OPTICAL WAVEGUIDE AMPLIFIER

Abstract: An optical fibre amplifier (50) comprises a SiO2-Al2O3-GeO2 single-mode optical fibre (52) doped with Er<3+>. It is pumped by 1.55 mu m and 1.47 mu m optical sources (54 and 56) whose optical outputs are combined by an optical coupler (58) and are then coupled to the fibre (52) by a further optical coupler (50). A source of optical signals to be amplified (62) is also coupled to the fibre (52) by the further coupler (60). The Er<3+> ions provide a three-level lasing scheme with a fluorescence peak at about 1.53 mu m. Low noise amplification of optical signals in the long-wavelength tail of the fluorescence spectrum with suppressed ASE at the fluorescence spectrums peak wavelength is obtained as follows. The 1.47 mu m pump provides some low noise amplification of the optical signal but preferentially amplifies the 1.55 mu m pump source. The resultant 1.55 mu m amplified pump signal then provides pump power to amplify the optical signal whilst suppressing ASE as it has a longer wavelength than the fluorescence peak wavelength.

Description: OPTICAL WAVEGUIDE AMPLIFIER This invention relates to optical waveguide amplifiers and in particular to those comprising an optical waveguide doped with an active species providing a three-level laser scheme associated with a primary fluorescence peak at a first wavelength. In this specification the term "optical" is intended to refer, to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. A three-level laser scheme comprises a lower lasing level or band of levels (LLL), that is the ground state or an energy level so close to it that it has a significant thermal population, or band of such levels, an upper lasing level or band of levels (ULL) and one cr more pump bands of higher energy than the bottom of the ULL. In such a scheme, absorption from the LLL band to the ULL band can take place in direct competition with the stimulated emission of photons obtained by transitions in the reverse direction. Pumping such an amplifier with an optical pump of wavelength shorter than the fluorescence peak associated with the lasing transition and corresponding to a pump band will produce, in well known fashion, a depopulation of the LLL and an increase in population of the ULL. The fractional population of the ULL is dependent on the pump power and the relative gain coefficient of the amplifier for a given signal wavelength is proportional to N 2 σ 2 (λ)-N,σ-(λ) where 1^ and N 2 are the fractional population densities of the LLL and ULL respectively and σ^ and σ~ are the absorption and emission cross-sections. Three-level laser scheme waveguide amplifiers can provide gain to signals at a wavelength in the long-wavelength tail of the fluorescence spectrum but there are problems connected with amplified spontaneous emission (ASE) at the gain-peak wavelength. According to the present invention an optical waveguide amplifier comprises an optical waveguide doped with an active species providing a three-level laser transition associated with a primary fluorescence peak at a first wavelength characterised in that the waveguide is coupled to at least a first optical pump source for providing optical pump power at a second wavelength which is longer than the first wavelength. Pumping the amplifier at a wavelength longer than the wavelength of the primary fluorescence peak eliminates gain and therefore ASE, at this wavelength. The invention is expected to find particular application in providing amplification of optical signals transmitted through optical fibre communications networks. A convenient class of waveguide amplifier for use with such silica-based optical fibre networks is the optical fibre doped with a rare-earth ion as it is readily incorporated in such a network. For example, a 3+ Si0 2 - l 2 0 3 -Ge0 2 optical fibre doped with Er ions can provide gain in the l.55*tm telecommunications window. Such an optical amplifier has been found to provide useful gain around 1.60μm for a pump wavelength of 1.55«m achieving a maximum useful spectral bandwidth of 1.57ym to l.βlym. The present invention is not restricted in its application to . . 3+ this specific silica-based Er doped lasing scheme. Other doped waveguides exhibiting three-level lasing schemes may be employed, for example planar silica or lithium niobate waveguides doped with a suitable dopant. Similarly other optical fibre hosts, such as fluoride fibres, or other appropriate dopants, for example ytterbium may be used with the pumping scheme of the present invention. Preferably the amplifier includes a second optical pump source for providing optical pump power at a wavelength shorter than the first wavelength. As will be explained in more detail below this provides superior noise performance while maintaining the ability to suppress ASE build-up at the fluorescence peak wavelength. For 3+ such a silica based Er doped, optical fibre