Coherent light generators – Optical fiber laser
Reexamination Certificate
1999-06-10
2001-12-04
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Optical fiber laser
Reexamination Certificate
active
06327278
ABSTRACT:
The invention refers to a diode laser pumped multimode waveguide laser, particularly a fiber laser.
BACKGROUND
Multimode lasers are employed in various spectroscopic applications, among others. Absorption spectroscopy requires a broad-band emitting light source, when narrow absorption lines are to be studied within the emission spectrum. In the field of highly sensitive spectroscopy, the method of intra-cavity spectroscopy has taken a wide application range in research activities. This purpose requires multimode lasers, the resonators thereof being equipped with a light amplifying medium and, in addition, an absorption cell with a narrow-band absorber.
In recent years, a great number of lasers suited for this type of spectroscopy have been developed. Mostly, these are dye lasers or solid state lasers pumped by an ion laser. Such arrangements may have a smooth emission spectrum since they emit in a plurality of independent laser modes at the same time, yet they are restricted to lab use, because dye lasers are very sensitive to external influences and ion lasers that are the pump source are difficult to transport, have a high energy consumption and must be connected to a cooling water line with a high flow rate. Further, most of the absorbers relevant for practical use have a well measurable absorption in the infrared range, whereas conventional multimode lasers mostly operate best in the visible spectral range.
In this respect, waveguide lasers, and in particular fiber lasers, promise substantial improvements since they are suited for compact designs, are mechanically stable enough and practically insensitive to external influences, and do not require complicated cooling. Moreover, the waveguide may readily be doped with different active ions, whereby the range of emission may be predetermined well and may be set to lie within the infrared range.
However, presently used waveguide lasers are disadvantageous in that they must be pumped by an ion laser. The application of a fiber laser, pumped by an argon ion laser, in intra cavity spectroscopy is known, for example, from a publication in OPTICS LETERS 1993, vol. 18, No. 22, p. 1955, entitled “Intracavity absorption spectroscopy with a Nd
3+
-doped fiber laser”. Yet, this arrangement is not suited for mobile use, due to the argon ion laser.
It is further known from the same publication that fiber lasers can be pumped by diode lasers. However, the emission spectra obtained thereby are, as of yet, not suited for absorption spectroscopy with a narrow-band absorber since they show substantial irregularities that make the resolution of the absorption lines impossible.
SUMMARY
It is the object of the present invention to develop a diode laser pumped wave guide laser such that it is suited for this kind of absorption spectroscopy. Compared to the conventional arrangements, the laser and its pump source should be simple and economical to produce, have dimensions that allow for an easy transport and should not be too demanding with a view to its energy supply.
According to the present invention, the object is solved with a diode laser pumped multimode wave guide laser, characterized in that the cutoff wavelength of the waveguide is smaller than the smallest wavelength of the emission spectrum of the wave guide laser.
Each laser mode in the wave guide laser has longitudinal and transverse components. Above the so-called cutoff wavelength, which is also referred to as the limit wavelength and is specific to the waveguide, only the longitudinal components and the transverse fundamental components of the waveguide are transmitted.
It has ben found that in waveguide lasers different transverse components of the modes interfere with each other, thereby causing spectral noises. However, since all laser modes have the same transverse components, namely the fundamental components, above the cutoff wavelength, no noises occur and the emission spectrum is not super-imposed by sharp interference structures that cover narrow absorption lines. Further, it is very sensitive to narrow-band absorption in the resonator, since the absorption suppresses only individual modes, taking no influence on the other modes. Preferably, the cutoff wavelength should be at least 20 nm below the smallest emission wavelength.
Preferably, the light amplifiers used are waveguides doted with ions from the group of rare earths, in particular Nd, Yb, Tm, Ho, Er or Pr.
Such a diode laser pumped wave guide laser becomes particularly handy if the waveguide is a fiber, for example a glass fiber. Depending on the doping, the fiber may be selected with an appropriate length and wound on a hub. Thus, one can construct a particularly small and handy laser.
Preferably, a so-called stepped index fiber is employed, where the fiber core and the fiber sheath have different refractive indices n
core
or n
sheath
with distinct limits. With such fibers, the cutoff wavelength &lgr;
cut
is calculated, for example, as follows:
&lgr;
cut
=2&pgr;
a NA/
2.405.
Here,
a is the radius of the fiber core;
NA is the numeric aperture,
obtained from
NA
=(
n
core
2
−n
sheath
2
)
½
.
So-called indiffused waveguides may also be used as the waveguides.
For the purposes of the highly sensitive intra-cavity spectroscopy, one end of the waveguide is reflective and the other end is non-reflective. The end of the resonator on the non-reflective side of the waveguide is formed by another external mirror. The beam path within the resonator extends unobstructed outside the waveguide between the non-reflective end of the waveguide and the external mirror. This space may be used for absorption spectroscopy at a narrow-band absorber. To this end, one either introduces an absorption cell into the resonator, or the absorber to be analyzed is inserted directly into the open resonator.
Preferably, the non-reflective end of the waveguide is also cut or broken in a beveled manner if no perfect non-reflective coating can be obtained. Thus, multiple reflections at the limit surface lying in the laser resonator are avoided, which could otherwise cause the spectrum to be superimposed by an interfering structure again. Preferably, the angle between the normal to the surface of the non-reflective end and the axis of the waveguide should be between 1° and 15°. The end may also be cut under the Brewster angle that depends on the waveguide material.
Further, it has been found that disturbing superimposed structures on the emission spectrum may be suppressed by optically decoupling the waveguide laser and the diode laser in the wavelength range of the emission. Some laser light always escapes not only through the outcoupling mirror of the waveguide, but also through the coupling mirror. In particular, if the waveguide laser is pumped collinearly by the diode laser, this laser beam can be reflected in the pumping diode or the collimating optics of the pumping diode and be returned into the waveguide laser. The occurring wavelength selection of the radiation fed back into the waveguide laser is superimposed on the emission spectrum in an interfering manner.
An optical decoupling may be obtained, for example, through a particularly high degree of mirroring on the reflective waveguide end for the emission wavelength, having a reflectivity of at least 98%, preferably more than 99%. At the same time, the reflectivity regarding the pumping wavelength should be as low as possible so as to guarantee a low pumping threshold energy of the diode laser.
According to another embodiment, an optical diode is provided between the laser diode and the waveguide laser. The optical diode is non-transmissive for the reflected light at the emission wavelengths of the waveguide laser. Laser light decoupled at the reflective end of the waveguide falls through the optical diode onto the collimating optics and the pumping diode, but is not reflected back into the waveguide laser by the optical diode.
In another embodiment, the optical decoupling is achieved by the collimating optics having a chromatic aberration between the pump
Baev Valerie M.
Boehm Rainer
Toscheck Peter E.
Arroyo Teresa M.
Inzirillo Gioacchino
Shumaker & Sieffert P.A.
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