Stimulated Raman Scattering Gain Research Articles

Raman based on-chip photonic quantizers for ADCs

In this work, a model for an on-chip all-photonic analog to digital converter is proposed. Here, the underlying quantization technique is based on slicing the supercontinuum generated inside the highly nonlinear nanocrystal embedded slotted photonic crystal waveguide at specifically chosen wavelengths. It is shown that although both the broadening and splitting of the spectrum can be used for four level quantization, only broadening can be used for designing eight (or more) level quantization. The highly nonlinear device system makes the eight level quantization scheme incremental within a footprint of 50×20µm2, which can convert 0–11 mW analog optical power to respective quantization levels. Although both Kerr and stimulated Raman scattering (SRS) contribute to the generation of the supercontinuum, the giant SRS gain of the silicon nanocrystal essentially outweighs that of the Kerr, with, consequently, unprecedented miniaturization in both operating power and footprint.

Non-reciprocal sub-micron waveguide Raman amplifiers, towards loss-less silicon photonics

Optical amplification in silicon photonic integrated circuits remains a challenge and stimulated Raman scattering has been proposed as a means of achieving optical gain in silicon. In this work, we experimentally investigate silicon Raman amplifiers optimized for fabrication with open-access foundry sub-micron silicon platform. We discuss and experimentally validate the importance of considering the non-reciprocal Raman-gain by using counter-propagating or co-propagating pumps and probes, different amplifier lengths, input pump powers and nonlinear loss values. We demonstrate a Raman-assisted loss-less optical circuit in a 1.2-cm-length waveguide that reaches zero net-gain with only 60 mW continuous-wave pumping. A 5.5 dB stimulated Raman scattering (SRS) gain yielding 0.5 dB net-gain is also demonstrated with a 115 mW pump in a 3.25-cm waveguide. Furthermore, we examine the nonlinear loss of silicon waveguides to estimate free carrier lifetime (FCL) with different bias voltages applied to the waveguide <i>p-i-n</i> structure. Accounting for non-reciprocal Raman amplification by using the two dataset of co- and counter-propagating SRS gain, we extract the Raman gain coefficient of this 220 nm thick silicon photonic waveguide. We perform the curve fitting over the whole input pump power range using the extracted FCL, and over the low power range where only linear loss is expected. We find a good agreement in the extracted Raman gain coefficients. We use these key parameters as input to a silicon photonic Raman amplifier model to find the optimum performance based on the available footprint and pump power.

All‐optical logic inverter for large‐scale integration in silicon photonic circuits

A new device architecture of an all-optical logic inverter (AOLI) has been proposed in this study. The AOLI functions based on stimulated Raman scattering (SRS) in silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW). The operating power and footprint size of the proposed device have significantly been scaled down to ∼1.5 mW and ∼ 8.5 × 42 μ m 2 , respectively, by combining the benefits of giant SRS gain of silicon nanocrystal and ultra-high spatio-temporal confinement of the SPCW. Cascadability of the proposed AOLI has been investigated using the maximum product criterion of the noise margin. The successful operation of the AOLI has been demonstrated for the repetition rate of the pulses as high as 100 Gbps. Finally, tolerance of performance of the design under different types of fabrication imperfections has also been calculated to establish its sustainability in a practical scenario.

Raman mediated ultrafast all-optical NOR gate.

A new device architecture has been proposed in this paper implementing the all-optical cascadable logic NOR functionality. The device functions based on stimulated Raman scattering (SRS) in silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW). Substantial miniaturizations both in operating power and overall footprint of the device have been achieved owing to the ultrahigh SRS gain of silicon nanocrystal and strong spatio-temporal confinement of the SPCW. Successful operation of the device has been demonstrated at a pulse rate that is as high as 125 Gbps.

A single-frequency intracavity Raman laser

A continuous-wave (CW) single-longitudinal-mode (SLM) intracavity Raman laser is demonstrated for the first time, by virtue of the spatial hole-burning free nature of stimulated Raman scattering (SRS) gain. By using a single etalon in the Nd:GdVO4 fundamental laser cavity, the spectral linewidth of the multimode fundamental field is suppressed below the Raman linewidth of Raman crystal BaWO4; hence power in all the longitudinal modes of fundamental field can be extracted by one single Stokes mode. Therefore, the hole-burning free SRS gain exhibits a spectral cleanup effect whereby a stable SLM Stokes field is derived from the multimode fundamental field within a simple standing-wave cavity arrangement. The low-threshold SLM Raman laser delivered 3.42 W SLM Stokes and 1.53 W SLM one-way yellow harmonic at the guide-star wavelength of 589.16nm. The results here provide a new approach to SLM laser operation with good simplicity and power dynamic range. Further engineering for power scaling and better stability is also discussed.

