Mode-locked Semiconductor Disk Laser Research Articles

Real-time spectral transient dynamics measurement of mode-locked semiconductor disk lasers with DFT

The real-time measurement of transient dynamics of a semiconductor disk laser (SDL) was demonstrated using the dispersive Fourier transform (DFT) technology. Operating in a stable mode-locking state with a repetition rate of 425 MHz and a pulse width of 2.18 ps, the SDL exhibits significant changes in both spectral shape and pulse profile in the process of the establishment and extinction of mode locking. The pulse-resolved spectral evolution was captured, which includes relaxation oscillation, Q-switched oscillation and mode-locking states. Notably, we observed a progressive shift in the central wavelength—from 983.8 nm during initial mode-locking to 982.5 nm in the stabilization phase, and finally to 980 nm at extinction. The shift of mode locked wavelength can be attributed to thermal effect. To the best of our knowledge, this is the first time to observe the mode-locked dynamics in SDLs using a DFT technology.

Low repetition rate passive mode-locked semiconductor disk laser

Semiconductor disk lasers (SDLs) have advantages of high output power and good beam quality. Their flexible external cavity provides convenience for inserting additional optical element to start mode locking and produce ultra-short pulse train with duration from picosecond to femtosecond. However, the very short lifetime in a range from about a few nanoseconds to tens of nanoseconds of the carrier in semiconductor gain medium limits the decrease of pulse repetition rate, thus restricting the increase of peak power of the mode-locked laser pulse to some extent. In this work, by using the relatively shallow In<sub>0.2</sub>GaAs quantum wells, which have a relatively long carrier lifetime in the active region of gain chip, as well as the particularly designed semiconductor saturable absorption mirror (SESAM) that has a relatively small saturation flux, a passively mode-locked SDL with low repetition rate and high peak power is demonstrated. The used six-mirror cavity has a spot radius of about 200 μm on the chip and a 40 μm spot on the SESAM, and the total cavity length is about 1.92 m. The SESAM passively mode-locked SDL produces a stable pulse train with a lowest repetition rate of 78 MHz. When the temperature is 12 ℃ and the transmittance of the output coupler is <i>T</i> = 3%, an average output power value of 2.1 W and a pulse duration of 2.08 ps are achieved. The corresponding pulse peak power reaches 12.8 kW, which is about twice the reported highest peak power in an SESAM mode-locked SDL. When <i>T</i> = 2% and <i>T</i> = 5%, the obtained average output power values are 1.34 W and 1.62 W respectively, and the corresponding pulse peak power values are 8.17 kW and 9.88 kW. Based on the values reported in the literature and the results of pulse repetition rate in our experiments, the estimated lifetime of the carriers of the In<sub>0.2</sub>GaAs quantum wells in the active region of the gain used chip is 16.4 ns. This high peak power mode-locked semiconductor disk laser has important potential applications in biomedical photonics, chemistry, and nonlinear microscopy.

Optimizing the Cavity-Arm Ratio of V-Shaped Semiconductor Disk Lasers

Passively mode-locked semiconductor disk lasers have received tremendous attention from both science and industry. Their relatively inexpensive production combined with excellent pulse performance and great emission-wavelength flexibility make them suitable laser candidates for applications ranging from frequency-comb tomography to spectroscopy. However, due to the interaction of the active medium dynamics and the device geometry, emission instabilities occur at high pump powers and thereby limit their performance potential. Hence, understanding those instabilities becomes critical for an optimal laser design. Using a delay-differential equation model, we are able to detect, understand, and classify three distinct instabilities that limit the maximum achievable pump power for the fundamental mode-locking state and link them to characteristic positive-net-gain windows. We furthermore derive a simple analytic approximation in order to quantitatively describe the stability boundary. Our results enable us to predict the optimal laser-cavity configuration with respect to positive-net-gain instabilities and therefore may be of great relevance for the future development of passively mode-locking semiconductor disk lasers.

Thermal Characteristics of GHz Passively Mode-Locked Semiconductor Disk Laser

A 1.3 GHz repetition rate optically pumped passively mode-locked semiconductor disk laser with 5.5 ps pulse duration and 204 mW average output power at 978 nm wavelength was demonstrated. Main factors preventing the mode-locked laser from higher output power, such as the epitaxy structure of wafer and thermal characteristics of gain chip were investigated, and corresponding methods to improve the thermal performance of the laser, e.g., growing the wafer in a reversed sequence, removing substrate entirely and using a diamond as heat spreader, were proposed. The experimental results showed that both the microcavity mode of laser and the emission wavelength of QWs redshifted when the pump power increased, but their redshift rates were different. This suggested us to consider a pre-offset between the emission wavelength of QWs at room temperature and the target wavelength of laser at an intense pump, and the pre-offset values of the wavelength should be designed deliberately to guarantee the resonant periodic gain structure working properly when the temperature in the active region approaches the desired value.

