The present invention relates to a light source for use in optical sensors such as fibre optic gyroscopes.

A number of optical sensors, in particular the fibre optic gyroscope (FOG), require a low.teπporal-coherence source for optimum operation and, in general, this implies a spectrally broad-band source. In the case of the FOG, a low teπporal-coherence source is required in order to overcame the detrimental ef ects of coherent backscatter within the fibre coil, as well as gyro bias due to the optical Kerr effect. Both of these error sources in the FOG reduce considerably when a low teπporal-coherence source is used.

One possible broadband source is the super-luminescent diode (SLD) which exhibits an optical spectrum around 40πm in width and. hence provides the required low temporal coherence for the FOG. However, in common with other goniconductor sources, SLDs show a marked shift in anitting wavelength with temperature, typically 0.3πm/°C i.e. 0.02%/°C, for devices operating-at a wavelength of 1300nm. Since the gyro scale factor (gyro output/rotatio rate) is inversely proportional to the source wavelength, variations in the latter with teiperature are detrimental to performance, particularly in high-rotation rate applications where- ery high stability in scale factor over a wide teπperature range is often required. The required scale factor stability of a medium-performance FOG will be typically of the order 0.01%, which implies that the SLD will require temperature stabilisation to better than a fraction of a degree Celsius. Higher performance devices will require even greater scale factor stability. For applications requiring instant start-up, temperature regulation is undesirable, since a warm-up period is necessary. A further problem is that SLDs age and their .emission wavelength cannot be predicted throughout their lifetime. An alternative source for the FOG based on superfluourescence in rare-earth-doped optical fibres is described in published European Patent Application No. 179,320. The device described

is claimed to provide optical radiation at high intensity with the potential of greater spectral stability with temperature than the SLD.

In accordance with the invention an optical waveguide light source is characterised in that the guiding structure of the waveguide is formed f om a material containing phosphorus and a rare-earth-material capable of absorbing light of one wavelength and emitting light at one or more other wavelengths.

Two light sources in accordance with the invention will now be described in detail, by way of example, with reference to the drawings, in which:

Figure 1 shows means output wavelength as a function of pump wavelength for a light source using a Nd-doped alumino-phospho- silica fibre;

Figure 2 shows mean output wavelength as a function of pump wavelength for a Nd-doped germania-silica fibre;

Figure 3 shows mean output wavelength as a function of pump wavelength for a Nd-doped phospho-silica fibre at both room temperature and liquid nitrogen temperature;

Figure 4 shows mean output wavelength as a function of pump wavelength for Nd-doped phosphate glass;

Figure 5 shows the fluorescence spectrum for a pump wavelength of 810nm of the Nd-doped __lιιmino-ptospho-silica fibre referred to above;

Figure 6 shows the fluorescence spectrum at a pump ^' wavelength of 820nm of Nd-doped phosphate glass fibre referred to above;

Figure 7 shows diagraπmatically an optical fibre light source according to the invention; and

Figure 8 shows diagrairmatically a planar waveguide light source according to the invention.

Experimental results show that although rare-earth-doped fibres themselves do indeed show little sensitivity to temperature, variations in the shape of the fluorescence spectrum with pump wavelength can have a marked effect on the effective emission wavelength. This is an .important consideration, as the most convenient pump source for a practical rare-earth-doped fibre superfluorescent source is a semiconductor laser diode which

unfortunately exhibits a temperatτ__re-dependent output: wavelength. Thus, although a superfluorescεnt rare-earth-c_ ped fibre gives improved spectral stability over SLDs, for -many applications of the FOG the stability will still be insufficient. More specifically, unpredictable variations in semiconductor laser output wavelength with time will adversely affect the rare-earth-doped fibre emission wavelength. As with SLDs, in FOG applications which require very high scale-factor stability virtually instantaneously after switch- on, the use of temperature compensation to overcome the pump wavelength dependence on ambient temperature is precluded.

We have appreciated that a specific choice of host glass material for the rare-earth-doped fibre leads to a superfluoresceπz source which can have substantially greater spectral stability with pump wavelength, and thus with temperature, than that of previously- disclosed fibre types. Fabrication of a rare-earth-doped fibre superf luorescent source from such a material offers significant advantages in spectal stability, in particular with respect to ageing and switch-on criteria.

