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Time-stretch analog-to-digital converter

January 01, 1970

The time-stretch analog-to-digital converter (TS-ADC),[1][2][3] also known as the time-stretch enhanced recorder (TiSER), is an analog-to-digital converter (ADC) system that has the capability of digitizing very high bandwidth signals that cannot be captured by conventional electronic ADCs.[4] Alternatively, it is also known as the photonic time-stretch (PTS) digitizer,[5] since it uses an optical frontend. It relies on the process of time-stretch, which effectively slows down the analog signal in time (or compresses its bandwidth) before it can be digitized by a standard electronic ADC.

Background edit

There is a huge demand for very high-speed analog-to-digital converters (ADCs), as they are needed for test and measurement equipment in laboratories and in high speed data communications systems.[citation needed] Most of the ADCs are based purely on electronic circuits, which have limited speeds and add a lot of impairments, limiting the bandwidth of the signals that can be digitized and the achievable signal-to-noise ratio. In the TS-ADC, this limitation is overcome by time-stretching the analog signal, which effectively slows down the signal in time prior to digitization. By doing so, the bandwidth (and carrier frequency) of the signal is compressed. Electronic ADCs that would have been too slow to digitize the original signal can now be used to capture and process this slowed down signal.

Operation principle edit

 
A time-stretch analog-to-digital converter (with a stretch factor of 4) is shown. The original analog signal is time-stretched and segmented with the help of a time-stretch preprocessor (generally on optical frontend). Slowed down segments are captured by conventional electronic ADCs. The digitized samples are rearranged to obtain the digital representation of the original signal.
 
Optical frontend for a time-stretch analog-to-digital converter is shown. The original analog signal is modulated over a chirped optical pulse (obtained by dispersing an ultra-short supercontinuum pulse from a mode-locked laser, MLL). Second dispersive medium stretches the optical pulse further. At the photodetector (PD) output, stretched replica of original signal is obtained.

The time-stretch processor, which is generally an optical frontend, stretches the signal in time. It also divides the signal into multiple segments using a filter, for example, a wavelength-division multiplexing (WDM) filter, to ensure that the stretched replica of the original analog signal segments do not overlap each other in time after stretching. The time-stretched and slowed down signal segments are then converted into digital samples by slow electronic ADCs. Finally, these samples are collected by a digital signal processor (DSP) and rearranged in a manner such that output data is the digital representation of the original analog signal. Any distortion added to the signal by the time-stretch preprocessor is also removed by the DSP.

An optical front-end is commonly used to accomplish this process of time-stretching. An ultrashort optical pulse (typically 100 to 200 femtoseconds long), also called a supercontinuum pulse, which has a broad optical bandwidth, is time-stretched by dispersing it in a highly dispersive medium (such as a dispersion compensating fiber). This process results in (an almost) linear time-to-wavelength mapping in the stretched pulse, because different wavelengths travel at different speeds in the dispersive medium. The obtained pulse is called a chirped pulse as its frequency is changing with time, and it is typically a few nanoseconds long. The analog signal is modulated onto this chirped pulse using an electro-optic intensity modulator. Subsequently, the modulated pulse is stretched further in the second dispersive medium which has much higher dispersion value. Finally, this obtained optical pulse is converted to the electrical domain by a photodetector, giving the stretched replica of the original analog signal.

For continuous operation, a train of supercontinuum pulses is used. The chirped pulses arriving at the electro-optic modulator should be wide enough (in time) such that the trailing edge of one pulse overlaps the leading edge of the next pulse. For segmentation, optical filters separate the signal into multiple wavelength channels at the output of the second dispersive medium. For each channel, a separate photodetector and backend electronic ADC is used. Finally the output of these ADCs are passed on to the DSP which generates the desired digital output.

Impulse response of the photonic time-stretch (PTS) system edit

The PTS processor is based on specialized analog optical (or microwave photonic) fiber links[5] such as those used in cable TV distribution. While the dispersion of fiber is a nuisance in conventional analog optical links, time-stretch technique exploits it to slow down the electrical waveform in the optical domain. In the cable TV link, the light source is a continuous-wave (CW) laser. In PTS, the source is a chirped pulse laser.

 
Capture of a 95 GHz RF tone using the photonic time-stretch digitizer. The signal is captured at an effective sample rate of 10 terasamples per second.

In a conventional analog optical link, dispersion causes the upper and lower modulation sidebands, foptical ± felectrical, to slip in relative phase. At certain frequencies, their beats with the optical carrier interfere destructively, creating nulls in the frequency response of the system. For practical systems the first null is at tens of GHz, which is sufficient for handling most electrical signals of interest. Although it may seem that the dispersion penalty places a fundamental limit on the impulse response (or the bandwidth) of the time-stretch system, it can be eliminated. The dispersion penalty vanishes with single-sideband modulation.[5] Alternatively, one can use the modulator's secondary (inverse) output port to eliminate the dispersion penalty,[5] in much the same way as two antennas can eliminate spatial nulls in wireless communication (hence the two antennas on top of a WiFi access point). This configuration is termed phase-diversity.[6] Combining the complementary outputs using a maximal ratio combining (MRC) algorithm results in a transfer function with a flat response in the frequency domain. Thus, the impulse response (bandwidth) of a time-stretch system is limited only by the bandwidth of the electro-optic modulator, which is about 120 GHz—a value that is adequate for capturing most electrical waveforms of interest.

Extremely large stretch factors can be obtained using long lengths of fiber, but at the cost of larger loss—a problem that has been overcome by employing Raman amplification within the dispersive fiber itself, leading to the world's fastest real-time digitizer.[7] Also, using PTS, capture of very high-frequency signals with a world record resolution in 10-GHz bandwidth range has been achieved.[8]

Comparison with time lens imaging edit

Another technique, temporal imaging using a time lens, can also be used to slow down (mostly optical) signals in time. The time-lens concept relies on the mathematical equivalence between spatial diffraction and temporal dispersion, the so-called space-time duality.[9] A lens held at a distance from an object produces a magnified image of the object. The lens imparts a quadratic phase shift to the spatial frequency components of the optical waves; in conjunction with the free space propagation (object to lens, lens to eye), this generates a magnified image. Owing to the mathematical equivalence between paraxial diffraction and temporal dispersion, an optical waveform can be temporally imaged by a three-step process of dispersing it in time, subjecting it to a phase shift that is quadratic in time (the time lens itself), and dispersing it again. Theoretically, a focused aberration-free image is obtained under a specific condition when the two dispersive elements and the phase shift satisfy the temporal equivalent of the classic lens equation. Alternatively, the time lens can be used without the second dispersive element to transfer the waveform's temporal profile to the spectral domain, analogous to the property that an ordinary lens produces the spatial Fourier transform of an object at its focal points.[10]

In contrast to the time-lens approach, PTS is not based on the space-time duality – there is no lens equation that needs to be satisfied to obtain an error-free slowed-down version of the input waveform. Time-stretch technique also offers continuous-time acquisition performance, a feature needed for mainstream applications of oscilloscopes.

Another important difference between the two techniques is that the time lens requires the input signal to be subjected to high amount of dispersion before further processing. For electrical waveforms, the electronic devices that have the required characteristics: (1) high dispersion to loss ratio, (2) uniform dispersion, and (3) broad bandwidths, do not exist. This renders time lens not suitable for slowing down wideband electrical waveforms. In contrast, PTS does not have such a requirement. It was developed specifically for slowing down electrical waveforms and enable high speed digitizers.

Relation to phase stretch transform edit

The phase stretch transform or PST is a computational approach to signal and image processing. One of its utilities is for feature detection and classification. phase stretch transform is a spin-off from research on the time stretch dispersive Fourier transform. It transforms the image by emulating propagation through a diffractive medium with engineered 3D dispersive property (refractive index).

Application to imaging and spectroscopy edit

In addition to wideband A/D conversion, photonic time-stretch (PTS) is also an enabling technology for high-throughput real-time instrumentation such as imaging[11] and spectroscopy.[12][13] The first artificial intelligence facilitated high-speed phase microscopy is demonstrated to improve the diagnosis accuracy of cancer cells out of blood cells by simultaneous measurement of phase and intensity spatial profiles.[14] The world's fastest optical imaging method called serial time-encoded amplified microscopy (STEAM) makes use of the PTS technology to acquire image using a single-pixel photodetector and commercial ADC. Wavelength-time spectroscopy, which also relies on photonic time-stretch technique, permits real-time single-shot measurements of rapidly evolving or fluctuating spectra.

Time stretch quantitative phase imaging (TS-QPI) is an imaging technique based on time-stretch technology for simultaneous measurement of phase and intensity spatial profiles. In time stretched imaging, the object's spatial information is encoded in the spectrum of laser pulses within a pulse duration of sub-nanoseconds. Each pulse representing one frame of the camera is then stretched in time so that it can be digitized in real-time by an electronic analog-to-digital converter (ADC). The ultra-fast pulse illumination freezes the motion of high-speed cells or particles in flow to achieve blur-free imaging.[15][16]

References edit

  1. ^ A. S. Bhushan, F. Coppinger, and B. Jalali, "Time-stretched analogue-to-digital conversion," Electronics Letters vol. 34, no. 9, pp. 839–841, April 1998. [1]
  2. ^ A. Fard, S. Gupta, and B. Jalali, "Photonic time-stretch digitizer and its extension to real-time spectroscopy and imaging," Laser & Photonics Reviews vol. 7, no. 2, pp. 207-263, March 2013. [2]
  3. ^ Y. Han and B. Jalali, "Photonic Time-Stretched Analog-to-Digital Converter: Fundamental Concepts and Practical Considerations," Journal of Lightwave Technology, Vol. 21, Issue 12, pp. 3085–3103, Dec. 2003. [3]
  4. ^ Mahjoubfar, Ata; Churkin, Dmitry V.; Barland, Stéphane; Broderick, Neil; Turitsyn, Sergei K.; Jalali, Bahram (June 2017). "Time stretch and its applications". Nature Photonics. 11 (6): 341–351. Bibcode:2017NaPho..11..341M. doi:10.1038/nphoton.2017.76. ISSN 1749-4885. S2CID 53511029.
  5. ^ a b c d J. Capmany and D. Novak, "Microwave photonics combines two worlds," Nature Photonics 1, 319-330 (2007). [4]
  6. ^ Yan Han, Ozdal Boyraz, Bahram Jalali, "Ultrawide-Band Photonic Time-Stretch A/D Converter Employing Phase Diversity," "IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES" VOL. 53, NO. 4, APRIL 2005 [5]
  7. ^ J. Chou, O. Boyraz, D. Solli, and B. Jalali, "Femtosecond real-time single-shot digitizer," Applied Physics Letters 91, 161105 (2007). [6]
  8. ^ S. Gupta and B. Jalali, "Time-warp correction and calibration in photonic time-stretch analog-to-digital converter," Optics Letters 33, 2674–2676 (2008). [7]
  9. ^ B. H. Kolner and M. Nazarathy, "Temporal imaging with a time lens," Optics Letters 14, 630-632 (1989) [8]
  10. ^ J. W. Goodman, "Introduction to Fourier Optics," McGraw-Hill (1968).
  11. ^ K. Goda, K.K. Tsia, and B. Jalali, "Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena," Nature 458, 1145–1149, 2009. [9]
  12. ^ D. R. Solli, J. Chou, and B. Jalali, "Amplified wavelength–time transformation for real-time spectroscopy," Nature Photonics 2, 48-51, 2008. [10]
  13. ^ J. Chou, D. Solli, and B. Jalali, "Real-time spectroscopy with subgigahertz resolution using amplified dispersive Fourier transformation," Applied Physics Letters 92, 111102, 2008. [11]
  14. ^ C. Chen, A. Mahjoubfar, & B. Jalali, "Deep Learning in Label-free Cell Classification," Scientific Reports 6, 21471 (2016) doi:10.1038/srep21471. [12]
  15. ^ Chen, Claire Lifan; Mahjoubfar, Ata; Tai, Li-Chia; Blaby, Ian K.; Huang, Allen; Niazi, Kayvan Reza; Jalali, Bahram (2016). "Deep Learning in Label-free Cell Classification". Scientific Reports. 6: 21471. Bibcode:2016NatSR...621471C. doi:10.1038/srep21471. PMC 4791545. PMID 26975219.published under CC BY 4.0 licensing
  16. ^ Michaud, Sarah (5 April 2016). "Leveraging Big Data for Cell Imaging". Optics & Photonics News. Full text download available: The Optical Society. Retrieved 8 July 2016.

Further reading edit

  • G. C. Valley, "Photonic analog-to-digital converters," Opt. Express, vol. 15, no. 5, pp. 1955–1982, March 2007. [13]
  • Photonic Bandwidth Compression for Instantaneous Wideband A/D Conversion (PHOBIAC) project.
  • Short time Fourier transform for time-frequency analysis of ultrawideband signals

January 01, 1970
time, stretch, analog, digital, converter, time, stretch, analog, digital, converter, also, known, time, stretch, enhanced, recorder, tiser, analog, digital, converter, system, that, capability, digitizing, very, high, bandwidth, signals, that, cannot, capture. The time stretch analog to digital converter TS ADC 1 2 3 also known as the time stretch enhanced recorder TiSER is an analog to digital converter ADC system that has the capability of digitizing very high bandwidth signals that cannot be captured by conventional electronic ADCs 4 Alternatively it is also known as the photonic time stretch PTS digitizer 5 since it uses an optical frontend It relies on the process of time stretch which effectively slows down the analog signal in time or compresses its bandwidth before it can be digitized by a standard electronic ADC Contents 1 Background 2 Operation principle 3 Impulse response of the photonic time stretch PTS system 4 Comparison with time lens imaging 5 Relation to phase stretch transform 6 Application to imaging and spectroscopy 7 References 8 Further readingBackground editThere is a huge demand for very high speed analog to digital converters ADCs as they are needed for test and measurement equipment in laboratories and in high speed data communications systems citation needed Most of the ADCs are based purely on electronic circuits which have limited speeds and add a lot of impairments limiting the bandwidth of the signals that can be digitized and the achievable signal to noise ratio In the TS ADC this limitation is overcome by time stretching the analog signal which effectively slows down the signal in time prior to digitization By doing so the bandwidth and carrier frequency of the signal is compressed Electronic ADCs that would have been too slow to digitize the original signal can now be used to capture and process this slowed down signal Operation principle edit nbsp A time stretch analog to digital converter with a stretch factor of 4 is shown The original analog signal is time stretched and segmented with the help of a time stretch preprocessor generally on optical frontend Slowed down segments are captured by conventional electronic ADCs The digitized samples are rearranged to obtain the digital representation of the original signal nbsp Optical frontend for a time stretch analog to digital converter is shown The original analog signal is modulated over a chirped optical pulse obtained by dispersing an ultra short supercontinuum pulse from a mode locked laser MLL Second dispersive medium stretches the optical pulse further At the photodetector PD output stretched replica of original signal is obtained The time stretch processor which is generally an optical frontend stretches the signal in time It also divides the signal into multiple segments using a filter for example a wavelength division multiplexing WDM filter to ensure that the stretched replica of the original analog signal segments do not overlap each other in time after stretching The time stretched and slowed down signal segments are then converted into digital samples by slow electronic ADCs Finally these samples are collected by a digital signal processor DSP and rearranged in a manner such that output data is the digital representation of the original analog signal Any distortion added to the signal by the time stretch preprocessor is also removed by the DSP An optical front end is commonly used to accomplish this process of time stretching An ultrashort optical pulse typically 100 to 200 femtoseconds long also called a supercontinuum pulse which has a broad optical bandwidth is time stretched by dispersing it in a highly dispersive medium such as a dispersion compensating fiber This process results in an almost linear time to wavelength mapping in the stretched pulse because different wavelengths travel at different speeds in the dispersive medium The obtained pulse is called a chirped pulse as its frequency is changing with time and it is typically a few nanoseconds long The analog signal is modulated onto this chirped pulse using an electro optic intensity modulator Subsequently the modulated pulse is stretched further in the second dispersive medium which has much higher dispersion value Finally this obtained optical pulse is converted to the electrical domain by a photodetector giving the stretched replica of the original analog signal For continuous operation a train of supercontinuum pulses is used The chirped pulses arriving at the electro optic modulator should be wide enough in time such that the trailing edge of one pulse overlaps the leading edge of the next pulse For segmentation optical filters separate the signal into multiple wavelength channels at the output of the second dispersive medium For each channel a separate photodetector and backend electronic ADC is used Finally the output of these ADCs are passed on to the DSP which generates the desired digital output Impulse response of the photonic time stretch PTS system editThe PTS processor is based on specialized analog optical or microwave photonic fiber links 5 such as those used in cable TV distribution While the dispersion of fiber is a nuisance in conventional analog optical links time stretch technique exploits it to slow down the electrical waveform in the optical domain In the cable TV link the light source is a continuous wave CW laser In PTS the source is a chirped pulse laser nbsp Capture of a 95 GHz RF tone using the photonic time stretch digitizer The signal is captured at an effective sample rate of 10 terasamples per second In a conventional analog optical link dispersion causes the upper and lower modulation sidebands foptical felectrical to slip in relative phase At certain frequencies their beats with the optical carrier interfere destructively creating nulls in the frequency response of the system For practical systems the first null is at tens of GHz which is sufficient for handling most electrical signals of interest Although it may seem that the dispersion penalty places a fundamental limit on the impulse response or the bandwidth of the time stretch system it can be eliminated The dispersion penalty vanishes with single sideband modulation 5 Alternatively one can use the modulator s secondary inverse output port to eliminate the dispersion penalty 5 in much the same way as two antennas can eliminate spatial nulls in wireless communication hence the two antennas on top of a WiFi access point This configuration is termed phase diversity 6 Combining the complementary outputs using a maximal ratio combining MRC algorithm results in a transfer function with a flat response in the frequency domain Thus the impulse response bandwidth of a time stretch system is limited only by the bandwidth of the electro optic modulator which is about 120 GHz a value that is adequate for capturing most electrical waveforms of interest Extremely large stretch factors can be obtained using long lengths of fiber but at the cost of larger loss a problem that has been overcome by employing Raman amplification within the dispersive fiber itself leading to the world s fastest real time digitizer 7 Also using PTS capture of very high frequency signals with a world record resolution in 10 GHz bandwidth range has been achieved 8 Comparison with time lens imaging editAnother technique temporal imaging using a time lens can also be used to slow down mostly optical signals in time The time lens concept relies on the mathematical equivalence between spatial diffraction and temporal dispersion the so called space time duality 9 A lens held at a distance from an object produces a magnified image of the object The lens imparts a quadratic phase shift to the spatial frequency components of the optical waves in conjunction with the free space propagation object to lens lens to eye this generates a magnified image Owing to the mathematical equivalence between paraxial diffraction and temporal dispersion an optical waveform can be temporally imaged by a three step process of dispersing it in time subjecting it to a phase shift that is quadratic in time the time lens itself and dispersing it again Theoretically a focused aberration free image is obtained under a specific condition when the two dispersive elements and the phase shift satisfy the temporal equivalent of the classic lens equation Alternatively the time lens can be used without the second dispersive element to transfer the waveform s temporal profile to the spectral domain analogous to the property that an ordinary lens produces the spatial Fourier transform of an object at its focal points 10 In contrast to the time lens approach PTS is not based on the space time duality there is no lens equation that needs to be satisfied to obtain an error free slowed down version of the input waveform Time stretch technique also offers continuous time acquisition performance a feature needed for mainstream applications of oscilloscopes Another important difference between the two techniques is that the time lens requires the input signal to be subjected to high amount of dispersion before further processing For electrical waveforms the electronic devices that have the required characteristics 1 high dispersion to loss ratio 2 uniform dispersion and 3 broad bandwidths do not exist This renders time lens not suitable for slowing down wideband electrical waveforms In contrast PTS does not have such a requirement It was developed specifically for slowing down electrical waveforms and enable high speed digitizers Relation to phase stretch transform editThe phase stretch transform or PST is a computational approach to signal and image processing One of its utilities is for feature detection and classification phase stretch transform is a spin off from research on the time stretch dispersive Fourier transform It transforms the image by emulating propagation through a diffractive medium with engineered 3D dispersive property refractive index Application to imaging and spectroscopy editIn addition to wideband A D conversion photonic time stretch PTS is also an enabling technology for high throughput real time instrumentation such as imaging 11 and spectroscopy 12 13 The first artificial intelligence facilitated high speed phase microscopy is demonstrated to improve the diagnosis accuracy of cancer cells out of blood cells by simultaneous measurement of phase and intensity spatial profiles 14 The world s fastest optical imaging method called serial time encoded amplified microscopy STEAM makes use of the PTS technology to acquire image using a single pixel photodetector and commercial ADC Wavelength time spectroscopy which also relies on photonic time stretch technique permits real time single shot measurements of rapidly evolving or fluctuating spectra Time stretch quantitative phase imaging TS QPI is an imaging technique based on time stretch technology for simultaneous measurement of phase and intensity spatial profiles In time stretched imaging the object s spatial information is encoded in the spectrum of laser pulses within a pulse duration of sub nanoseconds Each pulse representing one frame of the camera is then stretched in time so that it can be digitized in real time by an electronic analog to digital converter ADC The ultra fast pulse illumination freezes the motion of high speed cells or particles in flow to achieve blur free imaging 15 16 References edit A S Bhushan F Coppinger and B Jalali Time stretched analogue to digital conversion Electronics Letters vol 34 no 9 pp 839 841 April 1998 1 A Fard S Gupta and B Jalali Photonic time stretch digitizer and its extension to real time spectroscopy and imaging Laser amp Photonics Reviews vol 7 no 2 pp 207 263 March 2013 2 Y Han and B Jalali Photonic Time Stretched Analog to Digital Converter Fundamental Concepts and Practical Considerations Journal of Lightwave Technology Vol 21 Issue 12 pp 3085 3103 Dec 2003 3 Mahjoubfar Ata Churkin Dmitry V Barland Stephane Broderick Neil Turitsyn Sergei K Jalali Bahram June 2017 Time stretch and its applications Nature Photonics 11 6 341 351 Bibcode 2017NaPho 11 341M doi 10 1038 nphoton 2017 76 ISSN 1749 4885 S2CID 53511029 a b c d J Capmany and D Novak Microwave photonics combines two worlds Nature Photonics 1 319 330 2007 4 Yan Han Ozdal Boyraz Bahram Jalali Ultrawide Band Photonic Time Stretch A D Converter Employing Phase Diversity IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES VOL 53 NO 4 APRIL 2005 5 J Chou O Boyraz D Solli and B Jalali Femtosecond real time single shot digitizer Applied Physics Letters 91 161105 2007 6 S Gupta and B Jalali Time warp correction and calibration in photonic time stretch analog to digital converter Optics Letters 33 2674 2676 2008 7 B H Kolner and M Nazarathy Temporal imaging with a time lens Optics Letters 14 630 632 1989 8 J W Goodman Introduction to Fourier Optics McGraw Hill 1968 K Goda K K Tsia and B Jalali Serial time encoded amplified imaging for real time observation of fast dynamic phenomena Nature 458 1145 1149 2009 9 D R Solli J Chou and B Jalali Amplified wavelength time transformation for real time spectroscopy Nature Photonics 2 48 51 2008 10 J Chou D Solli and B Jalali Real time spectroscopy with subgigahertz resolution using amplified dispersive Fourier transformation Applied Physics Letters 92 111102 2008 11 C Chen A Mahjoubfar amp B Jalali Deep Learning in Label free Cell Classification Scientific Reports 6 21471 2016 doi 10 1038 srep21471 12 Chen Claire Lifan Mahjoubfar Ata Tai Li Chia Blaby Ian K Huang Allen Niazi Kayvan Reza Jalali Bahram 2016 Deep Learning in Label free Cell Classification Scientific Reports 6 21471 Bibcode 2016NatSR 621471C doi 10 1038 srep21471 PMC 4791545 PMID 26975219 published under CC BY 4 0 licensing Michaud Sarah 5 April 2016 Leveraging Big Data for Cell Imaging Optics amp Photonics News Full text download available The Optical Society Retrieved 8 July 2016 Further reading editG C Valley Photonic analog to digital converters Opt Express vol 15 no 5 pp 1955 1982 March 2007 13 Photonic Bandwidth Compression for Instantaneous Wideband A D Conversion PHOBIAC project 14 Short time Fourier transform for time frequency analysis of ultrawideband signals Retrieved from https en wikipedia org w index php title Time stretch analog to digital converter amp oldid 1186548831, wikipedia, wiki, book, books, library,

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