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Characterization of a THz CW spectrometer pumped at 1550 nm
Accepted Manuscript Characterization of a THz CW spectrometer pumped at 1550nm Woon-Gi Yeo, Niru K. Nahar PII: DOI: Reference:
S1350-4495(15)00054-7 http://dx.doi.org/10.1016/j.infrared.2015.02.009 INFPHY 1752
To appear in:
Infrared Physics & Technology
Received Date:
6 February 2015
Please cite this article as: W-G. Yeo, N.K. Nahar, Characterization of a THz CW spectrometer pumped at 1550nm, Infrared Physics & Technology (2015), doi: http://dx.doi.org/10.1016/j.infrared.2015.02.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characterization of a THz CW spectrometer pumped at 1550nm Woon-Gi Yeo and Niru K. Nahar
Abstract—We present an evaluation of a cost-effective THz CW spectrometer pumped at 1550 nm wavelengths with a fixed delay line. To study the spectral competence of the spectrometer, transmission data is obtained for various organic and inorganic samples. Spectral comparisons of the samples are presented by using THz time domain spectroscopy and vector network analyzer (VNA). Despite the capability of highly resolved transmission spectroscopy, our current system reveals the uncertainty in interferometric output data for phase analysis. Here, we identify the effect of fringing space of raw output data toward frequency resolution, phase analysis, and data acquisition time. We also propose the proper delay line setup for phase analysis for this type of spectrometers. Index Terms—terahertz (THz), continuous wave (CW) spectrometer, validation, frequency resolution, fixed delay line, fringing space
I. INTRODUCTION
R
ecent research in terahertz (THz) technologies has pioneered a new era toward various application areas from security to food inspection based on the exotic properties of THz wave. Among them, the unique “fingerprint” THz spectra has demonstrated that various chemical, biological, and semiconducting materials can be characterized in THz band through their macromolecular vibrations [1, 2]. For instance, the unique absorption spectra in THz have been reported for explosives and drugs such as C-4, RDX, TNT, Aspirin, and Acetaminophen, etc [3,4]. Owing to extremely high sensitivity to water content, THz spectroscopy has also been utilized to diagnose biological tissues including malignant and cancerous regions [5-7]. Furthermore, THz spectra have been described inherent characteristics of semiconducting materials such as carrier dynamics, phase and spin-state transitions [8-10]. However, most of the previous research and progress has been established in the field of THz pulsed spectrometer, which has some limitations such as size, cost, and frequency resolution. Thus, in many Woon-Gi Yeo is with the ElectroScience University, 1330 Kinnear Rd. Columbus, [email protected]). Niru K. Nahar is with the ElectroScience University, 1330 Kinnear Rd. Columbus, [email protected]).
Laboratory at the Ohio State OH 43212 USA (e-mail: Laboratory at the Ohio State OH 43212 USA (e-mail:
THz applications, continuous wave (CW) spectrometer has been desired for highly resolved THz spectroscopy such as gas phase analysis [11, 12]. The recent progress of photo-mixing technologies allows the generation of THz CW source with MHz-scale frequency resolution. However, most of the THz CW systems published in contemporary literatures adopted GaAs-based emitters and detectors accompanied by laser sources at either 790 nm or 850 nm wavelength [12-14]. As a result, the systems have remained relatively expensive and large. In this study, to retain the availability of high cost-effectiveness, two 1550 nm distributed feedback (DFB) lasers are utilized for our CW system with antenna-integrated photomixers made by InGaAs grown on InP substrate. Despite the increasing number of available state-of-the-art THz spectrometers, the spectral measurements have been rarely compared by experimental methods and the optical properties of samples have rarely been presented by CW spectral measurements. Therefore, the purpose of this paper is twofold. First, we present experimental validation for CW spectral measurements through comparison with a THz time domain spectrometer and an electronic-based vector network analyzer (VNA) with Virginia Diode’s frequency multipliers (VNA-VDI). In particular, we investigate transmission spectroscopy of solid-state samples (Į-lactose monohydrate pellets and soda-lime-silica glasses), biological samples (wild and farm-raised salmons), and a metamaterial device. Secondly, we present the significance of fringing space in determining the optical properties of the samples using the CW spectrometer. We characterize the effect of interference pattern alteration towards frequency resolution, phase analysis and data acquisition time in a fixed delay line setup. II. EXPERIMENTAL SETUP Figure 1 describes the TOPTICA Photonics Inc. THz CW spectrometer that is used in this work. The system employs two tunable DFB lasers operating around 1539.9 and 1544.6 nm, respectively. The laser frequencies are tuned by both current (~0.6 GHz/mA) and temperature (~14 GHz/K in the temperature range between 276.15 K and 321.15 K) controls. As such, the CW system is capable of collecting THz frequency spectrum between about 100 GHz and 1000 GHz with a minimum of 10 MHz frequency step.
