Multi-wavelength Laser Research Articles

Theoretical derivation and application of empirical Harvey scatter model.

Starting from the Rayleigh-Rice perturbation theory, this paper derives the empirical Harvey scatter model and ABg scatter model applied extensively in optical analysis software packages and verifies the shift-invariant behavior of the scattered radiance in direction cosine space. Using data obtained from multi-wavelength laser scatterometer on carbon nanotube black coating and pineblack coating, we establish the polynomial model based on the sine of the scattering angle plus the sine of the specular reflection angle, i.e., sin θs+sin θ0 and the dual-Harvey model based on sin θs-sin θ0 , respectively. The models are in good accordance with the experimental data and further extend the valid range of empirical models.

Agile THz-range spectral multiplication of frequency combs using a multi-wavelength laser

A breakthrough technology, on-chip frequency comb sources offer broadband combs while being compact, energy-efficient, and cost-effective solutions for various applications from lidar to telecommunications. Yet, these sources encounter a fundamental trade-off between controllability and bandwidth: broadband combs, generated in microresonators, lack free-spectral range or spectral envelope control, while combs generated with electro-optic modulators can be carefully tailored but are limited in bandwidth. Here, we overcome this trade-off through agile spectral multiplication of narrowband combs. Exploiting the nonlinear dynamics of a multi-wavelength laser under modulated optical injection, we achieve spectral multiplication at frequency offsets from 26 GHz to 1.3 THz. Moreover, on-chip control allows for nano-second switching of the frequency offset. Compatible with generic platforms, our approach can be scaled up to cover several THz. When combined with THz photomixers, our system could enable low-cost, compact, and power-efficient THz comb sources, paving the way towards a new generation of THz applications.

Multiwavelength fiber laser based on enhanced bidirectional SOA utilizing Lyot filter

We demonstrated a multiwavelength fiber laser (MWFL) based on two bidirectional semiconductor optical amplifiers (SOAs) and a Lyot filter. The structure is enhanced by placing two -SOAs in a bidirectional region to improve the lasing performance. This resulted in a higher extinction ratio (ER) of 36.05 dB, as earlier work only achieved a maximum extinction ratio (ER) of 17 dB utilizing an SOA based on bidirectional configuration. The performance of the MWFL in unidirectional and bidirectional configurations, as well as different parameter settings of SOA current and polarization controller (PC) angle is examined. Using a unidirectional configuration decreases the amplification on one side of the SOA, hence reducing the wavelength bandwidth by 2 nm, which is equivalent to six lasing lines. According to the wavelength bandwidth of 13 nm and channel spacing of 0.22 nm, the optimal performance of MWFL employing bidirectional configuration is 56 lasing lines within 3 dB bandwidth. Consequently, we decreased each SOA's operating current, resulting in lower lasing lines. Changing the PC angle modifies the polarization degree and state, hence impacting multiwavelength flatness and wavelength bandwidth performance. When the polarizer is removed, the ER and lasing lines are decreased to 42 dB and 9 nm, respectively. The MWFL has high stability with a peak power fluctuation of 0.25 dB in 120 min.

Sagnac-interferometer-based multiwavelength SOA fiber laser assisted by an intensity-dependent loss mechanism.

We demonstrate a Sagnac-interferometer-based multiwavelength fiber laser with an intensity-dependent loss (IDL) mechanism in the L-band region using a semiconductor optical amplifier (SOA) as the gain medium. The IDL mechanism flattens and stabilizes the multiwavelength spectrum. We also investigate the effect of rotation angles of polarization controllers (PCs) at different polarization devices on multiwavelength performance. At best settings, 31 lasing lines within 3dB uniformity were generated with an extinction ratio (ER) of 17dB. Adjusting the half-wave plate of PC1 and PC2 from 0° to 90° shifted the multiwavelength output by 0.01nm and 0.072nm, respectively. PC2 adjustment also affects the multiwavelength flatness as compared to PC1. Furthermore, the number of lasing lines and the ER were directly influenced by the SOA current.

Tunable and switchable multi-wavelength random fiber laser based on random distributed feedback and improved Lyot filter

Tunable and switchable multi-wavelength random fiber laser based on random distributed feedback and improved Lyot filter

Highly-reflective facet-coated multi-wavelength DFB laser array with exact wavelength spacings.

