1. INTRODUCTIONConventional terahertz (THz) time domain spectrometer(TDS) systems [1] are based on a femtosecond (fs) laserand a mechanical delay line to control the temporal delay be-tween the optical pulses in the detector and emitter arms ofthe spectrometer. Commonly used delay lines are mechanicaltranslation stages or rotating mirrors [2]. They comprise freespace optics and moveable components and are demanding interms of mechanical stability and optical adjustment. Typi-cally, the scanning speed of these mechanical delay lines islimited to several tens of hertz with a range of a few picose-conds. For a new class of fiber-coupled THz-TDS systems, fi-ber stretchers have been used [3]. Here, new questions arise inregard to performance due to the effects of polarizationscrambling and the need for dispersion compensation.
Alternatively, asynchronous optical sampling (ASOPS)overcomes these drawbacks, as it does not require any mech-anical delay lines. The technique relies on two synchronizedpulsed fs lasers that are slightly detuned in the repetition rate[4–6]. However, such systems need two pulsed laser sourcesand additional stabilization electronics, which considerablyincreases system complexity.
Recently optical sampling by cavity tuning (OSCAT) [7] wasdemonstrated, a technique that enables direct control of thetime delay between two pulses by varying the laser repetitionrate. The OSCAT technique requires just one fs laser, butshares the main advantages with the ASOPS technique: noexternal moveable delay line, capability of high-speed mea-surements, and long scanning ranges, eliminating the needfor precise temporal adjustment of the experimental setup.Mechanical scanning inside the laser cavity is still required,but over a much reduced scan range and with built-in controlvia the repetition frequency of the laser. As we showed in ourprevious work [8], there is perfect agreement between time-resolved experiments with a conventional delay line and theOSCAT technique. It is worth mentioning that similar tech-niques have been employed in the past for distance mea-
surements in an unbalanced Michelson interferometer byYamaoka et al. [9] and Ye [10].
Here, we apply the OSCAT technique to THz-TDS [11,12].We present a fully fiber-coupled THz-TDS based on a1:55 μm fs fiber laser, LT-InGaAs antennas, and the OSCATscheme. Furthermore, we have further extended the scanningrange, now covering basically the full temporal delay betweentwo adjacent pulses of 4 ns.
2. THEORETICAL FOUNDATIONSIn a standard THz-TDS, a pulsed laser beam is split into twoparts. One part is sent to the THz emitter antenna, the otherpart is directed to the receiver or detector antenna. At the de-tector antenna, the THz pulse is superimposed with the opticalgating pulse. Typically, delays in a system are set so that thesame pulse is used in both arms of the interferometer, makingthe setup completely immune to timing jitter of the pulsed la-ser. However, for an fs laserwith a negligibly small timing jitter,it is possible to superimpose pulses that originate not from thesame but from two adjacent or even further separated opticalpulses (i.e., when spectrometer arms are of unequal length thatoptical pulses i and iþ a reach the detector antenna at thesame time). For such an unbalanced spectrometer, the tempor-al shift between the pulses i and iþ a is determined by the re-petition rate of the fs laser, as shown in Fig. 1. A change of therepetition rate f rep by Δf leads to a temporal Δτ shift of
Δτ ¼ a
�1
f rep−
1f rep þΔf
�: ð1Þ
The necessary length of the passive delay line is determinedby the index a, the laser repetition rate f rep, the tuning range,and the refractive index of the delay line medium n. For agiven scanning range Δτ, the required length of the passivedelay line ld is
ld ¼ Δτ · c0 · ðf rep þΔf ÞΔf · n
: ð2Þ
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3. EXPERIMENTAL SETUPThe fs fiber laser used in the experiment (model M-Fiber,Menlo Systems GmbH) delivers sub-90 fs pulses at a repetitionrate of 250MHz. The repetition rate can be tuned over 2:5MHzand 3:5 kHz by a precise intracavity stepper motor and a fastpiezoactuator, respectively. In order to cover the completedistance between the two adjacent pulses from the 250MHzpulse train, a scanning range of 4 ns is required. For the givenrepetition rate and a tuning range of 2:5MHz, the passive delayline has to delay optical pulses by the index a ¼ 101, as givenby Eq. (1). The corresponding length calculated by Eq. (2) of121m in free space would be impractical. Instead, we haveengineered a passive fiber delay line of approximately 80m.It consists of two types of fiber in order to achieve zerogroup-velocity dispersion operation: a standard single modefiber and a telecom dispersion shifted fiber. The passive delayline does not have any significant influence on the pulse lengthand its shape. In order to protect the 80m-long fiber link formthe surrounding acoustic noise, the fiber is spooled and pro-tected by an enclosure.
