Hybrid bistable devices at 10.6 µm

We present a new hybrid bistable device which depends on the frequency of a modulation carried by the output optical beam. Experiments have been carried out with a CW CO2laser at 10.6 μm wavelength and an electrooptic CdTe amplitude modulator. The necessary nonlinearity for switching was provided by the tuning characteristics of a resonant circuit. This resonant driver circuit allows application of reduced voltages on the CdTe crystal to obtain bistability. Two feedback loops have been considered using either synchronous (at 0.117 MHz) or envelope detection (at 0.853 MHz).     

Optical single sideband generation at 10.6 µm

The indicatrix ellipsoid of an electrooptic crystal with a threefold rotation axis under the influence of an applied microwave field circularly polarized in a plane normal to the axis intersects this plane in a rotating ellipse of constant shape. A circularly polarized optical field in this plane produces an electric polarization at one sideband frequency only, upshifted if the optical field rotates in the opposite direction the indicatrix rotates, downshifted if the rotations are the same. Ideally, 100 percent conversion of the optical incident field into the up- or downshifted field may be achieved. A square waveguide containing a CdTe crystal is investigated as the structure appropriate for phase-matched single sideband conversion. Experiments are reported yielding a 67 percent conversion efficiency for shifting 10.6 μm laser radiation by 17 GHz. The efficiency was limited by crystal length and/or microwave breakdown. For longer crystals conversion efficiencies approaching 100 percent are extrapolated. Scaling laws for other microwave and laser frequencies are presented.     

Efficient low-field Stark interaction at 10.78 µm

We describe a new Stark interaction in the 10μm region which is resonant at zero Stark field and produces large absorption changes at low voltages. The resonance is due to the interaction between the R(18) line at 10.78μm from a¹³CO2laser and the asQ     

Low-loss polarization-independent electrooptical switches at λ = 1.3µm

Polarization-independent single-mode optical cross points with low loss have been fabricated forlambda = 1.3-mum wavelength. Waveguide design, coupling strength equalization for the two polarizations, fabrication tolerances, and the performance of several Ti:LiNbO3directional-coupler switches are presented. Two 2 × 2 switches have been made with fiber pigtails. The switches have an insertion loss of ∼3- andsim-14-dB crosstalk isolation with a 70-V operating voltage. A 4 × 3 switch had similar performance but higher insertion loss.     

Lateral confinement InGaAsP superluminescent diode at 1.3 µm

A superluminescent diode (SLD) has properties, based on the degree of coherence, that are bounded by those of the light emitting diode and the laser diode. The SLD can be designed to meet a wide range of optical system needs. By introducing ridge-waveguide lateral confinement, a good anti-reflection coating at one end and a high reflectivity mirror at the other, we have demonstrated an SLD that allows 30 percent coupling efficiency into a lensed 0.23 NA, 50 μm diameter graded index core fiber. The power in the fiber is 550 μW at 250 mA and 20°C. It is possible to maintain a constant power level in the fiber greater than 250 μW over the temperature range 0 to 35°C by adjusting the current. The spectral width is 300 Å and the modulation bandwidth 350 MHz. PCM with 400 Mbit/s rate has been observed. These devices are relatively easy to fabricate from ridge-waveguide lasers or any other lateral confinement laser.     

Low-threshold InGaAsP ridge waveguide lasers at 1.3 µm

The ridge waveguide configuration is shown to provide reliable low-threshold fundamental-transverse-mode lasers that are readily fabricated. Two variants are described: in the simple ridge laser, the 1.3 μm bandgap active layer is sandwiched between InP layers and in the cladded ridge, the active layer is surrounded by 1.1 μm bandgap InGa AsP. Thresholds as low as 34 mA and efficiencies as high as 66 percent are observed. Output power is linear to more than 12 mW. Several lasers have been operated at 30°C for over 1500 h without measurable degradation. Selected lasers exhibit stabilized longitudinal mode behavior over extended temperature and current ranges. The potential manufacturability of this device is its most attractive feature.     

Amplifying medium characteristics in optimized 190 µm/195 µm DCN waveguide lasers

Extensive study of the CW, dischaxge-pumped, DCN laser operating on the (22⁰0-09¹0) 190 and 195 μm lines has been undertaken in order to develop a new source of radiation suitable for plasma diagnostics. The optimum values of the parameters of the discharge, the unsaturated gain, and the saturation intensity are given as a function of the tube diameter. Scaling laws predicting the maximum output power as a function of the tube dimensions and of the cavity loss are established. Such lasers compare favorably with optically pumped submillimeter lasers, since 250 mW are available from a 3 m long, 5 cm diameter waveguide DCN laser.     

