JOURNAL OF APPLIED PHYSICS 107, 033111 共2010兲

Continuously tunable, phase-locked, continuous-wave terahertz generator
based on photomixing of two continuous-wave lasers locked to two
independent optical combs
Takeshi Yasui,1,a兲 Hisanari Takahashi,2,3 Yutaka Iwamoto,2,3 Hajime Inaba,2 andKaoru Minoshima2,3
1

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 5608531, Japan
2
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 11-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan
3
Faculty of Science and Technology, Tokyo University of Science, 2641 Yamasaki, Noda, Chiba 278-8510,
Japan

共Received 16 November 2009; accepted 29 December 2009; published online 9 February 2010兲
A continuously tunable, phase-locked, single-frequency, continuous-wave 共cw兲 terahertz generator
has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band
unitraveling-carrier photodiode 共UTC-PD兲 used as a photomixer in this study. This cw-terahertz
generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser inthe optical frequency region, phase locked to two independent optical combs. The continuous tuning
range of the presented method was three orders of magnitude around 0.1 THz and two orders around
1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked
to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD.
The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation
was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This
cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency
metrology. © 2010 American Institute of Physics. 关doi:10.1063/1.3305324兴
I. INTRODUCTION

Terahertz electromagnetic radiation, lying at the boundary between optical and electrical waves, has emerged as aninnovative mode for sensing, spectroscopy, andcommunication.1 Along with recent progress in terahertz
technology, the requirements of terahertz frequency metrology have increased in various applications due to the factthat frequency is a fundamental physical quantity of electromagnetic waves. The terahertz frequency metrology can beestablished by realizing precise frequency measurement
equipments 共terahertz spectrum analyzers兲, signal sources ofstandard frequency 共terahertz clocks兲, and accurate, tunable,
single-frequency signal generators 共terahertz synthesizers兲.
The establishment of the terahertz frequency metrology will
enable the expansion of the scope of terahertz applications
based on its high reliability because frequency accuracy inthe terahertz region is relatively lower than that in the opticaland electrical regions. For example, when the terahertz radiation is used as a carrier wave for broadband wireless communications, its transmission frequency should be highly accurate and stable in order to secure necessary and sufficient
bandwidth for broadband communication without interferences with other applications, such as astronomy or sensing.
The terahertz clocks will be used to generate the accurate andstable transmission frequency. Also, the terahertz frequency
metrology will play an important role in frequency calibration of various types of commercial terahertz instruments,a兲Electronic mail: t-yasui@me.es.osaka-u.ac.jp.

0021-8979/2010/107共3兲/033111/7/$30.00

such as sources, detectors, and systems. The absolute frequency of the terahertz source is accurately determined by
the terahertz spectrum analyzers whereas frequency scale ofthe terahertz spectrometer is precisely calibrated by the terahertz synthesizers. The precisely calibrated terahertz spectrometer increases identification power in spectroscopic applications based on terahertz spectral fingerprints. For
example, high-resolution spectroscopy in gas sensing and astronomy requires high accuracy for the frequency scale because many absorption lines of molecular gasses jostle in theterahertz frequency range.
Recently, terahertz spectrum analyzers based on a photocarrier terahertz frequency comb2–4 and a nonsynchronizedelectro-optic sampling technique5 have been proposed andeffectively applied to measure the absolute frequency andspectral shape of continuous-wave terahertz 共cw-terahertz兲
radiation. On the other hand, accurate, stable, and tunablecw-terahertz sources are still lacking. Frequency-stabilized
cw-terahertz sources have been achieved by active control ofa terahertz quantum cascade laser 共terahertz-QCL兲;6,7 however, it is difficult to tune the frequency in a broad range dueto the restricted tunable range of the terahertz-QCL, typicallya few hundreds gigahertz depending on its band structure.8
One promising alternative method for widely tunable cwterahertz sources is photomixing of two cw near-infrared lasers of adjacent wavelengths with a photomixer, such as aunitraveling-carrier photodiode 共UTC-PD兲9 or a photoconductive antenna 共PCA兲.10 With this method, however, it isdifficult to obtain accurate, stable cw-terahertz radiation because the conventional photomixing techniques are based on107, 033111-1© 2010 American Institute of Physics

