WO2005103782A1

WO2005103782A1 - Optical time delay line circuit, in particular for true time delay generation of microwave phase array antennas

Info

Publication number: WO2005103782A1
Application number: PCT/EP2005/051824
Authority: WO
Inventors: Zhiqiang Zhao
Original assignee: Thales International Asia Holding Pte Ltd
Priority date: 2004-04-27
Filing date: 2005-04-22
Publication date: 2005-11-03

Abstract

The present invention relations to an optical multi-bit delay line circuit, for example a delay integrated photonic circuit. The optical multi-bit delay line comprises a concatenation of 2x2 switches (2), each switch (2) having an associated delay unit (43). Each delay unit (43) includes a waveguide forming a delay loop, each delay loop consisting of two symmetrically arranged waveguides (31, 32) in a racetrack topography, wherein each racetrack has an inward spiral and an outward spiral, the inward spiral and the outward spiral being concentrically arranged. The first output (52) of one of the switches (2) is connected to the input of the associated delay unit (43), the first input (51) of the same switch (2) is connected to the output of the delay unit. A main waveguide (40) connects a second output (53) of one of the switches to the second input (54) of the following switch in the concatenation of switches. By choosing an appropriate switch state, an optical signal can either be routed through the associated delay unit (43), or directly be passed on to the following delay unit (43). Each delay unit (43) corresponds to a bit in the multi-bit delay line, and the waveguide lengths of successive delay units (43) increase by a factor of two. The invention is applicable in particular to the generation of true time delays for microwave phased array antennas.

Description

OPTICAL TIME DELAY LINE CIRCUIT , IN PARTICULAR FOR TRUE TIME DELAY GENERATION OF MICROWAVE PHASE ARRAY ANTENNAS

The present invention relates to an optical multi-bit delay line circuit, for example a multi-bit delay integrated photonic circuit. It is applicable in particular to the creation of true time delay generation of microwave phased array antennas. 10 A microwave phased array antenna comprises many radiating elements that ensure both transmission and reception of an ultrahigh frequency signal. A transmission or reception beam is formed by all the signals transmitted or received by each element. To orient a beam in a given direction it is 15 necessary to create time delays between signals transmitted or received by the various radiating elements. The use of optical techniques allows to control the radiating elements directly through time delays. It is known to use a binary switched delay line to create required time delays. An optical carrier of the microwave signal is optically routed through N fiber 20 segments whose lengths increase successively by a power of 2. The fiber segments are addressed using a set of N optical 2x2 switches. Since each switch allows the signal to either go through or bypass a fiber segment, a delay T may be inserted which can take any value, in increments of ΔT, up to a maximum value T_maχ given by : 25

The required length of the delay line for phased array antenna is in the order of magnitude of about 1 meter. For the delay time resolution of 5 to 8 bits for 30 example, the elementary segment of the delay line should be in the order of magnitude of about 1 cm. And, in order to have the accuracy of required bits control, all the segments should have the length tolerance of the elementary segment, that means below 1 cm. It is difficult to get such a tolerance if optical fibers are used for making the delay line segments, and the good 35 solution should be using optical waveguides, which is easy to achieve much better length tolerance and can be integrated with 2x2 optical switches. There is a prior art to make optical planar waveguides for such delay lines, which is in spiral shape for compactness. In order to make the input and output of the waveguide in the same plane without crossing, a structure shown in the figure 2 is adopted, with a S shape in the center of the spiral to change the direction, and the waveguides in the opposite directions are laid alternatively. A drawback of this prior art is that the input and the output of the optical waveguides are in opposite direction, which is not suitable for connecting with a 2x2 switch. Furthemore, such a structure goes against the miniaturization of the circuit if some extra lead is made to fit the 2x2 switch.

One goal of the invention is more particularly to overcome aforementioned drawbacks. To this end, an object of the invention is an optical multi-bit delay line circuit comprising a main waveguide connected to delays units through optical 2x2 switches. Each delay unit comprises a waveguide forming two groups of symmetrical loops which do not intersect, the input of the waveguide being connected to the first output of its associated switch, the output of the waveguide being connected to the first input of the switch, the main waveguide being connected to the second input and second output of the switch.

Advantageously, the loops of each group have approximately the same length. In order to allows a multi-bit controls, the waveguide lengths of successive units are increased in the sequence of power of 2.

In an optional embodiment, a circuit according to the invention comprises a first unit having a circle shape waveguide. Furthermore, it can comprises a second unit having a square shape with rounded corners, which is twice as long as the first one.

