Amirhossein Tehranchi

Design of Novel Unapodized and Apodized Step-Chirped Quasi-Phase Matched Gratings for Broadband Frequency Converters Based on Second-Harmonic Generation

The unapodized and apodized step-chirped gratings (SCGs) for broadband frequency converters based on quasi-phase-matched second-harmonic generation with pump depletion in lithium niobate waveguides have been theoretically modeled and simulated as a function of the number of sections, and compared with the linearly chirped gratings (LCGs) for the first time to our knowledge. It is shown that for the same length, using fewer sections with more segments and larger amounts of chirp, the efficiency and bandwidth of an SCG approach over that of an LCG and can be extensively improved with apodization. Moreover, the increasing chirp period and duty cycle for the SCG structure may provide a more convenient method for fabrication and poling. In addition, we present useful relations for the band-widths that help to find the appropriate number of segments in the proposed SCGs of a given length.

Novel Designs for Efficient Broadband Frequency Doublers Using Singly Pump-Resonant Waveguide and Engineered Chirped Gratings

Novel engineered step-chirped gratings (SCG) for broadband frequency doublers based on quasi-phase matched second harmonic generation with and without singly pump-resonance in lithium niobate waveguides have been theoretically modeled and simulated taking pump depletion into account, for the first time to our knowledge. It is shown that apodized SCGs flatten the conversion bandwidth which can be improved extensively with apodization ratio. Also, it is demonstrated numerically for an ASCG that by means of singly pump-resonant waveguide, the conversion efficiency of quasi-continuous waves shown with the envelope of responses increases substantially especially for low loss. The optimized values of back facet reflectivity and input power to achieve the maximum efficiency, for several waveguide loss parameters have also been calculated. Furthermore, the ASCG structure may provide a more convenient method for fabrication and poling as it increases the chirp period and duty cycle.

Engineered gratings for flat broadening of second-harmonic phase-matching bandwidth in MgO-doped lithium niobate waveguides

Novel engineered step-chirped gratings (SCG) for broadband frequency converters based on quasi-phase matched second harmonic generation in MgO-doped lithium niobate waveguides have been theoretically modeled and simulated. It is shown mathematically that engineered apodized gratings can flatten the efficiency response. Also, it is verified that the bandwidth and flatness of an apodized SCG can be improved extensively with decreasing the number of segments and increasing the apodization ratio, respectively. Further, we show enhancing the minimum width of the line of the gratings to 1 micron for easing fabrication, almost all the beneficial effect on the efficiency response of an apodized SCG are maintained. The possibility of increasing the width of the poled lines and the increase in the chirp step due to use of the SCG structure, may provide more convenient route for fabrication and poling.

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.

Efficient flattop ultra-wideband wavelength converters based on double-pass cascaded sum and difference frequency generation using engineered chirped gratings

High-efficiency ultra-broadband wavelength converters based on double-pass quasi-phase-matched cascaded sum and difference frequency generation including engineered chirped gratings in lossy lithium niobate waveguides are numerically investigated and compared to the single-pass counterparts, assuming a large twin-pump wavelength difference of 75 nm. Instead of uniform gratings, few-section chirped gratings with the same length, but with a small constant period change among sections with uniform gratings, are proposed to flatten the response and increase the mean efficiency by finding the common critical period shift and minimum number of sections for both single-pass and double-pass schemes whilst for the latter the efficiency is remarkably higher in a low-loss waveguide. It is also verified that for the same waveguide length and power, the efficiency enhancement expected due to the use of the double-pass scheme instead of the single-pass one, is finally lost if the waveguide loss increases above a certain value. For the double-pass scheme, the criteria for the design of the low-loss waveguide length, and the assignment of power in the pumps to achieve the desired efficiency, bandwidth and ripple are presented for the optimum 3-section chirped-gratings-based devices. Efficient conversions with flattop bandwidths > 84 nm for lengths < 3 cm can be obtained.

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.

Improved cascaded sum and difference frequency generation-based wavelength converters in low-loss quasi-phase-matched lithium niobate waveguides

We numerically evaluate the wavelength converters based on cascaded sum and difference frequency generation in quasi-phase-matched lithium niobate waveguides with and without loss. A technique is also proposed to flatten the response by increased detuning of the pump wavelength. We present the criteria for the design of waveguide length and the assignment of pump power to achieve the desired efficiency, ripple and bandwidth, assuming a large pump wavelength difference of 75 nm.

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.

Temporal characterization of a multi-wavelength Brillouin–erbium fiber laser

This paper provides the first detailed temporal characterization of a multi-wavelength-Brillouin–erbium fiber laser (MWBEFL) by measuring the optical intensity of the individual frequency channels with high temporal resolution. It is found that the power in each channel is highly unstable due to the excitation of several cavity modes for typical conditions of operation. Also provided is the real-time measurements of the MWBEFL output power for two configurations that were previously reported to emit phase-locked picosecond pulse trains, concluded from their autocorrelation measurements. Real-time measurements reveal a high degree of instability without the formation of a stable pulse train. Finally, we model the MWBEFL using coupled wave equations describing the evolution of the Brillouin pump, Stokes and acoustic waves in the presence of stimulated Brillouin scattering, and the optical Kerr effect. A good qualitative consistency between the simulation and experimental results is evident, in which the interference signal at the output shows strong instability as well as the chaotic behavior due to the dynamics of participating pump and Stokes waves.

Efficient wavelength converters with flattop responses based on counterpropagating cascaded SFG and DFG in low-loss QPM LiNbO 3 waveguides

A wavelength converter based on counterpropagating quasi-phase matched cascaded sum and difference frequency generation in lossy lithium niobate waveguide is numerically evaluated and compared to a single-pass scheme assuming a large pump wavelength difference of 75 nm. A double-pass device is proposed to improve the conversion efficiency while the response flattening is accomplished by increasing the wavelength tuning of one pump. The criteria for the design of the low-loss waveguide length, and the assignment of power in the pumps to achieve the desired efficiency, ripple and bandwidth are presented.