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Abstract
We present a mode-locked semiconductor laser oscillator that emits few picosecond pulses (5-8ps at a repetition rate of 379MHz and wavelength of 1064nm) with record peak power (112W) and pulse energy (0.5nJ) directly out of the oscillator (with no amplifier). To achieve this high power performance we employ a high-current broad-area, spatially multi-mode diode amplifier (0.3×5mm), placed in an external cavity that enforces oscillation in a single spatial mode. Consequently, the brightness of the beam is near-ideal (M2 = 1.3). Mode locking is achieved by dividing the large diode chip (edge emitter) into two sections with independent electrical control: one large section for gain and another small section for a saturable absorber. Precise tuning of the reverse voltage on the absorber section allows to tune the saturation level and recovery time of the absorber, providing a convenient knob to optimize the mode-locking performance for various cavity conditions.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Semiconductor lasers represent the industry standard for sources of coherent light due to their high electrical-to-optical efficiency, simple and robust construction, high optical power, tunability, wide range of available wavelengths (covering the entire VIS-NIR range), and low cost. Thus, the generation of ultrashort optical pulses directly from semiconductor lasers with high peak power and high spatial beam quality (brightness) is highly desired for many applications, such as: 3D micromachining of polymer materials [1–3], advanced methods of nonlinear and fluorescence microscopy [4–7], precision measurement and frequency metrology [8–10], frequency conversion [11,12], direct material processing [13,14] and medical applications [15]. However, the peak power of laser diodes oscillators so far is not up to par with standard solid-state or fiber lasers, such as mode-locked Ti:Sapphire lasers or mode-locked Er/Yb fiber lasers, which offer the more prevalent solution in the market currently, despite their high complexity, limited wavelength availability and higher cost.
Since the optical power of a laser diode is directly dictated by its physical area, broad area laser diodes (BAL) are an attractive route for high-power lasers competitive with solid-state lasers, while maintaining high customizability and simplicity – requiring only electrical pumping. Unfortunately, the high power from BAL diodes normally comes at the expense of degraded spatial coherence, since the wide cross-section of the diode wave-guide is inherently multi-mode spatially along the slow axis of the BAL.
To mitigate the degradation of brightness, many techniques were developed, ranging from shaping the spatial profile of the chip of the diode laser by special fabrication techniques [16,17] to configurations of coherent beam combining [18–20]. However, the simplest approach that maintains the simplicity of the multimode BAL chip is to place the BAL gain medium in an external cavity [21–23] that shapes the spatial profile of the beam to enforce single-mode operation. The external cavity approach is advantageous primarily due to its simplicity, flexibility, and the use of off-the-shelf components. An obstacle for high-power operation that must be addressed is self-lasing of the diode: At high powers, even small parasitic reflections from the diode surface may induce self-lasing inside the diode due to the high gain of the chip, bypassing the external feedback. In this work, we circumvent this limitation by a simple and flexible design based on an angle-cut diode ($3.6\deg$ angle between the face and the beam inside the chip), which strongly suppresses self lasing. This allows us to obtain high-power mode locked operation, as reported hereon.
In a previous paper [23], we have employed an external cavity to combine power and brightness from a single diode oscillator in continuous-wave operation. Here we implement a similar concept with a passively mode locked semiconductor laser to obtain both high pulse-energy and spatial coherence in a single oscillator. We employ the soft-aperture filtering, where the diode itself acts as an effective spatial filter for the intra-cavity beam, such that higher-order spatial modes are mismatched with the active area of the diode and suffer diffraction losses, leading to operation in a single spatial mode. For further details on the method see [23]. We achieve record results for a single diode oscillator, delivering $0.55\rm {nJ}$ pulse-energy and $112 \rm {W}$ peak power together with a $4.9\rm {ps}$ pulse duration, all in a highly pure spatial mode ($M^2 = 1.3$). This is achieved in a simple design that uses no special fabrication techniques, only off-shelf components and no subsequent amplification stages. Of course, amplification and custom-diodes are compatible with our design, and could be used to push the laser power even further. Mode locking is obtained using a diode chip with two-sections that are electrically controlled independently: a large section of the diode is pumped with forward current to act as the gain medium, and a smaller section of the diode that is biased with a reverse voltage to act as a saturable absorber with tunable saturation [24–26]. We demonstrate mode-locking at even-harmonics operation [27–29], characteristic of colliding-pulse mode-locking [30,31], and characterize the spatial and temporal performance of the laser for different harmonics.
