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| United States Patent |
4,873,690
|
|
Adams
|
October 10, 1989
|
Optical switch
Abstract
Using a semi-conductor laser amplifier in reflection, an optical switch is
achieved which can be applied as either an AND/OR, or a NAND/NOR, logic
gate. The amplifier responds to an optical input switching signal and the
logical characteristics of its response can be controlled by selection of
the drive current to the amplifier. The switch finds application in
optical logic or communication systems.
| Inventors:
|
Adams; Michael J. (Ipswich, GB)
|
| Assignee:
|
British Telecommunications public limited company (GB3)
|
| Appl. No.:
|
230837 |
| Filed:
|
August 10, 1988 |
| PCT Filed:
|
December 14, 1987
|
| PCT NO:
|
PCT/GB87/00905
|
| 371 Date:
|
August 10, 1988
|
| 102(e) Date:
|
August 10, 1988
|
| PCT PUB.NO.:
|
WO88/04791 |
| PCT PUB. Date:
|
June 30, 1988 |
Foreign Application Priority Data
| Current U.S. Class: |
372/8; 372/96 |
| Intern'l Class: |
H01S 003/30 |
| Field of Search: |
372/26,8,44,18,19,9-13
307/311
|
References Cited [Referenced By]
U.S. Patent Documents
| 4359773 | Nov., 1982 | Swartz | 372/26.
|
| 4382660 | May., 1983 | Pratt Jr. et al. | 372/18.
|
| 4468773 | Aug., 1984 | Seaton | 372/18.
|
| 4689793 | Aug., 1987 | Liu et al. | 372/8.
|
| Foreign Patent Documents |
| 0110388 | Jun., 1984 | EP.
| |
| 0169567 | Jan., 1986 | EP.
| |
| 2492995 | Apr., 1982 | FR.
| |
Other References
Olsson et al., "Optoelectronic Logic Operations by Cleaved-Coupled Cavity
Semiconductor Lasers", IEEE Journal of Quantim Electronics vol. QE-19, No.
11 Nov. 1983, pp. 1621-1625.
Sharfin et al., "Femtojoule Optical Switching in Nonlinear Semiconductor
Laser Amplifiers"Applied Physics Letters, vol. 48, No. 5, 3 Feb. 1986.
|
Primary Examiner: Scott, Jr.; Leon
Attorney, Agent or Firm: Nixon & Vanderhye
Claims
I claim:
1. An optical switch comprising:
a semi-conductor or optical amplifier having optical input and output ports
located at a common end of an amplifier cavity,
means for coupling a detuned optical input switching signal to the
amplifier, and
means for applying a driving electrical current to the amplifier, which
driving current is selected to have either one of at least two different
values, at a first value of which driving current the optical amplifier is
operated as an AND/OR logic gate for input optical signals and at a second
value of which driving current the optical amplifier is operated as a
NAND/NOR logic gate for input optical signals.
2. An optical switch according to claim 1 wherein the amplifier comprises a
laser and the driving current lies below the lasing threshold current of
that laser.
3. An optical switch according to claim 2 wherein the amplifier comprises a
passive Fabry-Perot cavity laser and the switching signal is detuned from
a cavity resonance of the laser.
4. An optical switch according to claim 2 wherein the amplifier comprises a
laser having optical feedback strutures distributed along an optical
lasing cavity and the switching signal is detuned from an output peak on
the short wavelength side of a stop band of the laser, said stop band
being a range of optical wavelengths which, when input to said amplifier,
are substantially reflected therefrom.
5. An optical switch as in any of the preceding claim wherein an end facet
of the amplifier cavity opposite said input and output ports is coated
with a high-reflectivity coating.
6. An optical time-division switch comprising an optical switch according
to any one of the preceding claims 1, 2, 3, or 4.
7. An optical signal regenerator comprising an optical switch according to
any one of the preceding claims 1, 2, 3, or 4.
8. An optical switching method utilizing an optical switch providing AND/OR
logical operations on input optical signals in a first condition and
providing NAND/NOR logical operations on input optical signals in a second
condition, said method comprising:
providing a semi-conductor optical signal amplifier having an amplifying
cavity with a common facet providing both an optical signal input port and
an optical signal output port;
inputting optical logic level signals to said input port;
establishing an electrical driving current of a first magnitude through
said semi-conductor optical signal amplifier to establish said first
condition wherein AND/OR logic operations are performed on said input
optical signals to provide output optical signals from said facet; and
alternatively establishing an electrical driving current of a second
magnitude through said semi-conductor optical signal amplifier to
establish said second condition wherein NAND/NOR logic operations are
performed on said input optical signals to provide output optical signals
from said facet.
