Frequency and Phase Stabilization of an HCN Laser by Locking to 13 a Synthesized Reference A
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DATE DUE "S-QQ-^/OtU */ ^1^1' XMd ; W-z >M* f ' I ' Demco, Inc. 38-?93 NITED STATES MTMENT OF NBS TECHNICAL NOTE 620 MMERCE National Bureau of Standard ILICATION Library, E-01 Admin. Bldg. OCT 6 1981 131102 *Ntmo» / /CO Frequency and Phase Stabilization of an HCN Laser by Locking to a Synthesized Reference U.S. PARTMENT OF OMMERCE National Bureau QC > f loo ls 15753 o.4>10 973 NATIONAL BUREAU OF STANDARDS 1 The National Bureau of Standards was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. 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'it ~ • . 6 jOO Frequency and Phase Stabilization of an HCN Laser by Locking to 13 a Synthesized Reference a Joseph S. Wells Quantum Electronics Division Donald Halford Electromagnetics Division Institute for Basic Standards ^UJ- National Bureau of Standards Boulder, Colorado 80302 - £ 7e d /> s> /c &< ^* ?A - j s?& && & NBS Technical notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be listed or referred to in the open literature. c<^ U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director Issued May 1973 National Bureau of Standards Technical Note 620 Nat. Bur. Stand.(U.S-), Tech. !\ote 620. 56 pages (Ma) 1973) CODEN: NBTNAE For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D. C. 20402 (Order by SI) Catalog !\o. C 13.46:620). 60 cents CONTENTS Page 1. INTRODUCTION . 2 2. LASER PARAMETERS . 3 3. STABLE FREQUENCY REFERENCE 11 3.1. Short Term Stability Requirements 13 4. ACTIVE TECHNIQUES 24 4. 1. Frequency Lock Loop 24 4. 2. Phase Lock Loop 26 5. DISCUSSIONS 40 6. REFERENCES 48 in LIST OF FIGURES Page Figure 1. Block diagram of experimental scheme for 4 an HCN laser frequency measurement. Figure 2a. Schematic drawing of a frequency controllable " HCN laser. Figure 2b. Ring anode and resulting stable plasma 6 configuration. Figure 3. RF power spectrum of beat note between 10 HCN laser and 12th harmonic of 74- GHz klystron. Figure 4. Frequency synthesis scheme for stabilization 12 of HCN laser frequency. Figure 5. Beat notes of a free running HCN laser versus 14 12th harmonic of 74- GHz klystron. Figure 6. RF power spectrum of 0. 891 vs (12 x 0. 074 THz) 23 beat note when 74- GHz klystron is locked to a phase locked klystron (a) and a cavity- stabilized klystron (b). Figure 7. Block diagram of laser stabilization scheme. 25 Figure 8. Response of the current controller- laser 29 combination to a fixed amplitude modulation signal. Figure 9. 74- GHz vs HCN laser beat notes. 32 Figure 10. Quality of phase lock of HCN laser. 33 Figure H. HCN laser stability as measured with a computing 35 frequency counter. IV LIST OF FIGURES (Continued) Page Figure 12. Block diagram of scheme for measuring 37 line width and stability of a phase locked HCN laser. Figure 13. RF power spectrum on analyzer B of beat 39 between laser and multiplier chain B when laser is (a) free running and (b) phase locked to chain A. Figure 14. Spectrum of phase lock correction signal 41 under various experimental conditions more fully described in text. Figure 15. Phase detector circuit used in HCN stabilization. 45 Figure 16. Filter and impedance matching circuit connecting 46 phase detector and series current controller. Figure 17. Circuit for series current controller (SCC). 47 Frequency and Phase Stabilization of an HCN Laser by Locking to a Synthesized Reference by Joseph S. Wells and Donald Halford Infrared frequencies as high as 88 THz have recently been synthesized using diode harmonic mixers with accuracies of parts in 1 . Stabilized lasers are needed to make frequency measure- ments of higher accuracy. The HCN laser is the lowest frequency basis laser used in these synthesis schemes and its stabilization has been the subject of recent interest. The laser is stabilized by locking it to a phase locked microwave reference chain. Two servo loops are utilized. The first loop is a relatively slow frequency lock loop with the correction applied to a P ZT driver. This loop not only accommodates thermal expansion of the laser, but also serves as an acquisition aiding loop for the second servo. The latter is a phase locked system with the cor- rection applied to the laser discharge current. Details of the laser design and some noise considerations relative to the microwave reference chain are presented along with some experimental data which indicate the results of the stabilization techniques. Data regarding the system stability and improved fast linewidth are included. Key words: Fast linewidth; Frequency noise; HCN laser; Infrared frequency synthesis; Laser frequency measurements; Laser linewidth; Laser stabilization; Phase locked laser. 1 1. INTRODUCTION The HCN laser is currently the basis laser of lowest fre- quency that has been used in infrared frequency synthesis (IFS) [1 2 3] schemes. The line of greatest interest (337 pm) can be made to oscillate in a 0. 000 007 - THz bandwidth centered at 0. 890 76 THz. In order to improve the accuracy of subsequent