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Tunable Optical Filter for Wavelength Division Network

IP.com Disclosure Number: IPCOM000121595D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 4 page(s) / 149K

Publishing Venue

IBM

Related People

Bowen, DF: AUTHOR [+2]

Abstract

A compact, high speed tunable optical filter with a large tuning range can significantly improve the performance of wavelength division multiplexed system. Currently available devices are limited either by the tuning speed, filtering bandwidth or size of the device. Here, we propose a novel compact tunable device that can potentially be tuned over a range from 1 to 2 mm with tuning speed of Z 10 ms and high spectral resolution Z 1 o . The dimension of the device could be around a Z few cm.

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Tunable Optical Filter for Wavelength Division Network

      A compact, high speed tunable optical filter with a large
tuning range can significantly improve the performance of wavelength
division multiplexed system.  Currently available devices  are
limited either by the tuning speed, filtering bandwidth or size of
the device. Here, we propose a novel compact tunable device that can
potentially be tuned over a range from 1 to 2 mm with tuning speed of
Z 10 ms and high spectral resolution Z 1 o . The dimension of the
device could be around a Z few cm.

      Fig. 1 shows the schematics of the proposed design, which is
based on the well known Lyot-Ohman filter design. Instead of the
birefringent material interspersed between polarizers as in
Lyot-Ohman filter, we propose the use of magneto-optically active
material.  By applying an external magnetic field, the polarization
of the incident light will change (Faraday's rotation) and the
appropriate wavelength can be transmitted through the polarizers.
The filter is positioned inside an inductive coil through which the
electrical current is passed.

      The detail of the tunable filter is described as below: Similar
to the Lyot-Ohman filter, our proposed filter has a series of N
parallel magneto-optic plates of thickness tr,
interspersed between N+1 polarizers whose planes of polarization are
parallel. For a given applied field, the rotation of the plane of
polarization of the incident radiation in the rth crystal plate is
where H is the magnetic field strength and V is the Verdet constant
and is wavelength dependent. Clearly, if r1 = (2M + 1)/2f, the
linearly polarized light at that particular g entering it will have
its polarization direction turned 90o and will be completely blocked
by the second polarizer. However, for other g which satisfy the
condition r1 =Mf, transmission will be allowed.  The transmittance of
the rth element of the filter, consisting of one crystal and one
polarizer is
for incident light plane-polarized parallel to the azimuth of the
polarizer.  The complete transmittance of the complete filter for
unpolarized light is, therefore,
To depends on the light losses within the filter.   The transmittance
curve represented by the above equation consists of a series of
narrow transmission bands (see Fig. 2). Successive bands or Free
Spectral Range is wgZgf/V(g)Ht and the full width at half maximum is
Wg3dbZgf/ V(g)HtN . Fig. 2 shows the resulting transmission of 5 such
plates. Typically, V(g) varies as g-2...