9. An optical signal logic switch capable of accepting plural combined
logic level optical signals at an optical input port and of providing a
corresponding AND/OR or NAND/NOR Boaleon logic optical resultant signal at
an optical output, said switch comprising:
a semi-conductor optical amplifier having an electrical drive bias current
path therethrough and also having an optical amplifying cavity with common
facet which acts both as an optical signal input port and an optical
signal output port; and
bias current control means connected to establish a predetermined
electrical bias current through said path which bias current magnitude
determines whether an optical output signal represents logical AND/OR or
logical NAND/NOR operations upon the optical input signals.
Description
The present invention relates to an optical switch. It finds particular
application in optical logic and signal processing.
It is known to use semiconductor optical devices as switches in optical
logic and signal processing. They are advantageous in that they can be
designed to operate at low power levels, physically take up little space
in a signal processing system, operate at wavelengths compatible with
those common in optical communications, and potentially can be
monolithically integrated with other optical components.
Either passive or amplifying devices can be used as switches. The inherent
gain of an amplifying device, such as a laser, reduces the need for
additional amplification in a system and therefore gives amplifying
devices an advantage compared with passive devices. Further, amplifying
devices can be designed to switch at power levels typically of the order
of 10.sup.3 times lower than those required for passive devices, and are
readily available.
A semiconductor laser commonly comprises a wafer grown from materials
containing combinations of elements from the III and V groups of the
Periodic Table. The layers of the wafer are selectively doped to provide a
p-n junction, in the vicinity of which lies an active region. Photons can
be generated in the active region by radioactive recombination of
electron-hole (carrier) pairs under a driving current applied across the
junction. By variation in the refractive index of the wafer materials
and/or by control of the current distribution in the photodiode, the
generated photons are guided to move in a waveguiding region along the
photodiode. Feedback is provided to the waveguiding region for instance by
reflective end facets of the laser (a Fabry-Perot laser) or by
corrugations in an interface which lies near the active region (an example
of a distributed feedback laser).
A factor in the choice of materials for optical devices is the fact that
silica optical fibres, widely used in today's communications systems, have
loss minima at 0.9 .mu.m, 1.3 .mu.m and 1.55 .mu.m approximately.
Accordingly there is an especial need for devices which show favourable
characteristics when operated using optical radiation in the wavelength
range from 0.8 to 1.65 .mu.m, and especially in the ranges from 0.8 to 1.0
.mu.m and from 1.3 to 1.65 .mu.m. (These wavelengths, like all the
wavelengths herein except where the context indicates otherwise, are in
vacuo wavelengths). Materials which have been found suitable for the
manufacture of optical switches with such favourable chracteristics
comprise the III-V semiconductor materials, including gallium arsenide,
indium gallium arsenide, gallium alluminium arsenide, indium phosphide,
and the quaternary materials, indium gallium arsenide phosphides (In.sub.x
Ga.sub.1-x As.sub.y P.sub.1-y). With regard to the quaternary materials,
by suitable choices of x and y it is possible to lattice-match regions of
different ones of these materials to neighbouring III- V materials in a
device while being able to select the associated band gap equivalent
wavelength.
If optical radiation is input to the active region of a semiconductor laser
and a driving current applied, amplification of the radiation occurs even
when the driving current is below the lasing threshold current necessary
for lasing action to occur. The relationship between input and output
radiation intensity is non-linear and can show bistability, the output
intensity switching rapidly between two values as the input intensity
reaches a relevant switching level. The non-linearity arises from changes
in the refractive index of the material of the active region. The input
radiation in undergoing amplification reduces the free carrier
concentration and hence the gain. The refractive index varies with the
gain according to the Kramers-Kronig relationship. In turn, the degree of
amplification of the input radiation is dependent on a relationship
between input wavelength and the refractive index of the active region
material. Hence if the refractive index changes but the input wavelength
remains constant, the degree of amplification will change and therefore
the output radiation intensity.
The relationship between input and output radiation intensity is
complicated by another factor which affects the refractive index of the
active region material: temperature. Both the laser drive conditions and
the input radiation have an effect on temperature. Overall, the
interaction of gain, refractive index, driving current and input radiation
is complicated and difficult to specify for a specific device.
