Low Polarization Sensitivity Research Articles

Wide-Band Polarization-Insensitive High-Output-Power SemiconductorOptical Amplifier Based on Thin Tensile-Strained Bulk InGaAs

A polarization-insensitive semiconductor optical amplifier (SOA) with avery thin active tensile-strained InGaAs bulk has been fabricated. Thepolarization sensitivity of the amplifier gain is less than 1 dB over both the entire range of driving current and the 3 dB optical bandwidthof more than 80 nm. For optical signals of 1550 nm wavelength, the SOAexhibits a high saturation output power +7.6 dBm together with a lownoise figure of 7.5 dB, fibre-to-fibre gain of 11.5 dB, and lowpolarization sensitivity of 0.5 dB. Additionally, at the gain peak1520 nm, the fibre-to-fibre gain is measured to be 14.1 dB.

Tunable planar polymer Bragg gratings having exceptionally low polarization sensitivity

We report fabrication of planar polymer Bragg gratings that can be thermally tuned 20 nm with very low polarization sensitivity, (/spl lambda//sub TE/-/spl lambda//sub TM/)</spl plusmn/0.05 nm, throughout the tuning range. Polarization sensitivity in a planar film is caused by an anisotropic effective refractive index in the film typically resulting from strain. Strain in polymer Bragg gratings is caused by the photopolymerization and the thermal annealing processes utilized for the grating fabrication. For the case of polymer Bragg gratings, we show for the first time that fabrication-induced strain can be nearly eliminated by laser etching parallel trenches into the polymer layers on each side of the grating. The improvement in polarization sensitivity depends strongly on the trench separation, the thermal annealing process, and the grating tuning method.

High-output-power polarization-insensitive semiconductor optical amplifier

A high-output-power 1550 nm polarization-insensitive semiconductor optical amplifier (SOA) was developed for use as a compact in-line optical amplifier. A very thin tensile-strained bulk structure was used for the active layer and active width-tapered spot-size converters (SSCs) were integrated on both input and output sides. The SOA module exhibited a high saturation output power of +17 dBm together with a low noise figure of 7 dB, large gain of 19 dB, and low polarization sensitivity of 0.2 dB for optical signals of 1550 nm wavelength. For the amplification of optical signals modulated at 10 Gb/s in the nonreturn-to-zero (NRZ) format, a good eye pattern without waveform distortion due to the pattern effect was obtained at an average output power of up to +12 dBm. Additionally, good amplification characteristics were demonstrated for the signal wavelength range corresponding to the C-band.

Experimental and theoretical studies of flame hydrolysis deposition process for making glasses for optical planar devices

We have here presented experimental and theoretical studies of a flame hydrolysis deposition (FHD) process for making glasses for optical planar devices. FHD involves deposition of soot particles generated in the flame on a planar surface to form a porous layer. In order to function as a waveguide core, the porous soot deposit must be sintered at high temperatures to form a dense glass. However, these temperatures are high enough to cause the dopants to volatilize at the surfaces of the deposit. As a result, dopant concentration gradients and compositional inhomogeneities are created in the densified glass layer which result in inferior optical properties. If the layers could be deposited pre-sintered during laydown, these problems could be minimized or avoided. To understand the FHD process and its effect on the morphology of the resulting deposit, a number of models have been developed. We present models to predict the structure of the soot-laden flame along with the methods to estimate soot number density and mean size at different locations in the flame. A simple method is presented to predict morphology of the suspended soot and soot deposit formed during deposition. Motivated by the predictions of the model, process changes were made which resulted in the morphology of the core layers to change from sooty to pre-sintered during deposition. These process changes resulted in core glasses with significantly improved core roughness, index uniformity and thickness uniformity which helped reduce the straight waveguide losses from 0.2–0.3 to 0.02–0.05 dB/cm. Details of underclad and overclad glasses which resulted in low warpage and polarization sensitivity of 0.1 nm for phasar or phasar-arrayed waveguide devices are also presented.

Micro-electro-mechanical system-based digitally controlled optical beam profiler.

An optical beam profiler is introduced that uses a two-dimensional (2-D) small-tilt micromirror device. Its key features include fast speed, digital control, low polarization sensitivity, and wavelength independence. The use of this 2-D multipixel device opens up the important possibility of realizing several beam profile measurement concepts, such as a moving knife edge, a scanning slit, a moving pinhole, a variable aperture, and a 2-D photodiode array. The experimental proof of the optical beam profiler concept using a 2-D digital micromirror device to simulate the 2-D moving knife edge indicates a small measurement error of 0.19% compared with the expected number based on a Gaussian beam-propagation analysis. Other 2-D pixel arrays such as a liquid-crystal-based 90 degrees polarization rotator sandwiched between crossed polarizers can also be exploited for the optical beam whose polarization direction is known.

