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In this chapter, the first section gives the reviews of all-optical switch (AOS), including the significance, basic principle, classification method, application requirement, recent problem, and development direction.

This chapter studies the phenomenon of nonlinear interaction between the laser pulse and the propagation fiber. At first, we will deduce the nonlinear Schrodinger equation for describing the propagation of light pulse in the fiber. Under this foundation, we will analyze the two effects of dispersion and self-phase modulation how to affect the propagation of light pulse in the fiber and how to form the time optical soliton in both combined action.

In this chapter, firstly we will study light intensity induced medium refractive-index change phenomenon, i.e., optical Kerr effect. Mainly study on the self-phase modulation optical Kerr effect, the cross-phase modulation optical Kerr effect, and related light induced birefringence effect. Secondly, we will study the self-focusing effect induced by transverse strength inhomogeneous distribution of Gaussian beam, including steady-state and dynamic self-focusing, time and space self-phase modulation phenomena.

Nonlinear light absorption is a basic phenomenon of the interaction between the light and the matter.

A new M–Z interferometer all-optical switch (AOS) consisting of a single nonlinear fiber and a linear long-period fiber grating (LPFG) pair is investigated. The switching principle is based on shifting the transmission spectrum over half of the subspectrum width by the pump light. In this paper, two types of such devices are simulated. The first one concerns use of the usual silica fiber based on the Kerr effect, the switching power of which is 471 W, consisting of the experimental result3; the second one concerns the use of an erbium-doped fiber (EDF) based on the resonant nonlinearity, the switching power of which is reduced by less than 25 mW. The transmissions of the AOS in the EDF-LPFG pair for different parameters of LPFG are also discussed.

Low-power 1×2 optical switching in a double coupler ring resonator (DCRR) made by a silicon nanoscale waveguide based on two photon absorption (TPA) is analyzed theoretically. The TPA originates from a femtosecond pump light at 400 nm, which enters the DCRR together with a CW signal light at 1.55 μm through the input port. TPA makes the silicon free-carrier concentration change, which is proportional to the change of reflective index. Our numerical simulation shows that when average pump power reaches 2 mW, it will induce the 10. 3 refractive-index change and the π-phase shift of signal light, after which 1×2 all-optical switching can be realized.

In this paper, we study on optical bistability behaviors in a double-coupler ring resonator, in which an erbium-doped fiber amplifier (EDFA) is inserted into the ring for counteracting the loss by the gain of EDFA. The two groups of transmitted and reflected optical bistability loops under different parameters are investigated symmetrically. According to our optimum design of this new optical bistable device, the switching power can be less than 60 mW.

A pseudospectral time-domain approach (PSTD) is proposed for the first time to treat one-dimensional photonic crystal with bi-isotropic chiral media. It is proved that PSTD method has the computational advantages over finite-difference time-domain (FDTD) method. The switching schemes based on the defect and the sidebands are summarized, and the idea of utilizing the smoothed sideband is also introduced. Compared with several apodization types, the envelop functions are emphasized, which benefits a switch with enough contrast below a certain limited threshold.

We propose a novel Sagnac interferometer (SI) optical switch consisting of a pumped erbium-doped fiber 3dB coupler for a signal at 1.55 μm and a short fiber loop. The switching between the reflected and transmitted output powers can be accomplished by two approaches: using 980nm-pump power in the cross-pumping mode or using an additional signal power in the self-pumping mode. In the two modes, the switching powers are 40mW and 20mW, and the switching times are sub-nanoseconds and milliseconds, respectively. In the both cases, the switching powers are independent of the fiber-loop length, so that the power-length products for proposed SI switch can be smaller than that of other kind of nonlinear SI switches.

Eye-protection glasses against YAG laser injury based on band gap reflection of one-dimensional photonic crystal (PC) is designed and manufactured in this paper. The laser beam (wavelength 1.06 μm) is reflected by the one-dimensional PC (with the transmission 10−7) and absorbed by the phosphatic glass substrate (with the transmission 1% for 1.06 μm), so the transmission of the device for wavelengths of1.06 μm can reach 10−9. The glasses have enough capabilities to protect the eyes from injury of ns-YAG lasers whose energy density is 1 J/cm2 for all incident angles, and also to avoid a second injury to others from the reflected laser beams. The transmission of the glasses is beyond 70% for the visible lights. The testing data of the eye-protection glasses agree well with the theoretical predictions.

