KR101540541B1 - Apparatus for generating femtosecond laser and apparatus for measuring sprayed coating thickness thereof - Google Patents

Abstract

The present invention is an apparatus for measuring the thickness of a coating film using a terahertz wave having a high transmittance with respect to a coating film. The present invention dramatically reduces the driving range of a driving apparatus required for obtaining a sufficient scanning range, To a femtosecond laser generator and a coating film thickness measuring device in cooperation with the femtosecond laser generator.

Description

TECHNICAL FIELD [0001] The present invention relates to a femtosecond laser generating apparatus and a coating film thickness measuring apparatus that cooperates with the femtosecond laser generating apparatus,

The present invention relates to an apparatus for measuring the thickness of a coating film using a terahertz wave having a high transmittance with respect to a coating film, and more particularly, to an apparatus for measuring the thickness of a coating film by dramatically reducing the driving range of a driving apparatus required for obtaining a sufficient scanning range, To a femtosecond laser generator capable of terahertz wave scanning and a coating film thickness measuring apparatus in cooperation with the femtosecond laser generator.

2. Description of the Related Art In recent years, various technologies using terahertz wave have been proposed by advances in quantum electronics and the semiconductor industry. The terahertz wave is an electromagnetic wave having a frequency of about 0.01 to 100 kW (wavelength of 3 to 30 mm). As an example of such a terahertz wave application, measurement of characteristics of various materials (measurement target) in the terahertz region is attempted.

Generally, since the characteristic of the measurement object is a characteristic value in the frequency domain, for example, a transmittance spectrum, the frequency spectrum of the terahertz wave to be irradiated to the measurement object is preferably as wide as possible. It is also desirable that the frequency spectrum of the terahertz wave can be appropriately changed depending on the measurement object.

In the technique of oscillating and detecting terahertz waves, a femtosecond laser is divided into an excitation beam and a probe beam by using a beam splitter, and then the excitation beam and the probe beam are incident on the generation and detection elements, respectively, By changing the beam path of the beam, it is possible to obtain a terahertz wave. In this case, a scanning range of several mm or more is required in consideration of the distance, refractive index, and incident angle of the THz waves. The most simple way to implement a light path changing device to obtain a long scanning range is to install a retroreflector on a stage equipped with a motor. A method of installing two mirrors or a single retroreflector on a motor stage, in which the displacement of the optical delay line is exactly twice the travel distance of the motor. In the case of a stage equipped with such a motor, it takes a few tens of seconds to several minutes.

 One of the methods for obtaining a long optical path length at a high speed is to make the difference in the optical path between the excitation beam and the probe beam very large to several tens of meters or more so that different pulse trains meet and then change the repetition rate of the femtosecond laser at a high speed to be. In this case, since the position of the two pulses shifts on the time axis sensitive to the change in the repetition rate, it is a method capable of obtaining a very long scanning range from the change of the resonator length of several tens of 탆 to the order of several tens of millimeters.

If a PZT element (piezoelectric element) is used as a resonator length adjusting device, it has a sampling rate of several hundreds Hz. However, since this technique has a limited path difference between the two beams that can be designed, a sufficient scanning range can be secured only by using a laser having a high repetition rate of several hundred MHz. That is, there is a problem that a sufficient scanning range can not be secured for using a femtosecond laser.

Korean Patent Registration No. 10-0987386 (registered on October 6, 2010) Korean Patent Publication No. 10-2009-0098458 (published on September 17, 2009)

It is an object of the present invention to provide a femtosecond laser capable of achieving miniaturization and high-speed terahertz wave scanning by drastically reducing the driving range of a driving unit required to obtain a sufficient scanning range. And to provide a coating film thickness measuring device in cooperation with the apparatus.

Another object of the present invention is to provide a femtosecond laser generating apparatus in which two reflectors and a resonator are combined in an obliquely aligned manner and a terahertz system composed of an excitation beam and an optical fiber for making the difference in optical path between the probe beam and the probe beam to be several tens of meters or more, The present invention also provides a femtosecond laser generator capable of measuring the thickness of a coating film through a plurality of femtosecond laser generators.

