GB2212651A - Lasers with external optical feedback - Google Patents

Abstract

A pulsed injection laser 20 provided with a reflector, eg external mirror 22, that produces spectral broadening of the laser emission is also provided with means for generating an optical seed signal of predetermined amplitude and frequency in the source at the commencement of a pulse. The optical seed signal may be provided by a second laser 31 for optically seeding the laser emission in order to make more deterministic the amplitude and frequency changes that occur at least in the initial stages of the spectrally broadened emission. Alternatively optical seeding may be provided by initially operating laser 20 while external reflector 22 is rendered inoperative by a Q-switching device. The external reflector and Q-switching device may be combined by the use of an active mirror. Use of the deterministic changes is made in an optical fibre reflectometer with correlating coherent light detection. The device may also be used in certain spread spectrum secure transmission systems. <IMAGE>

Description

LASERS WITH EXTERNAL OPTICAL FEEDBACK It is known that optical feedback external of the optical cavity of a laser can affect the operation of that laser, and this phenomenon has been used to produce line narrowing of the spectral output of injection lasers. Typically for line narrowing purposes such feedback has been provided by the back-scatter in a long length of optical fibre coupled with the injection laser. It is also known that under certain circumstances optical feedback can have the opposite effect and lead to spectral broadening. This is described for instance in the paper by H. Olesen et al entitled 'Non Linear Dynamics and Spectral Behaviour for an External Cavity Laser', IEEE Journal of Quantum Electronics Vol. QE-22 No. 6 June 1966 pp. 762-773.In the situation of feedback providing spectral broadening, the instantaneous values of emission wavelength and amplitude change rapidly somewhat after the manner that is mathematically referred to as 'chaos'. However, insofar as it is possible to argue that the emission behaviour of a spectrally broadened emission laser is not chaotic in view of the effects of spontaneous emission, it is preferred to characterise this spectral broadening effect as 'cacophony'.

The present invention is concerned with the operation of a laser with optical feedback in a cacophonous manner in which at least for a limited period of time the frequency and phase its output changes in an at least partially deterministic manner.

According to the present invention there is provided a pulsed cacophonous laser source provided with a spectral broadening external reflector in its optical feedback path, and means for generating an optical seed signal of predetermined amplitude and frequency in the source at the commencement of each pulse.

This determinism may be made use of in applications involving the optical correlation of two signals. Such correlation is for instance required in certain form of spread spectrum secure transmission systems in which a signal is employed to modulate a carrier that is frequency hopping in a deterministic manner and is coherently detected at a remote location using a local oscillator that frequency hops in the same manner. Correlation may also be used in reflectometry, particularly optical fibre reflectometry.

There follows a description of reflectometers incorporating lasers embodying the invention in preferred forms. The description refers to the accompanying drawings in which: Figure 1 is a schematic diagram of the reflectometer.

Figure 2 depicts a laser with an external reflector and Figures 3 and 4 depict alternative forms of laser source suitable for use in the reflectometer of Figure 1.

Coherent detection has already found application in optical time domain reflectometers to enhance the sensitivity of the detection. With the reflectometer, components and waveguides are characterised by the returned signal from a short optical pulse which is analysed versus time to evaluate the backscatter and intervening losses. With the low transmission loss long wavelength single mode fibre the backscatter is very small (else it would not be low loss fibre) and hence little signal is returned. The resolution is increased as pulse length is reduced, and so for high resolution very little energy per pulse may be injected.The combined effect of these factors is that a high resolution reflectometer for single-mode long-wavelength fibres is hard to conceive, and yet this is exactly what is required to inspect an optical local area network where optical components are connected by relatively short lengths of fibre waveguide. A solution to this problem is to arrange for a reflectometer not to use short duration pulses but instead to inject into the fibre longer pulses of modulated light having a sharp and distinct autocorrelation function providing a single sharp correlation peak. The return signal is then cross-correlated with a delayed version of the injected source signal to find the contribution to the total return from a particular point along the waveguide determined by the length of the delay existing between the injected signal and its delayed version.Since the input is a relatively long pulse the total integrated return signal is greater than for a short pulse input.

