Mode Fiber Coupling Research Articles

Integration of a superconducting nanowire detector into a confocal microscope for TRPL-mapping: Sensitivity and time resolution.

Superconducting Nanowire Single Photon Detectors (SNSPDs) were introduced to the market in 2003 and are used in multiple quantum optics applications, due to their outstanding properties such as fast instrument response and high quantum efficiency. The high quantum efficiency is important for materials science applications with emission in IR-range >1000 nm, where other available single photon detectors have a low sensitivity, high dark noise, and slow time response. We have integrated a SNSPD (Single Quantum) into a confocal photoluminescence lifetime microscope MicroTime 100 (PicoQuant) in order to compare the performance of different SNSPD designs with a standard IR-PMT for time-resolved photoluminescence (TRPL) measurements and imaging of IR sample e.g., CIGS device. One used SNSPD channel was a classical single mode fiber coupling to guide the light onto the sensor, the other channel used an internal multimode fiber instead. We achieved a significant increase in photoluminescence sensitivity in both designs compared to a standard IR-PMT, as well as a several times higher sensitivity of the multimode-fiber coupled nanowire compared to the singe-mode fiber one, despite comparable photon quantum efficiencies in this wavelength range. The increased sensitivity combined with the lower dark count rate resulted in an increase of signal-to-noise ratio by more than 2 orders of magnitude compared to the IR-PMT. The high sensitivity of SNSPDs combined with high temporal resolution (instrument response function of this setup was below 100 ps) allows to identify and investigate highly quenched signals of samples even at low illumination levels. The integration of the SNSPD detector to a scanning confocal microscope will also enhance the capabilities in field of single oxygen sensing and cell imaging in NIR by using NIR emitting dyes or nanoparticles without autofluorescence.

Demonstration of 100 Gbps coherent free-space optical communications at LEO tracking rates

Free-space optical communications are poised to alleviate the data-flow bottleneck experienced by spacecraft as traditional radio frequencies reach their practical limit. While enabling orders-of-magnitude gains in data rates, optical signals impose much stricter pointing requirements and are strongly affected by atmospheric turbulence. Coherent detection methods, which capitalize fully on the available degrees of freedom to maximize data capacity, have the added complication of needing to couple the received signal into single-mode fiber. In this paper we present results from a coherent 1550 nm link across turbulent atmosphere between a deployable optical terminal and a drone-mounted retroreflector. Through 10 Hz machine vision optical tracking with nested 200 Hz tip/tilt adaptive optics stabilisation, we corrected for pointing errors and atmospheric turbulence to maintain robust single mode fiber coupling, resulting in an uninterrupted 100 Gbps optical data link while tracking at angular rates of up to 1.5 deg/s, equivalent to that of spacecraft in low earth orbit. With the greater data capacity of coherent communications and compatibility with extant fiber-based technologies being demonstrated across static links, ground-to-low earth orbit links of Terabits per second can ultimately be achieved with capable ground stations.

Experimental optimization of the fiber coupling efficiency of GaAs quantum dot-based photon sources

We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.

Fusion and Elongation Method Integrated with Vacuum System to Fabricate Single-Mode Fiber Couplers

. The single mode fiber coupler has been successfully fabricated in various power ratio based on fusion and elongation method. Two single mode fibers were joined by heating them under the torch flame injected by H2 Gas with the temperature range of 1200°C - 1500°C. Coupling mode theory was determined to simulate the power characteristics of the fiber coupler which is then combined with the experimental results. The measurement of power output characteristics shows that good agreement between power ratio characteristic of experimental and theoretical simulation was obtained when the elongation process runs with a pulling speed of 180 μm/s. However, good performance of fiber couplers interm of coupling loss with typical of 0.05 dB has been obtained by integrating a vacuum system as the fiber holders inside the slot of pulling stages. Experimental results found that the suitable pressure of vacuum system operating during heating process is 120 mbar.