Bistable switching action in both passive and amplifying devices in
response to changes in input radiation can be exploited in optical logic
as .-+.AND/OR" or "NAND/NOR" gates. If the bistability comprises a sudden
increase in output radiation intensity in response to increasing input
radiation, then the device is suitable for use as an "AND/OR" gate. If the
bistability comprises a sudden decrease, then the device is suitable for
use as a "NAND/NOR" gate. Devices are known which will operate with one or
other of these characteristics. For instance, a simple, passive
Fabry-Perot cavity (etalon) is known to be capable of acting as an
"AND/OR" gate in transmission, while a laser is known to be capable of
acting as a NAND/NOR gate in reflection. ("In transmission" describes the
case where the input and output ports of a device are at opposing ends of
the device while "in reflection" describes the case where the input and
output ports are at the same end).
It has now been discovered that a single amplifying device can be used
either as an AND/OR, or as a NAND/NOR logic gate.
It is an object of the present invention to provide an optical switch which
can be used either as an AND/OR, or as a NAND/NOR, logic gate.
According to the present invention, there is provided an optical switch
comprising a semiconductor laser amplifier for use in reflection, means
for coupling an detuned optical switching signal to the amplifier, and
means for applying a driving current of less than the lasing threshold
current to the amplifier, which driving current can be selected to have
one of at least two different values, the amplifier being operable as an
AND/OR logic gate at a first of those values, and operable as a NAND/NOR
logic gate at the second of those values.
The amplifier may comprise a Fabry-Perot or a distributed feedback (DFB)
laser. In each case, the switching signal should be detuned in that its
wavelength should be such as to avoid a resonance of the laser.
Preferably, the wavelength should be one for which the amplifier shows
strong gain however. In the case of a Fabry-Perot laser, the switching
signal should be detuned from a cavity resonance of the laser. In the case
of a DFB laser, the switching signal should be detuned from an output peak
on the short wavelength side of a stop band.
The term stop band is used here, in the usual way, to describe a range of
wavelengths of an input signal to a DFB device for which the Bragg
conditions are satisified and the device acts to reflect rather than
transmit most or all of the input signal.
Optical switches according to embodiments of the present invention can
benefit from an advantage known to be associated with the use of passive
devices in reflection, that is lower critical input intensities using
reflective coatings on the back facet.
Because the devices are active rather than passive, there is considerable
control available over the operating parameters used.
An optical switch according to an embodiment of the present invention will
now be described, by way of example only, with reference to the
accompanying Figures in which:
FIG. 1 shows a schematic representation of the optical switch, means for
supplying a switching signal to the switch, and means for detecting the
response of the switch to the switching signal;
FIGS. 2a and 2b show in schematic graph form the optical output response of
the switch of FIG. 1 to optical switching signals when being operated as
an AND/OR gate and as a NAND/NOR gate respectively;
FIG. 3 shows a relationship between input and output radiation intensities
for the optical switch of FIG. 1;
FIG. 4 shows in schematic form the optical output response of the switch of
FIG. 1 to an optical switching signal for a special case of the operating
conditions;
FIG. 5 shows a spectral response of the switch of FIG. 1;
FIGS. 6 to 8 show the response of the switch of FIG. 1 to a high frequency
sinusoidal optical input signal; and
FIG. 9 shows a schematic representation of an optical time--division
multiplexing arrangement comprising switches as shown in FIG. 1.
Referring to FIG. 1, the switch comprises a double channel planar buried
heterostructure (DCPBH) laser amplifier 1 used in reflection. An optical
switching signal is provided by a tunable laser source 2, in combination
with an attenuator 7.
A beam splitter 3 mounted between the source 2 and the amplifier 1 deflects
a portion of the optical output of the source 2 to a switching signal
monitor 6, and a portion of the output of the amplifier 1 to an output
signal monitor 5. Interaction between the source 2 and the amplifier 1 is
prevented by an isolator 8, between the beam splitter 3 and the source 2,
and an attenuator 7 is used to modify the output of the source 2 to
produce a controllable switching signal. (The source 2 and the attenuator
7 are provided in the present embodiment to mimic an incoming signal which
would in practice comprise a signal carried by for instance an optical
communications system in operational use).
The amplifier 1 is a Fabry-Perot DCPBH laser without anti-reflection
coatings, 200 .mu.m long, comprising Inp with and InGaAsp active layer.
