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We demonstrate the generation of frequency combs with a low repetition rate of 450 MHz, using a hybrid integrated InP-Si3N4 diode laser with more than 60 cm optical roundtrip length on-chip. Comb generation is achieved without a saturable absorber, based on four-wave mixing in combination with gain-index coupling and spectrally filtered feedback; both passive and active mode-locking is observed. External modulation reduces the RF linewidth from several hundreds of kHz to the ten-kHz level.

We demonstrate the generation of frequency combs with a low repetition rate of 450 MHz, using a hybrid integrated InP-Si3N4 diode laser with more than 60 cm optical roundtrip length on-chip. Comb generation is achieved without a saturable absorber, based on four-wave mixing in combination with gain-index coupling and spectrally filtered feedback; both passive and active mode-locking is observed. External modulation reduces the RF linewidth from several hundreds of kHz to the ten-kHz level.

U ovome diplomskom radu eksperimentalno se istraživala interakcija hladnog oblaka atoma rubidija 87Rb sa svjetlosti unutar optičkog rezonatora visoke finese, te mogućnost rezonatorskog hlađenja u procesima analognim sub-Dopplerovom hlađenju atoma pomoću kontinuiranih lasera. Atomski oblak, predhlađen na sub-Dopplerove temperature, pomoću standardnih tehnika hlađenja s kontinuiranim laserima u magneto optičkoj stupici, uveden je u centar optičkog rezonatora a potom je ispušten iz stupice kako bi slobodno interagirao s optičkim potencijalom unutar rezonatora. Interakcija je ispitivana slikanjem fluorescencije oblaka u metodi vremena proleta. Ispitivanja interakcije u režimu longitudinalnog pumpanja s kontinuiranim laserom, blizu frekvencije prijelaza atoma, pokazala su efekt djelomičnog zatočenja atoma iz oblaka, no neovisnost o pomaku frekvencije lasera od linije rezonatora upućuje na odsustvo dodatnog efekta hlađenja. Eksperiment je ponovljen s pumpanjem rezonatora frekventnim češljem blizu atomske rezonancije rubidija. Opažen je potpis interakcije atoma u rezonatoru i frekventnog češlja, no u fluorescenciji oblaka nije zabilježen efekt hlađenja atoma multimodnim pobuđenjem rezonatora frekventnim češljem. In this thesis, we experimentally tested the interactions of cold clouds of rubidium atoms 87Rb with light inside a high-fineness optical resonator, and the possibility of resonator cooling in processes analogous to sub-Doppler cooling of atoms using continuous wave lasers. An atomic cloud, pre-cooled to sub-Doppler temperatures, using standard continuous laser cooling techniques in a magneto-optical trap, was introduced into the center of the optical resonator and then released from the trap to interact freely with the optical potential inside the resonator. The interaction was examined by imaging of cloud fluorescence in the time of flight method. Tests of the interaction, in the longitudinal pumping regime, with a continuous wave laser near atomic resonance have shown the effect of partial trapping of atoms from the cloud, but lack of sensitivity on laser detuning from the resonator line indicates the absence of additional cooling effects. The experiment was repeated by pumping the cavity with a frequency comb with frequency near rubidium atomic resonance. Signature of comb-atom interaction in the cavity was recorded, but no cooling effect on the atoms by multimode comb excitation of the cavity was observed in the cloud fluorescence.

In this article, we study the effect of various environmental factors on intra-atomic frequency comb (I-AFC) based quantum memory. The effect of the environment is incorporated as random fluctuations and non-uniformity in the parameters such as comb spacing and the optical depth, of the frequency comb. We found that the I-AFC is viable for photon storage even for very large fluctuations in the parameters of the frequency comb, which makes I-AFC a robust platform for photon storage. Furthermore, we show that the non-uniform frequency combs without any fluctuations in the comb parameters can also yield efficient quantum memory. Since the intra-atomic frequency combs found in natural atomic systems are often non-uniform, our results suggest that a large class of these systems can be used for I-AFC based efficient quantum memory.

A scheme, using two-single drive Mach–Zehnder modulators (SD-MZMs) to generate flat optical frequency comb (OFC) by an analogue driving signal, is experimentally demonstrated. The modulator is used...

Frequency comb swept lasers or wavelength stepped swept lasers with comb filters have been proposed in optical domain subsampling or circular-ranging optical coherence tomography (OCT) to achieve high axial scan rate and extended imaging range with a given acquisition bandwidth. However, the comb filters lack flexibility in varying the free spectral range (FSR). In this article, we propose and demonstrate a novel discrete Fourier domain mode locked (FDML) laser by using intracavity optical intensity modulation. Swept signals with comb lines uniformly distributed in the frequency domain is generated with tailor-made pulse patterns according to the sweep trace of the swept filter. Discrete k -space uniform swept signals with different FSRs and pulse durations are generated in the same laser. The FDML laser discretized by temporal modulation is more stable than that with a comb filter. The discrete FDML lasers will be potential sources of OCT and the fast-developing swept source LiDAR.

