Continuous-wave Raman Fiber Laser Research Articles

1.7-$\mu$m Tm/Ho-Codoped All-Fiber Pulsed Laser Based on Intermode-Beating Modulation Technique

A 1.7- $\mu$ m pulsed laser is of tremendous interest for biomedical imaging applications due to its low scattering loss in bio-tissues, but is relatively difficult to generate in a fiber laser. In this paper, we propose and demonstrate a compact 1.7- $\mu$ m Tm/Ho-codoped fiber pulsed laser based on a novel intermode-beating modulation (IM) technique. A 1211-nm continuous-wave (CW) Raman fiber laser is especially homemade to pump a $\sim$ 20-cm short-length Tm/Ho fiber for obtaining $\sim$ 1.7- $\mu$ m optical gain. A pair of fiber end-facet mirrors with 1.7–1.8- $\mu$ m reflective band is used to form the all-fiber linear cavity. 1.76- or 1.78- $\mu$ m lasing is successfully achieved, and surprisingly exhibits stable self-gain-switching operation with $\sim$ 1- $\mu$ s pulse duration. Different from the conventional gain-switching requiring pulsed pumping, herein the gain modulation in the 1.7- $\mu$ m laser is induced by strong intermode-beating of the CW pump source. The pulse repetition rate with an increasing pump power is fixed at 145 kHz, which exactly coincides with the longitudinal-mode beat frequency of the 1211-nm pump laser. Our results may provide a new type of optical modulation (i.e., intermode-beating modulation technique) for a low-cost, compact pulsed laser.

Intensity statistics in a long random fiber Raman laser.

We study the output emission statistics of a random continuous-wave Raman fiber laser. The signal evolution is governed by a generalized nonlinear Schrödinger equation (NLSE) with inserted gain. The statistics are close to the Rayleigh one, and the deviations are caused by the Kerr nonlinearity. To characterize the deviations, we analytically find the mean of the squared output signal intensity, based on the kinetic theory. We show qualitative agreement with available experimental data and supplement the results with numerical calculations. With the limit of small gain, the kinetic theory gives a finite answer for the mean of squared intensity in the first and the second order with respect to small nonlinearity. The result is consistent with the fact that the NLSE is integrable in the case of zero gain and is applicable to any generalized NLSE if the inserted terms are effectively small.

Amplification of picosecond pulses and gigahertz signals in bismuth-doped fiber amplifiers

We demonstrate the capability for amplification of picosecond pulses in two bismuth-doped alumosilicate fibers. A spectrally filtered supercontinuum source is used to provide a train of picosecond pulses at discrete wavelengths within the gain bandwidth of bismuth fiber amplifiers. With a 30 m length of active fiber, a small signal gain at 1160 nm of over 20 dB is observed. In addition, we assess the viability of amplification of high repetition rate signals in such amplifiers, applying a 10 GHz modulation to a continuous wave Raman fiber laser operating at 1178 nm, finding that such signals are amplified without noticeable distortion.

Nelder-Mead simplex method for modeling of cascaded continuous-wave multiple-Stokes Raman fiber lasers

We propose and demonstrate an effective and computation- ally compact Nelder-Mead simplex method for the design and modeling of cw cascaded Raman fiber lasers. The Nelder-Mead method is efficient for finding a local minimum of a function of several variables. We employ this classical powerful local descent algorithm to solve the multidimen- sional problem for the modeling of n'th-order cascaded Raman fiber la- sers. With our proposed method, we investigate a linear cascaded Ra- man fiber laser with a pump wavelength of 1064 nm. The convergence of the proposed method solving the rate equations with boundary condi- tions is easily and correctly achieved. Our simulation results verify that the proposed method has good computational speed without losing simulation accuracy. © 2010 Society of Photo-Optical Instrumentation Engineers.

150 W highly-efficient Raman fiber laser

We report a more than 150 W spectrally-clean continuous wave Raman fiber laser at 1120 nm with an optical efficiency of 85%. A approximately 30 m standard single mode silica fiber is used as Raman gain fiber to avoid second Stokes emission. A spectrally asymmetric resonator (in the sense of mirror reflection bandwidth) with usual fiber Bragg gratings is designed to minimize the laser power lost into the unwanted direction, even when the effective reflectivity of the rear fiber Bragg grating becomes as low as 81.5%.

Observation of high-contrast, fast intensity noise of a continuous wave Raman fiber laser

We investigate the intensity noise of a continuous-wave Raman fiber laser based on a novel technique where the noise is sampled with a low repetition rate signal in a low walk-off Raman amplifier configuration. With this method, we experimentally demonstrate that continuous-wave Raman fiber lasers exhibit high contrast intensity fluctuations on a timescale of several tens to hundreds of GHz. In fact, the power of the Raman laser fluctuates between nearly zero and a high multiple of the average power. Numerical simulations are used to study the role of pump depletion on the noise transfer process. Our study validates models of pumps used in some recent continuous wave supercontinuum generation simulations.

