Raman spectroscopy system with balanced detection

A method for measuring a Raman signal, the method comprising:

• producing, by a pump light source, a pump beam of light at a pump frequency;
• producing, by a Stokes light source:
• a Stokes beam of light at a Stokes frequency, wherein the pump and Stokes frequencies are offset by a frequency offset Ω;
• and
• a Stokes reference beam of light;
• directing the pump and Stokes beams of light to a sample;
• collecting (i) a Raman signal produced by the sample in response to the pump and Stokes beams of light and (ii) residual light from the Stokes beam of light after the Stokes beam of light has interacted with the sample;
• detecting the Raman signal, comprising:
• producing, by a probe light source:
• a probe beam of light at a probe frequency;
• and
• a probe reference beam of light;
• producing, by a signal detector, a signal photocurrent corresponding to the Raman signal, the probe beam of light, and the residual Stokes beam of light, wherein a portion of the signal photocurrent corresponds to coherent mixing between the Raman signal and the probe beam of light;
• producing, by a reference detector, a reference photocurrent corresponding to the probe reference beam of light and the Stokes reference beam of light;
• and
• determining, by a subtraction module, a subtraction signal that equals a difference between a signal corresponding to the signal photocurrent and a signal corresponding to the reference photocurrent;
• and
• determining, by a processor, a characteristic of the subtraction signal.

One or more computer-readable non-transitory storage media embodying software that is operable when executed to:

• produce a pump beam of light at a pump frequency;
• produce (i) a Stokes beam of light at a Stokes frequency, wherein the pump and Stokes frequencies are offset by a frequency offset Ω and (ii) a Stokes reference beam of light;
• direct the pump and Stokes beams of light to a sample;
• collect (i) a Raman signal produced by the sample in response to the pump and Stokes beams of light and (ii) residual light from the Stokes beam of light after the Stokes beam of light has interacted with the sample;
• detect the Raman signal, comprising:
• produce (i) a probe beam of light at a probe frequency and (ii) a probe reference beam of light;
• produce a signal photocurrent corresponding to the Raman signal, the probe beam of light, and the residual Stokes beam of light, wherein a portion of the signal photocurrent corresponds to coherent mixing between the Raman signal and the probe beam of light;
• produce a reference photocurrent corresponding to the probe reference beam of light and the Stokes reference beam of light;
• and
• determine a subtraction signal that equals a difference between a signal corresponding to the signal photocurrent and a signal corresponding to the reference photocurrent;
• and
• determine a characteristic of the subtraction signal.

A system comprising:

• a pump light source configured to produce a pump beam of light at a pump frequency;
• a Stokes light source configured to produce:
• a Stokes beam of light at a Stokes frequency, wherein the pump and Stokes frequencies are offset by a frequency offset Ω;
• and
• a Stokes reference beam of light;
• one or more optical elements configured to:
• direct the pump and Stokes beams of light to a sample;
• and
• collect (i) a Raman signal produced by the sample in response to the pump and Stokes beams of light and (ii) residual light from the Stokes beam of light after the Stokes beam of light has interacted with the sample;
• an optical receiver configured to detect the Raman signal, the optical receiver comprising:
• a probe light source configured to produce:
• a probe beam of light at a probe frequency;
• and
• a probe reference beam of light;
• a signal-detection channel comprising:
• a signal detector configured produce a signal photocurrent corresponding to the Raman signal, the probe beam of light, and the residual Stokes beam of light;
• a signal-photocurrent amplifier configured to produce a signal-voltage output corresponding to the signal photocurrent;
• and
• a signal digitizer configured to produce a digital representation of the signal-voltage output;
• and
• a reference-detection channel comprising:
• a reference detector configured to produce a reference photocurrent corresponding to the probe reference beam of light and the Stokes reference beam of light;
• a reference-photocurrent amplifier configured to produce a reference-voltage output corresponding to the reference photocurrent;
• and
• a reference digitizer configured to produce a digital representation of the reference-voltage output;
• and
• a processor configured to:
• determine a subtraction signal corresponding to a difference between a signal corresponding to the digital representation of the signal-voltage output and a signal corresponding to the digital representation of the reference-voltage output;
• and
• determine a characteristic of the subtraction signal.