Quantum Optics

Entangled photons exhibit correlations that are unattainable with classical light. Their stronger-than-classical correlations make them desirable in applications ranging from secure communications to high-speed computation. Although, many degrees of freedom for entanglement exist, our research focuses on developing novel techniques for controlling time-frequency entangled photons (“biphotons”) that would be applicable in quantum communication. Most of our manipulation schemes have utilized broadband biphotons generated from spontaneous parametric down-conversion in periodically poled lithium niobate waveguides. However, we are also now exploring photon pair generation using silicon nitride microring resonators, a platform that could lead us to on-chip time-frequency manipulations.

Shaping of Entangled Photons

Depiction of experimental setup for pulse shaping of entangled photons.

Fourier-transform pulse shaping is a long-standing technique for manipulating ultrashort light pulses, which by the uncertainty principle are broadband in nature. In 2004, the Silberberg group showed the first application of pulse shaping at the single-photon level. As a result of our expertise in femtosecond pulse shaping, we have recently dedicated some of our efforts to studying pulse shaping of broadband entangled photons (“biphotons”)--generated from spontaneous parametric down-conversion (SPDC). Some of our contributions to this field include the demonstration of nonlocal dispersion cancellation for arbitrary orders of spectral phase, encoding and decoding of entangled photons, generation of biphoton correlation trains, the first verification of the temporal Talbot effect for entangled photons. These experiments on shaping biphoton correlations could lead to the development of quantum communication protocols that rely on preexisting classical pulse shaping technologies.

In addition to our work on biphoton pulse shaping, we are also investigating shaping of entangled photons through control of the SPDC pump. In this regard, we have developed a novel scheme for controlling the relative delay between an entangled photon pair through pump frequency tuning.

Lastly, we are exploring electro-optic modulation of these biphoton states. In a recent paper, we discuss how we employed intensity modulation to obtain a resolution improvement in temporal correlation measurements using slow single photon detectors.

Photon-Pair Generation in Silicon Nitride Microring Resonators

Depiction of microring resonator for generating comb-like photon pairs.

Recently, we started using silicon nitride microresonator cavities to generate comb-like time-frequency entangled photons (“biphoton frequency comb”) through spontaneous four-wave mixing. Our main interest here lies in the spectral processing of these chip-generated photon-pairs. We have successfully measured the spectro-temporal correlations of our comb-like photon pairs and conducted two-photon interference measurements to observe key signatures of our source. In addition, we have demonstrated nonlocal dispersion cancellation using these photon-pairs, suggesting their potential for long-distance secure communications. This platform could contribute to future developments in quantum information processing technologies.

Nonlocal Detection of Spectral-Phase-Shaped Biphotons

Although electronic single photon detectors have come a long way in being used to efficiently measure the temporal correlation of entangled photons, the rise time and timing jitter of these detectors still limit measurement of very fast temporal features. Pushing beyond these limits, we are working on a nonlocal high resolution detection scheme based on sum frequency generation of our entangled photons with ultrashort optical pulses. Potentially, this leads to an attainable resolution of hundreds of femtoseconds compared to the tens-hundreds of picosecond resolution of commercial single-photon detectors.

Kerr Comb Generation in High-Q Microresonators

Optical frequency combs consist of a series of equidistantly spaced, discrete spectral lines in the optical domain. Optical frequency combs have become a valuable tool in spectroscopy, optical atomic clocks, optical frequency metrology etc. Conventional optical frequency combs emit from mode-locked lasers; here we study the Kerr combs generation in high-Q microresonators, which can be integrated on silicon chips and provides for a compact way for comb generation. The integrated silicon-nitride microresonators generate a series of comb lines with a single cw pump. The generation dynamics of Kerr combs is complicated, therefore we study the fundamental physics in Kerr comb generation to better control it. With Kerr combs, we also investigate the applications in optical arbitrary waveform generation, optical communications, and microwave filtering etc.

Integrated Kerr Comb Generation

Illustration of comb generation in a microring resonator.

For typical Kerr comb generation, the pump source is usually an optical fiber based high power amplifier. To fulfill the potential of integrability of Kerr combs, we are working to realize fully integrated Kerr combs, with an integrated cw diode laser as the pump. We are collaborating with Infinera, which is a world leading company on integrated photonics, to realize the integrated Kerr combs. The available pump power for on-chip diode lasers is limited to ~200 mW. To achieve broadband Kerr combs with this power level, we work to increase the Q-factor of our silicon-nitride microresonators. Together with Prof. Qi’s group at Purdue, we recently realized silicon-nitride microresonators with Q-factor of 17 million, which is the highest Q-factor for silicon-nitride microresonators to date and is an order of magnitude higher than typical silicon-nitride microresonators.

Kerr Comb Based Optical Communications

Kerr combs provide a series of widely spaced spectral lines, which can be used as the light source for optical communications. Together with Prof. V. Torres-Company, we are working on Kerr combs based optical communications. For optical communications, the signal-to-noise ratio of the optical carrier is a critical criterion to achieve high performance. This requires the generated comb lines to have high power, which means we should achieve high conversion efficiency from the pump. Here, we use the normal-dispersion microresonators to generate the Kerr combs for optical communications. We recently achieved a conversion efficiency of ~30% in normal dispersion Kerr combs. This is higher than their anomalous dispersion counterpart.

