Digitally controlled distributed phase shifter

A digitally controlled distributed phase shifter is comprised of N phase shifters. Digital control is achieved by using N binary length-weighted electrodes located on the top surface of a waveguide. A control terminal is attached to each electrode thereby allowing the application of a control signal. The control signal is either one or two discrete bias voltages. The application of the discrete bias voltages changes the modal index of a portion of the waveguide that corresponds to a length of the electrode to which the bias voltage is applied, thereby causing the phase to change through the underlying portion of the waveguide. The digitally controlled distributed phase shift network has a total phase shift comprised of the sum of the individual phase shifters.

FIELD OF THE INVENTION 
This invention relates generally to digital distributed phase shifters and 
relates particularly to a digitally controlled distributed phase shifter 
network wherein each phase shifter within the network has a binary 
weighted length. 
BACKGROUND OF THE INVENTION 
Phase shifters delay a signal so that the phase of the signal is shifted by 
a desired amount, typically measured in degrees or radians. A distributed 
phase shifter is one in which the phase shift occurs along the entire 
length of the device. Controlled phase shifters are used in numerous 
optical and microwave applications such as modulation schemes in 
coherent-optical communication systems, control of microwave phased array 
antennas, and signal processing applications in optics and microwave 
systems. 
Analog phase shifters typically utilize a varying DC bias level to achieve 
a desired phase change of a radiation signal propagating through a 
waveguide. The relationship between DC bias level and phase shift, 
however, it generally not a linear one. 
Digitally controlled phase shifters typically utilize a digital to analog 
converter to produce an analog signal required for phase modulation. The 
utilization of digital to analog converters, however, increases 
complexity, cost, and power consumption. 
It is thus an object of the invention to provide a digitally controlled 
distributed phase shifter that does not require the use of digital to 
analog converters. 
It is a further object to provide a digital distributed phase shifter that 
linearly varies the phase of a radiation signal with changes in a digital 
control word applied to the phase shifter. 
SUMMARY OF THE INVENTION 
The foregoing problems are overcome and other advantages are realized by a 
digitally controlled distributed phase shifter network comprised of a 
number of phase shifters, each having a binary weighted length. 
For an optical phase shifter embodiment, the digital control is achieved by 
using N-length binary-weighted electrodes. The electrodes are comprised of 
a metal contact conductively coupled to a p+ semiconductor layer of a 
ridge-type waveguide. The p+ semiconductor layer provides improved 
conductivity and is located on the top-surface of the waveguide. The 
binary weighting of the electrode's length takes advantage of the 
distributed nature of the phase shift element, allowing N-bit digital 
control of the phase shift. A control terminal is attached to each 
electrode thereby allowing the application of a control signal of either 
one or two discrete bias voltages. The application of the discrete bias 
voltages changes the modal index of refraction of the portion of the 
waveguide that corresponds to the electrode to which the bias voltage was 
applied, thereby causing the phase of the radiation to change as it 
propagates through that portion of the waveguide. 
The digitally controlled distributed phase shifter has a total phase shift 
comprised of the sum of the phase shift provided by each discrete phase 
shifter. The technique guarantees the perfect linearization of the phase 
control as only of two discrete bias voltages are applied to the 
electrodes. 
Any nonlinearity between the two voltage states used is irrelevant. The 
device is inherently linear provided that the number of electrodes, N 
where 1.ltoreq.n.ltoreq.N, are related in length by L.sub.n 
=(L.sub.n-1)/2. The length of each electrode is defined 
photolighographically so that any errors in length are minimized.

DETAILED DESCRIPTION O THE INVENTION 
FIGS. 1c-1c illustrate the fundamental operation of the present invention. 
FIG. 1a shows an analog distributed phase shifter comprised of an 
electromagnetic waveguide 50 and an analog phase shifter 59. It is assumed 
that the device is uniform along its length so that the phase shift 
produced by any given length along the line is the same as any other. It 
is also assumed that the distributed phase shifter produces a normalized 
phase shift of 360.degree. with an applied voltage of, for example, 5 
volts and 0.degree. with, for example, 0 volts applied. The L.sub.2.pi. 
represents the length of the analog phase shifter required to produce the 
360.degree. (2.pi.) phase shifter. 
FIG. 1b shows the phase shifter from FIG. 1a partitioned into N discrete 
sections. A length of each section is related by a geometric relationship 
to the power of 2. 
Assuming that the Figure-Of-Merit, the FOM, is known for any optical or RF 
waveguide. The FOM is defined as the change in phase shift 
(.DELTA..theta.) per unit length per applied voltage change, i.e., 
##EQU1## 
where L is the length of .DELTA.V is the voltage change. For a selected 
total phase shift, .DELTA..theta..sub.T, and a desired voltage 
differential, .DELTA.V, e.g., for 0 to 5 volts, .DELTA.V=5V, the length, 
L.sub.T, of the waveguide required to produce the total phase shift is: 
##EQU2## 
The length of any given section n is L.sub.n =L.sub.T /2.sup.n. Note that 
for n=1, the electrode length is L.sub.T /2 and is longest; but where n=N, 
and , in the case of FIG. 1c, where N=8, the electrode length is L.sub.8 
=L.sub.T /256 and is the shortest. 
