Parallel beam frequency doubling of 170 fs, NIR pulses is demonstrated by placing a thin beta barium borate (BBO) nonlinear crystal after a spatial light modulator. Computer-generated holograms applied to the spatial light modulator create 18 parallel diffracted beams at the fundamental wavelength λ=775 nm, then frequency doubled to λ=387 nm and focussed inside the poly(methyl methacrylate) (PMMA) substrate for refractive index structuring. This procedure, demonstrated for the first time in PMMA, requires careful attention to phase matching of multiple beams and opens up dynamic parallel processing at UV wavelengths where nematic liquid crystal devices are more sensitive to optical damage. By overlapping filamentary modifications, an efficient, stable volume phase grating with dimensions 5×5×2.0 mm3 and pitch Λ=15 μm was fabricated in 18 minutes and reached a first-order diffraction efficiency of 70 % at the Bragg angle.

Refractive index structuring of poly(methyl methacrylate) (PMMA) by femtosecond (fs) laser irradiation is discussed, including writing conditions defined by wavelength, pulse duration, and associated photochemistry. The aim is to determine optimal conditions for refractive index modification, Delta n without doping for photosensitivity. The work presented here forms a generic methodology for other polymers. Nanostructuring using holographic optics and precise control of beam parameters has versatile application for three-dimensional (3D) photonic devices. Self-focusing and filamentation at various depths below the surface of bulk PMMA are discussed together with parallel processing using a spatial light modulator. Applications of refractive index structures in polymers include microfluidics, labon-a-chip, organic optoelectronic devices, and gratings in polymer optical fibres.

Spatial light modulator addressed with computer generated holograms is used to diffract femtosecond laser pulses into > 15 parallel beams and focused simultaneously inside poly(methyl methacrylate) (PMMA) for high throughput 3D refractive index modification. Uniform modification throughout the structure is demonstrated and used to create a 19 mu m pitch, 5 x 5 x 3.5 mm(3) thick volume grating in < 50 minutes with first order Bragg diffraction efficiency > 65%. Thick volume phase gratings are created by carefully stitching filamentary modifications. PMMA shows a remarkable time dependent Delta n, related to the photochemistry during and after exposure.

We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates irradiated by a femtosecond Ti: Sapphire laser. In order to investigate the distribution of the refractive index change produced by the femtosecond laser irradiation, we performed a two-dimensional visualization and spatially resolved optical analysis of the induced refractive index profile by using a digital holographic technique and an adaptive-iterative algorithm for wavefront reconstruction. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated samples, providing information how the fabrication process depends on the laser irradiation.

We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates by a femptosecond Ti: Sapphire laser. As for their optical characterization, a spatially resolved microscopy interferometric technique is used to investigate the two-dimensional distribution of the refractive index change produced by the irradiation process. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated PMMA, providing information on how the fabrication process depends on the laser irradiation.

Parallel femtosecond refractive index laser inscription of clinical grade poly(methyl methacrylate) (PMMA) at 775 nm, 170 fs pulselength is demonstrated with multiple low fluence beams generated with the aid of a spatial light modulator. Using optimised computer-generated holograms (CGHs), 16 diffracted near identical beams were focused simultaneously within bulk PMMA to create a series of 19 μm pitch, 5 mm×5 mm×1–4 mm thick volume phase gratings at high speed. First order diffraction efficiency rises with grating thickness in accord with diffraction theory, reaching 75% at the first Bragg angle (4 mm thick) with fabrication time around 1 hour. By carefully stitching filamentary modifications while eliminating effects such as pulse front tilt during inscription, gratings exhibit high uniformity, which has not been achieved previously using femtosecond inscription. Highly uniform modification is exhibited throughout the material consistent with the observed excellent angular selectivity and low background scatter and quantitative comparison with first order diffraction theory is satisfactory. The diffraction efficiency and hence refractive index profile shows a temporal behaviour related to the material response after exposure. Simultaneous 3D modification at different depths is also demonstrated, highlighting the potential of creating complex 3D integrated optical circuits at high speed through the application of CGHs.

Femtosecond laser pulses are of particular interest for internal modification of transperent materials as they enable nonlinear absorption due to the extremly high intensity in the focal volume. Since output from commercial laser sources currently exceeds single beam process requirements, parallel processing with multiple beams could provide a route to up-scaling processing speed and establish cost-effectiveness.The use of spatial light modulators, driven by fast computer-generated holograms for splitting a parent laser beam into a number of beamlets and digitally manipulate their positions and the laser intensity is demonstrated. With successful blocking of the zero order beam and subsequent focusing of the diffracted beams inside transperant materials, high throughput dynamic 2D/3D refractive index modification of polymer and glass substrates with a gain factor G > 20 has been achieved. Fundamental IR (775nm) femtosecond laser pulses were employed to produce optical components. For example, thick volume gratings written with more than 20 beams have 1st order diffraction efficiency η > 60%, indicating a refractive index change Δn ≈ 1.6×10−4.Characterization by microscopic examination and light coupling tests revealed the extent of resolution, process quality and assisted quantification of the process speed gain. The benefits and current limitations of this technique are discussed in detail.

