[Paper]Serial time-encoded amplified microscopy

Serial time-encoded amplified microscopy ( STEAM ) ，連續時間編碼顯微鏡，是目前世界上最高速的影像擷取技術，其快門速度可以達到100ps (10的-15次方秒)，以其錄製的影片fps可以達兆赫等級，光學影像的增益則是能夠達到30dB。換句話說，一秒鐘可以拍攝610萬張照片，快門速度為440萬億分之一秒，而在這短時間內，光只前進了不到一公分的距離。

Serial time-encoded amplified imaging/microscopy (STEAM) is a fast real-time optical imaging method that provides MHz frame rate, ~100 ps shutter speed, and ~30 dB (× 1000) optical image gain. 

在時間延遲顯微術中，STEAM是目前世界紀錄上，保有最快快門速度(shutter speed)以及幀率(frame rate)。

An example of time-stretch microscopy, STEAM holds world records for shutter speed and frame rate in continuous real-time imaging. 

STEAM 系統將光子傳遞的時間延遲連同光學影像一同放大，藉此來解決幾乎所有的光學成像系統以及傳感系統中會靈敏度與速度之間的權衡問題。

STEAM employs the photonic time stretch along with optical image amplification to circumvent the fundamental trade-off between sensitivity and speed that affects virtually all optical imaging and sensing systems.

該系統採用單一像素探測器(single-pixel photodetector )，省去了使用陣列式的讀取時間，也使得架構免去使用陣列式探測器。省去了陣列式探測器的讀取時間又透過放大器放大在短時間內能夠接收到的有限光強，STEAM的快門速度比現今最快的CCD、CMOS相機還要快上一千倍。

Avoiding this problem and featuring the optical image amplification for dramatic improvement in sensitivity at high image acquisition rates, STEAM's shutter speed is at least 1000 times faster than the state-of-the-art CCD and CMOS cameras.

此概念最早是由合田圭介教授以及他的同事們，在2007年於美國加州大學洛杉磯分校中的光電實驗室中所提出。幾個月後，該研究團隊( Keisuke Goda, Kevin Tsia, and Bahram Jalali )展示了一維的STEAM系統。

In 2007, the concept was conceived by Keisuke Goda and co-workers at Photonics Laboratory directed by Bahram Jalali in the Electrical Engineering Department at University of California, Los Angeles. A few months later, a team that consists of Keisuke Goda, Kevin Tsia, and Bahram Jalali demonstrated the one-dimensional version.

STEAM 系統能夠將二維的空間圖像轉換成連續的時域波形，反之亦然。一開始，時域的脈衝光經過二維空間分散器後成為二維的光陣列，之後經由觀察物的反射，循原光路再次進入二為空間分散器，將二維影像譯碼成一個寬頻脈衝頻譜( spectrum of a broadband pulse)，之後進入光學循環器(Optical circulator)將從物體反射回來的脈衝光導入一個光學式傅立葉色散放大器(optically amplified dispersive Fourier transformer)中。

The STEAM camera maps a 2D spatial image into a serial time-domain waveform. It starts by encoding the 2D image into the spectrum of a broadband (continuum) pulse (Methods). The optical circulator directs the pulse reflected off the object into an optically amplified dispersive Fourier transformer (inset).

在該放大器中，使用了含鉺(erbium)光纖放大器(EDFA)，將影像先放大10dB倍。隨後影像進入了色散光纖(dispersive fibre)中，再放大15dB倍，同時也將其頻譜映射到時域中。在色散光纖中，利用了分散式的拉曼放大器，而拉曼放大器是由分波多路復用器(wavelength-division multiplexer,WDM)所推動。

 An erbium-doped fibre amplifier (EDFA) pre-amplifies the image by 10 dB.The image then enters a dispersive fibre, where it is further boosted by 15 dB using a distributed Raman amplifier pumped by multiple powerful lasers through wavelength-division multiplexers (WDMs), and its spectrum is simultaneously mapped into the time domain.

