We construct highly repetitive low-coherence interferometer using time-stretch technique and confirm its basic characteristics. The experimental system consists of a mode-locked laser diode (MLLD), a time-stretcher, an optical interferometer, a photodiode (PD), and a real-time oscilloscope. The ultra-short pulse from MLLD was passed through a dispersion flat fiber to generate supercontinuum light with 23.5 nm wavelength bandwidth. Repetition frequency of the laser pulse is set to be 10 MHz by the LiNbO3 modulator, and then it feeds to a time-stretcher composed of a dispersion compensation fiber (DCF) with a wavelength dispersion of 8959 ps2. The pulse width after passing through the timestretcher is 28.2 ns. The fiber-optic Michelson interferometer consists of a 50:50 optical fiber coupler, two collimating lenses, an objective lens and two reflective mirrors. Interference signals are detected by a photodiode (32 GHz) and recorded by a real-time oscilloscope (16 GHz, 50 GS/s). The temporal profile of the recorded interference signal is converted to an optical frequency profile. The optical path length difference is determined by Fourier transform of the spectrum. We demonstrate a preliminary measurement on the experimental system. The calculated path length difference agrees well with the actual set values. It is confirmed that the optical path length difference can be measured at a high repetition rate of 10 MHz. It is shown that the degradation of the interference signal can be prevented by considering the second order of the group delay of the DCF.
Pulse-by-pulse single-shot spectrum measurement can be realized using the time stretch dispersive Fourier Transform technique. In this paper, we report a fully integrated and modular pulse-by-pulse single-shot optical spectrum analyzer (PS-OSA) system. Many modules including a picosecond gain-switched laser, multiple fibers with different chromatic dispersions, high-power optical amplifiers, a clock-generator, a photodetector and a digitizer are integrated into a main-frame. These modules are standard in size and can be replaced to improve the performance or changed to adapt the system to different applications. Here, we present the specification of this single-platform PS-OSA and successfully demonstrate high throughput measurement of the transmission spectrum of electroabsorption optical modulators with 11.7 ns refresh time.
The time stretch dispersive Fourier Transform (TS-DFT) technique based on a fiber chromatic dispersion is a powerful tool for pulse-by-pulse single-shot spectrum measurement for highrepetition rate optical pulses. The distributed feedback laser diode (DFB-LD) with the gain switch operation can flexibly change the pulse repetition frequency (PRF). In this paper, we newly introduce a semiconductor gain-switched DFB-LD operating from 1 MHz up to 1 GHz PRF into the TS-DFT based spectrum measurement system to improve the flexibility and the operability. The pulse width can be below 2 ps with a pulse compression technique. We successfully measure the spectrum of each optical pulse at 1 GHz, 100 MHz, and 10 MHz PRF, and demonstrate the flexibility of the measurement system.
Single-shot and long record length spectrum measurements of high-repetition-rate optical pulses are essential for research on nonlinear dynamics as well as for applications in sensing and communication. To achieve a continuous measurements we employ the Time Stretch Dispersive Fourier Transform. We show single-shot measurements of millions of sequential pulses at high repetition rate of 1 Giga spectra per second. Results were obtained using -100 ps/nm dispersive Fourier transform module and a 50 Gsample/s real-time digitizer of 16 GHz bandwidth. Single-shot spectroscopy of 1 GHz optical pulse train was achieved with the wavelength resolution of approximately 150 pm. This instrument is ideal for observation of complex nonlinear dynamics such as switching, mode locking and soliton dynamics in high repetition rate lasers.
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