GB2220789A

GB2220789A - Low-coherence optical source

Info

Publication number: GB2220789A
Application number: GB8816898A
Authority: GB
Inventors: Terry Bricheno; Robert Anthoney Baker
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-07-15
Filing date: 1988-07-15
Publication date: 1990-01-17
Anticipated expiration: 2008-07-15
Also published as: GB2220789B; GB8816898D0

Abstract

A low coherence optical source includes a length of fluorescent fibre (1), an optical pump source (2) and means (4) at the pump end of the cavity for suppressing reflection of optical signals reflected thereto from a system (3) with which the low coherence source is employed. Thus formation of a laser cavity is prevented and broad-band operation is maintained. The reflection suppression means may be a wavelength selective attenuator comprising a uniformally tapered reduced diameter portion (1a) of the fluorescent fibre (1) and absorbing cladding (5) provided thereon. The longer wavelength reflected signals are absorbed whilst the shorter wavelength pump signals are transmitted. <IMAGE>

Description

LOW-COHERENCE OPTICAL SOURCE This invention relates to low coherence optical sources and in particular to such sources based on optically pumped super-radiant optical fibre.

The operation and application of a laser pumped super fluorescent fibre source is described in a paper by K.Lui et al entitled wlOmw superfluorescent single-mode fibre source at 1060no. Electronics Letters 23 (24) p.1320-1 (1987). The paper is particularly concerned with broad-spectrum optical sources with stable spectra for fibre-optic gyroscopes in order to minimise coherent back-scattering noise and zero rotation drift due to the Kerr effect. The source described in the paper comprised a length of GeO2/Sio2 telecommunications type fibre but which had been additionally doped with approximately 1 mole P205 and 300 parts in 106 Nd. The fibre was single mode at 1060nm but bimoded at the pump wavelength (810nm). The ends of the fibre where mounted in capillary tubes and mechanically polished.The fibre was end pumped near 810nm by a dye laser coupled to the fibre through a reflector (high reflectance at 1060nm, 80% transmissive at 810 nm) at one end of the fibre. The output at the other end of the fibre was substantially at 1060nm. To avoid resonant oscillation between the reflector and the output fibre end the latter was polished at a 100 angle. The source was broadband emitting at up to 10mW, with a mode free spectrum and an essentially powerindependent linewidth of around 17 nm at 5-10mW output.

The maintenance of the required broadband operation relies upon suppressing laser action in the source. This suppression was achieved in the K. Lui et al paper by polishing the output end of the fibre at a 0 10 angle.

In practical applications, the source must be connected into some associated optical system, for example a fibre optic gyroscope, and the connected system forms a further potential source of reflections which would combine with unwanted reflections at the pump end of the fibre to form the unwanted resonant cavity and leading to laser action, line narrowing and loss of the low coherence property. The present invention is aimed at overcoming such problems.

According to the present invention there is provided a low coherence optical source including a length of fluorescent fibre, as hereinafter defined, an optical source associated with one end of the fluorescent fibre for optically pumping the fluorescent fibre, and including means associated with the one end for suppressing reflection of optical signals reflected thereto in use of the low coherence optical source from a system with which the low coherence optical source is coupled.

The expression fluorescent fibre" employed in this specification is defined as a fibre comprising or containing species exhibiting fluorescent properties capable of being exploited to yield optical amplification by a mechanism of stimulated emission.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates schematically a first embodiment of a low coherence source, and Fig. 2 illustrates schematically a second embodiment of a low coherence source.

The low coherence sources of the present invention are based on optically pumped fluorescent optical fibre. The fibre may be neodymium doped GO2/SiO2 laser fibre as in the aforementioned paper, alternatively the fibre may contain other rare earth dopants (e.g. erbium, holmium, samarium etc.) or transition metal dopants (e.g. chromium) which yield fluorescence centred on a wavelength suitable for the system for which the low coherence source is to be used.

Fig. 1 shows a first embodiment of low coherence source. It basically comprises a length of a fluorescent fibre 1 pumped by a pump source 2. In order to avoid the formation of a resonant cavity, reflections as indicated by the arrows due to an optical system 3 coupled to the output end of fibre 1 are prevented from reflection at the pump end of the fibre 1 by means of a wavelength selective fibre attenuator 4. For the fibre dopant neodymium typically the output is at 1060 nm and the pump is at the shorter wavelength of 810 nm.

The attenuator 4 may be provided by forming an extended uniformally tapered region la in the fibre 1 such as by the differential pulling technique disclosed in our Patent Specification No. 2 150 703 B and controlling the degree of tapering such that the longer wavelength (super-radiant) signals are subject to loss while the shorter wavelength pump is transmitted without loss. The extra attenuation results from absorption by the absorbing cladding 5 at the outer surface of the tapered section, or mode-converting structural perturbation or both. This embodiment is particularly effective in removing the reflected longer wavelength signals.

As an alternative to the tapered wavelength selective attenuator of Fig. 1, a wavelength selective coupler as indicated in Fig. 2 may be employed. It is, however, not as effective as the attenuator arrangement of Fig. 1. The pump source 2 is in this case coupled to the fluorescent fibre 1 via one arm 7 of a 2 x 2 wavelength selective fibre coupler 6. The laser fibre could alternatively comprise the one arm 7 of the coupler. The fibre coupler is manufactured, such as by the differential pulling technique described in our aforementioned patent specification in order to cross couple the longer wavelengths of the reflected-by-the system super-radiant signals to arm 8 of the coupler 6, which arm may be terminated by an antireflection element 9, but not to cross-couple at the shorter pump wavelength. Thus the pump signal is transmitted without loss, but reflected-from-the system super-radiant signals are coupled out.

In both of the described embodiments the formation of an unwanted laser cavity is prevented and low coherence is maintained.

Claims

1. A low coherence optical source including a length of laser fluorescent fibre, as hereinbefore defined, an optical source associated vith one end of the fluorescent fibre for optically pumping the fluorescent fibre, and including means associated with the one end for suppressing reflection of optical signals reflected thereto in use of the low coherence optical source from a system with which the low coherence optical source is coupled.

2. A low coherence optical source as claimed in claim 1, wherein said reflection suppression means comprises a wavelength selective attenuator which attenuates the reflected signals but transmits the optical pump signals.

3. A low coherence optical source as claimed in claim 1 wherein the wavelength selective attenuator is comprised by a uniformally tapered reduced diameter portion of the fluorescent fibre adjacent the one end and absorbing cladding provided on the fluorescent fibre at said tapered reduced diameter portion.

4. A low coherence optical source as claimed in claim 1 wherein said reflection suppression means comprises a wavelength selective coupler which couples out the reflected signals but transmits the optical pump signals.

5. A low coherence optical source as claimed in claim 4 wherein the fluorescent fibre is coupled to one end of one arm of a 2 x 2 coupler and the optical pump source is coupled to the other end of the one arm, and the other arm of the coupler is terminated by non-reflective means.

6. A low coherence optical source as claimed in claim 4 wherein a length portion at the one end of the fluorescent fibre comprises one arm of a 2 x 2 coupler and the optical pump source is coupled to the one end of the fluorescent fibre, the other arm of the coupler being terminated by non-reflective means.

7. A low coherence optical source substantially as herein described with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.