>Supercontinuum laser applications
>Serial time-coded magnification microscope based on stable picosecond supercontinuum light source
是实现高性能超快串行时间编码放大显微镜(STEAM)的关键因素。 The temporal stability of a broadband light source (such as supercontinuum ) is a key factor in achieving a high-performance ultrafast serial time-coded magnification microscope (STEAM). Because the generation of long-pulse (picosecond to nanosecond) supercontinuum (SC) usually results in an ultra-wideband spectrum with significant pulse-to-pulse fluctuations, so far only ultrashort with better time stability has been used in STEAM Pulsed (femtosecond) SC light source. This article introduces a simple method to achieve active control of the stability of picosecond SC, and helps to extend the applicability of SC in STEAM from femtosecond to picosecond or even nanosecond. We use a continuous wave (CW) triggered picosecond SC light source to experimentally achieve stable single-shot STEAM imaging at a frame rate of 4.9 MHz. This stable SC can greatly reduce lens-to-lens fluctuations, thereby significantly improving STEAM image quality.
Fig. 1 Schematic diagram of STEAM system based on CW stable picosecond SC light source.
Figure 1 (a) is the CW triggering mechanism (bottom) for SC generation. During SC generation, this is achieved by injecting additional weak CW light (~ 50dB weaker than the pump) into the modulation instability (MI) gain spectrum. In this CW triggering method, MI growth can be caused by controllable CW rather than noise, which in turn triggers subsequent soliton fission in a more deterministic manner.
Figure 1 (b) is a space-wavelength mapping module, which uses a diffraction grating (600 lines / mm) and an imaging lens (50 mm focal length) to transmit a spatially dispersed SC beam onto a sample, and then uses the spatial information of the sample to backscatter The SC beam is encoded.
Figure 1 (c) is a wavelength-time mapping module that maps the spectrally encoded image of a sample into a time waveform by enlarging the dispersive Fourier transform (ADFT), which is then captured by a photodetector and digitized by a real-time oscilloscope. ADFT is essentially a wavelength-time mapping process using group velocity dispersion (GVD) and Raman optical amplification in dispersion-compensated fiber (DCF) (GVD: -356ps / nm). Not only does it have the advantage of compensating for the inherent losses associated with GVD, it also provides optical gain to enhance detection sensitivity in STEM.
Figure 2. Spectrum and time characteristics of a CW-triggered SC source.
A strong pumping pulse (pulse width 5.8 ps and peak power 22 W) and a wavelength tunable CW source (: 80 μW) provided by a picosecond mode-locked fiber laser were coupled to a 50 m long highly nonlinear dispersion-shifted fiber (HNL- DSF) (zero dispersion wavelength: 1554nm, dispersion slope: 0.035ps / nm2 / km, nonlinear coefficient: 14W-1km-1).
Figure 2 (a) is a graph of the measured SC spectrum as a function of the CW trigger wavelength. When the weak CW trigger wavelength is tuned to the MI gain sideband (1500-1510nm and 1610-1620nm), the SC spectrum is greatly expanded.
Figure 2 (b) shows the amplitude histogram of 781 filtered SC pulses (1620-1650 nm) obtained by real-time pulse amplitude statistical measurement using a real-time oscilloscope (4 GHz, 20 GS / s). Without triggering, it shows a clear long tail distribution. In contrast, when the CW trigger is added, the SC amplitude statistics are almost Gaussian and the standard deviation is reduced by 50%. This improvement in stability can also be seen in real-time pulse traces in the case of un-triggered and CW-triggered cases (inset of Figure 2 (b)). This CW stable picosecond SC pulse facilitates high-speed single-pulse STEAM imaging.
A STEAM system based on a stable CW triggered SC light source was used to image the test barcode pattern, as shown in Figure 3. The test barcode is printed on a transparent film.
Figure 3 (a) is the comparison between the image-coded time waveform (generated from the wavelength-time mapping, blue top) captured by the oscilloscope and the image-coded spectrum (generated from the space-wavelength mapping, red bottom) measured by the spectrum analyzer. It is clear that the shape of the spectrum is very similar to the shape of the time waveform.
Figure 3 (b) is a time waveform encoded with another test barcode.
In order to further study the influence of the temporal stability of the SC source on the STEAM image quality, the STEAM of the un-triggered SC light source and the CW-triggered SC light source is used to image the USAF-1951 standard resolution target. The STEAM system operates in a single-ray scan mode (along the x direction) at a rate of 4.9 MHz and obtains a two-dimensional (2-D) image by translating the samples in an orthogonal direction. This 1-D (line scan) STEAM configuration is suitable for applications involving microfluidic flow cell imaging.
Figure 4.STEAM image of target discrimination using untriggered SC and CW triggered SC
Figure 4 (a) (c) is a STEAM image of a resolved target (USAF-1951) of a SC light source triggered by CW;
Fig. 4 (b) (d) is a STEAM image of a resolved target using an untriggered SC light source.
Obviously, the STEAM image captured by the CW-triggered SC has better image quality in terms of image noise and image contrast than the image captured by the untriggered SC. This is mainly due to the severe temporal instability of untriggered SCs.
Figure 5 STEAM image of lens cleaning paper: (a) SC light source not triggered, (b) SC light source triggered by CW. (c) is a bright-field microscope image of the same area shown in (a) (b).
A STEAM image of the swab paper is also performed in the article. For the case of STEAM using untriggered SC, the fibrous structure in the lens paper is hardly visible, and the image suffers from severe noise pollution (Figure 5 (a)). In contrast, the STEAM image (Figure 5 (b)) taken by the CW-triggered SC is sharper in contrast.
The stable long-pulse SC (picosecond) source used by STEAM is achieved through a simple active CW triggering scheme. This triggering method can enhance and stabilize the long pulse SC without complicated techniques (such as precise delay tuning, phase lock, and dedicated feedback control). More importantly, this demonstration extends the applicability of SC in STEAM from femtoseconds to picoseconds or even nanoseconds. In addition, the system can use a shorter near-infrared (NIR) wavelength range, where CW-triggered SC can be achieved with highly nonlinear fibers (such as photonic crystal fibers) and off-the-shelf CW laser diodes. This shorter wavelength range can achieve better diffraction-limited resolution, which is more favorable for STEAM's cell imaging. The current stable SC is also suitable for stable, real-time and ultra-fast optical measurements, such as ADFT-based spectroscopy and optical time-spread signal processing.
From <C. Zhang, Y. Qiu, R. Zhu, KKY Wong, and KK Tsia, “Serial time-encoded amplified microscopy (STEAM) based on a stabilized picosecond supercontinuum source,” Opt. Express 19, 15810-15816 (2011 ).>