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Erbium doped fibersRelated Patent Categories: Coherent Light Generators, Particular Active MediaErbium doped fibers description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060153260, Erbium doped fibers. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The invention relates to using erbium doped fibres for the production of green light signals, and particularly using such erbium doped fibres in amplifiers and lasers. [0002] Erbium doped fibres (EDFs) have been used in various applications for some time. In particular, EDFs have been used in amplifiers in telecommunication networks. Such erbium doped fibre amplifiers (EDFAS) amplify traffic-carrying signals in the wavelength region of approximately 1520 mm to approximately 1610 nm, i.e. around 1550 nm. Amplification is achieved by the interaction of photons of the traffic/carrying signals with electrons of erbium ions in a metastable state, approximately 1550 nm above the erbium ion ground state. The interaction causes stimulated emission by the electrons, producing photons in phase with and of approximately the same wavelength as those of the traffic-carrying signals. Each photon of the traffic-carrying signals which interacts in this way thus becomes two photons, and amplification occurs. [0003] For efficient amplification by the above process, the traffic/carrying signals need to encounter an EDF where more erbium ions are in the metastable state than in the ground state. This is achieved by a process known as pumping, in which signals from a pump source are coupled into the EDF where they are absorbed by electrons of erbium ions in the ground state populating the metastable state. Pump signal wavelengths of approximately 980 nm or 1480 nm are widely used, as the absorption spectra of EDFs exhibit peaks around these wavelengths and pump sources having these wavelengths are available. When 980 nm pump signals are used these cause ground state absorption (GSA) in the EDF, i.e. the 980 nm pump signals are absorbed by electrons of the erbium ions in the ground state raising them to a higher, pump state with subsequent decay to the metastable state. In addition to GSA of 980 nm pump signals another process, known as excited state absorption (ESA), also occurs within the EDF. In this, electrons of the erbium ions in the pump state absorb 980 nm pump signals raising them to yet higher, excited states from which they decay emitting photons having wavelengths in the range approximately 520 nm to approximately 560 nm, i.e. green light signals. Such ESA has heretofore been considered a negative effect, which results in a decrease of the efficiency of amplification in an EDFA by decreasing the population of the metastable state, and EDFA conditions have previously been chosen to minimise this effect and hence the production of green light signals. However, it has been realised by the inventors that such ESA-produced green light signals have a number of advantageous applications, e.g. they can be used to bring about an increase in the amplification of traffic-carrying signals in an EDFA. The invention therefore proposes the enhancement of green light signal production in an EDF, and applications of such green light signals. [0004] According to a first aspect of the invention there is provided a method of producing green light signals, comprising coupling pump signals from at least one pump source into at least one erbium doped fibre (EDF) which cause ground state absorption (GSA), and excited state absorption (ESA) in erbium ions of the EDF, which produces green light signals, wherein the majority of the pump signals have a wavelength at which the probability of occurrence of ESA in the EDF is greater than the probability of occurrence of GSA in the EDF. [0005] According to a second aspect of the invention there is provided a device for producing green light signals, comprising [0006] at least one erbium doped fibre (EDF), coupled to at least one pump source to receive pump signals therefrom, which cause ground state absorption (GSA), and excited state absorption (ESA) in erbium ions of the EDF, which produces green light signals, the majority of which pump signals have a wavelength at which the probability of occurrence of ESA in the EDF is greater than the probability of occurrence of GSA in the EDF. [0007] By green signals, it is meant signals which each have a wavelength which lies in the range approximately 520 nm to approximately 560 nm. [0008] By choosing such pump signal wavelengths, the occurrence of ESA is privileged with respect to the occurrence of GSA in the EDF. Once some ground state erbium ions have undergone GSA, further incident pump signals will be more likely to be absorbed in ESA of these pump state erbium ions than in GSA of other ground state erbium ions. The enhanced occurrence of ESA will result in enhanced production of green light signals, with regard to that of known EDFs. As already stated, it has been realised by the inventors that green light signals have a number of advantageous applications, and that the production of these signals is therefore desirable. [0009] Preferably, at least 60% of the pump signals have a wavelength at which the probability of occurrence of ESA in the EDF is greater than the probability of occurrence of GSA in the EDF. [0010] There are a number of pump signals wavelengths which may be used, at which the probability of occurrence of ESA within the EDF will be greater than the probability of occurrence of GSA within the EDF. The majority of the pump signals may have a wavelength in the range approximately 