HARMONIC GENERATION: Frequency-doubled poled silica fiber is widely tunable

Silica fibers have been used for frequency doubling of infrared light, with the added benefit of tunability.

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Silica fibers have been used for frequency doubling of infrared light, with the added benefit of tunability. Now, a tuning range of nearly 45 nm at the fundamental wavelength has been reported by the Optoelectronics Research Centre (ORC) at the University of Southampton, England. The team claims that the resulting tuning range of the second-harmonic (SH) light is the broadest reported so far for a uniform periodically poled device.

Normally, the inherent macroscopic inversion symmetry of amorphous silica prohibits second-order nonlinear processes such as frequency doubling. However, through a process called thermal poling, a permanent second-order nonlinearity (SON) can be introduced into the silica-glass matrix. Thermal poling involves heating the sample to an optimum temperature while simultaneously applying a high direct-current electric field. The high temperature mobilizes ions while the electric field relocates them, allowing a region depleted of like charges to form near the anode. The sample is cooled to room temperature with the electric field still applied, enabling the change in charge distribution to be frozen in. This frozen-in field combines with the inherent third-order nonlinearity of the glass to produce an effective SON.

Ultraviolet erasure

The induced SON is significantly lower than that of bulk crystalline materials, but the benefits offered by the extended interaction lengths and ease of integration make a silica-fiber-based frequency-doubling system attractive. The phase-velocity mismatch between the fundamental and SH waves in the fiber can be compensated through quasi phase matching (QPM). This can be achieved by ultraviolet periodic erasure of the induced SON in uniformly poled fibers, producing periodically poled silica fibers (PPSFs).

The ORC group has increased the usefulness of PPSFs by adding broad wavelength tuning of the generated SH light through mechanical compression of the periodic QPM structure (mechanical compression also has use in tuning fiber Bragg gratings). In this new work, a specially designed silica fiber with a numerical aperture of 0.20 was used as the basis of the PPSF. Once thermally poled, the SON was periodically erased using a frequency-doubled argon-ion laser at a QPM period matched to a fundamental wavelength of around 1594.5 nm. A 4 cm length of the PPSF was embedded in a compression stage capable of delivering an adjustable strain uniformly along the length of the device.

The device was characterized using the broadband supercontinuum generated by a fiber laser. The high-power pulsed laser consists of a seed laser followed by a series of cascaded fiber amplifiers in a master-oscillator power-amplifier configuration, and delivers 10 ns pulses at a repetition rate of 450 kHz. The source produces high power in the erbium gain range and a lower-power supercontinuum in the 1400 to 1600 nm range (see figure). An inherent birefringence in the fiber results in two slightly different phase-matching conditions for orthogonal polarizations, giving rise to two SH peaks of slightly different center wavelengths.

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Different compressive tuning adjustments produce different SH output spectra (top). The idle point is the SH spectrum measured with no strain applied to the QPM grating. The wavelength range at the input of the PPSF is broad (bottom). (Courtesy of the ORC)
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“PPSFs are ideal for frequency doubling high-power, ultrashort-pulse fiber-laser sources, because of the high damage threshold and the straightforward adaptability to all-fiber systems,” says ORC researcher Albert Canagasabey. “Adding wavelength tunability increases their functionality enormously. Using the tuning mechanism from fiber-Bragg-grating technology, we have managed to achieve the largest second-harmonic wavelength tuning range seen so far for single quasi-phase-matched periodically poled devices. This discovery allows access to wavelengths within fibers that were not accessible before, giving us a platform to generate new wavelengths which can be used in medical, industrial, and sensing applications.”

Bridget Marx

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