Download Temperature Tuning Characteristics of Periodically Poled Lithium

Temperature Tuning Characteristics of Periodically Poled Lithium
Niobate for Second Harmonic Generation at 490 nm
Movva Sai Krishna*a, U.S. Tripathia, Ashok Kaula, K. Thyagarajan b, M.R. Shenoyb
a
Photonics Division, Instruments Research & Development Establishment, Dehradun, INDIA
b
Department of Physics, Indian Institute of Technology Delhi, New Delhi, INDIA
ABSTRACT
Blue light at 490 nm was generated by second harmonic generation in periodically poled lithium niobate (PPLN) using
the quasi phase matching (QPM) scheme. A fiber coupled high power laser diode operating at 980 nm wavelength was
used as a pump source. Experiments were carried out to find the temperature tuning curve and the phase matching
temperature for the PPLN sample. These results were then compared with theoretical simulations, which show good
agreement.
Keywords: Nonlinear optics, second harmonic generation, temperature tuning, quasi phase matching
1. INTRODUCTION
Blue lasers are attractive light sources for display applications because of their high brightness and complete colour
saturation. Blue lasers also find applications in optical data storage, optical recording, for DNA sequencing in
1
biotechnology, submarine communications, in warfare as dazzlers and reprographics . We have generated blue light at
490 nm by nonlinear interactions in periodically poled lithium niobate by using a high power fiber-coupled laser diode
(FC-LD) at 980 nm as the pump source. In this paper, we present the temperature tuning characteristics of PPLN for
second harmonic generation (SHG) at 490 nm taking into account the source spectrum. Initially, we outline the principle
of quasi phase matching (QPM) followed by basic theory involved in SHG. We then discuss the experimental setup for
generating blue light at 490 nm and find the temperature tuning characteristics of the PPLN sample. Finally, using
theoretical simulation, we explain the nature of the temperature tuning curve obtained experimentally.
2. QUASI PHASE MATCHING TECHNIQUE FOR SECOND HARMONIC GENERATION
Mature nonlinear optical materials are limited, and development cycle for newer ones is expensive and time consuming.
QPM is an important technique for extending the useful wavelength range of available nonlinear optical materials. It has
been used quite frequently in the development of new coherent laser sources. QPM is an alternative phase matching
technique to the birefringent phase matching (BPM) technique, in which the accumulated phase mismatch is reset
periodically as the interacting waves travel through the crystal. QPM is usually implemented by a periodic modulation
of the nonlinear susceptibility. In lithium niobate, QPM is achieved by periodically reversing the polarity of ferroelectric
domain when the phase mismatch between the pump and the second harmonic wave reaches π, and the material
prepared in this way is referred to as PPLN. PPLN has advantages over standard lithium niobate due to absence of walkoff and it is less susceptible to photorefractive effects. An additional advantage of PPLN is its design flexibility afforded
by QPM. The ability to engineer the domain-periodicity for nonlinear frequency conversion in the whole transparency
range (0.35–5 µm) of lithium niobate makes PPLN a versatile and important material2. Planar processing methods from
the microelectronics industry, in particular, lithography followed by E-field poling can be used to produce PPLN in
large numbers 3. Since this is chip-based processing, the production costs are low. Engineering more complex behavior,
such as broadband phase matching and fanned gratings, involves nothing other than designing a more sophisticated
lithographic mask.
In the case of frequency doubling or SHG, the phase matching condition is given by
4π (nSH − n F )
∆ k = k SH − 2k F =
λF
*[email protected]; phone 91-135-2780854; fax 91-135-2787161
(1)
where kSH and kF are the magnitudes of the wave vectors; nSH and nF are the refractive indices at the second harmonic
(SH) and fundamental waves, respectively, and ∆k is the phase mismatch term.
In BPM, both the waves travel with the same phase velocity when nSH = nF and this makes ?k zero. The electric field of
the second harmonic wave then grows linearly along the propagation direction and the second harmonic power
quadratically. If the phase matching condition is not satisfied i.e. ?k ? 0, then the propagating SH wave, after a
characteristic distance of Lc, will become out of phase with the locally generated SH field and energy starts to couple
back from the SH to the fundamental wave. This is a periodic process with half-period Lc, called the coherence length,
and is defined as
π
Lc =
(2)
(k SH − 2 k F )
In QPM, the grating period Λ, which is used to compensate for the phase slip, has a length corresponding to an
accumulated phase difference, between the fundamental and the SH wave, that is a multiple of 2π and is given as
2 mπ
mλF
Λ=
=
(3)
∆k
2( nSH − nF )
where m is the order of the periodic modulation. A first order grating (m=1) is one where the locally generated SH is out
of phase by π with respect to the propagating SH wave after the half-period. For QPM, the phase matching condition
becomes
4π (n SH − nF ) 2π
∆ k = k SH − 2 k F − K =
−
(4)
λF
Λ
Figure 1: Phase Matching Curves: BPM, QPM and Non-Phase Matched Case
Figure 1 shows the calculated efficiencies of SHG for three cases of phase matching in Lithium Niobate: BPM
corresponds to perfect phase matching, leading to the highest efficiency; QPM represents quasi phase matching with
first order grating (m=1), and the last curve is that of a non-phase matched case (Note: The last curve has been
multiplied by a factor of 15 to make it visible on the same scale).