Ltd.) and VNA (N5242A, Agilent Technologies Inc.). THz pulsed spectrometer exhibits more than 45 dB dynamic range up to 3 THz and the latter includes frequency multipliers (Virginia Diodes, Inc.) with 100 dB dynamic range in the frequency range from 325 GHz to 750 GHz. III. VALIDATION OF THZ TRANSMISSION SPECTRAL MEASUREMENTS A. Transmission data analysis in CW spectrometer
Fig. 1. Block diagram of THz CW spectrometer with a fixed delay line
In particular, the heterodyne signal from the lasers is fibercoupled into the photo-mixers in both THz detector and emitter. Only the difference (THz) of the signal is radiated via a hyper-hemispherical silicon lens by broadband bow-tie antenna integrated within the photo-mixer. THz wave is collimated and focused into the sample position, and the transmitted THz wave is collimated and focused onto the detector. Finally, the detected THz signal is superimposed with the heterodyne signal, and is fed into a lock-in amplifier. To alleviate noise signal level, the lock-in integration time can be set from 21 to 650 ms. The effective optical path length difference between LA and LB plus LC (depicted in Fig. 1) is about 91 cm, which yields 165 MHz spectral resolution. Later, we investigate the effect of optical path length variation towards output interference pattern, data analysis and data acquisition time. For this purpose, the optical path length between the lasers and the THz detector (LB in Fig. 1) was elongated by additional fibers with the length from 30 to 200 cm.
Fig. 3. Detected photocurrent as an interference pattern for ambient air
Based on the coherent detection principle, the detected photocurrent of the THz CW system, Iph, is shown in Fig.3 as an interference pattern. The detected photocurrent is determined by the amplitude of the THz electric field, ETHz, and phase difference, ǻij, between the THz wave and the IR laser signal [14]: I ph ∝ E THz cos( ∆ ϕ ) = E THz cos (
2π f ∆ L eff ) c
where f denotes the THz frequency, c is the speed of light, and the effective optical path difference is ∆ L eff = n eff ∆ L = n eff L A − L B + L C
Fig. 2. Dynamic range of the CW spectrometer from 100 GHz to 1200GHz
The dynamic range of this CW system varies with frequency due to the efficiency of the photo-mixer (-40 dB/decade). Depicted in Fig. 2, the actual signal to noise ratio (S/N) with 300 ms lock-in integration time is 50 dB at 100 GHz and drops to 10 dB at 1 THz. Figure 2 also shows clearly resolved absorption lines for water vapor around at 560 GHz, 750 GHz, and 990 GHz. To validate CW spectral data, the respective measurements for each sample were also performed by THz pulsed spectrometer (TPS3000, Teraview
(1)
(2)
where neff is the effective refractive index for overall system path length in the fixed delay line system. As described in Fig. 1, LA (200 cm) and LB (210 cm) are the optical paths from the laser source to the emitter and from the laser source to the detector, respectively. LC represents the THz path from the emitter to the detector. From Eq. (1) and (2), the actual THz data can be obtained by extracting the envelop function, ETHz, as shown in Fig. 4 (a). Thus, the transmittance can be determined by § E sam T ( w ) = 20 log ¨¨ THz ref © E THz sam
· ¸¸ ¹
(3)
ref
where ETHz and ETHz denote the extracted envelopes of the sample and the reference data, which are the extrema of the interference patterns. Here, we note that the extrema of sample data are positioned at different frequencies from that
of the reference data, as illustrated in Fig. 4 (b). Therefore, to compare the envelopes of both data, the extrema of the sample data need to be interpolated on equally spaced frequency grid of the extrema of the reference data.
Fig. 6. Transmittance of soda-lime-silica glass windows for 1 mm thickness (left) and 3.15 mm thickness (right)
C. Biological sample (a) (b) Fig. 4. Examples of the extracted THz data from the absolute value of the detected photocurrent
B. Solid state samples The Į-lactose monohydrate is a standard reference sample for both THz CW and pulsed spectrometers due to its wellknown absorption properties at 530 GHz [15]. Thus, we prepared two lactose samples to demonstrate the sensitivity as well as the specificity of the CW spectrometer to chemicals. One of the samples is a 10% Į-lactose monohydrate plus 90% polyethylene pellet, which is transparent to the THz band. The other one is a pure Į-lactose monohydrate pellet. As exhibited in Fig. 5, the measurements for both samples show excellent agreements between the transmission data from VNA-VDI, CW, and TPS3000. Moreover, the transmittance spectra for two samples reveal the relative difference of the lactose compounds as about -20 dB, which corresponds to 10% in percentage scale.
Fig. 5. Transmittance of Į-lactose monohydrate pellets: 10% Į-lactose monohydrate plus 90% polyethylene with 3.5 mm thickness (left) and pure Įlactose monohydrate with 0.82 mm thickness (right)
The CW spectrometer was also used to characterize the relative thickness of solid materials. Two soda-lime-silica glass windows were measured for different thickness. As seen in Fig. 6, about -7 dB and -22 dB power losses was observed at between 100 GHz and 500 GHz for 1 mm and 3.15 mm windows, respectively. From the results, the relative thickness of the windows (1:3.14) can be estimated since both have the same material properties such as absorption coefficient (unit: dB/cm).
