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Dual Wavelength Chirped Pulse Regenerative Amplifier Based on Yb3+:LuAlO3 Crystal for Terahertz Applications
11 1 2 Alexander Rudenkov , Viktor Kisel , Anatol Yasukevich , Karine Hovhannesyan , 21 Ashot Petrosyan , Nikolai Kuleshov
1Center for Optical Materials and Technologies, Belarusian National Technical University, Nezavisimosty Ave., 65, Minsk 220013, Belarus
2Institute for Physical Research, National Academy of Sciences of Armenia, 0203, Ashtarak-2, Armenia
Received 30.01.2020
Accepted for publication 06.03.2020
Abstract
Compact diode-pumped chirped pulse regenerative amplifier systems with pulse repetition rate of hundreds kilohertz based on Yb3+-doped crystals are of practical importance for wide range of applications such as materials processing, medicine, scientific research, etc. The aim of this work was to study the Yb3+:LuAlO3 crystal based dual wavelength chirped pulse regenerative amplifier.
Perovskite-like aluminate crystals have unique spectroscopic properties that allowed to use amplifier active element gain spectrum as an amplitude filter for amplified pulse spectrum and even obtained dual wavelength amplification without any additional components.
In our work a simple way to obtain dual-wavelength operation of chirped pulse regenerative amplifier by using the active medium gain spectrum as an amplitude filter for the formation of the amplified pulses spectrum demonstrated for the first time to our knowledge. Maximum output power of 5.4 W of chirped pulses (3.8 W after compression) and optical-to-optical efficiency of 22.5 % have been obtained for Yb:LuAP E//b-polarization at 200 kHz repetition rate. Compressed amplified pulse duration was about 708 fs while separate spectral components durations were 643 fs and 536 fs at 1018.3 nm and 1041.1 nm central wavelengths, respectively. Performed investigations show high potential of Yb3+:LuAP crystals as active elements of compact diode pumped chirped pulse regenerative amplifiers
Keywords: chirped pulse regenerative amplifier, ytterbium ions, diode pumping, lutetium aluminate crystals, gain narrowing effect.
DOI: 10.21122/2220-9506-2020-11-1-7-14
Адрес для переписки:
Александр Руденков
Центр оптических материалов и технологий, Белорусский национальный технический университет, пр-т Независимости, 65, г. Минск 220013, Беларусь e-mail: a.rudenkov@bntu.by
Address for correspondence:
Alexander Rudenkov
Center for Optical Materials and Technologies, Belarusian National Technical University, Nezavisimosty Ave., 65, Minsk 220013, Belarus e-mail: a.rudenkov@bntu.by
Для цитирования:
Alexander Rudenkov, Viktor Kisel, Anatol Yasukevich, Karine Hovhannesyan, Ashot Petrosyan, Nikolai Kuleshov. Dual Wavelength Chirped Pulse Regenerative Amplifier Based on Yb3+:LuAlO3 Crystal for Terahertz Applications. Приборы и методы измерений. 2020. - Т. 11, № 1. - С. 7-14. DOI: 10.21122/2220-9506-2020-11-1-7-14
For citation:
Alexander Rudenkov, Viktor Kisel, Anatol Yasukevich, Karine Hovhannesyan, Ashot Petrosyan, Nikolai Kuleshov. Dual Wavelength Chirped Pulse Regenerative Amplifier Based on Yb3+:LuAlO3 Crystal for Terahertz Applications. Devices and Methods of Measurements. 2020, vol. 11, no. 1, pp. 7-14. DOI: 10.21122/2220-9506-2020-11-1-7-14
УДК 621.373.826
Регенеративный усилитель чирпированных импульсов
3+
на кристалле УЬ :ЬиА1О3 с усилением отдельных спектральных компонент для применений
в терагерцовой области спектра
11 12 Александр Руденков , Виктор Кисель , Анатолий Ясюкевич , Карин Ованесьян ,
21 Ашот Петросян , Николай Кулешов
1 Центр оптических материалов и технологий, Белорусский национальный технический университет, пр-т Независимости, 65, Минск 220013, Беларусь
2Институт физических исследований Национальной академии наук Армении, 0203, Аштарак-2, Армения
Поступила 30.01.2020 Принята к печати 06.03.2020
Компактные регенеративные усилители чирпированных импульсов с диодной накачкой, обеспечивающие частоту повторения усиленных импульсов в сотни килогерц, построенные на материалах, легированных ионами Yb3+, представляют практический интерес для широкого ряда научных, промышленных и биомедицинских применений. Целью данной работы являлось исследование регенеративного усилителя на кристалле Yb3+:LuAlO3 с усилением отдельных спектральных компонент импульсов задающего лазера.
