raman spectroscopy

it's one of those days when you just can't get yourself to start writing anything technical so you doodle on your graph instead and hope you remember everything you'd like to say on your paper when you actually write it later on.

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photovoltaic energy conversion and sustainable energy

Written Part:
Q1. In Jan 2004 NASA's Mars Exploration Rover landed on the surface of Mars in search of answers about the history of water on the planet
(a) What is the solar irradiance (Wm^-2) entering the mars atmosphere, knowing R_sun=696 Mm, T_sun=5762 K, r_mars=230 Gm, σ=5.67x1^0-8 Wm^-2K^-4?
(b) What is the solar irradiance on the horizontally oriented solar panels (n=0°) of the Mars rover when the sun is in the zenith (z=0°), using the following properties of the Mars atmosphere: optical thickness αd=0.5 and s=0.8?
(c) The rover gets its energy from 1.3m² of solar cells with 27.5% overall conversion efficiency and it needs 100 W electrical power to drive. Calculate the direct, indirect, and total irradiation (Wm^-2) as well as the electrical output power when z=55° and panel is tilted towards the sun with n=10°. Will it be able to drive?

A1. 
(a) Calculate dilution factor, f, then get solar irradiance by Q=fσT⁴
(b) Calculate direct, indirect, and total irradiance.
(c) Calculate direct, indirect, and total irradiance and compare with Q_tot from η=W/Q_totA.

Q2. For the study of solar conversion we know that the spectral distribution n(E) follows from the emissivity ε(E), the number of photon modes h(E)dE, and the occupation probability f(E).
(a) Explain the greybody spectrum. What is emissivity?
(b) Derive h(E).
(c) Show that you can obtain the photon occupation probability from the electron occupation probability.

A2. See long derivation in course notes.

Q3. Forgot. Something about tandem cells and fill factors and output powers.

Q4. All about the window effect for heterojunction solar cells.
(a) Define the general current efficiency including all necessary parameters.
(b) Calculate the current efficiency as a function of u=amt. of light that can be absorbed in layer 2/amt. of light that can be absorbed in layer 1.
(c) Explain how the window effect comes to take place from (b).
(d) Sketch a graph of the current efficiency as a function of u.

A4. See long derivation in course notes.

Oral Part:
Q1. Derive and explain the equation for calculating a planet's temperature from the sun's temperature.
A1. See derivation in course notes.

Q2. Derive the (3) factors that affects the absolute efficiency, η^*, of a solar cell.
A2. See derivation in course notes.

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photonics in optogenetics

i enjoyed the recent trends in photonics course a lot particularly because it didn't require much effort owing to the fact that you only need to be present during the lectures, give a lecture on one topic of interest, and make a paper about it. on the other hand, you would need to read, read, read some more, study and talk about your chosen topic but you do learn a lot and get stimulated to think of new ideas in the process. taking the course with someone who works at the military defense industry is an added bonus especially when he drops out hints of some highly classified projects by the military every now and again.

our own topic was on optogenetics, i've read at least twenty papers on the photonics part yet i'm not even close to knowing a quarter of what there is to know about this field even though it started barely twenty years ago. however, i had such a great time reading up and i'm happy we've stirred up some discussion after our talk. for instance, following the example on sight restoration using optogenetics technology, pieterjan questioned the possibility of implanting IR-sensitive opsins to the human eye. i never even thought about that possibility as i have already discarded the use of IR light since it would cause a lot of heating in the human tissue. but of course, what are engineers for but to solve such kind of problems, right? :P i can imagine all sorts of applications for such technology already!

i am still in the process of writing the report in journal format but the slides we used for the presentation are as follows:

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optical convolution processor

so i've been sifting through my old photos and uncovered some images which, having been unfiled, didn't find it's way into the lab reports last semester. this one in particular is for the optical convolution processor under the session on fourier optics.

Equipment

HeNe Laser
Lenses
Filters
Pinhole
CCD

Set-up

errata: "image" → should be "object". that is, the image plane is the plane with the ccd.

