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Reverse saturation of absorption is a relatively rare phenomenon in light-matter interaction, as it requires a few conditions to be fulfilled. We observe that ruby exhibits a very strong reverse saturation of absorption at 473 nm. Furthermore, we measure the group velocity of a modulated laser beam in ruby and observe that the peaks of the pulses appear more than a hundred microseconds earlier than the reference signal. A theoretical model based on coherent population oscillation would suggest a fast-light effect with an extremely large and negative group index of -(1.7 ± 0.1) × 10 6 in consistency with the observed temporal advancement. We propose that this pulse advancement can also be described by time-dependent absorption of ruby. Our study helps to understand the nature of the fast- and slow-light effects in transition-metal-doped crystals such as ruby and alexandrite with potential applications in optical memories and delay lines.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Bianisotropic media can be used to engineer absorbance, scattering, polarization, and dispersion of electromagnetic waves. However, the demonstration of a tunable light-induced bianisotropy at optical frequencies is still lacking. Here, we propose an experimentally feasible concept for a light-induced tunable bianisotropic response in a homogeneous sphere made of an epsilon-near-zero (ENZ) material. By exploiting the large linear absorption and the large possible intensity-dependent changes in the permittivity of ENZ materials, the direction-dependent scattering and absorption cross sections could be obtained. Our findings pave the way for further studies and applications in the optical regime requiring full dynamic control of the bianisotropic behavior.

Multiresonant metasurfaces could enable many applications in filtering, sensing, and nonlinear optics. However, developing a metasurface with more than one high-quality-factor or high-Q resonance at designated resonant wavelengths is challenging. Here, we experimentally demonstrate a plasmonic metasurface exhibiting different, narrow surface lattice resonances by exploiting the polarization degree of freedom where different lattice modes propagate along different dimensions of the lattice. The surface consists of aluminum nanostructures in a rectangular periodic lattice. The resulting surface lattice resonances were measured around 640 nm and 1160 nm with Q factors of ∼50 and ∼800, respectively. The latter is a record-high plasmonic Q factor within the near-infrared type-II window. Such metasurfaces could benefit such applications as frequency conversion and all-optical switching.

We experimentally investigate the tunable Doppler shift in an 80 nm thick indium-tin-oxide (ITO) film at its epsilon-near-zero (ENZ) region. Under strong and pulsed excitation, ITO exhibits a time-varying change in the refractive index. A maximum frequency redshift of 1.8 THz is observed in the reflected light when the pump light has a peak intensity of ∼140 G W / c m 2 and a pulse duration of ∼580 f s , at an incident angle of 40°. The frequency shift increases with the increase in pump intensity and saturates at the intensity of ∼140 G W / c m 2 . When the pump pulse duration increases from ∼580 f s to ∼1380 f s , the maximum attainable frequency shift decreases from 1.8 THz to 0.7 THz. In addition, the pump energy required to saturate the frequency shift decreases with the increase in pump pulse duration for ∼ x <1 p s and remains unchanged for ∼ x >1 p s durations. Tunability exists among the pump pulse energy, duration, and incident angle for the Doppler shift of the ITO-ENZ material, which can be employed to design efficient frequency shifters for telecom applications.

In this Letter, we propose a tunable coherent perfect absorber based on ultrathin nonlinear metasurfaces. A nonlinear metasurface is made of plasmonic nanoantennas coupled to an epsilon-near-zero material with a large optical nonlinearity. The coherent perfect absorption is achieved by controlling the relative phases of the input beams. Here, we show that the optical response of the nonlinear metasurface can be tuned from a complete to a partial absorption by changing the intensity of the pump beam. The proposed nonlinear metasurface can be used to design optically tunable thermal emitters, modulators, and sensors.

