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A frequency-modulated continuous-wave (FMCW) lidar is demonstrated with heterodyne detection. The lidar transmitter utilizes an electro-optic I∕Q modulator for the first time to generate carrier-suppressed and frequency-shifted FM modulation. This eliminates the need for an acousto-optic frequency shifter commonly used in heterodyne lidar transmitters. It also allows the use of a much wider modulation bandwidth to improve the range resolution. The capability of complex optical field modulation of the I∕Q modulator provides an additional degree of freedom compared with an intensity modulator, which will benefit future lidar applications. © 2012 Optical Society of America OCIS codes: 060.2300, 280.3640, 120.4640.Lidar systems have been widely used for measuring range, velocity, vibration, and air turbulence [1][2][3]. Coherent detection has become a major detection mechanism in lidar systems because of the much improved receiver sensitivity compared with direct detection. While homodyne detection is susceptible to the phase noise of the optical carrier and the associated signal fading, heterodyne detection is a more robust detection scheme for many practical lidar systems [4,5]. In a heterodyne receiver, mixing between the optical signal and the local oscillator (LO) shifts the optical signal to an intermediate frequencyIn practical implementation of a lidar system, the signal and the LO are usually split from the same laser. While the signal is modulated by an electro-optic modulator, the LO has to go through an acousto-optic modulator (AOM) to create the required frequency shift f IF . In addition to the increased complexity, the frequency shift of an AOM is usually not more than 1 GHz. Although the bandwidth of a practical electro-optic modulator can be higher than 40 GHz, to avoid spectral aliasing, the usable modulation bandwidth of a heterodyne detection lidar is limited by the IF frequency, which is determined by the AOM. In recent years, the rapid advance in coherent fiber-optic communication has created high-speed electro-optic in-phase/ quadrature (I∕Q) modulators, which are capable of modulating the amplitude and the phase of the optical carrier independently. Despite the popular application of I∕Q modulators in optical communication systems, their application in lidar has not been reported. In this paper, we demonstrate a frequency-modulated continuous-wave (FMCW) lidar system using heterodyne detection. An I∕Q modulator is used to simultaneously generate the linear FM chirp required for pulse compression and the IF frequency shift required for heterodyne detection. Therefore, the optical frequency of the LO does not have to be shifted, and the AOM is not required. Based on a commercial transmitter originally designed for a 10 Gb∕s optical communication system, the lidar system allows unprecedented chirping bandwidth to ensure the fine range resolution, while the optical system configuration is simplified.For an FMCW lidar system [6], a linear frequency chirp is applied across each optical pulse. The r...

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A coherent lidar system based on the measurement of complex optical field is demonstrated for the first time. An electro-optic inphase/quadrature (I/Q) modulator is used in the lidar transmitter to realize carrier-suppressed complex optical field modulation in which the positive and the negative optical sidebands can carry independent modulation waveforms. A fiber-optic 90° hybrid is used in the lidar receiver for coherent heterodyne detection and to recover the complex optical field. By loading a constant modulation frequency on the lower optical sideband and a wideband linear frequency chirp on the upper sideband, vector velocity and target distance can be measured independently. The wide modulation bandwidth of this lidar system also enabled unprecedented range resolution and the capability of measuring high velocity unambiguously.

C(sp 3 )ÀHf unctionalization ·c arbene analogues ·carbocations ·C À Hinsertion ·v inyl cations Carbocations are important reactive intermediates for Highlights Highlights

Accurate quantum-chemical computations,c ombined with SAPT and NBO analyses,u nveiled the intermolecular interactions occurring in cycloalkene-water,C n H 2nÀ2 -H 2 O (n = 3-7), adducts,thus pointing out the dominant O À H···p C=C hydrogen bond. For n ! 5, the members of this series show very similar intermolecular interactions and dissociation energies,w ith the primary contact being also assisted by aw eaker CÀH···O hydrogen bond. Based on this consideration, the cyclopentene-water complex was subsequently fully characterized by combining rotational spectroscopy in supersonic expansion with the capability of state-of-the-art quantum-chemical computations in accurately determining structural and energetic properties.T he assignment of the rotational spectra for four isotopologues allowed the determination of an accurate semi-experimental equilibrium structure for the heavy-atom backbone of the molecular adduct.

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