Download Efficient yellow-green light generation at 561 nm by frequency

6672
OPTICS LETTERS / Vol. 39, No. 23 / December 1, 2014
Efficient yellow-green light generation at 561 nm
by frequency-doubling of a
QD-FBG laser diode in a PPLN waveguide
Ksenia A. Fedorova,1,* Grigorii S. Sokolovskii,2,3 Maksim Khomylev,4 Daniil A. Livshits,4 and Edik U. Rafailov1
1
School of Engineering and Applied Science, Aston University, Aston Triangle, Birmingham B4 7ET, UK
2
Ioffe Physico-Technical Institute, 26 Polytechnicheskaya Str., St. Petersburg 194021, Russia
3
Saint-Petersburg State Polytechnical University, St. Petersburg 195251, Russia
4
Innolume GmbH, 11 Konrad-Adenauer-Allee, Dortmund 44263, Germany
*Corresponding author: [email protected]
Received September 1, 2014; revised October 20, 2014; accepted October 24, 2014;
posted October 24, 2014 (Doc. ID 222071); published November 24, 2014
A compact high-power yellow–green continuous wave (CW) laser source based on second-harmonic generation
(SHG) in a 5% MgO doped periodically poled congruent lithium niobate (PPLN) waveguide crystal pumped by
a quantum-dot fiber Bragg grating (QD-FBG) laser diode is demonstrated. A frequency-doubled power of
90.11 mW at the wavelength of 560.68 nm with a conversion efficiency of 52.4% is reported. To the best of our
knowledge, this represents the highest output power and conversion efficiency achieved to date in this spectral
region from a diode-pumped PPLN waveguide crystal, which could prove extremely valuable for the deployment
of such a source in a wide range of biomedical applications. © 2014 Optical Society of America
OCIS codes: (140.3515) Lasers, frequency doubled; (230.4320) Nonlinear optical devices; (230.7370) Waveguides;
(140.5960) Semiconductor lasers; (250.5590) Quantum-well, -wire and -dot devices.
http://dx.doi.org/10.1364/OL.39.006672
Compact continuous wave (CW) lasers in the visible
spectral region at around 560 nm are attractive for a variety of cutting-edge applications in many diverse fields.
In particular, such lasers have already found valuable applications in cosmetic dermatology for photodynamic
rejuvenation therapy [1] and cancer phototherapy [2,3]
for a selective treatment of cancer cells without toxic repercussions on normal cells by driving a controlled photorelease of drugs with the 561 nm irradiation. The
yellow–green lasers of this wavelength also play an important role in ophthalmology [4] and are ideal for treatments around the macula because of the high absorption
in hemoglobin and a negligible absorption by melanin in
the retinal pigment epithelium and macular xanthophyll.
Furthermore, the lasers near 561 nm have found an application in flow cytometry [5,6], Laser Doppler velocimetry [7], and confocal microscopy [8], and are used to
excite phycoerythrin and its tandem, as well as red fluorescent proteins such as mCherry, mStrawberry, DsRed,
and dTomato. Recently, such lasers have also attracted
attention for their application in interferometry [9] and
spectroscopy [10].
Since the yellow region of the optical spectrum, where
a range of important fluorescent proteins are optimally
excited, remains inaccessible to conventional diode lasers, frequency-doubling and sum-frequency generation
in second-order nonlinear crystals using infrared laser
sources are traditionally used. Several systems generating yellow–green light at ∼561 nm wavelength were recently reported. In particular, CW laser sources at
561 nm using frequency doubling of an Nd:YAG laser
in an LBO crystal have been demonstrated with output
power of 1.2 W [11], 2.3 W [4], and 60 W [12], and optical-to-optical conversion efficiency of 13.3%, 10.6%, and
6.1%, respectively. Second harmonic generation (SHG) at
561 nm using an Nd:YAG laser and a KTA crystal with
0146-9592/14/236672-03$15.00/0
output power of 55 mW [13] and 2.6 W [14], and corresponding conversion efficiency of 23% and 8.7%, has also
been reported. Output power of 0.5 W [7] and 8.1 W [15],
and optical-to optical efficiency of 17.9% and 5.5%, respectively, from frequency doubled Nd:YAG/KTP lasers
at 561 nm has been recently obtained. Up to 570 mW
of output power at 561 nm with an optical-to-optical conversion efficiency of 3.3% has also been achieved from an
Nd:KGdWO4 2 self-Raman laser by intracavity sum-frequency mixing of fundamental and first-Stokes wavelengths in an LBO crystal [16]. Recently, Yang et al.
demonstrated a laser source at 561 nm with output
power of 10 mW and conversion efficiency of 2% by
frequency doubling of an Nd:YAG monoplane laser in
a PPLN [10].
