Variable time delay of microwave signals using high dispersion fibre

A programmable time delay of 0-5 ns has been demonstrated for a 3 GHz signal using a tunable semiconductor diode laser and 1 km of a new high-dispersion singlemode fibre with negative chromatic dispersion. Because it is fibre based and uses direct detection, this system is resistant to vibration effects.<>     

Population-inversion and gain estimates for a semiconductor TASER

We have investigated a solid-state design advanced (see Soref et al. in SPIE Proceedings, vol.3795, p.516, 1999) to achieve a terahertz-amplification-by-the-stimulated-emission-of-radiation (TASER). The original design was based on light-to heavy-hole intersubband transitions in SiGe-Si heterostructures. This work adapts the design to electron intersubband transitions in the more readily available GaAs/Ga _1-xAl_xAs material system. It is found that the electric-field induced anti-crossings of the states, derived from the first excited state with the ground states of a superlattice in the Stark-ladder regime, offers the possibility of a population inversion and gain at room temperature     

Voltage-controlled optical/RF phase shifter

A new fiber-optic device designed to steer the radiation beam of a phased-array antenna has been demonstrated. A radiofrequency (RF) signal at 125 MHz is generated via photomixing at the output of a single-mode fiber-optic interferometer. The phase of the electrical signal is shifted over several cycles in direct proportion to a voltage applied to an optical controller. The controller consists of a Pockels-type optical phase modulator located in one arm of the heterodyne interferometer. Rapid changes in RF phase are feasible. A miniature low-voltage version of the device, based upon integrated optics, is proposed.     

Multimode 2 × 2 optical crossbar switch

A monolithic electro-optical 2 × 2 crossbar has been developed. The multimode network consists of channel waveguides and four crosspoint switches integrated in a LiTaO3 plate. Matrix switching loss was 11 dB and crosstalk was ?16 dB for TM-polarised 0.07-n.a. light.     

Sensitive fieldable photoelastic fiber-optic hydrophone

A multimode fiber-optic intensity-modulated hydrophone has been constructed that appears capable not only of matching the sensitivity of conventional hydrophones but in fact has the potential for deployment at ocean depths and the potential for surviving severe hydrostatic shocks. This approach uses a transduction element comprised of a channel of photoelastic material that becomes optically birefringent when subjected to a uniaxial stress. The birefringence in turn acts cooperatively with polarizers to modulate the light beam traveling through the material in a manner that is in accordance with the applied acoustic stress signal.     

Theory of the strain-symmetrized silicon-based Ge-Si superlatticelaser

A unipolar p-i-p silicon-based intersubband laser consisting of a symmetrically strained GeSi superlattice on a relaxed Si_0.5Ge _0.5 buffer layer is modeled and analyzed. The strain-symmetrization removes the limitation on the size of the superlattice. The procedure for calculating the in-plane energy dispersion is extended to a superlattice. Analysis of the in-plane energy dispersion shows that the population inversion is local-in-k-space. For an 11 ML/11 ML superlattice (15.4 Å/15.4 Å), interminiband lasing between HH2 and HH1 is predicted at λ=2.2 μm. From the envelope functions and material properties, the miniband lifetimes and laser gain are calculated. For a current density of 10 kA/cm², a gain of G_L=96/cm is calculated. Alternate structures with larger expected gains are considered. Quantum-parallel, quantum-cascade, and quantum-staircase lasing are examined     

Quantum parallel laser: a unipolar superlattice interminiband laser

Designs are given for parallel (rather than cascade) intersubband lasing in superlattice structures of group IV and III-V materials. Structures with population inversion due to coupled wells have minibandwidths too narrow for coherent transport and the incoherent interperiod rates are too small for population of a significant number of periods. A single-well period with local population inversion gives operation in the coherent regime. An example is given for the III-V materials. Both SiGe-Si and Si-ZnS are discussed for intersubband lasing at 1-20 /spl mu/m.     

Interdigital twisted-nematic displays

A new field-effect liquid crystal display is reported. Display patterns are derived from arrays of grating-like electrodes on one plate of the display package. Since the opposed plate is not electroded, construction of the display is somewhat simplified compared to conventional twisted-nematic displays.