Threshold current at room temperature is 15.7 mA and the emission
wavelength 1508 nm.
The laser has an active cross section of 0.4 .mu.m.sup.2.
The source 2 is a grating tuned external cavity laser which provides a
single-mode signal. This laser is an anti-reflection coated ridge
waveguide laser, tunable in the range from 1450 to 1580 nm inclusive,
again comprising InP with an InGaAsp active layer.
The isolator 8 is provided by two isolating devices, giving together 60 dB
isolation. Maximum coupled powers from the source 2 to the amplifier 1 of
a few hundred .mu.W can be obtained, as deduced from the resultant
photocurrent induced in the amplifier 1. The beam splitter 3 comprises a
simple uncoated glass slide, and a fast PIN-pre amp combination (not
shown) provides temporal resolution of 100 psecs to allow switching speed
measurements by directly modulating the tunable source 2.
Methods of operating the switch will now be described, and results
discussed.
The source 2 is tuned to produce a signal detuned from a cavity resonance,
showing strong gain, of the amplifier 1 by an amount corresponding to a
single-pass (ie non-reflected) pahse change of -0.3.pi., or 30% of the
difference between adjacent cavity modes. By applying selected
combinations of driving current and switching signals, the amplifier 1 can
be caused to show optical bistability in three different manners.
Referring to FIG. 2a, using a drive current which produces a material gain
in the amplifier 1 of 0.95 times the lasing threshold gain, the amplifier
1 shows behaviour of a first type which can be exploited as a logic AND/OR
gate. In the Figure, the amplifier output signal intensity "Io" is plotted
against the input switching signal intensity "Ii", both intensities being
normalised using a scaling intensity "Is". It can be seen that "Io/Is"
shows an anticlockwise hysteresis loop 10 in response to "Ii/Is", which
loop 10 includes a step increase 9 in response to increasing "Ii/Is". If
"Ii/Is" increases from a value below the step increase 9 (less than B) to
a value above it (more than C), "Io/Is" will switch from a value in a low
range (less than b) to a value in a relatively high range (more than c).
These two ranges for "Io/Is" can then be used to represent "logic 0" and
"logic 1" outputs respectively.
If "Ii/Is" represents the sum of two incoming binary logic signals, (i) and
(ii), the values of the incoming signals representing "1" and "0" inputs
can be selected as follows:
1. AND gate operation
(i) and (ii) each have zero or insignificant intensity for a "0" input;
(i) and (ii) each alone have an intensity equivalent to "Ii/Is" in a range
(A to B) which lies within the hysteresis loop 10 for a "1" input such
that (i) and (ii) "1" inputs summed have an intensity equivalent to
"Ii/Is" more than C;
giving the following logic table:
______________________________________
(i) (ii) "Io/Is"
______________________________________
0 0 0
0 1 0
1 0 0
1 1 1
______________________________________
2 OR gate operation
(i) and (ii) each have zero or insignificant intensity for a "0" input;
(i) and (ii) each alone have an intensity equivalent to "Ii/Is" more than C
for a "1" input;
giving the following logic table;
______________________________________
(i) (ii) "Io/Is"
______________________________________
0 0 0
0 1 1
1 0 1
1 1 1
______________________________________
It can be seen that the step increase 9 occurs as part of a hysteresis loop
10 which includes a step decrease 11. The step decrease 11 applies when
"Io/Is" has dropped to a critical value c'. If the range of values for
"Io/Is" greater than c' is taken to represent a "logic 1" output, then it
can be seen that the switch offers AND gate operation as above but with a
memory characteristic. This is because once "Io/Is" has shown a "logic 1"
output if only one of the incoming signals drops to a 0" input, "Io/Is"
will continue to show the "logic 1" output. Only when both incoming
signals have dropped to a "0" input will "Io/Is" reach the step decrease
11 and switch to a "logic 0" output. This is represented by the following
logic table;
3. AND gate operation with memory characteristic
(i) and (ii) having values in the sequential order of combinations shown;
______________________________________
(i) (ii) "Io/Is"
______________________________________
0 0 0
0 1 0
1 0 0
1 1 1
1 0 1
0 0 0
______________________________________
It will be understood that after the 1-1 combination of values for (i) and
(ii), the combinations 1-0 and 0-1 are interchangeable. Only the 1-0
combination has been shown here however, and in following logic tables
with memory characteristics, to reflect the practical danger that
switching sequentially from 1-0 to 0-1 might take the amplifier through a
0-0 condition, and so cancel the memory characteristic.