An attractive way to develop a compact, efficient, and
low-cost yellow laser source is SHG in a periodically
poled nonlinear crystal containing a waveguide [17]
which because of the high pump light intensities over
a long interaction length allows highly efficient frequency
conversion even at low and moderate pump power levels. In this respect, a PPLN waveguide crystal, which offers high nonlinearity along with strong confinement of
light, is one of the best candidates for highly efficient
nonlinear wavelength conversion. Recent progress in
the fabrication of good quality waveguides in PPLN crystals in combination with the availability of low-cost,
highly efficient semiconductor diode lasers allows compact CW laser sources in the visible spectral region to be
realized [18]. Today, the emphasis in laser technology has
moved to novel quantum-dot (QD) lasers which offer the
flexibility in choice of semiconductor material compounds allowed for a wide spectral range to be covered.
Laser sources based on InAs/GaAs QD structures, offering the coverage of a broad wavelength range between
1 and 1.3 μm [19,20], are especially useful in this regard.
© 2014 Optical Society of America
December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS
6673
In recent years, several laser sources generating visible
light in periodically poled lithium niobate waveguide
crystals have been demonstrated. In particular, blue light
generation with output power up to 189 mW [21] and conversion efficiency up to 52% [22] by SHG in a PPLN waveguide has been obtained. Frequency doubling in an MgO:
PPLN waveguide was also used to demonstrate the
generation of green light at 530 nm [23] and 556 nm [24]
with output power of 107 and 11.8 mW and conversion
efficiency of 51% and 52.5%, respectively. Generation
of yellow light at 575 [25] and 578 nm [26] in a PPLN
waveguide with output powers of 40 and 10 mW, and
conversion efficiencies of 7% and 29.6%, respectively,
has been reported. The output power of 494 mW with
overall conversion efficiency of 41% at 589 nm by
sum–frequency generation in a Zn:PPLN ridge waveguide
pumped by two Nd:YAG lasers at 1064 and 1319 nm was
also recently obtained [27]. However, to date, only 32 mW
of yellow–green emission at 560 nm with conversion efficiency of 32% generated by SHG from a broad area laser
diode and PPLN waveguide has been demonstrated [28].
Here, we report on a compact frequency-doubling
scheme generating over 90 mW of CW yellow–green light
at 561 nm with a conversion efficiency in excess of 52% in
the periodically poled lithium niobate waveguide pumped
by a quantum-dot fiber Bragg grating (QD-FBG) laser.
The experimental setup consisted of a QD-FBG pump
laser (Innolume GmbH) operating at ∼1122 nm and a
PPLN waveguide crystal (as illustrated in Fig. 1). The
QD-FBG pump laser consisted of a 6 mm long gain chip
with a ridge waveguide normal to the facets. An active
region of the chip was similar to that described in [20]
and contained InAs QD layers, incorporated into
Al0.35 Ga0.65 As cladding layers and grown by molecular
beam epitaxy (MBE). The front facet of the gain chip
was anti-reflective (AR) coated guaranteeing the back reflection below 0.03%, and the back facet had high reflective coating. The gain chip was embedded in a 14-pin
butterfly package with the laser output coupled into a
single-mode polarization maintaining fiber PM-980. The
external cavity was closed with the fiber Bragg grating
(FBG) of 10% peak reflection at 1122 nm, which was located in 100 cm from the chip. The QD-FBG laser was
mounted on a copper heatsink, and its temperature
was controlled by a thermo-electric cooler. The temperature of the laser was set at 25°C with a wavelength of
1121.4 nm. The maximum output power of the QD-
FBG laser diode used in our experiments was 350 mW
(limited mainly by the thermal rollover). The output of
the laser was collimated with a 30× (NA ∼ 0.50) ARcoated aspheric lens and then coupled into the PPLN
waveguide using a 40× (NA ∼ 0.55) AR-coated aspheric
lens with ∼50% laser-to-waveguide coupling efficiency.