amplifier, the second optical pump source is preferably a semiconductor laser providing pump power at about 1.47ym. An optical waveguide amplifier according to the present invention may comprise a first optical waveguide amplifier comprising an optical waveguide doped with an active species providing a three-level laser scheme associated with a primary fluorescence peak at a first wavelength coupled to a first optical pump source for providing optical pump power at a second wavelength which is longer than the first wavelength and a second optical waveguide amplifier comprising a waveguide doped with the same active species as the first optical waveguide amplifier and including a third optical pump source for providing optical pump power at a wavelength which is shorter than the first wavelength, an optical coupling means for coupling portions of an optical signal to a respective one of the first and second optical waveguide amplifiers, and an optical combining means for combining the portions of the optical signal after propagation along the first and second optical waveguide amplifiers. Such an amplifier provides a composite amplifier having a broader bandwidth than a single fibre amplifier doped with the active species. According to a further aspect of the present invention an optical transmission system comprises an optical waveguide amplifier according to present invention to which is coupled a source of optical signals at a wavelength longer than the second wavelength. According to a yet further aspect of the present invention a method of amplifying optical signals comprises coupling the optical signals to an optical waveguide amplifier according to the present invention whilst providing pump power from each optical pump source, the optical signals having a wavelength longer than the second wavelength. The present invention and its operation will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic diagram of an optical Er 3+-doped fibre amplifier according to the present invention configured to permit its amplifying behaviour to be studied; Figure 2 is a graph of the relative absorption and emission cross-sections for the doped fibre of Figure 1; Figure 3 is a graph of the relative gain spectrum of the doped fibre of Figure 1 for a fractional inversion of 0.35? Figure 4 is a graph of the small signal gain spectra for various lengths of doped-fibre in the embodiment of Figure 1; Figure 5 is a graph showing the pump power dependence of the small signal gain for a 150m length of doped fibre in the embodiment of Figure 1; Figure 6 is a graph of the signal output power against signal input power for a 150m length of doped fibre in the embodiment of Figure 1; and Figure 7 is a schematic diagram of a further embodiment of the present invention; Figure 8 is a schematic diagram of a further embodiment of the present invention using two optical pumps. Referring to Figure 1, an optical waveguide amplifier 2 includes a Si0 2 - l 2 0 3 -Ge0 2 optical fibre 4 having a core diameter of 5.5/.m and Δn of 0.015 doped with Er ions to a level corresponding to an absorption of 5.5dB/m at the 1.53wm absorption peak (see Figure 2). It had a background loss measured at l.lum of less than 0.01 dB/m. Figure 2 shows the relative absorption and emission cross-sections as a function of wavelength for zero inversion (i.e. unpu ped fibre) and full inversion of the three-level laser transition scheme associated with the 1.53am fluorescence peak. As discussed in an article entitled "High-gain, broad spectral bandwidth erbium-doped fibre amplifier pumped near 1.5am" by C.G. Atkins, J.F. Massicott, J.R. Armitage, R. Wyatt, B.J. Ainslie and s.P. Craig-Ryan, Electronics Letters 6th July 1989 Vol 25 No. 14 pp 910-911, the maximum population inversion for intense pumping at any wavelength can be calculated from the absorption and emission cross-sections assuming equal absorption and emission rates at the pump wavelength. Incomplete inversion affects both the peak value of the gain obtainable for any given fibre length and also the shape of the gain spectrum. The result for a fractional inversion of 0.35 is shown in Figure 3. The invention in its broadest aspect is based on the appreciation by the applicants that operation of the fibre amplifier at a suitably low fractional population inversion will provide gain at wavelengths in the long-wavelength tail of the fluorescence spectrum of the laser transition whilst suppressing gain at the peak wavelength and, in particular, this can be achieved by pumping the waveguide at a second wavelength longer than the fluorescence peak wavelength. The actual fractional population to avoid ASE depends on the particular waveguide and dopant. Referring again to Figure l, a stabilised Nd:ϊAG pumped colour centre laser 6 operating with a NaCl:OH crystal and tunable from <1.45am to >1.65αm provides the optical pump power to pump the doped fibre waveguide 4. An