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Fabry-Perot Interferometers Astronomy 6525 Literature: C.R. Kitchin, “Astrophysical Techniques” Born & Wolf, “Principles of Optics”

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Fabry-Perot Interferometers

Astronomy 6525

Literature: • C.R. Kitchin, “Astrophysical Techniques”• Born & Wolf, “Principles of Optics”

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Theory: 1Fabry-Perots are best thought of as resonant cavities formed between two flat, parallel highly reflecting mirrors. The pair of mirrors is called an etalon.

Let: d = spacing between the mirrorsτ = amplitude transmissivityρ = amplitude reflectivity

For a collimated beam with amplitude, A, incident at angle θ, the phase shift (difference) between successive transmitted rays is:

θλπ

δ cos22⋅= d

θδ

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Theory: 2Here we ignored the phase shift on reflection (this just acts like a change in the cavity size) and assumed that the index of refraction between the mirrors is 1.

The reflection phase shift can be included by letting:d → d + ∆d

where:

and φ is the reflection phase shift (excepting the first ray).The complex amplitude for the nth transmitted ray is:

ψn = Aτ2ρ2(n-1)ei(ωt-(n-1)δ)

and the total amplitude is:

ψ = ∑ ψn = Aτ2eiωt/(1 - ρ2e-iδ)n = 1

θπλφ cos

2⋅

−=∆d

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Theory: 3 (Airy Function)So that the transmitted intensity is:

I ≡ ψ*ψ = A2τ4/(1 - 2ρ2cosδ + ρ4)

Rewriting in terms of the mirror intensity coefficients:t ≡ τ2 and r ≡ ρ2

The transmitted intensity of the etalon is:

I(δ)/Io = Tmax/1 + [4r/(1 – r)2]·sin2(δ/2)

where: Tmax ≡ t2/(1 – r)2

The form above is that of the Airy function. The incident intensityis given by:

Io = A2

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Theory: 4 (Transmission Peaks)In the limit of no absorption:

t = 1 – r and Tmax = 1

The Airy function has peak when the phase shift, δ, is an integermultiple of 2π, e.g.

δ = 2πm m = 0, 1, 2, … = order of etalon

where: m = 2d/λfor normal incidence.

(m-1) σ m σ (m+1) σ

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Theory: 5 (Peak Width)

Set:I(δ)/Io = Tmax/1 + [4r/(1 – r)2]·sin2(δ/2) = Tmax/2

Solve for δ, resulting in δHWHM

Then, the full-width at half-maximum (FWHM) of a single peak is (phase):

δFWHM = 2 δHWHM = 4 sin-1((1 – r)/2√r) ≈ 2(1 – r)/√r

!

δFWHM

σ

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Theory: 6 (Free Spectral Range)

The spacing between transmission peaks (in units of wavelengths) is called the free spectral range. We have:

mλ = (m – 1)·(λ + ∆λFSR)

⇒ ∆λFSR = λ/m

Thus: ∆λFSR = λ/m = λ2/(2d)

ΔλFSR

σ

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Theory: 7 (Reflectivity Finesse)The finesse, F, of an FPI is the ratio of the distance between peaks, to their FWHM.

F≡ 2π/δFWHM ≈ π√r/(1 – r)

This approximation is accurate to within 1% for r > 0.65.For normal incidence, we can compute the single peak width, from the definition of δ (=4πd/λ):

∆δ = (4πd·∆λFWHM)/λ2 ≡ δFWHMhence:

so that:F = λ2/(2d·∆λFWHM) = λ/(m·∆λFWHM)

The finesse along with the transmission, is the primary figure of merit for an FPI.

rr

dFWHM πλ

λ−

⋅≈∆1

2

2

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Theory: 8 (Resolving Power)

The resolving power:RFPI ≡ λ/∆λFWHM

= m·(λ/(m∆λFWHM)) = m·F

Since the resolving power in an interferometer is determined by the largest difference between interfering rays, F can be thought of as a path length multiplier, and is roughly the number of reflections an average photon makes before being transmitted through the system.

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Theory: 9 (Resolution Limitation)

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Theory: 10 (Contrast)Another figure of merit for the FPI is the contrast, which is the ratio of the maximum to minimum transmission:

C ≡ Tmax/Tmin

= (1 + r)2/(1 – r)2

= 1 + (2F/π)2

For a F > 15, C > 100, which is adequate.

C is a strong function of F: F > 45 ⇒ C > 1000

Graf 1986 Phd Thesis, Cornell

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Limits to Performance: I1. Mirror Absorption

If a is the single pass intensity absorption coefficient, then themaximum transmitted intensity is:

Tmax = t/(1 - r)2

= (1 – r – a)/(1 – r)2 = 1 – a / (1 - r)2

The reflectivity finesse is independent of the absorptioncoefficient. Note that for small a (≤ 2%), the transmission is1–aF, since F ~ the number of reflections. For aF/(π√r) << 1:

Tmax ~ 1 - 2aF/(π√r)

Since F = π√r/(1-r)

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Limits to Performance: IIMirror Absorption

Transmission of an FPIas a function of a = 1- t- r. Typically a ~ 1%,for which a good FPIwill have F ~ 30 to 50(r = 0.90 → 0.94)

It is important to keepTmax close to unity,since large F is notuseful.

