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We provide a perspective review over the recent development of short-pulsed Raman fiber lasers (RFLs), which can provide laser emissions with flexible wavelengths for a variety of applications as well as an excellent platform to investigate various nonlinear pulse dynamics behaviors that cannot be captured in conventional rare-earth (RE) doped counterparts. Various pulse generation techniques have been explored in RFLs. However, the output pulse performance in terms of the pulse energy, duration and stability from short-pulsed RFLs is still inferior to their RE-doped counterparts despite significant advances made over the past few decades. Therefore, more efforts are required to improve these targets. In this review, we present a detailed overview of the short-pulsed RFLs based on different mechanisms from the principle to the experiment, including the Q-switching, gain-switching, mode-locking, synchronous pumping and other innovative techniques. In addition, Raman-induced pulse dynamics in ultrafast RFLs and RE-doped mode-locked fiber lasers (MLFLs) are briefly reviewed. Finally, a perspective outlook for the future ultrafast RFLs is provided based on their potential applications in industrial and scientific research areas.

Raman-based technologies have enabled many ground-breaking scientific discoveries related to surface science, single-molecule chemistry and biology. For example, researchers have identified surface-bound molecules by their Raman vibrational modes and demonstrated polarization-dependent Raman gain. However, a surface-constrained Raman laser has yet to be demonstrated because of the challenges associated with achieving a sufficiently high photon population located at a surface to transition from spontaneous to stimulated Raman scattering. Here, advances in surface chemistry and in integrated photonics are combined to demonstrate lasing based on surface stimulated Raman scattering (SSRS). By creating an oriented, constrained Si–O–Si monolayer on the surface of integrated silica optical microresonators, the requisite conditions for SSRS are achieved with low threshold powers (200 μW). The expected polarization dependence of SSRS due to the orientation of the Si–O–Si bond is observed. Owing to the ordered monolayer, the Raman lasing efficiency is improved from ~5% to over 40%.Surface stimulated Raman scattering-based lasing is achieved using just a monolayer of molecules on silica optical microresonators.

A continuous-wave Raman silicon laser with a photonic-crystal nanocavity less than ten micrometres in size and an unprecedentedly low lasing threshold of one microwatt is demonstrated, showing that the integration of all-silicon devices into photonic circuits may be possible. Spotlight on silicon Silicon is the workhorse of the microelectronics industry, but its performance as a 'photonic' material is not exceptional. Nevertheless, much progress has been made in imparting useful optical properties to silicon, culminating in the realization of an all-silicon laser. Yasushi Takahashi and colleagues now present a new architectural twist on the silicon laser, showing how the incorporation of a photonic-crystal nanocavity into such a structure can drastically reduce both the size and the threshold power (the power at which it starts to behave as a laser) of the resulting device — both features that are essential for large-scale integration with other photonic and electronic circuitry. The application of novel technologies to silicon electronics has been intensively studied with a view to overcoming the physical limitations of Moore’s law, that is, the observation that the number of components on integrated chips tends to double every two years. For example, silicon devices have enormous potential for photonic integrated circuits on chips compatible with complementary metal–oxide–semiconductor devices, with various key elements having been demonstrated in the past decade 1 , 2 , 3 , 4 , 5 , 6 . In particular, a focus on the exploitation of the Raman effect has added active optical functionality to pure silicon 7 , 8 , 9 , 10 , culminating in the realization of a continuous-wave all-silicon laser 11 . This achievement is an important step towards silicon photonics, but the desired miniaturization to micrometre dimensions and the reduction of the threshold for laser action to microwatt powers have yet to be achieved: such lasers remain limited to centimetre-sized cavities with thresholds higher than 20 milliwatts 12 , even with the assistance of reverse-biased p–i–n diodes. Here we demonstrate a continuous-wave Raman silicon laser using a photonic-crystal, high-quality-factor nanocavity without any p–i–n diodes, yielding a device with a cavity size of less than 10 micrometres and an unprecedentedly low lasing threshold of 1 microwatt. Our nanocavity design exploits the principle that the strength of light–matter interactions is proportional to the ratio of quality factor to the cavity volume and allows drastic enhancement of the Raman gain beyond that predicted theoretically 13 , 14 . Such a device may make it possible to construct practical silicon lasers and amplifiers for large-scale integration in photonic circuits.

