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Heterodyne Detection of Phase-Shift Encoded Data in a Time Domain Spectral Hole-Burning Data Storage System

IP.com Disclosure Number: IPCOM000117317D
Original Publication Date: 1996-Feb-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 106K

Publishing Venue

IBM

Related People

Jefferson, M: AUTHOR [+2]

Abstract

The idea of using phase-shift encoded data storage in a persistent spectral hole-burning system has been previously disclosed (1). This technique requires some form of local oscillator (a laser) serving as the optical phase reference to retreive the data using the stimulated echo. This local oscillator must reproduce the optical phase/frequency jitters of the laser field used in the recording process, but with a constant frequency offset. The bit error rate of the storage/retreival process is thus dependent upon the phase and frequency stability of the laser source. The optical phase and frequency jitter of the laser field is mainly caused by environmental disturbances, but also are due to the spontaneous emission of the photons in the gain medium of the laser, especially in diode lasers.

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Heterodyne Detection of Phase-Shift Encoded Data in a Time Domain
Spectral Hole-Burning Data Storage System

      The idea of using phase-shift encoded data storage in a
persistent spectral hole-burning system has been previously disclosed
(1).  This technique requires some form of local oscillator (a laser)
serving as the optical phase reference to retreive the data using the
stimulated echo.  This local oscillator must reproduce the optical
phase/frequency jitters of the laser field used in the recording
process, but with a constant frequency offset.  The bit error rate of
the storage/retreival process is thus dependent upon the phase and
frequency stability of the laser source.  The optical phase and
frequency jitter of the laser field is mainly caused by environmental
disturbances, but also are due to the spontaneous emission of the
photons in the gain medium of the laser, especially in diode lasers.
Although phase stable laser sources of the required specifications
can be built, their complexity, fragility, and great expense
prohibits their consideration for a practical storage device.

      In order to retreive the phase-shift encoded data in a time
domain spectral hole-burning data storage system, it is vital that
the phase jitter of the recording laser be recorded at the same time
that the data is recorded.  This phase reference echo pulse can then
serve as local oscillator during readback and heterodynes the data
echo signal to the difference frequency between the two optical
fields, with the relative phase shifts between the two fields (the
data) appearing as phase shifts in the (radio frequency) beat note
output by the detector.

The temporal dependence of the optical field of the data pulse which
is to be stored in the medium can be written as:
  eqno lbracket Eq.1 rbracket E(t)=THETA (t) lbrace E sub 1 cos
   lbracket omega sub 1 t + psi (t) + phi sub 1 (t) rbracket plus
   E sub 2 cos lbracket omega sub 2 t + psi (t) + phi sub 0 + phi sub
   2 (t) rbracket rbrace
  where THETA (t) describes the amplitude control of the laser
field.  THETA (t)=1  when the data pulse is on, otherwise THETA
(t)=0 .  E sub 1 and E sub 2 are the amplitudes of the field at
optical frequencies omega sub 1 and omega sub 2 respectivly, psi (t)
is the optical phase noise, phi sub 0 is a constant phase, and phi
sub 1 and phi sub 2 are the phase shifts due to the data coding.  The
constant difference frequency OMEGA = omega sub 2 - omega sub 1 falls
into the radio or microwave frequency regime, where very stable
oscillators are  available.  OMEGA is chosen to be larger than both
the data rate and 1  over <T sub 1> or 1 over <T sub 2> of the
material.  OMEGA  should also  be chosen to avoid problems due to the
hyperfine levels, if a rare earth  ion doped storage medium is used.
The encoding process is designed such  that the phase difference phi
sub 2 (t)- phi sub 1 (t) represents the data.

      The field described abo...