We demonstrate the first semiconductor mode-locked lasers for ultrashort pulse generation at the 760 nm waveband. Multi-section laser diodes based on an AlGaAs multi-quantum-well structure were passively mode-locked, resulting in the generation of pulses at around 766 nm, with GHz (with different laser cavity lengths of 1.8 mm and 1.5 mm, respectively). The influence of the bias conditions on the mode-locking characteristics was investigated for these new lasers, revealing trends which can be ascribed to the interplay of dynamical processes in the saturable absorber and gain sections. It was also found that the front facet reflectivity played a key role in the stability of mode-locking and the occurrence of self-pulsations. These lasers hold significant promise as light sources for multi-photon biomedical imaging, as well as in other applications such as frequency conversion into the ultraviolet and radio-over-fibre communications.
© 2014 Optical Society of America
In recent years, there has been a major effort in the development of compact and low-cost ultrafast laser systems, with the aim of replacing bulky and expensive sources in applications such as biomedical multi-photon imaging. In this quest, semiconductor edge-emitting mode-locked laser diodes (MLLDs) have shown significant promise as seed oscillators in ultrafast laser systems incorporating optical amplifiers, which can be deployed for multi-photon imaging of live cells and tissues. Different wavebands have been demonstrated in this context, ranging from 1.26 μm, based on a combination of InAs/GaAs picosecond quantum-dot laser and amplifier , down to the near-infrared at 783 nm, which was demonstrated with a picosecond all-semiconductor master-oscillator power-amplifier based on a GaAs quantum-well (QW) structure . This latter work represents also the shortest wavelength generated so far directly from a MLLD in the red/near-infrared part of the electromagnetic spectrum – in fact, in order to access shorter wavelengths with a GaAs QW approach, the QW would have to be less than 3nm, which becomes increasingly challenging to control reliably across a wafer during the growth process. Therefore, and to the best of our knowledge, semiconductor ultrafast lasers addressing the 760 nm waveband have not been reported up until now. This spectral region is of significance for multi-photon bio-imaging applications as it is particularly suitable for the excitation of key endogenous fluorophores such as keratin and elastin, as well as leading to the efficient second harmonic generation in collagen . It is therefore a key spectral band for enabling functional multi-photon imaging of skin and muscular tissues without the use of any external fluorophores, which is of primary relevance for clinical applications . In order to address this need, previous work has relied on the use of fiber-amplified infrared gain-switched laser diodes with additional second-harmonic generation schemes to convert their infrared output into the 774 nm  or 766 nm  spectral wavebands.
In this paper, we demonstrate the first semiconductor mode-locked laser generating ultrashort pulses directly on the 760 nm spectral waveband. Access to this spectral region was enabled via the development of an AlGaAs multi-quantum-well (MQW) structure, from which two-section edge-emitting lasers were fabricated. These passively mode-locked monolithic lasers enabled picosecond pulse generation at either 19.4 GHz or 23.2 GHz (from two different laser cavity lengths). The results of our investigation have also revealed that the operating bias conditions and the front facet reflectivity play a key role on the mode-locking (ML) regime. These lasers are of high relevance not only for multi-photon imaging applications, but also for radio-over-fibre communications, THz generation and second harmonic generation into the UV spectral range.
2. Devices structure and fabrication
The MLLDs were based on an AlGaAs MQW graded-index separate-confinement-heterostructure (GRIN-SCH), which was grown on a heavily n-doped GaAs substrate via metal organic chemical vapor deposition (MOCVD), using a single-step growth. The layout of the lasers and wafer structure is shown in Fig. 1.
As previously demonstrated in the literature, a high concentration of aluminium in the laser material can lead to the creation of deep-level defects which act as centers of non-radiative recombination, due to aluminium’s sensitivity to impurities such as oxygen – this is the case even for aluminium-free active regions, which still contain aluminium in the cladding layers . It has been shown that a number of measures can be taken during material growth to enable high optical quality wafer structures with high concentrations of aluminium (up to 75%) . Taking this into consideration, we have used a relatively low flow rate of arsine (AsH3), while the growth temperature was kept at relatively high values , to address the challenge inherent to the high concentration of aluminium. Accordingly, the growth temperature for the GaAs buffer layer was 754 °C, and 830 °C for the Al0.6Ga0.4As cladding layer, while the other layers were grown at 750 °C.
The internal optical loss was estimated to be αi = 6.7 cm−1, comparable to that reported for 760 nm laser diodes based on AlGaInAs quantum-dot structures (αi ~6 cm−1) . It is noteworthy to mention that the optical loss is, to some extent, a manifestation of the concentration of defects present in the material [10,11], and as such, the value obtained provides a positive indication of the optical quality of the material.