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free-running cw lasers without frequency control.3 If the optical frequencies of the cw lasers for photomixing could bewell stabilized in the optical region, photomixing techniques
would enable accurate, stable, and tunable cw-terahertz generation.
One attractive frequency reference for simultaneous control of two cw lasers is an optical frequency comb.11 A stable
millimeter-wave synthesizer has been realized by photomixing two cw lasers locked to an optical comb generator basedon an electro-optic modulator.12 Following it, accurate,
stable, phase-locked cw-terahertz radiation has been discretely generated at four different frequencies 共0.30, 0.56,
0.84, and 1.1 THz兲 by photomixing of two cw lasers locked
to a broadband optical comb based on a mode-locked Ti:sapphire laser.13 Furthermore, the output frequency was tuned
continuously by scanning the frequency interval of the optical comb while locking the cw lasers to the comb; however,
the range of continuous tuning was limited to several kilohertz. In the case where the two cw lasers share a single
optical comb, when scanning the frequency interval of thecomb, the optical frequencies of the two cw lasers change
simultaneously, like bellows of an accordion. This commonmode behavior cancels most of the optical frequency change
in the two cw lasers. As a result, the continuous tuning rangeof the cw-terahertz radiation is much smaller than that of the
optical frequency in the cw lasers. If the optical frequency of
one cw laser is made accurately tunable while that of theother cw laser is fixed at a certain value, the continuous
tuning range of the cw-terahertz radiation would be greatly
enhanced, equaling that of the optical frequency in the tunable cw laser.
In the study described in this paper, we demonstrated
continuous tuning of phase-locked cw-terahertz radiation
based on photomixing of an accurately tunable cw laser anda tightly fixed cw laser in the optical frequency region. The
two cw lasers were, respectively, phase locked to two independent optical combs, based on mode-locked Er-doped fiber
lasers,14 to increase the range of continuous tuning of the
cw-terahertz radiation while maintaining high-frequency accuracy and stability. To evaluate the potential of the proposed
method for application to a terahertz clock and synthesizer,
tight locking and continuous tuning of the cw-terahertz radiation were observed in real time with a fiber-based,terahertz-comb-referenced spectrum analyzer.3 The currently
demonstrated tuning range was over 16 MHz, which is limited by the electrical bandwidth of the terahertz spectrum
analyzer used for the measurement. In the current cwterahertz generation system, however, there are no obstacles
to achieving continuous tuning of 110 GHz without losing
phase locking to the comb, thus fully covering the available
spectral bandwidth of the F-band UTC-PD photomixer usedin this study.
II. PRINCIPLE OF OPERATIONLet us compare the continuous tuning range ofphotomixing-based cw-terahertz radiation between the previous method13 and our proposed method. We consider theprevious method shown in Fig. 1共a兲. Two cw lasers 共CWL1

J. Appl. Phys. 107, 033111 共2010兲

FIG. 1. 共Color online兲 Continuous tuning of the phase-locked cw-terahertz
radiation based on photomixing of 共a兲 two cw lasers 共CWL1 and CWL2兲
referenced to a single optical comb 共OFC兲 and 共b兲 two cw lasers 共CWL1 and
CWL2兲 referenced to two independent optical combs 共OFC1 and OFC2兲.

