Linewidth reduced cascaded Raman fiber lasers and their harmonic conversion for visible laser sources.

Cascaded Raman Fiber Lasers (CRFLs) are wavelength versatile sources that can provide power at any wavelength in the Near-Infrared (NIR) region. Conventional CRFLs with broadband feedback are widely wavelength tunable but have broad line widths. A feedback mechanism must be used to reduce the broadening of the linewidth without compromising the wavelength tunability. Here, we propose to use a dual feedback mechanism that combines broadband feedback at all wavelengths, using a flat cleave, with filtered feedback at a desired wavelength due to a grating filter. This allows substantial linewidth reduction of CRFLs up to the 6th Raman shifts, from 1100 nm to 1500 nm, and can be extended further. Significantly reduced linewidth with multi-watt in-band output power is achieved with fine wavelength tuning within each Raman Stokes band using a fixed wavelength pump. As an application of linewidth narrowed output, we performed frequency doubling of CRFL output to generate over 100 mW of wavelength tunable yellow-green and yellow output with enhanced efficiency.

Cascaded Raman fiber lasers with ultrahigh spectral purity.

Random distributed feedback (RDFB) cascaded Raman fiber lasers (CRFLs) are simple, wavelength agile, and enable high-power fiber lasers outside emission bandwidths of rare-earth doped fiber lasers. However, the spectral purity, defined as the percentage of total output power in the desired Stokes wavelength band, and relative intensity noise (RIN) of these systems are limited due to the intensity noise of the pump source used for Raman conversion. RIN gets amplified and transferred to Raman Stokes orders which causes incomplete Raman conversion and hence limits the spectral purity. Here, we demonstrate a low-intensity noise (<-100 dBc/Hz from 9 kHz to 10 GHz) CRFL with a record spectral purity of ∼99%, tunable over six Stokes orders, using a very low-intensity noise, narrow linewidth Yb-fiber amplifier as a pump source.

Enhanced nonlinear spectral broadening and sub-picosecond pulse generation by adaptive spectral phase optimization of electro-optic frequency combs.

We utilize adaptive optimization to enhance the spectral broadening of an amplified electro-optic frequency comb with a 25 GHz repetition rate in a highly nonlinear fiber and subsequently generate sub-picosecond pulses. The spectral phase of the comb is adaptively optimized by a Fourier pulse shaper in a closed control loop with the HNLF output spectrum as the process variable to be optimized. Enhanced spectral broadening also increases the stimulated Brillouin scattering threshold allowing increased power scaling and thereby boosting the bandwidth by a factor of more than 13 times over the initial comb. System versatility to varying conditions is demonstrated by achieving consistent bandwidth enhancement (nearly or more than 100 lines) in varying operating conditions that distort the temporal profile of the comb. In all cases, the optimization yields a near transform limited pulse that enters the nonlinear fiber. Sub-picosecond pulse generation is achieved with a short length of single mode fiber post the nonlinear fiber.

Microwave power induced resonance shifting of silicon ring modulators for continuously tunable, bandwidth scaled frequency combs.

We demonstrate a technique to continuously tune center frequency and repetition rate of optical frequency combs generated in silicon microring modulators and bandwidth scale them. We utilize a drive frequency dependent, microwave power induced shifting of the microring modulator resonance. In this work, we demonstrate center frequency tunability of frequency combs generated in silicon microring modulators over a wide range (∼8nm) with fixed number of lines. We also demonstrate continuously tunable repetition rates from 7.5GHz to 15GHz. Further, we use this effect to demonstrate a proof-of-principle experiment to bandwidth scale an 8-line (20dB band) comb generated from a single ring modulator driven at 10GHz to a comb with 12 and 15 lines by cascading two and three ring modulators, respectively. This is accomplished by merging widely spaced ring modulator resonances to a common location, thus coupling light simultaneously into multiple cascaded ring modulators.

Octave-spanning, continuous-wave supercontinuum generation with record power using standard telecom fibers pumped with power-combined fiber lasers: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 1172 (2020).OPLEDP0146-959210.1364/OL.384690.

