Due to the steady evolution of terahertz (THz) time-domain spectroscopy (TDS) [1,2] systems, THz technology is steadily growing in importance for real-world applications [3–6]. From the very beginning of THz-TDS systems, the complexity, performance, and costs were mainly driven by the laser engine. The essential requirement of the laser engine is to provide two optical femtosecond pulses with an adjustable temporal separation to generate and detect THz transients in a pump-probe-measurement scheme. The architecture of common laser engines can be classified into two categories: single-laser and dual-laser approaches. In the single-laser approach, the output of one laser is split into the pump arm and probe arm. The delay is either established by changing the optical path length difference of the two arms {e.g., optical delay unit (ODU) [7], acousto-optic delay (AOD) [8]} or by fast modulation of the cavity length of the used femtosecond laser (optical sampling by cavity tuning [OSCAT] [9]). While the ODU approach suffers from the limited measurement speed determined by the inertia of the moving masses, the AOD approach shows small delay windows, which could be a problem for adapting the system to different measurement tasks. OSCAT seems to be a potential candidate for the reduction of system costs, but suffers from the necessary tuning range of the resonator cavity length and the inherently fixed location of the scan window. However, in the dual-laser approach, two distinct laser cavities with different optical lengths are used. Here the temporal delay is inherently established due to the mismatch of the repetition rates, which either could be stabilized to a fixed repetition rate difference {asynchronous optical sampling (ASOPS) [10]} or controlled to a variable repetition rate difference {electronically controlled optical sampling (ECOPS) [11]}. The main advantage of ECOPS is the ability to adapt the scanning range and start position of the scan window, which increases the measurement efficiency in comparison to ASOPS [12]. It is noteworthy to apply ECOPS it is mandatory to change the repetition rate difference ($\mathrm{\Delta }{f}_{\mathrm{Rep}}$) of the two cavities from positive to negative values. Hence, in terms of flexibility, performance, and cost, it would be highly beneficial to have a combination of the single-laser ODU and dual-laser ECOPS approach. One way to achieve this goal is by using only one cavity to emit two pulse trains with different repetition rates. In a very early contribution, Kieu and Mansuripur [13] showed that it is feasible to achieve the mode-lock operation of two pulse trains while using only one cavity. In this setup, the two pulse trains are circulating counter-clockwise through a ring resonator. In another contribution from Gong et al. [14] it was shown that two pulse trains with different and adjustable repetition rates can be generated from only one laser cavity by making use of the polarization of light. Most recently in [15], a free-space (FS) single-cavity setup with a birefringent element is used to change the optical length for the orthogonally polarized pulse trains. Nevertheless, in all contributions, it was not shown that the system is capable of changing the $\mathrm{\Delta }{f}_{\mathrm{Rep}}$ from positive to negative values, preventing it to be used in an efficient way in THz-TDS. Note that linear cavity setups [16,17] are not suited to be used for single-laser ECOPS systems due to the occurring passive synchronization of the pulse trains caused by cross-phase modulation while approaching $\mathrm{\Delta }{f}_{\mathrm{Rep}}=0\mathrm{Hz}$. Here we present a laser setup, combining the advantages of a single-laser system with that of a dual-laser approach. All components are polarization-maintaining (PM), which guarantees stable and robust performance, even in harsh environments. The key component is the laser, which is able to emit two pulse trains with adjustable repetition rates using only one gain section and one pump diode. With this design, we are able to apply our concept similar to ECOPS, using just one single-laser source. We call this concept single-laser polarization-controlled optical sampling (SLAPCOPS).