Each section produces a phase shift of .DELTA..phi..sub.T /(2.sup.n), where 
.DELTA..phi..sub.T is the total phase shift desired which, in general, can 
be any desired value, commonly 360.degree., produced by length L.sub.T. 
The total phase shift of the device is expressed as: 
##EQU3## 
where V.sub.n is the voltage to the nth section. The above equation 
applies only if the reference voltage is 0 volts and V is the other 
applied voltage such that V.sub.n /.DELTA.V is equal to 0 or 1 only, hence 
the digital control. As long as the lengths of the sections are related by 
L.sub.T /(2.sup.n), the phase shift produced (.DELTA..phi.) is controlled 
linearly by the applied binary word with maximum phase shift 
[.DELTA..phi..sub.T (2.sup.N-1)]/2.sup.N, where, in this case, N=8. 
FIG. 1c shows the phase shifter from FIG. 1a partitioned into eight 
discrete sections. It is assumed that .DELTA..phi..sub.T =360.degree. and 
for illustrative purposes L.sub.T =456 .mu.m. A length of each section is 
related by a geometric relationship to the power of 2. The phase shifter 
is comprised of the electromagnetic waveguide 50 and discrete phase 
shifters 51, 52, 53, 54, 55, 56, 57, 58. Numbers above the phase shifters 
51, 52, 53, 54, 55, 56, 57, 58 represent the lengths of the phase shifters 
51, 52, 53, 54, 55, 56, 57, 58. The largest section is of length L.sub.T 
/2 because L.sub.T =256, L.sub.T /2=128. When an applied DC voltage is 
changed from 0 to 5 volts, the L.sub.T /2 section produces 180.degree. of 
phase shift. The next largest section is of length L.sub.T /4. When a 
voltage of 5 volts is applied to this section, it produces 90.degree. of 
phase shift. Numbers below each phase shifter represent the phase shift 
produced by the respective phase shifter. The smallest section has a 
length of L.sub.T /2.sup.N, where N is the total number of discrete phase 
shifter sections. Therefore, the smallest section has a length of 1 
micrometer. By appropriately applying 0 or 5 Volts (0 or 1's) to the 
sections of such a structure any phase shift between 0.degree. to 
360.degree., with a resolution of 360.degree./(2.sup.N), is produced. 
Furthermore, with the most significant bit, MSB, of an 8 bit word coupled 
to the largest section, the least significant bit, LSB, attached to the 
smallest section, and all other bits connected correspondingly in between, 
the binary value (J) applied to a phase shifter section produces a linear 
phase shift in proportion to the binary value (J360.degree./(2.sup.N)). 
Optical waveguide phase modulators operate by changing the modal index of 
refraction of the waveguide, resulting in a change in the phase velocity 
of an optical signal passing through the guide, with a relative phase 
shift accumulating with length. Four detailed physical phenomenon 
responsible for the net shift in the modal index of the waveguide are: 
(1) the linear electrooptic effect, related to the biaxial birefringence of 
the material under an applied electric field; 
(2) the electrorefractive effect, or Franz-Keldysh effect, which causes a 
red-shift of the absorption edge under an applied electric field which 
corresponds to a refractive index change via Kramers-Kronig relations; 
(3) the plasma effect, due to free-carrier adsorption altering the 
refractive index as free-carriers are removed from the material by the 
depletion edges of a p-n junction; and 
(4) the band-filling effect, which causes a red-shift of the fundamental 
adsorption edge and an increase in the refractive index upon depletion of 
free carriers from doped material. 
All four effects contribute to the total shift in modal index of the 
waveguide, although for energies well below the band-edge of the material 
the linear electrooptic effect dominates, followed by band-filling, 
electrorefraction, and the plasma effect, in that order. As a result, it 
is possible to adjust the phase shift at a fixed bias by changing the 
length of the phase modulator. Therefore, for example, if a 1 mm long 
device operates at 72.degree./V-mm (phase shift per unit length, per volt 
applied: .DELTA..phi./V.sub.a L) figure-of-merit (FOM), there is obtained 
approximately 360.degree. phase shift of 5 volts; while a 500 micrometer 
long structure yields approximately 180.degree. phase shift at 5 volts, 
and a 250 micrometer long device yields approximately 90.degree. at 5 
volts. 
Because the total phase shift adds with series connected modulators, the 
invention couples together N modulators whose lengths vary as L.sub.2.pi. 
