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Abstract
Blue-violet optical pulses of 9-kW peak power and 150-fs duration were obtained from a GaInN master oscillator power amplifier system using a nonlinear dispersion compensator. Seed pulses from a dispersion-compensated GaInN mode-locked semiconductor laser diode were stretched to 3-ps duration using a nonlinear dispersion compensator with a spatial light modulator that added second-order phase dispersion to an optimized nonlinear phase dispersion compensating the higher-order dispersion of the optical pulses. The stretched phase-optimized pulses were efficiently amplified to 3.0 nJ by a GaInN semiconductor optical amplifier. The amplified pulses were subsequently compressed using a linear pulse compressor, yielding 1.4-nJ femtosecond pulses. The obtained results show the highest peak-power ever reported for an electrically-pumped semiconductor gain medium.
© 2017 Optical Society of America
1. Introduction
Ultrashort optical pulses are widely utilized in various fundamental sciences and engineering applications, such as volumetric optical recording, three-dimensional microfabrication, and nonlinear optical bio-imaging [1–3]. For these applications, mode-locked solid-state lasers, of which the mode-locked Ti:Sapphire laser is the most common, have been conventionally used. However, solid-state light sources are bulky and expensive, and thus semiconductor-based ultrashort pulse lasers are expected to develop into a compact and cost-effective alternative.
In order to develop semiconductor-based ultrashort pulse lasers into application-ready devices, both a high pulse energy and short pulse duration are required. To achieve a high pulse energy, the master-oscillator power amplifier (MOPA) scheme has often been adopted for edge-emitting laser diodes by combining a mode-locked semiconductor laser diode (MLLD) and a semiconductor optical amplifier (SOA). To achieve short pulses, intracavity dispersion-compensation is a basic technique to broaden the oscillation bandwidth of MLLDs [4,5]. Using this technique, 158-fs optical pulses at 850 nm were generated by an intracavity dispersion managed MLLD with a subsequent pulse compression. The seed pulses from the MLLD were then amplified to 6.5-kW peak power in a MOPA configuration [6]. The pulse characteristics of the MLLDs could be further controlled by introducing self-optimization function of the intracavity dispersion [7,8].
Vertical external cavity surface emitting lasers (VECSELs) are another candidate rapidly attracting attention as semiconductor-based ultrashort pulse lasers [9–12]. In VECSELs, an optically-pumped semiconductor gain medium serves as one of the mirrors in the optical cavity so that a transverse optical mode is defined by the external cavity. A top-emission configuration of the semiconductor gain medium is advantageous to dissipate the heat that accompanies with high-power optical excitation. Wilcox et al. reported a VECSEL capable of generating 400-fs pulses at 1013 nm with a peak power of 4.3 kW and a repetition rate of 1.67 GHz [13]. The peak power of 6.5 kW was recently reported by configuring the multiple incidence angles to a VECSEL to manage intracavity group delay dispersion [14].
Wide-gap semiconductors based on GaN-related materials can directly emit photons in the visible to ultraviolet range, where conventional mode-locked lasers require wavelength conversion. Although GaN-based vertical cavity surface emitting lasers (VCSELs) have been reported recently [15,16], GaN-based edge-emitting lasers are more practical for generating ultrashort pulses. We have developed ultrashort pulse lasers based on GaInN semiconductor laser diodes for applications such as volumetric optical recording [17,18]. Recently, we reported the generation of 2.2-nJ picosecond pulses from a GaInN MOPA system that combines a GaInN MLLD and a GaInN SOA [19]. The obtained pulse energy was comparable to that of frequency-doubled pulses of a mode-locked Ti:Sapphire laser. The saturation energy of the SOA was increased by reducing the confinement factor of the tapered amplifier. We obtained 3.0-ps seed pulses by adjusting the driving current and reverse bias voltage to the saturable absorber (SA) for the MLLD in order to efficiently amplify the seed pulses. However, since the spectral widths of the seed and amplified pulses were 0.08 nm and 0.26 nm, respectively, the spectral bandwidth had to be broadened to increase the peak power by shortening the pulse duration.
