Silicon Raman Amplifiers Research Articles

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.

Design of Silicon Photonic Crystal Waveguides for High Gain Raman Amplification Using Two Symmetric Transvers-Electric-Like Slow-Light Modes

We designed silicon photonic crystal (PhC) waveguides (WGs) for efficient silicon Raman amplifiers and lasers. We adopted narrow-width WGs to utilize two symmetric transvers-electric-like (TE-like) guided modes, which permit efficient external coupling for both the pump and Stokes waves. Modifying the size and shape of air holes surrounding the line-defect WG structures could tune the frequency difference between these two modes, at the Brillouin-zone edge, to match the Raman shift of silicon. Thus, small group velocities are also available both for pump and Stokes waves simultaneously, which results in a large enhancement of Raman gain. The enhancement factor of the Raman gain in the designed structure is more than 100 times that reported previously.

Frequency Tuning and Bandwidth Enhancement of Raman Amplification in SiG-OI Waveguides

In this paper, some patents on silicon Raman amplifiers are investigated. These patents emphasize on the length reduction of amplifiers compared with the silica ones. However, the bandwidth of the amplifier in these patents is very low. In addition, while the pump wavelength is constant, the central frequency remains unchanged. The germanium doping leads to tuning of central frequency and bandwidth enhancement of Raman gain spectrum. Thus, the effect of germanium doping on the characteristics of major patents on silicon Raman amplifiers is investigated in this paper. Amplifier gain depends on various parameters such as the Raman gain coefficient. The effects of germanium doping on the impressive parameters of the silicon Raman amplifiers are also presented. By usage of the variable germanium concentration waveguides, the bandwidth of amplifiers is enhanced which has a great effect on communication systems. Furthermore, the characteristics of waveguide are estimated in order to achieve a flat gain amplification in silicon germanium on insulator (SiGe-OI) waveguides. Keywords: Continuous wave pumping, free carrier absorption, free carrier lifetime, pulsed pumping, Raman amplification, ring resonator, silicon germanium (SiGe), two photon absorption, SiG-OI Waveguides, Frequency, SiGe Raman Amplifiers, Amplifiers, resonator, insulator, waveguides

Maximization of Gain in Slow-Light Silicon Raman Amplifiers

We theoretically study the problem of Raman gain maximization in uniform silicon photonic-crystal waveguides supporting slow optical modes. For the first time, an exact solution to this problem is obtained within the framework of the undepleted-pump approximation. Specifically, we derive analytical expressions for the maximum signal gain, optimal input pump power, and optimal length of a silicon Raman amplifier and demonstrate that the ultimate gain is achieved when the pump beam propagates at its maximum speed. If the signal’s group velocity can be reduced by a factor of 10 compared to its value in a bulk silicon, it may result in ultrahigh gains exceeding 100 dB. We also optimize the device parameters of a silicon Raman amplifier in the regime of strong pump depletion and come up with general design guidelines that can be used in practice.

Theoretical analysis on gain characteristics of siliconRaman amplifiers

We report the new theoretical analysis method to obtain net gain in silicon waveguide by stimulated Raman scattering, and the key parameters of the free carrier lifetime and the pump light intensity are analyzed. The free carrier lifetime threshold and pump light intensity threshold are deduced. The gain characteristics of silicon Raman amplifiers with different pumping schemes are investigated, and the results show that the gain can be significantly enhanced in the bidirectional pump scheme.

Spectral compression and group delay of optical pulses in silicon Raman amplifiers

We present a general study of pulse amplification in silicon Raman amplifiers, with special emphasis on the effects of spectral compression and group delay due to the Raman gain dispersion. We use the undepleted-pump approximation to analytically calculate the dynamics of an arbitrary pulse spectra and find the temporal profile of the pulse at the amplifier's output. We show that cw-pumped silicon waveguides are extremely inefficient in amplification of subpicosecond optical pulses but provide large net gains and controllable group delays for pulses with widths of ∼10 ps.

Analytical study of pulse amplification in silicon Raman amplifiers

The nonlinear process of stimulated Raman scattering is important for silicon photonics as it enables optical amplification and lasing. To understand the dynamics of silicon Raman amplifiers (SRAs), a numerical approach is generally employed, even though it provides little insight into the contribution of different SRA parameters to the signal amplification process. In this paper, we solve the coupled pump-signal equations analytically under realistic conditions, and derive an exact formula for the envelope of a signal pulse when picosecond optical pulses are amplified inside a SRA pumped by a continuous-wave laser beam. Our solution is valid for an arbitrary pulse shape and fully accounts for the Raman gain-dispersion effects, including temporal broadening and group-velocity reduction (a slow-light effect). It can be applied to any pumping scenario and leads to a simple analytic expression for the maximum optical delay produced by the Raman dispersion in a unidirectionally pumped SRA. We employ our analytical formulation to study the evolution of optical pulses with Gaussian, exponential, and Lorentzian shapes. The ability of a Gaussian pulse to maintain its shape through the amplifier makes it possible to realize soliton-like propagation of chirped Gaussian pulses in SRAs. We obtain analytical expressions for the required linear chirp and temporal width of a soliton-like pulse in terms of the net signal gain and the Raman-dispersion parameter. Our results are useful for optimizing the performance of SRAs and for engineering controllable signal delays.

