Slow chirp compensation for enhanced signal bandwidth and transmission performances in directly modulated lasers
View Patent ↗The frequency chirp modulation response of a directly modulated laser is described using a small signal model that depends on slow chirp amplitude s and slow chirp time constant τ s . The small signal model can be used to derive an inverse response for designing slow chirp compensation means. Slow chirp compensation means include electrical compensation, optical compensation, or both. Slow chirp electrical compensation can be implemented with an LR filter or other RF circuit coupled to a direct modulation source (e.g., a laser driver) and the directly modulated laser. Slow chirp optical compensation can be implemented with an optical spectrum reshaper having a rounded top and relatively large slope (e.g., 1.5-3 dB/GHz). The inverse response can be designed to under-compensate, to produce a flat response, or to over-compensate.
1. An optical transmitter with slow chirp compensation, comprising:
an optical signal source adapted to be directly modulated and to produce a first signal, the first signal being frequency modulated, phase modulated, or both; and
slow chirp compensation means for introducing an inverse response of the slow chirp into the first signal;
wherein frequency chirp of the optical signal source is modeled by the formula:
v
m
(
f
)
=
A
(
1
+
s
1
+
2
π
jf
τ
s
)
I
m
(
f
)
;
where:
v m (f) is frequency chirp;
A is adiabatic chirp;
s is slow chirp amplitude;
j is the imaginary unit;
f is modulation frequency;
τ s is slow chirp time constant; and
I m (f) is modulation current.
2. The optical transmitter of claim 1 , wherein the slow chirp compensation means includes one or more of electrical compensation and optical compensation.
3. The optical transmitter of claim 2 , wherein electrical compensation includes a filter coupled to the optical signal source and to a laser driver configured to directly modulate the optical signal source, the filter including one or more of: an inductor-resistor filter, a resistor-capacitor filter, and a resistor-inductor-capacitor filter.
4. The optical transmitter of claim 2 , wherein optical compensation includes an optical spectrum reshaper adapted to reshape the first signal into a second signal, the second signal being amplitude modulated and frequency modulated.
5. The optical transmitter of claim 4 , wherein slope and bandwidth of the optical spectrum reshaper are selected to configure the optical spectrum reshaper to provide both slow chirp compensation and dispersion tolerance.
6. The optical transmitter of claim 4 , wherein the optical spectrum reshaper includes a rounded top and a slope in the range of 1.5 dB/GHz to 3 dB/GHz.
7. The optical transmitter of claim 1 , wherein the optical signal source is a directly modulated semiconductor laser.
8. The optical transmitter of claim 7 , wherein slow chirp in the first signal results from intrinsic properties of the directly modulated semiconductor laser, including one or more of carrier transport and spatial hole burning.
9. The optical transmitter of claim 7 , wherein the optical signal source is a distributed feedback laser.
10. A method for reducing slow chirp, comprising:
directly modulating an optical light source to generate an optical signal that is frequency modulated, phase modulated, or both;
compensating slow chirp in the optical signal by introducing an inverse response of the slow chirp into the optical signal; and
prior to directly modulating the optical light source, modeling frequency of the optical light source according to the formula:
v
m
(
f
)
=
A
(
1
+
s
1
+
2
π
jf
τ
s
)
I
m
(
f
)
;
where:
v m (f) is frequency chirp;
A is adiabatic chirp;
s is slow chirp amplitude;
j is the imaginary unit;
f is modulation frequency;
τ s is slow chirp time constant; and
I m (f) is modulation current.
11. The method of claim 10 , further comprising, prior to compensating slow chirp, calculating the inverse response of the slow chirp using the equation of claim 11 .
12. The method of claim 11 , wherein the calculated inverse response of the slow chirp is modeled using the following equation:
H
c
(
f
)
=
k
1
+
s
1
+
2
π
jf
τ
s
;
wherein:
H c (f) is the calculated inverse response of the slow chirp; and
k is a frequency independent compensation coefficient.
13. The method of claim 12 , wherein compensating slow chirp in the optical signal includes providing passive electrical slow chirp compensation, and wherein the value of k is less than 1.
14. The method of claim 10 , wherein compensating slow chirp in the optical signal includes introducing an inverse response of the slow chirp into the optical signal electrically by a radio frequency circuit coupled between the optical light source and a direct modulation source.
15. The method of claim 10 , further comprising converting frequency modulation, phase modulation, or both, of the optical signal to amplitude modulation of the optical signal using an optical spectrum reshaper following the optical light source, wherein the optical spectrum reshaper is configured to optically introduce an inverse response of the slow chirp into the optical signal.
16. The method of claim 10 , wherein the inverse response of the slow chirp is configured to over-compensate or under-compensate slow chirp.
17. The method of claim 16 , wherein the introduction of an inverse response of the slow chirp is performed by an optical spectrum reshaper that reduces bandwidth of the optical signal, and wherein over-compensation of slow chirp generates over shoot that partially compensates the bandwidth reduction from the optical spectrum reshaper.
18. The method of claim 10 , wherein compensating slow chirp improves one or more of:
frequency chirp modulation response of the optical signal;
jitter of the optical signal;
eye mask margin of the optical signal;
receiver sensitivity to the optical signal; and
dispersion tolerance of the optical signal,
compared to not compensating slow chirp.
19. The optical transmitter of claim 4 , wherein the optical spectrum reshaper includes a coupled multi-cavity filter or a bandpass filter.