Кристаллы алюминатов со структурой перовскита обладают уникальными спектроскопическими свойствами, что позволяет использовать спектр усиления активной среды регенеративного усилителя в качестве амплитудного фильтра и усиливать отдельные участки спектра импульсов задающего лазера без каких-либо дополнительных оптических компонентов.
В данной работе впервые исследован простой подход, позволяющий использовать спектр усиления активной среды регенеративного усилителя как амплитудный фильтр для формирования спектра усиленного импульса в виде спектра состоящего из отдельных полос. Максимальная средняя выходная мощность 5,4 Вт (3,8 Вт после компрессора) с оптической эффективностью 22,5 % получена для поляризации излучения параллельной оси Ь кристалла Yb:LuAP при частоте следования импульсов 200 кГц. Длительность сжатых импульсов составила 708 фс при учете влияния всех спектральных компонент, и 643 фс и 536 фс отдельно для спектральных компонент с центральными длинами волн 1018,3 нм и 1041,1 нм. Проведённые исследования показывают высокий потенциал использования кристалла Yb3+:LuAP в качестве активного элемента компактных регенеративных усилителей чирпи-рованных импульсов с диодной накачкой.
Ключевые слова: регенеративный усилитель чирпированных импульсов, ионы иттербия, диодная накачка, кристалл лютециевого алюмината, эффект сужения спектра усиления.
БОТ: 10.21122/2220-9506-2020-11-1-7-14
Адрес для переписки:
Александр Руденков
Центр оптических материалов и технологий, Белорусский национальный технический университет, пр-т Независимости, 65, г. Минск 220013, Беларусь e-mail: a.rudenkov@bntu.by
Address for correspondence:
Alexander Rudenkov
Center for Optical Materials and Technologies, Belarusian National Technical University, Nezavisimosty Ave., 65, Minsk 220013, Belarus e-mail: a.rudenkov@bntu.by
Для цитирования:
Alexander Rudenkov, Viktor Kisel, Anatol Yasukevich, Karine Hovhannesyan, Ashot Petrosyan, Nikolai Kuleshov. Dual Wavelength Chirped Pulse Regenerative Amplifier Based on Yb3+:LuAlO3 Crystal for Terahertz Applications. Приборы и методы измерений. 2020. - Т. 11, № 1. - С. 7-14. DOI: 10.21122/2220-9506-2020-11-1-7-14
For citation:
Alexander Rudenkov, Viktor Kisel, Anatol Yasukevich, Karine Hovhannesyan, Ashot Petrosyan, Nikolai Kuleshov. Dual Wavelength Chirped Pulse Regenerative Amplifier Based on Yb3+:LuAlO3 Crystal for Terahertz Applications. Devices and Methods of Measurements. 2020, vol. 11, no. 1, pp. 7-14. DOI: 10.21122/2220-9506-2020-11-1-7-14
Introduction
Compact diode-pumped chirped pulse regenerative amplifier (CPRA) systems with pulse repetition rate of hundreds kilohertz based on Yb3+-doped crystals are of practical importance for wide range of applications such as materials processing, medicine, scientific research, etc.
One of the rapidly developing areas of research is the generation of THz radiation. Radiation of this spectral range is used in imaging and airport security inspection, cosmology and radio astronomy, atmospheric remote sensing, material measurements, medical sensing, etc.
Due to sub-picosecond pulse durations, ^J-level pulse energies and hence tens of MW pulse peak power levels CPRA systems can be successfully used for THz radiation generation by means of optical rectification [1-3] or difference frequency generation (DFG) techniques [4, 5]. In the case of DFG technique it is necessary to use laser source that emits at least two spectral components. For this purpose can be used Q-switched lasers [6, 7] which provide compact THz sources, optical parametric oscillators [8, 9] with complex cavities, two color semiconductor lasers [10] providing cost effective THz sources with relatively low power levels, etc. It is evident that application of ultrafast chirped pulse regenerative amplifier systems allowed power scaling of the generated THz radiation.
Unique spectroscopic properties of Perovskite-like aluminate crystals are well studied in our previous works, such as spectroscopy and high power CW [11], actively Q-switched [12] lasers, broad-band femtosecond chirped pulse regenerative amplifier with amplified pulse spectral broadening [13].
