Short-cavity Laser Research Articles

Experimental study on linewidth compression based on Rayleigh backscattering in 1064 nm fiber laser

Abstract Attributed to the unique energy level structure of the Yb 3 + , it is arduous to effectively compress the linewidth of Ytterbium-doped fiber (YDF) laser by conventional methods. Herein, we investigated Rayleigh backscattering (RBS) characteristics in single mode fiber-1060-XP (SMF-1060-XP) and the linewidth compression efficacy of RBS in 1064 nm laser system. A single longitudinal mode (SLM) fiber laser operating at 1064 nm has been obtained, and the 3 dB linewidth can be compressed from 8.01 kHz to 1.35 kHz thereby yielding a side mode suppression ratio as high as ∼ 50 dB provided the pump power is set to 390 mW. The theoretical prediction suggests the feasibility to actualize a linewidth of hundreds of Hertz provided the experimental setup is further optimized. The investigation of our work has a potential application in the study of linewidth compression of short cavity lasers or DFB lasers operating at a 1064 nm wavelength.

Temporal Stability and Spectral Accuracy Enhancement of the Spectro-Temporal Analyzer

The advent of the time-stretch technology, together with the high-speed photodetector, enables ultrafast spectrum analysis in the form of spectro-temporal analyzer, such as the amplified dispersive Fourier transform (ADFT) and the parametric spectro-temporal analyzer (PASTA). Leveraging the MHz to GHz frame rate, such a spectro-temporal analyzer is capable of observing certain ultrafast or non-repetitive events, such as the birth of mode-locking. However, since it always requires a large amount of dispersion, the spectral accuracy will be greatly degraded by the temporal instability, including the temperature-induced timing jitter and the cavity-induced frequency jitter, which hinder the practical application of such an ultrafast spectroscopy technique. To enhance the temporal stability and thereby spectral accuracy, two active control schemes are introduced here: the temperature feedback control for the long fiber link, and the phase-locked loop for the short laser cavity. As a result, the temporal stability is greatly enhanced, and the spectral accuracy is improved from 1.6 to 0.04 nm over a 5-h period of time. This stabilization process is an essential step for the practical application of spectro-temporal analyzers, such as the ADFT and particularly the PASTA.

Direct experimental measurement of single-mode and mode-hopping dynamics in frequency swept lasers.

A time-resolved study is presented of the single-mode and mode-switching dynamics observed in swept source vertical cavity surfing emitting lasers and swept wavelength short external cavity lasers. A self-delayed interferometric technique is used to experimentally measure the phase and intensity of these frequency swept lasers, allowing direct examination of the modal dynamics. Visualisation of the instantaneous optical spectrum reveals mode-hop free single mode lasing in the case of the vertical cavity laser, with a tuning rate of 6.3 GHz/ns. More complex mode-switching behaviour occurs in the external cavity laser, with the mode-hopping dynamics found to be dominated by the deterministic movement of the spectral filter. Evidence of transient multi-mode operation and mode-pulling is also presented.

PbSrS/PbS mid-infrared short-cavity edge-emitting laser on Si substrate

A PbSrS/PbS short-cavity edge-emitting laser consisting of a SrS/PbS short period superlattice was prepared on the Si (111) substrate. The laser with a cavity length of 25 μm and a cavity width of 3 μm operated up to 313 K around 3 μm in wavelength under pulsed laser excitation. Maximum peak output power was 40 mW at 260 K which indicated a high quantum efficiency.

Q-switched mode-locking and continuous-wave mode-locking operation in a short-cavity erbium-doped fiber laser

We demonstrate a short-cavity Q-switched mode-locking (QML) and continuous-wave mode-locking (CWML) erbium-doped fiber (EDF) laser based on graphene-oxide (GO) as a passive ssaturable absorber (SA). By film-drying and inserting GO between two fiber connectors, passive GO SA is constructed. The GO SA is then incorporated into a short EDF laser cavity. The laser can operate both on QML pattern and CWML pattern just by adjusting the pump power. The mode-locking pulse duration can be as short as ∼472fs. And the fundamental repetition frequency can be 111.4MHz.

