Efficient second harmonic generation in the optical
telecom S-band using non-segmented PPKTP waveguides
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Citation
Nejadmalayeri, A.H. et al. “Efficient second harmonic generation
in the optical telecom S-band using non-segmented PPKTP
waveguides.” Lasers and Electro-Optics, 2009 and 2009
Conference on Quantum electronics and Laser Science
Conference. CLEO/QELS 2009. Conference on. 2009. 1-2. ©
2009 IEEE.
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Institute of Electrical and Electronics Engineers
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http://hdl.handle.net/1721.1/60060
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© 2009 OSA/CLEO/IQEC 2009
Efficient second harmonic generation in the optical telecom
S-band using non-segmented PPKTP waveguides
Amir H. Nejadmalayeri1, Franco N. C. Wong1, Tony D. Roberts2, and Franz X. Kärtner1
1
Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics,
Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA
2
AdvR, Inc., 2310 University Way, Bozeman, Montana 59715, USA
nejadmal@mit.edu
Abstract: Continuous channel waveguides in periodically poled KTiOPO4 with guided orthogonal
polarizations are used to demonstrate type-II background-free second harmonic generation at 1505 nm with
1.2 %/(W cm2) normalized efficiency.
©2008 Optical Society of America
OCIS codes: (190.0190) Nonlinear optics; (070.4550) Correlators; (270.2500) Fluctuations, relaxations, and
noise; (230.7370) Waveguides.
1. Introduction
It was recently demonstrated that balanced optical cross correlation can be superior to microwave techniques in
measuring the timing jitter of mode locked lasers. A bulk periodically poled potassium titanyl phosphate (PPKTP)
crystal under type-II phase matching was used to obtain background-free second harmonic generation (SHG)−at
0.006%/(W cm2)–of ultrashort laser pulses in the telecom band [1]. We later utilized the method to measure the
timing jitter of free running mode locked lasers down to a resolution of 470 attosecond (as) [2]. It seems feasible to
extend the resolution limit of the technique to the 100-as level by using laser sources with higher output energies and
optimizing the experimental setup. However, measurements at the few attosecond level on high repetition-rate laser
sources require two to three orders of magnitude improvement in second harmonic (SH) signal detection that is
currently not feasible with the available input power levels and photodetector sensitivities. As a result, we seek to
use nonlinear waveguides instead of bulk crystals to significantly improve the efficiency of the frequency
upconversion process in order to reach a resolution of a few attoseconds.
For cross correlation applications in which background fluctuations may limit the measurement precision, PPKTP is
the nonlinear material of choice because of its room temperature operation, ultra wide type-II phase matching
bandwidth in the telecom C band for ultrashort pulse SHG [3]. More importantly, waveguides in PPKTP can support
both polarizations at comparable loss levels, thus enabling the extension of background free operation to guided
wave devices. In this work we demonstrate efficient type-II SHG in a PPKTP waveguide. Although type-II guided
wave SHG in KTP has been previously demonstrated [4], those studies used segmented waveguides. To the best of
our knowledge, this is the first time that continuous (non-segmented) waveguides in electric field poled KTP are
utilized to demonstrate type-II SHG. The process exhibits 1.2 %/(W cm2) normalized conversion efficiency, a 200×
improvement over previous bulk crystal results [1], that will pave the way to few and eventually even subattosecond resolution in timing jitter measurements of mode locked lasers, as well as femtosecond timing
distribution [5].
2. Experiment
The waveguides were fabricated by Rb+ ion exchange and the sample was diced and polished to a final length of 1
cm. The quasi phase matching grating in KTP was formed by electric field poling at a period of ~ 114 µm. An
external cavity tunable laser, covering the range of 1460–1600 nm, was used as the input source. The beam was
mechanically chopped at 270 Hz and launched into one of the waveguides using a lensed fiber. At the output the
beams were collected by a microscope objective and then dispersed using a prism to spatially separate the
fundamental and the second harmonic output. The two spatially separated beams were then detected using InGaAs
and Si detectors, and the signals were obtained using two separate lock-in amplifiers. The apparatus was capable of
measuring SH signals at the few pW level.