and CWL2兲 are respectively phase locked to two different
modes 共m1 and m2兲 of a single optical frequency comb
共OFC兲 共frequency interval of comb modes= f rep兲 and are then
optically heterodyned by a photomixer to generate cwterahertz radiation. The frequency of the cw-terahertz radiation 共f THz兲 is determined by the optical frequency separationof the two cw lasers. Therefore, it is possible to generate f THz
discretely in increments of f rep within the spectral bandwidthof the photomixer by selection of the targeted modes m1 andm2. Furthermore, f THz can be tuned continuously by scanningf rep while locking the two cw lasers to the comb. In this case,the range of continuous tuning 共⌬f THz兲 is determined by the
product of the scanning range of f rep 共⌬f rep兲 and the numberof comb lines between the two cw lasers 共⌬m = m2 − m1兲. For
example, ⌬m = 20, 000 when f THz = 1 THz and f rep= 50 MHz, and ⌬m = 2000 when f THz = 0.1 THz and f rep= 50 MHz. Therefore, when ⌬f rep = 500 kHz, which is 1% of
f rep, ⌬f THz = 10 and 1 GHz, for f THz = 1 and 0.1 THz, respectively.
In our method, consider that the two cw lasers 共CWL1and CWL2兲 are respectively locked to two independent optical combs 共OFC1 and OFC2兲, as shown in Fig. 1共b兲. We
assume that the carrier-envelope offset frequencies 共f ceo1 andf ceo2兲 of the two combs are fixed by respective laser control.
If the repetition frequency of OFC2 共=f rep2兲 is tunable 共tun-

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FIG. 2. 共Color online兲 Experimental setup. OFC1, OFC2, and OFC3: opticalfrequency combs; CWL1 and CWL2: cw near-infrared lasers; ␭ / 2: halfwave plate; ␭ / 4: quarter-wave plate; UTC-PD: unitraveling-carrier photodiode for photomixing; L: lenses; PCA: photoconductive antenna; SHG crystal: second-harmonic-generation crystal; UTC-NMIJ: coordinated universal
time operated by the National Metrology Institute of Japan.

able range= ⌬f rep2兲, whereas that of OFC1 共=f rep1兲 is fixed,
the tuning range of the cw-terahertz radiation 共⌬f THz兲 is determined by the product of ⌬f rep2 and the comb mode 共m2兲 towhich the cw laser used for tuning 共CWL2兲 is locked. The
value of m2 reaches 3 880 000 when the optical frequency of
the CWL2 is 194 THz 共corresponding wavelength= 1550 nm兲 and f rep2 is 50 MHz, which is two orders ofmagnitude higher than the value of ⌬m for photomixing withf THz = 1 THz. Furthermore, when f THz is set to 0.12 THz, as
demonstrated in the following experiments, the value of m2
is three orders of magnitude higher than the value of ⌬m. In
this method, with ⌬f rep = 500 kHz, the frequency tuning
range of the terahertz radiation, ⌬f THz, could reach 2 THz.
Therefore, our method enables a large increase in the continuously tuning range, ⌬f THz, of the cw-terahertz radiationcompared with the previous method.
III. EXPERIMENTAL SETUP

The experimental setup of the proposed method is shownin Fig. 2. We prepared two cw lasers operating at a wavelength of 1542 nm for photomixing. One was a distributed
feedback fiber laser 共CWL1; Koheras A/S, Inc., AdjustiKE15-PM兲, and the other was an external cavity laser diode共CWL2; Optical Comb, Inc., LT-5001兲. The optical frequency difference between them was set to approximately
120 GHz by using an optical wavemeter 共Advantest Corp.,
Q8326兲. CWL1 and CWL2 were respectively phase locked
to two independent optical combs of mode-locked Er-doped
fiber lasers operating at a center wavelength of 1550 nm
共OFC1 with f rep1 = 48.683 MHz, and OFC2 with f rep2
= 51.150 MHz兲. Details of the phase-locking process are
given elsewhere.15 The frequencies f rep1, f rep2, and f ceo2 werephase locked to microwave references synthesized from ahydrogen maser linked to coordinated universal time, oper-