/2.sup.n, where n ranges from 1 to N inclusive and for a total phase shift 
range of 0.degree. to 360.degree., to form a N-bit digital phase modulator 
with a fixed voltage of V.sub.n =0 or 
360.degree.(FOM.multidot.L.sub.2.pi.).sup.-1. In this example the 
modulator is designed to provide a maximum relative phase shift of 360 and 
L.sub.2.pi. is the total modulator length needed to achieve 360.degree. 
phase shift at a fixed voltage V.sub.n. There is thus obtained a 
binary-digital phase modulator whose total phase shift is given by the 
following equation: 
##EQU4## 
where .multidot. denotes multiplication, L denotes length, .DELTA.Vn 
denotes applied voltage change, and FOM denotes the figure of merit of the 
waveguide used. 
FIG. 3 shows a cross sectional view of a ridge-type waveguide phase 
modulator utilized by the present invention. The waveguide 280 is of a 
GaAs/AlGaAs double heterostructure design having a p-n junction 100 
centrally located between GaAs waveguide layers 179 and 181. Both layers 
179 and 181 are the "higher-index" layers of the optical waveguide 280. 
The light cannot distinguish carrier type so that both layers 179 and 181 
simply appear as GaAs to the light. The shallow etched-ridge waveguide 
design allows direct transitions from the phase modulator into directional 
couplers or other elements with a minimum of process steps. 
The waveguide 280 shown in FIG. 3 maximizes modulation efficiency by using 
a 0.25 micrometer GaAs waveguide composed of n-type GaAs layer 179 and 
p-type GaAs layer 181 region interposed between an n-type AlGaAs cladding 
layer 178 and a p-type AlGaAs cladding layer 177. A p-n junction 100 is 
formed at the common edge of the n- and p-type GaAs layers 179 and 181. 
The total thickness of the GaAs layers 179 and 181, and the Al mole 
fraction of the cladding layers 177 and 178 are chosen to minimize the 
width of the optical mode perpendicular to the layers, parallel to the 
applied electric electric field. The lateral extent of the waveguide does 
not influence the modulation behavior, and is designed as a single-mode 
ridge-type guide with low loss. 
The phase modulator operates by changing the modal index of refraction of 
the waveguide 280 under varying reverse bias, but the local refractive 
index of the material is only changed in the depletion zone of the p-n 
junction 100. The refractive index of a wave-transmission medium is the 
ratio between the phase velocity in free space and in the medium. 
Therefore, it is preferred to place the p-n junction 100 in the middle of 
the waveguide 280 to maximize the confinement of the optical mode in the 
reverse-biased depletion zone so that as much of the optical energy as 
possible of the region of the junction swept by the depletion zone under 
changing bias conditions. This maximizes the influence of the applied 
electric field and the depletion-edge translation on the modal index. 
Referring to FIG. 3, a semiconductor layer 175, which is a heavily p.sup.+ 
doped material, is placed over the top surface of the p-type AlGaAs 
cladding layer 177. A planarizing layer of polyimide 176 is deposed over 
the portion of cladding layer 177 not covered by semiconductor layer 175. 
A metal contact 190 comprised of, for example, Ti/Au, is deposited over 
semiconductor layer 175 forming electrode 201 (see FIG. 5). Contact to the 
n-type layer 178 is made at the bottom of the n+ substrate 150, via a 
common contact layer 180 that is typically comprised of Au, Ge and Ni. 
Referring now to FIG. 5, the phase modulator is fabricated in such a manner 
that semiconductor layer sections 175 and the overlying metalization 190 
are differentiated by at etching process. The resulting electrodes 201 are 
arranged in series along the top of the waveguide and are electrically 
isolated from one another. 
It will be appreciated by those skilled in the art that, while the 
invention herein has been described using specific types of electrical 
conductivity within specific layers, the types of electrical conductivity 
within those layers can be reversed, so long as the relationship between 
the layers is preserved, i.e., a layer with p-type semiconductor may be 
replaced with a n-type semiconductor so long as any other p-type is 
replaced with an n-type. 
A digitally controlled distributed phase shifter 195, based on these 
principles, is shown in FIG. 2. FIG. 2 shows a top view, not to scale, of 
a preferred layout of a distributed optical phase shifter in accordance 
with the invention. The phase shifter is shown with 8 bits of resolution. 
Electrodes 201a-h of appropriate lengths, such as 256, 128, 64, 32, 16, 8, 
4, and 2 micrometers, are fabricated on the optical waveguide. The largest 
electrode 201a, which is connected to the MSB, is 256, (2.sup.8), 
micrometers in length, and the shortest electrode 201h, which is connected 
to the LSB, is 2(2.sup.1) micrometers in length. When a bias voltage is 
applied to a particular electrode 201a-h, via control terminal 400a-h, 
respectively, a variation in the modal index is caused in the portion of 
the waveguide 280 corresponding to that particular electrode. Referring to 
FIG. 5, if a bias voltage is applied to the electrode having length 
L.sub.T /2, via control terminal 400a (see FIG. 2 for control terminal 
400a), the portion of waveguide 280 undergoing a change in modal index is 
represented by the area between the two vertical dotted lines 500a and 500 
b. 
