GB2178591A

GB2178591A - Laser feedback system

Info

Publication number: GB2178591A
Application number: GB08518785A
Authority: GB
Inventors: Dr John Philip Dakin; Dr Philip Barclay Withers
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-07-25
Filing date: 1985-07-25
Publication date: 1987-02-11
Also published as: GB2178591B; GB8518785D0

Abstract

A semiconductor or other laser arrangement for producing a low phase noise output having good single mode characteristics, in which the laser 1 is arranged to launch emitted light into an optical transmission medium such as an optic fibre 2 having a plurality of partially reflective mirrors, splices 11,12 or equivalent means which are positioned at locations spaced apart by different predetermined distances along its length and which produces optical feedback to the laser 1 which constrains the laser to produce light predominantly at a single frequency. <IMAGE>

Description

SPECIFICATION Improvements relating to lasers This invention relates to lasers and relates more specifically to semiconductor or other lasers having low phase noise and corresponding highly coherent or single mode output characteristics.

Such high coherent lasers are necessary in interferometric optical sensors and coherent communication systems utilising interferometers (e.g. Mach-Zehnder or Michelson interferometers) incorporating optical fibre paths of unequal lengths, otherwise signal-noise degradation of the interferometer output will occur due to fluctuations in the phase or frequency of the laser light source.

According to one known arrangement for reducing phase noise in the output of a semiconductor laser to an acceptable level the laser drive current on which the frequency of the laser output is strongly dependent is actively controlled by feedback signals derived from a frequency discriminator interferometer which monitors the laser output.

In another known arrangement for reducing phase noise in semiconductor lasers a small proportion of the laser emission may be controllably reflected back into the laser by means of a single mirror (or dual mirror) external cavity. Although significant phase noise reductions have been achieved using this arrangement multimode laser outputs tend to be produced which necessitate tuning of the external cavity when the laser is used with interferometric sensors. This is to ensure good fringe visibility in the interferometer.

Yet another known arrangement for reducing phase noise semiconductor laser outputs utilises Rayleigh back-scattering of the laser emission from inherently inhomogeneous regions along a relatively long standard optical fibre into one end of which the laser emission is launched. As the back-scattered light returns along the optical fibre to the laser the laser locks on to peaks in the back scatter/frequency response characteristic. This arrangement suffers from the disadvantage that the optical fibre needs to be of considerable length (e.g. 100m-1km) to provide sufficient back-scatter intensity and, moreover, the random occurrence of scattering regions gives a relatively large number of possible wavelengths at which the laser could operate and represents a relatively uncontrolled means of achieving the desired reflective cavity.It is limited in the peak reflectivity which may be achieved.

In all of the foregoing known arrangements described there must be efficient attenuation of light reflected back into the semiconductor laser source, otherwise rapid variations in the laser output (i.e. mode shifting or mode hopping) and multimode operation will result. Such unstable behaviour will cause problems when the lasers are used for examples with interferometric optical sensors having optical fibre sensor arms of unequal lengths.

According to the present invention there is provided a semiconductor or other laser arrangement for producing a low phase noise output having good single mqde characteristics, in which the laser is arranged to launch emitted light into a optical transmission medium having a plurality of partially reflective mirrors, splices or equivalent means which are positioned at locations spaced apart by different predetermined distances along its length and which produce optical feedback to the laser which constrains the laser to produce light predominantly at a single frequency.

In carrying out the present invention the transmission medium preferably comprises a series of random lengths of optical fibre coupled together so that the splices between adjacent lengths of optical fibre constitute the aforesaid partially reflective means. A small proportion of the light emitted by the laser will be reflected back to the laser by the splices and the laser will lock on to an optimum frequency within its gain curve at which the light reflected from the splices adds coherently and thereby renders the laser relatively insensitive to other reflections (e.g. back-scattered light from inherent impurity regions along the optical fibre).

In an alternative contemplated arrangement according to the present invention the optical transmission medium comprises an air space whilst the partially reflective means are defined by an array of mirrors which are spaced apart in accurate predetermined relationship so that the light reflected back to the laser by the mirrors adds coherently whereby the laser locks on to a single frequency and provides a single mode output having low phase noise.