Underwater Channel Characteristics and Wireless Optical Communications Using a Blue Mode-Locked Semiconductor Disk Laser

Underwater Channel Characteristics and Wireless Optical Communications Using a Blue Mode-Locked Semiconductor Disk Laser

Underwater Channel Characteristics and Wireless Optical Communications Using a Blue Mode-Locked Semiconductor Disk Laser

Underwater Channel Characteristics and Wireless Optical Communications Using a Blue Mode-Locked Semiconductor Disk Laser

Stable fundamental and dual-pulse mode locking of red-emitting VECSELs

We present a thorough characterization of a red-emitting mode-locked semiconductor disk laser producing the first stable fundamental mode-locked pulse train in the red spectral range. The achievable temporal FWHM pulse length of the Lorentzian-shaped pulses is just below 3 which represents the shortest pulses obtained in a fully-resonant absorber and gain structure design. We also include an intracavity birefringent filter to stabilize and tune the emission wavelength between and . This, in combination with a real-time sampling of the emitted pulse train, allows us to distinguish three regimes according to their round-trip dynamics. The first regime resembles stable fundamental mode locking and is observed for wavelengths below while for an emission wavelength of the continuous wave regime is observed. These two operating regimes enclose the dual-pulse mode locking regime appearing between and . This regime is characterized by an energy exchange between the individual pulses of the dual-pulse mode locking regime and we clearly relate the side-peaks in the radio frequency spectrum to the dual-pulse energy-exchange frequency and the temporal interpulse spacing to the individual arm lengths of the gain-folded external cavity. Overall, we attribute these different operating regimes to near-bandgap operation of the SESAM which allows for dispersion of saturation fluence and modulation depth.

Continuous repetition rate tuning from 960 MHz to 1.72 GHz of a sub-300 femtosecond mode-locked semiconductor disk laser

Dataset supports: Chen Sverre, T. et al (2018). Continuous repetition rate tuning from 960 MHz to 1.72 GHz of a sub-300 femtosecond mode-locked semiconductor disk laser. Applied Physics Letters, 113(16), [161106].Vertical External Cavity Surface Emitting semiconductor Lasers rely on Semiconductor Saturable Absorbing Mirrors for mode-locking, allowing laser cavities to be designed far from stability limits. We have harnessed this feature to study repetition rate tunability. The cavity element separations were determined by a coded cavity design protocol based on cavity round trip matrix calculations. We produced a sub-300-fs near transform-limited pulse train with a repetition rate of 0.96 MHz - 1.72 GHz, at 1035 nm and average power of 50 mW.

Optical efficiency and gain dynamics of modelocked semiconductor disk lasers.

Compact optically pumped passively modelocked semiconductor disk lasers (SDLs) based on active quantum wells (QWs) such as vertical external-cavity surface-emitting lasers (VECSELs) or modelocked integrated external-cavity surface-emitting lasers (MIXSELs) are wavelength-versatile sources that offer a unique combination of gigahertz pulse repetition rates and short pulse durations. In this paper, we present record-short pulses of 184 fs from a gigahertz MIXSEL emitting at a center wavelength of 1048 nm. This result comes at the expense of low optical-to-optical pump efficiency (<1%) and average output power limited to 115 mW. We experimentally observe that shorter pulses significantly reduce the macroscopic gain saturation fluence and develop a QW model based on rate equations to reproduce the gain saturation behavior and quantitatively explain the VECSEL and MIXSEL modelocking performances. We identify spectral hole burning as the main cause of the reduced gain at shorter pulse durations, which in combination with the short lifetime of the excited carriers strongly reduces the optical pump efficiency. Our better understanding will help to address these limitations in future ultrafast SDL designs.

High-power 100 fs semiconductor disk lasers

Optically pumped passively modelocked semiconductor disk lasers (SDLs) provide superior performance in average output power, a broad range of operation wavelengths, and reduced complexity. Here, we present record performance with high average power and pulse durations as short as 100 fs with a semiconductor saturable absorber mirror (SESAM) modelocked vertical external-cavity surface-emitting laser (VECSEL) at a center wavelength of 1034 nm. A comprehensive pulse characterization confirms fundamental modelocking with a close to transform-limited output pulse of 128 fs and with negatively chirped output pulses as short as 107 fs, which are externally compressed to 96 fs with a single path through a 2-mm-thick ZnSe plate. For the “96 fs result” the pulse repetition rate is 1.6 GHz, the average output power is 100 mW, and the pulse peak power is 560 W. The transform-limited optical spectrum could in principle support pulses as short as 65 fs with higher order dispersion compensation. We measured the most relevant spectral and nonlinear VECSEL and SESAM parameters and used them as input parameters for our pulse formation simulations. These simulations agree well with our experimental results and provide an outlook for further performance scaling of ultrafast SDL technology.