In this paper we study chirped pulse regenerative amplification of broad-band pulses in (Yb:LuAP) crystal based system in presence of strong gain narrowing effect which led to dual wavelength shaping of the amplified pulse spectrum for the first time to the best of our knowledge.
Crystal growth
LuAP is a biaxial crystal of the "distorted perovskite" type (space group D2h16-Pbnm). Unlike the stable aluminates based on large-size rare-earths (RAlO3, R = Gd-Er), the end-member orthorhombic aluminates of smaller-size rare-earths (RAlO3, R = Tm, Yb, Lu) are considered as metastable compounds, i. e., they have no stability regions in
the subsolidus and cannot be sintered employing traditional solid state reaction techniques. In the flux growth, TmAlO3 and YbAlO3 phases have been observed together with the corresponding garnet phases, while LuAlO3 could not be obtained; instead of this, Lu3Al5O12 and Lu2O3 phases have been recognized. The first reported LuAlO3 single crystals were grown from the melt by Czochralski method. Phase equilibria studies in the Lu2O3-Al2O3 system [14, 15] have shown that the range of stability of the perovskite phase is quite narrow and its nucleation may occur only on cooling from the molten state, from temperatures well above the liquidus temperature. Based on solidification behavior of the LuAP melts, the schemes were developed for single crystal growth, the details of which can be found in [16]. LuAP:Yb single crystals for the present studies were grown by the vertical Bridgman method (or vertical directional crystallization) [16, 17] under Ar/H2 atmosphere (5 vol% of H2) using molybdenum containers 14 mm in diameter. Hugh purity Lu2O3, Yb2O3 and crystalline Al2O3 were used as starting components; the selected concentrations of Yb were 2, 5 and 10 at.%. Due to a very small size mismatch between Yb3+ (rVIII = 0.985 A) and Lu3+ (rVIII = 0.977 A) ions [18] (around 0.8 % with respect to Lu3+), the distribution coefficient of Yb3+ ions in LuAlO3 is close to unit and practically all Yb ions amount added to the melts is being incorporated into the lattice.
Spectroscopy
Polarized absorption spectra of Yb3+(2 at.%):LuAP (corresponding ytterbium concentration was 4.02*1020 cm3) at room temperature were registered by a Varian CARY-5000 spectrophotometer. Absorption cross-section spectra for three light polarizations parallel to the a, b and c crystallographic axes are shown in Figure 1.
Strong absorption is found for E//c light polarization with the peak absorption cross-section at 978.5 nm of about 6.6*10-2° cm2 and spectral bandwidth FWHM of 4 nm.
It is well known that radiation trapping strongly affects the measured lifetime of Yb-doped materials because of significant overlap of the absorption and emission bands [16, 19]. The comparatively high index of refraction of LuAP (n0 = 1.923) also increases the probability of reabsorption even in optically thin samples because of the total internal reflection. Thus the special methods discussed in the
literature [16, 19] should be used to determine the luminescence lifetime accurately. In our experiments we used a fine powder ofYb:LuAP crystal immersed in glycerin. The diameter of the powder particles was measured to be approximately 30-40 pm, several times lower than absorption length of the most heavily doped Yb3+(10 at.%):LuAP crystal (75 pm at 978.5 nm). The Yb ions contents in the samples were 2, 5 and 10 at.%. The samples were excited by 20 ns pulses at 978.5 nm and luminescence kinetics was registered with the use of a 0.3-m monochromator, fast Ge-photodiode with a rise time of < 20 ns and a 500 MHz digital storage oscilloscope. All the samples exhibited single exponential decays (see Figure 2a). Starting from certain powder content, the lifetime remained constant despite further dilution (Figure 2b), thus indicating that reabsorption effects became negligible. Emission lifetime for 10, 5, and 2 at.% Yb-doped crystals was measured to be 310 ± 10 ps, 380 ± 10 ps and 475 ± 10 ps, respectively (see Figure 2c). Taking into account that similar concentration quenching was observed for Yb:YAP starting from about 4 at.% of Yb doping concentration [20], we believe that the measured value of (475 ± 5) ps corresponds to the radiative lifetime of Yb3+-ions in LuAP.
