1.Terahertz electromagnetic pulse

Ref. : http://www.advancedphotonix.com

Boundary between light wave and electric wave

Excellent transmittance due to less scattering,free-space propagation,coherent beam,low energy,
broadband spectrum,
Spectral fingerprint(gas,drug,vitamin)

2.Application of THz spectroscopy
for monitoring of gas moleculeGas analysis in air is required for air pollution,
global warming,and ozone depletion

Absorption

dry air
water vapor

Frequency [THz]

Ref.Terahertz technology reportHigh accuracy, high resolusion THz spectroscopy is required!

3.THz time-domain spectroscopy (THz-TDS)
F.T.

Temporal waveform
Resolution = inverse of temporal window

Fourier spectrum
Accuracy = precision of time delay

Spectral scaling based on moving ofmechanical stage(1) Trade-off between spectral
resolution and measurement time(2) Spectral accuracy depends onpositioning precision of stage
Stage-scanning THz-TDS

4.Principle of AOS-THz-TDS
Temporal magnification factor

1f
M= Δ = 11Δf1
Mechanical stage for time delay is unnecessary!

Sampling interval
11Δ
S=− =f 2 f1
f1 f 2time domainfrequency domain

Frequency range
1f1 f 2
F .Range = =SΔ
Frequency resolution ＝ f1
Temporal overlap of THz pulse and probe pulse
automatically shifted at every pulse!

Scan rate = Δ	

5.Ｐｒｏｂｌｅｍ of previous AOS-THz-TDS system
Previous AOS laser source

Δ: stabilized f1,f2: free-running

Temporal magnification factor

1f1
Δ
M==1
Δf1

fluctuated by free-running laser
fixed by laser control

M fluctuates
depending on f1

Fluctuation on frequency scaledepending on f1

Decrease of accuracy and resolution in THz-TDS

6.New AOS laser source
M is fixed and selectable

f1,f2,Δ: stabilized
Δcan be set at arbitary frequency(260.000kHz)

f1=81,834,630,000Hz, f2=81,834,630,100HzΔ = f1-f2 = 100Hz

10-2

Δ	

(=100Hz,free-run)

10-4
10-6

-10

f1(free-run)

10-12
0.01

CLEOdf/f-rbNdf/f-A,freedf/f-F,freedf/f-D,freedf/f-D,stabi

f2(free-run)

for-8

10
10

Δ(stabilized)
10-1210-1010-810-610-40.010.010.1110100data

Frequency instability

7.Frequency instability of f1,f2,and Δ

Rb fr

f1f2(stabilized)
0.11Gate time[sec]10eq. standa

100rd8.Experimental setup

9. Experimental Results(1)0.4THz Metal Hole Array
φd

SUS304 plate (t=0.5mm) d=0.75mm,φ=0.4mm

Temporal waveform

Amplitude Spectrum

10. Experimental Results(2)Water vapor in room air
Temporal waveform

Amplitude Spectrum

10GHz

27GHz

Broadening of absorption line caused
by insufficient stability of Δ？	

11.Effect of timing jitter between two lasers
Insufficient stability of Δ
Effect of signal averaging
Timing jitter between two lasers·Inefficient signal averaging (low SNR)
·Smoothing effects due to random timing-jitter(Blurring of fine structures in THz signal)·Decrease of THz spectral bandwidth·Decrease of accuracy and resolution in THzTDS

Future(1) Further improvement of stability in Δ
(2) Select of larger Δ value(fluctuation of Δ is independent of Δ value
in new AOS laser source)

Conclusion(1) New AOS laser source
Stability of f1 and f2 = 10-11 @10sec(≒Stability of Rb frequency standard)(2)Application for water vapor and MHA
Frequency resolution = 27GHz@0.56THz (50sec)(3)Further improvement of laser stability is needed
for high accuracy, high resolusion AOS-THz-TDS