JP6934748B2 - Laser device and frequency shift amount identification method - Google Patents

Description

The present invention relates to a laser apparatus capable of specifying a frequency shift amount of a modulated laser beam and a method for specifying a frequency shift amount.

Conventionally, a laser that stabilizes the oscillation frequency of laser light to a specific saturated absorption line by changing the resonator length based on the saturation absorption line contained in the light output signal obtained by irradiating the absorption cell with laser light. Devices are known (see, eg, Patent Document 1 and Non-Patent Document 1).

Japanese Unexamined Patent Publication No. 2012-134371

Keiichi Tanaka, "Frequency Stabilization of He-Ne Laser", Report of National Research Laboratory of Metrology, 1975, Vol. 24, No. 4 Reprint

As a method for improving the accuracy of the oscillation frequency of the laser beam, a method using an inverting ram dip of an iodine saturated absorption line is known. The inverted ram dip is a depression caused by absorption saturation by the pump light, which occurs on the transmission spectrum of the probe light when the pump light and the probe light are incident on a two-level system atom having a Doppler spread. By using the inverted ram dip as a reference for the oscillation frequency, a laser with high frequency stability can be realized.

The difference between the absorption coefficient of the inverted ram dip and the absorption coefficient near the inverted ram dip is very small and cannot be detected directly. Therefore, the position of the inverted ram dip is specified by frequency-modulating the laser and detecting the differential signal of the inverted ram dip by the lock-in amplifier. At this time, since the maximum frequency shift of the modulated laser must be about half the width of the inverted ram dip (several MHz), it is necessary to drive the mirror at one end of the laser resonator in a sine wave with an amplitude of sub-nanometer.

A piezo element is used to drive the sine wave of the resonator mirror, and the maximum frequency shift amount is determined based on the magnitude of physical displacement of the piezo element by the AC voltage applied to the piezo element. However, if the relationship between the voltage applied to the piezo element and the amount of displacement changes due to aging of the piezo element, the maximum frequency shift amount of the modulated laser light output by the laser device exceeds the permissible range. If the maximum frequency shift amount exceeds the permissible range, the characteristics of the modulated laser light deteriorate. Therefore, it is required to measure the relationship between the voltage applied to the piezo element and the maximum frequency shift amount with high accuracy.

In order to specify the relationship between the maximum frequency offset amount and the piezo applied voltage, the maximum frequency offset amount when a predetermined voltage is applied to the piezo element must be measured with high accuracy. As a method for measuring the maximum frequency shift amount, a heterodyne detection method using two modulated lasers as a reference laser and a heterodyne detection method using one unmodulated laser as a reference laser are known. However, when the heterodyne detection method using the reference laser is used, maintenance of the reference laser is required and heterodyne detection is required, so that there is a problem in terms of cost and required time.

Therefore, the present invention has been made in view of these points, and a laser light source adjusting device and a laser light source adjusting method capable of easily specifying the maximum frequency deviation amount of the modulated laser light output from the laser device are provided. The purpose is to provide.

The laser apparatus of the first aspect of the present invention includes a semiconductor laser that outputs excitation laser light, a laser medium crystal that outputs laser light when excited by the excitation laser light output by the semiconductor laser, and the laser. A reflector that reflects the laser light output by the medium crystal and forms a resonator with the laser medium crystal, a nonlinear optical element that outputs the second harmonic of the laser light, and filtering the second harmonic into a single mode. The optical element, the piezoelectric element that changes the position of the reflecting mirror based on the applied applied voltage, the voltage control unit that controls the applied voltage input to the piezoelectric element, and the position of the reflecting mirror change. A specific unit that specifies the maximum frequency shift amount of the modulated laser light based on the center frequency of the modulated laser light at the time when the inversion ram dip is detected in the modulated laser light in which the laser light is modulated. Has.

The specific unit may specify the maximum frequency shift amount of the modulated laser light in the modulated laser light by referring to the relationship between the predetermined maximum frequency shift amount specified in advance and the inversion ram dip.

