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Laser Phys. Lett. 8, No. 7, 520–524 (2011) / DOI 10.1002/lapl.201110028 Abstract: We have demonstrated a simple and compact tunable dual-wavelength Yb3+ :KGd(WO4 )2 (Yb:KGW) laser. The laser is diode pumped through an optical bifurcated fiber and an adjustable beamsplitter. Thanks to two transversally chirped volume Bragg gratings (TCVBG) used as output couplers, the frequency difference between the two collinear waves is continuously tunable from 0 up to 7.8 THz with 110 GHz bandwidth. The frequency range is only limited by non optimized optical components and not by the principle of functioning. 150 Output power, mW 520 100 50 o-laser, R' = 90% e-laser, R' = 90% o-laser, R' = 80% e-laser, R' = 80% 0 1020 1030 1040 1050 Wavelength, nm Spectral dependence of the e- and o-output powers obtained from Y-moving the PTR glass c 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA Tunable THz frequency difference from a diode-pumped dual-wavelength Yb3+:KGd(WO4)2 laser with chirped volume Bragg gratings A. Brenier ∗ Laboratoire de Physico-Chimie des Matériaux Luminescents, UMR CNRS 5620, Université Lyon 1, 69622 Villeurbanne, France Received: 25 February 2011, Revised: 9 March 2011, Accepted: 12 March 2011 Published online: 2 May 2011 Key words: dual wavelength; tunable laser; terahertz; volume Bragg grating 1. Introduction Terahertz waves, which are safe for human on the contrary to X-rays, have a huge potential for non-destructive imaging of concealed objects and free-space communications [1,2]. They can be produced directly with quantum cascade lasers at low temperature, gas lasers or Shottcky and Gunn diodes. As an indirect way, nonlinear optical crystals such as GaP, LiNbO3 , ZnGeP2 , and GaSe oriented for adequate phase matching can also be used [3,4] in difference frequency mixing, mainly in the nanosecond regime basically provided by Q-switch Nd:YAG lasers pumping ∗ optical parametric oscillators. Recent efforts in this channel have led to efficient THz generation [5]. Other indirect sources of THz radiations are based on the photocurrent induced inside a photoconductive device (photomixer) associated with an antenna [6,7]. The device is illuminated by a femtosecond laser or by two continuous wave (CW) laser beams at neighbouring frequencies whose beatnote modulates the photocurrent in the THz range. Dual wavelength semi-conductors driven by a diffraction grating can be used but solid-state lasers with good beam quality and high output power are also of interest. The spectral tunability of the frequency difference is obtained by inserting a selective optical element, Corresponding author: e-mail: [email protected] c 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA Laser Phys. Lett. 8, No. 7 (2011) 521 Nm Transversally-chirped reflective Bragg gratings 45° TCBGE Np Optical axis TCBGO Focusing lens Ng ve wa Entrance mirror Pump leg 2 e- Pump leg 1 o-wave KGW:Yb laser crystal YVO4 crystal Partialy reflective mirror Y-displacement Figure 1 (online color at www.lphys.org) General laser cavity setup of the tunable Yb:KGW two-frequency laser typically an etalon or a diffraction grating, in the optical path of each laser beam. This was accomplished with the KGd(WO4 )2 (KGW) crystal with frequency difference in the 0 – 3 THz range [8] with the help of two etalons and a half-wave plate in order to work with the best polarization (gain optimization) inside the gain medium. With the unusual c-orientation of a Nd:GdVO4 crystal a dualwavelength laser was built and 0.56 THz radiation was produced from a GaSe nonlinear crystal [9]. Very recently a dual-wavelength Cr3+ :LiCaAlF6 laser with tunable frequency difference between 0 and 9 THz was demonstrated using two rotating diffraction gratings in a Littrow mounting [10] and supplying 63 mW of total output power but no power spectral dependence is provided. Let us indicate also THz radiation from two-color filaments in air due to induced plasma [11]. A general and recent review of THz generation by means of optical lasers can be found in [12]. The Yb:KGW or Yb:KYW crystals have the advantage to be favorably evaluated as laser materials [13] thanks to a small quantum defect, a high absorption near 981 nm and a high peak emission cross-section, and to be commercially available. Their broadband emission can be exploited for efficient ultra-short pulse generation [14–17]. The challenging laser emission at the 981 nm zero-line transition has been demonstrated recently [18]. The usual polarization axis for lasing is Nm because it corresponds to the highest emission cross-section, but lasing along Ng can be beneficial for compact Q-switched and Raman lasers. A diode-pumped Ng -polarized laser has been demonstrated with Yb:KGW [19]. Recently we succeeded in active Q-switching a two-wavelength Yb:KGW laser with spectrally-free outputs, that is to say with no tunability and no optical component for wavelength selection [20]. www.lphys.org On the other hand since a few years a new optical component has been proved to be efficient for the spectral tuning of the solid-state lasers: the volume Bragg gratings (VBG) based on the modulation of the refractive index of a photo-thermo-refractive glass [21]. From rotation of such a VBG a Yb:KYW laser tunable from 997 up to 