GB2239125A - Optical fibre superfluorescent source - Google Patents

Abstract

A superfluorescent source comprises an optical fibre waveguide of erbium-doped glass. The pump light intensity is sufficient to saturate the gain of the waveguide medium at the fibre output end and cause superfluorescence in the linear region of the pump input/fluorescence output characteristic.

Description

IMPROVED OPTICAL FIBRE SUPERFLUORESCENT NT SOURCE The present invention relates to a light source, of the kind whose output has low temporal coherence.

A number of optical sensors, in particular the fibre optic gyroscope, (hereafter referred to as the FOG) require a low temporal-coherence source for optimum operation. Use of a low temporal-coherence source overcomes detrimental light interference effects associated with reflections from surfaces or other refractive index perturbations such as scattering in the light path within the sensor. In the case of the FOG, non-reciprocity in the light path of the fibre coil due to the optical Kerr effect is also considerably reduced with a broadband source. In addition, many optical sensor devices require a light source which gives high optical intensities in order to maximise the signal-to-noise ratio obtained from the sensor.

Light-emitting diodes (LED) exhibit low temporal coherence, but, in general are not considered optimum light sources for sensors owing to the low power that can be coupled into a single mode fibre, typically, luW. Superluminescent diodes (SLD) on the other hand show low temporal coherence and are capable of injecting considerably higher power levels into a single-mode fibre, around 1mW. However, in conanon with all semiconductor light sources both LEDs and SLDs show a marked shift in operating wavelength with temperature (typically 100-400 ppm/OC). This is a severe limitation in a number of optical sensor applications, most particularly the FOG, where very high wavelength stability is required.The wavelength of the light source directly determines the scale factor of the FOG, where scale factor is defined as the gyro output for a given rotation rate. An uncertainty in the scale factor of the gyro leads to an error in the perceived orientation of the gyro, particularly when sensing high rotation rates.

It is clear from the above that a requirement exists for a light source which shows high intensity, low temporal coherence and high spectral stability with respect to environmental changes. One means of achieving this is to pump optically a rare-earth doped glass fibre on any one of a number of absorption lines in order to achieve superfluorescent (sometimes called super-radiant or superluminescent) emission associated with the high gain of such a fibre structure. This concept has previously been described in European Patent Application No. 0179320 (Snitzer and Ezekiel) with neodynium identified as the optimum rare-earth species in the amplifying medium.

Despite the fact that erbium is a three-level laser medium which would normally lead to poor spectral stability, we have found that under certain operating conditions an erbium-doped superfluorescent fibre source can be made unexpectedly stable.

In accordance with the invention there is provided a fluorescent light source comprising a rare earth-doped glass waveguide which exhibits 3-level characteristics, the waveguide being pumped optically with light at a wavelength corresponding to one of the absorption bands of the rare earth dopant, the intensity of the pump light being sufficient to cause amplified stimulated emission at a level which saturates the gain of the waveguide medium at the output end of the waveguide and causes superfluorescence in the linear region of the pump input/fluorescence output characteristic. Saturation is defined as occurring when the magnitude of the stimulated emission is equal to, or exceeds, that of the spontaneous emission. Under saturated operating conditions, the waveguide fluorescence output is linearly related to the pump intensity.

An embodiment of the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 is a schematic diagram of an optical fibre superfluorescent source in accordance with the invention; Figure 2 is a schematic diagram showing the energy level structure of erbium incorporated into a glass matrix; Figure 3 shows the variation of fibre output superfluorescence power with coupled pump power using a pump wavelength of 980no and an erbium-doped germano-silicate fibre; Figure 4 shows the variation of the mean wavelength of emitted light with pump power for the same arrangement as Figure 3; Figure 5 shows the variation in emission wavelength with pump wavelength at pump power above 15mW;; and Figure 6 is a schematic diagram of a preferred superfluorescent source in accordance with the invention.

As will become apparent from the following, it proved that, owing to the three level nature of the erbium species, large variations in the optical spectrum do indeed occur with temperature, as expected. However, under certain specific operating conditions it has been found that excellent spectral stability can be obtained, along with high light intensity and high energy conversion efficiency.

A preferred form of optical fibre source according to the invention uses an erbium-doped optical waveguide, such as a silicabased optical fibre in which the erbium concentration in the waveguide may be in the range 0.001% to 10%. The waveguide may alternatively be of planar geometry, such as a glass rib waveguide on a suitable substrate, or a diffused buried waveguide in a glass substrate.

