We demonstrate an electro-optic imaging system for measuring millimeter-wave propagation along a coplanar waveguide (CPW). A polarization-resolved microscope images small electro-optic effects due to millimeter-wave voltages between the signal and ground of the CPW. Dual electro-optic frequency combs demonstrate time-domain waveform imaging on-wafer with >100 GHz of bandwidth. A second configuration demonstrates continuous wave optically-derived vector network analysis with measurement planes on-wafer, filling the role of multiple electronic network analyzers but with instantaneous bandwidth and time-domain capability that cannot be achieved otherwise.
We demonstrate ultrawideband mmWave waveform generation based on optical frequency combs and additive frequency synthesis. We generate an electro-optic frequency comb, apply fine amplitude and phase control to pairs of comb lines (beat notes), and demultiplex the beat notes to multiple uni-traveling carrier photodiodes, which are connected on-wafer to a mmWave frequency combiner.
Using multi-tier microwave S-parameter and power calibrations, we demonstrate on-wafer mmWave power levels up to +12.7 dBm and fine amplitude and phase control with 0.1 dB and 25 mrad resolutions respectively. We apply this fine resolution to arbitrary waveform generation with 100 GHz of instantaneous bandwidth on a single photodiode. We also summarize new results with five photodiodes connected to an on-wafer frequency combiner and our efforts to integrate NIST’s high-power amplifiers up to 500 GHz and beyond.
In this work, we propose a novel approach for stabilizing frequency comb for stable microwave generation. This approach does not require stabilization of the offset frequency, but instead employs processing the heterodyne beats from an electro-optic comb and two continuous-wave lasers that are locked to an ultrastable compact cavity. We used a free-running laser and electro-optical modulators to generate a 10 GHz frequency comb spanning over 1.3 THz, matching the frequency separation of the CW lasers. Servo control of the 10 GHz modulation frequency reduces the 10 GHz phase noise to -140 dBc/Hz phase noise at 10 kHz offset frequency. At the same time, the 10 GHz signals show frequency instability at the 10^(-13) level at integration time below 1~s. In ongoing work, we seek to implement the system using integrated laser sources, chip-scale frequency combs and millimeter-scale optical cavities.
KEYWORDS: Radio optics, Calibration, Heterodyning, Frequency combs, Laser radiometry, Radiometry, Near infrared, Optical calibration, Spectroscopy, Iron
Using laser heterodyne radiometry with a resolution of 100 MHz we have observed a selection of iron lines in the solar spectrum near 1565 nm with an approximate resolving power of 2,000,000 million. With a laser frequency comb as the wavelength calibration source we have achieved absolute frequency instability on measurements of single lines at better than 13 MHz in a 10 second averaging window, and SNR of over 5000 when averaging up to a 1 hour exposure. An amplitude servo on the laser power creates a stable, repeatable background level, allowing for simple extraction of the line depth.
Radial velocity (RV) surveys supported by high precision wavelength references (notably ThAr lamps and I2 cells) have
successfully identified hundreds of exoplanets; however, as the search for exoplanets moves to cooler, lower mass stars,
the optimum wave band for observation for these objects moves into the near infrared (NIR) and new wavelength
standards are required. To address this need we are following up our successful deployment of an H band(1.45-1.7μm)
laser frequency comb based wavelength reference with a comb working in the Y and J bands (0.98-1.3μm). This comb
will be optimized for use with a 50,000 resolution NIR spectrograph such as the Penn State Habitable Zone Planet
Finder. We present design and performance details of the current Y+J band comb.
The search for extrasolar planets is an exciting new field of astronomy. Since detection of a planet orbiting the
sun-like star 51 Peg,1 the field of planet finding has pushed the limits of sensitivity and accuracy in astronomical
photometry and spectroscopy. To date 455 exoplanets have been detected*, of which the radial velocity technique
is responsible for nearly 80%.2 Radial velocity measurements are also an important complement to photometric
missions such as Kepler and CoRoT, which survey vast numbers of stars simultaneously but which require follow
up measurements for positive identification of planets.
The chief objective in the search for exoplanets is the identification of habitable Earth-like planets in close
proximity to our solar system. Of the currently detected exoplanets, only a few are Earth-like,3 the vast majority
being giants in close orbits. While it is possible that these planets are the most common type, it is likely that
an inherent selection bias in planet finding techniques is the cause. Simply, large radial velocity shifts and
high contrast occultations are the most detectable by radial velocity spectroscopy and photometry, and so we
primarily observe planets capable of inducing them.
A novel scheme for recognition of electronic bit-sequences is demonstrated. Two electronic bit-sequences that are to be
compared are each mapped to a unique code from a set of Walsh-Hadamard codes. The codes are then encoded in
parallel on the spectral phase of the frequency comb lines from a frequency-stabilized mode-locked semiconductor laser.
Phase encoding is achieved by using two independent spatial light modulators based on liquid crystal arrays. Encoded
pulses are compared using interferometric pulse detection and differential balanced photodetection. Orthogonal codes
eight bits long are compared, and matched codes are successfully distinguished from mismatched codes with very low
error rates, of around 10-18. This technique has potential for high-speed, high accuracy recognition of bit-sequences, with
applications in keyword searches and internet protocol packet routing.
