Spatial Hole Burning Effect Research Articles

An 8-channel narrow linewidth multi-wavelength laser hybrid integrated by photonic wire bonding structures for DWDM systems

An 8-channel monolithically integrated narrow linewidth multi-wavelength laser array (NL-MLA) combined with the commercial 8 × 1 planar lightwave circuit (PLC) coupler by the photonic wire bonding (PWB) technique is demonstrated in this paper. A distributed feedback (DFB) laser with the high-reflection and anti-reflection coatings (HR-AR) and the tapered asymmetric corrugation-pitch-modulated (TACPM) grating structure is proposed. The TACPM grating is achieved by the reconstruction equivalent chirp (REC) technique, which is low-cost and of high wavelength accuracy. According to the numerical simulation results, under the unavoidable impact of the facet phase of the HR end, the TACPM DFB laser suppresses the longitudinal spatial hole burning (LSHB) effect significantly, while having better single-mode performance and control of wavelength compared with the conventional HR-AR DFB laser. An 8-channel HR-AR TACPM DFB NL-MLA is fabricated with the 0.8 nm wavelength spacing design. The hybrid integration between the 8-channel NL-MLA and the PLC chip is achieved using the PWB technique. Both good single-mode performance and accurate wavelength interval of 0.8 nm can be obtained when all channels work simultaneously. The laser linewidth is below 273.8 kHz with good uniformity, and the relative intensity noise is under −159.6 dB/Hz for each channel.

Impact of Grating Duty-Cycle Randomness on DFB Laser Performance

The duty-cycle randomness (DCR) of the Bragg grating of the distributed feedback (DFB) lasers introduced by the fabrication process is inevitable, even with state-of-the-art technologies such as electron beam lithography and dry or wet etching.This work investigates the impact of grating DCR on DFB laser performance through numerical simulations. The result reveals that such randomness causes a reduction in the side mode suppression ratio (SMSR), and deteriorates the noise characteristics, i.e., broadens the linewidth and increases the relative intensity noise (RIN). With the grating DCR, the effective grating coupling coefficient decreases as evidenced by the reduced Bragg stopband width. However, the longitudinal spatial hole burning (LSHB) effect in the DFB lasers can somewhat be diminished by the grating DCR. The seriousness of these effects depends on different grating structures and their coupling strengths. Our simulation shows that a degradation of 17 dB can be brought to the SMSR of the uniform grating DFB lasers with their duty cycles taking a deviation of ±25% in a uniformly distributed random fashion. It also broadens the linewidth of the quarter-wavelength phase-shifted DFB lasers by more than 2.5 folds. The impact of this effect on the RIN is moderate—less than 2%. All the performance deteriorations can partially be attributed to the effective reduction in the grating coupling coefficient of around 20% by such a DCR.

Exploring the spatial hole burning effect on the mode-locking characteristics of self-mode-locked Nd:YVO4 lasers

The spatial hole burning (SHB) effect on the mode-locking characteristics of Nd:YVO4 self-mode-locked (SML) lasers are investigated. By varying the cavity length, pump power, and crystal temperature, we manipulate the SHB effect on the longitudinal mode distributions and the temporal waveforms of the SML lasers. The experimental results indicate that, at a pump power of 3395 mW and a cavity length of 23 mm, the SHB effect is minimal, the temporal waveform remains stable and the longitudinal mode spacing equals the fundamental longitudinal mode interval (FLMI). As increasing the cavity length to 75 mm, the experimental longitudinal mode spacing becomes 1 to 5 times the FLMI and the temporal waveforms are then multi-pulse mode-locked state, indicating an aggravation in the SHB effect of the SML laser. With the cavity length of 23 mm, as the pump power increases from 3.4 W to 9.7 W, the SHB effect on the mode-locking characteristics gradually also aggravates. The laser gradually changes from the fundamental mode-locked state to the spaced mode-locked state. By adjusting the crystal temperature from 25 °C to 35 °C, the aggravated SHB effect caused by the increase in pump power is counteracted.

