End-face Reflection Research Articles

Trapezoidal Angled Fiber Coupler for Microsphere Resonator With Enhancement of Fano Resonance

We propose and demonstrate a trapezoidal angled fiber coupler for excitation of whispering-gallery mode microsphere resonator, with enhanced Fano resonance. The reflection from the angled fiber is enhanced when the oblique angle is greater than critical angle, which is obtained as 30° theoretically for angled fiber. To enhance the endface reflection, the front tip of the angled fiber is polished flat to achieve the trapezoidal angled fiber. Compared with angled fiber, the proportion of Fano lineshape in spectra for trapezoidal angled fiber is nearly doubled and the slope is greatly improved, which reaches 304.55 dB/nm. Moreover, through the coupling of dual microspheres, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor is improved severalfold and the thermal tuning efficiency is increased by 50%. With the advantage of low cost, easy fabrication procedure and reflection mode operation, the proposed device shows a great potential for sensing, optical tuning and optical switching applications.

Reducing the influence of direct reflection on return signal detection in a 3D imaging lidar system by rotating the polarizing beam splitter

The direction rule of the laser beam traveling through a deflected polarizing beam splitter (PBS) cube is derived. It reveals that, due to the influence of end-face reflection of the PBS at the detector side, the emergent beam coming from the incident beam parallels the direction of the original case without rotation, with only a very small translation interval between them. The formula of the translation interval is also given. Meanwhile, the emergent beam from the return signal at the detector side deflects at an angle twice that of the PBS rotation angle. The correctness has been verified by an experiment. The intensity transmittance of the emergent beam when propagating in the PBS is changes very little if the rotation angle is less than 35 deg. In a 3D imaging lidar system, by rotating the PBS cube by an angle, the direction of the return signal optical axis is separated from that of the origin, which can decrease or eliminate the influence of direct reflection caused by the prism end face on target return signal detection. This has been checked by experiment.

End-face reflected LiNbO3 waveguide based stationary miniature Fourier transform spectrometer with two-fold enhanced spectral resolution

Miniature Fourier transform spectrometer (FTS) has attracted considerable interest because of its important application in spaceborne spectroscopy and as a portable analytical tool for rapid on-site chemical/biochemical detection. In a previous paper, a stationary miniature FTS constructed with an electro-optic (EO) modulator of a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) containing push-pull electrodes was demonstrated. This stationary miniature FTS is operated in the near-infrared region with either nonlinear or linear scanning of the modulating voltage. The simple and mirrorless structure renders the device compact, vibration resistant, and cost-effective. However, the spectral resolution of the proposed prototype FTS was not satisfactory due to the limited optical pathlength difference (OPD), thereby limiting the device application. To improve its spectral resolution, the factors affecting the spectral resolution of the LN waveguide-based FTS is investigated in this paper. Findings show that the spectral resolution is inversely proportional to the maximum OPD, which is proportional to the length of the EO modulating region. A simple method for two-fold enhancement of the spectral resolution of the FTS is proposed based on the end-face reflection in LN waveguide interferometer. With the end-face reflection geometry the guided mode can propagate back and forth in the LN waveguide, making the mode passing through the EO modulating region twice and consequently leading to two times enhancement of the OPD. Therefore, the end-face reflection geometry enables to double the maximum OPD of the interferometer without increasing the device size and thus to offer the device a two-fold enhanced spectral resolution according to the equation for FTS resolution. Two prototypes of FTS with and without the end-face reflection structure are prepared using the same commercial LN waveguide EO modulator. The spectral resolutions in terms of the full-width at half maximum (FWHM) at different wavelengths for the two prototypes of FTS are measured using a series of distributed feedback lasers. The FWHM measured at a specific wavelength with the end-face reflection structure is half as large as that obtained without the end-face reflection structure. Experimental results are in excellent agreement with the theoretical data, demonstrating the applicability of the end-face reflection method to the spectral resolution enhancement.

Online reflectivity measurement of an ultra-weak fiber Bragg grating array

An online measurement method is introduced to ensure the reflectivity of an arbitrary grating in a large-scale ultra-weak fiber Bragg grating (FBG) array. The measurement errors were investigated by theoretical analysis and experiment. In the proposed method, the reflected spectrum of one single FBG was addressed and obtained from the grating array by time domain interrogation, and the FBG reflectivity was calculated by the reference of the end-face reflection in anhydrous ethanol. A measurement error of less than 0.5 dB and a 20 dB dynamic range were obtained in a larger-scale array of 1000 FBGs with an average peak reflectivity of −43 dB, which demonstrated the feasibility of this online measurement method.

