Increasing Cavity Length Research Articles

Influence of intracavity-air loss on 946-nm Nd:YAG laser performance

We report on the impact of humidity in a cryogenically cooled Nd:YAG laser operating at 946-nm. Performance degrades with increasing cavity length, which is attributed to absorption by water vapour, an additional intracavity loss.

The study of N-polar GaN/InAlN MOS-HEMT and T-gate HEMT biosensors

The sensing performance of N-polar GaN/InAlN MOS-HEMT biosensors for neutral biomolecules was investigated and compared with the Ga-polar MOS-HEMT and N-polar T-gate HEMT by numerical simulation. The results indicate that the N-polar GaN/InAlN MOS-HEMT biosensor has higher sensing sensitivity than the Ga-polar MOS-HEMT and N-polar T-gate HEMT biosensors. Furtherly, to improve the sensing performance of N-polar MOS-HEMT, the influence of cavity dimensions, GaN channel layer thickness, and InAlN back barrier layer thickness on device performance was investigated. It is demonstrated that the sensitivity of the biosensor increases as the cavity height decreases and the cavity length increases. Therefore, the sensing performance of the N-polar MOS-HEMT device will be enhanced by thinning the GaN channel layer thickness or increasing the InAlN back barrier thickness, which can be mainly attributed to the variation of the energy band structure and two-dimensional electron gas concentration in the HEMT heterostructure. Finally, the highest sensitivity can be obtained for the N-polar MOS-HEMT with 6 nm-thick GaN channel layer, 30 nm-thick InAlN back barrier layer, and two 0.9 μm-long and 5 nm-high cavities. This work provides structural optimal design guidance for the N-polar HEMT biosensor.

Topological rainbow based on coupling of topological waveguide and cavity.

Topological photonics and topological photonic states have opened up a new frontier for optical manipulation and robust light trapping. The topological rainbow can separate different frequencies of topological states into different positions. This work combines a topological photonic crystal waveguide (topological PCW) with the optical cavity. The dipole and quadrupole topological rainbows are realized through increasing cavity size along the coupling interface. The flatted band can be obtained by increasing cavity length due to interaction strength between the optical field and defected region material which is extensively promoted. The light propagation through the coupling interface is built on the evanescent overlapping mode tails of the localized fields between bordering cavities. Thus, the ultra-low group velocity is realized at a cavity length more than the lattice constant, which is appropriate for realizing an accurate and precise topological rainbow. Hence, this is a novel release for strong localization with robust transmission and owns the possibility to realize high-performance optical storage devices.

Ultrashort cavity length effects on the performance of GaInP multiple-quantum-well laser diode

The cavity length of a semiconductor laser plays a key role in its operation and dynamic behaviour. Moreover, downsizing the quantum well (QW) laser chip is required for specific applications, such as hard disk drives (HDDs). Therefore, it is vital to study laser operation at various cavity lengths in micro-scale dimensions. Here, the operation of a GaInP/GaAs multiple-quantum-well laser with different short cavity lengths (i.e., 200, 300, 400, and 500 µm) at room temperature is demonstrated. The threshold current (Ith), differential efficiency, emission spectra, and near-field profile images are all experimentally reported in this study. Likewise, the modal gain threshold and mirror facet coating effects on the operation of the samples are also discussed. The Ith decreases with the increase of cavity length, whereas emission spectra exhibited a redshift by about 3 nm when the laser length rises from 200 to 500 µm. The near-field images showed a reduction in profile width when the laser length is shorter, which is attributed to a reduction of the spreading current along the laser stripe. The short cavity of the 200 µm sample showed high efficiency, low spreading current, and fast carrier dynamic but a slightly high Ith which can be reduced by coating its facets. This could be a promising small-footprint candidate laser chip for next-generation HDDs and for downsizing photonic applications.

