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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.

Stimulated Raman scattering is a third-order nonlinear optical effect that is not only effective for wavelength converting laser output, but also for single longitudinal-mode output due to the absence of spatial hole burning. Diamond is a prominent Raman-active medium that has significant potential for linewidth narrowing and wavelength converting lasers at high power levels due to its high thermal conductivity, long Raman frequency shift and wide spectral transmission range. In this work we utilize diamond in a resonantly mode-matched external cavity to achieve cascaded Raman conversion of a 1064 nm laser. By fine-tuning the length of this external cavity, we can obtain narrow linewidth emission at 1240 and 1485 nm. When operating at maximum power, the measured linewidths were more than twofold narrower than the linewidth of the fundamental field. In addition, the noise levels of the Stokes fields are lower than that of the fundamental field throughout the entire noise frequency range, and the intrinsic linewidth of the second Stokes field, which is expressed at the hertz level (~3.6 Hz), is decreased by approximately three orders of magnitude compared to that of the pump. This work represents the first measurement and analysis of the linewidth and noise characteristics of cascaded diamond Raman lasers and, significantly, offers a new means by which high-power, narrow linewidth laser output can be produced from wavelength-converted laser systems.

A diode-pumped acousto-optic Q-switched crystal Raman laser operating at multi-wavelength is reported, in which a c-cut Nd:YVO 4 crystal was used as the self-Raman medium and a c-cut GdVO 4 crystal was used as the other Raman medium. Benefited from the overlapping gain at the Raman shift of 259 cm −1 in the two crystals, the corresponding cascade Stokes light efficiently oscillated, and the 1177, 1178 nm lights corresponding to the primary Raman-shifts of the two crystals oscillated with the 1097 nm laser. The laser configuration was investigated at pulse repetition frequencies (PRFs) of 10, 20, and 30 kHz. With an incident pump power of 8.62 W, the highest average Raman laser output of 1.31 W composed of 1097, 1129, 1177 and 1178 nm lights was obtained at the PRF of 20 kHz. The 1163 nm light corresponding to the third-Stokes light of the Raman shift of 259 cm −1 only arose at the PRF of 10 kHz when the incident pump power exceeded 6.37 W.

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.

Linearly-polarized (LP) fiber lasers, which could find wide potential applications such as coherent detection, coherent/spectral beam combining, nonlinear frequency conversion, have been a research focus in recent years. In this paper, we will present a general review on the achievements of various kinds of high power LP fiber laser and its applications for the first time. The recent progress in high power LP fiber oscillator, including fiber oscillator based on active fiber, Raman fiber laser and Random distributed feedback fiber laser are summarized. Power scaling of LP fiber laser by using active-fiber based power amplifier, passive-fiber based Raman amplifier, and active/passive fiber based hybrid fiber amplifier has been achieved. Polarization-maintained active fiber based power amplifier and non-polarization-maintained active fiber based power amplifier incorporating active polarization control are specially introduced in detail. High power LP fiber laser with diversified property, such as narrow-linewidth, wavelength-tunable and ultrashort pulse operation, are summarized. Various kinds application of high power LP fiber laser, including beam combining, supercontinumm generation, mid-infrared lasing, structured light field and ultrasonic generation, are presented at the end of this paper.