Fast Tunable Laser Research Articles

Development of an Optical-Burst Switching Node Testbed and Demonstration of Multibit Rate Optical Burst Forwarding

With the rapid growth in Internet traffic, demands for a high-capacity switching network with low end-to-end latency are being felt. Optical-label switching, in general, and optical burst switching (OBS), in particular, can provide such a solution with realizable device technology. This paper is a summary of the Photonic Network Project undertaken in Japan, wherein we have implemented a prototype OBS-node testbed based on novel device concepts. A three-node OBS core testbed and edge nodes operating over payload bit rates of 10 Gb/s to 40 Gb/s were developed, demonstrating feasibility of the OBS concept. We report newly developed key photonic building blocks of the OBS node, namely: a fast matrix switch based on lead-lanthanum-zirconate-titanate, a bit-rate transparent tunable wavelength converter based on a monolithically integrated wavelength converter; a fast tunable wavelength laser diode; compact arrayed waveguide amplifiers; and athermal, flat-top multiplexers. Together, with an optical-label recognition scheme implemented in a field-programmable gate array (FPGA), we have integrated the building blocks into an optical burst switching-node prototype that can potentially be scaled up to 64 times 64 ports. We have confirmed error-free optical burst forwarding for more than 10 hops in an OBS network with variable length, asynchronous burst arrival. We have also achieved a mixed bit-rate optical burst forwarding and contention resolution with wavelength conversion. We have also demonstrated coordination of OBS with upper-layer protocols by implementing an Ethernet-frame-to-optical-burst converter edge node, which successfully transmitted real application data over the OBS network at 40 Gb/s. Similar results were achieved in a multinode testbed under the field fiber environments, with a low frame error rate of less than 1 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> .

Monolithic AWG-based Discretely Tunable Laser Diode With Nanosecond Switching Speed

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel concept for an arrayed-waveguide-grating (AWG)-based fast tunable laser is presented. It is fabricated in the InP–InGaAsP monolithic integration technology. Laser peaks have a sidemode suppression ratio of 30–40 dB. The wavelength switching speed is in the order of a few nanoseconds and switching is achieved by a 1-mA bias current. The switching between AWG channels is discrete and no laser operation takes place at wavelengths corresponding to other channels during the tuning process. </para>

Optimization of Wavelength-Locking Loops for Fast Tunable Laser Stabilization in Dynamic Optical Networks

This paper presents a theoretical model of a wavelength-locking loop for stabilization of the output wavelength in multisection tunable lasers for their application in future dynamic optical burst-switched networks. The linearized theoretical model was used to derive expressions for the optimal PID coefficients of a control circuit for etalon-based wavelength-locking control loop as a function of the key loop parameters. The validity of the model was then experimentally verified by measurements of locking performance of a digital supermode-distributed Bragg reflector tunable laser based on the proposed model.

Submicrosecond Scan-Angle Switching Photonic Beamformer With Flat RF Response in the C and X Bands

A wideband (>10 GHz) beamformer, based on a photonic true-time-delay, with submicrosecond scan-angle switching is reported. The smooth microwave transmission (ripples <0.5 dB and <3deg) and superb uniformity among the elements (<0.1 dB and <0.5deg) are then used for the processing of 1 GHz-wide linear frequency modulation (LFM) signals in both the C and X bands with excellent performance. This performance is also maintained under dynamic operation, where a fast tunable laser is employed to provide <300 ns wavelength-controlled angle scanning. Based on these characteristics, an optimized architecture, where a photonic beamformer feeds a series of classical subarrays, can offer high performance in both the time and spatial domains for large, wideband phased-array antennas, with wide scan angles.