Laser fabrication was subsequently carried out, whereby a ridge waveguide with a width of 3.7 μm and a depth of 1 μm was defined by conventional lithography and inductively coupled plasma (ICP) dry-etching. A silicon dioxide insulating layer was grown by plasma-enhanced chemical vapor deposition (PECVD), followed by the deposition of Ti/Au metal p-contacts and Au/Ge/Ni n-contacts. Multi-section lasers with two different cavity lengths were fabricated: one with a total length of 1815 μm, consisting of a 1728 μm-long gain section and a 72 μm long saturable absorber; the other with a total length of 1515 μm, comprising a 1440 μm long gain section and a 60 μm long saturable absorber. Both laser geometries had a similar absorber-to-total-length ratio of around 4%, which has previously shown to be a suitable value enabling a broad range of conditions for stable ML in QW-based MLLDs [12,13]. Electrical isolation between the gain and absorber electrodes was achieved via a 15 μm wide and 350 nm deep gap in the contact layer. Unless otherwise stated, the front and rear facets of the MLLDs were coated with reflectivities of 13.6% and 93.9%, respectively. The MLLDs were mounted epilayer-up, operated at 10 °C using a thermoelectric temperature controller, and biased under CW conditions. The gain section was forward biased, while the saturable absorber was reverse-biased in order to speed up the absorption recovery time. A non-collinear autocorrelator based on second harmonic generation was used to measure the pulse duration.
3. Mode-locking regime characteristics
The light–current characteristics of the 1815 μm long MLLD were evaluated for different values of reverse-bias applied to the saturable absorber, as shown in Fig. 2(a). The distinct effect of optical absorption can be seen in the power drop between −2 V and −3 V. Indeed, a stable ML regime could be achieved for forward bias currents higher than 250 mA and values of reverse bias ranging between −2.3 V and −3 V (depending on the applied current). Typical characteristics of the pulsed output observed under a stable ML regime are shown in Figs. 2(b)-2(d), depicting a pulse duration of 5.7 ps, the optical spectrum centered at 766 nm and the corresponding RF spectrum, exhibiting a dynamic contrast of over 30 dB.
For these bias conditions, the time-bandwidth product (TBWP) was 2.04, revealing that the pulse is strongly chirped. The chirp in pulses generated from passively MLLDs is typically a result of the interplay between gain and absorption saturation mechanisms, which lead to self-phase modulation effects in both gain and absorber sections . For example, as a pulse propagates in the gain section, the carrier density and thus the gain is depleted across the pulse. This leads to a dynamic increase of the refractive index, which then introduces a phase modulation on the pulse, changing the instantaneous frequency across the pulse (a more in-depth discussion can be found on ).
The average power was 11.3 mW, corresponding to a peak power of 90 mW (assuming a sech2 pulse shape). It is worth noting that these values of peak power and pulse duration are of a similar order to those previously demonstrated for an external-cavity MLLD oscillator at 783 nm , operating at 500 MHz and incorporating a multi-section gain chip with a 440 µm length (total external-cavity length of 300 mm).
The effect of the reverse bias on the ML regime was then investigated for a fixed current of 265 mA applied to the gain section. Figure 3 depicts the variation of pulse duration and TBWP with reverse bias. The decrease in pulse duration up until a reverse bias of −2.6 V is consistent with a reduction of the absorption recovery time and thus the net gain window with increasing reverse bias . Indeed, previous pump-probe investigations reported an exponential decrease of absorption recovery time in AlGaAs QW structures . Beyond −2.6 V, the pulse duration does not decrease further, even though the absorption recovery time could possibly continue to reduce. This trend is in agreement with previously reported simulations [17,18], which suggested that the ultimate limitation in the pulse duration with increasing reverse bias would be linked to a less effective saturation mechanism, eventually leading to the occurrence of self-pulsations, as shown in Fig. 3 (inset).
It is also worthy to note that the increase in reverse bias also leads to a red-shift of the saturable absorber’s band-edge due to quantum-confined Stark effect (QCSE), which can limit the ML region as the mismatch between the gain peak and band-edge increases . The absorption increases and becomes increasingly less saturable which, along with the concomitant reduction of intracavity power, leads to greater difficulty in saturating the absorber. At some point, this would also affect the ML regime, which would become more prone to self-pulsations.
The effect of the current applied to the gain was also investigated with a fixed reverse bias, as represented in Fig. 4. With increasing current, the optical spectrum widens, leading to shorter pulses until the edge of the stable ML region. Accordingly, the shortest pulse duration achieved was 4.7 ps. The optical spectra also red-shifts as the current is increased, becoming increasingly more asymmetric and developing additional peaks on the red side of the spectra, due to an increase of self-phase modulation with increasing gain . Eventually, at 275 mA the mode-locking regime stability collapses as shown by the autocorrelation, which exhibits a coherence spike on a strong background level - a signature for continuous noise .
