Optical head employing a concentric-circular focusing grating coupler

An optical head employing a concentric-circular grating coupler (CGC) and a concentric-circular focusing grating coupler (CFGC) is proposed, and its operating principle and characteristics are reported. Satisfaction with a prerequisite for the head, i.e., the removal of aberrations caused by deviations in wavelength and the effective index, is theoretically achieved by application of the concept of optimization of an annular aperture. With CGC and CFGC fabricated by an electron-beam-writing method, we experimentally confirmed its fundamental characteristics of light input, waveguiding, output, and convergence, with an elliptical focusing spot converging at half-intensity widths of 1.8 and 4.0μm.     

Optical switching in a symmetric three-waveguide nonlinear directional coupler

In this paper we study optical switching in a system of a three-waveguide nonlinear directional coupler. We consider a waveguide structure with three parallel waveguides in a linear, symmetric coupling configuration. We work in the case that the (absolute value of the) difference of the propagation constants of the outer and centre waveguides is much larger than the linear coupling constants. When one of the outer waveguides is initially excited, we show that for small values of the initial power the system behaves as a two-waveguide nonlinear directional coupler where only the two outer waveguides are involved in the switching. For larger values of the initial power, transfer of light between the initially excited waveguide and the centre waveguide is observed.     

Optical communication noise rejection using correlated photons

This paper describes a completely new way to perform noise rejection using a two-photon sensitive detector and taking advantage of the properties of correlated photons to improve an optical communications link in the presence of uncorrelated noise. In particular, a detailed analysis is made of the case where a classical link would be saturated by an intense background, such as when a satellite is in front of the sun, and identifies a regime where the quantum correlating system has superior performance.     

Focusing waveguide grating coupler using a diffractive doublet

A novel focusing waveguide grating coupler comprising an integrated uniform grating coupler and binary-phase-only diffractive lens is proposed, designed, and fabricated. Experimental results are also presented to demonstrate the device performance. This device is in direct competition with single-element focusing grating couplers defined by direct-write electron-beam lithography and its structure is attractive because the fabrication procedure is better suited for mass production.     

Optical retrodirective tracking system approach using an array of phase conjugators for communication and power transmission

A new concept for a retrodirective tracking system applicable for communication and power transmission is proposed. In the proposed concept, the power transmitter utilizes a receiver's pilot signal to obtain information about its direction by conjugating the signal's phase inside a nonlinear medium. Power is therefore transmitted back to the receiver by the phase-conjugated signal beam. The power can be concentrated by an array of phase conjugators, which provides a large aperture so that the intensity can be increased on the receiver's photovoltaic panels compared to a single element. Controlling the phase and the direction of the readout beams in the four-wave-mixing process provides control over the interference pattern, its position, and its size. A numerical analysis is given for the phase and spot size control, and measurements with two Co-doped Sr(x)Ba(1-x)Nb(2)O(6) (Co:SBN) crystals confirm the occurrence of interference that is achieved for the case of two beams.     

Fabrication of a concentric-circular focusing grating coupler by a conic-wave-front interference method and light-convergence experiments using the coupler

An interference method utilizing conic-wave-front light for the fabrication of a concentric-circular and chirped grating is proposed. The design method and fabrication of an interference lens that generates conic-wave-front light are also shown. A focusing element is constructed from a concentric-circular grating coupler with a 0.4-mm diameter and a concentric-circular focusing grating coupler with an annular aperture of 2-mm focal length and 2.0-4.0-mm diameter. Light-convergence experiments using the focusing element were able to obtain a focusing spot of 0.5 μm × 0.7 μm at half-intensity widths for a wavelength of 820 nm in combination with liquid-crystal polarization elements.     

A novel reflector-filter using a SAW waveguide directional coupler

The SAW waveguide technique was used to form a novel reflector-filter for the realization of compact IF filters in CDMA handsets. The reflector-filter design is an attractive technique to obtain characteristics with steep skirts in a short device length, because the SAW propagation path is folded and the frequency response is synthesized by utilizing both IDT and reflector responses. In this paper, we propose a new reflector-filter structure using a SAW waveguide directional coupler. For the implementation of the proposed reflector-filter, the key technology is the design of SAW waveguides. We have formed SAW waveguides using Al gratings loaded on the surface of the substrate. The pitch of the Al gratings has been chosen unequal to half the acoustical wavelength to avoid the occurrence of the gratings stopband at the filters passband position. Using the proposed reflector-filter structure, PCS-CDMA IF filters were fabricated on quartz substrates. The filter exhibited an insertion loss of 8.5 dB, a 5 dB bandwidth of 1.45 MHz, and a rejection of more than 33 dB at the center frequency +/-1.25 MHz with the package size of 4.8x9.1 mm (2). i.e., half the size of a conventional transversal filter was achieved using the proposed reflector-filter technique.     