Development of a novel label-free all-fiber optofluidic biosensor based on Fresnel reflection and its applications

A novel, compact, cost-effective, and robust label-free all-fiber optofluidic biosensor (LF-AOB) based on Fresnel reflection mechanism was built through integrating single-multi mode fiber coupler and highly sensitive micro-photodetector. The Fresnel reflection light intensity detected by the LF-AOB greatly depended on the RI change on the end-surface of the fiber probe according to experimental and simulation results. The capability of the LF-AOB for real-time in situ detection in optofluidic system were verified by measuring salt and protein solution, and the lowest limit of detection was 1.0 × 10−6 RIU. Our proposed theory can effectively eliminate the influence of light intensity fluctuation, and one-point calibration method of sensor performance is conducive for rapid and convenient detection of targets. Label-free sensitive detection of SARS-Cov-2 Spike protein receptor-binding domain (S-RBD) and the binding kinetics assay between S-RBD and anti-S-RBD antibody were achieved using the LF-AOB. These contributed to the elegant design of all-fiber optical system with high efficiency, high resolution and sensitivity of micro-photodetector, and enhanced interaction between the light and the samples at the liquid-sensor interface because of the large surface area of the multi-mode fiber probe. The LF-AOB can be extended as a universal sensing platform to measure other factors associated with refractive index because its high sensitivity, low sample consumption (∼160 nL), and capability of real-time in situ detection.

Mode analysis of atmospheric turbulence mitigation with Multi-Plane Light Conversion

Developing standard and accessible atmospheric turbulence mitigation is of paramount importance to enable laser communication network deployment. Multi-Plane Light Conversion (MPLC) [1]–[3] is proving to be an effective technique to improve coupling efficiency of a perturbated wave-front profile towards a single-mode fiber [4]–[6]. However, to date, experimental studies are restricted to a fixed number of modes. In this work, we numerically and experimentally study performance improvement of the MPLC technique and mode collection compared to direct single mode fiber coupling for several D/r0 ratio, D being the receiver aperture diameter and r0 the Fried parameter. We calculate the coupling efficiency, represented in Fig 1. Collection for SMF only is shown in Fig 1-a while the summation of the 15 first Hermite-Gaussian modes is represented in Fig 1-b. Collection for each mode for D/r0=4 is displayed in Fig 1-c. We demonstrate that 15 modes MPLC appears to be a good compromise between number of modes and complexity of the device. This configuration typically improves the collection efficiency by >7 dB in the case of strong turbulence when D/r0 >4. More important, the minimum collection efficiency that would correspond to a link failure is dramatically increased compared to SMF fiber alone. MPLC thus shows high robustness to turbulence fluctuation.

Novel defocus hologram aberration compensation method in digital holography integrated pathology microscope for label free 3-D imaging

In this paper we report successful integration of digital holography in off- axis transmission configuration into a commercially available pathology microscope (PathologyDHM), where Mach-Zehnder interferometric configuration was achieved in pathology microscope using 1 × 2 single mode fiber coupler and custom built optical modules. A smartphone/webcam positioned at the image plane of the finite conjugate microscope captures the interference pattern generated by the system. Using PathologyDHM, we have demonstrated a novel and simple method for compensation of aberration due to microscope objective (MO). The proposed method is called defocus hologram method and it is particularly useful in cases, where samples do not have specimen free or background flat region. Proper validation of the method was done by imaging a transmissive grating (commercial compact disc, CD-R), which doesn't have a flat background region and comparing phase profile of CD-R computed by our proposed method with self-reference conjugated hologram method. For reiterating its biological applications, phase profile of a biological sample, red blood cell smear (RBC smear), was computed with our defocus hologram method and compared with already established reference conjugated hologram method. Phase profiles of CD-R and RBC smear computed with defocus hologram method agree with self-reference and reference conjugated hologram methods respectively, within the phase error limits of the system. Integration of digital holography in to commercial pathology microscope enables 3D imaging, which coupled with artificial intelligence may prove useful in disease diagnostics and monitoring response of cells to treatment.

