Mode selection in lasers

This is a tutorial review on the subject of mode selection in lasers. We begin with a historical review. After an introduction to the subject of modes in laser resonators and a brief review of the theory of laser gain saturation, the main body of the paper is devoted to a discussion of various mode-selection techniques, many which can be used to produce single-frequency laser operation. We discuss some systems for frequency stabilization of single-frequency lasers, and conclude with examples of laser applications where mode-selection techniques are required.     

Room-temperature color center lasers

A survey of results is presented on the development of room-temperature tunable lasers using color centers in ionic crystals and on using color center crystals as nonlinear filters and passive Q switches for neodymium lasers     

Spectroscopy of ion lasers

A brief review of the history of laser oscillation in gaseous ions is given, including a tabulation of wavelengths, classifications, and references for the 230 reported ion laser transitions. Selection rules and excitation mechanisms yielding oscillation are discussed.     

Continuous-duty argon ion lasers

The problems associated with the construction of continuous-duty dc excited ion lasers are indicated. Two solutions to the thermal dissipation and ion bombardment problem which have been experimentally tested and appear promising are described. These involve the use of axial magnetic fields with high current, low pressure, constricted arcs in large bore tubing and the use of metal-walled discharge vessels. The influence of an axial magnetic field on the properties of an ion laser is discussed, and the optical power levels which have been obtained are given.     

Hollow cathode metal ion lasers

A new class of metal ion lasers with significant CW output power in the UV (220-320 nm) and near IR (800-2000 nm) spectral regions is described. In a hollow cathode discharge the upper laser levels are excited via charge transfer collisions between ground state buffer gas ions and ground state metal atoms. At the present stage of development, hollow cathode metal ion lasers are shown to be comparable in UV output power to rare-gas ion lasers but with lower threshold currents by a factor of more than twenty. Visible output powers are lower than rare-gas ion lasers. In the text we present device progress to date, measurements of important plasma parameters, and an outline of potential applications of hollow cathode metal ion lasers.     

High-power pulsed xenon ion lasers

Output and threshold characteristics of small-bore pulsed xenon ion lasers are presented in detail as a function of current and gas pressure for ranges of these parameters that are consistent with high optical power output in the green-blue spectral region. It has been found that six wavelengths characteristic of xenon exhibit peak output powers greater than 100 watts, from a 5-foot laser tube over a limited (8-24 mtorr) range of xenon tube pressure. Laser action has also been obtained at high peak powers for longer current pulse (5-50 mus) operation of the tube. In addition, observation of three new laser wavelengths 5340, 5501, and 5590 Å is reported. These lines are only observed at very low tube pressures and very high peak currents.     

Electrical pumping of color center lasers

This paper presents an assessment of the feasibility of using electrical pumping to achieve laser action in color centers in the alkali halides. We discuss mainly the self-trapped exciton in the alkali halides, which is a prime candidate for such a pumping scheme because it is known to produce strong electroluminescence. The theory of electrical contacts to insulating crystals is reviewed, as is the theory of space-charge-limited currents in insulators. It is shown that sufficient energy can be delivered to an alkali halide crystal by a space-charge-limited current to reach the threshold for laser action. We present experimental evidence which demonstrates that when ohmic contact is made to a KI single crystal, large space-charge-limited currents can flow. Finally, from the measured value of the electroluminescent intensity we infer the value of the population inversion achieved through electrical pumping of KI with blocking contacts.     

Polarization selection in birefringent vertical-cavity surfaceemitting lasers

An analysis has been undertaken of the effect of refractive index anisotropies or birefringence on the selection of polarization states in vertical-cavity surface emitting lasers. Attention is focused on the competition between the two orthogonal polarization states of the fundamental transverse mode. It is shown that refractive index differences of 0.005% are adequate for ensuring preferential excitation of a specific polarization state. It is found that at low injection currents both polarizations coexist with the same optical power. The dominant polarization state at higher injection currents is found to depend upon the geometry of the current injection in the active region     

Single-frequency operation of argon ion lasers

An expression is derived for the gain coefficient of a laser mode sustained by excited ions produced in a dc gas discharge. Included in the derivation are the effects of the left-right gain asymmetry introduced by ion drift. From this expression, laser power versus frequency curves are obtained which are fit to experimental data obtained on the 4880-Å and 5145-Å transitions of singly ionized argon. From the curve fitting procedure, radiation interaction widths and saturation intensities for the transitions are determined. In addition, it is shown that the fraction of population inversion utilized during single-frequency operation of these lasers can be quite large.     

