GB2352529A - Optic fibre grating made using phase mask and variable filter - Google Patents

Abstract

An optic fiber grating is made using phase mask. A laser source is arranged to scan along the phase mask parallel to the fiber, and the resultant diffraction pattern within the fiber produces a fiber grating. A variable filter is located between the laser and the fiber, so that control of the variable filter during the scan allows/ permits control of the average refractive index profile and the amplitude profile of the grating. Optionally, the beam can be scanned along the fiber a second time, with the phase mask removed, or replaced with a different mask, so that a desired average refractive index profile can be obtained.

Description

2352529 TECHNIQUE FOR PRODUCING CUSTOM PROFILE FIBER GRATINGS The present

invention relates to a method and apparatus for producing a fiber grating.

Fiber gratings can be used in a wide variety of roles, ranging from sensing applications to the compensation of chromatic dispersion. Accordingly, the characteristics of fiber gratings vary greatly, depending upon the intended application. The performance characteristics of fiber gratings are primarily defined by the grating length, grating pitch, the average refractive index and the amplitude of the induced periodic refractive index change. Of particular interest herein are the profiles of the average refractive index and the amplitude of the refractive index change that, for specialised gratings, must be carefully tailored if the correct performance characteristics are to be achieved. The current invention aims to provide a technique for producing custom average refractive index profile and custom amplitude profile fiber gratings.

There are many situations in which fiber-optic gratings provide a simple and effective solution. For example, fiber Bragg gratings are ideal transducers in sensing systems due to their sensitivity to temperature, pressure and strain. It is important that gratings used for this purpose have a narrow bandwidth. Fiber Bragg gratings can also serve as band-pass and/or band-stop 2 filters, depending upon whether the grating is used in reflective or transmissive mode. Gratings used for this purpose must have low side-lobes in reflection and high loss and steep edges in transmission.

Non-Bragg gratings, more commonly known as chirped gratings, have an optical pitch that varies along the length of the grating. Specific physical locations along the chirped grating reflect specific wavelengths, according to the pitch at that location. This property imbues the grating with a broad optical bandwidth, which is useful for applications as diverse as compensation of chromatic dispersion, gain flattening of erbium doped fiber amplifiers (EDFAs) and channel filtering in wavelength division multiplexing (WDM) systems In most of the listed examples the performance of the grating can be improved if a custom amplitude profile is applied to the grating. The amplitude profile of a grating describes the strength of the induced refractive index variations as a function of the position along the grating. For example, a grating with a constant amplitude profile will have index variations of constant amplitude. A grating with a cosine amplitude profile will have index variations that follow a cosinusoidal envelope. Both amplitude profile examples are illustrated in Figure 1. It is beneficial to have complete control over the amplitude profile of a grating, so that profiles of any shape and length can be 3 fabricated.

As an example of the usefulness of a custom amplitude profile, the deleterious side-lobes of a fiber Dragg grating used in reflection as a band-pass filter 5 can be significantly reduced by applying a cosine apodisation amplitude profile to the grating. This helps to reduce crosstalk in wavelength division multiplexing systems. A cosine apodisation amplitude profile is also beneficial for a chirped dispersion compensation grating.

In this case, the profile reduces the amount of noise in the delay response of the grating. As a further example, the task of an EDFA gain flattening chirped grating is to induce a wavelength dependent transmission loss, so that the EDFA amplification at high-gain wavelengths is reduced to the same level as that at low-gain wavelengths, thus ensuring that signals at all wavelengths are amplified equally. In order to achieve this aim the strength of the coupling coefficient, which defines the transmission loss, must be dependent upon the position along the grating. A strong coupling coefficient at a given location will induce high loss at the corresponding wavelength, and vice versa. The strength of the coupling coefficient is controlled via the amplitude of the refractive index variations, and therefore a custom amplitude profile can create the required wavelength dependent loss.

The "average refractive index profile" refers to the 4 average refractive index within each period of the grating, as a function of the position of that period along the grating length. Figure 2 shows the average refractive index profiles that correspond to the constant amplitude 5 profile and cosine amplitude profile gratings from Fig. 1.

