Keunwoo Lee

Time-of-flight measurement with femtosecond light pulses

Scientists improve the precision of time-of-flight measurements from several hundreds of micrometres to the nanometre regime by timing femtosecond pulses through phase-locking control of the pulse repetition rate using the optical cross-correlation technique. This result looks set to benefit synthetic aperture imaging for future space missions of formation-flying satellites and remote experiments involving the general theory of relativity.

Absolute distance measurement by dual-comb interferometry with adjustable synthetic wavelength

Absolute distances were measured using two femtosecond lasers of different pulse repetition rates by revisiting the dual-comb interferometric method proposed by Coddington et al (2009 Nature Photon. 3 351–6). The apparatus built for experiments was designed to eliminate the dead zones in the measurement range by separating the measurement pulses from the reference pulses using orthogonal polarization. In addition, the pulse repetition rate of the signal laser was made tunable in order to extend the non-ambiguity range (NAR) by adaptively adjusting the synthetic wavelength in consideration of the de facto measurement stability in the air. Actual tests performed in the open air proved that a target distance of 69.3 m is measured without interruptions at a 200 µs update rate in the presence of a ~170 µm drift of the optical path length caused by the fluctuation of the refractive index of air. The proposed hardware system design for effective NAR extension will facilitate the use of dual-comb interferometry for various terrestrial applications.

High precision laser ranging by time-of-flight measurement of femtosecond pulses

Time-of-flight (TOF) measurement of femtosecond light pulses was investigated for laser ranging of long distances with sub-micrometer precision in the air. The bandwidth limitation of the photo-detection electronics used in timing femtosecond pulses was overcome by adopting a type-II nonlinear second-harmonic crystal that permits the production of a balanced optical cross-correlation signal between two overlapping light pulses. This method offered a sub-femtosecond timing resolution in determining the temporal offset between two pulses through lock-in control of the pulse repetition rate with reference to the atomic clock. The exceptional ranging capability was verified by measuring various distances of 1.5, 60 and 700 m. This method is found well suited for future space missions based on formation-flying satellites as well as large-scale industrial applications for land surveying, aircraft manufacturing and shipbuilding.

Absolute distance measurement with extension of nonambiguity range using the frequency comb of a femtosecond laser

Abstract. We revisit the method of synthetic wavelength interferometry (SWI) for absolute measurement of long distances using the radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser. Our intention here is to extend the nonambiguity range (NAR) of the SWI method using a coarse virtual wavelength synthesized by shifting the pulse repetition rate. The proposed concept of NAR extension is experimentally verified by measuring a ∼13-m distance with repeatability of 9.5 μm (root-mean-square). The measurement precision is estimated to be 31.2 μm in comparison with an incremental He–Ne laser interferometer. This extended SWI method is found to be well suited for long-distance measurements demanded in the fields of large-scale precision engineering, geodetic survey, and future space missions.

Position-controlled hydrothermal growth of ZnO nanorods on arbitrary substrates with a patterned seed layer via ultraviolet-assisted nanoimprint lithography

A novel technique for the position-controlled growth of ZnO nanorods is established, by combining ultraviolet-assisted nanoimprint lithography (UV-NIL) and hydrothermal growth. Various ZnO nanorod arrays were obtained on silicon substrates, by UV-NIL of ZnO seed patterns with lines of 200 nm wide at a pitch of 1000 nm from a photosensitive ZnO precursor, followed by a position-controlled hydrothermal growth step with varied growth times. It was found that the aspect ratio of ZnO nanorods increased from 2.7 to 11.8 as the growth time was increased from 2 to 6 h. Selected area electron diffraction (SAED) analysis indicates that the root of studied ZnO nanorods consists of both amorphous and polycrystalline phases whereas the stem shows a single-crystalline nature with a preferred (002) growth. ZnO nanorod arrays were also routinely obtained on transparent glass and flexible polyethylene terephthalate (PET). In all cases, ZnO nanorods were observed on both the sidewalls and top surfaces of the ZnO seed patterns with a nanoflower-like structure regardless of substrate substances. This technique offers an alternative method for integrating ZnO nanorods at low temperatures and free of high vacuum, potentially useful in applications such as nanophotonics, photovoltaics and flexible nanoelectronics.

Femtosecond laser pulses for fast 3-D surface profilometry of microelectronic step-structures.