Limited-size receiver (Rx) apertures and transmitter-Rx (Tx-Rx) misalignments could induce power loss and modal crosstalk in a mode-multiplexed free-space link. We experimentally demonstrate the mitigation of these impairments in a 400 Gbit/s four-data-channel free-space optical link. To mitigate the above degradations, our approach of singular-value-decomposition-based (SVD-based) beam orthogonalization includes (1) measuring the transmission matrix H for the link given a limited-size aperture or misalignment; (2) performing SVD on the transmission matrix to find the U, Σ , and V complex matrices; (3) transmitting each data channel on a beam that is a combination of Laguerre-Gaussian modes with complex weights according to the V matrix; and (4) applying the U matrix to the channel demultiplexer at the Rx. Compared with the case of transmitting each channel on a beam using a single mode, our experimental results when transmitting multi-mode beams show that (a) with a limited-size aperture, the power loss and crosstalk could be reduced by ∼8 and ∼23dB, respectively; and (b) with misalignment, the power loss and crosstalk could be reduced by ∼15 and ∼40dB, respectively.

We establish the ultimate limits that quantum theory imposes on the accuracy attainable in optical ellipsometry. We show that the standard quantum limit, as usually reached when the incident light is in a coherent state, can be surpassed with the use of appropriate squeezed states and, for tailored beams, even pushed to the ultimate Heisenberg limit.

We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the L P 11 group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the L P 11 group. We employ two output OAM modes l =+1 and l =-1 as resultant linear combinations of degenerate L P 11 a and L P 11 b modes inside the L P 11 group of a ∼0.6- k m GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis within L P 11 group, which is a subset of the full LP TM of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results for l =+1 and l =-1 modes show, respectively, that (i) intra-modal-group crosstalk is reduced by >5.8 d B and >5.6 d B and (ii) near-error-free bit-error-rate performance is achieved with a penalty of ∼0.6 d B and ∼3.8 d B , respectively.

We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameter ${r&#95;0} = 0.4,\,1.0\;{\rm mm}$r 0 =0.4,1.0mm) are mitigated. The experimental results show the following. (1) Crosstalk between OAM $l = + 1$l=+1 and $l = - 1$l=-1 modes can be reduced by $ {\gt} {10.0}$>10.0 and $ {\gt} {5.8}\;{\rm dB}$>5.8dB, respectively, under the weaker turbulence (${r&#95;0} = 1.0\;{\rm mm}$r 0 =1.0mm); crosstalk is further improved by $ {\gt} {17.7}$>17.7 and $ {\gt} {19.4}\;{\rm dB}$>19.4dB, respectively, under most realizations in the stronger turbulence (${r&#95;0} = 0.4\;{\rm mm}$r 0 =0.4mm). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be ${\sim}{0.7}$∼0.7 and ${\sim}{1.6}\;{\rm dB}$∼1.6dB under weaker turbulence, while measured to be ${\sim}{3.2}$∼3.2 and ${\sim}{1.8}\;{\rm dB}$∼1.8dB under stronger turbulence for OAM $l = + 1$l=+1 and $l = - 1$l=-1 mode, respectively.

We propose and demonstrate a simple and easy-to-implement projective-measurement protocol to determine the radial index p of a Laguerre-Gaussian (LGpl) mode. Our method entails converting any specified high-order LGp0 mode into a near-Gaussian distribution that matches the fundamental mode of a single-mode fiber (SMF) through the use of two phase screens (unitary transforms) obtained by applying a phase-retrieval algorithm. The unitary transforms preserve the orthogonality of modes before the SMF and guarantee that our protocol can, in principle, be free of crosstalk. We measure the coupling efficiency of the transformed radial modes to the SMF for different pairs of phase screens. Because of the universality of phase-retrieval methods, we believe that our protocol provides an efficient way of fully characterizing the radial spatial profile of an optical field.

We experimentally demonstrate a 10 Mbit/s free-space quantum communication link using data encoding on orthogonal Laguerre-Gaussian (LG) modes with the same azimuthal index but different radial indices. Data encoding on two LG ℓp modes (i.e., for ℓ=0, we encode ["0", "1"] as [p=0, p=1], and for ℓ=1, we encode ["0", "1"] as [p=0, p=1]) is demonstrated by employing directly modulated laser diodes and helical phase holograms. The quantum symbol error rate (QSER) of <5% is achieved at an encoding rate of 10 Mbit/s. Moreover, the influence of the circle radius (R) of the receiver phase pattern on registered photon rates and QSERs is investigated. Our results show that a receiver phase pattern whose R does not match the beam size of the LG modes would induce higher cross talk between the two encoded quantum branches.