The mechanisms of refractive index change in poly(methyl methacrylate) by frequency doubled femtosecond laser pulses are investigated. It is demonstrated that positive refractive index modificaton can be caused by a combination of depolymerization and crosslinking.

There is currently great interest in writing photonic structures into polymers, and femtosecond (fs) lasers can fabricate micronsized, 3-D, permanent refractive index structures in transparent media for photonic devices, but work to date has been carried out in glasses. Polymethyl methacrylate (PMMA) or perspex is a polymeric transparent material exploited in integrated optics, microfluidics and polymer optoelectronics due to its low processing temperature, biocompatibility and ease of injection moulding and extrusion. Pulse duration, which has to date received little attention in material modification below the ablation threshold, is shown to be important for efficient modification of poly (methyl methacrylate) and writing conditions determined by wavelength and pulse length and the associated photochemistry for refractive index modification were investigated. Refractive index modification of pure PMMA is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. At 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. It is suggested that three- and two-photon absorption mechanisms are responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation using short pulses at low laser fluence, facilitate photo-modification control via incubation, reducing bulk damage. The paper concludes that efficient fs laser modification relies on a suitable combination of laser pulse duration and wavelength in relation to the material bandgap.

Polymethyl methacrylate (PMMA) or perspex is of great interest as a photonic material for integrated optics, microfluidics and polymer optoelectronics due to its low processing temperature, biocompatibility and ease of injection moulding and extrusion. Gratings written into polymer optical fibre (POF) have increased strain and temperature sensitivity by 14% compared with those written in silica optical fibre. Previously reported work has optimised 3-D photonic structures in undoped, bulk PMMA using femtosecond laser writing techniques. This paper evaluates the optical properties of a diffraction grating structure in bulk PMMA, when subjected to strain and deformation. A 20 ??m period grating was written into undoped commercial grade PMMA (ICI Perspex) using a 387nm, 180fs laser at 0.8??J. A He-Ne laser beam was diffracted by the grating, and the centroids of the diffracted beams, (0 and ) were measured using a CCD array. Under compression, the periodicity of the grating was perturbed, causing angular displacement of the orders, enabling loaded stress to be measured. The measurement was complicated by deformation of the sample surface, changing the angles of incidence and diffraction of all orders. Methods of distinguishing bend or tilt from compression, and the grating behaviour under extension will be presented, enabling the performance of PMMA based devices to measure strain and temperature to be predicted and unwanted effects of environmental strain and temperature on optical properties of photonic structures in PMMA to be evaluated.

There is currently great interest in writing photonic structures into polymers, and femtosecond (fs) lasers can fabricate micronsized, 3-D, permanent refractive index structures in transparent media for photonic devices, but work to date has been carried out in glasses. Polymethyl methacrylate (PMMA) or perspex is a polymeric transparent material exploited in integrated optics, microfluidics and polymer optoelectronics due to its low processing temperature, biocompatibility and ease of injection moulding and extrusion. Pulse duration, which has to date received little attention in material modification below the ablation threshold, is shown to be important for efficient modification of poly (methyl methacrylate) and writing conditions determined by wavelength and pulse length and the associated photochemistry for refractive index modification were investigated. Refractive index modification of pure PMMA is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. At 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. It is suggested that three- and two-photon absorption mechanisms are responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation using short pulses at low laser fluence, facilitate photo-modification control via incubation, reducing bulk damage. The paper concludes that efficient fs laser modification relies on a suitable combination of laser pulse duration and wavelength in relation to the material bandgap.

Refractive index modification of pure poly(methyl methacrylate) (PMMA) is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. It is observed that at 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. Results suggest that three- and two-photon absorption is responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation with short pulses of low laser fluence allows control of the photomodification via incubation, thus reducing bulk damage.

Ultrafast laser micro-structuring of ceramic alumina, titanium and stainless steel are presented confirming that precision, melt free micro-structuring is indeed possible with intense femtosecond optical pulses. While metals are best machined at a fluence a few times ablation threshold, ceramic alumina, on the other hand, shows excellent structuring with no evidence of melt at high fluence F ∼ 21Jcm−2. In PMMA, at 387nm with pulselength ∼ 200fs, we demonstrate refractive index modification within the bulk induced by two photon absorption to generate phase gratings. By using a holographic technique combined with 387nm femtosecond radiation and 0.5NA UV objective, sub-micron pitch periodic structures were ablated on stainless steel, titanium and silicon. With a lower (0.15NA) objective and focussing the periodic optical field distribution within bulk PMMA, micron pitch phase gratings can be generated.

Femtosecond, subablation threshold photomodification of poly(methyl methacrylate) (PMMA) at 387 nm is explored to enable fabrication of optical components. Volatile fragment analysis (thermal desorption gas chromatography-mass spectrometry) and molecular weight distribution monitoring (size exclusion chromatography) suggest photochemical modification, involving direct cleavage of the polymer backbone and propagation via chain unzipping under formation of monomers, similar to the pyrolytic degradation of PMMA. Waveguides were produced in undoped, clinical-grade PMMA, showing an increased refractive index in the laser focal region (Delta n(max)=4 X 10(-3)). (c) 2006 Optical Society of America.