Nanostructure Fusion Region of Single Mode Fiber Coupler

This paper reported the characteristic of the fusion region of single mode fiber coupler. Theoretical consideration of the coupling model such as the degree of fusion and the profile of the coupling region has been investigated. The degree of fusion was inversely proportional with the separation distance between the fiber axis and the diameter of the fusion region. Fusion and elongation process also has also changed the geometrical structure of the single mode fiber. FESEM image describes that the Silicon distribution was defined as a spherical crystalline which were well-dispersed. The surface of coupling region fiber coupler has the average size of the spherical-shape crystalline varies between 300 – 500 nm. However, by increasing the elongation speed and the temperature will reduce the numbers of Si per unit area, so that the density becomes fewer. It also causes the distance between the spherical shapes become longer.

Probability enhancement of fiber coupling efficiency under turbulence with adaptive optics compensation

Abstract Statistical distribution of single mode fiber (SMF) coupling efficiency η c with atmospheric compensation is important to evaluate the bit error rate performance of the satellite to ground laser communication (SGLC). The statistical distribution of SMF coupling efficiency with adaptive optics (AO) compensation under different turbulence strength is investigated. When the AO is turned on, the signal beam wavefront root mean square (RMS) value decreases from 0.42 λ to 0.04 λ under the turbulence strength D / r 0 = 7. Simultaneously, the average of coupling efficiency increases from 2.2% to 35.4%. The statistical distribution of SMF coupling efficiency with and without AO correction is consistent with the gamma distribution. In addition, the AO correction can help greatly improve SMF coupling efficiency with high probability. For example, the cumulative probabilities of coupling efficiency P ( η c > 15 % ) is 56.4%, 35.8%, 14.8%, 3.0% and 0.1% without AO correction when the D / r 0 is 3, 5, 7, 9 and 11, respectively. The value is improved to 100%, 100%, 99.9997%, 99.72% and 97.95% with AO correction in the corresponding D / r 0 . The results provide a quantifiable parameter to evaluate the SGLC link performance with AO correction.

Method to enhance the single mode fiber coupling efficiency for obscured receiver by beam shaping

Since the aperture obstruction contributes to an energy leak from the core to the sidelobe of the far field spot, the Gaussian mode of the single-mode fiber (SMF) will receive less energy leading to the coupling efficiency (CE) degradation. A method based on diffraction optical element (DOE) and wavefront corrector is introduced to reshape the plane wave with hollow uniform intensity into the plane wave with uniform intensity to improve the core energy of spot. The SMF CE with and without DOE for different central obscuration ratio is studied by numerical simulation. The DOE is effective to improve the SMF CE. Without the DOE, the CE decreases from 81% to 54% when the central obscuration ratio increases from 0 to 0.4. Compared with 54% CE at the of 0.4, the CE can increase to 72%, 78% and 80% with 4 level, 8 level and 16 level quantized DOE, respectively. It is not necessary to use the DOE beam shaper when the central obscuration ratio is small. The 4 level, 8 level and 16 level quantized DOE can obtain benefits of the CE when the central obscuration ratio is above 21.6%, 11.5% and 7%, respectively.

Review of the technology of a single mode fiber coupling to a laser diode

Abstract In this paper, the technology of a single mode fiber coupling to a semiconductor laser diode has been summarized and the latest developments in the bulk optics coupling scheme and the microlens fiber coupling scheme have been reviewed. The review has focused on optimizing the optical structure and the coupling parameters to improve the coupling efficiency and packaging performance. The advanced manufacturing technology as well as common modeling methods and applications of coupling systems have also been reviewed. Finally, the paper has summarized the key technologies of a single mode fiber coupling to a laser diode and its direction of development in the future.

