Optical Coupling Technique Research Articles

Optical Coupling Technique Based on Fiber Side-Polishing Without Service Interruption

We propose an optical coupler insertion technique based on fiber side-polishing that does not interrupt service. We clarify that fiber side-polishing with an insertion loss of 0.64 dB or less is possible by polishing while monitoring the loss and halting the polishing when the loss reaches the target value. Experiments on fabricated couplers confirm coupling efficiencies of up to 95% when another side-polished fiber is set close to the polished in-service fiber.

Cryogenic packaging of an optomechanical crystal.

There is an increasing need to simplify optical coupling techniques for low-temperature integrated photonics experiments. Various promising and scalable photonic packaging techniques have been under development, but few methods compatible with low-temperature operation have been reported. Here, we demonstrate 25% coupling efficiency from an optical fiber to a silicon optomechanical crystal at 7 mK in a dilution refrigerator without in-situ optical alignment at cryogenic temperatures. Our coupling scheme uses angle-polished fibers glued to the surface of the chip. The technique paves the way for scalable integration of optical technologies at low temperatures, circumventing the need for optical alignment in a highly constrained cryogenic environment. The technique is broadly applicable to studies of low-temperature optical physics and to emerging quantum photonic technologies.

Demonstration of the optical AC coupling technique at the advanced LIGO gravitational wave detector

Lasers for gravitational wave detectors need to fulfill tight requirements in amplitude stability, which can only be met by means of feedback control loops. Ultimately, power stabilization control loops are limited by the shot noise of their sensor. The power noise increases linearly with the amount of detected power, while the shot noise grows with the square root. Increasing the detected power is therefore a suitable means to reach a lower sensing noise but it is limited by the power handling capabilities of the photodiodes.An alternative way of improving the sensitivity is the optical AC coupling technique, which exploits the high pass behavior of an optical resonator to reduce the optical power on the detector without compromising its sensitivity above the corner frequency.In this paper we investigate the optical AC coupling technique at the aLIGO Livingston gravitational wave detector. We measured an optical AC coupling gain of 10 dB in the gravitational wave detection band, which offers the potential to improve the laser power stability by the same factor.

Mid-Infrared Waveguide Array Inter-Chip Coupling Using Optical Quilt Packaging

A MEMS-based mid-infrared (MIR) chip-to-chip optical coupling technique, optical quilt packaging (OQP), is described. Numerical simulations are performed to predict performance and establish fabrication tolerances. The OQP fabrication process is described in detail and MIR inter-chip optical coupling between two waveguide arrays joined by OQP is characterized. The coupling loss between Ge-on-Si passive MIR waveguides is found to be ~ 4.1 dB, which is the lowest butt-coupling loss reported between two chips.

A method to achieve spatial linearity and uniform resolution at the edges of monolithic scintillation crystal detectors

We have performed Monte Carlo simulations of the scintillation light transport between adjacent monolithic LYSO crystals that are optically coupled together using coupling media of varying refractive index. The scintillation light from the crystals was read out by SiPM arrays from the large crystal face. Scintillation event positioning results show that this optical coupling technique preserves the shape of the light spread function near and across the interface between the two crystals in order to substantially reduce the edge-artifacts observed in monolithic scintillation crystals, while not degrading the timing performance.

Development of silicon photonic devices for optical interconnects

Silicon photonic devices based on complementary-metal-oxide-semiconductor (CMOS) compatible technologies have shown attractive performances on very-large-scale monolithic optoelectronic integration, high speed modulation and switching, and efficient off-chip optical coupling. This paper presents the recent progress on fast silicon optical modulation, wavelength-insensitive optical switching and efficient optical coupling techniques in our group. Several CMOS-compatible silicon optical couplers with different structures have been developed, showing the highest coupling efficiency of 65%. Broadband silicon-based optical switches with sub-nanosecond switching on-off time are experimentally realized. Silicon modulators with novel PN junctions are demonstrated with the speed up to 50 Gb s−1.

Evanescent field coupling between two parallel close contact SMS fiber structures

We proposed a novel optical coupling technique based on two parallel singlemode-multimode-singlemode (SMS) fiber structures. This technique utilizes one SMS structure to excite multiple cladding modes within an output singlemode fiber. The excited multiple cladding modes will be coupled to the input SMF in the second SMS structure by placing the two SMS fiber structures in parallel and in close contact each other. The coupled cladding modes will be re-coupled to a guided core mode by the second SMS fiber structure. Theoretical analysis for such technique was provided and experimentally we have achieved a pass band spectral response with an extinction ratio higher than 20 dB and a maximum coupling efficiency of 5.9%.

