Hysteresis Loop Effect on Intensity for Horizontal and Vertical Polarization by Optical Feedback

The effect of the optical feedback on the polarization flipping point and hysteresis loop was studied. The polarization flipping occurred at all angles between the polarizer axis and the laser polarization. The polarization flipping point changed by an optical feedback occurred at angles from 0° to 90°. Ability of choosing or controlling the laser polarization was determined by changing the direction of vertical and horizontal polarization by polarizer rotation in the external cavity from 0° to 90°.

Raman scattering is a powerful technique for quantitatively probing high temperature and high speed flows. However, this technique has typically been limited to clean hydrogen flames because of the broadband fluorescence interference which occurs in hydrocarbon flames. Fluorescence can also interfere with the Raman signal in clean hydrogen flames when broadband UV lasers are used as the scattering source. A solution to this problem has been demonstrated. The solution to the fluorescence interference lies in the fact that the vibrational Q-branch Raman signal is highly polarized for 90 deg. signal collection and the fluorescence background is essentially unpolarized. Two basic schemes are available for separating the Raman from the background. One scheme involves using a polarized laser and collecting a signal with both horizontal and vertical laser polarizations separately. The signal with the vertical polarization will contain both the Raman and the fluorescence while the signal with the horizontal polarization will contain only the fluorescence. The second scheme involves polarization discrimination on the collection side of the optical setup. For vertical laser polarization, the scattered Q-branch Raman signal will be vertically polarized; hence the two polarizations can be collected separately and the difference between the two is the Raman signal. This approach has been used for the work found herein and has the advantage of allowing the data to be collected from the same laser shot(s). This makes it possible to collect quantitative Raman data with single shot resolution in conditions where interference cannot otherwise be eliminated.

We present a numerical study of the effects of optical feedback on the static and dynamic characteristics of vertical-cavity surface-emitting lasers (VCSELs) under both single-mode and two-mode operations. Our model includes transverse effects such as carrier diffusion and spatial hole burning (SHB) and is therefore capable of including gain-saturation-induced coupling among transverse modes. For short external cavities (/spl sim/30 /spl mu/m), optical feedback can completely suppress higher-order transverse modes. For relatively long external cavities (/spl sim/1 cm), VCSELs follow a period-doubling route to chaos. Numerical results show that multimode VCSELs exhibit self-pulsations at higher feedback levels compared with the single-mode case.

The effect of isotropic optical feedback from an extremely short external cavity on the emission properties of vertical-cavity surface-emitting lasers (VCSELs) is experimentally investigated. When changing the external cavity length a modulation of the total power and the wavelength emitted by the VCSEL is observed with a period of half the wavelength of operation of the device. When biasing the VCSEL around its solitary (without optical feedback) polarization switching (PS) current, PS between the two linearly polarized modes is observed by changing the external cavity length. This PS is found to occur through a hysteresis region. Our experimental results show a modulation of the PS currents and the hysteresis width when changing the external cavity length with a period of half the wavelength of operation. The experimental results are well explained by a two mode rate equation model.

A frequency-stabilized diode laser is widely used for applications in laser cooling and high-resolution spectroscopy. In this work, the 780-nm external cavity diode laser was constructed and subsequently frequency-controlled by three parameters, i.e., temperature, injection current and optical feedback. The laser frequency was measured with respect to the 5S1/2 → 5P3/2 (D2-lines) transition of Rubidium, while the laser mode was characterized by a Fabry-Perot interferometer. The laser temperature was passively controlled to a single value between 20 ̊C and 25 ̊C while the injection current was investigated in combination with course and fine adjustments of optical feedback. Only data relevant to a single-mode laser operation was collected. It was found that as the current increased, the laser frequency shifted linearly with slopes approximately 0.5-0.8 GHz/mA. Optical feedback from the external cavity was tuned by the voltage applied to the piezoelectric transducer, yielding a linear frequency response of approximately 0.2 GHz/V. The measured parameters were rearranged to represent the island of stability of the laser, suggesting suitable conditions that yielded single-mode operation, at a desirable laser frequency. The results were important for a design of an active feedback, in order to further reduce the frequency linewidth and intensity noise of the laser.

