Laser Induced Refractive Index Change (LIRIC) uses femtosecond laser pulses to locally modify a material’s refractive index below material damage thresholds. This technique has been successfully applied to create refractive correctors in wet ophthalmic materials such as hydrated contact lenses. However, applying LIRIC in wet contact lens materials to mass produce refractive correctors would require drastic changes to the current manufacturing infrastructure. To integrate LIRIC in the production line in a cost-effective manner, applying this technique in a dry contact lens is necessary. Therefore, this study seeks to understand how the absence of water affects LIRIC writing mechanisms. Our experiments have shown that LIRIC writing using 400nm laser light produced rough, non-uniform regions of index change in dehydrated Hydroxyethyl Methyl Acrylate based hydrogels. However, in dehydrated silicone hydrogels, LIRIC successfully induced greater index changes than when the materials are in hydrated state.
We consider the difficult problem of ground-based propagation of 90 femtosecond laser pulses through the turbulent atmosphere in the range of 1 mile, where the atmospheric properties change significantly according to weather conditions. Our experimental data indicates that use of femtosecond laser pulses for optical communications at up to 1.35 Gb/sec is significantly advantageous compared to CW lasers, offering much reduced error rates and higher consistency in transmission in diverse atmospheric conditions. We develop a new model using Gaussian beamlets to simulate the effects of propagation in different refractive index conditions within the beam. The results are quite consistent with the experimental observations indicating significantly less scintillation and small- scale fluctuations in received data when fs laser pulses at 1540 nm are used. In the new model, we simulate the transmitter as a coherent sum of Gaussian beamlets (5-500) and propagate them with slightly different index of refraction, corresponding to the normal variations in average index of refraction due to changing atmospheric conditions. For instance, when transmitting over one mile of air, a temperature difference within the beam of only 0.02 °C causes the Gaussian beamlets to arrive at the receiver with time delays differing by about 200 fs, exceeding the coherence time of the 90 fs laser pulses, thereby causing incoherent summation at the detector.
Intraocular lenses (IOLs) are widely used to treat cataracts and restore vision, however, the accuracy of manufacture and surgical implantation of IOLs is subject to some limitations. Femtosecond laser micromachining relies on tightly focused, ultrashort laser pulses to locally modify the properties of bulk materials, and has been recently applied in the field of vision correction. In this study, multi-layer dense line patterns were inscribed into Tecnis IOLs using a femtosecond laser operating at 8.3 MHz at a wavelength of 405 nm. Below the damage threshold, uniform phase changes could be obtained within each pattern, and its magnitude increased nonlinearly with laser power. To explore the mechanisms underlying the refractive index (RI) changes, microstructural changes of the phase pattern were quantified by confocal micro-Raman spectroscopy using sectioned IOLs. A significant decrease of the integral intensity of 2988 cm-1 band (v(C-H)) that confined in the written layer was observed from the lateral scan profile of IOL cross section. We posit that the positive RI changes in IOLs were likely associated with localized photochemical depolymerization, which includes broken of chemical bonds and diffusion of molecular fragments. These findings enhance our understanding of femtosecond micromachining as a new method to customize high visual-quality IOLs.
Laser induced refractive index change (LIRIC) technique has been demonstrated as a non-invasive way to alter optical refractive powers of ophthalmic materials including hydrogel-based contact lenses, exercised corneal tissues and even corneas in vivo, via modifying the refractive index through multiphoton absorption process. In our previous work, blue femtosecond laser pulses at 405 nm with a repetition rate of 80 MHz were focused tightly into the stromal region to achieve refractive corrections in live cats via inscribing phase wrapped structures. In order to improve the efficacy of LIRIC, we here demonstrate that blue femtosecond laser pulses at a lower repetition rate range induce larger amounts of phase change with lower laser powers and higher scan speeds owing to a higher pulse energy density deposition. In comparison to a high repetition rate writing at 80 MHz, higher phase change can be attained at 8.3 MHz at the same average power. Furthermore, one wave of phase change measured at 543 nm was attained in rabbit corneas ex vivo for the first time by inscribing single LIRIC layer at 8.3 MHz with a power of 200 mW and a scan speed of 100 mm/s, corresponding to a refractive index change of 0.025 after the layer thickness was estimated to be 20 μm. Accordingly, the optimum laser repetition rate used in femtosecond micromachining corneas can be determined to be around 8.3 MHz, as arbitrary phase structures can be manufactured by wrapping the phase between 0 and 1 wave.
