A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

在一个磁通量子内dc Josephson电流的阶梯效应(Ⅱ)——大尺寸Josephson结

大尺寸的Josephson结放置于Q值足够高的谐振腔中,当其Josephson频率ω=2cV₀/h激励起腔的第r个本征模谐振时,这个振荡将反馈辐照结,必须考虑反馈场本征模的空间部分在结上是不同的。理论上得到了和文献[2]不同的结果,给出了阶跃电流随磁场的变化,分析了腔的反馈场本征模和Josephson结的辐射电磁波空间匹配的条件,解释了为什么大尺寸的Josephson结不存在周期小于φ₀的阶梯效应的物理原因。 关键词：     

Influences of geometry parameter on mode suppression ratio of fiber grating external cavity semiconductor laser

According to equivalent external cavity approximation model, after taking into account the joint contribution of semiconductor laser, external cavity and fiber grating (FG) to the phase condition, the mode distribution of the fiber grating external cavity semiconductor laser (FGESL) can be determined. As a result, the effect of the FG external cavity length (L) on the side mode suppression ratio (SMSR) of the FGESL is investigated theoretically. The results show that with the injected current and the coupling efficiency increase of the SMSR has taken on rise at all. For strong feedback (R₂ = 10^?4), the SMSR become more flattened with more than 40 dB, but, for weak feedback condition, The SMSR have lesser than 35 dB by an oscillation during rising course. Under the condition of short external cavity, the SMSR is in deep relation to the external cavity length, but the SMSR of longer external cavity is smaller than the SMSR of shorter external cavity on the whole and for 8–11 mm of the external cavity length, the SMSR of the FGESL has better (SMSR > 40.8 dB), and the SMSR become more flattened.     

Analyses of two-cavity gyroklystron with feedback

Making use of the linear kinetic theory and the perturbation approach, a gyroklystron with feedback, in which a low order (TE₀₁₁) mode is presented in the bunching cavity by the feedback of the output cavity, and the low order mode in the bunching cavity is coupled by the electron beam to a high order (TE₀₂₁) mode in the output cavity, has been investigated in detail. And electron beam to wave interaction in each cavity and electron bunching mechanism in the bunching cavity have been analysed. The formulas of electron beam to wave interaction power and starting current etc. have been derived. Then some calculations are carried out.     

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.     

Improvement of the beam quality of a broad-area diode laser using asymmetric feedback from an external cavity

In this work, the use of an asymmetric feedback technique for improving the beam quality of a broad-area diode laser is investigated using numerical simulations. A mirror stripe is placed in the external cavity to select lateral mode and provide asymmetric feedback. The width and the position of the mirror stripe are optimized to improve the beam quality. The simulation results show a good beam quality of 0.7° FWHM and M ² value of 2.69.     

Observations on the dynamics of external cavity semiconductor lasers

Dynamics of external cavity semiconductor lasers is known to be a complex and uncontrollable phenomenon. Due to the lack of experimental studies on the nature of the external cavity semiconductor lasers, there is a need to theoretically clarify laser dynamics. The stability of laser dynamics in the present paper, is analyzed through plotting the Lyapunov exponent spectra, bifurcation diagrams, phase portrait and electric field intensity time series. The analysis is preformed with respect to applied feedback phase C_p, feedback strength η and the pump current of the laser. The main argument of the paper is to show that the laser dynamics can not be accounted for through simply a bifurcation diagram and single-control parameter. The comparison of the obtained results provides a very detailed picture of the qualitative changes in laser dynamics.     

Optomechanical dynamics in detuned whispering-gallery modes cavity

The dynamics of a microresonator in detuned whispering-gallery modes (WGM) cavity opto-mechanical system are investigated by the quantum Langevin equation. A WGM cavity coupling to two parallel waveguides is devised to study the transmission and reflection of this system. In single mode WGM cavity, without optomechanical coupling, both the transmission and reflection of the cavity present a Lorentzian dip and peak. When the coupling between the cavity mode and mechanical mode is considered, the transmission and reflection of the optomechanical cavity show “W” and “M” shape mode splitting. Moreover, under the action of a controlling and a probe laser, the output field at the probe frequency presents electromagnetically induced transparency (EIT)-like spectrum in the system. We give the physical origin of EIT-like and the pump-probe response for the WGM shares all the features of the Λ system in atoms. Further, due to backscattering, the two traveling waves in WGM are coupled with a rate γ. The transmission and reflection of the optomechanical cavity display three modes splitting in the spectra with optomechanical coupling between the two cavity modes and the mechanical mode.     

