a first embodiment of the invention relates to a fabrication method developed for ptp of d - shaped germanosilicate optical fibres . for the first embodiment , the wavelength of interest is 1 . 5 μm , and the non - linear process considered is forward three - wave mixing . poling periods ranging between about 10 to 100 μm are therefore needed . the method of the first embodiment starts from a d - shaped germanosilicate fibre with numerical aperture of ˜ 0 . 19 , core diameter of ˜ 6 μm , outer diameter of ˜ 300 μm and initial flat surface / core distance of 13 μn . the outer diameter is about twice that of common fibres , thus allowing better photolithographic processing and lower breakdown probability during thermal poling . the d - shaped fibre was preliminary etched uniformly using the apparatus sketched in fig1 which allows to control in real time the transmission at around 1 . 5 μm while the fibre is placed in a hf buffered solution ( etching rate of ˜ 0 . 1 μm / min ). fig2 shows the transmission of the fibre against the etching depth . the etching is stopped when the transmission starts decreasing , so that the flat surface / core distance is about 10 μm . this distance is , on the one hand , sufficiently small that the core region is within range of the subsequent thermal poling induced by a surface electrode , while , on the other hand , sufficiently large to avoid significantly increased optical propagation losses in the waveguide . fig3 shows the next step of the method . namely , after uniform etching and cleaning , the fibre is placed on a metal substrate with v - groove and taped at the ends ( or alternatively glued ) for standard planar lithography to define a structured surface and subsequently deposit a metal electrode to use as anode for ptp . fig4 a and fig4 b show the next steps of the method . namely , instead of direct patterning of an anode electrode after the uniform etching , another technique is used in which the surface is structured with the idea of improving the modulation of the second order non - linearity . preliminary uniform etching reduces the flat surface / core distance to ˜ 10 μm , as mentioned above . a photoresist pattern is then deposited on the plane face of the d - fibre and periodic etching in a hf buffered solution is performed . the region not covered by the resist is etched away by about 5 μm . subsequently , al was evaporated and finally the resist removed . fig4 a and fig4 b show the results of this process . the geometry ( including the mark - to - space ratio ) of the initial resist pattern has to take account of the fact that the etching is fairly isotropic , so that material is removed under the resist itself , thus altering the mark to space ratio of the structure . ( alternatively , anisotropic etching may be considered to avoid this additional factor ). the al lines are connected by a lateral longitudinal back - bone electrode , which ensures electrical contact along the whole length ( see fig4 b ). conventionally , modulation of the effective non - linearity in the case of direct al patterning after uniform etching ( without periodic etching ) is due to the modulation of the electric field during poling . by contrast , in the present embodiment where al patterning is performed after periodic etching , an improved modulation of the effective non - linearity is provided because spreading effects are reduced . in fact , when periodic etching is performed , al patterning is not necessary to periodically modulate the non - linearity . after periodic etching , the resist can be removed and al evaporated over the whole plane face of the d - fibre . a periodic second order non - linear pattern will be created in the core purely by virtue of the varying distance between the waveguiding core and the blanket surface electrode that will follow the profile of the structured surface of the d - shaped fibre . the extent of the non - linearity induced by the thermal poling ( described further below ) is shown highly schematically in fig4 b by the shading which can be seen to extend through the core region where the surface and thus the anode electrode is closer to the core region , but not to extend to the core region where the surface and thus the anode electrode is farther from the core region . the reason lies in the aforesaid change in the effective non - linearity , and is attributable to the fact that the non - linear layer can be 5 - 20 μm thick under the anodic electrode , following the surface profile . a careful design of the fibre geometry therefore provides periodic longitudinal alternance of poled and weakly poled core segments . effectively the overlap integral between the interacting modes and the non - linear layer is periodically varied to provide the desired quasi - phase - matching condition . avoiding an al or metal pattern by using a continuous , blanket electrode could also help to reduce the fringe effects at the electrodes edges which lead to high electric field points with consequent enhanced breakdown probability and damage of the electrode . according to our findings , in the case of directly patterned al electrodes , the use of rounded corners for the electrode structure , significantly decreases the occurrence of electrical breakdown , improving the reliability of the poling process . however , use of an unpatterned electrode , which is only possible as a result of the structured surface , avoids the problem altogether . fig5 shows the next step in the method . namely , the structured fibre is placed in a high vacuum chamber for ptp . ptp is performed by applying 3 - 5 kv voltage at 270 - 280 ° c . for 10 - 20 minutes . the al surface electrode was the positive electrode during poling ( anode ) while a silicon wafer placed under the curved face was used as the grounded electrode ( cathode ). the vacuum atmosphere reduces the