LIRIC (laser induced refractive index change) employs femtosecond laser pulses to inscribe local refractive index (RI) changes in corneal tissue via multi-photon absorption. Using a near-infrared wavelength (810 nm) offers a decreased risk of photochemical damage to the retina for clinical applications of LIRIC for refractive vision correction compared to shorter wavelengths that have been used, however the multi-photon order is higher, limiting the amount of RI change achievable before damage. Excised rabbit corneal tissue samples were soaked in 0.25% and 0.50% solutions of either riboflavin or sodium fluorescein, photosensitizers with large two-photon absorption cross-sections at 810 nm that are biologically compatible and safe for use in the cornea. Near-infrared LIRIC was performed on these doped cornea samples as well as on undoped samples using two different laser sources: a Ytterbium fiber laser at 5 MHz and a Ti:Sapphire oscillator at 80 MHz, with average powers in the laser focal volume up to 140 mW and 950 mW, respectively. For the native cornea, similar RI change was induced at both repetition rates for a scanning speed of 10 mm/s (5 MHz) and 20 mm/s (80 MHz). Doping the cornea with either sodium fluorescein or riboflavin allowed for a 10x increase in scanning speed at 5 MHz and a 5x increase in scanning speed at 80 MHz while producing a similar magnitude of RI change to that of the native cornea. With all samples, using the lower repetition rate allowed for a large reduction in the average power needed to induce a similar amount of RI change.
Intra-Tissue Refractive Index Shaping (IRIS) uses a 405 nm femtosecond laser focused into the stromal region of the cornea to induce a local refractive index change through multiphoton absorption. This refractive index change can be tailored through scanning of the focal region and variations in laser power to create refractive structures, such as gradient index lenses for visual refractive correction. Previously, IRIS was used to create 2.5 mm wide, square, -1 D cylindrical refractive structures in living cats. In the present work, we first wrote 400 μm wide bars of refractive index change at varying powers in enucleated cat globes using a custom flexure-based scanning system. The cornea and surrounding sclera were then removed and mounted into a wet cell. The induced optical phase change was measured with a Mach- Zehnder Interferometer (MZI), and appeared as fringe displacement, whose magnitude was proportional to the refractive index change. The interferograms produced by the MZI were analyzed with a Fourier Transform based algorithm in order to extract the phase change. This provided a phase change versus laser power calibration, which was then used to design the scanning and laser power distribution required to create -1.5 D cylindrical Fresnel lenses in cat cornea covering an area 6 mm in diameter. This prescription was inscribed into the corneas of one eye each of two living cats, under surgical anesthesia. It was then verified in vivo by contrasting wavefront aberration measurements collected pre- IRIS with those obtained over six months post-IRIS using a Shack-Hartmann wavefront sensor.
Blue intratissue refractive index shaping (blue-IRIS) is a method with potential to correct ocular refraction noninvasively in humans. To date, blue-IRIS has only ever been applied to cat corneas and hydrogels. To test the comparability of refractive index change achievable in cat and human tissues, we used blue-IRIS to write identical phase gratings in ex vivo feline and human corneas. Femtosecond pulses (400 nm) were focused ∼300 μm below the epithelial surface of excised cat and human corneas and scanned to write phase gratings with lines ∼1 μm wide, spaced 5 μm apart, using a scan speed of 5 mm/s. Additional cat corneas were used to test writing at 3 and 7 mm/s in order to document speed dependence of the refractive index change magnitude. The first-order diffraction efficiency was immediately measured and used to calculate the refractive index change attained. Our data show that blue-IRIS induces comparable refractive index changes in feline and human corneas, an essential requirement for further developing its use as a clinical vision correction technique.
