University of Tokushima	

Osaka University	THz frequency synthesizer
based on dual optical comb
T. Yasui1, 2), H. Inaba3), K. Minoshima3),and T. Araki1)
1)Osaka Univ., 2) Univ. Tokushima,and 3) AIST, Japan
Optical Comb Symposium @ Tsukuba (2011.9.26)

University of Tokushima	

Background

Osaka University	

Accurate, stable, tunable CW-THz source(THz clock and synthesizer)(freq. metrology, high-resolution spectroscopy, local oscillator for heterodyne
receiver, carrier wave for wireless communications,etc)

Photomixing of two CW NIR lasers
with photomixer
FreerunningRef) Laser&Photon. Rev. 3, 123 (2009).

University of Tokushima	

Osaka University	

Photomixing of two stabilized CW lasers
Optical frequency comb is attractive frequency
reference to control two CW lasers for photomixing
Conventional study	

Photomxing of two CW lasers phase-locked to single optical combRef) Opt. Lett. 30, 3231 (2005).

ΔfTHz=Δm × Δfrep
ΔfTHz: tuning range of CW-THz wave
Δm: the number of comb modeΔfrep: scanning range of frep

ΔfTHz < 1% of fTHz when fTHz=1THz
ΔfTHz <0. 1% of fTHz when fTHz=0.1THz

Common-mode change of CWL1 and CWL2
cancels most of optical frequency change!

University of Tokushima	

Osaka University	

Intensity

Optical Frequency Synthesizer (OFS)fCWfm

Optical frequency comb

f m = m ⋅ f rep + f ceo

frep

fceo

frep and fceo arephase-lockedFrequency

Precise freq. ruler€
in optical region
Phase-locking single-mode CW laser to optical comb	
fbeat

f ofs = m ⋅ f rep + f ceo + f beatPhase-locked to microwave
freq. standard	

Determined at uncertainty of frequency standard	
Continuous tuning by scanning frep	ref) Takahashi et al., CLEO2009, CTuK4

University of Tokushima	

Osaka University	

Performance of OFS	
Tuning characteritic-1

10-910-2

10-1010-3-1110-121010-4

100.010.1

1Frequency fluctuation [Hz]Frequency instability

Stability of fopt

Gate time [s]ref) Takahashi et al., CLEO2009, CTuK4

THz synthesizer based on photomixing oftwo independent OFSs

University of Tokushima	

Osaka University	ref) Opt. Express 19, 4428 (2011)University of Tokushima	

Osaka University	

Experimental setup
fofs1=fceo1+m1frep1+fbeat1

fofs2=fceo2+m2frep2+fbeat2

Freq.=90~140 GHz
Power=250µWref) Opt. Express 16, 13052 (2008); Opt. Express 17, 17034 (2009)University of Tokushima	

Osaka University	

Spectrum of CW-THz wave measured by
THz-comb-referenced spectrum analyzer
fTHz = 92 GHz

fTHz = 140 GHz

OFS1: Δfofs1=280 kHz OFS2: Δfofs2=600 kHz

Assignment of absolute frequency to
CW-THz spectrum around 140 GHz

University of Tokushima	

Osaka University	

Parameters of OFSs	
fceo (Hz)	 

m	 

frep (Hz)	 

fbeat (Hz)	 

fofs (Hz)	 

OFS1	  10,683,000	  3,889,264	  49,985,129.0	  -21,384,000	  194,405,352,054,056	 
OFS2	  10,683,000	  3,811,224	  50,971,884.1	  69,960,000	  194,265,348,650,138	 

Center frequency ofCW-THz wave (fTHz)
140,003,403,918 Hz(uncertainty = 10-12)	

University of Tokushima	

Osaka University	

Incremental tuning of CW-THz wave around 133
GHz by scanning frep2 at 0.2 Hz intervals

University of Tokushima	

Osaka University	Consecutive tuning of fTHz overrange of 1GHz around 131.71GHz@@@ GHz+0.2GHz+0.4GHz+0.6GHz+0.8GHz+1GHz

University of Tokushima	

7106

ΔfTHz (Hz)1010

Osaka University	

Tuning characteristic

13

ΔfTHz = m2 Δf rep 2 = 3,813,624 × Δf rep 251041031020.0

Presented method (dual optical comb)

Ref) OL 30, 3231 (2005).

Previous method (single optical comb)

ΔfTHz = ΔmΔf rep = 2,575 × Δf rep 20.5

1.0

1.5

2.0

Δfrep2, Δfrep (Hz)ΔfTHz can achieve 1.7 THz when Δfrep2 = 450 kHz

University of Tokushima	

Osaka University	

Summary
Continuously tunable THz synthesizer
traceable to hydrogen maser(1) Frequency uncertainty of 10-12(2) Linewidth = 600 kHz(3) Discrete tuning = 50 GHz,
limited by available BW of UTC-PD(4) Continuous tuning = 1.26 GHz(5) Maximum tuning range = 0.99 THz
if broadband photomixer is used