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Spectral shaping is critical to many fields of science. In astronomy forexample, the detection of exoplanets via the Doppler effect hinges on theability to calibrate a high resolution spectrograph. Laser frequency combs canbe used for this, but the wildly varying intensity across the spectrum can makeit impossible to optimally utilize the entire comb, leading to a reducedoverall precision of calibration. To circumvent this, astronomical applicationsof laser frequency combs rely on a bulk optic setup which can flatten theoutput spectrum before sending it to the spectrograph. Such flatteners requirecomplex and expensive optical elements like spatial light modulators and havenon-negligible bench top footprints. Here we present an alternative in the formof an all-photonic spectral shaper that can be used to flatten the spectrum ofa laser frequency comb. The device consists of a circuit etched into a siliconnitride wafer that supports an arrayed-waveguide grating to disperse the lightover hundreds of nanometers in wavelength, followed by Mach-Zehnderinterferometers to control the amplitude of each channel, thermo-optic phasemodulators to phase the channels and a second arrayed-waveguide grating torecombine the spectrum. The demonstrator device operates from 1400 to 1800 nm(covering the astronomical H band), with twenty 20 nm wide channels. The deviceallows for nearly 40 dBs of dynamic modulation of the spectrum via theMach-Zehnders , which is greater than that offered by most spatial lightmodulators. With a superluminescent diode, we reduced the static spectralvariation to ~3 dB, limited by the properties of the components used in thecircuit and on a laser frequency comb we managed to reduce the modulation to 5dBs, sufficient for astronomical applications.

Flattening laser frequency comb spectra with a high dynamic range, broadband spectral shaper on-a-chip