Cavity Mode Spacing Research Articles

Wideband finely tunable, ultralow-phase noise microwave generation in a Brillouin cavity.

A novel, to the best of our knowledge, approach to generate frequency-tunable microwave sources with low-phase-noise based on a Brillouin laser frequency comb is proposed and experimentally demonstrated. The Brillouin laser frequency comb is generated by combining stimulated Brillouin scattering, frequency shifting optical injection locking, modulation sideband optical injection locking (MSOIL), and four-wave mixing effects. By beating the generated comb lines, the microwave is generated with an extremely low-level phase noise of -120 dBc/Hz at a 10-kHz offset. The frequency of the microwave signal can be finely tuned in steps of a Brillouin cavity mode spacing (i.e., 2 MHz) and coarsely adjusted to integer times the applied RF signal frequency in the MSOIL unit. Remarkably, the phase noise of the microwave source can be kept at almost the same low level during the whole tuning process over the frequency range of 30-75 GHz. The proposed tunable low-phase-noise microwave generation approach has great potential applications in communications, radars, and metrology.

A versatile platform for gas-phase molecular polaritonics.

Cavity coupling of gas-phase molecules will enable studies of benchmark chemical processes under strong light-matter interactions with a high level of experimental control and no solvent effects. We recently demonstrated the formation of gas-phase molecular polaritons by strongly coupling bright ν3, J = 3 → 4 rovibrational transitions of methane (CH4) to a Fabry-Pérot optical cavity mode inside a cryogenic buffer gas cell. Here, we further explore the flexible capabilities of this infrastructure. We show that we can greatly increase the collective coupling strength of the molecular ensemble to the cavity by increasing the intracavity CH4 number density. In doing so, we can tune from the single-mode coupling regime to a multimode coupling regime in which many nested polaritonic states arise as the Rabi splitting approaches the cavity mode spacing. We explore polariton formation for cavity geometries of varying length, finesse, and mirror radius of curvature. We also report a proof-of-principle demonstration of rovibrational gas-phase polariton formation at room temperature. This experimental flexibility affords a great degree of control over the properties of molecular polaritons and opens up a wider range of simple molecular processes to future interrogation under strong cavity-coupling. We anticipate that ongoing work in gas-phase polaritonics will facilitate convergence between experimental results and theoretical models of cavity-altered chemistry and physics.

Multimode-polariton superradiance via Floquet engineering

We consider an ensemble of ultracold bosonic atoms within a near-planar cavity, driven by a far detuned laser whose phase is modulated at a frequency comparable to the transverse cavity mode spacing. We show that a strong, dispersive atom-photon coupling can be reached for many transverse cavity modes at once. The resulting Floquet polaritons involve a superposition of a set of cavity modes with a density excitation of the atomic cloud. The mutual interactions between these modes lead to distinct avoided crossings between the polaritons. Increasing the laser drive intensity, a low-lying multimode Floquet polariton softens and eventually becomes undamped, corresponding to the transition to a superradiant, self-organized phase. We demonstrate the stability of the stationary state for a broad range of parameters.

Study of the Effects of Cavity Mode Spacing on Mode-Hopping in III–V/Si Hybrid Photonic Crystal Lasers

We present a design methodology for hybrid lasers to realise mode-hop free operation by controlling the cavity mode spacing. In this study, a compact hybrid photonic crystal laser (H-PhCL) was employed which allowed a reduction of the Fabry–Perot length of the laser cavity and eliminated the need for an active mode stabilisation mechanism in order to realise mode-hop free operation. The H-PhCL was formed by butt-coupling a reflective semiconductor optical amplifier (RSOA) with a two-dimensional silicon (Si) photonic crystal (PhC) cavity. Continuous stable single frequency operation with >40 dB side-mode suppression ratio (SMSR) of the laser was achieved for gain currents of up to 100 mA, i.e., up to four times the threshold current. The shorter length of the laser cavity enabled single frequency operation due to the selection of a single longitudinal mode by the PhC narrowband reflector. Various longitudinal mode spacing regimes were studied to explain the mode-hop free characteristics of the H-PhCL. The proposed hybrid laser design methodologies can be adapted to eliminate mode-hopping in laser wavelength.