Low-power nonlinear Fabry-Perot reflection in CdHgTe at 10 µm

Measurements of the intensity dependence of CO2laser radiation reflected from a polished Cd0.23Hg0.77Te etalon at 77 K results in nonlinear features at power levels on the order of 1 mW, in good agreement with a theory which includes bandgap resonant nonlinearities and Auger carrier recombination. A high-reflectivity rear coating enhances these features, giving a characteristic which demonstrates differential optical gain. The effects of heating are discussed in this unusual case in which the electronic and thermal contributions to the intensity dependence have the same sign.     

Electron beam direct writing technology applied to 512 kbit ROM with 1 µm geometry

A 512 kbit read-only memory (ROM) to store Chinese ideographs has been fabricated using variable-shaped electron beam and dry-etching lithography. 1.0-µm minimum line width was used to delineate device area spacings smaller than those obtained with conventional design rules using photoimaging techniques. SiO2, Si3N4, and polysilicon etchings were accomplished by reactive sputter techniques with CF4+ H2and CCl3F gases using negative electron beam resist PGMA and positive resist AZ-2400. Al etching was carried out by plasma with CCl4gas using negative electron beam resist NER-1. The alignment marks detectability and their locating accuracy were improved by properly using the basis arithmetic operations, subtraction and summation, in backscatter signal processings. 6.6 mm × 8.9 mm chip-by-chip alignment yielded about 0.2-µm level-to-level registration accuracy. Memory cell size and chip size are 5.2 µm × 8.4 µm and 6.6 mm × 8.9 mm, respectively; access time and power dissipation are 400 ns and 800 mW, respectively.     

Diffracted light spectroscopy of the 3.39 µm CH4system

A differential analysis of the line shapes of the intracavity saturated absorption peak of methane, observed simultaneously on both the output beam of the laser and on the diffracted beam, points out frequency shifts on the order of several tens of kHz, and also asymmetries of opposite signs. The value and sign of the shift on the principal output beam depends essentially on the cavity geometry. A "critical geometry," analogous to that found for the classical Lamb dip, is proposed for elimination of these important shifts related to the Gaussian character of the beams and for obtaining symmetric line profiles.     

Operation of an Er:YLF laser at 1.73 µm

The performance, characterization, and optimization of an Er:YLF laser operating at 1.73 μm is described. The lifetimes of the upper and lower laser level were measured which allowed the optimum Er concentration and the ultimate performance to be predicted. In addition, the pulse-forming network and the mirror transmissivity were optimized. The laser output energy in the normal mode and theQ-switched mode as well as theQ-switched pulselength were measured. In a separate, more efficient, laser arrangement, an output energy of 55 mJ was obtained with an overall efficiency of 0.0024.     

Free electron laser small signal gain measurement at 10.6 µm

Small signal gain has been observed for 10.6 μm radiation using a constant (untapered) wiggler. The permanent magnet wiggler was 2.65 m long, had a period of 3.56 cm (75 periods) and a magnetic field on axis of 2.67 kG. The gain measured was 1.5 ± 0.4 percent agreeing well with the predicted gain corresponding to the experimental electron beam conditions (E = 25MeV,DeltaE/E = 0.75percent,epsilon = 8pimm . mrad, andI = 10A).     

Multiphoton ionization of rare gases at 1.06µ and 0.53µ

The interaction between an intense focused beam of optical photons and matter in gaseous form at low pressure (10^-3torr) brings into play some strongly nonlinear processes. These multiphoton processes occur through the simultaneous absoprtion of several quanta by an atom that may be thus either excited or ionized. The orders of nonlinearity of the interaction of a multimodeQ-switched laser beam with rare gas atoms were measured with laser intensities up to 10¹³W ċcm^-2at 1.06μ and up to 10¹²W ċcm^-2at 0.53μ. The energy of the number of quanta corresponding to the order of nonlinearity is always close to the energy of an atomic level. The results seem to emphasize the particularly important role performed by bound states during the ionization process. Thus a two-stage ionization process seems far more probable than a single direct transition between the ground state and the continuum spectrum. Experimental values of multiphoton ionization probabilities are also given after having precisely determined the spatiotemporal intensity distribution function.     

Distributed feedback laser emitting at 1.3 µm for gigabit communication systems

A distributed feedback (DFB) laser emitting at 1.3 μm for gigabit lightwave communication systems has been developed. The distributed feedback structure has been introduced in a newly developed buried heterostructure and designs for stable single-mode operation, high speed modulation, and low noise have been done. Threshold current of 10-15 mA, differential efficiency of around 0.28 mW/mA, low noise, small signal modulation bandwidth of 13.9 GHz, and satisfactory modulation waveform at 5-Gbit/s NRZ modulation have been attained with high single-mode operation yield.