J. Appl. Phys. 107, 033111 共2010兲

ated by the National Metrology Institute of Japan 共UTCNMIJ兲. Although f ceo1 is also an important parameter for the
final characteristics of this cw-terahertz generator as discussed later, it was not stabilized in the presented system due
to financial limitation. However, principal objective of thisstudy is to confirm the principle and usefulness of the continuously tunable cw-terahertz generator based on two independent optical combs. We consider that its principle andusefulness can be confirmed even though f ceo1 is not stabilized. CWL1, which was phase locked to OFC1, was used asa fixed cw light source in our method, whereas CWL2,
which was phase locked to OFC2, was used as a tunable cw
light source. In the present system, the optical frequency of
m2 in OFC2 could be continuously tuned over 1.7 THz bymaking full use of the scanning range of f rep2 共⌬f rep2
= 450 kHz兲. On the other hand, continuously tuning range of
CWL2 without the mode hopping was limited to 110 GHz
due to the stroke of a piezoelectric actuator used to tilt adiffraction grating for wavelength tuning. As a result, theoptical frequency of the tunable light source could be continuously tuned over 110 GHz without losing phase locking
of CWL2 to OFC2 by varying the repetition frequency over
30 kHz with a variable optical delay line module in OFC2.15
If we use a cw laser having broader tunability for CWL2, itshould be possible to extend tuning range of the tunable lightsource to 1.7 THz. Work is in progress to extend the tuning
range over 1 THz.
The outputs of CWL1 and CWL2 were combined with afiber coupler and then photomixed by an F-band UTC-PD
共NTT Electronics, available frequency= 90– 140 GHz兲
equipped with a horn antenna. The polarization of CWL1
was well aligned with that of CWL2 by adjusting a halfwave plate 共␭ / 2兲 and a quarter-wave plate 共␭ / 4兲. The photocurrent of the UTC-PD was set to 6.9 mA by controllingthe output powers of CWL1 and CWL2. This resulted in
cw-terahertz radiation with an average power of 250 ␮W at
a frequency of 120 GHz. This power is almost equal to thatof the traditional system based on photomixing of two freerunning cw lasers with the UTC-PD.
To evaluate the spectral characteristics of the proposed
cw-terahertz generator, we used a fiber-based, terahertzcomb-referenced spectrum analyzer 共spectral range= 0.075– 1.5 THz, frequency accuracy= 2.4⫻ 10−11, spectral
resolution= 2.3 Hz at a gate time of 1 s, detection limit ofterahertz power= 11 nW兲.2,3 The free-space-propagating cwterahertz radiation passing through a pair of terahertz lenses
共Pax Co., Tsurupica兲 was made incident on a bow-tie-shaped,
low-temperature-grown GaAs PCA in the terahertz spectrum
analyzer. The PCA was triggered by second-harmonicgeneration 共SHG兲 light 共center wavelength= 775 nm兲 fromanother repetition-frequency-locked fiber-laser comb 共OFC3
with f rep3 = 49.996 MHz兲. This resulted in the generation of aphotoconductive self-beat signal of OFC3 in the PCA,
namely, a photocarrier terahertz comb without frequency
offset.4 The spectrum analyzer was based on a frequency
beat measurement between the cw-terahertz radiation and thephotocarrier terahertz comb. The beat current signal from the
PCA was amplified by a high-gain current preamplifier
共bandwidth= 10 MHz and sensitivity= 105 V / A兲 and was

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In the frequency measurement with the terahertz-combreferenced spectrum analyzer, the absolute frequency of the
cw-terahertz radiation 共=f THz兲 is given byf THz = nf rep3 ⫾ f b ,FIG. 3. 共Color online兲 Spectra of f b beat signal when f rep3 is set at
49 996 105 Hz 共red plots兲 and 49 994 495 Hz 共blue plots兲 共sweep time
= 189 ms, RBW= 10 kHz, number of integrated signals= 100兲. The fitting
result for the red plots is shown as a green curve for comparison.

measured with a rf spectrum analyzer. Details of theterahertz-comb-referenced spectrum analyzer are given
elsewhere.2,3 The absolute frequency and spectral shape ofthe cw-terahertz radiation were measured by the terahertzcomb-referenced spectrum analyzer referenced to the frequency standard.

