Fiber-optic Communication Links Research Articles

The Effects of an Axial Index on a Low-nonlinear and Nonzerodispersion Wideband Optical Fiber Link containing an Er-doped Dispersion Compensating Optical Fiber Amplifier

We present the simulation results of the effects of axial index dip of an optical fiber amplifier used in an optical fiber communication link. The results are based on the mode field diameter, chromatic dispersion, zero-dispersion length of dispersion compensating optical fiber amplifier, minimum pump power requirement, noise figure, bit rate and channel capacity.

High-speed digital modulation characteristics of oxide-confined vertical-cavity surface-emitting lasers-numerical simulations consistent with experimental results

The vertical-cavity surface-emitting laser (VCSEL) is a preferred light source for short-distance high-speed fiber-optic communication links. We simulate the digital modulation behavior of typical oxide-confined VCSELs under realistic working conditions with a comprehensive model that includes the detailed geometry when calculating the optical fields and that accurately accounts for the dynamic effects of carrier density and temperature on the modal distributions. The intrinsic output characteristics of single- and multimode VCSELs were studied as functions of bias and modulation depth under a 2/sup 7/-1 pseudorandom bit sequence current modulation at 2 and 10 Gb/s. The data were used to create numerical eye diagrams that show, e.g., the significant impact of the bit pattern history and the noise on the timing jitter in both single- and multimode VCSELs. For the single-mode VCSEL, the choice of the bias current and modulation depth was less critical due to its higher damping of the relaxation oscillations. The simulated VCSELs were fabricated and experimentally evaluated. The measured eye diagrams showed the same characteristic features as those in the simulations.

Power penalty analysis for realistic weight functions using differential time delay with higher-order dispersion

In this paper, the power penalty analysis for approximate and realistic weight functions has been presented for combating the pulse broadening effects of group-velocity dispersion in a fiber-optic communication link using differential time delay method including higher-order dispersion terms. The expressions for root mean square (RMS) phase deviation, optimum chirp factor and figure of merit have been evaluated for approximate and realistic systems. We show that the optimum value of chirp factor corresponds to dispersion compensation. The power penalty graphs for second-, third-, and fourth-order dispersion and their combinations have been presented for distance up to 300 km for this chirp factor for different weight functions. It is observed that the power penalty for realistic weight functions is less in comparison with the approximated weight function. It has also been shown that it is possible for a short pulse to propagate without significant broadening over the lengths many times longer than the usual dispersion length of fiber.

A tunable, high-effective optical isolator based on Nd:Fe:B magnets

We report on a tunable, high-effective optical isolator based on Nd:Fe:B permanent magnets. The device can operate in a wide spectral range and may be used in high-power laser systems, fiber-optic communication links and optical interferometry.

Wavelength-division multiplexing isolation filter using concatenated chirped long period gratings

We propose the use of concatenated chirped long period gratings as an efficient wavelength-division multiplexing (WDM) isolation filter. The proposed filter not only works in the transmission mode but also has almost equispaced pass bands with high isolation at the stop bands. The filter has negligible dispersion effects in the pass band, which should make such a device suitable for use in WDM fiber optic communication links.

Optical fibers with low nonlinearity and low polarization-mode dispersion for terabit communications

Refractive-index nonlinearities have negligible effect on the performance of short-haul fiber-optic communication links utilizing electronic repeaters. However, in long links, nonlinearities can cause severe signal degradations. To mitigate nonlinear effects, a new generation of fibers, referred to as large effective-area fibers, have been introduced in recent years. This paper reviews the latest research and development work on these fibers conducted by several research groups around the world. Attention is focused on a class of large effective-area fibers that are based on a depressed-core multiple-cladding design. Another important issue in long-haul and high capacity fiber optic systems is the polarization-mode dispersion (PMD) which has been recognized as a serious limiting factor. In this paper, an improved fiber design is proposed which, in addition to providing large effective-area and low bending loss, eliminates PMD due to elliptical deformation in the single-mode wavelength region. Furthermore, this design is allowed to provide a small chromatic dispersion about few ps/ nm km , in order to overcome four-wave mixing effects.

Dispersion compensation in transmission using uniform long period fiber gratings

It is proposed that the high dispersion at the transmission band edges of uniform long period gratings (LPG) fabricated on relatively high Δ fibers can be used for efficient dispersion compensation. Since the transmission of LPG varies with length of the grating or refractive index modulation, we show that it is possible to tailor the transmission spectrum to obtain high transmission with constant dispersion and negligible delay ripple over a reasonable bandwidth. Since the proposed structure works in transmission it should be suitable for fiber optic communication links.

