Abstract:
A device for producing an optical delay in an optical signal having variable optical frequency, the optical delay varying with the optical frequency, comprises a chirped Bragg reflector formed in an optical fiber and a directional coupler for separating the reflected signal from the input signal. One application of the device is for chromatic dispersion equalization. Various methods of manufacturing the chirped Bragg reflector are described.

Description:
This application is a continuation of application Ser. No. 749,050, filed June 26, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to optical fibre transmission systems and in particular, but not exclusively, to overcoming chromatic dispersion problems therein. 
     Chromatic dispersion in optical fibre presents serious problems when using light sources whose spectrum is non-ideal, for example broad or multi-spectral-line. This problem has previously been resolved, at least partially, in two ways. Firstly, by operating at or close to the optical frequency at which the chromatic dispersion is a minimum, for example at a wavelength of 1.3 micron in conventional silica fiber. The frequency does not generally correspond with the frequency of minimum transmission loss and attempts to modify the fibre to shift its frequency of minimum chromatic dispersion usually result in some loss penalty. The second way of overcoming the problem is to use a source with a near ideal spectrum. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative means to overcome chromatic dispersion problems in optical fibre systems. 
     According to one aspect of the present invention there is provided a device for producing an optical delay in an optical signal having variable optical frequency, which delay varies with the optical frequency, comprising a chirped Bragg reflector and directional coupler means whereby to apply the optical signal to the Bragg reflector and to separate the optical signal as reflected by the Bragg reflector therefrom, the distance the optical signal travels through the Bragg reflector before reflection varying with the optical frequency and being determined by the chirp of the Bragg reflector. 
     According to another aspect of the present invention there is provided a method of making a chirped Bragg reflector formed in optical fibre, comprising forming a Bragg reflector in each of a plurality of optical fiber sections whereby each optical fibre Bragg reflector operates at a respective wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 shows, schematically, a chromatic dispersion equaliser according to the present invention in use in an optical fibre transmission system; 
     FIG. 2 shows an optical fibre dispersion characteristics graph; 
     FIG. 3 shows characteristics of the chromatic dispersion equaliser of the present invention; 
     FIG. 4a shows, schematically, a chromatic dispersion equaliser according to the present invention, whilst FIG. 4b shows on an enlarged scale a portion of the Bragg reflector thereof; 
     FIGS. 5a, 5b, 5c, and 5d illustrate, schematically, four possible versions of directional coupler which can be used in the chromatic dispersion equaliser of the present invention, and 
     FIG. 6 illustrates a cross-section through a mandrel on which an optical fibre is wound. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As mentioned above chromatic dispersion problems are generally overcome, or partially overcome, by operating at optical frequencies at which the chromatic dispersion is a minimum, or by using a source with a near ideal spectrum. There is, however, an alternative, that is to equalise the dispersion with an element of equal and opposite dispersion. Since the system is linear such an element 1 (FIG. 1) can be placed at any position along the length of a dispersive single mode optical fibre 2 between the light source 3, for example an imperfect laser source, and a receiver 4. The equaliser element 1 which will be described in greater detail hereinafter basically comprises a directional coupler 5 and a Bragg reflector 6. 
     FIG. 2 shows the fibre dispersion characteristic for a conventional single mode silica fibre, that is a graph of relative delay τ versus wavelength λ (micron). The minimum chromatic dispersion is achieved at a wavelength of 1.3 microns whereas minimum transmission loss can be achieved at a wavelength in the vicinity of 1.55 micron. The spectrum is unstable over the marked wavelength range A with corresponding range of delay fluctuation B. What is required, therefore, is a means of correcting the delay versus optical frequency characteristic, in the region having ranges A and B, that is equalising it. This may be achieved by a device, with at least two ports, which is such that the delay versus optical frequency characteristic may be preset to compensate for the chromatic dispersion inherent in the optical transmission path. FIG. 3 illustrates the characteristics of the equaliser. The dashed line C indicates the relative delay versus wavelength characteristic for the dispersive fibre which is to be equalised. The dotted line D indicates the relative delay versus wavelength characteristic for the equaliser and the solid line E indicates the ratio of optical power in to optical power out versus wavelength, which is a maximum over the equalised wavelength range, as indicated. 
