Fundamental Linewidth Research Articles

321 W high-efficiency continuous-wave green laser produced by single-pass frequency doubling of narrow-linewidth fiber laser.

In this study, a high-power continuous-wave green laser for copper processing is investigated. The laser is produced by single-pass second-harmonic generation (SHG) of a narrow-linewidth fiber laser. Based on Sellmeier equations, the factors (e.g., fundamental wave linewidth, temperature, and angle) that decide LBO noncritical phase matching are theoretically analyzed for optimizing the SHG setup. A narrow-linewidth polarization-maintained fiber laser is used as the fundamental laser with a linewidth of 20 GHz and polarization extinction ratio of greater than 15 dB. Type I noncritical phase-matching LBO is used as the nonlinear crystal in the SHG. A green laser (up to 321 W) is obtained from a 784 W fundamental laser with a conversion efficiency of 40.9%. The green laser is near-diffraction-limited and has a $ M^{2} $ factor of 1.07.

Single-longitudinal-mode cascaded crystalline Raman laser at 1.7 µm.

A single-longitudinal-mode crystalline Raman laser in the 1.7 µm wave band was reported for the first time, to the best of our knowledge. The YVO4 Raman laser, which was intracavity-pumped by an actively Q-switched 1314 nm Nd:YLF laser, demonstrated the cascaded Stokes oscillation at 1715 nm. By inserting an etalon in the fundamental resonator, linewidth narrowing and power scaling of the second-Stokes laser were realized based on the spatial-hole-burning-free Raman gain. With an optimal pulse repetition frequency of 4 kHz, the maximum single-longitudinal-mode average output power of 1.8 W was acquired with the spectrum linewidth of ∼340MHz. Further increasing the incident pump power, the second-Stokes laser transitioned to multimode regime, and the maximum average output power reached 2.7 W with the peak power as high as ∼380kW.

(Invited) Temperature Stable III-V/Si/Si3N4 Heterogeneous Integrated Laser

Silicon nitride (Si3N4) is a widely used complementary metal–oxide–semiconductor (CMOS) compatible material in modern IC industry and recent years have witnessed tremendous development of Si3N4 in photonics. The benefits of Si3N4 in photonics include low loss, low thermo optic coefficient and negligible nonlinear absorption at telecom wavelengths. Devices based on Si3N4 waveguides innovates in the field of nonlinear photonics, sensing, microwave photonics and so on. However, they are all dependent on the excellent passive properties of Si3N4. There are no successful demonstrations of fully integrated laser with Si3N4 yet.For an external cavity laser design, it is beneficial to have a low loss waveguide. A low loss waveguide platform can enable a long optical cavity which can greatly reduce the laser phase noise. Butt-coupled hybrid integrated lasers are extensively studied using Si3N4 cavity, but they are based on separate chips and are not scalable for large volume integration.Heterogeneous integration of III-V material and Si has achieved great success in the past years, offering the silicon photonics industry with low-cost, high-performance silicon photonic integrated circuits fully integrated with lasers. For the heterogeneous integration of III-V material with Si3N4, the biggest challenge is the large index mismatch. Typical III-V epi used for heterogeneous lasers is about 2 μm thick and extreme tapering cannot facilitate a high coupling efficiency between the III-V layer and Si3N4 layer.In this work, we used an intermediate Si layer to bridge the index mismatch between the III-V and Si3N4 layer. By using multilayer heterogeneous integration shown in Fig.1 (a), we enable a III-V/Si/Si3N4 structure to efficiently couple the mode between III-V/Si and Si/Si3N4. Based on this structure, we designed the III-V/Si/Si3N4 laser using an external Si3N4 spiral grating as the feedback cavity. This spiral grating provides the narrow-band optical feedback and determines the laser wavelength. The laser configuration is shown in Fig. 1(b).The laser performance is summarized in Fig. 1 (c)-(e). The lasing wavelength is at 1546 nm and the laser has over 58 dB side mode suppression ratio (SMSR). This shows the excellent mode selectivity from the 20 mm long Si3N4 spiral grating. We measured the frequency noise of the laser. With optimized loop mirror reflectivity, the laser fundamental linewidth is about 4 KHz. This linewidth can be further improved by lowering the Si3N4 waveguide loss. We anticipate the laser fundamental linewidth can be reduced to below 100 Hz using this design.Another advantage of integrating Si3N4 into the laser cavity is to have much higher temperature stability. By comparison, III-V/Si/Si3N4 laser wavelength shifts by only 0.47 nm for 45 oC change, while for a III-V/Si DBR laser this wavelength shift is 3.3 nm. This is over 7x difference and is a result of the low thermo-optic coefficient of Si3N4. When combined with Si3N4 based arrayed waveguide gratings, our laser can be potentially used in dense wavelength-division-multiplexing (DWDM) systems.The successful demonstration of this multilayer heterogeneous integration will enable a whole new class of devices that require low loss waveguides and are presently not integrated. This is a key step in fully exploiting the capabilities of wafer-bonding technologies. We anticipate this platform can take narrow-linewidth semiconductor laser performance to a new level. Figure 1

Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

Monolayer Transition Metal Dichalcogenides (TMDS) are highly luminescent materials despite being sub-nanometer thick due to the ultra-short ($<1$ ps) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $(\hbar cQ)$ as the generalized lower bound for allowed radiative recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the non-radiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of $1.54\pm0.17\ $meV, owing purely to finite radiative lifetime in the absence of non-radiative dephasing processes. As a direct consequence of the large radiative limit, we find a surprisingly large ($\sim 0.27 $ meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the non-radiative linewidth broadening at $T = 0$ K.

Wavelength tuning and power enhancement of an intracavity Nd:GdVO4-BaWO4 Raman laser using an etalon.

We report the wavelength tuning, linewidth narrowing and power enhancement of a continuous-wave intracavity Raman laser by incorporating solid etalons in the high-Q fundamental resonator. With a-cut Nd:GdVO4 and a-cut BaWO4 serving as the laser and Raman crystals respectively, tilting of a 50 μm-thick etalon in the high-Q fundamental cavity enabled the fundamental to be tuned from 1061.00 nm to 1065.20 nm. This gave rise to Stokes output which was tunable from 1176.46 nm to 1181.63 nm whilst the narrowed fundamental linewidth resulted in higher effective Raman gain and as a consequence enhanced output power, as well as the narrow-linewidth Stokes output. Frequency-doubling of the Stokes field resulted in yellow output tunable from 588.23 nm to 590.81 nm, which covers the guide star wavelength of 589.16 nm.

Fundamental noise dynamics in cascaded-order Brillouin lasers

The dynamics of cascaded-order Brillouin lasers make them ideal for applications such as rotation sensing, highly coherent optical communications, and low-noise microwave signal synthesis. Remarkably, when implemented at the chip scale, recent experimental studies have revealed that Brillouin lasers can operate in the fundamental linewidth regime where optomechanical and quantum noise sources dominate. To explore new opportunities for enhanced performance, we formulate a simple model to describe the physics of cascaded Brillouin lasers based on the coupled mode dynamics governed by electrostriction and the fluctuation-dissipation theorem. From this model, we obtain analytical formulas describing the steady-state power evolution and accompanying noise properties, including expressions for phase noise, relative-intensity noise, and power spectra for beat notes of cascaded laser orders. Our analysis reveals that cascading can enhance laser noise, resulting in a broader emission linewidth and larger intensity fluctuations with increased power. Consequently, higher-coherence laser emission can be achieved if indefinite cascading can be prevented. In addition, we derive a simple analytical expression that enables the Stokes linewidth to be obtained from spectra of beat notes between distinct cascaded laser orders and their relative powers. We validate our results using stochastic numerical simulations of the cascaded laser dynamics.

Phonon-Limited-Linewidth of Brillouin Lasers at Cryogenic Temperatures.

Laser linewidth is of central importance in spectroscopy, frequency metrology, and all applications of lasers requiring high coherence. It is also of fundamental importance, because the Schawlow-Townes laser linewidth limit is of quantum origin. Recently, a theory of stimulated Brillouin laser (SBL) linewidth has been reported. While the SBL linewidth formula exhibits power and optical Q factor dependences that are identical to the Schawlow-Townes formula, a source of noise not present in conventional lasers, phonon occupancy of the Brillouin mechanical mode is predicted to be the dominant SBL linewidth contribution. Moreover, the quantum limit of the SBL linewidth is predicted to be twice the Schawlow-Townes limit on account of phonon participation. To help confirm this theory the SBL fundamental linewidth is measured at cryogenic temperatures in a silica microresonator. Its temperature dependence and the SBL linewidth theory are combined to predict the number of thermomechanical quanta at three temperatures. The result agrees with the Bose-Einstein phonon occupancy of the microwave-rate Brillouin mode in support of the SBL linewidth theory prediction.

