Watt-level single-frequency tapered amplifier laser using a narrowband interference filter.

We demonstrate a tunable external cavity tapered amplifier laser (ECTAL) using a narrowband interference filter as the wavelength discriminator. The laser is tunable over a wavelength range from 1006 to 1031 nm with an output power of ∼1 W. The amplified stimulated emission of the laser system is suppressed to better than 32 dB. The laser is applied to study the saturation spectroscopy on the R(39) 57-0 line of iodine molecule, which, to our best knowledge, is the first measurement of this line close to the dissociation limit. The linewidth of the a1 component is ∼2 MHz at the iodine vapor pressure of ∼11 Pa, and the pressure-broadening coefficient is ∼156 kHz/Pa. This laser system is also used for the injection seeding of a 1030 nm disk laser to perform hyperfine spectroscopy of muonic hydrogen. To reach a satisfactory condition for disk laser use, the ECTAL is successfully stabilized to the iodine Doppler-free spectroscopy of the P(26) 43-0 line near 515 nm, with continuous locking over 48 h.

Absolute frequency measurement of molecular iodine hyperfine transitions at 647 nm.

We report absolute frequency measurements of molecular iodine P(46) 5-4 a1, a10, and a15 hyperfine transitions at 647 nm with a fiber-based frequency comb. The light source is based on a Littrow-type external-cavity diode laser. A frequency stability of 5×10-12 at a 200 s integration time is achieved when the light source is stabilized to the P(46) 5-4 a15 line. The pressure shift is determined to be -8.3(7) kHz/Pa. Our determination of the line centers reached a precision of 21 kHz. The light source can serve as a reference laser for lithium spectroscopy (2S→3P).

Precision saturated absorption spectroscopy of H3.

In our previous work on the Lamb-dips of the ν2 fundamental band transitions of H3+, the saturated absorption spectrum was obtained by third-derivative spectroscopy using frequency modulation with an optical parametric oscillator (OPO). However, frequency modulation also caused errors in the absolute frequency determination. To solve this problem, we built a tunable offset locking system to lock the pump frequency of the OPO to an iodine-stabilized Nd:YAG laser. With this improvement, we were able to scan the OPO idler frequency precisely and obtain the saturated absorption profile using intensity modulation. Furthermore, ion concentration modulation was employed to subtract the background noise and increase the signal-to-noise ratio. To determine the absolute frequency of the idler wave, the OPO signal frequency was locked to an optical frequency comb. The absolute frequency accuracy of our spectrometer was better than 7 kHz, demonstrated by measuring the wavelength standard transition of methane at 3.39 µm. Finally, we measured 16 transitions of H3+ and our results agree very well with other precision measurements. This work successfully resolved the discrepancies between our previous measurements and other precision measurements.

Iodine-stabilized single-frequency green InGaN diode laser.

A 520-nm InGaN diode laser can emit a milliwatt-level, single-frequency laser beam when the applied current slightly exceeds the lasing threshold. The laser frequency was less sensitive to diode temperature and could be finely tuned by adjusting the applied current. Laser frequency was stabilized onto a hyperfine component in an iodine transition through the saturated absorption spectroscopy. The uncertainty of frequency stabilization was approximately 8×10-9 at a 10-s integration time. This compact laser system can replace the conventional green diode-pumped solid-state laser and applied as a frequency reference. A single longitudinal mode operational region with diode temperature, current, and output power was investigated.

Cavity-enhanced Faraday rotation measurement with auto-balanced photodetection.

Optical cavity enhancement for a tiny Faraday rotation is demonstrated with auto-balanced photodetection. This configuration is analyzed using the Jones matrix formalism. The resonant rotation signal is amplified, and thus, the angular sensitivity is improved. In the experiment, the air Faraday rotation is measured with an auto-balanced photoreceiver in single-pass and cavity geometries. The result shows that the measured Faraday rotation in the single-pass geometry is enhanced by a factor of 85 in the cavity geometry, and the sensitivity is improved to 7.54×10(-10) rad Hz(-1/2), which agrees well with the Jones matrix analysis. With this verification, we propose an AC magnetic sensor whose magnetic sensitivity is expected to achieve 10 pT Hz(-1/2).

Optimal power split ratio for autobalanced photodetection.

The noise suppression of the autobalanced photoreceiver devised by Hobbs [Proc. SPIE1376, 216 (1990)] had been determined to depend on the photocurrent ratio of reference beam to signal beam under the condition of constant signal beam photocurrent, and the best noise cancellation was suggested at a ratio close to 2. But in most applications, the available optical power has a limit. Therefore, to optimize the sensitivity of measurements, we should consider how to allocate the beam power in the case of fixed total optical power. In this paper, we measure the air Faraday rotation at different azimuth angles of beam polarization, which correspond to different photocurrent ratios. The signal-to-noise ratio at each photocurrent ratio is determined, and the best sensitivity appears at the photocurrent ratio of 1. This best sensitivity achieved is 3.02×10(-8) rad Hz(-1/2), which is about 1.3 times the shot noise limit. Our results are useful for sensitive optical measurements with the autobalanced photoreceiver.

