Demonstration of a 1-W injection-locked continuous-wave titanium:sapphire laser

We report on a 1-W injection-locked cw titanium:sapphire ring laser at 846 nm. Single-frequency operation requires only a few milliwatts of injected power. This relatively simple and inexpensive system can be used for watt-level single-frequency lasers across most of the titanium:sapphire gain region. A brief review of injection-locking theory is given, and conclusions based on this theory indicate ways to improve the performance of the system.     

Pulse colliding in a self-mode-locked ring-cavity ti:sapphire laser

A new self-mode-locked ring-cavity Ti:sapphire laser is described that is self-mode locked in both unidirectional and bidirectional operations. We found that clockwise and counterclockwise pulses collide with each other at the Ti:sapphire rod when the laser is mode locked in a bidirectional operation. Spectrum narrowing and pulse broadening were found in bidirectional rather than unidirectional mode-locked operation. This is explained by the performance of a transient grating in the gain medium that restricts oscillation to a narrow spectral range.     

Enhancement of spectral purity of injection-seeded titanium:sapphire laser by cavity locking and stimulated Brillouin scattering

We report improvements to and better characterization of the spectral purity of a diode laser injection-seeded, cavity-locked titanium sapphire laser that serves as the source for a previously reported rubidium vapor spectrally filtered Thomson scattering apparatus at 780.24 nm. In a detailed set of measurements the spectral purity P of the laser, defined as the ratio ofthe narrowband component of the laser output to the total output, has been studied as a function of frequency mismatch between the seed laser frequency and the central frequency of the unseeded cavity. It is found that spectral purity exceeding 0.999 can be obtained for a seed-cavity mismatch as high as +/- 0.25 nm, corresponding to approximately 950 cavity longitudinal-mode spacings and as high as approximately 0.9999 for a cavity-seed mismatch in the range +/- 0.10 nm (380 mode spacings). It is also shown that the addition of an external-cavity stimulated Brillouin-scattering phase-conjugate mirror increases both the spectral purity, to a minimum of 0.99999, and the cavity-seed mismatch range, to +/- 0.25 nm, for which this maximum effective purity is obtained.     

Intracavity absorption spectroscopy with a tunable multimode traveling-wave ring Ti:sapphire laser

A tunable multimode unidirectional traveling-wave Ti:sapphire laser was developed to measure in situ the atmospheric absorption spectra using intracavity absorption spectroscopy. The effective absorption path length was 2100 km. O2 and H2O vapor lines in atmosphere with absorption coefficients of 10(-6)-10(-8) cm(-1) were measured with uncertainties <5%, and the absorption coefficients were in agreement with those estimated from the HITRAN database. By tuning the wavelength, a weak absorption line with an absorption coefficient of 10(-9) cm(-1) was measured with a sensitivity of 2×10(-10) cm(-1). The sensitivity was limited by the residual parasitic variation that appeared in the spectrum.     

Fluid velocity measurements in a microchannel performed with two new optical heterodyne microscopes

Two laser Doppler microscopes (LDMs) based on an optical heterodyne interferometer have been developed for measuring fluid velocity in a microchannel. One of LDMs receives light from a Zeeman laser, and one easily obtains the standard heterodyne signal because a polarizer is set in front of a photomultiplier tube. The other LDM, with light from a He-Ne laser, employs a diffractive grating as a frequency shifter that is modulated in a sinusoidal movement by a piezoelectric transducer stack. By this modulation the nonstandard heterodyne signal is further processed by a new synthetic heterodyne algorithm. Finally, the phase shift related to the fluid velocity in both LDMs is demodulated by digital postprocessing in fast-Fourier-transform, bandpass filtering, inverse-fast-Fourier-transform, and arctangent algorithms.     

Absolute distance measurement with heterodyne optical feedback on a Yb:Er glass laser

Absolute distance measurement based on optical feedback using a single-frequency Yb:Er glass laser is demonstrated via the combination of heterodyne detection and frequency sweep. The technique allows for the enhancement of the sensitivity of the laser response to self-mixing thanks to resonant excitation close to the relaxation-oscillation frequency peak. The experimental results on noncooperative targets are in good agreement with the theory, and the shape of the resulting signal is analyzed in both the temporal and the frequency domains considering the specific dynamic of the class B solid-state laser. Suggestions are provided for further improvements on the signal processing.     

Optical heterodyne arrangement with a two-frequency radiation source for mechanical vibration measurements

Conclusions An experimental model of an optical heterodyne arrangement has been developed and fabricated for the measurement of mechanical vibrations having a complicated form. The metrological characteristics of the arrangement were determined experimentally. The relative measuring error for the velocity of harmonic vibrations over the range from 0.2 to 20 mm/sec, referred to the upper limit, is 1.2%. The arrangement can reproduce an analog of mechanical vibrations having a complicated form with a dynamic error of no more than 4%.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 39–41, September, 1973.     

Self-seeding of a pulsed double-grating Ti:sapphire laser oscillator

A self-seeded pulsed double-grating Ti:sapphire laser oscillator consisting of a grazing incidence cavity geometry with a pair of gratings and a standing-wave cavity pumped by a frequency-doubled Nd:YAG laser was developed and characterized. With self-seeding, narrow-linewidth single-longitudinal-mode (SLM) operation and SLM scanning were possible with a reduced lasing threshold, which was desirable for the intended applications.     

