1. Introduction

Nonlinear optical devices using ferroelectric materials such as LiNbO₃ and LiTaO₃ have made great technological breakthrough and achieved broad public acceptance in the past decade. The reason is simple: every required frequency can be generated from IR through UV light region [1–3] as if a made-to-order system by quasi phase matching (QPM). The technology can be applied to the field of optical wavelength switching for future wavelength division multiplexing (WDM) network in telecommunication bands. Besides the QPM, highly nonlinear fiber (HNLF) [4] is a well-known wavelength converter by four wave mixing (FWM), however its weak third order nonlinearity χ⁽³⁾ caused low conversion efficiency, even putting a few tens- or hundreds-meters-long HNLF. Recently, although a new material chalcogenide glass (As₂S₃) [5,6] is also known, the narrow waveguides result in high coupling/propagation loss and its conversion efficiency reduction. On the other hand, a gain medium, semiconductor optical amplifier (SOA) is adequate for placing in WDM system using cross-phase-modulation (XPM), cross-gain-modulation (XGM) and FWM effects [7,8]. Nevertheless, the semiconductor device causes several serious problems with XPM and XGM effects: frequency chirping, signal-to-noise ratio degradation, narrow input dynamic range and limited channel number.

Thus, in order to achieve the compatibility between high efficiency and high quality in the wavelength conversion, periodically poled magnesium doped LiNbO₃ (Mg:LN) material is very attractive for future network vision, due to advantages of its large second order nonlinearity χ⁽²⁾ (d₃₃ = 25 pm/V) and ultrafast parametric conversion process. An adhered-ridge-waveguide (ARW) structure [9] that we have presented is also a key role to enhance the conversion performance. When comparing with the previously known waveguide fabrication techniques such as proton exchange [10–12] and Ti diffusion [13], ARW structure allow to achieve a tighter confinement and higher overlap for interacting modes by the step index profile (Δn~1), and suppress unnecessary effects such as degradations for nonlinear coefficient and low durability for long term operation. In our previous report [9], second harmonic generation (SHG) normalized conversion efficiency of 370%/Wcm² in 11.4 mm (480%/W) was demonstrated with a combination of QPM and ARW by dicing approach for waveguides, but the method yielded moderate propagation loss due to sidewall roughness that limits interaction length of the device. This work, with our recent developments and optimizations in fabrication processes, it has become possible to attain low-insertion-loss, high-design-flexibility and efficient channel conversion that containing optical parametric amplification (OPA) in difference frequency generation (DFG) using a 50 mm-long QPM-ARW device.

2. Device fabrication

All fabrication processes can be separated as wafer-bonding and -polishing, periodical-poling, and waveguide-dry-etching steps. First of all, a Z-cut 500 µm-thick 5.0 mol% Mg:LN with a sputtered SiO₂ layer was adhered to a 500 µm-thick non-doped congruent LN (CLN) substrate by epoxy, and precisely lapped to 4 µm thickness by mechano-chemical polishing (CMP). Each of SiO₂ and epoxy adhesive layers is approximately 0.5 µm thick. All adhered and polished wafers underwent measurements by interferometer to certain that the thickness distribution of Mg:LN layer is no greater than 5%. Following the typical wafer cleaning processes, periodic photoresist patterns of 16.5, 16.7 and 16.9 µm were formed on the +Z face by photolithography, and cut the wafer to chip size by a dicing saw. After evaporation of a metal film on the both sides as electrodes, we proceeded with the electric field periodical poling step. For the reason of coercive field difference between Mg:LN and CLN materials, just the polarization of Mg:LN layer was reversed when applying 9.0 kV/mm electric field and 1200 pulses in a vacuum chamber at 100 °C. A long application time corresponds to the charge transfer time to stabilize polarization reversal. Finally, we obtained a uniform periodically poled structure over 50 mm length. As discussed already in our previous report [9], the poling process after adhesion and polishing enables to avoid excess loss on the top surface in comparison with poling process before adhesion. For dry etching process, we firstly cleansed the layers of metal and photoresist on the both sides, then formed a new mask pattern on the +Z face as an etching mask, and fabricated ridge waveguides by electron cyclotron resonance-reactive ion etching (ECR-RIE) which provides waveguides design with more flexibility like taper couplers and folding type integrations.

3. Results and discussions

An overview photomicrograph of ARWs is shown in Fig. 1 . The ridge height, width and length of the ARWs were approximately 3 µm, 3.5-12.0 µm and 50 mm, respectively. Figure 2 shows a scanning electron microscope (SEM) cross-sectional photograph of 9, 8 and 7 µm-wide waveguides: each waveguide width corresponds to a dimension of photomask design. With the improvement of a fabrication approach for waveguides from dicing to ECR-RIE, we were placing our hopes for reducing the propagation loss, and actually confirmed the sidewall roughness was fairly reduced in the observation by SEM.

 

Fig. 1 Overview photomicrograph from the top face of ARWs (26 waveguides per each section: total 52 waveguides in the above photograph).

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Fig. 2 SEM cross-sectional photograph of 9, 8 and 7 µm-wide waveguides. An insulator-sandwich structure is fabricated by adhesion and CMP processes.

