Waveguide Array Fiber Laser Volume 4, Number 5, October 2012 Qing Chao Darren D. Hudson J. Nathan Kutz S. T. Cundiff, Senior Member, IEEE
DOI: 10.1109/JPHOT.2012.2209411 1943-0655/$31.00 ©2012 IEEE
IEEE Photonics Journal
Waveguide Array Fiber Laser
Waveguide Array Fiber Laser Qing Chao, 1;2 Darren D. Hudson, 1;3 J. Nathan Kutz, 4 and S. T. Cundiff,1;2;3 Senior Member, IEEE 1
JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440 USA 2 Department of Electrical, Computer and Energy Engineering, University of Colorado, Boulder, CO 80309-0425 USA 3 Department of Physics, University of Colorado, Boulder, CO 80309-0390 USA 4 Department of Applied Mathematics, University of Washington, Seattle, WA 98195-2420 USA DOI: 10.1109/JPHOT.2012.2209411 1943-0655/$31.00 Ó2012 IEEE
Manuscript received June 13, 2012; accepted July 10, 2012. Date of publication July 19, 2012; date of current version August 6, 2012. This work was supported by the National Institute of Standards and Technology. The work of J. N. Kutz was supported by the National Science Foundation (NSF) (DMS0604700) and by the US Air Force Office of Scientific Research (AFOSR) (FA9550-09-0174). Corresponding author: S. Cundiff (e-mail:
[email protected]).
Abstract: A waveguide array fiber laser is demonstrated. An AlGaAs waveguide array is used as a saturable absorber to achieve passive mode-locking in an erbium-doped fiber laser. The output consists of a very regular train of pulses. Autocorrelations show that the pulses consist of noise bursts due to incomplete mode-locking. Index Terms: Fiber lasers, mode-locking, spatial solitons.
1. Introduction Mode-locked lasers are extremely versatile tools for femtosecond physics, chemistry, and ultrafast science [1]–[3]. Passive mode-locking techniques using effective saturable absorbers, such as Kerr lensing [4] and nonlinear polarization rotation [5], [6], generate the shortest pulses, ranging from tens to hundreds of femtoseconds. As has been demonstrated theoretically, a waveguide array can be used as an effective saturable absorber, effectively producing a discrete version of Kerr-lens mode-locking [7]. In this paper, we experimentally demonstrate an erbium-doped fiber laser incorporating an AlGaAs waveguide array as a passive fast saturable absorber. The output of the laser is a very regular train of pulses, which is evidence that the laser is mode-locking. The autocorrelation is consistent with a train of noise bursts, indicating that the mode-locking is incomplete. Waveguide arrays were used in the first demonstration of discrete spatial solitons [8], which had been predicted theoretically [9]. Light propagating through the waveguide structure self-focuses at high intensities but undergoes discrete diffraction at low intensities. Other novel spatial phenomena using a waveguide array have been reported including discrete modulation instability [10] and optical discrete surface solitons [11]. Temporal effects in the waveguide array were largely ignored in the early experiments. Recent experiments have studied x-wave formation [12], temporal pulse reshaping [13], chirp clamping effects [14], and multiphoton absorption [15] in AlGaAs waveguide arrays. The results presented in this paper show that incorporating a waveguide array in a fiber can result in mode-locking and thus provide an avenue to developing a new type of mode-locked laser. Further parametric studies are needed to perfect the design and optimize the mode-locked performance. The results also suggest that waveguide arrays could be used for mode-locking other types of lasers, for example, diode lasers [16].
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Fig. 1. Laser setup. Inset 1, scanning electron microscope image of the front view of the waveguide array and inset 2, multiphoton absorption in the center waveguide (a bright stripe) when the laser is mode-locked.
