Electro-optic integration of embedded electrodes and waveguides in LiNbO_3 using a femtosecond laser

Abstract

We describe the fabrication of a Mach-Zehnder optical modulator in LiNbO 3 by femtosecond laser micromachining, which is composed of optical waveguides inscripted by a femtosecond laser and embedded microelectrodes subsequently fabricated using femtosecond laser ablation and selective electroless plating. A half-wave voltage close to 19 V is achieved at a wavelength of 632.8 nm with an interaction length of 2.6 mm. This simple and cost-effective technique opens up new opportunities for fabricating integrated electro-optic devices. © 2008 Optical Society of America OCIS codes: 130.0250, 140.3390, 230.2090 Lithium niobate ͑LiNbO 3 ͒ is one of the most widely used nonlinear optic materials in integrated optics owing to its excellent nonlinear optical and electrooptic (EO) properties. Integrated EO devices based on waveguiding structures such as optical switches [1] and modulators [2,3] have gained significant attention. Conventionally, the waveguide fabrication is based on titanium diffusion or proton exchange, which permits fabrication of channel waveguides only close to the surface [4]. Recently, it has been demonstrated that buried optical waveguides in LiNbO 3 can be fabricated by femtosecond laser inscription [5], which opens the possibility to write 3D optical circuits in the crystal. Various fabrication parameters have been optimized by several groups [6][7][8], such as wavelength, repetition rate, polarization, pulse width, pulse energy, focusing geometry, and scanning speed. To realize integrated EO devices, it is further crucial to design and fabricate microelectrodes, which are usually obtained by use of lithographic methods. However, owing to the inherently planar nature of the lithographic process, this technique is limited in its capability to produce 3D structures. Furthermore, because the optical waveguides fabricated by femtosecond laser inscription usually are deeply buried in the crystal, the conventional surface electrodes give rise to weak EO interaction. As a simple and costeffective technique, laser-induced selective electroless deposition has been widely investigated, because it does not require fabrication steps involved in the traditional lithographic technique, such as deposition of thin layers of metals, pattern etching, and so on [9,10]. Recently, we developed a technique for selective metallization in LiNbO 3 using femtosecond-laser ablation and femtosecond-laser-assisted selective electroless plating, which allows for fabricating microelectrodes deeply embedded in LiNbO 3 [11]. In this Letter, we report the integration of embedded microelectrodes and optical waveguides in LiNbO 3 using a femtosecond laser. Based on this technique, a Mach-Zehnder interferometer (MZI) EO modulator in an x-cut LiNbO 3 crystal is demonstrated.For the fabrication of waveguides and electrodes, a Ti:sapphire laser system (Legend USP, Coherent Inc.) with an operation wavelength of 800 nm, a pulse width of ϳ40 fs, and a repetition rate of 1 KHz was used. Commercially available MgO-doped x-cut LiNbO 3 crystals...

Self-frequency-doubling of ultrafast laser inscribed neodymium doped yttrium aluminum borate waveguides

“…This is, indeed, in accordance with previous works reporting on the nonlinear properties of ultrafast laser inscribed WGs in nonlinear crystals. [8][9][10][11] The unexpected increment of the SH back intensity observed at damage tracks has been tentatively attributed ͑in accordance with previous works͒ to the presence in those volumes of a defect-assisted back-scattering enhancement. 12 Thus, the confocal images of Fig.…”

On-Chip Tuning of the Resonant Wavelength in a High-Q Microresonator Integrated with a Microheater

“…More details on the selective metallization can be found elsewhere. [16,20] The reason to fabricate the microheater in prior to the fabrication of microresonator is for avoiding the contamination on the microresonator during the electroless plating. …”

“…[12][13][14][15][16] By tightly focusing a femtosecond laser beam in dielectric materials such as glass and crystals, the interior of the material can be modified in a space-selective manner through multiphoton absorption. This technique therefore provides an ideal solution for integrating multifunctional microcomponents in a single chip, such as a microresonator integrated with a microheater which allows rapid tuning of the resonant wavelength.…”

“…[17][18][19] Meanwhile, selective metallization has also been achieved in transparent substrates such as glass and crystal based on femtosecond laser direct writing, which has enabled fabrication of microelectrodes and on-chip microheaters. [16,20,21] Nevertheless, these techniques have not been synergistically employed for achieving on-chip tuning of the resonant wavelength of a microresonator. Here, we demonstrate fabrication of a microresonator integrated with a microheater on a single fused silica chip.…”

Progress in ultrafast laser processing and future prospects

“…Ultrafast laser processing has been extensively applied to fabricate 3D photonic devices such as optical couplers and splitters [84], volume Bragg gratings [85], diffractive lenses [86], Mach-Zehnder interferometers (MZIs) [87], and waveguide lasers [88,89]. Such photonic devices rely on the writing of 3D optical waveguides that can be easily inscribed in transparent materials such as glass, crystalline materials, and polymers by inducing permanent refractive index changes in the focal volumes of tightly focused ultrafast laser pulses [90].…”