A 2-Ghz Discrete-Spectrum Waveband-Division Microscopic Imaging System

Optics Communications 338 (2015) 22–26

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Optics Communications

journal homepage: www.elsevier.com/locate/optcom

A 2-GHz discrete-spectrum waveband-division microscopic imaging system

Fangjian Xing, Hongwei Chen n, Cheng Lei, Minghua Chen, Sigang Yang, Shizhong Xie

Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, P.R. China article info abstract

Article history: Limited by dispersion-induced pulse overlap, the frame rate of serial time-encoded amplified microscopy Received 26 June 2014 is confined to the megahertz range. Replacing the ultra-short mode-locked pulse laser by a multi-wa- Received in revised form velength source, based on waveband-division technique, a serial time stretch microscopic imaging sys- 1 September 2014 tem with a line scan rate of in the gigahertz range is proposed and experimentally demonstrated. In this Accepted 4 September 2014 study, we present a surface scanning imaging system with a record line scan rate of 2 GHz and 15 pixels. Available online 22 October 2014 Using a rectangular spectrum and a sufficiently large wavelength spacing for waveband-division, the Keywords: resulting 2D image is achieved with good quality. Such a superfast imaging system increases the single- Real-time imaging shot temporal resolution towards the sub-nanosecond regime. Ultrafast & 2014 Elsevier B.V. All rights reserved. Discrete spectrum Wavelength-to-space-to-time mapping GHz imaging

1. Introduction elements with a broadband mode-locked laser to achieve ultrafast single pixel imaging. An important feature of STEAM is the spec- Most of the current research in optical microscopic imaging trum of the broadband laser source. The spectral characteristics of aims to overcome the diffraction limit of spatial resolution; how- the mode-locked ultra-short pulse laser are not well controlled, ever, there are numerous applications that demand high temporal including its spectral bandwidth and shape [14–18]. Moreover, resolution (i.e., frame rate). The pursuit of high temporal resolu- limited by temporal dispersion, increasing the repetition rate of tion in an optical imaging system without sacrificing detection the broadband mode-locked pulse laser source is not available, sensitivity is considerably restricted by the charge-coupled device which makes adjacent pulses overlap, e.g., an image of good (CCD) or complementary metal oxide semiconductor (CMOS) quality cannot be obtained due to the small spectral bandwidth imaging techniques [1], especially in those applications requiring [19]. As a result, the frame rate of STEAM is confined to the temporal resolution of less than 1 μs, such as flow cytometry, Megahertz range, which impedes investigations of the phenomena microfluidics and surface detection [2–5]. In scientific research, an in the sub-nanosecond time range. In photonic time stretching, a ultrafast single-shot diffraction imaging system using X-rays was continuous-time large-bandwidth time-stretched signal is seg- proposed, but with the CCD acting as the detection device, which mented and interleaved into multiple parallel channels using vir- can only capture repetitive events [6]. In addition, a streak camera tual time gating (VTG), which ensures via wavelength division synchronized to a pulsed laser can be used to measure the time- multiplexing that no temporal overlap occurs for all of the chan- fl of- ight of a light signal between the camera and the subject for nels [20,21]. each point of the image and further obtain 2D images [7]. How- In this paper, we propose a waveband-division imaging system ever, this technique is only suitable for capturing an event that can based on a multi-wavelength source, which creates a record 2 GHz be recreated exactly the same way multiple times. Recently, serial line scan rate. Unlike the mode-locked ultra-short pulse source, fi time-encoded ampli ed microscopy (STEAM) was demonstrated the advantage of waveband-division with a multi-wavelength as a novel optical imaging technique that can achieve a high frame source is that there is no loss for each discrete wavelength. Using – rate ( 10 MHz) in real time [8 13]. STEAM is an optical method wavelength division multiplexing, all of the wavelengths can be that uses a combination of spatially and temporally dispersive carved into two-channels to avoid pulse overlap induced by dispersion. Sufficient dispersion is a key element for wavelength-to- n Corresponding author. time mapping. Compared to the Gaussian shape of the mode- E-mail address: [email protected] (H. Chen). locked laser, the spectral shape of our multi-wavelength source http://dx.doi.org/10.1016/j.optcom.2014.09.077 0030-4018/& 2014 Elsevier B.V. All rights reserved. F. Xing et al. / Optics Communications 338 (2015) 22–26 23 can be designed to be rectangular, which is beneficial to achieve a intensity of the infinite set of field contributions corresponding to higher-quality image. Additionally, the wavelength spacing and each optical frequency of the light source. In the particular case the number of wavelengths can be appropriately chosen. With this when m(t) is short compared to the variations of the spectral novel method, we achieved a surface scanning imaging system at a density, one obtains [22] record scan rate of 2 GHz with 15 discrete wavelengths. As far as ·· ·· ·· we know, this is the first time that the temporal resolution of a Itout()∝ StRt()() /ΦΦΦ / / , (4) real-time line scan imaging system reaches 500 ps based on the Eq. (4) links the average temporal intensity of the waveform waveband-division technique. after gating and chromatic dispersion to the spectral density of the incoherent process via frequency-to-time mapping. In such an imaging system, the time parameters are determined by the fol- 2. Working principle lowing equations:

