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65

EDIT updated (improved) description of phase detection circuit There are two principles used in these systems. The first is the time-of-flight principle. As you noted, if you wanted to get down to 3 mm accuracy, you need timing resolution of 20 ps (20, not 10, because you would be timing the round trip of the light). That's challenging - certainly not ...


24

The timing circuit doesn't have to run that fast. It just needs a time-to-digital converter which has a high enough resolution (0.1ns is nearly trivial with off the shelf CMOS technology) and then it can average many pulses (hundreds or thousands) to get the resolution improved by another order of magnitude. These are all fairly standard engineering ...


20

You don't have to run a clock that fast, and you don't need any new physical principles either, just some clever electronic design, mixing analog and digital components and making a few critical parts (switches, in essence) very fast. One simple technique, as described here on wikipedia, is a two-slope ramp. At the start of the time to be measured, you ...


10

Instead of attempting to time the round-trip of individual pulses (which depends on a good way to separate reflected pulses from ambient noise), you can also build a phase-locked loop. Control the sending of outgoing pulses by a voltage-controlled oscillator, sending one pulse at each rising zero crossing. Whenever you see an incoming pulse just before the ...


7

My bet is that your fiber is very short (something like one meter or so) and that the fluctuations you see on the output mode are due to cladding modes, i.e. a part of the injected light propagating into the cladding of the fiber instead of the core. The resulting fluctuations are due to external perturbations of the fiber (thermal fluctuations or you ...


6

The temperature limit for laser cooling is not related to gravity but to the always-present momentum kick during absoprtion/emission of photons. Ultracold atom experiments typically use laser cooling at an initial stage and afterwards evaporative cooling is used to reach the lowest temperatures. In evaporative cooling the most energetic atoms are discarded ...


6

Laser is coherent light, so with a technique called interferometry you can actually measure distance with a resolution of less than a micro-meter, regardless of your timing resolution. It should be noted that the measurement produced by interferometry has half-wavelength periodicity (e.g. 200-350nm for visible light). This means that in order to absolutely ...


5

Lasers used in medicine can have a range of different wavelengths depending on their application, and these wavelengths will be absorbed differently depending on which part of you they are being used on... Unfortunately this means that it isn't enough to just consider power when it comes to laser safety, but also the wavelength, duration of the light pulses ...


4

I did a couple of experiments with a DSLR and a cheap red home improvement laser level. The most revealing factor is that a direct reflection of the laser on the CCD sensor gives a distinct square diffraction pattern. This is pretty much what one would expect from a camera that has square pixels. The reflections have sharp peaks that are spaced fairly far ...


4

This sounds as though the aberration in the laser's output could be fluctuating owing to "mode hopping" (where several of the laser's cavity modes are active and playing a time varying role) so that, even at a constant output power, the aberration of the output beam varies with time. Wavefront aberration is roughly the Fourier-dual of Strehl ratio. This ...


3

I have a lot of experience with CVD and sputtering, but limited experience in PLD; however, several of my colleagues did this all the time in our shared a laser lab. When they were attempting to reproduce a specific result all of the parameters had to be systematically varied, from the laser fluence to the substrate conditioning and temperature, and more. ...


2

I have no experience with either CVD or PLD, but it was interesting to think about this question. In a humble attempt to build on Peter Diehr's answer, here is some theory (at a very heuristic and simplified level). The deposition of each new layer can be thought of as being governed at large scales by some mixture of Laplacian and Eden growth in two ...


2

The first thing you should do is cover your detector with an optical filter that only allows through light with the same wavelength as the laser you are looking for.


2

If cosmological expansion applies on the scale of the earth moon system, then in some short period of time $\delta t$ the distance between the earth and moon increases from $r$ to $r+\delta r$. So the force of gravity between the bodies changes to: $F+\delta F=\frac{GMm}{(r+\delta r)^2}\approx\frac{GMm}{r^2}\left(1+\frac{\delta ...


2

Interesting question. Taking a stab at it - not absolutely sure this is correct, but let the comments begin. In the frame of reference of Earth, the light travels straight out to the reflector, and straight back. You are asking about the case where an observer is in a reference frame that is moving with respect to Earth/moon, and the picture would have to ...


2

I think that the following might help: The laser modes of a cavity of length $L$ have the following (angular) frequency spacing: \begin{equation} \Delta \omega = 2\pi c/(2L) = 2\pi/(T_c) \end{equation} Here, $T_c$ is the cavity roundtrip time. The frequencies of the cavity take the following form: \begin{equation} \omega_n = \omega_{\text{offset}} + ...


2

Some other effects that might be at play: 1. Reflections from the end-faces of the fiber causing interference 2. Brillion Scattering 3. Check to see if in fact the fiber you're using has a cut-off wavelength shorter than the wavelength you're actually using.


2

It seems like some kind of Newton rings formed. Does your mobile camera has a flat thin sheet covering the lense. If so, the newton rings formed is captured in the sensor. Having said that, I can't explain why there are multiple of them, that too in a lattice formation. May be if the distance between the thin sheet and lense increases we have various newton ...


2

I have talked to the manufacturer of the laser. They told me that one must use safety goggles at all times. Of course, they have to say this by law. Even if there is some mechanism that makes the laser safe at usual operation, they have to be careful. They ship the device with safety goggles. On my last visit I checked them closer and they are class 5 to 7 ...


1

It depends upon the shape of your laser pulse. The time bandwidth product is defined as: $$\Delta v \Delta t \geqslant K $$ Where both $\Delta v$ and $\Delta t$ are measured as FWHM (full width half max). The factor $K$ depends upon the shape of your laser pulse. $K = 0.441$ for a Gaussian shaped pulse, $0.315$ for a sech pulse, $0.142$ for a Lorentzian ...


1

Hint: Use the final equation from my other answer here: \begin{equation} E(0,t) = E_0\exp(-i\bar{\omega}t)\frac{\sin[(N/2)\Delta\omega t]}{\sin[(1/2)\Delta\omega t]}. \end{equation} This is already a function of time. Now, $I(0,t)\propto |E(0,t)|^{2}$, so just square that function for the intensity (normalize to remove constants if you like). That will ...


1

Your problem is more math related than physics. To find proper full-width at half maximum $\Delta t$ and $\Delta \omega$ you have to equate the functions to $x$, with some proportionality coefficient so that when $x=1$ you're at the maximum of the function. This way after solving for $t$ or $\omega$ you'll find the half-width half-maximum by plugging in ...


1

The diameter of the output aperture of a laser is not necessarily the same as the Gaussian beam waist, the latter is typically less than the former, as otherwise you won't get a Gaussian beam.


1

Regarding your 1st question, the two options you give appear to be the same. Using the wave picture, the emitted wave is in phase with the incident wave, and the two waves (which have the same frequency and are in phase) interfere constructively to make a wave of larger amplitude. The emitted wave is in phase with the incident wave because the excited atom ...


1

Semiconductor light emitters are made of such materials, which have quite large index of refraction. This makes it hard for light to exit the emitter — due to Fresnel equations and low index of refraction of air. In a laser the light mostly goes back and forth between two mirrors, and reflections only help the lasing. So the light either exits from a tiny ...



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