Threshold Gains Research Articles

Threshold Gain Reduction in Tandem Semiconductor Nano-Lasers

It is shown that a significant reduction in the threshold gain of electrically pumped semiconductor nano-lasers may be achieved in bridge-connected tandem semiconductor nano-lasers. Optimization of the design is achieved by exploring the impact of bridge length and width on the threshold gain. In addition, a detailed examination is also made of the emission patterns of the structure. It is found that a trade-off emerges between threshold gain and beam quality where multi-lobed far field emission may be associated with the lowest threshold gains.

Optimization of a quantum cascade laser cavity for single-spatial-mode operation via machine learning

Neural networks, trained with the ADAM algorithm followed by a globally convergent modification to Newton’s method, are developed to predict the threshold gain of the fundamental and first higher-order modes as functions of the refractive-index profile in a quantum cascade laser cavity. The networks are used to optimize the design of a refractive-index profile that provides essentially single-spatial-mode performance in a nominally multi-moded cavity by maximizing the threshold-gain differential between the modes. The use of neural networks allows the optimization to be performed in seconds, instead of days or weeks which would be required if Maxwell’s equations were repeatedly solved to obtain the threshold gains.

Aging Resets the Spontaneous Baroreflex Control of Sympathetic Action Potential Subpopulations in Humans

Currently, the effect of aging on sympathetic baroreflex control is equivocal, with some studies finding that integrated sympathetic baroreflex threshold gain is not affected by age, and others finding that it is blunted in older compared to young adults. While these studies evaluated the effect of aging on baroreflex control of integrated sympathetic burst frequency, it remains unclear how aging modulates the baroreflex control of postganglionic action potentials (AP). In young adults, the arterial baroreflex exerts non-uniform control over varying-sized APs spontaneously firing within bursts of muscle sympathetic nerve activity (MSNA), with the strongest control over medium sized AP clusters, and less control over the smallest and largest APs. Therefore, we tested the hypothesis that resting baroreflex control of AP subpopulations and integrated sympathetic bursts would be blunted in older compared to young adults. Baroreflex threshold relationships for integrated MSNA bursts and the underlying subpopulations of AP clusters (obtained using microneurography and a continuous wavelet transform) were assessed during 5 minutes of supine rest in eleven older (45-75 years, 5 females) and eleven young (21-30 years, 5 females) adults. Baroreflex threshold was quantified as the slope of the linear regression between AP probability (%) and MSNA burst probability (%) versus diastolic blood pressure (DBP, mmHg; Finger plethysmography). AP discharge was greater in older compared to young adults (Older: 614 ± 327 spikes/100 beats vs. Young: 211 ± 107 spikes/100 beats, P < 0.001), and the baroreflex control of AP subpopulations was reset towards greater firing probabilities in older adults (group-by-cluster effect: P < 0.001). Specifically, the arterial baroreflex threshold gain of medium AP clusters was greater in older compared to young adults (e.g. Cluster 4, Older: -7.8 ± 3.1 %/mmHg vs. Young: -4.1 ± 2.9 %/mmHg, P = 0.010), whereas the baroreflex threshold gains for the smallest (e.g. Cluster 1, Older: -1.2 ± 1.1 %/mmHg vs. Young: -0.7 ± 0.4 %/mmHg, P = 0.197) and largest (e.g. Cluster 10, Older: -0.5 ± 0.4 %/mmHg vs. Young: -0.6 ± 0.6 %/mmHg, P = 0.673) AP clusters were not modified by aging. Additionally, integrated sympathetic burst threshold gain was greater in older compared to young adults (Older: -7.9 ± 3.8 %/mmHg vs. Young: -3.9 ± 1.8 %/mmHg, P = 0.005), and the operating point was shifted rightward towards a higher DBP (Older: 76 ± 8 mmHg vs. Young: 68 ± 7 mmHg, P = 0.021). Contrary to our hypothesis, the strength of baroreflex control over medium sized AP clusters as well as integrated sympathetic baroreflex threshold gain was greater in this group of older adults compared to young adults.