A half-wave plate was used to adjust the polarization
for optimal SHG in the PPLN crystal. The frequencydoubled output was then collimated by a 30× (NA∼
0.50) AR-coated aspheric lens onto a power meter after
a suitable filter at the fundamental wavelength.
The frequency-doubling crystal waveguide used in this
work was a 5% MgO-doped Y-cut congruent lithium niobate (HC Photonics Corp) which was periodically poled
for SHG at ∼1122 nm. The crystal facets were optically
polished at ∼5.4° and had an AR-coating for 1122 nm
(R < 0.5%) and 561 nm (R < 1%) on both input/output
facets. The PPLN crystal was 10 mm in length and contained a channel waveguide with a cross-sectional area of
∼4 × 5 μm2 which was covered by an additional lithium
niobate layer on top of the waveguide to support a more
symmetric mode profile. For quasi-phase-matching, the
temperature of the crystal was stabilized at 25°C.
The measured second harmonic output power and
conversion efficiency as functions of the launched pump
power are shown in Fig. 2. The output power is quadratically increased as pump power increases, as expected.
The maximum frequency-doubled power of 90.11 mW
at the wavelength of 560.7 nm from 172 mW of pump
power was obtained. This corresponded to an opticalto-optical conversion efficiency of 52.4% and a wall-plug
efficiency of 4.3%. To the best of our knowledge, the demonstrated output power and conversion efficiency are the
highest at this wavelength reported until now from a frequency-doubled diode laser in a PPLN waveguide crystal.
The obtained maximum SHG output power was only limited by the available pump power from the QD-FBG laser.
No degradation of the nonlinear waveguide crystal was
observed in our experiments (although, no long-term
stability tests were performed). The experimental results
Fig. 1. Simplified schematic of the experimental setup (including quantum-dot fiber bragg grating (QD-FBG) laser, aspheric
lenses, half-wave plate (λ∕2), 5% MgO doped periodically poled
congruent lithium niobate waveguide crystal (PPLN), and filter
at 1122 nm.
Fig. 2. Dependences of the frequency-doubled output power
and conversion efficiency on the launched pump power. The
curves are the numerical tanh2 -fit of the experimental data according to the depleted pump approximation with normalized
conversion efficiency of 500%/W.
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OPTICS LETTERS / Vol. 39, No. 23 / December 1, 2014
Fig. 3. Optical spectrum of the second harmonic generated
output at 560.7 nm. Inset: the optical spectrum of the pump
QD-FBG laser (1121.4 nm).
were numerically fitted according to the depleted pump
approximation with normalized conversion efficiency of
500%/W (Fig. 2).
The optical spectrum of the yellow–green laser emission at 560.7 nm was measured with an Ocean Optics
spectrometer (Fig. 3) and exhibited a spectral width of
around 0.5 nm (limited only by the instrumental resolution of the spectrometer used). The inset of Fig. 3 depicts
the measured optical spectrum of the pump QD-FBG laser at 1121.4 nm with a spectral width of ∼0.3 nm, which
was dominated by the resolution limit of the optical spectrum analyzer (OSA Advantest Q8384) used to monitor
the laser spectrum. No degradation of the beam quality
of the yellow–green light was observed in the whole
range of output powers.
To summarize, a highly efficient frequency-doubling of
a CW QD-FBG laser was demonstrated using a 5% MgOdoped PPLN waveguide crystal. Yellow–green CW light
at 561 nm with more than 90 mW of output power was
generated with conversion efficiency in excess of 52%.
To the best of our knowledge, this is the highest output
power and conversion efficiency at the wavelength
around 560 nm reported until now from a frequencydoubled diode laser in a PPLN waveguide crystal. Compactness, simplicity, high power, and relatively low cost
make this yellow–green laser well suited as a source for
many emerging biomedical applications. The work on
further improvement of SHG output power and conversion efficiency is currently underway.
This work was partly supported by the EU FP7 programme through the FAST-DOT project (contract
no. 224338) and RFBR grant no. 14-02-01160. The authors
wish to acknowledge HC Photonics Corp. for the fabrication of the PPLN waveguide crystal.
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