Highly stable semiconductor lasers and sensors for III-V and silicon photonic integrated circuits
Building blocks are provided for on-chip chemical sensors and other highly-compact photonic integrated circuits combining interband or quantum cascade lasers and detectors with passive waveguides and other components integrated on a III-V or silicon. A MWIR or LWIR laser source is evanescently coupled into a passive extended or resonant-cavity waveguide that provides evanescent coupling to a sample gas (or liquid) for spectroscopic chemical sensing. In the case of an ICL, the uppermost layer of this passive waveguide has a relatively high index of refraction that enables it to form the core of the waveguide, while the ambient air, consisting of the sample gas, functions as the top cladding layer. A fraction of the propagating light beam is absorbed by the sample gas if it contains a chemical species having a fingerprint absorption feature within the spectral linewidth of the laser emission.
1. A dual comb spectroscopy chemical sensor, comprising: an interband cascade laser (ICL) wafer having a first highly stable ICL frequency comb laser, a second highly stable ICL frequency comb laser, an extended passive sensing waveguide, a second passive waveguide, and an interband cascade detector (ICD) waveguide formed thereon,
a comb spacing of the first frequency comb laser being different from a comb spacing of the second frequency comb laser,
each of the frequency comb lasers having a corresponding laser cavity having a first highly reflective end and a second highly reflective end, each laser cavity comprising a corresponding gain section and saturable absorber section,
the gain section of each of the frequency comb lasers having a corresponding active narrow-ridge waveguide, and
a reflectivity of the first and second ends of each of the laser cavities preventing laser light emitted from each of the frequency comb lasers from being fed back into the corresponding laser cavity following interactions of the laser light with external optical elements;
the extended passive sensing waveguide receiving output from the first frequency comb laser, a top surface of the extended passive sensing wave guide being exposed to an ambient sample gas or liquid to provide evanescent coupling of a propagating optical mode from the first frequency comb laser to a sample gas or liquid;
the second passive waveguide receiving output from the second highly stable ICL frequency comb laser, the second passive waveguide having a length that is shorter than a length of the first passive waveguide; and
the interband cascade detector (ICD) waveguide receiving inputs from both the extended passive sensing waveguide and the second passive waveguide and having an active ICD waveguide similar to or the same as the active gain waveguide in the laser cavities of the two highly stable ICL frequency comb lasers, the active ICD wave guide further including a first top electrical contact configured to detect a photocurrent resulting from light propagating in the ICD waveguide; and
wherein a multi-heterodyne beating in the photocurrent flowing through the ICD detector waveguide provides information about at least one spectral characteristic of an absorption of laser light from the first and second frequency comb lasers by the sample gas or liquid.
2. The dual comb spectroscopy chemical sensor according to claim 1 , wherein each of the active narrow-ridge waveguides of the first and second highly stable ICL frequency comb lasers are formed from an ICL wafer material that includes:
an n + -GaSb substrate;
an n-InAs/AlSb superlattice bottom clad on an upper surface of the substrate;
a bottom GaSb separate confinement layer (SCL) on an upper surface of the bottom clad;
at least one active gain stage on an upper surface of the bottom GaSb SCL;
an n-InAs/AlSb superlattice top clad on an upper surface of the active gain stage; and
an n + -InAs or n + -InAsSb top contact layer on an upper surface of the top clad;
the active narrow-ridge waveguides of each of the first and second highly stable ICL frequency comb lasers further including sidewalls of the narrow ridges etched to a depth below the at least one active stage of the ICL wafer and further including a dielectric layer deposited on the sidewalls of the ridges and a metallization layer deposited on the upper surface of the ridges to provide a top electrical contact;
wherein the n + -InAs or n + -InAsSb top contact layers, the n-InAs/AlSb superlattice top clad layers, and the active gain stages of the ICL wafer material are etched away to form the extended passive sensing waveguide and the second passive waveguide, with the etch stopping near the top of the bottom GaSb SCL so as to leave an n + -GaSb substrate, an n-InAs/AlSb superlattice bottom clad, and some or all of the bottom GaSb separate confinement layer; and
wherein the extended passive sensing waveguide and second passive waveguide comprise ridges formed by etching the sidewalls of the extended passive sensing waveguide and second passive waveguide to a depth stopping near a top of the n-InAs/AlSb superlattice bottom clad or near a bottom of the bottom GaSb SCL.
3. The dual comb spectroscopy chemical sensor according to claim 1 , wherein output from a first ICL frequency comb laser waveguide is provided by coupling to a corresponding extended passive sensing waveguide running parallel to the first ICL frequency comb laser waveguide at a predetermined distance from the first ICL frequency comb laser waveguide over a predetermined length of the laser cavity;
wherein a predetermined fraction of light from the first ICL frequency comb laser waveguide is tunably passed to the extended passive sensing waveguide via evanescent coupling, the evanescent coupling to the extended passive sensing waveguide providing the only pathway for coupling light into or out of the laser cavity to an optical element outside the laser cavity; and
wherein an extent of the evanescent coupling between the first ICL frequency comb laser and the extended passive sensing waveguides and corresponding output from the laser cavity is tuned by tuning the distance between the first ICL frequency comb laser and extended passive sensing waveguides or by tuning the length of the laser cavity over which the first ICL frequency comb laser and extended passive sensing waveguides run in parallel; and
wherein output from the second ICL frequency comb laser waveguide is provided by coupling to the second passive waveguide running parallel to the second ICL frequency comb laser waveguide at a predetermined distance from the second ICL frequency comb laser waveguide over a predetermined length of the laser cavity;
wherein a predetermined fraction of light from the second ICL frequency comb laser waveguide is tunably passed to the second passive waveguide via evanescent coupling, the evanescent coupling to the second passive waveguide providing the only pathway for coupling light into or out of the laser cavity to an optical element outside the laser cavity; and
wherein an extent of the evanescent coupling between the second ICL frequency comb laser and the second passive waveguides and corresponding output from the laser cavity is tuned by tuning the distance between the second ICL frequency comb laser and second passive waveguides or by tuning the length of the laser cavity over which the second ICL frequency comb laser and second passive waveguides run in parallel.
4. The dual comb spectroscopy chemical sensor according to claim 1 , further comprising a distributed feedback (DFB) interband cascade laser whose output evanescently couples to an output of each of the first and second ICL frequency comb lasers:
wherein the DFB interband cascade laser locks a frequency of the first ICL frequency comb laser locked to a frequency of the second ICL frequency comb laser.