The Hermite-Gaussian (HG) modes, sometimes referred to as transverse electromagnetic modes in free space, form a complete and orthonormal basis that have been extensively used to describe optical fields. In addition, these modes have been shown to be helpful in enhancing information capacity of optical communications as well as achieving super-resolution imaging in microscopy. Here we propose and present the realization of an efficient, robust mode sorter that can sort a large number of HG modes based on the relation between HG modes and Laguerre-Gaussian (LG) modes. We experimentally demonstrate the sorting of 16 HG modes, and our method can be readily extended to a higher-dimensional state space in a straightforward manner. We expect that our demonstration will have direct applications in a variety of fields including fiber optics, classical and quantum communications, as well as super-resolution imaging.

By optimizing the dispersion curve of a parallelogram-based 2D photonic crystal superprism for constant angular group velocity dispersion over a broad bandwidth, we designed a device capable of experimentally demonstrating linear dispersion from 1500 to 1600 nm with clear separation of as many as eight channels, while maintaining a compact footprint.

Optical fiber links and networks are integral components within and between cities' communication infrastructures. Implementing quantum cryptographic protocols on either existing or new fiber links will provide information-theoretical security to fiber data transmissions. However, there is a need for ways to increase the channel bandwidth. Using the transverse spatial degree of freedom is one way to transmit more information and increase tolerable error thresholds by extending the common qubit protocols to high-dimensional quantum key distribution (QKD) schemes. Here we use one type of vortex fiber where the transverse spatial modes serves as an additional channel to encode quantum information by structuring the spin and orbital angular momentum of light. In this proof-of-principle experiment, we show that two-dimensional structured photons can be used in such vortex fibers in addition to the common two-dimensional polarization encryption, thereby paving the path to QKD multiplexing schemes.

We experimentally demonstrate spatial multiplexing of an orbital angular momentum (OAM)-encoded quantum channel and a classical Gaussian beam with a different wavelength and orthogonal polarization. Data rates as large as 100 MHz are achieved by encoding on two different OAM states by employing a combination of independently modulated laser diodes and helical phase holograms. The influence of OAM mode spacing, encoding bandwidth, and interference from the co-propagating Gaussian beam on registered photon count rates and quantum bit error rates is investigated. Our results show that the deleterious effects of intermodal crosstalk effects on system performance become less important for OAM mode spacing Δ≥2 (corresponding to a crosstalk value of less than -18.5  dB). The use of OAM domain can additionally offer at least 10.4 dB isolation besides that provided by wavelength and polarization, leading to a further suppression of interference from the classical channel.

We show that standard approximations in nonlinear optics are violated for situations involving a small value of the linear refractive index. Consequently, the conventional equation for the intensity-dependent refractive index, n(I)=n 0 +n 2 I, becomes inapplicable in epsilon-near-zero and low-index media, even in the presence of only third-order effects. For the particular case of indium tin oxide, we find that the χ (3) , χ (5) , and χ (7) contributions to refraction eclipse the linear term; thus, the nonlinear response can no longer be interpreted as a perturbation in these materials. Although the response is non-perturbative, we find no evidence that the power series expansion of the material polarization diverges.

Slow light photonic crystal waveguides tightly compress propagating light and increase interaction times, showing immense potential for all-optical delay and enhanced light-matter interactions. Yet, their practical application has largely been limited to moderate group index values (<100), due to a lack of waveguides that reliably demonstrate slower light. This limitation persists because nearly all such research has focused on a single photonic crystal lattice type: the triangular lattice. Here, we present waveguides based on the kagome lattice that demonstrate an intrinsically high group index and exhibit slow and stopped light. We experimentally demonstrate group index values of >150, limited by our measurement resolution. The kagome-lattice waveguides are an excellent starting point for further slow light engineering in photonic crystal waveguides.