For this purpose, we developed a dispersion-compensated GaInN MLLD and obtained 200-fs optical pulses after subsequent spectral filtering [20]. We found that the SA played an important role for a soliton-like mode-locking in generating femtosecond pulses. However, the output spectrum before spectral filtering exhibited a nonlinear frequency chirp due to combination of self-phase modulation in the gain section and the intracavity group velocity dispersion (GVD) [21]. The output optical pulses over the entire spectrum were compressed without spectral filtering using a nonlinear pulse compressor, resulting in 140-fs pulses with 60-W peak power [22]. These results demonstrated that dispersion-compensated MLLDs are a good candidate for providing spectrally broadband seed pulses for a MOPA system to obtain shorter duration and higher peak-power optical pulses.
In this paper, we report the generation of 150-fs pulses of 9-kW peak power using a GaInN MOPA system. This is the highest peak power ever reported for optical pulses generated by an electrically-pumped semiconductor gain medium. Seed pulses were stretched to ps-duration using a nonlinear dispersion compensator with a spatial light modulator (SLM). The seed pulses were then amplified in a GaInN SOA and subsequently compressed using a linear pulse compressor. In order to minimize the duration of the compressed SOA output pulses, the nonlinear chirp of the seed pulses was carefully compensated using the nonlinear dispersion compensator.
In our previous publication about the volumetric optical recording, voids were created in a disk medium with 0.3-nJ picosecond pulses using nonlinear absorption [18]. Thus, we expect higher efficiency for nonlinear absorption with 1.4-nJ femtosecond pulses delivered from the present MOPA system. The obtained pulses also enable the MOPA system to be applied for a next-generation refractive index surgery in cornea using nonlinear absorption of blue-violet femtosecond pulses [23].
2. Experimental
The devices used for the GaInN MOPA system were based on GaInN quantum wells grown on an n-type GaN substrate by metal-organic chemical vapor deposition. The bisectional laser diode (BS-LD) for the MLLD was 600-μm long, and consisted of a 550-μm long gain section and a 40-μm long SA section. The output facet on the gain section had an anti-reflection coating, and the other facet had a high-reflection coating. The BS-LD was passively mode-locked in a dispersion-compensated external cavity. The bias current applied to the gain section was 80 mA and the reverse bias voltage applied to the SA section was 6.6 V. The GVD of the external cavity was set to a negative value in order to have a broad oscillation spectrum. The average output power was 13.3 mW and the pulse repetition rate was 917 MHz.
The GaInN SOA was a ridge waveguide optical amplifier with a lowered confinement factor in order to increase the saturation power [19]. The waveguide consisted of a linear preamplifier and a tapered main amplifier. The total length of the SOA was 3.0 mm and the input and output waveguides were 1.4-μm and 118-μm wide, respectively. The temperature of the SOA was kept at 15 °C using a Peltier thermoelectric element. We applied a pulsed current to the SOA to avoid gain saturation due to thermal effects. The frequency and duty ratio of the current pulses were 100 kHz and 20%, respectively. Amplified optical pulses were obtained at 917 MHz while 2 μs duration of the current pulses and the maximum peak current was 8 A.
The experimental configuration of the MOPA system is shown in Fig. 1. After the MLLD, the optical pulses are sent to a nonlinear dispersion compensator with a SLM, which is a reflection-type liquid crystal on silicon device (LCOS, Hamamatsu, X10468-05) positioned at the focal plane of a Fourier lens (f = 150 mm). The dispersive element in the nonlinear dispersion compensator is a holographic grating with 4320 grooves/mm. An arbitrary amount of phase dispersion can be introduced into the optical pulses using the SLM without affecting the beam shape and position; thus, the optical coupling to the SOA is kept constant. After the nonlinear dispersion compensator, the optical pulses are injected into the SOA. An optical isolator after the nonlinear dispersion compensator prevents the amplified spontaneous emission (ASE) from the SOA from entering the MLLD and from re-entering the SOA itself after diffraction at the LCOS. After the optical isolator, a half-wave plate rotates the polarization direction of the seed pulses. The optical pulses amplified by the SOA are then sent to a grating pulse compressor with a holographic grating (4320 grooves/mm), where they are compressed by adjusting the distance between the grating and the Fourier lens (f = 150 mm). The optical outputs from the MLLD and the SOA were characterized using an optical spectrum analyzer (Yokogawa, AQ-6315) and a laboratory-made intensity autocorrelator [24]. The output power from the SOA was measured using a thermal sensor (Coherent, PowerMax-USB PS19Q).