Light Coupling Between a Singlemode- Multimode-Singlemode (SMS) Fiber Structure and a Long Period Fiber Grating

We propose a novel optical coupling technique based on evanescent field coupling between a singlemode- multimode-singlemode (SMS) fiber structure and a long period fiber grating (LPFG). By parallel placement of the two fiber sections in close proximity to each other, the excited multi-cladding modes from the SMS fiber section can be selectively coupled to the guided mode in the LPFG, and vice versa. A theoretical analysis based for such a structure is undertaken and the simulated results are verified by experiments demonstrating a maximum coupling efficiency of up to 1.66% (which could be improved to 27.5% in theory) over a broadband resonance (42 nm with a 3 dB bandwidth).

Laser power noise detection at the quantum-noise limit of 32 A photocurrent

The power noise of a cw Nd:YAG laser system was measured at radio frequencies using the optical ac coupling technique. An additional mode cleaner in the setup allowed a high optical ac coupling amplification of 62.3. For the first time, to our knowledge, a sensitivity of 1.1×10⁻¹⁰ Hz(-1/2) relative power noise was achieved corresponding to an equivalent detected photocurrent of 32 A. High precision optics experiments can utilize this scheme to improve the sensitivity of their photodetectors.

Spherical Cavity Glass Lasers for Multiwavelength Emission

This review takes a look at small-sized resonators, especially, spherical glass lasers of micrometer size. Two optical coupling techniques developed in our laboratories are described: (1) terrace microspheres (high-refractive index glass spheres having a light entrance portion made from organic–inorganic hybrid materials) and (2) a half-polished fiber coupler. We tried to pump several tens of micrometer-sized high-index glass spheres (nD=1.93) in BaO–TiO2–SiO2 glass system and containing a few ppm of Nd3+ for laser demonstrations. Using two optical coupling techniques with CW Ti:sapphire laser, we carried out overlapped emission of both of the wavelength regions originating from Raman scattering (840–890 nm) and fluorescence of Nd3+ (860–940 nm). A potential for a multiwavelength laser having >100 emission peaks was demonstrated with thresholds of several mW order.

New concepts and results in laser power stabilization

In the recent decade the development of high-power, high-sensitivity photodetectors and low-noise sensing techniques lead to achievable relative power stabilities at the level of 10−9 Hz−1/2 for frequencies between about 10 Hz and 10 kHz. A new power stabilization concept involving the novel optical ac coupling technique complements these high-sensitivity photodetectors. This technique improves the sensitivity of a photodetector by about one order of magnitude. Furthermore, it beats the quantum limit of traditional power stabilization concepts and opens a whole new range of attainable power stabilities that seemed to have been inaccessible due to technical limitations before. In this article we review various laser power stabilization experiments in the field of ground-based gravitational wave detectors, present the best results and discuss their limits.

Quantum limit of different laser power stabilization schemes involving optical resonators

Three different laser power stabilization schemes are compared: a traditional power stabilization, a traditional one with subsequent optical resonator, and a power stabilization with the novel optical ac coupling technique. The performance of the schemes is evaluated using the theoretical quantum limit and the power stability achieved considering technical limitations. The scheme with optical ac coupling is superior to the other ones especially at high laser power levels that will be used in future interferometric gravitational wave detectors.

Reduction of CM Noise Emissions From PLC Modem Using Optically Coupled Signaling

This paper investigates the feasibility of implementing an optical coupler in the analog front-end (AFE) circuit of a power line communication (PLC) modem to reduce the common mode (CM) noise coupling onto the power lines. Based on a two-current-probe measurement approach, an equivalent CM circuit model is developed to predict the expected CM current generated by the PLC modem when it is plugged onto the power line network. The model provides insight into the CM noise coupling mechanism from any PLC modem onto the power line network, so that the effectiveness of using an optical coupling technique in reducing the CM current from the power line network can be properly quantified

Realization of an On-Line Fiber-Optic Bending Loss Measurement System

In this paper, an on-line bending loss measurement system is studied and realized. To avoid complicated calculation of fiber bending loss, we propose a simplified theoretical model for on-line evaluation. A specific fiber adaptor is developed for light signal feeding and picking up through a free space optical fiber coupling technique. Relevant factors, including free space fiber misalignment, are considered. The signal-to-noise ratio of the whole system is improved by optimizing the spatial perturbation length of the fiber and the deformation of the bending fiber. The use of an on-line monitoring technique for measuring the bending loss of optical fibers is presented in this paper. With this technique, manufacturing efficiency can be improved, and the cost of maintaining fiber quality control can be reduced.

