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Relief Bragg grating reflectors inscribed on channel waveguides may be used in optical communications as add-drop wavelength multiplexers, gain-flattening filters, distributed feedback laser mirrors, or in sensing technology as high sensitivity devices for precise monitoring of chemical or biomedical processes. We present strong Bragg grating reflectors in Ta2O5 thin oxide films overlaid on potassium ion exchanged channel waveguides in BK-7 glass, inscribed using 248nm excimer laser holographic ablation. The grating pattern was created employing two-beam interference using a modified Mach-Zehnder interferometric cavity and the output of a narrow-lined injection cavity 248nm excimer laser. The experimental data presented are divided into two sections: the first section refers to the study of the grating ablation process of thin Ta2O5 films with respect to the exposure conditions; and the second focuses in the implementation of those relief grating in functional waveguide devices.

Sn-doped silica optical fibres are temperature resistant, have high photosensitivity and low loss at 1.55 µm, properties which make them desirable for use in telecom components and sensors. Results of the fabrication of Bragg gratings in Sn-doped fibres using KrF excimer and UV copper vapour lasers will be presented. 1. Introduction Since the first observation in 1978 by Hill and co-workers [l], photosensitivity has launched many new fields in telecom and fibre optic based devices. In the last decade many papers have been published showing enormous progress both in grating writing techniques and fibre fabrication. New materials have been studied to increase the photosensitivity of telecom optical fibres that would otherwise be insufficient for many applications. SnO2-doping was proposed as a convenient co-dopant [2,3,4,5] because of its low-absorption in the third telecom window at 1.55 µm and high photosensitivity when exposed to 248 KrF excimer laser (KEL) radiation. Recently it was demonstrated that the reflectivity of gratings written with frequency-doubled copper vapour laser (CVL) is similar to the reflectivity of the gratings written with the frequency-doubled argon ion laser [6] . Moreover, the CVL can be used to write simultaneous multiple gratings with identical reflectivity [6,7]. In this work we study the photosensitivity of tin-doped fibres with the CVL and compare it to the photosensitivity achieved with the KEL.

Optically assisted surface microstructuring of iron doped and undoped LiNbO3 based on preferential etching has been achieved by optical addressing, either laser induced surface damage or optically induced space charge field.

Bragg gratings have been written in SiO2:SnO2:Na2O optical fibres, where the presence of Na2O allows for higher concentrations of SnO2. Significant refractive index modulations (up to 6.2 x 10-4) have been achieved with enhanced temperature stability (no erasure up to temperatures >600°C).

Relief gratings overlaid on optical waveguides may be used as high-extinction ratio wavelength filters with applications in integrated optics and optical communications. Cross-patterned, high contrast relief grating waveguide structures may potentially perform as 2-D photonic band gaps if properly designed and fabricated. A new waveguide grating design process which relies on a novel grating patterning method and new materials will be presented in this paper. This design consists of the combination of a potassium ion-exchanged singlemode channel waveguide with a high-index film (InOx) overlaid on its upper surface [1]. The high-index overlayer is deposited over a part of the waveguide length. A relief grating is patterned on the InOx layer using 248nm interferometric excimer laser ablation [2].

We report recent developments on materials and techniques to improve the photosensitivity of glass based optical fibres and waveguides. These will include thermal treatments and use of dopants such as tin. Measurements on structural changes and defect formation will be presented.

High quality relief gratings of period 500nm have been patterned in InOx and Ta2O5 thin films using interferometric 248nm excimer laser ablation. Details of the ablation process and the morphology of the gratings are presented.

A study of the photosensitivity of lead germanate glasses grown by pulsed laser deposition is presented. Lead germanate films were grown from stoichiometric bulk targets, on borosilicate glass substrates in an oxygen atmosphere, to form optical waveguides. The films grown exhibited high absorption in the ultra violet spectral area (around 240 nm) which indicated the possibility of high photosensitivity. The photosensitivity of the materials was examined by phase mask and interferometric grating recording over a wide ultra violet spectral region (325nm-193nm) using various continuous and pulsed laser sources. Very high refractive index changes (up to 1%) have been observed along with variable sign of the refractive index change depending on the oxygen background pressure used during growth and recording intensity.

High quality fibre Bragg gratings have been written through the fibre coating using both 244nm frequency-doubled Ar+-ion and 248nm KrF excimer lasers for the first time. A standard off-the-shell coating was used, and 92% reflectivity gratings were obtained with an index change of 2.4 x 10-4.

Fibre Bragg gratings have been written through the fibre coating using a 248nm excimer laser for the first time. A standard off-the-shelf coating was used, and 92% reflectivity gratings were obtained with an index change of 2.4x10-4.