A distributed feedback (DFB) laser array of twenty wavelengths with highly reflective and anti-reflective (HR-AR) coated facets is both theoretically analyzed and experimentally validated. While the HR facet coating enhances high wall-plug efficiency, it inadvertently introduces a random facet grating phase, thereby compromising the lasing wavelength's predictability and the stability of the single-longitudinal-mode (SLM). In this study, two key advancements are introduced: first, the precisely spaced wavelength is achieved with an error of within ±0.2 nm using the reconstruction-equivalent-chirp (REC) technique; second, the random grating phase on the HR-coated facet is compensated by a controllable distributed phase shift through a two-section laser structure. The SLM stability can be improved while the wavelength can be continuously tuned to the standard wavelength grid. The overall chip size is compact with an area of 4000 × 500 µm2. The proposed laser array has a light power intensity above 13 dBm per wavelength, a high side mode suppression ratio above 50 dB, and low relative intensity noise under -160 dB/Hz. These attributes make it apt for deployment in DWDM-based optical communication systems and as a light source for optical I/O.

Tunable broadband multi-wavelength Brillouin-Raman random fiber laser based on high-order Raman pump

Tunable broadband multi-wavelength Brillouin-Raman random fiber laser based on high-order Raman pump

Multi-wavelength lasers coupling system used in fluorescent microscopy

Multi-wavelength lasers coupling system used in fluorescent microscopy

Highly efficient tunable multi-wavelength laser properties of Er3+,Yb3+,Pr3+ triple-doped BaLaGa3O7 crystal at ∼2.7 μm

Co-doping with sensitizer Yb3+ and deactivator Pr3+ can effectively improve laser efficiency in the Er/Yb/Pr:BLGO crystal; the maximum output power of 0.81 W was obtained at ∼2.7 μm.

Multiwavelength laser doppler holography (MLDH) in spatiotemporal optical coherence tomography (STOC-T)

Spatiotemporal optical coherence tomography (STOC-T) is the novel modality for high-speed, crosstalk- and aberration-free volumetric imaging of biological tissue in vivo. STOC-T extends the Fourier-Domain holographic Optical Coherence Tomography by the spatial phase modulation that enables the reduction of spatial coherence of the tunable laser. By reducing the spatial coherence of the laser, we suppress coherent noise, and, consequently, improve the imaging depth. Furthermore, we remove geometrical aberrations computationally in postprocessing. We recently demonstrated high-speed, high-resolution STOC-T of human retinal imaging in vivo. Here, we show that the dataset produced by STOC-T can be processed differently to reveal blood flow in the human retina in vivo. To render the blood flow, we first pre-process STOC-T holographic data to access the approximated information about the Doppler-shifted optical field backscattered from the sample. Then, we analyze it using methods from the laser Doppler flowmetry, namely, by analyzing the Doppler broadening caused by moving light scatterers (red blood cells). However, contrary to conventional approaches, we use multiple illumination wavelengths. This enables us to render the structural volumetric and blood flow images from the same dataset concurrently. Our method, denoted as multiwavelength laser Doppler holography (MLDH), links laser Doppler flowmetry with multiwavelength holographic detection to enable noninvasive visualization and possible blood flow quantification at different human retina layers at high speeds and high transverse resolution in vivo.

Scattering characteristics of various nodular defects in a dichroic beam splitter.

Dichroic beam splitters are widely used in multi wavelength laser systems, and their scattering loss affects the signal-to-noise ratio and performance of the system. In this study, we investigate forward and backward scattering induced by nodular defects in a dichroic beam splitter. The seed size, seed position, and geometric constants of nodules exhibited distinct effects on the scattering characteristics. The modeling and simulation provide valuable insights into the relationship between the structural parameters of nodules and their scattering characteristics, offering practical guidance for various high-performance optical multilayer coatings and systems.

Integrated WDM-compatible optical mode division multiplexing neural network accelerator

On-chip photonic neural networks (PNN) are emerging as an attractive solution for artificial neural networks due to their high computing density, low energy consumption, and compact size. Matrix-vector multiplication (MVM) plays a key role in on-chip PNN, and can achieve high-speed multiply-accumulate operation. Most current schemes implement MVM by adopting wavelength division multiplexing technology to accumulate the power of different wavelengths together. This requires multiple laser sources. Additionally, both positive and negative domain MVM are inevitable for realizing precise PNNs, but because of the innate limitations of light, effective solutions to perform negative value computing are still inadequate. Here, we propose and demonstrate a PNN accelerator based on mode division multiplexing technology to reduce the use of multi-wavelength lasers. We show that it can satisfactorily tackle real-number-field computing (including positive and negative domains) based on a novel, to our knowledge, transformation mapping approach. As a proof-of-concept, we demonstrate a fabricated accelerator for image convolution and letter pattern detection, achieving a computing density of 1.37TOPS/mm2 under the 22.38 Gbaud modulation rate.