The specific unit may specify the relationship between the applied voltage and the maximum frequency shift amount.
In addition, the specific unit may specify the maximum frequency shift amount based on a plurality of adjacent inverted ram dips in the modulated laser light.

The specific unit may specify the maximum frequency shift amount based on the maximum value in a state where the differential signals of the plurality of inverted ram dips are overlapped. The specific unit may detect the inverted ram dip by scanning the center frequency of the modulated laser beam by changing the applied voltage.

The laser device further has a storage unit that stores information indicating an allowable range of the maximum frequency shift amount in association with the applied voltage, and the specific unit has the specified maximum frequency shift amount. It may be determined whether or not the voltage control unit is included in the permissible range stored in the storage unit in relation to the applied voltage input to the piezoelectric element.

The specific unit determines that the specified maximum frequency shift amount is not included in the allowable range stored by the storage unit in association with the applied voltage input by the voltage control unit to the piezoelectric element. When this happens, the applied voltage that the voltage control unit inputs to the piezoelectric element may be changed.

In the method for specifying the amount of frequency shift according to the second aspect of the present invention, the step of controlling the applied voltage input to the piezoelectric element that changes the position of the reflector forming the resonator with the laser medium crystal and the position of the reflector are set. Based on the step of detecting the inverted ram dip in the modulated laser light in which the laser light output by the laser medium crystal is modulated by the change, and the center frequency of the modulated laser light at the time when the inverted ram dip is detected. The step includes a step of specifying the maximum frequency shift amount of the modulated laser light.

According to the present invention, there is an effect that the maximum frequency shift amount of the modulated laser light output from the laser apparatus can be easily specified.

It is a figure which shows the structure of the laser apparatus 100 which concerns on 1st Embodiment. It is a figure for demonstrating the relationship between the change of the maximum frequency shift amount and the spread of the differential signal of an inversion ram dip. It is a figure which shows the relationship between the change of the maximum frequency shift amount and the change of the 2nd derivative signal of two or more inversion ram dips.

[Structure of Laser Device 100]
FIG. 1 is a diagram showing a configuration of a laser device 100 according to the first embodiment. The laser device 100 is a 532 nm iodine stabilized laser. The laser device 100 is a solid-state laser in the 532 nm region of continuous wave oscillation using an Nd: YV04 crystal excited by a semiconductor laser as a gain medium, and is used as a standard length. The laser apparatus 100 can obtain high wavelength stability by controlling the oscillation wavelength at the center of the inverted ram dip of the saturated absorption line of the iodine molecule by using the spectroscopic technique of the absorption line of the iodine molecule.

The laser device 100 includes a semiconductor laser unit 1 for excitation, a laser resonator housing 2, an optical system for stabilizing iodine, and a controller 4.
The excitation semiconductor laser unit 1 has a semiconductor laser 11 that outputs an excitation laser beam, and the excitation laser beam in the 808 nm band emitted from the semiconductor laser 11 is incident on the laser resonator housing 2.

The laser cavity housing 2 includes an Nd: YV04 crystal 21, a KTP crystal 22, an etalon 23, a reflector 24, and a piezo element 25. The Nd: YV04 crystal 21 is a laser medium crystal that outputs a laser beam by being excited by the excitation laser beam emitted from the excitation semiconductor laser unit 1. Specifically, the excitation laser light emitted from the excitation semiconductor laser unit 1 is incident on the Nd: YV04 crystal 21, and the Nd: YV04 crystal 21 excites the multimode laser light having a wavelength of 1064 nm. The end face of the Nd: YV04 crystal 21 is coated so as to reflect light of 1064 nm, and the 1064 nm multimode laser light is amplified by the resonator composed of the Nd: YV04 crystal 21 and the reflector 24. The 1064 nm laser light output by the Nd: YV04 crystal 21 is converted into a multimode laser light having a wavelength of 532 nm, which is the second harmonic, in the KTP crystal 22, which is a nonlinear optical element that outputs the second harmonic of the laser light. NS.