Octave-spanning, continuous-wave supercontinuum generation with record power using standard telecom fibers pumped with power-combined fiber lasers.

We have demonstrated a record output power of ∼72W, octave-spanning, nearly single-mode, continuous-wave supercontinuum with a bandwidth of ∼1050nm using standard telecom fiber as the nonlinear medium in an all-fiber architecture. We have utilized the recently proposed nonlinear power combining architecture by which power scaling is achieved using multiple independent Ytterbium lasers operating at different wavelengths. In this Letter, Raman conversions in the fiber assist in combining multiple input laser lines into a single wavelength which then undergoes supercontinuum generation. The architecture is based on the recently proposed grating-free, cascaded Raman lasers based on distributed feedback. Here all Raman conversions are well seeded, thereby enhancing the efficiency of supercontinuum generation to ∼44%. In this Letter, we have obtained power spectral densities (PSDs) of >3mW/nm from 850 to 1350 nm and a high PSD of >100mW/nm from 1350 to 1900 nm. Here we have also investigated the power-combined supercontinuum generation for different pump wavelength combinations demonstrating the flexibility of this technique.

Optical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers.

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase modulated electro-optic 25 GHz frequency combs sourced from individual seed carriers offset by 100 GHz, to within 6.7 MHz of each other. We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb. We obtain $\sim 86 $∼86 lines in a 20 dB band spanning over 2 THz.

Visible light generation in the cladding of optical fibers carrying near-infrared continuous-wave lasers due to Cherenkov-phase matched harmonic conversion.

In this work, we report and analyze the cause of the surprising observation of visible light generation in the cladding of silica-based continuous-wave (CW), near-infrared fiber lasers. We observe a visible rainbow of hues in a cascaded Raman fiber laser, which we attribute to second and third harmonic conversion of the different wavelength components propagating in the core of the fiber. The light in the cladding of the fiber occurs through Cherenkov-type phase matching, and a mathematical analysis is presented to estimate the power of the harmonic light generated. We then extend this theory to visible light generation in other types of fiber lasers. Specifically, we analyze the case of a CW supercontinuum generated in standard telecom fibers, and verify our theoretical predictions with experimental results through visible spectra collected.

High-power, cascaded random Raman fiber laser with near complete conversion over wide wavelength and power tuning.

Cascaded Raman fiber lasers based on random distributed feedback (RDFB) are proven to be wavelength agile, enabling high powers outside rare-earth doped emission windows. In these systems, by simply adjusting the input pump power and wavelength, high-power lasers can be achieved at any wavelength within the transmission window of optical fibers. However, there are two primary limitations associated with these systems, which in turn limits further power scaling and applicability. Firstly, the degree of wavelength conversion or spectral purity (percentage of output power in the desired wavelength band) that can be achieved is limited. This is attributed to intensity noise transfer of input pump source to Raman Stokes orders, which causes incomplete power transfer reducing the spectral purity. Secondly, the output power range over which the high degree of wavelength conversion is maintained is limited. This is due to unwanted Raman conversion to the next Stokes order with increasing power. Here, we demonstrate a high-power, cascaded Raman fiber laser with near complete wavelength conversion over a wide wavelength and power range. We achieve this by culmination of two recent developments in this field. We utilize our recently proposed filtered feedback mechanism to terminate Raman conversion at arbitrary wavelengths, and we use the recently demonstrated technique (by J Dong and associates) of low-intensity noise pump sources (Fiber ASE sources) to achieve high-purity Raman conversion. Pump-limited output powers >34W and wavelength conversions >97% (highest till date) were achieved over a broad - 1.1µm to 1.5µm tuning range. In addition, high spectral purity (>90%) was maintained over a broad output power range (>15%), indicating the robustness of this laser against input power variations.

High-power, widely wavelength tunable, grating-free Raman fiber laser based on filtered feedback.