Effects of chromatic dispersion on microwave and millimetre wave carrier transmission in optical fibres

A new formalism for the analysis of optical (phase) noise in fibre-optic communication links is presented. The formalism is applied to an amplitude modulated millimetre-wave (or microwave) link with chromatic dispersion. A realistic link model is described and rigorously solved. In general, the power spectrum of the output signal consists of a ‘comb’ of spectral components with centre separation equal to the modulation frequency. Each spectral component contains a smooth ‘noise’ curve and a narrow spike which represents a modulated field component (carrier). With the new method it is possible to compute each spectral component and, consequently, the full power spectrum. From the power spectrum one can compute quantities relevant to the link design, in particular the carrier-to-noise ratio. Several graphs are presented, showing the power spectrum and the carrier-to-noise ratio for different values of the modulation frequency, link dispersion and modulation index. Several conclusions regarding the millimetre-wave fibre-optic link are dis-cussed.

Analysis of spatial–temporal converters for all-optical communication links

We analyze parallel-to-serial transmitters and serial-to-parallel receivers that use ultrashort optical pulses to increase the bandwidth of a fiber-optic communication link. This method relies on real-time holographic material for conversion of information between spatial and temporal frequencies. The analysis reveals that the temporal output of the pulses will consist of chirped pulses, which has been verified experimentally. When the signal pulses are transmitted along with a reference pulse, the distortions of the received signal, caused by dispersion and other factors in the fiber, are canceled because of the phase-conjugation property of the receiver. This self-referencing scheme simplifies the receiver structure and ensures perfect timing for the serial-to-parallel conversion.

Higher-Order Dispersion Compensation by Differential Time Delay

A novel approach to combating the pulse broadening effects of group-velocity dispersion in a fiber-optic communication link due to higher-order dispersion terms has been presented. It has been shown that it is possible for a short pulse to propagate without significant broadening over lengths many times longer than the usual dispersion length of fiber. Root mean square phase deviation, dimension-free chirp parameter, and figure of merit have been evaluated for ideal and realistic optical communication systems using higher-order dispersion terms together. It has been shown that dispersionless propagation length can be enhanced to 5 and 6.5 times if the compensation is performed by using second- and third-order and third- and fourth-order dispersion terms together, respectively.

Degenerate parametric amplification in an optoelectronic GaAs CPW-to-slotline ring resonator

Nonlinear optical-microwave mixing is performed in an uniplanar CPW-to-slotline ring resonator on a semi-insulating GaAs substrate, in which a Schottky photodetector is monolithically integrated as a coupling gap. The parametric amplification effect of the mixer occurs when the capacitive reactance of the detector is modulated. In this structure, a parametric amplification gain of 20 dB, without the applied bias in the radio frequency (RF) signal, is obtained. This microwave optoelectronic mixer can be used in fiber-optic communication links.

A packaged broad-band monolithic variable gain amplifier implemented in AlGaAs/GaAs HBT technology

Broad-band amplifiers find application in fiber-optic communication link and instrumentation or serve as generic components in high-speed electronic test laboratories. With an advanced AlGaAs/GaAs heterojunction bipolar transistor (HBT) technology, we have designed and fabricated broad-band monolithic variable gain amplifiers (VGAs). High-speed packages for the VGAs were carefully designed to minimize insertion and return losses and to suppress undesired package cavity resonances. Accurate and efficient models for the packages were obtained based on experimental data so that their parasitic effects could be considered in the VGA design. The packaged VGAs provided 10-16 dB adjustable gain with approximately /spl plusmn/1 dB gain variations and constant group delay in the DC-26 GHz band, and showed better than 10 dB input/output return losses in the amplifier passband. When the packaged VGAs were inserted in a 30 Gb/s electronic link, error-free operation was achieved for a 2/sup 31/-1 input pseudorandom bit sequence. These VGAs can be used in fiber-optic transmission systems with data rates up to 30 Gb/s.

Wavelength multiplexed fiber optics resonant ring hydrophone array

A hydrophone array employing a plurality of laser sources coupled onto a single optical fiber communications link. A matching plurality of optical fiber resonant rings, each having a resonance peak at a wavelength substantially equal to the center wavelength of one of the lasers, is selectively coupled to the optical fiber communication link so that substantially only optical energy in a narrow bandwidth around one of the laser center wavelengths is coupled to the corresponding one of the optical fiber resonant rings. Corresponding photodetectors are also wavelength selectively coupled to the optical fiber communications link so that each detector receives only optical energy corresponding to the wavelength of one of the lasers and its corresponding resonant ring, to provide an output signal indicating variations in acoustic pressure at individual resonant rings in the array.