Semiconductor nanowire lasers

Semiconductor nanowires (or other wire-like nanostructures, including nanoribbons and nanobelts) synthesized by bottom-up chemical growth show single-crystalline structures, excellent geometric uniformities, subwavelength transverse dimensions, and relatively high refractive indices, making these one-dimensional structures ideal optical nanowaveguides with tight optical confinement and low scattering loss. When properly pumped by optical or electrical means, lasing oscillation can be readily established inside these high-gain active nanowires with feedback from endface reflection or near-field coupling effects, making it possible to realize nanowire lasers with miniature sizes and high flexibilities. Also, the wide-range material availability bestows the semiconductor nanowire with lasing wavelength selectable within a wide spectral range from ultraviolet (UV) to near infrared (IR). As nanoscale coherent light sources, in recent years, nanowire lasers have been attracting intensive attention for both fundamental research and technological applications ranging from optical sensing, signal processing, and on-chip communications to quantum optics. Here, we present a review of the status and perspectives of semiconductor nanowire lasers, with a particular emphasis on their optical characteristics categorized in two groups: (1)&#xA0;waveguiding related properties in Section&#xA0;3, which includes waveguide modes, near-field coupling, endface reflection, substrate-induced effects, and nanowire microcavities, and (2)&#xA0;optically pumped semiconductor nanowire lasers in Section&#xA0;4, starting from principles and basic types of UV, visible, and near-IR nanowire lasers relying on Fabry&#x2013;Perot cavities, to advanced configurations including wavelength-tunable, single-mode operated, fiber-coupled, and metal-incorporated nanowire lasing structures for more possibilities. In addition, the material aspects of semiconductor nanowires, including nanowire synthesis and electrically driven nanowire lasers, are briefly reviewed in Sections&#xA0;2 and 5, respectively. Finally, in Section&#xA0;6 we present a brief summary of semiconductor nanowire lasers regarding their current challenges and future opportunities.

Experimental measurement of the temperature-birefringence characteristics of birefringent photonic crystal fiber filled with ethanol

The birefringence value of the birefringent photonic crystal fiber (PCF) is not sensitive to the change of temperature, while the refractive index of ethanol will be changed with the varied temperature. So if the air-holes of PCF are filled with ethanol, the birefringence value will surely change along with the varied temperature, which can be used as a sensing principle to measure temperature. In this paper, a method called End Face Reflection of the optical fiber was used to measure the refractive index against the change of temperature. The experimental results indicated that, on the condition of the incident wavelength of 1550nm, the refractive index of ethanol changed from 1.3691 to 1.3602 within the temperature ranging from 24.5°C to 44.5°C. The birefringence value of the filled PCF changed from 1.8900e-04 to 2.1500e-04. That is to say, the temperature sensitivity coefficient could reach 10e-6 order of magnitude.

Measurement of end-face reflection property of micro-nano fibers

The end-face reflection property of micro-nano fiber (MNF) is one of its important transmission features, which plays remarkable roles in a variety of practical applications of MNF. In this paper, we propose a novel method to study the MNF end-face reflection property using an optical circulator, which overcomes the deficiencies of the direct measurement relying on a 3 dB coupler. On the basis of this method, we further introduce the aerogel fixing and power compensation techniques to effectively eliminate the errors caused by MNF pigtail floating, the light source output instability and internal loss. Therefore, the whole experimental design improves the measurement accuracy. The MNF end-face reflections with respect to the diameter of MNF and the wavelength of the guided light are investigated. The experimental results are in good agreement with simulation results, showing that this new approach is reliable for measuring the reflectivity of specific MNF. This detailed investigation on the end-face reflection versus various factors will be conducive to the constructions of versatile micro-nano scale devices based on the MNF, such as the MNF lasers, MNF amplifiers, MNF couplers and MNF filters.

Reflectivity measurement of weak fiber Bragg grating (FBG)

One method based on the end-face reflection as a reference was used to measure weak grating reflectivity of 0.01%–1%. For measuring ultra-weak grating reflectivity, the grating group with an identical wavelength was used. By using anhydrous alcohol and deionized water as the reflecting medium, the singlepulse FBG reflectivity of a highly doped-Ge photosensitive fiber was calculated to be about 0.747% and 0.739%, respectively; and the single-pulse FBG reflectivity of ordinary SMF fiber was about 0.016% and 0.015%, respectively. The consistent results by the two different reflecting media showed that the end-face reflection method on the grating group was feasible.

Preparation of a planar multimode interference coupler with a graded-index profile using polysilane with photobleaching properties

To investigate the viability of a planar graded-index multimode waveguide fabricated using a new method (to our knowledge) employing polysilane with photobleaching properties, we designed a planar two-way multimode power splitter based on multimode interference, with a core possessing a vertically parabolic and horizontally uniform index profile. We carried out mode propagation analysis using the Marcatili model and optimization using a three-dimensional beam-propagation method. The prototype was fabricated by means of multiple dicing after UV exposure and heat treatment in air. Index profile measurements using a special end-face reflection method showed it to have a vertically graded-index profile with relative index difference of 0.0456. Output beam profile observation using a CCD camera showed that it successfully divided the beam planewise.

Epitaxial Nd:YLF linear waveguide laser

We report laser operation in planar and linear Nd:YLF waveguide structures grown by liquid-phase epitaxy (LPE). We could reduce the relatively high threshold of 115-mW incident pump power observed for the planar waveguide to 8 mW in a ridge-type structure that we obtained by mechanical polishing and subsequent growth of a cladding layer. Substantial improvements seem possible with reduction of the losses caused by imperfect end-face reflection and scattering, as well as by use of techniques capable of producing smaller structures than the 40-microm-wide triangular ridge cross section that was obtained with polishing. Since the LPE technique is applicable to other activator ions as well, it might offer a route to development of low-threshold upconversion lasers.