Guided-wave second harmonics in Nd:YCOB optical
waveguides for integrated green lasers
Yingying Ren1, Yuechen Jia1, Ningning Dong1, Lilong Pang2, Zhiguang Wang2, Qingming Lu3, and Feng Chen1,*
1School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and Particle Irradiation (MOE),
Shandong University, Jinan 250100, China
2Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 73000, China
3School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
*Corresponding author: firstname.lastname@example.org
Received November 9, 2011; revised November 28, 2011; accepted November 29, 2011;
posted November 30, 2011 (Doc. ID 157943); published January 13, 2012
We report on guided-wave second-harmonic generations in nonlinear Nd:YCa4O?BO3?3(Nd:YCOB) optical wave-
guides that are produced by the low-fluence swift Ar8?ion irradiation. The guided-wave second harmonics are
realized through the frequency doubling and the self-frequency-doubling of the waveguides under the optical
pumps at wavelengths of 1064 and 810 nm, respectively. By virtue of the self-frequency-conversion configuration,
the Nd:YCOB waveguides are promising candidates as novel, compact, miniature green laser sources.
tical Society of America
OCIS Codes:230.7390, 190.4390, 140.3515.
© 2012 Op-
The on-chip integration of diverse photonic components
is intriguing for the exciting possibilities of realization of
enhanced functionality in compact and robust circuits
[1,2]. Particularly, there is strong interest on the de-
velopment of integrated green laser systems on single,
miniature photonic chips [3,4]. For the generation of
nonlinear crystal” (e.g., Nd:YVO4? KTiOPO4), a concei-
vable solution is to use self-frequency-doubling crystal
. By virtue of enhanced light intensity inside the wave-
guide structures, some performances relevant to the
bulks may be considerably improved, which may result
in the occurrence of various nonlinear processes or laser
actions at low light powers [1,2,6,7]. With the combina-
tion of the advantages of waveguide geometry and the
excellent features conferred by the bulk materials, it is
possible to achieve highly efficient miniature photonic
devices. Compared with the configuration of distributed
feedback (DFB) or a distributed Bragg reflector laser
plus periodically poled lithium niobate (PPLN) nonlinear
waveguides for compact green laser generation, the
self-frequency-doubling (SFD) waveguide laser system
may possess the possibility of higher-grade integration
[8,9]. Neodymium-doped yttrium calcium oxyborate
[Nd:YCa4O?BO3?3or Nd:YCOB] is an attractive crystal
with a combination of excellent fluorescence features
of incorporated Nd3?ions and the nonlinear properties
of a YCOB matrix, which is intriguing for the green light
generation through the SFD process [10–12]. Such an in-
tracavity nonlinear process of the generated IR lasers
from the activations of the Nd3?ions in the YCOB host
matrix produces the green laser oscillations without re-
quirement of additional nonlinear optical components.
Benefiting from the advantages of the waveguide’s com-
pact geometry as well as the excellent features of the
Nd:YCOB crystals, the Nd:YCOB waveguides may offer
a unique platform to realize the self-generated green
integrated lasers by the simple optical pump of laser
diodes. To achieve this purpose, the optical waveguiding
structures supporting both TE and TM modes [for re-
quirement of the birefringent phase matching of funda-
mental and second-harmonic (SH) waves] must be fabri-
cated. Ion irradiation/implantation is a powerful techni-
que to fabricate optical waveguides in a wide range of
materials [13–19]. High-energy heavy ions modify the re-
fractive indices of the substrates through the electronic-
excitation-induced damages. The Nd:YCOB waveguide
has been produced by the low-fluence irradiation of swift
Ar8?ions, and efficient planar waveguide lasers at
1061 nm has been realized . In this Letter, we report
on nonlinear performance of the Nd:YCOB waveguide by
focusing on the second-harmonic generation (SHG) for
miniature green lasers in the developed nonlinear system
through the guided-wave frequency doubling (FD) and
The 7.7 mm long Nd:YCOB (doped by 5 at. % Nd3?
