Lisheng Chen

Sub-Doppler molecular-iodine transitions near the dissociation limit (523-498 nm).

A widely tunable and high-resolution spectrometer based on a frequency-doubled Ti:sapphire laser was used to explore sub-Doppler transitions of iodine molecules in the wavelength range 523-498 nm. The wavelength dependence of the hyperfine transition linewidth of iodine was mapped out in this region, and the narrowest linewidth was ~4 kHz near 508 nm. The hyperfine-resolved patterns were found to be largely modified toward the dissociation limit. The observed excellent signal-to-noise ratio should lead to high-quality optical frequency standards that are better than those of the popular 532-nm system.

Measurement and control of residual amplitude modulation in optical phase modulation.

Residual amplitude modulation is one of the major sources of instability in ultra-sensitive optical detections based on frequency modulation. Using a MgO·LiNbO(3) electro-optic crystal, we systematically measure the temperature and polarization dependence of residual amplitude modulation and our experimental results are in good agreement with a previous theoretical analysis. After optical phase modulation, two independent arms including optical detection and frequency demodulation are employed to closely examine the instability of the residual amplitude modulation. Residual amplitude modulation below 25 ppm is obtained with an active cancellation scheme in which the crystal temperature is varied so as to zero the baseline drifts with different origins. Possible improvements for better suppression and stability are discussed.

Hyperfine interactions and perturbation effects in the B0u+(3Πu) state of 127I2

We report new measurements of the hyperfine spectra of B←X transitions in the wavelength range 500–517 nm. Four effective hyperfine parameters, eqQB, CB, dB, and δB, are determined for an extensive number of rovibrational levels spanning the intermediate region 42≤v′≤70 in the electronically excited B0u+(3Πu) state. Second-order perturbation accounts for most of the observed rovibrational dependence of the hyperfine interactions. In addition, it was found that, near vibrational levels v′=57–60, the 1g(1Πg) electronic state strongly perturbs the B state through rotational coincidence, leading to effects such as abnormal variations in the hyperfine parameters and strong u–g mixing recorded for the transition P(84)60–0. Various perturbation effects in the B state identified so far are summarized. Also, the radial dependence of the hyperfine interactions was examined by removal of the vibrational average in the hyperfine parameters.

Characterization of the molecular iodine electronic wave functions and potential energy curves through hyperfine interactions in the B0+_u(3Pi_u) state

We present a high-resolution analysis of the six electronic states that share the same dissociation limit with the second excited electronic state B in molecular iodine. These six states are coupled to the B state via hyperfine interactions. The four hyperfine parameters, C_B, delta_B, d_B, and eqQ_B, are calculated with the available potential energy curves and wave functions constructed from the separated-atom basis set. We obtain a maximum separation of the respective contributions from all six electronic states and compare each individual contribution with high-precision spectroscopic data, providing an independent verification of the relevant electronic structure.

Prototyping a compact system for active vibration isolation using piezoelectric sensors and actuators

Being small in size and weight, piezoelectric transducers hold unique positions in vibration sensing and control. Here, we explore the possibility of building a compact vibration isolation system using piezoelectric sensors and actuators. The mechanical resonances of a piezoelectric actuator around a few kHz are suppressed by an order of magnitude via electrical damping, which improves the high-frequency response. Working with a strain gauge located on the piezoelectric actuator, an auxiliary control loop eliminates the drift associated with a large servo gain at dc. Following this approach, we design, optimize, and experimentally verify the loop responses using frequency domain analysis. The vibration isolation between 1 Hz and 200 Hz is achieved and the attenuation peaks at 60 near vibration frequency of 20 Hz. Restrictions and potentials for extending the isolation to lower vibration frequencies are discussed.

Developing a narrow-line laser spectrometer based on a tunable continuous-wave dye laser

We present the development of a dye-laser-based spectrometer operating at 550-600 nm. The spectrometer will be used to detect an ultra-narrow clock transition ((1)S0-(3)P0) in an Ytterbium optical lattice clock and perform high-resolution spectroscopy of iodine molecules trapped in the sub-nanometer channels of zeolite crystal (AlPO4-11). Two-stage Pound-Drever-Hall frequency stabilization is implemented on the tunable continuous-wave dye laser to obtain a reliable operation and provide stable laser radiations with two different spectral linewidths. In the first-stage frequency locking, a compact home-built intracavity electro-optic modulator is adopted for suppressing fast frequency noise. With an acquisition time of 0.1 s the 670-kHz linewidth of the free-running dye laser is reduced to 2 kHz when locked to a pre-stabilization optical cavity with a finesse of 1170. When the pre-stabilized laser is locked to a high-finesse optical cavity, a linewidth of 1.4 Hz (2 s) is observed and the frequency stability is 3.7 × 10(-15) (3 s). We also measure and analyze the individual noise contributions such as those from residual amplitude modulation and electronic noise. The ongoing upgrades include improving long-term frequency stability at time scales from 10 to 100 s and implementing continuous frequency scan across 10 GHz with radio-frequency precision.

Systematical Measurement of Molecular Iodine Hyperfine Transitions in the Wavelength Range of 500-517 Nm

We report new measurement of the hyperfine spectra of molecular iodine BlarrX transitions in the wavelength range of 500-517 nm. Four effective hyperfine parameters, eqQB, CB, dB , and deltaB are determined for an extensive number of rovibrational levels in the B state, yielding rich information about the rotational and vibrational dependence of the hyperfine interactions. In particular, abnormal variations in the hyperfine parameters and strong u-g mixing at several levels are observed in this intermediate region

Ultra-Precise Phase Control of Short Pulses - Applications to Nonlinear Spectroscopy

Precise phase control of femtosecond lasers has become increasingly important as novel applications utilizing the femtosecond laser-based optical comb are developed that require greater levels of precision and higher degrees of control [1]. Improved stability is beneficial for both “frequency domain” applications, where the relative phase or “chirp” between comb components is unimportant (e.g. optical frequency metrology), and, perhaps more importantly, “time domain” applications where the pulse shape and/or duration is vital, such as in nonlinear optical interactions [2]. For both types of applications, minimizing jitter in the pulse train and noise in the carrier-envelope phase is often critical to achieve the desired level of precision. Phase-stabilized mode-locked femtosecond lasers have played a key role in recent advances in optical frequency measurement [3,4], carrier-envelope phase stabilization [2,5,6], alloptical atomic clocks [7,8], optical frequency synthesizers [9], coherent pulse synthesis [10], and ultra-broad, phase coherent spectral generation [11]. The capability of absolute optical frequency measurements in the visible and IR spectral regions adds a new meaning to the term of precision molecular spectroscopy. Understanding of molecular structure and dynamics often involves detailed spectral analysis over a broad wavelength range. Such a task can now be accomplished with a desired level of accuracy uniformly across all relevant spectral windows, allowing precise investigations of minute changes in the molecular structure over a large dynamic range. For example, absolute frequency measurement of vibration overtone transitions and other related

Absolute frequencies, linewidths, and hyperfine structures of iodine transitions near the dissociation limit (523 to 498 nm)

A widely tunable and high-resolution spectrometer based on a frequency doubled Ti:sapphire laser is used to explore sub-Doppler transitions of iodine molecules in the wavelength range of 523-498 nm. The wavelength-dependence of the hyperfine transition linewidth of iodine is mapped out in this region. Towards the dissociation limit near 500 nm, the hyperfine-resolved patterns are found to be largely modified. A femtosecond optical comb is used to determine the absolute frequency of the 514.7 nm optical standard with more than a 100-fold improvement in precision.