I would simply make the following observations in response:
a) The author cites standoff applications in the 1 to 100m range. Yes certainly such exist, for example detection of residual pesticides on agricultural produce, factory floor monitoring for release of TIC's, breathalyzer for detection of human exhalations associated with say cancer and so on. Explosive detection at 1 to 100m? No thanks, if you are within 1 to 100m of it and it is a reasonable charge with shrapnel, and it detonates, you may as well just have touched it. So meaningless. Also there is the question of CONOPS. If you are processing a crowd, what do you do? Process them one by one with an inter individual spacing of 100m per person. Such crowd processing is not practical.
b) Agreed, MIR there is no eye safe issue per se as there is no retinal transit, and the MPE of the eye is rather high.
c) Cost/wafer scale manufacturing? I just looked up QCLs on Thor Labs site. 9um they weigh in at many 10,000's of $ per Watt. That is just the diode, excluding PSU and external cavity say should one be required. By contrast a DPSS hybrid, capable of tuning near throughout the 10um atmospheric window, and producing burst sequences of 300mJ per 100ns FWHM pulse, would weigh in at <1000$/W. So I would question the whole idea of it being so cost effective. This latter particularly if you lump in the number of units you would require to command the volume of space the DPSS hybrid could command. Additionally, the DPSS hybrid can be protected from contamination, after all it would sense a threat with ample time for relocation. With these QCL units, if you can even sense at 100m, then a threat plume moving towards you at 5mph would give you ~ 45seconds to react. That is about enough time to grab your atropine, and perhaps start running, but nothing else. You would abandon all your equipment.
d) Agreed, there is a distinction between the form factor for a QCL and a DPSS hybrid of the capability mentioned. But there is a very substantial difference between those capabilities. Nonetheless, a DPSS hybrid of the characteristics mentioned would be man portable, involving a backpack and a rifle sized laser element. So also not a problem. Having as a lad been in a foreign military, and having run around with full kit and rifle ( sometimes more ), I would not hesitate to carry a standoff detection system which would give me better than 1/2 an hour warning on an incoming threat plume at say 5mph. Tactically, one can relocate, protect equipment and so on. A huge plus.
e) They are not mass producible. Well, unless your QCL is a stand alone chip, without external cavity and PSU, then they are also not mass producible.
f) Yes, using CW permits certain applications - yet it is also restrictive. A pulsed high power system for standoff at significant range permits use of an optical switch prior to return signal sensor. Basically gating optically sensor open only for a limited time at a desired time. That yields range gating plus minimizes background interference/contribution. More or less the only significant noise of concern is intrinsic sensor noise.
g) The author mentions 5.9 to 10.9um, and 3 to 20um. In real standoff, the 5.9um to ~8.2um is completely irrelevant. Only output within atmospheric windows is significant. Similarly ~4.2um to 4.7um is worthless. The very fact that these atmospheric absorption band wavelengths are mentioned is an exclusionary to true standoff detection.
Please note, I am not decrying QCL's as a technology. I am questioning the whole CONOPS whereby 1 to 100m is considered 'standoff' of any use in the event of hostile action. That in turn brings into question the kind of technology that is being leveraged then to achieve a goal which does not seem very useful in the specific threat case I was discussing. Seems to me the argument of use for civil ( or military )defense and so on does not really hold up to examination as a motivation for the effort. Other motivations do exist as pointed out.
1/16/2016 2:08:13 PM
Sure, standoff is strictly any non-contact range, but, within the application space, the term referred to >250m. Understood, there is no official definition, but history shows this to be what was meant - and proximate covered the range <250m.
This reclassification/general understanding change, is indicative of an industry redefining terms to lower the bar. Industry starts with bold goals, in this case, development of lasers for standoff (i.e. true standoff >250m and out to kilometers/miles). After much money has been spent without success, the money flow is still required, so the bar is lowered, in this case, standoff become a closer and closer distance, to the point that QCLs can actually meet the requirement. Like BobHope1, QCLs per se are not my issue, rather the surreptitious redirection that results in funds for the real, worthwhile goal of real standoff out to miles being directed towards the reduced capability of less than 100m.
1/16/2016 3:49:06 PM