Towards standards for measuring greenhouse gas fluxes from agricultural fields using instrumented towers

This is a discussion of the available technology for measuring turbulent fluxes using instrumented towers. This review focuses on the flux measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) for agricultural systems and the development of standards and protocols for measuring them. Agroecosystems present unique challenges because they undergo large leaf area index (LAI) and canopy architecture changes in a relatively short period of time (i.e., months) coupled with the fact that many of the greenhouse gas sources are diffuse. This review examines all aspects of the theory and application of the micrometeorological techniques, with focus on the flux gradient, eddy accumulation and eddy covariance techniques. Instrument placement, sens or response and noise characteristics are also explored. Innovative applications of micrometeorological methods are discussed for closed- and open-path trace gas sensors and commonly used meteorological instrumentation. The use of fast response single-pass optical tunable diode laser (i.e., CH4, N2O) and infrared gas analyzers (i.e., CO2, H2O) is described. Consideration is also taken of the trace gas sensors’flow system design, mixing ratio measurement, and data acquisition and reduction requirements for micrometeorological flux measurement. Procedures are outlined for the meteorological instrumentation necessary for eddy covariance-based energy budget measurement including ultrasonic anemometry. Key words: Tower-based greenhouse gas flux measurements, nitrous oxide, methane, carbon dioxide, tunable diode laser

Demonstration of an agile hybrid IP-optical packet construction mechanism in wavelength routed optical packet switched networks

This paper presents a novel architecture for an ingress edge OPS router and demonstrates a mechanism that maps Internet traffic onto optical packets. The architecture utilises a high-speed reconfigurable hardware platform and a fast tunable laser and also supports the user network interface (UNI) functionality by fully interacting with the physical and higher network layers. The main design issues, including wavelength agility, traffic aggregation based on the class of service and variable length optical packet construction and transmission are also discussed.

Low-loss wavelength-multiplexed optical scanners using volume Bragg gratings for transmit-receive lasercom systems

Low-loss, no moving parts, free-space wavelength-multiplexed optical scanner (W-MOS) modules for transmit-receive lasercom systems are proposed and experimentally demonstrated. The proposed scanners are realized using volume Bragg gratings stored in dichromated gelatin (DCG) coupled with high-speed wavelength selection, such as by a fast tunable laser. The potential speed of these scanners is in the gigahertz range using present-day state of the art nanosecond tuning speed lasers. A 940-lines/mm volume Bragg grating stored in dichromated gelatin is used to demonstrate the scanners. Angular dispersion and diffraction efficiency of the volume Bragg grating used for demonstration are studied, versus wavelength and angle of incidence to determine the free-space W-MOS angular scan and insertion loss, respectively. Experimental results show that a tunable bandwidth of 80 nm, centered at 1560 nm, delivers an angular scan of 6.25 deg. The study also indicates that an in-line scanner design realized using two similar Bragg gratings in DCG delivers a 13.42-deg angular scan, which is more than double the angular scan available from the free-space W-MOS using a single volume Bragg grating. Furthermore, a free-space W-MOS using a single Dickson grating features low (<0.15 dB) polarization-dependent loss and an average scanner insertion loss of only 0.4 dB over the 70-nm wavelength band around 1550 nm.

Fast tunable laser based on Fabry-Perot lasers with optical injection

We have proposed and experimentally demonstrated a new tunable laser structure, which is based on Fabry-Perot (FP) lasers with external lightwave injection. The wavelength tuning can be obtained by adjusting the bias currents of FP lasers. The wavelength tuning time of 19 dB have been achieved experimentally. In addition, the SMSR performance has also been investigated. This tunable laser has the advantage of simple architecture, potentially low cost, data direct modulation, and fast wavelength tuning, and is expected to benefit the applications of fast wavelength tuning.

Fast Wavelength-Tunable Laser Technique Based on a Fabry–PÉrot Laser Pair With Optical Interinjection

A new fast tunable laser technique, which is based on a Fabry-Pe/spl acute/rot (FP) laser pair with optical interinjection, has been proposed and experimentally demonstrated. By adjusting different bias currents of FP lasers, the wavelength tuning can be realized. In the experimental demonstration, a 2.3-nm tuning range with three different wavelengths, the sidemode suppression ratio (SMSR) of >20 dB, and the wavelength switching time of <6.8 ns have been reached. In addition, the SMSR performance versus injection power level has also been investigated. This tunable laser has the advantage of simple architecture, data direct modulation, and fast wavelength tuning.

