Electro-optic Filter Research Articles

Rapidly tunable optical add-drop multiplexer utilising relaxed beam splitters

A polarisation-independent four-port electro-optic tunable filter in the 1530 nm wavelength regime utilising non-polarising relaxed beam splitters and strain-induced polarisation converters on LiNbO3 with 16 nm tuning range and 46 ns tuning speed is reported.

Duty cycle dependence of a periodically poled LiNbO_3-based electro-optic Solc filter

We demonstrate that the performance of a periodically poled LiNbO3- (PPLN-) based electro-optic Solc filter is dependent on the duty cycle of the crystal. This may limit the performance of the device for applications such as add-drop filtering and switching, owing to the deterioration of the extinction ratio. It is shown that by adding a retarder to the Solc filter it is possible to improve the extinction ratio; thus the dependence of the filter on the duty cycle can be reduced. Using Jones calculus, we analyzed the effect of a variable retarder that can also be rotated on the extinction ratio. We experimentally observed a 6 dB increase in the extinction ratio when we used a half-wavelength retarder.

Polarization dependence of the coupling ratio in LiTaO_3 directional couplers

The effect of design and fabrication parameters on the polarization dependence of the splitting ratio in directional couplers produced in LiTaO3 by Zn diffusion has been investigated experimentally at a wavelength of 1558 nm. The directional couplers featured various combinations of waveguide width, separation gaps between waveguides, bending angle, and diffusion conditions. In each case the coupling region was 3.5 mm long. Of particular interest was the identification of parameter sets for which the sum of power splitting ratios from TE and TM inputs equals unity at the output, as needed for electro-optic tunable filters with relaxed beam-splitter requirements.

Flattop tunable dense-wavelength division-multiplexing filter-modulator

For spectral filters with subnanometer bandwidth, precise control of the device components’ dimensions (error<λ/1000, where λ is the center wavelength) is critical. Theoretical results from the transfer-matrix method show that an electro-optic thin-film filter can have a subnanometer bandwidth and still tolerate a root-mean-square error in the thickness of each layer up to λ/100. Illustrative examples of an electro-optic thin-film spectral filter with 0.05-nm (6-GHz), −3-dB bandwidth are presented.

Guided wave electrooptic and acoustooptic tunable filter apparatus and method

A two-port guided wave tunable filter in a birefringent electrooptic and/or acoustooptic substrate material includes two 3-port, symmetric Y-branch beam splitters connected by two waveguide sections in which phase-matched polarization coupling occurs, with an input port and an output port. The optical path difference between the beam splitters is half an optical wavelength, and the polarization coupling regions between the beam splitters are relatively displaced by an odd integral multiple of half the spatial period of the perturbation responsible for the coupling. In one embodiment, an electrooptic tunable filter, the polarization coupling in the waveguides is caused by a spatially periodic strain-inducing film and tuning results from an applied electric field. In another embodiment, an acoustooptic tunable filter, polarization coupling results from a surface acoustic wave and tuning is accomplished by changing the acoustic frequency. Alternatively, four port electrooptic and acoustooptic tunable filters are formed by replacing the 3-port beam splitters with 4-port directional couplers, where in each of the directional couplers the splitting ratio for TE input polarization plus the splitting ratio for TM input polarization is substantially equal to one.

Polarization dependence in Ti-diffused LiNbO3 directional couplers.

The effect of design and fabrication parameters on the dependence on polarization of the splitting ratio in directional couplers produced by Ti diffusion in LiNbO3 has been investigated experimentally at a wavelength of 1.54 microm. The parametric study was carried out at a diffusion temperature of 1025 degrees C, which was found to suppress outdiffusion effects. The directional couplers were produced by use of various combinations of waveguide separation, Ti film thickness, and diffusion time. Of particular interest was the identification of parameter sets for which the sum of TE and TM splitting ratios equals unity, as required for electro-optic and acousto-optic tunable filter designs with relaxed beam-splitter requirements. Directional couplers that closely match this criterion were obtained by diffusion of 3.5-mm-long, 7-microm-wide Ti strips separated by 11 microm for 11 h. It was found that a bending angle of 0.6 degrees for input and output waveguides produces lower transmission loss for both polarizations than a 1.0 degree bend angle (>1-dB loss reduction).

