Numerical analysis of integrated acoustooptic tunable filters withweighted coupling

A numerical approach based on the finite element method (FEM) and the beam propagation method (BPM) is newly formulated for design and modeling of integrated acoustooptic tunable filters (AOTF's) with weighted coupling on a piezoelectric substrate. First, acoustical field profiles in the substrate region and on the substrate surface are separately calculated with FEM and BPM, respectively. These two-dimensional (2-D) profiles are used for constructing three-dimensional (3-D) strain and electric field profiles. Then, from the strains and electric fields the refractive index changes due to acoustooptic (AO) and electrooptic (EO) effects are derived. Finally, by analyzing modified optical waveguides with AO8- and EO-induced changes in refractive indexes using again BPM, optical filter characteristics of AOTF's can be evaluated. Numerical examples are shown for AOTF's with a tapered acoustical directional coupler on a LiNbO₃ substrate     

Laser detection utilizing tunable acoustooptic filters

A laser detection technique was developed utilizing frequency modulated tunable acoustooptic filters. CW laser radiation measuring about5(10^{-7})mW was detected in the presence of an incoherent background irradiance of 0.1 W/cm².     

Demonstration of narrow-band acoustooptic tunable filters ondispersion-enhanced single-mode fibers

The physical parameters affecting the bandwidth of acoustooptic tunable filter on single-mode (SM) fiber are investigated. Compared to standard step-index SM fiber, we found that the bandwidth-length product can be reduced by a factor of 1.3 and 4 for nonzero-dispersion-shifted and dispersion-compensating fibers (DCFs), respectively. Record narrow bandwidth of 0.4 nm is measured for a 28-cm-long DCF (D of - 100 ps/nm/km at 1550 nm). The measured results are in good agreement with calculated values     

Dynamic erbium-doped fiber amplifier based on active gainflattening with fiber acoustooptic tunable filters

We describe the first demonstration of a dynamic erbium-doped fiber amplifier (EDFA) based on automatic feedback control of active gain equalizing filters. The filters are all-fiber acoustooptic tunable filters capable of generating controllable filter shapes. Wide dynamic-range gain/power control is achieved with <0.6-dB signal ripple over 30 nm in various operating conditions. We also show, by numerical simulation, an important advantage of the dynamic EDFA over a conventional EDFA in cascaded structures     

Actively gain-flattened erbium-doped fiber amplifier over 35 nm byusing all-fiber acoustooptic tunable filters

We demonstrate an actively gain-flattened erbium-doped fiber amplifier (EDFA) using an all-fiber gain-flattening filter with electronically controllable spectral profiles. A good gain flatness (<0.7 dB) over a broad wavelength span (>35 nm) is achieved for a wide range of operational gain levels as well as input signal and pump powers     

Integrated acousto-optical mode-convertors with weighted coupling using surface acoustic wave directional couplers

Acoustic directional couplers for guided surface acoustic waves have been developed for application in a weighted acousto-optical TE-TM convertor in LiNbO/sub 3/. Tapering of the coupling strength is achieved by longitudinally sin/sup 2/-varying acoustic power providing a filter response with strongly suppressed sidelobes.<>     

Miniature and tunable filters using MEMS capacitors

Microelectromechanical system (MEMS) bridge capacitors have been used to design miniature and tunable bandpass filters at 18-22 GHz. Using coplanar waveguide transmission lines on a quartz substrate (/spl epsiv//sub r/ = 3.8, tan/spl delta/ = 0.0002), a miniature three-pole filter was developed with 8.6% bandwidth based on high-Q MEMS bridge capacitors. The miniature filter is approximately 3.5 times smaller than the standard filter with a midband insertion loss of 2.9 dB at 21.1 GHz. The MEMS bridges in this design can also be used as varactors to tune the passband. Such a tunable filter was made on a glass substrate (/spl epsiv//sub r/ = 4.6, tan/spl delta/ = 0.006). Over a tuning range of 14% from 18.6 to 21.4 GHz, the miniature tunable filter has a fractional bandwidth of 7.5 /spl plusmn/ 0.2% and a midband insertion loss of 3.85-4.15 dB. The IIP/sub 3/ of the miniature-tunable filter is measured at 32 dBm for the difference frequency of 50 kHz. The IIP/sub 3/ increases to >50 dBm for difference frequencies greater than 150 kHz. Simple mechanical simulation with a maximum dc and ac (ramp) tuning voltages of 50 V indicates that the filter can tune at a conservative rate of 150-300 MHz//spl mu/s.     

An electronically wavelength-tunable mode-locked fiber laser using an all-fiber acoustooptic tunable filter

We have demonstrated a novel scheme of a continuously wavelength-tunable and passively mode-locked fiber laser by using the sliding frequency effect of an all-fiber acoustooptic tunable filter. Stable and nearly transform-limited pulses of 800-fs pulsewidth were produced with this laser scheme for a wavelength tuning range of 17 nm at around 1550 nm.     

Adaptive weighted highpass filters using multiscale analysis

We propose a general framework for studying a class of weighted highpass filters. Our framework, based on a multiscale signal decomposition, allows us to study a wide class of filters and to assess the merits of each. We derive an automatic procedure to tune a filter to the local structure of the image under consideration. The entire algorithm is fully automatic and requires no parameter specification from the user. Several simulations demonstrate the efficacy of the proposed algorithm.     

