Fabrication of Optical Modulator Based on Proton Exchange

For the investigation of optical modulator, the optical wave-guide was fabricated on x-cut LiNbO 3 su strate using proton exchange method with self-aligned electrode. The electrode pattern was designed using a self-aligned thin film electrode method. After proton exchange process, the wave-guide could be prepared by annealing process to control the width and depth of the optical wave-guide. The initial crossover state of the fabricated 1 ×2 optical switch was observed with controlling the annealing process variables and the structure of self-aligned thin film electrodes. As the results in the present work, the measured cross talk and minimum detectable switching voltage were obtained at the values of -29.5dB and 8.0V, respectively, with good merits. Eurasian ChemTech Journal 2 (2000) 175-180  2000 al-Farabi Kazakh St.Nat. University *corresponding author. E-mail: vlsi1@wh.myongji.ac.kr Introduction Signal processing, optical communication and computer technologies require broadband optical modulators operating with low power consumption and fast speed or high frequency range. Optical waveguides, due to the confinement of the optical beams over long distances, could make modulation with long interaction length possible. As a result, the power required to drive a modulator can be significantly reduced. The bandwidth of traveling wave modulators is determined, on one hand, by the velocity difference between the optical and electrical waves and, on the other hand, by the dispersive properties of the transmission line resulting from electrical dispersion, and skin effect losses. The invention of semiconductor laser and development of optical electron technology leads to taking advantage of laser light source instead of electricity. In order to transfer optical signal in conversion of electrical signal using photodiode and the method of going steps of optical signal utilized laser diode and switching, the profits of optical switch could be employed. LiNbO 3 uni-axial, ferroelectric crystal satisfies this requirement and can be fitted to make clause optical wave in low guided loss and to obtain single mode optical wave-guide on LiNbO3 substrate. Electrical field through electrode to electrode on LiNbO3 substrate can change refractive index of wave-guide channel by electro-optic effect. Optical switch can make good use of this refractive index alteration, and hence optical beam can be easily modulated or switched by electric field applied to electrodes on LiNbO3 substrate. It is also proceeding actively to study integrated optics utilized concept of guided wave for supplying the light signal source [1]. Research and development of coherent optical fiber communication systems have been accelerated because of the possibility of sensitive receiver improvement reaching 25dB and the possibility of frequency division multiplex (FDM). However, the practical application of the systems has not been developed, mainly because of the poor spectral purity and frequency stability of semiconductor lasers and the system complexity. In this research, optical switch having several input and output ports for light beam signal transport into wave-guide was investigated and fabricated to overcome the limiting factors for coherent optical communications. It was fabricated using new method of self-aligned thin film electrode method. The optical switch operated based on the switching phenomenon of optical beam power by electro-optic effect between two close channels of optical wave-guide [2]. It was using forward giving and taking optical beam power between two channels of optical wave-guide. Applying input power into input port of optical switch, the measurements of switching voltage, cross-talk, and switching rate were analyzed for investigation of optical switch characteristics.


Introduction
Signal processing, optical communication and computer technologies require broadband optical modulators operating with low power consumption and fast speed or high frequency range. Optical waveguides, due to the confinement of the optical beams over long distances, could make modulation with long interaction length possible.
As a result, the power required to drive a modulator can be significantly reduced. The bandwidth of traveling wave modulators is determined, on one hand, by the velocity difference between the optical and electrical waves and, on the other hand, by the dispersive properties of the transmission line resulting from electrical dispersion, and skin effect losses. The invention of semiconductor laser and development of optical electron technology leads to taking advantage of laser light source instead of electricity. In order to transfer optical signal in conversion of electrical signal using photodiode and the method of going steps of optical signal utilized laser diode and switching, the profits of optical switch could be employed. LiNbO 3 uni-axial, ferroelectric crystal satisfies this requirement and can be fitted to make clause optical wave in low guided loss and to obtain single mode optical wave-guide on LiNbO 3 substrate. Electrical field through electrode to electrode on LiNbO 3 substrate can change refractive index of wave-guide channel by electro-optic effect.
Optical switch can make good use of this refractive index alteration, and hence optical beam can be easily modulated or switched by electric field applied to electrodes on LiNbO 3 substrate.
It is also proceeding actively to study integrated optics utilized concept of guided wave for supplying the light signal source [1]. Research and development of coherent optical fiber communication systems have been accelerated because of the possibility of sensitive receiver improvement reaching 25dB and the possibility of frequency division multiplex (FDM). However, the practical application of the systems has not been developed, mainly because of the poor spectral purity and frequency stability of semiconductor lasers and the system complexity.
In this research, optical switch having several input and output ports for light beam signal transport into wave-guide was investigated and fabricated to overcome the limiting factors for coherent optical communications. It was fabricated using new method of self-aligned thin film electrode method. The optical switch operated based on the switching phenomenon of optical beam power by electro-optic effect between two close channels of optical wave-guide [2]. It was using forward giving and taking optical beam power between two channels of optical wave-guide. Applying input power into input port of optical switch, the measurements of switching voltage, cross-talk, and switching rate were analyzed for investigation of optical switch characteristics.

