Investigation of tellurium for frequency doubling with CO2lasers

Second-harmonic generation (SHG) of CO2laser radiation with tellurium has been investigated. The nonlinear coefficient was measured (d_{11} =1.6 times 10^{-6}ESU), as were the peak and average power damage thresholds (2-4 MW/cm²and 150 W/cm²). Finally, the SHG conversion efficiency was found to saturate due to increased absorption of the fundamental.     

Gain of high-pressure CO2lasers

Gain formulas are shown for the 10.4- and 9.4-μm CO2bands for the high-pressure broadening case to point out a new regime of operation for this laser. Computer-simulated results of this gain are presented as a function of wavenumber, with density equivalent pressure and population inversion ratio as parameters. The results show gain curves that suggest continuously tunable lasers over the entire 9.4- and 10.4-μm bands with no rotational structure.     

Gain of high-pressure CO2lasers

Previous theories of high-pressure CO2gain spectra have been modified to include gain Contributions from the newly discovered 00°2 and 00°3 sequence bands. It is shown that the inclusion of these bands has important consequences for TE CO2lasers. At a pressure of 14 atm, the sequence bands typically cause a 40 percent increase in calculated gain at all frequencies. Even at pressures as low as 1 atm, the presence of the sequence lines leads to anomalous gain coefficients on many of the regular 00°1 laser lines.     

Review of CW high-power CO2lasers

Various forms of CO2lasers have achieved CW powers in the 60-kW range, operating efficiencies approaching 30 percent, pulse energies of approximately 2000 J, pulsewidths less than 1 ns, peak pulse powers in excess of 10⁹W, a frequency stability of a few parts in 10¹², and sealed-off tube lifetimes of many thousands of hours. In addition, the laser can be easily Q-switched as well as gain-switched and has been electrically, optically, gas-dynamically, and chemically pumped. In addition to all these attributes, the CO2laser output wavelength lies within one of the best atmospheric windows. It should be no surprise then that during the last eight years, the CO2laser has firmly established itself as a candidate for recognition as the most important among the numerous laser devices presently known. Depending on the gas pressure, gas flow rate, pumping mechanisms, gas mixture, etc., CO2lasers can exhibit a wide range of noise, bandwidth, gain, and power saturation characteristics. This flexibility enables a designer to optimize the performance of CO2laser stable-frequency master oscillators; power oscillators; low-noise high-gain preamplifiers; intermediate-power or high-power amplifiers. As a result, CO2laser oscillator-amplifier chains can be designed utilizing guidelines similar to those which have been extensively applied in the design of transmitters in the RF and microwave region of the electromagnetic spectrum.     

Characteristics of sealed-off waveguide CO2lasers

Gain and output power of sealed-off waveguide CO2lasers are presented as a function of gas mixtures and total gas pressure. Experimental data on circular-bore and square-channel waveguide lasers are presented. Output power per unit length of 0.2 W/cm is achieved for both types of lasers in agreement with gas-discharge scaling laws which are presented. Saturation intensities as high as 24 kW/cm²are inferred from the data. The effects of the optical properties of the waveguide wall material on the waveguide losses are discussed and theoretical waveguide loss versus wavelength is presented for BeO, Al2O3, and fused silica.     

Compact sealed TEA CO2lasers with corona-discharge preionization

Optimization of the performance of compact sealed TEA CO2lasers employing a novel version of the corona-discharge preionization scheme is reported. Performance with high oxygen concentrations, long-life operation with organic additives, limits of arc-free operation with gas mixtures without helium, and in the presence of high CO2concentrations have been investigated. Our results show that devices whose construction is considerably simplified by employing this preionization technique are capable of producing performance superior to more complex systems.     

Properties of high-energy electrically pulsed CO2lasers

The output properties of an electrically pulsed CO2laser have been investigated as a function of partial gas pressure and discharge voltage. The output pulse became sharper with increasing CO2pressure. The output pulse energy increased with increasing helium pressure up to a pressure where the output became independent of further increases. Output pulses of 24 joules at 3.6 MW and 3.6 percent efficiency were obtained at 1/10 atmosphere in a 3.5-meter-long 5-cm-bore laser tube.     

Analysis of CH3OH far infrared laser lines optically pumped by sequence band and isotopic CO2lasers

Assignments are presented for seven far infrared (FIR) laser lines of CH3OH pumped by theS-9P(31),18-10R(24),13-9R(26), and13-9P(16)CO2laser lines, plus an interesting speculation for the FIR line pumped by the18-9P(12)CO2line. Frequencies have been deduced to a substantially improved accuracy of ±0.001 cm^-1from IR and FIR spectroscopic combination differences for most of the assigned lines as well as three other predicted transitions. In addition, accurate frequencies are given for 13 predicted FIR laser transitions which are expected from the IR spectrum to be pumped by three¹⁶O¹²C¹⁸O laser lines.     

