Optics Communications 255 (2005) 248–252
www.elsevier.com/locate/optcom
Intracavity sum-frequency generation of 3.23 W
continuous-wave yellow light in an Nd:YAG laser
Aicong Geng a,b,*, Yong Bo a, Xiaodong Yang a,b, Huiqing Li a,b, Zhipei Sun a,b,
Qinjun Peng a,b, Xuejun Wang a,b, Guiling Wang a, Dafu Cui a, Zuyan Xu a
a
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Group CL06, P.O. Box 603,
Chinese Academy of Sciences, Beijing 100080, China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100080, China
Received 6 April 2005; received in revised form 12 June 2005; accepted 13 June 2005
Abstract
By carefully designing a co-folding-arm plane–plane symmetrical L-shaped Nd:YAG laser cavity, both same waist
width and same waist position of 1064 and 1319 nm beam are obtained in an intracavity type II KTP crystal. After
optimizing the intracavity power ratio of two beams by choosing proper pump power of the two beams, continuous-wave yellow light at 589 nm is obtained with output power of 3.23 W. To the best of our knowledge, this is the
highest output power at 589 nm by an intracavity sum-frequency Nd:YAG laser.
2005 Elsevier B.V. All rights reserved.
PACS: 42.55.Xi; 42.60.By; 42.65.Ky; 42.60.Jf
Keywords: Yellow light; Nd:YAG laser; Sum frequency; Solid-state laser; Diode-pumped laser
Powerful yellow light generation has attracted
increased attention in the recent years. The laser
has been required by many applications, such as
medicine [1], Bose–Einstein condensation [2], lidar
measurement in the atmosphere region 15–100 km
[3], and such source for creating guide stars
*
Corresponding author. Tel.: +86 10 82648093; fax: +86 10
82649542.
E-mail address: [email protected] (A. Geng).
through resonance fluorescence in the mesosphere
sodium layer [4]. An interesting coincidence of nature is that the sum frequency of two lines of
Nd:YAG lasers operating near 1064 and 1319 nm
may be made resonant with the sodium D2 transition wavelength [5]. In 1994, Danailov confirmed
the possibility of producing 589 nm light by an
intracavity sum-frequency generation (SFG) in
an Nd:YAG laser, operated simultaneously at
two lines: 1064 and 1319 nm [6]. After this,
0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2005.06.026
A. Geng et al. / Optics Communications 255 (2005) 248–252
Nd:YAG lasers have already been used for the
SFG of yellow light operating as both Q-switched
and continuous wave (c.w.) [7–11]. c.w. SFG is of
interest especially because of its potential to yield
very narrowband radiation [8]. Moreover, intracavity frequency mixing with a nonlinear optical
crystal exploits higher intracavity intensities and
ensures a high degree of spatial overlap between
the beams [9]. However, high power c.w. yellow
light was yielded only by complex injection-locked
lasers [10], and only 510 mW c.w. yellow light at
589 nm has been obtained by using an intracavity
SFG Nd:YAG laser [11].
In this paper, by using a co-folding-arm plane–
plane symmetrical L-shaped cavity, c.w. yellow
light at 589 nm with output power of 3.23 W is obtained, to the best of our knowledge, which is the
highest output power at 589 nm by an intracavity
SFG Nd:YAG laser. The advantage of this approach lies primarily on its simplicity: the 1064
and 1319 nm lines are generated simultaneously
from an Nd:YAG laser, and at the same time the
intracavity SFG at 589 nm is generated with a type
II KTP crystal. In order to ensure a high degree of
spatial overlap between two fundamentals, the
cavity is carefully designed to offer both same
waist width and same waist position of two beams.
Moreover, according to the law of conservation of
energy, the ratio between the powers of two beams
is optimized.