external cavity semiconductor laser 8 tunable from 1.56αm to 1.65αm is used to provide signals to be amplified, which signals (probe signals) are passed through a precision optical attenuator 10 followed by a polarisation insensitive optical isolator 12 both fibre-coupled and connected by splicing. The pump and probe signals are combined by a dichroic fibre coupler 14 through which maximum pump transmission occurred at 1.55αm and maximum signal transmission at 1.6am. The combined pump and probe signals are coupled to the doped fibre 4 by a precalibrated l°/o coupler tap 16 that allows monitoring of the pump and probe power levels at the input of the fibre 4 by a monitor photodiode 18. The output signal from the fibre 4 was passed through a fixed fibre attenuator 20 of 30dB and measured on an optical spectrum analyser 22. Preliminary optimisation of the pump wavelength between 1.54αm and 1.57αm revealed 1.55αm as the best compromise achieving maximum spectral bandwidth of 1.57αm to 1.61am. This was used for all the following measurements. For the measurement requiring a fixed probe wavelength, 1.6αm was chosen. The maximum pump and signal powers launched into the fibre 4 were 160m and O.lmW (-lOdBm), respectively. The residual pump power was always less than lO W. Figure 4 shows the small signal gain spectra obtained for 150m, 175m and 200m of fibre pumped with 140m . The input signal level was held constant at -28dBm. For 150m of fibre 4 a broadband gain response in excess of 25dB was measured between 1.57αm and 1.61αm, with a maximum value of 31dB. The shift in the gain spectra to shorter wavelengths as the fibre is cut back is consistent with a higher nett fractional inversion in the fibre. However, a much sharper than expected and length-independent cut-off in the gain " in 1.61αm is seen which is consistent with excited state absorption measurements. Figure 5 shows the pump power dependence of the small signal gain in the 150m length of fibre 4. The curve departs from linearity for launched pump powers greater than 80m . This is probably the result of gain saturation due to amplified spontaneous emission. At this optimum point 0.33dB of gain per mW of pump is available. Signal output against input power for 140mW of pump power is shown in Figure 6. Saturation of the gain begins to occur at signal input levels of -l8dBm. For the maximum signal launched the gain was 25dB and for an increased pump power of 160mW, an output signal of 45m was obtained. 3+ The above demonstrates the use of Er doped silica fibre as a 1.6αm broadband amplifier, obtaining in excess of 25dB of small signal gain between 1.57αm and 1.61αm by pumping at 1.55αm. An optimum small signal gain of 0.33dB/mw and a saturated output of 45mW for 160mW of pump power have been achieved. Referring now to Figure 7 an optical fibre amplifier 3+ compπses an Er doped silica-based optical fibre amplifier 30 optimised for amplification at 1.55αm in known manner in parallel 3+ with an Er doped silica-based optical fibre amplifier 32 pumped by an optical pump source at 1.55αm, as in the embodiment of Figure 1. A source of optical signals 34 is coupled to the amplifiers 30 and 32 by means of an optical fibre coupler 36. The portions of the signals amplified by the respective amplifiers 30 and 32 are combined by the optical coupler 38 for onward transmission along an fibre 40 spliced to the coupler 38. The arrangement of Figure 7 can provide a bandwidth of about 70nm making it suitable for amplification in wavelength division multiplexed systems. Such amplifiers may not, in general have an optimum noise performance. The noise figure for a three level laser transition scheme which is pumped directly into the metastable upper lasing level depends on the pump wavelength. A minimum value is obtained for high pump powers in which case Fmin = 2/(I-(R pump /R signal )) the ratios of emission to pump and signal wavelengths, respectively. The R value for 1.47αm is typically 0.34, for 1.55αm, typically 1.7, and for 1.58αm, typically 3 for a SiC -Al 2 0,-Geθ 2 fibre 4 of Figure 1. Using the noise figure equation for a 1.47αm-pumped amplifier amplifying signals at l.55αm, one obtains a theoretical minimum noise figure of 3.9dB, while if one takes the case of a 1.55αm-pumped amplifier amplifying at at l.58αm, one obtains a value of 6.6dB, so this case is clearly worse. This is due to the much closer proximity of the pump wavelength to the signal wavelength. This proximity is necessary according to the teaching of the present invention to obtain a suitably tailored gain spectrum, while at the same time suppressing lasing and ASE nearer the gain peak. A 1.47αm-pumped, 1.58αm amplifier would be capable of a noise figure of 3.3dB if a way could be found to suppress the effects of ASE at the gain peak. A way to obtain the benefit of shorter pumping wavelengths with their superior noise performance while maintaining the