| As one of the key devices of the next generation all-optical switching network,the reconfigurable optical add-drop multiplexer(ROADM)should have the capability of flexible-grid management,so that flexibly bandwidth adaption and path slection can be satisfied for different applications.Wavelength selective switch(WSS)is a key component of ROADM,which can be regarded as a programmable spectral processor(PSP)with the switching function.To meet the challenge of flexible grid bandwidth allocation,liquid crystal on silicon based programmable spectral processor was studied,focusing on high resolution spectral separation and high-performance wavelength routing.The main contents and results are as follows:(1)The phase modulation of liquid crystal spatial light modulator(LC-SLM)were characterized for accurate phase operation.The least square method(LSM)and Zernike polynomial fitting method were used to eliminate the additional phase distortion caused by the aberration of the optical device and the tilted phase caused by residual carrier frequency,respectively.The inherent phase distortion was precisely calibrated and the results indicate that the peak-to-valley value of inherent phase distortion was 1.55 rad,and the root mean square value was ?/34.Subsequently,the phase distortion caused by the spatial phase non-uniformity of LC-SLM was compensated by preloading a phase pattern,and the phase accuracy of LC-SLM is improved.Besides,the fringing field effect of LC-SLM was modeled and characterized by the point spread function model.(2)A specific double-grating configuration is proposed to achieve multiple round-trip diffractions for both promoting the angular dispersion and harnessing the spot size of emergent light by two gratings with appropriate included angle.The angular dispersion is repeatedly enlarged by multiple round-trip diffractions,while the spot size of emergent light remains unchanged due to the reversibility of multiple diffractions,finally leading the enhancement of optical spectral resolution.The specific double-grating configuration realizes the improvement of spectral resolution.With the commercial grating parameters,theoretical results indicate that the spectral resolution can reach 6.60 pm,almost 19-time promotion than that based on the single diffraction grating.Due to the beam leakage restricted by the grating size,the experimental spectral resolution is 13.04 pm,indicating of nearly ten-time enhancement.(3)Special optical systems were designed to realize polarization-insensitive and high spectral resolution for PSP.The LCOS is divided into two parts to treat the incident light beam twice with polarization operation in the spectral domain.The extinction ratio(ER)of the designed system can reach 46.4 d B.The polarization dependent loss over C-band is less than 1 d B,and the channel isolation of multi-channel filtering can reach 40 d B.Then,we demonstrate a LCOS-based spectral processor with double-grating configuration.Its spectral roll-off bandwidth can realzie 1.63 GHz and its addressable resolution reaches 240 MHz,which indicates that the optical filter based on the proposed configuration can distinctly promote the capability of the optical signal processing with fine spectral granularity.Moreover,the high-resolution spectral processor can be applied for wavelength selective switch and Nyquist pulse generation.(4)The phase distortion in LC-SLM was compensated to improve the performance of WSS based on LC-SLM.A four-channel wavelength selective switch was designed and constructed.The phase distortion of the optical system was compensated by the model-based wavefront correction method.The insertion loss(IL)and the channel crosstalk were respectively reduced by 1.41 d B and 3.09 d B after phase distortion compensation.Finally,the central wavelength tuning and bandwidth tuning of the system were characterized,and its channel switching function is verified. |
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