Analysis & Discussion

An optical convolution processor, or 4f processor, is an optical setup which enables real time implementation on the mathematical cross-correlation and convolution methods. It states that the convolution of two images (f(x,y) and h(x,y))



can be expressed as a simple product of the fourier transforms of both images:



Given a certain image, we first would like to view its fourier transform. Using the setup shown above, we obtain the following images at the specified planes:

object mask placed on the object plane.

image at fourier plane: "A" decomposed in it's spatial frequencies.

image at CCD without filter at fourier plane

if a low pass filter, like this, is placed at the fourier plane, we can clearly see that light corresponding to the high frequencies are filtered out. hence, in the image plane we see the image below.

low pass filter

image at CCD with a low-pass filter at fourier plane

on the other hand, placing a high-pass filter on the fourier plane generates the following image:

image at CCD with a high-pass filter at fourier plane

when a pinhole is set in the fourier transform plane, intuitively we would expect it to give a better, clearer output of the image at the observation plane. this is not the case that was observed, however. since the lenses that make up the system are imperfect, then their imperfection adds to the noise which is contained in the high frequencies. decreasing the aperture removes the ambient noise hence we can see the image become clearer. further decreasing the aperture removes more higher frequencies hence the image becomes more blurred at the edges.

these are simple examples of spatial filtering. spatial filtering is a technique for filtering out certain spatial frequencies usually in order to improve input image caused by scattering by defects or particles in the air. when focusing a beam, the image of the source composed of low frequencies is concentrated at the centre while higher order frequencies, noise, are focused further away.by employing a pinhole at the fourier transform plane, the higher order frequencies will be filtered out thus, theoretically, giving a clean spatial profile at the output.

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a little music a day keeps insanity at bay

aww yisss. optical sensors lab 1, prev. sem.: optical transducer using fiber bragg gratings/recording sound through optical fibers. photo c/o yuting.

Background

Acoustic vibrations are generated by musical instruments either by plucking the strings mounted on an instrument, blowing air into a cavity, and various other means. These vibrations are transmitted through the air and are received by the ear. In some cases amplification and/or recording of these acoustic vibrations is desired and is typically done using piezoelectric transducers that are sensitive to mechanical vibration in the acoustic frequency (from 5Hz to 20kHz). In some cases, magnetic pickups are used which record vibrations using electromagnetic induction. 

A drawback from these methods, however, is that the system involves electrical cables connecting the musical instrument to the recording instrument which generates noise and significant audio signal delays. In order to address some of these issues optical pickups have been developed being potentially insensitive to electrical interference.

Here, we recreate the study performed by Loock, et. Al. on an optical pickup which uses a fiber bragg grating as a sensor. As discussed in Lab 1, an FBG is a periodically modulated structure of the refractive index along the core of a single-mode fiber. Any environmental change that changes the parameters of the FBG leads to a change in the reflection spectrum. By exploiting the sensitivity of an FBG to mechanical strain caused by acoustic vibrations we are able to monitor and transduce those signals.

Experiment 

Figure 1 shows a schematic diagram of the experimental setup. The FBG is pre-strained and fixed on a flat portion of the guitar. The reflection spectrum of the grating is shown in Figure 2 and shows the peak to be at 1520.296 nm. Light signal from a tunable semiconductor laser is guided through an optical circulator to the FBG and is tuned to the steep flank along the long wavelength range of the FBG spectrum.

Fig. 1. Schematic diagram of the experimental set-up. The light from the tunable light source is guided through a cirulator towards the optical pickup (FBG). The third arm of the circulator is connected to a photodiode which monitors the change in transmission.

By doing so, the intensity of the transmitted light would vary depending on how much the FBG peak has shifted. The change in transmission is monitored using a photodiode. The setup is connected to a PC and uses the software Matlab for processing the recorded signal.

Fig. 2. Reflection spectra of the FBG (lower spectrum, yellow) and laser diode (upper spectrum, white).

Results and Discussion 

By slightly adjusting the spectrum of the laser source and changing the gain of the photodiode we can tune the system in order to get a recording with the best signal to noise ratio. Figure 3 shows sample waveforms using different gain values and laser diode wavelength peaks.

