Laser system tolerating disturbances using multiple modes

Details Laser system tolerating disturbances using multiple modes Laser system tolerating disturbances using multiple modes

1999-01-07

2001-04-24

Lee, John D. (Department: 2874)

Coherent light generators

Particular beam control device

Mode discrimination

C372S009000, C372S029020

Reexamination Certificate

active

06222860

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a laser system comprising a gain medium for providing and amplifying a laser beam within an optical resonator.
Molecules, and in particular gas molecules, are mainly investigated by spectroscopy. Two different spectroscopic methods, i.e. infrared absorption and Raman scattering, are generally applied for airway gas monitoring. The most common and widely spread measuring type is the infrared absorption, since it provides a robust and simple system with reliable accuracy. Disadvantageous, however, is that the infrared absorption is not flexible for upgrading to other molecules. Raman scattering overcomes that disadvantage because each molecule provides its own characteristic scattering signal. In addition and in contrast to the infrared absorption, the wavelength of the excitation light can be chosen flexibly. The drawback of the Raman scattering, however, lies in its minor effect, meaning that an excitation power of a light beam will create only a very low Raman signal (e.g. an excitation power of 1 W will create a Raman signal of 1 pW).
For medical purposes, such as respiratory or anesthetic gas monitoring, Raman scattering has been investigated as shown e.g. by Van Wagenen et al in “Gas Analysis by Raman scattering”, Journal of Clinical Monitoring, vol. 2 No. 4, October 1986.
Because of the minor effect in Raman scattering, the optical output power of the excitation light should be selected as high as possible. In addition, to achieve a good resolution of the molecule spectra, the excitation light source should be a narrow band source with a good wavelength and power stability. Thus, laser sources are commonly used as excitation light sources, whereby for reasons of compactness, lifetime and price, semiconductor lasers are normally superior to solid state or gas lasers. However, semiconductor lasers exhibit, in contrast to solid state and gas lasers, the disadvantage of a low internal circulating optical power and a low coupled out optical power.
A known solution for increasing excitation power for Raman scattering is disclosed in U.S. Pat. No. 5,153,671 and U.S. Pat. No. 5,245,405 for a gas analyzing system. A gas analysis cell employing Raman scattering is positioned within a single optically resonant cavity. The gas flow is directed into the cavity and analyzed within the gas analysis cell.
FIG. 1A
shows in principle such a laser system
10
in the gas analyzing system of U.S. Pat. No. 5,153,671. The laser system
10
comprises a laser cavity
20
between a first mirror
30
and a second mirror
40
. A gain medium
50
provides and amplifies a laser beam
60
which serves as an excitation beam in a gas analysis cell
70
within the laser cavity
20
. The first mirror
30
may also be part of the gain medium
50
.
In a more sophisticated solution for increasing excitation power, in particular when semiconductor lasers are used as excitation sources for Raman scattering, the optical output power of the excitation laser is coupled into an external resonator as shown e.g. in U.S. Pat. No. 5,642,375 or U.S. Pat. No. 5,684,623 by the same applicant.
FIG. 1B
shows in principle such a coupled laser system
80
. The coupled laser system
80
comprises the laser cavity
20
between the first mirror
30
and the second mirror
40
and the gain medium
50
providing and amplifying the laser beam
60
. An external cavity
90
is provided between the second mirror
40
and a third mirror
95
, and is optically coupled to the laser cavity
20
. The laser beam
60
serves as excitation beam in the gas analysis cell
70
within the external cavity
90
. By applying low loss mirrors with different reflection coefficients for the mirrors
30
,
40
and
95
, as described e.g. in U.S. Pat. No. 5,642,375, a very high built-up optical power inside the resonator of the external cavity
90
can be achieved. For example, a 10 mW semiconductor laser beam
60
is capable of pumping the external cavity
90
up to several hundreds of Watts. U.S. Pat. No. 5,432,610 further discloses a passive, purely optical locking of a laser diode on an external resonator.
Using such an external pumped resonator, as depicted as the laser system
80
in
FIG. 1B
, for probing an unknown gas sample in the external cavity
90
will in particular provide enough optical power to excite a Raman signal well above the sensitivity limit of optical sensors. Optical sensors can simply be photodiodes, charged coupled devices or other image sensors for more sophisticated applications.
As well in the single cavity laser system
10
as in the coupled cavity laser system
80
, the gas analysis cell
70
represents the principal possibility of probing a gas sample, whereby the gas sample can be analyzed in a specific (separated) environment or directly in the respective cavity. Probing the gas sample can either be accomplished ‘offline’, i.e. the gas sample is taken and analyzed later (e.g. in a defined environment), or ‘online’, i.e. the gas sample is directly provided to the gas analysis cell
70
and analyzed. The latter case, in particular, allows monitoring of a gas flow such as a respiratory or anesthetic gas. Online gas monitoring, however, requires an increased effort with respect to stabilizing the laser system.
If there are no changes of the applied active and passive components of the laser system, e.g. laser system
10
or
80
, and as long as the environmental conditions remain unchanged, the light beam
60
(in the laser cavity
20
of
FIG. 1A
or in the external cavity
90
in
FIG. 1B
) will substantially remain at constant power. The light beam
60
circulating in the laser cavity
20
and the external cavity
90
comprises one or more (longitudinal) optical modes determined by the components of the laser system and the specific environmental circumstances within the respective cavity/cavities. Associated with each optical mode are a defined wavelength and a defined roundtrip phase shift. The gain medium
50
supports the optical mode(s) that match(es) the required wavelengths and provides the necessary phase, thus leading to a high intensity build-up light beam
60
at the supported optical mode(s).
It is to be understood that semiconductor type lasers generally only support one optical mode at a time, while other laser types (e.g. gas laser) may support more than one optical mode concurrently. For the sake of simplicity, only semiconductor type lasers, supporting only one optical mode at a time, shall be considered in the following. However, it is clear that the principals as illustrated herein are applicable for multi-mode concurrently supporting lasers accordingly.
In the coupled cavity system of
FIG. 1B
, a locking mechanism between the two resonators has to take place. To achieve substantial amplification and stability in the external cavity, the feedback of the external cavity into the laser cavity has to be adjusted, so that the laser diode radiation emits coherent radiation with a bandwidth and a wavelength to actively support the external cavity
90
at a cavity resonant frequency. This process is called hereinafter “optical locking”.
When a change of the applied active and passive components (e.g. of the optical path length) of the laser system occurs and/or the environmental conditions change, the currently supported optical mode does not match anymore the required wavelength and phase shift, and the laser system has to ‘find’ another optical mode matching the changed resonating conditions within the laser system. Thus, the light beam
60
can suddenly extinguish (albeit temporarily) until a new optical mode is built up fitting to the changed cavity properties. This leads to a (significant and in most cases unwanted) variation of the optical power of the optical beam
60
over the time.
FIG. 2
shows an example of a variation of the optical power over the time in the external cavity
90
in an arrangement according to FIG.
1
B. The power variations in the measurement example of
FIG. 2
were possibly induced by temperature or other envir

Affiliated with

Also associated with

Rating

Laser system tolerating disturbances using multiple modes does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Laser system tolerating disturbances using multiple modes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Laser system tolerating disturbances using multiple modes will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2466283