Coherent light generators – Particular beam control device – Tuning
Patent
1997-12-11
2000-03-14
Lee, John D.
Coherent light generators
Particular beam control device
Tuning
372 98, 372 99, 372101, H01S 3105
Patent
active
060382392
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a tunable laser light source which involves an uncomplicated arrangement and is remarkable for its high optical stability and specific suppression of the broadband spontaneous emission (ASE: amplified spontaneous emission) and the side modes. One field of application for such a light source is the Raman spectroscopy, among others.
2. Related Art
Tunable laser light sources are known in many variants. FIG. 1 exemplifies a semiconductor laser arrangement. Appropriately modified, this configuration is also used in dye lasers. Essentially, it consists of the laser diode LD, a collimator KO, a diffraction grating GI for dispersing the laser radiation, and a plane mirror SP rotatable in the dispersion direction of the grating. The laser radiation forming an essentially parallel beam after the collimator KO is diffracted at the grating GI and reaches the rotatable mirror SP. Only those laser wavelengths diffracted in such a fashion as to strike the mirror SP substantially vertically return on the same path with sufficient accuracy and are imaged on the active laser facet, thereby generating an optical back-coupling. Thus, the wavelength range coupled back and hence, the emission major wavelength of the arrangement may be selected by merely rotating the mirror SP in the dispersion direction of the grating GI.
The usable laser radiation, on the other hand, is decoupled by the zero order diffraction of the grating, for which purpose the radiation is focussed, e.g., by a lens O in an optical waveguide LWL. Independently of the wavelength adjustment, the usable radiation always appears at the same position.
On the one hand, these arrangements which are widely used in the form of the above or a similar type, are disadvantageous in that they are highly sensitive even to minor maladjustment. For example, because the optically effective facet of a semiconductor laser is very small, the configuration is required to have particular stability with respect to inclinations of the optical path vertically to the dispersion direction. This involves the bearing of the rotatable mirror, the stability of grating support, laser and collimator. In such configurations, three degrees of freedom must be controlled, only one of which being imperatively required, namely, the rotation of the mirror for wavelength tuning. The displacement of the laser vertically to the dispersion direction of the dispersing element, as well as the displacement of the laser chip along the optical axis, which is required for focussing, must be held in optimum position without permitting any of the starting parameters to be changed. One aspect that complicates the problem is that these two independently adjustable coordinates cannot be optimized separately, so that naturally, there is only one optimum position in this two-dimensional method of adjusting. Frequently, a special procedure is used for this object.
Another drawback of such arrangements is that rather than spectrally purified radiation, a fraction of the entire radiation mix in the resonator, including the spontaneous emission and more or less strong side modes, is taken from the resonator as usable radiation. In addition, since decoupling of the radiation takes place through a path different from that of the back-coupling, barely avoidable reflections from the external arrangement wherein the laser radiation is used may have massive influence on the radiation in the resonator, unless additional means are employed, such back-coupling normally not taking place in a wavelength-selective fashion. Inherently, this is the case if an optical image of the laser facet is present on an at least partially reflecting, not necessarily mirroring surface (e.g., optical waveguide, receiver surfaces), because such arrays act as retroreflectors.
Solutions are known which either allow to obtain a radiation which is spectrally pure to a large extent, and those which increase the adjusting tolerance of such a laser resonator by using special
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