Abstract: The temperature dependency of an EDFA is compensated in a wavelength of 1570 to 1600 nm to realize a wavelength division multiplexing transmission. First and second long period gratings different in grating period from each other are formed in an optical fiber. The peak wavelength of the waveform of the light transmission loss characteristic of the first long period grating is formed on the shorter wavelength side than a transmission band. The peak wavelength of the waveform of the light transmission loss characteristic of the second long period grating is formed on the longer wavelength side than a transmission band.

Abstract: An optical amplifier module that is insensitive to polarization comprises a first fiber, a second fiber, a semiconductor optical amplifier (SOA), and a polarization dependent loss (PDL) unit. The first fiber provides for optical input to the SOA. The SOA amplifies the optical signal received and outputs the amplified signal. The output of the SOA is optically coupled to the unit. The PDL unit provides polarization dependent loss and the loss is preferably selected to match the polarization dependent gain of the SOA such that when two are coupled there is no overall polarization dependence. The output of the PDL is optically coupled to the fiber for transmission output.

Abstract: A gain-clamped, optical amplifier includes a rare-earth doped fiber and a source of pump energy coupled to the rare-earth doped fiber. The doped fiber serves as a gain medium that is optically pumped by the pump source. A wavelength-selective optical feedback loop is coupled between input and output ports of the rare-earth doped fiber. The feedback loop supports a compensating lasing signal, which is a pulsed signal located at a wavelength different from a signal wavelength.

Abstract: An optical fiber communication system according to the present invention has, for example, first and second phase conjugators. The first phase conjugator converts a signal beam from a first optical fiber into a first phase conjugate beam. The first phase conjugate beam is supplied to the second phase conjugator by a second optical fiber. The second phase conjugator converts the first phase conjugate beam into a second phase conjugate beam. The second phase conjugate beam is transmitted by a third optical fiber. The second optical fiber is composed of a first portion located between the first phase conjugator and a system midpoint and a second portion located between the system midpoint and the second phase conjugator. The total dispersion of the first optical fiber substantially coincides with the total dispersion of the first portion, and the total dispersion of the second portion substantially coincides with the total dispersion of the third optical fiber.

Abstract: An optical amplifier makes use of a co-propagating fiber amplifier stage and a counter propagating fiber amplifier stage. One of the stages is preferably a low noise stage, while the other is a high gain stage, typically the output stage. The low noise stage may have a small core fiber while the high gain stage has a large core fiber. Pumping energy is introduced into the amplifier by coupling a plurality of laser sources in the same wavelength range using a narrow channel wavelength division multiplexer coupler. In each stage of the amplifier, some portion of the pump energy is transferred from the pumping wavelength to the signal wavelength. In a single polarization optical signal generator embodiment, the optical signal is developed in a distributed Bragg reflector oscillator having an output Bragg grating embedded in a polarization perserving fiber. A polarization controller is used at the amplifier output to restore the single polarization which is developed in the oscillator.

Abstract: In a light amplification apparatus in which a part of a light output from a light amplification medium is transmitted through a branching means to a light detection means so that light power of the light output is detected by the light detection means whereby at least one of an excitation source and a return means is controlled through a control means so that light branch from a branching means is returned as a return light to the light amplification medium through the branching means again in a state in which light power is adjusted by the return means, and, as a result, the light power of the light output from the light amplification medium can be kept in a desired value while the occurrence of a light surge is suppressed, a light amplification medium control method and a light amplification apparatus wherein a signal light input can be amplified while the occurrence of a light surge is suppressed by a simple structure and with good control response, and a system using the same.

Abstract: The present invention relates to a method for transmitting a supervisory optical signal. A first optical amplifier for a first band (e.g., a conventional band: C band) determining a noise characteristic and a gain efficiency is provided. A second optical amplifier for a second band (e.g., a long wavelength band: L band) determining a noise characteristic and a gain efficiency is provided. The supervisory optical signal is processed in relation to one of the first and second optical amplifiers superior in at least one of the noise characteristic and the gain efficiency to the other. According to this method, undue degradation in the noise characteristic and the gain efficiency can be prevented to maintain a good transmission quality of a main signal.

