Osaka University	

CMI5

Terahertz-Comb-Referenced
Spectrum Analyzer
Takeshi Yasui
Osaka University, Japan
CLEO2009@Baltimore (2009.6.1)

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Collaborations
Mode-locked Er-doped fiber laser
THz clock and synthesizer
Drs. Hajime Inaba and Kaoru MinoshimaNat. Inst. of Adv. Ind. Sci. and Tech. (AIST),Japan

Photomixing of two CW lasers
with UTC-PD
Prof. Tadao Nagatsuma
Osaka University, Japan

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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-THzsources (THz-QCL, photomixing
with UTC-PD, RTD etc)

Precise frequency measurement ofCW-THz wave is required!

However, techniques of frequency measurement in THzregion are still lacking.

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Conventional method
Electrical heterodyned method

Optical interferometric method

Cryogenic cooling
Obstacle topractical use!

ΔL=(λ/2)*N

Difficult 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!

Simple, broadband selectivity, high spectral purity,offset free, and absolute frequency calibration

Osaka University	Principle
Freq. domain?

measurefx=mf ± fb

m: order of comb modef: ML frequency
fb: beat frequencyRef) Yokoyama et al, "Terahertz spectrum analyzer based on a terahertz frequency
comb", Opt. Express 16, pp. 13052-13061 (2008).

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Determination of m and sign of fb
Shift of ML freq. by δf(f → f+δf)

δf b
m=
δf
Change of beat freq. by δfb(fb → fb+δfb)€f = mf − fxbf x = mf + f b(δf b /δf > 0)(δf b /δf < 0)

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ML-freq.-stabilized fs Er-doped fiber laser
Compact, robust, inexpensive
Direct coupling with PCA

PZT

Temp. control (Peltier)ref) H. Inaba et al, Opt. Express 14, 5223 (2006)

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Control system of ML frequency

1.122GHz

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Photograph and basic performance	

EDFA

Pavg = 90 mW
λc = 1550nm
Δt = 41 fs

Optical spectrum

OSC

Autocorrelation signal

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Instability of ML frequency of fiber laser

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Experimental setup

THz-comb-referenced spectrum analyzer	

Osaka University	THz detection unit

1

Bowtie-type LT-GaAs-PCA(0.8µm@6mW)

2

Bowtie-type LT-GaAs-PCA(1.5µm@20mW)

3

Bowtie-type LT-InGaAs-PCA(1.5µm@30mW)

4

1mm-ZnTe-FSEOS(0.8µm@8mW)

Osaka University	Comparison of SNR of fb beat signal

Output of test source = 5.4 mW @ 80 GHz

Detection limit

6,800nW

540nW 11nW

4,300nW

Detection sensitivity

0.22%

10%

0.86%

100%

Osaka University	Linewidth of fb beat signal
Output freq.=80 GHz

LinewidthFrequency fluctuation

Linewidth = 10 mHz at 1 sec

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Determination of absolute frequency~Shift of ML frequency by 25Hz~
δf b −35,651.8m==≈ 1426
δf25
δf b −35,651.8=<0δf25€€€

f x = mf + f b= 1426 * 56,124,408 + 354,191.122= 80,033,759,999.122 Hz

error=0.878Hz

Setting freq. of test source=80,033,760,000 Hz

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Frequency tuning of test source

Mean precision=2.2*10-11

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Test source based on
photomixing of two free-running CW lasers
Free-running condition
Freq. fluctuation<100MHz/hour

Osaka University	Real-time monitoring of beat signal
Freq. span = 1.5 MHz
Output=100µW
Freq.=120GHz

Sweep time=20 ms
RBW=10 kHz

Large fluctuation of beat frequency
caused by two fee-running CW lasers

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Test source based on
photomixing of two stabilized CW lasers
Phaselocking

Optical comb 1

Optical comb 2

See CWB7,
in Topics in Optical Metrology I,
Optical Metrology@2009.6.3

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Real-time monitoring of beat signal
THz clock(freq. =121,845,771,520 Hz)
4MHz

THz synthesizer(tuning range = 90~140 GHz )16MHz

See CWB7, in Topics in Optical Metrology I, Optical Metrology@2009.6.3

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Optical comb and THz comb

Many CW-THz waves
having freq. spacing of f

Test source with multiple frequencies
ranging from sub-THz to THz

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Experimental setup

PC-THzcomb
f1=81,834,630 Hz
f2=81,834,635 Hz
Δ= f2- f1=5 Hzref) Yasui, APL 88, 241104 (2006).

Yokoyama, CMR5, CLEO2007.

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Spectrum of EM-THz comb

Re-scaling(x f1/Δ)

Spectral range over 1THz !

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Observation of EM-THz comb mode
Mode-locked freq.
f1=81,834,630Hz
Measurement time
=0.5sec

f1

f1

f1

f1

RBW=10Hz

Average power of each mode < 1nW
High sensitivity in THz region

Osaka University	THz frequency metrology based onfrequency comb techniques
Transfer ofaccuracy
Coherent linkof frequency

Same accuracy as microwave and optical regions