Real-time monitoring of continuous-wave
terahertz radiation using a fiber-based,terahertz-comb-referenced spectrum analyzer
Takeshi Yasui,1,* Ryotaro Nakamura,1 Kohji Kawamoto,1 Atsushi Ihara,1,2
Yoshihide Fujimoto,1 Shuko Yokoyama,1,3 Hajime Inaba,2 Kaoru Minoshima,2Tadao Nagatsuma,1 and Tsutomu Araki1
1

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan
2
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology,
1-1-1 Umezono, Tsukuba, Ibaraki, 305-8563, Japan,
3
Micro-Optics Co. Ltd., 2-26 Oe-Tsukahara, Nishikyoku, Kyoto, 610-1105, Japan*t-yasui@me.es.osaka-u.ac.jp

Abstract: We propose a fiber-based, terahertz-comb-referenced spectrum
analyzer which has the advantages of being a portable, alignment-free,robust, and flexible apparatus suitable for practical use. To this end, weconstructed a 1550-nm mode-locked Er-doped fiber laser whose modelocked frequency was stabilized precisely by referring to a rubidium
frequency standard, and used it to generate a highly stable terahertz (THz)frequency comb in a photoconductive antenna or an electro-optic crystal.
By standardizing the THz comb, we determined the frequency accuracy ofan active-frequency-multiplier-chain (AFMC) source to be 2.4 × 10−11.
Furthermore, the potential of the THz spectrum analyzer was effectively
demonstrated by real-time monitoring of the spectral behavior of the AFMC
source and a photomixing source of two free-running CW lasers at adjacent
wavelengths.
©2009 Optical Society of America
OCIS codes: (120.3930) Metrological instrumentation; (120.6200) Spectrometers andspectroscopic instrumentation; (300.6320) Spectroscopy, high-resolution; (300.6495)
Spectroscopy, terahertz.

References and links
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8.

S. Yokoyama, R. Nakamura, M. Nose, T. Araki, and T. Yasui, “Terahertz spectrum analyzer based on a terahertz
frequency comb,” Opt. Express 16(17), 13052–13061 (2008),
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-17-13052.
T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, and T. Araki, “Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy,”
Appl. Phys. Lett. 88(24), 241104 (2006).
P. Gaal, M. B. Raschke, K. Reimann, and M. Woerner, “Measuring optical frequencies in the 0–40 THz range
with non-synchronized electro–optic sampling,” Nat. Photonics 1(10), 577–580 (2007).
B. Sartorius, H. Roehle, H. Künzel, J. Böttcher, M. Schlak, D. Stanze, H. Venghaus, and M. Schell, “All-fiber
terahertz time-domain spectrometer operating at 1.5 microm telecom wavelengths,” Opt. Express 16(13), 9565–
9570 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-13-9565.
J. Ward, E. Schlecht, G. Chattopadhyay, A. Maestrini, J. Gill, F. Maiwald, H. Javadi, and I. Mehdi, “Capabilityof THz sources based on Schottky diode frequency multiplier chains,” in Proceedings of IEEE MTT-S
International Microwave Symposium (Institute of Electrical and Electronics Engineers, Fort Worth, 2004), pp.
1587–1590.
T. Nagatsuma, H. Ito, and T. Ishibashi, “High-power RF photodiodes and their applications,” Laser, Photon.
Rev. 3(1-2), 123–137 (2009).
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, “Long-term measurement of optical frequencies using a simple, robust and lownoise fiber based frequency comb,” Opt. Express 14(12), 5223–5231 (2006),
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-12-5223.
M. Tani, K.-S. Lee, and X.-C. Zhang, “Detection of terahertz radiation with low-temperature-grown GaAs-based
photoconductive antenna using 1.55 µm probe,” Appl. Phys. Lett. 77(9), 1396–1398 (2000).

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9.

T. Löffler, and T. May, “C. am Weg, A. Alcin, B. Hils and H. G. Roskos. “Continuous-wave terahertz imaging
with a hybrid system,” Appl. Phys. Lett. 90, 091111 (2007).
10. T. Yasui, H. Takahashi, Y. Iwamoto, H. Inaba, and K. Minoshima, “Widely tunable, phase-locked CW-THz
radiation by photomixing of two CW lasers locked to two independent fiber combs,” in Conference on Lasers
and Electro-Optics (CLEO) 2009, Technical Digest (CD) (Optical Society of America, 2009), paper CWB7.
http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2009-CWB7.
11. M. Ashida, R. Akai, H. Shimosato, I. Katayama, T. Itoh, K. Miyamoto, and H. Ito, “Ultrabroadband THz field
detection beyond 170THz with a photoconductive antenna,” in Conference on Lasers and Electro-Optics(CLEO) 2008, Technical Digest (CD) (Optical Society of America, 2008), paper CTuX6.
http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2008-CTuX6.

