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All-solid-state continuous-wave yellow laser

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All-solid-state continuous-wave yellow laser
at 561 nm under in-band pumping
Impact Factor: 1.97 · DOI: 10.1364/JOSAB.30.000095
Jing Gao
Xianjin, Eric, Dai
Chinese Academy of Sciences
University of Florida
Haixuan Sun
Xiaodong Wu
Chinese Academy of Sciences
Suzhou Institute of Biomedical Engineerin…
Available from: Jing Gao
Retrieved on: 08 November 2015
Gao et al.
Vol. 30, No. 1 / January 2013 / J. Opt. Soc. Am. B
All-solid-state continuous-wave yellow laser at 561 nm
under in-band pumping
Jing Gao,* Xianjin Dai, Long Zhang, Haixuan Sun, and Xiaodong Wu
Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, Suzhou,
Jiangsu 215163, China
*Corresponding author: [email protected]
Received October 17, 2012; revised November 5, 2012; accepted November 12, 2012;
posted November 12, 2012 (Doc. ID 178246); published December 6, 2012
We demonstrate a continuous-wave (cw) yellow laser at 561 nm generation by intracavity frequency doubling of a
direct in-band pumped Nd:YAG laser on the R1 → Y 6 transition. By using an inserted solid glass etalon and a
special coating design, the 1123 nm single line oscillation is realized. With an absorbed pump power of
∼21.8 W, the maximum output power of ∼2.3 W is achieved, with an optical-to-optical efficiency of ∼10.6%. This
is, to the best of our knowledge, the highest cw output power at 561 nm of a diode-pumped frequency doubled
Nd:YAG laser. © 2012 Optical Society of America
OCIS codes: 140.3580, 140.3530, 140.3480, 140.3515.
Yellow lasers around 560 nm are highly interesting for their
potential in biomedicine and biotechnology [1–4]. These
sources have higher absorption in hemoglobin, better penetration through the nuclear sclerotic cataracts, lower absorption
in lutein of the macula lutea, less heat dispersion in the neurosensory retina, and less discomfort to the patient. Thus, this
type of laser plays an important role in ophthalmology [5–7].
Especially, the 561 nm emission can excite phycoerythrin and
its tandems and red fluorescent proteins, such as DsRed and
dTomato; as a result, this laser has become a standard for confocal microscopy and flow cytometry and is being used for
laser Doppler velocimetry [8,9].
Frequency doubling of the R1 → Y 6 transition at 1123 nm in
the Nd:YAG gain medium has been proven to be an efficient
way of generating a 561 nm yellow laser. Nd:YAG single crystal, the most widely used laser gain medium until now, has
become the most promising candidate due to its excellent
combination of mechanical, thermo-optical, and spectral
properties. The first laser diode (LD) pumped 561 nm laser
with 0.5 mW continuous-wave (cw) output was reported by
Guo et al. in 2004, employing a diode-end-pumped structure
with an 808 nm LD and an extracavity frequency doubled
scheme with a periodically poled lithium niobate (PPLN) crystal, respectively [10]. In 2008, Räikkönen et al. demonstrated a
diode-end-pumped passively Q-switched Nd:YAG laser, and its
extracavity frequency doubling into the yellow range by using
a potassium titanyl arsenate (KTA) crystal. The pulsed repetition rate was 12 kHz, and the average power was 55 mW [11].
In 2010, Yao et al. presented a 561 nm laser with 1.2 W cw
output power by intracavity frequency doubling of a diodeend-pumped Nd:YAG laser with an 808 nm LD and a lithium
triborate (LBO) crystal, resulting in an optical-to-optical efficiency (ηo-o ) of 13.3% and M 2 ∼ 1.67 [6]. Georges et al. demonstrated a diode-end-pumped single frequency 561 nm laser
with an 808 nm LD and a potassium titanyl phosphate
(KTP) crystal, giving a 0.5 W cw yellow output with ηo-o ∼
18% and M 2 ∼ 1.1 [9]. In 2012, as high as 60 W of 561 nm laser
average output was acquired by acousto-optical Q-switching
and intracavity frequency doubling in an 808 nm diodeside-pump structure, with ηo-o ∼ 6.1% and M 2 ∼ 30 [7].