2. Laser Design To demonstrate mode-locking due to self-focusing in a waveguide array, we built a fiber laser that incorporates a waveguide array as a mode-locking element. The waveguide array consisted of two Al0:24 Ga0:76 As cladding layers and a 1.5-m-thick Al0:18 Ga0:82 As core layer, grown by molecularbeam epitaxy on a GaAs substrate. To produce waveguides, we etched 1-m-tall ridges into the top cladding layer. The ridges are about 3 m wide, and the center-to-center distance is 8 m. Fig. 1 inset 1 shows a scanning electron microscope image of the front view of this waveguide array. There are a few advantages of choosing AlGaAs over silica for the waveguide material. The nonlinear coefficient of AlGaAs is 300 times higher than silica, which suggests discrete selffocusing can occur at lower intensities. Another advantage is that the band gap of Alx Ga1 x As is tunable by adjusting the aluminum concentration. For Al0:18 Ga0:82 As, input light of 1550-nm wavelength is below half band gap, which means two-photon absorption will be suppressed around this wavelength. The measured insertion loss of this waveguide array is about 58%. The input and output facets of the waveguide array are antireflection coated with hafnium oxide. We have not characterized the polarization properties of the waveguide. We built fiber sections around the waveguide array and used free-space optics to couple light from the fiber sections to the center waveguide. The setup is shown in Fig. 1. In this ring cavity configuration, light from the fiber collimator is coupled into the center waveguide using a standard 40x microscope objective. The light emerging from the center waveguide is symmetrically coupled back to the fiber through a second objective lens and fiber collimator. If a pulse is launched into the center waveguide, at low input intensity, most of the power couples into neighboring waveguides and is ejected from the cavity. In contrast, for higher intensity, the low-intensity wings of the pulse are evanescently coupled into neighboring waveguides, while the high-intensity peak self-focuses and stays inside the center waveguide. Simply put, high-intensity input light self-focuses in the waveguide array while weak light diffracts. Due to a complex interplay between discrete diffraction, discrete spatial soliton formation, and three-photon absorption, at high enough input intensity, the waveguide array sets the output chirp to a fixed value independent of the input chirp [14], [17]. At low intensity, the measured dispersion is approximately 15 000 fs2 , which includes material and modal dispersion; however, the nonlinear effects outweigh the linear dispersion but cannot be characterized by a simple dispersion. At high input intensity, the output chirp should be clamped to 12 500 fs2 by the waveguide array. Thus, we used 227 cm of SMF28, 85 cm of erbium gain fiber, and 152.5 cm of Flexcor fiber in the cavity, such
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Fig. 2. (a) Optical spectrum, (b) RF spectrum, and (c) pulse train of the waveguide array mode-locked laser.
that the input chirp to the center waveguide is around 17 000 fs2 . Hence, the cavity dispersion is anomalous. This value is chosen based on studying the pulse shortening as a function of input chirp, which shows a maximum for an anomalous input chirp due to competition between the discrete self-focusing, dispersion, and three-photon absorption [17]. The gain fiber is reverse pumped with a 600-mW laser diode operating at 980 nm. It is fusion spliced to the cavity through a wavelength-division multiplexer. An erbium-doped fiber with an absorption coefficient of 80 dB/m provides the necessary gain for lasing. The optical fiber isolator sets the direction of light propagation inside the cavity. To monitor the laser output, we output couple 5% of the light that circulates inside the cavity. We observe the output with an optical spectrum analyzer and a fast photodiode that is connected to a radio frequency (RF) spectrum analyzer and a high-speed oscilloscope.
3. Experiment Mode-locking is achieved when light coming out from the center waveguide is carefully coupled back into the fiber because only high-intensity light is self-focused in the central waveguide. The mode-locking can be monitored using an optical spectrum analyzer and a fast photodiode that are connected to the laser cavity by the 5% fiber output coupler. Fig. 2 shows the optical spectrum and the RF spectrum and pulse train of the signal from the fast photodiode. The 3-dB bandwidth of the optical spectrum is 28 nm, and it is centered at 1556 nm. The broad and smooth optical spectrum is consistent with the laser being mode-locked. Further evidence for mode-locking is the observation of a stable pulse train on the oscilloscope. The RF spectrum shows a narrow line with a resolutionlimited 3-dB width of 1 kHz at the fundamental frequency of 38.8 MHz. The narrow RF linewidth shows the stability of the repetition rate and the pulse energy. The average power measured at the 5% output coupler is 7.5 mW, which means the intracavity power is 150 mW. The FWHM bandwidth corresponds to a transform-limited sech2 pulse of 91 fs. Luminescence due to multiphoton absorption in the center waveguide is observed using a silicon camera when the laser is mode-locked, shown in Fig. 1 inset 2. As described in Section 1, the bandgap engineering of Alx Ga1 x As can suppress two-photon absorption. However, at high input intensities, three-photon absorption becomes non-negligible [15], [17]. Carriers created by threephoton absorption relax to the band edge of the Alx Ga1 x As and then luminesce. Therefore, when the input intensity to the center waveguide is high enough, multiphoton absorption is visible with a silicon camera. Multiphoton absorption is an intensity-dependent loss mechanism; it is essentially an inverse saturable absorption in AlGaAs waveguide arrays and thus is deleterious for modelocking. Nevertheless, the observation of multiphoton absorption provides further evidence that the laser is mode-locked as the luminescence is only observed when the laser is producing a broad optical spectrum. Next, we measure the output pulse with intensity autocorrelation. The result is shown in Fig. 3. The autocorrelation trace reveals more information about the laser output pulses. First, the trace shows a coherence spike at time delay zero on top of a broad pedestal. This type of autocorrelation
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Fig. 3. Autocorrelation trace.
