cavity-enhanced, frequency-agile rapid scanning spectroscopy: measurement principles j.t. hodges,...

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Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell, R.D van Zee, D.F. Plusquellic National Institute of Standards and Technology, Gaithersburg, MD [email protected] 68 th Ohio State University International Symposium on Molecular Spectroscopy June 17-21, 2013, Columbus OH 250 spectra in 0.7 s

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Page 1: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles

J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell, R.D van Zee, D.F. Plusquellic

National Institute of Standards and Technology, Gaithersburg, MD

[email protected]

68th Ohio State University InternationalSymposium on Molecular SpectroscopyJune 17-21, 2013, Columbus OH

250 spectra in 0.7 s

Page 2: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Single-mode excitation with locked cavity

Cavity ring-down spectroscopy (CRDS)

Fabry-Pérot resonator

incident laser beam

recirculating field

detector

low-loss dielectric mirror

Attributes:compact volume insensitive to atmospheric absorption and laser intensity noiselong effective pathlength, leff = lcav(Finesse/)potentially high spectral resolution & negligible instrumental broadeningreadily modeled from first principlesspectra based on observation of time and frequency

Page 3: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

A little history …multi-mode cavity ring-down spectroscopy (CRDS)signal with pulsed excitation

Signals are dominated by transverse and longitudinal mode beating effects, resulting in suboptimal statistics and severely compromised frequency resolution.

transverse mode beats

transform-limited pulse

Page 4: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Excitation bandwidth << free-spectral range (FSR)

cavity mode spectrum

0 20 40 60 80tim e (s)

-0.006

0

0.006-0.006

0

0.006

0

0.4

0.8

1.2

Rin

g-do

wn

sign

al (

V) 300 shot average:

= 3 .7097(53) s

= 6 .3783(82) s

Re

sid

ua

ls (

V)

empty-cavity

absorbing medium

single-mode decay signals

CRDS with continuous wave lasers

Page 5: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

cw-CRDS scanning methodstechnique RD signal

amp; acq. rate (Hz)

frequency detuning

meas.

frequency res.

other

dither cavity length,step tune laser viacurrent, pzt or temp

low; 10 - 100 externaletalon, l-meter

laser bandwith, >> cavity linewidth

std. approach,slow scan

dither laser frequency through FSR at fixed cavity length, step tune laser viacurrent, pzt or temp

low; 10 - 100 externaletalon, l-meter

cavity mode spacing,>> cavity linewidth

slow scan,no cav. pzt req’d

rapidly sweep laser frequency via current tuning

low; ~5 kHz mode spacing

cavity line width

RD signal distortion

optical feedback lock oflaser to cavity, scan cavity to drag laser frequency

high; ~5 kHz pzt tuning ofcavity mirror

cavity line width

can’t use 2-mirror cavity, non-linear tuning axis

Page 6: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Frequency-stabilized Cavity Ring-Down Spectroscopy (FS-CRDS)

frequency-stabilizedreference laser

cw probe laser

cavity stabilization servo

pztoptical resonator

decay signal

(a)

(b)

time

stabilized comb of resonant frequenciesFSR = 200 MHz

absorption spectrum

frequency

Enables high-fidelity and high-sensitivity measurements of transition areas, widths & shapes, positions and pressure shifts

Page 7: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

High-spectral fidelity of FS-CRDS

Saturation dip spectroscopyof blended H2O spectra

Systematic errors arise from overly simplistic line shapes

Voigt Profile

Galatry Profile

Line shape effects in O2

Page 8: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

The problem of slow frequency tuning

optical frequency

To record a spectrum in FS-CRDS you typically tune the laser frequency by

using a grating, pzt-actuated mirror or by temperature tuning

These approaches limit the spectrum acquisition rate to ~5 s/jump

Page 9: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Rapid Step Scanning of Laser Frequency

Page 10: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Frequency-agile, rapid scanning (FARS) spectroscopy

Advantages:• Overcomes slow mechanical and thermal scanning • Links optical detuning axis link to RF and microwave standards• Wide frequency tuning range (> 90 GHz = 3 cm-1)

Method:• Use waveguide electro-optic phase-modulator (PM) to generate tunable sidebands• Drive PM with a rapidly-switchable microwave (MW) source• Fix carrier and use ring-down cavity to filter out all but one selected side band

MW source

phase modulator

cw laserring-down cavity

side-band spectrum

Detector

gas analyte

Page 11: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

nC+d+2FSR

nC+d+FSR

nC+d

cavityresonances

frequencyscanning

FSR

FARS measurement principle

Page 12: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

d

carrierselected sideband

FSR

Lowest order of a spurious sideband close to a cavity mode is1- N where,

N = Round(R=FSR/ d)

How well does the cavity filter out sidebands?

