[Show abstract][Hide abstract] ABSTRACT: We present a new technical implementation of a high-speed broadband frequency sweep of continuous-wave terahertz (THz) radiation. THz frequency sweeping with a kHz sweep rate and a THz sweep range is implemented using THz photomixing in which an optical beat source consists of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection employing signal averaging for noise reduction, which can give time-of-flight information via fast Fourier transform. Multiple reflections in a Si wafer and the thickness of the wafer are measured to demonstrate the potential of this method for fast THz tomography and thickness measurement.
[Show abstract][Hide abstract] ABSTRACT: High-speed terahertz (THz) reflection three-dimensional (3D) imaging is demonstrated using electronically-controlled optical sampling (ECOPS) and beam steering. ECOPS measurement is used for scanning an axial range of 7.8 mm in free space at 1 kHz scan rate while a transverse range of 100 × 100 mm2 is scanned using beam steering instead of moving an imaging target. Telecentric f-θ lenses with axial and non-axial symmetry have been developed for beam steering. It is experimentally demonstrated that the non-axially symmetric lens has better characteristics than the axially symmetric lens. The total scan time depends on the number of points in a transverse range. For example, it takes 40 s for 200 × 200 points and 10 s for 100 × 100 points. To demonstrate the application of the imaging technique to nondestructive testing, THz 3D tomographic images of a glass fiber reinforced polymer sample with artificial internal defects have been acquired using the lenses for comparison.
[Show abstract][Hide abstract] ABSTRACT: We present experimental implementation of high-speed broadband frequency sweep of CW THz radiation applicable to high-speed THz spectroscopy and tomography. A wavelength-swept laser and a distributed-feedback laser diode constitute a high-speed frequency-swept optical beat source used for photomixing.
Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2013 38th International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: We demonstrate high-speed terahertz (THz) reflection three-dimensional (3D) imaging based on electronically controlled optical sampling (ECOPS). ECOPS enables scanning of an axial range of 9 mm in free space at 1 kHz. It takes 80 s to scan a transverse range of 100 mm × 100 mm along a zigzag trajectory that consists of 200 lines using translation stages. To show applicability of the imaging system to nondestructive evaluation, a THz reflection 3D image of an artificially made sample is obtained, which is made of glass fiber reinforced polymer composite material and has defects such as delamination and inclusion, and is compared with an ultrasonic reflection 3D image of the sample.
[Show abstract][Hide abstract] ABSTRACT: We propose a rapidly frequency-swept optical beat source for continuous wave (CW) THz generation using a wavelength swept laser and a fixed distributed feedback (DFB) laser. The range of the sweeping bandwidth is about 17.3 nm (2.16 THz), 1541.42-1558.72 nm. The achieved side mode suppression ratio for both wavelengths within the full sweeping range is more than 45 dB. We observe CW THz signals for tunable optical beat sources using a fiber coupled CW THz measurement system to confirm the feasibility of using our frequency swept optical beat source as a CW THz radiation source. The THz output signal falls to the thermal noise level of the low-temperature grown (LTG) InGaAs photomixer beyond 1.0 THz. The rapidly frequency-swept optical beat source will be useful for generating high-speed tunable CW THz radiation.
[Show abstract][Hide abstract] ABSTRACT: Recently reported asynchronous-optical-sampling terahertz (THz) time-domain spectroscopy enables high-resolution spectroscopy due to a long time-delay window. However, a long-lasting tail signal following the main pulse is often measured in a time-domain waveform, resulting in spectral fluctuation above a background noise level on a high-resolution THz amplitude spectrum. Here, we adopt the wavelet power spectrum estimation technique (WPSET) to effectively remove the spectral fluctuation without sacrificing spectral features. Effectiveness of the WPSET is verified by investigating a transmission spectrum of water vapor.
Journal of the Optical Society of Korea 04/2011; 15(1). DOI:10.3807/JOSK.2011.15.1.103 · 0.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The birefringence of zinc oxide (ZnO) in the terahertz (THz) frequency range is measured using a parallel-polarization configuration THz time-domain spectrometer and compared with the result of an ab initio calculation. The measured birefringence of 0.180 at 1 THz shows good agreement with the calculated value of 0.170 from full phonon consideration, both of which are about 20 times larger than the birefringence in the visible range. It is found that the difference of the transverse optical and longitudinal optical (TO--LO) phonon splitting between the optical phonon branches (A1 and E1) predominantly contributes to the huge birefringence of ZnO in the THz frequency region.
Japanese Journal of Applied Physics 03/2011; 50(3). DOI:10.1143/JJAP.50.030203 · 1.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a high-speed frequency-scanning optical beat source using a wavelength swept laser and a tunable laser for continuous THz wave generation. The beat frequency is scanned at 1 kHz over a range with a 2.16 THz width.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a frequency swept optical beat source for high speed continuous wave THz generation. The range of the sweeping bandwidth is about 2.16 THz (17.3 nm). And we observe CW THz waveforms for several static optical beat sources using a fiber coupled CW THz measurement system in order to confirm the feasibility of our frequency swept optical beat source as a CW THz radiation source.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate high-speed terahertz (THz) time-domain spectroscopy based on electronically controlled optical sampling (ECOPS). The ECOPS system utilizes two synchronized Ti:sapphire femtosecond lasers with a 100 MHz repetition frequency. The time delay between the two laser pulses is demonstrated to be rapidly swept at a scan rate of 1 kHz on a time delay window of 77 ps by using an external offset voltage applied to a locking electronics. It is shown that a THz pulse can be exactly measured by ECOPS, as is done by asynchronous optical sampling (ASOPS), and the measurement time is shortened by a factor of 50 by using ECOPS compared with ASOPS in the case of employing 100 MHz repetition-rate lasers.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a terahertz (THz) spectrum analyzer based on frequency and power measurement. A power spectrum of a continuous THz wave is measured through optical heterodyne detection using an electromagnetic THz frequency comb and a bolometer and power measurement using a bolometer with a calibrated responsivity. The THz spectrum analyzer has a frequency precision of 1x10(-11), a frequency resolution of 1Hz, a frequency band up to 1.7THz, and an optical noise equivalent power of approximately 1 pW/Hz(1/2).