We have designed and implemented a novel acoustic lens based focusing technology into a prototype photoacoustic imaging camera. All photoacoustically generated waves from laser exposed absorbers within a small volume get focused simultaneously by the lens onto an image plane. We use a multi-element ultrasound transducer array to capture the focused photoacoustic signals. Acoustic lens eliminates the need for expensive data acquisition hardware systems, is faster compared to electronic focusing and enables real-time image reconstruction. Using this photoacoustic imaging camera, we have imaged more than 150 several centimeter size ex-vivo human prostate, kidney and thyroid specimens with a millimeter resolution for cancer detection. In this paper, we share our lens design strategy and how we evaluate the resulting quality metrics (on and off axis point spread function, depth of field and modulation transfer function) through simulation. An advanced toolbox in MATLAB was adapted and used for simulating a two-dimensional gridded model that incorporates realistic photoacoustic signal generation and acoustic wave propagation through the lens with medium properties defined on each grid point. Two dimensional point spread functions have been generated and compared with experiments to demonstrate the utility of our design strategy. Finally we present results from work in progress on the use of two lens system aimed at further improving some of the quality metrics of our system.
We report the optical system design of a novel speckle-free ultrafast Red-Green-Blue (RGB) source based on angularly multiplexed simultaneous second harmonic generation from the efficiently generated Stokes and anti-Stokes pulses from a commercially available photonic crystal fiber (PCF) with two zero dispersion wavelengths (TZDW). We describe the optimized configuration of the TZDW fiber source which supports excitations of dual narrow-band pulses with peak wavelengths at 850 nm, 1260 nm and spectral bandwidths of 23 nm, 26 nm, respectively within 12 cm of commercially available TZDW PCF. The conversion efficiencies are as high as 44% and 33% from the pump source (a custom-built Yb:fiber master-oscillator-power-amplifier). As a result of the nonlinear dynamics of propagation, the dual pulses preserve their ultrashort pulse width (with measured autocorrelation traces of 200 fs and 227 fs,) which eliminates the need for dispersion compensation before harmonic generation. With proper optical design of the free-space harmonic generation system, we achieve milli-Watt power level red, green and blue pulses at 630 nm, 517 nm and 425 nm. Having much broader spectral bandwidths compared to picosecond RGB laser sources, the source is inherently speckle-free due to the ultra-short coherence length (<37 μm) while still maintaining an excellent color rendering capability with >99.4% excitation purities of the three primaries, leading to the coverage of 192% NTSC color gamut (CIE 1976). The reported RGB source features a very simple system geometry, its potential for power scaling is discussed with currently available technologies.
Photonic crystal fiber (PCF) with two closely spaced zero dispersion wavelengths (TZDW) offers a unique route to efficient energy transfer to two spectrally localized continua beyond either side of the ZDWs, which we have employed in previous work for mid-IR difference frequency generation and speckle-free red-green-blue generation. In this manuscript, we report the interferometric coherence characterization and radio frequency (RF) noise measurements of the Stokes side TZDW component. With a custom-built 1.3 W, 1035 nm, 40 MHz, 240 fs Yb:fiber chirped pulse amplifier as the pump source, we use 12 cm of commercially available TZDW PCF to excite the dual narrow-band continua from which the Stokes pulse is filtered out with a 1180 nm long wave pass filter. We achieve 0.8 to 3 nJ of narrow-band pulses within the spectral range of 1200 – 1315 nm at an average power conversion efficiency of 33%. Employing an un-balanced Michelson interferometer, measured mutual spectral coherence of the Stokes pulse is in excess of 0.76 with pump Soliton order as high as N ~70. Its measured RF noise spectrum at the first harmonic of the laser repetition rate shows less than 8 dBc/Hz increase in relative intensity noise (RIN) compared to that of the power amplifier, which is consistent with reported studies employing sub-100 fs pulses from relatively low noise oscillators. In contrast to the broadband continuum from a single ZDW PCF wherein severe de-coherence is found with pumping at high soliton order and longer pump pulse width, the reported TZDW fiber source shows preservation of intensity stability and phase coherence against variation in pump pulse parameters, which not only attests to the stability of our reported method for mid-IR generation, but also shows promising potential towards an all-fiber, efficient and low noise ultrafast source that can be helpful for applications such as biomedical deep-tissue imaging.
