Hole Burning Research Articles

Modeling and analysis of narrow-linewidth distributed feedback lasers based on apodized laterally coupled gratings.

A physical model is demonstrated to optimize narrow-linewidth distributed feedback lasers based on apodized laterally coupled gratings (AG-DFB). The structure can effectively suppress the longitudinal spatial hole burning as well as remove the regrowth process during fabrication by using the apodized grating geometry. The studies include numerical simulations of the AG-DFB laser for its static and dynamic behaviors at different cavity lengths and facet coating conditions. The results show that the proposed device can achieve narrow linewidth, high slope efficiency, and broad modulation bandwidth, as compared to λ/4 phase-shifted DFB lasers.

Some open challenges in polymer physics*

AbstractThree subjects in polymer and soft matter physics are outlined in the present article. The first relates to concepts of an ideal glass transition. We describe work on ultra‐stable glass, either a 20‐million‐year‐old amber or a vapor deposited amorphous fluoropolymer, which examines the temperature dependence of the dynamics in a window between a low fictive temperature and the glass transition temperature. From this “finesse” of the problem of the geological time scales in sub‐glass temperature systems strong evidence that the divergence of the relaxation times at finite temperature from WLF‐types of extrapolation is not correct. The second topic is polymer nonlinear viscoelasticity as it relates to dynamic heterogeneity. We show results from mechanical hole burning experiments that suggest that dynamic heterogeneity arises from the nature of specific relaxation mechanisms rather than heterogeneous changes in, for example, the fictive temperature. Included is a discussion of large amplitude oscillatory shear testing and how it relates to characterization of nonlinear viscoelastic materials. The last topic addressed is the rheology of circular macromolecules. Here, we take a historical perspective and describe important new results from several laboratories that seem inconsistent. New works from our own studies on circular DNA are also discussed.

Laser guide star return-flux gain from frequency chirping

ABSTRACT Spectral hole burning reduces sodium laser guide star efficiency. Due to photon recoil, atoms that are initially resonant with the single-frequency laser get Doppler shifted out of resonance, which reduces the return flux. Frequency-chirped (also known as frequency-swept) continuous-wave lasers have the potential to mitigate the effect of spectral hole burning and even increase the laser guide star efficiency beyond the theoretical limit of a single-frequency laser. We investigate the return flux of frequency-chirped laser guide stars and its dependence on environmental and chirping parameters. On-sky measurements of a frequency-chirped, single-frequency laser guide star are performed at the Roque de los Muchachos Observatory on La Palma. A fast photon-counting receiver system is employed to resolve the return-flux response during laser frequency sweeps gaining insights into the population dynamics of the sodium layer. At a launched laser power of 16.5 W, we find a maximum gain in return flux of 22 per cent compared to a fixed-frequency laser at a chirping amplitude of the order of 150 MHz and a chirping rate of 0.8 MHz µs−1. Time-resolved measurements during the chirping period confirm our understanding of the population dynamics in the sodium layer. These are the first measurements of return-flux enhancement for laser guide stars excited by a single-frequency-chirped continuous-wave laser. For higher laser powers, the effectiveness of chirping is expected to increase, which could be highly beneficial for telescopes equipped with high-power laser guide star adaptive optics systems.

Ultra-slow electron spin-lattice relaxation in a low magnetic field enables massive optical spin polarization in the 4A2 ground state of Cr3+ in ruby* *This paper is dedicated to my late friend and mentor Dr Alex Szabo, the originator of transient spectral hole-burning and fluorescence line narrowing in the solid state, whose knowledge of the spectroscopy of ruby is unsurpassed.

Ruby (α-Al2O3 doped with Cr3+) has been an archetypal material in the development of optical spectroscopy of the solid state for the last 150 years and was the first material that was demonstrated to lase. Notwithstanding the vast literature on ruby, one effect was somehow missed: in a magnetic field B ∥c ∼ 235 mT, the spin-lattice relaxation time T 1 for the |+3/2⟩ level in the 4A2 ground state is massively lengthened to ∼12 s at 1.4 K as demonstrated in this study. This very long relaxation time enables optical pumping of the |+3/2⟩ level via the R 1(±1/2) lines and a considerable +3/2 spin polarization of ∼95% is readily achieved. The observed magnetic field dependence can be quantitatively described using the one-phonon relaxation process.

