Hole Burning Research Articles

Spatially Resolved Brillouin Spectral Hole Burning in PMF and SMF

We observe the spectral hole burning effect in a polarization maintain fiber (PMF) and a single-mode fiber (SMF) through the interaction between a continuous wave (CW) beam and a high power pulse with two frequency components located around the Stokes and anti-Stokes frequencies of the CW beam. The linewidth of the spectral burning hole is much narrower than that of the intrinsic Brillouin gain spectrum. We experimentally study the position dependent dip formation process in a PMF and an SMF, and explore the possibility of using this narrowed spectral width for distributed temperature and strain sensing with a higher accuracy. A hole burning peak with a linewidth of 9 MHz in a Brillouin gain spectrum with a full-width at half-maximum (FWHM) of 50 MHz is observed in a PMF. Since the frequency measurement accuracy of Brillouin sensor is inversely proportional to the FWHM of detected spectrum, the spectrum hole burning represents a potential for improving the Brillouin peak detection accuracy of distributed Brillouin sensor.

Dual-wavelength single-longitudinal-mode erbium-doped fiber laser with Mach–Zehnder interferometer and saturable absorber loop

A stable dual-wavelength erbium-doped fiber laser (DW-EDFL) in the single longitudinal mode (SLM) with a Mach–Zehnder interferometer (MZI) and a saturable absorber loop (SAL) is proposed and demonstrated experimentally. Dual wavelengths are obtained with different MZI arm lengths. After the first mode selection of the tunable fiber Bragg grating, SLM operation of the dual-wavelength is acquired by the SAL based on the Vernier effect and spatial hole burning, which induce an auto-tracking narrow bandwidth filter with an optical coupler and a 2 m unpumped erbium-doped fiber (EDF). The output of the DW-EDFL shows good performance, achieving 1.08 kHz at dual wavelengths of 1558.67 nm and 1558.77 nm. The interval of the dual wavelength is 0.1 nm, which has a good agreement with the theoretical value, and the peak powers are −5.508 dBm and −5.993 dBm, respectively under 170 mW pump power. Meanwhile, because of the automatic tracking of the wavelength of the SAL, the optical power instability and beating frequency fluctuation of the dual wavelength are less than 0.5 dB and 0.5 MHz, respectively, showing favorable stability.

Mechanical spectral hole burning of an entangled polymer solution in the stress-controlled domain.

Nonresonant spectral hole burning has proven to be a versatile method to characterize the dynamics of complex fluids, including polymers. Early work focused on dielectric susceptibility measurements in glass forming liquids, while recent work from our lab used mechanical viscoelastic measurements to investigate polymer melts and solutions. While the observed results were similar, the interpretations were different, with the former being interpreted by attributing the "holes" in the response as being due to dynamic heterogeneity in the system that is related to the glass or other transition, while the latter was interpreted in a way that suggested that the observed holes depend on the type of dynamics (Rouse, terminal, etc.) rather than an identifiable spatial heterogeneity. In this work, we have expanded mechanical spectral hole burning (MSHB) into the stress-controlled domain and carried out experiments in the rubbery regime of a polystyrene solution, similar to one which was tested previously with strain-controlled MSHB. The effects of pump stress amplitude, pump frequency, and waiting time were investigated. The mechanical holes in both directions (vertical and horizontal) were successfully burned, unlike the strain-controlled MSHB experiments on the same polystyrene solution, in which vertical holes were at best incomplete. The hole intensity exhibits a linear relationship with the amount of energy dissipated in the system during the large mechanical pump modification. The results suggest that the stress-controlled MSHB can be combined with strain-controlled MSHB to build a more complete framework to investigate the dynamics of polymeric materials and is consistent with the dynamic heterogeneity being related to the type of dynamics rather than to localized heating effects.

Elimination of Spatial Hole Burning in Microlasers for Stability and Efficiency Enhancement

Optical gain and loss are considered as a pair of feuds. It is widely believed that optical loss is the most detrimental factor in laser systems that decreases the laser efficiency. The hitherto approach is to minimize or even avoid any loss in designing any laser systems. From an opposite vantage point, however, we demonstrate that harnessing additional optical losses enables an energy-efficient microlaser with a robust single-mode lasing action even in the nonlinear regime above lasing threshold. This is due to elimination of the undesired spatial hole burning effect by tailoring the complex index modulations at an exceptional point (EP), thereby spatially maximizing the interaction of optical gain material with light field in a homogeneous fashion. Approximately 2× enhancement in the laser slope efficiency is observed, enabled by elimination of spatial hole burning. This result is expected to reposition a strategic role of optical losses in laser systems, advancing fundamental laser physics and promisi...

