Methods and apparatus for direct multiplication Fourier transform millimeter wave spectroscopy

1. A method, comprising:

generating at least one pulse of microwave electromagnetic energy;

frequency-multiplying the at least one pulse to generate at least one frequency-multiplied pulse without up-converting the at least one filtered pulse;

filtering at least one spurious harmonic of the at least one frequency-multiplied pulse to generate at least one filtered pulse, the at least one spurious harmonic generated by frequency-multiplying the at least one pulse;

exciting a sample using the at least one filtered pulse; and

detecting an emission from the sample, the emission elicited at least in part by the at least one filtered pulse.

2. The method of claim 1 , wherein generating the at least one pulse includes generating at least one chirped pulse within a frequency range from about 1 GHz to about 3 GHz.

3. The method of claim 1 , wherein the at least one frequency-multiplied pulse has a frequency in a range from about 75 GHz to about 110 GHz.

4. The method of claim 1 , wherein the filtering includes filtering the at least one spurious harmonic using a cavity bandpass filter.

5. The method of claim 1 , wherein the generating includes generating the at least one pulse in response to a trigger signal.

6. The method of claim 5 , wherein the detecting includes detecting the emission in response to the trigger signal.

7. The method of claim 1 , wherein the at least one pulse comprises a first pulse, further comprising:

generating a second pulse of microwave electromagnetic energy, the second pulse synchronized with the first pulse; and

frequency-multiplying the second pulse generate a second frequency-multiplied pulse,

the detecting including mixing the second frequency-multiplied pulse and the emission to generate a downconverted emission signal within the bandwidth of an analog-to-digital converter (ADC).

8. The method of claim 1 , wherein the detecting includes heterodyne detection.

9. An apparatus, comprising:

a signal generator configured to generate at least one pulse of microwave electromagnetic energy;

a frequency multiplier circuit operably coupled to the signal generator, the frequency multiplier circuit configured to frequency-multiply the at least one pulse to generate at least one frequency-multiplied pulse without up-converting the at least one pulse;

a filter circuit operably coupled to the frequency multiplier circuit, the filter circuit configured to filter at least one spurious harmonic of the at least one frequency-multiplied pulse to generate at least one filtered pulse, the at least one spurious harmonic generated by frequency-multiplying the at least one pulse, the filter circuit further configured to excite a sample using the at least one filtered pulse; and

an emission detector configured to detect an emission from the sample, the emission elicited at least in part by the at least one filtered pulse.

10. The apparatus of claim 9 , wherein the signal generator is further configured to generate the at least one pulse as at least one chirped pulse within a frequency range from about 1 GHz to about 3 GHz.

11. The apparatus of claim 9 , wherein the at least one frequency-multiplied pulse has a frequency in a range from about 75 GHz to about 110 GHz.

12. The apparatus of claim 9 , the filter circuit including a cavity bandpass filter.

13. The apparatus of claim 9 , wherein the signal generator is further configured to generate the at least one pulse in response to a trigger signal.

14. The apparatus of claim 13 , wherein the emission detector is further configured to detect the emission in response to the trigger signal.

15. The apparatus of claim 9 , wherein the at least one pulse comprises a first pulse,

wherein the signal generator is further configured to generate a second pulse of microwave electromagnetic energy, the second pulse synchronized with the first pulse,

wherein the frequency multiplier circuit is further configured to frequency-multiply the second pulse to generate a second frequency-multiplied pulse, and

wherein the emission detector is further configured to mix the second frequency-multiplied pulse and the emission to generate a downconverted emission signal within the bandwidth of an analog-to-digital converter (ADC).

16. The apparatus of claim 9 , wherein the emission detector is further configured to detect via heterodyne detection.

17. A method, comprising:

generating, with a free-running signal generator, at least one pulse of microwave electromagnetic energy;

frequency-multiplying the at least one pulse to generate at least one frequency-multiplied pulse;

exciting a sample using the at least one frequency-multiplied pulse; and

detecting an emission from the sample, the emission elicited at least in part by the at least one frequency-multiplied pulse.

18. The method of claim 17 , further comprising filtering at least one spurious harmonic of the at least one frequency-multiplied pulse to generate at least one filtered pulse, the at least one spurious harmonic generated by frequency-multiplying the at least one pulse,

wherein the exciting includes exciting the sample using the at least one filtered pulse.

19. The method of claim 17 , wherein the generating includes generating the at least one pulse in response to a trigger signal from a trigger signal source external to the signal generator.

20. The method of claim 19 , wherein the detecting includes detecting the emission in response to the trigger signal.