Optical Heterodyne Beat Research Articles

Multi-channel 28-GHz millimeter-wave signal generation on a silicon photonic chip with automated polarization control

We propose and experimentally demonstrate an integrated silicon photonic scheme to generate multi-channel millimeter-wave (MMW) signals for 5G multi-user applications. The fabricated silicon photonic chip has a footprint of 1.1 × 2.1 mm2 and integrates 7 independent channels each having on-chip polarization control and heterodyne mixing functions. 7 channels of 4-Gb/s QPSK baseband signals are delivered via a 2-km multi-core fiber (MCF) and coupled into the chip with a local oscillator (LO) light. The polarization state of each signal light is automatically adjusted and aligned with that of the LO light, and then 7 channels of 28-GHz MMW carrying 4-Gb/s QPSK signals are generated by optical heterodyne beating. Automated polarization-control function of each channel is also demonstrated with ~7-ms tuning time and ~27-dB extinction ratio.

Offset-free mid-infrared frequency comb based on a mode-locked semiconductor laser.

We demonstrate a carrier-envelope offset-free frequency comb in the mid-wavelength infrared (MWIR) based on a passively mode-locked vertical external cavity surface emitting laser (VECSEL) operating at a 1.6GHz repetition rate. The 290mW output spanning 3.0-3.5μm is generated through difference frequency generation (DFG) in periodically poled lithium niobate. The VECSEL pulse train is centered at 1030nm and amplified up to 11W in a Yb fiber amplifier system. The output is split to generate a second pulse train at 1560nm through nonlinear broadening in a Si3N4 waveguide followed by amplification in an Er gain fiber. DFG between the 1030 and 1560nm pulse trains results in a coherent and offset-free MWIR frequency comb, verified with optical heterodyne beat note measurements. Active stabilization of the VECSEL repetition rate provides a fully stabilized high repetition rate frequency comb in the MWIR, uniquely suited for applications in molecular spectroscopy.

High-detectivity optical heterodyne method for wideband carrier-envelope phase noise analysis of laser oscillators.

Broadband characterization of the carrier-envelope phase (CEP) noise spectral density of free-running mode-locked lasers is essential for advanced low-noise optical frequency comb designs. Here we present a direct method that utilizes an optical heterodyne beat between a pair of repetition-rate-locked mode-locked lasers for CEP noise characterization, without requiring an f-2f interferometer or nonlinear optical conversion steps. A proof-of-principle experiment in a femtosecond Yb-fiber laser achieves CEP noise spectral density characterization with >270 dB dynamic range over a Fourier frequency range from 5mHz to 8MHz. The measurement noise floor is well below 1 μrad/√Hz, enabling dependable detection down to a quantum-limited noise floor. The method can resolve various noise mechanisms that cause specific CEP noise spectral shapes. The underlying mechanisms are further analyzed in terms of spurious temporal correlation to distinguish between technical and stochastic noise signatures. Moreover, a Hadamard deviation analysis reveals a varying degree of frequency stability in the measured CEP time series.

698-nm diode laser with 1-Hz linewidth

Two diode lasers at 698 nm are separately locked to two independent optical reference cavities with a finesse of about 128,000 by the Pound–Drever–Hall method. The more accurate coefficient between voltage and frequency of the error signal is measured, with which quantitative evaluation of the effect of many noises on the frequency stability can be made much more conveniently. A temperature-insensitive method is taken to reduce the effect of residual amplitude modulation on laser frequency stability. With an active fiber noise cancellation, the optical heterodyne beat between two independent lasers shows that the linewidth of one diode laser reaches 1 Hz. The fractional Allan deviation removed linear frequency shift less than 30 mHz/s is below 2.6×10−15 with 1- to 100-s average time.

Active Faraday optical frequency standard.

We propose the mechanism of an active Faraday optical clock, and experimentally demonstrate an active Faraday optical frequency standard based on narrow bandwidth Faraday atomic filter by the method of velocity-selective optical pumping of cesium vapor. The center frequency of the active Faraday optical frequency standard is determined by the cesium 6 (2)S(1/2) F=4 to 6 (2)P(3/2) F'=4 and 5 crossover transition line. The optical heterodyne beat between two similar independent setups shows that the frequency linewidth reaches 281(23) Hz, which is 1.9×10(4) times smaller than the natural linewidth of the cesium 852-nm transition line. The maximum emitted light power reaches 75 μW. The active Faraday optical frequency standard reported here has advantages of narrow linewidth and reduced cavity pulling, which can readily be extended to other atomic transition lines of alkali and alkaline-earth metal atoms trapped in optical lattices at magic wavelengths, making it useful for new generation of optical atomic clocks.

