Dual-polarization mode-locked Nd:YAG laser

A mode-locked solid-state laser containing a birefringent element is shown to emit synchronously two frequency combs associated to the two polarization eigenstates of the cavity. An analytical model predicts the polarization evolution of the pulse train, which is determined by the adjustable intracavity birefringence. Experiments realized with a Nd:YAG laser passively mode locked by a semiconductor saturable absorber mirror are in perfect agreement with the model. Locking between the two combs arises for particular values of their frequency difference, e.g., half the repetition rate, and the pulse train polarization sequence is then governed by the relative overall phase offset of the two combs.     

A diode-pumped, frequency-doubled, tunable single-mode quasi-continuous-wave Yb:YAG laser emitting 70 W peak power at 515 nm

An efficient pumping scheme for a quasi-continuous-wave diode-pumped Yb:YAG laser is presented. Single-mode operation and fine wavelength tuning are assured by the use of a rubidium titanyl phosphate (RTP) Fabry–Perot étalon. When frequency doubled, the 200–420 μs duration pulses reach a peak power of 70 W at a wavelength of 515 nm. The TEM₀₀ beam is nearly diffraction limited with an M ² factor of 1.06 at full power. The tuning range spans from 512 to 520 nm and the pulse to pulse frequency stability is on the order of ±10 MHz. Laboratoire Aimé Cotton is associated with Université Paris Sud 11.     

Impact of size upon lasing in ZnO microtetrapods

High optical quality, well end leg faceted ZnO microtetrapods with leg length between 1 and 12 μm have been grown by carbothermal chemical vapor deposition. Lasing with mode quality factors of 2500–3000 is demonstrated. The origin of laser resonator cavity is discussed as a function of the tetrapod size. It is shown that in big tetrapods with legs of 12 μm in length the laser emission lines are well explained by longitudinal Fabry–Pérot modes generated in cavities formed by individual tetrapod legs. The dispersion of the ZnO refractive index is experimentally determined from the position of lasing modes in the temperature interval from 10 to 300 K. It is shown that the lasing mode structure is seriously affected by the decrease of the tetrapod size. For a small tetrapod with a leg length of 1 μm, the lasing modes cannot be explained anymore by the formation of longitudinal Fabry–Pérot modes in separate tetrapod legs, and the generation of guided modes by multiple total internal reflections in single tetrapod legs or in pairs of legs should be taken into account. The correlations between the lasing threshold and the tetrapod size are discussed.     

Stabilization of a multiwavelength erbium-doped fiber laser using a nonlinear silicon waveguide

We propose and demonstrate a multiwavelength erbium-doped fiber laser stabilized by four-wave mixing (FWM) in a nonlinear silicon-on-insulator (SOI) waveguide. The optical gain was provided by an erbium-doped fiber amplifier, and the wavelength selectivity was achieved by a Fabry–Pérot comb filter in the ring cavity. The FWM in the SOI waveguide was enhanced by applying a reverse-biased p-i-n diode structure to reduce free-carrier absorption. Making use of the nonlinearity of the SOI waveguide, a multiwavelength laser with six output wavelengths at 0.8 nm spacing was achieved. The power difference among modes was equalized within a range of 1.8 dB. The power fluctuation of each mode was stabilized to <0.65 dB during 20 min observation at room temperature.     

Diode laser spectrum of the ν6 band of CH3I using a novel étalon as a calibration scale

The ΔK = Δl = +1 transitions of the ν₆ band of the CH₃I molecule were measured by a tunable diode laser spectrometer. A newly designed Fabry-Perot étalon with a free spectral range of 0.00945 cm^?1 was used to calibrate the line position very precisely. The present data were analyzed together with the previously reported transition wavenumbers, and precise molecular constants have been obtained.     

Self-mode-locked pulsed monomode laser

We predict the existence of a new pulsed-laser operation regime, when the phases and polarizations of the two coupled cold-cavity eigenstates of a monomode solid-state laser are taken into account in the derivation of the Maxwell-Bloch equations. This monomode pulsed regime is experimentally observed, without any normal mode locking or Q switching occurring inside the cavity. We obtain close agreement between experiments and theory, even in the simple case of a Nd:YAG microchip laser, for which sech(2) pulses at nearly megahertz repetition rates are readily observed.     