Silicon bipolar chipset for SONET/SDH 10 Gb/s fiber-optic communication links

High-speed multiplexers, demultiplexers, frequency dividers, mixers, and amplifiers are key electronic components in high-speed fiber-optic communications systems such as SONET/SDH. In this paper, we present several important digital and analog integrated circuits (IC) which have been developed for use in SONET/SDH 10 Gb/s optical communication links. The circuits have been fabricated in MOSAIC 5E, an advanced silicon bipolar technology (f/sub T/=26 GHz). The resulting chipset which amounts to a total of 10 IC's consists of multiplexers, demultiplexers, a regenerative frequency divider (2:1), a dual output limiting amplifier, and two different types of mixers for clock extraction. Specifically, the design and performance of these IC's and a hybrid clock recovery module are discussed. The high performance and potential low cost of this research chipset show that advanced silicon bipolar circuit technology can play an important role in future multigigabit fiber-optic communication systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dispersion compensation with phase-sensitive optical amplifiers

Recently, we proposed a novel approach to combating the pulse-broadening effect of group-velocity dispersion (GVD) in a fiber-optic communication link (IEEE Photon. Technol. Lett., vol. 5, pp. 669-672, 1993). In our scheme, linear loss in the fiber is balanced by a chain of periodically spaced, phase-sensitive optical amplifiers. In this paper, we present a detailed analysis of pulse propagation in such a fiber line showing that, due to attenuation in the quadrature orthogonal to the amplified quadrature, it is possible for a pulse to propagate without significant broadening over lengths many times longer than the usual dispersion length of the fiber. An averaged equation for pulse evolution over distances longer than the amplifier spacing is derived. The effect of optical-phase fluctuations between the propagating pulse and the amplifiers is also considered. Since our proposed scheme does not rely on the formation of solitons in the fiber, it can be implemented in both the positive and negative GVD regions, and it does not require a minimum peak power for the pulses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Unified Method for Design and Analysis of Analog/Digital Fiber Optic Communication Links

The design of Fiber Optic Communication (FOC) systems requires many optical and electronic operational parameters for specifying and optimizing the selection of components in a communication link. An analysis method is developed using the noise effective temperature concept. This approach allows a unified Noise- Bandwidth parametric analysis of analog, digital or combined modulation formats in FOC systems. A typical real life system analysis is presented, demonstrating the system bandwidth dependence on various noise sources contributions, originating from the electrical and optical parts. To this end, simple models for both optical and electronic components are used

Combating dispersion with parametric amplifiers

A novel approach to combating the pulse broadening effect of group-velocity dispersion in a fiber-optic communication link is presented. In the scheme linear loss in the fiber is balanced by a chain of periodically spaced, phase-sensitive, degenerate optical parametric amplifiers. Analysis of pulse propagation in such a fiber line shows that, due to attenuation in the quadrature orthogonal to the amplified quadrature, it is possible for a pulse to propagate without significant broadening over lengths much longer than the usual dispersion length of the fiber. >

OPTICAL WAVEGUIDES IN ANALYTICAL CHEMISTRY: RECENT ADVANCES

Achievements in guided wave optics have had a great influence on many areas of technology for several years. Fibre optic communication links, sensors for various parameters, recently developed distributed temperature sensors, integrated optical switches, etc. are all applications that are commercially available. The field of analytical chemistry is no exception in this growing technology. In order to compete with well‐established chemical‐sensing instrumentation, optical waveguide chemical sensors (OWCSs) must show all the qualities of such instrumentation. OWCSs combine well‐known features of sensors, based on waveguide optics, with optical methods of chemical analysis and offer advantages over other types of chemical sensor. OWCSs are electrically passive, corrosion‐resistant, can respond to analytes for which other chemical sensors are not available, and referencing can be carried out optically. They allow multicomponent measurements at several wavelengths, have a common technology for fabrication of sensors for different chemical and physical parameters and are easily compatible with telemetry etc. Further, only OWCSs are capable of distributed sensing. However, interference from ambient light, temperature, long‐term instability, relatively slow response time, and limited dynamic range may be a problem for some types of OWCS. These disadvantages can be considerably reduced using various methods.

Pushing the limits of standard CMOS

An improved clocking scheme and sharper circuit design and logic selection, which have yielded a five-to-tenfold increased in the speed of standard CMOS ICs, are discussed. The clocking strategy relies on a true single-phase clock, device sizes are varied to optimize their speed, and a precharged logic style reduces capacitive loads. The tradeoff is roughly a doubling of circuit area. The high-speed CMOS technique has been demonstrated experimentally, with good results. For example, ripple counters in 3 and 2 mu m CMOS processes reached input frequencies of 400 and 750 MHz, respectively, or nearly 80 and 70% of the intrinsic speeds of these processes. Pipelined accumulators in 2 and 1.2 mu m CMOS processes operated at up to 430 and 700 MHz clock frequencies, respectively. The data rate of an error correcting encoder designed for an optical fiber communication link was measured as 1.2 Gb/s, while the corresponding decoder was simulated at the same speed. In general, the circuits were found to be as robust as CMOS circuits designed in a conventional way.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical equalization to combat the effects of laser chirp and fiber dispersion

Optical equalizers capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communication links at 1.5 mu m are described. They consist of both reflective and transmissive cavity structures. The equalizers are adaptive in the sense that the position of their periodic frequency responses are optimally adjusted in real time. The equalizers are evaluated by using computer simulation routines based on the rate equations for intensity-modulated lasers. It is shown that, by using equalizers of the type proposed, the receiver data eyes remain open well beyond where they would close without equalization. >