| He-Ne gas laser has an excellent optical quality for its high monochromaticity,narrow linewidth, small diverging angle, good frequency stability and high intensitystability. After being modulated, it is used extensively in the field of opticalmeasurement, optical sensing and optical research etc.. External electro-optical (EO)modulation is generally employed for He-Ne gas laser. Unfortunately, due to the highhalf-wave voltage of EO crystal which is the critical element in a modulation system,external EO modulation has the disadvantage of limited modulation depth and lowmodulation efficiency. Semiconductor laser can be modulated easily, but compared withHe-Ne gas laser, its optical quality and stability is relatively bad, which limits itsapplication. Combining the merits of He-Ne gas laser and semiconductor laser, aninternal modulated He-Ne gas laser with deep modulation depth, high modulationefficiency, easy modulation method and excellent optical quality, would account for theapplication of modulation laser. One typical of such applications is the cavity ring downmeasurement, in which the modulation voltage of internal EO modulation technique ismuch lower than that of the external modulation method. In this paper, an internal EOmodulation He-Ne laser is proposed and investigated systematically, including thestructural scheme, the elements choice, the modulation theory, the optimal configurationand so on. And a cavity ring down loss measurement system is established based onsuch an internal modulation He-Ne gas laser. The main contents and results of thispaper are listed as follows:1. An internal EO modulation He-Ne gas laser is designed with BBO (β-BaB2BO4)EO crystal, Brewster window and laser constructed with Zerodur crystallite glass.Suitable modulation elements and design scheme are selected according to theirperformance and the application requirement. The internal EO modulation theory ofHe-Ne laser is deduced, which confirms the high modulation efficiency and deepmodulation depth of the internal modulation technique.2. All factors that influence the modulation efficiency of internal modulation laserare analyzed systematically, including the number of Brewster window, distribution ofmodulation elements and orientation of EO crystal. Through experimental andtheoretical investigation, we found that the tilt of EO crystal with respect to cavity axisof laser has a significant influence on modulation efficiency. Take switching-off voltageof laser beam as an example, only several minutes of tilt angle may cause theswitching-off voltage change from hundreds of volts to thousands of volts. Thepermitted mounting error of the EO crystal for certain modulation requirement could beevaluated by the theoretical analysis. In the end, an optimal configuration of the internalmodulation laser is obtained, namely, the number of Brewster window is one, the optical axis of the EO crystal is in parallel to the cavity axis of laser resonance, and theincidence plane of Brewster window is in parallel or perpendicular to the x axis of theEO crystal. For a certain EO crystal, it is validated that the switching-off voltage of theinternal modulation method is only~1/25of the external modulation method.3. By employing the rate equation of He-Ne gas laser, the transition of turn-on andturn-off of the laser light is simulated. It is found that the switching-off time of theHe-Ne laser, which is generally longer than10ns, is much longer than the relaxationtime of EO effect. Besides, it is discovered and validated by experiments that internalmodulation laser could reduce the influence of ringing effect of voltage applied on EOcrystal on switching-off light signal. When the internal modulation laser is used incavity ring down loss measurement, this effect could greatly reduce the requirement ofthe voltage signal applied on EO crystal.4. Based on the conclusion of theoretical and experimental analysis, the designparameters of internal modulation He-Ne gas laser are optimized. And according to theengineering experience, an integration internal modulation He-Ne gas laser is designedwith convenient mounting structure for EO crystal and stable output laser beam.5. The cavity ring down loss measurement system with external modulation laser isstudied preliminarily. It is known that a bottleneck of this technique is the requirementof the voltage applied on EO crystal. A scheme of high voltage generator is found toaccommodate the requirement with amplitude of thousands of volts and rise or fall timeof nanosecond. In addition, it has been studied by PSpice simulation. The reason for theeasy-fault of such a circuit is analyzed, and a solution has been put forward.6. A cavity ring down loss measurement system with internal modulation He-Negas laser is built. The constituent part of the system is introduced in detail. Especially,the influence of the bandwidth of the photodetector on loss measurement result isanalyzed. The working circuit of the system is designed and tested to ensure the properoperation of the system. Two passive resonant cavities, namely, a linear cavity and afold cavity are constructed, and their cavity losses are measured respectively. Comparedwith the results that measured by a semiconductor laser, it is found that the two resultsare close to each other. In addition, the variances of the results measured with He-Nelaser are smaller. The loss of one film is measured as77.423ppm, which corresponds toa reflectivity of99.99225%. |
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