Submicrosecond speed variable optical attenuator usingacoustooptics

A novel variable optical attenuator (VOA) is introduced that can provide high dynamic range and high-resolution all at fast submicrosecond speeds. The VOA design exploits an in-line one-to-one imaging optical architecture operating with Bragg-mode acoustooptic Bragg cells. Optical attenuation is achieved via frequency and drive voltage control of the Bragg cells, leading to flexible and precise attenuation-level settings. VOA demonstrated a 45-dB dynamic range with a 175-ns switching time. Other measured parameters include 1.14-dB/MHz resolution, 1.56-dB polarization dependent loss, and an excess loss of 3.55 dB     

MOEMS variable optical attenuator with improved dynamic characteristics based on robust design

The design and preliminary data of a novel microoptoelectromechanical systems variable optical attenuator (VOA) driven by a pair of V-beam electrothermal actuators is described. This VOA deploys a face-to-face arranged pair of 45/spl deg/ tilted mirrors in front of two coaxially aligned lensed fibers to form retro-reflection planar light path for attenuation. The initial insertion loss is 0.7 dB at 1550 nm and the maximum dynamic range of attenuation is 50 dB, respectively. The polarization-dependent loss is measured as 0.15 dB at 20-dB attenuation. The dynamic attenuation deviation is less than /spl plusmn/0.36 dB at 20-dB attenuation with respect to 20-G shock of periodical mechanical vibration at 1 K Hz, in which it complies with requirements of the Telcordia GR1221 regulations.     

Segmented-clad fiber design for tunable leakage loss

This paper proposes a novel segmented-clad fiber (SCF) design in which the spectral variation of leakage loss of the fundamental mode can be finely tuned by varying the fiber parameters. A fiber with such optimized spectral variation of leakage loss should find application in the realization of inherently gain-flattened optical fiber amplifiers, wavelength filters, wavelength-flattened attenuators, etc. In this paper, the authors present SCF designs optimized for realizing inherently gain-flattened S-band erbium-doped fiber amplifier (EDFA) and high-gain discrete Raman fiber amplifier (RFA). Amplifier characteristics have been modeled in both cases, and simulations show that a 20-dB gain with /spl plusmn/0.9 dB of gain ripple over a 30-nm bandwidth in S-band is achievable with the designed EDFA based on the optimized SCF. In case of discrete RFA based on SCF, a 20-dB net gain with /spl plusmn/0.5-dB ripple is shown to be achievable over a 28-nm wavelength range in S-band.     

A MEMS VOA Using Electrothermal Actuators

A comprehensive study of electrothermally driven microelectromechanical system (MEMS) variable optical attenuator (VOA) devices using an H-shaped structure is presented in this paper. Based on its unique structural design, a retroreflection-type VOA of smaller footprint is realized. The repeatability and stability of the static and transient characteristics of attenuation behavior at various ambient temperatures are characterized. The fluctuation of attenuation curves under the same driving voltage at the same ambient temperatures is less than plusmn0.1 dB. Again, comparing the attenuation curves measured at 25 degC to 75 degC and at 25 degC to 12.5 degC, the deviation of attenuation under the same driving voltage is within the 0.6-dB range. Within the 40-dB attenuation range, the measured switching time from nonattenuation state to a particular attenuation state or between two attenuation states is less than 10 ms. This electrothermally actuated MEMS VOA also demonstrates the state-of-the-art dynamic attenuation stability that complies with the Telecordia GR1221 regulations, where the dynamic fluctuation of attenuation at 20 dB is less than plusmn0.36 dB under a vibration testing condition of 20 G periodical shocks with frequency from 20 Hz to 2 kHz     

Broadband All-Digital Variable Fiber-Optic Attenuator Using Digital Micromirror Device

To the best of our knowledge, we propose the first broadband variable fiber-optic attenuator (VFOA) using a digital micromirror device (DMD). The VFOA design is based on a unique double reflection geometry structure that counters the strong spectral attenuation inherent when implementing single-mode-fiber light coupling via a pixelated blazed grating device such as a DMD. The demonstrated VFOA features ~C-band 1530- to 1560-nm broadband attenuation operation with a plusmn0.17-dB in-band variation and a 25-dB attenuation dynamic range. This high repeatability all-digital VFOA can be useful in broadband optical source controls, test instrumentation, telecommunications, signal processing, and sensor systems.     

