Low-loss Cavity Research Articles

Mode-locked fiber laser with a fiber phase modulator

FM mode-locked operation of a single-mode Nd(3+)-doped fiber laser has been achieved with an integrated fiber phase modulator. The technique permits a low-loss cavity configuration, resulting in low threshold and high slope efficiency. Pulses of ~200-psec duration are observed at a repetition rate of 417 MHz with an average output power of 15 mW.

Losses in fibre laser cavities

A passive cavity ring-down technique has been used to measure the intrinsic losses of a fibre laser cavity. The results show that very low cavity losses can be obtained using conventional mirrors butted to the fibre. Optimum output coupling for a typical cavity intrinsic loss is derived.

Intracavity Gain Detection Applied to Transient Inversions of IF

A CW dye laser has been used to study transient optical gain on the B →X electronic transition of iodine monofluoride. By placing the gain generator inside the CW dye laser cavity, gain was readily observed even at levels too small to support lasing in a low loss cavity. Transient phenomena, including V–T energy transfer, were easily observable. An analysis of the intracavity gain detection technique revealed a method for estimating optical gain. The technique should be readily applicable to other systems.

Results of cold testing a quasioptical cavity for a two-stage FEL

A low-loss quasioptical cavity consisting of a cylindrical waveguide and two spherical end mirrors has been proposed to support the intense millimeter-wavelength pump field in a two-stage FEL [1]. The loss due to mode mismatch between the guided and open sections of the cavity is serious for a cavity of reasonable length unless mode matching elements are used [2]. In order to determine the adequacy of proposed designs and fabrication techniques, the losses in a model cavity have been measured at KMS. A comparison is made between losses from a quasioptical cavity with a single spherical end mirror and a cavity with a two-mirror end configuration. Data for both cavity configurations are found to agree with theoretical predictions.

Coupling efficiency of waveguide laser resonators formed by flat mirrors: Analysis and experiment

We present a new and more efficient method of calculating the losses of a waveguide laser resonator consisting of a hollow circular dielectric waveguide and flat mirrors, taking into account the effects of waveguide modes up to order HE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</inf> . Both symmetric and asymmetric cavities are considered. We show that low cavity losses, only slightly exceeding the HE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</inf> waveguiding losses, are predicted to be possible for much larger mirror distances than had previously been suspected, provided that an optimum total cavity length is chosen. The low losses arise when the HE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</inf> and HE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> modes emerge from the guide with relative amplitudes and phases such that the returning diffraction patterns interfere to produce a narrow beam with low aperture losses. The theoretical predictions were checked experimentally for CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> lasers having various waveguide dimensions. Good qualitative agreement was found, but the optimum total cavity lengths were typically 3-5 percent longer than predicted. Possible explanations of this discrepancy are discussed. We also predicted and experimentally verified that variations of the cavity length over a few centimeters can exert a coarse wavelength selectivity sufficient to determine the band and branch on which a CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser oscillates; conversely, that for a grating tuned laser, the cavity length must be varied by a similar amount as the wavelength is tuned in order to maintain low cavity losses over the entire wavelength range.

LED end-pumped Nd:YAG lasers

Experimental results are reported on the room-temperature operation of Nd:YAG lasers end pumped with an LED. The radiation from a 10-percent-efficient 0.46-mm-diam domed LED was coupled to the end of a 0.46-mm-diam × 5.0-mm-long laser rod with a large hemispherical reflector. At 20°C, a multimode laser power of 0.25 mW was obtained at an LED current of 250 mA. By measuring the variation of threshold pump power with rod temperature and the laser power versus pump power just above threshold, both the laser cavity loss and the output mirror transmission were determined. A round-trip cavity loss as low as 0.022 percent was measured. A calculation of the fractional pump power absorbed in the rod for the LED spectrum gave 56 percent for a 5-mm-long rod while measurements showed that 30 to 35 percent of the LED power was being absorbed indicating an LED-to-laser rod coupling efficiency of 54 to 63 percent for this arrangement. With such efficient absorption of pump power and low cavity loss, end-pumped Nd:YAG lasers with high slope efficiencies above threshold are possible.

Waveguide CO2laser gain: Dependence on gas kinetic and discharge properties

Using a simple rate equation approach we examine the gas kinetic and discharge properties of waveguide CO 2 lasers. We calculate the dependence of the population inversion and laser small-signal gain on gas pressure, gas mixture, pumping rate (discharge current), tube bore diameter, and wall temperature. The results indicate, for example, that at a pressure of 50 torr and a tube-bore diameter of 0.125 cm, the gain is optimized with a gas mixture in the ratio CO 2 :N 2 :He of 1:0.75: 1.5. At higher pressures the gain is optimized by using more helium-rich mixtures. We also calculate the dependence of laser tunability on the gas kinetic properties and cavity losses. We find that for low-loss cavities the laser tunability may substantially exceed the molecular full width at half-maximum. Furthermore, the more helium-rich gas mixtures give greater tunability when cavity losses are small, and less tunability when cavity losses are large. The roles of the various gases in the waveguide CO 2 laser are the same as those in conventional devices. By contrast with conventional lasers, however, the waveguide laser transition is homogeneously broadened. Thus the dependence of gain on gas pressure and other kinetic properties differs substantially from that predicted by scaling results from conventional low-pressure lasers.

Cw chemical transfer CO2 laser

The combustion of Mg vapor in N2O with He and Co2 diluents in a low-loss coaxial laser cavity has resulted in cw oscillation at 10.6 μm. The performance characteristics of the flame laser in a coaxial geometry were found to differ significantly from those expected on the basis of previous gain measurements taken in a transverse flow configuration.

Cylindrical laser resonators*

A general class of cylindrical laser resonators is described in which the radiation propagates partially in the radial direction. There is a strong focusing of energy at the axis of such resonators. The low-loss cavity modes are related to ordinary gaussian beam modes. Two limiting forms are the disk laser and the tube laser, and some applications are considered.

Theory of intracavity optical second-harmonic generation

An analysis of optical second-harmonic generation internal to the laser cavity is presented. It is shown that the maximum second-harmonic power generated in this way is equal to the maximum fundamental power available from the laser. Further, it is found that there exists a value of nonlinearity that optimally couples the harmonic out for all power levels of the laser. The magnitude of the nonlinearity required for optimum coupling is shown to be proportional to the linear losses at the fundamental and inversely proportional to the saturation parameter for the laser transition. For the YAlG:Nd laser at 1.06 μ using Ba 2 NaNb 5 O 15 as the nonlinear material, the required crystal length for optimum coupling is given by l\min{c}\max{2}(cm)\simeq 2.7 \times 10^{2}L/f where L is the linear round-trip loss and f is the ratio of the fundamental power density in the nonlinear crystal to that in the laser medium. For low-loss cavities, optimum coupling can thus be achieved for crystal lengths of 1 cm or less. The use of a mirror or mirrors within the cavity, reflecting at the harmonic, is considered as a means to couple out the total harmonic in one direction. Considerations of temperature stability and the finite oscillating linewidth of the laser are shown to favor a configuration with a single harmonic mirror located on the same surface as the fundamental mirror.

Absolute Measurement of Microwave Power in Terms of Mechanical Forces

A LOW-LOSS cavity having input and output wave guides, and containing a movable element, can, in principle, be used for measuring microwave power absolutely.