Martin Schnack

Experimental realization of femtosecond transverse mode conversion using optically induced transient long-period gratings

We present the experimental realization of transverse mode conversion in an optical fiber via an optically induced long-period grating. The transient gratings are generated by femtosecond laser pulses, exploiting the Kerr effect to translate intensity patterns emerging from multimode interference into a spatial refractive index modulation. Since these modulations exist only while the pump beam is present, they can be used for optical switching of transverse modes. As only a localized part of the grating was written at a time and the probe beam was co-propagating with the pump beam the required pulse energies could be reduced to 120 nJ which is about a factor of 600 lower than in previous quasi-continuous-wave experiments. Accompanying numerical simulations allow a better understanding of the involved effects and show excellent agreement to the experimental results.

Ultrafast, low-power, all-optical switching via birefringent phase-matched transverse mode conversion in integrated waveguides

We demonstrate the potential of birefringence-based, all-optical, ultrafast conversion between the transverse modes in integrated optical waveguides by modelling the conversion process by numerically solving the multi-mode coupled nonlinear Schroedinger equations. The observed conversion is induced by a control beam and due to the Kerr effect, resulting in a transient index grating which coherently scatters probe light from one transverse waveguide mode into another. We introduce birefringent phase matching to enable efficient all-optically induced mode conversion at different wavelengths of the control and probe beam. It is shown that tailoring the waveguide geometry can be exploited to explicitly minimize intermodal group delay as well as to maximize the nonlinear coefficient, under the constraint of a phase matching condition. The waveguide geometries investigated here, allow for mode conversion with over two orders of magnitude reduced control pulse energy compared to previous schemes and thereby promise nonlinear mode switching exceeding efficiencies of 90% at switching energies below 1 nJ.

Ultrafast, all-optical control of modal phases in a few-mode fiber for all-optical switching

The phase differences between the transverse modes of an optical fiber can be altered all-optically by intermodal cross-phase modulation. In this Letter, we experimentally demonstrate this effect with ultrashort laser pulses. An ultrashort probe pulse, guided in both modes of a two-mode fiber, is co-propagating and temporally overlapping with an ultrashort control pulse, guided in the fundamental mode only and centered at a separate wavelength. The use of ultrashort pulses allows for a notable phase shift at a 33-fold reduced control pulse energy and a 173-fold reduced fiber length, compared to previous experiments. A total phase shift of 0.285π between the two probe modes was achieved at a 9 nJ control pulse energy in a 19 cm long two-mode graded-index fiber. Additionally, the capability of this scheme to switch ultrashort pulses in an all-optical manner was investigated. A modulation depth of 50% was achieved, limited by temporal nonlinear effects.

Ultrafast, all-optical phase tuning between transverse fiber modes for all-optical switching

With the development of new fiber types, such as few-mode graded-index fibers, transverse modes in optical fibers have gained increasing interest in the recent years. As it became easier to address individual modes, transverse modes could be exploited for new fundamental nonlinear optical effects, like intermodal four-wave mixing [1], as well as for new applications, like spatial division multiplexing [2].

All-optical switching using transverse modes in integrated waveguides

Low-power, ultrafast, all-optical switches have been developed by tuning the phase difference between transverse modes of a probe beam via intermodal cross-phase modulation (XPM) with a second beam as the control beam [1]. Switching energies in the nanojoule-regime were achieved in few-mode fibers [2].

Experimental verification of femtosecond transverse mode conversion induced by non-permanently written long-period gratings

Summary form only given. Mode conversion in few-mode fibers has been excessively studied and utilized, e.g., for dispersion compensation. For this purpose long-period gratings (LPG) have been permanently written into a fiber using UV or pulsed radiation. Recently, mode conversion has been demonstrated by transiently writing the grating structure into the fiber core via the optical Kerr effect with counter-propagating high power nanosecond pulses. An energy transfer between two probe modes with an efficiency of about 50% could be measured, limited by the damage threshold of the fiber front facet. A femtosecond laser source offers the possibility to reach the necessary peak powers at by a factor of 1000 reduced pulse energy. Therefore, we demonstrate the use of femtosecond pulses for writing optically induced long-period gratings (OLPG) to convert energy from one transverse probe mode to another. Due to the short interaction length of counter-propagating femtosecond pulses, our approach is changed to a co-propagating one, compared to our earlier work, using polarization to distinguish between probe and write beam.