Fast, precise 3-D measurement of discontinuous step-structures fabricated on microelectronic products is essential for quality assurance of semiconductor chips, flat panel displays, and photovoltaic cells. Optical surface profilers of low-coherence interferometry have long been used for the purpose, but the vertical scanning range and speed are limited by the micro-actuators available today. Besides, the lateral field-of-view extendable for a single measurement is restricted by the low spatial coherence of broadband light sources. Here, we cope with the limitations of the conventional low-coherence interferometer by exploiting unique characteristics of femtosecond laser pulses, i.e., low temporal but high spatial coherence. By scanning the pulse repetition rate with direct reference to the Rb atomic clock, step heights of ~69.6 μm are determined with a repeatability of 10.3 nm. The spatial coherence of femtosecond pulses provides a large field-of-view with superior visibility, allowing for a high volume measurement rate of ~24,000 mm3/s.

Wafer-scale surface roughening for enhanced light extraction of high power AlGaInP-based light-emitting diodes

A new approach to surface roughening was established and optimized in this paper for enhancing the light extraction of high power AlGaInP-based LEDs, by combining ultraviolet (UV) assisted imprinting with dry etching techniques. In this approach, hexagonal arrays of cone-shaped etch pits are fabricated on the surface of LEDs, forming gradient effective-refractive-index that can mitigate the emission loss due to total internal reflection and therefore increase the light extraction efficiency. For comparison, wafer-scale FLAT-LEDs without any surface roughening, WET-LEDs with surface roughened by wet etching, and DRY-LEDs with surface roughened by varying the dry etching time of the AlGaInP layer, were fabricated and characterized. The average output power for wafer-scale FLAT-LEDs, WET-LEDs, and DRY3-LEDs (optimal) at 350 mA was found to be 102, 140, and 172 mW, respectively, and there was no noticeable electrical degradation with the WET-LEDs and DRY-LEDs. The light output was increased by 37.3% with wet etching, and 68.6% with dry etching surface roughening, respectively, without compromising the electrical performance of LEDs. A total number of 1600 LED chips were tested for each type of LEDs. The yield of chips with an optical output power of 120 mW and above was 0.3% (4 chips), 42.8% (684 chips), and 90.1% (1441 chips) for FLAT-LEDs, WET-LEDs, and DRY3-LEDs, respectively. The dry etching surface roughening approach developed here is potentially useful for the industrial mass production of wafer-scale high power LEDs.

Active compensation of large dispersion of femtosecond pulses for precision laser ranging.

We describe an active way of compensation for large dispersion induced in the femtosecond light pulses travelling in air for laser ranging. The pulse duration is consistently regulated at 250 fs by dispersion control, allowing sub-micrometer resolution in measuring long distances by means of time-of-flight measurement. This method could facilitate more reliable applications of femtosecond pulses for satellite laser ranging, laser altimetry and active LIDAR applications.

Space radiation test of saturable absorber for femtosecond laser

We report a space radiation test performed on a semiconductor-type saturable absorber (SA) for its use in outer space as a key mode-locking component of fiber-based femtosecond pulse lasers. Gamma-ray effects on the nonlinear transmission behavior of the SA were evaluated by configuring a pump-probe experiment with femtosecond light pulses. Test results revealed that when the total ionizing dose of gamma-ray exposure reaches 120 krad, the SA encounters a sudden failure with its modulation depth and saturation fluence deteriorating from 11% to 6% and 40 to 110u2009u2009μJ/cm2, respectively. On the other hand, no notable degradation was observed in its temporal response of absorption recovery.

Polarization maintaining linear cavity Er-doped fiber femtosecond laser

We present a polarization-maintaining (PM) type of Er-doped fiber linear oscillator designed to produce femtosecond laser pulses with high operational stability. Mode locking is activated using a semiconductor saturable absorber mirror (SESAM) attached to one end of the linear PM oscillator. To avoid heat damage, the SESAM is mounted on a copper-silicon-layered heat sink and connected to the linear oscillator through a fiber buffer dissipating the residual pump power. A long-term stability test is performed to prove that the proposed oscillator design maintains a soliton-mode single-pulse operation without breakdown of mode locking over a week period. With addition of an Er-doped fiber amplifier, the output power is raised to 180 mW with 60 fs pulse duration, from which an octave-spanning supercontinuum is produced.