ECE 5616Curtis

Using Stock Optics

• What shape to use• X & Y parameters

• Please use achromatics• Please use camera lens• Please use 4F imaging systems• Others things• Data link

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Stock OpticsSome comments

• Advantages– Time and cost– Can still choose custom coatings (typically)– If vendor is good can eventually transfer quality measurement to

vendor after process is established.• ALL optics have to be inspected before being used for quite some

time.• Disadvantages

– Limited performance choices in EFL, lens diameter, and function.– Most lenses are design for infinite conjugates and visible

wavelengths– Part can be obsolete even if you are buying it. Must confirm supply

if used in production and guarantee transition time if part is to be discontinued.

– Vendor can go out of business.– Vendor quality can vary dramatically with time as they change

suppliers. Even the most simple things can be messed up. The vendor’s ability for SQE can vary a lot.

ECE 5616Curtis

What shape lens should I use ?Aberrations using Thin Lenses

Shape Parameter X

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Aberrations using Thin Lenses

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Spherical Aberration for a thin lens

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Thin Lenses approximation

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Summary Chart from Smith

Note direction of flat side of plano convex lens – toward the focus. This is the correct way to use it.

ECE 5616Curtis

When ever possible use camera lensNikon AF Micro-Nikkor

105mm f/2.8

Wavelength range is ~450 to 680nmUse in 4F system for imaging, collimators, or imaging at distance

ECE 5616Curtis

Camera Lenses in 4F

• Put film side of lens (BFP) toward image/object and set focus in infinity• Alignment – back propagate Plane Wave to put focus on Image/Object and set distance between lenses with a PW though both lens and a shear plate.• Most camera lenses have FFP INSIDE lens if EFL is less than ~60mm• Magnification use different EFL for first and second lens.

ObjectImage

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Stock LensesUse 4F system for imaging

• Two achromatic doublets corrected for infinite conjugate– Narrow field flatter surface towards image/object– Wider fields reverse orientation works better

• Limited Field with really good performance• Ratio of Focal length is magnification• Why 4F systems ??? - symmetrical systems about the stop

(between the lens) then the system is free of coma, distortion, and lateral color.

• Also 4F is telecentric and is insensitive to image/object position.

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Stock LensField Lens

Use a field lens to flatten the field (Eliminates Petzval Field Curvature with little effect on other aberrations). Use a negative lens to flatten field resulting from positive elements.

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Stock LensesSplitting elements

Reduce SA by factor of 5 by using two lenses together

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Stock LensesPlacement of Stop

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Stock LensesHigher NA – alignment of lenses

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Stock LensesGeneral considerations

• Please use achromatic doublets. Much better SA – at F#2 can get diffraction limited focus from achromat but not PC lens.

• Accuracy of EFL is typically better for achromatic doublet lens than singlet.

• Remember to get correct AR coating.• Pick correct material for lens depending on wavelength (BK-7 or fused

Si are typical choices)• Look at scratch and dig (quality) and surface quality (λ/8 or better for

laser apps) needed for your application.• Can measure actual lens performance and thickness and re-optimize

mechanics…

For demanding laser and imaging systems where minimizing scattered light is criticalHigh20-10

Excellent for laser and imaging systems with focused beams that can tolerate little scattered lightModerate40-20

Used for low power laser and imaging applications where scattered light is not as critical as costLow60-40

ApplicationsCostScratch-Dig

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Stock LensesSpatial Filtering – use microscope objective

• When a positive lens of focal length F focuses a Gaussian beam, the image at the focal plane (the Optical Power Spectrum, or OPS) will be an inverted “map” of spatial wavelengths present in the beam. Shortwavelength noise (dn) will appear in an annulus of radius Fλ/dn centered on the optic axis. The long spatial wavelength of an ideal Gaussian profile will form an image directly on the optic axis.

A pinhole centered on the axis can block the unwanted noise annulus while passing most of the laser’s energy. The fraction of power passed by a pinhole of diameter D is:

This passes 99.3% of the total beam energy and blocks spatial wavelengths smaller than 2a, the diameter of the initial beam. Since dn is always much smaller than the beam diameter, the filtered beam is very close to the ideal profile.

A pinhole of diameter Dopt:

and the minimum noise wavelength transmitted by the pinhole is

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Stock OpticsBeam splitters or plates

• Place in collimated beam if possible. Even tilted plate in collimated beam will not introduce astigmatism.

• If not in collimated space place close to image/object plane (like field lens) to have minimal impact.

• Remember to align lenses with the BS or plate in the system to adjust spacing.

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ZEMAX

Click on Len to get

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ZEMAX

Click on the down arrow to select a vendor of you choice

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ZEMAX

Vendor Melles Griot

EFL between 80 to 100 mm

Diameter 15 to 25 mm

Lens can only be doublet

and have all the type of shapes selected

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ZEMAX

Click on Insert to place doublet lens

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ZEMAX

Need to enter EPD using Gen Button and use the marginal ray solve to find the paraxial focus

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Data Link Design

Amnon Yariv, Optical Electronics

As an example of the use of some of the basic concepts, lets consider the problem of designing an optical communication link using an optical fiber for transmission of binary digital data over a distance of 5km. The system requirements are

1) Data rate 5x108 bits/s2) Error probability after amplification at the receive must be less than 10-9.3) Parameters: Loss is 4dB/km, λ=1.04um, total capacitance of diode

(junction plus package) is 3x10-12f, noise figure of amplifier is 6dB, QE of detector is 50%.

System is shown below. How much power do we need?

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Detecting PulsesThreshold Detection

Noiseless data

Detected data with threshold

Data stream after detection

in< -is(1-k), 1 turns into 0in> isk, 0 turns into 1

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Detecting PulsesThreshold Detection

Let the noise current be random Gaussian, then σ is the RMS of the noise current, σ2 = in2 . Setting k=1/2 give probability of error Pe of

Using

Gives

Pe

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Detecting PulsesThreshold Detection

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Data TransmissionPower Needed

Dominant noise sources are amplifier and Johnson noise in resistor, so

Signal current is given by

Ps is the peak pulse power incident on detector

SNR after amp is We need this to be >11.89

So we need to know Te, Δν, and RL to find needed Ps and then cascade that power back to the diode to find required diode output power.

2/1)/4(/

Le

s

N

s

RkTheP

ii

ννη

Δ=

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Data TransmissionPower Needed

K2901 o

eT

TF +≡

)/(2 πτν =Δ

Te is obtained from the amplifiers noise figure.

Te= 290 + (4-1)290 = 1160. Lecture 21

Bandwidth of signal is given as 5x108 pulses per second (binary data).Thus, conservatively

Where τ is 2x10-9s from data rateΔν = 3.18 x108 Hz

Now the RL must not exceed the value below to support this rate

CRL υπΔ

=2

1

So using the given value of C, RL< 167 ohms

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Data TransmissionPower Needed

Plug in these values into equation below and solve for Ps

89.11)/4(

/2/1=

Δ Le

s

RkTheP

ννη

Ps ≅ 10-5 watts

Now loss to detector include 5km of fiber (20dB) and assume another 4dB coupling loss into the fiber for a total of 24dB (factor of 251).

Therefore the required laser power must exceed

Plaser > 10-5 x 251 = 2.51 mW

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Reading

W. Smith “Modern Optical Engineering”

Chapter 21

See “Optical Resources” on class website.