Intra-cavity frequency-doubled mode-locked semiconductor disk laser at 325 nm.

We present a passively mode-locked semiconductor disk laser (SDL) emitting at 650nm with intra-cavity second harmonic generation to the ultraviolet (UV) spectral range. Both the gain and the absorber structure contain InP quantum dots (QDs) as active material. In a v-shaped cavity using the semiconductor samples as end mirrors, a beta barium borate (BBO) crystal is placed in front of the semiconductor saturable absorber mirror (SESAM) for pulsed UV laser emission in one of the two outcoupled beams. Autocorrelation (AC) measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps. With a repetition frequency of 836MHz, the average output power is 10mW per beam for the red emission and 0.5mW at 325nm.

Recent advances in ultrafast semiconductor disk lasers

AbstractThe performance of ultrafast semiconductor disk lasers has rapidly advanced in recent decades. The strong interest from industry for inexpensive, compact, and reliable ultrafast laser sources in the picosecond and femtosecond domains has driven this technology toward commercial products. Frequency metrology and biomedical applications would benefit from sub-200-femtosecond pulse durations with peak powers in the kilowatt range. The aim of this review is to briefly describe the market potential and give an overview of the current status of mode-locked semiconductor disk lasers. Particular focus is placed on the ongoing efforts to achieve shorter pulses with higher peak powers.

Compact, temperature-stable multi-gigahertz passively modelocked semiconductor disk laser**Project supported by the National Natural Science Foundation of China (Grant No. 61177047) and the Key Project of the National Natural Science Foundation of China (Grant No. 61235010).

We present a compact passively mode-locked semiconductor disk laser at 1045 nm. The gain chip without any post processing consists of 16 compressively strained InGaAs symmetrical step quantum wells in the active region. 3-GHz repetition rate, 4.9-ps pulse duration, and 30-mW average output power are obtained with 1.4 W of 808-nm incident pump power. The temperature stability of the laser is demonstrated to have an ideal shift rate of 0.035 nm/K of the lasing wavelength.

Exploring the influence of high order transverse modes on the temporal dynamics in an optically pumped mode-locked semiconductor disk laser.

We quantitatively investigate the influence of high-order transverse modes on the self-mode locking (SML) in an optically pumped semiconductor laser (OPSL) with a nearly hemispherical cavity. A physical aperture is inserted into the cavity to manipulate the excitation of high-order transverse modes. Experimental measurements reveal that the laser is operated in a well-behaved SML state with the existence of the TEM(0,0) mode and the first high-order transverse mode. While more high-order transverse modes are excited, it is found that the pulse train is modulated by more beating frequencies of transverse modes. The temporal behavior becomes the random dynamics when too many high-order transverse modes are excited. We observe that the temporal trace exhibits an intermittent mode-locked state in the absence of high-order transverse modes.

193-GHz 53-W Subpicosecond Pulse Source

We present a light source that can generate a pulse train with an extremely high repetition rate, tens of watts of average output power, and a low-divergence output beam. This unique combination of system characteristics is achieved with single-stage amplification of a passively harmonically mode-locked semiconductor disk laser in a tapered Yb-doped double-clad fiber. With the short-length tapered fiber amplifier an amplification factor >17 dB is reached, while preserving the 930-fs pulse duration of the semiconductor disk laser. The demonstrated pulse source with a beam quality factor <2 enables efficient coupling into single-mode fibers for nonlinear optical experiments and ultracoarse frequency comb generation.

Self-mode-locking semiconductor disk laser.

The development of mode-locked semiconductor disk lasers received striking attention in the last 14 years and there is still a vast potential of such pulsed lasers to be explored and exploited. While for more than one decade pulsed operation was strongly linked to the employment of a saturable absorber, self-mode-locking emerged recently as an effective and novel technique in this field - giving prospect to a reduced complexity and improved cost-efficiency of such lasers. In this work, we highlight recent achievements regarding self-mode-locked semiconductor devices. It is worth to note, that although nonlinear effects in the active medium are expected to give rise to self-mode-locking, this has to be investigated with care in future experiments. However, there is a controversy whether results presented with respect to self-mode-locking truly show mode-locking. Such concerns are addressed in this work and we provide a clear evidence of mode-locking in a saturable-absorber-free device. By using a BBO crystal outside the cavity, green light originating from second-harmonic generation using the out-coupled laser beam is demonstrated. In addition, long-time-span pulse trains as well as radiofrequency-spectra measurements are presented for our sub-ps pulses at 500 MHz repetition rate which indicate the stable pulse operation of our device. Furthermore, a long-time-span autocorrelation trace is introduced which clearly shows absence of a pedestal or double pulses. Eventually, a beam-profile measurement reveals the excellent beam quality of our device with an M-square factor of less than 1.1 for both axes, showing that self-mode-locking can be achieved for the fundamental transverse mode.