Precision frequency metrology of helium 2(1)S(0)→2(1)P(1) transition.

We report a precision frequency measurement of the (4)He 2(1)S(0)→2(1)P(1) transition at 2058 nm. The saturated absorption spectroscopy is performed in a rf discharge sealed-off cell with a volume Bragg grating-based Tm:Ho:YLF laser. The absolute transition frequency measured using a fiber optical frequency comb is 145 622 892 822 (183) kHz with a relative uncertainty of 1.3×10(-9). Our result is ten times more precise than current best theoretical calculations and is in reasonable agreement with the calculated values. However, the ionization energy of the 2(1)P(1) state, derived from our result and other precisely measured transitions, shows a discrepancy of approximately 3.5σ with the most precise atomic theory. We have also determined the isotope shift between (3)He and (4)He to be 4248.7 (5.3) MHz, which is more precise than the previous measurement by one order of magnitude.

Widely tunable difference frequency generation source for high-precision mid-infrared spectroscopy.

We have developed a widely tunable mid-infrared difference frequency generation (DFG) source by mixing ~ 1 W Ti:sapphire laser and 6 W Nd:YAG laser beams in a 50-mm MgO-doped long periodically poled lithium niobate (MgO:PPLN). The power of the DFG source is > 2 mW over the tuning range of 2.66-4.77 µm and its free-running linewidth is about 100 kHz. Combining various frequency stabilisation schemes for the Nd:YAG laser and the Ti:sapphire laser, the DFG frequency can be precisely controlled. Besides, its frequency can be determined better than 12 kHz by measuring the Ti:sapphire laser frequency using an optical frequency comb. Two high resolution spectroscopic studies on (12)C(16)O(2) molecule are demonstrated using this DFG source. The saturation spectra of R(18) and R(60) transitions of 00(0)1 ← 00(0)0 fundamental band at 4.2 µm and P(20) transition of [10(0)1, 02(0)1](I) ← 00(0)0 band at 2.7 µm have been observed and their absolute transition frequencies are measured with an accuracy better than 30 kHz.

Precise frequency measurements of iodine hyperfine transitions at 671 nm.

We report absolute frequency measurements on the a(1), a(10), and a(15) hyperfine components of the R(78) 4-6 line of (127)I(2). An external-cavity diode laser system at 671 nm is frequency-stabilized to the saturated absorption center obtained by modulation transfer spectroscopy in an iodine vapor cell. Its absolute frequency is measured by an optical frequency comb. The effect of pressure shift is investigated to obtain the absolute transition frequency at zero pressure. Our determination of the line centers reaches a precision of better than 40 kHz and will provide useful input for theoretical calculations. This frequency-stabilized laser can be used as a reference laser for the spectroscopy of lithium D lines.

Saturation spectroscopy of CO2 and frequency stabilization of an optical parametric oscillator at 2.77 µm.

We report the frequency stabilization of a CW single-frequency, singly resonant optical parametric oscillator (OPO) to the saturation absorption center of the (12)C(16)O2[10°1,02°1](II)>←00°0 P(14) line at 2.77 µm. The CO2 molecules were excited by the OPO idler wave, and the absorption signal was monitored through the fluorescence at 4.3 µm using a gold-coated longitudinal cell. The idler frequency was stabilized onto the line center by wavelength modulation method. The linewidth of the saturation dip was estimated to be 4.7 MHz, and the achieved frequency stability was 3.9 kHz (3.6×10(-11)).

High-resolution sub-Doppler Lamb dips of the ν2 fundamental band of H3(+).

The high-resolution sub-Doppler Lamb dips of the ν2 fundamental band transitions of H3(+) have been observed using an extended negative glow discharge tube as an ion source and a periodically poled lithium niobate optical parametric oscillator as a radiation source. The absolute frequency of the R(1,0) transition was measured to be 81,720,371.550 MHz with an accuracy of 250 kHz using an optical frequency comb. In addition, we have investigated the linewidth of the Lamb-dip signal of the R(3,0) transition systematically and obtained its pressure-broadening parameter, which may shed some light on the reaction of H3(+) with H2. This is the first observation of the infrared saturated spectrum and the first determination of the pressure-broadening parameter of the ro-vibrational transitions of a molecular ion.

Single frequency 1070 nm Nd:GdVO4 laser using a volume Bragg grating.

We demonstrate a single frequency diode-pumped Nd:GdVO(4) laser at 1070 nm using a volume Bragg grating as the output coupler of a short plano-concave cavity. The TEM(00) output had a maximum power of 300 mW and a linewidth less than 23 MHz. The beam propagation parameter M2 and the divergence angle at 200 mW were 1.2 and 0.37°, respectively. The single frequency tuning range was 5.1 GHz at 100 mW. Upon locking the laser frequency to a confocal reference cavity, a relative stability of 7.58 kHz was achieved. If frequency doubled, such as using a periodically-poled lithium niobate (PPLN) crystal, this laser offers an excellent light source for parity non-conservation experiments of atomic thallium.