A Coupled Optoelectronic Oscillator With Performance Improved by Enhanced Spatial Hole Burning in an Erbium-Doped Fiber

A coupled optoelectronic oscillator (COEO) with performance improved by the enhanced spatial hole burning (SHB) effect in an unpumped erbium-doped fiber (EDF) is proposed and demonstrated. In the proposed COEO, a strong standing-wave forming structure is incorporated to enable large SHB effect in the unpumped EDF, which significantly improves the supermode suppression ratio of the generated optical pulse, and the sidemode suppression ratio and phase noise performance of the generated electrical signal. A 10-GHz COEO is established. A stable optical pulse train is successfully generated with a supermode suppression ratio as high as 75 dB. The system is observed in the lab environment for 1.5 h with no significant variation. Without introducing any long fiber or multiloop structure in the optoelectronic oscillator loop, the sidemode suppression ratio of the 10-GHz RF output reaches 90 dB and the phase noise is about −130 dBc/Hz at 10-kHz offset. The influence of the optical power injected into the EDF, and the effects of the absorption coefficient and length of the EDF on the key parameters of the COEO are investigated and discussed.

Impact of Single-Point Mutations on the Excitonic Structure and Dynamics in a Fenna–Matthews–Olson Complex

Hole burning (HB) spectroscopy and modeling studies reveal significant changes in the excitonic structure and dynamics in several mutants of the FMO trimer from the Chlorobaculum tepidum. The excited-state decay times ( T1) of the high-energy excitons are significantly modified when mutation occurs near bacteriochlorophyll (BChl) 1 (V152N mutant) or BChl 6 (W184F). Longer (averaged) T1 times of highest-energy excitons in V152N and W184F mutants suggest that site energies of BChls 1 and 6, believed to play an important role in receiving excitation from the baseplate BChls, likely play a critical role to ensure the femtosecond (fs) energy relaxation observed in wild-type FMO. HB spectroscopy reveals preferentially slower T1 times (about 1 ps on average) because fs times prohibit HB due to an extremely low HB quantum yield. Uncorrelated (incoherent) excitation energy transfer times between monomers, the composition of exciton states, and average, frequency-dependent, excited-state decay times ( T1) are discussed.

Modeling of pump wavelength-dependent spectral hole-burning in EDFA

Throughout this paper, considering inhomogeneous broadening in amplification and pump bands, the dynamical behavior of an erbium-doped fiber amplifier is investigated. To this end, equations of propagation and populations are obtained with the erbium atoms being treated as three-level atoms. The governing equations comprise a system of infinitely many PDEs which reduce to finitely many PDEs using some mathematical tricks. The phenomena of spectral hole-burning and pump-mediated inhomogeneity are inspected by solving these equations through using the finite-difference method and calculating the gain difference spectrum.

Photoluminescence and gain/absorption spectra of a driven-dissipative electron-hole-photon condensate

We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.

Temporal characteristics of quantum cascade laser frequency modulated combs in long wave infrared and THz regions.

We consider here a time domain model representing the dynamics of quantum cascade lasers (QCLs) generating frequency combs (FCs) in both THz and long wave infrared (LWIR λ = 8-12µm) spectral ranges. Using common specifications for these QCLs we confirm that the free running laser enters a regime of operation yielding a pseudo-randomly frequency modulated (FM) radiation in the time domain corresponding to FCs with stable phase relations in the frequency domain. We provide an explanation for this unusual behavior as a consequence of competition for the most efficient regime of operation. Expanding the model previously developed in [Opt. Eng. 57(1), 011009 (2017)] we analyze the performance of realistic THz and LWIR QCLs and show, despite the vastly different scale of many parameters, that both types of lasers offer very similar characteristics, namely FM operation with an FM period commensurate with the gain recovery time and an FM amplitude comparable with the gain bandwidth. We also identify the true culprit behind pseudo-random dynamics of the FM comb to be spatial hole burning, rather than the more pervasive spectral hole burning.

What Causes the Pulse Power Saturation of GaAs-Based Broad-Area Lasers?

With short current pulses, the output power of semiconductor lasers is limited by non-thermal internal loss mechanisms. Two-photon absorption (TPA) was recently proposed as dominating photon loss process. We investigate this claim by advanced numerical laser simulation that includes all relevant mechanisms self-consistently. Using common material parameters, we obtain good agreement with the measured performance up to 90-W output power. Carrier leakage and free-carrier absorption are identified as the main saturation mechanisms, followed by longitudinal spatial hole burning. Contrary to earlier studies, TPA and gain compression are found negligible.

