Longitudinal Mode Operation Research Articles

Self-injection locked blue laser

We demonstrate a 446.5 nm GaN semiconductor laser with sub-MHz linewidth. The linewidth reduction is achieved by locking the laser to a magnesium fluoride whispering gallery mode resonator characterized with 109 quality factor. Self-injection locking ensures single longitudinal mode operation of the laser.

Multi-Wavelength Brillouin Random Fiber Laser via Distributed Feedback From a Random Fiber Grating

Multi-wavelength (MW) laser sources with high optical signal-to-noise ratios (OSNR) are of great interest for optical communications with ultrahigh data capacity as well as microwave and terahertz photonics. In this paper, we report the first demonstration of MW Brillouin random fiber laser in the telecom spectral window around 1.5 μm based on a unique random fiber grating (RFG). Random feedback by enhanced distributed Rayleigh scattering from the RFG basically enables a high-efficiency random lasing resonance of the Stokes wave via stimulated Brillouin scattering in optical fibers. A subfiber loop eventually delivers the cascading process for the high-efficiency generation of up to a 14-order Stokes’ comb with an ultrahigh OSNR of 41 dB and an optimized peak power discrepancy of 3.4 dB. Thanks to Rayleigh scattered random feedback from the RFG, single longitudinal mode operation of the Stokes laser emission is achieved.

Temperature dependence of characteristic parameters of 1550 nm vertical-cavity surface-emitting laser

Compared with conventional edge-emitting semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) exhibit many advantages such as low power consumption, low threshold current, single longitudinal-mode operation, circular output beam with narrow divergence, on-wafer testing capability, high bandwidth modulation, low cost and easy large-scale integration into two-dimensional arrays, etc. VCSELs have been widely adopted in various applications such as optical communication, optical storage, parallel optical links, etc. At the same time, the rich dynamic characteristics of VCSELs have always been one of the frontier topics in the field of laser research, and many theoretically and experimentally investigated results have been reported. For theoretically investigating the dynamical characteristics of VCSELs, the spin-flip model (SFM) is one of most commonly and effectively used methods. In order to accurately predict the nonlinear dynamical performance of a 1550 nm-VCSEL, six characteristic parameters included in the rate equations of the SFM need to be given accurately. The six characteristic parameters are the decay rate of field k, the decay rate of total carrier population N, the linear anisotropies representing dichroism a, the linear anisotropies representing birefringence p, the spin-flip rate s, and the linewidth enhancement factor . In this work, through experimentally analyzing the output performances of a 1550 nm-VCSEL under free-running and parallel polarized optical injection, such six characteristic parameters included in the SFM are extracted first in the case that the temperature of the VCSEL is set to be 20.00℃. Furthermore, through gradually increasing the temperature of the 1550 nm-VCSEL from 10.00℃ to 30.00℃, the dependence of the six characteristic parameters on the temperature of the 1550 nm-VCSEL is investigated emphatically. The results show that with the increase of temperature of the 1550 nm-VCSEL, the linear anisotropy representing birefringence p behaves as an increasing trend, and the linewidth enhancement factor shows a decreasing trend. However, the other four characteristic parameters present complex varying trends with the increase of the temperature of the 1550 nm-VCSEL. The research in this paper is helpful in accurately understanding and controlling the dynamical characteristics of the VCSEL, and we hope that it can give a guidance for practical applications.

