Ingo Nee

Role of iron in lithium-niobate crystals for the dark-storage time of holograms

The dark decay of holograms stored in iron-doped photorefractive lithium-niobate crystals is studied for samples containing up to 0.25 wt% Fe2O3 (iron concentration 71×1018 cm−3). The oxidation/reduction state of the crystals, i.e., the concentration ratio of Fe2+ and Fe3+ ions, is changed in a wide range by thermal annealing. The dark decay is attributed to two effects: An ionic dark conductivity arising from mobile protons and an electronic dark conductivity caused by tunneling of electrons between iron sites. The latter is proportional to the effective trap density, i.e., to the density of charge carriers which can be moved between the iron sites. The proportionality factor is the specific dark conductivity which increases exponentially with the third root of the entire iron concentration.

Ionic and electronic dark decay of holograms in LiNbO3:Fe crystals

The lifetimes of nonfixed holograms in LiNbO3:Fe crystals with doping levels of 0.05, 0.138, and 0.25 wt % Fe2O3 have been measured in the temperature range from 30 to 180 °C. The time constants of the dark decay of holograms stored in crystals with doping levels of 0.05 and 0.25 wt % Fe2O3 obey an Arrhenius-type dependence on absolute temperature T, but yield two activation energies: 1.0 and 0.28 eV, respectively. For these crystals, two different dark decay mechanisms are prevailing, one of which is identified as proton compensation and the other is due to electron tunneling between sites of Fe2 + and Fe3 + . The dark decay of holograms stored in crystals with the doping level of 0.138 wt % Fe2O3 is the result of a combination of both effects.

Holographic recording of Bragg gratings for wavelength division multiplexing in doped and partially polymerized poly(methyl methacrylate)

Bragg gratings are recorded in doped and partially polymerized poly(methyl methacrylate) with green light (wavelength, 532 nm) in transmission geometry, and the gratings are read in reflection geometry with infrared light (wavelength, approximately 1550 nm). Diffraction efficiencies of more than 99% with a wavelength bandwidth of approximately 1 nm are obtained for single gratings with a typical length of 15 mm. Superposition of four gratings in a volume sample has been demonstrated as well. The material is promising for use in the fabrication of add-drop filters, attenuators, switches, and multiplexers-demultiplexers for optical networks that use wavelength division multiplexing.

Multichannel wavelength-division multiplexing with thermally fixed Bragg gratings in photorefractive lithium niobate crystals

The transmission capacity of fiber communication networks is enhanced by usage of dense wavelength division multiplexing (WDM). This technique requires wavelength filters for multiplexing of the channels. We report on the realization of a multiplexer device based on superimposed volume-phase gratings in a single lithium niobate crystal. The gratings are recorded through the photorefractive effect by interference of two green laser beams. Thermal fixing is employed to increase the lifetime of the gratings. Each grating diffracts light of a certain WDM channel (wavelengths of ∼1500 nm). Simultaneous multiplexing of many channels is achieved by suitable arrangement of the gratings in the crystal. We present the basic concept of this technology as well as recent advances: (1) refined experimental methods about tailored recording of many-channel multiplexers, (2) characterization of the multiplexers for up to sixteen WDM channels (1-dB bandwidth up to 0.1 nm, channel spacing down to 0.4 nm), and (3) construction of a two-channel multiplexer device.

Development of thermally fixed holograms in photorefractive lithium-niobate crystals without light

Summary form only given. Holographic gratings are recorded at room temperature in photorefractive iron-doped congruently-melting LiNbO/sub 3/ crystals. Afterwards the diffraction efficiency of these gratings is measured from time to time to monitor the dark decay. Samples with different iron contents are investigated, and the Fe/sup 2+//Fe/sup 3+/ concentration ratio is varied. Time constants /spl tau/ of the decay range from minutes to years, and the dark conductivities /spl sigma//sub d/=/spl epsi//spl epsi//sub 0///spl tau/ are deduced, where /spl epsi/=28 is the dielectric constant. The major outcomes are: (1) There is a small, iron independent background conductivity (/spl tau/=1 year) which arises from mobile ions. (2) For Fe concentrations in excess of about 20/spl times/10/sup 18/ cm/sup -3/ (0.05 wt.% Fe/sub 2/O/sub 3/): the dark conductivity is proportional to the effective trap density N/sub eff/=(1/C(Fe/sup 2+/)+1/C(Fe/sup 3+/)/sup 9/-1) and the normalized dark conductivity /spl sigma//sub d//N/sub eff/ rises exponentially with (C/sub Fe/)/sup 1/3/.

