Laser frequency combs are fast becoming critical to reaching the highest
radial velocity precisions. One shortcoming is the highly variable brightness
of the comb lines across the spectrum (up to 4-5 orders of magnitude). This can
result in some lines saturating while others are at low signal and lost in the
noise. Losing lines to either of these effects reduces the precision and hence
effectiveness of the comb. In addition, the brightness of the comb lines can
vary with time which could drive comb lines with initially reasonable SNR's
into the two regimes described above. To mitigate these two effects, laser
frequency combs use optical flattener's.
Flattener's are typically bulk optic setups that disperse the comb light with
a grating, and then use a spatial light modulator to control the amplitude
across the spectrum before recombining the light into another single mode fiber
and sending it to the spectrograph. These setups can be large (small bench
top), expensive (several hundred thousand dollars) and have limited stability.
To address these issues, we have developed an all-photonic spectrum flattener
on a chip. The device is constructed from optical waveguides on a SiN chip. The
light from the laser frequency comb's output optical fiber can be directly
connected to the chip, where the light is first dispersed using an arrayed
waveguide grating. To control the brightness of each channel, the light is
passed through a Mach-Zehnder interferometer before being recombined with a
second arrayed waveguide grating. Thermo-optic phase modulators are used in
each channel before recombination to path length match the channels as needed.
Here we present the results from our first generation prototype. The device
operates from 1400-1800 nm (covering the H band), with 20, 20 nm wide channels.
Comment: 7 pages, 5 figures, conference
Subjects: Astrophysics - Instrumentation and Methods for Astrophysics; Physics - Instrumentation and Detectors; Physics - Optics