What is claimed is: 1. A cavity ringdown system capable of providing optical heterodyne detection of a ringdown signal including: an optical-signal generator capable of generating an optical beam comprising a plurality of modes, the modes being chopped out of phase with respect to a chopping cycle such that no more than one of the chopped modes is dominant in the output, an optical resonator optically coupled to the signal generator capable of receiving the optical beam, the resonator being capable of containing an intra-cavity absorber onto which at least one of the modes may be tuned, and a heterodyne detector coupled to the optical resonator, the heterodyne detector capable of receiving a plurality of the modes and demodulating a heterodyne-beat waveform between the modes to produce a heterodyne-beat amplitude, the heterodyne detector generating a difference signal of the heterodyne-beat amplitude between adjacent chopping cycles, which contains the information of intra-cavity absorption. 2. The cavity ringdown system recited in claim 1 wherein the optical-signal generator includes an intensity-stabilized continuous-wave laser. 3. The cavity ringdown system recited in claim 1 wherein the optical-signal generator includes a tunable cw laser. 4. The cavity ringdown system recited in claim 1 wherein the optical-signal generator includes an electro-optic modulator capable of providing RF sidebands to the optical beam. 5. The cavity ringdown system recited in claim 1 wherein the optical-signal generator includes a plurality of acousto-optic modulators capable of splitting the beam into a plurality of modes having a relative frequency offset. 6. The cavity ringdown system recited in claim 1 wherein the optical resonator is characterized by a free-spectral range and the optical-signal generator is adapted to generate modes having a frequency offset that is an integer multiple of the free-spectral range. 7. The cavity ringdown system recited in claim 1 wherein the optical-signal generator includes a chopping system capable of chopping the modes out of phase. 8. The cavity ringdown system recited in claim 7 wherein the chopping system includes a plurality of acousto-optic modulators. 9. The cavity ringdown system recited in claim 1 further comprising: a pre-fixed cavity resonance photodetector, a polarized beam splitter and a quarter-wave plate capable of steering the entire reflected optical beam from the optical resonator onto the pre-fixed photodetector, said photodetector optically coupled to the beam splitter, the photodetector capable of receiving the reflected optical beam and converting the received beam to an electrical signal, and a laser-cavity locking loop electrically coupled to the photophotodetector, the laser-cavity locking loop capable of receiving the electrical signal and generating a feedback signal to maintain a lock between the optical-signal generator and the optical resonator. 10. The cavity ringdown system recited in claim 9 wherein the lock between the optical-signal generator and the optical resonator is maintained using the Pound-Drever-Hall technique. 11. The cavity ringdown system recited in claim 9 wherein the optical-signal generator includes an electro-optic modulator capable of providing RF sidebands to the optical beam. 12. The cavity ringdown system recited in claim 9 wherein the optical-signal generator generates an off-resonance continuous mode that is processed by the laser-cavity locking loop to maintain a lock between the optical-signal generator and the optical resonator. 13. The cavity ringdown system recited in claim 12 wherein the heterodyne photodetector is adapted to filter out contributions of the continuous mode to the heterodyne-beat waveform. 14. The cavity ringdown system recited in claim 1 wherein the optical resonator is characterized by a decay time relative to an empty cavity, and the optical-signal generator chops the modes with respect to a chopping cycle suitable for the decay time. 15. The cavity ringdown system recited in claim 1 wherein the optical resonator is a high-finesse cavity. 16. The cavity ringdown system recited in claim 1 wherein the optical resonator is adapted to contain an intra-cavity sample gas. 17. The cavity ringdown system recited in claim 1 wherein the cavity reflection photodetector includes a p-i-n diode in a resonant matching circuit. 18. The cavity ringdown system recited in claim 1 wherein the heterodyne photodetector includes a suitable detector, such as an avalanche photodiode, resonant matched p-i-n photodiode or photomultiplier. 19. The cavity ringdown system recited in claim 1 wherein the heterodyne photodetection system includes an RF demodulation component. 20. The cavity ringdown system recited in claim 1 wherein the optical signal generator includes an adjustable power system capable of being responsive to increased intra-cavity absorption in the optical resonator for increasing power of the optical beam. 21. The cavity ringdown system recited in claim 1 wherein the optical signal generator includes an adjustable chopping system capable of being responsive to increased intra-cavity absorption in the optical resonator by decreasing duration of the chopping cycle. 22. A cavity ringdown system capable of providing optical heterodyne detection of a ringdown signal including: an optical source capable of generating an optical beam with a plurality of modes having a predetermined relative frequency offset between the modes, a high-finesse optical cavity capable of receiving the modes, a chopping system for chopping the modes out of phase such that only one dominant mode is coupled into the cavity at any time, and a heterodyne detector optically coupled to the cavity capable of detecting an optical signal coupled out of the cavity, the optical signal including the modes, which modes overlap to produce a heterodyne-beat waveform, the heterodyne detector including: a demodulation unit capable of demodulating the waveform to produce a heterodyne-beat amplitude, and a difference-signal analyzer coupled to the demodulator, the difference-signal analyzer capable of generating a difference signal from adjacent chopping cycles of the heterodyne-beat amplitude. 23. The cavity ringdown system recited in claim 22 wherein the optical source includes an intensity-stabilized continuous-wave laser. 24. The cavity ringdown system recited in claim 22 wherein the optical source includes a tunable cw laser. 25. The cavity ringdown system recited in claim 22 wherein the optical source includes an electro-optic modulator capable of providing RF sidebands to the optical beam. 26. The cavity ringdown system recited in claim 22 wherein the optical source includes a plurality of acousto-optic modulators capable of splitting the beam into a plurality of modes having a relative frequency offset. 