The cascaded Raman fiber laser is a proven technology that provides wavelength agile high-power fiber lasers outside the rare-earth emission windows. However, conventional cascaded Raman fiber lasers lack wavelength agility due to the use of fixed wavelength fiber Bragg gratings. Recently, proposed cascaded Raman fiber lasers based on random distributed feedback have provided a grating-free solution enabling wavelength agility. With these lasers, wide wavelength tunability has been achieved. However, there are still limitations in scaling output power while maintaining high spectral purity of wavelength conversion. Spectral purity is characterized by the in-band power ratio, which is the ratio of the output power in the required wavelength to the total power. The origin of this limitation arises from the inability to efficiently terminate the Raman cascade at a specific wavelength with increasing power. In this Letter, we propose a novel filtered distributed feedback mechanism to terminate the Raman cascade at any desired wavelength, enabling power scaling with high spectral purity. Output power up to 28 W has been achieved with >85% in-band power ratio and >400 nm tuning range from 1118 to 1535 nm.

High-power continuous-wave supercontinuum generation in highly nonlinear fibers pumped with high-order cascaded Raman fiber amplifiers.

A novel method for efficient generation of a high-power, equalized continuous-wave supercontinuum source in an all-conventional silica fiber architecture is demonstrated. Highly nonlinear fiber is pumped in its anomalous dispersion region using a novel, high-power, L-band laser. The L-band laser encompasses a sixth-order cascaded Raman amplifier which is pumped with a high-power Ytterbium-doped fiber laser and amplifies a low-power, tunable L-band seed source. The supercontinuum generated 35 W of power with ∼40% efficiency. The supercontinuum spectrum was measured to have a high degree of flatness of better than 5 dB over 400 nm of bandwidth (1.3-1.7 µm, limited by spectrum analyzer range) and a power spectral density in this region of >50 mW/nm. The extent of the SC spectrum is estimated to be up to 2 µm.

High power, high efficiency, continuous-wave supercontinuum generation using standard telecom fibers.

We demonstrate a simple module for octave spanning continuous-wave supercontinuum generation using standard telecom fiber. This module can accept any high power ytterbium-doped fiber laser as input. The input light is transferred into the anomalous dispersion region of the telecom fiber through a cascade of Raman shifts. A recently proposed Raman laser architecture with distributed feedback efficiently performs these Raman conversions. A spectrum spanning over 1000nm (>1 octave) from 880 to 1900nm is demonstrated. The average power from the supercontinuum is ~34W with a high conversion efficiency of 44%. Input wavelength agility is demonstrated with similar supercontinua over a wide input wavelength range.

Generation of tunable, high repetition rate optical frequency combs using on-chip silicon modulators.

We experimentally demonstrate tunable, highly-stable frequency combs with high repetition-rates using a single, charge injection based silicon PN modulator. In this work, we demonstrate combs in the C-band with over eight lines in a 20-dB bandwidth. We demonstrate continuous tuning of the center frequency in the C-band and tuning of the repetition-rate from 7.5GHz to 12.5GHz. We also demonstrate through simulations the potential for bandwidth scaling using an optimized silicon PIN modulator. We find that the time varying free carrier absorption due to carrier injection, an undesirable effect in data modulators, assists here in enhancing flatness in the generated combs.

High-power, fixed, and tunable wavelength, grating-free cascaded Raman fiber lasers.

Cascaded Raman lasers enable high powers at various wavelength bands inaccessible with conventional rare-earth-doped lasers. The input and output wavelengths of conventional implementations are fixed by the constituent fiber gratings necessary for cascaded Raman conversion. We demonstrate here a simple architecture for high-power, fixed, and wavelength tunable, grating-free, cascaded Raman conversion between different wavelength bands. The architecture is based on the recently proposed distributed feedback Raman lasers. Here, we implement a module which converts the ytterbium band to the eye-safe 1.5 µm region. We demonstrate pump-limited output powers of over 30 W in fixed and continuously wavelength tunable configurations.

All passive architecture for high efficiency cascaded Raman conversion.