Use of a continuous wave Raman fiber laser in graded-index multimode fiber for SRS beam combination

We report using a Raman fiber laser (RFL) based on a multimode graded-index fiber as a novel method for beam combination of two continuous wave pump beams. Due to stimulated Raman scattering, the RFL generates a Stokes beam which can be up to 300% brighter than the pump beams. Up to 5.8 W of Stokes power is generated with an optical conversion efficiency of 56%.

Reply to comment on "Theoretical analysis of nth-order cascaded continuous-wave Raman fiber lasers II Optimization and design rules"

The comments by Liu and Zhou [J. Opt. Soc. Am. B24, 28-29] about our theoretical analysis of nth-order cascaded Raman fiber laser [J. Opt. Soc. Am. B22, 772 (2005)] are acknowledged to be correct. In particular, some approximations used in the design rules, such as the depleted-pump approximation, may fail for low numbers of Stokes waves.

Comment on "Theoretical analysis of nth-order cascaded continuous-wave Raman fiber lasers II Optimization and design rules"

Recently, a model of theoretical analysis for nth-order cascaded Raman fiber laser was proposed by Burgoyne et al. [J. Opt. Soc. Am. B22, 764 (2005)], and it is very useful for designing a cascaded Raman fiber laser with a high efficiency. Nevertheless, we deduced all of the equations of the paper and found several mistakes in it. We would like to correct them in order so that one can properly design a cascaded Raman fiber laser using the equations in the papers.

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

We describe the effect of temperature variations on supercontinuum (SC) generation in optical fibers using a continuous-wave (CW) Raman fiber laser as a pump. We achieve supercontinuum generation by pumping only ∼2W of power into a 7km-long nonzero dispersion-shifted fiber (NZDSF) in the region of small anomalous dispersion. In these conditions, the supercontinuum builds up basically on modulational instability and Raman. At room temperature, the supercontinuum covers effectively the S, C and L transmission bands defined by the International Telecommunication Union (ITU). Temperature tuning of the fiber environment provides a means of tuning the fiber dispersion, and thus a means of changing the width and shape of the supercontinuum spectrum. We demonstrate a 27% increase in the 10-dB SC width. We believe that the application of this new tuning mechanism to other experimental configurations using pulsed sources might be used to produce extremely broad supercontinuums.

Effect of cavity structure on the spectrum of the supercontinuum generated in a linear cavity pumped by a continuous-wave Raman fiber laser

With a 10-km single-mode fiber and a pair of fiber Bragg gratings (FBGs), we design four different linear cavity structures for supercontinuum (SC) generation using a cw Raman fiber laser (RFL) as pump. The output characteristics of the generated SC in these different constructs are compared when the RFL reaches its maximum output power. The broadest SC spectrum of 100 nm and the highest output power of 1.1 W are obtained when the nested cavity structure is employed. The influences of the feedback FBGs and cavity constructs on the SC generation are analyzed by considering the Raman effect and four-wave mixing.

Theoretical analysis of nth-order cascaded continuous-wave Raman fiber lasers II Optimization and design rules

Using the analytical model we developed for an nth-order cascaded Raman laser, we find the design rules for such lasers. We determine analytical expressions for the cavity length and the output-coupler reflectivity that maximize the output power and minimize threshold power. Simple expressions are obtained in the depleted-pump approximation. Deviations from these expressions when the pump is not completely depleted are shown to be different depending on the parity of the number n of Stokes shifts (cascades). The design rules show that the mirror reflectivity is a critical factor in the laser quality and that the ultimate slope efficiency epsilon is found to be g_n/g_0. We also find a condition to determine if P-doped fibers are more useful than Ge-doped fibers in Raman fiber lasers based only on the Raman shift and absorption of the fibers.

Theoretical analysis of nth-order cascaded continuous-wave Raman fiber lasers I Model and resolution

We develop an approximate analytical solution to the steady-state equations that describe a nested fiber, Raman laser cavity that incorporates Bragg reflectors. From this solution the output power, the threshold power, and the efficiency are found. Numerical simulation of a six-step cascade shows that the approximate analytical solution is accurate within 1.5%. It appears that the laser behaves differently depending on whether it has an even or odd number of Stokes waves.

High-power supercontinuum generation in highly nonlinear, dispersion-shifted fibers by use of a continuous-wave Raman fiber laser.

High-power supercontinua are demonstrated in highly nonlinear, dispersion-shifted fibers with a continuous-wave Raman fiber laser. Supercontinuum growth is experimentally studied under different combinations of fiber length and launch power to show output powers as high as 3.2 W and bandwidths greater than 544 nm. Modulation instability (MI) is observed to seed spectral broadening at low launch powers, and the interplay between MI and stimulated Raman scattering plays an important role in the growth of the continuum at high launch powers. The effect on continuum generation of parametric four-wave mixing coupled with the higher-order dispersion properties of the fiber is investigated.