55 GHz Bandwidth Distributed Reflector Laser

We have demonstrated a 1300-nm short-cavity distributed reflector (DR) laser having 55 GHz bandwidth (BW), and successful 112-Gb/s transmission using four-level pulse-amplitude-modulation (PAM-4), without any pre-equalization for the transmitter. Two effects were realized in the design and operation of the DR laser: photon–photon ( P–P ) resonance and detuned-loading. The P–P resonance effect was realized between the DFB and distributed Bragg reflector (DBR) modes that coexisted in the cavity of the DR laser. The detuned-loading effect was used to effectively enhance the differential gain through the dynamic change in the mirror reflectivity that occurs on the flank of the DBR mirror due to the frequency chirp under modulation. Despite a limited RC cutoff frequency of 22 GHz, a wide modulation BW of 55 GHz was achieved. It is shown that the RC limitation was counteracted by the combined effects of the detuned-loading, which reduces the damping of relaxation oscillations, and an in-cavity FM–AM conversion effect that created a high-pass filter effect in the modulation response. This will be discussed by way of simulations and experimentally observed eye diagrams for 10 Gb/s NRZ and 28 Gbd PAM-4. The short-cavity DR laser achieved 112-Gb/s PAM-4 transmission over links having a range of dispersions from –28 to +7 ps/nm without precompensation on the transmitter.

Low Threshold Single Mode ZnO Nanowire Nanolasers

With a wide bandgap of 3.36 eV at room temperature and large exciton binding energy of 60 meV (excitons being stable up to room temperature), ZnO holds promise for use in blue and ultraviolet optical devices [1], including ultraviolet microlasers. Lasing has been demonstrated with epitaxial and microcrystalline thin films [2], and in arrays of ZnO nanorods [3] and nanowires [4], for example. Usually, the feedback mechanism in nanorod and nanowire-based lasers is related to the formation of Fabry–Perot cavities for longitudinal modes [5], or to the structures ability to couple and facilitate guided modes [6]. Discriminating between the feedback mechanisms in ZnO nanostructures is not an easy issue, especially in microstructures consisting of nanorods with various orientations. Additionally, nanoscale systems also suffer from impure spectral characteristics and multimodal emission. Strategies to improve emission characteristics have seen limited success due to the sensitivity of nanoscale resonator cavities to optical properties dependant on structural shape and details of the crystallinity. Whispering gallery modes and complicated multimodal emission with a spread in gain factors compound the issue of spectrally pure emission in nanoscale systems. High crystallinity is a prerequisite for short-cavity ZnO nanorod lasers due to expected coexistence of heavy mirror losses and scattering losses caused by the intrinsic structure and surface defects typically affecting ZnO. Here, arrays of vertically aligned ZnO nanorods were fabricated by CVD, with very high structural purity and quality. Characterization of aluminum deposition as surrounding layers on nanorods of 2-5 μm in length and 150-200 nm in width will be presented, and we detail the effect of selectively coated individual nanorods on the waveguiding and emission quality of the nanorod laser. The aluminum deposition alters the hexagonal cavity properties by reducing the guided modes and lowering the gain of the higher order modes so that a better, more spectrally pure emission is possible. [1] U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, M. Morkoc, J. Appl. Phys. 98, 041301 (2005). [2] Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72, 3270 (1998). [3] G. Visimberga, C. C. Faulkner, M. Boese and C. O'Dwyer, ECS Trans., 45, 51 (2012). [4] S. P. Lau, H. Y. Yang, S. F. Yu, H. D. Li, M. Tanemura, T. Okita, H. Hatano, H. H. Hng, Appl. Phys. Lett. 87, 013104 (2005). [5] Y. Zhang, R. E. Russo, S. S. Mao, Appl. Phys. Lett. 87, 043106 (2005). [6] H. Zhou, M. Wissinger, J. Fallert, R. Hauschild, F. Stelzl, C. Klingshirn, H. Kalt, Appl. Phys. Lett. 91, 181112 (2007).

Phase-locked laser arrays through global antenna mutual coupling

Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold. Here, we show a conceptually novel phase-locking mechanism based on ‘antenna mutual coupling’ in which laser elements interact through far-field radiations with definite phase relations. This allows a long-range global coupling among the array elements to achieve a robust phase locking in two-dimensional laser arrays. The scheme is ideal for lasers with a deep subwavelength confined cavity, such as nanolasers, whose divergent beam patterns could be used to achieve a strong coupling among the elements in the array. We demonstrated experimentally such a scheme based on subwavelength short-cavity surface-emitting lasers at terahertz frequencies. More than 37 laser elements that span over ∼8 λo were phase locked to each other, and delivered up to 6.5 mW (in a pulsed operation) single-mode radiation at ∼3 THz, with a maximum 450 mW A–1 slope efficiency and a near-diffraction-limited beam divergence. Two-dimensional arrays of short-cavity surface-emitting THz quantum cascade lasers are phase-locked to each other via mutual coupling. A directive beam on the order of 10° divergence and a maximum slope efficiency of 450 mW A−1 is achieved.