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

We explore the use of the spatial domain as a degree of freedom for data encoding and channel hopping. We experimentally demonstrate data encoding at 20  Gbit/s using four possible orbital angular momentum (OAM) modes. The influence of mode spacing and time misalignment between modal channels on the switching crosstalk and bit-error rates is investigated. We find that the use of adjacent modes with a mode spacing of one introduces an extra power penalty of 3.2 dB compared with a larger mode spacing. Moreover, we demonstrate reconfigurable hopping of a 100  Gbit/s quadrature-phase-shift-keying (QPSK) data channel between four OAM modes with a 2 ns switching guard time. The results show that the power penalties for different hopping rates and mode spacings are less than 5.3 dB.

We investigate the sensing of a data-carrying Gaussian beacon on a separate wavelength as a means to provide the information necessary to compensate for the effects of atmospheric turbulence on orbital angular momentum (OAM) and polarization-multiplexed beams in a free-space optical link. The influence of the Gaussian beacon's wavelength on the compensation of the OAM beams at 1560 nm is experimentally studied. It is found that the compensation performance degrades slowly with the increase in the beacon's wavelength offset, in the 1520-1590 nm band, from the OAM beams. Using this scheme, we experimentally demonstrate a 1 Tbit/s OAM and polarization-multiplexed link through emulated dynamic turbulence with a data-carrying beacon at 1550 nm. The experimental results show that the turbulence effects on all 10 data channels, each carrying a 100 Gbit/s signal, are mitigated efficiently, and the power penalties after compensation are below 5.9 dB for all channels. The results of our work might be helpful for the future implementation of a high-capacity OAM, polarization and wavelength-multiplexed free-space optical link that is affected by atmospheric turbulence.

Silicon photonic crystal waveguides have enabled a range of technologies, yet their fabrication continues to present challenges. Here, we report on a post-processing method that allows us to tune the operational wavelength of slow-light photonic crystal waveguides in concert with optical characterization, offsetting the effects of hole-radii and slab thickness variations. Our method consist of wet chemical surface oxidation, followed by oxide stripping. Theoretical modelling shows that the changes in optical behavior were predictable, and hence controlled tuning can be achieved by changing the number of processing cycles, where each cycle removes approximately 0.25 nm from all exposed surfaces, producing a blueshift of 1.6±0.1  nm in operating wavelength.

We demonstrate that a |q|=1/2 plate, in conjunction with appropriate polarization optics, can selectively and switchably excite all linear combinations of the first radial mode order |l|=1 orbital angular momentum (OAM) fiber modes. This enables full mapping of free-space polarization states onto fiber vector modes, including the radially (TM) and azimuthally polarized (TE) modes. The setup requires few optical components and can yield mode purities as high as ∼30  dB. Additionally, just as a conventional fiber polarization controller creates arbitrary elliptical polarization states to counteract fiber birefringence and yield desired polarizations at the output of a single-mode fiber, q-plates disentangle degenerate state mixing effects between fiber OAM states to yield pure states, even after long-length fiber propagation. We thus demonstrate the ability to switch dynamically, potentially at ∼GHz rates, between OAM modes, or create desired linear combinations of them. We envision applications in fiber-based lasers employing vector or OAM mode outputs, as well as communications networking schemes exploiting spatial modes for higher dimensional encoding.

A stochastic-parallel-gradient-descent algorithm (SPGD) based on Zernike polynomials is proposed to generate the phase correction pattern for a distorted orbital angular momentum (OAM) beam. The Zernike-polynomial coefficients for the correction pattern are obtained by monitoring the intensity profile of the distorted OAM beam through an iteration-based feedback loop. We implement this scheme and experimentally show that the proposed approach improves the quality of the turbulence-distorted OAM beam. Moreover, we apply phase correction patterns derived from a probe OAM beam through emulated turbulence to correct other OAM beams transmitted through the same turbulence. Our experimental results show that the patterns derived this way simultaneously correct multiple OAM beams propagating through the same turbulence, and the crosstalk among these modes is reduced by more than 5 dB.

