Narrow-linewidth Diode Laser Research Articles

Frequency comb-referenced narrow line width diode laser system for coherent molecular spectroscopy

We analyze in detail the frequency noise properties of a grating enhanced external cavity diode laser (GEECDL). This system merges two diode laser concepts, the grating stabilized diode laser and the diode laser with resonant optical feedback, thus combining a large tuning range with an excellent short-term frequency stability. We compare the frequency noise spectrum of a GEECDL to that of a grating stabilized diode laser and demonstrate a 10-fold reduction of the frequency noise linear spectral density. The GEECDL is phase locked to a similar laser and to a fs-frequency comb with a servo loop providing an open-loop unity-gain frequency of only 237 kHz, which is a tenth of the bandwidth typically required for grating stabilized diode lasers. We achieve a residual rms phase error as small as 72 mrad (≈ 200 mrad) for stabilization to a similar laser (to the fs-frequency comb). We demonstrate that the novel diode laser can phase-coherently track a stable optical reference with an instability of 1.8×10-16 at 1 s. This laser system is well suited for applications that require phase locking to a low-power optical reference under noisy conditions. It may also be considered for the implementation of optical clock lasers.

Optical frequency measurements of6sS1∕22−6pP1∕22(D1)transitions inCs133and their impact on the fine-structure constant

High resolution laser spectroscopy of the $6s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6p\phantom{\rule{0.2em}{0ex}}^{2}P_{1∕2}$ transition (${D}_{1}$ line) in neutral $^{133}\mathrm{Cs}$ is performed in a highly collimated thermal atomic beam by use of a femtosecond laser frequency comb and narrow-linewidth diode laser. The diode laser is offset locked to a single frequency component of the femtosecond laser frequency comb and probes the optical transitions between selected pairs of ground-state and excited-state hyperfine components. A photodiode detects the excited-state decay fluorescence, and a computerized data acquisition system records the signal. The Doppler shift is eliminated by orienting the laser beam in a direction perpendicular to the atomic beam to within a precision of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{rad}$. Optical frequencies for all four pairs of hyperfine components are measured independently, from which the ${D}_{1}$ line centroid and excited-state hyperfine splitting are obtained by least-squares minimization with the ground-state splitting as a fixed constraint. We find the ${D}_{1}$ line centroid to be ${f}_{{D}_{1}}=335\phantom{\rule{0.2em}{0ex}}116\phantom{\rule{0.2em}{0ex}}048\phantom{\rule{0.2em}{0ex}}748.1(2.4)\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$, and the $6p\phantom{\rule{0.2em}{0ex}}^{2}P_{1∕2}$ state hyperfine splitting to be 1 167 723.6(4.8) kHz. These results, in combination with the results of an atom interferometry experiment by Wicht et al. [Phys. Scripta T 102, 82 (2002)], are used to calculate a new value for the fine-structure constant.

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.

Resonant Frequency Control of a Microspherical Cavity by Temperature Adjustment

We demonstrate frequency tuning of whispering-gallery mode resonances in a fused-silica microsphere by means of temperature control. Scattering spectra of the microsphere are measured using a tunable narrow-linewidth diode laser, and the frequency scale is precisely calibrated from a simultaneously observed spectrum of rubidium vapor. The resonant frequencies of whispering-gallery modes can be varied linearly by temperature elevation and lowering, at a rate of -2.6 GHz/K for a 190 µm sphere.

Development of an optical frequency comb around 1556 nm referenced to an Rb frequency standard at 778 nm

To transfer and expand in the optical telecommunication range the accurate 5S1/2(F = 3) – 5D5/2 (F = 5) rubidium frequency reference at 778 nm (385.285 THz), we employ a second harmonic generation (SHG) process coupled to an optical frequency comb (OFC) both based on the use of a low power and narrow linewidth laser diode operating around 1556 nm. The Rb optical frequency reference is known with an accuracy of 2.6 × 10−12 and exhibits a frequency stability of 4 × 10−13τ−1/2, with a Flicker plateau of 2 × 10−14 for τ > 200 s. With 20 mW of laser power at 1556 nm, we perform a SHG process in a periodically poled lithium niobate (PPLN) crystal and we obtain 6.5 μW of harmonic radiation at 778 nm, used to phase lock the IR laser against a Rb optical frequency standard (OFS). This Rb-referenced fundamental radiation at 1556 nm is duplicated as 34 lines spaced by 10 GHz using an electro-optic phase modulator (EOM). This preliminary result is mainly limited by unexpected high optical losses in the Fabry Perot cavity, which comprises the EOM.