Er/Yb共掺光纤直写短腔激光器实验研究

Er/Yb共掺光纤直写短腔激光器实验研究

Lasing of multiperiod quantum-cascade lasers in the spectral range of (5.6–5.8)-μm under current pumping

The lasing of multiperiod quantum-cascade lasers in the spectral range of (5.6–5.8)-μm under current pumping are demonstrated. The quantum-cascade laser heterostructure is grown by molecular-beam epitaxy technique. Despite the relatively short laser cavity length and high level of external loss the laser shows the lasing in the temperature range of 80–220 K. The threshold current density below 4 kA/cm2 at 220 K with the characteristic temperature T0 = 123 K was demonstrated.

A Model for a Quantum-Dot Gain-Lever Mode-Locked Laser

An alternative explanation of achieving passive mode-locking in a quantum-dot (QD) laser without a dedicated saturable absorber section is proposed. The method employs an analytical model based on the net gain modulation within the cavity. Two differentially pumped sections, operated in forward bias, may utilize the differential gain contrast between them to enable pulse formation. The large differential gain contrast available in QD media and the relative ease with which such devices can be fabricated extend the utility of this methodology for the development of a QD gain-lever mode-locked laser as a next generation optical source. Alternative cavity configurations that exclude a dedicated saturable absorber section and the concomitant reduction of intracavity losses provides another path in the development of short cavity lasers with high pulse repetition rates.

Single-Longitudinal-Mode Dual Wavelength-Switchable Fiber Laser Based on Superposed Fiber Bragg Gratings

In this work, a simple switchable dual-wavelength short-cavity fiber laser operating in a single-longitudinal-mode regime at room temperature and based on superposed fiber Bragg gratings is proposed and experimentally demonstrated. Only by introducing stress into one of the overwritten FBGs, single- or dual-wavelength laser oscillations can be attained. Either single- or multimode operation of the laser can be easily achieved. Single-longitudinal-mode emission has been verified in two different ways: the first corroboration has been conducted by the heterodyne detection of the output signal, and the second one has been carried out by using a high-resolution optical spectrum analyzer.

1064-nm DFB laser diode modules applicable to seeder for pulse-on-demand fiber laser systems

Semiconductor DFB (Distributed feedback) laser diodes with an operating wavelength of 1064nm, which is suitable for pulse-on-demand fiber laser, have been developed. The stable performance of CW and nanosecond/picosecond pulsed operation is reviewed. By applying gain-switching operation with a simple direct modulation technique, 50-ps pulse generation with a stable spectral single-mode property was obtained. For the efficient amplification of the obtained 50-ps pulse, a monolithic semiconductor optical amplifier (SOA) was integrated into the DFB lasers. An improved peak power of 300mW at 50-ps pulse was observed with limited optical noise injection when the synchronous modulation technique of the DFB and the SOA was employed. Short cavity lasers showed a high-frequency response compared to the original DFB lasers and achieved a short pulse width of 13ps by standard gain-switched operation.

Extra-broadband wavelength-tunable actively mode-locked short-cavity fiber ring laser using a bismuth-based highly nonlinear erbium-doped fiber

We demonstrate an ultra-wideband wavelength-tunable actively mode-locked short-cavity laser employing a 151-cm-long bismuth-based highly nonlinear erbium-doped fiber (Bi-HNL-EDF). A wavelength tuning range of 87nm from 1533nm to 1620nm can be achieved because the Bi-HNL-EDF has an ultra-wide gain bandwidth. High nonlinearity of the Bi-HNL-EDF also collaborates with spectral filtering by an optical bandpass filter to suppress the supermode noise quite effectively. Total length of the fiber ring cavity is as short as 16m. Thus, stable and clean 5.6–6.1ps pulses with a repetition rate of 10GHz are successfully obtained over the wavelength tuning range almost completely covering both the conventional wavelength band (1530–1565nm) and the longer wavelength band (1565–1625nm). The bismuth-based short-cavity fiber laser also shows good performance in the back-to-back bit-error-rate measurements, and maintains bit-error-free mode-locking operation throughout the entire wavelength tuning range.

High Speed 1.55 μm InAs/InGaAlAs/InP Quantum Dot Lasers

We report static and dynamic characteristics of InAs/InP quantum dot (QD) lasers emitting near 1.55 μm. The gain section was optimized for a high speed operation using a unique spatially resolved model. The measured modulation capability dependence on structural parameters (barrier width and the number of QD layers) is consistent with the model predictions. Short cavity lasers with a modal gain of more than 10 cm-1 per dot layer exhibit a small signal modulation bandwidth above 9 GHz and large signal modulation at up to 22 Gb/s with an on/off ratio of 3 dB.