We proposed an active double-coupler ring resonator all-optical switch, in which half of the fibre ring is made by a 980 nm laser-pumped erbium-doped fibre to form an erbium-doped fibre amplifier (EDFA). The switching is based on self-modulation by the signal light at 1.55 µm and has milliwatt switching power and nanosecond switching time for a 0.2 m ring length. Because the gain of the EDFA compensates the losses in the ring and the EDF has high nonlinearity, the switching power can be reduced dramatically. Both switching time and switching power of such a switch can be controlled by adjusting the 980 nm pumping power to make a trade-off between both parameters.

A novel all-optical switch based on nonlinear polarization mechanism using polarization-maintaining fiber ring with a polarization rotator is proposed. Optical switching with low threshold of mW order and optical limiting with broader limiting range, less fluctuation, higher damage threshold and response speed are demonstrated numerically. The deterioration of switching and the improvement of limiting originating from losses are also studied. Considering the tradeoff between switching power and bandwidth, the way to increase bandwidth is discussed.

We report a high-quality interleaver with frequency spacing of 25 GHz, which is made by a Michelson interferometer with a mirror replaced by a Gires–Tournois etalon. Our optimal design and elaborate manufacture assure its excellent performance. We present the analysis of performance parameters, design of structural parameters, the unique fabrication technology and testing results of this interleaver in this paper.

A novel configuration of all-optical switch in Mach-Zehnder interferometer with two-arm-sharing nonlinear ring resonator is proposed. Self- and pump-controlled switching are investigated numerically. The simulation results show the reduction of switching threshold down to 0.1 mW and 4 mW correspondingly and the high stability on the switch on/off states. The feasibility of ps-order ultrafast all-optical switching is also discussed.

Optical bistability occurs when the effects of nonlinear behavior of materials cause hysteresis in the transmission and the reflection of a device. An optical bistable device based on one-dimensional photonic crystal waveguide is designed and fabricated. Doped semiconductor CdS x Se 1 − x glass, which has large nonlinear refractive index coefficient and fast nonlinear responding time, is used. The threshold light intensity of the optical bistability is 1.65 × 10 5 W/cm 2 and the optical switching time is 63 ps.

We fabricate a 3 dB nonlinear coupler for a signal at 1550 nm by fusing together two single-mode Er 3+ -doped fibres. Pump light at 980 nm passes through one of the two fibres, changing the coupling ratios of the coupler. Our experiment shows that when the pump power increases from 0 to 20 mW, the bar coupling ratio changes by 40%. Using the coupled mode equations and resonant nonlinear polarization theory, we obtain the relationship between the coupling ratio and the pump power. The theoretical curve agrees well with the experimental data.

A dynamical addressing device for optical interconnection based on one-dimensional nonlinear photonic crystal made on the planar waveguide was designed and fabricated. The dynamical interconnection is performed by controlling the power density of the control light. In our device, when increasing the power density of the control light from 0 to 2.60 × 105 W/cm2, the angle of the signal beam can be changed by about 2°. This kind of devices is promising for use in all-optical interconnection, optical information processing and optical communication.

Optical bistability device based on nonlinear one-dimensional photonic crystals is designed and manufactured. The device has 20 periods and is made from ZnS and ZnSe layers alternately. When the incident power density reach threshold power density 1.0 × 105 W/cm2, the Ar ion laser, whose wavelength is 514.5 nm, is shifted out of the optical band gap of the one-dimensional nonlinear photonic crystals, so an optical switch is made. It also can be made into an optical bistability device. The threshold power density of the optical bistability is 1.38 × 105 W/cm2 and its switching time is about 100 ps in experiment. The experimental results are in good accordance with the theoretical ones.

We propose a dynamic all-optical switch in a high-nonlinear and 3cm long micro-ring resonator side coupled with a M-Z interferometer (MZI) through a nonlinear erbium-doped coupler. This switching device works in the 1.55 μm wavelength. One of its two channels is pumped by a light at 980 nm. When changing the pump power to control the coupler reflectivity, the working parameters of the switching device can be changed in a wide range. The optimum parameters of the switching devise with 0.6 mW switching power and 9 ns switching time can be obtained.

In order to get the lowest switching power in the ring-resonator-coupled M-Z interferometer switching devices, the loss in the ring resonator has to approach zero. However, in practice, the loss in the ring and coupler cannot be neglected, as it might make the switch unworkable. Therefore, we suggest inserting a gain-controllable optical amplifier into the ring to compensate for the losses and enhance the ring finesse to reduce the switching power. By adjusting the gain, a tradeoff between the switching power and the switching time can be obtained.