Rate-equation theory of a feedback insensitive unidirectional semiconductor ring laser.

For our recently designed continuous-wave and single-frequency ring laser with intra-cavity isolator, we have formulated a rate-equation theory which accounts for two sources of mutual back-scattering between the clockwise and counterclockwise modes, i.e. induced by side-wall irregularities and due to inversion-grating-induced spatial hole burning. With this theory we first confirm that for a ring laser without intra-cavity isolation, from sufficiently large pumping strength on, the inversion-grating-induced bistable operation (i.e. either clockwise or counterclockwise) will overrule the back-reflection-induced coupled-mode operation (i.e. both clockwise and counterclockwise). We then analyze the robustness of unidirectional operation in case of intra-cavity isolation against the intra-cavity back-reflection mechanism and grating-induced mode coupling and derive for this case an explicit expression for the directionality in the presence of external optical feedback, valid for sufficiently strong isolation. The predictions posed in the second reference remain unaltered in the presence of the mode coupling mechanisms here considered.

Rate equation reformulation including coherent excitation: application to periodic protocols based on spectral hole-burning

A large number of signal-processing protocols are based on recording a spectral pattern via spectral hole-burning in an inhomogeneously broadened absorption profile. We present a simulation method specifically designed for periodic excitation sequences leading to the creation of a spectral pattern. This method is applicable to any multilevel atomic structure. The atomic variables’ coherent dynamics are solved for a single temporal excitation step. The result is expressed as an equivalent population transfer rate. This way, the whole sequence is described as a matrix product and the steady state of the system under periodic excitation is easily derived. The propagation through the atomic medium is fully decoupled from the temporal evolution. We apply this method to the engraving of a spectral grating in a large-absorption Tm:YAG sample for wideband spectral analysis.

Controlled hole burning for a cavity field with a single atom

We present a simple method of realizing controlled hole burning for a cavity mode in Fock space by using a single two-level atom. In the scheme, the atom is driven by a classical field and dispersively interacts with the cavity mode. Due to the photon number-dependent atomic transition frequency shift, the atom is excited only for a specific Fock state. As a result, the detection of the atom remaining in the ground state projects the cavity to the state with a desired hole in Fock space. An important feature of the scheme is that multiple holes can be burnt at the same time with a single atom driven by a pulse composed of multiple frequency components.

Deep tissue imaging with acousto-optical tomography and spectral hole burning with slow light effect: a theoretical study.

Biological tissue is a highly scattering medium that prevents deep imaging of light. For medical applications, optical imaging offers a molecular sensitivity that would be beneficial for diagnosing and monitoring of diseases. Acousto-optical tomography has the molecular sensitivity of optical imaging with the resolution of ultrasound and has the potential for deep tissue imaging. Here, we present a theoretical study of a system that combines acousto-optical tomography and slow light spectral filters created using spectral hole burning methods. Using Monte Carlo simulations, a model to obtain the contrast-to-noise ratio (CNR) deep in biological tissue was developed. The simulations show a CNR > 1 for imaging depths of ∼5 cm in a reflection mode setup, as well as, imaging through ∼12 cm in transmission mode setups. These results are promising and form the basis for future experimental studies.

Time-dependent population inversion gratings in laser frequency combs

In standing-wave lasers, spatial hole burning induces a static grating of the population inversion, enabling multimode operation with several independent lasing modes. In the presence of a mode-locking mechanism, these modes may become correlated, giving origin to a frequency comb. Quantum cascade lasers, owing to their ultrafast gain dynamics, are ideally suited to achieve comb operation. Here we experimentally demonstrate that the modes of a quantum cascade laser frequency comb coherently beat to produce time-dependent population inversion gratings, which spatially modulate the current in the device at frequencies equal to the mode separation and its higher harmonics. This phenomenon allows the laser to serve as a phased collection of microwave local oscillators and is utilized to demonstrate quadrature amplitude modulation, a staple of modern communications. These findings may provide for a new class of integrated transmitters, potentially extending from the microwave to the low terahertz band.