The measured autocorrelation signal after passing throughthe fiber link is shown in Fig. 2. The measured pulse width is77 fs, assuming a Gaussian pulse shape. Figure 3 shows thepulse width and output power stability over the entire
2:5MHz tuning range of the M-Fiber laser. The pulse widthstability is better than the �2:5 fs accuracy of the interferenceautocorrelator, which was used in the experiment. The outputpower variation was measured with a Thorlabs PM100Apower meter and a S132C sensor with an accuracy of �5%.Also, in this case, the variation of the output power cannotbe properly resolved. Therefore, we conclude that the M-Fiberlaser and the presented OSCAT scheme is equivalent to thetwo-laser ASOPS setup in terms of pulse width and outputpower stability.
The pulse train will be delayed by the index a ¼ 98 afterpassing through the 80m-long fiber link. According toEq. (1), the OSCAT technique will cover basically the full dis-tance (more than 97%) between two consecutive pulses and,therefore, offers full temporal pulse control functionality.
The experimental setup is shown in Fig. 4. The light fromthe laser is split into two fiber output ports labeled A and Busing a fiber-coupled 50=50 splitter. Both ports are precom-pensated for 3m of optical fiber. The light from port A is sentvia fiber directly to a fiber-coupled LT-InGaAs/AlInAs multi-quantum well detector antenna with a dipole length of25 μm and a 10 μm gap matching the fiber mode field diameter(Tera15-DP25-FC, Menlo Systems GmbH) [13]. The light fromport B is sent to the 80m-long fiber delay line and subse-quently illuminates the fiber-coupled emitter stripline antenna(Tera15-SP25-FC, Menlo Systems GmbH). Both antennae areilluminated with 25mWat λ ¼ 1560 nm. The emitter antenna isbiased with a bipolar square wave with an amplitude of 10V at15kHz. The THz beam is collimated and refocused on the de-tector antenna via two polymethylpentene polymer lenseswith an effective focal length of 54mm. The repetition rate
Fig. 1. Principle of operation of optical scanning by cavity tuning.Solid and dashed curves show pulse train for f rep and f rep þΔf ,respectively.
Fig. 2. (Color online) Autocorrelation signal after passing through an80m-long fiber link.
Fig. 3. (Color online) (a) Pulse width and (b) output power stabilityof the M-Fiber laser over entire 2:5MHz tuning range.
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is measured with a frequency counter (53181A, Agilent). Itsinput signal is derived from a photodiode integrated in thelaser head. The signal from the detector antenna is measuredwith a lock-in amplifier (LIA-BV-150-H, Femto GmbH). 4. RESULTS AND DISCUSSION
Figure 5(a) shows the measured THz pulse for the OSCAT re-petition rate scan between 249,679,920 and 249; 741; 710Hzwith a frequency step of 60Hz. The resulting scanning rangein the time domain equals to 97ps. The corresponding spec-trum is shown in Fig. 5(b). The measurement was performedwith the intracavity stepper motor and took about 2 min. Theintracavity piezoactuator is capable of 5 ps scans with a rate of100Hz. However, with a larger piezoactuator and a longer pas-sive delay line, a scanning range of several tens of picose-conds at several hundred hertz would be possible. The THzOSCAT measurement agrees very well with the referencepulse of the same antenna types measured using a conven-tional TDS with mechanical delay line, as presented by Roehleet al. [13]. Figure 6 shows the measured refractive index of a550 μm-thick reference high-resistivity Si wafer. The solidcurve represents measurement, including Fabry–Perot ef-fects. The dashed curve was calculated from the temporaldata cut just before the first echo occurs. The mean valueof 3.42 agrees with the measurements reported in the litera-ture [14]. The timing jitter of modern fiber lasers is small en-ough to use the OSCAT method and delayed pulses by indicesa up to several tens of thousands.
5. CONCLUSIONIn summary, we have presented an all-fiber THz-TDS withoutan external mechanical delay line. The spectrometer is basedon an ultrafast fiber laser, two fiber-coupled LT-InGaAs anten-nae, and a passive fiber delay line. The temporal delay is di-rectly controlled by solely varying the laser’s repetition rate.We have shown that the THz OSCAT spectrometer shares themain advantages of an ASOPS system. Contrary to the ASOPSscheme, OSCAT requires only one fs laser source. It allows fora compact and turnkey system. It is based on telecom compo-nents and is therefore capable of maintenance-free long-termoperation. This represents a major step toward real-life appli-cations of THz technology.
Fig. 5. (a) Measured THz pulse and (b) corresponding spectrum.
Fig. 6. Measured refractive index of 550 μm-thick high-resistivitySi wafer.
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