Single-mode Light Research Articles

Performance characteristics of external cavities to generate deep-ultraviolet coherent lights resonant to 3p3P14s3P0 cyclic transition of 28Si

We developed a robust 252 nm single-mode coherent continuous-wave light source through two-stage highly efficient frequency conversion systems using external cavities. In the first stage, we obtained the second harmonics (SH) power of 630 mW from the LBO-installed external cavity by frequency doubling of a 746 nm Ti: sapphire laser with an efficiency of almost 50% [Y. Asakawa, H. Kumagai, K. Midorikawa, M. Obara, Opt. Commun. 217 (2003) 311.]. In the second stage, 252 nm light power was obtained by doubly resonant sum-frequency mixing of 373 nm light from the first-stage conversion system and 780 nm light from another Ti:sapphire laser. In the sum-frequency external cavity, the maximum 252 nm light power of over 150 mW is obtained, which is about three times larger than that obtained in our previous experiment.

Dark spatial optical solitons in planar gradient waveguides in the Z-cut of the 3m symmetry crystals

The propagation of light beams is studied in a planar photorefractive waveguide fabricated by high-temperature diffusion of metal ions in the Z-cut substrate of the 3m symmetry crystal. The wave equations are obtained for single-mode light beams with TE and TM polarisations in planar diffusion waveguides, which take into account the two-dimensional distribution of the optical field. Expressions are found for a nonlinear change in the refractive index when the photovoltaic mechanism makes a dominant contribution to the photorefractive effect. The propagation of single-mode light beams is analysed numerically for a Ti:Fe:LiNbO3 waveguide fabricated by the successive diffusion of titanium and iron into lithium niobate. It is shown that single-mode light beams with a smooth amplitude envelope can propagate without significant changes in the region of a dip in the intensity modelling a dark soliton. The relations between the amplitude and width of a dark spatial soliton are obtained for the TM modes of a photorefractive planar waveguide.

Efficient sum-frequency generation of continuous-wave single-frequency coherent light at 252 nm with dual wavelength enhancement

Highly efficient frequency conversions were conducted to obtain deep-ultraviolet single-mode coherent light by use of two-stage external cavities. A power of 154 mW at approximately 252 nm was obtained with a conversion efficiency of more than 8% by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a single-mode Ti:sapphire laser. The output performance of the deep-ultraviolet light source is sufficient for use in the laser cooling of neutral silicon atoms.

Experimental investigation of a coherent quantum measurement of the degree of polarization of a single-mode light beam

A novel method for the direct measurement of the degree of polarization is described. It is one of the first practical implementations of a coherent quantum measurement, the projection on the singlet state. Our first results demonstrate the successful operation of the method. However, because of the nonlinear crystals used at present, its application is limited to spectral widths larger than about 8 nm.

Precise Beam Shaping of Diode Laser Direct Application of LD Light for Material Processing

Current techniques developed for beam shaping of multi-mode laser diode (LD) output are reviewed and a new approach for the direct micro-processing with a single-mode LD is reported. The advantage of single-mode LD for high brightness focusing, the source of aberration in LD light, and the method to improve the brightness are described. The experiment on the laser ablative shaping (LAS) for single-mode laser light indicated that the initial wavefront distortion of 0.9 λ was improved to 0.4 λ. Possible applications of LD micro-marking system are described.

Experimental investigation of a coherent quantum measurement of the degree of polarization of a single-mode light beam

Abstract A novel method for the direct measurement of the degree of polarization is described. It is one of the first practical implementations of a coherent quantum measurement, the projection on the singlet state. Our first results demonstrate the successful operation of the method. However, because of the nonlinear crystals used at present, its application is limited to spectral widths larger than about 8 nm.

Hollow fibre guides for metastable helium atoms

The realisation of an atomic waveguide will be an important component for the development of future atom optic devices such as large angle atom interferometers. Hollow optical fibres that employ repulsive (blue detuned) evanescent light fields to guide the atoms are one approach to this technology. We report here on multimode guiding of metastable helium ( 2 3S 1 ) atoms through hollow optical fibres that support multimode light fields. In order to produce efficient single-mode atom guiding, hollow fibres supporting single light mode propagation will be required to generate uniform evanescent light fields inside fibre wall. We have recently developed such a fibre and report on the suitability of its light transmission characteristics for atom waveguiding.

Critical Power of Stimulated Brillouin Scattering in Multimode Optical Fibers

A set of coupled-wave equations for stimulated Brillouin scattering (SBS) in multimode optical fibers excited by single-mode laser light is derived. The analytical solution of the critical pump power is also derived. The solution explains the experimental results well.

