Optical resonating device

Details Optical resonating device Optical resonating device

1988-01-25

1989-06-13

Scott, Jr., Leon

Coherent light generators

Optical fiber laser

372 92, 350 9615, H01S 330

Patent

active

048398985

DESCRIPTION:

BRIEF SUMMARY
OPTICAL RESONATING DEVICE

This invention relates to optical devices employing a resonant cavity.
Optical filters employing resonant cavities are known, for example, from U.S. Pat. No. 3,589,794 ("Optical Circuits", Enrique A. J. Marcatili, assigned to Bell Telephone Laboratories). That patent discloses a variety of optical filter devices based on resonant cavities employing an optical waveguide loop to form the end reflectors of the cavity.
Depending on the configuration of the device, the resonance of the cavity permits a variety of filter functions to be implemented such as, for example, band-pass filters, band-stop filters, channel-dropping filters and so forth. The devices described there are considered advantageous since they can readily be formed from optical waveguides embedded in a substrate.
Another form of an optical resonant device is known from GB No. 2180392A (R. J. Mears et al.) which discloses a laser device based on an optical fibre structure. The device described consists of an optical resonant cavity formed by the fibre and terminating in a mirror at each end. The device operates as an optically pumped laser light source, with a pump wavelength different to the lasing wavelength.
According to the present invention, a fibre based laser device comprises an optical fibre resonator having at least one optical fibre loop termination comprising a directional coupler having adjacent ports connected by a looped optical fibre path arranged to be reflective at the chosen lasing wavelength of the laser device and to be at least partially transmissive at the chosen pump wavelength of the device.
The reflectivity of the fibre loop termination can be selected by appropriately choosing the the coupling ratio of the directional coupler at the wavelength concerned: if the coupling ratio is 50:50 then the loop termination will theoretically be 100% reflective if the ratio is 100:0 or 0:100 then the loop termination will be completely transparent; any other degree of reflectivity can be obtained by using couplers with coupling ratios intermediate these extremes.
Preferably, the reflectivity of the termination at the lasing and pump wavelengths can be selected by employing a directional coupler which has wavelength-selective coupling properties such that its coupling ratio is close to 50:50 at the lasing wavelength and different from this even ratio at the pump wavelength.
An optical fibre based laser device having a fibre loop termination as reflector at the lasing wavelength is attractive for several reasons: coupling looses which would otherwise occur between the cavity and the reflectors are eliminated or greatly reduced, and the optical waveguide reflectors are temperature and mechanically stable. Moreover, the reflectivity of the fibre loop termination can be made at least very close to 100% at the lasing wavelength while at the same time providing good transmission of light at the pump wavelength into the cavity.
Similarly the reflectivity of the other termination can be readily adjusted to provide the required degree of transmission at the lasing wavelength in order to pass the optical output of the laser.
It is envisaged that in certain situations, a hybrid device could be produced in which one of the reflectors comprises a conventional mirror. Preferably, however, each of first and second reflectors comprises an optical fibre loop termination.
One of the most advantageous features of the invention is that the whole device, including its pump power supply path and its output path comprises a continuous optical fibre waveguide, which may, however, consist of several optical fibre sections spliced together. Furthermore, in comparison with the coventional alternatives, a wholly fibre based device offers a significant saving in production costs.
In one example, the loop termination forming one of the reflectors is substantially 100% reflecting at the laser wavelength (assuming zero losses) and the reflectivity of the other reflector is chosen for optimum output. The device can be pumped by a band of wa

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