Iodine-stabilized high resolution dual-frequency Ti : sapphire laser : THz generation

A dual frequency Ti: sapphire laser is presented in which two lines operate simultaneously with the same intensity on two TM00 longitudinal modes. This operation is obtained by means of the I2 molecules contained in an intra-cavity cell, pumped by a lowpower laser. Its properties are remarkable: the two lines lase simultaneously, they overlap spatially, their spectral width is very narrow, their frequency is automatically locked to molecular frequencies and can serve as a frequency standard, the frequency difference is adjustable over the thousands of rotational or vibrational molecular energy gaps in the 0.1-0.9 THz and 3-6 THz domains and is computable with high precision. THz wave production is demonstrated as a first application. © 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (140.3590) Lasers, titanium; (190.4223) Nonlinear wave mixing; (190.7070) Two-wave mixing. References and links 1. M. Tani, O. Morikawa, S. Matsuura, and M. Hangyo, “Generation of terahertz radiation by photomixing with dualand multiple-mode lasers,” Semicond. Sci. Technol. 20(7), S151–S163 (2005). 2. Y.M. Aĭvazyan, V.M. Baev, V.V. Ivanov, S.A. Kovalenko, and É.A. Sviridenkov, “Kinetics of emission spectra of multimode lasers and its influence on the sensitivity of intracavity laser spectroscopy,” Sov. J. Quantum Electron. 17(2), 168 (1987). 3. T. Yasui, Y. Kabetani, E. Saneyoshi, S. Yokoyama, and Tsutomu Araki, “Terahertz frequency comb by multifrequency-heterodyning photoconductive detection for high-accuracy, high-resolution terahertz spectroscopy,” Appl. Phys. Lett. 88, 241104 (2006). 4. M. Alouini, M. Brunel, F. Bretenaker, M. Vallet, and A. Le Floch, “Dual tunable wavelength Er:Yb:Glass laser for Terahertz beat frequency generation,” IEEE Photon. Technol. Lett. 10, 1554-1556 (1998). 5. F. Pallas, E. Herault, J. Zhou, J-F. Roux, and G. Vitrant, “Stable dual-wavelength microlaser controlled by the output mirror tilt angle,” Appl. Phys. Lett. 99, 241113 (2011). 6. J.P. Pique, “Bi-frequency laser emission system,” Patent WO 2016087124 A1. 7. S. Gerstenkorn, and, P. Luc, “Absolute iodine (I2) standards measured by means of Fourier transform spectroscopy,” Rev. Phys. Appl. 14, 791-794 (1979); S. Gerstenkorn, and P. Luc, “Atlas du Spectre d'Absorption de la Molécule d'Iode entre 14 800-20 000 cm,” Editions du CNRS, Paris, (1978). 8. H. Katô, M. Baba, S. Kasahara, K. Ishikawa, M. Misono, Y. Kimura, J. O'Reilly, H. Kuwano, T. Shimamoto, T. Shinano, C. Fujiwara, M. Ikeuchi, N. Fujita, Md H. Kabir, M. Ushino, R. Takahashi, and Y. Matsunobu, “Doppler-free high resolution spectral atlas of iodine molecule 15 000 to 19 000 cm,” (2000), http://web1.kcn.jp/kansha-kansha/AtlasofI2.html. 9. F. Martin, R. Bacis, S. Churassy, and J. Vergès, “Laser-induced-fluorescence Fourier transform spectrometry of the XOg state of I2: Extensive analysis of the BOu → XOg fluorescence spectrum of I2,” J. Mol. Spec. 116(1), 71-100 (1986). 10. J.P. Pique, and S. Farinotti, “Efficient modeless laser for a mesospheric sodium laser guide star,” J. Opt. Soc. Am. B 20, 2093-2101 (2003). 11. J.P. Pique, V. Fesquet, and S. Jacob, “Pulsed frequency-shifted feedback laser for laser guide stars,” Appl. Opt. 50, 6294-6301 (2011). 12. J.B. Koffend, F.J. Wodarczyk, and R.W. Field, “ CW optically pumped molecular iodine laser,” in High-Power Lasers and Applications, Springer Series in Optical Sciences 9, K.L. Kompa, and H. Walther, eds (Springer, 1978). 13. V.R. Mironenko, and V.I. Yudson, “Quantum noise in intracavity laser spectroscopy,” Opt. Comm. 34(3), 397403 (1980). 14. J. P. Pique, F. Stoeckel, and A. Camgarque, “High sentitivity intracavity stimulated emission pumping,” Appl. Opt. 26(15), 3103-3107 (1987). 15. H-J. Song, and T. Nagatsuma, Handbook of Terahertz Technologies: Devices and Applications (CRC Press, 2015), Chap. 1.4.2. 16. J.L. Coutaz, Optoélectronique térahertz (EDP Sciences, 2008).