Further, the memory characteristic can be modified by applying an optical
bias (iii) to the amplifier 1, and changing the values of (i) and (ii) for
a "1" input, as follows:
4. AND gate operation with biased memory characteristic
Optical bias (iii) has an intensity equivalent to "Ii/Is" slightly above A;
(i) and (ii) each have zero or insignificant intensity for a "0" input;
(i) and (ii) each alone has an intensity for a "1" input such that (iii)
plus that intensity is equivalent to "Ii/Is" less than B but (iii) plus
two times that intensity is equivalent to "Ii/Is" more than C;
giving the following logic table;
______________________________________
bias (iii) (i) (ii) "Io/Is"
______________________________________
on 0 0 0
on 0 1 0
on 1 0 0
on 1 1 1
on 1 0 1
on 0 0 1
off 0 0 0
______________________________________
An optical bias (iii) can also be used to produce OR gate operation with a
memory characteristic as follows:
5. OR gate operation with biased memory characteristic
Optical bias (iii) has an intensity as under 4 above;
(i) and (ii) each have zero or insignificant intensity for a "0" input;
(i) and (ii) each alone have intensity for a "1" input such that (iii) plus
that intensity is equivalent to "Ii/Is" more than C;
giving the following logic table;
______________________________________
bias (iii) (i) (ii) "Io/Is"
______________________________________
on 0 0 0
on 0 1 1
on 1 0 1
on 1 1 1
on 1 0 1
on 0 0 1
off 0 0 0
______________________________________
Referring to FIG. 2b, using a drive current which produces a material gain
in the amplifier 1 of 0.65 times the lasing threshold gain, the amplifier
1 shows behaviour of a second type which can be exploited as a logical
NAND/NOR gate. Again, in the figure, "Io" is plotted against "Ii", both
being normalised against "Is". It can be seen that in this case "Io/Is"
shows a clockwise hysteresis loop 12 in response to "Ii/Is", which loop 12
includes a step decrease 13 in response to increasing "Ii/Is", and a step
increase 14 in response to decreasing "Ii/Is".
By selecting ranges of values of "Io/Is" as "logic 1" and "logic 0"
outputs, for instance from G to H and from E to F respectively, it can be
seen that the amplifier 1 will act as a switch as described above but with
reversed logical outputs. Because of the direction, and position with
regard to zero "Io/Is", of the hysteresis loop 12, it is preferable to use
an optical bias (iv). Without an optical bias (iv), there are two ranges
of values of "Ii/Is", g to h and g' to h', which will produce a "logic 0"
output value of "Io/Is". To avoid ambiguity, in case for instance of noise
or malfunction, the optical bias (iv) should be equivalent to a value of
"Ii/Is", greater than h', which will bring the switch into an operating
range excluding the range of values of "Ii/Is", g' to h', which introduces
ambiguity. Employing an optical bias (iv), the amplifier offers the
following modes of operation:
6. NAND gate operation
Optical bias (iv) has an intensity equivalent to "Ii/Is", just above e'
which can produce a "logic 1" output of "Io/Is", but lies well below the
hysteresis loop 12;
incoming binary logic signals (i) and (ii) each have zero or insignificant
intensity for a "0" input;
(i) and (ii) each alone have an intensity for a "1" input such that (iv)
plus that intensity is equivalent to "Ii/Is" in the range, e' to e, which
can produce a "logic 1" output of "Io/Is" but lies below the hysteresis
loop 12, but such that (iv) plus two times that intensity is equivalent to
"Ii/Is" in the range, g to h, which produces a "logic 0" output of
"Io/Is";
giving the following logic table;
______________________________________
bias (iv) (i) (ii) "Io/Is"
______________________________________
on 0 0 1
on 0 1 1
on 1 0 1
on 1 1 0
______________________________________
By changing the relative values of the bias (iv) and each incoming binary
logic signal (i) and (ii), NAND gate operation with a memory
characteristic can also be achieved, as follows:
7. NAND gate operation with biased memory characteristic
Optical bias (iv) has an intensity as under 6 above;
incoming binary logic signals (i) and (ii) each have zero or insignificant
intensity for a "0" input;
(i) and (ii) each alone have an intensity for a "1" input such that (iv)
plus that intensity is equivalent to "Ii/Is" in a range, e to f, within
