Method and apparatus for monitoring modulation depth of dither signal and optical transmitter
Embodiments of this disclosure provide a method and apparatus for monitoring a modulation depth of a dither signal and an optical transmitter. The apparatus includes a detector to detect an optical signal output by a Mach-Zehnder modulator to obtain an electrical signal, a dither signal at a determined frequency is superimposed on a direct current bias voltage of the Mach-Zehnder modulator; a frequency-doubling dither signal synchronization detection module to perform synchronization detection on the electrical signal and a frequency-doubling dither signal at a frequency twice the determined frequency, to obtain an amplitude of a signal component contained in the electrical signal at a frequency identical to the frequency of the frequency-doubling dither signal A signal processor is to calculate a modulation depth of the dither signal superimposed on the direct current bias voltage according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.
1. An apparatus comprising:
a detector to detect an optical signal output by a Mach-Zehnder modulator to obtain an electrical signal, wherein a dither signal at a determined frequency is superimposed on a direct current bias voltage of the Mach-Zehnder modulator;
a frequency-doubling dither signal synchronization detection module to perform synchronization detection on the electrical signal output by the detector and a frequency-doubling dither signal at a frequency twice the determined frequency, to obtain an amplitude corresponding to a signal component contained in the electrical signal output by the detector at a frequency identical to the frequency of the frequency-doubling dither signal; and
a signal processor to calculate a modulation depth of the dither signal superimposed on the direct current bias voltage according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.
2. The apparatus according to claim 1 , wherein,
the signal processor calculates the modulation depth according to a responsivity of the detector, power of a laser output by a laser providing an optical carrier for the Mach-Zehnder modulator, the signal component and the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.
3. The apparatus according to claim 2 , wherein the signal processor calculates the modulation depth according to relation (9) below:
coef
(
2
f
d
)
·
16
π
2
RP
0
〈
cos
(
2
φ
sl
)
〉
≈
α
V
π
local
;
(
9
)
where, R is the responsivity of the detector, P 0 is the power of the laser, cos(2φ SI ) is the signal component, and Coef(2f d ) is the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.
4. The apparatus according to claim 1 , further comprising:
an original frequency dither signal synchronization detection module to perform synchronization detection on the electrical signal output by the detector and an original frequency dither signal at a frequency that is the determined frequency, to obtain an amplitude of a signal component contained in the electrical signal output by the detector at a frequency identical to the frequency of the original frequency dither signal; and
the signal processor calculates the modulation depth of the dither signal superimposed on the direct current bias voltage according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal and the amplitude of the signal component at the frequency identical to the frequency of the original frequency dither signal.
5. The apparatus according to claim 4 , wherein,
the signal processor calculates the modulation depth according to the responsivity of the detector, the power of the laser outputted by the laser providing the optical carrier for the Mach-Zehnder modulator, the signal component, the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal and the amplitude of the signal component at the frequency identical to the frequency of the original frequency dither signal.
6. The apparatus according to claim 5 , wherein the signal processor calculates the modulation depth by solving relation (7a) below:
(
RP
0
〈
cos
(
2
φ
sl
)
〉
)
2
(
πα
V
π
local
)
4
-
(
coef
(
fd
)
·
8
)
2
(
πα
V
π
local
)
2
-
(
coef
(
2
fd
)
·
16
)
2
=
0
;
(
7
a
)
where, R is the responsivity of the detector, P 0 is the power of the laser, cos(2φ SI ) is the signal component, and ω d is a circular frequency to which the frequency of dither signal corresponds.
7. The apparatus according to claim 1 , wherein,
the signal processor further calculates a target adjustment amplitude of the dither signal according to the calculated modulation depth and a current amplitude and an initial modulation depth of the dither signal.
8. An optical transmitter comprising the apparatus for monitoring a modulation depth of a dither signal as claimed in claim 1 .
9. A method comprising:
by at least one processor,
detecting an optical signal output by a Mach-Zehnder modulator to obtain an electrical signal, wherein a dither signal at a determined frequency is superimposed on a direct current bias voltage of the Mach-Zehnder modulator;
performing synchronization detection on the electrical signal and a frequency-doubling dither signal at a frequency twice the determined frequency, to obtain an amplitude corresponding to a signal component contained in the electrical signal at a frequency identical to the frequency of the frequency-doubling dither signal; and
calculating a modulation depth of the dither signal superimposed on the direct current bias voltage according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal.
10. The method according to claim 9 , wherein the method further comprises:
performing synchronization detection on the electrical signal and an original frequency dither signal at a frequency that is the determined frequency, to obtain an amplitude of a signal component contained in the electrical signal at a frequency identical to the frequency of the original frequency dither signal;
wherein the modulation depth of the dither signal superimposed on the direct current bias voltage is calculated according to the amplitude of the signal component at the frequency identical to the frequency of the frequency-doubling dither signal and the amplitude of the signal component at the frequency identical to the frequency of the original frequency dither signal.