共1兲

where n is the order of the terahertz comb mode nearest in
frequency to the cw-terahertz radiation, f rep3 is the frequency
interval of OFC3, and f b is the lowest beat frequency.2 A
portion of the output light from OFC3 was detected with aphotodetector 共not shown in Fig. 2兲, and f rep3 was measured
to be 49 996 105 Hz with a rf frequency counter. The valueof f b was calculated to be 5.264 MHz by fitting the red plots
in Fig. 3 to a Lorentzian function, in which uncertainty of⫾2 kHz was achieved using the regression analysis based onLevenberg–Marquardt algorithm. To determine n and thesign of f b, f rep3 was changed by ␦ f rep3 共=−1.610 kHz兲. This
resulted in a change of the beat frequency by ␦ f b
共=3.924 MHz兲, as shown in the blue plots in Fig. 3. Since兩␦ f b兩 is equal to 兩n␦ f rep3兩, n is determined fromn=

兩3.924 ⫻ 106兩兩 ␦ f b兩= 2.437 ⫻ 103 ,=兩␦ f rep3兩 兩− 1.610 ⫻ 103兩

共2兲

Since the sign of ␦ f b / ␦ f rep3 共negative in this case兲 is opposite
the sign of f b, f THz was determined as follows:f THz = nf rep3 + f b = 2.437 ⫻ 103 ⫻ 4.999 610 5 ⫻ 107

IV. RESULTS
A. Terahertz clock

To evaluate the potential performance of the presented
system as a terahertz clock, we generated frequency-locked
cw-terahertz radiation by photomixing two fixed lightsources. Here, the combination of CWL2 and OFC2 in Fig. 2
was also used as the second light source. The red plots in
Fig. 3 illustrates the linear-scale spectrum of a beat signal at
the lowest frequency f b, namely, the f b beat signal, measured
by the rf spectrum analyzer 关sweep time= 189 ms, resolution
bandwidth 共RBW兲 = 10 kHz, number of integrated signals= 100兴. The linewidth of the cw-terahertz radiation was
1.093⫾ 0.008 MHz when a Lorentzian function was fitted tothe spectral shape by regression analysis based onLevenberg–Marquardt algorithm. The fitting result is shown
as a green curve in Fig. 3 for comparison. The reason why
we use the Lorentzian fit is that spectrum of the cw laser
usually indicates the Lorentzian shape although the terahertz
spectrum looks like the Gaussian shape. We consider that theGaussian-like spectral shape is caused by fast fluctuations of
optical frequency in the two cw lasers. CWL1 and CWL2
had linewidths of 0.890 and 1.07 MHz when phase locked to
OFC1 and OFC2, respectively. Since the convolution ofthem gave a linewidth of 0.97 MHz, slight broadening of thelinewidth occurred. We consider that the free-running f ceo1
was the source of this broadening. The linewidth of the cw
lasers used in the present system was limited by the frequency response of piezoelectric transducers used to control
their optical frequencies in addition to the spectral linewidth
in the free-running mode. Fast feedback control of cw lasers
further decreases the linewidth of the cw-terahertz radiation.

+ 5.264 ⫻ 106 = 121.845 772 GHz.

