A laser-based sensor system-on-a-chip for real-time monitoring of a blood constituent concentration level in a subject by means of tunable laser absorption spectroscopy, the system-on-a-chip comprising: a tunable hybrid III-V/IV laser, configured to perform a laser wavelength sweep across a spectral range corresponding to an absorption band of a target molecule; a fiber-optic interface coupled to the laser, to couple the emission of the tunable III-V/IV laser to a fiber probe, the probe being configured to be placed in optical communication with a subject in such a manner that blood of the subject is illuminated by the wavelength swept laser light; a discrete III-V photodiode coupled to the fiber-optic interface, configured to detect light of the wavelength swept laser emission that is reflected from the subject; and sensor control electronics including a microcontroller configured to convert the electrical signal from the photodiode that corresponds to the detected reflected light into a calibrated blood constituent level, wherein the system is so configured that during use (i) the laser is remote from the subject and the probe is in optical communication with the subject and (ii) the discrete III-V photodiode is disposed remotely from the tunable laser. constituent concentration level in a subject by means of tunable laser absorption spectroscopy, the system-on-a-chip comprising: a tunable hybrid III-V/IV laser, configured to perform a laser wavelength sweep across a spectral range corresponding to an absorption band of a target molecule; a fiber-optic interface coupled to the laser, to couple the emission of the tunable III-V/IV laser to a fiber probe, the probe being configured to be placed in optical communication with a subject in such a manner that blood of the subject is illuminated by the wavelength swept laser light; a discrete III-V photodiode coupled to the fiber-optic interface, configured to detect light of the wavelength swept laser emission that is reflected from the subject; and sensor control electronics including a microcontroller configured to convert the electrical signal from the photodiode that corresponds to the detected reflected light into a calibrated blood constituent level, a tunable hybrid III-V/IV laser, configured to perform a laser wavelength sweep across a spectral range corresponding to an absorption band of a target molecule; a fiber-optic interface coupled to the laser, to couple the emission of the tunable III-V/IV laser to a fiber probe, the probe being configured to be placed in optical communication with a subject in such a manner that blood of the subject is illuminated by the wavelength swept laser light; a discrete III-V photodiode coupled to the fiber-optic interface, configured to detect light of the wavelength swept laser emission that is reflected from the subject; and sensor control electronics including a microcontroller configured to convert the electrical signal from the photodiode that corresponds to the detected reflected light into a calibrated blood constituent level, wherein the system is so configured that during use (i) the laser is remote from the subject and the probe is in optical communication with the subject and (ii) the discrete III-V photodiode is disposed remotely from the tunable laser.