# High-Efficiency Intracavity Continuous-Wave  Green-Light Generation by Quasiphase Matching in  a Bulk Periodically Poled MgO:LiNbO3 Crystal

Authors: Shaowei Chu; Ying Zhang; Bin Wang; Yong Bi
Journal: Advances in OptoElectronics
 (2008)

---

## Abstract

908 mW of green light at 532 nm were generated by intracavity quasiphase matching in a bulk periodically poled MgO:LiNbO3 (PPMgLN) crystal. A maximum optical-to-optical conversion efficiency of 33.5% was obtained from a 0.5 mm thick, 10 mm long, and 5 mol% MgO:LiNbO3 crystal with an end-pump power of 2.7 W at 808 nm. The temperature bandwidth between the intracavity and single-pass frequency doubling was found to be different for the PPMgLN. Reliability and stability of the green laser were evaluated. It was found that for continuous operation of 100 hours, the output stability was better than 97.5% and no optical damage was observed.


---

## Body

## 1. Introduction

Compact and efficient  green laser
light sources have  numerous applications such as laser displays, material
processing, biological investigations, and optical communications. There are many methods to achieve coherent
green light; however, second harmonic generation (SHG) by the
quasi-phase-matching (QPM) technique has been an attractive method to obtain
compact and high-efficiency laser [1]. The QPM technique based on
periodically poled lithium niobate (PPLN) has significant advantages including
phase matching of an arbitrary wavelength by the use of an appropriate period
of polarization inversion and a higher nonlinear coefficient than KTP, LBO, and
BIBO.So far, the single-pass SHG scheme
is a popular solution for achieving CW green laser light. However, this scheme
requires a high nonlinear coefficient and a long interaction length to achieve
high conversion efficiency, which can be satisfied by  employing PPLN
crystals. CW green power of 2.7 W has been obtained in a 50 mm long PPLN single-pass crystal pumped by a 6.5 W Nd:YAG laser [2]. Due to its higher photorefractive damage
threshold and lower green-induced infrared absorption as compared with PPLN [3],
periodically poled
MgO:LiNbO3 (PPMgLN) has replaced the PPLN. A maximum power of 1.18 W at 531 nm with 16.8%
conversion efficiency has been obtained from a 2 mm thick, 25 mm long PPMgLN single-pass crystal pumped by a 7 W Nd:GdVO4 laser [4]. Periodically
poled Mg-doped stoichiometric lithium tantalate (PPMgSLT) is usually used as an alternative material for high-power
generation. 7 W of SHG green light with 35.4% conversion efficiency in a 2 cm long
PPMgSLT single-pass crystal pumped by a 19.6 W, 1084 nm Yb-doped fiber laser have recently been
reported [5].Intracavity second harmonic generation
(ISHG) of Nd-doped lasers has always been an attractive method for producing
green light [6]. Second
harmonic generation in a bulk PPLN crystal was demonstrated using the
intracavity scheme for the first time in 1995 [7]. In a later
experiment, 53μW of green ISHG at 541 nm were generated with a pump power of
300 mW, indicating 0.02% optical-to-optical conversion efficiency in 1997 [8]. A maximum output power of 740 mW of blue light has also been
generated with an optical-to-optical efficiency of 5.7% at a pump power of
13.5 W [9].In this paper, we report highly efficient continuous-wave
green-light generation based on intracavity frequency doubling, in a quasi-phase-matched
PPMgLN bulk crystal. With an end-pump power of 2.7 W at 808 nm, a
maximum green output power of 908 mW at 532 nm is achieved with a high
optical-to-optical conversion efficiency of 33.5%.


## 2. Experiments

The experimental setup of a high-efficiency
CW, laser-diode (LD), and end-pumped green laser with an intracavity SHG scheme is shown schematically in Figure1. YVO4 doped with 1% Nd with a size of 3×3×3mm3 was used as the gain
medium. A 0.5 mm
thick, 10 mm long, 2 mm wide, and 5 mol% PPMgLN crystal provided by the C2C Link Corporation, Canada, was used as a frequency doubler. Both
sides of the PPMgLN crystal were antireflective (AR), coated at 532 nm and 1064 nm. Temperature of the PPMgLN was controlled
by a thermoelectric cooler (TEC).Figure 1
Experimental setup used for ISHG.The laser cavity consisted of a
high-reflection (HR) coating at 1064 nm and AR coating at 808 nm on the pumping
side of the Nd:YVO4 crystal, as well as a 50 mm radius of curvature mirror (M) with HR coating at 1064 nm and AR coating at
532 nm, which were used, respectively, for folding the fundamental laser beam
and for the second harmonic output. The
other side of the Nd:YVO4 crystal
was AR-coated at 1064 nm  and HR-coated at 532 nm. The optical end-pump was a CW-2.7 W-laser diode whose end-face was imaged into the pump side of the Nd:YVO4 crystal by a graded index lens
(GRIN). The GRIN with a size of 1.8×5(R×L)mm3 was AR-coated at 808 nm.