Comparative study of effects of group-velocity dispersion on midinfrared quantum-cascade lasers with Fabry–Perot and ring cavities

We focused on the effects of group-velocity dispersion (GVD) on the coherent pulse progression in midinfrared quantum-cascade lasers (QCLs). We carried out the study on Fabry–Perot (FP) cavities. Comparison of GVD effects on the two kinds of typical QCL cavities, i.e., FP and ring cavities, brings insight into the interaction between the GVD and spatial hole burning (SHB) effect, which is only supported by FP cavities but not ring cavities. The theoretical model is built based on the Maxwell–Bloch formulism accounting for two-way propagations of electric field and polarization as well as the couplings among the electric field, polarization, and population inversion. The pulse evolution in time–spatial domains is simulated by the finite-difference method with prior nondimensionalization, which is necessary for a convergent solution. Results predict that the SHB could broaden the QCL gain bandwidth and induce additional side modes closely around the central lasing mode with an intensity more pronounced than that of GVD associated side modes. Moreover, owing to the SHB, the lasing instability caused by GVD is weaker in an FP cavity than a ring cavity.

Spatial hole burning in thin-disk lasers and twisted-mode operation.

Spatial hole burning prevents single-frequency operation of thin-disk lasers when the thin disk is used as a folding mirror. We present an evaluation of the saturation effects in the disk for disks acting as end mirrors and as folding mirrors, explaining one of the main obstacles toward single-frequency operation. It is shown that a twisted-mode scheme based on a multi-order quarter-wave plate combined with a polarizer provides an almost complete suppression of spatial hole burning and creates an additional wavelength selectivity that enforces efficient single-frequency operation.

Photodissociation Spectroscopy of Cold Protonated Synephrine: Surprising Differences between IR-UV Hole-Burning and IR Photodissociation Spectroscopy of the O-H and N-H Modes.

We report the UV and IR photofragmentation spectroscopies of protonated synephrine in a cryogenically cooled Paul trap. Single (UV or IR) and double (UV-UV and IR-UV) resonance spectroscopies have been performed and compared to quantum chemistry calculations, allowing the assignment of the lowest-energy conformer with two rotamers depending on the orientation of the phenol hydroxyl (OH) group. The IR-UV hole burning spectrum exhibits the four expected vibrational modes in the 3 μm region, i.e., the phenol OH, Cβ-OH, and two NH2+ stretches. The striking difference is that, among these modes, only the free phenol OH mode is active through IRPD. The protonated amino group acts as a proton donor in the internal hydrogen bond and displays large frequency shifts upon isomerization expected during the multiphoton absorption process, leading to the so-called IRMPD transparency. More interestingly, while the Cβ-OH is a proton acceptor group with moderate frequency shift for the different conformations, this mode is still inactive through IRPD.

Generation of double giant pulses in actively Q-switched lasers

Generation of a second giant pulse in a longitudinal mode neighbouring to the longitudinal mode possessing minimal losses is theoretically and experimentally studied in actively Q-switched lasers. A mathematical model is suggested for explaining the giant pulse generation in a laser with multiple longitudinal modes. The model makes allowance for not only a standing, but also a running wave for each cavity mode. Results of numerical simulation and data of experiments with a Nd : YLF laser explain the effect of second giant pulse generation in a neighbouring longitudinal mode. After a giant pulse in the mode with minimal losses is generated, the threshold for the neighbouring longitudinal mode is still exceeded due to the effect of burning holes in the population inversion spatial distribution.

Gain investigation of Perylene-Red-doped PMMA for stimulated luminescent solar concentrators.

Luminescent solar concentrators (LSCs) utilizing stimulated emission by a seed laser are a promising approach to overcome the limitations of conventional LSCs, with a significant reduction of the photovoltaic material. In our previous work, we demonstrated the principle of a stimulated LSC (s-LSC) and correspondingly developed a model for quantifying the output power of such a system, taking into account different important physical parameters. The model suggested Perylene Red (PR) dye as a potential candidate for s-LSCs. Here, we experimentally investigate the gain of PR-doped polymethyl methacrylate (PMMA) required for s-LSCs using a single pump wavelength (instead of the solar spectrum) as a proof of principle. The results found from the experiment are well matched with the previously developed numerical model except for gain saturation, which occurs at a comparatively small seed laser signal power. To investigate the gain saturation, two approaches were taken: investigating (i) spectral hole burning and (ii) triplet state absorption. Experimental investigation of spectral hole burning with PR dyes showed a small effect on the gain saturation. We developed a general state model considering triplet state absorption of the PR dyes for the second approach. The state model suggests that the PR dyes suffer from significant triplet state absorption loss, which obstructs the normal operation of the PR-based s-LSC system.