此時的光譜被轉換成時域上的一連串訊號，如此一來就能夠使用單像素探測器以及示波器來捕捉二維影像資訊。該方法使該系統不必使用CCD或是CMOS，同時也放大25dB倍的光域圖像。而一連串的放大，使得該系統能夠以即時影像來呈現動態事件。另外，STEAM也能夠做為共焦顯微鏡來使用。

The optical spectrum appears as a serial sequence in time, allowing the image to be captured with a single-pixel photodiode and an oscilloscope.This scheme eliminates the need for a CCD and also performs an image amplification of 25 dB in the optical domain. The amplification enables fast real-time imaging of dynamic events.The STEAM camera can also be used as a confocal microscope .

完整版影片如下：

STEAM系統中的主要硬體設備有Continuum Pulse Laser、2D Spatial Disperser、Optical Circulator、Amplified dispersive Fourier transform、Single-pixel photodiode。

Optical source for the STEAM camera

The optical source used for the STEAM camera was a mode-locked femtosecond laser. A high-nonlinearity fibre was used to generate a supercontinuum with a bandwidth of about 40 nm centred around 1,590 nm. The laser was pulse-picked to reduce the repetition rate to 6.1 MHz using a gated Mach–Zehnder amplitude modulator.

2D spatial disperser

The 2D spatial disperser consisted of a pair of orthogonally oriented spatial dispersers (a virtually imaged phased array (VIPA) and diffraction grating) that produced a 2D spectral pattern which we refer to as a ‘spectral shower’. An input pulse is spatially dispersed and separated by the dispersers into many subpulses of different colours. The VIPA was essentially a tilted Fabry–Pérot cavity with a highly reflective coating (R = 99.9%) on one surface except for an uncoated window area and a partially reflective coating (R ≈ 95%) on the other surface. A collimated laser beam was focused by a cylindrical lens (f = 200 mm) onto the highly reflective surface of the VIPA at a small angle (~3°) with respect to the propagation direction through the uncoated window area. The multiple interference that occurred inside the VIPA resulted in angular dispersion that is over an order of magnitude larger than that of typical diffraction gratings. The VIPA with a free spectral range (FSR) of 67 GHz and a linewidth of 550 MHz dispersed the spectrum of the pulse in one direction. Wavelengths that differed by integer multiples of the FSR were spatially overlapped with each other. An orthogonally placed diffraction grating with a groove density of 1,200 lines per millimetre was used to separate this degeneracy in the other direction. It resulted in the spectrum of the incident pulse being mapped into a 2D spectral pattern in space, resembling a spectral shower.

上圖是一維的空間分散器，而二維的則是在Beam Expander 以及Diffractioin Grating 中再加上virtually imaged phased array(VIPA)，藉由VIPA以及Diffraction Grating，將脈衝光擴展成二維的Spectral Shower。

Amplified dispersive Fourier transform

Before the amplified dispersive Fourier transformation was performed, the spectrum of the back-reflected spectral shower was pre-amplified using an L-band EDFA and filtered using a band-pass filter centred at 1,590 nm with a pass band of 15 nm. The Raman-amplified dispersive element consisted of (1) a DCF with a total group-velocity dispersion of -3.3 ns nm-1, (2) four ~300-mW continuous-wave pump lasers, positioned at regular length intervals along the DCF, with respective wavelengths of 1,470 nm, 1,480 nm, 1,490 nm and 1,480 nm, and (3) wavelength-division multiplexers that coupled the pump lasers into and out of the DCF. These pump wavelengths were chosen to produce a uniform Raman gain profile across the optical filter bandwidth with ~1-dB variation. The amplified 1D temporal data stream was captured by a single-pixel photodetector with a 10-GHz bandwidth (Discovery Semiconductors) and digitized using a high-speed digital oscilloscope with a 16-GHz bandwidth and a sampling rate of 5 × 1010 samples per second (Tektronix).