Figure 1 depicts a length of optical fibre 10 connected for reflectometry measurement to a reflectometer. The light source for this reflectometer is a cacophonous laser 11 driven by a pulser 12 that produces pairs of pulses. Light from this cacophonous laser 11 is directed via first and second beam splitters 13 and 14 for injection into one end of the optical fibre 10. Light that is backscattered in fibre 10 is directed back to beam splitter 14 from where a proportion is directed to a further beam splitter 15.

Also directed to this beam splitter 15, for combining with the backscattered signal, is a 'local oscillator' signal derived from beam splitter 13. An output of beam splitter 15 directs a proportion of the combined signal on to a photodetector 16 which is followed by a low pass filter 17. Conveniently, the beam splitters 13, 14 and 15 are single mode optical fibre beam splitters such as can be made by the method described and claimed in GB 2 150 703 B. Under these circumstances the light that is directed between the beam splitters is also propagated by way of single mode fibre.

The principle of operation of this reflectometer is that a first pulse of light is launched from the cacophonous laser into the fibre under test to produce a backscattered signal which is interfered in beam splitter 15 with the light derived from the second pulsing of the laser. The frequency of the light within each laser pulse is changing in a rapidly non-cyclic manner and hence, if two pulses from the laser are interfered with each other for coherent detection, the resultant beat frequency in the output of the photodetector will similarly change in a rapidly non-cyclic manner when the two pulses, though overlapping in time, are significantly out of synchronism with each other. As synchronism approaches, a beat frequency will begin to appear, this frequency falling off to zero when exact synchronism is reached.

In the case of the reflectometer, the local oscillator signal is being interfered, not with a single backscattered pulse for a single backscattering located at a single point in the fibre under test, but with many backscattered pulses from many scattering points located at different distances along the fibre. For any given delay between the two pulses, only those backscattering points in the immediate vicinity of a certain specific distance along the fibre will be giving rise to backscattered light sufficiently close to synchronism with the local oscillator pulse to generate a low frequency output from the detector. Examination of the backscatter from points at different distances along the fibre under test is achievable by adjustment of the pulser 12 in order to sweep the time interval it provides between the two pulses of each pair of pulses it generates.

For this reflectometer to work as a precision instrument the cacophonous laser must produce a signal whose frequency and amplitude changes in a noise-like way in the sense of having close to zero autocorrelation at all delays away from zero, and a narrow maximum at that point. Furthermore such changes must be substantially deterministic in order for successive pulses to be sufficiently closely similar for the correlation function between successive pulses not to be severely degraded in comparison with that provided by autocorrelation of a single pulse.

Figure 2 schematically depicts a semiconductor laser chip 20 provided with a lens or lens system 21 and an external reflector 22 whose positioning and reflectivity are such as to produce spectral broadening of the laser emission. In the case of a Fabry-Perot type injection laser manufactured by Hitachi under the designation HLP 3400 the mirror may for instance be a total reflector positioned at about 300mm from the nearer face of the laser chip as to provide a round trip reflection time of 2ns and a 1% power return. Similar conditions are also suitable when using a DFB lase chip.The lens or lens system 21 may be positioned to collimate the light emerging from the laser or, if it is preferred to reduce the contribution of reflections from other planar surfaces along the optical path, such as plano lens surfaces, the lens system may be positioned to bring the emitted light to a focus at the reflector 22. Offsetting the alignment of the reflector can be used to reduce the effective relectivity seen by the laser.

When the laser chip 20 of Figure 2 is driven above its lasing threshold by a current pulse, it starts with 2ns of unperturbed lasing such as would occur in the absence of the external reflector 22. This is followed by 2ns during which the emission is interfered with the unperturbed laser signal reflected from the external mirror. In the next 2ns time interval the emission is interfered with the perturbed laser signal produced in the preceding 2ns, and with each succeeding 2ns increment the optical signal becomes more convoluted, leading to deep amplitude modulation with noise-like appearance and a bandwidth of around 2GHz.

Successive electrical pulses of the laser chip 20 induce a similar build up of the noise like pattern in 2ns steps, but the exact pattern of this noise-like pattern is not exactly replicated from pulse to pulse.

