Tunable External Cavity Diode Laser Research Articles

Real-time displacement measurements with a Fabry-Perot cavity and a diode laser

We present the basic operating principles of a traceable measurement system suitable for use with atomic force microscopes (AFMs) and nanometer-resolution displacement sensors. Our method is based upon a tunable external-cavity diode laser system which is servo-locked via a phase-modulated heterodyne locking technique to a Fabry-Perot interferometer cavity. We discuss mechanical considerations for the use of this cavity as a displacement metrology system and we describe methods for making real-time (sub 10 ms sampling period) measurements of the optical heterodyne signals. Our interferometer system produces a root-mean-squared (RMS) displacement measurement resolution of 20 pm. Two applications of the system are described. First, the system was used to examine known optical mixing errors in a heterodyne Michelson interferometer. Second, the Fabry-Perot interferometer was integrated into the Z axis of a commercial AFM scanning stage and used to produce interferometer-based images of a 17 nm step height specimen. We also demonstrate atomic resolution interferometer-based images of a 0.3 nm silicon single atomic step-terrace specimen.

633-nm tunable external-cavity diode laser with an intracavity glass plate as the fine-tuning device

The application of beatwave Fabry-Perot interferometers (BFPIs) to nonlinearity error calibration is limited by the short tuning range of the slave He-Ne laser. In this paper, a 633-nm tunable externalcavity diode laser (ECL) is proposed as the slave laser of a BFPI. The ECL consists of an antireflection-coated quantum-well laser diode, a collimating lens, and a diffraction grating in Littrow configuration. In addition, an intracavity glass plate is adopted to provide cavity fine tuning, which makes the ECL more convenient, as it is easier to align the light in the cavity. At an optical length of 45 mm, a coarse-tuning range of 4 nm around 633 nm is obtained by rotating the grating. It is also demonstrated that this ECL can operate in a single-mode regime and can be continuously tuned over a frequency range of 1.8 GHz by tilting the intracavity glass plate.

Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure.

Widely tunable continuous-wave Raman laser in diatomic hydrogen pumped by an external-cavity diode laser

What is to the authors' knowledge the first experimental demonstration of a nonresonant cw Raman laser pumped by a tunable external-cavity diode laser (ECDL) is presented. The ECDL is phase-frequency locked to a high-finesse Raman laser cavity containing diatomic hydrogen (H(2)) by the Pound-Drever-Hall locking technique. The Stokes lasing threshold occurs at a pump power of 400 +/- 30 muW, and a maximum photon conversion efficiency of 12.0 +/- 1.3% is achieved at 1.6 mW of pump power. A 40-nm tuning range of the cw Stokes emission, 1174-1214 nm, is obtained by tuning of the wavelength of the ECDL pump source.

Electronically tunable external-cavity laser diode

We present a new concept for an electronically tunable diode laser. It is based on an external-cavity configuration with simultaneous feedback and intracavity spatial separation of the laser's spectral components. The electronical tunability is achieved by insertion of a liquid-crystal array as an electronically controlled aperture into the region of spatial separation of the spectral components. Wavelength tunability without mechanical movement over a range of 10 nm and two-color operation are demonstrated with a 670-nm laser diode.

Frequency tracking and stabilization of a tunable dual-wavelength external-cavity diode laser

We show a unique dual-wavelength external-cavity laser diode with frequency tracking capability and obtain a stable beat frequency between the dual-wavelength output. By using a Fabry–Perot interferometer as the frequency discriminator and the time-gating technique in a servo loop, the peak-to-peak frequency fluctuations were stabilized, with respect to the Fabry–Perot cavity, to 86 kHz in the dual-wavelength output at 802.5 and 804.5 nm, and to 17 kHz in their 0.9 THz beat signal. Similar performance was achieved for tuning of the dual wavelength separation ranging from 0.2 to 4 nm.

On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference-frequency laser spectrometer

The design and application of a novel automated room-temperature laser spectrometer are reported. The compact instrument is based on difference-frequency generation in bulk LiNbO(3). The instrument employs a tunable cw external-cavity diode laser (795-825 nm) and a pulsed diode-pumped Nd:YAG laser (1064 nm). The generated mid-IR nanosecond pulses of 50-microW peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 microm, are coupled into a 36-m multipass cell for spectroscopic studies. On-line measurements of methane are performed at concentrations between 200 ppb (parts in 10(9) by mole fraction) and approximately 1%, demonstrating a large dynamic range of 7 orders of magnitude. Furthermore computer-controlled multicomponent analysis of a mixture containing five trace gases and water vapor with an overall response time of 90 s at an averaging time of only approximately 30 s is reported. A minimum detectable absorption coefficient of 1.1 x 10(-7) cm(-1) has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH(4), C(2)H(6), H(2)CO, NO(2), N(2)O, HCl, HBr, CO, and OCS.