[Show abstract][Hide abstract] ABSTRACT: We propose a widely tunable dual-wavelength Erbium-doped fiber laser that uses two micro-heater-integrated Fabry-Perot laser diodes (FP-LDs) and two fiber Bragg gratings (FBGs) for tunable continuous-wave (CW) terahertz (THz) radiation. Each wavelength can be independently tuned by using an FP-LD and an FBG. The wavelength fine tuning is achieved by simultaneously applying current to the micro-heater on the FP-LD and strain to the FBG. The side-mode suppression ratio is more than 35 dB for both wavelengths. The wavelength spacing of the dual wavelength can be continuously tuned from 3.2 nm to 9.6 nm. Continuous frequency tuning of the CW THz radiation is also successfully achieved using an InGaAs-based photomixer with our dual-wavelength fiber laser as the optical beat source. The emitted CW THz radiation is continuously tuned from 0.3 to 0.8 THz.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate and characterize both asynchronous optical sampling terahertz time-domain spectroscopy (AOS THz-TDS) and terahertz frequency comb spectroscopy (TFCS) as high-speed, high-resolution terahertz (THz) spectroscopy. Two mode-locked femtosecond (fs) lasers with slightly different repetition frequencies are used without a mechanical delay stage to generate and detect THz pulses. The repetition frequencies of the two fs lasers are stabilized by use of two phase-locked loops sharing the same reference oscillator. For AOS THz-TDS, the difference frequency between the repetition frequencies is optimized, and the signal-to-noise ratio is measured as a function of the measurement time. The spectra of a THz frequency comb and its individual modes are measured using TFCS. A spectral resolution of 100 MHz is obtained in both types of spectroscopy.
Journal- Korean Physical Society 02/2010; 56(1):7-57. DOI:10.3938/jkps.56.255 · 0.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a widely tunable injection-locked dual-wavelength Erbium-doped fiber laser using two micro-heater-integrated Fabry-Perot laser diodes and two fiber Bragg gratings. The wavelength spacing can be continuously tuned from 2.4 nm to 9.0 nm.
[Show abstract][Hide abstract] ABSTRACT: We present high-speed terahertz time-domain spectroscopy based on electronically controlled optical sampling (ECOPS). The time delay between the two laser pulses is demonstrated to be rapidly swept at a scan rate of 1 kHz on a time delay window of 77 ps. It is shown that a measurement time is shortened by a factor of 50 by using ECOPS compared to asynchronous optical sampling.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a continuous THz emission using a low-temperature-grown InGaAs photomixer and a widely tunable dual-wavelength erbium-doped fiber laser as an optical beat source. The wavelength spacing can be tuned from 2.3 nm to 8.8 nm which corresponds to the CW THz frequency from 0.3 to 1.1 THz.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate pulse-echo mode terahertz (THz) reflectance tomography, where scattered THz waveforms are measured using a high-resolution asynchronous-optical-sampling THz time domain spectroscopy (AOS THz-TDS) technique, and 3-D tomographic reconstruction is accomplished using a compressed sensing approach. One of the main advantages of the proposed system is a significant reduction of acquisition time without sacrificing the reconstruction quality, thanks to the sufficient incoherency in the pulse-echo mode-sensing matrix and the fast sampling scheme in AOS THz-TDS.
[Show abstract][Hide abstract] ABSTRACT: We present a method of using one-dimensional dielectric multilayer structures for designing terahertz frequency spreading filters. The interference of terahertz pulses in these structures composed of alternating weak and strong refractive materials allows design of well-separated THz frequency components within a modulation-limited THz spectral envelope. The design characteristics of these coarse THz combs are limited by the saturation effect and also by the deformation of the THz pulse time-traveling within the structure. The details of the designed THz waveform synthesis from these THz multilayer spectral filters are verified by experiments using time-domain terahertz pulsed spectroscopy.
Journal of the Optical Society of Korea 09/2009; 13(3):398-402. DOI:10.3807/JOSK.2009.13.3.398 · 0.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on a monolithic dual-mode semiconductor laser operating in the 1550-nm range as a compact optical beat source for tunable continuous-wave (CW) terahertz (THz) generation. It consists of two distributed feedback (DFB) laser sections and one phase section between them. Each wavelength of the two modes can be independently tuned by adjusting currents in micro-heaters which are fabricated on the top of the each DFB section. The continuous tuning of the CW THz emission from Fe(+)-implanted InGaAs photomixers is successfully demonstrated using our dual-mode laser as the excitation source. The CW THz frequency is continuously tuned from 0.17 to 0.49 THz.