Pulse width stretcher and chirped pulse amplifier including the same

Provided are a pulse width stretcher and a chirped pulse amplifier including the same. The pulse width stretcher includes first and second multiple reflection mirrors, and a pulse group-delay dispersion block disposed between the first and second multiple reflection mirrors and configured to refract a pulse laser beam to stretch a pulse width of the pulse laser beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application Nos. 10-2014-0166594, filed on Nov. 26, 2014, and 10-2015-0153791, filed on Nov. 3, 2015, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure herein relates to an optical amplifier, and more particularly, to a pulse width stretcher that stretches the pulse width of a pulse laser beam, and a chirped pulse amplifier including the same.

In the mid-1980s, there was an important advance of optical technology that overcomes the output power limit of a pulse laser beam according to the damage threshold of a gain medium. That is a chirped pulse amplification technique. The chirped pulse amplification technique may provide a pulse laser beam having output power significantly higher than a general pulse amplification technique. A conventional chirped pulse amplifier may generate a pulse laser beam having maximum output power lower than or equal to about 1 GW. The chirped pulse amplifier may control the pulse width of a pulse laser beam to minimize the effect of the damage threshold of an optical medium. Thus, the chirped pulse amplifier may generate a pulse laser beam having maximum output power equal to higher than about 1 TW.

SUMMARY

The present disclosure provides a pulse width stretcher that has multiple paths of a pulse laser beam.

The present disclosure also provides a pulse width stretcher that may maximize spatial efficiency, and a chirped pulse amplifier including the same.

An embodiment of the inventive concept provides pulse width stretchers including a first multiple reflection mirror comprising a first large area mirror and a first small area mirror in the first large area mirror, the first large area mirror reflecting a pulse laser beam; a second multiple reflection mirror comprising a second large area mirror and a second small area mirror in the second large area mirror, the second large area mirror disposed to face the first large area mirror; and a pulse group-delay dispersion block disposed between the first multiple reflection mirror and the second multiple reflection mirror, and configured to refract the pulse laser beam to stretch a pulse width of the pulse laser beam.

In an embodiments of the inventive concept, a chirped pulse amplifier includes an oscillator configured to generate a pulse laser beam; a pulse width compressor separated from the oscillator and configured to compress a pulse width of the pulse laser; a pulse amplifier disposed between the pulse width stretcher and the oscillator and configured to amplify intensity of the pulse laser beam; and a pulse width stretcher disposed between the pulse amplifier and the oscillator, and configured to stretch the pulse width of the pulse laser beam. The pulse width stretcher includes a first multiple reflection mirror comprising a first large area mirror and a first small area mirror in the first large area mirror, the first large area mirror reflecting the pulse laser beam; a second multiple reflection mirror comprising a second large area mirror a second small area mirror in the second large area mirror, the second large area mirror disposed to face the first large area mirror; and a pulse group-delay dispersion block disposed between the first multiple reflection mirror and the second multiple reflection mirror, and configured to refract the pulse laser beam to stretch the pulse width of the pulse laser beam.

DETAILED DESCRIPTION

Exemplary embodiments of the inventive concept are described below in detail with reference to the accompanying drawings. The effects and features of the inventive concept, and implementation methods thereof will be clarified through following embodiments to be described in detail with reference to the accompanying drawings. However, the inventive concept is not limited to embodiments to be described below but may also be implemented in other forms. Rather, these embodiments are provided so that this disclosure can be thorough and complete and fully convey the scope of the inventive concept to a person skilled in the art, and the inventive concept is only defined by the scopes of claims. The same reference numerals throughout the disclosure refer to the same components.

The terms used herein are only for explaining embodiments and not intended to limit the inventive concept. The terms in a singular form in the disclosure also include plural forms unless otherwise specified. The terms used herein “comprises” and/or “comprising” do not exclude the presence or addition of one or more additional components, steps, operations and/or elements other than the components, steps, operations and/or elements that are mentioned. Also, the terms a chamber, a thin film, and coating in the disclosure may be understood as general semiconductors and device terms. Since the following description presents an exemplary embodiment, the reference numerals presented according to the order of the description are not limited to the order.

FIG. 1shows a chirped pulse amplifier10according to the inventive concept.

