Production of optical pulses at a desired wavelength utilizing higher-order-mode (HOM) fiber

Abstract

An apparatus and method for producing optical pulses of a desired wavelength utilizes a section of higher-order-mode (HOM) fiber to receive input optical pulses at a first wavelength, and thereafter produce output optical pulses at the desired wavelength through soliton self-frequency shifting (SSFS) or Cherenkov radiation. The HOM fiber is configured to exhibit a large positive dispersion and effective area at wavelengths less than 1300 nm.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Applications 60 / 863,082, filed Oct. 26, 2006, and 60 / 896,357, filed Mar. 22, 2007, both provisional applications herein incorporated by reference.FIELD OF THE INVENTION[0002]The present invention relates to the production of optical pulses at a desired wavelength using higher-order-mode fibers and, more particular, to the utilizing of HOM fiber with a positive dispersion and large effective area sufficient to generate high energy, short pulses at wavelengths below 1300 nm, considered useful for numerous applications.BACKGROUND OF THE INVENTION[0003]Mode-locked femtosecond fiber lasers at 1.03 μm and 1.55 μm have been improving significantly in the last several years, particularly with respect to the achievable output pulse energy (increasing from 1 to ˜10 nJ). Even higher pulse energy can be achieved in femtosecond fiber sources based on fiber chirped pulse amplification. However, fe...

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Benefits of technology

[0018]It is an advantage of the present invention that the HOM module is capable of achieving these characteristics at wavelengths below 1300 nm, heretofore not accomplished in an all-silica (non-holey) fiber.

[0019]Further, the HOM module of the present invention is designed such that the difference between the effective index neff of the mode in which signal propagation is desired is separated from that of any other guided mode of the fiber by greater than 10−5, thus providing for enhanced modal stability of the signal.

[0020]In one embodiment, the input comprises a single mode fiber (SMF) spliced to the HOM fiber before mode conversion, with the properties of the splice ensuring that signal propagation in the HOM fiber occurs predominantly in the LP01 mode, further enhancing modal stability for the signal.

Problems solved by technology

However, femtosecond fiber sources, including lasers, have seen only limited applications in multiphoton imaging.

The main reason is that they offer very limited wavelength tunability (tens of nanometer at best), severely restricting the applicability of these lasers, making them only suitable for some special purposes.

In addition, existing femtosecond fiber sources at high pulse energy (>1 nJ) are not truly “all fiber,” i.e., the output is not delivered through a single mode optical fiber.

The problem of tunability remains an issue for these arrangements capable of creating Cherenkov radiation.

Unfortunately, the pulse energy required to support stable Raman-shifted solitons below 1300 nm in index-guided PCFs and air-core PBGFs is either on the very low side, a fraction of a nJ for silica-core PCFs, or on the very high side, greater than 100 nJ (requiring an input from an amplified optical system) for air-core PBGFs.

The low-energy limit is due to high nonlinearity in the PCF.

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