Downhole optical communication

A well system, comprising:

• at least one optical waveguide extending in a wellbore;
• a downhole optical wavelength conversion device excitable to convert light of a first wavelength from the optical waveguide into coherent light of a second wavelength;
• and
• a downhole optical modulator which phase modulates the coherent light of the second wavelength, wherein the coherent light has a relatively narrow line width that is less than 10 MHz.

A method of optical communication in a well, the method comprising:

• launching light at a first wavelength into an optical waveguide extending in a wellbore, the light launched into the optical waveguide having information modulated thereon using a first modulation technique;
• converting, with a downhole optical wavelength conversion device, the light at the first wavelength to light at a second wavelength that is different from the first wavelength;
• modulating the light at the second wavelength in the wellbore, using an optical modulator comprising a potassium titanyl phosphate crystal, the modulating comprising modulating information using a second modulation technique different from the first modulation technique;
• transmitting the light modulated in the wellbore via the same optical waveguide.