Combless broadband terahertz generation with conventional laser diodes

We present a novel technique to generate a continuous, combless broadband Terahertz spectrum with conventional low-cost laser diodes. A standard time-domain spectroscopy system using photoconductive antennas is pumped by the output of two tunable diode lasers. Using fine tuning for one laser and fine and coarse tuning for the second laser, difference frequency generation results in a continuous broadband THz spectrum. Fast coarse-tuning is achieved by a simple spatial light modulator introduced in an external cavity. The results are compared to multi-mode operation for THz generation. © 2011 Optical Society of America OCIS codes: (300.6495) Spectroscopy, terahertz; (140.2020) Diode lasers. References and links 1. D. Grischkowsky, S. Keiding, M. v. Exter, and C. Fattinger, “Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors,” J. Opt. Soc. Am. B 7, 2006–2015 (1990). 2. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995). 3. A. G. Markelz, A. Roitberg, and E. J. Heilweil, “Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz,” Chem. Phys. Lett. 320, 42–48 (2000). 4. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B 13, 2424–2436 (1996). 5. D. Molter, F. Ellrich, T. Weinland, S. George, M. Goiran, F. Keilmann, R. Beigang, and J. L éotin, “High-speed terahertz time-domain spectroscopy of cyclotron resonance in pulsed magnetic field,” Opt. Express 18, 26163– 26168 (2010). 6. O. Morikawa, M. Tonouchi, and M. Hangyo, “A cross-correlation spectroscopy in subterahertz region using an incoherent light source,” Appl. Phys. Lett. 76, 1219–1521 (2000). 7. M. Scheller and M. Koch, “Terahertz quasi time domain spectroscopy,” Opt. Express 17, 17723–17733 (2009). 8. C. Brenner, M. Hofmann, M. Scheller, M. K. Shakfa, M. Koch, I. C. Mayorga, A. Klehr, G. Erbert, and G. Tr änkle, “Compact diode-laser-based system for continuous-wave and quasi-time-domain terahertz spectroscopy,” Opt. Lett. 35, 3859–3861 (2010). 9. E. R. Brown, J. E. Bjarnason, A. M. Fedor, and T. M. Korter, “On the strong and narrow absorption signature in lactose at 0.53 THz,” Appl. Phys. Lett. 90, 061908 (2007).


Introduction
Since the first demonstration of coherent generation and detection of terahertz (THz) radiation using photoconductive antennas in conjunction with continuous wave or pulsed lasers [1] and its promising wide-spread field of applications [2,3], the search for cost-effective systems has been an ongoing process.The major component dominating the price of THz time-domain spectroscopy (TDS) systems has always been the pump laser.Several types of lasers have been proven to be suitable for THz TDS, reaching from mode-locked Titanium-Sapphire [4] to modern compact femtosecond fiber lasers [5].At the beginning of the past decade, the applicability of multi-mode diode lasers for TDS-like, so-called cross-correlation spectroscopy (CCS) was demonstrated [6] and recently recalled into the focus of attention [7,8].This technique provides in principle the same time-domain signal as femtosecond pumped systems, based on the mixing of the laser modes in a photoconductive antenna (emitter) and cross-correlating the generated THz radiation with the same multi-mode laser pattern in a second photoconductive antenna (detector).The disadvantage of these systems, when driven by a standard non AR-coated diode laser, is the inherent discrete THz spectrum consisting of spectral lines typically spread by tens of GHz.This is because the length of the laser cavity of the pump source is only in the range of some millimeters.In principle, this can be overcome by AR-coating the laser-active medium and using a larger external cavity, resulting in a decrease of the spectral spacing of the laser modes, leading to a tightened THz frequency comb.However, the generated THz spectrum is still not continuous.In this paper we propose and demonstrate a cost-effective technique based on standard, non AR-coated laser diodes, resulting in a continuous THz spectrum.The output of two external cavity diode lasers, which are driven in different spectral modulation modes, is combined on a photoconductive antenna for difference frequency generation.

External cavity diode laser
The experimental setup of the used Fourier transform external cavity diode lasers is shown in Fig. 1.Fig. 1.Scheme of the external cavity diode laser used in our experiments.The laser diode itself is not AR-coated, therefore only discrete spectral components can be fed back by the external cavity.A variable attenuator before the grating ensures the maximum available power in the experiment, while still sending enough light to the external cavity for spectral control of the laser.
A commercially available laser diode (Thorlabs Inc., Model L785P100) mounted on a temperature stabilized holder was used as laser source.This laser diode is specified to be a multimode source, capable of delivering at least 90 mW of optical power at a center wavelength of 785 nm.The divergent radiation was collimated by a standard AR-coated lens with a focal length of 4.5 mm.A variable attenuator, consisting of a half-wave plate and a polarizing beam splitter cube, was used to adjust the power level directed to the external cavity to the minimum required power for spectral control.The external cavity consists of a blazed grating with 1200 rules per millimeter, followed by a lens with a focal length of 400 mm.This lens parallelizes the spectral components (modes) spread by the grating and focuses each beam of a specific mode.So in the Fourier plane of this lens, spatially separated foci of different modes of the diode laser are accessible.Finally, a dielectric end mirror was used to feed the radiation back.In front of it, several types of apertures serve as spatial light modulators (SLM), enabling a selective spectral feedback to the laser diode (see Fig. 2).The overall length of the external cavity is about 1 m.