High-sensitivity mid-infrared heterodyne spectrometer with a tunable diode laser as a local oscillator

A new mid-IR heterodyne spectrometer, which is intended to be applied for atmospheric and astrophysical studies, is presented. The spectrometer uses a frequency-stabilized tunable diode laser as a local oscillator. Owing to the low output power of available single-mode diode lasers, a newly developed confocal-ring resonator, the diplexer, is used to superimpose the source signal efficiently with that of the local oscillator. Additionally, the diplexer serves as an optical filter that establishes controlled optical feedback between the laser diode and the detector, which allows stable laser operation with linewidths of the order of 1 MHz. The heterodyne signal from the HgCdTe detector is analyzed by means of a 1.4-GHz acousto-optical spectrometer. With this setup we find system temperatures as low as 4400 K (double sideband), that is, approximately a factor of 6 of the quantum limit.     

Generation of Fourier-transform-limited 35-ns pulses with a ramp-hold-fire seeding technique in a Ti:sapphire laser

We report on an injection-seeded Ti:sapphire laser pumped by the second harmonic of a Nd:YAG laser. The resonance between the low-power seed laser and the slave cavity is achieved by a ramp-hold-fire technique. Because of the triangular cavity design, the spatial beam profile is excellent; and combined with the narrow-linewidth pulses, the conversion efficiencies for nonlinear frequency generation are excellent.     

Single-mode tunable Ti:sapphire laser over a wide frequency range

We report a design of a tunable Ti:sapphire laser capable of operating in the range between 700 and 800 nm. The continuous, single-mode tuning is achieved by a pseudoexternal cavity consisting of highly reflective mirrors and a diffraction grating. The advantages of this laser include low operational threshold, a simple configuration that involves only four optical elements, and fine-tuning capabilities. The single longitudinal mode of operation was demonstrated at wavelengths between 695 and 725 nm and was limited by the choice of end mirrors in the laser cavity.     

Silver-mirror-based multipass preamplifier for a broadband terawatt Ti:sapphire laser

We have developed a silver-mirror-based multipass preamplifier for a broadband amplification in a terawatt Ti:sapphire laser. With the extremely broad bandwidth of the silver mirrors, a very broad amplified spectrum can be generated at an amplified energy of 4 mJ; the amplified spectral width is 65 nm at half maximum and 160 nm at -25 dB without any spectral shaping technique. Such a broad amplification can be explained well by the simulation that includes gain narrowing and gain saturation. Even after a further amplification to an energy of 600 mJ, the amplified spectrum is broad enough to support an approximately 20 fs transform-limited pulse duration.     

Simultaneous absolute measurements of principal angle and phase retardation with a new common-path heterodyne interferometer

This study demonstrates a new method for simultaneously measuring both the angle of the principal axis and the phase retardation of the linear birefringence in optical materials. We used a circular common-path interferometer (polariscope) as the basic structure modulated by an electro-optic (EO) modulator. An algorithm was developed to simultaneously measure the principal axis and the phase retardation of a lambda/4 or lambda/8 plate as a sample. In the case of a lambda/4 plate, the average absolute error of the principal axis is approximately 3.77 degrees, and that of the phase retardation is approximately 1.03 degrees (1.09%). The retardation error is within the 5% uncertainty range of a commercial wave plate. Fortunately, the nonlinear error caused by the reflection phase retardation of the beam splitter dose not appear in the new system. Therefore the error could be attributed to misalignment and defects in the EO modulator or the other optical components. As for the repeatability of this new common-path heterodyne interferometer, the average deviation for the principal axis is 0.186 degrees and the phase retardation is 0.356 degrees. For the stability, the average deviation for the principal axis is 0.405 degrees and the phase retardation is 0.635 degrees. The resolution of this new system is estimated to be approximately 0.5 degrees, and the principal axis and phase retardation could be measured up to pi and 2pi, respectively, without ambiguity.     

Flow-velocity measurements with a laser diode array

To measure flow velocity, a new technique, laser diode array velocimetry, generates multiple laser spots (four or more) separated by short distances (20-100 mum) at the probe volume. Particles that pass through the probe volume generate a signal that is analyzed by a digital signal processor for frequency content. The product of frequency and laser-to-laser spot separation determines the velocity. Rotating wire and wind-tunnel experiments confirm the performance of the new technique. An error analysis showed that the uncertainties in the processed diode array velocimetry signal frequency were less than 0.3%, and uncertainties in the beam-to-beam separation were less than +/-0.05%.     

Demodulation of intensity and shot noise in the optical heterodyne detection of laser interferometers for gravitational waves

Demodulation of intensity noise in the optical heterodyne detector is analyzed for application in interferometric gravitational-wave detectors. The correlation function and the power spectral density of the demodulated intensity noise are derived, taking into account the effect of bandpass filtering at the photodiode and an arbitrary demodulation waveform. The analysis includes demodulation of the rf-modulated shot noise as a special case of the intensity noise. For shot-noise-limited detection, the signal-to-noise ratio is found as a function of the modulation parameters, and the optimization of the signal-to-noise ratio with respect to the demodulation phase is described.     

Ti:sapphire laser cavity mode and pump-laser mode calculations

Comprehensive calculations of the cavity mode size throughout a Ti:sapphire laser, made with the ABCD Gaussian beam formalism are reported. These calculations show that the beam is not collimated, in general, in what are normally referred to as the collimated arms of the laser cavity. Additionally, the mode size and volume (in the gain medium) of the argon-ion laser, which is used to pump the Ti:sapphire laser optically, are evaluated for different focusing geometries, and graphs that can be used to select suitable mode-matching optics are produced. It is concluded that an appropriate strategy for mode matching the pump beam to the Ti:sapphire laser mode is to use a zoom telescope to tailor the collimated pump-laser beam diameter to an optimum value. Finally, comparisons of the pump-laser mode and the Ti:sapphire laser mode are presented for selected pumping geometries.