Lithium isotope separation with tunable diode lasers.

A laser-isotope-separation study of lithium has been performed with two-step excitation involving UV laser radiation and a visible tunable-diode laser. The method yields a high degree of selectivity by tuning the narrow-linewidth diode laser to the D1 or D2 levels of the lithium atom. Selective laser excitation is simplified by the use of the tunable diode laser and the overall approach benefits from the application of a compact mass selector that includes a precision magnetic sector and an ion beam that is designed specifically for light atoms such as lithium.

Characterization of a high-power tapered semiconductor amplifier system

We have characterized a semiconductor amplifier laser system which provides up to 200 mW output after a single-mode optical fiber at 780 nm wavelength. The system is based on a tapered semiconductor gain element, which amplifies the output of a narrow-linewidth diode laser. Gain and saturation are discussed as a function of operating temperature and injection current. The spectral properties of the amplifier are investigated with a grating spectrometer. Amplified spontaneous emission (ASE) causes a spectral background with a width of 4 nm FWHM. The ASE background was suppressed to below our detection limit by a proper choice of operating current and temperature and by sending the light through a single-mode optical fiber. The final ASE spectral density was less than 0.1 nW/MHz, i.e. less than 0.2% of the optical power. Related to an optical transition linewidth of Γ/2π=6 MHz for rubidium, this gives a background suppression of better than -82 dB. An indication of the beam quality is provided by the fiber coupling efficiency of up to 59%. The application of the amplifier system as a laser source for atom-optical experiments is discussed.

Spatial gain measurements in a chemical oxygen iodine laser (COIL)

The spatial distribution of small signal gain has been investigated on the RADICL device, a supersonic chemical oxygen-iodine laser (COIL). A frequency-stabilized, narrow linewidth diode laser system operating on the F=3/spl rarr/F=4 hyperfine levels of the (/sup 2/P/sub 1/2/) to (/sup 2/P/sub 3/2/) spin-orbit transition in atomic iodine was used as a small signal probe. A peak gain of 1.2%/cm was measured along the horizontal centerline of the single-slit, supersonic nozzle, which is about two times greater than measurements made on ReCOIL by Hager et al. (1988) and compares favorably with measurements made on the RotoCOIL device by Keating et al. (1990). Gain distribution was investigated under three I/sub 2/ flow conditions. Scans across the supersonic expansion indicate a gradient in gain distribution due to higher gas temperatures along the walls and mixing phenomena. >

1.244-Gb/s 32-channel transmission using a shelf-mounted continuous-phase FSK optical heterodyne system

A total capacity of 40 Gb/s is achieved using a shelf-mounted continuous-phase frequency-shift-keying (CPFSK) optical heterodyne frequency-division-multiplexing (FDM) transmission system with 32 optical channels and a bit-rate of 1.244-Gb/s per channel. For achieving a stable bit-error-rate (BER) characteristics with high-sensitivity, narrow-linewidth laser diodes, a channel-spacing stabilization circuit, and an optical tuner are developed. The obtained sensitivity at a BER of 10/sup -9/ for fiber transmission over 121 km ranges from -45.1 to -44.2 dBm, which is 9.8-10.7 dB lower than the shot-noise-limited sensitivity. The crosstalk penalty is suppressed to within 0.1 dB. The developed system has feasibility achieving a distribution system which can distribute more than 250 HDTV (high definition television) signals or 1250 current-standard TV signals to about 8000 subscribers 10 km from the office, or a 40-Gb/s trunk-line system with a fiber span of more than 50 km. >

Narrow linewidth laser diodes with broad continuous tuning range

A fast electronic frequency stabilisation scheme is presented which improves the coherence properties of near IR laser diodes substantially. The control loop is effective even under fast, wideband frequency tuning conditions, which opens up new fields of application in spectroscopy, range finding and data communication.

Injection behavior and modeling of 100 mW broad area diode lasers

The output from a single-frequency, narrow-linewidth GaAlAs diode laser was injected into a 100-mW broad-area diode laser. The injected laser emitted in a single longitudinal mode with up to 80 mW in a single, diffraction-limited, 0.5 degrees FWHM far-field lobe. The angle of the far-field lobe steered with injection frequency and broad-area laser bias current, displaying behavior similar to that observed with injected gain-guided laser arrays. A Fabry-Perot amplifier model which explains the injection behavior in terms of Gaussian beam propagation in a large optical cavity is proposed. The model provides criteria for optimizing locking bandwidth, beam steering, and injection efficiency. >