The overall layout of the laser system is shown
in Fig. 1. The mirrors M1, M3, M4, M5 and M6
form the cavity for 1319 nm resonation, while the
mirrors M2, M5 and M6 form that for 1064 nm
resonation. Mirrors M5 and M6 form the co-folding-arm. All the mirrors are plane. M1 and M2 are
coated with high-reflection (HR) at 1319 and
1064 nm, respectively. M3 and M4 are identical
M1
1319nm laser
LH1
QR1 LH2
M3
M6
KTP
1064nm laser
M2
LH3
QR2 LH4
M4
M5
589nm
Fig. 1. Overall layout of the laser system; LH, laser head; QR,
quartz rotator.
249
and HR for the vertically (s-) polarized 1319 nm
field and antireflection (AR) for the parallel (p-)
polarized 1064 nm field. M5 is HR for the s-polarized 1319 nm field, HR for the p-polarized
1064 nm field and AR for the p-polarized 589 nm
field. M6 is HR for all three wavelengths. This system contains four laser heads (LHs), each consisting of nine 20-W laser diode arrays (LD) and an
Nd:YAG rod, respectively, (3 mm in diameter,
65 mm in length, Nd doping of 1.0%). Both end
facets of the Nd:YAG rods of LH1 and LH2 are
AR at 1319 nm, while those of LH3 and LH4
are AR at 1064 nm. A 90 quartz rotator (QR)
at 1319 nm is inserted between LH1 and LH2 to
compensate for the polarization-dependent birefringence, which makes the stable regions of different polarizations overlap each other, thus
improving the output power and beam quality
[12,13]. Another 90 QR at 1064 nm is inserted between LH3 and LH4.
Calculated by SNLO [14], a 3 mm · 4 mm ·
20 mm flux grown KTP crystal cut at h = 78.5,
/ = 0 is located in the co-folding-arm. This device
permits type II critical phase matching for
1064 nm (o) and 1319 nm (e), producing 589 nm
(o). A high effective nonlinear coefficient of
deff = 3.42 m/V, and relatively small 1.1 walkoff
angle of the 1319 nm beam from the phase-matching direction are expected. The z–x plane is tilted
45 from vertical so the fundamentals propagate
as both o and e components, thus only half of each
fundamental may contribute to the type II SFG
output. The phasematching temperature is calculated to be 37 C, in agreement with our experimental results. The KTP crystal is HR coated for
all three wavelengths on both facets.
In order to ensure a high degree of spatial overlap between two fundamentals in the nonlinear
crystal, both same waist width and same waist position of 1064 and 1319 nm resonation are needed.
It can be easily known that being a plane–plane
cavity, both beam waists of two fundamentals locate on mirror M6. Therefore, the KTP crystal is
located as close to mirror M6 and, consequently,
as close to the beam waist as possible.
However, accurately measuring the beam width
in a resonator by experiments is difficult, so standard ABCD ray propagation matrix is used as
A. Geng et al. / Optics Communications 255 (2005) 248–252
an auxiliary tool [15]. Owing to the different thermal lensing effects produced by the Nd:YAG rod
AR at 1064 and 1319 nm, different round trip
matrices from the mirror M6 can be obtained [15]
Ai B i
;
ð1Þ
Mi ¼
C i Di
where the subscript i = 1064 nm, 1319 nm and denote those parameters for 1064 and 1319 nm laser,
respectively. Ai ,Bi ,Ci and Di are functions of the
thermal focal length and the cavity length. Therefore, the fundamental mode waist widths on the
mirror M6 can be given by [15]
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
u
4 ðAi þ Di Þ2 ukBi
;
t
ð2Þ
xi ¼ 1 Ai D i
2p
where x1064 nm and x1319 nm are functions of
the thermal focal length and the cavity length
too. That is x1319 nm = F(f1319 nm ,L1319 nm), and
x1064 nm = F(f1064 nm ,L1064 nm), where f1319 nm and
f1064 nm present the thermal focal length of the
Nd:YAG rod AR at 1319 nm and 1064 nm, while
L1319 nm and L1064 nm present the cavity length of
1319 nm laser and 1064 nm laser, respectively. In
order to obtain same waist width on the mirror
M6, the fundamental mode waist widths need to
satisfy the following equation
F ðf1319 nm ; L1319 nm Þ ¼ F ðf1064 nm ; L1064 nm Þ.
