Questions tagged [photonics]

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Test for correlation in photon detection

I have data from an experiment as follows. $x^A_t = [0,0,1,0,2,0,3,...]$ and $x^B_t = [0,1,1,0,2,2,0,...]$. These numbers are photon counts measures in two different photon-counting detectors. I have ...
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Iridescent duck and photonics in nature. Why duck's head is only green?

We all know and adore pure beauty of mallard male: I know that such iridescent colours are usually a result of feathers forming a photonic crystal with layers of air sandwiched between layers of ...
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Are electrons quantum confined in two-dimensional semiconductor materials?

In the case of superlattice devices, such as HEMT, the carriers are confined to a two-dimensional quantum well and form what we call 2DEG. Hence, when calculating the carrier density, we need to ...
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Temporal vs Spatial Contrast in Speckle Contrast Imaging

I have been reading about Speckle Contrast Imaging recently. Its quite an interesting topic given that it should be possible with relatively inexpensive equipment. The basic premise from what I ...
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Wavelength-dependence of optical elements in integrated photonic circuits

I am interested in integrated photonic circuits or silicon photonic circuits. In the long run, I would like to analyze these circuits using a rigorous and analytical mathematical approach, taking the ...
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How to calculate eigenmodes from the intensity profile of Wave Propogation Method in a single mode waveguide

I am trying to calculate the eigenmodes of a single mode waveguide. I have found a python module diffractio https://diffractio.readthedocs.io/en/latest/index.html which allwos to set up a WPM/BPM ...
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Can laser diode characteristics comparison be based on laser of one length?

As we know, e.g. from this website, to know the laser Transparency Threshold Current and Internal Quantum Efficiency, we need the LI curves from lasers of various widths. However sometimes it is ...
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Refractive Index Ellipsoid for Circular Birefringence Materials

When light travels through an anisotropic medium, its refraction may depend on both its polarization and propagation. Such materials may be classified in terms of their (Linear) Birefringence as ...
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How is the physical angle between two complex electromagnetic fields defined?

If I have two electromagnetic field vectors, which are in general complex, how do I sensibly define an angle between these two fields? Some books refer to the angle between electromagnetic fields, ...
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1 answer
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Calculate the effective refractive index from group index

I'm currently trying to calculate the effective refractive index $n_{eff}$ of waveguides using the (measured) group index $n_g$ and the wavelength $\lambda$. I started with the relation between $n_{...
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2 answers
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Uncertainty principle and Fock states

While studying about single photon sources, I came across the following sentence on the related Wikipedia page The Heisenberg uncertainty principle dictates that a state with an exact number of ...
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How does changing path length to align phase of two pairs of down-converted photons automatically create a GHZ quantum state between pairs?

The problem At least one StackExchange physicist suggests "In an ideal laser, there is no entanglement whatsoever between successive emerging photons. This occurs because a laser state is ...
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1 answer
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Why light radiates out of the bend in dielectric waveguides?

Why EM wave radiates out of the waveguide. Is this similar to centrifugal force?
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Is a room-temperature & pressure single-photon sensor theoretically possible?

In the realm of photonic quantum computing, single-photon sensors are required for determining the outputs of each chip. Unlike most forms of QC, photonic chips themselves may be run at room ...
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Minimizing, or correcting for, beam divergence for long-range LIDAR applications

I'm performing a literature review centred around LIDAR for space-debris detection. One issue that I've been unable to find any information on is what factors limit the range and resolution of such ...
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Why is wavefront quality important, and what is the relationship between the coherence length of a laser and its wavefront quality (if any)?

I am currently studying laser interferometry. I understand that the coherence length of a laser, $L_\text{coherence} = c T_\text{coh}$, where $c$ is the speed of light and $T_\text{coh}$ is the ...
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1 answer
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What is the difference between this "spectral width" and the laser linewidth (FWHM)?

I am currently looking at the 1550nm fibre-coupled DFB laser diodes on Alibaba for use in interferometry experiments. These are commonly used in optical communications applications, but it seems to me ...
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Why do two-beam interferometers have a relatively broad "fringe width" compared to single-beam interferometers?