Semiconductor lasers with internal wavelength selection

Laser diodes with a deep built-in trapezoidal large-period diffraction grating have been developed on the basis of InGaAs/GaAs/AlGaAs heterostructures. The electroluminescence and stimulated emission spectra and light-current characteristics of diffraction-grating laser diodes have been studied. A substantial narrowing of both the luminescence and stimulated emission spectra was achieved due to the spectral selectivity of the grating. The maximum output optical power was 1 W at a drive current of 4 A. At the highest power, the lasing spectrum had a FWHM of ～2 Å. A narrowing of the lasing spectrum of the diffraction-grating laser by tens of times was demonstrated, compared with that of a laser with a Fabry-Perot cavity.     

Negative ion production rates in rare gas-halide lasers

This paper reports on dissociative electron attachment in F2, NF3, Cl2, and I2. The principle of the method is to produce a short burst of photoelectrons from a photocathode by means of light from an argon-fluoride laser. Subsequently, by studying the motion of electrons and negative ions in a constant electric field (E) region, information is obtained about drift velocities and effective attachment cross sections. Helium, argon, and nitrogen were used as buffer gases. Of particular interest is a very strong temperature dependence of the attachment coefficient in I2. Measurements were taken from 35 to 110°C, covering anE/Nrange of 1-50 Townsend. An explanation based on vibrational excitation is presented.     

New cold cathode for pulsed ion lasers

A simple, reliable high-current indium cold cathode for pulsed ion lasers is described. Currents in excess of 2000 amperes and pulse durations of 10^-2second have been obtained. Pulse repetition rates and operational lifetime appear to be limited only by the dynamics and lifetime of the discharge tube.     

Transverse-mode selection of rotating-mirror Q switched lasers

The TEM00 mode has been selected in a rotating-mirror Q switched Nd +3 : CaWO4 laser by insertion of a slit and an edge into the resonator. Shot-to-shot repeatability was enhanced, and multiple pulsing was suppressed. The same technique has been successfully applied to a CO2 laser.     

Two-segment cavity theory for mode selection in semiconductor lasers

A simple three-mirror composite laser cavity is analyzed and optimum structures proposed in terms of the position of the central reflector and the value of its reflectivity in order to give single longitudinal mode operation. Two optimum structures are discussed, the first corresponding to relatively long cavities where a large period in the gain modulation is required in order to avoid a jump to the next mode with a high effective reflectivity, usually in a second mode group (as defined by a neighboring period of the gain modulation). The second corresponds to shorter lasers where jumps to neighboring modes should be avoided, since the spectral gain variation prevents jumps to other mode groups. The effects of mode discrimination due to external feedback have been analyzed and minimum values of reflectivity for the third mirror derived in order to preserve the internal mode selectivity.     

Off-Axial mode selection in solid-state lasers due to nonuniformities

It is shown that the nonuniformity of excitation produced in circular cross-section solid-state lasers due to the focusing of pump power leads to a coupling of certain modes and reduction in their threshold of oscillation. These modes correspond in plane-wave representation to simple closed paths in the cavity. The off-axial radiation observed in the form of rings by Collins and Giordmaine and others is explained on the basis of the theory.     

连续运转Ar~+激光器的最佳气压分析

本文从 Tonks-Langmuir 低气压放电正柱理论出发,推导氩离子激光器输出功率和充气压力的关系,由此求得最佳气压,讨论了最佳气压的影响因素,并与实验结果相比较。     

Mode selection in GaAs injection lasers resulting from fresnel reflection

In several recent papers, attention has been given to the Fresnel reflectivity associated with cleaved facets of the laser. The small dimensions of the heterostructure waveguide give rise to a considerable angular spread in the energy incident on the facet. As a result, the mode reflectivity is not given simply by the Fresnel equation, which is only valid for an infinite plane wave. Rather, it is a properly weighted average over the plane-wave distribution of the mode. The differences in mode reflectivity with respect to TE and TM modes, as well as the variations with mode number, have been offered as a possible explanation for the predominant appearance of TE modes and preference for higher order modes in the large optical-cavity (LOC) geometry. However, the considerations to date have ignored the finite extent of the field in the junction plane. This situation is rectified in this paper. It is shown that the splitting in the mode reflectivity values between TE (electric field in the junction plane) and TM (electric field perpendicular to the junction plane) modes is reduced, and under certain conditions TM modes can be favored. In particular, it is shown that if a TE mode is oscillating, then there is a preference for a lowest order mode in the plane of the junction and a highest order mode in the transverse plane. Conversely, if for some reason a TM mode is oscillating, then the preference is for highest order modes in the plane of the junction and lowest order modes in the transverse plane.     

Radiation losses in distributed feedback lasers and longitudinal mode selection

We show that seemingly symmetric longitudinal modes symmetrical distributed feedback (DFB) lasers utilizing higher orc Bragg reflection actually have differing thresholds and resonant wavelength shifts from the Bragg condition as a result of power radiated by the grating.