Diode-laser absorption measurements of hydrazine and monomethylhydrazine

Absorption measurements of pure hydrazine, N 2H 4, and monomethylhydrazine, CH 3N 2H 3 (MMH), were recorded in an optical absorption cell (3 passes, 50 cm/pass) at low pressure (0–35 Torr) using a tunable external-cavity diode laser (ECDL), operating in the range from 6350 to 6650 cm -1 (1.49 to 1.58 μm). Peak absorption cross-section measurements of hydrazine and MMH, as determined from fixed wave-length absorption measurements, were found to be α e( ν)=0.163±0.008 cm -1atm -1 at 6495 cm -1 ( λ=1539.6 nm) and α e( ν)=0.152±0.003 cm -1atm -1 at 6560 cm -1 ( λ=1524.4 nm), respectively.

Highly Sensitive Cavity-Enhanced Sub-Doppler Spectroscopy of a Molecular Overtone Band with a 1.66 µm Tunable Diode Laser

Sensitive sub-Doppler resolution molecular spectroscopy developed by Hall and co-workers [J. Opt. Soc. Am. B 15 (1998) 6] has been applied to the 1.66 µm region using a widely tunable external-cavity diode laser. We have recorded saturated absorption spectra of the 2ν3 vibrational overtone band of methane with a sensitivity of 9.5×10-11 cm-1 at a time constant of 1.25 s and a spectral linewidth of 320 kHz (full width at half maximun; FWHM) over the tunable range of 1.63 to 1.68 µm.

Compact gas sensor using a pulsed difference-frequency laser spectrometer

We present a novel compact pulsed laser spectrometer based on difference-frequency mixing of a cw tunable external-cavity diode laser (795-825 nm) and a pulsed Nd:YAG laser (1064 nm) in bulk LiNbO(3) . The pulsed mid-IR source is continuously tunable from 3.16 to 3.67microm and exhibits a linewidth of only 154 MHz, a peak power of approximately 50microW , and a pulse duration of 6 ns at a 6.5-kHz repetition rate. Spectra of methane in room air and formaldehyde have been recorded at room-temperature operation in a multipass cell with deduced detection limits of 10 and 40 parts in 10(9) , respectively.

Simple high-coherence rapidly tunable external-cavity diode laser

We describe a simple electro-optically activated external-cavity diode laser designed to provide high-speed, high-coherence tuning over gigahertz frequency ranges. Tuning as fast as 23 GHz/ micros is demonstrated. Coherence measurements indicate transform-limited quiescent laser linewidth in observation windows as wide as ~100 micros and transform-limited chirps. A self-heterodyne, cross-correlation-based coherence diagnostic is developed for characterizing phase coherence during high-speed chirps.

Distributed measurement of static strain in an optical fiber with multiple Bragg gratings at nominally equal wavelengths

The Fourier transform relationship between the reflected light froma Bragg grating and the complex spatial modulation of the Bragg grating is used to produce a distributed strain sensing system. A tunable external cavity diode laser along with a reference reflector in anoptical fiber are used to produce a measurement of the phase and amplitude of the reflected light from the modulated Bragg grating as a function of wavelength. The system is demonstrated with 22 Bragg gratings in a single fiber on a cantilever beam and compared with foil strain gauge readings.

High-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter

A method of measuring strain over 30-cm intervals to an accuracy of10 microstrain in unaltered low-loss communications-grade single-modeoptical fiber is presented. The method uses a tunable external cavity diode laser to measure the reflected intensity of a reflector-fiber system as a function of wavelength. This measurement is performed with no strain applied to the fiber to produce a reference and then again after a strain has been induced. Cross correlation of the Rayleigh scatter spectra from a selected section of fiber in the strained and unstrained states determines the spectral shift resulting from the applied strain.

Second harmonic generation in a domain-inverted MgO-doped LiNbO3 waveguide by using a polarization axis inclined substrate

We demonstrate quasiphase matched second harmonic generation by use of annealed proton-exchanged 87° Z-cut MgO 5 mol % doped LiNbO3 (MgO:LN) waveguides supporting a TE-mode guided wave. Formation of a domain inversion was achieved by applying electric fields in 3° Y-cut and 87° Z-cut MgO:LN. In 3° Y-cut and 87° Z-cut MgO:LN substrates, a polarization axis inclines at an angle θ equal to 3° to the surface. The domain inversion was formed in a direction parallel to the polarization axis like needles stuck into the substrate. The depth of the domain inversion was 2.5 μm with a period of 4.75 μm. The depth was about 2.5 times deeper than that in X- and Y-cut substrate at an angle θ equal to 0°. By forming a waveguide on the domain-inverted substrate, a blue light of wavelength 475 nm, and a power of 37 mW has been obtained and a normalized second harmonic conversion efficiency was 300%/W cm2 with interaction length of 10 mm, using a tunable external cavity laser diode as a fundamental wave source.