3-
2
2-
ш со to
< to
1« 1« 1 ' 1 / « ' / " ** - - " " I\ ~ -7 1 | E//a \ \ >v
1, 1, 1 , 1 , 1 1 1 , / l\x / \ / E//b M 4 ^w/ \
- - •'J^Us' E//c
ft / v ^ / --Absorption -Stimulated emission
ч J" ^—
10
E о
4
2
30 25 20 15 10
5 0
ф о it 0) О
о с о
920 940 960 980 1000 1020 1040 1060 Wavelength, nm
Figure 1 - Polarized absorption and stimulated emission cross-section spectra of Yb3+:LuAlO3 crystal (the spectra were obtained for Yb3+(2 at.%):LuAlO3)
Figure 2 - Kinetics of luminescence decay (a), measured lifetime for different weight content of Yb(2 at.%):LuAP crystalline powder in glycerin suspension (b) and measured Yb excited state lifetime for LuAP with different concentrations (c)
The stimulated-emission cross sections were calculated by use of the modified reciprocity method in which it is not necessary to know the Stark level structure of the Yb3+ manifolds (2F5/2 and 2F7/2) [21]:
(X)=
3 ■ exp(- hcj (kT X))
о /IDC
■ c ZjxAL(X)exp(-hcl(kTX))dX
(X),(1)
а
c
where Trad is the radiation lifetime of an active center; c is the light velocity; h and k are Planck and Boltzmann constants, respectively; T is the crystal temperature; n is the refractive index of a crystal; a and P denote the polarization state; and cabs is the ground-state absorption cross section.
The stimulated-emission cross section spectra calculated with this method are presented in Figure 1.
The most intensive stimulated emission (SE) cross-section band at 999.6 nm has peak value of about 3.74-10-20 cm2 for E//c-polarization.
Continuous wave laser experiment
For laser operation the most interesting polarization states in the crystal are E//c and E//b (a and b are crystallographic axes) due to high stimula-ted-emission cross sections values. In comparison with Yb-doped YAP, the crystal ofYb:LuAP exhibits slightly higher stimulated-emission cross section, a close radiative lifetime and a comparable stimulated emission bandwidth [22].
For a CW laser experiments a set up with X-folded cavity design was used (see Figure 3). It consisted of two curved mirrors Ml and M2 and two plane mirrors: OC and HR.
pump light was formed by a set of lenses into the spot with a diameter of about 180
A 2 mm long Yb(2 at.%):LuAlO3 crystal was used as a gain medium. The crystal was a-cut to provide E//b and E//c polarized laser output. It was a slab with dimensions 2(a)x5(b)x1.5(c) mm3; both 5x2 mm2 lateral faces were maintained at 15 °C by means of copper plates (indium foil was used to improve thermal contact) and thermo-electrical cooling elements with water-cooled heat sink, while 1.5x5 mm2 working faces were antireflection coated for pump and laser radiation.
The dependencies of the laser output power on the absorbed pump power for E//b- and E//c-polarized outputs and different OCs are shown in Figure 4. Absorbed pump power was real-time measured during the laser action.
Figure 3 - Experimental setup of continuous wave diode-pumped Yb:LuAP laser: HR-highly reflective mirror; M1, M2-concave mirrors; OC-output coupler; LD-laser diode
The calculated TEM00 mode diameter in the crystal was about 180 ^m. As a pump source, a multiple single emitter InGaAs fiber-coupled laser diode (0105 ^m, NA = 0.15) with a maximum output power of about 25 W was used. An "off-axis" pump layout was used for longitudinal pumping of the active element (see Figure 3). This pump arrangement was successfully tested in our previous work [11-13] and the main advantage of such a pump scheme is that all the cavity mirrors have highly reflecting coating at 900-1100 nm. The
Figure 4 - CW laser performance of Yb:LuAP crystal for different polarizations and output coupler transmittances
The maximum CW output power of 9.6 W at absorbed pump power of 15.2 W with slope efficiency of 73.5 % was demonstrated for E//b polarization with 5 % OC transmittance. With output coupler transmission of 10 % and 20 % the laser output power slightly decreased to 8.6 W and 6.6 W, respectively, while the corresponding slope efficiencies increased to 76.4 % and 84.5 %. Similar output powers were demonstrated for E//c laser output. With 10 % output coupler transmittance 9.1 W of output power was
obtained at 14.5 W of absorbed pump power with 77.9 % slope efficiency. Output powers of 8.3 W and 7.2 W with slope efficiencies of 62.5 % and 73.6 % were obtained for 5 % and 20 % OCs, respectively.