We propose an adaptive optics compensation scheme to simultaneously compensate multiple orbital angular momentum (OAM) beams propagating through atmospheric turbulence. A Gaussian beam on one polarization is used to probe the turbulence-induced wavefront distortions and derive the correction pattern for compensating the OAM beams on the orthogonal polarization. By using this scheme, we experimentally demonstrate simultaneous compensation of multiple OAM beams, each carrying a 100  Gbit/s data channel through emulated atmospheric turbulence. The experimental results indicate that the correction pattern obtained from the Gaussian probe beam could be used to simultaneously compensate multiple turbulence-distorted OAM beams with different orders. It is found that the turbulence-induced crosstalk effects on neighboring modes are efficiently reduced by 12.5 dB, and the system power penalty is improved by 11 dB after compensation.

We observe experimentally the self-phase modulation of a surface plasmon-polariton (SPP) propagating along a gold film bounded by air in a Kretschmann-Raether configuration. Through analyzing the power dependence of the reflectance curve as a function of the incidence angle, we characterize the complex-valued nonlinear propagation coefficient of the SPP. Moreover, we present a procedure that can further extract the complex value of the third-order nonlinear susceptibility of gold from our experimental data. Our work provides direct insights into nonlinear control of SPPs utilizing the nonlinearity of metals, and serves as a practical method to measure the complex-valued third-order nonlinear susceptibility of metallic materials.

Gaussian profile fiber Bragg gratings exhibit narrow-bandwidth transmission peaks with significant group delay at the edge of their photonic bandgap. We demonstrate group delays ranging from 0.2 to 5.6 ns from a 1.2 cm structure. Simulations suggest such a device would be capable of enhancing the field intensity of incoming light by a factor of 800. Enhancement is confirmed by photothermally induced bistability of these peaks even at sub-milliwatt input powers with as much as a four-fold difference in the magnitude of their responses. The strong field intensities of these modes could significantly enhance desired nonlinear optical responses in fiber, provided the impact of absorption is addressed.

We experimentally investigate the performance of an orbital angular momentum (OAM) multiplexed free space optical (FSO) communication link through emulated atmospheric turbulence. The turbulence effects on the crosstalk and system power penalty of the FSO link are characterized. The experimental results show that the power of the transmitted OAM mode will tend to spread uniformly onto the neighboring mode in medium-to-strong turbulence, resulting in severe crosstalk at the receiver. The power penalty is found to exceed 10 dB in a weak-to-medium turbulence condition due to the turbulence-induced crosstalk and power fluctuation of the received signal.

A phase-only hologram applies a modal transformation to an optical transverse spatial mode via phase encoding and intensity masking. Accurate control of the optical field crucially depends on the method employed to encode the hologram. In this Letter, we present a method to encode the amplitude and the phase of an optical field into a phase-only hologram, which allows the exact control of spatial transverse modes. Any intensity masking method modulates the amplitude and alters the phase of the optical field. Our method consists in correcting for this unwanted phase alteration by modifying the phase encryption accordingly. We experimentally verify the accuracy of our method by applying it to the generation and detection of transverse spatial modes in mutually unbiased bases of dimension two and three.

We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.

We propose a multi-aperture slow-light laser radar with two-dimensional scanning. We demonstrate experimentally that we can use two independent slow-light mechanisms, namely dispersive delay and stimulated Brillouin scattering, to dynamically compensate the group delay mismatch among different apertures, while we use optical phase locking to control the relative phases of the optical signals emitted from different apertures, as the system steers the beam in two dimensions.

We observe experimentally that the reflectances of metal-dielectric nanocomposite films in the Kretschmann configuration show different characteristics, depending on the metal fill fraction f, that fall into one of three distinct regimes. In the "metallic" regime, in which f is large, the film supports conventional surface-plasmon polaritons (SPPs), and one can tailor the properties of the SPPs by controlling the value of f. In the "dielectric" regime, in which f is small, the film does not support any surface modes. In the intermediate "lossy" regime, the nanocomposite film supports a SPP mode that is different from that of a "metallic" film. These results are explained by using an anisotropic effective medium model and mode analysis.