If the average refractive index profile of a grating is a straight line with constant non-zero gradient, the grating becomes chirped. This is due to the refractive index in adjacent periods becoming progressively higher, so that the optical pitch of the grating becomes progressively larger. (This technique for chirping a grating is distinct from the physical chirping technique, whereby the physical pitch of the grating is made progressively larger along the grating) For certain other grating types, for example WDM band- pass filters and physically chirped dispersion compensating gratings, it is advantageous for the average refractive index profile to be flat, i.e. the average refractive index is the same at all positions.

Depending upon the technique used to fabricate the grating, the average refractive index profile will often follow the shape of the amplitude profile. If the amplitude profile does not match the required average refractive index profile, it is necessary to alter the average refractive index profile alone. The current invention provides the ability to arbitrarily control the average refractive index profile. Historically, fabrication of fiber gratings has been achieved using one of two methods: the phase mask technique or the holographic technique. In recent years the phase mask technique has emerged as the preferred method, due to its ease of set-up and repeatability. The current invention is applicable to fiber grating fabrication systems based upon the phase mask technique.

In its simplest form the phase mask technique requires relatively little hardware. The basic requirements are a suitable UV wavelength laser, the phase mask and various positioning components for holding the phase mask and target fiber. A typical arrangement is illustrated in Figure 3. The laser is used to irradiate the phase mask, behind which the target fiber is closely positioned. In the simplest possible configuration the width of the UV inscription beam is sufficiently large to cover the entire phase mask, and no beam scanning facilities are required. The phase mask is typically manufactured from fused silica, which is transparent to UV wavelengths, and has very fine parallel grooves etched into its surface. The grooves cause the phase mask to act as a diffraction grating, behind which is formed an interference pattern. The core of the target f iber is arranged to lie in this pattern and is irradiated with a periodic UV intensity that causes the refractive index of the core to assume a matching periodic pattern, thus creating a fiber grating. More 6 complexity, and therefore more control over the grating characteristics, can be introduced into the fabrication process in the form of a moving phase mask, a moving target fiber or a moving UV inscription beam.

Custom fiber grating profiles can be generated by using complex variations of the basic phase mask technique. Many such variations have been demonstrated and reported, and a summary of a number of the most important is given below:

Variable velocity double scan: The UV inscription beam width is significantly smaller than the length of the uniform phase mask in this example, and therefore must be scanned in order to use the entire length of the mask. The UV inscription beam is scanned over the phase mask and target fiber twice. For the first scan the velocity of the beam movement is varied as a function of position along the mask, thus controlling the UV exposure time and the subsequent grating strength as a function of grating position to create the amplitude profile. A side-effect of the first scan is that the average refractive index profile matches the amplitude profile. For the second variable velocity scan the phase mask is removed so that only the average refractive index profile is altered. In this way the average refractive index profile can be tailored and the grating can be chirped if required. Arbitrary profile gratings can be fabricated with this technique.

7 Moving fiber/phase mask (see W.H. Loh, M.J. Cole, M. N. Zervas, S. Barcelos, and R. I. Laming, "Complex grating structures with uniform phase masks based on the moving fiber-scanning beam technique", Optics Letters, Vol. 20, No. 20, October 1995, p2051-2053): The target f iber is moved relative to the phase mask as the UV inscription beam is scanned. The movement can either be a periodic jitter, which washes out the grating at the point being written, or it can be a small translation that imposes a chirp on the grating at the point being written. In this way chirped gratings with arbitrary profiles can be fabricated. The average refractive index profile does not follow the amplitude profile with this method.

Dual-purpose phase mask (see J. Albert, K. 0. Hill, B. Malo, S. Theriault, F. Bilodeau, D. C. Johnson and L.

E. Erickson, "Apodisation of the spectral response of fiber Bragg gratings using a phase mask with variable diffraction efficiency", Electronics Letters Vol 33, No.

3, February 1993, p222-223; and B. Malo, S. Theriault, D.

