Talk:Optical heterodyne detection

Adding a section for applications

simply adding links to one-off applications of this is gratuitous linksmanship. However the article is large enough now that it does deserve to have a section listing all the myriad applications. - user cems Here is a start in accumulating these:

• microsopy
• micorwave generation
• remote sensing

− The optical heterodyne technique is also employed in microscopy, with medical applications.

− The technique can also be applied to the convenient generation of stable microwave signals.

— Preceding unsigned comment added by 192.12.184.7 (talk) 22:22, 19 April 2018 (UTC) 

References

Nonlinearity

Optical heterodyne detection is the implementation of heterodyne detection principle using a nonlinear optical process. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" (LO) that is set at a close-by frequency.

The mixing itself can be mathematically described by just (linearly!) adding two time dependent electric fields. The beat frequency is then detected by photo diodes that perform something like squaring and integrating the already mixed signal. Nonlinear optical mixing would be Sum-Frequency-Generation or Difference-Frequency-Generation for example but these processes are not required to perform optical heterodyne detection. --herrkami —Preceding undated comment added 12:02, 14 February 2013 (UTC)

@herrkami I would fundamentally disagree with what you just wrote here. First "mixing" is a term of art in fields and waves that descibes this sum and difference frequency by non-linear operations, usually squaring or rectification. And also contrary to your statement difference frequency mixing is pretty much the entire basis of heterodyne detection so I'm confused by your claim that neither non-linear processes or Non-linear processes are required. Perhaps one could try to artificially measure raw frequencies and digitally construct the difference frequency or perhaps some sort of beat frequency envelope detection, but that would be completely out of the realm of Optical heterodyne detection. So no, without more specificty your initial argument is easily rejected I feel. — Preceding unsigned comment added by 65.125.35.61 (talk) 18:24, 9 January 2019 (UTC)

When I shine a laser pointer at a black wall, and it gets absorbed, is that a nonlinear optical process? In one sense, yes, absorption goes as electric field squared. In another sense, when optics professionals talk about "nonlinear optical processes", they are never talking about light absorption, but rather the processes that mix light waves to produce other light waves at different frequencies, which typically only happens at high light intensities. So "nonlinear optical processes" is a term that is commonly used by practitioners in a way that doesn't quite line up with its literal meaning—hence herrkami's confusion. (Optical heterodyne detection is indeed typically implemented as just plain old absorption: overlapping two light beams onto a fast photodetector.) Anyway, the quoted sentence is no longer in the article, so it's a moot point. --Steve (talk) 17:54, 10 January 2019 (UTC)

How mixing happens

There seems to have emerged a dispute on the fine-points of the origin of mixing. A recent edit changes from a simple notion of square law detection:

In optical detection, the desired non-linearity is embedded in the photon absorption process itself. Conventional light detectors—so called "Square-law detectors"-- respond to the photon energy to free bound electrons , and since the energy flux scales as the square of the electric field, so does the rate at which electrons are freed. A difference frequency only appears in the detector output current when both the LO and signal illuminate the detector at the same time, causing the square of their combined fields to have cross term or "difference" frequency modulating the average rate at which free electrons are generated.

to a more sophiticated but obfuscated discussion of how electric fields interact with matter, that is not only questionable in it's physics assetions it contains no citations of these provocative assertions

At the higher frequencies of optical detection, the electric field drives changes in oscillatory motion of bound electrons in atoms, molecules. The rate of photon absorption from one source modifies rate of absorption from other sources (because the magnitude of the electric field is modulated), resulting in a periodic variation in the detected power. This periodic variation oscillates at the difference frequency between the two sources, known as the beat frequency. A common misconception among engineers is that the non-linear response of an optical detector to the strength of the electric field is a sufficient condition to achieve mixing, which in the RF case generates sum and difference frequencies. Mixing in optical detection is not necessary to explain the observed beat frequency in an optical detector, (since the modulation of photon absorption explains this effect) and mixing at optical frequencies only occurs when other (phase matching) conditions are met.

Trying to sort this out I note that the simple explanation is not wrong. The issue is one of semantics and sophistication. To examine electronic mixing at all levels one needs to consider higher order electrodynamics which is way beyond the intended discussion level here. One can often simplify full blown electrodynamics to the standard Quantum Electronics point of view as a practical level of explanation. At that level of discussion one is arguing about distinctions between induced polarizations of media and absorption. One can often make arguments that the light was not abosrbed per se but rather the polarization was modulated leaving variable absorption. This sort of argument is merely confusing because the net result is the same.

That is to say, at the end of the black box, energy is absorbed in a square law fashion, the rest just depends on the expansion you chose to represent it as to what modulated what and what polarized what.

--cems1

Fortunately there is a simple resolution to this conflicting point of view in reach. Further down in the article, there is a brief mention in the coherence section that the naive treatment of squarelaw detection of single frequencies is entirely incorrect but happens to give the correct result. This is nice segue to link to another wiki pedia page (that does not yet exist) that does a much more formal job of disccusion both coherence and polarization modulation in Quantum electronic terms. Thus the author favoring the more sophisticated explanation is invited to create such a page.

