Optical Power Levels Research Articles

Comparative analysis of triple band S-C-L erbium-doped fibre and semiconductor optical amplifier for CWDM applications

The authors present a comparative analysis between a triple-band S-C-L erbium-doped fibre amplifier and a commercial semiconductor optical amplifier in a CWDM application scenario. Both technologies were characterised for gain and noise figures from 1480 to 1610 nm (S, C and L bands) and their systemic performances were evaluated in terms of bit error rate measurements for a wide range of optical power levels.

High-performance carbon nanotube coatings for high-power laser radiometry

Radiometry for the next generation of high-efficiency, high-power industrial lasers requires thermal management at optical power levels exceeding 10 kW. Laser damage and thermal transport present fundamental challenges for laser radiometry in support of common manufacturing processes, such as welding, cutting, ablation, or vaporization. To address this growing need for radiometry at extremely high power densities, we demonstrate multiwalled carbon nanotube (MWCNT) coatings with damage thresholds exceeding 15 000 W/cm2 and absorption efficiencies over 90% at 1.06 μm. This result demonstrates specific design advantages not possible with other contemporary high-power laser coatings. Furthermore, the results demonstrate a performance difference between MWCNTs and single-walled carbon nanotube coatings, which is attributed to the lower net thermal resistance of the MWCNT coatings. We explore the behavior of carbon nanotubes at two laser wavelengths (1.06 and 10.6 μm) and also evaluate the optical-absorption efficiency and bulk properties of the coatings.

All-Optical Modulation in a Silicon Waveguide Based on a Single-Photon Process

All-optical, low-power modulation is a major goal in photonics. Because of their high mode-field concentration and ease of manufacturing, nanoscale silicon waveguides offer an intriguing platform for photonics. So far, all-optical modulators built with silicon photonic circuits have relied on either two-photon absorption or the Kerr effect. Both effects are weak in silicon, and require extremely high (~5 W) peak optical power levels to achieve modulation. Here, we describe an all-optical Mach-Zehnder modulator based on a single-photon absorption (SPA) process, fabricated entirely in silicon. Our SPA modulator is based on a process by which a single photon at 1.55 mum is absorbed and an apparently free-carrier-mediated process causes an index shift in silicon, even though the photon energy does not exceed that of silicon's bandgap. We demonstrate all-optical modulation with a gate response of 1deg/mW at 0.5 Gb/s. This is over an order of magnitude more responsive than typical previously demonstrated devices. Even without resonant enhancement, further engineering may enable all optical modulation with less than 10 mW of gate power required for complete extinction, and speeds of 5 Gb/s or higher.

Decreasing EDFA transients by power shaping

Multi-wavelength optical networks make extensive use of erbium-doped optical amplifiers (EDFAs) to offset the effects of fiber attenuation and signal power splitting. As optical networking moves towards burst and packet switching, the effects of changes of optical power levels on one wavelength, or channel, can have an impact on the behavior of other wavelengths passing through the same amplifier and impair network performance. We have observed in our simulations of EDFA operation that both the amplitude and duration of EDFA transients will decrease when gradual changes are made to the input power of the EDFA. It was also verified in [D.H. Richards, J.L. Jackel, M.A. Ali, A theoretical investigation of dynamic all-optical automatic gain control in multichannel EDFA’s and EDFA cascades, J. Select. Top. Quantum Electron. 3 (4) (1997)] that the switching time of the channel dropping operation can influence the amplitude and duration of EDFA transients. We show that this effect can be exploited to successfully suppress EDFA transients by power shaping. Our approach is to increase/decrease the power of an added/dropped channel gradually, rather than abruptly. Power shaping can be implemented by adapting existing link layer protocols. It is general and can be applied to amplifiers using different technologies, including EDFAs, solid state and Raman amplifiers.

Accurate measurement of scattering and absorption loss in microphotonic devices

We present a simple measurement and analysis technique to determine the fraction of optical loss due to both radiation (scattering) and linear absorption in microphotonic components. The method is generally applicable to optical materials in which both nonlinear and linear absorption are present and requires only limited knowledge of absolute optical power levels, material parameters, and the structure geometry. The technique is applied to high-quality-factor (Q=1x10(6) to Q=5x10(6)) silicon-on-insulator (SOI) microdisk resonators. It is determined that linear absorption can account for more than half of the total optical loss in the high-Q regime of these devices.

Nanomechanical displacement detection using fiber-optic interferometry

We describe a fiber-optic interferometer to detect the motion of nanomechanical resonators. In this system, the primary technical challenge of aligning the fiber-optic probe to nanometer-scale resonators is overcome by simply monitoring the scattered light from the devices. The system includes no free-space optical components, and is thus simple, stable, and compact with an estimated displacement sensitivity of ∼0.3pm∕Hz at optical power levels of ∼0.75mW.