In this paper, we demonstrate a compact, diode-endpumped 561 nm laser with ∼2.3 W cw output power by using
an 885 nm LD and a diffusion bonded composite crystal. The
optical-to-optical efficiency with respect to the absorbed
pump power is ∼10.6%. To the best of our knowledge, the
cw output power represents the highest level ever reported
for diode-pumped frequency doubled Nd:YAG lasers
at 561 nm.
Figure 1 presents the diagram of the typical energy levels in
Nd:YAG crystal. To realize efficient yellow laser operation, the
first step is to obtain the fundamental infrared 1123 nm transition (the red line). However, there are two factors should be
considered. First, the stimulated-emission cross section (σ) of
the 1123, 1064, 1318, 1338, and 946 nm transitions are determined as 3.0, 45.8, 8.7, 9.2, and 5.1 × 10−20 cm2 , respectively
[12]. In this case, the other transitions with higher gain should
be suppressed to ensure the efficient oscillating of 1123 nm
line. A special coating design is an effective solution for this
problem [6,7]. Second, there are three lines in an extremely
narrow spectral range (∼7 nm), i.e., the 1112, 1116, and
1123 nm lines. Considering that the σ for these lines are
equivalent [12], it is very difficult to realize single line oscillating at 1123 nm simply by optical coating. To solve this question, a solid etalon is introduced into the resonator.
When an etalon is inserted into the resonator, the round-trip
phase change of the light beam can be given by [13]
· 2nd · cos θ;
where n is the refractive index of the etalon, λ is the wavelength of the light, d is the thickness of the etalon, and θ is
© 2013 Optical Society of America
J. Opt. Soc. Am. B / Vol. 30, No. 1 / January 2013
Gao et al.
the 1112 and 1116 nm lines can be restrained due to the higher
insertion losses. We note that, when θ 0.049 rad, the transmission for the 1123 nm line is about 100%, and moreover, the
transmissions for the 1112 and 1116 nm lines are about 85%
and 95%, respectively. Therefore, this angle can achieve low
insertion loss at 1123 nm under the premise of suppressing the
1112 and 1116 nm lines.
In terms of the pumping scheme, we believe that the
in-band pumping concept is one of the most efficient schemes
[14–16]. Seen from Fig. 1, the in-band pumping directly into
the 4 F3∕2 level changes the quantum defect ratio from 0.72
for the pump at 809 nm to 0.788 for the pump at 885 nm in
case of the 1123 nm oscillation. This induces a reduction of
the fractional thermal load by ∼25% for the efficient laser
emission at 1123 nm, from 0.28 to 0.21, which is beneficial
to power scaling of the fundamental 1123 nm line and also
the frequency doubled 561 nm laser [16].
Fig. 1. (Color online) Diagram of the energy levels of Nd:YAG with a
description of the main pumping and transition lines.
the incident angle. Therefore, the transmission can be
expressed as
T 1
· sin
1 − r2
where r is the reflectivity of the etalon surface. Consequently,
the transmission curves of the 1112, 1116, and 1123 nm lines
versus the tilt angle of the etalon can be calculated and shown
in Fig. 2. Typical parameters are listed as follows: n 1.46,
d 0.13 mm, and r 0.04. As can be seen from Fig. 2, the
three curves are clearly separated. This allows us to adjust
the inserting losses at the three lines, which is important
for the control of the corresponding net gain inside the cavity
and the consequential efficient single 1123 nm line oscillation.
By tuning the etalon to a proper angle, at which the transmission at 1123 nm is close to the peak value of the solid blue line,
Fig. 2. (Color online) Transmission curves for 1112, 1116, and
1123 nm lines varied with the tilt angle of the etalon.
