Optical threshold and comparison devices and methods

Details Optical threshold and comparison devices and methods Optical threshold and comparison devices and methods

2001-07-27

2004-07-20

Robinson, Mark A. (Department: 2872)

Optical waveguides

Integrated optical circuit

C250S227210, C359S344000

Reexamination Certificate

active

06766072

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to optical devices and methods for optical threshold determination and for performing comparison functions, as are used in signal regeneration, pattern recognition and optical computing applications.
FIG. 1
of the accompanying drawings shows an optical device as proposed by Gray et al [
1
]. The proposed device is an amplifier made of an active gain medium such as a rod used for a solid state laser. In operation, one signal I
1
is injected straight through the gain medium and another signal I
2
is injected in a zigzag path defined by total internal reflections from the sides of the gain medium. The zigzag path is longer than the straight path and thus has a higher net gain. Its output is thus more sensitive to variations in the gain of the gain medium than the output from the straight path. This allows a signal injected in the lower gain straight path to control the gain of a signal travelling in the zigzag path by cross saturation of the gain, i.e. cross gain modulation (XGM). Light input into the amplifier along the straight path thus represents a control input, whereas light input along the zigzag path represents a signal input to be modulated.
FIG. 2
of the accompanying drawings shows another optical device proposed by Gray et al [
1
] (see FIG.
10
(
a
) of that reference) which is an optical differential comparator based on two of the above-described amplifiers connected in a feedback configuration. A portion of the signal output from a first amplifier
1
is fed back to the control input of a second amplifier
2
by means of a partially reflecting mirror
3
and further routing components
4
which may be a sequence of waveguides or mirrors. The feedback action results in operation analogous to that of an electrical differential amplifier or comparator, as illustrated in FIG.
10
(
b
) of reference [
1
]. Threshold and logic functions are also discussed in reference [
1
].
FIG. 3
of the accompanying drawings shows a further prior art optical device according to Parolari et al [
2
]. This device may be considered to be a development of the free-space device of
FIG. 2
proposed in reference [
1
]. The device comprises two amplifier elements
120
and
130
analogous to the corresponding elements
1
and
2
of FIG.
2
. These are implemented as semiconductor optical amplifiers (SOA's). A portion of the output signal from the first SOA
120
is fed back to the input of the second SOA
130
. Similarly, a portion of the output signal from the second SOA
130
is fed back to the input of the first SOA
120
. Input and output side three-way couplers
123
,
121
,
133
and
131
(C
11
, C
21
, C
12
and C
22
) are provided for this purpose, as illustrated.
In the prior art free-space device of
FIG. 2
, the two beams are separated spatially and thus do not interact. By moving to the waveguide implementation of
FIG. 3
, spatial separation is lost. To overcome this problem, spectral separation is provided by using two different wavelength &lgr;
1
, and &lgr;
2
and inserting first and second optical filters
129
and
139
after their respective SOA's in order to separate the interacting signals. The first optical filter
129
is transmissive at the wavelength &lgr;
1
of the first SOA
120
but absorptive at the wavelength &lgr;
2
of the second SOA
130
. This prevents the feedback signal P
feedback
(&lgr;
2
) being transmitted to the coupler
121
and on to the device output together with the output signal P
out
(&lgr;
1
). Similarly, the second optical filter
139
is transmissive at the wavelength &lgr;
2
of the second SOA
130
but absorptive at the wavelength &lgr;
1
of the first SOA
120
. The filters
129
and
139
may be bandpass or cut-off filters. Bragg reflectors could be used, for example.
In addition, optical isolators
124
and
134
are inserted before both SOA's. This helps suppress problems arising from the use of SOA's as the non-linear active gain medium. These SOA problems may arise from: (a) the high small signal gain for unit length; (b) parasitic oscillations which may occur due to reflections in the circuit decrease the gain available for compression; and (c) the total device efficiency. Coupling ratios are chosen from a trade off between the feedback and output powers.
In summary, the device of
FIG. 3
may be considered to be a waveguide development of the free-space device of
FIG. 2
in which two wavelengths &lgr;
1
and &lgr;
2
are used instead of one, in conjunction with optical filters
129
and
139
, and the isolators
124
and
134
.
The maximum bit rates that can be handled by the prior art devices of reference [
1
] and reference [
2
] would be limited by the carrier lifetime in the active gain medium. In addition, the inventors have also realized that the maximum bit rates are also limited by the propagation time associated with the feedback paths. The propagation time is defined by the device topology and would limit the ultimate maximum speed of any such device, if the active gain medium response time is very short.
The XGM effect is also discussed by Fatehi et al [
3
] in relation to Erbium doped fiber amplifiers (EDFA's). Moreover, it has been proposed to exploit the related effect of cross phase modulation (XPM) in various interferometer devices [
4
][
5
][
6
].
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided an optical device comprising:
(a) a first active medium having a first propagation path for traversal of a first optical signal in a first forward direction;
(b) a second active medium having a second propagation path for traversal of a second optical signal in a second forward direction; and
(c) a feedback path connecting the first and second active media so as to route at least a portion of the first and second optical signals, after traversing the first and second active media, to the second and first active media as respective second and first optical control signals which travel along the second and first propagation paths in second and first reverse directions that are opposite to the second and first forward directions respectively.
According to a second aspect of the invention there is provided a method of modulating an optical signal, comprising:
(a) providing first and second active media;
(b) supplying first and second optical signals to traverse the first and second active media in first and second forward directions; and
(c) routing at least a portion of the first and second optical signals, after traversing the first and second active media, to the second and first active media as second and first optical control signals respectively, wherein the first and second optical control signals are supplied through the first and second active media in first and second reverse directions opposed to the first and second forward directions so that the first and second optical control signals vary the modulation experienced by the first and second optical signals.
The device and method of the first and second aspects of the invention thus fundamentally differ from those of references [
1
] and [
2
] in that the optical control signal is supplied to the active medium in the opposite direction to the optical signal to be modulated, instead of in the same direction.
Whether the optical control signal is supplied in a co-propagating configuration, as in the prior art, or in the counter-propagating configuration according to the invention has little influence on the XGM effect. However, counter-propagation of the optical control signal means that separation of the optical signal and optical control signal can be obtained by the direction of propagation alone. Consequently, the optical signal and optical control signal can be the same wavelength if desired. Moreover, there is no requirement to separate out the optical control signal from the optical signal at the output,

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