Effect of free carriers on pump-to-signal noise transfer in silicon Raman amplifiers

We study noise transfer from pump to signal in silicon Raman amplifiers, with particular emphasis on the regimes of strong cumulative free-carrier absorption and heavy pump depletion. We calculate the relative intensity noise (RIN) transfers in copumped and counterpumped amplifiers and provide intuitive explanations for RIN peculiarities. We show that noise transfer at low frequencies may be suppressed by carefully choosing the pump intensity, effective free-carrier lifetime, or amplifier length, but only at the expense of a rise in noise at high frequencies.

Nonlinear Silicon Photonics: Analytical Tools

Since the recent demonstration of chip-scale, silicon-based, photonic devices, silicon photonics provides a viable and promising platform for modern nonlinear optics. The development and improvement of such devices are helped considerably by theoretical predictions based on the solution of the underlying nonlinear propagation equations. In this paper, we review the approximate analytical tools that have been developed for analyzing active and passive silicon waveguides. These analytical tools provide the much needed physical insight that is often lost during numerical simulations. Our starting point is the coupled-amplitude equations that govern the nonlinear dynamics of two optical waves interacting inside a silicon-on-insulator waveguide. In their most general form, these equations take into account not only linear losses, dispersion, and the free-carrier and Raman effects, but also allow for the tapering of the waveguide. Employing approximations based on physical insights, we simplify the equations in a number of situations of practical interest and outline techniques that can be used to examine the influence of intricate nonlinear phenomena as light propagates through a silicon waveguide. In particular, propagation of single pulse through a waveguide of constant cross section is described with a perturbation approach. The process of Raman amplification is analyzed using both purely analytical and semianalytical methods. The former avoids the undepleted-pump approximation and provides approximate expressions that can be used to discuss intensity noise transfer from the pump to the signal in silicon Raman amplifiers. The latter utilizes a variational formalism that leads to a system of nonlinear equations that governs the evolution of signal parameters under the continuous-wave pumping. It can also be used to find an optimum tapering profile of a silicon Raman amplifier that provides the highest net gain for a given pump power.

Silicon Raman Amplifiers With Ring-Resonator-Enhanced Pump Power

In this paper, design rules for optimal silicon Raman amplifiers operating at standard telecommunications wavelengths are formulated. For given material and waveguide parameters, and a specified upper pump-power limit, explicit expressions are derived for the optimal length and maximum possible gain of single-pass-pumped amplifiers. For ring-resonator (RR) amplifiers, the optimization problem is reduced to finding a zero of a fifth-order polynomial. Due to resonant pump-power enhancement, RR amplifiers with positive gain can be realized for any pump power, however small it is. Finally, for small pump powers, the maximum possible gain is most effectively increased by reducing the linear waveguide losses as opposed to the effective area or the effective free-carrier lifetime.

Influence of Pump-to-Signal RIN Transfer on Noise Figure in Silicon Raman Amplifiers

The effect of the relative intensity noise (RIN), transferred from the pump to the signal, in 1-cm-long chip scale silicon Raman amplifiers is investigated in the presence of nonlinear losses. We show that due to the short waveguide length, the reduction in fluctuations that normally occurs due to ldquowalk-offrdquo between pump and signal waves in fiber amplifiers is inefficient in chip scale Raman amplifiers. In the counterpropagating pump configuration, which leads to minimum frequency RIN transfer, fluctuations up to 1.5 GHz are transferred from the pump to the signal. As a case study, the noise figure degrades by as much as 11 dB in the silicon waveguide with the free carrier life time of 0.1 ns, when it is pumped with a laser with a RIN value of -125 dB/Hz.

Gain Enhancement in Cladding-Pumped Silicon Raman Amplifiers

Core-pumped silicon Raman amplifiers reported so far have exhibited gains of 4 dB or less in continuous-wave operation, limited by free-carrier absorption. Here we propose a device design that mitigates the problem of free-carrier absorption and leads to significantly higher gain values. In the new design, we surround the silicon waveguide with an additional rib-shaped passive cladding with a refractive index between that of silicon and the silica buffer. By injecting the pump power into the cladding, instead of directly into the silicon core, the overlap of the pump power with the silicon core is reduced and thus the effect of free-carrier absorption on the pump is weakened. The amplifier may therefore be made much longer, and higher total gains be achieved than in conventional core-pumped amplifiers. We present a detailed analysis of a design where a total gain as high as 18 and 34 dB may be achieved with pump powers of 300 mW and 1 W, respectively. We describe the model we have developed to design and optimize cladding-pumped silicon Raman amplifiers. Explicit expressions are formulated for the nonlinear effective areas for stimulated Raman scattering and two-photon absorption in silicon waveguides, taking into account the differing tensorial structures of these nonlinearities.