The schematic diagram of the experimental setup is shown in
Fig. 3. A fiber coupled laser diode array [LDA, (DILAS, Inc.)]
was employed as the pump source, which delivers a maximum
output power of 40 W at 885 nm from the end of a fiber with a
core diameter of 400 μm and a NA of 0.22. The FWHM linewidth of ∼1.4 nm is favorable to achieve good absorption efficiency for the pump light due to an excellent overlap
between the absorption spectra of the gain medium and the
diode laser’s emission spectra. The pump radiation was injected into the gain medium through two coupling lenses with
a waist spot diameter of ∼400 μm. A diffusion bonded YAG/
Nd:YAG crystal, which consists of an 8 mm long, 1.0 at. %
Nd-doped part and a 4 mm long undoped end cap with a cross
section of 3 mm × 3 mm, was used as the gain medium. Both
surfaces of the crystal were well polished and coated for high
transmission (HT) at 1123 nm (T > 99.8%) and the pump
wavelength (T > 99%). When the pump wavelength was tuned
to match the absorption peak of the laser crystal, an average
absorption coefficient of ∼1.1 cm−1 was obtained at the maximum pump level. The frequency doublers were antireflection
(AR) coated at both 1123 and 561 nm to reduce the cavity loss.
A thermal-electric cooler with a stability of 0.1 °C was used
to control the temperature of both the laser crystal and the
frequency doubler. The laser output power was tested by a
power sensor L50(300)A and recorded by a power meter
VEGA (Ophir, Inc.). A thin solid fused silica etalon was inserted inside the laser cavity to enforce the laser operation
on the 1123 nm line. The etalon had a thickness d 0.13 mm and a refractive index n 1.46.
A V-type cavity insensitive to the thermal lens was designed
to favor the frequency doubling. The geometric lengths of arm
Fig. 3. (Color online) Schematic diagram of the experimental setup
of the 561 nm laser.
Gao et al.
Vol. 30, No. 1 / January 2013 / J. Opt. Soc. Am. B
L1 and L2 were selected to be 70 and 35 mm, respectively.
Mirrors M2 and M3 have radii of curvature of 50 and
400 mm, respectively. With the approximation of a thin lens
in the middle of the laser crystal, the mode radii in the laser
crystal and the LBO were calculated by an ABCD matrix formalism with M 2 ∼ 2 and determined as ∼180 μm and ∼80 μm,
respectively [17].
To suppress the more efficient transitions including 1064,
1318, 1338, 946 nm lines in the laser crystal and achieve efficient yellow laser output, a special coating for the cavity mirrors was accomplished. M1 was coated for high reflection
(HR) (R > 99.8%) at 1123 nm, HT at 1064 nm (T > 85%)
and 946 nm (T > 95%), and AR at 885 nm (T > 95%). M2
was coated for HR (R > 99.8%) at 1123 nm, and HT at
561 nm (T > 85%). M3 was coated for HR at 1123 and 561 nm
(R > 99.8%) and HT at 1318 and 1338 nm (T > 90%).
With respect to the high damage threshold, the small walk-off
angle, and the wide spectral and angular acceptance bandwidths with a modest effective nonlinear coefficient, LBO
seems to be the most promising candidate for the frequency
doubler of the high-power, high-brightness yellow laser [6,7].
Therefore, two different LBO crystals cut for type-I
critical matching were employed as the frequency doublers.
One is cut at θ 90° and φ 8.3° with dimensions of
3 mm × 3 mm × 10 mm; the other is cut at θ 90° and φ 7.5° with dimensions of 3 mm × 3 mm × 15 mm. The dependence of the yellow output power on the absorbed pump
power is shown in Fig. 4. The threshold of the yellow laser
was measured about 2.5 W. At an absorbed pump power of
∼21.8 W, the maximum yellow laser outputs of ∼2.3 W with
the 10 mm long LBO and ∼1.8 W with the 15 mm long LBO
were achieved, yielding optical-to-optical efficiencies versus
the absorbed pump power of ∼10.6% and 8.3%, respectively.
