Browse Prior Art Database

Cascaded Wedge Etalon Filter

IP.com Disclosure Number: IPCOM000121232D
Original Publication Date: 1991-Aug-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 7 page(s) / 222K

Publishing Venue

IBM

Related People

Bowen, DF: AUTHOR [+2]

Abstract

This article generally relates to a method of narrowband filtering of optical radiation, as used in optical communication or spectroscopy. More specifically, this invention describes a compact and low-cost optical filter with extreme stability, precision and high resolution. The filtering mechanism is based on the mechanical positioning of wedge etalons (Fabry-Perot interferometers) along the input light propagation path. The as-described invention can be a spectrometer used in the scientific laboratory or used as an optical filter in wavelength division multiaccess communication network. Both applications require extreme accuracy and high resolution. The latter application, in particular, is sensitive to cost.

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Cascaded Wedge Etalon Filter

      This article generally relates to a method of narrowband
filtering of optical radiation, as used in optical communication or
spectroscopy.  More specifically, this invention describes a compact
and low-cost optical filter with extreme stability, precision and
high resolution.  The filtering mechanism is based on the mechanical
positioning of wedge etalons (Fabry-Perot interferometers) along the
input light propagation path.  The as-described invention can be a
spectrometer used in the scientific laboratory or used as an optical
filter in wavelength division multiaccess communication network.
Both applications require extreme accuracy and high resolution. The
latter application, in particular, is sensitive to cost.

      The existing tunable optical filters provide either a large
bandwidth with low resolution capability, such as a grating-based
spectrometer, or a low bandwidth but with high resolution capability,
such as a piezoelectrically tuned Fabry-Perot interferometer.  The
conventional spectrometer is not only unable to deliver the spectral
resolution (resolution up to Z 0.1 o at 0.5 mm) required in most
applications, but is usually bulky and expensive.  The
piezoelectrically tuned Fabry-Perot filter is limited to small
bandwidth (Z 10 nm at 1.55  mm, for example) and is susceptible to to
mechanical vibrations and thermal drift. There is an instrument
available, the optical spectrum analyzer, which combines the above
technologies but is very expensive and bulky in size.

      The present invention makes use of many single cavity etalons.
Each of these etalons has a wedge such that the thickness varies the
diameter or diagonal of the etalon. By positioning these wedge
etalons along the beam path, the pass bands of the etalons in which
the beam propagates through will filter the appropriate wavelengths
and reflects the rest.  The positioning of the wedge etalons can be
done by a digitally-controlled stepper motor.

      Single- and multi-cavity etalons are described in [1]. The use
of two separately tuned single cavity etalons in optical
communication applications is described in [2].

      The use of wedges goes all the way back to Fizeau, but in the
so-called Fizeau interferometer, the two surfaces of the wedge have
only low reflectivity so that interference takes place between only
two reflected beams of light, one from the near surface and one from
the far surface.  In a true wedge etalon, both facing surfaces of the
wedge are reflective-coated so that the light makes many bounces, as
in any Fabry-Perot interferometer (etalon).  The use of wedge etalons
as optically tunable devices has been described in [3,4].

      The invention of [5] involves deflection followed by a single
wedge. The deflection is mechanically performed by rotating a
polystyrene block placed in the light path.  This single-wedge
configuration, not the multiple wedges in this invention, lacks...