Fig. 3. (a)1516.900nm, gain: 50dB, (b)1516.900nm, gain: 40dB, (c) 1516.956nm, gain: 40dB, (d) 1516.960nm, gain: 40dB

Although we can clearly notice the presence of noise in some waveforms, we cannot make a direct association between the recorded waveforms and it’s signal to noise ratio. The quality of the recording is measured by ear and a gain of 40 with a wavelength peak at 1516.956 nm seems to yield the best signal to noise ratio.

The sensitivity of the response depends on the slope of the laser attenuation spectrum where the FBG peak is tuned. It is noticeable that even a very small change in the wavelength settings, in the order of picometers, leads to a considerable change in the quality of the recording. In certain settings we notice a relatively good signal to noise ratio but there exists high harmonics in the recordings (Fig.3a) that may be caused by the FBGs nonlinear response to strain.

The quality of the recordings also depends greatly on the initial strain induced on the fiber as well as the relative distance on which the fiber is affixed on the instrument. When unstrained, acoustic vibrations would not induce large changes in the fiber properties resulting in poor quality recording. This phenomena was observed yet unquantified for this demonstration.

From this we can remark that FBGs can indeed be used as an optical transducer.

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a roll of tissue a day keeps the booger monsters away

ladies and gentlemen, let's all cheer for nosey who's been running a marathon since yesterday! forgive her for being a little slow because she's dragging fever, headache, sore throat, and all kinds of body aches along with her. gah, pamper me, pleaaase! on the brighter side, running a fever is the perfect excuse to wear winter clothes on a 20 degree weather.

arf!

..and roll in bed for the whole day. not very exciting when your skull feels like cracking, but still.
..and eat anything i want so i could, you know, regain my strength and all :P
..and try out new apps like this photo editor which can do doodles. xperia's own photo editor had the doodle feature before but it's no longer in their latest update. bad move, sony, bad move :P

..and rediscover stuff on my phone, like this one, haha.

there's no time stamp but written probably around the end of may if i haven't figured out how to get the receiver sensitivity yet. the professor didn't have major comments on the project so i believe we did okay. o, ha! quite proud of it, actually. his only question was why the sensitivity we mentioned was about 0.15 dB different from the figure presented. while, yes, it could be that we decided to be a little conservative with the estimate to ensure a lower BER, the real reason was actually less dramatic, that is, we missed updating the figure after the final optimization change. bug! to think it even passed unnoticed through virginie's reviews. OC, that one, i tell you. even her notes are immaculate. REPORT

..and watch a mowbee!
..and get miserable over the fact that the recent solar flare did not produce the aurora borealis i was expecting over belgium T_T nobody really prays for solar storms to happen but i hope we get another huge one within next year. just huge enough to incite an aurora over here again, please, i don't want to feel too responsible for the end of the world.

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optical sensors

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optical design with ray tracing

Q1. describe how you would define the following structure on ASAP. in addition, create an ASAP script which generates the surface, the only necessary condition being n1 != n2.

A1. the provided volume could be defined using 5 surfaces: elliptical air-n1 interface, tubular air-n1 interface, elliptical n1-n2 interface, tubular air-n2 interface, and elliptical air-n2 interface. the material composition is defined by defining a database of material parameters, particularly the refractive index. the INTERFACE command is necessary to define not only the material refractive index but the reflection, transmission, and absorption properties as well.

then    
SYSTEM NEW
RESET
UNITS MM
MISSED 0.5
WAVELENGTH 1550 NM
SPLIT 1
 
MEDIA
  1.4 'N1'

MEDIA
  1.5 'N2'
DIA=2
L1=2
L2=2
!! Grin substrate
SURFACE                              
  PLANE Z 0 ELLIPSE (DIA/2) (DIA/2)
OBJECT 'N1.EDGE'              
INTERFACE 0 1 N1 AIR
SURFACE                                                
  TUBE Z 0 (DIA/2) (DIA/2) (L1) (DIA/2) (DIA/2) 0
OBJECT 'N1.TUBE'      
INTERFACE 0 1 N1 AIR
             