SI-traceable molecular transition frequency measurements at the 10−12 relative uncertainty level

Accurate and spectroscopic measurements of molecular transition frequencies are increasingly being employed in a variety of rigorous tests of physics, including the validity of quantum electrodynamics, the proton–electron mass ratio, and the dipole moment of the electron. Near-infrared molecular transitions may also underpin secondary frequency standards for length metrology and provide convenient wavelength standards for telecommunication and spectroscopy. To report progress in this field, we describe measurements of near-infrared Doppler-broadened carbon dioxide line positions. Our experiment uses cavity ring-down spectroscopy in the linear domain and provides an accurate International System of Units (SI)-traceable frequency axis via active locking of the probe laser and ring-down cavity to a Cs-clock-referenced optical frequency comb. The approach has several key attributes: the ability to rapidly scan the probe laser while maintaining a phase lock to the optical frequency comb, continuously tunable spectrum frequency axes that are not limited by the cavity mode spacing, and high signal-to-noise-ratio spectra resulting in stationary statistics amenable to long-term averaging. This performance is achieved by phase locking a single-mode probe laser to the optical frequency comb, coherent electro-optical phase modulation for rapid cavity mode-to-mode shifting of the probe laser, and sub-kilohertz-level frequency stabilization of the ring-down cavity relative to the probe laser. We report vacuum transition frequencies of five transitions in the ( 30012 ) ← ( 00001 ) vibrational band of 12 C 16 O 2 near 1.57 µm and achieve combined standard uncertainties as low as 212 Hz, corresponding to a relative combined standard uncertainty near 10 − 12 .

Investigation of Cavity Length and Mode Spacing Effects in Dual-Mode Sensor

Utilizing the principles of intracavity absorption spectroscopy, a biomedical sensor has been developed. The unique feature is the deployment of dual-mode competition which renders characteristics of feasibility, cost, and investigation in various domains. Insight of the high sensitivity of this sensor, relative intensity noise (RIN) is being explored as one of the detection parameters. Cavity length and mode spacing effects of the sensor on average RIN have been investigated to optimize these parameters for improvement in RIN response of the sensor. It has been found that operating the sensor for increased mode spacing improves its RIN response by 24%. In this mode of operation, change in cavity length has almost negligible effect on RIN response of the sensor. This fact in the proposed sensor system can be used to detect various trace gases with minute concentration levels thereby revealing important information about specific diseases in human body.

Application of a Self-Sweeping Yb-Doped Fiber Laser for High-Resolution Characterization of Phase-Shifted FBGs

Application of a self-sweeping self-pulsing laser for high-resolution spectral analysis of π-shifted FBGs has been demonstrated for the first time. It is shown that the self-sweeping laser generates nearly single-frequency radiation during a single pulse. This fact allows us to characterize the shift-induced dip of 14 MHz width in the FBG reflection spectrum. Nonlinear deformation of the measured ultranarrow spectral structure with increasing incident power is observed. Sampling interval during the scan is shown to be limited by pulse to pulse frequency shift, which is equal to the cavity mode spacing of 6 MHz. The main feature of the applied technique is its simplicity due to the absence of special elements for frequency scanning.

Cavity-enhanced storage—preparing for high-efficiency quantum memories

Cavity-assisted quantum memory storage has been proposed for creating efficient (close to unity) quantum memories using weakly absorbing materials. Using this approach, we experimentally demonstrate a significant (∼20-fold) enhancement in quantum memory efficiency compared to the no cavity case. A strong dispersion originating from absorption engineering inside the cavity was observed, which directly affects the cavity line width. A more than three orders of magnitude reduction of cavity mode spacing and cavity line width from GHz to MHz was observed. We are not aware of any previous observation of several orders of magnitude cavity mode spacing and cavity line width reduction due to slow light effects.