the hysteresis loop 12, while (iv) plus two times that intensity is
equivalent to "Ii/Is" in the range, g to h, which produces a "logic 0"
output of "Io/Is";
giving the following logic table, (i) and (ii) having values in the
sequential order of combinations shown;
______________________________________
bias (iv) (i) (ii) "Io/Is"
______________________________________
on 0 0 1
on 0 1 1
on 1 0 1
on 1 1 0
on 1 0 0
on 0 0 1
______________________________________
8. NOR gate operation
Optical bias (iv) has an intensity equivalent to "Ii/Is" in the range e'
to e, just below the hysteresis loop 12;
(i) and (ii) each have zero or insignificant intensity for a "0" input;
(i) and (ii) each alone have intensity for a "1" input such that (iv) plus
that intensity, or plus two times that intensity, is equivalent to "Ii/Is"
in the range, g to h, which produces a "logic 0" output of "Io/Is";
giving the following logic table;
______________________________________
bias (iv) (i) (ii) "Io/Is"
______________________________________
on 0 0 1
on 0 1 0
on 1 0 0
on 1 1 0
on 1 0 0
on 0 0 1
______________________________________
It will be noticed that NOR gate operation as above shows no memory
characteristic. However, by introducing a higher value of the optical bias
(iv), both NAND and NOR operation show a memory characteristic, as
follows:
9. NAND gate operation with modified biased memory characteristic
Optical bias (iv) has an intensity equivalent to "Ii/Is" in the range, e to
f, which lies within the hysteresis loop 12;
(i) and (ii) each have intensities as under 7 above;
giving the following logic table, (i) and (ii) having values in the
sequential order of combinations shown;
______________________________________
bias (iv) (i) (ii) "Io/Is"
______________________________________
on 0 0 1
on 0 1 1
on 1 0 1
on 1 1 0
on 1 0 0
on 0 0 0
off 0 0 reset
______________________________________
It will be seen that when the optical bias (iv) and both (i) and (ii) are
at zero, "Io/Is" is merely reset rather than giving an output value since
the ranges of "Io/Is" selected for "logic 0" and "logic 1" outputs E to F
and G to H, do not include zero.
10. NOR gate operation with memory characteristic
Optical bias (iv) has an intensity as under 9 above;
(i) and (ii) each have intensities as under 8 above;
giving the following logic table;
______________________________________
bias (iv) (i) (ii) "Io/Is"
______________________________________
on 0 0 1
on 0 1 0
on 1 0 0
on 1 1 0
on 1 0 0
on 0 0 0
off 0 0 reset
______________________________________
Referring to FIG. 3, it is thought that the switching behaviour described
above is derived from a relationship between "Io/Is" and "Ii/Is", as
affected by the driving current supplied to the amplifier 1. This
relationship has been plotted for a range of values of driving current,
represented by the ratio of the amplifier gain in operation to the lasing
threshold gain of the amplifier, g/gth.
It can be seen that for higher values of g/gth of 0.85 and above, the
relationship shows an open, upwards loop 15. For lower values, of 0.7 and
below, the relationship shows an open, downwards loop 16. In fact the
downwards loop 16 should occur for values of g/gth of up to and including
0.74. It is thought to be these loops 15, 16, within which the value of
Io/Is is bistable, which introduce the hysteresis loops 10,12 of FIGS. 2a
and 2b. (Bistability rather than tristability occurs because the linking
portions of the loops 15, 16, with opposite Ii/Is direction, are
unstable).
For a range of values of g/gth lying between 0.74 and 0.85, the
relationship shows a closed loop 17, and the amplifier 1 shows behaviour
of a third type. Referring to FIG. 4, this closed loop 17 indicates a
double hysteresis loop 18 with two step decreases in "Io/Is", a first part
19 of the loop 18 being traversed in a clockwise direction (for increasing
input) and a second part 20 being traversed in an anticlockwise direction
(for decreasing input).
To convert the normalised intensities of FIGS. 2 to 4 to optical input
power levels, for the amplifier 1 described above the scaling intensity Is
has a value of about 8.times.10.sup.5 W/cm.sup.2. Over the active cross
section of the amplifier diode, this corresponds to a factor
8.times.10.sup.9 .times.0.4.times.10.sup.12 W, or 3.2.times.10.sup.3 W.