SURFACE                              
  PLANE Z (L1) ELLIPSE (DIA/2) (DIA/2)
OBJECT 'N1N2INTERFACE'              
INTERFACE 0 1 N1 N2
SURFACE                                                
  TUBE Z (L1) (DIA/2) (DIA/2) (L1+L2) (DIA/2) (DIA/2) 0
OBJECT 'N2.TUBE'      
INTERFACE 0 1 N2 AIR
 
SURFACE                              
  PLANE Z (L1+L2) ELLIPSE (DIA/2)
OBJECT 'N2.EDGE'              
    INTERFACE 0 1 N2 AIR

!! DETECTOR
SURFACE
PLANE Z (L1+L2+1) 0 ELLIPSE 2@((DIA)/2)
OBJECT 'DETECTOR'

                   
!!SOURCE
WINDOW Y Z
PLOT FACETS OVERLAY
EMITTING DISK Z 0 -(DIA/2) (DIA/2) 100 5 5
!!SOURCE DIR 0 0 1
FLUX TOTAL 100

TRACE PLOT
CONSIDER ONLY DETECTOR
WINDOW  X Y        
!!SPOTS POS
!!PLOT FACETS OVERLAY
!!STATS
$VIEW

Q2. (a script for a light pipe is provided) what is the intensity at the output of the light pipe? calculate the etendue at the entrance and end facets of the light pipe.

A1. the etendue is a pure geometric concept that does not say anything about the energy distribution of the light but determines the positional and angular extent of a bundle of light. it is important for calculating how much light can be coupled from a source to a detector theoretically. the etendue is a  function of the refractive index, detector/emitter area, as well as the acceptance/exit solid angle. the light pipe exit angle could be extracted from the simulation by using the ANGLES command.

then
SYSTEM NEW
RESET  
UNITS MM 'WATT'
MEDIA
  1.48 'ACRYLIC'    

PIXELS 101
SPLIT 1 MONTECARLO
FRESNEL AVE

SURFACE
  PLANE Z 0 ELLIPSE 1 1
OBJECT 'FACE_ENTRANCE'
INTERFACE COATING BARE_SUBSTRATE AIR ACRYLIC        
SURFACE
  TUBE Z 0 1 1 10 2 2 0 0
OBJECT 'TUBE'
INTERFACE COATING BARE_SUBSTRATE AIR ACRYLIC

SURFACE
  PLANE Z 10 ELLIPSE 2 2
OBJECT 'FACE_EXIT'
INTERFACE COATING BARE_SUBSTRATE AIR ACRYLIC

EMITTING DISK Z -0.01 1 1 500000 90 90          
FLUX TOTAL 100
WINDOW Y Z
PROFILES OVERLAY
TRACE PLOT 10000

!!TRINIDAD START
PIXELS 101
WINDOW X Y
CONSIDER ONLY FACE_EXIT
!!SELECT ONLY COS[25] C
SPOTS DIR 

STATS
DISPLAY
  ANGLES
  PICTURE
RETURN
!!TRINIDAD END
$VIEW

Q3. (this question was about a comparison of the simulation time between SPLIT and SPLIT MONTECARLO commands using the light pipe scripts previously provided) which simulation model would you use for optimization?

A1. (add $TIC command to script to get quantitative values of the simulation time) the SPLIT command would generate much more rays than the SPLIT MONTECARLO because it could generate more than one child rays with power distributions according to INTERFACE and MATERIAL settings. SPLIT MONTECARLO, on the other hand, generates only the child ray with the highest probability of occurrence and in fact conserves the number of rays created within the simulation. this being the case, it also follows that SPLIT MONTECARLO simulations are faster than normal SPLIT commands. for design and optimization of light pipes, since we are only interested in getting a homogeneous distribution of light at the output, using SPLIT MONTECARLO would be acceptable since it's faster and gives us the information we need, anyway. on the other hand for ghost analysis, for example, one should use the SPLIT command in order to study where unwanted rays could come from or arrive on your detector.

