Techniques for measuring difference of an optical property at two wavelengths by modulating two sources to have opposite-phase components at a common frequency

Apparatus for measuring a difference between the value of an optical property of a sample at a first wavelength and the value of the optical property of the sample at a second wavelength, and a difference between the value of the optical property of the sample at the second wavelength and the value of the optical property of the sample at a third wavelength, the apparatus comprising:

• first, second, and third optical sources emitting light at the first, second, and third wavelengths;
• modulation means for modulating the optical power of each of said sources so that (a) the optical powers of said first and second sources have respective first and second components that vary periodically with a common frequency, wherein the periodic variation of said first component is of opposite sign to the periodic variation of said second component, and (b) the optical powers of said second and third sources have respective third and fourth components that vary periodically with an additional common frequency, wherein the periodic variation of said third component is of opposite sign to the periodic variation of said fourth component;
• means for combining light from said first, second, and third sources and directing said light to engage the sample;
• detection means, responsive to said light, so combined, after engagement with the sample, for providing an electrical signal proportional to the total optical power of said light after engagement with the sample;
• control means, responsive to a signal at a control input, for changing the amplitude of power variation of said first component relative to said second component;
• additional control means, responsive to a signal at an additional control input, for changing the amplitude of power variation of said third component relative to said second component;
• means, responsive to said electrical signal, for generating a control signal proportional to the amplitude of a component of said electrical signal at said common frequency and for communicating said control signal to said control input;
• and
• means, responsive to said electrical signal, for generating an additional control signal proportional to the amplitude of a component of said electrical signal at said additional common frequency and for communicating said additional control signal to said additional control input;
• said control means responding to said control signal so as to minimize the amplitude of said component of said electrical signal at said common frequency, whereupon said control signal is proportional to the difference between the value of the optical property of the sample at the first wavelength and the value of the optical property of the sample at the second wavelengths;
• said additional control means responding to said additional control signal so as to minimize the amplitude of said component of said electrical signal at said additional common frequency, whereupon said additional control signal is proportional to the difference between the value of the optical property of the sample at the second wavelength and the value of the optical property of the sample at the third wavelength.