Soliton waveguide arrays in LiNbO3 generated with blue-violet lasers for ultrafast parallel coupling 1
S. T. Popescu1*, A Petris1, V. I. Vlad1, E. Fazio2 Natl. Institute for Laser, Plasma and Radiation Physics, Bucharest-Magurele, Romania 2 University “La Sapienza” of Rome, Dept. of Energetics, Italy e-mail:
[email protected]
ABSTRACT We experimentally demonstrate generation of bright spatial solitons in lithium niobate crystals, with a blueviolet laser diode, at λ=405nm. This wavelength is near the short wavelength absorption edge of the crystal and allows the rapid (minute) generation of solitons and soliton waveguides (SWGs) with low laser powers (~ 5µW). Using a serial writing procedure, we have built a SWG array of 10 fiber-like waveguides in approx. 10 minutes. The IR guiding capability of these SWGs recorded in lithium niobate is tested by guiding femtosecond laser pulses at λ=1030nm and 1550nm and shows low absorption and low dispersion. These waveguides can be very useful for ultrafast parallel coupling applications in integrated photonics. Keywords: bright spatial solitons, lithium niobate, soliton waveguides, waveguide array, femtosecond pulse guiding, blue ray technology INTRODUCTION In the past two decades, spatial optical solitons have attracted much attention. After the first experimental demonstration of photorefractive solitons [1], many types of photorefractive solitons were investigated. A particular type is the bright screening soliton, which can be obtained with low intensity beams by applying an external electrical field [2]. Lithium niobate (LN) is a very good material for optoelectronic applications due to its electro-optic, photovoltaic, piezoelectric and pyroelectric properties. Its large production makes LN cheap, with reproducible parameters and a good platform for integrated photonics. The successful writing of soliton waveguides in the volume of LN makes possible many applications in integrated photonics [3]. We show here that SWGs can be written rapidly using a cheap, blue-violet laser diode at λ=405nm (near the short wavelength absorption edge of the crystal). The fast writing of SWGs opens the possibility of creating large SWG arrays, with thousands waveguides, which can be recorded in few hours, if the input power is suitable selected. We used femtosecond pulses in IR region at λ=1030nm and 1550nm to show the guiding capability and robustness of these waveguides. These SWGs can transport powers of several orders of magnitude higher in IR and can be very useful in ultrafast coupling. EXPERIMENTAL SETUP The experimental setup (Fig. 1) is similar to that used in [4], where the first screening spatial solitons are experimentally proved in LN crystals.
Figure 1. Experimental setup for SWG writing A beam at λ=405nm from a blue-violet laser diode is focused on the input face of a LN crystal to a spot size of ~8µm at FWHM. This gives around 12 Rayleigh diffraction lengths for the given size of the crystal (10mm) on the propagation direction. The SWG formation is visualized using a CCD camera. A high voltage power supply is used to apply an external bias electrical field, Eb.
SWG WRITING The SWG writing process is characterized by many parameters like input beam spot size, beam intensity, external electrical field, beam polarization, crystal length and background light intensity. Writing time dependence on the input beam intensity, for a constant electrical field, is shown in Fig. 2.a and its dependence on the electrical field, for constant beam intensity, is presented in Fig 2.b. From Fig. 2.a, one can see that SWG can be recorded very fast at low intensity. Using an intensity of 2 W/cm 2 (beam power of ~1µW) the writing time (tw) is around 250 seconds, but increasing the intensity (powers of tens of µW) writing times of few seconds can be obtained.
Figure 2. SWG writing time dependence of the intensity (a) and of the external field (b) using extrarodinary polarized light. Light polarization has a significant influence on the writing time due to different electro-optic coefficients for different polarizations. For the light intensity of 5.5W/cm2 and an external field of 45kV/cm, the writing time can vary between 100 seconds for extraordinary polarization and 240 seconds for ordinary polarization. It is known that writing with ordinary polarization leads to SWGs with a larger refractive index contrast, which provides better guiding [5].
Figure 3. Time evolution of normalized output beam diameter at FWHM. In Fig.3, the time evolution of the output beam diameter is shown. The 2D beam confinement into a bright spatial soliton is reached after ~100s. The soliton remains stable for another ~100s (depending on the beam intensity) and if the writing exposure continues, a soliton beam filamentation occurs. This short stability time is due to the absence of a stronger background ilumination. However, this kind of SWG, recorded without background illumination, can have a very long stability time of the order of a year in the dark [6].
Writting SWG very fast is an important advantage when writing large SWG arrays which can be used for ultrafast parallel coupling.