The terahertz (THz) band has opened up a new frontier for high-speed, wireless communication, high-resolution radars, and highly precise remote sensing. Identifying the low-loss atmospheric windows is vital for these applications. In this study, a long-path THz communication channel is characterized in the frequency range of 0.32–1.1 THz using a custom, silicon-based THz pulse radiator chip. The chip radiates 1.7-ps pulses via an on-chip antenna at a repetition rate of 8 GHz, resulting in a broadband 0.1–1.1 THz frequency comb. It is fabricated in 130-nm SiGe BiCMOS process and consumes 45 mW of dc power. A specular link was created using the impulse radiator, parabolic reflector antennas, a plane mirror, and a downconverter mixer. The THz channel was characterized up to a distance of 110 m. The measurement results demonstrate channel path loss, atmospheric absorption, and low-loss frequency windows suitable for wireless links in the THz range. The results correspond well with the HITRAN database [1].

Dissipative Kerr soliton (DKS) featuring broadband coherent frequency comb with compact size and low power consumption, provides an unparalleled tool for nonlinear physics investigation and precise measurement applications. However, the complex nonlinear dynamics generally leads to stochastic soliton formation process and makes it highly challenging to manipulate soliton number and temporal distribution in the microcavity. Here, synthesized and reconfigurable soliton crystals (SCs) are demonstrated by constructing a periodic intra-cavity potential field, which allows deterministic SCs synthesis with soliton numbers from 1 to 32 in a monolithic integrated microcavity. The ordered temporal distribution coherently enhanced the soliton crystal comb lines power up to 3 orders of magnitude in comparison to the single-soliton state. The interaction between the traveling potential field and the soliton crystals creates periodic forces on soliton and results in forced soliton oscillation. Our work paves the way to effectively manipulate cavity solitons. The demonstrated synthesized SCs offer reconfigurable temporal and spectral profiles, which provide compelling advantages for practical applications such as photonic radar, satellite communication and radio-frequency filter., Here the authors demonstrate an on-demand generation of perfect soliton crystal using synthesized potential field. The individual solitons can also be controlled, for example oscillate around their equilibrium position, by the external field.

Electro-optic modulators are used in a wide variety of photonic systems, and their operation at the 1064 nm wavelength is notable for its applications such as frequency comb generation and optical interconnects. This work demonstrates the first (to the best of our knowledge) thin film lithium niobate electro-optic modulator operating at a wavelength of 1064 nm. The modulator was fabricated from crystal-ion-sliced lithium niobate wafers procured from NanoLN. Electron beam lithography was used to define the waveguides, and laser lithography was used to define the electrodes, forming a ridge waveguide in the lithium niobate, with a width of 900 nm and an etch depth of 90 nm, by inductively coupled plasma etching. The modulator was characterized using a polarization rotation technique, in which two modes of the waveguide were excited and both were shown to exhibit phase modulation. This modulator exhibits a half-wave voltage of 2.73 volts, with a 7 mm long interaction region and 1.91 V-cm modulation efficiency.

We propose a novel point-to-point (P2P) coherent optical link based on optical frequency comb and injection-locking optical process, featuring single wavelength and wavelength division multiplexing (WDM) full-duplex coherent transmission over a fiber link up to 100-km distance. With light sources and local oscillators that are injection locked to an optical frequency comb, simultaneous down-stream and up-stream operations at the same wavelength using full-duplex coherent optical transceivers have been experimentally demonstrated. 32-GBd dual-polarization quadrature phase shift keying (DP-QPSK) and dual-polarization 16 quadrature amplitude (DP-16QAM) modulation formats have been investigated in the setup. When using QPSK, full-duplex operation up to 100 km has been achieved, which introduces 2.2 dB penalty at the hard-decision forward-error-correction (HD-FEC) and 1.3 dB penalty at the soft-decision forward-error-correction (SD-FEC) threshold. When using 16QAM, full-duplex operation up to 80 km has been successfully achieved and as large as 2 dB penalty is observed at the HD-FEC threshold, and 1 dB penalty at the SD-FEC threshold. The penalty mainly comes from continuous and discrete reflection impairments.

We propose a comb-locked multi-channel transmitter with high-speed VCSELs injection-locked by an optical frequency comb. VCSEL dynamics are analyzed with various injection parameters including number of comb tones, frequency detuning, and injection ratio. We evaluate the performance of filtering and amplification effect when multiple comb tones injection into single VCSEL and estimate the transmission performance considering the reflection from VCSEL. Compared to intrinsic parameters of VCSEL, a 2-fold increase in frequency response and a $2.6\times 10^4$ -fold reduction in linewidth have been verified experimentally when the bandwidth-limited VCSEL locked to an incident comb line. 14-Gb/s single-lane transmission is demonstrated through 20 km single-mode fiber based on a 3.4-GHz VCSEL without any dispersion compensation. The similar performance can be achieved for all generated 25 comb tones, verifying a potential high-capacity transmitter with distinct feature of two-dimensional VCSELs.

