Seongheum Han

Testing of a femtosecond pulse laser in outer space

We report a test operation of an Er-doped fibre femtosecond laser which was conducted for the first time in outer space. The fibre-based ultrashort pulse laser payload was designed to meet space-use requirements, undergone through ground qualification tests and finally launched into a low-earth orbit early in 2013. Test results obtained during a one-year mission lifetime confirmed stable mode-locking all the way through although the radiation induced attenuation (RIA) in the Er-doped gain fibre caused an 8.6% reduction in the output power. This successful test operation would help facilitate diverse scientific and technological applications of femtosecond lasers in space and earth atmosphere in the near future.

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.

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.

Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser by sweeping the pulse repetition rate

Femtosecond lasers allow for simultaneous detection of multiple absorption lines of a specimen over a broad spectral range of infrared or visible light with a single spectroscopic measurement. Here, we present an 8-THz bandwidth, 0.5-GHz resolution scheme of Fourier-transform spectroscopy using an Er-doped fiber femtosecond laser. A resolving power of 1.6 × 104 about a 1560-nm center wavelength is achieved by sweeping the pulse repetition rate of the light source on a fiber Mach-Zehnder interferometer configured to capture interferograms with a 0.02-fs temporal sampling accuracy through a well-stabilized 60-m unbalance arm length. A dual-servo mechanism is realized by combining a mechanical linear stage with an electro-optic modulator (EOM) within the fiber laser cavity, enabling stable sweeping control of the pulse repetition rate over a 1.0-MHz scan range with 0.4-Hz steps with reference to the Rb clock. Experimental results demonstrate that the P-branch lines of the H13CN reference cell can be observed with a signal-to-noise ratio reaching 350 for the most intense line.

Parallel determination of absolute distances to multiple targets by time-of-flight measurement using femtosecond light pulses

Distances to multiple targets are measured simultaneously using a single femtosecond pulse laser split through a diffractive optical element. Pulse arrival from each target is detected by means of balanced cross-correlation of second harmonics generated using a PPKTP crystal. Time-of-flight of each returning pulse is counted by dual-comb interferometry with 0.01 ps timing resolution at a 2 kHz update rate. This multi-target ranging capability is demonstrated by performing multi-degree of freedom (m-DOF) sensing of a rigid-body motion simulating a satellite operating in orbit. This method is applicable to diverse terrestrial and space applications requiring concurrent multiple distance measurements with high precision.

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 110  μJ/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.

Development of fiber femtosecond lasers for advanced metrological space missions

Femtosecond pulse lasers offer breakthroughs in precision metrology particularly in the fields of time, distance and spectroscopy. This advance attracts much attention to extend todays space missions by improving the precision of remote sensing and control capabilities. In this presentation, we introduce how femtosecond lasers are being investigated for space explorations in the near future. Special emphasis is given to address the noble methods for absolute distance measurements and multi-functional spectroscopy, which are expected to be soon available with enhanced resolutions and ranges for the next generation space missions.

Space radiation effects on a semiconductor saturable absorber

The gamma-ray induced behavior of a saturable absorber in outer space was tested by pump-probe experiments using a femtosecond laser. The test reveals that after gamma-ray exposure of ~120 krad, the modulation depth and saturation fluence of the saturable absorber vary from 6 to 3 % and 40 to 105 μJ/cm2, respectively, without no conceivable change in the relaxation time.

Real-time monitoring and control system for femtosecond pulse lasers

Real-time monitoring and control (RMC) of the operational state of a femtosecond laser is essential for its practical uses with reliability. For the purpose, we present a compact all-fiber-based RMC system which automatically performs the task of supervising multiple key pulse parameters at a same time in both the temporal and frequency domain in situ with the femtosecond laser being in operation.