MLLDs with a shorter cavity length of 1515 µm were also successfully mode-locked. The light-current characteristics for such a MLLD are depicted in Fig. 5(a), for various values of reverse bias applied to the saturable absorber section. Stable ML was observed with currents ranging from 210 mA to 235 mA, and reverse bias varying from −2.76 V to −3.04 V. This regime resulted in pulses with spectra centered at around 766 nm, at a repetition rate of ~23.2 GHz and an approximate pulse duration of 4 ps. The characteristics of a typical pulse from the identified stable ML region are shown in Figs. 5(b)-5(d), depicting an RF peak at least 30 dB above the noise floor. Assuming a sech2 pulse shape, the pulse duration was estimated to be 4.17 ps. With the optical spectrum centered at 766.31 nm and with a bandwidth of 0.65 nm, a TBWP of 1.39 was achieved. In this case, the average power of 8.3 mW corresponded to a peak power of 75 mW.
In a similar fashion to the longer MLLD, this 1515 µm chip tended to produce pulses of a shorter duration with increased reverse bias and increased forward current. However, the region of stable ML for the shorter cavity length was quite narrow, often only ranging a few tens of mV of reverse bias for a given driving current. This narrower bias region could be due to the stronger constraints imposed on the absorption/gain processes by the higher pulse repetition rate. On one hand, to satisfy the background stability criterion, the reverse bias should be high enough in order to speed up the absorption recovery time, which should be shorter than both the gain recovery time and the pulse cavity round trip time (~43 ps). On the other hand, the increase in reverse bias leads to a QCSE-induced shift in absorption and the increase in absorption saturation energy, thus imposing an upper bound limit to the reverse bias that enables a stable ML regime.
In a bid to investigate the possibility of extracting a higher output power from these MLLDs, lasers with 1515 and 1815 µm lengths were manufactured with a front facet coating of 6.6%, as opposed to 13.6%. The 6.6% coated MLLDs did display a higher average output power under the same bias conditions as their 13.6% coated counterparts. However, robust ML was not observed under any bias conditions. The RF spectra displayed a peak corresponding to the pulse repetition rate with significant noise and a dynamic range of 20 dB or less, always accompanied by self-pulsations at lower frequencies. On the other hand, the autocorrelation traces displayed prominent coherence spikes atop a strong background level, similar to that shown on the top graph in Fig. 4(b) (for a current of 275 mA). This trade-off can be understood in the light of conditions for ML versus self-pulsations, as a higher intracavity optical power associated with higher reflectivity mirrors is favorable towards the ML regime, while lowering the probability of self-pulsations . A higher reflectivity of the front facet could be implemented in future work, as it has been shown theoretically that this could lead to shorter pulses, due to a higher intracavity peak power which leads to a more efficient saturation of the saturable absorber . In future work, the epitaxial structure and device design could also incorporate some of the elements used in the successful approach demonstrated by Tandoi et al.  - whereby a reduced optical confinement factor, a large vertical optical mode and the inclusion of non-absorbing mirrors led to the generation of sub-picosecond pulses with a high peak power from a GaAs/AlGaAs laser at 795 nm.
We have demonstrated the first mode-locked semiconductor lasers operating at the spectral waveband of 760 nm. Two different cavity lengths of 1815 µm and 1515 µm were tested and both enabled the generation of ultrashort pulses with a central wavelength at 766 nm, with pulse durations down to ~4 ps. The ML characteristics were investigated as a function of the bias, which can be explained by the interplay of dynamic gain and loss processes. The role of the front facet reflectivity was also found to be instrumental in ensuring a stable ML regime without self-pulsations. This spectral waveband is of relevance for multi-photon imaging of skin and muscular tissue, and as such, this demonstration paves the way for the future development of an all-semiconductor master-oscillator power amplifier, which would allow the use of such a compact laser system in a clinical setting.
This work was partly supported by the State Key Lab on Integrated Optoelectronics Open Project (Grant No.:2011KFB002) and the National 863 project (Grant No.: 2012AA012203). Y.D. also acknowledges financial support from FP7, through a Marie Curie International Incoming Fellowship (contract no. 273362). M.A.C. also acknowledges financial support through a Royal Academy of Engineering/EPSRC Research Fellowship, as well the Philip Leverhulme Prize (Leverhulme Trust, PLP-2011-172). A.F., D.B. and S.E.H. also acknowledge support from the EPSRC Doctoral Training Account, the BBSRC (A.F.) and Wellcome Trust (S.E.H.) Doctoral Training Accounts and the Leverhulme Trust (D.B.).
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