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Browse Prior Art Database

Multiple-Phase Generation for Parallel RF-Injected Acousto-Optic Tunable Filter

IP.com Disclosure Number: IPCOM000104911D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 4 page(s) / 144K

Publishing Venue

IBM

Related People

Choy, MM: AUTHOR [+4]

Abstract

Disclosed is a scheme to generate the phases required by an acousto-optic tunable filter (AOTF) using a parallel RF-injection scheme.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 48% of the total text.

Multiple-Phase Generation for Parallel RF-Injected Acousto-Optic Tunable Filter

      Disclosed is a scheme to generate the phases required by an
acousto-optic tunable filter (AOTF) using a parallel RF-injection
scheme.

      The tuning speed of a conventional acousto-optic tunable filter
on a Li Nb O sub 3 substrate is limited by the time required by the
surface acoustic wave to fill up the whole substrate.  For an AOTF up
to 2 cm, the tuning speed is thus limited to <=4 &mu.s, using the
propagation speed of the acousto-optic wave in the [*]  direction of
Li Nb O sub 3 6.57 times 10 sup 5 cm/sec [*].  A parallel injection
scheme in which more than one transducer is placed in sequence on the
Li Nb O sub 3 substrate so that the RF signals can be injected into
the filter at multiple points has thus been proposed to increase the
tuning speed of an AOTF.  However, specific implementation for
generating the precise phases in the right sequence for multiple
transducers remained a technological challenge and will be addressed
in this article.

      In order to support a tuning range of 100 nm at 1.55 &mu.m, the
wavelength of the surface acoustic wave of a Li Nb O sub 3 AOTF needs
to be tuned from 101 &mu.m at 65 MHz to 146 &mu.m at 45 MHz.
Assuming the transducers are placed at integer multiples of the
nominal surface acoustic wavelength N lambda sub SAW,o (where lambda
sub SAW,o = 119 mu m), all of the transducers require the same phase
for the nominal RF frequency at 55 MHz.  When the RF source for the
AOTF is detuned to a lower or higher frequency, different phase
correction would be required at different transducer positions.  When
the RF source is detuned, for example, to 50 MHz, a phase correction
of ( 36 degree times N ) mod 360 is required for the first
transducer, ( 36 degree times 2N ) mod 360 is required for the second
transducer, etc.

      In this invention, a switchable multiple phase generation
architecture, as shown in Fig. 1, is used to drive the parallel
injection points.  The major components in this invention includes a
voltage-controlled oscillator (VCO), a multiple phase generator, an
analog switch, and an on-chip cache which stores the interconnection
patterns.

      A ring oscillator with a phase detector can be used to generate
various phases of the RF signals, with the delay of each tunable
delay element controlled by a phase detector.  The amount of delay of
each tunable delay element is determined by the current flowing
through the element.  The nominal delay of these delay elements is
designed so that the total delay through the ring equals the nominal
period of the RF frequency (18 ns for 55 MHz).  (For example, the
nominal delay of each delay element would be 1.125ns if there are 16
delay elements.)  The increase or decrease of the delay of each delay
element is controlled by the phase detector at the end of the ring
and is determined by the comparison of the phase between the output...