Making light with silicon Last month, Intel researchers reported a notable advance in optoelectronics ( Nature 433, 292–294; 2005). They had produced an all-silicon laser on a single chip, making silicon, the foundation of modern microelectronics, a real prospect for optical applications. Now Intel's labs report the first experimental demonstration of a continuous wave laser in a silicon waveguide cavity on a single chip. This is another step towards silicon-based optoelectronic circuits for applications in communications and computing. Achieving optical gain and/or lasing in silicon has been one of the most challenging goals in silicon-based photonics 1 , 2 , 3 because bulk silicon is an indirect bandgap semiconductor and therefore has a very low light emission efficiency. Recently, stimulated Raman scattering has been used to demonstrate light amplification and lasing in silicon 4 , 5 , 6 , 7 , 8 , 9 . However, because of the nonlinear optical loss associated with two-photon absorption (TPA)-induced free carrier absorption (FCA) 10 , 11 , 12 , until now lasing has been limited to pulsed operation 8 , 9 . Here we demonstrate a continuous-wave silicon Raman laser. Specifically, we show that TPA-induced FCA in silicon can be significantly reduced by introducing a reverse-biased p-i-n diode embedded in a silicon waveguide. The laser cavity is formed by coating the facets of the silicon waveguide with multilayer dielectric films. We have demonstrated stable single mode laser output with side-mode suppression of over 55 dB and linewidth of less than 80 MHz. The lasing threshold depends on the p-i-n reverse bias voltage and the laser wavelength can be tuned by adjusting the wavelength of the pump laser. The demonstration of a continuous-wave silicon laser represents a significant milestone for silicon-based optoelectronic devices.

We propose a technique to design highly-efficient and -unidirectional DFB Raman fiber lasers based on the engineering of the grating's coupling coefficient including a π-phase shift position in the fiber. For this purpose, first the ideal intra-cavity signal powers for different pump power levels are determined for given fiber lengths. Then, the sum and difference between the counter-propagating wave intensities at each small segment within fiber lengths are calculated resulting in determining the ideal grating's coupling functions for co- and contra-directional-pumping. The steady-state behavior of the laser using realistic parameters is finally simulated for modified coupling functions considering the Kerr nonlinearity. For a 10 W co-directional-pumped, ∼1 m long single-mode super-efficient DFB, a ∼50% increase in the laser efficiency, more than 44 dB reduction in contra-directional lasing power, ∼15 times drop in the peak power of intra-cavity signal and ∼38% decrease in the unused-pump power are found, compared to those in a standard DFB with the same coupling-length product. Although an enhanced nonlinear refractive index due to thermal gradient reduces the output power of such lasers, it is shown that the super-efficient laser presents a better performance than the standard one, under such conditions.

In this study, we developed a novel “see-and-treat” theranostic system named “surface-enhanced Raman scattering (SERS) imaging-guided real-time photothermal therapy” for accurate cancer detection and real-time cancer cell ablation using the same Raman laser. Facilely synthesized polydopamine-encapsulated gold nanorods (AuNRs), which possess excellent biocompatibility and enhanced stability, were used as multifunctional agents. Under near-infrared (NIR) laser irradiation, polydopamine-encapsulated AuNRs show strong SERS effect and high photothermal conversion efficiency simultaneously. After immobilization of antibodies (anti-EpCAM), polydopamine-encapsulated gold nanorods show high specificity to target cancer cells. Tumor margins could be distinguished facilely by a quick SERS imaging process, which was confirmed by H&E staining results. By focusing the exciting light on detected cancer cells for a prolonged time, cancer cells could be ablated immediately without the need of other procedure. This “see-and-treat” theranostic strategy combining SERS imaging and real-time photothermal therapy using the same Raman laser is proposed for the first time. Experimental results confirmed the feasibility of our “SERS imaging-guided real-time photothermal therapy system.” This novel theranostic strategy can significantly improve the efficiency of cancer therapy in clinical application, allowing the effective ablation of cancer cells with no effects on surrounding healthy tissues. Graphical abstract ᅟ