Fig. 1 Block diagram of the optical setup of the GaInN MOPA system. (a) Dispersion-compensated GaInN MLLD, (b) Nonlinear dispersion compensator using a SLM, (c) GaInN SOA, and (d) Grating pulse compressor. M: Mirror, L: Lens, CL: Cylindrical lens, G: Grating, SLM: Spatial light modulator, I: Optical isolator, W: Half-wave plate.
3. Results and discussion
An intensity autocorrelation trace of the optical pulses measured directly after the MLLD is presented by the gray trace in Fig. 2(a). The trace exhibits a sharp peak at zero time delay. Similar pulse shapes were typical of the output pulses from a GaInN MLLD with a negative intracavity GVD [20]. Since we performed a cross-correlation measurement of the MLLD output in our previous report [20], this shape suggests that the optical pulses had a steep leading edge with a rise time of few hundred fs and a slow trailing tail with a decay time of about 1 ps. The rise time of the leading edge was as fast as the time scale of intra-band carrier relaxation, which indicates that the fast pulse components will be less efficiently amplified by the SOA [25]. The 1-ps trailing edge was nonlinearly chirped due to self-phase modulation and the intracavity negative GVD. This nonlinear chirp can be transmitted to the amplified optical pulses that are not efficiently compressed by the linear pulse compressor. To avoid these problems, we first optimized the nonlinear phase dispersion of the seed pulses using the nonlinear dispersion compensator and then added or subtracted a constant second-order phase dispersion to stretch the pulse duration [22]. The optimized nonlinear GVD in the present experiment is presented as the dashed line in Fig. 2(b). The intensity autocorrelation trace after the optimization is also presented as the black trace in Fig. 2(a). The full width at half maximum (FWHM) of the intensity autocorrelation trace was 270 fs. The pulse shape was not reproduced by a sech2 function but we estimated the pulse duration to be 180 fs assuming a sech2 pulse shape for convenience.
Fig. 2 (a) Intensity autocorrelation traces measured directly after the MLLD (gray trace) and after the nonlinear dispersion compensator with the optimized nonlinear phase dispersion (black trace). Inset: Intensity autocorrelation trace of the compressed MLLD output in an expanded scale (solid trace) and a sech2 function (dotted trace). (b) Optical spectrum of the MLLD output (solid trace) and the nonlinear GVD given by the nonlinear dispersion compensator to compress the MLLD output (dashed trace, referred to the right axis).
As we mentioned above, we stretched the seeding pulses by adding or subtracting a constant GVD, which we call the GVD shift, to the optimized nonlinear GVD. The output power of the SOA as a function of the GVD shift is shown in Fig. 3(a). The output power was minimum when the GVD shift was −0.05 ps2, and increased rapidly with a change in the GVD shift in this vicinity. The pulse durations of the seed and amplified pulses are also plotted in Fig. 3(b). Since the pulse shapes were not always Gaussian or sech2, we plotted the FWHM values of the intensity autocorrelation traces. The pulse durations of the seed and amplified pulses increased linearly with the GVD shift and the duration of the amplified pulses was consistently longer than that of the seed pulses. Around a GVD shift of −0.05ps2, the SOA output power increased rapidly with change in the GVD shift. However, the output power was less dependent on the seed pulse duration when the GVD shift was large.