Planar optical waveguides consisting of layers from different materials created at elevated temperatures usually exhibit substantial stresses. These stresses are caused by thermal-induced strains that originate from the bonding of the layers in addition to intrinsic strains. For the first time, the analytical form of thermal stress formula is derived for the waveguide glass layer of the silicon-based silica waveguide in bilayer structures by the thin-film approximation and under the strain compatibility and the force equilibrium conditions. These conditions address the composite nature of the waveguide glass layer containing the waveguide core layer and the cladding layers within the optical planar waveguide. The developed formula reveals that temperature parameter, material parameters, and structural parameters affect the distribution of the thermal stress. By applying the formula, we demonstrate that it is possible to achieve the thermal stress-free, and, hence, the stress-induced birefringence-free waveguide devices by proper waveguide designs.

In this paper we propose a new sensor based on a periodically modulated fiber grating. The fiber grating is fixed on a PZT plate, which is modulated by an alternating voltage. In order to measure strains, we only use an oscilloscope and a single-mode-laser source to measure the change of the reflected-light pulse period. Comparing with the original method of strain measurement that using an optical spectrum analyzer and a broadband light source to measure the wavelength shift of reflected power peak, this method is much simpler and has lower cost. Our research results show that two methods are equivalent, and theoretical analysis well consists with experimental results.

A novel optical fiber sensing technique for length and temperature measurement with accuracy of sub-nanometer and 10-4° C , respectively, is proposed. The new kind of sensor is a combination of a conventional interferometric sensor and a hybrid optical bistable device. The measurement is accomplished by counting the number of electrical pulses, provided by switching on and switching off of the optical bistability periodically. An optical bistable fiber sensor with a March–Zehnder interferometer is analyzed in this paper. The main advantages of this technique lie in the adjustable high accuracy and ability to distinguish the variation direction of quantities to be measured.

We have studied the nonlinear refractive properties of the organometalic fullerene-C 60 derivatives fac- and mer- [bis (1,2-diphenylphosphino) ethane] (tricarbonyl)(η 2 -fullerene-C 60 ) chromium and molybdenum using the Z-scan method. These compounds change from being self-defocusing to self-focusing as the input light intensity increases. The experimental results have been interpreted using rate-equation theory. Critical conditions for the occurrence of this kind of transition have been elucidated for the steady-state situation.

A novel optical limiting device by sandwiching a nonlinear liquid between two specially designed diffraction gratings is proposed and demonstrated. Under low intensity of light, the device has a high transmittance because of index matching between the liquid and grating materials. Strong light beam can induce the nonlinear effects such as reverse saturable absorption, nonlinear scattering and self-defocusing to reduce the transmittance of device. On the other hand, the strong light beam can also change the index of the liquid to induce index mismatch and reactivate the gratings to diffract the output light away from the observed direction to protect eyes. In this paper, the different gratings with suppressed zero-order diffraction and nonlinear liquids are analyzed. The new device provides an effective protection of eyes against the ns-pulsed laser in the visible range.

The nonlinear refraction property of excited-state nonlinear optical materials was studied based on rate-equation theory. The effects of the ratios of excited-state absorption cross section and refrangibility to those of ground state upon their nonlinear refraction properties were analyzed. The nonlinear optical properties of ( m -MeC 6 H 4 ) 5 C 60 H-toluene solution were investigated by using Z -scan method with 8 ns 532 nm laser pulses. The solution was proved to possess strong nonlinear absorption and weak nonlinear refraction.

The optical nonlinearity of a new pentanuclear cluster WS 4 Cu 4 I 2 (py) 6 was investigated by using the Z-scan method. The cluster has a strong nonlinear absorption and self-focusing property. The optical limiting power of the cluster in DMF was stronger than that of C60–toluene and the reported cluster with the same structure and skeleton atoms but different peripherical ligands. The experimental results were analyzed based on the rate-equation theory.

In this paper, we have applied a perturbation expanding method to the hopping model, and studied the coupling effect of modulation depth between two photorefractive phase gratings stored in one point. The coupling equations of modulation depth and their steady solutions have been derived. It has been found that the modulation depth of one of the two gratings seriously affects the spatial-charge field of the other grating.

A theoretical steady-state rate-equation analysis is presented for investigating the critical conditions for the conversion from reverse saturable absorption to saturable absorption. The intensity threshold is defined and seen as an important factor in the interpretation of the reverse saturable absorption (RSA) to saturable absorption (SA) conversion. A comparison between steady-state and dynamic results is made and the application of intensity threshold in optical limiting is proposed.