Vibrational energy transfer in CO2under laser conditions with and without water vapor

It is shown that in the case of a dry 1.5-torr CO2gas fill the upper laser level is indirectly excited by vibrationally excited CO produced during the discharge, whereas in the case of a 1.5-torr CO2and 0.2-torr H2O mixture the upper laser level is directly excited by the electrons in the discharge. The collision relaxation times measured under laser conditions for the symmetric valence vibration of CO2in a CO2-H2O mixture and in a CO2-CO mixture as produced during a discharge of an initially pure CO2fill were 19 and 73 μs, respectively. If the reasonable assumption was taken that half of the CO2was dissociated into CO then this result shows that H2O was 14 times more effective in depopulating the lower laser level than CO. The growth in laser intensity for the dry fill was shown to be due to the CO (nu = 1) transfer of energy to the asymmetric vibration of CO2(00°1) with a characteristic increase that was exponential strictly only for a time short compared with the relaxation time of the symmetric vibration. The characteristic transfer time for excitation of the asymmetric vibration was dependent upon the fraction of CO present. If we make the assumption of 50 percent dissociation, the intermolecular energy transfer time between CO and CO2was found to be 40 torr-μs. Results obtained with N2and He added to the laser mixture indicated that He was not more effective in relaxing the lower laser level than N2or CO and was less effective than H2O.     

Role of helium in TEA CO2lasers

The role of helium in TEA CO2lasers differs from that in low-pressure CO2lasers. TE CO2laser output is nearly proportional to the gain-bandwidth product, and, at a total pressure of 250 torr, it is independent of whether helium is present or not. Above 250 torr, the laser could not be operated without helium.     

Far infrared laser frequencies of 13CD3OH

We have measured 30 far-infrared laser frequencies of optically pumped ¹³CD₃OH, 13 of which are new lines. The frequencies range from 0.5 to 4.9 THz with the majority between 0.75 and 1.5 THz. Two frequency stabilized CO₂ lasers were used as standards for the heterodyne measurements     

Mechanism of passiveQswitching in CO2lasers

PassiveQswitching of a CO2laser by saturable absorbers is analyzed in terms of a four-state kinetic model previously used to interpret infrared saturations double resonance, and pulse transmission. Good agreement is found between the predictions of this model and the experimental dependence of such variables as pulsewidth, repetition frequency, and peak power on the presence of buffer gases mixed in with the absorber.     

Operation of TEA CO2lasers with rough surface electrodes

In contrast with the normally accepted concepts in the construction of transversely excited atmospheric (TEA) CO2lasers it is found that homogeneously roughened electrodes perform better then polished ones.     

Life and parameter studies on sealed CO2lasers

The life and power outputs of sealed CO2lasers with xenon and hydrogen additives have been investigated. The results confirm the usefulness of hydrogen for achieving long life, but suggest that the power increases previously observed with the addition of hydrogen may be due to changes in CO2concentration rather than to relaxation effects.     

Design and short-term stability of single-frequency CO2lasers

This paper describes a family of sealed-off, single-frequency, TEM00q-mode CO2lasers with output powers up to 15 watts. Short-term frequency stability[1],[2] measurements of the beat note of two free-running lasers in typical laboratory setups are discussed thereafter. The preliminary measurements indicate a short-term stability of about 5 parts in 10¹²for an observation time of 0.05 seconds, and about 5 parts in 10¹³, disregarding the discrete spectral lines resulting from 60-Hz modulation by power supply ripple. One or two orders-of-magnitude improvement is predicted, leading to the possibility of measuring the linewidth limit imposed by quantum noise.     

Hybrid injection locking of higher power CO2lasers

Frequency stabilization of a higher power CO2ring laser by locking with a stable low-power reference laser is described. Successful locking is achieved by employing a novel hybrid injection-locking technique. In the hybrid mode, the higher power laser is tuned to operate on a self-oscillation line different from that of the reference laser. Within the frequency-locking range, the self-oscillation of the locked laser is completely quenched, and only the amplified drive power of the locking laser appears. Operation of this hybrid technique relies on the line competition in the homogeneously broadened CO2medium. The hybrid technique, which is easily implemented with a simple hill-climbing servo, permits stabilized operation over a variety of laser lines. An analysis of injection locking that explicity includes the saturation of the homogeneous medium is presented. Expressions are obtained for the gain and the phase relationships within and outside the locking region. To demonstrate the potential of the hybrid injection-locking technique, a 60-W CO2laser was locked in frequency to a 0.5-W stable oscillator. The experimental data are in close agreement with theory.     

Effects of time-delayed amplification in TEA CO2lasers

A novel technique is reported for studying the behavior of the time-dependent gain in a TEA CO2amplifier. The method involves the incorporation of an additional amplifier tube into a laser cavity already containing a laser gain tube. The two tubes are independently operated, but so arranged that they can be fired with a controllable time delay between the discharge current pulses. This system permits effects of the additional gain tube on the lasing properties to be investigated as the time delay is varied. In particular, there is a time delay between the discharge current pulse of the laser tube and the onset of lasing. The variations in this delay produced by the firing of the additional amplifier tube have been investigated. The observed time-delay changes can be related to a simple theory for the time-dependent gain. The analysis of the measurements can be used to determine parameters describing the time-dependent gain. This method has been used to measure decay times of the gain for various gas mixtures. The techniques reported here can also be used to study other time-dependent effects within laser amplifiers.     

Signature variations with mirror separation for small sealed CO2lasers

A detailed experimental study of laser signature as a function of laser mirror distance variations is reported. This was done for small sealed low-power CO2lasers (≈ 1 watt) of the type useful in laser communication. It is shown that most laser-output profiles change drastically from half-wavelength to half-wavelength except the strong P(20) line, which seems to remain relatively constant. Frequency width is given for many of the laser output profiles appearing in the various signatures observed. Line competition is also discussed.