Generation Dynamics of Kerr Combs

Since Kerr combs can involve hundreds of comb lines, the interplay between these lines can be quite complicated. To better use the Kerr combs for applications, we should understand the generation dynamics to control the comb generation confidently. We study the Kerr comb generation dynamics in both normal dispersion and anomalous dispersion silicon-nitride microresoators. For normal dispersion microresonators, we demonstrate the first mode-locked dark pulse broadband combs. Dark pulse combs have also been shown to have higher conversion efficiency for Kerr comb generation. Furthermore, we unveiled the role of mode-interaction in the Kerr combs initialization in normal-dispersion microresonators. We also demonstrated the generation of breather solitons in anomalous dispersion microresonators.

Microwave and Millimeter Wave Photonics

Our lab has focused attention on Ultrawideband (UWB) and millimeter-wave (MMW) technologies leading to our extensive research in the processing of such signals via RF photonic filters, waveform generation, propagation estimation, and dispersion compensation. Our lab has pushed the boundaries of UWB systems, and has added to the field of this emerging tool for future high-speed wireless communication systems.

UWB RF Arbitrary Waveform Generation (AWG) and Multipath Channel Characterization and Compensation

Figure depicting multipath environment.

UWB and MMW signals offer significant potentials for various applications from ultrahigh-speed communications to high-resolution ranging and electromagnetic imaging. With RF-AWG, spread spectrum sounding experiments for indoor multipath wireless channel over frequency band spanning from baseband 12 GHz can be performed. In such a scenario, channel multipath dispersion is the main reason for delay spreads.

By having the channel impulse response from a sounding experiment, both time reversal and phase compression pre-filtering techniques are utilized to both investigate the accuracy of channel characterization and to compensate the multipath dispersion, which will achieve, at the receiver end, a largely enhanced signal to noise ratio with better anti-jamming performance for wireless communication. Based on the fact that the channel decorrelates wavelength away, both time and space focusing are realized by compensating the delay spreads. Combined with fast data acquisition and processing of a FPGA, the channel sounding and compensation are applied to time-changing dynamic channel with real-time update.

Photonics Assisted RF AWG (Radio-Over-Fiber)

State-of-the-art electronics suffer from limited digital-to-analog conversion speeds, which in turn constrain the frequency range and bandwidth of the systems. As a result, robust photonic assisted RF-AWG has been conducted to address these shortcomings. In photonics-assisted RF-AWG techniques, a short optical input pulse is first shaped in the optical domain abd then converted into the microwave and MMW domain through high-speed photodetection. Taking advantage of Professor Weiner's expertise in his pioneering work in Fourier Transform Optical Pulse Shaping, our lab has performed extensive research in this area.

Frequency-to-time mapping (FTM) method is utilized in our work. In FTM, the target waveform is first programmed on a pulse shaper. After passing through a dispersive fiber, the waveform in the time domain is a stretched replica of the shaped power spectrum. A down-chirp RF waveform over frequencies from baseband to ~41 GHz with time aperture of ~6.8ns has been achieved in our work. Multipath wireless channel sounding and compensation have also been achieved with photonics assisted RF AWG over the frequency band spanning 2-12 GHz.

RF Photonic Filtering

Figure showing sample (A) amplitude and (B) group delay response of an RF photonic filter.

Our lab's interest in photonic processing of these signals stems from the fact that they offer many unique advantages. Such as large channel capacity and strong anti-jamming ability. Our research in Electro-Optic comb generation allows us to develolp RF photonic filters employing the multi-tap configuration for the processing of such signals. RF photonic filters are attractive by taking advantage of photonic processing such as, large bandwidth, immunity to electromagnetic interference, tunability, and programmability.

The implementation of the RF photonic filter we are concerned with is the tapped delay line structure. Multiple CW lasers at different wavelengths are used as the source. By controlling the power of each of the CW lasers, the weight of each tap can be controlled. The light at the different wavelengths are combined, and the RF signal is then modulated onto the optical carriers. The optical carriers then travel through a dispersive element that introduces a frequency dependent delay. This delay spreads the different optical carriers, thus making them different filter taps. The optical signal is transformed into an electrical signal via a photodiode. This electrical signal is the output RF signal after filtering. The tapped delay structure results in a finite impulse response (FIR) filter. This type of filter has the advantage of being flexible in the implementation of both amplitude and phase filters.

By switching the source from an array of CW lasers at different wavelengths to a frequency comb, the system now possesses some advantages. The system now only has one CW source, and the frequency comb offers a large nukmber of taps and tunability. Our group has demonstrated RF photonic filtering employing a frequency comb as the source. The implemenation of the tapped delay line structure with a frequency comb, as the source varies little from the original implementation presented before. The input RF signal is modulated onto the frequency comb generated from our home built Electro-Optic comb generators. This creates a single sideband on each of the individual comb lines. The individual optical carriers then travel through the dispersive fiber. This dispersive fiber induces a different delay on each of the optical carriers, i.e. the individual comb lines. The signal is converted back into the RF domain through a photodetector. Due to the delay induced by the dispersive fiber, the individual comb lines function as independent taps.

By varying the quadratic phase applied by a pulse shaper, the phase response of the filter can be matched to the input RF signal. This will allow for pulse compression. a chirped pulse is compressed and the comparison of the ideal autocorrelation trace and the measured compressed pulse show that the filter response matches the chirp well.

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