Abstract: An optical fiber amplifier comprises an optical fiber for light loss which is provided in a later stage than an optical fiber amplification section or midway therein, the amount of light absorption and the wavelength dependency of the absorption of the optical fiber changing according to the power of input light; whereby the optical fiber amplifier can maintain flat gain in a wide wavelength band, the gain flatness not changing over a wide dynamic range and being maintained irrespective of variation in the power of input signal light.

Abstract: An optical communication system having a transmitting station for outputting WDM(wavelength-division multiplexing) signal light, an optical fiber transmission line, a receiving station, and an optical repeater including an optical amplifier. The transmitting station includes a supervisory circuit for detecting the number of channels of the WDM signal light and transmitting supervisory information including the number of channels to the optical repeater. The optical repeater further includes a circuit for controlling the optical amplifier so that the output level of the optical amplifier becomes a target level. The target level is set according to the supervisory information. According to the structure, it can be possible to provide a system which can easily respond to a change in the number of WDM channels.

Abstract: A method and apparatus for determining control signals of a filter for optimization of the gain profile of an amplifier, particularly though not exclusively for use in optical communications systems, and a system incorporating the same. The method is for calculating at least one control signal for at least one periodic filter arranged to alter the output gain profile of an amplifier, in which at least one of the phase and amplitude of the filter response are tuneable.

Abstract: An optical brancher 110 branches an input optical signal into two. An optical detector 120 converts one optical signal branched by the optical brancher 110 into an electrical signal. A first controller 122 generates a control electrical signal having a waveform obtained by inverting the envelope of the electrical signal. Based on the control electrical signal, an optical signal generator 124 produces a dummy optical signal having a waveform &lgr;d and an amplitude &agr;/2. The other signal branched by the optical brancher 110 is delayed by a delay unit 112 for a predetermined time, and then multiplexed by an optical multiplexer 114 with the dummy optical signal from the optical signal generator 124. An optical amplifier 116 amplifies a multiplexed optical signal. An optical filter 118 separates an optical signal of a wavelength &lgr;1 from the amplified optical signal. Thus, optical signal amplification can be carried out without optical surges.

Abstract: This invention relates to a method for filtering and to a variable slope optical filter for in-line use with an optical amplifier signal. The filter has a wavelength response that is substantially linear in slope within a band of operation wavelengths of the amplifier; the slope of the filter in an operating band of wavelengths is between zero and only one of a positive or negative number; furthermore the filter has an amplitude response that has an opposite and counter slope as a function of wavelength to that of a gain tilt of the amplifier within the operation band of wavelengths. The filter having means of operating on polarized light such that different wavelengths within a band of wavelengths within the operation band are subject to different levels of attenuation according to their polarization state. A wave plate having a predetermined thickness provides a relative polarization difference between two wavelengths of an incoming beam in a controlled manner.

Abstract: An optical amplifier which suppresses the occurrence of a nonlinear optical phenomena, such as four-wave mixing. The optical amplifier includes an optical fiber doped with a rare earth element, and a light source providing excitation light to the fiber so that a wavelength division multiplexed (WDM) light is amplified as the WDM light travels through the fiber from a first point to a second point along the fiber. A third point exists along the fiber between the first and second points at which a power of the WDM light substantially reaches a level producing a nonlinear optical phenomenal. A first portion of the fiber is between the first point and the third point, and a second portion of the fiber is between the third point and the second point. The second portion suppresses the occurrence of the nonlinear optical phenomena as compared to the first portion. The second portion can have various different constructions to provide the required suppression effect.

Abstract: An optical communication system having a transmitting station for outputting WDM (wavelength-division multiplexing) signal light, an optical fiber transmission line, a receiving station, and an optical repeater including an optical amplifier. The transmitting station includes a supervisory circuit for detecting the number of channels of the WDM signal light and transmitting supervisory information including the number of channels to the optical repeater. The optical repeater further includes a circuit for controlling the optical amplifier so that the output level of the optical amplifier becomes a target level. The target level is set according to the supervisory information. According to the structure, it can be possible to provide a system which can easily respond to a change in the number of WDM channels.

Abstract: An optical transmission system has been designed to optimize the use of the spectral emission range of rare-earth-doped fiber amplifiers. The system includes a wide band of channels in the spectral emission range of erbium-doped fiber amplifiers, which is split into two sub-bands, a low sub-band corresponding to the low end of the range and a high sub-band corresponding to the high end of the range. The two sub-bands are separately amplified and optimized, and then recombined without significant competition between the two sub-bands. In addition, an equalizing filter, such as a specialized Bragg filter or interferential filter, is applied to the low sub-band instead of the entire band of channels, thus greatly reducing any equalization need or unequalization effects.