(a)
(b) (c) Fig. 7. Measurements of dried salmon tissues: (a) sample preparation, transmittance of (b) wild salmons and (c) farm-raised salmons
Next, we tested the accuracy of our spectrometer for biological tissue characterization. Based on the availability, we chose to investigate wild and farm-raised salmon tissues to contrast of their lipid contents and chemical compositions [16, 17]. For these experiments, eight specimens were taken from both wild and farm-raised salmons, and the results were averaged. We also note that the salmon tissues were air-dried in between two glass slides to remove highly absorptive water to maximize measurable frequency range. As depicted in Fig. 7, even though transmission values may vary due to the surface roughness of the tissues and measurement positions, the spectra exhibit excelent agreement between CW and VNA-VDI systems. More importantly, the spectra demonstrate the potential to differentiate wild salmons from farm-raised salmons with higher lipid content, which is more transparent in THz band. D. Metamaterials The characterization of artificially constructed metamaterials such as Frequency Selective Surfaces (FSS) can be one of major application arena of THz spectrometer. Here, we measured a cross-dipole FSS band-pass filter resonating at 508.5 GHz, as presented in Fig. 8. The spectra from the CW and the pulsed system demonstrate the exact expected resonant frequency and also demonstrate excellent agreement up to 800 GHz.
can be originated either from jitter noise or coarse number of data to generate the sinusoidal interference pattern in our delay line setup. Thus, the extrema of the raw data can be detected at undesired positions. More importantly, the deviation shown in Fig. 9 may be more dominantly caused by the ambiguous
(a) (b) Fig. 8. Measurements of a metamaterial device: (a) Frequency Selective Surfaces (FSS) band-pass filter resonating at 508.5 GHz (Lake Shore Cryotronics, Inc.) and (b) measured transmittance
IV. EFFECT OF FRINGING SPACE TOWARD OPTICAL PROPERTY EXTRACTION AND DATA ACQUISITION TIME A. Extraction of Optical Properties in CW spectrometer Traditionally, the way to extract the phase information of the THz signal is to use a mechanical delay stage or an optical phase modulator to vary ǻL in Eq. (1) section III. Alternatively, in the fixed delay line system, the relative phase difference can be varied by scanning THz frequency [14]. In particular, as addressed in Eq. (1), the frequency and the optical path difference determine the oscillation period of the detected photocurrent, which refers to the fringing space. Here, for the reference measurement, THz wave travels through air with the constant refractive index (nair § 1), thus the fringing space is nearly constant independent of frequencies. As such, the position of extrema can be defined as: ref f extrema ,m =
mc , 2 ∆ Lref eff
m = 1,2 ,3 ,!
Fig. 9. Deviations of the refractive index for Į-lactose monohydrate pellet by different relative order, m, of the extrema for reference and sample signals
relative order, m, of the extrema for both the reference and the sample data. For example, as described in Fig. 10 (a), the determination of the relative order, m, for reference and sample signals is not straightforward if the narrow frequency range is scanned. Although the wide frequency range is explored to examine the in-phase frequency where both signals have the same order [14], the ambiguity is still present since a group of the sample signals are in phase with the reference signal, see Fig. 10 (b). Therefore, here, to analyze this problem, we present the experimental study of the effect of fringing space of the practical phase analysis and the data acquisition time by utilizing additional fibers in the system.
(4)
In the case of the sample measurement, the effective optical sam
path difference, ∆Leff , can be changed with the frequency dependent refractive index, nsam, and the thickness, d, of the sample. Thus, compared to the reference signal, the extrema of the sample signal are not equally spaced in frequency, and the fringing pattern is shifted, as depicted in Fig. 10(a). Based on this fact, by comparing the extrema of the sample and the reference data, the relative phase difference can be achieved, thus the refractive index of the sample can be calculated using the following equation [14],
(n sam
ref § f extrema · ,m ref − n air )d = ∆ Lsam = ¨ sam − 1 ¸ ∆ Lref eff − ∆ L eff ¨ f ¸ eff © extrema ,m ¹
(
)
(a) (b) Fig. 10. Examples of the extracted THz data from the absolute value of the detected photocurrent
B. Effective optical path difference versus fringing space In general, the frequency resolution of the CW system is regarded as the fringing space of the interference pattern. For the fixed delay line system, the fringing space in the air, ǻf, is determined by the effective path difference, ǻLeff:
(5).
However, in practical phase analysis, two major difficulties hinder the extraction of the relevant refractive indices from CW raw data compared to the pulsed system, as demonstrated in Fig. 9. First, the detected photocurrent is not perfectly sinusoidal, and the constant fringing space between the extrema is unattainable even from the reference data. This
∆f =
c c = 2 ∆ Leff 2 n eff L A − L B + LC
(6)
Theoretically, as we increase effective path difference, the fringing space becomes smaller and therefore better frequency resolution can be achieved. However, the frequency resolution can be limited by the measurable frequency step size and the stability of the laser frequency tuning module in the system.
In particular, as described in Fig. 11, the fringing space in Zone 3 is less than 75 MHz, and the available number of data for a half cycle of the interference pattern is less than eight with the minimum 10 MHz frequency step. Thus, the detected photocurrent is too coarse to exhibit relevant transmission spectra and refractive indices.