A multiband dual-polarized omnidirectional antenna for 2G/3G/Long Term Evolution (LTE) mobile communications is proposed in this letter, which consists of horizontal polarization (HP) and vertical polarization (VP) element with separate feeds. The VP element consists of three polygonal radiation patches with three equally spaced legs shorted to the ground plane. The HP element consists of three wideband slot loop structures, which is nested on the top of the VP element. Three slot loop structures provide a 360° coverage for HP and enhance its bandwidth. Both the simulated and measured results indicate that the frequency bands of 1650-2900 MHz for HP and 780-2700 MHz for VP can be achieved. The reflection of VP improves the gain of the HP element at least 1 dBi after nesting. The gain of HP element is more than 3 dBi for LTE, and the gain of VP element is more than 5 dBi in the LTE band and 1.5 dBi in the 2G band. Port isolation larger than 30 dB and low-gain variation levels are also obtained. The proposed antenna can be applied in mobile communications.

The electromagnetic signal propagation with vertical or horizontal polarization from a meteorological radar and the interaction with precipitation particles were studied by [1]. As a result of the studies, the researchers identified how a vertical polarized signal interaction with a precipitation particle differs from that of a horizontal polarized signal. This was also examined by [2]. In their work it was found that the attenuation experienced, horizontal polarization tended to be slightly larger than that undergone by vertical polarization [2], where the main cause of this phenomenon was identified as the raindrops distortion when falling. For heavy rain events, raindrops distortion is more noticeable than in light rain, since the raindrop diameter is extremely small and the shape is spherical, while, for heavy rain the diameter increases and the shape becomes oblate.

This paper describes the effects of optical feedback on the sensitivity of VCSEL tunable-diode laser spectroscopy (TDLS). Three VCSELs, emitting at different wavelengths in the near-infrared, were used. A TDLS system, subjected to optical feedback, exhibited a common signal-to-noise ratio profile for all three lasers. A catastrophic degradation of TDLS sensitivity occurred when feedback exceeded a level which we associate with coherence collapse. The TDLS system had a CH4 minimum detection limit of 7.5 ppmm without optical feedback. Optical feedback of less than ten percent reduced this sensitivity by two orders of magnitude. This reduction of system sensitivity was accompanied by a second-harmonic absorption signal baseline shift which degraded the system accuracy.

On-chip integration of semiconductor lasers have shown a growing interest in recent years, especially for the development of photonic integrated circuits (PICs) which are of paramount importance for high-speed communication within and between data centers, and fast on-board data exchanges. For all these applications, a key challenge remains the stability of the laser sources integrated on a PIC in presence of external optical feedback with the view to avoid integrated bulky and costly optical isolation. In this study, the effects of external optical feedback are investigated in hybrid InAs/InP quantum dot comb lasers on silicon. The design of the cavity includes a semiconductor optical amplifier section, a saturable absorber and an on-chip external cavity incorporating a vertical coupler. We measured the resulting feedback properties with respect to the operation conditions (bias current and voltage) and to the length of the saturable absorber. We show that under most operating conditions, the laser remains stable against optical feedback, only few regimes of operation occur, which either improve or degrade the frequency comb and/or the radio-frequency beatnote power of the laser.

We present an experimental study on the effect of optical feedback in both ground and excited emission of a GaAs quantum dot passively mode locked laser. The experimental setup consisted of a long external cavity with variable cavity length and feedback level ranging from –50 to –20dB. The obtained experimental results show dependence of the emission properties on the cavity length regarding both the ground and excited state. In addition a strong tolerance of the laser operation to feedback at the excited state operation regime is observed.

A low-profile omnidirectional dual-polarised antenna for 2.4 GHz wireless local area network (WLAN) applications is proposed. The vertically/horizontally dual-polarised radiation patterns are achieved by combining a modified monopole antenna with a printed arc dipole array. The overall volume of the presented antenna is less than 0.63 λ 0 × 0.63 λ 0 × 0.057 λ 0 ( λ 0 is the free-space wavelength at central frequency). To verify the design, a prototype antenna is fabricated and tested. The measured results show that the -10 dB impedance bandwidths for vertical polarisation (VP) and horizontal polarisation (HP) have both covered the required band of 2.4-2.48 GHz and the isolation, in the operating band, is more than 24 dB. The measured cross-polarisation levels within the working plane for VP and HP are lower than -18 dB. Stable antenna gains are greater than 0.46 and 0.44 dBi for VP and HP over the operating frequency band, respectively.