Figure 4. Array of 2x5 SWG Our previous results [6] show the possibility to write large SWG arrays in lithium niobate and suggest different methods for this process. A SWG array of 2 by 5, recorded at 405nm, is shown in Fig. 4. This array was recorded in ~10 minutes using a serial procedure. If the input power of the writing beam is selected properly to ensure the writing time of ~3 seconds, one can write arrays of 1000 waveguides in less than one hour. SWG GUIDING PROPERTIES The guiding properties of these SWGs are demonstrated by propagating femtoseconds pulses in IR produced by a Yb-doped fiber laser, at λ=1030nm and an Er-doped fiber laser, at λ=1550nm. The peak intensity for these pulses is of the order of GW/cm2. In Fig. 5 the propagation of 1030nm pulses is shown in a SWG written with I ≈3W/cm2, extraordinary polarization and Eb ≈ 45kV/cm. One can see that good guiding can be obtained with SWGs written with extraordinary polarization. Also, it was shown [7] that waveguide dispersion is very small and the total pulse broadening is given by the material dispersion, which decreases at telecom wavelengths. For extraordinary polarized pulses of 300 fs, at λ=1.55 µm, propagating through 1cm long LN crystal, the pulse broadening is ~2 fs.
Figure 5. Beam spot at the input face of the LN crystal (a), output face of the LN crystal in free propagation (b) and in propagation through a SWG (c), for laser pulses with duration of ~100fs. The results shown prove good guiding properties of SWGs recorded with blue-violet light at low intensities and their capability to guide IR femtosecond pulses with several orders of magnitude larger intensities. CONCLUSIONS We have investigated the bright soliton generation and soliton waveguide writing process in the volume of the lithium niobate crystals using blue-violet (405nm) c.w. laser radiation (near the short wavelength absorption edge of the crystal). At this wavelength, one can obtain short SWG writing times of few seconds with powers of tens of µW. The good guiding capability of these SWG has been proved by propagating femtosecond laser pulses at λ=1030nm and 1550nm. The fast formation of soliton waveguides at this wavelength can considerably reduce the time and the input power needed to write large SWG arrays in the volume of lithium niobate crystals with applications in ultrafast parallel coupling and integrated photonics.
ACKNOWLEDGEMENTS This work has been supported by the PNCDI 2 - IDEI project #572 (CNCSIS). A. Petris thanks the Abdus Salam International Centre for Theoretical Physics, Trieste, Italy for the research visits in the Centre as Regular Associate Member. REFERENCES [1] G. C. Duree, Jr., J. L. Shultz, G. J. Salamo, M. Segev, A. Yariv, B. Crosignani, E. J. Sharp, R. R. Neurgaonkar, P. Di Porto: Observation of self-trapping of an optical beam due to the photorefractive effect, Phys. Rev. Lett., vol 7. pp. 533-536, Jul. 1993. [2] M. Segev, G. C. Valley, B. Crosignani, P. Di Porto, A. Yariv, Steady-State Spatial Screening Solitons in Photorefractive Materials with External Applied Field, Phys. Rev. Lett., vol. 73. 3211-3214, Dec. 1994. [3] C. Denz, A. Desyatnikov, P. Jander, J. Schröder, D. Träger, M. Belic, M. Petrovic, A. Strinic, J. Petter: Photonic applications of spatial photorefractive solitons - soliton lattices, bidirectional waveguides and waveguide couplers, Photorefractive Effects, Materials, and Devices, P. Delaye, C. Denz, L. Mager, and G. Montemezzani, eds., Vol. 87. of OSA Trends in Optics and Photonics (Optical Society of America), Jun. 2003, paper 382. [4] E. Fazio, F. Renzi, R. Rinaldi, M. Bertolotti, M. Chauvet, W. Ramadan, A. Petris, V. I. Vlad: Screeningphotovoltaic bright solitons in lithium niobate and associated single-mode waveguides, Appl. Phys. Lett., vol. 85. pp. 2193-2195, Sept. 2004. [5] J. Safioui, M. Chauvet, F. Devaux, V. Coda, F. Pettazi, M. Alonzo, E. Fazio: Polarization and configuration dependence of beam self-focusing in photorefractive LiNbO3, J. Opt. Soc. Am. B, vol. 26. pp. 487-492, Mar. 2009. [6] S. T. Popescu, A. Petris, V. I. Vlad, E. Fazio, Arrays of soliton waveguides in lithium niobate for parallel coupling, J. Optoelectronics and Adv. Materials, vol. 1. pp. 19-23, Jan. 2010. [7] A. Petris, A. Bosco, V.I. Vlad, E. Fazio, M. Bertolotti: Laser induced soliton waveguides in lithium niobate crystals for guiding femtosecond light pulses, J. Optoelectronics and Adv. Materials, vol. 7. pp. 2133-2140, Aug. 2005.