Advantageously, the main waveguide has a ring shape.

A circuit according to invention can be implemented with photonic polymer technology. The optical switches are made of electro-optical polymer. The waveguides are made of low loss polymer material. The invention can be applied for true time delay generation of Microwave Phased Array Antennas.

The main advantages of the invention are that it allows connections with 2x2 optical switches without crossing and it allows arrangements which can make the circuit more compact.

Other features and advantages of the invention will become clear in connection with the description that follows given in relation to the appended drawings, of which :

- the figure 1 shows the concept of a binary switched delay line according based on which the invention is made ; - the figure 2a shows a planar waveguide delay line according to the prior art ; - the figure 2b shows more particularly the mode of winding of a planar waveguide delay line ; - the figure 3 shows an example of delay unit structure used in a multi- bit delay line according to the invention ; - the figure 4 shows an example of an optical multi-bit delay line according to the invention ; - the figure 5 shows a connection of a delay unit and a 2x2 switch to the delay line ; - the figures 6 and 7 show two other arrangements of a mutli-bit delay line according to the invention.

Referring to figure 1, the concept of an optical binary switched delay line is shown. Such a delay line is for example connected via an electro-optic interface to a radiating element of a phased array antenna. Then it applies a given time delay to the microwave signal transmitted or received by this radiating element. To this end the optical carrier of the microwave signal is optically routed through or bypass N optical waveguide delay line segments 1 , for example optical fiber segments. The lengths of the waveguide segment 1 increase successively by a power of 2. The waveguide segments 1 are addressed using a set of N 2x2 optical switches 2. In fact the delay line of figure 1 is consisted of time delay unit connected in series, each delay unit comprising a 2x2 switch 2 and an optical waveguide segment 1 of a given length. Since the lengths of the segments 1 increase successively by a power of 2, the first unit applies a ΔT delay, the second unit a 2ΔT delay and the n^th unit a 2^π"1 ΔT delay.

A 2x2 switch comprises two inputs and two outputs terminals and works in two states. In a first state the first input is connected to the first output and the second input is connected to the second output. We may refer to this state as the "Through" state. In the second state, the first input is connected to the second output and the second input is connected to the first output. We may refer to this state as the "Cross" state. In the delay line of figure 1 a waveguide segment 1 is connected for example between the first input and the first output of a 2x2 switch. Then input and output of a time delay unit are respectively the second input and the second output of the 2x2 switch. In the Through state the signal bypass the waveguide 1 , then no delay is applied. In the Cross state the signal goes through the waveguide 1 and a delay is applied. The input and output of the delay line for the optical carrier are respectively the second input of the first time delay unit and the second output of the last time delay unit. The 2x2 switch are controlled by digital control means 3. To this end, each switch is linked to this means 3, for example via a control bus 4.

As aforementioned, since each switch allows the signal to either go through or bypass a waveguide segment 1 , a delay T may be inserted which can take any value, in increments of ΔT, up to a maximum value T_maχ given by equation (1 ).

Figure 2a shows a planar waveguide delay line 1 of the prior art. The waveguide is an optical line forming as many loops as necessary to insert a given time delay. In particular the loops cannot intersect due in particular to the planar structure. The loops are concentric and do not intersect. To this end the line is wound round an inside loop 21 as shown in figure 2b, the inside loop 21 comprising the middle point of the line. As a result, the optical signal circulating in two adjacent loops 22, 23 are in opposite direction. The inside loop 21 has a S shape which changes the direction of the winding. The loops are for example very closed to each other in order to make the delay unit compact. The shape of the waveguide allows to easily control the delays by controlling the number of loops. However, a drawback of such a waveguide delay line is that the input and the output are in opposite direction which is not suitable for connecting to a 2x2 switch.

Figure 3 shows a waveguide delay line 1 used in a time delay circuit according to the invention. The waveguide consists of two symmetrical delay line structures having the structure shown in figures 2a or 2b. That means the waveguide forms two groups of loops 31 , 32. The two groups do not intersect. The two groups of loops 31 , 32 are symmetrical in regard of an axis 33. Each group has the same structure as the structure illustrated by figures 2a and 2b.

The input of the waveguide is the input of the first group 31 of loops and the output of the waveguide is the output of the second group 32 of loops. The output 31 of the first group is also the input of the second group 32. As shown by the figure 3, in this shape, the input and output of the waveguide are in one line and in the same direction, which can easily connect with the input and output ports of a 2x2 switch, and without crossing.