Fig. 11. Calculated relation between effective optical path difference and fringing space
C. Fringing space analysis method To characterize the delay line setup in detail, we tested our CW system by utilizing additional fibers in various lengths, which enabled to determine the unknown parameters such as LC and neff. Five fibers with the lengths from 30 cm to 200 cm were added to the existing fiber between a diode laser and the detector, thus LB became lengthen. The fringing spaces were obtained from the detected photocurrents. Based on the theoretical relation described in Eq. (6), respective effective optical path differences, ǻLeff, was evaluated, as summarized in Table I. Here, the unknown parameters can be obtained by algebraic methods in the Eq. (2), and the averaged values are 1.614 and 66.773 cm for neff and LC, respectively. Based on the calculated values, the relation between fringing spaces and additional fiber lengths can be established as shown in Fig. 13. The fringing space can be predicted for the additional fiber with the certain length. As a result, the desired frequency resolution can be achieved by considering the appropriate number of data points for the phase analysis.
(a) (b) Fig. 12. Detected photocurrents after data smoothing: (a) Zone 1 (ǻf § 360 MHz) and (b) Zone 3 (ǻf § 65 MHz)
More importantly, for the extrema detection in data analysis, data smoothing is an inevitable process due to jitter noise. However, the lack of data points constrains the span size in this process, thus it is hard to obtain intact sinusoidal signal for the extrema detection, as shown in Fig. 12(b). Likewise, Zone 2 including our system setup also imposes the restrictions particularly on the phase analysis. Furthermore, since the minimum frequency step should be required in the systems for Zone 2 and Zone 3, more than 10 hours of data acquisition time is needed for a full frequency range scan with 300ms integration time. In contrast, the measurements in Zone 1 can guarantee to obtain enough number of data points (more than 30) and can resolve the problems, see Fig. 12. Thus, it is desired to adopt Zone 1 not only for more reliable data analysis with sub-GHz frequency resolution but also for more economic data acquisition time. TABLE I EXPERIMENTAL RESULTS FOR FRINGING SPACE BY PATH LENGTH VARIATION Ladd (cm)
LB (cm)
Leff (cm)
ǻf (GHz)
0 30 60 100 130 200
210 240 270 310 340 410
~ 91 ~ 42 ~ 3.35 ~ 70 ~ 120 ~ 231
~ 0.165 ~ 0.36 ~ 4.516 ~ 0.215 ~ 0.125 ~ 0.065
Fig. 13. Calculated relation between additional fiber length and fringing space based on the experimental results (marked with X).
V. CONCLUSION We demonstrated the overall performances of a THz CW spectrometer employing a fixed delay line and two 1550nm tunable distributed feedback diode lasers. The validation of THz CW transmission spectroscopy was evaluated by the comparative measurements of the chemical and biological samples as well as the metamaterial device. The spectral efficiency of the CW system showed excellent agreement with the TPS300 and the VNA-VDI. Ambiguities of the phase analysis in the current system setup were described in terms of the fringing space, which correlates with the delay line setup. To alleviate the ambiguities, we suggested a methodology to set the proper optical delay configuration in CW system with fixed delay line. REFERENCES [1] [2]
M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, “Terahertz Spectroscopy,” J. Phys. Chem. B, Vol. 106, pp. 7146-7159, 2002 D. F. Plusquellic, K. Siegrist, E. J. Heilweil, and O. Esenturk, “Applications of Terahertz Spectroscopy in Biosystems,” ChemPhysChem, Vol. 8, pp. 2412-2431, 2007
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W. R. Tribe, D. A. Newnham, P. F. Taday, and M. C. Kemp, “Hidden object detection: security applications of terahertz technology,” Proc. SPIE, Vol. 5354, pp. 168-176, 2004 J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security applicationsexplosives, weapons, and drugs,” Semicond. Sci. and Technol., Vol. 20, pp. S266-S280, 2005. C. H. Zhang, G. F. Zhao, B. B. Jin, Y. Y. Hou, H. H. Jia, J. Chen, and P. H. Wu, “Terahertz Imaging on Subcutaneous Tissues and Liver Inflamed by Liver Cancer Cells,” Terahertz Science and Technology, Vol. 5, No.3, 2012 P. C. Ashworth, E. Pickwell-MacPherson, E. Provenzano, S. E. Pinder, A. D. Purushotham, M. Pepper, and V. P. Wallace, “Terahertz pulsed spectroscopy of freshly excised human breast cancer,” Optics Express, Vol. 17, No. 15, 2009 M.-A. Brun, F. Formanek, A. Yasuda, M. Sekine, N. Ando, and Y. Eishii, “Terahertz imaging applied to cancer diagnosis,” Phys. Med. Biol., Vol. 55, 2010 P. U. Jepsen, “Metal-insulator phase transition in a VO2 thin film observed with terahertz spectroscopy,” Physical Review B, Vol. 74, 2006 B. Viquerat, J. Degert, M. Tondusson, J.