Optical fiber monitored by a directional coupler for delivering laser radiation in medical treatments

The safety of therapeutic laser treatments could be strongly improved by introducing real-time monitoring to the fiber delivery system. Since any fiber damage leads to a significant variation of the backscatter, its detection could be exploited to monitor the system, in particular the fiber trip. To assess this potential, the backscattered modal power distribution (BMPD) from flat, bulb, and damaged fiber tips were investigated. The BMPD detection was accomplished by using a conventional beam-splitter method and two directional couplers: a prism-fiber and a fiber-fiber lapped coupler. Unlike the conventional method, use of the couplers allows for the separate detection of backscattered and forward transmitted signals. Therefore variations in the backscatter that are due to only a change in the laser pulse can be determined. Moreover, the directional couplers, because of the amplification of high-index modes, allow for an increase in sensitivity of the method. This was particularly evident in our tests on fiber tips that had been dipped into water to obtain a better simulation of the real working conditions. Finally, the influence of the target on the BMPD was investigated as a function of its distance from the fiber tip. All the tests confirm that the target must be taken into account if the distance is <1 mm, but the monitoring system can also be used when the fiber tip works in contact with the tissue wall provided that one can ensure contact by pushing the tip against the target wall.     

Realization of an optical communication tunable filter using a diffraction-based optical system

Optical bandpass tunable filters are key elements for dense WDM optical communication systems. Recently, a realization of optical linear transformation using a spatial diffraction optical system was presented and theoretically analyzed [J. Opt. Soc. Am. A21, 732 (2004)]. We present an optical bandpass tunable filter based on this optical system and a spatial light modulator. The mathematical analysis and additional computer simulations for testing the filter's basic parameters using the Fresnel diffraction theory are presented. Some newly obtained laboratory experimental results of the device in the optical visible spectrum will be shown.     

Temperature sensor using an optical fiber coupler with a thin film

A temperature sensor was demonstrated and fabricated by coating thermosensitive film around a fiber coupler. Based on the multicladding equivalent method, the coated fiber coupler was simplified to a conventional one. With the high thermo-optical coefficient of organic-inorganic solgel material, a good sensing result was achieved. The range of temperature measured is from -50 to 100 degrees C. The resonant wavelength has a shift of about 25 nm. A sensitivity of 0.17 nm/degrees C is achieved. With the advantages of having a simple structure and being unaffected by the instability of the light source, the proposed fiber coupler temperature sensor will find wide applications.     

Optical electric-power-sensing system using faraday and pockels cells

We present what to our knowledge is a novel optical electric-power-sensing system that uses a cascade of a Faraday cell and a Pockels cell. According to Jones-matrix analysis, the electric-power information is contained in the intensity of the emerging light. An instantaneous ac electric-power signal has been observed in real time, and the active power has been measured in the 0-1-kW range. The experimental results are in good agreement with the theoretical analysis and prove the feasibility of the optical sensing system.     

Optical generation of millimeter-wave pulses using a fiber Bragg grating in a fiber-optics system

A scheme is proposed to transform an optical pulse into a millimeter-wave frequency modulation pulse by using a weak fiber Bragg grating (FBG) in a fiber-optics system. The Fourier transformation method is used to obtain the required spectrum response function of the FBG for the Gaussian pulse, soliton pulse, and Lorenz shape pulse. On the condition of the first-order Born approximation of the weak fiber grating, the relation of the refractive index distribution and the spectrum response function of the FBG satisfies the Fourier transformation, and the corresponding refractive index distribution forms are obtained for single-frequency modulation and linear-frequency modulation millimeter-wave pulse generation. The performances of the designed fiber gratings are also studied by a numerical simulation method for a supershort pulse transmission.     

Experimental study on a two-stage rotary desiccant cooling system

The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COP_th. It has found that the required regeneration temperature of TSRDC system is low and COP_th of the system is high. Regeneration temperatures from 65 °C to 80 °C, 65 °C to 75 °C and 80 °C to 90 °C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study.