Figure 4 shows an example of a structure of a multi-bit integrated delay line according to the invention. The line has a ring structure. Delay units 41, 42,

43 are located along a main optical waveguide forming the ring 40, each unit having the shape of the line of figure 3. The units 41, 42, 43 are preferably regularly located. The unit are connected to the ring through optical 2x2 switches 2. The waveguide lengths of successive units are increased in the sequence of power of 2. For the first unit 41 and the second unit 42, the lengths are short, so they do not need to have the structure shown in figure 3, and a shape of circle or square with rounded corners should be used. For the successive units with longer and longer lengths, the shape of figure 3 is used and the number of loops are increasing, for the required lengths. So, if the first waveguide 41 has a length , the n^th waveguide 43 has a length 2^π"1 , corresponding to the elementary delay ΔT. For the example of figure 4 where the muti-bit delay line circuit is a 8-bit delay line, the lengths of the waveguides are successively, , 2L₀, 4L₀, 8L₀, 16l_o, 32l_o, 64 , 128Lo.

Figure 5 shows the connection of a delay unit 43 to the ring 40 through a 2x2 switch 2. The line of the ring 40 meets the second input 54 of the switch. It is connected either to the input of waveguide 43 through a first output 52 of the switch or to the next section of the ring through the second output 53. The output of the waveguide 43 is linked to the first input 51 of the switch 2.

The delay line of figure 4 works with a control electronic circuit 44 located for example inside the ring 40. This circuit 44 controls the states of the 2x2 optical switches. To this end it is linked to each switch 2, either through direct lines or through a bus. For a given control signal in a binary form fed to the switches, a specific delay time will be set up between the input and the output of the multi-bit integrated delay line.

A structure of the integrated circuit as illustrated by figure 4 can be relatively compact. The circuit of figure 4 is a 8-bit structure. This is just an example and the actual number of bits should be determined by the application requirements. The ring structure of figure 4 has in particular the advantage to be compact. However, other structures can be used such as for example straight line structures as shown in figures 6 and 7. Figure 6 shows a rectangular line 60 where the delay units 41 , 42, 43 are inside the rectangular line 60. Figure 7 shows a mere straight line 70, the units 41 , 42, 43 being are arranged on each side of the line.

The invention can be implemented with photonic polymer technology. The switch parts are for example made of electro-optical polymer. The delay line parts are for example made of low loss polymer waveguide material. The sizes of the waveguide units are determined by the minimum curvature radius of the waveguide limited by the bend loss. The maximum length of the waveguide is in particular limited by the attenuation coefficient of the waveguide. The nano-imprinting technology should be used to make mass production.

A circuit according to the invention can be used for controlling a phased array antenna, in particular for true time delay generation of a Microwave Phased Array Antenna. Obviously it can be used for other optical or microwave applications which required true time delays.

Claims

1. Optical multi-bit delay line circuit, comprising a main waveguide (40) connected to delays units (41 , 42, 43) through optical 2x2 switches (2), each delay unit comprising a waveguide forming two groups of symmetrical concentric loops (31 , 32) which do not intersect, the input of the waveguide (43) being connected to the first output (52) of its associated switch, the output of the waveguide (43) being connected to the first input (51) of the switch, the main waveguide (40) being connected to the second input (54) and second output of the switch (53).

2. Optical multi-bit delay line circuit according to claim 1, characterised in that the loops of each group (31 , 32) have approximately the same length.

3. Optical multi-bit delay line circuit according to claim 2, characterised in that the waveguide lengths of successive units (41 , 42, 43) are increased in the sequence of power of 2.

4. Optical multi-bit delay line circuit according to anyone of the previous claims, characterised in that it further comprises a first unit (41) comprising a waveguide having a circle shape.

5. Optical multi-bit delay line circuit according to claim 4, characterised in that it further comprises a second unit having a square shape with rounded corners.

6. Optical multi-bit delay line circuit according to anyone of previous claims, characterised in that the main waveguide (40) has a ring shape.

7. Optical multi-bit delay line circuit according to anyone of previous claims, characterised in that it is implemented with photonic polymer technology.

8. Optical multi-bit delay line circuit according to claim 7, characterised in that the optical switches (2) are made of electro-optical polymer.

9. Optical multi-bit delay line circuit according to any of claims 7 or 8, characterised in that the waveguides (40, 41, 42, 43, 44) are made of low loss polymer material.

10. Optical multi-bit delay line circuit according to anyone of previous claim, characterised in that it is used for true time delay generation of a Microwave Phased Array Antenna.