Enhanced UV photosensitivity and increase in defects' concentration have been observed in GeO2-doped optical fibres thermally treated using CO2 laser.

Photorefractive fibre gratings are devices that are rapidly being taken up by the telecommunications and fibre sensor industries due to their wide range of uses. They can be thought of as one-dimensional holograms written directly into the core of a conventional optical fibre which have the capability of reflecting light of a specific wavelength back along the fibre. Like holograms, they are essentially permanent and are written using specialised lasers. They rely on the fact that the refractive index of the germania-doped silica fibre core can be changed by exposure to intense ultraviolet (UV) light, however the exact nature of the photorefractive change is not yet fully understood. In general, it is possible to produce an absolute refractive index change of 10-6 - 10-3 in most fibres, though it is necessary to use specially designed fibres in order to achieve the highest index change.

Wavelength-division-multiplexing (WDM) systems is the key technology for the next generation high speed optical fibre systems. Technologies for routing on a channel to channel basis in such systems are under intensive research. All-fibre channel add/drop filters with low insertion loss are required. The dominant approach is a device based on two Bragg gratings in the arms of two concatenated 3 dB fused fibre couplers [1]. The position of the two identical gratings has to be accurately controlled to provide in-phase reflection in the two arms of each coupler. This interferometric set-up requires path-length trimming during device fabrication and the correct optical phase needs to be maintained during the device lifetime.

A new method for suppression of coupling from guided optical modes into cladding modes in an optical fiber Bragg grating using a fiber with a strongly depressed cladding is proposed. Strong suppression of the coupling has been demonstrated both theoretically and experimentally.

All-fibre versions of bulk devices are very attractive in fibre systems where low insertion loss is always desirable. We have greatly improved the performance of a recently reported [1] Bragg-grating-based acousto-optic superiattice modulator (AOSLM) by reducing the fibre diameter by HF etching. This improves the overlap of acoustic and optical power, resulting in modulator efficiencies as high as 90%. The modulator produces narrow-band amplitude-adjustable reflections that can be tuned over several nm, which is attractive for WDM and laser applications. In the device, an extensional acoustic wave is launched along a fibre Bragg grating. The ensuing superiattice modulation causes additional bands of reflection to appear on both sides of the Bragg condition. The device works by coupling together the forward and backward propagating Bloch modes in the grating (a principle first proposed in 1986 [2,3]) and operates in reflection mode. This differentiates the AOSLM from all previously reported acousto-optic fibre devices, which all rely on intermodal coupling, examples being LP01 to LP11 coupling in dual-mode [4] and twin-core [5] fibres, four-port null-couplers [6], polarisation mode couplers in Hi-Bi fibre [7] and guided-to-cladding mode coupling in standard telecom fibre [8]. In all these devices the intermodal beat length L B = λ /(n1 – n2) at optical wavelength λ between two modes with phase indices n1, and n2 is matched by the acoustic wavelength v s / f s where v s and f s are the acoustic phase velocity and frequency.

We have demonstrated both theoretically and experimentally strong suppression of coupling of the guided fundamental mode into cladding modes in a Bragg grating by introducing a depressed cladding around the core region. We have also studied the effect for different depression depths.

Refractive index changes as high as ~5 x 10-4 in a phosphorus-doped germanosilicate fibre were observed for the first time without hydrogen loading during grating formation with a 193 nm laser. Dynamics was studied and it was found that Type IIa gratings was easily formed in this fibre.

We have demonstrated a technique capable of controllably producing fibre Bragg gratings with any arbitrary chirp profiles. The technique involves making a taper on the outer cladding of a section of fibre by differential etching along the length of the fibre. A grating is then written over the taper in the usual manner. A strain gradient develops over the taper when a tension is applied to the fibre and this strain gradient is used to introduce a chirp in the gratings. The chirp can be introduced either during or after grating writing. Tension can be used to tune the central wavelength and chirp the gratings if desired. Linearly chirped gratings with bandwidth up to 4.8nm have been produced to demonstrate the controllability of the technique. These gratings are ideal for use in dispersion compensation and optical pulse shaping.

Index change (~1.4 x 10-3) of 3 times larger than in germanosilicate fibres is demonstrated. Both fibre loss at 1.55µm and high temperature stability of the gratings are much improved comparing to those in B-codoped germanosilicate fibres.

