Josef Schroefel

Novel low-temperature Er3+ doping of lithium niobate

Low temperature doping procedure is employed to make erbium doped lithium niobate substrates. The doping process is studied for X-, Y- and Z-cuts in both virgin and proton exchanged wafers. The amount as high as 9 weight percent of erbium was found in X-cuts when the doping was performed from a mixture of molten nitrates containing 10 weight percent of erbium slat after 5 hours diffusion at 400 degrees C. The content of erbium in Z- and Y-cuts as well as in the proton exchanged X-cuts was found to be much lower, below 0.5 percent. In-diffused erbium ions concentration is localized in a very thin layer, which can be extended by a subsequent annealing. Annealed proton exchanged waveguides were fabricated in the erbium doped wafers without any deterioration of the samples surfaces.

Localized moderate-temperature Er3+doping into optical crystals

Possibility of localized doping by Er3+ diffusion at moderate (less than 500 degree(s)C) temperature is demonstrated for lithium niobate and sapphire. The doping is achieved by immersing the substrate wafers into reaction melt containing small amounts of erbium salt. A crucial point of the presented technology is a crystallographic orientation of the used wafers. Though in the Z- and Y-cuts of lithium niobate the content of incorporated erbium did not exceed the concentration achieved using standard high temperature (or high energy) approaches, lithium niobate X-cuts contained up to 10 weight % of erbium. Similar results were obtained also for the corresponding cuts of sapphire. This strong anisotropy of the doping is explained on the basis of crystal structure of the particular cuts. The in-diffused layers in all the cuts are rather shallow, however, erbium can be diffused deeper into the substrates by post-diffusion annealing in air. The moderate-temperature approach enabled us to fabricate APE waveguides in erbium doped lithium niobate without deteriorating the samples surfaces. The samples were characterized by RBS, SEM, NDP and mode spectroscopy.

Thin carbon and carbon nitride films for passive and active optical waveguides

Thin carbon and carbon nitride films exhibit specific optical and mechanical properties which make them promising materials for integrated optics. For this purpose we have investigated the preparation and optical properties of carbon and carbon nitride (CN x ) films deposited on silicon substrates. The contribution deals with the study and fabrication of planar optical waveguides on semiconductor (silicon) substrates by the method of Plasma Assisted Chemical Vapor Deposition (PACVD). This waveguide is a carbon or carbon nitride layer deposited in a PACVD apparatus on a layer of silicon oxide, which also provides optical shielding of the substrate and is prepared by the oxidation of a silicon substrate wafer. The carbon as well as carbon nitride layers were fabricated by the reaction of methane, methane and nitrogen or methane and ammonia, respectively, in the PACVD apparatus. The fabricated films were characterized by optical ellipsometry and standard mode spectroscopy at 633 nm. The attenuation of the best sample was less than 0.3 dB/cm. It was found that optical and mechanical properties of films fabricated on the positive and negative electrodes were substantially different. The films deposited on the negative electrode were harder and their refractive index was higher compared with those deposited on the negative electrode. The refractive index of the harder films ranged from 2.2 to 2.6, while for the softer films it ranged from 1.6 to 1.8. We proved that it is in principle possible to dope the deposited layers with erbium ions so that the resulting structures can also be used as active waveguides.

Properties of the APE waveguides fabricated in Er:LiNbO3 and (Er+Yb):LiNbO3

We present a study of the annealed proton exchanged waveguides fabricated in erbium and in a mixture of erbium and ytterbium (RE) bulk doped lithium niobate. Waveguiding properties and composition of the RE doped waveguides were not substantially changed compared with those fabricated in pristine lithium niobate. However, presence of the doping ions decreases the r33, but a carefully designed APE technology can increase the r33 almost to the value of the pristine LiNbO3. According to our results the proton exchange need not necessarily change the efficiency of the 1,5 μm emission and certainly does not lower the concentrations of the RE.

Optical spectroscopic properties of Er3+ ions in LiNbO3 planar waveguides produced by annealed proton exchange

We investigated the optical properties of Er3+ ions in LiNbO3 planar waveguides produced by annealed proton exchange (APE) using the site-selective method of combined excitation-emission spectroscopy at low temperature. The spectroscopic results obtained for the luminescence in the green spectral region (≈ 550nm) under the direct laser excitation at 450 nm and under two step laser excitation at 980 nm (up-conversion process) are compared with bulk material and LiNbO3 waveguides produced by Ti-diffusion. Notable differences have been found in the kind of defect sites, in their number distribution, and in the inhomogeneous broadening of the optical transitions.

Carbon layers for integrated optics

Study of fabrication and properties of the carbon layers by using the PACVD (Plasma Assisted Chemical Vapor Deposition) apparatus is reported. The layers were grown on silicon substrates with methane as the precursor and were then doped with the erbium ions by treating the fabricated samples in glycerin or in the solution of erbium nitrate. To obtain deeper erbium containing carbon layers (up to 1 μm) the “sandwich method” was used based on repetition (three times) of carbon deposition and subsequent diffusion of erbium after which followed annealing in vacuum oven. The obtained results proved that it is in principle possible to fabricate the erbium containing carbon thin optical layers.

Erbium localized doping into various cuts of lithium niobate and sapphire: a comparative study

Medium temperature (350 °C) localized doping of Er3+ was studied in lithium niobate (LN) and sapphire single crystal wafers that were cut in various crystallographic directions. It was found that the efficiency of the doping was connected with orientations of the substrate wafers of both LN and sapphire, and with the presence of mobile lithium ions in the structure of LN. The basic interstitial mechanism of erbium incorporation into the structure of sapphire and LN is in the latter accompanied with erbium for lithium ion exchange. While the rate of the interstitial diffusion was higher in the wafers oriented perpendicularly towards the cleavage planes of the crystals, ion exchange process was significant in the wafers cut in cleavage planes. Waveguiding properties in erbium doped lithium niobate originated rather from presence of erbium in the structure of the crystals than being a consequence of a weak proton exchange. Luminescence properties of the fabricated samples are also presented.

Low-loss optical waveguides fabricated by Li+ for Na+ ion exchange in Na2O rich glass substrates

The aims of our work is to project and to test a material and technological solution of a preparation of planar and channel optical waveguides in substrates made of special glasses using Li+

Possibility of tailoring n e vs. c Li relations in lithium niobate optical waveguides for sensors applications

ARLR Na+ ion exchange. This method should ensure to these waveguides wide parameter variability and low losses. We paid special attention to relations between technologic parameters and final waveguide properties.

Education in photonics at the technical university level

We present results of our study of concentration profiles of lithium (cLi) in annealed proton exchanged (APE) waveguiding layers as measured by the neutron depth profiling method. This non-destructive method is based on the 6Li(n,(alpha) )3H reaction induced by thermal neutrons and allowed easy monitoring of cLi profiles in a large number of samples fabricated under various fabrication conditions. Our systematic study revealed that there was no linear relationship which unambiguously attributed (Delta) ne to (Delta) cLi, on contrast with up to now generally accepted opinion. Every particular waveguide has very similar mirror-shaped ne as well as cLi depth profiles, but, generally, all the waveguides can not be characterized with the same ne vs. cLi relationship. The most important fabrication step has appeared to be the post-exchange annealing, during which lithium atoms are transported towards the sample surfaces. The annealing regime pre-destined not only the depth distribution of lithium atoms, but as a consequence of it, also other properties of the waveguiding region. We have formulated ne vs. cLi semi-empirical relationship and listed a set of case-dependent empirical constants for our fabrication system. That allows us to fabricate the APE waveguides with a priori given properties for a wide range of special applications.