Efficiency of SHG can be calculated using the equation given below 1:
2π 2 d eff2 L 2 (I 1 ) 2
 ∆kL 
I2 =
sin c 2 
(5)

2 2
λF n1 n2 ε0 c
 2 
where I2 is the intensity of the second harmonic generated, I1 is the intensity of the pump beam, deff is the effective
nonlinear coefficient for LiNbO3, ∆k is the phase mismatch term, L is the length of the LiNbO3 sample, λF is the
wavelength of the pump beam, n1 and n2 are the refractive indices of the LiNbO3 sample at the pump and the second
harmonic wavelengths, respectively; ε0 is the dielectric permittivity of free space and c is the velocity of light in free
space. From the above, the efficiency of SHG is given as ηSHG = (I2/I1). We have used the above equation for plotting
the curves in Fig.1 and the same equation will be used later in the paper to explain the nature of temperature tuning
curve using theoretical calculations.
3. EXPERIMENTAL SETUP
The experimental set up used for obtaining SHG at 490 nm in PPLN using a FC-LD is shown in Fig. 2. As shown in the
figure light emanating from a fiber-coupled laser diode (Thales Optronics, France) is first collimated using a microscope
objective and the parallel beam is then focussed using a suitable lens into the PPLN sample. The PPLN chip, with a
periodicity of 5 µm and designed for generating SHG (@ 490 nm), was housed in a small micro-oven with a
temperature controller having an accuracy of ± 0.1°C. The pump beam, which was focussed into the sample, interacts
with the PPLN and generates blue output at 490 nm, which is measured by a detector. Here we have used a colour filter
that allowed blue light to pass through while blocking the pump beam from reaching the detector.
Micro-Oven
MO
Lens
980 nm FC-LD
Blue light detector
with filter
PPLN Chip
Temperature
Controller
Figure 2: Experimental setup for obtaining blue light at 490 nm from 980 nm FC-LD
With the above setup, temperature tuning was carried out on the PPLN sample by varying the temperature of the micro
oven from 80–140°C in steps of 1°C, initially, but as the blue light power being detected increased and the temperature
was approaching phase matching temperature the step size was reduced to 0.1°C. All the above measurements were
made at fixed input pump power of FC-LD. Maximum blue light was generated when the temperature was at 113°C,
which is the phase matching temperature obtained experimentally for our PPLN chip.
4. RESULTS AND DISCUSSION
The normalized temperature tuning results are shown in Fig.3. It can be seen that the measured temperature tuning curve
has a broadband profile (shown as full dot s in Fig. 3).
Figure 3: Variation of normalized efficiency with temperature of the PPLN chip. The solid line corresponds to the experimental curve
and the circles correspond to theoretical simulation of the temperature tuning curve
The phase matching temperature for this PPLN sample (with a periodicity of 5 µm) was calculated theoretically using
the temperature dependent Sellemier equations4 and was found to be 113.14°C, which is in close agreement with the
experimentally obtained value of 113°C. However, the broadband nature of the tuning curve was unexpected in view
the analytical result for the efficiency of second harmonic generation (cf. Eq.5), which indicates that the efficiency curve
should be a sharply peaked curve centered at the phase matching temperature (sinc2 function). On further investigation
on the spectral purity of the pump source, it was found that the FC -LD was having a finite spectral linewidth of around
3.76 nm. It was also found that the peak wavelength was at 973.9 nm at that particular pump power (see Fig.4).
We can attribute the broadband nature of the measured temperature dependence of SHG to the finite spectral width of
the pump beam. To ascertain the above, we simulated theoretical curves using an asymmetric Gaussian profile for the
power spectrum of the input pump beam from the FC-LD. The left side of the Gaussian profile was assumed to have a
width of 2 nm from the peak wavelength, and on the right side of the profile, the width was taken as 1.76 nm from the
peak wavelengt h (see Fig.4). We used Eq. (5) for calculating the normalized efficiency of second harmonic generation
at 490 nm; while performing the efficiency calculations, we have incorporated the asymmetric Gaussian distribution of
the pump power as described above. Both the theoretical and experimental efficiency curves are shown in Fig.3 (after
normalization) for comparison of the results. It can be seen that the theoretical and the experimental results are in
excellent agreement. Thus, we conclude that the broadband nature of the temperature tuning curve is because of the
finite spectral width of the pump source.
Figure 4: Wavelength spectrum of the FC-LD pump source
5. CONCLUSION
Blue light at 490 nm was generated in PPLN using a high power FC -LD at 980nm as the pump source. Phase matching
temperature, and temperature tuning curve were obtained experimentally. The broadband profile of the measured
temperature tuning curve was explained using theoretical simulation by taking into account the finite spectral width of
the FC -LD source. Further experiments on blue light generation using PPLN are in progress.
ACKNOWLEDGEMENTS
The authors would like to thank Sh. J.A.R. Krishna Moorty, Director, IRDE for permitting them to publish this work.
They would also like to thank Director, LASTEC for providing the 980 nm fiber coupled laser diode.
REFERENCES
1.
2.
3.
4.
W.P. Risk, T.R. Gosnell & A.V. Nurmikko, Compact Blue Green Lasers, Ch.1, Cambridge Univ. Press, U.K.,
(2003)
M. M. Fejer, G.A. Magel, D.H. Jundt and R.L. Byer, IEEE J. Quantum Electronics, vol. 28, No.11, 2631 (1992)
L.E. Myers, R.C. Eckardt, M.M. Fejer and R.L. Byer, Proc. of SPIE, vol. 2379, 154 (1995)
G.J. Edwards and M. Lawrence, Opt. & Quantum Electronics, vol. 16, 373 (1984)