Ameneh Bostani

Agile multicasting based on cascaded χ (2) nonlinearities in a step-chirped periodically poled lithium niobate

We experimentally demonstrate the possibility of agile multicasting for wavelength division multiplexing (WDM) networks, of a single-channel to two and seven channels over the C band, also extendable to S and L bands. This is based on cascaded χ(2) nonlinear mixing processes, namely, second-harmonic generation (SHG)-sum-frequency generation (SFG) and difference-frequency generation (DFG) in a 20-mm-long step-chirped periodically poled lithium niobate crystal, specially designed and fabricated for a 28-nm-wide SH-SF bandwidth centered at around 1.55 μm. The multiple idlers are simultaneously tuned by detuning the pump wavelengths within the broad SH-SF bandwidth. By selectively tuning the pump wavelengths over less than 10 and 6 nm, respectively, multicasting into two and seven idlers is successfully achieved across ~70 WDM channels within the 50 GHz International Telecommunication Union grid spacing.

Engineering of effective second-order nonlinearity in uniform and chirped gratings

The dependence of second harmonic generation conversion efficiency resulting from the position of poled regions of the second-order nonlinearity in uniform and chirped gratings in ferroelectric materials has been studied analytically and numerically. The displacement of the poled region’s position from a specific location in second-order nonlinear materials can introduce a wavelength shift in a uniform grating’s second harmonic intensity peak and strongly influences the bandwidth and ripple in the harmonic conversion response of chirped gratings. We propose that the poled regions should be located at specific positions within the single period to minimize the ripples and achieve a desired nonlinearity function, and also to significantly improve tolerance to fabrication errors.

Tailoring and tuning of the broadband spectrum of a step-chirped grating based frequency doubler using tightly-focused Gaussian beams

We demonstrate theoretically and experimentally, that the non-uniform spectra of second harmonic generation (SHG) from an unapodized step-chirped periodically poled nonlinear optical grating can be apodized utilizing tightly-focused Gaussian beams to suppress the ripple in its wideband response. In our example, by increasing focusing, a ripple-free response is progressively achieved over a 6-dB bandwidth of >5 nm, with a beam waist of 20 µm. With this tight focusing arrangement, a continuous tuning of 11-nm is also demonstrated by simply changing the focal point by 5.8 mm within the step-chirped grating based APPLN.

Super-tunable, broadband up-conversion of a high-power CW laser in an engineered nonlinear crystal

A specially-designed chirped periodically poled lithium niobate nonlinear crystal was fabricated with a phase-matching bandwidth as large as 50 nm for sum frequency generation to operate at room and higher temperatures. This device also benefits from insensitivity to laser frequency drift and fine alignment. The loosely-focused beam position of a high-power CW laser at around 1550 nm is optimized within the grating for maximum up-conversion efficiency, to realize a super-tunable source in the range of 770–778 nm by tuning a narrowband control signal over 30 nm in the communication band. This device is demonstrated to be fully phased-matched simultaneously for both second-order nonlinear up-conversion processes, namely second harmonic generation and sum frequency generation. The measurement of the generated sum-frequency power versus wavelength agrees well with the theory. The device allows for the creation of tunable broadband CW sources at shorter wavelengths with potentially high power.

Tunable single-to-dual channel wavelength conversion in an ultra-wideband SC-PPLN

We experimentally demonstrate tunable dual channel broadcasting of a signal over the C-band for wavelength division multiplexed (WDM) optical networks. This is based on cascaded χ(2) nonlinear mixing processes in a specially engineered, 20-mm-long step-chirped periodically poled lithium niobate with a broad 28-nm second harmonic (SH) bandwidth in the 1.55-μm spectral range. A 10-GHz picosecond mode-locked laser was used as a signal along with a CW pump to generate two pulsed idlers, which are simultaneously tuned across the C-band by detuning of the pump wavelength within the broad SH bandwidth. Variable-input, variable-output scheme of tuned idlers is successfully achieved by tuning the signal wavelength. Pump or signal wavelength tuning of ~10 nm results in the idlers spreading across 30 nm in the C-band.

Design, fabrication and characterization of a specially apodized chirped grating for reciprocal second harmonic generation

A specially-designed apodized chirped PPLN based on particular positioning of poled regions within the periods has been realized theoretically and experimentally to demonstrate the reciprocal response in the SHG spectra over a 30-nm bandwidth, for up-chirp and down-chirp directions. The simulation results are compared with another apodized chirped PPLN for which the placement of poled regions is deviated from optimum positions. The average power difference is less than 0.75 dB and the standard deviations of extrema on second harmonic power responses are 1.34 dB and 1.64 dB for two up-chirp and down-chirp directions respectively.

Flexible All-Optical Wavelength Shifters Using Strong Focusing in a Wideband Engineered PPLN

Wavelength shift of modified second-harmonic bandwidth in a 20-mm-long engineered chirped periodically poled lithium niobate is experimentally demonstrated by focalpoint displacement of a strongly focused pump beam of a 30-μm beam waist. Utilizing this non-confocal arrangement in a cascaded second harmonic generation and difference frequency generation process, a CW probe as a signal wavelength is variably shifted within an 8-nm span by pump tuning using a ~7-nm second-harmonic bandwidth.

Study of apodization of aperiodically poled lithium niobate (APPLN) for second harmonic generation (SHG)

Apodized chirped aperiodically poled lithium niobate is designed to generate smooth second harmonic of a pump wave over a wide bandwidth. The effect of different apodization functions and the ratio of apodization have been examined for the reduction of ripples in the efficiency response of APPLN.

Full bandwidth frequency doubling of a high-power CW fiber laser using a bulk aperiodically poled LN

Temperature- and pump-frequency-shift- insensitive, full-bandwidth frequency doubling of a high-power CW fiber laser is realized using a bulk step-chirped PPLN. Quadratic second harmonic generation with respect to the pump pump has been demonstrated.

Tunable wavelength shifters using tight focusing in a wideband engineered PPLN crystal

Enhanced tuning of pump bandwidth of an efficient broadband engineered PPLN is realized with a non-confocal focusing to variably shift signal wavelengths in the C-band based on cascaded second-harmonic and difference-frequency generation.