The multimode laser light output by the KTP crystal 22 is incident on the reflecting mirror 24 after being converted to single mode by the wavelength filter Etalon 23. The etalon 23 is an optical element that filters second harmonics into a single mode. The reflecting mirror 24 reflects the laser light output by the Nd: YV04 crystal 21 and forms a resonator with the Nd: YV04 crystal 21. The reflecting mirror 24 reflects the incident laser light having a wavelength of 1064 nm and transmits the laser light having a wavelength of 532 nm and 808 nm.

The piezo element 25 is a piezoelectric element that displaces the position of the reflector 24 due to the distortion generated by the voltage applied from the controller 4. By applying a sinusoidal AC voltage to the piezo element 25, the distance between the Nd: YV04 crystal 21 and the reflector 24 is displaced in synchronization with the AC voltage cycle, so that the laser light is frequency-modulated. It is applied and the modulated laser light is output.

The iodine stabilization optical system 3 includes a beam splitter 31 and a stabilization signal detection unit 32. The beam splitter 31 separates the laser light input from the laser resonator housing 2 into the emitted laser light L1 and the wavelength control laser light L2. The wavelength control laser beam L2 is input to the stabilization signal detection unit 32.

The stabilization signal detection unit 32 has, for example, a lock-in amplifier, and detects a differential signal of an inverted ram dip based on the wavelength control laser beam L2 input from the beam splitter 31. The stabilization signal detection unit 32 inputs the detected differential signal of the inverted ram dip to the controller 4.

The controller 4 has a CPU (Central Processing Unit) and a storage unit 41. The storage unit 41 has a storage medium such as a ROM (Read Only Memory) and a RAM (Random Access Memory). The storage unit 41 stores a program executed by the CPU. Further, the storage unit 41 stores information indicating an allowable range of the maximum frequency shift amount in frequency modulation in relation to the voltage applied to the piezo element 25.

By executing the program stored in the storage unit 41, the CPU functions as a current control unit 42, a temperature control unit 43, a voltage control unit 44, and a frequency shift identification unit 45.
The current control unit 42 controls the laser current supplied to the semiconductor laser 11.

The temperature control unit 43 detects the values of the semiconductor laser 11, the laser resonator housing 2, the KTP crystal 22, the etalon 23, and the temperature sensor provided adjacent to the stabilization signal detection unit 32, which are the control targets, and each of them A temperature changing device such as a Pelche element installed in the fixed holder of the controlled object is driven to control each controlled object to reach the target temperature.

The voltage control unit 44 controls the voltage applied to the piezo element 25 to control the resonator length, which is the distance between the Nd: YV04 crystal 21 and the reflector 24, and stabilizes the laser wavelength. Specifically, the voltage control unit 44 detects a signal component based on the oscillation wavelength from the optical output signal output from the stabilization signal detection unit 32, and controls the amount of displacement of the piezo element 25 to which the reflector 24 is attached. As a result, the position of the reflector 24 is changed to control the resonator length. The voltage control unit 44 periodically changes the cavity length by periodically changing the magnitude of the voltage applied to the piezo element 25, and the laser light output from the semiconductor laser 11 is modulated. Is output from the laser cavity housing 2.

The frequency deviation specifying unit 45 specifies the maximum frequency deviation amount of the wavelength control laser light L2, which is the modulated laser light, based on the differential signal of the inverted ram dip output from the stabilizing signal detecting unit 32. The frequency shift specifying unit 45 changes the center frequency of the modulated laser light by changing the voltage of the voltage control unit 44, for example, and the modulated laser at the time when the inverted ram dip is detected in the wavelength control laser light L2. The maximum frequency shift amount of the wavelength control laser beam L2 is specified based on the central frequency of the light. The frequency shift identification unit 45 is the voltage applied to the piezo element 25 by the voltage control unit 44 and the maximum frequency shift in response to the activation of the test mode for specifying the maximum frequency shift amount of the modulated laser light. The relationship with quantity may be specified. Details of the method by which the frequency shift specifying unit 45 specifies the maximum frequency shift amount will be described later.