ions) wafer used in this Letter is cut along the direction
to satisfy the 1061 nm fundamental wave to 531 nm SHG
(with θ ? 150.3°, φ ? 0°). The 170 MeV Ar8?ion irradia-
tion at an ultralow fluence of 2 × 1012cm−2is performed
on the sample surface, producing a buried planar wave-
guide layer (∼34 μm thick) with positive index changes
of Δn ∼ 3 × 10−3. The nonlinear performances of the
waveguides are characterized by using an end-face cou-
pling system. For the FD experiments of the Nd:YCOB
waveguide, both CW and pulsed laser beams at wave-
length of 1064 nm, as the fundamental wave, are coupled
into the waveguide by a convex lens with focal length
f ? 25 mm, respectively. The pulsed laser produces
∼80 μJ pulses with width of 11.05 ns at a repetition of
∼5kHz. The light emerging from the output of the wave-
guide is captured using a microscope objective lens
(N.A. ? 0.4). To separate the laser from the pump beam,
a few mirrors, which are antireflective at the waveguide
laser wavelength and highly reflective at the pump wave-
length, are placed during the light path after the outcou-
pling objective. The SFD of the Nd:YCOB waveguide is
realized by using similar optical system, using a CW po-
larized beam as pump source [at approximately 810 nm
from a Ti:sapphire laser (Coherent MBR 110)]. The laser
cavity consists of an input mirror highly reflective (HR)
at around 1061 nm and highly transmittive (HT) at about
244OPTICS LETTERS / Vol. 37, No. 2 / January 15, 2012
0146-9592/12/020244-03$15.00/0© 2012 Optical Society of America
810 nm and an output mirror HR at ∼1061 and 810 nm and
HT at ∼530nm. The generated green light is separated
from the 1061 nm emission and the pump 810 nm light
detected by using appropriate filters in front of the
spectrometer. The transmitted light signals are imaged
by CCD and characterized by the spectrometer and
For the FD experiments of the Nd:YCOB waveguide,
the fundamental waves are found to be TE polarized with
zero-order mode, and the corresponding output waves
are with the first TM mode, which means that the SHG
from 1064 → 532 nm is corresponding to the TE0ωto
TM02ωconversion process. Figure 1(a) shows the SH
green laser power as a function of the absorbed funda-
mental wave power under the CW configuration. The ex-
perimental data (in solid circles) are well fitted by the
curve function (solid line). The maximum SH light power
reaches 0.9 mW at 504 mW of pump power. The effi-
ciency of the mode-to-mode conversion η is estimated
to be 0.35% W−1. Under the pulsed 1064 nm laser pump,
the maximum output power (peak power) of the pulsed
green light is ∼4.4 W with pump power of ∼3.6 kW, lead-
ing to a conversion efficiency of η ? 0.12% [see Fig. 1(b)
for the dependence of the SH power on that of the funda-
mental wave]. The inset of Figs. 1(a) and (1b) show the
spectra of output light under the CW and pulsed laser
pumping, respectively, from which we can clearly see
that, in both cases, the wavelength of generated emission
is centered at ∼532 nm. It should be noted that the SHG
conversion efficiency of the waveguide is relatively low,
which may be due to the modification of the lattices
(resulting in a certain degradation of the nonlinear
properties) in the waveguide region induced by the ion
irradiation and the considerable absorption of Nd:YCOB
crystalsat 532nm (with
cient of ∼0.5 cm−1) . Nevertheless, the capability of
the Ar8?ion irradiated Nd:YCOB waveguides is com-
parable with those of the ion implanted nonlinear wave-
For the realization of the intracavity SFD waveguide
lasers, we use the 810 nm CW laser as the optical pump.
The linearly polarized pump beam is coupled into the
sample, exciting the TE0mode of the waveguide. It
has been found that the fundamental wave (i.e., the gen-
erated IR light at 1.06 μm) is with the same polarization of
the pump beam, while the SFD green laser is with TM
polarization. This is also correlated to the TE0ωto
TM02ωconversion. Figure 2 shows the generated SFD
green laser spectrum of the 170 MeV Ar8?ion irradiated
Nd:YCOB waveguide. As one can see, the output la-
ser spectrum is peaked at ∼530.8 nm, which is in good
agreement with the SFD green lasers from the bulk gain
medium. The modal profile of the output SFD green
waveguide laser is shown in Fig. 2 as an inset, which con-
firms that the generated laser emission is with the
Figure 3 depicts the SFD laser power at ∼530.8 nm as a
function of absorbed pump power at ∼810 nm of the
Nd:YCOB waveguide. The maximum output power ob-
tained from the intracavity SFD is ∼36 μW under the
pump power of ∼65mW, denoting an optical conversion
efficiency of η ? 0.85% W−1from the 810 nm pump light
to 531 nm SFD light. The Nd:YCOB SFD green waveguide
laser is comparable to the newly developed Nd:YAB
waveguide system  while is much better than the early
reported Nd:LiNbO3waveguides . Compared with the
DFB ? PPLN waveguide laser system, the obtained SFD
waveguide laser in the present Letter is much lower [8,9];
however, one could expect performance improvement of
the SFD green waveguide laser of the Nd:YCOB system
guide under (a) CW and (b) pulsed laser pumping. Insets show
the corresponding spectra of the output SH laser radiations.
(Color online) FD output power of Nd:YCOB wave-
generated from the Nd:YCOB waveguide in the SFD experi-
ment. The inset shows the near field distribution of the output
SH laser centered at a wavelength of ∼530.8nm.
(Color online) Emission spectrum of the green laser
January 15, 2012 / Vol. 37, No. 2 / OPTICS LETTERS 245
by further optimizing the two-dimensional guiding Download full-text
structure (such as ridge waveguides) instead of the
In summary, we have realized the guided-wave FD and
SFD of the Nd:YCOB waveguide produced by the low-
fluence 170 MeV Ar8?ion irradiation with the guidance
of both TE and TM polarized light. The SHG of the CW
green light through the intracavity frequency conversion
shows advantage over the extracavity FD. The future
work would be focused on the optimization of the
Nd:YCOB waveguide system in order to achieve a highly
efficient compact SFD green waveguide laser source.
This work is supported by the National Natural
Science Foundation of China (10925524) and the 973 pro-
ject (2010CB832906 and 2010CB832902) of China.
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waveguide laser as a function of absorbed pump power at
(Color online) Output power at ∼530.8nm of the SFD
246OPTICS LETTERS / Vol. 37, No. 2 / January 15, 2012