Limitations imposed by birefringence uniformity on narrow-linewidth filters based on mode coupling

Integrated tunable narrow-linewidth wavelength filters based on TE-TM mode conversion require a high quality processing. The dis- cussion is based on numerical simulations of typical technology-induced defects and on a comparison with experimental results. We show that uniformity of the LiNbO3 crystal growth currently limits the linewidth to about 0.6 nm for spectral filters operating at 1.55mm. Moreover, such a filter would require a technology with strict conditions of uniformity over several centimeters. The discussion is focused on electro-optic filters but the results can be extended to any device based on codirectional mode coupling, such as acousto-optic filters. © 2001 Society of Photo-Optical Instru- mentation Engineers. (DOI: 10.1117/1.1418711)

Electro-optic spectral filter based on optical superlattice LiNbO3

Electro-optic (EO) effect of optical superlattice LiNbO3 (OSLLN), a single crystal with periodic ferroelectric domain structure was studied theoretically and experimentally. Because the electro-optic coefficient has different signs in different domains, the refractive index ellipsoid deforms periodically in the presence of an electric field applied along the Y-axis, which results in the coupling between the extraordinary wave and ordinary wave. Under specific conditions, the polarization of a light may be rotated to 90°, which means that the optical material could act as a precise spectral filter. The influence on the transmission spectrum of an OSLLN EO filter when there is another electric field along the Z-direction was also studied. It was demonstrated that the transmission peak could move with the electric field along the Z-axis if the duty cycle of the periodic structure is not 50%. Based on this effect, a tunable filter may be constructed.

A tunable filter with electrooptical TE–TM mode conversion in a GaAs/AlAs multiquantum-well waveguide

In this paper, the theoretical study of a new electrooptic tunable filter based on TE–TM mode conversion, and the high form birefringence that occurs in a GaAs/AlAs multiquantum-well waveguide is presented. Then TE–TM conversion can be achieved with a relatively low voltage (30 V). Such a filter shows off a tuning rate voltage ranging around 7 V/channel, and as regards selectivity a 1.3 nm filter bandwidth can be obtained.

Guided wave acousto-optic and electro-optic tunable filter designs with relaxed beam-splitter requirements

New designs for acousto-optic tunable filters and electro-optic tunable filters with the Mach-Zehnder configuration are proposed. The new designs differ from conventional designs in three respects: (1) polarizing beam splitters are not required, (2) the optical path difference for the waveguides between the beam splitters is a half-wavelength, and (3) the relative positions of the polarization coupling regions in the two waveguides are displaced in the propagation direction by half of the spatial period of the perturbation responsible for the coupling. Because the new designs provide an additional degree of freedom in achieving the required beam-splitter performance, they are expected to be much easier to fabricate than conventional filter designs with polarizing beam splitters.

A class of centralized high-performance protocols for single-hop lightwave networks

A new class of protocols for WDM star networks is introduced. According to the proposed protocols, two or more stations share each wavelength and transmit their packets in a random access fashion. An array of tunable electro-optic filters or tunable wavelength converters, placed at the network hub, allows at most one packet per wavelength to pass to the star coupler, at each time slot. Furthermore, the selection of the passing packets is implemented in such a way that receiver conflicts are avoided. When wavelength converters are used instead of electro-optic filters, the hub is capable of uniformly distributing the incoming packets to the available wavelengths. Thus, the load is balanced between the wavelengths, resulting in a further performance improvement. The performance of the proposed protocols is studied via analytical and simulation results which indicate that a WDM star network operating under these protocols achieves a high performance under any load conditions. Copyright © 1999 John Wiley & Sons, Ltd.

Centralized Packet Filtering protocols: a new family of MAC protocols for WDM Star Networks

The small number of available wavelengths has been a limiting factor in the development of WDM Broadcast-and-Select Star Networks, since most protocols require the number of wavelengths be equal to the number of users. A new protocol which overcomes this limitation is introduced in this paper. According to the proposed Centralized Packet Filtering (CPF) protocol, more than one station share each wavelength and transmit their packets in a random access fashion. An array of electrooptic tunable filters, which is placed at the network hub, allows at most one packet per wavelength to pass to the Star Coupler, at each time slot. In this way, channel collisions are eliminated. Furthermore, the selection of the passing packets is implemented in such a way that receiver conflicts are avoided. Extensive analytical and simulation results are presented which indicate that a WDM Star Network operating under the CPF protocol achieves a high performance under any load conditions.

Tunable optical filters for dense WDM networks

WDM is currently taking over as the leading technology in point-to-point transmission links. For optical implementation of WDM networks, logical functionalities such as wavelength (channel) selection should be carried out in the wavelength domain; thus, the development of dynamic optical devices is required. One key device is a tunable optical filter. Important features of such a filter include low insertion loss, narrow bandwidth, high sidelobe suppression, large dynamic range, fast tuning speed, a simple control mechanism, small size, and cost effectiveness. Here, an extensive overview of the different technologies used to produce tunable optical filters is presented. Among them, fiber filters such as fiber Bragg gratings and fiber Fabry Perot are the most commercialized, yet inherently limited in their dynamic speeds. For high demanding dynamics, micro-machined and acousto-optic filters can offer a good solution for microsecond tuning speeds. Faster tunable devices, in nanosecond tuning speeds, might emerge out of microresonators, electrooptic filters, and active DBR filters.