The erbium ions in the waveguide may be excited into higher energy states by injecting pump light into the fibre at any one of a number of wavelengths corresponding to absorptive transitions of erbium. The preferred pumping wavelengths are in the range 965995nm and 1.45-1.50fm. Both of these wavelength ranges correspond to absorptive transitions which are relatively free from excited state absorption which is known to reduce the pumping efficiency of erbiumraixped glass media. In addition, semiconductor laser sources are available in both these wavelength bands, so compact and practical devices can be envisaged.

As shown in Figute 1, light from the puEp source, for example a laser diode 10, may be longitudinallyy coupled into the core of an optical fibre waveguide 12 using conventional fibre coupling techniques, such as a coupling lens 14 or, alternatively, a graded index rod coupling. The fibre 12 is preferably single mode at both pump and emission wavelengths, but may be multimode at one or both.

With pump light at sufficiently high intensity in the core region, high single-pass gain is obtained and as is well known, this leads to substantial output of broadband light by superfluorescence.

The output end or port of the optical fibre superfluorescent source is preferably provided with a termination 16 such as to prevent substantial feedback of the light. This can be achieved by polishing the waveguide end at an angle to prevent reflected light being directed back down the fibre. Alternatively, index matching or fusion splicing directly to the fibre sensor may be employed.

At the pump input end of the waveguide a rubber of configurations are possible. Incorporation of a mirror 18 which is reflective at the superfluorescent wavelength of around 1.54Fm but substantially transmissive at the pump wavelength ensures that the majority of the backward generated superfluorescence is returned to the fibre for further airplification, whereupon it emerges from the output port of the device. This mirror 18 may usefully be anywhere in the reflectivity range 0-1008 depending on the application, so the Fresnel reflection associated with normal incidence of a glass/air interface (4%) can sometimes be used. On the other hand, for applications where substantial feedback of source light from the sensor back into the source is possible, for example in the FOG, it is preferable to terminate this waveguide end in a manner similar to that of the output end to eliminate reflection altogether and thus prevent double-pass feedback which may induce undesirable effects such as laser oscillation. In this case, to ensure proper superfluorescent operation a high single pass gain is required, since the spontaneous emission does not have the benefit of two traversals of the gain medium.

Figure 2 shows schematically the energy level structure of erbium when incorporated into a glass matrix. The three level nature of the 4I13/2-4Il5/2 transition emitting at 1.54Fm means that the lower level of the transition is also the unexcited or ground state of the ion and thus will in general be significantly populated, leading to re-absorption of light at the source wavelength. This is in contradistinction to a 4-level laser transition (such as the 1.06)rm transition in neodymium doped glasses) in which the lower level of the transition is substantially depopulated at normal temperatures.In addition, it should be noted from Figure 2 that both the upper level of the erbium transition and the ground state are heavily split into a number of sub-levels known as Stark levels, the relative populations of which are temperature dependent. It was previously expected therefore that both the emission and re-absorption spectra of erbium-doped glass would be temperature sensitive, and a superfluorescent source constructed from it would exhibit a variation in emission spectrum with temperature.

Evaluation of the output power and output emission spectrum of an erbium-doped germano-silicate fibre was performed experimentally using a configuration similar to that shown in Figure 1, using a tunable dye laser operating at 980nm as the purrp source rather than the laser diode 10. The fibre 12 was characterised by a refractive index between core and cladding of 0.01, erbium dopant concentration 60ppm and second mode cutoff of 950nm. A mirror with > 98% reflection at 1.54 but > 70% transmission at 980nm was butted up to one end of the 6.2m length of fibre and the pump light was launched into the fibre 12 through this mirror. From the other end of the fibre the source output power and spectra were recorded.Figure 3 shows the variation of fibre output superfluorescence power at 1.54um with coupled pump power and indicates a conversion efficiency of approximately 13%.

In Figure 4 the variation of the mean wavelength of the emission against pump power is plotted. Data were recorded with the fibre at room temperature and at liquid nitrogen temperature (77K) in order to assess the stability of the spectrum with temperature.

As can be seen from Figure 4, at low pimp powers ( < 15mW) there exists a large variation with pimp power as well as a variation with fibre temperature. The variation with pump power is seen to be approx. 1600ppmZmW and the variation with temperature is around 60ppm2 C. Both of these figures indicate inadequate spectral stability for gyro applications. Surprisingly however, above 15mew pimp power (when the output exceeds 0.5mW) excellent stability with respect to pump power and temperature is observed ( < lppm/mW & BR< < IOpF8n/OC). In addition, once in this stable regime it was found that the emission wavelength did not vary significantly with pump wavelength as shown in Figure 5.Thus shifts in pump wavelength due for example to temperature variations of a semiconductor pump source will have a negligible effect on spectral stability.