On the generation of a generalized superposition of displaced squeezed states

A feasible scheme is presented to generate a generalized superposition of displaced squeezed states, cosθ|α,z〉±sinθ|-α,z〉, in a single mode of the electromagnetic field inside a microwave cavity. The scheme employs a two-level (Rydberg) atom driven by an external classical field. The success probability and the interaction time of such generation are also considered.     

On the dynamical structure of the Dicke maser model

For suitable states of the Dicke maser model we study the time evolution of the mean photon number n(t) in the limit N → ∞. Here N is the number of maser active atoms. Our starting point is a well-tuned cavity with only one mode of the radiation field excited. Introducing new dynamical variables we are able to exploit fully the conservation laws so as to get a simple but completely rigorous solution. We find that n(t) is periodic, and given by a Weierstrassian elliptic function, provided a net polarization is present in the cavity. No approximation is involved.     

Complete chaotic synchronization characteristics of the linear-polarization mode of vertical-cavity surface-emitting semiconductor lasers with isotropic optical feedback

Based on the SFM model [M. San Miguel, Q. Feng, J.V. Moloney, Phys. Rev. A. 52 (1995) 1728.], complete chaotic synchronization characteristics of linear-polarization (LP) Mode of the vertical-cavity surface-emitting semiconductor lasers (VCSELs) with isotropic optical feedback are numerically investigated. When the propagation time τ_c of light from the transmitter to the receiver equals to the external cavity round-trip time τ, and the central frequency of the T_VCSEL matches that of the R_VCSEL, in large scale region of the feedback coefficient and the injection current where the x-polarization mode as well as the y-polarization mode obtains high synchronization quality at the same time. However, in the central frequency mismatch regions where the x- and y-polarization mode can obtain inferior synchronization quality at same time. But for τ_c ≠ τ, each LP mode of the mixed polarization modes achieved inferior synchronization quality at the same time. Besides, with big enough injection current, the output of the system with the same central frequency in two lasers is entirely governed by the y-polarization mode when the feedback coefficient and Δτ(=τ_c − τ) fixed at a certain value. So the system can realize steadily good synchronization of the completely dominant y-polarization mode. But the synchronization quality of the completely dominated y-polarization mode can be deteriorated by increasing absolute frequency detuning value. Besides, in positive frequency mismatch regions where the influence of the same frequency detuning value on the synchronization quality of the entirely governed y-polarization mode becomes smaller than in negative frequency detuning regions. At last, when τ is given and the central frequency of the T_VCSEL matches that of the R_VCSEL, the influence of the τ_c is not a simple time-shift in the time-evolution of the receiver VCSEL, which has serious influence on the complete synchronization quality of each LP mode.     

Atomic Spatial Decoherence in a Squeezed Cavity Field

A moving two-level atom with momentum p is put into a single mode of a cavity field in the squeezed coherent state. An approximate expression of decoherence factor of the space is obtained. Our results show that the decoherence factor is mainly determined by two elements: the coherent parameter β and the squeezed parameter r, and the effects of them are discussed in detail, respectively.     

Feedback coupling loss in single mode fiber lasers

Based on the expansion of the fundamental mode LP₀₁ in single mode fiber in terms of Laguerre-Gaussian free space modes, the feedback coupling losses for two different types of cavity mirror, i.e., a curvature mirror and a combing mirror of a lens and a plane mirror, are numerically calculated for the first time. The results show that, for the curvature mirror, the lowest coupling loss is obtained when its curvature radius matches the wavefront curvature. In particular, if a plane mirror is used as the cavity mirror, it has to be placed close to the fiber end to obtain the low coupling loss. For the combing mirror, the lowest coupling loss can be obtained when the plane mirror is placed at the back focal plane of the lens, and the variation of the coupling loss is insensitive to the mirror positions for the lens with longer focal length. Finally, the plane mirror and the combing mirror of a lens and a plane mirror are suggested to be the cavity mirror in the practical construction of the high power fiber lasers.     