Enhanced stimulated Raman scattering by suppressing stimulated Brillouin scattering in liquid water

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium.

Evolution of non-frequency shift components of pulse tail in normal dispersion region of highly nonlinear fiber

Supercontinuum generated in normal dispersion region of highly nonlinear fiber (HNLF) is widely used in signal processing and communication benefiting from its good flatness and high coherence. Because of the normal dispersion, optical wave breaking (OWB) occurs when non-frequency shift components and frequency shift components caused by self-phase modulation (SPM) overlap in time domain, and ends when non-frequency shift components disappear. The evolution of non-frequency shift components at the front and rear edge of optical pulse play an essential role in the supercontinuum generation process. In this paper, the evolution of non-frequency shift components in normal dispersion region is numerically calculated and analyzed based on generalized nonlinear Schrödinger equation. The results demonstrate that non-frequency shift components shrink gradually as the pulse propagates in the normal dispersion region. Cross-phase modulation (XPM) and stimulated Raman scattering (SRS) play a major role in this process, while the third-order dispersion imposes little effect on it. Because of XPM, non-frequency shift components at the front and rear edge shrink gradually, and keep red shifting and blue-shifting respectively. The influence of XPM on the non-frequency shift components at both edges is symmetrical. However, the influence of SRS on the evolution of non-frequency-shift components at both edges is asymmetric. At the front edge, SRS transfers the energy from non-frequency shift component to frequency shift component, which is opposite to that at the rear edge. At the front edge, SRS accelerates the shrinking process of the non-frequency shift component, while it slows down the shrinking process at the rear edge. And this asymmetric effect is more obvious when the peak power of the pulse is higher and SRS is more efficient. The evolution of the non-frequency shift components of chirped pulses propagating in the normal dispersion region is studied. Comparing with the unchirped pulse, the non-frequency shift components at the front and rear edge of the chirped pulse have different wavelengths. For the negative chirped pulse, the wavelength spacing between the overlapped frequency-shift components and non-frequency shift components is larger, which is easier to satisfy the SRS gain range. Therefore, the evolution of non-frequency-shift components at the front and rear edge of the negative chirped pulse are more asymmetric due to the higher SRS efficiency. For positive chirped pulses, the wavelength spacing between the overlapped components is difficult to satisfy the SRS gain range. The evolution of non-frequency-shift components in the positive chirped pulses is more symmetrical due to the lower SRS efficiency.

Integrable all‐optical pass switch

A completely new design for an all-silicon all-optical pass switch has been proposed based on the stimulated Raman scattering (SRS) in silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW). Substantial miniaturisation in the device footprint and operating power has been attained by merging the benefits of the giant SRS gain and the ultra-high optical confinement of the SPCW. The proposed switch offers a tremendously high contrast ratio of ∼ 27 dB between the logic levels and an extraordinary bit rate of 125 Gbps.

Channel Power Optimization of WDM Systems Following Gaussian Noise Nonlinearity Model in Presence of Stimulated Raman Scattering

The impact of interchannel stimulated Raman scattering (SRS) on optimization of channel powers to maximize the minimum channel margin is examined using a discrete Gaussian noise model for the Kerr nonlinearity. The simultaneous consideration of these two nonlinear effects is found to be incompatible with the goal of a convex SNR expression that can be optimized globally. A sequence of convex optimizations is employed to obtain a locally optimal solution, along with a bound on the degree of suboptimality. Optimization results obtained are most accurate for Gaussian-distributed signals, such as probabilistically shaped high-order-modulated signals. In a dispersion-uncompensated 4000-km fiber system utilizing the full C -band with perfect per-span SRS gain compensation, power optimization yields benefits of 0.25 to 2 dB over optimal spectrally flat power allocations. In systems including both C- and L -band, an optimization method that accounts for both SRS and Kerr nonlinearity effects provides a 0.23 to 0.60 dB margin benefit over a method compensating for SRS gain alone. In a system spanning only the C -band, per-span SRS gain compensation is not critical, as the maximum benefit is a 0.14 dB gain in minimum margin for optimized power allocations. By contrast, in a system spanning both C- and L -band, per-span SRS gain compensation provides a gain of up to 1.23 dB with optimized power allocations and larger gains with suboptimal power allocations.