1. Introduction
A spectrum analyzer is a fundamental frequency measurement instrument widely used forradio-frequency wave, microwave, and millimeter wave. However, it has been difficult to use
in the terahertz (THz) region, lying at the boundary between the infrared and millimeter-wave
bands. Recently, new types of spectrum analyzers using a mode-locked Ti:Sapphire laser
have been proposed and developed which can measure the absolute frequency and spectral
shape of continuous-wave (CW) THz radiation without the need for cooling. One such device
is a THz-comb-referenced spectrum analyzer [1] using a frequency comb of photocarriers(PC-THz comb) generated in a photoconductive antenna (PCA) [2]. By precisely stabilizing alaser mode-locked frequency and hence the PC-THz comb by referring to a rubidium (Rb)frequency standard, the absolute frequency of CW-THz radiation in the sub-THz and THz
regions could be measured to a precision of 10−11. A second type of spectrum analyzer isbased on a non-synchronized electro-optic sampling technique using a free-running
Ti:Sapphire laser, and has been applied to frequency measurements of a CW CO2 laser at 28
THz [3].
From the standpoint of practical use, such as spectral analysis of various CW-THz
sources, portable, alignment-free, robust, flexible, and inexpensive devices are ideal.
However, the use of the Ti:Sapphire laser in those spectrum analyzers is an obstacle toachieving their characteristics. One promising alternative laser source for THz spectrum
analysis is a mode-locked erbium-doped (Er-doped) fiber laser with a center wavelength of1550 nm, which has the advantages of being compact, stable, and relatively inexpensive.
Also, use of the Er-doped fiber laser enable us to benefit by generation of stable THz comb
because it does not employ a free-space cavity but a fiber cavity tolerant of surrounding
disturbances. Furthermore, direct coupling between the laser output and the PCA using an
optical fiber, namely fiber-coupled PCA, eliminates free-space propagation of the laser lightwith many mirrors and lenses and enables the measurement head to be placed at any position[4]. Such fiber-based THz spectrum analyzers are likely to become portable, alignment-free,robust, and flexible measurement tools. However, highly stabilized fiber lasers to meet therequirement of the THz-comb-referenced spectrum analyzer are still lacking.
In this paper, we first construct the mode-locked Er-doped fiber laser whose mode-locked
frequency is stabilized precisely by referring to the Rb frequency standard, and use it togenerate the THz comb in a PCA or an electro-optic crystal. By standardizing the THz comb,
we achieved a fiber-based, THz-comb-referenced spectrum analyzer. To evaluate a potentialof the fiber-coupled PCA, we compare signal-to-noise ratio among four kinds ofphotoconductive detections and free-space electro-optic sampling detection. After evaluationof the basic performance of the frequency measurement and comparison with the previous
Ti:Sapphire-laser-based spectrum analyzer, we perform real-time monitoring of CW-THz
radiation generated by an active frequency multiplier chain [5] and by photomixing of two
free-running CW lasers with a uni-traveling-carrier photodiodes (UTC-PD) [6].
2. Experimental setup
2.1 THz-comb-referenced spectrum analyzer
Figure 1 shows a schematic diagram of the experimental setup. The THz-comb-referenced
spectrum analyzer was composed of a mode-locked-frequency-stabilized, Er-doped fiber#114490 - $15.00 USD(C) 2009 OSA