920 nm to approximately 980 nm. The majority of the pump signals may have a wavelength in the region of 960 nm. [0011] The probability of occurrence (or cross section) of GSA and the probability of occurrence (or cross section) of ESA versus wavelength spectra for EDFs exhibit a number of regions of enhanced cross section, in the form of peaks. A GSA cross section peak and an ESA cross section peak occur together at a number of particular wavelengths, with the ESA cross section peak generally down-shifted in wavelength with regard to the GSA cross section peak. For example, there is a peak in both the GSA and ESA cross section versus wavelength spectra in the region of 980 nm. For each GSA/ESA cross section peak set, because of the ESA down-shift, at a particular wavelength the upper side of the ESA peak will intersect the lower side of the GSA peak, i.e. a crossover point between the GSA and ESA peaks exists at a crossover wavelength. The actual crossover wavelength will vary from EDF to EDF and will depend, for example, on the composition of the EDF. At wavelengths less than the crossover wavelength, the cross section of ESA will be greater than the cross section of GSA, i.e. ESA will be privileged with respect to GSA at these wavelengths. The majority of the pump signals may have a wavelength less than the crossover wavelength of an EDF GSA and ESA cross section peaks crossover point. For the GSA/ESA cross section peak pair in the region of 980 nm, the crossover wavelength may occur between 920 nm and 980 nm, depending, for example, on the EDF, and the majority of the pump signals may have a wavelength in this range. [0012] A pump source may be coupled to an EDF such that the pump signals are coupled into the EDF to propagate therealong in a first direction. Additionally, a pump source may be coupled to an EDF such that the pump signals are coupled into the EDF to propagate therealong in a second direction, opposite to the first direction. Two or more EDFs may be provided in a chain. A pump source may be coupled to a first EDF in the chain such that the pump signals are coupled into the EDF to propagate therealong in a first direction, and/or a pump source may be coupled to a last EDF in the chain such that the pump signals are coupled into the EDF to propagate therealong in a second direction, opposite to the first direction. The or each pump source may be coupled to an EDF using a pump coupler. [0013] The or each or some of the pump sources may comprise a laser diode. The or each or some of the pump sources may comprise a distributed feedback (DFB) laser. The or each or some of the pump sources may comprise a Fabry-Perot laser. The or each or some of the Fabry-Perot lasers may output pump signals having wavelengths in the range approximately 940 nm to approximately 1000 nm. The or each or some of the pump sources may output a power level in the range approximately 50 mW to approximately 1 W, or higher. The level of green light signal production is directly proportional to the output power level of the or each pump source. [0014] The method of producing green light signals may comprise reflecting at least some pump signals escaping from the or each or some of the EDFs back into the EDF. This may comprise placing a pump signal reflector at a first end of the or each or some of the EDFs, and/or placing a pump signal reflector at a second end of the or each or some of the EDFs. Two or more EDFs may be provided in a chain, and this may comprise placing a pump signal reflector at an outer end of a first EDF in the chain, and/or placing a pump signal reflector at an outer end of a last EDF in the chain. In such an arrangement, the or each pump signal reflector may comprise part of a device for producing green light signals, or may be placed outside the device. The or each or some of the pump signal reflectors may reflect pump signals having wavelengths in the range approximately 920 nm to approximately 980 nm. The or each or some of the pump signal reflectors may comprise a grating. Using such reflectors will increase the production of the green light signals. [0015] The or each or some of the EDFs may be in the form of a coil of fibre. The or each or some of the EDFs may comprise different dopants, the concentration of which is chosen to enhance the occurrence of ESA and result in enhanced production of green light signals, with regard to that of known EDFs. The or each or some of the EDFs may have one or more of the following characteristics: absorption peak at approximately 1530 nm of approximately 6 dB/m to approximately 8 dB/m, mode field diameter of approximately 5.3 cm, numerical aperture of approximately 0.25 to approximately 0.29, a cut-off in the range of approximately 850 nm to approximately 970 nm. The or each or some of the EDFs may have a length in the range of approximately 5 cm, e.g. with a heavy dopant concentration or using different bulk material, to approximately 80 m, which may depend on the absorption coefficient of the fibre. [0016] According to a third aspect of the invention there is provided a method of amplification of traffic-carrying signals in an erbium doped fibre amplifier (EDFA), comprising, pumping the EDFA with green light signals produced by the method according to the first aspect of the invention. [0017] According to a fourth aspect of the invention there is provided an erbium doped fibre amplifier (EDFA) for amplifying traffic/carrying signals, which is pumped by green light signals produced by the method according to the first aspect of the invention. [0018] The absorption versus wavelength spectra of EDFs exhibit a peak in