Micron-scale Raman amplifier: another progressive step towards the maturity of silicon photonics

Notice of redundant publication "Micron-scale Raman amplifier: another progressive step towards the maturity of silicon photonics" It has been brought to the attention of the editors that this article contains redundancies when compared with an earlier article published by the authors in Datta T and Sen M 2017 Superlattices Microstruct. 109 107–16. The original article was not referenced. These concerns were identified after the article appeared as accepted but before publication of the final version. As a member of COPE this was been investigated in accordance with the COPE guidelines and it was agreed a notice of redundant publication should be applied. A new model for micron scale all-silicon amplifier has been proposed based on stimulated Raman scattering (SRS) in a slotted photonic crystal waveguide (SPCW). Silicon nanocrystal material is considered to be embedded in the slot of the SPCW to exploit its giant Raman gain coefficient. Moreover, the gain has further been enhanced by using the ultra-high optical confinement of the SPCW. Results of simulations show that the threshold power of a 3 μm long SPCW based Raman amplifier is in the order of merely 500 μW, while the CW Stokes is launched with a power of 50 μW. The simulations also show that a merely 1.5 μm long SPCW can offer ≈3 dB gain even at the 400 Gbps pulse repetition rate. However, pulse shape becomes disrupted by the SRS induced temporal broadening which has further been reduced considerably by operating the Stokes at a different resonance peak in the SRS gain spectrum. With these tremendous performances, the SPCW based Raman amplifier proves its potentiality for application in on-chip optical interconnects and also opens up a pavement for realizing the long desired micron-scale all-silicon Raman laser.

Comparison of Raman‐active crystals as a narrowband probe light source for picosecond three‐color vibrational CARS thermometry

The feasibility of N2‐based vibrational three‐color coherent anti‐Stokes Raman scattering (CARS) thermometry employing narrowband stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman‐active crystal to provide the wavelength‐shifted probe radiation has been investigated. This shift of the probe beam wavelength is aimed to enable temperature measurements in sooting flames by using the established technique of N2 vibrational CARS spectroscopy in the range free from the interference of the detected radiation with the absorption/emission bands of the flame intermediate radicals C2. The use of solid‐state Raman wavelength shifters may be an inexpensive solution providing reliable temperature measurements. The comparative study of the employment of two high SRS gain Raman‐active crystal materials [namely, Ba(NO3)2 and KGd(WO4)2] pumped by a 100‐ps frequency‐doubled Nd:YAG laser pulse has been performed. To test and compare the operation of each of the crystals in a three‐color CARS experiment, we have performed temperature measurements at ambient conditions and in a fuel‐rich sooting ethylene/air flame. Despite the comparable conversion efficiencies into the first‐order Stokes SRS radiation provided by both crystals, the achievable pulse energies were four to five times larger for the KGd(WO4)2 crystal, and the vibrational CARS spectra at flame temperatures had a significantly better signal‐to‐noise ratio. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Determination of the stimulated raman scattering threshold for a pump pulse of arbitrary width

A theoretical solution to the problem of determining the stimulated Raman scattering (SRS) threshold has been found within the undepleted pump approximation for a pump pulse of arbitrary width, which distinguishes it from the known solutions for the limiting cases of very short (highly transient SRS) and very long (quasi-steady-state SRS) pump pulses with respect to the oscillation dephasing time of the SRS medium. The general formula of the theoretical estimate of SRS threshold, in dependence of not only the pump radiation intensity and the SRS interaction length but also the pump-pulse width, is obtained based on the found solution. The theoretical estimate of the SRS threshold has been shown to be in good agreement with the experimental results on the excitation of picosecond SRS in crystals, which justifies the new express method for estimating the SRS gain in experimental measurements of the picosecond SRS threshold.

Image formation using stimulated raman scattering gain

Theoretical analysis of the spatial, noise, and energy characteristics of an amplifier has been performed in the mode of spectral and time selection using subnanosecond stimulated Raman Scattering gain of weak echo signals in crystalline active media that are known for high (up to 10–1 cm/MW) gain coefficients. The possibility to reach high gain values has been demonstrated for weak signals from objects at acceptable angular sizes of the field of vision of an amplifier. To provide a signal-to-noise ratio that exceeds unity over the entire field of vision, the number of photons at the input to an amplifier that is required has to exceed the number of its resolution elements. Accurate determination of the possibilities of recording of weak echo signals and quality of images of targets that are obtained using amplifiers under stimulated Raman Scattering requires additional special experiments.