To the best of our knowledge, the 2.3 W cw yellow output represents the highest level ever reported in a diode-pumped
frequency doubled Nd:YAG laser at 561 nm.
The yellow output power P SHG at 561 nm as a function
of the absorbed pump power P abs was evaluated based on
the plane-wave model introduced by Smith [18]. With this
approach, the yellow output power can be written as [18–20]
P SHG πω21 2
Is − k i
1∕2 2
Li 2
4 2K c P abs − Li ;
Fig. 4. (Color online) CW output power of 561 nm laser versus the
absorbed pump power with different frequency doublers. The solid
blue line represents the modeling with loss Li ∼ 0.014. The inset is
the lasing spectrum of the 561 nm laser at the maximum output.
medium and ω2 in the LBO nonlinear crystal have been determined in Section 3. Based on the Findlay–Clay analysis [21],
the pump coupling coefficient K c and the resonator loss Li
were determined as ∼0.0079 W−1 and ∼0.014, respectively,
when using the 10 mm long LBO as the frequency doubler.
From Fig. 4, it can be seen that the agreement between the
experimental result (the red line, squares) and the model
(the solid blue line) is good.
A fiber spectrometer (HR4000, Ocean Optics, Inc.) with a
resolution of ∼0.24 nm was used to monitor the laser spectrum. The inset of Fig. 4 presents the optical spectrum of
the frequency doubled laser at the highest output power. It
is noticed that, thanks to the special coating design and the
inserted etalon, only one single laser line at 561 nm can be
The transverse spatial profile and the beam propagation
factors of the yellow laser beam were also measured with
where I s is the saturation intensity of the 1123 nm transition in
the gain medium. The parameter k is the effective nonlinearity
of the LBO crystal:
4π 2 ω21 d2eff l2c
Z 0 2 3 β;
ω2 n
where Z 0 377 Ω is the vacuum impedance, lc denotes the
LBO crystal length, n ∼ 1.6 is the refractive index, deff 0.84 pm∕V is the nonlinear coefficient of LBO, and β is a factor
that accounts for the phase mismatch between the fundamental wavelength λω and the second harmonic wavelength in
the second pass of LBO. The laser beam radius ω1 in the gain
Fig. 5. (Color online) Beam quality measurements of the 561 nm
laser under the maximum output. The inset is its three-dimensional
intensity profile.
J. Opt. Soc. Am. B / Vol. 30, No. 1 / January 2013
Fig. 6. (Color online) Stability of the yellow laser power at the maximum output level.
a M 2 -200s-FW (Ophir–Spiricon, Inc.) at the maximum output
power (see Fig. 5). The beam propagation factors M 2 were
found to be about 2.0 and 1.7 in the X and Y directions,
respectively. Considering that the Nd:YAG single crystal
is an optically isotropic material, we assume that the astigmatism was mainly ascribed to the folded angle in the cavity and
the thermally induced birefringence in the gain medium [22].
Using the StarLab2.40 software from Ophir, Inc., We tested
the short-term power stability of the 561 nm laser at the maximum output power. The power fluctuation value was calculated according to the standard deviation divided by the mean
value of the yellow output. Figure 6 depicts the power fluctuation of the yellow laser, which was determined to be ∼5.6%
within the given 20 min.
In summary, we have reported a compact diode-end-pumped
frequency doubled Nd:YAG/LBO laser at 561 nm. By employing an 885 nm LD and a diffusion bonded Nd:YAG crystal,
efficient power scaling was achieved. With an absorbed pump
power of ∼21.8 W, we obtained up to ∼2.3 W of cw output
power at 561 nm. This represents, to the best of our knowledge, the highest cw output power from a diode-pumped
solid-state frequency doubled Nd:YAG laser at 561 nm so far.
This work was supported by the Knowledge Innovation
Program of the Chinese Academy of Sciences (CAS) under
contract KGCX2-YW-910, the Industry-Academic Joint
Technological Innovations Fund Project of Jiangsu Province
with grant BY2011183, and a grant from the Youth Innovation
Promotion Association, CAS.
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