-F. Letard, C. Mauriac, and E. Freysz, “Time-Domain Spectroscopy of Spin State Transition in Polymeric Spin Crossover compounds,” 36th IRMMW-THZ, 2-7 Oct. 2011 S. Balci, W. Baughman, D. S. Wilbert, L. Butler, P. Kung, and S. M. Kim, “Temperature Dependent THz Time-Domain Spectroscopy of Carrier Dynamics in GaN Thin Film,” 36th IRMMW-THZ, 2-7 Oct. 2011 R. Guo, K. Akiyama, and H. Minamide, “Frequency-agile terahertz-wave spectrometer for high-resolution gas sensing,” Applied Physics Letter, Vol. 90, 2007 D. Rigourd, A. Cuisset, F. Hindle, S. Matton, R. Bocquet, G. Mouret, F. Cazier, D. Dewaele, and H. Nouali, “Multiple component analysis of cigarettle smoke using THz spectroscopy, comparison with standard chemical analytical methos,” Applied Physics B-Laser and Optics, Vol. 86, pp. 579-586, 2007 T. M. Korter and D. F. Plusquellic, “Continuous-wave terahertz spectroscopy of biotin: vibrational anharmonicity in the far-infrared,” Chemical Physics Letters, Vol. 385, 2004 A. Roggenbuck, H. Schmitz, A. Deninger, I. Camara Mayorga, J. Hemberger, R. Gusten, and M. Gruninger, “Coherent broadband continuous-wave terahertz spectroscopy on solid-state samples,” New Journal of Physics, Vol. 12, 2010. E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53THz,” Applied Physics Letter, Vol. 90, 2007 M. C. Hamilton, R. A. Hites, S. J. Schwager, J. A. Foran, B. A. Knuth, D. O. Carpenter, “Lipid Composition and Contaminants in Farmed and Wild Salmon,” Environmental Science & Technology, Vol. 39, pp. 86228629, 2005 J. Exler and P. R. Pehrsson, “Nutrient contents and variability in newly obtained salmon data for USDA nutrient data base for standard reference,” USDA, 2007
Highlights We present an evaluation of a cost-effective THz CW spectrometer pumped at 1550 nm wavelengths with a fixed delay line. We present experimental validation of THz CW transmission spectra for various organic and inorganic samples. We identify the effect of output fringing space to frequency resolution, phase analysis, and data acquisition time. We propose the proper delay line setup for phase analysis for the THz CW spectrometers.
S1350-4495(15)00054-7 http://dx.doi.org/10.1016/j.infrared.2015.02.009 INFPHY 1752
To appear in:
Infrared Physics & Technology
Received Date:
6 February 2015
Please cite this article as: W-G. Yeo, N.K. Nahar, Characterization of a THz CW spectrometer pumped at 1550nm, Infrared Physics & Technology (2015), doi: http://dx.doi.org/10.1016/j.infrared.2015.02.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characterization of a THz CW spectrometer pumped at 1550nm Woon-Gi Yeo and Niru K. Nahar
Abstract—We present an evaluation of a cost-effective THz CW spectrometer pumped at 1550 nm wavelengths with a fixed delay line. To study the spectral competence of the spectrometer, transmission data is obtained for various organic and inorganic samples. Spectral comparisons of the samples are presented by using THz time domain spectroscopy and vector network analyzer (VNA). Despite the capability of highly resolved transmission spectroscopy, our current system reveals the uncertainty in interferometric output data for phase analysis. Here, we identify the effect of fringing space of raw output data toward frequency resolution, phase analysis, and data acquisition time. We also propose the proper delay line setup for phase analysis for this type of spectrometers. Index Terms—terahertz (THz), continuous wave (CW) spectrometer, validation, frequency resolution, fixed delay line, fringing space
I. INTRODUCTION
R
ecent research in terahertz (THz) technologies has pioneered a new era toward various application areas from security to food inspection based on the exotic properties of THz wave. Among them, the unique “fingerprint” THz spectra has demonstrated that various chemical, biological, and semiconducting materials can be characterized in THz band through their macromolecular vibrations [1, 2]. For instance, the unique absorption spectra in THz have been reported for explosives and drugs such as C-4, RDX, TNT, Aspirin, and Acetaminophen, etc [3,4]. Owing to extremely high sensitivity to water content, THz spectroscopy has also been utilized to diagnose biological tissues including malignant and cancerous regions [5-7]. Furthermore, THz spectra have been described inherent characteristics of semiconducting materials such as carrier dynamics, phase and spin-state transitions [8-10]. However, most of the previous research and progress has been established in the field of THz pulsed spectrometer, which has some limitations such as size, cost, and frequency resolution. Thus, in many Woon-Gi Yeo is with the ElectroScience University, 1330 Kinnear Rd. Columbus, [email protected]). Niru K. Nahar is with the ElectroScience University, 1330 Kinnear Rd. Columbus, [email protected]).
