Alan C. G. Nutt

Domain inversion in LiNbO3 using direct electron‐beam writing

A 3‐μm periodic domain‐inverted pattern on the C− face of LiNbO3 has been written using direct electron‐beam writing for first‐order gratings used in second harmonic generation. The domain formation mechanism has been investigated, especially the shape of the domain and need for a conductor on the C+ face during the electron‐beam writing process.

Domain inversion in KTiOPO4 using electron beam scanning

Domain inversion in c‐cut KTiOPO4 was produced by scanning an electron beam on the −c face. The domain reversal occurred through the 1 mm thickness of the sample. Second‐harmonic conversion efficiency of 7×10−5 was measured for a fifth‐order grating by focusing the beam to a 7 μm spot in a 500‐μm‐long domain‐inverted KTP crystal. This efficiency is close to the theoretical value of 9×10−5. The measured phase matching bandwidth was 1.9 nm, which is in agreement with the theoretical value of 1.5 nm indicating that the domain‐inverted grating is reasonably uniform over its entire length.

Second‐harmonic generation in bulk and waveguided LiTaO3 with domain inversion induced by electron beam scanning

Bulk quasi‐phase‐matched frequency doubling experiments are reported for a lithium tantalate crystal which was periodically poled through its 0.5 mm thickness by electron beam scanning. The measured phase‐matching bandwidth of 1 nm for the 2‐mm‐long domain inverted section was close to the theoretical value of 0.4 nm, although the conversion efficiency was lower than theoretically expected. Chemical etching revealed domain duty‐cycle variations and incomplete inversions which are likely to have caused the lower conversion efficiency. Frequency doubling in waveguides fabricated in this material produced a normalized conversion efficiency of 290%/W cm2 and a phase matching bandwidth of 0.3 nm for a 1‐mm‐long domain inverted region.

Periodic domain inversion in Z-cut LiTaO3 by electron-beam scanning assisted by an internal field within the crystal

The coercive field for domain reversal in a 0.5 mm Z-cut LiTaO3 was measured to be 21 KV/mm. However, if the polarization of the crystal is reversed using an electric field of 21 KV/mm, the coercive field for a second reversal is found to be 11 KV/mm. This asymmetry is due to the presence of an in-built internal field of 5 KV/mm in the virgin crystal. If the electron beam writing is done on the new C-face of the polarization reversed region of the crystal, (1) no cracking was observed, (2) the writing required less charge for domain inversion because of the reduced coercive field, thereby facilitating shorter scan times. We have obtained uniform 3.6 micrometers period domain inversion grating using this technique.

One-step stripe waveguide fabrication process for polymer integrated optics

Single-mode stripe optical waveguides in poled organic thin films have been fabricated by excimer laser photoablation using wavelengths at 248 nm and 193 nm. The process uses a quartz proximity photomask as a stencil to the collimated laser light. The process also requires relatively low light densities (approximately 20 mJ/cm2) allowing large areas to be processed with currently available excimer lasers. This one-step process allows the definition of intricate structures without altering film alignment properties necessary to utilize the inherent electro-optic effect. Possible distortion of waveguide structures caused by diffraction elements (such as in waveguide Y-junctions) is examined. Active devices exhibit electro-optic coefficients similar to equivalent planar waveguide devices.

1- to 100-channel waveguide beam splitter and TE to TM polarization converter

This paper describes a channel waveguide beam splitter (CWBS) in which a single laser beam can be split into many beams using glass stripe waveguides and localized surface relief gratings. The grating element situated on top of individual 90 degree(s) T-branches couples a fraction of the backbone light down a side channel and acts as a miniature mode converter. Single-mode TE light in the backbone is converted into single-mode TM light in the side channel. The laser wavelength used in these experiments was 830 nm. Each individual grating was only a few microns in length and so the grating acceptance exhibited a large bandwidth. The total excess loss coupling the input waveguide optical power partially into 100 waveguide branches was only 0.25 dB.