GB2219127A

GB2219127A - Lasers and optical amplifiers

Info

Publication number: GB2219127A
Application number: GB8812649A
Authority: GB
Inventors: Robert Baker
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-05-27
Filing date: 1988-05-27
Publication date: 1989-11-29
Also published as: GB8812649D0

Abstract

A fibre laser in whose laser cavity is arranged a resonant coupler (15) which is laser wavelength selective. The laser cavity is defined between a broadband reflector (17) at one end of a laser fibre (10) and a broadband reflector (16) at one end of an optical fibre (14). An optical pump signal is applied to the other end (12) of the laser fibre (10) and a laser output is obtained from the other end (18) of the optical fibre (14). The fibre laser is thus effectively pumped internally of the cavity. Only simple mirrors are required for the reflectors (16, 17) instead of the dichroic mirrors of conventional fibre lasers which involve alignment problems. The arrangement acts as an optical amplifier for an input signal, namely the pump signal referred to above. <IMAGE>

Description

LASERS AND OPTICAL AMPLIFIERS This invention relates to lasers and in particular to fibre lasers, and to optical amplifiers.

A fibre laser may comprise a length of single mode laser fibre, such as neodymium, erbium or other rare earth doped single mode silica, or other host, fibre, disposed in a laser cavity. A conventional cavity arrangement for a fibre laser is illustrated in Fig. 1 of the accompanying drawings. Dichroic mirrors 1 and 2 butted against the ends of a laser fibre 3 form a laser cavity. An optical pump signal, such as provided by a laser diode 4, is coupled into the cavity via mirror 1, whereas the laser output is obtained at mirror 2. The mirrors are transmissive at the pump wavelength to allow the pump light into the laser cavity and invert the population of the laser fibre. The mirrors are highly reflective at the laser wavelength, forming a high Q cavity but also allowing a small percentage of laser emission to be coupled out of the cavity at the output mirror 2.The mirror 1 may be totally (100%) reflective at the laser wavelength and 90% transmissive at the pump wavelength, whereas the mirror 2 may be 88% reflective at the laser wavelength and 80% transmissive at the pump wavelength. This conventional arrangement suffers from alignment problems in view of the use of dichroic mirrors, which latter involve complex dichroic coatings and are relatively expensive. The arrangement may be considered to comprise an optical amplifier or a fibre laser.

According to one aspect of the present invention there is provided a fibre laser having a laser cavity in which is arranged a resonant coupler.

According to another aspect of the present invention there is provided a fibre laser comprising a laser fibre and a resonant coupler, a laser cavity being defined between a reflective member at one end of the laser fibre and a reflective member at one end of an optical fibre of the resonant coupler.

According to a further aspect of the present invention there is provided an optical amplifier comprising a laser fibre and a laser-wavelengthselective resonant fibre coupler, one arm of the coupler being comprised by part of the laser fibre, an input port for an optical signal to be amplified being comprised by one end of the laser fibre, an output port for the amplified optical signal being comprised by one end of the other arm of the coupler, a lasing cavity being defined between the other end of the laser fibre and the other end of the other arm of the coupler.

According to yet another aspect of the present invention there is provided an optical amplifier comprising a laser fibre and a laser-wavelength-selective resonant fibre coupler, an input port for an optical signal to be amplified being comprised by one end of one arm of the coupler, the other end of the one arm being coupled to one end of the laser fibre, an output port for the amplified optical signal being comprised by one end of the other arm of the coupler, a lasing cavity being defined between the other end of the laser fibre and the other end of the other arm of the coupler.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates the conventional fibre laser/optical amplifier referred to above; Fig. 2 illustrates schematically a fibre laser/optical amplifier according to an embodiment of the present invention, and Fig. 3 illustrates inclusion of an acousto-optic Q switch within part of the cavity.

The fibre laser/optical amplifier illustrated in Fig. 2 comprises a length of single mode laser fibre 10 which is pumped by an input signal, for example the optical output of an optical pump signal source 11, which output is coupled directly into one end 12 of the fibre 10, such as by means of a lens 13. A part of the fibre 10 together with a part of a single mode optical fibre 14 comprise the two arms of a wavelength selective resonant coupler 15. The coupler may be a biconical taper coupler manufactured by twisting the two fibre parts together and heating and stretching them using the differential pulling technique disclosed in our GB Patent specification No. 2150703B (T. Bricheno 8-1-1).

At one end of fibre 14 is a mirror 16. At the opposite end of the laser fibre 10 to the source 11 is a mirror 17. The coupler 15 is manufactured so as to cross couple at the laser wavelength but not at the pump wavelength. Using such a coupler with a high cross transfer at the laser wavelength, for example 95%, a laser cavity is thus defined between mirrors 16 and 17.