Current fibre optic communication systems owe their high-capacity abilities to the wavelength-division multiplexing (WDM) technique, which combines data channels running on different wavelengths, and most often requires many individual lasers. Optical frequency combs, with equally spaced coherent comb lines derived from a single source, have recently emerged as a potential substitute for parallel lasers in WDM systems. Benefits include the stable spacing and broadband phase coherence of the comb lines, enabling improved spectral efficiency of transmission systems, as well as potential energy savings in the WDM transmitters. In this paper, we discuss the requirements to a frequency comb for use in a high-capacity optical communication system in terms of optical linewidth, per comb line power and optical carrier-to-noise ratio, and look at the scaling of a comb source for ultra-high capacity systems. Then, we review the latest advances of various chip-based optical frequency comb generation schemes and their applications in optical communications, including mode-locked laser combs, spectral broadening of frequency combs, microresonator-based Kerr frequency combs and electro-optic frequency combs.

Two-dimensional electronic spectroscopy (2DES) is a powerful method to probe the coherent electron dynamics in complicated systems. Stabilizing the phase difference of the incident ultrashort pulses is the most challenging part for experimental demonstration of 2DES. Here, we present a tutorial review on the 2DES protocols based on active phase managements which are originally developed for quantum optics experiments. We introduce the 2DES techniques in box and pump-probe geometries with phase stabilization realized by interferometry, and outline the fully collinear 2DES approach with the frequency tagging by acoustic optical modulators and frequency combs. The combination of active phase managements, ultrashort pulses and other spectroscopic methods may open new opportunities to tackle essential challenges related to excited states.

$$^{171} \mathrm{Yb}^{+}$$ ions were trapped in a quadrupole ion trap. A mode-locked pulse laser at 355 nm was used to generate a frequency comb to drive a Raman transition between the ground hyperfine qubit states of the Doppler cooled ion. Qubit transitions were driven in co-propagating and counter propagating laser beam geometries. In the co-propagating geometry, the coherence time and the fidelity of gate operations were obtained. By taking advantage of the phonon number sensitivity of the Rabi frequency in the counter-propagating geometry, we measured the center of mass temperature of the trapped ion near the Doppler limit which can be used for general thermometry in the ion trap experiment.

Optical sinc-shaped Nyquist pulses are widely used in microwave photonics, optical signal processing, and optical telecommunications due to their numerous advantages, like rectangular shape in the frequency domain, the orthogonality and the consequential possibility to use these pulses to transmit data with the maximum possible symbol rate. Ideal sinc pulses with the rectangular spectrum are just a mathematical construct. However, high-quality sinc pulse sequences offer the same advantages and can be generated by a phase-locked rectangular frequency comb with mode-locked lasers, intensity modulators, and integrated devices. Nevertheless, any non-idealities in the pulse and comb generation might lead to a degradation of the system performance, especially for metrology. Here, we investigate and analyze the effect of three major non-idealities, namely, the roll-off factor, the side band suppression ratio (SSR), and the ripple of sinc-shaped reconfigurable optical Nyquist pulse sequences based on 3, 5, and 9-line optical phase-locked frequency combs. We compare these results with the existing literature for the three-line comb followed by the experimental verification of the simulation results. We illustrate that by increasing the number of comb lines, the pulse sequences have superior performance and contribute to lesser root-mean-square (r.m.s.) error. We also discuss the trade-off between the r.m.s. error and the optical power loss for increasing the SSR.

Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. The demonstration of soliton formation via self-injection locking of the pump laser to the microresonator has significantly relaxed the requirement on the external driving lasers. Yet to date, the nonlinear dynamics of this process has not been fully understood. Here, we develop an original theoretical model of the laser self-injection locking to a nonlinear microresonator, i.e., nonlinear self-injection locking, and construct state-of-the-art hybrid integrated soliton microcombs with electronically detectable repetition rate of 30 GHz and 35 GHz, consisting of a DFB laser butt-coupled to a silicon nitride microresonator chip. We reveal that the microresonator’s Kerr nonlinearity significantly modifies the laser diode behavior and the locking dynamics, forcing laser emission frequency to be red-detuned. A novel technique to study the soliton formation dynamics as well as the repetition rate evolution in real-time uncover non-trivial features of the soliton self-injection locking, including soliton generation at both directions of the diode current sweep. Our findings provide the guidelines to build electrically driven integrated microcomb devices that employ full control of the rich dynamics of laser self-injection locking, key for future deployment of microcombs for system applications., Self-injection locking of the pump laser for a soliton microcomb has significantly relaxed the requirements for laser drives. Here the authors study self-injection locking in experiment and theory and reveal that the soliton formation is feasible with detunings unreachable according to previous theories.

Quantum cascade laser frequency combs are nowadays well-appreciated sources for infrared spectroscopy. Here their applicability for free-space optical communication is demonstrated. The spontaneously-generated intermodal beat note of the frequency comb is used as carrier for transferring the analog signal via frequency modulation. Exploiting the atmospheric transparency window at 4 $\mu$m, an optical communication with a signal-to-noise ratio up to 65 dB is realized, with a modulation bandwidth of 300 kHz. The system tolerates a maximum optical attenuation exceeding 35 dB. The possibility of parallel transmission of an independent digital signal via amplitude modulation at 5 MS/s is also demonstrated., Comment: 17 pages, 13 figures