     The equaliser element 1 comprises a means for producing an optical delay which varies rapidly with optical frequency, in such a manner as to achieve chromatic equalisation, and is constituted by a chirped distributed Bragg grating formed in a fibre 5 and a directional coupler 6 for separating forward and reverse propagating waves. The element is shown in FIGS. 4a and 4b. The directional coupler 6 may comprise, for example, an optical circulator or isolator or a simple fibre coupler. Any type of optical coupler may be used e.g. a half-silvered mirror or an integrated optics system for example of lithium niobate. Specific examples of directional coupler techniques are illustrated in FIG. 5. Ideally the coupler is low loss, for example this may be achieved by use of an optical circulator (FIG. 5a) or isolator (FIG. 5b). Both of these techniques use Faraday rotation to separate counter propagating energy. The simple fibre coupler (FIG. 5c) may be used but it would introduce a minimum loss of 6 dB (3 dB for each transition). If the input light is in a stable state of polarisation then counter propagating energy could be separated using a quarter wave section 7 and a polarisation splitting element 8 (FIG. 5d). A circular polarisation state would then pertain in the Bragg reflector with a linear polarisation state for the input. This quarter wavelength coupler is also low loss. As indicated in FIG. 4a, the distance which light travels along the Bragg reflector fibre 5 before being reflected varies with optical wavelength λ (optical frequency ω) and is determined by the chirp of the grating. 
     It has previously been demonstrated (see, for example, &#34;Fiber-optic integrated interference filters&#34; J. Lapierre et al. OPTICS LETTERS January 1982 Vol. 7 No. 1 pp 37-39) that it is possible to &#34;write&#34; or &#34;record&#34; Bragg reflectors in optical fibre simply by launching into it a high optical power level beam and ensuring that there is sufficient end reflection to produce a standing wave in the fibre. After a short time (seconds to minutes) the reflectivity at the employed pumping frequency increases dramatically because a Bragg reflector, precisely matched to the pumping frequency, is formed. This grating is permanent and continues to operate at any power level. As illustrated schematically in FIG. 4b after the grating is made the fibre 5, having core 9 and cladding 10, has a periodic structure due to a photo-induced refractive index change, the period being approximately half the wavelength of the employed pump source, and Bragg reflection from this periodic structure is obtainable. The length of the grating formed is dependent on the writing power used, it may be as short as 0.1 cm with very high writing powers or several meters with low writing powers. 
     For use in an equaliser element a Bragg reflector is required in which the periodicity, and thus the optical frequency at which it reflects, is varied along the fibre length in a predetermined manner. This variation may be achieved in a number of ways, of which the following are examples. 
     Using a tunable high power laser, several separate sections of optical fiber are exposed to different optical frequencies and subsequently joined (spliced) together to form a single fiber. The chirp thus achieved would be discontinuous, but provided the number of sections is sufficiently large, little penalty will result. 
     Alternatively, a single frequency laser may be employed to expose several separate sections of optical fibre whilst the sections are extended in length to different extents, by for example stress, strain, change in temperature, or any combination thereof. When a fibre section is returned to its normal state the induced grating will have altered its resonant frequency. A number of such fibre sections can thus be joined to make a single fibre. Alternatively the reverse procedure may be applied, that is the fibre may be stretched or heated after formation of the grating, that is whilst in use, however this is less attractive for reasons of fibre fatigue. It is considered that it would be acceptable to fine tune the chirp prior to use by applying low levels of strain. 
     A further possibility comprises exposing a single continuous length of optical fiber using a single frequency laser whilst a strain/stress/temperature gradient is maintained along its length. When the perturbing gradient is subsequently removed after induction of the grating, the grating will have acquired a chirp. A stress/strain gradient can be applied to an optical fibre 11 by winding the fibre onto a deformable mandrel. Such a mandrel 12 is illustrated in FIG. 6. The cavity 13 is so shaped that when pressure is introduced to expand the mandrel 12 the fibre 11 wound thereon is stretched, there being a strain gradient along the length of the fibre in view of the variation in wall thickness of the mandrel. The pressure in the cavity 13 is maintained whilst the grating is being &#34;written&#34; in the fibre. Similarly the chirp may be adjusted after recording (i.e. for use) by applying a controllable strain gradient. 
     In the case of a system having a fiber to be equalised which has the characteristics shown in FIG. 3 (dashed line) the chirped Bragg reflector will be required to allow the shorter wavelengths to travel further along the reflector fibre before reflection than the longer wavelengths thereby to compensate for the different delay values. The actual chirp required will be determined by the particular optical transmission path. 
     The equaliser elements proposed by the present invention enable very high dispersion to be achieved in a guided wave structure of low overall size and in use provide an optical delay which varies rapidly with applied optical frequency and can by appropriate construction of the chirped distributed Bragg reflector be preset to compensate for the chromatic dispersion inherent in an optical transmission path. 
     Whereas the device has been described above in terms of chromatic equaliser applications it is not to be considered as so limited. Any predetermined amount of optical delay in an optical signal having variable optical frequency, which delay varies with optical frequency, can be achieved in dependence on the chirp of the grating and the chromatic equaliser is only a particular case thereof. The device offers the production of very high dispersion in a guided wave structure of low overall size. Such a device may also be used to achieve optical pulse compression/expansion. A chirped laser may thus produce narrow pulses of much higher peak power.