Chirped fibre gratings are of particular interest for compensating the dispersion (~17ps/nm.km @ 1.55µm) of installed step index (SI) fibre links since they are compact, low-loss, polarisation-insensitive and offer high negative-dispersion. In this paper we present a detailed investigation of the bandwidth-dispersion trade-off for a fixed (40mm) length tunable linearly-chirped fibre grating. In addition we demonstrate that such a grating can precisely compensate the dispersion in a 10Gbit/s transmission experiment for SI fibre lengths in the range 103-216km.

Strong photosensitive gratings of both type I and II have been demonstrated in germanium-free tin-doped phosphosilicate fibre. An index change of ~1.2 x 10-3 has been achieved in 40 seconds of exposure. This is the first time that such strong gratings were written in a phosphorous-containing silica fibre and that type II gratings were written in a germanium-free fibre.

The dispersion characteristics of linearly chirped fibre gratings have been studied in detail and design criteria established. It is shown that in order to compensate the linear dispersion of 100km of standard fibre, over a certain FWM bandwidth, the required fibre-grating length is 17.7cm/nm. Dispersion Compensation measurements for both a 200km, 2.5Gbit/s direct-modulated transmitter and a 216km, 10Gbit/s externally-modulated transmitter experiment confirm these predictions.

In the recent years a large number of techniques have been developed to produce linearly chirped Bragg gratings. A novel technique based on tapering the fiber cladding in an etching bath has been recently reported, the variation of the fiber cross section results in a variation of the strain along the grating when the fiber is under tension, this strain gradient is used to chirp the grating. This technique has several advantages: apart from a good controllability resulting in high quality chirped gratings, it does not require active control, and it is highly versatile in producing a variety of chirp profiles. In this paper we report on the resultant chirp structures and applications from the versatility of this technique.

The development of chirped fibre grating filters for dispersion compensation in long fibre telecommunications links requires full and accurate amplitude and time delay characterisation of these devices. We have recently demonstrated a wavelength scanning interferometric technique with sub-picosecond time-delay and 3µm wavelength resolutions to provide such measurements. The system is based on an all-fibre Michelson interferometer. The reference arm of the interferometer is phase-modulated to generate an electric signal at the photodetector which carries the optical phase and amplitude information of the reflective fibre device under test, which is included in the signal arm. The amplitude response of the interferometer is directly proportional to the field reflection coefficient whilst the time delay is given by the derivative of the relative phase with respect to wavelength. A high wavelength resolution tunable laser source (HP8168A/C) is used in conjunction with a fully automated set-up to provide sub-picosecond resolution.

Use of a reduced laser diode length with a fibre Bragg reflector leads to decreased mode-hopping and a factor of 3 improvement in temperature stability of the lasing wavelength over Δ T=22°C. Single frequency output power of 1.7 mW in the fibre with 45 dB side mode suppression was obtained.

A wavelength scanning interferometric technique has been used to provide phase dispersion and time delay measurements of photorefractive fibre gratings with sub-picosecond time-delay and 3µm wavelength resolutions for the first time. Chirped fibre grating filters for dispersion compensation in long fibre telecommunications links have been fully characterised.

Passively modelocked long-pulse lasers were made using fiber gratings, MQW nonlinear reflectors and active fiber media. Singlemode Nd- and Er-doped fibers and cladding-pumped fiber lasers operated in high-stability and multi-pulsing regimes.

A dispersion tunable 40mm fibre grating has been employed in a 10Gbit/s, step-index fibre link. Dispersion is effectively compensated for span lengths in the range 103-216km. However, at the longer spans the increased dispersion of the grating results in a reduced bandwidth and therefore its centre wavelength is found to be critical (± 0.005nm).

To date, most research into Bragg fibre gratings has centred on conventional silica-germania optical fibre. Often, fibres co-doped with boron or containing high concentrations of germania are used in order to increase the achievable refractive index modulation.

Presentation slides

Fibre gratings are rapidly becoming an important component for use in fibre optic telecommunication and quasidistributed fibre sensor systems. They can be used as filters, mirrors, wavelength-selective taps and sensing elements for strain, temperature and pressure. They are also used extensively as tuning and line-narrowing devices in rare-earth doped fibre lasers, eliminating the need for bulk, external cavity components. Recent advances in the use of KrF excimer lasers for fibre grating fabrication will be described, including the production of single pulse gratings with reflectivities approaching 100%, the development of gratings for use in harsh environments and the production of gratings 'on-the-fly' as the fibre is drawn.

An Yb-doped silica fibre has been developed for cladding pumping at 374nm. Cladding-pumped laser action has been demonstrated with slope efficiencies up to 80%.

We report the first DFB fiber laser. When pumped with a 980nm diode laser, the 2cm-long laser has an output power of 2mW at 1.534µm and a RIN of < -156dB/Hz.