The broad spectral linewidth source described below consists of an optical waveguide, either an optical fibre or a planar waveguide which cont ins a proportion of rare-earth dopant ions (e.g. Nd ^oτ ) within the guiding structure of the waveguide, i.e. in the core and/or cladding of the waveguide. The waveguide is preferably made from glass, but could also be a crystalline material in fibre or planar-guide form. When pumped at suitable wavelengths, e.g. 820nm from a laser diode, electrons in the rare-earth species are excited to higher energy states and can return to their original state by emitting a photon of light. The photons thus emitted constitute fluorescence from the material which occuirs with random i rection, i.e. overall, the fluorescence is isotrcpically radiated. The waveguide is able to capture a proportion of this fluorescence lich corresponding to that proporτ±on which is emitted into the modes of the waveguide. The output from a device operating in such a mode • would be termed fluorescence and typical output, spectra are shown in Figures 5 and 6. Subsrantial amplification of the fluorescence signal can also take place is the pump light intensity is high enough to give rise to a significant single-pass gain within the

device. This situation can be readily-achieved with optical fibres and the output in this case is termed superfluorescence or aπplified spontaneous emission (ASE) . If the fibre is fabricated to have a single transverse mode at the fluorescence wavelength, the fluorescent or superfluorescent output of the device will be in a single transverse mode.

The effective source wavelength λ _e seen by a FOG is dete__mined by a weighted average of the source spectrum according to the relation

cence signal at wavelengt A .

Figures 1-3 show the stability of the weighted-average fluorescent wavelength Α _e with pump wavelength for three different neodymium-dαped silica fibre types. The fibres were made by the process disclosed in "Solution-doping technique'for fabrication of rare-earth-doped optical fibres", J.E. Townsend, S.B. Poole and D.N. Payne, (Electron. Lett., Vol. 23, No. 7, pp. 329-331, 1989) and had core compositions of:

Fibre 1 : 1300 ppm Nd ³⁺ in 3% P ₂ 0 ₅ , 4.3% A1 ₂ 0 ₃ , 92.7% Si0 ₂

Fibre 2 : 225 ppm Nd ³⁺ in 10% Ge0 ₂ , 90% Si0 ₂

Fibre 3 : 125 ppm Nd ³⁺ in 15% 0 ₅ , 85% Si0 ₂

(percentages and concentration are molar) The results show a marked variation of \ _e with pump wavelength, particularly for fibres 1 and 2. Figure 3 shows data for Fibre 3 which also contains a large proportion of ₂ 0 ₅ (i.e. a phosphosilicate) taken both at roam t____ιperature and with the fibre at liquid-nitrogen temperature. The difference in the characteristics show that fibre temperature does have an effect on the output spectrum. However, the temperature change is large (223°C) and even at the pump wavelength where the fibre is roost sensitive (815nm) the difference corresponds to only -lOppmPc. Thus ■the sensitivity of the output spectrum to fibre temperature is small.

These results demonstrate that the fluescence output spectrum

.

centred around a wavelength of 1.06um for a Nd ³⁺ -dαped fibre is stable with teπperature, but varies with the wavelength of the pump source.

Figure 4 shows the characteristic of an optical fibre fabricated with a core of neodymium-doped phosphate glass (Schott LG750 core material) a glass which exhibits very small changes in weighted emission wavelengthΛ _e with pump wavelength.

Over the pump wavelength range 800-830nm the variation inA _e in parts per million (ppm) for the different fibre types is: Silica Fibre 1 : 3700 ppm Silica Fibre 2 : 3600 ppm Silica Fibre 3 : 1700 ppm Phosphate Glass Fibre : 250 ppm The phosphate glass fibre type clearly shows the most stable output spectrum.

If a value of 0.3nm/°C wavelength variation with teπperature is assured for a typical AlGaAs laser-diode pump-source the worst-case sensitivity of each fibre type to temperature variations can be examined. Considering the steepest portion of the curves in Figures 1-4:

Silica Fibre 1 : 120 ppm/°C Silica Fibre 2 : 90 pptn°C Silica Fibre 3 : 70 ppm/°C Phosphate Glass Fibre : <20 pprn/°C Again, the phosphate glass fibre shows considerably greater spectral stability than the silica-based fibres. However, over the more restricted wavelength range of 810-830r_rπ, the phospho-silicate fibre (Fibre 3) shows stability approaching that of the phosphate glass. From this, it can be inferred that the incorporation of phosphorous has a stabilising effect on the fluorescence spectrum.

In the context of this specification a "silicate glass" is defined as a glass which has silica (Si0 ₂ ) as the main glass former, usually in the region of 20-100% molar. ^" Similarly "phosphate" glass has P ₂ θ5 as the main glass former in proportions of 20-100%. A ^• glass containing both Si0 and P ₂ 0 ₅ as glass formers (sometimes referred to as "network formers") is referred to as a phospho- silicate glass.