Fiber parametric amplifiers for wavelength band conversion

By using a loop configuration formed by a polarization beam splitter, we experimentally demonstrate that the existing wavelength-division multiplexing (WDM) sources in C-band can be wavelength converted to the S-band with low polarization sensitivity and low crosstalk. Using a fiber parametric amplifier as a band converter, we achieve experimentally 4.7-dB conversion efficiency over 30-nm conversion bandwidth in 315 m of fiber. Compared to the conventional straight fiber wavelength conversion scheme, a more than 2-dB improvement in polarization sensitivity is measured. In addition to the polarization insensitivity, channel crosstalk is measured to be <-27 dB in 315 m of high nonlinearity fiber. In a detailed experimental study, the pattern of crosstalk in longer fiber lengths and the coupling between the polarization sensitivity and crosstalk are measured. For example, with a 430-m fiber length, we measure the degradation in polarization sensitivity to be /spl sim/4 dB for 12-dB increased signal power. The experimental results are also confirmed by theoretical calculations. Moreover, in a 32 channels systems simulation, the signal-to-noise ratio (SNR) of the converted signals after 800-km propagation is calculated to be only 0.8-dB degraded compared to using laser diodes with the same initial SNR values. Furthermore, we calculate the effect of pump noise and show that the relative intensity noise of the pump is transferred to the converted signals with an additional 8-dB/Hz degradation.

Liquid crystal-based self-aligning &lt;inline-formula&gt;&lt;math display="inline" overflow="scroll"&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;mo&gt;×&lt;/mo&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/math&gt;&lt;/inline-formula&gt; wavelength routing module

A novel retroreflective polarization insensitive 2x2 fiber optic switch (FOS) using programmable quarter waveplates (QWPs) is introduced. Key features include low polarization sensitivity and a selfaligning module via a compact retroreflective structure. A proof of concept 2 x 2 FOS using ferroelectric liquid crystal (FLC)-based programmable QWPs is demonstrated, providing a response speed of 394 ?s and a low 10.14-mW total electrical power consumption for the worst case. Multiwavelength switch operation in the 1550-nm wavelength region indicates an average optical loss of 6.02 dB with a ±0.18-dB loss fluctuation, and an optical coherent crosstalk of -21.6 dB with a ±0.84 dB variation over a 60-nm bandwidth. The temperature dependence of our switch is investigated over a 17°C range, indicating an average 5.80-dB optical loss with a ±0.13-dB fluctuation, and an average optical coherent crosstalk of -20.5 dB with a fluctuation of ±0.42 dB. An average polarization dependent loss of 0.4 dB is also measured. A time-multiplexed FLC device addressing technique is introduced for FOS to achieve long duration operation.

An asymmetric twin-waveguide high-bandwidth photodiode using a lateral taper coupler

We describe a high-bandwidth high-responsivity low-polarization sensitivity p-i-n photodiode based on an integratable asymmetric twin-waveguide structure. Incident light is collected by a diluted large-fiber guide followed by transfer to a thin-coupling waveguide using a low-loss lateral-taper coupler. The light is finally absorbed by the uppermost In/sub 0.53/Ga/sub 0.47/As layer. The device has a responsivity of (0.75/spl plusmn/0.03) ampere per watt and a polarization sensitivity of /spl les/0.4 dB. The measured electrical 3 dB bandwidth is /spl ges/40 GHz. The responsivity is comparable with the best 40-GHz waveguide p-type-intrinsic-n-type photodiodes, while the twin-waveguide design provides a single-epitaxial growth step and a simple means of fabrication with possibility for monolithic integration of the photodiodes with other optical components, such as semiconductor optical amplifiers and in-plane waveguide filters.

Compact wide-band polymer wavelength-division multiplexers

Wide-band wavelength-division multiplexers (WDMs) have been developed for applications in short-haul and satellite communications. Device designs based on Mach-Zehnder interferometers (MZIs) and arrayed waveguide gratings (AWGs) have been fabricated using AlliedSignal Inc. low-loss acrylate polymer materials. Low channel-counts and channel separations of 5-30 nm result in compact devices with relaxed processing tolerances, thus minimizing the cost and increasing the yields of the devices. Operating at around 1550 nm, the multiplexers offer crosstalk of 20-25 dB, on-chip loss of <10 dB, thermal stability within several degrees and low polarization sensitivity.

3.7 ps pulse generation at ≥ 30 GHz by dual-driveelectroabsorption modulator

The authors describe optical short pulse generation at frequencies ≥ 30 GHz by a dual-drive scheme of a high-saturation power travelling-wave electroabsorption modulator. Sub-4 ps almost-transform-limited pulses are achieved with &gt; 20 dB dynamic extinction ratio and low polarisation sensitivity.

