Generation of Orthogonally Polarized
Mode-Locked Lasers at Wavelength of 1342 nm
Volume 9, Number 5, October 2017
Hsing-Chih Liang
Feng-Lan Chang
Tai-Wei Wu
Cheng-Lin Sung
Yung-Fu Chen
DOI: 10.1109/JPHOT.2017.2728859
1943-0655 © 2017 IEEE
Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C.
DOI:10.1109/JPHOT.2017.2728859
1943-0655C 2017 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
Manuscript received March 24, 2017; revised July 13, 2017; accepted July 14, 2017. Date of publication July 19, 2017; date of current version August 23, 2017. This work was supported by the Ministry of Science and Technology under Contract No. MOST 105-2112-M-019-002-. Corresponding author: H. C. Liang (e-mail: [email protected]).
Abstract: A mode-locked laser with orthogonal polarization at the wavelength of 1342 nm is experimentally achieved by using the natural birefringence of the Nd:YVO4
crystal. A total output power of 2.64 W could be obtained at an incident power of 14.4 W and the pulse durations are measured to be 15.1 and 16.9 ps with pulse repetition rates of 6.0 and 6.45 GHz for the laser output along theπ- andσ-polarization, respectively. The mode-locked output is further found to exhibit a complex temporal trace with beat frequencies originated from the different central frequencies and pulse repetition rates.
Index Terms:Infrared lasers, diode-pumped lasers, solid state lasers, mode-locked lasers.
1. Introduction
Diode-Pumped solid-state lasers at 1.3μm are attractive for many applications such as telecommu-nication, range finding, data storage, and scientific researches. Nowadays, Nd-doped gain media are widely used to efficiently generate 1.3-μm lasers at continuous-wave or pulsed operation [1]–[5]. For some applications, high-power passively mode-locked lasers at wavelength of 1.3μm are prerequisite [6]. Among various pulsed lasers, Kerr-lens mode locking has been identified as a simple and practical technique to generate ultrafast pulses. The physical mechanism is com-prehended to be associated with the self-focusing of laser beam and the amplitude modulation produced by either a hard aperture or a soft gain aperture [7]–[9]. Various experimental results reveal that a short cavity round-trip time and high Kerr nonlinearity can beneficially achieve the mode-locked operation [10]–[12]. By employing Nd-doped vanadate crystals with the large Kerr nonlinearities, diode-pumped self-mode-locked (SML) lasers at 1.3μm have been demonstrated in short cavities with a linear polarization output [13], [14].
In addition to linearly polarized mode-locked lasers, orthogonally polarized dual-wavelength mode-locked lasers [15]–[20] have aroused considerable attention for various applications including
Fig. 1. Experimental setup for the orthogonally polarized self-mode-locked Nd:YVO4lasers at 1342 nm.
laser spectroscopy [21], asynchronous optical sampling [22], magnetization metrology controlling [23], and secure communication [24]. It has been reported that the orthogonal polarization output can be achieved in a single cavity with an identical repetition rates (synchronous) or different repe-tition rates (asynchronous) by exploiting the isotropic or anisotropic gain crystal, respectively. With a synchronous scheme, the tiny birefringence induced by the thermal stress in the gain medium can lead to laser emission with orthogonal polarized states [25]–[27]. Alternatively, with an asyn-chronous scheme, orthogonally polarized lasers can be realized by either using a birefringent crystal as the gain medium or inserting a birefringent element into the cavity [28], [29]. Since the vector mode-locked light sources show great potentials for extensive applications, it is desirable to develop orthogonally polarized SML lasers at 1.3 um.
In this work, we have explored the temporal dynamics of an asynchronously mode-locked laser with orthogonally polarized states. By utilizing the natural birefringence of a wedged Nd:YVO4
gain crystal, the simultaneously SML laser at 1.34 μm with orthogonal polarization output was successfully achieved for the first time. Since the optical paths for the two orthogonally-polarized states are slightly different inside the resonator, the output polarization component can be easily manipulated by precisely tuning the orientation of the output coupler [30]. More importantly, the experimental result reveals that there exists a specific tilted angle of the output coupler for achieving a stable orthogonally-polarized SML output with balanced intensities for each polarized state. Under the optimal condition for the balanced orthogonal-polarization intensities, the total average output power of 2.64 W can be obtained with an incident pump power of 14.4 W. The pulse durations are measured to be 15.1 ps and 16.9 ps with repetition rates of 6.0 GHz and 6.45 GHz for the output beam along theπ- and σ-polarizations, respectively. It is also found that the SML output composed of two orthogonal polarization components displays a complex beating modulation in temporal traces. We have also theoretical reconstructed the complex pulse train in an asynchronous mode-locked laser different central frequencies.