The principle of the proposed scheme is illustrated in Fig. 1. The Δ≈ΔτλDT0 = N Δ τ complex electric ﬁeld of a multi-wavelength source at each optical where Δτ is the pulse width, Δλ is the wavelength spacing of the frequency ω can be described as follows [22]: i multi-wavelength source, D is the dispersion value, N is the ⎡ ⎤ ⎣ ⎦ number of wavelengths and T0 is the repetition period of the pulse Et0(,)ωωϕωii=−− S ()exp ji ()exp(ii jwt ), (1) train. In our scheme, the relationship between the time-stretched where S(ωi) is the energy density spectrum of the laser source and pulse duration T and the pulse train period T0 complies with the

ϕωi()i is the frequency uncorrelated spectral phase. Multi-wave- following: length continuous waves (CW) sources are combined and feed into TT00<<2 T an amplitude modulator driven by a pulse train. A certain amplitude signal m(t) is used for on-off control at all the wavelengths This inequality indicates that the adjacent pulses after time synchronously. The electric field after modulation can be ex- stretching exhibit temporal overlap. This condition of the time- pressed as follows: stretched pulse width being less than the pulse period is different ⎡ ⎤ than the condition of our previously reported paper [13]. Here, the Et(ωω , )=−− Smtj ( ) ( )exp⎣ ϕω ( )⎦ exp( jwt ). 1 ii i ii (2) time-domain overlapping part (encoded with the image) cannot The modulated multi-wavelength pulses enter into the free be detected directly to recover the real image. Two filters are used space optical link. In this part, the information of the targets is to carve every image-encoded pulse into two different wavebands mapped onto the discrete spectrum of each of the multi-wave- (as shown in Fig. 1). The number of wavelengths filtered from each length pulses. This mapping is essentially a process of space-to- channel is N/2. Next, the temporal width of the filtered pulse in wavelength mapping. Next, the image-encoded pulses are broa- each channel is less than T0, which indicates that there is no dened due to chromatic dispersion. As mentioned in Ref. [23] the temporal overlap in a single channel. Finally, the signal from both final average time domain intensity waveform can be written as channels is detected using high-speed photodetector and is digi- follows: tized using a high-speed oscilloscope.

⎡ ·· ·· ⎤ 2 It()∝⊗⎢ StRt /ΦΦ (/⎥ mt () , out ⎣ () )⎦ (3) 3. Experimental setup and results where R(ωi) is the reﬂective intensity from the samples corre- ·· 2 2 sponding to each frequency ωi,GVD≡Φ ¼−∂Φω/ ∂ (Φ is the Fig. 2 shows the experimental setup. A distributed feedback spectral phase transfer function of the dispersive medium along (DFB) laser array is used as the optical source, which contains 15 ·· the entire optical bandwidth), t≡Φω, ⊗ denotes a convolution and discrete wavelengths. An array waveguide grating is used as a the symbol ∝ denotes proportionality. According to Eq. (3), the wavelength multiplexer to combine all of the CW laser beams. average optical intensity can be simply calculated as the sum in These beams are fed into an amplitude modulator driven by an

Fig. 1. Principle of wavelength division time-stretch imaging system with a line scan rate of gigahertz. CW: continuous wave, WC: wavelength combiner, AM: amplitude modulator, CD: chromatic dispersion, PD: photodetector. 24 F. Xing et al. / Optics Communications 338 (2015) 22–26

