What is claimed is: 1. A method of cross-correlating radiation pulses, comprising the steps of: focusing a first radiation pulse on a photodiode, the first radiation pulse having a first intensity and a first wavelength; focusing a second radiation pulse on the photodiode where the first and second radiation pulses illuminate a common point on the photodiode, the second radiation pulse having a second intensity and a second wavelength, wherein the first wavelength and second wavelength are different; converting the first and second radiation pulses into a photocurrent using a photodiode, wherein a product of the first and second intensities is proportional to the photocurrent; and detecting an amplitude of the photocurrent while delaying at least one of the first and second radiation pulses. 2. The method of cross-correlating radiation pulses of claim 1, wherein the first and second radiation pulses simultaneously illuminate the common point. 3. The method of cross-correlating radiation pulses of claim 1, wherein the first wavelength and second wavelength differ by at least five parts in eighty. 4. The method of cross-correlating radiation pulses of claim 1, further comprising the step of delaying at least one of the first and second radiation pulses so that the first radiation pulse is coincident in time with the second radiation pulse at the common point of the photodiode. 5. The method of cross-correlating radiation pulses of claim 1, further comprising the step of determining the product of the first intensity and the second intensity from the amplitude of the photocurrent. 6. The method of cross-correlating radiation pulses of claim 1, further including the steps: modulating the first radiation pulse with a first frequency; modulating the second radiation pulse with a second frequency; and demodulating at least one of: a sum of the first frequency and the second frequency, and an absolute value of a difference between the first frequency and the second frequency. 7. The method of cross-correlating radiation pulses of claim 1, further comprising the step of chopping at least one of the first radiation and second radiation pulses to allow for more accurate determination of an amplitude of the photocurrent. 8. A cross-correlation method including the steps of: providing a first radiation pulse and a second radiation pulse, wherein the first and second radiation pulses have different wavelengths; processing the first and second radiation pulses in a non-linear way such that a two-photon process occurs; and generating a signal representative of a first radiation pulse intensity and a second radiation pulse intensity, wherein the processing and generating steps are performed by the same photodiode. 9. The cross-correlation method of claim 8, further comprising the step of delaying at least one of the first and second radiation pulses so that the first radiation pulse is coincident in time with the second radiation pulse at a common point on the photodiode. 10. The cross-correlation method of claim 8, further including the steps: modulating the first radiation pulse with a first frequency; modulating the second radiation pulse with a second frequency; and demodulating at least one of: a sum of the first frequency and the second frequency, and an absolute value of a difference between the first frequency and the second frequency. 11. A multi-wavelength cross-correlation system for radiation pulses, comprising: a first radiation pulse; a second radiation pulse, wherein the first and second radiation pulses have different wavelengths; a radiation detection means having a surface, the radiation detection means converting energy from the first radiation pulse and the second radiation pulse into a current, the first radiation pulse contacting a first area on the surface of the radiation detection means and the second radiation pulse contacting a second area on the surface of the radiation detection means wherein the first and second areas overlap on the surface; and wherein a product of intensities of the first and second radiation pulses is proportional to said current; and a current sensing means which determines an amplitude of the current. 12. The multi-wavelength cross-correlation system of claim 11, wherein the overlap of the first and second areas is located on a p-type layer of the photodiode. 13. The multi-wavelength cross-correlation system of claim 11, wherein: the first and second radiation pulses have first and second photon energies, respectively; the photodiode has a band gap energy; and each of the first and second photon energies is less than the band gap energy. 14. The multi-wavelength cross-correlation system of claim 11, wherein the multi-wavelength cross-correlation system is free of a non-linear crystals. 15. The multi-wavelength cross-correlation system of claim 11, further comprising an interferometer, in communication with at least one of the first and second radiation pulses. 16. The multi-wavelength cross-correlation system of claim 11, wherein the photodiode operates as both a non-linear element and detector. 17. The multi-wavelength cross-correlation system of claim 11, wherein a power of the first and second radiation sources varies by at least two orders of magnitude. 18. The multi-wavelength cross-correlation system of claim 11, further comprising an optical delay means operatively engaging at least one of the first radiation and second radiation pulses for altering the length of an optical path of at least one of the first and second radiation pulses for adjusting a temporal alignment of the first radiation pulse with the second radiation pulse. 19. The multi-wavelength cross-correlation system of claim 11, further comprising a radiation chopping means operatively engaging at least one of the first radiation and second radiation pulses, the radiation chopping means being coupled to the current sensing means. 20. The multi-wavelength cross-correlation system of claim 11, wherein the radiation detection means is comprised of a plurality of photodiodes, wherein each photodiode has different band gap characteristics.