Sweep Control of an Optical Heterodyne Measurement System
Described herein is a system and method of controlling an optical heterodyne measurement system ( 1 ). The measurement system ( 1 ) has a tunable laser ( 9 ) for generating a local oscillator signal, an optical input ( 5 ) for receiving an input optical signal ( 7 ) and a mixing module ( 13 ) for mixing the local oscillator signal with the input optical signal to generate an output optical measurement signal. One embodiment provides a method including the steps of: a) receiving an input electrical drive signal for driving the tunable laser ( 9 ) to produce a laser output having a spectral linewidth and peak central frequency; b) coupling the input electrical drive signal with an electrical linewidth control signal ( 54 ) to selectively broaden the spectral linewidth; and c) during a measurement period, selectively tuning the central frequency of the laser in a stepwise manner across a predetermined frequency spectrum at predefined tuning increments.
1 . A method of controlling an optical heterodyne measurement system, the measurement system having a tunable laser for generating a local oscillator signal, an optical input for receiving an input optical signal and a mixing module for mixing the local oscillator signal with the input optical signal to generate an output optical measurement signal; the method including the steps of:
a) receiving an input electrical drive signal for driving the tunable laser to produce a laser output having a spectral linewidth and peak central frequency, an initial spectral linewidth of the laser output having a first spectral width;
b) coupling the input electrical drive signal with a linewidth control signal to selectively broaden the spectral linewidth to a second spectral width; and
c) during a measurement period, selectively tuning the central frequency of the laser in a stepwise manner across a predetermined frequency spectrum at predefined tuning increments.
2 . A method according to claim 1 wherein the tuning increment is defined based on the second spectral width.
3 . A method according to claim 2 wherein the tuning increment is in the range of 0.5 to 1.5 times the second spectral width.
4 . A method according to claim 3 wherein the tuning increment is equal to the second spectral width.
5 . A method according to claim 1 wherein the linewidth control signal is based on user input indicative of a desired scan time or refresh rate of the optical heterodyne measurement system.
6 . A method according to claim 1 wherein the linewidth control signal includes a pseudo random bit function.
7 . A method according to claim 1 wherein the linewidth control signal includes a repeating triangular function.
8 . A method according to claim 1 wherein the linewidth control signal includes a sinusoidal function.
9 . A method according to claim 1 wherein the input electrical drive signal controls the gain of the tunable laser.
10 . A method according to claim 1 wherein the electrical drive signal controls the phase of the tunable laser.
11 . A method according to claim 1 wherein the second spectral width is proportional to the amplitude of the linewidth control signal.
12 . A method according to claim 1 wherein the second spectral width is 5 to 100 times greater than the first spectral width.
13 . A method according to claim 1 wherein the linewidth control signal also modifies a spectral profile of the laser.
14 . A method according to claim 13 wherein the linewidth control signal flattens the spectral profile of the laser.
15 . A method according to claim 1 wherein the tuning increment is variable over a given frequency range.
16 . A method according to claim 1 wherein step b) includes modulating the input electrical drive signal with the linewidth control signal.
17 . (canceled)
18 . A method according to claim 17 wherein the linewidth control signal varies as a function of the central frequency of the tunable laser so as to vary the second spectral width during the measurement period.
19 . A control system for an optical heterodyne measurement system, the measurement system having a tunable laser for generating a local oscillator signal, an optical input for receiving an input optical signal and a mixing module for mixing the local oscillator signal with the input optical signal to generate an output optical measurement signal, the control system including:
a drive module for producing an input electrical drive signal for driving the tunable laser to produce an optical output having a spectral linewidth and peak central frequency, an initial spectral linewidth of the optical output having a first spectral width;
a linewidth tuning module for coupling the input electrical drive signal with a linewidth control signal to selectively broaden the spectral linewidth to a second spectral width; and
a tuning module for selectively tuning the central frequency of the laser in a stepwise manner across a predetermined frequency spectrum.
20 . A control system according to claim 19 wherein the tuning module selectively tunes the central frequency at integer multiples of a tuning increment, wherein the tuning increment is defined based on the second spectral width.
21 . (canceled)
22 . A method of calibrating a tunable laser including the steps of:
a) performing a first measurement of a spectral linewidth of an output of the laser;
b) coupling the input electrical drive signal with an electrical control signal to selectively broaden the spectral linewidth;
c) performing a second measurement of the spectral linewidth;
d) defining a tuning increment based on the second measurement such that, during operation, the central frequency of the laser is incrementally tuned in a stepwise manner across a predetermined frequency spectrum at integer multiples of the tuning increment.
23 . (canceled)