Search

Erbium-doped modclocked fiber lasers provide a potentially attractive short pulse source1 near 1.5 microns for applications to high speed fiber optic communication systems and interconnection networks.

We present a micromechanical optical interference device that produces changes in the slope (in dB) of its transmitted spectrum via a single voltage control, without changes in attenuation level, a Spectrally Linear Optical Power Equalizer, or SLOPE device. Simple linear power equalization is sometimes all that is required in optical networks, especially in optical amplifiers, and this device can perform this function inexpensively and with easier control than multi-element equalizers. The device element is a vertically moving membrane similar to the MARS modulator. The reflectivity of the element is varied by the adjustment of the air gap via a bias applied to the element's electrode. The element can be thought of as a thin-film optical stack where one of the films (the air gap) is variable. Its reflectivity is transmitted via a dual fiber ferrule arrangement, in which the light comes into the device in one fiber, is reflected off the device and focused onto the outgoing fiber.

Many optical devices, but particularly vertical-cavity surface emitting lasers (VCSEL's), require high reflectivity mirrors. These mirrors are typically stacks of quarter-wavelength thick layers (as measured in the material), of alternating high and low index layers, placed on the device either by epitaxial techniques, or by dielectric deposition techniques such as evaporation. For bottom emitters, the thickness of these mirrors can be greatly reduced by capping the stack with a low optical loss metal (the thickness of the last dielectric layer would be adjusted for optical phase matching).

We describe a WDM/SDM network where each bit of a word-parallel link is carried on a different wavelength. Each processor node has an "transceiver" which can transmit and receive optical data on single mode fiber. The fibers are linked through a central switch. The transceivers and switch use CMOS/SEED technology, where MQW modulators and detectors are flip chip bonded onto the surface of silicon CMOS circuits. The center of the network is AMOEBA, an asynchronous multiprocessor optoelectronic bit-sliced array switch using K identical switches with optical input and output to route K-bit words.

Current state-of-the-art Ti:sapphire laser systems operate in a dispersion-limited regime at around 10 fs pulsewidth. This remarkable progress has been made as a result of intracavity dispersion optimization, as initial results in self-focusing modelocking produced pulses of /spl sim/90 fs duration. The recently developed technique of Frequency-Domain Dispersion (FDD) can provide measurements of the complete intracavity group delay under operating conditions. Since dispersion is a linear optical property, one might wonder whether it is meaningful to measure the dispersion inside a nonlinear system while it is operating. In the paper, the author discusses the magnitude and origin of nonlinear corrections to the measured intracavity group delay in FDD measurements. The author also raises the general questions: (a) since a modelocked laser is a nonlinear optical system, is it correct to measure its dispersion when it's lasing ?; and (b) since a modelocked laser is a nonlinear system, is it correct to measure the dispersion when it's not lasing?. >

We present an application of femtosecond laser pulses that is not directly related to high speed, but rather makes use of the large spectral bandwidth of these pulses to address a systems issue in wavelength-division-multiplexed (WDM) communication links. One inherent difficulty of currently proposed WDM systems is the need for multiple lasers operating at different wavelengths, which hence have to be wavelength-selected and stabilized at that particular wavelength for years to come. We demonstrate a WDM system based on a single short-pulse laser source that covers all WDM channels simultaneously, hence eliminating many of the source problems of conventional WDM systems.

We report generation of coherent terahertz electromagnetic transients from GaAs/Al0.3Ga0.7As quantum wells in perpendicular fields. Although, at low temperature, the quantum well barriers suppress the transport current perpendicular to the layers by at least two orders of magnitude compared to bulk, we observe terahertz signals that are comparable in strength to those generated from bulk GaAs surfaces. This directly proves that the field‐induced polarization of photoexcited electron‐hole pairs is an important mechanism for the generation of terahertz radiation at semiconductor surfaces.

The basic mechanisms of self-focusing modelocking (also called Kerr-lens modelocking, KLM) in broadband solid state gain media are now well established [1]. Recent advances in dispersion management and cavity design have led to the generation of pulses in the sub 10 fs regime [2]. Here we show that, because self-phase modulation and self-focusing have the same underlying origin, there is an intimate connection between wavelength and spatial mode in KLM lasers.