Patterning of wave-guide
The fabrication of optical switch is the method for making a small, low loss optical wave-guide for control the refractive index, which is the most important problem in the fabricating conditions of optical modulator [3].
The refractive index could be changed and decided under the annealing process of the substrate with enough quantity of oxygen flow into furnace (400°C) during 10 to 15 minutes for deep diffusion of proton (H + ) exchange at surface. The typical fabricating methods of wave-guide are illustrated at Fig.1, where Ti-in-diffusion and proton exchange methods were used [4]. As the method of making optical wave-guide with low loss by changing or increasing refractive index on substrate, LiNbO 3 is to be a useful substrate material for use of proton exchange and titanium indiffusion processing.
of LiNbO 3 under the Ti strip increased and the channel of the optical wave-guide was formed with proper thickness and width by the processing conditions.
The proton exchange process is one of ways to make optical wave-guide on the LiNbO 3 with use of open-window on metal mask intending only to make channel portion on x-cut LiNbO 3 substrate. The Li + ion of x-cut LiNbO 3 can move away in melted acid or hydrate solution and ion exchange can occur with H + ion supplied from acid or hydrate melt under the proper temperature condition. Therefore, the chemical reaction of proton exchange goes like as following equation:

Patterning of electrode
The mask of proton exchange for patterning optical wave-guide was shown in the Fig. 2 [5]. The selfaligned thin film electrode structure was fabricated on 1×2 optical switch without using mask aligner [6,7]. In order to use only the necessary parts of electrodes, the four extra gaps between electrodes should be designed. Since the extra gaps for separation are connected to the end of channels and so can operate as a wave-guide, the propagation loss of optical power could occur in these regions. Therefore, in order to minimize the propagation loss as it can be ignored, the extra gaps were to be positioned vertical to the channels and to be narrower than 4µm [8,9].
The process of titanium in-diffusion was carried out by diffusion of Ti atoms penetrating into LiNbO 3 forming a channel depth for several hours in high temperature and then, by annealing process after lifting up with Ti strip to be formed optical wave-guide. After the Ti-in-diffusion processing, the refractive index

Output characteristics
To obtain the characteristics of the optical modulation phenomenon from the fabricated optical device,   the modulated output power was measured from the output ports of channels W 1 and W 2 by applying 8.0V of square wave to the electrodes, after irradiating the incident light from light source (He-Ne Laser, λ=0.6328 µm) into the input ports of channels ( Fig.3(a) and (b)). The frequency range of driving voltage was from 100kHz to 400MHz in the measuring system. The length of switching area in the wave-guide is called transfer-like length. The maximum input power transferred to neighbor channel is cross-over state. However, a certain degree of input voltage under the maximum optical power transferred is in cross-over state, or because of the phase mismatch between channels of optical wave-guide under the zero input voltage. The existing of optical phase mismatch between the channels can occur at the beginning point of wave-guide by the technical error in fabrication. In the case of the switching length is equal to transfer-like length without phase mismatch between two optical wave-guides (1×2) at the beginning of switching area, the transfer length could have been used instead of transfer-like length by using the heat treatment of annealing process. However, although the limitation of outside conditions reduced the optical phase mismatch in the experiment, the remained optical power could exist at the output port of input waveguide canceling the optical power in the wave-guide in cross-over state. This way also left the optical power acting up to cross-talk. Fig. 4 shows the magnification of Fig. 3(a) from 2 V/div to 100 µs/div for measuring the cross-talk -29.5dB of remain output power of W 1 at crossover state, when the input light was irradiated to W 1 . Fig.5 shows the modulation characteristics. It appeared that optical power from 1×2 wave-guide driving with 8.0V square wave input voltage of frequency. It can appear with variation of optical power of 1×2 output counted with theory controlled from 100kHz to 400MHz, former of phase mismatched 1×2 output accompanying input voltage. The measurement system is shown Fabrication of Optical Modulator Based on Proton Exchange modulator or directional couplers with forming electrode by self aligned thin film method will obtain very high effect with the method of going down cross-talk. Also to confirm the process of forming optical waveguide by proton exchange is sure getting surface single optical wave-guide.
Table shows the results of measuring the characteristics of the 1×2 optical switch light switching on the conditions of Fig. 3. It gives the relationship among the optical output power (by photo detector), the cross-talk at the cross-over state and the bar-state of each channel (W 1 and W 2 ). of pattern-mask was used for proton exchange and self-aligned thin film electrodes method.
The following results were obtained from the analysis of characteristics by measuring optical outputs of transferred and remained power with crosstalk in wave-guides after applying the input light to the wave-guide of the 1×2 optical switch.
1) With the input light applied to W 1 of the waveguide and the drive voltage to the electrodes, it was obtained that transferred power, remained power and cross-talk value were 3.5 V of W 1 , 3.9 mV of W 2 , and -29.5 dB of cross talk.
2) The switching phenomenon was obtained by proton exchange method to form wave-guide channel on LiNbO 3 substrate. The conditions of proton exchange were 200°C, 60min for reaction temp. and time, and 400°C, 10min for annealing temp. and time, respectively.
3) The best switching phenomenon, V S = 8.0 V was obtained with 4 mm of switching length.
The performance of the proposed coupled optical modulator was very superior to that of reported. It was concluded that a stable condition and accurate annealing time of process of the wave-guide were necessary for making a device with less cross-talk and better switching voltage.

Conclusion
The 1×2 electro-optical switch fabricated on x-cut LiNbO 3 optimized the efficiency of electrodes and electric field. For using by electrode only necessary part