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Parametric THz Generation Pumped by Q-Switched Fiber Lasers in GaSe Crystal Wei Shi*, Matt Leigh, Jie Zong, Shibin Jiang NP Photonics Inc., 9030 S. Rita Road, Tucson, AZ, 85747, Tel: 520-799-7413; Fax: 520-799-7411 E-mail: [email protected]

Abstract: Two fiber lasers were simultaneously Q-switched by using one piezo to modulate the intracavity polarization-dependent loss, which were amplified as pump sources. Coherent and single-frequency THz radiation has been achieved by using all-fiber Q-switched laser pumps based on difference-frequency generation in GaSe crystal, for the first time to the best of our knowledge. The conversion efficiency was measured to be 1.26 × 10-7.  2007 Optical Society of America

OCIS Codes: (190.2620) Nonlinear optics: Frequency conversion; (140.3070) Lasers and laser optics: Infrared and farinfrared lasers; (060.2320) Fiber optics amplifiers and oscillators.

1. Introduction Among all the schemes for THz generation, optical parametric processes such as difference-frequency generation (DFG) in nonlinear optical (NLO) crystals are quite promising [1-3]. Recently, a GaSe crystal was used to generate coherent THz waves tunable from 58.2 µm to 3540 µm pumped by a pulsed Nd:YAG laser beam and the idler beam of an optical parametric oscillator (OPO) pumped by the third harmonic of the same Nd:YAG laser [1]. This THz source has an extremely wide tuning range, and the maximum output peak power can reach several hundred watts. However, much of the complexity of this THz source are only associated with the pump laser and OPO system. In order to achieve a compact THz source, we demonstrate the combination of all-fiber Q-switched lasers and optical parametric processes that produce coherent, high repetition rate and high power THz radiation. 2. Experiments and results

Fig. 1

Schematic of a compact THz source pumped by all-fiber Q-switched lasers based on DFG in a GaSe crystal.

Fig. 1 illustrates the schematic of this compact THz source. In this system, all fibers and fiber-based components are polarization maintaining (PM). In this design, two fiber lasers were simultaneously Q-switched by using one piezo to modulate the intracavity polarization-dependent loss, for the first time to the best of our knowledge. The wavelength of the fiber lasers was chosen around 1550 nm because there are many commercial and mature fiberbased components near this wavelength due to the telecom industry. Each of laser chains in Fig. 1 consists of a 2-cm long Er/Yb co-doped phosphate glass fiber spliced between two silica-based fiber Bragg gratings (FBGs). This phosphate glass fiber has high doping concentration of active ions owing to the high solubility of the phosphate

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glass, enabling efficient lasers with active fibers only a few centimeters long. One FBG has high reflectivity (HR) and imprinted on a standard non-PM silica fiber. The other FBG with a low reflectivity is written on a PM fiber, splitting the reflection wavelength for different polarizations, each having approximately 10 GHz of bandwidth. The reflection band of the HR-FBG is matched to one of the reflection bands of the output coupler, making the laser cavity polarization dependent. The HR-FBG on the standard fiber is cleaved and spliced at a few millimeters away from the FBG, leaving room for the fiber to be stressed to produce birefringence [4]. The longitudinal mode spacing is 2.5-3 GHz, therefore there are only a few longitudinal modes supported in the reflection band of the output coupling FBG. Single-frequency operation can be maintained by proper adjustment of the temperature of the FBGs as well as the entire cavity. Spectral tuning of the laser cavity can be realized by adjusting the temperatures of the FBGs as well as the whole laser cavity [5]. In order to modulate the loss internal to the resonator, we clamped a PZT on the non-PM fiber to apply stress from the side of the fiber in the section between the splice and the HR-FBG, introducing the birefringence in the fiber. Due to the polarization dependence of the resonator, the loss of the resonator can be modulated. Each laser chain was pumped by two commercial fiber-pigtailed, single mode, 976-nm diodes, respectively. Two Q-switched fiber lasers have pretty close performance under the simultaneously Q-switching by using the same piezo shown in Fig. 1. We tuned the repetition rate for two Q-switched lasers from 50 Hz to 650 KHz, adjusted pulse widths from 15 ns to 70 ns, observed the average power from 3 mW to 56 mW and peak power from 1 W to 12 W without any external amplifier. The temporal overlap of pulses from two Q-switched lasers can be easily obtained by using this scheme, which is very important for optical parametric processes or DFG in order to generate THz radiation. Typically, Fig. 2 shows the pulse shape and the temporal overlap of pulses from the two Q-switched lasers when the repetition rate is 80 KHz.

80 KHz

Fig. 2

Pulse shape and temporal overlap of pulses from two simultaneously Q-Switched fiber lasers.