Terahertz cross-correlation spectroscopy system
The TDS-like THz CSS system is shown in Fig. 3.The radiation of the two lasers is directly fiber coupled into the branches of a fiber-optic 50:50 coupler.The use of fibers ensures a perfect overlap of the laser radiation and reduces the complexity of the system drastically.In contrast to the use of femtosecond pulses with fibers, no special dispersion compensation techniques have to be applied here.The collimator in the emitter arm is mounted on a standard motor driven linear stage serving as a delay line.A standard 60 µm dipole antenna based on low-temperature gallium arsenide (LT-GaAs) serves as emitter and is biased by a 2.7 kHz modulated 40 V RMS voltage, enabling Lock-In technique as detection method.Another 20 µm dipole antenna on LT-GaAs is used as detector and connected to a standard transimpedance amplifier followed by a Lock-In amplifier, whose time constant is set to 300 ms in the measurements presented here.The emitter and detector antennas are illuminated with optical powers of 25 mW and 40 mW respectively.The THz path consists of two off-axis parabolic mirrors enabling the measurement of samples in a collimated THz beam.

Spectral modulation of the lasers
The key principle of the proposed method to generate a continuous broadband THz spectrum is the simultaneous modulation of the two lasers used.Generally, a THz CCS system delivers the same type of signal, when either pumped by a single multi-mode laser diode (see Fig. 4 a) or pumped by two lasers, where one is single-mode and the other one is coarse (but not modehopfree) tuned (Fig. 4 b).As long as the tuning frequency is higher than the corresponding cutoff frequency set by the Lock-In integration time, the signals are very similar and both consist of discrete THz frequencies.As non AR-coated diode lasers cannot be tuned modehop-free over a wide spectral range, one has to introduce a continuous tuning of the second laser (Fig. 4 c).The modes of a laser diode can be fine-tuned by modulation of the injection current up to a frequency shift of +/-half the mode separation before a modehop occurs.Typical tuning sensitivities are about 3 GHz/mA.The laser diodes used in this work have a mode spacing of approx.40 GHz, resulting in the need of a current modulation of about 13 mA of one of the lasers.The current modulation of only one laser results in a modulation of the optical power in the experiment.To minimize this effect and to reduce the amplitude of the modulation for one laser, the two lasers are modulated in opposite directions.So the optical power in the experiment, which consists of the sum of the two lasers is nearly constant.The result of these combined modulation techniques can be described either in the frequency or time domain.In the frequency domain, the coarse modulation generates a THz frequency comb, which is shifted during the integration time of the Lock-In steadily by one FSR of the laser diode by the injection current modulation, resulting in a continuous overlap.In the time domain, the cross-correlation peaks adjacent to the center peak are modulated in their phase, resulting in a destructive interference during the Lock-In integration time.So the proposed method is based on the integration over an appropriate fast modulation of the difference frequency.While the instantaneous THz spectrum is not continuous, the finite integration time used in the measurement results in the measurement of a combless spectrum.