All quantum dot mode-locked semiconductor disk laser emitting at 655 nm

We present a semiconductor disk laser mode-locked by a semiconductor saturable absorber mirror (SESAM) with emission in the red spectral range. Both the gain and the absorber structure are fabricated by metal-organic vapor-phase epitaxy in an anti-resonant design using quantum dots as active material. A v-shaped cavity is used to tightly focus onto the SESAM, producing pulses with a duration of about 1 ps at a repetition rate of 852 MHz.

Gigahertz self-referenceable frequency comb from a semiconductor disk laser.

We present a 1.75-GHz self-referenceable frequency comb from a vertical external-cavity surface-emitting laser (VECSEL) passively modelocked with a semiconductor saturable absorber mirror (SESAM). The VECSEL delivers 231-fs pulses with an average power of 100 mW and is optimized for stable and reliable operation. The optical spectrum was centered around 1038 nm and nearly transform-limited with a full width half maximum (FWHM) bandwidth of 5.5 nm. The pulses were first amplified to an average power of 5.5 W using a backward-pumped Yb-doped double-clad large mode area (LMA) fiber and then compressed to 85 fs with 2.2 W of average power with a passive LMA fiber and transmission gratings. Subsequently, we launched the pulses into a highly nonlinear photonic crystal fiber (PCF) and generated a coherent octave-spanning supercontinuum (SC). We then detected the carrier-envelope offset (CEO) frequency (f(CEO)) beat note using a standard f-to-2f-interferometer. The f(CEO) exhibits a signal-to-noise ratio of 17 dB in a 100-kHz resolution bandwidth and a FWHM of ≈10 MHz. To our knowledge, this is the first report on the detection of the f(CEO) from a semiconductor laser, opening the door to fully stabilized compact frequency combs based on modelocked semiconductor disk lasers.

Pulse repetition rate scaling from 5 to 100 GHz with a high-power semiconductor disk laser

The high-power semiconductor laser studied here is a modelocked integrated external-cavity surface emitting laser (MIXSEL), which combines the gain of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in a single semiconductor layer stack. The MIXSEL concept allows for stable and self-starting fundamental passive modelocking in a simple straight cavity and the average power scaling is based on the semiconductor disk laser concept. Previously record-high average output power from an optically pumped MIXSEL was demonstrated, however the long pulse duration of 17 ps prevented higher pulse repetition rates and many interesting applications such as supercontinuum generation and broadband frequency comb generation. With a novel MIXSEL structure, the first femtosecond operation was then demonstrated just recently. Here we show that such a MIXSEL can also support pulse repetition rate scaling from ≈5 GHz to >100 GHz with excellent beam quality and high average output power, by mechanically changing the cavity length of the linear straight cavity and the output coupler. Up to a pulse repetition rate of 15 GHz we obtained average output power >1 W and pulse durations <4 ps. Furthermore we have been able to demonstrate the highest pulse repetition rate from any fundamentally modelocked semiconductor disk laser with 101.2 GHz at an average output power of 127 mW and a pulse duration of 570 fs.

Amplitude Noise and Timing Jitter Characterization of a High-Power Mode-Locked Integrated External-Cavity Surface Emitting Laser

We present a timing jitter and amplitude noise characterization of a high-power mode-locked integrated external-cavity surface emitting laser (MIXSEL). In the MIXSEL, the semiconductor saturable absorber of a SESAM is integrated into the structure of a VECSEL to start and stabilize passive mode-locking. In comparison to previous noise characterization of SESAM-mode-locked VECSELs, this first noise characterization of a MIXSEL is performed at a much higher average output power. In a free-running operation, the laser generates 14.3-ps pulses at an average output power of 645 mW at a 2-GHz pulse repetition rate and an RMS amplitude noise of 0.15% [1 Hz, 10 MHz]. We measured an RMS timing jitter of 129 fs [100 Hz, 10 MHz], which represents the lowest value for a free-running passively mode-locked semiconductor disk laser to date. Additionally, we stabilized the pulse repetition rate with a piezo actuator to control the cavity length. With the laser generating 16.7-ps pulses at an average output power of 701 mW, the repetition frequency was phase-locked to a low-noise electronic reference using a feedback loop. In actively stabilized operation, the RMS timing jitter was reduced to less than 70 fs [1 Hz, 100 MHz]. In the 100-Hz to 10-MHz bandwidth, we report the lowest timing jitter measured from a passively mode-locked semiconductor disk laser to date with a value of 31 fs. These results show that the MIXSEL technology provides compact ultrafast laser sources combining high-power and low-noise performance similar to diode-pumped solid-state lasers, which enable world-record optical communication rates and low-noise frequency combs.