共3兲We next measured the optical frequencies of CWL1 andCWL2 using an optical wavemeter. Since the resulting frequencies were 194.405 46 and 194.526 55 THz, respectively,f THz was 121.09 GHz. Therefore, there was a discrepancy of
760 MHz, or 0.63%, between the terahertz spectrum analyzer
measurement and the optical wavemeter measurement. Since
the optical frequency resolution of the wavemeter
共=10 MHz兲 was much smaller than this discrepancy, the error caused by the wavemeter measurement is likely to be
negligible. Since the terahertz spectrum analyzer had thesame frequency accuracy as that of the reference frequency
standard, we considered the two major components for theuncertainty of the cw-terahertz generator induced in the terahertz spectrum analyzer measurement as follows. One component originated in the peak analysis used to determine f b
by the curve-fitting analysis of the spectral shape. The actual
uncertainty in this peak analysis was estimated to be 2 kHz
for the f b value of 5.264 MHz, which is caused by the distorted spectral shape and broad linewidth of the beat signal.
This causes an error of ⫾2 when determining the n value
based on Eq. 共2兲. Therefore, the uncertainty in the peakanalysis would lead to an uncertainty of 100 MHz, or
0.083%, in the f THz value obtained based on Eq. 共3兲. The
other component was fluctuation of f ceo1 because it was notstabilized. To evaluate the fluctuation of f ceo1, we measured
the frequency deviation of the f b beat signal during a 4 min
period, which was equal to the measurement time required
for the experiment shown in Fig. 3, using the terahertz spectrum analyzer. The resulting frequency deviation was 25.5
kHz. This f b deviation leads to a deviation of 800 MHz, or
0.65%, in the value of f THz obtained based on Eqs. 共2兲 and

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FIG. 5. 共Color online兲 Continuous tuning of the phase-locked cw-terahertz
radiation around 121.85 GHz by scanning f rep2 at 0.14-Hz intervals 共sweep
time= 189 ms, RBW= 10 kHz, number of integrated signals= 100兲.
FIG. 4. Real-time monitoring of the phase-locked cw-terahertz radiation
when locking its output frequency at 121.845 772 GHz 共full span= 4 MHz,sweep time= 97 ms, and RBW= 510 kHz兲 共enhanced online兲. 关URL:
http://dx.doi.org/10.1063/1.3305324.1兴

共3兲. Taking the root-sum-square of these two major components, the total uncertainty was estimated to be 810 MHz, or
0.65%. Therefore, the discrepancy between the terahertz
spectrum analyzer measurement and the optical wavemetermeasurement was within the estimated uncertainty of the cwterahertz generator. If f ceo1 were further stabilized in thesame manner as f ceo2, the relative standard uncertainty in theterahertz spectrum analyzer measurement would be substantially decreased. Further improvement is expected if the linewidth of the cw-terahertz radiation can be narrowed by fast
feedback control of the cw lasers.
Real-time monitoring of the frequency-locked cwterahertz radiation was demonstrated and is shown as amovie in Fig. 4 共full span= 4 MHz, sweep time= 97 ms, and
RBW= 510 kHz兲. The raised background floor was due tothe noise characteristics of the current preamplifier used in
the terahertz spectrum analyzer whereas the small peak beside the cw-terahertz radiation resulted from electromagnetic
noise propagating in free space. Although signal-to-noise ratio of the terahertz spectrum is a little poor due to insufficient
gain of the current preamplifier, it is important to note thatthe terahertz-comb-referenced spectrum analyzer captures
the spectrum of the cw-terahertz radiation in real time without the need for cryogenic cooling of detector or mixer. As aresult, spectral behavior of the cw-terahertz radiation can beinvestigated in detail. For example, the center frequency ofthe cw-terahertz radiation was tightly fixed at a certain value
although its spectral shape was a little fluctuated by instability of optical frequency in the two light sources for photomixing. We previously measured the frequency spectrum of
the cw-terahertz radiation generated by photomixing of two
free-running cw lasers and confirmed that its spectrum
largely fluctuated due to instability of the optical frequencies
in the two free-running cw lasers.3 Comparison with thepresent results clearly shows the superiority of the proposed
method even though f ceo1 was not stabilized. If f ceo1 wasfurther stabilized, this cw-terahertz generator could be used

as an F-band terahertz clock with accuracy and stability
equal to those of the reference frequency standard used.
B. Terahertz synthesizer