Stimulated laser Raman processes in low-density Rydberg matter: Wave number and intensity blueshifts

Wave-number blueshifts of single-mode laser light in low-density Rydberg matter are found to be due to stimulated Raman processes. The Rydberg matter is mainly formed from Rydberg states of K atoms. The lasers used are mW continuous-wave lasers in the ir, lead-salt tunable diode lasers. Of special significance is the blueshifting observed as shifts of Fabry-Perot etalon fringes of the order of 0.05 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This blueshifting is explained as due to complete conversion to anti-Stokes radiation through higher-order stimulated Raman shifting by extremely small quanta from the near continuum of energy levels in the metallic Rydberg matter phase. Complicated strong shifts of intensity to larger wave numbers in the laser modes are also observed, due to coupling of laser side modes to the anti-Stokes waves. The high efficiency of the stimulated Raman process is partly due to the weak coupling to other degrees of freedom.

Development of a high-power deep-ultraviolet continuous-wave coherent light source for laser cooling of silicon atoms

We developed a deep-UV single-mode coherent light source through two-stage highly efficient frequency conversions by use of external cavities. In the first stage, second-harmonic power of 500 mW was obtained by frequency doubling of a 746-nm Ti:sapphire laser with a conversion efficiency of 40%. In the second stage, 50-mW power at ~252nm was obtained by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a diode laser. The output performance of this deep-UV light source is sufficient for laser cooling of neutral silicon atoms.

Waveguide properties of a four-layer planar structure with an absorbing coating layer

An exact solution is obtained and a problem of waveguide light propagation in a scalar asymmetric four-layer waveguide with an absorbing coating layer is analyzed numerically. The periodicity of modal characteristics with change in the coating layer thickness is established. The feasibility of achieving a single-mode light propagation regime at the expense of the absorbing coating layer is demonstrated.

The spatial behavior of nonclassical light

Nonclassical effects such as squeezing, antibunching, and sub-Poissonian statistics of photons have been attracting attention in quantum optics over the last decade. Up to now most theoretical and experimental investigations have been carried out exclusively in the time domain while neglecting the spatial aspects by considering only one spatial mode of the electromagnetic field. In many situations such an approximation is well justified. There are, however, problems that do not allow in principle a single-mode consideration. This is the case when one wants to investigate the quantum fluctuations of light at different spatial points in the plane perpendicular to the direction of propagation of the light beam. Such an investigation requires a complete description of quantum fluctuations of light in both time and space and cannot be done within a single-mode theory. This space-time description brings about a natural generalization into the spatial domain of such notions as the standard quantum limit, squeezing, antibunching, etc. It predicts, for example, the possibility of generating a light beam with sub-Poissonian statistics of photons not only in time but also in the beam's transverse plane. Of particular relevance to the applications is a situation in which the cross section of the light beam contains several nonoverlapping areas with sub-Poissonian statistics of photons in each. Photodetection of such a beam produces several sub-shot-noise photocurrents depending on the number of independent areas with sub-Poissonian statistics. This is in marked contrast to the case of a single-mode sub-Poissonian light beam in which any attempt to collect light from only a part of the beam deteriorates the degree of shot-noise reduction. This property of multimode squeezed light opens a range of interesting new applications in optical imaging, optical parallel processing of information, parallel computing, and many other areas in which it is desirable to have a light beam with regular photon statistics across its transverse area. The aim of this review is to describe the recent development in this branch of quantum optics.

Statistical uncertainty in quantum-optical photodetection measurements

Abstract We present a complete statistical analysis of quantum-optical measurement schemes based on photodetection. Statistical distributions of quantum observables determined from a finite number of experimental runs are analytically characterized with the help of the generating function, which is derived using the exact statistical description of raw experimental outcomes. We use the developed formalism to point out that the statistical uncertainty results in substantial limitations on the information determined about the quantum state; although a family of observables characterizing the quantum state can be safely evaluated from experimental data, its further use to obtain the expectation value of some operators generates exploding statistical errors. These issues are discussed using the example of phase-insensitive measurements of a single light mode. We study reconstruction of the photon number distribution from photon counting and from random-phase homodyne detection. We show that utilization of the reconstructed distribution to evaluate a simple well behaved observable, namely the parity operator, encounters difficulties owing to accumulation of statistical errors. As the parity operator yields the Wigner function at the phase space origin, this example also demonstrates that transformation between various experimentally determined representations of the quantum state is quite a delicate matter.

Optimal receiver for quantum cryptography with two coherent states

A setup for discriminating between arbitrary two coherent states of a single light mode with the highest success rate allowed by quantum mechanics is presented. Its application to time-multiplexed quantum key distribution is discussed.

Amplitude-squared squeezing in two-photon resonance fluorescence

Interaction of single-mode coherent light with dipole-forbidden two-level atoms via non-resonant virtual level is studied in this paper. It is shown that, in the case of two-photon resonance fluorescence, the atomic system generates three bands of correlated photon pairs with the carrier frequencies ω k 0 , ω k 0 ± Γ, where Γ=[Ω 2+(Δ/2) 2] 1/2 , 2Ω is the two-photon Rabi frequency and Δ is the detuning from the two-photon resonance. It is established that amplitude-squared squeezing occurs from the sidebands of the two-photon resonance fluorescence spectrum. When the atom number increases, the amplitude-squared squeezing also increases.

