Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication.

THz waves are promising wireless carriers for next-generation wireless communications, where a seamless connection from wireless to optical communication is required. In this study, we demonstrate carrier conversion from THz waves to dual-wavelength NIR light injection-locking to an optical frequency comb using asynchronous nonpolarimetric electro-optic downconversion with an electro-optic polymer modulator. THz wave in the W band was detected as a stable photonic RF beat signal of 1 GHz with a signal-to-noise ratio of 20 dB via the proposed THz-to-NIR carrier conversion. In addition, the results imply the potential of the photonic detection of THz waves for wireless-to-optical seamless communication.

Amplification and phase noise transfer of a Kerr microresonator soliton comb for low phase noise THz generation with a high signal-to-noise ratio.

Optical injection locking is implemented to faithfully transfer the phase noise of a dissipative Kerr microresonator soliton comb in addition to the amplification of the Kerr comb. Unlike Er-doped fiber and semiconductor optical amplifiers, the optical injection locking amplifies the comb mode without degrading the optical signal-to-noise ratio. In addition, we show that the residual phase noise of the optical injection locking is sufficiently small to transfer the relative phase noise of comb modes (equivalent to the repetition frequency) of low phase noise Kerr combs, concluding that the optical injection locking of a Kerr comb can be an effective way to generate low phase noise terahertz (THz) waves with a high signal-to-noise ratio through an optical-to-electronic conversion of the Kerr comb.

Thermal control of a Kerr microresonator soliton comb via an optical sideband.

We report the thermal control of a dissipative Kerr microresonator soliton comb via an optical sideband generated from an electro-optic modulator. Same as the previous reports using an independent auxiliary laser, our sideband-based (S-B) auxiliary light also enables access to a stable soliton comb and reduces the phase noise of the soliton comb, greatly simplifying the set-up with an auxiliary laser. More importantly, because of the intrinsically high frequency/phase correlation between the pump and S-B auxiliary light, the detuning between the pump and resonance frequency is automatically almost fixed, which allows an 18 times larger "effective" soliton existence range than the conventional method using an independent auxiliary laser, as well as a scanning of the soliton comb of more than 10 GHz without using microheaters.

Frequency-scanned microresonator soliton comb with tracking of the frequency of all comb modes.

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µs, 50 GHz in 125µs, and 25 GHz in 50µs (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at two different wavelengths enables estimation of the frequency shift of all comb modes.

Low noise, self-referenced all polarization maintaining Ytterbium fiber laser frequency comb: erratum.

We provide a corrected figure of our previous publication [Opt. Express25, 18017 (2017)10.1364/OE.25.018017].

Investigation of the phase noise of a microresonator soliton comb.

Optical frequency combs generated from microresonators (especially microresonator soliton combs) have been attracting significant attentions because of the potential to be fully chip-scale. Among various promising applications of soliton combs, coherent optical communications and mm/THz wireless communications require low phase noise of the comb modes and low relative phase noise between the comb modes, respectively. Here, we measure the phase noise of a soliton comb, investigating how the thermorefractive noise of a microresonator influences on the phase noise. We observe the quadratic increase of the phase noise of the comb modes, as the comb mode number, counted from the wavelength of a pump cw laser, increases. In addition, we measure the relative phase noise between the comb modes, showing less influence of the phase noise of pump cw lasers by comparing soliton combs generated from pump cw lasers with low and large phase noise.

Continuous scanning of a dissipative Kerr-microresonator soliton comb for broadband, high-resolution spectroscopy.

Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) are promising to realize chip-scale integration of full soliton comb systems providing high precision, broad spectral coverage, and a coherent link to the micro/mm/THz domain with diverse applications coming on line all the time. However, the large soliton comb spacing hampers some applications. For example, for spectroscopic applications, there are simply not enough comb lines available to sufficiently cover almost any relevant absorption features. Here, we overcome this limitation by scanning the comb mode spacing by employing Pound-Drever-Hall locking and a microheater on the microresonator, showing continuous scanning of the soliton comb modes across nearly the full free-spectral range of the microresonator without losing soliton operation, while spectral features with a bandwidth as small as 5 MHz are resolved.

Control of Kerr-microresonator optical frequency comb by a dual-parallel Mach-Zehnder interferometer.

A technique to integrate key functions of a Kerr-microresonator optical frequency comb into one device, i.e., a dual-parallel Mach-Zehnder interferometer (DP-MZI), is proposed. In the technique, a DP-MZI enables the control of carrier envelope offset frequency (fceo), as well as repetition frequency (frep), in addition to generating a stable dissipative Kerr soliton. In experiments, influences on fceo and frep by pump frequency and power modulation via a DP-MZI are investigated, followed by a demonstration of long-term full stabilization of a microresonator soliton comb to a fiber-based optical frequency comb. As another example demonstration, timing jitter of a microresonator soliton comb is significantly suppressed by referencing to a fiber through a two-wavelength delayed self-heterodyne interferometer (TWDI).

Low-noise millimeter-wave synthesis from a dual-wavelength fiber Brillouin cavity.

