Transcript SorgentiLaser_Napoli - Istituto Nazionale di Ottica

Nonlinear generation of radiation by periodically poled
LiTaO3 crystals
I. Ricciardi, M. De Rosa, A. Rocco, P. Ferraro, P. De Natale
CNR-INOA, Istituto Nazionale di ottica Applicata, Sezione di Napoli
A. Vannucci, P. Spano
Andromedra s.r.l
In the field of nonlinear frequency generation Lithium tantalate (LiTaO3) represents an interesting and suitable choice for high power applications in the visible and IR
range: its nonlinear coeffcient is comparable to KTP one, its transparency window extends in the UV region, it is less sensitive to absorption respect to other similar
crystals and shows a high resistence against photorefractive damage. Moreover quasi phase-matching (QPM) in periodically poled ferroelectric crystals has become a
well assesed and versatile technique for efficient nonlinear generation. By the use of periodically poled Lithium Tantalate we have first realized a mW level CW laser
source by Sum Frequency Generation at 355 nm in single pass configuration and then tested a cavity enhanced Second Harmonic Generation with conversion
efficiencies from 50% to 80%.
Single pass UV generation
Intracavity Second Harmonic Generation
We tested an experimental configuration where SHG is generated in a first crystal,
starting from a fiber laser emitting up to 10 W at 1064 nm; then the resulting radiation
at 532 nm and the remaining radiation at the fundamental frequency are sent to a
second crystal for sum frequency generation (SFG) at 355 nm. The experimental set
up including both non linear stages is shown below.
532 nm
355 nm
The cavity is made by four mirrors in a bow-tie configuration, with two curved mirrors
M1 and M4, 200 mm radius of curvature, and two plane mirrors M2 and M3.
Relevant parameters:
 total cavity length : 1.22 m
 free spectral range : 245 MHz
 waist in the crystal: 60 μm
 locking scheme: Hänsch–Couillaud
 input mirror M1 reflectivities: 88%, 94%, 97%
 mirrors M2, M3, M4 reflectivities: 99.9 %
1064 nm
1064 nm
HWP: half wave plate
PBS: polarizing beam splitter
OSA:Opticcal Spectrum Analyzer
L1,L2: collimation lenses
SHG, SFG: non linear crystal
DM: dichroic mirror
PD: photodiode
IF: interference filter
PZT
Bow-tie cavity
Fig. 1: Schematic for non linear generation at 355 nm: SHG represents the stage
for Second Harmonic while SFG respresents the stage for Sum Frequency
Generation
 Single
pass Second Harmonic Generation
OI: Optical Isolator
L1,L2: collimation lenses
SM: steering mirror
M1: input/output cavity mirror (R= 200mm)
M4: concave cavity mirror (R=200mm)
M2, M3: plane cavity mirror
We performed single pass SHG using two different periodically poled crystals:
BS: beam splitter
PBS: polarizing beam splitter
QWP: quarter wave plate
PD1,2: photodiode
PZT : piezoelectric actuator
 z-cut stochiometric LiTaO3 crystal, 1% MgO-doped, 30 mm long, 1 mm thick, with
a 7.97 μm period.
 z-cut congruent LiNbO3, 5% MgO doped, 30 mm long, 0.5 mm thick, with a 6.92
µm period.
Crystal
PPMgOLiTaO3
PPMgOLiNbO3
Maximum pump
power (W)
8.5
8.5
Maximum second
harmonic (W)
1.4
2.6
Efficiency
( % W-1 cm-1)
0.65
1.2
Photo of the cavity
for second harmonic
generation.
Bow tie cavity
Second
harmonic
power at 532 nm as a
function of the pump
power for LiNbO3 and
LiTaO3 crystals
Second harmonic power as a function of the pump
power for the three different input cavity mirrors
 Single
Second harmonic conversion efficiencies as a
function of the pump power for the three different
input cavity mirrors
pass Sum Frequency Generation
We used a z-cut stochiometric LiTaO3 crystal, 1% MgO-doped, 15 mm long,
0.5 mm thick, with a 6.48 μm period (third order quasi phase matching ).
 Thermal
effects
The cavity suffers from thermal effects due to the heating of the mirrors and the
crystal, which in the highest finesse configurations affect the stability of the cavity
dynamics. A detailed study of these effects is still in progress.
Photothermal effect
Sum frequency power
at 355 nm as a
function
of
the
fundamental
pump
power
Distorsion of green peak
?