GB2339921A - Writing Bragg gratings in optical fibres using pulsed laser - Google Patents

Description

2339921 THE USE OF A LASER PULSE EXTENDER TO INCREASE THE SPEED FOR

WRITING BRAGG GRATINGS IN OPTICAL FIBRES This invention relates to the use of a pulse extender to increase the speed at which Bragg gratings can be written in optical fibres. It is widely applicable for use with all types of pulsed laser sources and is not specific to the method adopted for delivering or shaping the laser beam incident on the fibre during writing of the grating.

For the purposes of this invention 'exposure time' and 'time for exposure' refer to the time elapsed between when a material is first to when it ceases to be illuminated by pulsed laser radiation used for its processing. Exposure time is often a major contributor which determines the maximum throughput of pulsed laser-based systems used for material processing. A requisite energy dose to which a workpiece needs to be exposed is set by the nature of the laser beam-material interaction and the degree of material modification desired. In many instances the minimum time in which a dose is delivered is limited by either or both the maximum pulse repetition frequency of the laser source, or the maximum pulse energy fluence to which a part can be exposed without damaging it. For example, when writing Bragg gratings into the core of fused silica (SiO2) optical fibres with ultraviolet (uv) light from pulsed laser sources, the fibre needs to be exposed to time-integrated energy fluence doses in the range I - 100kJ/cm2. In order to prevent damage to the fibre with the limited pulse repetition frequency beams available from high power laser sources, such doses can take many minutes to deliver. By incorporating a pulse extender into the beam delivery system between the laser and the fibre, this invention allows exposure times to be reduced with ensuing improvements to the rate parts can be processed by the system.

For the purposes of this invention apulse extender'is defined as an optical device that produces from a single input pulse of laser radiation an output beam comprising of a timed sequence of multiple pulses. Prior to this invention, pulse extenders have been used to reduce the risk of damaging surfaces and bulk materials during their exposure to single short-duration high-intensity laser pulses(l).

When an extender is designed so the interpulse separation of output pulses is longer than the input laser pulse duration, an output beam consisting of a train of sequenced pulses is produced for each input pulse. As illustrated by the oscillogram in Figure 1, for every pulse input from the laser a burst of lower intensity pulses is produced. In this case a single 20nsec full-width at half-maximum (FWHM) pulse from a KrF laser is converted into a beam comprising of a sequence of 10 subpulses each having 20nsec duration separated by 40nsec in an overall 350nsec FWHM pulse envelope. Eight damped afterpulses, extend the duration of thecomplete train to 6OOnsec. Since each pulse from the laser produces 10 or more subpulses, the repetition frequency of pulses in the beam exiting from such an extender is increased. Since the pulse repetition frequency within each burst is 25MHz for the extender which produced Figure 1, in one second > 2x103 pulses are produced when used in conjunction with a laser providing pulses at a rate of 200pulses/sec.

Fibre Bragg gratings (FBG's) several millimeters in length and with a pitch of -0.5gm recorded in optical fibres can produce reflection filters having bandwidths of less than 100GHz (5 0.8nm) at - I - telecommunication wavelengths near 1.5gm. They provide a key component for efficiently separating (demultiplexing) closely spaced wavelengths which copropagate in fibres used in wavelength division multiplexed (WDM) telecommunication systems(2). 11ey can also be used for other signal management operations such dispersion compensation in long-haul communications fibre, gain flattening in fibre amplifiers, frequency and mode-selective mirrors for erbium fibre and diode lasers, narrow- bandwidth transmission filters and wavelength-selective taps for network monitoring. In addition, FBG's can be used as optical sensors for the remote detection of local stress, strain and temperature in engineering and medical environments. Their small size, corrosion resistance, chemical inertness, electrical nonconductivity and immunity to electromagnetic interference make them attractive for embedding in civil engineering structures like buildings, bridges and dams; transportation systems like aircraft, trains and boats; military systems like sonar arrays; terrestrial intruder alarms and medical catheters. Reflected wavelength shifts of <Ipm at 1.5= correspond to grating optical length changes of <10-6% induced by local strain and temperature fluctuations of < 1 ILstrain and <0. 1 OC respectively.