Laboratory at the Ohio State OH 43212 USA (e-mail: Laboratory at the Ohio State OH 43212 USA (e-mail:
THz applications, continuous wave (CW) spectrometer has been desired for highly resolved THz spectroscopy such as gas phase analysis [11, 12]. The recent progress of photo-mixing technologies allows the generation of THz CW source with MHz-scale frequency resolution. However, most of the THz CW systems published in contemporary literatures adopted GaAs-based emitters and detectors accompanied by laser sources at either 790 nm or 850 nm wavelength [12-14]. As a result, the systems have remained relatively expensive and large. In this study, to retain the availability of high cost-effectiveness, two 1550 nm distributed feedback (DFB) lasers are utilized for our CW system with antenna-integrated photomixers made by InGaAs grown on InP substrate. Despite the increasing number of available state-of-the-art THz spectrometers, the spectral measurements have been rarely compared by experimental methods and the optical properties of samples have rarely been presented by CW spectral measurements. Therefore, the purpose of this paper is twofold. First, we present experimental validation for CW spectral measurements through comparison with a THz time domain spectrometer and an electronic-based vector network analyzer (VNA) with Virginia Diode’s frequency multipliers (VNA-VDI). In particular, we investigate transmission spectroscopy of solid-state samples (Į-lactose monohydrate pellets and soda-lime-silica glasses), biological samples (wild and farm-raised salmons), and a metamaterial device. Secondly, we present the significance of fringing space in determining the optical properties of the samples using the CW spectrometer. We characterize the effect of interference pattern alteration towards frequency resolution, phase analysis and data acquisition time in a fixed delay line setup. II. EXPERIMENTAL SETUP Figure 1 describes the TOPTICA Photonics Inc. THz CW spectrometer that is used in this work. The system employs two tunable DFB lasers operating around 1539.9 and 1544.6 nm, respectively. The laser frequencies are tuned by both current (~0.6 GHz/mA) and temperature (~14 GHz/K in the temperature range between 276.15 K and 321.15 K) controls. As such, the CW system is capable of collecting THz frequency spectrum between about 100 GHz and 1000 GHz with a minimum of 10 MHz frequency step.
Ltd.) and VNA (N5242A, Agilent Technologies Inc.). THz pulsed spectrometer exhibits more than 45 dB dynamic range up to 3 THz and the latter includes frequency multipliers (Virginia Diodes, Inc.) with 100 dB dynamic range in the frequency range from 325 GHz to 750 GHz. III. VALIDATION OF THZ TRANSMISSION SPECTRAL MEASUREMENTS A. Transmission data analysis in CW spectrometer
Fig. 1. Block diagram of THz CW spectrometer with a fixed delay line
In particular, the heterodyne signal from the lasers is fibercoupled into the photo-mixers in both THz detector and emitter. Only the difference (THz) of the signal is radiated via a hyper-hemispherical silicon lens by broadband bow-tie antenna integrated within the photo-mixer. THz wave is collimated and focused into the sample position, and the transmitted THz wave is collimated and focused onto the detector. Finally, the detected THz signal is superimposed with the heterodyne signal, and is fed into a lock-in amplifier. To alleviate noise signal level, the lock-in integration time can be set from 21 to 650 ms. The effective optical path length difference between LA and LB plus LC (depicted in Fig. 1) is about 91 cm, which yields 165 MHz spectral resolution. Later, we investigate the effect of optical path length variation towards output interference pattern, data analysis and data acquisition time. For this purpose, the optical path length between the lasers and the THz detector (LB in Fig. 1) was elongated by additional fibers with the length from 30 to 200 cm.
Fig. 3. Detected photocurrent as an interference pattern for ambient air
Based on the coherent detection principle, the detected photocurrent of the THz CW system, Iph, is shown in Fig.3 as an interference pattern. The detected photocurrent is determined by the amplitude of the THz electric field, ETHz, and phase difference, ǻij, between the THz wave and the IR laser signal [14]: I ph ∝ E THz cos( ∆ ϕ ) = E THz cos (
2π f ∆ L eff ) c
where f denotes the THz frequency, c is the speed of light, and the effective optical path difference is ∆ L eff = n eff ∆ L = n eff L A − L B + L C
Fig. 2. Dynamic range of the CW spectrometer from 100 GHz to 1200GHz
The dynamic range of this CW system varies with frequency due to the efficiency of the photo-mixer (-40 dB/decade). Depicted in Fig. 2, the actual signal to noise ratio (S/N) with 300 ms lock-in integration time is 50 dB at 100 GHz and drops to 10 dB at 1 THz. Figure 2 also shows clearly resolved absorption lines for water vapor around at 560 GHz, 750 GHz, and 990 GHz. To validate CW spectral data, the respective measurements for each sample were also performed by THz pulsed spectrometer (TPS3000, Teraview
(1)
(2)
where neff is the effective refractive index for overall system path length in the fixed delay line system. As described in Fig. 1, LA (200 cm) and LB (210 cm) are the optical paths from the laser source to the emitter and from the laser source to the detector, respectively. LC represents the THz path from the emitter to the detector. From Eq. (1) and (2), the actual THz data can be obtained by extracting the envelop function, ETHz, as shown in Fig. 4 (a). Thus, the transmittance can be determined by § E sam T ( w ) = 20 log ¨¨ THz ref © E THz sam
· ¸¸ ¹
(3)
ref
where ETHz and ETHz denote the extracted envelopes of the sample and the reference data, which are the extrema of the interference patterns. Here, we note that the extrema of sample data are positioned at different frequencies from that
of the reference data, as illustrated in Fig. 4 (b). Therefore, to compare the envelopes of both data, the extrema of the sample data need to be interpolated on equally spaced frequency grid of the extrema of the reference data.