Fig. 3 (a) Average SOA output power as a function of the GVD shift to the optimized nonlinear GVD. (b) FWHM pulse durations of the seed pulses (filled black squares, referred to the left axis) and the amplified SOA output (filled red squares, referred to the left axis) as a function of the GVD shift to the optimized nonlinear GVD. FWHM pulse duration of the compressed SOA output (open red circles, referred to the right axis) as a function of the GVD shift. Inset: Intensity autocorrelation trace of the compressed SOA output at a GVD shift of 0.0 ps2.
Regardless of the sign of the GVD shift, the SOA output power saturated at a similar value with increasing seed pulse duration; however, the spectrum of the SOA output changed drastically depending on the sign of the GVD shift. Figure 4 shows optical spectra of the SOA output measured for various GVD shifts. At zero GVD shift, the SOA spectra can be divided into two parts; a broad spectral peak on the short wavelength side and a strong sharp peak on the long wavelength side. This is interpreted as being the results of differences in amplification efficiency depending on the temporal characteristics of the spectral components of seed pulses. When the GVD shift was negative, the SOA spectra were broader and more symmetric compared with that of the seed pulses shown in Fig. 2(b). In this case, the seed pulses were negatively chirped, so the amplification started from the short wavelength side. Since the carrier density was lower at the shorter wavelengths, the gain saturation was possible when the incident pulse was negatively chirped. The carriers at higher energy could relax to lower energy to form gain components at the lower energy sides; however, the carriers with higher energy could be depleted in the amplification process. Periodic structures observed in the SOA spectrum at a GVD shift of −0.10 ps2 suggest self-phase modulation due to the carrier depletion during the amplification. In contrast, the SOA spectra observed at positive GVD shifts were comparatively not as broad. The spectral peak was observed at the wavelengths close to the spectral peak of the seed pulses. Compared to when the GVD shift was negative, the amplified spectrum was not much different than that of the seed pulses. At positive GVD shifts, the long wavelength spectral components underwent amplification in the SOA first, so the carriers were less depleted compared with the case of the negative GVD shift.
Since the SOA spectra were more broadened at negative GVD shifts, shorter pulse durations by compressing the SOA output were expected. The pulse duration after the linear compressor was found to be dependent on the GVD shift of the seed pulses. As expected, for negative GVD shifts, the pulse duration become shorter as the GVD shift was increased. The pulse duration (FWHM) of the compressed SOA output is plotted in Fig. 3(b). At GVD shifts between −0.15 ps2 and 0.10 ps2, autocorrelation traces showed ps-long pedestals, as shown in the autocorrelation trace at 0.0 ps2 presented in the inset of Fig. 3(b). At positive GVD shifts, the pulse duration was not as short as that observed for negative GVD shifts, as expected due to the narrower spectral bandwidth.
From −0.5 ps2 to −0.05 ps2, the SOA output power decreased with the GVD shift so that there was an optimized GVD shift where the highest peak power of the compressed pulse was obtained. Figure 5(a) shows the optical spectra of the SOA output for a GVD shift of −0.20 ps2, the ASE, and the MLLD output. The optical spectrum of the SOA output was broader than that of the MLLD spectrum, and the spectrum width was 2.7 nm. The SOA output and ASE spectra are plotted using the same vertical scale and the spectral components at the short wavelength tail of the SOA spectrum were less intense than those of the ASE spectrum. This difference is due to the finite time needed for recovery of the carrier density after pulse amplification. Figure 5(b) shows the intensity autocorrelation traces of the seed pulses, the SOA pulses, and the compressed SOA pulses. The pulse durations (FWHM) of the seed and SOA were 3.0 ps and 4.2 ps, respectively. The seed pulse shape could not be reproduced by a Gaussian or sech2 function. However, the SOA output pulses show an ideal pulse shape, and the compressed SOA pulses were well reproduced by a sech2 function. The measured FWHM of the compressed pulse was 230 fs yielding 150 fs pulse duration assuming a sech2 function.
