25 research outputs found
Electric propulsion using C.sub.60 molecules
Fullerene propellants, which are stable carbon cage structures composed of even numbers of carbon atoms in the range of about 32 to 200 atoms, particularly a combination of conveniently obtainable C.sub.60 and C.sub.70, may be carried in solid form in a spacecraft, sublimated to produce the appropriate molecular propellant such as C.sub.60 or C.sub.70, which may then be ionized by DC discharge or RF radiation to efficiently produce specific impulses in the range above 1000 lbf -s/lbm
Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification
Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spacing is demonstrated by interleaved difference-frequency-generation. The method, applied to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse microcomb spectra
Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification
Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spacing is demonstrated by interleaved difference-frequency-generation. The method, applied to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse microcomb spectra
An all-photonic, dynamic device for flattening the spectrum of a laser frequency comb for precise calibration of radial velocity measurements
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, conferenc
Flattening laser frequency comb spectra with a high dynamic range, broadband spectral shaper on-a-chip
Spectral shaping is critical to many fields of science. In astronomy for
example, the detection of exoplanets via the Doppler effect hinges on the
ability to calibrate a high resolution spectrograph. Laser frequency combs can
be used for this, but the wildly varying intensity across the spectrum can make
it impossible to optimally utilize the entire comb, leading to a reduced
overall precision of calibration. To circumvent this, astronomical applications
of laser frequency combs rely on a bulk optic setup which can flatten the
output spectrum before sending it to the spectrograph. Such flatteners require
complex and expensive optical elements like spatial light modulators and have
non-negligible bench top footprints. Here we present an alternative in the form
of an all-photonic spectral shaper that can be used to flatten the spectrum of
a laser frequency comb. The device consists of a circuit etched into a silicon
nitride wafer that supports an arrayed-waveguide grating to disperse the light
over hundreds of nanometers in wavelength, followed by Mach-Zehnder
interferometers to control the amplitude of each channel, thermo-optic phase
modulators to phase the channels and a second arrayed-waveguide grating to
recombine the spectrum. The demonstrator device operates from 1400 to 1800 nm
(covering the astronomical H band), with twenty 20 nm wide channels. The device
allows for nearly 40 dBs of dynamic modulation of the spectrum via the
Mach-Zehnders , which is greater than that offered by most spatial light
modulators. With a superluminescent diode, we reduced the static spectral
variation to ~3 dB, limited by the properties of the components used in the
circuit and on a laser frequency comb we managed to reduce the modulation to 5
dBs, sufficient for astronomical applications.Comment: 15 pages, 10 figures. arXiv admin note: substantial text overlap with
arXiv:2209.0945