Fig. 6. Transmittance of soda-lime-silica glass windows for 1 mm thickness (left) and 3.15 mm thickness (right)
C. Biological sample (a) (b) Fig. 4. Examples of the extracted THz data from the absolute value of the detected photocurrent
B. Solid state samples The Į-lactose monohydrate is a standard reference sample for both THz CW and pulsed spectrometers due to its wellknown absorption properties at 530 GHz [15]. Thus, we prepared two lactose samples to demonstrate the sensitivity as well as the specificity of the CW spectrometer to chemicals. One of the samples is a 10% Į-lactose monohydrate plus 90% polyethylene pellet, which is transparent to the THz band. The other one is a pure Į-lactose monohydrate pellet. As exhibited in Fig. 5, the measurements for both samples show excellent agreements between the transmission data from VNA-VDI, CW, and TPS3000. Moreover, the transmittance spectra for two samples reveal the relative difference of the lactose compounds as about -20 dB, which corresponds to 10% in percentage scale.
Fig. 5. Transmittance of Į-lactose monohydrate pellets: 10% Į-lactose monohydrate plus 90% polyethylene with 3.5 mm thickness (left) and pure Įlactose monohydrate with 0.82 mm thickness (right)
The CW spectrometer was also used to characterize the relative thickness of solid materials. Two soda-lime-silica glass windows were measured for different thickness. As seen in Fig. 6, about -7 dB and -22 dB power losses was observed at between 100 GHz and 500 GHz for 1 mm and 3.15 mm windows, respectively. From the results, the relative thickness of the windows (1:3.14) can be estimated since both have the same material properties such as absorption coefficient (unit: dB/cm).
(a)
(b) (c) Fig. 7. Measurements of dried salmon tissues: (a) sample preparation, transmittance of (b) wild salmons and (c) farm-raised salmons
Next, we tested the accuracy of our spectrometer for biological tissue characterization. Based on the availability, we chose to investigate wild and farm-raised salmon tissues to contrast of their lipid contents and chemical compositions [16, 17]. For these experiments, eight specimens were taken from both wild and farm-raised salmons, and the results were averaged. We also note that the salmon tissues were air-dried in between two glass slides to remove highly absorptive water to maximize measurable frequency range. As depicted in Fig. 7, even though transmission values may vary due to the surface roughness of the tissues and measurement positions, the spectra exhibit excelent agreement between CW and VNA-VDI systems. More importantly, the spectra demonstrate the potential to differentiate wild salmons from farm-raised salmons with higher lipid content, which is more transparent in THz band. D. Metamaterials The characterization of artificially constructed metamaterials such as Frequency Selective Surfaces (FSS) can be one of major application arena of THz spectrometer. Here, we measured a cross-dipole FSS band-pass filter resonating at 508.5 GHz, as presented in Fig. 8. The spectra from the CW and the pulsed system demonstrate the exact expected resonant frequency and also demonstrate excellent agreement up to 800 GHz.
can be originated either from jitter noise or coarse number of data to generate the sinusoidal interference pattern in our delay line setup. Thus, the extrema of the raw data can be detected at undesired positions. More importantly, the deviation shown in Fig. 9 may be more dominantly caused by the ambiguous
(a) (b) Fig. 8. Measurements of a metamaterial device: (a) Frequency Selective Surfaces (FSS) band-pass filter resonating at 508.5 GHz (Lake Shore Cryotronics, Inc.) and (b) measured transmittance
IV. EFFECT OF FRINGING SPACE TOWARD OPTICAL PROPERTY EXTRACTION AND DATA ACQUISITION TIME A. Extraction of Optical Properties in CW spectrometer Traditionally, the way to extract the phase information of the THz signal is to use a mechanical delay stage or an optical phase modulator to vary ǻL in Eq. (1) section III. Alternatively, in the fixed delay line system, the relative phase difference can be varied by scanning THz frequency [14]. In particular, as addressed in Eq. (1), the frequency and the optical path difference determine the oscillation period of the detected photocurrent, which refers to the fringing space. Here, for the reference measurement, THz wave travels through air with the constant refractive index (nair § 1), thus the fringing space is nearly constant independent of frequencies. As such, the position of extrema can be defined as: ref f extrema ,m =
mc , 2 ∆ Lref eff
m = 1,2 ,3 ,!
Fig. 9. Deviations of the refractive index for Į-lactose monohydrate pellet by different relative order, m, of the extrema for reference and sample signals
relative order, m, of the extrema for both the reference and the sample data. For example, as described in Fig. 10 (a), the determination of the relative order, m, for reference and sample signals is not straightforward if the narrow frequency range is scanned. Although the wide frequency range is explored to examine the in-phase frequency where both signals have the same order [14], the ambiguity is still present since a group of the sample signals are in phase with the reference signal, see Fig. 10 (b). Therefore, here, to analyze this problem, we present the experimental study of the effect of fringing space of the practical phase analysis and the data acquisition time by utilizing additional fibers in the system.