The current cw-terahertz generator can fully cover theavailable spectral bandwidth of the F-band UTC-PD photomixer because CWL2 achieves continuous tuning of 110
GHz without losing phase locking to COMB2.15 We show
here the result of continuous tuning of the cw-terahertz radiation around 121.85 GHz by scanning f rep2 at 0.14 Hz intervals. The resulting consecutive spectra of the cw-terahertz
radiation are shown in Fig. 5 共sweep time= 189 ms, RBW= 10 kHz, number of integrated signals= 100兲, in which thehorizontal coordinate is scaled by the terahertz spectrum analyzer. The reason why the amplitude of the terahertz spectrum decreases as the frequency increases is frequency dependence of the gain in the current preamplifier rather thanthat of the output power from the UTC-PD. We fitted theirspectral shapes to Lorentzian functions and determined thecenter frequencies of the cw-terahertz radiation 共f THz兲 in nineconsecutive spectra. The relationship between the change off rep2 共⌬f rep2兲 and that of f THz 共⌬f THz兲 is shown in Fig. 6. A
linear relationship was found in the form ⌬f THz = m2⌬f rep2,
where m2 = 3 757 241, with a regression coefficient of
0.999 92. Although ⌬f rep2 was changed by only 1.12 Hz in

FIG. 6. 共Color online兲 Relationship between change of f rep2 共⌬f rep2兲 and thatof f THz 共⌬f THz兲.

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ranging from 90 to 140 GHz. Furthermore, if we use a broadband photomixier, such as PCA,10 and a CWL2 having thecontinuous tunability over a few terahertz, it will be possible
to extend tuning range of this cw-terahertz generator to 1.7
THz.

V. CONCLUSION

FIG. 7. 共Color online兲 Real-time monitoring of the phase-locked cwterahertz radiation while tuning its output frequency continuously around121.85 GHz 共full span= 16 MHz, sweep time= 97 ms, and RBW= 510 kHz兲
共enhanced
online兲.
关URL:
http://dx.doi.org/10.1063/1.3305324.2兴

Fig. 6, the slope m2 is maintained in full range of ⌬f rep2
共=450 kHz兲. This results in continuous tuning of 110 GHz.
It is important to note that the slope m2 of the present method
was three orders of magnitude larger than the slope ⌬m ofthe previous method13 when f THz was set to around 0.12
THz. This resulted in a large expansion of the continuous
tuning range of the cw-terahertz radiation. From the opticalwavemeter measurement, we also determined m2 to be
3 803 061. Although there is a discrepancy of 45 820, or
1.2%, in these m2 values, we attributed the main cause of thisdiscrepancy to the fluctuation of f ceo1, as discussed in theprevious section. The acquisition time for all spectra in Fig. 5
was about 25 min. The frequency drift of the measured f bbeat signal indicated that f ceo1 changed by 250 kHz in 25
min. Since this deviation of f ceo1 shifts ⌬f THz by 250 kHz
between the beginning and end of the experiment of Fig. 5, it
leads to an error in m2 of 238 679, or 6.3%. Therefore, thediscrepancy between the two m2 values is within the uncertainty of the cw-terahertz generator. This can be further improved if the range of the frequency scanning is increased,
together with stabilizing f ceo1.
Finally, we demonstrated real-time monitoring of the
cw-terahertz radiation while tuning its output frequency continuously around 121.85 GHz. The resulting movie is shownin Fig. 7 共full span= 16 MHz, sweep time= 97 ms, and
RBW= 510 kHz兲. Smooth tuning of the cw-terahertz radiation was achieved over 16 MHz in this demonstration, although the signal power became smaller at higher frequencies due to the frequency response of the current preamplifier
used in the terahertz spectrum analyzer. It should be emphasized that the tuning range of this demonstration is not limited by the proposed cw-terahertz generator but by the electrical bandwidth of the current preamplifier. Therefore, we
believe that it should be possible to extend the tuning range
to 110 GHz in the current system because we have alreadydemonstrated continuous tuning of 110 GHz in the opticaldomain using the same setup.15 This tuning range can cover
the frequency bandwidth of the F-band UTC-PD photomixer,

