COMB-BASED SPECTROSCOPY WITH SYNCHRONOUS SAMPLING FOR REAL-TIME AVERAGING

• US 20110069309A1
• Filed: 09/16/2010
• Published: 03/24/2011
• Est. Priority Date: 09/18/2009
• Status: Active Grant

• Current Assignee: National Institute of Standards and Technology (Government of the United States of America)
• Original Assignee: National Institute of Standards and Technology (Government of the United States of America)
• Inventors: Swann, William C., Newbury, Nathan R., Coddington, Ian
• Granted Patent: US 8,564,785 B2
• Time in Patent Office: Days
• US Class Current: 356/326
• CPC Class Codes: G01J 3/453 by correlation of the amplitudes

Abstract

A method of comb-based spectroscopy with synchronous sampling for real-time averaging includes measuring the full complex response of a sample in a configuration analogous to a dispersive Fourier transform spectrometer, infrared time domain spectrometer, or a multiheterodyne laser spectrometer. An alternate configuration of a comb-based spectrometer for rapid, high resolution, high accuracy measurements of an arbitrary cw waveform.

Claims

1. A method of comb-based spectroscopy with synchronous sampling for real-time averaging comprising:

generating a source comb that transmits a source pulse train through a sample to provide a signal pulse train;

generating a local oscillator (LO) comb that transmits a LO pulse train at a different repetition rate than the source pulse train so that the difference in repetition rates between the combs is Δ

f_r;

establishing a base coherence between the source comb and the LO comb wherein each Nth pulse from the LO pulse train will have identical overlap with each Nth+1 pulse from the source pulse train in terms of both the pulse envelope and the carrier phase;

detecting the signal pulse train through linear optical sampling against the LO pulse train in which consecutive samples of an overlap between a signal pulse and a LO pulse from the LO pulse train yields a measurement of the signal pulse;

digitizing the overlap between the signal pulse train and the LO pulse train synchronously with the LO pulse train; and

real time summing of every Nth digitized sample to generate an averaged signal pulse.

2. A method as in claim 1, wherein the coherence between the LO comb and source comb is established by phase locking a first tooth of the source comb and LO comb to a first CW reference laser and a second tooth of the source comb and the LO comb to a second CW reference laser with the number of intervening teeth for the source comb being N1 and the number of intervening teeth for the LO comb being N2 such that N2/(N1−

N2) is an integer N.

3. A method as recited in claim 1, further comprising:

optically filtering the source and LO comb outputs so that Δ

f_r can be high enough to avoid 1/f noise without violating the Nyquist sampling constraints;

successively shifting the center filter position across the full LO and source comb spectra to measure the signal pulse in different spectral regions;

coherently reconstructing the signal pulse by concatenating the measurement at different filter positions in the frequency domain;

shifting one of the source comb and the local oscillator (LO) comb at each filter position by a multiple of Δ

f_r such that the carrier frequency of the averaged signal pulse is moved away from base band or Nyquist.

4. A method as recited in claim 1, wherein:

a portion of the source pulse train that does not pass through the sample provides a reference pulse train;

delaying the reference pulse train relative the signal pulse train by a percentage of a period of the signal pulse train such that the reference pulse train and the signal pulse train do not overlap to provide a resultant interleaved signal pulse train and reference pulse train; and

detecting the resultant interleaved signal pulse train and reference pulse train through linear optical sampling against the LO pulse train as in claim 1 to generate a measurement of the interleaved signal and reference pulses.

5. A method as recited in claim 4, wherein:

separating the resultant measurement of the interleaved signal pulse and reference pulse to normalize the signal pulse by the reference pulse yielding the time-domain response of the sample.

6. A method as recited in claim 5, wherein the Fourier transform of the normalized signal pulse yields the complex frequency domain response of the sample.

7. A method as recited in claim 1, wherein:

a portion of the source pulse train that does not pass through the sample provides a reference pulse train; and

alternately detecting the signal pulse train and the reference pulse train through selectively blocking each of the signal pulse train and the reference pulse train to normalize the signal pulse by the reference pulse yielding the time-domain response of the sample.

11. A method as recited in claim 8, wherein the phase coherence between the source and LO combs is established as recited in claim 2.

8. A method of comb-based spectroscopy for measuring a CW source at time-bandwidth limited resolution by using frequency combs with a high degree of mutual coherence (<

1 radian phase noise) comprising;

generating a first comb that transmits a first pulse train that is optically combined with a CW source;

detecting the overlap of each pulse of the first comb with the CW source in a first photodetector;

digitizing a photodetector response from the first photodetector for each pulse of the first pulse train;

generating a second comb that is optically coherent with the first comb, said second comb transmits a second pulse train that has a pulse period that differs by Δ

T and a pulse repetition frequency that differs by Δ

f_r from the first pulse train, and is separately combined with the CW source;

detecting the overlap of each pulse of the second comb with the CW source in a second photodetector;

digitizing a photodetector response from the second photodetector for each pulse of the second pulse train;

multiplying the digitized samples from the first comb times the digitized samples of the second comb over a time (1/Δ

f_r) to generate a data point record of length (f_r/Δ

f_r) where f_r is the repetition rate;

rescaling the point separation for the data record therebetween to be Δ

T; and

Fourier transforming the product of the digitized samples to yield a wideband spectrum of the CW source at a resolution given by the comb repetition rate and an ambiguity given by 1/(2Δ

T) to provide an absolute frequency of the CW source with respect to a CW reference laser to which the respective first comb and the second are locked.

9. A method as recited in claim 8, further comprising:

generating a third comb by splitting comb 2 into two parts and frequency-shifting one part to create a third comb with the same repetitition rate but an offset frequency shift equal to a fraction, X, of the repetition rate;

detecting the overlap of each pulse of this third frequency-shifted comb with the CW source in a third photodetector;

processing the digitized samples from the third comb and the digitized samples from the first comb as described in claim 8 to yield a wideband spectrum of the CW source at a resolution given by the comb repetition rate and but an offset frequency that differs by X/Δ

T from the wideband spectrum generated from the product of the first and second combs in claim 8.

Combining the two wideband spectrum to provide an absolute frequency of the CW source with respect to a CW reference laser to which the respective first comb and the second are locked to an overall ambiguity 1/(Δ

T) and without ambiguities related to the Nyquist frequency 1/(2Δ

T)

10. A method as recited in claim 8, further comprising:

individually Fourier transforming the digitized samples from either of the first comb or the second comb or the third comb to generate a spectrum of the CW source with time-bandwidth limited resolution.