According to an aspect of the present invention, there is provided a femtosecond laser generating apparatus including a pump light source for emitting pump light, a wavelength division multiplexer for multiplexing and outputting pump light received from the pump light source, A gain medium optical fiber which is excited by the pump light output from the wavelength division multiplexer to generate and output light of a predetermined wavelength band; and a gain medium optical fiber which outputs light input from the gain medium optical fiber to an output stage, A first resonator including an isolator for blocking light traveling in a reverse direction to the first resonator,

A beam splitter for splitting the light reflected from the single mode optical fiber (SMF) connected to the wavelength division multiplexer and reflecting the returned light; a first optical reflector for reflecting the incident light; a second optical reflector disposed to incline at an incident angle? and incident on the beam splitter at an incident angle? of a predetermined magnitude, a moving device for moving the first optical reflector in a straight line, A second resonator unit including a piezoelectric transducer (PZT) attached to the reflector, and an optical coupler coupling the light input from the first resonator unit and the second resonator unit to output a femtosecond pulse.

The femtosecond laser generating apparatus according to another aspect of the present invention is a femtosecond laser generating apparatus for converting a pump light input from a single mode optical fiber (SMF) connected to a wavelength division multiplexer into a parallel light, (SMF), a first optical reflector for reflecting incident light, and a second optical reflector provided so as to incline at a predetermined angle (?) From the first optical reflector and incident from the collimator at an incident angle? A second resonator unit including a piezoelectric transducer (PZT) attached to the first optical reflector, a second optical reflector for reflecting the light, a second optical reflector for linearly moving the first optical reflector, And an optical coupler coupling the light input from the first resonator and the second resonator to output a femtosecond pulse.

A film thickness measuring apparatus used for measuring a coating film thickness in cooperation with a femtosecond laser generating apparatus according to another aspect of the present invention includes a first single mode optical fiber (SMF) connected to an optical coupler, With a beam splitter to separate. A terahertz wave generator for receiving an excitation beam from a second single mode optical fiber (SMF) connected to the beam splitter and generating a terahertz wave, and a probe beam from a third single mode optical fiber (SMF) connected to the beam splitter And a terahertz wave detecting unit for detecting a reflected terahertz beam after the terahertz beam generated from the terahertz wave generating unit is incident on the object to be measured. The length of the third single mode optical fiber (SMF) 2 single mode optical fiber (SMF) with a length of 10 m or more and 99 m or less.

According to the present invention, the following effects can be obtained.

First, the present invention dramatically reduces the driving range of a driver required to obtain a sufficient scanning range, thereby enabling downsizing and high-speed terahertz wave scanning.

Second, the present invention relates to a femtosecond laser generating device in which two reflectors and a resonator are combined at an angle, and a terahertz system composed of an excitation beam and an optical fiber for allowing a probe beam to have a difference in optical path of at least several tens of meters, The thickness of the coating film can be measured.

FIG. 1 is an overall configuration diagram of a femtosecond laser generating apparatus 100 according to a first embodiment of the present invention,
FIG. 2 is an overall configuration diagram of a femtosecond laser generating apparatus 100 according to a second embodiment of the present invention,
3 is an exemplary diagram for explaining a process of amplifying an optical path in a second resonator according to the present invention,
FIG. 4 is an overall configuration diagram of an apparatus 400 for measuring a film thickness according to the present invention,
FIG. 5 is a result of acquiring a scanning range using the femtosecond laser generating apparatus 100 according to the present invention and a coating film thickness measuring apparatus 400 cooperating therewith.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout.

1, the femtosecond laser generating apparatus 100 according to the first embodiment of the present invention includes a pump light source 105, a wavelength division multiplexer 107, a first resonator 110, 2 resonator unit 120, an optical coupler 131, and a mode lock unit 132. [ In Fig. 1, two reflectors 125 and 127, which are arranged at an angle in the second resonator portion 120, amplify the optical path length.

The pump light source 105 oscillates the pump light, and may be, for example, a semiconductor array diode laser. The pump light emitted from the pump light source 110 may have a wavelength of, for example, 980 nm. The wavelength division multiplexer 107 multiplexes the pump light received from the pump light source 105 and outputs the multiplexed light.

The first resonator unit 110 includes a gain medium optical fiber 113 that excites the pump light output from the wavelength division multiplexer 107 to generate and output light of a predetermined wavelength band and a gain medium optical fiber 113 that is excited by the gain medium optical fiber 113 And an isolator 115 for outputting the input light to the output terminal and blocking the light traveling in the reverse direction from the output terminal to the gain medium optical fiber 113. The gain medium optical fiber 113 stimulates light within a predetermined wavelength range determined according to characteristics of a gain medium and amplifies the emitted light.