We investigate the performance of S-band amplification for Tm3+-doped fluoride fiber pumped at 787 nm with cooperative lasing at 1.8 mum through numerical analysis based on a theoretical model. The simulated results show that the signal gain at 1470 nm in a 3-m-long Tm3+-doped fluoride fiber with the pumping of 0.5-W 787-nm laser diode can be enhanced by 15.66 dB when 1.8-mum laser oscillates.

A modified equal correlation peak (ECP) optimization is proposed and introduced into the filter-feature-based joint transform correlator (FFB JTC) used for distortion-invariant pattern recognition. Instead of one fixed and arbitrarily chosen point in the classical ECP optimization, the maximum point in the correlation output plane is selected in our method. The modified ECP optimization is quite efficient when involving filter modulation. When compared with other ECP optimization methods related to the filter-modulation, our optimization method is linear and can easily obtain an exact solution. Simulation results show that after ECP optimization the stability of the correlation output of the FFB JTC may be greatly improved.

We report on a method of determining the C 60 molecular polarizability α e of the excited singlet state under 23 picosecond excitation at 532 nm using the Z -scan technique. Based on the Clausius–Mossotti equation, the sign and the magnitude of the real part and the imaginary part of α e are respectively measured to be 1.2×10 −22 and 7.2×10 −23 esu. We also obtain the nonlinear refractive cross section σ r of 4.2×10 −17 cm 2 .

The chiral photonic crystal (PC) with layered structure is investigated by using the finite-difference time-domain method. The bandgap separation for right-hand/left-hand circularly polarized light in chiral PC is demonstrated by comparing to the achiral case. The effect of layer number and the optical nonlinearity on bandgap structure is analyzed. Optical switching with the help of the change in bandgap structure is discussed. A novel scheme of an all-optical switch based on sideband improved by apodization is proposed

Two kinds of fiber-grating-pair all-optical switches are proposed. They consist of two silica gratings (FBG and LPFG) connected with a nonlinear fiber. The switching powers of these two switches are few milliwatt.

Nonlinear absorption at 532 nm in a series of metallo-porphyrin-like compounds using 8 ns and 23 ps laser pulses is reported. The experiments show that for picosecond pulses, reverse saturable absorption occurs only at low fluences, and saturable absorption occurs at high fluences, for nanosecond pulses, only reverse saturable absorption occurs. The density matrix equations of motion for a six-level model are deduced to explain these nonlinear absorption effects, and numerical results calculated by the model agree well with the experimental results.

The dynamic and steady-state processes of the reverse saturable absorption (RSA) and the related optical limiting (OL) and optical bistability (OB) in metallophthalocyanine have been studied in detail using rate equations and the light-propagation equation. The experiments for demonstrating RSA, OL, and OB in a copper phthalocyanine (CuPc) solution with a Q-switched, frequency-doubled neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser were accomplished. The experimental results were consistent with the dynamic theoretical simulations, also approximately in accordance with the expression of RSA derived from the same equations at the steady-state condition.

To optimize the functions of the surface-relief diffractive optical elements for optical limiters and other applications, we present a systematic design and analysis with numerical illustrations of the transmission properties of different surface-relief phase gratings and Fabry–Perot elements. The spectral response and the tolerance of fabrication errors have been included. An analysis shows that the blazed grating and the echelon grating, a multilevel element as a substitute of the blazed grating, can make the on-axis transmittance very low (less than 1% with one grating) over the broad visible band with a large tolerance of fabrication errors. The results are highly significant for the design of optical-limiting devices.

An optical pulse generator based on instability is realized by using a bistable light-emitting diode (BILED) circuit. The circuit is constructed with two feedback signals having different relaxation time constants. To obtain square-wave pulses, a ZnS interference filter is employed in pulse shaping.

We review low-power all-optical switches based on active interferometers proposed by us, such as double-coupler ring resonator, ring-coupled M-Z interferometer, and Sagnac interferometer, in which an EDFA is contained in the ring or the coupler.

This paper aims to study mesoporous silicon with diameter range of 10-100 nm. The mesoporous Si samples, obtained by electrochemical etching, exhibit negative birefringence. Transmission measurements show effective two-photon absorption and self-focusing which depends on the porosity of the samples.

Air-guiding photonic bandgag (PBG) fibers are new kind fibers which are quite different from conventional correspondence ones. In this paper, we studied the light transmission properties and light wave modes of hexagonal air-guiding PBG fibers. The fiber core is 10μm diameter air hole, surrounded by a cladding of 71.5μm diameter. The outer-cladding of the fiber is about 185μm. The experimental results show that when the wavelength ranges from 1430 to 1600 nm, there is merely no attenuation. It means that in these new fibers, light wavelength ranges widely. In addition, light wave mode and fiber core are the same in symmetrical structure. The theoretical results coincide with experimental results exactly.