Multi-wavelength thulium-doped fiber laser at 2.05 μm incorporating a superimposed polarization-maintaining fiber Bragg grating

A switchable multi-wavelength thulium-doped fiber laser with a high optical signal-to-noise ratio (OSNR) and stability is proposed and experimentally demonstrated at 2.05 μm. For the first time, a superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG) was utilized as a multi-channel narrow bandwidth filter to select the lasing wavelengths in the laser cavity in the 2-μm band. It also introduced polarization hole burning (PHB) to inhibit the strong wavelength competition induced by homogeneous gain broadening in the thulium-doped fiber. By adjusting the two polarization controllers (PCs), four stable switchable single-wavelength operations were obtained. The OSNR reached 72 dB at a pump power of 2.89 W, and the maximum wavelength and power fluctuations were 0.01 nm and 0.162 dB, respectively. Four combinations of dual-wavelength operations were obtained by adjusting the PCs. The polarization states between each dual-wavelength lasing were orthogonal. The stability of the dual-wavelength operation was investigated in detail. The proposed laser also has the capability for three- or four-wavelength operation. The excellent performance enables this fiber laser to be used in atmospheric sensing and free-space optical communication.

High-Brightness, High-Speed, and Low-Noise VCSEL Arrays for Optical Wireless Communication

The development of high-speed and high-brightness vertical-cavity surface-emitting lasers (VCSELs), which can serve as an efficient light source for optical wireless communication (OWC), play a crucial role in growth of the next generation of wireless communication networks, e.g., 6 G and satellite communications. In this work, by optimizing the size of the Zn-diffusion and oxide-relief apertures in a high-speed 850 nm VCSEL, we obtain record-high brightness (2.9 MW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {cm}}^{-2}{\mathrm {sr}}^{-1}$ </tex-math></inline-formula> at 10 mW output) with single polarized and (quasi-) single-mode (SM) outputs under continuous wave (CW) operation. However, such high brightness output comes at the cost of spatial hole burning (SHB) effect and degraded quality of 25 Gbit/sec eye patterns. In addition, an SM VSCEL array structure is usually needed to further boost the total available optical power for long-reach OWC. Here, a novel (quasi-) SM VCSEL array structure is demonstrated which releases the trade-off between the performances of brightness and eye-pattern quality. Our demonstrated array has a special crisscross mesa connecting neighboring VCSEL units and an extra electroplated copper substrate integrated on the backside of the chip. Compared to the reference array without the copper substrate and connected active mesas, the demonstrated array exhibits a higher (quasi-) SM output power, narrower divergence angle, larger orthogonal polarization mode suppression ratio (OPSR), and flatter E-O response. This in turn leads to smaller jitter and less noise in the measured 12.5 Gbit/sec eye-patterns. The demonstrated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7\times 7$ </tex-math></inline-formula> array exhibits a maximum SM power of around 90 mW with a 1/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{e}^{2}$ </tex-math></inline-formula> divergence angle as narrow as 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> (FWHM: 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> ), single polarized output (10 dB OPSR), decent relative intensity noise performance (<−130 dB/Hz) and clear 12.5 Gbit/sec eye-opening. Such new device with remarkable static/dynamic performances has strong potential to further improve the product of the linking distance and data rate in the next generation of OWC channels.

Burning and reading ensembles of spectral holes by optical frequency combs: Demonstration in rare-earth-doped solids and application to laser frequency stabilization

We describe a spectroscopic method applicable to inhomogeneously broadened transitions. As a test system we use suitable rare-earth-doped solids in which persistent spectral holes can be burned on weakly allowed electronic transitions. These systems are of interest as frequency references for the stabilization of laser frequencies. In all experiments so far, spectral holes are burned and read out individually. Here we present a demonstration of simultaneous burning and readout of sets of spectral holes, containing up to 1024 individual spectral holes, by a comb of laser frequencies. The simultaneous readout of such a set leads to an effective compound signal that has a higher signal-to-noise ratio. Here, an improvement by a factor of 20 was obtained for a set of 512 spectral holes. We use the method to stabilize the frequency of a laser and characterize its stability.

Switchable and tunable linear cavity erbium-doped fiber laser based on FBGs embedded in Sagnac rings

A switchable and tunable erbium-doped fiber laser with a linear cavity based on fiber Bragg gratings embedded in Sagnac rings is proposed and experimentally verified. Due to the stress birefringence effect and the polarized hole burning effect, which are introduced into the single-mode fiber in the polarization controllers (PCs) by the PCs, the designed laser can achieve seven kinds of laser-states output including three kinds of single-wavelength laser states, three kinds of dual-wavelength laser states and one kind of triple-wavelength laser state. The optical signal-to-noise ratios of the output wavelengths are all higher than 52 dB, and the wavelength shifts are all less than 0.04 nm. Furthermore, the temperature tuning of the wavelength range is also researched, which is about 1.2 nm. Due to advantages, such as low cost, simple structure, easy switching and multiple laser states, the designed laser has great application potential in laser radar, optical fiber sensing and so on.

Study the plasmonic property of gold nanorods highly above damage threshold via single-pulse spectral hole-burning experiments

Intense femtosecond laser irradiation reshapes gold nanorods, resulting in a persistent hole in the optical absorption spectrum of the nanorods at the wavelength of the laser. Single-pulse hole-burning experiments were performed in a mixture of nanorods with a broad absorption around 800 nm with a 35-fs laser with 800 nm wavelength and 6 mJ/pulse. A significant increase in hole burning width at an average fluence of 106 J/m2 has been found, suggesting a tripled damping coefficient of plasmon. This shows that the surface plasmonic effect still occurs at extremely high femtosecond laser fluences just before the nanorods are damaged and the remaining 10% plasmonic enhancement of light is at the fluence of 106 J/m2, which is several orders of magnitude higher than the damage threshold of the gold nanorods. Plasmon–photon interactions may also cause an increase in the damping coefficient.

Coherent Surface Plasmon Hole Burning via Spontaneously Generated Coherence.

Surface plasmon (SP)—induced spectral hole burning (SHB) at the silver-dielectric interface is investigated theoretically. We notice a typical lamb dip at a selective frequency, which abruptly reduces the absorption spectrum of the surface plasmons polaritons (SPP). Introducing the spontaneous generated coherence (SGC) in the atomic medium, the slope of dispersion becomes normal. Additionally, slow SPP propagation is also noticed at the interface. The spectral hole burning dip is enhanced with the SGC effect and can be modified and controlled with the frequency and intensity of the driving fields. The SPP propagation length at the hole-burning region is greatly enhanced under the effect of SGC. A propagation length of the order of 600 µm is achieved for the modes, which is a remarkable result. The enhancement of plasmon hole burning under SGC will find significant applications in sensing technology, optical communication, optical tweezers and nano-photonics.

Dynamics and its Effects on Saturation Region in Semiconductor Optical Amplifiers

We focus on studying the dynamics of bulk semiconductor optical amplifiers and their effects on the saturation region for short pulse that differ, however there is the same unsaturated gain for both dynamics. Parameters like current injection, fast dynamics present by carrier heating (CH), and spectra hole burning (SHB) are studied for regions that occur a response to certain dynamics. The behavior of the saturation region is found to be responsible for phenomena such as recovery time and chirp for the pulse under study.

Measurement of the thulium ion spin Hamiltonian in an yttrium gallium garnet host crystal

We characterize the magnetic properties for thulium ion energy levels in the Y$_3$Ga$_5$O$_{12}$ (Tm:YGG) lattice with the goal to improve decoherence and reduce line-width broadening caused by local host spins and crystal imperfections. More precisely, we measure hyperfine tensors for the lowest level of the, $^3$H$_6$, and excited, $^3$H$_4$, states using a combination of spectral hole burning, absorption spectroscopy, and optically detected nuclear magnetic resonance. By rotating the sample through a series of angles with an applied external magnetic field, we measure and analyze the orientation dependence of the Tm$^{3+}$ ion's spin-Hamiltonian. Using this spin-Hamiltonian, we propose a set of orientations to improve material properties that are important for light-matter interaction and quantum information applications. Our results yield several important external field directions: some to extend optical coherence times, another to improve spin inhomogeneous broadening, and yet another that maximizes mixing of the spin states for specific sets of ions, which allows improving optical pumping and creation of lambda systems in this material.

Data Storage in a Nanocrystalline Mixture Using Room Temperature Frequency-Selective and Multilevel Spectral Hole-Burning

Data Storage in a Nanocrystalline Mixture Using Room Temperature Frequency-Selective and Multilevel Spectral Hole-Burning

Triple-wavelength EDF laser based on phase-shifted fiber grating and 50 μm MMF

We propose and experimentally demonstrate a novel switchable triple-wavelength erbium-doped fiber laser using linear cavity structure. It is composed of two Sagnac loops that act as cavity mirrors in the resonant cavity. The left loop consists of a 3 dB fiber coupler (FC), a three-ring polarization controller (PC), and a phase-shifted fiber grating. The right loop consists of a same 3 dB FC, a same PC and a length of multimode fiber with 50 µm core diameter. The two loops based on different operation principles become two comb filters to jointly excite the multiwavelength output. On the role of the polarization hole burning effect, after adjusting the two PCs, the laser can switch flexibly to output three kinds of different modes from single-wavelength to the triple-wavelength. All lasing lines with a narrow linewidth less than 0.4 nm have the optical signal-to-noise ratio of more than 28 dB.

Localized modes revealed in random lasers

In sufficiently strong scattering media, light transport is suppressed and modes are exponentially localized. Anderson-like localized states have long been recognized as potential candidates for high- Q optical modes for low-threshold, cost-effective random lasers. Operating in this regime remains, however, a challenge since Anderson localization is difficult to achieve in optics, and nonlinear mode interaction compromises its observation. Here, we exhibit individually each lasing mode of a low-dimension solid-state random laser by applying a non-uniform optical gain. By undoing gain competition and cross-saturation, we demonstrate that all lasing modes are spatially localized. We find that selective excitation significantly reduces the lasing threshold, while lasing efficiency is greatly improved. We show further how their spatial locations are critical to boost laser power efficiency. By efficiently suppressing the spatial hole burning effect, we can turn on the optimally outcoupled random lasing modes. Our demonstration opens the road to the exploration of linear and nonlinear mode interactions in the presence of gain, as well as disorder-engineering for laser applications.

Molecular-Level Photo-Orientation Insights into Macroscopic Photo-Induced Motion in Azobenzene-Containing Polymer Complexes.

As part of continuing efforts to deepen the understanding of photo-induced mass transport in azo-containing polymers, we compared the diffraction efficiency (DE) during surface-relief grating (SRG) inscription, photo-induced molecular orientation (<P2>), and thermal stability in two sets of supramolecular azopolymer complexes, namely, hydrogen-bonded (H-bonded) and ionically bonded (i-bonded) complexes, both as a function of the polymer degree of polymerization (DP). To that end, poly(4-vinylpyridine) (P4VP) polymers with DPs of 41, 480, and 1900 were H-bonded at an equimolar ratio with 4-hydroxy-4'-dimethylaminoazobenzene (azoOH), and the fully quaternized derivatives of the three P4VPs (P4VPMe) were i-bonded via ion exchange to sodium 4-[(4-dimethylamino)-phenylazo]benzene sulfonate (azoSO3), also known as methyl orange, where the OH functionality of azoOH is replaced by a sulfonate group. The i-bonded complexes show much better DE performances and <P2> levels than those of H-bonded complexes, which we relate to the liquid crystal structure of the former complexes. Fitting the <P2> curves by a biexponential equation leads to two parameters associated with a fast trans-cis or angular hole burning (AHB) process and a slow angular redistribution (AR) process of the azo, respectively. It is found that AHB is predominant in the H-bonded complexes, whereas the AR contribution is much greater in the i-bonded complexes, assuring their superior SRG efficiency that is enabled by the anisotropic free volume created mainly by the AR process. In each set of complexes, the SRG efficiency is much better for the lowest DP complex, while the AR contribution is constant (and low) for the H-bonded complexes and increases roughly linearly with the decrease in DP for the i-bonded complexes. The latter difference might be related to the presence of entanglements in the complexes with DPs 480 and 1900, which slow down the macroscopic movement during SRG inscription but not the molecular-scale movement in photo-orientation.

Research on the Asymmetric Corrugation-Pitch-Modulated HR-AR DFB Lasers With Sampled Gratings

A distributed feedback (DFB) laser for high-speed modulation with asymmetric corrugation-pitch-modulated (ACPM) grating structure and high-reflection and anti-reflection coating (HR-AR) coating is demonstrated theoretically and experimentally in this paper. The ACPM gating structure is fabricated by the reconstruction equivalent chirp (REC) technology, which is with low cost. The impact of the longitudinal spatial hole burning (LSHB) effect and the facet phase on the characteristics of the ACPM DFB laser is discussed in detail. The numerical simulation results show that the proposed ACPM DFB laser can suppress the LSHB effect significantly. Comparing with the conventional HR-AR DFB laser, the ACPM DFB laser can achieve higher output efficiency, better single-mode yield, and wider modulation bandwidth. The modulation eye diagrams of HR-AR DFB lasers with different grating structures are also presented. The experimental results show that the LSHB effect in the HR-AR DFB laser with ACPM structure is suppressed effectively. The laser with ACPM structure fabricated by the REC technique has high slope efficiency, stable single longitudinal mode operation, and wide modulation bandwidth.

Semiconductor Laser Linewidth Theory Revisited

More and more applications require semiconductor lasers distinguished not only by large modulation bandwidths or high output powers, but also by small spectral linewidths. The theoretical understanding of the root causes limiting the linewidth is therefore of great practical relevance. In this paper, we derive a general expression for the calculation of the spectral linewidth step by step in a self-contained manner. We build on the linewidth theory developed in the 1980s and 1990s but look from a modern perspective, in the sense that we choose as our starting points the time-dependent coupled-wave equations for the forward and backward propagating fields and an expansion of the fields in terms of the stationary longitudinal modes of the open cavity. As a result, we obtain rather general expressions for the longitudinal excess factor of spontaneous emission (K-factor) and the effective α-factor including the effects of nonlinear gain (gain compression) and refractive index (Kerr effect), gain dispersion, and longitudinal spatial hole burning in multi-section cavity structures. The effect of linewidth narrowing due to feedback from an external cavity often described by the so-called chirp reduction factor is also automatically included. We propose a new analytical formula for the dependence of the spontaneous emission on the carrier density avoiding the use of the population inversion factor. The presented theoretical framework is applied to a numerical study of a two-section distributed Bragg reflector laser.

Spacially resolved coupled mode analysis for TMI threshold powers in quantum and Rayleigh scattering limits.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability (TMI) threshold powers in Yb-doped fiber amplifiers is presented in this paper. Threshold powers are computed in the quantum and thermal Rayleigh scattering limits and are compared with those calculated by other coupled mode analyses. Quantum-limited threshold powers are found to be more than three times greater than those calculated with coupled-mode analyses that use uniform and/or average gain approximations. The analysis presented here includes pump depletion, gain saturation, and transverse hole burning. Simulations are applied to co-, cnt-, and bidirectionally pump amplifier configurations. The appearance of TMI is attributed to the formation of a dynamic thermal grating, which enables the exchange of optical power between the fundamental mode (FM) and higher-order mode (HOM). The sole approximation made is that the power in the HOM is much less than that in the FM. A distributed thermal Rayleigh scattering model is introduced that includes a ray-optic representation of the fiber mode structure that relates the Rayleigh power captured by the HOM to the waveguide structure. The location and strength of the thermal gratings are identified to assist in the application of mitigation techniques.

Comb operation in terahertz quantum cascade ring lasers

Ring resonators are an interesting alternative cavity solution to the commonly used ridge-type waveguide for terahertz (THz) quantum cascade lasers. They either support a standing-wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. Here, we report on ring-shaped THz quantum cascade lasers emitting between 3.2 and 4.1 THz operating in four different emission regimes. The presence of defects in the cavities force the THz quantum cascade laser into a standing-wave pattern. The measurements show a complex behavior highlighting the effect of strong confinement and the optical nonlinearities leading to the generation of a harmonic state, as well as to a fundamental comb, exhibiting over 30 equidistant modes and covering a bandwidth of 622 GHz. The results are explained by numerical calculations based on the Maxwell–Bloch formalism, including the linewidth enhancement factor and reflection points. The compact geometry and high output power (4 mW detected) make these devices extremely appealing for on-chip frequency comb applications in the terahertz region.