Misao Ohi

Frequency Stabilization of a PbSnTe Laser on a Methane Line in the v4 Band

A PbSnTe diode laser in a small glass dewar for liquid nitrogen cooling was utilized for observation of absorption lines of methane in the v4 band around 7.7 µm. Some of the lines were identified and frequency differences between two lines were naeasured as well as the pressure broadening of the lines. The PbSnTe laser was stabilized on one of the methane lines by controlling the laser drive current. Frequency stability of the laser was estimated by Allan variance and the minimum value of 4.3×10-11 was obtained at an averaging time of 15 s.

Absorption of Rb-D Limes by Rb Filter Cell

The effects of absorption of Rb-D lines are studied with the combination of natural Rb lamps and natural Rb filter cells containing nitrogen at the pressures of 100 Tori and 152 Torr. It is found that the hyperfine components of D lines effective for optical pumping is obtained with Rb atoms of natural abundance both for lamps and for cells. The optimum temperature of the filter cell is observed to be near at 60°C. The absorption coefficient of the filter cell is less than 1.5 cm-1. The absorption cross sections obtained for three broad peaks consisting of hyper finecomponents are measured to be 10-12~10-13 cm2, which is 102~103 times larger than the atomic cross section of Rb in its ground state. The absorption cross sections for the filter cell with N2 100 Torr have values a few times larger than those for the filter cell with N2 152 Torr.

Measurements of Frequency Shifts in the 3.39 µm Ne Transition with the Aid of CH4 Lamb Dip

Isotope shift between 20Ne and 22Ne, pressure shift due to He and Ne, and shift due to discharge current of the Ne(3s2–3p4) transition were measured by the use of a 3.39 µm He-Ne laser with a CH4 cell. These shifts are obtained by observing the shift of an inverted Lamb dip produced in the laser output power curve with respect to its center for two isotopes, and as functions of pressure and discharge current. The observed isotope shift is (90.3±1.6) MHz, in which the frequency of 22Ne falls on the higher frequency side. The coefficient of the pressure shift due to He is (25.6±0.6) MHz/torr and that due to He-Ne mixture at 9: 1 is (24.3±0.3) MHz/torr. The pressure shift due to Ne is estimated to be (12.4±4.9) MHz/torr. The coefficient of the shift due to the discharge current is about -1.4 MHz/mA.

Frequency stabilization under very small modulation and its stability estimation of a PbSnTe diode laser

The frequency of a PbSnTe diode laser around 10.6 μm was stabilized under very small frequency modulation to one of the absorption lines of C 2 H 4 in the ν 7 band. The frequency control and modulation were performed by the injection current. The minimum modulation width in this experiment was analytically determined to be 1.1 MHz. From analysis of a linear approximate model for the experimental system, the frequency fluctuation, which represents peak-to-peak width during 5 min, including the modulation width, was estimated to be 3.8 \times 10^{-8} . This value was about two orders larger than the value obtained by the ordinary method, which is a simple conversion of the control error signal.

Unmodulated-Type Frequency Stabilization of a PbSnTe Diode Laser

The frequency of a PbSnTe diode laser around 10.6 µm was stabilized to one of the absorption lines of C2H4 without frequency modulation by controlling the injection current. Two different methods were tried: one used light after absorption and the other used light after absorption and reference light for comparison. From the analysis of the linear approximate models of the experimental systems, the frequency fluctuations, which here are taken to represent the peak-to-peak widths over a period of 5 minutes, were estimated to be 9.3×10-8 in the former case and 2.2×10-8 in the latter. The analysis also clarified that frequency fluctuations are underestimated when obtained by the usual method, which is a simple conversion of the control error signals. These values were over one order smaller than the values obtained from the linear approximate models.

Wavelength Measurement of an H2CO-Stabilized He-Xe Laser at 3.51 µm

The vacuum wavelength of an H2CO (51,5(ground state)→60,6(v5=1))-stabilized He-Xe laser was measured with respect to the CH4-stabilized He-Ne laser at 3.39 µm by using a pressure-scanning Fabry-Perot interferometer. The average value H2CO of the vacuum wavelength obtained was 3 507 979.48 pm. The standard deviation σn-1 was 0.39 pm with an accuracy of σn-1/H2CO=1.1×10-7. The estimated values of the systematic errors were -0.24 pmΔλH2CO,10, ΔλH2CO,2=0.004 pm, and ΔλH2CO,3=0.02 pm, respectively, where ΔλH2CO,1, ΔλH2CO,2, and ΔλH2CO,3 represent the errors caused by diffraction, nonparallelism between the interferometer mirrors, and nonparallelism between the two laser beams, respectively.

Profile Measurements of Methane Saturated Absorption Using a Methane-Stabilized Laser Spectrometer

A laser spectrometer with a high resolution of several kHz has been constructed with a methane-stabilized laser to study saturated absorption profiles of methane in a pressure range of 0.5–120 mTorr (0.07–16 Pa). The profiles are almost Lorentzian; the broadening due to methane pressure is 14.8 kHz/mTorr (111 kHz/Pa) and that due to air pressure is 11.2 kHz/mTorr (84 kHz/Pa). From analysis of the profiles, the frequency shift of our methane stabilized laser due to spurious voltage in the servo system is estimated to be within 3 kHz.

Transient Behavior of He-Ne Lasers

An experimental study on He-Ne laser (1.15 µ) under pulsed H.F. excitation reveals that there exist delay in oscillation build-up and a peak in the output at the beginning of oscillation. From qualitative discussions on the behavior of photon density and inversion density on the basis of coupled rate equations, these transient phenomena are shown reasonable. Contributions of various factors related to lasing mechanism are clarified from numerical calculations of simplified rate equations by the use of electronic computers. Both loss terms for the photon and the inversion effect strongly the shape of the curve of the solution. The increase of the former produces an oscillating build-up, while the contribution of the latter is opposite. Resultant contribution of the loss terms is also analysed. A reasonable set of the coefficients is found to lead to a solution having the delay which agrees with the experiment.

Profile Analysis of Spectral Line by Fabry-Perot Interferometer

Some of problems concerning the half-width measurement in the profile analysis of a spectral line by a Fabry-Perot interferometer are discussed. The apparent line shape and its half-width are treated and the two figures showing the relations among the apparent width, apparatus width and pure line width for the Gaussian and Lorentzian line are obtained. These curves are useful for the correction of apparatus width in the Fabry-Perot interferometer. The combination of the Doppler and Lorentz type broadening is also discussed and the figures showing the relation among the Doppler, Lorentz and resulting half-width are obtained. With the aid of these curves, the unknown width can be evaluated, if the other two widths are known. These results are applied to the analysis of the 5461A line emitted by Hg198 isotope lamps.