Important accomplishments concerning an integrated laser source based on stimulated Raman scattering (SRS) have been achieved in the last two decades in the fields of photonics, microphotonics and nanophotonics. In 2005, the first integrated silicon laser based upon SRS was realized in the nonlinear waveguide. This breakthrough promoted an intense research activity addressed to the realization of integrated Raman sources in photonics microstructures, like microcavities and photonics crystals. In 2012, a giant Raman gain in silicon nanocrystals was measured for the first time. Starting from this impressive result, some promising devices have recently been realized combining nanocrystals and microphotonics structures. Of course, the development of integrated Raman sources has been influenced by the trend of photonics towards the nano-world, which started from the nonlinear waveguide, going through microphotonics structures, and finally coming to nanophotonics. Therefore, in this review, the challenges, achievements and perspectives of an integrated laser source based on SRS in the last two decades are reviewed, side by side with the trend towards nanophotonics. The reported results point out promising perspectives for integrated micro- and/or nano-Raman lasers.

Bright future for ‘optical’ silicon With the growing use of optoelectronics in information technology, manipulating light is almost as important as manipulating electrons. Unfortunately silicon, workhorse of modern microelectronics, is next to useless in optical applications. There has been a massive effort to overcome silicon's inadequacies, and ways of coaxing silicon to handle light are under development but a key component — the laser — has been problematic. Last year a silicon laser was produced, but it involved metres of optical fibre. Now workers in Intel's research labs have come up with an all-silicon laser on a single chip. The device is compact and readily integrated with other silicon components. The possibility of light generation and/or amplification in silicon has attracted a great deal of attention 1 for silicon-based optoelectronic applications owing to the potential for forming inexpensive, monolithic integrated optical components. Because of its indirect bandgap, bulk silicon shows very inefficient band-to-band radiative electron–hole recombination. Light emission in silicon has thus focused on the use of silicon engineered materials such as nanocrystals 2 , 3 , 4 , 5 , Si/SiO 2 superlattices 6 , erbium-doped silicon-rich oxides 7 , 8 , 9 , 10 , surface-textured bulk silicon 11 and Si/SiGe quantum cascade structures 12 . Stimulated Raman scattering (SRS) has recently been demonstrated as a mechanism to generate optical gain in planar silicon waveguide structures 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 . In fact, net optical gain in the range 2–11 dB due to SRS has been reported in centimetre-sized silicon waveguides using pulsed pumping 18 , 19 , 20 , 21 . Recently, a lasing experiment involving silicon as the gain medium by way of SRS was reported, where the ring laser cavity was formed by an 8-m-long optical fibre 22 . Here we report the experimental demonstration of Raman lasing in a compact, all-silicon, waveguide cavity on a single silicon chip. This demonstration represents an important step towards producing practical continuous-wave optical amplifiers and lasers that could be integrated with other optoelectronic components onto CMOS-compatible silicon chips.

We report here an all-fiber structure tunable gas Raman laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs). An all-fiber gas cavity is fabricated by fusion splicing a 49 m high-pressure deuterium-filled HC-PCF with two solid-core single-mode fibers at both ends. When pumped with a pulsed fiber amplifier seeded by a tunable laser diode at 1.5 μm, Raman lasers ranging from 1643 nm to 1656 nm are generated. The maximum output power is ~1.2 W with a Raman conversion efficiency of ~45.6% inside the cavity. This work offers an alternative choice for all-fiber lasers operating at 1.6–1.7 μm band.

Raman lasers based on multimode graded-index (GRIN) fibers directly pumped by laser diodes are the object of intensive research, promising to produce high-power, high-quality beams at new wavelengths. In this paper, we demonstrate a pulsed operation of such a laser based on a 1 km 100/140 GRIN fiber in which a resonator is formed by a pair of FBGs: a high-reflective broadband input FBG and a tunable narrowband output FBG inscribed by fs laser pulses in the fundamental mode area of the core. In addition to beam quality improvement, the output FBG modulated by a piezoelectric nanopositioner with a frequency of 20–180 Hz generates laser pulses with a duration of 23–1.2 ms, respectively. The maximum power reached is 22 watts, and the signal spectrum widens significantly with increased pumping (>2 nm from the central wavelength of 976 nm). The pulse generation method used in this work introduces wavelength chirp in the individual pulse, which can be used in sensing and other applications.