Counter-propagating Configuration Research Articles

Long-range distributed temperature and strain optical fibre sensor based on the coherent detection of spontaneous Brillouin scattering with in-line Raman amplification

We report an extended range distributed temperature and strain optical fibre sensor based on the coherent detection of spontaneous Brillouin scattering combined with Raman amplification. The Raman amplification was achieved within the sensing fibre using either co- or counter-propagating Raman pump configuration with respect to the probe pulse and experiments were conducted to investigate the optimum pump and probe power combination. Using Brillouin frequency shift measurements with co-propagating Raman pump configuration, a temperature resolution of 1.7 °C with a 20 m spatial resolution at 100 km was achieved. With the counter-propagating pump configuration, a temperature resolution of 5 °C with a 50 m spatial resolution at 150 km was achieved. Measuring both the power and frequency of the Brillouin signal, a simultaneous temperature and strain measurement was performed over 50 km using co-propagating Raman pump. Temperature and strain resolutions of 3.5 °C and 85 με with 5 m spatial resolution were achieved.

Studies on the material transparent light in semiconductor optical amplifiers

We study the relationship of the material transparency wavelength with a semiconductor optical amplifier's (SOA's) bias current and length theoretically. The dependence of the gain recovery and phase shift, under transparent assist light injection, on the bias current, SOA length and assist light's power are also analysed with special attention to the differences between co- and counter-propagating configurations. It is shown that counter-propagating configurations have a lower recovery lifetime and a bigger phase shift. To get noticeable fast gain recovery, the transparent light's power should not be less than 15 dBm. For a long SOA (longer than 1.5 mm), co-propagating transparent light slows down the gain recovery and decreases the phase shift. When employing a long SOA driven at high current densities for high-speed signal processing, transparent assist light injection is unnecessary, while gain region assist light injection is optimal.

Performance analysis on terahertz optical asymmetric demultiplexer with assist light injection

Operating characteristics of terahertz optical asymmetric demultiplexer (TOAD), under gain and transparent region assist light injection, are investigated in theory. We have analyzed the demultiplexing performance for three OTDM systems: 10, 40, 100Gb/s. Though the switching window is distorted, the resilience to timing jitter is improved under transparent light injection. In practical application, counter-propagating configuration should be taken. Gain region assist light should not be used in TOAD. Some key parameters are given through numerical analysis and simulations.

Gain recovery acceleration in semiconductor optical amplifiers employing a holding beam

Experimental characterization of gain recovery is presented for a semiconductor optical amplifier saturated by a holding beam in co- and counter-propagating configurations at various wavelengths. The gain recovery is studied in detail for a holding beam at a wavelength near the gain transparency where an increase of the recovery time with the injected power is observed for the co-propagating configuration. The measurements confirm previous modelling results that are also used to explain this behaviour.

Comprehensive Analysis of Two Cascaded Semiconductor Optical Amplifiers for All-Optical Switching Operation

This paper presents numerical calculations and experimental results for two cascaded semiconductor optical amplifiers (SOAs) in a counterpropagating configuration for an all-optical switching operation. A detailed theoretical analysis with regard to the gain evolution and the extinction ratio (ER) through the SOAs' cavities show that the two cascaded SOAs have improved performance over the single SOA configuration. A measured ER up to 20 dB is obtained for close contra-directional signal wavelengths and for a wide range of optical input powers. An all-optical switching operation is realized for 2.5-GHz data pulses, and its feasibility is demonstrated with an ER higher than 12 dB over a wide range of wavelength.

Raman amplification in ultrasmall silicon-on-insulator wire waveguides

We measure stimulated Raman gain at 1550 nm in an ultrasmall SOI strip waveguide, cross-section of 0.098 microm2. We obtain signal amplification of up to 0.7 dB in the counter-propagating configuration for a sample length of 4.2 mm and using a diode pump at 1435 nm with powers of <30 mW. The Raman amplifier has a figure-of-merit (FOM) of 57.47 dB/cm/W. This work shows the feasibility of ultrasmall SOI waveguides for the development of SOI-based on-chip optical amplifiers and active photonic integrated circuits.

