Peter R. Herman

Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides

A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a smooth transition from diffusion-only transport at 200 kHz repetition rate to strong heat accumulation effects at 0.5 to 2 MHz was observed and shown to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200 mW average power and approximately 15-mm/s scan speed was discovered across the range of 200 kHz to 2 MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at approximately 1 microJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. A comprehensive characterization of waveguide properties is presented for laser writing in the thermal diffusion and heat accumulation regimes. The waveguides are shown to be thermally stable up to 800 degrees C and can be written in a convenient 520 microm depth range with low spherical aberration.

Vacuum ultraviolet laser spectroscopy. V. Rovibronic spectra of Ar2 and constants of the ground and excited states

Vibronic spectra of three band systems of 40Ar2 and 36Ar40Ar at 108, 107, and 105 nm were recorded and analyzed. Isotope shifts of bandheads were used to establish vibrational numbering of the observed bands. Rovibronic structure was examined at resolving powers up to 5×105, yielding band centers, rotational constants, and providing evidence of Hund’s case (c) coupling for Ar2. From band system II at 107 nm, with bands involving v’=20–27 and v‘=0–5, improved constants for the ground X 0+g state were obtained: ω‘e=30.68(8), ωex‘e=2.42(5), ωey‘e=−0.062(13), ωez‘e=0.010(2), and D‘e=99.2(10) cm−1, B‘e=0.059 65(8) cm−1, and Re=3.761(3) A. Similarly, constants for v’=20–27 of the B 0+u were evaluated, with estimates of T’e=88 210(400) and De=5640(400) cm−1. Band system I at 108 nm, led to constants for the v’=23–30 levels of the A1u state and to estimates of T’e=87 458(500) and De=5786(500) cm−1. Analysis of band system III at 105 nm yielded new and improved data for levels v’=0–9 of the C 0+u state, including ...

Waveguide writing in fused silica with a femtosecond fiber laser at 522 nm and 1 MHz repetition rate

We report on waveguide writing in fused silica with a novel commercial femtosecond fiber laser system (IMRA America, FCPA microJewel). The influence of a range of laser parameters were investigated in these initial experiments, including repetition rate, focal area, pulse energy, scan speed, and wavelength. Notably, it was not possible to produce low-loss waveguides when writing with the fundamental wavelength of 1045 nm. However, it was possible to fabricate telecom-compatible waveguides at the second harmonic wavelength of 522 nm. High quality waveguides with propagation losses below 1 dB/cm at 1550 nm were produced with 115 nJ/pulse at 1 MHz and 522 nm.

Laser shaping of photonic materials : deep-ultraviolet and ultrafast lasers

Optical materials are especially challenging to process with conventional lasers simply because of their high transparency. We are exploiting two extremes in laser technology — ultrafast lasers and very short wavelength F2 lasers — to microsculpt surfaces and to control refractive index in transparent glasses. These lasers drive fundamentally different interactions, many-photon and ‘big’ photon, respectively, that offer distinct advantages and limitations for shaping photonic devices in fused silica. Comparisons of surface morphology, shock-induced microcracking, resolution, and photosensitivity responses are presented.

Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser.

For the first time to our knowledge, high-strength (>30 dB) first-order Bragg grating waveguides were fabricated in bulk fused silica glass in a single-scanning step by modulating a high-repetition-rate femtosecond fiber laser with an external acousto-optic modulator. The modulation induced a waveguide segmentation by delivering controlled bursts of laser pulses to define an array of partially overlapped refractive index voxels. With appropriate choice of modulation frequency and sample scanning speed, low loss waveguides could be formed at high writing speeds to yield sharp Bragg spectral resonances tunable over the 1300 to 1550 nm telecom band. Effects of acousto-optic modulation duty cycle on propagation loss and grating strength are characterized. This modulation method offers facile control and integration of multiwavelength Bragg grating devices to enhance overall functionality of optical circuits in three-dimensional geometries.

Design and holographic fabrication of tetragonal and cubic photonic crystals with phase mask: toward the mass-production of three-dimensional photonic crystals

We present design and holographic fabrication of the woodpile-type photonic crystals through phase mask techniques. Three-dimensional photonic crystal structures with tetragonal or cubic symmetries are fabricable by exposing the photoresist to the interference patterns generated by the phase masks. These photonic crystals have full photonic band gaps as large as 27% of the gap center frequency if made from silicon. The realized photonic crystal in SU-8 photoresist shows overlapped thus more stable woodpile-type structures. The phase-mask based lithography for the fabrication of the photonic crystals, together with the computer-controlled fabrication process, could take advantage of the standard tools of the electronics industry, leading toward the mass-production of the three-dimensional photonic crystals.

Vacuum ultraviolet laser spectroscopy. IV. Spectra of Kr2 and constants of the ground and excited states

Krypton dimers formed by supersonic jet expansion have been excited with tunable, and monochromatic VUV radiation leading to isotopically resolved fluorescence excitation spectra of three band systems at 125, 124, and 117 nm. Unambiguous upper‐state vibrational numbering was possible, resulting in evaluation of the spectroscopic constants for the three lowest excited states. From system I at ∼125 nm, with bands v’=32 to 38 and v‘=0, constants for the A 3Σ+u state of Kr2 were found to be T’e =74 303.9(8.0), ωe =244.60(43), ωex’e =2.6045(60), and De=5807.0(8.0) cm−1. Band system II at ∼124 nm, with bands v’=30 to 38 and v‘=0, led to constants for the B 1Σ+u state, T’e =75 426.8(1.7), ωe =219.50(10), ωex’e =2.2325(15), and De =5629.1(2.5) cm−1. Improved values for constants of the C 1Σ+u state were obtained from analysis of band system III at ∼117 nm (with bands v’=0 to 5, and v‘=0 to 2): T’e =85 520.60(10), ωe =43.82 (10), ωex’e =1.812(18), and De =465.3(2.0) cm−1. Vibrational constants for the ground state...

Ultrafast laser waveguide writing: Lithium niobate and the role of circular polarization and picosecond pulse width.

For the first time to our knowledge, ultrafast laser writing has generated room-temperature stable guided-wave optics in bulk lithium niobate for the telecommunication spectrum. Among a seven-dimensional parameter space for waveguide optimization, two frequently overlooked parameters, pulse duration and polarization, were found to be key in overcoming undesired nonlinear optical responses imposed by this material. Single-mode waveguides were best formed with circularly polarized light having a relatively long pulse duration of approximately 1.0 ps. The waveguides were highly polarization dependent and guided in both telecommunication bands near 1300 and 1550 nm, exhibiting losses as low as 0.7 dB/cm.

Femtosecond laser written optofluidic sensor: Bragg Grating Waveguide evanescent probing of microfluidic channel.

Microfluidic channels and Bragg Grating Waveguides (BGWs) were simultaneously fabricated inside fused silica glass by means of femtosecond laser exposure followed by chemical etching. Evanescent field penetration of the waveguide mode into the parallel microfluidic channel induced Bragg resonant wavelength shifts to enable refractive index characterization of the fluidic medium in the 1 to 1.452 range. Laser exposure was optimized to fabricate devices with optically smooth channel walls and narrow Bragg resonances for high sensing response at 1560 nm wavelength. Reference gratings were also employed in the optical circuit for temperature and strain compensation. These devices open new directions for optical sensing in three-dimensional optofluidic and reactor microsystems.