We report a fiber based approach to broadly tunable femtosecond mid-IR source based on difference frequency mixing of the outputs from dual photonic crystal fibers (PCF) pumped by a femtosecond fiber laser, which is a custom-built Yb-doped fiber chirped pulse amplifier (CPA) delivering 1.35 W, 300 fs, 40 MHz pulses centered at 1035 nm. The CPA output is split into two arms to pump two different types of PCFs for generation of the spectrally separated pulses. The shorter wavelength pulses are generated in one PCF with its single zero dispersion wavelength (ZDW) at 1040 nm. Low normal dispersion around the pumping wavelength enables spectral broadening dominated by self-phase modulation (SPM), which extends from 970 to 1092 nm with up to 340 mW of average power. The longer wavelength pulses are generated in a second PCF which has two closely spaced ZDWs around the laser wavelength. Facilitated by its special dispersion profile, the laser wavelength is converted to the normal dispersion region of the fiber, leading to the generation of the narrow-band intense Stokes pulses with 1 to 1.25 nJ of pulse energy at a conversion efficiency of ~30% from the laser pulses. By difference mixing the outputs from both PCFs in a type-II AgGaS2 crystal, mid-IR pulses tunable from 4.2 to 9 μm are readily generated with its average power ranging from 135 – 640 μW, corresponding to 3 – 16 pJ of pulse energy which is comparable to the reported fiber based mid-IR sources enabled by the solitons self-frequency shift (for example, 3 – 10 μm with 10 pJ of maximum pulse energy in [10]). The reported approach provides a power-scalable route to the generation of broadly tunable femtosecond mid-IR pulses, which we believe to be a promising solution for developing compact, economic and high performance mid-IR sources.
Two-color nonlinear absorption imaging has been previously demonstrated with endogenous contrast of hemoglobin and melanin in turbid media using transmission-mode detection and a dual-laser technology approach. For clinical applications, it would be generally preferable to use backscattering mode detection and a simpler single-laser technology. We demonstrate that imaging in backscattering mode in turbid media using nonlinear absorption can be obtained with as little as 1-mW average power per beam with a single laser source. Images have been achieved with a detector receiving backscattered light at a 45-deg angle relative to the incoming beams' direction. We obtain images of capillary tube phantoms with resolution as high as 20 µm and penetration depth up to 0.9 mm for a 300-µm tube at SNR ~1 in calibrated scattering solutions. Simulation results of the backscattering and detection process using nonimaging optics are demonstrated. A Monte Carlo-based method shows that the nonlinear signal drops exponentially as the depth increases, which agrees well with our experimental results. Simulation also shows that with our current detection method, only 2% of the signal is typically collected with a 5-mm-radius detector.
Photonic crystal fiber (PCF) is considered as a special type of 2-dimensional photonic
crystal structure. As the result of its large design flexibility, there are numerous different
types of PCFs exhibiting vastly different optical properties. Among them, the small-core
PCFs have extremely high effective nonlinearity, which makes them an ideal nonlinear
medium for new frequency generation. However significant nonlinear process in fiber only
occurs when the phase matching condition is satisfied. Therefore, the dispersion properties
of PCFs are also critical.