Retro-Axial VOA Using Parabolic Mirror Pair

This letter presents a design of a microelectromechanical systems variable optical attenuator (VOA) that employs a pair of parabolic mirrors as the retro-reflector, which has obtained a linear relationship over a 62-dB range between the attenuation (in decibels) and the mirror rotation angle (in degrees). The insertion loss measures 0.6 dB thanks to the three-dimentional optical coupling design. The linearity comes from the simultaneous shift and defocus of the laser beam. Compared with the conventional coaxial and cross-axial VOAs, such retro-axial design has the two fibers arranged on the same side and thus facilitates the use of standard packaging formats     

Digital optical space switch based on micromotor grating scanners

We present the design of a new class of optical space switch based on the use of grating beam deflectors mounted on electrostatic micromachined micromotors. The micromotors are used to position the beam deflectors according to the selected output channels. Due to the nature of the micromotor movement, switching is digital. Switching speeds of up to 12 ms have been achieved, with a maximum crosstalk of 61.9- and 22.94-dB attenuation. This high attenuation is mainly due to the simple grating fabrication process used, and could be greatly reduced with a more specialized process. Calculations of scalability are also presented.     

BOTDA-nondestructive measurement of single-mode optical fiberattenuation characteristics using Brillouin interaction: theory

A theoretical investigation of Brillouin optical-fiber time-domain analysis (BOTDA) is described. BOTDA uses Brillouin interaction in optical fibers to analyze the attenuation characteristics of the optical fibers nondestructively. The dynamic range performance of BOTDA is approximately 10-dB greater than that of conventional optical time-domain reflectometry     

Broad-Band Microwave Measurements on GaAs "Traveling-Wave" Transistors

Instantaneous gain, noise figure, reverse attenuation, and gain and phase control measurements in the frequency range 8-18 GHz have been performed on GaAs traveling-wave transistors. The broad-band high-gain nature of the device together with the requirement for several bias connections precluded the use of standard test fixtures, and resulted in a package design exhibiting less than 1-dB insertion loss over the band together with 75- to 90-dB internal isolation. Untuned X-band gain, noise figure, and reverse attenuation were 12 dB, 18 dB, and 32 dB, respectively, and the gain and phase could be electronically varied over a 35-dB and 360/spl deg/ range. When RF tuning was employed, the gain, on the average, improved by 10 dB.     

Design of bidirectional communication systems with opticalamplifiers

The performances of four different configurations implementing bidirectional communication systems with erbium-doped fiber optical amplifiers in a 622-Mb/s IM/DD system are compared. The transmission span and the tolerance to variation in link attenuations (attenuation margin) are studied as a function of the amplifiers' position in the link. Operating the amplifiers slightly into saturation (0.4 dB) turns the configurations robust with respect to changes in link attenuations (4.0-dB improvement in the tolerable increase of attenuation, for a span of 76.9 dB)     

Design of a 4-MHz analog integrated CMOStransconductance-C bandpass filter

A continuous-time eighth-order fully integrated CMOS transconductance-C bandpass filter is presented for operation at 4 MHz with 800 kHz bandwidth and 0.5 dB pass-band ripple. A phase-locked loop for frequency tuning and four-point amplitude-locking loop for Q-factor tuning at the reflection zeros of the filter are implemented. The transfer characteristics were found to be essentially within specifications: less than 1-dB passband attenuation, 75-dB stopband attenuation and S/N ratio, and 0.5% harmonic distortion for 0.5-V_p-p signal were observed. Offset of the transconductances was internally controlled by an offset-control loop to less than 4 mV. A temperature-insensitive transconductance design and the noise characteristics of the filter building blocks are also discussed     

Fiber-optic multigigabit GaAs MIC front-end circuit with inductorpeaking

The design and performance of a multigigabit optical front-end circuit are discussed. Inductor peaking is applied to the GaAs MIC preamplifiers and a 3-dB down bandwidth of 6.5 GHz, 15.5-pA/√Hz averaged input equivalent noise current density from 10 MHz to 6.5 GHz, and transimpedance gain of 57 dB are achieved. A 3-dB down bandwidth of 6.1 GHz is achieved in an optical front-end circuit with a InGaAs p-i-n photodiode. This performance indicates that the optical front-end circuit with inductor peaking is promising for multigigabit optical receivers     

All-optical controlled variable optical attenuator using photochromic sol gel material

In this letter, we report an all-optical controlled polarization-dependent variable optical attenuator using a photochromic sol gel material developed by us. Optical attenuation is achieved by means of photochromic phase modulation in a Mach-Zehnder interferometric waveguide device fabricated with the sol gel film. A 5-mW/mm/sup 2/ light illumination at 532 nm on a 3-mm-long sol gel arm was found to produce a complete 2/spl pi/ phase shift between the two arms of the device, resulting in a 15-dB attenuation dynamic range for transverse-electric-polarization light. The response time of the device is around 50 ms. The wavelength-dependent performance of the device has been characterized across the entire C-band.     