[Overview of how to identify the maximum frequency shift]
When a predetermined button (not shown) provided on the laser device 100 is operated, the test mode is activated. When the test mode is activated, the frequency shift identification unit 45 scans the center frequency of the modulated laser light by instructing the voltage control unit 44 to change the voltage level applied to the piezo element 25. At this time, if the frequency shift specifying unit 45 increases the maximum frequency shift amount of the modulated laser light, the spread of the differential signal of the inverted ram dip becomes large.

FIG. 2 is a diagram for explaining the relationship between the change in the maximum frequency shift amount and the spread of the differential signal of the inverted ram dip. The horizontal axis of FIG. 2 is the frequency, and the vertical axis indicates the presence or absence of the differential signal of the inverted ram dip. The thick broken line in FIG. 2 indicates the center frequency of the modulated laser beam. As shown in FIG. 2, when the maximum frequency shift amount becomes large, the modulated laser light starts from a time when the difference between the center frequency of the modulated laser light and the frequency of the inverted ram dip is larger than when the maximum frequency shift amount is small. Begins to overlap the inverted ram dip. As a result, the inverted ram dip is detected when the center frequency of the modulated laser beam is farther from the frequency of the inverted ram dip.

FIG. 2A is a diagram when the maximum frequency shift amount is relatively small, and FIG. 2B is a diagram when the maximum frequency shift amount is relatively large. When the frequency shift specifying unit 45 scans the center frequency of the modulated laser light by changing the voltage applied to the piezo element 25 by the voltage control unit 44, the modulated laser light is emitted at a position away from the center frequency of the modulated laser light. It begins to overlap with the inverted ram dip.

When the maximum frequency shift amount is large as shown in FIG. 2 (b), the center frequency when the frequency range of the modulated laser light starts to overlap with the inverted ram dip starts to overlap with the inverted ram dip in the case of FIG. 2 (a). Lower than the center frequency of time. Utilizing this fact, the frequency shift identification unit 45 can specify the maximum frequency shift amount based on the center frequency when the modulated laser light starts to overlap with the inverted ram dip. In other words, the frequency shift identification unit 45 can specify the maximum frequency shift amount based on the spread of the differential signal of the inverted ram dip when the center frequency of the modulated laser light is scanned.

However, since the spread of the differential signal of the inverted ram dip is small, it is difficult to specify the maximum frequency shift amount with high accuracy. Therefore, the frequency shift identification unit 45 utilizes the fact that when two or more inverted ram dips are close to each other, the differential signals are superposed between them, and the change due to the spread of each differential signal is remarkable. Then, by specifying the maximum frequency shift amount based on a plurality of adjacent inverted ram dips, the accuracy of specifying the maximum frequency shift amount is improved.

FIG. 3 is a diagram showing the relationship between the change in the modulation voltage corresponding to the maximum frequency shift amount and the change in the second derivative signal of two or more inverted ram dips. 3 (a), 3 (b), 3 (c) and 3 (d) show the iodine saturation absorption line R [56] 32 when the modulation voltage is 30 mV, 40 mV, 50 mV and 60 mV, respectively. The second derivative signals a11 to a14 of the inverted ram dip of −0 are shown. The horizontal axis of FIG. 3 shows the data count at the time of wavelength scanning, and corresponds to the applied DC voltage. The vertical axis of FIG. 3 shows the magnitude of the second derivative signal of the inverted ram dip.

According to FIG. 3, when the maximum frequency shift amount changes, the center frequency of the modulated laser light when the peak appears in the second derivative signal of the inverted ram dip changes. From this, it can be seen that the frequency shift specifying unit 45 can specify the maximum frequency shift amount based on the center frequency of the modulated laser light when the inverted ram dip appears.

Focusing on a11 and a12 in the figure, as the maximum frequency shift amount increases, the level of the minimum value between the peaks of a11 and a12 increases, and in FIGS. 3 (c) and 3 (d). Has a maximum value. The maximum value indicates that the maximum frequency shift amount is so large that both a11 and a12 are detected at the same time. Therefore, the frequency deviation specifying unit 45 may use this phenomenon to specify the maximum frequency deviation amount with high accuracy based on the maximum value in the state where the differential signals of a plurality of inverted ram dips are overlapped. ..
Hereinafter, two methods for specifying the maximum frequency shift amount using this phenomenon will be described.