FBG's are currently fabricated using a spatially-modulated ultraviolet laser beam to directly expose the fibre core through the side cladding(3. 4). Optical arrangements such as two-beam interferometers, transmission phase masks, biprisms and mask projectors are used to create a spatiallymodulated power density (intensity) distribution incident on the side of the fibre. Although not fully understood, it is believed the intense uv light breaks oxygen-vacancy defect bonds in the germanium or cerium-doped fused silica core. Liberated electrons migrate and retrap at other color center sites leading to permanent intensity-dependent changes of up to 1% of the refractive index. Compaction of the size and spacing of the oxygen ions by the uv light may also contribute to the index change. The speed for writing FBG's is determined by the time taken to accumulate a requisite integrated dose of uv radiation to which the fibre is exposed. Depending on the photosensitivity of the fibre core determined by various doping and postprocessing treatments, time-integrated uv exposure fluences of 1 - 1OOkJ/cm2 are generally required for recording FBG's. On the other hand, uv laser exposure fluences in excess of -2J/cm2/pulse cause catastrophic damage and breakage to the fibre when the pulse duration is 100nsec or less. Hence when using some of the more common pulsed uv sources such as 193 and 248nm wavelength excimer, 266nm fourth- harmonic neodymiurn (Nd) and 244nm frequency-doubled dye lasers, FBG writing times can range from minutes to hours.

Figure 2 illustrates a common method used for manufacturing FBG's. A concentrated ultraviolet beam (1) from a laser (2) is incident on a transmission phase mask (3) placed in close proximity to the side of the optical fibre (4) in which the FBG (5) is to be recorded(5). In this case the spatially-modulated laser power density distribution incident on the side of the fibre is created by the interference between transmitted 1 diffracted orders (6) from a phase mask (3) designed to suppress light in the zerothdiffracted order. T"his invention applies only to the use of pulsed lasers for writing FBG's and not to cases in which continuous-wave uv sources such as frequency-doubled Ar+ lasers are used.

Although not reliant upon, to appreciate better the benefits of this invention a discussion of some of the scaling parameters which relate to the laser-material interaction physics involved in writing gratings in fused silica fibres is appropriate. It is well known that exposure of Si02 to pulses of uv laser radiation induces changes to its refractive index An. For low-defect material, index changes due to oxygen-ion compaction scale with an approximate proportionality given by(6,7): N =,F An E oor C where N,,r and E are the total number, duration and energy fluence of each pulse incident. Although the core of fused silica optical fibres is doped with impurities and so does not necessarily constitute lowdefect material, it is reasonable to assume a similar scaling of the laser- induced material modification parameter dependances. Thus when recording a grating in a fused silica fibre with pulses incident at a frequency f, the exposure time T = N required to induce a given refractive index modulation is likely to f scale with a proportionality given by:

T cc f Hence if the exposure time is to be minimized, T should be made as short and f and E as large as possible. In practice, E cannot be made larger than the single pulse fluence which leads to catastrophic damage to the fibre core-cladding combination. Transmissive materials like fused silica are damaged by single uv laser pulses at a threshold fluence Ed., which for durations in the 0. 1 - I OOnsec range scales with a proportionality (8):

E&,n - IF Thus when carrying out extensive multiple pulse exposures such as is used for FBG writing, a value of E << Ed. should be chosen with some suitably conservative safety margin like E = O.OlEdam. Then:

T oc f

Claims (2)

1. Hence the only practical way of reducing the FBG writing time T is to

   increase the pulse repetition frequency f of the beam incident on the fibre. This invention relates to a method for increasing the repetition frequency of pulses incident on the fibre without increasing the rate pulses are emitted from the laser. A claim of this invention (Claim 1) is that the speed for writing fibre Bragg gratings in optical fibres with pulsed lasers can be increased substantially by incorporating a pulse extender into the beam delivery system. The effect of such an extender (Claim 2) is to increase the repetition frequency of exposure pulses incident on the fibre.

