A Simple Method for Single-Frequency Operation

Appl. Phys. B 26, 193-195 (1981)AppliedPhysics9 Springer-Verlag 1981A Simple Methodfor Single-Frequency Operationand Stabilization of a He-Ne LaserJ. KauppinenDepartment of Physics, University of Oulu, SF-90570 Oulu, FinlandReceived 8 December 1980/Accepted 30 June 1981Abstract. A simple method for single-frequency operation and stabilization of a low powerHe-Ne laser has been described and used. With a proper resonator length very weak sideband modes on each side of the strong mode produce a feedback signal to a piezoelectricpusher which controls the resonator length. The long-term frequency stability wasestimated to be about _ 10 MHz.PACS: 42.55The aim of this work was to construct a singlefrequency stabilized He-Ne laser for the measurementof long optical path differences in the Fourier transform spectrometer at the University of Oulu [1].In the Michelson interferometer adjacent modes of themultimode laser will be exactly out of phase, when theoptical path difference is an odd multiple of the opticallength of the laser resonator and the modes producedeconstructive interference minima.

If the laser hasonly two modes equal in intensity the amplitude of theinterferogram due to the laser light decreases to zero atthose minima. This makes it more difficult to measurelong optical path differences in the interferometer. Inaddition, high frequency stability is also needed toachieve sufficient precision in the optical pathdifference.An excellent way to overcome these problems in themeasurement of long optical path differences is toconstruct a stabilized, single-frequency gas laser usingcommercial components.GainUnsaturated ~gain envelope ~/Intensity~resholdIfFig.

1. Output frequenciesin a low power gas laser0340-3793/81/0026/0193/$01.00194J. KauppinenUnsaturated /f_i!llation thresholdloseroutput\\\N,,fFig. 2. Output spectrum of the constructedlaser1. Basic PrinciplesAs we know, the frequencies of the longitudinal modesin the gas laser obey the following formula:kcf k - 2L'(1)where c is the speed of light, k is an integer, and L is theoptical length of the resonator. However, the modesM3Jln~rfer~IJM2,~~ffItvoltagel scon L ~are active only around the emission line of amplifyinggas, as shown in Fig. 1.There are many methods of obtaining single-frequencyoperation and frequency stabilization [2-5] of a gaslaser, for example: a short resonator, a short intracavity etalon, or a long etalon outside theresonator.In this work the optical length L of the resonator iscontrolled by feedback so that we have a situationshown in Fig, 2.

The intensities of the side-band modesare equal and less than 1% of the strong mode at themidpoint. The optical length of the resonator is about50 cm in this case. The stabilization is accomplished bykeeping the side-band modes equal in intensity. Thearrangement of the laser system is shown in Fig. 3.

Thelaser tube used is from Spectra-Physics (model 120).Theintensity difference between the side-band modesA and B produces an error signal which is integratedand amplified, then connected to the piezoelectricpusher to correct the length of the resonator. Theintensities of the side-band modes are measured by a//M4p=piezoetectricpusherspectrum]Ifeedbocki (., ,.,'~,Fig. 3. Arrangementof the constructedsingle,frequencylaser__mOptical I~thlength.....=time tO"5l........Ltime tFig. 4. Optical path length and theoutput voltage versus time in theFabry-Perot interferometerdrivenbyff~escan generatorSingle-FrequencyOperation and Stabilizationof a He-Ne Laser195_Llk-7.sv.-A~-12V=A_L .*zsv0,47lk-~[MC14066 ~HtOHVOLTAGE9OUTAMPLIFIERti-12V=-12VFig.

5. Circuit of the feedbackelectronicsused for stabilizationfast-scanning Fabry-Perot interferometer (SpectraPhysics model 470). The scan generator (SpectraPhysics 476) drives the mirror in the Fabry-Perotinterferometer so that the optical path length and thephoto detector output of the Fabry-Perot interferometer change in the manner shown in Fig. 4.The circuit of the feedback electronics is presented inFig.

5. The strong mode of the laser triggers a monostable multivibrator T~ (MC 14528) followed by twomonostable multivibrators T z and T 3. The timingperiods are adjusted so that these multivibrators, usingswitches (MC t4066), allow one of the t5 nF capacitorsto charge by a tiny mode A and the other by B. Thedifference of the voltages across the 15 nF capacitors isconnected to an integrator through an inverting amplifier during a short time interval. This is repeatedafter every strong mode (Fig.

4). The output of theintegrator is amplified before the high voltageamplifier.The points C and D are connected to a circuit whichdischarges quickly the 6.8 gF capacitor if the outputvoltage goes beyond a certain voltage range. At suchtimes the laser performs very fast mode hopping. Thisis because the range of the piezoelectric pusher may betoo small to compensate for the greatest temperaturechanges of the resonator.2. Test and ResultsThe frequency stability of the constructed laser wasestimated comparing its frequency with the frequencyof a reference laser.

Tropel 100 (Coherent), a singlefrequency He-Ne laser, with a long-term frequencystability of better than • 10MHz was used as areference laser. The frequency difference of the laserswas estimated looking at the simultaneous interferencefringe pattern due to the laser beams at the output ofMichelson interferometer [1]. With an optical pathdifference of 1.5 m the frequency difference of 100 MHzis just from one fringe to another in the interferencepattern. Modulating one of the laser beams by amechanical chopper it is possible to find frequencydifferences of about 5MHz. Using the method described above we estimated that the frequency difference between the lasers was about 30_+ 5 MHz duringthree days.References1. J.Kauppinen: Appt.

Opt. 18, t788-1796 (1979)2. A.L.Bloom:Gas Lasers (John Wiley& Sons, New York 1968)3. A.LBloom: In Progress in Optics IX, ed. by E.Wolf (NorthHolland, Amsterdam1971)4. K.M.Baird,G.R.Hanes: Rep. Prog. Phys. 37, 927 (1974)5. N.Umeda,M.Tsukiji,H.Takasaki: Appl.Opt. 19, 442--450(1980).