Gain-flattening of fiber Raman amplifiers using non-linear pump spectral broadening

We demonstrate experimentally that gain flattening of single-pump fiber Raman amplifiers is possible by use of non-linear broadening of the pump spectrum. In our experiments, pump broadening of a continuous-wave Raman fiber laser tuned at 1455 nm is achieved by propagation in a non-zero dispersion-shifted fiber with small anomalous dispersion at the pump wavelength. The broadened pump spectrum can reach a full width at half maximum of as much as 28 nm. Our results demonstrate that an acceptable flatness of the Raman gain spectrum can be obtained in the 1560–1640 nm region while avoiding pump-to pump non-linear interactions. On the dark side, our measurements show that this configuration introduces more noise than the conventional, non-broadened pump configuration since a non-negligible fraction of the broadened pump power falls within the band of amplification. However, we believe that further work and optimization should demonstrate that gain-flattened fiber Raman amplifiers using a single pump can be achieved over the C (1525–1565 nm) and L (1565–1625 nm) bands.

Supercontinuum generation using a continuous-wave Raman fiber laser

A supercontinuum, nearly flat spectrum covering the S, C and L transmission bands defined by the International Telecommunication Union is obtained by pumping a conventional nonzero dispersion-shifted fiber with a continuous-wave Raman fiber laser tuned to the region of small anomalous dispersion. By using a long fiber with low loss, complete power transfer from the pump to the supercontinuum is reached. We demonstrate that this supercontinuum can be used to perform accurate, long-range (>200 km) measurements of polarization mode dispersion in fibers.

Supercontinuum generation using Raman fiber laser

Using a phosphosilicate-doped continuous-wave Raman fiber laser, a 350-m single-mode fiber, and a 50% feedback fiber Bragg grating at 1483.4 nm, we obtained a high-efficiency ultra-broadband (1434–1527 nm) supercontinuum (SC) centered at 1483.4 nm with an average output power of 2.1 W, a spectral intensity of 22.3 mW nm-1, and a nonlinear conversion efficiency of 94%. Increasing the fiber length in the Raman cavity to 1840 m gave simultaneous SC centered at 1483.4 nm and 1239 nm. An average power of 2.3 W in the bandwidth of 1434–1544 nm and 0.2 W in the bandwidth of 1224–1257 nm is obtained. The temperature dependence of the SC was also investigated.

Pulsed and continuous-wave supercontinuum generation in highly nonlinear, dispersion-shifted fibers

Supercontinuum generation in a highly nonlinear, dispersion-shifted fiber at 1550 nm is discussed. Spectrum generation under both pulsed and continuous-wave conditions is considered. With a few meters of highly nonlinear, dispersion-shifted fiber and a femtosecond erbium fiber laser, an octave-spanning supercontinuum is demonstrated. Kilometer lengths of nonlinear fiber pumped by a continuous-wave Raman fiber laser are shown to generate a continuum with a bandwidth greater than 247 nm. A nonlinear Schrodinger-equation model is used to investigate the effect of varying the dispersion on the pulsed continuum and noise effects in the continuous-wave continuum.

Output Characteristics of High-Power Continuous Wave Raman Fiber Laser at 1484 nm Using Phosphosilicate Fiber

A continuous wave (CW) high-power Raman fiber laser (RFL) with maximum output power of 2.24 W and slope efficiency of 32.8% at 1484 nm is obtained using a CW 8.4 W/ 1064 nm Yb-doped double-clad fiber laser as a pump, 700 m phosphosilicate fiber, and cascaded cavities with two pairs of fiber Bragg grating mirrors for the first and the second Stokes orders. Theoretical simulation of the RFL using a very efficient and rapidly converging collocation method is done to understand and optimize the fiber length to obtain maximum second Stokes power. RFL performance for the 300, 700 and 1150 m phosphosilicate fiber lengths was experimentally investigated by observing maximum output power, slope efficiency, threshold power, and full-width at half maximum at the second Stokes wavelength.

Simultaneous Double-Color Continuous Wave Raman Fiber Laser at 1239 nm and 1484 nm Using Phosphosilicate Fiber

Simultaneous double-color high-power continuous wave (CW) Raman fiber laser at 1239 nm and 1484 nm is demonstrated which uses CW 8.4 W Yb-doped double-clad fiber laser at 1064 nm as a pump, 1 km phosphosilicate fiber, and cascaded cavities consisting of two pairs of fiber Bragg grating (FBG) mirrors. Maximum output powers are 0.65 W at 1239 nm and 0.97 W at 1484 nm with a 50%/50% output mirror reflectivity pair, and 0.37 W at 1239 nm and 1.06 W at 1484 nm with a 75.5%/50% pair. The output characteristics of this laser for different FBG mirror reflectivities are reported.