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laser, a THz detection unit, and RF frequency instruments. The principles of operation havebeen given in detail elsewhere [1]. The output of the fiber laser was delivered to a THz
detection unit by an optical fiber. The CW-THz radiation from a test source propagated infree space through a pair of THz lens (Pax Co., Tsurupica), and was then incident on the THz
detection unit. Photoconductive, or electro-optical, heterodyne mixing between the CW-THz
radiation and the THz comb in the detection unit generates beat signals in the RF region. Thebeat signal with the lowest frequency (freq. = fb), referred to as fb beat signal, is measured
with an RF spectrum analyzer (Agilent E4402B with a frequency range of 100 Hz to 3 GHz)and an RF frequency counter (Agilent 53132A with a frequency range to 225 MHz) to
determine its spectral shape and center frequency. A portion of the laser light is detected witha photodetector (PD), and its mode-locked frequency (freq. = f) is measured using the RF
frequency counter. The RF spectrum analyzer and RF frequency counter are synchronized toa Rb frequency standard (Rb frequency standard 1; Stanford Research Systems FS725 withfrequency = 10 MHz). When accuracy is defined as the degree of closeness of a measured
quantity to its true value, the frequency accuracy of the Rb frequency standard 1 is ± 5 × 10−11
at shipment in specifications. Aging of the frequency standard is less than 5 × 10−11 permonth.

Fig. 1. Experimental setup.

2.2 Mode-locked-frequency-stabilized, Er-doped fiber laser
We constructed a self-starting, stretched-pulse mode-locked Er-doped fiber laser oscillator
and amplifier [7] for use as a laser source to generate the PC-THz comb. The laser oscillator
was a ring resonator that employed nonlinear polarization rotation as a passively modelocking mechanism. To quickly fine-control the mode-locked frequency by adjustment of thecavity length, a portion of the fiber cavity was rolled on a drum-type piezoelectric actuator.
Furthermore, slow, coarse control of the mode-locked frequency could be achieved byvarying the temperature of the fiber cavity with a Peltier heater. The 20th harmoniccomponent of the mode-locked frequency was filtered, amplified, and used to stabilize themode-locked frequency using a proportional and integral (PI) control system referred to the
Rb frequency standard 1.
Figure 2 shows the instability of the mode-locked frequency in the free-running condition(black plots) and the stabilized condition (red plots). Here, frequency instability is the degree
to which further frequency measurements show the same results. It is clear from a comparison
between them that the mode-locked frequency is highly stabilized by the laser control system.
For comparison, frequency instability of the Rb frequency standard is also shown in Fig. 2(see green plots). The frequency instability with the stabilized control is almost equal to thatof the Rb frequency standard. One reason why the frequency instability of the stabilized
mode-locked frequency somewhat exceeds that of the referenced Rb frequency standard is
likely due to difference of the experimental conditions between them. Another reason is#114490 - $15.00 USD(C) 2009 OSA

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systematic errors caused by the frequency counter and/or frequency synthesizer used in thepresent setup, due for example to insufficient frequency calibration of the instruments. The
excellent stability achieved with the laser control enabled us to generate a stable and accurate
PC-THz comb, and perform frequency measurements with high precision.

Fig. 2. Frequency instability of mode-locked frequency with and without the laser control (redand black plots), and the Rb frequency standard (green plots).

The output light from the laser oscillator was amplified by a home-built Er-doped fiber
amplifier, and the resulting laser output had a center wavelength of 1550 nm, an average
power of 90 mW, a pulse duration of 40 fs, and a mode-locked frequency of 56,122,206 Hz.
2.3 THz detection unit
We used the four types of THz detection units shown in Fig. 3 and compared their signal-tonoise ratio. The first unit is a bowtie-shaped, low-temperature-grown GaAs PCA (LT-GaAsPCA; bowtie length = 1 mm, maximum input power = 10 mW for 800-nm light) equippedwith a hemispherical silicon lens. In this unit, the laser light is converted by second-harmonicgeneration (SHG) to half its original wavelength using a nonlinear optical crystal (β-BaB2O4,
thickness = 8 mm), as shown in Fig. 3(a). The resulting SHG light (center wavelength = 775
nm, average power = 6 mW) is focused using a lens onto an antenna gap in the PCA. This
results in the generation of a PC-THz comb in the PCA. The output current signal from the
PCA is amplified by a high-gain current preamplifier (AMP; bandwidth = 1 MHz andsensitivity = 4 × 106 V/A). On the other hand, use of the PCA without the need forwavelength conversion is attractive for achieving direct coupling between the laser output and
the PCA using an optical fiber. The above LT-GaAs-PCA has a little sensitivity even for1550-nm laser light whose energy does not exceed the optical band gap of GaAs (1.43 eV atroom temperature, corresponding wavelength of 867 nm) [8]. Recently, a low-temperaturegrown InGaAs/InAlAs PCA (LT-InGaAs/InAlAs-PCA) has been further developed for
detecting laser light around 1550 nm [4]. Therefore, we used bowtie-shaped LT-GaAs-PCA(bowtie length = 1 mm, maximum input power = unknown for 1550-nm light) and LTInGaAs/InAlAs-PCA (bowtie length = 3 mm, maximum input power = 30 mW for 1550-nm
light), directly triggered by the 20 mW and 30 mW fundamental lights from the fiber laser, as
the second and third THz detection units [see Figs. 3(b) and (c)]. The fourth unit is based on
the free-space electro-optic sampling (FSEOS) method because the FSEOS enables electrooptically heterodyne beat-down based on the THz comb [9]. Therefore, a 1 mm-thick (110)
ZnTe crystal was used in this unit [see Fig. 3(d)]. The SHG light (center wavelength = 775
nm, average power = 8 mW) was detected with an auto-balanced photodetector (bandwidth =125 kHz) after passing through the polarizer (PL), a quarter waveplate (λ/4), ZnTe crystal,#114490 - $15.00 USD(C) 2009 OSA