Abstract: A repeater station 10 detects whether or not an abnormality exists in each of signal light and monitor light having arrived from a terminal station 1 or a repeater station 20, and controls, according to the result of detection, operations of optical amplifiers 11 and 12 and the monitor light. In the case where both of the signal light and monitor light arriving from the terminal station 1 via the optical transmission line for the downstream direction are abnormal, the optical amplifier 11 for the downstream direction stops its amplifying operation by itself, whereas a monitor control apparatus 13 stops the amplifying operation of the other optical amplifier for the upstream direction and also stops transmitting the monitor light to the terminal station 1 in the upstream direction.

Abstract: An optical amplification apparatus for adjusting optical power of wavelength-multiplexed signal light at respective wavelengths and for adjusting the optical output power at the respective wavelengths and a deviation of the optical output power between the wavelengths. An optical power adjusting unit for receiving inputted light having signal light at a plurality of different wavelengths (&lgr;1, &lgr;2, &lgr;3, . . . , &lgr;N) multiplexed thereon, amplifying or attenuating light at least one wavelength included in the received light independently of light at different wavelengths from the wavelength of the light, is provided before or after an optical amplifier unit for amplifying the light having the signal light at the plurality of different wavelengths multiplexed thereon. Further, a control unit is provided for controlling the gain of amplification or attenuation performed by the optical power adjusting unit and the gain of amplification performed by the optical amplifier unit, respectively.

Abstract: An erbium doped fiber amplifier (EDFA) suitable for amplifying a long wavelength light signal in the range in the 1570 to 1610 nm band, includes a pump light source for generating pump light in the 980 or 1480 nm band, a light coupler for receiving the pump light from the pump light source and coupling the same to the received light signal, first and second optical fibers doped with erbium ions, for amplifying the received light signal by the pump light, and an isolator positioned at a point between the first and second erbium doped optical fibers (EDFs). The gain and noise figure of the EDFA for amplifying received light signal in a long wavelength band can be improved. Also, an EDFA suitable for amplifying received light signal in the 1570 to 1610 nm band can be attained.

Abstract: A device with an optical fiber amplifier, wherein a control circuit is provided for stabilizing the amplification. In the control circuit, a supplemental optical signal (z) is supplied to the input of the fiber amplifier within its optical amplification bandwidth. The level of the former is controlled by a control member (Q), to whose first input a first measuring signal (m1) is supplied, which represents the level of an optical signal, amplified by the fiber amplifier (V), from a first area of the amplification bandwidth. A second measuring signal (m2) is present at the second input of the control member, which represents the level of a not yet amplified optical signal from the second area of the amplification bandwidth.

Abstract: A light amplification system for amplifying a selected wavelength of light used for communicating information signals and having a microcontroller for substantially automatic start-up and shutdown and continuous monitoring of the critical parameters of the system.

Abstract: A system is provided for partially or completely offsetting optical fiber amplifier gain spectrum variations due to temperature by setting the optical pump signal leading into the amplifier to a desired wavelength value. Optical fiber amplifiers have gain spectrum variations associated with temperature and gain spectrum variations associated with the optical pump signal wavelength. By controlling the optical pump signal wavelength supplying the amplifier, the optical fiber amplifier gain variations due to temperature can be partially or completely offset, which helps minimize total amplifier spectral gain variations. The invention is applicable to dense wavelength division multiplexing (DWDM) systems.

Abstract: This is disclosed an optical amplifier comprising: an optical waveguide for amplifying signal light incident thereon and outputting thus amplified signal light while pumping light is being supplied; a transmission device, substantially cascaded to said optical waveguide, for transmitting therethrough the signal light with a predetermined transmission characteristic; pumping device for outputting the pumping light and supplying the pumping light to the optical waveguide; temperature detecting device for detecting a temperature of or near the optical waveguide; and control unit for controlling the transmission characteristic of the transmission device according to the temperature detected by the temperature detecting device.