is obtained for a noise burst [3]. Thus, we conclude that there are substantial pulse-to-pulse fluctuations in the pulse shape and phase profile due to incomplete mode-locking. The FWHM of the pedestal wings below the coherence spike corresponds to the FWHM pulsewidth, which is about 380 fs. This width is larger than the Fourier-transform limit, most likely because the output pulse is chirped. No attempt has been made to compensate the chirp on the output pulse. The intracavity pulse is certainly chirped at the location of the 5% coupler, and there will be chirp due to the fiber between the 5% coupler and the fiber-to-free-space coupler. Second, random Bblinks[ are captured along the time axis in the trace, although they are mostly suppressed in Fig. 3 by averaging multiple traces together. These blinks suggest that the laser occasionally drops out from mode-locking. However, the fact that the autocorrelation intensity recovers immediately implies the laser is selfstarting and robust. The pulse-to-pulse fluctuations in envelope and phase profile lead to difficulties for the femtosecond frequency-resolved optical gating (FROG) algorithm to correctly retrieve the original FROG trace, because FROG assumes laser output pulses are identical; thus, we cannot characterize the pulse using FROG. We attribute the blinks, the dropouts of mode-locked state, to mechanical vibrations of the 3-axis waveguide translation stages of the laser testbed and environmental noise. This laser instability can be mitigated by eliminating the free-space section of the cavity. This is achievable by employing advanced laser packaging techniques or fiber butt-to-butt coupling to the center waveguide. However, the optical and RF spectra show that the laser is mode-locked even though the coherence spike on the autocorrelation shows that the mode-locking is incomplete so that there are pulse-topulse fluctuations in the pulse shape and/or phase profile. Therefore, our results demonstrate that the nonlinear spatial self-focusing behavior of waveguide arrays can be used to construct a modelocked laser. This is essentially a discrete version of Kerr-lens mode-locking in a fiber laser. However, unlike a typical Kerr-lens cavity, the waveguide array laser is self-starting. Adjusting the strength of the saturable absorption by, for example, using a waveguide array with different spacing or length may result in a decrease in the fluctuations and a cleaner autocorrelation trace. Based on our prior single-pass experiments [13], we estimate the self-amplitude modulation in the waveguide array to be approximately 45%. Optimizing the saturable absorption is a nontrivial task because it requires fabricating a set of waveguide array chips with systematically varying structural parameters, characterizing their nonlinear optical parameters, and then inserting them into the laser. This task will be addressed in future work. An advantage of the waveguide array fiber laser we observed is its ability to remain mode-locked when polarization of the cavity is perturbed by twisting the fiber. In contrast, traditional nonlinear polarization rotation mode-locked lasers stop mode-locking when the light polarization is perturbed. We believe that this polarization robustness of the waveguide array fiber laser has the potential to vastly increase the use and applicability of mode-locked fiber lasers as tools in demanding work environments.
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4. Conclusion A waveguide array fiber laser is experimentally demonstrated. Measurements provide evidence that the laser is mode-locking, although the mode-locking appears to be incomplete, resulting in pulseto-pulse fluctuations in the pulse shape and/or phase profile. The results show that waveguide arrays can be used as a fast saturable absorber to generate a discrete version of Kerr-lens modelocking [4] in a passive erbium-doped fiber laser. Unlike lasers mode-locked using nonlinear polarization rotation [5], [6], this laser self-starts and is polarization robust. Self-starting and polarization robustness have also been achieved using semiconductor saturable absorbers [18], [19]; however, waveguide arrays present potential advantages in that the effect is instantaneous and that its properties can be controlled by geometry (waveguide spacing and length) rather than just material properties. The results suggest that on-chip mode-locking technology is feasible using a waveguide array structure. This first demonstration of using the AlGaAs waveguide array for saturable absorption is a promising step toward building future highly compact mode-locked laser cavities. Such laser cavities can be engineered with standard technologies and should be polarization insensitive, self-starting, and made from commercially viable materials.
Acknowledgment The authors acknowledge D. Christodoulides (CREOL) and R. Morandotti (INRS) for enlightening discussions and encouragement, A. Funk (JILA) for stimulating discussions, and R. P. Mirin (NIST) for providing the epitaxially grown wafer.
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