Oliver Beyer

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.

Femtosecond time-resolved absorption processes in lithium niobate crystals

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.

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.

Femtosecond holography in lithium niobate crystals

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.

Femtosecond recording and time-resolved readout of spatial gratings in lithium niobate crystals

Experimental and theoretical results on recording of spatial gratings with interfering femtosecond pulses at 388 nm and time-resolved Bragg-matched readout at 776 nm are presented for LiNbO3 crystals. These include dependences of the diffraction intensity on the time delay between probe and pump pulses, on the pump intensity and angle, and on the crystal composition. The grating buildup involves instantaneous and quasi-permanent changes of the refractive index and the absorption coefficient. These changes are due to Kerr and two-photon absorption effects and the modulation of photoexcited carriers, respectively. A good agreement between theory and experiment is achieved. Our analysis provides an understanding of nonlinear optical phenomena in LiNbO3 crystals on the femtosecond time scale. The theory is applicable to a large range of optical materials with modestly wide, ≈3to5 eV, bandgap.

Enhanced temporal resolution in femtosecond dynamic-grating experiments

Recording of gratings by interference of two pump pulses and diffraction of a third probe pulse is useful for investigating ultrafast material phenomena. We demonstrate, in theory and experiment, that the temporal resolution in such configurations does not degrade appreciably even for large angular separation between the pump pulses. Transient Kerr gratings are generated inside calcium fluoride (CaF2) crystals by two interfering femtosecond (pump) pulses at 388 nm and read out by a Bragg-matched probe pulse at 776 nm. The solution to the relevant coupled-mode equations is well corroborated by the experimental results, yielding a value of the Kerr coefficient of ~ 4.4×10^(–7) cm^2/GW for CaF2.

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).