Received 21 Jul 2009; revised 2 Sep 2009; accepted 2 Sep 2009; published 9 Sep 200914 September 2009 / Vol. 17, No. 19 / OPTICS EXPRESS 17037

and a Rochon prism (RP). Difference of the SHG power between setups of Figs. 3(a) and (d)is due to the condition of the fiber laser, such as pulse pedestal or polarization purity, whichchanges the conversion efficiency to the SHG light.

Fig. 3. THz detection unit. (a) bowtie-shaped LT-GaAs-PCA triggered by 775-nm, 6-mW
light, (b) bowtie-shaped LT-GaAs-PCA triggered by 1550-nm, 20-mW light, (c) bowtieshaped LT-InGaAs/InAlAs-PCA triggered by 1550-nm, 30-mW light, and (d) FSEOS of 1mmthick ZnTe crystal probed by 775-nm, 6-mW light. AMP: high-gain current preamplifier; PL:polarizer; λ/4: quarter waveplate; RP: Rochon prism.

2.4 CW-THz test source
Two kinds of CW sources emitting in the low-frequency THz region were used as test sourcesin this experiment. The first was an active frequency multiplier chain (Millitech AMC-10R0000 with multiplication factor = 6, tuning range = 75–110 GHz), referred to as an AFMC
source, as shown in Fig. 4(a). The AFMC source multiplies output frequency of a frequency
synthesizer (Agilent E8257D with frequency = 12.5–18.33 GHz, and linewidth < 0.1 Hz) by
six, and then radiates the resulting electromagnetic wave in free space via a horn antenna.
Since this AFMC source is synchronized to another Rb frequency standard (Rb frequency
standard 2; Stanford Research Systems FS725 with frequency = 10 MHz, accuracy atshipment = ± 5 × 10−11 and aging < 5 × 10−11 per month in specifications), it acts as an
accurate, stable, tunable test source. The second test source was produced by photomixing of
two CW near-infrared lasers at adjacent wavelengths (CWL1 and CWL2) with a UTC-PD [6],as shown in Fig. 4(b), which can be used for broadband wireless communications andspectroscopic sensing. The two CW lasers were external cavity wavelength-tunable laser
diodes with an emission wavelength of 1550 nm (Koshin Kogaku Co., LS-601A-15S1,
spectral linewidth ≤ 100 kHz) and operated in free-running mode (frequency fluctuation <100 MHz/hour). The optical frequency difference between them was set to be approximately
120 GHz. The outputs of the lasers were combined using a fiber coupler, amplified with an
Er-doped fiber amplifier, and then photomixed by a F-band UTC-PD (available frequency =90-140 GHz) equipped with a horn antenna. The polarization of CWL1 could be made tosignificantly overlap with that of CWL2 by adjusting a half waveplate (λ/2) and a quarter
waveplate (λ/4). The output power was set to be 100 µW at a frequency of 120 GHz.

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Fig. 4. Test source for CW-THz radiation. (a) Active frequency multiplier chain (output power= 5.4 mW at 80 GHz and tuning range = 75–110 GHz) driven by a frequency synthesizer and(b) photomixing source (output power = 100 µW and output frequency = 120 GHz) of two
free-running near-infrared lasers (CWL1 and CWL2) with a F-band uni-traveling-carrier
photodiodes (UTC-PD).