Cochlear Implants in Subjects Over Age 65: Quality of Life and Audiological Outcomes.

BackgroundCochlear implants (CIs) have been recognized as a safe and effective means for profound hearing loss rehabilitation in children and adults and recently their use has been extended to subjects over 65 years of age. The aim of this paper was to assess indices related to changes in the quality of life (QoL) in elderly CI recipients.Material/MethodsA case-control paradigm was used to assess the effects of CIs on the QoL. Forty-two subjects were assigned to the Case group and 15 subjects to the Control group. All 57 subjects were affected by profound hearing loss and had received a CI. Audiological data were collected from both groups at: (i) 1 month pre-implantation [T1]; (ii) 1 day pre- implantation [T2]; (iii) 30 days post-implantation, with CI used in free field [T3]; and (iv) 12 months post-implantation, with CI used in a free field [T4]. The QoL was assessed via a Glasgow Benefit Inventory (GBI) questionnaire, adapted to otolaryngology. To compare subjects across different ages with varying degrees of speech development, a perception parameter was used from the Speech Perception Categories test developed by Geers and Moog.ResultsHearing performance was considerably improved after CI. In relation to the hearing performance at time T1, statistically significant threshold gains were observed in both groups in the T3 and T4 observation windows. At time T4, a threshold gain of 70 dB HL in the Case group and a gain of 84 dB HL in the Control group were observed. With speech therapy rehabilitation, a perception level of 6 was reached by 80.0% of patients in the Case group and by 100% of patients in the Control group. In terms of QoL, both groups showed improved post-CI scores. Statistical differences were observed between the 2 groups, with the Control group outperforming the Case group in all but the social section.ConclusionsDespite age-related changes in auditory system and prolonged hearing deprivation, CIs offer audiological and QoL benefits in the elderly.

Numerical investigation of side emission from large-area vertical-cavity surface-emitting lasers.

For large-area ion-implanted vertical-cavity surface-emitting lasers (VCSELs), side emission from the edges of a chip disturbs the laser emission of a VCSEL mode, and suppression of it is fundamental. In this paper, we present results of a numerical investigation of the side emission from large-area VCSELs. We have modeled a VCSEL structure by an infinitely broad layer structure with mirror loss at the edge surfaces. Estimated threshold gains indicate that laser emission occurs either in a VCSEL mode or in an edge-emitting Fabry-Perot (EEFP) mode. Calculated emitter length dependence of the threshold gain of these modes shows good agreement with experimental results, and the side emission is verified to be the laser emission of the EEFP mode. We have also discussed the way to suppress the side emission and confirmed that our recent achievement of over 200 W quasi-continuous-wave output from an ion-implanted VCSEL array is due to antireflection coatings of the edges and introduction of optical losses in ex-emitter regions.

Engineering the Lateral Optical Guiding in Gallium Nitride-Based Vertical-Cavity Surface-Emitting Laser Cavities to Reach the Lowest Threshold Gain

In order to improve the current injection in GaN-based blue vertical-cavity surface-emitting lasers (VCSELs) a dielectric aperture is generally used in combination with an indium–tin-oxide (ITO) layer on the top intracavity p-contact layer. The most straightforward way to realize this introduces a depression of the structure near the optical axis and we show, by using a two-dimensional (2D) effective index method and a three-dimensional (3D) coupled-cavity beam propagation method, that this typically results in optically anti-guided structures with associated high optical losses and thus very high threshold gains. Remarkably, the threshold gain reduces with increased negative guiding, which is due to improved lateral confinement and reduction of lateral leakage. Still, moderately positively guided designs should be preferred to avoid the detrimental effect of lateral leakage and high diffraction loss. To ensure positive index guiding, we propose to planarize the structure or introduce an elevation near the optical axis by additional processing, with an associated reduction in threshold material gain from 6000 to 2000 cm-1 for the studied structures.

Precise Lateral Mode Control in Photonic Crystal Vertical-Cavity Surface-Emitting Lasers

We demonstrate an oscillatory behavior of resonant wavelengths and threshold gains of the fundamental and first-order transverse mode of photonic crystal vertical-cavity surface-emitting laser (PhC VCSEL) with the PhC etching depth. To this aim we use a full vectorial, 3-D optical model (plane wave admittance method) combined with a thermal model and an exemplary 1300 nm quantum-dot VCSEL structure. Such oscillatory behavior poses a strong restriction on the etching depth precision: about 0.1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\mu{\rm m}$</tex></formula> in order to achieve a threshold gain not higher than 10% the threshold gain minimum and twice higher precision for simultaneous strong discrimination of the first-order transverse mode.

Threshold gain analysis in GaN-based photonic crystal surface emitting lasers

We have analyzed threshold gains and lasing modes in GaN-based photonic crystal (PC) surface emitting lasers (PCSELs) by using the multiple scattering method (MSM) for triangular-lattice PC patterns. Moreover, GaN-based PCSELs with different boundary shapes have been fabricated and measured. The lasing mode at the Γ band edge of GaN-based PCSELs can be identified by using the angled resolved spectroscopy and matched well to the results calculated by MSM. Threshold conditions in the GaN-based PCSELs with different boundary shapes are obtained by optical pumping and agree well with simulation results.

Pure tone auditory thresholds can change according to duration of interrupted tones in patients with psychogenic hearing loss

Conclusion: Pure tone auditory thresholds can change according to duration of interrupted tones in patients with mild to severe psychogenic hearing loss (PHL). Objectives: To examine how the duration of stimulus tones affects the hearing thresholds of patients with PHL. Methods: Twelve patients with PHL (21 ears) were enrolled in this study. We initially measured their hearing thresholds using interrupted tones with a duration of 2 s and equal length of on-time and off-time, 225 ± 35 ms, respectively. After a 10 min interval, we measured their hearing thresholds using the same interrupted tones conditions lasting 5 s. The average threshold gains (2 s thresholds minus 5 s thresholds) were compared to those of 15 control subjects with normal hearing (25 ears), 15 patients with cochlear hearing loss (23 ears), and 4 patients with retrocochlear lesions (4 ears). Patients with profound PHL (4 patients, 6 ears) were analyzed separately. Results: The average threshold gain of PHL patients (excluding profound PHL patients) at all frequencies was 18.3 dB, which was significantly larger than that of other groups: 0.3 dB (profound PHL patients), 3.8 dB (controls with normal hearing), 3.0 dB (patients with cochlear hearing loss), and 3.2 dB (patients with retrocochlear lesions).

Multi-mode hopping in Nd:YAG lasers with optical feedback

The multi-modes hopping in Nd:YAG lasers induced by optical feedback is presented. The threshold gains of different longitudinal modes are modulated by optical feedback. The mode which has the lowest threshold gain can oscillate. During the tuning of the external cavity length, when the threshold gain of the oscillating mode is higher than that of the adjacent mode, mode hopping occurs, and the intensity modulation curve fluctuates correspondingly. The different positions of hopping modes in the gain curve lead to different mode hopping points and intensity modulation curves. The closer to the center frequency, the larger intensity amplitude and the longer oscillating period the mode has. When the frequencies of two hopping modes are symmetric about the center frequency, the intensity amplitudes of them are equal, and their oscillating periods are equal too.

Application of algebraic invariants to full-wave simulators - rigorous analysis of the optical properties of nanowires

We show an application of four states algebraic invariants to the extraction of the main optical properties of anisotropic semiconductor thin nanowires such as the complex propagation constant of the guided modes and the reflectivity at the waveguide terminations. In order to provide a numerical example, we calculate the modal and threshold gains for the first few guided modes. This has been done in conjunction with the use of a full-wave finite-element method simulator without the isotropic approximation of passive GaN. Moreover, the device has been directly analyzed in his active state, i.e., with a complex dielectric tensor: in principle, a rigorous complex resonance for simultaneously calculating the lasing frequency and the threshold gain may be performed.