2. Experiment
Figure 1(a) shows an image of the diode chip (5x0.3mm), which is cut at an angle to mitigate parasitic feedback from the diode facets that may cause self-lasing. The diode is comprised of a large gain section and the small saturable absorber section with independent electrical contacts, as marked on the image. The gain section is driven in forward current, in order to provide amplification, whereas the absorber section is driven in reverse voltage to tune its saturation level to induce passive mode-locking and optimize its performance. This diode chip was then placed in the external cavity configuration of Fig. 1(b), comprised of two identical cavity arms around the diode chip. The length of the arms ($\approx 400$mm) acts as a soft spatial filter [23] that enforces oscillation in a single spatial mode. the right arm includes also a variable output coupler, implemented by a polarizing beam-splitter and a rotating $\lambda /4$ wave-plate for optimizing the output power and mode-locking performance. To stabilize the cavity for both the fast and slow axes, the diode facet is located slightly before the focal plane of a spherical "fast" lens ($f_s=9mm$), thereby imaging the diode facet in the fast axis onto the end mirror of the cavity (at 400mm distance). An additional cylindrical "slow" lens of a longer focus ($f_c=75mm$) is placed to form a telescope for the slow axis (together with the spherical lens). This arrangement stabilizes the slow-axis mode of the cavity and enforces single mode operation.
Fig. 1. (a) Close up image of our angle-cut diode, with the two different sections highlighted. The diode is cut at an angle of $3.6\deg$ to the beam within the diode. Externally, the diode is placed at an angle of $13.7\deg$ to the laser beam in the cavity (to ensure refraction at $3.6\deg$ internally). The longer section ($4.4\rm {mm}$) is operated with forward current and acts as a gain medium, while the shorter one ($0.4\rm {mm}$) is driven in reverse voltage and acts as a variable saturable absorber. The diode facet is both AR-coated (<0.2% reflection) and angle-cut to effectively mitigate self lasing. (b) Simplified scheme of our external cavity configuration, comprised of a two-section angle-cut diode positioned between two arms of the external cavity. Each arm includes a spherical lens and a cylindrical lens to stabilize the fast and the slow axes of the beam, while the left arm also contains a variable output coupler (V.O.C), composed of a polarizing beam-splitter and a quarter-wave plate.
We evaluated the performance of this laser in time, frequency and space. The temporal width of the ultrashort pulse was measured using an intensity auto-correlator (model FR-103XL with a resolution of $0.1 \rm {ps}$) and to evaluate the spatial beam profile we employed a simple high-resolution CCD camera (Thorlabs model CS165MU with spatial resolution of $3.45\mu m$). To measure the pulse-train we used a fast, DC-coupled detector (20GHz bandwidth), coupled to either a digital oscilloscope (6GHz bandwidth) to observe the train in time, or to an RF spectrum analyser (Keysight model N9020A, 13GHz bandwidth) to characterize the repetition rate and its stability.
3. Results
Table 1 summarizes the measured performance of our laser for various harmonics of the fundamental repetition rate (2nd-to-8th with repetition rates of $380-1520\rm {MHz}$). Since pulses were formed only for even harmonics of the repetition rate, we suspect that the interaction between colliding pulses in the gain medium was involved in the mode-locking mechanism [30,31], but further studies will be needed to verify this assumption. For the 2nd harmonic we obtained high pulse energy of $550 \rm {pJ}$ and $4.9 {\rm ps}$ pulse duration. The spatial purity of the beam was practically single-mode with $M^2=1.2-1.4$ and fractional power in the main spatial lobe of $86-88{\% }$ for all the harmonics. The reverse voltage on the saturable absorber is the major parameter that affects the harmonic order of the pulse repetition. Specifically, at low voltage the harmonic order is high (8th), and as we increase the reverse voltage, the harmonic order is pushed down (finally to the 2nd harmonic). Conversely, the average power drops with reverse voltage, but the pulse energy remains roughly the same, which is indicative of soliton-type mode locking.
Figure 2 provides a comparison of the obtained pulse performance to previously published systems in the literature. As evident, our pulse energies surpass previously published work by 1-2 orders of magnitude, while our peak power is 1-2 orders of magnitude higher than all ps-range sources, comparable only to fs-range oscillators (marked by a blue dashed circle on the figure) that have much lower pulse energy. Note that the pulse duration of our laser was far from minimal and not transform limited (with time-bandwidth product of 5.7 for the lowest harmonic) since our laser configuration did not include any measure for dispersion compensation, whereas the works circled with dashed blue in the figure did. Hopefully, proper dispersion compensation can push our pulse duration down as well, allowing to achieve higher peak powers at similar pulse energies.