Demonstration of stable wavelength switching on a fast tunable laser transmitter

We describe a new technique to investigate continuous wavelength switching between 32 ITU channels in a tunable laser transmitter by switching the laser once every microsecond. We examine the effect of different switching schedules on the switching of the laser, an important issue for commercial switch fabrics for optical packet routing. Switching to all 32 channels of the laser is characterized in terms of a "packet-loss rate" (PLR) due to wavelength switching. PLRs are studied as a function of the switching-time window for a laser to tune to an ITU channel. Error-free wavelength switching in less than 50 ns is demonstrated for 26 hours.

Dynamic demonstration of diffractive optic analog-to-digital converter scheme.

Dynamic behavior of an analog-to-digital converter (ADC) based on diffractive optical element(s) (DOE)(s) was studied and found to be in agreement with predictions. The analog signal was translated to an angular deflection of a laser beam by means of an acousto-optic (AO) cell. The number of bits in this experimental demonstration was three, using an eight-element DOE array. The maximum sample rate was found to be 2.5 MS/s, the limiting factor being the transit time for the acoustic wave across the width of the laser beam in the AO cell. The study is intended as a first dynamic demonstration of a proposed ADC scheme previously demonstrated in a quasi-static version. The full potential of the ADC scheme will require the use of a fast tunable diode laser to replace the AO deflection scheme used here.

HORNET: a packet-over-WDM multiple access metropolitan area ring network

Current metropolitan area networks (MANs) based on the SONET transport are not developing at the rate required to support the phenomenal increase in data traffic. To address the needs of future MANs, the Optical Communications Research Laboratory at Stanford University and Sprint Advanced Technology Laboratories are building HORNET (Hybrid Optoelectronic Ring NETwork). HORNET has a multiple access architecture, in which nodes access any WDM channel using a novel media access control protocol and fast tunable laser transmitters. HORNET transports data packets directly over the WDM ring, eliminating the SONET transport. This paper presents the HORNET architecture, the node design consisting of novel packet-over-WDM components, and the experimental testbed with results.

Fast response tunable diode laser spectroscopy for trace gas flux measurements

A fast tunable diode laser spectrometer was applied for measurements of N 2O emissions from a harvested wheat field located in Sealand, Denmark. The dual-channel instrument uses two tone frequency modulation and signal detection at 11 MHz in conjunction with rapid (1 ms) scanning of the laser. A dichroic beam combiner and a mechanical chopper allow time multiplexing. The absorption signals are recorded and analysed on-line. The combination of a software, current and temperature controlled line locking scheme results in high stability of the instrument. The time response (200 ms) of the spectrometer is limited by the measurement-cell gas exchange. For eddy correlation measurements, the trace gas mixing ratio and wind data were sampled synchronously at a rate of 10 Hz using an interrupt-driven algorithm. Mean N 2O flux detection limits were 6 pptv m s −, corresponding to a deposition velocity of 2 × 10 −5 m s − for a half hour measurement time. In addition, concentration gradients were determined from measurements at four different inlet heights at 0.1 ppbv precision in a shared integration time of 30 min. These measurements resulted in a flux detection limit of 7 pptv m s −1 ror the boundary layer mixing conditions encountered.

Measurements of N2O fluxes from fertilized grassland using a fast response tunable diode laser spectrometer

