Real-time Fourier-domain optical vector oscilloscope.

To meet the constant demands of high-capacity telecommunications infrastructure, data rates beyond 1 terabit per second per wavelength channel and optical multiplexing are widely applied. However, these features pose challenges for existing data acquisition and optical performance monitoring techniques because of bandwidth limitation and signal synchronization. We designed an approach that would address these limitations by optically converting the frequency limit to an unlimited time axis and combining this with a chirped coherent detection to innovatively obtain the full-field spectrum. With this approach, we demonstrated a real-time Fourier-domain optical vector oscilloscope, with a 3.4-terahertz bandwidth and a 280-femtosecond temporal resolution over a 520-picosecond record length. In addition to on-off keying and binary phase-shift keying signals (128 gigabits per second), quadrature phase-shift keying wavelength division-multiplexed signals (4 × 160 gigabits per second) are simultaneously observed. Moreover, we successfully demonstrate some high-precision measurements, which indicate them as a promising scientific and industrial tool in high-speed optical communication and ultrafast optical measurement.

Experimental observation of the spectral self-imaging effect with a four-wave mixing time lens.

Here we use a four-wave mixing time lens to demonstrate the spectral self-imaging effect for a frequency comb. The time lens is built by imposing a temporal quadratic phase modulation onto the input signal pulses, which corresponds to a frequency comb in the Fourier spectrum. The modulation is implemented by a Gaussian pump pulse propagating in an external single-mode fiber. Both the signal and pump pulses are injected into a highly nonlinear fiber and four-wave mixing Bragg scattering occurs. We observe periodic revivals of the input frequency comb as the pump pulse propagates periodic distances. The comb-spacing is squeezed at fractional ratios to its original value. Meanwhile, the central-frequency undergoes redshifts and blueshifts subject to the scattered frequencies. We also find that the envelope width of input pulses has an effect on the output spectrum width. The study may find great applications in spectral reshaping and frequency metrology used for optical communication and signal processing.

Large group delay and low loss optical delay line based on chirped waveguide Bragg gratings.

On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23 nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87 ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57 dB/ns and the total insertion loss of the device is 6 dB at the central wavelength near 1550 nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.

Fast and high-resolution spectroscopy based on asynchronous optical sampling.

Dispersive time stretch has made many ultrafast applications possible owing to its high frame rate, as compared to conventional spectroscopies. By further introducing a converging time lens, this spectroscopy can resolve arbitrary emission spectra within the aperture. However, a spectral resolution of tens of picometers hinders its high-precision application. There are two limitations: the temporal aperture of the acquired signal and the actual acquisition bandwidth. To overcome these restrictions, two approaches were developed. First, a large-aperture time lens, with higher-order dispersion compensation, is used to overcome the fundamental limit of the time-bandwidth product. Second, asynchronous optical sampling, based on two frequency combs, overcomes the technical limit of the acquisition bandwidth. As a result, in this study, time-stretch spectroscopy achieved a 1-pm spectral resolution, 24-nm observation bandwidth, and 1-kHz frame rate. Moreover, it was used to observe some spectral dynamics of the random lasing process and devices with narrow spectral widths. This scheme provides essential improvement for time-stretch spectroscopy to achieve high precision.

Time-division-multiplexed observation bandwidth for ultrafast parametric spectro-temporal analyzer.

Parametric spectro-temporal analyzer (PASTA) has been demonstrated as a powerful tool for ultrafast spectrum measurement with superior frame rate and resolution. Compared with other time-stretch-based counterparts, the temporal focusing mechanism enlarges the initial condition and enables the observation of arbitrary waveform, especially the emission spectrum. However, due to the limited conversion bandwidth of the parametric mixing-based time-lens, the observation bandwidth of PASTA is constrained within the C (conventional) band, which hinders its practical applications. To overcome this constraint, both stokes and anti-stokes conversions of the parametric mixing process are leveraged, and the concept of time division multiplexing (TDM) is introduced to ensure their separability. Therefore, the TDM-based PASTA system successfully demultiplexes the C band and L (long) band spectra in two adjacent temporal frames. It is capable of reconstructing the wavelength-to-time sequence for arbitrary waveform over a record 58-nm observation bandwidth, which can be further improved by optimizing the filters and amplifiers. Meanwhile, both of these two bands achieve 20-pm resolution, 10-MHz frame rate, and -30-dBm sensitivity. Moreover, this TDM concept can also be applied to other parametric mixing-based temporal imaging systems to enlarge the working wavelength band, such as temporal magnification.

Ultrafast discrete swept source based on dual chirped combs for microscopic imaging.

An inertial-free, ultrafast frequency comb source based on two chirped optical frequency combs (OFCs) is proposed and experimentally demonstrated. The high linearity frequency sweeping is realized by the Vernier effect between the two OFCs rather than any mechanical motion component, so that good stability and reliability are ensured and no recalibration or resampling process is required. Swept rate up to 1 MHz is realized while keeping a narrow instantaneous linewidth of 0.03 nm, thanks to the extra-cavity frequency sweeping method. The wavelength step is proportional to the swept rate (3.8 pm at 10 kHz), and can be tuned by changing the repetition rate difference between the two OFCs. This swept source is applied for high-speed wavelength encoded imaging and achieves 4.4-µm spatial resolution at a 329-kHz frame rate. Compared with the traditional time-stretch microscopy, the signal acquisition bandwidth decreased from 3.8 GHz to below 90 MHz to achieve the same spatial resolution. Furthermore, the exposure time for a specific wavelength is much longer due to the discrete sweeping feature, which is a benefit for higher sensitivity. This discrete swept source provided a promising low-cost option for high-speed biomedical imaging systems and high-accuracy spectroscopy.