Numerical investigation on the characteristics of cavity-transpiration combined cooling for hypersonic vehicle leading edge

During the flight of hypersonic vehicles, the wedge-shaped leading edge is exposed to severe thermal loads, so a reliable cooling system is required to provide protection. Transpiration cooling has been shown to be an efficient thermal protection technique for hypersonic vehicles. In this paper, the cavity-transpiration combined cooling is applied to the wedge-shaped leading edge of vehicles and an optimization scheme of lip blunting for the combined cooling is proposed. The effects of flight angle of attack, aspect ratio of the cavity and coolant injection mode are studied using the full field coupled numerical method. And the cooling characteristics of cavity-transpiration combined cooling with lip blunting are analyzed. The results reveal the following novel and valuable phenomena: (1) The existence of the cavity reduces the lift-drag ratio of the leading edge, and the lift drag ratio increases with the increase of the angle of attack. (2) When no coolant is injected, the temperature and pressure of the stagnation point decrease with the increase of the length and diameter of the cavity. When the coolant is injected at a constant mass flow rate, the cooling efficiency at stagnation point decreases with the increase of cavity length and diameter, but the driving force decreases. When the coolant is injected at a constant pressure, the mass flow of the coolant increase, the cooling efficiency at the stagnation point is increased by 9%. (3) The temperature at the stagnation point of lip blunting structure is decreased by 94.78 K compared with no blunting, and the cooling efficiency is increased by 9.8%.

Distributed feedback lasers up to the 400th Bragg order with an organic active layer

The output characteristics and lasing threshold behavior of higher order Bragg lasers are explored using an organic active layer spin-cast over substrate-defined fused-silica gratings. Gratings ranging from 1st to the 400th Bragg order of varying duty cycle are fabricated with standard e-beam lithography. Distinct diffraction orders are observed at lower Bragg orders but smear out toward higher orders due to overlapping diffracted orders. Significant variation in thresholds is observed with duty cycle for most Bragg orders. A dramatic reduction in threshold is observed with increasing cavity length. The lowest lasing thresholds obtained for 4th and 400th order distributed feedback lasers are ∼1.4 and 4 μJ cm−2, respectively, using F80.9BT0.1 as an active layer. 400th order Bragg lasers are fabricated with direct-write photolithography using a UV laser diode, with comparable thresholds to e-beam lithography fabricated devices.

Study on the spectral characteristics of noise-like pulses in Erbium-doped fiber lasers

We demonstrate an Erbium-doped fiber laser (EDFL) based on nonlinear polarization rotation (NPR) mode-locking. The operating characteristics of noise-like pulse (NLP) with different cavity lengths and pump powers has been studied. For the first time, we experimentally demonstrate that the maximum spectral width of NLP narrows with the increase of cavity length under the same pump power, the maximum spectral width of NLP increases with the increase of pump power with the same cavity length.

Experimental investigation of stripe cavity length effect on threshold current density for InP/AlGaInP QD laser diode

InP/AlGaInP QD laser diodes with various cavity lengths grown on GaAs substrate were synthesized by (MOVPE) technique. L-I characteristics at various temperatures (190–390K) were measured for InP/AlGaInP QD laser diodes to investigate the effect of the various stripe cavity length on threshold current density and characteristic temperature. It is found that the threshold current increases with temperature, reaches a peak, then drops as temperature increases, before it increases again at temperatures over 300K due to carrier redistribution in the quantum dot states with temperature. It is found also that the threshold current density decreases with increasing cavity length due to the increasing threshold gain with the cavity length. However, as the cavity length increases beyond certain length, the rate of the decrease of the threshold current with cavity length decreases. One important finding is that the characteristic temperature of threshold current decreases significantly with the cavity length at the operating temperature range from 190K to 360K.

Experimental and numerical evaluation of thermodynamic effect on NACA0015 hydrofoil cavitation in hot water

In this study, the cavitation in hot water, which implies tight interaction of thermodynamic effect, phase change phenomena, and flow behavior, was studied by a combination of experiment and numerical simulation. The experiment in water up to 90°C was performed in the high temperature and high-pressure water tunnel with NACA0015 as a cavitator. The temperature inside the cavity was measured using the high-accuracy thermistor probe. According to the result, the temperature depression in the cavity was increased proportionally with the increase of freestream temperature. The inverse thermodynamic effect was observed with the increase of cavity length when temperature increased. The maximum temperature depression of about 0.41°C was measured in the water at around 90°C. The temperature drop was reasonably captured in simulation by coupling our simplified thermodynamic model with our cavitation model and governing equations. The tendency of temperature depression in the cavity agreed well with experimental data under different flow conditions.

On the voltage–current optimization in high-harmonic gyrotrons

In low-power short-wavelength gyrotrons with weak intensity of the electron–wave interaction, determining of the optimal parameters of the system is a result of a trade-off between the enhancement of the electron efficiency and the increase in the Ohmic loss share with an increasing cavity length. In this paper, we investigate the question of whether it is more advantageous to use operating regimes with a higher current at a fixed electron beam power, or whether it is better to increase the accelerating voltage. The answer to the question is determined strictly by the number of the operating cyclotron harmonic of the gyrotron.