Abstract: The invention relates to optical fiber apparatus comprising an amplifier having optimal gain excursion over a given wavelength band for given input power, and a filter having a bell-shaped transfer function, with the center wavelength of the filter being tunable over a range that is sufficient to enable the gain excursion of the amplifier over said wavelength band to be reduced when the power received by the amplifier is different from said given input power. The invention makes it possible to match amplifier apparatuses to line losses while minimizing overall loss.

Abstract: An apparatus and method are described for exploiting almost the full almost 25 THz of bandwidth available in the low-loss window in optical fibers (from 1430 nm to 1620 nm) using a parallel combination of optical amplifiers. The low-loss window at about 1530 nm-1620 nm can be amplified using erbium-doped fiber amplifiers (EDFAs). However, due to the inherent absorption of the erbium at shorter wavelengths, EDFAs cannot be used below about 1525 nm without a significant degradation in performance. For the low-loss window at approximately 1430-1530 nm, amplifiers based on nonlinear polarization in optical fibers can be used effectively. A broadband nonlinear polarization amplifier (NLPA) is disclosed which combines cascaded Raman amplification with parametric amplification or four-wave mixing. In particular, one of the intermediate cascade Raman order wavelengths &lgr;r should lie in close proximity to the zero-dispersion wavelength &lgr;0 of the amplifying fiber.

Abstract: An apparatus and method are described for exploiting almost the full almost 25TH% of bandwidth available in the low-loss window in optical fibers (from 1430 nm-1620 nm) using a parallel combination of optical amplifiers. The low-loss window at about 1530 nm-1620 nm can be amplified using erbium-doped fiber amplifiers (EDFAs). However, due to the inherent absorption of the erbium at shorter wavelengths, EDFAs cannot be used below about 1525 nm without a significant degradation in performance. For the low-loss window at approximately 1430-1530 nm, amplifiers based on nonlinear polarization in optical fibers can be used effectively. A broadband nonlinear polarization amplifier (NLPA) is disclosed which combines cascaded Raman amplification with parametric amplification or four-wave mixing. In particular, one of the intermediate cascade Raman order wavelengths &lgr;r should lie in close proximity to the zero-dispersion wavelength &lgr;0 of the amplifying fiber.

Abstract: The present invention relates generally to the field of optical communication and particularly to optical communication techniques which compensated for dispersion such as that in optical fibers. Compensation of optical dispersion is achieved with a polarization beam splitter with first, second and third ports, a dispersion compensating fiber, a first return means that changes the mode of polarization of a returned signal and a second return means. One end of the dispersion compensation means is connected to the third port of the polarization beam splitter, the other end is connected to the first return means. The second return means is connected with the second port of the polarization beam splitter, while the first port of the polarization beam splitter forms input and output. As the signal passes through the dispersion compensating fiber four times, the length of the dispersion compensating fiber can be greatly reduced.

Abstract: In a conventional photo signal amplification transmission system, since control is performed to make constant an output power, total output powers of all wavelengths are made constant when a multi-wavelength transmission is made. An output power of one wavelength is decreased and deviated from an optimum power. To solve the problem, a transmission photo amplifier 3, a relay photo amplifier 4 and a receiving photo amplifier 5 are provided with wavelength number information management sections 11, 12 and 13, respectively. An amplifying means is controlled in accordance with the number of wavelengths of the multi-wavelength signal which is obtained by the wavelength number information management sections. The wavelength number information management sections 11 to 13 multiplex a wavelength number information signal onto an optical fiber cable 6 and renews the signal.

Abstract: An optical amplifier with improved output power and reduced noise is disclosed. A first optical fiber amplifier is pumped by a first pumping light and a second optical fiber amplifier is pumped by a second pumping light. A direction-dependent filter is connected between the first and second optical fiber amplifiers. Only input signal light and the first pumping light are passed through into the second optical fiber amplifier. Only the second pumping light is passed through into the first optical fiber amplifier.

Abstract: An optical transmission path monitoring and controlling method including the steps of monitoring a probe light beam and other multiplexed light beams which are included in a multiplex light ray propagated through a transmission path in an optical transmission system, detecting values of the monitored probe light beam and other multiplexed light beams, while the other multiplexed light beams are in a multiplexed state, in relation to predetermined values, respectively. A determination is made based upon the detected value of whether or not at least one of an optical transmission path trunk line, the probe light beam per se, the other multiplexed light beams per se, an optical amplifier and a transmitter is normal.