Statistical uncertainty in quantum-optical photodetection measurements

We present a complete statistical analysis of quantum-optical measurement schemes based on photodetection. Statistical distributions of quantum observables determined from a finite number of experimental runs are analytically characterized with the help of the generating function, which is derived using the exact statistical description of raw experimental outcomes. We use the developed formalism to point out that the statistical uncertainty results in substantial limitations on the information determined about the quantum state; although a family of observables characterizing the quantum state can be safely evaluated from experimental data, its further use to obtain the expectation value of some operators generates exploding statistical errors. These issues are discussed using the example of phase-insensitive measurements of a single light mode. We study reconstruction of the photon number distribution from photon counting and from random-phase homodyne detection. We show that utilization of the reconstructed distribution to evaluate a simple well behaved observable, namely the parity operator, encounters difficulties owing to accumulation of statistical errors. As the parity operator yields the Wigner function at the phase space origin, this example also demonstrates that transformation between various experimentally determined representations of the quantum state is quite a delicate matter.

Synchro-scanned rotating tunable optical disk filter for wavelength discrimination

We propose a novel wavelength-discrimination scheme using a rotating tunable optical disk filter with a circularly graded cavity thickness. The transmittance peak of the filter varies linearly with the angular position of the beam on the disk while the bandwidth remains constant. The rotation is synchronized to an electrical signal, enabling temporal measurement of an optical spectrum. The first experimental demonstration was performed at 670 r/m using a filter that ranged from 1560 to 1615 nm under a 3-dB bandwidth of 5 nm. Single-mode laser light was correctly discriminated. Good linearity of the transmittance-peak variation was confirmed by analyzing the measured trace waveform.

Accurate Stabilization of a 3 mW Single-Mode Output He-Ne Laser by Intermittent Frequency Offset Locking to an Iodine Stabilized He-Ne Laser

A high-power, unmodulated frequency standard laser light with low running cost is a powerful tool for industrial measurement involving laser calibration and interferometry in length and shape. To obtain such a laser light, we developed an intermittent frequency offset locked laser (IFOL laser). This laser system, composed of an iodine stabilized He-Ne laser (I2SL) and a high-powered symmetric three-mode He-Ne laser (HP-3ML, optical output=5 mW), provides two stabilization operational modes: (1) independent mode (I-mode) and (2) slave mode (S-mode). Utilizing the frequency pulling effect, HP-3ML is stabilized at the condition of symmetric three-mode oscillation, and a 3 mW single-mode light with frequency instability of 10-12 is obtained (I-mode). When the optical frequency deviates from the allowed limit during I-mode, HP-3ML is temporarily offset-locked to I2SL (S-mode) with frequency resettability of 10-10 and frequency instability of 10-12. After completion of the drift reset in S-mode, HP-3ML is stabilized in I-mode again, and then I2SL is turned off. In a typical case of allowed drift in ±1 MHz, repetitive operation of I-mode for 10 days and S-mode for 5 min was satisfactory. Such an intermittent operation reduces the working time of I2SL dramatically, resulting in low running cost that enables long-term use in industrial measurement.

Dynamic light scattering with single-mode receivers: partial heterodyning regime

A frequent source of errors in dynamic light-scattering experiments is partial heterodyning caused by scattering on large particles or imperfections of the sample cell. With a conventional two-pinhole receiver it is impossible to distinguish its effect from the effects of a finite detector area and detector nonlinearity. However, an accurate data analysis is feasible when a single-mode light receiver is employed. We present formulas for single-mode autocorrelation and cross-correlation functions that include a local oscillator and an incoherent background of arbitrary strength and take into account detector nonlinearity (e.g., dead time) up to second order. A simple but accurate method for the determination of the nonlinearity parameters and the effective number of receiver modes is also provided. The success of the data-evaluation procedure is demonstrated by the measurement of the hydrodynamic radius of latex in the presence of deliberately added local-oscillator or incoherent-background contributions.

Intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser

There is a need for an intense, unmodulated single-frequency stabilized laser light that guarantees absolute optical frequency in a rapid laser calibration or an ultra-high resolution interferometer. To obtain such a light, we developed a new laser system that uses an intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser. The proposed laser system provides two operational modes: (1) independent and (2) slave mode. In the independent mode, frequency of the three-mode laser is stabilized via control of frequency difference between two intermode beats. The resultant output is a single longitudinal mode light of maximum intensity that locates at the top of the gain curve. Frequency instability of 8X10-12 (at a sampling time of 100 s) which is better than conventional stabilized lasers is attained in the independent mode. Slow optical frequency drift during the independent mode is periodically corrected by the offset lock to the iodine stabilized laser (slave mode), resulting in accurate reset of the frequency drift. After reset of the frequency deviation, the three-mode laser is again operated in the independent mode. Due to such intermittent offset lock, duty factor of the iodine stabilized laser was reduced to a few % of continuous operation.