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Detection (or demodulation) of the phase modulation is a critical operation in different photonic applications, in particular, in interferometric fiber sensors, material evaluation via laser induced ultrasound, coherent communications etc. Different techniques were proposed to overcome the main problem of conventional interferometers: necessity of stabilization of the quadrature operation point and compensation of the complex spatial structure of the detected wave. Here one can mention, active operation point stabilization via external optoelectronic loop [1], adaptive dynamic gratings based systems [2], self-reference (i.e. without external reference wave) confocal Fabry-Perot cavities [3], and configuration based on quantum phase memory of an ensemble of two-level atoms/molecules [4]. All these seemingly different approaches have the same common feature — they are based on utilization of a narrow absorption/gain spectral profile, artificial, as in the first two above-mentioned techniques, or natural, as in the two last cases. Here we analyse a general case of passing of the phase modulated wave through a narrow absorption/gain spectral line as a technique for its demodulation (i.e. its partial transformation into the intensity modulation wave) with the following detection by conventional photodetector.

Lithium niobate (LN) is one of basic materials of integrated optics used, for example, for electro and acousto-optical modulators, switches, diffraction gratings, nonlinear optical wavelength converters, and others [1]. LN-based ion exchange or Ti-indiffused photolithographic techniques are established technologies, however, some modern applications, such as high-purity suppressed carrier synthesis or high extinction ratio pulse generation for precision measurement systems require the use of additional active trimming electrodes [2] and a corresponding servo loop because of finite tolerances of optical elements. Recently developed direct laser writing (DLW) with femtosecond/ultrafast laser pulses [3] can be employed to address the problem, but this method is problematic for mass production and suffer from high insertion loss in optical waveguides (typically ∼ 1 dB/cm compared to

Studies of magneto-optic properties of Fe 3 O 4 ferrofluid (colloidal solution of nanosized particles) in a pulsed and AC magnetic field are reported. It is shown that a microstructured optical fiber with cladding holes filled with ferrofluid is sensitive to the external magnetic field vector and that operating speed of magnetically controlled optofluidic devices can be increased by using a transverse biasing.

Saturated optical absorption, electromagnetically induced transparency, and slow/fast light propagation on their basis have recently been reported for the acetylene (C2H2) filled Hollow Core Photonic Crystal Fibers (HC-PCF) in the 1530nm spectral region (see e.g. [1-3]). Here we report experimental observation of the nonstationary coherent effects, such as optical nutation and free induction, in the acetylene filled HC-PCF. For experiments we used an allfiber pumped-through setup [3] with a 2.6 m-long HC-PCF with the capillary core diameter 10 μm. The gas pressure was 0.35 Torr, and the wavelength (1530.37 nm) corresponded to the P9 acetylene absorption line. Fig.1 presents typical profiles of the 15 ns input and output pulses observed for different incident light peak powers. The ratio between the output and input profiles (i.e., transmission coefficient) demonstrates pronounced temporal oscillations, which we attribute to the optical nutation effect caused by coherent Rabi oscillations [4]. The trailing edges of the input/output pulses shown in the inset to Fig.1 demonstrate another coherent effect i.e. the free optical induction [4].

Rare-earth doped fibers, and in particular Er-doped fibers (EDFs), are demonstrated as promising substitutes of photorefractive crystals in adaptive interferometers based on transient two-wave-mixing (TWM) [1]. The dynamic population gratings are recorded in these fibers by local saturation of the optical absorption/gain in the bright fringes of the interference pattern formed by two counter-propagating mutually coherent recording waves [2]. When one of the waves is phase modulated, the TWM signal is observed as an intensity modulation in the output waves. For the recording wavelength close to the maximum of fundamental absorption of EDF (∼ 1532 nm), the transient TWM response depends quadratically on the modulation amplitude, which corresponds to the unshifted absorption grating recorded in the saturable doped fiber.

Adaptive detection of phase modulated optical signal in polarization insensitive configuration based on two-wave mixing in Er-doped fiber is proposed. Experiments were performed in a birefringent 0.5 m long Er-doped fiber at lambda=1492 nm.

Simplified schematic of an all-fiber adaptive interferometer in a two-wave mixing (TWM) configuration is applied for detection of mechanical vibrations. This experiment used OFS-Fitel single-mode Er-doped optical fiber HG980. To improve the adaptive properties of the interferometer more, optical pumping of the Er-doped fibers (at 980 nm) could be done, or rare-earth doping elements with shorter spontaneous relaxation times of the metastable level could be utilized.