Pulse Narrowing and RF Linewidth Reduction of Integrated Passively Mode-Locked Laser in Anticolliding Design by Means of Spectral Tuning

In this paper, we present the first tunable extended cavity integrated passively mode-locked laser realized in an anticolliding design. The laser is realized as an InP-based photonic integrated circuit. A detailed study of the laser performance under various operating conditions is presented. The evolution of the radio-frequency (RF) spectrum and optical spectrum with the injection current to the optical amplifier is investigated. Tuning over 9 nm is achieved by injecting current in a distributed Bragg reflector section. We demonstrate that tuning of the optical spectrum toward shorter wavelengths, which increases absorption in the saturable absorber section, leads to an improvement of the mode-locked laser performance in terms of reduction of fundamental RF linewidth and reduction of the pulsewidths.

Modeling and analysis of an ultra-stable subluminal laser

We describe a subluminal laser which is extremely stable against perturbations. It makes use of a composite gain spectrum consisting of a broad background along with a narrow peak. The stability of the laser, defined as the change in frequency as a function of a change in the cavity length, is enhanced by a factor given by the group index, which can be as high as 105 for experimentally realizable parameters. We also show that the fundamental linewidth of such a laser is expected to be smaller by the same factor. We first present an analysis where the gain profile is modeled as a superposition of two Lorentzian functions. We then present a numerical study based on a physical scheme for realizing the composite gain profile. In this scheme, the broad gain is produced by a high pressure buffer-gas loaded cell of rubidium vapor. The narrow gain is produced by using a Raman pump in a second rubidium vapor cell, where optical pumping is used to produce a Raman population inversion. We show close agreement between the idealized model and the explicit model. A subluminal laser of this type may prove to be useful for many applications.

Fundamental frequency noise and linewidth broadening caused by intrinsic temperature fluctuations in quantum cascade lasers

We theoretically investigate fundamental thermal frequency noise and linewidth broadening caused by intrinsic temperature fluctuations in both terahertz (THz) and mid-infrared (IR) quantum cascade lasers (QCLs) for the first time. The analytical derivation is based on the Green function analysis and the Van Vliet-Fassett theory. The results show that the fundamental frequency noise caused by temperature fluctuations is prominent in the low frequency range (below a few kHz) and is sensitive to the temperature, heat conductivity, and the thickness of the active region/substrate. It also shows that this fundamental frequency noise does not show a 1/$f$ trend in the whole frequency spectra for both THz and mid-IR QCLs. For THz QCLs, considering only the refractive-index variation caused by the current-induced device self-heating, we calculate the linewidth broadening to be only around 1 Hz, which is comparable to the value of 3 Hz caused by spontaneous emission, stimulated emission, and blackbody radiation in high power THz QCLs. For mid-IR QCLs this frequency noise leads to the linewidth broadening from 14.74 Hz to 62.02 Hz as the temperature increases from 200 K to 400 K. When the microscopic features of the refractive-index variations associated with the intersub-band gain transition, the self-heating-induced thermal expansion and energy-level broadening in mid-IR QCLs are considered, an estimation shows that the linewidth broadening increases greatly by a factor of at least four times.

A Narrow-Linewidth On-Chip Toroid Raman Laser

In this paper, we report a narrow-linewidth on-chip toroid Raman laser. Under free-running condition, we have obtained the minimum fundamental linewidth of 3 Hz and the maximum unidirectional output power of 223 μW. Lasing under the same condition in continuous-wave mode over 90 min is achieved at an average power level of 21 μW and a standard deviation of 0.17 μW. We further derived the frequency noise spectrum and identified an enhancement of frequency noise due to Kerr non linearity. In addition, we have observed the shifting of relaxation oscillation frequency as a consequence of weak mode splitting.

Frequency-doubling of a CW fiber laser using PPKTP, PPMgSLT, and PPMgLN

Second-harmonic generation (SHG) in PPKTP, PPMgSLT, and PPMgLN crystals is analyzed by frequency-doubling CW light from a 1064 nm fiber laser over a range of powers up to 10 W. Data for fundamental powers less than 3 W is used to determine the effects of the fundamental laser linewidth on SHG and to identify imperfections in the periodicity and boundary sharpness of the crystals' poled domains which can reduce SHG. Data for fundamental powers greater than 3 W is used to diagnose and model limiting effects on SHG such as pump depletion and thermal dephasing. Thermal dephasing was found to reduce second-harmonic power by 25% or more for input fundamental powers approaching 10 W.

Theory of the fundamental linewidth of a two-mode laser

Starting from the field master equation for a laser oscillating simultaneously in two modes, we probe the dependence of its fundamental phase-diffusion linewidth on the degree of coupling between the two modes. We find that the heterodyned intrinsic linewidth shows the usual decrease when the output power of each mode increases with an increase in the gain, while the nonlinear self-saturation and cross-coupling coefficients are held constant. The linewidth also decreases with increase in the cross-coupling between the modes for constant gain when the output power of each mode is kept constant by decreasing the self-saturation coefficient. The linewidth, however, increases with increasing cross-coupling when (a) the gain and the self-saturation coefficient are held constant and hence the output power in each mode decreases and (b) the gain is increased keeping the output power and the self-saturation of each mode constant.

