Terahertz spectrum analyzer based on a terahertz frequency comb

Photonic comb-rooted synthesis of ultra-stable terahertz frequencies

Stable terahertz sources are required to advance high-precision terahertz applications such as molecular spectroscopy, terahertz radars, and wireless communications. Here, we demonstrate a photonic scheme of terahertz synthesis devised to bring the well-established feat of optical frequency comb stabilization down to the terahertz region. The source comb is stabilized to an ultra-low expansion optical cavity offering a frequency instability of 10-15 at 1-s integration. By photomixing a pair of comb lines extracted coherently from the source comb, terahertz frequencies of 0.10-1.10 THz are generated with an extremely low level of phase noise of -70 dBc/Hz at 1-Hz offset. The frequency instability measured for 0.66 THz is 4.4 × 10-15 at 1-s integration, which reduces to 5.1×10-17 at 65-s integration. Such unprecedented performance is expected to drastically improve the signal-to-noise ratio of terahertz radars, the resolving power of terahertz molecular spectroscopy, and the transmission capacity of wireless communications.

Method for the generation of microwave frequency combs based on a Vernier optoelectronic feedback loop

Microwave frequency combs (MFCs) with flexible tunability and prominent phase noise performance are of importance to many applications, including consumer electronic product, fundamental research and military defence. It is difficult for traditional electronic signal sources to meet the imperative demand in terms of high frequency scale, due to a challenging problem of deteriorating phase noise performance with increasing frequency. Photonics-assisted methods have capacity of implementing the generation of microwave signals with high frequency and low phase noise. Here we report a novel photonics-assisted MFC generation method utilizing an optoelectronic feedback loop with a Vernier configuration. The proposed MFC generation system features self-sustained oscillation, inherent multiple-mode oscillation and low phase noise level. In the proof-of-principle experiment, the MFC generation system based on a dual-path Vernier optoelectronic feedback loop is demonstrated, and the comb spacing tuning from 3.072 to 4.710 GHz and the single sideband phase noise of -99.60 dBc/Hz at 10 kHz offset from the carrier are achieved.

High-precision millimeter-wave frequency determination through plasmonic photomixing

We present a technique for high-precision millimeter-wave frequency determination through plasmonic photomixing. Our technique utilizes a plasmonic photomixer pumped by an optical frequency comb with a high-stability millimeter-wave beat frequency. The plasmonic photomixer down-converts the millimeter-wave signal to the radio frequency regime at which high-accuracy frequency counters are available. The precision of this technique is determined by the frequency stability of the optical beat frequency, which can be directly characterized in the presented experimental setup. We demonstrate frequency measurement precision as low as 3.9×10^-10 at 95 GHz through plasmonic photomixing without phase-locking the optical frequency comb.

High precision frequency measurement of terahertz waves using optical combs from a Mach-Zehnder-modulator-based flat comb generator

Using a frequency-tunable optical comb generated from a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a nonlinear optical fiber, we demonstrated a frequency measurement of continuous terahertz wave sources with the frequency of 0.1 and 0.6 THz by an electro-optic sampling method. We clearly observed beat signals between the terahertz source and the optical two-tone extracted from the optical comb, allowing us to determine the absolute frequency. Owing to the wide comb spacing of the MZ-FCG, this method has a high potential for the high-speed measurement of the frequency of terahertz wave sources.

Dual terahertz comb spectroscopy with a single free-running fibre laser

Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications.

Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser

A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). Two independent mode-locked laser beams with different wavelengths and different repetition frequencies were generated from this laser and were used to generate dual PC-THz combs having different frequency spacings in photoconductive antennae. Based on the dual PC-THz combs, the absolute frequency of CW-THz radiation was determined with a relative precision of 1.2 × 10⁻⁹ and a relative accuracy of 1.4 × 10⁻⁹ at a sampling rate of 100 Hz. Real-time determination of the absolute frequency of CW-THz radiation varying over a few tens of GHz was also demonstrated. Use of a single dual-wavelength mode-locked fibre laser, in place of dual mode-locked lasers, greatly reduced the size, complexity, and cost of the measurement system while maintaining the real-time capability and high measurement precision.

Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.

Real-time absolute frequency measurement of continuous-wave terahertz radiation based on dual terahertz combs of photocarriers with different frequency spacings

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) radiation is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz radiation involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 10(-11) was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications.

High-resolution terahertz spectroscopy with a single tunable frequency comb

We report an improvement of three orders of magnitude in the spectral resolution of a recently proposed single-comb terahertz spectroscopy [Opt. Lett.39, 5669 (2014)]. The improvement is achieved by using a femtosecond optical pulse train with a tunable repetition rate. Terahertz comb with tunable spectral line spacing generated by the train is detected via nonlinear mixing with a harmonic of a CW signal from a microwave frequency synthesizer. By applying this technique to the low-pressure gas spectroscopy, we achieved a 100 kHz spectral resolution in measuring separate absorption lines of the rotational manifold of fluoroform (CF₃H).