3. Results
We evaluated the basic performance of the fiber-based, THz-comb-referenced spectrum
analyzer using the AFMC source as a test source because it is synchronized to the Rb
frequency standard and hence is highly accurate and stable. The output frequency of the
AFMC source was set to be 80 GHz. We first compared the signal-to-noise ratio (SNR) of the
fb beat signals obtained by four THz detection units: LT-GaAs-PCA triggered by a 775-nm, 6mW light [(a) 775-nm, 6-mW LT-GaAs-PCA], LT-GaAs-PCA triggered by a 1550-nm, 20mW light [(b) 1550-nm, 20-mW LT-GaAs-PCA], LT-InGaAs/InAlAs-PCA triggered by a1550-nm, 30-mW light [(c) 1550-nm, 30-mW LT-InGaAs/InAlAs-PCA], and FSEOS of a 1mm ZnTe crystal probed by a 775-nm, 8-mW light [(d) 775-nm, 8-mW ZnTe-FSEOS]. Figure
5 shows a comparison of the logarithmic-scale spectra of the fb beat signal for the four units,measured by the RF spectrum analyzer with a sweep rate of 43.5 kHz/s and a resolution
bandwidth (RBW) of 1 kHz. The highest SNR of 57 dB was achieved with the (a) 775-nm, 6mW LT-GaAs-PCA. The (b) 1550-nm, 20-mW LT-GaAs-PCA and the (c) 1550-nm, 30-mW
LT-InGaAs/InAlAs-PCA, capable of direct coupling with the fiber laser output via the opticalfiber, yielded a SNR of 31 dB and 40 dB, respectively. Conversely, the lowest SNR of 29 dB
was obtained for the (d) 775-nm, 8-mW ZnTe-FSEOS. Since the output power of the AFMC
source was 5.4 mW ( = + 7.3 dBm) at 80 GHz, the detection limit for THz power is estimatedto be 11 nW for the (a) 775-nm, 6-mW LT-GaAs-PCA, 4300 nW for the (b) 1550-nm, 20mW LT-GaAs-PCA, 540 nW for the (c) 1550-nm, 30-mW LT-InGaAs/InAlAs-PCA, and6800 nW for the (d) 775-nm, 8-mW ZnTe-FSEOS. However, this comparison was performed
under different probe power conditions. Differences in probe power can be corrected byassuming that signals measured in the PCA and FSEOS are proportional to the power of theprobe beam. Normalized to the result for the (a) 775-nm, 6-mW LT-GaAs-PCA, the corrected
detection sensitivities are 0.077% for the (b) 1550-nm, 20-mW LT-GaAs-PCA, 0.41% for the(c) 1550-nm, 30-mW LT-InGaAs/InAlAs-PCA, and 0.12% for the (d) 775-nm, 8-mW ZnTeFSEOS. Thus, from the viewpoint of high SNR, the (a) 775-nm, 6-mW LT-GaAs-PCA is thebest choice at this stage although it requires the wavelength conversion optics, which areslight obstacles to direct PCA coupling. Conversely, to benefit by the direct coupling
capability while keeping moderate SNR, the (c) 1550-nm, 30-mW LT-InGaAs/InAlAs-PCA
is an attractive THz detection unit. In subsequent experiments, we used the (c) 1550-nm, 30mW LT-InGaAs/InAlAs-PCA for the high-power AFMC source and the (a) 775-nm, 6-mW
LT-GaAs-PCA for the lower-power photomixing source as the THz detection unit.

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Fig. 5. Comparison of signal-to-noise ratio of the fb beat signal obtained by four THz detection
units (RBW = 1 kHz and sweep rate = 43.5 kHz/s). A test source is the AFMC source with
output power of 5.4 mW at frequency of 80 GHz.

We next evaluated the spectral linewidth of the fb beat signal when the output frequency of
the AFMC source is set to be 100 GHz. Figure 6(a) shows a linear-scale spectrum of thissignal, measured by the RF spectrum analyzer with a sweep rate of 43.5 Hz/s and an RBW of
1 Hz. The resulting linewidth of the beat signal was 1.8 Hz. However, since this linewidth
may be limited by the RBW of the RF spectrum analyzer (minimum RBW = 1 Hz), we next
evaluated the linewidth in detail by measuring the frequency fluctuation of the fb beat signal
with the RF frequency counter. Figure 6(b) show the fluctuation of the beat frequency,
represented by the standard deviation, with respect to various gate times. Such narrow
linewidth of the beat signal enables us to perform the frequency measurement with highprecision.