2. Experimental Setup
A schematic diagram of the laser experiment is shown in Fig. 1. The cavity configuration is a simple flat-flat resonator. The active medium is an a-cut 0.25 at.% Nd:YVO4 crystal with dimensions of
3×3×8 mm3 and the c axis (πaxis) of crystal was arranged to be along the x axis. One facet of the laser crystal was normal to the crystal axis and was high-reflection coated at 1342 nm (>99.8%) and high transmission coated at 808 nm. The second facet was antireflection coated at 1342 nm and wedged 0.5°to eliminate the Fabry-Perot etalon effect. The cavity length,Lcav, was initially designed to be 15.6 mm. With the refractive indices of nπ = 2.154 and nσ = 1.949, the optical path length was set to be approximately 25.15 mm and 23.1 mm forπ- andσ-polarizations,
of OC. (b) Average output power versus the incident pump power forσ- andπ-polarized optimization and balanced orthogonal-polarization intensities. (c) Polarization-resolved output beam profiles of the two orthogonal emissions.
respectively. The laser crystal was wrapped with indium foil and mounted in a water-cooled cooper holder. The water temperature was maintained at 16°C to ensure stable laser output. A wedged flat mirror with 7% transmission at 1342 nm and high transmission at 1064 nm was used as an output coupler. In order to suppress the excitation of high-order transverse modes, a physical aperture with aperture radius of 175μm was inserted into the cavity. The pump source was a 16-W, 808-nm fiber-coupled laser diode with a core diameter of 400 μm and a numerical aperture of 0.22. A focusing lens with 25-mm focal length and 90% coupling efficiency was used to reimage the pump beam into the laser crystal. The average pump diameter was approximately 220μm.
3. Experimental Results and Discussions
First of all, we explore the dependence of output performance with respect toπ- andσ-polarization emissions in the Nd:YVO4laser on the orientation of the output coupler. As shown in Fig. 2(a), the
output polarization state can be easily controlled by precisely tilting the orientation of output coupler. Note that the tilting angleφis defined as the included angle with respect to the orientation of output coupler corresponding to the maximum output power atπ-polarization. At the pumped powerPin =
10.5 W, the angular separationθspebetween theπ- andσ-polarization under the individual maximum output power is found to be 1.8 mrad, as shown in Fig. 2(a). The intrinsic angular separation for the two orthogonal polarizations mainly results from the different refractive indices at 1342 nm for the a-cut Nd:YVO4 crystal withnπ = 2.154 andnσ = 1.949 which corresponding to different
optical axes for the wedged crystal. With the Snell’s law under the small-angle approximation, we can theoretically derive the angular separation to beθspe = (nπ − nσ)θw ∼1.79 mrad which is in a good agreement with the experimental results. The output power with respect to incident power for theσ- andπ-polarizations of the laser system is indicated in Fig. 2(b). The pump threshold is around 4.5 and 2.0 W, and the maximum average output power is found to reach 2.13 and 3.53 W under the pump power of 14.4 W for the σ- andπ-polarizations, respectively. More importantly, by precisely controlling the tilting angle, the laser can operate under balanced orthogonal-polarization intensities. Fig. 2(b) shows experimental results for the average output power of the SML NdYVO4laser versus
incident pump power under balanced orthogonal-polarization intensities, corresponding to the tilted angle φ = 1.17 mrad. At a pump power of 14.4 W, the maximum output power of 2.64 W can be achieved, as shown in Fig. 2(b). Due to the contribution of both σ- and π-polarizations, the average output power under balanced orthogonal-polarization operation is even higher than σ -polarization output emission. It is believed that this configuration is not only simple but also high efficient. Although the two orthogonal polarizations were to be emitted in different directions, the beam profiles still can be seen well-superimposed over a fine distance behind the output coupler.
Fig. 3. Polarization-resolved output intensity I0◦ (t) with the time span of: (a) 5μs and (b) 5 ns;
polarization-resolved output intensityI90◦(t) with the time span of: (a) 5μs and (b) 5 ns.
The polarization-resolved output beam profiles of the two orthogonal output were experimentally observed to be well-superimposed at the position of 100 mm behind the output coupler and the lasing modes are near TEM00 fundamental mode with the beam quality better than 1.3 under
dual-polarized operation, as shown in Fig. 2(c).
The temporal dynamics was detected by a high-speed InGaAs photodetector (Electro-optics Technology Inc. ET-3500 with rise time of 35 ps), whose output signal was connected to a digital oscilloscope (Agilent DSO 80000) with a 10 GHz electrical bandwidth and a sampling interval of 25 ps. For the sake of description, the direction ofπ- andσ-polarization emissions are denoted as the x- and y-axes. Fig. 3 shows the polarization-resolved output intensitiesIθ(t) atθ = 0◦andθ = 90◦ with two different timescales under balanced orthogonal-polarization intensities at the pumped powerPin = 10.5 W, whereθis the analyzer angle with respect to the x axis. It can be seen that the
time traces exhibit a full modulation and complete mode-locking is achieved in bothI0◦(t) andI90◦(t),
as observed in our previous work [13], [14]. The pulse repetition rates ofπ-polarization emission andσ-polarization emission were found to be 6.0 GHz and 6.45 GHz which are consistent with the optical path lengths, respectively.