I recently overheard someone discussing the basics of interferometry setups. They said that two-beam interferometers have a relatively broad "fringe width" compared to single-beam ...
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Coupling a light source into an optical fiber ring

My question is whether the following scenario is realistic. Imagine coupling a source of photons (say a laser) into a ring made of an optical fiber using a y-junction. (see picture) If the ...
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Why do we get rings in the Spontaneous Parametric Down conversion (SPDC) process?

In the SPDC process, the phase-matching conditions between the o-rays and/or e-rays have to hold for the down-conversion process to take place. Say we have a non-collinear phase-matching at the plane ...
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Is spatial coherence more important for interferometry, or is temporal coherence more important for interferometry?

I am currently going back through my textbook Laser Systems Engineering by Keith Kasunic. Chapter 1.2 Laser Engineering says the following: One of the primary goals of laser engineering is to meet ...
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1 answer
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What aspects of the design of a beamsplitter cube determine wavefront error/distortion?

I was recently looking into having some custom beamsplitter cubes made. I require good wavefront quality, so I wanted to ensure that this was a primary consideration of the design (see how Thorlabs ...
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1 answer
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Is it possible for an atom to contain multiple electrons in the same excited state?

This question mostly stems from my quite rudimentary knowledge of laser physics. As far as I understand it, all the 3,4,..-level-systems regard the electrons being in more or less excited states. ...
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What are the factors that decide which type of SPDC (Type I or II) occurs in a Non-Linear Crystal?

I have been reading through some literature on the theory of Spontaneous Parametric Down conversion (SPDC), their types, the phase-matching condition, etc. To my understanding, the downconverted ...
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1 vote
1 answer
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What is the wavefront quality of VCSELs? How does the wavefront quality of VCSELs compare to Fabry-Perot laser diodes and DPSS lasers?

I am currently looking at this VCSEL. According to the datasheet, it has a narrow spectral linewidth and emits a Gaussian beam, which are both properties that I find interesting. But I'm also ...
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What does it mean by "the apparent linewidth is limited by the measurement resolution, which is $7.5 \ \text{GHz} \ (0.25 \ \text{cm$^{-1}$})$"?

The datasheet for this VCSEL has the following graph: What does it mean by "the apparent linewidth is limited by the measurement resolution, which is $7.5 \ \text{GHz} \ (0.25 \ \text{cm$^{-1}$})...
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1 answer
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What photodetector parameters should I be focusing on, and in what priority, to maximise the performance of such an interferometer?

I'm trying to select a photodetector for an interferometry experiment. The interferometer will be trying to measure subtle vibrations occurring at a relatively high frequency (high kHz to low MHz). It ...
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2 votes
1 answer
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Squeezed quantum states of light

I am a beginner in quantum optics and started from reading the Fox's book. I got to Ch.7, where there is a discussion about the amplitude-squeezed states. I am really puzzled by the effect of phase ...
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Do interferometers require polarizing beam splitters, or do they require non-polarizing beam splitters?

I want to build an interferometer. In order to do so, I need to buy some optical components, such as beam splitter cubes. These beam splitter cubes, and other optical components, come in a polarizing ...
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3 votes
1 answer
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Comparison of beam divergence between laser diode and DPSS lasers

I'm currently comparing a pulsed laser diode and a pulsed DPSS laser. For the DPSS laser, the beam divergence is listed as "beam divergence (80% of total energy): $<4 \text{mr}$". The ...
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1 vote
1 answer
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Interferometry: effect of aspheric lens on self-mixing effect

I want to perform self-mixing interferometry experiments using a laser diode, as was done here. I'm going to use a low-power laser diode. However, I want to improve the performance of the device, ...
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2 answers
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What is meant by a 'diffraction-limited value with a focusing objective' and a 'free space propagation value with some divergence $\theta$'?

I am currently studying laser interferometry. I read that we can describe the spot size at the target as (1) a diffraction-limited value with a focusing objective or as (2) a free space propagation ...
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1 vote
1 answer
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How can I find the angle after the last lens from in the last lens?

I want to find the angle after the last lens when the beam diffracts. How can I do that? Is it the field of view? But for the field of view formula it takes account the size of the sensor. I want to ...
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0 votes
1 answer
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What is 'optical phaseshift'?

