Coherent THz light source
based on photo-mixing
with a UTC-PD andASE-free tunable diode lasers
D. Fukuoka, K. Muro, K. Noda

9747-42SPIE OPTO 2016

UTC-PD is …
High-Speed PhotodiodeUTC-PD : Uni-Traveling-Carrier PhotodiodeTHz-Wave Photomixer Module
Waveguide-Output Type

Produced by NTT-EL
Air Space Output TypeOur 1mm-Band External Cavity Diode Laseris used as Mixing Light Sources• High Power ASE-Free Output : 50 mW• Wide-Band Mode-Hop-Free Tuning : 50 nm• Ultra High Resolution Control : 0.01 pm

Outline of THz Light Source
ν1ν1 ,ν2ISO

ECDL 1 (Tune)

ECDL 2 (fix)Pmix = 45 mW at UTC-PD head
Iph = -5.8 mA, Vbias = -1.5 V

Mode Monitor(10GHz Etalon)

ν2ISO

2x2 PM fiber ν ,ν1 2combiner

Mixing Light Source in Single Chamber
ν, TEC, Power Control

ECDL Controller

Continuous THz-waves output
UTC-PDν1 - ν2Frequency Calibration(270MHz FSR Etalon)THz-Waves Detection

Off-Axis Parabolic Mirror
Continuous THz-Waves
Cold Filter Window

Mix. Lights

Mixing Light Source
UTC-PD

Function Generator
Mod. 80 Hz, 0/-1.5 V
Ref.

Lock-in Sig.

Vbias Mod.

Lock-in-Amplifier
Modulated THz-Wave Sig.

Ge Bolometerin LHe Chamber

Emission Spectra from UTP-PD Photo-Mixer
Detected Power (mW)

10101010101010Iph = 5.8 mA @ Pmix = 45 mW
0 - 1.5 V bias mod. @80Hz
L.I.A Det. with 0.1 sec
Frequency Step : 0.5 GHz

0-1-2-3-4-5-6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Frequency (THz)• THz-Waves Emission is observed up to 4.5 THz• 100 nW at 0.9 THz, 1 nW at 3 THz

4.55.0High Resolution Measurement ofWater Vapor Absorption
Detected Power (mW)

10101010-2-3-4-5

Noise Floor
10-6

1.63

1.64

1.65

1.66

1.67

1.68

1.69

1.70

1.71

1.72

1.73

Frequency (THz)• Dynamic range reaches 40 dB (in power ratio) against ~2 pW Noise Floor.
→ may be ascribed to ASE-free mixing light source.

Frequency Calibration
PBS

QWP

~ 555 mmTransmitted Signal
Heat Sink Case

Reflected Signal

FSR ≈ 270 MHz

Etalon Interference Fringe

RF Noise at ν1 = ν2
Difference Frequency “Zero Point”

0.8Signal (V)ν1, ν2 locked at Dark/Bright Fringe

1.00.6
0.4
0.2
0.0
-1.0

-0.5

0.00.5Different Frequency (GHz)1.0

Duel Frequency Locking
1.0Signal (V)0.8

Etalon Interference Signal

0.6
0.4
0.2

Error Signal
by Current Modulation

0.0
-0.2

-0.4

-0.2

0.0

0.2

0.4

Different Frequency (GHz)• Mixing frequencies are locked into an Etalon
by using error signal with current modulation.
Modulation frequency : 10 kHz (ν1), 1 kHz (ν2)
Modulation Amplitude : 20 MHz (0.5 mA)

Difference Frequency Tuning
under Dual Frequency Locking
ν1 : TuningLocking to the 270 MHz Etalon

Signal (V)

0.8
0.6
0.4
0.2

ν2 : Locked02468

Mode Monitor Signal(10 GHz Etalon)
10Different Frequency (GHz)• Fixed ECDL (ν2) is locked at a fixed mode.
• Tuning ECDL (ν1) is scanned, and locked at fringe by fringe.
 Beat noise at ν1 ≈ ν2

Indicates Zero Point of Difference Frequency

 Data are obtained at each fringewith an accuracy of etalon FSR estimation error.

Etalon FSR Estimation
8Mode Monitor Signal(10 GHz Etalon)

282.2

ν1 : Tuning

282.1
6Signal (V)Frequency Calibration Signal(270 MHz Etalon)

281.9

4281.82281.7 Fitting
Errors fromSlope = 0.2736812 GHz/Fringe

001000Fringe Number

FSR = 0.2736812 GHz (Estimation error is 1 x 10-7 GHz)fitting error (MHz)Frequency (ν1) obtained by
Wavelength meter(771A-NIR, Bristol)

Frequency (THz)

282.0

20

281.6

02000-2001000

Fringe Number

2000

Frequency Resettable Tuning

1010-3

Wavelength (nm)

10First scan
Second scan-2-4-5

1062.3670

0.00

1062.3668

0.05

0.10

1062.3666

0.15

1062.3664

0.20

1062.3662

0.25

10

20.0-6

1.240

1.250

1.260

Frequency (THz)

1.270

1.280

20.2

20.4

20.6

Frequency Shift (MHz)Detected Power (mW)

10

20.8

Temperature (°C)

Thermal Change Ratio of FSR = 1.15 x 10-6/℃≈ 𝜶𝒊𝒏𝒗𝒂𝒓 (1.2 x 10-6/℃)• Twice scans show good agreement.
Frequency Resettability is ascribed to less than 1.2 MHz @ 1 THz

Summary
Photomixing with UTC-PD and ASE-Free ECDL (λ-Master) Provides・ Wide-Tuning Range ：4.5 THz
・ High Resolution Frequency Control ：3MHz・ High-Purity THz-Waves： > 40dB
Dual Frequency Locking to an athermal etalon enables
・ High Accuracy and Resettability Spectroscopy : 1 MHz @ 1 THz