Spectral behavior of high-power distributed feedback lasers

The mode hopping behavior of high-power distributed feedback lasers emitting near 780,hbox {nm} is studied. The lasers have highly reflective rear and anti-reflection coated front facets. The influence of the phase of the grating at the rear facet is investigated by means of numerical simulations in the time domain taking into account spatial hole burning and self-heating effects. The simulation results are in agreement with experimental findings.

Spectral narrowing and broadening of Cr:ZnS/Se laser oscillation due to mode competition and spatial hole burning in the gain element.

In this paper, we demonstrate the laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective unpolarized, linearly polarized, and twisted mode cavities. Lasers were based on post-growth diffusion-doped, commercially available antireflective-coated Cr:ZnSe and Cr:ZnS polycrystals with a length of 9 mm. The spectral output of lasers based on these gain elements in non-selective unpolarized and linearly polarized cavities was measured to be broadened to ∼20-50 nm due to the spatial hole burning (SHB) effect. SHB alleviation in the same crystals was realized in the "twisted mode" cavity, with linewidth narrowing to ∼80-90 pm. Both broadened and narrow-line oscillations were captured by adjusting the orientation of intracavity waveplates with respect to facilitated polarization.

Graphene saturable absorber mirror for passive mode-locking of mid-infrared QCLs

Passive mode-locking in quantum cascade lasers (QCLs) remains one of the huge challenges because of the fast relaxation time of the excited carriers which is typically in the range of sub-picoseconds. The use of conventional techniques such as the semiconductor saturable absorber mirror is inefficient because the spatial hole burning effect dominates the carrier dynamics. To overcome this effect, longitudinal transition structures with relaxation time around $$50~\mathrm {ps}$$ were proposed. However, mode-locking is assured with an external modulation at a cavity roundtrip frequency. In this paper, we demonstrate that a single-layer graphene used as a saturable absorber permits to generate stable pulses in such structures. The graphene is integrated with a highly reflective mirror to increase the internal electric field and achieve the saturation intensity. The dynamic of the QCL is modeled with Maxwell-Bloch equations while Maxwell-Ampere equation is used for the graphene layer by considering a nonlinear conductivity. This system of equations is solved using the one-dimensional Finite-Difference Time-Domain ( FDTD) method. To model the single-layer graphene of a $$0.33~\mathrm {nm}$$ thickness, a specific sub-cell is implemented based on Maloney method. Simulation results of a $${\mkern 1mu} 6.2~\mu {\text{m}}$$ QCL with a diagonal radiative transition show a generation of isolated pulses with a peak electric field of $$80~\mathrm {MV.m^{-1}}$$ and a duration of $$51~\mathrm {fs}$$ . The mode-locking remains stable for QCLs with a vertical transition having a relaxation time below $$5~\mathrm {ps}$$ .

Active Modelocking Improvement of MIR-QCL by Integrating Graphene as a Saturable Absorber

The active mode-locking technique of quantum cascade lasers is highly affected by the bias current level and the magnitude of the modulated signal because of the spatial hole burning effect. The laser generates a stable pulse train near the threshold current and for a high modulation magnitude. We show in this paper an improvement of these conditions with an integration of a single-layer graphene which works as a saturable absorber. The light-matter interaction in the laser active region is described by the two-level Maxwell-Bloch equations and in graphene layer by the Maxwell-Ampere equation and Maxwell-Bloch equations. These system equations are solved using the Finite-Difference Time-Domain (FDTD) method which has the main advantage of not involving any approximation. The weakly coupled splitting scheme is adopted for the time discretization of Maxwell and Bloch equations. The graphene saturable absorption is modelled by a saturable conductivity in Maxwell-Ampere equation and with the dipole moment and carrier density in Maxwell-Bloch equations. For a QCL structure with a gain recovery time of 50 ps, simulation results show better stability of the mode-locking over a broad range of injection current and for smaller magnitude of the modulated signal. QCL structure with gain recovery time close to 1 ps reveals the generation of two pulses per roundtrip linked to spatial hole burning. The carrier grating strength can be reduced by changing the boundary conditions of the cavity such as with a high reflectivity mirror in front of graphene layer.

