Light source for swept source optical coherence tomography based on cascaded distributed feedback lasers with engineered band gaps
The present invention is a tunable semiconductor laser for swept source optical coherence tomography, comprising a semiconductor substrate; a waveguide on top of said substrate with multiple sections of different band gap engineered multiple quantum wells (MQWs); a multiple of distributed feedback (DFB) gratings corresponding to each said band gap engineered MWQs, each DFB having a different Bragg grating period; and anti-reflection (AR) coating deposited on at least the laser emission facet of the laser to suppress the resonance of Fabry-Perot cavity modes. Each DFB MQWs section can be activated and tuned to lase across a fraction of the overall bandwidth as is achievable for a single DFB laser and all sections can be sequentially activated and tuned so as to collectively cover a broad bandwidth, or simultaneously activated and tuned to enable a tunable multi-wavelength laser. The laser hence can emit either a single lasing wavelength or a multiple of lasing wavelengths and is very suitable for swept-source OCT applications.
1. A swept source OCT system comprising:
a tunable light source;
a beam splitter for dividing the light along a sample and a reference path;
a photodetector for receiving light returned from both the sample and the reference paths and generating output signals as a function of time as the wavelength of the source is tuned;
a processor for analyzing the output signals to derive a reflectance distribution along the sample path and wherein the tunable light source includes an elongated optical waveguide structure, with one end thereof defining a laser output facet;
a linear series of distributed feedback gratings formed along the waveguide structure to define a series of resonant cavities;
a series of semiconductor gain structures formed within the waveguide structure and aligned with the gratings, with the bandgap energy of the gain structures increasing towards said output facet; and
a power supply for supplying current to the gratings and the gain structures in a manner to generate laser output from each resonant cavity and for wavelength tuning the output.
2. An OCT system as recited in claim 1 , wherein a common current is supplied to a grating and the associated gain structure.
3. An OCT system as recited in claim 1 , wherein current is independently supplied to a grating and the associated gain structure.
4. An OCT system as recited in claim 1 , wherein current is simultaneously supplied to all the gratings and the gain structures and wherein the current is tuned so that a tuned, multi-wavelength output is generated.
5. An OCT system as recited in claim 1 , wherein current is supplied to the gain structure of each of the associated resonant cavities, said current being controlled in manner so that each of the gain sections are sequentially activated and tuned one at a time so that a tuned, narrow-band output is generated.
6. A method of evaluating the reflectance distribution within a sample using swept source OCT comprising the steps of:
providing a tunable light source, said light source including an elongated optical waveguide structure, with one end thereof defining a laser output facet, a linear series of distributed feedback gratings formed along the waveguide structure to define a series of resonant cavities, a series of semiconductor gain structures formed within the waveguide structure and aligned with the gratings, with the bandgap energy of the gain structures increasing towards said output facet, and a power supply for supplying current to the gratings and the gain structures in a manner to generate laser output from each resonant cavity and for wavelength tuning the output;
dividing the light along a sample and a reference path;
measuring light returned from both the sample and the reference paths and generating output signals as a function of time as the wavelength of the source is tuned; and
analyzing the output signals to derive a reflectance distribution along the sample path and wherein the light source is operated by supplying current to the gratings and the gain structures in a manner to generate laser output from each resonant cavity and for wavelength tuning the output.
7. A method as recited in claim 6 , wherein current is simultaneously supplied to all the gratings and the gain structures and wherein the current is tuned so that a tuned, multi-wavelength output is generated.
8. A method as recited in claim 6 , wherein current is supplied to the gain structure of each of the associated resonant cavities, said current being controlled in manner so that each of the gain sections are sequentially activated and tuned one at a time so that a tuned, narrow-band output is generated.