Experiments on transverse detection of transient fluorescence excited by oscillating interference pattern in Er-doped fiber in spectral range 1480-1600 nm are reported. This technique is used for evaluation of local amplitude of the recorded population grating.

We demonstrated vibration measurement using photo-electromotive-force detector and gain-switched laser diode. This configuration can lead to laser vibrometer with easy-to-setup optical train, low average optical power, compact design, and higher vibration frequency.

In this paper, we propose relatively simple experimental procedure for measuring the diffusion coefficient D and drift mobility in a single experimental configuration, which can be used for verification of the Einstein relation in amorphous photoconductors. Our method is based on utilisation of the nonsteady state photo-EMF effect excited in the bulk of a photoconductor by a vibrating light interference pattern. Experimental results obtained in a photoconductive polyvinylcarbazole-based polymer in reflectance configuration are presented. In our analysis we consider a monopolar photoconductor with arbitrary structure of trapping centres and no saturation of these centres during formation of the space charge field.

Polarization insensitive detection of optical phase modulation can still be considered as one of important problems, which is facing, in particular, particular, practical utilization of coherence optical communication lines. We consider novel polarization insensitive detection configurations, utilizing adaptive photo-detectors, which can be based on photorefractive two-wave mixing (TWM), or photo-EMF configurations.

Summary form only given. Earlier we have reported application of the photo-EMF based adaptive detectors for coherence measurements of the laser beams, including those reflected from rough surfaces. Later, application of this technique for laser profilometry of as-processed metal surfaces was experimentally demonstrated. Here we present for the first time, micron resolution monitoring of in-depth distribution of the reflectance i.e., another step to application of this technique to Optical Coherence Tomography.

Summary form only given. Due to simplicity and robustness adaptive photodetectors based on the photo-EMF effect are considered promising for various applications, and in particular for laser ultrasonic detection. The most important drawback of these photodetectors is relatively low current response:/spl les/10/sup -3/ A/W for typical GaAs devices with several tens of interference fringes in the interelectrode spacing. Since the dominating noise of the detection configuration is the current noise of the transimpedance amplifier (/spl sim/ 1pA//spl radic/Hz for operational amplifiers with bipolar transistors), this reduces dramatically the signal-to-noise ratio in the detected signal. Various changes were proposed to increase the current response, and in particular, longitudinal configuration of the device and asymmetric interdigitated planar system of contacts. We demonstrate that external DC bias can also increase the response of the GaAs photo-EMF detectors significantly.

Summary form only given.Summary from only given. Recently the GaAs multiple quantum well (MQW) transverse-field structures were investigated by means of DC photo-EMF effect, which is excited by moving interference pattern at /spl lambda/=1064 nm. Below we present the first results on investigation of similar and longitudinal-filed structures by means of alternating photo-EMF currents (excited by vibrating interference pattern) at shorter wavelengths, which ensure efficient generation of photocarriers of both types.

In several holographic experiments with photorefractive crystals (PRCs), detailed information concerning space-charge gratings in the near vicinity of the surface is needed. Examples include beam coupling in photorefractive surface waveguides, holographic recording with highly absorbed (e.g., UV) light in ferroelectric PRCs, or photoelectromagnetic flow signals (EMF) excitation in semiconductors with visible light. We show that direct observation of light diffraction from gratings formed at the surface of the PRCs can give the necessary information.

Linear detection of the optical phase modulation in a randomly polarized laser beam is highly important, in particular, for optical fiber sensors and coherent optical communication systems. This type of detection cannot be performed properly by means of conventional techniques, and the problem is that the original optical signal beam has, in general, two orthogonally polarized components with randomly fluctuating phases and relative intensities. We propose a simple original configuration based on utilization of adaptive photodetectors to overcome this problem. The system consists of two independent channels for coherent (with the reference waves) detection of two orthogonally polarized components of the signal beam and by following summation of the output electric signals. The data presented demonstrate applicability of this configuration for polarization-independent coherent photodetection. In general, the adaptive systems in consideration also solve the problem of compensation for the general phase drifts and irregularities of the detected wavefronts. Moreover, if the cut-off frequency of the adaptive detector is high, it can also compensate for the discrepancy between the optical frequency of the signal wave and that of the local oscillator.