Page 13: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

In general for unwanted sideband orders,local detuning/cavity linewidth = e*finesse/N where e = R – N (non-integer remainder)

In the absence of dispersion, this level of discrimination does not change as the modulation frequency is stepped in increments of the FSR

If there is a spurious overlap, one can readily change carrier detuning to avoid this situation

Cavity filtering (fixed TEM)

Page 14: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

R = 203.076 (MHz)/13 (MHz) = 15.621 so that N = 16 and epsilon = 0.3788, meaning that the m = -15 sideband would be the first one to come near a resonance of the cavity.

For our finesse of 20,000, the local detuning would be about 485 times the cavity line width, showing that we have nearly perfect frequency discrimination (assuming perfect mode matching into TEM00).

We have never observed any evidence of spurious coupling into other sidebands.

Sideband filtering for our spectrometer

Page 15: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Independent methods for characterizing frequency axisof PDH-locked FARS-CRDS setup

1. Measure frequency, f, of probe laser with optical frequency comb and count change in mode order, q. Gives absolute frequencies and cavity free spectral range (FSR).

2. Measure FSR from differences in microwave frequencies corresponding to transmission resonance peaks.

3. Measure FSR with dual sideband method of Devoe & Brewer.

Methods 2 & 3 give agreement in FSR at 2 Hz level and yield dispersion

Absolute frequencies are ~5 kHz and are limited by 10 kHz stability of I2-stabilized HeNe reference laser

Page 16: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Devoe & Brewer, PRA 30, 2827 (1984).

Dual-sideband FSR measurement scheme

w0 –(w1+w2)w0 -w1

w0 –(w1-w2) w0 -w2 w0 +w2

w0w0 +(w1-w2) w0 +w1+w2

w0 +w1

qq-1 q+1

D-d D D+d

Two sets of sidebands:w1 at FSR= sw2 for PDH lock D = w0 – qsd = w1 – s

Demodulation of heterodyne beat at w1 - w2 givesdispersion signal g(d) centered about =0d , whered = w1 – .s

Page 17: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Accuracy of FARS-CRDS frequency axis

cavity dispersion

gDD 40 fs2

Page 18: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Due to the quality of our frequency axis we can record the shape and width of individual cavity resonances

The width of the resonances provides an equivalent measure of the absorption in the frequency domain,α = Δω1/2/c

~130 Hz relative laser linewidth

Uncertainty of the fitted resonance frequency ~1 Hz

Uncertainty of the fitted width of the resonances ~0.04%

Measuring losses in terms of cavity line width

Page 19: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Effect of beam extinction ratio on ring-down time measurement statistics

Page 20: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Extinction ratio = 10 log( Id/Il)

t0

Il = “leakage” intensity

Id = “decay” intensity

Id

Il

cavity decay signal = Idexp(-t/t)

cw leakage signal = Il

Ideal case (infinite extinction ratio): Il = 0, exponential decay

Actual case: leakage intensity interferes with decay signalto yield noisier and/or non-exponential decay

Page 21: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Measured FARS-CRDS decay signals

Noise in residuals is insensitive to extinction ratio (phase-locked case)

Systematic deviations become important for extinction ratios < 50 dB

Page 22: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

Huang & Lehmann, Appl. Phys. B 94, 355 (2009)

This work

With DFB laser leakage intensity introduces excess noise in ring-down signal

phase locked case, small amount of excess noise

Effect of extinction ratio on measurement precision

/ = 8x10-5

Page 23: Cavity-Enhanced, Frequency-Agile Rapid Scanning Spectroscopy: Measurement Principles J.T. Hodges, D.A. Long, G.W. Truong, K.O. Douglass, S.E. Maxwell,

FARS-CRDS has been demonstrated with:

1) distributed feedback diode laser (DFB)

2) single-mode fiber laser

3) external cavity diode laser (ECDL) with high-bandwidth Pound-Drever-Hall lock

waveguide electro-optic phase modulator