半外腔Nd:YAG和Nd:YVO4微片固体激光器的腔调谐特性研究

微片固体激光器具有体积小、寿命长等优点,是精密测量仪器的重要光源。构建了平-平、半外腔的Nd:YAG和Nd:YVO₄微片激光器,通过控制压电陶瓷伸缩改变激光器的谐振腔长,同时使用F-P扫描干涉仪和波长计观察纵模和波长。研究了这2种微片固体激光器的腔调谐特性,包括腔长与光功率的关系,激光纵模扫过出光带宽过程的光功率和单、双纵模变化的规律。实验结果表明：腔调谐过程中单、双纵模交替出现,腔长和泵浦电流共同影响激光器的输出模式和光功率。     

Role of stationary photon statistics in a high-Q cavity mode while it is exciting the active laser medium

It has been shown by Yu. M. Golubev, M. I. Kolobov, and I. V. Sokolov, Zh. éksp. Teor. Fiz. 111, 1579 (1997) [JETP 84, 864 (1997)], that when an optical cavity is excited by external radiation from a sub-Poisson laser the cavity mode may be in either a sub-Poisson or a Poisson stationary state. This is not important for a resonant medium which is excited into the upper laser level while interacting with this mode inside the cavity. The degree of regularity of the excitation will be identical to that of the initial light flux incident on the cavity, and this ultimately ensures the same sub-Poisson lasing as for strictly regular pumping of the resonant medium. Zh. éksp. Teor. Fiz. 113, 1223–1234 (April 1998)     

Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback

This paper demonstrates the influence of external optical feedback on the polarization state of longitudinal modes in quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the polarization state of longitudinal modes in quasi-isotropic lasers relies strongly on the intracavity anisotropy loss and mode competition. When the intracavity anisotropy loss is small, external optical feedback can cause polarization switching and strong mode competition between two orthogonal linearly polarized eigenstates of one laser longitudinal mode, which leads to the distortion of laser intensity modulation waveform. The polarization switching is independent of the initial external cavity length. By increasing the intracavity anisotropy loss, one polarization eigenstate can be suppressed and the laser works in single-polarization state. A theoretical analysis based on the compound cavity model is presented, which is in good agreement with the experimental results. The results offer guidance to the development of laser feedback interferometers.     

Microbead dynamics in optical trap assisted nanopatterning

An optically pumped ZnO nanowire laser with a 10-period SiO₂/SiN _x distributed Bragg reflector (DBR) was demonstrated. Stimulated emissions with equally distributed Fabry–Pérot lasing modes were observed at pumping powers larger than 121 kW/cm². This result, when compared to nanowires of the same length and without a DBR structure, shows that a lower threshold of pumping power, higher quality factor, and larger cavity finesse can be achieved due to the high reflectivity of the DBR in the designed wavelength range. A coexistence of stimulated and spontaneous emissions was also observed above threshold and was attributed to partially confined waveguide modes in nanowires with diameters smaller than 100 nm.     

Generalized Rotating-Wave Approximation for the Quantum Rabi Model with Optomechanical Interaction

The spectrum of energy and eigenstates of an hybrid cavity optomechanical system, where a cavity field mode interacts with a mechanical mode of a vibrating end mirror via radiation pressure and with a two level atom via electric dipole interaction are investigated. In the spirit of approximations developed for the quantum Rabi model beyond rotating-wave approximation (RWA), the so-called generalized RWA (GRWA) to diagonalize the tripartite Hamiltonian for arbitrary large couplings is implemented. Notably, the GRWA approach still allows to rewrite the hybrid Hamiltonian in a bipartite form, like a Rabi model with dressed atom-field states (polaritons) coupled to mechanical modes through reparametrized coupling strength and Rabi frequency. A more accurate energy spectrum for a wide range of values of the atom-photon and photon–phonon couplings, when compared to the RWA results is found. The fidelity between the numerical eigenstates and its approximated counterparts is also calculated. The degree of polariton-phonon entanglement of the eigenstates presents a non-monotonic behavior as the atom-photon coupling varies, in contrast to the characteristic monotonic increase in the RWA treatment.     