Integrated optical multi-/demultiplexer using acoustooptic effectfor multiwavelength optical communications

Optical integrated switching systems using collinear acoustooptic (AO) interactions for wavelength division multiplexing (WDM) systems are discussed. The AO switch module has excellent advantages compared to electrooptic switches. Optical WDM signals can be wavelength-selectively switched by frequency multiplexed surface acoustic waves. WDM switching systems are proposed as an application of this AO switch, and optical insertion loss, crosstalk, and optical frequency shift are discussed. Wavelength-selective characteristics in the AO module are established. The crosstalk from other wavelengths is found to be reduced by decreasing slightly the switching transfer efficiency. The wavelength-selective switching was experimentally demonstrated with a slab-type module of a 4-mm interaction region, where the WDM interval of 25.2 Å was realized. This wavelength interval can be decreased by increasing the interaction length     

Tunable acoustooptic filters and equalizers for WDM applications

We describe a high performance acoustooptic tunable filter for add-drop application and for signal equalization in WDM telecommunication crossconnects. It results from a thorough investigation in TeO/sub 2/ of bulk collinear interaction, the geometry of which, particularly the direction of propagation of the acoustic wave, has been chosen in order to obtain the best compromise between the spectral resolution of the device and the acoustooptic figure of merit. Less than 40 mW of electric power is needed either to deviate 100% of a selected light wavelength /spl lambda/ at resonance, or to induce a 30-dB attenuation of its intensity. The sidelobes practically vanish for this configuration and the resolution is equal to 0.75 nm (or 94 GHz) for /spl lambda/=1.55 /spl mu/m. Polarization splitters combined with half-wave plates allow to completely get rid of polarization sensitivity problems. The use of optical fibers to collect the signal at the filter outputs, actually contributes to the high performance of the device as a whole. Experiments have been performed by multiplexing three signals in the input fiber, separated by 4, 2, and 1 nm. The transmission of the filter has been examined through the bar and cross state.     

A photonic wavelength-division switching system using tunable laserdiode filters

A photonic wavelength-division switching system using semiconductor tunable wavelength filters is proposed. A switching system using wavelength switches and multistage switching networks is discussed. A crucial point in developing this switching system is to achieve a large number of wavelength-division channels. The potential of 100 wavelength-division channels in such switching systems is estimated, based on InP optical integrated circuits. A wavelength network synchronization which permits the network to utilize such a large number of wavelength-division channels without wavelength misalignment and drift is proposed. An eight-channel wavelength-division switching experiment, using phase-shift-controlled distributed feedback laser diodes as tunable wavelength filters, is reported     

Compact VDTA-based current-mode electronically tunable universal filters using grounded capacitors

This paper presents a compact current-mode three-input single-output (TISO) type universal filter. Only one voltage differencing transconductance amplifier (VDTA) and two grounded capacitors are employed in the proposed filter. The circuit can realize lowpass, bandpass, highpass, bandstop and allpass biquadratic filter outputs by connecting the appropriate inputs, and offers electronic control of the natural angular frequency (ω₀) and quality factor (Q) by means of adjusting the transconductance gain of the VDTA. In addition, by slight modification of the proposed scheme, another more useful TISO construction with orthogonal ω₀–Q tuning has been obtained. Both the discussed universal filters have been shown to have low incremental active and passive sensitivities. To demonstrate the performances of the filters and verify the theoretical analysis, computer simulations are accomplished with the PSPICE program.     

100% efficient narrow-band acoustooptic tunable reflector usingfiber Bragg grating

This paper reports a 100% efficient Bragg-grating-based acoustooptic superlattice modulator in a fiber whose diameter is reduced by HF etching. The acoustically induced reflections, which appear on both sides of the Bragg stop-band, are tunable by altering the acoustic frequency, and have bandwidths corresponding to weak versions of the permanent Bragg grating. These unique properties may lead to important applications in, e.g., reconfigurable wavelength division multiplexing, Q-switching, amplitude modulation, and frequency shifting     

Characterization of wavelength-tunable single-frequency fiber laser employing acoustooptic tunable filter

This paper demonstrates and characterizes a novel wavelength-tunable single-frequency erbium-doped fiber ring laser incorporating an all-fiber acoustooptic tunable bandpass filter and a self-constructed saturable absorption grating (SAG). Stable single-longitudinal-mode operation was achieved over the wavelength range of 48 nm with a sidemode suppression ratio higher than 50 dB. The wavelength tuning characteristics, and the laser dynamics in wavelength switching and sweeping are analyzed in detail. A theoretical analysis on the effect of the SAG is also described.     

Electronic wavelength tuning using acoustooptic tunable filter with broad continuous tuning range and narrow channel spacing

The use of acoustooptic tunable filters as tunable-wavelength filters in a multiwavelength network is reported. Fast ( approximately=3 mu s access time), broad, continuous tuning (1.3-1.56 mu m) and narrow channel spacing (4.5 nm) in a nine-channel distribution system were demonstrated. The combination of broad tuning range and narrow channel spacing allows channel selection in a distribution system with hundreds of wide-band wavelength-division-multiplexed channels. The wavelength switching time, measured as approximately=3 mu s, is important in many high-speed switching applications.<>     

Thin-film tunable optical filtering using anisotropic and noncollinear acoustooptic interaction in LiNbO3waveguides

Theoretical and experimental studies of optical filtering which utilize anisotropic and noncollinear acoustooptic interaction in aY-cut LiNbO3waveguide are reported. A guided-light beam from either an He-Ne laser at 6328 Å or an argon laser at around 5000 Å, propagating at an angle centering around 70° from theZ(c)axis of the LiNbO3crystal, was Bragg diffracted by the surface acoustic wave with 500 MHz center frequency and propagating at 16° from thexaxis of the crystal. High optical resolving power, large optical angular aperture, and good spatial separation between the filtered light and the unfiltered light have been simultaneously achieved. An optical passband of 16 Å and an angular aperture of 9° have been measured for the case in which the undiffracted and diffracted light propagate in TE0mode and TM0mode, respectively. Furthermore, by varying the acoustic frequency from 350 to 670 MHz, the filtered optical wavelength may be tuned from 8300 to 5000 Å.