Periodically poled titanium-diffusion waveguide was fabricated in near-stoichiometric MgO:LiNbO3 wafers. The characteristics of the device were examined by pump-probe second harmonic generation (SHG). The device shows very high resistance to photorefractive damage at room temperature. The wavelength tuning of the converted difference frequency (DF) wave can be achieved from 1450 to 1542 nm by tuning pump wave and signal wave. The wavelength conversion efficiency was measured to be -7.3 dB with coupled pump and signal power are 150 mW and 50 mW, respectively.
©2009 Optical Society of America
Wavelength converters using quasi-phase-matched (QPM) LiNbO3 waveguides are recognized as key devices for future wavelength division multiplexing (WDM) systems. These converters offer several distinct advantages such as the simultaneous conversion of WDM channels, a large signal bandwidth and transparency as regards modulation format. However, congruent LiNbO3 is known to be exhibit photorefractive damage in the presence of radiation at wavelengths below 1000 nm . To suppress photorefractive damage effect, congruent LiNbO3 is typically operated at temperatures around 120°C , making this material impractical for telecom products.
Highly Mg-doped LiNbO3 is known to be more resistance to photorefractive damage than congruent LiNbO3. However, unexpected leakage current and the strong effect of domain widening out of the electrode area impose limitations on the period of short-pitch domain inversion patterns . Near-stoichiometric LiNbO3 crystals doped with MgO (SMgLN) at lower concentration show better photorefractive damage resistance than congruent LiNbO3 and conventionally highly MgO doped congruent LiNbO3, and poling field for ferroelectric domain inversion can be decreased significantly as well . Therefore, periodically poled SMgLN (PPSMgLN) has been anticipated as the most practical QPM material.
Anneal/proton-exchange (APE) is widely used techniques for waveguide fabrication in LiNbO3. However, APE waveguide supports only extraordinarily-polarized guide modes . In contrast, waveguides made of titanium- diffusion can support both ordinary and extraordinary modes, and it has been shown that Ti diffusion waveguide is effective for polarization-independent operation . In this work we report the first preparation of a QPM Ti diffusion waveguide in PPSMgLN (Ti:PPSMgLN), and we examine the wavelength conversion properties based on the QPM difference frequency generation (DFG) in the waveguide at room temperature. For a variable signal wavelength, a converted DF wavelength can be obtained by properly tuning the pump wavelength. In addition, the characteristics of the Ti:PPSMgLN waveguide was tested by SHG.
2. Device fabrication
We prepared a Z-cut 40×6×1 mm3 near-stoichiometric LiNbO3 doping 2mol% MgO wafer as a substrate. The QPM structure for nonlinear optic devices using this SMgLN substrate can be realized by periodic inversion of the spontaneous polarization, which is associated with the sign inversion of the nonlinear optic coefficients. Formation of periodically poled domain inversion structures is the key step of the device fabrication. PPSMgLN was performed on the substrates using the conventional method of the voltage pulse application technique . Domain grating period of 16μm to 18μm in PPSMgLN permitted QPM of second harmonic generation having fundamental wavelengths around 1.5 μm at operating room temperature. We can tune the pump wavelength or signal wavelength by using waveguides with different QPM periods at room temperature.
Fabricating Ti-diffused waveguide is more important to achieve wavelength conversion. We fabricated Ti-diffused SMgLN waveguides for the 1.5 μm band by using sputtered Ti as a diffusion source. A Ti film of ~0.1 μm thickness was deposited on +Z surface of LiNbO3 crystal by RF sputtering, and patterned into channels with 6.5 μm width by a conventional photolithography technique. Ti diffusion was carried out at 1049 □ in an O2-H2O gas mixture. Wet oxygen flowed through the quartz tube after bubbling through a glass vessel filled with deionized water at room temperature. The flow of oxygen was 2.4 l/min, the total gas pressure was 1 atm. The wafers were placed on a support made of platinum. The duration of annealing was eight to ten hours. In order to erase the influence of high temperature for PPSMgLN structures, we fabricated the Ti diffusion waveguide at first, and then prepared domain inverted structure. Figure 1 shows the scanning electron microscope (SEM) image of the fabricated Ti:PPSMgLN device following 5 minutes etching in HF:HNO3 mixture.
3.1. Characteristics of Ti:PPSMgLN
As a preliminary test, we measured second harmonic generation (SHG) characteristics of the device by CW operation. The experiment was carried out at room temperature and a fundamental wave from a Nd:YAG laser pumped Gr:YAG with 100 mW output at 1530 nm . The mode size was adjusted with a telescope to provide an appropriate mode match to the waveguide. A SH wave for input of 1530 nm was obtained in a channel with period is 17.3 μm. Intensity profile of the SH mode was shown in Fig.2. The SHG efficiency dependent on the fundamental power was measured and is shown in Fig.3. The slope of the line in the figure shows that the normalized SHG efficiency was 630%/W. It was 2 times as high as that obtained in a wavelength converter using Ti:PPLN waveguide (319%/W) .