Table 1. Summary of the obtained pulse parameters in our experiments. The first column denotes the pulse repetition-rate, which was either the 2nd, 4th, 6th or 8th harmonic of the fundamental repetition rate ($f_1=189.5$MHz). TBP denotes the time-bandwidth product of the pulses.
Fig. 2. Comparison of our results (circled in red) to previously published results (diode edge emitter oscillator with an external cavity and no additional amplification stages) [32–46]. The axes indicate pulse-duration and pulse energy, while the straight lines indicate constant peak power.
Figure 3 provides details of the measured pulses in both time and optical frequency. Figure 3(a) shows the auto-correlation traces of the different rep-rate harmonics and Fig. 3(b) shows the optical spectrum of each harmonics, as well as the spontaneous-emission spectrum, shown in Fig. 3(c), which reflects the spectral curve of the gain. Clearly, the pulses are not transform limited as summarized in the Table 1 above.
Fig. 3. Time and Frequency characterization of the pulses: (a) Temporal intensity auto-correlation traces of the pulses for the various repetition rate harmonics: The pulse-lengths of Table 1 are estimated assuming a Gaussian pulse, which yield a factor of 0.7 between the optical pulse and the auto-correlation trace. (b) The measured spectra of the pulses for different harmonics, which together with the auto-correlation measurements allows to calculate the time-bandwidth product of the pulses (TBP). (c) The spectrum of the amplified spontaneous emission, indicating the gain profile of the diode.
The stability and purity of the generated pulse train, sa detected by a fast photo-detector was observed in both time and RF-frequency, as illustrated in Fig. 4. A clean trace of the RF spectrum is obtained for 2nd,4th and 6th harmonics, showing only the oscillation rep-rate (and multiples) without any sub-harmonics or spurious frequencies (down to 50dB) below the carrier, which indicates a clean, stable pulse train, whereas the 8th harmonics started showing rep-rate instabilities. Detailed RF spectra of the rep-rate frequency are shown in Figs. 4(b)-(e), where the observed line-widths is primarily due to the passive stability of the cavity length (which is not stabilized here at all). Fig. 4(f) shows the pulse-train in time, comparing the pulse reading on the fast scope (blue) to the dark level of the blocked detector (red). Evidently, no DC background is observed between the pulses, indicating that the average power arises solely from the pulsed operation and that the pulse-energy can be safely deduced from measured average-power and repetition rate.
Fig. 4. Stability of the pulse train: RF spectrum of the photo-detected pulses. (a) Full span of the spectrum, the measurement resolution bandwidth is 10kHz. (b)-(e) Narrow span ($10 \rm {kHz}$) around the n-th mode, with frequencies $f_{2}=379\rm {MHz}$, $f_{4}=758\rm {MHz}$, $f_{6}=1.137\rm {GHz}$, $f_{8}=1.516\rm {GHz}$ (resolution bandwidth of 10Hz). (f) shows the measured output intensity in time from a fast, DC-coupled detector (20GHz) as recorded on a digital oscilloscope (6GHz bandwidth). The blue graph shows the pulse reading compared to the dark level of the blocked detector (red). Evidently, the DC level outside the pulse is negligible, and the power can be safely attributed to the pulsed operation only.
Finally, our high-energy pulses are achieved with high spatial coherence, as illustrated in Fig. 5. Specifically, $M^2=1.2-1.3$ is achieved for all the harmonics, indicating a near-ideal spatial mode with high brightness and focusability.
Fig. 5. Beam intensity profile across the slow axis normalized by the peak intensity. The beam profiles were taken after filtering out the low intensity side lobes, which amount to 12${\% }$–14.5${\% }$ of the total power. The width marked across the graphs is the width for which the distribution drops to 13.5${\% }$ of the peak. (red) Far-field: the beam intensity profile as seen on the end-mirror. (black) Near-field: the beam intensity profile at the Fourier plane of the imaging system. The calculated beam quality is varied between $M^2\!=\!1.2-1.4$.
4. Conclusions
In conclusion, we present an external cavity configuration that implements passive mode-locking in a broad-area dual-section diode - a gain section with forward current and an integral saturable absorber section with a voltage controlled saturation level. The diode chip is cut at an angle to mitigate self-lasing at the diode facets. Our results demonstrate ps-range pulses with record performance in terms of pulse energy and peak power. The simplicity of the external cavity configuration that produces such high-energy and high-brightness ultrafast pulses is attractive for a wide range of applications, including laser micromachining, precision metrology, and nonlinear optics.
The possibility to produce even shorter, sub-ps pulses by incorporation of dispersion control in the cavity is fascinating and will be the focus of future research.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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