Thus, we have appreciated from this that it is possible to provide a superfluorescent source emitting at a wavelength of 1.535 m which is spectrally stable with respect to pump power, temperature and pump wavelength. Provided that the source is operated in the linear regime of the input/output characteristic (see Figure 3), i.e. when the source power is sufficient to saturate the gain at the output end of the fibre a wavelength-stable source can be fabricated using erbium-doped optical fibres. In the case of the data presented here, this regime of operation occurs for source output powers in excess of 0.5mW.

Figure 6 shows a preferred implementation of the invention. A semiconductor source 20 emitting light at 980nm is coupled into a germano-silicate fibre 22which is doped with erbium ions in the concentration range 100-SOOppm. The erbium ions are incorporated into the whole of the fibre core region or alternatively a smaller volume centred on the fibre axis. The latter is known to improve the overlap of the pimp and emission mcde fields and thus improve pimp efficiency. The fibre should ideally be single-mxled with a large index difference between core and cladding. This ensures a small core size and a high pump intensity, which, in turn, provides a low pimp power for saturated operation.However, consideration should also be given to matching the emission spot size to that of the fibre sensor in order to obtain optimum coupling for splicing purposes.

In order to prevent back reflection into the fibre 22 a pigtailed optical isolator 24 is coupled to the output end of the fibre 22 by means of a fusion splice 23.

An expression relating the source signal power at which saturation takes place can be written:

where: P = source power = = photon energy n1,n2 = refractive index of the waveguide core and cladding Tf = upper laser level lifetime o' = stimulated cross section of the erbium ions V = normalised frequency of the source light = = source wavelength It can be seen from this expression that increasing the refractive index difference between the core and cladding materials of the waveguide will reduce the power level at which saturation and hence spectrally stable emission takes place.

Typical values for refractive index differences will be in the range 0.003 - 0.05.

The normalised frequency of the source light V is kept in the range 1-2.4 in order to establish single mode operation of the source. The fibre is either cleaved normally or terminated to prevent optical feedback at the pump input end and can be coupled to a polarisation insensitive optical isolator at the output end. The purpose of the latter is to minimise the degree of feedback from reflections into the high-gain superfluorescent source and prevent laser action. The degree of feedback into the source must be kept ideally to less than hv.sss where hm is the energy of the source photons and Ao is the optical bandwidth measured in Hertz, a typical value for h9.9 being 0.5nW. A level of feedback in excess of this value will adversely affect the generation of light in the source.

The coupling of pimp light into the fibre may be achieved by butting the fibre end directly to the laser diode facet 20 or using conventional micro-optics, such as a coupling lens 21.

The fibre length is chosen such that efficient conversion between pimp light and source light is obtained while still maintaining good source stability. A typical pump absorption ratio will be 80-90% of the pump light coupled into the fibre. A fibre length which is shorter than the optimum will give rise to reduced output power. A length greater than that of the optimum will give rise to reduced output power and degraded spectral stability with respect to temperature.

Although the invention has been described with reference to the use of erbium-doped media, it will be appreciated that media doped with other rare earth elements exhibiting similar characteristics may be used and are within the scope of the invention.

Claims (13)

ClAIMS

1. A superfluorescent light source comprising a rare earth-doped glass waveguide which exhibits 3-level characteristics, the waveguide being pumped optically with light at a wavelength corresponding to one of the absorption bands of the rare earth dopant, the intensity of the pump light being sufficient to cause amplified stimulated emission at a level which saturates the gain of the waveguide medium at the output end of the waveguide and causes superfluorescence in the linear region of the pimp input/fluorescence output characteristic.

2. A light source according to claim 1 in which the waveguide is monomode at the pimp wavelength and/or the source wavelength.

3. A light source according to claim 1 or 2 in which the waveguide is an optical fibre.

4. A light source according to claim 3 in which the waveguide is formed in planar geometry by deposition of a glass rib waveguide or by diffusion into a substrate.

5. A light source according to any preceding claim in which the waveguide is of silicate or phosphate glass.

6. A light source according to claim 5 in which the waveguide is of germano-silicate, alumino-silicate or phospho-silicate glass.

7. A light source according to any preceding claim in which the rare earth dopant is erbium.

8. A light source according to claim 7 in which the erbium concentration is in the range 0.01-10%.

9. A light source according to claim 7 or 8 in which the pump wavelength is in the range 965-995nm or 1.45-1.50

10. A light source according to any preceding claim in which the waveguide is terminated at either end to prevent internal feedback of the source light.

11. A light source according to any preceding claim in which the pump light intensity is greater than 15mW.

12. A light source according to any preceding claim in which the source output power is in the range 0.5-50mW.

13. A superfluorescent light source substantially as herein before described with reference to the drawings.