The laser spectra of two Q-switched lasers were measured by using a scanning Fabry-Perot interferometer with a free spectral range (FSR) of 1 GHz. The observed spectrum shows bursts of pulses at the pulse repetition rate only under transmission peaks of the Fabry-Perot. This observation confirms that the Q-switched laser operate in a single frequency. From the envelope of the pulse train in the scanning Fabry-Perot spectrum, the linewidth of the Qswitched lasers is about 35 MHz, which is very close to the Fourier transform limit. Then two Q-switched lasers were amplified by two identical fiber-based 3-stage cascade amplifiers shown in Fig. 1. The final amplified pulses by the 3-stage cascade amplifier have a peak power of about 600 W without any observable nonlinear effects. The pulse width and the linewidth of the amplified pulses are the same as those before amplification. The peak-ASE ratio is more than 10 dB. The output mode is single mode (M2 ~ 1.2) and the polarization extinction ratio (PER) is about 10 dB. In the optical parametric process (DFG), the two amplified beams were collimated into a GaSe crystal through a polarization beam combiner (PBC) shown in Fig. 1. After the crystal, the generated THz radiation was

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collimated and focused into a liquid-helium cooled bolometer by two parabolic mirrors, and the residual pumps were blocked by using a black polyethylene filter. The detected THz signal was recorded by using a lock-in amplifier combined with an optical chopper in Fig. 1. The THz output power can be determined by the calibrated bolometer and verified by using the responsibility of the bolometer. According to the phase matching (PM) conditions, typeoee collinear PM can be achieved for the two pump wavelengths (λ1 = 1550.67 nm and λ2 = 1538.74 nm) in GaSe crystal [6], where e and o indicate the polarization directions of beams inside the crystal. The calculated external PM angle (θ) of 14.8° is in very good agreement with the experimental external PM angle θ = 15° in our experiments. The azimuthal (ϕ) angles were chosen such that |cos3ϕ|=1 according to the effective nonlinear optical coefficient, deff = d22 cos2θ cos3ϕ, for type-oee collinear PM. Fig. 3 shows the THz output power versus the pump average power. The average power for the generated THz radiation can reach 57 nW when the pump average power is 451 mW, corresponding to a peak power of 71.2 µW and a conversion efficiency of 1.26 × 10-7. The THz wavelength of 200 µm can be easily obtained by the difference frequency of two pump beams, which was verified by using a metal-mesh etalon.

THz average Power (nW)

60 50 40 30 20 10 0 0

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300

400

500

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Pump average power (mW)

Fig. 3 shows the THz output power versus the pump peak power.

3. Conclusions In conclusion, we have implemented a new compact coherent THz source based on all-fiber Q-switched lasers for he first time. In this system, we pioneered the use of one piezo to simultaneously Q-switch two fiber lasers that were amplified by two identical fiber-based 3-stage cascade amplifiers. The average power for the generated THz radiation can reach 57 nW when the pump average power is 451 mW, which corresponds a conversion efficiency of 1.26 × 10-7. 4. References 1. 2. 3. 4. 5. 6.

W. Shi and Y. J. Ding, A monochromatic and high-power THz source tunable in the ranges of 2.7−38.4 µm and 58.2-3540 µm for variety of potential applications, Appl. Phys. Lett. 84, 1635-1637 (2004) F. Jr. Zernike and P. R. Berman, Generation of Far Infrared as a Difference Frequency, Phys. Rev. Lett. 15, 999 (1965). K. Kawase, T. Hatanaka, H. Takahashi, K. Nakamura, T. Taniuchi and H. Ito, Tunable terahertz-wave generation from DAST crystal by dual signal-wave parametric oscillation of periodically poled lithium niobate, Opt. Lett. 25, 1714 (2000). Y. Kaneda, Y. Hu, C. Spiegelberg, J. Geng, S. Jiang, Single-frequency, all-fiber Q-switched laser at 1550-nm, OSA Topical Meeting on Advanced Solid-State Photonics 2004, Postdeadline paper PD5: February 2, 2004. Ch. Spiegelberg, J. Geng, Y. Hu, T. Luo, Y. Kaneda, J. Wang, W. Li, M. Brutsch, S. Hocde, M. Chen, J. Babico, K. Barry, W. Eaton, M. Blake, D. Eigen, I. Song, and S. Jiang, " Compact 100 m W fiber laser with 2 kHz linewidth,"Optical Fiber Communication Conference 2003, postdeadline paper PD45, 2003. W. Shi, Y. J. Ding, N. Fernelius, and K. Vodopyanov, "Efficient, tunable, and coherent 0.18-5.27-THz source based on GaSe crystal," Opt. Lett. 27, 1454-1456 (2002)