Fig. 6. (a) Spectra of the fb beat signal of the AFMC source measured by the RF spectrum
analyzer (RBW = 1 Hz and sweep rate = 43.5 Hz/s). (b) Frequency fluctuation of the fb beatsignal measured by the RF frequency counter.

In the THz-comb-referenced spectrum analyzer measurement, the absolute frequency ofthe measured CW-THz radiation ( = fTHz) is given as follows

fTH z = mf ± f b .

(1)

where m is order of the comb mode nearest in frequency to the CW-THz radiation, f is modelocked frequency of the fiber laser, and fb is the lowest frequency of beat signals [1]. The f#114490 - $15.00 USD(C) 2009 OSA

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and fb values were measured to be 56,122,206.03 Hz and 356,156 Hz with the RF frequency
counter, respectively. To determine the value of m and the sign of fb, the frequency f waschanged by δf ( = 25 Hz) using the laser control system. This resulted in a change of the beatfrequency by δfb ( = 44,549 Hz). Since |δfb| is equal to |mδf |, m is determined asm=δ fb
44,549== 1, 781.96 ≈ 1, 782.
25
δf

(2)

Since the sign of δfb/δf (positive in this case) is opposite to that of fb, the value of fTHz wasdetermined as follows
fTH z = mf − f b= 1, 782 * 56,122, 206.03 − 356,156(3)= 100, 009, 414,989.46 H z.
Since the actual set frequency of the AFMC test source was 100,009,414,988.9 Hz, theerror between the set and measured frequencies was only 0.56 Hz. When the frequency
accuracy of the test source is defined as the ratio of the error to the fTHz, the corresponding
accuracy was 8.7 × 10−10. To evaluate the frequency accuracy of the AFMC source inavailable frequency range, we determined the absolute frequency of the source while tuning
its output frequency from 75 to 110 GHz at 5-GHz intervals. The resulting accuracy for eight
different measurement frequencies was shown as black plots in Fig. 7. A mean accuracy of
2.4 × 10−11 was obtained for this source. Here, let us consider the reason why frequency
accuracy of the THz spectrum analyzer exceeds that of the Rb frequency standard. Althoughfrequency accuracy of 5 × 10−11 for the Rb frequency standard is the value at shipment, agingof the frequency standard deviates its output frequency slowly from the shipment value. If thefrequency drift caused by the aging makes the output frequency apart from a true value of 10
MHz, the actual frequency accuracy becomes worse than the shipment value. Conversely, if it
goes close to the true value, the actual accuracy becomes better. We consider that the two Rb
frequency standards used in this experiment come under the latter case. Therefore, thefrequency accuracy of the THz spectrum analyzer exceeds that of the Rb frequency standard
at shipment.

Fig. 7. Frequency accuracy of the AFMC source measured with the fiber-based, THz-combreferenced spectrum analyzer (black plots) and the Ti:Sapphire-laser-based one (red plots).

For comparison, we indicated the frequency accuracy of the same test source measured
with the previous spectrum analyzer using a Ti:Sapphire laser [1] as red plots in Fig. 7. A
comparison between them reveals a little improved frequency accuracy in the case of the#114490 - $15.00 USD(C) 2009 OSA

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present spectrum analyzer. We consider that this improvement is due to stable THz comb
generated by the fiber laser robust to surrounding disturbances. We next compare the present
spectrum analyzers with the previous one in terms of simplicity and convenience. The
previous spectrum analyzer has often required precise alignments of the laser cavity andoptical components to maintain the laser for best condition and optimize the PCA,
respectively. Conversely, the fiber laser used in the present spectrum analyzer has needed a
few adjustments of the laser cavity although it has been working in continuous operation
more than several months. Such the continuous operation of the fiber laser also enables us tobenefit by stable long-term stability. Furthermore, rigid Poynting vector of the laser beamsaves us a lot of labor to adjust the optical components. Most importantly, use of fibercoupled PCA achieves easy handling of the measurement head and robustness to thesurrounding disturbances. Therefore, the present fiber-based spectrum analyzer surpasses theprevious one in frequency accuracy, simplicity of operation, robustness, and long-term
stability.
Finally, real-time monitoring of the frequency spectrum for two kinds of test sources was
carried out by measuring the fb beat signal with the RF spectrum analyzer. Since the output
frequency of the AFMC source is highly accurate and stable, we evaluated the effectivenessof the stabilized control of the PC-THz comb by observing the fb beat signal with the laser
control turned off and on. Figures 8(a) and (b) illustrate five consecutive spectra of the beatsignal without and with the laser stabilization measured at 6-sec intervals, respectively (sweep
rate = 16.5 kHz/s, and RBW = 100 Hz). Also, temporal changes of the beat spectra without
and with the laser stabilization are shown as movies in Media 1 and 2 (full span = 10 kHz,sweep time = 605 ms, and RBW = 100 Hz). The beat frequency is tightly pinned to a certain
value under laser stabilization, whereas it fluctuates over a range of several kHz without thestabilization. This difference reflects that of the mode-locked frequency instability shown in
Fig. 2. Therefore, stabilization of the mode-locked frequency is indispensable for precise
determination of the absolute frequency of the CW-THz radiation.

