Active mid-infrared ring resonators

Performances of anoptical frequency comb generator and nonlinear optical frequency conversionof semiconductor lasers were reviewed. They are studied for realizing frequency referencegrids for an accurate and wideband optical frequency sweep. Semiconductor lasers wereused as light sources by improving their frequency stability to 2.1×10-12 and by reducing theirfield spectral linewidth to 7 Hz. A precise and continuous frequency sweep for 64GHz wasrealized by heterodyne-type optical phase locking. Frequency refrence grids with aspan of 0.1THz was realized by the optical frequency comb generator. Second harmonics, sum-frequencyand difference frequency signals were generated by using nonlinear optical crystals for the 180-577 THzrange, i. e., the wavelength range of 0.52-1.67μm.

We review the frequency comb generation process, main microresonator parameters such as free spectral range (FSR) and Q-factor, previously used optical frequency comb (OFC) generator parameters and resulting frequency combs, as well as the implementation of OFC for optical data transmission. An optical frequency comb is produced in a setup based on a tapered fibre and a SiO2 microsphere. The generated frequency comb has a frequency spacing of 2 nm or 257 GHz. During the fabrication of a tapered fibre from SMF28, use is made of the transmission signal to control the taper pulling process. The final measured tapered fibre transmission is ∼96%. A microsphere whispering gallery-mode resonator (WGMR), exhibiting a Q-factor of at least 2 × 107, is fabricated from an optical fibre with a thicker core than SSMF. Moreover, for future experiments, a frequency comb generator based on a free-space setup consisting of lenses, a prism, and a microsphere is developed, and the Q-factor dependence on different distances between the prism and the microsphere is investigated.

Using mathematical modelling we have studied the conditions of low-threshold collinear optical frequency comb generation under transient (picosecond) stimulated Raman scattering (SRS) and parametric four-wave coupling of SRS components in crystals. It is shown that Raman-parametric generation of an octave-spanning optical frequency comb occurs most effectively under intermediate, transient SRS at a pump pulse duration exceeding the dephasing time by five-to-twenty times. We have found the optimal values of not only the laser pump pulse duration, but also of the Raman crystal lengths corresponding to highly efficient generation of an optical frequency comb from the second anti-Stokes to the fourth Stokes Raman components. For the (high dispersion) and (low dispersion) crystals pumped at a wavelength of and a pulse duration five or more times greater than the dephasing time, the optimum length of the crystal was 0.3 and , respectively, which is consistent with the condition of the most effective Stokes – anti-Stokes coupling , where is the wave detuning from phase matching of Stokes – anti-Stokes coupling, determined by the refractive index dispersion of the SRS medium.

We study optical frequency comb (OFC) generation using cascaded injection of semiconductor lasers in this work. The OFC generation system is operated in two cascaded optical injection stages. When a master laser optically injects into the first stage with proper injection power and frequency, period-one (P1) dynamics are invoked in an optically injected semiconductor laser of the first stage. Another semiconductor laser in the second stage is then optically injected by the P1 dynamics. With proper injection power adjusted in the second stage, the P1 dynamics are regenerated, and the semiconductor laser relaxation oscillations (ROs) become undamped so that subharmonic oscillations appear. Because a subharmonic oscillation frequency is half of an oscillation frequency of the P1 dynamics, extra optical frequency components appear in the middle of the adjacent optical frequency components of the P1 dynamics, thus signaling OFC generation. The OFC signals exhibit at least 15 comb lines, resulting in a bandwidth greater than 140 GHz. Microwave comb signals are obtained after photodetection, although the microwave linewidth is on the order of a few megahertz because of the semiconductor laser noise. Thus, we propose a cascaded injection-locking scheme to stabilize the P1 dynamics and OFC signals. We have demonstrated pure microwave generations with a linewidth of less than 3 Hz and low phase noise.

Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Substantial advances in mid-infrared frequency comb generation have been made in recent years based on nonlinear frequency conversion, microresonator Kerr frequency combs, quantum cascade lasers and mode locking regimes. Here we demonstrate a simple, yet effective method for the direct generation of mid-infrared optical frequency combs in the region from ${2.5-4~\mu{\rm m}}$, i.e. ${2500-4000~{\rm cm}^{-1}}$ covering a large fraction of the functional group region, directly from a conventional and compact erbium-fiber-based femtosecond laser in the telecommunication band (i.e. ${1.55~\mu{\rm m}}$). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in large-cross-section and dispersion-engineered silicon nitride (${\rm Si_3N_4}$) waveguides. The long-wavelength dispersive wave, with its position lithographically determined, performs as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such a simple and versatile approach to mid-infrared frequency comb generation is suitable for spectroscopic applications in the first mid-infrared atmospheric window. Moreover, the compactness and simplicity of the approach have the potential to realize compact dual-comb spectrometers. The generated combs have a fine teeth-spacing, making them also suitable for gas phase analysis.

We design, fabricate and characterize first on-chip prism-like waveguide (prism-waveguide) coupler to high-Q (Q>1010) micro-resonator with record 1.1dB coupling loss at 1550nm and demonstrate an integrated optical frequency comb (OFC) generation unit based on this coupler.

The integration of a leaky-wave antenna with a ring resonator is presented based on the leaky-wave theory and integrated waveguide models. The device consists of a ring resonator fed by a directional coupler, where the ring resonator path includes a segment that is turned into a leaky-wave antenna. We devise an analytical model of the guided wave propagation along a directional coupler and the ring resonator path, including the antenna and non-radiating segments. The resonator integration provides two main advantages: 1) the high-quality factor resonance ensures effective control of radiation intensity by controlling the resonance conditions and 2) the efficient radiation from a leaky-wave antenna even when its length is much smaller than the propagation length of the leaky wave. The tradeoffs regarding the quality factor of resonance and the antenna efficiency of such a design are reported in terms of the coupler parameters, leaky-wave constant, and radiation length. Finally, a CMOS-compatible OLWA suitable for the ring resonator integration is designed where silicon is utilized as the waveguide material for a possible electronic control of radiation intensity. The simulation results together with the analytical model show that slight variations in the leaky wave's propagation constant, realized through excitation of excess carriers in the Si domain, is sufficient to control the far-field radiation modulation with high extinction ratio.

Optical ring or disk resonators can trap traveling optical waves in a closed path, for example, in a waveguide loop or silicon disk cavity. Energy can be added to or extracted from the cavity with the help of waveguide directional couplers. Ring/disk resonators can be fabricated with high quality factor (Q) in diverse material platforms. The high Q of these devices makes them very sensitive to small variations in (i) optical path length (phase), (ii) optical path loss (attenuation), and (iii) coupling coefficient. Any change in the optical path length can cause a shift in the resonance frequency of the ring. Whereas, the change in path loss and coupling coefficient can lead to a change in the Q of the resonator. The micro or nano opto-electro-mechanical systems (MOEMS/NOEMS) provide unique flexibility with a wide change in the tuning parameters. The tuning is obtained by either (i) displacement of the detached parts or (ii) strain in the waveguide/disk. Displacement of a structure detached from the ring/disk closer to the evanescent field can increase the effective refractive index or attenuation. The displacement of a waveguide in the directional coupler altering the gap can change the coupling coefficient. Strain from various sources can lead to refractive index (phase) and absorption changes, in addition to changing the dimensions. Mechanical, electrothermal, piezoelectric, electrostatic, and magnetic actuators have been demonstrated with MOEMS. Mechanical latching has been used to achieve nonvolatile tuning. Multi-beam supported, freestanding, and diaphragm supported structures have shown applications in sensing, optical switching, and energy conversion. MOEMS/NOEMS has recently been shown to provide the desired properties of modern sensors and photonic integrated circuits (PICs) such as low optical power consumption, high resolution, wide tuning range, low complexity, small footprint, low actuator power consumption, and compatibility with standard fabrication technologies. This chapter will start by briefly covering the basics of MOEM tuned ring/disk resonators. Then, the current technology aspects such as tuning mechanism, actuator types, and the fabrication technology will be discussed along with recent innovations. Furthermore, the evolution into NOEMS with specialized applications and materials will be emphasized along with emerging trends.

