Design, fabrication and characterization of a specially apodized chirped grating for reciprocal second harmonic generation
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Abstract:
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.Keywords:
Chirp
Apodization
Harmonic
Reciprocal
We discuss spectrotemporal measurements of laser diode pulses performed with a streak camera and a grating monochromator to yield a precise evaluation of chirping effects in Q-switched multimode emissions. We experimentally illustrate several causes of errors, depending on the grating size and period as well as on the adjustment of the collimating lens at the monochromator output. An analytical formula is derived that allows us to relate the chirp amplitude to the inclination of the modal structures in the streak image. Two configurations are proposed for a practical determination of the chirp amplitude in multimode emissions. Illustrations are provided with Q-switched AlGaAs laser diodes that exhibit chirp amplitudes larger than the mode spacing.
Monochromator
Streak camera
Chirp
Streak
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We present experimental results of complex grating structures fabricated with uniform phase masks by the moving fiber-scanning beam approach. Pure apodized gratings with side-mode-suppression levels in excess of 40 dB, self-apodized linearly chirped gratings, and phase shifted gratings with narrow-band transmission peaks have all been realized.
Apodization
Fiber Bragg Grating
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Fiber gratings have a growing impact on the fiber optic communication industry. The simulation result of the reflectance of the uniform and apodized fiber bragg grating (FBG) are presented. Various apodization technique is useful to reduce secondary lobes or side lobs of reflection spectrum of fibre bragg grating. The effect of FBG length and apodization profile are presented Fiber Bragg Gratings (FBGs) are most commonly used as wave- length selective reflector. Fiber Bragg gratings are spectral fil- ters based on the principle of Bragg reflection. They typically reflect light over a narrow wavelength range and transmit all other wavelengths. When light propagates by periodically alternating regions of higher and lower refractive index, it is partially reflected at each interface between those regions. If the pitch of the rating is properly designed, then all partial reflections add up in phase and can grow to nearly 100%, for a specific wavelength even if the individual reflections are very small. The condition for high reflection is known as Bragg condition. For all other wavelengths the out of phase reflec- tions end up cancelling each other, resulting in high transmis- sion. Fiber grating can be classified into two types. First one is Bragg Grating and another is Transmission Grating. Bragg grating favors coupling between travelling in opposite directions. They are also called reflection gratings or short-period gratings. On the other hand, in transmission grat- ings, coupling occurs between modes travelling in the same direction. Transmission gratings are also referred to as long period gratings. The reflected wavelength is mainly determined by the period of the grating. Most common applications of fiber gratings in fiber optic communications are as add-drop filters in WDM systems, gain flatteners and pump stabilizers for EDFA's, wavelength selective reflectors for Raman amplifiers, Dispersion compensa- tors for long-haul systems, encoder for CDMA systems.
Fiber Bragg Grating
Apodization
PHOSFOS
Long-period fiber grating
Reflection
Ultrasonic grating
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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.
Chirp
Apodization
Harmonic
Reciprocal
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To produce a compact low-cost tunable filter required for WDM optical communications, a polymeric Bragg reflection filter with an apodized grating structure is proposed. A high-contrast polymeric waveguide is incorporated in order to obtain high reflectivity from a short Bragg grating. To overcome the bandwidth broadening, an apodized grating with a gradually changing depth of surface relief grating along the propagation direction is fabricated through the dry etching with a shadow mask. The apodized polymer grating exhibits 3-dB, 20-dB bandwidths of 0.36 nm, and 0.72 nm, respectively with a 95% reflection. The reflection wavelength is tunable over 14 nm for an applied thermal power of 500 mW.
Apodization
Reflection
Fiber Bragg Grating
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We conduct a detailed theoretical analysis of ultrashort pulse propagation through waveguide long-period grating (LPG) structures operating in the linear regime. We first consider the case of uniform LPGs and we also investigate the impact of the typical grating non-uniformities, e.g. grating profile apodization, and grating period chirping, in the spectral and temporal behaviour of LPG structures. Besides its intrinsic physical interest, our study reveals the strong potential of LPG-based devices for optical pulse re-shaping operations in the sub-picosecond regime.
Apodization
Chirp
Picosecond
Waveguide
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An experimental acousto-optic tunable filter that has a narrow spectral bandwidth (0.2 nm at 1550 nm) and a fast (10-micros) tuning capability with a continuous tuning range of approximately 50 nm is described. The tunable filter consists of an acousto-optic beam deflector with a diffraction grating whose grating vector is transverse to the direction of light propagation.
Diffraction efficiency
Ultrasonic grating
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We conduct a detailed theoretical analysis of ultrashort pulse propagation through waveguide long-period grating (LPG) structures operating in the linear regime. We first consider the case of uniform LPGs, and we also investigate the effect of the typical grating nonuniformities, e.g., grating profile apodization, grating period chirping, and discrete phase shifts, on the spectral and temporal behavior of LPG structures. The two interacting modes are analyzed separately, and advanced representation tools, namely, space-wavelength and space-time diagrams (where space refers to the longitudinal grating dimension), are used to provide a deeper insight into the physics that determines the pulse evolution dynamics through the grating structures under analysis. In addition to its intrinsic physical interest, our study reveals the strong potential of LPG-based devices for optical pulse reshaping operations in the subpicosecond regime.
Waveguide
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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.
Apodization
Wideband
Chirp
Narrowband
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An investigation of the effects of apodization on a holographic demultiplexer that is based on a photopolymer grating is presented. Uniform and Gaussian apodized gratings are fabricated in a DuPont HRF-150-38 photopolymer. From the theoretical and experimental results, the spectral response of the apodized grating has a larger main lobe but lower sidelobes than those in the uniform-grating case. A 42-channel demultiplexer that is based on the Gaussian apodized grating with an 0.4-nm channel spacing is demonstrated. A cross-talk level of -30 dB and an interchannel uniformity of 1.5 dB are archived in the wavelength range of approximately 1550 nm.
Apodization
Demultiplexer
Channel spacing
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