In consequence the laser system of Figure 2 is not suitable in that form to constitute the laser 11 of Figure 1.

The exhibition of variability is attributed to the fact that when the laser system of Figure 2 is first taken above its lasing threshold the lasing oscillation is started by amplification of spontaneous emission, and this spontaneous emission is a random process. It can be demonstrated that the variability can be much reduced by optically seeding the laser emission with light of a predetermined amplitude and frequency. Under these circumstances the laser emission can be induced to be substantially deterministic at least over a period of several, up to a few tens of, round trip reflection times to the extent that correlation between consecutive pulses can successfully be performed over this time period.Over longer periods of time the effects of spontaneous emission are liable to have built up to the extent of masking the deterministic changes of amplitude and frequency with random changes.

Various ways are possible of providing this optical seeding to make a laser spectrally broadened by an external reflector operate satisfactorily to function as the laser 11 of Figure 1. In the arrangement depicted schematically in Figure 3 a beam splitter 30 is placed between the laser chip 20 and its external reflector 22 by means of which light from a seed laser 31 is directed into the laser chip 20. The seed laser is arranged to be operating in a stable (spectrally narrow) manner at the time the current drive to laser chip 20 is taken above its lasing threshold. If desired, this seed laser may be switched off once laser chip 20 has commenced lasing, but this is not necessary.

The arrangement of Figure 3 uses a separate laser for seeding, but alternatively the laser chip 20 itself may be used as the source of the seeding light.

Thus in the arrangement of Figure 4 the laser chip 20 is driven above its lasing threshold while the external reflector 21 is rendered inoperative by a Q-switching device 40. Initially, the laser chip 20 operates in a stable spectrally narrow manner until the Q-switching device is pulsed to provide sufficient optical coupling between the external mirror and the laser chip to lead to the commencement of cacophonous operation of the laser. The function of external reflector and Q-switching device can conveniently be combined by the use of an active mirror whose reflectivity can be switched rapidly from a relatively low value to a relatively high one. A particularly convenient form of such an active mirror is a second laser chip with an anti-reflection coated facet facing the main laser chip 20. When this second laser chip is biased below threshold the device reflectivity is low because the front facet reflectivity is low and light is absorbed before reaching the relatively high reflectivity rear facet. When biased at lasing threshold this absorption is removed. Use of an active mirror of this sort also affords the possibility of introducing further modulation of the cacophonous emission by modulation of the mirror reflectivity during this emission.

Although the foregoing examples of pulsed cacophonous laser sources have each employed a single external reflector for inducing spectral broadening of the emission it should be appreciated that the spectral broadening function can alternatively be effected with a reflection system comprising a set of co-operating reflectors.

Claims (9)

CLAIMS:

1. A pulsed cacophonous laser source provided with a spectral broadening external reflector in its optical feedback path, and means for generating an optical seed signal of predetermined amplitude and frequency in the source at the commencement of each pulse.

2. A pulsed cacophonous laser source as claimed in claim 1, which source includes a first laser optically coupled with said spectrally broadening external reflector, and wherein the means for generating the optical seed signal includes a second laser together with means for injecting the emission of the second laser into the first laser.

3. A pulsed cacophonous laser source as claimed in claim 1, which source includes a laser optically coupled with said spectrally broadening external reflector via a Q-switching device.

4. A pulsed cacophonous laser source as claimed in claim 1, wherein the spectrally broadening external reflector is constituted by an active mirror the value of whose reflectivity is electrically controllable.

5. A pulsed cacophonous laser source as claimed in any preceding claim wherein said spectral broadening external reflector is one reflector of a spectral broadening external reflector system comprising a plurality of reflector elements.

6. A pulsed cacophonous laser source substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

7. An optical reflectometer whose optical source is provided by a pulsed cacophonous laser source as claimed in any preceding claim.

8. A reflectometer as claimed in claim 7 which reflectometer is an optical fibre reflectometer.

9. An optical fibre reflectometer substantially as hereinbefore described with reference to the accompanying drawings.