The photorefractive damage characteristics of the fabricated device was examined by observing the second harmonic generation spectra when a high power 780 nm pump light was lunched into the waveguide. To monitor the output wavelength shift caused by the photorefractive damage, we used a high power signal light at 1550 nm and measured the generated SHG spectra with the spectrum analyzer. Fig.4 shows the peak wavelength shift against the pump wave irradiation time. These experiments were carried out at room temperature and a 780 nm light with a power of 200 mW was launched into the waveguide. Figure 4 also shows the result for the device made of congruent LiNbO3 for comparison. The wavelength shift of the Ti:PPLN device shows an increase at the beginning of the irradiation and stabilizes at 4 nm after 10 min. In contrast, the Ti:PPSMgLN device shows no wavelength shift even after 10 min irradiation. This characteristic of the Ti:PPSMgLN device will be useful for the precise control of the phase matching wavelength of the QPM wavelength conversion process.
3.2. DFG optical wavelength conversion
The experimental setup is shown schematically in Fig.5. The pump laser was a cw Ti:sapphire laser that was tunable from 700 to 870 nm with an output power of several hundreds of milliwatts. The signal laser was a Nd:YAG laser pumped Gr:YAG that was tunable from 1400 to 1700 nm with an maximum output power of more than one hundred milliwatts. The two laser beams, after being combined by a dichroic mirror (high transmission near 1550 nm), were then focused by a microscope objective, and then lunched into end-polished QPM Ti:PPSMgLN waveguide. The output spectrum from the waveguide was analyzed in an optical spectrum analyzer. To obtain high frequency conversion efficiency, the pump and signal laser beams have to be spatially overlapped within Ti:PPSMgLN waveguide. Two apertures were placed along the laser beam to make the two beam spots as equal as possible.
We performed multichannel wavelength conversion with coupled pump and signal power is 150 mW and 50 mW, respectively. A pump power of ~50 nW launched inside the waveguide, and a signal power of ~10 nW launched inside the waveguide. Figure 6 illustrates the measured output spectrum from the QPM DFG based wavelength converter. The signal wavelength is tuned at 1550nm. The pump wavelength is chosen at 770nm to meet the QPM condition for the DFG process. As shown in Fig.6, the DF wave λ=1530 nm was obtained.
The QPM period of the Ti:PPSMgLN was 17.5μm, and the waveguide loss was determined to be 0.12 dB/cm at a 1550 nm wavelength. The wavelength conversion efficiency for converting the signal wavelength to the newly generated DF wave is about -7.3 dB.
Figure 7 illustrates the tunable performance of the QPM DFG based wavelength conversion for different signal wave and pump wave. The signal wavelength is tuned at 1470 nm and 1578 nm. Two pump wavelengths are tuned at 730 and 780 nm respectively to satisfy the QPM condition for the DFG process. It is found that the output DF wavelength can be tuned from 1450 nm to 1542 nm as the pump wavelength is changed from 730 to 780 nm and the signal wavelength is changed from 1470 to 1578 nm.
The optical spectra of wavelength conversion experiments are shown in Figs. 6 and 7. The coupled power levels of pump wave and signal waves were 150 mW and 50 mW, respectively. The conversion efficiency from the signal to the generated DF wave was measured to be approximately 17%. According to Ref., the theoretically predicted conversion efficiency is 109%; our experimental conversion efficiency was relatively low in comparison with the optimized one. The difference between the calculated and the experimental conversion efficiencies might be caused by such factors as propagation loss and spatial overlap between the waveguide modes of the waves involved in the nonlinear interaction. We believe that the conversion efficiency could be further increased by using a pump and signal laser fiber combiner, optimizing the waveguide design and fabrication conditions, and minimizing the propagation loss.
The QPM Ti:PPSMgLN wavelength conversion device has been demonstrated for the first time. For a variable signal wavelength, a converted DF wavelength can be obtained by properly tuning the pump wavelength. The wavelength tuning of the converted DF wave can be achieved from 1450 to 1542 nm by tuning pump wave and signal wave, which produces DFG process. The conversion efficiency of the signal to the DF wave was approximately -7.3 dB with the pump power of 150 mW and the signal power of 50 mW at room temperature. The characteristics of the Ti:PPSMgLN waveguide was tested by second harmonic generation, the study has revealed the potential for our device to operate at room temperature without exhibiting any wavelength shift caused by photorefractive damage. These results indicate that the use of Ti:PPSMgLN waveguide may contribute appreciably to the development of practical wavelength converter devices suitable for future all-optical communication applications.
The authors would like to thank Dr. B. B. Zhou and Dr. Z. Y. Wei for assistant with the measurements and for fruitful discussions. This work was supported by the National Natural Science Foundation of China under Grant No. 60878033, the Foundation of Beijing Jiaotong University under Grant No.2008RC058 and “863” Project of China No.2006AA03Z423.
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