In this Letter, a photonic system is proposed to generate millimeter waves with low phase noise and ultra-high frequency stability. By locking two free-running CW lasers to the same fiber cavity whose free-spectral range is actively stabilized, millimeter waves can be synthesized in a wide frequency range with fine-tuning capability. Exploiting the spectral narrowing effect of stimulated Brillouin scattering, the generated millimeter waves exhibit low phase noise that does not scale up as the frequency increases. In the experimental demonstration, up to ∼300 GHz millimeter waves are generated, with a phase noise of <-90 dBc/Hz at 10 kHz offset limited by the local oscillator and an in-loop 60 min frequency RMS drift of 0.43 mHz. The output frequency of the system can be readily increased to sub-THz region by replacing one of the pump CW lasers.

Photonics-enabled wideband microwave burst detection.

In this Letter, to the best of our knowledge, a novel wideband microwave burst detection system is realized by utilizing photonics-assisted wavelength and time division multiplexing, in conjunction with optical storage. Deploying a coherent electro-optical dual comb and a recirculating optical frequency shifter with ∼1.28 µs round-rip delay, the proof-of-concept experimental system demonstrates the interrogation of ∼1 µs radio frequency (RF) bursts with up to an 8 GHz bandwidth and arbitrary hopping pattern at 1 MHz resolution and a refresh rate of ∼60 kHz using an 80 MHz RF detection unit. The proposed system can be easily upgraded to higher bandwidths and longer burst time apertures with suitable hardware.

Fast ultra-wideband microwave spectral scanning utilizing photonic wavelength- and time-division multiplexing.

A high-speed ultra-wideband microwave spectral scanning system is proposed and experimentally demonstrated. Utilizing coherent dual electro-optical frequency combs and a recirculating optical frequency shifter, the proposed system realizes wavelength- and time-division multiplexing at the same time, offering flexibility between scan speed and size, weight and power requirements (SWaP). High-speed spectral scanning spanning from ~1 to 8 GHz with ~1.2 MHz spectral resolution is achieved experimentally within 14 µs. The system can be easily scaled to higher bandwidth coverage, faster scanning speed or finer spectral resolution with suitable hardware.

Low noise, self-referenced all polarization maintaining Ytterbium fiber laser frequency comb.

We report the implementation of a self-referenced optical frequency comb generated by a passively mode-locked all polarization maintaining (PM) Yb fiber laser based on a nonlinear amplifying loop mirror (NALM). After spectral broadening the optical spectrum spans from 650 nm to 1400 nm, allowing for the generation of an optical octave and carrier envelope offset frequency (fceo) stabilization through a conventional f-2f interferometer. We demonstrate for the first time the stabilization of the fceo of such a PM Yb system with an in-loop fractional frequency stability scaled to an optical frequency of low 10-19 at 1 second averaging time, offering a great potential for applications in optical atomic clock metrology.

Low noise electro-optic comb generation by fully stabilizing to a mode-locked fiber comb.

A fully stabilized EO comb is demonstrated by phase locking the two degrees of freedom of an EO comb to a low noise mode-locked fiber comb. Division/magnification of residual phase noise of locked beats is observed by measuring an out-of-loop beat. By phase locking the 200 th harmonics of the EO comb and a driving cw frequency to a fiber comb, a record low phase noise EO comb across +/- 200 harmonics (from 1544.8 nm to 1577.3 nm) is demonstrated.

Static FBG strain sensor with high resolution and large dynamic range by dual-comb spectroscopy.

We demonstrate a fiber Bragg grating (FBG) strain sensor with optical frequency combs. To precisely characterize the optical response of the FBG when strain is applied, dual-comb spectroscopy is used. Highly sensitive dual-comb spectroscopy of the FBG enabled strain measurements with a resolution of 34 nε. The optical spectral bandwidth of the measurement exceeds 1 THz. Compared with conventional FBG strain sensor using a continuous-wave laser that requires rather slow frequency scanning with a limited range, the dynamic range and multiplexing capability are significantly improved by using broadband dual-comb spectroscopy.

Injection locking of Yb-fiber based optical frequency comb.

We demonstrated the synchronization of offset and repetition frequency between two independent Yb-doped fiber mode-locked lasers by injection locking. By injecting master-laser pulse-train into slave laser cavity, stability and accuracy of master frequency comb are transferred to slave comb. Passive stabilization of frequency comb offers robust and convenient way to duplicate frequency comb that can be applied to long-distance comb transfer. Injecting master pulse would also help to initiate and stabilize mode-locking of high repetition rate or ultrabroadband frequency combs. Additionally, we also demonstrated even more robust synchronization of combs can be achieved with the help of active stabilization of relative offset frequency difference.

Self-compensation of third-order dispersion for ultrashort pulse generation demonstrated in an Yb fiber oscillator.

Compensation of the intracavity dispersion in the mode-locked oscillator is known to be one of the most important factors for ultrashort pulse generation. However, recent investigations of a Yb-doped fiber mode-locked oscillator revealed that precise third-order dispersion (TOD) compensation is not always necessary for ultrashort pulse generation, owing to the strong nonlinearity that compensates residual TOD without reducing its spectral bandwidth. The origin of the nonlinear TOD compensation has remained unclear. To investigate the process in detail, we studied the pulse evolution inside a 30 fs Yb-doped fiber mode-locked oscillator both experimentally and numerically, and we found that the nonlinear phase shift with a temporally asymmetric pulse shape introduces an appropriate amount of TOD that exactly cancels the residual cavity dispersion.