The remainder of the laser radiation, 5% for the quoted example, is coupled out of the cavity and output from the other end 18 of the fibre 14. The end 12 of fibre 10 and the end 18 of fibre 14 comprise the input and output ports of the coupler or rather of the overall fibre laser/optical amplifier. In each round trip in the cavity, the optical signal (laser signal) passes through the coupler 15 twice and the total output coupling is 10%, comprised by 5% down each port 12 and 18. As a result the wavelength selective coupler 15 acts as a dichroic element.

The mirrors 16 and 17 need only to be broadband reflectors and can, for example, be achieved by direct deposition on the associated fibre ends, using, for example, silver mirror reduction from aqueous solution.

These mirrors are not only of a simple design but they also free the system from the alignment problems associated with the conventional arrangement (Fig. 1) where fibre ends are butted up against dichroic mirrors.

The cavity of a fibre laser thus includes a single wavelength resonant coupler and this allows the use of very simple mirrors at the cavity ends rather than complex dichroic mirrors. The laser fibre may be considered as being pumped by a pump signal coupled thereto at a point within the laser cavity (internally pumped) rather than coupled to the laser cavity from the exterior thereof by being transmitted through one of the dichroic mirrors as in the conventional arrangement.

There is thus provided an internally-pumped laser source comprising a single mode laser fibre and a resonant coupler, with only simple mirrors being required to define the laser cavity, the output wavelength being dependent on the dopant employed for the laser fibre 3+ (1.55um for Er doped silica). Typically the single mode fibre 14 is of GeO2 doped SiO2.

Whereas the fibre laser/optical amplifier, illustrated in Fig. 2 and described above employs a length of laser fibre 10 which extends between port 12 and mirror 17, alternatively the laser fibre 10 may extend only between point 19 and mirror 17, the remainder of the length being comprised by another single mode optical fibre extending between port 12 and port 19 and being twisted together with fibre 14 etc. as described above to provide the coupler 15.

Examples of typical laser fibres are as follows, the length depending on dopant concentration; erbium doped fibre (1.55pm): dopant concentration 300ppm, length of the order of three metres; neodymium doped fibre (1.08cm): dopant concentration 300ppm, length of the order of ten metres. The fibre may have a numerical aperture of 0.24, core radius 2.3cm and a cut off wavelength of 1.3pm.

In order to provide high intensity short duration pulses a "shear wavew acousto-optic switch 20 can be disposed between the end of laser fibre 10 and mirror 17, together with a lens 19 for focussing purposes.

Claims

CLAIMS:

1. A fibre laser having a laser cavity in which is arranged a resonant coupler.

2. A fibre laser comprising a laser fibre and a resonant coupler, a laser cavity being defined between a reflective member at one end of the laser fibre and a reflective member at one end of an optical fibre of the resonant coupler.

3. A fibre laser as claimed in claim 1 wherein the resonant coupler is a laser-wavelength selective biconical taper fibre coupler comprised by portions of a first optical fibre and a laser fibre, or a second optical fibre coupled to the laser fibre.

4. A fibre laser as claimed in claim 3 wherein a respective reflective member is provided at one end of the laser fibre and a respective reflective member is provided at one end of the first optical fibre, which reflective members define the laser cavity.

5. A fibre laser as claimed in claim 4 wherein in use an optical pump signal is coupled to the other end of the laser fibre, or to the second optical fibre coupled thereto, and a laser output is obtained at the other end of the first optical fibre.

6. A fibre laser as claimed in any one of claims 2, 4 and 5 wherein the reflective members comprise broadband reflectors.

7. A fibre laser as claimed in claim 6 wherein the reflective members comprise mirrors provided directly on the ends of the fibres.

8. A fibre laser as claimed in any one of the preceding claims wherein the resonant coupler cross couples at a laser wavelength but not at a pump wavelength.

9. A fibre laser as claimed in claim 8 wherein the resonant coupler was manufactured by a method involving twisting two fibres together, and heating and stretching using a differential pulling technique.

10. A fibre laser substantially as herein described with reference to Fig. 2, with or without reference to Fig. 3, of the accompanying drawings.

11. An optical amplifier comprising a fibre laser as claimed in any one of the preceding claims.

12. An optical amplifier comprising a laser fibre and a laser-wavelength-selective resonant fibre coupler, one arm of the coupler being comprised by part of the laser fibre, an input port for an optical signal to be amplified being comprised by one end of the laser fibre, an output port for the amplified optical signal being comprised by one end of the other arm of the coupler, a lasing cavity being defined between the other end of the laser fibre and the other end of the other arm of the coupler.

13. An optical amplifier comprising a laser fibre and a laser-wavelength-selective resonant fibre coupler, an input port for an optical signal to be amplified being comprised by one end of one arm of the coupler, the other end of the one arm being coupled to one end of the laser fibre, an output port for the amplified optical signal being comprised by one end of the other arm of the coupler, a lasing cavity being defined between the other end of the laser fibre and the other end of the other arm of the coupler.

14. An optical amplifier as claimed in claim 12 or claim 13 wherein broad band reflectors are provided at the said ends between which the lasing cavity is defined.