1050 nm has been demonstrated with 0.1 nm bandwidth and good quality beam [22]. Other interesting results (996 – 1048 nm tunability) on a similar system are reported in [23]. In these works the spatial period of the VBG is constant all over the glass volume and the peak reflectivity is high, about 0.97. The spectral tunability can also result from a transversally chirped volume Bragg grating (TCVBG) used as the output coupler. With a spatial period varying at 1 nm/mm rate and a 20 mm long component the tunability of the laser obtained in [24] was in the 997 – 1016 nm range. The present work is devoted to a continuous wave tunable diode pumped Yb:KGW dual-wavelength laser. The tunability of the two frequencies is obtained with two TCVBG with up to several hundreds of mW, the frequency difference being tunable in the 0 – 7.6 THz range. Thanks to the TCVBG the device is simple and compact. 2. Laser cavity setup The laser cavity (Fig. 1) is constituted with two plane mirrors and is stabilized by a convergent lens. The entrance dichroic mirror has a high transmission at the 981 nm pump wavelength and a high reflectivity at the laser wavelength and the output mirror is partially reflective at the laser wavelength. The lens has 5 cm focal length and is anti-reflection coated in the 1000 – 1100 nm range. The distance between the lens and the entrance mirror is 6.5 cm and it is 17 cm between the lens and the output coupler. c 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA A. Brenier : Tunable THz frequency difference from a Yb3+ :KGd(WO4 )2 laser 3. Tunable frequency difference in the THz frequency range In Fig. 1 we can see that close to the output mirror two TCVBG are drawn. In a first step we omit these two optical components as the cavity description was given in Sec. 2. The goal of the non-standard orientation of the Yb:KGW laser crystal used in this work is to exploit the anisotropy of the crystal in order that the laser gains of the two orthogonally polarized modes propagating perpendicularly to the crystal face have a different spectral dependence. The o- and e-laser gains, based on the polarized emission and absorption spectra, are represented in Fig. 2a in the wavelength range of interest with the reasonable value of 10% population inversion. With a 90% reflectivity output coupler we obtained free-running laser emissions simultaneously in o- and epolarizations peaking at 1038.7 and 1028.7 nm, respectively, with about 1 nm bandwidth at half maximum. With c 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA (a) o-polarization 1.0 Laser gain, a.u. 0.8 e-polarization 0.6 0.4 0.2 0 1020 1030 1040 1050 Wavelength, nm 0 grating e grating o (b) 5 Y-position, mm Then the laser waists inside the cavity can be calculated to be about 60 and 160 μm on the entrance and output mirrors, respectively. The Yb doping of the KGW laser crystal, provided by MolTech GmbH, was 5%, its size was 5×5 mm2 and its thickness was 1.7 mm. Its two faces were broadband antireflection coated near 1040 nm. The crystal was mounted in a copper heat sink maintained near 10◦ C temperature. We chose a Yb:KGW crystal oriented as it is described in Fig. 1, likely to emit a Np -polarized ordinary (o) wave and an extraordinary (e) wave polarized in the Nm – Ng principal plane, both waves propagating in this plane at 45◦ from the Nm and Ng axis. Inserting a 0.7 mm thickness plate of YVO4 birefringent crystal allows to collapse the two o- and e-waves in a lone laser path. Of course a birefringent angle occurs also inside the KWG crystal for the e-wave but this detail is omitted in Fig. 1 not to make it cumbersome. The pumping beam was provided by a fiber-coupled laser diode from LIMO (25 W, N.A. = 0.22, fiber core diameter 100 μm) and was first injected in a home-made beam-splitter described with the whole pump setup in [20]. The beam-splitter allows the adjustment of the ratio of its two output powers, the ratio being controlled by a manual micrometric actuator. The two outputs are connected to the two entrance channels of an Avantes optical bifurcated fiber with 100 μm core diameter each channel. The two circular outputs of the bifurcated fiber were focused into the Yb:KGW crystal in the 1:1 ratio by two 60 mm focal length doublets, the diameter of each focal point, measured by the knife method, being about 120 μm. The measured distance between the centers of the two focal points was 130 μm. Due to the 0.7 mm thickness of the YVO4 plate the two beam pumping (leg 1 and leg 2 in Fig. 1) optimize the overlap with the two o- and e-laser waves for the best laser gains. 