We show theoretically that high-order thermal ghost imaging has considerably higher visibility and contrast-to-noise ratio than conventional thermal ghost imaging, which utilizes the lowest-order intensity cross correlation of the object and the reference signal. We also deduce the optimal power order of the correlation that gives the best contrast-to-noise ratio.

We analyze the influence of atmospheric turbulence on the propagation of an optical vortex beam having the form V(r,theta)=A(0)e(imtheta). The probability that a detected photon after propagating through the atmosphere has the same value of the orbital angular momentum as the launched photon is found to be given by s(0)=[1+(1.845D/r(0))(2)](-1/2), where D is the aperture diameter and r(0) is the Fried coherence diameter. These vortex beams behave very similarly to Laguerre-Gauss beams under the influence of atmospheric turbulence. These results have important implications for atmospheric laser communication systems that employ quantum encryption.

We measure the nonlinear susceptibility of Bi(3.25)La(0.75)Ti(3)O(12) (BLT) thin films grown on quartz substrates using the Z-scan technique with picosecond laser pulses at a wavelength of 532 nm. The third-order nonlinear refractive index coefficient gamma and absorption coefficient beta of the BLT thin film are 3.1 x 10(-10) cm(2)/W and 3 x 10(-5) cm/W, respectively, which are much larger than those of most ferroelectric films. The results show that the BLT thin films on quartz substrates are good candidate materials for applications in nonlinear optical devices.

A slow-light medium based on multiple, closely spaced gain lines is studied. The spacings and relative strengths of the gain lines are optimized by using the criteria of gain penalty and eye-opening penalty to maximize the fractional delay defined in terms of the best decision time for random pulse trains. Both numerical calculations and experiments show that an optimal design of a triple-gain-line medium can achieve a maximal fractional delay about twice that which can be obtained with a single-gain-line medium, at three times higher modulation bandwidth, while high data fidelity is still maintained.

We demonstrate experimentally that the spectral sensitivity of an interferometer can be greatly enhanced by introducing a slow-light medium into it. The experimental results agree very well with theoretical predictions that the enhancement factor of the spectral sensitivity is equal to the group index n(g) of the slow-light medium.

When a pulse superposed on a cw background propagates through an erbium-doped fiber amplifier with a negative group velocity, either pulse broadening or pulse compression can be observed. These effects can be explained in terms of two competing mechanisms: gain recovery and pulse spectrum broadening. The distortion of the pulse shape caused by these effects depends on input pulse width, pump power, and background-to-pulse power ratio. With the proper choice of these three parameters, we can obtain significant pulse advancement with minimal pulse distortion.

We describe observations of various transverse instabilities that occur when two laser beams intersect in nonlinear optical liquids. Patterns that we observe include two types of conical emission and the generation of a linear array of spots. These results can be understood in terms of the physical processes of self-diffraction, two-beam-excited conical emission, and seeded modulational instability.

We have constructed and characterized several optical microring resonators with scale sizes of the order of 10 microm. These devices are intended to serve as building blocks for engineerable linear and nonlinear photonic media. Light is guided vertically by an epitaxially grown structure and transversely by deeply etched air-clad sidewalls. We report on the spectral phase transfer characteristics of such resonators. We also report the observation of a pi-rad Kerr nonlinear phase shift accumulated in a single compact ring resonator evidenced by all-optical switching between output ports of a resonator-enhanced Mach-Zehnder interferometer.

We show that, when two channel waveguides are coupled by a sequence of periodically spaced microresonators, the group-velocity dispersion is low in the vicinity of the gap associated with the resonant frequency of the resonators. This low dispersion permits the excitation of a gap soliton with much lower energy than in a gap of similar width caused by Bragg reflection.

We report the influence of photonic stopgaps on two-photon excited emission from highly efficient nonlinear chromophores infiltrated into high-quality photonic crystals. We have observed a sharp decrease (filter effect) in emission within the frequency range and direction of the stopgap as well as sharp enhancement of the two-photon excited emission associated with the stopgap's edge. This effect may be important for the development of low-threshold upconversion lasers.