the region of 550 nm, which is larger than the peak in the region of 980 nm. Thus the absorption of green light signals by an EDF will be high, and, in particular, will be higher than the absorption of 980 nm signals. When 980 nm ESA occurs within an EDFA, this populates a number of excited states. Some of the electrons in these states will decay to the ground state spontaneously resulting in the production of green light signals. Some of these signals will interact with electrons in the excited states causing them to decay to lower states, i.e. stimulated emission occurs, and this results in the production of photons having wavelengths of approximately 800 nm, 1480 nm and 1530 nm. The 800 nm and 1480 nm photons are absorbed by the EDFA and result in the population of the metastable state (i.e. behave as pump signals), and subsequent amplification of traffic-carrying signals having wavelengths around 1550 nm. The 1530 nm photons amplify the traffic/carrying signals in a direct way. So, it can be seen that green light signals, when absorbed in an EDFA, will provide amplification of traffic/carrying signals having wavelengths around 1550 nm. In pumping the EDFA with green light signals produced by the method of the first aspect of the invention, as the production of the green light signals is enhanced over that which would occur in known EDFs, and as the absorption of such green light pump signals in EDFs is higher than the absorption of other, e.g. 980 nm, pump signals, the amplification produced by the above method will be greater than that of known EDFAs. [0019] The green light signals may be produced substantially externally to the EDFA, and may be coupled into the EDFA. The green light signals may be produced using one or more devices according to the second aspect of the invention, coupled to the EDFA. A device may be coupled to a first end of the EDFA, and/or a device may be coupled to a second end of the EDFA. The EDFA may comprise one or more EDFs. One or more devices may be coupled to the or each or some of the EDFs. A co-directional device may be coupled to an EDF, i.e. green light signals are coupled into the EDF to propagate therealong in the same direction as the traffic/carrying signals. Additionally or alternatively, a counter-directional device may be coupled to an EDF, i.e. green light signals are coupled into the EDF to propagate therealong in the opposite direction to the traffic-carrying signals. The EDFA may comprise two or more EDFs in a chain, and a co-directional device may be coupled to a first EDF in the chain, and/or a counter-directional device may be coupled to a last EDF in the chain. The or each device may be coupled to an EDF using a coupler. [0020] The green light signals may be produced substantially within the EDFA. This may comprise pumping the EDFA with one or more pump sources coupled to the EDFA, in which the majority of the pump signals have a wavelength at which the probability of occurrence of ESA in the EDFA is greater than the probability of occurrence of GSA in the EDFA. Preferably, at least 60% of the pump signals have a wavelength at which the probability of occurrence of ESA in the EDF is greater than the probability of occurrence of GSA in the EDF. The majority of the pump signals may have a wavelength in the range approximately 920 nm to approximately 980 nm. The majority of the pump signals may have a wavelength in the region of 960 nm. The majority of the pump signals may have a wavelength less than the crossover wavelength of an EDF GSA and ESA cross section peaks crossover point. A pump source may be coupled to a first end of the EDFA, and/or a pump source may be coupled to a second end of the EDFA. The EDFA may comprise one or more EDFs. One or more pump sources may be coupled to the or each or some of the EDFs. A co-directional pump source may be coupled to an EDF, i.e. pump signals are coupled into the EDF to propagate therealong in the same direction as the traffic/carrying signals. Additionally or alternatively, a counter-directional pump source may be coupled to an EDF, i.e. pump signals are coupled into the EDF to propagate therealong in the opposite direction to the traffic-carrying signals. The EDFA may comprise two or more EDFs in a chain, and a co-directional pump source may be coupled to a first EDF in the chain, and/or a counter-directional pump source may be coupled to a last EDF in the chain. The or each pump source may be coupled to an EDF using a pump coupler. Using one or more such pump sources, some of the pump signals will give rise to GSA, ESA and green light signal production, and subsequent amplification of traffic-carrying signals. Other pump signals will give rise to GSA and amplification of traffic/carrying signals. The two amplification processes work together resulting in an EDFA with even greater amplification than known EDFAs. [0021] The or each or some of the pump sources may comprise a laser diode. The or each or some of the pump sources may comprise a distributed feedback (DFB) laser. The or each or some of the pump sources may comprise a Fabry-Perot laser. The or each or some of the Fabry-Perot lasers may output pump signals having wavelengths in the range approximately 940 nm to approximately 1000 nm. The or each or some of the pump sources may output a power level in the range approximately 50 mW to approximately 1 W, or higher. The level of green light signal production and therefore amplification is directly proportional to the output power level of the or each pump source. Continue reading about Erbium doped fibers... 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