Quantum Interpretation of Light Normalization at the Coupling Region Single Mode Fiber Coupling

This paper describes light normalization phenomenon as a quantum interpretation at the coupling region optical fiber coupler. Two single mode fibers (SMF) were succefully fabricated together based on fusion method at heating temperature of 1350°C and stopped automatically when the pre-set coupling ratio of 50% and 50% obtained. During fusion process, 1 mW laser diode was inserted to the one of the two input ports, and it was gradually transferred to the second fiber as function of the coupling coefficient. The properties of light intensity in term of electromagnetic wave also was determined based on quantum mechanics theory by utilizing Bassel and Hankel’s functions. The results show that the composite fiber has higher normalization propagation constant than other fiber for wavelength below 1320 nm. It found that to get lossless fiber coupler the wavelength operation should be below than 1320 nm so that few powers at cores transferred and propagated at the cladding.

Research of temperature on polarization properties of 2 × 2 single mode fiber coupler based on degree of polarization

from the point of degree of polarization (DOP), influence of environmental temperature on the polarization properties of 2 × 2 single mode fiber coupler is studied in detail. The mathematical expression of DOP for the throughout arm is given and temperature effect on DOP is analyzed; the numerical simulation shows the DOP reached 0 and 1 at −24 °C and 206 °C respectively, and between them the DOP change with temperature is nearly linear. Finally the temperature on DOP is experimentally investigated, and results are in accordant with numerical simulation. The above results also applies to the coupled arm. This research is helpful to further analyze the optical system incorporating 2 × 2 single mode fiber coupler.

Performance of mode diversity reception of a polarization-division-multiplexed signal for free-space optical communication under atmospheric turbulence.

We investigated the performance of mode diversity reception of a polarization-division-multiplexed (PDM) signal with few-mode-fiber (FMF) coupling for high-speed free-space optical communications under atmospheric turbulence. Optical propagation through eigenmodes of a FMF yields coupling between different linearly polarized (LP) modes in orthogonal polarizations, which causes power imbalance and loss of the orthogonality of multiplexed signals within each individual LP mode. Due to this phenomenon, the architecture of mode diversity combining affects the receiver performance. We numerically simulated the power fluctuation coupled to each LP mode after atmospheric propagation and FMF propagation in the condition of an optical downlink from a low-Earth-orbital satellite to the ground. We found that full receiver-side multiple-input multiple-output (Rx-MIMO) architecture in three-mode diversity reception improved the performance by 5 dB compared with selection combining (SC) of signals decoded individually in LP modes, and that it mitigated the required transmitted power by 6 dB compared with reception with single mode fiber (SMF) coupling. We also experimentally confirmed in three-mode diversity reception of a 128 Gb/s PDM-quadrature phase-shift keying with a diffuser plate as a turbulence emulator, that full Rx-MIMO with adaptive filters could work under severe fading and that it outperformed SC.

Robust, high brightness, degenerate entangled photon source at room temperature

We report on a compact, simple and robust high brightness entangled photon source at room temperature. Based on a 30-mm-long periodically-poled potassium titanyl phosphate crystal, the source produces non-collinear, type-0, phase-matched, degenerate photons at 810 nm with spectral brightness as high as ~0.41 ± 0.02 (~0.025 ± 0.02) MHz/mW/nm for multi (single) mode fiber coupling. So far, this is the highest number of degenerate photons generated using a continuous-wave laser pumped bulk crystal and detected using multimode fiber. We have studied the dependence of pump focusing on the brightness of the generated photons collected using both multimode, and single mode fibers. For a fixed pump power and crystal parameters, the SPDC source has an optimum pump waist radius producing maximum number of paired photons. Combining the crystal in a novel system architecture comprised with Sagnac interferometer and polarizing optical elements, the source produces polarization entangled photon states with high spectral brightness. Even in the absence of any phase compensation, the entangled photon states detected using single mode fiber have a Bell’s parameter, S = 2.63 ± 0.02, violating the Bell’s inequality by nearly 32 standard deviations and fidelity of 0.975. The compact footprint, robust design, and room temperature operation, make our source ideal for various quantum communication experiments.