In interferometry, it is said that lasers can be used to measure the 'optical phaseshift' of something. I am familiar with the concept of phase in the context of waves, but I don't understand exactly ...
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2 votes
2 answers
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Explanation of 'modal gain' (especially in the context of laser diodes)

I have seen the phrase 'modal gain' used in the context of photonics. Researching this phrase, I found this resource, which explains it as follows: Modal Gain: which is the material gain adjusted to ...
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What is the minimum diameter of the spot a monochromatic beam can be focused to in relation to the beam wavelength?

Can the beam be focused down to a spot with diameter less than two times the wavelength or even less than a single wavelength?
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2 votes
1 answer
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Is energy lost when a photon bounces of an electron?

It was a long time ago I studied atomic physics but I remember that when a photon does not have enough energy to excite the electron to the next energy level it will "bounce off". This is ...
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3 votes
1 answer
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Change in refractive index due to the acousto-optic effect, and the use of acousto-optic modulators for the sole purpose of frequency shifting

Chapter 3 Acousto-Optics of the textbook Crystal Optics – Properties and Applications by Ashim Kumar Bain says the following: In general, acousto-optic effects are based on the change in the ...
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2 votes
1 answer
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Optical Waveguide Mode Profile

SE, I have a question about a mode profile chart from Synopsis RSOFT. After following an example silicon Y-Splitter waveguide below with the waveguide width = 0.45um and height=0.22 um , I simulated ...
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Affect of using a pattern generator with a laser?

I've been thinking about laser pattern generators and the implications of using them. If we consider a normal laser diode with a Gaussian beam, then we can imagine a dot being projected onto a target. ...
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1 answer
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Why does CW imaging not provide the same data as Pulsed imaging?

"Unlike pulsed THz imaging, the CW imaging (...) only yields intensity data and does not provide any depth, frequency-domain or time-domain information (...)." [1] Why does a pulsed signal ...
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1 answer
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What happens to the power of the beam passing through a lens?

Sorry for the fundamental and basic question, Does the power of the beam that is falling on lens is equal to the power of the beam after passing through lens. Suppose If I used convex lens to converge ...
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1 vote
3 answers
180 views

Can photons only exist in the state of motion in a medium?

Photons are known to travel at a speed of $\rm 299 \ 792 \ 458 \ m / s$ in vacuum. Photons can be absorbed, or absorbed and re-emitted by matter. They slow down to $\rm 225,000,000 \ m/s$ in water ...
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2 votes
1 answer
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About two photon interference in nanophotonics

Within the standard model, photons are point particles, i.e., with no spatial distribution. On the other hand, classical electromagnetic modes have field distributions. Suppose there are two different ...
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1 vote
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What are the "frustrated rotations" of molecules?

What are the frustrated rotation of molecules and how is this any different from the normal rotation of molecules? This question originated from reading: A promising diagnostic method: Terahertz ...
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Why does coherent enhancement cannot lead to a power improvement better than the square of interaction length?

I'm studying for an advanced course on photonics. The professor gave us a bunch of reference questions for the exam, like the following: Why does coherent enhancement cannot lead to a power ...
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1 vote
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Pulsed laser data sheets: "repetition frequency" in Hertz and "repetition rate" in "pps" (assuming pulse(s) per second) are equivalent concepts?

When I read the data sheets for pulsed lasers, some specify "repetition frequency" in Hertz, and others specify "repetition rate" in "pps" (assuming pulse(s) per second). ...
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1 vote
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Normalization of Phase Noise / RF Power Spectrum to 1 Hz RBW

I used a Rohde & Schwarz Phase Noise Analyzer to take some phase noise and RF spectrum measurements. All in all, I'm trying to compute the linewidth of a laser from a measurement. I cannot detail ...
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14 votes
3 answers
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What happens to the non-axial photons of a laser cavity?

When a laser cavity is pumped, the axial photons are reflected back and forth by the cavity mirrors and so contribute to stimulated emission, but the non-axial photons do not have this opportunity. ...
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1 vote
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Does laser coherence length impact phase noise?

I am trying to simulate laser phase noise numerically by creating a random phase $\phi(t) = \sqrt{2\pi\Delta\nu_L} \int_{0}^{t} dW(t)$, where $\Delta\nu_L$ is the laser linewidth in Hz and $\int_{0}^{...
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