Bradford 2001 Phd Thesis, Cornell

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Limits to Performance: III

2. Etalon Tip - Walk OffTipping the etalon causes beams to “walk” off the mirrors. Thebeam will walk a distance w before it is transmitted (after Freflections) given by: w = F⋅d⋅sin(θTip) = R⋅λ/2⋅θsky(Dp/DC)

Things get bad when the central ray has traveled a distance equalto the beam radius across the etalon, the constructive interferenceis significantly reduced:

DC/2 ≈ FTip·d·sin(θTip) ≈ FTip·d θTip⇒ FTip ≈ DC/(2d·θTip)

where DC is the beam diameter.

θTip

d

DC

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Limits to Performance: IV

Etalon Tip

Notice that tipping the etalon shifts the central wavelength oftransmission to shorter wavelength:

θTip = 0: 2d/λo = m

θTip small: 2d/λ1(cosθ) = m

⇒ 2d/λ1(1 - θ2/2) = m

⇒ λ1 - λo ≡ ∆λ = -λoθ2Tip/2

⇒ ∆λ = -λoθ2Tip/2 (blue shift)

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Limits to Performance: V

3. Mirror ParallelismIf one mirror is tipped with respect to the other at an angle θ,then the problem is much more severe than a mere tipping ofthe entire etalon. Not only does the reflected angel grow witheach reflection, but the cavity resonates at a differentwavelength at different positions!!

d

DC

θpar

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Limits to Performance: VI

Mirror ParallelismTo make an estimate of this effect, lets assume resonance atλo at one end of the etalon, and at λ1 at the other. Themaximum resolution of the etalon is then:

Rmax = (λo + λ1)/2(λo - λ1) = FPar· m

Now, if ∆ is the gap difference, i.e. ∆ = DC sinθpar (where DC=beam diameter, θpar=deviation from parallelism), then:

(λo - λ1) = 2d/m – 2(d + ∆)/m = -2∆/mor, letting (λo + λ1)/2 → λ, then:

Fpar·m = mλ/(2∆) ⇒ FPar = λ/(2∆)

If the mirrors are parallel to λ/n then FPar = n/2⇒ you need parallelism to better than λ/200 or so!

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Limits to Performance: VII

4. Aperture EffectsThe resonant condition is: 2d·cosθ/λ = m

normal ray: 2d/λo = moff-axis ray: 2d·cosθFP/(λ1) = m

⇒ 2d/λo ≈ 2d/λ1 (1 - θFP2/2) = m

⇒ λ1 - λo ≡ -∆λ = - λoθFP2/2

⇒ ∆λ/λo ≡ 1/R = θFP2/2 ≡ 1/(Faperture·m)

⇒ Faperture = R/m = 2π/mπ(θFP2) = πλo/(d·ΩFP)

⇒ R = 2π/ΩFP, or R·ΩFP = 2π

θFP

d

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Limits to Performance: VIIIAperture EffectsNow, since etendue is conserved:

AFPΩFP = APrimaryΩPrimary

⇒ ΩPrimary·APrimary/AFP · R = 2π⇒ Rmax = 2πAFP/APrimary/ΩPrimary

5. Diffraction EffectsDiffraction results in a minimum angle within the beam given by: θTeldiff ~ λ/DTel. Plugging this into our aperture effects equation:

Rdiff = 2πAFP/APrimary/π/4·(λ/DPrimary)2 = 8⋅(DFP/λ)2

where: Ω = π/4·(λ/D)2, DFP is the diameter of the beam through the etalon.

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Limits to Performance: IX

6. Surface Defects

Irregularities in the surfaces of the mirrors will cause irregularities in the path lengths for different regions of the cavity. Suppose we have an rms gap variation that is given by ∆s, and is distributed in a Gaussian manner. It can be shown that:

Fsurf = λ/(32(ln2))1/2∆s ≈ λ/(4.7∆s)

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Limits to Performance: X

Effective Finesse and Transmission

Each effect reduces both the FPI transmission, and the finesse. The total effective finesse is given by the inverse quadratic sum of each finesse term above with the reflective finesse (Cooper, J. and Greig, J.R. 1963 J. Sci. Instrum., 40, 433.):

1/Feff2 = Σ 1/Fi

2

The peak transmission is reduced, since some of the radiation is absorbed, some of it is forced out of the beam, and some of it interferes destructively. The etalon transmission is reduced by the ratio of the effective finesse to the reflective finesse:

TFPI = Tmax·Feff/Fref = [(1 – r – a)/(1 – r)]2·Feff/Fref

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Limits to Performance: XIOptimizing PerformanceIn a background limited situation, one can characterize the optimal performance of an FPI by the product of the finesse times the transmission:

NEP≡ PS|S/N=1 ∝ T-1/2·R-1/2 ∝ (F·T)-1/2

(where PS|S/N=1 is the signal power from the sky that is equivalent to the background noise)Therefore, we would wish to maximize the product:

Q ≡ F·TNow: Tmax ≈ 1 – (2aF)/(π√r)

⇒ Q ≈ 1 – (2aF)/(π√r)·F⇒ dQ/dF ≈ 1 – (4aF)/(π√r) = 0

For example: Foptimal ≈ π/4a ≈ 80 for a = 1%⇒ r ≈ 96%

√r ~ 1

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Applications of Fabry-Perot Interferometers

Use a Fabry-Perot with high resolution as the spectral element that defines the resolution

To filter out unwanted transmission peaks of a Fabry-Perot: Use several Fabry-Perots in series Use additional edge filters and band pass filters Some detectors are not sensitive to wavelengths outside of

their range The Fabry-Perot that determines the spectral resolution is

scanned over a wavelength range, by changing the plate separation to obtain a spectrum at each spatial position on the sky => resulting data is a 3D data cube:

2 spatial dimensions (from 2D detector array) 3rd dimension is spectral dimension (scanning time)

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Example: The UC Berkeley Tandem FPIOptical filtration train:Tunable high order FPI (HOFPI) is the resolution achieving device • n ~ 20 to 600 ⇒ R ~ 1000 to 36,000

(F varied from 30 to 60)• HOFPI fringes evenly spaced

frequency spaceFixed filters (low-order FPI, LOFPI)• Set in 1st to 4th ⇒ R ~ 30 to 260

(optimize F for line of interest)• One filter for each line of interest• Up to 6 filters on a wheelSalt Filters• Act as low frequency pass filters• Block typically > 20 µm• Can be put on wheel (specialized)Transmitted profile is the product of each element of the train.

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Examples of Fabry-Perot Instruments

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Kuiper Widefield Infrared Camera: KWIC• KWIC is an Imaging Spectrometer/Photometer

Tandem FPI’s1. R > 2000 (up to 10,000): Spectral Lines2. R ~ 35, 70, 105: Dust Continuum

• Wide field of view with diffraction limited beams using a 128 × 128 pixel Si:Sb BIB Array ⇒ 5.8’ × 5.8’ FOV

• Far-IR (18 µm < λ < 38 µm)Lines: [SIII] 18 and 33 µm

[SiII] 35 µm[NeIII] 36 µmetc.

Continuum: Tplanck ~ 85 K at 38 µm

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Optical Layout For KWIC: I

Latvakoski, H. 1997 PhD Thesis Cornell

Si:Sb BIB detector Bottom View• Beam enters at left at f/12.6• Passes through calibration

unit• Gas cell for spectral

calibration• Hot chopped blackbody for

flatfielding• HOFPI in collimated beam to

minimize aperture effects on R

• LOFPI in focus – makes parallelizing easy

• Reimaging optics to match f/# to pixel sizeTo KAO telescope

HOFPI

LOFPI

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Fabry-Perot Interferometer in the Mid-IR to Submm Wavelength Regime

In the mid-IR (≥25 µm) and up to submm wavelengths Fabry-Perot “plates” can be made using free-standing metal meshes

Scanning and fixed Fabry-Perot Interferometers with free-standing metal meshes have been used in many instruments

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Fixed Wavelength Filters

Fixed λ filters are used to sort the orders of the HOFPI:• Made from free standing metal mesh• Stretched onto stainless steel rings• Push and pull screws to align and fix FPI• Optically flat to about ¼ HeNe wavelength• Finesse typically 30 to 60, orders between 1 and 5, ⇒ R ~ 30 to 300• Peak transmissions from 25 to 90%• Quite robust, cycled dozens of times over many years

Lugten, J.B. 1986 PhD Thesis, UCB

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Properties of Mesh FPI: Finesse vs. Wavelength

• Worked with a variety of mesh grid spacings and optical transmissions

• Finesse typically scales as λ2

• Select mesh to keep F reasonable ensuring good transmission.

Lugten, J.B. 1986 PhD Thesis, UCB

∝ λ2

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Example of a Fabry-Perot Design: KWIC LOFPI KWIC LOFPI

• Flex vane based translation stage ensures parallelism• Mesh rings are magnetic stainless, held in place with magnets• Rough adjustment via externally accessible fine-adjustment screw• Scanning PZT for fine adjustment and scanning • Spacing measured with a capacitance bridge fed back to PZTs, and held to < 14 nM• Liquid Helium temperature operation• Tilt PZT’s travel ~ 2 µm

Latvakoski, H. 1997 PhD Thesis Cornell