The precise measurement of difference frequencies between two lasers in a terahertz (THz) region is a basic technique for optical frequency standards and for many other applications, such as dense WDM communication systems. Frequency difference measurements based on the beat signal between two lasers become quite difficult when the frequency difference exceeds the detector bandwidth. An optical frequency comb (OFC) generator with an electro-optic (EO) phase modulator inside a Fabry-Perot cavity can overcome this problem. The OFC generator creates a large number of sidebands, which can be applied for frequency markers. In conventional OFC generation, modulation sidebands appears on both sides of the carrier, which is a disadvantage for the efficient use of frequency bandwidth. The limit of the frequency difference measurement with this scheme depends on the power degradation in the sidebands. We propose an alternative technique for generating OFC which employs a dual-electrode Mach-Zehnder Modulator (D-MZM) inside a fiber recirculating loop to generate optical single sideband (SSB). This enables the efficient use of the frequency bandwidth. An optical amplifier is used in the loop to compensate for the losses.

Flexible broadband optical frequency comb generation assisted by injection locking of semiconductor laser

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10 dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.

A synchronously pumped Kerr cavity is proposed and studied for power-efficient frequency comb generation in optical microring resonators. The system is modeled using the Lugiato-Lefever equation. Analytical solutions are provided for an ideal case and extended by numerical methods to account for optical loss and higher orders of dispersion. It is shown that the average power requirement is reduced by the duty cycle of the pulse with respect to the conventional continuous-wave-pumped microrings, and it is significantly lower than the pulsed pumping of straight waveguides.

We present an ultrabroadband supercontinuum (SC) frequency comb generation covering the optical spectrum more than 1.2 octave from 570 nm to 1350 nm with a near flat-top power spectrum by using a mode-locked Yb fiber laser at 1030 nm as a master oscillator (MO). The repetition rate of the MO that has a spectral-width of 28 nm is 152 MHz and is phase-locked to a reference frequency synthesizer. A power amplifier is used to boost up the output power up to 1.3 W and the positively chirped amplified pulses are compressed by using a transmission grating pair. We used a commercially-available 10-cm long photonic crystal fiber that has two zero-dispersion wavelengths at 800 nm and 1093 nm, respectively, to generate an ultrabroadband SC frequency comb. Optical spectrum depending on the frequency chirp and pulse-width of the amplified pulses are analyzed to find an optimum coupling power as well as an optimum chirp parameter. We found that a dimensionless chirp parameter of 8.6 and a pulse-width of 517 fs for a chirped Gaussian pulse resulted in the optimum spectrum width of 1.2 octave.

Broadband optical frequency comb (OFC) generation based on a single electro-absorption modulator (EAM) is proposed. The EAM is driven by a radio frequency (RF) multi-frequency signal generated by a multiplication coupler composed of an electrical power splitter and an arithmetic circuit. Thus the number of comb-lines of the generated OFC can be increased. A complete theoretical model of OFC generation by an EAM driven by nth power of the RF source is established, and the performance of the OFC is analyzed by using OptiSystem software. The results show that, the number of comb-lines of the OFC is positively correlated with the number of multiplication of the RF source signal. The frequency spacing of the comb-lines is twice the frequency of the RF source signal and is tunable by adjusting the frequency of the RF source signal. Increasing chirp factor and modulation index of EAM could increase the number of comb-lines of the generated OFC. The amplitude of the RF source signal had little impact on the flatness of the OFC and the average OFC power. The scheme developed is not only simple and low-cost, but also can produce a large number of comb-lines.Graphical

A wideband optical frequency comb (OFC) generator using a fiber re-circulating loop (FRL) including a phase modulator and optical amplifier, cascaded with a spectrum expander containing a high-power erbium-doped fiber amplifier and highly nonlinear fiber (HNLF) is proposed and experimentally demonstrated, in which the FRL is used as a source to generate seed comb lines, and then generated seed comb lines are extended to a wideband optical frequency comb using cascaded four-wave mixing in the HNLF. Only using one stage spectrum expander, a stable 20-GHz optical comb with 201 comb lines within 30-dB power deviation, spanning 40-nm bandwidth, is achieved. The results show that the HNLF has unique ability to broaden output spectrum from the FRL. In addition, an optical frequency comb is dramatically broadened, which may be predicted by optimizing gain and dispersion of the FRL or configuration of the spectrum expander, or both.