We show that the nonlinear phase shift produced by a ring resonator constructed from a given nonlinear optical material can be greater than the phase shift produced by a single pass through an infinite length of the same material when linear and nonlinear absorption are taken into consideration. The figure of merit (defined by the phase shift times the throughput) also improves for the ring resonator over that of the native nonlinear absorbing material. We finally show that these benefits of using the ring resonator as a nonlinear phase-shifting element can enhance the switching characteristics of a Mach-Zehnder interferometer.

Stable spatial laser patterns were observed in a high-finesse Fabry-Perot cavity containing up to 2 atm of CO(2) and O(2). The gases displayed the same sequence of patterns that obey a scaling law of the form P(beta)p(2), where P is the power stored in the cavity, p is the pressure of the gas, and beta is a material-dependent parameter.

The thermal contribution to the nonlinear refractive index of air at 1.064mum was measured with a high-finesse Fabry-Perot cavity and a 500-mW cw laser beam. At room temperature and pressure, the nonlinear refractive-index coefficient of air was found to be n(2)((th))=(-1.9+/-0.2)x10 (-14) cm(2)/W for a beam waist radius of 0.23 mm and was found to be independent of the relative humidity. The thermal nonlinearities of N(2) , O(2) , and CO(2) were also measured, and it was found that the dominant contribution to air is its O(2) content.

All known polarizers operate through a separation of orthogonal electric field components, one of which is subsequently discarded. As a result, 50% of the unpolarized incident light is wasted in the process of conversion to polarized light. We demonstrate a new method by which we use the optical power in the ordinarily discarded component as the pump to amplify the retained component through photorefractive two-beam coupling to achieve greater than 50% throughput.

Metals typically have very large nonlinear susceptibilities (~10(6) times larger than those of typical dielectrics), but because they are nearly opaque their nonlinear properties are effectively inaccessible. We demonstrate numerically that a multilayer metal-dielectric structure in which the metal is the dominant nonlinear [chi((3))] material can have much larger intensity-dependent changes in the complex amplitude of the transmitted beam than a bulk sample containing the same thickness of metal. For 80 nm of copper the magnitude of the nonlinear phase shift is predicted to be as much as 40 times larger for the layered copper-silica sample, and the transmission is also greatly increased. The effective nonlinear refractive-index coefficient n(2) of this composite material can be as large as (3+6iota)x10(-9) cm (2)/W , which is among the largest values for known, reasonably transmissive materials.

We predict dramatically reduced switching thresholds for nonlinear optical devices incorporating fiber ring resonators. The circulating power in such a resonator is much larger than the incident power; also, the phase of the transmitted light varies rapidly with the single-pass phase shift. The combined action of these effects leads to a finesse-squared reduction in the switching threshold, allowing for photonic switching devices that operate at milliwatt power levels in ordinary optical fibers.

The electrostrictive contribution to the nonlinear refractive index is investigated by use of frequency-dependent cross-phase modulation with a weak unpolarized cw probe wave and a harmonically modulated pump copropagating in optical fibers. Self-delayed homodyne detection is used to measure the amplitude of the sidebands imposed upon the probe wave as a function of pump intensity for pump modulation frequencies from 10 MHz to 1 GHz. The ratio of the electrostrictive nonlinear coefficient to the cross-phase-modulation Kerr coefficient for unpolarized light is measured to be 1.58:1 for a standard step-index single-mode fiber and 0.41:1 for dispersion-shifted fibers, indicating a larger electrostrictive response in silica fibers than previously expected.

Coherent soliton packets generated in a passively mode-locked fiber laser are transmitted through 23km of dispersion-decreasing fiber. We observe a shift of the phase difference between solitons that is due to intrapulse Raman scattering. We attribute the stability in propagation of these trains to a trade-off between minimizing soliton-soliton interactions by reduction of the pulse width and minimizing this Raman-induced phase migration, which can force the solitons into a deleterious attractive phase relationship. We are thus able to demonstrate the propagation of 177-Gbit/s soliton packets over a distance of 123 soliton periods.