Microstructure fiber is a new kind of optical fiber. An arrangement of air holes running along the full length of the fiber provides the confinement and guidance of light. The light is confined in the silica core along the fiber axis. This fiber has endlessly single mode, special dispersion characteristics and so on. Using a vectorial effective-index numerical method, we investigated the dispersion characteristics of a microstructure fiber with honeycomb cladding structures. The dispersion, dispersion slope, and Kappa parameter can be designed neatly by changing the air hole size and the separation of the holes in fiber cladding. We demonstrated the Microstructure fiber with large absolute value of normal dispersion and negative dispersion slope at the wavelength of 1550nm. Conventional communication non-shifted single-mode fibers and nonzero-dispersion shifted fibers (NZ-DSF) can be compensated efficiently using the microstructure fiber at the wavelength around 1550nm.

A simple optical setup that uses a single pulse width laser is proposed to identify different kinds of nonlinear optical absorption mechanisms. It can easily distinguish reverse saturable absorption from two photon absorption, and direct three photon absorption from two photon absorption followed by one photon absorption from the excited level. Both theoretical and experimental results are reported.

A new type of deconvolver using a hybrid optical-electronic joint transform correlator is proposed. The inverse intensity filter used for deconvolution can be synthesized with this hybrid architecture in real time mode. A computer simulation is performed to demonstrate its capability to deconvolute the convolution of two functions.

Mach-Zehnder interferometer (MZI) 2×2 thermo-optic switches, based on parabolically taped MMI couplers, have been designed in this paper. The theory of the parabolically tapered MMI coupler is studied to estimate the optimal length, and then the effective index method (EIM) and the beam propagation method (BMP) are utilized to simulate these devices to find out the optimal length. The values of waveguide parameters are set as: the refractive index difference is chosen to be 0.75%, the width of the MMI field is 30 micron, the distance between the MZI arms is 100 micron and the separation between the access (input/output) waveguides is 250 micron. The simulated results show that the performances of the tapered coupler is better, whose the second width is equal to 25 micron. MMI coupler is also simulated by BPM. Although the tapered coupler can not offer performances as well as those of other MMI coupler, it can offer compact devices. The switches, based on conventional MMI couplers and tapered couplers, are simulated at the same of access waveguide separation and MMI width. Simulated results show that although the performances of this new structure are not as good as those of the switches based on conventional MMI couplers, but can reduce the length of the device effectively. This compact characteristic is important for integrated planar optical circuit.

Nano-scale thin films are of wide interest. The distribution of angles between the grain-boundary plane and the surfaces of such films can affect their mechanical behaviour. A transmission electron microscopy method is described to measure the distribution of such angles. This method is based on the well-known double-diffraction effect from two overlapping grains, but its implementation at multiple interfaces is made practically feasible by recent advances in computer and digital imaging techniques, as demonstrated in this paper.

We proposed two types of silica-based all-optical switches with pumped erbium-doped ring resonator, and obtained the milliwatt switching power and nanosecond switching time for a centimeter-long ring, both parameters can be adjusted by pumping light.

Low power all-optical switches based on M-Z interferometer side-coupled with a ring resonator for get π -phase shift have been proposed [1]. However, the losses inside the ring and the coupler strongly affect the characteristics of switches. We suggest inserting a gain-controllable optical amplifier SOA into the ring to compensate all of the losses in the ring resonator as shown in Fig. 1.

The purpose of this paper is to present a grating-assisted nonlinear limiting method based on the principle of index mismatching and grating diffraction. In addition, various nonlinear organic materials have been tested in the optical limiting system. A few examples with experimental results are presented. Using a YAG nanosecond pulse laser at 532 nm as a simulated threat laser source, we tested the optical limiting characteristics of the methyl-red doped nematic liquid crystal films. In the meantime, we observed some quite interesting optical phenomena, such as self-defocusing diffraction rings, dark spots in the far-field behind the cell. Our experimental results presented in the paper show that the methyl-red doped nematic liquid crystal films posses a good optical limiting performance. When the input energy focused on the cell increases from approximately zero micro Joule to approximately 200 micro Joule, the output energy on the far-field exit plane is kept under 1 micro Joule. We will describe a completed optical limiting system with Q-switched frequency-doubled 12- ns Nd:YAG laser, dual-channel detectors, and computer processing units based on Lab-view software and its design considerations.

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