Hence it can be seen from FIG. 3 that at the higher value of g/gth, 0.95,
the open, upwards loop 15 occurs between values of approximately 16 .mu.W
and 64 .mu.W for Ii. However, at the lower value of g/gth, 0.7, the open,
downwards loop 16 occurs between values of approximately 100 .mu.W and 120
.mu.W for Ii. It can be expected that these higher values of Ii required
to obtain optical bistability are a result of the associated lower value
of g/gth. There is, here, a trade-off. If the source 2 is tuned to produce
a signal which is closer to a cavity resonance of the amplifier 1, optical
bistability is achieved at lower values of Ii but the loops 15,16 are
reduced in size. Conversely, if the detuning of the source 2 is increased,
optical bistability is achieved at higher values of Ii but the loops 15,16
are larger.
Another factor which affects the size of the loops 15,16 and the values of
Ii associated with optical bistability is the reflectivity of the
reflecting facet of the amplifier 1. That is, the "back" facet of the
amplifier 1 which acts neither as an input nor as an output port. By
increasing the reflectivity of this facet, for instance by the use of a
high-reflectivity coating, the depth of the optical bistabilities produced
can be increased. That is, the ranges of values of "Io/Is" taken to
represent "logic 0" and "logic 1" outputs can be selected to lie further
apart. However, at the same time the values of Ii at which optical
bistability occurs increase. (Uncoated, as in the arrangement of FIG. 1,
the reflecting facet of the amplifier 1 has a relectivity of about 30%).
Further information on the hysteresis loops of FIG. 3 can be obtained from
the calculated spectral response of the amplifier for fixed input power.
Referring to FIG. 5, the spectral response curves plotted for a range of
values of .phi.o, the input signal phase detuning from a cavity resonance
of the amplifier, for a fixed "Ii/Is" of 0.03, and for values of g/gth as
marked, show loops 21 associated with the hysteresis loops of FIG. 3 at
gains g/gth of 0.8 and 0.9. For gains g/gth of 0.7 and 0.6, the spectral
response curves resemble those seen for passive Fabry-Perot devices.
The speed of switching of an optical switch according to an embodiment of
the present invention can be assessed by looking at the device response to
an optical input signal varying sinusoidally with time.
Referring to FIGS. 6, 7 and 8, in each case calculated optical output
response is shown for a steady state input signal (graph (a)), and for
sinusoidal input signals of periods equal to 4, 8 and 12 times the carrier
recombination time of about 1.7 nsecs (graphs (b) to (d) respectively). (A
steady state signal in this context is a signal whose repetition rate is
of the order of KHz or less. The value of g/gth is different for each
Figure, being 0.9, 0.8, and 0.7 for FIGS. 6, 7 and 8 respectively. The
first graph (a) of each Figure thus corresponds to one of the plotted
curves on FIG. 3.
From the observed device response to the sinusoidal signals, it would seem
that the switching time between stable gain states, at least where the
optical switch is being used as an AND/OR gate, is likely to be of the
order of the carrier recombination time. Hence the maximum clock rate at
which the switch will operate will be limited by nanosecond switching
times.
Intensity spikes 22 can be observed on the device response curves. These
are understood to stem from changes in the material refactive index of the
amplifier 1 due to changes in the optical input intensity. In the case of
FIG. 8, it can be seen that these spikes 22 are directed downwards and are
only vestigial.
Referring to FIG. 9, optical switches 26 according to embodiments of the
present invention can be used in an array for instance in optical
time-division switching. Using LiNbO.sub.3 directional coupler matrices to
provide a read gate 23 and a write gate 24, four 64 Mb/s digitally encoded
colour video signals which are time multiplexed in bit-interleaved form
are applied to a time switch 25 comprising optical switches 26 according
to embodiments of the present invention. The write gate 24 suplies the 256
Mb/s time-multiplexed signal to each switch 26 in turn. The switches 26
each store the optical signals for a frame period. The stored signals can
then be read out according to a required sequence and time switching has
been accomplished.
The optical switches 26 can be reset by the use of optical clock signals.
In another application, optical switches according to embodiments of the
present invention could be used in optical regenerators. Using an incoming
digital signal and clock pulses as the two incoming binary logic signals
(i) and (ii) referred to above, the incoming digital signal can in effect
be regenerated and sent onwards.
It is not necessary that an embodiment of the present invention should
include all the features described above. In particular it is not
necessary that a Fabry-Perot laser diode be used as the amplifier 1.
Instead, for instance, a DFB laser diode may be used.
* * * * *
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