Variable delay apparatus for optical signals

In order to reduce its switching time, the delay apparatus comprises a coupler and a multiplexer provided with n inlets set respectively to n distinct wavelengths. Inlets of the multiplexer are coupled respectively to outlets of the coupler via wavelength conversion apparatuses, and via delay lines all of which are different. Each conversion apparatus has an amplifying optical gate function and delivers to the corresponding inlet of the multiplexer a wave carried by the wavelength associated with said inlet. The invention also concerns apparatus for synchronizing channels of a wavelength multiplex using the variable delay apparatus. Application in particular to synchronizing optical packets in a switched optical network.

The invention relates to the field of data transmission using optical 
links, and it relates more particularly to optical delay apparatus. Such 
apparatus is used in particular in "all-optical" switched networks to 
solve problems of packet synchronization with a view to managing 
contention in the switching systems. 
BACKGROUND OF THE INVENTION 
A first known type of delay apparatus consists in placing optical switches 
and calibrated delay lines in cascade and in alternation. The path 
travelled by a wave applied to the inlet of the apparatus is a function of 
the commands applied to the optical switches. Unfortunately, that system 
suffers from the drawback of imparting interference noise. 
Another type of delay apparatus consists in placing a tunable wavelength 
converter, an optical demultiplexer, and an optical multiplexer in 
cascade, the outlets of the demultiplexer being coupled to the 
corresponding inlets of the multiplexer via delay lines all having 
different lengths. Thus, as a function of its wavelength, the wave 
delivered by the converter is directed to a single one of the delay lines. 
That solution offers the advantage of being simple, but it is difficult to 
implement if very short switching times are desired. Moreover, means need 
to be provided for dynamically tuning the carrier of the converter. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the invention is to remedy the above drawbacks. To this end, 
the invention provides apparatus for creating a variable delay between an 
output optical wave and an input optical wave, the waves being modulated 
between low and high power levels. 
According to an exemplary embodiment, a coupler is provided with at least 
one inlet for receiving the input optical wave, and with n outlets for 
delivering portions of said input wave. 
In addition, a multiplexer is provided with at least one outlet for 
delivering an output optical wave, and n inlets set respectively to n 
associated different wavelengths. The inlets of the multiplexer are 
coupled respectively to the outlets of the coupler via associated 
wavelength conversion apparatuses, and, for at least n-1 inlets of the 
multiplexer, via respective delay lines all of which are different. 
Furthermore each conversion apparatus has an amplifying optical gate 
function for selectively delivering a converted wave modulated as a 
function of the input wave, and carried by a wavelength equal to that 
associated with the inlet. 
By means of the presence of amplifying optical gates, i.e. optical 
amplifiers generally optimized for enabling gain to be varied rapidly as a 
function of an electrical command, switching can be very rapid, typically 
less than 300 picoseconds, unlike solutions using tunable wavelength 
converters. Furthermore, the carriers are stable because each converter 
transmits with a fixed wavelength. Furthermore, the gains of the 
amplifiers can be adjusted, which makes it possible to adjust the power of 
the output signal and to make it independent of the delay applied. 
Finally, the use of optical gates having a high isolation ratio and 
associated with a multiplexer guarantees spectral separation, which avoids 
creating interference noise. 
According to another aspect of the invention, each wavelength conversion 
apparatus comprises a semiconductor optical amplifier coupled firstly to a 
respective one of said outlets of the coupler and secondly to a laser 
oscillator delivering a probe wave. The amplifier is organized to operate 
under saturated conditions when the input optical wave is at its high 
power level. In addition, the modulated converted wave is the probe wave 
as amplified by the amplifier. 
In this aspect, the semiconductor optical amplifier acts both as an 
amplifying optical gate, and as a wavelength converter. 
The invention also provides apparatus for synchronizing channels of a 
wavelength multiplex using the variable delay apparatus of the invention.

