Applied Physics B

, Volume 29, Issue 4, pp 279–285

Harmonic detection with tunable diode lasers —two-tone modulation

  • D. T. Cassidy
  • J. Reid
Contributed Papers


The minimum absorption detectable by a tunable diode laser spectrometer utilising harmonic techniques is often limited by interference fringes generated by scattered light. The sensitivity of the spectrometer to absorption can be increased by applying a jitter modulation. In this paper, the theory of harmonic response for single- and two-tone modulation over optical fringes and Lorentzian absorption lines is developed and compared to experimental measurements. A simple analytic expression for the two-tone harmonic line shape is derived. This expression provides a physical understanding of the effects of the second modulation, and a means to unravel the effects of the second modulation on the linewidth and line shape. For a specific choice of the jitter frequency and phase, it is possible to simultaneously minimise the fringe signal and increase the harmonic absorption signal. The results of this investigation are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic techniques are used.


07.65 42.80 


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  1. 1.
    J. Reid, J. Shewchun, B.K. Garside, E.A. Ballik: Appl. Opt.17, 300–307 (1978)Google Scholar
  2. 2.
    J. Reid, B.K. Garside, J. Shewchun, M. El-Sherbiny, E.A. Ballik: Appl. Opt.17, 1806–1810 (1978)Google Scholar
  3. 3.
    D.T. Cassidy, J. Reid: Appl. Opt.21, 1185–1190 (1982)Google Scholar
  4. 4.
    J. Reid, D. Labrie: Appl. Phys. B26, 203–210 (1981)Google Scholar
  5. 5.
    M.D. Miller, A. Pires, D. Chen, S.K. Poultney: Poster paper presented at the conference on High Resolution Infrared Applications and Developments, National Bureau of Standards, Gaithersburg, Maryland, June 23–25 (1980)Google Scholar
  6. 6.
    J. Reid, M. El-Sherbiny, B.K. Garside, E.A. Ballik: Appl. Opt19, 3349–3354 (1980)Google Scholar
  7. 7.
    Although the change in tuning rate affects both modulations equally when measured as a percentage change, the absolute change of the jitter modulation is insignificant in terms of the number of fringes modulated over. This feature renders the minimum of the two-tone harmonic response less sensitive to changes in the tuning rateGoogle Scholar
  8. 8.
    R.S. Loughurst:Geometrical and Physical Opties, 3rd ed. (Wiley, New York 1973) p. 180Google Scholar
  9. 9.
    G. Arfken:Mathematical Methods for Physicists, 2nd ed. (Academic Press, New York 1970) Chap. 11Google Scholar
  10. 10.
    R. Arndt: J. Appl. Phys.36, 2522–2524 (1965)Google Scholar

Copyright information

© Springer-Verlag 1982

Authors and Affiliations

  • D. T. Cassidy
    • 1
  • J. Reid
    • 1
  1. 1.Departments of Physics and Engineering PhysicsMcMaster UniversityHamiltonCanada

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