Power instability of singly resonant optical parametric oscillators: Theory and experiment

We present a theoretical model on the effects of mechanical perturbations on the output power instability of singly-resonant optical parametric oscillators (SR-OPOs). Numerical simulations are performed based on real experimental parameters associated with a SR-OPO designed in our laboratory, which uses periodically-poled LiNbO3 (PPLN) as the nonlinear crystal, where the results of the theoretical model are compared with the measurements. The out-coupled power instability is simulated for a wide range of input pump powers the SR-OPO oscillation threshold. From the results, maximum instability is found to occur at an input pump power of ~1.5 times above the OPO threshold. It is also shown theoretically that the idler instability is susceptible to variations in the cavity length caused by vibrations, with longer cavities capable of generating more stable output power. The validity of the theoretical model is verified experimentally by using a mechanical vibrator in order to vary the SR-OPO resonator length over one cavity mode spacing. It is found that at 1.62 times threshold, the out-coupled idler suffers maximum instability. The results of experimental measurements confirm good agreement with the theoretical model. An intracavity etalon is finally used to improve the idler output power by a factor of ~2.2 at an input pump power of 1.79 times oscillation threshold.

Behavior of the relaxation oscillation frequency in vertical cavity surface-emitting laser with external feedback

The influence of weak optical feedback on the dynamics of a single longitudinal mode vertical-cavity surface-emitting laser (VCSEL) is studied experimentally under conditions when the feedback power is sufficiently low so as to avoid the onset of chaos. The VCSEL dynamics divides into two regimes depending on the ratio of the relaxation oscillation frequency to the external cavity mode spacing. In the long cavity regime, the modulation frequency response of the VCSEL exhibits resonances corresponding to the frequencies of the external cavity modes, while for the short cavity regime the modulation response of the VCSEL at the cavity resonances remains very low while the relaxation oscillation frequency is shifted and the modulation response at the shifted frequency exhibits a sharpened and enhanced resonance.

Enhancement in microwave modulation efficiency of vertical cavity surface-emitting laser by optical feedback

We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of −16dBm of rf modulation power compared to +1.25dBm needed in the absence of the external cavity.

Photonic crystal beamsplitting laser diode

A description of a novel Y-branch photonic crystal laser diode operating at 1.55 m is presented. Optical gain calculations within an InGaAsP-InP multiple-quantum-well MQW gain medium predict a threshold current of 0.5 mA, which is lower than a typical ridge laser. The resonant structure of the beamsplitting laser is modeled using a scatter- ing matrix approach incorporating the effects of multiple internal reflec- tions on the cavity mode structure and mode spacing. Phase-matching conditions are derived, taking into account all the different resonant sub- structures. The results from the phase-matching analysis show an oper- ating mode that reduces threshold current beyond that of a geometrically similar conventional ridge laser. This Y-branch photonic crystal laser, when cascaded as a multichannel resonant structure, produces numer- ous optical intensity levels that can be used in an optical analog-to-digital converter ADC. © 2006 Society of Photo-Optical Instrumentation

Experimental and Theoretical Study of Semiconductor Laser Dynamics Due to Filtered Optical Feedback

We report experimental results on the nonlinear dynamical response of a semiconductor laser subjected to time-delayed (>5 ns), frequency selective, optical feedback from a Fabry-Pe/spl acute/rot interferometer type of filter. Three regimes of interest, based on the relative value of the filter bandwidth with respect to the relevant laser parameters (relaxation oscillation frequency and external cavity mode spacing), are identified, viz. a wide filter case, an intermediate filter width case, and a narrow filter case. The dynamical response of the laser is shown to be quite different in each of these regimes. The principal results are 1) the laser's linewidth enhancement factor, coupled with the nonlinear response of the filter, can be exploited to induce nonlinear dynamics in the instantaneous optical frequency of the laser light on a time scale related to the time-delay of the feedback, 2) a mode mismatch effect which arises from a detuning between the filter center frequency and the nearest external cavity mode and manifests itself in a reduction of the maximum light available for feedback, and 3) a reduction in, or even disappearance of, relaxation oscillations in the laser dynamics when a filter of appropriate width is chosen. More generally, it is observed that certain dynamics that occur due to unfiltered optical feedback may be suppressed when the feedback light is spectrally filtered.