A 10 bit digitally controlled distributed optical phase shifter would use 
segmented electrodes of 512, 256, 218, 64, 32, 16, 8, 4, 2 and 1 
micrometers length placed in series over a single waveguide. As a result, 
a logic one signal applied to any combination of electrodes directly 
results in a digitally-coded phase shaft with 0.35.degree. resolution. 
The digitally controlled distributed optical phase shifter is inherently 
linearized. The phase shift derived at the LSB stage of the phase shifter 
network is equal to the resolution of the phase shifter network. Each 
individual section of the phase shifter only experiences applied biases of 
one of two possible state. Therefore, an individual section produces a 
corresponding phase shift of one of two possible amounts. Any nonlinearity 
between the two voltage states used is irrelevant. Thus, the device is 
inherently linear provided only that the N sections are related in length 
by L.sub.n =L.sub.n-1 /2 and that the waveguide FOM is uniform across the 
total length. Because the length of each section is defined 
photolithographically, length errors are minimized. 
Although the present invention is directed at digitally controlled 
distributed optical phase shifters, the technique of the present invention 
is equally applicable to distributed microwave phase shifters. Virtually 
any application of distributed microwave phase shifters may utilize 
digital drive, thus indicating the utility of the present invention. 
FIG. 4 shows a schematic of an embodiment of a digitally controlled 
distributed microwave phase shifter. The rectangular elements 300, 308 and 
310 represent distributed radio frequency voltage-controlled phase 
shifters and are related in length by L.sub.n =L.sub.n-1 /2 wherein n has 
values ranging from 1 to N. N is the number of phase shifters within the 
phase shift network and L.sub.1 is the length of the longest electrode. 
Each section is comprised of a distributed microwave phase shifter, a 
blocking capacitor 302, and bias network 350 which may be comprised of, 
for example, inductor 307 and capacitor 320. One section is required for 
each bit of resolution desired. The bi-state bias voltage is applied to 
the control terminal 306 of each phase shifter. Each section is 
capacitively coupled via capacitor 302 to the next and bias networks 
350a-c, are used to independently control the bias to each section. The 
length of an electrode is chosen for an rf phase shifter analogously with 
the optical phase shifter. Except for the mechanism of phase shift, all 
design considerations presented for the optical digital phase shifter are 
identical for microwave phase shifters. The voltage-controlled distributed 
microwave phase shift could be produced by several means available to one 
skilled in the art. One possible approach would be the use of a microwave 
transmission line fabricated on a suitable semiconductor surface. The 
transmission line structure could be, for example, of microstrip or 
coplanar-waveguide type. It could further be designed to be slow-wave, 
which would enhance the phase shift per unit length. The semiconductor 
would preferably be configured to be a p-n junction along the length of 
the transmission line within the region of maximum electric field from the 
said transmission line. As the bias is then varied on the transmission 
line, the depletion layer thickness will vary within the p-n junction, 
thus varying the capacitance per unit length of the transmission line. 
Because the velocity of propagation along an electrical transmission line 
is 
##EQU5## 
a change in capacitance causes a change in velocity and hence a change in 
phase shift through the transmission line. Thus, a voltage-controlled 
distributed phase shift is formed. 
Other approaches to distributed phase control are easily envisioned, i.e. 
microwave waveguides loaded with ferrites thereby requiring magnetic 
control, analogous to electrical control. In short, any distributed phase 
control structure can be made digitally controlled and linear by the 
technique used in our invention. 
Phase control of optical rf/microwave signals has numerous applications. 
For example, in communications, information is often applied to a carrier 
signal by phase modulation (PM). In particular, in digital communications, 
digital data is commonly represented by phase changes of the carrier 
signal (PSK-Phase-Shift Keying). The digital control technique of the 
present invention is useful for such modulation schemes. 
In coherent optical systems, e.g., coherent optical telephone, cable TV and 
data systems, phase shifts for modulation and signal "trimming" are 
necessary. Since all such systems typically use computers for control, 
digital control of these phase shifters is advantageous. An example of a 
specific rf/microwave application is in the phase control for phased-array 
antennas where the phase of each antenna element is controlled by a 
digital means, in general, phase control is critical in virtually any 
coherent communication/signal processing application. 
Based on the foregoing teaching those having ordinary skills in the art may 
derive modifications to the embodiments of the invention disclosed above. 
The invention is therefore not to be construed to be limited only to these 
disclosed embodiments, but it is instead intended to be limited only as 
defined by the breadth and scope of the appended claims.