   An example of an embodiment of this invention is the incorporation of a pulse extender (7 in Figure 2) into the beam delivery system between a krypton fluoride (KrF) laser (2) and a single-mode telecommunications optical fibre (4) for the purposes of increasing the speed of writing a FBG (5). Typically KrF laser sources produce pulsed beams of uv radiation at 248nm wavelength with energies of! IOOmJ/pulse in durations of 20nsec at repetition frequencies of!! 200pulses/sec. Telecommunications fibre usually consists of a -3.5gm diameter germanium or cerium-doped fused silica central core with a -125gm diameter cladding region housed inside a protective polymer sleeving. After removal of the sleeving, writing the FBG in the core proceeds by uv laser beam (6) exposure through a transmission phase mask (3) placed adjacent to the sidewall of the fibre (4).

   The threshold fluence for catastrophically damaging fused-silica material with single pulses of KrF laser radiation Edam is < I0J/cm2/pulse(6). Hence to minimize the risk of damaging the fibre by the incident high intensity laser radiation, the exposure fluence used for writing FBG's is restricted to E - 0.0 1 Edm <_ 0.lJ/cm2/Pulse. Because an accumulated exposure dose of typically I - 100kJ/cm2 is needed to induce the approximate I% change to the refractive index necessary for recording efficient reflective gratings at 248nm wavelength, between 104 - 106 pulses are required to achieve the exposure level with such a laser. Hence grating recording times can range from around a minute to more than an hour. Furthermore, since the area of the beam used to illuminate the fibre along its length is typically:5 0.0 1 CM2, the directed energy used for FBG writing is:5 1 mJ/pulse or!5 1 % of the total pulse energy available from the laser. By using an extender to redistribute in time the energy contained in a single laser pulse, more efficient use is made of the optical power delivered by the laser which allows the exposure time to be reduced and the speed for writing FBG's increased. The increase in writing speed is a consequence of the higher frequency of pulses incident on the fibre. In this example, with an extender producing an output train of pulses like that shown in Figure 1, the time to accumulate the 104 106 pulses required for recording a FBG can be reduced by up to a factor of ten compared to when writing the grating with the same laser source without an extender.

   This invention is widely applicable for improving pulsed laser writing speeds of FBG's and is not specific to the laser type used or the method adopted for generating the modulated power density distribution of the beam incident on the fibre. The speed for writing FBG's is increased by virtue of the higher frequency of pulses which are made incident on the fibre. With a suitably designed pulse extender integrated into the laser beam delivery system a tenfold or greater reduction in exposure time can be achieved.

   References I. M C Gower. 'Excimer lasers: principles of operation and equipmene in 'Laser Processing in Manufacturing', Eds. R C Crafer & P J Oakley, Chapman & Hall, (1993) 2. J P Ryan & M Steinberg. "WDM and optical networks: Market Directions", Optics & Photonics News, 9, 25 (Feb 1998) 3. G Meltz, W W Morey and W H Glenn. 'Formation of Bragg gratings in optical fibers by a transverse holographic method, Optics Letters, 14, 823 (1989) 4. W W Morey, G A Ball, G Meltz. Thotoinduced Bragg gratings in optical fibers', Optics & Photonics News, 5, 8 (Feb 1994) 5. K 0 I-fill, B Malo, F Bilodeau, D C Johnson and J Albert. 'Bragg gratings fabricated in monomode photosensitive opticalfiber by uv exposure through a phase mask', Applied Physics Letters, 62, 1035(1993) 6. P Schermerhorn. 'Excimer laser damage testing of optical materials', Proc SPIE, 1835, 70, (1992) 7. M Rothschild. 'Photolithography at wavelengths below 200nm'Proc SPIE, 3274, 222 (1998) 8. F Rainer, W H Lowdermilk and D Milam. 'Bulk and surface damage thresholds of crystals and glasses at 248nm', Optical Engineering, 22, 431, (1983) CLAIMS 1) The speed for writing fibre Bragg gratings in optical fibres with pulsed lasers can be increased substantially by incorporating a pulse extender into the beam delivery system. With a suitably designed extender a tenfold or greater increase in writing speed can be achieved.
2. 2) With this invention, the speed for writing FBG's is increased by the reduced exposure time that results from the higher frequency of pulses incident on the fibre.