3. Results
The generated SH signal shows a phase matching bandwidth of ~10 nm, centered at a wavelength of 1505 nm within
the optical telecom S-band (1490–1530 nm). The SH signal as a function of the fundamental harmonic (FH) power
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© 2009 OSA/CLEO/IQEC 2009
(both measured at the output of the waveguide, henceforth ignoring the linear attenuation in the waveguides) is
shown in Figure 1. The plot shows an excellent quadratic relation with a conversion efficiency of 2.9 %/(W cm2).
Using the experimental parameters of our device and assuming a nonlinear coefficient of d24 = 3.6 pm/V [6], and
estimating (using Fabry-Perot technique) the linear loss values of 3.1 and 3.4 dB/cm for TM and TE guided modes
(depicted in Figure 1b), we numerically solved the coupled mode equations for guided mode type-II phase matching
in the non-depleted pump regime [7] and obtained a theoretical normalized efficiency of ≈ 1.2 %/(W cm2) for
conversion of the total FH power (both polarizations) at the input of the waveguide into SH power at the output. The
conversion efficiency of output FH to output SH power using this numerical simulation was 2.8 %/( W cm2), in
good agreement with the measured result of 2.9 %/(W cm2) in Figure 1, suggesting that the process of waveguide
formation and periodic poling has had practically no effect on the KTP d24 nonlinearity. In the limit of lossless
waveguides, the 2.9 %/(W cm2) would be the maximum conversion efficiency that could be attained in converting
input FH into output SH. The overall conversion efficiency of 1.2 %/( W cm2) (including the effect of linear losses)
shows a 200× improvement over the bulk crystal version at 0.006%/(W cm2) [2], making this device a natural
substitute in the next generation of optical timing detectors.
Figure 1 (a) Type-II second harmonic (SH) signal vs. fundamental harmonic (FH), both measured at the output of the 1-cm long
PPKTP waveguide. (b) Waveguide spatial mode profiles, from left to right: FH TE, FH TM, SH TE.
4. Conclusions
Channel waveguides in electric field poled KTP were used to generate background-free second harmonic in the
optical telecom S-band. The guided wave version exhibits over two orders of magnitude higher conversion
efficiency in comparison with its bulk crystal counterpart. Together with improved implementation (particularly
lower noise detectors and higher input powers) the PPKTP waveguide can potentially enable measurement of timing
jitters of optical signals with precisions in the few to sub-attosecond level at low average power levels.
References
[1] Kim, J., et al., Long-term femtosecond timing link stabilization using a single-crystal balanced cross correlator. Optics Lett, 2007. 32(9),
1044-1046.
[2] Kim, J., et al., Attosecond-resolution timing jitter characterization of free-running mode-locked lasers. Optics Lett., 2007. 32(24), 35193521.
[3] Konig, F. and F.N.C. Wong, Extended phase matching of second-harmonic generation in periodically poled KTiOPO4 with zero groupvelocity mismatch. Appl. Phys. Lett., 2004. 84(10): p. 1644-1646.
[4] Sundheimer, M.L., et al., Cascading nonlinearities in KTP waveguides at communications wavelengths. Electron. Lett., 1994. 30(17), 14001401.
[5] Kim, J., et al., Drift-free femtosecond timing synchronization of remote optical and microwave sources. Nature Photon., 2008. 2(12), 733736.
[6] Vanherzeele, H. and J.D. Bierlein, Magnitude of the nonlinear optical coefficients of KTiOPO4. Opt. Lett., 1992. 17(14), 982-984.
[7] Suhara, T. and M. Fujimura, Waveguide Nonlinear-Optic Devices. Springer Series in Photonics. Vol. 11. 2003, Berlin: Springer-Verlag.