ability to suppress ASE build-up at the gain peak is to utilise two pump wavelengths. The first pump, at 1.47αm say, is a strong pump, and provides a few dB of low noise gain for the 1.58αm signal. The second pump is near 1.55αm and has a power of O.lm -lmW at the fibre input. The 1.47αm pump, at a wavelength less than the fluorescence peak of the three level laser scheme, acts as a pump for this 1.55αm second pump for which the fibre has higher gain than for the 1.58αm signal. Hence, after some distance down the fibre, the amplified " 1.55αm secondary pump will deplete the power from the primary pump, and hence prevent the further build up of ASE at the gain peak. Transfer of power to the secondary pump will be very efficient and the amplified 1.55αm will now act as a new primary pump for the 1.58αm signal See "Efficient, high power, high gain, erbium doped silica fibre amplifier", J.S. Hassicott, R. Wyatt, B.J. Ainslie, S.P. Craig-Ryan, Electronics Letters Vol 26 No. 14 ppl038-9. Because this signal is now at a higher level, the noise penalty from the close pump wavelength is less significant. This pumping scheme effectively utilises the front end of the fibre, which is pumped at 1.47αm, as a built-in, low-noise preamplifier. In this way one can obtain the combined benefits of low noise, and high gain. An embodiment of the present invention utilising this dual pumping scheme is shown in Figure 8 in which an 3+ Si0 2 -Al 2 0„-Geθ 2 , Er doped, optical fibre 52 is pumped by both 1.55αm and 1.47αm sources 54 and 56 whose outputs are combined by an optical fibre coupler 58. The combined output and an optical signal source 62 are coupled to the fibre by a further optical fibre coupler 60. Dielectric, dichroic beam combiners may be used instead of the couplers 58 and 60 to obtain a flatter pass band but at the possible expense of increased insertion loss.

Claims: CLAIMS 1. An optical waveguide amplifier comprising an optical waveguide doped with an active species providing a three-level laser transition associated with a primary fluorescence peak at a first wavelength characterised in that the waveguide is coupled to at least a first optical pump source for providing optical pump power at a second wavelength which is longer than the first wavelength. 2. An amplifier as claimed in claim 1 in which the waveguide comprises a silica-based optical fibre doped with a rare-earth, ion. 3. An amplifier as claimed in claim 2 in which the waveguide comprises a SiC -A C -GeCk optical fibre doped and the active species is Er . 4. An amplifier as claimed in claim 3 in which second wavelength is between 1.54αm and 1.57αm. 5. An amplifier as claimed in claim 4 in which the second wavelength is 1.55αm. 6. An amplifier as claimed in any preceding claim in which there is included a second optical pump source for providing optical pump power at a third wavelength which is shorter than the first wavelength. 7. An amplifier as claimed in claim 6 as dependant on either one of claims 4 and 5 in which the second optical pump source is a semi-conductor laser providing pump power at about l.47αm. 8. An optical waveguide amplifier comprising a first optical waveguide amplifier as claimed in any one of claims 1 to 7; a second optical waveguide amplifier comprising a waveguide doped with the same active species as the first optical waveguide amplifier and including a third optical pump source for providing optical pump power at a wavelength which is shorter than the first wavelength; an optical coupling means for coupling portions of an optical signal to a respective one of the first and second optical waveguide amplifiers, and an optical combining means for combining the portions of the optical signal after propagation along the first and second optical waveguide amplifiers. 9. An optical waveguide amplifier as claimed in claim 8 as dependent on any one of claims 2 to 7 in which the optical coupling means and optical combining means comprise optical fibre couplers. 10. An optical transmission system comprising an optical waveguide amplifier as claimed in any preceding claim to which is coupled a source of optical signals at a fourth wavelength which is longer than the second wavelength. 11. A method of amplifying optical signals comprising coupling optical signals to an optical waveguide amplifier as claimed in any one of claims 1 to 9 whilst providing pump power from each optical pump source, the optical signals having a wavelength which is longer than the second wavelength. 12. An optical amplifier as hereinbefore described with reference to Figure 1 or Figure 7. 13. A method of amplifying optical signals as hereinbefore described with reference to the drawings.

Inventors: ARMITAGE JONATHAN RICHARD, MASSICOT JENNIFER FRANCES, WYATT RICHARD, ARMITAGE, JONATHAN, RICHARD, MASSICOT, JENNIFER, FRANCES, WYATT, RICHARD
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: H01S3
IPC codes: H01S3