Introduction
Many applications use dual-frequency lasers: THz wave generation, spectroscopy, lidar-radar Doppler, telemetry, underwater detection, very high speed network ... A dual-frequency laser system running on two longitudinal modes of a single cavity is an elegant solution for various reasons.Firstly it requires only one laser and is therefore technologically simpler and more reliable.Moreover, the two modes are spatially overlapped, which is essential for two-photon applications.But an even more important reason is the stability of the frequency difference of two longitudinal modes of the same cavity, which is intrinsically very robust.Indeed, unlike solutions that consist in using two different lasers, the frequency difference of the longitudinal modes of the same cavity is much more stable than the absolute frequency of a single mode because thermal, mechanical, acoustic or atmospheric disturbances are compensated up to a certain order [1].
However, there is a fundamental difficulty related to mode competition in laser cavities [2]: relative intensities of two modes can fluctuate temporally by 100%.This is a major problem for any application using two simultaneous photons.Different solutions have been proposed in the literature in order to force two modes to emit simultaneously with the same intensity.A recent solution is to use the frequency comb of a mode-locked laser, which by definition has relative intensities that no longer fluctuate [3].But, mode blocking is a collective non-linear effect of a set of a large number of modes that are simultaneously lasing, so it is not possible to lock only two longitudinal laser modes.It is therefore necessary to develop methods outside the laser cavity to use only two selected modes.This significantly restricts applications and is at the expense of useful intensity.Another elegant solution [4] consists in introducing a phase anisotropy into a laser cavity that causes the laser to oscillate along two spatially separated orthogonal polarization eigenstates in the cavity, and then to recombine them.However, since the optical paths of the two intra-cavity modes are different, the frequencies of the two modes are not subject to the same disturbances and their difference can therefore fluctuate.Finally, let us quote a solution that consists in favoring two modes by means of a Fabry-Pérot étalon of well-chosen free spectral range.To avoid mode competition the output mirror is tilted with a critical angle [5].The two modes are then no longer coupled because their paths no longer coincide in the amplifying medium.But then we have the previous problem of uncompensated fluctuations due to different optical paths.Furthermore the output mirror tilt induces critical instabilities and the two output lines are not collinear.
In this paper, we propose a solution [6] that does not reduce adjacent modes intensity by introducing losses but rather by favoring two selected modes by generating enough intracavity additional photons at the two corresponding frequencies simultaneously.Here we present experimental results and an application to THz wave production.A numerical model will be presented in a forthcoming paper.

Iodine-gain-filter
The method (Fig. 1 and Fig. 2) consists in exciting, with a "pump" laser, iodine 127 I 2 molecules contained in an intra-cavity cell which then emits photons in the form of rotational PR doublets (dump) in the gain curve of the laser amplifying medium (here Ti: sapphire).Other molecules could be used, but the iodine molecule has many advantages.At room temperature, a very large number of transitions are easily accessible in the visible and near infra-red.The spectrum is sufficiently resolved to allow selection of a single rotational PR doublet.In addition, the spectroscopy of the homo-nuclear isotope 127 I 2 is well documented and its transitions are calculable with very high precision [7][8][9].Finally a low power "pump" laser can saturate the transitions of the iodine molecules, which is a fundamental point, as will be seen later.