Continuously tunable, phase-locked, 120 GHz cw radiation was demonstrated by photomixing of two stabilized cw
lasers referenced to two independent fiber combs. The spectral behavior of the tight locking and continuous tuning of
the cw-terahertz radiation was evaluated in detail with aterahertz-comb-referenced spectrum analyzer. The evaluation
clearly indicated the high potential of the proposed method
for application to a terahertz clock and synthesizer. Althoughcontinuous tuning of the cw-terahertz radiation is limited
within the F-band due to the available spectral bandwidth of
the UTC-PD photomixer 共=90– 140 GHz兲 in this study, it
should be possible to achieve it in the frequency region over1 THz by using a high-frequency photomixer, such as a
PCA,10 in place of the UTC-PD. The combination of thedeveloped cw-terahertz generator with the terahertz-combreferenced spectrum analyzer will pave the way for establishment of frequency metrology in the terahertz region. Furthermore, it is expected to find applications in carrier wave
generation for broadband wireless communications, in local
oscillators for heterodyne receivers, and in high-resolution
spectroscopy.

ACKNOWLEDGMENTS

This work was supported by Grants-in-Aid for Scientific
Research Grant Nos. 20560036, 21360039, and 21650111
from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan. We also acknowledge financial
support from the Tateisi Science and Technology Foundation,
Japan.
M. Tonouchi, Nat. Photonics 1, 97 共2007兲.
S. Yokoyama, R. Nakamura, M. Nose, T. Araki, and T. Yasui, Opt. Express 16, 13052 共2008兲.
3
T. Yasui, R. Nakamura, K. Kawamoto, A. Ihara, Y. Fujimoto, S.
Yokoyama, H. Inaba, K. Minoshima, T. Nagatsuma, and T. Araki, Opt.
Express 17, 17034 共2009兲.
4
T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, and T. Araki, Appl.
Phys. Lett. 88, 241104 共2006兲.
5
P. Gaal, M. B. Raschke, K. Reimann, and M. Woerner, Nat. Photonics 1,
577 共2007兲.
6
A. L. Betz, R. T. Boreiko, B. S. Williams, S. Kumar, Q. Hu, and J. L.
Reno, Opt. Lett. 30, 1837 共2005兲.
7
A. A. Danylov, T. M. Goyette, J. Waldman, M. J. Coulombe, A. J. Gatesman, R. H. Giles, W. D. Goodhue, X. Qian, and W. E. Nixon, Opt. Express
17, 7525 共2009兲.
8
S. P. Khanna, M. Salih, P. Dean, A. G. Davies, and E. H. Linfield, Appl.
Phys. Lett. 95, 181101 共2009兲.
9
T. Nagatsuma, H. Ito, and T. Ishibashi, Laser Photonics Rev. 3, 123
共2009兲.
10
S. Matsuura, M. Tani, and K. Sakai, Appl. Phys. Lett. 70, 559 共1997兲.
11
Th. Udem, R. Holzwarth, and T. W. Hänsch, Nature 共London兲 416, 233
共2002兲.
12
M. Musha, A. Ueda, M. Horikoshi, K. Nakagawa, M. Ishiguro, K. Ueda,
and H. Ito, Opt. Commun. 240, 201 共2004兲.
1
2

Downloaded 22 Mar 2010 to 133.1.55.214. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

033111-7

J. Appl. Phys. 107, 033111 共2010兲

Yasui et al.

13

Q. Quraishi, M. Griebel, T. Kleine-Ostmann, and R. Bratschitsch, Opt.
Lett. 30, 3231 共2005兲.
14
H. Inaba, Y. Daimon, F.-L. Hong, A. Onae, K. Minoshima, T. R. Schibli,
H. Matsumoto, M. Hirano, T. Okuno, M. Onishi, and M. Nakazawa, Opt.

15

Express 14, 5223 共2006兲.
H. Takahashi, Y. Nakajima, H. Inaba, and K. Minoshima, Conference on
Lasers and Electro-Optics 共CLEO兲 2009, Technical Digest 共CD兲, OpticalSociety of America, 2009 共unpublished兲, Paper No. CTuK4.

Downloaded 22 Mar 2010 to 133.1.55.214. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp