# How is an digital coherent receiver able to achieve 100 Mhz line-scan rate for detecting coherent light in the presence of incoherent light?

Recently, I read an October 2006 IEEE JOURNAL OF LIGHTWAVE TECHNOLOGY, article titled, High-Sensitivity Detection of Narrowband Light in a More Intense Broadband Background Using Coherence Interferogram Phase written by the esteemed Ricardo C. Coutinho, Member, IEEE, David R. Selviah, Member, IEEE, and Hugh D. Griffiths, Fellow, IEEE from highly respected University College of London.

In this excellent article which is available publicly, it describes that the frame rate of the scan is 150 microns per seconds in steps of 0.1 micron which is equivalent to 1500 seconds time duration for detecting the presence of a coherent laser light in an incoherent light background with averaging

Page 3656 "A 10-μm scan length, corresponding to a 20-μm path difference range, was chosen, and this was scanned at a speed of 150 μm/s in steps of 0.1 μm."

Then, tonight I read in the July 25 2016 Optics Express journal, an article titled "Optical digital Coherent detection technology enabled flexible and ultrafast quantitative phase imaging" by Yuan-Hua Feng et al. from Peking University Electrical Engineering Department. This article is currently open and availabe at the Optical Society Of America web portal.

Dr. Feng and his colleagues report that "we present a digital coherent time-stretch microscopy system that is capable of delivering ultrafast QPI of fixed cells in a scanning rate up to 100 MHz without averaging" . In addition, these researchers state that "bebefiting from a digital coherent receiver, we can retrieve intensity and phase information with high quality simultaneously. Because of the improved Signal to Noise (SNR) of the proposd imaging method, the amplitude and phase images were processed without averaging. So we can achieve an effective line-scan rate as high as 100 MHz."

Both of these papers are very important to use because we wish to protect pilot's vision from increasingly sophisticated laser pointers on the ground 150 meters away aimed at the Boeing 747-787 cockpit windows. If we can detect the presencen of coherent laser light in an incoherent background within 10 milliseconds of arrival , we can then activate an electro-optic shuttter to block the laser ray and then funnel the recovered incoherent light signal to an Planar(@TM) transparent display.

My question is which of the two papers is most effective for accomplishing the aforementioned purpose. In addition, how is the digital coherent receiver able to achieve line-scan rates as high as 100 MHz? Please use equations to explain this if necessary.

Any help is greatly appreciated.

You are wrong about that calculation. Pay attention to the unit analysis!

If you have 150 µm/s and 0.1 µm/step, then you divide the two to get the step rate:

$$\frac{150 \frac{\mu m}{s} }{ 0.1 \frac{\mu m}{step}} = 1500 \frac{step}{s}$$

Alternatively, the time taken per step is:

$$\frac{ 0.1 \frac{\mu m}{step}}{150 \frac{\mu m}{s} } = 0.667 \frac{ms}{step}$$

• Thank you for your excellent answer. So how long does it take to scan a 10-μm scan length, corresponding to a 20-μm path difference range? I am upvoting your answer right now. – Frank Jul 30 '16 at 11:54
• Do the math! I'm not going to spoon-feed you this stuff. – Dave Tweed Jul 30 '16 at 11:56
• A 10-μm scan length is the same as 100 steps. At your calculation of 0.667 milliseconds per step, that translates into 66.7 miilliseconds which is too long an interval for pilots to tolerate without permanent damage to their retinas . I read your stellar resume. I went to high school at Mount Pleasant Green Knights in Wilmington Delaware and studied at the University Of Delaware for 2 years. – Frank Jul 30 '16 at 12:05
• How is a digital coherent receiver able to achieve line-scan rates as high as 100 MHz? Thank you. – Frank Jul 31 '16 at 9:56