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Applied Optics

Applied Optics


  • Editor: James C. Wyant
  • Vol. 47, Iss. 28 — Oct. 1, 2008
  • pp: 5163–5166

Piezo-locking a diode laser with saturated absorption spectroscopy

J. E. Debs, N. P. Robins, A. Lance, M. B. Kruger, and J. D. Close  »View Author Affiliations

Applied Optics, Vol. 47, Issue 28, pp. 5163-5166 (2008)

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We demonstrate modulation-based frequency locking of an external cavity diode laser, utilizing a piezo-electrically actuated mirror, external to the laser cavity, to create an error signal from saturated absorption spectroscopy. With this method, a laser stabilized to a rubidium hyperfine transition has a FWHM of 130 kHz over seconds, making the locked laser suitable for experiments in atomic physics, such as creating and manipulating Bose–Einstein condensates. This technique combines the advantages of low-amplitude modulation, simplicity, performance, and price, factors that are usually considered to be mutually exclusive.

© 2008 Optical Society of America

OCIS Codes
(020.1335) Atomic and molecular physics : Atom optics
(020.1475) Atomic and molecular physics : Bose-Einstein condensates
(020.3320) Atomic and molecular physics : Laser cooling

ToC Category:
Atomic and Molecular Physics

Original Manuscript: March 18, 2008
Revised Manuscript: August 17, 2008
Manuscript Accepted: August 22, 2008
Published: September 25, 2008

J. E. Debs, N. P. Robins, A. Lance, M. B. Kruger, and J. D. Close, "Piezo-locking a diode laser with saturated absorption spectroscopy," Appl. Opt. 47, 5163-5166 (2008)

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  16. See Piezomechanik GmbH, http://piezomechanik.com/f/core/frontend/http/http.php?dl=50-file-1.
  17. This estimate is calculated based on a typical saturated absorption signal with 300 μW of power focused onto the photodetector. We assume a Lorenzian absorption profile and use Beer's law with Eq. to calculate a theoretical error signal for a modulation frequency of 100 kHz. The absolute lower limit of 1 Å is based on a feedback bandwidth of 200 Hz, a value that is typical for two of our three BEC lasers, whereas a value of 10 Å is the lower limit required for a feedback bandwidth of 20 kHz. Both of these values result in a signal-to-noise ratio of approximately 5 relative to the shot noise, leading to theoretical stability in the lock point of 200 kHz in the laser output frequency.
  18. Diode laser purchased from TOPTICA Photonics AG, Model DL 100; see http://www.toptica.com/page/scientific_lasers.php.
  19. This value is based on a 50 mm focal length for the lens in Fig. and a tilt angle of 50 μrad, calculated assuming a 200 nm arclength due to the tilt. We feel this estimate of arclength is a more than generous number, given the previously measured piezo displacements referenced in the paper.
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  22. The self-heterodyne beat measurement uses an AOM to scatter a portion of the laser beam into the first order, producing a frequency-shifted beam (~200 MHz for our AOM). The unscattered zeroth order is then launched into a single-mode optical fiber before being mixed with the first order on a beam splitter. The length of the fiber should be significantly longer than the coherence length of the laser, rendering the zeroth-order beam incoherent relative to the first order. Upon mixing the two beams, a beat signal is obtained at the AOM drive frequency, and as long as the two beams are sufficiently incoherent, this results in a reliable measurement of the laser linewidth without the need for locking two identical lasers. Note that a laser with a 100 kHz linewidth has a coherence length of about 2 km, which is significantly shorter than the 3 km of fiber used in our setup.

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