Fig. 2. Experimental setup. DFB: distributed feedback, AWG: array waveguide grating, MZM: Mach-Zehnder modulator, EDFA: Erbium doped fiber amplifier, SMF: single mode fiber. arbitrary waveform generator to produce a pulse train at a repetition rate of 2 GHz with the pulse width of 70 ps (as shown in Fig. 3(a)). Every pulse carries 15 discrete wavelengths over the range from 1549.32 nm to 1560.52 nm at a wavelength spacing of 0.8 nm (as shown in Fig. 3(b)). Next, the modulated pulse train enters the free space optical link, which performs the wavelength- to-space mapping. In this part, the collimated laser pulses are incident onto a spatial diffraction grating (1200 lines/mm) that dis- perses the spectrum in one-dimensional (1D). Subsequently, a singlet spherical lens (focal length of 100 mm) is used to focus the dispersed spectrum into a line focus. The 1D rainbow line is ver- tically incident onto the sample placed at focal plane of the lens. The focused rainbow line is measured using a beam profiler (Thorlabs: BP209IR/M), as shown in Fig. 4. The size of the beam is 3000 μm 250 μm and it contains 15 bright spots corresponding to the 15 discrete wavelengths. As shown in Fig. 4, each bright spot represents one wavelength and indicates an image pixel. The distance between adjacent spots is approximately 210 μm, which represents the lateral spatial resolution of this imaging system. Fig. 4. Intensity distribution of the optical rainbow spots in the scan direction at the focal plane of the lens. The colorful spots show the beam profile measured by The rainbow pulses encoded with the information of the sample beam profiler. The inset shows the single wavelength spot size and its intensity are reflected back and directed via an optical circulator towards a distribution. 5-km-long single mode fiber (SMF). From the time stretching with dispersion of 85 ps/nm, the walk-off of approximately 68 ps be- filters have an intermediate spectral range between transmission tween adjacent wavelengths occurs, which is similar to the and reflection spectra, which will result in the loss in this range. modulated pulse duration of 70 ps. This step represents the wa- Here the intermediate range of both filters is less than 10 GHz, velength-to-time mapping. Here, the modulated pulse with a which is far less than 100 GHz (discrete wavelength spacing). The 12 nm bandwidth is linearly chirped, resulting in a pulse width of spectral characteristic of this multi-wavelength source is not af- 950 ps, which is wider than the repetition period (500 ps) of the fected by filtering and is superior to the use of a mode-locked laser modulated pulse train. The temporal overlapped pulse train is source, which is shown in Fig. 5. The discrete wavelength spacing divided into two channels using a 3-dB coupler. The designed pass is far beyond the intermediate range of the filter and an appro- bandwidth of filter 1 is from 1549.2 nm to 1555.2 nm, and that of filter 2 is from 1555.2 nm to 1561.2 nm. It should be clear that the priately decreasing wavelength spacing is available. As shown in

Fig. 3. (a) Pulse train pattern modulated from an arbitrary waveform generator, with a pulse width of 70 ps and a period of 500 ps and (b) optical spectrum of the multi- wavelength source; the wavelength spacing is 0.8 nm. F. Xing et al. / Optics Communications 338 (2015) 22–26 25

Fig. 5. (a) The spectrum output from filter 1. (b) The spectrum output from filter 2. The rectangular shapes indicate the design of the filters.

Fig. 4 (inset), the 3-dB width of every bright spot is only 30 μm due to the diffraction limit, which is far less than the distance between adjacent spots (210 μm). These two conditions indicate that the number of wavelengths can be increased to obtain more pixels. For channel 1, the number of wavelengths passed from filter 1 is 8 (from 1549.32 nm to 1554.92 nm) and the pulse duration is less than 500 ps. The rest of wavelengths passing through filter 2 carry the other half of the sample information, and the pulse duration is also less than the repetition period of the pulse train. Both channels still operate at the 2 GHz scan rate but with half of the spectral bandwidth of the multi-wavelength source. At the receiver, the signal from both channels has a power of 10 dBm, which is detected using high-speed photodetectors with the detection bandwidth of 10 GHz and digitized using a high-speed oscilloscope at the sampling rate of 40 Gs/s. By moving the sample using a stepper motor in the vertical direction, the 1D spectral Fig. 7. Reconstructed 2D images of USAF-1951 with Group 0 element 4. shower can be used to interrogate and line-scan the sample to acquire the two-dimensional (2D) image. In this instance, the sample used in experiment is standard USAF1951 resolution test Unlike conventional optical microscopes, the imaging resolu- target that consists of transparent and opaque regions. Finally, the tion of this system depends not only on the diffraction-limited entire image is acquired by means of the combination of the both- optical link, but also on two other factors: the spectral resolution channels' signal fragments. of the time-stretch defined by the group velocity dispersion (GVD) To demonstrate this superfast waveband-division microscopic of the fiber and the temporal resolution of the optical detection imaging system performance, we performed a test by imaging the system (i.e., photodetector and electronic digitizer). All three USAF-1951 standard resolution target within the scope of group limitations are also present in STEAM, [24,25] though there is 0 element 4. The line-width of 0–4 is 355 μm. The present system some difference in our system due to the use of discrete wave- operates in the single-shot line scan mode at the rate of 2 GHz (as lengths. The optimal spectral resolution is limited by the discrete shown in Fig. 6). With the translation stage moved, the single-shot wavelength spacing. In our experiment, the first limitation is de- line scan waveform is reconstructed to be the 2D image (as shown fined by the grating, lens and wavelength spacing to be 210 μm (as in Fig. 7). The vertical distance is read out from stepper motor and shown in Fig. 4). For the second limitation, the walk-off between the space of every step is 100 μm. The dashed line in Fig. 7 in- adjacent wavelengths is equivalent to the modulated pulse dura- dicates the boundary of both channels. The section of the image tion. Every pixel (equivalently, the discrete wavelength) can be above dashed line is detected from channel 1 and the other part is distinguished after the time stretch. Unlike STEAM, the dispersion from channel 2. Note that the absence of any channel causes the amount of 85 ps/nm is sufficient for the wavelength spacing of image to no longer be complete. 0.8 nm and each single frequency can be distinguished from the

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