Fig. 8. Spectra of the fb beat signal of the AFMC source (a) without (Media 1) and (b) with thelaser stabilization (Media 2). Five consecutive spectra of the beat signal were measured at 6sec intervals. RBW = 100 Hz and sweep rate = 16.5 kHz/s.

We next used the THz spectrum analyzer for real-time monitoring of the photomixing
source. The resulting image and movie of the beat spectrum are shown in Fig. 9 (sweep rate =75 MHz/s, and RBW = 1 kHz) and Media 3 (full span = 1.5 MHz, sweep time = 19.3 ms, and
RBW = 10 kHz), respectively. In contrast to the stable AFMC source, the beat frequency ofthe photomixing source exhibits large fluctuations within a spectral window of 1.5 MHz. Thisis because the two CW lasers used for the photomixing are operated in free-running mode
without any frequency control. Work is in progress to suppress the frequency fluctuation of
the CW-THz radiation by precisely controlling the optical frequencies of the two CW lasers[10].
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Fig. 9. Spectrum of the fb beat signal of the photomixing source (Media 3). RBW = 1 kHz andsweep rate = 75 MHz/s.

4. Conclusion

We have developed a fiber-based, THz-comb–referenced spectrum analyzer using a modelocked Er-doped fiber laser to achieve a portable, alignment-free, robust, and flexible
apparatus suitable for practical use. The lowest detection limit of 11 nW for THz power wasachieved when using the (a) 775-nm, 6-mW LT-GaAs-PCA as a THz detection unit.
Furthermore, a fiber-coupled, (c) 1550-nm, 30-mW LT-InGaAs/InAlAs-PCA can be used formeasurement of CW-THz sources with a power of the order of µW. Conversely, the (d) 775nm, 8-mW ZnTe-FSEOS is less attractive than the PCA because of its low SNR and difficultyof the direct coupling due to complicated optical systems for wavelength conversion andauto-balanced detection. We determined the absolute frequency of the AFMC source infrequency range from 75 to 110 GHz and obtained the mean frequency accuracy of 2.4 ×
10−11. Finally, we demonstrated real-time monitoring of the spectral behavior of the AFMC
source and the photomixing source comprising two free-running CW lasers. These sources
exhibited contrasting behavior in the frequency domain, in that the former was highly stable
whereas the latter exhibited large fluctuations. Although the frequencies of the test sourceswere in the low-frequency THz region due to the limitation of our available CW-THz sources,
it is natural to believe that the THz spectrum analyzer can be extended to the higher frequency
THz region in the same manner as the THz spectrum analyzer based on non-synchronizedelectro-optic sampling [3]. This is because the spectral bandwidth of the THz spectrum
analyzer is limited mainly by spectral sensitivity of the PCA or FSEOS. For example, thespectral characteristics of the LT-GaAs-PCA as a THz detector have been investigated using
THz time-domain spectroscopy, for which a bandwidth of 170 THz was achieved [11].
Therefore, the THz-comb-referenced spectrum analyzer can cover the entire THz spectral
region. Real-time monitoring with such a THz spectrum analyzer is a powerful technique fordetailed characterization of the spectral behavior of various CW-THz sources. Furthermore, it
paves the way for establishment of frequency metrology in THz region.
Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research Nos. 20560036, 21360039,
and 21650111 from the Ministry of Education, Culture, Sports, Science, and Technology ofJapan. We also gratefully acknowledge financial support from Tateisi Science andTechnology Foundation, Japan.

#114490 - $15.00 USD(C) 2009 OSA

Received 21 Jul 2009; revised 2 Sep 2009; accepted 2 Sep 2009; published 9 Sep 200914 September 2009 / Vol. 17, No. 19 / OPTICS EXPRESS 17043