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.
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.
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.
Prototype: MDFC and MNFFC are validated using an Nd:YAG-laser
Patents granted: US7692502 and DE102004028252
Patent Applicant: University of Göttingen public law foundation
Dipl.-Phys. Winfried Willemer
Patent Manager Physics und IT
Tel.: +49 551 30724 153