Sensitive Faraday rotation measurement with auto-balanced photodetection.

A magneto-optic polarimetry based on auto-balanced photodetection is investigated. In this experiment, a commercial auto-balanced photoreceiver is adopted to measure the Faraday rotation of air. With a proper setup to utilize its noise cancellation capability, the measurement can be flexible and sensitive. The angular sensitivity is 2.99×10(-8) rad Hz(-1/2), which is about 2.7 times the shot noise limit. The measured Verdet constant of air is +1.39×10(-9) rad G(-1) cm(-1) at 634.8 nm. Significantly we applied a small AC current to induce the magnetic field, so there was no heating in the coil. In addition, a double current modulation scheme was used to demonstrate that there was no zero drift and amplifier instability in the measurement. The possibility of improvement of the angular sensitivity and the potential applications are also discussed.

In-situ monitoring of pattern filling in nano-imprint lithography using surface plasmon resonance.

Nano-imprint lithography possesses the advantages of high throughput, sub-10-nm feature and low cost. In NIL, the mold filling is subjected to the applied imprinting pressure, temperature and time. Incomplete mold filling causes a detrimental effect on the final imprinted pattern dimensions. The monitoring system of imprinting is essential to control the imprinting parameters properly. Up to now, no high-sensitivity monitoring of filling rate and end point has ever been proposed. In this study, the authors apply the surface plasmon resonance to monitor the filling rate and end point during imprint process. The mold contains a layer of glass of high refractive index, a metal thin film and the pattern of low refractive index. In addition, the imprinted polymer is selected considering its refractive index, which should be lower than the glass layer of mold. When the filling rate varies, it will affect the SPR behavior, including the measurable reflectivity change and resonance angle shift. The analysis results reveal that the resonance angle is truly proportional to the filling rate. When the filling rate varies from 50% to 100%, the SPR angle shifts more than 5 degree. The analysis demonstrates this innovative method for monitoring of filling rate is effective with high sensitivity.

Broadband micro-Michelson interferometer with multi-optical-path beating using a sphered-end hollow fiber.

We demonstrate a high-sensitivity broadband (1250-1650 nm) fiber micro-Michelson interferometer using a single-mode fiber end-spliced with a sphered-end hollow-core fiber. The hollow core is slightly smaller than the solid core of a single-mode fiber, so the fractional power of the core mode is converted into cladding modes. The excited cladding modes propagate at distinct optical paths along the hollow-core fiber and have individual foci outside the spherical lens. The reflected core mode, generated at the solid core-air interface, and the reflected cladding modes, generated at external material, interfere with each other to produce beating in the interference signals.

Frequency stabilization of an external cavity diode laser to molecular iodine at 657.483 nm.

The saturation spectrum of the P(84) 5-5 transition of 127I2 at 657.483 nm is obtained with the third-harmonic demodulation method using an external cavity diode laser. The laser frequency is modulated by modulating the diode current instead of modulating the cavity length with a piezoelectric transducer (PZT). Current modulation allows a modulation frequency that is higher than PZT modulation. The signal-to-noise ratio of 1000 is better than previous results presented in the literature. The laser is frequency stabilized to the hyperfine component o of the P(84) 5-5 transition with a frequency stability of better than 10 kHz (2.2 x 10(-11) relative stability).

Absolute frequency measurement of rubidium 5S-7S two-photon transitions with a femtosecond laser comb.

The absolute frequencies of rubidium 5S-7S two-photon transitions at 760 nm are measured to an accuracy of 20 kHz with an optical frequency comb based on a mode-locked femtosecond Ti:sapphire laser. The rubidium 5S-7S two-photon transitions are potential candidates for frequency standards and serve as important optical frequency standards for telecommunication applications. The accuracy of the hyperfine constant of the 7S1/2 state is improved by a factor of 5 in comparison with previous results.

Iodine stabilization of a diode laser in the optical communication band.

The iodine molecule has frequently been used as a frequency reference from the green to the near-infrared wavelength region (500-900 nm). We describe the frequency locking of the second-harmonic signal of a 197.2-THz (1520.25-nm) distributed-feedback diode laser to the absorption lines of the iodine hyperfine structure; a frequency jitter below 0.1 MHz was achieved at a 300-ms time constant. This scheme provides a simple, compact, and high-performance frequency reference in the optical communication band.

Frequency-stabilized 1520-nm diode laser with rubidium 5S(1/2) --> 7S(1/2) two-photon absorption.

A 760-nm light source of >10 mW is obtained from a frequency-doubled external-cavity diode laser by use of using an erbium-doped fiber amplifier and a periodically poled lithium niobate waveguide. The 5S(1/2) --> 7S(1/2) two-photon transitions of rubidium are observed with such a light source. This laser frequency is locked to the Rb two-photon transitions with an instability of 10 kHz (1 s). Our experimental scheme provides a compact, high-performance frequency, standard in the S band (1480-1530 nm) for fiber-optic communication and sensing applications.