For example, the gain medium optical fiber 113 absorbs pump light having a wavelength of 980 nm or 1480 nm and transmits an erbium doped fiber or a pump light having a wavelength of 980 nm, which forms a gain of a bandwidth of several ten nanometers at a center wavelength of 1550 nm And Ytterbium doped fiber which absorbs 1030 nm and forms a gain of a bandwidth of several tens of nm with a central wavelength.

The second resonator unit 120 includes a beam splitter 123 for passing the light input from the single mode optical fiber (SMF) 121 connected to the wavelength division multiplexer 107 and dividing the reflected and reflected light, a beam splitter And an optical reflector 127 provided so as to incline at a predetermined angle? From the optical reflector 125 and the optical reflector 127 are arranged in a straight line To a mobile device (129). The light reflectors 125 and 127 may be implemented as a plane mirror, for example.

For example, the light reflector 127 may be configured to have a vibrating element attached thereto so that the traveling speed of the light can be moved at a very high speed. The vibration element may be, for example, a piezoelectric transducer (PZT). A cylindrical piezoelectric transducer (PZT) is a device that exhibits a piezoelectric phenomenon. When a pressure is applied in a certain direction, positive and negative charges appear on both sides of the crystal plate in proportion to the external force.

As shown in Fig. 1, a moving device 129 for moving the optical reflector 127 in a straight line may be provided. The moving device 129 can change the length of the optical path by moving the optical reflector 127. [ The mobile device 129 may be embodied as a motor, for example.

The optical coupler 131 couples light input from the first resonator unit 110 and the second resonator unit 120 and outputs a femtosecond pulse. The optical coupler 131 may be implemented as an optical coupler or a wavelength division multiplexer (WDM). The femtosecond pulse output from the optical coupler 131 is input to the first single mode optical fiber (SMF) 405 of the film thickness measuring apparatus 400 shown in FIG.

The mode locking device 132 is a device for outputting a femtosecond pulse and includes a wave plate and a polarizing plate. The mode locking device 132 receives a femtosecond laser output from the beam splitter 123 of the second resonator part 120, An automatic polarization controller for controlling the polarization of the mode locking by adjusting a rotation angle of the femtosecond laser, and a mode lock detecting unit for detecting a spectrum difference before and after mode locking of the femtosecond laser output from the beam splitter 123 of the second resonator unit 120, And a controller for controlling the automatic polarization controller and the mode lock detector by a program to lock the mode lock signal when the mode lock signal is detected.

The mode lock sensing unit may include an optical filter for filtering a portion of the optical beat corresponding to a certain wavelength band of the light output from the beam splitter 123 of the second resonator unit 120, An optical filter for detecting an RF signal corresponding to an arbitrary repetition rate of the light filtered through the optical filter, an electronic filter for filtering only the first repetition rate in the light filtered through the optical filter, An A / D converter for converting a corresponding analog signal into a digital signal; and a frequency divider for converting a frequency to be detected by the A / D converter to a frequency lower than that of the analog signal to process the digital signal.

2, the femtosecond laser generating apparatus 100 according to the second embodiment of the present invention mainly includes a pump light source 105, a wavelength division multiplexer 107, a first resonator 110, 2 resonator unit 120, an optical coupler 131, and a mode lock unit 132. [ In Fig. 2, the same components as those in Fig. 1 are denoted by the same reference numerals.

2, the second resonator 120 converts the pump light input from the single mode optical fiber (SMF) 121 connected to the wavelength division multiplexer 107 into parallel light and outputs the reflected parallel light And includes an optical circulator 122 that is disposed between the collimator 124 and the wavelength division multiplexer 107. The optical circulator 122 is a single mode optical fiber (SMF)

The collimator 124 collectively refers to a photometer or a collimator, and is an optical system that converts light emitted from a minute light source into parallel rays or collects parallel rays that have propagated. Since the output of the laser has a diameter larger than the diameter of the optical fiber, the light is collected on the end surface of the optical fiber by using a thinner lens which passes through the lens and ignores the movement of the light beam. It can be made into a light beam.