Other rare-earths containing phosphorous compounds such as lithium neodymium pentaphosphate (liNP), although not glassy in nature, may also be expected to show stable fluorescence spectra with variations in pump wavelength.

Figures 5 and 6 show fluorescence spectra of silica Fibre 1 and the Nd:phosphate glass fibre. The emission spectra are similar in shape, with the phosphate glass giving a more symmetric spectrum and a peak at a somewhat shorter wavelength.

In addition to greater spectral stability with pump wavelength, the relative syππietry of the spectrum leads to another advantage for phosphate glass fibres over silica-based fibre in relation to the power dependence of the emission spectrum. For a spectral line which is asymmetric (a situation encountered with rare-earth materials in a number of silicate glasses) the superfluorescent output spectrum has a weighted average wavelength which will not in general coincide with that of the fluorescence spectrum owing to preferential amplification of the fluorescence on the peak of the spectral line. As a result of this, the spectral shape of the emission changes with the degree of superfluorescence present and therefore the weighted average wavelength will become power dependent. Thus for a practical application where laser diode pumping is used, the inevitable variations in laser diode pump power with temperature will induce variations in.the mean position of the superfluorescent output spectrum. This is undesirable for a number of applications of such an optical source, in particular when used in the FCG. As seen in Figure 6, the spectral line of the favourable 1.054um transition in neodyπ um-doped phosphate glass is almost symmetric and the power dependence of the ASE spectrum will therefore be substantially reduced in the case of phosphate glass fibres.

Two examples of light sources which incorporate phosphate or phospho-silicate fibres in a super lurescent and fluorescent configureation are described in detail below.

Example 1: Superfluorescent source using neodymium-doped phosphate or phospho-silicate optical fibre.

Figure 7 shows a light source utilising an optical fibre in accordance with the invention. An ODtical fibre 10 which.is chosen

to be single-transverse mode at the emitted wavelength is used as the waveguide. The fibre 10 is manufactured by conventional chemical vapour deposition (CVD) techniques in the case of a phospho-silicate based fibre, or by similarly conventional rod-in- tube techniques in the case of the phosphate glass fibre. Preferably the fibre is also chosen to be single-transverse mode at the pump wavelength bo avoid possible variations in output power with pump launching conditions. Pump light frαπ a laser diode source 12 is coupled into the fibre 10 either by an appropriate lens arrangement 14 or by direct butting of the laser to the fibres. Some of the fluorescence is captured by the fibre 10, amplified by the population inversion which exists in the fibre and directed to the output port of the device. A dichroic mirror 16 positioned at the pump input end of the device enables the rearward directed ASE to be reflected and further aπpli ied on its way toward the output port. This mirror can be discarded in order to reduce the sensitivity of the device to external reflections, but at the expense of reduced superfluorescent output.

Example 2: Fluorescent source using neodymium-doped phosphate or phospho-silicate optical fibre.

Figure 8 shows a planar-waveguide .implementation to the device which operates similarly to the fibre ___tιpleπ___ntation described above. In this i_ftpl-_mentation, a planar waveguide is constructed using material similar to that used in Fibre 3 described above, that is, 125 ppm Nd ³⁺ in 15% 0 ₅ , 85% Si0 ₂ . The waveguide 20 is formed from a silica substrate 21 with a waveguiding region 23 of the neodymium- doped, phosphorus containing glass referred to above. A laser diode 22 is coupled directly to the waveguide 20 to pump light into it.

The pump power directed into the waveguide 20 is, in this case, insu ficient to establish a substantial single-pass gain in the waveguide and the output is consequently fluorescence rather than superfluorescence. Fluorescence increases quasi-linearly with increased pump power rather than exponentially as with small-signal . superfluorescence. The difference between these two regiires of operation will be obvious to persons skilled in the art. For many applications the fluorescence emission is sufficient as a source and offers several advantages over a superfluorescent device, πamelv:

(a) The absence of substantial single-pass gain in the device means that source light reflected back from the sensor is not amplified. The source is thus feedback insensitive and this obviates the need to use an optical isolator.

(b) The fluorescent signal will be shot-noise limited so obviating the requirement to employ the noise compensation techniques which are required to gain benefit from higher power superfluorescent signals.

(c) Compact and reliable inexpensive laser diodes with low power requirements can be used as pump sources.

All of these three advantages indicate that a fluorescent source based on the use of rare-earth-doped phospho-silicate or phosphate glass fibres will provide a substantially lower cost source than a suDerfluorescent source.