MORE DETAILED DESCRIPTION 
The variable delay apparatus Dx shown in FIG. 1 includes a coupler C 
provided with an inlet P and n outlets A1, A2, Ai, An. The inlet P serves 
to receive an input optical signal Ex. This signal is in the form of a 
power-modulated carrier of wavelength .lambda.x'. The first outlet A1 of 
the coupler C is connected to an inlet B1 of a multiplexer M via a 
wavelength conversion apparatus K1. Each of the other outlets A2, Ai, An 
of the coupler is connected to a respective inlet B2, Bi, Bn of the 
multiplexer M via a respective delay line L2, Li, Ln, and via a respective 
wavelength conversion apparatus K2, Ki, Kn. The outlet Q of the 
multiplexer M delivers the output signal Sx which is also in the form of a 
power-modulated carrier. 
If an output signal of determined wavelength .lambda.x is desired, the 
outlet Q is connected to an output wavelength converter kx organized to 
deliver a converted output signal Sx' carried by said wavelength 
.lambda.x. 
In a first embodiment, each conversion apparatus, e.g. Ki comprises a 
wavelength converter ki coupled to an amplifying optical gate G that can 
be controlled by a control signal ri. Each converter ki receives a portion 
ai of the input wave Ex via the associated delay line Li or directly for 
the converter k1. The optical gates G receive from the respective 
converters k1, k2, ki, kn respective converted waves carried by respective 
wavelengths .lambda.1, .lambda.2, .lambda.i, .lambda.n. These wavelengths 
are those to which the respective inlets B1, B2, Bi, Bn of the multiplexer 
M are set. 
In operation, the control signals r1, r2, ri, rn are applied selectively to 
the gates G of the wavelength conversion apparatuses K1, K2, Ki, Kn. When 
a modulated wave Ex is applied to the inlet P of the coupler C, the active 
gate(s) G deliver delayed waves b1, b2, bi, bn to the corresponding inlets 
of the multiplexer M with delays that are functions of the associated 
delay lines L2, Li, Ln. The multiplexer M then delivers via its outlet Q 
the output signal Sx which is then composed of the delayed wave(s). 
Optionally, the output converter kx converts the signal Sx into a converted 
output signal Sx' carried by a determined wavelength .lambda.x that may be 
equal to or different from the wavelength .lambda.x' carrying the input 
signal Ex. 
The conversion apparatuses and the delay lines may in principle be placed 
in any relative positions. 
The above-described wavelength conversion apparatuses use wavelength 
converters and separate amplifying gates. However, the two components may 
advantageously be replaced with the apparatus shown in FIG. 2. 
In this embodiment, each conversion apparatus Ki includes a semiconductor 
optical amplifier G, one face of which receives a pump wave PRi delivered 
by a laser oscillator Lo tuned to deliver the wavelength .lambda.i. The 
opposite face of the amplifier is connected to a first port of a 
three-port circulator Ci. The circulator is disposed so as to direct the 
wave portion ai received from the coupler C via its second port towards 
the amplifier G, and to direct the wave received from the amplifier G 
towards a third port. When the wavelength .lambda.x of the wave portion ai 
is different from the wavelength .lambda.i delivered by the laser LO, the 
third port may be coupled to a stop filter F set to the wavelength 
.lambda.x. The filter F then outputs the delayed wave bi carried by the 
wavelength .lambda.i. 
The amplifier G is dimensioned so that, when it is fed with current, it 
operates under saturated conditions, if the wave portion ai is at a high 
power level, i.e. when the input signal Ex is at its high power level. 
In the embodiment shown, the wave portion ai, and the probe wave PRi 
injected into the amplifier have opposite propagation directions. In a 
variant, it is possible to apply these two waves co-directionally. 
However, the configuration shown provides better elimination of the 
wavelength .lambda.x of the delivered wave bi. 
The above-described delay apparatus may be used advantageously to implement 
synchronization apparatus for synchronizing optical channels making up a 
wavelength multiplex. Such synchronization apparatus is shown in FIG. 3. 
This apparatus includes an inlet demultiplexer M1 and an outlet multiplexer 
M2. The inlet demultiplexer M1 is provided with an inlet port P1, and with 
a plurality of outlet ports Aa, Ab, Ax, Az. The outlet multiplexer M2 is 
provided with an outlet port P2, and with a plurality of inlet ports Ba, 
Bb, Bx, Bz. The demultiplexer M1 is organized to deliver via its outlets 
Aa, Ab, Ax, Az the channels Ea, Eb, Ex, Ez respectively carried by the 
wavelengths .lambda.a', .lambda.b', .lambda.x', .lambda.z' of an input 
multiplex We received via the inlet port P1. 
The outlets Aa, Ab, Ax, Az of the demultiplexer M1 are connected 
respectively to the inlets Ba, Bb, Bx, Bz of the multiplexer M2 via 
variable delay apparatuses Da, Db, Dx, Dz. Each of the delay apparatuses 
is implemented in accordance with the invention, as in the variant that 
includes an output converter kx. The multiplexer M2 and the delay 
apparatuses Da-Dz are organized so that the wavelengths .lambda.a, 
.lambda.b, .lambda.x, .lambda.z delivered respectively by the delay 
apparatuses Da, Db, Dx, Dz correspond respectively to the wavelengths 
allowed at the inlets Ba, Bb, Bx, Bz of the multiplexer M2. 
In operation, the delay apparatuses Da, Db, Dx, Dz respectively receive the 
commands Ra, Rb, Rx, Rz that define the delays to be applied respectively 
to the corresponding channels Ea, Eb, Ex, Ez. Thus, each of the various 
channels Ea-Ez of the inlet multiplex We may be delayed by a respective 
determined value, and the multiplexer M2 outputs an output multiplex Ws 
made up of these selectively delayed channels. 
The channels of the output multiplex Ws are carried by the wavelengths 
.lambda.a-.lambda.z imposed by the multiplexer M2. The wavelengths 
.lambda.a-.lambda.z may be identical to or different from the wavelengths 
.lambda.a'-.lambda.z' of the input multiplex We.