Mode splitting of a cavity with a high-density birefringence rubidium vapor in the superstrong coupling regime

The mode splitting in a system with Doppler-broadened high-density two-level atoms in the presence of magnetic field inside a relatively long optical cavity is studied in the superstrong coupling regime(atoms-cavity coupling strength g√N is near or larger than the cavity free-spectral range?FSR).The effect of a magnetic field applied along the quantization axis is used to break the polarization degeneracy of the cavity and thereby introduce birefringence(or Faraday rotation)into the medium.The cavity modes are further split in the presence of the magnetic field compared with the normal case of the multi-normal-mode splitting of the two-level system near the D2 line of87Rb.The dependence of the mode splitting on the magnetic field and the temperature is studied.The theoretical analysis according to the linear dispersion theory can provide a good explanation.     

Locking the frequency of lasers to an optical cavity at the 1.6×10−17 relative instability level

We stabilized the frequencies of two independent Nd:YAG lasers to two adjacent longitudinal modes of a high-finesse Fabry–Pérot resonator and obtained a beat frequency instability of 6.3 mHz at an integration time of 40 s. Referred to a single laser, this is 1.6×10^?17 relative to the laser frequency, and 1.3×10^?6 relative to the full width at half maximum of the cavity resonance. The amplitude spectrum of the beat signal had a FWHM of 7.8 mHz. This stable frequency locking is of importance for next-generation optical clock interrogation lasers and fundamental physics tests.     

Fabry–Pérot filter cavities for wide-spaced frequency combs with large spectral bandwidth

We use low-finesse Fabry–Pérot cavities in series to generate frequency combs with a large mode spacing in a way that allows its application to a large optical bandwidth. The attenuation of laser modes closest to the pass bands of the cavity exceeds 70 dB for a filter ratio of m=20 relative to the resonant modes centered within the pass bands. We also identify the best cavity geometry to suppress spurious transmission of higher order transversal modes. Such a thinned out frequency comb can be used to calibrate traditional spectrographs for precision astronomy. In the time domain mode filtering generates a pulse train with a multiplied repetition rate. High-fidelity filtering, as described here, implies small variations of the pulse energies.     

Difficulties with massless particles?

Some difficulties with sharp momentum (one-particle) states for massless particles are indicated, in the framework of unitary irreducible representations of the Poincaré group. It is shown that a Poincaré covariant set of such states requires the introduction, in the spatial direction opposite to the point stabilized, of momentum generalized eigenstates which (when the helicity is nonzero) have a nontrivial orbital transformation. The relevance of these generalized momentum eigenstates for massless theories is then shown.     

Fast and stable enhancement of the far-field peak power by use of an intracavity deformable mirror

We demonstrate that an intracavity deformable mirror (DM) is capable of compensating for the aberrations of a continuous-wave (CW) Nd:YAG solid-state laser and improving its far-field peak power fast and stably. The deformable mirror is controlled by a stochastic parallel gradient descend (SPGD) algorithm. Experimental results reveal that high-order-like transverse modes can be transformed into Gaussian-like modes quickly without any pinhole added in the laser cavity. At the same time, the far-field peak powers are also increased to different extents.     

A 658-W VCSEL-pumped rod laser module with 52.6% optical efficiency

A high-efficiency and high-power vertical-cavity surface-emitting laser (VCSEL) side-pumped rod Nd:YAG laser with temperature adaptability are demonstrated. The VCSEL side-pumped laser module is designed and optimized. Five VCSEL arrays are symmetrically located around the laser rod and a large size diffused reflection chamber is designed to ensure a uniform pump distribution. Furthermore, the absorbed pump power distribution of the rod is simulated to verify the uniformity of the pump absorption. Finally, a proof-of-principle experiment is performed in short linear cavity laser with single laser module. A continuous-wave output power of 658 W at 1064 nm is obtained, the corresponding optical-to-optical efficiency is 52.6%, and the power variations are ±0.7% over 400 s and ±3.1% over the temperature range from 16 ℃ to 26 ℃. To the best of our knowledge, this is the highest output power and the highest optical-to-optical efficiency ever reported for VCSEL pumped solid-state lasers. By inserting a telescopic module into the cavity and optimizing the TEM₀₀ mode volume, the average beam quality is measured to be M²=1.34 under an output power of 102 W. The experimental results reveal that such a high power rod laser module with temperature stability is appropriate or field applications.     