27. The cavity ringdown system recited in claim 22 wherein the optical cavity is characterized by a free-spectral range and the optical source is adapted to generate modes having frequency offsets that are related to the free-spectral range. 28. The cavity ringdown system recited in claim 22 wherein the chopping system includes a plurality of acousto-optic modulators. 29. The cavity ringdown system recited in claim 22 further comprising: a cavity resonance photodetector, a polarized beam splitter and a quarter-wave plate capable of steering the entire reflected optical beam from the optical resonator onto the cavity resonance photodetector, said photodetector optically coupled to the beam splitter, the photodetector capable of receiving the reflected optical beam and converting the received beam to an electrical signal, and a laser-cavity locking loop electrically coupled to the photodetector, the laser-cavity locking loop capable of receiving the electrical signal and generating a feedback signal to maintain a lock between the optical-signal generator and the optical resonator. 30. The cavity ringdown system recited in claim 29 wherein the lock between the optical source and the optical cavity is maintained using the Pound-Drever-Hall technique. 31. The cavity ringdown system recited in claim 29 wherein the optical source includes an electro-optic modulator capable of providing RF sidebands to the optical beam. 32. The cavity ringdown system recited in claim 29 wherein the optical source generates an additional off-resonance continuous mode that is processed by the laser-cavity locking loop to maintain a lock between the optical source and the optical cavity. 33. The cavity ringdown system recited in claim 32 wherein the heterodyne detector is adapted to filter out contributions of the continuous mode to the heterodyne-beat waveform. 34. The cavity ringdown system recited in claim 22 wherein the chopping system chops the modes with respect to a chopping cycle that is suitable for the decay time characterizing the optical cavity when empty. 35. The cavity ringdown system recited in claim 22 wherein the optical cavity is adapted to contain a sample gas. 36. The cavity ringdown system recited in claim 22 wherein the cavity reflection photodetector includes a p-i-n diode in a resonant matching circuit. 37. The cavity ringdown system recited in claim 22 wherein the heterodyne photodetector includes a suitable detector, such as an avalanche photodiode, resonant matched p-i-n photodiode or photomultiplier. 38. The cavity ringdown system recited in claim 22 wherein the heterodyne photodetection system includes an RF demodulation component. 39. The cavity ringdown system recited in claim 22 wherein the optical source includes an adjustable power system capable of being responsive to increased intra-cavity absorption in the optical cavity for increasing power of the optical beam. 40. The cavity ringdown system recited in claim 22 wherein the chopping system is an adjustable chopping system capable of being responsive to increased intra-cavity absorption in the optical cavity by decreasing the duration of the chopping cycle. 41. A method of cavity ringdown spectroscopy including: providing for generation of an optical beam, providing for generation of a plurality of modes from the optical beam, the modes having a predetermined relative frequency offset, providing for out-of-phase chopping of the modes with respect to a chopping cycle such that the optical beam consists of only one dominant mode when incident upon an optical resonant cavity, providing for resonating of the modes in the cavity, and providing for heterodyne detection of an optical signal coupled out of the cavity, the optical signal including the modes, which modes overlap to produce a heterodyne-beat waveform, the heterodyne detection including: providing for demodulation of the waveform to produce a heterodyne-beat amplitude, and providing for generation of a difference signal from adjacent chopping cycles of the heterodyne-beat amplitude. 42. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of an optical beam includes generating an intensity-stabilized continuous-wave laser beam. 43. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of an optical beam includes generation of an optical beam by a cw tunable laser. 44. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of an optical beam includes electro-optic modulation of the beam for providing RF sidebands to the optical beam. 45. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of a plurality of modes includes acousto-optic modulation of the beam into a plurality of modes having a relative frequency offset. 46. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of a plurality of modes includes providing a frequency offset to the modes that is related to the free-spectral range that characterizes the cavity. 47. The cavity ringdown spectroscopy method recited in claim 41 further comprising: providing for coupling the reflected optical beam from the cavity to a transducer for providing an electrical signal in response to the reflected beam, and providing for locking between the optical-beam generation and the mode resonance. 48. The cavity ringdown spectroscopy method recited in claim 47 wherein providing for locking is performed using the Pound-Drever-Hall technique. 49. The cavity ringdown spectroscopy method recited in claim 47 wherein providing for generation of a plurality of modes includes generating an off-resonance continuous mode that is processed by the locking step. 50. The cavity ringdown spectroscopy method recited in claim 49 wherein the providing for heterodyne detection is adapted to filter out contributions of the continuous mode to the heterodyne-beat waveform. 51. The cavity ringdown spectroscopy method recited in claim 41 wherein the chopping cycle providing for out-of-phase chopping of the modes is suitable for the cavity decay time. 52. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for resonating of the modes includes coupling the beam into a high-finesse cavity. 53. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for resonating of the modes includes providing an intra-cavity sample gas having an absorption line to which at least one of the modes is tuned. 54. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for heterodyne detection includes RF demodulation. 55. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for generation of the beam includes increasing power to the beam in response to increased intra-cavity absorption in the cavity. 56. The cavity ringdown spectroscopy method recited in claim 41 wherein providing for out-of-phase chopping of the modes includes decreasing duration of the chopping cycle in response to increased intra-cavity absorption in the cavity.