High-resolution broadband terahertz spectroscopy via electronic heterodyne detection of photonically generated terahertz frequency comb

We report an alternative approach to the terahertz frequency-comb spectroscopy (TFCS) based on nonlinear mixing of a photonically generated terahertz pulse train with a continuous wave signal from an electronic synthesizer. A superlattice is used as a nonlinear mixer. Unlike the standard TFCS technique, this approach does not require a complex double-laser system but retains the advantages of TFCS-high spectral resolution and wide bandwidth.

Phase-slope and phase measurements of tunable CW-THz radiation with terahertz comb for wide-dynamic-range, high-resolution, distance measurement of optically rough object

Phase measurement of continuous-wave terahertz (CW-THz) radiation is a potential tool for direct distance and imaging measurement of optically rough objects due to its high robustness to optical rough surfaces. However, the 2π phase ambiguity in the phase measurement of single-frequency CW-THz radiation limits the dynamic range of the measured distance to the order of the wavelength used. In this article, phase-slope measurement of tunable CW-THz radiation with a THz frequency comb was effectively used to extend the dynamic range up to 1.834 m while maintaining an error of a few tens µm in the distance measurement of an optically rough object. Furthermore, a combination of phase-slope measurement of tunable CW-THz radiation and phase measurement of single-frequency CW-THz radiation enhanced the distance error to a few µm within the dynamic range of 1.834 m without any influence from the 2π phase ambiguity. The proposed method will be a powerful tool for the construction and maintenance of large-scale structures covered with optically rough surfaces.

Microwave synthesis from a continuous-wave terahertz oscillator using a photocarrier terahertz frequency comb

We report low-noise microwave synthesis from radiation with a frequency of 0.3 THz using a photocarrier frequency comb in a photoconductive antenna. The synthesized microwave signal at 1 GHz is phase coherent to the 0.3 THz radiation and has a fractional instability of 1×10(-15) within 300 s averaging times and single-sideband phase noise of -105 dB/Hz at a 100 Hz offset from the carrier. This terahertz (THz)-to-microwave synthesizer is capable of being a THz frequency divider, which would be indispensable to not only THz metrology but also future high-speed wireless networks.

Measurement of optical-beat frequency in a photoconductive terahertz-wave generator using microwave higher harmonics

A new method for measuring optical-beat frequencies in the terahertz (THz) region using microwave higher harmonics is presented. A microwave signal was applied to the antenna gap of a photoconductive (PC) device emitting a continuous electromagnetic wave at about 1 THz by the photomixing technique. The microwave higher harmonics with THz frequencies are generated in the PC device owing to the nonlinearity of the biased photoconductance, which is briefly described in this article. Thirteen nearly periodic peaks in the photocurrent were observed when the microwave was swept from 16 to 20 GHz at a power of -48 dBm. The nearly periodic peaks are generated by the homodyne detection of the optical beat with the microwave higher harmonics when the frequency of the harmonics coincides with the optical-beat frequency. Each peak frequency and its peak width were determined by fitting a Gaussian function, and the order of microwave harmonics was determined using a coarse (i.e., lower resolution) measurement of the optical-beat frequency. By applying the Kalman algorithm to the peak frequencies of the higher harmonics and their standard deviations, the optical-beat frequency near 1 THz was estimated to be 1029.81 GHz with the standard deviation of 0.82 GHz. The proposed method is applicable to a conventional THz-wave generator with a photomixer.

Widely and continuously tunable terahertz synthesizer traceable to a microwave frequency standard

We constructed a widely and continuously tunable terahertz frequency synthesizer traceable to a hydrogen maser linked to coordinated universal time. Photomixing of two optical frequency synthesizers, linked to the hydrogen maser via dual optical frequency combs, gave this THz synthesizer frequency uncertainty of 10⁻¹². To demonstrate the potential of wide and continuous tunability in the THz synthesizer, we tuned its output frequency up to 50 GHz discretely and 1.26 GHz continuously in the F-band while maintaining the unprecedented frequency uncertainty by using a uni-traveling-carrier photodiode as a photomixer. This THz synthesizer will be a powerful tool for broadband, high-precision THz spectroscopy and THz frequency metrology.

Terahertz spectrum analyzer based on frequency and power measurement

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).

Real-time monitoring of continuous-wave terahertz radiation using a fiber-based, terahertz-comb-referenced spectrum analyzer

We propose a fiber-based, terahertz-comb-referenced spectrum analyzer which has the advantages of being a portable, alignment-free, robust, and flexible apparatus suitable for practical use. To this end, we constructed a 1550-nm mode-locked Er-doped fiber laser whose mode-locked frequency was stabilized precisely by referring to a rubidium frequency standard, and used it to generate a highly stable terahertz (THz) frequency comb in a photoconductive antenna or an electro-optic crystal. By standardizing the THz comb, we determined the frequency accuracy of an active-frequency-multiplier-chain (AFMC) source to be 2.4 x 10(-11). Furthermore, the potential of the THz spectrum analyzer was effectively demonstrated by real-time monitoring of the spectral behavior of the AFMC source and a photomixing source of two free-running CW lasers at adjacent wavelengths.