二维电子气等离激元太赫兹波器件

固态等离激元太赫兹波器件正成为微波毫米波电子器件技术和半导体激光器技术向太赫兹波段发展和融合的重要方向之一。本综述介绍AlGaN/GaN异质结高浓度和高迁移率二维电子气中的等离激元调控、激发及其在太赫兹波探测器、调制器和光源中应用的近期研究进展。通过光栅和太赫兹天线实现自由空间太赫兹波与二维电子气等离激元的耦合,通过太赫兹法布里-珀罗谐振腔进一步调制太赫兹波模式,增强太赫兹波与等离激元的耦合强度。在光栅-谐振腔耦合的二维电子气中验证了场效应栅控的等离激元色散关系,实现了等离激元模式与太赫兹波腔模强耦合产生的等离极化激元模式,演示了太赫兹波的调制和发射。在太赫兹天线耦合二维电子气中实现了等离激元共振与非共振的太赫兹波探测,建立了太赫兹场效应混频探测的物理模型,指导了室温高灵敏度自混频探测器的设计与优化。研究表明,基于非共振等离激元激发可发展形成室温高速高灵敏度的太赫兹探测器及其焦平面阵列技术。然而,固态等离激元的高损耗特性仍是制约基于等离激元共振的高效太赫兹光源和调制器的主要瓶颈。未来的研究重点将围绕高品质因子等离激元谐振腔的构筑,包括固态等离激元物理、等离激元谐振腔边界的调控、新型室温高迁移率二维电子材料的运用和高品质太赫兹谐振腔与等离激元器件的集成等。     

Possibility of suppressing quantum light fluctuations when excess photon fluctuations occur inside a cavity

Using the optical excitation of a high-Q cavity as an example, it is shown that when light is observed at the output of this cavity, effective suppression of the photocurrent shot noise below the quantum limit is in general independent of the parameters of the stationary state of the field oscillator (in particular, it is independent of the rms photon fluctuations) inside the cavity and can occur not only at any allowed negative value but even at a positive value of the Mandel parameter. It was assumed in solving the problem that the cavity is optically excited by superimposing the radiation of a sub-Poisson laser and a laser with excess photon noise. A formal solution was obtained in terms of the kinetic equation for the density matrix of the actual fields (inside the laser cavities and the empty cavity), which is derived here on the basis of the Heisenberg-Langevin quantum equations, taking into account directed propagation of the field from the laser cavities inside the empty cavity. The resulting kinetic equation can also be used to solve other physical problems, since it is applicable to optical systems that contain, in principle, an arbitrary number of coupled cavities and interference mixers. Zh. éksp. Teor. Fiz. 111, 1579–1600 (May 1997)     

Dual-wavelength external cavity laser with a sampled grating formed in a silica PLC waveguide for terahertz beat signal generation

We propose and demonstrate a dual-wavelength external cavity laser (ECL). In our design, a Fabry–Pérot laser diode (FP-LD) is hybrid-integrated with a sampled Bragg grating written in a silica planar lightwave circuit (PLC). The grating selects two laser wavelengths that share the same laser cavity. The dual-wavelength oscillation with a side mode suppression ratio >32 dB has been demonstrated experimentally. Experimental results indicate that the hybrid-integrated dual-wavelength ECL exhibits strong dual-wavelength emission corresponding to beat frequency of 1 THz. Simulations of this system also indicate good mutual coherence of the two modes and stability of the 1 THz beat signal. PACS 42.79.Dj; 42.60.By; 42.55.Px