Gain dynamics of semiconductor optical amplifiers and three-wavelength devices

A numerical model that accounts for the effects of the amplified spontaneous emission (ASE) on the carrier dynamics in a travelling wave semiconductor optical amplifier is presented. The ASE is modeled using effective parameters that are derived starting from the spontaneous emission and gain models. The gain dynamics are then analyzed using the parameters extracted from measurements on a real device to explain the overshoot in the gain recovery. The model is also used to simulate the gain recovery in a three-wavelength device configuration for various injected powers and wavelengths. The recovery time when the injected beam is at the device transparency wavelength is also analyzed with particular attention to the differences between co- and counter-propagating configurations.

Laser frequency stabilization by Doppler-free magnetic dichroism

We report a method of laser stabilization to an atomic transition frequency, which is based on a magnetically induced nonlinear circular dichroism in the counter-propagating pump-probe configuration. The proposed method does not require any modulation of the laser frequency. In comparison to the linear, Doppler-broadened dichroic lock, the proposed version provides a precise locking to a well-defined atomic resonance at a cost of a smaller capture range. A simple model description of the locking signal is presented and two experimental realizations are shown: for a dye laser at λ=589.6 nm (Na D1 line) and for a diode laser at 795 nm (Rb D1).

Polarization dependence of gain in discrete Raman amplifiers with dispersion compensating fibres

Polarization dependence of stimulated Raman scattering is a fundamental physical effect, which influences the design and application of Raman amplifiers for optical networks. We report experimental data of the study of polarization dependence of Raman gain in dispersion compensating fibres (DCF). The investigation displays a strong variation of the polarization dependence of Raman gain versus average polarization mode dispersion (PMD) values of DCF for both co-propagating and counter-propagating pump configurations. The reported data show, for the first time to our knowledge, that, for large PMD values, such a dependence may be as small as to be comparable with the accuracy of the measurements. The results of the study may be used to simplify the design of Raman amplifiers to avoid depolarization of pump sources that are currently used to decrease polarization dependence of Raman gain.

Correction to "quasi-phase-matched (QPM) difference frequency generation in a mirrorless counterpropagating configuration"

Correction to "quasi-phase-matched (QPM) difference frequency generation in a mirrorless counterpropagating configuration"

Quasi-phase-matched (QPM) difference frequency generation in a mirrorless counterpropagating configuration

We present a theoretical analysis of second-order nonlinear difference frequency generation (DFG) in a generalized mirrorless quasi-phase-matching (QPM) frame, aimed at a comparison of counterpropagating DFG configuration (CDFG) to other DFG schemes, in view of all-optical processing applications. Field nonlinear coupling equations have been numerically solved under the hypothesis of phase-matched interaction. The evolution of propagating fields within the material and the wavelength conversion efficiency have been calculated in dependence of operating parameters. The increased complexity in the evolution of amplitude and phase for fields interacting in CDFG with respect to forward-propagating DFG (FDFG) is at the basis of a dramatic increase in the wavelength conversion efficiency under particular settings of device parameters.

Solution of Generalized Optical Bloch Equations by the Method of Matrix Continued Fraction

A numerical method is developed for solving the optical Bloch equations in the center-of-mass momentum space for a closed V system in a counterpropagating field configuration. The method consists of an iterative procedure based on the matrix continued fraction and a transformation by which the optical Bloch equations can be organized into the tridiagonal matrix recurrence form.

Coherent-to-incoherent light conversion for optical correlators

The authors present a novel coherent-to amplified spontaneous emission (ASE) converter based on SOA's cross-gain modulation (XGM) in a counterpropagating configuration. Experimental characterizations of the ASE converter with bit-error rate (BER) measurements at 2.5 and 5 Gb/s are shown. The device capabilities are exploited in a delay lines optical recognizer at 2.5 Gb/s. Recognition is based on a coherent-to-incoherent light conversion followed by a fiber array correlation filter. Experimental evidence here reported shows performance improvement in comparison with a scheme which adopts a standard telecom source.

Measurement of the effective area of non-linear power transfer in single-mode fibers due to stimulated Raman scattering

A method to measure the non-linear effective area in single-mode fibers is described. It is based on a Raman non-linear effect characterization set-up that uses a continuous-wave double beam arrangement in counter-propagating configuration. Non-linear effective areas are theoretically evaluated and experimentally determined for various types of fibers (standard single-mode fiber, dispersion-shifted fiber and large effective area dispersion-shifted fiber) and a correction factor that relates these effective areas with the mode field diameters of the fibers is introduced for taking into account the departures of the power distributions from the Gaussian shape.