Order controllable multi-wavelength laser utilizing cascaded diamond Raman conversion

Controllable multi-wavelength gets worse efficiency and quality at high orders and high energy, remaining cascade Raman laser a challenge. Here, we establish an order controllable multi-wavelength cascaded diamond Raman laser to achieve the first-order to fifth-order Raman lasers output with wavelengths of 573.0, 620.3, 676.1, 743.0 and 824.7 nm. At a 532.8 nm pump energy of 1.60 mJ, the total Stokes output energy of 0.51 mJ with a slope efficiency of 36.5% has been achieved. Meanwhile, a Stokes output threshold model of all order is proposed under short pulse pumping for the first time. The threshold energy of the first-order to fifth-order was 256, 298, 320, 494, 1100 μJ separately, which is in consistent with the simulation results perfectly.

Sub-micron inline thickness measurement of cold-rolled metal strips by multi-wavelength interferometry and laser triangulation.

Thin metal foils of thicknesses below 100 µm are finding increasing use in high-tech applications. For such foils it is essential that production be controlled inline with sub-micron accuracy in highly challenging environments. An optical thickness gauge combining laser triangulation with multi-wavelength interferometry has now been developed for this purpose. Modulation-based 2f-3f-interferometry was used to realize a compact and robust sensor. A thorough measurement uncertainty analysis of the complete thickness measurement process yielded an expanded measurement uncertainty of U=(0.30μm)2+4π R a2, which is dependent on the roughness average Ra. The influence of oil remnants on measurement results is significantly weaker in the interference measurement than in geometric optical systems. Verification measurements against tactile reference measurements support the derived measurement uncertainty, and initial measurements in actual rolling mill environments have proven the real-world capability of this measurement technique over relevant process time scales at metal strip speeds of 200 m/min.

Electrically Tunable Two-Color Cholesteric Laser.

Two-color lasing emission from an asymmetric structure, consisting of two dye-doped cholesteric liquid crystal (DD-CLC) layers separated by a transparent interlayer, is demonstrated. The DD-CLC mixtures have different reflection bands with long-wavelength band edges located at the green and red wavelengths of the visible spectrum, respectively. For the laser action, the CLC hosts provide the feedback, and the fluorescent laser dyes represent the active medium. When the stacked structure is optically pumped above the threshold, two simultaneous laser lines separated by 123 nm are observed at the long-wavelength band edges of the DD-CLC mixtures. The influence of an electric field on lasing behavior is also analyzed and discussed in terms of the reflection spectrum and laser action. The results show a reversible tuning of the reflection band, accompanied by a modification of the lasing characteristics under the application of an external field. Above a specific threshold voltage, one of the emission lines is suppressed and the other is conserved. With a further increase in the voltage, both laser emissions are entirely inhibited. The investigated structure demonstrates a simple technique to obtain an electrically tunable multi-wavelength laser, which might pave the way for a new generation of organic laser sources.

High-power dual-wavelength intracavity diamond Raman laser

Diamond crystals have garnered significant attention for applications in photonics, owing to their characteristics of ultra-high thermal conductivity, extremely broad spectral transmission range, and high Raman gain coefficient. These characteristics are particularly advantageous for the development of high-power, multi-wavelength lasers. In this work, we utilize single crystal diamond as a Raman gain medium, for intracavity wavelength-conversion of emission from a side-pumped, electro-optically Q-switched Nd:YAG module. Cascaded Raman conversion is demonstrated and dual-wavelength, pulsed laser emission at wavelengths of 1.2 μm (first-Stokes wavelength) and 1.5 μm (second-Stokes wavelength) is achieved (with a maximum combined pulse energy of 6.19 mJ). The relative ratio of output energy at the two Stokes orders could be varied by tuning the characteristics of the output coupling mirror and the length of the laser resonator. When operating at maximum pump power, the pulse width of the second-Stokes field was approximately half that of the first-Stokes field (second-Stokes ∼10 ns, first-Stokes ∼18 ns), resulting in maximum peak powers of 174 kW (at 1.2 μm) and 220 kW (at 1.5 μm). To the best of our knowledge, this is the highest reported peak power generated from a dual-wavelength, intracavity diamond Raman laser. This work highlights the diversity and utility of diamond as an effective wavelength-conversion component in high-power lasers, and stands as the current state-of-the-art in high pulse-energy, high peak-power, multi-wavelength intracavity Raman lasers.