The optical spectrum of laser output was recorded with a Michelson optical interferometer (Advantest, Q8347) with a resolution 0.003 nm that is also able to perform optical spectral analysis by Fourier transformation of the first-order field autocorrelation. Fig. 4 depicts the lasing spectrum and autocorrelation trace for the two orthogonally polarized states at an incident power of 10.5 W. It can be seen that the central wavelengths for the two polarization states are observed to be nearly at 1342.15 nm with tiny difference was 0.012 nm corresponding to a central frequency difference of 1.7 GHz. The full width at half maximum (FWHM) values of the lasing spectra at π- and σ -polarization states are 0.18 nm and 0.16 nm, respectively. The longitudinal mode spacings within each spectral band are found to be 0.036 nm and 0.039 nm that are consistent with the pulse repetition rates of 6.0 GHz (π-polarization) and 6.45 GHz (σ-polarization). A commercial autocorre-lator (APE Pulse Check, Angewandte Physik and Elektronik GmbH) was employed to measure the mode-locked pulses. Fig. 4(c) and (d) show the autocorrelation traces of the single pulse for two orthogonally polarized emissions. Assuming the Gaussian-shape temporal profile, the pulse widths were evaluated to be 15.1 ps forπ-polarization and 16.9 ps forσ-polarization. Although the optical spectra have a slight asymmetry, the experimental results of digital oscilloscope and autocorrelation traces are revealed that the mode-locking are clean. As a result, the time-bandwidth products of the mode-locked pulses were estimated to be 0.453 and 0.45, respectively. Both values are slightly greater than the Fourier-limit value of 0.441 which indicates there exist little frequency chirps in the present pulses. Even though the polarization-resolved output intensities for θ = 0◦ andθ = 90◦ display the temporal feature of a conventional mode-locked laser, the pulse train of the
polarization-Fig. 4. Experimental optical spectrum of the orthogonally polarized Nd:YVO4lasers (a)π-polarization;
(b)σ-polarization; Autocorrelation traces of the output pulses (c)π-polarization; (d)σ-polarization.
resolved intensity along θ = 45◦ is observed to exhibit a complex temporal dynamics, as shown in Fig. 5(a) and (b) with two timescales. In order to elucidate the complex temporal profiles more quantitatively, we numerically reconstructed the complex temporal traces. The normalized complex electric field for mode-locked laser form by N longitudinal modes can be expressed as:
E(t)= 1 Ne i2πf0t e−iπ(N+1)frept N n=1
ei2πnfrept = ei2πf0t sin(Nπfrept)
Nsin(πfrept)
(1) wheref0is the central frequency andfrepis the longitudinal mode spacing. Considering the optical
axis for light propagation is along z-direction, the polarization state of the mode-locked laser can be described as the vector sum of the x-(horizontal) and y-(vertical) components along the prin-cipal axes. For the in-phase and asynchronous two orthogonally polarized mode-locked laser, the polarization-resolved output intensity is then given by
I(t)=cos(θ)Ex(t)+sin(θ)Ey(θ) 2 (2) Ej(t)= ei2πf0,jt sin(Njπfrep,jt) Njsin(πfrep,jt) , j = x, y (3)
Substituting the experimental parameters withfr ep,x = 6.0 GHz,fr ep,y = 6.5 GHz,f0 = 1.7 GHz,
Nx = 8, andNy = 7 into Eq. (2) and Eq. (3), the complex pulse train could be numerically recon-struct, as shown in Fig. 5(c) and (d). It can been seen that numerical calculations agree very well with the experimental results. The excellent consistency between the experimental and numerical results validates that the present analysis can well describe the complex temporal traces observed
Fig. 5. Experimental polarization-resolved output intensity I45°(t) with the time span of: (a) 200 ns and
(b) 20 ns; Numerical reconstructed polarization-resolved output intensity I45°(t) with the time span of:
(a) 200 ns and (b) 20 ns.
Fig. 6. Beat frequency of orthogonally polarized Nd:YVO4laser as a function of the geometric length of
cavity.
in an asynchronous mode-locked laser with two orthogonally polarized states corresponding to different central frequencies and pulse repetition rates. Note that the complex temporal traces can be improved with fine-tuning of the cavity length to reduce the difference in central frequencies. Furthermore, the difference in the pulse repetition rates between two orthogonally polarized pulse trains can be calculated to befr ep = fr ep,y − fr ep,x = nLcry(c/2L2cav), whereLcavis the geometric
length of the cavity, n and Lcry are the natural birefringence and length of gain crystal. It can be clearly seen that difference of repetition rate can be tuning by adjusting the geometric length of cavity. Fig. 6 shows both experimental and calculated results for the beat frequency versus the geometric length of cavity. Consequently, the repetition rate differencefr ep can be approximately in the range of 500–350 MHz for the geometric length of cavity within 14–18 mm.
4. Conclusion
In conclusion, we have experimentally realized an asynchronous self-mode-locking of two orthogo-nally polarized emissions in a Nd:YVO4laser at the wavelength of 1342 nm by exploiting the natural
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