Chalcogenide optical fiber couplers made by FBT method

We report on developing and characterization of chalcogenide FBT single mode fiber coupler for Mid-IR fiber optics applications. Single mode 6.3μm core/123μm cladding chalcogenide fibers of IHPS RAS with homemade polishing and standard PC/AFC connectors were utilized. Two fiber coupler geometries with angles α1=70° and α2=40° between the fibers were investigated and nature of excess loss has been studied. Excess loss and coupling ratios on time for both geometries were determined. Excess losses were measured to be 7dB and 5dB for α1 and α2 geometries respectively. For α1 geometry 70:30 and for α2 geometry 55:45 coupling ratios were achieved. For α2 geometry coupling ratio at wavelength 2.64μm was 63:37.

Profile of Single Mode Fiber Coupler Combining with Bragg Grating

This paper describes a numerical experiment of design and operation of a fiber coupler between single mode fiber and fiber Bragg grating (FBG). Both components are coupled depending with optical waveguide and source parameters. A characterization of fiber coupler is simulated by varying long grating of 10 mm to 60 mm using transfer matrix method based on coupled mode equation. The wave peak, transmission, and dispersion parameters are analyzed to determine the performance of the fiber coupler. The transmission spectrum showed the wave peaks rise to any increase in the grating length on channel 1 and channel 2. Transmission on channel 1 and channel 2 decreased from the wavelength range of 1.45μm–1.55μm and rised in the range of 1,55μm–1,65μm for each increment in length of grating. The dispersion showed the zero dispersion at specific wavelength for each increase in length of grating. This component can be applied for controlling information signal in wide range communication.

A Tuneable Multi-Core to Single Mode Fiber Coupler

We demonstrate a low loss, wideband multi-core to single-mode fiber coupler using a modified side-polishing technique. The coupler was designed to access light from a single core of the multi-core fiber without disrupting the signal propagation in the remaining cores. The coupling ratio between the two fibers can be continuously tuned over the entire spectral band via a simple mechanical displacement method. We expect that such couplers will be of use for monitoring, splitting, and multiplexing applications in future multi-core communication and sensing systems.

Mode coupling in hybrid square-rectangular lasers for single mode operation

Mode coupling between a square microcavity and a Fabry-Pérot (FP) cavity is proposed and demonstrated for realizing single mode lasers. The modulations of the mode Q factor as simulation results are observed and single mode operation is obtained with a side mode suppression ratio of 46 dB and a single mode fiber coupling loss of 3.2 dB for an AlGaInAs/InP hybrid laser as a 300-μm-length and 1.5-μm-wide FP cavity connected to a vertex of a 10-μm-side square microcavity. Furthermore, tunable single mode operation is demonstrated with a continuous wavelength tuning range over 10 nm. The simple hybrid structure may shed light on practical applications of whispering-gallery mode microcavities in large-scale photonic integrated circuits and optical communication and interconnection.

50 W low noise dual-frequency laser fiber power amplifier.

A three-stage dual-frequency laser signal amplification system is presented. An output from a narrow-linewidth Nd:YAG nonplanar ring-oscillator (NPRO) is split into two parts, one of them is frequency shifted by an acoustooptic modulator (AOM) then coupled into a single mode optical fiber. The other part is coupled into another single mode fiber then combined with the frequency-shifted beam with a 2 to 1 single mode fiber coupler. The combined beam has a power of 20 mW containing two frequency components with frequency separation of 150 ± 25 MHz. The dual-frequency signal is amplified via a three-stage Yb3+-doped diode pumped fiber power amplifier. The maximum amplified power is 50.3 W corresponding to a slope efficiency of 73.72% of the last stage. The modulation depth and signal to noise ratio (SNR) of the beat signal are well maintained in the amplifying process. The dual-frequency laser fiber power amplifier provides robust optical carried RF signal with high power and low noise.