Extreme Double/Triple Asymmetric Epitaxial Structure Based Diode Lasers for High Powers and High Efficiencies

In edge-emitting high power laser diodes, the longitudinal spatial hole burning (LSHB) effect induced by the inhomogeneous photon and carrier distributions and the non-linear process, two-photon absorption (TPA), are demonstrated to the key causes of power saturation at the high injection level even in the absence of thermal roll-over. Laser diodes based on extreme, double/triple asymmetric (EDAS/ETAS) epitaxial structures show great potentials in high-power and high-efficiency applications owing to the reduced series resistance, low optical loss, and suppressed power saturation. In this paper, a novel semi-analytical calculation method is proposed to systematically analyze the impact of LSHB and TPA effects on the output characteristics of EDAS/ETAS-based broad area lasers (BALs). By leveraging the feature that it is flexible to adjust the confinement factor of ETAS epitaxial structure without compromising the internal loss and resistance level, the laser parameters are first optimized to minimize power saturation of ETAS-based BALs under pulsed and Continuous Wave (CW) operation.

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.

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.

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.

Single-longitudinal-mode operation of a 2.09 μm holmium laser with anti-misalignment corner cube ring cavity

A single-longitudinal-mode (SLM) 2.09 μm Ho:YAG laser with anti-misalignment corner cube ring cavity was demonstrated. Unidirectional components composed of a faraday rotator and half-wave plate were used to organize the travelling wave beam propagation and to eliminate the spatial hole-burning effect. The maximum SLM power was 728 mW. The corresponding slope efficiency achieved 23.42% and the optical conversion efficiency was 7.16%. Widely tunable wavelength outputs from 2090.43 nm to 2098.37 nm have been achieved by employing a Fabry–Perot etalon. With the advantage of long cavity length and small volume, this structure of the Ho:YAG SLM laser with corner cube ring cavity is a potentially useful way to ensure narrow linewidth output and strong anti-misalignment stability.

Wavelength-flexible all-polarization-maintaining self-sweeping fiber laser based on intracavity loss tuning

We reported a wavelength-flexible all-polarization-maintaining self-sweeping fiber laser based on the intracavity loss tuning brought by the bent optical fiber. The bidirectional cavity structure achieved the self-sweeping effect due to the appearance of the dynamic grating in the active fiber with the spatial hole burning effect. Under this, a section of fiber was bent into a circle for adjusting the loss of the cavity. With a descending diameter of bent fiber circle, the sweeping range moves to the shorter wavelength and covers a wide range from 1055.6 to 1034.6 nm eventually. Both the initial wavelength of self-sweeping regime and the threshold of the fiber laser show exponential correlation with the diameter of the circular fiber. Our work provides a compact and low-cost way to achieve the broad wavelength-flexible self-sweeping operation.

Competition of whispering gallery lasing modes in microwire with hexagonal cavity

The competition characteristics of different orders of whispering-gallery lasing modes in a single ZnO microwire with hexagonal cavity are investigated comprehensively. As the scale of the cavity increases, the mode competition would be emerging strongly. The high-order whispering-gallery mode (WGM) lasing is indicated to be transverse electric modes with polarization dependence of about 86%. In addition, the competition of whispering-gallery lasing modes depends strongly on the diameter of cavities and the excitation intensities due to the effects of spatial hole burning. The Power Fourier transfer gives the distinct path length of fundamental and high-order optical field. Moreover, the finite difference time domain presents the optical field of high-order WGM lasing that tends to be closer to the center of hexagonal cavities. Remarkably, the Purcell factors of WGM are enhanced strongly with the reduction in diameter of hexagonal cavities, and the maximum value of F p approaches 20.

High power continuous wave operation of single mode quantum cascade lasers up to 5 W spanning λ∼3.8-8.3 µm.