Period Doubling in a Fabry–Perot Laser Diode Subject toOptical Pulse Injection

Experimental study and numerical simulations of the period doubling ofinjected optical pulses in Fabry–Perot laser diodes are presented. Inour experiments, the period doubling is achieved within a wide inputfrequency range and the period doubling of the injected optical pulseswith 6.32 GHz repetition rate is investigated in detail. The obtainedexperimental results indicate that period doubling occurs at anappropriate injected optical power level when the bias current of theFabry–Perot laser diode is located in lower ranges. Moreover, theexperimental observed features have been numerically demonstrated byusing a coupled rate-equation model. Numerical simulations areconsistent with the experimental results.

이득 포화된 광섬유증폭기를 사용하는 기상에 둔감한 무선광통신

We present a weather-insensitive optical free-space communication method supporting optical packet channels. It operates optical fiber amplifiers in gain-saturation regions. When the propagation loss gets too high, it decreases the average packet rate, or the average packet length, or both, to increase the optical power level launched into the free-space. As a demonstration, we transmit Gigabit Ethernet channels over a terrestrial distance of 2.4 km. One gain-saturated free-space optical repeater is used at the halfway point.

Quantum dots as self-referenced optical fibre temperature probes for luminescent chemical sensors

The use of semiconductor nano-particles as temperature probes in luminescence chemical sensing applications is addressed. Temperature changes the intensity, the peak wavelength and the spectral width of the quantum dots luminescent emission in a linear and reversible way. Results are presented that show the feasibility of implementing a self-referenced intensity-based sensor to perform temperature measurements independent of the optical power level in the sensing system. A resolution of 0.3 °C was achieved. In addition, it is demonstrated that self-referenced temperature measurements at multiple points could be performed using reflection or transmission based optical fibre configurations.

Two-wavelength switching with a distributed-feedback semiconductor optical amplifier (DFBSOA)

Switching of a signal beam by another control beam at different wavelength is demonstrated experimentally using the optical bistability occurring in a 1.55 /spl mu/m-distributed feedback semiconductor optical amplifier (DFBSOA) working in reflection. Counterclockwise (S-shaped) and reverse (clockwise) bistability are observed in the output of the control and the signal beam respectively, as the power of the input control signal is increased. With this technique an optical signal can be set in either of the optical input wavelengths by appropriate choice of the powers of the input signals. The switching properties of the DFBSOA are studied experimentally as the applied bias current is increased from below to above threshold and for different levels of optical power in the signal beam and different wavelength detunings between both input signals. Higher on-off extinction ratios, wider bistable loops and lower input power requirements for switching are obtained when the DFBSOA is operated slightly above its threshold value.

Improved characteristics of a terahertz set-up built with an emitter and a detector made on proton-bombarded GaAs photoconductive materials

We report the coherent generation and detection of terahertz radiation from antenna-type devices made by using proton-bombarded GaAs photoconductive materials. Our combined emitter/detector set-up allows us to obtain a large bandwidth going from 0.1 up to 2 THz. We compare the performance of antenna emitters fabricated using mono- and multi-energy proton implantation in semi-insulating GaAs. Improved emission of terahertz radiation with a comparable bandwidth has been obtained using multi-energy proton implantation. Our results show that creating more defects in the optical absorption region gives rise to higher damage threshold biasing and larger saturation optical pumping power levels.

Weather-insensitive optical free-space communication using gain-saturated optical fiber amplifiers

This paper presents a weather-insensitive optical free-space communication (OFSC) method supporting optical packet channels. It operates optical fiber amplifiers (OFAs) used as boosters in gain-saturation regions. When the propagation loss gets too high, it decreases the average packet rate, or the average packet length, or both, to increase the optical power level launched into the free space. As a demonstration, 8 /spl times/ 10 Gigabit-Ethernet (GbE) channels were transmitted over a terrestrial distance of 2.4 km. One gain-saturated free-space optical repeater is used at the halfway.

Mechanical Failure of Bent Optical Fiber Subjected to High Power

As optical fiber penetrates further into the communications infrastructure and comes closer to the home or business, higher optical power levels are expected. Several studies have shown that sharply bent optical fiber will fail prematurely when exposed to high optical power levels. In an extreme case, where the fiber is bent to a maximum bend stress on the order of 2 GPa and subjected to a power level of 1–2 W in the near‐infrared wavelength window, optical fiber will fail in minutes. Time to failure decreases with increasing bend stress and optical power. A recent report suggests that power levels in the range of a few hundred milliwatts may be enough to induce delayed failure in bent fiber. This study explores the progression of events leading to failure. Light that escapes the core of bent fiber passes into the coating, where a small amount is absorbed and converted to heat. The coating heats to a stable temperature and visually darkens with time. This is followed by an abrupt rise in temperature, which occurs as the coating transforms to a highly absorptive material, consistent with thermal runaway. The abrupt rise in coating temperature stimulates viscoelastic deformation of the glass. Glass deformation is explained in terms of the ability of highly quenched glass to experience viscous flow at temperatures well below the glass transition range (i.e. sub‐Tg aging or relaxation). As the glass portion of the fiber moves toward a “kinked” configuration, it concentrates more power on a smaller region of coating, resulting in further temperature increase. There is no evidence of the fiber fuse effect in the lower viscosity glass core. The final kinked configuration of the glass fiber leads to complete attenuation of the light and failure is complete. Coating decomposition is self‐limiting with no visible flame. A coating with a refractive index near or below that of silica was found to virtually eliminate this failure mode.