The SLAPCOPS resonator shown in Fig. 1 consists of two ring resonators (ring 1 and ring 2) sharing one gain section from the cavity. Each ring contains one semiconductor saturable absorber mirror (SESAM), a three-port circulator (CIR) and a 90/10 fiber output coupler to extract the pulse trains. The SESAM has a modulation depth of 13%, a recovery time of 2 ps, a saturation fluence of $90\mathrm{\mu J}/{\mathrm{cm}}^2$, and non-saturable losses of 8%. Besides the introduction of the fiber-coupled SESAM, the CIR acts additionally as an optical isolator and determines the propagation direction of the pulses in the corresponding ring. To easily compensate for the length mismatch and account for ring-related losses due to splice variations, an FS section is introduced in each ring. For the necessary fast control of the repetition rate, a short piece of optical fiber is glued on a piezo stack with 175 nF and maximum displacement of 9 μm. All passive fibers used have a group velocity dispersion (GVD) value of $-22{\mathrm{ps}}^2/\mathrm{km}$ and a specified beat length of 3–5 mm. The gain medium is a 1 m PM erbium-doped fiber with GVD of $-20{\mathrm{ps}}^2/\mathrm{km}$ at 1550 nm and a core-absorption coefficient of $55\mathrm{dB}/\mathrm{m}$ at 1530 nm. The gain section is operated polarization multiplexed, by using two polarization beam combiners (PBCs). Thus, the colliding pulses in the gain section have orthogonal polarization, as well as opposite propagation direction, to reduce cross-phase modulation effects between the pulses. Additionally, this design efficiently suppresses crosstalk between the rings caused by non-proper polarization maintenance in the polarization-multiplexed part [18]. A 976 nm continuous-wave laser diode (pump) with maximum output power of 950 mW unidirectionally pumps the gain medium through a wavelength-division multiplexer (WDM). The optical length of each ring is approximately 3.8 m, leading to a calculated round-trip cavity dispersion of $-0.1{\mathrm{ps}}^2$.

 

Fig. 1. Schematic setup of the SLAPCOPS laser using only one pump diode and one laser-active section.

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The self-mode-locked operation of both rings is initiated by applying an optical pump power of 120 mW. Under this condition, multiple pulses occur in both rings. By reducing the pump power, the number of pulses continuously decreases until only one pulse is circulating per ring at 52 mW. In this state, the repetition rates are 51.8 MHz measured with a radio frequency spectrum analyzer (Anritsu MS2830A), and the average output powers are 420 μW for ring 1 and 440 μW for ring 2. Figure 2 illustrates our experimental THz-TDS setup with the SLAPCOPS resonator.

 

Fig. 2. Schematic diagram of the experimental setup for THz-TDS with a SLAPCOPS system.

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The output pulses are split up by two 90/10 tap couplers. The 90% parts are further amplified by a forward pumped, dispersion-compensated optical fiber amplifier and guided to the THz setup. The other part drives the phase control electronics. The amplifier contains 2 m of PM erbium-doped fiber with a core diameter of 3 μm and a core-absorption coefficient of $80\mathrm{dB}/\mathrm{m}$ at 1530 nm enclosed by two WDMs. Identically to the arrangement in the resonator, the optical amplifier is used polarization multiplexed by placing it between two PBCs. In this configuration, we achieved output powers of 8.4 mW and 8.2 mW at a pump power of 350 mW at the output of the second PBC. The optical spectra (measured with an optical spectrum analyzer (OSA) AQ6370D) and corresponding intensity autocorrelation signals (measured with an APE Pulse Check) are shown in Fig. 3. The ${\mathrm{sech}}^2$-fitted pulse widths are 280 fs for ring 1 and 350 fs for ring 2. Due to the high peak power and the small mode field diameter of the gain fiber, spectral broadening caused by nonlinear effects [19] can be observed for both polarizations simultaneously.

 

Fig. 3. (a) Optical spectra measured at the output of the amplifier. (b), (c) Corresponding ${\mathrm{sech}}^2$-fitted intensity autocorrelation signals of the optical pulses. The red trace indicates the amplified signal from ring 1, and the black trace indicates the amplified signal from ring 2.