The optical circulator 122 is an irreversible element that changes the position of the output port along the traveling direction of the light. The light passing through the irreversible crystal experiences a different path difference depending on the traveling direction. Using this phenomenon, And output to another port. In order to eliminate the polarization dependence, the two polarized light beams are basically divided to give another path and recombine. The optical circulator 122 may be implemented, for example, by including a beam displacer, a rotator, and a collimator.

3 is an exemplary diagram for explaining a process of amplifying an optical path in the second resonator unit 120 according to the present invention. The first optical reflector 125 and the second optical reflector 127 of the second resonator unit 120 are positioned at the angle θ and the light is incident on the first optical reflector 125 at an incident angle α ₁ . At this time, the light reflected by the first light reflector 125 travels toward the opposite second light reflector 127 and re-enters. The angle of re-incident is? _{1 -} ? By the law of reflection. The light reflected by the second light reflector 127 travels toward the opposite first light reflector 125 at an incident angle of? ₁ -2 ?. This repetitive reflection continues until the angle incident on the first light reflector 125 or the second light reflector 127 is less than zero and the incident angle of the N (N is an integer) Satisfies.

In the above equation, when α ₁ satisfies θ × N, α _N becomes 0, which means that the light is incident perpendicular to the light reflector, so that light is reflected back to the incident path again. The path of this light is kept constant irrespective of the distance between the two light reflectors if only the angle &thetas; of the two light reflectors and the angle &thetas; ₁ incident at the _first are fixed. When the distance between the two light reflectors is changed, Is multiplied by the number of times of retroreflection, a long optical path length change can be obtained even at a small displacement of the reflector. The number of multiple reflections increases as the angle of incidence first increases and as the angle between the two light reflectors decreases.

As shown in FIG. 3, when the first optical reflector 125 is fixed and the position of the second optical reflector 127 is moved away by d, the displacement amplification factor L is expressed by the following equation.

From the above equation, the displacement amplification factor L can be calculated as a function of the angle between the moving distance d of the light reflector, the light incident angle alpha, and the angle between the two light reflectors, Since the amplification factor L is in a linear relationship, the calibration can easily and accurately predict the length.

As shown in FIG. 4, the film thickness measuring apparatus 400 used for measuring the film thickness in cooperation with the femtosecond laser generating apparatus according to the present invention includes a beam splitter 410, a terahertz wave generating unit 420, And a terahertz wave detecting unit 430, as shown in FIG.

The beam splitter 410 splits the beam input from the first single mode optical fiber (SMF) 405 connected to the optical coupler 131 of the femtosecond laser generator 100 into an excitation beam and a probe beam. The terahertz wave generating unit 420 receives the excitation beam from the second single mode optical fiber (SMF) 421 connected to the beam splitter 410, and generates a terahertz wave.

The terahertz wave detector 430 receives the probe beam from the third single mode optical fiber (SMF) 431 connected to the beam splitter 410 and drives the terahertz wave detector 430. The probe beam serves as a kind of switching for allowing a current to flow through the detecting element. The terahertz wave detecting unit 430 detects the reflected terahertz beam after the terahertz beam generated from the terahertz wave generating unit 420 is incident on the object P to be measured. Here, the length of the third single mode optical fiber (SMF) 431 is longer than the length of the second single mode optical fiber (SMF) 421 by 10 m or more and 99 m or less.

The apparatus 400 for measuring a coating thickness according to the present invention includes a dispersion compensating optical fiber (DCF), a beam splitter 410 and a compensation optical fiber 420 for compensating pulse width spreading between a beam splitter 410 and a second single mode optical fiber (SMF) 421 And a dispersion compensating optical fiber (DCF) for compensating the pulse width spreading may be installed between the third single mode optical fibers (SMF) 431.

The scanning range amplification factor K in the film thickness measuring apparatus 400 according to the present invention is expressed by the following equation.

Here, n : refractive index of the fiber,

l _d : Length difference between excitation beam and probe beam

f _rep : laser repetition rate,

c _o It is the speed of light.

Accordingly, the optical path length displacement is firstly amplified using multiple reflectors and a piezoelectric transducer (PZT) provided in the second resonator of the femtosecond laser generator of the present invention, which are arranged at an angle, It is possible to obtain an amplification ratio of several hundred times or more even at a low repetition rate by secondarily amplifying the excitation beam and the probe beam through the long optical path difference in the film thickness measuring apparatus of the invention.