(4)
In the case of the sample measurement, the effective optical sam
path difference, ∆Leff , can be changed with the frequency dependent refractive index, nsam, and the thickness, d, of the sample. Thus, compared to the reference signal, the extrema of the sample signal are not equally spaced in frequency, and the fringing pattern is shifted, as depicted in Fig. 10(a). Based on this fact, by comparing the extrema of the sample and the reference data, the relative phase difference can be achieved, thus the refractive index of the sample can be calculated using the following equation [14],
(n sam
ref § f extrema · ,m ref − n air )d = ∆ Lsam = ¨ sam − 1 ¸ ∆ Lref eff − ∆ L eff ¨ f ¸ eff © extrema ,m ¹
(
)
(a) (b) Fig. 10. Examples of the extracted THz data from the absolute value of the detected photocurrent
B. Effective optical path difference versus fringing space In general, the frequency resolution of the CW system is regarded as the fringing space of the interference pattern. For the fixed delay line system, the fringing space in the air, ǻf, is determined by the effective path difference, ǻLeff:
(5).
However, in practical phase analysis, two major difficulties hinder the extraction of the relevant refractive indices from CW raw data compared to the pulsed system, as demonstrated in Fig. 9. First, the detected photocurrent is not perfectly sinusoidal, and the constant fringing space between the extrema is unattainable even from the reference data. This
∆f =
c c = 2 ∆ Leff 2 n eff L A − L B + LC
(6)
Theoretically, as we increase effective path difference, the fringing space becomes smaller and therefore better frequency resolution can be achieved. However, the frequency resolution can be limited by the measurable frequency step size and the stability of the laser frequency tuning module in the system.
In particular, as described in Fig. 11, the fringing space in Zone 3 is less than 75 MHz, and the available number of data for a half cycle of the interference pattern is less than eight with the minimum 10 MHz frequency step. Thus, the detected photocurrent is too coarse to exhibit relevant transmission spectra and refractive indices.
Fig. 11. Calculated relation between effective optical path difference and fringing space
C. Fringing space analysis method To characterize the delay line setup in detail, we tested our CW system by utilizing additional fibers in various lengths, which enabled to determine the unknown parameters such as LC and neff. Five fibers with the lengths from 30 cm to 200 cm were added to the existing fiber between a diode laser and the detector, thus LB became lengthen. The fringing spaces were obtained from the detected photocurrents. Based on the theoretical relation described in Eq. (6), respective effective optical path differences, ǻLeff, was evaluated, as summarized in Table I. Here, the unknown parameters can be obtained by algebraic methods in the Eq. (2), and the averaged values are 1.614 and 66.773 cm for neff and LC, respectively. Based on the calculated values, the relation between fringing spaces and additional fiber lengths can be established as shown in Fig. 13. The fringing space can be predicted for the additional fiber with the certain length. As a result, the desired frequency resolution can be achieved by considering the appropriate number of data points for the phase analysis.
(a) (b) Fig. 12. Detected photocurrents after data smoothing: (a) Zone 1 (ǻf § 360 MHz) and (b) Zone 3 (ǻf § 65 MHz)
More importantly, for the extrema detection in data analysis, data smoothing is an inevitable process due to jitter noise. However, the lack of data points constrains the span size in this process, thus it is hard to obtain intact sinusoidal signal for the extrema detection, as shown in Fig. 12(b). Likewise, Zone 2 including our system setup also imposes the restrictions particularly on the phase analysis. Furthermore, since the minimum frequency step should be required in the systems for Zone 2 and Zone 3, more than 10 hours of data acquisition time is needed for a full frequency range scan with 300ms integration time. In contrast, the measurements in Zone 1 can guarantee to obtain enough number of data points (more than 30) and can resolve the problems, see Fig. 12. Thus, it is desired to adopt Zone 1 not only for more reliable data analysis with sub-GHz frequency resolution but also for more economic data acquisition time. TABLE I EXPERIMENTAL RESULTS FOR FRINGING SPACE BY PATH LENGTH VARIATION Ladd (cm)
LB (cm)
Leff (cm)
ǻf (GHz)
0 30 60 100 130 200
210 240 270 310 340 410
~ 91 ~ 42 ~ 3.35 ~ 70 ~ 120 ~ 231
~ 0.165 ~ 0.36 ~ 4.516 ~ 0.215 ~ 0.125 ~ 0.065
Fig. 13. Calculated relation between additional fiber length and fringing space based on the experimental results (marked with X).
V. CONCLUSION We demonstrated the overall performances of a THz CW spectrometer employing a fixed delay line and two 1550nm tunable distributed feedback diode lasers. The validation of THz CW transmission spectroscopy was evaluated by the comparative measurements of the chemical and biological samples as well as the metamaterial device. The spectral efficiency of the CW system showed excellent agreement with the TPS300 and the VNA-VDI. Ambiguities of the phase analysis in the current system setup were described in terms of the fringing space, which correlates with the delay line setup. To alleviate the ambiguities, we suggested a methodology to set the proper optical delay configuration in CW system with fixed delay line. REFERENCES [1] [2]
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Highlights We present an evaluation of a cost-effective THz CW spectrometer pumped at 1550 nm wavelengths with a fixed delay line. We present experimental validation of THz CW transmission spectra for various organic and inorganic samples. We identify the effect of output fringing space to frequency resolution, phase analysis, and data acquisition time. We propose the proper delay line setup for phase analysis for the THz CW spectrometers.