Antiphase complex-coupled surface-emitting distributed feedback diode lasers with absorptive gratings

Theoretical analysis of antiphase-type complex-coupled, surface-emitting distributed feedback (CC-SE-DFB) diode lasers with absorptive gratings is presented and discussed. Two different designs are studied: one utilizing a semiconductor-based second-order loss and index grating placed at the metal-semiconductor (p-side) interface, and the other employing a combination of metallic and semiconductor materials for the second-order loss and index grating. For certain design parameters, these two types of absorptive-grating structure are shown to select lasing in the symmetric mode (i.e., orthonormal emission in a single-lobe beam pattern). By comparison to metal-grating surface-emitting devices, the threshold gains for these structures are lower by factor of 3 to 4. For 500-μm-long gratings, the symmetric-mode is favored to lase with threshold gain values as low as 18 cm/sup -1/ and differential quantum efficiency as high as 34%.

Two-dimensional theory of distributed feedback semiconductor lasers

A theory which takes account of the two dimensional waveguide structure of the distributed-feedback (DFB) laser is presented. Laser threshold conditions in the case of no external reflections are calculated for a three-layer model in which one cladding layer has a periodically changing dielectric constant. In contrast with the coupled-wave theory the threshold conditions are found to be asymmetric with respect to the Bragg frequency. The longitudinal mode which lies below and nearest the Bragg frequency has the lowest threshold gain. The difference between the threshold gains of the two adjacent longitudinal modes straddling the Bragg frequency has a maximum as a function of the coupled strength of the grating.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of radiation loss on the performance of second-order DFB semiconductor lasers

The effects of radiation loss on the performance of second-order distributed-feedback (DFB) semiconductor lasers with a symmetrical grating are analyzed systematically. The threshold gains of the two lowest modes, the differential quantum efficiency, and the spectral linewidth are calculated as a function of the complex coupling coefficient. The comparison is also made between a conventional DFB and a quarter-wave phase-shifted DFB laser. It is shown that the effects of radiation loss are different in these two laser types. In addition, a simple approximate expression for threshold gain is derived. By way of example, a DFB laser with a rectangular grating is analyzed. It is shown that the performance of a second-order DFB laser is not sensitive to the grating pitch over a fairly wide range, provided that it is close to the pitch which gives the maximum coupling coefficient.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis and design of coupled-cavity lasers - Part I: Threshold gain analysis and design guidelines

The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts. In this first part, the focus is on two-section laser design and control using threshold gains. In the second part numerical analysis of the transient behavior is given. The present treatment begins by using the poles of a linear transfer function for the coupled-cavity laser to obtain mode wavelengths and wavelength dependent threshold gains. A general wave scattering matrix describes the intercavity coupling, so that simple waveguide discontinuities or complex arrays of discontinuities can be modeled. Numerical examples are given. Design relationships obtained primarily from the scattering analysis, together with numerical examples, show the dependence and interdependence of the laser mode wavelengths and threshold gain minima on various parameters such as cavity and coupler lengths, indexes of refraction, and gains. These relationships show optimum or preferred coupling gap lengths and cavity length ratios for various design criteria. For example, for optimum mode and spurious rejection stability a short ( \lsim40 \mu m) cavity can be coupled via a gap approximately an integer number of half wavelengths long to a second longer cavity ( \sim 100- 250 \mu m). Conversely, for maximum tunability (or sensitivity to input changes) together with good mode selectivity, two medium length cavities ( \sim 100-250 \mu m) differing in length by a small amount ( \lsim40 \mu m) should be chosen.

Threshold gain characteristics of the asymmetrical distributed bragg-reflector laser

The asymmetrically distributed Bragg-reflector (DBR) laser is analyzed by using the coupling-mode theory. Threshold solution for the required laser gains are presented. Numerical computations show that the asymmetrical geometry of the laser considered here allows for optimization of the laser characteristics such as lowering the threshold gains and providing the unidirectional operation.