Mechanisms of the dark decay of holograms in LiNbO/sub 3/:Fe crystals

Summary form only given. Two mechanisms of dark decay, proton compensation and electron tunneling have been identified. In LiNbO/sub 3/:Fe with doping levels less than 0.05 wt%, proton compensation dominates the dark decay, while in crystals with doping levels as high as 0.25 wt%, electron tunneling dominates. For crystals with doping levels between 0.05 wt% and 0.25 wt%, both mechanisms contribute significantly to the dark decay, and the single Arrhenius law does not hold anymore with a single activation energy.

Advanced wavelenth division multiplexing with thermally fixed volume-phase gratings in iron-doped lithium niobate crystals

Fiber communication networks utilize wavelength-division- multiplexing (WDM) to enhance the transmission capacity of fiber-optical networks. This technique requires narrowband wavelength filters for multiplexing and de-multiplexing of the channels. We report on realization of an advanced multiplex/demultiplex device based on superimposed volume- phase gratings in lithium-niobate crystals. The gratings are recorded via the photorefractive effect by interference of two green laser beams. Thermal fixing is employed to increase the lifetime of the recorded gratings. Infrared light in the telecommunication wavelength region around 1500 nm is diffracted from the gratings. Each grating reflects light of a certain WDM channel. The selected wavelengths and the propagation directions of the diffracted beams are determined by spatial frequency and orientation of the gratings in the crystal. We will present the basic concept of this technology as well as recent advances : (1) construction and testing of a two-channel demultiplexer prototype (fiber to fiber insertion loss 5-6 dB, crosstalk less than -25 dB, channel spacing 0.8 nm), (2) simultaneous demultiplexing of 8 channels (separation 0.8 nm).

Thermally-fixed volume-phase holograms in lithium-niobate crystals for optical interconnects

Summary form only given. Volume holographic components enable free-space optical interconnects with outstanding wavelength and angular selectivity. An application of particular importance is dense wavelength-division multiplexing (WDM). From each hologram light of one WDM channel is diffracted. Orientations and spatial frequencies of the gratings determine the directions of the diffracted light beams. A free-space arrangement allows simultaneous demultiplexing of several channels. The light can be re-coupled into fibers by, for example, gradient-refractive-index (GRIN) lenses. Photorefractive iron-doped lithium niobate crystals (LiNbO/sub 3/) are used for recording and superposition of the required holograms. Inhomogeneous illumination builds up space-charge fields which modulate the refractive index via the electrooptic effect. Thermal fixing is employed to store the holograms permanently. The crystal is heated during or after recording, protons become mobile and compensate for the space-charge fields. After cooling down to room temperature, the fixed hologram can be developed by homogeneous illumination. We report on the realization of a two-channel demultiplexer by superposition of two thermally fixed volume phase reflection holograms in a LiNbO/sub 3/ crystal.

Optimization of iron-doped photorefractive lithium niobate crystals

Summary form only given. Photorefractive iron-doped lithium niobate crystals (LiNbO/sub 3/:Fe) are still the best choice for many applications, such as holographic data storage, outstanding interference filtering, or wavelength division multiplexing. Iron occurs in LiNbO/sub 3/ in the valence states Fe/sup 2+/ and Fe/sup 3+/, only. Holographic investigations are performed by an (argon-ion laser) and with near-infrared light (wavelength 760 nm, Ti:sapphire laser). Gratings are recorded by interference of two plane waves. One of the recording waves is blocked and the other one is diffracted from the grating. The amplitude of the refractive-index modulation is calculated from the measured diffraction efficiency.