ð3Þ
From the above equation, it can be seen that
once f1319 nm and f1064 nm are known, the relationship between L1319 nm and L1064 nm is obtained.
Therefore, the thermal focal length of single
Nd:YAG rod AR at 1064 nm or 1319 nm laser
was measured by a quasi steady-state cavity [16],
as shown in Fig. 2. It can be seen that the thermal
focal length of the Nd:YAG rod AR at 1064 nm is
longer than that AR at 1319 nm at the same LD
pump power. In the Nd:YAG rod pumped at
808 nm, the quantum defect or Stokes shift is
24% for 1064 nm laser and 39% for 1319 nm laser.
Therefore, the total heat dissipated in the
Nd:YAG rod for 1064 nm laser is less than that
for 1319 nm laser at the same LD pump power,
which results in a longer thermal focal length of
the Nd:YAG rod AR at 1064 nm [17].
650
Thermal lens length (mm)
250
AR at 1064 nm
AR at 1319 nm
600
550
500
450
400
130
140
150
160
170
LD pump power (W)
Fig. 2. Thermal focal length of a Nd:YAG rod AR at 1064 nm
or 1319 nm versus the LD pump power.
Moreover, according to the law of conservation
of energy: hm1064 nm + hm1319 nm = hm589 nm [17], one
photon of 1064 nm and one photon of 1319 nm
produce one photon of 589 nm. Therefore, in theory the optimal ratio between the intracavity powers of 1064 and 1319 nm becomes
P in1064 nm hm1064 nm
¼
¼ 1.24.
P in1319 nm hm1319 nm
ð4Þ
It is well known that the intracavity power can be
worked out by the equation [17]
P ini ¼ P outi 1 þ Ri
;
1 Ri
ð5Þ
where Pout and R represent the output power and
the reflectivity of the output coupler, respectively.
Therefore, when same reflectivities of the output
couplers of two lasers are chosen, the following
equation can be obtained
P in1064nm P out1064nm
¼
.
P in1319nm P out1319nm
ð6Þ
Because the output power of 1064 and 1319 nm
laser are associated with their LD pump powers,
by choosing the ratio of the LD pump powers of
two lasers, the optimal ratio of the intracavity
powers of two fundamentals in the nonlinear crystal can be realized. Therefore, the maximal LD
pump power of single 1064 nm laser head is chosen
as 1.24 times that of single 1319 nm laser head. In
addition, the LD pump power is also associated
A. Geng et al. / Optics Communications 255 (2005) 248–252
0.5
1064 nm beam
1319 nm beam
Waist width (mm)
0.4
0.3
0.2
0.1
400
500
600
700
800
900
1000
Thermal focallength (mm)
Fig. 3. Calculated fundamental waist width of 1064 and
1319 nm beam on mirror M6 as a function of the thermal
focal length.
0.5
1064 nm beam
1319 nm beam
0.4
Beam width (mm)
with the thermal focal length of the Nd:YAG rod.
As shown in Fig. 2, at the maximal LD pump
power of 171 W at 808 nm, the thermal focal
length f1319 nm is 410 mm where maximal output
power at 1319 nm will be obtained. Then the
shortest thermal focal length f1064 nm is chosen as
590 mm where the LD pump power is 138 W at
808 nm.
Since two shortest thermal focal lengths are
determined, the cavity lengths L1319 nm and
L1064 nm can be estimated to be 865 mm and
1225 mm, respectively, by investigating the stable
regions of the cavity as shown in Fig. 3, where
the calculated fundamental waist width of 1319
and 1064 nm beam on mirror M6 is shown as a
function of the thermal focal length. In this case,
the beam width in the co-folding-arm of two lasers
at their full pump power is calculated as shown in
Fig. 4. It can be seen from Fig. 4 that this geometry yields a waist of x = 57 lm for 1064 nm beam,
which is very close to that for 1319 nm beam
(x = 59 lm). Moreover, a relatively high degree
of spatial overlap between two beams within the
nonlinear crystal (within 20 mm from M6) is
obtained.