Dual wavelength chirped pulse regenerative amplifier experiment
Experimental setup of chirped pulse regenerative amplifier is shown in Figure 5. As a seed source laser diode-pumped Yb:KYW oscillator was used which provided 100 fs pulse train with 70 MHz PRF and 25 nJ single pulse energy. The seed pulse spectrum was about 12.5 nm wide (FWHM) and centered at 1038 nm. A 10 m-long single mode 09/125 ^m telecom fibre was used for pulse spectral broadening and temporal stretching
(tpuise ~ 7 5 ps). After passing through double-BaB2O4 Pockels cell based pulse picker and Faraday isolator, the seed pulse was injected into the RA. The isolator was employed to protect the seeder from high-intensity back reflections and, at the same time, for separating the amplified output pulse from the seed oscillator. The RA setup chosen for this experiment is quite common, employing a 40 mm-long double-BaB2O4 Pockels cell for pulse injection and ejection. Pulse repetition frequency (PRF) was chosen to be 200 kHz to prevent damage of the optical elements. For chirped pulse regenerative amplifier experiments was used the same active element as for CW experiments. The last unit of the amplifier system is compressor based on transmission diffraction grating with 1000 grooves per millimetre.
Figure 5 - Experimental setup of chirped pulse regenerative amplifier: AE - active element; M - concave mirror; HR - highly reflecting mirror; Polarizer - in this case is used polarizing cube; TFP - thin film polarizer; PC - Pockels cell
As one can see from the stimulated emission cross-section spectrum (Figure 1) for E//b polarization separate peaks around 1020 and 1040 nm are observed. Using broad-band seed source and taking into account strong gain narrowing effect we can estimate spectral shaping of the amplified pulses in such manner that the output pulse spectrum will consist of separate parts corresponding to SE cross-section spectrum of the amplifier gain medium. In the following experiment we studied of this idea. Seed and amplified pulse spectra obtained during the experiment are shown in Figure 6.
1,0
0,6
0,4
0,2
0,0
aa=2.9nm л 1л?.=5.2пт
¿.0=1018.3nmJ X / Seed laser: B<s / a^=60nm x0=1032nm/^. _Amplified__J^w >.0=1041.1nm
pulse I § spectra I
j x | \------
1000
1020
1040
1060
Wavelength, nm Figure 6 - Seed and amplified pulse spectra
The maximum uncompressed average output power of 5.4 W was obtained for E//b- polarized light after 112 roundtrips (RT) of the pulse through the amplifier cavity, while the pulse spectral width (FWHM) was 2.9 nm at 1018.3 nm (short wavelength part) and 5.2 nm at 1041.1 nm (long wavelength part). When the incident pump power was 24 W, the optical-to-optical efficiency was as high as 22.5 %.
Then we adjust compressor unit in the following order: adjust compressor for best compression of short wavelength part of the amplified pulse spectrum separately, made sure that the long-wavelength part of the spectrum is also satisfactorily compressed separately and then adjust relative offset of the pulses relative to each other by moving the rear mirror of the compressor. Measured autocorrelation traces of short and long wavelength parts separately are shown in Figure 7.
1,04
0,!
с
13 .Q
CD 0,6H
« 0,4-CD
Shortwave component at=1.414*643fs av*at=0.54
0,20,0 U—»
-1000 -500 0
500 1000
Time, fs
b
Figure 7 - Measured autocorrelation traces of short (a) and long (b) wavelength parts
After the adjusting of the compressor unit we measured autocorrelation trace of both spectral parts (Figure 8).
Figure 8 - Measured autocorrelation traces spectral parts
of both
During the experiments compressed pulse durations of 643 fs for short wavelength part and 536 fs for long wavelength part were achieved. Pulse duration of about 708 fs was obtained from autocorrelation trace of both spectral components demonstrating good overlap between components.
After simple calculations, one can expect that as a result of the difference frequency generation process, a radiation of about 6.5 THz spectral region can be obtained.
Conclusion
In conclusion, a simple way to obtain dual-wavelength operation of chirped pulse regenerative amplifier by using the active medium gain spectrum as an amplitude filter for the formation of the amplified pulses spectrum demonstrated for the first time to our knowledge. Maximum output power of 5.4 W of chirped pulses (3.8 W after compression) and optical-to-optical efficiency of 22.5 % have been obtained for Yb:LuAP E//b-polarization at 200 kHz repetition rate. Compressed amplified pulse duration was about 708 fs while separate spectral components durations were 643 fs and 536 fs at 1018.3 nm and 1041.1 nm central wavelengths, respectively.
Performed investigations show high potential of Yb3+:LuAP crystals as active elements of compact diode pumped chirped pulse regenerative amplifiers.
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