Theorical analysis of a monolithic all-active three-section semiconductor laser

We propose a novel semiconductor laser structure. It is composed of three cascaded active sections: a Fabry-Pérot laser section sandwiched between two gain-coupled distributed feedback (DFB) laser sections. We have modeled this multi-section structure. The simulation results show that compared with index- and gain-coupled DFB lasers, a significant reduction in the longitudinal spatial-hole burning can be obtained with the proposed device, and that this leads to a stable single longitudinal mode operation at relatively high optical power with a SMSR exceeding 56dB. Full Text: PDF ReferencesL.A. Coldren, "Monolithic tunable diode lasers", IEEE J. Select. Topics Quant. Electron. 6, 988 (2000) CrossRef O. Kjebon, R. Schatz, S. Lourdudoss, S. Nilsson, B. Stalnacke, L. Backbom, "30 GHz direct modulation bandwidth in detuned loaded InGaAsP DBR lasers at 1.55 [micro sign]m wavelength", Electron. Lett. 33(6), 488 (1997). CrossRef N. Kim, J. Shin, E. Sim, C.W. Lee, D.-S. Yee, M.Y. Jeon, Y. Jang, K.H. Park, "Monolithic dual-mode distributed feedback semiconductor laser for tunable continuous-wave terahertz generation", Opt. Expr. 17(16), 13851 (2009). CrossRef M.J. Wallace, R. ORreilly Meehan, R.R Enright, F. Bello, D. Mccloskey, B. Barabadi, E.N. Wang, J.F. Donegan, "Athermal operation of multi-section slotted tunable lasers", Opt. Expr. 25(13), 14426 (2017). CrossRef J.E. Carroll, J.E.A. Whiteaway, R.G.S. Plumb, "Distributed Feedback Semiconductor Lasers", Distributed feedback semiconductor lasers (IEE and SPIE, 1998). CrossRef H. Ghafour-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters (Wiley, 2003). CrossRef D.D. Marcenac, Ph.D dissertation (University of Cambridge, 1993). DirectLink L.M. Zhang, J.E. Carroll, C. Tsang, "Dynamic response of the gain-coupled DFB laser", IEEE J. Quant. Electr. 29, 1722 (1993). CrossRef W. Li, W.-P. Huang, X. Li, J. Hong, "Multiwavelength gain-coupled DFB laser cascade: design modeling and simulation", IEEE J. Quant. Electro. 36(10), 1110 (2000). CrossRef B.M. Mehdi, H. Rachid, in Proc. 3rd Intern. Conf. on Embedded Systems in Telecomm. and Instrument., Annaba, Algeria (2016). DirectLinkC. Henry, "Theory of the linewidth of semiconductor lasers", IEEE J.Quant. Electr. QE-18, 259 (1982). CrossRef K. Takaki, T. Kise, K. Maruyama, N. Yamanaka, M. Funabashi, A. Kasukawa, "Reduced linewidth re-broadening by suppressing longitudinal spatial hole burning in high-power 1.55-/spl mu/m continuous-wave distributed-feedback (CW-DFB) laser diodes", IEEE J. Quant. Electr. 39, 1060 (2003) CrossRef

Experimental Demonstration of the Distributed Feedback Semiconductor Laser With S-Bent Waveguide and Sampled Grating

A distributed feedback (DFB) semiconductor laser based on s-bent waveguide and sampled grating is theoretically studied and experimentally demonstrated. The proposed laser operates with a high side mode suppression ratio (SMSR) when the bias current is changed from 50 to 130 mA. When the bias current is fixed at 130 mA, the SMSR is larger than 55 dB, which is benefit from the suppressed spatial hole burning, while the SMSR of the equivalent phase shifted DFB laser decreases to 39 dB. The wavelength spacing error between two proposed DFB lasers is measured to be 0.089 nm compared with the designed value. When temperature is increased from 20 °C to 42 °C, the presented DFB laser achieves stable single longitudinal mode (SLM) operation with SMSR >51 dB and the wavelength varies from 1562.24 to 1564.6 nm with a slope of 0.11 nm/°C. The s-bent waveguide and sampled grating can be fabricated simply by the common holography exposure and photolithograph technique resulting in a low cost. Besides, the wavelength accuracy and SLM yield can be improved significantly, which is very beneficial for the multiwavelength DFB laser array.

Robustness and mode selectivity in parity-time (PT) symmetric lasers

We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode.

Bifurcation structure of cavity soliton dynamics in a vertical-cavity surface-emitting laser with a saturable absorber and time-delayed feedback

We consider a wide-aperture surface-emitting laser with a saturable absorber section subjected to time-delayed feedback. We adopt the mean-field approach assuming a single longitudinal mode operation of the solitary VCSEL. We investigate cavity soliton dynamics under the effect of time- delayed feedback in a self-imaging configuration where diffraction in the external cavity is negligible. Using bifurcation analysis, direct numerical simulations and numerical path continuation methods, we identify the possible bifurcations and map them in a plane of feedback parameters. We show that for both the homogeneous and localized stationary lasing solutions in one spatial dimension the time-delayed feedback induces complex spatiotemporal dynamics, in particular a period doubling route to chaos, quasiperiodic oscillations and multistability of the stationary solutions.

Far-field analysis of third-order distributed feedback terahertz quantum cascade lasers

The single lobe far-field patterns produced from terahertz quantum cascade lasers (QCLs) are greatly demanded for various applications, such as imaging, data transmission, etc. However, for a ridge waveguide terahertz QCL, the far-field beam divergence is large due to the fact that the waveguide aperture is far smaller than the terahertz wavelength. This is the case typically for double-metal waveguide terahertz QCL which emits terahertz photons in almost every direction in the space. Even for a single plasmon waveguide terahertz QCL, the divergence angle is as large as 30 in both horizontal and vertical direction. Here, in this work we design and fabricate a double metal third-order distributed feedback terahertz QCL emitting around 4.3 THz, and investigate the characteristics of the longitudinal and transverse modes. This work aims to achieve high beam quality for terahertz QCL by exploiting the third-order distributed feedback geometry, and in the meantime to achieve single longitudinal mode operation. The electromagnetic field distribution in the waveguide is modelled by employing a finite element method. The mode selection mechanism is studied by using the eigen frequency analysis, and the far-field beam is simulated by applying the near-field to far-field Fourier transform technique. The QCL active region used in this work is based on the resonant-phonon design, which is grown by a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The wafer bonding and traditional semiconductor device fabrication technology, i.e., optical lithography, electron beam evaporation, lift-off, wet and dry etching, are used to process the MBE-growth wafer into the third-order distributed feedback geometry with double-metal waveguides. By carefully designing the grating structures and optimizing the fabrication process, we achieve third-order distributed feedback terahertz QCL with quasi-single-longitudinal mode operation and single lobe far-field beam pattern with low beam divergence in both vertical and horizontal directions. The effect of grating duty cycle on the far-field beam divergence is systematically studied theoretically and experimentally. By the simulation, we finally achieve the divergence angle of 1213 for a third-order distributed feedback laser with a grating duty cycle of 12% that results in an effective refractive index close to 3. The experimental results show good agreement with the simulation. There is still room to further reduce the beam divergence of third-order distributed feedback terahertz QCL by improve the accuracy of the simulation and the fabrication.

Single longitudinal mode diamond Raman laser in the eye-safe spectral region for water vapor detection.

We report a narrowband and tunable diamond Raman laser generating eye-safe radiation suitable for water vapor detection. Frequency conversion of a tunable pump laser operating from 1063 to 1066 nm to the second order Stokes component in an external standing-wave cavity yielded 7 W of multimode output power in the wavelength range from 1483 to 1488 nm at a conversion efficiency of 21%. Stable single longitudinal mode operation was achieved over the whole tuning range at low power (0.1 W), whereas incorporation of a volume Bragg grating as an output coupler enabled much higher stable power to be attained (0.5 W). A frequency stability of 40 MHz was obtained over a minute without active cavity stabilization. It was found that mode stability is aided via seeding of the second Stokes by four-wave mixing, which leads to a doubling of the mode-hopping interval. The laser was employed for the detection of water vapor in ambient air, demonstrating its potential for remote sensing applications.

Compact two wavelength Brillouin fiber laser sensor with double Brillouin frequency spacing

A compact two wavelength Brillouin fiber laser (BFL) sensor with double Brillouin frequency spacing is proposed and demonstrated. In the experiment, 20 m polarization maintaining fiber is used as the sensing element and Brillouin gain medium. This short cavity configuration not only guarantees single longitudinal mode operation of two Stokes wavelengths, but also can effectively reduce external perturbations, complexity and noise of BFL in the absence of an erbium-doped fiber amplifier in intra-cavity. In experiment, about 2 MHz/°C sensitivity of beat frequency between the pump and the 2nd-order Stokes wavelength keep in good agreement with the theoretical value. Meanwhile, 0.2 °C temperature stability and ±0.1 dB power fluctuation are better than the traditional structure. The system is simple and stable, making it convenient for more applications.

Enhancement of asymmetry in light output from front/rear facets in resonance-shifted DFB-LDs

To enhance asymmetry in light output from front/rear facets in a resonance-shifted DFB-LD maintaining single longitudinal mode (SLM) operation, lasing characteristics of resonance-shifted DFB-LDs with asymmetric regions are simulated. The laser cavity of the resonance-shifted DFB-LD is divided into two regions. Each region has uniform diffraction gratings; a corrugation pitch in Region 1 is different from a corrugation pitch in Region 2; length L1 of Region 1 is different from length L2 of Region 2. By adjusting the corrugation pitches of the two regions, a resonance mode in Region 1 and a resonance mode in Region 2 are overlapped. At the overlapped resonance mode, SLM operation is obtained. When a corrugation pitch difference is 0.2nm with κL1=2κL2=3 where κ is a grating coupling coefficient, a normalized threshold gain difference is 0.413, which is large enough for SLM operation. Under this condition, the light output ratio from the front/rear facets is 67.6, which is larger than the reported highest value of 51 with SLM operations.

Dynamics of Stokes Waves and Pulses Generated by Stimulated Brillouin Scattering in a Resonator Including Highly Nonlinear Fiber

We experimentally and theoretically investigate the generation of Stokes waves and the dynamics of pulses produced by cascaded stimulated Brillouin scattering (SBS) in a resonator, including a highly nonlinear fiber (HNLF) and an in-cavity erbium-doped fiber amplifier (EDFA) forming a multi-wavelength Brillouin erbium-doped fiber laser system. Using coupled wave equations with the Brillouin pump, Stokes and acoustic waves, propagation in the presence of the SBS and optical Kerr effect, the modeled system simulates the evolution of participating pump and Stokes waves to verify the experimental results. The dependence of the number of generated Stokes waves on EDFA pump and Brillouin pump powers is studied showing that the proper simultaneous change (increase or decrease) of the two pump powers over few hundred of milliwatts (using the power design diagram) results in generating the same number of Stokes waves reaching steady state. Dynamics of the interference signal at the resonator output are also presented for a long HNLF (33 m) showing strong instability due to the existence of a few longitudinal modes within the SBS gain bandwidth with the possibility of random mode hopping. For a short HNLF (3 m), a single longitudinal mode operation is possible and the stability of generated pulses is expected but still depends on the phase relationships per round trip of the participating pump and Stokes waves. A good consistency between simulation and experiment is found.

Intrinsically stable high-power single longitudinal mode laser using spatial hole burning free gain

A fundamental advantage of lasers is their ability to produce a large number of photons in a single optical mode, yet this is achieved in only a minor fraction of devices due to the instability mechanism called spatial hole burning. Here, we exploit the spatial hole burning free nature of a stimulated scattering gain medium to demonstrate single longitudinal mode (SLM) operation in a generic standing wave cavity. A continuous wave diamond Raman oscillator with multi-Watt-level output power and a frequency stability of 80 MHz is demonstrated without use of additional mode-selective elements. Mode stability is addressed by considering the coupling of the Stokes power with thermally induced optical path length changes in the gain medium. The results foreshadow a novel approach for greatly extending the power and wavelength range of SLM laser sources, and with potential advantages for achieving sub-Poissonian intensity noise and sub-Schawlow–Townes linewidths.

Enhancement of resonance frequency in a DFB-LD with internally incident modulated light

To achieve high speed modulation of a semiconductor laser maintaining single longitudinal mode operation, enhancement of resonance frequency in a DFB-LD with internally incident modulated light is proposed and its lasing characteristics are simulated. The laser cavity of the DFB-LD is divided into two regions. Each region has phase-shifted diffraction gratings with the phase-shift of −π, and the corrugation pitch of Region 1 is different from that of Region 2. Region 1 and Region 2 form a coupled cavity and a sub mode contributes to enhance the resonance frequency for a main mode. When the corrugation pitch difference is 1.6nm with κL1=κL2=1.2 where the grating coupling coefficient κ is 40cm−1; length L1 of Region 1 and length L2 of Region 2 are 300μm; the injected current is 3.5 times the threshold current, the resonance frequency is enhanced to 43.7GHz and the 3-dB down bandwidth is 61.5GHz.

Resonance-shifted DFB-LD for asymmetric light output from front/rear facets

To obtain asymmetric light output from front/rear facets in a semiconductor laser maintaining single longitudinal mode (SLM) operation, a resonance-shifted DFB-LD is proposed and its lasing characteristics are simulated. The laser cavity of the resonance-shifted DFB-LD is divided into two regions. Each region has uniform diffraction gratings, and the corrugation pitch of region 1 is different from that of region 2. By adjusting the corrugation pitches of the two regions, a resonance mode in region 1 and that in region 2 are overlapped. At the overlapped resonance mode, SLM operation is obtained. When the corrugation pitch difference is 0.3nm with κL1=κL2=3 where κ is the grating coupling coefficient; length L1 of region 1 and length L2 of region 2 are about 560μm, the normalized threshold gain difference was 0.509, which is large enough for SLM operation. Under this condition, the light output ratio from the front/rear facets is 5.69. This light output ratio is larger than the highest value of 2.6, which was achieved in a phase-shifted DFB-LD with asymmetrically-pitch-modulated gratings.