Investigation of influence of compact actuators on air cavity in water flow under recessed hull

Air cavity drag reduction is one promising method for reducing power consumption of ships. Its current practical applications are rather limited, owing largely to the fact that air cavity size and shape change drastically in response to variations in ship attitude, motions and speed, as well as sea conditions. This study explores how deployment of moveable hydrodynamic actuators near the air cavity on a small-scale simplified hull form can effectively increase the air cavity size in adverse hull positions. Experimentally investigated actuators included an adjustable plate in the front part of an air cavity, a stern spoiler, and a hydrofoil with regulated attack angle and streamwise position beneath the hull. In the cases of significant hull trims that are challenging for maintenance of long air cavities, optimal actuator placement increased cavity length by nearly 110% from its degraded state at negative trim and by 24% at positive trim. Actuator effects were more pronounced at higher water speeds.

Fabry-Perot vector curvature sensor based on cavity length demodulation

Abstract We have demonstrated a high-sensitivity Fabry-Perot curvature sensor. The main components of its sensing fiber are hollow silica tube and single-mode fiber (SMF). When the sensor is bent, the length of the FPI changes with curvature. Hereby, we can achieve vector curvature measurement through cavity length demodulation. The convex and concave bending sensitivities are 151.30 nm/m−1 and −333.89 nm/m−1, respectively. Besides, the curvature sensitivity is optimized with the increase of air cavity length. The packaging of the sensing fiber increases its robustness while increasing its sensitivity from 151.30 nm/m−1 to 251.54 nm/m−1. Meanwhile, the structure is insensitive to axial stress and refractive index (RI). This sensor has potential applications in engineering health detection and small-scale structural deformation detection.

High resolution, fast response fiber-optic temperature sensor with reduced end conduction effect

We report a fiber-optic silicon Fabry-Perot temperature sensor with high speed by considering the end conduction effect, which refers to the unwanted heat transfer between the sensing element and the fiber stub delaying the sensor from reaching thermal equilibrium with the ambient environment. The sensor is constructed by connecting the narrow edge surface of a thin silicon plate to the edge of the microtube attached to the fiber tip. Compared to the traditional design where the silicon plate is attached to the fiber end face on its large plate surface, the new sensor design minimizes the heat transfer path to the fiber stub for improved sensor speed. It has the additional benefit of increased cavity length for improved resolution. We show that, compared with the sensor of traditional design, the sensor of the new design shortened the characteristic response time in still air from 83 ms to 13 ms and improved the sensor resolution by a factor of 12, from 0.15 K to 0.012 K.

Effect of recovery time of nonlinear absorber saturated losses on the soliton pulse structure in a fibre laser with different cavity lengths

Using numerical simulation methods, we have calculated the dependences of the pulse profile and spectrum of a ring fibre laser in the region of anomalous group velocity dispersion on the length of the laser cavity and the recovery time of the nonlinear absorber saturated losses. It is found that with increasing cavity length the laser pulse acquires a pedestal, the positions of its edges corresponding to the frequencies of the Kelly sidebands. It is shown that the finiteness of the recovery time of the saturable absorber leads to an asymmetry in the profile and spectrum of the laser pulse.

Electrical and Thermal Design of a $W$ -Band Gyrotron Interaction Cavity

In this paper, the design of an interaction cavity for a $W$ -band gyrotron with a 55-kV, 5-A beam has been presented. For the above purpose, a parametric design code has been developed. The mode of interaction has been selected as TE6,2. The large-signal analysis was carried out using the code Gyro-K for four differed lengths of the cavity, such as 24, 25, 26, and 27 mm to estimate the output power. The diffractive quality factor and ohmic loss have also been computed for all these four lengths of cavity. For the sake of cooling the cavity, 16 numbers of cooling channels are proposed surrounding the cavity with deionized water flowing through the channels at an ambient temperature of 20 °C with the flow-rate of 8 L/min. As the cavity length increases, the diffractive quality factor and the output power increase. However, with the increase of cavity length, the ohmic loss and thermal load also increase. And this eventually leads to higher cooling requirements. The steady-state temperature distribution for the cavity of four different lengths is computed using FloEFD software. Also, the transient analysis was carried out for a pulselength of 100 ms and 10% duty with and without cooling. In view of these results, the cavity length of 26 mm is proposed as the optimum length of the cavity.