Q4. describe how you would design an hot mirror. no need to write a script.

A1. a hot mirror is a reflective filter, composed of layers of dielectric materials, which reflects IR light (around 750-1200 nm) and allows visible light to pass. as a starting point we can first define a low-pass filter with central wavelength around 750nm or so. this generates not a perfect low pass filter but some sort of a band pass filter with a bandwidth of about 200nm. since the range of visible light is larger (380-750nm) we need to increase the bandwidth to allow all visible light to pass. this can be done by adding another set of layers of dielectric material to create low pass filters centered on the wavelengths you wish to suppress. ripples in the spectrum may be smoothed out by optimizing the widths of the quarter-wave plates. furthermore, hot mirrors are extremely sensitive to the incident angle of light. in order to account for this sensitivity in the design (and minimize it as much as possible), one may use materials of higher refractive index.

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asap

just some cool stuff one can do with ASAP:

Dove Prism

POFcoupler1_Shao_Trinidad

POFcoupler2_Shao_Trinidad

Free-form TIR reflector_Shao_Trinidad

Tapered Light Pipe

view from the light sources (4 LEDs) and the homogeneous distribution of light at the tapered end.

Projector_Shao_Trinidad

GhostAnalysis

LCD Diffuser Design

and a lot of cooler ones like modeling interference..by ray tracing!

MICHELSON_INTERFEROMETER

SLITDIFFPATTERN

ProjectionEngine_Pro6_Shao_Trinidad

red (bottom), green (left), and blue (top) light sources focused on a polarizing beam splitter designed to transmit s-polarized light.

very good homogeneity of red, green, and blue light at the detector.

SELFOC

see the parabolic bending of light in the lens? beautiful! boyang did ask why we had to let the light propagate in free space instead of just using one GRIN lens to couple light into two fibers and i was, like, so we'd have something to optimize, of course! haha. indeed, using just one lens would be very advantageous as you can get rid of some of the alignment issues, for one. seriously, though, things like these actually exist in the market but they've fixed the alignment pretty well.

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ocs

prof. van erps sure does take his time during exam interviews, they've even been at it with floris for around 45min. and since it's a closed book exam i had time to rewrite the questions for lack of anything else to do while waiting for my turn. for my alone time. with my crush. aw. hahaha.

Q1. you were hired as a telecom operator to design a trans-oceanic link spanning 4000km operating at a bitrate of 800Gb/s. your employer has proposed the following components to be used in the link:
- directly modulated DFB laser operating at 1550nm
- gradient index multimode fiber
- semiconductor optical amplifier
- silicon PIN photodetector
give a comprehensive assessment on whether or not these devices are suitable for the link. propose a solution otherwise. provide figures, equations, and a discussion of the underlying physical phenomena.

A1. (just a summary because i'm lazy and..how do you write down equations on blogspot?)
- use an externally modulated laser because a DM laser would generate chirp. at very high bitrate, like 800Gb/s, inter-symbol interference would be so profound you can no longer differentiate between signals/data. DFB laser is a good choice because a narrow linewidth of ~0.001nm is achievable which would reduce the effect of intramodal dispersion in the fiber. how to code 800Gb/s? WDM. the operating wavelength of 1550 nm is also a good choice. it falls in the middle of the 3rd transmission window and has the lowest loss among the three transmission windows. attenuation of 0.2dB/km is easily achievable at 1550 nm.
- use a single mode fiber for long haul transmission to avoid intermodal dispersion.
- use an optical fiber amplifier instead of a SOA because it has lower loss and is data transparent. EDFA is used for amplification at 1550nm.
- depending on the receiver sensitivity and power budget, PIN detector may or may not be a good choice. should the receiver sensitivity fall below the required value, this could be increased by either adding a pre-amplifier, using an APD instead of a PIN detector, or doing coherent detection instead of direct detection. the material also determines at which wavelength you can detect photons. for 1550 nm, a suitable choice would be InGaAsP as silicon is transparent at that wavelength.

Q2. give and derive the dispersion criterion for chromatic intramodal dispersion. what can you learn from it?