Cascaded Raman fiber lasers have offered a convenient method to obtain scalable, high-power sources at various wavelength regions inaccessible with rare-earth doped fiber lasers. A limitation previously was the reduced efficiency of these lasers. Recently, new architectures have been proposed to enhance efficiency, but this came at the cost of enhanced complexity, requiring an additional low-power, cascaded Raman laser. In this work, we overcome this with a new, all-passive architecture for high-efficiency cascaded Raman conversion. We demonstrate our architecture with a fifth-order cascaded Raman converter from 1117nm to 1480nm with output power of ~64W and efficiency of 60%.

Simultaneous Raman based power combining and wavelength conversion of high-power fiber lasers.

We present a technique for simultaneous power-combining and wavelength-conversion of multiple fiber lasers into a single, longer wavelength in a different band through Raman-based, nonlinear power combining. We illustrate this by power combining of two independent Ytterbium lasers into a single wavelength around 1.5micron with high output powers of upto 99W. A high conversion efficiency of ~64% of the quantum limited efficiency and a high level of wavelength conversion with >85% of the output power in the final wavelength is demonstrated. The proposed method enables power-scaling in various wavelength bands where conventional fiber lasers are unavailable or limited in power.

Power scaling of high-efficiency 1.5 µm cascaded Raman fiber lasers.

High-power fiber lasers operating at the 1.5 µm wavelength region have attractive features, such as eye safety and atmospheric transparency, and cascaded Raman fiber lasers offer a convenient method to obtain high-power sources at these wavelengths. A limitation to power scaling, however, has been the lower conversion efficiency of these lasers. We recently introduced a high-efficiency architecture for high-power cascaded Raman fiber lasers applicable for 1.5 µm fiber lasers. Here we demonstrate further power scaling using this new architecture. Using numerical simulations, we identify the ideal operating conditions for the new architecture. We demonstrate a high-efficiency 1480 nm cascaded Raman fiber laser with an output power of 301 W, comparable to record power levels achieved with rare-earth-doped fiber lasers in the 1.5 µm wavelength region.

A high efficiency architecture for cascaded Raman fiber lasers.

We demonstrate a new high efficiency architecture for cascaded Raman fiber lasers based on a single pass cascaded amplifier configuration. Conversion is seeded at all intermediate Stokes wavelengths using a multi-wavelength seed source. A lower power Raman laser based on the conventional cascaded Raman resonator architecture provides a convenient seed source providing all the necessary wavelengths simultaneously. In this work we demonstrate a 1480nm laser pumped by an 1117nm Yb-doped fiber laser with maximum output power of 204W and conversion efficiency of 65% (quantum-limited efficiency is ~75%). We believe both the output power and conversion efficiency (relative to quantum-limited efficiency) are the highest reported for cascaded Raman fiber lasers.

Stimulated Brillouin scattering thresholds in optical fibers for lasers linewidth broadened with noise.

Phase and/or intensity modulation techniques to broaden the Linewidth of an optical source are well known methods to suppress stimulated Brillouin scattering (SBS) in optical fibers. A common technique used to achieve significant bandwidth enhancement in a simple fashion is to phase modulate with a filtered noise source. We will demonstrate here that, in this case the stochastic nature of noise requires an inclusion of length dependent corrections to the SBS threshold enhancement. This effect becomes particularly significant for short fiber lengths common to most high power fiber amplifiers.

Fully programmable two-dimensional pulse shaper for broadband line-by-line amplitude and phase control.

We introduce a fully programmable two-dimensional (2D) pulse shaper, able to simultaneously control the amplitude and phase of very fine spectral components over a broad bandwidth. This is achieved by aligning two types of spectral dispersers in a cross dispersion setup: a virtually imaged phased array for accessing fine resolution and a transmission grating for achieving broad bandwidth. We take advantage of the resultant 2D dispersion profile as well as introduce programmability by adding a 2D liquid crystal on silicon spatial light modulator at the masking plane. Our shaper has a resolution of ~3 GHz operating over the entire 'C' band of >5.8 THz. Experimental evidence is provided that highlights the full programmability, fine spectral control, and broad bandwidth operation (limited currently by the bandwidth of the input light). We also show line-by-line manipulation of record 836 comb lines over the C-band.