Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
  • H01S3/067—Fibre lasers
  • H01S3/06795—Fibre lasers with superfluorescent emission, e.g. amplified spontaneous emission sources for fibre laser gyrometers

Definitions

• 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 POG) 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.
• non-reciprocity in the light path of the fibre coil due to the optical Kerr effect is also considerably reduced with a broadband source.
• 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.
• LED Light-emitting diodes
• SID Superluminescent diodes
• both LEDs .and SLDs show a marked shift in operating wavelength with temperature (typically 100-400 ppm/°C) . 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.
• scale factor is defined as the gyro output for a given rotation rate.
• 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.
• FIG. 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 incorporat»sd into a glass matrix
• Figure 3 shows the variation of fibre output superfluorescence power with coupled pump power using a pump wavelength of 980nm .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.
• Figure 6 is a schematic diagram of a preferred superfluorescent source in accordance with the invention.
• a preferred form of optical fibre source according to the invention uses an erbium-doped optical waveguide, such as a silica- based 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 965- 995nm .and 1.45-1.50um. 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 erbium-doped glass media.
• semiconductor laser sources are available in both these wavelength bands, so compact and practical devices can be envisaged.
• light from the pump source 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 subst-antial 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 substanti»al 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 ⁇ atching or fusion splicing directly to the fibre sensor may be employed.
• a number of configurations are possible. Incorporation of a mirror 18 which is reflective at the superfluorescent wavelength of around 1.5 um but substantially trans issive at the pump wavelength ensures that the majority of the backward generated superfluorescence is returned to the fibre for further .amplification, whereupon it emerges from the output port of the device.
• This mirxor 18 may usefully be anywhere in the reflectivity range 0-100% depending on the application, so the Fresnel reflection associated with normal incidence of a glass/air interface (4%) can sometimes be used.
• Figure 2 shows schematically the energy level structure of erbium when incorporated into a glass matrix.
• the three level nature of the ⁇ _/_ ⁇ __/2 ⁇ a__? ___ emitting at 1.54um means that the lower level of the transition is also the unexcited or ground state of the ion and thus will in general be signi icantly populated, leading to re-absorption of light at the source wavelength.
• This is in contradistinction to a 4-level laser transition (such as the 1.06pm transition in neodymium doped glasses) in which the lower level of the transition is substantially depopulated at normal temperatures.
• a superfluorescent source emitting at a wavelength of 1.535um which is spectrally stable with respect to pump power, temperature and pump wavelength.
• 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.
• FIG. 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 100-500ppm.
• 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 pump and emission mode fields and thus improve pump efficiency.
• the fibre should ideally be single- oded 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 pump power for saturated operation.
• a pigtailed optical isolator 24 is coupled to the output end of the fibre 22 by me-ans of a fusion splice 23.
• V normalised frequency of the source light - source wavelength
• 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>. ⁇ v where hv is the energy of the source photons and ⁇ > is the optical bandwidth measured in Hertz, a typical value for . ⁇ 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 pump 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 pump 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.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)

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• 1989
  • 1989-09-15 GB GB8921006A patent/GB2239125A/en not_active Withdrawn
• 1990
  • 1990-09-13 WO PCT/GB1990/001418 patent/WO1991004594A1/en not_active Application Discontinuation
  • 1990-09-13 EP EP19900913856 patent/EP0452430A1/de not_active Withdrawn

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