Diode laser cavity ring down spectroscopy

We recently demonstrated how in cativy ring down spectroscopy (CRDS) a CW single frequency dye laser may be conveniently employed in place of the pulsed laser of standard CRDS. Here we extend this result to external cavity tunable diode lasers. Compact spectroscopic devices with extreme sensitivity (2 × 10 −10/cm) become a reality. To demonstrate the instrumental resolution we obtained high quality NO 2 spectra in a supersonic slit jet, with a residual Doppler width of about 250 MHz.

Coherent anti-Stokes Raman scattering spectroscopy using an optical parametric oscillator

A -barium borate optical parametric oscillator (OPO) pumped by the third harmonic of a pulsed Nd:YAG laser was developed and its potentials and limits to carry out coherent anti-Stokes Raman scattering spectroscopy are studied. Spectral properties of the OPO operated in a noncollinear phase-matching configuration are detailed, demonstrating that the OPO bandwidth can be controlled and considerably enlarged from the classical value of 1 nm up to 40 nm. The broadband signal wave emitted in this case fulfils the requirements of the tunable source needed for coherent anti-Stokes Raman scattering experiments in liquids. For gas phase studies, where the Raman linewidths are narrower than the distance separating the OPO cavity modes, scanning CARS is preferable. Injection seeding of the OPO with an external-cavity tunable diode laser reduces the signal wave linewidth to less than 500 MHz. Using this seeded OPO and its continuous tunability over 100 GHz, coherent anti-Stokes Raman scattering spectroscopy experiments are performed and molecular hydrogen Q(5) line profiles at different pressures and temperatures are measured.

Continuously tunable external-cavity diode laser with a double-grating arrangement

A new external-cavity diode laser system based on an extended version of the Littman configuration is investigated. In our laser system the tuning mirror is replaced with a second grating mounted at the Littrow angle. This double-grating design provides a smaller passive bandwidth than the grating-mirror configuration, which is used to compensate for the large angle-of-incidence-dependent losses of the grazing-incidence grating. This results in a broader continuous tuning range with an improved mode stability. Because two laser diodes, emitting at 820 and 775 nm, respectively, are used, the external-cavity laser is continuously tunable without mode hops across 35 nm at 820 nm and 27 nm at 775 nm. Such a laser was used to measure the absorption lines of the oxygen A band around 762 nm. The complete R - and P -rotational branches of the b summation operator(g)(1) (+)(nu(??)=0)?X (3) summation operator(g)(-)(nu (?)=0) transition were recorded in a single-wavelength scan.

A novel tunable dual-wavelength external-cavity laser diode array and its applications

We review the principles and characteristics of a novel tunable external-cavity laser diode source. It can operate as either a continuous wave (CW) single-wavelength, a CW dual-wavelength, a single-wavelength mode-locked, or a tunable dual-wavelength mode-locked laser. The gain medium is a commercial laser diode array. The telescopic external cavity consists of a grazing-incidence grating, a lens and a stripe-mirror. The external cavity provides functions of wavelength selection, dispersion compensation and spatial mode control. The characteristics of the laser under CW or mode-locked operation are described. The dual-wavelength laser output exhibits beat note beyond 7 THz. The tunable multi-terahertz beat frequency is characterized by a non-collinear intensity autocorrelation technique. A dual-wavelength interferometer that uses a tunable dual-wavelength laser as the light source is demonstrated. Finally, theoretical analysis is presented, which demonstrates that the dual-wavelength output of the laser is stable if the modes at the two wavelengths correspond to different array lateral modes.

Sequential TE/TM polarization emission from a tunable external cavity diode laser

A tunable external cavity diode laser that emits in a closely spaced sequence of alternating TE and TM wavelength intervals in the range between 1256 and 1282 nm is reported. In each interval, stable single longitudinal mode TE and then TM emission occurs that can be tuned in wavelength. Two consecutive intervals are separated by a wavelength region where emission instabilities are observed. The wavelength spacing between two successive intervals belonging to the same polarization state corresponds to the mode spacing of the solitary diode laser. As the laser is tuned beyond 1282 nm and up to 1316 nm, the emitted radiation is only TM polarized. The above-mentioned phenomena are based on the composite cavity gain modulation with the wavelength, and on the comparable losses of the system for the TE and TM polarization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tunable external cavity diode laser that incorporates a polarization half-wave plate

The threshold current and specttal properties of a grating tuned external cavity semiconductor laser are improved by utilizing an intracavity half-wave plate to optimize the grating feedback.