Lithium niobate electro-optic tunable filter with high sidelobe suppression

The first demonstration of a high sidelobe suppression, tunable, electro-optic filter fabricated in lithium niobate for 1.55 μm is reported. Weighting of the mode convenor structure using a Taylor distribution resulted in a −3 dB passband width of 1.6 nm and nearest sidelobes of −25 dB. A continuous, linear electro-optic tuning rate of ±30 V/nm is also reported.

Wavelength-tunable optical filters: applications and technologies

The authors review noncoherent, frequency-tunable filter (receiver) technologies. They describe three basic mechanisms of wavelength filtering: filters that are based on the wavelength dependence of interferometric phenomena, with emphasis on Fabry-Perot interferometer filters; filters that are based on the wavelength dependence of coupling between optical fields (modes) induced by external perturbations (both acoustooptic and electrooptic filters are described); and filters that are based on resonant amplification of optical signals in semiconductor laser diode devices (these devices provide gain in addition to wavelength selectivity). For each technology the authors explain briefly the principles of operation and quantify the relevant system parameters: tuning range, channel separation, number of channels, crosstalk isolation, gain and distortion, speed of wavelength tuning, and complexity. They present a unified picture of filtering mechanisms in an appendix. >

Broad-band tuning of Al2O3:Ti3+laser radiation using an electrooptic filter

An electrooptic filter was used for broad-band tuning (720–900 nm) of a flashlamp-pumped Al2O3:Ti3+ crystal laser. Some characteristics of the use of electrooptic filters in tunable laser resonators are analyzed. It is shown that the observed deviation of the Al2O3:Ti3+ laser tuning characteristics from a linear dependence on the voltage across the filter is due to a combination of two factors: the wavelength dependence of the active medium gain and the width of the filter transmission band.

Tunable acoustooptic reflection filters in LiNbO/sub 3/ without a Doppler shift

A proposal is presented for an acoustooptic filter in LiNbO/sub 3/ without the Doppler shift, which is normally present in acoustooptic filters and which is undesirable as an intracavity element. This tunable filter has a significantly wider tuning range than previously demonstrated electrooptic filters. A structure incorporating an interdigital acoustic transducer, polarization filter, and single-mode optical waveguide is shown. The design presented works by cascading two acoustooptic filters with an intermediate polarizer, where the Doppler shift experienced in the first filter is precisely compensated for in the second filter. Hence, the output light of the two cascaded acoustooptic filters is not shifted in frequency, thus making the device suitable for applications inside a laser cavity. The filter with intermediate polarizer can be integrated with a single-mode waveguide and requires only a single interdigital acoustic transducer. Crystal symmetry and acoustic power considerations are treated in detail. >

Design of a liquid crystal based tunable electrooptic filter

Procedures for designing a tunable liquid crystal (LC) electrooptic filter are illustrated. Selective filtering of one of the two mixed laser lines is used as an example for calculation. It is demonstrated that a LC filter can select a desired beam to pass while rejecting the other one, or vice versa, by switching the voltage biased on the LC cell. The switching times are determined by the liquid crystal properties, thickness, and biased voltage. A multilayered structure is proposed to improve the response times and power loading capability.

Full-field modeling of the longitudinal electro-optic probe

Optical polarization changes and mode coupling due to spatially varying anisotropic perturbations in caused by electro-optic, acousto-optic, and other effects have been widely studied for both plane-wave and fiber mode propagation. A new optical S-parameter analysis of these effects, applicable to arbitrary optical field distributions, is presented. It is applied to evaluating the performance of the longitudinal electro-optic probe used for noninva-sively examining GaAs integrated circuits. Error in probe measurements of circuit voltage distributions can be characterized by considering the probe as a scanned electro-optic spatial filter.

Rapid-frequency-scan laser for instantaneous line-profile measurements in fluorescence spectroscopy

Rapid-frequency-scan (RAFS) dye lasers have been developed for determination of instantaneous line profiles of neutral atoms and molecules. Two types of rapid-frequency scanners are examined and their scanning performances are compared. One scanner is an étalon driven by a PZT piezoelectrical element, and the other is an electro-optical (EO) filter with a KDP crystal. The PZT scanner takes 4–5 μs to scan over the free spectral range (FSR), and the EO scanner can make faster scanning. The maximum scanning range is limited by FSR, and the static bandwidth is approximately 1/100 of FSR in both cases. In fast scanning, however, the dynamic bandwidth increases. The error in the single shot measurement is evaluated, based on an experimental result of the line-profile measurement of Fe atoms.