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1. Introduction

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Background

Frequency is a fundamental physicalquantity of electromagnetic wave
Maintenance of THz frequency
metrology is required for various
THz applications

Advent of practical CW-THz
sources (THz-QCL, UTC-PD,RTD, etc)

Precise frequency measurement ofCW-THz wave is required!

However, techniques of frequency measurement in THz
region have been underdeveloped yet.

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Conventional method (1)Frequency calibration based on water vapor absorptionref) http://spec.jpl.nasa.gov/Simple
Pressure broadening of absorption line, discrete distribution

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Conventional method (2)Electrical heterodyned method

Optical interferometric method

ΔL=(λ/2)*N

coolingcoolingDifficult to cover all frequency region of THz wave (0.1~10THz)→Requirement of new method optimized for THz wave!

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Optical comb and THz comb
Frequency ruler in THz region!
Photocarrier THz comb (PC-THz-comb)

Accurate, stable, broadband selectivity, high spectral purity,
and absolute frequency calibration

Osaka University	Principle(freq. = fx)

Freq. domain(ML freq. = f)measuref b = f x − mfsign

m: order of comb modef: ML freq.
fb: beat freq.

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Determination of order of m
Shift of ML freq. by δf(f ® f+δf)δf bm=
δfChange of beat freq. by δfb(fb ® fb+δfb)

f x = mf − f b
f x = mf + f b(δf b /δf > 0)(δf b /δf < 0)

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2. Ti:S-laser-based
THz spectrum analyzer

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Laser source for PC-THz comb

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Stability of mode-locked frequency

Stability and accuracy of PC-THz comb is equal tothose of Rb freq. standard

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Experimental setup

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Observation of beat signal (1)（90GHz freq. multiplier source）	
f=81.6MHZ

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Observation of beat signal (2)（90GHz freq. multiplier source）	

Linewidth
RBW=1Hz, sweep time=2.294s

fb signal<0.6Hz@test source

Linewidth < 1Hz

Signal-to-noise ratio
RBW=1kHz, sweep time=773ms>50dB

2.5mW@
test source

Detection limit = 25nW

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Observation of beat signal (3)（90GHz freq. multiplier source）	
Laser control : OFF

Laser control : ON

RBW=100Hz, sweep time=165ms, full span=2kHz

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Observation of beat signal (4)（freq. multiplier source wave）	

Freq. modulation of test source(100GHz±87.6kHz@0.5Hz)

RBW=3kHz, sweep time=50ms,full span=300kHz

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Determination of absolute frequency~Shift of ML frequency by 100Hz~

δf b
δf454,027.976 − 333,027.731=81,823,857 − 81,823,757= 1210.00245

m=f x = mf + f b= 1210 * 81,823,757 + 454,027.976= 99,007,119,997.976 Hz

error=2.004Hz
Setting freq. of test source=99,007,200,000 Hz

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Frequency tuning of test source

Precision=2.8*10-11

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3. Fiber-laser-based
THz spectrum analyzer~simple, compact, inexpensive,and robust system~

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Er-doped fiber laserref) H. Inaba et al, Opt. Express 14, 5223 (2006)

PZT

Temp. control (Peltier)Osaka University	

Stabilization system of ML frequency

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Photograph and cost of home-build
fiber laser	

45 cm

COSTEDFAOSCOSC¥ 700,000

EDFA¥ 650,000

Controlelectronics¥ 400,000

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Autocorrelation signal and spectrum

Average power = 90mW@1550nm or 10mW@775nm
Mode-locked frequency = 56MHz

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Stability of ML frequency of fiber laser

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Experimental setup4mW

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Observation of beat signal (1)（80GHz active frequency multiplier）	

Linewidthfb signal

Signal-to-noise ratio<0.6Hz@test source

Linewidth < 1Hz

2.5mW@
test source

60 dB

Detection limit = 2.5nW

Osaka University	Comparison of fb beat signal
Bowtie-type LTG-GaAs PCA

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Frequency tuning of test source（active frequency multiplier）	

Mean precision=2.2*10-11

Osaka University	Experimental setup for UTC-PD

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Real-time monitoring of beat signal
80GHz active freq. multiplier(2.5 mW)

120GHz UTC-PD(0.1 mW)

Freq. span = 2 kHz

Freq. span = 1.5 MHz

Sweep time=165 ms, RBW=100 Hz

Sweep time=20 ms, RBW=10 kHz

Osaka University	Comparison with conventional method