in which an optical fiber coupler is used as a branching filter. An example of a pulsed light monitoring device using a beam splitter as a duplexer is shown in FIG. In addition, the code | symbol attaches | subjects the same code | symbol to the component same as FIG.

  The monitored pulsed light 60 is incident on the collimator lens, collimated, and demultiplexed by the beam splitter 31. The demultiplexed pulsed light enters the first photodetector 10 and the second photodetector 20, respectively. The first signal 11 from the first photodetector and the second signal 21 from the second photodetector are respectively input to the signal arithmetic processor 40, and the signal 41 subjected to the arithmetic processing similar to the above is output as the output device. The time width of the pulsed light can be monitored.

  As described above, the pulse light monitoring apparatus and the pulse light monitoring method according to the present invention do not have a mechanically moving structure unlike the conventional autocorrelator, and only perform an arithmetic processing on an electrical signal. Thus, the time width state of the pulsed light can be monitored. Therefore, since the measurement time is as short as microsecond order, the time width state of the pulsed light can be monitored substantially in real time.

  As described above, the pulsed light monitoring apparatus and pulsed light monitoring method according to the present invention are configured without using a nonlinear optical element, and therefore do not require adjustment of the nonlinear optical element. Furthermore, since a Michelson interferometer or the like is not required, the configuration of the measurement system can be simplified.

The pulse light monitoring device described above is preferably combined with a laser generator.
When the pulse light monitoring device according to the present invention is combined with a laser generator, it is possible to monitor the time width of the pulsed light by changing the laser generation conditions in real time, so that the optimum laser generation conditions can be easily obtained. Can do.

  Further, when the output device of the pulse light monitoring device and the control unit of the laser generator are connected, the laser generation conditions and the time width of the pulse light can be associated and displayed. Therefore, it is preferable because optimum laser generation conditions can be obtained quickly.

  For example, in a mode-locked fiber laser generator, the wavelength and polarization direction of a short pulse laser in the fiber are constantly changing. When the present invention is applied to a laser generator including such an optical fiber, optimum laser generation conditions can be obtained quickly, which is particularly useful.

  The pulsed light monitoring device according to the present invention can be used as a time width feedback monitor in a laser generator. In addition, it is possible to monitor the time width of the pulsed light due to the time change accompanying the change in the dispersion of the laser device and the short-time pulse optical system in real time, and is ideal as a feedback monitor for the dispersion compensation device. .

100 Pulse light monitoring device 30 Demultiplexer (fiber optic coupler)
31 Beam splitter 32 Input port 33, 34 Output port 10 First photodetector (InGaAsPD)
11 First signal 12 Condensing lens 13 Filter 20 Second photodetector (SiAPD)
21 Second signal 22 Condensing lens 23 Filter 24 Collimator lens 202 Waveguide type semiconductor (LD)
40 arithmetic processing unit 41 processing signal 50 output unit 51 display unit 60 pulse light

Claims (13)

A demultiplexer that demultiplexes the laser pulse light to be measured into two lights;
A first photodetector and a second photodetector, each receiving two demultiplexed lights;
A signal processor,
An output device,
The first photodetector converts the energy of light received using one-photon absorption into a first signal, and the second photodetector detects the energy of light received using two-photon absorption as a second. Converted into a signal,
The signal arithmetic processing unit obtains a quotient obtained by dividing the second signal by the square of the first signal, and has an arithmetic function that represents a time width state of the pulsed light from the quotient.
A pulse light monitoring apparatus, wherein a signal representing a state of a time width of the pulse light is output to the output device and monitored. The pulsed light monitoring device according to claim 1,
The pulse light monitoring apparatus, wherein the time width state of the pulsed light is a change in the time width of the pulsed light. The pulsed light monitoring device according to claim 1,
The time width state of the pulsed light is the time width of the pulsed light,
The signal arithmetic processor is a pulsed light monitoring device that monitors the time width of the pulsed light based on the time width of the pulsed light and the quotient relational expression obtained in advance. The monitoring device for pulsed light according to claim 3,
The signal arithmetic processor is a pulsed light monitoring device that monitors the time width of the pulsed light according to the relational expression calibrated using the time width of the pulsed light measured in advance. The pulse light monitoring device according to any one of claims 1 to 4,
A pulsed light monitoring device in which a condensing lens is provided between the duplexer and the second photodetector. The pulse light monitoring device according to any one of claims 1 to 4,
A pulsed light monitoring device in which a condensing lens is provided between the duplexer and the first photodetector. The pulse light monitoring device according to any one of claims 1 to 4,
An apparatus for monitoring pulsed light, wherein an input intensity ratio of pulsed light in the first photodetector and the two photodetectors is in the range of 1: 9 to 1: 999. The pulse light monitoring device according to any one of claims 1 to 4,
A pulsed light monitoring device in which a variable loss device is provided between the duplexer and the first photodetector. The pulse light monitoring device according to any one of claims 1 to 4,
The first photodetector is indium gallium arsenide / photodiode,
The second photodetector is a pulsed light monitoring device which is an avalanche photodiode using silicon. In the peak power monitoring device according to any one of claims 1 to 4,
The second light detector is a peak power monitor device which is a laser diode capable of emitting light having a wavelength shorter than the wavelength of laser pulse light to be measured. The pulse light monitoring device according to any one of claims 1 to 4,
The duplexer is a pulsed light monitoring device which is an optical fiber coupler. The pulse light monitoring device according to any one of claims 1 to 4,
The branching filter is a pulsed light monitoring device which is a beam splitter. A step of splitting the laser pulse light to be measured into two lights;
In the first photodetector and the second photodetector that receive the two demultiplexed lights, respectively, the first photodetector converts the energy of the received light into a first signal using one-photon absorption. And the second photodetector converts the energy of light received using two-photon absorption into a second signal;
An arithmetic processing step for obtaining a quotient obtained by dividing the second signal by the square of the first signal, and representing a time width state of the pulsed light from the quotient;
An output step of outputting a signal representing a time width state of the pulsed light to the output unit and monitoring the signal, and monitoring the pulsed light.

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