Abstract: Described is a noise figure measurement for optical amplifiers by disabling a signal of a channel before the measurement of the amplified spontaneous emission for that channel and by distributing a power equivalent which is substantially equal to the power of the signal of the disabled channel, at at least one other wavelength. The noise figure measurement can be used as well in multi-channel applications such as WDM, as in a single channel operation.

Abstract: A partial optical regenerator for use in high bit-rate optical systems and useful in WDM applications employing soliton transmission. The regenerator essentially comprises a saturable optical element in a main optical path and a resonant optical circuit coupled to the element. A resonant pulse can be generated from an input data pulse in a number of ways. (A) an input data pulse may be coupled directly to the resonant optical circuit before entering the element; (B) the input data pulse may first saturate the element, resulting in the creation of a counter-propagating pulse; (C) the data pulse may pass through the element and be coupled to the resonant optical circuit upon exit. The resonant pulse travels around the resonant circuit and repeatedly returns to the saturable optical element after an integer multiple of the symbol duration or after an odd integer multiple of half the symbol duration.

Abstract: A long-band optical fiber amplifier with enhanced power conversion efficiency including a first optical fiber part provided with a pumping light source, a second optical fiber part connected with the first optical fiber part, the second optical fiber part being not provided with the pumping light source, and a reuse circuit coupled between the first optical fiber part and the second optical fiber part for utilizing amplified spontaneous emission (ASE) as a secondary pumping light source from the first optical fiber part. Preferably, the reuse circuit comprises a WDM coupler connected between the first and second optical fiber parts, and a light pumping device connected with the WDM coupler to provide the pumping light source. The first and second optical fiber parts consist of erbium-doped optical fiber, and the erbium-doped optical fiber is adjusted so as to produce a gain in the L-band.

Abstract: A fast means of detecting that a fiber has been disconnected at one end of a bidirectional optical amplifier by monitoring for several signature effects simultaneously. Upon detection of a loss of signal, the pump laser in the corresponding direction is shut off quickly to prevent oscillation from occurring should the other end of the amplifier be subsequently disconnected.

Abstract: The invention is directed to a multistage optical amplifier having a gain stage between a GFF and an optical attenuator. More specifically, in accordance with an illustrative embodiment of the present invention, an optical amplifier system comprises multiple gain stages arranged serially for providing gain to an optical signal propagating therein. The system also includes a gain flattening filter for reducing a variation in gain of the optical amplifier system in a particular wavelength band and an optical attenuation element. One of the gain stages is located between the gain flattening filter and the optical attenuation element to reduce an overall noise factor of the optical amplifier system. Illustratively, each gain stage is a pumped segment of erbium doped optical fiber. The optical attenuation element may precede the gain flattening filter in a direction of propagation of an optical signal in the amplifier system.

Abstract: The present invention relates generally to the field of optical transmission and particularly to a method and a system for controlling optical amplification in wavelength division multiplex optical transmission. WDM systems currently under development will have eighty or more channels, i.e., modulated optical signals with different wavelengths (known as Dense Wavelength Division Multiplexing, DWDM). These DWDM systems are demanding optical amplifiers which, especially considering the cascadation of a plurality of optical amplifiers along the transmission path of the DWDM system, have only very limited tolerances in certain parameters. Among these parameters gain flatness and gain tilt are of special importance. The present invention discloses a method and a system which make use of auxiliary optical signals having a given power level which is equal for all auxiliary signals.

Abstract: The invention provides an optical amplifying system having a structure with two amplification levels to amplify received optical signals. The optical amplifying system includes a Michelson interferometer, a preamplifier, and a post amplifier. In the preamplifier and the post-amplifier, respectively have a light pumping source, a wavelength multiplexer, and an optical gain media. The Michelson interferometer includes an optical coupler and an optical grating set composed of a pair of optical gratings. The optical gratings are located different optical passes, and its central wavelength is equal to the intended wavelength. The optical grating can be formed on a piezoelectric substrate or an electrothermal substrate so that the central wavelength can be changed by applying stress or heat. The intended wavelength reflected by the Michelson interferometer can therefore be adjusted. The preamplifier and the post-amplifier are coupled together through the optical coupler of the Michelson interferometer.