C. Johnson, F. Bilodeau, J. Albert and K. 0. Hill, "Apodised in-fibre Bragg grating reflectors photoimprinted using a phase mask", Electronics Letters, Vol 33, No. 3, February 1995, p223-225): Specialist uniform or chirped phase masks can be constructed which have a variable groove depth. The depth of the grooves defines the strength of the interference pattern created 8 by the mask, and hence defines the amplitude profile. However, the UV power transmitted by the mask is constant along its length and therefore creates a flat average refractive index profile. This technique is inflexible, since it requires different phase masks for different grating profiles.

UV inscription beam intensity control by attenuating mask: The target fiber is exposed twice. During the first exposure a profiled spatial filter is placed over the phase mask to create the amplitude profile. For the second exposure the phase mask and spatial filter are removed, and replaced by a custom profile spatial filter in order to control the average refractive index profile. This technique is inflexible, since it requires different masks for different grating profiles.

The present invention seeks to provide a new method for manufacturing fiber gratings, preferably one which, in comparison to existing techniques, is simpler to implement and produces results that are more flexible, repeatable and accurate.

The current invention particularly relates to the phase mask technique for manufacturing fiber gratings. It requires relatively simple additions to be made to the basic phase mask set-up, and enables the fabrication of fiber gratings with an arbitrary custom amplitude profile and average refractive index profile. In its simplest 9 form, the current invention is not capable of producing chirped fiber gratings from uniform phase masks, but can be used in conjunction with physically chirped phase masks to produce physically chirped gratings with custom 5 profiles.

In a first aspect, the present invention proposes a fiber grating apparatus comprising:

a phase mask, which can be positioned in relation to a target fiber; an EM radiation source (e.g. a UV inscription laser) for generating a beam of EM radiation which impinges on the phase mask, thereby generating a radiation pattern (e.g. diffraction pattern) within the fiber; means for scanning the beam along the phase mask substantially parallel to the length direction of the fiber; and a variable filter arranged between the radiation source and the phase mask, whereby the intensity of the radiation pattern within the fiber can be controlled by control of the variable filter.

The current invention operates by controlling the intensity of the UV inscription beam. The width of the UV beam is preferably much smaller than the length of the phase mask; therefore the UV beam must be scanned over the mask in order to use the entire length of the mask. As the LTV beam is scanned over the phase mask and target f iber the intensity of the beam is varied to create an amplitude profile, the shape of which can be arbitrarily def ined.

In a second aspect, the present invention provides a method of fabricating a fiber grating, comprising the steps of:

positioning a target fiber in relation to a phase mask, scanning a beam of EM radiation along the phase mask, parallel to the length direction of the fiber to generate a radiation pattern within the fiber; and during the scanning step, varying the intensity of the beam using a variable filter.

After the scan is complete, the grating is left with an average refractive index profile that matches the amplitude profile. A second exposure of the target fiber, without the phase mask, using the inverse of the original beam intensity profile may be performed in order to flatten the average refractive index profile of the grating.

Alternatively, the second exposure may use a custom beam intensity profile to create a custom average refractive index profile. The present proposal differs from the existing solutions in the way that the intensity of the UV inscription beam is controlled.

As mentioned above, control of the intensity of the UV inscription beam is achieved by placing a variable 11 f ilter in the path of the beam, between the laser and the phase mask. For example, the filter may be mounted on a motor that has a remote link to a control computer, via which the attenuation introduced by the filter can be changed. The computer is simultaneously in contact with the beam scanning equipment, so that the UV beam intensity control can be accurately synchronised with the position along the grating. This permits the creation of accurate custom profiles. The profiles can have arbitrary shape, and are calculated and stored in computer memory in array form.

An embodiment of the present invention will now be described, for the purpose of example only, with reference to the figures in which:

Fig. 1 illustrates two examples of amplitude profiles of two respective gratings; Fig. 2 illustrates the average refractive index of the gratings shown in Fig. 1; Fig. 3 illustrates a conventional phase mask technique for producing a f iber grating; and Fig. 4 illustrates a preferred configuration of components in an embodiment of the present invention.

