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

A 40 Gb/s chip-to-chip interconnect for 8-socket direct connectivity using integrated photonics

We present an O-band any-to-any chip-to-chip (C2C) interconnection at 40 Gb/s suitable for up to 8-socket direct connectivity in multi-socket server boards, utilizing integrated low-energy photonics for the transceiver and routing functions. The C2C interconnect exploits an Si-based ring modulator as its transmitter and a co-packaged photodiode/transimpedance amplifier enabled receiver interconnected over an 8 x 8 Si-based arrayed waveguide grating router, allowing for a single-hop flat-topology interconnection between eight nodes. A proof-of-concept demonstration of the C2C interconnect is presented at 25 and 40 Gb/s for eight possible routing scenarios, revealing clear eye diagrams at both data rates with extinction ratios of 4.8 +/- 0.3 and 4.38 +/- 0.31 dB, respectively, among the eight routed signals

Monolithically Integratable Colliding Pulse Modelocked Laser Source for O-CDMA Photonic Chip Development

We demonstrate modelocking of a colliding-pulse mode-locked laser formed by 3-μm-deep etched-mirrors on an InP platform for integration with passive waveguide components. Timing jitter of 243 fs and pulse width of 10 ps were measured

Monolithically Integratable Colliding Pulse Modelocked Laser Source for O-CDMA Photonic Chip Development

We demonstrate modelocking of a colliding-pulse mode-locked laser formed by 3-μm-deep etched-mirrors on an InP platform for integration with passive waveguide components. Timing jitter of 243 fs and pulse width of 10 ps were measured

Optical-CDMA in InP

This paper describes the InP platforms for photonic integration and the development on these platforms of an optical code division multiple access (O-CDMA) system for local area networks. We demonstrate three building blocks of this system: an optical pulse source, an encoder/decoder pair, and a threshold detector. The optical pulse source consists of an integrated colliding pulse-mode laser with nearly transform-limited 10 Gb/s pulses and optical injection locking to an external clock for synchronization. The encoder/decoder pair is based on arrayed waveguide gratings. Bit-error-rate measurements involving six users at 10 Gb/s showed error-free transmission, while O-CDMA codes were calibrated using frequency resolved optical gating. For threshold detection after the decoder, we compared two Mach--Zehnder interferometer (MZI)-based optical thresholding schemes and present results on a new type of electroabsorber-based MZI

Semiconductor optical amplifiers in a non-linear Mach-Zehnder interferometer

Semiconductor Optical Amplifiers (SOAs) are widely used as non-linear elements for optical data processing. For optimal use, high values of the phase change should accompany low changes in gain. The gain itself should be as high as possible. The relation between these two effects is described by the linewidth enhancement factor (a-factor). Here a method is proposed to unambiguously determine it on-chip. The method uses an integrated SOA in a Mach-Zehnder Interferometer with unequal power distribution. The MZI output depends on the gain saturation and the phase shift, due to self-phase modulation. Analyzing this signal gives information about the a-factor