In this paperr, three different approaches are taken to extend the tunability of an
Yb-fiber-based femtosecond source. All of the schemes are enabled by the unique
dispersion and nonlinear properties of photonic-crystal-fibers (PCFs). In the first approach,
by adopting the newly available PCF into an optical parametric oscillator (FOPO) and
combining it with our newly-developed mode-locked Yb-fiber laser [1], we extend the
tunability of this fiber-based system to over 200-nm around 1 μm [2]. The second scheme
uses Raman soliton self-frequency shifting effect in PCFs. Femtosecond soliton pulses
tunable from 1100 nm to 1300 nm are generated. In the third approach, by taking
advantage of broadly variable phase matching point for the Cherenkov radiation in PCFs,
broadly tunable femtosecond visible pulses from 450 nm to 630 nm are achieved [3].
How can you best prepare yourself for a career in Optics in an uncertain and exciting world? First, I would say - you must find your passion - the thing that truly gets you excited. You should make sure that you get a solid, broad education in fundamentals, with lots of lab experience, and prepare yourself to be flexible. Then, as your career progresses, wherever you are, you should continuously ask yourself - "what do I really want to be doing in 5 or 10 years?" And then, ask yourself "are the things that I am doing now consistent with what I want to be doing in 5 or 10 years?"
Dispersion properties of the novel tapered air-silica microstructure fibers are measured between 1.3 and 1.65 micrometers by white-light interferometry. Dispersion values ((beta) 2) of -181 ps2/km and -152 ps2/km were obtained for 2.2 micrometers and 3 micrometers core sizes, respectively, at (lambda) equals1.55 micrometers .
Passively modelocked fiber lasers operating in the soliton regime can generate pulses at multi-gigahertz harmonic repetition rates. The lasers are modelocked with an ultrafast saturable absorber and the low loss cavities support the formation of multiple equally spaced soliton pulses. These sources are potentially attractive for applications in high speed fiber optic communications systems. The design and construction of these laser sources as well as their application to spectrally sliced wavelength division multiplexed transmission is described.
There have recently been significant advances in compact sources of ultrafast optical pulses. We review our work on developing fiber lasers that are modelocked using a saturable Bragg reflector. These lasers exhibit interesting polarization dynamics due to the lack of any explicit intracavity polarizers. The use of these lasers as a source in a wavelength division multiplexing transmitter is discussed.
The advent of large-scale, free-space, opto-electronic interconnections, as demonstrated in recent system prototypes, requires new sampling methods to reveal diagnostic information. Several factors contribute to the difficulty of probing optical communications channels without disrupting their operation. High-speed electronic connections to the chip periphery are not available in sufficient number and would contribute an undesirable thermal load. Electronic and optical physical contact probes would obscure many of the optical channels that are relayed to a common surface of the chip in current systems. Optical sampling provides the better method although many standard techniques are either too time consuming or complex to implement. We describe a tool we developed that delivers diagnostic information on a large number of high-speed, optical data channels simultaneously and operates analogously to the conventional sampling electronic oscilloscope. The optical oscilloscope is constructed using CCD cameras and video capture boards that are controlled by a software application resident in a personal computer. Sampling is based on a stroboscopic method of using short pulsed laser probe beam synchronized to a data stream to illuminate optical modulators within the optoelectronic circuit. We have demonstrated and discuss the tool's capability of simultaneously monitoring arrays of broadband optoelectronic devices operating at speeds from several hundred Megabit/s to a few Gigabit/s.
We demonstrate mode-locked two-wavelength synchronized operation of a single, cw pumped Ti:Sapphire laser. There is no active feedback or external cavity. The measured cross correlation is 105 fs. Wavelengths as much as 57 nm apart can be simultaneously cross-mode- locked. We then investigate the nonlinear interactions between the pulses. We show the nonlinear interaction caused by self focusing can correct for a difference in cavity lengths of up to +/- 3 micrometers corresponding to 20% of the pulse width on each round trip. Further, we show that the concept of mean group delay can be extended to a pair of pulses with disparate spectra and spatial distribution in a nonlinear regenerative system.
We discuss the possibility of generating, propagating and detecting THz electrical pulses
in semiconductor quantum well structures and present preliminary experimental results.
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