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In optical characteristics, we firstly measured the insertion loss for different device length to determine the propagation loss using a CW tunable diode laser at 1550 nm and a CW

Ti:sapphire laser at 780 nm. The insertion loss of the 8 µm-wide waveguide was 4.4 dB at 1550 nm and 3.2 dB at 780 nm, in the 50 mm-long device, for the TM-mode without anti-reflection coating. Under the same conditions, the propagation loss was approximately <0.5 dB/cm at 1550 nm and <0.3 dB/cm at 780 nm, respectively. These results exhibit light scattering from sidewall was considerably improved, and become identical in comparison with already known waveguide techniques such as Ti diffusion and proton exchange. To investigate the confinement in the waveguide, near-field mode profiles of both lights at the same waveguide width were represented in Fig. 3 . Measured each mode field diameter at 1/e² intensity is as follows: 5.40/3.03 µm (horizontal/vertical) at 1550 nm (Fig. 3(a)) and 4.75/3.05 µm at 780 nm (Fig. 3(b)), respectively.

 

Fig. 3 Near-field mode profiles at 8 µm-wide waveguide for each wavelength: (a) 1550 nm, (b) 780 nm.

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After evaluating these basic properties for ARWs, here we will discuss about wavelength conversion characteristics. We carried out a measurement of SHG tuning curve as shown in Fig. 4 . Through the use of a CW tunable laser diode source, the focused beam by lens was coupled into the waveguide, a maximum internal normalized conversion efficiency of 4600%/W was achieved at a fundamental wavelength of 1576.5 nm, where the SHG and fundamental powers were 0.36 mW and 2.8 mW at the inside of the output surface, respectively. The each inside power was estimated from the measured output power and the Fresnel reflection loss at the wavelength. The waveguide width, QPM period, and interaction length were 8 µm, 16.7 µm, and 50 mm, respectively, and the device was operated at 25 °C by a thermo-electric cooler (TEC). Note that, the insertion loss and wavelength conversion efficiency were characterized with the same waveguide, and the efficiency is assumed zero propagation loss.

 

Fig. 4 SHG tuning curve shows that the maximum normalized conversion efficiency is approximate 4600%/W at fundamental wavelength of 1576.5 nm. The side robes correspond to fluctuations of the waveguide’s dimensions.

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By numerical calculations using the measured mode field diameters at 1550/780 nm and following expression as Eq. (1) [14],

we estimated that an SHG normalized conversion efficiency around 6000%/W could be accomplished. However, under experimental circumstance, several remarkable side lobes in the tuning curve indicate non-uniformity of waveguide’s dimensions for the whole device length. When finishing the dry etching process, several interference fringes can be identified with observation from the top surface of the device. Therefore, we supposed that the waveguide-defining process is a main factor to cause them comparing with the other possibilities such as fluctuations of composition in Mg:LN. However, note that these phenomena imply that the efficiency will increase significantly, when we can improve the processes of the photolithography and ECR-RIE. We also compared a calculated FWHM of a 50 mm-long device with the measured FWHM of the highest peak in the device; both of them are approximately 0.2-0.3 nm. Since the value is quite small, the experimental FWHM could include accuracy and resolution deviations of wavelength in the tunable laser.

Figure 5 provides DFG spectra with the same device, that contained two curves without and with a Ti:sapphire CW pump light of 160 mW. The DFG process was demonstrated in the L-band from wavelength 1570 nm to 1583 nm at 25 °C with TEC control. When input signal light power was fixed at −15 dBm, the signal and converted lights reached +9.5 dB (890%) and +7.6 dB (570%) with OPAs, being able to cover all losses in this device and fiber pigtail module [15,16]. We plotted the dependences of signal and converted lights power on pump in Fig. 6 that include the above measured plots of pump power 160 mW. From the order of pump power 10 mW, OPAs caused a gain rise for both lights, having a correspondence with our estimation that was calculated from three-wave-coupled mode equations using measured mode field diameters.

 

Fig. 5 DFG spectra comparison of just signal light (dashed line: −15 dBm) and coupling with 160 mW pump of a Ti:sapphire CW laser (solid line). Clear OPAs appear in the both lights. The central peak corresponds to the 2nd order diffraction of the pump light in an optical spectrum analyzer.

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Fig. 6 Dependences of signal (white) and converted (black) lights power on Ti: Sapphire CW pump power. Each upper right plot of signal and converted lights correspond with the spectra in Fig. 5 for 160 mW pump.

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In addition, we also measured an input power dynamic range in DFG, and over 50 dB was obtained. Such a wide range enables to convert a multi-value modulated signal with high extinction ratio [17,18], to handle irregular input signal variations.

4. Conclusion

We reported recent technology progresses of QPM-ARW, showing low insertion loss, flexible design, and highly efficient gain by OPA in channel conversion. It has been recognized that optical channel conversion for practical applications needs erbium-doped fiber amplifier (EDFA) or SOA to amplify the both of signal and converted lights as pre-/post- amplifiers. However, our study derived a loss-less and gain device as a result of enhancing the device performance up to 4600%/W internal SHG normalized conversion efficiency. We have an expectation for ultra-high-speed wavelength switching with QPM-ARW in all optical nodes.