Tunable laser assembly and method of control
A tunable laser assembly housed in a single enclosure and a method of control is described wherein the tunable laser, pump and semiconductor optical amplifier do not share a common optical axis but are all aligned to optical waveguides on an intervening planar lightwave circuit (PLC). Wavelength monitoring circuitry is included on the PLC to enable monitoring and control of the tunable laser center wavelength and optical bandwidth. The design of the PLC does not introduce perturbations into the swept-source laser output spectrum that would cause artifacts in imaging applications such as optical coherence tomography (OCT).
1. A method for closed-loop controlling an absolute wavelength and an optical bandwidth of a swept-source tunable laser, wherein a closed-loop control uses timing information from a signal generated by a reference wavelength filter and an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers,
wherein said reference wavelength filter and said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a fiber Bragg grating (FBG) with multiple reflection peaks,
wherein said FBG with multiple reflection peaks has one main reflection peak that is higher in amplitude than all the other reflection side peaks and serves as said wavelength reference filter, all other reflection side peaks generate said signal pulses corresponding to nearly equally spaced wavenumber.
2. The method of claim 1 , wherein the said timing information from a signal generated by a reference wavelength filter is used for absolute wavelength control and said timing information from an optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers is used for optical bandwidth control.
3. The method of claim 2 , wherein the said timing information is comprised of a timer starting point generated by said reference wavelength filter and a second timing marker generated by said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
4. The method of claim 3 , wherein said second timing marker is generated by selecting the appropriate ‘Nth’ pulse from said optical element that generates signal pulses corresponding to nearly equally spaced wavenumbers.
5. The method of claim 1 , wherein said main reflection peak has a reflectance of greater than 90% and said reflection side peaks have a reflectance of less than approximately 40%.
6. The method of claim 1 , wherein said main reflection peak is located within +/−20 nm of the center wavelength of the tunable laser.
7. The method of claim 1 , wherein said reflection side peaks have a free spectral range (FSR) in the range of 0.1 nm to 10 nm.
8. The method of claim 1 , wherein said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises an etalon.
9. The method of claim 1 , wherein said reference wavelength filter comprises one of a fiber Bragg grating (FBG), notch filter, or bandpass filter.
10. The method of claim 1 , wherein said reference wavelength filter and said optical element that generates signal pulses corresponding to nearly equally spaced wavenumber comprises a planar lightwave circuit (PLC) with integrated wavelength monitoring circuit (WMC).
11. The method of claim 10 , wherein said WMC comprises at least one selected from a list of: a Bragg grating, a micro-ring resonator (MRR), and a Mach-Zehnder interferometer (MZI).
12. The method of claim 11 , wherein said at least one MRR or at least one MZI generates said signal pulses corresponding to nearly equally spaced wavenumber used for optical bandwidth control.
13. The method of claim 12 , wherein said at least one MRR or at least one MZI has a FSR in the range of 0.1 to 10 nm.
14. The method of claim 11 , wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates signal pulses used for absolute wavelength control.
15. The method of claim 14 , wherein said at least one selected from a list of: a Bragg grating, a MRR or a MZI generates said signal pulses in the range of 1 to 10 pulses as the tunable laser sweeps over its wavelength range.
16. The method of claim 10 , wherein said WMC comprises at least one Bragg grating.
17. The method of claim 16 , wherein said Bragg grating comprises said wavelength reference filter.
18. The method of claim 16 , wherein said at least one Bragg grating has a main reflection peak located within +/−20 nm of the center wavelength of the tunable laser.