10 15 20 1020 1030 1040 1050 Wavelength, nm 100 (c) e-wave o-wave 80 Laser intensity, a.u. 522 60 40 20 0 1020 1030 1040 1050 Wavelength, nm Figure 2 (online color at www.lphys.org) (a) – spectral dependence of the laser gains in o- and e-polarizations of the Yb:KGW crystal pumped for 10% population inversion, (b) – transverse Y-position of the PTR glass and corresponding resonant wavelengths of the two TCVBG, and (c) – spectral dependences of the couples of the o- and e-laser lines obtained from Y-moving the PTR glass according to (b) www.lphys.org Laser Phys. Lett. 8, No. 7 (2011) 523 Output power, mW 150 100 50 o-laser, R' = 90% e-laser, R' = 90% o-laser, R' = 80% e-laser, R' = 80% 0 1020 1030 1040 1050 Wavelength, nm Figure 3 (online color at www.lphys.org) Spectral dependence of the e- and o-output powers obtained from Y-moving the PTR glass 150 Output power, mW o-laser e-laser 100 λe = 1026.4 nm λo = 1042.4 nm 50 0 1.0 1.5 2.0 Absorbed pump power, W Figure 4 (online color at www.lphys.org) Output powers versus absorbed pump powers of two selected e- and o-laser lines at 1026.4 and 1042.4 nm, respectively an 80% reflectivity output coupler the o-wave remained close to the previous value and the e-wavelength decreased up to 1024 nm. We found that the laser peaking wavelengths are extracted from the laser gains spectral behavior (Fig. 2a) and depend on the cavity losses, i.e., the output mirror transmission. From now on we insert close to the output mirror the two TCVBG working in reflection. To the contrary on it is drawn in Fig. 1 the two TCVBG are in realty recorded and are overlapped inside a lone piece of a photo-thermorefractive (PTR) glass (they are separated in Fig. 1 for a www.lphys.org better visualization). They have been provided by OptiGrate with the following requirements. At one side, corresponding to Y-position that equals to 0 in Fig. 2b and bottom in Fig. 1, the spatial period of the two gratings are close and the two reflected resonant wavelengths are about 1033 nm. Then increasing the value of the Y-transverse position, the grating labeled TCVBGO has its spatial period increasing in such a way that its reflected resonant wavelength increases at 1 nm/mm chirp rate up to about 1051 nm. The grating labeled TCVBGE has its spatial period decreasing and its reflected wavelength decreases with 1 nm/mm chirp rate up to 1015 nm. The correspondence between the Y-position, controlled experimentally with a micrometric actuator, and the reflected resonant wavelength is represented in Fig. 2b for the two gratings. The resonant diffraction efficiencies (RDE) are 53% and 59%, respectively, for the TCVBGO and TCVBGE . These values are too low to lead to reasonable laser thresholds at the two resonant wavelengths in our experimental conditions. In order to decrease these thresholds each TCVBG, playing the role of a virtual plane mirror, and the plane output coupler with the reflectivity R are used together, forming a composite mirror whose the maximum reflectivity R can be obtained as the one of a Fabry-Perrot resonator with unequal mirrors [25,26]: √ √ 2 R + RDE R = √ 2 . 1 + R RDE The calculated R value with R = 0.90 and RDE = 0.53 is 0.98 and is 0.96 with R = 0.80. In these two cases we expect lasing at the two resonant wavelengths. This is what we observed: after careful alignment of the PTR glass device, the two above-mentioned free running laser emissions disappeared completely and were replaced by two laser lines in o- and e-polarizations at the resonant wavelengths selected by the transverse Y-position. Several couples of such laser lines are represented in Fig. 2c in the 1020 – 1033 nm range for the e-waves and in the 1033 – 1047 nm for the o-waves. We obtained a frequency difference continuously tunable up to 7.6 THz. The relative e/o-laser power depends on the relative leg 2/leg 1 pump power. With the fixed absorbed pump powers leg 2 is equal to 2.97 W and leg 1 to 2.72 W we obtained e- and o-laser powers visualized in Fig. 3 (symbols link up with solid lines) versus wavelength, using R = 0.90 and R = 0.80. At fixed laser wavelengths we have also studied the laser output powers versus the absorbed pump power in each channel. This is visualized for the resonant wavelengths λo = 1042.4 nm and λe = 1026.4 nm in Fig. 4, using R = 0.80 and the relative leg 2/leg 1 pump power fixed closed to 1. Finally we have measured the spectral dependence of the laser lines with a high resolution HRS2 Jobin Yvon monochromator equipped with a1 μm blazed grating and a R1767 photomultiplier. This step was necessary because c 2011 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA A. Brenier : Tunable THz frequency difference from a Yb3+ :KGd(WO4 )2 laser 524 References 1.5 Spectral widths, a.u. ×103 Free lasing VGB lasing 1.0 0.5 0 1021 1022 1023 1024 1025 Wavelength, nm Figure 5 (online color at www.lphys.org) Comparison of the spectral widths of the free lasing (after 1 nm shift down) and of the VGB lasing in Fig. 2c the laser lines were broadened because we used a low resolution HR2000 monochromator equipped with a CCD in order to get a fast acquisition. The high resolution result is visualized in Fig. 5 (solid curve) for the ewave selected at 1023 nm. Its full width at half-maximum (FWHM) is 110 GHz, that is to say slightly lower than the TCVBG width 140 GHz. For comparison, the width of the free lasing wavelength is 240 GHz at 1024 nm and is represented by the dashed curve on the same Fig. 4 after 1 nm translation for an easier visualization. 4. Conclusion We have demonstrated a simple and compact tunable dualwavelength Yb:KGW laser. The laser is diode pumped through an optical bifurcated fiber and an adjustable beamsplitter. Thanks to two TCVBG used as output couplers, the frequency difference between the two collinear waves is continuously tunable from 0 up to 7.8 THz with 110 GHz bandwidth. The frequency range is only limited by non optimized optical components and not by the principle of functioning. 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