## 3. Experimental Results

The CW green laser output power versus the pump power is shown in Figure2. To obtain the data, the 808 nm pump
LD power was varied by changing the injection current of the LD. When the pump power was up to 2.7 W, the green laser delivered  908 mW with 33.5% optical-to-optical conversion efficiency at 35.2°C. To the best of our knowledge, this is the
highest optical-to-optical conversion efficiency reported to date for a green
laser at low pump power (<5 W). This conversion efficiency is even higher
than that obtained for CW single-pass SHG in a bulk PPMgLN with a 1064 nm pump
power of 7 W [4]. It is worth
noting that the AR coating of the PPMgLN crystal has not been optimized since
multiple beam spots were observed in the far-filed pattern of the green laser.
We believe that the efficiency can further be enhanced by optimizing the AR
coating conditions.Figure 2
532 nm green output power and the optical-to-optical
conversion efficiency versus 808 nm input power at phase-matching temperature.In order to measure the uniformity of the
PPMgLN crystal at
different transverse positions,
we shifted the PPMgLN
crystal
transversely when the temperature was set at the phase-matching temperature,
while all other conditions are held constant.
As shown in Figure3, change of the green output power is less than 3%,
indicating high uniformity of the PPMgLN crystal. Therefore, it is possible that several beams
can pass a single PPMgLN crystal at the same time, which can further enhance the optical-to-optical conversion efficiency.Figure 3
532 nm output power versus transverse position of PPMgLN crystal at an optical input power of 2.7 W.Maximum green output power can be obtained at
the phase-matching temperature for PPMgLN crystal. In the previous reports, the
single-pass phase-matching temperature has been investigated, but the phase-matching
temperature of the PPMgLN in a laser cavity has not been reported. In this
paper, the phase-matching temperature of the PPMgLN in the laser cavity was investigated
by changing the crystal temperature. As shown
in Figure4, the phase-matching temperature of PPMgLN intracavity frequency
doubling is 35.2°C, and the output power is very sensitive to
the crystal temperature. The temperature
change of 1°C could cause nearly 100 mW output drop. In contrast, in the single-pass scheme, the
phase-matching temperature is 38°C, which is 2.8°C higher than that in the intracavity scheme.Figure 4
532 nm output power versus temperature for SHG
at an optical input power of 2.7 W.To evaluate the reliability and
stability of our green laser, continuous operation for 100 hours was carried
out. During that period, no drop in green output
power was observed, implying that the photorefractive damage is negligible in
our experiments. As shown in Figure5,
the change of the green output power is less than 2.5% for 100 hours,
indicating that the PPMgLN crystal is a practical material to use in generating
stable green laser light. Longer-time
experiment for evaluating the stability of the PPMgLN crystal is in process.Figure 5
532 nm output power versus work time for ISHG at
phase-matching temperature.


## 4. Conclusions

CW power of 908 mW at 532 nm with
33.5% optical-to-optical conversion efficiency has been
obtained from a 0.5 mm thick, 10 mm long PPMgLN crystal in an intracavity
frequency-doubling scheme. It has been shown that the efficiency of the
intracavity scheme can be much higher than that of single-pass frequency-doubling scheme, indicating
that we can obtain higher efficiency and power by employing the intracavity
scheme. At the phase-matching
temperature, the output power has shown stable operation for more than 100 hours, and the change of the output power is less than 3% at the different transverse
positions of the PPMgLN. The experiment results clearly
indicate that practically compact and highly efficient green lasers can be realized based on bulk PPMgLN crystals if the uniformity of the
crystal is high. We expect that optical-to-optical
conversion efficiency of more than 40% could be achieved if the AR coating of
the PPMgLN crystal is improved.



---

*Source: 151487-2008-10-30.xml*