The enhancement of the reflection coefficient of the laser renders the laser arrangement relative insensitive to reflections at other frequencies.

In addition to semiconductor lasers the invention has application to crystal and glass host lasers and dye lasers.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figures 1 and 2 shown schematic diagrams of alternative embodiments of the invention; and Figure 3 shows a representative reflection amplitude/frequency characteristic for the embodiment of Fig. 1 or Fig. 2.

Referring to Fig. 1 of the drawings, there is shown a low phase noise semiconductor laser arrangement comprising a semiconductor laser 1 the light output from which is launched into one end of an optical fibre 2 through a convex lens 3. The optical fibre 2 consists of a plurality of random lengths of optical fibre in dicated at 4 to 10 coupled together to provide partially reflective splices, such as the splices 11 and 12, between adjacent lengths of optical fibre. A small proportion of the light emitted by the laser 1 is reflected back along the optical fibre 2 to the laser. The laser then locks on to the optimum frequency within its own gain curve at which the reflections from the splices add coherently. This action on the part of the laser arrangement can be readily appreciated from Fig. 3 of the drawings.As can be seen from the reflection amplitude/frequency characteristic, the reflected light from the splices adds coherently to provide reflection peaks 13 to 17 at different frequencies.

The laser 1 receiving these reflections will select the optimum reflection frequency within its gain curve and will lock on to that frequency thus operating in single mode. In Fig.

3 the gain curve for the laser 1 is shown at 18 and this gain curve contains within it the frequency peak 14 at which the splice reflections add coherently. Thus the laser will operate at this single optimum frequency to provide a low noise coherent single mode output.

By locking on to the single frequency the laser will be rendered insensitive to other reflections such as due to inherent impurity regions distributed along the optical fibre.

Referring now to Fig. 2 of the drawings, this shows a low phase noise laser arrangement comprising a semiconductor laser 19 the emitted light from which is collimated by a collimating lens 20 and optically transmitted through an air-space 21 in which are located a series of partially reflective mirrors 22 to 29.

Light is reflected back to the laser 19 from the mirrors 22 to 29, the spacing between the mirrors being predetermined so that reflected light adds coherently to enable the laser to lock on to the optimum frequency within its gain characteristic. The laser accordingly produces light at that single frequency with low phase noise. The locking action of the laser which makes the laser less sensitive to reflections at other frequencies is similar to that already described with reference to Figs.

1 and 3.

The single frequency light output from the laser may be derived from the end of the laser cavity remote from the mirrors or it may be derived from a beam splitter introduced in between the collimating lens and the array of mirrors or it may be derived from the far end of the mirror array.

Claims

1. A semiconductor or other laser arrangement for producing a low phase noise output having good single mode characteristics, in which the laser is arranged to launch emitted light into an optical transmission medium having a plurality of partially reflective mirrors, splices or equivalent means which are positioned at locations spaced apart by different predetermined distances along its length and which produce optical feedback to the laser which constrains the laser to produce light predominantly at a single frequency.

2. An arrangement as claimed in claim 1, in which the transmission medium comprises a series of random lengths of optical fibre coupled together so that the splices between adjacent lengths of optical fibre constitute the said partially reflective means, a small proportion of light emitted by the laser being reflected back to the laser by the splices and the laser being locked on to an optimum frequency within its gain curve at which the light reflected from the splices adds coherently and thereby renders the laser relatively insensitive to other reflections.

3. An arrangement as claimed in claim 1, in which the optical transmission medium comprises air whilst the partially reflective means is defined by an array of mirrors which are spaced apart in accurate predetermined relationship so that the light reflected back to the laser by the mirrors adds coherently whereby the laser locks on to a single frequency and produces a single mode output having low phase noise.

4. A laser arrangement substantially as hereinbefore described with reference to Figs.

1 and 3 of the accompanying drawings.

5. A laser arrangement substantially as hereinbefore described with reference to Figs.

2 and 3 of the accompanying drawings.