In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.

Narrow-linewidth distributed feedback moiré-grating laser for high-speed optical communications.

A time-domain traveling wave model is described to simulate a novel distributed feedback moiré-grating (DFM) laser, which can suppress the longitudinal spatial hole burning (LSHB) effect and improve the spectral linewidth for high-speed optical communications. The longitudinal photon distribution, linewidth, and dynamic modulation characteristics of the DFM lasers are calculated and compared with the state-of-the-art distributed feedback lasers. We found that the DFM laser can also improve the light-current slope efficiency and modulation bandwidth by introducing multiple $\unicode{x03C0} $π-phase shifts at the moiré envelope index changes while effectively suppressing the LSHB.

Mid-infrared frequency comb from a ring quantum cascade laser

The effect of spatial hole burning on mode proliferation has been shown both theoretically and experimentally to be important for frequency comb operation in quantum cascade lasers (QCLs). It is therefore of great interest to investigate the frequency comb operation from integrated ring cavities where this effect is absent. Here we report a mid-infrared quantum cascade laser based on a buried heterostructure ring cavity that exhibits unidirectional lasing with a frequency comb spectrum displaying a predominantly s e c h 2 profile over a bandwidth of ∼ 10 − 1 . A single beat note with 3 dB linewidth of &lt; 900 H z was observed over the last 85% of the whole current dynamic range. To further verify the frequency comb operation, a multiheterodyne experiment was carried out using a standard Fabry–Pérot QCL comb as a reference laser. This measurement allowed the retrieval of the spectral phases, indicating an operation with strong amplitude modulation.

Equivalent Circuit Model of High-Performance VCSELs

In this work, a general equivalent circuit model based on the carrier reservoir splitting approach in high-performance multi-mode vertical-cavity surface-emitting lasers (VCSELs) is presented. This model accurately describes the intrinsic dynamic behavior of these VCSELs for the case where the lasing modes do not share a common carrier reservoir. Moreover, this circuit model is derived from advanced multi-mode rate equations that take into account the effect of spatial hole-burning, gain compression, and inhomogeneity in the carrier distribution between the lasing mode ensembles. The validity of the model is confirmed through simulation of the intrinsic modulation response of these lasers.

Single-Mode 940 nm VCSELs With Narrow Divergence Angles and High-Power Performances for Fiber and Free-Space Optical Communications

Using the Zn-diffusion and oxide-relief techniques with the optimized aperture sizes, we demonstrate a novel single-mode 940 nm vertical-cavity surface-emitting laser (VCSEL) with high brightness performance. The highly single-mode (SM) output optical spectra (SMSR>50 dB) can be sustained under a full range of bias currents and from room temperature (RT) to 85°C operation. Under RT operation, the maximum SM power can be as high as 7.1 mW with a moderate threshold current ($\text{I}_{\mathrm {th}}$ : 1.1mA) and narrow divergence angles in the far-field pattern (FWHM: 5°, 1/e 2 : 7-8°). Furthermore, the maximum 3-dB E-O modulation bandwidth of this high-power SM VCSEL can reach 15 GHz without the low-frequency roll-off induced by spatial hole burning effect. By using this novel device as the transmitter, we can achieve 25 Gbit/sec error-free (bit-error-ratio (BER) $ 0.3 km) data communications as well as in free-space optical communication.

Theory of Frequency-Modulated Combs in Lasers with Spatial Hole Burning, Dispersion, and Kerr Nonlinearity

Frequency-modulated (FM) frequency combs constitute an exciting alternative to generate equidistant spectra. The full set of Maxwell-Bloch equations is reduced to a single master equation for lasers with fast gain dynamics to provide insight into the governing mechanisms behind phase locking. It reveals that the recently observed linear frequency chirp is caused by the combined effects of spatial hole burning, group velocity dispersion, and Kerr nonlinearity due to asymmetric gain. The comparison to observations in various semiconductor lasers suggests that the linear chirp is general to self-starting FM combs.