High-density and alignment-tolerant integration of monitoring photodetector arrays onto polymeric guided-wave components

We report on a hybrid integration concept which allows high-density and alignment-tolerant surface mount assembly of top-illuminated photodetector (PD) arrays onto polymeric guided-wave components by means of a micromachined submount with an integrated mirror. This research aims at the development of polymeric waveguide based devices with an embedded optical power monitoring capability for applications in optical fiber communication networks. A prototype integrated multichannel optical power monitor comprising an eight-channel PD module and a polymeric waveguide splitter array was demonstrated which exhibited an average insertion loss of 5.2 dB, a PD coupling efficiency of -2.6 dB, and a crosstalk value of -17.1 dB between adjacent channels. Low polarization and wavelength sensitivity (/spl sigma/<0.1 dB) was confirmed over a spectral range of 40 nm in both telecom wavelength windows.

Deformation of ferroelectric short-pitch helical liquid crystal by transverse electric field: Application for diffraction-based light modulator

The electro-optic effect in helical ferroelectric liquid crystal deformed by transverse electric field is exploited to realize a diffraction-based light modulator. Experiments show that a contrast ratio better than 100:1 is easily achievable for the short-pitch liquid crystal aligned homeotropically with oblique readout breaking the initial symmetrical geometry. The modulation obtains low polarization sensitivity for selected directions of the readout.

Simple method of fabricating polarisation-insensitiveand very low crosstalkAWG grating devices

A simple method of fabricating polarisation insensitive and very low crosstalk dense-WDM devices has been developed. The matching of the thermal coefficient of expansion (TCE) of the overcladding layer to the silicon substrate, for a 16 channel arrayed-waveguide demultiplexer, resulted in very low polarisation sensitivity (0.03 nm). The TCE of the undercladding and the core layers, on the other hand, were found to have negligible effect on the polarisation sensitivity. PECVD was used for depositing all the oxide layers.

80 Gbit/s OTDM using electroabsorption modulators

An 80 Gbit/s (8 × 10 Gbit/s) OTDM system, constructed entirely with electroabsorption modulators (EAM) as the active components, is presented. A tandem EAM pulse source with a simple dual frequency drive generates transform limited 4.5 ps pulses. At the receiver, the 10 Gbit/s channels are demultiplexed from 80 Gbit/s in a single step using a low polarisation sensitivity EAM. Error free operation with a sensitivity of –21.5 dBm is achieved.

Low polarisation sensitivity electroabsorption modulators for 160 Gbit/s networks

Packaged low polarisation sensitivity InGaAs/InAlAs MQW electroabsorption modulators employing a novel ridged deeply etched buried heterostructure design are described. The feasibility of using them in 160 Gbit/s OTDM systems is experimentally assessed.

Silicon-on-insulator (SOI) phased-array wavelength multi/demultiplexer with extremely low-polarization sensitivity

We demonstrate the first phased-array wavelength multiplexer fabricated in the silicon-on-insulator (SOI) waveguide technology. The four-channel wavelength division multiplexer (WDM) has a channel spacing of 1.9 nm centered at 1550-nm wavelength and a 3-dB channel bandwidth of 0.72 nm. The crosstalk to neighboring channels is less than -22 dB and the on-chip insertion loss is below 6 dB for all channels. The TE-TM shift is less than 0.04 nm which is the smallest attained without compensation techniques in any integrated optic technology.

All-fiber acoustooptic filter with low-polarization sensitivity and no frequency shift

An acoustooptic tunable filter based on an all-fiber null coupler, exhibiting low polarization sensitivity and zero net frequency shift, is demonstrated. The polarization dependence has been eliminated by simultaneously applying two acoustic waves that independently satisfy the phase-matching condition for the two eigenpolarization states. A simple double-pass arrangement is employed to exactly cancel the net frequency shift and to provide an enhanced spectral sidelobe suppression of better than -17 dB.

Polarization diversity waveguide grating receiver with integrated optical preamplifiers

We present an 8/spl times/100 GHz waveguide grating multifrequency receiver in InP with low polarization sensitivity with integrated optical preamplifiers and detectors. The polarization insensitivity is achieved through a special polarization diversity scheme in which both inputs share the same waveguide grating and photodetectors. Also, we show theoretically how chirping the waveguide grating in optically preamplified receivers can improve the sensitivity.

All-optical 1300-nm to 1550-nm wavelength conversion using cross-phase modulation in a semiconductor optical amplifier

All-optical 1300-nm to 1550-nm wavelength converters may be important components in lightwave networks which use both the 1300-nm and the 1550-nm low-loss transmission windows of silica optical fiber. We describe a new all-optical 1300-nm to 1550-nm wavelength converter, based on cross-phase modulation in a 1300-nm semiconductor optical amplifier. We demonstrate operation of the wavelength converter at 1.25 Gb/s, and present bit-error rate measurements. The wavelength converter demonstrated here potentially operates at high speed, with low input power and low polarization-sensitivity.

Regenerative 20 Gbit/s wavelength conversion anddemultiplexing using asemiconductor laser amplifier nonlinear loop mirror

A nonlinear loop mirror containing a semiconductor laser amplifier is used to perform all-optical wavelength conversion and demultiplexing at 20 Gbit/s. This technique has a low polarisation sensitivity and offers a regenerative function where jitter in the received data is considerably reduced.