Analysis of frequency noise in ultra-stable optical oscillators with active control of residual amplitude modulation

Two 1,064-nm Nd:YAG lasers frequency stabilized by high-finesse optical cavities are developed to investigate various noise mechanisms in ultra-stable optical oscillators. Active control of residual amplitude modulation using a separate sensing path is implemented and its effectiveness in the presence of a resonant optical cavity is theoretically analyzed and experimentally verified by measuring the rejection ratios in optical heterodyne beat between a perturbed laser and a stable reference. Laser frequency noises originated from vibration, residual amplitude modulation, quantum-limited shot noise, and electronic noise are experimentally analyzed. With active control, residual amplitude modulation is suppressed to below 1 x 10(-6) at 0.02-1,000 s, reaching a minimum of 2 x 10(-7) at similar to 2 s. A frequency stability of 2 x 10(-15) is obtained from 0.1 to 10 s, and the optical heterodyne beat of the two Nd: YAG lasers shows 1-Hz linewidth with a measurement time of 4.096 s. In addition, the experimentally determined linewidths agree well with the calculation according to a simplified relationship between the linewidth and the underlying flicker noise that modulates the laser frequency.

Optical Spectrum Analyzer with Quantum-Limited Noise Floor

Interactions between atoms and lasers provide the potential for unprecedented control of quantum states. Fulfilling this potential requires detailed knowledge of frequency noise in optical oscillators with state-of-the-art stability. We demonstrate a technique that precisely measures the noise spectrum of an ultrastable laser using optical lattice-trapped 87Sr atoms as a quantum projection noise-limited reference. We determine the laser noise spectrum from near dc to 100 Hz via the measured fluctuations in atomic excitation, guided by a simple and robust theory model. The noise spectrum yields a 26(4) mHz linewidth at a central frequency of 429 THz, corresponding to an optical quality factor of 1.6×10(16). This approach improves upon optical heterodyne beats between two similar laser systems by providing information unique to a single laser and complements the traditionally used Allan deviation which evaluates laser performance at relatively long time scales. We use this technique to verify the reduction of resonant noise in our ultrastable laser via feedback from an optical heterodyne beat. Finally, we show that knowledge of our laser's spectrum allows us to accurately predict the laser-limited stability for optical atomic clocks.

Multichannel 120-Gb/s Data Transmission Over 2$\,\times\,$2 MIMO Fiber-Wireless Link at W-Band

We experimentally demonstrated a seamlessly integrated fiber-wireless system that delivers multichannel 120-Gb/s data through 80-km fiber and 2-m 2×2 multiple-input multiple-output (MIMO) wireless link at 92-GHz W-band adopting polarization division multiplexing quadrature phase shift keying (PDM-QPSK) modulation. The three-channel 3×40-Gb/s optical PDM-QPSK signals with 12.5-GHz channel spacing are simultaneously upconverted to 92-GHz wireless carrier by optical polarization-diversity heterodyne beating and then transmitted and received by two pairs of transmitter and receiver antennas, which form a full 2×2 MIMO wireless link. At the wireless receiver, a two-stage analog and digital downconversion is performed. Polarization and wireless 2×2 MIMO demultiplexing are realized by constant modulus algorithm based on digital signal processing. The bit-error ratio performance for the 120-Gb/s PDM-QPSK signal is measured after 80-km single-mode fiber-28 and 2-m wireless transmission.

Fiber-wireless transmission system of 108 Gb/sdata over 80 km fiber and 2×2multiple-input multiple-output wireless links at 100 GHz W-band frequency

We experimentally demonstrate a seamlessly integrated fiber-wireless system that delivers a 108 Gb/s signal through 80 km fiber and 1 m wireless transport over free space at 100 GHz adopting polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation and heterodyning coherent detection. The X- and Y-polarization components of the optical PDM-QPSK baseband signal are simultaneously upconverted to 100 GHz wireless carrier by optical polarization-diversity heterodyne beating, and then independently transmitted and received by two pairs of transmitter and receiver antennas, which form a 2×2 multiple-input multiple-output wireless link. At the wireless receiver, two-stage downconversion is performed firstly in the analog domain based on balanced mixer and sinusoidal radio frequency signal, and then in the digital domain based on digital signal processing (DSP). Polarization demultiplexing is realized by the constant modulus algorithm in the DSP part at the receiver. The bit-error ratio for the 108 Gb/s PDM-QPSK signal is less than the pre-forward-error-correction threshold of 3.8×10(-3) after both 1 m wireless delivery at 100 GHz and 80 km single-mode fiber-28 transmission. To our knowledge, this is the first demonstration to realize 100 Gb/s signal delivery through both fiber and wireless links at 100 GHz.

Seamless integration of 572-Gb/s signal wireline transmission and 100-GHz wireless delivery

We experimentally demonstrated the seamless integration of 57.2-Gb/s signal wireline transmission and 100-GHz wireless delivery adopting polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulation with 400-km single-mode fiber-28 (SMF-28) transmission and 1-m wireless delivery. The X- and Y-polarization components of optical PDM-QPSK baseband signal are simultaneously up-converted to 100 GHz by optical polarization-diversity heterodyne beating, and then independently transmitted and received by two pairs of transmitter and receiver antennas, which make up a 2x2 multiple-input multiple-output (MIMO) wireless link based on microwave polarization multiplexing. At the wireless receiver, a two-stage down conversion is firstly done in analog domain based on balanced mixer and sinusoidal radio frequency (RF) signal, and then in digital domain based on digital signal processing (DSP). Polarization de-multiplexing is realized by constant modulus algorithm (CMA) based on DSP in heterodyne coherent detection. Our experimental results show that more taps are required for CMA when the X- and Y-polarization antennas have different wireless distance.