A2. dispersion criterion: B・L・|D|・dlambda < 1. basically tells you the relationship between different factors affecting transmission. for example, in order to transmit at longer lengths, L, the laser linewidth, dlambda, should be significantly small..etc.

Q3. which nonlinear effects play a role in optical telecom systems? how do you avoid them?

A3. nonlinear effect occurs because of the intensity dependence of the refractive index. generally, to avoid these effects, modulate the input power and amplifier gain.
SPM - use a power source with as narrow a linewidth as possible.
XPM - nonlinearity occurs in MMF. use non-zero dispersion shifted fiber.
FWM - use non-zero dispersion shifted fiber.

Q4. what is the cut-off wavelength of a fiber? where does it play a role?

A4. (see notes)

Q5. give an example of how WDM can be used to facilitate communication in a local area network.

A5. (see notes. what i did, though, was give different LAN topologies and explained where multiplexing and demultiplexing can take place as well as what kind of lasers are best suited at each node.)

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summer school (dummy post)

spss 2014

M05

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fourier transform phase component

hahahahahaha.

"We generally do not display PHASE images because most people who see them shortly thereafter succumb to hallucinogenics or end up in a Tibetan monastery."

 sorry, this isn't very helpful and i can't even explain why i find it funny but i'm laughing my head off, hahaha.

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optical activity and how not to work with lasers

optical activity is a property of certain materials where they cause rotation of incoming linearly polarized light around the direction of propagation. If the polarization direction rotates clockwise when viewing from the light source, the material is called dextrorotatory. otherwise, it is called levorotatory. A most common example of an optically active material is a sugar solution, which is what we're measuring above. Optical activity happens in chiral substances and sugars are mostly dextrorotatory, which is why sucrose is also known as "dextrose".

moving on to what we're doing above, the set-up is for measuring the rotatory power of a sugar solution. from the right we have the light source (HeNe laser), then the substance to be measured, an analyzer, and a power meter at the end. the rotatory power (theta_rot) is given by the formula,

theta_2 - theta_1 = theta_rot * l * P / 100

the HeNe laser used here already gives off linearly polarized light due to the brewster window incorporated in the manufacturing. we, therefore, only need to measure the following: theta_1 (azimuth angle of the analyzer of diminishing intensity without test solution), theta_2 (azimuth angle of the analyzer of diminishing intensity with test solution), l (length of the "tube"), and P (concentration of the sugar solution). so what i was doing above when yuting took the photo was actually changing the angle of the polarizer until we get a minimum transmission.

and there you have it! i still have to collate my data into a report but that's basically how you measure rotatory power with two polarizers instead of a saccharimeter.

oh, and, HeNe lasers are high-powered lasers which is why laser safety dictates one should wear safety glasses when working with these type of lasers. tsktsk ishi. reflections are not very harmful, though, but it's still advisable to work with goggles all the time in case of accidents.

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on-chip mach-zehnder interferometer

i've been studying interferometers for some time now and though i understand why fabry-perot interferometers and ring resonators give a resonance pattern, somehow, i couldn't quite grasp the reasoning behind "resonance" patterns in mach-zehnder interferometers. in other words, what my question really is IS, unlike FP and ring resonators, light doesn't get coupled back into an MZI, so how does it even produce resonance?

in the end i realized i have been trying to understand the "resonance" graph all wrong. first of all, it would be wrong to describe the graph above as "resonances" as you can see dips instead of resonances but the previous TA always uses the term "resonance" which probably added to my confusion. the graph above, in fact, does not show resonance peaks but rather interference.

so, basically, the dips in the wavelength vs attenuation graph above shows the wavelengths at which destructive interference occurs due to the difference in arm length of the interferometer. changing the arm length difference results in (wavelength shift or FSR change?). furthermore, as can be easily noticed, the graph (regardless of the dips) does not follow a straight line. in fact, it is relatively bent with a minimum value of about 12dBm at 1540nm. this bending is due to the attenuation of the waveguide itself which shows it has been optimized for 1540nm use.

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