probability of breakdown , which would certainly occur in air when these voltages are used over such a small electrodes gap . fig6 shows a study of thermal poling used to optimise the ptp process . the results show sh signal ( and the value of the non - linear susceptibility in the inset ) as a function of thermal poling time , obtained using a q - switched and mode - locked nd : yag laser , for fused silica samples after thermal poling in air for different sample thickness . the smaller the samples thickness the higher the value of the non - linearity , a clear indication that part of the voltage during the thermal poling process drops across the undepleted ( not poled ) sample region . it seems convenient to reduce the electrodes distance during poling , so that the non - linearity is increased . however it has to be pointed out that to form an efficient depletion layer ( where most of the voltage drops and is stored in the material when the sample is cooled down to room temperatures ) is necessary to have a material thickness ( hence the electrode distance ) greater than the depletion layer thickness . for example in vacuum the depletion layer thickness ( which corresponds to the thickness of the non - linearity ) is already ˜ 10 - 20 μm even for short poling times [ 15 ], therefore the electrode distance in any fibre and waveguide structure for thermal poling in vacuum should probably be in the range of 50 - 100 μm for the temperature and voltage conditions used in these studies . the optimum material thickness for thermal poling is also likely to be dependent on material composition ( for example the germanosilicate fibre is made of layers with different proportion of geo 2 and sio 2 ) and poling conditions ( atmosphere , poling temperature and voltage , etc .). for example for thermal poling in air atmosphere the minimum material thickness ( electrode distance ) could be smaller that that required for thermal poling in vacuum given the fact that the evolution of the depletion layer in air is slower [ 12 ]. these results show that the distance between the electrodes ( anode and cathode ), i . e . the thickness of the material subjected to the poling process , is an important parameter for the thermal poling process which needs careful consideration . fig7 shows in cross - section a different fibre geometry for thermal poling , alternative to that of fig5 . in this alternative geometry , the d - shaped fibre is provided with a hole for electrode insertion placed along the two - fold symmetry axis of the d - shaped fibre between the core and the curved surface of the “ d ”. a metal wire of gold , au , plated tungsten , w , is inserted into the hole of the fibre and is used as the cathode during thermal poling . the anode electrode is fabricated on the flat surface of the “ d ” as described above . in this way electrical breakdown problems during thermal poling are avoided without needing to provide a high vacuum atmosphere . by contrast , when a planar ‘ pressed contact ’ cathode is used in contact with the round face of the d - fibre ( see fig5 ), breakdown can only be avoided by thermal poling in high vacuum . the embedded electrode structure reduces effectively the breakdown probability because any conductive path between the two electrodes has to be mainly through the bulk of the material . the thermal poling of this type of fibre has a much higher reliability . for example higher voltages can be applied without encountering breakdown problems . the fibre has been tested for sh generation ( shg ) and produced promising results . in principle this fibre could be poled in air , however , according to our studies , the vacuum atmosphere is necessary in order to avoid spreading of the poled sections , at least if a patterned electrode is used , thus maintaining a deep modulation of the non - linearity . fig8 shows results of second harmonic generation ( shg ). examples are shown of quasi - phase - matching curves ( second harmonic power as a function of fundamental wavelength ) for two different gratings : sample a ( period of 56 . 45 μm and length of 75 mm ) and sample b ( period of 56 . 5 μm and length of 60 mm ). the numerical aperture and the core radius of the fibres are 0 . 191 and 3 μm respectively . the results were taken to assess the quality of the waveguide structures and used a high power tuneable er - doped fibre amplifier source in the low power regime to avoid saturation and higher order non - linear effects . typical qpm curves ( sh power versus fundamental wavelength ) are shown in the figure for sample a and sample b . the bandwidths of the qpm curves compare favourably with the expected values for perfect gratings of the same length , thus indicating that the whole grating length is contributing to the non - linear interaction . from the value of the second harmonic efficiency it is possible to estimate the non - linear coefficient [ 9 ]. fig9 shows a qpm curve for parametric fluorescence experiment in sample b . the bars indicate the bandwidth of the generated fluorescence . parametric fluorescence at around 1 . 