All-optical wavelength tuning in S ˇ olc filter based on Ti:PPLN waveguide Y.L. Lee, N.E. Yu, C.-S. Kee, D.-K. Ko, Y.-C. Noh, B.-A. Yu, W. Shin, T.-J. Eom, K. Oh and J. Lee All-optical wavelength tuning in a waveguide-type S ˇ olc filter based on a Ti:PPLN waveguide has been demonstrated for the first time by ultra- violet illumination. The measured wavelength tuning rate as a function of the UV intensity was about 226.42 nm/W cm 22 . Introduction: Although the S ˇ olc filter was proposed more than 50 years ago [1], difficulties with the fabrication technology in making a large number of birefringent plates stack has prevented the appearance of a prac- tical narrowband S ˇ olc filter. Recently, the advance in electric-field poling technology [2] in LiNbO 3 allowed a new type of narrowband S ˇ olc filter based on periodically poled LiNbO 3 (PPLN). After the first PPLN S ˇ olc filter was demonstrated [3], several researchers have reported on the wave- length tuning [4, 5] and transmission control properties [6]. The wave- length tuning methods can be classified into the temperature control [4] and light illumination method [5]. Both tuning methods give an almost linear wavelength tuning curve as a function of the control parameters such as temperature and light intensity. In the case of the temperature control method, some results have already been achieved in bulk PPLNs [4, 6] and Ti:PPLN waveguides [7, 8]. However, up to now, the light illu- mination method was only adopted in a bulk PPLN S ˇ olc filter [5]. There has been no research into the dependence of waveguide-type PPLN S ˇ olc filter characteristics on light illumination. In this Letter, all-optical wave- length tuning in a waveguide-type S ˇ olc filter based on a Ti:PPLN wave- guide by ultraviolet (UV) illumination is demonstrated for the first time. Experiments: The experimental setup to perform the all-optical wave- length tuning in the S ˇ olc filter based on a Ti:PPLN waveguide is shown in Fig. 1. An optical signal from the wavelength swept fibre laser (WSFL) [8] with an average power of 10 mW was polarised in TE-polarisation by a fibre u-bench polarisation controller and butt coupled into the Ti:PPLN waveguide through a single mode fibre. The output signal from the Ti:PPLN waveguide was filtered by an analyser (TM-polarisation) and the spectrum was measured by an optical spectrum analyser (OSA). A UV source (Moritex Co.) with a centre wavelength of about 360 nm was irradiated onto the Z-face of the Ti:PPLN while measuring the spectrum. The physical length of the Ti:PPLN waveguide was about 78 mm and the periodicity of the quasi-phase-matching (QPM) grating was 16.6 mm. Detailed information about the Ti:PPLN wave- guide and the WSLF are listed in Tables 1 and 2, respectively. Fig. 1 Experimental setup for all-optical wavelength tuning in Ti:PPLN S ˇ olc filter by UV illumination PC: fibre u-bench polarisation controller Analyser: bulk-type linear polariser Table 1: Specifications of Ti:PPLN waveguide Physical size (mm) (length  width  thickness) 78  10  0.5 Propagation loss: TM-mode (at 1280 nm) TE-mode  0.11 dB/cm , 0.1 dB/cm Mode size (FWHM) filtered signal TM , at 1280 nm 6.64 mm  5.15 mm SHG efficiency (3 dB bandwidth) 544 %/W(0.2 nm) Table 2: Specifications of WSFL Output power (average) 10 mW Wavelength swept frequency 15 kHz Wavelength tuning range 1260  1340 nm The centre wavelength of the Ti:PPLN S ˇ olc filter is defined as l 0 ¼ðn o  n e Þ L=ð2m þ 1Þ ð1Þ where n o and n e are the refractive indices of the ordinary and extraordi- nary waves, respectively, L is the periodicity of QPM-grating, and m is the order of the bandpass wavelength (in our case m ¼ 0). When unpo- larised UV light irradiates onto the Z-face of Ti:PPLN, the centre wave- length of the Ti:PPLN S ˇ olc filter shifts according to the variation of the refractive index difference between n o and n e . This refractive index change results from the electric field generated by the photovoltaic effect (PVE) along the z-axis [9]. The variation of the refractive index difference between n o and n e can be described as follows [10]: dn ¼ðn o  n e Þðn 3 o g 33  n 3 e g 13 ÞE PVE =2 ð2Þ where g 33 and g 13 are nonlinear coefficients and E PVE is given by E PVE ¼ak 31 I =s ð3Þ where a and k 31 represent the absorption coefficient and photovoltaic constant of Ti:PPLN, respectively, I is the light intensity, and s is the conductivity of Ti:PPLN. The measured transmission spectrum of the Ti:PPLN waveguide is shown in Fig. 2. The scatter and the solid line indicate the experimental data measured at room temperature (238C) and the theoretical curve [8, 11], respectively. The measured 3 dB bandwidth of the filter was about 0.21 nm (theoretical value: 0.22 nm) which is narrow enough to be used as a tunable wavelength filter in an optical communication system. Also, the filter shows the signal-to-noise ratio of 13 dB over the tuning range (1260  1340 nm). Fig. 3 shows the centre wavelength of the filter as a function of the UV illumination intensity. As the inten- sity increases, the centre wavelength of the filter shifts to a shorter wave- length, because the second term of (2) increases as a function of the intensity of UV. The measured wavelength tuning rate of the filter was about 226.42 nm/W cm 22 which shows more slant than that of a bulk PPLN S ˇ olc filter (19.23 nm/W cm 22 ) [5]. The right axis of Fig. 3 indicates the amount of change in the refractive index difference (dn) between n o and n e as a function of UV intensity. These results indi- cate that a Ti:PPLN S ˇ olc filter gives a wider wavelength tuning range than that of a bulk PPLN S ˇ olc filter. Fig. 2 Measured transmission spectrum of Ti:PPLN S ˇ olc filter at room temperature (238C) Fig. 3 Centre wavelength of Ti:PPLN S ˇ olc filter aganst UV illumination intensity ELECTRONICS LETTERS 3rd January 2008 Vol. 44 No. 1 Authorized licensed use limited to: Yonsei University. Downloaded on May 10, 2009 at 22:12 from IEEE Xplore. Restrictions apply.