Results
In the left column of Fig. 5, the THz time signal from the system driven by only one of the lasers run with broadband spectral feedback (no inserted SLM) is shown.As known well by literature [6][7][8], this results in a periodic time-domain signal (upper plot) corresponding to discrete frequencies in the spectrum (lower plot).The spreading of these frequencies in this case is approx.40 GHz, in agreement with the FSR of the laser diode.The spectrum directly shows the disadvantage of such a system driven by a non AR-coated diode.In spectroscopic applications, narrow (below 40 GHz) absorption features of potential samples cannot be resolved or even detected.
Fig. 5. Comparison between the single multi-mode laser driven system (left column) and the new approach introduced here (right column).By the combination of a coarse and a fine frequency-tuned diode laser, a continuous, combless THz spectrum is generated.A slight mismatch of the fine-tuning results in a little remaining echo about 25 ps after the main pulse.
When applying the method already described, the repetitions of the THz pulse vanish, resulting in a truly continuous, combless spectrum (right column in Fig. 5).The modulation frequencies for the coarse and fine modulation in these measurements are 924 Hz and 17 Hz, respectively, but do not need to be very stable or referenced.A slightly mismatched modulation depth of the injection current results in a small remaining echo about 25 ps after the main pulse here.The steep cutoff at about 1 THz originates from the bandwidth of the used SLM blade.
A first spectroscopic application of this THz CCS system utilizing the generation of a con-tinuous THz spectrum is shown in Fig. 6.A lactose pellet is inserted in the THz beam path between the two off-axis parabolic mirrors.The prominent absorption line around 530 GHz (see e.g.[9]) is clearly detected.Application to narrow absorption measurement tasks have not yet been performed, but are expected to be possible with the proposed setup.A measurement of two silicon (Si) wafers of different thicknesses is shown in Fig. 7 and proves the principle applicability of the introduced method to layer thickness measurement tasks.The main cross-correlation peak is shifted in time and additional echoes occur in agreement with results from femtosecond pumped THz TDS systems.The method introduced here and used in the measurements of Fig. 5, Fig. 6 and Fig. 7 was performed with the fine tuned laser spectrally emitting at one edge of the spectrum of the SLMmodulated laser.However, the used principle in general allows for the generation of an arbitrary THz spectrum, if appropriate photoconductive emitters and detectors are available.To demonstrate the flexibility of our simple system, the center frequency of the fine modulated laser was shifted by translating the slit-SLM of its external cavity.The result of eight measurements is shown in Fig. 8.One can clearly see the potential of our proposed method.By increasing the frequency separation of the spectra of the tuned lasers, the THz spectrum is shifted to higher frequencies.In our experiment the low efficiency of our photoconductive switches for high frequencies causes the small THz signal for widely separated laser spectra.Nevertheless, this measurement proofs the feasibility to generate predefined THz spectra using more sophisticated spectral control techniques.and THz spectrum (right).By increasing the frequency separation between the laser spectra, the THz frequency is shifted towards higher values.The low efficiency of the used photoconductive antennas for higher frequencies causes the almost vanishing signal for widely separated laser spectra.

Conclusion
We have proposed and demonstrated a THz cross-correlation spectroscopy (THz CCS) system based on conventional non AR-coated laser diodes.A combination of two tuning methods results in the generation of a broadband, truly continuous THz spectrum.The applicability of the system was demonstrated by a spectroscopy measurement of lactose, reproducing a prominent absorption feature well known from measurements with conventional THz TDS systems as well as a measurement of Si wafers of different thicknesses.The pump lasers and additional equipment needed for this setup are very cost-effective, compared to systems pumped by femtosecond lasers or high accuracy frequency stabilized lasers.Further improvements by tailoring the emitter and detector characteristics to the frequency range to be generated and optimizing the tuning process of the two diode lasers are expected.

Fig. 2 .
Fig. 2. Different types of SLM apertures.Option a) is used for generation of tunable two color operation by one laser.b) pins the lasing frequency.c) controls the laser frequency and makes it tunable by vertical displacement.d) is the scheme of the used periodical modulation.The SLM consists of repetitions of the scheme shown in c) arranged around a rotational center.The SLM used in our work was mounted on a standard chopper head.

Fig. 3 .
Fig. 3. Experimental setup consisting of the two fiber coupled lasers, a 50:50 fiber optic splitter, one collimator on a linear translation stage as delay line and two photoconductive dipole antennas as emitter and detector.

Fig. 4 .
Fig. 4. Laser frequency over time (left column) and resulting frequency mixing probabilities (right) column of different methods.a) describes the multi-mode operation of a THz spectroscopy system with and arbitrary kind of laser mode drift and modulation.Due to the nearly fixed FSR of the laser diode, the possible difference frequencies do not vary in contrast to the individual modes.The scheme for one fixed laser and a second coarsely tuned laser (mode hop modulation with a SLM) is shown in b).The resulting THz spectrum still consists of separated modes.The proposed and demonstrated principle of this work is depicted in c).The coarse fixed laser 2 is injection current modulated, resulting in continuous shifts of the lasing mode.In combination with the coarse tuned laser 1, this results in a continuous mixing spectrum.

Fig. 6 .
Fig. 6.Measurement of a lactose pellet with the presented setup.

Fig. 7 .
Fig. 7. Measurement of two Si wafers of different thickness.(Note that this measurement was performed with a different set of antennas compared to the results presented above, resulting in slightly different cross-correlation peak shapes.)

Fig. 8 .
Fig.8.Measurements of the optical spectrum (left), the THz time domain signal (middle) and THz spectrum (right).By increasing the frequency separation between the laser spectra, the THz frequency is shifted towards higher values.The low efficiency of the used photoconductive antennas for higher frequencies causes the almost vanishing signal for widely separated laser spectra.