Referring toFIG. 1, the chirped pulse amplifier10may include a pulse oscillator20, a pulse width stretcher30, a pulse amplifier40, and a pulse width compressor50. The pulse oscillator20may generate a pulse laser beam100. For example, the pulse laser beam100may produce an ultra-short (picosecond or femtosecond) pulse102. The pulse width stretcher30may stretch the pulse width104of the pulse laser beam100. The pulse width104may be defined as a time interval that intensity and/or amplitude become ½ at the rise time and fall time of the pulse102. The intensity of the pulse102may vary at the pulse width stretcher30, the pulse amplifier40, and the pulse width compressor50. The pulse width stretcher30may stretch the pulse width104at each wavelength range of the pulse laser beam100. The intensity of the pulse102may decrease. For example, the pulse width stretcher30may decrease the intensity of the pulse laser beam100to be lower than or equal to the damage threshold of a second gain medium46(inFIG. 6) of the pulse amplifier40. The pulse amplifier40may amplify the intensity of the pulse laser beam100. The pulse width compressor50may compress the pulse width104of the pulse laser beam100. For example, the intensity of the pulse laser beam100at the pulse width compressor50may increase by about 105to 106times or more than that of the pulse laser beam100at the pulse oscillator20.

FIG. 2shows an example of the pulse oscillator20inFIG. 1.

Referring toFIG. 2, the pulse oscillator20may include a first pump laser21, a first resonator24, a first chirped mirror28, and a first output mirror29. The first pump laser21may generate a first pump light21a. The first pump light21amay be provided to the first resonator24by a pump light focusing lens22. The first resonator24and the first chirped mirror28may generate the pulse laser beam100from the pump light21a. For example, the first resonator24may include first and second concave mirrors23and25, and a first gain medium26. The first and second mirrors23and25may reflect the pulse laser beam100. Alternatively, the first and second mirrors23and25may amplify the intensity of the pulse laser beam100. The first gain medium26may be disposed between the first and second concave mirrors23and25. The first gain medium26may oscillate the pulse laser beam100. The first chirped mirror28may be disposed outside the extended line of the first and second concave mirrors23and25and the first gain medium26. For example, the pulse laser beam100may be transmitted to between the second concave mirror25and the first chirped mirror28. The first chirped mirror28may generate the pulse102of the pulse laser beam100. The first concave mirror23may provide the pulse laser beam100to the first output mirror29. The first output mirror29may output the pulse laser beam100to the pulse width stretcher30.

FIGS. 3 and 4show an example of the pulse width stretcher30inFIG. 1.

Referring toFIGS. 3 and 4, the pulse width stretcher may include a first multiple reflection mirror32, a second multiple reflection mirror34, and a group-delay dispersion block38. The first and second multiple reflection mirrors32and34may reflect the pulse laser beam100many times. The group-delay dispersion block38may be disposed between the first and second multiple reflection mirrors32and34. The group-delay dispersion block38may increase the pulse width104of the pulse laser beam100.

The first and second multiple reflection mirrors32and34may be apart from each other. The first and second multiple reflection mirrors32and34may perform multiple reflections on the pulse laser beam100. For example, the first and second multiple reflection mirrors32and34may reflect the pulse laser beam100about 24 times. “1” to “24” ofFIG. 3may correspond to the reflection points of the pulse laser beam100. The distance between the first and second multiple reflection mirrors32and34may be about 33.5 cm. The pulse laser beam100may travel a distance of about 8 m (24*33.5 cm). The pulse laser beam100may pass through the group-delay dispersion block38. The group-delay dispersion block38may have the multipath106of the pulse laser beam100. The multipath106may not intersect with the group-delay dispersion block38and may be spatially parallel thereto. The multipath106may extend the transmission and/or refraction length of the pulse laser beam100in the group-delay dispersion block38without the length extension of the group-delay dispersion block38. Thus, the first and second multiple reflection mirrors32and34and the group-delay dispersion block38may maximize the spatial efficiency of the pulse width stretcher30.

The first multiple reflection mirror32may include a first large area mirror31and a first small area mirror33. According to an example, the first large area mirror31may include a concave mirror. The first large area mirror31may have the radius of curvature of about 5 m. The first large area mirror31may have a first side hole31a. The first small area mirror33may be disposed in the first large area mirror31. The first small area mirror33may be disposed in the first side hole31a. For example, the first small area mirror33may include a flat mirror. The pulse laser beam100may pass the first multiple reflection mirror32through the first side hole31a.

The second multiple reflection mirror34may include a second large area mirror35and a second small area mirror36. According to an example, the second large area mirror35may include a flat mirror. The second large area mirror35may a second side hole35a. The second small area mirror36may have a smaller area than the second large area mirror35. The second small area mirror36may be disposed in the second side hole35a. For example, the second small area mirror36may include a flat mirror. The pulse laser beam100may pass the second multiple reflection mirror34through the second side hole35a.