双信道偏振复用保密通信系统的完全混沌同步的操控性研究

针对双信道偏振复用保密通信系统,利用线性电光效应,提出了一种新的完全混沌同步的控制方案.在该方案中,每一个线性偏振模的完全混沌同步质量随外加电场成准周期性变化.其变化规律:完全混沌同步?剧烈振荡;固定一定的外加电场,电光调制促使其完全混沌同步质量对偏置电流和反馈强度的稳健性获得极大的加强.在较大的偏置电流和反馈强度范围内,每一个线性偏振模能够实现完全混沌同步,并且调制到每一个线性偏振模的加密信号基本上能够恢复.     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Quantum Speed Limit Time of a Two-Level Atom under Homodyne-Mediated Feedback and Classical Driving

International Journal of Theoretical Physics - We study the QSLT of the two-level system coupling to a single mode cavity which is driven by a laser field under the feedback control, and focus on...     

Generation and Preparation of the Sustained Optimal Entropy Squeezing State of a Motional Atom Inside Vacuum Cavity

Considering two two-level atoms initially in Bell state, we send one atom into a vacuum cavity while leaving the other outside, and consider the motion of atom inside the cavity. Using quantum information entropy squeezing theory, the time evolution of the entropy squeezing factor of atom inside the cavity is discussed for two cases, i.e., before and after performing rotation operations and measuring atom outside, the influences of the field mode structure and atomic motions on the atomic entropy squeezing are evaluated. It is shown that atom inside the cavity has no entropy squeezing phenomenon before operating atom outside the cavity. However, the optimal entropy squeezing phenomenon of period T = 2π/p emerges and constant entropy squeezing phenomenon can occur by adjusting rotation operation to R(π/4), and setting the field mode structure parameter 0 < p < 50. In particular, if choosing p > 50, a sustained optimal entropy squeezing state (SOESS) can be generated. We also present the schematic circuit diagram of preparation of SOESS. Our proposal provides a possible way for the initial decoherent state recovering into sustained maximal coherent superposition state of single atom in the quantum noise environment.     

Solvable dynamics of N driven two-level atoms coupled to a dissipative cavity mode

We solve exactly the dynamics of N strongly driven two-level atoms equally coupled on resonance to a dissipative cavity mode. Analytical results are derived on decoherence, entanglement, purity, atomic correlations and cavity field mean photon number. Decoherence-free subspaces are predicted for the whole system and the N-qubit subsystem. Multi-partite entangled states and cavity cat-like states can be conditionally generated. The decay of quantum coherence and purity can be monitored by joint measurements on atomic populations. Atoms prepared in states invariant under permutation of any two components evolve within the subspace spanned by the completely symmetric Dicke states. Applications to N = 3, 4 are discussed.     

二极管激光阵列在Talbot外腔中同相模的选择

 从理论和实验上对二极管激光阵列在Talbot外腔中的锁相进行了研究。在同相模和异相模近场分布的基础上，利用1维情况下的菲涅耳衍射公式计算了其远场分布。根据同相模和异相模在Talbot腔中的分布特性，采用1/2 Talbot腔并将外腔镜倾斜一个角度a的方法既能选择同相模，又能使模式的功率损耗最小。二极管激光阵列芯片采用CD金刚石材料,“三明治”结构对其进行封装，明显地减小了阵列的“smile”效应。在实验中实现了二极管激光阵列同相模的锁相输出，远场单瓣模的半高全宽为0.11 mm。     

Micromechanical Photonics: Microresonator Displacement Measurement by an Extremely-Short-Length External Cavity Laser Diode

A composite cavity laser diode was used to monitor the displacement of the external cavity microresonator from the phase difference in the near field. The carrier-to-noise ratio was very high (more than 45 dB) because of the lack of mode hopping noise due to the extremely short (less than 3 μm) external cavity length and strong optical feedback. The small distance (near field) allows a lensless system, making it easier to integrate on a gallium arsenide (GaAs) substrate.