Noise Figure of Silicon Raman Amplifiers

The noise figure of silicon Raman amplifiers in the presence of nonlinear losses is calculated. The impact of two-photon absorption (TPA) and free-carrier scattering on the noise figure is quantified using the quantum formulation of the Langevin approach. It is found that TPA-induced free-carrier loss degrades the noise figure by an amount that depends on the carrier lifetime. For example, in a 1-cm-long waveguide pumped at 200 MW/cm2, the noise figure is 5.2 dB for a lifetime of tau = 1.6 ns and is reduced to 3.7 dB for tau = 0.1 ns. The reduction in the noise figure along with a concomitant increase in Raman gain from 2 to 8 dB suggests that lifetimes on the order of 0.1 ns or less are needed to create a useful silicon Raman amplifier that operates in the continuous-wave mode. It is also shown that in devices that use a p-n junction for carrier sweep-out, the screening of the junction field by generated free carriers results in a sharp increase in the noise figure at high-pump intensities. These results apply to operation in the near-infrared communication wavelengths. For mid-infrared wavelengths above the two photon absorption band-edge (2.3 nm), the absence of TPA and pump-induced free-carrier absorption ensures that the amplifier has a low-noise figure.

Application of defect engineering to silicon Raman lasers and amplifiers

Silicon Raman lasers and amplifiers are the only silicon-based, monolithic device structures in which optical gain has been unambiguously observed. The main limitation on this gain is optical absorption due to free carriers, generated by two photon absorption. Here we explore a means to mitigate carrier effects via defect engineering. The optical and electrical properties of the defected silicon waveguides are modeled for both a uniform defect distribution and a remote, localized defect distribution. Simulation results indicate that a uniform defect distribution provides no improvement with any increase in net gain provided solely by surface modification. In contrast, for devices with remote defect volumes the reduction of carrier lifetime and limited optical absorption results in a significant improvement to net gain.

Demonstration of a Mid-infrared silicon Raman amplifier

We demonstrate, for the first time, a mid infrared silicon Raman amplifier. Amplification of 12 dB is reported for a signal at 3.39 micron wavelength. The active medium was a 2.5 cm long silicon sample that was pumped with 5ns pulses at 2.88 micron. Such a technology can potentially extend silicon photonics' application beyond data communication in the near IR and into the mid-IR world of remote sensing, biochemical detection and laser medicine. Challenges faced in the mid-IR regime such as a higher free carrier scattering rate longer lifetime in mid-IR waveguides are also discussed.

Self-imaging silicon Raman amplifier

We propose a new type of waveguide optical amplifier. The device consists of collinearly propagating pump and amplified Stokes beams with periodic imaging of the Stokes beam due to the Talbot effect. The application of this device as an Image preamplifier for Mid Wave Infrared (MWIR) remote sensing is discussed and its performance is described. Silicon is the preferred material for this application in MWIR due to its excellent transmission properties, high thermal conductivity, high damage threshold and the mature fabrication technology. In these devices, the Raman amplification process also includes four-wave-mixing between various spatial modes of pump and Stokes signals. This phenomenon is unique to nonlinear interactions in multimode waveguides and places a limit on the maximum achievable gain, beyond which the image begins to distort. Another source of image distortion is the preferential amplification of Stokes modes that have the highest overlap with the pump. These effects introduce a tradeoff between the gain and image quality. We show that a possible solution to this trade-off is to restrict the pump into a single higher order waveguide mode.

Energy harvesting in silicon Raman amplifiers

Continuous-wave Raman amplification in silicon waveguides with negative electrical power dissipation is reported. It is shown that a p-n junction can simultaneously achieve carrier sweep-out leading to net continuous-wave gain and electrical power generation. The approach is also applicable to silicon Raman lasers and other third-order nonlinear optical devices.

Multi-mode Mid-IR Silicon Raman Amplifiers

ABSTRACTThis paper discusses the prospects of silicon as a Mid-wave Infra-red Raman crystal. As a specific example of novel devices that can be realized, we introduce the concept of multi-mode silicon Raman amplifiers and their application in infrared image pre-amplification. The same technology also has application in incoherent beam combining.

Limitations of active carrier removal in silicon Raman amplifiers and lasers

The lifetime of two-photon generated carriers has been established as the critical parameter that determines the performance of silicon Raman lasers and amplifiers since it determines the optical loss. Here, we investigate the intensity dependence of the carrier lifetime in the case where the carriers are swept out by means of a p-n junction. Numerical simulations show that at sufficiently high pump intensities, the generated carriers screen the applied electric field and therefore result in a higher lifetime and hence a lower net Raman gain. We also quantify the electrical power dissipation necessary to maintain low optical losses.

Demonstration of 11dB fiber-to-fiber gain in a silicon Raman amplifier

We report the demonstration of 11dB fiber-to-fiber optical gain in a silicon Raman amplifier. Pulsed pumping is employed to reduce the TPA induced free carrier losses resulting in net signal amplification. The influence of free carriers is elucidated by observing the dependence of gain on pulse energy.