Broadband phase noise suppression in a Yb-fiber frequency comb

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7 dB out to Fourier frequencies of 100 kHz with a unity-gain crossing of ∼700 kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100 kHz is due to amplified spontaneous emission or other quantum noise sources.

Nd:YAG lasers at 1064 nm with 1-Hz linewidth

Two Nd:YAG lasers are tightly frequency-stabilized to separately located, vertically mounted ultrastable cavities, which are connected by single-mode optical fibers employing fiber phase noise cancellation. The optical heterodyne beat between two independent lasers shows that the linewidth of each laser reaches 1 Hz and the frequency drift is less than 0.3 Hz/s.

The quantum-limited comb lineshape of a mode-locked laser: Fundamental limits on frequency uncertainty

We calculate the quantum-limited shape of the comb lines from a mode-locked Ti:sapphire laser using experimentally-derived parameters for the linear response of the laser to perturbations. The free-running width of the comb lines is found across the laser spectrum. By modeling the effect of a simple feedback loop, we calculate the spectrum of the residual phase noise in terms of the quantum noise and the feedback parameters. Finally, we calculate the frequency uncertainty in an optical frequency measurement if the limiting factor is quantum noise in the detection of the optical heterodyne beat.

Two-hertz-linewidth Nd:YAG lasers at 1064 nm stabilized to vertically mounted ultra-stable cavities

Two Nd:YAG lasers operating at 1064 nm are separately servo-locked to two vertically mounted ultra-stable cavities. The optical heterodyne beat between two cavity-stabilized lasers shows that the linewidth of each laser reaches 2 Hz and the average frequency drift reduces to less than 1 Hz/s.

Characterization of frequency noise on a broadband infrared frequency comb using optical heterodyne techniques

We measure the frequency noise across a Cr:forsterite infrared frequency comb through the optical heterodyne beat of different comb teeth against stable continuous wave (CW) lasers. This sensitive measurement shows strong correlations of the frequency noise between spectral components of the comb, relative to a fixed optical frequency near the 1.3 micron carrier of the Cr:forsterite laser. The correlated frequency fluctuations are shown to arise from amplitude noise on the pump laser. We also report a preliminary comparison of excess noise that occurs during supercontinuum generation in both highly nonlinear fiber and an extruded glass microstructured fiber.

Experimental simulation of two-particle quantum entanglement using classical fields.

We experimentally demonstrate simulation of two entangled quantum bits using classical fields of two frequencies and two polarizations. Multiplication of optical heterodyne beat signals from two spatially separated regions simulates coincidence detection of two particles. The product signal so obtained contains several frequency components, one of which can be selected by bandpass frequency filtering. The bandpassed signal contains two indistinguishable, interfering contributions, permitting simulation of four polarization-entangled Bell-like states. These classical field methods may be useful in small scale simulations of quantum logic operations that require multiparticle entanglement without collapse.

Side-mode injection locking characteristics of 150 mW AlGaAs semiconductor lasers

A high-power AlGaAs semiconductor laser was side-mode injection locked for laser cooling with frequency 1300 GHz away from free-running states. The injection locking properties of the slave laser were measured with saturated absorption spectrum and optical heterodyne beat technique. The results show that the mode properties of master laser were completely transferred to the slave laser. The dynamics of injection locking with respect to the driven current of the slave laser and injection light power was studied. The relationship of the locking bandwidth versus injection light power is consistent with the theoretical calculation based on the multi-mode rate equation with an injection term.

投影露光装置用ＴＴＬアライメント法 ２光束入射を用いて色収差を解決したＴＴＬアライメント

This paper presents a novel TTL (through the lens) alignment method, called SMART (separated marks TTL alignment), for optical lithography system. This alignment optical system could be accomplished without any complicated chromatic aberration compensation mechanisms. It was clarified that the relative displacement between the reticle and the wafer can be directly detected by measuring the intensity change from detector or the phase difference in the optical heterodyne beat signals. It is also demonstrated that the alignment signal is independent from the wafer focus variation, and can always be detected, even during exposure. An alignment signal stabilization method, in which part of the diffraction lights from the reticle marks are used as reference beat signal, has been adapted to obtain high positioning accuracy. As a result, alignment signal variation of less than ± 12 nm (0.012 μm) has been obtained.

Precise optical heterodyne beat frequency from laser diodes locked to atomic resonances

Semiconductor lasers locked to atomic resonances provide precise optical frequencies with very good long-term stability. This property can be used in coherent optical communications for obtaining a well defined IF frequency (or frequencies) at the receiver. The letter deals with the optical heterodyne detection of a precise 6.54 GHz frequency offset derived from the hyperfine structure of rubidium 87