Experim
The experime laser medium mirror.The N absence of se therefore over 250 MHz.On spectral band.about 1 THz molecules are through the o adapted to th laser.A spect doublet.We u oriented and longitudinal m    To demonstrate the spectral possibilities of the system we chose to excite the P(J)16-1 (v'=16, v"= 1) transitions of iodine molecule at about 580 nm.All transitions for J ranging from 10 to 150 are experimentally accessible thanks to the precise calculation of the corresponding frequencies of reference [7,8] and the use of a lambda-meter (Fig. 2).Here, we only present the spectra of the laser beam when J varies from 10 to 10 (Fig. 3).As mentioned above, by means of the spectral selection of the "pump" laser, the quartz plate and the iodine molecule, a line-width of 10 GHz for the "pump" laser is sufficient to force the Ti:Sa laser to emit on a single PR doublet.The adjustment of the intensity of the two modes is obtained by rotation of the quartz plate around an axis perpendicular to its surface.In addition, tilting this quartz plate optimizes the coincidence of the frequencies of the lines PR with that of the two modes.Adjustment of the cavity length has the same effect.Finally the temperature of the iodine cell can also be adjusted to increase the thermal population and the molecular density for a more effective excitation.A "pump" laser operating in the mW range is sufficient to obtain the dual-frequency regime easily.The calculation of the frequency difference ν 2 -ν 1 depends only on the energy difference of the levels (v, J-2) and (v, J) of the fundamental electronic state (Fig. 1).It is calculated from the very precise data of references [8,9].Using the same notation we have: where E v (J) is the energy of the rovibrational level (v, J).B v , D v , H v and L v are the Dunham coefficients.It should be noted that these coefficients depend on the vibrational quantum number v of the ground electronic state + Σ g X .As a first approximation, the difference frequency ν 2 -ν 1 varies linearly with J as can be seen in Fig. 3.
The measurements made with our high resolution monochromator confirm this relationship.The confocal Fabry-Pérot of free spectral range 1 GHz used at the monochromator output analyses the spectral purity of each of these two lines ν 1 or ν 2 .With a Fabry-Pérot finesse of about 100 and a free spectral range of the Ti:Sa cavity of 250 MHz, we observed that the two lines are separately single-mode and have a spectral width of a few tens of MHz.This width is compatible with the fact that the length of our cavity is free.It could be locked by conventional methods to achieve very high resolution which, in pulse mode, would be limited only by the duration of the laser pulse and, in cw regime, by the characteristics of the amplifying medium.As mentioned above, the frequency difference ν 2 -ν 1 has intrinsically a very high resolution (probably of the order of kHz in the cw regime).

Pulse characteristics
Fig. 4. "pump" laser pulse in blue and Ti:Sa dual-frequency laser pulse in red.The "pump" pulse starts in the Ti:Sa built-up.
In the pulsed regime we use the same Nd: YAG laser to pump the Ti:Sa crystal and the dye of the "pump" laser.The duration of the pulse of the Nd:YAG laser is 50 ns, the lifetime of the excited state of Ti: Sa is 7 μs, the lifetime of the dye excited state of the order of ns and the lifetime of the iodine molecule excited state of about 1 μs.Considering these characteristic times, we set up a delay line to optimize the pumping conditions.Experimentally we found that the best condition in terms of intensity was that shown in Fig. 4: the "pump" laser pulse must be synchronized in the build-up of the Ti:Sa amplifying medium, a hundred ns before the rise of the intensity of the Ti:Sa laser.This result is in good agreement with numerical simulations.However the value of this delay is not very critical.As the lifetime of the excited states of Ti:Sa and I 2 are in the µs range, a more detailed study with longer pulse durations remains to be done.

Intensity Fluctuations
The non-linear BBO crystal generates the frequencies 2ν 2 , ν 1 +ν 2 and 2ν 1 .Observation of the spectral component ν 1 +ν 2 shows that photons at the frequencies ν 1 and ν 2 are emitted simultaneously but does not measure correctly the relative fluctuations of the two frequencies.
Then we spatially separated the two frequencies ν 1 and ν 2 using the monochromator.The corresponding two pulses are then detected on two fast photodiodes, and their intensities are recorded pulse by pulse for a large number of successive pulses.Their statistics are analysed using the following quantity: -500 0 500 1000 1500 a.u.