[First identification method]
(Step 1)
First, the center frequency of the reference modulation laser whose relationship between the maximum frequency shift and the piezo applied voltage is identified is scanned, and the differential signal level and the maximum between adjacent inverted ram dips (for example, a11 and a12 in FIG. 3) are scanned. Identify the relationship with frequency shift. The identified relationship is stored, for example, in the storage unit 41 of the controller 4.

(Step 2)
When the test mode for executing the operation for specifying the relationship between the magnitude of the voltage applied to the piezo element 25 and the amount of frequency shift is activated in the laser device 100, the voltage control unit 44 moves the center of the modulated laser light. Scan the frequency to determine the differential signal level between adjacent inverted ram dips.

(Step 3)
The voltage control unit 44 refers to the relationship between the differential signal level of the reference modulation laser stored in the storage unit 41 and the maximum frequency shift amount in step 1, and the maximum frequency deviation corresponding to the differential signal level specified in step 2. Identify the transfer. In this way, the voltage control unit 44 can specify the maximum frequency shift amount with high accuracy.

[Second identification method]
(Step 1)
First, the center frequency of the reference modulation laser in which the relationship between the maximum frequency shift amount and the piezo applied voltage is specified is scanned, and based on the differential signal obtained when the maximum frequency shift amount is set to a predetermined value, Identify the frequency of the inverted ram dip.

(Step 2)
Identify the relationship between the frequency-by-frequency absorption coefficient of the iodine saturated absorption line (known) and the differential signal level between the adjacent inverted ram dips of each maximum frequency shift. The identified relationship is stored, for example, in the storage unit 41 of the controller 4.

(Step 3)
When the test mode for executing the operation for specifying the relationship between the magnitude of the voltage applied to the piezo element 25 and the amount of frequency shift is activated in the laser device 100, the voltage control unit 44 moves the center of the modulated laser light. Scan the frequency to determine the differential signal level between adjacent inverted ram dips.

(Step 4)
The voltage control unit 44 refers to the relationship between the absorption coefficient (known) for each maximum frequency shift amount of the reference modulation laser stored in the memory in step 2 and the differential signal level between the adjacent inversion ram dips, and steps. It is specified which maximum frequency deviation amount corresponds to the differential signal level of the inverted ram dip specified based on the differential signal level specified in 3.

[Failure detection process in laser device 100]
The frequency shift specifying unit 45 can specify the maximum frequency shift amount in a state where a predetermined voltage is applied to the piezo element 25 by the above method. In the laser apparatus 100, the range of the maximum frequency shift amount allowed when a predetermined voltage is applied to the piezo element 25 at the time of shipment from the factory is stored in the storage unit 41. When the test mode is activated, the frequency shift identification unit 45 stores the maximum frequency shift amount specified by the storage unit 41 in association with the applied voltage input by the voltage control unit 44 to the piezo element 25. It is determined whether or not the frequency shift amount is within the permissible range.

When the specified maximum frequency offset amount is not within the allowable range of the maximum frequency offset amount stored in the storage unit 41, the frequency shift identification unit 45 is the optimum maximum stored in the storage unit 41. Calculate the difference between the frequency offset amount and the current maximum frequency offset amount. Then, the frequency shift identification unit 45 changes the voltage applied to the piezo element 25 by the voltage control unit 44 so as to cancel the calculated difference. By doing so, the laser apparatus 100 can output the modulated laser light having a desired characteristic by canceling the influence of the aging change even when the characteristic of the piezo element 25 changes over time.

Further, in the frequency shift specifying unit 45, the characteristics of the piezo element 25 changed when the specified maximum frequency shift amount was not within the allowable range of the maximum frequency shift amount stored in the storage unit 41. Warning information indicating that may be output. By doing so, the user of the laser apparatus 100 can grasp that the characteristics of the piezo element 25 have changed, so that it becomes easy to take necessary measures such as replacement of the piezo element 25.

Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various changes or improvements can be made to the above embodiments. It is clear from the description of the claims that such modified or improved forms may also be included in the technical scope of the present invention.

1 Excitation semiconductor laser unit 2 Laser resonator housing 3 Electroelement stabilization optical system 4 Controller 11 Semiconductor laser 21 Nd: YV04 Crystal 22 KTP crystal 23 Etalon 24 Reflector 25 Piezo element 31 Beam splitter 32 Stabilization signal detection unit 41 Storage unit 42 Current control unit 43 Temperature control unit 44 Voltage control unit 45 Frequency shift identification unit 100 Laser device

Claims (9)

A semiconductor laser that outputs excitation laser light and
A laser medium crystal that outputs a laser beam by being excited by the excitation laser beam output by the semiconductor laser, and
A reflector that reflects the laser light output by the laser medium crystal and forms a resonator with the laser medium crystal.
A non-linear optical element that outputs the second harmonic of the laser beam,
An optical element that filters the second harmonic to a single mode,
A piezoelectric element that changes the position of the reflector based on the input applied voltage,
A voltage control unit that controls the applied voltage input to the piezoelectric element, and
The maximum frequency shift of the modulated laser light is based on the center frequency of the modulated laser light at the time when the inverted ram dip is detected in the modulated laser light in which the laser light is modulated by changing the position of the reflector. A specific part that specifies the amount and
A storage unit that stores an allowable range of the maximum frequency shift amount of the modulated laser beam in association with the voltage applied to the piezoelectric element.
Have a,
The voltage control unit changes the applied voltage so as to reduce the difference between the maximum frequency shift amount specified by the specific unit and the maximum frequency shift amount corresponding to the allowable range stored in the storage unit. Laser device. The specific unit specifies the maximum frequency shift amount of the modulated laser light in the modulated laser light by referring to the relationship between the predetermined maximum frequency shift amount specified in advance and the inversion ram dip.
The laser device according to claim 1. The specific unit specifies the relationship between the applied voltage and the maximum frequency shift amount.
The laser device according to claim 1. The identification unit identifies the maximum frequency shift amount based on a plurality of adjacent inverted ram dips in the modulated laser beam.
The laser device according to any one of claims 1 to 3. The specific unit specifies the maximum frequency shift amount based on the maximum value in a state where the differential signals of the plurality of inverted ram dips are overlapped.
The laser device according to claim 4. The specific unit detects the inverted ram dip by scanning the center frequency of the modulated laser beam by changing the applied voltage.
The laser apparatus according to any one of claims 1 to 5. Whether or not the specified maximum frequency shift amount is included in the allowable range stored in the storage unit in association with the applied voltage input by the voltage control unit to the piezoelectric element. To judge,
The laser apparatus according to any one of claims 1 to 6. It is determined that the maximum frequency shift amount specified by the specific unit is not included in the allowable range stored by the storage unit in association with the applied voltage input by the voltage control unit to the piezoelectric element. When this is done, the voltage control unit changes the applied voltage input to the piezoelectric element.
The laser apparatus according to claim 7. A step of controlling the applied voltage input to the piezoelectric element that changes the position of the reflecting mirror that forms a resonator with the laser medium crystal, and
A step of detecting an inverted ram dip in a modulated laser beam in which the laser beam output by the laser medium crystal is modulated by changing the position of the reflecting mirror.
A step of specifying the maximum frequency shift amount of the modulated laser light based on the center frequency of the modulated laser light at the time when the inverted ram dip is detected.
The maximum frequency deviation corresponding to the specified maximum frequency shift amount and the allowable range stored in the storage unit that stores the allowable range of the maximum frequency shift amount of the modulated laser light in relation to the voltage applied to the piezoelectric element. The step of changing the applied voltage so as to reduce the difference between the transfer amount and
A method for identifying a frequency shift amount having.

Images