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5µm written directly into a 2-cm-long Er3+-doped fiber codoped with Yb3+. We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

Yb3+-doped germanosilicate fibre has been demonstrated to be a versatile host for laser action at wavelengths from 1 to 1.2µm. It can be pumped at wavelengths from 800-1064nm, and the emission band extends from 970-1200 nm, including wavelengths of interest for specific applications, e.g., 1020nm for pumping of 1.3µm Pr3+-doped fibre amplifiers, 1140nm for pumping of Tm3+-doped fluoride fibre upconversion lasers, and 1083nm for optical pumping of 3He.

A novel thermally-compensated bending gauge using surface-mounted fibre Bragg gratings is demonstrated. The gauge configuration involves two fibre Bragg gratings, surface-mounted on opposite surfaces of a bent mechanical structure. Experimental results using a cantilever beam are presented, showing a strain resolution of 9 pstrain.

We present the first report of Bragg gratings with reflectivities exceeding 99% written in germania-fibre optical fibres by UV exposure. This result has important implications for the production of ultrashort single-frequency fibre lasers.

Laser action at 491, 520, and 635nm from Pr3+-doped ZBLANP fibre is reported for pumping at 840 and 1020nm with the output of an 840nm-pumped Yb-doped silica fibre laser.

Spectral behaviour of strain and thermal sensitivities of two superimposed fibre gratings of two different Bragg wavelengths has been studied. This involves monitoring the Bragg wavelength as a function of strain and temperature on the gratings. The results show that the ratio of sensitivity at two different Bragg wavelengths (850 nm and 1300 nm) is different with strain and temperature, which can be used for simultaneous measurement of these parameters in fibre grating sensors.

Recent advances in the operation of Yb3+-doped silica fibre lasers will be presented. including very efficient (up to 80%) cladding-pumped operation at 1020, 1040 and 1140nm, and the development of single-frequency sources for spectroscopic applications.

Single-frequency Er3+-doped Fabry-Perot fiber lasers using fiber-grating Bragg reflectors are emerging as interesting alternatives to distributed-feedback (DFB) diode lasers for use in future high-capacity wavelength-division multiplexed (WDM) communication systems. They are fiber compatible, scaleable to high output powers (>1W), and have low noise and kilohertz linewidths. Fiber lasers are only robustly single frequency provided that the laser length is reduced to a few centimeters to increase the axial mode spacing and the grating bandwidth is kept below approximately 0.2 nm. It is also necessary to keep the Er3+ concentration low to avoid a reduction in efficiency by ion-pair quenching, and this combination of factors means that the pump absorption in the laser length can be as low as a few percent. Hence the slope efficiency of these lasers is very low. It is possible to increase the output power by using the residual pump power to pump an erbium-doped fiber amplifier (EDFA) following the fiber laser. However, the presence of the amplifier amplified spontaneous emission (ASE) increases the output noise. In addition, it has been reported that these lasers are susceptible to strong self-pulsation.

Following the report by Grubb we describe an all solid state, single wavelength pumped, cw, room temperature, upconversion laser which has operated in the blue between 475nm and 483nm or in the near infrared at 778nm. We have pumped this device at both longer and shorter wavelengths than those reported by Grubb. The system uses Yb-doped silica fibre to convert the output from a diode pumped Nd:YLF laser to a wavelength in the range 1.05µm to 1.18µm. This is used to pump a Tm-doped ZBLAN fibre giving rise to an upconversion process which involves the sequential absorption of three pump photons to populate the 1G4 upper laser level as shown in Fig 1.

A novel all-fibre channel-dropping filter based on a new physical principle, , is demonstrated. A prototype device with 0.7 nm bandwidth, 13 dB isolation and 0.2 dB excess loss is reported.

Yb-doped fibre lasers have been previously reported as versatile, efficient laser sources in the 1 µspectral region. The very broad Stark splitting of Yb energy levels in silica results in wide pump (830 - 1064 nm) and emission (975 - 1160 nm) bands. The emission band includes a number of wavelengths of interest for specific uses; examples include 1020 nm, the optimum pump wavelength for the Pr:ZBLAN amplifier and upconversion laser, and 1128 nm. which has been utilised to pump a Tm:ZBLAN upconversion laser.

A fibre optic sensor for monitoring high pressures, using an in-fibre grating, has been constructed. The method involves tracking the wavelength of peak reflection from an in-fibre grating as it is pressurised. Experimental trials, under the influence of hydraulic pressure, have been carried out and a wavelength shift of 0.22 nm at 70 MPa pressure was observed. The sensor should be an attractive choice for ultra-high pressure monitoring.