Stimulated Raman scattering detection for chemically specific time-resolved imaging of gases.

A stimulated Raman scattering (SRS) imaging technique based on spatial modulation of the pump beam has been used to study gases. The SRS gain signal was separated from the Stokes beam background in the spatial frequency domain. The SRS signal shows linear behaviour with the gas pressure at a range from 1.0 to 8.0 bars. The signal is linearly proportional to the pump beam intensity while it is enhanced with increasing the Stokes beam intensity to a certain limit than it saturates. Further, the chemical specificity of the technique has been investigated. Two sharp peaks with line width at half maximum of about 0.30 nm have been obtained at Stokes beam wavelengths of 629.93 nm and 634.05 nm corresponding to the methane and ethylene gases, respectively. The results show that SRS imaging is a promising technique to provide chemical specificity as well as spatial and temporal information of gaseous species.

Enhancement of stimulated Raman scattering in terrace-microspheres

Terrace-microspheres of high-index glass (BaO-SiO2-TiO2 glass, nD=1.93) containing 0.3 ppm of Nd3+ were used to investigate the interaction between Raman scattering due to a glass matrix and fluorescence due to Nd3+. The terrace-microspheres were pumped with a tunable CW Ti:sapphire laser (790 nm–830 nm wavelength) for changing pumping wavelengths. With pumping in the 800–830 nm wavelength region, there was a spectral overlap between Raman scattering and Nd3+ fluorescence. Under such conditions, Nd3+ fluorescence works as a seeding and an amplifier of stimulated Raman scattering (SRS), resulting in SRS enhancement. With pumping of 20 mW power at around 830 nm wavelength, the terrace-microspheres showed the strongest SRS gain, 5–6 times of that of 790 nm wavelength pumping. SRS thresholds of the terrace-microspheres were improved from 3 mW(790 nm wavelength pumping) to 0.3 mW (830 nm pumping) due to the enhancement effect. The potential application for a multiwavelength Raman laser with a low threshold was demonstrated in the near-IR region (λ=840–940 nm).

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.

Light-induced frequency shift of stimulated Raman scattering in Bessel light beams

The medium susceptibility related to stimulated Raman scattering (SRS) is known to be a complex value whose imaginary part determines the SRS gain coefficient whereas the real part constitutes a small fraction of the refractive index at the frequencies of the scattered emission that is proportional to the intensity of the exciting beam. Strong transverse nonuniformity of this beam causes focusing of high-frequency components of the scattered light and defocusing of low-frequency ones that inevitably affect their amplification efficiency. The effect for Gaussian pumping is responsible for a slight shift of the SRS gain spectrum towards higher frequencies. The present work analyzes theoretically SRS excited by a Bessel light beam and demonstrates that the axial component of the scattered emission experiences a frequency shift that is much greater than that for the case of Gaussian beam pumping.

Semiclassical model of stimulated Raman scattering in photonic crystals

We study the stimulated Raman scattering (SRS) of light from an atomic system embedded in a photonic crystal and coherently pumped by a laser field. In our study, the electromagnetic field is treated classically and the atomic system is described quantum mechanically. Considering a decomposition of the pump and Stokes fields into the Bloch modes of the photonic crystals and using a multiscale analysis, we derive the Maxwell-Bloch equations for SRS in photonic crystals. These equations contain effective parameters that characterize the SRS gain, the nonlinear atomic response to the electromagnetic field, and the group velocity and that can be calculated in terms of the Bloch modes of the unperturbed photonic crystal. We show that if the pump laser frequency is tuned near a photonic band edge and the atomic system is carefully chosen such that the Stokes mode matches another photonic band edge, low-threshold, enhanced Raman amplification is possible. Possible physical realizations of SRS in photonic crystals are also discussed.

New Crystals for Raman Lasers

Steady-state and transient stimulated Raman scattering (SRS) in crystals is analyzed. The basic laws of an increase in the SRS gain in crystals are revealed, and methods for searching and creating new SRS laser materials are developed. New crystals for picosecond and nanosecond SRS lasers are proposed, fabricated, and characterized. These materials have the highest SRS cross section, a low SRS threshold, and a wide spectral range of operation.

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.