Ultraviolet Lasing Phenomenon of Zinc Oxide Hexagonal Microtubes

Prismatic zinc oxide (ZnO) hexagonal microtubes have been fabricated on silicon substrate using vapor transport. Two morphologies, bell-mouthed and uniform hexagonal microtubes, are obtained from the fabrication process. The ZnO hexagonal microtubes have demonstrated ultraviolet (∼393 nm) lasing emission under 355 nm optical excitation. It is found that the threshold pump intensity and cavity mode spacing of the microtube lasers are ∼500 kW/cm2 and ∼0.7 nm, respectively. The lasing characteristics of the microtube lasers are also investigated. It is shown that high threshold pump intensity of the microtube lasers is due to the high radiation loss arising from the hollow hexagonal structure near the prismatic facets.

Numerical analysis of a dual polarization mode-locked laser with a quarter wave plate

Dynamical behaviors and frequency characteristics of an active mode-locked laser with a quarter wave plate (QWP) are numerically studied by using a set of vectorial laser equation. Like a polarization self-modulated laser, a frequency shift of half the cavity mode spacing exists between the eigen-modes in the two neutral axes of QWP. Within the active medium, the symmetric gain and cavity structure maintain the pulse's circular polarization with left-hand and right-hand in turn for each round trip. Once the left-hand or right-hand circularly polarized pulse passes through QWP, its polarization is linear and the polarized direction is in one of the directions of ±45° with respect to the neutral axes of QWP. The output components in the directions of ±45° from the mirror close to QWP are all linearly polarized with a period of twice the round-trip time.

Numerical analysis of a dual polarization mode-locked laser with a quarter wave plate

Dynamical behaviors and frequency characteristics of an active mode-locked laser with a quarter wave plate (QWP) are numerically studied by using a set of vectorial laser equation. Like a polarization self-modulated laser, a frequency shift of half the cavity mode spacing exists between the eigen-modes in the two neutral axes of QWP. Within the active medium, the symmetric gain and cavity structure maintain the pulse's circular polarization with left-hand and right-hand in turn for each round trip. Once the left-hand or right-hand circularly polarized pulse passes through QWP, its polarization is linear and the polarized direction is in one of the directions of ±45° with respect to the neutral axes of QWP. The output components in the directions of ±45° from the mirror close to QWP are all linearly polarized with a period of twice the round-trip time.

Design and performance of a frequency chirped external cavity diode laser

The design of a frequency chirped external cavity diode laser (CECDL) in the Littrow configuration with an intra-cavity electro-optic crystal is presented and its tuning and noise performance is studied. The CECDL has a center wavelength of 793 nm with a 20 nm tuning range by mechanical rotation of the feedback grating about a pivot point. Rapid tuning of the CECDL is achieved electronically by voltage control of the electro-optic crystal. The electro-optic tuning response of this laser is 2.01 MHz/V and linear frequency chirps of 800 MHz ranging in duration from 3 to 337 μs are demonstrated. The maximum electro-optic tuning is set by the external cavity mode spacing of 2.4 G Hz for this laser.

Geometrical optics model of Mie resonances

The geometrical optics model of Mie resonances is presented. The ray path geometry is given and the resonance condition is discussed with special emphasis on the phase shift that the rays undergo at the surface of the dielectric sphere. On the basis of this model, approximate expressions for the positions of first-order resonances are given. Formulas for the cavity mode spacing are rederived in a simple manner. It is shown that the resonance linewidth can be calculated regarding the cavity losses. Formulas for the mode density of Mie resonances are given that account for the different width of resonances and thus may be adapted to specific experimental situations.

A multistate external cavity laser diode

A semiconductor laser diode in a photon cavity containing a mirror whose optical bandwidth is less than the classical cavity mode spacing can have multiple lasing states at the same bias current. The physics determining the behavior of this nonlinear multistate device is self-consistency between photon number, carrier number, and refractive index.

Theoretical stability analysis of single-mode operation in uncontrolled mode-selection semiconductor lasers

An uncontrolled mode-selection laser is a laser containing an intracavity filter under which the cavity mode alignment is uncontrolled. We present a theoretical stability analysis of the single-mode operation of uncontrolled mode-selection semiconductor lasers. We find that to maximize the stability the cavity length and intracavity filter bandwidth must be minimized, and a specific, nonzero, linewidth enhancement parameter must be chosen. For a given cavity mode spacing to filter bandwidth ratio, a shorter cavity is more stable.