An efficient all-fiber variable optical attenuator via acoustoopticmode coupling

A novel broad-band variable optical attenuator is realized using acoustooptic mode coupling on a cladding etched single-mode fiber. Broad-band coupling bandwidth with low attenuation ripples is achieved by matching both the group index and dispersion parameter between the core and the LP₁₁^cl cladding modes. A single-section device exhibits a 10-dB dynamic range with attenuation ripples of <0.5 dB over a spectral range of >100 nm, a response time of 20 μs, an insertion loss of 0.3 dB, and a maximum power consumption of only 3.4 mW. Attenuation of 22 dB is obtained for a cascaded two-section device     

Monolithic ultra-broadband transimpedance amplifiers usingAlGaAs/GaAs heterojunction bipolar transistors

Monolithic ultra-broadband transimpedance amplifiers are developed using AlGaAs/GaAs HBTs. To realize good amplifier performances, two factors are mentioned: an affordable HBT fabrication process using the self-aligned method and an optimized circuit design considering large signal operations. The developed HBT fabrication process achieves excellent uniformity in DC characteristics and the effect on amplifier microwave performances, derived from the discrete device uniformity, is estimated. Amplifier circuit configurations are designed by harmonic balance simulation using the extracted large signal device parameters The fabricated amplifier exhibits a DC to 13.4-GHz bandwidth with an 18.1-dB gain. Fairly good uniformity is also achieved for the amplifier microwave performances. An optical receiver module is constructed mounting the developed HBT amplifier and InGaAs p-i-n photodiode chips. The optical receiver module provides a 9.4-GHz bandwidth and an optical receiver sensitivity of -15.7 dBm at 10-Gb/s data rate     

Packaging technology for 40-Gb/s optical receiver module with anMU-connector interface

A compact 40-Gb/s optical receiver module with an MU-connector interface has been developed. Its packaging has three main technical features. (1) Coplanar waveguide (CPW) patterns of the waveguide photodiode (WG-PD) and of the preamplifier IC in the facing area of the flip-chip structure are optimized for impedance matching. (2) A film carrier is used to connect the preamplifier IC to an electrical coaxial connector for electrical signal output. (3) An MU-connector is used as the optical interface to reduce the module size. Optimum design enabled a module size of 14.0 mm wide, 40.4 mm long, and 9.65 mm high. Measurements showed a 3-dB down bandwidth of the optical/electrical response of at least 50 GHz and a clear open eye pattern for a 40-Gb/s nonreturn-to-zero (NRZ) signal input. This optical receiver module is suitable for large-capacity communication network systems     

High-Speed High-Isolation 2$, times ,$2 Fiber-Optic Switch for Wideband Radar Photonic Beamforming Controls

A new high-performance 2$, times ,$2 fiber-optic switch is designed and demonstrated for wideband radar photonic beamforming controls. The switch deploys two bulk acoustooptic deflectors (AODs) in an imaging free-space symmetric optical design that exploits image inversion control via a Dove prism to form a 2$, times ,$2 fully reversible low crosstalk noise high-speed switching structure. Experiments at the 1550-nm test wavelength show the switch to handle 0.5-W level optical input powers, $≪ {hbox{2.2-}}mu$s switching time, $≪ $2.6-dB fiber-to-fiber optical loss, better than 56-dB optical crosstalk levels, and $≪$0.2-dB polarization-dependent loss (PDL).      

Designing IIR filters with a given 3-dB point

Often in infinite impulse response (IIR) filter design, our critical design parameter is the cutoff frequency at which the filter's power decays to half (-3 dB) the nominal passband value. This article presents techniques that aid in the design of discrete-time Chebyshev and elliptic filters given a 3-dB attenuation frequency point. These techniques place Chebyshev and elliptic filters on the same footing as Butterworth filters, which traditionally have been designed for a given 3-dB point. The result is that it is easy to replace a Butterworth design with either a Chebyshev or an elliptic filter of the same order and obtain a steeper rolloff at the expense of some ripple in the passband and/or stopband of the filter.     

Modeling and design of GaAs traveling-wave electrooptic modulators based on the planar microstrip structure

GaAs traveling-wave electrooptic modulators based on the planar microstrip structure (PMS) have been thoroughly characterized theoretically and a near-optimal design is proposed. The optical, microwave, and modulator performance are investigated in detail. The structure is simple to fabricate and so should provide cost advantages. An electrical 3-dB bandwidth of about 15 GHz (optical 3-dB bandwidth of about 40 GHz) is obtained for our best design.     

Rayleigh scattering influence on performance of 10 Gb/s opticalreceiver with Er-doped optical fiber preamplifier

The achievement of -30.8 dBm (630 photon/bit) receiver sensitivity at 10 Gb/s, with an Er³⁺-doped optical fiber preamplifier, is discussed. This is an 8.3-dB sensitivity improvement over the avalanche-photodiode/FET receiver. Power penalties caused by a noise increase due to Rayleigh backscattering by the transmission optical fiber have been evaluated. Approximately -30-dB Rayleigh scattering from a 20-km optical fiber resulted in a 3.5-dB power penalty for a 25-dB-gain optical amplifier