Graphene filled optical fiber Fabry‐Pérot cavity tunable filter

AbstractBefore saturated absorption, graphene possesses optical nonlinear refractive index, which is a photoinduced refractive index effect. Taking advantage of this effect, a wavelength tunable filter is realized based on a graphene filled all optical fiber Fabry‐Pérot microcavity. Experimental results show that the interference wavelength of the filter exhibits linear red‐shift with the increase of the tunning optical power and tunning efficiency increases with the increase of cavity length and filled graphene thickness. In this study, the maximum wavelength vs power tunning efficiency can get 66 pm/mW. This filter has the advantages of high tunning efficiency, simple structure, antienvironmental interference good mechanical performance, and repeatability.

Reducing the pulse repetition rate of picosecond dissipative soliton passively mode-locked fiber laser.

This paper proposes and demonstrates a method to reduce the repetition rate of all- polarization-maintaining (PM) linear-cavity picosecond dissipative soliton passively mode-locked fiber lasers. An optical coupler (OC) is inserted into the cavity to extract pulse energy, and the cavity length is increased using a low-nonlinear coefficient large-mode field fiber at the rear end of the OC, where the propagated pulse has lower energy. This enables the nonlinear phase shift to be within the tolerated value of the single pulse mode-locking even with a considerably increased cavity length; this allows reducing the laser repetition rate considerably without substantially changing the pulse characteristics. Using the proposed method, for a 0.3-nm filter bandwidth, the laser repetition rate is successfully reduced to 1.77 MHz with a nearly Fourier-transform limited pulse duration of 10 ps; it can be further reduced by optimizing the OC split ratio. The proposed method can be applied to reduce the repetition rate for a picosecond dissipative soliton passively mode-locked fiber laser with an arbitrary bandwidth filter.

Wavelength-adjustable mode-locked Tm-Ho co-doped fiber laser from 1839 nm to 1876 nm

Abstract We demonstrate a passively mode-locked Tm-Ho co-doped fiber laser using GaAs saturable absorber (SA). The laser has a very broad output spectrum covering from 1700 nm to 2100 nm with output wavelengths adjustable from 1839 nm to 1876 nm. The significant broadening produced by mode-locking is due to the laser-induced self-phase modulation (SPM). Results show that the peak wavelength is red-shifted with the increase of pump power. On the other hand, with the increase of the cavity length, the peak wavelength is blue-shifted. The red-shift generated by the increasing pump power may be due to the reabsorption of the short wavelength part of the laser by Ho3+ ions, while the increase in the blue shift due to the increasing cavity length is due to the absorption loss in the long wavelength direction inside the optical fiber at the broad-band laser since the long wave part of the output has already entered the absorption band of the fiber material. As the length of the fiber increases, the longer wavelength side of the laser output spectrum is absorbed more, causing a blue-shift of the peak wavelength. Therefore, the laser peak wavelength can be adjusted by changing the pump power or cavity length.

Enhanced Amplitude Noise Tolerance of a Self-Seeded RSOA Laser Using Balanced Detection

Self-seeded cavity lasers appear as attractive colorless sources for dense wavelength division multiplexed passive optical networks. Although this technology has already demonstrated high performance in a number of experiments, the strong multimode behavior of these kilometer-long cavities results in large relative intensity noise (RIN) that can become the main limiting factor for transmission. In this letter, we correlate the transmission performance degradation to the RIN increase when the cavity length increases. We evidence a better tolerance to amplitude noise when a combination of Manchester encoding and balanced detection is used in a proof-of-concept experiment. Using this combination, the cavity length can be four times longer than when using standard non-return-to-zero encoding for the same bit-error-rate performance.

Exploiting concave-convex linear resonators to design end-pumped solid-state lasers with flexible cavity lengths: Application for exploring the self-mode-locked operation.

The characteristics of a convex-concave linear resonator under the thermal lensing effect are theoretically analyzed to find an analytical model for designing end-pumped solid-state lasers with flexible cavity lengths. By exploiting the design model, the power scaling for continuous-wave operation under strong thermal lensing can be easily achieved in the proposed resonator with different cavity lengths. Furthermore, the proposed resonator is applied to explore the exclusive influence of cavity length on the self-mode-locked (SML) operation. It is discovered that the lasing longitudinal modes will split into multiple groups in optical spectrum to lead to a multi-pulse mode-locked temporal state when the cavity length increases. Finally, a theoretical model is derived to reconstruct the experimental results of SML operation to deduce a simple relationship between the group number of lasing modes and the cavity length.