WDM-PON system based on the laser light injected reflective semiconductor optical amplifier

This paper proposes a WDM-PON system which uses the laser light injected reflective semiconductor optical amplifier (R-SOA) as the optical transmitter. In the system, optical carrier of laser lights is supplied from the central office (CO) to the R-SOA both in CO and in the subscriber's side. The optical carrier consists of a pair of the perpendicularly polarized laser lights to avoid the polarization dependence of R-SOA. The optical carrier can be shared by several WDM-PON systems to save the cost. We measured the transmission performance of R-SOA which modulates the polarization mixed injection laser at 1.25 Gbps. It is shown that the combined effect of Rayleigh backscattering and the intensity noise induced by the feedback of optical signals into the R-SOA increases the required level of optical injection power into the R-SOA.

Thermal analysis of bond layer influence on performance of an all-active vertically coupled, microring resonating laser

The vertical coupling of active InP based ring resonators and passive feeding waveguides necessitates the use of a waferbonding technology in the fabrication process. The required bond material (BCB) has a low thermal conductivity and will strongly influence the operating temperature and thus the performance of the ring resonator through its insulating effect. A comprehensive thermal analysis of a proposed vertically coupled ring resonator of 50 μm outer radius is undertaken during the design phase to determine the thermal impact of: the design of the wafer bond, the design of the passivation layer and the optical power levels. Thermal abatement strategies for semiconductor lasers are presented.

Luminescence-Based Optical Fiber Chemical Sensors

A scheme for the simultaneous determination of temperature and analyte concentration for application in luminescence-based chemical sensors is proposed. This scheme is applied to an optical oxygen sensor, which is based on the quenching of the fluorescence of a ruthenium complex. Temperature measurement is performed using the excitation radiation and an absorption long-pass filter. Preliminary results are presented that show the viability of an oxygen measurement that is independent of temperature and optical power level. The possibility of self-referenced temperature measurements with semiconductor nanoparticles is also investigated. In order to optimize the sensor design, several different optical fiber probe geometries for oxygen sensing are tested and compared, including different methods of coupling radiation into the optical fiber system. Polyvinyl alcohol (PVA) and polyacrylamide membranes are tested as supports for sensor immobilization in fiber-optical pH sensing devices in aqueous solution. Some results are presented that show the feasibility of using fiber-optical pH indicators for remote monitoring.

Development of GPON Upstream Physical-Media-Dependent Prototypes

This paper presents three new gigabit-capable passive optical network (GPON) physical-media-dependent (PMD) prototypes: a burst-mode optical transmitter, an avalanche photodiode/transimpedance amplifier (APD-TIA), and a burst-mode optical receiver. With these, point-to-multipoint (P2MP) upstream transmission can be realized in a high-performance GPON at 1.25 Gb/s. Performance measurements on the new burst-mode upstream PMD modules comply with GPON uplink simulations. The laser transmitter can quickly set and stabilize the launched optical power level over a wide temperature range with better than 1-dB accuracy. A burst-mode receiver sensitivity of -32.8 dBm (BER=10/sup -10/) is measured, combined with a dynamic range of 23 dB at a fixed APD avalanche gain of 6. Full compliance is achieved with the recently approved ITU-T Recommendation G.984.2 supporting an innovative overall power-leveling mechanism.

Analysis of a demodulation system for Fiber Bragg Grating sensors using two fixed filters

The analysis of a demodulation system for Fiber Bragg Grating sensors based on two fixed spectral filters has been carried out both theoretically and experimentally. Different system configurations were analyzed by modifying the spectral position of the filters as well as the optical power-level of the signal reaching the two photo-detectors. Measurements with integration times that varied from 0.01 to 1 s have been compared with the low-frequency limit predicted for long-term operation. Comparisons between simulated and experimental results show good agreement, and extrapolations indicate that it should be possible to achieve an operating spectrum range of the order of 7 nm, with uncertainties equivalent to less than 2 pm in measurements of the sensor peak position.

DC-coupled burst-mode transmitter for 1.25 Gbit/s upstream PON

An innovative DC-coupled burst-mode optical transmitter is presented for gigabit PON upstream transmission at 1.25 Gbit/s. Experimental results show that this burst-mode transmitter meets the specifications of the recently approved ITU-T Recommendation G.984.2. The chip can stabilise and control the launched optical power level with accuracy better than 1 dB over a wide temperature range.

Optical Performance Monitoring Using Nonlinear Detection

A definitive goal for optical performance monitoring in an optical communications network is to provide comprehensive signal quality information in a cost-effective manner. This paper explores in detail the possibility of using nonlinear optical detection to achieve this goal. Sensitive nonlinear detection techniques commonly used in the field of ultrafast optics are applied to the problem of performance monitoring and are shown to allow quantitative measurements to be made of quantities such as accumulated chromatic dispersion, polarization-mode dispersion impairment, optical signal-to-noise ratio, and extinction ratio. Experiments performed on a 40-Gb/s transmission system demonstrate the immediate viability of this approach for measuring these quantities of interest at practical optical power levels.