The group-delay dispersion block38may be disposed between the first and second multiple reflection mirrors32and34. According to an example, the group-delay dispersion block38may include a dielectric cylinder. The group-delay dispersion block38may include silicon oxide. The group-delay dispersion block38may have a positive group-delay dispersion value for the pulse laser beam100. For example, the time required for transmission may vary at each wavelength of the pulse laser beam100, when the pulse laser beam100passes through the group-delay dispersion block38. The reason is that the group-delay dispersion block38has different refractive indexes for each wavelength of the pulse laser beam100. The group-delay dispersion block38may have the group velocity dispersion of the wavelength of the pulse laser beam100. The long wavelength of the pulse laser beam100may be stretched toward the front portion of the pulse102with respect to a time axis and the short wavelength of the pulse laser beam100may be stretched toward the rear portion of the pulse102. Thus, the pulse width104may be stretched over the entire wavelength of the pulse laser beam100.

The group velocity dispersion may correspond to the phase shift of the pulse laser beam100in the group-delay dispersion block38. When the phase shift of the pulse laser beam100is the function ψ(ω) (for the frequency component of the pulse laser beam100, the passage time τ(ω) of the pulse laser beam100of each frequency component may be expressed by Equation (1) as follows:

When Taylor's expansion is performed on the central frequency of the pulse laser beam100, τ(ω) may be expressed by Equation (2) as follows:

The first term ψ0is the initial phase of the pulse laser beam100and may be the absolute phase of the central frequency of the pulse laser beam100. The second term

ⅆ2⁢ψⅆω2⁢|ω⁢⁢0⁢(ω-ω0)2
may be the group velocity of the pulse laser beam100, i.e., a time taken for the pulse laser beam100of the central frequency to pass the group-delay dispersion block38. The third term

12⁢ⅆ2⁢ψⅆω2⁢|ω⁢⁢0⁢(ω-ω0)2
is a term representing the linear variation of τ(ω) according to the frequency of the pulse laser beam100and may be a group delay dispersion (GDD) value. The GDD may be in proportion to the linear variation of τ(ω) according to the frequency of the pulse laser beam100. For example, the larger the GDD is, the greater the linear variation of τ(ω) may be. The group-delay dispersion block38may determine the GDD of the pulse laser beam100. The entire group-delay dispersion of the pulse laser beam100may correspond to the multiplication of the GDD of the group-delay dispersion block38and the travel distance of the pulse laser beam100in the group-delay dispersion block38. Although not shown, the function ψ(ω) may include third order dispersion to Nth order dispersion. When the GDD is calculated, the third order dispersion to the Nth order dispersion may have little effect on the variation of the group velocity of the pulse laser beam100.

The pulse102of the pulse laser beam100may have bell-shaped Gaussian distribution. The stretched pulse width Δt of the pulse laser beam100by the group-delay dispersion block37may be calculated from Equation (3) and is represented by the GDD and the input pulse width τ0:

Δ⁢⁢t=r0⁢1+(4⁢⁢log⁢⁢(2)⁢GDD)2r02(3)
where the input pulse width τ0may correspond to

cλ02⁢Δ⁢⁢λ×Δ⁢⁢t=0.441
according to uncertainty principle. λ0may be the central wavelength of the pulse laser beam100. C may be the speed of light, 3×108 m/s. Δλ may be the full width half maximum of the wavelength of the pulse laser beam100. The stretched pulse width Δt may be calculated by the central wavelength λ0and the full width half maximum Δλ of the pulse laser beam100. For example, when the pulse laser beam100having a central wavelength λ0of about 800 nm and full width half maximum Δλ of about 100 nm passes the group-delay dispersion block38, the pulse width104of the pulse laser beam100may be stretched to about 388 picosecond (ps). Thus, the pulse width stretcher30may use the multipath106of the group-delay dispersion block38to effectively stretch the pulse width104.

FIG. 5shows an example of the pulse oscillator20inFIG. 1.

Referring toFIG. 5, the first multiple reflection mirror32may include a plurality of first small area mirrors33. The second multiple reflection mirror34may include a plurality of second small area mirrors35. The first and second large area mirrors31and35and the group-delay dispersion block38may be the same as inFIG. 3.

For example, the first and second multiple reflection mirrors32and34of the pulse width stretcher30may reflect the pulse laser beam100about 70 times. “1”-“70” may correspond to the reflection points of the pulse laser beam100. The pulse laser beam100may pass through the group-delay dispersion block38seventy-two times. The pulse laser beam100may pass about 24 m (72*33.5 cm) in the group-delay dispersion block38. When the pulse laser beam100having a central wavelength λ0of about 800 nm and full width half maximum Δλ of about 100 nm passes 24 m in the group-delay dispersion block38, the pulse width104of the pulse laser beam100may be stretched to about 1,164 ps.

FIG. 6shows an example of the pulse amplifier40inFIG. 1.