The embodiment of the current invention is a custom profile fiber grating fabrication facility. The embodiment is a variation on the basic phase mask set-up, as illustrated in Figure 3, and incorporates the constituent components of the current invention. The 12 novel configuration is illustrated in Figure 4. The additional components (i.e. additional in comparison to the conventional phase mask set-up) are a circular freespace variable filter, a dc motor, a phase mask and fiber holding assembly, a linear translation stage (used to perform scanning), a fixed beam steering mirror and a control computer. The target fiber is required to be exposed to the UV inscription beam twice; the first exposure writes the grating with the desired amplitude profile, the second exposure, without the phase mask, creates the desired refractive index profile.

The laser emits a cw laser lightwave at a wavelength of 248nm, with a focused beam width of 0.5mm. Typical phase masks have lengths in the range 10mm. - 100mm. The lightwave passes through the circular freespace variable filter, whereupon it is attenuated. The circular filter introduces an attenuation in the range 0 - 20dB, that is a function of its angular orientation. The f ilter is mounted on a dc motor assembly that is capable of rotating the filter with an angular accuracy of the order of one milli-radian, which is equivalent to significantly greater than O. OldB attenuation accuracy. Having passed through the filter, the lightwave is deflected (for convenience only) through an angle of 90 by a fixed beam-steering mirror, before being normally incident upon the phase mask and target fiber holding assembly. The phase mask and target fiber are both 13 f irmly attached to the assembly, so that they cannot move relative to each other. The assembly, in turn, is firmly attached to a linear translation stage that is capable of performing linear movements along an axis perpendicular to the incident inscription beam, with an accuracy of imm over a range of 450mm. The translation stage scans the phase mask and target fiber through the stationary UV inscription beam. During the first exposure scan the movements of the translation stage and the variable filter are co-ordinated remotely by the control computer to create the desired amplitude profile. After the grating has been written it is necessary to remove the phase mask from the holding assembly, and the target fiber is re-exposed with a different UV exposure scan profile, in order to create the desired average refractive index profile of the grating.

The task of the human operator of the system is to enter the shapes of the amplitude profile and the average refractive index profile into the control computer, to position and align the components, to mount the phase mask and target fiber in the holding assembly before the first scan and to remove the phase mask before the second scan. The two scans themselves are fully automated.

14

Claims (4)

1. An apparatus for fabricating a fiber grating, comprising; a phase mask, for positioning in relation to a target fiber; an EM radiation source, for generating a beam of EM radiation which impinges on the phase mask, thereby generating a radiation pattern within the fiber; means for scanning the beam along the phase mask along the length direction of the fiber; and a variable filter arranged to attenuate the beam before it impinges on the phase mask, whereby the intensity of the radiation pattern within the fiber can be controlled by control of the variable filter.

2. An apparatus according to claim 1, wherein the beam is a UV inscription beam, having a width smaller than the length of the phase mask parallel to the length direction of the fiber.

3. An apparatus according to claim 1 or claim 2 further including a control computer which controls the variable filter. 4. An apparatus according to claim 1, claim 2 or claim 3 further comprising an electric motor, for moving the variable filter, whereby the level of attenuation of the beam by the variable filter is varied. 5. An apparatus according to claim 3 or claim 4, in which the control computer further controls the scanning means, in coordination with the control of the variable filter, to provide a desired profile of the intensity along the length of the fiber, whereby a desired amplitude profile and/or average refractive index profile of the fiber grating is generated.

6. A method of fabricating a fiber grating, comprising the steps of:

positioning a target fiber in relation to a phase mask, scanning a beam of EM radiation along the phase mask, parallel to the length direction of the fiber to generate a radiation pattern with the fiber; and during the scanning step, varying the intensity of the beam using a variable filter.

7. A method according to claim 6, further including a step of replacing the phase mask with a different phase mask, and repeating the scanning step, with optional simultaneous variation of the intensity of the beam using the variable scanner.

8. A method according to claim 6, further including a step of removing the phase mask, and scanning the beam along the target fiber, with optional simultaneous variation of the intensity of the beam using the variable scanner.

9. A method of fabricating a fiber grating substantially as described herein, with reference to Fig.

4.

16 10. An apparatus for fabricating a fiber grating substantially as described herein with reference to Fig. 4.