Abstract: An optical amplifier is provided in which optical channels, each at a respective wavelength, make two passes through a segment of erbium-doped optical fiber. After the first pass, certain optical wavelengths lying in the high gain spectrum (“the high gain wavelengths”) of the erbium-doped optical fiber are amplified more than other optical wavelengths lying in the low gain spectrum (“the low gain wavelengths”). The optical channels are then reflected with a reflective element back to the segment of erbium-doped optical fiber for the second pass. The reflective element selectively attenuates the high gain wavelengths to compensate for the excessive gain of the erbium-doped optical fiber at these wavelengths. As a result, after the second pass, the optical power at the high and low gain wavelengths is substantially the same and gain flattening is achieved.

Abstract: The invention concerns a wavelength-division multiplex soliton signal fiber optic transmission system; to reduce the effects of collision jitter on the soliton channels, it proposes quasi-distributed amplification combining lumped amplification and distributed amplification. In this way it is possible to combine pumping of a lightly doped line fiber an dumped amplification by an EDFA in the repeaters. Reduncant and common pumping means are advantageously used for the lumped amplification and the distributed amplification and the gain of the lumped amplification is advantageously controlled automatically.

Abstract: The invention relates to an optical amplifier; more particularly, the invention provides an optical amplifier that contains a front-end optical amplifier and a back-end optical amplifier, and that is equipped with an automatic compensation function for automatically detecting and compensating for signal loss caused by a dispersion compensator inserted between the front-end and back-end optical amplifiers. The optical amplifier, which contains a front-end optical amplifier 31 and a back-end optical amplifier 32, comprises: a loss detection means 36 for detecting optical signal loss occurring between the front-end optical amplifier and the back-end optical amplifier; and a gain control means 37 for compensating, based on the loss detected by the loss detection means, for a variation in optical output power of the entire optical amplifier including the front-end optical amplifier and back-end optical amplifier.

Abstract: A stabilized gain temperature compensated optical amplifier having automatic gain control and automatic level control. An optical amplifier with a temperature dependent gain wavelength characteristic is disposed along an optical transmission line and compensated with a temperature sensitive gain controller cascade connected with the temperature sensitive optical amplifier. The temperature sensitive gain controller has a temperature sensitive loss wavelength characteristic which complements the temperature sensitivity of the amplifier gain wavelength characteristic. The gain controller includes a plurality of grating portions provided with mutually different loss wavelength characteristics, the loss wavelength characteristics having mutually different temperature dependencies. An automatic gain control, independent of the gain controller maintains the gain of the temperature sensitive optical amplifier at a constant level.

Abstract: An optical amplifier having a substantially flat response over a multi-channel bandwidth for signals which have different input levels. It utilizes input and output photodetectors for detecting amplitude variations. The gain can be determined by a microprocessor receiving signals derived from the photodetectors, which causes change in the gain of the amplifier. The microprocessor calculates output ASE noise content at different amplifier input levels and subtracts that noise from the total power received by the output detector thereby determining an accurate amplifier gain value. The gain setting offered by the amplifier will then be the same as what is desired by a network management system.

Abstract: An optical amplifying apparatus which includes an optical amplifier, an optical attenuator and a controller. The optical amplifier amplifies a light signal having a variable number of channels. The optical attenuator passes the amplified light signal and has a variable light transmissivity. Prior to varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator so that a power level of the amplified light signal is maintained at an approximately constant level that depends on the number of channels in the light signal prior to the varying the number of channels. While the number of channels in the light signal is being varied, the controller maintains the light transmissivity of the optical attenuator to be constant.

Abstract: A method of manufacturing an optical filter that can restrain the wavelength dependence of the gain of an optical amplifier. The input signal light power, output power of pump light source (8), and the output signal light power of an optical amplifier (4) are set to the power values used for optical communication systems, and the total power of the simulation input light of a plurality of wavelengths (&lgr;1, &lgr;2, . . . &lgr;n) from light sources (1a) and an input light (probe light) from light sources (1m) are set equal to the total power of the input signals of a plurality of wavelengths used in wavelength division multiplex transmission systems.