Referring toFIG. 6, the pulse amplifier40may include second pump lasers41, first mirrors42, and a second resonator44. The second pump lasers41may be disposed at the opposite sides of the second resonator44and the first mirrors42. The second pump lasers41may generate a second pump light41a. The first mirrors42may provide the second pump light41ato the second resonator44. The second resonator44may amplify the intensity of the pulse laser beam100. According to an example, the second resonator44may include first mirrors43, second mirrors45, and the second gain medium46. The first mirrors43and the second mirrors45may be disposed so that they face each other. The second gain medium46may be disposed between the first mirrors43and the second mirrors45. The second gain medium46may include the same material as the first gain medium26. Each time the pulse laser beam100passes through the second gain medium46, the intensity of the pulse laser beam100may gradually increase. On the contrary, the pulse widths104of the pulse laser beam100in the pulse width stretcher30and the pulse amplifier40may be the same.

Referring toFIGS. 3, 4, 7, and 8, the pulse width compressor50may include third and fourth multiple reflection mirrors52and54. The third and fourth multiple reflection mirrors52and54may be disposed so that they face each other. According to an example, the third and fourth multiple reflection mirrors52and54may include group-delay dispersion mirrors that reflect the pulse laser beam100. The third and fourth multiple reflection mirrors52and54may have the GDD of the pulse laser beam100which is opposite to the GDD of the group-delay dispersion block38. For example, the third and fourth multiple reflection mirrors52and54may have negative GDD.

According to an example, each of the third and fourth multiple reflection mirrors52and54may include low-refractive dielectric layers62and high-refractive dielectric layers64. The low-refractive dielectric layers62may have the same refractive index as the group-delay dispersion block38. The low-refractive dielectric layers62may include silicon oxide (SiO2) that has a refractive index of about 1.4. The high-refractive dielectric layers64may be disposed between the low-refractive dielectric layers62. The high-refractive dielectric layers64may have a higher refractive index than the low-refractive dielectric layers62. The high-refractive dielectric layers64may include titanium oxide (TiO2) that has a refractive index of about 1.9. The thicknesses of the low-refractive dielectric layers62and the high-refractive dielectric layers64may be different from each other. For example, the low-refractive dielectric layers62may have a thickness of about ¼ compared to the wavelength of the pulse laser beam100. The high-refractive dielectric layers64may have a thickness of about ½ compared to the wavelength of the pulse laser beam100. When the pulse laser beam100has a wavelength of about 800 nm, the low-refractive dielectric layers62may have a thickness of about 200 nm and the high-refractive dielectric layers64may have a thickness of about 400 nm.

The third multiple reflection mirror52may include a third large area mirror51and third small area mirrors53. According to an example, the third large area mirror51may include a concave mirror. The third large area mirror51may have the same radius of curvature as the first large area mirror31. The third large area mirror51may have a third hole51a. The third small area mirrors53may be disposed in the third large area mirror51. The third small area mirrors53may be fixed in the third hole51a.

The fourth multiple reflection mirror54may include a fourth large area mirror55and fourth small area mirrors56. According to an example, the fourth large area mirror55may include a flat mirror. The fourth large area mirror55may have a fourth hole. The fourth small area mirrors56may be disposed in the fourth large area mirror55. The fourth small area mirrors56may be fixed in the forth hole55a.

The pulse laser beam100may pass through the third hole51aand be reflected from the fourth large area mirror55. Alternatively, the pulse laser beam100may be reflected from the third small area mirror53and then pass through the fourth hole55a.

For example, when the pulse laser beam100has a waveform of about 800 nm, the pulse laser beam100may have GDD of about −2,000 fs2each time it is reflected from the third and fourth large area mirrors51and55and the third and fourth small area mirrors53and56. The third and fourth large area mirrors51and55and the third and fourth small area mirrors53and56may reflect the pulse laser beam100about seventy times. The third and fourth large area mirrors51and55and the third and fourth small area mirrors53and56may compress the pulse width of the pulse laser beam100to GDD of a total of about −144,000 fs2. The pulse width104of the pulse laser beam100having full width half maximum Δλ of about 100 nm may be compressed to about −42 ps.

As described above, the pulse width stretcher according to an embodiment of the inventive concept may include the group-delay dispersion block that refracts the pulse laser beam reflected between the first and second multiple reflection mirrors to stretches the pulse width of the pulse laser beam. The group-delay dispersion block may have the multiplath of the pulse laser beam between the first and second multiple reflection mirrors. The first and second multiple reflection mirrors and the group-delay dispersion block may maximize the spatial efficiency of the pulse width stretcher.

While embodiments of the inventive concept are described with reference to the accompanying drawings, a person skilled in the art may understand that the inventive concept may be practiced in other particular forms without changing its technical spirits or essential characteristics. Therefore, the above-described embodiments and applications should be understood as illustrative and not limitative in every aspect.