FIG. 5 is a result of acquiring a scanning range using the femtosecond laser generating apparatus 100 according to the present invention and a coating film thickness measuring apparatus 400 cooperating therewith.

The displacement of the piezoelectric transducer (PZT) attached to the optical reflector was 45 μm when the voltage of 10 V was applied, and the displacement amplification factor L in the second resonance portion was 14.06. At the femtosecond laser repetition rate ( f _rep ) of 60 MHz, the scanning range amplification factor (K) was 20. Thus, the final displacement amplification factor was 281 times, and the final scanning range obtained a scanning range of 12.645 mm (42.15 ps), which is 281 times the displacement of the piezoelectric transducer (PZT) of 0.045 mm.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Will be clear to those who have knowledge of.

100: femtosecond laser generator 105: pump light source
107: wavelength division multiplexer 110: first resonator unit
120: second resonance part
121: Single mode optical fiber (SMF) 122: Optical circulator
123: beam splitter 124: collimator
125: first light reflector 127: second light reflector
129: Mobile device
131: optical coupler 132: mode locking device
400: Film thickness measuring device
405: first single mode optical fiber (SMF)
410: beam splitter 420: terahertz wave generator
430: terahertz wave detection unit
421: second single mode optical fiber (SMF)
431: third single mode optical fiber (SMF)

Claims (3)

A pump light source for emitting pump light; A wavelength division multiplexer for multiplexing and outputting pump light received from the pump light source;
A gain medium optical fiber which is excited by the pump light output from the wavelength division multiplexer to generate and output light of a predetermined wavelength band; and a gain medium optical fiber which outputs light input from the gain medium optical fiber to an output stage, A first resonator including an isolator for blocking light traveling in a reverse direction to the first resonator;
A beam splitter for splitting the light reflected from the single mode optical fiber (SMF) connected to the wavelength division multiplexer and reflecting the returned light; a first optical reflector for reflecting the incident light; a second optical reflector disposed to incline at an incident angle? and incident on the beam splitter at an incident angle? of a predetermined magnitude, a moving device for moving the first optical reflector in a straight line, A second resonator including a piezoelectric transducer (PZT) attached to the reflector; And
A photocoupler for combining light input from the first resonator and the second resonator to output a femtosecond pulse;
And a femtosecond laser generator for generating a femtosecond laser beam. A pump light source for emitting pump light; A wavelength division multiplexer for multiplexing and outputting pump light received from the pump light source;
A gain medium optical fiber which is excited by the pump light output from the wavelength division multiplexer to generate and output light of a predetermined wavelength band; and a gain medium optical fiber which outputs light input from the gain medium optical fiber to an output stage, A first resonator including an isolator for blocking light traveling in a reverse direction to the first resonator;
A collimator for converting the pump light input from the single mode optical fiber (SMF) connected to the wavelength division multiplexer into parallel light and converging the reflected parallel light to the single mode optical fiber (SMF) A first optical reflector that reflects incident light; and a second optical reflector that is provided to incline at a predetermined angle? With respect to the first optical reflector and is incident at an incident angle? Of a predetermined magnitude from the collimator, A second resonator including a piezoelectric transducer (PZT) attached to the first optical reflector, a second optical reflector for reflecting the light, a moving device for moving the first optical reflector in a straight line, and a piezoelectric transducer (PZT) And
A photocoupler for combining light input from the first resonator and the second resonator to output a femtosecond pulse;
And a femtosecond laser generator for generating a femtosecond laser beam. A film thickness measuring apparatus used for measuring film thickness in cooperation with the femtosecond laser generating apparatus according to any one of claims 1 to 2,
A beam splitter for splitting a beam input from a first single mode optical fiber (SMF) connected to the optical coupler into an excitation beam and a probe beam;
A terahertz wave generator for receiving an excitation beam from a second single mode optical fiber (SMF) connected to the beam splitter and generating a terahertz wave; and a third single mode optical fiber (SMF) connected to the beam splitter for inputting a probe beam And a terahertz wave detector for detecting the reflected terahertz beam after the terahertz beam generated from the terahertz wave generator is incident on the object to be measured,
Wherein the length of the third single mode optical fiber (SMF) is longer than the length of the second single mode optical fiber (SMF) by 10 m or more and 99 m or less.

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