Abstract: Disclosed is a fiber optic filter that includes a central core, a ring core concentric with the central core, an inner cladding region of refractive index ni between the central and ring cores, and a cladding layer of refractive index n, surrounding the ring core. The maximum refractive index n, of the central core and the maximum refractive index n2 of the ring core are greater than nc and ni. The propagation constants of one core mode and one ring mode are different at wavelengths except for at least one wavelength &lgr;0 whereby power transfers between the two cores at &lgr;0. At least a portion of the fiber optic filter fiber is wound around a reel to subject it to a continuous curvature, the radius of which determines the amplitude of the attenuation. Such fibers are useful in fiber amplifiers in which the central core contains active dopant ions.

Abstract: An optical fiber used as the active amplifying medium in a fiber laser is arranged to have a high insertion loss at an undesired frequency, while retaining a low insertion loss at a desired lasing frequency. In one embodiment, loss at a Raman-shifted frequency is introduced by using an optical fiber which has multiple claddings with an index profile that includes an elevated index region located away from the core, but within the evanescent coupling region of the core. A distributed loss, which can be enhanced by bending, is produced at the Raman frequency which effectively raises the threshold at which Raman scattering occurs in the fiber and therefore results in a frequency-selective fiber. In another embodiment, an absorbing layer is placed around the core region. The absorbing layer is chosen to have a sharp absorption edge so that it absorbs highly at the Raman-shifted wavelength, but minimally at the desired lasing wavelength.

Abstract: A waveguide device for optical signals in an optical communication system relies upon photonic bandgap material to guide the optical signal via a plurality of pathways. A two-dimensional photonic bandgap material is formed in a planar slab of dielectric medium by a two-dimensional lattice in which discontinuities in the lattice region define waveguides. A waveguide array formed in the device allows multiple Mach-Zehnder interferometer devices to be constructed including small radius turns without significant losses. An optical signal equalizer formed of such a device relies upon the transfer function defined by a multiple arm Mach-Zehnder having arms of different length. By applying controlled modulation to propagation through the arms, an adaptive equalizer is formed. The adaptive equalizer has application in correcting gain tilt in line amplifiers of optical communications systems.

Abstract: The invention relates to an optical fiber having a core based on silica with at least one fluorescent dopant distributed over the entire section of the core, a central zone co-doped with a co-dopant chosen from Al and P, and a peripheral zone co-doped with the other co-dopant chosen from Al and P. The invention can be used in an optical fiber amplifier, and it guarantees good amplifier gain flatness over a wavelength band suitable for WDM transmission.

Abstract: An apparatus and method for controlling the gain in an erbium-doped fiber amplifier (EDFA) incorporated into a multi-wavelength communication system so as to amplify each of the wavelength signals. The amplifier operates near to saturation so that, if one or more of the multi-wavelength signals is removed from the transmission, the remaining channels are increasingly amplified, leading to problems with other components in the system which depend upon intensity. According to the invention, an optical signal at a wavelength that is not within any of the transmission channels is selectively fed back around the amplifier and caused to lase in a wavelength-filtered ring-laser configuration. The lasing signal governs the saturation of the amplifier such that any gain shed by a disappearing data signal is predominately used by the lasing signal, not by the remaining data signals. Thereby, the data signals do not experience gain variations dependent upon the number of data signals being amplified.

Abstract: There is disclosed a technique regarding an optical amplifier in an optical communications system. In order to effectively suppress optical surges during amplification of signal light, through a simple structure, there is provided an amplifying medium which amplifies input light and outputs the amplified light, and reflecting members are connected to the input and output sides of the amplifying medium. The reflecting members can reflect light with a predetermined optical wavelength different from a signal light wavelength contained in the input light.

Abstract: An apparatus which receives an input light having a spectrum, and determines a momental wavelength of the spectrum. The apparatus includes a decoupling unit, a weighting unit and a computation unit. The decoupling unit decouples a portion of the received input light, to provide a first signal representing the input light with the portion decoupled therefrom, and a second signal representing the decoupled portion. The weighting unit weights the second signal. The computation unit determines the momental wavelength from the power of the first signal and the power of the weighted second signal. An optical amplifier is also provided which determines the momental wavelength of an amplified light, and controls a gain tilt parameter of the optical amplifier in accordance with the determined momental wavelength, to reduce gain tilt. An optical communication system is also provided which includes a plurality of such optical amplifiers sequentially arranged along an optical transmission line.