Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
1998-12-03
2001-01-23
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S519000, C372S032000
Reexamination Certificate
active
06178002
ABSTRACT:
The invention relates to a method of and a device for laser frequency measurement and stabilization by means of an interferometer, on the basis of the generation of two signals shifted in phase by roughly 90° relative to each other, which signals are obtained from two component beams of the laser beam to be measured.
PRIOR ART
As a function of the intended application of a laser it may be important to maintain the laser frequency at a constant level, to allow for a desired behavior in frequency de-tuning, the so-called scans, or to provide an appropriate possibility to check the scan behavior or the longitudinal single-mode nature of the laser. To this end the use of optical interferometers, particularly Michelson, Mach-Zehnder and Fabry-Perot interferometers, is common for generating an electrical signal varying as the laser frequency varies. On account of the periodic structure of this signal, however, the laser frequency is not yet fixed because the frequency can be determined only within a period, the so-called free spectral range of the interferometer—which will be briefly referred to as FSR in the following. The resonator mode, i.e. the rough frequency, must then be determined by a further element, e.g. by means of an optical diffraction grating or another interferometer with a correspondingly wider FSR. Moreover, misinterpretation may occur in signal analysis as a result of variations of the intensity of the laser light because the measured signals are normally proportional to the intensity. Therefore, an intensity normalization and a compensation for possible offsets must be realized for precise measurements. Finally, the sinusoidal signal profile in measurement of an individual signal results in the fact that a distinction between the positive and the negative signal edge cannot be made and that the frequency resolution in the maximum and minimum ranges is substantially lower than in the range of the signal edges. These problems can be solved by using two practically identical signals which are, however, shifted through roughly 90° relative to each other.
Such phase-shifted signals can be generated, for instance, by means of a so-called sigmameter. A sigmameter is a modified Michelson interferometer where the specific phase jump characteristics of total reflection are utilized in order to generate two sinusoidal signals shifted by 90°. The measuring beam is detected in the two potential planes of polarization independently of each other. A highly similar principle is the basis of the so-called Koesters interferometer. The essential disadvantage of these systems is the fact that their optical structure consists of a great number of high-quality components. This involves high costs of the system and a great expenditure incurred for mechanical and thermal stabilization provisions. Moreover, a compact design is hardly possible.
Other methods of laser frequency measurement operate on piano-parallel Fabry-Perot interferometers or such devices having a small angle between the reflecting surfaces, so-called Fizeau interferometers. In both cases the laser beam to be measured is spatially expanded and the transmitted or reflected one or two-dimensional structure is detected by a detector array. A digital computer has the function of determining the laser frequency. What is expedient in such a system is the wide frequency range which can be unambiguously calculated within one stage of such a system. Moreover, the frequency spectrum can be determined to a certain extent in longitudinal multi-mode operation. What is inexpedient in these systems is again the high expenditure incurred by the optical elements, particularly the beam expanders, however mainly the detection rate is strongly restricted by the processing by means of the digital computer and the read-out of the detector array. Compared against the method proposed here, a further disadvantage consists in the critical dependence on the spatial structure of the beams to be detected, the transverse mode structure. This structure must be very well known or it must be additionally detected and compensated for high-resolution measurements.
INVENTION
The present invention is based on the problem of proposing a method of laser frequency and mode control which allows for a simple, low-cost and compact structure and simultaneously for a high-speed high-precision detection of the laser frequency. This problem is solved by the method defined in claim 1. The signals so obtained can be used to perform a desired high-speed correction of the laser frequency.
The invention is based on the further problem of providing a device for application of the inventive method. This problem is solved by the device defined in claim 14.
Depending on the angle of incidence of a laser beam on a Fabry-Perot interferometer a different phase shift &PHgr; is obtained between the component beams interfering with each other in the interferometer. This shift is defined by
Φ
(
α
,
d
,
λ
)
=
2
π
2
n
d
λ
1
-
sin
2
(
α
)
n
2
wherein n is the refractive index within the interferometer, d represents the thickness of the interferometer, &agr; indicates the angle of incidence, and &lgr; corresponds to the wavelength of the incident light. This phase shift &PHgr; determines the fraction of the reflected light versus the transmitted light. It is therefore possible to generate two component beams PBA and PBB having detected intensities of the reflected or transmitted components which present a phase difference of &PHgr;
A
−&PHgr;
B
=&pgr;/2, i.e. 90°, when the wavelength &lgr; varies. This can be achieved by an appropriate selection of the angles of incidence &agr;
A
and &agr;
B
and by a suitable choice of the optical thickness d
A
and d
B
of the interferometer at the site of the component beams.
On principle, the inventive method of laser frequency measurement now consists in the aspect that two component beams PBA and PBB, which are to be spatially separated or via their polarization properties, are passed through an interferometer FPI and are reflected by the latter as a function of the laser frequency. The reflected or transmitted components are detected by photosensitive sensors and converted into electronic signals, with the signals being intended to undergo a mutually relative phase shift through roughly 90° when the wavelength varies. This is achieved either by slightly different angles of incidence &agr;
A
and &agr;
B
or a slightly different thickness d
A
and d
B
of the interferometer at the site of the component beams PBA and PBB or by different refractive indices n
A
and n
B
for the two component beams, e.g. by the effect of birefringence.
The interferometer is preferably a Fabry-Perot interferometer which, compared against the other interferometer types, allows for a highly compact structure which is easy to stabilize in both thermal and mechanical terms.
A particularly simple and compact structure is achieved when a substrate consisting of a transparent material is employed as Fabry-Perot interferometer, a so-called etalon.
When the reflecting surfaces present a low degree of reflection the reflected and the transmitted signal display an approximately sinusoidal structure with a mutually relative shift through 180° over the wavelength. Specifically the reflected signal presents a wide scope of contrast and is therefore preferably used for detection.
It is expedient to generate one respective or one common normalization signal for the two component beams, which is proportional to the intensity of the respective component beam, with intensity-independent quadrature signals being generated from the electronic signals by normalization by means of the normalization signals.
With an appropriate summation such an intensity-proportional signal can be generated from the transmitted and the reflected signal of an interferometer for the purpose of normalizing the intensity, without the need for a further beam splitter.
A particular advantage is the fact tha
Birch & Stewart Kolasch & Birch, LLP
Turner Samuel A.
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