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Intensity Stabilization of Miniaturized cw-Lasers

For a variety of metrological and analytical tasks there is a need for highly stable cw-lasers. Often besides a high long time stability of the intensity output, a high stability with regard to very short time scales is necessary. But in particular compact lasers often show highly modulated chaotic fluctuations on a time scale of microseconds. Such fluctuations are known as so-called 'green noise' of cw-DPSS. Similar chaotic fluctuations can be observed in diode lasers, too.
In particular for frequency doubled DPSS-lasers, the chaotic behaviour is caused by a mutual reaction of the different excited laser modes. Because of this, highly stable cw-lasers are designed most often to be single mode.

Using the method patented by the University of Göttingen it is now possible to stabilize multiple mode cw-lasers with almost no loss down to a noise of far better than 1% pp relative, measured at a time resolution corresponding to a high MHz-frequency. The method is easy to implement. There is no need for a special laser resonator design.

  

Schematic view of multiple delay feedback control (MDFC) of a frequency doubled Nd:YAG-laser; Source: Prof. Dr. U. Parlitz, Dr. A. Ahlborn
Schematic view of multiple delay feedback control (MDFC) of a frequency doubled Nd:YAG-laser; Source: Prof. Dr. U. Parlitz, Dr. A. Ahlborn; An electric current source (top left) provides via a bias element (below the current source) the power for a laser

Challenge

Highly stable compact cw-lasers are yet available but they cost many times more than simple compact cw-lasers because of the high effort for building single mode lasers. Besides this the power of such lasers is limited to the power of one single laser mode. The innovative method aimes for providing highly stable compact lasers with lesser technical effort; additionally such lasers should take advantage of using the power of multiple excited laser modes.

Our Solution

The patented multiple delay feedback control  (MDFC)  is as an essential improvement of state of the art methods to stabilize chaotic processes as e.g. the time delay auto synchronisation (TDAS).
In the practical laser application of the very more general method, the laser power is measured and from the measured signal a feedback signal is generated; the feedback signal consists additively of components with at least to different time delays relative to the measured laser power signal.
Essentially equivalent is the multiple notch filter feedback control (MNFFC or short NFF). At least two notch filters of different frequencies are used to generate the feedback signal. The filter frequencies can be derived from the delay terms by Fourier theory.

The applicability of this method in both variants, MDFC and MNFFC, to stabilize the output intensity of cw-lasers is successfully demonstrated using the example of a frequency doubled multiple mode Nd:YAG-laser.

  

Successfull laser intensity stabilization by MDFC: signal of the monitor diode.
Successfull laser intensity stabilization by MDFC: the signal of the monitor diode used in a set up as shown above is displayed; the range of the time scale is from about -750 µs to +750 µs; at t=0 the control is activated, at negative times the laser is

Advantages

  • high intensity stability, proven better 1% pp relative
  • competitive due to simple laser design
  • easy to implement
  • applicable to laser resonators without changing their designs
  • higher optical power due to use of multiple modes
  • suppression of periodic oscillations easy to integrate

Applications

  • Measurement technology in particular in the fields:

    • life sciences
    • medical technology
    • analysis
    • interferometry

  • Furthermore applicable to:

    • RGB projection systems
    • holographic displays

Developmental Status

Prototype: MDFC and MNFFC are validated using an Nd:YAG-laser

Patent Status

Patents granted: US7692502 and DE102004028252
Patent Applicant: University of Göttingen public law foundation

References

  1. Stabilisierung und Kontrolle komplexer Dynamik durch mehrfach zeitverzögerte Rückkopplung (Stabilization and control of complex dynamics using multiple delay feedback); A. Ahlborn, Göttingen, Univ., Diss., 2007 (http://webdoc.sub.gwdg.de/diss/2007/ahlborn/)
  2. Controlling spatiotemporal chaos using multiple delays; A. Ahlborn and U. Parlitz; PHYSICAL REVIEW E 75, 065202R (2007)
  3. Stabilizing Unstable Steady States Using Multiple Delay Feedback Control; A. Ahlborn and U. Parlitz; PRL 93, 264101 (2004)
  4. Chaos control of an intracavity frequency-doubled Nd:YAG laser; A. Ahlborn and U. Parlitz CP742, Experimental Chaos: 8th Experimental Chaos Conference, edited by S. Boccaletti et al. (2004 American Institute of Physics)
  5. Controlling dynamical systems using multiple delay feedback control; A. Ahlborn and U. Parlitz; PHYSICAL REVIEW E 72, 016206 (2005)
  6. Laser stabilization with multiple-delay feedback control; A. Ahlborn and U. Parlitz; OPTICS LETTERS Vol. 31, No. 4, p465-467 (2006 )
  7. Chaos Control using Notch Filter Feedback; A. Ahlborn and U. Parlitz, PRL 96, 034102 (2006)

Contact

Dr. Alexander Brinkmann
Patent Manager Physics
E-Mail: abrinkmann(at)sciencebridge.de
Tel.: +49 551 30724 159
Reference: CPA-0532-SUG

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