Single-frequency distributed Bragg reflector Nd doped silica fiber laser at 930 nm.

We report a single-frequency distributed Bragg reflector (DBR) fiber laser at 930 nm for the first time, to the best of our knowledge. A ∼2.5 cm long commercial highly neodymium-doped silica fiber was utilized as the gain medium to achieve ∼1.9 mW laser output. The single longitudinal mode operation of this laser was verified by a scanning Fabry-Perot interferometer. This fiber laser is suited for seeding high-power 930 nm narrow-linewidth laser amplifiers, which can be used to generate coherent single-frequency pure blue light through frequency doubling.

Broad-stripe single longitudinal mode laser based on metal slots

Single-longitudinal-mode end-emitting laser with 10 periods of metal slots at around 956nm has been fabricated. 100μm wide broad-stripe and ten periods of 9.5μm periodicity metal slots are defined by i-line lithography and dry etching. Experimentally, continuous-wave power of 213mW has achieved, at a slope efficiency of 520mW/A, having a 3dB spectrum width of less than 0.04nm at 900mA, and operating in a stable single longitudinal mode with the side-mode suppression ratio (SMSR) of 42dB. We prove that metal slots introduce sufficient loss into the cavity to filter out the wanted mode, and is more efficient on our chip structure than traditional slot laser. This paper provides a new method for the realizing high power broad-stripe (~100μm) laser and array with single longitudinal mode operation.

Transient mode competition in directly modulated DFB semiconductor laser

A new effect of transient mode competition in directly modulated DFB laser based on equivalent phase-shift (EPS) technique is presented and studied. Since there are multi-order reflections in EPS structure and if the 0th order subgrating is properly designed, the transient lasing of 0th order will occur during the rising time of the injection current. As a result, transient mode competition between -1st order (main mode) and 0th order will occur accordingly. This can consume redundant carrier and suppress the transient relaxation oscillation, which may be applied in some areas like on-off switching modulation of DFB semiconductor lasers. As an example, an equivalent π phase shift (π-EPS) is carefully designed to realize the effect. In such a laser the 0th order wavelength is in the margin of the material gain region and the -1st order wavelength is around the gain peak, while the stable single longitudinal mode (SLM) operation of the -1st order is guaranteed. The simulation investigation is performed. Good results with suppressed relaxation oscillation and 1.25 Gb/s directly on-off modulation (32 dB extinction ratio) are demonstrated. We believe it provides a new kind of method for on-off switching with high extinction ratio and weak relaxation oscillation.

Tunable diode pumped Rb laser with single longitudinal and transverse mode operation

In this paper, a tunable single frequency Rb laser with TEM00 mode is realized. The linewidth is less than 1GHz, with tuning range of 5GHz, and central wavelength drift of less than 95MHz (0.2pm). The laser power is 1.4W, with power fluctuation of less than 2%. Such kinds of tunable DPALs could be potential in applications for laser spectroscopy and quantum optics etc.

Suppressing longitudinal spatial hole burning with dual assisted phase shifts in pitch-modulated DFB lasers

An equivalent corrugation pitch-modulated (ECPM) distributed feedback (DFB) semiconductor laser with two equivalently assisted phase shifts (EAPS) was theoretically studied and experimentally demonstrated. The simulated results showed that the longitudinal photon density distribution of the ECPM + EAPS DFB lasers was much more uniform than that of a sole ECPM DFB laser without EAPS, and the longitudinal spatial hole burning was therefore suppressed more effectively. The results of experiment showed that good single longitudinal mode operation was achieved, with side mode suppression ratio (SMSR) being over 35dB, and the wavelength range was from 1,559.64 to 1,563.02nm when the operation current was from 50 to 160mA at the temperature of 25 °C, and the SMSR increased to 46.29dB when the injection current was 130mA.