5 μm was produced by pumping with 300 mw from a continuous wave ti : sapphire laser ( lasing wavelength = 766 nm ). a photon production rate of ˜ 150 mhz and a gain bandwidth ˜ 100 nm were achieved . the figure shows the signal and idler wavelength against pump wavelength for the parametric fluorescence experiment . some device design considerations are now discussed . typically the grating periods required to phase - match the second - order parametric interactions are in the range of tens of microns for forward interactions . this , very importantly , enables one to use the surface structuring technique with standard relatively large feature size processing techniques . some devices using ptp structures fabricated according to the above described methods are now described . fig1 a and fig1 b show a forward - three - wave - mixing ( ftwm ) fibre device . the illustrated ftwm device provides a transparent and effective frequency converter , which would be largely employed in wdm optical telecommunication systems . the pump beam interacting with the non - linear fibre and with the incoming signal , produces a forward travelling idler which carries the same modulation as the signal at a different wavelength such that : where ω i , ω s , ω p denote idler , signal and pump frequency respectively . at the same time the signal wave is also amplified . the phase - matching condition is provided by the use of a periodic non - linearity achieved in the core by means of the above - described ptp methods . the advantage of ftwm consists in the use of non - linear gratings possessing a period of tens of microns , however due to the co - propagating nature of the waves involved , optical filtering is necessary to separate idler from signal and to suppress the residual pump signal . fig1 a and fig1 b show another device , namely an optical parametric oscillator . ftwm can be used to implement optical parametric oscillators ( opos ). however suitable optical feedback must be provided by means of mirrors that can be external or directly fabricated onto the polished ends of the non - linear fibre . opos find large use as sources of coherent radiation possessing a very broad tuneability range , which find useful application for example in spectroscopy , material and laser science . fig1 a and fig1 b show another device . this device is a photon - pair generator based around a fibre possessing periodic second - order non - linearity is a particular case of twm . here a forward configuration is considered . the pump enters the fibre device and interacting with the non - linear medium provides amplification for the photons originated by quantum noise . each pump photon is then split into two photons , according to equation ( 1 ) above . the two photons represent a pair and possess special correlation properties , which exhibit non - local behaviour . this can be exploited for example in fibre - based quantum key distribution system where the photon pair generator would represent the light source fig1 a and fig1 b show another device , namely a second harmonic generator . this device is another case of twm ( here considered forward ). the pump beam at frequency ƒ self - beats to produce a second harmonic beam at frequency 2ƒ . sh generation is useful to generate new wavelengths by doubling the output of readily available powerful lasers , in particular fibre lasers . a fibre second harmonic generator would be straightforward to integrate with existing high power fibre lasers / amplifiers , by means of direct splicing or micro - optics coupling . fig1 shows another device which is a power dependent optical switch . in this device the non - linearity is cascaded , by which it is meant the pump at frequency ω is frequency doubled and then converted back to ω . in this process , the pump wave acquires a phase shift , which is power dependent and that can be used in combination with an interferometer , e . g . mach - zehnder , in order to implement a power dependent switch . if the power is high the signal will exit port p high , otherwise p low . an optical fibre possessing periodic second order non - linearity would allow the implementation of a filly fiberised version of such a device , thus improving its integratability with existing fibre optics networks . 1 . j . a . armstrong , n . bloembergen , j . ducuing , and p . s . pershan , ‘ interactions between light waves in a non - linear dielectric ’, physical review 127 , 1918 ( 1962 ) 2 . r . a . myers , n . mukhexjee , and s . r . j . brueck , ‘ large second - order non - linearity in poled fused silica ’, optics lett . 16 , 1732 ( 1991 ) 3 . r . kashyap , ‘ phase - matched periodic electric - field - induced second - harmonic generation in optical fibres ’, j . opt . soc . am . b 6 , 313 ( 1989 ) 4 . r . a . myers , n . mukheree , and s . r . j . brueck , ‘ large second - order non - linearity in poled fused silica ’, optics lett . 16 , 1732 ( 1991 ) 5 . r . kashyap , g . j . veldhuis , d . c . rogers , and p . f . mckee , ‘ phase - matched second - harmonic generation by periodic poling of fused silica ’, appl . phys . lett . 64 , 1332 ( 1994 ) 6 . p . g . kazansky , v . pruneri and p . st . j . russell , ‘ blue - light generation by quasi - phase - matched frequency doubling in thermally poled optical fibres ’, optics lett . 20 , 843 ( 1995 ) 7 . v . pruneri and p . g . kazansky ‘ frequency doubling of picosecond pulses in periodically poled d - shape silica fibre ’, electronics lett . 33 , 318 ( 1997 ) 8 . v . pruneri , g . bonfrate , p . g . kazansky , c . simnonneau , p . vidakovic , and j . a . levenson , ‘ efficient frequency doubling of 1 . 5 μm femtosecond laser pulses in quasi - phase - matched optical fibres ’, applied phys . lett . 72 , 1007 ( 1998 ) 9 . v . pruneri , g . bonfrate , p . g . kazansky , d . j . richardson , n . g . broderick , c . simonneau , p . vidakovic , j . a . levenson , ‘& gt ; 20 %- efficient frequency doubling of 1532 nm nanosecond pulses in quasi - phase - matched germanosilicate optical fibres ’, optics lett . 24 , 208 ( 1999 ) 10 . u . s . pat . no . 5 , 617 , 499 : s . r . j . brueck , x . c . long ‘ technique for fabrication of a poled electro - optic fiber segment ’ 11 . m . scalora et al ., ‘ pulsed second - harmonic generation in non - linear , one - dimensional , periodic structures ’, physical review a 56 , 3166 ( 1997 ) and j . w . haus et al ., ‘ enhanced second - harmonic generation in media with a weak periodicity ’, physical review a 57 , 2120 ( 1998 ) 12 . v . pruneri , f . samoggia , g . bonfrate , p . g . kazansky , and g . m . yang , applied phys . lett 74 , 2423 ( 1999 )