Airborne passive microwave signatures collected in Northern Finland during EMAC-95 are analyzed with the emphasis on forested areas and dry snow conditions. The microwave signatures cover the 6.8-18.7-GHz frequency range and were acquired at both vertical and horizontal polarizations. The analysis is carried out with respect to the forest-stem volume data and comprises three different snow-depth situations. Emissivities approach saturation limit with the increasing stem volume. At 10.65 GHz, the saturation level was found to be linearly related to the snow-water equivalent. On the basis of passive-microwave measurements, an empirical forest transmissivity model is developed. The model is valid at vertical polarization 50/spl deg/ incidence angle, and it accounts for microwave frequency and forest-stem volume effects in the range of 6.8-94 GHz and 0-150 m/sup 3//ha, respectively.

Effects of optical feedback on period-one nonlinear dynamics of an optically injected semiconductor laser are numerically investigated. The optical feedback can suppress the period-one dynamics and excite other more complex dynamics if the feedback level is high except for extremely short feedback delay times. Within the range of the period-one dynamics, however, the optical feedback can stabilize the period-one dynamics in such a manner that significant reduction of microwave linewidth and phase noise is achieved, up to more than two orders of magnitude. A high feedback level and/or a long feedback delay time are generally preferred for such microwave stabilization. However, considerably enhanced microwave linewidth and phase noise happen periodically at certain feedback delay times, which is strongly related to the behavior of locking between the period-one microwave oscillation and the feedback loop modes. The extent of these enhancements reduces if the feedback level is high. While the microwave frequency only slightly changes with the feedback level, it red-shifts with the feedback delay time before an abrupt blue-shift occurs periodically. With the presence of the laser intrinsic noise, frequency jitters occur around the feedback delay times leading to the abrupt blue-shifts, ranging from the order of 0.1 GHz to the order of 1 GHz.

The purpose of this study was to determine the effect of visual feedback and verbal encouragement on concentric peak torque of the quadriceps and hamstrings muscles of males and females at an isokinetic velocity of 60 deg/s. All testing was performed on the Biodex B-2000 isokinetic dynamometer in three sessions, separated by 7 to 14 days. Fifteen males and 15 females (age = 25.4 ± 2.4 yrs, wt = 76.6 ± 16.5 kg, ht = 173.61 ± 9.5 cm) were tested under the following conditions: (a) visual feedback, (b) verbal encouragement, (c) combined visual feedback and verbal encouragement, and (d) no feedback (control). Testing order was counterbalanced to control for possible learning effects. Two separate 2 x 4 repeated measure analyses of variance (ANOVA) were used to examine the difference among test conditions. Examination of quadriceps force production revealed that subjects generated greater peak torque when visual feedback was provided than when verbal encouragement or no feedback were provided. Similarly, quadriceps force production was greater when combined visual feedback and verbal encouragement was provided than when verbal encouragement or no feedback were provided (p < 0.05). Examination of hamstrings force production revealed that subjects generated greater peak torque when combined visual feedback and verbal encouragement was provided than when verbal encouragement, and no feedback were provided. Additionally, hamstrings force production was greater when visual feedback was provided than when no feedback was provided (p < 0.05). These results indicate that the use of visual and combined visual and verbal feedback increased quadriceps and hamstrings force production when compared to the control condition where no feedback was provided.

A broadband dual-polarized omnidirectional antenna is presented in this letter for the 2G/3G/LTE/WiFi communication systems. It consists of one vertically polarized (VP) element and one horizontally polarized (HP) element. The VP element is composed of a ground plane and a center-fed discone coupled by a circular patch with four shorting legs, while the HP element is composed of a printed dipole array, a feeding network, and four pairs of parasitical branches. Through such deliberate design, a broad bandwidth from 820 MHz to more than 6 GHz (about more than 152%) for VP and from 1530 to 2950 MHz (about 63%) for HP can be achieved while keeping the antenna compact. Good port isolation, omnidirectional patterns in horizontal plane, along with acceptable cross polarization and gain are also obtained. Details of the antenna design as well as experimental results are presented and discussed.