Broadband multi-wavelength Brillouin-Raman fiber laser with frequency switching

We purpose and demonstrate the switchable multi-wavelength Brillouin–Raman fiber laser (MBRFL) through a bi-directional Raman pumping scheme. The laser structure is arranged in a linear cavity by including a physical mirror at one side of the cavity. The switching operation for MBRFL with single- and double-wavelength spacing is implemented by optimizing the Raman power distribution through a variable optical coupler. This effect on feedback power of the physical mirror provides the difference between odd- and even-order Stoke lines’ maximum power on different sides of the cavity with 10 GHz and 20 GHz spacing. A 90/10 coupler is found to be the optimal. Up to 460 flat-amplitude lines within only a 0.5-dB flatness range, average −5 dBm Stokes peak power (SPP), 10 GHz frequency spacing, and an average optical signal-to-noise ratio (OSNR) of 26 dB are observed. All the counted laser lines are spread across a 37 nm bandwidth. Simultaneously, 170 Stoke lines with overall −2 dBm SPP, 28 dB OSNR, and 20 GHz frequency spacing are attained on other side of the cavity. These are achieved when the Raman pump power is set only at 900 mW. To date, this is the simplest cavity design with the flattest spectrum and highest output power for both wavelength spacing and excellent OSNR achieved in multi-wavelength fiber lasers that incorporate a single low-power Raman pump unit.

Rayleigh backscattering-based simultaneous linewidth narrowing of a multi-wavelength DFB laser array with an arbitrary wavelength spacing.

Simultaneous linewidth narrowing of a multi-wavelength laser array with an arbitrary wavelength spacing based on Rayleigh backscattering is experimentally demonstrated. Rayleigh backscattering from a single 30 m high numerical aperture fiber (HNAF) is employed to simultaneously narrow the linewidths of a DFB laser array consisting of four distributed feedback (DFB) semiconductor lasers with different wavelengths. Experimental results show that the instantaneous linewidths of the four DFB lasers can be simultaneously narrowed from megahertz to kilohertz no matter whether the wavelength spacing between the lasers is equally spaced or not, verifying the self-adaptivity of Rayleigh backscattering on laser linewidth narrowing. The method demonstrated here is also applicable for on-chip waveguides without wavelength dependence, providing a more compact narrow linewidth laser array for the wavelength-multiplexing division system and other promising applications.

Enhancement of the stimulated Raman scattering of aqueous sulfuric acid solutions via cascaded resonance

In the present study, we propose a scheme for generating multi-mode resonance-enhanced stimulated Raman scattering (SRS) of H2O in a low concentration (6.5 mol/L) sulfuric acid aqueous solution. The SRS of H2O exhibits a prominent vibrational feature peak at 3371 cm−1 and a secondary peak at 3325 cm−1. The closeness of the vibrational frequencies for the H–O stretching modes in different water structures allows for resonance interaction. The enhanced vibrational mode can serve as a “new starting point” for a cascaded resonance process, resulting in the generation of eight SRS Stokes signals with a normal distribution centered on the characteristic Raman peak at 3371 cm−1. This research holds potential implications for the development of multi-wavelength and significant frequency shift Raman lasers.

Multi-wavelength sub-100 Hz linewidth Brillouin fiber laser with cascaded SBS polarization parity-time symmetry

A multi-wavelength sub-100 Hz linewidth Brillouin fiber laser (BFL) with cascaded stimulated Brillouin scattering (SBS) polarization parity-time (PT) symmetry is proposed and experimentally investigated. Multi-order Stokes wave generation using a structure consisting of a main fiber ring (MFR) and a sub fiber loop (SFL). The MFR is used to excite the Stokes laser and to guarantee single longitudinal mode (SLM) operation of the generated stokes laser of each order and the SFL cascades each Stokes lasing emission to act as a Brillouin pump for the next order of Stokes generation. A Sagnac loop in the MFR consisting of optical coupler (OC), polarization beam splitter (PBS) and two polarization controllers (PCs), is used to achieve cascaded SBS polarization PT symmetry. By tuning the PC to control the polarization state of each order linearly polarized light injected into PBS at each order, thus controlling the gain and loss of the loop corresponding to the two backpropagating light waves. When the gain and loss of the coupling loops are the same in magnitude and greater than the coupling coefficient, the cascaded SBS polarization PT symmetry is broken, resulting in SLM operation for each order of Stokes waves. Experimental results reveal that thirteen Stokes waves were obtained from two different output ports at an erbium-doped fiber amplifier (EDFA) output power of 28 dBm. The measured linewidths at — 20 dB power were 0.34, 0.39, 0.33, 0.4, 0.39 and 0.43 kHz for the 1st-6th order Stokes emission, corresponding to linewidths of 17.1, 19.6, 16.6, 20.1 19.6 and 21.6 Hz, respectively, while the maximum side mode suppression ratios (SMSRs) were measured to be 47, 45, 49, 44, 41 and 41 dB for the 1st-6th order Stokes emission. Within 60 min of the stability experiment, the wavelength and power stability were ±2 pm and ±0.6 dB, respectively. Compared with previous studies, the present design guarantees the operation of SLM for each order of Stokes waves with sub-100 Hz laser linewidth.