Fig. 5 (a) Optical spectra measured at a GVD shift of −0.20 ps2: SOA output (solid red trace), ASE (solid blue trace), ASE multiplied by a factor to estimate ASE portion (dashed blue trace), and MLLD (dashed gray trace). All spectra except that of MLLD are plotted on the same vertical scale. (b) Intensity autocorrelation traces measured at a GVD shift of −0.20 ps2: Input MLLD pulses (solid gray trace), SOA output pulses before compression (dashed red trace), and SOA output pulses after compression (solid red trace). Inset: The SOA output pulses after compression plotted using an expanded time scale. Dotted black trace shows a sech2 function.
The average power of the SOA output was 617 mW for an input power of 4.4 mW. The ASE power was 361 mW. The average power of the SOA output after exiting the linear pulse compressor was 282 mW. In order to estimate the pulse energy, the fraction of ASE in the amplified output needs to be estimated since the carriers in the SOA have a finite lifetime ranging from tens of picoseconds to several nanoseconds. We can conventionally estimate the amplified portion of the optical pulses by subtracting the ASE power from the average SOA output. In this case, the amplified power can be underestimated because the ASE power requires a finite time to recover after the generation of an amplified pulse. In our previous report, the ASE portion was estimated by connecting the shorter and longer wavelength sides of the ASE by linear interpolation [19]. This spectral method was appropriate since the bandwidth of the amplified spectrum (0.26 nm) was narrower than that of the ASE spectrum. However, in the present study, the bandwidth of the amplified spectrum was as broad as that of the ASE spectrum so the ASE portion can be underestimated by interpolating the ASE spectrum. To avoid these problems, we multiplied the ASE spectrum by a factor to produce overlap between the ASE and SOA output spectra at their short wavelength edges (blue dashed trace in Fig. 5(a)). We estimated the contribution of SOA output power to the amplified pulses to be 88% by subtracting the area of the blue dashed spectrum from that of the SOA output spectrum. Using the amplified pulse contribution, p, with the pulse repetition rate of the MLLD, r, and the duty ratio of the current pulses driving the SOA, d, the pulse energy of the SOA output was estimated from the average power using the following equation;
The pulse energy before compression was estimated to be 3.0 nJ using the above equation. Thus, the peak power was estimated to be 1.1 kW for a pre-compression pulse duration of 2.7 ps assuming a sech2 function. The pulse energy after compression including the above factors was calculated to be 1.4 nJ, which corresponds to a peak power of 9 kW for a pulse duration of 150 fs. This is the highest peak power ever reported for an electrically-pumped semiconductor gain medium. The spectral width of the amplified output was 2.7 nm and the time-bandwidth product was estimated to be 0.75. We may need to consider higher order dispersion at the last pulse compression stage in order to obtain Fourier transform-limited pulses.4. Conclusion
We generated 150-fs blue-violet optical pulses of 9-kW peak power using a GaInN master oscillator power amplifier system with a nonlinear dispersion compensator. The pulse repetition rate of the MLLD was 917 MHz and the SOA was driven with current pulses of 8 A at a repetition rate of 100 kHz and with a duty ratio of 20%. The obtained pulses show the highest peak power ever reported for an electrically-pumped semiconductor gain medium. The output pulse energy obtained was 1.4 nJ, which is comparable to a pulse energy for a frequency-doubled mode-locked Ti:Sapphire laser. Blue-violet femtosecond optical pulses of about 5 nJ are available from a frequency doubler for a mode-locked Ti:Sapphire laser with fundamental femtosecond pulses of about 15 nJ at 80-MHz repetition rate. Since the Ti:Sapphire laser in this category requires a 10-W-class green laser consuming several tens of amperes for its pump LDs, the present GaInN MOPA system is highly energy efficient. The results of this study demonstrate that a GaInN MOPA system with nonlinear dispersion compensators is a novel source for blue-violet ultrashort pulses for applications like micro processing.
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