Characterization of energy-efficient and colorless ONUs for future TWDM-PONs

The Full Services Access Network group has recently selected the time and wavelength division multiplexed passive optical network (TWDM-PON) as the base technology solution for next-generation PON stage-2 (NG-PON2). Meeting the core requirements of NG-PON2 necessitates the following additional features in the transceivers of the optical network unit (ONU) that is located at subscriber premises: (a) legacy system compliant; (b) wavelength tunable; (c) cost-efficient; and (d) energy-efficient. To address these features, we investigate the properties of short-cavity vertical-cavity surface-emitting lasers (SC-VCSELs) for implementation as colorless ONU transmitters in future TWDM-PONs. Specifically, we investigate the tunability and transmission performance of the SC-VCSEL across the C-minus wavelength band for legacy system compliance. We report on error-free transmission across a 800 GHz tuning range with a potential aggregate upstream capacity of 80 Gbps over a system reach of 40 km and with a split ratio of 1:128 per wavelength channel. Results were achieved without dispersion compensation and electronic equalization. We also evaluate the energy efficiency of the SC-VCSEL in active, doze, and sleep mode. When in active mode, the SC-VCSEL transmitter block consumes 91.7% less power than a distributed feedback (DFB) laser transmitter block. When transitioning between doze and active modes, the transmitter block has a short settling time of only 205 ns, thus increasing the power-saving duration and consequently reducing the overall power consumption of the ONU. Through numerical analysis, evaluation of the energy-savings of the SC-VCSEL ONU over the DFB ONU under various modes of operation, demonstrates up to 84% of energy-savings. The capacity, tuning range, split ratio, system reach, and energy-savings arising from SC-VCSEL ONU implementation as reported in this work, exceed the minimum requirements of NG-PON2 for future TWDM-PON deployments.

Enhancement of thermal damage threshold of carbon-nanotube-based saturable absorber by evanescent-field interaction on fiber end

We present a scheme of fiber-connector-type carbon-nanotube-based saturable absorber (CNT-SA) with enhanced thermal damage threshold, in which the CNTs are deposited on the fiber connector end in a ring pattern for evanescent-field interaction instead of direct interaction. The thermal damage threshold of such CNT-SA is found to be increased by around 130% compared with an evenly deposited one. An all-fiber Fabry-Perot (FP) linear cavity passively mode-locked laser is further constructed incorporating the prepared CNT-SA, where the optical power is confined in a relatively short laser cavity to investigate the thermal damage threshold and the performance of the CNT-SA. Stable output pulses with a fundamental repetition rate of 211.84 MHz and a pulse width of 680 fs are generated from the fiber laser. The mode-locking operation can be maintained an intra-cavity average power of 30 mW, indicating that the CNT-SA can withstand a relatively high optical power without performance degradation.

Micro-cavity 2-μm GaSb-based semiconductor disk laser using high-reflectivity SiC heatspreader

An optically pumped GaSb-based semiconductor disk laser (SDL) emitting at 2.05 μm has been realized with a very short (380 μm long) laser cavity by high-reflectivity coating the intra-cavity SiC heatspreader, which then serves as the outcoupling mirror. Room-temperature output powers in excess of 750 mW have been demonstrated in multimode operation and still 100 mW in TEM00 emission, which is a more than 100× increase in output power compared to previous reports on GaSb-based micro-cavity (μC) SDLs. Mode-hop-free tunable single-frequency emission with linewidths <7 MHz has been achieved which makes this type of miniaturized SDLs attractive for sensing applications requiring small-size 2-μm laser sources.

1.3-μm Range Metamorphic InGaAs Laser With High Characteristic Temperature for Low Power Consumption Operation

We demonstrated a 1.3-μm-range laser with a high characteristic temperature ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 220 K) and the highest operating temperature (200 °C) ever reported for a metamorphic laser. The temperature characteristics were greatly improved by inserting a p-InGaAlAs electron stopper layer. In addition, we realized a high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> value with a low injection current using a short cavity ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> = 200 μm) laser operating at a high temperature. The bias current at 10 Gbit/s and 85 °C was only 15 mA. These results show that a laser with a metamorphic buffer on a GaAs substrate is a good candidate as an uncooled light source with low power consumption.

Colourless operation of short‐cavity VCSELs in C‐minus band for TWDM‐PONs

The tunability feature of short-cavity vertical-cavity surface-emitting lasers in the C-minus band to provide low-power, cost-efficient, colourless, and legacy system compliant transmitters for future time and wavelength division multiplexed passive optical networks is investigated. For the first time, a report is presented on error-free performances across a 800 GHz tuning range with a potential aggregate upstream capacity of 80 Gbit/s over a system reach of at least 40 km and with a 1:64 split ratio.