Phase noise compensation system, and method
A system for compensating for phase noise, with particular application in lidar, includes a compensation interferometer that receives a signal from a source, and splits it into a first and second path, with a path length difference Δτ between them. Typically the path length is significantly less than that of the return distance to a target. The output of the compensation interferometer, which consists of phase noise generated in time Δτ is vectorially summed during a time similar to a signal flight time to a target, and the result used to reduce phase noise present on measurements of a target. It further includes means for selecting Δτ such that competing noise elements are reduced or optimised.
1. A system comprising:
a) a signal generator for providing a signal,
b) a splitter for splitting the signal from the signal generator and directing it to a measurement path and a compensation path, wherein:
i) the measurement path comprises a measurement interferometer having a splitter for splitting the signal into a first and a second part, the measurement interferometer being arranged to transmit the first part to a remote target, and to receive reflections therefrom, and to combine the received reflections with the second part in a measurement interferometer mixer, and
ii) the compensation path comprises a compensation interferometer arranged to split its input signal into a first, delayed path, and a second, undelayed path, and to combine the first and second paths in a compensation interferometer mixer to produce an output bearing information pertaining to a phase difference between the two paths, the relative delay between the first and second paths being a predetermined time Δτ;
c) a processor configured to calculate a phase compensation figure by digitally measuring a phase change across n successive passes through the compensation interferometer over a time nΔτ, where nΔτ is equal to n times the predetermined time Δτ, and subtracting the calculated phase compensation figure from the output of the measurement interferometer,
wherein the system is arranged to calculate a range L/2 to the remote target based upon the compensated output from the measurement interferometer, said range giving a time delay τ to the received reflections as compared to the second part, and wherein the phase compensation figure is calculated under the condition that |τ−nΔτ|≤Δτ/2.
2. A system as claimed in claim 1 wherein the system is arranged to select a value for n a based on an estimate of the range to the target.
3. A system as claimed in claim 1 wherein the delay time Δτ of the compensation interferometer is chosen based upon a combination of errors associated with multiple measurement of the phase error from the compensation path, and the size of Δτ/2.
4. A system as claimed in claim 1 wherein the values of n and Δτ are chosen to reduce residual phase noise following the processing carried out in step (c).
5. A system as claimed in claim 4 wherein the length of delay coil ΔL in the compensation interferometer, and hence the values of n and Δτ are chosen, to produce a minimum value for σ phase in the equation:
σ
phase
=
(
[
∫
f
Low
f
High
Φ
(
f
)
2
df
]
1
/
2
Y
)
2
+
L
Δ
L
σ
meas
2
where σ phase is the RMS residual phase error, Φ(f) is the laser phase noise spectrum expressed in radians per root Hertz referred to 1 m optical path difference which is integrated over the relevant frequency band f Low →f High , ΔL is the optical path length of the delay coil (with time equivalent Δτ), Y represents the maximum uncompensated path length, (with a maximum value of ΔL/2), a σ meas is the RMS phase error associated with a single differential phase measurement in the compensator interferometer, and L is the target return optical path length.
6. A system as claimed in claim 1 wherein the compensation interferometer is arranged to have a delay time Δτ of less than 10% of the flight time of the signal traversing twice an expected in-use target range.
7. A system as claimed in claim 1 wherein the compensation interferometer is arranged to have a delay time Δτ of less than 1% of the flight time of the measurement signal traversing twice an expected in-use target range.
8. A system as claimed in claim 1 wherein the compensation interferometer mixer provides a complex output allowing in-phase (I) and quadrature (Q) signals to be extracted.
9. A system as claimed in claim 1 wherein the measurement interferometer mixer provides a complex output allowing in-phase (I) and quadrature (Q) signals to be extracted.
10. A system as claimed in claim 1 wherein the measurement path contains a modulator for modulating the signal before it is transmitted to the target.
11. A system as claimed in claim 1 wherein the signal generator is a laser.
12. A system as claimed in claim 11 , wherein the measurement path contains a modulator for modulating the signal before it is transmitted to the target, wherein the modulator is an acousto-optic modulator.
13. A system as claimed in claim 1 wherein the signal generator is a radio frequency signal generator.
14. A system as claimed in claim 1 wherein the delay path of the compensation interferometer comprises an optical fibre.
15. A system as claimed in claim 11 wherein the delay path comprises of a coaxial cable.
16. A system as claimed in claim 1 wherein the system is a LIDAR system or a radar system.
17. A system as claimed in claim 16 wherein the system is arranged to have a slant range of at least 1 km.
18. A system as claimed in claim 1 wherein values of phase noise compensation are calculated for different target ranges, and are used to correct the output of the measurement interferometer at different ranges.
19. A method for compensating for phase noise in a LIDAR or radar, comprising the steps of:
i) generating a signal;
ii) splitting the signal into a measurement signal and a compensation signal and directing the measurement signal to a measurement path and directing the compensation signal to a compensation path;
iii) transmitting a first part of the measurement signal in the measurement path to a remote target, and mixing returns from the remote target with a second part of the measurement signal in a measurement interferometer mixer to down-convert the returns;
iv) passing the compensation signal into a compensation interferometer arranged to split its input signal into a first, delayed path, and a second, undelayed path, wherein a relative delay between the first and second paths being a predetermined time Δτ;
v) calculating a phase compensation figure by measuring a phase change across n successive passes through the compensation interferometer over a time nΔτ, where nΔτ is equal to n times the predetermined time Δτ;
vi) subtracting the calculated phase compensation figure from the down-converted output of the measurement interferometer mixer of step (iii); and
vii) calculating a range L/2 to the target based upon the compensated output from the measurement interferometer, wherein said range giving a time delay τ to the received reflections as compared to the second part, and wherein the phase compensation figure is calculated under the condition that |τ−nΔτ|≤Δτ/2.