Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-01-28
2001-07-31
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S043000, C385S007000, C385S088000, C359S385000
Reexamination Certificate
active
06269206
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention relates to microscopes and, in particular, microscopes with a short pulse laser which is coupled in via a light-conducting fiber.
b) Description of the Related Art
Short pulses, depending on pulse length, have a determined spectral bandwidth. In dispersive media such as, e.g., the glass of an optical fiber, the pulse length changes when passing through the medium due to the influence of group velocity dispersion (GVD).
This occurs as a result of the temporal splitting of the individual frequency components of the pulses because in normal dispersive media (glass) the red-shifted frequency components have a higher group velocity than the blue-shifted frequency components. The spectrum remains unaffected by this.
This pulse widening can be compensated by means of a suitable pre-chirping unit (e.g., comprising gratings or prisms or a combination thereof) as in DE-GM 29609850.
For this purpose, the spectral components of the pulses are arranged with respect to time in the pre-chirping unit in such a way that the blue-shifted frequency components run in advance of the red-shifted frequency components in comparison with the center frequency. When subsequently coupled into a dispersive medium (e.g., an optical glass fiber), this temporal splitting of the pulse frequency components is eliminated again. The pulses accordingly appear at the end of the optical medium (glass fiber) in their original form, i.e., in the form in which they came out of the laser.
In addition to these linear effects, however, nonlinear effects, i.e., effects depending on the intensity of the laser radiation, also occur in optical media. These effects (SPM, XPM, etc.) have an effect on the spectral width or pulse profile.
In most cases, they limit the minimum pulse length that can be achieved in a pre-chirping unit. These effects are undesirable for coupling of a short pulse laser.
These effects can be prevented in principle by limiting the intensity of the laser radiation below a critical value (I
crit
). In a short pulse laser, the intensity (I) is determined by the pulse length (T), repetition rate (f), average output (P
avg
) and by the beam cross section (A) by the following equation:
I
=
P
Avg
τ
·
f
·
A
<
I
crit
In mode-conserving and polarization-conserving glass fibers, the cross-sectional surface is determined by the wavelength of the laser radiation to be coupled and the repetition rate is determined by the laser system that is used.
The change in pulse length of a pulse sent through a pre-chirping unit beforehand when passing through the glass fiber is shown in the upper part of FIG.
2
. It will be seen that the pulse length at the end of the fiber is minimal. Accordingly, the intensity increases toward the end of the fiber when the average output remains constant.
At the same time, there is also an increase in the influence of the nonlinear effects (SPM) at the end of the fiber. This is shown in the lower part of
FIG. 2
by the change in the spectral width.
Accordingly, at a given pulse length at the output of the glass fiber, the average output that can be coupled into the microscope is limited by the nonlinear effects.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to minimize the influence of nonlinear effects on the pulse profile. This object is met by a microscope with a short pulse laser which is coupled into the illumination beam path via light-conducting fiber, particularly with pulse lengths in the subpicosecond or picosecond range comprising an optical arrangement for wavelength-dependent temporal delay of the laser pulses being provided between the laser and light-conducting fiber. Further, means for increasing the average output of the radiation coupled into the microscope by at least one dispersive element is arranged following an end of the light-conducting fiber in the illumination direction.
In order to prevent nonlinear effects at the end of the fiber, the intensity is reduced according to the invention. This can be carried out in an advantageous manner, for example, by enlarging the cross-sectional surface A (see equation above). This enlargement must be carried out at that point at which the pulse length, and accordingly the intensity, reaches the critical value. The pulse is accordingly further compressed in a region with greater cross-sectional surface so that nonlinear effects can be prevented. The following applies in this case, the greater the dispersion in the region with large cross section, the greater the average output that can be coupled.
A fiber whose core diameter increases at the end of the fiber is shown in FIG.
4
. Instead of a fiber with increasing cross section, two or more fibers which can also advantageously be connected one into the other can also be used, wherein the cross section of the individual fibers increases in the direction of illumination.
However, it is particularly advantageous when a highly dispersive element is installed in the laser scanning microscope. A highly dispersive element of this kind is, for example, any type of prism or grating compressor. Further, special glass materials or crystals (e.g., TeO2) such as those installed, e.g., in acousto-optic devices, are suitable.
FIG. 1
shows an arrangement using an acousto-optic modulator (AOM).
By using these AO devices (AOM; AOD (acousto-optic device); AOTF (acousto-optic tunable filter)), the nonlinear effects can be prevented (i.e., the average output that can be coupled at a given pulse length) and all of their advantages in laser scanning microscopy can be made use of at the same time.
For example:
scanning of the laser beam
continuous attenuation
rapid switching in ms range
and delay of the phase.
Especially when used as an AOM, this crystal can also be traversed repeatedly to increase the dispersion. This is carried out in a particularly simple manner by using the zeroth order and by reflecting out the 1 st order in a corresponding manner.
REFERENCES:
patent: 4342499 (1982-08-01), Hicks, Jr.
patent: 5119385 (1992-06-01), Aoshima et al.
patent: 5852702 (1998-12-01), Nishida et al.
patent: 5862287 (1999-01-01), Stock et al.
patent: 5920425 (1999-07-01), Yoo et al.
patent: 5952668 (1999-09-01), Baer
patent: 5995281 (1999-11-01), Simon et al.
patent: 6016376 (1999-07-01), Ghaemi et al.
Simon Ulrich
Wolleschensky Ralf
Bovernick Rodney
Carl Zeiss Jena GmbH
Kim Ellen E.
Reed Smith LLP
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