Laser scanning microscopy combined with two-photon excitation of fluorescence holds great promise in imaging biological systems. This two-photon excitation laser scanning microscopy (TPLSM) [1] yields intrinsic submicron three-dimensional resolution with much reduced background fluorescence and thus reduced photodamage. Although the advantages of TPLSM as compared to wide field fluorescence microscopy and confocal microscopy have been demonstrated in a number of applications [2], the large cost and utility requirements of mode locked Ti:sapphire laser systems and other femtosecond light sources have kept TPLSM out of reach for most biology labs. We demonstrate here that a recently developed compact solid state laser that is mode locked with a Saturable Bragg Reflector (SBR) [3] is well-suited for TPLSM. A SBR-modelocked Cr:LiSAF laser was pumped with a 0.5 W, 670 nm diffraction-limited MOPA (SDL), providing 90 fs pulses at 860 nm with CW power of 25-44 mW per beam (Fig. la). A single beam was directed into a laser scanning microscope consisting of a pair of galvomirrors, a relay lens, a dichroic mirror, a Zeiss water-immersion objective (63 x 0.9 NA), and a photomultiplier tube for the detection of fluorescence photons [2]. Rat cortical brain slices (300 μm thick) were prepared using standard techniques. For anatomical imaging, neocortical pyramidal cells that were deeply embedded in the tissue were dialyzed and voltage clamped using whole-cell electrodes containing 500 μM fluorescein dextran (MW = 3 kD). TPLSM imaging at low magnification (Fig. 1B) revealed primary and secondary dendrites and the initial segment of the axon. At high magnification single dendritic spines, the smallest neuronal compartments, became apparent (Fig. 1C, arrow). A series of images acquired at different focal planes (Δz = 1.6 μm) demonstrates the sectioning capabilities of the microscope (Fig. 1D-F). For functional imaging of physiological calcium responses, neurons were dialyzed with electrodes containing the calcium indicator Ca-green-1 (300 μM, Molecular Probes). Ca-green is a fluorophore that undergoes a large fluorescence intensity change in response to Ca2+ binding. Intracellular free calcium concentration changes evoked by single action potentials could easily be detected (Fig. 1G).

Two-photon excitation laser scanning microscopy (TPLSM) allows high resolution fluorescence imaging with minimal photodamage and photobleaching (for a review see [1]). TPLSM has recently been used to study, for example, information processing in dendritic spines, cell division in developing embryos, and calcium dynamics in auditory hair cells. TPLSM yields intrinsic submicron three-dimensional resolution (Fig. 1) and optical sectioning equivalent to one-photon confocal microscopy simply by spatial confinement of excitation. This eliminates counterproductive absorption outside the focal slice. In highly scattering samples, such as nervous tissue, the red or near-infrared light used for illumination in TPLSM leads to a much improved penetration depth and the absence of a detector pinhole permits ballistic and diffuse photons to contribute to the signal [1]. Because two-photon absorption depends on the square on the instantaneous light intensity, ultrashort pulses are essential for TPLSM to achieve efficient excitation while using low average power. The main impediment to a more widespread use of TPLSM have been the cost, utility, and space requirements for femtosecond modelocked lasers that are based on large-frame argon-ion pump sources. The use of passive modelocking schemes that directly produce pulses of < 100 fs duration is desirable to reduce overall complexity and optimize performance.

Surface second harmonic generation (SSHG) from the Si-SiO2 interface is of considerable interest because it is sensitive to surface steps [1], straining of subsurface bonds [2] and surface roughness [3], all of which are important in the fabrication of integrated circuits. By using 10 fs pulses we are able to obtain acceptable signal to noise ratio using less than 30 mW, as compared to the hundreds of mW necessary for 100 fsec pulses [1,3]. The p-polarized pulses (45° incident angle) are generated by a Kerr-lens-modelocked Ti: sapphire laser [4]. Additionally the very broadband nature of these pulses allows us to observe resonance behavior in the SSHG by simply spectrally resolving the SSHG, i.e., tuning the laser is not required.