Mode-locked lasers have applications in signal processing and communications such as analog to digital conversion,
arbitrary waveform generation and wavelength division multiplexing. For such applications low noise and phase
coherent frequency stabilized optical combs are needed. In this work we report a low noise, Pound-Drever Hall
frequency stabilized, semiconductor mode-locked laser at 10.287GHz centered at 1550nm with 1000-Finesse sealed,
ultralow insertion loss intracavity etalon. The output optical power of the mode locked laser is ~5mW.
Harmonically mode-locked semiconductor lasers with external ring cavities offer high repetition rate pulse trains while
maintaining low optical linewidth via long cavity storage times. Continuous wave (CW) injection locking further
reduces linewidth and stabilizes the optical frequencies. The output can be stabilized long-term with the help of a
modified Pound-Drever-Hall feedback loop. Optical sidemode suppression of 36 dB has been shown, as well as RF
supermode noise suppression of 14 dB for longer than 1 hour. In addition to the injection locking of harmonically mode-locked
lasers requiring an external frequency source, recent work shows the viability of the injection locking technique
for regeneratively mode-locked lasers, or Coupled Opto-Electronic Oscillators (COEO).
It is critical to know the free spectral range (FSR) of an etalon for telecommunication applications. In this
paper, we have improved the Pound-Drever-Hall (PDH) based technique for measuring the FSR of an
etalon by 2 orders of magnitude. This improved technique results 1 part in 106 precision. To our knowledge
this is the most precise measurement of FSR.
We are investigating optical frequency comb generation by direct modulation of CW light. Our scheme is based on three cascaded modulators; one amplitude modulator and two phase modulators. The modulation scheme is optimized for flatness and power efficiency. A stable optical spectrum has been generated with ~100 comb lines with 0.625 GHz spacing and 3 dB flatness. We also investigate comb generation via phase only modulation.
Mode-locked laser designs for both time and frequency domain based applications are presented. It is shown that for
strictly time domain applications, simple laser cavity designs can produce pulse trains at 10 GHz with sub-5 fs relative
timing jitter (1 Hz-100 MHz) using only commercially available components. Frequency stabilized sources maintain
low timing jitter while achieving 1MHz maximum frequency deviations for optical spectra consisting of ~200 10 GHz
spaced frequencies. Methods of characterizing pulse-to-pulse timing jitter by examining the photodetected spectrum are
reviewed. The effects of the phase noise of an rf source used to drive an actively mode-locked laser on the laser's
amplitude and timing fluctuations is also discussed.
Frequency skewed optical pulses are generated via both a composite cavity structure in a fiberized semiconductor optical amplifier ring laser and a frequency skew loop outside the laser cavity. The composite cavity technique is similar to rational harmonic mode-locking, however it is based on cavity detuning rather than frequency detuning. These frequency skewed pulses are ideal for range detection applications since their interference results in a range dependent RF signal. The intracavity frequency skewed pulse train showed superior performance in both stability and signal quality.
We report on supermode noise suppression of a harmonically modelocked laser by optical injection. The modelocked
laser was injection locked to a CW narrow linewidth source. Injection locking selects a single supermode group
reducing the supermode noise spurs in the photodetected signal by 20 dB to a level of -130 dBc/Hz.
A novel method incorporating time division multiplexing technique with optical parabolic phase modulation has been introduced to overcome the limitations on optical generation of chirped RF signals. Extension of the frequency span and frequency sweep time of a RF chirp signal has been experimentally realized. A chirped RF signal with a center frequency of 100 MHz, frequency span of 20 MHz and sweep time of 200 ns has been generated via this novel method. This chirp signal agrees well with the chirp signal generated by conventional methods.
Frequency stabilized modelocked lasers have recently garnered much attention owing to their potential in metrology, communications, and signal processing applications. The possibility of optical source technology that is economical, compact, and electrically efficient suggests that semiconductor gain media could allow frequency stabilized ultrafast sources to rapidly gain a foothold in communication and signal processing applications. This work will summarize recent work in the area of stabilized modelocked semiconductor diode lasers, and highlight unique features that will impact photonic signal processing applications.
Using an intracavity Pound-Drever-Hall technique, simultaneous optical frequency comb stabilization within ±3 MHz range and supermode phase noise suppression were demonstrated for a 10 GHz harmonically modelocked semiconductor ring laser resulting in timing jitter of 63.5 fs integrated from 10 Hz to 10 MHz.
A novel time division multiplexing technique has been incorporated with optical serrodyne phase modulation to generate a single microwave tone with up to 1 MHz tunability. Sideband spurs due to finite phase reset time in ordinary serrodyne phase modulation have been suppressed to more than 40 dB below the carrier. These results show that endless phase modulation can easily be achieved using this novel technique.
We report on low noise performance of a 10 GHz actively mode-locked laser. The laser is a fiberized ring laser using a commercially available semiconductor optical amplifier (SOA) as the gain medium. The noise properties as a function of cavity length and optical spectrum are investigated. It is found that supermode noise is reduced when the cavity length is increased past a certain threshold. Best performance is achieved with a 20 meter cavity whose pulses are down chirped. No active feedback control is utilized to reduce the noise, yet the integrated jitter is only 29 fs (10Hz-100MHz).
This paper discusses use of optical frequency combs generated by modelocked semiconductor lasers for coherent photonic signal processing applications. Key in our approach is a high Q cavity, supermode suppression and low spontaneous emission. Targeted applications of the stabilized optical frequency combs lie in areas of metrology, optical sampling, arbitrary waveform generation and communications using coherent detection.
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