Wavelength-tunable light source and wavelength control method for the same
A wavelength-tunable light source includes a wavelength-tunable laser including a first region and a second region each of which includes at least one of heaters, a frequency locker configured to receive output light of the wavelength-tunable laser and output two electric control signals whose phases are mutually different by 90° and having frequency period with respect to frequency of the output light, a thermal electric cooler on which the wavelength-tunable laser and the frequency locker are mounted, and a controller configured to control temperature of the heaters, and the thermal electric cooler on the basis of any one of the two electric control signals.
1. A wavelength control method for a wavelength-tunable light source comprising:
a wavelength-tunable laser including a first region and a second region each of which includes at least one of heaters;
a frequency locker configured to receive output light of the wavelength-tunable laser and to output an electric control signal having frequency period with respect to frequency of the output light;
a thermal electric cooler on which the wavelength-tunable laser and the frequency locker are mounted; and
a controller configured to include a look-up table, the method comprising:
acquiring a target oscillation frequency;
selecting a reference frequency closest to the target oscillation frequency from among a plurality of reference frequencies preliminarily stored in the look-up table;
calculating Δf which is a difference between the target oscillation frequency and the reference frequency;
calculating temperature T NEW of the thermal electric cooler such that a lock point of the output property of the frequency locker matches to the target oscillation frequency;
setting and maintaining temperature of the thermal electric cooler to and at the temperature T NEW ;
calculating a first power P k NEW as an initial value such that the oscillation frequency of the wavelength-tunable laser matches to the target frequency, where k is a natural number; and
supplying each of the heaters with the first power P k new .
2. The wavelength control method according to claim 1 , wherein
the value T NEW is indicated by a formula: TNEW=T+Δf×C LOCK , where T is temperature set in the thermal electric cooler at the reference frequency, and C LOCK is an inverse of a temperature coefficient of the electric control signal output from the frequency locker, and
the first power P k new is indicated by a formula: P k new =P k +C HTk ×(1−C Lock /C LD )×Δf, where P k is power supplied to a corresponding heater from among the heaters at the reference frequency, C HTk are inverses of a change rate of oscillation frequency of the wavelength-tunable laser with respect to the power supplied to the respective heaters, and C LD is an inverse of a change rate of the oscillation frequency of the wavelength-tunable laser which changes depending on a temperature change of the thermal electric cooler.
3. The wavelength control method according to claim 2 , wherein in the setting temperature of the thermal electric cooler, a value obtained by adding (T MON -T CAL )/C LOCK_AMB from the value T NEW is regarded as the temperature of the thermal electric cooler, and
in the supplying each of the heaters with the initial value, a value obtained by subtracting C HTk ×(T MON −T CAL )/C LOCK_AMB from the first power P k new is regarded as the initial value, where T MON is a current ambient temperature of the wavelength-tunable light source, T CAL is an ambient temperature of the wavelength-tunable light source at a time when the look-up table is created, and C LOCK_AMB is an inverse of a change rate of the electric control signal output from the frequency locker with respect to an ambient temperature of the wavelength-tunable light source.
4. The wavelength control method according to claim 2 , wherein
in the setting temperature of the thermal electric cooler, a value obtained by adding (T MON −T CAL )/C LOCk_AMB from the value T NEW is regarded as the temperature of the thermal electric cooler, and
in the supplying each of the heaters with the initial value, a value obtained by subtracting C HTk ×(T MON −T CAL )/C LOCK_AMB from the first power P k new and adding C HTk ×(T MON −T CAL )/C LOCK_AMB is regarded as the initial value, where T MON is a current ambient temperature of the wavelength-tunable light source, T CAL is an ambient temperature of the wavelength-tunable light source at a time when the look-up table is created, C LOCK_AMB is an inverse of a change rate of the electric control signal output from the frequency locker with respect to an ambient temperature of the wavelength-tunable light source, and C LD_AMB is an inverse of temperature coefficient of an oscillation frequency of the wavelength-tunable laser with respect to an ambient temperature of the wavelength-tunable light source.
5. The wavelength control method according to claim 1 , wherein
the electric control signals of the frequency locker include two control signals whose phases are mutually different by 90°, the two control signals having frequency period with respect to frequency of the output light of the wavelength-tunable laser, and
the wavelength control method further comprises:
selecting one of the two control signals corresponding to the reference frequency.
6. The wavelength control method according to claim 1 , further comprising:
performing feedback control on the first power P k new to converge the oscillation frequency of the wavelength-tunable laser with the target oscillation frequency.