Next-generation Space Mission

(1) Broadband spectroscopy LIDAR based on femtosecond laser for space mission

As an ultrafast femtosecond laser (10^-15 s pulse width) is considered as 21st future core technology, it plays a leading role in high speed and high precision metrology. By applying this unique technology in outer space, it is expected to significantly improve performance in various applications such as space LIDAR, laser altimeter and satellite operation by replacing existing RF signal and short-wavelength laser technology. Therefore, development of fundamental principles of absolute ranging and spectroscopy with ultra-precision based on femtosecond laser will enable a wide variety of applications in future space missions.

<Promising futuristic technology in space based on ultrafast optics>

For spectroscopy and calibration of spectrometer in wide spectral range, novel technologies including nonlinear spectral broadening and its amplification will proceed. Furthermore, the principles of precision spectroscopy and calibration based on femtosecond laser would be developed and verified. As a result, The development of the precision spectroscopy with unprecedented resolution and reliability based on femtosecond laser will revolutionize the LIDAR technology for earth observation and planetary exploration.

<Phase-locked fr-sweeping Fourier transform spectroscopy for broadband femtosecond LIDAR>

In the field of Remote sensing LIDAR, Fourier-transform spectrometer has been offering powerful analytical tool for broadband spectroscopic LIDAR over other infrared spectrometer for remote sensing applications including planetary exploration and global environmental monitoring. However, it has limited performance due to the instrumental limit of mechanical scanning. So, we propose the different type of the Fourier-transform spectroscopic technique by sweeping repetition rate of the femtosecond laser pulses instead of the obsolete mechanical scanning for solving this challenge. Here, we demonstrate that it enables longer scanning range amplified by the ratio between the propagation time through the delay line and the average cavity round-trip time than conventional Fourier-transform spectrometer. Moreover, it reduces system complexity and allows robust spectrometer system compared to conventional Fourier-transform spectrometer by eliminating mechanical scanning instrument in the interferometer. We present a repetition rate sweeping based Fourier-transform spectrometer using fiber femtosecond laser and it comprise all-fiber based Mach-Zender interferometer with a differential arm length of 60 m. To demonstrate the system performance and measuring accuracy, the interferogram is measured to resolve absorption lines of HCN gas cell spanning 60 nm while the repetition rate is scanned over 200 kHz covering an OPD of 500 ps, which leads to a spectral resolution of 2 GHz. Consequently, we may achieve efficient spectroscopic LIDAR system with unprecedented performance by alleviating the prevalent difficulties in overcoming the performance limitation of the Fourier-transform spectrometer based on mechanical scanning in remote sensing application.

(2) Optical coherent communication in free space

Space is the special place to excite the curiosity of human. Each country has planned the space missions to research and observe the space environment and already launched various space missions. Because the development and diversity of these space missions require faster data transmission speed and bigger data transmission capacity, the novel communication system is demanded. Nowadays common data transmission method is based on rf communication. But rf communication system implement the radio frequency which has some limitations as low transmission speed and narrow spectrum resource. In this research, our research group is developing the free space optical communication system based on femtosecond laser. This technique will promise increased data capacity and transmission speed in space and evolve the space mission to next stage. The femtosecond laser has a broad spectrum in 1.5 um which is c-band for communication. And we already constructed the optical frequency generator based on femtosecond laser as monochromatic light source for communication. OFG system can generate multiple monochromatic sources in simultaneous with high frequency stability, accuracy and narrow linewidth. In addition, we also compensate phase/frequency noise in real time with frequency modulator. By integrating these whole method, we can develop novel type optical coherent communication in free space which can overcome present bottleneck in space communication.

(3) High Speed, Highly Efficient, Highly Stable Fiber Femtosecond Laser for Space Missions

The passively mode-locked femtosecond pulse lasers has led to remarkable advances in many fields; high resolution spectroscopy, broad band calibration of astronomical spectrographs, time/frequency transfer over long distances, absolute laser ranging, precision strain sensing, and inter-comparison of atomic clocks. Now the ultrashort laser is anticipated to be applied directly to space missions in the near future; examples include experimental validation of the theory of general relativity, high precision mapping of the geo-potential and large synthetic aperture imaging. Above all, fiber-based femtosecond lasers are preferential choice due to low cost, small foot print, light weight, their susceptibility to optical misalignment, and power conversion efficiency.