A novel bidirectional optical coupling module for subscribers

To overcome drawbacks and limitations of planar lightwave circuit based modules for bidirectional communications, such as the demand for several chips and in consequence more packaging efforts, we have recently developed a novel optical coupling technique using our unique 155 Mbps bidirectional laser chip. Since the chip is structured with a pin-photodiode monolithically integrated on a laser diode's waveguide, the optical coupling requires only the alignment of the chip with a fiber. To optically couple the laser diode and photodiode simultaneously with a single fiber, we have designed an unusual coupling structure using a fiber having a cleaved surface whose normal is 35/spl deg/ angled to the fiber core axis, and using an index-controlling medium with a refractive index of /spl sim/1.3. The bidirectional chip is flip-chip bonded and the fiber is passively aligned using a V-groove on the same substrate of 2.5/spl times/1.3 mm/sup 2/ in size. Even with this extremely small and simple scheme for bidirectional optical coupling, we could obtain an optical output power of -7/spl sim/-10 dBm and a responsivity of <-30 dBm, which are satisfactory to the STM-1 level telecommunications specifications.

Bidirectional optical coupling of transceiver chipfor subscribers

To reduce the size and cost of a bidirectional optical module, a novel optical coupling technique has been developed using a new 155 Mbit/s transceiver chip. Since the chip is structured such that a pin-photodiode is monolithically integrated on a laser diode, bidirectional optical coupling is simply achieved by passively aligning an angle faceted singlemode fibre on the V-groove of a silicon optical bench. In a packaged size of 2.5 × 1.3 mm2, an output power of –7 to –10 dBm and responsivity of 0.45 ± 0.05 A/W can be obtained.

Secondary Electron Detectors, Image Quality &amp; Contrast

Abstract The Secondary Electron Detector (SED) is the first link in the chain of image formation after the initial illumination of a specimen in a Scanning Electron Microscope (SEM), therefore it is most crucial that the efficiency of the SED be as great as possible in order to maximize Signal-to- Noise Ratio (S/N) as any imperfections, inadequacies, and noise are then amplified along with the signal by the subsequent electronic amplifiers in the signal chain. In the Everhart-Thornley SED, matching the scintillator's output spectrum to the photomultiplier tube's photocathode spectral sensitivity is one factor along with other characteristics of the PM tube, like quantum efficiency and noise characteristics. Light-pipe material along with as optical coupling techniques are another factor which determines the overall efficiency of converting SE electron strikes on the scintillator to a video output signal.

IC compatible optical coupling techniques for integration of ARROW with photodetector

New structures for efficient coupling of light from an antiresonant reflecting optical waveguide (ARROW) to integrated photodiode are proposed and analyzed. Both end-fire and leaky-wave types of coupling have been considered and it is found that high-coupling efficiencies can be achieved by exploiting the intrinsic properties of the ARROW structure without requiring additional non-IC compatible materials. The coupling structures were simulated using a simple ray optics model as well as the beam propagation method (BPM). Fabrication process conditions and measurement results for various coupling structures are presented.

Excitation of surface electromagnetic waves on water

Excitation of surface electromagnetic waves (SEW) on water was studied using optical coupling techniques at microwave frequencies. Excitation of SEW was also achieved using direct horn antenna coupling. The transmitted SEW power was increased by adding acid and salt to water. The horn antenna gave the maximum excitation efficiency 70%. It was increased to 75% by collimating the electromagnetic beam in the vertical direction. Excitation efficiency for the prism (0 degrees pitch angle) and grating couplers were 15.2% and 10.5% respectively. By changing the prism coupler pitch angle to +36 degrees , its excitation efficiency was increased to 82%.

Optoelectronics as applied to functional electronic blocks

The combination of a GaAs light source and a silicon photodetector in a Functional Electronic Block (FEB) provides signal coupling with electrical isolation. Circuit functions not previously available in microelectronic form can be realized with optical coupling. The characteristics of the GaAs light source, the silicon photodetector and optical coupling techniques of importance in FEB design are discussed. Several useful FEB's which employ optical coupling to achieve electrical isolation are described; these include a multiplex switch, an isolated gate p-n-p-n-type switch, a photon-coupled transistor and an optoelectronic pulse amplifier.