Ultrafast technology has created new opportunities for investigating the properties of many-body systems. For example, an intense pump optical pulse tuned in the transparency regime induces virtual charge fluctuations between the conduction and valence bands that renormalize all the semiconductor parameters. Augmented by correlation effects, such processes result in a "dressed" interacting system whose excitation spectrum can be measured by a weak probe pulse. The exciton absorption in undoped semiconductors was investigated with this technique in [1]. As the pump intensity increases, the bound state resonance shifts to the blue; this is called the AC (optical) Stark effect. Unlike atomic systems, in semiconductors the optically excited electrons and holes interact with each other; and many-body effects need to be addressed [2]. A narrow-band pump pulse blue-shifts the resonance and slightly enhances its oscillator strength [3,4]. The oscillator strength is significantly reduced if the pulse is ultrashort (100 fs or less) [5].

AC-Stark shifts of the electronic states are observed when a semiconductor is excited by ultrashort, below band-gap laser pulses. So far, all the studies have been performed in undoped samples [1,2], and the effect has been called "excitonic AC-Stark effect". It has been explained by means of the interaction of "virtual" excitons created by the pump beam with the real excitons created by the weak probe beam [3]. In this talk, we will describe our work on the AC-Stark shifts observed in the absorption spectra of modulation doped quantum wells (MDQW). This effect occurs in a new regime, as the absorption edge in MDQW samples at low temperatures is governed by the "Fermi edge singularity" (FES).

The exciton in quantum wells is a sensitive detector of electric fields. This field response has been used recently to detect ultrafast electrical transients in ultra-thin quantum well coplanar striplines with time resolution as fast at 180 fs [1]. In these experiments, it is found that the field in the gap is not uniform [2]. In such as case, it is interesting to use the spatial resolution of the excitonic electroabsorption effect to obtain space-time resolved electrical transient information. In the experiments, we use an infrared CPM dye laser operating at 820 nm wavelength [3], which is coincident with the exciton energy. The pump beam is focused into a 10 micron coplanar stripline which is patterned on the sample. The sample consists of a 0.4 micron thick layer of GaAs-AlGaAs quantum wells and a 0.35 micron thick AlGaAs stop etch layer. The substrate is etched out, leaving a 0.75 micron thick free-standing film. This structure has interesting high frequency propagation characteristics such as propagation speed of 82 % the speed of light, and low dielectric loss, radiation losses and modal dispersion [1,4]. The pump beam creates an ultrafast electrical transient which propagates in the coplanar stripline (Figure 1, inset). We probe the signal by focusing a delayed probe pulse through the stripline further down the line at the desired distance. In the present case, we investigate the spatial distribution of the field transient, as viewed by the quantum well excitons, which give a microscopic reading of the conditions in the structure. Figure 1 shows a result of focusing the probe beam to a 1 micron diameter spot and scanning the time delay at various positions in the gap.

The physics of passively modelocked Ti:Sapphire femtosecond lasers is a topic of current interest. In the broadband case (short pulses) the net intracavity dispersion is an important limit in determining the pulse width and tuning behavior. We report a remarkably simple and powerful technique for the characterization of dispersion in broadband tunable passive modelocked lasers. A CW-pumped passive modelocked laser chooses its cavity roundtrip time independent of the pump laser cavity length, of course. If we simply note that the cavity roundtrip time at any particular frequency is exactly the total group delay of a quasi-monochromatic wave packet at that frequency, then clearly by accurately measuring the cavity repetition rate as a function of frequency while tuning the laser, we obtain the group delay as a function of frequency directly. The changes are small: a 100 fs group delay change in a 10 ns period corresponds to a frequency shift of only 1 kHz, which is easily measured with a high precision frequency counter. Using this simple technique, we demonstrate the effect of translating a Brewster prism, and show the magnitude of the cumulative cubic phase in an actual operating laser. The signals contain contributions from mirrors, materials, air and refraction. This appears to be a powerful and simple technique which should provide useful information for understanding of the new class of broadband tunable femtosecond lasers.