In our investigation, a fiber femtosecond laser was operated in space as an attempt ever made for the first time to our knowledge. The femtosecond laser, named FSO in short, was carefully designed so as to meet space-use requirements, undergone through ground qualifications and finally launched into an orbit. As illustrated in following figure, the hardware of FSO was basically comprised of optical and electrical components constituting an all-fiber ring-type oscillator. An Er-doped fiber was used as the gain medium. Passively mode-locked Er-doped fiber lasers have received considerable recent attention for various space-based laser applications because of eye-safe wavelength and wavelength compatibility with scientifically important atmospheric trace gas(e.g. H₂O, CO₂, CH₂, O₂) spectral features

The femtosecond laser employed for space missions should be capable of surviving high-g launching acceleration and should be robust enough under harsh thermal-vacuum environment of outer space. However, The performance of passively mode-locked fiber lasers can be sensitive to temperature, pressure and mechanical perturbation. In this respect, the most effective way to avoid these problems is to construct passively mode-locked fiber laser systems using polarization-maintaining fiber and fiber components. Then, we have the plan to develop all-PM fiber mode-locked femtosecond pulse laser systems. The systems are expected to play important roles for various space missions in the near future.

(4) Frequency-comb-referenced synthetic wavelength interferometer for formation flying of satellites.

Absolute distance measurement (ADM) refers to the attempt to determine distance by a single measurement without ambiguity, which is not the case when measuring distance by means of wavelength-dependent interferometry using light or radio-frequency signals. ADM has long been conducted with several well-established principles relying on the time-of-flight measurement of a pulsed laser, intensity modulation of a continuous-wave (cw) laser, frequency sweeping of a cw laser and cross-correlation of pseudo-random micro-wave signals. Recently, in response to increasing demands on the measurement precision and range beyond conventional limits, femtosecond lasers begun to draw attention as a new light source for various advanced ADM principles. Examples include synthetic wavelength interferometry, multi-wavelength interferometry, dispersive interferometry, dual-comb interferometry and pulse repetition rate-tuned time-of-flight measurement. In this investigation, we use the principle of synthetic wavelength interferometry (SWI) to measure long distance using the radio-frequency waves produced by radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser.

Following figure shows the measurement system configured in our investigation. The light source used here is an Er-doped fiber femtosecond laser emitting 150 fs pulses at a 100 MHz repetition rate. The output comb of the light source has a spectral bandwidth of 40 nm centered about a 1560 nm wavelength. The inter-mode beating within the comb produces radio-frequency synthetic wave signals which are integer-multiple harmonics of the pulse repetition rate, f_r. The nth-order harmonic wavelength Ln is expressed as

L_n = c / (nf_r)

where c is the velocity of light in air. The fundamental 1st-order harmonic wavelength L₁ is 3.0 m in our measurement system while the 10th-order harmonic wavelength L₁₀ is 300 mm. The output beam from the light source – containing all the f_r-induced harmonics – splits into two beams through a fiber coupler; one beam goes to the reference photo-detector (PD) and the other beam to the measurement PD after being reflected from the target retro-reflector. The radio-frequency harmonic signals detected by the two PDs are integer multiples of 100 MHz which are in fact too fast for precise phase measurement. Thus, the detected harmonic signals are frequency-down-converted to a 10 kHz range by means of super-heterodyning with the clock signals provided from a local oscillator. The interference phase between the reference and measurement signals is then quantified using a lock-in amplifier with a 0.01 degree resolution. For a given target distance D_abs, the interference phase Φ_n of the nth-order harmonic wavelength L_n is expressed by

Φ_n= 4πD_abs / L_n

The measured value of Ln is wrapped by 2π, so the non-ambiguity range (NAR) of Dabs is limited to L_n/2; the NAR becomes 1.5 m for the 1st-order harmonic and it decreases in proportion to the harmonic order. Thus, the longest distance to be measurable without ambiguity is restricted by the NAR of the 1st-order harmonic wavelength, which is denoted here by L_NAR for convenience. Naturally, distances shorter than L_NAR can be measured by employing two separate harmonic wavelengths, preferably L₁ and L₁₀, so that the measurement resolution can be enhanced to 4.2 μm by L₁₀. On the other hand, for longer distances exceeding L_NAR, a priori measurement using an auxiliary coarse tool of a larger non-ambiguity range has to be made so that the distance resolutions provided by the harmonic wavelength chain from L₁ to L₁₀, are exploited without ambiguity.

The frequency-comb-referenced synthetic wavelength interferometer (SWI) built in this work provided a repeatability of 9.5 μm when a 13 m distance was measured over 4000 s. The Allan deviation decreases for shorter averaging time which is found to be ~1 μm at 0.25 s averaging and ~5.0 μm at 5 s averaging. In addition, comparative measurements with an incremental laser interferometer showed a maximum discrepancy of 31.2 μm in peak-to-valley. The test results imply that the SWI system proposed with an extension scheme of the non-ambiguity range here could be adopted for large-scale precision engineering, geodetic survey and possibly for future space missions.