t (ns)
dump "pump" where I 1 and I 2 represent the integrated pulse intensity of modes ν 1 and ν 2 .Initially, to test the systems using the intra-cavity Fabry-Pérot described in the introduction [5], we replaced the iodine cell with an étalon of free approximate spectral range 0.4 THz.The time-integrated output spectrum over several pulses is similar to those in Fig. 3.
In this situation, the blue curve in Fig. 5 represents β.It clearly shows that the intensity fluctuations vary by 100%, which means that the laser operates most of its time on one line.This phenomenon is well understood and is described in the literature: it is due to competition of the intra-cavity longitudinal modes.This is a major drawback for two-photon applications, but this mode competition has been cleverly exploited for very high sensitivity spectroscopy [2,[13][14].
In view of the difficulties mentioned in the literature, to force the two frequencies to be emitted simultaneously with equal intensity, we inserted into the cavity, instead of a Fabry-Pérot, our "iodine-gain-filter" system described in paragraph 2. Beyond a certain "pump" laser intensity, which we believe to correspond to the saturation of the I 2 transition, the spectacular result is shown in red in Fig. 5: the two lines are always emitted simultaneously with the same intensity and with very small relative intensity fluctuations.We estimate that these residual fluctuations correspond to instrumental disturbances.

THz generation
The above findings and the high peak power of our pulsed dual-frequency laser are well suited for generating THz radiation outside the Ti:Sa laser cavity.Fig. 6 shows the corresponding arrangement.The laser beam is split into two arms for homodyne detection.For the emitter and the receiver we used two array iPCA antennas produced by BATOP.By using an array of 1000 micro-lens with a filling factor of 73.5 % only every second gap between the finger structures is excited by the laser beam with photon energy larger than the energy gap.The gap size is 5 µm and the chip is made of a GaAs fast absorbing layer.The coherent excitation of the single emitters, located at every micro-lens spot results in a constructive interference of the radiated THz waves in the far field.Silicon lenses directly bonded to the substrates allow the THz radiation to be focused on the receiver.But we have    7.The repetition rate of the Ti: Sa laser is 3 kHz and the chopper frequency is 80 Hz.To improve the signal to noise ratio, each point is an integral of 6000 pulses.Points correspond to the experiment and full-lines to the fit.The black dots are the excitation of the transition P(25)16-1 of I 2 and the red dots that of the transition P(101)16-1.The PR doublet corresponds to the vibrational numbers v '= 16 and v = 25 (see Fig. 1 and 3).

Conclusion
We have demonstrated that an intra-cavity gain filter, such as our iodine-gain-filter, enables the construction of very stable and high resolution dual-frequency lasers.The two modes are spatially overlapped inside and outside the cavity, and emit simultaneously.The iodine molecule provides more than 6000 different absolute dual-frequencies not only in the 0.1-0.9THz band by the generation of rotational PR doublets but also in the 3-6 THz band by the generation of vibrational doublets.These molecular-dual-frequencies can serve as frequencystandards.As the spectroscopy of the fundamental electronic state of the 127 I 2 molecule has been widely studied, their calculation with an accuracy of a few MHz is already available, but higher accuracy could be obtained by very high resolution spectroscopy techniques [8].Another advantage of the system presented in this paper is the possibility of concentrating high power (continuous or pulsed) on two laser lines emitted in the same beam simultaneously.This could be done by adding an amplifier to the system described here.The system is generalizable to other all-solid state laser media and could possibly be miniaturized.Finally, note that our system also works as Dual Frequency Shifted Feed Back laser using an intra-cavity acousto-optical shifter.This could lead to new and interesting applications.

Fig
Fig. 1 sho fundamental e excited throug selection rule the fundamen doublet.The p overlap (Fran population of vibrational lev Optimization cell.The freq the excitation parameters [9 judicious cho the anharmon

Fig. 3 .
Fig. 3. Spectrum of the dual-frequency laser beam as a function of the rotational quantum number J of 127 I 2 molecule excitation.

Fig.
Fig.7.The repetition rate of the Ti: Sa laser is 3 kHz and the chopper frequency is 80 Hz.To improve the signal to noise ratio, each point is an integral of 6000 pulses.Points correspond to the experiment and full-lines to the fit.The black dots are the excitation of the transition P(25)16-1 of I 2 and the red dots that of the transition P(101)16-1.The PR doublet corresponds to the vibrational numbers v '= 16 and v = 25 (see Fig.1 and 3).