Optical frequency up-conversion of femtosecond pulses into targeted single bands in the visible and ultraviolet
View Patent ↗An apparatus and methods for generating a substantially supercontinuum-free widely-tunable multimilliwatt source of radiation characterized by a narrowband line profile. The apparatus and methods employ nonlinear optical mechanisms in a nonlinear photonic crystal fiber (PCF) by detuning the wavelength of a pump laser to a significant extent relative to the zero-dispersion wavelength (ZDW) of the PCF. Optical phenomena employed for the selective up-conversion in the PCF include, but are not limited to, four-wave mixing and Cherenkov radiation. Tunability is achieved by varying pump wavelength and power and by substituting different types of PCFs characterized by specified dispersion properties.
1. A method for generating broadly tunable visible or ultraviolet optical pulses, substantially free of non-frequency-specific supercontinuum emission, the method comprising:
a. providing a pulsed source for generating pump pulses characterized by a spectrum having a central emission wavelength λ 0 , a bandwidth, and an average power;
b. providing a photonic crystal fiber characterized by a short-wavelength zero dispersion wavelength; and
c. pumping the fiber with the pump pulses at a pump wavelength, thereby exciting a nonlinear process in such a manner as to produce an isolated spectral band in the visible or ultraviolet characterized by a central wavelength that is tuned over a range of greater than 20 nm, wherein the isolated band in the visible or ultraviolet is further characterized by a 10 dB bandwidth of less than 50 nm, and a power spectral density exceeding that of any continuum emission adjacent to it by at least 10 dB,
wherein the average power in the isolated spectral band exceeds 0.1 mW.
2. A method in accordance with claim 1 , wherein the central wavelength of the isolated spectral band is fine tuned by varying the average power characterizing the pump pulses.
3. A method in accordance with claim 1 , wherein the zero dispersion bandwidth is displaced by more than 100 nm from λ 0 .
4. A method in accordance with claim 1 , wherein the nonlinear process is four-wave mixing.
5. A method in accordance with claim 1 , wherein the nonlinear process is dispersive wave generation.
6. A method in accordance with claim 1 , wherein the pulsed source is a pulsed laser.
7. A method in accordance with claim 1 , wherein the pump pulses are substantially transform-limited in bandwidth.
8. A method in accordance with claim 1 , further comprising tuning the central wavelength of the isolated spectral band in the visible or ultraviolet by at least one of varying the wavelength of the pump source and switching photonic crystal fibers with different dispersion properties.
9. A method in accordance with claim 1 , further comprising generating a self-referenced optical frequency comb based at least on the frequency up-converted isolated spectral band.