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The THz setup contains two fiber-coupled photo-conductive antennas as the THz emitter and detector [20] in a standard transmission setup [21]. The distance between the emitter and detector is approximately 30 cm. The measurement is conducted in ambient atmosphere containing water vapor. The output signal of the THz detector is amplified via a transimpedance amplifier (TIA) and is acquired with a 16 bit, 1 MHz data acquisition card (DAQ). To control the phase relation and, by this means, the delay for the TDS measurement, a similar setup as in [12] is used. The optical signals of the pulse trains are measured with two fiber-coupled InGaAs photo diodes (DET08CFC) with a $-3\mathrm{dB}$ bandwidth of 5 GHz. In the first stage, the 97th harmonic of the repetition rate is down-mixed to approximately 10 MHz via two double-balanced mixers (Mini Circuits ZX05-153-S+) and an RF signal generator (Agilent Technologies E8257D). The down-mixed signals are fed into a 50 MHz high-speed lock-in amplifier (Zuerich Instruments HF2LI) which resolves the phase relation between both inputs. This phase signal acts as the measured output of the control loop and is subtracted from the reference signal, which defines the desired phase difference. The error signal is then sent into a proportional–integral–derivative filter and is further amplified with a high-voltage amplifier to drive the piezo of ring 2. To adjust the phase relation between the pulse trains, a digital signal generator is attached to the reference port of the control loop. By applying a waveform to this port, the control loop will force the system’s phase relation to follow this waveform accordingly. To calibrate the time axis of the THz transient, the phase signal retrieved by the lock-in amplifier is acquired simultaneously with the DAQ card.

A typical THz waveform measured by the system is plotted against the time delay in Fig. 4(a). Here we choose a maximum time delay of 100 ps with a scan rate of 40 traces per second with an averaging time of 1 s. The spectrum with a bandwidth of approximately 1.5 THz and a 50 dB dynamic range (DR) can be seen in Fig. 4(b). Clearly visible are the spectral features coming from water vapor absorption. A small ripple on top of the spectrum indicates the existence of a weak additional optical pulse. We think that this is related to the extinction ratio of the optical amplifier which is not optimal. In this measurement, the necessary change in cavity length by the piezo is approximately 50 nm. Thus, up to 2 kHz scan rates at a scan window of 100 ps should be feasible with this setup. Furthermore, by an improved dispersion and gain control of the amplifier stage, we expect broader THz bandwidth of the system, similar to results obtained with a standard TDS setup [7].

 

Fig. 4. THz transient (upper graph) and spectrum (lower graph) obtained with the SLAPCOPS system with a measurement rate of 40 Hz and an averaging time of 1 s.

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In conclusion, we presented a single-laser, all-PM THz-TDS setup without an external delay line. The system is based on a novel fiber laser design, consisting of two ring resonators sharing one polarization-multiplexed gain section. Similar to ECOPS, the phase relation between the emitted pulse trains is adjusted via the control of the optical length difference of the ring resonators. With this laser, we have been able to measure THz transients with a bandwidth of approximately 1.5 THz and a DR of 50 dB at a scan rate of 40 Hz and an averaging time of 1 s. We have shown that SLAPCOPS shares the same advantages of an ECOPS system, while using only one laser-active medium and one pump diode in the resonator. Further improvement of the system’s performance in terms of timing jitter and resolution can be expected by reducing the overall cavity dispersion of the ring resonator by substituting the currently used anomalous dispersion gain fiber with a normal dispersion gain fiber. Besides this, the impact of the usage of even higher harmonics to retrieve the time axis of the THz transient is the scope of future investigations. Additionally, the currently deployed FS section in the cavity will be removed to have all the benefits of a PM fiber laser. These steps will be covered in future work. We believe that this laser design will also find potential applications in other pump-probe measurement systems. The work in this Letter is done in collaboration with HÜBNER GmbH & Co. KG, Division Hübner Photonics, Kassel, Germany.