From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals flat islands, shaped like truncated pyramids, with a very homogeneous height measured at 2.4 nm. A fundamental level linewidth of 22 meV at 8 K is associated to this very narrow height distribution. Such low photoluminescence linewidth values are believed to be mainly due to the propensity of the InAs/InP(001) system to produce flat InAs islands with discrete height fluctuation.

Forward and backward waves in Cherenkov flux-flow oscillators

Josephson flux-flow oscillators (FFOs) have been used as an on-chip local oscillator at frequencies up to 650 GHz. An autonomous FFO linewidth of about 1 MHz was measured in the resonant regime at Vb<950 µV for niobium-aluminium oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose an on-chip Cherenkov radiation flux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow-wave transmission line that modifies significantly the junction dispersion relation. Two superconductor-insulator-superconductor junction detectors are connected to both the long Josephson junction and the slow-wave line to determine the available microwave power. The power is measured at different CRFFO biasing conditions. Both a forward wave and a backward wave oscillation regime are observed. An FFO and a CRFFO with the same junction parameters are compared.

High Spectral Coherence in Long-Pulse and Continuous Free-Electron Laser: Measurements and Theoretical Limitations

Single mode operation was exhibited on the Israeli tandem free-electron laser (FEL). This enabled judicious measurements of narrow spectral linewidth, frequency chirp, and relaxation-oscillation effects. Exact 3D simulations of the FEL oscillator showed good agreement with the measurements, and permitted an estimation of the fundamental Schawlow-Towns linewidth limit of the FEL (including `` $\ensuremath{\alpha}$ effect'') as well as technical noise linewidth limits. We estimate that with voltage-controlled stabilization high-power (10 kW) tunable (over $60%$ bandwidth) quasi-cw coherent $[(\ensuremath{\Delta}\ensuremath{\nu}/\ensuremath{\nu}{)}_{\mathrm{rms}}\ensuremath{\approx}{10}^{\ensuremath{-}10}]\mathrm{mm}$-FIR (far infrared) radiation is attainable in the tandem FEL.

Design and fabrication of Cherenkov flux-flow oscillator

The Josephson Flux-Flow Oscillator (FFO) has been used as an on chip local oscillator at frequencies up to 650 GHz. The FFO linewidth of about 1 MHz was measured in the resonant regime V<915 /spl mu/V for niobium-aluminum oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose a novel on chip Cherenkov radiation flux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow wave transmission line that modifies essentially the junction dispersion relation. Two SIS detectors are connected both to the long Josephson junction and the transmission line to evaluate available microwave power. The output power coming both from the long junction and the transmission line is estimated at different bias conditions.

Electrical method of spectral filtering within the Schawlow-Townes laser linewidth

An electrical technique is reported that allows spectral filtering within the fundamental laser linewidth with much higher precision than can be achieved using optical methods. The intensity fluctuations of a singlemode semiconductor laser are shown to increase dramatically after such filtering.

Measurement of the spontaneous emission factor of a 3.39 μm HeNe laser

The spontaneous emission factor N sp = N 2 (N 2 − N 1) of a 3.39 μm HeNe laser is determined from spontaneous emission light radiated from the side of the gain tube. N sp is shown to depend linearly on the intra-cavity optical intensity. This linear power dependence of N sp will give rise to a power-independent contribution to the quantum-limited spectral linewidth of the laser. We predict that at an output power of 190 μW this contribution will already make up half of the fundamental laser linewidth.

Spectral filtering within the Schawlow-Townes linewidth as a diagnostic tool for studying laser phase noise

We report on spectral filtering within the fundamental or Schawlow-Townes linewidth of a laser. Intensity correlation functions of the filtered laser light have been calculated assuming the phase-diffusion model (PDM), the phase-jump model (PJM) and the phase-increment model (PIM) as the model underlying the statistics of the laser phase noise. Experiments have been performed on semiconductor lasers; either a one-line or a two-line filter has been positioned underneath the Schawlow-Townes linewidth of the semiconductor laser. We observe that a one-line filter converts (amplitude-stable) laser light into chaotically fluctuating light in the limit that the filter linewidth is much smaller than the Schawlow-Townes linewidth. Two such narrow filters, when sufficiently separated spectrally, lead to two independent chaotic sources. Distribution functions and correlation functions of the filtered light have also been measured as a function of the filter bandwidth. In this case the various models for laser-phase noise lead to different theoretical predictions. We find that the experimental results agree with the phase-diffusion model and not with the other models.