In our experiment, by replacing the total reflector M1 with a flat output coupler (with the transmittance of 10% at 1319 nm), the 1319 nm
operation is performed only with LH1 and LH2
working. Similarly, replacing M2 by another with
251
0.3
0.2
0.1
0.0
0
10
20
30
40
50
60
Distance from mirror M6 (mm)
Fig. 4. Calculated fundamental beam width of 1064 and
1319 nm beam in the co-folding-arm versus the distance from
mirror M6 at full LD pump power.
10% reflectivity for 1064 nm performs the
1064 nm operation only with LH3 and LH4 working. The experimental results are shown in Fig.
5(a), from which the optimal ratio of the LD pump
powers of two fundamentals can be found. The
experimental results agree well with the calculated
results. The concaves of the output power are resulted from the dissymmetrical cavity and different
polarizations, which has been discussed in detail
elsewhere [18].
Then the total reflectors M1 and M2 replace the
output couplers. With four laser heads working
simultaneously, 1319 nm laser and 1064 nm laser
are summed in the KTP crystal. The 589 nm yellow light generated in two directions of the KTP
crystal is extracted from mirror M5 in one direction, which not only improves the utilization efficiency of the nonlinear crystal, but also avoids
the absorption of the yellow beam in Nd:YAG
rods and then reduces the generation of waste
heat. After optimizing the ratio of the LD pump
powers of two fundamentals, the experimental
output power at 589 nm is obtained as shown in
Fig. 5(b). At LD pump power of 558 W, 3.23 W
yellow light is generated. It is obvious that the
slope efficiency increases sharply near the full
pump power, and the reason is exactly that the laser configuration is designed at the point of full
pump power.
252
A. Geng et al. / Optics Communications 255 (2005) 248–252
our knowledge, which is the highest output power
at 589 nm obtained by one intracavity SFG Nd:
YAG laser. In the following work, we will further
optimize the cavity design to improve the output
power of the yellow light and resonate the light
with the sodium D2 line.
40
Output power (W)
a
At 1064 nm
At 1319 nm
30
20
Acknowledgments
10
This work was supported by the Knowledge
Innovation Program of the Chinese Academy of
Sciences under Grant No. KJCX1-05, the
National High Technology Development Program
of China under Grant Nos. 2002AA731052 and
2002AA311040, the National Key Basic Research
and Development Program of China under Grant
No. G1998061413, and the Council for Science
and Technology of Beijing under Grant No.
H020420060060110.
0
100
150
200
250
300
350
LD pump power (W)
Output power at 589 nm (W)
3.5
b
3.0
Experimental results at 589 nm
Linear fit of the experimental results
2.5
2.0
1.5
1.0
References
0.5
0.0
200
250
300
350
400
450
500
550
600
LD pump power (W)
Fig. 5. Experimental output power as a function of the LD
pump power; (a) at 1064 and 1319 nm, (b) at 589 nm.
In conclusion, only by using a simple co-foldingarm plane–plane symmetrical L-shaped cavity,
simultaneous 1064 and 1319 nm emission from
an Nd:YAG laser is summed with an intracavity
type II KTP crystal. By combining experimental
results with the ABCD ray propagation matrix, a
cavity is designed with both same fundamental
mode waist width and same waist position of
two fundamentals, which ensures a high degree
of spatial overlap between two fundamentals.
Moreover, by choosing the ratio of the LD pump
powers of two lasers, the optimal ratio of the intracavity powers of two fundamentals is realized in
the nonlinear crystal. In the experiment, 3.23 W
yellow light at 589 nm is obtained, to the best of
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