The nonlinear behavior of near-bandgap electronic excitations in semiconductors is dominated by anharmonicities in exciton-exciton and exciton-photon interactions which depend critically on dimensionality1. Upon application of a magnetic field perpendicular to quantum wells, the constant density of states above bandgap becomes singular in the discrete spectrum of the magneto-exciton (Coulomb correlated Landau levels). At very high magnetic fields, the size of the in-plane wavefunctions is reduced to less than the excitonic Bohr radius, so that the states are strongly confined in all three dimensions. Investigation of the nonlinear optical response of such systems allows measurement of the exciton-exciton and exciton-photon interactions within and between magneto-exciton states in the quasi-zero dimensional limit. We report the first observation of femtosecond time resolved optical nonlinearities in quasi-zero-dimensional magneto-excitons at fields up to 12 T, and at cryogenic temperatures.

Excitonic electroabsorption in thin GaAs-AlGaAs free-standing films is a novel approach to femtosecond optoelectronics in which limits of femtosecond electrical pulse generation, propagation and detection can be investigated [1]. The use of ultrathin substrates as a propagation medium for femtosecond electrical signals appears to be a interesting new feature. As the substrate becomes thinner, the TE cutoff frequency increases, until the field lines ultimately propagate in air above and below the lines. This drastically reduces the modal dispersion and phonon losses. The ultimate limit for this technique is achieved when the substrate thickness is only one optical absorption length; for thinner substrates, no electroabsorption is expected. We have fabricated a sample which is only 0.75 microns thick, for which the calculated TE cutoff frequency is 30 THz, and the propagation speed is almost the speed of light. This sample is transparent in the visible spectral range. We investigate the excitonic nonlinear response to resonant excitation, then apply a DC field and measure the propagation of femtosecond electrical signals.

Optoelectronic sampling based on the Pockels' effect1 has become an important technique for the measurement of electrical signals with the highest time resolution, currently at 300 fs. We present first results obtained using a new technique for femtosecond electrical pulse measurement: excitonic electroabsorption sampling (EES). We have previously shown that excitons exhibit a femtosecond electroabsorption response, however the device which was used did not facilitate propagation studies over macroscopic distances2. In our new embodiment, a coplanar stripline is fabricated on a GaAs multiple quantum well mesa ridge structure (Fig. 1). We thus obtain optical modulation by parallel-field electroabsorption, which is due to lifetime broadening by field ionization of the excitons3. The detection sensitivity is about 1%/volt in a 10 micron structure. We etch the GaAs substrate down to a 1 micron AlGaAs stop-etch layer in a 1×2 mm area and leave the stripline free-standing on the 1 micron thick film, thus obtaining an extremely low dispersion structure to test the EES concept. We use an infrared dye laser which produces femtosecond pulses at a wavelength of 805 nm4 at 82 MHZ repetition rate. The exciton energy is temperature-tuned to the laser with a Peletier device, in this case operating at about 5 degrees above ambient temperature. At 300 fs pulsewidth the laser spectrum is already comparable to the exciton linewidth, and we expect that shorter pulses will provide reduced sensitivity relative to the DC response. We expect that time resolution of 100 fs or less may be possible with this technique. We note that electroabsorption is a purely electronic phenomenon, with no ionic lattice contribution such as that of LiTaO3.

The investigation of vertical transport in superlattices (SL's) and multiple quantum wells (MQW’s) has recently attracted much attention. Intense research was focused on basic quantum effects such as Bloch transport of electrons and holes in minibands,1 coherent and incoherent, resonant, sequential and Zener tunneling2 in double barriers,3,4 SL’s and MQW’s of GaAs/AlAs,5,6 GaAs/AlGaAs,7 and InGaAs/InP.2,8 In addition to the academic interest, the understanding of the mechanisms of escape from and travel through quantum wells is vital for the recently developed and constantly growing family of electro-optical devices using semiconductor quantum wells. Most of them are based on the quantum-confined Stark effect (QCSE), and they include bistable selfelectro-optic effect devices (SEED’s), tunable detectors, electro-absorption modulators and optical logic elements.9 The basic unit of all of these is an epitaxially grown p-i-n diode, with the quantum well layers in the intrinsic region. By reverse biasing the diode, an electrical field is applied on the quantum wells, controlling its optical absorption spectrum. Since operating wavelengths are close to the excitonic absorption peaks, photoexcited charge is created in the quantum wells and is transported to the electrodes. The mechanisms by which this transport occurs are ultimately responsible for the intrinsic maximum speed or operating intensity of these devices.