A fast response tunable diode laser spectrometer was used to make N2O flux measurements by both eddy correlation and concentration gradient techniques during a methods intercomparison field program in April 1992 at a site in Stirling, Scotland. A description of the site and the results of the intercomparison are presented in companion papers. Sufficient instrumental precision and time resolution for N2O flux determination using both techniques were obtained by application of the recently developed two‐tone frequency modulation coupled with fast scanning of the laser. The use of a dedicated digital signal processor allowed zero‐overhead on‐line data handling at a rate of 10 Hz such that the time response of the system was only limited by the gas exchange time in the multipass sample cell (200 ms). Vertical concentration gradients that lead to a difference of ≤1 part per billion by volume in the N2O mixing ratio at 0.06‐ and 1.05‐m elevation were statistically resolved within 1 min. Eddy correlation measurements with intake heights of 2.25 m and 2.75 m were made in conjunction with two different sonic anemometers. The software developed for reduction and analysis of the 10‐Hz eddy correlation data was based on time efficient FFT methods and performed time‐base matching of the data set, drift correction, coordinate rotation, and evaluation of the covariances and the frequency power distributions. N2O fluxes determined with this technique were in the range of 38–113 ng N m−2 s−1. An unusual shape of the N2O concentration covariance function can be ascribed to fetch inhomogeneities, and we conclude that the eddy correlation data set contains information that can be used to characterize the spatial variability in N2O emission in addition to its spatial integral.

New advances on optical components needed for FDM optical networks

Two critical components, needed for optical networks based on frequency division multiplexing (FDM) and frequency reuse technologies, are presented. One is a fast tunable laser, yielding 24 discrete frequencies regularly spaced by 40 GHz around 1.53 mu m. The laser can be switched randomly in less than 8 ns among these frequencies. Frequency routing of a 3-Gb/s bit stream in 10-ns packets between ten different destinations has been demonstrated with this laser. The second component is a broadband wavelength shifter capable of switching multigigabit data between optical frequencies in the 1.5- mu m region. The device is tunable and cascadable, provides conversion gain, and is nearly polarization insensitive. Data degradation after wavelength shifting is negligible. Such a device provides the means of reusing the limited set of frequencies given by the lasers and thus of realizing large size networks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fast optical packet switching based on WDM

An optical switch capable of routing a 3-Gb/s bit stream in 10-ns packets between ten different destinations is demonstrated. Switching time between destinations is 2 ns. The switch is based on wavelength division multiplexing (WDM) technology. It uses a fast tunable laser capable of addressing 24 discrete optical frequencies spaced by 40 GHz. >

Discretely-tuned N -frequency laser for packet switching applications based on WDM

A fast tunable laser for packet switching applications based on wavelength-division multiplexing (WDM) is presented. The laser is tunable to 16 discrete frequencies regularly spaced by 43 GHz over a tuning range of 645 GHz centred around a wavelength of 1.53 mu m. It can be switched randomly in less than 5 ns among these frequencies. Importantly, the same comb of frequencies is obtained from different lasers fabricated by the same lithographic process. This greatly simplifies the implementation of optical WDM systems.

Pulsed photothermal radiometry in turbid media: internal reflection of backscattered radiation strongly influences optical dosimetry

The integrated irradiance (energy fluence rate) within tissue can exceed the incident irradiance due to backscattered and multiply reflected light near the sample surface. This was studied quantitatively using pulsed photothermal radiometry, which measures blackbody radiation emitted by a sample during and after absorption of an optical pulse. Aqueous gels containing absorbing dye with or without various scattering materials were studied using a fast sensitive IR detector system and 1-micros tunable pulsed dye laser. For nonscattering samples, the temperature transient (T-jump) due to absorption of a laser pulse was consistent with Beer's law for homogeneous absorbing media. When scattering was present, increases of up to almost an order of magnitude in the T-jump were observed. For a given absorption coefficient, there was a proportional relationship between the increase in the T-jump and the sample's diffuse reflectance. A model describing the reflectance of diffuse radiation at the sample boundary was derived to explain this result. To test the model, the refractive index was varied with air as the external medium and was also matched to that of BaF(2) as the externalmedium. The subsurface fluence is, to areasonable approximation, given by E congruent with E(0)(l + 2bR),where E(0) is the incident fluence of an infinitely wide collimated beam, b is a coefficient strongly dependent on only the refractive index, and R is the measured diffuse reflectance of the sample. This study shows that irradiance within tissues can greatly exceed the irradiance of incident collimated light, an effect that should be accounted for in photomedical dosimetry or research.