Method for optimizing laser diode operating current
View Patent ↗A method for optimizing a dc operating drive current of a laser diode is presented. Lasers of a particular type are characterized in terms of the dependency of certain parameters on temperature and of the minimum bias current required for adequate laser speed. The minimum bias current is determined using the operating bias current minus the threshold current all normalized by the threshold current. For individual laser diodes, the threshold current and slope efficiency are obtained at various temperatures to allow calculation at operating temperatures of interest. The minimum laser bias current for speed can then be determined over the operating temperature range. This may be done by setting the optical power to the highest value calculated using this minimum acceptable bias criterion over the operating temperature range. The ac and dc laser driver current sourcing requirements may be computed to ensure laser and module compatibility.
1. A method for determining a laser diode dc bias current for operation at a required speed over a target temperature range, the method comprising:
determining a functional dependence of performance parameters on temperature for a group of laser diodes, at least one of the performance parameters including threshold current (I th );
determining a parameterized fractional increase above I th that results in a required operating speed for the group, where a minimum bias current (I op ) is based on the required operating speed;
determining a minimum k value for select laser diodes in the group using the minimum bias current (I op ), wherein a k value is defined as a bias current offset above a threshold current normalized by the threshold current ((I op −I th )/I th ); and
selecting a bias current for an individual laser diode in the group based on the minimum k value, the selected bias current corresponding to a predetermined optical power level.
2. The method of claim 1 further comprising determining the output power level of the individual laser diode based on a slope efficiency of the laser diode and a fiber coupling efficiency.
3. The method of claim 1 , wherein selecting the bias current is further based on X(T 1 , T 2 ), wherein
X
(
T
1
,
T
2
)
≡
k
T
2
Ith
T
2
SE
T
2
k
T
1
Ith
T
1
SE
T
1
,
where k T1 is a k value corresponding to a minimum bias current required at a first temperature (T 1 ) for the group, wherein k T1 ≧the minimum k value determined for select laser diodes in the group,
k T2 is a k value corresponding to a minimum bias current required at a second temperature (T 2 ) for the group,
Ith T1 is a threshold current at T 1 for the individual laser diode in the group,
Ith T2 is a threshold current at T 2 for the individual laser diode,
SE T1 is a slope efficiency at T 1 for the individual laser diode, and
SE T2 is a slope efficiency at T 2 for the individual laser diode.
4. The method of claim 3 , wherein selecting the bias current is further based on calculating a bias current required at T 1 to satisfy operating speed requirements at T 2 , calculated as:
(1+X(T 1 ,T 2 )k T1 )Ith T1 .
5. The method of claim 1 further comprising:
determining threshold currents (Ith T1 ,Ith T2 ), slope efficiencies (SE T1 , SE T2 ) and k values (k T1 , k T2 ) at multiple temperatures T 1 and T 2 for select laser diodes in the group;
calculating X(T 1 , T 2 ) for each of the selected laser diodes, wherein
X
(
T
1
,
T
2
)
≡
k
T
2
Ith
T
2
SE
T
2
k
T
1
Ith
T
1
SE
T
1
wherein k T1 and k T2 are determined for the group of laser diodes and Ith T1 , Ith T2 , SE T1 , and SE T2 are determined for individual laser diodes in the group; and
categorizing the laser diodes according to values of X.
6. The method of claim 5 wherein T 1 is room temperature and T 2 is approximately a highest expected operating temperature.
7. The method of claim 5 wherein T 1 is room temperature and T 2 is a temperature at which the laser diodes typically require a highest optical output power while satisfying minimum bias current conditions needed for laser response.
8. The method of claim 1 wherein determining the functional dependence comprises taking measurements at different temperatures within a target temperature range.
9. The method of claim 1 farther comprising optically attenuating an output from the individual laser diode to achieve the predetermined power level.
10. The method of claim 1 farther comprising setting the individual laser diode's output power to a maximum required value over the target temperature range.
11. The method of claim 1 further comprising:
building one of the laser diodes into a transmitter optical subassembly (TOSA); and
testing the TOSA to verify compliance with the required speed over the target temperature range.
12. The method of claim 1 further comprising:
building one of the laser diodes into a module; and
testing the module to verify compliance with the required speed over the target temperature range.
13. The method of claim 1 further comprising predicting ac and dc drive current requirements for a minimal frequency response at various temperatures(T), wherein the ac drive current (I ac ) and dc drive current (I dc ) are computed as:
I
ac
(
T
)
=
k
(
T
)
·
Ith
·
(
10
(
ER
/
10
)
-
1
)
(
10
(
ER
/
10
)
+
1
)
,
where k(T) is a k value corresponding to a minimum bias current required at a temperature T and ER is a required extinction ratio between 1 and 0 bit power levels in dB, and
I dc ( T )=(1+ k ( T )) Ith ( T ).
14. The method of claim 13 further comprising determining whether the ac and dc drive currents are supported by a particular laser driver at the predetermined power level.
15. A method of determining a dc bias current for a laser diode, the method comprising:
grouping laser diodes into bins of similar performance over temperature (T) based on a dimensionless parameter
X
(
T
1
,
T
2
)
≡
k
T
2
Ith
T
2
SE
T
2
k
T
1
Ith
T
1
SE
T
1
,
wherein k T1 =(I op −I th (T 1 ))/I th (T 1 ) for the group,
k T2 =(I op −I th (T 2 ))/I th (T 2 ) for the group,
I op =minimum dc bias current that provides target operating speed,
Ith T1 =threshold current at T 1 for an individual laser diode in the group,
Ith T2 =threshold current at T 2 for the individual laser diode,
SE T1 =slope efficiency at T 1 for the individual laser diode,
SE T2 =slope efficiency at T 2 for the individual laser diode, and
T 1 and T 2 are first and second temperatures, respectively; and
determining a dc bias current I op (T 1 )=(1+X(T 1 , T 2 )k T1 )Ith T1 for the laser diodes in the bin.
16. The method of claim 15 further comprising mounting one of the laser diodes in a transmitter optical subassembly (TOSA) that is used in an electronics package transmitter or transceiver module.