Download Tunable THz frequency difference from a diodepumped

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
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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]
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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].
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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.
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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
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(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)
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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
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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
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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. In particular progress improving the
reflectivity of the TCVBG could help to suppress the additional partial reflective output mirror.
Acknowledgements Y. Guillin and L. Grosvallet are gratefully
acknowledged for technical assistance. This work was supported
by PRES Université de Lyon and Lyon Science Transfert under
grant no. L326.
c 2011 by Astro Ltd.
Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA
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