Guiding of metastable helium atoms through hollow optical fibres

Atoms can be transported through hollow optical fibres using laser light blue-detuned from the atomic resonance, which propagates through the fibre to create a repulsive evanescent light field at the inner fibre wall. We report here the first transmission of metastable helium (2 3S1) atoms through such hollow optical fibres. Using fused silica capillaries with several different geometries, laser radiation that is blue-detuned from the 2 3S12 3P2 transition was focused into the body of the capillary via total internal reflection from a 45° bevel polished at one end. Laser fields were used with both co- and counter-propagating configurations to the atomic beam.

Quadratic solitary waves in a counterpropagating quasi-phase-matched configuration

We demonstrate the possibility of self-trapping of optical beams by use of quasi phase matching in a counterpropagating configuration in quadratic media. We also show the predominant stability of these beams and estimate the power level required for their observation.

Performance analysis of wavelength converters based on cross-gain modulation in semiconductor-optical amplifiers

An analytical theory describing all-optical wavelength converters based on cross-gain modulation (XGM) in semiconductor-optical amplifiers is derived. Our theory consists of two parts: a large-signal analysis yielding the transmission function for the signal, and a small-signal analysis in order to describe the transformation of the signal and probe intensity noise. Both the large-signal as well as the small-signal theory reveal similar performance for the co- and the counterpropagating injection scheme for bit rates up to 2.5 Gb/s. This is confirmed by computer simulations. Consequently, the counterpropagating configuration is preferable because the implementation is simpler and conversion to the same wavelength is possible. In order to increase the conversion efficiency it is better to reduce the average signal power than to increase the probe power, which additionally reduces the output power range. However, there is a tradeoff between conversion efficiency and output extinction ratio. According to the small-signal analysis, the relative-intensity noise (RIN) due to the probe and due to the amplified spontaneous emission is negligible. Moreover, the converted signal has a lower RIN than the input signal.

Development of a Novel Fluorimeter Based on Superluminescent Light-Emitting Diodes and Acousto-Optic Tunable Filter and its Application in the Determination of Chlorophylls a and b

A novel, compact, inexpensive fluorimeter that has high sensitivity and no moving parts has been developed by using super luminescent (bright-blue) light-emitting diodes (SLEDs) and an acousto-optic tunable filter (AOTF). In this instrument, the recently developed gallium nitride SLEDs were used in a counterpropagating configuration to provide excitation light. These SLEDs provide not only high intensity (several milliwatts) but also wide spectral bandwidth in the blue region (from 370 to 570 nm). The AOTF can be placed before the sample to facilitate the measurements of excitation spectra or after the sample for the emission spectra measurements. This fluorimeter is suitable for the sensitive and general fluorescent analysis of a variety of compounds. It has been used, as an example, for the sensitive and simultaneous determination of chlorophylls a and b. Detection limits of 2.30 × 10−9 and 1.10 × 10−9 M have been achieved for chlorophyll a and chlorophyll b, respectively. Index Headings: Superluminescent light-emitting diode; Acousto-optic tunable filter; Fluorescence; Chlorophyll.

Two-photon effects in continuous-wave electromagnetically-induced transparency

Electromagnetically-induced transparency (EIT) in a cascade three-level scheme is studied in rubidium vapour using continuous-wave titanium sapphire lasers. A counter-propagating experimental configuration significantly reduces the coupling laser power requirements and a reduction in absorption of over 90% is observed. The hyperfine structure of the upper level is seen within the EIT feature and the application of EIT to high-resolution two-photon spectroscopy is discussed. Simultaneous measurements of the excitation to the upper state are presented and clearly show Autler-Townes splitting and power broadening.

Amplification of light pulses in the visible through stimulated Raman scattering

Pulses from a colliding pulse modelocked dye laser operating around 546 nm have been amplified within an optical fibre through stimulated Raman scattering in a counterpropagating configuration. Gain values of up to 25 dB have been achieved at a repetition rate of 1 kHz.