Harald Herrmann

A transport network layer based on optical network elements

An approach to the realization of a broadband, flexible, multiwavelength transport network employing an optical network layer. The design methodology for a network demonstrator is presented, and the transmission, switching, line, and management/supervisory subsystems and components are described. >

Integrated Optical Devices in Lithium Niobate

Lithium niobate offers incredible versatility as a substrate for integrated optics. Researchers have developed an array of new optical devices based on this material, including waveguide structures, electro-optical wavelength filters and polarization controllers, lasers with remarkable properties, nonlinear frequency converters of exceptional efficiency, ultrafast all-optical signal processing devices and single photon sources.

Polarization-independent integrated optical, acoustically tunable double-stage wavelength filter in LiNbO/sub 3/

We report a polarization-independent integrated acousto-optical double-stage wavelength filter in X-cut, Y-propagating LiNbO/sub 3/. The device consists of a combination of four acousto-optical mode converters, a TE- and a TM-pass polarizer, and two polarization splitters; their specific design and properties are discussed. The filter has a bandwidth of 1.4 nm and a tuning range of 76 nm around /spl lambda/1.55 /spl mu/m. Maximum filter transmission is achieved with a RF drive power of only 80 mW; it excites via a single unidirectional transducer a guided surface acoustic wave driving the four polarization converters of both stages. Fiber-to-fiber insertion loss is 4.6 dB for TM- and 4.8 dB for TE-polarized waves. Multiwavelength filtering has been demonstrated. >

Quasi-phase-matched difference-frequency generation in periodically poled Ti:LiNbO 3 channel waveguides

Mid-infrared radiation near 2.8 mum was generated by difference-frequency generation in an 80-mm-long periodically poled Ti:LiNbO(3) channel waveguide by pump radiation near 1.55 mum (tunable external-cavity laser) and a signal radiation of 3.391 mum (HeNe laser). We obtained a normalized conversion efficiency of 105% W(-1) , which is to our knowledge the highest value ever reported.

Source of sidelobe asymmetry in integrated acousto‐optic filters

The often‐observed extreme asymmetry in the sidelobe spectra of integrated acousto‐optic filters exacerbates cross talk between wavelength‐multiplexed optical channels. We explain the source of this asymmetry in terms of a systematic even‐order variation of the effective waveguide birefringence as a function of distance from the device center. Calculations show what degree of nonuniformity in Ti‐stripe thickness, width, or diffusion temperature are required to account for typical asymmetries. We describe two experiments in which temperature gradients were applied to low‐power acousto‐optic filters in order to induce sidelobe suppression on either side of the optical transmission function, while greatly enhancing sidelobes on the opposite side.

Tapered acoustical directional couplers for integrated acousto-optical mode converters with weighted coupling

Weighted coupling for strong sidelobe suppression of integrated acoustooptical mode converters in LiNbO/sub 3/ using acoustical directional couplers has been studied theoretically and experimentally. A parameter free model for the propagation of surface acoustic waves in guiding structures has been developed based on a step-like variation of the acoustic velocity. Comparisons of theoretical results with experimental ones for acoustic waveguides and directional coupler structures confirm the applicability of the model. A coupled mode description of the acousto-optical polarization conversion in converters with acoustical directional couplers has been developed and applied to several tapered acoustical directional couplers. The model reveals that the conversion characteristics are usually strongly asymmetric. If the directional coupler is appropriately designed, a sidelobe suppression of about 30 db can be achieved. First experimental results with tapered directional couplers confirm within some limits the theoretical predictions. >

Low-loss tunable integrated acoustooptical wavelength filter in LiNbO/sub 3/ with strong sidelobe suppression

An integrated acoustooptical double-stage wavelength filter in LiNbO/sub 3/ has been developed using acoustooptical polarization converters and polarization splitters as basic components integrated on the same chip. Ti-indiffusion technology is used throughout to fabricate the device. The packaged and pigtailed filter has low insertion losses (4.1 dB fiber-to-fiber), low-polarization dependence (0.1 dB) and a strong sidelobe and baseline suppression (>30 dB). Its tuning range exceeds the gain bandwidth of erbium-doped fiber amplifiers around 1550 nm.

Polarization independent, integrated optical, acoustically tunable wavelength filters/switches with tapered acoustical directional coupler

Enhanced sidelobe suppression of the filter characteristics of tunable acousto-optical mode converters in LiNbO/sub 3/ has been achieved using for the first time tapered acoustical directional couplers. A sidelobe suppression of more than 15.5 dB could be demonstrated in a single stage device. By combining the mode converter with two integrated optical polarization splitters a polarization independent filter/wavelength selective switch has been fabricated with less than 3 dB intrinsic insertion loss, a polarization dependence of less than 1 dB, and a sidelobe suppression exceeding 14 dB.<<ETX>>

Post-selection free, integrated optical source of non-degenerate, polarization entangled photon pairs.

We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.

All-optical Ti:PPLN wavelength conversion modules for free-space optical transmission links in the mid-infrared

Data-format-independent all-optical transmitter and receiver modules for free-space optical communications in the 3.8 mum region have been developed, essentially consisting of Ti-indiffused, periodically poled LiNbO(3) waveguides. Using these modules, conversion of C-band radiation to/from the mid-infrared (MIR) via difference-frequency generation was demonstrated. More than 10 mW of MIR power could be generated with the transmitter unit; the internal low-power conversion efficiency was 69%/W. A free-space MIR link was set up with a -41 dB fiber-to-fiber loss; only -15 dB are due to the parametric processes.