Both of these explanations are semiclassical. As such, neither is strictly correct. The simple explanation is just fine for the level of this article. Common optical detectors respond to the energy falling on them. Thus, their response is to the square of the E field, or, more precisely, the Poynting vector. However, any mention of electric fields dooms the discussion to a classical or semiclassical one. The simpler one is preferable, in my opinion. Drphysics (talk) 02:52, 16 October 2009 (UTC)

Expanding the articles content while keeping it focused

This article is centered around the synthetic array heterodyne method, as if it were the standard method used. In fact, this method is seldom used because of several disadvantages that are not mentioned, most notably, that it has more shot noise per speckle than the conventional method. This method also requires more electronic bandwidth for the same signal bandwidth than the conventional method. The conventional technique, far from being limited by problems that were solved by synthetic array methods, is the principal method in current use.

This article needs revision to reflect these facts, as well as put this alternate method in better perspective. --Drphysics (talk) 03:06, 16 October 2009 (UTC)

DrPhysics, I don't think this article is the place to discuss the pro and con of specific techniqies in detail. Those can be placed in other pages and the linked to. To keep this short and pedantic, I think emphasis should be on short pithy descriptions of what is special about optical heterodyne (as opposed to heterodyne in general), reveal of the consequent unique optical issues and then to illustrate those considerations concretely through a brief revelation of some techniques for their solution or exploitation. SAHD is used as a workhorse example to expose some of the practical problems that arise. As more techniques are added to this article it will naturally be diluted in relative appearance.

Thus the article could be improved, exactly as you did, by illustrating the need for tight frequency control with two methods of achieving that. (you will note I expanded your lead in a new section)

Thus I'm advocating that the decision to discuss a technique herein or not should be judged on how well it serves to illustrate a practical aspect of optical heterodyne rather than to teach how to implement that technique or discuss it's pitfalls. While the points you make about the issues that arise in SAHD are correct, this article itself would not be a good place to discourse extensively on the higher order details of that or any other illustrative technique. For example, the section on line narrowing for coherent addition should not extensively go into the limitations of slow wavelength tuning or excess amplitude noise control that injection lasers typically suffer from, just as an extensive SNR bandwith analysis of SAHD would be too dense and further overweight SAHD in the panoply of methods. ( I note we might want a section of say frequency hopping difficulties for rapidly scanned wavelengths (e.g. for use in multi spectral coherent dial) or a section on types on noise control in laser systems relevant to this, but those should be relevant to optical heterodyne issues and not get bogged in arguing the merits of specific techniques.)

Linking out to their own pages where those method specific limitations can be leisurely discussed would be a good idea. Indeed I've been wanting to do it, but I wanted to see the article mature a little first before that. This is a good discussion and I hope you continue to contribute. I'm quite thrilled to finally see someone else with expertise on coherent detection show up.

--cems1

Synthetic array heterodyne detection

Synthetic array heterodyne detection redirects here. Please can someone who knows about the subject either create a section in this article with that title or create a new starter article with that name in place of the redirect. Thanks. PeterEastern (talk) 07:55, 24 February 2011 (UTC)

Inadequately sourced?

I think this article is inadequately sourced. Most sources seems to be of poor quality, or rather old, which is surprising given that this technique is said to have been established 20 years ago. Among them, I couldn't identify a textbook or readily available reasonably modern source in a peer-reviewed journal. Source [1] is a commercial web page, not credible, and should be removed. [2] is behind a paywall, and old. [3[ is very old. [4] is a conference proceeding, and [5] and [6] lead to the same document, which only says the technique is well known, citing a 1976 book of conference proceedings. [8] looks interesting, but is behind a paywall, and my usual ports are down. I tried to find better sources, but found it to be difficult. Where are the relevant textbooks for budding engineers?

I'm not yet convinced that the technique described here is actually the standard option called "coherent" in the widely applied LiDAR (aka LADAR) systems. (That article wiki-links to this one.) For example, I found this "review", which seems to indicate that coherent LiDAR now a well-developed standard technique, but doesn't give the basics. But then I found [1] from 1973. It sez:

The technique involves modulating the intensity of the laser beam at a frequency that changes rapidly and linearly with time. A portion of the transmitted signal is mixed electronically with the light reflected from the targets in a device similar to a radio receiver. Each target appears at a particular frequency. By tuning the radar's receiver to these target frequencies, the operator can measure both the range and the opacity of semitransparent targets over distances of 100 to 200 meters. 

This makes a lot more sense to me than trying to generate two laser wavelengths corresponding to "frequencies" stably differing by of order a GHz or less for electronic processing. The latter seems very difficult and expensive, but seems to be what this article is presently trying to describe: "nonlinear optical process". Maybe the LiDAR article links here by mistake if LiDAR really uses modulated light as described above. If this article is really about optical heterodyning, I would like to see a source showing experimental results including spectrum analyses of signals from a simple photodetector under at least 3 conditions: light from only the signal source, light from only the "local oscillator", and light from both. This article leads us (or me at least) to believe that only the third spectrum will show a nice peak at the difference "frequency", which has no relation to frequencies of any modulation. That would indeed be impressive. Has anyone seen such? Layzeeboi (talk) 05:38, 12 February 2017 (UTC)

After working on this and related articles, and adding sources and material, I now feel that I understand it. Layzeeboi (talk) 06:10, 16 February 2017 (UTC)